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

Abstract

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.

Selective Synthesis and Thermodynamic Relations of Polymorphic Co(NCS)2‐4‐Dimethylaminopyridine Coordination Compounds

“…Unfortunately, the energy difference between possible polymorphs or isomers is frequently small, which can lead to the formation of different products under kinetic or thermodynamic control . In this context, a large number of polymorphic and isomeric coordination compounds were reported in the last years , . From a viewpoint of material synthesis this might be of disadvantage but there is also some benefit from these phenomena.…”

Design and Study of Structural Linear and Nonlinear Optical Properties of Chiral [Fe(phen)3]2+Complexes

“…Notably, we have been investigating the combination of SCO and chirality for some years now, 26 a combination sometimes obtained by serendipity 27 and obtained rationally only using optically active ligands. [27][28][29][30][31][32][33][34] Due to their extensively studied photophysics, the model complexes [Fe(bipy)3] 2+ , [Fe(phen)3] 2+ , or [Fe(terpy)2] 2+ were evident targets for attractive low-cost non-centrosymmetric materials showing non-linear optical properties, and for the study of their eventual dependence with the spin state of the complex. Indeed some data were recently published on a SCO compound, but puzzlingly the reported crystalline structure is centrosymmetric, and the magnitude of the effect reported was quite weak.…”

Spin-crossover materials: Getting the most from x-ray crystallography

“…The chirality functionality, if interrelated to the SCO behavior, offers remarkable potential properties and applications such as electro-optic effects, nonlinear optical properties, chiroptical switching, magneto-optic properties, spin filtering or spin polarization in molecular spintronics. 52,53 The chiral functionality may emerge from different possibilities : use of chiral ligands (such as macrocyclic ligands with asymmetric substituents) in a mononuclear complex, 54 use of chelating ligands which twist around the metal center (such as for the [Fe(phen) 3 ] complex), 52,53,55 confining SCO molecules in chiral host framework, 56 or co-crystalizing SCO molecules with chiral counterions. 57 Chiral SCO materials of different architectures have been reported…”

“…in the literature: chiral SCO metal-organic frameworks, 58 chiral SCO mononuclear complexes [52][53][54][55]57 , chiral SCO polymeric chains. 59 Synthesis with equimolar racemic composition of the two enantiomers usually pack in a centrosymmetric space group, or crystallize as inversion twins.…”