Laure Benhamou

Joffrey Wolf

Vincent César

Agnès Labande

Rinaldo Poli

Noël Lugan

Guy Lavigne

Chelation-Assisted Reactions of Phosphine-and Olefin-Tethered Imidazolium Derivatives and Their Affiliated N-Heterocyclic Carbenes with Roper's Complex Ru(CO) 2 (PPh3)3

published or not. The documents may come

Chelation-assisted reactions of phosphine-and olefintethered imidazolium derivatives and their affiliated Nheterocyclic carbenes with Roper's complex Ru(CO) 2 (PPh 3 ) 3 Laure Benhamou, a Joffrey Wolf, a Vincent César, a Agnès Labande, a Rinaldo Poli, a,b Noël Lugan, a and Guy Lavigne* a a CNRS ; LCC (Laboratoire de Chimie de Coordination) ; 205, route de Narbonne, F-31077 Toulouse, France, and Université de Toulouse ; UPS, INPT ; LCC ; F-31077 Toulouse, France. b Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France. Email: Lavigne@lcc-toulouse.fr

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ABSTRACT. Complex Ru(CO)2(PPh3)3, 1, is a suitable starting compound for the generation of Nheterocyclic carbene complexes of Ru (0). Though monodentate NHCs are totally unreactive toward 1, phosphine-or olefin-functionalized N-heterocyclic carbenes, as well as their imidazolium precursors, react with 1 under chelation assistance where the phosphine or the olefin are acting as directing groups.

Reactions of 1-mesityl-3-(2-diphenylphosphinoeth-1-yl)-imidazolium bromide, [HL 1a ] + Br - 1-mesityl-

Introduction

During the past fifteen years, N-heterocyclic carbenes (NHCs) have gained considerable significance in synthetic organometallic chemistry and catalysis, 1 essentially because of their ability to function as powerful ancillary ligands in a variety of catalytically active metal complexes, 2 and, albeit to a lower extent, as reactive intermediates in certain transition-metal catalyzed transformations of their imidazolium precursors. 3,4 In the chemistry of ruthenium(II), representative evidences for their major benefits as ligands are found in the development of "new generations" of Grubbs and Grubbs/Hoveyda catalysts. 5 By contrast, studies of the interaction of NHCs with basic mono-or polynuclear carbonyl or carbonyl/phosphine derivatives of Ru( 0) are only beginning to emerge. In the footsteps of the pioneering work of Lappert on reactions of the dimeric 1,3-dialkylimidazolin-2-ylidene with metal carbonyls, 6 several authors have recently revisited the substitution reactions of Ru3(CO)12, now using various stable N-heterocyclic carbenes as incoming ligands. 7,8,9 Such reactions were found to be very dependent on the carbene steric and electronic properties, the faster and more efficient substitutions being observed with the more basic and less bulky ligands. 7 Cabeza and co-workers were the first to report the isolation of a simple monosubstituted 1,3-dimesitylimidazol-2-ylidene tri-ruthenium carbonyl derivative, Ru3(CO)11(IMes), obtained in 36% yield and exhibiting a normal coordination of the carbene through the C2 atom. 7b Quite unexpectedly, Whittlesey 8 observed that a parallel substitution reaction involving the bulkier carbene 1,3-di-tert-butylimidazol-2-ylidene (ItBu) gives the substituted derivative Ru3(CO)11(ItBu*) (81% yield) in which the ligand is bound to the metal through the backbone C4 atom of the heterocycle, thus reflecting the occurrence of an "abnormal" C-H activation 10 accompanied by transfer of the activated H to the available C2 site.

Beyond the scope of cluster chemistry, where further irreversible transformations of these NHCs at contiguous metal centers are seen to occur via subsequent C-H activation reactions, 7,8 the above trinuclear IMes complexes can be alternatively degraded with an excess of ligand to give Ru(NHC)(CO)4 or Ru(NHC)2(CO)3 in low to moderate yields. 8b,9 Better than such a multistep "cluster" route, efficient methods for the incorporation of N-heterocyclic carbenes into mononuclear ruthenium carbonyl complexes have been proposed. They involve phosphine (or arsine) displacement from various Ru(II) precursors such as RuH(CO)Cl(PCy3)2, 11 RuH2(CO)(PPh3)3 and RuH2(CO)(AsPh3)3, 12 or direct addition of the NHC to [Ru(CO)2Cl2]n. 13 Further reduction of the resulting NHC/Ru(II) complexes to Ru(0) appears to be problematic and has been observed only under CO, again giving only access to the tricarbonyl derivative Ru(NHC)2(CO) 3. 8b,12 However, just like their phosphine analogues Ru(PR3)2(CO)3, such complexes are reluctant to CO loss, hence, they remain poorly reactive and of limited practical utility.

The particularly reactive benchmark derivative Ru(CO)2(PPh3)3, known as Roper's complex, 14 might appear as a suitable starting complex for the direct generation of Ru(0)/NHC complexes, especially since it is now readily available in good yield through a fast preparative procedure. 15 This complex is fluxional and exists as a mixture of two rapidly interconverting isomers with a very low activation energy barrier (chart I). 14,16 Its intrinsic high substitutional lability has been rationalized in terms of the transient generation of an unsaturated 16 e -species "Ru(CO)2(PR3)2", 17 (chart 1) which has been even isolated in the case of the bulky phosphine PMe t Bu2, and is prone to add a variety of 2e -donor substrates S within the time of mixing, giving simple substituted derivatives of the type Ru(CO)2(PR3)2(S) (S = basic phosphine, alkyne, olefin). 17,18 With H2 or HX type substrates possessing reactive H-element bonds (S = HCl, H-CCR, H-SiR3), the transient Ru(0) adduct "Ru(CO)2(PR3)2(HX)"

is not intercepted, since rapid oxidative addition of the H-element bond to the metal gives directly the Ru(II) species Ru(H)(X)(CO)2(PR3)2. 18 The complex Ru(CO)2(PPh3)3 was originally used as a pre-catalyst for the Murai reaction, 19 an early example of chelation-assisted catalytic functionalization of substrates possessing unreactive C-H bonds. 20 In this context, our recent experimental modeling of a stepwise Ru-mediated stoichiometric hydro-acylation of an alkyne with a tethered aldehyde 21 provides a hint that the same concept might be transposable to a broader range of coupling reactions.

In a preliminary set of experiments, it was found that monodentate 1,3-disubstituted imidazol-2ylidenes (incorporating mesityl, cyclohexyl, or methyl substituents) are totally reluctant to react with Ru(CO)2(PPh3)3. Indeed, IR monitoring indicated that no reaction occurs at room temperature, whereas sacrificial transformation of 1 into the thermodynamically more stable complex Ru(CO)3(PPh3)2 occurs at high temperatures. We were thus prompted to examine whether the functionalization of one of the two heterocyclic nitrogen atoms by a potentially coordinating side arm would assist the incorporation of the heterocycle into the metal's coordination sphere.

Two parallel approaches are presented here. They deal respectively with two categories of hybrid heterocyclic ligands (see Chart II) differing in the nature of the "directing group", namely, (i) a phosphine-tethered imidazolium and its affiliated NHC derivative, and (ii) an olefin-tethered imidazolium, and its NHC analog.

The propensity of a phosphine to act as a directing group susceptible to assist the interaction of an Nheterocycle with a metal center has been established for various metals, 22 but rarely applied to the case of ruthenium. 22b In parallel, the ability of an olefin -a chemically reactive substrate -to play such a role, was proposed to account for the transition-metal catalyzed annulation of heterocycles, reported by Ellman 4 for rhodium complexes, and by Cavell 3 for nickel complexes. In light of such precedents, it was of interest to examine the possible transposition of such chelation-assisted reactions to the case of a benchmark ruthenium(0) complex.

Results and discussion

A. Phosphine-functionalized imidazolium derivatives

Both the mesityl (a) and 2 Abnormal C-H activation of imidazolium cations has been observed in various instances and is now well documented. 10 In their studies of the coordination of hybrid imidazolium pyridine ligands to iridium, Crabtree and Eisenstein 10e reported the observation of an anion-dependent switch in selectivity between the activation of C2-H and C4-H positions, which was tentatively ascribed to the different acidities of the two sites. However, such an explanation cannot be transposed to the present case since we do observe that the reaction of [HL 1a ] + BF 4 -with 1 still involves selective activation at the C4 position (as observed with complexes of other metals, particularly Ir(I)), 22d,23e possibly reflecting a slightly better steric accessibility of such a site.

B. Phosphine-functionalized NHC derivatives.

In line with the above results, we became interested in synthesizing the first Ru(0) analogs of Roper's complex Ru(CO)2(PPh3)3 (1) incorporating a chelating N-heterocyclic carbene/phosphine ligand. The above phosphine-functionalized imidazolium derivatives [HL 1a-b ] + Br -were thus deprotonated by KOtBu in THF for 10 minutes and subsequently transferred to a solution of complex ). Whereas this IR pattern corresponds to a di-carbonyl ruthenium(0) complex possessing two CO ligands in cis position, the position of these bands at very low wavelength provides convincing evidence for the presence of a strongly basic ligand within the metal's coordination sphere. 16,17 The new complex was subsequently isolated and indeed unambiguously formulated as Ru{L 1b }(CO)2(PPh3) (3b) (92% yield) based on NMR data. The occurrence of a normal coordination of the NHC through the C2 center was inferred from the 13 C{ 1 H} NMR spectra showing a doublet of doublet at  = 185.9 ppm for C2 and singlets at  =120.5 ppm and  =123.6 ppm for the backbone atoms C4 and C5. The relative position of the NHC ligand with respect to the two phosphorus centers was deduced from the magnitude of the 2 JC2P coupling constants of 63 Hz, and 35 Hz associated with that signal, and from selective hetero-nuclear decoupling experiments revealing that the carbene is in a trans position relative to the PPh3 ligand, and, inherently, in a cis position relative to the RPPh2 arm, the latter two P nuclei being in a mutual cis position ( 2 JPP = 30 Hz). Taken altogether, the spectroscopic data are fully consistent with the structure shown in Scheme 2 for 3b. It should be mentioned that when the reaction was carried out with the mesityl-substituted ligand L 1a , the corresponding Ru(0) complex Ru{L 1a }(CO)2(PPh3) (3a)

was also obtained in good yield (92%), but, curiously, the complex, apparently existing as a mixture of two inseparable isomeric forms as revealed by IR spectroscopy, 17d appeared to be rather unstable, which precluded its full characterization by NMR spectroscopy. This complex represents the first analogue of Roper's complex incorporating an N-heterocyclic carbene.

Scheme 2. Chelation-assisted reaction of Ru(CO)2(PPh3)3 with a phosphine-functionalized Nheterocyclic carbene.

C. Olefin-functionalized imidazolium derivatives.

It was also of interest to determine whether an olefin -a chemically reactive donor ligand -could be used as a directing group for the chelation-assisted cleavage of an unreactive C-H bond onto a Ru center, with a view to the possibility of exploiting this for the annulation of heterocycles, known to be catalyzed by certain transition metals. 3,4 We were thus led to prepare several olefin-functionalized imidazolium ligands, namely, 3-allyl-1-mesitylimidazolium bromide, and 3-(but-3-enyl)-1mesitylimidazolium bromide ([HL 2a ] + Br -), by simple nucleophilic substitution of allyl or homoallyl bromide respectively by 1-mesitylimidazole. We also prepared the saturated equivalent of the latter, namely, 3-butyl-1-imidazolium bromide, which we used in a preliminary blank experiment aimed at verifying that no C-H bond activation occurs when the nitrogen substituent of the imidazolium is a saturated aliphatic arm.

Preliminary assays using 3-allyl-1-mesityl imidazolium bromide as a potential substrate, did not lead to the expected C-H activation. Instead, its reaction with 1 afforded the known cationic allyl derivative [Ru(C3H5)(CO)2(PPh3)3] + Br -via C-N bond cleavage 24 and concomitant recovery of 1mesitylimidazole. With the aim to avoid such a splitting, we were thus prompted to start from the homo-allylic imidazolium salt, [HL 2a ] + Br -, possessing one more carbon atom in its aliphatic chain.

The latter was effectively found to react cleanly with Ru(CO)2(PPh3)3 (1) in toluene solution at 110°C over a period of two hours, giving a colourless complex which was fully characterized in solution and isolated in crystalline form (77% yield) (see Scheme 3). The reaction was monitored by infrared spectroscopy, following the disappearance of the (CO) stretching bands of complex 1 (1907, 1857 cm -1 ) and the appearance of two (CO) bands at 2012 and 1948 cm -1 indicative of the formation of a ruthenium(II) complex containing two carbonyl ligands in a cis position. Here, the disappearance of the imidazolium C2-H signal in the 1 H NMR spectra indicated that "normal" C-H oxidative addition at the C2 position had taken place. This is corroborated by the 13 C{ 1 H} NMR spectrum showing a doublet at  = 183.2 ppm for the carbene carbon atom, with an associated 2 JCP coupling constant of 89

Hz indicative of a trans arrangement of the carbene relative to the remaining PPh3 ligand.

Furthermore, the absence of any hydride signal and the emergence of the characteristic signal of a Ru-CHR group in the 13 C NMR spectra at  = 41.0 ppm ( 2 JCP = 6.0 Hz) strongly suggested that olefin insertion into the transient Ru-H bond had taken place. The new complex was unambiguously formulated as the hybrid NHC/alkyl derivative Ru{Ar(N2C3H2)CH2C(H)(CH2CH3)}(CO)2( PPh3)Br (4a) on the basis of an X-ray structure analysis. An ORTEP drawing of the complex is shown in Figure 1, along with a selection of relevant interatomic distances and bond angles, whereas relevant crystallographic data are set out in Table 1. The X-ray analysis confirms the cis arrangement of the CO ligands, and the trans arrangement of the carbene carbon atom C3 relative to the remaining PPh3 ligand. The coordination sphere of the ruthenium center is completed by a bromine atom trans to a first CO ligand, and an alkyl fragment trans to the second one. Clearly, it appears that the imidazolium/olefin ligand L 2a was converted into a chelating NHC/alkyl ligand forming a fivemembered metallacycle with the ruthenium through additional bonding with C7. The five-membered metallacycle adopts an envelope conformation, C7 pointing away by 0.592 Å from the Ru1 / C3 / N2 / C6 plane. It appears that the carbene moiety is significantly tilted away from the ideal octahedral basis set (P1-Ru1-C3 = 169.67(7)°) reflecting both the steric strain within the five-membered metallacyle, and the steric repulsion between the mesityl ring and the carbonyl ligands in the equatorial plane.

Given that 4a possesses two stereogenic centers, namely, Ru1 and the alkyl carbon atom C7, the generation of two diastereoisomers can be anticipated. Actually, although the 1 H spectra appear relatively limpid, the 31 P NMR spectra show, besides the main singlet at 26.0 ppm, an additional singlet at 26.4 ppm in an approximate 1:9 ratio, while some signals in the 13 C spectra show a very weak parent resonance (see experimental). Since the elemental analysis is reasonably good, this impurity may actually be the second diastereoisomer of 4a. The isomer corresponding to the solidstate structure reported here is presumably the thermodynamically more stable one, where the ethyl group of the alkyl moiety is directed away from the bulky bromide ligand.

The formation of complex 4 could be reasonably understood in terms of a stepwise sequence involving: i) chelation-assisted activation of the C2-H bond of the cationic 3-(but-3-enyl)-1mesitylimidazolium pro-ligand to give an elusive cationic NHC/olefin-hydrido-complex; ii) reversible hydride migration to the olefin giving an alkyl species; iii) migration of the double bond along the chain by reversible -H elimination to afford the observed five-membered metallacycle; iv) capture of the bromide anion by the unsaturated cationic alkyl intermediate.

D. Olefin-functionalized NHC derivatives.

In further work, we were interested in the synthesis of a potentially hemilabile NHC/olefin Ru(0) complex. [START_REF] Hahn | For examples of relevant hemilabile coordination of hybrid NHC/olefin ligands with other transition metals[END_REF] In a one-pot reaction, the 3-(but-3-enyl)-1-mesitylimidazolium cation [HL 2a ] + Br was deprotonated in situ by addition of KOtBu in THF at 0°C for a period of one hour, [START_REF] Fürstner | NHCs were shown to be stable against olefins[END_REF] then allowed to react with 1 at room temperature. The reaction, monitored by following the evolution of IR (CO) bands, appeared to be spectroscopically quantitative within one hour, producing a white solid, which was unambiguously identified on the basis of spectroscopic and crystallographic data as the new complex Ru{L 2a }(CO)2(PPh3) (5a) (96% yield) (Scheme 4).

The multiplicity and position of the (CO) bands for 5a are indicative of the formation of a dicarbonyl Ru(0) complex, in which the CO ligands are in a cis position (two bands: 1947(m), 1879(s) cm -1 ). In the associated 2 JCP coupling constant of 87 Hz clearly shows that it occupies a trans position relative to the remaining PPh3 ligand. NMR data are otherwise in agreement with the persistence of a but-3-ene-1-yl moiety (see experimental). Resonances attributable to the olefinic termination, however, appear to be significantly shifted up-field, as compared with the corresponding ones in the pro-ligand The structure of this unique NHC/olefin Ru(0) 18 e -complex is shown in Figure 2, along with a selection of interatomic distances and bond angles, whereas relevant crystallographic data are set out in Tables 1. The complex exhibits a trigonal bipyramidal geometry where the NHC unit and the remaining triphenylphosphine ligand occupy the two opposite apexes, whereas the two mutually cis carbonyl ligands and the  2 terminal butenyl moiety occupy the three equatorial coordination sites. The carbeneto-ruthenium bond length in the present Ru( 0) complex (Ru1-C3 = 2.0775(13) Å) is not significantly different from the corresponding bond in 4a (4a: Ru1-C3 = 2.071(2) Å), and actually appears to fall within the typical range for Ru / NHC complexes. 7,8,12 The butenyl C=C termination is bound in a slightly dissymmetrical fashion to Ru, the internal Ru-C bond being the longest one (Ru1-C8 = 2.2076( 14) Å, Ru1-C9 2.1838(13) Å), whereas its coordination plane is slightly tilted off the equatorial plane ({Ru1-C1-C2-CENTC8C9}-{Ru1-C8-C9} = 11.3°). As expected, due to coordination, the C8-C9

bond distance (C8-C9 = 1.430(2) Å) is significantly longer than one would expect for an isolated C=C bond (1.299 Å). 27 Noticeably, the carbonyl ligand C1O1, which is located below the mesityl ring, is significantly bent away from the ring (Ru1-C1-O1 = 172.91 (12); O1-C1-Ru1-C11 = -170.1(1)°)

probably due to steric repulsion.

For comparative purposes, we remind that several olefin adducts of Roper's complex are known. 14,28 In particular Kakiuchi 28c previously reported a parallel reaction of 1 with an olefin-functionalized 1965(m), 1899 (s) cm -1 (CDCl3)), corroborating the fact that the N-heterocyclic carbene acts as a better electron-donor than the phosphine. The observation that such frequencies are yet relatively high, illustrates the fact that the olefin plays an important role as a  acceptor, which also results in the above mentioned lengthening of the C=C double bond. It appears that the Ru(CO)2(NHC)(PPh3) fragment of the present complex relies heavily on its -basicity for binding the ethylene, as previously noted by

Caulton in the case of the unsaturated species Ru(CO)2(P t Bu2Me)2. 16,17 This may account for the relatively high stability of 5a, as indicated, for example, by the fact that, unlike Roper's complex, it does not react with molecular hydrogen at atmospheric pressure. Such a behaviour is also in sharp contrast with the reported case of an olefin-functionalized NHC complex of Iridium, where the olefinic arm could be readily hydrogenated under one atmosphere of H 2 at 20°C. 25b

Since 5a is the first NHC olefin complex of Ru( 0), it appears as a convenient model to trace the elementary transformations of an olefin in the vicinity of a strongly basic Ru(0) center. In the context of the present work, we were interested in the identification of elusive intermediates generated through the metal-mediated transformation of the salt [HL 2a ] + Br -right after the key C-H bond activation step, on the way to the alkyl species 4a (Scheme 3). Considering that C-H bond cleavage is expected to produce a cationic carbene-hydrido-derivative as the first putative intermediate undetectable under thermal activation, it seemed obvious that such a complex could alternatively be generated at low temperature by simple protonation of 5a, which effectively proved to be the case.

The addition of HBF4 to 4a was performed at -80°C, and monitored by NMR spectroscopy. The initial complex generated at low temperature was spectroscopically identified as the cationic hydrido NHC/olefin derivative [RuH{L 2a }(CO)2(PPh3)] + BF4 -, [6a] + BF4 -(Scheme 5) exhibiting a 31 P NMR signal at  = 39.1 ppm. 1 H NMR data revealed, in particular, the occurrence of (i) two signals at  = -0.66 ppm and  = -0.11 ppm corresponding to the two terminal hydrogen atoms of the olefinic substituent of the NHC, and (ii) a hydride signal at  = -7.80 ppm. Very characteristically, the latter three signals appeared to be relatively broad, as compared with those of the mesityl signals for instance, suggesting that the corresponding three protons were engaged in a fast exchange process on the NMR timescale. This was established by a selective 1D DPFGSE-NOE experiment 29 revealing a selective enhancement of the terminal olefinic proton signals upon irradiation of the hydride signal at -7.80 ppm (Figure 3). 30 Examination of the coalescence between the hydride signal and the terminal olefinic protons signals (. = 3850 Hz, Tc  255 K) [START_REF]between Z and E isomers different from 1:1 reflects the fact that the formation of one isomer is kinetically favored over the second one[END_REF] led to an estimated activation barrier of ca. 10 kcal.mol -1

for the relevant dynamic exchange process, which can be rationalized in terms of the transient (Tm = 100 ms) (upper spectrum) providing evidence for the exchange process between the terminal olefinic protons ( -0.11 and -0.66 ppm ) and the hydride ligand ( -7.80 ppm). 30 During the course of NMR experiments aimed at determining the activation barrier of the above process, we were led to observe that complex 6a further evolves above -20°C in an irreversible manner. Interestingly, and even though none of the above unsaturated cationic alkyl intermediates [7a] + or

[9a] + is directly observable, their existence could be inferred from trapping experiments, taking advantage of their intrinsic unsaturation. As shown in scheme 6, we were effectively led to observe that the addition of bis-triphenylphosphineiminium chloride (PPNCl), to the final mixture containing [8a] + , results in quasi-instantaneous chloride capture by the elusive unsaturated alkyl intermediate [9a] + with formation of the neutral alkyl species 4a', the chloro-equivalent of the previously isolated bromocomplex 4a, obtained in a 1:1 mixture of diastereoisomers, different from that obtained through the thermal reaction (Scheme 3). Thus, the entire sequence displayed here provides a suitable model for the observed generation of the NHC/alkyl species upon reaction of 1 with [HL 2a ] + Br -.

Scheme 6: Trapping of the elusive cationic alkyl intermediate by halide capture leading to the neutral alkyl species 4a'.

Logically, addition of the halide ion Cl -prior to the isomerisation of the olefinic arm, i.e. when [6a] + is still present in solution, should give a neutral alkyl complex resulting from halide capture by [7a] + .

This effectively happens when the protonation of 5a is achieved at 0°C with HCl, since the potentially coordinating halide anion is immediately liberated in solution. As expected, (Scheme 7), quenching the vacant site by coordination of the halide also quenches any possibility of -elimination, thus blocking the isomerisation process. Quite surprisingly, however, the anticipated neutral alkyl intermediate is not intercepted due to its immediate conversion into the 16 e -NHC-acyl-Ru(II) complex Scheme 7. Protonation of the hybrid NHC/olefin complex 5a by HCl and generation of the NHC/acyl complex 10a.

At first sight, the spontaneous conversion of an 18 e -Ru(II) alkyl carbonyl complex into the corresponding formally unsaturated 16 e -acyl derivative might appear as unexpected, though there are precedents for such a transformation in the literature, and in particular, in the case of comparable carbonyl phosphine complexes. 33 Besides, the reluctance of the closely related alkyl complex 4a' to undergo migratory CO insertion is probably due to the high thermodynamic stability of its fivemembered metallacyclic ring. Importantly, chelation assistance was also found to be a prerequisite to observe C-H activation from the parent hybrid imidazolium/phosphine or imidazolium/olefin ligands. Kinetic preference for abnormal vs normal C-H activation of the heterocycle was observed in the case of the imidazolium/phosphine ligand, but such a preference may not be systematic since it can be determined by the steric bulk of the nitrogen substituents. Normal coordination at the C2 center takes place if the imidazolium precursor is de-protonated prior to complexation. Normal coordination was also observed with the imidazolium/olefin ligand bearing the same substituents. In that case, the elusive hydrido intermediate was not intercepted under the temperature conditions of such an activation reaction, because of rapid H transfer to the olefin to give a new NHC/alkyl Ru(II) complex. Nevertheless, the principal cationic isomeric hydrido olefinic intermediates sequentially generated in such a transformation were identified in an NMR experiment following the evolution of Ru(CO)2[NHC/olefin](PPh3) in the presence of a proton source. It was shown that the isomerisation of the metal-mediated olefinic arm takes place through elusive high energy cationic intermediates which are not directly observable, but, because of their unsaturation, can be quenched by incoming nucleophilic anions. It is noteworthy that the interconverting intermediate complexes detected here are closely related to those postulated by Cavell in the nickel-catalyzed annulation of heterocycles, except that the final CC bond forming reaction involving the carbene center does not occur in the case of ruthenium. Unlike what was observed with nickel, once an N-heterocyclic carbene becomes coordinated to Ru, it systematically behaves as an excellent strongly stabilizing ancillary ligand and is thus unlikely to be subsequently engaged in any further reductive CC bond forming reaction involving its carbene center.

Conclusion

Experimental

General considerations. All manipulations were performed under an inert atmosphere of dry nitrogen by using vacuum line and Schlenk tube techniques. THF and diethyl ether were distilled from sodium/benzophenone and toluene was distilled from sodium. Pentane, hexane and dichloromethane were dried over CaH2 and subsequently distilled. Deuterated solvents (dichloromethane, benzene and chloroform) were dried over CaH2, vacuum distilled, degassed by three freeze-pump-thaw cycles, and stored under a nitrogen atmosphere in Teflon valve ampoules. NMR spectra were recorded on Bruker AC200, WM250, AV300, AV400, or AV500 spectrometers. 1 H and 13 C chemicals shifts (δ) are given in ppm (the residual peak of deuterated solvents was used as reference). [START_REF]between Z and E isomers different from 1:1 reflects the fact that the formation of one isomer is kinetically favored over the second one[END_REF] P chemicals shifts are reported in ppm. Infrared spectra were obtained on a Perkin Elmer 1725 FT-IR spectrometer for solutions and on a Perkin Elmer GX 2000 FT-IR spectrometer (ATR mode) for solids. Microanalyses were performed by the micro analytical Service of the Laboratoire de Chimie de Coordination and MS spectra by the mass spectrometry service of the Paul Sabatier University. Melting points were obtained with a Stuart Scientific Melting Point apparatus SMP1. Compounds Ru(CO)3Cl2(thf), 34 and Ru(CO)2(PPh3)3, 15 were prepared using our previously published synthetic procedures. 1-Mesitylimidazole was prepared according to a literature procedure. 35 The hybrid phosphine/imidazolium salts 1

-mesityl-3-(2- diphenylphosphinoeth-1-yl)-imidazolium bromide, [HL 1a ] + Br -1-mesityl-3-(2-diphenylphosphinoeth-1- yl)-imidazolium tetrafluoroborate, [HL 1a ] + BF4 - and 1-(2,6-diisopropylphenyl)-3-(2-
diphenylphosphinoeth-1-yl)-imidazolium bromide [HL 1b ] + Br -were prepared using procedures established by some of us. 233-Allyl-1-mesitylimidazolium bromide. 3-Bromopropene (370 µL, 4.25 mmol) was added to a solution of 1-mesitylimidazole (750 mg, 4 mmol) in toluene (5 mL). The mixture was heated in an oil bath at 100°C for 20 hours during which time a white precipitate appeared. After cooling, Et2O (10 mL)

was added and the overlaying solution was filtered through cannula. The white solid residue was washed with Et2O (2 x 10 mL) and dried in vacuo to yield the imidazolium salt as a white powder (928 mg, 75%). mp: 225°C; 1 2972, 1608, 1557, 1541, 1486, 1443, 1202, 1162, 1067, 1043, 1021, 990, 937, 924, 877, 760, 729, 669, 638, 582, 559, 526 cm -1 .

3-Allyl-1-mesitylimidazolium tetrafluoroborate. 3-Allyl-1-mesitylimidazolium bromide (150 mg, 0.49 mmol) and sodium tetrafluoroborate (108 mg, 0.98 mmol) were placed in an Erlenmeyer. CH2Cl2 (20 mL) and H2O (10 mL) were added. The mixture was poured into a separatory funnel and vigorously stirred. The organic layer was extracted, dried over Na2SO4, filtered and evaporated to dryness by rotary evaporation. The white residue was washed with Et2O (5 mL) to give a white solid (131 mg, 85 %). 1 3-(But-3-enyl)-1-mesitylimidazolium bromide, [HL 2a ] + Br -4-Bromobut-1-ene (210 µL, 2.05 mmol) was added to a solution of 1-mesitylimidazole (382 mg, 2.05 mmol) in THF (5 mL). The mixture was heated in an oil bath at 70°C for 20 hours.The product precipitates along the reaction course. After cooling, Et2O (10 mL) was added and the overlaying solution was evacuated through a filtering cannula.

The white solid residue was washed with Et2O (2 x 10 mL) and dried in vacuo to yield the imidazolium salt as a white powder (584 mg, 89%). mp: 143°C; 1 3055, 1561, 1546, 1487, 1459, 1433, 1202, 1158, 1067, 1039, 1001, 993, 931, 875, 838, 762, 667 cm -1 . 3-Butyl-1-mesitylimidazolium bromide. 4-Bromobutane (290 µL, 2.7 mmol) was added to a solution of 1-mesitylimidazole (500 mg, 2.7 mmol) in THF (5 mL). The mixture was heated in an oil bath at 70°C for 20 hours during which time a white precipitate appeared. After cooling, Et2O (10 mL) was added and the overlaying solution was filtered through cannula. The white solid residue was washed with Et2O (2 x 10 mL) and dried in vacuo to yield the imidazolium salt as a white powder (251 mg, 30 %). mp: 174-175°C; 1 2932, 2872, 1560, 1543, 1488, 1460, 1449, 1376, 1200, 1158, 1110, 1066, 1037, 1047, 863, 751, 668, 639, 583 cm -1 .

3-(But-3-enyl)-1-mesitylimidazolium tetrafluoroborate, [HL 2a ]

Dicarbonylhydrido with pentane (2 x 10 mL) and dried to yield the complex as a white powder (84 mg, 93%). 1 Note: complex 4a', the chloro-analog of 4a, was not fully characterized; it was just identified on the basis of the close similarity of its 1 H NMR and IR spectra with those of 4a, listed above. were removed in vacuo. The residue was washed with pentane (15 mL) and the overlaying solution was filtered through cannula. The product was then dissolved in CH2Cl2 and filtered through Celite. Solvents were evaporated and the residue was washed with pentane (2 x 10 mL) to obtain a white solid (258 mg, 96 %). Colourless crystals suitable for an X-ray diffraction study were obtained by slow diffusion of hexane in a solution of complex in toluene. X-ray Diffraction Studies. Crystals of 4a, and 5a suitable for X-ray diffraction were obtained through recrystallization from CH2Cl2/pentane (4a), or toluene/pentane (5a) solutions. Data were collected on a Bruker D8 APEX II diffractometer. All calculations were performed on a PC-compatible computer using the WinGX system. 36 Full crystallographic data are given in Table 1. The structures were solved by using the SIR92 program, 37 which revealed in each instance the position of most of the non-hydrogen atoms. All remaining non-hydrogen atoms were located by the usual combination of full matrix least-squares refinement and difference electron density syntheses by using the SHELXL97

Dicarbonyl

program. [START_REF]SHELX97 Includes SHELXS97, SHELXL97, CIFTAB[END_REF] Atomic scattering factors were taken from the usual tabulations. Anomalous dispersion terms for Ru, Br, and P atoms were included in Fc. All non-hydrogen atoms were allowed to vibrate a full list of interatomic bond lengths and angles for the reported complexes 4a, and 5a. This material is available free of charge via the Internet at http://pubs.acs.org.

The phosphine and olefinic arms of functionalized imidazolium cations serve as directing groups in the chelation-assisted C-H bond cleavage of such ligands onto a Ru(0) center. In the olefin case, all cationic intermediates involved in the subsequent Ru-mediated H transfer to the olefinic arm have been identified by NMR in a model reaction carried out on the new Ru(0) complex Ru{NHC/olefin}(CO)2(PPh3) at low temperature.

  3-(2-diphenylphosphinoeth-1-yl)-imidazolium tetrafluoroborate, [HL 1a ] + BF4 - and 1-(2,6diisopropylphenyl)-3-(2-diphenylphosphinoeth-1-yl)-imidazolium bromide, [HL 1b ] + Br - with 1 give cationic hydrido species formulated as [RuH{L* 1a-b }(CO)2(PPh3)] + X -[2a-b] + X -(a, Ar = mesityl; b, Ar = 2,6 diisopropylphenyl), in which abnormal activation (symbolized by the asterisk) at the C(4) position of the heterocycle has taken place to yield the bidentate ligands L* 1a-b . Deprotonation of [HL 1b ] + with KOtBu gives the corresponding NHC/phosphine bidentate ligand, which reacts with 1 to give the chelated NHC/phosphine complex Ru{L 1b }(CO)2(PPh3) (3b), the first analog of Roper's complex incorporating an NHC moiety. The olefin-functionalized imidazolium ligand 3-(but-3-enyl)-1mesitylimidazolium bromide [HL 2a ] + Br -reacts with 1 via chelation-assisted C-H activation and H transfer to the olefin giving Ru{Ar(N2C3H2)CH2C(H)(CH2CH3)}(CO)2(PPh3)Br (4a). Deprotonation of [HL 2a ] + Br -gives L 2a , which reacts with 1 to give Ru{L 2a }(CO)2(PPh3) (5a). Its protonation with HBF4 at -80°C gives a cationic NHC/olefin-hydrido-complex [RuH{L 2a }(CO)2(PPh3)] + BF4 -, [6a] + BF4 -. NMR data indicate the occurrence of a dynamic process involving a fast exchange between the hydride and the two terminal hydrogen atoms of the coordinated olefin, which can be rationalized in terms of N2C3H2)CH2CH2C(H)CH3)}(CO)2(PPh3)] + BF4 - [7a] + BF4 -. At temperatures above -20°C, [6a] + BF4 -is irreversibly converted into the isomerized NHC/olefin-hydrido-complex [RuH{Ar(N2C3H2)CH2CH=C(H)CH3}(CO)2(PPh3)] + BF4 - [8a] + BF4 - Here again, NMR data reveal a dynamic process involving fast exchange between the hydride and the terminal hydrogen atom of the coordinated olefin, now through the intermediacy of the elusive cationic NHC/alkyl species [Ru{Ar(N2C3H2)CH2C(H)CH2CH3}(CO)2(PPh3)] + BF4 - [9a] + BF4 - Though none of the above unsaturated cationic alkyl intermediates [7a] + or [9a] + was observed, their occurrence could be inferred from trapping experiments. Indeed, the addition of LiCl or [PPN]Cl to the above mixture after equilibration at 25°C leads to the formation of the chloride analog of 4a. Protonation with HCl instead of HBF4 allows capture of the first elusive intermediate [7a] + by the halide, which quenches the isomerization process and promotes a migratory CO insertion yielding the NHC/alkyl derivative Ru{Ar(N2C3H2)CH2CH2C(H)(CH3)C=O}(CO)(PPh3), 10a. The X-ray structure analyses for 4 and 5 are included.

Scheme 1 .

 1 Scheme 1. Chelation-assisted reaction of Ru(CO)2(PPh3)3 with phosphine-tethered imidazolium salts,

Scheme 3 .

 3 Scheme 3. Chelation-assisted reaction of Ru(CO)2(PPh3)3 with an olefin-functionalized imidazolium

Figure 1 :

 1 Figure 1: Perspective view of the alkyl complex 4a. Ellipsoids are shown at the 50% probability level. Selected interatomic distances (Å) and bond angles (°): Ru1-Br1 2.5695(14); Ru1-P1 2.4220(14); Ru1-C1 1.937(3); Ru1-C2 1.843(3); Ru1-C3 2.071(2); Ru1-C7 2.197(3); C1-O1 1.126(3); C2-O2 1.126(3); N1-C3 1.354(3); N1-C4 1.393(3); N1-C11 1.431(3); N2-C3 1.344(3); N2-C5 1.382(3); N2-C6 1.457(3); C4-C5 1.337(4); C6-C7 1.531(4); C7-C8 1.524(4); C8-C9 1.535(4); Br1-Ru1-P1 91.01(3); Br1-Ru1-C1 88.94(7); Br1-Ru1-C3 84.81(6); Br1-Ru1-C7 86.93(7); P1-Ru1-C1 91.76(7); P1-Ru1-C2 91.91(8); P1-Ru1-C7 93.26(7); C1-Ru1-C2 93.37(11); C3-Ru1-C7 77.12(9); Ru1-C3-N1 138.35(17); Ru1-C3-N2 117.04(16); C3-N2-C6 118.6(2); N2-C6-C7 108.3(2); Ru1-C7-C6 107.65(16); Ru1-C7-C8 116.23(17); C6-C7-C8 110.8(2); Ru1-C1-O1 178.2(2); Ru1-C2-O2 179.9(3).

[

  Scheme 4. Chelation-assisted reaction of Ru(CO)2(PPh3)3 with an olefin-functionalized N-heterocyclic

Figure 2 :

 2 Figure 2: Perspective view of complex 5a. Ellipsoids are shown at the 50% probability level. Selected interatomic distances (Å) and bond angles (°): Ru1-P1 2.3467(3); Ru1-C1 1.9148(14); Ru1-C2 1.8959(13); Ru1-C3 2.0775(12); Ru1-C8 2.2076(14); Ru1-C9 2.1838(13); N1-C3 1.3578(16); N1-C4 1.3910(19); C4-C5 1.336(2); N2-C5 1.3824(19); N2-C3 1.3611(17); N2-C6 1.4602(19); C6-C7 1.516(2); C7-C8 1.527(2); C8-C9 1.430(2); Ru1-C1-O1 172.91(12); Ru1-C2-O2 175.66(13); Ru1-C9-C8 71.90(8); Ru1-C8-C9 70.10(8); Ru1-C8-C7 120.25(9); C1-Ru1-C2 108.09(6); C1-Ru1-C9 103.39(5); C2-Ru1-C8 111.10(5); C1-O1 1.1484(18); C2-O2 1.1515(19); P1-Ru1-C1 89.08(4); C1-Ru1-C3 97.52(5); C3-Ru1-C8 86.15(5); C3-Ru1-C9 83.02(5); P1-Ru1-C8 87.54(4); P1-Ru1-C9 93.65(4); Ru1-C3-N2 122.48(9); N1-C3-N2 104.05(11).

Figure 3 : 1 H

 31 Figure 3: 1 H NMR reference spectrum for [6a] + at 195 K and 1D selective DPFGSE-NOE 29 experiment

  Indeed, as the temperature is raised above -20°C, NMR data actually indicate the progressive and gradual disappearance of [6a] + BF4 -with concomitant generation of a new hydrido NHC/olefin species characterized by a characteristic 31 P{ 1 H} NMR signal at  = 35.2 ppm (25°C). The new complex was formulated as [RuH{Ar(N2C3H2)CH2CH=C(H)CH3}(CO)2(PPh3)] + BF4 - [8a] + BF4 -on the basis of both 1 H and 13 C{ 1 H} NMR data. As shown in scheme 5, [8a] + is a new isomeric form of the previous species, differing from its antecedent [6a] + by the position of the olefinic double bond along the functionalized arm. The 1 H NMR spectra, at 298K are indeed consistent with the presence of the coordinated olefinic arm "CH2CH=C(H)Me" as revealed by the occurrence of an ABX pattern centered irradiation exchange at  = 4.50 ppm (2H, -CH2CH=), a complex multiplet at  = 2.55 ppm (-CH2CH=), a relatively sharp triplet at  = 0.79 ppm for the methyl group, and a very broad signal at  = -1.26 ppm for the remaining proton in geminal position. The corresponding hydride ligand appears as a very broad singlet at  = -4.80 ppm. The formation of this complex reflects the fact that, as the temperature is raised above 255 K, -elimination from the intracyclic carbon atom of the elusive intermediate [7a] + BF4 -(Scheme 5)becomes more favourable than -elimination from the exocyclic carbon atom. Noticeably, the observation of concurrent broadening of the resonances due to the terminal olefinic hydrogen atom and the hydride ligand indicates that these nuclei are, here also, involved in an exchange process, which was established by a 2D NOESY experiment. The present exchange process can be rationalized in terms N2C3H2)CH2C(H)CH2CH3}(CO)2(PPh3)] + BF4 - [9a] + BF4 -(Scheme 5), offering, like for 6a, a low activation energy pathway for the E/Z isomerisation of the coordinated olefinic arm. Very characteristically, by lowering the temperature down to 178 K, the exchange process can be slowed down to an extent allowing the observation of the two isomers of 8a, corresponding to the generation of E and Z isomers of the coordinated olefin. Indeed, we do observe a splitting of the[START_REF]between Z and E isomers different from 1:1 reflects the fact that the formation of one isomer is kinetically favored over the second one[END_REF] P resonance at  = 35.2 ppm into two sharp singlets at  = 36.7 ppm and  = 34.4 ppm ( 5:1 ratio), 32 along with a concomitant splitting of the hydride resonances into two still relatively broad signals at  = -6.89 ppm and  = -7.15 ppm ( 10:1 ratio) Examination of the de-coalescence/coalescence behavior of the 31 P NMR signals ( = 484 Hz; Tc  230 K) finally gives an estimate of the free activation enthalpy of the exchange process  10 kcal.mol -1 . 31

  Chelation assistance is known to play an important role in stoichiometric and catalytic transformations of substrates possessing sterically hindered donor atoms or unreactive C-H bonds. In the present account, we have shown that phosphine-functionalized N-heterocyclic carbenes and, more surprisingly, olefin-functionalized N-heterocyclic carbenes, react cleanly with Ru(CO)2(PPh3)3 via pre-coordination of their functionalized arm, to give, respectively, chelating NHC/phosphine or NHC/olefin complexes of general formula Ru(CO)2[NHC/phosphine](PPh3) and Ru(CO)2[NHC/olefin](PPh3) in good to excellent yields. These new Ru(0) species represent the first isolated congeners of Roper's complex incorporating a strongly stabilizing N-heterocyclic carbene ligand. Their intrinsic reactivity deserves to be explored in future work.

  triphenylphosphine)ruthenium(II) bromide, [2a] + Br -. Compounds [HL 1a ] + Br -(48 mg, 0.10 mmol) and 1 (94 mg, 0.10 mmol) were reacted in THF (10 mL) at room temperature. The light yellow mixture was stirred for 2 h, and THF was then removed in vacuo. The resulting residue was washed

  KOtBu (48 mg, 0.43 mmol) and 3-(but-3-enyl)-1-mesitylimidazolium bromide (130 mg, 0.40 mmol) were weighed and placed in a Schlenk tube. THF (20 mL) was syringed into the Schlenk tube and the mixture was stirred for 1 hour at 0°C. Then, the complex Ru(CO)2(PPh3)3 (384 mg, 0.40 mmol) was added and the cool bath was taken off. After 1 hour at room temperature, volatiles

13 C{ 1

 1 H} NMR (100.6 MHz, CD2Cl2): δ 242.8 (d, JCP = 6.0 Hz, C=Oacyl), 200.7 (d, JCP = 13.8 Hz, CO), 184.0 (d, JCP = 99.0 Hz, N2C), 139.0 (Cpara), 136.0 (Cortho), 135.8 (Cipso-mes), 135.7 (Cortho), 134.0 (d, JCP = 11 Hz, CHo-PPh3), 133.6 (d, JCP = 37 Hz, Cipso PPh3), 129.6 (d, JCP = 3.0 Hz, CHp-PPh3), 129.2 (CHmes), 129.1 (CHmes), 127.9 (d, JCP = 9.1 Hz, CHm-PPh3), 122.6 (d, JCP = 3.6 Hz, CHIm-4/5), 122.1 (d, JCP = 2.4 Hz, CHIm-4/5), 59.8 (d, JCP = 1.9 Hz, CH3-CH), 48.0 (N-CH2), 37.2 (CH-CH2), 21.0 (CH3 mes), 18.6 (CH3 mes), 18.5 (CH3 mes), 7.1 (CO-CH3). MS (FAB, MNBA matrix): m/z (%) 660 (1) [M -Cl] + , 630 (6) [M -CO -2H] + , 602 (17) [M -2CO -2H] + , 241 (100) [NHC(olefin)] + . FT-IR ((CO), ATR) 1930 (s), 1644 (m) cm -1 .

  anisotropically. All the hydrogen atoms -except hydrogen atoms attached to C8 and C9 in the structure of 5a -were set in idealized position (R3CH, C-H = 0.96 Å; R2CH2, C-H = 0.97 Å; RCH3, C-H = 0.98 Å; C(sp 2 )-H = 0.93 Å; Uiso 1.2 or 1.5 time greater than the Ueq of the carbon atom to which the hydrogen atom is attached) and their position were refined as "riding" atoms. The olefinic hydrogen atoms attached to C8 and C9 in 5a were inferred from a Fourier difference map calculated in the final stage of the refinement. Their position were ultimately refined holding their Uiso 1.2 time greater than the Ueq of the associated carbon atom; this led to acceptable C-H bond lengths.ACKNOWLEDGMENT: This research was supported by the CNRS. We thank the Ministère de l'Education Nationale for a fellowship to L. B., the RDR2 network of the CNRS for additional funding, and Johnson Matthey for regular complimentary gifts of RuCl3•nH2O.SUPPORTING INFORMATION PARAGRAPH: CIF files giving crystallographic data and including

  H NMR (400 MHz, CDCl3): δ 10.19 (br s, 1H, CHIm-2), 8.10 (br s, 1H, CHIm-4/5), 7.17(s, 1H, CHIm-4/5), 6.95 (s, 2H, CHmes), 5.94 (m, 1H, JHH = 7.2 Hz,

	JHH = 10.1 Hz, =CH), 5.02 (br s, 1H, =CH2), 4.91 (br d, 1H, JHH = 7.4 Hz, =CH2), 4.80 (t, 2H, JHH = 6.3
	Hz, N-CH2), 2.70 (td, 2H, JHH = 6.3 Hz, JHH = 6.4 Hz, =C-CH2), 2.27 (s, 3H, CH3 para), 1.98 (s, 6H, CH3
	ortho).	13 C{ 1 H} NMR (100.4 MHz, CDCl3): δ 141.2 (Cpara), 137.7 (CHIm-2), 134.2 (Cortho), 132.9 (=CH),
	130.7 (Cipso), 129.8 (CHmes), 123.5 (CHIm-4/5), 123.1 (CHIm-4/5), 119.4 (=CH2), 49.1 (CH2), 34.9 (CH2),
	21.1 (CH3 para), 17.5 (CH3 ortho). MS (ESI): m/z (%) 241.3 (100) [M -Br] + , 187.3 (34) [M -Br -
	homoallyl] + . FT-IR (ATR):

  H NMR (250 MHz, CDCl3): δ 10.50(s, 1H, CHIm-2), 7.83 (br s, 1H, CHIm-

	4/5), 7.21 (t, 1H, JHH = 1.7 Hz, CHIm-4/5), 7.08 (s, 2H, CHmes), 4.75 (t, 2H, JHH = 7.2 Hz, N-CH2), 2.35 (s,
	3H, CH3 para), 2.08 (s, 6H, CH3 ortho), 1.99 (m, 2H, CH2), 1.46 (m, 2H, CH2), 0.99 (t, 3H, JHH = 7.3 Hz,
	CH3). 13 C{ 1 H} NMR (62.9 MHz, CDCl3): δ 141.3 (Cpara), 138.4 (CHIm-2), 134.2 (Cortho), 130.7 (Cipso),
	129.9 (CHmes), 123.2 (CHIm-4/5), 122.9 (CHIm-4/5), 50.1 (CH2), 31.5 (CH2), 21.1 (CH3 para), 19.4 (CH2),
	17.6 (CH3 ortho), 13.6 (CH3). MS (ESI): m/z (%) 243.3 (100) [M -Br] + . FT-IR (ATR): 3056, 3005, 2958,

mesityl-3-(3-methyl-4-oxo-but-1-yl-κC 2 )-imidazol-2-ylidene- κC 2 ](triphenylphosphine)ruthenium(II), 10a

  . A solution of compound 5a(30 mg, 0.045 mmol) in THF(2.5 mL) was cooled to 0°C. To this solution, HCl (1M in Et2O) was slowly added. The light yellow mixture was stirred for an hour and then evaporated to dryness. The resulting residue was JHH = 4.5 Hz, JHH = 14.2 Hz, N-CH2), 3.16 (m, 1H, CH2-CH), 2.42 (s, 3H, CH3 mes), 2.22 (s, 3H, CH3 mes), 2.05 (m, 1H, CH-CH2), 1.98 (s, 3H, CH3 mes), 1.81 (dt, 1H, JHH = 3.2 Hz, JHH = 12.6 Hz, CH-CH2), 0.93 (d, 3H, JHH = 6.5 Hz, CO-CH3). 31 P{ 1 H} NMR (121.5 MHz, CD2Cl2): δ 32.3 (s, PPh3).

	92-7.98 (m, 6H, CHPPh3), 7.47-7.50 (m, 2H, CHPPh3), 7.27 (s, 1H, CHPPh3), 6.93-7.16 (m, CHPPh3 + mes), 6.22 (d, 1H, JHH = 1.5 Hz, CHIm-4/5), 6.09 (d, 1H, JHH = 1.5 Hz, CHIm-4/5), 4.17 (td, 1H, 2 JH1b/H1a = 12.2 Hz, 3 JH1b/H2a = 12.8 Hz, 3 JH1b/H2b = 2.5 Hz, N-CH2), 3.39 (td, 1H, 2 JH1a/H1b = 12.2 Hz, 3 JH1a/H2a = 2.9 Hz, 3 JH1a/H2b = 2.3 Hz, N-CH2), 2.79 (m, 1H, =CH), 2.53 (tdd, 1H, 3 JH2b/H1a= 2.3 Hz, 3 JH2b/H3 = 9.3 Hz, 2 JH2b/H2a = 13.5 Hz, 3 JH2b/H1b = 2.5 Hz, =CH-CH2), 2.32 (s, 3H, CH3 mes), 2.26 (s, 3H, CH3 mes), 2.11 (s, 3H, CH3 mes), 1.62 (tdd, 1H, 2 JH2a/H2b = 13.5 Hz, 3 JH2a/H1a = 2.9 Hz, 3 JH2a/H3 = 3.6 Hz, 3 JH2a/H1b = 12.8 Hz, =CH-CH2), 1.50 (ddd, 1H, 2 JH4b/H4a = 1.5 Hz, JH4bP = 6.6 Hz, 3 JH4b/H3 = 8.6 Hz, =CH2), 1.28 (ddd, 1H, 2 JH4a/H4b = 1.5 Hz, JH4a/P = 5.5 Hz, 3 JH4a/H3 = 9.6 Hz, =CH2). 31 P{ 1 H} NMR (121.5 MHz, C6D6): δ 58.4 (s, PPh3). 13 C{ 1 H} NMR (75.4 MHz, CD2Cl2): δ 211.7 (d, JCP = 14.9 Hz, CO), 208.8 (d, JCP = 12.3 Hz, CO), 185.8 (d, JCP NMR (400 MHz, CD2Cl2): δ 7.42 (m, 3H, CHPPh3), 7.34 (m, 12H, CHPPh3), 7.25 (s, 1H, CHIm-4/5), 7.06 (br s, 2H, CHmes, CHIm-4/5), 6.99 (s, 1H, CHmes), 5.11 (dt, 1H, JHH = 4.0 Hz, JHH = 13.9 Hz, N-CH2), 3.98 = 87.0 Hz, N washed with Et2O (2 x 5 mL). The product was isolated as a bright yellow powder (29 mg, 92%). 1 H (dd, 1H,

1

H NMR (300 MHz, C6D6): δ 7.2 C), 138.8 (C mes ), 136.9 (C mes ),17.8 (CH 3 mes ), 136.2 (C mes ), 135.2 (d, J CP = 38.3 Hz, C ipso PPh3), 134.0 (d, JCP = 10.9 Hz, CHo-PPh3), 133.7 (d, JCP = 19.6 Hz, CHo-PPh3), 129.1 (d, JCP = 1.95 Hz, CHp-PPh3), 128.8 (s, CHmes), 128.7 (CHmes), 127.5 (d, JCP = 9.2 Hz, CHm-PPh3), 120.7 (d, JCP = 3.2 Hz, Carbonylchloro[1-

Table 1 .

 1 Crystal data and structure refinement parameters, for complexes 4a, and 5a.

	Complex	4a	5a.C7H8
	Empirical formula	C36H36BrN2O2PRu C43H43N2O2Ru
	Mr	740.61	751.83
	T/ K	180	180
	 /Å	0.71073	0.71073
	Crystal system	monoclinic	monoclinic
	Space group (no.)	P 21/n (# 14)	P 21/n (# 14)
	a/ Å	13.534(7)	9.7482(4)
	b/ Å	12.260(6)	16.8095(7)
	c/ Å	19.840(2)	22.2652(9)
	 	93.96(3)	95.755(2)
	V/ Å 3	3284(2)	3630.0(3)
	Z	4	4
	Dc/ g.cm -3	1.498	1.376
	/ mm -1	1.777	0.515
	F(000)	1504	1560
	max/ °	30.50	30.51
	Completeness to  max (%)	0.99	0.99
	Index range, hkl	-19<h<19	-13<h<13
		-17<k<17	-24<k<24
		-28<l<28	-31<k<31
	Reflections collected	56655	247950
	Independant reflections	10026	11056
	Data/restraints/parameters	10026/0/392	11056/0/456
	GOF	0.964	1.11
	R [I>2(I)]	0.0342	0.0247
	Rw [I>2(I)]	0.0909	0.0588

C{1 H} NMR spectra, the carbene carbon atom in 5a is observed at  = 185.8 ppm and, as for 4a, the

RuCl{C(O)CH(CH3)CH2CH2(N2C3H2)Mes}(CO)(PPh3) (10a) via a migratory CO insertion reaction (Scheme 7).


Chantal Carayon

Christiane André-Barrès

Nadine Leygue

Nathalie Saffon-Merceron

Martial Boggio-Pasqua

Suzanne Fery-Forgues

The role of the synthetic chromophore of GFP in generating polymorphismdependent on/off photoluminescence

Polymorphism, i.e. the ability of a solid material to exist in more than one crystal form, gives unique opportunity to study the correlation between the solid-state molecular arrangement and the photoluminescence properties, and allows optimizing the performances of materials with reduced synthesis effort. Compound 1, which results from the combination of 2phenylbenzoxazole (PBO) and benzylideneimidazolinone (BDI), was known to be weakly fluorescent in solution and in the solid state. Time-dependent density functional theory (TD-DFT) and spin-flip TD-DFT calculations were first used to revisit the behaviour of the isolated molecule in solution. It was shown that the relaxation pathway in the lowest singlet excited state leads to a thermally-accessible conical intersection (CI), which favours nonradiative deactivation. Then, crystallizing molecule 1 under precise experimental conditions allowed four distinct polymorphs and one solvate to be obtained. Remarkably, forms 1a and 1b exhibited strong photoluminescence (PL), while polymorphs 1c-e were virtually not emissive. Recrystallization allowed passing reversibly from one crystalline species to another. Mechanical stimuli, exhibition to solvent vapours and heating were not very effective, but a non-trivial effect of PL enhancement upon freezing was observed for 1a and 1b. The nonemissive polymorphs were characterized by the presence of a centrosymmetric dimer Manuscript File structured by hydrogen bonds, in which paired molecules vibrate with the same mode and are in turn subjected to maximum deformation, according to DFT calculations. Based on the whole set of results, possible deactivation mechanisms were proposed to account for the opposite PL behaviour of the two types of crystalline species. It is shown that the emergence of polymorphism and of a rare on/off PL mechanism may be attributed to the presence of the BDI unit in the molecule.

Introduction

Organic crystalline materials that exhibit tunable and switchable photoluminescence (PL) emission find promising applications in the fields of sensors, data storage and optoelectronic devices [START_REF] Zhang | Organic solid fluorophores regulated by subtle structure modification: color-tunable and aggregation-induced emission[END_REF][START_REF] Yang | Smart On-Off Switching Luminescence Materials with Reversible Piezochromism and Basichromism[END_REF]. There is considerable research interest in controlling their optical properties through non-covalent routes, by modulating molecular conformation, as well as the collective arrangement and interaction mode of molecules in the solid [START_REF] Anthony | Organic Solid-State Fluorescence: Strategies for Generating Switchable and Tunable Fluorescent Materials[END_REF][START_REF] Varughese | Non-covalent routes to tune the optical properties of molecular materials[END_REF][START_REF] Wang | Molecular conformation and packing: their critical roles in the emission performance of mechanochromic fluorescence materials[END_REF]. The smartest way to achieve this goal is probably to take advantage of polymorphism, i.e. the ability of a solid material to exist in more than one crystal form [START_REF] Lu | Recent advances in photofunctional polymorphs of molecular materials[END_REF][START_REF] Tan | Polymorphism-Dependent and Mechanochromic Luminescent Molecules[END_REF][START_REF] Nangia | Conformational Polymorphism in Organic Crystals[END_REF][START_REF] Yan | Molecular crystalline materials with tunable luminescent properties: from polymorphs to multi-component solids[END_REF]. Since this approach considers exactly the same molecule, it gives unique opportunity to make precise structure-property correlations, and leads to optimized performances of materials with reduced synthesis effort.

Moreover, as polymorphs generally differ by a small amount of energy, the transition between them may be easily achieved under ambient conditions, and this opens the door to stimuliresponsive materials and their applications [START_REF] Varughese | Non-covalent routes to tune the optical properties of molecular materials[END_REF][START_REF] Wang | Molecular conformation and packing: their critical roles in the emission performance of mechanochromic fluorescence materials[END_REF][START_REF] Yan | Molecular crystalline materials with tunable luminescent properties: from polymorphs to multi-component solids[END_REF][START_REF] Ma | Mechanically Induced Multicolor Change of Luminescent Materials[END_REF][START_REF] Du | Reversible luminescence color switching in the crystal polymorphs of 2,7bis(2′-methyl-[1,1′-biphenyl]-4-yl)-fluorenone by thermal and mechanical stimuli[END_REF]. There are many examples of polymorphs that exhibit very different emission colour and intensity [START_REF] Du | Reversible luminescence color switching in the crystal polymorphs of 2,7bis(2′-methyl-[1,1′-biphenyl]-4-yl)-fluorenone by thermal and mechanical stimuli[END_REF][START_REF] Yoon | Polymorphic and mechanochromic luminescence modulation in the highly emissive dicyanodistyrylbenzene crystal: secondary bonding interaction in molecular stacking assemblyJ[END_REF][START_REF] Yang | Three polymorphs of one luminogen: how the molecular packing affects the RTP and AIE properties?[END_REF][START_REF] Pick | Polymorph-Selective Preparation and Structural Characterization of Perylene Single Crystals[END_REF][START_REF] Mallia | Crystallization induced enhanced emission in conformational polymorphs of a rotationally flexible molecule[END_REF][START_REF] Gu | Polymorphism-Dependent Emission for Di(p-methoxylphenyl)dibenzofulvene and Analogues: Optical Waveguide/Amplified Spontaneous Emission Behaviors[END_REF][START_REF] Zhan | Donor-acceptor π-conjugated quinoxaline derivatives exhibiting multi-stimuli-responsive behaviors and polymorphism-dependent multicolor solidstate emission[END_REF][START_REF] Sarkar | Aggregation-Induced and Polymorphism-Dependent Thermally Activated Delayed Fluorescence (TADF) Characteristics of an Oligothiophene: Applications in Time-Dependent Live Cell Multicolour Imaging[END_REF][START_REF] Huang | Manipulation of Molecular Aggregation States to Realize Polymorphism, AIE, MCL, and TADF in a Single Molecule[END_REF] while total or almost total on/off switching of emission has rarely been reported [START_REF] Mutai | Reproducible on-off switching of solid-state luminescence by controlling molecular packing through heat-mode interconversion[END_REF][START_REF] Liu | Polymorph-Dependent Luminescence Response to Acid Vapors and Its Application in Safety Protection of File Information[END_REF][START_REF] Shida | Sterically induced polymorphism: ON-OFF control of excited-state intramolecular proton transfer (ESIPT) luminescence of 1-methyl-2-(2′-hydroxyphenyl)benzimidazole[END_REF][START_REF] Borbone | On-Off Mechano-responsive Switching of ESIPT Luminescence in Polymorphic N-Salicylidene-4-amino-2-methylbenzotriazole[END_REF][START_REF] Devi | Mechano-responsive luminescent emissions of an organic molecular crystal: effects of aromatic stacking interactions and solid state packing[END_REF][START_REF] Yoshida | Solid-state fluorescence of zwitterionic imidazolium pyridinolates bearing long alkyl chains: Control of emission properties based on variation of lamellar alignment[END_REF][START_REF] Tchoń | Polymorphism and resulting luminescence properties of 1-acetylpyrene[END_REF].

Over the last few years, our group introduced new molecules [START_REF] Ghodbane | Effects of the benzoxazole group on green fluorescent protein chromophore crystal structure and solid state photophysics[END_REF][START_REF] Carayon | Conjugates of Benzoxazole and GFP Chromophore with Aggregation-Induced Enhanced Emission: Influence of the Chain Length on the Formation of Particles and on the Dye Uptake by Living Cells[END_REF] resulting from the combination of 2-phenylbenzoxazole (PBO) [START_REF] Carayon | 2-Phenylbenzoxazole derivatives: a family of robust emitters of solid-state fluorescence[END_REF] and benzylideneimidazolinone (BDI), i.e. a synthetic derivative of the chromophore of the well-known Green Fluorescent Protein [START_REF] Walker | Fluorescence imaging using synthetic GFP chromophores[END_REF] (Fig. S1). All these molecules had the same electron conjugated system and were weakly fluorescent in organic solutions. Those bearing an alkyl chain (from C4 to C16) on the BDI nitrogen atom showed strong aggregation-induced emission enhancement (AIEE) effect, [START_REF] Mei | Aggregation-Induced Emission: Together We Shine, United We Soar![END_REF] and were good solid-state emitters. Curiously, the first member of the series bearing a methyl group (1, Fig. 1) was almost not emissive in the aggregated and solid states. This discrepancy prompted us to undertake the present work. First, the photophysical mechanism that explains the weak fluorescence in solution was revisited using spin-flip time-dependent density functional theory (SF-TD-DFT) and TD-DFT calculations. Then, it was shown that not only compound 1 exists as many polymorph forms, an unprecedented case for PBO derivatives, but polymorphism controls on/off solid-state PL. In conjunction with the results of IR spectroscopy and various quantum chemical calculations, an original mechanism was proposed to rationalize these data, and to explain how the presence of the BDI unit endows the molecules with this exceptional behaviour.

Experimental section

Materials and methods

Methanol (HPLC grade) and petroleum ether (reagent grade) were from Fisher Chemicals. Chromasolv HPLC grade dichloromethane and n-heptane, and reagent grade chloroform, were from Sigma-Aldrich. HPLC grade acetonitrile and analytical grade THF were from SDS. All solvents were used without further purification.

Compound 1 was synthesized as previously described [START_REF] Ghodbane | Effects of the benzoxazole group on green fluorescent protein chromophore crystal structure and solid state photophysics[END_REF] and purified on silica column chromatography using petroleum ether/CH 2 Cl 2 as eluent, resulting in yellow microcrystalline powder. Purity was checked by thin layer chromatography. Single crystals were grown by dissolving an aliquot (3 mg) of the pristine powder in an organic solvent with gentle heating, and then evaporating the solvent in conditions indicated in Table S1.

Apparatus

The melting points were measured on a Stuart Automatic SMP40 apparatus. The photoluminescence spectra were recorded using a Xenius SAFAS spectrofluorimeter equipped with a BaSO 4 integrating sphere. They were corrected using a home-made [START_REF] Zhang | Organic solid fluorophores regulated by subtle structure modification: color-tunable and aggregation-induced emission[END_REF] correction curve. Low temperature measurements were done on powder samples dispersed in KBr. For the measurement of quantum yields, the excitation source was scanned in order to evaluate the reflected light for the empty sphere (L a ), the samples facing the source light (L c ) and the sample out of the irradiation beam (L b ). The luminescence spectra were recorded with the sample facing the source light (E c ) and out from direct irradiation (E b ). The PM voltage was adapted to the measurement of reflected light and emission spectra, respectively, and proper correction was applied to take into account the voltage difference. The absolute photoluminescence quantum yield values (Φ PL ) were calculated by a method based on the one developed by de Mello et al. [START_REF] Mello | An improved experimental determination of external photoluminescence quantum efficiency[END_REF] using the formula:

Φ PL = [E c -(1-α)E b ]/L a α (1)
with α = 1-L c /L b . The error on the Φ PL value was estimated to be about 20%. Fourier transform infrared (FTIR) spectra were recorded using a Nexus ThermoNicolet FTIR spectrophotometer equipped with a diamond ATR.

Crystallography

Crystal data were collected at 193(2)K (1b, 1c and 1d) and 293( 2)K (1e) on a Bruker-AXS APEX II diffractometer equipped with a 30 W air-cooled microfocus source (ImS) with focusing multilayer optics (1b, 1c and 1d) and on a Bruker-AXS D8-Venture diffractometer equipped with a CMOS detector (photon100), using MoKα radiation (λ = 0.71073 Å). The structures were solved using an intrinsic phasing method (ShelXT) [START_REF] Sheldrick | SHELXT -Integrated space-group and crystal-structure determination[END_REF]. All non-hydrogen atoms were refined anisotropically using the least-squares method on F 2 [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF]. Molecular graphics were generated using the program MERCURY [START_REF] Macrae | Mercury: visualization and analysis of crystal structures[END_REF]. Molecular views and crystal data of all polymorphs are given in Fig. S2 and Table S2, respectively. CCDC supplementary crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures.

The X-ray powder diffraction patterns were recorded in the "Nano X platform of CEMES-CNRS". The powders were placed in a sample holder with no background noise. The measurements were made in reflection mode using a Bragg-Brentano configuration on a Bruker D8 Advance diffractometer equipped with a Copper anticathode (Cu Kα1 = 1.54059 Å and Kα2 = 1.54439 Å) and a 1D Lynx eye detector.

DFT, TD-DFT and SF-TD-DFT calculations

DFT calculations of the harmonic normal modes were performed at the B3LYP/6-31+G(d,p) level of theory [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF] for dimers 1a, 1b, 1c, 1d, 1e obtained from X-ray data (Table S5). The frequencies were calculated using a scaling factor of 0.9613 [START_REF] Andersson | New Scale Factors for Harmonic Vibrational Frequencies Using the B3LYP Density Functional Method with the Triple-ζ Basis Set 6-311+G(d,p)[END_REF]. The ground-and first excited-state potential energy surfaces of a model isolated monomer were explored at the TD-DFT and SF-TD-DFT levels of theory. TD-DFT and SF-TD-DFT calculations were performed with the PBE0 [START_REF] Adamo | Toward reliable density functional methods without adjustable parameters: The PBE0 model[END_REF] functional and BH&HLYP [START_REF] Becke | A new mixing of Hartree-Fock and local density-functional theories[END_REF] functionals, respectively, and the 6-31+G(d,p) basis set. The SF-TD-DFT calculations used a reference triplet state obtained from a restricted open-shell DFT calculation. The excited-state transition state was optimized at the TD-DFT level only, for which analytical second derivatives of the energy with respect to nuclear coordinates could be computed. On the other hand, the twisted minimum and conical intersection could only be optimized with SF-TD-DFT, which allows a correct description of the potential energy surface topology where the ground-and first excited state couple [START_REF] Bernardi | Potential energy surface crossings in organic photochemistry[END_REF]. DFT and TD-DFT calculations were performed with Gaussian [START_REF] Frisch | Gaussian 16, Revision B.01[END_REF] and Gamess [START_REF] Barca | Recent developments in the general atomic and molecular electronic structure system[END_REF] quantum chemistry codes, respectively. The geometries (Cartesian coordinates) of optimized structures obtained at TD-DFT and SF-TD-DFT levels are given in Table S6.

Results and discussion

Revisiting the spectroscopic behaviour in solution: Spin-flip TD-DFT calculations

It was shown in our previous work that all compounds of this series emit weak blue light in organic solvents (e.g. λ em max = 448 nm, Φ F = 0.02 for 1 in ethanol) [START_REF] Ghodbane | Effects of the benzoxazole group on green fluorescent protein chromophore crystal structure and solid state photophysics[END_REF]. To explain this behaviour, it was proposed that the two molecular moieties that rotate around the vinylene link lead to dispersion of the excitation energy. The PL enhancement observed for the long-chain analogues was classically attributed to rigidification of the molecular backbone and restriction of internal rotations [START_REF] Mei | Aggregation-Induced Emission: Together We Shine, United We Soar![END_REF]. However, regarding recent findings in this research field [START_REF] Shi | Solid State Luminescence Enhancement in π-Conjugated Materials: Unraveling the Mechanism beyond the Framework of AIE/AIEE[END_REF][START_REF] Crespo-Otero | Exploring Potential Energy Surfaces for Aggregation-Induced Emission-From Solution to Crystal[END_REF], it seemed necessary to us to better understand what happens in solutions. To do so, time-dependent density functional theory (TD-DFT) and its spin-flip variant (SF-TD-DFT), which allows the correct description of conical intersections (CIs) between ground and excited states [START_REF] Nikiforov | Assessment of approximate computational methods for conical intersections and branching plane vectors in organic molecules[END_REF], were used to investigate the lowest singlet excited state (S 1 ) relaxation pathway by first considering an isolated model system in vacuum. The model system was built by replacing the two methyl groups of the imidazolinone moiety by hydrogen atoms.

Fig. 2 shows that upon excitation to the S 1 state, the system first relaxes to a planar structure (Min-S 1 -planar) which is assigned to the emissive species. The calculated emission wavelength at the TD-DFT level (435 nm) is consistent with that observed in solution. This structure is characterized by a slight shortening of 0.03 Å of the C-C bond between the benzoxazole and BDI moieties, a more substantial shortening of 0.06 Å of the phenyl-bridge C-C bond length and a lengthening of 0.06 Å of the imidazolinone-bridge C-C bond length.

However, this minimum is not the lowest energy excited-state structure on the S 1 potential energy surface. Along the torsion coordinate of the imidazolinone moiety, a lower energy S 1 minimum (Min-S 1 -Im-twist) can be found in which the torsion dihedral angle of the imidazolinone moiety is 94° with respect to the rest of the molecule. A slight pyramidalization (ca. 5°) of the carbon atom linked to the bridge is also involved. This global minimum on the S 1 potential energy surface is located 6.5 kcal/mol below the S 1 planar minimum, and can be reached by overcoming a small excited-state potential energy barrier of about 6 kcal/mol from Min-S 1 -planar. Interestingly, upon further imidazolinone torsion to 102° and pyramidalization of the imidazolinone carbon atom linked to the bridge to 50°, a low-lying S 0 /S 1 minimum energy conical intersection (S 0 /S 1 -MECI Im-twist ) is reached requiring only an activation energy of 8.5 kcal/mol from Min-S 1 -Im-twist. This CI is located at a similar energy to the S 1 planar minimum and below the Franck-Condon region. This is reminiscent of the potential energy surface topology found in the neutral green fluorescent protein chromophore belonging to the BDI family [START_REF] Polyakov | Potential Energy Landscape of the Electronic States of the GFP Chromophore in Different Protonation Forms: Electronic Transition Energies and Conical Intersections[END_REF]. This CI, i.e. a critical structure of the potential energy surfaces where the S 1 and S 0 states are degenerate and the probability of nonradiative internal conversion is most efficient [START_REF] Carayon | 2-Phenylbenzoxazole derivatives: a family of robust emitters of solid-state fluorescence[END_REF], is most likely responsible for the very efficient non-radiative decay of 1 in solution, where molecules can move freely.

Preparation and characterization of the various crystalline forms

To study the properties of 1 in the solid state, this compound was synthesized according to a variant of our previously described procedure and single crystals suitable for X-ray analysis were grown by evaporation of organic solutions under various experimental conditions (Table S1). In some cases, two types of crystals co-existed and were sorted manually. The X-ray analysis led to identification of one acetonitrile solvate (1b) and four polymorphs (1a, 1c-e) including one previously reported structure [START_REF] Ghodbane | Effects of the benzoxazole group on green fluorescent protein chromophore crystal structure and solid state photophysics[END_REF]. It is obvious that not only the solvent, but also the temperature and speed of solvent evaporation, played a key role in the preparation of these various crystalline forms. Regarding outward appearance, the crystals were roughly divided into two groups. Forms 1a and 1b had needle or rod-like shapes, the other ones (1c-e) were irregular plates of varying thickness (Fig. 3). The crystal colour ranged from pale yellow to yellow-orange. The melting points were between 214.7 and 216.0 °C, with one exception (207.4°C) for solvate 1b. The very close energy levels explain the formation of concomitant polymorphs and the easy transition from one form to another that was observed in the course of the preparation of crystals. The examination of the X-ray structures provides a lot of precious information on the geometry of molecules and mutual interactions between them. Crystallographic data are collected in Table S2 and molecular structures are given in Fig. S2. The inversion between oxazole N and O atoms is frequently found in crystallographic structures of PBO [START_REF] Carayon | 2-Phenylbenzoxazole derivatives: a family of robust emitters of solid-state fluorescence[END_REF]. In the present case, this trend and the dissymmetry brought by the BDI unit lead to the emergence of two conformers, named syn and anti depending on the relative position of the benzoxazole and imidazolinone oxygen atoms on the same side or on the opposite side with respect to the long axis of the molecule (Fig. S3). For instance, crystals of 1d (and possibly 1a and 1c) contained only molecules in the anti conformation, while 1b and 1e contained more than 80% of molecules in the syn conformation.

Molecules were almost planar in solvate 1b, where packing forces are weak, and in 1a.

They were moderately twisted in polymorphs 1c-e, with the benzoxazole and imidazolinone moieties forming a dihedral angle below or equal to 20° (Table S3). It must be underlined that this twist is distributed over the entire molecular structure, and not localized between two adjacent aromatic rings. If the relative position of the oxazole N and O atom is not considered, the superimposition of all molecules reveals that other geometrical changes are not very significant (Fig. S4). Two types of molecular arrangements can be distinguished as illustrated in Fig. 4. In crystals of 1a and 1b, molecules showed a crossed arrangement, typical of many PBO derivatives [START_REF] Carayon | 2-Phenylbenzoxazole derivatives: a family of robust emitters of solid-state fluorescence[END_REF]. In the other crystals, molecules were aligned along their long axis, with the PBO moieties situated in parallel (1c and 1e) or slightly different planes (1d). In every case, the nearest neighbours were distant by ~3.3 Å in the stacking direction, and were antiparallel to each other. Several π-π interactions were detected for every crystalline form (see Table S3 and Fig. 5). Noticeably, the overlap of the aromatic moieties was pronounced for 1a, slightly weaker for 1b. The strongest π-π interactions are between the benzoxazole ring of one molecule and the imidazolinone ring of a neighbouring molecule in the stacking direction. In contrast, no or very weak overlap of the aromatic moieties was observed for 1c-e, due to slippage of molecules in the long molecular axis direction (Fig. S5), and π-π interactions take place between two imidazolinone rings. At least one CH- interaction involving one hydrogen atom borne by the methyl group of the imidazolinone cycle and a 6-membered ring can also be found for each crystalline species. Regarding hydrogen bonds between 2.4 and 2.8 Å (Table S4), the only commonality between all crystals is the involvement of the imidazolinone oxygen atom in an intermolecular H-bond that strongly structures the network.

In 1a and 1b, this bond is directed towards one hydrogen atom of the benzoxazole moiety of the neighboring molecule, leading to a compact arrangement. In contrast, for 1c-e, the H-bond involves the hydrogen atoms of the phenyl and vinyl groups, so that molecules form sort of centrosymmetric dimers. The overall result is that the BDI moieties of the molecules seem to be tightly bound as columns, while the benzoxazole moieties have a looser environment.

Other types of short contacts were encountered, or not, in the various structures, involving for example the oxazole nitrogen and oxygen atoms.

Infrared spectroscopy and DFT calculations

To examine the H-bonds formed by the imidazolinone oxygen atom in greater detail, the FTIR spectra of the five crystalline species were recorded. The peaks corresponding to the C=O stretching vibrations were situated around 1701 cm -1 for 1a and 1b, and around 1707 cm -1 for 1c-e (Fig. S7). As the same molecule is considered, these results suggest that the order of the covalent bond is weaker, and hence the H-bond is stronger, for 1a and 1b than for 1c-e. At lower frequencies, in the fingerprint region, peaks were at 749 and 733 cm -1 , respectively (Fig. S8). They seem to indicate that deformations of C-H bonds were stronger in 1c-e than in 1a and 1b. The differences between the frequency values are small, but it is clear that the two types of crystals have a distinct IR signature.

To get a deeper insight, DFT frequency analyses were first performed on the HBDI unit in the gas phase used as reference, and then on the five crystalline species (Table S5).

Adjacent molecules linked by H-bonds involving the C=O(imid) group were considered in each case on the basis of X-ray data. The choice was made to keep the constraints imposed by the crystal packing, instead of calculating the equilibrium geometries, which do not reflect what is actually taking place in the crystals. In this case, the calculation of harmonic frequencies is not exact. Consequently, the results collected in Table 1 must be considered with some caution, but we think that they provide precious elements of comparison between the crystalline species. For the centrosymmetric dimers of 1c, 1d and 1e, the H-bonds formed between the two C=O(imid) and either H-C(phen) or H-C(vinyl) were examined.

Remarkably, for every calculated frequency, two very close values were found, respectively associated with very high and near-zero intensity. This indicates that the normal modes of vibration are degenerated, the two molecules vibrating together with the same mode. A consequence is the dramatic overall deformation of the dimer, the two molecules of which undergo maximal deformation alternately. In contrast, for 1a and 1b, each C=O(imid) group presented its own stretching vibration of moderate intensity. Therefore, molecular deformation was weak, as for HBDI in the gas phase. Films showing the stretching vibrations of carbonyl groups, and related molecular deformations, are consigned as supplementary data.

Calculations suggest that the associated intensities play a far more important role than the actual frequency values. These data are not accessible experimentally, because they are hidden by the envelope of IR spectra, as shown by the calculated IR spectra (Fig. S10).

Table 1

Calculated C=O(imid) … H-C frequencies in cm -1 with a scaled factor of 0.9613, and associated intensities between brackets.

Spectroscopic properties

The microcrystalline powder of 1a appeared to be strongly emissive in the green region, with maximum near 502 nm and photoluminescence quantum yield of 0.22 (Fig. 6).

Agglomerates of microcrystals had a slightly more yellow emission, probably due to reabsorption effects [START_REF] Carayon | Mechanofluorochromic Properties of an AIEE-Active 2-Phenylbenzoxazole Derivative: More than Meets the Eye?[END_REF]. With aging, the sample exhibited a shoulder near 550 nm (Fig. 7b), but no distinct crystalline species could be characterized. Solvate 1b emitted in the yellow-orange region (582 nm), with a weaker but still significant quantum yield (0.11).

The differences observed between the emission properties of 1a and 1b can be explained as follows. The sharp green emission of 1a could be assigned to excitons [START_REF] Majumdar | Tuning Solid-State Luminescence in Conjugated Organic Materials: Control of Excitonic and Excimeric Contributions through π Stacking and Halogen Bond riven Self-Assembly[END_REF][START_REF] Gierschner | Solid-state optical properties of linear polycon ugated molecules: π-stack contra herringbone[END_REF] or individual molecules that interact with each other in the crystalline environment [START_REF] Presti | Understanding Aggregation-Induced Emission in Molecular Crystals: Insights from Theory[END_REF], while the broad long-wavelength emission of 1b suggests the additional contribution of excimers as previously proposed for the long chain analogues [START_REF] Ghodbane | Effects of the benzoxazole group on green fluorescent protein chromophore crystal structure and solid state photophysics[END_REF]. At the opposite, polymorph 1d was very weakly emissive, and 1c and 1e were virtually not emissive. Passing from 1a and 1b to 1c-e by recrystallization in appropriate solvent, and vice versa, resulted in turning photoluminescence on and off. The process was reversible many times, although the non-emissive polymorphs were more and more difficult to obtain over time, probably due to their lower stability and to the unavoidable presence of germs of the emissive polymorphs in the laboratory [START_REF] Dunitz | Disappearing Polymorphs[END_REF]. Other methods of switching from one crystalline form to another have been tried. Upon grinding, the shape of the PL spectrum of 1a was unchanged. In contrast, the PL spectrum of 1b was shifted to the blue till 500 nm. Exposition of the ground powder to acetonitrile vapours during 48 h led to a marked decrease of intensity, accompanied by a broadening of the PL spectrum due to the appearance of a band in the orange region (Fig. S9). This partially reversible mechanofluorochromic effect was rather attributed to amorphization of the sample and then to partial restoration of the crystalline structure due to solvent vapours. Grinding also led to the appearance of a weak PL signal for the non-emissive polymorphs, which rapidly faded when the sample was left to stand alone at room temperature. Efforts were not pursued in this direction because the effects were not strong enough to be harnessed. Finally, heating samples of 1a and 1d at 90°C for 24 h did not alter their emission properties, suggesting that this temperature rise did not allow interconversion between polymorphs.

However, an interesting effect was noticed when studying the behaviour of our compounds at various temperatures. In fact, dropping a flask of 1a into liquid nitrogen resulted in a sharp increase of PL, without noticeable change of the initial colour (Fig. 7a).

Since our fluorimeter did not allow controlling the temperature of solid samples, measurements started when the sample was just removed from the liquid nitrogen, and continued when progressively returning to room temperature. In these conditions, the maximum PL intensity recorded for the samples at low temperature (LT) was 3.2 times more intense than at room temperature, and this difference would probably be greater if the sample could be maintained at -196 °C. The intensity change was consistent during many thermal cycles (Fig. 7b). No alteration of the spectrum shape was observed (Fig. 7c), indicating that there is no change in the crystal structure (i.e. crystal packing release followed by large structural relaxation [START_REF] Naito | Highly-efficient solid-state emissions of anthracene-o-carborane dyads with various substituents and their thermochromic luminescence properties[END_REF][START_REF] Wu | Highly-efficient solid-state emission of tethered anthracene-o-carborane dyads and their visco-and thermo-chromic luminescence properties[END_REF] or interconversion of polymorph forms by heating and cooling [START_REF] Ohtani | A Flexible, Fused, Azomethine-Boron Complex: Thermochromic Luminescence and Thermosalient Behavior in Structural Transitions between Crystalline Polymorphs[END_REF][START_REF] Wei | Reversible Thermochromism of Aggregation-Induced Emission-Active Benzophenone Azine Based on Polymorph-Dependent Excited-State Intramolecular Proton Transfer Fluorescence[END_REF]) or photophysical processes [START_REF] Wu | Multifunctional luminescent molecules of o-carborane-pyrene dyad/triad: flexible synthesis and study of the photophysical properties[END_REF][START_REF] Železný | The Temperature Dependence of Emission Spectra in α-9, 10-Dichloroanthracene[END_REF] with changing temperature. Very close results

were obtained for 1b, with the difference that the long-wavelength region of the spectrum was narrower for the frozen sample than at room temperature. In contrast, no significant PL enhancement was detected at low temperature for samples of 1c-e.

Towards an understanding of the contrasting solid-state spectroscopic behaviour

The chemical structure of molecule 1 was unchanged in the various crystalline samples, which nevertheless can be divided in two groups with opposite emission behaviours.

Explaining the emission of 1a and 1b by evoking the restriction of molecular motions in the solid state is obviously insufficient, because rigidification also occurs for the non-emissive species. The origin of the spectroscopic variations must be sought in the crystal structure.

Obviously, the emission properties do not depend on the syn or anti molecular conformation.

The planarity of 1a and 1b was a good indication for PL, but the twist observed for 1c-e is not enough to account for the extinction of luminescence. In fact, the butyl and hexadecyl analogues of 1 have dihedral angles close to 14° and 31° (Fig. S6), respectively, and are very emissive in the solid state [START_REF] Ghodbane | Effects of the benzoxazole group on green fluorescent protein chromophore crystal structure and solid state photophysics[END_REF][START_REF] Carayon | Conjugates of Benzoxazole and GFP Chromophore with Aggregation-Induced Enhanced Emission: Influence of the Chain Length on the Formation of Particles and on the Dye Uptake by Living Cells[END_REF].

It was shown above that the presence of a thermally-accessible conical intersection is probably responsible for the non-radiative release of excitation energy of the molecule in solution. It is reasonable to assume that it also opens the main non-radiative deactivation pathway of 1 in the solid state. The CI may be located at higher energies due to steric constraints in solids, impeding the non-radiative deactivation pathway and enhancing the PL emission with respect to solutions. This restricted access to conical intersection (RACI) is now recognized for underlying the behaviour of numerous SLE-active molecules [START_REF] Crespo-Otero | Exploring Potential Energy Surfaces for Aggregation-Induced Emission-From Solution to Crystal[END_REF]. To explain why the two types of polymorphs have such a different behaviour, it is tempting to say that the CI is higher in energy and thus less accessible in 1a and 1b than in 1c-e.

Optimizing the CI structures and finding their energy position in the different polymorphs are beyond the aim of this work, although sophisticated computational strategies have been proposed to take the crystal environment into account in such calculations [START_REF] Rivera | Fromage: A Library for the Study of Molecular Crystal Excited States at the Aggregate Scale[END_REF]. Here, we will simply use qualitative arguments to consider the possibility of molecular motions within the crystals to provide some clues as to whether the CI is accessible or not.

In 1a and 1b, the overlap of the aromatic moieties is significant and π-π interactions take place at short distances. The π-π stacking and the strong packing due to short contacts limit molecular motions [START_REF] Tchoń | Polymorphism and resulting luminescence properties of 1-acetylpyrene[END_REF], so that rotation is impeded and the CI becomes inaccessible.

Only the relaxation pathway leading to the planar minimum Min-S 1 -planar responsible for PL would be available for molecules situated in the crystal bulk, thus explaining the strong PL of these samples. Moreover, in the present case, the molecules situated at the crystal surface could have more freedom to rotate, and act as PL traps. This would be in line with the observations made at low temperature. Indeed, the PL increase observed at LT most probably arises from the reduced rate of thermally-activated processes, which encompass molecular motions [START_REF] Gierschner | Solid-state optical properties of linear polycon ugated molecules: π-stack contra herringbone[END_REF][START_REF] Pei | Highly fluorescence emissive 5, 5′-distyryl-3, 3′-bithiophenes: Synthesis, crystal structure, optoelectronic and thermal properties[END_REF][START_REF] Hariharan | Tunable and Switchable Solid State Fluorescence: Alkyl Chain Length-Dependent Molecular Conformation and Self-Reversible Thermochromism[END_REF][START_REF] Schweikart | Highly luminescent oligo(phenylenevinylene) films: The stereochemical approach[END_REF], as well as exciton migration and quenching by non-emissive surface defects [START_REF] Gierschner | Highly Emissive H-Aggregates or Aggregation-Induced Emission Quenching? The Photophysics of All-Trans para-Distyrylbenzene[END_REF]. For solvate 1b, the narrowing of the long-wavelength region of the spectrum of the frozen sample also suggests the involvement of excimers, whose emission can be reduced at LT [START_REF] Dimitriev | Excimer Emission in J-Aggregates[END_REF].

For 1c-e, at first glance, the BDI units seem to be quite rigid in the crystals due to both stacking of the imidazolinone moiety and formation of the centrosymmetric dimers. However, the BDI units are stacked between one another, whereas in 1a-b, they are facing the benzoxazole units. This would possibly allow for a concerted torsion of the imidazolinone moieties allowing access to the CI in the solid state. Another option would be that the CIs involved in the non-radiative decay process in the aggregate phase of 1c-e are different from that computed for the isolated monomer. The steric constraints imposed by the crystal environment may forbid the access to the fairly large amplitude motion necessary to reach S 0 /S 1 -MECI Im-twist , and could possibly involve a more volume-conserving motion, as observed for example in some acenimides [START_REF] Crespo-Otero | Exploring Potential Energy Surfaces for Aggregation-Induced Emission-From Solution to Crystal[END_REF].

Finally, another possibility is that the extinction of PL is correlated with the presence of several hydrogen bonds. Assuming that the deformations observed for 1c, 1d and 1e in the ground state also take place in the excited state, they could be partly responsible for the extinction of PL via dispersion of the excitation energy along the C=O … H-C vibrations. It can be noticed that total PL extinction was observed for samples 1c and 1e, where deformation is the strongest. Such a deactivation pathway would be particularly original because H-bonds are generally considered to rigidify the molecular structures and activate the SLE processes of numerous fluorophores [START_REF] Saccone | Mesogens with Aggregation-Induced Emission Formed by Hydrogen Bonding[END_REF][START_REF] Wang | A switch-on fluorophore using water molecules via hydrogen bonding and its application for bio-imaging of formaldehyde in living cells[END_REF][START_REF] Wang | Multiple Hydrogen Bonds Promoted ESIPT and AIE-active Chiral Salicylaldehyde Hydrazide[END_REF][START_REF] Zhang | Phenothiazine modified triphenylacrylonitrile derivates: AIE and mechanochromism tuned by molecular conformation[END_REF], although the opposite behaviour was recently reported by us for hydroxy-substituted PBO derivatives [START_REF] Bremond | Effect of substitution on the solid-state fluorescence properties of styrylbenzoxazole derivatives with terminal dicyanomethylene group[END_REF]. Clearly, dimeric structures like the ones studied in the present work would deserve more attention regarding their solid-state photophysical properties.

Conclusion

In conclusion, five distinct crystalline forms were identified for the same PBO derivative, which is unprecedented, and a rare on/off fluorescence switching effect was reported. In the literature, non-emissive polymorphs of a solid-state fluorescent compound result from a molecular twist [START_REF] Mutai | Reproducible on-off switching of solid-state luminescence by controlling molecular packing through heat-mode interconversion[END_REF][START_REF] Liu | Polymorph-Dependent Luminescence Response to Acid Vapors and Its Application in Safety Protection of File Information[END_REF][START_REF] Shida | Sterically induced polymorphism: ON-OFF control of excited-state intramolecular proton transfer (ESIPT) luminescence of 1-methyl-2-(2′-hydroxyphenyl)benzimidazole[END_REF][START_REF] Borbone | On-Off Mechano-responsive Switching of ESIPT Luminescence in Polymorphic N-Salicylidene-4-amino-2-methylbenzotriazole[END_REF], or from a detrimental packing mode, including edgeto-face interactions [START_REF] Tchoń | Polymorphism and resulting luminescence properties of 1-acetylpyrene[END_REF] or face-to-face - stacking [START_REF] Borbone | On-Off Mechano-responsive Switching of ESIPT Luminescence in Polymorphic N-Salicylidene-4-amino-2-methylbenzotriazole[END_REF][START_REF] Devi | Mechano-responsive luminescent emissions of an organic molecular crystal: effects of aromatic stacking interactions and solid state packing[END_REF][START_REF] Yoshida | Solid-state fluorescence of zwitterionic imidazolium pyridinolates bearing long alkyl chains: Control of emission properties based on variation of lamellar alignment[END_REF]. To the best of our knowledge, the involvement of CI was proposed here for the first time.

Obviously, both polymorphism and PL switching are related to the presence of the small BDI unit, which is also responsible for the SLE behavior of compound 1. It is now important to check whether the BDI moiety may confer this set of properties to other types of molecules, which would make it a precious "polymorphogenic" building block. In addition to their interest for practical applications, such BDI derivatives would be ideal tools for exploring the link between molecular arrangement, vibrational modes and excited states in photoluminescent organic crystals.

Furthermore, a fast screening with more than 30 commercially-available dyes in the solid state indicated that PL enhancement at LT is a non-trivial process, which was only observed for one sixth of the investigated dyes (i.e. Pyrene, Coumarin 6, Coumarin 153, Rhodamin B, DCM). Remarkably, no significant PL enhancement at LT was detected for dyes that do not emit at room temperature. It could be instructive to know what these dyes have in common with compound 1, from a photophysical point of view. This could enable further developments towards new thermochromic materials.

Fig. 1 .

 1 Fig. 1. Chemical structure of compound 1.

Fig. 2 .

 2 Fig. 2. Schematic potential energy profile for the isolated model molecule of 1 along the main S 1 excited-state relaxation pathway. Energies are in kcal/mol with respect to the ground-state minimum (Min-S 0 ). SF-TD-DFT values are in normal font and TD-DFT values are in parentheses. Geometries are obtained at SF-TD-DFT level, except for TS-S 1 -Im-twist (optimized with TD-DFT). Relevant bond lengths (Å) and angles for torsion and pyramidalization are shown.

Fig. 3 .

 3 Fig. 3. Optical microscopy images of solvate 1b and polymorphs 1a, 1c, 1d and 1e.

Fig. 4 .

 4 Fig. 4. Molecular arrangement of the five crystalline forms of 1, showing the short contacts that involve the imidazolinone oxygen atom (blue ink).

Fig. 5 .

 5 Fig. 5. Molecular views showing - interactions for the five crystalline forms of 1. Distances in Å.

Fig. 6 .

 6 Fig. 6. Top: Photographs of microcrystalline powders of 1a, 1b and 1c illuminated by day light and UV light (365 nm). Bottom: PL spectra of the five crystalline forms. λ ex = 380 nm, maximum intensity proportional to quantum yield.

Fig. 7 .

 7 Fig. 7. a) Comparative emission of microcrystalline powder 1a illuminated by UV light (365 nm) at room temperature and after cooling in liquid nitrogen. b) Variation of the PL spectrum just after cooling (purple line) and during progressive return to room temperature (one spectrum every 50 s). The arrow indicates the sense of the variation. The dashed line corresponds to the initial spectrum recorded at 20°C. c) Variation of the emission intensity just after cooling the sample (arrows) and during progressive return to room temperature (λ ex = 380 nm, λ em = 550 nm).

  PhotoluminescenceInt.(AU)

Acknowledgements

ANR is gratefully acknowledged for funding (SUPERFON project # ANR-17-CE07-0029-03). We are indebted to Mr. Nicolas Ratel-Ramond (CEMES-CNRS) for the measurement of XRPD patterns, Ms. Grace Bauger for help in synthesis and Dr. Corinne Routaboul (Service commun IR, ICT) for FTIR spectra.

Electronic supplementary data

Conditions for the preparation of the various crystalline forms, physico-chemical characteristics, crystallographic data, molecular views, optical microscopy images, IR spectra, DFT calculations of the H-bond frequencies and videos showing molecular deformations. CCDC: 1997405, 1997406, 1997407, 1997408.

Keywords

Photoluminescence; Polymorphism; Organic Crystal; Green Fluorescent Protein; Conical intersection