Manganese- and Lanthanide-Based 1D Chiral Coordination Polymers

Article pubs.acs.org/IC

Manganese- and Lanthanide-Based 1D Chiral Coordination Polymers as an Enantioselective Catalyst for Sulfoxidation Munendra Yadav, Asamanjoy Bhunia, Salil K. Jana, and Peter W. Roesky* Institut für Anorganische Chemie, Karlsruher Institut für Technologie (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: The chiral 1D-coordination polymers (CP) {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n [Ln = Pr (1), Nd (2), Sm (3), and Gd (4)] were synthesized by the reaction of N,N′-bis(4-carboxysalicylidene)cyclohexanediamine (H4L) with [MnCl2·4(H2O)] and [Ln(NO3)3·x(H2O)] in the presence of dmf/pyridine at 90 °C. The polymers consist of manganese-salen-based moieties having carboxylate linkers connected to rare earth atoms in a 1D-chain structure. The polymers are very easily accessible. A one-step synthesis for the ligand and a second step for the preparation of the 1D coordination polymers starting from commercially available material are needed. The solid state structures of 1−4 were established by single-crystal X-ray diffraction. Compounds 1−4 were investigated as heterogeneous catalysts for the sulfoxidation reaction of various alkyl and aryl sulfides. The influence of various solvents and oxidizing agents on the catalytic reaction was examined. It was found that the catalysts were active for more than one reaction cycle without significant loss of activity. For phenylsulfide with 1 mol % of the catalyst 4, a maximum conversion 100% and a chemoselectivity 88% were observed.

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reactions.2,4,11,24−26,35,38 The stability of the MOF used plays a key role maintaining its structural integrity during catalysis. In contrast, 1D CPs have been far less studied for catalytic applications.28,39−41 Their relative ease of formation and the good accessibility of the catalytic active metal centers make them promising candidates for heterogeneous catalysis. By altering the metal within the 1D CPs scaffold, the scope of catalytic application may easily be varied. Chiral sulfoxides are valuable subunits for the pharmaceutical industry37 and biological systems.42’43,44 Thus, considerable progress has been made developing catalysts for asymmetric sulfoxidation over the past few decades.45,46 Sulfoxides were synthesized starting from achiral molecules in the presence of enantiomeric pure catalyst.37,42,47−51 Mainly homogeneous catalysts derived from transitional metals, such as titanium,52,53 vanadium,44 chromium,47 manganese,54−57 and iron,47,58 ligated by chiral moieties and a few metal-free systems59 were used in the past. Exceptional efficiency in the enantioselective oxidation of sulfides and olefins has been observed by using these homogeneous systems. They show high activities and selectivities for most of the organic reactions, but their practical application remains limited because of problems concerning catalytic stability, separation, and recyclability.25,37,47,60−63 Therefore, the development of new asymmetric heterogeneous catalysts remains a continuing challenge.48,50,61,64 In this regard, the synthesis of chiral metal organic frameworks (CMOFs)65,66 and chiral coordination polymers (CCPs)67

INTRODUCTION Recently, large research activity has been observed in the area of metal organic frameworks (MOF’s). MOFs have been widely used for applications, such as gas adsorption, catalysis, drug delivery, separation, and imaging.1−21 The structural diversity of MOFs needed for these applications can be realized by choosing the desired functionality within the linker or the metal knots.2,4,8,22−26 Besides the well-structured three-dimensional MOFs, also one- and two-dimensional coordination polymers (CP) have been studied intensively.27−29 The structures, properties, and applications of 1D coordination polymers have been reviewed by J. J. Vidal recently.28 In contrast to three-dimensional MOFs, one-dimensional coordination polymers (1D CP) have a much simpler topology. The relative ease of formation by self-assembly and the simplicity of the 1D CPs facilitates the incorporation of functional groups at the metal centers or in the backbone of the organic linkers.28,30 The variation of the structures of 1D CP ranges from linear chains, over zig chains and helicates to complicated structures. Their ease of assembly makes them interesting for material science. Molecular-based ferromagnets, synthetic metallic conductors, nonlinear optical materials, and ferroelectrics represent several applications of low-dimensional coordination polymers.31 1D CPs were also deposited onto surfaces as single molecule devices32 and as molecular wires for potential nanoelectronics.29,32 The well-structured three-dimensional MOFs have widely been used in heterogeneous catalysis.2,6,9,11,13,15,33−38 One advantage of using MOFs for catalysis is the uniform and homologous pore size, which may allow shape selective © XXXX American Chemical Society

Received: September 29, 2015

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DOI: 10.1021/acs.inorgchem.5b02234 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Details of 1−4 chemical formula formula mass crystal system a (Å) b (Å) c (Å) β (deg) unit cell vol (Å3) temp (K) space group no. of formula units per unit cell, Z radiation type absorption coefficient, (μ/mm−1) no. of reflns measured no. of independent reflns Rint final R1 values (I > 2σ(I)) final wR(F2) values (all data) GOF on F2 Flack parameter

1

2

3

4

C62H84Cl2Pr2Mn2N12O27 1891.99 monoclinic 32.726(7) 10.721(2) 25.497(5) 114.88(3) 8116(3) 150(2) C2 4 Mo Kα 1.629 58171 17201 0.0400 0.0755 0.1906 1.255 0.077(6)

C62H84Cl2Nd2Mn2N12O27 1898.65 monoclinic 32.763(7) 10.803(2) 25.554(5) 114.82(3) 8210(3) 150(2) C2 4 Mo Kα 1.688 45633 16582 0.1013 0.0820 0.1761 0.977 0.186(11)

C62H84Cl2Sm2Mn2N12O27 1910.89 monoclinic 32.571(7) 10.734(2) 25.464(5) 114.71(3) 8087(3) 150 C2 4 Mo Kα 1.882 31488 17138 0.0581 0.0980 0.2409 1.199 0.092(15)

C62H84Cl2Gd2Mn2N12O27 1924.67 monoclinic 32.393(7) 10.749(2) 25.390(5) 114.29(3) 8058(3) 150 C2 4 Mo Kα 2.077 138229 21739 0.0578 0.0642 0.1764 1.048 0.043(4)

has been an attractive research area.68 One method for the generation of CMOFs is the use of chiral metalloligands as linkers between the knots. Among others, enantiomeric pure salen complexes have been used as metalloligands.25,37,61,69−72 These chiral metalloligands within the CMOFs have been employed as chiral catalytically active centers to generate heterogeneous catalysts for various enantioselective reactions.25,35,37,61,69−74 The enantioselective sulfoxidation has only been studied with some CMOFs.25,34,37,74,75 Mostly, zinc, copper, or titanium were used as catalytic active centers. To the best of our knowledge, 1D CCPs have not been employed as heterogeneous catalysts for sulfoxidation so far. Previously, we reported a series of 1D achiral coordination polymers (CPs) based on manganese and lanthanide metal atoms.40 The polymers consist of manganese-salen-based metalloligands having carboxylate linkers connected to lanthanide atoms to form 1D CPs. The CPs were used as heterogeneous oxidation catalysts for the epoxidation reaction. Air was used as oxidant. Herein we report now chiral 1D coordination polymers derived from chiral salen manganese metalloligands and lanthanide knots. These polymers are very easily accessible. We present a one-step synthesis for the ligand starting from commercially available material. Only a second step is needed to prepare the 1D coordination polymers. These polymers were used as catalysts for the enantioselective sulfoxidation reaction of aryl and alkyl sulfides.76

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X-ray powder diffraction patterns (XRD) for different samples of compound 4 were measured on a STOE STADI P diffractometer (CuKa1 radiation, Germanium monochromator, Debye−Scherrer geometry) in sealed glass capillaries. The theoretical powder diffraction pattern was calculated on the basis of the atom coordinates obtained from single crystal X-ray analysis by using the program package Mercury by CCDC. Synthesis of N,N′-Bis(4-Carboxysalicylidene)cyclohexanediamine (H4L). H4L was synthesized by condensation of 4-formyl-3hydroxybenzoic acid (99.6 mg 0.6 mmol) and (1R, 2R)-(−)-1,2diaminocyclohexane (34.2 mg 0.3 mmol) in 15 mL of ethanol. The reaction mixture was stirred for 4 h at 80 °C and then cooled to room temperature. The resulting yellow solid was filtered and washed with cold ethanol and dried in vacuum. Yield: 75.4 mg (61.3%). EI-MS: m/ z = 410.11. Anal. Calcd for C22H22N2O6: C, 64.38; H, 5.40; N, 6.83. Found: C, 64.39; H, 5.42; N, 6.18. 1H NMR (300 MHz, DMSO-d6): δ 8.55 (s, 2H, −CHN−), 7.43 (s, 2H aromatic-H), 7.37 (m, 2H, aromatic-H), 7.31 (m, 2H, aromatic-H), 3.40 (m, 2H, cylohexane), 1.20−1.90 (m, 8H). 13C{1H} NMR (75 MHz, DMSO-d6): δ 167.8, 165.0, 160.3, 136.5, 131.8, 121.5, 119.7, 117.4, 71.8, 32.8, 24.0. IR (ATR): 2944 (m), 2859 (w), 1689 (m), 1629 (s), 1605 (m), 1561 (s), 1543 (s), 1449 (m), 1384 (m), 1366 (w), 1346 (w), 1286 (s), 1203 (m), 1148 (w), 1133 (w), 1096 (w), 1049 (w), 985 (w), 958 (m), 895 (m), 864 (w), 835 (w), 783 (w), 770 (w), 759 (s), 693 (w), 606 (w), 557 (m), 447 (w) cm−1. General Procedure for the Synthesis of {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (1−4). H4L (22.5 mg, 0.055 mmol) and MnCl2·4(H2O) (10.8 mg, 0.055 mmol) were stirred in dmf (2 mL) for 5 min. Then, Ln(NO3)3·(H2O)m (0.1 mmol) and pyridine (0.15 mL) were added to the reaction mixture. The resulting black solution was stirred for 2 h at room temperature and then filtered in a 10 mL glass vial. To get the single crystals, the reaction solution was heated in an oven at 90 °C and then was cooled to room temperature. Red block shaped crystals suitable for single-crystal X-ray diffraction were collected and washed three times with dmf, followed by diethyl ether and dried in air for further analysis. {[Pr2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (1). Yield: 20 mg (21% as single crystalline material). Anal. Calcd for C56H70Cl2Mn2N10Pr2O25 (corresponds to loss of two dmf molecules and x = 1): C, 38.53; H, 4.04; N, 8.02. Found: C, 37.96; H, 4.89; N, 8.52. IR (ATR): 2939 (w), 2861 (w), 1613 (m), 1559 (m), 1515 (m), 1479 (m), 1397 (s), 1338 (s), 1300 (s), 1269 (s), 1198 (m), 1140 (w), 1109 (w), 1022 (w), 977 (m), 895 (w), 862 (w), 819 (w), 777 (s), 736 (w), 637 (w), 606 (s), 520 (w), 493 (w), 467 (w), 441 (w) cm−1.

EXPERIMENTAL SECTION76

General Considerations. IR spectra were obtained on a Bruker FTIR Tensor 37 via the attenuated total reflection method (ATR). Elemental analyses were carried out with an Elementar vario EL or Vario Micro Cube. NMR spectra were recorded on Bruker Avance II 300 MHz NMR spectrometer. Chemical shifts are referenced to internal solvent resonances and were reported to tetramethylsilane. The enantiomeric ratios of sulfoxides were recorded using high performance liquid chromatography (HPLC) Agilent Technologies 1200 series with a CHIRALPAK-IA column. TGA measurements were made on a Netzsch STA 429 instrument. Aldehydes, amines, sulfides, and solvents were purchased from commercial sources and used without further purification. B


Article

Inorganic Chemistry Scheme 1. Synthesis of 1−4

97.77 Further non-hydrogen atoms were located from successive Fourier difference map calculations. The refinements were carried out using full-matrix least-squares techniques on F2, minimizing the function (F0 − Fc)2, where the weight is defined as 4F02/2(F02) and F0 and Fc are the observed and calculated structure factor amplitudes using the program SHELXL-97.77 The hydrogen atoms were placed in their calculated positions without refinement. The final values of refinement parameters are given in Table 1. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities in all case were of no chemical significance.

{[Nd2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (2). Yield: 19 mg (20% as single crystalline material). Anal. Calcd for C59H77Cl2Mn2N11Nd2O26 (corresponds to loss of one dmf molecule and x = 1): C, 38.82; H, 4.25; N, 8.44. Found: C, 38.12; H, 4.85; N, 8.59. IR (ATR): 2940 (w), 2861 (w), 1647 (m), 1615 (s), 1565 (s), 1521(w), 1476 (w), 1389 (s), 1337 (w), 1299 (m), 1269 (m), 1198 (m), 1139 (w), 1106 (w), 1023 (w), 974 (m), 897 (w), 860 (w), 817 (m), 775 (s), 736 (w), 662 (w), 634 (s), 601 (w), 520 (w), 494 (w), 467 (w), 442 (w), 418 (w) cm−1. {[Sm2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (3). Yield: 20 mg (21% based on single crystals). Anal. Calcd for C58H77Cl2Sm2Mn2N11O26 (corresponds to loss of one dmf molecule and x = 1): C, 38.56; H, 4.22; N, 8.38. Found: C, 38.31; H, 4.43; N, 8.91. IR (ATR): 2939 (w), 2861 (w), 1651 (m), 1617 (s), 1564 (s), 1519 (w), 1477 (w), 1391 (s), 1337 (w), 1299 (m), 1269 (m), 1197 (m), 1139 (w), 1106 (w), 1022 (w), 975 (m), 897 (w), 860 (w), 813 (m), 777 (s), 737 (w), 635 (s), 602 (w), 520 (w), 494 (w), 467 (w), 441 (w) cm−1. {[Gd2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (4). Yield: 18 mg (18% based on single crystals). Anal. Calcd for C53H63Cl2Gd2Mn2N9O24 (corresponds to loss of three dmf molecules and x = 1): C, 37.33; H, 3.72; N, 7.39. Found: C, 36.75; H, 4.52; N, 7.81. IR (ATR): 2939 (w), 2861 (w), 1647 (m), 1616 (s), 1566 (s), 1521(w), 1477 (w), 1394 (s), 1337 (w), 1300 (m), 1270 (m), 1198 (m), 1139 (w), 1107 (w), 1023 (w), 975(m), 898 (w), 861 (w), 819 (m), 776 (s), 738 (w), 667 (w), 634 (s), 602 (w), 519 (w), 494 (w), 467 (w), 443 (w), 419 (w) cm−1. General Procedure for the Catalytic Reactions. The catalyst {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·H2O}n [Ln = Pr (1), Nd (2), Sm (3), and Gd (4); x = 1] (1−4) (20 mg) and the affiliated sulfide (1 mmol) were combined in acetonitrile (10 mL) followed by addition of iodosobenzene (1 mmol). The combined reaction mixture was stirred for 16 h at room temperature. The reaction progress was monitored by TLC (eluent = EtOAc/hexane). The catalyst was separated out from the reaction mixture by ordinary filtration. The catalyst was recycled and performed in the similar way. The products were analyzed by NMR and HPLC. The ratio sulfide/sulfoxides/sulfone was determined by integration of the 1H NMR spectra. X-ray Crystallographic Studies of 1−4. A suitable crystal was covered in mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to a cold stream of a STOE IPDS II diffractometer. All structures were solved using the program SHELXS-

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RESULTS AND DISCUSSION Synthesis and Structures. The new ligand N,N′-bis(4carboxysalicylidene)cyclohexanediamine (H4L) was obtained in one step from 4-formyl-3-hydroxybenzoic acid and (1R, 2R)(−)-1,2-diaminocyclohexane in ethanol. The reaction of H4L with [MnCl2·4(H2O)] and [Ln(NO3)3·x(H2O)] in the presence of dmf/pyridine at elevated temperature led to the formation of chiral 1D-cordination polymers (CPs) {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·H2O}n [Ln = Pr (1), Nd (2), Sm (3), and Gd (4) in a second step. The resulting products were obtained as red single crystals. Only single crystals were used for the subsequent analysis and catalytic reactions. The solid state structures of 1−4 were established by single-crystal X-ray diffraction and characterized by standard analytical/ spectroscopic techniques. Compounds 1−4 were insoluble in common solvents and due to their paramagnetic behaviors no NMR data could be acquired. The IR spectra for the carboxylate group show stretching bands at 1613 cm−1 for 1; 1615 cm−1 for 2; 1617 cm−1 for 3 and 1616 cm−1 for 4. The absence of absorption at 1689 cm−1 also proves the complete deprotonation of carboxyl groups of the salen ligand. Compounds 1−4 crystallized in the monoclinic chiral space group C2. All compounds are isostructural (see also Scheme 1 and Table 1). The molecular structure of compound 4 is shown in Figure 1. The basic building blocks of all structures consist of C


Article

Inorganic Chemistry

network, the number of solvent molecules could not be determined without doubt. In the lattice we determined one water molecule, which corresponds to x = 1 in the sum formula. Therefore, an uncertainty remains concerning the sum formula of compounds 1−4. BET measurements show that solvent is in the void space because the observed N2 uptake curve at 77 K shows a surface absorption only (Figure S1). The manganese ions in the salen units [MnLCl(NO3)] are in the center of a distorted octahedral coordination polyhedron, which is built by the ONNO atoms of the salen ligand, chloride and a nitrate ion. The chloride and nitrate occupy the apical positions. The ONNO atoms of the salen ligand form an equatorial plane around the Mn(III) ion. 6-fold-coordinated Mn(III) ions within salen ligands have been reported earlier {[Ln2(MnLCl)2(NO3)2(dmf)5] (H4L = N,N′-bis(4-carboxysalicylidene)ethylenediamine).40 The Mn−salen units [MnLCl(NO3)] are similar to each other in the structural parameters and the environment near to Mn1 and Mn2 ions. Thus, only the representative structural parameters of complex 4 are discussed in detail. The Mn1−Cl1 and Mn2−Cl2 bond distances are similar (Mn1−Cl1 2.600(6) Å and Mn2−Cl2 2.600(4)). The Mn1−Ophenolate bond distances are Mn1−O3 1.880(8) Å and Mn1−O4 1.884(8) Å, which are comparable to the Mn2−Ophenolate distances Mn2−O9 1.892(8) Å and Mn2− O10 1.904(8) Å. The Mn1−N1 1.998(8) Å and Mn1−N2 1.997(8) Å bond distances are also close to Mn2−N3 1.997(9)

Figure 1. Solid-state structure of the compound 4. The coordination arrangement of a gadolinium dimer is shown, and hydrogen atoms are omitted for clarity.

a {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n chain. The asymmetric unit is formed by two Mn-salen units [MnLCl(NO3)], two rare earth ions (Ln1 and Ln2), six dmf, and two water molecules. However, because of the potentially porous

Figure 2. Solid-state structure of compound 4. SBU unit is highlighted (top) and cutout of the polymeric structure (bottom). All hydrogen atoms and most of the solvent molecules have been omitted for clarity. D


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Inorganic Chemistry

release of additional three dmf molecules in the temperature range of 100−295 °C (obsd 11.61%, calcd 11.47%). By increasing the temperature, the frameworks started to decompose. For compound 4, a rapid weight loss is found in the temperature range of 30−70 °C, corresponding to the loss of one dmf and three water molecules (obsd 5.88%, calcd 6.8%). In the temperature range of 100−136 °C, another dmf is lost (obsd 3.89%, calcd 3.80%). After that, the four remaining dmf molecules were released (obsd 16.43%, calcd 15.19%) in the temperature range of 170−305 °C. Finally, compound 4 is destroyed by further heating. From the above TGA plot of compounds 1−4, we concluded that compounds 1−4 released the solvent molecules in the temperature range of 30−315 °C and then started to decompose. Catalytic Studies of Compounds 1−4. Mn(III)-based salen complexes are known to be highly active catalysts in epoxidation and sulfoxidation reactions.40,33,37 Previously, we reported the epoxidation of trans-stilbene with air by using 1D coordination polymers as catalysts. We observed that Mn-Lnbased polymers were active for this epoxidation reaction.40 Motivated by these results, we examined the chiral-1D coordination polymers 1−4 as catalysts for sulfoxidation reactions. The synthesis of enanatiomerically pure sulfoxides is challenging as they are useful drugs in some cases.37 Compounds 1−4 were used for the oxidation of aryl and alkyl sulfides with iodosobenzene, which act as an oxidizing reagent. We explored the utilization of compounds 1−4 as heterogeneous catalysts and the reaction resulted in the formation of aryl and alkyl sulfoxides as main products and sulfone as side products. First, we examined compounds 1−4 for the oxidation of methyl p-tolylsulfide. Conversions of 71−77% indicate that the choice of the lanthanide does not have a significant impact on the sulfoxidation reaction. Therefore, we focused the catalytic screening test on only one catalyst and used compound 4 for detailed studies. The reaction parameters were varied to optimize the conversion and selectivity for the methyl ptolylsulfide. The parameters shown in Scheme 2 are the

Å and Mn2N4 1.980(8) Å. The MnOnitrate bond distances are Mn1−O16 2.346(10) and Mn2O13 2.36(2). The Mn-salen unit acts as a linker to form the 1D coordination polymers. Each carboxylate group of the Mn−salen unit connects two lanthanide atoms. Thus, the two lanthanide atoms (Ln1 and Ln2) are coordinated together by four Mn−salen units, each through four bridging carboxylate groups and six dmf and two water molecules, as it is expected because of the large size of the lanthanide atoms. Two of these coordinated dmf molecules and two water molecules are disordered. The lanthanide atoms Ln1 and Ln2 form the secondary building units (SBUs). Each SBU is based on a [(Ln)2(μ-O2CR)4(dmf)6(H2O)2] building block. These SBUs can be considered as a distorted square paddlewheel built from two rare earth ions which are bridged by four carboxylate groups. The basic structural arrangement of compounds 1−4 is comparable to our previously reported 1D-cordination polymers {[Ln2(MnLCl)2(NO3)2(dmf)5] (H4L = N,N′-bis(4-carboxysalicylidene)ethylenediamine).40 The thermal stability of compounds 1−4 was proven by TGA measurements (Figure 3). Although compounds 1−4 are

Figure 3. TGA curve of 1−4 in the temperature range of 20−1000 °C under N2 atmosphere.

Scheme 2. Sulfoxidation of Aryl and Alkyl Sulfides with PhIO Catalyzed by Compounds 1−4

isostructural, they show slightly different thermal behavior. The TGA curve of compound 1 shows a gradual weight-loss step of 3.14% in the temperature range of 30−60 °C, corresponding to the escape of three water molecules from the frameworks (calcd. 2.85%) followed by a plateau in the temperature range of 60−100 °C. A gradual weight loss of 14.65% (calcd 15.45%) is observed in the temperature range of 100−285 °C, which is assigned to the loss of four dmf molecules. Another two dmf molecules are lost rapidly in the temperature range of 290−310 (obsd 8.22%, calcd 7.72%), after that the compound started to decompose. In compound 2, there is a gradual weight loss of 6.54% between 30 and 70 °C, corresponding to loss of three water and one dmf molecules (calcd 6.69%). A similar type of decomposition behavior as observed for compound 1 is seen: gradual weight loss followed by rapid weight loss due to release of additional five dmf molecules in the temperature range of 100−315 °C (obsd 18.37%, calcd 19.25%) before the frameworks starts to collapse. The TGA curve of compound 3 shows a rapid weight loss in the temperature range of 100− 170 °C (obsd 13.16%, calcd 14.30%), corresponding to the loss of three water and three dmf molecules. Afterward, a gradual weight loss followed by rapid weight loss is found due to the

optimized conditions. We examined air, tBuOOH, hydrogen peroxide and iodosobenzene as oxidizing agents (Table S2). We found that hydrogen peroxide is destroying the polymeric structure. Compared to the other oxidizing reagents the conversion was significantly higher by using idosobenzene (Table S2). By using iodosobenzene as oxidizing agent there is no difference if the reaction is performed under air or under N2 atmosphere (Table S2, entries 1−6). We thus, conclude that molecular oxygen from air is not significantly involved in the reaction. The reaction is almost not running in the absence of 4 (Table 2, entry 14, and Table S1). The catalytic reactions were also examined in the presence of different solvents, such as acetone, hexane, dichloromethane and acetonitrile but conversion was highest when acetonitrile was used as a solvent. Furthermore, we examined different stoichiometric ratios for the oxidizing agent. 1eq of idosobenzene was found most useful for maximum conversion of the substrates. A higher amount of E


Article

Inorganic Chemistry Table 2. Catalytic Oxidation of Sulfidesa

(100% conversion; Table 2 entry 5) the yield is significantly lower. This trend cannot be generalized. The para-brom aryl ethyl and methyl sulfides (Table 2, entries 7 and 11) showed almost the same reactivity. When a n-butyl group was introduced in para position (n-BuPhSEt), the conversion decreased to 36% (Table 2, entries 12). This is probably due to the slower mass-diffusion of the bulky substrate in the potentially porous media. Whereas the yields of the reaction are very good, the enantioselectvities were found to be quite low. A comparative experiment was carried out to prove the heterogeneity of the catalytic reaction and leaching of the catalyst. By using methyl p-tolylsulfide as substrate, we performed two catalytic reactions under similar reaction conditions. For the first reaction, the catalyst was removed from the reaction mixtures after 4 h, for the second reaction, it was removed after 8 h. The conversion rates of the 4 and 8 h reactions were compared with the standard 16 h reaction (Table 2, entries 4). When the standard reaction was carried out with reaction time of 4 h, a conversion of 21% with 100% chemoselectivity was observed. Thereafter, the solid catalyst was separated out from the reaction and fresh iodosobenzene was added to the reaction mixture, whereas the other reaction conditions were maintained as shown in Scheme 2. After an additional 14 h, the conversion had barely increased (22% conversion; 100% chemoselectivity). Second, a standard reaction was carried out for a reaction time of 8 h and a conversion of 44% with 93% chemoselectivity was obtained. Then, the solid catalyst was separated out and fresh idosobenzene was added to the reaction mixture. After an additional 14 h, the conversion of methyl p-tolylsulfide had slightly increased (50% conversion; 90% chemoselectivity). These results undoubtedly proved that the removal of the catalyst after filtration through a regular filter afforded very little additional oxidation product. Thus, no active species is leached out from the reaction system as the conversion rates of the catalysis after 4, 8, and 16 h under standard conditions were close to each other. Some blinds tests were performed by running the catalysis without a catalyst (Table 2, entry 14 and Table S1), for example, for 4-bromophenyl methyl sulfoxide, under the standard reaction conditions of Table 2, 4% yield of sulfoxide was obtained after 16 h without adding a catalyst (Table 2, entry 14). The stability of the catalyst by recycling was investigated for the oxidation of methyl phenylsulfide. The catalyst was recycled after the first run just by ordinary filtration and reused under exactly the same reaction conditions. After the first, second, and third run, the conversion for the methyl phenylsulfide oxidation was found to be 100%, 98%, and 90%. Thus, the catalyst was not significantly deactivated under optimized reaction conditions after 3 runs. The reused catalyst was characterized by PXRD (Figure 4). The experimental powder pattern of compound 4 shows a good agreement with the simulated pattern from single crystal, proving the crystalline purity of the compound. A comparison of the PXRD pattern of compound 4 before and after catalysis shows that the structure remains unchanged except from some few minor structural rearrangements of the 1D CP, which may be due to the loss of some dmf and water molecules from its lattice. The reused catalyst was found to be similar to the dry catalyst except from small deviations. This data proves the stability of compound 4 during catalysis.

a

Reaction conditions: A reaction solution of aryl or alkyl sulfides (1 mmol), PhIO (1 mmol), acetonitrile (10 mL), and catalyst (1 mol %) was stirred for 16 h. The ee of the resulted sulfoxide was determined by HPLC with Chiralpak-A column. The conversion and selectivity were determined by 1H NMR. *n.d.: sulfoxides not measured for ee.

an oxidizing agent led to sulfone formation. As optimized conditions we established acetonitrile as solvent, iodosobenzene (1 equiv) as an oxidizing agent and 1 mol % of the catalyst at room temperature for the oxidation of methyl p-tolylsulfide. All other reactions were performed under these optimized reaction conditions. The sulfoxidation of methyl p-tolylsulfide and methyl phenylsulfide gave for compound 4 a conversion of 76% and 100%, respectively (Table 2, entries 4 and 5), while the 4chlorophenyl methyl sulfide and 4-bromophenyl methyl sulfide led to conversion of 78% and 81%, respectively (Table 2, entries 6 and 7). When the phenyl ring was functionalized with the strong electron-withdrawing −NO2 and −CN groups, the same reaction conditions led to a conversion of 64% and 71%, respectively (Table 2, entries 8 and 9). Introduction of an ethyl group in aryl sulfide (PhSEt) instead of a methyl resulted in a conversion of 79% (Table 2, entry 10). Compared to PhSMe F


Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DFG-funded transregional collaborative research center SFB/TRR 88 “3MET”. Ms Sibylle Schneider is acknowledged for measuring TGA. We would like to thank Dr. Olaf Fuhr for helpful discussion regarding the Xray diffraction solution, and Dr. Andreas Eichhöfer for the PXRD measurement.

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Figure 4. Powder X-ray diffractograms of compound 4.

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SUMMARY In summary, we synthesized and structurally characterized the manganese- and lanthanide-containing chiral 1D CPs 1−4 by using trivalent lanthanide nitrates and manganese chloride along with the salen ligand (H4L) in a one-step procedure. The new manganese−lanthanide compounds, {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·(H2O)} (Ln = Pr, Nd, Sm, Gd), are polymeric materials with defined structures. The polymers consist of manganese−salen-based metalloligands having carboxylate linkers connected to lanthanide atoms forming the chiral 1D CPs. A twisting of the chains is observed and potentially porous structures are formed, which possess large free void space. The manganese−gadolinium compound 4 was used as catalyst for various sulfides by employing iodosobenzene as an oxidant to give the corresponding sulfoxides. Compounds 1−3 displayed comparable activities for the oxidation of methyl p-tolylsulfide. A low catalyst concentration of 1 mol % in acetonitrile emerged as optimized parameters. In the presence of compound 4, a maximum conversion of 100% methyl phenylsulfoxide with 88% selectivity was obtained after 16 h. Under these optimized reaction conditions, 1−4 showed a remarkable activity. A simple filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous pathway cannot be completely excluded. Compound 4 could be reused three times without significant loss of activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02234. Catalytic oxidation of sulfides, NMR data of sulfoxides, HPLC analyses of sulfoxides, and BET measurement of 4 (PDF) X-ray crystallographic files in CIF format for the structure determinations of 1 (CIF) X-ray crystallographic files in CIF format for the structure determinations of 2 (CIF) X-ray crystallographic files in CIF format for the structure determinations of 3 (CIF) X-ray crystallographic files in CIF format for the structure determinations of 4 (CIF) G


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Manganese- and Lanthanide-Based 1D Chiral Coordination Polymers

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