Solid-State Redox Switching of Magnetic Exchange and Electronic Conductivity in a Benzoquinoid-Bridged Mn<sup>II</sup> 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 (<sup>N,O</sup>LH<sub>4</sub>) with Mn<sup>III</sup> affords the chain compound Mn­(<sup>N,O</sup>L)­(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn<sup>II</sup> centers bridged by <sup>N,O</sup>L<sup>2–</sup> ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp<sub>2</sub>Co, Mn­(<sup>N,O</sup>L)­(DMSO) undergoes a ligand-centered solid-state reduction to [Mn­(<sup>N,O</sup>L)]<sup>−</sup>, 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 <i>J</i> = −0.38(1) to −15.6(2) cm<sup>–1</sup>, and a 10,000-fold increase in electronic conductivity, from σ = 2.33(1) × 10<sup>–12</sup> S/cm (<i>E</i><sub>a</sub> = 0.64(1) eV) to 8.61(1) × 10<sup>–8</sup> S/cm (<i>E</i><sub>a</sub> = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp<sub>2</sub>Fe]<sup>+</sup> 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.