Pyrrole-Based New Diphosphines: Pd and Ni Complexes Bearing the PNP Pincer Ligand

A new class of diphosphine PNP pincer ligand, 2,5-bis(diphenylphosphinomethyl)pyrrole 2, was synthesized by the reaction between Ph2PH and 2,5-bis(dimethylaminomethyl)pyrrole in 90% yield. The analogous reaction of Ph2PH with 1,9-bis(dimethylaminomethyl)diphenyldipyrrolylmethane readily afforded a PNNP type diphosphine ligand, 1,9-bis(diphenylphosphinomethyl) diphenyldipyrrolylmethane 5 in 92% yield. These phosphine compounds were oxidized with H2O2 and S-8 to give the corresponding phosphoryl and thiophosphoryl compounds 6-9 in very good yields. The reaction of the PNP pincer ligand 2 with [PdCl2(PhCN)(2)] in the presence of Et3N afforded the mononuclear Pd(II) complex, [PdCl{C4H2N-2,5-(CH2PPh2)(2)-kappa(PNP)-P-3}] 10 in 87% yield. Conversely, treatment of 2 with [PdCl2(PhCN)(2)] in the absence of Et3N gave the dinuclear Pd(II) complex [Pd2Cl4{mu-C4H3N-2,5-(CH2PPh2)(2)-kappa(PP)-P-2}(2)], the structure which is proposed based on the spectroscopic data. When 2 was treated with Pd(0) precursor [Pd-2(dba)(3)]center dot CHCl3 the dinuclear Pd(I) complex [Pd-2{mu-C4H2N-2,5-(CH2PPh2)(2)-kappa(PN)-P-2,kappa P-1}(2)], 12, was obtained in 23% yield. The formation of complex 12 is solvent dependent, which transforms into complex 10 in CDCl3 as studied by Variable temperature H-1 and P-31 NMR methods. Treatment of 2 with [Ni(OAc)(2)]center dot 4H(2)O gave the mononuclear Ni(II) pincer complex [Ni(OAc){C4H2N-2,5-(CH2PPh2)(2)-kappa(PNP)-P-3}], 13, which upon treatment with an excess of LiCl or LiBr or KI afforded the respective halide ion substituted Ni(II) complexes, [NiX{C4H2N-2,5-(CH2PPh2)(2)-kappa(PNP)-P-3}] (X = Cl, Br, and I), 14-16, in very good yields. The structures of 5, 2,5-bis(diphenylphosphorylmethyl)pyrrole 6, 10, 12, and 14-16 were determined by the single crystal X-ray diffraction method. In the structure of 12, two short contacts between the diagonally positioned Pd and P atoms are observed. To understand these weak interactions, density functional theory (DFT) calculations were done and an interaction MO diagram is presented.