Synthesis and Molecular Structure of a Novel Compound Containing a

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Synthesis and Molecular Structure of a Novel Compound Containing a Carbonate-Bridged Hexacalcium Cluster Cation Assembled on a Trimeric Trititanium(IV)-Substituted Wells−Dawson Polyoxometalate Takahiro Hoshino, Rina Isobe, Takuya Kaneko, Yusuke Matsuki, and Kenji Nomiya* Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan S Supporting Information *

ABSTRACT: A novel compound containing a hexacalcium cluster cation, one carbonate anion, and one calcium cation assembled on a trimeric trititanium(IV)-substituted Wells−Dawson polyoxometalate (POM), [{Ca6(CO3)(μ3-OH)(OH2)18}(P2W15Ti3O61)3Ca(OH2)3]19‑ (Ca7Ti9Trimer), was obtained as the Na7Ca6 salt (NaCa-Ca7Ti9Trimer) by the reaction of calcium chloride with the monomeric trititanium(IV)-substituted Wells−Dawson POM species “[P2W15Ti3O59(OH)3]9−” (Ti3Monomer). Ti3Monomer was generated in situ under basic conditions from the separately prepared tetrameric species with bridging Ti(OH2)3 groups and an encapsulated Cl − ion, [{P 2 W 1 5 Ti 3 O 5 9 (OH) 3 } 4 {μ 3 -Ti(H 2 O) 3 } 4 Cl] 2 1 − (Ti 1 6 Tetramer). The Na 7 Ca 6 salt of Ca7Ti9Trimer was characterized by complete elemental analysis, thermogravimetric (TG) and differential thermal analyses (DTA), FTIR, single-crystal X-ray structure analysis, and solution 183W and 31P NMR spectroscopy. X-ray crystallography revealed that the [Ca6(CO3)(μ3-OH)(OH2)18]9+ cluster cation was composed of six calcium cations linked by one μ6-carbonato anion and one μ3-OH− anion. The cluster cation was assembled, together with one calcium ion, on a trimeric species composed of three triTi(IV)-substituted Wells−Dawson subunits linked by Ti−O−Ti bonds. Ca7Ti9Trimer is an unprecedented POM species containing an alkaline-earth-metal cluster cation and is the first example of alkaline-earth-metal ions clustered around a titanium(IV)-substituted POM.

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INTRODUCTION Polyoxometalates (POMs) are discrete metal oxide clusters that are attracting interest as soluble metal oxides. The coordination chemistry of POMs with transition-metal cations (TMC) has been extensively investigated in the past few decades,1−9 and in particular, POM-supported organometallic and transition-metal complexes have been intensely studied as molecular models of metal oxide supported transition-metal catalysts.10,11 The relatively low reactivities of the bridging and terminal oxygen atoms in many POMs prevent the formation of TMC derivatives. However, two methods are available to improve the nucleophilicity of the surface oxygen atoms in POMs: (i) the use of lacunary species of POMs in which the most nucleophilic oxygen atoms constitute the vacant sites and (ii) increase in the overall negative charge of the POMs.12 For example, the former approach has provided monolacunary Keggin and Wells−Dawson POM-based [Ru(arene)]2+ complexes,12−14 while the latter involves replacement of one or several MoVI and WVI centers by cations with a lower charge (VV, NbV, TiIV, ...) or reduction of one or several of the main metals (MVI = MoVI, WVI) constructing the POM framework.15−19 Indeed, tri-M(V)-substituted Wells−Dawson POMsupported organometallic complexes (M = NbV, VV) have been formed by covalent binding of cationic organometallic species, © 2017 American Chemical Society

utilizing the fact that the tri-M-substituted Wells−Dawson POM anion [P2W15M3O62]9− has three extra negative charges in comparison with the parent anion [P2W18O62]6−. For example, [(Cp*Rh)P2W15Nb3O62]7− and [(Cp*Rh)2P2W15V3O62]5− have been reported.15−18 Similarly to the substituted POM, the reduced POM [XV2MmnMVI18−nO62]{6+(6−m)n}− (X = P, As, etc.; M = W, Mo) has one or several external negative charges and forms capped architectures with Cu(2,2′-bpy)2+ and As 3+ : for example, [{Cu(2,2′-bpy)} 3 {As 2 V Mo 2 V Mo16VIO62}]2−, [AsIII(As2VMo2VMo16VIO62)]5−, [As2III(As2VMo3VMo15VIO62)]3−, and [As3III(As2VMo3VMo15VIO62)].19 The trititanium(IV)-substituted Wells−Dawson POM has been reported to exist as two stable tetrapod-shaped tetramers formed by the reaction of the trilacunary Wells−Dawson POM [P2W15O56]12− with TiIV,20−27 though not as the monomeric form “[P 2 W 1 5 Ti 3 O 6 2 ] 1 2 − ”. One of the tetramers, [{P2W15Ti3O57.5(OH)3}4Cl]25− (Ti12Tetramer), is composed of four Wells−Dawson units linked through intermolecular Ti− O−Ti bonds and one encapsulated Cl− ion.21 The other tetramer, [{P 2 W 15 Ti 3 O 59 (OH) 3 } 4 {μ 3 -Ti(H 2 O) 3 } 4 Cl] 21− (Ti16Tetramer), is also composed of four Wells−Dawson Received: April 26, 2017 Published: August 7, 2017 9585

DOI: 10.1021/acs.inorgchem.7b01043 Inorg. Chem. 2017, 56, 9585−9593

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units and one encapsulated anion (Cl−, Br−, I−, NO3−), but it shows four additional μ3-Ti(OH2)3 bridging groups.22−24 These Wells−Dawson POM-based tetrapod-shaped tetramers are actually generated as stable forms by self-assembly due to the rapid formation of Ti−O−Ti bonds by the monomer “[P2W15Ti3O62]12−.” Consequently, the monomeric form of the trititanium(IV)-substituted Wells−Dawson POM has so far never been obtained by the direct reaction of the trilacunary Wells−Dawson POM [P2W15O56]12− with TiIV. However, we recently found that this monomeric species is generated in situ by hydrolysis of Ti16Tetramer,25 though not by hydrolysis of Ti12Tetramer. In our preliminary experiments, we isolated the “[P2W15Ti3O59(OH)3]9−” monomer (Ti3Monomer) in the solid state as a water-soluble sodium salt of the form protonated at the Ti−O−Ti sites (Na-Ti3Monomer). However, it was always contaminated with soluble unidentified titanium(IV) compounds, i.e., it has never been obtained in pure form.27 It should also be noted that Na-Ti3Monomer has never been transformed to an organic-solvent-soluble form, such as a Bu4N+ salt. Moreover, Ti3Monomer readily reverts to the tetrameric species under acidic or neutral conditions and is stable only under basic conditions. Nevertheless, Ti3Monomer is expected to react with cationic complexes, because of its high negative surface charge. In fact, we recently isolated the sodium/potassium salt of the trititanium(IV)-substituted Wells−Dawson POM-supported organometallic complex [{P2W15Ti3O60(OH)2}2(Cp*Rh)2]16− in analytically pure form, by the reaction of in situ generated Cp*Rh2+ species with Ti3Monomer contaminated with soluble titanium(IV) species.27 X-ray crystallography revealed that this complex took an unusual dimeric form, composed of the two trititanium(IV)substituted Wells−Dawson POMs bridged by two Cp*Rh2+ groups. On the other hand, there is little information on complexes between alkaline-earth-metal (AEM) cations and modified POMs. Some AEM cluster cations with organic ligands have been reported,28−32 but there are only a few compounds that contain AEM cluster cations combined with POMs: for example, (1) {[Ca(H 2 O)]6 [P 4 M 6 O 34 ] 2 } 12− (M = W VI , MoVI),33 i.e., the hexacalcium cluster cation {[Ca(H2O)]6}12+ sandwiched by two [P 4 M 6 O 34 ] 12− anions, (2) {{[Ca(H2O)]6(H2O)}[H2As4W6O34]2}8−,34 i.e., a hexacalcium cluster cation with an encapsulated water molecule, {[Ca(H2O)]6(H2O)}12+, sandwiched by two [As4W6O34]12− anions, and (3) {Ba10(NMP)14(H2O)8[V12O33]4Br}5− (NMP = Nmethyl-2-pyrrolidone),35 i.e., a POM-based heterometallic cluster consisting of the decabarium cluster {Ba10(NMP)14Br}19+ and four [V12O33]6− anions. Here, we present full details of the synthesis and characterization of a novel compound consisting of a hexacalcium cluster cation containing one carbonate anion and one calcium cation assembled on a trimeric trititanium(IV)-substituted Wells− Dawson polyoxometalate (POM), i.e., [{Ca6(CO3)(μ3-OH)(OH2)18}(P2W15Ti3O61)3Ca(OH2)3]19‑ (Ca7Ti9Trimer) isolated as a sodium/calcium salt (NaCa-Ca7Ti9Trimer), by means of the reaction of in situ generated Ti3Monomer and calcium chloride. The compound was characterized by complete elemental analysis, including oxygen, TG/DTA, FTIR, single-crystal X-ray crystallography, and solution 183W and 31P NMR spectroscopy. Both the solid-state and solution structures of this POM were clarified.

Article

EXPERIMENTAL SECTION

Materials. The following reactants were used as received: CaCl2· 2H2O, Na2CO3, and 1 M NaOH(aq) (Wako) and D2O (Isotec). The sodium salt of Ti16Tetramer, Na19H2[{P2W15Ti3O59(OH)3}4{μ3Ti(OH2)3}4Cl]·92H2O (Na-Ti16Tetramer), was prepared by a modification of the reported method22−24 and characterized by TG/ DTA, FTIR, and solution 31P NMR in D2O (see the Supporting Information). Instrumentation/Analytical Procedures. Complete elemental analysis was carried out by Mikroanalytisches Labor Pascher (Remagen, Germany). A sample was dried at room temperature under 10−3−10−4 Torr overnight before analysis. IR spectra were measured on a JASCO 4100 FTIR spectrometer in KBr disks at room temperature. TG and DTA measurements (air flow 50 mL/min, heating rate 4 °C/min) were acquired using a Rigaku Thermo Plus 2 series TG 8120 instrument. The 31P NMR (160 MHz) spectra of samples in D2O solution were recorded in 5 mm outer diameter tubes on a JEOL JNM-ECA 400 FT-NMR spectrometer and a JEOL ECA400 NMR data processing system. The 31P NMR spectra were referenced to an external standard of 25% H3PO4 in H2O in a sealed capillary. The 31P NMR signals in commonly used 85% H3PO4 are shifted by +0.544 ppm from our data obtained in 25% H3PO4. The 183 W NMR (25.0 MHz) spectra were recorded in 10 mm outer diameter tubes, on a JEOL JNM-ECZ 600 FT-NMR spectrometer equipped with a JEOL NM-60T10L low-frequency tunable probe and a JEOL ECZ-600 NMR data processing system. These spectra were referenced to an external standard (saturated Na2WO4−D2O solution) by the substitution method. Chemical shifts were reported on the δ scale with resonances upfield of Na2WO4 (δ 0) shown as negative. The 183 W NMR signals are shifted by −0.787 ppm by using a 2 M Na2WO4 solution as a reference instead of saturated Na2WO4 solution. Pre paration of Na 7 Ca 6 [{Ca 6 (CO 3 )(μ 3 -OH)(OH 2 ) 1 8 }(P 2 W 15 Ti 3 O 61 ) 3 Ca(OH 2 ) 3 ]·62H 2 O (NaCa-Ca 7 Ti 9 Trimer). NaTi16Tetramer (2.0 g, 0.11 mmol) was dissolved in water (15 mL) to afford a colorless aqueous solution, which was added to the NaOH(aq) (pH 11.0, 80 mL) with vigorous stirring for more than 45 min. In this workup, the pH of the mixed solution of the Ti16Tetramer solution and the NaOH(aq) was maintained using 1 M NaOH(aq) in the range of pH 10.7−11.0. The slightly cloudy solution was concentrated to 40 mL using a rotary evaporator at 35 °C, and a solution of 0.5 M CaCl2(aq) (3.48 mL) was added to it. The mixture was left to stand for 24 h at room temperature. The resulting white precipitate was separated by centrifugation and collected on a folded filter paper (Whatman #5). The filtrate was slowly evaporated at room temperature. After 1 day, colorless clear plate crystals formed, which were collected on a membrane filter (JG 0.2 μm) and dried in vacuo for 2 h. The yield was 0.83 g (41% yield based on Na-Ti16Tetramer). The obtained crystalline sample (NaCa-Ca7Ti9Trimer) was slightly soluble in water. The sample was dried overnight at room temperature under 10−3−10−4 Torr before complete elemental analysis. Anal. Found: C, 0.07; H, 0.83; O, 25.30; Na, 1.32; Ca, 3.95; P, 1.43; Ti, 3.26; W, 63.90; a total of 100.06%. Calculated values were fitted within allowed error values for n = 5−10 in Na7Ca6[{Ca6(CO3)(μ3OH)(OH2)18}(P2W15Ti3O61)3Ca(OH2)3]·nH2O. Calcd for n = 5 or C1H53Ca13Na7O213P6Ti9W45: C, 0.09; H, 0.41; O, 26.12; Na, 1.23; Ca, 3.99; P, 1.42; Ti, 3.30; W, 63.42% and calcd for n = 10 or C1H63Ca13Na7O218P6Ti9W45: C, 0.09; H, 0.48; O, 26.55; Na, 1.23; Ca, 3.97; P, 1.41; Ti, 3.28; W, 62.98. A weight loss of 7.10% (water of solvation) was observed during overnight drying at room temperature, at 10−3−10−4 Torr before analysis, suggesting dehydration of 52−57 water molecules (calcd 6.65−7.29%). Separately, TG/DTA under the atmospheric conditions showed a weight loss of 10.63% below 399.9 °C, with endothermic peaks at 67.5, 80.9, 92.0, 115.2, and 149.8 °C; calcd 10.61% for x = 62 in Na7Ca6[{Ca 6(CO3)(μ3-OH)(OH2) 18}(P 2W15Ti3 O61 )3Ca(OH2 )3]· xH2O. The carbonate release as carbon dioxide cannot be observed as clear weight loss in TG/DTA (one CO2 (ca. 0.3%) in the formula weight ca. 14000), because of low contents of the carbonate. IR (KBr) (polyoxometalate region): 1625 (s), 1472 (w), 1447 (w), 1087 (vs), 9586

DOI: 10.1021/acs.inorgchem.7b01043 Inorg. Chem. 2017, 56, 9585−9593

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Figure 1. 31P NMR spectra of reaction solutions of in situ generated Ti3Monomer, CaCl2 ,and Na2CO3 in H2O in the following molar ratios: (a) 3:4:0; (b) 3:6:0; (c) 3:7:0; (d) 3:10:0; (e) 3:4:1; (f) 3:6:1; (g) 3:7:1; (h) 3:10:1. X-ray Crystallography. A colorless clear block crystal of NaCaCa7Ti9Trimer (0.16 × 0.11 × 0.05 mm3) was placed in liquid paraffin (Paratone-N) to prevent degradation and analyzed at 130(2) K. All measurements were performed with a Rigaku MicroMax-007HF with Saturn CCD diffractometer. The structure was solved by direct methods (SHELXS-97),36 followed by difference Fourier calculations and refinement by a full-matrix least-squares procedure on F2 (program SHELXL-97).36 Absorption correction was performed with the CrystalClear program (empirical absorption correction).37 The composition and formula of the POM, which contained many countercations and many molecules of water of crystallization, were determined by complete elemental analysis and TG/DTA analysis. In X-ray crystallography, the polyoxoanion of Ca7Ti9Trimer, seven sodium and six calcium cations, and 84 hydrated water molecules per formula unit were identified in the crystal structure. All of the atoms in the polyoxoanion, all countercations, and some of the hydrated water molecules were refined anisotropically, while the rest (as solvents of crystallization) were refined isotropically. Refinements of the positions of many of the counterions and solvent molecules in the POM were limited because of disorder. This is generally the case in POM crystallography. Crystal data for NaCa-Ca7Ti9Trimer: CCa13Na7O293P6Ti9W45; Mr = 14272.15; triclinic, space group P1̅; a = 24.971(5) Å, b = 26.137(5) Å, c = 26.494(5) Å, α = 60.59(3)°, β = 85.79(3)°, γ = 87.46(3)°, V = 15021(5) Å3, Z = 2, Dc = 3.155 g cm−3, μ(Mo Kα) = 17.752 mm−1; R1 = 0.0674, wR2 = 0.1526 (for all data); R1 = 0.0536, wR2 = 0.1424 (for I > 2σ(I)); Rint = 0.0917, GOF = 0.998 (207283 total reflections, 53295 unique reflections where I > 2σ(I)). Further details of the crystal structure investigations may be obtained from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif., quoting the depository number CCDC-1548467 for NaCa-Ca7Ti9Trimer (Iidentification code RiI1201).

1065 (m), 1010 (m), 944 (vs), 916 (vs), 822 (vs), 784 (vs), 682 (vs), 600 (vs), 564 (vs), 524 (vs), 468 (s) cm−1. 31P NMR (21.9 °C, D2O): δ −6.87, −14.32 ppm. 183W NMR (21.7 °C, D2O): δ −153.21 (1W), −159.22 (2W), −180.53 (2W), −192.26 (2W), −197.22 (2W), −231.69 (2W), −231.92 (2W), −232.85 (2W) ppm. The 183W NMR measurement was carried out as follows. The sodium/calcium salt of Ca7Ti9Trimer (0.5 g) was suspended in D2O (3.0 mL), and cation exchange resin (Amberlite IR 120B NA) in Na+ form was added until the suspension turned clear. The resin was filtered off with a glass filter (P40), and the colorless filtrate was used for 183W NMR measurement at room temperature (Figure 4). The 31P NMR spectrum was also measured using the same solution (δ −6.76, −14.32 ppm); the results indicated that the cation-exchange procedure had no effect on the structure of the original compound. Because of the low solubility in water of Ca7Ti9Trimer and low contents (one CO32− ion (C, 0.09%) in the formula weight ca. 14000), the 13C NMR measurement of the coordinated carbonate ion was very difficult. Thus, we tried 13C NMR measurement of the more highly concentrated solution using the cation-exchange resin in Na+ form, just as for the 183W NMR measurement, and increased accumulation times (27000 scans for 15 h). However, we were not able to obtain the 13 C NMR signal of the coordinated carbonate ion. Control Experiment: 31P NMR Measurements of Reaction Solutions with Various Molar Ratios of CaCl2, Na2CO3, and in Situ Generated Ti3Monomer. Na-Ti16Tetramer (2.0 g, 0.11 mmol) was dissolved in water (15 mL), and the colorless aqueous solution was added to NaOH(aq) (pH 11.0, 80 mL) with stirring for at least 45 min, while the pH of the solution was maintained in the range of 10.7−11.0. The slightly cloudy solution was concentrated to 40 mL in a rotary evaporator at 35 °C and divided into four aliquots. 31P NMR samples containing in situ generated Ti3Monomer:CaCl2:Na2CO3 in the ratios (a) 3:4:0, (b) 3:6:0, (c) 3:7:0, and (d) 3:10:0) were prepared by adding 0.29, 0.44, 0.51, and 0.73 mL of 0.5 M CaCl2(aq) to aliquots of the above solution. In addition, 31P NMR samples containing Ti3Monomer:CaCl2:Na2CO3 in the ratios (e) 3:4:1, (f) 3:6:1, (g) 3:7:1, and (h) 3:10:1 were prepared by adding 0.36 mL each of 0.1 M Na2CO3(aq) to samples a−d. The 31P NMR spectra of samples a−h were measured after 1 day (Figure 1). 31P NMR data: (a) δ −5.12, −5.82, −6.70, −14.36, −14.48, −14.61 ppm; (b) δ −5.08, −6.56, −6.64, −14.10, −14.17, −14.24, −14.52 ppm; (c) δ −5.06, −6.55, −6.61, −6.62, −14.04, −14.13, −14.19, −14.44 ppm; (d) δ −6.82, −7.74, −14.12, −14.19, −14.36 ppm; (e) δ −5.14, −5.82, −6.72, −14.38, −14.49, 14.62 ppm; (f) δ −5.24, −6.78, −14.38, −14.51, −14.62 ppm; (g) δ −5.29, −6.79, −14.21, −14.36, −14.62 ppm; (h) δ −5.33, −6.84, −7.72, −14.21, −14.37, −14.64 ppm).

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RESULTS AND DISCUSSION Synthesis and Compositional Characterization. NaCaCa7Ti9Trimer was obtained by reaction of a 12:3 molar ratio of calcium chloride and monomeric trititanium(IV)-substituted Wells−Dawson POM (Ti3Monomer) that was generated in situ by base hydrolysis of Ti16Tetramer. The composition and molecular formula of NaCa-Ca7Ti9Trimer, or Na 7 Ca 6 [{Ca 6 (CO 3 )(μ 3 -OH)(OH 2 ) 18 }(P 2 W 15 Ti 3 O 61 ) 3 Ca(OH2)3]·62H2O, were determined by complete elemental analysis (C, H, O, Na, Ca, P, Ti, and W), FTIR, and single9587

DOI: 10.1021/acs.inorgchem.7b01043 Inorg. Chem. 2017, 56, 9585−9593

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Figure 2. FTIR spectra in the polyoxoanion region (1800−400 cm−1), measured in KBr disks, of (a) Ca7Ti9Trimer, (b) Ti3Monomer, and (c) Ti16Tetramer.

In the complete elemental analysis, the total of the found values of eight elements (C, H, O, Na, Ca, P, Ti, and W) was 100.06%, indicating the high purity of Ca7Ti9Trimer. The presence of one carbonate ion in Ca7Ti9Trimer was confirmed by the elemental analysis data, as well as FTIR (Figure 2) and X-ray crystallography (Figure 3). The solid-state FTIR spectrum (Figure 2a) of Ca7Ti9Trimer showed characteristic spectral patterns of the Wells−Dawsontype “[P2W18O62]6−” POM framework.38 In the case of Ca7Ti9Trimer, the IR bands of the coordinating carbonate ion were observed at 1472 and 1447 cm−1, whereas such bands were not observed for Ti 16 Tetramer (Figure 2c) or Ti3Monomer (Figure 2b). Ca7Ti9Trimer, like Ti16Tetramer, showed a Ti−O−Ti intersubunit vibration band at 682 cm−1, indicating that Ca7Ti9Trimer is not a monomeric Wells− Dawson unit but is an oligomer of Wells−Dawson units. Molecular Structure of Ca7Ti9Trimer. X-ray crystallography revealed that the Ca7Ti9Trimer polyoxoanion exists as a dimeric entity (Figure 3c), in which two tripod-shaped species (Figure 3a) are linked through two Ca−O−W bonds. The structure of Ca 7 Ti 9 Trimer is shown as a polyhedral representation in Figures 3a,b (Figure 3a, top view; Figure 3b, side view), with its three components, {Ca6}, {Ti9W45}, and {Ca1}. Figure 3d−f shows a partial structure of the {Ca6} unit, the {Ca1} unit, and the {Ti9W45} unit, respectively. Bond

crystal X-ray crystallography. In particular, the hydrated water molecules, or 62H2O, are based on TG/DTA data. The formation of the polyoxoanion of Ca7Ti9Trimer can be represented by eq 1. This reaction was performed under basic conditions, because Ti3Monomer was unstable under neutral or acidic conditions, where it was readily transformed to Ti12Tetramer. The CO32− group present in Ca7Ti9Trimer would be derived from CO2 in air under the basic conditions. 3“[P2W15Ti3O59(OH)3 ]9 − ” + 7Ca 2 + + 6OH− + CO2 + 14H 2O Ti3Monomer

→ [{Ca6(CO3)(μ3 ‐OH)(OH 2)18 }(P2W15Ti3O61)3 Ca(OH 2)3 ]19 − Ca 7Ti 9Trimer

(1)

CO32−

We consider that the group stabilizes the structure of Ca7Ti9Trimer. 31P NMR measurements (Figure 1) of the reaction solutions containing various molar ratios of CaCl2, Na2CO3, and in situ generated Ti3Monomer showed the significant effect on the signals of Ca7Ti9Trimer (31P NMR; δ ca. −6.8 and −14.3 ppm) and Ti3Monomer (δ ca. −5.1 and −14.5 ppm). The influence of the increasing calcium chloride and sodium carbonate was observed as decreased signals of Ti3Monomer and increased signals of Ca7Ti9Trimer. The minor peaks were much more decreased by the increasing sodium carbonate (Figure 1f−h). 9588

DOI: 10.1021/acs.inorgchem.7b01043 Inorg. Chem. 2017, 56, 9585−9593

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Ca3, O1F, O2F, O3F, O2Z; for Ca4, O1G, O1Z; for Ca5, O1H, O2H; for Ca6, O1I, O2Z) (Ca−O distances 2.320(8)− 2.712(9) Å (average 2.482 Å)) (Figure 3d and Tables S1 and S2 in the Supporting Information). This calcium cluster cation {Ca6} is unique and has not been found in any previously reported AEM cluster cations formed by organic or inorganic ligands.28−35 In the {Ca6} cluster unit, oxygen atoms of the μ6carbonato group are linked to six calcium atoms (Ca1, Ca2, Ca3, Ca4, Ca5, and Ca6) in such a way that each O atom acts as a μ3-bridge among three Ca centers, and the oxygen atom of the μ3-OH− group (O1Y) is linked to three calcium atoms (Ca4, Ca5, and Ca6). All calcium atoms in the {Ca6} cluster are eight-coordinated. The Ca−Ca distances (Ca1−Ca2, 4.705(6) Å; Ca2−Ca3, 5.166(5) Å; Ca3−Ca1, 4.970(6) Å (average 4.947 Å)) among the three calcium atoms around the CO32− group in the {Ca6} cluster were longer than those (Ca4−Ca5, 3.877(6) Å; Ca5−Ca6, 3.909(6) Å; Ca6−Ca4, 3.865(5) Å (average 3.884 Å)) among the three calcium atoms around the μ3-OH− group. Since all of the Ca−Ca distances are different, the {Ca6} cluster is asymmetric (i.e., C1 symmetry) in the solid state. On the other hand, the hexacalcium cluster cation in {[Ca(H2O)]6[P4M6O34]2}12− (M = WVI, MoVI)33 (Ca−Ca distance: M = WVI, average 4.056 Å; M = MoVI, average 4.076 Å) and the hexacalcium cluster cation in {{[Ca(H2O)]6(H2O)}[H2As4W6O34]2}8− 34 (Ca−Ca distance: average 3.718 Å) have Ci symmetry in the solid state. The second component, i.e., the {[(P2W15Ti3O61)3]30−} or {Ti9W45} moiety, is a tripod-shaped trimer consisting of three trititanium(IV)-substituted Wells−Dawson units (designated as A−C). The O atoms (O18A, O19A, O20A, O18B, O19B, O20B, O18C, O19C, and O20C) of the intramolecular Ti−O− Ti bonds in each Wells−Dawson unit are deprotonated (BVS value 1.984−2.092 (average 2.037), Ti−O distance 1.806(8)− 1.937(8) Å (average 1.875 Å)), and the O atoms (O2A, O2B, and O2C) of the intermolecular Ti−O−Ti bonds between two Wells−Dawson units are not protonated (BVS value 1.991− 2.000 (average 1.997), Ti−O distance 1.805(8)-1.837(8) Å (average 1.820 Å)) (Figure 3d and Table S1 and S2 in the Supporting Information). The intermolecular Ti−O−Ti angles (average 141.7°) in the {Ti9W45} moiety were narrower than those in two Ti12Tetramer species21,25 (intermolecular Ti−O− Ti angles: average 153.8°21 for Ti12Tetramer with encapsulated Cl− ion; average 148.7°25 for Ti12Tetramer with encapsulated NH4+ ion). The third component is the monocalcium cation (Ca7), i.e., {Ca(OH2)3} or {Ca1} moiety, which is attached to the three oxygen atoms (O19A, O19B, and O19C) of the intramolecular Ti−O−Ti bonds in each Wells−Dawson unit (Figure 3e). In addition, X-ray crystallography showed that the monocalcium cation was coordinated to three disordered water molecules (O1JA, O2JA, and O3JA or O1JB, O2JB, and O3JB) (Ca−O distance 2.30(3)-2.43(3) Å (average 2.37 Å)) (Figure 3e and Table S1 and S2 in the Supporting Information). The monocalcium cation is six-coordinated, whereas the six calcium atoms in the {Ca6} cluster are eight-coordinated. In this work, by the reaction of CaCl2 and Ti3Monomer, Ti3Monomer was not assembled to Ti12Tetramer, but assembled to Ca7Ti9Trimer. This can be attributed to formation of the {Ca6} cluster blocking one of the positions in the tetrapod and preventing the fourth trititanium(IV)substituted Wells−Dawson subunit leading to the Ti12Tetramer from assembling. As related compounds, the

Figure 3. (a) Top view of the polyoxoanion [{Ca6(CO3)(μ3OH)(OH2)18}(P2W15Ti3O61)3Ca(OH2)3]19− (Ca7Ti9Trimer) consisting of the three components {Ca6}, {Ti9W45}, and {Ca1} represented by polyhedral and ball-and-stick models. (b) Side view of the polyoxoanion. (c) Dimeric entity in which two Ca7Ti9Trimer units are linked through two Ca−O−W bonds. (d) Partial structure around the {Ca6}unit, in which the C atom of the carbonate is shown in sky blue. (e) Partial structure around the {Ca1} unit. (f) Partial structure around one Wells−Dawson framework containing the intermolecular and intramolecular Ti−O−Ti bonding modes in the {Ti9W45} unit. Color code: Ca, orange; O, red; P, yellow; C, sky blue; Ti, blue; W, gray.

valence sum (BVS) calculations of the C, O, Ca, P, Ti, and W atoms using the observed bond lengths are presented in Table S1 in the Supporting Information,39−42 and selected bond lengths (Å) and angles (deg) around the C, Ca, and Ti centers in the Ca7Ti9Trimer are presented in Table S2 in the Supporting Information. The first of the three components, i.e., the {[Ca6(CO3)(μ3OH)(OH2)18]9+} or {Ca6} moiety, contains 1 μ6-carbonato group (C1, O1X, O2X, and O3X), 1 μ3-OH− group (O1Y), and 18 coordinating water molecules (for Ca1, O1D, O2D, O3D, O4D, O1Z; for Ca2, O1E, O2E, O3E, O4E, O5E; for 9589

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Figure 4.

183

W NMR spectra in D2O of (a) Ca7Ti9Trimer, (b) Ti3Monomer, and (c) Ti16Tetramer.

tripod-shaped trimers based on A-site trititanium(IV)-substituted Keggin POM have been also reported.43−45 Solution 183W and 31P NMR Measurements. The 183W NMR spectra in D2O of both Ti3Monomer (Figure 4b) and Ti16Tetramer (Figure 4c) showed three line peaks with 1:2:2 intensity ratio (Ti3Monomer at −158.11, −193.51, and −233.90 ppm;27 Ti16Tetramer at −145.0, −181.5, and −201.4 ppm22). These spectra are consistent with the molecular structures of Ti3Monomer with C3v symmetry and Ti16Tetramer with Td symmetry, respectively. On the other hand, the 183W NMR spectrum (Figure 4a) of Ca7Ti9Trimer in D2O showed eight lines at −153.40 (1W), −159.05 (2W), −180.63 (2W), −193.19 (2W), −198.97 (2W), −231.62 (2W), −231.92 (2W), and −232.79 (2W) ppm. The eight-line 183W NMR spectrum in D2O comes from eight inequivalent W atoms in the tri-Ti-substituted Wells− Dawson subunit of the monomeric form of the Ca7Ti9Trimer with C3v symmetry (Figure 5). The solid-state dimeric form with Ci symmetry due to the Ca−O−W bonds of the Ca7Ti9Trimer contains more than 15 inequivalent W atoms (Figure S1 in the Supporting Information). 183W NMR studies of Ca7Ti9Trimer in D2O suggested that the monomeric molecular structure of the Ca7Ti9Trimer with approximate C3v symmetry is formed in solution, resulting from hydrolysis or hydration of of the Ca atom in the Ca−O−W bond, when the dimeric form of the Ca7Ti9Trimer with Ci symmetry was dissolved in water. The 31P NMR spectrum (Figure 6a) of Ca7Ti9Trimer in D2O shows two lines at −6.87 and −14.32 ppm, indicating that the three Wells−Dawson units in the {Ti9W45} moiety are

Figure 5. Polyhedral representations of (a) the monomeric structure of Ca7Ti9Trimer in solution and (b) Wells−Dawson unit A in the monomeric structure (a). In (a), the oxygen atoms of water molecules coordinated to calcium atoms are omitted for clarity.

equivalent in D2O. The lower-field signal at −6.87 ppm was assigned to the P atom closest to the trititanium(IV)9590

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Figure 6. 31P NMR spectra in D2O of (a) Ca7Ti9Trimer, (b) Ti3Monomer, and (c) Ti16Tetramer.

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substituted side, while the upper-field signal at −14.32 ppm was assigned to that on the opposite side. The precursors, Ti3Monomer and Ti16Tetramer, also showed two-line spectra: Ti3Monomer at −5.04 and −14.69 ppm (Figure 6b); Ti16Tetramer at −7.04 and −13.77 ppm (Figure 6c). The lower-field signal of Ca7Ti9Trimer was located between the signals of Ti3Monomer and Ti16Tetramer, and the upper-field signal of Ca7Ti9Trimer was also located in an intermediate position.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01043. Synthesis of the tetrameric trititanium(IV)-substituted Wells−Dawson POM with bridging Ti(OH2)3 groups (Na-Ti16Tetramer), polyhedral representations of (a) the Ca−O−W bonding dimeric structure of the Ca7Ti9Trimer units in the solid state and (b) two types (X and Y) of Wells−Dawson units in the dimeric structure, bond valence sum (BVS) calculations of C, O, Ca, Ti, P, and W atoms for [{Ca6(CO3)(μ3-OH)(OH2)18}(P2W15Ti3O61)3Ca(OH2)3]19− in Ca7Ti9Trimer, and selected bond lengths (Å) and angles (deg) around the C, Ca, and Ti centers for [{Ca6(CO3)(μ 3 -OH)(OH 2 ) 18 }(P 2 W 15 Ti 3 O 61 ) 3 Ca(OH 2 ) 3 ] 19− in Ca7Ti9Trimer (PDF)

CONCLUSION

During studies of the chemistry of trititanium(IV)-substituted Wells−Dawson POMs, we synthesized the Na7Ca6 salt of Ca7Ti9Trimer, consisting of a hexacalcium cluster cation containing a carbonate anion and a monocalcium cation assembled on a trimeric trititanium(IV) substituted Wells− Dawson POM, in analytically pure form. The compound was obtained by the reaction of in situ generated monomeric Ti3Monomer with calcium chloride under basic conditions. Xray crystallography and 31P NMR showed that the CO32− group stabilized the structure of Ca7Ti9Trimer, composed of three components: i.e., {Ca6}, {Ti9W45}, and {Ca1}. The calcium cluster cation {Ca6} is distinct from previously reported AEM cluster cations formed by organic and inorganic ligands. 31P and 183 W NMR studies of Ca7Ti9Trimer in D2O suggested that the monomeric molecular structure of the Ca7Ti9Trimer with approximate C3v symmetry is formed in solution, resulting from hydrolysis or hydration of the Ca atom in the Ca−O−W bond, when the dimeric form of the Ca7Ti9Trimer with Ci symmetry was dissolved in water. Studies of clusterization of other alkaliearth cations (strontium, barium, etc.) and the role of coordination of the carbonate anion using Ti3Monomer derived from Ti16Tetramer are in progress.

Accession Codes

CCDC 1548467 contains 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 data_ [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 Author

*K.N.: tel, 81-463-59-4111; fax, 81-463-58-9684; e-mail, [email protected]. ORCID

Kenji Nomiya: 0000-0003-0225-877X Notes

The authors declare no competing financial interest. 9591

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(17) Nomiya, K.; Nozaki, C.; Kaneko, M.; Finke, R. G.; Pohl, M. The All-Sodium Salt of a Polyoxoanion-Supported Organometallic Complex: Synthesis and Characterization of Na7[(η5-C5Me5)Rh· PW2W15Nb3O62]·7DMSO·5H2O. J. Organomet. Chem. 1995, 505, 23−28. (18) Nomiya, K.; Hasegawa, T. Synthesis and Spectroscopic Characterization of a Dawson Trivanadium-Substituted Polyoxotungstate-Supported {(Cp*Rh)2}4+ Complex; (Bun4N)5[(Cp*Rh)2P2W15V3O62]. Chem. Lett. 2000, 29, 410−411. (19) Zhang, H.; Yu, K.; Wang, C.; Su, Z.; Wang, C.; Sun, D.; Cai, H.; Chen, Z.; Zhou, B. pH and Ligand Dependent Assembly of Wells− Dawson Arsenomolybdate Capped Architectures. Inorg. Chem. 2014, 53, 12337−12347. (20) Nomiya, K.; Arai, Y.; Shimizu, Y.; Takahashi, M.; Takayama, T.; Weiner, H.; Nagata, T.; Widergren, J. A.; Finke, R. G. Synthesis and Characterization of the Tetrameric, Tri-Titanium(IV)-Substituted Wells−Dawson-Substructure Polyoxotungstate, [(P2W15Ti3O60.5)4]36−: the Significance of Ultracentrifugation Molecular Weight Measurements in Detecting Aggregated, Anhydride Forms of Polyoxoanions. Inorg. Chim. Acta 2000, 300−302, 285−304. (21) Sakai, Y.; Yoza, K.; Kato, C. N.; Nomiya, K. A First Example of Polyoxotungstate-Based Giant Molecule. Synthesis and Molecular Structure of a Tetrapod-Shaped Ti−O−Ti Bridged Anhydride Form of Dawson Tri-Titanium(IV)-Substituted Polyoxotungstate. Dalton Trans. 2003, 3581−3586. (22) Sakai, Y.; Yoza, K.; Kato, C. N.; Nomiya, K. Tetrameric, Trititanium(IV)-Substituted Polyoxotungstates with an α-Dawson Substructure as Soluble Metal-Oxide Analogues: Molecular Structure of the Giant “Tetrapod” [(α-1,2,3-P 2 W 1 5 Ti 3 O 6 2 ) 4 {μ 3 -Ti(OH)3}4Cl]45−. Chem. - Eur. J. 2003, 9, 4077−4083. (23) Kortz, U.; Hamzeh, S. S.; Nasser, A. Supramolecular Structures of Titanium(IV)-Substituted Wells−Dawson Polyoxotungstates. Chem. - Eur. J. 2003, 9, 2945−2952. (24) Sakai, Y.; Yoshida, S.; Hasegawa, T.; Murakami, H.; Nomiya, K. Tetrameric, Tri-Titanium(IV)-Substituted Polyoxometalates with an α-Dawson Substructure as Soluble Metal Oxide Analogues: Synthesis and Molecular Structure of Three Giant “Tetrapods” Encapsulating Different Anions (Br−, I− and NO3−). Bull. Chem. Soc. Jpn. 2007, 80, 1965−1974. (25) Sakai, Y.; Ohta, S.; Shintoyo, Y.; Yoshida, S.; Taguchi, Y.; Matsuki, Y.; Matsunaga, S.; Nomiya, K. Encapsulation of Anion/ Cation in the Central Cavity of Tetrameric Polyoxometalate, Composed of Four Trititanium(IV)-Substituted α-Dawson Subunits, Initiated by Protonation/Deprotonation of the Bridging Oxygen Atoms on the Intramolecular Surface. Inorg. Chem. 2011, 50, 6575− 6583. (26) Sakai, Y.; Kitakoga, Y.; Hayashi, K.; Yoza, K.; Nomiya, K. Isolation and Molecular Structure of a Monomeric, Tris[peroxotitanium(IV)]-Substituted α-Dawson Polyoxometalate Derived from the Tetrameric Anhydride Form Composed of Four Tris[titanium(IV)]-Substituted α-Dawson Substructures and Four Bridging Titanium(IV) Octahedral Groups. Eur. J. Inorg. Chem. 2004, 2004, 4646−4652. (27) Matsuki, Y.; Hoshino, T.; Takaku, S.; Matsunaga, S.; Nomiya, K. Synthesis and Molecular Structure of a Water-Soluble, Dimeric TriTitanium(IV)-Substituted Wells−Dawson Polyoxometalate Containing Two Bridging (C5Me5)Rh2+ Groups. Inorg. Chem. 2015, 54, 11105−11113. (28) Yadav, S.; Swamy, V. S. V. S. N.; Gonnade, R.; Sen, S. S. Benz− amidinato Stabilized a Monomeric Calcium Iodide and a Lithium Calciate(II) Cluster featuring Group 1 and Group 2 Elements. ChemistrySelect 2016, 1, 1066−1071. (29) Westerhausen, M. Recent Developments in the Organic Chemistry of Calcium-An Element with Unlimited Possibilities in Organometallic Chemistry? Z. Anorg. Allg. Chem. 2009, 635, 13−32. (30) Fromm, K. M.; Gueneau, E. D.; Bernardinelli, G.; Goesmann, H.; Weber, J.; Mayor-López, M.-J.; Boulet, P.; Chermette, H. Understanding the Formation of New Clusters of Alkali and Alkaline

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant No. 22550065 and also by funds from the Strategic Research Base Development Program for Private Universities (Ministry of Education, Culture, Sports, Science and Technology of Japan).

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