Nanoparticle Nucleation Is Termolecular in Metal and Involves Hydrogen: Evidence for a Kinetically Effective Nucleus of Three {IrH·PWNbO} in Ir(0) Nanoparticle Formation From [(1,5-COD)Ir·PWNbO] Plus Dihydrogen.

The nucleation process yielding Ir(0) nanoparticles from (BuN)Na[(1,5-COD)Ir·PWNbO] (abbreviated hereafter as (COD)Ir·POM, where POM = the polyoxometalate, PWNbO) under H is investigated to learn the true molecularity, and hence the associated kinetically effective nucleus (KEN), for nanoparticle formation for the first time. Recent work with this prototype transition-metal nanoparticle formation system ( J. Am. Chem. Soc. 2014 , 136 , 17601 - 17615 ) revealed that nucleation in this system is an apparent second-order in the precatalyst, A = (COD)Ir·POM, not the higher order implied by classic nucleation theory and its nA ⇌ A, "critical nucleus", A concept. Herein, the three most reasonable more intimate mechanisms of nucleation are tested: bimolecular nucleation, termolecular nucleation, and a mechanism termed "alternative termolecular nucleation" in which 2(COD)Ir and 1(COD)Ir·POM yield the transition state of the rate-determining step of nucleation. The results obtained definitively rule out a simple bimolecular nucleation mechanism and provide evidence for the alternative termolecular mechanism with a KEN of 3, Ir. All higher molecularity nucleation mechanisms were also ruled out. Further insights into the KEN and its more detailed composition involving hydrogen, {IrHPOM}, are also obtained from the established role of H in the Ir(0) formation balanced reaction stoichiometry, from the p(H) dependence of the kinetics, and from a D/H kinetic isotope effect of 1.2(±0.3). Eight insights and conclusions are presented. A section covering caveats in the current work, and thus needed future studies, is also included.