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

Our recent kinetic and mechanistic studies of the formation of Bu4N+ and P2W15Nb3O629- polyoxoanion-stabilized Ir(0)(similar to 300) nanoclusters led to the elucidation of a new mechanism for nanoclusters synthesized from metal salts under H-2: slow, continuous nucleation, rate constant hi, then autocatalytic surface growth, rate constant k(2). 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 (=k(2)[nanocluster active sites]/k(1)), 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, M-13, M-55 M-147, M-309, M-561, M-923 (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)(similar to 150), Ir(0)(similar to 300), Ir(0)(similar to 560), and Ir(0)(similar to 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.