Is it homogeneous or heterogeneous catalysis? Identification of bulk ruthenium metal as the true catalyst in benzene hydrogenations starting with the monometallic precursor, Ru(II)(η⁶-C₆Me ₆)(OAc)₂, plus kinetic characterization of the heterogeneous nucleation, then autocatalytic surface-growth mechanism of metal film formation

A reinvestigation of the true catalyst in a benzene hydrogenation system beginning with Ru(II)-(η⁶-C₆Me₆)(OAc) ₂ as the precatalyst is reported. The key observations leading to the conclusion that the true catalyst is bulk ruthenium metal particles, and not a homogeneous metal complex or a soluble nanocluster, are as follows: (i) the catalytic benzene hydrogenation reaction follows the nucleation (A → B) and then autocatalytic surface-growth (A + B → 2B) sigmoidal kinetics and mechanism recently elucidated for metal(0) formation from homogeneous precatalysts; (ii) bulk ruthenium metal forms during the hydrogenation; (iii) the bulk ruthenium metal is shown to have sufficient activity to account for all the observed activity; (iv) the filtrate from the product solution is inactive until further bulk metal is formed; (v) the addition of Hg(0), a known heterogeneous catalyst poison, completely inhibits further catalysis; and (vi) transmission electron microscopy fails to detect nanoclusters under conditions where they are otherwise routinely detected. Overall, the studies presented herein call into question any claim of homogeneous benzene hydrogenation with a Ru(arene) precatalyst. An additional, important finding is that the A → B, then A + B → 2B kinetic scheme previously elucidated for soluble nanocluster homogeneous nucleation and autocatalytic surface growth (Widegren, J. A.; Aiken, J. D., III; Özkar, S.; Finke, R. G. Chem. Mater. 2001, 13, 312-324, and ref 8 therein) also quantitatively accounts for the formation of bulk metal via heterogeneous nucleation then autocatalytic surface growth. This is significant for three reasons: (i) quantitative kinetic studies of metal film formation from soluble precursors or chemical vapor deposition are rare; (ii) a clear demonstration of such A → B, then A + B → 2B kinetics, in which both the induction period and the autocatalysis are continuously monitored and then quantitatively accounted for, has not been previously demonstrated for metal thin-film formation; yet (iii) all the mechanistic insights from the soluble nanocluster system (op. cit.) should be applicable to metal thin-film formations which exhibit sigmoidal kinetics and, hence, the A → B, then A + B → 2B mechanism.