Thermolabile Organotitanium Monoalkyl Phosphates: Synthesis

Article pubs.acs.org/IC

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

Thermolabile Organotitanium Monoalkyl Phosphates: Synthesis, Structures, and Utility as Epoxidation Catalysts and Single-Source Precursors for TiP2O7 Gulzar A. Bhat,† Sonam Verma,† Antony Rajendran, and Ramaswamy Murugavel* Department of Chemistry, Indian Institute of Technology Bombay (IIT-Bombay), Powai, Mumbai 400076, India

Downloaded via UNIV OF TOLEDO on June 16, 2018 at 12:39:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The reaction of [Cp*TiCl3] (Cp* = C5Me5) with monoalkyl phosphates (RO)PO3H2 (R = Me, Et, and i Pr) in tetrahydrofuran (THF) at 25 °C leads to the formation of binuclear complexes [Cp*2Ti2(μ-O2P(OH)OR)2(μ-O2P(O)OR)2] [R = Me (1), Et (2), and iPr (3)]. On the other hand, the reaction of (tBuO)2PO2K with [Cp*TiCl3] in acetonitrile or THF results in isolation of either the dinuclear [Cp*2Ti2(μ-O2P(OH)OtBu)2(μ-O2P(O)OtBu)2] (4) or the trinuclear titanophosphate [Cp*3Ti3(μ-O3POtBu)2(μ-O)2(μO2P(OtBu)2)] (5), respectively. The formation of compounds 4 and 5 is facilitated by partial hydrolysis of the tert-butoxy groups of (tBuO)2PO2K. New titanophosphates 1−5 have been characterized by spectroscopic and analytical methods, and the molecular structures have further been confirmed by single-crystal X-ray diffraction studies. Thermal decomposition studies of 1−5 reveal the initial loss of thermally labile alkyl substituents of the organophosphate ligands, followed by the loss of C5Me5 groups to form an organic-free amorphous titanophosphate in the temperature range 300−500 °C. This material transforms to highly crystalline titanium pyrophosphate TiP2O7 at 800 °C. Compounds 1−5 and the TiP2O7 materials obtained at 300, 500, and 800 °C through the thermal decomposition of 3 have been employed as efficient homogeneous catalysts for the alkene epoxidation reaction. Using hydrogen peroxide as the oxidant in an acetonitrile medium, these catalysts exhibit >90% alkene conversion with >90% epoxide selectivity in 4 h at temperatures below 100 °C.

■

INTRODUCTION Titanium-incorporated silicates, phosphates, and phosphonates find useful applications in the field of catalysis, covering the oxidation of alcohols,1 epoxidation of alkenes,2 conversions of amines to oximes,3 and polymerization.4 Apart from catalysis, interest in solid-state titanium-containing ceramic materials continues to be driven by their varied technological applications. For example, Nasicon-type [KTi2(PO4)3] is being used as a fast ion conductor and low thermal expansion ceramics.5 Similarly, potassium titanyl phosphate (KTP or KTiOPO4) has been explored as a very useful nonlinear optical material,6 while layered titanophosphates have been employed as ion exchangers for various alkali- and alkaline-earth-metal ions and as electrode materials.7 Molecular metal silicates and phosphates are structural and functional mimics for the catalytic sites of many heterogeneous titanium-doped catalytic materials and, hence, they have the potential to act as models of metal species bound on oxo surfaces of heterogeneous catalysts.3b,8 Since the isolation of the first molecular titanophosphate,9 interest in molecular titanophosphates has gained momentum because of their potential applications as single-source molecular precursors for structurally controlled inorganic materials.10 Significantly, © XXXX American Chemical Society

Tilley et al. have reported on the synthesis of titanophosphates [Ti(OR)3O2P(OtBu)2]n (R = Et and iPr) and [Ti2K(OEt)8O2P(OtBu)2]2 starting from di-tert-butyl phosphate, which, in principle, can serve as ideal precursors for ceramic phosphates including KTP.10 Apart from the thermally labile di-tert-butyl phosphate, the dimethyl and dibenzyl counterparts have also been incorporated in molecular titanium phosphates.11 While there have been no reported synthetic protocols, until recently,12 for the synthesis of multigram quantities of stable monoalkyl phosphates, our group has been employing a lipophilic monoaryl phosphate, 2,6-diisopropylphenyl phosphate (dippH2), for over a decade to synthesize a large number of molecular metal phosphates,13 which also include examples of tri-, tetra-, and pentanuclear titanophosphate clusters.14 However, the use of monoalkyl phosphates (instead of monoaryl phosphates) would offer the unique advantage of yielding organic-soluble molecular systems that are also thermally highly labile and decompose at relatively low temperatures to produce ceramic materials. Received: March 8, 2018

A

DOI: 10.1021/acs.inorgchem.8b00611 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of Dimeric Titanophosphates 1−3

Scheme 2. Synthesis of Dimeric (4) and Trimeric (5) Titanophosphates

remove Et3N·HCl, followed by crystallization of the obtained residual solid in toluene at −30 °C, resulted in the isolation of dinuclear titanium(IV) phosphates [Cp*2Ti2(μ-O2P(OH)OR)2(μ-O2P(O)OR)2] [R = Me (1; CCDC 1827592), Et (2; CCDC 1827593), and iPr (3; CCDC 1827594)] as red crystals (see Scheme 1). Compounds 1−3 have been characterized by spectroscopic and analytical methods. Their molecular structures have been confirmed by single-crystal X-ray diffraction studies. In the Fourier transform infrared (FT-IR) spectra of 1−3, bands observed in the range 3000−2850 cm−1 correspond to C−H stretching of the alkyl groups. The absorption bands appearing at 1157, 1114, and 1061 cm−1 for 1, 1127, 1047, and 999 cm−1 for 2, and 1143, 1123, and 991 cm−1 for 3 are assigned to P O and P−O−M stretching (antisymmetric and symmetric) frequencies, respectively (Figure S1). The 1H NMR spectra obtained for 1−3 (Figures S2−S4) are consistent with their molecular structures obtained from single-crystal X-ray diffraction studies described below. In the 1H NMR spectrum of 1, the singlet appearing at 3.71 ppm corresponds to the methyl protons, while the methylene and methyl protons of 2 resonate at 4.07 ppm (as a quartet) and at 1.31 ppm (as a triplet), respectively (Figures S2 and S3). Similarly, the septet appearing at 4.68 ppm in 3 corresponds to the methine proton of iPr substituents on (iPrO)PO3H2, while the doublet appearing at 1.34 ppm is assigned to the methyl protons (Figure S4). The singlets appearing at 2.16 ppm in 1, 2.15 ppm in 2, and 2.14 ppm in 3 correspond to methyl protons of the Cp* ligand. Further evidence for the formation of dititanium phosphates 1−3 comes from the observation of a single

In this context, we envisaged that the recent isolation and structural characterization of a series of monoalkyl phosphates, (RO)PO3H2 (R = Me, Et, iPr, and tBu),12 can be effectively exploited in molecular titanophosphate chemistry by synthesizing molecules that contain M:P stoichiometries that are different from the earlier reported examples. This can lead to the realization of single-source precursors for less-common variants of titanophosphate ceramics. This is particularly important in the current context, where the relatively less investigated form of titanium phosphate, viz., the pyrophosphate TiP2O7, has been extensively investigated as a possible electrode material in lithium/sodium-ion-type batteries.7c,g,15 Given this background on titanophosphate chemistry, we have investigated in the present study the reactions of the organotitanium precursor [Cp*TiCl3] with monoalkyl phosphates (RO)PO3H2 (R = Me, Et, and iPr) and the potassium salt of dialkyl phosphate (tBuO)2PO2K to produce the first examples of titanium monoalkyl phosphates. The new titanium oxo clusters were further employed as homogeneous catalysts for alkene epoxidation using hydrogen peroxide (H2O2) as the oxidant. They also serve as ideal starting points for the preparation of TiP2O7 ceramic materials at relatively low temperatures. The results obtained are detailed below.

■

RESULTS AND DISCUSSION Synthesis and Characterization of Dimeric Titanium Monoalkyl Phosphates. Monoalkyl phosphates (RO)PO3H2 (R = Me, Et, and iPr) react with [Cp*TiCl3] in a tetrahydrofuran (THF) medium in the presence of triethylamine (Et3N) as an HCl scavenger. Standard workup to B


Article

Inorganic Chemistry

Figure 1. Molecular structures of (a) 3 and (b) 4 (C−H hydrogen atoms have been omitted for clarity).

at 3432 cm−1 in 4 confirms the presence of free P−OH groups. On the other hand, the spectrum of 5 is devoid of any such absorption in this region, suggesting complete neutralization of all P−OH groups (Figure S1). The observed 1H NMR spectral patterns of 4 and 5 and the ratio of their integrated intensities are consistent with their molecular structures, as confirmed by single-crystal X-ray diffraction (Figures S12 and S13). The singlet appearing at 1.5 ppm in 4 corresponds to the tert-butyl protons, whereas the two singlets of similar intensities appearing at 1.57 and 1.62 ppm in 5 confirm the presence of tert-butyl protons in different chemical environments in accordance with the crystal structure. The singlet appearing at around 2.13 ppm in 4 corresponds to methyl protons of Cp*, while two singlets of different intensities (2:1) appearing at around 2.20 and 2.29 ppm in 5 are assigned to methyl groups of the Cp* ligand. The most convincing evidence for the formation of 4 comes from the observation of a single resonance in the 31P NMR spectrum at −1.54 ppm; two resonances of different intensities (1:2) that appear at −14.76 and −0.91 ppm in 5 indicate that the three phosphate ligands attached to the three titanium centers are present in two different chemical environments (Figure S5). These compounds have further been characterized by 13C NMR spectroscopy, which also strongly supports the structure assigned for these compounds (Figures S14 and S15). Compound 4 remains intact under positive-ion-mode ESIMS spectral conditions and, hence, exhibits the molecular-ion peak (M + H)+ at m/z 977 (Figure S16). On the other hand, cluster 5 appears to be unstable under ESI-MS conditions and, hence, does not yield the molecular-ion peak. Compounds 4 and 5 feature partial hydrolysis of the alkyl substituents on the phosphate ligand; in 4, the phosphate ligand has become mono-tert-butyl phosphate rather than the originally used ditert-butyl phosphate. From the 31P NMR data, it is clear that the resonance undergoes a downfield shift with a decrease in the bulkiness of the alkyl groups on the phosphate ligand (Figure S5). Molecular Structures of the Dinuclear [Cp*2Ti2(μO2P(OH)OR)2(μ-O2P(O)OR)2] [R = Me (1), Et (2), iPr (3), and tBu (4)]. Red single crystals of 1−3 were obtained from a toluene solution by cooling in a deep freezer at −30 °C for 1

resonance in the 31P NMR spectra for 1−3 at 3.13, 1.96, and 1.37 ppm, respectively (Figure S5), due to the hydrogen-bondaided fast exchange of the P−OH protons between the two phosphate ligands. Compounds 1−3 have further been characterized by 13C NMR spectroscopy, which is also consistent with the molecular structures shown in Scheme 1 (Figures S6−S8). The electrospray ionization mass spectrometry (ESI-MS) spectra of 1−3 show the molecular-ion peaks [M + H]+ at m/z 809, 865, and 921 respectively. The simulated and experimental isotopic patterns for all three compounds are in good agreement with each other, further confirming the stability and structural integrity of these compounds in solution (Figures S9−S11). Because the tert-butyl phosphate (tBuO)PO3H2 is unstable under ambient conditions (although it can be isolated in the solid state),12 in order to synthesize the tert-butyl analogue of the dinuclear complexes 1−3, we have employed the more stable potassium salt of di-tert-butyl phosphate (tBuO)2PO2K as the starting material. However, the reactions of [Cp*TiCl3] with (tBuO)2PO2K has been found to be sensitive to the solvent medium used. For example, when acetonitrile was employed as the solvent, the dititanium tert-butylphosphate complex [Cp*2Ti2(μ-O2P(OH)OtBu)2(μ-O2P(O)OtBu)2] (4; CCDC 1827595) was obtained as the sole product. When the solvent medium is switched to THF, the same reaction yields the trititanium phosphate [Cp*3Ti3(μ-O3POtBu)2(μ-O)2(μO2P(OtBu)2)] (5; CCDC 1827596), possessing mono- and ditert-butyl phosphate ligands. Both 4 and 5 were obtained as single crystals when crystallized from CDCl3 and toluene, respectively (Scheme 2). Compounds 4 and 5 were characterized by various spectroscopic and analytical techniques, apart from the confirmation of their molecular structures by single-crystal Xray diffraction studies. In the FT-IR spectra of 4 and 5, the presence of the alkyl groups is confirmed by two sharp bands observed in the region 2850−3000 cm−1 due to C−H stretching (Figure S1). The peaks observed at 1183, 1068, and 1040 cm−1 in 4 and at 1115, 1019, and 990 cm−1 in 5 are assigned to PO and P−O−M stretching (antisymmetric and symmetric) frequencies, respectively. The broad peak centered C


Article

Inorganic Chemistry

Figure 2. Core structure of the cage cluster Ti2O12P4 for 1−4 (C−H hydrogen atoms have been omitted for clarity).

Table 1. Comparison of the Structural Parameters of 1−4 with the Earlier Reported Dinuclear Titanium Phosphates bond distance [Å]

bond angle [deg]

formula

Ti−O

P−O

P−O−Ti

O−P−O

[(C5Me4Et)TiF(μ-F){μ-O2P(OSiMe3)2}]2 [{Ti(C5Me4Et)(μ-O2P(OH)Me)(μ-O2P(O)Me)}2] [[Cp*TiF(μ-F)(OPOPh2)]2 [Cp*2Ti2(μ-O2P(OH)OMe)2(μ-O2P(O)OMe)2] (1) [Cp*2Ti2(μ-O2P(OH)OEt)2(μ-O2P(O)OEt)2] (2) [Cp*2Ti2(μ-O2P(OH)O iPr)2(μ-O2P(O)O iPr)2] (3) [Cp*2Ti2(μ-O2P(OH)OtBu)2(μ-O2P(O)OtBu)2] (4)

2.042(2) 1.978(3) 2.035(2) 2.017(3) 2.000(3) 2.009(4) 2.011(3)

1.532(2) 1.523(3) 1.515(3) 1.530(3) 1.525(3) 1.526(4) 1.529(3)

132.48(2) 139.09(1) 136.36(1) 114.83(2) 115.14(2) 114.82(2) 114.42(2)

111.29(2) 112.53(1) 114.12(1) 107.94(2) 107.96(2) 108.2(2) 107.79(2)

ref 17 18 19 this this this this

work work work work

The inner inorganic core Ti2O12P4 is shielded by the hydrophobic sheaths of titanium-bonded Me5C5 ligands and four alkyl groups of phosphates, which increases the solubility of 1−4 in organic solvents. Selected bond lengths and angles along with a comparison with similar titanium complexes are presented in Table 1. The phase purities of the bulk samples were confirmed by powder X-ray diffraction (PXRD). The positions of the diffraction peaks of both the experimental and simulated (from single-crystal X-ray diffraction data) PXRD patterns match well (Figures S18−S21), confirming the purities of the bulk samples. Molecular Structure of 5. Yellow single crystals of 5 were obtained in toluene at −30 °C. Compound 5 is an asymmetrical trinuclear titanophosphate cluster that crystallizes in the centrosymmetric P1̅ space group. Compound 5 is made up of three pentamethylcyclopentadienyltitanium units, three phosphate ligands, and two μ-oxo groups (Figure 3). The adventitious water responsible for producing the oxo ligands in 5 comes from the small amounts of hydrogen-bonded/ coordinated water present in the potassium salt of di-tertbutylphosphate. The three titanium(IV) ions in 5 are bridged by one di-tertbutyl phosphate, two mono-tert-butyl phosphates, and two μoxo ligands, which are surrounded by three hydrophobic titanium-bounded Cp* ligands. Out of the three phosphate ligands, the two mono-tert-butyl phosphates connect to all

week, whereas red crystals of 4 were obtained from a CDCl3 solution under similar conditions. Single-crystal X-ray diffraction studies revealed that compounds 1−4 are centrosymmetric dititanium clusters composed of two pentamethylcylopentadienyltitanium units and four monoalkyl phosphates. Compounds 1 and 4 crystallize in the orthorhombic Pccn and monoclinic P21/n space groups, whereas 2 and 3 crystallize in the triclinic space group P1̅. Perspective views of the molecular structures of 1−4 are shown in Figure 1 or in Figure S17. Both titanium(IV) centers in these four compounds have similar coordination environments in which two pentamethylcylopentadienyltitanium units are bridged by two bidentate monoanionic and two dianionic monoalkyl phosphates that exhibit similar coordination behavior (Harris notation [2.110]),16 resulting in the formation of a cage cluster. The geometry around the titanium(IV) ions is distorted octahedral, a geometry commonly observed for other Cp*−TiIV complexes reported in the literature.14a Strong intramolecular hydrogen-bonding interactions were observed between the uncoordinated P−OH groups of monoanionic phosphates with phosphoryl oxygen atoms of dianionic phosphates [O7−H7···O3 = 2.523(5) Å, 175.69(3)°], which contributes to the stability of these cage complexes. The inorganic cage cluster Ti2O12P4 of 1−4 (Figure 2) contains one nonplanar Ti2O2 four-membered ring, two TiO2P four-membered rings, and one Ti2O4P2 eight-membered ring. D


Article

Inorganic Chemistry

Figure 3. Molecular structure of 5 (C−H hydrogen atoms have been omitted for clarity).

Figure 4. View of the cage cluster Ti3O10P3 of 5.

absorption bands can be attributed to π−π* and n−π* electronic transitions, respectively. The absorption maxima of the π−π* transition for 1−4 occur at 253, 254 255, and 257 nm, respectively, and the n−π* electronic transitions occur at 442, 440, 435, and 415 nm, respectively. The absorption maxima for the pure ligands (RO)PO3H2 (R = Me, Et, and iPr) and (tBuO)2PO2K occur at 230, 229, 228, and 260 nm, respectively (Figure S24). Thermal Decomposition Studies. Metal complexes of ditert-butyl phosphate undergo facile low-temperature decomposition due to inherent β-elimination, leading to the formation of pure ceramic phosphates along with the evolution of volatile isobutene as the byproduct.20 Titanium monoalkyl phosphates 1−4 should also be amenable as single-source precursors because they contain lesser organic content on the phosphorus centers. Thermal decomposition studies of 1−5 were carried out under a flow of dry nitrogen gas at a heating rate of 10 °C min−1. Because all four compounds show similar decomposition behavior, decomposition of 3 is discussed in detail, while the thermogravimetric analysis (TGA) traces of the other compounds are presented in Figures S25−S28. The weight loss of ∼25−27% in the temperature range 100−230 °C can be attributed to the loss of isopropyl substituents on phosphate as volatile CH3CHCH2 gas due to β-elimination. Upon a further increase in temperature (up to 400 °C), a loss of the pentamethylcyclopentadiene groups and water molecules (due to condensation of the P−OH groups produced) occurs to produce TiP2O7 as the final ceramic material at around 550 °C, which is found to be stable at least up to 900 °C under the TGA conditions employed (Figure 5). A suggested decomposition pathway is schematically shown in Scheme S1. The differential scanning calorimetry trace of 3 also supports this decomposition pathway (Figure S29). Further, in order to determine the carbon-based impurities in the decomposition products of 3, its TGA isotherm has been carried out under air for a period of 6 h by holding the samples at 300, 500, and 800 °C (Figure S30). The purities of the products were further confirmed by microanalysis data, which indicate that the sample heated at 300 °C shows 9.39% carbon while that heated at 500 °C contains very little carbon (0.19%). The sample heated at 800 °C is, however, completely free from any carbonaceous material (Figures S36−S38 and Table S9).

three titanium(IV) centers with Harris notation [3.111]. On the other hand, the lone di-tert-butyl phosphate ligand bridges Ti1 and Ti2 only with Harris notation [2.110].16 The μ-oxo ligands O1 and O2 bridge Ti1 with Ti2 and Ti3. As in the case of compounds 1−4, the titanium ions exhibit a distorted square-pyramidal-type geometry, where the metal ion sits above the plane formed by four oxygen atoms (0.751 Å for Ti1). While two of the three titanium ions, Ti2 and Ti3, exhibit very similar coordination environments, a slightly different coordination geometry is observed for Ti1. Coordination around Ti1 is provided by two oxygen atoms of the bidentate mono-tert-butyl phosphate and two bridging μ-oxo ligands, O1 and O2, besides the capping pentamethylcyclopentadienyltitanium unit. The coordination environments around both Ti2 or Ti3 are provided by two oxygen atoms from bridging dianionic bidentate tert-butyl phosphate and one oxygen atom from bridging di-tert-butyl phosphate, besides the capping pentamethylcyclopentadienyltitanium unit. The inorganic Ti3O10P3 cage in 5 is made of the fusion of four nonplanar Ti2O3P six-membered rings, two puckered Ti2O4P2 eight-membered rings, and one Ti3PO4 eightmembered ring (Figure 4). Thus, there are three types of Ti···Ti distances in 5 (Ti1···Ti2 3.381 Å, Ti1···Ti3 3.367 Å, and Ti2···Ti3 4.003 Å). Similarly, the Ti···P distances also vary over the range 3.018−3.324 Å. The periphery of this core is surrounded by a hydrophobic sheath of pentamethylcyclopentadienyl and tert-butoxy groups of the phosphate ligand, which elucidates its high solubility in organic solvents. The Ti−O bond lengths are comparable with similar cage clusters reported in the literature.14a The phase purity of 5 has been confirmed by a comparison of the PXRD data of the bulk sample with that of the simulated spectrum from the single-crystal X-ray diffraction data (Figure S22). A comparison of selected bond lengths, angles, and hydrogenbonding interactions of 1−5 is presented in Tables S1−S7. Absorption Spectra. Absorption spectra for compounds 1−4 were recorded in methanol under ambient conditions. Because of the instability of 5 under aerobic conditions, its spectrum could not be studied. The absorption spectra of 1−4 exhibit two prominent absorption bands, which are observed in the ranges 253−257 and 415−442 nm (Figure S23). These E


Article

Inorganic Chemistry

Figure 5. TGA of compound 3.

To verify the above in situ decomposition process and to further characterize the product(s) formed during decomposition under TGA conditions, bulk thermolysis of 3 was carried out in a tubular furnace by heating samples of 3 at 300, 500, and 800 °C, respectively, for an identical period of time (6 h). The samples formed were examined by PXRD (Figure 6)

Figure 7. SEM images of the decomposition (800 °C) product of 3 at (a) 20000× and (b) 350000×. (c) Energy-dispersive X-ray spectrum with elemental mapping. The inset represents a mixture of all elements.

electrocatalysis.7c,d,15b,21 The synthesis of this material in the present study from organometallic precursors in an amorphous form at temperatures as low as 400 °C and in crystalline form at 800 °C would pave the way for additional applications, of which we have demonstrated their use in an alkene epoxidation reaction as efficient catalysts (vide infra). Catalytic Studies. In spite of the advances made during the last several decades on the titanium-catalyzed olefin epoxidation reaction,22−25 there is still considerable interest in unraveling newer titanium-based homo- and heterogeneous catalysts that use environmentally friendly H2O2 as the oxidant. For example, titanium oxo clusters have generally been employed in epoxidation reactions compared to other titanium compounds for the reason that titanium oxo clusters are considered to be model compounds for bulk TiO2.26 Recently, we have employed the hexameric titanium phosphonate cluster [Ti6(acac)6(μ-O)2(OiPr)2(μ-tBuPO3)6] for cyclohexene epoxidation using cumene hydroperoxide as the oxidant.27 In a quest to prepare novel titanium oxo clusters that can act as good epoxidation catalysts in the presence of environmentally benign oxidants like H2O2, in the present investigation, we have investigated the catalytic activity of 1−5 for epoxidation in the presence of a less toxic and more economical oxidant H2O2. Keeping better conversion and epoxide selectivity in mind, we ran a number of trial reactions by varying the time, temperature, oxidants, and solvents. Because compound 3 is obtained in better yield (62%), it is chosen as the catalyst for performing the optimization reactions. The time-dependent study of cyclooctene epoxidation (Scheme 3) is carried out by analyzing the aliquots of the catalytic reaction mixture at different time intervals. The maximum conversion (96%) and epoxide selectivity (97%) are reached at 4 h, and any further prolongation of the reaction time does not produce any further conversion or selectivity (Figure 8). The optimum temperature

Figure 6. PXRD pattern of as-synthesized 3, the decomposition products obtained after pyrolysis at 300, 500, and 800 °C, and the JCPDS data for TiP2O7.

and IR spectroscopy (Figure S31), besides morphological characterization by scanning electron microscopy (SEM) analysis (Figures 7 and S32 and S33). The Brunauer− Emmett−Teller surface area of the decomposed products was measured at 77 K and 1 bar. The surface areas were found to be 0.0, 83.8, and 10.6 m2 g−1 for samples pyrolyzed at 300, 500, and 800 °C, respectively (Figure S34). The thermolysis products obtained after heating the samples at 300 and 500 °C have been found to be amorphous, while the white powder obtained after heating 3 at 800 °C is highly crystalline, as established by the PXRD studies. The PXRD pattern of the pyrolyzed sample at 800 °C matches exactly with the JCPDS data reported for TiP2O7 (Figure 6).7g It is instructive to note that this phase of titanium phosphate has been implicated in recent years as electrode materials for batteries as well as F


Article

Inorganic Chemistry

presence of different oxidants like H2O2, tert-butyl hydroperoxide, and cumene hydroperoxide, as shown in Figure 10.

Scheme 3. Cyclooctene Epoxidation Catalyzed by TitaniumBased Catalysts

Figure 10. Epoxidation of cyclooctene using catalyst 3 in the presence of different oxidants.

Figure 8. Epoxidation of cyclooctene at different time intervals using catalyst 3 (cyclooctene, 5 mmol; H2O2, 10 mmol; catalyst, 0.01 mmol; acetonitrile, 10 mL; temperature, 70 °C).

The results revealed that H2O2 is a better oxidant compared to other organic peroxides. The added advantage with H2O2 is that it leaves only water at the end of the epoxidation. We have also observed that, in the absence of an oxidant, 3 does not exhibit any notable epoxidation activity (Figure 10). On the basis of these studies, the catalytic activities of 1−5 in cyclooctene epoxidation were investigated using the optimized reaction conditions. It is found that all of the catalysts 1−5 exhibit almost equal catalytic activities and epoxide selectivities (Figure 11). The catalytic activity of

for cyclooctene epoxidation has been achieved by carrying the reactions at different temperatures (30−100 °C), and it has been found that the maximum conversion and epoxide selectivity are obtained when the reaction is conducted at 70 °C (Figure 9). Beyond this temperature, it has been found that

Figure 9. Epoxidation of cyclooctene at different temperatures using catalyst 3 (cyclooctene, 5 mmol; H2O2, 10 mmol; catalyst, 0.01 mmol; acetonitrile, 10 mL; time, 4 h).

the epoxide is converted into a diol, thus decreasing the epoxide selectivity. Hence, 70 °C is set as the reaction temperature for further catalytic studies. In homogeneous catalysis, the role of the solvent is pivotal because it brings the catalyst and substrate together into a uniform phase. Screening of different solvents (water, acetonitrile, methanol, and toluene) led to the conclusion that acetonitrile is the best solvent medium, leading to maximum conversion (96%) and epoxide selectivity (97%). Solvent-free reaction conditions produce many side products along with the desired epoxide, thus reducing the catalyst selectivity (Table S8). Oxidant optimization was achieved by performing the cyclooctene epoxidation reaction in the

Figure 11. Comparison of the catalytic activities of 1−5 and decomposition products of 3 after heating at 300, 500, and 800 °C.

thermally decomposed products of 3 (at 300, 500, and 800 °C) has also been explored under heterogeneous conditions. As can be seen from Figure 11, these heterogeneous systems perform as well as the parent homogeneous catalyst 3. Besides cyclooctene epoxidation, the substrate scope of the epoxidation has been investigated by employing 1-hexene, cyclohexene, norbornene, α-pinene, styrene, and trans-stilbene under the same optimized reaction conditions, and the results are given in Table 2. It is obvious that the current catalytic system yields G


Article

Inorganic Chemistry

isolated by tuning the reaction conditions. In our previous contribution to titanium chemistry of monoaryl phosphates, we have shown that hierarchical titanium phosphate clusters can be isolated by the proper choice of titanium metal precursors and organophosphate esters. Thus, there is a possibility that titanium clusters of higher nuclearity can also be isolated by employing these monoalkyl phosphates, and currently we are investigating these possibilities. Catalytic studies reveal that these clusters are promising epoxidation catalysts with 96% selectivity in the presence of less toxic H2O2 rather than the commonly used toxic organic peroxides. Thermal decomposition studies of 1−5 reveal facile conversion of these molecular species at low temperatures to carbon-free titanium pyrophosphate TiP2O7. It appears that the use of less expensive and more thermally labile precursors such as titanium alkoxides and β-diketonates in place of Cp*TiCl3 may result in clusters that would more cleanly yield such pyrophosphate ceramics through both thermolysis and hydrolysis pathways. We are currently investigating these possibilities.

Table 2. Epoxidation of Various Alkene Substrates Using Catalyst 3 substrate

product

yield (%)

1-hexene cyclohexene

1-hexene oxide cyclohexene oxide cyclohex-2-en-1-ol cyclooctene oxide cyclooctane-1,2-diol norbornene oxide 5-norbonen-2-ol α-pinene oxide verbenol verbenone styrene oxide benzaldehyde phenylacetaldehyde trans-stilbene oxide benzaldehyde

6 91 4 93 3 89 10 51 13 7 9 20 4 6 12

cyclooctene norbornene α-pinene

styrene

trans-stilbene

selectivity (%) 100 96 4 97 3 90 10 73 18 9 27 61 12 33 67

total yield (%)

TON

6 95

30 475

96

480

99

495

71

355

33

165

18

90

■

promising results for the cyclohexene, cyclooctene, and norbornene epoxidation; contrary to this, only less conversion is noticed in the case of 1-hexene (6%) and trans-stilbene (18%) epoxidation. The obtained product yield (71%) and epoxide selectivity (73%) are satisfactory in the epoxidation of α-pinene. In the case of epoxidation of styrene, less epoxide selectivity (27%) was observed. On the basis of these results, we can conclude that our catalytic system is more suitable for the epoxidation of cycloalkenes rather than the epoxidation of linear alkenes. This is also supported by a recent review on titanium silicates (TSs) as catalysts for alkene epoxidation by Přech, where the author has compared the activity of all well-known TS catalysts, and our results are much better than the ones already reported in this review.28 The other well-known titanium-based epoxide catalysts are titanium silsesquioxanes. Our titanium alkyl phosphates also exhibit comparable activity in terms of both conversion and epoxide selectivity with titanium silsesquioxane catalysts.29 Thus, we believe that the present titanium alkyl phosphate catalysts may be greatly emphasized for the epoxidation of cyclohexene/cyclooctene/norbornene in the future. We further propose that the mechanism of epoxidation in the present case is similar to that proposed earlier for titaniumcatalyzed olefin epoxidation reactions conducted in the presence of other organic peroxides.30 In such a mechanism, the formation of titanium peroxo complex is regarded as the key step. To test the applicability of this proposal, the absorption spectrum of 3 was recorded in acetonitrile in the presence of H2O2, producing the characteristic ligand-to-metal charge-transfer band of titanium hydroperoxo species at 370 nm in the UV−vis spectrum (see the Supporting Information). On the basis of this observation, a four-step mechanism can be proposed that involves (1) the formation of a titanium hydroperoxo complex, (2) olefin activation, (3) transfer of oxygen to olefin, and (4) epoxide formation and removal of water from H2O2.

EXPERIMENTAL SECTION

Methods and Materials. All of the experiments were carried out under dry nitrogen using Schlenk techniques by rigorously excluding air and moisture. Solvents were purified according to standard procedures prior to their use. Starting materials such as Cp*TiCl3 were procured from Sigma-Aldrich, whereas the potassium salt of ditert-butyl phosphate was synthesized by using the reported literature procedure.31 (RO)PO3H2 (R = Me, Et, and iPr) were synthesized by a procedure recently reported by us.12 Melting points were measured in glass capillaries and are reported uncorrected. IR spectra were obtained on a PerkinElmer Spectrum One FT-IR spectrometer as KBr diluted disks. Microanalyses were performed on a Thermo Finnigan (FLASH EA 1112) microanalyzer. NMR studies were performed on Bruker Avance DPX-400 and 500 MHz spectrometers. TGA was carried out on a PerkinElmer Pyris thermal analysis system under a stream of nitrogen gas at a heating rate of 10 °C min−1. ESI-MS studies were carried out on a Bruker MaXis impact mass spectrometer. Powder X-ray diffraction was performed on a Philips X’pert Pro (PANalytical) diffractometer using Cu Kα radiation (λ = 1.54190 Å). Bulk thermolysis studies were carried out under air in a tubular furnace at three difference temperatures (300, 500, and 800 °C) by heating the samples for a period of 6 h. Synthesis and Characterization of 1−3. To a stirred solution of Cp*TiCl3 (290 mg, 1 mmol) in THF at room temperature was added dropwise a solution containing (RO)PO3H2 [R = Me (1; 224 mg, 2 mmol), Et (2; 252 mg, 2 mmol), and iPr (3; 280 mg, 2 mmol)] and Et3N (202 mg, 2 mmol) in THF under a positive flow of nitrogen. The red solutions obtained were allowed to stir overnight. After removal of the solvent containing Et3N·HCl, the residue obtained was dissolved in toluene and filtered under a nitrogen atmosphere. Compounds 1−3 crystallize from the filtrate at −30 °C in a deep freezer. Analytically pure red block-shaped crystals were obtained within 1 week in each case. 1. Yield: 42% based on Cp*TiCl3. Mp: >200 °C Anal. Calcd for C24H44O16P4Ti2 (Mr = 808.27; found): C, 35.81 (35.67); H, 5.45 (5.49). FT-IR (as KBr pellets, cm−1): 3444(br), 2957(s), 2919(vs), 1634(s), 1384(s), 1157(s), 1061(s), 992(s), 798(vs), 615(s), 508(s). 1 H NMR (CDCl3, 400 MHz): δ 3.71 (s, 12H), 2.16 (s, 30H). 31P NMR (CDCl3, 202 MHz): δ 3.13. 13C NMR (CDCl3, 125 MHz): δ 137.17 (C), 53.57 (CH3), 12.86 (CH3). ESI-MS. Calcd for C24H44O16P4Ti2 [Mr = 808.27; (M + H)+]: m/z 809.05. Found: m/ z 809.02. 2. Yield: 48% based on Cp*TiCl3. Mp: >200 °C Anal. Calcd for C32H60O16P4Ti2 (Mr = 864.37; found): C, 38.91 (38.96); H, 5.81 (6.06). FT-IR (as KBr pellets, cm−1): 3434(br), 2922(s), 2851(vs),

■

CONCLUSIONS It has been demonstrated in the present study that the dinuclear (1−4) and trinuclear (5) titanophosphates, the first examples of organotitanium monoalkyl phosphates, can be H


Inorganic Chemistry

■

1633(s), 1441(s), 1374(s), 1127(vs), 1047(s), 999(s), 787(vs), 616(s), 505(s). 1H NMR (CDCl3, 400 MHz): δ 4.53 (s, 2H), 4.07 (q, 8H), 2.15 (s, 30H), 1.31 (t, 12H). 31P NMR (202 MHz, CDCl3): δ 1.96. 13C NMR (CDCl3, 125 MHz): δ 136.95 (C), 62.67 (CH2), 16.46 (CH3), 13.02 (CH3). ESI-MS. Calcd for C32H60O16P4Ti2 [Mr = 864.37; (M + H)+]: m/z 865.12. Found: m/z 865.11. 3. Yield: 50% based on Cp*TiCl3. Mp: >200 °C Anal. Calcd for C28H52O16P4Ti2 (Mr = 920.42; found): C, 42.09 (41.76); H, 6.50 (6.57). FT-IR (as KBr pellets, cm−1): 3435(br), 2977(s), 1634(s), 1384(vs), 1313(s), 1123(s), 1034(s), 991(s), 762(s), 643(s), 506(vs). 1H NMR (CDCl3, 400 MHz): δ 4.68 (septet, 4H), 4.19 (s, 2H), 2.14 (s, 30H), 1.34 (d, 24H). 31P NMR (CDCl3, 202 MHz): δ 1.37. 13C NMR (125 MHz, CDCl3): δ 136.35 (C), 70.61 (CH), 23.99 (CH3), 12.89 (CH3). ESI-MS. Calcd for C28H52O16P4Ti2 [Mr = 920.42; (M + H)+]: m/z 921.20. Found: m/z 921.20. Synthesis and Characterization of 4. Cp*TiCl3 (290 mg, 1 mmol) in acetonitrile (20 mL) was added to a stirred solution of (tBuO)2PO2K (744 mg, 3 mmol) in acetonitrile (30 mL). The color of the reaction mixture changed to yellow after stirring for 24 h at 25 °C. The volume of the solution (90 mL) was reduced slowly under vacuum. The yellow precipitate obtained was washed with petroleum ether, later dissolved in chloroform, and filtered through a glass frit under inert conditions. Red microcrystals of 4 were obtained from CDCl3 after 6 days. Yield: 40% based on Cp*TiCl3. Mp: >200 °C. Anal. Calcd for C36H68O16P4Ti2 (Mr = 976.58; found): 43.63 (44.28); H, 7.68 (7.02). FT-IR (as KBr pellets, cm−1): 3432(br), 2971(s), 2925(vs), 1745(s), 1368(s), 1253(s), 1115(s), 1041(s), 1019(s), 803(vs), 694(s), 510(s). 1H NMR (400 MHz, CDCl3): δ 3.03 (s, 2H), 2.13 (s, 30H), 1.50 (s, 36H). 31P NMR (CDCl3, 202 MHz): δ −1.54 (s). 13C NMR (CDCl3, 125 MHz): δ 135.42 (C), 80.04 (C), 30.15 (CH3), 12.83 (CH3). ESI-MS. Calcd for C36H68O16P4Ti2 [Mr = 976.58; (M + H)+] m/z 977.26. Found: m/z 977.32 Synthesis and Characterization of 5. To Cp*TiCl3 (290 mg, 1 mmol) in THF (20 mL) was added (tBuO)2PO2K (744 mg, 3 mmol) in THF (30 mL), and the resulting solution was stirred for 24 h at 25 °C. The solvent was removed in vacuum, and the residue was dissolved in toluene (10 mL), filtered, and left for crystallization. Yellow single crystals of 5 were obtained after 4 days at −30 °C. Yield: 46% based on Cp*TiCl3. Anal. Calcd for C46H81O14P3Ti3 (Mr = 1094.71; found): C, 51.61 (50.47); H, 8.57 (7.46). FT-IR (as KBr pellets, cm−1): 2976(s), 2914(vs), 1367(s), 1249(s), 1183(s), 1040(s), 1012(s), 729(s), 590(s). 1H NMR (400 MHz, C6D6): δ 2.29 (s, 15H), 2.20 (s, 30H), 1.62 (s, 18H), 1.57 (s, 18H). 31P NMR (C6D6, 202 MHz,): δ −0.91 (s), −14.76 (s). 13C NMR (100 MHz, C6D6): δ 124.37 (C), 78.34 (C), 78.25 (C), 30.41 (CH3), 30.36 (CH3), 12.65 (CH3), 12.52 (CH3). Single-Crystal X-ray Diffraction Studies. X-ray diffraction data were collected on a Rigaku Saturn 724+ CCD diffractometer fitted with microfocus Mo Kα (λ = 0.7107 Å) and Cu Kα (1.5418 Å) radiation sources at 150 K. Rigaku Crystal Clear-SM Expert software was used for data collection. Data integration and indexing were performed with the CrysAlisPro software suite. WinGX module was used to perform all of the calculations.32 The structures were solved by direct methods (SIR-92).33 The final structure refinement was carried out using full least-squares methods on F2 using SHELXL2014.34 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were refined isotropically as rigid atoms in their idealized locations. Crystal data and structure refinement details for 1−5 are given in Table S10. Epoxidation of Alkenes. A 25 mL two-neck, round-bottomed flask was sequentially charged with solvent, catalyst, alkene, and oxidant. The resultant catalytic mixture was magnetically stirred at 70 °C for 4 h. From this catalytic reaction mixture, aliquots were taken out and diluted with ethyl acetate prior to analysis using an Agilent Technologies 5975C gas chromatography−mass spectrometry system. Two blank experiments were also carried out (one without oxidant and one without catalyst).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00611. Synthesis, crystallographic details, additional figures, and spectral characterization (PDF) Accession Codes

CCDC 1827592−1827596 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.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ramaswamy Murugavel: 0000-0002-1816-3225 Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by DST Nanomission (Grant SR/ NM/NS-1119/2011) and IIT-Bombay Bridge Funding. R.M. thanks SERB (Grant SB/S2/JCB-85/2014), New Delhi, India, for a J. C. Bose Fellowship, G.A.B. and S.V. thank UGC New Delhi for a research fellowship. A.R. thanks the Department of Science and Technology, SERB, for a National Postdoctoral Fellowship (Grant NPDF/2016/00037).

■

REFERENCES

(1) (a) Murugavel, R.; Roesky, H. W. Titanosilicates: recent developments in synthesis and use as oxidation catalysts. Angew. Chem., Int. Ed. Engl. 1997, 36, 477−479. (2) (a) Fujiwara, M.; Wessel, H.; Hyung-Suh, P.; Roesky, H. W. Formation of titanium tert-butylperoxo intermediate from cubic silicon−titanium complex with tert-butyl hydroperoxide and its reactivity for olefin epoxidation. Tetrahedron 2002, 58, 239−243. (b) Murugavel, R.; Shete, V. S.; Baheti, K.; Davis, P. A novel cyclic titanasiloxane derived from [Ph2Si(OH)]2O: synthesis and crystal structure of [Cp*Ti(Cl)(OSiPh2OSiPh2OSiPh2O)]. J. Organomet. Chem. 2001, 625, 195−199. (c) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Titanium-containing mesoporous molecular sieves for catalytic oxidation of aromatic compounds. Nature 1994, 368, 321−323. (d) Kresge, C.; Leonowicz, M.; Roth, W.; Vartuli, J.; Beck, J. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710−712. (e) Sankar, G.; Thomas, J. M.; Catlow, C. R. A.; Barker, C. M.; Gleeson, D.; Kaltsoyannis, N. The three-dimensional structure of the titanium-centered active site during steady-state catalytic epoxidation of alkenes. J. Phys. Chem. B 2001, 105, 9028−9030. (f) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Heterogeneous catalysts obtained by grafting metallocene complexes onto mesoporous silica. Nature 1995, 378, 159. (g) Notari, B. Titanium silicalites. Catal. Today 1993, 18, 163− 172. (h) Reddy, J. S.; Kumar, R.; Ratnasamy, P. Titanium silicalite-2: Synthesis, characterization and catalytic properties. Appl. Catal. 1990, 58, L1−L4. (3) (a) Fujiwara, M.; Wessel, H.; Park, H. S.; Roesky, H. W. A Sol− Gel Method Using Tetraethoxysilane and Acetic Anhydride: I


Article

Inorganic Chemistry Immobilization of Cubic μ-Oxo Si−Ti Complex in a Silica Matrix. Chem. Mater. 2002, 14, 4975−4981. (b) Voigt, A.; Murugavel, R.; Montero, M. L.; Wessel, H.; Liu, F. Q.; Roesky, H. W.; Usón, I.; Albers, T.; Parisini, E. Soluble molecular titanosilicates. Angew. Chem., Int. Ed. Engl. 1997, 36, 1001−1003. (4) (a) Murugavel, R.; Davis, P.; Shete, V. S. Reactivity Studies, Structural Characterization, and Thermolysis of Cubic Titanosiloxanes: Precursors to Titanosilicate Materials Which Catalyze Olefin Epoxidation. Inorg. Chem. 2003, 42, 4696−4706. (b) Davis, P.; Murugavel, R. Recent Developments in the Chemistry of Molecular Titanosiloxanes and Titanophosphonates. Synth. React. Inorg., Met.Org., Nano-Met. Chem. 2005, 35, 591−622. (c) Zhang, H.; Ji, S.; Cai, X. Synthesis of Some Novel Half Titanocene Catalysts and Their Catalytic Performance for Copolymerization of Ethylene-Propylene. Procedia Eng. 2011, 18, 422−426. (5) Woodcock, D.; Lightfoot, P. Comparison of the structural behaviour of the low thermal expansion NZP phases MTi2(PO4)3 (M= Li, Na, K). J. Mater. Chem. 1999, 9, 2907−2911. (6) (a) Munowitz, M.; Jarman, R.; Harrison, J. Theoretical study of the nonlinear optical properties of potassium titanyl phosphate (KTiOPO4): an extended Hueckel/sum-over-states approach. Chem. Mater. 1992, 4, 1296−1304. (b) Hagerman, M. E.; Poeppelmeier, K. R. Review of the structure and processing-defect-property relationships of potassium titanyl phosphate: A strategy for novel thin-film photonic devices. Chem. Mater. 1995, 7, 602−621. (c) Harrison, W. T.; Phillips, M. L.; Stucky, G. D. Single-Crystal Structures and Optical Properties of the Nonlinear Optical KTiOPO4-Type Materials K0.55Li0.45TiOPO4 and K0.54Li0.46TiOAsO4 Prepared by Molten Salt Ion-Exchange. Chem. Mater. 1997, 9, 1138−1144. (7) (a) Zhang, W.; Koivula, R.; Wiikinkoski, E.; Xu, J.; Hietala, S.; Lehto, J.; Harjula, R. Efficient and selective recovery of trace scandium by inorganic titanium phosphate ion-exchangers from leachates of waste bauxite residue. ACS Sustainable Chem. Eng. 2017, 5, 3103−3114. (b) Alvarez, C.; Llavona, R.; Garcia, J. R.; Suarez, M.; Rodriguez, J. Lamellar inorganic ion exchangers. Hydrogen (1+)/ calcium (2+) ion exchange in. gamma.-titanium phosphate. Inorg. Chem. 1987, 26, 1045−1049. (c) Wen, Y.; Chen, L.; Pang, Y.; Guo, Z.; Bin, D.; Wang, Y.-g.; Wang, C.; Xia, Y. TiP2O7 and Expanded Graphite Nanocomposite as Anode Material for Aqueous Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 8075−8082. (d) Yee, G.; Shanbhag, S.; Wu, W.; Carlisle, K.; Chang, J.; Whitacre, J. F. TiP2O7 exhibiting reversible interaction with sodium ions in aqueous electrolytes. Electrochem. Commun. 2018, 86, 104−107. (e) Singh, B.; Bhardwaj, A.; Gautam, S. K.; Parkash, O.; Kumar, D.; Jadhav, H. S.; Song, S.-J. Synthesis and characterization of MnO-doped titanium pyrophosphates (Ti1-xMnxP2O7; x = 0 − 0.2) for intermediatetemperature proton-conducting ceramic-electrolyte fuel cells. Ionics 2017, 23, 1675−1684. (f) Zhang, S.; Liu, Y.; Zhang, N.; Zhao, K.; Yang, J.; He, S. Development of a macroporous-spherical polyanionic compound (TiO)2P2O7 as a novel anode material for sodium ion batteries. RSC Adv. 2016, 6, 53296−53302. (g) Senguttuvan, P.; Rousse, G.; Oró-Solé, J.; Tarascon, J.; Palacín, M. A low temperature TiP2O7 polymorph exhibiting reversible insertion of lithium and sodium ions. J. Mater. Chem. A 2013, 1, 15284−15291. (8) (a) Voigt, A.; Murugavel, R.; Chandrasekhar, V.; Winkhofer, N.; Roesky, H. W.; Schmidt, H.-G.; Usón, I. Facile and rational route for high-yield synthesis of titanasiloxanes from aminosilanetriols. Organometallics 1996, 15, 1610−1613. (b) Walawalkar, M. G.; Horchler, S.; Dietrich, S.; Chakraborty, D.; Roesky, H. W.; Schäfer, M.; Schmidt, H.-G.; Sheldrick, G. M.; Murugavel, R. Novel organic-soluble molecular titanophosphonates with cage structures comparable to titanium-containing silicates. Organometallics 1998, 17, 2865−2868. (c) Stauber, J. M.; Cummins, C. C. Terminal Titanyl Complexes of Tri-and Tetrametaphosphate: Synthesis, Structures, and Reactivity with Hydrogen Peroxide. Inorg. Chem. 2017, 56, 3022−3029. (9) Thorn, D. L.; Harlow, R. L. Phosphato-titanium coordination chemistry. New phosphato-bridged chlorotitanium, imidotitanium, and oxotitanium compounds. Inorg. Chem. 1992, 31, 3917−3923.

(10) Lugmair, C. G.; Tilley, T. D. Di-tert-butyl Phosphate Complexes of Titanium. Inorg. Chem. 1998, 37, 1821−1826. (11) (a) Mehring, M.; Guerrero, G.; Dahan, F.; Mutin, P. H.; Vioux, A. Syntheses, Characterizations, and Single-Crystal X-ray Structures of Soluble Titanium Alkoxide Phosphonates. Inorg. Chem. 2000, 39, 3325−3332. (b) Guerrero, G.; Mehring, M.; Mutin, P. H.; Dahan, F.; Vioux, A. Syntheses and single-crystal structures of novel soluble phosphonato-and phosphinato-bridged titanium oxo alkoxides. J. Chem. Soc., Dalton Trans. 1999, 1537−1538. (12) Bhat, G. A.; Kalita, A. C.; Murugavel, R. Intriguing structural chemistry of neutral and anionic layered monoalkylphosphates: singlesource precursors for high-yield ceramic phosphates. CrystEngComm 2017, 19, 5390−5401. (13) (a) Dar, A. A.; Sharma, S. K.; Murugavel, R. Is single-4-ring the most basic but elusive secondary building unit that transforms to larger structures in zinc phosphate chemistry? Inorg. Chem. 2015, 54, 4882−4894. (b) Kalita, A. C.; Murugavel, R. Fluoride Ion Sensing and Caging by a Preformed Molecular D4R Zinc Phosphate Heterocubane. Inorg. Chem. 2014, 53, 3345−3353. (c) Kalita, A. C.; Gogoi, N.; Jangir, R.; Kuppuswamy, S.; Walawalkar, M. G.; Murugavel, R. Ab Initio Chemical Synthesis of Designer Metal Phosphate Frameworks at Ambient Conditions. Inorg. Chem. 2014, 53, 8959−8969. (d) Dar, A. A.; Bhat, G. A.; Murugavel, R. Dimensionality Alteration and Intraversus Inter-SBU Void Encapsulation in Zinc Phosphate Frameworks. Inorg. Chem. 2016, 55, 5180−5190. (e) Kalita, A. C.; Gupta, S. K.; Murugavel, R. A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions. Chem. - Eur. J. 2016, 22, 6863−6875. (f) Bhat, G. A.; Maqbool, R.; Dar, A. A.; Ul Hussain, M.; Murugavel, R. Selective formation of discrete versus polymeric copper organophosphates: DNA cleavage and cytotoxic activity. Dalton Trans. 2017, 46, 13409− 13420. (g) Bhat, G. A.; Rajendran, A.; Murugavel, R. Dinuclear Manganese(II), Cobalt(II), and Nickel(II) Aryl Phosphates Incorporating 4′-Chloro-2,2′:6′,2″-Terpyridine Coligands − Efficient Catalysts for Alcohol Oxidation. Eur. J. Inorg. Chem. 2018, 2018, 795−804. (h) Gupta, S. K.; Kalita, A. C.; Dar, A. A.; Sen, S.; Patwari, G. N.; Murugavel, R. Elusive Double-Eight-Ring Zeolitic Secondary Building Unit. J. Am. Chem. Soc. 2017, 139, 59−62. (i) Murugavel, R.; Kuppuswamy, S. Octameric and decameric aluminophosphates. Angew. Chem., Int. Ed. 2006, 45, 7022−7026. (14) (a) Murugavel, R.; Kuppuswamy, S. Organic-soluble tri-, tetra-, and pentanuclear titanium (IV) phosphates. Inorg. Chem. 2008, 47, 7686−7694. (b) Kalita, L.; Kalita, A. C.; Murugavel, R. Organotitanium phosphates with free P−OH groups: Synthesis, spectroscopy and solid state structures. J. Organomet. Chem. 2014, 751, 555−562. (15) (a) Sun, Y.; Gai, L.; Zhou, Y.; Zuo, X.; Zhou, J.; Jiang, H. Polyhierarchically structured TiP2O7/C microparticles with enhanced electrochemical performance for lithium-ion batteries. CrystEngComm 2014, 16, 10681−10691. (b) Rai, A. K.; Gim, J.; Song, J.; Mathew, V.; Anh, L. T.; Kim, J. Electrochemical and safety characteristics of TiP2O7−graphene nanocomposite anode for rechargeable lithium-ion batteries. Electrochim. Acta 2012, 75, 247−253. (16) Coxall, R. A.; Harris, S. G.; Henderson, D. K.; Parsons, S.; Tasker, P. A.; Winpenny, R. E. Inter-ligand reactions: in situ formation of new polydentate ligands. J. Chem. Soc., Dalton Trans. 2000, 2349−2356. (17) Pevec, A.; Demšar, A.; Pinkas, J.; Necas, M. Synthesis of Organotitanium (IV) Fluoride Phosphates and the Crystal Structure of [(C5Me4Et)TiF(μ-F){μ-O2P(OSiMe3)2}]2. Acta Chim. Slov. 2012, 59, 203−206. (18) Pevec, A.; Demšar, A.; Pinkas, J.; Necas, M. Synthesis, spectroscopic and X-ray characterization of new molecular organotitanium(IV) phosphonate. Inorg. Chem. Commun. 2008, 11, 5−7. (19) Gindl, J.; Said, M. A.; Yu, P.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H. G. Synthesis and Structure of New Dimeric Cyclopentadienyl Titanium Fluorine−Oxygen Systems:[Cp*TiF(μ-F)(μOPOPh2)]2,[Cp*TiF(μ-F)(μ-OSO2-p-C6H4Me)]2 and [Cp*TiF2(μOMe)]2. Isr. J. Chem. 1999, 39, 125−128. J


Article

Inorganic Chemistry (20) (a) Murugavel, R.; Walawalkar, M. G.; Dan, M.; Roesky, H. W.; Rao, C. N. R. Transformations of Molecules and Secondary Building Units to Materials: A Bottom-Up Approach. Acc. Chem. Res. 2004, 37, 763−774. (b) Murugavel, R.; Sathiyendiran, M.; Walawalkar, M. G. Di-tert-butyl phosphate complexes of cobalt (II) and zinc (II) as precursors for ceramic M(PO3)2 and M2P2O7 materials: synthesis, spectral characterization, structural studies, and role of auxiliary ligands. Inorg. Chem. 2001, 40, 427−434. (c) Pothiraja, R.; Sathiyendiran, M.; Butcher, R. J.; Murugavel, R. Cobalt and Manganese Nets via Their Wires: Facile Transformation in Metal− Diorganophosphates. Inorg. Chem. 2004, 43, 7585−7587. (d) Ahn, H. S.; Tilley, T. D. Electrocatalytic water oxidation at neutral pH by a nanostructured Co(PO3)2 anode. Adv. Funct. Mater. 2013, 23, 227− 233. (e) Lugmair, C. G.; Tilley, T. D.; Rheingold, A. L. Zinc Di(tertbutyl)phosphate Complexes as Precursors to Zinc Phosphates. Manipulation of Zincophosphate Structures. Chem. Mater. 1997, 9, 339−348. (21) Wu, W.; Shanbhag, S.; Wise, A.; Chang, J.; Rutt, A.; Whitacre, J. F. High performance TiP2O7 based intercalation negative electrode for aqueous lithium-ion batteries via a facile synthetic route. J. Electrochem. Soc. 2015, 162, A1921−A1926. (22) (a) Ramaswamy, V.; Awati, P.; Ramaswamy, A. Epoxidation of indene and cyclooctene on nanocrystalline anatase titania catalyst. Top. Catal. 2006, 38, 251−259. (b) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen, K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N. Highly selective hydrolytic kinetic resolution of terminal epoxides catalyzed by chiral (salen)CoIII complexes. practical synthesis of enantioenriched terminal epoxides and 1, 2diols. J. Am. Chem. Soc. 2002, 124, 1307−1315. (23) (a) Petasis, N. A.; Hu, Y.-H.; Lu, S.; Bzowej, E.; Fu, D.; Staszewski, J.; Akritopoulouzanze, I.; Patane, M. Cyclopentadienyl titanium derivatives in organic synthesis. Curr. Org. Chem. 1997, 1, 249−286. (b) Antonova, N. S.; Carbó, J. J.; Kortz, U.; Kholdeeva, O. A.; Poblet, J. M. Mechanistic insights into alkene epoxidation with H2O2 by Ti-and other TM-containing polyoxometalates: Role of the metal nature and coordination environment. J. Am. Chem. Soc. 2010, 132, 7488−7497. (c) Thomas, J. M.; Raja, R. The advantages and future potential of single-site heterogeneous catalysts. Top. Catal. 2006, 40, 3−17. (d) Ratnasamy, P.; Srinivas, D.; Knözinger, H. Active sites and reactive intermediates in titanium silicate molecular sieves. Adv. Catal. 2004, 48, 1−169. (24) (a) Fraile, J. M.; García, J. I.; Mayoral, J. A.; Vispe, E. Silicasupported titanium derivatives as catalysts for the epoxidation of alkenes with hydrogen peroxide: A new way to tuneable catalytic activity through ligand exchange. J. Catal. 2000, 189, 40−51. (b) Chang, C.-C.; Jin, F.; Jang, L.-Y.; Lee, J.-F.; Cheng, S. Effect of the grafting agent on the structure and catalytic performance of TiMCM-22. Catal. Sci. Technol. 2016, 6, 7631−7642. (25) (a) Corma, A.; Díaz, U.; Fornés, V.; Jordá, J. L.; Domine, M.; Rey, F. Ti/ITQ-2, a new material highly active and selective for the epoxidation of olefins with organic hydroperoxides. Chem. Commun. 1999, 779−780. (b) Jeong, S.-M.; Burri, A.; Jiang, N.; Park, S.-E. Microwave synthesis of hydrophobic Ti-TUD-1 mesoporous silica for catalytic oxidation of cycloolefins. Appl. Catal., A 2014, 476, 39−44. (c) Wu, P.; Tatsumi, T.; Komatsu, T.; Yashima, T. Postsynthesis, characterization, and catalytic properties in alkene epoxidation of hydrothermally stable mesoporous Ti-SBA-15. Chem. Mater. 2002, 14, 1657−1664. (d) Blasco, T.; Corma, A.; Navarro, M.; Pariente, J. P. Synthesis, characterization, and catalytic activity of Ti-MCM-41 structures. J. Catal. 1995, 156, 65−74. (26) Jiménez-Lozano, P.; Skobelev, I. Y.; Kholdeeva, O. A.; Poblet, J. M.; Carbó, J. J. Alkene Epoxidation Catalyzed by Ti-Containing Polyoxometalates: Unprecedented β-Oxygen Transfer Mechanism. Inorg. Chem. 2016, 55, 6080−6084. (27) Sharma, K.; Antony, R.; Kalita, A. C.; Gupta, S. K.; Davis, P.; Murugavel, R. Complex Structural Landscape of Titanium Organophosphonates: Isolation of Structurally Related Ti4, Ti5, and Ti6 Species and Mechanistic Insights. Inorg. Chem. 2017, 56, 12848− 12858.

(28) Přech, J. Catalytic performance of advanced titanosilicate selective oxidation catalysts − a review. Catal. Rev.: Sci. Eng. 2018, 60, 71−131. (29) (a) Ventura, M.; Tabernero, V.; Cuenca, T.; Royo, B.; Jiménez, G. Cyclopentadienyl−Silsesquioxane Titanium Catalysts: Factors Affecting Their Formation and Activity in Olefin Epoxidation with Aqueous Hydrogen Peroxide. Eur. J. Inorg. Chem. 2016, 2016, 2843− 2849. (b) Zhang, L.; Abbenhuis, H. C.; Gerritsen, G.; Bhriain, N. N.; Magusin, P. C.; Mezari, B.; Han, W.; van Santen, R. A.; Yang, Q.; Li, C. An Efficient Hybrid, Nanostructured, Epoxidation Catalyst: Titanium Silsesquioxane−Polystyrene Copolymer Supported on SBA-15. Chem. - Eur. J. 2007, 13, 1210−1221. (c) Crocker, M.; Herold, R. H. M.; Guy Orpen, A. Synthesis and structural characterisation of tripodal titanium silsesquioxane complexes: a new class of highly active catalysts for liquid phase alkene epoxidation. Chem. Commun. 1997, 2411−2412. (d) Ventura, M.; Mosquera, M. E.; Cuenca, T. s.; Royo, B.; Jiménez, G. Cyclopentadienyl− silsesquioxane titanium complexes: highly active catalysts for epoxidation of alkenes with aqueous hydrogen peroxide. Inorg. Chem. 2012, 51, 6345−6349. (e) Klunduk, M. C.; Maschmeyer, T.; Thomas, J. M.; Johnson, B. F. The influence of steric congestion on the catalytic performance of TiIV active centers in the epoxidation of alkenes. Chem. - Eur. J. 1999, 5, 1481−1485. (f) SkowronskaPtasinska, M. D.; Vorstenbosch, M. L.; van Santen, R. A.; Abbenhuis, H. C. Titanium silsesquioxanes grafted on three-dimensionally netted polysiloxanes: catalytic ensembles for epoxidation of alkenes with aqueous hydrogen peroxide. Angew. Chem., Int. Ed. 2002, 41, 637− 639. (30) (a) Raboin, L.; Yano, J.; Tilley, T. D. Epoxidation catalysts derived from introduction of titanium centers onto the surface of mesoporous aluminophosphate: Comparisons with analogous catalysts based on mesoporous silica. J. Catal. 2012, 285, 168−176. (b) Donoeva, B. G.; Trubitsina, T. A.; Antonova, N. S.; Carbó, J. J.; Poblet, J. M.; Al-Kadamany, G.; Kortz, U.; Kholdeeva, O. A. Epoxidation of Alkenes with H2O2 Catalyzed by DititaniumContaining 19-Tungstodiarsenate (III): Experimental and Theoretical Studies. Eur. J. Inorg. Chem. 2010, 2010, 5312−5317. (c) Antonova, N. S.; Carbó, J. J.; Kortz, U.; Kholdeeva, O. A.; Poblet, J. M. Mechanistic Insights into Alkene Epoxidation with H2O2 by Ti- and other TM-Containing Polyoxometalates: Role of the Metal Nature and Coordination Environment. J. Am. Chem. Soc. 2010, 132, 7488− 7497. (d) Yoon, C. W.; Hirsekorn, K. F.; Neidig, M. L.; Yang, X.; Tilley, T. D. Mechanism of the Decomposition of Aqueous Hydrogen Peroxide over Heterogeneous TiSBA15 and TS-1 Selective Oxidation Catalysts: Insights from Spectroscopic and Density Functional Theory Studies. ACS Catal. 2011, 1, 1665−1678. (e) Talsi, E. P.; Samsonenko, D. G.; Bryliakov, K. P. Titanium Salan Catalysts for the Asymmetric Epoxidation of Alkenes: Steric and Electronic Factors Governing the Activity and Enantioselectivity. Chem. - Eur. J. 2014, 20, 14329−14335. (31) Zwierzak, A.; Kluba, M. Organophosphorus estersI: t-Butyl as protecting group in phosphorylation via nucleophilic displacement. Tetrahedron 1971, 27, 3163−3170. (32) Farrugia, L. WinGX suite for small-molecule single-crystal crystallography. J. Appl. Crystallogr. 1999, 32, 837−838. (33) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. Completion and refinement of crystal structures with SIR92. J. Appl. Crystallogr. 1993, 26, 343−350. (34) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8.

K


Thermolabile Organotitanium Monoalkyl Phosphates: Synthesis

Recommend Documents