Systematic Study of Spin Crossover and Structure in [Co(terpyRX)2](Y)2 Systems (terpyRX = 4′-alkoxy-2,2′:6′,2′′-terpyridine, X = 4, 8, 12, Y = BF4−, ClO4−, PF6−, BPh4−)

A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)2](Y)2·nH2O (X = 4, 8, 12 and Y = BF4−, ClO4−, PF6−, BPh4−) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)2](ClO4)2, [Co(terpyR8)2](BF4)2·H2O, and [Co(terpyR4)2](PF6)2·3H2O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF4− family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)2](BF4)2·H2O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn−Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).