Autocatalytic Surface Growth Research Articles

Supported-Nanoparticle Heterogeneous Catalyst Formation in Contact with Solution: Kinetics and Proposed Mechanism for the Conversion of Ir(1,5-COD)Cl/γ-Al2O3to Ir(0)∼900/γ-Al2O3

A current goal in heterogeneous catalysis is to transfer the synthetic, as well as developing mechanistic, insights from the modern revolution in nanoparticle science to the synthesis of supported-nanoparticle heterogeneous catalysts. In a recent study (Mondloch, J. E.; Wang, Q.; Frenkel, A. I.; Finke, R. G. J. Am. Chem. Soc. 2010, 132, 9701-9714), we initialized tests of the global hypothesis that quantitative kinetic and mechanistic studies, of supported-nanoparticle heterogeneous catalyst formation in contact with solution, can provide synthetic and mechanistic insights that can eventually drive improved syntheses of composition-, size-, and possibly shape-controlled catalysts. That study relied on the development of a well-characterized Ir(1,5-COD)Cl/γ-Al(2)O(3) precatalyst, which, when in contact with solution and H(2), turns into a nonaggregated Ir(0)(~900)/γ-Al(2)O(3) supported-nanoparticle heterogeneous catalyst. The kinetics of the Ir(1,5-COD)Cl/γ-Al(2)O(3) to Ir(0)(~900)/γ-Al(2)O(3) conversion were followed and fit by a two-step mechanism consisting of nucleation (A → B, rate constant k(1)) followed by autocatalytic surface growth (A + B → 2B, rate constant k(2)). However, a crucial, but previously unanswered question is whether the nucleation and growth steps occur primarily in solution, on the support, or possibly in both phases for one or more of the catalyst-formation steps. The present work investigates this central question for the prototype Ir(1,5-COD)Cl/γ-Al(2)O(3) to Ir(0)(~900)/γ-Al(2)O(3) system. Solvent variation-, γ-Al(2)O(3)-, and acetone-dependent kinetic data, along with UV-vis spectroscopic and gas-liquid-chromatography (GLC) data, are consistent with and strongly supportive of a supported-nanoparticle formation mechanism consisting of Ir(1,5-COD)Cl(solvent) dissociation from the γ-Al(2)O(3) support (i.e., from Ir(1,5-COD)Cl/γ-Al(2)O(3)), solution-based nucleation from that dissociated Ir(1,5-COD)Cl(solvent) species, fast Ir(0)(n) nanoparticle capture by γ-Al(2)O(3), and then subsequent solid-oxide-based nanoparticle growth from Ir(0)(n)/γ-Al(2)O(3) and with Ir(1,5-COD)Cl(solvent), the first kinetically documented mechanism of this type. Those data disprove a solid-oxide-based nucleation and growth pathway involving only Ir(1,5-COD)Cl/γ-Al(2)O(3) and also disprove a solution-based nanoparticle growth pathway involving Ir(1,5-COD)Cl(solvent) and Ir(0)(n) in solution. The present mechanistic studies allow comparisons of the Ir(1,5-COD)Cl/γ-Al(2)O(3) to Ir(0)(~900)/γ-Al(2)O(3) supported-nanoparticle formation system to the kinetically and mechanistically well-studied, Ir(1,5-COD)·P(2)W(15)Nb(3)O(62)(8-) to Ir(0)(~300)·(P(2)W(15)Nb(3)O(62)(8-))(n)(-8n) solution-based, polyoxoanion-stabilized nanoparticle formation and stabilization system. That comparison reveals closely analogous, solution Ir(1,5-COD)(+) or Ir(1,5-COD)Cl-mediated, mechanisms of nanoparticle formation. Overall, the hypothesis supported by this work is that these and analogous studies hold promise of providing a way to transfer the synthetic and mechanistic insights, from the modern revolution in nanoparticle synthesis and characterization in solution, to the rational, mechanism-directed syntheses of solid oxide-supported nanoparticle heterogeneous catalysts, also in contact with solution.

Dimethylammonium Hexanoate Stabilized Rhodium(0) Nanoclusters Identified as True Heterogeneous Catalysts with the Highest Observed Activity in the Dehydrogenation of Dimethylamine−Borane

Herein we report the discovery of a superior dimethylamine-borane dehydrogenation catalyst, more active than the prior best heterogeneous catalyst (Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 9776) reported to date for the dehydrogenation of dimethylamine-borane. The new catalyst system consists of rhodium(0) nanoclusters stabilized by C(5)H(11)COO(-) anions and Me(2)H(2)N(+) cations and can reproducibly be formed from the reduction of rhodium(II) hexanoate during dehydrogenation of dimethylamine-borane at room temperature. Rhodium(0) nanoclusters in an average particle size of 1.9 +/- 0.6 nm Rh(0)(approximately 190) nanoclusters) provide 1040 turnovers over 26 h with a record initial turnover frequency (TOF) of 60 h(-1) (the average TOF value is 40 h(-1)) in the dehydrogenation of dimethylamine-borane, yielding 100% of the cyclic product (Me(2)NBH(2))(2) at room temperature. The work reported here also includes the full experimental details of the following major components: (i) Characterization of dimethylammonium hexanoate stabilized rhodium(0) nanoclusters by using TEM, STEM, EDX, XRD, UV-vis, XPS, FTIR, (1)H, (13)C, and (11)B NMR spectroscopy, and elemental analysis. (ii) Collection of a wealth of previously unavailable kinetic data to determine the rate law and activation parameters for catalytic dehydrogenation of dimethylamine-borane. (iii) Monitoring of the formation kinetics of the rhodium(0) nanoclusters by a fast dimethylamine-borane dehydrogenation catalytic reporter reaction (Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382) at various [Me(2)NH.BH(3)]/[Rh] ratios and temperatures. Significantly, sigmoidal kinetics of catalyst formation was found to be well fit to the two-step, slow nucleation and then autocatalytic surface growth mechanism, A --> B (rate constant k(1)) and A + B --> 2B (rate constant k(2)), in which A is [Rh(C(5)H(11)CO(2))(2)](2) and B is the growing, catalytically active rhodium(0) nanoclusters. (iv) Mercury(0) and CS(2) poisoning and nanofiltration experiments to determine whether the dehydrogenation of dimethylamine-borane catalyzed by the dimethylammonium hexanoate stabilized rhodium(0) nanoclusters is homogeneous or heterogeneous catalysis.

Monitoring Supported-Nanocluster Heterogeneous Catalyst Formation: Product and Kinetic Evidence for a 2-Step, Nucleation and Autocatalytic Growth Mechanism of Pt(0)nFormation from H2PtCl6on Al2O3or TiO2

A pressing problem in supported-metal-nanoparticle heterogeneous catalysis--despite the long history and considerable fundamental as well as industrial importance of such heterogeneous catalysts--is how to monitor such catalysts' formation more routinely, rapidly, and in real time. Such information is needed to better control the size, shape, composition, and thus resultant catalytic activity, selectivity, and lifetime of these important catalysts. To this end, a study is reported of the formation of supported Pt(0)(n) nanoparticles by H(2) reduction of H(2)PtCl(6) on Al(2)O(3) (or TiO(2)) to give 6 equivalents of HCl plus supported Pt(0)(n)/Al(2)O(3) (or Pt(0)(n)/TiO(2)), all while in contact with a solution of EtOH and cyclohexene. The HCl and Pt(0)(n) products were confirmed, respectively, by the stoichiometry of HCl formation using pH(apparent) measurements, appropriate standards, and by TEM and EDX measurements. The hypothesis of this research is that the kinetics of formation of this supported heterogeneous catalyst could be successfully monitored by a fast cyclohexene hydrogenation catalytic reporter reaction method first worked out for monitoring transition-metal nanoparticle formation in solution (Watzky, M. A. and Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382-10400). Significantly, sigmoidal kinetics of Pt(0)(n)/Al(2)O(3) catalyst formation were in fact successfully monitored by the catalytic hydrogenation reporter reaction method and then found to be well fit to the Finke-Watzky (hereafter F-W) 2-step, slow continuous nucleation and then autocatalytic surface growth mechanism, A --> B (rate constant k(1)) and A + B --> 2B (rate constant k(2)), respectively, in which A is the H(2)PtCl(6) and B is the growing, catalytically active Pt(0) nanoparticle surface. The finding that the F-W mechanism is applicable is significant in that it, in turn, suggests that the > or = 8 insights from studies of the mechanisms of soluble nanocluster formation can likely also be applied to supported heterogeneous catalyst synthesis, including a recent equation that gives nanocluster size vs time in terms of k(1), k(2), [A](o) and other parameters (Watzky, M. A., Finney, E. E. and Finke, R. G. J. Am. Chem. Soc. 2008, 130, 11959-11969 ). Also presented are the use of the catalytic reporter reaction to reveal H(2) gas to-solution mass-transfer-limitations (MTL) in the system of H(2)PtCl(6) on TiO(2), results relevant to a recent communication in this journal. The use of the F-W 2-step nucleation and autocatalytic growth kinetic model to fit 3 literature examples of heterogeneous catalyst formation, involving H(2) reduction of both supported or bulk M(x)O(y) (i.e., and in gas-solid reactions), are also presented as part of the Supporting Information. A conclusion section is then provided summarizing the insights and caveats from the present work, as well as some needed future studies.

General Synthesis of Uniform Metal Sulfide Colloidal Particles via Autocatalytic Surface Growth: A Self-Correcting System

After decomposition of thiourea (TU) in the presence of various metal cations to form metal sulfide nanoclusters, an autocatalytic reaction between TU and the surface of the as-formed metal sulfide dominates the further growth of these metal sulfide nanoclusters. This autocatalytic surface growth is self-correcting, leading to formation of uniform metal sulfide colloidal particles. The size (from nanometer to micrometer) and shape of the particles are able to be tuned simply by varying the reactant concentration, reaction time, and temperature. More complex anisotropic particles can also be prepared in the autocatalytic surface growth system.

Hydroquinone Synthesis of Silver Nanoparticles: A Simple Model Reaction To Understand the Factors That Determine Their Nucleation and Growth

A simple experimental model system based on the reaction between silver(I) and a hydroquinone has been used to investigate the synthesis of silver nanoparticles (NPs) in aqueous media. Progressive nucleation and autocatalytic surface growth were found to be the relevant processes that control the morphology. The effect of synthetic parameters (such as the chemical nature of the metallic precursor, hydroquinone concentration, stabilization agents, and ligands) on the NP nucleation and growth are discussed on the basis of the optical properties, morphological characterization, and electrodynamics calculations. A very important dependence of the final morphology of the NPs on the procedure of mixing of reactants was found to be connected to the nature of the metallic precursor. Ammonia solutions containing strong stabilization agents were demonstrated to help to control shape and size by quenching autocatalytic surface growth.

Fitting Neurological Protein Aggregation Kinetic Data via a 2-Step, Minimal/“Ockham's Razor” Model: The Finke−Watzky Mechanism of Nucleation Followed by Autocatalytic Surface Growth

The aggregation of proteins has been hypothesized to be an underlying cause of many neurological disorders including Alzheimer's, Parkinson's, and Huntington's diseases; protein aggregation is also important to normal life function in cases such as G to F-actin, glutamate dehydrogenase, and tubulin and flagella formation. For this reason, the underlying mechanism of protein aggregation, and accompanying kinetic models for protein nucleation and growth (growth also being called elongation, polymerization, or fibrillation in the literature), have been investigated for more than 50 years. As a way to concisely present the key prior literature in the protein aggregation area, Table 1 in the main text summarizes 23 papers by 10 groups of authors that provide 5 basic classes of mechanisms for protein aggregation over the period from 1959 to 2007. However, and despite this major prior effort, still lacking are both (i) anything approaching a consensus mechanism (or mechanisms), and (ii) a generally useful, and thus widely used, simplest/"Ockham's razor" kinetic model and associated equations that can be routinely employed to analyze a broader range of protein aggregation kinetic data. Herein we demonstrate that the 1997 Finke-Watzky (F-W) 2-step mechanism of slow continuous nucleation, A --> B (rate constant k1), followed by typically fast, autocatalytic surface growth, A + B --> 2B (rate constant k2), is able to quantitatively account for the kinetic curves from all 14 representative data sets of neurological protein aggregation found by a literature search (the prion literature was largely excluded for the purposes of this study in order provide some limit to the resultant literature that was covered). The F-W model is able to deconvolute the desired nucleation, k1, and growth, k2, rate constants from those 14 data sets obtained by four different physical methods, for three different proteins, and in nine different labs. The fits are generally good, and in many cases excellent, with R2 values >or=0.98 in all cases. As such, this contribution is the current record of the widest set of protein aggregation data best fit by what is also the simplest model offered to date. Also provided is the mathematical connection between the 1997 F-W 2-step mechanism and the 2000 3-step mechanism proposed by Saitô and co-workers. In particular, the kinetic equation for Saitô's 3-step mechanism is shown to be mathematically identical to the earlier, 1997 2-step F-W mechanism under the 3 simplifying assumptions Saitô and co-workers used to derive their kinetic equation. A list of the 3 main caveats/limitations of the F-W kinetic model is provided, followed by the main conclusions from this study as well as some needed future experiments.

Nanocluster nucleation and growth kinetic and mechanistic studies: A review emphasizing transition-metal nanoclusters

A review of the literature of kinetic and mechanistic studies of transition-metal nanocluster nucleation and growth is presented; the focus is on nucleation processes. A brief survey of nucleation theory is given first, with an emphasis on classical nucleation theory, as this is the logical starting point of transition-metal nanocluster nucleation and growth studies. The main experimental methods for following nanocluster formation are examined next—dynamic light scattering, UV–visible spectroscopy, electron microscopy, and X-ray spectroscopies—with special attention paid to their strengths and weaknesses. Several specific examples of transition-metal nanocluster formation are then given, beginning with LaMer's classic sulfur sol system and including the Finke–Watzky mechanism of slow continuous nucleation A → B followed by fast autocatalytic surface growth A + B → 2 B . Finally, brief overviews of semiconductor nanoparticle preparations, solid-state nucleation studies—emanating from Avrami's work—and protein agglomeration mechanistic studies are also provided, as these processes are relevant, conceptually and in a general sense, to the field of transition-metal nanocluster nucleation and growth mechanisms.

Nanocluster Nucleation, Growth, and Then Agglomeration Kinetic and Mechanistic Studies: A More General, Four-Step Mechanism Involving Double Autocatalysis

The discovery of the four-step, double autocatalytic mechanism by which transition-metal organometallic and metal-salt precursors self-assemble into zerovalent transition-metal nanoclusters under reductive conditions is reported. The prototype system investigated is (1,5-COD)PtCl2 reduction under hydrogen plus 2 equiv of Bu3N and 2 equiv of Proton Sponge (1,8-bis(dimethylamino)naphthalene). The reaction stoichiometry is established by TEM, XPS, NMR, and GLC. A concomitant, fast, cyclohexene hydrogenation reporter reaction is employed to monitor the kinetics of Pt0 product/catalyst formation and agglomeration. After 15 alternative mechanisms were ruled out, a minimalistic (“Ockham's Razor”) mechanism is proposed consisting of four steps: slow continuous nucleation, A → B (rate constant k1), fast autocatalytic surface growth, A + B → 2B (rate constant k2), bimolecular agglomeration, B + B → C (rate constant k3), and a new, unprecedented autocatalytic agglomeration step between small (B) and larger, bulk-metal-like (C) particles, B + C → 1.5C (rate constant k4). The results provide the following: a rare case of a mechanism with two autocatalytic steps in the same reaction scheme (“double autocatalysis”); the most general mechanism to date by which transition-metal nanoparticles nucleate, grow, and agglomerate to bulk metal under reductive conditions; probably the best understood self-assembly mechanism to date for such a large system in which the extensive kinetic studies required for reliable mechanistic deduction also exist; kinetic curves that can have step-function-like shapes; and insights for the synthesis of nanoclusters vs bulk-metal films (notably that higher temperature and lower concentrations favor nanocluster formation, while the opposite conditions favor bulk-metal production). A summary section details the main conclusions plus caveats and remaining questions/future research goals.

Transition-Metal Nanocluster Kinetic and Mechanistic Studies Emphasizing Nanocluster Agglomeration: Demonstration of a Kinetic Method That Allows Monitoring of All Three Phases of Nanocluster Formation and Aging

A kinetic method following the development, and then the loss, of catalytic activity is used to monitor the nucleation, growth, and then pyridine or acetonitrile-induced agglomeration (also called aggregation, coagulation, or flocculation) of modern, catalytically active Ir(0) nanoclusters stabilized by P2W15Nb3O629- polyoxoanions and Bu4N+ cations in acetone. The agglomeration kinetics of the Ir(0) nanoclusters are shown to fit an often assumed, but rarely experimentally determined, bimolecular aggregation step, B + B → C, rate constant k3, where B is the active catalyst and C is the deactivated catalyst. When this agglomeration step is added to the previously determined slow, continuous nucleation A → B, rate constant k1, and then autocatalytic surface growth, A + B → 2B, rate constant k2, steps (A is the (1,5-COD)Ir(I)+ in the precatalyst), the full steps of nucleation, growth, and then agglomeration are followed and treated kinetically for the first time for a modern, prototype transition-metal nanocl...

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)(eta 6-C6Me6)(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)(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.

Transition-Metal Nanocluster Stabilization Fundamental Studies: Hydrogen Phosphate as a Simple, Effective, Readily Available, Robust, and Previously Unappreciated Stabilizer for Well-Formed, Isolable, and Redissolvable Ir(0) and Other Transition-Metal Nanoclusters

This work tests the hypothesis that tridentate oxoanions are especially effective stabilizers of transitionmetal nanoclusters when the O-O distance of the anions matches closely the M- M( M) metal) distance atop the nanocluster surface. Specifically, we test the hypothesis that HPO4 2- with its 2.5 A O-O distance is a very simple, effective, but previously unrecognized anion for the stabilization of transition-metal(0) nanoclusters such as those of Ir(0), where the Ir-Ir surface distance is ca. 2.6-2.7 A. This hypothesis is tested by the five criteria we recently developed. These criteria emphasize the ability of a given nanoclusterstabilizing anion to allow the formation of highly kinetically controlled, near-monodisperse (e(15%) size distributions of nanoclusters and then to allow isolable and fully redissolvable nanoclusters that exhibit, once redispersed into solution, good catalytic activity and long catalytic lifetimes. The previously unknown precursor complex {[Bu4N][(1,5-COD)Ir‚HPO4]}n, 1, was prepared and shown to be a preferred precursor for the reproducible formation of hydrogen phosphate- and tetrabutylammonium-stabilized transitionmetal Ir(0) nanoclusters. The nanocluster formation reaction was shown to follow the slow continuous nucleation (A f B, rate constant k1) followed by fast autocatalytic surface growth (A + B f 2B, rate constant k2) mechanism uncovered previously; this finding was then exploited by showing that nanocluster size control could be achieved as expected by adding excess HPO4 2- to lower the k2/k1 ratio, resulting in the formation of smaller nanoclusters. A relatively rare experimental demonstration of the balanced reaction for nanocluster formation is also provided. Proton Sponge [i.e., 1,8-bis(dimethylamino)naphthalene] is shown to be a preferred scavenger of the 1 equiv of H+ byproduct formed from the H2 reduction of the (1,5-COD)Ir(I) + moiety in the nanocluster precursor to Ir(0) plus H + ; positive effects of Proton Sponge on the resultant nanocluster catalytic lifetime are also demonstrated. Transmission electron microscopy (TEM) of the postcatalysis nanoclusters shows that agglomeration is a catalysis-inhibiting deactivation reaction. Overall, the results show that HPO4 2- is an effective anion for the formation, and then stabilization, of Ir(0) transition-metal nanoclusters in acetone and with Bu4N + countercations. More specifically, HPO4 2rates alongside citrate 3- in the developing series of anion efficacy for nanocluster formation, stabilization and catalytic activity: polyoxoanions > HPO4 2- citrate 3- > other commonly employed nanoclusterstabilizing anions. Since a reasonable match between the tridentate O-O distance in HPO4 2- and the M-M distances is present for the metals Fe, Co, Ni, Ru, Rh Ir, Pd, Re, Os, and Pt [i.e., the lattice sizematching criterion is fulfilled; O ‹ zkar, S.; Finke, R. G. Coord. Chem. Rev. 2003 (submitted for publication)], our results imply that HPO4 2- merits consideration for nanocluster synthesis and stabilization any time M(0) nanoclusters of the above list of metals are planned. The additional advantages of HPO4 2- are also presented and briefly discussed, namely, its thermal robustness, its high resistance to reduction or oxidation, its valuable 31 P NMR handle, and its inexpensive and readily available nature. Finally, it is briefly discussed

Additional Investigations of a New Kinetic Method To Follow Transition-Metal Nanocluster Formation, Including the Discovery of Heterolytic Hydrogen Activation in Nanocluster Nucleation Reactions

A few years ago we developed a new kinetic method for following transition-metal nanocluster formation in which the resultant nanocluster's catalytic activity was used as a reporter reaction via the pseudoelementary step concept. This method in turn yielded insights into a new, broadly applicable mechanism of nanocluster formation under H2 consisting of (a) slow, continuous nucleation, A → B, followed by (b) fast autocatalytic surface growth, A + B → 2B (A = the nanocluster precursor, [Bu4N]5Na3[(1,5-COD)Ir·P2W15Nb3O62], B = the resultant nanocluster's surface metal atoms), in which the nanocluster behaves as a “living metal polymer”. Herein, this new kinetic method is investigated and tested further: (i) by following the Ir(0)∼300 nanocluster's kinetics of formation more directly via the H2 uptake reaction of the [Bu4N]5Na3[(1,5-COD)Ir·P2W15Nb3O62] precursordoes this also show an autocatalytic H2 uptake curve?; (ii) by seeing if the predicted initially small, then larger (past the induction period) size...

Nanocluster Formation Synthetic, Kinetic, and Mechanistic Studies. The Detection of, and Then Methods To Avoid, Hydrogen Mass-Transfer Limitations in the Synthesis of Polyoxoanion- and Tetrabutylammonium-Stabilized, Near-Monodisperse 40 ± 6 Å Rh(0) Nanoclusters

Previously we reported P2W15Nb3O629- polyoxoanion- and Bu4N+-stabilized, 20 ± 3 Å, Ir(0)∼300 nanoclusters. These nanoscopic materials are synthesized from the reduction of [(C4H9)4N]5Na3[(1,5-COD)M·P2W15Nb3O62] (M = Ir) by H2 in acetone and are isolable, highly catalytically active, with an unprecedented catalytic lifetime in solution. However, an initial attempt to synthesize Rh(0) nanoclusters from the analogous M = Rh precursor, and under conditions otherwise identical to the M = Ir synthesis, led to polydisperse 16 to 116 Å Rh(0) nanoclusters. The above results led, in turn, to the discovery that H2 gas-to-solution mass-transfer limitations (MTL) were responsible for the failure of the initial synthesis, an important finding since H2 is one of the most common reducing agents in syntheses of modern, near-monodisperse transition metal nanoclusters. The finding of a hydrogen MTL regime is fortified by stirring-rate-dependence data, kinetic data [demonstrating a catalyst nondependent (MTL) regime, and a catalyst-dependent (chemical-reaction rate-limiting) regime], and transmission electron microscopy used as a mechanistic probe. The results provided evidence for a growth mechanism involving parallel autocatalytic surface growth (leading to near-monodisperse nanoclusters) in competition with diffusive agglomeration (leading to polydisperse nanoclusters). The above mechanistic insights were then used, in turn, to design conditions where only the autocatalytic surface-growth pathway occurs, conditions which led to the successful synthesis of the desired near-monodisperse, 40 ± 6 Å Rh(0) P2W15Nb3O629- polyoxoanion- and Bu4N+-stabilized nanoclusters. The resultant Rh(0) nanoclusters are only the second example of polyoxoanion- and Bu4N+-stabilized transition metal nanoclusters.

Nanocluster Size-Control and “Magic Number” Investigations. Experimental Tests of the “Living-Metal Polymer” Concept and of Mechanism-Based Size-Control Predictions Leading to the Syntheses of Iridium(0) Nanoclusters Centering about Four Sequential Magic Numbers

Our recent kinetic and mechanistic studies of the formation of Bu4N+ and P2W15Nb3O629- polyoxoanion-stabilized Ir(0)∼300 nanoclusters led to the elucidation of a new mechanism for nanoclusters synthesized from metal salts under H2: slow, continuous nucleation, rate constant k1, then autocatalytic surface growth, rate constant k2. This mechanism contains four key, previously unverified predictions: (i) that the nanoclusters are “living-metal polymers” and, hence, that a series of increasing size nanoclusters can be synthesized by design; (ii) that the ratio of rates of growth to nucleation, R (=k2[nanocluster active sites]/k1), should correlate with and should be useful to predict the size of new nanoclusters; (iii) that the autocatalytic surface growth should tend to favor so-called “magic-number” size (i.e., closed shell; higher stability) nanoclusters; and, overall, (iv) that it should be possible to prepare, for the first time, a sequential series of nanoclusters centering about the transition metal magic-number nanocluster sizes, M13, M55 M147, M309, M561, M923 (and so on). These mechanism-based predictions are tested via the present work. The end result is the synthesis of an unprecedented sequential series of Ir(0)n nanocluster distributions centering about four sequential transition-metal magic numbers, specifically Ir(0)∼150, Ir(0)∼300, Ir(0)∼560, and Ir(0)∼900. Also discussed is another, as-yet unverified, prediction of the autocatalytic surface-growth mechanism and its living-metal polymer phenomenon, namely, that one can in principle rationally design and then synthesize all possible geometric isomers of bi-, tri-, and higher multimetallic transition-metal nanoclusters, each in an initially known, layered, “onionskin” structure.

Transition Metal Nanocluster Formation Kinetic and Mechanistic Studies. A New Mechanism When Hydrogen Is the Reductant: Slow, Continuous Nucleation and Fast Autocatalytic Surface Growth

Following an overview of the primitive state of mechanistic studies of the formation of nanoclusters, with a focus on LaMer's classic work on the formation of sulfur sols, kinetic and mechanistic studies of the formation of our recently reported novel P2W15Nb3O629- polyoxoanion- and Bu4N+- stabilized Ir∼190-450 (hereafter, Ir(0)∼300) nanoclusters are presented. The work reported consists of the full experimental and other details of the following eight major components: (i) development of an indirectbut easy, continuous, highly quantitative and thus powerfulmethod to monitor the formation of the Ir(0) nanoclusters via their catalytic hydrogenation activity and through the concept of pseudoelementary reaction steps; (ii) application of the appropriate kinetic equations for nucleation and autocatalysis, and then demonstration that these equations fit the observed, sigmoidal-shaped kinetic curves quantitatively with resultant rate constants k1 and k2; (iii) confirmation by a more direct, GLC method that the method in (i) indeed works and does so quantitatively, yielding the same k1 and k2 values within experimental error; (iv) collection of a wealth of previously unavailable kinetic and mechanistic data on the effects on nanocluster formation of added olefin, H2 pressure, anionic nanocluster stabilizer ([Bu4N]9P2W15Nb3O62 in the present case), H2O, HOAc, and temperature; (v) careful consideration and ruling out of other hypotheses, notably that particle-size rate effects alone might account for the observed sigmoidal shaped curves; and then (vi) distillation of the results into a minimalistic mechanism consisting of several pseudoelementary steps. Also presented as part of the Discussion are (vii) a concise but comprehensive review of the literature of transition metal nanocluster formation under H2 as the reducing agent, an analysis which provides highly suggestive evidence that the new mechanism uncovered is a much more general mechanismif not a new paradigmfor transition metal nanoclusters formed under H2 (and, the data argue, probably also for related reducing agents); and (viii) a summary of the seven key predictions of this new mechanism which remain to be tested (four predictions are the expected predominance of magic-number size nanoclusters; designed control of nanocluster size via the living-metal polymer concept; the synthesis of onion-skin structure bi-, tri-, and higher-metallic nanoclusters; and the use of face-selective capping agents as a way to block the autocatalytic surface growth and, thereby, to provide designed-shape nanoclusters). Overall, it is hoped that the resultsthe first new mechanism in more than 45 years for transition metal nanocluster formationwill go far toward providing a firmer mechanistic basis, and perhaps even a new paradigm, for the designed synthesis of new transition metal nanoclusters of prechosen sizes, shapes, and mono- to multimetallic compositions.