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Jan 20, 2014 - Cu(II) paddle wheel secondary building unit controlled formation of a catalytically active metal−organic polyhedron (MOP) derived fro...
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Secondary Building Unit (SBU) Controlled Formation of a Catalytically Active Metal−Organic Polyhedron (MOP) Derived from a Flexible Tripodal Ligand Mithun Paul,†,§ N. N. Adarsh,‡ and Parthasarathi Dastidar*,†,§ †

Department of Organic Chemistry, Indian Association for the Cultivation of Science (IACS), 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, West Bengal, India S Supporting Information *

ABSTRACT: A nanosized truncated octahedron-shaped metal−organicpolyhedron (MOP) namely [{Cu12(TMBTA)8(DMA)4(H2O)8}·8H2O·X] MOP-TO (X = 48 H2O molecules as per SQUEEZE calculation and TGA data) was successfully derived from a flexible C3-symmetric ligand (2,4,6trimethylbenzene)-1,3,5-triacetic acid (TMBTA) and Cu(NO3)2 in dimethyl acetamide (DMA) and EtOH under solvothermal conditions. The nanocage was well-characterized by single crystal X-ray diffraction (SXRD) and electron spray ionization mass spectrometry (ESI-MS). Remarkably, the nanocage molecules could be seen under high-resolution transmission electron microscope (HR-TEM). It was evident that the Cu(II) paddle wheel secondary building unit (CPWSBU) was responsible for the formation of the nanocage, as the corresponding reactions of TMBTA with other metal ions (e.g., Co(II) and Zn(II) resulted in the formation of two coordination polymers, namely [{Co(μ-TMBTA)(H2O)4}]∝ TMBTA-Co(II) and [{(H2O)2Zn(μ-TMBTA)Zn·K}·2H2O]∝ TMBTA-Zn(II). Interestingly, the nanocage MOPTO was exploited in catalyzing (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO)-assisted aerobic oxidation of benzyl alcohol to benzaldehyde.

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and face-directed self-assembly approaches have been exploited.15 Pyridyl-based donors and acceptors like capped metal centers [e.g., Pd(II) or Pt(II)] having predefined geometry were used to achieve various MOPs in most of the cases.16 Paddle-wheel cluster derived from Cu(II) acetate, Cu2(CO2)4, represents one of the most successful SBUs in designing highly microporous functional MOFs17 wherein polytopic carboxylate linkers were used; CPWSBU is comprised of two square planar Cu(II) metal centers connected by four carboxylate linkers offering an octahedral node for generating such microporous MOFs. In polyhedras like truncated octahedron, truncated cuboctahedron, and truncated icosidodecahedron, the vertices of the parent polyhedras are replaced by squares, and CPWSBU can be represented as a square considering the plane connected by the four C atoms of the carboxylate linkers connected to the Cu(II) metal centers (Scheme 1). Thus, CPWSBU has been exploited in generating MOPs.14 In the present work, a conformationally flexible tripodal ligand namely (2,4,6-trimethylbenzene)-1,3,5-triacetic acid (TMBTA) has been utilized to synthesize a discrete truncated

uilding complex architecture with function is one of the major goals in supramolecular chemistry.1 Various noncovalent interactions such as hydrogen bonding, charge-assisted hydrogen bonding, halogen bonding, charge transfer interactions, π−π stacking, etc. have been exploited to generate intriguing supramolecular entities that often displayed useful functions such as sensing,2 catalysis,3 immobilizing liquids (gelation),4 opto-electronic properties,5 drug-delivery,6 biomedical applications,7 etc. One of the most widely used supramolecular tools to construct intriguing supramolecular architecture is metal−ligand coordination. For example, construction of highly porous and/or catalytic metal−organic frameworks (MOFs) also known as coordination polymers is based on the exploitation of metal−ligand coordination.8 Discrete supramolecular species such as metal−organic polyhedra9 (MOP, also known as nanocage, nanoballs, etc.), on the other hand, has attracted much attention during the last decade or so because of their inherent structural complexity, aesthetic appeal and consequently synthetic challenges, and various potential applications such as recognition,10 storage,11catalysis,12 their relevance in biological self-assembly,13 etc. Usually MOPs are synthesized by spontaneous self-assembly of a suitably chosen metal center or secondary building unit (SBU) comprised of a metal−ligand cluster having predefined spatial geometry and rigid ditopic or tritopic ligands with ∼90° or ∼120° ligating topology, respectively.14 Both edge-directed © 2014 American Chemical Society

Received: December 9, 2013 Revised: January 20, 2014 Published: January 20, 2014 1331

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Scheme 1. Truncated Octahedron Containing Six Square Vertices Derived from CPWSBU

octahedron MOP, namely [{Cu12(TMBTA)8(DMA)4(H2O)8}· 8H2O·48H2O] MOP-TO by exploiting CPWSBU. Both single crystal X-ray diffraction (SXRD) and ESI-MS supported the formation of MOP-TO. The diameter of the polyhedral capsule MOP-TO was around 2.6 nm as revealed both by SXRD and HR-TEM. Syntheses of two coordination polymers namely [{Co(μ-TMBTA)(H2O)4}]∝ TMBTA-Co(II) and [{(H2O)2Zn(μ-TMBTA)Zn·K}·2H2O]∝ TMBTA-Zn(II) derived from Co(II) and Zn(II), respectively, emphasized the role of CPWSBU in generating MOP-TO. Remarkably, MOP-TO was able to catalyze TEMPO-assisted aerobic oxidation of benzyl alcohol to benzaldehyde.

Figure 1. Two different conformations syn-syn-syn and syn-syn-anti of the ligand TMBTA.

All Cu(II) metal centers embedded within the cage were coordinated by water molecules, whereas the peripheral Cu(II) were coordinated by DMA and water molecules. Furthermore, eight uncoordinated water molecules were found within the molecular cage, sustained by participating in hydrogen-bonding interactions [O···O = 2.910(6) − 2.948(8) Å] with the adjacent coordinated water molecules. One of the peripherally coordinated DMA molecules was found to be disordered (Figure 2). The cage molecules were packed in 2D running parallel to the a−c plane, sustained by the C−H···π interactions (3.868 Å) involving the benzyl CH and aromatic ring of the adjacent molecules. Such 2D layers were further packed in a parallel fashion via dispersion forces. Overall packing of the molecules revealed the existence of continuous channels running down the a axis, having an approximate size of 7 × 7 Å (Figure 2). Unaccounted electron density peaks were observed within such channels during the final cycles of refinement, which could not be fit into any reasonable model. SQUEEZE18 calculations revealed that there were 481 electrons per asymmetric unit, which were attributed to ∼48 water molecules. Thermogravimetric data analyses revealed a weight loss of 35.5% that occurred within a temperature range of 40.6−238.5 °C; this was attributed to the weight loss of both coordinated and uncoordinated water [16 H2O (located in the difference Fourier map) + 48 H2O (according to SQUEEZE calculation)] and 4 DMA molecules (calculated weight loss of 34.7%). This data corroborated well with the SQUEEZE calculations (Figure S1 of the Supporting Information). Remarkably, the discrete molecule of the nanoball MOP-TO could be seen in HR-TEM. When a dilute solution (0.0597 × 10−2 M) of freshly grown crystals of MOP-TO was drop-casted on a carbon-coated gold grid and observed under HR-TEM, discrete objects having a dimension of ∼2−2.6 nm could be observed (Figure 3). SXRD data was in good agreement with this observation as the dimension of each molecule was found to be in the same range (1.9 × 2.6 nm). Electron diffraction in HR-TEM on the observed nanoparticle did not show any diffraction, indicating that the nanoparticles observed in the HR-TEM were representing a single molecule of the nanocage MOP-TO. Subsequent EDX data showed the existence of the Cu element on the nanoparticle supporting further the molecular nature of the nanoparticles (Figure S4 of the Supporting Information). Existence of the nanocage molecule was also established by

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RESULTS AND DISCUSSION Cambridge Structural Database (CSD) Search. CSD (version 1.15) search revealed that there were 36 crystal structures of MOPs involving CPWSBU (Table S1 of the Supporting Information). Out of these 36 MOPs, 10 structures were of the type of cryptand (having two CPWSBU connected by four ditopic ligands) and 26 structures were of the type of cages or nanoballs (having more than two CPWSBU and many ditopic or tritopic ligands). Majority of the MOPs found in this CSD search represented truncated cuboctahedron (TCO) (16 nos.), and half of that represented truncated octahedron (TO) structures. Close examination of the CSD search pointed to the fact that in most of the cases, the organic ligands used were rigid and had mostly 120° angle between the functional groups. Only in a few cases (4 structures), conformationally flexible ligands were used. Since conformationally flexible ligands have large numbers of degrees of freedom, it is difficult to achieve one particular conformation required for cagelike MOP architecture. The ligand TMBTA, we chose to work with, should display two different types of conformations, namely syn-syn-syn and syn-synanti (Figure 1). Understandably, the syn-syn-syn conformation is required for MOP formation. Single Cryrstal X-ray Crystallography. Single crystals of all the compounds namely MOP-TO, TMBTA-Co(II), and TMBTA-Zn(II) were obtained under solvothermal (experimental) conditions and were subjected to SXRD (Table 1). [{Cu12(TMBTA)8(DMA)4(H2O)8}·8H2O·48H2O]MOP-TO. Reaction of TMBTA with Cu(NO3)2 in DMA and EtOH under solvothermal condition resulted in green-colored block-shaped single crystals. SXRD revealed that the crystals of MOP-TO belonged to the centrosymmetric orthorhombic space group Ccca. The crystal structure of MOP-TO was composed of large discrete molecules constructed from Cu(II)-paddle-wheel SBU (square SBU) connected by the tripodal ligand TMBTA displaying syn−syn−syn conformation. The structure may best be described as a truncated octahedron, wherein 6 vertices and 8 faces of an octahedron were replaced by the squares arising from the paddle-wheel SBUs and tripodal ligands, respectively. 1332

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Table 1. Crystallographic Parameters crystal parameters

MOP-TO

TMBTA-Co(II)

TMBTA-Zn(II)

CCDC no. empirical formula formula weight crystal size (mm) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dcalc/g cm−3 F(000) μ Mo Kα (mm−1) temperature (K) Rint range of h, k, l θmin/max (deg) reflections collected/unique/observed [I > 2σ(I)] data/restraints/parameters goodness of fit on F2 final R indices [I > 2σ(I)]

962728 C136H188Cu12N4O68 3729.38 0.36 × 0.16 × 0.28 orthorhombic Ccca 26.138(3) 35.912(3) 22.594(2) 90.00 90.00 90.00 21208(4) 4 1.168 7696 1.247 100(2) 0.0950 −27/27, −37/38, −23/23 1.13/22.18 75118/6661/3807 6661/10/488 1.035 R1 = 0.0711 wR2 = 0.2172 R1 = 0.1140 wR2 = 0.2417

963230 C15H24CoO10 415.21 0.28 × 0.08 × 0.40 orthorhombic Pnma 27.730(3) 11.6608(11) 5.1180(5) 90.00 90.00 90.00 1654.9(3) 4 1.699 852 1.092 293(2) 0.0266 −32/32, −13/13, −6/6 1.47/25.00 14465/1532/1433 1532/0/129 2.097 R1 = 0.0689 wR2 = 0.2403 R1 = 0.0713 wR2 = 0.2428

963231 C15H23KO10Zn 467.80 0.72 × 0.12 × 0.05 monoclinic C2/c 15.437(5) 15.985(5) 16.475(6) 90.00 90.145(13) 90.00 4065(2) 8 1.529 1936 1.461 293(2) 0.1174 −17/17, −17/17, −17/18 1.83/23.39 15532/2956/1926 2956/0/250 1.079 R1 = 0.0627 wR2 = 0.1734 R1 = 0.0955 wR2 = 0.1916

R indices (all data)

Figure 3. (a) The TEM micrograph of the discrete nanocage MOPTO. (b) Selected area electron diffraction of the nanocage.

3344.97)}. Remarkably, the water molecules within the cage remained intact during ionization in mass spectrometry. Thus, it was clear that one of the two possible conformations of TMBTA had to be locked in syn-syn-syn conformation in order to self-assemble into the nanocage MOP-TO via metal− ligand coordination. To probe the role of metal cluster such as CPWSBU in controlling the conformation of the ligand TMBTA and, subsequently, formation of the nanocage, we further reacted TMBTA with Co(II) and Zn(II) salts. In fact, reaction of TMBTA with Co(NO3)2 and ZnCl2 under solvothermal conditions resulted in two coordination polymers, namely, TMBTA-Co(II) and TMBTA-Zn(II), instead of any MOP. [{Co(μ-TMBTA)(H2O)4}]∝. Pink-colored blocked-shaped crystals of TMBTA-Co(II) belonged to the centrosymmetric orthorhombic space group Pnma. The ligand TMBTA displayed syn−syn−anti conformation and was located on a

Figure 2. Crystal structure illustration of MOP-TO. (a) Discrete nanocage, (b) MOP-TO molecule displaying the space (yellow sphere) within the nanocage, (c) 2D packing of the cage molecules sustained by C−H···π interactions, and (d) overall 3D packing of the molecules, showing continuous channels down the a axis. The inset is a photograph of the single crystal.

ESI-MS. When a dilute solution of MOP-TO crystals was further diluted in MeCN and subjected to ESI-MS, a signal at 3345.22 was observed consistently. The signal was attributed to the nanocage without the peripheral coordinated solvent molecules {i.e., [(MOP-TO)-(4DMA+2H 2 O)] + (calcd. 1333

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metal centers, two lattice included water, and one K+ ion were located. Inclusion of K+ ion was due to KOH used in the synthesis, and also, it was a requirement to achieve charge balance. The tripodal ligand adopted syn−syn−syn conformation. All the carboxylates participated in coordination with the Zn(II) metal center that displayed two different coordination geometries: tetrahedral (on a 2-fold axis) and octahedral (on a center of inversion). The overall coordination network was 3D, wherein a looped-chain type of architecture was formed due to extended coordination of the two carboxylates to tetrahedron Zn(II) metal center. The other carboxylate bridged such looped-chain via octahedral Zn(II) coordination, generating an overall 3D coordination network. The lattice occluded water molecules were found to be participating in hydrogen bonding via O−H···O interactions [O···O = 2.725(9)−2.930(14) Å] involving the metal-coordinated water and carboxylate O atoms. Weak interactions such as C−H···π (3.364 Å) involving the methyl C−H and π cloud of the adjacent aromatic rings and cation−π (2.996 Å), involving K+ and the aromatic ring were also observed (Figure 5). Thermogravimetric data analyses

mirror plane. Two carboxylate moieties (syn to each other) were found to be coordinated in an extended fashion to octahedral Co(II) metal centers; the other carboxylate remained free from coordination. The metal center Co(II) was located on a mirror symmetry and displayed slightly distorted octahedral geometry; the equatorial positions were coordinated by O atoms of two carboxyates and water, and the axial sites were occupied by water molecules. The structure may best be described as a 1D coordination polymer, wherein the syn-related carboxylates coordinated the adjacent metal centers, resulting in a 1D polymeric chain. The free carboxylate participated in hydrogen bonding with the coordinated water molecule of the neighboring chain (O···O = 2.747(3)−3.013(3) Å]. The coordinated carboxylates also participated in hydrogen bonding with the coordinated water molecule of the neighboring chain [O···O = 2.849(5) Å] resulting in an overall 3D hydrogenbonded network (Figure 4). Thermogravimetric data analyses

Figure 5. Crystal structure illustration of TMBTA-Zn(II). (a) 1D polymeric-looped chain displaying tetrahedral (light green) and octahedral (blue) Zn(II) metal centers, (b) cation−π interactions involving the K+ ion and phenyl ring, and (c) overall 3D network containing lattice-occluded water molecules (red balls). The inset is a photograph of the single crystal.

Figure 4. Crystal structure illustration of TMBTA-Co(II). (a) Parallel packing of 1D polymeric chain displaying H-bonding interaction. (b) Overall 3D hydrogen-bonded network. The inset is a photograph of the single crystal.

(Figure S3 of the Supporting Information) revealed a weight loss of 15.9% that occurred within a temperature range of 28.1−105.2 °C; this was attributed to the weight loss of two coordinated and two lattice-occluded water molecules (calculated weight loss of 15.4%). This observation was in good agreement with the single crystal structure of TMBTA-Zn(II). The crystalline phase purity of both the coordination polymers were established by powder X-ray diffraction (PXRD) (Figure S6−S7 of the Supporting Information). It is understandable that if the axial coordinating ligands are removed from CPWSBU, the Cu(II) center might act as a Lewis acid catalyst in catalyzing important organic transformations.19 In fact, in a recent report, it was shown that a MOF having CPWSBU was able to catalyze aerobic oxidation of benzyl alcohol to benzaldehyde, wherein a continuous

(Figure S2 of the Supporting Information) revealed a weight loss of 17.4% that occurred within a temperature range of 55.5−156.2 °C; this was attributed to the weight loss of the four coordinated water molecules (calculated weight loss of 17.0%) corroborating well with the single crystal structure of TMBTA-Co(II). [{(H 2 O) 2Zn(μ-TMBTA)Zn·K}·2H2 O] ∝. Colorless plateshaped crystals of TMBTA-Zn(II) were found to be crystallized in the centrosymmetric monoclinic space group C2/c. In the asymmetric unit, one fully occupied ligand, two Zn(II) metal centers located on special positions (center of inversion and 2-fold axis), two water molecules coordinated to one of the 1334

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catalysis, the nanocage molecule could retain the integrity of its molecular structure (Figure S8 of the Supporting Information). Interestingly, the FT-IR of the recovered catalyst after the second cycle (that displayed 99% completion of the reaction after 14 h instead of 12 h in the first cycle) showed significant differences, indicating disintegration of the molecular structure of MOP-TO after the second cycle. It may be mentioned here that since MOP-TO crystals were unstable outside its mother liquor due to fast evaporation of the lattice-occluded solvents, monitoring the fate of the crystalline phase of the catalyst under various conditions by PXRD was not possible. However, it was possible to obtain the PXRD pattern of the freshly grown bulk crystals of MOP-TO by placing the crystals in the diffractometer almost instantaneously after removing them from the mother liquor. Remarkably, this PXRD pattern matched quite well with the simulated pattern, indicating the crystalline phase purity of MOP-TO. Upon exposure to the environment for even a short period of time, MOP-TO resulted in an amorphous PXRD pattern (Figure S5 of the Supporting Information).

channel exposing the catalytic metal center Cu(II) was available.20 On the other hand, MOPs are unlikely to pack in such a way that continuous channels exposing the catalytic metal centers are formed. To demonstrate the Lewis acid behavior in catalysis of CPWSBU in MOP, Zhou et al. functionalized a CPWSBU-based MOP to make it soluble in a noncoordinating solvent to achieve homogeneous catalysis.21 With this background, we decided to carry out the possibility of exploiting the Lewis acid behavior of Cu(II) in CPWSBU in MOP-TO in a heterogeneous fashion. For this purpose, we chose to study the aerobic oxidation of benzyl alcohol to benzaldehyde using TEMPO as a radical initiator (Scheme 2). Scheme 2. Oxidation of Benzyl Alcohol to Benzaldehyde using MOP-TO with Oxygen

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CONCLUSIONS A truncated octahedron-shaped metal-organic-polyhedron derived from a tripodal C3-symmetric tricarboxylate ligand and Cu(NO3)2 was synthesized solvothermally and characterized by SXRD and ESI-MS. Interestingly, MOP-TO molecules could be seen under HR-TEM as well. Subsequent formation of two coordination polymers namely TMBTACo(II) and TMBTA-Zn(II) derived from the same ligand under similar solvothermal conditions emphasized the role of the Cu(II) paddle wheel SBU in generating the nanocage structure MOP-TO. The Lewis acid character of the Cu(II) metal center in MOP-TO was exploited to catalyze aerobic oxidation of benzyl alcohol to benzaldehyde assisted by TEMPO. MOP-TO belonged to the rare class of metal− organic cage molecule derived from flexible ligand like TMBTA.

In the crystal structure of MOP-TO (vide supra), the nanocage molecule itself did not have any porosity. Packing of the MOP molecules resulted in a continuous channel running down the a axis, which did not expose the Cu(II) metal center. In fact, the continuous channels were occupied by solvent molecules. In order to access the Cu(II) metal center for catalysis, we first dispersed MOP-TO crystals in CHCl3 by repeated washing (for 3 days), followed by heating under vacuum (100 °C, 2 h) with the hope that axially free random aggregation of the nanocage molecules would be formed in the bulk solid. MOP-TO so activated was then used in catalytic amount (3 mol % of the substrate) in TEMPO to catalyze aerobic oxidation of benzyl alcohol. GC/MS analyses revealed that the reaction was 99.0% complete within 12 h (Table S2 of the Supporting Information). To probe the nature of the catalysis (homogeneous or heterogeneous), we allowed the reaction to proceed up to 38% conversion (in 3 h), and after filtering off the solid catalyst, we continued the reaction for another 9 h, which resulted in another 10% conversion. In the control experiment without any catalyst, the reaction did not proceed, even after 9 h, thereby indicating that some amount of Cu(II) was leached out into the solution. In fact, atomic absorption spectroscopy (AAS) of the reaction mixture revealed that 0.4% Cu was leached out into the solution that presumably contributed to the 10% conversion observed during the leaching experiment (Supporting Information). These results indicated that the reaction was mainly heterogeneously catalyzed. We tried to monitor the fate of the catalyst during reaction by FT-IR. A comparison FT-IR spectra comprised of freshly grown crystals of MOP-TO, activated MOP-TO, and MOP-TO after the reaction revealed that they were almost identical, indicating nondegradation of the molecular structure of MOP-TO; in the spectra, metal-bound antisymmetric and symmetric stretching of COO− could be seen within the range of 1609−1622 and ∼1400 cm−1, respectively. The O−H stretching frequency of the occluded water molecules within the nanocage could also be observed at ∼3570 cm−1 in all these spectra, thereby indicating that during washing, activation, and

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EXPERIMENTAL SECTION

Materials and Method. All the chemicals were commercially available and used without further purification. Ligand TMBTA was synthesized by following reported procedures with significant modification.22 Elemental analyses ware carried out using a PerkinElmer 2400 Series-II CHN analyzer. FT-IR spectra were recorded using Perkin-Elmer Spectrum GX, and TGA analyses were performed on a SDT Q Series 600 Universal VA.2E TA Instruments. Powder Xray diffraction patterns were recorded on a Bruker AXS D8 Advance Powder (Cu Kα1 radiation, λ = 1.5406 Å) diffractometer. The TEM were recorded in the Jeol instrument using carbon-coated 300 mesh Au grid at 200 KV. The mass spectrum was recorded on QTOF Micro YA263. NMR spectra (1H and 13C) were recorded using a 300 MHz Bruker Avance DPX300 spectrometer. GC/MS measurements were carried out with a Perkin-Elmer Clarus 680GC and SQ8T MS, using column Elite 5 MS (30 m × 0.25 mm × 0.25 μm) with a maximum temperature of 300 °C. The copper content in a sample was estimated by using a Shimadzu AA-6300 atomic absorption spectrometer (AAS) fitted with a double beam monochromator. Characterization Ddata of (2,4,6-Trimethylbenzene)-1,3,5Triacetic acid (TMBTA). (Yield: 486 mg, ∼97%). M.p: 268−269 °C after recrystallization from H2O/MeOH (1: 2 v/v). Anal. Data calcd for C15H18O6: C, 61.22; H, 6.16. Found: C, 61.41; H, 6.33. 1H NMR (300 MHz, DMSO-d6): δ = 2.16 (s, 9H), 3.63 (s, 6H), 12.24 (s, 3H). ESI-MS: calcd for C15H18O6 295.20 (M+1). Found: [M + Na]+ 317.10. FT-IR (KBr, cm−1): 3003, 3082, 1693s, 2974, 2897, 2638m, 1335

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sample was prepared by making a thin film of finely powdered sample (∼30 mg) over a glass slide. The experiment was carried out with a scan speed of 0.3 s/step (step size = 0.02°) for the scan range of 5− 35° 2θ.

2360m, 1411m, 1392m, 1286m, 1240m, 1188m, 1018m, 923m, 825m, 680m, 653m cm−1. [{Cu12(TMBTA)8(DMA)4(H2O)8}·8H2O·48H2O] MOP-TO was synthesized under solvothermal condition from a DMA−ethanol solution of a mixture of TMBTA (97 mg, 0.33 mmol), Cu(NO3)2· 3H2O (241.6 mg, 1 mmol), and pyridine (79 μL, 1 mmol). The mixture was taken carefully in a 25 mL sealed glass tube, heated at 85 °C for 60 h, and allowed to cool slowly (5 °C per hour in 12 h). After cooling, good-looking block-shaped green crystals were obtained. The crystals were carefully characterized by SXRD, elemental analysis, PXRD, and FT-IR. Yield: ∼32% (100 mg, 0.321 mmol). FT-IR (KBr, cm−1): 3412, 1633, 1616, 1402s, 2933m, 1284m, 1201m, 1022s, 835m, 756m, 713s, 642m, 590m cm−1. Due to the gradual loss of lattice occluded solvents, elemental analysis of the crystals of MOP-TO was inconsistent and, thus, not reported. [{Co(μ-TMBTA)(H2O)4}] ∝TMBTA-Co(II) was synthesized under solvothermal condition from a DMF−ethanol solution of a mixture of TMBTA (97 mg, 0.33 mmol), Co(NO3)2·6H2O (291 mg, 1 mmol), and pyridine (79 μL, 1 mmol). The mixture was taken carefully in a 25 mL sealed glass tube, heated at 80 °C for 72 h, and allowed to cool slowly (5 °C per hour in 12 h). After cooling, goodlooking block-shaped pink crystals were obtained. The crystals were washed with ethanol−water and carefully characterized by SXRD, elemental analysis, PXRD, and FT-IR. Yield: ∼59% (250 mg, 0.590 mmol). Elemental analysis calcd for C15H24CoO10 (%): C, 42.56; H, 5.72. Found: C, 42.07; H, 5.43. FT-IR (KBr, cm−1): 3358, 1718, 1560, 1649, 1381, 2947, 2204m 1267s, 1201m, 1157s, 1026m, 831m, 750m, 792m, 686m cm−1. [{(H2O)2Zn(μ-TMBTA)Zn·K}·2H2O]∝ TMBTA-Zn(II) was synthesized under solvothermal condition from a water−ethanol solution of a mixture of potassium salt of TMBTA (60 mg, 0.1468 mmol), ZnCl2 (57 mg, 0.42 mmol). The mixture was taken carefully in a 25 mL Teflon-coated autoclaved bomb, heated at 120 °C for 48 h, and allowed to cool slowly. After cooling, good-looking plate-shaped colorless crystals were obtained. The crystals were washed with ethanol−water and carefully characterized by SXRD, elemental analysis, PXRD, and FT-IR. Yield: ∼49% (50 mg, 0.1468 mmol). Elemental analysis calcd for C15H23KO10Zn (%): C, 38.51; H, 4.96. Found: C, 38.04; H, 4.88. FT-IR (KBr, cm−1): 3396, 3240, 1604, 1556s, 1384s, 2978brs, 1271s 1228m, 1188m, 1022m, 904m, 831m, 812m, 744m, 692s, 615m, 542m cm−1. Catalysis. In a three-neck round-bottomed flask, an appropriate amount of activated catalyst MOP-TO (50 mg, 3 mol %), sodium carbonate (53 mg, 1 eqiv.), and TEMPO (39.5 mg, 0.5 equiv) were taken. To this mixture, 5 mL of acetonitrile was added, followed by the addition of benzyl alcohol (52 μL, 1 equiv). The reaction mixture was stirred vigorously at 60 °C under oxygen atmosphere. The progress of the reaction was monitored by GC/MS of the aliquots, taken after a certain interval. After the reaction was over, the mixture was cooled to room temperature. The catalyst was centrifuged out, and the supernatant organic part was characterized by GC/MS. Single-Crystal X-ray Crystallography. Single-crystal X-ray diffraction data were collected using Mo Kα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with a CCD area detector. Data collection, data reduction, and structure solution/ refinement were carried out using APEX II. All the structures [MOPTO, TMBTA-Co(II), and TMBTA-Zn(II)] were solved by direct methods and refined in a routine manner. In all the cases, nonhydrogen atoms were treated anisotropically. Whenever possible, the hydrogen atoms were located on a difference Fourier map and refined. In other cases, the hydrogen atoms were geometrically fixed. CCDC no. 962728, 963230, and 963231 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, U.K, by fax: (+44) 1223−336−033, or via e-mail:deposit@ ccdc.cam.ac.uk). Powder X-ray diffraction. PXRD data were collected using a Bruker AXS D8 Advance Powder (Cu Kα1 radiation, λ = 1.5406 Å) diffractometer equipped with a super speed LYNXEYE detector. The

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ASSOCIATED CONTENT

S Supporting Information *

Cambridge Structural Database, molecular plots, and Hbonding parameters of MOP-TO, TMBTA-Zn(II), and TMBTA-Co(II), TGA, PXRD, EDX, AAS, FT-IR, and CIF check reports. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ‡

Institute of Condensed Matter and Nanosciences,Université Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-laNeuve, Belgium. § Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, West Bengal, India Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.D. thanks the Department of Science & Technology (DST), New Delhi, India, for financial support. M.P. and N.N.A. thank CSIR and IACS for research fellowships, respectively. Single crystal X-ray diffraction was performed at the DST-funded National Single Crystal Diffractometer Facility at the Department of Inorganic Chemistry, IACS.

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