Substituent effects on cobalt diglyoxime catalysts for hydrogen evolution.

The design of efficient, robust, and inexpensive hydrogen evolution catalysts is important for the development of renewable energy sources such as solar cells. Cobalt diglyoxime complexes, Co(dRgBF(2))(2) with substituents R, are promising candidates for such electrocatalysts. The mechanism for hydrogen production requires a series of reduction and protonation steps for various monometallic and bimetallic pathways. In this work, the reduction potentials and pK(a) values associated with the individual steps were calculated for a series of substituents. The calculations revealed a linear relation between the reduction potentials and pK(a) values with respect to the Hammett constants, which quantify the electron donating or withdrawing character of the substituents. Additionally, the reduction potentials and pK(a) values are linearly correlated with each other. These linear correlations enable the prediction of reduction potentials and pK(a) values, and thus the free energy changes along the reaction pathways, to assist in the design of more effective cobaloxime catalysts.