Dimethylammonium Hexanoate Stabilized Rhodium(0) Nanoclusters Identified as True Heterogeneous Catalysts with the Highest Observed Activity in the Dehydrogenation of Dimethylamine−Borane

Abstract

Herein we report the discovery of a superior dimethylamine-borane dehydrogenation catalyst, more active than the prior best heterogeneous catalyst (Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 9776) reported to date for the dehydrogenation of dimethylamine-borane. The new catalyst system consists of rhodium(0) nanoclusters stabilized by C(5)H(11)COO(-) anions and Me(2)H(2)N(+) cations and can reproducibly be formed from the reduction of rhodium(II) hexanoate during dehydrogenation of dimethylamine-borane at room temperature. Rhodium(0) nanoclusters in an average particle size of 1.9 +/- 0.6 nm Rh(0)(approximately 190) nanoclusters) provide 1040 turnovers over 26 h with a record initial turnover frequency (TOF) of 60 h(-1) (the average TOF value is 40 h(-1)) in the dehydrogenation of dimethylamine-borane, yielding 100% of the cyclic product (Me(2)NBH(2))(2) at room temperature. The work reported here also includes the full experimental details of the following major components: (i) Characterization of dimethylammonium hexanoate stabilized rhodium(0) nanoclusters by using TEM, STEM, EDX, XRD, UV-vis, XPS, FTIR, (1)H, (13)C, and (11)B NMR spectroscopy, and elemental analysis. (ii) Collection of a wealth of previously unavailable kinetic data to determine the rate law and activation parameters for catalytic dehydrogenation of dimethylamine-borane. (iii) Monitoring of the formation kinetics of the rhodium(0) nanoclusters by a fast dimethylamine-borane dehydrogenation catalytic reporter reaction (Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382) at various [Me(2)NH.BH(3)]/[Rh] ratios and temperatures. Significantly, sigmoidal kinetics of catalyst formation was found to be well fit to the two-step, slow nucleation and then autocatalytic surface growth mechanism, A --> B (rate constant k(1)) and A + B --> 2B (rate constant k(2)), in which A is [Rh(C(5)H(11)CO(2))(2)](2) and B is the growing, catalytically active rhodium(0) nanoclusters. (iv) Mercury(0) and CS(2) poisoning and nanofiltration experiments to determine whether the dehydrogenation of dimethylamine-borane catalyzed by the dimethylammonium hexanoate stabilized rhodium(0) nanoclusters is homogeneous or heterogeneous catalysis.