Phospholes as ligands: Palladium(II) complexes of 1-substituted 3,4-dimethylphospholes. Structure and properties

Article abstract of DOI:10.1021/ic50205a026

A series of palladium(II) complexes of the type L₂PdX₂ (L = 1-R-3,4-dimethylphosphole; R = -CH₃, -n-C₄H₉, -t-C₄H₉, -C₆H₅, -CH₂C₆H₅; X = Cl^-, Br^-, N₃^-) have been prepared and characterized by elemental analyses, physical properties, conductance measurements, infrared and electronic spectroscopy, and ¹H, ³¹C{¹H}, and ³¹P{¹H} NMR spectroscopy. The complexes all appear to be cis in the solid state. Several isomerize at room temperature to equilibrium mixtures of cis and trans isomers in chloroform solution. The ³¹P coordination chemical shifts for the complexes behave in a fashion similar to that of phosphine complexes. The crystal structure of cis-dichlorobis(1-phenyl-3,4-dimethylphosphole)palladium(II) was determined from three-dimensional X-ray diffraction techniques. The molecule crystallizes in the monoclinic space group Cc in a unit cell of dimensions a = 10.427 (4) ?, b = 15.070 (6) ?, c = 15.910 (6) ?, β = 92.88 (2)°, ρ_calcd = 1.48 g/cm³, and ρ_obsd = 1.48 (1) g/cm³. Least-squares refinement converged at final values of R = 0.042 and R_w = 0.047. The phosphole ring is planar, indicating the existence of some intracyclic electron delocalization within the complexed phosphole. Short Pd-P (2.241 ? average) and Pd-Cl (2.353 ? average) bond distances indicate strong Pd-P bonds containing some π component. All the data suggest that Pd-phosphole bond strengths are somewhat greater than the Pd-P bond strengths in structurally similar Pd-phosphine complexes. Palladium-phosphole bonds are strong due to three contributing factors: (a) the phosphole does not undergo structural reorganization upon coordination, (b) phospholes are sterically undemanding, and (c) phospholes have a moderate π-acceptor capacity.