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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Synthesis, Structures, and Photocurrent Responses of PolyoxoTitanium Clusters with Oxime Ligands: From Ti4 to Ti18 Shuai Chen,†,‡ Wei-Hui Fang,*,† Lei Zhang,*,† and Jian Zhang† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ University of Chinese Academy of Sciences, 100049 Beijing, China

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S Supporting Information *

ABSTRACT: Six crystalline polyoxo-titanium clusters (PTCs) derived from oximes, namely, Ti4(μ4-O)(OMe)6(L1)4 (PTC-125; H2L1 = salicylaldoxime), H[Ti 5 (μ 2 -O)(μ 3 -O) 2 (OMe) 3 (L1) 6 ] (PTC-126), Ti 6 (μ 2 -O)(μ 3 O)2(OiPr)10(OAc)2(L2)2 (PTC-127; H3L2 = salicylhydroxamic acid; HOAc = acetic acid), Ti7(μ3-O)2(OEt) 18(L2)2 (PTC-128), Ti12(μ2-O) 4(μ3O)4(OEt)20(L2)4 (PTC-129), and Ti18(μ2-O)10(μ3-O)8(μ4-O)2(OEt)30(L3)2 (PTC-130; HL3 = acetoxime) have been solvothermally synthesized and structurally characterized. Compared with the reported solid-state monomer, dimer, trimer, and tetramer titanium oximes, these compounds possess high nuclearity structures in the range Ti4, Ti5, Ti6, Ti7, Ti12, and Ti18 as a result of the presence of oxo bridges. Even though three examples of tetranuclear and hexanuclear derivatives have been documented, PTC-130 with a centrosymmetric Ti18 cluster core represents the currently highest PTC compound in the oxime system. In addition, Ti/oximate ratio by solvothermal synthesis in this work possessed a broad range from 0.83:1 to 9:1, different from the constant ratio of 1:1 in the reported ones. According to the solid-state UV−vis diffuse reflectance measurement, the existence of oxime ligands largely enhanced their visible light harvest. Furthermore, their photocurrent response properties are also studied.

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INTRODUCTION Nowadays crystalline polyoxo-titanium cluster (PTC) materials have attracted unprecedented attention.1−3 The potential motivation not only lies in the enhanced utilization of solar energy beyond commercial TiO24 but also comes from the acquisition of accurate structural information, which is helpful for further mechanism explanations and theoretical calculation.2,3 With worldwide joint effort, the development of PTC materials has achieved rapid progress. So far, the synthetic methods are not limited to strict inert conditions, and new approaches emerge one after another, such as solvothermal synthesis, 5−9 evaporative crystallization in the water phase,10−12 ionothermal synthesis, and so on.13 During the synthesis of PTCs, functional ligands are one of the most important raw materials and factors in connection with their photophysical performance and potential applications. For example, the employment of organic chromophores could affect the band gaps and subsequent visible light harvest of PTCs, while application of chiral ligand would transfer chirality to final PTC materials, which are able to be used in asymmetric catalysis.14−17 Oximes have a slightly basic nitrogen donor and a mildly acidic hydroxyl group. To a great extent, diverse coordination modes of oximes from side-on to end-on to bridging meet steric demand toward maximum stability.18−21 The use of oximes in late transition metal coordination compounds has © XXXX American Chemical Society

been intensively investigated, while less attention has been paid to oximate derivatives of earlier transition metals.21 Although a number of reactions of oximes with titanium species have been reported between the late 1960s and early 1980s, few publications with precise structure information are found.22−24 Since then, more research work on molecular structure, new binding modes, and chemical modification were published.25−27 It is worth mentioning that Schubert and coworkers have contributed much in this area. In 2009, they reported a dozen ketoximate derivatives of titanium alkoxides.20 In this study, they also structurally characterized three partially hydrolyzed derivatives including Ti6O6(OiPr)6(ONCMe2)6, which represents the highest nuclearity PTC derived from oximes. More recently, they reported relevant work on aromatic aldoxime, dioximate, and bis(salicylaldiminate) derivatives of titanium alkoxides.21,28 Recently, our group has devoted research efforts to the isolation of new crystalline PTCs and also the investigation of their properties.3 Environmentally friendly alcohols were used as solvents, which might help to reduce the toxicity of the obtained PTCs. We successfully made a beautiful fullerene-like Ti42 cage and a 3.6 nm PTC of Ti52, the largest of the PTCs.5,6 In addition, we also studied their band gap engineering and Received: March 21, 2018

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DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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further applications in conjunction with metal−organic frameworks.7,29,30 In the course of the preparation of the PTC materials, a wide range of organic ligands have been used involving carboxylates, phosphonates, sulfonates, azole functional ligands, and π-conjugated chromophores. In continuation of our research and in consideration of the above characteristics of oximes, we herein demonstrate the synthesis and structures of six PTCs derived from salicylaldoxime (H2L1), salicylhydroxamic acid (H3L2) and acetoxime (HL3) (Scheme 1): Ti4(μ4-O)(OMe)6(L1)4 (PTC-125), H[Ti5(μ2-

Article

EXPERIMENTAL SECTION

Materials and Physical Measurements. All the reagents and solvents employed were purchased commercially and used as received without further treatment. Salicylaldoxime, acetoxime, benzoic acid, salicylhydroxamic acid, and Ti(OiPr)4 were purchased from Adamas, succinic acid was acquired from Aladdin, and methyl alcohol (99.5%), ethyl alcohol (99.7%), isopropanol (99.7%), and acetic acid (99.5%) were bought from Sinopharm Chemical Reagent Beijing. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/ SDTA 851e analyzer in N2 with a heating rate of 10 °C min−1 from 20 to 800 °C. The Fourier transform infrared spectroscopy (FT-IR) data (KBr pellets) was recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer. Powder X-ray diffraction (PXRD) data was collected on a Rigaku Mini Flex II diffractometer using Cu Kα radiation (λ = 1.54056 Å) in the 2θ range of 5−45° with a scanning rate of 5° min−1. The UV−vis diffuse reflection data was recorded at room temperature using a powder sample with BaSO4 as a standard (100% reflectance) on a PerkinElmer Lamda-950 UV spectrophotometer and scanned at 200−800 nm. Synthesis of PTC-125 (Ti4). To a 20 mL glass vial were added salicylaldoxime (0.137 g, 1 mmol), benzoic acid (0.0488 g, 0.4 mmol), and methyl alcohol (5.0 mL). Then Ti(OiPr)4 (0.17 mL, 0.5 mmol) was quickly added. The glass vial was sealed with a polyethylene cap and agitated for few seconds and then placed in an 80 °C oven for 3 days. After cooling to room temperature, orange colored crystals of PTC-125 were obtained (yield 72% based on Ti(OiPr)4). Synthesis of PTC-126 (Ti5). To a 20 mL glass vial were added salicylaldoxime (0.205 g, 1.5 mmol), methyl alcohol (3.0 mL), and ethyl alcohol (2.0 mL). Then Ti(OiPr)4 (0.17 mL, 0.5 mmol) was quickly added. The glass vial was sealed with a polyethylene cap, agitated for few seconds, and then placed in an 80 °C oven for 3 days. After cooling to room temperature, orange colored crystals of PTC126 were obtained (yield 45% based on Ti(OiPr)4). Synthesis of PTC-127 (Ti6). To a 20 mL glass vial were added salicylhydroxamic acid (0.115 g, 0.75 mmol), acetic acid (0.17 mL), and isopropyl alcohol (5.0 mL). Then Ti(OiPr)4 (0.92 mL, 3 mmol) was quickly added. The glass vial was sealed with a polyethylene cap,

Scheme 1. Oxime Ligands Used in This Study

O)(μ 3 -O) 2 (OMe) 3 (L1) 6 ] (PTC-126), Ti 6 (μ 2 -O)(μ 3 O)2(OiPr)10(OAc)2(L2)2 (PTC-127; HOAc = acetic acid), Ti 7 (μ 3 -O) 2 (OEt) 18 (L2) 2 (PTC-128), Ti 12 (μ 2 -O) 4 (μ 3 O)4(OEt)20(L2)4 (PTC-129), and Ti18(μ2-O)10(μ3-O)8(μ4O)2(OEt)30(L3)2 (PTC-130). X-ray single crystal structural determination reveals that these compounds possess nuclearities in the range of Ti4, Ti5, Ti6, Ti7, Ti12, and Ti18. In addition, solid-state UV−vis spectra measurements were carried out. Furthermore, they exhibited rapid photoresponse and reproducible photocurrent.

Table 1. Crystal Data and Structure Refinement for PTCs Derived from Oximes formula Mr temp (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρ (g cm−3) μ (mm−1) F(000) collected reflns unique reflns (Rint) completeness GOF on F2 R1a/wR2b[I > 2(I)] R1a/wR2b(all data) CCDC number

PTC-125

PTC-126

PTC-127

PTC-128

PTC-129

PTC-130

C34H38N4O15Ti4 934.28 293(2) 0.71073 orthorhombic Ccca 11.260(3) 24.455(6) 14.591(3) 90 90 90 4017.6(16) 4 1.545 0.838 1912 13527 2278 (0.0322) 98.90% 1.043 0.0421/0.1178 0.0476/0.1212 1830681

C45H40N6O18Ti5 1192.33 293(2) 0.71073 triclinic P1̅ 12.994(7) 15.246(8) 15.606(8) 79.543(15) 74.847(9) 89.654(17) 2932(3) 2 1.351 0.719 1212 18678 9042 (0.1491) 98.30% 1.01 0.0899/0.2622 0.1196/0.2860 1830682

C48H76N2O23Ti6 1336.5 293(2) 0.71073 triclinic P1̅ 12.988(3) 13.896(4) 21.001(7) 82.680(13) 73.035(10) 64.184(12) 3263.5(17) 2 1.360 0.769 1388 38005 14847 (0.0364) 99.60% 1.042 0.0566/0.1582 0.0810/0.1798 1830683

C50H98N2O26Ti7 1478.6 293(2) 0.71073 monoclinic C2/c 22.9160(13) 18.1330(14) 17.7833(11) 90 101.229(6) 90 7248.1(8) 4 1.355 0.802 3096 12793 5104 (0.0349) 94.90% 0.985 0.0646/0.1764 0.1119/0.2041 1830684

C68H116N4O40Ti12 2204.44 293(2) 0.71073 monoclinic P21/c 19.3435(9) 22.9450(10) 23.7545(15) 90 107.695(6) 90 10044.3(10) 4 1.458 0.978 4544 35209 14394 (0.0644) 99.70% 0.992 0.0826/0.1915 0.1592/0.2296 1830685

C66H164N2O52Ti18 2680.18 293(2) 0.71073 triclinic P1̅ 17.0842(7) 17.9089(8) 23.7136(11) 68.064(4) 84.065(4) 65.995(4) 6137.9(5) 2 1.450 1.175 2772 43620 21502 (0.0575) 96.70% 1.14 0.1104/0.2958 0.1813/0.3431 1830686

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑[w(F02)2]}1/2.

a

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DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Summary of the Structural Characteristics of PTCs with Oximesa compound

coord no. of Ti

Ti4O2(OiPr)8(ONCMeCH2CMeNO)2 Ti6O6(OiPr)6(L3)6 Ti6O6(OBu)6(ONC5H8)6 Ti4(μ4-O)(OMe)6(L1)4 (PTC-125) H[Ti5(μ2-O)(μ3-O)2(OMe)3(L1)6] (PTC-126) Ti6(μ2-O)(μ3-O)2(OiPr)10(OAc)2(L2)2 (PTC-127) Ti7(μ3-O)2(OEt)18(L2)2 (PTC-128) Ti12(μ2-O)4(μ3-O)4(OEt)20(L2)4 (PTC-129) Ti18(μ2-O)10(μ3-O)8(μ4-O)2(OEt)30(L3)2 (PTC-130)

6 6 6 6 6 6 6 5, 6 5, 6

oxo bridges μ3-O μ3-O μ3-O μ4-O μ2-O, μ2-O, μ3-O μ2-O, μ2-O,

μ3-O μ3-O μ3-O μ3-O, μ4-O

coord mode of oximate

Ti/oximate ratio

μ3-η1:η1:η1:η1 μ2-η1:η1 μ2-η1:η1 μ2-η1:η1:η1 μ2-η1:η1:η1 μ3-η1:η2:η1:η1 μ3-η1:η2:η1:η1 μ3-η1:η2:η1:η1 μ3-η1:η2

1:1 1:1 1:1 1:1 0.83:1 3:1 3.5:1 3:1 9:1

ref 20 20 20 this this this this this this

work work work work work work

a Abbreviations: (HONCMeCH2CMeNOH) = 2,4-pentanedione dioxime; (ONC5H8) = cyclopentanone oxime; H2L1 = salicylaldoxime; H3L2 = salicylhydroxamic acid; HL3 = acetoxime.

agitated for few seconds, and then placed in an 80 °C oven for 3 days. After cooling to room temperature, yellow colored crystals of PTC127 were obtained (yield 55% based on Ti(OiPr)4). Synthesis of PTC-128 (Ti7). To a 20 mL glass vial were added salicylhydroxamic acid (0.115 g, 0.75 mmol) and ethyl alcohol (5.0 mL). Then Ti(OiPr)4 (0.92 mL, 3 mmol) was quickly added. The glass vial was sealed with a polyethylene cap, agitated for few seconds, and then placed in an 80 °C oven for 3 days. After cooling to room temperature, yellow colored crystals of PTC-128 were obtained (yield 42% based on Ti(OiPr)4). Synthesis of PTC-129 (Ti12). To a 20 mL glass vial were added salicylhydroxamic acid (0.1148 g, 0.75 mmol), succinic acid (0.118 g, 1 mmol), and ethyl alcohol (5.0 mL). Then Ti(OiPr)4 (0.92 mL, 3 mmol) was quickly added. The glass vial was sealed with a polyethylene cap, agitated for a few seconds, and then placed in an 80 °C oven for 3 days. After cooling to room temperature, yellow colored crystals of PTC-129 were obtained (yield 81% based on Ti(OiPr)4). Synthesis of PTC-130 (Ti18). To a 20 mL glass vial were added acetoxime (0.110 g, 1.5 mmol) and ethyl alcohol (5.0 mL). Then Ti(OiPr)4 (0.92 mL, 3 mmol) was quickly added. The glass vial was sealed with a polyethylene cap, agitated for few seconds, and then placed in an 80 °C oven for 3 days. After cooling to room temperature, white colored crystals of PTC-130 were obtained (yield 15% based on Ti(OiPr)4). Photocurrent Measurement. A solution coating method is used to prepare the photocurrent measurement electrodes of compounds PTC-125 to PTC-129; the crystals (5 mg) and Nafion (10 μL) were dissolved in 0.5 mL of ethyl alcohol; then 40 μL solutions were transferred by pipet and then dropped on the cleaned FTO glass (1.5 × 4.0 cm2, 50 Ω/cm2). The coating film was obtained after evaporation under ambient atmosphere. A 150 W high-pressure xenon lamp, located 20 cm away from the surface of the FTO electrode, was employed as a full-wavelength light source. The photocurrent experiments were performed on an IM6/Zennium electrochemistry workstation in a three-electrode system, with the sample coated FTO glass (0.25 cm) as the working electrode, a Pt plate as the auxiliary electrode, and a Ag/AgCl electrode as the reference electrode. An aqueous solution of Na2SO4 (0.2 mol L−1) was used as the medium. The lamp was kept on continuously, and a manual shutter was used to block exposure of the sample to the light. X-ray Crystallographic Analyses. Crystallographic data for PTC-125, PTC-126, and PTC-127 were collected on a Mercury single crystal diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structure determination of PTC-128, PTC-129, and PTC-130 was performed on the Xcalibur diffractometer using graphite-monochromated Mo K radiation. The structures were solved with the dual-direct methods using ShelxT and refined with the full-matrix least-squares technique based on F2 using the SHELXL-2014.31 Non-hydrogen atoms were refined by anisotropic thermal parameters. Non-hydrogen atoms were refined anisotropically, and all hydrogen atoms bonded to C were generated geometrically. The diffuse electron density arising from the guests in

PTC-126 was treated with SQUEEZE routine within the PLATON software package.32 The results of the SQUEEZE process were attached to the CIF file. To maintain reasonable geometry and atomic displacement parameters, some constraints (DFIX, SIMU, DELU, and PART) were applied to the isopropyl group refinement process in PTC-128, PTC-129, and PTC-130. The discrepancies in the moiety formulas come from the difficulties in theoretical hydrogenation because of the short intra-hydrogen distances after disorder treatment. All absorption corrections were performed using the multiscan program. Selected crystallographic data and refinement details for these six compounds are summarized in Table 1.

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RESULTS AND DISCUSSION Syntheses. According to the literature, inert synthesis conditions dominate the solid-state isolation of oximate derivatives of titanium alkoxides.20,25,27 As a result, most of the products turn out to be monomer ([Ti(OiPr)2{bis(salicylaldiminate)}]),28 dimer [Ti(OiPr)2(benzaldoximate)2]2,21 trimer (Ti3(salicylaldoxime)2(OiPr)8),25 or tetramer ([Ti4(OiPr)8(ONCMeCH2CMeNO)4]).28 Upon slow diffusion of water after two months, Schubert et al.20 successfully obtained three partially hydrolyzed derivatives, Ti4O2(OiPr)8(ONCMeCH2CMeNO)2, Ti6O6(OiPr)6(L3)6, and Ti6O6(OBu)6(ONC5H8)6. Solvothermal synthesis was usually adopted in the preparation of polyoxometalates and metal−organic frameworks. Herein we adopt solvothermal synthesis in the presence of carboxylic acid and alcohol expecting that water molecules generated by in situ esterification reactions would provide oxo bridges (Table 2). Different from the three reported partially hydrolyzed derivatives with Ti/oximate ratio of 1:1, the corresponding ratio by solvothermal approach in this work varied from 0.83:1 to 9:1. In addition, the presence of carboxylic acid also plays an important role even though the carboxylic acids have not appeared in the structures of the final products. For instance, if we remove benzoic acid from the reaction system of PTC-125, no crystalline products were obtained. In one more example, when we added succinic acid to the reaction system of PTC127, PTC-129 with higher nuclearity was generated. The reactions proved to be completely reproducible with high yields under the applied experimental conditions except for PTC-130. Subsequent attempts to reproduce PTC-130 still resulted in low yields and sometimes with byproducts of [Ti16O16(OEt)32]. The crystals were harvested and characterized. FT-IR spectroscopy was carried out to further analyze the structural characteristics of the five clusters. As observed in Figure S1, IR spectra for the crystals shows apparent C

DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry absorption bands in the range of 1545−1589 cm −1 corresponding to the asymmetric stretching vibration of the carboxylate ligand.33 The formation of Ti−O bond can also be confirmed through the strong peaks observed at approximately ∼720 cm−1.34 The thermal stabilities of the five clusters are studied by TGA experiments under nitrogen atmosphere. As shown in Figure S2, TGA curves indicate that the obtained clusters are thermally stable up to around 200 °C. To determine the band gaps of the oxime-derived PTCs, solid state UV−vis absorption spectra of PTC-125 to PTC-129 were measured (Figures S10−S14). Their optical band gaps were estimated to be 2.03, 2.03, 1.86, 2.23, and 2.33 eV, respectively, from the absorption band edge.35 Actually, all of these absorption values are red-shifted with respect to purephase TiO2 and discrete oxo−titanium clusters. It is also demonstrated that the incorporation of oxime ligands can significantly influence the light absorption of PTCs. Structure Description. Crystal Structures Derived from H2L1. Crystal structure analyses reveal that there are four Ti atoms in the cluster of PTC-125. Each Ti atom is octahedrally coordinated by two O and one N donor from two neighboring L1 ligands, a μ4-O, a terminal OMe, and a bridge OMe group (Figure 1a). The four equivalent Ti metal centers joined

Figure 2. (a) The coordination mode of L2 ligand. (b) Molecular structure of PTC-127. (c) Molecular structure of PTC-128. (d, e) Perspective and side view, respectively, of PTC-129. Terminal OEt groups are omitted for clarity. Atom color code: Ti, green; C, black; O, red; N, blue.

clusters organized by vertex-sharing tetrahedral cores,39 PTC128 can be regarded as two vertex-sharing triangles plus two additional Ti ions (Figure 2c). The central vertex-sharing triangles are linked to the two peripheral Ti ions through four μ2-OEt ligands and two L2 ligands. Compared with the compact Keggin-type Ti12 cage,40 PTC-129 is a looser structure. Intuitively, it can be viewed as an overall butterfly shaped structure (Figure 2d,e). Two wings of the butterfly are associated by three bridging OEt groups and one μ2-O bridge. Each wing consist of a distorted trigonal bipyramid and an extra Ti ion linked by two L2 ligands and a bridging OEt group. Crystal Structures Derived from HL3. Crystal structure analyses reveal that PTC-130 possesses centrosymmetric Ti18 structure. Up to now, there are four documented condensed Ti18 PTCs,11,41−43 including Keggin-type (Figure 3a) and a water-soluble pentagonal-prismatic one (Figure 3b). The Ti18 inorganic core of PTC-130 is composed of two μ4-O bridged tetranuclear moieties and a pair of pentanuclear Ti moieties that are made up of two vertex-sharing triangles (Figure 3c).

Figure 1. (a, b) Molecular structures of PTC-125 and PTC-126, respectively. (c) Coordination mode of L1 ligand. Hydrogen molecules are omitted for clarity. Atom color code: Ti, green; C, black; O, red; N, blue.

together by a μ4-O bridge to give a tetrahedral geometry. Pentanuclear PTC-126 with an overall Ti5O2 core adopts a quasi-planar configuration (Figure 1b). Its structure can be viewed as two vertex-sharing Ti3O triangles. All of the five Ti ions in this structure are six coordinated. Notably, all the L1 ligands adopt the same coordination mode in both PTC-125 and PTC-126 (Figure 1c). Despite the triply bridging mode already reported by Davidson,25 the μ2-η1:η1:η1 bridging mode of the L1 ligand is not known for Ti(IV) prior to this study. Crystal Structures Derived from H3L2. Even though L2 ligand bridged metal complexes have been reported,36−38 its analogue chemistry with Ti(IV) has not been reported according to the Cambridge Structural Database. We herein introduce three PTCs derived from H3L2. It is worth noting that all of the L2 ligands exhibit the same coordination mode throughout the following three PTCs (Figure 2a). The structure of PTC-127 consists of two μ3-O-centered triangles linked by one μ2-O (Figure 2b). The inorganic core is surrounded by a pair of L2 ligands, two OAc groups, and ten OiPr ligands. Except the inorganic μ2-O bridge, the two μ3-Ocentered triangles are further connected and stabilized by a couple of L2 ligands in μ3-η1:η2:η1:η1 fashion and two OAc groups in bridging bidentate coordination mode. Moreover, each μ3-O-centered triangle is capped by five OiPr ligands with two equatorial bridging ones and three axially terminal ones (Figure S9). Different from the general heptanuclear Ti

Figure 3. (a) Keggin-type Ti18 in ref 40. (b) Water-soluble pentagonal-prismatic Ti18 in ref 11. (c) Polyhedral view of the Ti18 inorganic core of PTC-130. (d) Molecular structure of PTC-130 with emphases for the coordination model of L3 ligand. Atom color code: Ti, green; C, black; O, red; N, blue. D

DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX

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ligand linked PTCs, the current density of PTC-125 (2.0 μA cm−2) treated electrode is about 1.25 times that of the PTC126 (0.8 μA cm−2) at same bias potentials. Among the L2 ligand system, the photocurrent density of PTC-127 treated electrode (3.7 μA cm−2) exhibited the best response compared with PTC-128 (2.2 μA cm−2) and PTC-129 (1.5 μA cm−2). These results showed that either in L1 or L2 system, low nuclearity PTC presented better photocurrent responses. Although in the same order of magnitude, the best value of photocurrent densities of the L2 system was larger than those of the L1 system. Therefore, the L2 ligand can be considered as more photosensitive to display higher light harvesting efficiency than L1 ligand.

The connection between the two tetrahedral Ti moieties is realized by one μ2-O bridge, and the linkers between the two pentanuclear Ti moieties are four μ3-O and one μ2-O bridges. Then, the inorganic core is encircled by 30 OEt groups and two μ3-η1:η2 L3 ligands (Figure 3d). Although the side-on and bridging bidentate coordination mode of L3 ligand was reported, the triply bridging mode is first observed herein. In order to have more comprehensive understand of these compounds, we listed their binding mode geometries of oxides and oximes on the surface of the titanium clusters (Table 2). Till now, both slow diffusion of water and solvothermal method can successfully incorporate oxides and construct high nuclearity PTCs derived from oximes. In contrast to the reported compounds with all oxides presenting μ3-binding mode, the oxides in this work exhibited diverse modes, including μ2-, μ3-, and μ4-fashion. With respect to the binding modes of the oximes, each ligand presented the same modes in different PTCs, for example, L1 possessed μ2-η1:η1:η1 in PTC125 and PTC-126, and L2 adopted μ3-η1:η2:η1:η1 in PTC-127, PTC-128, and PTC-129. As a result of the diversity of the oxides and oximes, a number of high nuclearity of PTCs would be expected. Photocurrent Response Properties. To study the charge-separation efficiency, we measured the photocurrent responses of PTCs derived from H2L1 and H3L2. The photocurrent response properties are measured using a threeelectrode cell with cluster coated FTO working electrodes. All the photocurrent experiments were carried out in a 0.20 mol· L−1 Na2SO4 electrolyte solution under illumination upon on− off cycling irradiation with xenon light (intervals of 10 s). Upon repetitive irradiation, steady photocurrent responses were observed throughout all of the solid state electrode systems (Figure 4). Their photocurrent response is comparable to the reported ones.8,44 As shown in Figure 4a, for the L1

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CONCLUSIONS In summary, we have demonstrated the syntheses and structures of a series of crystalline PTC materials derived from oximes. Benefitting from the inclusion of oxo groups through solvothermal synthesis, basic trinuclear Ti3O and tetranuclear Ti4O structural motifs are extensively included in these PTCs. Unexpectedly, although carboxylic acid does not participate in the final complexes with Ti atoms, they still play an important role in the crystallization of PTCs. Through modulating the applied oxime ligands and synthetic conditions, the literature of titanium oximes has been extended to high nuclearity PTCs (from Ti4 to Ti18) for the first time. Moreover, photophysical studies confirm that the oxime ligands not only produce structural variety but also induce different light harvesting and photocurrent responses. Meanwhile, the obtained PTCs are also expected to have potential applications in bioinorganic chemistry and medicine because of the presence of oximes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00751. IR, PXRD, TGA, and UV spectra of PTC-125−130 (PDF) Accession Codes

CCDC 1830681−1830686 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lei Zhang: 0000-0001-7720-4667 Jian Zhang: 0000-0003-3373-9621

Figure 4. Photocurrent densities of PTC treated electrodes derived from H2L1 (a) and H3L2 (b) in a 0.2 M Na2SO4 aqueous solution under repetitive irradiation.

Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ACKNOWLEDGMENTS This work is supported by NSFC (Grants 21473202, 21673238, and 21771181) and Natural Science Foundation of Fujian Province (Grants 2017J06009 and 2017J05036).

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DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00751 Inorg. Chem. XXXX, XXX, XXX−XXX