Crystal Structures and Spin‐Crossover Behavior of Iron(II) Complexes with Chiral and Racemic Ligands

Sekimoto, Yusuke; Karim, Mohammad Razaul; Saigo, Naoto; Ohtani, Ryo; Nakamura, Masaaki; Hayami, Shinya

doi:10.1002/ejic.201601232

Cited by 16 publications

(6 citation statements)

References 27 publications

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“…Interestingly, the subtle differences in the structural features detected for each polymorph, essentially observed in their crystal packing, seems to be governed by the presence of either a racemic mixture (polymorph I) or a pure enantiomeric form (polymorph II) of the complexes within the structure. This observation makes the title compounds particularly interesting, since the examples of synergy between chirality and SCO so far reported are relatively scarce in the literature . Moreover, to the best of our knowledge, this is the first SCO polymorphic system where one of the polymorphs is chiral whereas the other represents a racemic mixture.…”

Section: Discussionmentioning

confidence: 93%

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

et al. 2017

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Understanding the origin of cooperativity and the equilibrium temperature of transition (T) displayed by the spin-crossover (SCO) compounds as well as controlling these parameters are of paramount importance for future applications. For this task, the occurrence of polymorphism, presented by a number of SCO complexes, may provide deep insight into the influence of the supramolecular organization on the SCO behavior. In this context, herein we present a novel family of mononuclear octahedral Fe complexes with formula cis-[Fe(bqen)(NCX)], where bqen is the chelating tetradentate ligand N,N'-bis(8-quinolyl)ethane-1,2-diamine and X = S, Se. Depending on the preparation method, these compounds crystallize in either the orthorhombic or the trigonal symmetry systems. While the orthorhombic phase is composed of a racemic mixture of mononuclear complexes (polymorph I), the trigonal phase contains only one of the two possible enantiomers (Λ or Δ), thereby generating a chiral crystal (polymorph II). The four derivatives undergo SCO behavior with well-differentiated T values occurring in the interval 90-233 K. On one hand, T is about 110 K (polymorph I) and 87 K (polymorph II) higher for the selenocyanate derivatives in comparison to those for their thiocyanate counterparts. These differences in T are ascribed not only to the higher ligand field induced by the selenocyanate anion but also to a remarkable difference in the structural reorganization of the [FeN] coordination core upon SCO. Likewise, the higher cooperativity observed for the thiocyanate derivatives seems to be related to their stronger intermolecular interactions within the crystal. On the other hand, T is about 53 K (thiocyanate) and 29 K (selenocyanate) higher for the trigonal polymorph II in comparison to those for the orthorhombic polymorph I. These differences, and the small changes observed in cooperativity, stem from the slightly different hetero- and homochiral crystal packing generated by the cis-[Fe(bqen)(NCX)] molecules, which determines subtle adaptations in the intermolecular contacts and the Fe coordination core.

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Section: Discussionmentioning

confidence: 93%

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

et al. 2017

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“…A check of the CCDC database reveals that only 6 of the 78 unique [Mn(R-Sal 2 323)] + complexes published before 2021 crystallize adventitiously in a chiral space group, − with a seventh example targeted by introduction of a chiral anion . The interplay between SCO and chirality − is increasingly recognized as an important route to switchable nonlinear optical (NLO) materials ,− and spin-state dependent changes in optical activity may also constitute an economic and low energy route to follow SCO in sensing applications. Here we use circular dichroism to confirm the spontaneous resolution in the case of the ClO 4 – complex ( 1 ) and to demonstrate that the complex is stable toward racemization in solution.…”

Section: Introductionmentioning

confidence: 99%

Homochiral Mn³⁺ Spin-Crossover Complexes: A Structural and Spectroscopic Study

et al. 2022

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Structural, magnetic, and spectroscopic data on a Mn 3+ spin-crossover complex with Schiff base ligand 4-OMe-Sal 2 323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal 2 323)]X where X = ClO 4 – ( 1 ), BF 4 – ( 2 ), NO 3 – ( 3 ), Br – ( 4 ), and I – ( 5 ) crystallize isotypically in the chiral orthorhombic space group P 2 1 2 1 2 with a range of spin state preferences for the [Mn(4-OMe-Sal 2 323)] + complex cation over the temperature range 5–300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes 4 and 2 with respective values: D S =1 = +23.38(1) cm –1 , E S =1 = +2.79(1) cm –1 , and D S =2 = +6.9(3) cm –1 , with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV–vis absorption spectra on high-spin complex 1 and mixed S = 1/ S = 2 sample 5 reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal 2 323)] + in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal 2 323 ligand.

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“…However, chiral‐at‐molecule SCO compounds are rare, with most studies involving just two series of iron(II) complexes: a family of bidentate Schiff base complexes formed from chiral amines; and, a macrocyclic ligand with an asymmetric pattern of backbone substituents . The influence of chirality on the SCO properties of such compounds has only recently begun to be investigated and, importantly, all data published thus far have been measured in the solid state . The effect of ligand structure on molecular spin states is better quantified in solution, in which the spin state of a complex is not influenced by lattice effects …”

Section: Introductionmentioning

confidence: 99%

“…Alternatively, chiral materials based on achiral SCO molecules have been prepared through doping them into chiral host frameworks, or crystallizing them with chiral counterions . However, chiral‐at‐molecule SCO compounds are rare, with most studies involving just two series of iron(II) complexes: a family of bidentate Schiff base complexes formed from chiral amines; and, a macrocyclic ligand with an asymmetric pattern of backbone substituents . The influence of chirality on the SCO properties of such compounds has only recently begun to be investigated and, importantly, all data published thus far have been measured in the solid state .…”

Section: Introductionmentioning

confidence: 99%

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

Burrows

McGrath

Kulmaczewski

et al. 2017

Chemistry A European J

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. 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-

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Crystal Structures and Spin‐Crossover Behavior of Iron(II) Complexes with Chiral and Racemic Ligands

Cited by 16 publications

References 27 publications

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

Homochiral Mn³⁺ Spin-Crossover Complexes: A Structural and Spectroscopic Study

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

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Crystal Structures and Spin‐Crossover Behavior of Iron(II) Complexes with Chiral and Racemic Ligands

Cited by 16 publications

References 27 publications

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study

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

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Homochiral Mn³⁺ Spin-Crossover Complexes: A Structural and Spectroscopic Study

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