PNNL

Abstract

The intra- and intermolecular deprotonation of [(CpC5F4N)Fe(PEtNR'PEt)H]+ (PEtNR'PEt = (Et2PCH2)2NR') complexes are rate-limiting processes in the electrocatalytic oxidation of hydrogen, and can be mediated by an amine base in the outer coordination sphere (OCS) by facilitating the matching of energy of deprotonation intermediates. A series of complexes with different N-substituents (R' = Me, Et, CH2OMe, CH2N(Ph)Me, and (CH2)nNMe2; n = 1–4) was examined computationally using density functional theory to determine the influence of the identity and proximity of the OCS base on the intramolecular deprotonation. Complexes with an OCS dimethylamino substituent were the most suitably energy matched. The number of carbon atoms between the secondary and OCS nitrogen atoms dictated the strength of the hydrogen bonding interaction and ring strain following deprotonation of the iron hydride. The previously reported [(CpC5F4N)Fe(PEtNR'PEt)H]+ (R' = (CH2)3NMe2) electrocatalyst was predicted to have the most favorable free energy (?G) and free energy of activation (?G‡). To verify the computational results, two of the iron precatalysts, (CpC5F4N)Fe(PEtN(CH2)nNMe2PEt)Cl (n = 2 and 4), were synthesized, and their electrocatalytic activity for the oxidation of hydrogen was determined under an atmosphere of H2 using DABCO as the exogenous base. Maximum turnover frequencies of 270 and 200 s-1 were measured when n = 2 and 4, respectively, validating the computational results.