Spin Crossover in a Family of Iron(II) Complexes with Hexadentate ligands: Ligand Strain as a Factor Determining the Transition Temperature

Abstract

This paper reports the synthesis of a family of mononuclear complexes [Fe(L)]X(2) (X=BF(4), PF(6), ClO(4)) with hexadentate ligands L=Hpy-DAPP ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine}), Hpy-EPPA ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and Hpy-DEPA ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}). The systematic change of the length of amino-aliphatic chains in these ligands results in chelate rings of different size: two six-membered rings for Hpy-DAPP, one five- and one six-membered rings for Hpy-EPPA, and two five-membered rings for Hpy-DEPA. The X-ray analysis of three low-spin complexes [Fe(L)](BF(4))(2) revealed similarities in their molecular and crystal structures. The magnetic measurements have shown that all synthesized complexes display spin-crossover behavior. The spin-transition temperature increases upon the change from six-membered to five-membered chelate rings, clearly demonstrating the role of the ligand strain. This effect does not depend on the nature of the counter ion. We discuss the structural features accountable for the strain effect on the spin-transition temperature.

Structure:function relationships in molecular spin-crossover complexes

“…113 The same ordering in T 1/2 was also found for the ClO 4 À and PF 6 À salts of the complexes, although those were not crystallographically characterised. 114 The consistent reduction in T 1/2 with increasing chelate ring size was attributed to an increase in ligand strain during spin-crossover. The larger sixmembered chelate rings will favour the less compact high-spin forms of the complexes on steric grounds, thus lowering T 1/2 as observed.…”

Ligand Strain and the Nature of Spin Crossover in Binuclear Complexes: Two‐Step Spin Crossover in a 4,4′‐Bipyridine‐Bridged Iron(II) Complex [{Fe(dpia)(NCS)2}2(4,4′‐bpy)] (dpia=di(2‐picolyl)amine; 4,4′‐bpy=4,4′‐bipyridine)

“…We paid special attention to the distortions of the [FeN 6 ] coordination polyhedron caused by ligand strain effects. Our recent study of the family of related monoA C H T U N G T R E N N U N G nuclear SCO compounds [23] unambiguously demonstrated that the strengthening of the ligand strain effects, which results in the enhancement of the [FeN 6 ] core distortion, decreases dramatically the spin transition temperature and in the limiting case stabilizes the HS state. Let us consider the effects of the ligand strain on the spin state and SCO process for several binuclear compounds.…”

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…Expanded chelate ring sizes should also promote the high spin states of complexes, by increasing the ligand conformational strain associated with the more compact low-spin form [71,72] [75,76], for example, although that remains to be confirmed by a solution-phase study. It is harder to elucidate trends from those compounds in the solid state, since their SCO is often very gradual and can be influenced by the salicyl ligand conformation in the crystal [68].…”