N-Acyclic carbene complexes supported by meso-Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2 (meso-dpmppm) as an asymmetric pincer ligand

Abstract

Reaction of [PtCl2(cod)] with a tetraphosphine meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (meso-dpmppm), afforded mononuclear PtII complexes, [PtCl(meso-dpmppm−κ3)]X (X = Cl (1a), PF6 (1b)); the meso-dpmppm ligand coordinates to the PtII ion tightly with two outer and one inner phosphorus atoms to form fused six- and four-membered chelate rings which is regarded as an asymmetric PPP pincer ligand bearing an uncoordinate inner phosphine unit. Complex 1 readily reacted with RNC in the presence of NH4PF6 to afford [Pt(meso-dpmppm−κ3)(RNC)](PF6)2 (R = Xyl (2a), Cy (2b), tBu (2c)). When 2a and 2b were reacted with excess of benzylamine (BnNH2), N-acyclic carbene complexes, [Pt(meso-dpmppm−κ3){C(NHR)(NHBn)}](PF6)2 (R = Xyl (3a), Cy (3c)), were obtained, and a similar treatment of 2a with n-octylamine (C8H17NH2) afforded [Pt(meso-dpmppm−κ3)- {C(NHXyl)(NHC8H17)}](PF6)2 (3b). In contrast, complex 2c was transformed into a cyanide complex, [Pt(CN)(meso-dpmppm−κ3)]PF6 (4), through NC(tBu) bond cleavage when heated at 80 °C with BnNH2 or PhNH2. Complexes 1–4 were characterized by 1H and 31P NMR and ESI-MS spectroscopies and X-ray diffraction analyses. The uncoordinate inner phosphine of 1b is readily reacted with [Cp*MCl2]2 to give heterodimetallic complexes, [PtCl(η5-Cp*MCl2)(μ−meso-dpmppm−κ3,κ1)]PF6 (M = Ir (5a), Rh (5b); Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl). Complex 2a also reacted with [Cp*IrCl2]2 to yield [Pt(η5-Cp*IrCl2)(μ−meso-dpmppm−κ3,κ1)(XylNC)]PF6 (6a) together with 5a, and however, 6a would not react with BnNH2, just releasing XylNC to result in 5a. Attachment of the metal fragment of {Cp*IrCl2} to the uncoordinated phosphine caused a crucial conformational change of the six-membered chelate ring from a stable chair conformation to a twist-boat structure, which concomitantly reduced reactivity of the isocyanide ligand toward nucleophilic attack of the amine by steric hindrance of meso-dpmppm pincer ligand. These results could be recognized as on/off switching of the asymmetric {PtII(meso-dpmppm−κ3)} pincer complex.