Spin crossover in iron(III) Schiff-base 1-D chain complexes

Abstract

A series of iron(III) 1-D polymeric materials of the general formula [Fe(III)(Schiff-base)(L)](BPh(4)).n(CH(3)OH) (Schiff base = N,N'-ethylenebis(salicylaldimine) (H(2)salen), N,N'-o-phenylenebis(salicylaldimine) (H(2)salophen) and N,N'-ethylenebis(acety1acetone)-2,2'-imine (H(2) acen); L = bridging di-pyridyl or di-imidazole ligand, n = 0-4) and analogues therein, have been synthesised and structurally and magnetically characterised. In this series, a range of structural motifs are observed including linear 1-D chains, hydrogen-bonded chains, a 'hybrid' 1-D chain- and -dimer compound and a hydrogen-bonded dinuclear material; all exhibit extensive intermolecular interactions. The magnetic consequences of varying both the equatorial Schiff-base ligands and axial bridging ligands have been investigated. Overall, we find that, independent of the axial bridging ligand employed, the salen equatorial ligand results in a high spin character and the acen ligand results in spin crossover character, generally with a spin transition of a gradual nature. Variations in magnetic behaviour can be rationalized, in part, in terms of the C(2)N(2) backbone conformation of the equatorial Schiff base ligand, which may either inhibit or allow a spin transition.

Spin-crossover Compounds with Wide Thermal Hysteresis

“…The importance of this hydrogen bond was demonstrated by deuteration of the imidazole NH groups in HT 4, which caused a reduction in ¦T to 66 K. 37 Notably, two recently published iron(III) complexes of type [Fe(L)(im) 2 ]X (im: imidazole), where L is a tetradentate N 2 O 2 -donor Schiff base and X ¹ is an anion, also adopt the pattern of NH£O hydrogen bonding in Figure 3 and show spin-crossover hysteresis with ¦T¯5 K. 38 Albeit much weaker than in 4, that hysteresis is still significant in the context of these iron(III) Schiff base complexes, which usually undergo spin crossover very gradually. 39 This supports the conclusion that the hydrogen-bonding pattern shown in Figure 3 can lead to strong intermolecular cooperativity.…”

Syntheses, three types of hydrogen-bonded assembly structures, and magnetic properties of [FeIII(Him)2(hapen)]Y·solvent (Him=imidazole, hapen=N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine, Y=BPh4−, CF3SO3−, PF6−, ClO4−, and BF4−)

“…2 (b) shows the orientations of the two imidazole rings, in which one imidazole ring bisects the angles defined by the two N-Fe-O diagonals, and the other imidazole ring is oriented nearly along the N-Fe-O diagonal. (2) the gaushe or planar conformation locks in high spin state [15]. Compounds 4 and 5 of high-spin complexes have a gaushe conformation, while the other compounds of SCO complexes 1-3 are closer to an envelope conformation, giving a distortion of two benzene moieties.…”

The Development of Spin‐Crossover Research

“…1.20). 104 The Fe III N 4 O 2 coordination geometry around each Fe III was similar to that employed in earlier work on SCO monomers of type [Fe(salen)(imidazole) 2 ](ClO 4 ). 15 Only in the acen series was SCO observed, in the form of gradual incomplete transitions in their χ M T plots, the salen and salophen compounds remaining HS.…”

“…The spin-transition for these compounds was very gradual, stretching from ∼ 20 K to 300 K, with μ eff values close to the HS-HS values even at 2 K, and with higher M values at 2 K and 7 T. We note that our recently reported series of 1D chain species containing tetradentate N 2 O 2 -Schiff base (salen, salophen, acen) Fe III centres bridged by 4,4 -bipy-type ligands, described later, had similar SCO magnetism to this dinuclear series, as did one dinuclear example. 104 The dinuclear CN-bridged Fe III Schiff base family is the one that required a new model for quantitative fitting of data. 48 Diamagnetic bridges such as Ni(CN) 4 2− , Pt(CN) 4 2− and Ag(CN) 2 − were also reported and formed heterotrinuclear species.…”

“…After consideration of intra-and inter-chain interactions of the H-bonding, π -stacking and phenyl-embrace (to BPh 4 − ) types, the main magnetostructural correlation to emerge was that of the ligand backbone C 2 N 2 conformation (envelope/meso/planar (umbrella)) playing a key role such that the spin-transition can be allowed or inhibited. Oshio et al 150 have reviewed Fe III SCO compounds, prior to the above 1D Schiff base studies, 104 including their LIESST features and hybrid properties, for example SCO and conductivity in double Fe III Schiff base/Ni(mnt) 2 − salts. 150…”