A stable five-coordinate platinum(IV) alkyl complex.

Coordinatively and electronically unsaturated species are shortlived intermediates in a large number of reactions involving late transition metal complexes.1 These species containing an “opensite” in the metal coordination sphere are notoriously difficult to characterize, owing to their inherent reactivity.2 In particular, fivecoordinate Pt(IV) alkyl species have been consistently proposed as key intermediates in reductive elimination/oxidative addition reactions to form or break C-C, C-H, C-O, and C-I bonds at Pt(IV)/Pt(II), but such species have never been isolated or unambiguously characterized.3,4 The intermediacy of these fivecoordinate complexes in bondbreaking and -making reactions of model Pt complexes has significant implications for their involvement in platinum-catalyzed selective alkane functionalization reactions.5 While it has been possible to calculate the geometry of five-coordinate Pt(IV) alkyl intermediates using quantum chemical methods,6 attempts to generate such species experimentally have invariably led to the observation that weakly coordinating anions or solvent molecules bind to the metal such that the examination of the truly coordinatively unsaturated species was not possible.7,8b No experimental evidence to establish the geometry of the five-coordinate Pt(IV) alkyl complexes has been reported.8 We report herein the first example of an isolable and crystallographically characterized five-coordinate Pt(IV) alkyl complex. The five-coordinate Pt(IV) complex, (L)PtMe3 (1), where Lis the anionic â-diimine ligand [{(o-Pr2C6H3)NC(CH3)}2CH] has been synthesized and fully characterized.9,10 Compound 1 was prepared in good yield (75%) from the potassium salt of the ligand11 and tetrameric trimethyl platinum(IV) triflate12 in dry pentane solution. X-ray quality crystals of (L)PtMe3 were obtained by reducing the volume of the pentane solution in vacuo. An ORTEP diagram of 1 is shown in Figure 1, with selected structural parameters contained in the figure legend. While coordination geometries ranging from square pyramidal to trigonal bipyramidal are available for a five-coordinate complex, the geometry about the platinum in 1 is clearly a square pyramid. The Pt atom deviates only 0.099(3) Å toward C2 from the perfect N1-N1′-C1-C1′ plane. Nothing is coordinating to the open site on the Pt atom. No solvent molecules are found in the crystal structure. The platinum center of the next molecule in the crystal lattice is 7.1 Å away. The closest approach of nonbonded groups to the open site are the isopropyl methine hydrogens at 3.0 Å. This is the same distance observed in the related Pt(II) complex 2 discussed below. In solution, no 195Pt satellites are seen for the 1H NMR signals of the isopropyl hydrogens. Thus, there is no indication of agostic interactions between the isopropyl moieties on the ligand and the metal. It is interesting that the methyl group trans to the open site shows only a slightly shorter Pt-C distance compared to methyl trans to nitrogen (2.038(7) versus 2.056(4) Å). A larger difference might have been expected based on trans influence arguments.13,14 The NMR spectral data9 impressively demonstrate the high solution-phase fluxionality of this five-coordinate complex. The apparent symmetry of the molecule is greater in solution than in the solid-state structure. The isopropyl groups above and below the NCCCNPt plane are equivalent by NMR. The same is observed for the aromatic hydrogens, and this pseudo-C2ν sym-