FULL PAPER DOI: 10.1002/ejic.200900169 Unconventional Spin Crossover in Dinuclear and Trinuclear Iron(III) Complexes with Cyanido and Metallacyanido Bridges Ivan S ˇ alitros ˇ, [a] Roman Boc ˇa,* [a] L’ubor Dlhán ˇ, [a] Milan Gembický, [b] Jozef Koz ˇís ˇek, [a] Jorge Linares, [c] Ján Moncol’, [a] Ivan Nemec, [a] Lucia Peras ˇínová, [a] Franz Renz, [d] Ingrid Svoboda, [e] and Hartmut Fuess [e] Keywords: Iron / Structure elucidation / Schiff bases / Spin crossover / Exchange interactions 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 sin- gle-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]·MeOH is low spin (S = 1/2). The dinuclear and trinuclear complexes show a kind of thermally induced spin crossover. The dinuclear complex [L 5 Fe III (CN)Fe III L 5 ](ClO 4 )·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 tem- perature; 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 Introduction The low-spin entity can be transformed into the high- spin species by heating when the energy gap ΔH is compar- able to the thermal energy. The conversion L (low-spin) to H (high-spin) states is an entropy-driven unimolecular reac- tion (ΔS  0): there exists a critical temperature T c = ΔH/ ΔS above which the conversion proceeds spontaneously. The degree of the conversion is characterized by the tem- perature dependence of the high-spin mole fraction, x H , [a] Institute of Inorganic Chemistry and Institute of Physical Chemistry, FCHPT, Slovak University of Technology, 81237 Bratislava, Slovakia E-mail: roman.boca@stuba.sk [b] Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260, USA [c] Groupe d’Etude de la Matière Condensée, CNRS-UMR 8635, University of Versailles, 78035 Versailles Cedex, France [d] Institute of Inorganic Chemistry, Leibniz University, 30167 Hannover, Germany [e] Material Sciences, Darmstadt University of Technology, 64287 Darmstadt, Germany Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.200900169. Eur. J. Inorg. Chem. 2009, 3141–3154 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3141 state. Also, the dinuclear complex [L 5 Fe III (CN)Fe III L 5 ](BPh 4 )· 2MeCN (L 5 = MeBu-salpet) exhibits a spin transition be- tween LH and HH spin pairs. The mixed-valence trinuclear complex [L 5 Fe III {Fe II (CN) 5 (NO)}Fe III L 5 ]·0.5MeOH·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 [L 5 Fe III {Ni(CN) 4 }Fe III L 5 ]·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, magne- tization, high-spin mole fraction obtained from the Möss- bauer spectra) on a common set of parameters. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) which defines the equilibrium constant K = x H /(1 – x H ); in an ideal case it follows the van’t Hoff formula, that is, ln K vs. 1/T is a linear function. [1] Spin crossover in mononuclear complexes is a case of electron bistability, associated with the existence of two mi- nima (L and H) of the Gibbs energy. In dinuclear com- plexes, four distinct spin pairs can exist: LL, LH, HL, and HH (if intermediate spin states are ignored). It is not im- plied, however, that the LH pair is identical to the HL one. The corresponding energies of these electronic reference states are separated from the common origin by E LL , E LH , E HL , and E HH gaps, of which three are independent. Which energy order is actually followed is ruled by the crystal-field strength of the donor set (in general by the orbital angular momentum) and not by the spin angular momentum alone (Figure 1). Spin–spin interactions cause the formation of spin multiplets that are further split in the applied magnetic field and thus they dominate the magnetic behavior. A floating nature of the reference energy levels, that is, the order of E LL , E LH , E HL , and E HH , was extensively mod- eled by DFT calculations. [2] It was found that in a series of structurally related dinuclear compounds [(NCX) 2 LFe II - bpym-Fe II L(NCX) 2 ] the order of energy levels is: (a) HH, LL(222); (b) LH, HH(68), LL(137); (c) LL, LH(12),