Effect of N-aryl substituents on the reactivity of electron-rich olefins as precursors of transition-metal–carbene complexes. The crystal and molecular structure of NN′N″N‴-tetraphenylbis(1,3-imidazolidin-2-ylidene), and its π-electron distribution

Abstract

The title compound (L^Ph₂) crystallises in space group P2₁/c with a= 10.854(8), b= 13.258(8), c= 8.225(6)Å, β= 98.19(5)°, and Z= 2, and the structure has been refined to R= 0.050 based on 1 233 counter reflections. The molecule has a non-planar geometry of approximate C_2h point-group symmetry which minimises steric strain between vicinal phenyl rings. The π-electron distribution in the molecule as calculated by Hückel theory shows that, relative to an N-methyl-substituted electron-rich olefin (L^Me₂) of similar geometry, the phenyl groups in L^Ph₂ have increased the positive π-electron charge on the nitrogen atoms. This supports the view that the reduced reactivity of L^Ph₂ relative to L^Me₂ in reactions with transition-metal complexes to form carbene complexes is due to inhibition of an initial rate-determining step of attack by the metal at one of the nitrogen atoms of the electron-rich olefin.