Preparation, via double oxidative addition, and characterization of bimetallic platinum and palladium complexes: Unique building blocks for supramolecular macrocycles. C-13 NMR analysis of the nature of the palladium-carbon bond

The high-yield preparation, by double oxidative addition, of nine novel platinum and palladium bis(trans-M(PR3)(2)X)aryl (M = Pt or Pd; R = PPh3 or PEt3; X = Br or I; aryl = 1,4-benzene, 4,4'-biphenyl, 4,4''-ter-p-phenyl, 4,4'-tolane, or 4,4'-benzophenone) complexes from the reaction of Pt(PPh3)4, Pt(PEt3)(4), or Pd(PPh3)(4) with the respective dihalo aromatic in toluene is described. These complexes were fully characterized by elemental analysis, mass spectrometry, and NMR (H-1, C-13{H-1} and P-31{H-1}) and vibrational (IR or Raman) spectroscopies. The single-crystal molecular structure of 4,4'-bis(trans -Pt(PEt3)(2)I)biphenyl (2a) was determined by X-ray crystallography. The key structural feature of this complex is the dihedral angle of 18.9 degrees between the two planes defined by the phenyl groups of the biphenyl linkage. The nature of the palladium-carbon bond is investigated by C-13{H-1) NMR spectroscopy; Taft's sigma(R) parameter is found to correlate in a linear fashion with [delta(C-ipso) - delta(C-0)] for these palladium complexes. These data indicate the C-13 chemical shift of C-ipso is linearly related to the amount of pi-electron density of the carbon bound to the palladium center. The potential utility of these bimetallic platinum and palladium complexes as subunits in the generation of organometallic macrocycles is described.