Investigation of transition metal-imido bonding in M(NBut)2(dpma)

A complete series down group 6 of the formula M(NBu(t))(2)(dpma) has been synthesized, where dpma is N,N-di(pyrrolyl-alpha-methyl)-N-methylamine. A fourth complex, Mo(NAr)(2)(dpma) (4), was also prepared, where Ar is 2,6-diisopropylphenyl. All four of these complexes display geometries in the solid state best described as square pyramidal with one imido ligand occupying the axial position and the other an equatorial site. In all cases, the axial imido ligand has a significantly smaller M-N(imido)-C bond angle with respect to the equatorial multiple-bond substituent. From the (1)H, (13)C, and (14)N NMR spectra, the axial (bent) imido appears to be more electron-rich than the equatorial and linear imido, with the differences becoming less pronounced down the column. The angular deformation energies for the axial imido ligands were studied by DFT in order to discern if and to what extent imido bond angles were important energetically. The electronic energies associated with straightening the axial imido ligand, while holding the remainder of the molecule at the ground-state geometry, for the Cr, Mo, and W derivatives were calculated as 4.5, 2.7, and 2.0 kcal/mol, respectively. A straight-line plot is found for deformation energies versus estimated electronegativity of the group 6 metals in the +6 oxidation state. The study suggests that the electronic differences between metal imido ligands of different angles are quite small; however, the effects may be more pronounced for metal centers with higher electronegativity, e.g. Cr(VI) with electron-withdrawing ligands.