Steric Effects in Metathesis and Reduction Reactions of Phosphinimines with Catechol- and Pinacolboranes

A series of catecholboryl-phosphinimide complexes with the general formula ((mu)-(R3PN)Bcat)x (cat = O2C6H4) have been synthesized via associative metathetical reactions. For R = Et, n-Bu, Ph, and i-Pr and R3 = n-Bu t-Bu2 X-ray crystallography as well as solution NMR spectroscopy and reactivity studies reveal that these species are dimeric. In the case of R = t-Bu, the steric congestion results in the monomeric species, t-Bu3PNBcat. Similarly, reactions of R(t-Bu)2PNH (R = n-Bu, t-Bu) and i-Pr3PNH with pinacolborane (HBO2C2Me4 = HBpin) led to the formation of n-Bu(t-Bu2)PNBpin, t-Bu3PNBpin, and i-Pr3PNBpin. Analogous reactions of smaller phosphinimines R3PNH (R = Et or n-Bu) with pinacolborane (HBpin) generated free phosphine and the boron-containing product HN(Bpin)2. In the related reactions of R3PNPh or R3PNAd (R = Et and n-Bu) and HBpin, the white crystalline solids PhHN(Bpin) or AdHN(Bpin) were isolated. HN(Bpin)2 was also derived from the reaction of Et3PNSiMe3 and HBpin. Kinetic studies showed this reaction is first order in both reagents with a rate constant of 1.3(7) × 10^-4 s^-1. A mechanism involving a 1:1 donor-acceptor interaction of the phosphinimine and borane affording reduction of the phosphinimine to phosphine with concurrent formation of borylamine is proposed. Computational studies were performed to probe the steric effects on these reactions of phosphinimine and borane. Model reactions involving t-Bu3PNH showed a lower activation barrier for protonolysis in comparison to phosphinimine reduction. In contrast, for the smaller phosphinimine H3PNH, the activation barriers for phosphinimine reduction are lower. The causes of these steric effects are considered.