Preparation of dihydroquinazoline carbohydrazone Fe(II) complexes for spin crossover

Dihydroquinazoline Fe^II complexes, namely [Fe(pq-R)₂](X)₂·xCH₃OH·yCH₃CN·zH₂O (R = 2-py: X = ClO₄^-, x = 1, y = 0, z = 2 (1), BF₄^-, x = 0, y = 2, z = 1.75 (2), CF₃SO₃^-, x = 1, y = 1, z = 0 (3); and X = ClO₄^-: R = 2-OCH₃, x = 0, y = 0, z = 1 (4), R = 3-OCH₃, x = 0, y = 0, z = 1 (5)), were prepared and the effects of the solvent, counteranion and ligand substituent on spin crossover properties were discussed. Comparison of X-ray diffraction data for these complexes revealed the sole presence of high-spin Fe^II at 298 K and the bond distances around the Fe^II center at low temperature fall much closer to those at high temperature, which is consistent with variable-temperature dc magnetic susceptibility data. However, the loss of solvent induced a significant change in the spin state of complexes 1 and 2. Moreover, fits to magnetic data of the desolvated samples provide crossover temperatures of T_1/2 = 182.9(6), 157.0(8) and 138.3(5) K for 1-des (ClO₄^-), 2-des (BF₄^-) and 3-des (CF₃SO₃^-), respectively, indicative of the anion-dependent transition temperature. The cooperativity operating in the complexes is thought to be mainly from the intermolecular π⋯π interactions between dihydroquinazoline rings on the neighboring molecules. The ligand substituent effect was also investigated on Fe^II SCO properties. This work revealed the spin crossover properties of new types of dihydroquinazoline Fe(ii) complexes, including the transition temperature, the degree of completion and the cooperative nature of the transition, which can be optimally designed when developing new spin-crossover materials.