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

Abstract

. Solid (R)-3·MeNO 2 exhibits an unusualv ery gradual, but discontinuoust hermals pin-crossover with an approximate T1 = 2 of 350 K. The discontinuity around 240 Kl ies well below T1 = 2 , and is unconnected to ac rystallographic phase change occurring at 170 K. Rather,i tc an be correlated with ag radual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-cross-

Pybox-Iron(II) Spin-Crossover Complexes with Substituent Effects from the 4-Position of the Pyridine Ring (Pybox = 2,6-Bis(oxazolin-2-yl)pyridine)

“…Furthermore, around T1/2 = 345 and 162-176 K, respectively, with attention concentrated on the solvent and counter anion dependence. Furthermore, Halcrow et al reported that chiral (S,S)-or (R,R)-4,4'-dimethylated derivatives were subjected to the SCO study [27]. On the other hand, we are now focusing on intramolecular substituent effects, because the introduction of substituents would bring about drastic SCO tuning through the covalent bonds, rather than through van der Waals interaction.…”

The speciation of homochiral and heterochiral diastereomers of homoleptic cobalt(II) and zinc(II) PyBox complexes

“…All four cobalt complexes are high-spin between 3-300 K in the solid state from magnetic susceptibility data, while (R)-3 and (RS)-3 are also fully high-spin in CD 3 CN solution over the liquid range of the solvent (Figure 1). Hence, in contrast to their iron(II) analogues [6], the different diastereomers of 3 have identical spin state properties.…”

“…2,6-Bis(oxazolinyl)pyridine (PyBox) derivatives (Scheme 1) are readily available in optically pure form [1], and are well established as tridentate supporting ligands in asymmetric catalysis [2]. More recently, their chirality has also been exploited in other aspects of coordination chemistry including molecular magnetism [3], supramolecular chemistry [4] and spin-crossover complexes [5,6]. During our continued studies of spin-crossover compounds based on tridentate diazolylpyridine derivatives [7], we recently demonstrated an unambiguous influence of chirality on molecular spin state, through solution measurements on diastereomers of [Fe(L Ph ) 2 ][ClO 4 ] 2 (Scheme 1) [8].…”

“…During our continued studies of spin-crossover compounds based on tridentate diazolylpyridine derivatives [7], we recently demonstrated an unambiguous influence of chirality on molecular spin state, through solution measurements on diastereomers of [Fe(L Ph ) 2 ][ClO 4 ] 2 (Scheme 1) [8]. That is, homochiral (rac) [Fe((R)-L Ph ) 2 ][ClO 4 ] 2 undergoes spin-crossover at 34 K lower temperature than heterochiral (meso) [Fe((R)-L Ph )((S)-L Ph )][ClO 4 ] 2 in CD 3 CN solution [6]. We attributed this to a more twisted coordination geometry adopted by the homochiral isomer, caused by an inter-ligand steric clash between phenyl substituents, which stabilises the more plastic high-spin form of that isomer to a small degree.…”

“…The same is true for salts of the analogous complexes [M((R)-L Ph )((S)-L Ph )] 2+ (M = Co [10,11], Zn [12], Cu [13] or another metal ion [9]). However, in contrast, heterochiral [Fe((R)-L iPr )((S)-L iPr )][ClO 4 ] 2 [6], and [Co((R)-L R )((S)-L R )] 2+ (R = Me or Bn) [10], do undergo partial racemisation under the same conditions. Hence, the solution stability of heterochiral [M(PyBox) 2 ] 2+ derivatives appears to depend on the PyBox 'R' substituents (Scheme 1).…”