The Role of Seven-Coordination in Ru-Catalyzed Water Oxidation

A family
of Ru complexes based on the pentadentate ligand t5a3– ((2,5-bis­(6-carboxylatopyridin-2-yl)­pyrrol-1-ide)
and pyridine (py) that includes {RuII(Ht5a-κ-N2O)­(py)3} (1HII(κ-N2O)), {RuIII(t5a-κ-N3O1.5)­(py)2} (2III(κ-N3O1.5)), and {RuIV(t5a-κ-N3O2)­(py)2}+ ({2IV(κ-N3O2)}+) has been prepared and thoroughly characterized.
Complexes 1HII(κ-N2O), 2III(κ-N3O1.5), and {2IV(κ-N3O2)}+ have been investigated
in solution by spectroscopic methods (NMR, UV–vis) and in the
solid state by single-crystal X-ray diffraction analysis and complemented
by density functional theory (DFT) calculations. The redox properties
of complex 2III(κ-N3O1.5) have been studied by electrochemical methods
(CV and DPV), showing its easy access to high oxidation states, thanks
to the trianionic nature of the t5a3– ligand. Under
neutral to basic conditions complex {2IV(κ-N3O2)}+ undergoes
aquation, generating {RuIV(OH)­(t5a-κ-N2O)­(py)2} (2IV(OH)­(κ-N2O)). Further oxidation of the complex forms {RuV(O)­(t5a-κ-N2O)­(py)2} (2V(O)­(κ-N2O)), which is a very
efficient water oxidation catalyst, reaching a TOFMAX value
of 9400 s–1 at pH 7.0, as measured via foot of the
wave analysis. The key to fast kinetics for the catalytic oxidation
of water to dioxygen by 2V(O)­(κ-N2O) is due not only to the easy access to high oxidation states
but also to the intramolecular hydrogen bonding provided by the noncoordinated
dangling carboxylate at the transition state, as corroborated by DFT
calculations.

一类基于五齿配体t5a3–（（2,5-双(6-羧基吡啶-2-基)吡咯-1-甲酰）和吡啶（py）的Ru配合物家族，包括{RuII(Ht5a-κ-N2O)­(py)3}（1HII(κ-N2O)）、{RuIII(t5a-κ-N3O1.5)­(py)2}（2III(κ-N3O1.5)）以及{RuIV(t5a-κ-N3O2)­(py)2}+（{2IV(κ-N3O2)}+）。该系列配合物已制备并进行了全面表征。1HII(κ-N2O)、2III(κ-N3O1.5)和{2IV(κ-N3O2)}+配合物已通过光谱法（NMR，UV-Vis）在溶液中进行研究，并在固态下通过单晶X射线衍射分析，辅以密度泛函理论（DFT）计算。通过电化学方法（CV和DPV）研究了配合物2III(κ-N3O1.5)的氧化还原性质，显示出其易于达到高氧化态，归因于t5a3–配体的三负离子特性。在近中性至碱性条件下，配合物{2IV(κ-N3O2)}+发生水解，生成{RuIV(OH)­(t5a-κ-N2O)­(py)2}（2IV(OH)­(κ-N2O)）。进一步氧化该配合物形成{RuV(O)­(t5a-κ-N2O)­(py)2}（2V(O)­(κ-N2O)），这是一种非常高效的水氧化催化剂，在pH 7.0下通过波谷分析测得的TOFMAX值为9400 s–1。2V(O)­(κ-N2O)催化水氧化生成氧气的快速动力学不仅得益于其对高氧化态的易于访问，还归因于过渡态中非配位悬挂羧酸根提供的分子内氢键，这一点已通过DFT计算得到证实。}