Electronic effects of ligand substitution on spin crossover in a series of diiminoquinonoid-bridged feⁱⁱ₂ complexes

A series of four isostructural Fe^II₂ complexes, [(TPyA)₂Fe₂(^XL)]²⁺ (TPyA = tris(2-pyridylmethyl)amine; ^XL^2- = doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone; X = H, Br, Cl, and F), were synthesized to enable a systematic study of electronic effects on spin crossover behavior. Comparison of X-ray diffraction data for these complexes reveals the sole presence of high-spin Fe^II at 225 K and mixtures of high-spin and low-spin Fe^II at 100 K, which is indicative of incomplete spin crossover. In addition, crystal packing diagrams show that these complexes are well-isolated from one another in the solid state, owing primarily to the presence of bulky tetra(aryl)borate counteranions, such that spin crossover is likely not significantly affected by intermolecular interactions. Variable-temperature dc magnetic susceptibility data confirm the structural observations and reveal that 54(1), 56(1), 62(1), and 84(1)% of Fe^II centers remain high-spin even below 65 K. Moreover, fits to magnetic data provide crossover temperatures of T_1/2 = 160(1), 124(1), 121(1), and 110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies of H = 11.4(3), 8.5(3), 8.3(3), and 7.5(2) kJ/mol, respectively. These parameters decrease with increasing electronegativity of X and thus increasing electron-withdrawing character of ^XL^2-, suggesting that the observed trends originate primarily from inductive effects of X. Moreover, when plotted as a function of the Pauling electronegativity of X, both T_1/2 and H undergo a linear decrease. Further analyses of the low-temperature magnetic data and variable-temperature Mössbauer spectroscopy suggest that the incomplete spin crossover behavior in [(TPyA)₂Fe₂(^XL)]²⁺ is best described as a transition from purely [Fe_HS-Fe_HS] (HS = high-spin) complexes at high temperature to a mixture of [Fe_HS-Fe_HS] and [Fe_HS-Fe_LS] (LS = low-spin) complexes at low temperature, with the number of [Fe_HS-Fe_HS] species increasing with decreasing electron-withdrawing character of ^XL^2-.