A multinuclear magnetic resonance study in solution and in the solid state of molybdenum dihydrobispyrazolylborates complexes

Fifteen complexes derived from dihydrobispyrazolylborates and molybdenum have been studied by H-1,C-13 and N-15 NMR spectroscopy both in solution (H and C-13 in CDCl3)and in the solid state (C-13 and N-15). The Mo bears two CO and one methallyl molecules. In one case, compound 15, the pyrazoles are actually 2H-indazoles. Besides the fluxional behavior of all the complexes, problems related to the positional isomerism (different substituents at positions 3 and 5 in the pyrazole ring) are observed. In H-1 NMR it was noted the diastereotopy of the BH2 protons, one of them forming an agostic bond B-H-Mo, being strongly shielded ((1)J(BH) similar to 90 Hz). The C-13 chemical shifts are linearly correlated to those of the corresponding NH-pyrazoles.