Spin-Crossover Behaviors Based on Intermolecular Interactions for Cobalt(II) Complexes with Long Alkyl Chains

A new family of terpyridine (terpy)‐based spin‐crossover cobalt(II) complexes, [Co(HO–Cn‐terpy)2](BF4)2 [n = 6 (1), 8 (2), 10 (3), 12 (4), and 14 (5)] that possesses a hydroxy‐appended long alkyl chain present on the terpyridine skeleton was synthesized and characterized by X‐ray structure analysis and temperature‐dependent magnetic susceptibility. In the solid state, the compounds 1·H2O, 4, and 5·H2O were crystallized in the triclinic P$\bar {1}$ space groups, and their central cobalt(II) ion was coordinated by six nitrogen atoms from two terpyridine units to form a pseudo‐octahedral symmetry. In the case of 1·H2O, one of the terminal hydroxy substituents was involved in hydrogen bonding with a water molecule. For compound 4, on the other hand, both of terminal hydroxy units form a short intermolecular contact with the BF4– anion. The temperature‐dependent magnetic behavior of 1 and 2 showed a gradual spin crossover, whereas 3 and 4 revealed an abrupt spin transition with a wide thermal hysteresis loop (ΔT = 21 K, T1/2↑ = 316 K, and T1/2↓ = 295 K for 3; ΔT = 40 K, T1/2↑ = 348 K, and T1/2↓ = 308 K for 4), and 5 exhibits “reverse spin transition” (ΔT = 17 K, T1/2↑ = 214 K, andT1/2↓ = 231 K). It has been suggested that compounds 3 and 4 form stronger intermolecular interactions through the fastener effect and hydrogen bonding than 1 and 2. On the basis of a comparison of the resulting magnetic behaviors of the compounds that contain long alkyl chains with a hydroxy group at the end, it was found that the long alkyl chains with the terminal hydroxy group could induce stronger intermolecular interactions between molecules. However, there are many short contacts through the counteranions for 5·H2O, but thermal motion of the long alkyl chains can be expected. Compound 5 exhibits “reverse spin transition” induced by a structural phase transition. Cobalt(II) complexes with hydroxy‐appended long alkyl chains at the end were synthesized. They were studied by single‐crystal X‐ray analysis and SQUIDmeasurement. For differences in magnetic behavior, we considered two factors: (i) a fastener effect and (ii) hydrogen bonding by the OH group.