text: The first catalysts for the dehydrogenation of ABs were derived from reduction of Rh(I) complexes to form the active colloidal heterogeneous catalyst. As in the case with the metal carbonyl catalysts, bulky secondary amine-boranes form monomeric aminoboranes. For RhL- and Rh(H)L-derived catalysts, the active species is a homogeneous catalyst, with the phosphine ligands interacting directly with the dehydrocoupling process. Changing the phosphine ligands from PPr to PBu significantly increases the turnover rate of the catalyst. Unlike other Rh(I) catalysts, the rhodium analogue of Wilkinson's catalyst RhCl(PHCy) (Cy=cyclohexyl) behaves like the RhL and Rh(H)L catalysts as a homogeneous species.
  In comparison to RhCl(PHCy), the iridium analogue has reduced catalytic activity on the dehydrogenation of non sterically hindered amine-boranes, and increased activity on more sterically hindered substrates. Dehydrocoupling of primary diborazanes NHR&mdash;BH&mdash;NHR&mdash;BH can be catalyzed by  via conversion to the metal-bound species MeNH&mdash;BH and subsequent polymerization/oligomerization. This same reaction has been found to occur in the absence of the iridium metal, upon heating of the reaction mixture. Dehydrogenation of ammonia-borane with Brookhart's catalyst results in quantitative formation of the cyclic pentamer [NHBH] rather than the typically seen cyclic dimers from other amine-borane dehydrogenations. When catalyzing ammonia-borane dehydrogenation, the catalyst acts homogeneously at a 0.5 mol% catalyst loading. Rather than the typical high temperatures needed for this dehydrogenation, the reaction proceeds cleanly at room temperature, with complete substrate conversion in 14min.