Cobalt complexes CpCo(PR3)2 (M; Cp = t?5-C5H5; R = Ph (1), Et (2), OMe (3)) and CpCoPh2PXPPh2 (X = C2H4 (4), C2H2(5), CH2 (6)) were transformed into the respective protonated salts [2H]PF6-[6H]PF6 with NH4PF6 in toluene/methanol. The electrochemistry of M as well as [MH]+ was studied at Pt, vitreous-carbon, and Hg electrodes in methylene chloride and propylene carbonate. Equilibrium constants KB = 3 X 106 M and 1.4 X 10s M were determined for the protonation of 3 and 5, respectively. In all cases a well-defined, chemically irreversible (EC¡), reduction peak, cathodic from the M+/0 couple but anodic from the reduction of protons at comparable concentration, was observed for the reduction of [MH]+. The reduction product MH either decays to M and H2 or can give H2 via proton-hydride reaction. In the presence of acid, M is reprotonated and a catalytic cycle for hydrogen evolution is formed. Thus, complex 3 is shown to act as a catalyst for hydrogen production at -1.15 V on a Hg cathode from an aqueous solution buffered to pH 5.

Recent developments on structural mimics for the oxygen-evolving complex of photosystem II are reviewed and discussed.

A recently reported synthetic complex with a MnCaO core represents a remarkable structural mimic of the MnCaO cluster in the oxygen-evolving complex (OEC) of photosystem II (Zhang et al., Science 2015, 348, 690). Oxidized samples of the complex show electron paramagnetic resonance (EPR) signals at g ≈ 4.9 and 2, similar to those associated with the OEC in its S state (g ≈ 4.1 from an S = / form and g ≈ 2 from an S = / form), suggesting similarities in the electronic as well as geometric structure. We use quantum-chemical methods to characterize the synthetic complex in various oxidation states, to compute its magnetic and spectroscopic properties, and to establish connections with reported data. Only one energetically accessible form is found for the oxidized "S state" of the complex. It has a ground spin state of S = /, and EPR simulations confirm it can be assigned to the g ≈ 4.9 signal. However, no valence isomer with an S = / ground state is energetically accessible, a conclusion supported by a wide range of methods, including density matrix renormalization group with full valence active space. Alternative candidates for the g ≈ 2 signal were explored, but no low-spin/low-energy structure was identified. Therefore, our results suggest that despite geometric similarities the synthetic model does not mimic the valence isomerism that is the hallmark of the OEC in its S state, most probably because it lacks a coordinatively flexible oxo bridge. Only one of the observed EPR signals can be explained by a structurally intact high-spin one-electron-oxidized form, while the other originates from an as-yet-unidentified rearrangement product. Nevertheless, this model provides valuable information for understanding the high-spin EPR signals of both the S and S states of the OEC in terms of the coordination number and Jahn-Teller axis orientation of the Mn ions, with important consequences for the development of magnetic spectroscopic probes to study S-state intermediates immediately prior to O-O bond formation.

A copper complex, [Cu(dpaq)](ClO 4 ) (1), of a monoanionic pentadentate amidate ligand (dpaq) has been isolated and characterized to study its efficacy toward electrocatalytic reduction of oxygen in neutral aqueous medium. The Cu(II) mononuclear complex, poised in a distorted trigonal bipyramidal structure, reduces oxygen at an onset potential of 0.50 V vs RHE. Kinetics study by hydrodynamic voltammetry and chronoamperometry suggests a stepwise mechanism for sequential reduction of O 2 to H 2 O 2 to H 2 O at a single-site Cu-catalyst. The foot-of-the-wave analysis records a turnover frequency of 5.65 × 10 2 s −1 . At pH 7.0, complex 1 undergoes a quasi-reversible mixed metal−ligand-based reduction and triggers the reduction of dioxygen to water. Electrochemical studies in tandem with quantum chemical investigation, conducted at different redox states, portray the active participation of ligand in completing the process of proton-coupled electron transfer internally. The protonated carboxamido moiety acts as a proton relay, while the quinoline-based orbital supplies the necessary redox equivalent for the conversion of complex 1 to Cu(II)-hydroperoxo species. Thus, a suitable combination of redox non-innocence and proton shuttling functionality in the ligand makes it an effective electron−proton-transfer mediator and subsequently assists the process of oxygen reduction.