Rh(0) Nanoclusters in Benzene Hydrogenation Catalysis: Kinetic and Mechanistic Evidence that a Putative [(C8H17)3NCH3]+[RhCl4]-Ion-Pair Catalyst Is Actually a Distribution of Cl-and [(C8H17)3NCH3]+Stabilized Rh(0) Nanoclusters

Abstract

A reinvestigation is reported of a prototype literature arene hydrogenation system, one previously believed to involve a [(C 8 H 17 ) 3 NCH 3 ] + [RhCl 4 ] - ion-pair catalyst. The methodology employed to uncover the true catalyst, and to deal with the classic and difficult mechanistic problem of “is it homogeneous or heterogeneous catalysis?”, is the four-step mechanistic approach developed previously in our laboratories. The data obtained (i) provide unequivocal TEM evidence that Rh(0) nanoclusters are formed under the reaction conditions and (ii) provide kinetic evidence that the benzene catalytic hydrogenation reaction follows the nucleation (A → B) and then autocatalytic surface-growth (A + B → 2B) mechanism elucidated recently for metal(0) nanocluster growth. These latter results require that “A” (i.e., [RhCl 4 ] - ) is not the catalyst to within the error limits (5−15%) of the fits of the data to the autocatalytic surface-growth mechanism; the kinetic results also provide some of the strongest possible evidence that “B” is the true catalyst, “B” being the Rh(0) nanoclusters. In addition, (iii) H/D exchange and (iv) Hg(0) poisoning data confirm that the Rh(0) nanoclusters are the only active catalysts since added Hg(0) poisons the arene hydrogenation completely. The results reported herein are of fundamental significance in five ways: (i) they are only the second use each of two new and powerful methodologies that were required for the success of the studies reported, the (a) more general 4-step methodology for testing “is it homogeneous or heterogeneous catalysis”, and (b) the pseudoelementary, catalytic reporter methodology for following the nanocluster growth kinetics. In addition, (ii) they correct the claim that [RhCl 4 ] - is a benzene hydrogenation catalyst, and identify soluble Rh(0) nanoclusters as the true catalyst; (iii) they call into question all previous claims of benzene hydrogenation but not anthracene or naphthalene arene hydrogenation by monometallic precatalysts; and (iv) they re-emphasize that, prior to any claim of a homogeneous catalyst in a reaction (such as arene hydrogenation) where a facile heterogeneous M(0) catalyst is well established, one must first rule out catalysis by even trace amounts of possibly highly active nanocluster catalysts (e.g., by using the methods utilized herein and any other applicable method). Overall, the studies presented herein (v) provide a definitive answer, at least for the specific Rh system studied, to the 34-year-old question, one controversial for the past 17 years, of “is benzene hydrogenation homogeneous or heterogeneous?”.