Hydridotris(3,5-dimethylpyrazolyl)borate Dimethylamido Imido Niobium and Tantalum Complexes: Synthesis, Reactivity, Fluxional Behavior, and C–H Activation of the NMe2 Function

The pseudo-octahedral dichlorido imido hydridotris­(3,5-dimethylpyrazolyl)­borate niobium and tantalum compounds [MTp*Cl2(NtBu)] (M = Nb (1a), Ta (1b); Tp* = BH­(3,5-Me2C3HN2)3) were prepared in better yields by treatment of equimolar quantities of MCl3(NtBu)­py2 and KTp* in toluene at reflux. Reactions of 1a,b with a small excess of LiNMe2 (1:1.2 ratio) in toluene gave the corresponding chlorido dimethylamido derivatives [MTp*Cl­(NMe2)­(NtBu)] (M = Nb (2), Ta (3)). Mixed methyl dimethylamido [MTp*Me­(NMe2)­(NtBu)] (M = Nb (4), Ta (5)) complexes were synthesized in good yields by heating for several days a mixture of 2 or 3 and MgClMe, in a 1:1 molar ratio. However, the reactions of 1a,b with excess LiNMe2 led to bis­(dimethylamido) complexes [MTp*­(NMe2)2(NtBu)] (M = Nb (6), Ta (7)) as unitary species. 4 and 5 reacted with B­(C6F5)3 to give the cation-like complexes [MTp*­(NMe2)­(NtBu)]+[BMe­(C6F5)3]− (M = Nb (8), Ta (9)), whereas in the case of complexes 6 and 7 the reaction led to [MTp*­(NMe2)­{N­(Me)CH2-κ1N}­(NtBu)]+[BH­(C6F5)3]− (M = Nb (10), Ta (11)) derivatives as result of the C–Hmethyl bond activation into a NMe2 function. The restricted rotation process of the NMe2 moiety around the M–Namido bond in complexes 2–7, the pseudo-rotation process of the Tp* ligand into the cationic species 8 and 9, and the CH2 terminal group around the NCH2 bond in compounds 10 and 11 were observed and studied by 1H DNMR spectroscopy. The isomerization of two enantiomers in the mixtures of 4 and 5 with B­(C6F5)3 was detected, and their mechanism was proposed. All compounds were studied by IR and multinuclear NMR (1H, 13C, and 15N) spectroscopy, and the molecular structures of complexes 1a,b and 3 were determined by X-ray diffraction methods.