Mössbauer Spectroscopy and Theoretical Studies of Iron Bimetallic Complexes Showing Electrocatalytic Hydrogen Evolution

Mössbauer spectroscopy and density functional theory (DFT) calculations are reported for the mononuclear Fe–nitrosyl complex [Fe­(N,N′-bis­(2-mercaptoethyl)-1,4-diazacycloheptane)­NO] {[Fe­(bme-dach)­(NO)] (1)} and the series of dithiolate-bridged dinuclear complexes M-Fe­(CO)­Cp [M = Fe­(bme-dach)­(NO) (1-A), Ni­(bme-dach) (2-A), and Co­(bme-dach)­(NO) (3-A)], in which M is a metallo-ligand to Fe­(CO)­Cp+ (Fe′Cp). The latter is an organometallic fragment in which Fe is coordinated by one CO and one cyclopentadienyl ligand. Complexes 1-A and 2-A were previously shown to have electrocatalytic hydrogen evolution activity. Mononuclear {Fe-NO}7 complex 1, with overall spin of 1/2, has an isomer shift of 0.23(2) mm/s [ΔEQ = 1.37(2) mm/s] and magnetic hyperfine couplings of {−38 T, −26.8 T, 8.6 T}. In complexes 2-A and 3-A, Fe′(CO)­Cp+ has a diamagnetic ground state and δ = 0.33(2) mm/s (ΔEQ ≈ 1.78 mm/s), consistent with a low-spin FeII site. In contrast, in complex 1-A, M = Fe­(bme-dach)­(NO) (i.e., complex 1) the magnetic hyperfine interactions of both metallo-ligand, M, and low-spin Fe′Cp are perturbed and Fe′Cp exhibits small magnetic hyperfine interactions, although its isomer shift and quadrupole splittings are largely unaltered. The DFT calculations for 1-A are in agreement with the paramagnetism observed for the Fe′(CO)­Cp+ iron site