Iana S Sinopalnikova

Tatyana A Peganova

Natalia V Belkova

Eric Deydier

Jean-Claude Daran

Elena S Shubina

Alexander M Kalsin

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

J.-C Daran

A N Nesmeyanov

Ruthenium p-cymene iminophosphonamide complexes: activation under basic conditions and transfer hydrogenation catalysis

), or with NaBHEt3 in THF. Partial conversion also occurs in iPrOH in the absence of base. 2 is stabilized by dihydrogen bonding with isopropanol, but attempts to isolate it induce isomerization by hydride migration to a ring CH position to yield a 16electron cyclohexadienyl derivative [{η 5 -p-C6H5(Me)(iPr)}Ru(NPN)], which has been crystallographically characterized as a disordered mixture of two regioisomers (4/4'). Complex 2 is able to release H2 upon treatment with medium strength proton donors (fluorinated alcohols), but also slowly with iPrOH. 2 is an active catalyst for the transfer hydrogenation of acetophenone to phenylethanol in isopropanol. The catalytic transformation is first order in acetophenone and first order in catalyst, with k = 117±10 M -1 h -1 at 40°C. The temperature dependence of the rate constant (25-80°C) gave the activation parameters ΔH ‡ = 9.6±1.3 kcal mol -1 and ΔS ‡ = -31±4 cal mol -1 K -1 . DFT calculations have validated the slow isomerization of 2 to 4/4' (high energy TS), the preference of the cyclohexadienyl system for 4/4' relative to the other isomers 4 Me and 4 iPr , where the hydride has migrated to the CMe or CiPr position, and suggest that the hydrogen transfer mechanism involves outer sphere hydride transfer to the ketone substrate with H-bonding assistance of isopropanol to yield a σ-complex intermediate [(η 6 -Cym)Ru + (NPN){H-C(Me)(Ph)O -}].

Introduction

During the last two decades catalytic transfer hydrogenation (TH) of ketones has become a powerful method for the production of chiral secondary alcohols featuring high chemo-and enantioselectivity under mild conditions. A number of highly efficient ruthenium-based catalysts have been developed to date, particularly [(η 6 -arene)RuCl(H2NCHPhCHPhNTs)] [1] by Noyori and Ikariya (A), cyclometallated complexes with κ 2 -PC and κ 3 -CNN ligands cis-[RuCl2(o-CH2C6H4PPh2)(aminomethylpyridine)] [2] (B) and [RuCl(PP)(CNN)] [3] (C) by Baratta, and zwitterionic complexes [(η 6 -arene)RuCl(κ 2 -P,N)] prepared from 1,2-substituted indenide with donor P,N groups [4] by Stradiotto (D). The mechanism of ketone transfer hydrogenation catalyzed by Noyori complexes has been scrutinized to discover that the origin of the high enantioselectivity is the ancillary NH-ligand assistance in the hydrogenation step, giving a highly ordered transition state. [5] The high enantioselectivities demonstrated by cyclometalated CNN ruthenium complexes were also attributed to the operation of NH bifunctional catalysis, although an alternative pathway via an alkoxide active species could not be excluded. [2][3] These three catalysts (A, B and C) involve the corresponding mono-hydrides Ru-H as the active species in the catalytic cycle. In sharp contrast, the catalysis by the zwitterionic P,N-indenide complex, which is unable to build ancillary Ru-NH interactions, proceeds via an undetermined mechanism, while the corresponding Ru-H species is completely inactive and its formation represents the catalyst deactivation pathway. [4] In this regard, we were interested in employing another type of zwitterionic ruthenium complex, having an iminophosphonamide (NPN) [6] ligand (E), for the reduction of ketones under transfer hydrogenation conditions. The only other ruthenium iminophosphonamide complex known before our studies, [(η 6 -Cym)Ru{(NH i Pr)2PPh(NH i Pr)}](BPh4) (Cym = pcymene, C6H4-1-i-Pr-4-Me), was reported to be inactive in ketone transfer hydrogenation. [7] This is not so surprising, as this 16ē complex is not able to add ligands except for strong π-acceptors like CO and CN -. We have shown that the 16ē ruthenium iminophosphonamides complexes are very stable species due to strong σ,π-donating ability of the NPN ligand. [8] At the same time, the high negative charges located at the nitrogen atoms of the NPN ligand render the corresponding 18ē ruthenium iminophosphonamides coordinatively labile and reactive, as we demonstrated by the facile insertion of the CO into the Ru-N bond in the [(C6Me6)Ru(CO){(NMe)2PPh2}](PF6) complex. [9] Hence, the 18ē ruthenium iminophosphonamide complexes, being isoelectronic to Noyori's catalysts, are promising for the catalyzed TH of ketones. The solution of 2 is also contaminated by very small amounts of the phosphine oxide Ph2P(O)NH(p-Tol) (NPO, δ 16.9) and the free protonated NPN ligand (p-Tol)NPPh2NH(p-Tol) (NPNH) (δ -5.5, cf. -3.5 [8] in CDCl3). Aminophosphine oxides Ph2P(O)NHAr have been shown to result from hydrolytic degradation of diaminophosphonium salts, Ph2P(NHAr)2 + , and iminophosphonamines, Ph2P(NAr)(NHAr); the related compound Ph2P(O)NH (2,6-C6H3Me2) was shown to exhibit a [START_REF] Dolg | [END_REF] P NMR resonance at δ 20.8. [10] Compound 2 slowly decomposes at room temperature (see Figure S3 for a longer timescale monitoring) and even faster upon warming. This degradation leads to the increase of the NPO and NPNH amounts, to the generation of two isomers of 2 with resonances at δ 50.6 (4) and 51.1 (4'), and two additional species characterized by [START_REF] Dolg | [END_REF] P{ 1 H} resonances at δ 27.4 and 26.2 (5 and 5'), which slowly accumulate within days (Figure S3) or upon attempt to isolate 2 (Figure S4). In 4 and 4', the hydride has migrated to the cymene ring to generate a cyclohexadienyl ligand, [{η 5 -C6H5(Me)(iPr)}Ru(NPN)]; further details on these two compounds will be shown below. Species 5 and 5' are asymmetric hydride complexes (see more details in the SI and further discussion below). The same treatment of 1 with iPrOH in C6D6 in the presence of a weaker base (DBU) led only to a slower and partial generation of 2 (60% yield in 30 minutes at room temperature), which decomposes upon heating for 2 h at 60°C to form mainly NPNH (δ -5.3) and NPO (δ 17.9). A minor amount of the same intermediate observed upon addition of NaHMDS (3, δ 38.4) is also formed (see [START_REF] Dolg | [END_REF] P{ 1 H} monitoring in Figure S5), but both 2 and 3 further decrease and nearly disappear after prolonged heating.

Interestingly, a similar trend is also observed upon heating the precursor complex 1 in the presence of iPrOH and in the absence of base. In neat iPrOH, long-term heating at 80°C leads to complete dissolution with generation of a new species characterized by a 31 P{ 1 H} resonance at δ 32.9, attributed to the 5 5'

4' 4

hydride compound 2 (Figure S6). The shift relative to the resonance observed in C6D6 is caused by H-bonding with iPrOH (vide infra). The spectrum also contains resonances of residual 1, phosphine oxide NPO (δ 19.2) and free protonated ligand NPNH (broad resonance at 4.0). Treatment with smaller amounts of iPrOH in C6D6 and warming to 60°C equally led to the slow generation of 2 (δ 33.0) and the same decomposition products NPO (δ 17.9) and NPNH (initially sharp at δ -5.3 and later broader and slightly downfield shifted to -3.9), see Figure S7. The chemical shift variability and width of the NPNH resonance indicate the presence of proton exchange equilibria (the 31 P NMR resonance of the protonation product [Ph2P(NH-p-C6H4Me)2]Br is reported at δ 28.0 [10] ). The 1 H NMR spectrum shows concomitantly the presence of the hydride resonance of 1 at δ -3.1, which is however evident only initially while the NPNH resonance is small and sharp (see below for a rationalization). The 31 P NMR spectrum also shows the growth of an additional sharp resonance at δ 40.4, which seems attributable to the isopropoxide complex 3 (cf. δ 38.5 in Figure 1). It would thus appear that 3, generated from 1 and iPrO -in a strongly basic medium, quantitatively releases acetone (observed by NMR) to yield 2 at room temperature, whereas 2 can react with iPrOH upon warming in the absence of base to regenerate 3 (and presumably release H2). The latter proposition is further suggested by the results of similar experiments with more acidic alcohols (vide infra).

Additional NMR spectra have been recorded for compound 2 in the presence of variable amounts of iPrOH and of the stronger proton donor trifluoroethanol (TFE), see Table 1. The results show quite clearly that the hydride resonance is affected by the establishment of a "dihydrogen bond", [11] namely an H-bond between the alcohol as a proton donor (HA) and the hydride ligand of 2 as a proton acceptor, [(η 6 -Cym)Ru(NPN)H•••HA], resulting in greater upfield shifts when the proton donor is either stronger (cf. entries 3 and 6) or present in greater amounts (entries 2-5). As expected, this effect is more notable on the 1 H hydride resonance and much less on the 31 P resonance. Formation of 2•iPrOH can be considered as a prerequisite for the observed H2 evolution from 2 and iPrOH, to yield 3. [11b] The IR spectrum of a C6D6 solution of the hydride complex 2, obtained as described above with NaHMDS/iPrOH, shows a well resolved ν(RuH) band at 1882 cm -1 and the bands of residual iPrOH (ν(OH) = 3604 cm -1 ) as well as of acetone (ν(CO) = 1716 cm -1 ). Upon addition of a nearly equimolar amount of acetophenone the ν(RuH) band shifts to 1887 cm -1 . This is probably the result of dihydrogen bond disruption caused by the formation of the stronger and competing PhMeCO•••HOiPr hydrogen bond (Figure S8).

The presence of acetone has no effect on the 1 H and 31 P resonances of 2 when added in small quantities. A large excess (Table 1, entry 8), on the other hand, leads to displacement of the 31 P resonance to lower fields and to a strong upfield shift of the hydride 1 H resonance. This phenomenon can be rationalized with the establishment of a hydride transfer equilibrium, probably involving an 18ē σ-complex [(p-Cym)Ru + (NPN)•••HCMe2O -] (6). Such a process should lead to increased shielding for the hydride ligand, similar to dihydrogen bond. [12] Complex 6 is a presumed intermediate on the way to the isopropoxide complex 3. A rapid equilibrium between 2 and 3 is excluded because both compounds are independently observed under other conditions (e.g. Figure 1, Figure S5 and Figure S7), thus their interchange is slow on the NMR timescale. [a] For all entries except 6, the compound was generated in situ in C6D6 from 1, NaHMDS and iPrOH as described in the experimental section. [b] Instrument lock with a C6D6 capillary. [c] Experiment run starting from a solution of 4/4' (vide infra).

On the basis of all the above evidence, the evolution of 1 in isopropanol can be summarized as follows (Scheme 2). In the absence of base at elevated temperatures, isopropanol reversibly substitutes a chloride ligand in 1, presumably following selfdissociation via the 16ē complex [7]Cl and the 18ē isopropanol adduct 8, the latter two compounds being probably present in small and unobservable equilibrium amounts. The generation of these two ionic intermediates should be facilitated by the polarity of isopropanol. The involvement of the 16ē [7]Cl complex as intermediate in the dissociative chloride-exchange process in polar solvents has previously been established and a salt of [7] + with the non-coordinating PF6 -anion has been fully characterized. [9] Formation of the observed hydride product 2 requires deprotonation of 8 to generate 3 followed by β-H elimination from the isopropoxide ligand. When an external base is not present, generation of a base may be imagined by decomposition of 2 via slow alcoholysis of one Ru-N bond and generation of the observed asymmetric hydride intermediates 5 and 5', followed by NPNH decoordination, presumably with solvent assistance, to yield a putative solvated isopropoxide complex 9. This product has not been directly detected, but we note that related [(arene)Ru(μ-OR)3Ru(arene)] + complexes have been described in the literature. [13] Although we have shown earlier that even weak bases such as Et2NH can deprotonate NPNH during the synthesis of 1, [8][9] this acid-base equilibrium is certainly solvent-dependent and the reverse process of NPN ligand decoordination from the ruthenium atom may become favorable in polar protic solvents. The formation of 3 is accompanied by the generation of the diaminophosphonium salt [Ph2P(NH-p-Tol)2] + Cl -(NPNH2 + Cl -), which can undergo rapid and Scheme 2. Proposed mechanism for the generation of hydride 2 from 1, and related chemical processes. degenerate proton transfer with NPNH, causing a broadening and a downfield shift for the [START_REF] Dolg | [END_REF] P NMR resonance with time (Figures S6 andS7). As stated above, the related [Ph2P(NH-p-Tol)2] + Br -salt exhibits a [START_REF] Dolg | [END_REF] P NMR resonance at δ 28.0 in CDCl3, [10] downfield shifted from that of NPNH. The observation of NPO may be rationalized through further degradation of NPNH and/or NPNH2 + by adventitious water with concomitant generation of p-TolNH2. [10] A base (B) is required to make the generation of isopropoxide 3 from 1 quantitative and rapid at room temperature. In this case, however, a direct associative exchange of Cl -with OiPr -seems more likely. [9] The β-H elimination in 3 to yield hydride 2 appears rather facile. However, compound 3 does not contain the open coordination site needed for a concerted intramolecular transformation to 2, while partial decoordination of the bidentate NPN and cymene ligands appears unlikely. We propose that this transformation follows the pathway of ionic dissociation followed by hydride transfer from the intermediate σ-complex 6. Additional evidence in support of this statement will be presented further below. This step thus appears competent to promote catalytic hydrogen transfer. Complex 2 appears relatively stable at room temperature in neat iPrOH and also in aromatic hydrocarbons (C6D6, toluene), provided iPrOH is present. Standing in benzene or toluene solutions without a sufficient amount of protecting alcohol induced isomerization to 4 and 4', as already shown in Figure 1 and in Figure S3. This conversion has further been investigated as follows. Generation of 2 from 1 and NaHMDS in neat iPrOH, followed by solvent removal and extensive drying, then extraction into ether and filtration to remove NaCl, further drying and 31 P NMR analysis of the residue in C6D6, revealed greater relative intensities for the two resonances at δ 50.6 and 51.1 of 4 and 4', relative to Figure 1a (see Figure 2a). When the same reaction was carried out in a toluene/iPrOH mixture and worked up in the same way, the corresponding spectrum showed 4 and 4' in even greater proportions (roughly 40% of the total, Figure 2b). Furthermore, generation of 2 by an alternative strategy (1 and NaBHEt3 in neat aromatic solvent, without any iPrOH), led again to a mixture of 2, 4 and 4' where the former was in minor proportion (Figure 2c). It can be noted that the 4:4' ratio varies depending on conditions. Compound 4 is initially generated in greater relative proportions (kinetic product) but later equilibrates with the other isomer 4' (vide infra).

Single crystals of a disordered mixture of 4 and 4' were obtained directly from neat iPrOH, as shown by the X-ray structural determination. An ORTEP view of the molecule is shown in Figure 3. The asymmetric unit contains two complete and independent molecules with the same ring conformation and very similar metric parameters. The crystal used for the structural analysis was rather poor, but the structure was of sufficient quality to establish the geometry and the metric parameters of the Ru(NPN) part. In spite of several subsequent crystallization attempts, better crystals could not be obtained. The more problematic part of the structure is the cyclohexadienyl ring, which needed to be restrained at the anisotropic level during refinement in order to be well-behaved. The most relevant distances and angles are reported in Table 2 and the full list is available in the SI (Table S1). Because of disorder in the ring, the ring centroid was defined as the center of gravity of all six C atoms and the parameters related to this centroid should not be considered reliable. The coordination geometry is a typical 2-legged piano stool with the cyclohexadienyl ring approximately orthogonal to the RuN2P plane. This is isoelectronic with the cation [7] + in the [7]PF6 salt, [9] and indeed the structural features of the two compounds (compared in Table 2) are rather similar. Interestingly, whereas the Cambridge Crystallographic database contains >2300 X-ray structures of (η 6 -p-cymene) complexes of transition metals, of which nearly all (>2100) are ruthenium complexes, this 4/4' mixture is apparently the first reported structure, for any transition metal, with a cyclohexadienyl ligand derived from p-cymene. It is also only the second reported structure of a cyclohexadienyl derivative of Ru II with a 16-electron configuration, the only other precedent being a compound where the cyclohexadienyl ligand is para-disubstituted by CF3 and by the β-diketiminate -CH(CMe=NAr)2 group (Ar = 2,6-C6H3 i Pr2), in which the bidentate anionic ligand is the same β-diketiminato (nacnac) ligand. [14] An interesting point of comparison between these two structures is that both have a para-disubstituted cyclohexadienyl ligand but, whereas 4 and 4' have the two substituents in the 1 and 4 positions, namely the hydride has added to a CH position and not to a CR (R = Me or iPr) position of the p-cymene ring, the previously published structure features these substituents in the 3 and 6 position, the sterically more encumbering β-diketiminate group occupying the sterically less congested 6 position (sp 3 C atom) anti relative to Ru. This difference may be related to the synthetic procedure, which consisted of external attack of the PhCF3 ligand by the β-diketiminate anion for the previously published compound, [14] [a] From ref. [9]. [b] The values pertaining to the two crystallographically independent molecules are shown in different columns. [c] Centroid = center of gravity of the six arene C atoms in the (η 6 -arene) and TS structures and in the experimental structure of 4; center of gravity of the five Ru-bonded C atoms in the DFToptimized (η 5 -cycloexadienyl) structures.

The structures of 4 and 4' in solution were confirmed by [START_REF] Dolg | [END_REF] P NMR, 1 H NMR, 2D COSY, 13 С, 2D H-P HMQC and 2D H-C HMQC spectra. Notably, only one set of signals for the NPN-moiety is found for each isomer in the 1 Н NMR spectrum while the cyclohexadienyl ligand signals differ for 4 and 4' (see details in the SI). In addition, [START_REF] Dolg | [END_REF] P EXSY NMR experiments carried out in C6D6 and toluene-d8 in the temperature range 25-80°C did not reveal any cross peak between 2, 4 and 4', showing that these isomeric compounds interconvert slowly. The thermal treatment also resulted in the appearance of a third peak in the same region of the cyclohexadienyl isomer, at δ 51.6, It appears reasonable to attribute this peak to one of the other possible isomers, 4 Me or 4 iPr and the DFT calculations (see below) suggest that this is most likely 4 iPr . The interconversion between these isomers was further monitored by [START_REF] Dolg | [END_REF] P{ 1 H} NMR in toluene-d8 at 80°C (see Figure 4). The initial solution, prepared from 1 and NaHMDS in neat iPrOH at room temperature as described above, shows a mixture of 2 (33%), the two cyclohexadienyl isomers 4 and 4' at δ 50.6 and 51.1 (42 and 21%, the δ 50.6 isomer being dominant), and a minor proportion of the third isomer at δ 51.6 (4%), Figure 4a. Warming to 80°C for 1.5 h resulted in a change of the relative isomer proportion and to appearance of yet another peak at δ 53.0, which could belong to the fourth isomer 4 Me . The intensity distribution changes to 2/4/4'/4 iPr /4 Me = 16/35/35/12/2% (Figure 4b). Hence, this thermal treatment resulted not only in the conversion of part of the hydride isomer 2 to the cyclohexadienyl structure but also to a redistribution of the cyclohexadienyl isomers among themselves, with an equilibration of the 4 and 4' proportion and a growth of the other two minor isomers 4 iPr and 4 Me . Cooling back to room temperature and recording the NMR spectrum after 24 h did not significantly change the relative intensity of the resonances with respect to the high temperature spectrum (2/4/4'/4 iPr /4 Me = 18/34/34/11/3%; Figure 4c). Therefore, the possible interconversion between the 2 and 4 isomers is a slow process. The stabilization of 2 relative to the cyclohexadienyl isomers by the presence of iPrOH may be attributed to a stabilization by the "dihydrogen bond" (see Scheme 2), as proposed above on the basis of the hydride NMR resonance shift in the presence of alcohols. We have therefore explored the possible rearrangement of the isomeric hexadienyl complexes back to 2 in the presence of stronger proton donors than iPrOH, namely trifluoroethanol, CF3CH2OH (TFE) and hexafluoroisopropanol, (CF3)2CHOH (HFIP). The addition of 10 equiv. of TFE to a C6D6 solution of 2, 4 and 4' ( 31 P NMR spectrum in Figure 5a), obtained as described above, induced a slight darkening from orange to orange-brown. The 31 P{ 1 H} NMR of this solution, recorded after 15 min at room temperature, is shown in Figure 5b. The (4+4')/2 intensity ratio does not decrease. It rather increases from 42:58% to 66:34%, whereas the 4/4' ratio changed slightly in favor of the minor resonance at δ 51.3 (from 25:75% to 36:64%). However, a new and broader resonance appears at δ 43.2 (ca. 35% of the total). This is the characteristic region of the [(η 6 -Cym)Ru(NPN)X] compounds (X = Cl, 1; H, 2; OiPr, 3). We therefore assign this resonance to the alkoxide product [(η 6 -Cym)Ru(NPN)(OCH2CF3)] (3') (Scheme 4). Note that while the 1 H and 31 P resonances of residual 2 are slightly shifted as discussed above (Table 1), because of the formation of the "dihydrogen bonded complex" [(η 6 -Cym)Ru(NPN)H•••HOCH2CF3], those of residual 4 and 4' remain at exactly the same chemical shifts. The change of relative intensity for (4+4') and 2 is therefore likely a consequence of the faster reaction of 2 with TFE to yield 3' than the isomerization of 4 and 4' to yield 2. Note also that this spectrum does not show any resonance above 60 ppm. Treatment of another 2/4/4' mixture, prepared in the same manner, in C6D6 with 10 equiv of HFIP under the same conditions, on the other hand, resulted in a completely different behavior. The color immediately changed from orange to dark blue and the 31 P{ 1 H} NMR spectrum indicated the total disappearance of 2 and 4/4' within 15 min with generation of a single product characterized by a resonance at δ 72.3 (Figure 5c). This clearly suggests the generation of the cationic complex [(η 6 -Cym)Ru(NPN)] + ([7] + ) by comparison with the resonance of the isolated PF6 -salt (a blue compound), which is reported as δ 72.3 in CDCl3. [9] The corresponding 1 H NMR spectrum (Figure S14) confirms this assignment. Furthermore, the generation of molecular hydrogen, resulting from the protonation of the hydride ligand by HFIP (Scheme 4), is confirmed by the observation of a signal at δ 4.48 in the 1 H NMR. Note that 3' and [7][OCH(CF3)2] have the same stoichiometry. The difference between these two reactions is notable in the kinetics, thermodynamics and nature of the product (immediate and quantitative for HFIP to yield [7] + , slow and partial for TFE to yield 3') as a result of the different proton donor strength of the two alcohols and of the lower coordinating power of the hexafluoroisopropoxide anion. However, both TFE and HFIP are sufficiently acidic to transfer their proton to the hydride ligand of complex 2. We recall here that even the weaker acid iPrOH has given evidence in favor of the transformation of 2 to the corresponding alkoxide 3 (Figure S5), although only partially and under forcing conditions. The rapid disappearance of 4 and 4' in the presence of HFIP suggests that this alcohol also has either a strong effect on the rate of the 2/4 interconversion, or that it is capable to directly abstract a hydride from the cyclohexadienyl ring. The above results have an interesting implication: the isopropoxide compound 3 is a neutral 18-electron complex that does not spontaneously ionize to the cation [(η 6 -Cym)Ru(NPN)] + plus isopropoxide, therefore the opposite process is spontaneous (equation 1); compound 3 irreversibly releases acetone to generate compound 2 (equation 2); finally, the latter reacts with HFIP to yield the [(η 6 -Cym)Ru(NPN)] + cation and hexafluoroisopropoxide (equation 3). Summing up the three equations yields equation 4, where HFIP delivers the proton to the methynic "hydride" of isopropoxide to produce H2 and acetone, rather than to the more basic oxygen atom to yield isopropanol. This is a slightly less thermodynamically favorable process (by ca. 3.3 kcal/mol, corresponding to the ΔG of the isopropanol dehydrogenation). [15] Nevertheless, this thermodynamic cycle consideration proves that it is a downhill process.

(CH3)2CHO -+ (CF3)2CHOH → (CH3)2C=O + H2 + (CF3)2CHO - (3)
(c) DFT calculations on the hydride complex 2 and cyclohexadienyl isomers

The DFT calculations were carried out with two specific objectives in mind: i) validate the high activation barrier for the 2/4 isomerizations; ii) validate the greater stability and kinetic accessibility of the 4 and 4' isomers relative to 4 iPr and 4 Me . For the first purpose, the calculations were carried out on a simplified model where p-cymene was replaced with a non-substituted benzene ring. The NPN ligand, on the other hand, was kept essentially unchanged, replacing only the tolyl p-Me groups, which are both electronically and sterically almost inconsequential, with H atoms. The Cartesian coordinates and energies of all optimized geometries are collected in Table S3. Initial exploration of the relative stability of the [(η 6 -C6H6)Ru(PhNPPh2NPh)H] (2B; B for benzene) and [(η 5 -C6H7)Ru(PhNPPh2NPh)] (4B) isomers were carried out with two standard functionals (M06L and B97D), all geometries being reoptimized with each functional. The calculations with the former functional were corrected for long-range dispersion interactions by Grimme's GD3 method, [16] whereas the B97D functional already considers dispersive forces. The calculations with both functionals were further corrected for the thermal parameters at 298.15 K and for solvation effects in isopropanol using a polarizable continuum model. The overall results were quite similar. Both show a slight preference for the cyclohexadienyl structure in terms of electronic energy (ΔEiPrOH), a bit greater with the M06L-D3 approach (see Table 2). This remains the preferred isomer also in terms of ΔGiPrOH at the M06L-D3 level, whereas the hydride becomes slightly preferred at the B97D level. The activation barrier for the 2/4 interconversion, involving H atom transfer between the Ru atom and a ring C atom, was also evaluated with both functionals, giving again similar geometries and energies for the transition state TSB, which were rather high relative to the starting and ending isomers 2B and 4B. The Mulliken charge on the H atom involved in the isomerization process goes from slightly negative in 2B (-0.094, M06L; -0.111, B97D) to slightly positive in TSB (0.050, M06L; 0.077, B97D) and then more positive in 4B (0.110, M06L; 0.134, B97D). The transition state is at high energy presumably because of the need to extensively distort the coordination geometry and because there is more extensive bond breaking than bond forming (e.g. the Ru•••H and H•••C bonds in TSB are 1.836 and 1.394 Å at the M06L level or 1.842 and 1.426 Å at the B97D level; vs. 1.601 or 1.575 Å for Ru-H in 2B and 1.094 or 1.098 Å for C-H in 4B). This result justifies the experimental observation of a very slow interconversion between 2 and the hexadienyl isomers, between the hexadienyl isomers among themselves. The isomerization of one hexadienyl isomer into another, e.g. from 4 to 4' or to 4 iPr , would presumably have to transit through the hydride isomer 2.

In terms of optimized geometries, the parameter of greatest interest is the pyramidalization of the N atoms in the NPN ligand, as evaluated by the sum of the three bond angles, C(Ar)-N-Ru, Ru-N-P and P-N-N(Ar) (indicated as ΣN in Table 2). This provides information on the delocalization of the N lone pair onto the aryl substituent and on the Ru atom (for the electronically unsaturated structure of 4/4'). The calculations indicate that ΣN is close to 360° (planarity) not only for the cyclohexadienyl structure 4B, but also for the 18-electron complex 2B, suggesting efficient delocalization of the N lone pair onto the phenyl substituent. This corresponds to the experimental evidence, not only for the 16-electron complexes 4/4' and [5] + PF6 -, but also for the 18-electron complex 1 (a structure for the hydride derivative could not be obtained, thus we compare with the isoelectronic chloride complex). On the other hand, related compounds with alkyl substituents on the N atom of the NPN ligand shown significant pyramidalization in 18-electron structures (e.g. ΣN = 344.4(8) and 357.6(8)° in [(η 6 -C6Me6)RuCl(MeNPPh2NMe)]. [8] In terms of the second objective, we have chosen to drastically simplify the NPN ligand for computational efficiency by replacing all Ph and Tol substituents with H atoms, since the nature of the NPN substituents should not affect to a great extent the relative stability of the different cyclohexadienyl isomers. The ring, on the other hand, was introduced in the calculations without any simplification. Because the four different isomers contain exactly the same number and type of bonds, we have only used one functional to evaluate their relative energy, selecting B97D. The results (see Figure 6) show that, in agreement with the experimental evidence, isomers 4H and 4H' (H for hydrogen on the NPN ligand) are more stable that the other two isomers. Their relative energies are quite close, but significantly lower than those of 4H iPr and 4H Me . This relative energy ordering suggests the assignment of the observed resonances in the 50-53 ppm region of the 31 P NMR spectrum as proposed in Figure 4: δ 53.0 for 4 Me , δ 51.6 for 4 iPr . Concerning the two resonances at δ 51.1 and 50.6, the relative 1:1 intensity after extensive warming in toluene at 80°C (vide supra) suggests that they should have the same energy, whereas the calculations show a slight preference for 4H'. The small discrepancy, which is well within the accepted margin of computational error, may in part be related to the simplification of the NPN ligand. The preferred generation at room temperature of the isomer characterized by the δ 50.6 resonance indicates kinetic control for the H atom transfer from the Ru atom to the ring C atom. We did not calculated the relative barriers for the H atom transfer to all four different types of C atom in the p-cymene ligand. Steric arguments, however, are consistent with the faster formation of 4 since the H ligand migrates to the C atom ortho to the less hindered CMe group.

In terms of optimized geometries, it is of interest to observe that the N atoms are significantly pyramidalyzed the ΣN parameter being in the 325.4-343.1° range (see Table 3). This difference relative to the PhNPPh2NPh models discussed above (2B, 4B and TSB) is related to the inability of the N atoms to delocalize their lone pair onto the H substituent, in spite of the possible partial delocalization to the Ru center in the 16-electron systems. 4 and all data are collected in Table S2.

Complex [(η 6 -Cym)RuCl2]2 does not catalyze the reaction in the absence of base (entry 1). Using compound 1 in the absence of base resulted in catalytic activity at 60°C (entry 2) with a 62.4 % yield of product in 24 h. It is relevant to point out that many other chloride precatalysts are not active unless transformed to the active hydride species by treatment with a strong base in the alcohol solvent. [17] The activity in this case may be attributed to the spontaneous generation of the hydride complex 2 in the system, even in the absence of base, as shown above in part a (Scheme 2).

Table 4. Results of the catalytic transfer hydrogenation of acetophenone. [a] Entry Precatalyst Solvent [b] B/Ru [c] T (°С) kobs (h -1 ) [d] 1 The kinetic analysis of entry 2 shows a relatively good first order behavior during the first 6 h (see Figure 7), but catalyst decomposition occurs at longer times as suggested by the observed deviation from linearity for the first-order plot after 24 h (Figure S15). This is in line with the observed thermal degradation of 2 (part a), even though increasing amounts of iPrOH were shown to have a protecting effect. A run carried out under the same conditions using [7]BF4 in place of 1 showed no significant activity (entry 3). In order to probe the catalyst stability, an additional run was carried out under the same conditions as run 2, except that the catalyst solution was kept at 60°C for 2 hours, instead of only 15 min, before introducing the substrate (entry 4). The dramatic reduction of activity (only 15.7% conversion after 6 h, vs. 36.2% for entry 2) confirms that the catalyst is degraded under these conditions. Note, however, that the linearity of the first order plot of entry 2 in the first 6 h (Figure S15) suggests a slower degradation in the presence of substrate. In other words, the substrate also seems to exert, like the isopropanol solvent, a slight protective role against catalyst decomposition. Since the stoichiometric studies presented in part a have indicated a solvent effect on the stability of the hydride complex 2, we have also carried out an additional test under the same experimental conditions, except for replacing iPrOH with a 1:1 mixture of iPrOH and benzene (entry 5). The result of this experiment are essentially identical to those of entry 2, including the activity reduction after the first 6 hours (Figure S15). This suggests that 50% of iPrOH in the solvent mixture is sufficient to provide a roughly equivalent protecting effect to the active catalyst. A final experiment in the absence of base was carried out at 80°C (entry 6). As expected, the activity was greater, the product yield reaching to 95% after 5 h. At 80°C, like at 60°C, the catalyst activity is maintained sufficiently to provide a linear first-order kinetic plot during the first 5 h (see Figure 7). 4).

The remainder of the catalytic tests were carried out in the presence of at least one equivalent of NaHMDS, which quantitatively transforms 1 to the hydride complex 2 (see part a). A control experiment with the precursor complex [(η 6 -Cym)RuCl2]2 (entry 7) shows low but non-negligible activity, with a 57.4 % yield of 2-phenylethanol in 24 h at 40°C and a TOF of 3.44 h -1 . Thus, the presence of base is essential for this precatalyst. The related C6Me6 and C6H3Me3 precatalysts [18] were shown to generate the corresponding dinuclear hydride complexes [(Arene)Ru(μ-H)2(μ-Cl)Ru(Arene)] + Cl -under the same conditions, as a result of β-H elimination from the corresponding μ-isopropoxide intermediate. [13] At the same temperature, use of compound 1 leads to a tenfold increase of kobs (entry 8), demonstrating the positive effect of the NPN ligand. However, thermal degradation was observed after the initial linear region of the first order plot (see Figure S16a), because conversion reaches only 93.8% after 24 h, whereas extrapolation of the initial activity should afford essentially quantitative conversion (> 99%) in less than 20 h. Increasing the temperature to 60°C results in a kobs increase to 0.81 h -1 (entry 9) and shows a faster onset of the catalyst decomposition because the linearity of the 1 st order plot is lost after the first 2 h (Figure S16b). Comparison of entries 2 and 9 in Table 4 shows that the base increases the catalytic activity by a factor of ca. 10, which we believe is related to the quantitative generation of catalytically active 2. At this temperature, an additional experiment was also carried out in a 50:50 iPrOHtoluene solvent mixture (entry 10), giving a similar result relative to the neat iPrOH solvent (Figure S16c). This result is similar to that of the above discussed base-free system, with the catalyst degradation being perhaps a bit faster in this case (onset before 2 h). Two additional experiments were carried out at higher (80°C, entry 11) and lower (25°C, entry 12) temperatures. At the higher temperature, essentially full conversion was attained in 4 h and the catalyst degradation phenomenon is evident once again after ca. 2 h (Figure S16d), whereas the slower catalyst decomposition at 25°C gave a linear 1 st order behavior up to 8 hours, but the slower conversion lead to ultimate decomposition with stagnation of the substrate conversion at ca. 82% after 30 h (Figure S16e). The 1 st order plots at the four different temperatures in the initial linear region are collected in Figure 8a. 4). (b) Eyring analysis of the observed 1 st order rate constants.

An Eyring analysis of the resulting 1 st order rate constants, shown in Figure 8b, yields the activation parameters ΔH ‡ = 9.6±1.3 kcal mol -1 (enthalpy span of the catalytic cycle) and ΔS ‡ = -31±4 cal mol -1 K -1 . The large negative activation entropy suggests a high level of ordering upon going from the resting state to the rate-determining transition state. A final experiment (entry 13) consisted in testing the [7]PF6 salt as precatalyst at 25°C, under the same conditions used for precatalyst 1 in run 12. It must be recalled that, in the absence of base, this compound shows essentially no catalytic activity (entry 3). However, in the presence of NaHMDS it gives rise to the same activity as precatalyst 1 (cf. entries 12 and 13 in Table 4), suggesting that the base transforms both compounds into the same catalytically active species. Quite clearly, the collective catalytic experiments, in combination with the stoichiometric studies of part (a), demonstrate the direct implication of the hydride complex 2 in the catalytic cycle. S2). (b) Plot of the observed 1 st order rate constant versus the catalyst concentration.

Additional catalytic runs were also carried out at 40°C and at different catalyst concentrations. The conversion data are collected in Table S2 and the full first order plots are in Figure S17. Once again, catalyst decomposition was evident from the 1 st order kinetic plots, but a sufficient number of points could be collected within the catalyst stability period (see Figure 9a) to derive observed rate constants, which are collected in Table 5. From the kobs vs. [Ru] plot (Figure 9b), a second order rate constant k = 117±10 M -1 h -1 was obtained.

Entry

[Ru] (M) Ru:substrate kobs (h -1 ) [b] 14 0.001 0.0025 0.086±0.002

(e) DFT study of the catalytic cycle

The combined catalytic, spectroscopic and chemical studies point rather firmly to the involvement of the hydride complex 2 as the catalytically active species. However, it is impossible to envisage either a Noyori-type (bifunctional and concerted outer-sphere proton/hydride transfer from catalyst to substrate) [19] or a coordination/insertion pathway for this species, because the complex does not contain deprotonatable ligands and is coordinatively saturated. The above described spectroscopic studies suggest facile accessibility of an isopropoxide species from 2 and acetone, either as a neutral O-bonded complex 3 or as an isomeric zwitterionic σ-complex 6. Hence, it is possible to envisage an outer-sphere hydride transfer from 2 to acetophenone via an intermediate 6', similar to 6, possibly in equilibrium with the O-bonded 1-phenylethoxide isomer, 3'. The 1-phenylethanol product may then be generated by proton exchange with the solvent, which may occur either directly or via the dissociated ion pair (complex [7] + and free alkoxide). The latter is suggested by the observation of free [7] + in the presence of the less coordinating hexafluoroisopropanolate. Complex 6 then completes the cycle by the reverse hydride transfer process. This catalytic cycle is summarized in Scheme 5. It is pertinent to remark that catalysts devoid of deprotonatable ligands have already been shown to operate by an outer-sphere ionic mechanism with preliminary hydride transfer, followed by subsequent protonation of the alkoxide intermediate, in ionic hydrogenation (the proton coming in this case from activated H2) [20] or in the reverse alcohol and formic acid dehydrogenation. [21] Even Noyori's catalyst itself, though containing a deprotonatable ligand, was suggested to have a similar, non-concerted mechanism with the proton delivered in a subsequent step by H2. [22] Scheme 5. Proposed mechanism for the catalyzed acetophenone transfer hydrogenation by isopropanol.

The feasibility of this process has been probed by DFT calculations. A first exploration, using the same computational level (B97D functional, basis set, thermal and solvation correction) as above in section (c), shows that the isopropoxide complex 3B (benzene in place of cymene) is more stable than 2B + acetone by 11.9 kcal/mol on the E scale in the gas phase, but less stable by 7.2 kcal/mol on the solvation-corrected G scale. Optimization of the benzene model 6B of the proposed σ-complex 6 was only possible after explicit introduction of three molecules of MeOH (computational model of the iPrOH solvent), which stabilize the negative charge of the isopropoxide O atom by Hbonding, to yield structure 6B•3MeOH. The importance of the explicit inclusion of alcohol molecules has previously been demonstrated for a related system. [20a] In order to compare systems with the same number of molecules, the Me2CO•3MeOH and (MeOH)3 cluster were also separately optimized. The results are shown in Figure 10. Note that acetone in the Me2CO•3MeOH cluster holds only two MeOH molecules directly H-bonded to the carbonyl O atom because this has only two available lone pairs. The third MeOH molecule is H-bonded to one of these MeOH molecules. The system composed of separate 3B and (MeOH)3 is placed at ΔGiPrOH,298K = 7.8 kcal/mol from the starting 2B + Me2CO•3MeOH system. The slight difference between this energy change and that from 2B + Me2CO to 3B (7.2 kcal/mol) is due to the different H-bonding situations within the Me2CO•3MeOH and (MeOH)3 clusters. As can be seen from Figure 10, 6B•(MeOH)3 is at a rather similar energy to the Obonded isopropoxide complex (3B and (MeOH)3), in fact it is even slightly preferred. Note, however, that the relative energy of 3B may be lowered by H-bonding interactions with MeOH molecules, which were not considered in our calculations. Thus, the calculations support the proposition that 6 is an accessible system and a minimum on the potential energy surface. Isopropoxide dissociation from 6B•(MeOH)3 was probed by calculating the separate ions 7 + B (benzene model of the cymene cationic complex 7 + ) and the cluster iPrO The transition state for the hydride transfer process from 2B to acetone could not be located/optimized. Attempts to optimize this stationary point on an even more simplified system (H atoms replacing the Ph and Tol substituents on the NPN ligand, symbolized as HB) also failed. Thus, we have explored the reaction pathway with a series of partial optimizations (relaxed scan), keeping the C-H distance at different fixed values. Starting from the optimized 6HB•3MeOH, the C-H distance was increased stepwise, resulting in an energy increase up to a distance of 1.688 Å, then to a decrease, see Figure 11 11. This local minimum is set as the zero energy reference point; the highest point along the reaction coordinate is higher by only 5.7 kcal/mol. Complex 6HB was also reoptimized in the presence of the same H-bonding environment to yield a local minimum, 6HB•(2+1)MeOH, at a slightly lower E, but slightly higher GiPrOH,298K. A new relaxed scan on the C-H distance was carried out starting from this point (purple points in Figure 11), yielding a qualitatively similar behavior with the highest energy point found at approximately the same C-H distance (1.677 Å), but at even lower energy, 2.5 kcal/mol. Since the transition states could not be located, the G ‡ iPrOH,298K values could not be determined. The calculations, however, clearly suggest that the transformations of 2 to 6 and vice versa require very low activation and should therefore be relatively rapid processes, in agreement with the above given interpretation of the NMR results in acetone[D6]. The other part of the cycle (proton transfer leading from 6' and iPrOH to 6 and PhCH(Me)OH) was not modelled by DFT, because of the computational challenge with the need to include a number of explicit solvent molecules in the system. Given the approximations, it would also be difficult to compare the intra-and intermolecular pathways. Processes involving pseudodegenerate proton transfers between alkoxide anions, however, are not expected to involve high energy barriers.

Conclusions

The present contribution has analyzed the action of an (η 6cymene)ruthenium chloride complex supported by a chelating iminophosphonamide ligand, [(η 6 -Cym)RuCl(NPN)] (1), as a precatalyst for ketone transfer hydrogenation by isopropanol, demonstrating that the active catalyst is a hydride species [(η 6 -Cym)RuH(NPN)] (2). Although this hydride species is unstable with respect to isomerization to a 16-electron cyclohexadienyl system by hydride migration to the arene ring, it is stabilized by dihydrogen bonding in isopropanol. The complex catalyzes efficiently the transfer hydrogenation of acetophenone and remains stable long enough in the 25-80°C temperature range to provide linear first order behavior to high conversion. The most relevant result of this study is the elucidation of the transfer hydrogenation mechanism by a combination of NMR investigations and DFT calculations. It constitutes, to the best of our knowledge, the first detailed mechanistic investigation of an outer-sphere transfer hydrogenation for a catalyst devoid of deprotonatable ligands, showing that the hydride ligand can be delivered to the outer-sphere ketone with H-bonding assistance by the isopropanol solvent, which then completes the process by protonation in a second step.

Experimental Section

General procedures. All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry argon. Solvents were purified by standard methods and distilled prior to use. 1 H, 31 P and 13 C NMR spectra were obtained on Bruker Avance 300 and Bruker AvanceIII 400 spectrometers and referenced to the residual signals of deuterated solvent ( 1 H and 13 C), and to 85% H3PO4 ( 31 P, external standard). Compounds [(η 6 -Cym)RuCl(NPN)] (1), [9] [(η 6 -Cym)Ru(NPN)}] + PF6 - ([7]PF6) [9] and [(η 6 -Cym)RuCl2]2 [23] were prepared according to described procedures.

Synthesis of [(η 6 -p-cymene)Ru{Ph2P(N-p-Tol)2}] + BF4 -([7]BF4).

Complex [7]BF4 was obtained by adapting the procedure previously reported for [7]PF6. [9] To a solution of 1 (0.27 g, 0.40 mmol) in CH2Cl2 (20 ml) solid TlBF4 (0.24 g, 0.81 mmol) was added causing the color to immediately change from red to deep blue. The reaction mixture was stirred for 3 h and then filtered through a bed of Celite. The solution was concentrated to 5 mL, further addition of Et2O (30 mL) resulted in precipitation of a blue-black microcrystalline, which was filtered off, washed with Et2O (2x5 mL) and dried in vacuum. Yield of [7]BF4: 0.27 g (93%). The 1 H and 31 P NMR properties of this compound perfectly match those reported previously for the analogous PF6 -salt. [9] In situ generation of [(η 6 -p-cymene)RuH{Ph2P(N-p-Tol)2}] (2). The hydride complex 2 was generated in situ in a NMR tube. To a solution of the chloride complex 1 (13 mg, 0.020 mmol) in C6D6 (0.5 mL) a NaHMDS solution (2.0 M in THF, 0.015 mL, 0.030 mmol) and isopropanol (0.80 mmol, 0.06 mL) were added. The resulting suspension was vigorously shaken for 15 min at room temperature to complete the dissolution of 1 yielding a yellow solution. According to the NMR spectra, the yield of complex 2 is 95-98%. [START_REF] Dolg | [END_REF] (4'). In a Schlenk tube containing a solution of NaHMDS (2.0 M in THF, 0.045 mL, 0.09 mmol) in isopropanol/toluene (2.5/2.5 mL), the chloride complex 1 (50 mg, 0.075 mmol) was added and the resulting suspension was stirred for 40 min at room temperature. The solvent was removed under reduced pressure, the solid residue was dried and then extracted with deuterated benzene (2 mL). The precipitate was filtered off and the filtrate was kept at 40°C for 1 day. According to the NMR spectra in C6D6, the product consists of a mixture of two regioisomers 4 and 4' and residual complex 2 in a ratio 4:4':2= 55:15:30. [START_REF] Dolg | [END_REF] Р NMR (C6D6): δ 50.6 (4), 51.1 (4'). The assignment of the resonances to compounds 4 and 4' was based on the detailed analysis of [START_REF] Dolg | [END_REF] P, 1 H, 13 C{ 1 H}, 1 H-1 H COSY and 1 H- 13 C HSQC NMR spectra (see details in the Supporting Information). In the 1 H NMR spectrum, the aromatic P-phenyl and N-tolyl resonances of the NPN ligands of compounds 4 and 4' overlap and are observed at δ 7.80 (m, 4Н, о-H), 6.97 (m, 6Н, m,p-H), 7.09 (d, 3 J = 7.8, 4Н, o'-H), 6.85 (d, 3 J = 7.8, 4Н, m'-H).. 1 Н NMR for 4 (C6D6): δ 6.03 (d, 3 J = 4.8, 1H, Hb), 3.93 (d, 3 J = 4.8, 1H, Нa), 2.92 (d, 3 J = 6.0, 1H, Нj), 2.67 (dd, 2 J = 13.2, 3 J = 6.6, 1H, Нen), 2.05 (s, 6Н, Меh), 1.94 (sept, 3 J = 6.6, 1Н, СНf), 1.38 (d, 3 J = 6.6, 3Н, Meg), 1.36 (s, 3H, Mee), 1.28 (d, 2 JHH = 13.2, 1H, Нex), 1.13 (d, 3 JНН = 6.6, 3Н, Meg'). 1 Н NMR for 4' (C6D6): δ 5.93 (d, 3 J = 4.8, 1H, Hb), 3.88 (d, 3 J = 4.8, 1H, Нa), 2.96 (d, 3 J = 6.0, 1H, Нj), 2.72 (dd, 2 J = 13.2, 3 J = 6.6, 1H, Нen), 2.04 (s, 6Н, Меh), 1.78 (s, 3H, Mee), 1.74 (m, 1Н, СНf), 1.71 (d, 3 J = 6.6, 3Н, Meg'), 1.02 (d, 2 J = 13.2, 1H, Нex), 0.90 (d, 3 J = 6.6, 3Н, Meg). 13 Alternative method for generation of 2. To a Schlenk tube containing a suspension of 1 (20 mg, 0.030 mmol) in toluene (3 mL), NaEt3BH (1.0 M in toluene, 0.036 mL, 0.036 mmol) was added, and the resulting mixture was stirred for 1 h at room temperature. The precipitate was filtered off, the filtrate was evaporated to dryness under reduced pressure and the residue was dried in vacuo. According to [START_REF] Dolg | [END_REF] P NMR (C6D6) the final product contains 2, 4 and 4' in a 35:57:8 ratio.

X-ray crystallography.

A freshly prepared mixture of 2, 4 and 4' was dissolved in neat iPrOH and the solution was allowed to stay for crystallization for 3 days at room temperature. A red crystals thus obtained was mounted under inert perfluoropolyether at the tip of glass fiber and cooled in the cryostream of a CAD4 Nonius diffractometer fitted with the APEX II CCD. The structure was solved by using the integrated spacegroup and crystal structure determination SHELXt software [24] and refined by least-squares procedures on F 2 using SHELXL-2014. [25] The asymmetric unit contains two independent Ru complexes with roughly similar geometry. The crystal was poorly diffracting and no significant intensities could be detected above 24° in theta. Moreover, the crystal is a mixture of two isomers as shown by NMR (see Results and Discussion). The occurrence of these two isomers induced a diffused disorder in the cyclohexadienyl rings, the conformation of which was difficult to define. It was impossible to correctly model the disorder. Therefore, the cyclohexadienyl rings were refined using severe restraints on the anisotropic thermal parameters to avoid large thermal ellipsoids. Owing to the poor quality of the data, the phenyl rings in the NPN ligands were treated as rigid group with idealized C-C distances of 1.39 Å and C-C-C angles of 120°, in order to minimize the number of refined parameters. As the space group is non centrosymmetric, the Flack parameter [26] was refined yielding a value of 0.21 (8), which indicates twinning by inversion. This phenomenon may also be related to the presence of a mixture of the two isomers. All H atoms attached to C atoms were introduced in the calculation at idealised positions and treated as riding models. The molecule drawing was realised with the help of ORTEP32. [27] Crystal data and refinement parameters are shown in Tables 1. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1583403. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Catalytic studies. In a typical experiment, the precatalyst 1 or [5] + (0.02 mmol) was suspended in iPrOH (5 mL) or iPrOH/toluene (2.5/2.5 mL) and then treated (if appropriate) with a 2.0 M solution of NaHMDS in THF in the amounts given in Table 4. In the presence of base, the mixture was preliminary stirred at room temperature for 40 minutes, then it was warmed to the desired temperature followed by the addition of dodecane (1 mmol, 0.225 ml) and acetophenone (2 mmol, 0.233 ml). Aliquots (0.1 mL) were taken at the times specified in Table 3, diluted with diethyl ether (0.8 mL) and filtered through a silica bed. The reaction samples were analyzed by GC equipped with J&W GC Column with a DB-1MS stationary phase (program: 25 min at 50 ˚C, with further heating rate 20 ˚C / min to 170 ˚C; retention times: τ(acetophenone) = 24 min, τ(1-phenylethanol) = 25 min, τ(dodecane) = 31 min).

Computational details. The calculations were carried out within the DFT approach with M06L [28] and 97D [29] functionals in Gaussian 09. [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF] All geometry optimizations were carried out using the LANL2DZ basis set and ECP for the Ru atom, [START_REF] Dolg | [END_REF] augmented with an f polarization function (α = 1.235), [32] and the 6-311G(d,p) [33] basis sets for all other atoms. Calculations were corrected for dispersion interactions by Grimme's D3 [16] method for M06L whereas the B97D functional already considers dispersion. The effect of the solvent was included by the SMD polarisable continuum [34] in isopropanol (ε = 19,264). All of the energies presented in the text are Gibbs energies in isopropanol (ΔGiPrOH). and the Russian Science Foundation (grant no. 14-13-00801) for financial support. This work was granted access to the HPC resources of IDRIS under the allocation 2017-086343 made by GENCI (Grand Equipement National de Calcul Intensif) and to the resources of the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP). I.S. thanks the French Ministry of Foreign Affairs for a Ph.D. scholarship through the French Embassy in Moscow.

Scheme 1 .

 1 Scheme 1. Generation of hydride 2 from 1.

Figure 1 .

 1 Figure 1. 31 P{ 1 H} NMR monitoring of the generation of 2 at room temperature: 0.5 mL of C6D6, 13 mg of 1 (0.02 mmol), 60 μL of iPrOH (iPrOH/1 = 40), 15 µL of NaHDMS (2 M in THF; HMDS/1 = 1.5). (a) 5 min from sample preparation, (b) 13 min, (c) 20 min, (d) 30 min.

Figure 2 .

 2 Figure 2. 31 P{ 1 H} spectra of samples of 2 obtained under various conditions. (a) From 1 and NaHMDS in neat iPrOH, followed by evaporation, extraction in Et2O, filtration, evaporation and analysis in C6D6. (b) Same protocol as for spectrum (a), except for running the reaction in toluene in the presence of iPrOH. (c) From 1 and NaBHEt3 in C6D6.

Figure 3 .Scheme 3 .

 33 Figure 3. An ORTEP view of the disordered structure of 4 and 4'. Only one of the two independent molecule is shown. The ellipsoids are drawn at the 30% probability level and hydrogen atoms are not shown for clarity.

whereas 4 and 4 '

 4 derive from an intramolecular migration of the hydride ligand from the Ru atom in 2 to the o or m C atoms of the ring. The preferential formation of 4 and 4' over the other two possible isomers 4 Me and 4 iPr (Scheme 3) may be kinetically favored by the greater partial positive charge of the sterically congested o and m C atoms. This point has been further investigated by DFT calculations (see next section).

Figure 4 .

 4 Figure 4. 31 P NMR spectra of the 2/4/4' mixture in C6D5CD3. The mixture was prepared from 1 (15 mg) and NaHMDS (12 µL, 1 equiv) in neat iPrOH (2 mL), followed by drying and redissolution in the NMR solvent. (a) At 25˚C. (b) After 90 min at 80°C. (c) At 25°C, after standing for 24 h after spectrum (b).

Figure 5 .

 5 Figure 5. 31 P NMR spectra of the reaction between the 2/4/4' mixture and fluorinated alcohols in C6D6. (a) Conditions for the synthesis: 13 mg (0.02 mmol) of 1 in 2 mL of iPrOH, 10 µL of 2 M NaHMDS solution in THF (1 equiv), followed by drying and dissolution in the NMR solvent. (b) From (a), 15 min after adding TFE (14 µL, 0.2 mmol, 10 equiv) at room temperature. (c) From (a), recorded 15 min after adding HFIP (20 µL, 0.2 mmol, 10 equiv) at room temperature.

Scheme 4 .

 4 Scheme 4. Reaction of 2/4/4' with fluorinated alcohols.

[(η 6 -

 6 Cym)Ru(NPN)] + + (CH3)2CHO -→ [(η 6 -Cym)Ru(NPN){OCH(CH3)2}] (1) [(η 6 -Cym)Ru(NPN){OCH(CH3)2}] → [(η 6 -Cym)Ru(NPN)H] + (CH3)2C=O (2) [(η 6 -Cym)Ru(NPN)H] + (CF3)2CHOH → [(η 6 -Cym)Ru(NPN)] + + H2 + (CF3)2CHO -

Figure 6 .

 6 Figure 6. Relative gas phase energies (ΔG and ΔE in parentheses, in kcal mol - 1 ) and views of the optimized geometries for the model compounds of the four different isomers 4, 4', 4 Me and 4 iPr .

Figure 7 .

 7 Figure 7. First order kinetics analyses of the acetophenone transfer hydrogenation in isopropanol catalyzed by 1 in the absence of base. Blue line (60°C), entry 2; red line (80°C), entry 6 (Table4).

Figure 8 .

 8 Figure 8. (a) First order kinetics analyses of the acetophenone transfer hydrogenation in isopropanol catalyzed by 1 in the presence of base at different temperatures: blue line, run 12; green line, run 8; orange line, run 9; red line, run 11 (Table4). (b) Eyring analysis of the observed 1 st order rate constants.

Figure 9 .

 9 Figure 9. (a) First order kinetics analyses of the acetophenone transfer hydrogenation in isopropanol catalyzed by 1 at different concentrations (data from TableS2). (b) Plot of the observed 1 st order rate constant versus the catalyst concentration.

  Standard conditions : 233 µL of acetophenone (2 mmol) and 226 µL of internal standard (dodecane) in 5 mL of solvent ([PhCOMe] = 3.66•10 -1 M), with the appropriate amount of compound 1 and NaHMDS/Ru = 1.5, T = 40°C; the catalytic mixture was conditioned at the reaction temperature for 15 min before substrate addition.[b] From the slope of the first order kinetics plot in the linear region

  -•(3MeOH), which maintains the same H-bonding arrangement as in 6B•(MeOH)3, with the proton of each MeOH molecule interacting with a separate isopropoxide lone pair. The resulting separate ion pair is at 10.7 kcal/mol from 2B + Me2CO•(MeOH)3 and at 3.8 kcal/mol from 6B•(MeOH)3 or 2.9 kcal/mol from 3B + (MeOH)3 on the ΔGiPrOH,298K scale. Without the three H-bonded MeOH molecules, alkoxide dissociation from 3B to yield 7 + B + iPrO -(both still considered with solvation correction in iPrOH) becomes uphill by 29.6 kcal/mol. These results clearly show that hydrogen bonding from the alcohol solvent provides the driving force toward alkoxide dissociation.

Figure 10 .

 10 Figure 10. Relative energies (ΔGiPrOH,298K, with gas phase ΔE in parentheses) of systems of relevance for the ketone transfer hydrogenation catalyzed by complex 2.

  (blue points). Full optimization starting from the final point at C-H = 1.938 afforded the van der Waals adduct of 2HB with the Me2CO•3MeOH cluster, where one MeOH molecule has shifted from the O atom of isopropanolate (which now become the acetone carbonyl O atom) to another MeOH , 2HB•Me2CO•(2+1)MeOH. The hydride ligand is at 2.56 and 2.58 Å from two H atoms and at 2.62 Å from the carbonyl C atom of acetone, as highlighted in Figure

Figure 11 .

 11 Figure 11. Computational exploration of the hydride transfer from 2HB to acetone leading to 6HB. The values indicated for the three local minima are the relative GiPrOH,298K (in bold characters) and relative gas phase E (in parentheses) in kcal/mol.

  Р NMR (C6D6): δ 33.2. 1 H NMR (C6D6): δ 8.16 (m, 2Н, о-HPh), 7.61 (ddd, 3 JHР = 11.0, 3 JHH = 8.0, 4 JHH = 1.6, 2Н, о'-HPh), 7.25 (m, 3Н, (p+m)-HPh), 6.82 (m, 3Н, (p+m)'-HPh), 6.79 (d, 3 JHH = 8.0, 4Н, (С6Н4)tol), 6.73 (dd, 3 JHH = 8.0, 4 JHP = 1.2, 4Н, (С6Н4)tol), 4.89 (d, 3 JНН = 5.6, 2Н, (C6H4)cym), 4.82 (d, 3 JНН = 5.6, 2H, (C6H4)cym), 2.65 (sept, 3 JНН = 6.8, 1Н, СН-Me2), 2.08 (s, 3H, Mecym), 2.08 (s, 6Н, Меtol), 1.29 (d, 3 JНН = 6.8, 6Н, CH-Me2), -3.07 (s, Ru-H). 13 C NMR (C6D6): δ 148.7 (d, 2 JСР = 5.1, i"-Ctol), 135.9 (d, 1 JCP = 92.4, i-CPh), 133.1 (d, 2 JCP = 9.2, o'-CPh), 132.6 (d, 2 JСР = 10.4, o-CPh), 131.8 (d, 4 JСР = 2.7, p-CPh), 131.4 (d, 4 JСР = 2.7, p'-CPh), 129.1 (s, m"-Ctol), 128.9 (d, 3 JСР = 11.5, m-CPh), 126.5 (d, 5 JСР = 1.1, p"-Ctol), 123.5 (d, 3 JСР = 11.1, o"-Ctol), 106.5 (s, Ccym), 99.7 (s, Ccym), 80.3 (s, CHcym), 76.2 (s, CHcym), 32.7 (s, CH-Me), 24.0 (s, СH-Me), 20.6 (s, Metol), 20.1 (s, Mecym). IR spectrum (C6D6): νRuH 1880 cm -1 . Transformation of 2 into a mixture of [(η 5 -C6H5-1-Me-4-i Pr)Ru{Ph2P(Np-Tol)2}] (4) and [(η 5 -C6H5-1-i Pr-4-Me)Ru{Ph2P(N-p-Tol)2}]

4 JСР

 4 C NMR for 4 (C6D6): δ 147.2 (d, 2 JСР = 5.3, i'-C), 132.4 (d, 2 JCP = 9.6, o-C), 131.9 (d, = 2.0, p-C), 131.8 (d, 1 JCP = 86.4, i-C), 129.2 (s, m'-C), 128.5 (d, 3 JСР = 11.0, m-C), 126.6 (s, p'-C), 125.0 (d, 3 JСР = 9.8, o'-C), 92.4 (s, Cd), 76.4 (s, CHa), 70.2 (s, CHb), 38.4 (s, CH2(k)), 37.5 (s, i-Cc), 32.7 (s, CHf), 29.2 (s, CHj), 25.3 (s, Mee), 24.9 (s, Meg'), 22.4 (s, Meg), 20.8 (s, Meh). 13 C NMR for 4' (C6D6): δ 147.0 (d, 2 JСР = 5.1, i'-C), 132.5 (d, 2 JCP = 9.6, o-C), 131.9 (overlapped, p-C), 129.3 (s, m'-C), 128.4 (d, overlapped, m-C), 126.6 (s, overlapped, p'-C), 125.1 (d, 3 JСР = 9.8, o'-C), 82.6 (s, Cd), 75.4 (s, CHa), 73.1 (s, CHb), 47.8 (s, Cc), 35.2 (s, CHf), 30.8 (s, CHj), 30.4 (s, CH2(k)), 22.1 (s, Meg'), 20.9 (s, Mee), 20.3 (s, Meg), 20.6 (s, overlapped, Meh). The detailed spectra are shown in the SI (Figures S8-S12).

Table 1 .

 1 NMR

		resonances for solutions of [(η 6 -Cym)Ru(NPN)H] (2) in the
	presence of alcohols. [a]
	entry	solvent	additive (amount)	31 P, ppm	1 H, ppm
	1	C6D6		33.2	-2.96
	2	C6D6	iPrOH (2 equiv)	33.2	-2.98
	3	C6D6	iPrOH (10 equiv)	33.2	-3.01
	4	C6D6	iPrOH (40 equiv)	33.2	-3.07
	5	iPrOH [b]		32.9	-3.14
	6	C6D6	TFE (10 equiv)	33.4	-3.16
	7	C6D6	Acetone (10 equiv)	33.2	-2.98
	8	CD3COCD3		33.9	-3.53

Table 2 .

 2 Relevant geometrical parameters and relative energies of the optimized structures of 2B, 4B and TSB, with comparison with the geometries of related experimental structures.

	Ru-N	2.167	2.164	2.145(2)	2.164	2.121	2.061(12)	2.060(12)	2.031(2)	2.173	2.132
		2.174	2.158	2.126(2)	2.135	2.124	2.090(13)	2.119(14)	2.017(3)	2.175	2.152
	Ru-(centroid) [c]	1.711	1.809		1.647	1.677	1.743	1.689	1.659(1)	1.687	1.714
						B. Angles (°)
	N-Ru-N	68.27	69.03	68.24(9)	69.50	70.66	70.2(5)	71.2(5)	72.42(10)	68.74	70.07
	N-P-N	96.99	96.32	96.73(13)	97.50	96.67	95.8(7)	97.7(7)	94.59(13)	98.05	97.12
	ΣN	354.4	355.8	357.2(6)	357.2	359.1	358.9	359.8	360.0(6)	359.7	357.1
		356.5	359.1	358.2(6)	358.6	358.4	359.7	353.9	358.9(5)	359.6	354.7
	(centroid)-Ru-P [c]	68.27	69.03	68.24(9)	69.50	70.66	70.2(5)	71.2(5)	72.42(10)	68.74	70.07
						C. Energies (kcal/mol)
	ΔEiPrOH	0.0	0.0		-6.7	-3.1				34.4	36.7
	ΔGiPrOH	0.0	0.0		-4.6	0.8				33.0	34.9

Parameter

2B (M06L) 2B (B97D) 1

[a]

4B (M06L) 4B (B97D) 4/4'

[b]

[7] + PF6 -[a] TSB (M06L) TSB (B97D)

A. Distances (Å)

Table 3 .

 3 Relevant geometrical parameters of the optimized structures of 4H

, 4H', 4H Me and 4H iPr using

  the B97D functional.

	parameter	4H	4H'	4H Me	4H iPr
	Distances (Å)
	Ru-N	2.218 2.212	2.215 2.211	2.210 2.204	2.162 2.135
	Ru-(centroid) [a]	1.747	1.759	1.757	1.654
	Angles (°)
	N-Ru-N	69.38	69.18	69.57	70.95
	N-P-N	99.92	99.50	99.71	99.14
	ΣN	319.0 319.1	322.7 317.9	320.5 319.1	329.9 339.5
	(centroid)-Ru-P [a]	159.94	160.99	159.06	165.15
	[a] Centroid = center of gravity of the five Ru-bonded C atoms.
	(d) Transfer hydrogenation catalysis
	Compounds 1 and [7] + (as the PF6 or BF4 salts) were tested as
	catalysts for the transfer hydrogenation of acetophenone by
	isopropanol, in comparison with the commercial precursor [(η 6 -
	Cym)RuCl2]2. The results obtained from the various experiments
	are listed in Table

Table 5 .

 5 Catalyzed transfer hydrogenation of acetophenone with 1/NaHMDS at different catalyst concentration.[a]

Acknowledgements

The authors thank the Centre National de la Recherche Scientifique (CNRS), the Institut Universitaire de France (IUF), Keywords: Ruthenium • Iminophosphonamide ligand • p-Cymene ligand • Transfer hydrogenation • DFT calculations

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A highly reactive arene ruthenium iminophosphonamide hydride complex catalyses acetophenone transfer hydrogenation by isopropanol via a proposed zwitterionic intermediate and rearranges to a cyclohexadienyl 16-electron complex in the absence of substrate.

Ruthenium p-cymene iminophosphonamide complexes: activation under basic conditions and transfer hydrogenation catalysis


Dr Kaili Xie

Alizée Glasser

Dr Shekhar Shinde

Zaicheng Zhang

Jean-Michel Rampnoux

Abdelhamid Maali

Dr Eric Cloutet

Georges Hadziioannou

Hamid Kellay
email: hamid.kellay@u-bordeaux.fr

Dr Z A Glasser

Dr J M Zhang

Dr A Rampnoux

Prof H Maali

Kellay

Dr A Glasser

Prof G Hadziioannou

Delamination and wrinkling of flexible conductive polymer thin films

Keywords: conductive polymer, flexible thin films, wrinkling instability, delamination and adhesion

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Introduction

Thin films of conductive polymers such as poly (3,4-ethylenedioxythiophene):polyelectrolytes (PEDOT: polyelectrolytes), have received much attention recently due to their flexibility, low sheet resistance, and good optical properties [1,2]. They are widely used as electrode layers in electronic devices such as organic light-emitting diodes (OLEDs) [3,4], organic photovoltaics (OPVs) [5], energy storage cells [6] and hold much promise in the manufacturing of flexible, all organic, devices and sensors. Another advantage of these polymers is that they are water soluble and can be used as water based inks for various deposition and coating purposes. The most widely used polyelectrolyte to stabilize the PEDOT in water is polystyrenesulfonic acid (PSS). An alternative approach [7] replaces PSS with (trifluoromethylsulfonyl)imide (TSFI) side groups attached to a polystyrene backbone (Figure 1a), poly(4-styrene trifuoromethyl (bissulfonylimide)) (PSTFSIK) (see Methods). The presence of hydrogen bonds in the PSTFSI polyanion simplifies the formulation of the conductive inks for various coating and thin film making processes such as doctor blading, inkjet and screen printing [8]. Stacking printed thin films layer by layer is a low cost and frequently-used technique to build structured devices with the desired electronic properties and functionalities [9,10]. The mechanical stability and adhesion between the adjacent layers is key in the performance and lifetime of such devices [11,12]. Thin flexible films are, however, prone to a number of instabilities: cracks [13,14], tears [15], or delamination and wrinkling [16,17]. This is a limiting factor for the performance of devices and a nuisance which needs to be avoided. On other occasions, however, this behavior can be harnessed to achieve additional functionality such as surface patterning at low cost [18][19][20][21] or to estimate the properties of materials [15,17,22]. Instabilities of thin films can be due to a variety of sources. For example, they may wrinkle or blister in the presence of a mismatch stress because of residual stresses [23], shrinking [24] or swelling as in the case of solvent-responsive polymers [25][26][27][28].

Here, we report a spontaneous wrinkling instability of organic flexible conductive thin polymer films initially deposited on rigid substrates. By simply depositing liquid drops, e.g. water, on the films, welldefined microscale wrinkles can be obtained due to swelling. By focusing on the properties of this instability and using a simple model, we rationalize the phase diagram of the instability and explain its main features. Our study offers a simple approach to avoid or harness such instabilities as well as a reliable one for the measurement of material properties, notably, the adhesion energies of thin films on different substrates.

Wrinkling phase diagram

Thin films of the water based polymer PEDOT:poly(4-styrene trifluoromethyl(bissulfonylimide)) (PE-DOT:PSTFSI) [7,29] (Figure 1a) are deposited onto glass substrates by a Doctor Blade technique [8] (Figure 1b). These films are dried at different temperatures and for different periods of time to evaporate the solvent. The prepared films are then brought naturally to room temperature (21°C) before additional characterizations are carried out. These films are robust and remain intact when folded (Figure 1b). However, when a solvent drop, e.g. water, is deposited onto the film at room temperature, either the film remains intact or a wrinkling instability occurs. When the wrinkling sets in, random wrinkles are observed in the area close to the center of the drop while finger-like wrinkles are observed near the contact line or front of the drop (Figure 1c and Video S1 in Supplementary Information). These fingerlike wrinkles near the edge of the drop grow preferentially perpendicular to the contact front and their number N depends on drop radius R s (micrographs in Figure 1c). Whether wrinkles arise depends on film preparation and in particular its drying conditions. This drying is used to evaporate the solvent remaining in the films. Figure 1d summarizes the instability phase diagram. Films dried at temperatures below 180 • C always exhibit wrinkling, even though the drying time is as long as 90 minutes. By contrast, the films remain stable when the drying temperature is higher than 260 • C. In the intermediate temperature range, the morphology depends on the drying time: Wrinkling is inhibited for longer drying times for example.

Dynamics of wrinkling

We now turn to the dynamics of the wrinkling. When a water drop with controlled size is deposited, it spreads with the appearance of spontaneous wrinkling (see Video S1 in Supplementary Information).

During the spreading, its radius R s and the number of wrinkles N change with time. Four stages can be defined (Figure 2a). In the early period, both the spreading radius and the number of wrinkles increase with time (denoted as Stage I). After this phase, a stationary regime (Stage II) sets in where the number of wrinkles as well as the radius of the drop remain roughly constant. This stationary state is a consequence of the maximum spreading of the drop. After this stage, the evaporation of the water sets in along with a decrease of the number of wrinkles while their local amplitude increases (Stage III). The final stage associated with the full drying of the drop leads to the formation of permanent folds (Stage IV). During the evaporation phase (Stage III), the structure coarsens: the initial small amplitude wrinkles disappear and larger amplitude ones become predominant [28]. These large ridges or wrinkles end up collapsing to give rise to the folds observed in Stage IV. These observations recall the delamination and self-folding observed in other types of thin films such as PDMS films [28] and hydrogel films [30].

The final thickness (far away from the folds) in the central area, previously covered by the drop, is smaller than the initial film thickness which indicates that the film has been stretched (section S1 in Supplementary Information). This alternatively confirms that the formation of the wrinkles and eventual folds [31] is due to the stretching and increase in area of the initial film under the drop. In what follows we will focus on the stationary regime where both the number of wrinkles and the area of the drop are stationary.

Our main concern is to understand what sets the number of wrinkles and what determines whether the instability sets in or not.

In the stationary regime, Figure 2b, 2c and 2d show a simplified schematic as well as the properties of the wrinkling pattern. In experiments, drops with different sizes have been deposited on films of different thicknesses. For each drop, once the stationary state has been reached, we measure the number of wrinkles near the front of the drop (see Figure 1c and section S2 in Supplementary Information). A measurement of the number of wrinkles along the perimeter of the drop gives an estimate of the wavelength of the wrinkling instability. The number of well-defined wrinkles has a linear relationship with the drop radius R s (Figure 2d). This linear relation assures that the wavelength of the instability is invariant as the drop diameter changes. However, as the thickness of the film increases, the slope of the plot of R s versus N , increases signalling that the wavelength λ ∼ R s /N , increases. Thus, the wavelength of the wrinkling pattern is independent of the drop size, but it increases with the film thickness. Previous work [32] has proposed that wrinkling is in part governed by the bending modulus of the film which increases with its thickness. This gives a qualitative explanation of our observations. In order to explain our observations quantitatively, different considerations enter into play. The formation of wrinkles necessarily arises from a competition between the bending of the film, through its bending modulus B, and its adhesion to the substrate [17,33,34]. The substrate (i.e. glass) is rigid, so for wrinkling to occur the film has to detach from the substrate, a process called delamination. Fur-ther, the polymer film used is hygroscopic and swells upon uptake of the solvent [35] as we will see be-low.

While we do not fully understand the effect of drying temperature and drying time on film swelling, it has been proposed that high temperatures, apart from removing the residual water, may also affect the conformational state of the PEDOT-polyelectrolyte complex and thus the structure of the film [35][36][37][38]. This change may then affect how the film responds to the presence of different liquids such as its own solvent.

This swelling increases the area of the film and introduces a mismatch stress between the film under the drop and the rest of the film which remains pinned to the substrate. We hypothesize that this swelling is the driving force for the delamination and the ensuing wrinkling instability. In addition, the drop liquid has to infiltrate beneath the film and wet the substrate and the film, creating two new interfaces, i.e. liquidsubstrate and liquid-film interfaces. The energy cost of this wettability needs to be taken into account [39,40]. We now propose a simple model taking into account these different contribu-tions to explain the observed wrinkling as well as its properties.

Rationalizing the wrinkling patterns

To understand the wrinkling and delamination induced by film swelling in our case, we consider a simple scaling approach based on a one-dimensional analysis following previous work on wrinkling [17,34]. For simplicity, the wrinkling pattern is assumed to be located mainly near the front of the drop and to have a simple periodic functional shape with wavelength λ and amplitude δ. The amplitude δ is assumed to be constant as a function of distance to the center of the drop, at least in the area where we count the wrinkles number. This assumption is difficult to justify a-priori but previous work suggests that this variation is weak [42]. Each wrinkle is also assumed to be under a biaxial compression Π (Figure 2c). The film takes up water and swells resulting in a mismatch strain ε m between the film underneath the drop and the film outside [START_REF] Freund | Thin film materials: stress, defect formation and surface evolution[END_REF].

If the mismatch stress is lower than the so called critical stress for de-lamination, the film is expected to remain adhered to the substrate [34]. The energy due to swelling is stored in the plane of the film [25], and once it overcomes the adhesion, delamination takes place releas-ing the stress. The swelling of the film is essential to the development of the instability comes from ad-ditional measurements of the swelling rate of the film (see section S6 in Supplementary Information and Figure 3c). Under the constraint of constant projected surface area, i.e. the area of the drop, the film can only bend out of the plane introducing (for a single wrinkle) a bending energy U b = Bπ 4 δ 2 /λ 3 (see section S3 in Supplementary Information). As mentioned above, the drop liquid fills the delaminated area introducing two additional surface energies i.e. liquid-film γ lf and liquid-substrate γ ls . The net in-terfacial energy change can be written as temperature. The films are dried for 3 and 30 minutes for the two cases shown. The strain is obtained by measur-ing the increase in the length of a strip upon water uptake. The strips are rectangular and the inset shows an example of the increase in length and width of these strips (see section S6 in Supplementary Information). Inset images show the film before and after swelling (scale bars are 500 µm). d, Wavelength of wrinkling. Each point here is taken from the slope of R s versus N using 4 to 8 drops as shown in Figure 2d. In total, around 190 drops have been tested. The color code gives the thickness of the films used.

U i = λ∆γ, (1)
Different symbols designate films dried at different temperatures and thus different swelling strains. The inset shows λ 2 /h 3 versus the strain ε s . The straight line is the prediction of our model using the same param-eters as the main plot. e, Estimates of the adhesion energy. Both water and glycerol drops are used to introduce wrinkling. The data from AFM measurement shown here is for PEDOT:PSTFSI films. The mechanical properties of PEDOT:PSS are taken from Ref. [41].

where ∆γ = Γ -γ l (cos θ s + cos θ f ) (see section S3 in Supplementary Information), Γ is the adhesion energy, γ l is the surface tension of the solvent, and θ s and θ f are the contact angles of the solvent on the substrate and the film respectively (see section S8 for measurements of the contact angles). The stiffness of the liquid, due to the hydrostatic pressure is neglected as the film is immersed inside the liquid rather than floating [START_REF] Vella | [END_REF]. At equilibrium, the swelling energy is expected to be totally released by bending and interfacial energy change, leading to total energy U t = U b + U i . By minimizing this energy with respect to the wavelength, ∂U t /∂λ = 0, we obtain

λ 4 = 2π 4 B ∆γ δ 2 , (2)
where the bending modulus of the film B = E * h 3 /12 with the effective Young's modulus E * = E/(1ν 2 ) using the Young's modulus E and the Poisson ratio ν. The amplitude of wrinkling δ is too small for measurements using optical methods [17]. On the other hand, the size of the wrinkles is not small enough to use AFM scanning [45]. From the geometrical relationship of the assumed wrinkling profile (Figure 2c), the amplitude δ can be related to the compressive strain by ε m = Π/λ = π 2 δ 2 /4λ 2 , where Π is the compression due to film swelling (see section S3 in Supplementary Information). We assume that the mismatch strain is equivalent to the swelling strain of the film, ε m = ε s because the swelling energy is totally released. Thus, by replacing the amplitude δ in Equation 2, we obtain,

λ = 2π 2 E * 3∆γ ε s h 3 1/2 . (3)
This scaling law is similar to the previously proposed predictions for the wrinkling or buckling wave-length due to a competition between bending and adhesion [17,34]. Our prediction however explicitly takes into account the swelling of the film at the origin of the compression. Note that Equation 3 for the wavelength uses only quantities that can be measured independently and can thus be tested directly. The Young's modulus and, a priori, the energy of adhesion can be obtained by independent measure-ments using AFM nanoindentation [46], as shown in Figure 3a and Figure 3b. The value of Young's mod-ulus E turns out to be near 0.4 GPa with no pronounced dependence on the film preparation temper-ature nor its thickness. The measurements of the adhesion energy on the other hand show much scat-ter from measurement to measurement and from different thicknesses. On average, this adhesion energy hovers around 0.16 J/m 2 with no systematic dependence on the film thickness or drying temperature (see section S7 in Supplementary Information).

The swelling strain can be estimated by independent measurements (see section S6 in Supplementary Information). Figure 3c illustrates that the swelling strain decreases linearly with the drying temperature for a given drying time, and almost zero swelling is detected when the temperature is high. This observation coupled to the prediction of our model explains the fact summarized in the phase diagram (Figure 1d) that the films under high drying temperatures show no buckling: The driving force due to the swelling strain is negligible for these films. Further, the phase diagram indicates that the longer the drying time, the smaller is the temperature for which stable films are obtained. This is in line with our measurements of the swelling strain which decreases as the drying time increases making for more sta-ble films at even lower drying temperatures (Figure 3c). In fact, the phase diagram may be related to the existence of a critical strain for instability (see Figure S8) as predicted recently, with the stable film region having strains below a threshold value [34].

In order to quantitatively compare our experimental results for the wavelength from different deposited drops and different film thicknesses with the theoretical prediction, we plot the wavelength λ versus the quantity (2π 2 E * ε s h 3 /3∆γ) 1/2 (Figure 3d). Here, we use the measured values of the strain and the Young's modulus, however, adhesion ∆γ or Γ, which is difficult to measure with good enough precision, is left as an adjustable parameter. In Figure 3d, the wavelength follows the predicted trend and the best fit cor-responds to a value of Γ = 0.138 J/m 2 . This is in good agreement with the one estimated using AFM measurements; the difference is roughly 14 percent. A summary of the adhesion energies obtained for different drying temperatures is shown in Figure 3e. The energy of adhesion obtained from the analysis of wrinkling is roughly independent of the drying temperature and is consistent with the values es-timated by AFM nanoindentation. To further test the model, we also plot in the inset to Figure 3d the dependence of the wavelength normalized by the thickness of the film, λ 2 /h 3 , on the strain ε s . The agree-ment with the expected result as illustrated by the straight line, using the same parameters as the main figure (Figure 3d), is good indicating that our model captures the essential features of our measurements namely the dependence on the thickness of the film and the dependence on the swelling strain. The pre-diction of the wavelength is thus quantitative and accounts for all the features observed: no dependence of the wavelength on drop radius, sensitivity to film swelling, and good agreement with material parame-ters notably the Young's modulus and the adhesion energy. As mentioned above, our scaling is based on the work of Reference [17,34], but other models have been proposed to explain the wrinkling or buckling instability of thin films. A comparison of our experimental data to other scaling laws using other models is shown in section S4 in Supplementary Information [47][48][49][50]. Because of different geometries and differ-ent driving forces for wrinkling or buckling, these models do not fully explain our experimental findings.

Discussion

This wrinkling instability using water drops can also be observed for other types of films and using other liquids. For example, the film wrinkling is also observed when glycerol drops are deposited on the film (Figure 4a). Compared with the water drop, the wrinkles are more ordered: straight and point outwards at the front of the drop. However, the emergence of this well ordered wrinkling pattern is much slower than that for water drops due to the difference in viscosity of the two liquids. In another example, we used the commercially available conductive polymer PEDOT:PSS, widely used in many applications [6, 51]. A closely similar pattern as that obtained with PEDOT:PSTFSI is observed (Figure 4b). Adhesion energies for these two experiments, obtained using the wrinkling pattern, remain in the range of values measured for PEDOT:PSTFSI films (Figure 3e). We are not aware of other measurements of the adhesion energy of PEDOT:polyelectrolytes polymer inks on glass. Nevertheless, the estimated value of (Γ ≈ 0.14 J/m 2 ) compares favourably at least in order of magnitude with previous measurements on closely related systems [22].

Besides providing a means to measure material properties and surface adhesion in particular, our findings also offer a simple way to pattern thin film surfaces. Figure 4c, 4d and 4e show two nearby drops as well as their respective engendered patterns. The proximity of the two drops leads to a connection between their respective wrinkles: Two neighbouring wrinkles merge with each other forming a connection (Figure 4d and Video S2 in Supplementary Information). As the drops dry, the connection persists making for a connected network of folds (Figure 4e). These folds consist of a few layers of material (section S1 in Supplementary Information). If these layers remain connected, which is the case in this example, a percolated network can then be obtained on the surface of the film if multiple drops are deposited.

We speculate that this network will lead to a better conductivity of the film since sheet resistance in the folded area is lower than in the film. For example, four-layer folds can lower the sheet resistance a factor of 6.8 for undoped films whereas the thickness only increases a factor of 4 (see section S5 in Supplementary Information). More electron pathways are thus created by folding. In the meantime, the film maintains its transparency because of the network remaining sparse.

In summary, we have described a wrinkling instability of thin polymer films. This instability is engendered by simple contact with a liquid (water) and swells the film giving rise to delamination. The observed wrinkling instability can be completely inhibited by using appropriate preparation: High enough drying temperatures or long drying times prevent the swelling of the film and thus the emergence of such buckling. The well-defined wrinkling patterns near the front of the drop, are rationalized using a simple model. The wavelength of the patterns is related to the film swelling, the mechanical properties of the film and the adhesion energy between the film and substrate. By coupling the model to the observations, the adhesion energy of the film-substrate system, a difficult quantity to measure with good precision but essential in the design of electronic devices, can be estimated reliably. The method is applicable to other types of polymer-substrate systems. Further, we show through an example that this wrinkling can be tamed to produce patterned surfaces with improved conductivity. All products were used without further purification. Water was used from a millipore water system (Pure-Lab Flex).

Experimental Section

Polymer synthesis: EDOT was stabilized with potassium salt of polyanion poly(4-styrene trifluoromethyl (bissulfonylimide)) (PSTFSIK) by ionic interaction during the oxidative dispersion polymerization of EDOT in water. The synthesis of PSTFSK followed previous protocols [7,8,52]. The yields of monomer STFSIK and polyanion PSTFSK were around 40% and 50%. Molecular weight of polyanion PSTFSIK used in our study was 100kDa. The complexation between EDOT and PSTFSIK was done according to Ref. 8, 29. For further processing, the ink solution was concentrated with a final concentration of PE-DOT around 1.2% which is very viscous at room temperature.

Substrate preparation: Prior to film deposition, microscope slides (RS France, 26 mm x 76 mm) were first cleaned with acetone, ethanol and isopropanol in an ultrasonic bath for 10 minutes each and then dried with clean nitrogen. To enhance the wettability, the slides were subsequently treated by UV/ozone for 15 minutes. After cleaning, we used the substrates immediately. The contact angle of liquid on the substrate was measured by static sessile drop method (DSA100, Krüss). For flexible substrates, kapton was cleaned with ethanol and isopropanol, and dried with nitrogen gas before depositing onto a glass substrate. The conductive polymer ink was then coated on the kapton surface.

Film deposition and processing: A rigid doctor blade (Erichsen) was used to deposit the film onto the pre-cleaned substrates (glass or glass covered by kapton). The thickness of the film can be tuned by adjusting the gap between the substrate and the blade and coating speed. Since the PEDOT:PSTFSI solution is shear-thinning (inset in Figure 1b, measured by a rheometer (TA Instruments)), we carefully chose the proper gap and speed. After deposition, the film was dried on a hot plate in ambient atmosphere under different temperatures and for different times in a clean room, followed by cooling to room temperature. The film thickness was then measured by a profilometer (Bruker Dektak) and interferometry (Horiba JY UVISEL) (section S1 in Supplementary Information). The contact angle of the liquid on the film was measured by a static sessile drop method (section S8 in Supplementary Information).

Four-point probe measurement: The sheet resistance of the film was measured by a four-point probe device (Lucas 302). A computer controlled Keithley was used to apply the current (10-100 µA) through the two outer probes while the voltage was measured by the two inner probes. The measurement accuracy is around 1%. Several different positions were measured for each sample.

Young's modulus, adhesion and swelling properties measurement: The Young's modulus of the film was estimated by performing nanoindentation measurements using AFM (D3100, Bruker). A silicon cantilever (LRCH 225 C3.0-R, Nanotec GmbH) with a radius of curvature of 840 nm and spring constant of 4.8 N/m was used. Several positions were tested for each sample. Two different ramp sizes, 500 nm and 1000 nm, were used for each force curve measurement. To fit the force-indentation trace curve, Johnson-Kendall-Robert (JKR) model [53] was considered. In our experiments, all the measurements were conducted at the relative humidity of 45-60% RH and room temperature of 21-22 • C.

To estimate the compressive strain due to swelling, we carefully cut the film into rectangular pieces with different sizes (at least 5 different pieces), and immersed the samples into the solvent bath. Once the swelling reaches equilibrium (a few seconds), images were captured by a camera (C11440, Hamamatsu) equipped on a microscope (AX10, Zeiss). The lengths of four sides before and after swelling were measured by ImageJ. The swelling strain is thus calculated as ε s = (L s -L 0 ) /L 0 , where L 0 and L s are the unswollen and swollen lengths, respectively. More details are given in section S6 in Supplementary Information.

Drop deposition: To deposit water drop with controllable size on the film, a needle (320 µm of diameter)

was connected to a precise syringe pump (Pump 11 Elite, Harvard Apparatus). Once the drop was generated, The needle was lowered slowly to approach the film surface. When the drop touched the surface, the needle was quickly removed out of the observation window, to avoid its influence on visualization (see the videos in Supplementary Information). For glycerol drops, since its viscosity is large, a larger needle size (820 µm in diameter) was used. All images were directly visualized with an inverted microscope (AX10, Zeiss) equipped with a high resolution camera (C11440, Hamamatsu). Image processing was done by a home-made Matlab program.

Figure 1 :

 1 Figure 1: Thin films preparation and wrinkling phase diagram. a, Chemical structure of the conductive polymer PEDOT:PSTFSI. The red dashed lines indicate probable hydrogen bonds. b, Film preparation: doctor-blade coating, drying on the hot plate and cooling to room temperature. Viscosity is measured by a shear rheometer (AR1000, TA Instruments). c, Radial organization of wrinkling. Water drops are deposited via a needle, and an inverted microscope is used to visualize the wrinkling patterns. The images show an increase of the number of wrinkles N counted near the edge of the drop, indicated by the dashed line, with the drop size R s . The average spacing between finger-like wrinkles is desig-nated as λ. The outer full circle indicates the edge of the drop while the dashed circle near the edge indicates the position where the number of wrinkles was counted. Scale bars: 500 µm. d, Phase diagram of surface stability of films dried at different temperatures and times. The shaded region indicates the transition zone between wrinkle formation and stable films. The presence of superimposed symbols (squares and circles) indicates situations where stable or wrinkled films can be obtained for different films suggesting sensitivity to experimental conditions. Two images are shown in the unstable and stable regions. The full circle in the image for T = 260 • C for which no wrinkles are observed represents the position of the water drop front in the stationary regime. Scale bars: 500 µm.

Figure 2 :

 2 Figure 2: Dynamics and properties of the wrinkling instability. a, Four stages are observed based on the state of the drop, its radius R s and the number of wrinkles N versus time t. The number of wrinkles here is counted near the drop front.Insets show images of the wrinkling patterns in the different stages. Scale bar: 500 µm. b, Schematic of the hypo-thetical three dimensional structure of the well-defined wrinkling pattern near the contact line. c, Hypothetical profile of a single wrinkle with a height δ and wavelength λ. The profile along the perimeter ξ(l) is expected to have a cosine shape (Equation S1) under biaxial compression Π when the wavelength λ is much smaller than the radius R s . d, Dependence of the number of wrinkles N versus the radius R s at the front of the drop in the stationary regime. Each data point repre-sents an independent measurement on a drop of radius R s . Different drop sizes and film thicknesses are used. Note that since the number of wrinkles measured near the edge of the drop increases linearly with drop radius R s , the wavelength of the wrinkling pattern is thus independent of drop size in our observation.

εFigure 3 :

 3 Figure 3: Characterization of the films and wrinkles wavelength. a, An example of a force-displacement curve for trace (approach) and retrace (retraction) from a nano-indentation test by AFM. The Young's modulus can be obtained by fitting the force-indentation curve with the JKR model, and the adhesion force F a can be estimated at the moment of separation between the tip and the film. Inset: SEM image of AFM tip (made of Silicon but covered with an SiO 2 layer, R c = 840 nm). b, Young's modulus of the films prepared under different drying temperatures. Inset: Young's modulus of films with different thicknesses. Poisson ratio is assumed as 0.4 here. c, Examples of swelling strain of the films as a function of drying

Figure 4 :

 4 Figure 4: Polymer film surface patterning. a, Periodic wrinkling induced by a glycerol drop on a PEDOT:PSTFSI film. b, Wrinkling pattern on a PEDOT:PSS film using a water drop. c-e, Interaction of wrinkling patterns induced by two water drops. A connection takes place when the wrinkles are close enough (the red rectangle) and it remains after drying.

Materials: Sodium 4 -

 4 vinylbenzenesulfonate, 2,2'-azobis(2-methylpropionitrile)(AIBN), hydrochloric acid, dichloromethane, potassium carbonate and iron III chloride were purchased from Sigma-Aldrich. Acetone, ethanol, isopropanol, methanol, diethyl ether, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF) and oxalyl chloride were purchased from Sigma-Aldrich as received. N,N-dimethylformamide (DMF) and dry-acetonitrile were purchased from Acros Organics. Sodium bicarbonate, triethylamine, ammonium persulfate and 3,4-Ethylenedioxythiophene (EDOT) were purchased from Fisher Scientific. Trifluoromethanesulfonamide and S-dodecyl-S -(α,α -dimethyl-α -acetic acid) trithiocarbonate (CTA) are from ABCR GmbH. Lewatit S100 KR/H and MP 62 WS were purchased from Lanxess. Commercial PEDOT:PSS (Clevios PH500) solution was purchased from Heraeus. Glycerol was purchased from VWR.

Acknowledgements

The authors gratefully acknowledge funding support from LabEx AMADEUS ANR-10-LABEX-0042-AMADEUS, the program of Excellence Initiative IdEx ANR-10-IDEX-003-02, ANR MULTISPOT ANR-17-CE09-0015. K.Xie thanks S. Khiev for SEM measurement, D. P. Mantione for fruitful discussion, and M. Bousquet for the help of polymer synthesis. The authors also thank the machine shop at LOMA for help with experiment setup. H. Kellay acknowledges support from Institut Universitaire de France.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the corresponding author.