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• In recent years, titanium oxo clusters (TOCs) with accurate atomic structures have attracted more attention because of their model application for titanium dioxide (TiO₂), and they also exhibit good photocatalytic properties such as photocatalytic degradation and photocatalytic decomposition of water^[1-14]. To date, the solvothermally synthetic approach by using alkoxy titanate as a titanium source has greatly promoted the development of TOCs, and a great number of TOCs have been synthesized and investigated^[15-16]. To be noted, compared with other transition metal complexes, the number of TOCs is still small, and the study of their properties is still worth further development^{[1, 17-19]}.

  To improve the photoelectric activities of titanium dioxide (TiO₂) materials, doping TiO₂ with nitrogen has proven to be an effective method mostly because of the modification of band gap energies and strengthened efficiencies resulting from the N—Ti bonds^[20]. Accordingly, many reported nitrogen-doped titanium oxo clusters (N-TOCs) also exhibited good photocatalytic activities due to the N—Ti bonds^[21]. Then, the TOCs including N—Ti bonds are expected to aim for photocatalytic properties. On the other hand, the light absorption range and band gap also affect the photocatalysis of TOCs. It is well known that the light absorption range and band gaps of TOCs can be enlarged and narrowed respectively when Ti⁴⁺ ions are coordinated by the types of C(sp²)—O or N(sp²)—O oxygen atoms^{[9, 22-26]}. By now, the band gaps of TOCs can be tuned precisely in a wide range by selecting the type and number of dye-functional ligands. However, it should be pointed out that the large band gap may limit light absorption of TOCs in a small range while the narrowed band gap may result in the photocarrier recombination.

  Based on the above-described background, the construction of N-TOCs with appropriate band gaps and investigation of their photoelectric activities are appealing. In this work, salicylaldoxime (H₂Saox) and acetohydroxamic acid (H₂Ahox) were selected as dye-functional ligands for the preparation of N-TOCs, since both the above two ligands can provide N-donor sites and C(sp²)—O or N(sp²)—O-donor sites. Here we have successfully synthesized two N-TOCs formulated as [Ti₆(μ₃-O)₄(Saox)₂(ⁱBuac)₄(OⁱPr)₈] (1) and [Ti₈(μ₃-O)₂(Ahox)₂(PhPO₃)₄(OⁱPr)₁₆] (2), where HⁱBuac=isobutyric acid, PhPO₃H₂=phenylphosphonic acid, and OⁱPr=isopropoxide. Their structures were determined by single-crystal X-ray diffraction and the phase purities were characterized by powder X-ray diffraction (PXRD). Moreover, their photoelectric properties were also investigated.

  1.   Experimental

  1.1   Material and methods

  The reagents involved in this work were AR grade and used directly. A Perkin Elmer Spectrum 100 FT-IR Spectrometer was used for IR spectra measurement. A Rigaku D/Max-2500 diffractometer (Cu Kα, λ= 0.154 2 nm) was used for PXRD patterns characterization at room temperature, and the parameters were 40 kV and 100 mA with a 2θ range from 5° to 50°. The UV-Vis diffuse-reflectance spectra were obtained on a TU-1901 spectrophotometer at solid-state with a scanning range of 240-800 nm. A CHI 660E was used as an electrochemical workstation for electrochemical measurements and the method was a standard three-electrode system in an ambient environment. The working electrode was indium-tin-oxide (ITO) glass coated with a 5 mg sample prepared as the reference reported^{[9, 21-22]}. The auxiliary electrode and the reference electrode were a Pt plate and an Ag/AgCl electrode respectively. The electrolyte was an aqueous solution of Na₂SO₄ (0.2 mol·L^-1). The light source was a 300 W xenon lamp used in a full wavelength for photocurrent measurement. The distance was 20 cm from the lamp to the surface of the ITO electrode. The irradiation intervals were 10 s for on-off cycling and the applied potential was located at 0.3 V.

  A photocatalytic H₂ production experiment was performed in a closed gas circulation system (Aulight Co.). 20 mg sample was used as a photocatalyst and 200 μL 0.1% H₂PtCl₆ solution was used as a cocatalyst. The above mixture was dispersed into an aqueous methanol solution (50 mL, 20%). After digestion thoroughly, the above suspension was then irradiated by a 300 W Xe lamp. The generated H₂ was monitored by online GC-7920A (Aulight Co., TDX-01 column, TCD, N₂ Carrier).

  1.2   Synthesis of complex 1

  Firstly, H₂Saox (1 mmol, 137.1 mg) and HⁱBuac (1 mmol, 88.1 mg) were added to a Teflon-lined stainless vessel (10 mL), and then 1 mL isopropanol and 6 mL acetonitrile were added and used as solvents. Secondly, the above solution was stirred vigorously and Ti(OⁱPr)₄ (1.63 mmol, 0.5 mL) was added dropwise. After 10 min the vessel was sealed and heated at 80 ℃ for 5 d under autogenous pressure. Lastly, the vessel was cooled to room temperature spontaneously. Orange-red crystals were prepared and washed thoroughly with acetonitrile. The yield was 0.166 g (46% based on HⁱBuac). Elemental analysis Calcd. (found) for C₅₄H₉₄N₂O₂₄Ti₆(%): C, 44.96 (45.08); H, 6.57 (6.52), N, 1.94 (1.99). IR (KBr, pellet, cm^-1): 2 974(s), 2 926(w), 2 871(w), 1 567(s), 1 473(m), 1 428(s), 1 293(m), 1 132(s), 1 003(s), 745(m), 572(s).

  1.3   Synthesis of complex 2

  Firstly, H₂Ahox (1 mmol, 75.1 mg) and PhPO₃H₂ (0.5 mmol, 79.1 mg) were added to a Teflon-lined stainless vessel (10 mL), and then 1 mL isopropanol and 6 mL acetonitrile were added and used as solvents. Secondly, the above solution was stirred vigorously and Ti(OⁱPr)₄ (1.63 mmol, 0.5 mL) was added dropwise. After 10 min the vessel was sealed and heated at 100 ℃ for 3 d under autogenous pressure. Lastly, the vessel was cooled to room temperature spontaneously. Yellow crystals were prepared and washed thoroughly with acetonitrile. The yield was 0.098 g (37% based on PhPO₃H₂). Elemental analysis Calcd. (found) for C₇₆H₁₃₆N₂P₄O₃₄Ti₈(%): C, 42.88 (42.76); H, 6.44 (6.38), N, 1.32 (1.41). IR (KBr, pellet, cm^-1): 2 970(s), 2 933(w), 2 861(w), 1 571(w), 1 433(w), 1 374(s), 1 139(s), 1 087(s), 1 014(s), 850(m), 699(s), 582(s).

  1.4   X-ray structure determination

  Single-crystal X-ray diffraction data for complexes 1 and 2 were measured on a Bruker APEX Ⅱ CCD diffractometer equipped with graphite-monochromatized Mo Kα radiation (λ=0.071 073 nm) using multi-scan techniques. The structures were solved using direct methods and refined with the full-matrix least-squares technique on the SHELXTL-2014 program. Nonhydrogen atoms were refined anisotropically, and the hydrogen atoms were added theoretically, riding on the concerned atoms and refined with fixed thermal factors. Because of part of the disordered carbon atoms from isopropyl and isobutyl groups several relatively large residual peaks in the structures were found near the carbon atoms. The main crystallographic data are shown in Table 1 and the selected bond lengths and bond angles are given in Table 2.

  Table 1

  

  Table 1.  Selected crystallographic data for complexes 1 and 2

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  Parameter	1	2
  Formula	C54H94N2O24Ti6	C76H136N2P4O34Ti8
  Formula weight	1 442.71	2 128.94
  Temperature/K	293	293
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a/nm	1.278 50(8)	1.259 17(8)
  b/nm	1.303 72(8)	1.326 76(9)
  c/nm	2.283 23(14)	1.942 39(18)
  α/(°)	95.405(3)	95.707(6)
  β/(°)	96.618(3)	103.170(5)
  γ/(°)	102.486(3)	115.559(4)
  V/nm3	3.663 0(4)	2.777 3(4)
  Z	2	1
  Dc/(g·cm-3)	1.308	1.273
  μ/mm-1	0.691	0.671
  F(000)	1 512	1 114
  Collected reflection	46 466	20 417
  Unique reflection (Rint)	17 747	9 729
  Completeness/%	99.5	99.5
  GOF on F2	1.056	1.027
  R1, wR2 [I > 2σ(I)]	0.061 7, 0.173 3	0.067 1, 0.195 3
  R1, wR2 (all data)	0.087 9, 0.200 5	0.096 3, 0.229 2

  Table 2

  

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

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  1
  Ti1—N1	0.218 0(3)	Ti1—O24	0.176 9(3)	Ti2—O7	0.193 7(2)
  Ti1—O3A	0.211 9(2)	Ti2—O1A	0.207 0(2)	Ti2—O8	0.195 8(2)
  Ti1—O8	0.187 7(2)	Ti2—O4	0.190 1(2)	Ti3—O2	0.204 0(2)
  Ti1—O9	0.190 1(3)	Ti2—O4A	0.213 3(2)	Ti3—O3	0.193 3(2)
  Ti1—O10	0.199 9(2)	Ti2—O6	0.177 5(2)	Ti3—O4	0.185 9(2)
  O3i—Ti1—N1	86.68(9)	O4—Ti2—O1A	93.07(9)	O2—Ti3—O11A	79.82(10)
  O24—Ti1—O8	99.74(12)	O4—Ti2—O4A	77.31(9)	O5—Ti3—O4	102.20(11)
  O8—Ti1—O3A	76.82(9)	O4—Ti2—O7	90.55(9)	O3—Ti3—O2	159.16(11)
  O8—Ti1—O10	95.24(10)	O4—Ti2—O8	153.82(9)	O3—Ti3—O8A	76.97(9)
  2
  Ti1—O1A	0.196 7(4)	Ti2—O5	0.199 4(3)	Ti3—O9	0.179 1(4)
  Ti1—O2	0.193 0(3)	Ti2—O6	0.196 0(3)	Ti3—O10	0.177 1(4)
  Ti1—O7	0.204 3(3)	Ti2—O7	0.216 1(3)	Ti3—O11	0.200 4(4)
  Ti1—O12	0.175 6(4)	Ti2—O8	0.189 1(3)	Ti4—O8	0197 3(3)
  Ti1—O13	0.198 0(4)	Ti3—N1	0.235 8(4)	Ti4—O11	0.199 6(4)
  Ti1—O14	0.201 3(4)	Ti3—O5	0.198 1(3)	Ti4—O15	0.193 1(4)
  Ti2—O3	0.197 5(3)	Ti3—O8	0.204 0(3)	Ti4—O16	0.174 2(4)
  O1A—Ti1—O7	90.24(15)	O4—Ti2—O5	98.77(16)	O9—Ti3—N1	168.89(16)
  O1A—Ti1—O13	87.48(16)	O4—Ti2—O6	97.29(17)	O8—Ti4—O11	76.62(14)
  O1A—Ti1—O14	172.74(15)	O4—Ti2—O7	176.26(16)	O15—Ti4—O8	88.52(15)
  O2—Ti1—O1A	87.41(15)	O5—Ti3—N1	78.32(13)	O15—Ti4—O11	153.28(17)
  O3—Ti2—O5	93.82(14)	O5—Ti3—O8	76.48(14)	O15—Ti4—O8	88.52(15)
  O3—Ti2—O7	83.95(13)	O5—Ti3—O11	150.56(15)	O16—Ti4—O8	105.81(19)
  Symmetry codes: A: -x+2, -y+1, -z+2 for 1; A: -x+1, -y+1, -z+2 for 2.

  2.   Results and discussion

  2.1   Structure description of complex 1

  Single-crystal XRD analysis reveals that complex 1 crystallizes in the triclinic system, space group P1. There are six Ti⁴⁺ ions, four μ₃-O ions, two Saox^2- anions, and eight OⁱPr groups in the molecular structure (Fig. 1a). The two Saox^2- anions present μ₂-η¹∷η¹∶η¹ coordination mode and bridge two Ti⁴⁺ ions together. Among the six Ti⁴⁺ ions, two of them show the same coordination environments of [TiO₅N], while four Ti⁴⁺ ions show octahedral [TiO₆] coordination environments. The skeleton Ti₆O₄ can be seen as two Ti₃(μ₃-O) units linked by two μ₃-O atoms via edge-sharing mode. Each Ti₃(μ₃-O) unit features pyramidal mode (Fig. 1b). The packing structure demonstrates the shortest distance of the adjacent clusters in 1 is 0.74 nm (Fig. 1c). To be noted, it is the fourth H₂Saox-based TOC among the reported TOCs^{[22, 27]}.

  Figure 1

  

  Figure 1.  (a) Crystal structure with 50% thermal ellipsoids of complex 1; (b) Polyhedral view and (c) packing diagram for 1

  Symmetry code: A: -x+2, -y+1, -z+2; Hydrogen atoms are omitted for clarity.

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  2.2   Structure description of complex 2

  Complex 2 crystallizes in the triclinic system, space group P1. The molecular structure consists of eight Ti⁴⁺ ions, two μ₂-O ions, two Ahox^2- anions, four phenyl phosphonates, and sixteen OⁱPr groups (Fig. 2a). There are two Ti⁴⁺ ions showing octahedral [TiO₄N₂] coordination environments. The two Ahox^2- anions present μ₃-η¹∷η¹∶η¹ coordination mode and bridge three Ti⁴⁺ ions together. The Ti₈ skeleton was formed by two Ti₄ units linked by two phenyl phosphonate groups. As is shown, phenyl phosphonate in the structure belongs to the tridentate ligand and the four phosphonate groups may greatly improve the hydrolytic stability of 2. The two Ti₃(μ₃-O) units in 2 feature nearly flat type mode (Fig. 2b). To the best of our knowledge, complex 2 is the first H₂Ahox-based TOCs among the reported TOCs. The packing structure demonstrates the shortest distance of the adjacent clusters in 2 is 0.99 nm (Fig. 2c).

  Figure 2

  

  Figure 2.  (a) Crystal structure with 50% thermal ellipsoids of complex 2; (b) Polyhedral view and (c) packing diagram for 2

  Hydrogen atoms are omitted for clarity.

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  The Ti—N bond length in 1 is 0.218 0(3) nm. The bond lengths of Ti—O in 1 are in a range of 0.176 2(3)-0.214 7(2) nm. The Ti—N bond length in 2 is 0.235 8(4) nm, longer than that in 1. The bond lengths of Ti—O range from 0.174 2(4) to 0.216 1(3) nm in 2. The Ti—O and Ti—N bond lengths in 1 and 2 are consistent with those in the literature^{[20, 28-30]}.

  2.3   PXRD analyses

  To determine the phase purity of complexes 1 and 2, the PXRD was conducted at room temperature. As shown in Fig. 3, the experimental PXRD patterns were in good agreement with the simulated ones from single-crystal X-ray data, which confirmed the pure phase of 1 and 2. The different intensities of the simulated and experimental patterns are mainly because of the powder size and variation in different orientations during the PXRD experiment^[31].

  Figure 3

  

  Figure 3.  XRD patterns of complexes 1 (a) and 2 (b)

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  2.4   Light absorption and band gaps

  The light absorption and band gaps of 1 and 2 were investigated by a UV-Vis diffuse-reflectance measurement in a solid state at room temperature. As shown in Fig. 4a, complex 1 exhibited a light-absorption range from 250 to 575 nm, consistent with its orange-red color. In contrast, the light absorption of complex 2 was located in a range from 250 to 525 nm, and complex 2 exhibited yellow color (Fig. 4b). The visible-light absorption of 1 and 2 can be ascribed to band absorption resulted from ligand-to-metal charge transfer. From Fig. 4c and 4d, it can be seen that the optical band gaps of 1 and 2 were estimated to be 2.43 and 2.61 eV respectively based on the Kubelka-Munk function^[32]. The narrower band gaps of complexes 1 and 2 than 3.2 eV of TiO₂ can be ascribed to the dye-functional ligands Saox^2- and Ahox^2-.

  Figure 4

  

  Figure 4.  Solid-state UV-Vis absorption spectra of complexes 1 (a) and 2 (b); Band gaps of 1 (c) and 2 (d) determined by Kubelka-Munk function

  Inset: the morphology and colors of 1 and 2.

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  2.5   Photoelectric properties

  TOCs belong to semiconductor materials which are expected to be studied for their photoelectric properties. As shown in Fig. 5a and 5b, by Mott-Schottky measurements at frequencies of 300, 500, and 1 000 Hz, the LUMO position values for complexes 1 and 2 were estimated to be -0.69 and -0.52 eV respectively, indicating their theoretical possibility for photocatalytic H₂ evolution. The electrochemical impedance spectroscopy (EIS) of complexes 1 and 2 is shown in Fig. 5c, which reveals nearly the same charge transfer resistance. The photocurrent densities (j) of 1 and 2 were also measured by using a typical three-electrode photoelectrochemical cell under 0.3 V bias potential (Fig. 5b), and the on-off illumination circles of photocurrent responses showed that the photocurrent densities of 1 and 2 could be quickly generated and decayed. The photocurrent densities were 0.035 and 0.046 μA·cm^-2, respectively. Both complexes 1 and 2 were used as photocatalysts for photocatalytic H₂ evolution experiments under UV-visible light illumination. However, only 2 had photocatalytic H₂ evolution activity and the H₂ generation rate could be rich to 140.2 μmol·g^-1·h^-1 and no H₂ can be produced without 2 (Fig. 5e). Complex 2 showed good photocatalytic stability and it could be reused at least three times and no significant loss of activity was observed (Fig. 5f), mainly because of the structural stability resulted from the four tridentate phosphonates ligands in the structure.

  Figure 5

  

  Figure 5.  Mott-Schottky plot for complexes (a) 1 and (b) 2 in 0.2 mol·L^-1 Na₂SO₄ aqueous solution; (c) Nyquist plots of 1 and 2; (d) Photocurrent responses of 1 and 2 under on/off cycle irradiation; (e) Photocatalytic H₂ evolution activity of 2 under UV-Vis light illumination; (f) Recyclability test of 2

  Inset: energy diagram of the HOMO and LUMO levels.

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  To further study the structural stability of 2 after being soaked in solution and photocatalysis, here we investigated the structural stability by PXRD measurements. As shown in Fig. 6, the PXRD patterns of 2 after being soaked in a methanol-aqueous solution (20% methanol) for 10 h were almost identical with the simulated patterns, which show the good hydrolytic stability of 2 in methanol-aqueous solution. The main PXRD peaks of 2 after photocatalysis for three circles could also be observed but the intensities were decreased than the simulated one, indicating the decreased crystallinity or some extent structural decomposition of 2. It is still important to improve the structural stabilities of metal oxygen clusters in photocatalytic reactions.

  Figure 6

  

  Figure 6.  PXRD patterns for investigation of the stability of complex 2 in solution and after photocatalysis

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  3.   Conclusions

  In conclusion, two N-TOCs with narrow band gaps have been successfully synthesized by using H₂Saox and H₂Ahox as dye-functional ligands, which can further enrich the types and number of TOCs. Both the two complexes have absorption in the visible light range. It has been proved that oxime and hydroximic acid are ideal dye-functional ligands for the construction of narrow band gaps-based N-TOCs. Moreover, complex 2 is the first reported TOCs based on H₂Ahox as ligands. Photocatalytic H₂ evolution experiment revealed that complex 2 has photocatalytic H₂ evolution activity.

  
  
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• 

  Figure 1  (a) Crystal structure with 50% thermal ellipsoids of complex 1; (b) Polyhedral view and (c) packing diagram for 1

  Symmetry code: A: -x+2, -y+1, -z+2; Hydrogen atoms are omitted for clarity.

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  Figure 2  (a) Crystal structure with 50% thermal ellipsoids of complex 2; (b) Polyhedral view and (c) packing diagram for 2

  Hydrogen atoms are omitted for clarity.

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  Figure 3  XRD patterns of complexes 1 (a) and 2 (b)

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  Figure 4  Solid-state UV-Vis absorption spectra of complexes 1 (a) and 2 (b); Band gaps of 1 (c) and 2 (d) determined by Kubelka-Munk function

  Inset: the morphology and colors of 1 and 2.

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  Figure 5  Mott-Schottky plot for complexes (a) 1 and (b) 2 in 0.2 mol·L^-1 Na₂SO₄ aqueous solution; (c) Nyquist plots of 1 and 2; (d) Photocurrent responses of 1 and 2 under on/off cycle irradiation; (e) Photocatalytic H₂ evolution activity of 2 under UV-Vis light illumination; (f) Recyclability test of 2

  Inset: energy diagram of the HOMO and LUMO levels.

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  Figure 6  PXRD patterns for investigation of the stability of complex 2 in solution and after photocatalysis

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  Table 1.  Selected crystallographic data for complexes 1 and 2

  Parameter	1	2
  Formula	C54H94N2O24Ti6	C76H136N2P4O34Ti8
  Formula weight	1 442.71	2 128.94
  Temperature/K	293	293
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a/nm	1.278 50(8)	1.259 17(8)
  b/nm	1.303 72(8)	1.326 76(9)
  c/nm	2.283 23(14)	1.942 39(18)
  α/(°)	95.405(3)	95.707(6)
  β/(°)	96.618(3)	103.170(5)
  γ/(°)	102.486(3)	115.559(4)
  V/nm3	3.663 0(4)	2.777 3(4)
  Z	2	1
  Dc/(g·cm-3)	1.308	1.273
  μ/mm-1	0.691	0.671
  F(000)	1 512	1 114
  Collected reflection	46 466	20 417
  Unique reflection (Rint)	17 747	9 729
  Completeness/%	99.5	99.5
  GOF on F2	1.056	1.027
  R1, wR2 [I > 2σ(I)]	0.061 7, 0.173 3	0.067 1, 0.195 3
  R1, wR2 (all data)	0.087 9, 0.200 5	0.096 3, 0.229 2

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

  1
  Ti1—N1	0.218 0(3)	Ti1—O24	0.176 9(3)	Ti2—O7	0.193 7(2)
  Ti1—O3A	0.211 9(2)	Ti2—O1A	0.207 0(2)	Ti2—O8	0.195 8(2)
  Ti1—O8	0.187 7(2)	Ti2—O4	0.190 1(2)	Ti3—O2	0.204 0(2)
  Ti1—O9	0.190 1(3)	Ti2—O4A	0.213 3(2)	Ti3—O3	0.193 3(2)
  Ti1—O10	0.199 9(2)	Ti2—O6	0.177 5(2)	Ti3—O4	0.185 9(2)
  O3i—Ti1—N1	86.68(9)	O4—Ti2—O1A	93.07(9)	O2—Ti3—O11A	79.82(10)
  O24—Ti1—O8	99.74(12)	O4—Ti2—O4A	77.31(9)	O5—Ti3—O4	102.20(11)
  O8—Ti1—O3A	76.82(9)	O4—Ti2—O7	90.55(9)	O3—Ti3—O2	159.16(11)
  O8—Ti1—O10	95.24(10)	O4—Ti2—O8	153.82(9)	O3—Ti3—O8A	76.97(9)
  2
  Ti1—O1A	0.196 7(4)	Ti2—O5	0.199 4(3)	Ti3—O9	0.179 1(4)
  Ti1—O2	0.193 0(3)	Ti2—O6	0.196 0(3)	Ti3—O10	0.177 1(4)
  Ti1—O7	0.204 3(3)	Ti2—O7	0.216 1(3)	Ti3—O11	0.200 4(4)
  Ti1—O12	0.175 6(4)	Ti2—O8	0.189 1(3)	Ti4—O8	0197 3(3)
  Ti1—O13	0.198 0(4)	Ti3—N1	0.235 8(4)	Ti4—O11	0.199 6(4)
  Ti1—O14	0.201 3(4)	Ti3—O5	0.198 1(3)	Ti4—O15	0.193 1(4)
  Ti2—O3	0.197 5(3)	Ti3—O8	0.204 0(3)	Ti4—O16	0.174 2(4)
  O1A—Ti1—O7	90.24(15)	O4—Ti2—O5	98.77(16)	O9—Ti3—N1	168.89(16)
  O1A—Ti1—O13	87.48(16)	O4—Ti2—O6	97.29(17)	O8—Ti4—O11	76.62(14)
  O1A—Ti1—O14	172.74(15)	O4—Ti2—O7	176.26(16)	O15—Ti4—O8	88.52(15)
  O2—Ti1—O1A	87.41(15)	O5—Ti3—N1	78.32(13)	O15—Ti4—O11	153.28(17)
  O3—Ti2—O5	93.82(14)	O5—Ti3—O8	76.48(14)	O15—Ti4—O8	88.52(15)
  O3—Ti2—O7	83.95(13)	O5—Ti3—O11	150.56(15)	O16—Ti4—O8	105.81(19)
  Symmetry codes: A: -x+2, -y+1, -z+2 for 1; A: -x+1, -y+1, -z+2 for 2.

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•