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-C₆Me₆)(OAc)₂, Plus Kinetic Characterization of the Heterogeneous Nucleation, Then Autocatalytic Surface-Growth Mechanism of Metal Film Formation

Abstract

A reinvestigation of the true catalyst in a benzene hydrogenation system beginning with Ru(II)(eta(6)-C(6)Me(6))(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; Ozkar, 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.

Organogel–quantum dots hybrid materials displaying fluorescence sensitivity and structural stability towards nitric oxide

“…The concentration of HBr increases as long as it is formed in each of the elemental reactions considered and decreases according to the process shown in (43).…”

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium Under Mild Reaction Conditions

“…offers the possibility to effectively control the formation of palladium nanoparticles: increasing numbers of P-N bonds in the ligands successively eases their water-induced degradation and consequently the formation of nanoparticles from the respective complexes. Accordingly, since palladium nanoparticles are the catalytically active form of 1 in the Heck reaction 35 , as indicated by sigmoidal-shaped kinetics 36,37 or the efficient inhibition of catalysis after addition of a large excess of metallic mercury to reaction mixtures of aryl bromide, olefin and catalyst, for example 38 , as well as their detection by analysis of reaction mixtures of exemplary Heck cross-coupling reactions by a transmission electron microscopy (TEM) equipped with an energy dispersive X-ray (EDX) analysator 35 successively increases the complex stability and hence, retards the (water-induced) formation of nanoparticles thereof. As a consequence, while dichloro-bis(1-(dicyclohexylphosphinyl)piperidine)palladium, is the catalyst of choice in the Heck reaction performed at 140 °C, the highest catalytic activity was obtained for dichloro{bis [ Even though the above described syntheses as well as the Heck reaction protocols are straight forward, some of the common troubleshooting procedures are: (a) make sure that the tetrabutylammonium bromide is newly purchased or properly stored (tetrabutylammonium bromide is hygroscopic), (b) make sure that dry solvents are used for the ligand synthesis when small amounts of ligand were prepared, (c) make sure that 1 is either freshly prepared or stored under an inert atmosphere, (d) make sure that the NMP or DMF are newly purchased, (e) make sure that the chemicals are either newly purchased or properly stored, (f) oven-dry all glassware and cool under vacuum.…”

Immobilized Co/Rh Heterobimetallic Nanoparticle‐Catalyzed Pauson–Khand‐Type Reaction

“…[26] The suppression of catalysis by Hg(0) is evidence for a heterogeneous catalyst; if Hg(0) does not suppress catalysis, the implication is that the catalysis is homogeneous. The Hg(0)-poisoning experiment is easy to perform, but is not definitive by itself, and not universally applicable because Hg(0) reacts with some single-metal complexes.…”