A {Co9}-Added Polyoxometalate for Efficient Visible-Light-Driven Hydrogen Evolution

A polyanion cluster H6Na8Cs3[Co9(μ3-OH)3(H2O)6(HPO4)2(B-α-PW9O34)3]Cl·40H2O (1) was made with the guidance of the lacunary directing strategy under hydrothermal conditions. Compound 1 was characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analysis, respectively. Single-crystal X-ray diffraction analyses showed that 1 consists of three anions [B-α-PW9O34]9− and a cyclic cationic [Co9(μ3-OH)3(H2O)6]15+ and two anions HPO42−. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Visible-light-driven hydrogen evolution tests demonstrated that 1 was an efficient water reduction catalyst with an H2 evolution rate of 1217.6 μmol h−1 g−1.

POMs have unique redox properties and modifiable Lewis acidity but their inherent poor stability and tendency to aggregate in solution have hindered further development [20]. TMAPs are widely used in photocatalysis [22,23] and electrocatalysis [24] fields due to their physicochemical properties and reversible multi-electron transfer ability. TMAPs have been widely studied due to their unique structural diversity, magnetic properties, and catalytic properties. Taking the above into account, TMAPs have demonstrated a massive potential for application for visible-light-driven Hydrogen evolution as heterogeneous water reduction catalysts (WRCs) [14,15,25].
Co-containing complexes were applied in photocatalytic water-splitting reactions with good reactivity [26] and Co 2+ can be induced by tri-lacunary POMs fragments {XW 9 O 34 } n− , thus improving the performance of Co-added POMs (CoAPs) as the catalytic active center in visible-light-driven H 2 evolution. Herein, a polyanion (HPO 4 ) 2 @-{Co 9 (PW 9 ) 3 } cluster was made with the guidance of the lacunary directing strategy under hydrothermal conditions. Interestingly, the HPO 4 groups of 1 originate from the central heteroatom of the tri-lacunary precursor [A-α-PW 9 O 34 ] 9− and assemble with the cyclic (HPO 4 ) 2 @{Co 9 } cluster to form CoAP. Moreover, visible-light-driven H 2 evolution tests have demonstrated that CoAP is an efficient WRC with the H 2 evlution rate of 1217.6 µmol h −1 g −1 , indicating that 1 is a potential heterogeneous WRC.

General Procedure
All chemicals were acquired commercially and used without further purification. The synthetic method of the Na 10 [A-α-PW 9 O 34 ]·7H 2 O ({PW 9 }) was derived from previous literature methods [27]. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance XRD diffractometer (Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54056 Å). FT-IR spectra were measured by using a Thermo Scientific Nicolet iS10 FT-IR spectrometer (Waltham, MA, USA) in the range of 400-4000 cm −1 with KBr pallets. Thermogravimetric analyses were conducted under air flowing on a Mettler−Toledo TGA/DSC 1000 (Zurich, Switzerland) with a heating rate of 10 • C min −1 from 25 to 1000 • C. UV-Vis absorption spectra were obtained using a Shimadzu UV3600 spectrometer (Kyoto, Japan).

X-ray Crystallography
A suitable crystal was selected and put on a Bruker APEX-II CCD diffractometer. This crystal was kept at 296.15 K during data collection. Using Olex2 [28], the structure was solved with the ShelXT [29] structure solution program using Intrinsic Phasing and refined with the ShelXL [30] refinement package using Least Squares minimization. The contribution of these disordered solvent molecules to the overall intensity data of the structure was treated using the SQUEEZE method [31] in PLATON (25 lattice water molecules in 1 are estimated by TGA). The crystallographic data structure refinement is listed in Table 1. Detailed crystallographic data of the title crystal were deposited on the Cambridge Crystallographic Data Centre with CCDC reference number 2213769 for 1. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html accessed on 29 June 2022, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; Fax: +44-1223 336 033.

Structure of 1
The structure of 1 crystallized in the hexagonal space group P63/m. The structure contains one [Co9

IR Spectrum and Optical Band Gap
FT-IR spectra were measured in the range of 400-4000 cm −1 with KBr pallets ( Figure  S3, Supplementary Materials). The strong peaks at around 3430 and 1630 cm −1 are dominated by the stretching and bending modes of the water molecules. The characteristic bands derived from the Keggin POM fragments in the 721-1030 cm −1 region. In detail, the peak at 1030 and 933 cm −1 for 1 is attributable to ν(P-Oa) and ν(W-Ot). The peak at 806, 721, and 692 cm −1 is attributable to ν(M-O-M) (M = W or Co), respectively. UV-Vis absorption and optical diffuse reflectance spectra of 1 were obtained in the wavelength range of 200-800 nm ( Figure S4, Supplementary Materials). Absorption peaks of 260 nm and 535 nm correspond to the charge transfer between O→W and the d-d charge transfer between Co 2+ , respectively (illustration). The band-gap energy (Eg) is obtained by extrapolating the linear part of the rising curve to zero [32]. The Eg of 1 was estimated as 2.58 eV.

PXRD and Thermogravimetric Analysis
The pure phase was confirmed by powder X-ray diffraction (PXRD) patterns ( Figure  S5

IR Spectrum and Optical Band Gap
FT-IR spectra were measured in the range of 400-4000 cm −1 with KBr pallets ( Figure S3, Supplementary Materials). The strong peaks at around 3430 and 1630 cm −1 are dominated by the stretching and bending modes of the water molecules. The characteristic bands derived from the Keggin POM fragments in the 721-1030 cm −1 region. In detail, the peak at 1030 and 933 cm −1 for 1 is attributable to ν(P-Oa) and ν(W-Ot). The peak at 806, 721, and 692 cm −1 is attributable to ν(M-O-M) (M = W or Co), respectively. UV-Vis absorption and optical diffuse reflectance spectra of 1 were obtained in the wavelength range of 200-800 nm ( Figure S4, Supplementary Materials). Absorption peaks of 260 nm and 535 nm correspond to the charge transfer between O→W and the d-d charge transfer between Co 2+ , respectively (illustration). The band-gap energy (Eg) is obtained by extrapolating the linear part of the rising curve to zero [32]. The Eg of 1 was estimated as 2.58 eV.

PXRD and Thermogravimetric Analysis
The pure phase was confirmed by powder X-ray diffraction (PXRD) patterns ( Figure S5
As shown in Figure 3a, the H 2 production increases with increasing radiation time. With the increasing concentrations of TEOA (5 mM, 10 mM, 15 mM, 20 mM) in the reaction system, the production of H 2 showed a trend of first increasing (11.88 µmol, 31.36 µmol, 36.53 µmol, respectively) and then decreasing (24.48 µmol) after 10 h of illumination. One possible explanation is that the addition of TEOA increases the pH value of the system, reducing the concentration of H + in the system, and thereby reducing the catalytic effect. The H 2 production (9.9 µmol, 36.5 µmol and 40.7 µmol, respectively) increases with the increasing quantity of catalyst (1, 3, 6 mg), and the H 2 evolution rate was 745.6 µmol h −1 g −1 , 1217.6 µmol h −1 g −1 and 745.6 µmol h −1 g −1 , respectively (Figure 3b). In terms of catalytic efficiency, the maximum H 2 production rate of 3 mg can reach 1217.6 µmol h −1 g −1 . Considering the catalytic system is heterogeneous catalysis, with the increase in the amount of catalyst, part of the light is scattered and the catalytic efficiency decreases. Hence, efficient catalysis may be achieved using the appropriate amount of catalyst. When the concentration of the photosensitizer (0.05 mM, 0.1 mM, 0.2 mM) was changed under the optimal conditions, the amount of the H 2 production was 20.6 µmol, 23.58 µmol and 36.53 µmol, respectively, and the amount of hydrogen production increased with the increase in the concentration of the photosensitizer (Figure 3c). The PXRD and FT-IR ( Figure S7, Supplementary Materials) tests before and after the photocatalytic reaction proved that the structure of 1 was basically unchanged and had good catalytic stability.
For heterogeneous catalytic systems, the stability and recyclability of catalysts are always important factors. In the cycling experiments ( Figure S8, Supplementary Materials), the yield of H 2 was detected by GC analysis, and the catalyst was isolated by centrifugation and washed with CH 3 CN after two hours of reaction at a catalyst dosage of 9 mg. The experiments were repeated for three successive cycles, the amount of H 2 production was similar in the first two cycles but decreased the third time. Such a decrease in activity might be ascribed to the loss of the catalyst sample during the isolating operation. The PXRD pattern and FT-IR spectra ( Figure S9, Supplementary Materials) of the catalyst before and after cycling experiments proved that 1 remains unchanged, which further demonstrated the recyclability and stability of complex 1.
In conclusion, the best reaction conditions explored were: 1 (3 mg), TEOA (15 mM) and photosensitizer (0.2 mM), and the best H 2 production efficiency can reach 1217.6 µmol h −1 g −1 . This value is comparable to that of previously reported results for H 2 evolution (Table S3, Supplementary Materials). Considering the factors of catalyst efficiency, 3 mg 1 was used for in-depth studies. As shown in Figure 3d, H 2 was produced only when the photosensitizer [Ir(ppy) 2 (dtbbpy)]-[PF 6 ] and TEOA electron sacrificial reagent were added with light irradiation, and a small amount of H 2 was produced when the equimolar amount of {PW 9 } and CoCl 2 ·6H 2 O was used as the control catalyst revealing the superiority of CoAPs in the visible-light-driven H 2 evolution. According to the reported literature [22,33,34], a possible photocatalytic mechanism for this visible-light-driven H 2 evolution process was proposed ( Figure S10, Supplementary Materials). In visible-light-driven catalytic systems, the photosensitizer's excited state can function as either an oxidant or reductant and thus can be quenched by an electron donor or an acceptor. A POMs-based catalyst and TEOA can oxidatively and reductively quench the excited state of [Ir(ppy) 2 (dtbbpy)] + * in the visible-light-driven H 2 evolution process catalyzed by 1. In conclusion, the best reaction conditions explored were: 1 (3 mg), TEOA (15 mM) and photosensitizer (0.2 mM), and the best H2 production efficiency can reach 1217.6 μmol h −1 g −1 . This value is comparable to that of previously reported results for H2 evolution (Table S3, Supplementary Materials). Considering the factors of catalyst efficiency, 3 mg 1 was used for in-depth studies. As shown in Figure 3d, H2 was produced only when the photosensitizer [Ir(ppy)2(dtbbpy)]-[PF6] and TEOA electron sacrificial reagent were added with light irradiation, and a small amount of H2 was produced when the equimolar amount of {PW9} and CoCl2·6H2O was used as the control catalyst revealing the superiority of CoAPs in the visible-light-driven H2 evolution. According to the reported literature [22,33,34], a possible photocatalytic mechanism for this visible-light-driven H2 evolution process was proposed ( Figure S10, Supplementary Materials). In visible-light-driven catalytic systems, the photosensitizer's excited state can function as either an oxidant or reductant and thus can be quenched by an electron donor or an acceptor. A POMs-based catalyst and TEOA can oxidatively and reductively quench the excited state of [Ir(ppy)2(dtbbpy)] + * in the visible-light-driven H2 evolution process catalyzed by 1.

Magnetic Properties
The variable-magnetic properties of 1 were measured in the temperature range of 2-300 K with a magnetic field of 5000 Oe. As shown in Figure 4a, the χmT value is 26.53 cm 3 mol −1 K at 300 K. The χmT value decreases continuously with decreasing temperature.

Magnetic Properties
The variable-magnetic properties of 1 were measured in the temperature range of 2-300 K with a magnetic field of 5000 Oe. As shown in Figure 4a, the χ m T value is 26.53 cm 3 mol −1 K at 300 K. The χ m T value decreases continuously with decreasing temperature. When the temperature drops below 50 K, the χ m T value decreases rapidly. The χ m T drops to 4.64 cm 3 mol −1 K at 2 K. The above behaviors suggest antiferromagnetic interactions in 1. The magnetic susceptibility data of 1 are in accordance with the Curie-Weiss law χ = C/(T−θ) (Figure 4b) in the temperature range of 50-300 K with a constant of θ = −28.25 K, which is consistent with the overall antiferromagnetic interaction in 1.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 9 When the temperature drops below 50 K, the χmT value decreases rapidly. The χmT drops to 4.64 cm 3 mol −1 K at 2 K. The above behaviors suggest antiferromagnetic interactions in 1. The magnetic susceptibility data of 1 are in accordance with the Curie-Weiss law χ = C/(T−θ) (Figure 4b) in the temperature range of 50-300 K with a constant of θ = −28.25 K, which is consistent with the overall antiferromagnetic interaction in 1.

Conclusions
In conclusion, a polyanion (HPO4)2@{Co9(PW9)3} cluster was made with the guidance of a lacunary directing strategy under hydrothermal conditions. The HPO4 of 1 originates from the central heteroatom of the tri-lacunary precursor [A-α-PW9O34] 9− and assembles with a cyclic (HPO4)2@{Co9} cluster to form CoAP 1. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Moreover, visible-light-driven H2 evolution tests demonstrated that 1 is an eco-friendly, efficient and stable WRC with an H2 evolution rate of 1217.6 μmol h −1 g −1 . This work further proves that the lacunary directing strategy is an

Conclusions
In conclusion, a polyanion (HPO 4 ) 2 @{Co 9 (PW 9 ) 3 } cluster was made with the guidance of a lacunary directing strategy under hydrothermal conditions. The HPO 4 of 1 originates from the central heteroatom of the tri-lacunary precursor [A-α-PW 9 O 34 ] 9− and assembles with a cyclic (HPO 4 ) 2 @{Co 9 } cluster to form CoAP 1. Variable-magnetic properties indicate antiferromagnetic interactions in 1. Moreover, visible-light-driven H 2 evolution tests demonstrated that 1 is an eco-friendly, efficient and stable WRC with an H 2 evolution rate of 1217.6 µmol h −1 g −1 . This work further proves that the lacunary directing strategy is an effective guideline for the synthesis of TMAPs under hydrothermal conditions. The photocatalytic performance showed that CoAP is an efficient WRC. In future work, the application of POMs in heterogeneous catalytic reactions is in progress.