Half-Sandwich Iminophosphonamide Rhodium Complexes as Highly Efficient Catalysts for Dehydrogenation of Dimethylamine-Borane

The dehydrogenation of dimethylamine-borane (DMAB) catalyzed by the iminophosphonamide rhodium(III) complexes [Cp*RhCl{Ph₂P(N–p-Tol)(NR)}] (Iа, R = p-Tol; Ib, R = Me) as well as their in situ formed fulvene [(η⁴-C₅Me₄CH₂)Rh(NPN)] (IIa, IIb) and diene [(η⁴-C₅Me₅H)Rh(NPN)] (IIIa, IIIb) rhodium(I) derivatives is studied. Catalysts IIIa and IIIb turn out to be the most active and demonstrate a TOF activity of 110 (IIIа) and 540 h^–1 (IIIb) at 40°С in toluene. The activity decreases significantly in more polar and coordinating THF. At the same time, the rate of DMAB dehydrogenation by complexes Iа and Ib is lower by 10–30 times, and fulvene complexes Iа and Ib are rapidly deactivated after the active initial period (<20% conversion). The kinetic studies show that the reaction has the first order with respect to the substrate and the catalyst. The model ¹¹В NMR experiments confirm that the reaction proceeds via the intermediate formation of a monomer Me₂N=BH₂, which rapidly dimerizes to (Me₂N–BH₂)₂. The mechanism of DMAB dehydrogenation with the formation of unstable hydride intermediate [Cp*RhH{Ph₂P(N–p-Tol)(NR)}] (IVa, IVb) is proposed on the basis of the preliminarily ³¹Р NMR results and published data.