Linear Free Energy Relationships in Hydrogen Evolution Catalysis by a Cobalt Tripeptide in Water

Water is the desired solvent for catalytic hydrogen production, but the presence of multiple proton donors in buffered water complicates analysis of reaction mechanisms. Here, we determine substrate-dependent rate constants for electrocatalytic hydrogen evolution by a cobalt tripeptide (CoGGH) in the presence of buffers of pKa 6.9 to 10.4. Catalytic rate constants in the presence of buffer (kHA) are 2 to 4 orders of magnitude higher than when water is the sole proton source, indicating that buffer acid outcompetes water as a proton donor. The rate of hydrogen evolution catalyzed by CoGGH is found to be dependent on the buffer-acid pKa and independent of pH (from pH 8 to 10). A Brønsted-type linear free energy relationship between kHA and buffer-acid pKa is found, supporting a concerted proton-coupled electron transfer with a buffer conjugate acid proton donor as a common rate-determining step for the buffers used.