Synthesis and X-ray Structural Characterization of Two Unbridged Diplatinum(III) Compounds:  cis- and trans-Bisbis(1-imino-1-methoxyethane)trichloroplatinum(III). Transient Species in the Oxidation of Platinum(II) to Platinum(IV)

Unbridged Pt III dimers, cis- and trans- PtCl 3 L 2 2 (L = ( E )-HN C(OMe)Me, cis- 2 and trans - 2 ), are formed, as transient species, in the oxidation with excess chlorine of the Pt II compounds cis- and trans- PtCl 2 L 2 ( cis- 1 and trans - 1 ) to the corresponding Pt IV species, cis- and trans- PtCl 4 L 2 ( cis- 3 and trans - 3 ). Complexes cis- 2 and trans - 2 are rather stable at low temperature (0.10 Å) or four bridging ligands (>0.20 Å). In cis - 2, the iminoether ligands are in equivalent pairs, and a 90° rotation of one platinum subunit with respect to the other about the Pt−Pt vector interchanges the ligand positions and converts the complex into its enantiomeric form. Both enantiomers are found in the crystal and in solution, where a Δ G * of 16.9 ± 0.2 kcal mol - 1 was calculated for the interconversion process ( 1 H NMR). The formation of intermediate Pt III dimers appears to be a relatively common, although overlooked, feature of platinum chemistry. The oxidation of Pt II to Pt IV requires the removal of two electrons by the oxidant (an acid), assisted by a ligand (a base) entering the coordination sphere from the opposite side. A second Pt II unit can act as a base contributing two d z 2 electrons and binding face-to-face from the axial direction. The resulting dimer completes the coordination sphere of the adjoined platinum unit by binding an extra ligand.