Dr Irene Mustieles Marín

Audrey Auffrant
email: audrey.auffrant@polytechnique.edu

Pr P Le Floch

Dr A Auffrant Lcm

Phosphasalen vs salen ligands: what does the phosphorus change?

Keywords: salen type ligands, redox non-innocent ligand, phosphorus oxidation, catalysis

Phosphasalen ligands can be viewed as the phosphorous analogues of salen, in which the imine functions were replaced by iminophosphoranes (P=N). Their divergent stereo-electronic properties compared to salen, i. e., larger electron-donation, higher flexibility, and larger steric hindrance due to the presence of two substituents on the tetrahedral phosphorus atoms, were exploited in coordination chemistry and catalysis. They were proved to be able to stabilize metals in unusual oxidation states; square planar copper(III) and nickel(III) complexes were isolated. Because of the redox noninnocent character of these ligands, the electronic structure of the one-electron oxidized complexes was studied in depth to determine the metallic vs ligand contribution in the overall delocalization. In addition, the catalytic activity of different phosphasalen complexes was proved in ring opening polymerization of lactide, and rare-earth phosphasalen complexes were shown to be powerful, rapid, controlled, and stereoselective initiators. Importantly, the stereochemical outcome of the reaction can be tuned by changing the ligand structure or the metal. In particular, efficient iso-selective initiators were developed.

Introduction

N,N-bis(salicylidine)ethylenediamines, generally called in short "salen", are a family of very successful ligands in coordination chemistry. [START_REF] Bellemin-Laponnaz | Coordination Chemistry and Applications of Salen, Salan and Salalen Metal Complexes[END_REF] Their applications encompass catalysis, [2] bioinorganic chemistry, [3] as well as material science. [3b, 4] Their popularity stems from their easy synthesis allowing facile structural variations and their ability to coordinate a wide range of metals in various oxidation states. However, among all the skeleton variations proposed, only few concern the introduction of a heteroatom in place of the carbon in the imine function, with the exception of some azo derivatives. [5] The introduction of phosphorus gave phosphasalen ligands, as we termed them, in which the imine groups have been substituted by iminophosphorane (P=N) functions (see Figure 1). First complexes of this family were reported in 2011. The group of P. Diaconescu described the coordination of a phosphasalen type ligand incorporating a ferrocene unit (labeled as Psalfen thereafter), to cerium and yttrium centers (see Figure 2). [6] Our group reported the synthesis and characterization of group 10 phosphasalen complexes. [7] The presence of the iminophosphorane functions in the salen backbone induces both electronic and geometric changes. Regarding the electronic properties, the most important difference is the absence of a π-system. In the iminophosphorane function the nitrogen atom possesses two lone pairs which are stabilized by delocalization in the -antibonding orbitals of the P-C bonds. [8] This so-called negative hyperconjugation induces a strong interaction between the phosphorus and the nitrogen atoms which is commonly pictured by a double bond. However, iminophosphorane functions can be better described as masked amides. Thus, they are strong -and -donors with almost no accepting ability, while imine functions are -donating and -accepting. Interestingly, the electron-donating properties of the iminophosphorane functions can be tuned via the nitrogen and phosphorus substituents. Those featuring an aryl group on the nitrogen atom are less electron-donating than those bearing an alkyl because of a partial delocalization of the nitrogen lone pairs on the aromatic ring. The substituents on the phosphorus atom influence its ability to stabilize the lone pairs of the nitrogen atoms. The negative hyperconjugation increases when the phosphorus bears aromatic substituents. The donation of the iminophosphorane is therefore enhanced by the presence of alkyl substituents on P or N atoms. Regarding the geometry, iminophosphoranes are not planar as the sp 2 hybridized imines since the phosphorus atom is engaged in four bonds. Indeed, the phosphorus bears two substituents while there is only one on the carbon of the imine. In addition, the bond lengths are not comparable (PN∼ 1.58 Å vs CN ∼ 1.28 Å) Therefore, phosphasalen complexes differ markedly from their carbon analogues. The purpose of this microreview is to give an overview of the recent results obtained with this new type of phosphorus based ligands. It will be shown how the presence of the heteroatom can influence the geometry, the electronic structure, as well as the catalytic ability, compared to salen analogues.

Synthesis, structure and electronic properties of phosphasalen complexes

Ligand synthesis, coordination and solid state structure

The structures of the first phosphasalen complexes are presented in Figure 2 together with that of their salen counterparts.

Scheme 1. Phosphasalen ligand synthesis.

The synthesis of the phosphasalen platform starts from ophosphinophenol. The introduction of the iminophosphorane function can then be achieved by two main strategies: the Staudinger reaction, [START_REF] Staudinger | [END_REF] consisting in the condensation of a phosphine and an azide, or the modified Kirsanov reaction, [10] involving the halogenation of a phosphine followed by the trapping of the intermediate by a primary amine in presence of a sacrificial base. The group of Diaconescu [6] relied on the first synthetic method to prepare the neutral H2Psalfen proligand (Scheme 1, top) because of the availability of 1,1'-diazidoferrocene. They were, by this way, able to isolate and coordinate the phosphazide intermediate (P=N-N=N) derivative. We preferred to rely on a modified Kirsanov reaction [7] allowing the use of diamines, thus avoiding the employ of potentially explosive azido compounds (Scheme 1, bottom). So, we isolated the corresponding bis(aminophosphonium) salts which are rather stable and can be stored almost indefinitely on the bench without any specific precautions. On the contrary, iminophosphorane derivatives are sensitive to moisture. Coordination reactions can be performed by mixing the neutral proligand with a metal precursor featuring basic ancillary ligands, as done for the Ce IV complex via the reaction between H2Psalfen and [Ce(OtBu)4(THF)2] (Scheme 2a). It is also possible to generate the dianionic ligand, using sodium or potassium hexamethyldisilazane or hydride, and make it react with the metal chloride precursor as done for Ce III , Y III , Pd II , and Ni II complexes (see Scheme 2b andc). In these syntheses, the anionic ligands were not isolated because of their sensitivity, but this was done in some other cases (vide section 4). The synthesis of rare-earth complexes was completed by metathesis with tBuOK. All diamagnetic complexes were characterized by multinuclear NMR spectroscopy. Coordination is evidenced by a deshielding of the 31 P nuclei compared to the corresponding proligands: P = 34. 6, 32.6, 33.7, and 32.8

ppm for [Ce(Psalfen)(OtBu)(THF)], [Y(Psalfen)(OtBu)], [Pd(Psalen H )],
and [Ni(Psalen H )] respectively, to compare with 27.1 ppm for H2Psalfen and 18.6 ppm for K2Psalen H . Scheme 2. Formation of phosphasalen complexes.

The characterization of [Ce(Psalfen)(OtBu)2], [Pd(Psalen H )], and [Ni(Psalen H )] by X-ray diffraction analysis allows discussing the geometrical modifications compared to the structures of the salen analogues. For these three complexes, the M-O and M-N distances measured are larger than in the corresponding salen complexes. The deviation from planarity also increases with the introduction of the phosphorus. In the cerium complex, the NCeOCO torsion angle (see Figure 3 for labeling) was measured at about 45 ° in the Psalfen complex [6] vs 4.5 ° in the salfen analogue. A larger distortion was also observed in [Pd(Psalen H )] and [Ni(Psalen H )] [7] compared to their salen counterpart: [11] the NMOCO torsion angle was measured at 25.64 and 22.57 ° in [Pd(Psalen H )] and [Ni(Psalen H )] respectively, while these are at 3.02 and 3.30 ° in the corresponding salen derivatives. This evidences the bending of the phenoxide rings relative to the main coordination plane. These geometrical differences arise from the tetrahedral arrangement of the phosphorus bearing one additional substituent compared to the imine carbon and the difference in bond lengths between C=N and P-N. These differences make phosphasalen ligands more flexible than salen ones.

Influence of iminophosphorane on electronic and magnetic properties

Phosphasalen ligands also differ a lot in terms of electronics as show by cyclic voltammetry experiments.

Table 1. Redox events for phosphasalen [6][7] and salen [12] complexes. F 4]; scan rate = 250 mV s -1 . c c = 1.5 mmol L -1 . d c = 2.0 mmol L -1 . e in DMF/CH3CN 60/40, with 0.12 mol L -1 NBu4BF4; scan rate = 100 mV s -1 . f c = 2.0 mmol L -1 . g c = 1 mmol L -1 in CH3CN with 0.1 mol L -1 NBu4ClO4 see ref.

Entry

[12b]. h c = 3.8 mmol L -1 i c = 1 mmol L -1 in CH3CN with 0.1 mol L -1 NEt4ClO4 see ref.

[12a] j anodic potential. k cathodic potential.

Data of yttrium, cerium, palladium and nickel phosphasalen complexes, and those of their salen analogues are gathered in Table 1. All potentials given are relative to Fc + /Fc for sake of comparison.

The oxidation turns out to be easier for phosphasalen complexes than for salen ones. Indeed, the potential measured for the oxidation of the ferrocene unit in yttrium(III) and cerium(IV) Psalfen complexes is lowered by 0.38 and 0.10 V compared to salfen ones (Table 1, entries 1 and 5 vs 2 and 6). [6] In palladium and nickel Psalen H complexes, the oxidation is lowered by about 0.2 V when introducing the iminophosphoranes (Table 1, entries 7-10). [7] Reduction was only observed with the cerium(IV) complexes (Ce IV /Ce III ), and is more difficult in presence of the Psalfen ligand than with the salfen one. [6] The better electron-donating ability of iminophosphoranes is responsible for the observed shifts in the redox potentials. The presence of the phosphorus also influences the magnetic properties. [Ni(Psalen H )] complex presents a low magnetic moment in solution, measured at 1.4 B at room temperature by the Evans method whereas [Ni(salen H )] did not, suggesting a low lying triplet state. To understand the origin of this behavior, which contrasts with the diamagnetism of [Ni(salen H )], DFT calculations were performed. They confirmed the small singlettriplet energy difference and demonstrated that the easier ionization of the Psalen H complex is responsible for the lowering of the singlet-triplet energy gap. [7]

One-electron oxidation of phosphasalen nickel and copper complexes

The oxidation of salen complexes was largely studied in order to bring a better understanding of reaction mechanisms in homogeneous catalysis or at the active site of metalloenzymes. Indeed, their N2O2 coordination sphere can in some cases be mimicked by the salen platform. From a fundamental perspective, the determination of the electronic structure of oneelectron oxidized salen complexes is challenging because of the redox-non innocence of the ligand. The oxidation can take place either on the ligand, leading to a phenoxyl radical or on the metal. As galactose oxidase (GO) was proved to incorporate in its active site a copper(II) center and a tyrosyl radical, the electronic structure of mono-oxidized copper(II) salen complexes was much investigated. They were generally characterized as a Cu II -ligand radical complex (Cu III was evidenced in only two cases). [3a, 13] Nickel complexes received also much attention, which may be explained by the vicinity of both metals in the periodic table and the involvement of Ni-based metalloenzyme, such as superoxide dismutase (SOD), in enzymatic oxidation reactions. [14] In that case, the catalytic relevant species was characterized as a Ni III complex. Salen proved capable to stabilize a high-valent Ni III center in an octahedral environment, [15] while mono-oxidation leads to nickel(II) phenoxyl radical complexes in absence of exogenous ligands. [16] Because of the geometric and electronic differences due to the presence of the phosphorus atom, the electronic structures of one-electron oxidized phosphasalen complexes merits further study. As the iminophosphorane function can be viewed as a phosphoniumnitrogen ylide (P + -N -), its incorporation within the ligand backbone should decrease the electron density on the phenolates (because of the presence of an electropositive phosphorus atom) as well as increase the electron donation from the nitrogen atoms. The phosphasalen platform should therefore favor a metal-centered oxidation (compared to salen) and may be able to stabilize the high-valent metal in a tetracoordinated environment. Copper and nickel complexes were investigated because of the precedent studies on salen derivatives and the recent interest for the synthesis and characterization of high-valent copper(III) [17] and nickel(III/IV) complexes. Examples of Ni IV complexes remain scarce, [18] while the Ni II /Ni IV redox couple [19] has started to be considered as an alternative to the Pd II /Pd IV couple in bond forming reactions. Ni III complexes have also recently found applications in synthesis, [20] and Cu III have been established as key intermediates in various catalytic reactions. [21] Most importantly, the precise determination of the electronic structure of a complex can be very complicated in presence of redox non-innocent ligands [22] such as those containing phenoxide units. Depending on the nature of the redox-active orbitals in such complexes, the redox processes can take place either on the metal or on the ligand. Therefore, different electronic configurations are possible for nickel and copper complexes. In the former, the oxidation leads to a paramagnetic complex which can be either a high-valent d 7 Ni III complex or a ligand radical complex (Figure 4). In the case of copper, the oxidation can give a diamagnetic Cu III complex (closed-shell singlet configuration), if the oxidation takes place on the metal. If the electron is removed from the ligand, the resulting d [START_REF] Staudinger | [END_REF] complex can be either diamagnetic, if the unpaired electron on the metal and the one on the ligand couple antiferromagnetically (open-shell singlet configuration), or paramagnetic if they couple ferromagnetically leading to a triplet (Figure 5). Figure 6 shows the structure of the four phosphasalen complexes investigated and their salen precedents (Figure 6). For this study a ligand featuring ortho and para-substituted phenoxide rings was employed, tert-butyl substituents were chosen for solubility reason (Psalen tBu ). [23] In order to determine the influence of the phenoxide substituents, the Psalen OMe ligand, where the para position on each phenoxide ring is occupied by a methoxy group, was also employed. [24] The electronic structure of the corresponding salen complexes was investigated: [Ni(salen tBu )] + was described as a delocalized radical complex in the absence of exogenous ligands, [15c, 25] while the introduction of a methoxy group ([Ni(1,2-salcn OMe )] + ) favors the formation of a localized phenoxyl radical complex. [13b] The same structure was established for [Cu(1,2-salcn OMe )] + , [13b] whereas [Cu(1,2-salcn tBu )] + was described as a Cu III complex in the solid state. In solution, a spin equilibrium between the highvalent and the ligand radical species was suggested. The phosphasalen ligands and complexes shown in Figure 6 were synthesized following the described methodology (see Schemes 1 and 2), and their electronic properties were investigated by cyclic voltammetry. Nickel phosphasalen complexes exhibit three oxidation waves: a first reversible, a second pseudo-reversible and a last one irreversible, which accounts for the possibility to oxidize the metal and the two phenoxide rings. The copper complexes display two reversible oxidations, which can correspond to the sequential oxidation of both phenoxide rings or that of the metal and one phenoxide. Table 2 gathers oxidation potential values for the phosphasalen complexes and their carbon analogues. As already noticed, oxidations are easier for phosphasalen complexes than salen ones. In the case of the nickel derivatives, the better electrondonating ability of Psalen ligands allows the observation of three oxidations, while in salen complexes, only two oxidation waves are observed. Moreover, oxidations are further facilitated by the electron-rich methoxy groups in the same extent for nickel and copper complexes (E = 0.07 and 0.06 V). The gap between the first and the second oxidation (E1-2) was used as an indication of the delocalization within the one-electron oxidized complex, translating how the first oxidation influences the second one. [15c] A large value in the case of [Ni(salen tBu )] (0.46 V) was ascribed to a strong electrochemical communication. This gap was measured at 0.81 and 0.53 V for [Ni(Psalen tBu )] and [Ni(Psalen OMe )], at 0.85 and 0.50 V for [Cu(Psalen tBu )] and [Cu(Psalen OMe )] respectively. These comparable E1-2 values for nickel and copper complexes when featuring the same ligand suggest similar electronic structure of the mono-oxidized species.

The gaps observed for phosphasalen complexes are much larger than those reported for the corresponding salen derivatives suggesting that the metal is more and more affected by the first oxidation on going from salen tBu , Psalen OMe to Psalen tBu ligands. a measured in CH2Cl2 and expressed in V versus Fc + /Fc. b c = 3 mmol L -1 for the complex, 0.12 mol L -1 NBu4PF6, see ref. [24]; c c = 1 mmol L -1 for the complex, 0.1 mol L -1 NBu4PF6. d ref.

[15c]. e ref.

[13b]. f ref. [26].

To investigate the electronic structure of the cationic phosphasalen complexes, [20b] they were synthesized in high yield using Ag I salts as oxidant (Scheme 3).

Scheme 3. Oxidation of phosphasalen complexes.

Their structure was studied employing various spectroscopic technics and quantum chemical calculations. X-ray structures of cationic nickel complexes showed a contraction of the coordination sphere, which is not identical for all atoms so that a long NNiO axis appears upon oxidation. The oxidation is accompanied by an amplification of the distortion relative to an ideal square planar geometry. Meanwhile, the phenoxy units do not experience much change; no quinoid pattern was observed. All these data therefore point towards the formation of highvalent Ni III complexes. In addition, the solid state magnetic measurements gave slightly different giso values: 2.22 for [Ni(Psalen tBu )](SbF6) and 2.16 for [Ni(Psalen OMe )](SbF6). In agreement with X-ray data, these g values indicate a substantial metallic character of the magnetic orbital. Nevertheless, the slightly decreased g value for [Ni(Psalen OMe )](SbF6) can be the sign of a lower metal character as suggested by the CV measurements. In solution, no 31 P resonance was observed for both complexes while their 1 H NMR spectra are typical of paramagnetic complexes, reinforcing the picture of d 7 complexes. However, subtle differences appear when considering their chemical shifts and in particular the isotropic paramagnetic shift ( para =  obs - dia ). The isotropic shifts are larger for [Ni(Psalen OMe )](SbF6) than for [Ni(Psalen tBu )](SbF6). This was proposed to be related to a larger spin density on the phenoxy ring in [Ni(Psalen OMe )](SbF6).

The cationic phosphasalen complexes were also characterized by EPR spectroscopy since Mono-oxidized nickel(II) complexes have a doublet multiplicity. An unresolved signal centered at g = 2.034 was observed for [Ni(salen tBu )] + at 200 K, [15c] while [Ni( [Ni(Psalen tBu )](SbF6) and [Ni(Psalen OMe )](SbF6) present different EPR spectra. At 290 K, [Ni(Psalen OMe )](SbF6) exhibits an isotropic complicated pattern centred at giso = 2.005 corresponding to an organic radical (Figure 7, top right). When decreasing the temperature, the spectrum broadens, and below 100 K a new rhombic signal appears (g1 = 2.22, g2 = 2.14, and g3 = 2.07, gav = 2.14, Figure 7, top left). It corresponds to a Ni III d 7 center in rhombic environment. A rhombic S = 1/2 signal was also observed for [Ni(Psalen tBu )](SbF6) at 5 K. For this complex, the signal broadens when increasing the temperature (see Figure 7, bottom right) without showing any other feature. For both complexes however, the solid state EPR spectrum agrees with a Ni III structure and no ligand signal was seen. The observation of both features (metallic and radical) in the EPR spectrum of [Ni(Psalen OMe )](SbF6) (Figure 7, bottom right) is not usual in these systems and can be explained by a strongly delocalized orbital, or a multi-configurational ground state with several configurations having either metallic or ligand character,

or by an incomplete valence tautomerism. [13c, 15c, d, 16e] It is very difficult to distinguish between these two last hypotheses. Nevertheless, a value of 1.9 B, exceeding the spin-only value, was obtained as the effective magnetic moment of this complex in solution. This may indicate an orbital contribution to this moment and seems to tip the balance in favor of the multiconfiguration rather than the valence tautomerism hypothesis.

For copper complexes, X-ray diffraction shows a contraction of the coordination sphere upon oxidation. This contraction is larger for [Cu(Psalen tBu )](SbF6) than for [Cu(Psalen OMe )](SbF6) which may be the sign of a different electronic structure. This contraction, together with the absence of significant variation in the metrics of the phenoxide, suggests in both case a metalcentred oxidation. Accordingly, these complexes are EPR silent between 77 and 293 K. However, EPR spectroscopy was poorly informative even for [Cu(1,2-salcn OMe )] + and [Cu(salen tBu )] + , for which oxidation occurs on the phenoxide. Their silent EPR spectra were explained by the nature of the coupling between the spin of the radical species and that of the Cu II center. [13b, 26] Nevertheless, the magnetic data for the cationic phosphasalen copper complexes do not fit with a pure d 8 configuration as they both present a temperature independent paramagnetism (TIP). This corresponds to a singlet ground state and a high energy triplet state, which is almost not populated at room temperature (gap beyond 1000 cm -1 ). Noteworthy, the  value measured on [Cu(Psalen tBu )](SbF6) crystals was much lower than that of [Cu(Psalen OMe )](SbF6) (0.0001(1) vs 0.00070(5) cm 3 mol -1 ), suggesting again some differences in the electronic structure of the two complexes. This was further evidenced by NMR studies. While [Cu(Psalen tBu )](SbF6) displays a diamagnetic behavior: 31 P and 1 H NMR signals were observed at room temperature with almost no variation with the temperature, [Cu(Psalen OMe )](SbF6) presents no 31 P signal at room temperature and a broader 1 H NMR spectrum with some resonances being particularly shifted.

The variable temperature NMR study evidenced a nonlinear evolution of the chemical shifts, which fitting allowed the determination of various parameters. The energy gap between the singlet and triplet state evaluated at 1420 cm -1 (~ 17 kJ mol - 1 ) is in agreement with that anticipated by magnetic measurements. The chemical shifts corresponding to the singlet state were also obtained; they slightly differ from those observed at -90 °C. DFT calculations were unable to give further indications about the electronic structure, since the computed HOMOs are very similar for all the oxidized complexes. CASSCF calculations were conducted pointing in each case to multi-configurational ground states in agreement with the experimental observations. This means that several configurations are necessary to accurately depict the ground state. It happens in a transition metal complex when, despite a decreased orbital overlap, the ligand and the metal remain close in energy so that bonding electrons can be located in different orbitals. [22b, d, e, 27] Very interestingly, the differences found depending on the ligands were well reproduced by these calculations. Thus, the main configuration obtained for [Ni(Psalen tBu )] + corresponds to a single electron in a metal based orbital. For [Ni(Psalen OMe )] + , the single electron is mainly localized in metal-ligand orbital with a strong contribution of the phenoxide orbitals. For [Cu(Psalen tBu )] + , the major configuration corresponds to a closed-shell singlet, while this configuration is minor in [Cu(Psalen OMe )] + , for which the open-shell singlet was calculated to be major. This larger ligand character found experimentally and confirmed by the calculations can be ascribed to an increased electron density on the phenoxide rings when substituted by a methoxy group. Nevertheless, aside electronic factors the different coordination geometry offered by the phosphasalen platform may explain the differences observed with respect to salen derivatives. A better orbital mixing may be possible in phosphasalen compared to salen, and even more for the Psalen OMe ligand than the Psalen tBu one. For the Psalen tBu and Psalen OMe nickel and copper complexes, the  and  angles (see Figure 8) range from 125 to 130 ° and 32 and 44 ° respectively. These values are comparable with those of galactose oxidase= 129 ° and  = 75 ° but differ much from those measured for salen complexes (180 ° and 90 °). Thus, in phosphasalen complexes the orbital overlap increases resulting in a delocalized bonding orbital with both metal and ligand character. Moreover, a slight difference in the  angle was found in Psalen OMe (44 ° on average) compared to Psalen tBu ligand (36 ° on average) which may be at the origin of the larger ligand radical character evidenced in [M(Psalen OMe )] + complexes. This geometrical factor may also play a role in the differences observed experimentally between solution and solid state measurements.

As this investigation of four mono-oxidized phosphasalen complexes points towards a multi-configurational ground state, in which the configuration corresponding to an oxidized metal is preponderant, further oxidations are assumed to be centered on the phenoxide rings. Nevertheless the precise electronic structure of [M(Psalen)] 2+ or even [Ni(Psalen)] 3+ would require a complete spectro-electrochemical study.

Moreover, the influence of other parameters such as nature of the N,N linker, as for salen ligands, but also that of the phosphorus substituents has to be investigated. This would impact the electronic structure, the reactivity of the complexes, and in fine the catalytic ability.

Catalytic application of phosphasalen ligands in (lactide) ring opening polymerization

Up to now, the only reported catalytic performances for phosphasalen complexes concern the ring opening polymerization (ROP), of lactide mostly. Polylactide (PLA) is a biodegradable polymer which has found numerous applications for packaging [28] and bio-medical materials. [29] The lactide derives from lactic acid which is produced from high starch content plant and can therefore offer a sustainable alternative to petrochemicals. That is why there is a growing interest in the preparation of well-defined PLA, which is efficiently obtained by a controlled polymerization of lactide. This can be catalyzed by enzymes, [30] organic molecules, [31] and metal amide or alkoxide complexes, [32] for which a careful design of the ancillary ligand may lead to high stereoselectivities. Indeed, lactide has two chiral centers and can exist as three stereoisomers: the Dlactide (combining two R-lactic acids), the L-lactide (containing two S-lactic acids) and the meso-lactide (a combination of R and S-lactic acids, Scheme 4). Initiators able to promote the stereoselective production PLA from rac-lactide are highly desirable. Most initiators yield atactic or heteroatactic-PLA (formed by the alternative incorporation of R,R and S,S-lactic acids) whereas isotactic PLA (resulting for the incorporation of the same enantiomer, either R,R or S,S) have superior thermal and mechanical properties. [33] Group 3, and in particular Y III based initiators, are efficient and can be stereoselective. [34] This has stimulated the evaluation of yttrium phosphasalen complexes for lactide ROP. The [Y(Psalfen)(OtBu)] complex described by Diaconescu and coworkers (Figure 9), catalyzes the ring opening polymerization of lactide; 74 % conversion is observed for L-lactide after 3 h (Table 2, entry 1) and 95 % for rac-lactide using 100 equivalents of monomer. Most interestingly, the catalytic ability can be switched off by oxidizing the ferrocene unit present in the ligand backbone; [35] [Y(Psalfen)(OtBu)](BAr F 4) was found unable to polymerize lactide. A stoichiometric experiment shows that the process stops with the insertion of one monomer molecule, because the propagation is hampered. In a reverse manner, they proved that trimethylcarbonate polymerization can be accelerated by oxidation in the case of an indium initiator ([In(Psalfen)(OPh)]). [35b] Yttrium phosphasalen initiators were also developed for rac-LA polymerization. As coordination of the Psalen H ligand to Y III gave dimers (Figure 9), Psalen tBu was used to avoid dimerization and improve solubility. [36] Complexes were obtained by coordination of the Psalen R ligands to [YCl3(THF)3.5] followed by metathesis of [Y(Psalen R )Cl] with R'OK (R' = tBu, Et), as depicted in Scheme 2. These complexes are rapid initiators for lactide polymerization and the process is well controlled (Table 4, entries 2-5). In particular, the performances of the monomeric [Y(Psalen tBu )(OtBu)] outstrip those of the dimers (Table 4, entries 2-4). This initiator is extremely active and shows a good control in presence of an exogenous alcohol as a chain transfer agent, as well as an excellent heteroselectivity at room temperature. It can be used at low loadings (down to 5000, see Table 4, entry 5), allowing the preparation of high molecular weight polylactide. Moreover, this initiator tolerates the use of unpurified lactide (Table 4,entry 6). [START_REF]Usually lactide used for ROP is sublimed and recrystallized several times to ensure a high level of purity. Impurities present are among other water and lactid acid. Lactide obtained from chemical supplier is indicated as 98% or 99% pure[END_REF] The control of the molecular mass is not optimal with the tertbutoxide complex but it was improved by adding one equivalent of isopropanol as a chain transfer agent (see Table 4, entries 4, 8, and 9). Most importantly, these yttrium based catalysts are stereoselective, leading to heterotactic polylactide. The best stereoselectivity in this series was observed for [Y(Psalen tBu )(OtBu)] with Ps around 0.9. These performances are much better than those of the salen analogues which present higher Lewis acidity. [38] This exceptional activity was ascribed to an easier migration of the alkoxide chain thanks to the higher electron-donating ability of the phosphasalen, which facilitates the propagation. This more than compensates the more difficile coordination of the monomer (due to the reduced Lewis acidity). Moreover, the flexibility of phosphasalen ligands may also be beneficial since the coordination sphere of the yttrium is expected to change much during the coordinationinsertion steps in the ring opening polymerization. [36] In order to improve the catalytic performances, the structure of the phosphasalen ligand was modified. Psalen OMe was employed (Figure 9) to further increase the electron density at the metal.

[Y(Psalen OMe )(OtBu)], is a slightly more rapid initiator than [Y(Psalen tBu )(OtBu)] (Table 4, entries 4 and 7); the polymerization control was also improved by adding one equivalent of isopropanol. Importantly, the tacticity control remains excellent. [39] Other yttrium complexes differing by the nature of the nitrogen linker were synthesized. The rate of polymerization as well as the heterotactic bias decrease when replacing the ethylene linker by a cyclohexene, phenylene , or propylene one. [40] However, the use of a pentadentate ligand incorporating a supplementary coordinating amine led to a powerful initiator. [39] Noteworthy, in [Y(PsalenN tBu )(OR')] (R' = Et, tBu) complexes (Figure 4), the yttrium has a different coordination environment since the phosphasalen adopts a cis- configuration and not a trans one. These complexes were found very active in the lactide ROP, although less rapid than [Y(Psalen tBu )(OR')] (Table 4, entries 4, 8, and 10). [39] Again, with a tert-butoxide initiator, the addition of an exogenous alcohol is necessary to ensure a good control of the polymerization (Table 4, entry 8 vs 10). The most striking point is the stereoselectivity, as [Y(PsalenN tBu )(OR')] initiators yield isotactic polylactide with Pi values of about 0.73 at 298 K to a maximum of 0.84 at 258 K (Table 4,entry 9). [39] This unexpected change of stereoselection by a simple modification of the ligand backbone was quite unprecedented and was explained by the more congested and constrained active site in hexacoordinated yttrium complexes. The modification of the ligand via the introduction of methoxy substituents allows an increased activity with a comparable isoselectivity (Table 4, entries 10 and12). [41] In attempt to further improve the catalytic performances, the group of C. K. Williams employed other metals (Figure 10). With lutetium, a lanthanide with a slightly smaller covalent radius than yttrium (1.87 vs 1.90 Å), the iso-selectivity of the polymerization was improved to reach Pi = 0.87 at room temperature (Table 4, entry 12) and 0.89 at 257 K. [42] This higher stereoselectivity was ascribed to a more constrained coordination sphere for the lutetium, which also induces a lower catalytic activity compared to yttrium ( Following this trend, no lactide polymerization was observed for the scandium based initiator ([Sc(PsalenN tBu )]). [41] But this initiator allows a slow but controlled polymerization of caprolactone. Conversely, lanthanum initiator incorporating a larger metal (covalent radius of 2.07 Å) gave a very rapid polymerization, with a moderate heteroselectivity (Ps = 0.72, Table 4, entry 14). [42] NMR studies evidenced a more fluxional structure for La compared to Y and Lu complexes, suggesting that the stereocontrol depends on the level of the rigidity dictated by the ligand. Aiming at developing more rapid and isoselective lactide polymerization initiators, the Williams' group investigated the combination of phosphasalen ligand and In 3+ cation. [43] As coordination experiments with the pentadentate PsalenN tBu ligand gave a mixture of products, the study was conducted with two tetradentate ligands featuring an ethylene linker, Psalen tBu and Psalen PhMe2 , which differ from the precedent by the replacement of tert-butyl groups on the phenoxide by cumyl ones (Figure 10). [43] Solid state structures were different for both complexes as [In(Psalen tBu )(OtBu)] was characterized as a monomer, and [In(Psalen PhMe2 )(OEt)] as a dimer with bridging ethoxide. These differences were related to the size of the alkoxide ligand. Nevertheless, in a coordinating solvent such as THF, both complexes were found mononuclear, and the same is presumed during polymerization. The best isoselectivity was observed with [In(Psalen PhMe2 )(OEt)] (Table 4, entry 15), giving Pi = 0.92 at 258 K. Interestingly, this catalyst allows a high degree of isoselectivity together with a rapid polymerization; it is as rapid as [Y(PsalenN tBu )(OtBu)] (Table 4, entries 8 and 15) and more selective than [Lu(PsalenN tBu )(OEt)] (Table 4, entries 13 and 15). Therefore, indium Psalen complexes show much higher polymerization rates than indium salen complexes with isoselectivity comparable to those of aluminium salen initiators. These first catalytic studies evidenced that the performances of the phosphasalen complexes can be nicely tuned by modifying the phenoxide substituents, the N,N linker, as well as the metals.

To date, no report deals with the influence of the phosphorous substituents on the catalytic ability. Moreover, considering the numerous applications of salen or salan ligands in catalysis, far beyond polymerization processes, this new family of ligands should find in the future other interesting applications.

Conclusion

Phosphasalen ligands were first described six years ago and have since shown interesting properties. Importantly, they are relatively easy to synthesize; the Kirsanov method have the advantage to use commercial diamine but the Staudinger reaction is also a valuable option when the diazide is stable and easy to access. As expected, phosphasalen exhibit better donating abilities compared to salen. They also offer a larger coordination sphere (longer M-N and M-O bonds). The P=N bond, which is longer than the imine one, and the tetrahedral geometry of the phosphorus compared to the sp 2 hybridization of the imine carbon in salen, play an essential role in the properties of phosphasalen. This leads to a more sterically hindered second coordination sphere and allow an increased flexibility compared to salen. The investigation of the one-electron oxidation of phosphasalen complexes showed that this ligand favors a metal-centered oxidation, allowing the stabilization of nickel(III) and copper(III) centers in a square planar environment. Thus, phosphasalen is a valuable alternative to salen when seeking to stabilize electron deficiency on the metal center. Moreover, the oxidation locus can be finely tuned by modifying the ligand structure of Psalen as exemplified with the introduction of methoxy groups. Both electronic and geometric considerations can explain the higher ligand radical character of the Psalen OMe mono-oxidized complexes. Most importantly, the contradictory experimental results as well as theoretical investigations point towards a multi-configurational ground state for these complexes. This is another contribution to the literature concerning the difficult description of the spectroscopic oxidation state in presence of redox non-innocent ligands. The peculiar coordination environment created by the phosphasalen platform was also exploited in catalysis for the ring opening polymerization, of lactide mainly. It was demonstrated that the reduced Lewis acidity of those complexes did not hamper their reactivity since it is compensated by an easier migration of the alkoxide, allowing a rapid propagation. Yttrium, lutetium and indium phosphasalen complexes are outstanding catalysts for the ROP of lactide exhibiting a high activity, a good control of the molecular mass, as well as a high stereoselectivity.

Heteroselective and isoselective catalysts were described. The switch in the stereochemical outcome of the reaction was, in some cases, done by modifying the N,N linker in the ligand backbone. These studies are very promising concerning the potential of phosphasalen complexes in catalysis, especially for reactions involving the transfer of a nucleophile from the metal to the substrate. As its carbon analogue salen, has been established over the years as a powerful ligand in catalysis and especially in asymmetric catalysis, developments of phosphasalen complexes in this area are expected in the future.

MICROREVIEW

Phosphasalen complexes featuring a tetradentate N2O2 ligand can be viewed as the phosphorous analogues of salen in which iminophosphoranes (P=N) replace imines. The presence of the phosphorus influences both the electronic properties and the steric hindrance. This was demonstrated by studies regarding the electronic structure of one-electron oxidized complexes as well as the catalytic abilities of phosphasalen complexes in ROP reactions.

Figure 1 :

 1 Figure 1: Salen and Phosphasalen general structure

Figure 2 .

 2 Figure 2. Structure of the first phosphasalen complexes and their salen analogues.

Figure 3 .

 3 Figure 3. Labeling for geometrical considerations.

Figure 4 .

 4 Figure 4. Representation of the possible electronic configurations for oneelectron oxidized nickel complexes in a square planar environment.

Figure 5 .

 5 Figure 5. Representation of the possible electronic configuration for oneelectron oxidized copper complexes in a square planar environment.

Figure 6 .

 6 Figure 6. Structure of Psalen tBu and Psalen OMe complexes and their salen analogues.

Figure 7 .

 7 Figure 7. EPR spectra of [Ni(Psalen OMe )](SbF6) (top: left at 10K, right at 290 K) and [Ni(Psalen tBu )](SbF6) (bottom: left at 5 K, right at 252 K) in CH2Cl2 solution.

Figure 8 .

 8 Figure 8. Definition of  and  angles.

Scheme 4 .

 4 Scheme 4. rac-Lactide polymerization.

Figure 9 .

 9 Figure 9. Yttrium phosphasalen initiators for lactide Ring Opening Polymerization.

Table 2 .

 2 Redox potentials for Psalen and salen nickel and copper complexes. a

	Compound	E 1 1/2	E 2 1/2	E1-2	Ea 3	E2-3
	[Ni(Psalen tBu )] b	0.001	0.82	0.81	1.07	0.26
	[Ni(Psalen OMe )] b	-0.06	0.47	0.53	1.00	0.53
	[Ni(salen tBu )] c,d	0.59	1.05	0.46	-	-
	[Ni(1,2-salcn OMe )] c,e	0.22	0.64	0.42	-	-
	[Cu(Psalen tBu )] b,	0.10	0.95	0.85	-	-
	[Cu(Psalen OMe )] b	0.04	0.54	0.50	-	-
	[Cu(salen tBu )] c,f	0.55	0.75	0.20	-	-
	[Cu(1,2-salcn OMe )] c,e	0.28	0.44	0.16	-	-

  1,2salcn OMe )] + gave a rhombic signal with g values of 2.015, 1.992, and 2.054 (gav = 2.02) at 100 K.[13b]

  Table 4, entries 10 and 13).

	Entry	Initiator	[LA]0 : [I](:[ i PrOH])	time	Conv.	Mn	Mn calc	PDI	Ps
						(%)	(kg mol -1 )	(kg mol -1 )
	1	[Y(Psalfen)(OtBu)] a		100:1	3 h	74	7.5	10.7	1.07	-
	2	[Y(Psalen H )(OEt)]2 b		200:1	56 min	90	27.1	25.9	1.36	0.78
	3	[Y(Psalen H )(OtBu)]2 b		200:1	40 s	91	57.6	26.2	1.42	0.78
	4	[Y(Psalen tBu )(OtBu)] b		1000:1:1	70 s	80	105	115	1.08	0.90
	5	[Y(Psalen tBu )(OtBu)] b		5000:1	30 min	98	720	700	1.23	0.88
	6	[Y(Psalen tBu )(OtBu)] b		1000:1 [c]	120 s	99	144	332	1.31	0.87
	7 Figure 10. Other phosphasalen initiators for ROP. [Y(Psalen OMe )(OtBu)] b	1000:1	20 s	88	165	128.7	1.50	0.87
	8	[Y(PsalenN tBu )(OtBu)] b		500:1:1	56 min	92	41	66.2	1.05	0.26
	9	[Y(PsalenN tBu )(OtBu)] b,d		100:1:1	3 h	77	16	11.1	1.01	0.16
	10	[Y(PsalenN tBu )(OEt)] b		500:1	48 min	85	61.7	61.2	1.04	0.28
	11	[Y(PsalenN tBu )(OEt)] b,d		500:1	3 h	73	13.5	10.5	1.01	0.19
	12	[Y(PsalenN tBu )(OEt)] b,d		500:1	30 min	85	48.6	61.2	1.08	0.22
	13	[Lu(PsalenN tBu )(OEt)] b		500:1	8.25 h	86	53.4	61.9	1.02	0.18
	14	[La(PsalenN tBu )(OtBu)] b		500:1:1	20 s	98	57.3	70.6	1.05	0.72
	15	[In(Psalen PhMe2 )(OEt)] b		500:1	1 h	96	65.3	69.2	1.15	0.13

Acknowledgements

A.A. thank T.-P.-A. Cao, who initiated the chemistry of phosphasalen ligands and persevered despite the initial synthetic difficulties. CNRS and Ecole polytechnique are acknowledged for their financial support as well as the ANR which funded the PsalenOx project regarding the oxidation of phosphasalen complexes (JCC project n°13-JS07-0001-01).


Nataliia Vereshchuk

Roc Matheu

Jordi Benet-Buchholz

Muriel Pipelier

Jacques Lebreton

Didier Dubreuil

Arnaud Tessier
email: arnaud.tessier@univ-nantes.fr

Carolina Gimbert-Suriñach

Mehmed Z Ertem
email: mzertem@bnl.gov

Antoni Llobet
email: allobet@iciq.es

Second Coordination Sphere Effects in an Evolved Ru Complex Based on a Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis

Keywords: Water oxidation catalysis, Ru complexes, water splitting, redox properties transition metal complexes

A new Ru complex containing the deprotoanted 2,2':6',2''-terpyridine,6,6''-diphosphonic acid (H 4 tPa) and pyridine (py) of general formula, [Ru II (H 3 tPa--N 3 O)(py) 2 ] + , 2 + , has been prepared and thoroughly characterized by means of spectroscopic, electrochemical techniques, X-ray diffraction analysis and with DFT calculations. Complex 2 + , presents a dynamic behavior in solution that involves the synchronous coordination and the decoordination of the dangling phosphonic groups of the tPa 4-ligand. However, at oxidation state IV complex 2 + , becomes seven coordinated with the two phosphonic groups now bonded to the metal center. Further at this oxidation state at neutral and basic pH, the Ru complex undergoes coordination of an exogenous OH -group from the solvent that leads to an intramolecular aromatic O-atom insertion into the CH bond of one of the pyridyl groups forming the corresponding pheoxoxo-phosohonate Ru complex [Ru III (tPaO-- 2 , where tPaO 5-is 3-hydroxo-[2,2':6',2''-terpyridine]-6,6''-diyl)bis(phosphonate) ligand. This new in situ generated Ru complex, 4 2-, has been isolated and spectroscopically and electrochemically characterized. In addition, a crystal structure has been also obtained using single crystal X-ray diffraction techniques. Complex 4 2-, turns out to be an exceptional water oxidation catalyst achieving record high TOF max in the of 16,000 s -1 . A mechanistic analysis complemented with DFT calculations has also been carried out showing the critical role of intramolecular second coordination sphere effects exerted by the phosphonate groups in lowering the activation energy at the rate determining step.

Introduction

Molecular water oxidation catalysis is a field that has been rapidly developing over the last decade mainly due to their potential application in new energy conversion schemes based on water splitting with sunlight. [START_REF] Berardi | Molecular Artificial Photosynthesis[END_REF][START_REF] Lewis | Research Opportunities to Advance Solar Energy Utilization[END_REF][START_REF] Alstrum-Acevedo | Chemical Approaches to Artificial Photosynthesis. 2[END_REF][START_REF] Kärkäs | Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation[END_REF][START_REF] Cook | Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds[END_REF][START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] De Luna | What Would It Take for Renewably Powered Electrosynthesis to Displace Petrochemical Processes?[END_REF] In particular, Ru complexes have been leading the field and today the amount of information extracted from these complexes has generated a deep understanding of the different parameters involved in the catalytic cycle. [START_REF] Garrido-Barros | How to Make an Efficient and Robust Molecular Catalyst for Water Oxidation[END_REF][START_REF] Matheu | The Development of Molecular Water Oxidation Catalysts[END_REF] This has been possible thanks to the spectroscopic characterization of the reaction intermediates as well as due to the kinetic characterization of the different steps involved and complemented with a thorough computational analysis. [START_REF] Sala | Molecular Water Oxidation Mechanisms Followed by Transition Metals: State of the Art[END_REF][START_REF] Gimbert-Suriñach | Structural and Spectroscopic Characterization of Reaction Intermediates Involved in a Dinuclear Co-Hbpp Water Oxidation Catalyst[END_REF][START_REF] Romain | Oxygen-Oxygen Bond Formation by the Ru-Hbpp Water Oxidation Catalyst Occurssolely via an Intramolecular Reaction Pathway[END_REF][START_REF] Shaffer | Bond Formation in Ruthenium-Catalyzed Water Oxidation: Single-Site Nucleophilic Attack vs. O-O Radical Coupling[END_REF] This wealth of information based on Ru complexes as water oxidation catalysts (WOCs) has also been extended to other transition metals as well as oxides reported as WOCs for their design and mechanistic proposals. [START_REF] Wang | Direct-Coupling O2 Bond Forming a Pathway in Cobalt Oxide Water Oxidation Catalysts[END_REF][START_REF] Mccrory | Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices[END_REF][START_REF] Hong | Charge-Transfer-Energy-Dependent Oxygen Evolution Reaction Mechanisms for Perovskite Oxides[END_REF][START_REF] Surendranath | Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral PH[END_REF] The best WOCs known today are based on the Flexible Adaptative Multidentate Equatorial (FAME) ligands [START_REF] Matheu | Seven Coordinated Molecular Ruthenium-Water Oxidation Catalysts: A Coordination Chemistry Journey[END_REF] containing polypyridyl carboxylate groups such as H 2 tda [START_REF] Matheu | Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex[END_REF] and H 2 bda [START_REF] Richmond | Supramolecular Water Oxidation with Rubda-Based Catalysts[END_REF][START_REF] Duan | A Molecular Ruthenium Catalyst with Water-Oxidation Activity Comparable to That of Photosystem II[END_REF][START_REF] Song | Base-Enhanced Catalytic Water Oxidation by a Carboxylate-Bipyridine Ru(II) Complex[END_REF] (see Chart I for the drawings of the ligands) and their related derivatives where the carboxylate groups have been partially or totally substituted by phosphonate groups. [START_REF] Xie | Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands[END_REF][START_REF] Kamdar | Ruthenium Complexes of 2,2′-Bipyridine-6,6′-Diphosphonate Ligands for Water Oxidation[END_REF][START_REF] Shaffer | Lability and Basicity of Bipyridine-Carboxylate-Phosphonate Ligand Accelerate Single-Site Water Oxidation by Ruthenium-Based Molecular Catalysts[END_REF][START_REF] Surendranath | Electrolyte-Dependent Electrosynthesis and Activity of Cobalt-Based Water Oxidation Catalysts[END_REF] In particular, the complex [Ru IV (OH)(tda--N 3 O)(py) 2 ] + (py for pyridine), 1 + , has been recently shown to achieve TOF max of 8,000 s -1 at pH 7.0. [START_REF] Matheu | Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex[END_REF] Further, the analysis of its performance shows a delicate balance among multiple equilibria at different oxidation states with diverse coordination number and degree of hydrogen bonding that are responsible for its performance as a catalyst. [START_REF] Matheu | Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex[END_REF][START_REF] Matheu | Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts[END_REF] Because of these delicate equilibria, the replacement of carboxylate groups in H 2 tda by phosphonate groups leading to the H 4 tPa ligand (Chart I), is expected to generate significant differences in its catalytic behavior. The electronic nature and geometry of the phosphonate with regard to those of the carboxylate group as well as an increase in steric will be responsible for the different catalytic behavior of the new Ru complexes based on H 4 tPa. [START_REF] Albrecht-Schmitt | Metal Phosphonate Chemistry[END_REF] In addition, the increased number of acidic protons with substantially different pK a 's are expected to not only facilitate remote proton-coupled electron transfer (PCET) events during redox leveling, [START_REF] Hammes-Schiffer | Catalysts by Design: The Power of Theory[END_REF][START_REF] Meyer | Mechanisms of Molecular Water Oxidation in Solution and on Oxide Surfaces[END_REF][START_REF] Huynh | Proton-Coupled Electron Transfer[END_REF][START_REF] Weinberg | Proton-Coupled Electron Transfer[END_REF][START_REF] Warren | Thermochemistry of Proton-Coupled Electron Transfer Reagents and Its Implications[END_REF][START_REF] Shaffer | Bond Formation in Ruthenium-Catalyzed Water Oxidation: Single-Site Nucleophilic Attack: Vs. O-O Radical Coupling[END_REF][START_REF] Hammes-Schiffer | Proton-Coupled Electron Transfer in Solution, Proteins, and Electrochemistry[END_REF] but also give rise to beneficial second sphere coordination effects such as hydrogen bonding interactions. Further, the phosphonate groups are expected to act as intramolecular proton acceptor, at the O-O bond formation step, significantly reducing its activation energies.

Here on we present a new family of Ru complexes derived from the H 4 tPa ligand together with a thorough characterization of their redox and water oxidation catalytic properties.

Results

Synthesis and structure of the precursor complex 2 +

The synthesis of the H 4 tPa ligand was carried out in three steps using 2,2':6',2''-terpyridine (trpy) as the starting material as described in the supporting information. Initially, the trpy ligand is oxidized to 2,2':6',2''-terpyridine-1,1''-dioxide using metachloroperbenzoic acid. The treatment of the latter with POEt 3 forms tetraethyl 2,2':6',2''-terpyridine-6,6''-diphosphonate that is hydrolyzed with trimethylbromosilane yielding the H 4 tPa ligand. The synthetic strategy for the preparation of the complexes described in this work is outlined in Figure 1. Complex [Ru II (H 3 tPa--N 3 O)(py) 2 ] + , 2 + , is prepared by refluxing stoichiometric amounts of H 4 tPa and [Ru(dmso) 4 Cl 2 ] in n-BuOH, followed by addition of excess of pyridine and further refluxing overnight. Proper column chromatography in silica generates pure 2Cl in moderate yields where the equatorial H 3 tPa  ligand acts in a monoanionic fashion. Crystals of 2PF 6 •3H 2 O sufficiently good for X-ray analysis were obtained by counteranion exchange crystallization from an acidic aqueous solution of 2Cl and KPF 6 . The ORTEP view of its solvated cationic moiety is presented in Figure 1. The Ru(II) center features an octahedral type of geometry where the H 3 tPa  ligand coordinates in the equatorial plane in a -N 3 O fashion leaving a dangling non-coordinated phosphonic acid group. The octahedral geometry is completed with two pyridyl groups in the axial position. The bonding parameters are unremarkable [START_REF] Laurent | Structure and Redox Properties of Ruthenium Complexes Containing the Tpm Facial and the Trpy Meridional Tridentate Ligands: Crystal Structures of[END_REF] except for a distortion due to the geometrical constraints of the HtPa 3 ligand at the equatorial zone, that produces a N-Ru-O angle of 115 o ; 25 o larger than the ideal O h geometry. The same angle for the [Ru II (tda)(py) 2 ] is 125 o , 19 that is 10 o higher which is a consequence of the different bonding parameters associated with the phosphonate and carboxylate moieties. [START_REF] Albrecht-Schmitt | Metal Phosphonate Chemistry[END_REF][START_REF] Kuppuraj | Factors Governing Metal-Ligand Distances and Coordination Geometries of Metal Complexes[END_REF] Finally, there are three H 2 O molecules in the unit cell, interacting with the phosphonic groups by hydrogen bonding as can be observed in Figure 1. Density functional theory (DFT) calculations at the M06 level of theory [START_REF] Zhao | The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Function[END_REF] in conjunction with SMD aqueous continuum solvation model [START_REF] Marenich | Supporting Information ( PART I ) Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions Contents[END_REF] (see computational methods for further details) have been carried out in order to complement the experimental work described here for 2 + . The computed structure for 2 + , shows a very good agreement with the experimental structure in terms of bonding parameters (Figure S62) revealing its good degree of reliability.

Addition of two equivalents of Ce(IV) generates the corresponding seven coordinated Ru(IV) complex [Ru IV (tPa--N 3 O 2 )(py) 2 ], 3, where the tPa 4 ligand now acts in a pentadentate manner and the metal center possesses a pentagonal bipyramidal geometry as can be observed in the computed structure shown in Figure 1. This complex is diamagnetic at room temperature (RT) with a high field low spin d 4 electronic configuration, (e 1 " ) 4 (e 2 ' ) 0 (a 1 ' ) 0 , analogous to related Ru(IV) complexes. [START_REF] Matheu | Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex[END_REF][START_REF] Hoque | Catalytic Oxidation of Water to Dioxygen by Mononuclear Ru Complexes Bearing a 2,6-Pyridinedicarboxylato Ligand[END_REF][START_REF] Matheu | The Role of Seven-Coordination in Ru-Catalyzed Water Oxidation[END_REF] .

Its 1 H, [START_REF] Shaffer | Bond Formation in Ruthenium-Catalyzed Water Oxidation: Single-Site Nucleophilic Attack vs. O-O Radical Coupling[END_REF] C and 31 P NMR are presented in the SI see Figures S27-29.

Spectroscopy and dynamic behavior of 2

The spectroscopic properties were investigated by means of UV-vis and NMR spectroscopy. The UVvis spectra of [Ru II (H 2 tPa--N 3 O)(py) 2 ], 2, and its one and two electron oxidized species [Ru III (H 2 tPa--N 3 O)(py) 2 ] + and [Ru IV (HtPa--N 3 O 2 )(py) 2 ] + were generated via redox titration using Ce(IV) as the oxidant at pH 1.0 in aqueous triflic acid (Figures S45-S46). Complex 2 is a diamagnetic low spin d [START_REF] Walter | Solar Water Splitting Cells[END_REF] ion with a typical (t 2g ) [START_REF] Walter | Solar Water Splitting Cells[END_REF] (e g ) 0 electronic configuration and its NMR spectra and assignment are presented in the Figure 2 and the supporting information (Figures S12-S26 andFigures S35-S36). At room temperature, the NMR spectrum displays eight resonances as if the complex had C 2v symmetry. However, at low temperature the resonances of the protons H a , H b , H c and H e split indicating the presence of a dynamic behavior (Figure 2). The dynamic behavior is associated with the synchronic coordination and decoordination of the dangling phosphonate that has an activation energy of 13.3 kcal/mol based on Eyring plots generated from VT-NMR (Scheme S2). By computational methods, an estimate activation energy of 7.9 kcal/mol (see Figure S37) was obtained for this dynamic behavior in line with the experimental observations. The value of the activation energy is similar to that measured for Ru-bda complexes involving coordination and decoordination of the dangling carboxylates. [START_REF] Matheu | Behavior of Ru-Bda Water-Oxidation Catalysts in Low Oxidation States[END_REF]

Redox properties of 2

The redox properties of 2 were investigated by electrochemical techniques in a three-electrode cell using glassy carbon as a working electrode and all potentials were measured using a Hg/Hg 2 SO 4 as a reference electrode and converted to vs NHE by adding 0.65 V to the measured potential. Cyclic Voltammetry of 2 were performed in aqueous solution as a function of pH that produces a different degree of protonation at the phosphonate groups. A cyclic voltammetry at pH 7.0 is shown in The four protons potentially available from the two phosphonic groups in 2, allow to obtain a rich family of complexes depending on the degree of oxidation and protonation as shown in the Pourbaix diagram. The pK a 's coincide with the dashed vertical lines and are deduced from the change of slope in the E o vs. pH plot. The first and second pK a 's due to the monodeprotonation of each phosphonic group are expected to occur below pH 1.0. [START_REF] Brauman | Equilibrium Acidities in Dimethyl Sulfoxide[END_REF][START_REF] Nazeeruddin | Determination of PKa Values of 4-Phosphonato-2,2':6',2' '-Terpyridine and Its Ruthenium(II)-Based Photosensitizer by NMR, Potentiometric, and Spectrophotometric Methods[END_REF][START_REF] Ordóñez | Stereoselective Synthesis of α-Amino-H -Phosphinic Acids and Derivatives[END_REF] Further, the pK a for the third and fourth proton loss are labeled, pK a and pK a' , and are defined in equations 1 and 2 respectively,

[Ru II (H 2 tPa--N 3 O)(py) 2 ] (aq) -> [Ru II (HtPa--N 3 O)(py) 2 ]   aq  + H + (aq) pK a (1) [Ru II (HtPa--N 3 O)(py) 2 ]  (aq) -> [Ru II (tPa--N 3 O)(py) 2 ] 2 (aq) + H + (aq) pK a' (2)
The latter have been independently calculated via spectrophotometric acid-base titration as shown in Figure S47. It is interesting to see how due to the different proton content of the species involved at a given pH, the difference between the IV/III-III/II redox couples changes considerably as a function of pH as can be graphically observed in Figure S49A. Indeed, at pH 7.0 this difference is only 100 mV but it increases up to 330 mV at pH 13.0 (Figure S49B).

Generation of the active Ru species via O-atom insertion

The synthesis of the active catalyst [Ru III (tPaO--N 2 O P O C )(py) 2 ] 2 , 4 2 , (see a drawing of the H 5 tPaO ligand in Chart 1; O P refers to oxygen atom coordinated to the Ru center via the oxygen atom from the phosphonic group and O C from the pyridyl group) was carried out electrochemically using

[Ru II (H 3 tPa--N 3 O)(py) 2 ] + , 2 + , ( 2
+
is predicted to convert to 2 at pH 7.7) as a precursor as outlined in Figure 1.

A sample of 2 + is dissolved in a pH 7.7 aqueous phosphate buffer solution and placed in a twocompartment electrochemical cell using a Pt mesh as a working electrode. Then a potential of 1.30

V is applied for 110 minutes. During this process, the initial 2 + precursor complex is quantitatively transformed into the final complex 4 2 , where an O atom insertion between the Ru center and one of the pyridyl groups takes place. One electron reduction of 4 2 yields the diamagnetic 4 3 complex that was characterized by 1 H, [START_REF] Shaffer | Bond Formation in Ruthenium-Catalyzed Water Oxidation: Single-Site Nucleophilic Attack vs. O-O Radical Coupling[END_REF] C and 31 P NMR spectroscopy and whose spectra are shown in the (Figure S39-S44). Electrochemically only the initial and final species are detected (Figure S50).

Single crystals of 4 3-where obtained by adding excess Cs+ to a solution fo 4 2-in MeOH. A crystal structure of 4 3 is displayed in Figure 1 showing the phenoxide bonding and the decoordination of the initial pyridyl-phosphonic group. The complex was further characterized by UV-vis spectroscopy using an Optically Transparent Thin Layer Electrochemical (OTTLE) cell (Figure S48).

In order to get further insight into the potential sequence of reactions that generate 4 2 from 2 + a computational analysis was carried out and the results obtained are summarized in Scheme 1. As the current flows at an applied potential of 1.30 V, the initial Ru(II) complex 2 + is initially converted to seven coordinate (CN7) Ru(IV) species, [Ru IV (tPa--N 3 O 2 )(py) 2 ], 3, in agreement with its redox properties described in the previous section. This complex then reacts with a water molecule with concomitant proton release inducing the decoordination of one of the pyridyl phosphonate groups

Water oxidation catalysis and mechanisms

A kinetic characterization of the electrocatalytic water oxidation reaction was carried out for 4 2 , based on Foot of the Wave Analysis (FOWA) [START_REF] Costentin | Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis[END_REF][START_REF] Rountree | Evaluation of Homogeneous Electrocatalysts by Cyclic Voltammetry[END_REF][START_REF] Matheu | Foot of the Wave Analysis for Mechanistic Elucidation and Benchmarking Applications in Molecular Water Oxidation Catalysis[END_REF] at pH 7.2 as shown in Figure S54. A TOF max = 1.6 10 4 (± 0.2•10 4 ) s -1 is obtained that is found to be independent of catalyst concentration between 0.5 and 3.0 mM. This first order kinetics behavior with regard to catalyst concentration points towards a water nucleophilic attack (WNA) [START_REF] Wasylenko | Electronic Modification of the [RuII(Tpy)(Bpy)(OH2)]2+ Scaffold: Effects on Catalytic Water Oxidation[END_REF] type of mechanism where the O-O bond formation between the Ru(V)=O species and the solvent is the rate determining step (rds). This is further supported by theoretical calculations as will be shown in the next section. The performance of 4 2 as WOC was further tested by a bulk electrolysis experiments described in detail in the supporting information (Figure S53). More than 42 million turnovers with Faradaic efficiencies above 93%. were obtained using Saveant's methodology that takes into account the concentration of the catalyst close to the electrode surface and manifest the remarkable stability of this catalyst. [START_REF] Duan | A Molecular Ruthenium Catalyst with Water-Oxidation Activity Comparable to That of Photosystem II[END_REF][START_REF] Song | Base-Enhanced Catalytic Water Oxidation by a Carboxylate-Bipyridine Ru(II) Complex[END_REF][START_REF] Xie | Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands[END_REF] It is also interesting to point out here that catalyst 4 2 is also active at lower pH albeit with lower efficiency (Figure S51).

A thorough mechanistic analysis was also carried out based on theoretical calculations and is summarized in Figure 4 top, where the different reaction intermediates and the computed Gs and redox potentials associated with each step are presented (see computational methods in the SI for further details). We propose that once the Ru(III) complex, 4 2 , is generated, water coordination together with a two electron oxidation process proceeds to form Ru V =O species, [Ru V (O)(HtPaO--N 3 O P )(py) 2 ]  , which will in turn react with a solvent H 2 O to generate the corresponding hydroxoperoxido derivative, as indicated in the equation below,

[Ru V =O] (aq) + H 2 O (l) -> [Ru-O---OH 2 ] ‡ -> [Ru III -OOH] (aq) + H + (aq) (3)
The optimized TS for WNA reaction is presented in Figure 4 bottom. It is interesting to realize here that the coordination number (CN) for the Ru(V) species is 6 with the tPaO 5-ligand acting in a -N 2 O C fashion and thus with a dangling phosphonate group. This dangling group is strategically situated so that it can act as an intramolecular proton acceptor to facilitate the WNA TS and to reduce the activation free energy (G ‡ = 22.1 kcal/mol). Once Ru(III)-OOH is formed then it suffers a PCET that forms the Ru(IV)-OO that spontaneously evolves dioxygen and generates the initial Ru(II) species completing the catalytic cycle.

Discussion

Detailed understanding of water oxidation catalysis is challenging due to the large number of reactions and intermediates involved in this complex process. The electronic demands imposed by the reaction intermediates accessed at each oxidation state at the different stages of the catalytic cycle involve a change in metal coordination number. Along this line, high oxidation states will prefer hard base ligands while lower oxidation states will prefer softer ones. Additional complexity arises because the variety of reactions handled by the catalyst including outer sphere electron transfers (OSET), PCET and chemical reactions that might not necessarily involve ET such as proton transfer or O-O bond formation. Moreover, non-desired pathways that derail these reactions from the catalytic cycle towards non-productive or decomposition products also need to be understood, [START_REF] Wang | Reinvestigation of Water Oxidation Catalyzed by a Dinuclear Cobalt Polypyridine Complex: Identification of CoOxas a Real Heterogeneous Catalyst[END_REF] to be able to avoid them and to obtain robust water oxidation catalysts. To control all the parameters indicated above, each step should occur with an activation barrier as low as possible so that efficient water oxidation catalysis can take place.

Geometrical and electronic consequences due to the presence of the phosphonate groups

Complex [Ru IV (O)(tda)(py) 2 ], 1, is an example of a catalyst that complies with the above requirements and up to now appears as the fastest water oxidation catalyst ever reported. [START_REF] Matheu | Seven Coordinated Molecular Ruthenium-Water Oxidation Catalysts: A Coordination Chemistry Journey[END_REF] The structural consequences of changing carboxylate by phosphonates can be graphically observed by comparing the structures of the diamagnetic seven coordinated catalyst precursors [Ru IV (tda-- S61 andS63). While the X-ray structure of

N 3 O 2 )(py) 2 ] 2+ and [Ru IV (tPa--N 3 O 2 )(py) 2 ] (Figures
[Ru IV (tda--N 3 O 2 )(py) 2 ] 2+
shows all the coordinating atoms of the tda 2-ligand nearly on the equatorial plane, the coordinating oxygen atoms of the phosphonate in [Ru IV (tPa--N 3 O 2 )(py) 2 ] complex are significantly off the equatorial plane as observed in the computed structure shown in Figure 1.

Furthermore, this distortion also breaks the planarity of the trpy moiety of the tPa 4-ligand diminishing the aromatic  delocalization. This distortion is due mainly to the different geometric parameters of the phosphonic acid group (pseudo T d ) as compared to the carboxylate (pseudo C 2v ), and results in weaker Ru-O bonds for the phosphonic complex. From an electronic perspective the phosphonate group acts as a stronger sigma donor than the carboxylate, leading in general to lower redox potentials and higher pKa (pK a 's are 3 and 11 for [Ru II (H 2 tPa)(py) 2 ] and 6.4 for [Ru III (HtPa)(py) 2 ]). [START_REF] Matheu | Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts[END_REF] The existence of a range of pK a values obtained at different oxidation states from the auxiliary ligands is interesting because it can promote remote PCET, that is a PCET where the proton is not associated with the Ru-OH 2 or Ru-OH group, but from auxiliary ligands such as the phosphonic ones. This is important because for typical polypyridyl Ru-aqua complexes such as [Ru(trpy)(bpy)(H 2 O)] 2+ , [START_REF] Moyer | Chemically Catalyzed Net Electrochemical Oxidation of Alcohols, Aldehydes, and Unsaturated Hydrocarbons Using the System (Trpy)(Bpy)Ru(OH2)2+/(Trpy)(Bpy)RuO2+[END_REF] the complex can access Ru IV =O species by two consecutive PCET processes from its aqua derivatives but access to reactive Ru(V) occurs as an ET only process and therefore requires high energy input and overpotential (1.8 V for [Ru(trpy)(bpy)(H 2 O)] 2+ ). [START_REF] Wasylenko | Supporting Information: Electronic Modification of the [Ru II (Tpy)(Bpy)(OH 2 )] 2+ Scaffold: Effects on Catalytic Water Oxidation[END_REF][START_REF] López | Oxo-Bridge Scenario behind Single-Site Water-Oxidation Catalysts[END_REF][START_REF] Takeuchi | Redox and Spectral Properties of Monooxo Polypyridyl Complexes of Ruthenium and Osmium in Aqueous Media[END_REF] Thus, the possibility to reach Ru(V) via three PCET processes can open up the access to a highly active Ru V =O species at much lower overpotentials.

The importance of the second coordination sphere effects

The two electron oxidation of the octahedral [

Ru II (H 2 tPa--N 3 O)(py) 2 ], 2, complex generates the CN7 diamagnetic [Ru IV (tPa--N 3 O 2 )(py) 2 ].
Here the importance of ligand flexibility and design is manifested by leaving a non-coordinated dangling phosphonic acid group at oxidation state II where Redox potentials (E) in units of volts (V) vs NHE, Gs and G ‡ in units of kcal/mol. Axial pyridyl ligands are omitted for clarity.

Figure 3 (

 3 Figure 3 (red trace) where two chemically reversible and electrochemically quasireversible redox processes can be observed. The first redox process at E 1/2 = 0.83 V is assigned to a PCET process to generate [Ru III (tPa--N 3 O 2 )(py) 2 ]  whereas the second wave at E 1/2 = 0.92 V is due to a one electron oxidation process to form [Ru IV (tPa--N 3 O 2 )(py) 2 ]. The one electron nature of the two processes is

  to generate[Ru IV (O)(HtPa--N 2 O)(py) 2 ] -. The latter then undergoes a PCET step to form[Ru V (O)(tPa--N 2 O)(py) 2 ]. This species is responsible for the intramolecular hydroxylation of the dangling pyridyl group that is highly favored from a thermodynamic perspective (G = 9.0 kcal/mol) and that finally leads to pure 4 2 following a deprotonation event (G = 54.6 kcal/mol). A transition state for the electrophilic oxygen insertion into the CH bond has also been located with a low free energy of activation (G ‡ ) of 14 kcal/mol. The redox and electrocatalytic properties of 42 were investigated by CV and Coulombimetric techniques. Figure3(black trace, left) shows the CV of 4 2 at pH 7.0 phosphate buffer solution where an electrochemically and chemically reversible wave appears at E 1/2 = 0.35 V that is assigned to the Ru(III)/Ru(II) couple. At higher anodic potentials a very large electrocatalytic current is observed at approximately 1.4 V that is due to the oxidation of water to dioxygen. This second wave is associated with a two electron process and oxido coordination, reaching the Ru(V)=O species that is responsible for the reaction with solvent water and O-O bond formation. A two electron process is proposed since no other waves are observed besides the III/II wave and the corresponding Ru(IV)=O species are not sufficiently reactive for fast water oxidation and further supported by DFT.

the

  Ru complex needs to be CN6. The ligand flexibility also allows for a dynamic behavior where the phosphonic groups synchronically coordinate and decoordinate fast at room temperature. Upon reaching Ru(IV) the dangling group immediately coordinates thanks to the flexibility of the ligands achieving CN7. At oxidation state IV in basic solution, [Ru IV (tPa--N 3 O 2 )(py) 2 ] undergoes OH  substitution to form either [Ru IV (OH)(tPa--N 2 O)(py) 2 ]  or [Ru IV (O)(HtPa--N 2 O)(py) 2 ]  . Further oxidation leads to the formation of highly reactive [Ru V (O)(tPa--N 2 O)(py) 2 ] -species. The generation of the Ru IV -OH (or Ru IV =O) occurs simultaneously with the breaking of both the Ru-N and Ru-O bonds of one of the pyridyl-phosphonato arms of the tPa 4 ligand (Scheme 1). This is due to the steric congestion generated by the phosphonate group that precludes the formation of the CN7 complexes [Ru IV (O)(HtPa--N 3 O)(py) 2 ]  and [Ru V (O)(tPa--N 3 O)(py) 2 ] -. Once the highly reactive Ru V =O species is generated then the dangling pyridyl-phosphonato arm has the perfect geometry to undergo intramolecular oxygen atom transfer to the non-coordinated pyridyl ring. This intramolecular O-atom insertion into the CH bond generates the catalytically active complex precursor [Ru III (tPaO--N 2 O P O C )(py) 2 ] 2 , 4 2 . Two electron oxidation of 4 2 forms the Ru V =O species that performs O-O bond formation via WNA and generates the Ru III -OOH intermediate. This step is the rate determining step (rds) of the catalytic cycle and is favored by the intramolecular proton transfer from the incoming water molecule to the phosphonate group that again has the right geometry to promote this step reducing the activation free energy. This renders 4 2 as an extremely powerful catalyst giving a TOF max of 16,000 s -1 with an overpotential of 530 mV at pH 7.0. It gives a higher TOF max (8.000 s -1 ) than 1 at lower overpotential at (75 mV.) In summary, the dangling phosphonate group is responsible for low energy pathway of the two key reactions: (i) The formation of active catalyst precursor [Ru III (tPaO--N 2 O P O C )(py) 2 ] 2 , 4 2 , via oxygen insertion and (ii) the intramolecular proton transfer from the incoming water molecule for the O-O bond formation step that is the rds in the WNA mechanism. These two key reactions occur intramolecularly thanks to the right positioning of the Ru V =O groups versus the dangling phosphonato group and thus are entropically highly favored.The present work highlights the importance of designing catalysts with the right second coordination sphere environment in the field of redox catalysis and in particular in the catalytic oxidation of water to dioxygen and the fate of the catalysts during turnover.

Figure 1 . 3 .

 13 Figure 1. Synthetic scheme and structures of the complexes described in this work. X-ray structure ORTEP views for {[Ru II (H 3 tPa--N 3 O)(py) 2 ](H 2 O) 3 } + , 2 + •3H 2 O, showing the 3 hydrogen bonded water molecules and for [Ru III (tPaO--N 2 O P O C )(py) 2 ] 2 , 4 2 . Calculated structure for [Ru IV (tPa--N 3 O 2 )(py) 2 ], 3. Color code: Ru, cyan; P, pink; N, blue; O, red; C, black; H, white.

9 [ 2 -Figure 2 .

 922 Figure 2. VT 1 H-NMR of [Ru II (H 2 tPa--N 3 O)(py) 2 ], 2 in CD 3 OD together with labeling.

Figure 3 .

 3 Figure 3. Left, cyclic voltammetry of 0.6 mM [Ru II (HtPa--N 3 O)(py) 2 ]  , 2  , at pH 7.0 in a 0.1 M phosphate buffer aqueous solution (red trace) and at pH = 1.0 in 0.1 M triflic acid (purple trace). Solid line black trace, cyclic voltammetry of 0.6 mM [Ru III (tPaO--N 2 O P O C )(py) 2 ] 2 , 4 2 , at pH 7.0 in a 0.1 M phosphate buffer aqueous solution, generated from the precursor 2  , by a 3h bulk electrolysis at 1.30 V. Right, Pourbaix diagram for 2. The dominant species as a function of pH and potential are indicated with the labels such as [Ru II (H 2 tPa)] where the oxidation state is indicated as well as the degree of protonation of the H 4 tPa ligand. The dashed vertical lines indicate the pK a of the species involved. The axial pyridyl ligands are omitted for clarity.

Figure 4 .

 4 Figure 4. Top, calculated catalytic cycle for catalyst [Ru V (O)(HtPaO--N 2 O C )(py) 2 ]  at pH 7.0 Redox potentials (E) (experimental in green and calculated in blue) in units of volts (V) vs NHE, Gs and G ‡ in units of kcal/mol. Axial pyridyl ligands are omitted for clarity. Bottom, optimized TS for the O-O bond formation step. Color code: Ru, cyan; P, orange; N, blue; O, red; C, black; H, white.

  SC0012704 with the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and utilized computational resources at the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, and the Scientific Data and Computing Center, a component of the Computational Science Initiative, at BNL under Contract No. DE-SC0012704. Drawing and labeling of ligands discussed in this work. Computed reaction pathway at pH 7.0 for the generation of the catalytically active species [Ru III (tPaO--N 2 O P O C )(py) 2 ] 2 , 4 2 , from the precursor complex [Ru II (H 2 tPa--N 3 O)(py) 2 ], 2.

	Scheme 1.
	4. Supporting Information. Additional experimental details, electrochemical and spectroscopic data. CIF files for complexes {[Ru II (H 3 tPa--N 3 O)(py) 2 ](H 2 O) 3 }(PF 6 ) and {[Ru II (tPaO--N 2 O P O C )(py) 2 ] (H 2 O) 8.5 (CH 3 OH)}Cs 3 , with CCDC numbers 1890782 and 1954776 respectively are available at https://www.ccdc.cam.ac.uk/. 5. Acknowledgements Support from MINECO, FEDER, "La Caixa" and AGAUR are gratefully acknowledged (CTQ2016-80058-R, CTQ2015-73028-EXP, SEV 2013-0319, ENE2016-82025-REDT, CTQ2016-81923-REDC, and 2017-SGR-1631). The work at Brookhaven National Laboratory (BNL; M.Z.E) was carried out under N N OH O O HO N N O OH OH O H 2 tda H 2 bda N N N P HO OH O P OH HO O N N P HO OH O N HO P HO OH O contract DE-Chart 1. N H 4 tPa H 5 tPaO

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