Synthesis, structure, spectroscopy and reactivity of new heterotrinuclear water oxidation catalysts

Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp–) and 2,2′:6′,2′′-terpyridine (trpy) of the general formula {[RuII(trpy)]2(μ-[M(X)2(bpp)2])}(PF6)2 have been prepared for the first time.


Introduction
Water oxidation catalysis is a eld that has expanded enormously over the last few years. [1][2][3][4][5][6][7][8][9][10] This development has been fuelled by interest in the topic for the achievement of new energy conversion schemes based on water and sunlight to obtain so-called solar fuel (such as H 2 ), mimicking natural photosynthesis. Water oxidation is the main reaction that occurs at the OEC-PSII, 11 yielding molecular oxygen, four protons and four electrons, which can later react further. The development of new water oxidation catalysts (WOCs) has been based mainly on mononuclear 12 and dinuclear 13,14 Ru complexes containing polypyridilic ligands. However, recently, a number of Ir and rst row transition metal complexes have also been reported to be able to oxidize water. 3,15,16 While mononuclear complexes are in general easier to prepare, targeted polynuclear complexes can present signicant synthetic challenges. Nevertheless, polynuclear complexes containing bridging ligands that can electronically couple the metal centers can attain important benets from the perspective of a WOC. For example, multiple electronically coupled redox active metal centers can cooperate during the four electron transfers needed for the water oxidation reaction. On the other hand, non-redox active centers can be of interest to exert electronic perturbation over the redox active center. 17,18 They can also provide aqua/hydroxo ligands which, when strategically situated, can help to lower activation energies by hydrogen bonding with key active species bonded to the active metal, or even participate in O-O bond formation, as has been proposed for the Ca-OH 2 moiety of the OEC-PSII on a number of occasions. 19,20 With these considerations in mind, we undertook the preparation of heterotrinuclear complexes where all of the metal centers are redox active and possess aqua ligands, to favor the achievement of high oxidation states via Proton Coupled Electron Transfer (PCET). Herein, we report the synthesis, structural, spectroscopic and electrochemical characterization of a new family of heterotrinuclear complexes containing Ru and Co or Mn as metal centers, together with their capacity to oxidize water to dioxygen.

Results and discussion
Synthesis and solid state structure The well-known Hbpp ligand is an asymmetric molecule which, once deprotonated, presents two equivalent coordination environments. The protonated ligand thus allows the preparation of mononuclear complexes that can be used as starting materials for the preparation of homo-21 or heterodinuclear, or heterotrinuclear 22 complexes. In the present report we use out-[Ru(Cl)(Hbpp)(trpy)] + , out-0, as the starting material for the preparation of heterotrinuclear complexes containing redox active metals, as displayed in Scheme 1.
The pyrazolato proton of out-0 is removed using NaOMe as a base, and then MnCl 2 or CoCl 2 salts are used to react with the vacant coordination sites of the bpp À ligand, as shown in eqn (1) for the case of cobalt.
Complex Ru 2 Co-Cl 2 is obtained in 58% yield, whereas a 70% yield is obtained for Ru 2 Mn-Cl 2 following a similar synthetic procedure. 22 Treatment of these trinuclear complexes with sodium acetate at 75 C in an acetone : water (5 : 1) solution replaces the chlorido bridging ligands by the more labile acetato ligands, as indicated in the following equation. È Finally, substitution of the acetato bridges by monodentate aqua ligands leads to the formation of the corresponding tetraaqua complex in neutral pH (eqn (3)). È In contrast, under acidic conditions, the trinuclear complexes decompose to the corresponding mononuclear in-[Ru(Hbpp)(trpy)(H 2 O)] 2+ , in-1, and free Co(II) (Fig. S27 †).
The synthetic strategies used for the preparation of Ru 2 Mn-OAc 2 and Ru 2 Mn-(H 2 O) 4 are analogous to those for the cobalt counterparts.
All of these new complexes have been characterized by analytic, spectroscopic or electrochemical techniques. Further, X-ray diffraction analysis has been carried out for the Mn complexes, Ru 2 Mn-Cl 2 and Ru 2 Mn-OAc 2 , and ORTEP views of their cationic moiety are presented in Fig. 1.
All the metal centers present pseudo-octahedral symmetry around the rst coordination sphere. In the Ru 2 Mn-Cl 2 case, the ruthenium atoms are coordinated by ve N-atoms, three from a meridional trpy ligand and two from bpp À , while the sixth coordination position is occupied by a chlorido ligand. The manganese center is coordinated by the same two bridging chlorido moieties, in a cis fashion, and by the two chelating bpp À ligands. The "Mn(bpp) 2 " moiety can thus be considered as a bridge between the two Ru centers (see Fig. 1 and Scheme 1). A very similar structure is obtained for Ru 2 Mn-OAc 2 where the bridging chlorido ligands have been substituted by bridging acetato ligands. For both structures, the bonding distances and angles presented by the Ru(II) and Mn(II) centers are unremarkable. 23,24 Redox properties and UV-vis spectroscopy in organic solvents for Ru 2 M-OAc 2 The cyclic voltammogram of Ru 2 Co-OAc 2 in CH 3 CN (Fig. 2) displays three successive reversible oxidation waves at E 1/2 ¼ 0.70 (DE ¼ 75 mV), 1.08 (DE ¼ 60 mV), and 1.22 V (DE ¼ 60 mV), and one reversible reduction wave at E 1/2 ¼ À1.23 V (DE ¼ 80 mV). All the redox potentials in this work are reported vs. NHE. Each of the three oxidation processes corresponds to the Scheme 1 Synthetic strategy followed for the preparation of trinuclear complexes and their labelling. exchange of one electron per molecule of complex, as evidenced by rotating disk electrode experiments (Fig. S19 †). The rst process is assigned to the oxidation of the cobalt Co(III)/Co(II) and the last two to the oxidation of the two Ru sites, Ru 2 (II,III)/ Ru 2 (II,II) and Ru 2 (III,III)/Ru 2 (II,III). The presence of two distinct one-electron redox processes, close in potential (DE 1/2 ¼ 140 mV), instead of a two-electron single wave, is in agreement with there being two identical electroactive centers in the molecule that can electronically communicate. 24 This is the case of the two Ru sites interacting through the conjugation of the bridging bpp À ligands and the acetato bridge via the central Co 3+ core. The two electron reduction wave at À1.23 V is assigned to the reduction of the terpyridine units of the Ru(II) centers. The shoulder observed at À1.18 V indicates that the two terpyridine ligands are not fully equivalent, which is in accordance with there being weak electronic coupling between the two Ru subunits.
The two oxidized forms of the complex, Ru 2 (II)Co(III) and Ru 2 (III)Co(III) are stable in CH 3 CN, as tested by successive electrolysis at E ¼ 0.85 V and E ¼ 1.40 V, which consume, respectively, one and two electrons (Fig. S18 †). Rotating disk electrode experiments conrm the quantitative formation of these species (Fig. S19 †). The electrogenerated solutions have also been analyzed by UV-visible (Fig. 2) and EPR spectroscopy (Fig. 4, see below).
The three stable oxidation states, Ru 2 (II)Co(II), Ru 2 (II)Co(III) and Ru 2 (III)Co(III), have distinct UV-visible signatures. The initial orange Ru 2 (II)Co(II) solution exhibits two intense visible bands at 366 and 500 nm (with a shoulder at 530 nm) and two less intense shoulders at 620 and 710 nm. The oxidation of the central Co(II) unit into Co(III) leads to a shi of the intense visible bands to 396 and 473 nm (shoulder at 500 nm) and of the two shoulders to 580 and 655 nm. A more pronounced color change of the solution is observed when the two Ru(II) species are oxidized into Ru(III), as indicated by the replacement of all the previous visible bands by new ones at 390 (shoulder) and 545 nm. An increase in absorption is also observed between 600 and 1000 nm. As the more evident changes occur aer the oxidation from Ru 2 (II)Co(III) to Ru 2 (III)Co(III), the intense visible absorption bands originate from the ruthenium units.
Back electrolysis of the nal solution conducted at 0.35 V (three electrons exchanged) restores the initial complex, Ru 2 (II) Co(II), quantitatively ( Fig. S20 †). This demonstrates the perfect stability of the different oxidation states of the trinuclear compound, and the reversibility of the processes.
The cyclic voltammogram of Ru 2 Mn-OAc 2 in CH 3 CN ( Fig. 3 and S21 †) also displays three successive one-electron reversible oxidation waves at E 1/2 ¼ 0.85 V (DE ¼ 60 mV), 0.96 V (DE ¼ 70 mV) and 1.47 V (DE ¼ 100 mV) at a scan rate of 50 mV s À1 and a reversible two-electron terpyridine-centered reduction wave at The two rst oxidation waves, very close in potential (DE 1/2 ¼ 110 mV), are thus assigned to   the oxidation of the Ru sites, Ru 2 (II,III)/Ru 2 (II,II) and Ru 2 (III,III)/ Ru 2 (II,III) and the last one to the Mn central unit, Mn(III)/Mn(II).
The stability of the two oxidized states, Ru 2 (III)Mn(II) and Ru 2 (III)Mn(III), has been evaluated by two successive electrolyses at E ¼ 1.11 V and 1.69 V. The rst electrolysis at 1.11 V consumed two electrons per molecule of initial complex and lead to the quantitative formation of the Ru 2 (III)Mn(II) species. An additional one electron oxidation carried out at 1.69 V leads to the formation of the fully oxidized form, Ru 2 (III)Mn(III). At this stage, the presence of an additional reversible process at E 1/2 ¼ 1.23 V with very small intensity should be pointed out, which is probably related to minor decomposition of the complex by decoordination of the Mn ion (less than 5%, Fig. S21 †). Indeed, the potential of this new process is similar to that of a RuN 6 mononuclear complex such as in-[Ru(Hbpp)(trpy)(CH 3 CN)] 2+ . Both electrolysis processes have been monitored by UV-vis and X-band EPR spectroscopy ( Fig. 3 and 4, respectively).
The UV-vis absorption spectrum of Ru 2 Mn-OAc 2 , with two intense visible bands at 365 and 497 nm (with a shoulder at 530 nm) and two less intense shoulders at 620 and 710 nm, is nearly identical to that of Ru 2 Co-OAc 2 (see Fig. S9 † for a comparison of the spectra). These observations conrm that the visible absorption bands originate mainly from the Ru units. Once the two Ru(II) units have been oxidized, formation of the Ru 2 (III) Mn(II) species leads to signicant changes with the replacement of the initial visible bands by new ones at 400 (shoulder) and 553 nm and two shoulders at 710 and 764 nm. For the fully oxidized solution, the oxidation of the central Mn(II) unit into Mn(III) leads to minor changes, with a shi of the band at 553 nm to 538 nm and a small increase of the absorption around 450 nm (Fig. 3).
The initial reduced state of the complex Ru 2 Mn-OAc 2 is restored almost quantitatively by a back electrolysis of the nal solution at 0.35 V (Fig. S22 †).

EPR properties
The X-band EPR spectra of the initial and electrochemically oxidized solutions of Ru 2 Co-OAc 2 have been recorded at low temperature (13 K) (Fig. 4). The initial solution of Ru 2 Co-OAc 2 shows an EPR signal characteristic of a Co(II) ion (d 7 ) in the high spin state (S ¼ 3/2), 25 arising from the central Co(II) unit, as both Ru units (Ru(II), d 6 ) are diamagnetic and thus EPR silent. The analysis of high spin Co(II) centers is difficult because the zero eld splitting (ZFS) energy is usually greater than the Zeeman interaction, leading to spectra insensitive to the magnitude of the axial term (D) of the ZFS. Consequently, only the real g-values (g real ) and the rhombicity (E/D ratio) of the system can be extracted from the simulation of the EPR spectra. Simulation of the experimental data provides the following spin-Hamiltonian parameters for the M s ¼ |AE1/2i: g real (x, y) ¼ 2.42, g real (z) ¼ 2.31, and E/D ¼ 0.23.
Aer the one-electron oxidation into Ru 2 (II)Co(III), the solution becomes EPR silent, in accordance with the formation of the low-spin Co(III) species (d 6 ) (S ¼ 0). 26 The EPR spectrum of the fully oxidized solution of Ru 2 (III) Co(III) displays two features ascribable to an effective S ¼ 1 spin state which results from a magnetic interaction between two S ¼ 1/2 spin states located on the Ru(III) metals. The characteristic rhombic signature of the low spin Ru(III) S ¼ 1/2 spin state (d 5 ) between 210 and 360 mT is retained in the spectrum. 27,28 A weak half-eld EPR line is also observed at 139 mT, which is consistent with the S ¼ 1 spin state induced by the magnetic interaction of two Ru(III) ions. EPR spectra with similar features have been previously reported for magnetically coupled dinuclear Ru(III)-Ru(III) complexes. 28 The EPR signal of the initial Ru 2 Mn-OAc 2 solution at 100 K exhibits a 6-line signal characteristic of a high-spin Mn(II) ion (3d 5 , S ¼ 5/2), related to the central Mn(II) unit of the complex (Fig. 4). 27 This signal fully disappears aer the oxidation of the two diamagnetic Ru(II) units into Ru(III). Although an EPR signal is expected for a magnetically coupled system involving two Ru(III) (S ¼ 1/2) ions and one Mn(II) (S ¼ 5/2), no EPR signal was detected regardless of temperature (from 13 K to 200 K), presumably due to fast relaxation. The fully oxidized solution of Ru 2 (III)Mn(III) is also EPR silent, which is in agreement with there being magnetic coupling between the two Ru(III) (S ¼ 1/2) and the Mn(III) (d 4 , S ¼ 2) ions, which must produce a integer spin ground state. The low-intensity signals observed around g ¼ 2 are attributed to some decomposition of the trinuclear Ru 2 (III)Mn(III) (OAc) 2 complex (less than 5%, as shown by electrochemistry) into "free" Mn(II) (six line feature centered at g ¼ 2.0, 339 mT) and the mononuclear [Ru III (bpp)(trpy)(CH 3 CN)] 3+ complex (S ¼ 1/2) (the two features at a low magnetic eld compared to the six line feature).

Magnetic properties
Magnetic susceptibility measurements were performed under a constant magnetic eld of 5000 Oe, in the temperature range of 2-300 K. Variable eld reduced magnetization at 2 K was performed for all complexes. Isoeld variable temperature reduced magnetization measurements were also performed under various elds. These experiments served as criteria of purity, since this chemistry oen leads to the formation of small impurities of Ru(0) particles that become very visible when the magnetic properties are examined.
The constant eld and variable temperature magnetic susceptibility results are represented in the form of cT vs. T curves (where c is the molar paramagnetic susceptibility) (Fig. 5). The cT products for the complexes Ru 2 Co-Cl 2 and Ru 2 Co-OAc 2 are, at 300 K, 3.44 and 2.96 cm 3 K mol À1 , respectively, which are signicantly higher than the expected values for spinonly Co(II) ions (calculated as 1.875 cm 3 K mol À1 for g ¼ 2.0 and S ¼ 3/2). This product decreases with an increasing rate as the temperature declines. Both observations are accounted for by the fact that octahedral high spin Co(II) exhibits an orbital angular momentum (L ¼ 1) in addition to the spin state (S ¼ 3/2). This causes the observation of an effective g value that is much higher than expected for a spin only system (here calculated as 2.51 and 2.71, respectively). The differences between both ions are caused by their slightly differing coordination geometries, as well as the varying ligand eld strength of some of the donors. 29 The depopulation of the J multiplets arising from the spin-orbit coupling explains the decline of cT upon cooling. The reduced magnetization curves measured at 2 K (Fig. S25 †) saturate for both compounds at values lower than expected for an S ¼ 3/2 system. This is because at such a low temperature, only the lowest J multiplet is populated. Isoeld magnetization curves (at temperatures below 7 K and under several elds) produce the same saturation values and show quasi-superposition of the various curves (Fig. S26 †). This shows that the anisotropy of these low energy states is very small. These results are consistent with the above characterization and with the interpretation of the EPR results.
At 300 K, the cT products for the complexes Ru 2 Mn-Cl 2 and Ru 2 Mn-OAc 2 are almost identical; 4.43 and 4.45 cm 3 K mol À1 , respectively, which correspond to the expected spin-only value for an isolated Mn(II) center with S ¼ 5/2 spin state and g ¼ 2.01 and 2.02, respectively, consistent with the EPR and electrochemical results. This value stays nearly constant until around 5 K, where a decline down to 3.98 and 4.04 cm 3 K mol À1 at 2 K, respectively, is observed. The low temperature decline can be due to a small value of ZFS, or weak antiferromagnetic intermolecular interactions. The variable eld reduced magnetization curves, measured at 2 K (Fig. S23 †), and the isoeld magnetization curves (Fig. S24 †) are the result of Mn(II) saturating near 5 Bohr magneton, and this is consistent with the above results. The fact that the isoeld lines superimpose with each other for the various elds employed conrms that the amount of ZFS is very small for both complexes.

Redox properties in aqueous solutions
The redox properties of the trinuclear complexes Ru 2 Co-(H 2 O) 4 and Ru 2 Mn-(H 2 O) 4 were investigated by means of CV and DPV in a mixture of a pH ¼ 7.0 phosphate buffer (50 mM) and CF 3 CH 2 OH (19 : 1), the use of the latter forced by the poor solubility of the complexes in water. While very nicely dened waves were observed in organic solvents for their acetato counterparts as shown previously, in aqueous solution, the waves are very wide (Fig. S28 †). Nevertheless, the DPVs in Fig. 6 allow the observation of three faradaic processes situated at approximately 0.6, 0.7 and 1.0 V for both Ru 2 Co-(H 2 O) 4 and Ru 2 Mn-(H 2 O) 4 , which are plotted together with the mononuclear aqua complex in-1 for comparative purposes. However, the most important feature is the large electrocatalytic wave displayed by these complexes, starting at 1.3 V for Ru 2 Co-(H 2 O) 4 and at around 1.5 V for Ru 2 Mn-(H 2 O) 4 and in-1, which is associated to water oxidation catalysis.
[Ru II (bpy) 3  The oxygen generation prole as a function of time is presented in Fig. 7    In light of the good results obtained in the photoactivated experiment, water oxidation catalysis with Ru 2 Co-(H 2 O) 4 , and with the mononuclear in-1 for comparison, was also investigated using oxone (KHSO 5 ) as a chemical oxidant. The oxygen evolution proles as a function of time are presented in Fig. 8. The Co containing complex generates 1.0 mmol of oxygen (0.41 mmol are subtracted due to the activity of the blank under the same conditions) that correspond to a TON of about 13.
Labelling experiments using H 2 18 O were also carried out in order to extract mechanistic information regarding the O-O bond formation event when using oxone as a chemical oxidant. The O 2 36/34/32 isotope ratio was followed by on-line Mass spectrometry. Two different degrees of H 2 O labelling were employed, and the results obtained are shown in Fig. 8. For discussion of the mechanism, we consider rst the experiment with 97% H 2 18 O, and then test our hypothesis on the data obtained from the experiment with 15% H 2 18 O. However, before making a hypothesis on the mechanism of oxygen evolution, it is paramount to obtain information on the labeling state of the system at the moment of the oxygen evolving event.
(a). Raman experiments carried out under the same conditions as the water oxidation labelling experiments showed that no 18 O exchange occurs at all between water and oxone for at least 10 h (Fig. S30 †). 30 (b). Substitution reactions of aqua ligands at low oxidation states (II) for Ru and Co complexes occur rapidly. As the precatalyst presents all acetate bridge ligands, we can assume that all the aqua ligands are initially present as H 2 18 O.
(c). Peroxide oxidations of Ru-aqua polypyridyl complexes occur through dehydrogenation pathways. 31 Thus, oxone will react with low oxidation states of Ru-aqua as follows (auxiliary ligands not shown): Ru II - 18 (e). Tautomeric equilibrium between the oxo-hydroxo complexes, as described in eqn (10), would produce labelling scrambling, as has been earlier proposed for related complexes. 35  At this point, the following conclusions are extrapolated from the oxygen data shown in Fig. 8.
(1). The practically negligible amount of 36  (2). The mononuclear complex in-1 does not generate sufficient oxygen to be signicant under the applied conditions and thus the inexistence of O 2 coming independently from Ru-aqua/ Ru-oxo moieties is ruled out.
(3). Cis O-O coupling within the same Co metal center would be compatible with the ratios of isotopic labelling obtained, although this mechanism has been discarded based on DFT in a number of examples. 36,37 (4). The 97% labelling experiment (Fig. 8 and S31 †) is consistent with the existence of a very fast tautomeric equilibrium where species A and B exist in a 2 : 1 ratio respectively (see Scheme 2). The origin of the higher stabilization of species A is proposed to come from the higher hydrogen bond capacity of isomer A with regard to that of B. Oxidation states of Ru and Co are tentatively assigned from the apparent removal of 4/5 electrons from the oxidation state II,II,II, as judged from the electrocatalytic wave displayed by the complex. Assuming this equilibrium, the reaction of oxone (XO) with A and B would generate molecular oxygen with a ratio of isotopes of 34 O 2 / 32 O 2 ¼ 2/1, which is what is found experimentally. (5). Finally, changing the ratio of labelled water to 15% 18 O ( Fig. 8 and S31 †), will generate molecular oxygen with a ratio of 34 O 2 / 32 O 2 ¼ 1/9. Experimentally, we obtained a ratio of 1/10, which is in very good agreement with the proposed mechanism.
Overall, the labelling experiments carried out together with the rest of the electrochemical and spectroscopic properties are in agreement with the presence of a water nucleophilic attack (WNA) mechanism occurring at the Co site of the trinuclear complex, with cooperative interaction of the two Ru sites via electronic coupling through the bpp À bridging ligand and via neighboring hydrogen bonding.
It is worth mentioning that in our previous work with related dinuclear Ru 21 and Co 3 complexes containing the Hbpp ligand, namely {[M(H 2 O)(trpy)] 2 m-[(bpp)} 3+ (M ¼ Ru or Co), the O-O bond formation occurred via an intramolecular mechanism (I2M). 38 However, with the new trinuclear complexes described here, given the new substantially different geometries and electronic interactions between the metal centers, the mechanism changes to a WNA.

Conclusions
We have prepared and isolated a family of trinuclear complexes, Ru 2 M-X 2 (M]Co or Mn; X ¼ Cl À or OAc À ), where the central Co or Mn atom, together with two bpp À ligands, acts as a bridge between the two external Ru moieties. These complexes have been characterized in the solid state by X-ray diffraction analysis and by magnetic measurements. In solution they have been characterized by spectroscopic (EPR, UV-vis) and by electrochemical (CV, DPV) techniques. Overall, all these experiments for the chlorido or acetato bridge complexes show the presence of a relatively weak electronic coupling between the metal centers, transmitted through the bridging ligands. In addition, it is very interesting to see how the redox pattern radically changes from the Mn complexes with regards to the Co complexes manifesting the intrinsically different electronic properties of the two transition metals. In aqueous acidic media the trinuclear complexes revert to their Ru mononuclear counterparts and free Co(II) or Mn(II). However at pH ¼ 7.0 the integrity of the complex is fully retained. Of particularly interest is the heterotrinuclear Ru 2 Co-(H 2 O) 4 complex, that is capable at this pH of catalytically oxidizing water to molecular dioxygen chemically, electrochemically and photochemically. Finally, while a few detailed H 2