Hydridotris(3,5-dimethylpyrazolyl)borate
Dimethylamido
Imido Niobium and Tantalum Complexes: Synthesis, Reactivity, Fluxional
Behavior, and C–H Activation of the NMe<sub>2</sub> Function
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Abstract
The pseudo-octahedral dichlorido imido hydridotris(3,5-dimethylpyrazolyl)borate
niobium and tantalum compounds [MTp*Cl<sub>2</sub>(N<i>t</i>Bu)] (M = Nb (<b>1a</b>), Ta (<b>1b</b>); Tp* = BH(3,5-Me<sub>2</sub>C<sub>3</sub>HN<sub>2</sub>)<sub>3</sub>) were prepared in
better yields by treatment of equimolar quantities of MCl<sub>3</sub>(N<i>t</i>Bu)py<sub>2</sub> and KTp* in toluene at reflux.
Reactions of <b>1a</b>,<b>b</b> with a small excess of
LiNMe<sub>2</sub> (1:1.2 ratio) in toluene gave the corresponding
chlorido dimethylamido derivatives [MTp*Cl(NMe<sub>2</sub>)(N<i>t</i>Bu)] (M = Nb (<b>2</b>), Ta (<b>3</b>)). Mixed
methyl dimethylamido [MTp*Me(NMe<sub>2</sub>)(N<i>t</i>Bu)]
(M = Nb (<b>4</b>), Ta (<b>5</b>)) complexes were synthesized
in good yields by heating for several days a mixture of <b>2</b> or <b>3</b> and MgClMe, in a 1:1 molar ratio. However, the
reactions of <b>1a</b>,<b>b</b> with excess LiNMe<sub>2</sub> led to bis(dimethylamido) complexes [MTp*(NMe<sub>2</sub>)<sub>2</sub>(N<i>t</i>Bu)] (M = Nb (<b>6</b>), Ta
(<b>7</b>)) as unitary species. <b>4</b> and <b>5</b> reacted with B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to give the
cation-like complexes [MTp*(NMe<sub>2</sub>)(N<i>t</i>Bu)]<sup>+</sup>[BMe(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup> (M = Nb (<b>8</b>), Ta (<b>9</b>)), whereas in the case
of complexes <b>6</b> and <b>7</b> the reaction led to
[MTp*(NMe<sub>2</sub>){N(Me)CH<sub>2</sub>-κ<sup>1</sup><i>N</i>}(N<i>t</i>Bu)]<sup>+</sup>[BH(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup> (M = Nb (<b>10</b>), Ta (<b>11</b>)) derivatives as result of the C–H<sub>methyl</sub> bond activation into a NMe<sub>2</sub> function. The
restricted rotation process of the NMe<sub>2</sub> moiety around the
M–N<sub>amido</sub> bond in complexes <b>2</b>–<b>7</b>, the pseudo-rotation process of the Tp* ligand into the
cationic species <b>8</b> and <b>9</b>, and the CH<sub>2</sub> terminal group around the NCH<sub>2</sub> bond in
compounds <b>10</b> and <b>11</b> were observed and studied
by <sup>1</sup>H DNMR spectroscopy. The isomerization of two enantiomers
in the mixtures of <b>4</b> and <b>5</b> with B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> was detected, and their mechanism
was proposed. All compounds were studied by IR and multinuclear NMR
(<sup>1</sup>H, <sup>13</sup>C, and <sup>15</sup>N) spectroscopy,
and the molecular structures of complexes <b>1a</b>,<b>b</b> and <b>3</b> were determined by X-ray diffraction methods
