Acetate-Bridged Platinum(III) Complexes Derived from Cisplatin

Oxidation of the acetate-bridged half-lantern platinum­(II)
complex cis-[PtII(NH3)2(μ-OAc)2PtII(NH3)2]­(NO3)2, [1]­(NO3)2, with
iodobenzene dichloride or bromine generates the halide-capped platinum­(III)
species cis-[XPtIII(NH3)2(μ-OAc)2PtIII(NH3)2X]­(NO3)2, where X is Cl in [2]­(NO3)2 or Br in [3]­(NO3)2, respectively. These three complexes, characterized
structurally by X-ray crystallography, feature short (≈2.6
Å) Pt–Pt separations, consistent with formation of a formal
metal–metal bond upon oxidation. Elongated axial Pt–X
distances occur, reflecting the strong trans influence of the metal–metal
bond. The three structures are compared to those of other known dinuclear
platinum complexes. A combination of 1H, 13C, 14N, and 195Pt NMR spectroscopy was used to characterize
[1]2+–[3]2+ in solution. All resonances shift downfield upon oxidation of [1]2+ to [2]2+ and [3]2+. For the platinum­(III) complexes, the 14N and 195Pt resonances exhibit decreased line
widths by comparison to those of [1]2+. Density
functional theory calculations suggest that the decrease in the 14N line width arises from a diminished electric field gradient
at the 14N nuclei in the higher valent compounds. The oxidation
of [1]­(NO3)2 with the alternative
oxidizing agent bis­(trifluoroacetoxy)­iodobenzene affords the novel
tetranuclear complex cis-[(O2CCF3)­PtIII(NH3)2(μ-OAc)2PtIII(NH3)­(μ-NH2)]2(NO3)4, [4]­(NO3)4, also characterized structurally by X-ray crystallography.
In solution, this complex exists as a mixture of species, the identities
of which are proposed.