2
1 An Overview of Metal Clusters and Their Reactivity
bioimaging, etc. [32–36]. Abundant metal nanoclusters (NCs) with precise atomiclevel structures have been synthesized [37–55], and the high thermodynamic, electronic and chemical stabilities of these nanoclusters are often found to be rooted in
bonding nature [56], degenerated electronic states and high symmetry of geometric
structure of the metallic core [57, 58]. In particular, abundant MPCs prefer core-shell
structures and similar building blocks of the kernel are frequently observed [59, 60],
such as tetrahedral M 4 [61, 62], octahedral M 6 [63], bi-tetrahedral M 8 [64], hollowcage M 12 [65–67], and particularly icosahedral M 13 [68]. For example, a vast of
MPCs including Au 18 [51], Au 20 [49, 50], Au 25 [45], Au 30 [42, 43], Au 36 [41, 69],
Au 38 [40], Au 55 [39, 70, 71], Au 60 [38], and Au 102 [37] etc., all have a 13-atom icosahedral inner core [72, 73]. More and more stable metal clusters that were ascertained
in gas phase have been successfully synthesized via wet chemistry approaches [74],
revealing the correspondences of gas-phase stability/reactivity and condense-phase
properties [48]. It is notable that the nature of the metallic core plays a determining
role in the cluster structure evolution [75], stability and electronic transition between
frontier orbitals.
Metal cluster reactivity in gas phase has been widely recognized of importance
to determine the correspondence between activity and electronic/geometric structure stability. In general, as indicated by the HOMO-LUMO gaps, binding energies,
ionization energies, etc., the metal cluster reactivity undergoes odd-even alternation
effect especially for the charge-transfer dominated reactions. Also, certain reactions
of the metal clusters could exhibit strong size-dependence and charge-state variation; some reactions are ADE (adiabatic detachment energy)-dependent with likely
long-range charge transfer pertaining to a harpoon mechanism. Besides, the multiple
valence states could account for the redox reactions of metal clusters with oxygenic
chemicals [76–78]. Typical gas-phase reactivity has been unveiled for metal clusters
reacting with water due to the importance of hydrogen evolution, where a mechanism of complementary active sites (Lewis acid/base) was demonstrated to account
for irregular charge distribution on the cluster surface and thus the size-dependent
reactivity of aluminum clusters with water. Similar hydrogen evolution reactions
have also been found for aluminum clusters reacting with alcohols [79], allowing
a competition and likely self-catalysis in the presence of multiple −OH reactants.
Besides, the reactivity of metal clusters provides a variety of insightful information
regarding the C–H bond activation [80–92], C–C bond cleavage [89, 93] C–N and
C–S bond activation [94, 95], as well as C–C cross coupling reactions [96–98].
As quantum confinement effects often govern the behavior of matter in tiny size
regime [2], studying the cluster reactivity provides fundamental insights into the
interplay of atomic structure, geometry, and electronic property, enabling to manipulate the chemical behaviors. The studies of metal cluster reactivity can help develop
tunable materials with possible catalytic or energetic qualities. This potential has been
brought to fruition through recent advances. For example, an insight into the gasphase reactivity experiments on aluminum clusters reacting with molecular oxygen
have led to the discovery of specific clusters mimicking elements of the periodic table.
This major finding, known as the superatom concept [9, 99–103], originated from
the experimental study of the reactivity of aluminum clusters by noting a dramatic
1 An Overview of Metal Clusters and Their Reactivity
bioimaging, etc. [32–36]. Abundant metal nanoclusters (NCs) with precise atomiclevel structures have been synthesized [37–55], and the high thermodynamic, electronic and chemical stabilities of these nanoclusters are often found to be rooted in
bonding nature [56], degenerated electronic states and high symmetry of geometric
structure of the metallic core [57, 58]. In particular, abundant MPCs prefer core-shell
structures and similar building blocks of the kernel are frequently observed [59, 60],
such as tetrahedral M 4 [61, 62], octahedral M 6 [63], bi-tetrahedral M 8 [64], hollowcage M 12 [65–67], and particularly icosahedral M 13 [68]. For example, a vast of
MPCs including Au 18 [51], Au 20 [49, 50], Au 25 [45], Au 30 [42, 43], Au 36 [41, 69],
Au 38 [40], Au 55 [39, 70, 71], Au 60 [38], and Au 102 [37] etc., all have a 13-atom icosahedral inner core [72, 73]. More and more stable metal clusters that were ascertained
in gas phase have been successfully synthesized via wet chemistry approaches [74],
revealing the correspondences of gas-phase stability/reactivity and condense-phase
properties [48]. It is notable that the nature of the metallic core plays a determining
role in the cluster structure evolution [75], stability and electronic transition between
frontier orbitals.
Metal cluster reactivity in gas phase has been widely recognized of importance
to determine the correspondence between activity and electronic/geometric structure stability. In general, as indicated by the HOMO-LUMO gaps, binding energies,
ionization energies, etc., the metal cluster reactivity undergoes odd-even alternation
effect especially for the charge-transfer dominated reactions. Also, certain reactions
of the metal clusters could exhibit strong size-dependence and charge-state variation; some reactions are ADE (adiabatic detachment energy)-dependent with likely
long-range charge transfer pertaining to a harpoon mechanism. Besides, the multiple
valence states could account for the redox reactions of metal clusters with oxygenic
chemicals [76–78]. Typical gas-phase reactivity has been unveiled for metal clusters
reacting with water due to the importance of hydrogen evolution, where a mechanism of complementary active sites (Lewis acid/base) was demonstrated to account
for irregular charge distribution on the cluster surface and thus the size-dependent
reactivity of aluminum clusters with water. Similar hydrogen evolution reactions
have also been found for aluminum clusters reacting with alcohols [79], allowing
a competition and likely self-catalysis in the presence of multiple −OH reactants.
Besides, the reactivity of metal clusters provides a variety of insightful information
regarding the C–H bond activation [80–92], C–C bond cleavage [89, 93] C–N and
C–S bond activation [94, 95], as well as C–C cross coupling reactions [96–98].
As quantum confinement effects often govern the behavior of matter in tiny size
regime [2], studying the cluster reactivity provides fundamental insights into the
interplay of atomic structure, geometry, and electronic property, enabling to manipulate the chemical behaviors. The studies of metal cluster reactivity can help develop
tunable materials with possible catalytic or energetic qualities. This potential has been
brought to fruition through recent advances. For example, an insight into the gasphase reactivity experiments on aluminum clusters reacting with molecular oxygen
have led to the discovery of specific clusters mimicking elements of the periodic table.
This major finding, known as the superatom concept [9, 99–103], originated from
the experimental study of the reactivity of aluminum clusters by noting a dramatic