Conspectus
Supramolecular chemistry is a major area of
chemistry that utilizes
weaker non-covalent interactions between molecules, including hydrogen
bonding, van der Waals, electrostatic, π···π,
and C–H···π interactions. Such forces
have been the basis of several molecular self-assemblies and host–guest
complexes in organic, inorganic, and biological systems. Atomically
precise nanoclusters (NCs) are materials of growing interest that
display interesting structure–property correlations. The evolving
science of such systems reaffirms their molecular behavior. This gives
a possibility of exploring their supramolecular chemistry, leading
to assemblies with similar or dissimilar cluster molecules. Such assemblies
with compositional, structural, and conformational precision may ultimately
result in cluster-assembled hybrid materials. In this Account, we
present recent advancements on different possibilities of supramolecular
interactions in atomically precise cluster systems that can occur
at different length scales. We first present a brief discussion of
the aspicule model of clusters, considering Au25(SR)18 as an example, that can explain various aspects of its atomic
precision and distinguish the similar or dissimilar interacting sites
in their structures. The supramolecular interaction of 4-tert-butylbenzyl mercaptan (BBSH)-protected [Au25(SBB)18]− NCs with cyclodextrins (CD) to form
Au25SBB18∩CD
n
(n = 1–4) and that of [Ag29(BDT)12]3– with fullerenes to form [Ag29(BDT)12(C60)
n
]3– (n = 1–9) (BDT = 1,3-benzenedithiolate)
are discussed subsequently. The formation of these adducts was studied
by electrospray ionization mass spectrometry (ESI MS), optical absorption
and NMR spectroscopy. In the subsequent sections, we discuss how variation
in intercluster interactions can lead to polymorphic crystals, which
are observable in single-crystal X-ray diffraction. Taking [Ag29(BDT)12(TPP)4]3– (TPP
= triphenylphosphine) clusters as an example, we discuss how the different
patterns of C–H···π and π···π
interactions between the secondary ligands can alter the packing of
the NCs into cubic and trigonal lattices. Finally, we discuss how
the supramolecular interactions of atomically precise clusters can
result in their hybrid assemblies with plasmonic nanostructures. The
interaction of p-mercaptobenzoic acid (p-MBA)-protected Ag44(p-MBA)30 NCs with tellurium nanowires (Te NWs) can form crossed-bilayer precision
assemblies with a woven-fabric-like structure with an angle of 81°
between the layers. Similar crossed-bilayer assemblies show an angle
of 77° when Au102(p-MBA)44 clusters are used to form the structure. Such assemblies were studied
by transmission electron microscopy (TEM). Precision in these hybrid
assemblies of Te NWs was highly controlled by the geometry of the
ligands on the NC surface. Moreover, we also present how Ag44(p-MBA)30 clusters can encapsulate gold
nanorods to form cage-like nanostructures. Such studies involved TEM,
scanning transmission electron microscopy (STEM), and three-dimensional
tomographic rec...
Two ligand‐protected nanoscale silver moieties, [Ag46(SPhMe2)24(PPh3)8](NO3)2 and [Ag40(SPhMe2)24(PPh3)8](NO3)2 (abbreviated as Ag46 and Ag40, respectively) with almost the same shell but different cores were synthesized simultaneously. As their external structures are identical, the clusters were not distinguishable and become co‐crystallized. The occupancy of each cluster was 50 %. The outer shell of both is composed of Ag32S24P8, which is reminiscent of fullerenes, and it encapsulates a well‐studied core, Ag14 and a completely new core, Ag8, which correspond to a face‐centered cube and a simple cube, respectively, resulting in the Ag46 and Ag40 clusters. The presence of two entities (Ag40 and Ag46 clusters) in a single crystal and their molecular formulae were confirmed by detailed electrospray ionization mass spectrometry. The optical spectrum of the mixture showed unique features which were in good agreement with the results from time‐dependent density functional theory (TD‐DFT).
We present the first example of polymorphism (cubic & trigonal) in single crystals of an atomically precise monolayer protected cluster, Ag29(BDT)12(TPP)43-. We demonstrate that C-Hπ interactions of the secondary ligands (TPP) are dominant in a cubic lattice compared to a trigonal lattice, resulting in a greater rigidity of the structure, which in turn, results in a higher luminescence efficiency in it.
We report an attempt to probe into the energy demand of the fragmentation of atomically precise silver clusters using collision induced dissociation mass spectrometry. Energy resolved collisions of several gas phase ions of clusters, Ag 29 (S 2 R) 12 , Ag 25 (SR) 18 , and Ag 44 (SR) 30 , reveal distinct fragmentation kinetics involving charge separation. The fragmentation pattern of [Ag 25 (SR) 18 ] − is found to be different from its structural analog, [Au 25 (SR) 18 ] − . Survival yield analysis has been used to establish a direct comparison between the stability of the ions of these clusters, which reveals that [Ag 29 (S 2 R) 12 ] 3− is the most stable cluster ion, followed by [Ag 25 (SR) 18 ] − and [Ag 44 (SR) 30 ] 4− . Gas phase stabilities reflect their solution phase stabilities, indicating that the molecular nature of the clusters is retained in the gas phase, too. We further report that fragmentation occurs in a stepwise fashion, conserving the closed shell electronic stability of the parent ion at each step. Such studies are important in understanding the electronic and geometric stability of cluster ions and their fragments.
We report the formation of naked cluster ions of silver of specific nuclearities, uncontaminated by other cluster ions, derived from monolayer-protected clusters. The hydride and phosphine co-protected cluster, [Ag(TPP)H] (TPP, triphenylphosphine), upon activation produces the naked cluster ion, Ag, exclusively. The number of metal atoms present in the naked cluster is almost the same as that in the parent material. Two more naked cluster ions, Ag and Ag, were also formed starting from two other protected clusters, [Ag(DPPE)H] and [Ag(DPPE)H], respectively (DPPE, 1,2-bis(diphenylphosphino)ethane). By systematic fragmentation, naked clusters of varying nuclei are produced from Ag to Ag selectively, with systematic absence of Ag, Ag, and Ag. A seemingly odd number of cluster ions are preferred due to the stability of the closed electronic shells. Sequential desorption of dihydrogen occurs from the cluster ion, AgH, during the formation of Ag. A comparison of the pathways in the formation of similar naked cluster ions starting from two differently ligated clusters has been presented. This approach developed bridges the usually distinct fields of gas-phase metal cluster chemistry and solution-phase metal cluster chemistry. We hope that our findings will enrich nanoscience and nanotechnology beyond the field of clusters.
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