The dimerisation of carbon disulfide on Fe centres. Some new tetrathiolene complexes of iron

Abstract

The thermolysis of [Fe(L)₂(CO)₂(η²-CS₂)] complexes (1) in refluxing benzene gives the blue tetrathiolene derivatives [Fe₂(L)₄(CO)₂(C₂S₄)] (2) when L = P(OPh)₃, P(OEt)₃, P(OPrⁱ)₃, PPh(OEt)₂, PPh₂(OEt). 2 are formed only when the phosphorus(III) ligands L readily dissociate from 1 and have cone angles between 109° and 133°. The two most stable are 2a {L = P(OPh)₃, cone angle 128°} and 2c {L = P(OPrⁱ)₃, cone angle 130°}. 2 have been characterised by IR, electronic and NMR spectroscopy and elemental analyses. Their UV-Vis spectra are dominated by an intense absorption band (ε = 13000–18000 dm³ mol⁻¹ cm⁻¹) with λ_max = 640–690 nm. The labile triphenylphosphite ligands in 2a can be replaced in part by CO. Spectroscopic data confirms that the unstable product, 3a, contains a ligand set similar to that of 2a. Electrochemical studies show that 2a undergoes a one-electron reversible reduction to [2a]˙⁻ and a one-electron, irreversible oxidation. The UV-Vis spectrum of [2a]˙⁻ shows a very low energy absorption band at 2556 nm (3912 cm⁻¹) with ε = ca. 3600 dm³ mol⁻¹ cm⁻¹ which is attributed to a charge transfer transition from the SOMO to an orbital with μ-C₂S₄ character. The ESR spectrum of [2a]˙⁻ is consistent with the SOMO being an orbital of primarily d_{x² − y²} character which is localised on one Fe atom. Under CO, one-electron reversible transfers are observed in the electrochemistry due to the redox series [3a]^−/0/+. 2a is oxidised by halogens to [L₂(OC)(X)Fe(S₂C₂S₂)Fe(X)(CO)L₂] derivatives.