Quantitative 1,10-Phenanthroline Catalyst-Poisoning Kinetic Studies of Rh(0) Nanoparticle and Rh₄Cluster Benzene Hydrogenation Catalysts: Estimates of the PoisonKassociationBinding Constants, of the Equivalents of Poison Bound and of the Number of Catalytically Active Sites for Each Catalyst

Quantitative catalyst poisoning studies are of fundamental interest and importance because (a) knowledge of the number of true active sites is required for calculation of the true turnover frequency = (moles of product)/(moles of actual active sites)(time), and because (b) quantitative catalyst poisoning is proving to be a key, required piece of data en route to distinguishing single metal (M1), small metal cluster (e.g., M4), or metal nanoparticle (Mn) catalysis. In evidence of the latter point, quantitative catalyst poisoning experiments using 1,10-phenanthroline as the poison proved to be crucial in the recent identification of Rh4 subnanometer clusters as the true benzene hydrogenation catalyst in a system beginning with [RhCp*Cl2]2 (Cp*: (η5-C5(CH3)5)) at 100 °C and 50 atm initial H2 pressure (Bayram et al. J. Am. Chem. Soc.2011, 133, 18889). However and despite the success of those quantitative poisoning studies, five questions about such poisoning studies remained unanswered, questions posed and then addressed herein. In addition, the analysis herein of the 1,10-phenanthroline poisoning of both Rh(0) nanoparticle and Rh4 subnanometer benzene hydrogenation catalysts results in kinetic models for, respectively, strong-binding and weak-binding poisons. Also provided are quantitiative estimates of the poison binding constants, of the number of equivalents required to completely poison each catalyst, and of the number of active sites on each catalyst. The weak-binding poison kinetic model is then shown to have immediate applicability toward analyzing extant literature data via its application to literature CS2 quantitative poisoning data for ammonia-borane dehydrocoupling beginning with a [Ru(cod)(cot)] (cod: cyclooctadiene and cot: cyclooctatriene) precatalyst. The significance of the results is then summarized in a Conclusions section.