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

Complexes formed between {Rh((PPr3)-Pr-i)(2)}(+) Or {Rh(H)(2)((PPr3)-Pr-i)(2)}(+) fragments and the amine- and dimeric aminoborane sigma ligands H3B center dot NMe3 and [H2BNMe2](2) have been prepared and their solution and solid-state structures determined: [Rh((PPr3)-Pr-i)(2)(eta(2)-H3B center dot NMe3)][BAr4F] (1), [Rh((PPr3)-Pr-i)(2){eta(2)-(H2B center dot NMe2)(2)}][BAr4F] (2), [Rh(H)(2)((PPr3)-Pr-i)(2)(eta(2)-H3B center dot NMe3)][BAr4F](3), and [Rh(H)(2)((PPr3)-Pr-i)(2)][eta(2)-(H2BN center dot Me-2)(2)][BAr4F](4) [Ar-F = C6H3(CF3)(2)], The last compound was only observed in the solid state, as in solution it dissociates to give [Rh(H)(2)((PPr3)-Pr-i)(2)] [BAr4F] and [H2B center dot NMe2](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 B-11 and (H) NMR spectroscopy in solution and the Rh center dot center dot center dot B distances measured in the solid state, the P'Pr3 complexes show tighter interactions with the a ligands compared to the (PBu3)-Bu-i complexes for the Rh(I) species and a greater stability toward H-2 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 H-2 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 interections, might also play a part. These tighter interactions and a slower H-2 loss are reflected in a catalyst that turns over more slowly in the dehydrocoupling of H3B center dot NHMe2 to give the dimeric amino-borane [H2BNMe2](2), when compared with the (PBu3)-Bu-i-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 (sigma-ligand and the formation of the adduct species trans-[Rh((PPr3)-Pr-i)(2)(NCMe)(2))[BAr4F] (with 1 and 2) and the previously reported [Rh(H)(2)((PPr3)-Pr-i)(2)(NCMe)(2)][BAr4F] (with 3).