Metal-Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex

The electronic structure of the bis(dioxolene) bridging ligand -SQTh- is responsive to metal-ligand magnetic exchange coupling. Comparison of the crystal structure of (NiSQ)Th to that of (ZnSQ)Th indicates an open-shell biradical ground state for the dinuclear Ni(II) complex compared to the closed-shell quinoidal character found in the dinuclear Zn(II) complex. Consistent with a comparison of bond lengths obtained by X-ray diffraction, the analysis of the variable-temperature magnetic susceptibility data for crystalline (NiSQ)Th yields reduced SQ-SQ radical-radical magnetic exchange coupling (J_SQ-SQ = -203 cm^-1) compared to that of (ZnSQ)Th (J_SQ-SQ = -321 cm^-1). The reduced SQ-SQ exchange coupling in (NiSQ)Th derives from an attenuation of the SQ spin densities, which in turn is derived from the Ni-SQ antiferromagnetic exchange interactions. This reduction in SQ--SQ exchange that we observe for (NiSQ)Th correlates with an effective lengthening of the bridge unit by ∼2.1 Å relative to that of (ZnSQ)Th. This magnitude of the effective increase in the bridge distance is consistent with the (NiSQ)Th J_SQ-SQ value lying between those of (ZnSQ)Th and (ZnSQ)Th. The ability to modulate spin populations on an organic radical via pairwise Ni-SQ magnetic exchange interactions is a general way to affect electronic coupling in the Th-Th bridge. Our results suggest that metal-radical exchange coupling represents a powerful mechanism for tuning organic molecular electronic structure, with important implications for molecular electronics and molecular electron transport.