Synthesis and X-ray Structural Characterization of Two Unbridged Diplatinum(III) Compounds:  cis- and trans-Bis[bis(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 and −20 °C for cis- 2 and trans - 2, respectively), while at higher temperature (20 °C) they readily oxidize to Pt IV (τ 1/2 = 80 and 5 min for cis- 2 and trans - 2, respectively). The structures of the Pt III species have been solved by single-crystal X-ray diffraction. The compounds are neutral dimers composed of two square-planar PtCl 2 L 2 units perpendicularly connected by a Pt−Pt bond and capped by axially coordinated chloride ligands. Bond distances and angles within the equatorial PtCl 2 L 2 units are normal. The axial Pt−Cl bond lengths are ca. 0.14 Å longer than the equatorial ones, indicating a strong trans influence of the intermetallic bond. The intermetallic bond distances are typical of either unbridged or pyrophosphite four-bridged Pt III dimers, while they are much longer than those found in other Pt III complexes with either two (>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.