Assembly of Atomically Precise Silver Nanoclusters into Nanocluster-Based Frameworks

Article abstract of DOI:10.1021/jacs.9b02486

Here, we demonstrate an approach to synthesizing and structurally characterizing three atomically precise anion-templated silver thiolate nanoclusters, two of which form one- and two-dimensional structural frameworks composed of bipyridine-linked nanocluster nodes (referred to as nanocluster-based frameworks, NCFs). We describe the critical role of the chloride (Cl^-) template in controlling the nanocluster's nuclearity with atomic precision and the effect of a single Ag atom difference in the nanocluster's size in controlling the NCF dimensionality, modulating the optical properties, and improving the thermal stability. With atomically precise assembly and size control, nanoclusters could be widely adopted as building blocks for the construction of tunable cluster-based framework materials.

Full text of DOI:10.1021/jacs.9b02486

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Article
Assembly of Atomically Precise Silver
Nanoclusters into Nanocluster-based Frameworks
Mohammad J. Alhilaly, Ren-Wu Huang, Rounak Naphade, Badriah Alamer,
Mohamed Nejib Hedhili, Abdul-Hamid Emwas, Partha Maity, Jun Yin, Aleksander
Shkurenko, Omar F. Mohammed, Mohamed Eddaoudi, and Osman M. Bakr
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02486 • Publication Date (Web): 28 May 2019
Downloaded from http://pubs.acs.org on May 28, 2019
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Assembly of Atomically Precise Silver Nanoclusters into Nanoclus-
ter-based Frameworks
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Mohammad J. Alhilaly,†,||, Ren-Wu Huang,†,|| Rounak Naphade,†,|| Badriah Alamer,†,|| Mohamed
⊥
Nejib Hedhili,‡ Abdul-Hamid Emwas,‡ Partha Maity,†,|| Jun Yin,† Aleksander Shkurenko,§ Omar
F. Mohammed,† Mohamed Eddaoudi,§ and Osman M. Bakr*,†,||
†Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
||KAUST Catalysis Center (KCC), Division of Physical Sciences and Engineering, King Abdullah University of Sci-
ence and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
‡Core Labs, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
§Functional Materials Design, Discovery and Development Research Group (FMD3), Advanced Membranes and Po-
rous Materials Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi
Arabia
⊥
Department of Physics, College of Sciences, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, 11623,
Saudi Arabia
ABSTRACT: Here, we demonstrate an approach for synthesizing and structurally characterizing three atomically precise
anion-templated silver thiolate nanoclusters, two of which form one- and two-dimensional structural frameworks com-
posed of bipyridine-linked nanocluster nodes (referred to as nanocluster-based frameworks, NCFs). We describe the critical
role of the chloride (Cl-) template in controlling the nanocluster’s nuclearity with atomic precision; and the effect of a single
Ag atom difference in the nanocluster’s size in controlling the NCF dimensionality, modulating the optical properties, and
improving the thermal stability. With atomically precise assembly and size control, nanoclusters could be widely adopted
as building blocks for the construction of tunable cluster-based framework materials.
INTRODUCTION
crystalline solids. In fact, NCs reported to date typically
crystallize into relatively trivial close-packed structures
that lack diversity. The ability to control the NCs’ coordi-
nation number affords the access to distinct secondary
building units (SBUs) with assorted geometries and con-
nectivity, defined by the arrangement of points of exten-
sions on the NCs, and the prospective to construct periodic
extended solids with fine-tuned porosity, reactivity, and
conductivity, as well as optical and mechanical proper-
ties.6-12
One primary aim of nanotechnology is the bottom-up
programmed assembly of materials from nano-sized build-
ing blocks with molecular precision and control. Unfortu-
nately, most nanoparticles are atomically inhomogeneous,
precluding their use as building blocks for such assembly.
Atomically precise nanoparticles, also known as nanoclus-
ters (NCs), are thus highly prized in the nanochemist’s
toolkit, as they make nanoparticles amenable to supramo-
lecular assembly, property control, and materials interro-
gation (e.g., periodic structures amenable to single-crystal
structure determination). Researchers have made tremen-
dous strides in synthesizing metal and metal chalcogenide
NCs with unique structures and distinct physicochemical
properties;1-5 for example, an impressive level of control
over the size and surface ligand chemistry of such NCs has
been achieved. However, relatively little progress has been
made in developing and understanding the supramolecu-
lar chemistry associated with the assembly of NCs into
Silver NCs stabilized with ligands (such as thiolate, al-
kynyl, or phosphine)13-27 are ideal candidates for developing
chemical approaches to the controlled self-assembly of sol-
ids. The size, composition, and surface chemistry of such
NCs are reasonably well understood; however, there are
enormous benefits to be gained from controlling the self-
assembly modes of these NCs, stemming from their pro-
spective applications in catalysis,28-30 fluorescence sens-
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ing,31 and bioimaging,32 as well as anti-microbials33. The as-
(one silver NC and two cluster-based assembled networks)
exhibits qualitatively distinct characteristics in photolumi-
nescence (PL) and thermal stability. Specifically, higher-di-
mensional structures show enhanced PL and higher ther-
mal stability.
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sembly of NCs into atomically precise NC-based frame-
works would combine advantages from both NCs and
metal-organic frameworks (MOFs), including: providing
high nuclearlity cluster nodes, high structure tunability,
highly connected frameworks with more structural com-
plexity, large channels, and more robustness. These poten-
tial advantages are attractive for a wide range of potential
applications including sensing, catalysis, and gas separa-
tion.6, 34 Recently, Ag thiolate NCs, such as Ag10, Ag12, and
Ag14,34-36 have been shown to assemble into one-, two-, and
three-dimensional frameworks in which the NCs form
nodes connected by pyridyl-type organic linkers. In addi-
tion, several assembled structures of anion-templated Ag
thiolate nanoclusters networked by short inorganic anions
have also been reported.37-38 The solvents used have also
been found to play a critical role in the assemblies’ mor-
phology.39 Despite advances in the synthesis of Ag-cluster-
based MOFs in terms of boosting stability and enhancing
optical properties, it remains challenging to produce atom-
ically precise cluster-based frameworks (which is also re-
ferred to here as nanocluster-based frameworks, NCFs)
with different dimensionalities. Indeed, studies on control-
ling cluster size in assembled networks with atomic preci-
sion or on the significant roles played by size and anion
templates40 – which are believed to affect the dimensional-
ity and resulting properties – have yet to be reported.
Our approach represents a new method for controlling
the nuclearity and dimensionality of self-assembled NCs,
leading to novel atomically precise cluster-based MOF-like
assembled materials, and at the same time modulating
their optical properties. With improved control and under-
standing of the structures’ self-assembly and size, we an-
ticipate that metal NCs will provide a myriad of modular
building blocks, with assorted shapes and connectivities,
for the practice of reticular chemistry.
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RESULTS AND DISCUSSION
Synthesis and characterization of 0-D NC, 1-D NCF
and 2-D NCF. As illustrated in Scheme 1, a zero-dimen-
sional Ag NC (0-D NC) and two Ag NC-based frameworks
(1-D NCF and 2-D NCF) were synthesized by a chemical
reaction of the precursors using a one-pot approach. The
chemical reaction initially occurred between the premade
[Ag-StBu]n complex with silver trifluoroacetate in CH3CN
solvent to produce a homogeneous mixture under stirring.
Then, a soluble dimethylformamide (DMF) solution of
tetraphenylphosphonium chloride (PPh4Cl) was added to
form the Ag NCs. It is noted that the DMF solvent plays a
critical role in facilitating the templating effect of Cl-. To
facilitate the assembly of the Ag clusters into one- and two-
dimensional networks (1-D NCF and 2-D NCF, respec-
tively), bpy was added to the reaction mixture and utilized
as an organic linker. By applying specific ratios of the pre-
cursors (see the experimental section for the synthesis de-
tails), rod-like green-emitting crystals of 2-D NCF were ob-
tained in high yield. By increasing the ratio of the inde-
pendent ligands (bpy and AgCF3COO precursors) to the Ag
thiolate complex, we found that the crystallization system
tended to grow predominantly large 1-D NCF crystals and
small 2-D NCF crystals (as a co-product). After the crystal-
lization of the resulting clear solutions of the filtrates by
slow evaporation at room temperature, high-quality single
crystals suitable for single-crystal X-ray diffraction
(SCXRD) were obtained after 7-12 days. The crystal struc-
tures of 0-D NC, 1-D NCF, and 2-D NCF were studied by
SCXRD, and the phase purities were evaluated by PXRD.
Furthermore, the optical properties and thermal stability
of the structures were studied. The synthesis and crystalli-
zation details of all three products can be found in the ex-
perimental section.
In this work, we present a strategy for extending the size
and dimensional control over Ag(I) thiolate NCs. In partic-
ular, we report three new crystal structures: a single Ag
nanocluster (0-D NC) and two Ag nanocluster-based
frameworks (1-D NCF and 2-D NCF). Our approach relies
on Cl- as a template for NC growth in conjunction with a
suitable organic linker, 4,4’-bipyridine (bpy), in the case of
1-D NCF and 2-D NCF. More specifically, when no organic
linker (no bpy) is used, a Ag16 cluster is attained, which
crystallizes as a discrete zero-dimensional structure (0-D
NC). When bpy is added, depending on the ratio of the lig-
ands (mainly the bpy linker) to the Ag thiolate complex,
we found that two distinct clusters form: Ag15 and Ag14,
each adopting a MOF-like structure with specific dimen-
sionality, i.e., 1-D NCF and 2-D NCF, respectively. The
three cluster units in 0-D NC, 1-D NCF, and 2-D NCF share
notable similarities; in particular, the units possess a simi-
lar geometrical structure, in which the skeletons of cluster
cores can be considered complete or incomplete square gy-
robicupolas (Figure 1) and the protecting ligand shell of
each cluster core contains a combination of the thiolate
(tBuS-) and trifluoroacetate (CF3COO-) ligands, as well as
coordinated solvent molecules. As the ratio of the ligands
to the Ag thiolate complex increases (in the presence of
bpy linker), clusters begin to distort more to accommodate
structures of higher connectivity (and, thus, confined spac-
ing). These distortions lead to a sequential loss of a single
surface Ag atom as the NC adopts one- and two-dimen-
sional assembled networks of Ag15 and Ag14 (1-D NCF and
2-D NCF), respectively. Each of all these three structures
Scheme 1. Synthesis of Ag NC and NC-based frameworks
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DMF molecules were observed in the Ag15 cluster of 1-D
Single-crystal structures of 0-D NC, 1-D NCF, and 2-
D NCF. [Ag16Cl(StBu)8(CF3COO)7(DMF)4(H2O)]1.5(DMF)
(0-D NC). First, a crystal of 0-D NC was evaluated by
SCXRD. Analysis of the crystallographic data showed that
0-D NC crystallizes in the monoclinic space group C2/c
(Table S1). The structure of 0-D NC is essentially a silver
cluster with a Ag16 skeleton welded by argentophilic inter-
actions; the Ag16 cluster is templated by one enclosed Cl- at
the center and forms a distorted square gyrobicupola (29th
Johnson solid, J29),41 which is a polyhedron with eight tri-
angular and ten distorted square faces (Figure 1A). Among
these polygons, each square face is capped by a tBuS- ligand
(except for the top and bottom faces) with a consistent μ4-
η1:η1:η1:η1-ligation mode to link four silver ions with Ag–S
distances in the range of 2.368(2)–2.598(2) Å (Figures 2,
top). The skeleton is also protected by seven CF3COO- aux-
iliary ligands (Figure S1). Five of those ligands bind to the
equatorial plane (octagonal Ag8) of the polyhedron
through Ag–O bonds adopting the μ2-η1:η1 mode (Ag–O:
2.19(3)–2.474(6) Å). The remaining two CF3COO- ligands
cap the upper (μ3-η1:η2 mode) and lower (μ2-η1:η1 mode)
square faces with Ag–O bonds in the ranges of 2.40(1)–
2.68(2) and 2.483(8)–2.69(3) Å, respectively. The protect-
ing ligand shell also contains four DMF molecules and one
H2O molecule coordinated in the equatorial plane (Figure
S4, left). In addition, the Ag16 cluster structure includes one
and a half molecules of the co-crystallized DMF solvents
(Figure S5, left). Interestingly, within the skeleton, the dis-
tances between the central Cl- and peripheral Ag atoms, in
the range of 2.789(2)–3.608(2) Å, are much longer than
those of conventional Ag−Cl bonds (approximately 2.6 Å).
A similar situation has been observed for alkynyl-protected
silver clusters.14, 42-43 The anion templating effect of Cl- is
quite important for the assembly of the skeleton structure.
It is worth noting that the argentophilic interactions also
play a crucial role in the stabilization of the silver cluster
skeleton. The observed Ag···Ag distances range from
2.919(2) to 3.286(2) Å, which are shorter than twice the van
der Waals radius of silver (3.44 Å), indicating significant
argentophilic interactions.
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NCF. Likewise in the previous structure, the Ag15 is co-pro-
tected by CF3COO- auxiliary ligands: four of them are co-
ordinated in the equatorial plane in the μ2-η1:η1 mode with
Ag–O distances in the range of 2.299(6)–3.054(8) Å; the lig-
ands are found in the upper (μ3-η1:η2 mode) and lower axial
positions (μ2-η1:η1 mode) with Ag–O distances in the range
of 2.450(8)–2.709(7) and 2.399(8)–2.60(2) Å, respectively.
The number of CF3COO- auxiliary ligands was also ob-
served to decrease proportionally as one Ag atom was lost
to balance the charge. Interestingly, this compound is
found to contain the substitutional disorder of one auxil-
iary ligand bonded to the equatorial Ag atom, with an oc-
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cupancy of 0.67 for CF3COO- and 0.33 for NO3 . The unan-
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ticipated NO3 may be derived from the residual silver ni-
trate in the [Ag-StBu]n complex precursor (Figure S4, mid-
dle). Finally, each Ag15 cluster is connected to three adja-
cent such clusters through four bpy linkers to form an in-
finite one-dimensional (1D) ladder-like structure. In this
structure, dual-cluster units, each consisting of two clus-
ters and two juxtaposed bpy monomer pillars, are further
combined by single-bpy bridges along the b-axis only. The
dual-cluster unit is inclined relative to the single-bpy
bridges at an acute angle of approximately 70° (Figure 3B).
Figure 1. Geometrical structures of the cores of 0-D NC (A),
1-D NCF (B), and 2-D NCF (C).
[Ag15Cl(StBu)8(CF3COO)5.67(NO3)0.33(bpy)2(DMF)2]4.3(D
MF)H2O (1-D NCF). Crystallographic data analysis of 1-D
NCF showed that its crystal structure contains a Ag15 clus-
ter also templated by Cl- and belongs to the triclinic space
group P–1 (Table S2). The skeleton of the Ag15 cluster core
is composed of an incomplete distorted square gyrobicu-
pola through the argentophilic interactions, with Ag···Ag
bond distances in the range of 2.912(1)−3.381(1) Å, due to
one missing Ag atom; the polyhedron contains six triangu-
lar and eight square faces (Figure 1B). The skeleton is pro-
tected by eight tBuS- ligands that adopt two types of coor-
dination modes—six adopt the μ4-η1:η1:η1:η1-ligation mode
and two adopt the μ3-η1:η1:η1-ligation mode—where the
Ag–S distances are in the range of 2.338(3)−2.576(2) Å. The
structure also features a clear distortion, as shown in Fig-
ure 2 (middle). In contrast to the four coordinated DMF
molecules in the Ag16 NC (0-D NC), only two coordinated
Figure 2. Top view vs. side view comparison between core
structures of 0-D NC (top), 1-D NCF (middle), and 2-D NCF
(bottom).
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Figure 3. Crystal structures of 0-D NC (A), 1-D NCF (B), and 2-D NCF (C). Free (co-crystallized) DMF molecules are not shown.
The green semi-transparent spheres in the silver clusters are shown purely as a guide. Hydrogens have been omitted for clarity.
[Ag14Cl(StBu)8(CF3COO)5(bpy)2(DMF)]2(DMF)
NCF). Crystallizing in the triclinic space group P–1 (Table
S3), 2-D NCF is formed by a Ag14 cluster that, similarly to
0-D NC and 1-D NCF, is also assembled by a templating
(2-D
and the entire two-dimensional framework is a 3-c, 2-peri-
odic edge transitive net called hcb (Figure S7C). Finally,
the adjacent 2D layers are further extended into a 3D su-
pramolecular structure via weak hydrogen bonds between
the terminal t-butyl and trifluoromethyl groups on the sur-
face of the adjacent layers (Figure S8). The layers are
slightly offset with respect to each other; thus, two types of
channels in the 2-D NCF crystal structure are observed: the
channels oriented along the direction [1 -1 0] are parallel to
the layers, whereas the channels oriented along the crys-
tallographic a-axis penetrates them. Two non-coordinated
DMF molecules were localized in the channels, and the
contribution of the significantly delocalized electron den-
sity was excluded from the refinement by the PLATON
SQUEEZE44 procedure.
chloride. The Ag14 skeleton is considered an incomplete
square gyrobicupola and assembles into a two-dimensional
structure of networked Ag14 cluster nodes (Figures 2, bot-
tom and 3C). Because two of the original 16 Ag atoms are
missing, the Ag14 skeleton features only four triangular and
six square faces constructed via sparse argentophilic inter-
actions (Ag···Ag distances in the range of 2.890(1)–
3.4072(8) Å) and is co-protected by eight tBuS- ligands, five
CF3COO- auxiliary ligands, and only one coordinated DMF
molecule (Figures S4 and S5, right). We found that the
eight tBuS- ligands adopt two types of coordination modes:
four adopt the μ4-η1:η1:η1:η1-ligation mode, and the other
four adopt the μ3-η1:η1:η1-ligation mode. The range of Ag–S
distances is 2.34(2)–2.558(2) Å. The five CF3COO- ligands
can also be classified into two groups. Similarly to the
structures described above, the equatorial and one axial
CF3COO- ligands are coordinated in the μ2-η1:η1 mode,
while the second axial ligand is coordinated in the μ3-η1:η2-
ligation mode. The Ag–O distances range from 2.288(6) to
2.93(1) Å. Furthermore, each Ag14 cluster coordinates to
four bpy linkers. Consequently, two clusters and two par-
allel bpy linkers are employed to construct a dual-cluster
unit similar to that in 1-D NCF. Each dual-cluster unit is
further connected to four others in the upper and lower
rows by single-bpy bridges to form a 2D framework, in
which the Ag14 clusters are regarded as the nodes of the 2D
framework (Figure 3C). In other words, two of the Ag14
clusters are directed towards the same neighbor, forming a
“dual-cluster unit”, as observed in 1-D NCF. From a topo-
logical point of view, the two linkers there act as one edge
(Figure S7B). Since the next two bpy spacers are directed
toward different Ag14 clusters, two-dimensional framework
formation is observed, where each Ag14 cluster is a 3-c node
Optical absorption and photoluminescence proper-
ties of 0-D NC, 1-D NCF, and 2-D NCF. We measured the
optical absorption spectra of 0-D NC in solution and solid-
state (Figure S9). Crystals of 0-D NC display a clear absorp-
tion band in the UV region (~200-350 nm), as shown in Fig-
ure S9(top). Crystals of 1-D NCF and 2-D NCF also exhibit
absorption bands in the UV region, but both show an ad-
ditional shoulder peak at ~420 nm (Figures S10 and S11).
This shoulder is only observed for 1-D NCF and 2-D NCF,
possibly due to residual solvent (DMF molecules) within
the networked cluster nodes, as the shoulder peak be-
comes less intense (or nearly disappears) with more effi-
cient drying of the crystals.
Crystals of 2-D NCF exhibit clear green emission at ~530
nm with excitation at 365 nm at room temperature (~298
K) under ambient conditions (Figure S12). In contrast, the
emissions of 0-D NC and 1-D NCF at room temperature are
not visually detectable; indeed, we could not observe any
steady-state PL under UV light (365 nm). Nevertheless, we
were able to detect weak emissions of 0-D NC and 1-D NCF
at lower temperatures using temperature-dependent pho-
toluminescence (TDPL). The maximum emission wavelen-
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Figure 4. Experimental absorption spectra (solid black lines) with calculated optical transition wavelengths and oscillator
strengths (vertical lines) using TDDFT method and the dominant natural transition orbital (NTO) hole and particle pairs
(isovalue = 0.02 eÅ-3) for the main singlet excited states of (A) 0-D NC, (B) 1-D NCF, and (C) 2-D NCF.
gth of the TDPL of 0-D NC occurs at ~535 nm, with no no-
table spectral shift with varying temperature; the spectrum
also shows a shoulder peak at ~430 nm (Figure S13). We
observed a clear, systematic decrease in the 0-D NC emis-
sion intensity with increasing temperature. 1-D NCF crys-
tals display similar emission spectra as 0-D NC crystals, ex-
hibiting the same emission at ~535 nm, with no clear shift
with changing temperature, and the shoulder peak (~430
nm) is also observed (Figure S14). However, 0-D NC shows
sensitivity to UV light; specifically, a change in color is ob-
served as a result of degradation induced by UV light (at
excitation wavelength 365 nm). We also examined the
TDPL of 2-D NCF crystals, which exhibit distinctly
stronger emission, as shown in Figure S15. A systematic
spectral red shift was observed with an increase in temper-
ature from 77 K to room temperature, with the emission
wavelength varying from ~511 to 540 nm under excitation
at 325 nm. Furthermore, given the appreciable PL signal of
2-D NCF crystals, a PL lifetime of 0.32 µs was measurable
(excitation at 372 nm), which may originate from the radi-
ative recombination of the excited triplet state (Figure
S16).34-35 The results of the optical investigation of the three
species clearly show that 2-D NCF, which possesses the
highest dimensionality among the three structures, dis-
plays enhanced PL properties. Such PL enhancement could
be attributed to the linker-to-cluster charge-transfer state
as explained from DFT calculations (vide infra).
DFT calculations. We performed time-dependent den-
sity functional theory (TDDFT) calculations to study the
major optical transitions of 0-D NC, 1-D NCF and 2-D NCF.
Overall, as shown in Figure 4, our calculated optical tran-
sition wavelengths and oscillator strengths are in reasona-
bly good match with the experimental absorption spectra.
For 0-D NC, most absorption peaks with a large oscillator
strength (S1, S2, S4, and S8) can be assigned to the electronic
transitions of a “hole” delocalized in the Ag16 cluster to the
“particle” localized partially in the cluster core using natu-
ral transition orbital (NTO) analysis. However, for both 1-
D NCF and 2-D NCF, the excitation induced by light irra-
diation can effectively shift the electronic distribution from
the hole/particle localized in organic linkers to the delocal-
ized particle/hole in the cluster core. In contrast to the
cluster-to-linker feature of the S1S0 transition in 1-D
NCF, the enhanced PL intensity observed for 2-D NCF
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could be attributed to linker-to-cluster charge-transfer ex-
citation.
Other characterizations. PXRD measurements con-
the Cl 2p3/2 and Cl 2p1/2 spin–orbit split components, respec-
tively (Figure S22, bottom). These binding energies corre-
spond to the ionic form of free chlorine (Cl-) in PPh4Cl.45
The shift in the Cl 2p3/2 component from low binding en-
ergy at 196.2 eV for the PPh4Cl precursor to high binding
energy at 198.2 eV for the 2-D NCF crystal sample indicates
that Cl- interacts strongly with the silver skeleton around
it in the 2-D NCF crystal compared with the free Cl- ions in
PPh4Cl.
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firmed the crystallinity and phase purity of the three struc-
tures based on the agreement between the experimental
and simulated spectra (Figure S17). Thermogravimetric
analysis (TGA) measurements were conducted to monitor
the thermal stability of the structures. As shown in Figure
S18, the main weight loss for 0-D NC occurs at ~124oC. This
loss corresponds to the complete decomposition of the
building blocks of Ag16 clusters. The TGA of 1-D NCF
showed a large drop corresponding to the main loss of
mass at ~150oC. We found that less distinct weight loss oc-
curs at low temperatures for better air-dried samples;
therefore, the initial loss is due to residual DMF solvent.
The main loss most likely corresponds to the complete de-
composition of the Ag cluster-building units. In 2-D NCF,
similar TGA behavior was observed, but the main loss oc-
curred at ~160oC, which indicates that the structure is more
robust. Thus, the TGA results indicate a clear improvement
in thermal stability and robustness with increasing dimen-
sionality from 0-D NC to 2-D NCF. Additionally, we evalu-
ated the thermal stability of NCFs through monitoring the
PL retention and recovery (from 0-100oC). The data show
that 2-D NCF have retained most of its PL, indicating that
2-D NCF is more thermally stable than 1-D NCF (Figure
S19).
Consistent with the SCXRD results, solid-state 13C nu-
clear magnetic resonance (NMR) spectroscopy was also
performed for the crystals of 0-D NC, 1-D NCF, and 2-D
NCF. Different DMF-CH3 peaks were observed at 31.16 and
32.22 ppm and at 45.99 and 47.91 ppm, which were associ-
ated with the bonded-DMF and free-DMF environments,
respectively. Moreover, the carbonyl C peak was observed
to split into two peaks at 161.74 ppm due to free DMF mol-
ecules and formed one extra peak at 165.34 due to coordi-
nated DMF. Due to F-C coupling, the CF3 peak split into
different lines at 114.68, 116.59, 118.71, and 122.25 ppm, while
a peak attributed to COOAg was observed at 159.82 ppm.
The strong peak observed at 36.88 ppm was assigned to
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t
methyl groups of BuS-, and a quaternary carbon peak ap-
peared at 51.09 ppm. A comparison of the 0-D NC spec-
trum with the 1-D NCF and 2-D NCF spectra shows extra
peaks in the aromatic region (122.31, 146.26, and 151.56
ppm), which can be assigned to the aromatic peaks of the
bpy linker (Figure S23).46
Fourier transform infrared (FTIR) spectroscopy was per-
formed on powder samples. The broad peak around 3400
cm-1 in the 0-D NC spectrum is assigned to the OH stretch-
ing vibration of water molecule (which confirms the coor-
dinated H2O in 0-D NC only). However, the peak between
2800 cm-1 and 3000 cm-1 is observed in all structures—0-D
NC, 1-D NCF, and 2-D NCF—and corresponds to the ter-
minal C–H of tert-butyl functional group in the thiolate lig-
and. The obtained sharp peak at ~1640 cm-1 is observed in
all the three structures which is assigned to the C=O
stretching vibration of the DMF molecules in the crystal
structures. In addition, an extra shoulder peak at ~1595 cm-
In addition, to further corroborate the molecular for-
mula and purity of 0-D NC, electrospray ionization mass
spectrometry (ESI-MS) measurements were performed on
the crystals of 0-D NC dissolved in CH3CN solvent (as
shown in Figure S24). The assignments of the mass peaks
demonstrate a clear match between isotopic patterns of
the experimental and simulated mass spectra (Figure S24,
a, b, and c). The deduced molecular formula from ESI-MS
is in good agreement with the crystal structure obtained by
SCXRD.
CONCLUSION
1
is only observed in 1-D NCF and 2-D NCF, which can be
To conclude, we report three novel structures of atomi-
cally precise chloride-templated Ag(I) thiolate nanoclus-
ters and nanocluster-based frameworks (0-D NC, 1-D NCF,
and 2-D NCF). By employing four main reaction levers (Cl-
anion template, DMF solvent, bpy linker, and reac-
tant/precursor ratio), we successfully achieved the con-
trolled assembly of a well-defined zero-dimensional Ag16
cluster (0-D NC) and two cluster-based assembled net-
works, Ag15 (1-D NCF) and Ag14 (2-D NCF) with atomic-
level control over cluster size and dimensionality. The
change in dimensionality offers a distinct way to tune the
optical and PL properties (as well as to improve the ther-
mal stability) of cluster-based frameworks. This study
paves the way for synthesizing new functional nanomateri-
als of cluster-based MOFs with full atomic precision and
utilizing their tunable properties for a wide range of poten-
tial applications.
assigned to the C–N stretching vibration from the bpy
linker (Figure S20).
Furthermore, X-ray photoelectron spectroscopy (XPS)
was performed to characterize the chemical composition
of the surface of a 2-D NCF crystal sample and to deter-
mine the oxidation state of chlorine. The survey spectrum
of the 2-D NCF crystal sample shows that Ag, S, F, Cl, O
and C were detected (Figure S21). The high-resolution XPS
spectrum of the Cl 2p core level was obtained from the 2-
D NCF crystal sample. The Cl 2p region shows one doublet
positioned at 198.2 eV and 199.8 eV corresponding to the
Cl 2p3/2 and Cl 2p1/2 spin–orbit split components, respec-
tively (Figure S22, top). For comparison, the high-resolu-
tion XPS spectrum of the Cl 2p core level was obtained
from the PPh4Cl precursor. The Cl 2p region shows one
doublet situated at 196.2 eV and 197.8 eV corresponding to
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Journal of the American Chemical Society
EXPERIMENTAL SECTION
ASSOCIATED CONTENT
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Materials. All the following chemicals and solvents were
purchased and used without further purification: silver ni-
trate (AgNO3, 99%, Aldrich), silver trifluoroacetate
(AgCF3COO, 98%, Aldrich), tetraphenylphosphonium
chloride (PPh4Cl, 98%, Aldrich), tert-butyl mercaptan
(99%, Aldrich), trimethylamine (99%, pure, ACROS), 4,4’-
bipyridine (bpy, 98%, Aldrich), dichloromethane (DCM,
HPLC grade, VWR), methanol (MeOH, HPLC grade,
VWR), N,N-dimethylformamide (DMF, HPLC grade, Al-
drich), and acetonitrile (CH3CN, HPLC grade, Aldrich).
Supporting Information. Instrumentation, UV−vis, steady-
state PL, PL lifetime, PXRD, XPS, TGA, FTIR, 13C NMR, ESI-
MS, and analyses of crystal structures of 0-D NC, 1-D NCF, and
2-D NCF. This material is available free of charge via the In-
ternet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
9
*osman.bakr@kaust.edu.sa
Notes
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Synthesis of [Ag-StBu]n complex. The complex was pre-
pared according to a previously reported method47 but
with some modifications. First, AgNO3 (3 mmol) was com-
pletely dissolved in 20 mL of MeOH by sonication. In an-
other vial, tert-butyl mercaptan (3 mmol) was dissolved in
10 mL of DCM and directly added to the methanolic solu-
tion of AgNO3. Then, triethylamine (~1 mmol) was added
quickly to the reaction mixture under stirring. The reaction
was continued at room temperature (RT) in the dark over-
night. The next day, the sample was centrifuged at 8000
rpm for 5 min; the white precipitate of the complex was
washed at least 5 times with MeOH and then stored under
vacuum for further use.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors would like to deeply thank the following people
for their help and dedication at various stages of this project:
Ayalew Assen, Karim Adil, and Osama Shekhah. The financial
support for this work was provided by KAUST.
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Synthesis and crystallization of 0-D NC. In a glass vial, ~0.3
mmol of [Ag-StBu]n and ~0.2 mmol of AgCF3COO were
mixed in 5 mL of CH3CN and stirred for ~5 min. Then, 4
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% (based on Ag).
Synthesis and crystallization of 2-D NCF. In a glass vial, 0.3
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for ~5 min. Then, 4 mL of a soluble DMF solution of PPh4Cl
(0.04 mmol) was added to the reaction mixture, and the
reaction was continued under stirring for ~2 h at room
temperature. The filtrate was then allowed to evaporate
slowly at RT in the dark under ambient conditions. Color-
less rod-like crystals were obtained after one week of crys-
tallization. Average yield of 2-D NCF: 55.6 % (based on Ag).
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