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.钳形d8构型一氢合铑配合物RhH{xant(PiPr2)2} [其中xant(PiPr2)2 = 9,9-二甲基-4,5-二(二异丙基膦基)呫吨] 可催化H3BNH3（氨硼烷，Ammonia Borane）与H3BNHMe2分别释放1当量氢气，50%转化率下的周转频率（TOF50%）分别为3150 h⁻¹与1725 h⁻¹，反应产物为[BH2NH2]n、[BH2NMe2]2并介导串联的氨硼烷脱氢-环己烯加氢反应。针对氨硼烷脱氢过程的密度泛函理论（DFT, Density Functional Theory）计算表明，该反应通过顺式κ2-PP型物种经四个阶段进行：(i) 氨硼烷的Shimoi型配位；(ii) 配位的B-H键发生均裂加成，生成五配位二氢化物-硼基-铑(III)中间体；(iii) 氨基团上的质子向其中一个氢化物配体发生分子内还原质子转移；(iv) 从所得平面正方形构型的氢化物二氢合铑(I)中间体中释放氢气。