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 S8 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-κ3PNP}] 10 in 87%
yield. Conversely, treatment of 2 with [PdCl2(PhCN)2] in the absence of Et3N gave the dinuclear
Pd­(II) complex [Pd2Cl4{μ-C4H3N-2,5-(CH2PPh2)2-κ2PP}2], the structure which is
proposed based on the spectroscopic data. When 2 was
treated with Pd(0) precursor [Pd2(dba)3]·CHCl3 the dinuclear Pd­(I) complex [Pd2{μ-C4H2N-2,5-(CH2PPh2)2-κ2PN,κ1P}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 1H and 31P NMR methods. Treatment of 2 with [Ni­(OAc)2]·4H2O gave the mononuclear Ni­(II)
pincer complex [Ni­(OAc)­{C4H2N-2,5-(CH2PPh2)2-κ3PNP}], 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-κ3PNP}] (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.