NMR studies of phosphine [sbnd]nickel(0) complexes of ethyl methacrylate Ni(PR 3) 2(CH 2=C(CH 3)COOC 2H 5)

Abstract

13C NMR data are given for a series of phosphine nickel(0) complexes of ethyl methacrylate (ema), Ni(PR 3) 2(CH 2=C(CH 3)COOC 2H 5) (PR 3 = PPh 3 (Ia), PEtPh 2 (Ib), PEt 2Ph (Ic), PMe 2Ph (Id), PEt 3 (Ie)). The olefinic carbon signals of ema shift upfield by 71.5–86.5 ppm on coordination, the magnitude of the upfield shift increasing with increase in the bacisity of the phosphine ligand. The effect of the basicity of PR 3 is discussed on the basis of the back-bonding from Ni to ema. Variable temperature 1H NMR studies reveal that the ema of Id, the complex having the least sterically demanding phosphine ligands, exchanges with free ema in toluene on the NMR time scale. The dependence of the rate of exchange on the concentration of ema shows that the exchange proceeds through an S N2 mechanism. The activation parameters are: Δ H 273 ≠ 2.75 kcal/mol, Δ G 273 ≠ 12.7 kcal/mol, Δ S 273 ≠ −37 e.u. The 31P NMR spectra of the complexes show two doublets when the exchange is frozen out, indicating the inequivalence of the two phosphine ligands in the ema-coordinated complex. The difference in the 31P chemical shifts of the two coordinated tertiary phosphines increases with increase in the basicity of the PR 3 ligand.