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

2009-09-21T00:00:00Z (GMT) by Mehmet Zahmakiran Saim Özkar
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 C5H11COO anions and Me2H2N+ 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)∼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 (Me2NBH2)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, 1H, 13C, and 11B 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 [Me2NH·BH3]/[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 k1) and A + B → 2B (rate constant k2), in which A is [Rh(C5H11CO2)2]2 and B is the growing, catalytically active rhodium(0) nanoclusters. (iv) Mercury(0) and CS2 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.