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

A nonsymmetrical triamine, 1,6-diamino-4-azahexane, was Schiff-condensed with (X-substituted) o-salicylaldehyde to yield pentadentate ligands X-L-5: salpet and MeBu-salpet. These ligands form mononuclear, dinuclear, and trinuclear Fe-III complexes, whose structures were determined by single-crystal X-ray analysis. Of the mononuclear complexes, [Fe-III(salpet)Cl] and [Fe-III(MeBu-salpet)Cl] are high spin (S = 5/2), whereas [Fe-III(salpet)CN]center dot MeOH is low spin (S = 1/2). The dinuclear and trinuclear complexes show a kind of thermally induced spin crossover. The dinuclear complex [(LFeIII)-Fe-5(CN)(FeL5)-L-III] (ClO4)center dot 2H(2)O (L-5 = salpet) is a mixed-spin assembly: the C-coordinated Fe-III center is low spin (L) and the N-coordinated Fe-III 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 [(LFeIII)-Fe-5(CN)(FeL5)-L-III(BPh4)center dot 2MeCN (L-5 = MeBu-salpet) exhibits a spin transition between LH and HH spin pairs. The mixed-valence trinuclear complex [(LFeIII)-Fe-5{Fe-II(CN)(5)(NO)}(FeL5)-L-III]center dot 0.5MeOH center dot 3.75H(2)O (L-5 = 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 (LFeIII)-Fe-5{Ni(CN)(4)}(FeL5)-L-III]center dot 2MeOH (L-5 = 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 Mossbauer spectra) on a common set of parameters. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)