English

• 0.   Introduction

  The rational design of titanium dioxide (TiO₂)-based photocatalysts is a research interest in the field of energy and environmental catalysis^[1-3]. Doping TiO₂ with transition metals can not only modify its band structure and expand the spectral absorption range, but also introduce transition metals as active sites for photocatalysis, thereby enhancing the photocatalytic performance of TiO₂ materials^[4-7]. However, these transition-metal-ion-doped TiO₂ materials have defects in imprecise structural information at the atomic level, such as uneven sizes, unclear inorganic-organic interface information, and uncertain surface composition. These defects may greatly limit the deeper research on such promising photocatalytic materials, particularly in terms of elucidating the catalytic reaction mechanisms, performing rational surface and interface modification, and establishing the structure-activity relationships^[8-9].

  Crystalline heterometallic titanium clusters (TCs) with well-defined structural information have attracted great attention in recent years^[10-16]. Due to their precise atomic positional information, clear ligand-cluster core binding modes, and tunable cluster core sizes, TCs can serve as ideal structural and reactivity models for in-depth investigations of TiO₂-based materials. The advantage of atomically precise structural information can not only facilitate the elucidation of their structure-activity relationships but also offer the possibility for expanding their functional applications^{[10, 17-20]}. Moreover, TCs exhibit good photocatalytic properties such as photocatalytic degradation and photocatalytic decomposition of water^[21]. To date, a great number of heterometallic TCs have been synthesized and investigated. As is well known, the divalent Mn²⁺ ions can act as the active centers for photocatalysis and are readily oxidized to trivalent Mn³⁺ species during the photocatalytic process, thus facilitating the electron transfer required for the photocatalytic reduction reaction. To date, a variety of heterometallic Mn/Ti clusters have been reported, while the research on the application of heterometallic Mn/Ti clusters in the photocatalytic CO₂ reduction reactions (CO₂RR) remains scarce^[22-27]. In a rare case, Zhang et al reported a Ti₄Mn₃-cluster which showed high efficiency for the photocatalytic conversion of CO₂ into CO, and also revealed that the undercoordinated active Mn sites can not only decrease the energy barrier but also possess a moderate adsorption strength of CO*, leading to the accelerated charge separation kinetics and enhanced CO₂ photoreduction activity^[28]. Then, aiming for high efficiency of TCs for photocatalytic CO₂RR, it is worthy to design and synthesizing heterometallic Mn/Ti clusters, especially in which the Mn sites are promised to be undercoordinated or weakly coordinated by solvent molecules. This is because the easy formation of unsaturated coordinated Mn centers is conducive to the enhancement of photocatalytic efficiency.

  Based on the aforementioned background, salicylic acid (H₂Sal) was chosen as the ligand for the construction of TCs. H₂Sal is a moderately strong acid with appropriate steric hindrance, which has been demonstrated to be a promising ligand for the fabrication of TCs. Meanwhile, Ti(OⁱPr)₄ was employed as the titanium source, and Mn(CH₃COO)₂·4H₂O was utilized as the precursor for introducing Mn²⁺ ions. Here we have successfully synthesized two heterometallic salicylate Mn/Ti clusters formulated as [Ti₃Mn₂(Sal)₈(Phen)(CH₃CN)(H₂O)] (1) and [Ti₄Mn₄(Sal)₁₂(CH₃CN)₂]·(CH₃CN)₄ (2). The crystal structures of the samples were determined by single-crystal X-ray diffraction, while their phase purities were characterized via powder X-ray diffraction (PXRD). Furthermore, the CO₂RR performance of the prepared samples was systematically investigated.

  1.   Experimental

  1.1   Materials and instruments

  All chemical reagents employed are of analytical reagent (AR) grade and utilized without further purification. Fourier-transform infrared (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 100 spectrometer. PXRD analyses were carried out at ambient temperature on a Rigaku D/Max-2500 diffractometer with Cu Kα radiation (λ=0.154 2 nm), covering a 2θ range of 5° to 50°, and the X-ray tube was operated at a voltage of 40 kV and a current of 100 mA. Solid-state UV-visible diffuse reflectance spectra (UV-Vis DRS) were acquired using a TU-1901 spectrophotometer over a scanning wavelength range of 250-800 nm.

  1.2   Synthesis of 1

  First, a mixture of Mn(CH₃COO)₂·4H₂O (0.3 mmol, 0.074 g), H₂Sal (1.5 mmol, 0.207 g), Phen (0.20 mmol, 0.090 g), and acetonitrile (8 mL) was placed in a 15 mL Teflon liner. The above mixture was stirred for 10 min, then Ti(OⁱPr)₄ (0.67 mmol, 0.2 mL) was added dropwise to the above mixture. Finally, the Teflon liner was heated to 120 ℃ and held for 72 h. After cooling to room temperature, red crystals were obtained and then washed thoroughly with acetonitrile. The yield (0.170 g) was 48% based on Ti(OⁱPr)₄. Elemental analysis Calcd. (Found) for C₇₀H₄₅Mn₂N₃O₂₅Ti₃(%): C, 53.16 (53.11); H, 2.87 (2.94); N, 2.66 (2.49). IR (KBr, pellet, cm^-1): 3 412(m), 3 066 (w), 1 601 (s), 1 586 (s), 1 550(s), 1 459 (s), 1 364(s), 1 234 (s), 1 143 (s), 1 033 (m), 893 (m), 841 (s), 760(m), 678 (m).

  1.3   Synthesis of 2

  First, a mixture of Mn(CH₃COO)₂·4H₂O (0.3 mmol, 0.074 g), H₂Sal (1.5 mmol, 0.207 g), and acetonitrile (8 mL) was placed in a 15 mL Teflon liner. The above mixture was stirred for 10 min, then Ti(OⁱPr)₄ (0.67 mmol, 0.2 mL) was added dropwise to the above mixture. Finally, the Teflon liner was heated to 120 ℃ and held for 72 h. After cooling to room temperature, red crystals were obtained and then washed thoroughly with acetonitrile. The yield (0.098 g) was 57% based on Mn(CH₃COO)₂·4H₂O. Elemental analysis Calcd. (Found) for C₉₆H₆₆Mn₄N₆O₃₆Ti₄(%): C, 50.33 (50.25); H, 2.90 (2.78); N, 3.67 (3.72). IR (KBr, pellet, cm^-1): 3 067 (w), 2 928 (w), 1 625 (w), 1 601 (s), 1 546 (s), 1 642 (s), 1 361 (s), 1 237 (s), 1 146 (s), 1 033 (m), 893 (m), 848(s), 760 (m), 675 (m).

  1.4   X-ray structure determination

  Single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a Bruker APEX Ⅱ CCD diffractometer, with the radiation source being graphite-monochromatized Mo Kα radiation (λ=0.071 073 nm), and the data collection strategy employing multi-scan techniques. The crystal structures were solved via direct methods and subsequently refined using the full-matrix least-squares method, with all calculations performed with the SHELXTL-2014 software package. Anisotropic refinement was applied to all non-hydrogen atoms, whereas H atoms were generated geometrically, fixed at idealized positions bonded to the parent atoms, and assigned fixed isotropic thermal parameters. The presence of disordered C atoms in the butyl substituents led to the appearance of several relatively large residual electron density peaks in the vicinity of these C atoms within the crystal structures. Key crystallographic parameters, along with selected bond lengths and bond angles, are summarized in Table 1 and 2, respectively.

  Table 1

  

  Table 1.  Selected crystallographic data for complexes 1 and 2

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  Parameter	1	2
  Formula	C70H45Mn2N3O25Ti3	C96H66Mn4N6O36Ti4
  Formula weight	1 581.67	2 290.90
  T / K	293	293
  Crystal system	Triclinic	Monoclinic
  Space group	P1	C2/c
  a / nm	1.273 17(9)	2.068 3(10)
  b / nm	1.489 77(10)	1.468 5(7)
  c / nm	1.897 07(14)	3.191 9(14)
  α / (°)	99.182(4)
  β / (°)	93.280(4)	97.407(8)
  γ / (°)	109.447(4)
  V / nm3	3.325 9(4)	9.614(8)
  Z	2	4
  Dc / (g·cm-3)	1.579	1.583
  μ / mm-1	0.801	0.913
  F(000)	1 604.0	4 640.0
  Collected reflection	128 548	36 896
  Rint	20 507(0.071 0)	12 923(0.034 5)
  Completeness / %	99.9	99.5
  GOF on F 2	1.054	1.046
  R1a, wR2b [I > 2(I)]	0.044 7, 0.119 8	0.045 8, 0.118 1
  R1a, wR2b (all data)	0.066 2, 0.132 4	0.075 8, 0.135 4
  a R1=∑||Fo|-|Fc||/∑|Fo|; b wR2={∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]}1/2.

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) in complexes 1 and 2

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  1
  Ti1—O7	0.203 56(15)	Ti2—O16	0.182 29(15)	Mn1—O13	0.213 00(16)
  Ti1—O8	0.185 15(17)	Ti2—O17	0.183 31(15)	Mn1—O21	0.226 54(17)
  Ti1—O9	0.186 30(18)	Ti3—O1	0.201 93(14)	Mn2—N1	0.229 15(18)
  Ti1—O10	0.186 64(16)	Ti3—O2	0.187 47(14)	Mn2—N2	0.232 3(2)
  Ti1—O11	0.202 67(17)	Ti3—O3	0.184 47(15)	Mn2—O20	0.212 28(16)
  Ti2—O14	0.201 99(15)	Mn1—N3	0.221 5(2)	Mn2—O22	0.213 93(16)
  Ti2—O15	0.197 02(14)	Mn1—O6	0.240 47(14)	Mn2—O23	0.211 47(15)
  O8—Ti1—O7	84.09(7)	O15—Ti2—O19	83.38(6)	O13—Mn1—O6	142.00(6)
  O8—Ti1—O9	97.57(8)	O16—Ti2—O17	103.42(7)	O25—Mn1—O21	94.37(6)
  O8—Ti1—O10	92.92(8)	O1—Ti3—O5	81.37(6)	N1—Mn2—N2	72.17(7)
  O8—Ti1—O11	96.17(8)	O2—Ti3—O6	96.34(6)	O20—Mn2—O22	97.45(7)
  O14—Ti2—O19	85.28(6)	O3—Ti3—O4	94.42(8)	O22—Mn2—N1	86.60(7)
  O15—Ti2—O14	86.62(6)	N3—Mn1—O6	95.16(8)	O22—Mn2—N2	158.42(6)
  O15—Ti2—O18	89.30(6)	O13—Mn1—N3	91.24(7)	O23—Mn2—O22	96.76(6)
  2
  Ti1—O1	0.184 50(19)	Ti2—O5	0.202 4(2)	Mn1—O9	0.216 28(19)
  Ti1—O2	0.210 3(2)	Ti2—O6	0.185 8(2)	Mn2—N1	0.224 9(3)
  Ti1—O10	0.198 14(19)	Ti2—O7	0.184 7(2)	Mn2—O3i	0.213 3(2)
  Ti1—O14	0.194 89(18)	Ti2—O8	0.207 63(18)	Mn2—O4i	0.223 9(2)
  Ti1—O15	0.184 9(2)	Mn1—O2	0.216 40(19)	Mn2—O13	0.209 1(2)
  Ti1—O18	0.198 47(18)	Mn1—O8	0.239 62(19)	Mn2—O17	0.210 31(19)
  O1—Ti1—O2	82.99(7)	O6—Ti2—O11	95.98(8)	O9—Mn1—O16i	96.36(7)
  O1—Ti1—O10	90.50(8)	O6—Ti2—O12	97.29(9)	N1—Mn2—O5i	87.66(10)
  O1—Ti1—O14	93.19(8)	O7—Ti2—O5	94.35(10)	O3A—Mn2—O4i	94.91(8)
  O1—Ti1—O15	100.56(8)	O2—Mn1—O8	102.55(6)	O13—Mn2—O17	94.60(8)
  O5—Ti2—O8	84.08(7)	O2—Mn1—O16i	103.25(6)	O17—Mn2—N1	83.36(10)
  O6—Ti2—O5	84.60(9)	O2—Mn1—O18	68.43(6)	O13—Mn2—O4i	96.66(8)
  Symmetry code: ⅰ -x+1, y, -z+3/2 for complex 2.

  1.5   Electrochemical measurements

  Electrochemical measurements were implemented on a CHI 660E electrochemical workstation based on a standard three-electrode configuration under ambient conditions. For electrode preparation, 5 mg of the thoroughly ground sample was uniformly coated onto an indium tin oxide (ITO) glass slide, which served as the working electrode^[17]. A platinum plate was used as the auxiliary electrode, while an Ag/AgCl electrode acted as the reference electrode. A 0.2 mol·L^-1 aqueous solution of sodium sulfate (Na₂SO₄) was selected as the electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were conducted on complexes 1 and 2 using a standard three-electrode photoelectrochemical cell under a bias potential of 0.2 V, and the frequency was set in the range of 10^-2 to 10⁵ Hz. Photocurrent measurements were conducted with a 300 W xenon lamp as the full-spectrum light source, positioned 20 cm away from the surface of the ITO electrode. Throughout the photocurrent tests, an applied potential of 0.25 V was maintained, and the light irradiation was switched on and off at 10 s intervals.

  1.6   Photocatalytic reduction of CO₂

  The photocatalytic reduction of CO₂ was carried out in a sealed gas-circulation system manufactured by Aulight Co. A total of 20 mg of the photocatalyst was dispersed into a mixed solvent consisting of 10 mL H₂O and 40 mL N,N-dimethylformamide, with the addition of 5 mL triethanolamine (TEOA) serving as the electron donor and 20 mg [Ru(bpy)₃]Cl₂·6H₂O as photosensitizer. The reaction temperature was stabilized at 280 K via a circulating cooling water system. Following complete dispersion of the photocatalyst in the solution, the resulting suspension was subjected to irradiation under a 300 W xenon lamp. The gaseous products (CO and CH₄) generated during the photocatalytic reaction were analyzed in real time using an online gas chromatograph (GC-7920A, Aulight Co.), which was equipped with a TDX-01 column, a flame ionization detector (FID), and nitrogen as the carrier gas.

  2.   Results and discussion

  2.1   Structure description of 1

  Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic system, space group P1. There are three Ti⁴⁺ ions, two Mn²⁺ ions, eight Sal^2- anions, one Phen molecule, one CH₃CN molecule, and one H₂O molecule in an asymmetric unit (Fig. 1a). All the Sal^2- anions present μ₂-η¹∶η¹∶η¹ coordination mode. All the Ti⁴⁺ ions show the same octahedral [TiO₆] coordination environments, and the coordinated O atoms are from the ligand salicylates. Interestingly, the two Mn²⁺ ions adopt different coordinated modes. The atom Mn1 presents a pentagonal bipyramid of [MnO₆N] coordination environments in which one O atom is from the coordinated water molecule and the N atom is from acetonitrile. However, Mn2 shows a six-coordinated mode of [MnO₄N₂] in which the four coordinated O atoms are from ligand Sal^2- anions, and the two N atoms are from one ligand Phen. The average bond length of Ti—O is 0.194 5 nm. While the average bond lengths of Mn—O and Mn—N are 0.218 8 and 0.227 7 nm, respectively. The bond lengths of Ti—O, Mn—O, and Mn—N are consistent with those in the literature^[28-29]. Ti1, Ti3, Mn1, and Mn2 are linked via a vertex-sharing mode. While the atom Ti2 is not directly connected to the above four metal atoms via vertex-sharing or edge- sharing mode (Fig. 1b), the packing structure is demonstrated in Fig. 1c. Notably, the coordinated molecular CH₃CN of Mn1 may be easily left when facing other strongly coordinated molecules due to its weak bond^{[28, 30]}.

  Figure 1

  

  Figure 1.  Crystal structure (a), polyhedral view (b), and packing diagram (c) for 1

  Atomic code: green, Ti; pink, Mn; red, O; blue, N; grey, C. Other atoms are omitted for clarity.

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  2.2   Structure description of 2

  Single-crystal X-ray diffraction analysis reveals that complex 2 crystallizes in the monoclinic system, space group C2/c. There are four Ti⁴⁺ ions, four Mn²⁺ ions, twelve Sal^2- anions, two coordinated CH₃CN molecules, and four free solvent acetonitrile molecules in the structure (Fig. 2a). Complex 2 exhibits central symmetry. Similar to 1, all the Sal^2- anions in 2 present μ₂-η¹∶η¹∶η¹ coordination mode. As well, all the Ti⁴⁺ ions show the same octahedral [TiO₆] coordination environments, and the coordinated oxygen atoms are from ligand Sal^2- anions. The four Mn²⁺ ions adopt different coordinated modes. Mn1 and Mn1A present octahedral [MnO₆] coordination environments in which the O atoms are from ligand Sal^2- anions. However, Mn2 and Mn2A show a six-coordinated mode of [MnO₅N] in which the five coordinated O atoms are from ligand Sal^2- anions, and the N atom is from one ligand CH₃CN. The average bond length of Ti—O is 0.195 1 nm. While the average bond lengths of Mn—O and Mn—N are 0.219 3 and 0.224 9 nm, respectively, which are consistent with those in 1. Ti1, Mn1, Ti1A, and Mn1A are linked via edge-sharing modes. While the atoms Mn2A and Ti2, Ti2 and Mn1 are connected via a vertex-sharing mode, respectively (Fig. 2b). The packing structure is demonstrated in Fig. 2c. Compared to 1, in the structure of 2, the Mn2 and Mn2A are coordinated by CH₃CN, which should also easily leave when facing other strongly coordinated molecules due to its weak bond.

  Figure 2

  

  Figure 2.  Crystal structure (a), polyhedral view (b), and packing diagram (c) for 2

  Atomic code: green, Ti; pink, Mn; red, O; blue, N; grey, C. Other atoms are omitted for clarification.

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  2.3   PXRD analyses

  It is crucial to verify the phase purity of complexes 1 and 2, which thus prompted the performance of PXRD measurements under ambient temperature conditions. As illustrated in Fig. 3, the experimentally obtained PXRD patterns exhibited a high degree of consistency with the simulated patterns derived from single-crystal X-ray diffraction data, thereby validating the phase purity of both complexes 1 and 2. It should be pointed out that the observed discrepancy in peak intensity between the simulated and experimental patterns is primarily attributable to the particle size distribution of the powder samples and the randomness of crystal orientation during the PXRD analysis process^[17].

  Figure 3

  

  Figure 3.  XRD patterns of complexes 1 and 2

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  2.4   Light absorption and band gaps

  Light absorption capacity and band gap energy are crucial parameters governing the performance of photocatalysts. In the present work, UV-Vis DRS measurements were performed on complexes 1 and 2 in the solid state at ambient temperature. As illustrated in Fig. 4a and 4b, both complexes 1 and 2 exhibited UV absorption in the wavelength range of 250-600 nm. This absorption phenomenon can be attributed to the combined contributions of aromatic ligands, titanium cores, and the incorporated Mn²⁺ ions. According to Fig. 4c and 4d, the optical band gaps of complexes 1 and 2 were calculated to be 1.93 and 2.15 eV, respectively, using the Kubelka-Munk function^[31]. This result confirms that both complexes possess semiconductor-like behaviors. Notably, the band gaps of complexes 1 and 2 were significantly narrower than that of TiO₂ (3.2 eV), which is presumably due to the introduction of dye-functionalized ligands and foreign metal ions.

  Figure 4

  

  Figure 4.  Solid-state UV-Vis DRS for (a) 1 and (b) 2; Band gaps for (c) 1 and (d) 2

  Inset: the morphologies and colors of 1 and 2.

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  2.5   Photoelectric properties

  The as-prepared complexes 1 and 2 are semiconductor materials with promising photoelectric properties that are worthy to be investigated. As illustrated in Fig. 5a, the on-off illumination cycles of the photocurrent responses for complexes 1 and 2 revealed rapid generation and decay of photocurrents, with the corresponding photocurrent densities of 0.305 and 0.367 μA·cm^-2, respectively. The EIS results demonstrated that the two complexes exhibited nearly identical charge transfer resistances (Fig. 5b), which can be attributed to their analogous ligand configurations. Mott-Schottky measurements were carried out at frequencies of 300, 500, and 1 000 Hz. As presented in Fig. 5c and 5d, the lowest unoccupied molecular orbital (LUMO) energies of complexes 1 and 2 were determined to be -0.83 and -0.73 eV, respectively. Combining the determined band gap values with the LUMO, the highest occupied molecular orbital (HOMO) energies were determined to be 1.10 and 1.42 eV for 1 and 2, respectively. Notably, the LUMO positions of both complexes are more negative than the potential thresholds required for the reduction of CO₂ to various carbonaceous products, which indicates that complexes 1 and 2 hold great potential as photocatalysts for CO₂RR.

  Figure 5

  

  Figure 5.  (a) Photocurrent responses of 1 and 2 under on/off cycle irradiation; (b) Nyquist plots for 1 and 2; Mott-Schottky plots for (c) 1 and (d) 2 in 0.2 mol·L^-1 Na₂SO₄ aqueous solution (inset: the energy diagram of the HOMO and LUMO levels); (e) Photocatalytic CO₂ activity of 1 and 2 under UV-Vis light illumination; (f) Recyclability test of 2

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  Both complexes 1 and 2 were employed as photocatalysts for the CO₂RR under UV-Vis light irradiation. As illustrated in Fig. 5e, both complexes exhibited evident photocatalytic activity toward CO₂RR, with CO being the sole gaseous product. Specifically, the CO evolution rates of complexes 1 and 2 were determined to be 7.21 and 14.39 μmol·g^-1·h^-1, respectively. No other gaseous by-products were detected in the gas phase, demonstrating the high selectivity of complexes 1 and 2 for CO generation during photocatalytic CO₂RR. The higher CO₂RR activity of 2 than 1 should be ascribed to the more active sites of Mn ions, especially those Mn ions coordinated by CH₃CN, which could easily leave during CO₂RR progress. Given that complexes 1 and 2 share the identical multidentate Sal^2- ligand, they are expected to exhibit comparable photocatalytic stability. Thus, complex 2 was selected as a representative to evaluate the photocatalytic stability. As illustrated in Fig. 5f, complex 2 demonstrated excellent photocatalytic stability, enabling reuse for at least three consecutive cycles without significant degradation in catalytic activity. This stability is primarily attributed to the robust structural integrity afforded by the protective multidentate Sal^2- anions ligands within the complex framework.

  Based on the photoelectric property analyses of complexes 1 and 2, combined with the literature-reported photocatalytic CO₂RR mechanisms of analogous clusters^{[28, 30]}, a plausible CO₂RR mechanism for complexes 1 and 2 is proposed herein (Fig. 6). Upon visible light irradiation, the excited photosensitizer [Ru(bpy)₃]²⁺ produces photogenerated electrons, which are subsequently transferred to the Mn active sites. With the dissociation of the coordinated CH₃CN ligand, CO₂ molecules are adsorbed onto the exposed Mn active centers, where they accept an electron to form the *CO₂^- intermediate. This intermediate then undergoes protonation to yield the *COOH species, which is further converted to CO with the assistance of additional protons and electrons from the reaction system. Finally, the oxidized photosensitizer [Ru(bpy)₃]³⁺ is reductively quenched by TEOA, thereby completing the entire photocatalytic CO₂RR cycle.

  Figure 6

  

  Figure 6.  Proposed photocatalytic mechanism for reduction of CO₂ to CO using 2

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  3.   Conclusions

  In summary, two heterometallic Mn/Ti TCs have been successfully fabricated via a straightforward and universal synthetic strategy, which is anticipated to enrich the structural diversity and quantity of TCs-based materials. Both complexes exhibited characteristic absorption bands of transition metal ions in the visible light region. Furthermore, complexes 1 and 2 possess photocatalytic CO₂RR activity, with prominent selectivity toward CO as the target product. This work is not only conducive to advancing the synthetic research on TCs but also lays a foundation for the exploration of TCs as promising photocatalysts for CO₂RR.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Crystal structure (a), polyhedral view (b), and packing diagram (c) for 1

  Atomic code: green, Ti; pink, Mn; red, O; blue, N; grey, C. Other atoms are omitted for clarity.

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  Figure 2  Crystal structure (a), polyhedral view (b), and packing diagram (c) for 2

  Atomic code: green, Ti; pink, Mn; red, O; blue, N; grey, C. Other atoms are omitted for clarification.

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  Figure 3  XRD patterns of complexes 1 and 2

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  Figure 4  Solid-state UV-Vis DRS for (a) 1 and (b) 2; Band gaps for (c) 1 and (d) 2

  Inset: the morphologies and colors of 1 and 2.

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  Figure 5  (a) Photocurrent responses of 1 and 2 under on/off cycle irradiation; (b) Nyquist plots for 1 and 2; Mott-Schottky plots for (c) 1 and (d) 2 in 0.2 mol·L^-1 Na₂SO₄ aqueous solution (inset: the energy diagram of the HOMO and LUMO levels); (e) Photocatalytic CO₂ activity of 1 and 2 under UV-Vis light illumination; (f) Recyclability test of 2

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  Figure 6  Proposed photocatalytic mechanism for reduction of CO₂ to CO using 2

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  Table 1.  Selected crystallographic data for complexes 1 and 2

  Parameter	1	2
  Formula	C70H45Mn2N3O25Ti3	C96H66Mn4N6O36Ti4
  Formula weight	1 581.67	2 290.90
  T / K	293	293
  Crystal system	Triclinic	Monoclinic
  Space group	P1	C2/c
  a / nm	1.273 17(9)	2.068 3(10)
  b / nm	1.489 77(10)	1.468 5(7)
  c / nm	1.897 07(14)	3.191 9(14)
  α / (°)	99.182(4)
  β / (°)	93.280(4)	97.407(8)
  γ / (°)	109.447(4)
  V / nm3	3.325 9(4)	9.614(8)
  Z	2	4
  Dc / (g·cm-3)	1.579	1.583
  μ / mm-1	0.801	0.913
  F(000)	1 604.0	4 640.0
  Collected reflection	128 548	36 896
  Rint	20 507(0.071 0)	12 923(0.034 5)
  Completeness / %	99.9	99.5
  GOF on F 2	1.054	1.046
  R1a, wR2b [I > 2(I)]	0.044 7, 0.119 8	0.045 8, 0.118 1
  R1a, wR2b (all data)	0.066 2, 0.132 4	0.075 8, 0.135 4
  a R1=∑||Fo|-|Fc||/∑|Fo|; b wR2={∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]}1/2.

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  Table 2.  Selected bond lengths (nm) and angles (°) in complexes 1 and 2

  1
  Ti1—O7	0.203 56(15)	Ti2—O16	0.182 29(15)	Mn1—O13	0.213 00(16)
  Ti1—O8	0.185 15(17)	Ti2—O17	0.183 31(15)	Mn1—O21	0.226 54(17)
  Ti1—O9	0.186 30(18)	Ti3—O1	0.201 93(14)	Mn2—N1	0.229 15(18)
  Ti1—O10	0.186 64(16)	Ti3—O2	0.187 47(14)	Mn2—N2	0.232 3(2)
  Ti1—O11	0.202 67(17)	Ti3—O3	0.184 47(15)	Mn2—O20	0.212 28(16)
  Ti2—O14	0.201 99(15)	Mn1—N3	0.221 5(2)	Mn2—O22	0.213 93(16)
  Ti2—O15	0.197 02(14)	Mn1—O6	0.240 47(14)	Mn2—O23	0.211 47(15)
  O8—Ti1—O7	84.09(7)	O15—Ti2—O19	83.38(6)	O13—Mn1—O6	142.00(6)
  O8—Ti1—O9	97.57(8)	O16—Ti2—O17	103.42(7)	O25—Mn1—O21	94.37(6)
  O8—Ti1—O10	92.92(8)	O1—Ti3—O5	81.37(6)	N1—Mn2—N2	72.17(7)
  O8—Ti1—O11	96.17(8)	O2—Ti3—O6	96.34(6)	O20—Mn2—O22	97.45(7)
  O14—Ti2—O19	85.28(6)	O3—Ti3—O4	94.42(8)	O22—Mn2—N1	86.60(7)
  O15—Ti2—O14	86.62(6)	N3—Mn1—O6	95.16(8)	O22—Mn2—N2	158.42(6)
  O15—Ti2—O18	89.30(6)	O13—Mn1—N3	91.24(7)	O23—Mn2—O22	96.76(6)
  2
  Ti1—O1	0.184 50(19)	Ti2—O5	0.202 4(2)	Mn1—O9	0.216 28(19)
  Ti1—O2	0.210 3(2)	Ti2—O6	0.185 8(2)	Mn2—N1	0.224 9(3)
  Ti1—O10	0.198 14(19)	Ti2—O7	0.184 7(2)	Mn2—O3i	0.213 3(2)
  Ti1—O14	0.194 89(18)	Ti2—O8	0.207 63(18)	Mn2—O4i	0.223 9(2)
  Ti1—O15	0.184 9(2)	Mn1—O2	0.216 40(19)	Mn2—O13	0.209 1(2)
  Ti1—O18	0.198 47(18)	Mn1—O8	0.239 62(19)	Mn2—O17	0.210 31(19)
  O1—Ti1—O2	82.99(7)	O6—Ti2—O11	95.98(8)	O9—Mn1—O16i	96.36(7)
  O1—Ti1—O10	90.50(8)	O6—Ti2—O12	97.29(9)	N1—Mn2—O5i	87.66(10)
  O1—Ti1—O14	93.19(8)	O7—Ti2—O5	94.35(10)	O3A—Mn2—O4i	94.91(8)
  O1—Ti1—O15	100.56(8)	O2—Mn1—O8	102.55(6)	O13—Mn2—O17	94.60(8)
  O5—Ti2—O8	84.08(7)	O2—Mn1—O16i	103.25(6)	O17—Mn2—N1	83.36(10)
  O6—Ti2—O5	84.60(9)	O2—Mn1—O18	68.43(6)	O13—Mn2—O4i	96.66(8)
  Symmetry code: ⅰ -x+1, y, -z+3/2 for complex 2.

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