Synthesis and characterization of palladium(II)-phosphole and -biphosphole complexes. Regulation of the homoleptic coordination environment of square-planar palladium(II)

A series of novel square-planar palladium(II)-phosphole complexes was synthesized in good yields by a ligand-substitution reaction of [Pd(CH3CN)(4)](BF4)(2) with 3,4-dimethyl-1-phenylphosphole (dmpp) and/or 3,3'4,4'-tetramethyl-1,1'-diphenyl-2,2'-biphosphole (bidmpp) in CH2Cl2; [Pd(dmpp)(4)](BF4)(2) (1), [Pd(bidmpp)(dmpp)(2)](BF4)(2) (2), and [Pd(bidmpp)(2)](BF4)(2) (3). Those complexes were characterized by H-1 and P-31 NMR spectroscopies and X-ray crystallography for 3. The crystal structure of 3 revealed that the configurations of the phosphorus atoms were (R, R, S, S), indicating that the structure was a meso form with an intramolecular pi-pi interaction between the phenyl groups. H-1 NMR spectroscopy suggested the structures of 1 and 2: Complex 1 has a water wheel-like C-4h structure and complex 2 is C-2 symmetric. In the P-31 NMR spectra of those complexes, each resonance showed a smaller downfield shift than that observed for [Pd(dppe)(2)]Cl-2 (dppe = 1,2-bis(diphenylphosphino)ethane), indicating that the palladium-phosphole interaction is governed by mainly a sigma-donation, and that there is a small contribution from a pi-back bonding interaction.