A Structural and Spectroscopic Investigation of Octahedral Platinum Bis(dithiolene)phosphine Complexes: Platinum Dithiolene Internal Redox Chemistry Induced by Phosphine Association

The complexes [Pt­(mdt)2] (4; mdt = methyldithiolene, [Me2C2S2]n−), [Pt­(adt)2] (5; adt = p-anisyldithiolene, [(MeO-p-C6H4)2C2S2]n−), and [Pd­(adt)2] (10) have been prepared in yields of ≥90% via transmetalation reactions with the corresponding [R2Sn­(S2C2R′2)] complexes (R = nBu, R′ = Me; R = Me, R′ = −C6H4-p-OMe, 3). Intraligand C–S and C–Cchelate bond lengths (∼1.71 and ∼1.40 Å, respectively) obtained by X-ray crystallography show these compounds to be comprised of radical monoanions mdt•– and adt•–. The six-coordinate octahedral adducts [Pt­(adt)2(dppe)] [6; dppe = 1,2-bis­(diphenylphosphino)­ethane], trans-[Pt­(adt)2(PMe3)2] (8), and trans-[Pt­(mdt)2(PMe3)2] (9) have also been prepared, and crystal structures reveal dithiolene ligands that are fully reduced ene-1,2-dithiolates (C–S and C–Cchelate = ∼1.77 and 1.35 Å, respectively). Reduction of the dithiolene ligand thus occurs to accommodate the +IV oxidation state typical of octahedral six-coordinate platinum. The cyclic voltammogram of 5 shows two fully reversible reductions at −0.11 and −0.84 V in CH2Cl2 (vs Ag/AgCl), attributed to successive (adt•– + e– → adt2–) processes, and a reversible oxidation at +1.01 V. The cyclic voltammogram of 9 shows two reversible oxidations at +0.38 and +0.86 V, which are assigned as successive (adt2– → adt•– + e–) oxidations. Consistent with their formulation as having fully reduced dithiolene ligands, the UV–vis spectra for 6, 8, and 9 show no low-energy absorptions below 700 nm, and the S K-edge XAS spectra of 6 and 8 show dithiolene sulfur that is reduced relative to that in 5. The introduction of PMe3 to 10 did not produce the palladium analogue of 8 but rather [Pd­(adt)­(PMe3)2] (11). The reaction of [PdCl2(PPh3)2] with Li2(mdt) produced a mixture of [Pd­(mdt)­(PPh3)2] (12, 20%) and [(Ph3P)­Pd­(μ-1,2-mdt-S,S′:S)2Pd­(PPh3)] (13, 28%), with the latter having C2 symmetry with a Pd2S2 core structure folded along the S···S axis.