ALKALI-METAL GATED VALENCE TAUTOMERISM IN A CROWN ETHER APPENDED COBALT–DIOXOLENE COMPLEX

Objective: Controlling intramolecular metal–ligand electron transfer with a chemically addressable, non-invasive input remains a central objective in the design of molecular electronic switches. Method: Here we report a crown-ether-appended cobalt–dioxolene complex, [Co(bpy-B18C6)(3,5-dbdiox)2] (1), in which a benzo-18-crown-6 receptor is tethered to a bipyridine ancillary ligand and positioned peripheral to the redox-active {Co(dioxolene)2} core. Recognition of alkali-metal cations (Li+, Na+, K+, Rb+, Cs+) at the remote crown ether, quantified by UV–Vis–NIR titration (K+: log Ka = 4.72 in CH2Cl2), reversibly biases this equilibrium toward the ls-Co(III)–cat state without the cation coordinating to cobalt. A crown-free control complex is essentially unresponsive to added cations, and broken-symmetry DFT reproduces the selective electrostatic stabilisation of the catecholate manifold. Sequestration of the bound cation with cryptand [2.2.2] restores the initial state, delivering a reversible supramolecular switch that retains ≥95% of its amplitude over six cycles. Results: The complex exhibits thermally accessible valence tautomerism between a low-spin Co(III)–catecholate (ls-Co(III)–cat) state and a high-spin Co(II)–semiquinonate (hs-Co(II)–SQ) state. Cation binding raises the valence-tautomeric transition temperature by up to 48 K, shifts the dioxolene-centred redox couple anodically by up to 86 mV, contracts the mean Co–O and Co–N bonds by ca. 0.12 and 0.17 Å, and lowers the room-temperature magnetic moment. The magnitude of the electronic perturbation tracks recognition strength but deviates measurably for the largest cation, indicating that the through-space charge–core distance, not binding affinity alone, governs the response. Novelty: The results establish alkali-metal recognition as a remote electrostatic gate for valence tautomerism.