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 P 2 W 15 Nb 3 O 62 9 - polyoxoanions and Bu 4 N + 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 k 3, 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 k 1, and then autocatalytic surface growth, A + B → 2B, rate constant k 2, 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 nanocluster. A pictorial scheme depicting these three pseudo-elementary steps is also presented. The treatment of agglomeration kinetics is significant since it should permit additional, important advances, specifically, the ready recognition of agglomeration, the measurement of the agglomeration rate constant k 3, and, especially, the quantitative study of agglomeration, k 3, as a function of all the variables that stabilize or destabilize transition-metal nanoclusters (e.g., added ligands, solvents of different dielectric constant or coordinating ability, different anions or cations, added polymers or dendrimers, and so on). Additional insights generated by these studies are also discussed. Last, presented as targets of future research are the unexplained observations: (a) that the Ir(0) nanoclusters are not agglomerated by the added salt of noncoordinating anions such Bu 4 N + BF 4 -, contrary to the salt-induced agglomeration observed for classical colloids where primarily DLVO (Derjaguin−Landau−Verway−Overbeek)-type Coulombic repulsion vs van der Waals attractive forces are operative; and (b) that the Ir(0) and other, selected modern transition-metal nanoclusters in organic solvents may be taken to dryness and then fully redissolved, that is, do not appear to possess a ccc (critical coagulation concentration) as do water-soluble, DLVO-stabilized classical colloids.