Synthesis and X-ray Structural Characterization of Two Unbridged Diplatinum(III) Compounds: <i>cis-</i> and <i>trans-</i>Bis[bis(1-imino-1-methoxyethane)trichloroplatinum(III)]. Transient Species in the Oxidation of Platinum(II) to Platinum(IV)

Unbridged PtIII dimers, cis- and trans-[PtCl3L2]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 PtII compounds cis- and trans-[PtCl2L2] (cis-1 and trans-1) to the corresponding PtIV species, cis- and trans-[PtCl4L2] (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 PtIV (τ1/2 = 80 and 5 min for cis-2 and trans-2, respectively). The structures of the PtIII species have been solved by single-crystal X-ray diffraction. The compounds are neutral dimers composed of two square-planar PtCl2L2 units perpendicularly connected by a Pt−Pt bond and capped by axially coordinated chloride ligands. Bond distances and angles within the equatorial PtCl2L2 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 PtIII dimers, while they are much longer than those found in other PtIII 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 ΔG* of 16.9 ± 0.2 kcal mol-1 was calculated for the interconversion process (1H NMR). The formation of intermediate PtIII dimers appears to be a relatively common, although overlooked, feature of platinum chemistry. The oxidation of PtII to PtIV 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 PtII unit can act as a base contributing two dz2 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.