Formation of a Highly Reactive Cobalt Nanocluster Crystal within a Highly Negatively Charged Porous Coordination Cage

Article abstract of DOI:10.1002/anie.201712372

Earth-abundant first-row transition-metal nanoclusters (NCs) have been extensively investigated as catalysts. However, their catalytic activity is relatively low compared with noble metal NCs. Enhanced catalytic activity of cobalt NCs can be achieved by encapsulating Co NCs in soluble porous coordination cages (PCCs). Two cages, PCC-2a and 2b, possess almost identical cavity in shape and size, while PCC-2a has five times more net charges than PCC-2b. Co²⁺ cations were accumulated in PCC-2a and reduced to ultra-small Co NCs in situ, while for PCC-2b, only bulky Co particles were formed. As a result, Co NCs@PCC-2a accomplished the highest catalytic activity in the hydrolysis of ammonium borane among all the first-row transition-metals NCs. Based on these results, it is envisioned that confining in the charged porous coordination cage could be a novel route for the synthesis of ultra-small NCs with extraordinary properties.

Full text of DOI:10.1002/anie.201712372

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Does Charge Matter? Formation of Highly Reactive Cobalt
Nanocluster Crystal within Highly Negative Porous Coordination
Cage
Authors: Yu Fang, Zhifeng Xiao, Jialuo Li, Christina Lollar, Lujia Liu,
Xizhen Lian, Shuai Yuan, Sayan Banerjee, Peng Zhang, and
Hongcai Zhou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712372
Angew. Chem. 10.1002/ange.201712372
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10.1002/anie.201712372
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Metal–Organic Polyhedron
Does Charge Matter? Formation of Highly Reactive Cobalt Nanocluster
Crystal within Highly Negative Porous Coordination Cage**
Yu Fang, Zhifeng Xiao, Jialuo Li, Christina Lollar, Lujia Liu, Xizhen Lian, Shuai Yuan, Sayan
Banerjee, Peng Zhang and Hong-Cai Zhou*
11]
Abstract: Earth abundant first-row transition-metal nanoclusters
(NCs) have been extensively investigated as catalysts. However,
their catalytic activity is relatively low compared with noble metal
NCs. In this work we show that enhanced catalytic activity of cobalt
NCs can be achieved by encapsulating Co NCs in soluble porous
coordination cages. Two cages, PCC-2a and 2b, possess almost
identical cavity in shape and size, while PCC-2a has five times more
net charges than PCC-2b. Co2+ cations were accumulated in PCC-
2a and reduced to ultra-small Co NCs in situ, while for PCC-2b,
only bulky Co particles were formed. As a result, Co NCs@PCC-2a
accomplished the highest catalytic activity in the hydrolysis of
ammonium borane among all the first-row transition-metals NCs.
Based on these results we envision that confining in the charged
porous coordination cage can be a novel route for the synthesis of
ultra-small NCs with extraordinary properties.
With an impending shortage of fossil fuels, new and sustainable
energy sources, such as “hydrogen energy”, are in high demand.
Ammonia-borane (NH3·BH3, AB), which has a high hydrogen
content of 19.6 wt%, is a promising hydrogen storage reagent for
chemical energy applications.[1] Nobel metal NCs are highly
efficient catalysts for the dehydrogenation of AB.[2] However, their
practical applications are retarded pertaining to the low abundance
and high cost. First-row transition metal NCs, such as Co, Ni, and
Cu NCs, are potential substitutes for noble metal NCs.[3]
Nevertheless, their catalytic activities are often low while they are
susceptible to amorphous form, catalyst poisoning, and
agglomeration.[4, 5]
difficult and has rarely been accomplished.[5,
Particularly,
preparing uniform ultra-small (mean diameter < 3.0 nm) NC crystals
immobilized in a porous material is still challenging. Moreover,
solid supports can do little to solve catalyst poisoning issue.[4] In
addition, immobilized NCs suffer from agglomeration as well owing
to the heterogeneous nature of the supports.[5] Therefore, it is
desirable to develop a novel synthetic route of first-row transition
metal NCs that can achieve uniform size distribution and enhanced
catalytic activity that is comparable to noble metal NCs.
Our group recently developed a new type of metal-organic hybrid
material, porous coordination cage (PCC), which is soluble in
common solvents and can be potentially used as support for metal
NCs. For cost-efficient purpose, we aim to test whether first-row
transition metal NCs with ultrahigh catalytic activities can be
obtained via encapsulation by PCCs. In addition, we tend to
investigate which features of PCC could affect the synthesis and
property of NCs.
Two PCCs, named as PCC-2a and 2b, were designed and
synthesized (Figure 1a). By reacting vertex ligand (Na4H4V1) and
panel ligand (H3L) with CoCl2 at 80 oC in methanol with 2 drops of
Et3N for 12 h, PCC-2a was readily formed. Single crystal X-ray
diffraction (SC-XRD) analysis reveals that PCC-2a adopted an
octahedral hollow cage structure in the trigonal R3 space group
(Figure 1b). Similarly, PCC-2b was prepared by reacting vertex
ligand (H4V2) and panel ligand (H3L) with CoCl2 at 120 oC in DMF
for 24 h. PCC-2b adopted a similar octahedral hollow cage structure
but in the tetragonal I4/m space group (Figure 1c). PCC-2b shares
the same structure as CIAC-106 reported by Liao Group.[12] Both
PCC-2a and 2b cages were assembled by six V ligand as vertices,
eight L ligands as faces, and six tetranuclear Co clusters as knots
between each four L ligands and one V ligand (Figure 1a). Each
cobalt atom in the cluster knot was held in a pseudo-octahdral
configuration by coordinating with one sulfonic oxygen and two
phenolate oxygen atoms from V, two carboxylate oxygens from two
adjacent L, and a 4-OH at the center of the cluster. The assignment
Immobilizing NCs on porous materials, such as metal-organic
frameworks (MOFs)[6], zeolites[7], three-dimensional graphene-
based framework (3DGFs)[8] and molecular cages[9, 10], have been
extensively studied. This strategy can promote the property and
performance of NCs. For example, size tuning of NCs and selective
catalysis by NCs have been achieved.[11] However, enhancement of
catalytic activity of first-row transition metal is proved to be very
of the central oxygen atom to be 4-OH or 4-O has remained
[]
Yu Fang, Zhifeng Xiao, Jialuo Li, Christina Lollar, Xizhen Lian,
Shuai Yuan, Sayan Banerjee, Peng Zhang, and Hong-Cai
Zhou
Department of Chemistry, Texas A&M University
College Station, Texas, 77843 (USA)
13]
controversial.[12,
Herein we assigned it 4-OH for both cages
because the basic synthetic condition with the presence of Et3N may
promote the formation of OH- anion. Based on the crystal structures
of two PCC-2, the longest inner-cavity distance are 25.1-24.6 Å (the
distance between two 4-OH of two opposite Co4 cluster), and the
largest outer diameter of them are 42.2-42.7 Å. From the
packing diagram, we revealed micro- to meso-porosity in the
solid state (Figure S3-5), which indicates that PCC-2 are porous
materials.
E-mail: zhou@chem.tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/zhou/
Lujia Liu
Department of Chemistry, Northwestern University
Evanston, Illinois, 60208 (USA)
Regardless of their structural similarity, the charge property of
PCC-2a and 2b is totally different. To the best of our knowledge,
PCC-2a is one of the most negatively-charged coordination
[] The project is supported by the Robert A. Welch Foundation
through an endowed chair to HCZ (A-0030). This material is
based upon work supported by the National Science
Foundation Graduate Research Fellowship under Grant No.
DGE: 1252521.
14
cages with a large internal cavity reported.[ ] For most cationic
or anionic coordination cages, the charge originates from the
coordination site. However, for PCC-2 series, the 48 positive
Supporting information for this article is available on the WWW
under http://www.angewandte.org.
1
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10.1002/anie.201712372
Angewandte Chemie International Edition
spectroscopy as the adsorption band of Co (II) from 600 to 700
15
]
nm disappeared upon the addition of NaBH4.[ Co NCs@PCC-
2a demonstrated excellent colloidal stability as it maintained
homogeneity even after 6 weeks storage at room temperature in
DMF solvent. Co NCs@PCC-2a solid can be precipitated by
adding excess MeCN into the solution and re-dissolved in DMF.
In contrast, for the solution containing PCC-2b, the addition of
NaBH4 led to instantaneous black precipitation, which is
indicative of NCs aggregation and a phase separation of NCs and
PCC-2b (Figure S8). The incapability of PCC-2b in terms of
NCs stabilization is probably because the 6- charge of PCC-2b
does not have strong interaction with Co2+ ions, thus the
reduction occurs outside the cavity of PCC-2b and is subjected
to instant agglomeration.
Figure 1. a) Cartoon of octahedron cage PCC-2 and the cage
components, b) crystal structure of PCC-2a, and c) crystal structure of
PCC-2b.
charges from the Co2+ metal ions are fully compensated by the
phenolate groups from V2 (24 negative charges) and the
carboxylate groups from panel ligand L (24 negative charges).
Thus, the negative charge of PCC-2b is only attributed to the six
4-OH groups in the Co cluster. For PCC-2a, the negative
charge is derived from a combination of sulfate groups from V1
ligand (24 additional negative charges) and hydroxyl group (6
negative charges) in the Co cluster. PCC-2a/b are soluble in
dimethylformamide (DMF) and dimethyl sulfoxide (DMSO).
Figure 2. a) Scheme of preparation of Co NCs by PCC-2a, b) UV-Vis
spectrum of PCC-2a, after adding Co salt and after adding NaBH4.
The as-synthesized Co NCs@PCC-2a was characterized by
Transmission Electron Microscopy (TEM). The Co NCs in the
coordination cages were uniform and well-dispersed, with an
average diameter of 2.5 nm (Figure 3a, and Figure S9-11). The
high resolution TEM (HRTEM) image (Figure 3b) revealed the
observed lattice spacing of Co NCs@PCC-2a to be 2.01 Å((111)
plane), which corresponds to fcc crytal face on Co NCs.[16]
Notably, although hcp type Co NCs are normally obtained from
low temperature synthesis, the fcc type of Co NCs is more
favorable when their size is below 20 nm.[16c] The size
distribution of Co NCs@PCC-2a is quite narrow and near the
diameter of the cage’s internal cavity (2.5 nm), which indicates
the formed Co NCs were encapsulated and regulated by PCC-2a
(Figure 3c). Energy Dispersive X-ray Spectroscopic (EDS) study
also confirmed that the Co NCs are entrapped within the cage
(Figure S14). Co NCs@PCC-2a was further characterized with
X-ray Photoelectron Spectroscopic (XPS). Peaks at 778.4 and
793.5 eV corresponded to Co 2p3/2 and 2p1/2 lines of elemental
Co and suggested that Co(0) was present in the complex (Figure
S15). Additionally, according to our simulation, only fcc-type
Co NCs with truncated octahedral shapes can be accommodated
by PCC-2a because of the shape matching requirements (Figure
S16 and S17). ICP-MS analysis indicated that the average atom
number of each Co NC in the complex was 780, which is
consistent with the estimation given by molecular simulation
(850 atom per NC within the cage). Overall, these data clearly
establishes the active role of PCC-2a in modulating the
formation of ultrasmall and well dispersed first-row transition
Electrospray
Ionization-Mass
Spectrometry
(ESI-MS)
unambiguously showed the ionic fragment distribution of
negatively charged PCC-2a (Figure S1, Table S1). Thermo-
Gravimetric Analysis (TGA) indicated that the PCC-2a is
thermally stable up to 240 °C (Figure S2). Molecular simulations
and N2 isotherm results visualized the porous nature of PCC-2a.
Based on the crystal structure of PCC-2a, the helium-accessible
void fraction is calculated to be an enormous 74.3%. This is in
good agreement with the simulated N2 uptake of 640 cm3/g (STP)
at 77 K and P/P0 = 0.94 (Figure S6 and S7). PCC-2b showed
identical property as with CIAC-106, a previously reported
12
]
structure.[
We then used PCC-2 cage, a potent cation container, for Co2+
cation encapsulation and NC stabilization. A 1.0 mL DMF
solution containing 0.44 × 10-3 mmol dried PCC-2a/b crystal
(5.2 mg and 5.0 mg for PCC-2a and PCC-2b, respectively) was
added 20.8 mg CoCl2. After 30 minutes incubation at ambient
temperature, the solution was added with a 1:2 DMF:MeOH
solution (total was 1.5 mL) containing 0.9 mg sodium
borohydride. The solution containing PCC-2a underwent an
immediate color change from green to dark brown without any
precipitate formation (Figure 2a). Similar sharp color change
was also observed when purple single crystal of PCC-2a was
soaked in the solution of Co NCs. These phenomenon indicated
the generation of well dispersed Co NCs mediated by PCC-2a.
The reduction of Co (II) to Co was monitored by UV−Vis
2
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10.1002/anie.201712372
Angewandte Chemie International Edition
metal NCs. On the other hand, Co NCs prepared in the presence
of PCC-2b, namely Co NCs/PCC-2b, yielded bulky (>100 nm)
and aggregated particles (Figure 3d and S12, 13).
According to literature, previous record-high result (TOF = 41)
was achieved by immobilizing Ni NCs on 3D-graphene.[8a]
Besides, PVP protected Co NCs gave a moderate TOF of 38.9
min-1. The less significant improvement of reactivity by PVP
was regarded as the alkane chain in PVP blocking the active site
of encapsulated NCs, which is
a common issue for
surfactants.[17] In contrast, Co NCs/PCC-2b catalyst only give a
TOF of 22.5 min-1. The main reason should be ascribed to the
incapability of encapsulating and stabilizing Co NCs by PCC-2b,
thus PCC-2b show no enhancement toward the catalytic
reactivity of Co NCs.
As a control, empty PCC-2a cage was added to the solution
of AB without adding any other Co source, and hydrogen gas
was barely generated. This result excluded the possibility that
PCC-2a cage itself accelerated the reaction. Since the most of
the negative charges of PCC-2a come from the vertex ligand
(Na4H4V1), we then replaced PCC-2a by Na4H4V1 in the catalytic
system. Na4H4V1 was simply mixed with CoCl2 and reduced by
NaBH4, followed by initiating reactions with AB. The TOF of
the reaction catalyzed by Na4H4V1 is only 19.1 min-1, which is
not comparable with PCC-2a. This fact excluded the possibility
that the surface charges of the vertex ligand stablized the NCs. In
addition, Co NCs@PCC-2a catalyst could be reused more than 5
times with negligible loss of activity (Figure S19) and the PCC-
2a cage remained intact, which was characterized by UV-Vis
spectroscopy and ESI-MS (Figure S20-S23) after reaction. This
phenomenon shows that the anionic feature of Co NCs@PCC-2a
is capable of preventing catalyst poisoning and agglomeration
during the kinetic process. Combining those results, we conclude
that the high negative charge, porous nature and cavity-effect of
PCC-2a is the key to encapsulate Co NCs and enhance the
catalytic property of NCs.
Figure 3. a) TEM images of Co NCs@PCC-2a (red cycles indicate
single NCs). b) HRTEM image of single NC of Co NCs@PCC-2a
showing the (1, 1, 1) crystal surface. c) Size distribution of Co
NCs@PCC-2a. d) TEM images of Co NCs/PCC-2b.
We then assessed the catalytic activity of Co NCs stabilized
by PCC-2a/b in the hydrolysis of ammonia borane (NH3·BH3,
AB) (Scheme 1). When the reaction was initiated by these
homogenous catalysts, hydrogen gas was gathered in a graduated
cylinder and the volume was record according to time (Figure 4).
The catalytic activity was evaluated and compared in Table 1. A
maximum turn-over frequency (TOF) of 90.1 min-1 was achieved
by Co NCs@PCC-2a, which is the highest among all catalysts
Table 1. Catalytic performance of PCC-2 and other compounds for
the hydrolysis of AB.
Catalyst
Particle
Size
Time
(min)
Catalyst
Percentage
Stabilizer
Percentage
TOF
(min-1)
(nm)
Co NPs@PCC-2a
Co NPs/PCC-2b
Co NPs@PVP
Ni@3D Graphene
PCC-2a
2.5
100
10-20
3.2-5.4
> 100
> 100
0.5
2.0
1.2
8.0
> 60
2.3
7%
7%
7%
9%
0.4%
7%
0.6%
0.6%
0.6%
4000%
100%
0.6%
90.1
22.5
38.9
41.7
<1
Scheme 1. Hydrolysis of ammonia borane.
Co NPs + Na4H4V1
19.1
In summary, we applied two porous coordination cage (PCC-
2a and 2b) sharing similar structure but with different charges to
study the charge effect in mediating Co NCs formation and
catalytic activity promotion. We found that, the highly negative
PCC-2a is efficient to stabilize the Co NCs, by trapping them in
the cavity of PCC-2a and disperse them by the intrinsic high
charge, whereas the PCC-2b hardly do. The composite Co
NCs@PCC-2a showed extraordinary catalytic activity in the
hydrolysis of NH3·BH3 that is superior to other first-row
transition metal NCs. This study demonstrate an efficient way to
trap and stabilize crystalline metal NCs in a soluble porous metal
organic polyhedron, followed by tuning the reactivity of
encapsulated catalyst via an interaction-engineering approach.
We envision that those remarkable features of PCCs allows
using them in other water mediated reactions involved by NCs.
Figure 4. Time course plots of H2 generation for the hydrolysis of AB
by Co NCs@PCC-2a, Co NCs/PCC-2b, and Co NCs@PVP.
Experimental Section
Full experimental details can be found in Supporting Information.
containing first-row transition metal for the hydrolysis of AB.[5]
3
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[14]
Keywords: metal-organic polyhedron · ammonia borane · hydrogen
generation· first-row transition metal· nanocluster
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Metal–Organic Frameworks
Y. Fang, Z. Xiao, J. Li, C. Lollar, L. Liu,
X. Lian, S. Yuan, S. Banerjee, P. Zhang,
and H.-C. Zhou __________ Page –
Page
Does Charge Matter? Formation of
Highly Reactive Cobalt Nanocluster
Crystal within Highly Negative Porous
Coordination Cage
Co Nanoclusters Encapsulated by PCCs: The 30- anionic PCC-2a is able to
absorb metal cluster precursors and reduce them within the cavity of PCC, thus
yielding high crystalline and ultrafine Co nanoclusters. In contrast, the 6- anionic
PCC-2b hardly encapsulates metal cluster precursors, by showing bulky and
aggregated nanoclusters. The highly reactive Co nanoclusters encapsulated by
PCC-2a, show considerable activity improvement for hydrolysis of ammonia
borane.
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