Temperature-Induced Spin-Transition in a Low-Spin Cobalt(II) Semiquinonate Complex

摘要

Spin crossover and valence tautomerism are examples of processes that can be utilized as a basis for achieving molecular switches. Whereas the spin-crossover process is characterized by a temperature-, pressure-, or light-induced change of the electronic state of the metal ion to one with a different spin multiplicity, valence tautomerism entails an intramolecular redox reaction between a metal ion and a coordinated ligand, which, in a few instances, is accompanied by a change in the spin state of the metal ion. Various reported low-spin cobalt(III) catecholate complexes, which can be transformed into high-spin cobalt(II) semiquinonate complexes by raising the temperature, provide excellent examples of the latter process. In contrast, spin-crossover chemistry is dominated by octahedral iron(II) complexes with a FeN6 coordination sphere; however, there are only very few known octahedral cobalt(II)-containing spin-crossover complexes. Herein we describe the first cobalt(II) semiquinonate complex that displays spin-crossover properties rather than valence tautomerism.