Self-Assembly of a Phosphate-Centered Polyoxo-Titanium Cluster: Discovery of the Heteroatom Keggin Family

Article abstract of DOI:10.1002/anie.201910491

Over the past 200 years, the most famous and important heteroatom Keggin architecture in polyoxometalates has only been synthesized with Mo, W, V, or Nb. Now, the self-assembly of two phosphate (PO₄^3?)-centered polyoxo-titanium clusters (PTCs) is presented, PTi₁₆ and PTi₁₂, which display classic heteroatom Keggin and its trivacant structures, respectively. Because Ti^IV has lower oxidate state and larger ionic radius than Mo^VI, W^VI, V^V, and Nb^V, additional Ti^IV centres in these PTCs are used to stabilize the resultant heteroatom Keggin structures, as demonstrated by the cooresponding theoretical calculation results. These photoactive PTCs can be utilized as efficient photocatalysts for highly selective CO₂-to-HCOOH conversion. This new discovery indicates that the classic heteroatom Keggin family can be assembled with Ti, thus opening a research avenue for the development of PTC chemistry.

Full text of DOI:10.1002/anie.201910491

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201910491
German Edition: DOI: 10.1002/ange.201910491
Polyoxometalates
Self-Assembly of a Phosphate-Centered Polyoxo-Titanium Cluster:
Discovery of the Heteroatom Keggin Family
+
+
Ning Li , Jiang Liu , Jing-Jing Liu, Long-Zhang Dong, Shun-Li Li, Bao-Xia Dong, Yu-He Kan,
and Ya-Qian Lan*
Abstract: Over the past 200 years, the most famous and
important heteroatom Keggin architecture in polyoxometalates
has only been synthesized with Mo, W, V, or Nb. Now, the self-
lacunary heteroatom Keggin species, where one or three
metal sites are taken off the plenary Keggin structure, also
have attracted increasing interest because of their well-
defined and highly reactive vacant sites that bring many
opportunities for the preparation of multifunctional high-
3
À
assembly of two phosphate (PO4 )-centered polyoxo-titanium
clusters (PTCs) is presented, PTi16 and PTi , which display
12
[
4]
classic heteroatom Keggin and its trivacant structures, respec-
nuclearity clusters and extended frameworks.
IV
tively. Because Ti has lower oxidate state and larger ionic
Despite those advantages mentioned above, the heter-
oatom Keggin family was only synthesized and developed in
VI
VI
V
V
IV
radius than Mo , W , V , and Nb , additional Ti centres in
these PTCs are used to stabilize the resultant heteroatom
Keggin structures, as demonstrated by the cooresponding
theoretical calculation results. These photoactive PTCs can
be utilized as efficient photocatalysts for highly selective CO2-
to-HCOOH conversion. This new discovery indicates that the
classic heteroatom Keggin family can be assembled with Ti,
thus opening a research avenue for the development of PTC
chemistry.
VI
VI
V
V
Mo -, W -, V -, and Nb -based systems in the past
[
5]
200 years. Considering their fascinating architectures with
unexplored functions, development of novel heteroatom
Keggin families by other elements has long been an attractive
but challenging research field. PTCs are an emerging class of
nanoclusters that have achieved the rapid growth in its
[
6]
structure library. Because of their intimate structural
[
7]
relationship with POMs, they are expected to have great
potential to construct more cluster structures as similar as
POMs. Nevertheless, the development of PTCs is severely
hindered by their daunting synthesis, in which Ti (group 4)
ions are prone to fast and spontaneous hydrolysis in hydrous
K
eggin and its lacunary structures constitute the most
important and extensive subunit of polyoxometalate (POM)
chemistry that features structural diversity and exhibits
desirable properties in catalysis, materials science, and
solvents to then generate uncontrolled TiO precipitation,
2
[
1]
energy conversion. The archetypical Keggin structure is
based on the well-known heteropolyoxometalate cluster
having the formula [X M O ] (X = P, Si, Ge, etc; M =
resulting in a sluggish progress in synthesizing new structur-
[6a]
es. Although diverse synthetic methods have been largely
3
À
[8]
developed in the past few years, there are still very few PTCs
1
12 40
VI
VI
[9]
Mo , W ), which is made up of four M O units that
that exhibit structural similarity as POMs. In particular, the
3
13
encapsulate a central heteroatom with a tetrahedral XO4
geometry (Supporting Information, Scheme S1). Compared
synthesis of famous heteroatom Keggin and its lacunary
structures has never been explored in this field, except for
very few homo-atom Keggin PTCs. Moreover, the homo-
VI
VI
[10]
to the original Mo - and W -based heteroatom Keggin
architectures, however, other Keggin analogues (including
isopolyoxometalates) composed of metal ions with low
accessible oxidation state and large ionic radius need more
metal centres (> 12) to effectively reduce overall cluster
atoms in all reported Keggin PTCs are Ti.
IV
Herein, Ti -based heteroatom Keggin and its trivacant
lacunary architectures, [Ti (OH) O (PO )(OiPr) ]·guests
1
6
4
20
4
16
(PTi , HOiPr= isopropanol) and [Ti O (PO )(iPrPO ) -
1
6
12 15
4
4 3
[
2,3]
charge, and thus a stable molecular cluster.
Besides,
(OiPr) ](CHA) (H O) (PTi12, CHA = cyclohexylammo-
12 3 2
nium), were first structurally synthesized by a facile solvo-
thermal approach, and they can be prepared easily in large
scale by increasing reactants ratios. Since the Ti has
[
+]
[+]
[
*] N. Li, Dr. J. Liu, J.-J. Liu, L.-Z. Dong, Prof. S.-L. Li, Prof. Y.-Q. Lan
Jiangsu Collaborative Innovation Centre of Biomedical Functional
Materials, School of Chemistry and Materials Science
Nanjing Normal University
IV
relatively low oxidation state and large ionic radius, addi-
tional Ti atoms are used to reduce overall PTC charge for
stabilizing the resultant Keggin structures, as demonstrated
by density functional theory (DFT) calculations. Additionally,
both of these PTCs can be treated as efficient photocatalysts
for CO2 reduction, and display very high selectivity and
activity for HCOOH production. Moreover, this is the first
report of isolated and soluble PTCs applied in photocatalytic
No.1, Wenyuan Road, NanJing 210023 (China)
E-mail: yqlan@njnu.edu.cn
[
+]
N. Li, Prof. B-.X. Dong
School of Chemistry and Chemical Engineering, Yangzhou University
Yangzhou 225002 (P. R. China)
Prof. Y.-H. Kan
School of Chemistry and Chemical Engineering Normal University
Huaian 223300 (P. R. China)
CO reduction reaction (CO RR).
2
2
+
[
Crystallographic structural analysis reveals that the PTi16
] These authors contributed equally to this work.
cluster crystallizes in the cubic space group PÀ43n and
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201910491.
presents a heteroatom a-Keggin structure (Figure 1a). As
with the classic heteroatom Keggin structure, the central
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Figure 1. The new heteroatom Keggin structures based on PTCs. a),b) Structural and polyhedral relationships between PTi and original
1
6
VI
VI
heteroatom Keggin structure (PM , M=Mo , W ). c),d) Structural and polyhedral relationships between PTi and original lacunary heteroatom
1
2
12
VI
VI
Keggin structure (PM , M=Mo , W ). C, H, and guests are omitted for clarity.
9
tetrahedron {PO } in PTi16 connects with twelve Ti atoms to
Information, Table S1. To better understand the structural
stability of these two Keggin clusters, we have performed
DFT calculations in combination with nucleus independent
chemical shifts (NICS), multicenter indices (MCI), and
natural bond orbital (NBO) analyses to investigate the
electron delocalization. As expected, the calculations indicate
4
form four triangular and edge-shared {Ti O } units by its m -O
3
13
3
atoms, and these {Ti O } units are corner-linked to the central
3
13
{
PO } (Supporting Information, Figure S1). Moreover, the
4
four square faces between trimetric {Ti O } units are capped
3
13
2
+
by four additional [TiO] units whose five-coordinated Ti
centers exhibit distorted square pyramid geometry (Fig-
ure 1b; Supporting Information, Figure S1b). The heteroa-
tom a-Keggin metal–oxo structure of PTi16 consists of sixteen
Ti atoms coordinated to oxo and isopropoxy groups and is
encapsulated by sixteen terminal isopropoxy groups. The high
formation tendency of spherical PTi16 is to optimize close-
packing of oxygen atoms and minimizes metal–metal repul-
sion (Supporting Information, Figures S1b, S2a).
2
+
that additional four/three [TiO] units inserted in the PTi /
1
6
PTi12 structure indeed play a key role in balancing overall
PTC charge (see computational details in the Supporting
Information).
The phase purity of PTi16 and PTi12 was first confirmed by
powder X-ray diffraction analysis (Supporting Information,
Figure S5). The P element in both PTCs was identified by
crystallographic structure, element mapping, X-ray photo-
3
1
By contrast, PTi12 cluster crystallizes in the orthorhombic
space group Pbca and exhibits an A-type trivacant heter-
oatom a-Keggin structure (Supporting Information, Fig-
ure S3a) in which three square faces between trimetric
electron spectroscopy (XPS), P solution NMR, and infrared
(IR) spectroscopy. As we can see from the element mapping
images (Figure 2a–h), O, P, and Ti elements are evenly
distributed in two PTCs. The corresponding full-scan spectra
of XPS also indicate the existence of above elements
including P, and the strong P 2p peak at 133.6/133.2 eV can
be assigned to the characteristic of PÀO bond (Figure 2i,k;
2
+
{
Ti O } units are capped by additional three [TiO] unit
3 13
(
Figure 1c; Supporting Information, Figure S3b). Interest-
ingly, the removed triad (that is, edge-shared {Ti O } unit)
3
13
from the plenary a-Keggin cluster is replaced by three
Supporting Information, Figure S6). Meanwhile, the high-
resolution Ti 2p spectra for PTi16 and PTi12 clearly exhibit two
regions of Ti 2p1/2 (ca. 464.4 eV) and Ti 2p3/2 (ca. 458.6 eV)
with about 5.8 eV binding energy difference, being indicative
esterifiable {iPrPO } tetrahedra (Supporting Information,
4
Figure S3), this kind of geometry has been observed in
trivacant lacunary polyoxotungstate and -niobate chemis-
[11]
4+
try. The central tetrahedron {PO } bonds to nine {TiO }
of the sole presence of typical Ti (Figure 2j,l). Besides, the
4
6
3
1
octahedra to form three triangular and edge-shared {Ti O }
liquid-state P NMR spectra of PTi16 and PTi12 also agree
with the structures determined by single crystal X-ray
diffraction (Supporting Information, Figure S7). For PTi16,
the spectrum only has one phosphate peak at d =+ 2.74 ppm
3
13
units, which is similar to PM9 (Figure 1c,d; Supporting
Information, Figure S4). The metal–oxo core of PTi12 is
assembled with twelve Ti atoms, one central {PO }, three m -O
4
2
and twelve m -O bridges, and is encapsulated by twelve
that corresponds to the central {PO } group in a-Keggin
3
4
3
1
terminal isopropoxy and three esterifiable {PO } groups
structure. No other P NMR resonances were detected,
indicating that there are no impurities or any minor phases
containing phosphorus. The spectrum of PTi12 features two
4
(
Supporting Information, Figure S4). More details about the
bond lengths and angles are listed in the Supporting
2
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phere to investigate their thermal stability (Supporting
Information, Figure S11).
It is well-recognized that titanium containing semicon-
ductors and solids can efficiently photocatalytic reduce CO
2
[
13]
into value-added chemicals.
However, the reports on
titanium-oxo clusters as photocatalysts for CO2 reduction
[14]
are scarce, except for a few titanium-based MOFs. Both of
PTi16 and PTi12 display strong absorption in the ultraviolet
region, from which the band gaps (E ) can be determined to
g
be 2.80 and 2.85 eV, respectively, indicating that they also
display semiconductor-like properties (Figure 4a,b). A cyclic
Figure 2. The morphology, elemental distribution, and XPS of PTi16
and PTi . a)–d) The SEM image and elemental mapping of PTi . e)–
1
2
16
h) The SEM image and elemental mapping of PTi . The XPS high-
1
2
resolution scans of P 2p and Ti 2p for PTi16 (i,j) and PTi12 (k,l).
phosphate resonances in a 3:1 ratio (integral peak areas
ratio), which are assigned to three esterifiable {iPrPO } and
4
one central {PO } groups in trivacant Keggin structure
4
stoichiometry. IR spectra were further used to identify
fingerprint peaks of multiple PÀO bands from different
{
PO } groups (Supporting Information, Figure S8). The two
4
À1
peaks at about 1067 and about 1005 cm are assigned to
esterifiable and central {PO } groups, respectively, which are
4
very similar to infrared studies on other PO -centered Keggin
4
[
11a,12]
and lacunary derivatives.
The liquid-state UV/Vis
Figure 4. a) UV/Vis spectra, b) Tauc plots, and c),d) band structure
diagrams (including LUMO and HOMO levels) for PTi16 and PTi12.
absorption spectra also display the characteristic of Keggin-
type structures (Supporting Information, Figure S9). Addi-
tionally, PTi16 and PTi12 clusters have good solubility in
conventional organic solvents and aqueous phase, which can
be demonstrated by electrospray ionization mass spectrom-
etry (ESI-MS) analysis (Figure 3; Supporting Information,
Figure S10). Thermogravimetric analysis (TGA) was further
voltammetry (CV) test and ultraviolet photoelectron spec-
troscopy (UPS) were performed to determine the LUMO and
HOMO levels of PTi16 and PTi12 (Supporting Information,
Figures S12, S13). Moreover, the HOMO and LUMO ener-
gies determined by CVs are very close to that of optical Tauc
plots associated with UPS, and they were converted to
electrochemical energy potentials in volts vs. normal hydro-
gen electrode (NHE) (Figure 4c,d). It is obvious that the
LUMO positions of PTi16 and PTi12 are more negative than
the reduction potentials of many photocatalytic products. We
thus conducted UV-light-driven photocatalytic reduction of
CO over PTi and PTi , with triethanolamine (TEOA) and
performed on their polycrystalline samples under N atmos-
2
2
16
12
CH CN as electron donor and solvent, respectively, which can
3
promote CO solubility and photo-generated electron trans-
2
[
15]
fer. During the photoreaction process, HCOOH is the only
liquid product detected by ion chromatography (IC; Support-
ing Information, Figure S14), while other gaseous products
including competitive H , CO, and CH are also observed by
2
4
gas chromatography (GC; Supporting Information, Fig-
ure S15). From Figure 5a, the production of HCOOH for
both titanium–oxo clusters has a continuous growth as the
irradiation time increases. After 18 h, the produced HCOOH
À1 À1
for PTi16 reaches up to 24.13 mmol (268.12 mmolg h ),
which is comparable to other reported titanium-containing
Figure 3. Enlarged m/z range of positive-mode ESI-MS spectra of
a) PTi16 and b) PTi12 dissolved in acetonitrile solution. Experimental
[
14,16]
semiconductors and MOF materials.
By contrast, PTi12
shows a relatively low HCOOH production of 9.21 mmol
(122.80 mmolg h ) after 15 hours. With the extended irrita-
(
black line) and simulated (red line) HRESI-MS spectra of PTi16 (c, m/
À1 À1
z 2194.8151) and PTi12 (d, m/z 2349.2224).
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Figure 5c and the Supporting Information, Figure S21, the
original reaction systems including photocatalyst and TEOA
in the dark under N atmosphere did not show any ESR
2
signal. When the reaction systems were irradiated by UV light
3
+
under identical condition, a clear signal of Ti (g = 1.944) can
4
+
be observed from their ESR spectra. This means that the Ti
3
+
within titanium–oxo clusters were reduced into Ti by
receiving photoexcited electrons transferred from O , while
the corresponding photo-generated holes were quenched by
2
À
3
+
TEOA. Moreover, the intensity of Ti signal increased with
the extended irradiation time. When the reaction system was
3
+
contacted with CO or O atmosphere, the ESR signal of Ti
2
2
3
+
disappeared, indicating that the photogenerated Ti
is
involved in CO reduction reaction. The ESR results reveal
2
4
+
that the Ti atoms within PTi16 and PTi12 are the photo-
catalytic active sites for the CO -to-HCOOH reduction, and
2
[14,18]
Figure 5. Photocatalytic characterizations over PTi16 and PTi12.
a) Amounts of HCOOH produced as a function of the time of UV light
irradiation. b) The yield distribution of different photocatalytic prod-
ucts. c) ESR spectra of PTi . d) The proposed reaction mechanism.
are in accordance with that in titanium-based MOFs.
Based on the above analysis, the mechanism for the
photocatalytic CO reduction can be illustrated as shown in
2
1
6
Figure 5d. Upon UV light irradiation, an excited charge
separation state (the generation of electrons and holes) occurs
in titanium–oxo cluster by transferring a photogenerated
2
À
4+
4+
3+
tion time, the production of gaseous products (H , CO, and
electron from O to Ti . Ti is thus reduced to Ti whereas
TEOA acts as an electron donor to consume the produced
2
CH ) also has a growth trend but low total yield, resulting in
4
3
+
that very high selectivity for CO -to-HCOOH photosynthesis
hole. Subsequently, the Ti donates one photogenerated
2
was obtained by PTi16 (93.4%) and PTi12 (83.9%) (Fig-
ure 5b). Moreover, this is the first report of titanium–oxo
electron to the absorbed CO for HCOOH production and
2
4
+
goes back to Ti . By this way, a complete photocatalytic cycle
3
+
cluster system applied to the CO photoreduction. To confirm
of CO -to-HCOOH reduction can be achieved by Ti in the
2
2
the photocatalytic activity of PTi16 and PTi , a series of
deletional control experiments were carried out in the
presence of TEOA as electron and hydrogen donors.
In summary, a new heteroatom Keggin family based on
PTCs has been synthesized. The DFT calculation results
reveal that the capped [TiO] units play a key role in
stabilizing the resultant Keggin clusters. Moreover, these
12
absence of photocatalysts, CO , TEOA, or UV-light illumi-
2
2
+
nation. Results revealed that no products can be detected by
IC and GC (Supporting Information, Table S2), corroborat-
ing the photocatalytic potential of these titanium–oxo clusters
for reducing CO2 into HCOOH. The UV/Vis absorption
spectra after photocatalytic reaction still displayed Keggin-
based character (Supporting Information, Figure S16), which
excludes the influence of the decomposed active components
from catalysts on the photocatalytic activity. Additionally,
soluble PTCs display high selectivity and activity for CO -to-
2
HCOOH photoconversion. It is predicted that this discovery
will not only enrich the heteroatom Keggin family but also
open a new research direction for the development and
application of novel PTC materials.
3
1
nearly unchanged P NMR spectra and ESI-MS after reac-
tion also prove the structural stability of PTi16 and PTi12 Acknowledgements
Supporting Information, Figures S17, S18). The isotopic
(
1
3
CO experiment was conducted under identical photocata-
This work was financially supported by NSFC (No. 21622104,
21871141, 21871142, 21701085, and 21901122); the NSF of
Jiangsu Province of China (No. BK20171032); the Natural
Science Research of Jiangsu Higher Education Institutions of
China (No. 17KJB150025 and 19KJB150011) and Project
funded by China Postdoctoral Science Foundation (No.
2018M630572 and 2019M651873). We thank the staff from
BL17B beamline of National Facility for Protein Science
Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility
for assistance during data collection.
2
lytic reaction condition to investigate the carbon source origin
of the produced HCOOH, and the resultant reaction filtrate
1
3
was characterized by C NMR spectroscopy. As shown in the
1
3
Supporting Information, Figure S19, the C NMR spectrum
À
gives a clear signal at 164.3 ppm, corresponding to HCOO ,
which is in line with that demonstrated in other important
[
14,17]
À
works.
Also, this HCOO signal cannot be observed
1
2
13
when using CO instead of CO , except for additional
2
2
peaks corresponding to CD CN and TEOA (Supporting
3
Information, Figure S20). These results unambiguously con-
À
firm that the produced HCOO indeed originates from CO ,
2
suggesting that both titanium–oxo clusters are very active and Conflict of interest
À
capable of reducing CO2 to HCOO under UV light
irradiation. Electron spin resonance (ESR) measurements
were further conducted to study the mechanism behind the
The authors declare no conflict of interest.
CO2 photoreduction over PTi16 and PTi . As shown in
12
4
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Keywords: CO photoreduction · Keggin structures ·
polyoxo-titanium cluster
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8] a) W.-H. Fang, L. Zhang, J. Zhang, J. Am. Chem. Soc. 2016, 138,
Manuscript received: August 16, 2019
7
480 – 7483; b) G. Zhang, C. Liu, D.-L. Long, L. Cronin, C.-H.
Accepted manuscript online: September 26, 2019
Version of record online: && &&, &&&&
Tung, Y. Wang, J. Am. Chem. Soc. 2016, 138, 11097 – 11100.
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
IV
Polyoxometalates
One of the family: A Ti -based heteroa-
tom Keggin and its trivacant lacunary
architectures were structurally synthe-
sized as a polyoxo-titanium cluster. They
exhibited a very high selectivity and
N. Li, J. Liu, J.-J. Liu, L.-Z. Dong, S.-L. Li,
B-.X. Dong, Y.-H. Kan,
Y.-Q. Lan*
&&&& — &&&&
activity for photocatalytic CO -to-
2
Self-Assembly of a Phosphate-Centered
Polyoxo-Titanium Cluster: Discovery of
the Heteroatom Keggin Family
HCOOH conversion.
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!