Synthesis and Characterization of Stable Cationic [Hydrotris(1-pyrazolyl)borato]Mo(CO)(NO)(η³-allyl) ComplexesSolid-State and Solution Evidence for an η²-Allyl Structure

From the corresponding TpMo(CO)₂(π-allyl) complexes, four symmetrically substituted TpMo(CO)(NO)(π-allyl)⁺ complexes (π-allyl = propenyl, 2-methylpropenyl, cyclohexenyl, and cyclooctenyl) were prepared and characterized by IR, by ¹H and ¹³C NMR spectroscopy, and in one case by X-ray crystallography. The BF₄^- salts of the cationic nitrosyl complexes were unstable in solution; however, using the noncoordinating counterion [(3,5-(CF₃)₂C₆H₃)₄B]^- (BAr‘₄^-) robust complexes were produced, permitting a thorough spectroscopic investigation. The crystal structure of [TpMo(CO)(NO)(η³-C₃H₅)][(3,5-(CF₃)₂C₆H₃)₄B] revealed a significant η³ → η² 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 η³ → η² distortion of the allyl complexes. ¹H NOE experiments were carried out in order to determine the conformation of the allyl fragment for each nitrosyl complex. [TpMo(CO)(NO)(η³-C₃H₅)][BAr‘₄] was formed as a mixture of exo/endo rotamers (5.2:1), while [TpMo(CO)(NO)(η³-C₄H₇)][BAr‘₄] existed exclusively as the endo rotamer in solution. Only the exo rotamer was observed for the cyclic complexes [TpMo(CO)(NO)(η³-C₆H₉)][BAr‘₄] and [TpMo(CO)(NO)(η³-C₈H₁₃)][BAr‘₄]. A plausible mechanism for the formation of the cationic nitrosyl complexes involves the electrophilic addition of NO⁺ to the neutral TpMo(CO)₂(η³-allyl) complex with concurrent slippage of the allyl form η³ to η¹ to generate a seven-coordinate cationic η¹-allyl complex. A deuterium labeling study using TpMo(CO)₂(η³-C₃H₄D) provided evidence for the η³ → η¹ mechanism responsible for the formation of exo/endo isomers.