Synthesis, Reactivity, and Structural Studies in Trimethylpalladium(IV) Chemistry, Including PdIMe3(bpy) and [MMe3((pz)3CH)]+ (M = Pd, Pt)

Abstract

The first synthetic, spectroscopic, and structural studies in (hydrocarbyl)palladium(IV) chemistry are reported. The fac-Pd(IV)Me3 complexes PdIMe3(L2) and [PdMe3(L3)]I are formed on oxidative addition of iodomethane to PdMez2(L2) (L2 = 2,2'-bipyridyl, 1,l0-phenanthroline, other planar bidentates with pyridin-2-yl (py) and 1-methylimidazol-2-y1 (mim) groups) and PdMe2(L3) (L3 = tripodal nitrogen donors containing pyrazol-1-yl (pz), py, and mim groups). The complexes PdIMe3(L2) (L2 = bpy, phen) are stable on storage at ca. -20 C, but in solution reductive elimination of ethane occurs with formation of PdIMe(L2). Palladium(IV) complexes with the other bidentate ligands could be detected in situ by 1H NMR spectroscopy but could not be isolated. 1H NMR studies in CD3CN show that the bidentate ligand complexes form an equilibrium between PdIMe3(L2) and [PdMe3(L2)(CD3CN)]+ and oxidative addition of CD3I or Me1 to PdMe2(bpy) in (CD3)2C0 at low temperature indicates that a cation is formed prior to formation of PdIMe2R(bpy). The complex PdIMe3(bpy) has octahedral geometry, with Pd-I = 2.834 (1) A, Pd-N = 2.188 (5), 2.173 (5) A, and Pd-C = 2.034 (7)-2.046 (7) A. The isolated complexes of the tripod ligands, [PdMe3(L3)]I, are stable at ambient temperature. In solution, the complexes containing pyridine and 1-methylimidazole donor rings as part of the tripod ligand, e.g. [PdMe3((py)(mim)2CH)]I, are more stable than [PdMe3((pz)3CH)]I, although the tetrafluoroborate derivative [PdMe3((pz)3CH)]BF4 appears to be indefinitely stable. The isostructural complexes [MMe3((pz)3CH)]I have essentially identical M-C bond distances, but the Pd-N distances are ca. 0.04 A longer than Pt-N. Crystals of PdIMe3(bpy) are monoclinic, space group P2,/c, with a = 7.917(4) A, b = 9.528(4) A, c = 20.207(8) A, beta = 104.28(4), and Z = 4. The complexes [MMe3((pz)3CH)]I (M = Pd, Pt) are monoclinic, space group C2/c, with Z = 8. Palladium [platinum] complex cells have a = 21.254(8) [21.253(8) A, b = 9.213(5) [9.099(4) A, c = 19.144(8) [19.284(8) A, and beta = 105.65(3) [105.63(3)].