Determining the kinetically dominant catalyst in a given catalytic system is a forefront topic in catalysis. The [RhCp*Cl-2](2) (Cp* = [eta(5)-C-5(CH3)(5)]) system pioneered by Maitlis and co-workers is a classic precatalyst system from which homogeneous mononuclear Rh-1, subnanometer Rh-4 cluster, and heterogeneous polymetallic Rh(0)(n) nanoparticle have all arisen as viable candidates for the true hydrogenation catalyst, depending on the precise substrate, H-2 pressure, temperature, and catalyst concentration conditions. Addressed herein is the question of whether the prior assignment of homogeneous, mononuclear Rh1Cp*-based catalysis is correct, or are trace Rh-4 subnanometer clusters or possibly Rh(0)(n) nanoparticles the dominant, actual cyclohexene hydrogenation catalyst at 22 degrees C and 2.7 atm initial H-2 pressure? The observation herein of Rh-4 species by in operando-X-ray absorption fine structure (XAFS) spectroscopy, at the only slightly more vigorous conditions of 26 degrees C and 8.3 atm H-2 pressure, and the confirmation of Rh., dusters by ex situ mass spectroscopy raises the question of the dominant, room temperature, and mild pressure cydohexene hydrogenation catalyst derived from the classic [RhCp*Cl-2](2) precatalyst pioneered by Maitlis and co-workers. Ten lines of evidence are provided herein to address the nature of the true room temperature and mild pressure cyclohexene hydrogenation catalyst derived from [RhCp*Cl-2](2). Especially significant among those experiments are quantitative catalyst, poisoning experiments, in the present case using 1,10-phenanthroline. Those poisoning studies allow one to distinguish mononuclear Rh-1, subnanometer Rh, duster, and Rh(0)(n) nanoparticle catalysis hypotheses. The evidence obtained provides a compelling case for a mononuclear, Rh1Cp*-based cyclohezene hydrogenation catalyst at 22 degrees C and 2.7 atm H-2 pressure. The resultant methodology, especially the quantitative catalyst poisoning experiments in combination with in operando spectroscopy, is expected to be more broadly applicable to the study of other systems and the "what is the true catalyst?" question.
