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

The doubly charged Lewis acid precursors [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 H-1 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 eta-1-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.