Influence of the Counterion and the Solvent Molecules in the Spin Crossover System [Co(4-terpyridone)2]Xp·nH2O

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Abstract
A series of new complexes belonging to the [Co(4-terpyridone)2]Xp·nS family (4-terpyridone = 2,6-bis(2-pyridyl)-4(1H)-pyridone) have been synthesized and characterized, using X-ray single crystal determination and magnetic susceptibility studies, to be X = [BF4]- (p = 2) and S = H2O for polymorphs 1 and 2, X = [BF4]- (p = 1) and [SiF6]2- (p = 0.5) and S = CH3OH for 3, X = [SiF6]2- (p = 1) and S = 3CH3OH and H2O for 4, X = [Co(NCS)4]2- (p = 1) and S = 0.5CH3OH for 5, X = I- (p = 2) and S = 5H2O for 6, X = [PF6]- (p = 1) for 7, and X = [NO3]- (p = 2) for 8. Compounds 1−7 can be grouped in three sets according to the space group in which they crystallize: (i) P1̄ triclinic (1, 3), (ii) P21 monoclinic (2), and (iii) P21/c monoclinic (4−7). The tridentate 4-terpyridone ligands coordinate the Co(II) ions in a mer fashion defining essentially tetragonally compressed [CoN6] octahedrons. The Co−N axial bonds involving the pyridone rings are markedly shorter than the Co−N equatorial bonds collectively denoted as Co−Ncentral and Co−Ndistal, respectively. The differences in the average Co−Ncentral or Co−Ndistal distances observed for 1−7 reflect the different spin states of Co(II). Complexes 7 and 4‘ are fully high spin (HS), while 5 and 6 are low spin (LS). However, the counterion [Co(NCS)4]2- in complex 5 is high spin. Complexes 1, 2, 3, and 8 exhibit spin-crossover behavior in the 400−100 K temperature region. Compounds 1 and 2 are polymorphs, and interestingly, 1 irreversibly transforms into 2 above 340 K because of a crystallographic phase transition which involves a drastic modification of the crystal packing. The relevant thermodynamic parameters associated with the spin transition of polymorph 2 have been estimated using the regular solution theory leading to ΔH = 3.04 kJ mol-1, ΔS = 20 J K-1 mol-1, and Γ = 0.95 kJ mol-1.