New neutral and ionic methyl and chloro palladium and platinum complexes containing hemilabile phosphorus-nitrogen ligands. Study of the insertion of carbon monoxide into the metal-methyl bond

Article abstract of DOI:10.1021/om00041a028

The synthesis and properties are described for the complexes (PAN)MCl₂ (M = Pd(II), Pt(II)), (PAN)₂PdCl₂, [(PAN)M(CH₃)]⁺Y^- (M = Pd(II); Y^- = Cl^-, SO₃CF₃^-, BF₄^-), and [(PAN)M(CH₃)(CH₃CN)]⁺Y^- (M = Pd(II), Pt(II); Y^- = SO₃CF₃^-, BF₄^-) and further (PC₃N)M(CH₃)Cl (M = Pd(II), Pt(II)) and [(PC₃N)Pd(CH₃)(CH₃CN)]⁺SO ₃CF₃^-, where PAN is the rigid ligand 1-(dimethylamino)-8-(diphenylphosphino)naphthalene and PC₃N the more flexible ligand 1-(dimethylamino)-3-(diphenylphosphino)propane. For (PAN)₂PdCl₂, in which PAN acts as a monodentate phosphorus-bonded ligand, and for (PAN)Pd(CH₃)Cl, in which PAN is a chelate, single-crystal X-ray structure determinations were carried out: (PAN)₂PdCl₂, space group P1, a = 12.837 (2) ?, b = 17.518 (3) ?, c = 10.788 (3) ?, α = 91.07 (2)°, β = 112.87 (2)°, γ = 100.27 (2)°, V = 2189.1 (6) ?³, Z = 2, R = 0.044, R_w = 0.065; (PAN)Pd(CH₃)Cl, space group P2₁/n, a = 11,418 (1) ?, b = 15.088 (1) ?, c = 12.434 (1) ?, β = 99.81 (1)°, V = 2110.7 (3) ?³, Z = 4, R = 0.025, R_w = 0.026. The investigations were mainly focused on the study of the carbonylation of the metal-carbon bond and in particular on the influence of the type of metal and ligand on the intimate steps of the mechanisms of the metal-acyl bond formation and on the magnitude of the rates. In the case of the reaction of (P-N)Pt(CH₃)Cl (CH₃ cis to P) evidence has been obtained for the formation of three intermediates with a terminal CO group in which the P-N (P-N = PAN, PC₃N) ligand temporarily functions as a monodentate phosphorus-bonded ligand, i.e. [trans P, cis CH₃ (Pt-CO)], [cis P, trans CH₃ (Pt-CO)], and [cis P, cis CH₃ (Pt-CO)], which for the first time were observed to occur together in one system. Subsequently the formation of the final metal-acyl product (P-N)Pt(C(O)CH₃)Cl (C(O)CH₃ cis to P) could be observed, in which the P-N ligand is again chelate-bonded. However, during the carbonylation of [(PAN)Pd(CH₃)]⁺Y^- and [(PAN)Pd(CH₃)(CH₃CN)]⁺Y^- (Y^- = SO₃CF₃^-, BF₄^-), only the intermediate [(PAN)Pd(CH₃)(CO)]⁺Y^- (CH₃ cis to P) could be detected, with the P-N ligand in the chelating form, before the formation of the acyl products [(PAN)Pd(C(O)CH₃)]⁺Y^- and [(PAN)Pd(C(O)CH₃)(CH₃CN)]⁺Y^-, in which the C(O)CH₃ group is cis to P. Several isomerization reactions are necessary to obtain these products. The reaction rates, in terms of half-life times t_1/2, were measured at 25 bar of CO in CD₂Cl₂ by means of NMR spectroscopy in a high-pressure tube and were compared with data for analogous bpy, dppp, and dppe complexes. The results show that generally the complexes with good leaving groups Y^- in noncoordinating solvents reacted much faster than the chloride complexes. The influence of the metal and the bidentate ligands on the insertion is reflected in a decrease in rates in the order Pd(II) > Pt(II) and bpy > PAN > PC₃N > dppp > dppe for (L-L)Pt(CH₃)Cl and bpy > dppp ?PAN > PC₃N > dppe for (L-L)Pd(CH₃)Cl, while for [(L-L)Pd(CH₃)(CH₃CN)]⁺SO₃CF ₃^- the rate decreased in the order bpy > dppp > PC₃N > PAN > dppe. Some general considerations are presented to rationalize the trends in the rates.