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

Kay E Burrows,Sarah E Mcgrath,Malcolm A Halcrow,Oscar Cespedes,Rafal Kulmaczewski,Simon A Barrett

doi:10.1002/chem.201700820

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 T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , 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 1 H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD3 CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Highlights

The phenomenon of spin-crossover (SCO) was first elucidated over fifty years ago,[1] SCO complexes and molecular materials derived from them continue to be heavily studied.[2,3,4] On the one hand, control of the temperature and form of an SCO transition is a challenging problem in molecular design and crystal engineering,[5] which impacts areas as diverse as bioinorganic chemistry,[6,7] catalysis[6,8] and solid state physics.[9]
The most noteworthy behaviour in the solid state is shown by (R)-3, which was purified as its nitromethane solvate.[36]
Bulk samples of (R)-3·MeNO2 are fully low-spin below 200 K from magnetic susceptibility data, but exhibit a gradual, non-hysteretic thermal spin-crossover at higher temperatures with a T1=2 of about 350 K, from its cMT value of 1.8 cm3molÀ1 K at that temperature (Figure 1)

Summary

Introduction

The phenomenon of spin-crossover (SCO) was first elucidated over fifty years ago,[1] SCO complexes and molecular materials derived from them continue to be heavily studied.[2,3,4] On the one hand, control of the temperature and form of an SCO transition is a challenging problem in molecular design and crystal engineering,[5] which impacts areas as diverse as bioinorganic chemistry,[6,7] catalysis[6,8] and solid state physics.[9]. Cespedes School of Physics and Astronomy, University of Leeds E.

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Results

Conclusion