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Phospholes as ligands: Palladium(II) complexes of 1-substituted 3,4-dimethylphospholes. Structure and properties
MacDougall, J. Jeffrey,Nelson, John H.,Mathey, Francois,Mayerle, James J.
, p. 709 - 718 (2008/10/08)
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)°, ρcalcd = 1.48 g/cm3, and ρobsd = 1.48 (1) g/cm3. Least-squares refinement converged at final values of R = 0.042 and Rw = 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.
