RH(0) NANOCLUSTERS IN BENZENE HYDROGENATION CATALYSIS - KINETIC AND MECHANISTIC EVIDENCE THAT A PUTATIVE [(C8H17)(3)NCH3](-) ION-PAIR CATALYST IS ACTUALLY A DISTRIBUTION OF CL- AND [(C8H17)(3)NCH3](+) STABILIZED RH(0) NANOCLUSTERS()[RHCL4]()

A reinvestigation is reported of a prototype literature arene hydrogen ation system, one previously believed to involve a [(C8H17)(3)NCH3](+) [RhCl4](-) 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 f our-step mechanistic approach developed previously in our laboratories . The data obtained (i) provide unequivocal TEM evidence that Rh(0) na noclusters are formed under the reaction conditions and (ii) provide k inetic evidence that the benzene catalytic hydrogenation reaction foll ows the nucleation (A --> B) and then autocatalytic surface-growth (A + B --> 2B) mechanism elucidated recently for metal(0) nanocluster gro wth. These latter results require that ''A'' (i.e., [RhCl4]-) is not t he catalyst to within the error limits (5-15%) of the fits of the data to the autocatalytic surface-growth mechanism; the kinetic results al so provide some of the strongest possible evidence that ''B'' is the t rue catalyst, ''B'' being the Rh(0) nanoclusters. In addition, (iii) H /D exchange and (iv) Hg(0) poisoning data confirm that the Rh(0) nanoc lusters are the only active catalysts since added Hg(0) poisons the ar ene hydrogenation completely. The results reported herein are of funda mental significance in five ways: (i) they are only the second use eac h of two new and powerful methodologies that were required for the suc cess 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 [RhCl4](-) is a benzene hydrogenation catalyst, and identify sol uble Rh(0) nanoclusters as the true catalyst; (iii) they call into que stion 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 cataly st in a reaction (such as arene hydrogenation) where a facile heteroge neous M(0) catalyst is well established, one must first rule out catal ysis by even trace amounts of possibly highly active nanocluster catal ysts (e.g., by using the methods utilized herein and any other applica ble method). Overall, the studies presented herein (v) provide a defin itive answer, at least for the specific Rh system studied, to the 34-y ear-old question, one controversial for the past 17 years, of ''is ben zene hydrogenation homogeneous or heterogeneous?''.