Accurate assembly of thiophene-bridged titanium-oxo clusters with photocatalytic amine oxidation activity

Designing and synthesizing well-defined crystalline catalysts for the photocatalytic oxidative coupling of amines to imines remains a great challenge. In this work, a crystalline dumbbell-shaped titanium oxo cluster, [Ti10O6(Thdc)(Dmg)2(iPrO)22] (Ti10, Thdc = 2,5-thiophenedicarboxylic acid, Dmg = dimethylglyoxime, iPrOH = isopropanol), was constructed through a facile one-pot solvothermal strategy and treated as a catalyst for the photocatalytic oxidative coupling of amines. In this structure, Thdc serves as the horizontal bar, while the {Ti5Dmg} layers on each side act as the weight plates. The molecular structure, light absorption, and photoelectrochemical properties of Ti10 were systematically investigated. Remarkably, the inclusion of the Thdc ligand, with the assistance of the Dmg ligand, broadens the light absorption spectrum of Ti10, extending it into the visible range. Furthermore, the effective enhancement of charge transfer within the Ti10 was achieved with the successful incorporation of the Thdc ligand, as opposed to PTC-211, where terephthalic acid replaces the Thdc ligand, while maintaining consistency in other aspects of Ti10. Building on this foundation, Ti10 was employed as a heterogeneous molecular photocatalyst for the catalytic oxidative coupling reaction of benzylamine (BA), demonstrating very high conversion activity and selectivity. Our study illustrates that the inclusion of ligands derived from Thdc enhances the efficiency of charge transfer in functionalized photocatalysts, significantly influencing the performance of photocatalytic organic conversion.


Introduction
Photocatalytic organic conversion stands out as a prospective method for achieving direct chemical bond functionalization in ambient conditions.6][7] Conventional approaches to imine preparation oen require harsh reaction conditions, involving high temperatures and pressures, leading to inevitable environmental pollution. 8,9][15] However, most of these catalysts always lack clear structural information and exhibit complex interfacial information, posing substantial challenges in comprehending the connection between structure and functionality.Therefore, the development of well-dened crystalline photocatalysts is an urgent need.
Titanium oxo clusters (TOCs), as molecular models of TiO 2 nanoparticles, have rapidly developed.TOCs not only exhibit similar catalytic activity to TiO 2 , but also readily grow into single crystals, facilitating structure identication.7][18][19][20] Specically, as an aggregate containing multiple metal ions, TOCs can generate multiple active metal sites by judiciously manipulating the coordination environment of metal ions. 21This, in turn, offers increased

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PAPER possibilities for enhancing catalytic activity and facilitating the application of photocatalytic reactions.3][24][25][26][27] However, to the best of our knowledge, there is a scarcity of reported TOCs employed in the photocatalytic oxidative coupling of amines to imines. 28For instance, Liu et al. explored the photocatalytic activity of TOCs in the oxidative coupling of amines, utilizing ferrocene-functionalized TOCs for the rst time. 29Therefore, more research efforts should be directed towards synthesizing novel TOCs for the photocatalytic oxidative coupling of amines.Nevertheless, numerous challenges persist in the utilization of conventional TOCs in photocatalytic applications.1][32] To address this, researchers focus on regulating the band gap through ligand modication, thereby tailoring the light absorption range. 33,34Therefore, the selection of suitable functionalized organic ligands becomes crucial for designing novel TOCs with broadened light absorption.
The derivatives of thiophene (Th), which serve as sulfurcontaining heterocyclic moieties, exhibit electron-rich characteristics, enabling their use as electron donors.6][37] For instance, polythiophene derivatives have been used in organic solar cells. 38,39owever, only a limited number of TOCs containing Th-derived functional ligands have been documented to date.
Based on the aforementioned factors, we designed and prepared a dumbbell-shaped TOC, [Ti 10 O 6 (Thdc)(Dmg) 2 (i PrO) 22 ] (Ti10, Thdc = 2,5-thiophenedicarboxylic acid, Dmg = dimethylglyoxime, i PrOH = isopropanol), employing solvent thermal methods.In this structure, Thdc serves as the horizontal bar, while the {Ti 5 Dmg} layers on each side act as the weight plates.Remarkably, the coordination of Thdc and Dmg ligands extends the light absorption range of Ti10 into the visible light region.Furthermore, the inclusion of the Thdc ligand notably enhances the photocurrent response of Ti10 in comparison to PTC-211.In PTC-211, terephthalic acid replaces the Thdc ligand, while other aspects remain consistent with Ti10.
Given these advantages, we proceeded with additional research on the photocatalytic oxidative coupling reaction of benzylamine (BA) using Ti10.Notably, Ti10 demonstrated exceptional light-driven photocatalytic efficiency, achieving a 99% yield and selectivity under ambient pressure.We further propose a plausible photocatalytic mechanism for this reaction.

Results and discussion
Syntheses and structure analyses of Ti10 Ti10 was synthesized via a solvothermal reaction of Ti( i PrO) 4 , Dmg, and Thdc ligands dissolved in isopropanol and reacted at 100 °C for three days (Scheme 1).Aer cooling to room temperature, the mixture was le to stand for one week, resulting in the formation of yellow crystals with a moderate yield (Fig. S1 †).It is well-known that solvents play a crucial role in inuencing the structure of metal clusters.Therefore, we have performed numerous parallel experiments in the synthesis of Ti10, changing the solvents, such as ethanol or acetonitrile, while keeping all other synthesis variables constant.However, no other crystalline products could be isolated, suggesting the irreplaceable role of isopropanol in the formation of Ti10.Single crystal X-ray diffraction (SCXRD) analysis revealed that Ti10 crystallizes in the monoclinic system with the P 1 space group.Its asymmetric unit contains one complete cluster.The overall structure of Ti10 appears dumbbell-shaped, with the Thdc ligand serving as the horizontal bar, and the {Ti 5 Dmg} layers on each side acting as the weight plates.Ti10 consists of 10 Ti atoms, 6 m 3 -O atoms, 1 Thdc ligand, 2 Dmg ligands, and 22 isopropanol molecules (Fig. 1a).This structure is similar to the PTC-211 previously reported by Zhang. 40The main difference is that we used sulfur-containing heterocyclic Thdc ligand, while they used linear ligands terephthalic acid (Fig. S2 †).Given that these clusters share a highly similar titanium-oxo core and are connected by various organic ligands, they could function as a model system for a comprehensive examination of the variations in properties resulting from distinct ligands.

General characterization of Ti10
The experimental powder X-ray diffraction (PXRD) pattern of Ti10 was well-matched with the simulated patterns derived from SCXRD data, providing evidence of high phase purity (Fig. S3 †).Thermal stability assessment of Ti10 was carried out through thermogravimetric analysis (TGA), and the corresponding results indicate its high structural integrity was maintained at temperatures below 190 °C (Fig. S4 †).Infrared (IR) spectroscopy of Ti10 shows that the peaks at 2974, 2934, and 2862 cm −1 can be assigned to n(C-H) stretching vibrations of isopropoxide groups.The vibrations at around 1527 cm −1 indicate the coordination vibrations of carboxyl groups of Thdc.The band at ∼1370 cm −1 represents the stretching vibrations of n(C]N) of Dmg (Fig. S5 †).Energy-dispersive X-ray spectroscopy (EDS) analysis and mapping for Ti10 were presented to determine the chemical composition (Fig. S6 and S7 †).Additionally, X-ray photoelectron spectroscopy (XPS) reveals that the titanium elements within Ti10 are found in the Ti(IV) oxidation state (Fig. S8 †).This result is consistent with the BVS analysis (Table S1 †).

ESI-MS of Ti10
3][44][45] To elucidate the solution behavior of Ti10, ESI-MS was performed in positive ion mode aer Ti10 was dissolved in CH 2 Cl 2 .As illustrated in Fig. 2 species (1c to 1a) can be considered as the result of the sequential loss of i PrO − molecules.The results reveal that fragmentation occurred during the ESI process, but the integrity of the metal skeleton is maintained.The loss of i PrO − molecules during ionization is attributed to the fact that they are weakly bound to the cluster surface compared to Dmg and Thdc ligands.

Photoelectric properties
Diffuse reectance spectroscopy analysis was employed to examine the UV-vis absorption of Ti10.Furthermore, we selected PTC-211 as a research representative because the structures of Ti10 and PTC-211 are isomorphic.This allows us to better study the property differences caused by different ligands.As illustrated in Fig. 3a, the solid-state UV-visible absorption spectra show that the absorption bands of Ti10, PTC-211, and Thdc can extend to 538, 498, and 369 nm, respectively.Ti10 exhibited a broader absorption than PTC-211, indicating that Thdc ligand inuences the light absorption capability of TOCs.Furthermore, Ti10 also demonstrated superior visible light absorption compared to the free Thdc ligand.As is known, the light absorption of titanium-oxo clusters with a relatively large band gap is primarily derived from O / Ti charge transfer transitions. 46Therefore, the broader absorption of Ti10 should be mainly attributed to the cocoordination of Thdc and Dmg.Based on the Kubelka-Munk function of (ahy) 1/2 = k(hy − E g ) (E g is the band gap (eV), h is the Planck's constant (J s), n is the light frequency (s −1 ), k is the absorption constant and a is the absorption coefficient), the optical band gaps of Ti10, PTC-211, and Thdc were estimated to be 2.37, 2.52, and 3.60 eV (Fig. 3b), respectively. 47he transient photocurrent response assessments were conducted to evaluate the capabilities of these TOCs in separating photogenerated electron-hole pairs.The photocurrent response experiments were carried out in a three-electrode cell, with TOC-coated ITO glass as the working electrode, Ag/AgCl as the reference electrode, and platinum wire as the counter electrode.All experiments were carried out in a 0.2 mol L −1 Na 2 SO 4 electrolyte solution, under the illumination of a 150 W xenon light source, with on-off cycling intervals of 20 s.As shown in Fig. 3c, a consistent and reproducible photocurrent response was observed when the Xe lamp was switched on and off, indicating their good photoelectric response and high stability.Remarkably, the transient photocurrent density of the Ti10 electrode (0.25 mA cm −2 ) was higher than that of the PTC-211 electrode (0.20 mA cm −2 ).This suggests that Ti10 has better photogenerated electron separation and transfer capabilities due to the introduction of Thdc ligand.
Mott-Schottky measurements at frequencies of 1000, 1500, and 2000 Hz were performed to determine the at-band potential of Ti10.As shown in Fig. 3d, the Mott-Schottky plot of Ti10 exhibits a positive slope, conrming its n-type semiconductor-like characteristic. 48The lowest unoccupied molecular orbital (LUMO) position of Ti10 was determined to be −0.72 V (vs.NHE, pH = 7).Combined with the band gap obtained from UV-Vis diffuse reectance spectra and LUMO, the highest occupied molecular orbital (HOMO) position of Ti10 was evaluated to be 1.65 V (vs.NHE).Evidently, Ti10 exhibits a signicantly higher HOMO value, rendering it suitable for potential applications in diverse photoinduced organic synthesis reactions, including amine oxidation.Additionally, the adequate band structure holds signicant importance for catalysts involved in photocatalytic reactions.It is apparent that the LUMO value of Ti10 is more negative than the theoretical potential for the reduction of O 2 to superoxide radical (−0.33 V vs. NHE), a key active species in organic oxidation reactions. 49his implies that Ti10 possesses theoretical viability for the photocatalytic transformation of O 2 into superoxide radical anions (O 2 c − ) intermediates (Fig. 3e).Additionally, Ti10 demonstrates a faster interfacial charge transfer process than PTC-211, as evidenced by electrochemical impedance spectroscopy (EIS) Nyquist plots (Fig. 3f).

Photocatalytic oxidative coupling reaction of benzylamine
Due to the outstanding photophysical characteristics outlined earlier, we carried out the photocatalytic oxidative coupling of BA.Through systematic optimization, we found that when subjecting 22 mg of Ti10 and 0.2 mmol of BA in 4 mL of CH 3 CN to 410 nm LED irradiation under O 2 at room temperature for 18 hours, Ti10 exhibited remarkable catalytic activity.The conversion efficiency reached an impressive 99%, accompanied by a selectivity of approximately 99% for N-benzylidenebenzylamine (BDA), which is comparable to the conversion rate of amines in most reported Ti-related materials (Table S6 Interestingly, the choice of solvent signicantly impacts the reaction (Fig. 4a and Table S2 †).While CH 3 CN promotes the generation of BDA, the employment of isopropanol, methanol, and n-hexane as solvents in the performance evaluations resulted in a marked decline in BA conversion, highlighting their signicant inuence on the reaction rate.Nevertheless, Additionally, the reaction is signicantly inuenced by the photosource.As depicted in Fig. 4b, the impact of photosources with different wavelengths on the reaction was examined (Table S3 †).The subsequent optimization results indicated that the 410 nm LED outperformed other photosources (435 nm and 480 nm LEDs), leading to the production of the target product BDA with a favorable yield and selectivity.Subsequently, Ti10 served as the catalyst, CH 3 CN as the solvent, and a 410 nm LED as the light source during the time screening process, leading to a 99% yield of BDA in 18 hours (Fig. 4c).The leaching of the catalyst in the experiment demonstrated the heterogeneous catalytic characteristics.
Fig. 4d illustrates the photocatalytic oxidative coupling of BA under various conditions.The results of the blank experiment indicated the indispensability of Ti10 and O 2 , as no product was detected in the absence of catalyst or O 2 .We have also performed the reaction directly in the air.The conversion efficiency is lower than normal (under an oxygen atmosphere), demonstrating the necessity of O 2 (Table S4, † entry 12).Additionally, under the dark condition, no product was detected, providing further evidence that the BA oxidation is facilitated by a photocatalytic process (Table S4, † entries 1-3).Moreover, a comparative experiment was conducted to assess the performance of TiO 2 and the physical mixture (TiO 2 , Thdc, and Dmg).It is evident that TiO 2 and the physical mixture exhibit inferior catalytic performance, achieving conversions of 42% and 51%, respectively (Table S4, † entries 4 and 5).This underscores the positive inuence of the binding between ligands and metals in facilitating the reaction.Additionally, the BA conversion rate of Ti10 surpasses that of PTC-211, providing evidence that the introduction of the Thdc ligand signicantly impacts the photocatalytic performance (Table S4, † entries 6 and 7).
Moreover, the cycling stability of Ti10 in the photocatalytic oxidative coupling of BA was examined (Fig. 4e).The results indicated that, aer 5 reaction cycles, the conversion rate exhibited a minor decrease, suggesting satisfactory cycling performance.Besides, the IR spectra of the recovered catalyst aligned with those of the fresh Ti10 catalyst aer ve cycles of reuse (Fig. S9 †).
In order to explore the mechanism of photocatalytic oxidative coupling of amine compounds, a series of controlled experiments were conducted (Fig. 4f).By introducing the electron scavenger AgNO 3 and the hole scavenger KI, the conversion rate was lowered to 56% and 45%, respectively.This highlights the affirmative contribution of photogenerated electrons and holes to this reaction.Furthermore, benzoquinone (BQ) and 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) were introduced into the mixture to act as scavengers for superoxide radicals  − as a primary reactive oxygen species in the synthesis of imines, which is similar to reported examples. 50Typically, photogenerated electrons drive the formation of reactive species cO 2 − and 1 O2 by initiating the activation of O 2 . 51ased on both the experimental results and relevant literature, 28,29,48,50,52,53 we propose a plausible mechanism for the photocatalytic oxidative coupling of BA.Upon irradiation, as depicted in Fig. 5, the Ti10 photocatalyst undergoes excitation, leading to the formation of electron-hole pairs.Photogenerated holes oxidized adsorbed benzylamine molecules, forming benzylamine radical cations (I), while photogenerated electrons reduced molecular oxygen, generating superoxide radicals (cO 2

−
).Following this sequence, intermediate I underwent a reaction with cO 2 − , giving rise to the generation of intermediate II.Subsequently, intermediate II is susceptible to attack by another free benzylamine molecule, ultimately leading to the formation of aminal (III).Finally, subsequent to the release of ammonia, the ultimate product, N-benzylidenebenzylamine (IV), was synthesized.

Conclusions
In summary, we have successfully constructed a dumbbellshaped crystalline titanium oxo cluster, Ti10, through a facial one-pot solvothermal strategy, and treated it as a catalyst for the photocatalytic oxidative coupling of amines.In this structure, Thdc serves as the horizontal bar, while the {Ti 5 Dmg} layers on each side act as the weight plates.Ti10 broadens its light absorption range into the visible spectrum as a result of the coordinated presence of Thdc and Dmg ligands.In contrast to PTC-211, the introduction of the Thdc ligand signicantly improves the charge transfer within the Ti10 structure.On the basis of these advantages, Ti10 and PTC-211 acted as photocatalysts to conduct the photocatalytic oxidative coupling reaction of BA.The Ti10 exhibits superior catalytic performance compared to PTC-211.Our study serves as a signicant example of the rational design of TOC-based photocatalysts with more precise functionalities to achieve effective photocatalytic conversion of organic compounds.

Fig. 2
Fig. 2 Positive-ion mode ESI-MS of Ti10 dissolved in CH 2 Cl 2 .Insets show the experimental (light green) and simulated (light coral) isotopic distribution patterns for species 1a-1c.

Fig. 4
Fig. 4 (a) The effects of various solvents on the reaction.(b) The effects of different photosources on the reaction.(c) Time profile for the reaction.(d) Photocatalytic performance of benzylamine conversion under different reaction conditions.(e) Catalytic durability of Ti10.(f) Photocatalytic conversion of benzylamine in the presence of radical scavengers (KI, AgNO 3 , BQ, and TEMPO).