Influence of donor and acceptor substitution on the MMCT properties of binuclear cyanide bridged Schiff base compounds

To explore the influence of the electron donating ability of a ligand on the acceptor and donor fragments for metal-to-metal charge transfer (MMCT), the series of asymmetric bimetallic cyanide-bridged complexes [L1Fe (NC)Y(dppe)L-2][CF3SO3] (Y = Fe, L-1 = saldien, L-2 = Cp, 1; Y = Fe, L-1 = Cl-2(saldien), L-2 = Cp 2; Y = Fe, L-1 = (NO2)(2)(saldien), L-2 = Cp, 3; Y = Fe, L-1 = (NO2)(2)(saldien), L-2 = Cp-5, 4; Y = Ru, L-1 = (NO2)(2)(saldien), L-2 = Cp, 5; Y = Ru, L-1 = (NO2)(2)(saldien), L-2 = Cp5, 6; saldien = 4-azaheptamethylene-1, 7-bis(salicylideneiminate); Cp = cyclopentadiene; Cp-5 = 1,2,3,4,5-pentamethyl-cyclopentadiene; dppe = 1,2-bis(diphenylphosphino)ethane) were synthesized and characterized. The experimental results suggest that for the Fe-Fe series of compounds, decreasing the electron donating ability of the ligand on the acceptor fragment or increasing the electron donating ability of the ligand on the donor fragment is conducive to MMCT; for the Ru-Fe series of compounds, with the enhancement of the electron donating ability of the ligand on the donor fragment, the energy of the MMCT decreases gradually; from the Fe-Fe series of compounds to the Ru-Fe series of compounds, by changing the metal center of the donor fragment the energy of the MMCT increases; all the six mixed-valence compounds belong to Class II systems, as confirmed by TDDFT/DFT calculations.