Towards Molecular Conductors with a Spin‐Crossover Phenomenon:Crystal Structures, Magnetic Properties and Mössbauer Spectra of[Fe(salten)Mepepy][M(dmit) 2 ] Complexes

Three new iron( III ) compounds of formula [Fe(salten)Mepepy][M(dmit) 2 ] · CH 3 CN (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– = 1,3‐dithiole‐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) 2 ] – 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 blabla S = 1/2 spin crossover of the Fe III ions. This phenomenon is confirmed by Mössbauer spectroscopy for the Ni compound, which also reveals a magnetic component at 4.5 K with a hyperfine magnetic field, H i , 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 ( Δ ) = 1536 cm –1 , entropy change (Δ S ) = 33 J K –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 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)