Control of Electronic Structure and Conductivity in Two-Dimensional Metal-Semiquinoid Frameworks of Titanium,...

The isostructural, two-dimensional metal-organic frameworks (H₂NMe₂)₂M₂(Cl₂dhbq)₃ (M = Ti, V; Cl₂dhbq^n- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) and (H₂NMe₂)_1.5Cr₂(dhbq)₃ (dhbq^n- = deprotonated 2,5-dihydroxybenzoquinone) are synthesized and investigated by spectroscopic, magnetic, and electrochemical methods. The three frameworks exhibit substantial differences in their electronic structures, and the bulk electronic conductivities of these phases correlate with the extent of delocalization observed via UV-vis-NIR and IR spectroscopies. Notably, substantial metal-ligand covalency in the vanadium phase results in the quenching of ligand-based spins, the observation of simultaneous metal- and ligand-based redox processes, and a high electronic conductivity of 0.45 S/cm. A molecular orbital analysis of these materials and a previously reported iron congener suggests that the differences in conductivity can be explained by correlating the metal-ligand energy alignment with the energy of intervalence charge-transfer transitions, which should determine the barrier to charge hopping in the mixed-valence frameworks.