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)<sub>2</sub>] (<b>4</b>; mdt = methyldithiolene, [Me<sub>2</sub>C<sub>2</sub>S<sub>2</sub>]<sup><i>n</i>−</sup>), [Pt­(adt)<sub>2</sub>] (<b>5</b>; adt = <i>p</i>-anisyldithiolene, [(MeO-<i>p</i>-C<sub>6</sub>H<sub>4</sub>)<sub>2</sub>C<sub>2</sub>S<sub>2</sub>]<sup><i>n</i>−</sup>), and [Pd­(adt)<sub>2</sub>] (<b>10</b>) have been prepared in yields of ≥90% via transmetalation reactions with the corresponding [R<sub>2</sub>Sn­(S<sub>2</sub>C<sub>2</sub>R′<sub>2</sub>)] complexes (R = <sup><i>n</i></sup>Bu, R′ = Me; R = Me, R′ = −C<sub>6</sub>H<sub>4</sub>-<i>p</i>-OMe, <b>3</b>). Intraligand C–S and C–C<sub>chelate</sub> bond lengths (∼1.71 and ∼1.40 Å, respectively) obtained by X-ray crystallography show these compounds to be comprised of radical monoanions mdt<sup>•–</sup> and adt<sup>•–</sup>. The six-coordinate octahedral adducts [Pt­(adt)<sub>2</sub>(dppe)] [<b>6</b>; dppe = 1,2-bis­(diphenylphosphino)­ethane], <i>trans</i>-[Pt­(adt)<sub>2</sub>(PMe<sub>3</sub>)<sub>2</sub>] (<b>8</b>), and <i>trans</i>-[Pt­(mdt)<sub>2</sub>(PMe<sub>3</sub>)<sub>2</sub>] (<b>9</b>) have also been prepared, and crystal structures reveal dithiolene ligands that are fully reduced ene-1,2-dithiolates (C–S and C–C<sub>chelate</sub> = ∼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 <b>5</b> shows two fully reversible reductions at −0.11 and −0.84 V in CH<sub>2</sub>Cl<sub>2</sub> (vs Ag/AgCl), attributed to successive (adt<sup>•–</sup> + e<sup>–</sup> → adt<sup>2–</sup>) processes, and a reversible oxidation at +1.01 V. The cyclic voltammogram of <b>9</b> shows two reversible oxidations at +0.38 and +0.86 V, which are assigned as successive (adt<sup>2–</sup> → adt<sup>•–</sup> + e<sup>–</sup>) oxidations. Consistent with their formulation as having fully reduced dithiolene ligands, the UV–vis spectra for <b>6</b>, <b>8</b>, and <b>9</b> show no low-energy absorptions below 700 nm, and the S K-edge XAS spectra of <b>6</b> and <b>8</b> show dithiolene sulfur that is reduced relative to that in <b>5</b>. The introduction of PMe<sub>3</sub> to <b>10</b> did not produce the palladium analogue of <b>8</b> but rather [Pd­(adt)­(PMe<sub>3</sub>)<sub>2</sub>] (<b>11</b>). The reaction of [PdCl<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] with Li<sub>2</sub>(mdt) produced a mixture of [Pd­(mdt)­(PPh<sub>3</sub>)<sub>2</sub>] (<b>12</b>, 20%) and [(Ph<sub>3</sub>P)­Pd­(μ-1,2-mdt-<i>S</i>,<i>S</i>′:<i>S</i>)<sub>2</sub>Pd­(PPh<sub>3</sub>)] (<b>13</b>, 28%), with the latter having <i>C</i><sub>2</sub> symmetry with a Pd<sub>2</sub>S<sub>2</sub> core structure folded along the S···S axis.