Electrochemical Kinetics Support a Second Coordination Sphere Mechanism in Metal‐Based Formate Dehydrogenase

Abstract

Metal-based formate dehydrogenases are molybdenum or tungsten-dependent enzymes that catalyze the interconversion between formate and CO 2 . According to the current consensus, the metal ion of the catalytic center in its active form is coordinated by 6 S (or 5 S and 1 Se) atoms, leaving no free coordination sites to which formate could bind to the metal. Some authors have proposed that one of the active site ligands decoordinates during turnover to allow formate binding. Another proposal is that the oxidation of formate takes place in the second coordination sphere of the metal. Here, we have used electrochemical steady-state kinetics to elucidate the order of the steps in the catalytic cycle of two formate dehydrogenases. Our results strongly support the "second coordination sphere" hypothesis.

Second Coordination Sphere Effects in an Earth-Abundant Monometallic Complex as Catalyst Dictate Highly Selective Photochemical Conversion of CO2 to HCOOH

“…The catalyst features the earth abundant transition metal cobalt, and uncoordinated amine groups commonly referred to as pendant amines on the ligand framework, providing a proton-exchange site, often present in the second coordination sphere of CO2 converting metalloenzymes. [17][18][19] The P R 2N R 2 macrocylic ligand (R and R denote the substituents on P and N atoms respectively) has been a subject of study in several reports on transition metal complexes for proton reduction and hydrogen oxidation. 20 23 To the best of our knowledge, it is by far the fastest and most selective molecular catalyst for electroreduction of CO2 to HCOOH, with a turnover frequency (TOF) of 1000 s -1 .…”