Alkynyl-Phosphine Substituted Fe₂S₂ Clusters: Synthesis, Structure and Spectroelectrochemical Characterization of a Cluster with a Class III Mixed-Valence [FeFe]³⁺ Core

Phosphinoalkynes P(C≡CC6H4Me-4)Ph2 (1) and P(C≡CC6H4C≡CC6H4Me-4)Ph2 (2) have been prepared from CuI catalysed reactions of the corresponding 1-alkyne and PClPh2. The trimethylamine-N-oxide promoted reaction of PPh3, 1 or 2 with [Fe2(μ-pdt)(CO)6] (pdt = propanedithiolate) affords derivatives [Fe2(μ-pdt)(CO)5{PRPh2}] [R=Ph (3), C≡CC6H4Me (5), C≡CC6H4C≡CC6H4Me (6)] or, at elevated temperatures, [Fe2(μ-pdt)(CO)4(PPh3)2] (4). The cyclic voltammograms of compounds 3 and 4 feature almost fully reversible one-electron oxidation processes and an irreversible reduction, whilst the electrochemical response of the alkynyl phosphine substituted complexes 5 and 6 is irreversible for both oxidation and reduction. IR spectroelectrochemical studies of 4 are consistent with an oxidation processes leading to a delocalized or (Class III) mixed valence [FeFe]3+ core in which the iron centers have an average oxidation state of 1.5. The molecular structures of the alkynyl phosphine substituted clusters 5 and 6 are also reported.