The cobalt(ii) complex incorporating π-conjugated substituent, [Co(Naph-C2-terpy)2](BF4)2 (1; Naph-C2-terpy = 4′-(2-naphthoxy(ethoxy))-2,2′:6′,2′′-terpyridine), exhibits an abrupt spin transition (ST) behavior (cooperative factor C = 0.91) while its solvated product, 1·2MeOH, shows gradual spin crossover (SCO) behavior (C = 0.49). Single crystal X-ray structural analyses demonstrated that the octahedral coordination core [CoN6] in 1 shows larger distortion in both high-spin and low-spin states than solvated 1·2MeOH or another two derivatives, [Co(R-terpy)2](BF4)2 (R = 2-naphthyl (2), 9-anthracenyl (3)). The respective distortion parameters (Σ) are compared with those for previously reported SCO cobalt(ii) compounds. The highly-distorted [CoN6] core in 1 (Σ = 126 in the HS state and 101.6 in the LS state) was stabilized by strong intermolecular interactions and observed an abrupt ST behaviour.
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