Transition-metal nanocluster stabilization versus agglomeration fundamental studies: Measurement of the two types of rate constants for agglomeration plus their activation parameters under catalytic conditions

摘要

A recent mechanistic study of transition-metal nanocluster formation and agglomeration (Besson, C.; Finney, E. E.; Finke, R. G. J. Am. Chem. Soc. 2005, 127, 8179) identified two types of agglomeration for the first time, specifically bimolecular agglomeration of nanoclusters B (i.e., B + B -> C; rate constant k(3)) and a new step of autocatalytic agglomeration of nanoclusters B with larger already somewhat agglomerated nanoclusters and/or bulk metal, C (i.e., B + C -> 1.5C; rate constant k(4)). Herein, this two-step, parallel-path agglomeration mechanism is independently tested by the temperature-induced agglomeration of the prototype preformed, pre-isolated P2W15Nb3O629--stabilized Ir(0)(similar to 900) transition-metal nanoclusters undergoing cyclohexene hydrogenation and concomitant agglomeration. The resulting k3 and k4 rate constants are measured as a function of the temperature, yielding the previously unavailable Delta H-double dagger and Delta S-double dagger for each type of nanocluster agglomeration under the specific reaction conditions, which include the presence of cyclohexene and H-2, Delta H-3(double dagger) = 6.2(3) kcal/mol, Delta S-3(double dagger) = -46(2) eu and Delta H-4(double dagger) = 18(1) kcal/mol and Delta S-4(double dagger) = -2.5(2) eu (standard state = 1 M). The interesting activation parameters suggest that the k(3) agglomeration step may be associatively activated, while the k(4) step appears to be dissociatively activated, for reasons discussed in the main text. Also reported is the attempted agglomeration of preformed, isolated Ir(0)(similar to 900) nanoclusters with added salt, Bu4NBF4. Reported primarily in the Supporting Information are extensive efforts attempting to achieve the in-principle ideal goal of measuring agglomeration kinetics (k(3) and k(4)) simultaneously with nucleation and growth kinetics (k(1) and k(2)) in the presence of pyridine as a known agglomerating agent. Overall, the successful kinetic measurements of the k(3) and k(4) agglomeration steps for pre-isolated Ir(0)(similar to 900) nanoclusters provided herein are significant in seven ways: (i) they independently verify the only recently discovered two types of agglomeration, bimolecular (k(3)) and autocatalytic (k(4)) agglomeration; (ii) the temperature dependence of the k(3) and k(4) processes provide the first activation parameters for these processes and yield the previously unavailable insights of their apparently associatively activated (k(3)) and dissociatively activated (k(4)) natures, at least under the reaction conditions with olefin and hydrogen present; and (iii) the two rate constants k(3) and k(4) define the term "nanocluster stability" in solution rigorously and in an experimentally testable way for the first time. In addition, (iv) the present studies serve as "proof of concept" that the measurement of agglomeration rate constants is a viable way to rank quantitatively and therefore distinguish transition-metal nanocluster stabilizers; (v) the results show that added salts, such as BU4NBF4, are ineffective in agglomerating at least highly stabilized P2W15Nb3O629--ligated Ir(0)(similar to 900) nanoclusers; and (vi) the results give some insight into the relative sizes of agglomerated C (>= Ir(0)(similar to 2000)) versus that of the starting Ir(0)(similar to 900) nanoclusters, B. Finally, (vii) the results also make apparent that the study of nanocluster agglomeration, by additional physical methods and using other, to-be-developed agglomerants, as a preferred way to quantitate nanocluster stability, remains as an important research challenge. Some thoughts about what additional physical methods may provide the best avenues for future studies are briefly discussed in the Summary.