Outer Coordination Sphere Proton Relay Base and Proximity Effects on Hydrogen Oxidation with Iron Electrocatalysts

The
intra- and intermolecular deprotonation reactions 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 (ΔG) and free energy
of activation (ΔG⧧). 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.