Spin-crossover in cobalt(II) compounds containing terpyridine and its derivatives

In this review article we discuss the unique and novel magnetic properties for the cobalt(II) compounds with a variety of terpy derivatives including substituents at the 4-position. These are also compared with the unsubstituted terpy cobalt(II) complex. Since the first SCO cobalt(II) complex with terpy ligand was reported, this system has been widely studied. SCO cobalt(II) complexes possessing terpy or OH-terpy ligand reveal incomplete or gradual SCO behavior. The pyterpy-appendecl cobalt(II) complex shows SCO depending on the guest molecules involved. Cobalt(II) complexes with long-alkylated terpy ligands, [Co(Cn-terpy)(2)](BF4)(2)(n = 16, 14 and 12) have been synthesized and some were characterized by single crystal X-ray analysis. Furthermore, variable-temperature magnetic susceptibility indicated that the non-solvated compounds [Co(Cn-terpy)(2)](BF4)(2) (n = 16. 14 and 12) exhibit "reverse spin transition" phenomenon with wide thermal hysteresis around room temperature. In addition, the solvated compound [Co(C12-terPY)(2)](BF4)(2 center dot)EtOH center dot 0.5H(2)O shows "re-entrant SCO" behavior. The long alkyl chains in SCO cobalt(II) complexes can lead to novel physical properties resulting from a synergetic effect between SCO and response of the flexibility toward external stimuli. (C) 2011 Elsevier B.V. All rights reserved.