The Lewis Basicity of Diaminocarbene – A Theoretical Study of Donor–Acceptor Complexes of C(NH2)2, NH3 and CO with the Lewis Acids EF3, ECl3 (E = B, Al, Ga, In), TiF4 and TiCl4

Abstract

Quantum chemical calculations at the MP2 level using large valence basis sets up to TZ+2P quality have been carried out in order to predict the geometries and bond energies of the title compounds. The nature of the donor–acceptor bond has also been investigated. The calculations show clearly that diaminocarbenes are much stronger Lewis bases than amines. The complexation energies of C(NH₂)₂ have been calculated to be 14–27 kcal/mol higher than those of NH₃. The most strongly bonded complex is Cl₃Al–C(NH₂)₂, which has a theoretically predicted Al–C bond energy D_o = 59.1 kcal/mol. In all the complexes, the strength of the Lewis bases is C(NH₂)₂ > NH₃ > CO, but the ordering of Lewis acid strength of EX₃ depends on the coordinated Lewis base. TiF₄ and TiCl₄ have similar Lewis acidities as BF₃, but the titanium tetrahalides may bind one or two donor molecules with almost the same bond strength. The investigated donor–acceptor bonds have a high degree of ionic character. The largest covalent contributions are found for the diaminocarbene complexes. The covalent character of the X₃E–CO bond increases on going from E = boron to the heavier Group 13 elements, while the opposite order is found for the X₃E–NH₃ and X₃E–C(NH₂)₂ bonds.