Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2]2+ Derivatives

Abstract

The following iron(II) complexes of 2,6-bis(oxazolinyl)pyridine (PyBox; LH) derivatives are reported: [Fe(LH)2][ClO4]2 (1); [Fe((R)-LMe)2][ClO4]2 ((R)-2; LMe=2,6-bis{4-methyloxazolinyl}pyridine); [Fe((R)-LPh)2][ClO4]2 ((R)-3) and [Fe((R)-LPh)((S)-LPh)][ClO4]2 ((RS)-3; LPh=2,6-bis{4-phenyloxazolinyl}pyridine); and [Fe((R)-LiPr)2][ClO4]2 ((R)-4) and [Fe((R)-LiPr)((S)-LiPr)][ClO4]2 ((RS)-4; LiPr=2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO2 exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T 1/2 of 350 K. The discontinuity around 240 K lies well below T 1/2‘, and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5–300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by 1H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T 1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD3CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.