Towards molecular conductors with a spin-crossover phenomenon:: Crystal structures, magnetic properties and Mossbauer spectra of [Fe(salten)Mepepy][M(dmit)2] complexes

Three new iron(iii) compounds of formula [Fe(salten)Mepepy)[M(dmit)(2)]-CH3CN (M = Ni, Pd, Pt; H(2)salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate); Mepepy = 1(pyridin-4-yl)-2-(N-methylpyrrol-2-yl) ethane; dmit(2)- = 1,3dithiole-2-thione-4,5-dithiolato) have been synthesised and the crystal structure of each compound has been solved at different temperatures. The structures consist of alternating layers of [M(dmit)21- units and [Fe(salten)Mepepy] cations. In the Ni compound photo-isomerisation of the Mepepy ligand can be observed in dichloromethane solution. The temperature dependence of the magnetic susceptibility of the compounds reveals a gradual S = 5/2 reversible arrow S = 1/2 spin crossover of the Fe-III ions. This phenomenon is confirmed by Mossbauer spectroscopy for the Ni compound, which also reveals a magnetic component at 4.5 K with a hyperfine magnetic field, Hi, of 442(3) Oe. Theoretical analysis of the thermal variation of the high-spin proportion allows us to calculate the thermodynamical parameters of the spin-crossover behaviour in this compound: energy-gap (Delta) = 1536 cm(-1), entropy change (Delta S) = 33 JK(-1) mol(-1) and a cooperative interaction, J, of 0 cm(-1) (non-cooperative spin crossover). The fractional oxidation state complex [Fe(salten)Mepepy][Ni(dmit)(2)](3) has been obtained after electrooxidation of [Fe(salten)Mepepy]-[Ni(dmit)(2)] in acetonitrile. It exhibits a gradual spin conversion coupled with antiferromagnetic interactions. Its room temperature electrical conductivity is 0.1 S cm(-1).