Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives.
Chemistry
; 23(38): 9067-9075, 2017 Jul 06.
Article
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| MEDLINE
| ID: mdl-28387453
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)-3â
MeNO2 exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350â
K. The discontinuity around 240â
K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170â
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 5-300â
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 Hâ
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 (244â
K) is 34â
K lower than for (RS)-3 (278â
K) in CD3 CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.
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Article