Spin-crossover cobalt(II) complexes exhibiting temperature- and concentration-dependent optical changes in solution.

Abstract

This work investigated the spin states of the cobalt(II) complexes [Co(L1)2](X)2 (1·X; L1 = 4'-(4-N,N'-diphenylaminophenyl)-2,2':6',2''-terpyridine, X = PF6, BPh4) and [Co(L2)2](X)2 (2·X; L2 = 4'-(4-N,N'-dimethylaminophenyl)-2,2':6',2''-terpyridine, X = PF6, BPh4) in the solid state and in solution. In the solid state, 1·PF6 and 2·PF6, both containing smaller PF6- counter anions, showed gradual spin-crossover. In contrast, 1·BPh4 and 2·BPh4 remained in the high-spin state over the temperature range of 5-400 K due to a lower degree of molecular cooperativity. Each of the cobalt(II) complexes exhibited effects of temperature and concentration on their absorption spectra that were related to the spin states in various organic solvents. This work provides new insights into the spectroscopic properties resulting from the spin states of cobalt(II) complexes in solution.