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

Abstract

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.