Ammonia Borane Dehydrogenation Promoted by a Pincer-Square-Planar Rhodium(I) Monohydride: A Stepwise Hydrogen Transfer from the Substrate to the Catalyst

The pincer d8-monohydride complex RhH­{xant­(PiPr2)2} (xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene)
promotes the release of 1 equiv of hydrogen from H3BNH3 and H3BNHMe2 with TOF50% values of 3150 and 1725 h–1, to afford [BH2NH2]n and [BH2NMe2]2 and the tandem ammonia borane dehydrogenation–cyclohexene
hydrogenation. DFT calculations on the ammonia borane dehydrogenation
suggest that the process takes place by means of cis-κ2-PP-species, through four stages including: (i)
Shimoi-type coordination of ammonia borane, (ii) homolytic addition
of the coordinated H–B bond to afford a five-coordinate dihydride-boryl-rhodium­(III)
intermediate, (iii) reductive intramolecular proton transfer from
the NH3 group to one of the hydride ligands, and (iv) release
of H2 from the resulting square-planar hydride dihydrogen
rhodium­(I) intermediate.