The Synthesis, Characterization and Dehydrogenation of Sigma-Complexes of BN-Cyclohexanes

The coordination chemistry of the 1,2-BN-cyclohexanes 2,2-R-2-1,2-B,N-C4H10 (R-2=HH, MeH, Me-2) with Ir and Rh metal fragments has been studied. This led to the solution (NMR spectroscopy) and solid-state (X-ray diffraction) characterization of [Ir(PCy3)(2)(H)(2)(eta(2)eta(2)-H2BNR2C4H8)][BAr4F] (NR2=NH2, NMeH) and [Rh(iPr(2)PCH(2)CH(2)CH(2)PiPr(2))(eta(2)eta(2)-H2BNR2C4H8)][ BAr4F] (NR2=NH2, NMeH, NMe2). For NR2=NH2 subsequent metal-promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine BNC4H8, via amino-borane and, tentatively characterized using DFT/GIAO chemical shift calculations, cycloborazane intermediates. For NR2=NMeH the final product is the cyclic amino-borane HBNMeC4H8. The mechanism of dehydrogenation of 2,2-H,Me-1,2-B,N-C4H10 using the {Rh(iPr(2)PCH(2)CH(2)CH(2)PiPr(2))}+ catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh-2(iPr(2)PCH(2)CH(2)CH(2)PiPr(2))(2)H-5][BAr4F]. Using the initial rate method starting from this dimer, a first-order relationship to [amine-borane], but half-order to [Rh] is established, which is suggested to be due to a rapid dimer-monomer equilibrium operating.