Molecular Engineering of Atomically Precise Silver Clusters into 2D and 3D Framework Solids

Two-
and three-dimensional (2D and 3D) atomically precise nanocluster
(NC)-based metal–organic frameworks (MOFs) with properties
richer than those of NCs themselves are emerging materials. However,
fabricating such materials with good stability has not been easy.
In this work, a facile synthetic strategy was employed for the creation
of silver NC–MOFs starting from [Ag12(TBT)7(TFA)4(CH3CN)6]+, facilitated
by heterocyclic amines, 4,4′-bipyridine (bpy) and pyrazine
(pyz), via metal–metal and metal-sulfide rearrangement reactions,
where TBT and TFA are tertiarybutylthiolate and trifluoroacetate,
respectively. In one of the reactions, the pyz ligand facilitates
the formation of a 2D framework with a trigonal crystal system, which
exhibits high stability and emits bright green luminescence at low
temperatures. Owing to its facile synthesis, good stability, efficient
luminescence, uniform porosity, and layered structure, the resultant
hexagonal 2D nanosheets can be efficiently exfoliated from parent
crystals. The 2D nanosheets are structurally similar to graphene.
A top-down approach was employed for the exfoliation of stable 2D
nanosheets with lateral dimensions in the range of 0.156 μm.
In another case, the bpy ligand induces the construction of a 3D framework
with an orthorhombic crystal system. Owing to its interpenetrated
AB···AB structure, robustness, and efficient green
luminescence at room temperature, the resultant 3D MOF is capable
of functioning as a high-performance luminescent sensor for selective
detection of explosive analogues, 2-nitrotoluene and 2,4-dinitrotoluene,
with excellent recyclability. However, in the absence of the heterocyclic
amines, a pristine AgNC was formed. Time-dependent density functional
theory calculations were employed to understand the mechanism of energy
transfer in AgNC-MOFs. Our strategy offers an unprecedented approach
in which heterocyclic amines facilitate intramolecular rearrangement
reactions, resulting in 2D and 3D atomically precise NC framework
materials. This work not only demonstrates the creation of 2D and
3D materials but also provides new insights into the critical surface
coordination chemistry controlled by heterocyclic amines for defining
the morphology and properties of cluster frameworks.