Reversible, Electrochemically Controlled Binding of Phosphine to Iron and Cobalt Bis(dithiolene) Complexes

The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M = Fe,
Co; R = p-anisyl) undergo two successive reductions to form
anions that display [M(S2C2R2)2]22- ↔ 2[M(S2C2R2)2]1- solution
equilibria. The neutral dimers react with Ph3P to form square
pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon
[M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at
negative potentials, consistent with Ph3P dissociation upon reduction. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)(S2C2R2)2]0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of
reversible, electrochemically controlled binding of Ph3P to
[M(S2C2R2)2]2 (M = Fe, Co; R = p-anisyl). All members of the
cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-,
[M(Ph3P)(S2C2R2)2]) for M = Fe, Co have been characterized by
crystallography. Square planar [Fe(S2C2R2)2]1- is the first such
iron dithiolene species to be structurally identified and reveals Fe−S
bond distances of 2.172(1) and 2.179(1) Å, which are appreciably
shorter than those in corresponding square planar dianions.