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

Carlos Bartual-Murgui; Lucía Piñeiro-López; F Javier Valverde-Muñoz; M Carmen Muñoz; Maksym Seredyuk; José Antonio Real

doi:10.1021/acs.inorgchem.7b02272

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

Inorg Chem. 2017 Nov 6;56(21):13535-13546. doi: 10.1021/acs.inorgchem.7b02272.

Authors

Carlos Bartual-Murgui¹, Lucía Piñeiro-López¹, F Javier Valverde-Muñoz¹, M Carmen Muñoz², Maksym Seredyuk^{1

3}, José Antonio Real¹

Affiliations

¹ Institut de Ciència Molecular (ICMol), Departament de Química Inorgànica, Universitat de València , C/Catedrático José Beltrán Martínez, 2, 46980 Paterna (Valencia), Spain.
² Departamento de Física Aplicada, Universitat Politècnica de València , Camino de Vera s/n, 46022 Valencia, Spain.
³ Taras Shevchenko National University , Department of Chemistry, Volodymyrska Str. 64, Kyiv 01601, Ukraine.

PMID: 29048915
DOI: 10.1021/acs.inorgchem.7b02272

Abstract

Understanding the origin of cooperativity and the equilibrium temperature of transition (T_1/2) 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^II complexes with formula cis-[Fe(bqen)(NCX)₂], 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_1/2 values occurring in the interval 90-233 K. On one hand, T_1/2 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_1/2 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 [FeN₆] 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_1/2 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)₂] molecules, which determines subtle adaptations in the intermolecular contacts and the Fe^II coordination core.