Nucleation Kinetics and Molecular Mechanism in Transition-Metal Nanoparticle Formation: The Intriguing, Informative Case of a Bimetallic Precursor, {[(1,5-COD)Ir-I center dot HPO4](2)}(2-)

摘要

Following a summary of the relevant literature on nucleation and applicable nucleation and growth models and mechanisms, the kinetics and molecular mechanism of nucleation are investigated in detail starting from [(1,5-COD)Ir-I(NCCH3)(2)][BF4], 1, which upon addition of HPO42- to 1 forms a neutral, phosphate-bridged species, {[(1,5-COD)Ir-I(NCCH3)](2)center dot HPO4}(0), 2, en route to {[(1,5-COD)(IrHPO4)-H-I center dot](2)[Bu4N](2)}, 3. Post a list of the seven advantages of the {[(1,5-COD)Ir-I center dot HPO4](2)}(2-) precatalyst as a novel bimetallic precursor for kinetic and mechanistic studies of nucleation, six important, previously unanswered questions are raised about the ill-understood but exceedingly broad and important topic of nucleation. H-1 NMR solution speciation and Signer apparatus solution molecular weight studies establish that in situ prepared {[(1,5-COD)Ir-I center dot HPO4](2)}(2-), 3, exists predominantly in its indicated, dimeric form. The NMR studies also identify {[(1,5-COD)Ir-I(NCCH3)](2)center dot HPO4}(0), 2, as an important, metastable species with one less HPO42-, formed in a dissociative equilibrium of 3 to 2 plus HPO42-. Kinetic studies reveal a first-order dependence of nucleation on the concentration of 3 and hence rule out the higher-order dependence implied by classical nucleation theory. Additional kinetic studies reveal a telling, inverse, quadratic dependence on added HPO42-, results that unveil the previously unavailable insights that a simple bimetallic, Ire precursor is sufficient to enable low-molecularity nucleation via {[(1,5-COD)Ir-I(NCCH3)](2)center dot HPO4}(0), 2, as a kinetically competent intermediate, a unique example of a nucleation mechanism known in molecular detail from a precisely defined molecular precursor that also includes spectroscopic detection of a kinetically competent intermediate. The results with {[(1,5-COD)Ir-I center dot HPO4](2)}(2-) in comparison to previous results with {[(1,5-COD)Ir-I center dot POM]}(8-) (POM = the polyoxometalate P2W15Nb3O629-) allow insights into the details of nucleation, notably that Ir-2 versus Ir-3 kinetically effective nuclei are controlled by the different HPO42- and POM8- anion's surface charge and resultant 2 versus 3 Ir-I(1,5-COD)(+) moieties they are able to bind to achieve surface-charge neutrality. The state-of-the-art nucleation results allow a total of nine insights, conclusions, and two new working hypotheses, insights that promise to help drive a deeper understanding of nucleation, not just in transition-metal nanoparticle formation but hopefully more broadly across nature.