Following an overview of the primitive state of
mechanistic studies of the formation of nanoclusters, with
a focus on LaMer's classic work on the formation of sulfur sols,
kinetic and mechanistic studies of the formation of
our recently reported novel
P2W15Nb3O62
9-
polyoxoanion- and Bu4N+- stabilized
Ir∼
190
-
450
(hereafter, Ir(0)∼300)
nanoclusters are presented. The work reported consists of the full
experimental and other details of the following
eight major components: (i) development of an indirectbut easy,
continuous, highly quantitative and thus
powerfulmethod to monitor the formation of the Ir(0)
nanoclusters via their catalytic hydrogenation activity
and
through the concept of pseudoelementary reaction steps; (ii)
application of the appropriate kinetic equations for
nucleation and autocatalysis, and then demonstration that these
equations fit the observed, sigmoidal-shaped kinetic
curves quantitatively with resultant rate constants
k
1 and k
2; (iii)
confirmation by a more direct, GLC method that
the method in (i) indeed works and does so quantitatively, yielding the
same k
1 and k
2 values
within experimental
error; (iv) collection of a wealth of previously unavailable kinetic
and mechanistic data on the effects on nanocluster
formation of added olefin, H2 pressure, anionic nanocluster
stabilizer
([Bu4N]9P2W15Nb3O62
in the present case),
H2O, HOAc, and temperature; (v) careful consideration and
ruling out of other hypotheses, notably that particle-size
rate effects alone might account for the observed sigmoidal
shaped curves; and then (vi) distillation of the results
into a minimalistic mechanism consisting of several pseudoelementary
steps. Also presented as part of the Discussion
are (vii) a concise but comprehensive review of the literature of
transition metal nanocluster formation under H2
as
the reducing agent, an analysis which provides highly suggestive
evidence that the new mechanism uncovered is a
much more general mechanismif not a new paradigmfor transition
metal nanoclusters formed under H2 (and, the
data argue, probably also for related reducing agents); and (viii) a
summary of the seven key predictions of this new
mechanism which remain to be tested (four predictions are the expected
predominance of magic-number size
nanoclusters; designed control of nanocluster size via the living-metal
polymer concept; the synthesis of onion-skin
structure bi-, tri-, and higher-metallic nanoclusters; and the use of
face-selective capping agents as a way to block
the autocatalytic surface growth and, thereby, to provide
designed-shape nanoclusters). Overall, it is hoped that
the
resultsthe first new mechanism in more than 45 years for transition
metal nanocluster formationwill go far toward
providing a firmer mechanistic basis, and perhaps even a new paradigm,
for the designed synthesis of new transition
metal nanoclusters of prechosen sizes, shapes, and mono- to
multimetallic compositions.