Unconventional Spin Crossover in Dinuclear and Trinuclear Iron(III) Complexes with Cyanido and Metallacyanido Bridges

Abstract

AbstractA nonsymmetrical triamine, 1,6‐diamino‐4‐azahexane, was Schiff‐condensed with (X‐substituted) o‐salicylaldehyde to yield pentadentate ligands X‐L5: salpet and MeBu‐salpet. These ligands form mononuclear, dinuclear, and trinuclear FeIII complexes, whose structures were determined by single‐crystal X‐ray analysis. Of the mononuclear complexes, [FeIII(salpet)Cl] and [FeIII(MeBu‐salpet)Cl] are high spin (S = 5/2), whereas [FeIII(salpet)CN]·MeOH is low spin (S = 1/2). The dinuclear and trinuclear complexes show a kind ofthermally induced spin crossover. The dinuclear complex [L5FeIII(CN)FeIIIL5](ClO4)·2H2O (L5 = salpet) is a mixed‐spin assembly: the C‐coordinated FeIII center is low spin (L) and the N‐coordinated FeIII center is high spin (H) at low temperature; an antiferromagnetic interaction occurs between them. This LH reference state is mixed with the LL one. Upon heating, the system shows an increasing content of the HH state. Also, the dinuclear complex [L5FeIII(CN)FeIIIL5](BPh4)·2MeCN (L5 = MeBu‐salpet) exhibits a spin transition between LH and HH spin pairs. The mixed‐valence trinuclear complex [L5FeIII{FeII(CN)5(NO)}FeIIIL5]·0.5MeOH·3.75H2O (L5 = salpet) shows spin crossover with a residual high‐spin fraction at liquid He temperature owing to the LL + LH ground state. The metallacyanido‐bridged complex [L5FeIII{Ni(CN)4}FeIIIL5]·2MeOH (L5 = MeBu‐salpet) contains a high‐spin pair, HH, over the whole temperature interval with a ferromagnetic exchange interaction. A theoretical model was outlined that allows simultaneous fitting of all available experimental data (magnetic susceptibility, magnetization, high‐spin mole fraction obtained from the Mössbauer spectra) on a common set of parameters. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)