A set of robust molecular cobalt catalysts for the generation of hydrogen from water is reported. The cobalt complex supported by the parent pentadentate polypyridyl ligand PY5Me(2) features high stability and activity and 100% Faradaic efficiency for the electrocatalytic production of hydrogen from neutral water, with a turnover number reaching 5.5 × 10(4) mol of H(2) per mole of catalyst with no loss in activity over 60 h. Control experiments establish that simple Co(II) salts, the free PY5Me(2) ligand, and an isostructural PY5Me(2) complex containing redox-inactive Zn(II) are all ineffective for this reaction. Further experiments demonstrate that the overpotential for H(2) evolution can be tuned by systematic substitutions on the ancillary PY5Me(2) scaffold, presaging opportunities to further optimize this first-generation platform by molecular design.

Introduction 4164 2. P 4 Activation Leading to P 1 , P 2 , and P 3 Products 4165 2.1. Terminal and Bridging P 1 Ligands from P 4 4165 2.2. Bridging P 2 Ligands from P 4 4166 2.3. Cyclo-P 3 Units Derived from P 4 4168 3. P 4 Activation Leading to P 4 Products 4169 3.1. Coordination of P 4 Tetrahedra 4169 3.2. Rearranged P 4 Products 4169 4. P 4 Activation Leading to P g5 Products 4171 5. Functionalization and Transfer Methodologies 4173 5.1. Functionalization and Subsequent Reactivity of Terminal Monophosphide and Bridging Diphosphorus Complexes Derived from P 4 4173 5.2. Reactivity and Transfer Chemistry of the Cyclo-P 3 Functional Group 4174 5.3. Functionalization and Transfer Chemistry of Polyphosphorus Ligands (P g4 ) Derived from P 4 4175 6. Concluding Remarks 4175 6.1. Conclusions 4175 6.2. Future Prospects 4176 7. Acknowledgments 4176 8. References 4176