Strong Lewis acids derived from molybdenum and tungsten nitrosyls containing the tri-2-pyridylmethane ligand. Dynamic NMR studies of their adducts with aldehydes, ketones, and esters

Article abstract of DOI:10.1021/ja00005a021

The doubly charged Lewis acid percursors [HC(py)₃M(NO)₂(CO)](SbF₆)₂ (M = Mo, W; HC(py)₃ = tri-2-pyridylmethane) are conveniently synthesized by reaction of HC(py)₃M(CO)₃ and 2 equiv of NOSbF₆. Facile loss of CO from the precursors generates the [HC(py)₃M(NO)₂](SbF₆)₂ Lewis acids. The Lewis acidity of the tungsten complex is greater than that of the molybdenum complex. With the ¹H NMR chemical shifts of bound crotonaldehyde as a qualitative assessment of relative acidity, the acidity of the tungsten species is comparable to that of BF₃ and AlCl₃, while that of the molybdenum species is similar to that of TiCl₄. Analysis of the NMR spectra of the Lewis acid-organic carbonyl base adducts, which include the adducts of aldehydes, ketones, and esters, showed that η¹-M(O=C) interactions dominate the chemistry. The barriers of rotation about the aldehyde C₁-C₂ bonds in the p-anisaldehyde adducts of the molybdenum and tungsten species were measured to be 12.8 and 13.7 kcal/mol, respectively, which are significantly higher than that for the free p-anisaldehyde. The exchange behavior between the E and Z isomers of the acetate adducts could be observed on the NMR time scale. The E to Z interconversion barriers of 12.2 ± 0.1 and 12.3 ± 0.1 kcal/mol for the methyl acetate and ethyl acetate complexes, respectively, were calculated from the results of variable-temperature proton NMR experiments. The free energy differences between the E and Z conformers of the methyl acetate and ethyl acetate adducts are 1.27 ± 0.01 and 0.96 ± 0.01 kcal/mol at 229 K, respectively.