Redox-Induced Structural Switching through Sporadic Pyridine-Bridged CoIICoII Dimer and Electrocatalytic Proton Reduction.

The homodinuclear CoII helicate complex [CoII(DQPD)]2 (1) was prepared by treating [Co(H2O)6](ClO4)2 with the deprotonated form of the ligand N2,N6-bis(quinolin-8-yl)pyridine-2,6-dicarboxamide (DQPDH2). Complex 1 represents a discrete homodinuclear helicate complex with two CoII centers having a distorted-octahedral geometry through an unprecedented pyridine bridge. Complex 1, upon treatment with H2O2, undergoes oxidation at one of the CoII centers followed by a structural deformation to generate the mixed-valence complex [CoIIICoII(DQPD)2](ClO4) (2·ClO4). In complex 2, the bridging through the central pyridine collapses along with the formation of Co(III) octahedral and Co(II) tetrahedral environments. Complexes 1 and 2 interconvert to one another. The effective magnetic moments for complexes 1 and 2 are respectively 5.88 and 4.30 μB. Complexes 1 and 2 have been employed for electrocatalytic proton reduction using AcOH as the proton source in 95/5 (v/v) DMF/H2O. A TOF of 30000 mmol of H2 h-1 (mol of 1)-1 at a potential of -1.7 V vs SCE was achieved. A resting-state analysis has been carried out to support the mechanism for the catalytic proton reduction.