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Amine− and Dimeric Amino−Borane Complexes of the {Rh(PiPr3)2}+ Fragment and Their Relevance to the Transition-Metal-Mediated Dehydrocoupling of Amine−Boranes

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journal contribution
posted on 01.02.2010, 00:00 by Adrian B. Chaplin, Andrew S. Weller
Complexes formed between {Rh(PiPr3)2}+ or {Rh(H)2(PiPr3)2}+ fragments and the amine− and dimeric amino−borane σ ligands H3B·NMe3 and [H2BNMe2]2 have been prepared and their solution and solid-state structures determined: [Rh(PiPr3)22-H3B·NMe3)][BArF4] (1), [Rh(PiPr3)22-(H2BNMe2)2}][BArF4] (2), [Rh(H)2(PiPr3)22-H3B·NMe3)][BArF4] (3), and [Rh(H)2(PiPr3)22-(H2BNMe2)2}][BArF4] (4) [ArF = C6H3(CF3)2]. The last compound was only observed in the solid state, as in solution it dissociates to give [Rh(H)2(PiPr3)2][BArF4] and [H2BNMe2]2 due to steric pressure between the ligand and the metal fragment. The structures and reactivities of these new complexes are compared with the previously reported tri-isobutyl congeners. On the basis of 11B and 1H NMR spectroscopy in solution and the Rh···B distances measured in the solid state, the PiPr3 complexes show tighter interactions with the σ ligands compared to the PiBu3 complexes for the Rh(I) species and a greater stability toward H2 loss for the Rh(III) salts. For the Rh(I) species (1 and 2), this is suggested to be due to electronic factors associated with the bending of the ML2 fragment. For the Rh(III) complexes (3 and 4), the underlying reasons for increased stability toward H2 loss are not as clear, but steric factors are suggested to influence the relative stability toward a loss of dihydrogen, although other factors, such as supporting agostic interactions, might also play a part. These tighter interactions and a slower H2 loss are reflected in a catalyst that turns over more slowly in the dehydrocoupling of H3B·NHMe2 to give the dimeric amino−borane [H2BNMe2]2, when compared with the PiBu3-ligated catalyst (ToF 4 h−1, c.f., 15 h−1, respectively). The addition of excess MeCN to 1, 2, or 3 results in the displacement of the σ-ligand and the formation of the adduct species trans-[Rh(PiPr3)2(NCMe)2][BArF4] (with 1 and 2) and the previously reported [Rh(H)2(PiPr3)2(NCMe)2][BArF4] (with 3).