Switchable cobalt coordination polymers: Spin crossover and valence tautomerism

Electronically labile, or switchable, cobalt coordination polymers exhibit reversible spin crossover (SCO) or valence tautomeric (VT) transitions upon the application of an external stimulus, such as temperature variation. Spin crossover transitions at pseudo-octahedral cobalt(II) centers with an appropriate ligand field involve a heating-induced transition from the low-spin to high-spin electronic configurations. Valence tautomeric transitions are most commonly observed for cobalt-dioxolene systems, which undergo an intramolecular electron transfer and concomitant spin transition at the cobalt center, from low-spin-cobalt(III)-catecholate at low temperature to high-spin-cobalt(II)-semiquinonate upon heating. The VT transition can also be induced by light, while the cobalt(II) SCO transition cannot. Incorporation of these switchable moieties into coordination polymers (CPs) is generally achieved using polytopic nitrogen-donor linking ligands. All but one of the switchable cobalt CPs that have been structurally characterized are 1D chains, most of which exhibit VT transitions rather than SCO. There is a single example of a switchable 2D cobalt CP where VT cobalt-dioxolene moieties are linked by a tetratopic nitrogen-donor bridging ligand. Efforts to confer the VT properties to compounds suitable for applications as materials or in devices have mainly focused on the incorporation of the 1D chain VT CPs into nano- and microparticles, in which the VT transition is generally maintained. Valence tautomeric nanoparticles synthesized from 1D chain CPs have been grafted on to gold surfaces as self-assembled monolayers.