ZHENG Ai-Ping GAO Mei-Yan FANG Wei-Hui KANG Yao

Two Novel {Ti6P2} Clusters Decorated with Inorganic Acids①

ZHENG Ai-Pinga, bGAO Mei-YanbFANG Wei-Huib②KANG Yaob②

a(350100)b(350002)

Two inorganic acids decorating titanium-oxo clusters (PTCs), Ti6O4(OiPr)10(O3P-Phen)2(NO3)2(PTC-251) and Ti6O4(OiPr)10(O3P-Phen)2(HSO4)2(PTC-252) (H2O3P-Phen = phenylphosphinic acid) have been synthesized under solvothermal conditions.As a result of the labile coordination sites of the {Ti6P2} unit, nitrite and sulfate adopt different capping mode.Besides, they also present different space packing.The photocatalytic H2evolution activities of these obtained PTCs have been studied, with sulfate decorating PTC-252 presenting a maximum H2production rate up to 110.95mol·g-1·h-1.

polyoxo-titanium clusters, inorganic acid, labile coordination sites, water-splitting;

1 INTRODUCTION

Nanoscale titanium oxide has been broadly used in solving energy crisis due to its abundance, low-cost, little toxicity, nice photostability, and high photocatalytic efficiency[1, 2].However, it is difficult to determine the reaction mechanisms while utilized as photocatalysis.Hence, crystalline polyoxo titanium clusters (PTCs) with accurate structures have appealed to researchers, and a great number of crystalline PTCs have been synthesized and characterized recently[3-9].One of the most important studies is bandgap engineering[10, 11].And one method to reduce the bandgap of titanium oxo clusters and enhance their visible light adsorption is the organic ligand modification[12-15].

In the family of PTCs, organophosphate-stabilized {Ti6P2} cluster with six labile coordination sites has been used as a platform for ligands substitution and metal incorporation[16-18].For example, in 2014, Schubertreported a series of stable Ti6O4(OiPr)10(O3PR)2(OAc)2(OAc = acetate) cluster with different phosphonate ligands[19].Subsequently, our group used carboxylates, phosphonates and sulfonates ligands to replace the active coordination sites of {Ti6P2} cluster, and demonstrated that high electron-withdrawing organic species can reduce the bandgaps of these complexes[20].However, there are still no studies on the inorganic acid-modified {Ti6P2} cluster.As is known to us all, inorganic acids like nitric or sulfuric acid are cheap, stable, and widely available.Thus, it is meaningful to study how inorganic acids occur in such {Ti6P2} structure.

As a continuation of our effort, we researched constructing {Ti6P2} cluster-based PTCs using nitric or sulfuric acid.Successfully, two complexes, namely, Ti6O4(OiPr)10(O3P- Phen)2(NO3)2(PTC-251) and Ti6O4(OiPr)10(O3P- Phen)2(HSO4)2(PTC-252) (H2O3P-Phen = phenylphosphinic acid), were synthesized and structurally characterized.As expected, the introduced inorganic acids indeed replace the organic carboxylates sites (like acetate, benzoic acid,.), and locate on the upper and bottom surfaces of {Ti6P2} cluster.Moreover, the bandgap properties and photocatalytic water-splitting hydrogen-evolution activities of these two complexes are also investigated.

2 EXPERIMENTAL

2.1 Materials and measurements

All chemicals except distilled water were purchased commercially and used without further purification.Distilled water was obtained by our laboratory.Ti(OiPr)4was purchased from Aladdin, and phenylphosphonic acid was purchased from Energy Chemical.Isopropyl alcohol, nitric acid and sulfuric acid were purchased from Sinopharm Chemical Reagent Beijing.Powder X-ray diffraction (PXRD) data were obtained by placing target crystals onto the flat sample holders using a MiniFlex2 X-ray diffractometer with Curadiation (= 0.1542 nm) in the 2range from 5° to 50° with a scanning rate of 5 °/min.The Fourier transform infrared (FTIR) spectroscopic data (KBr pellets) were obtained on a PerkinElmer Spectrum 100 FTIR Spectrometer.The diffuse reflectance ultraviolet (UV) data were collected on powder samples with BaSO4as standard (100% reflectance) with a PerkinElmer Lamda-950 UV spectrophotometer at room temperature.Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA 851eanalyzer at a heating rate of 10 ℃/min from 25 to 600 ℃ under a nitrogen atmosphere.

2.2 Synthesis

2.2.1 Ti6O4(OiPr)10(O3P-Phen)2(NO3)2(PTC-251)

Phenylphosphonic acid (0.9941 g, 7.0 mmol) and 4 drops of nitric acid were mixed in 5.5 mL anhydrous isopropanol.And then Ti(OiPr)4(0.92 mL, 3.0 mmol) was added rapidly into the mixture.The resultant solution was sealed and heated at 80 ℃ for 3 days.Colorless block crystals of PTC-251 were obtained after cooling to room temperature (Yield: 53% based on Ti(OPr)4).Important IR data (KBr, cm-1): 2970(m), 2932(w), 2864(w), 1627(w), 1555(s), 1474(w), 1438(w), 1366(w), 1275(s), 1034(m), 989(s), 928(m), 754(m), 684(m), 609(w), 551(w).

2.2.2 Ti6O4(OiPr)10(O3P-Phen)2(HSO4)2(PTC-252)

PTC-252 was prepared by the same procedure as that for PTC-251, except the nitric acid was replaced by sulfuric acid.Colorless blocks crystals of PTC-252 were obtained (60% yield based on Ti(OiPr)4).Important IR data (KBr, cm-1): 2973(m), 2930(w), 2866(w), 2311(w), 1625(m), 1445(s), 1363(m), 1327(w), 1202(m), 1140(w), 1038(w), 993(vs), 939(w), 756(m), 693(s), 621(w), 548(m), 430(w).

2.3 X-ray crystallography

3 RESULTS AND DISCUSSION

3.1 Crystal structures of PTC-251 and PTC-252

The organic modifications on {Ti6P2} cluster have been reported in 2014 and 2016[19, 20, 23].Herein, it’s the first time to decorate the {Ti6P2} cluster with different inorganic acids.The crystal belongs to monoclinic system with space group.{Ti3(3-O)} subunit is a common and low-nuclearity moiety existing in the hydrolysis of titanium[24-26].This fragment usually acts as a second building block generating larger aggregation[19, 27-33].In PTC-251, such {Ti3(3-O)} triangle block was equatorially bridged by isopropoxide groups (Fig.1).Five isopropoxide ligands alternately adopted bridging bidentate and monodentate coordination mode.Axially, it was then linked by a nitrate to form Ti3(3-O)(2- OiPr)2(OiPr)3(NO3)4+({Ti3}).Two parallel {Ti3} are connected through oxo bridges and phenylphosphinic ligands to form a {Ti6P2} cluster (Fig.2).Ti1 and Ti2 ions in the {Ti6P2} core are six-coordinated whilst Ti3 is only five-coordinated.

Fig.1.Perspective view of the {Ti3(3-O)} subunit in PTC-251.Color code: O red; C grey; Ti green; N blue

Fig.2.Molecular structure of PTC-251.Color code: O red; C grey; Ti green; N blue

The cluster core of PTC-252 is isostructural to that of PTC-251.However, the attachment of the “outer” Ti coordination environment is different.Sulfate took tridentate coordination mode capping on the {Ti3(3-O)} subunit (Fig.3).As a result, all of the Ti ions in PTC-252 are in octahedral coordination geometry (Fig.4).The Ti–O bond lengths between nitrate and sulfate were longer than the other Ti–O bond in the structure (Tables S1 and S2).These clusters packed differently largely attributed to the steric hindrance from the two outer faces (Fig.S1).

Fig.3.Perspective view of the {Ti3(3-O)} subunit in PTC-252.Color code: O red; C grey; Ti green; N blue

Fig.4.Molecular structure of PTC-252.Color code: O red; C grey; Ti green; S orange

3.2 Characterization

The experimental PXRD patterns of PTC-251 and PTC-252 are well-matched with their simulated PXRD patterns (Fig.S2), evidencing that the experimental samples are in good phase purity.The different reflection intensity between experimental and simulated is attributed to the variation of the powder sample in the preferred orientation.The TGA of PTC-251 and PTC-252 was analyzed in a dry air atmosphere from 25 to 600 ℃ (Fig.S3).The TGA of PTC-251 exhibits overall one-step weight loss while PTC-252 undergoes two stages of weight loss.The UV absorption spectra of PTC-251 andPTC-252 present bandgaps of 3.39 and 3.34 eV, respectively (Fig.S4).

The IR spectra of PTC-251 and PTC-252 have been recorded in the range of 4000～400 cm–1from solid samples palletized with KBr, which are presented in Fig.S5.In the high wavenumber region (> 1000 cm−1), the weak absorption bands at 3010～2910 cm−1are observed, which can be ascribed to the stretching vibration modes of C–H bonds in OiPr groups.The characteristic skeletal vibrations of benzene rings are observed at 1640～1430 cm−1.Besides, the peaks appearing at 1430～1260 cm−1and 1110～1040 cm−1can be respectively assigned to the characteristic bending vibrations ofC–Hand stretching vibration ofC–O.In the low wavenumber region (< 1000 cm−1), the absorptions in the region ca.800～642 cm−1can be attributed to the C–H in-plane or out-of-plane bends, ring breathing, and ring deformation absorptions of benzene rings.What’s more, the characteristic vibration of inorganic acid can be also observed at 861～827 cm−1for NO3–in PTC-251, 620～550 cm−1for HSO42–in PTC-252.Thus, the result of IR spectra coincides with that from the X-ray single-crystal structural analysis.

3.3 Photocatalytic properties

To evaluate the photocatalytic performances of PTC-251 and PTC-252, photocatalytic hydrogen production studies were carried out under UV-light.Although acetate decorating {Ti6P2} cluster didn’t show any hydrogen evolution activities[20], PTC-251 and PTC-252 with efficient photo- catalytic abilities in water-splitting hydrogen-evolution reactions can be observed.The nitrate decorating PTC-251 gives the hydrogen production of 14.5mol·g-1·h-1, while the sulfate decorating PTC-252 presents the higher hydrogen production of 110.95mol·g-1·h-1.The photocatalytic performance of PTC-252 was comparable to some PTCs and metal organic frameworks[34, 35].The rate trends of these two compounds are shown in Fig.5.Especially to PTC-252, the steady H2evolution rate indicates that the {Ti6P2} cluster decorated by sulfate is quite stable.These results also confirm that the photocatalytic H2evolution activity of PTCs can be influenced by modified ligands.

Fig.5.Comparison of H2evolution behaviors under UV-vis light-driven with PTC-251 and PTC-252

4 CONCLUSION

In summary, we have successfully utilized {Ti6P2} clusters as a platform and constructed two inorganic acids decoratingPTC-251 and PTC-252.The coordination modes of capped inorganic acid on {Ti6P2} clusters are inconsistent, which leads to totally different supramolecular packing.Moreover, the photocatalytic H2evolution activities of these two PTCs are also different.The H2evolution rate of PTC-252 can reach up to 110.95mol·g-1·h-1while that of PTC-251 is only 14.5mol·g-1·h-1.Our results not only enrich structures of organophosphate-stabilized PTCs but also provide an interes- ting model for better understanding the structure-property relationships of Ti–O materials.

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17 April 2020;

15 June 2020(CCDC 1995565 for 1 and 1995566 for 2)

① This project was supported by National Natural Science Foundation of China (21771181, 21935010 and 21973096) and Youth Innovation Promotion Association CAS (2017345)

E-mail: fwh@fjirsm.ac.cn and ky@fjirsm.ac.cn

10.14102/j.cnki.0254–5861.2011–2853