Synthesis and Characterization of Stable Cationic [Hydrotris(1-pyrazolyl)borato]Mo(CO)(NO)(η<sup>3</sup>-allyl) ComplexesSolid-State and Solution Evidence for an η<sup>2</sup>-Allyl Structure

From the corresponding TpMo(CO)<sub>2</sub>(π-allyl) complexes, four symmetrically substituted TpMo(CO)(NO)(π-allyl)<sup>+</sup> complexes (π-allyl = propenyl, 2-methylpropenyl, cyclohexenyl, and cyclooctenyl) were prepared and characterized by IR, by <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy, and in one case by X-ray crystallography. The BF<sub>4</sub><sup>-</sup> salts of the cationic nitrosyl complexes were unstable in solution; however, using the noncoordinating counterion [(3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>4</sub>B]<sup>-</sup> (BAr‘<sub>4</sub><sup>-</sup>) robust complexes were produced, permitting a thorough spectroscopic investigation. The crystal structure of [TpMo(CO)(NO)(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)][(3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>4</sub>B] revealed a significant η<sup>3</sup> → η<sup>2</sup> distortion of the allyl moiety. HETCOR and COSY NMR experiments were conducted in order to assign the chemical shifts of each of the allyl hydrogen and carbon atoms, unambiguously. These data also revealed the η<sup>3</sup> → η<sup>2</sup> distortion of the allyl complexes. <sup>1</sup>H NOE experiments were carried out in order to determine the conformation of the allyl fragment for each nitrosyl complex. [TpMo(CO)(NO)(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)][BAr‘<sub>4</sub>] was formed as a mixture of <i>exo</i>/<i>endo</i> rotamers (5.2:1), while [TpMo(CO)(NO)(η<sup>3</sup>-C<sub>4</sub>H<sub>7</sub>)][BAr‘<sub>4</sub>] existed exclusively as the <i>endo</i> rotamer in solution. Only the <i>exo</i> rotamer was observed for the cyclic complexes [TpMo(CO)(NO)(η<sup>3</sup>-C<sub>6</sub>H<sub>9</sub>)][BAr‘<sub>4</sub>] and [TpMo(CO)(NO)(η<sup>3</sup>-C<sub>8</sub>H<sub>13</sub>)][BAr‘<sub>4</sub>]. A plausible mechanism for the formation of the cationic nitrosyl complexes involves the electrophilic addition of NO<sup>+</sup> to the neutral TpMo(CO)<sub>2</sub>(η<sup>3</sup>-allyl) complex with concurrent slippage of the allyl form η<sup>3</sup> to η<sup>1</sup> to generate a seven-coordinate cationic η<sup>1</sup>-allyl complex. A deuterium labeling study using TpMo(CO)<sub>2</sub>(η<sup>3</sup>-C<sub>3</sub>H<sub>4</sub>D) provided evidence for the η<sup>3</sup> → η<sup>1</sup> mechanism responsible for the formation of <i>exo</i>/<i>endo</i> isomers.