Solid-State Redox Switching of Magnetic Exchange and Electronic Conductivity in a Benzoquinoid-Bridged Mn^II Chain Compound

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol (^N,OLH₄) with Mn^III affords the chain compound Mn(^N,OL)(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn^II centers bridged by ^N,OL^2- ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp₂Co, Mn(^N,OL)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn(^N,OL)]^-, as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm^-1, and a 10,000-fold increase in electronic conductivity, from = 2.33(1) × 10^-12 S/cm (E_a = 0.64(1) eV) to 8.61(1) × 10^-8 S/cm (E_a = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp₂Fe]⁺ regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity.