Novel Coordination Polymers with (Pyrazolato)-Based Tectons

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Novel Coordination Polymers with (Pyrazolato)-based Tectons: Catalytic Activity in the Peroxidative Oxidation of Alcohols and Cyclohexane Ivan Timokhin, Claudio Pettinari, Fabio Marchetti, Riccardo Pettinari, Francesca Condello, Simona Galli, Elisabete C. B. A. Alegria, Luísa M.D.R.S. Martins, and Armando J. L. Pombeiro Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00083 • Publication Date (Web): 23 Mar 2015 Downloaded from http://pubs.acs.org on March 31, 2015

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Crystal Growth & Design

Novel Coordination Polymers with (Pyrazolato)-based Tectons: Catalytic Activity in the Peroxidative Oxidation of Alcohols and Cyclohexane Ivan Timokhin,a Claudio Pettinari,*,a Fabio Marchetti,b Riccardo Pettinari,a Francesca Condello,a Simona Galli,*,c Elisabete C.B.A. Alegria,d,e Luísa M.D.R.S. Martins,d,e Armando J.L. Pombeiro*,e a

School of Pharmacy, Chemistry Section, University of Camerino, Via S. Agostino 1, 62032 Camerino, Italy.

b

School of Science and Technology, Chemistry Section, University of Camerino, Via S. Agostino 1, 62032 Camerino, Italy. c

Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy. d

Chemical Engineering Department, ISEL,

R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal. e

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049‐001 Lisboa, Portugal.

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to).

*Corresponding Authors

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ABSTRACT Coupling five, rigid or flexible, bis(pyrazolato)-based tectons with late transition metal ions allowed us the isolation of eighteen coordination polymers (CPs). As assessed by thermal analysis, all of them possess a remarkable thermal stability, their decomposition temperatures lying in the range 340-500 °C. As demonstrated by N2 adsorption measurements at 77 K, their Langmuir specific surface areas span the rather vast range of 135-1758 m2/g, in agreement with the porous or dense polymeric architectures retrieved by PXRD structure solution methods. Two representative families of CPs, built up with either rigid or flexible spacers, were tested as catalysts in i) the microwaveassisted solvent-free peroxidative oxidation of alcohols by t-BuOOH, and ii) the peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone by H2O2 in acetonitrile. Those CPs bearing the rigid spacer, concurrently possessing higher specific surface areas, are more active than the corresponding ones with the flexible spacer. Moreover, the two copper(I)-containing CPs investigated exhibit the highest efficiency in both reactions, leading selectively to a maximum product yield of 92% (and TON up to 1.5×103) in the oxidation of 1-phenylethanol and of 11% in the oxidation of cyclohexane, the latter value being higher than that granted by the current industrial process.

KEYWORDS Coordination polymers, pyrazole-based ligands, thermal stability, X-ray powder diffraction, catalytic peroxidative oxidation, alcohols, cyclohexane.


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INTRODUCTION Approximately 90% of the chemical processes implemented at the industrial level require the use of catalysts: as a matter of fact, about 60% of all consumer and industrial products are obtained by means of catalytic reactions. As such, catalysis generates annual global sales higher than 1.5 trillion dollars. Most catalytic processes are heterogeneous and rely on the use of transition metal particles dispersed on inert supports with high specific surface area. In this regard, (porous) coordination polymers (CPs) may be a versatile alternative to all-metal catalysts for a variety of organic reactions. A very attractive feature of porous CPs is the evidence that they can be used as nano-reactors in a number of catalytic processes in which size- and shapeselectivity is granted by the homogeneous dimension of the pores and the decoration of their walls.1 Another aspect of porous CPs possibly concurring to favor catalytic activity is the presence of exposed metal sites.2 The past two decades have known an incessantly growing interest for the self-assembly of CPs3 from (transition) metal ions and organic spacers, because of the outstanding functional properties that CPs might possess - e.g. in the fields of nonlinear optics, photosensitivity, magnetism, molecular recognition, gas adsorption - which are intimately related to their crystal and molecular structures. Several advances have been made in the design and rational prediction of CPs crystal structure. However, this predictive ability is primarily limited to CPs fabricated with rigid ligands. By contrast, flexible spacers4 may adopt a number of different conformations, also according to the restrictions imposed by the coordination geometry of the metal ion, thus complicating the estimation of the final structural architecture. For example, Sun and co-workers have recently designed and synthesized a family of CPs based on a series of flexible, imidazole-containing ligands, the different conformations of which resulted in different topologies.5 A key characteristic that CPs should possess for their practical application, irrespective of their functionality, is a remarkable chemical and thermal inertness. We have a long-standing experience in the synthesis of rigid or flexible poly(pyrazole)-based tectons6 which, coupled to transition metal ions, have typically lead to the formation of materials in which key structural topologies are coupled to high thermal stability7 and, in specific occasions, chemical resistance to harsh acidic and basic conditions or boiling water.8 As their poly(carboxylato)-containing counterparts, also poly(azolato)-based CPs have been tested in catalytic reactions. Examples of CPs containing nitrogen-based ligands which are suitable to catalyze oxidation reactions are related to the conversion of benzyl alcohol into benzaldehyde by means of molecular oxygen,9 to the synthesis of hydroxy ketones starting from silyl enolates,10 to the oxidation of ketones by means of molecular oxygen,11 and to the oxidation, performed by our


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group12 and by others,13 of cycloalkanes (cyclohexane and cyclopentane) to the corresponding ketones and alcohols by means of H2O2. In this context, following exclusively solvothermal routes, we have coupled five, rigid or flexible bis(pyrazole)-based spacers (Scheme 1) to late transition metal ions, isolating eighteen novel CPs (Table 1). In the following, for all of them, details about the syntheses, the thermal behavior and the adsorption performances toward N2 at 77 K are provided. The crystal and molecular structures of those CPs isolated as polycrystalline powders, retrieved by powder X-ray diffraction methods, are described. Finally, two representative families of CPs, one with a flexible spacer, the other with a rigid one, have been tested as catalysts in the microwave-assisted solventfree oxidation of a number of linear and cyclic alcohols with tert-butyl hydroperoxide (t-BuOOH), and of cyclohexane with hydrogen peroxide (H2O2). The efficiency of these reactions has been investigated. The influence of various parameters, such as reaction time, solvents, type and amount of catalyst, temperature and presence of additives, has been evaluated. The results of this extensive study are reported and discussed. EXPERIMENTAL SECTION Materials and Methods All the chemicals and reagents employed were purchased from Sigma Aldrich Co. and used as received without further purification. All the solvents were distilled prior to use. Melting points were recorded with an SMP3 Stuart instrument equipped with a capillary apparatus. Elemental analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNS-O elemental analyzer. 1H NMR spectra were recorded by using a Varian Mercury 400 Plus instrument operating at room temperature and 400 MHz; 1H chemical shifts are reported in the following in parts per million based on SiMe4 as standard. IR spectra were recorded from 4000 to 650 cm-1 with a Perkin-Elmer Spectrum 100 instrument by total reflectance on a CdSe crystal. Thermogravimetric analyses (TGA) were carried out in a N2 stream with a Perkin-Elmer STA 6000 simultaneous thermal analyzer with a heating rate of 8 °C/min. The catalytic tests under microwave (MW) irradiation were performed in a focused Anton Paar Monowave 300 microwave reactor fitted with a rotational system and an IR temperature detector, using a 10 mL capacity reaction tube with a 13 mm internal diameter. Gas chromatographic (GC) measurements were carried out using a FISONS Instruments GC 8000 series gas chromatograph equipped with a FID detector and a capillary column (DBWAX, column length: 30 m; column internal diameter: 0.32 mm) and run by the software JascoBorwin v.1.50. The temperature of injection was 240 °C. After the injection, the reaction temperature was maintained at either 100 °C (oxidation of cyclohexane) or 140 °C (oxidation of ACS Paragon Plus Environment

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alcohols) for 1 min, then raised, by 10 °C/min, either to 180 °C (oxidation of cyclohexane) or 220 °C (oxidation of alcohols) and held at this temperature for 1 min. Helium was used as the carrier gas. GC-MS analyses were performed using a Perkin Elmer Clarus 600 C instrument equipped with a 30 m × 0.22 mm × 25 µm BPX5 (SGE) capillary column, using helium as the carrier gas. The inductively coupled plasma (ICP) analyses were carried out by the Analytical Services of the Instituto Superior Técnico, Lisboa, Portugal. Synthesis of the Ligands. Bis(3,5-dimethyl-4-pyrazolyl)methane (H2L1), bis(3-methyl-4pyrazolyl)methane (H2L2) and bis(4-pyrazolyl)methane (H2L3) (Scheme 1) were synthesized according to the method originally reported by Trofimenko.14 The white solids thus obtained were further recrystallized from boiling water and characterized by elemental analysis, IR, 1H NMR, and measurement of the melting point. 1,4-Bis-4’-(3’,5’-dimethyl)-pyrazolylbenzene (H2L4) and 1,4bis-4’-(3’,5’-dimethyl-pyrazolyl)biphenyl (H2L5) (Scheme 1) were synthesized according to a general procedure already reported in the literature, i.e. by condensation of the proper bis(diketone) with hydrazine hydrate in water.15 Bis(diketones) were prepared following the procedure reported by Ramirez in 1967,16 involving the use of freshly synthesized oxyphosphoranes.17 The reader is addressed to the Supplementary Information for all the details upon the syntheses of each ligand. Synthesis of the CPs. CPs 1M-5M (Table 1) could be successfully synthesized by adopting the two different procedures reported in the following. Regardless the synthetic path followed, the batches obtained for a given material presented analytical data and specific surface areas in very close agreement. Low temperature synthetic path. A solution of the metal salt (nitrate or acetate) (0.25 mmol) in methanol (5 mL) was added to a solution of the bis(pyrazole)-based ligand (0.25 mmol) in dimethylformamide (DMF) (5 mL). The resulting clear solution was poured into a high-pressure tube equipped with a magnetic stirring bar. The tube was heated at 140 °C for 2 days under stirring. Then, it was cooled down to room temperature at a cooling rate of 4 °C/h. The resulting precipitate was filtered off, washed with DMF and CH2Cl2 and heated to reflux in benzene for 6 h. The excess of benzene was removed by decantation. In order to completely remove it, the wet CPs where frozen, lyophilized in vacuum and outgassed in high vacuum (0.005 mbar) at 60 °C overnight. In order to obtain single crystals suitable for the determination of the crystal structures, several syntheses were performed without stirring and heating from room temperature to 140 °C at a rate of 4 °C/h; in spite of this, no crystals of adequate quality were ever obtained.


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High temperature synthetic path. A solution of the metal salt (nitrate or acetate) (0.25 mmol) in methanol (2 mL) was added to a solution of the bis(pyrazole)-based ligand (0.25 mmol) in DMF (1 mL). The resulting clear solution was poured into a Teflon-lined steel autoclave. The reaction mixture was heated at 200 °C for 2 days and cooled down to room temperature at a cooling rate of 4 °C/h. The precipitated CPs were filtered off, washed with methanol and dried in vacuum (ca. 8 mbar), in a vacuum drying oven, at 100 °C. Analytical Data for CPs 1M. [Co(L1)], 1Co. Yield: 65% of blue solid. 1Co is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C11H14CoN4 (FW = 261.18 g/mol): C, 50.58; H, 5.40; N, 21.45%. Found: C, 50.33; H, 5.34; N, 21.35%. IR (cm-1): 1571, 1503, 1420, 1375, 1320, 1208, 1134, 987, 813, 765, 660. Langmuir specific surface area: 1021 m2/g. BET specific surface area: 730 m2/g. Main crystallographic details: C11H14CoN4, tetragonal, P4122, Z = 4, a = 9.2523(6) Å, c = 13.449(1) Å, V = 1151.3(2) Å3; Dc = 1.51 g/cm3, µ(CuKα) = 115.2 cm-1, F(000) = 540; Rp = 0.015, Rwp = 0.021 and RBragg = 0.82 for 4851 data and 29 independent parameters. [Cu2(L1)], 1Cu. Yield: 43% of white solid. 1Cu is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C11H14Cu2N4 (FW = 329.35 g/mol): C, 40.11; H, 4.28; N, 17.01%. Found: C, 39.96; H, 4.22; N, 16.93%. IR (cm-1): 1505, 1426, 1372, 1342, 1216, 1152, 994, 776. Langmuir specific surface area: 957 m2/g. BET specific surface area: 531 m2/g. Main crystallographic details: C11H14Cu2N4, orthorhombic, Pbca, Z = 8, a = 17.9734(4) Å, b = 17.2179(4) Å, c = 7.9530(4) Å, V = 2461.2(1) Å3; Dc = 1.78 g/cm3; µ(CuKα) = 41.1 cm-1, F(000) = 1328; Rp = 0.024, Rwp = 0.040 and RBragg = 3.30 for 4851 data and 46 independent parameters. [Zn(L1)], 1Zn. Yield: 70% of white solid. 1Zn is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C11H14N4Zn (FW = 267.65 g/mol): C, 49.36; H, 5.27; N, 20.93%. Found: C, 49.42; H, 5.21; N, 21.02%. IR (cm-1): 1571, 1506, 1424, 1377, 1330, 1211, 1150, 991, 765, 662. Langmuir specific surface area: 543 m2/g. BET specific surface area: 409 m2/g. [Cd(L1)], 1Cd. Yield: 68% of white solid. 1Cd is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C11H14CdN4 (FW = 314.67 g/mol): C, 41.99; H, 4.48; N, 17.81%. Found: C, 42.04; H, 4.40; N, 17.68%. IR (cm-1): 1497, 1418, 1310, 1206, 1127, 1022, 983, 801, 758, 751. Langmuir specific surface area: 135 m2/g. BET specific surface area: 21 m2/g. Main crystallographic details: C11H14CdN4, orthorhombic, Pcca, Z = 4, a = 16.9833(5) Å, b = 8.4671(2)


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Å, c = 7.9321(2) Å, V = 1140.63(6) Å3; Dc = 1.83 g/cm3; µ(CuKα) = 151.9 cm-1, F(000) = 624; Rp = 0.072, Rwp = 0.098 and RBragg = 5.62 for 4776 data and 35 independent parameters. Analytical Data for CPs 2M. [Co(L2)], 2Co. Yield: 75% of blue solid. 2Co is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C9H10CoN4 (FW = 233.14 g/mol): C, 46.37; H, 4.32; N, 24.03%. Found: C, 46.21; H, 4.25; N, 23.92%. IR (cm-1): 1502, 1421, 1376, 1313, 1206, 1132, 983, 897, 765, 755. Langmuir specific surface area: 1213 m2/g. BET specific surface area: 553 m2/g. [Cu2(L2)], 2Cu. Yield: 43% of white solid. 2Cu is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C9H10Cu2N4 (FW = 301.29 g/mol): C, 35.88; H, 3.35; N, 18.60%. Found: C, 35.61; H, 3.21; N, 18.43%. IR (cm-1): 1586, 1509, 1436, 1413, 1379, 1299, 1206, 1139, 1050, 1000, 834, 737, 669. Langmuir specific surface area: 678 m2/g. BET specific surface area: 462 m2/g. [Cd(L2)], 2Cd. Yield: 80% of white solid. 2Cd is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C9H10CdN4 (FW = 286.61 g/mol): C, 37.72; H, 3.52; N, 19.55%. Found: C, 37.56; H, 3.40; N, 19.37%. IR (cm-1): 1496, 1417, 1367, 1314, 1203, 1127, 1021, 980, 801, 751, 669. Langmuir specific surface area: 426 m2/g. BET specific surface area: 167 m2/g. Analytical Data for CPs 3M. [Co(L3)], 3Co. Yield: 78% of blue solid. 3Co is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C7H6CoN4 (FW = 205.09 g/mol): C, 41.00; H, 2.95; N, 27.32%. Found: C, 40.81; H, 2.84; N, 27.43%. IR (cm-1): 1558, 1441, 1397, 1361, 1282, 1151, 1059, 1009, 848, 832, 822, 747, 720. Langmuir specific surface area: 849 m2/g. BET specific surface area: 576 m2/g. Main crystallographic details: C7H6CoN4, monoclinic, C2/c, Z = 4, a = 9.5731(7) Å, b = 14.383(1) Å, c = 7.0945(5) Å, β = 117.792(4)°, V = 864.2(1) Å3; Dc = 1.58 g/cm3, µ(CuKα) = 151.8 cm-1, F(000) = 412; Rp = 0.016, Rwp = 0.022 and RBragg = 1.16 for 4726 data and 32 independent parameters. [Cu(L3)], 3Cu. Yield: 64% of pale green solid. 3Cu is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C7H6CuN4 (FW = 209.07 g/mol): C, 40.09; H, 2.88; N, 26.72%. Found: C, 40.25; H, 2.79; N, 26.37%. IR (cm-1): 1438, 1397, 1371, 1285, 1165, 1077, 1058, 1015, 823, 750, 720, 668. Langmuir specific surface area: 351 m2/g. BET specific surface area: 142 m2/g. Main crystallographic details: C7H6CuN4, orthorhombic, Cmcm, Z = 4, a = 8.8943(8) Å, b = 12.185(2) Å, c = 7.2837(9) Å, V = 789.4(2) Å3; Dc = 1.76 g/cm3, µ(CuKα) = 34.5 cm-1, F(000) = 420; Rp = 0.016, Rwp = 0.021 and RBragg = 1.33 for 4726 data and 27 independent parameters.


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[Zn(L3)], 3Zn. Yield: 78% of white solid. 3Zn is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C7H6N4Zn (FW = 211.54 g/mol): C, 39.74; H, 2.86; N, 26.49%. Found: C, 39.85; H, 2.80; N, 26.31%. IR (cm-1): 1561, 1398, 1371, 1285, 1158, 1072, 1012, 852, 835, 750, 718. Langmuir specific surface area: 347 m2/g. BET specific surface area: 203 m2/g. Main crystallographic details: C7H6N4Zn, monoclinic, C2/c, Z = 4, a = 9.5706(7) Å, b = 14.558(1) Å, c = 7.1107(6) Å, β = 118.259(4)°, V = 872.6(1) Å3; Dc = 1.61 g/cm3, µ(CuKα) = 35.0 cm-1, F(000) = 424; Rp = 0.058, Rwp = 0.075 and RBragg = 1.90 for 4751 data and 36 independent parameters. [Cd(L3)], 3Cd. Yield: 80% of white solid. 3Cd is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C7H6CdN4 (FW = 258.56 g/mol): C, 32.52; H, 2.34; N, 21.67%. Found: C, 32.37; H, 2.41; N, 21.54%. IR (cm-1): 1650, 1390, 1359, 1278, 1157, 1061, 1016, 992, 850, 831, 747. Langmuir specific surface area: 1150 m2/g. BET specific surface area: 911 m2/g. Main crystallographic details: C14H12Cd2N8, monoclinic, P21/a, Z = 4, a = 14.775(1) Å, b = 15.246(1) Å, c = 9.3099(7) Å, β = 79.504(6)°, V = 2062.0(3) Å3; Dc = 1.67 g/cm3, µ(CuKα) = 166.7 cm-1, F(000) = 992; Rp = 0.051, Rwp = 0.067 and RBragg = 3.68 for 4851 data and 58 independent parameters. Analytical Data for CPs 4M. [Co(L4)], 4Co. Yield: 80% of blue solid. 4Co is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C16H16CoN4 (FW = 323.26 g/mol): C, 59.45; H, 4.99; N, 17.33%. Found: C, 59.28; H, 5.08; N, 17.27%. IR (cm-1): 1573, 1491, 1423, 1377, 1317, 1283, 1122, 1046, 1012, 846, 788. Langmuir specific surface area: 1573 m2/g. BET specific surface area: 791 m2/g. Main crystallographic details: C16H16CoN4, tetragonal, P42/nnm, Z = 4, a = 18.6019(6) Å, c = 7.2745(5) Å, V = 2517.2(2) Å3; Dc = 0.85 g/cm3, µ(CuKα) = 54.0 cm-1, F(000) = 668; Rp = 0.036, Rwp = 0.059 and RBragg = 3.97 for 4951 data and 44 independent parameters. [Cu2(L4)], 4Cu. Yield: 50% of white solid. 4Cu is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C16H16Cu2N4 (FW = 391.42 g/mol): C, 49.10; H, 4.12; N, 14.31%. Found: C, 49.01; H, 4.15; N, 14.24%. IR (cm-1): 1559, 1497, 1425, 1372, 1344, 1291, 1053, 1014, 845, 667. Langmuir specific surface area: 1758 m2/g. BET specific surface area: 939 m2/g. [Zn(L4)], 4Zn. Yield: 64% of white solid. 4Zn is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C16H16N4Zn (FW = 329.72 g/mol): C, 58.28; H, 4.89; N, 16.99%. Found: C, 58.14; H, 4.74; N, 16.85%. IR (cm-1): 1568, 1495, 1430, 1379, 1341, 1285, 1167, 1113, 1048, 1013, 846, 815, 665. Langmuir specific surface area: 1028 m2/g. BET specific surface area: 647 m2/g.


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[Cd(L4)], 4Cd. Yield: 75% of white solid. 4Cd is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C16H16CdN4 (FW = 376.74 g/mol): C, 51.01; H, 4.28; N, 14.87%. Found: C, 50.88; H, 4.31; N, 14.76%. IR (cm-1): 1556, 1492, 1417, 1316, 1282, 1112, 1044, 1012, 860, 843, 782. Langmuir specific surface area: 1549 m2/g. BET specific surface area: 508 m2/g. Analytical Data for CPs 5M. [Co(L5)], 5Co. Yield: 80% of blue solid. 5Co is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C22H20CoN4 (FW = 399.35 g/mol): C, 66.17; H, 5.05; N, 14.03%. Found: C, 66.02; H, 4.94; N, 13.97%. IR (cm-1): 1570, 1492, 1423, 1377, 1316, 1284, 1260, 1045, 1012, 845, 807, 678. Langmuir specific surface area: 449 m2/g. BET specific surface area: 376 m2/g. [Zn(L5)], 5Zn. Yield: 72% of white solid. 5Zn is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C22H20N4Zn (FW = 405.81 g/mol): C, 65.11; H, 4.97; N, 13.81%. Found: C, 65.04; H, 4.83; N, 13.76%. IR (cm-1): 1551, 1490, 1426, 1324, 1284, 1048, 1011, 824, 756, 672. Langmuir specific surface area: 329 m2/g. BET specific surface area: 133 m2/g. [Cd(L5)], 5Cd. Yield: 70% of white solid. 5Cd is insoluble in all common solvents and in water. Elem. Anal. Calcd. for C22H20CdN4 (FW = 452.84 g/mol): C, 58.35; H, 4.45; N, 12.37%. Found: C, 58.32; H, 4.51; N, 12.24%. IR (cm-1): 1593, 1553, 1420, 1377, 1315, 1282, 1118, 1047, 1010, 823, 789, 670. Langmuir specific surface area: 510 m2/g. BET specific surface area: 427 m2/g. Main crystallographic details: C22H22CdN4O,18 orthorhombic, Pcc2, Z = 2, a = 15.2159(4) Å, b = 9.0611(3) Å, c = 7.7533(2) Å, V = 1068.98(6) Å3; Dc = 1.41 g/cm3, µ(CuKα) = 84.1 cm-1, F(000) = 386; Rp = 0.064, Rwp = 0.085 and RBragg = 3.77 for 5001 data and 27 independent parameters. Powder X-ray Diffraction Crystal Structures Determination. Polycrystalline samples of compounds 1Co, 1Cu, 1Cd, 3M, 4Co and 5Cd were gently ground in an agate mortar.19 Then, they were deposited in the hollow of an aluminum sample holder equipped with a zero-background plate. Diffraction data were collected at room temperature by means of overnight scans in the 2θ range 5105°, with steps of 0.02°, on a Bruker AXS D8 Advance diffractometer, equipped with Ni-filtered Cu-Kα radiation (λ = 1.5418 Å), a Lynxeye linear position-sensitive detector, and the following optics: primary beam Soller slits (2.3°), fixed divergence slit (0.5°), receiving slit (8 mm). The generator was set at 40 kV and 40 mA. A standard peak search, followed by indexing through the Singular Value Decomposition approach20 implemented in TOPAS-R,21 allowed to retrieve the approximate unit cell parameters of the nine species. The space groups were assigned on the basis of systematic absences. Structure solutions were performed by the simulated annealing technique,


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implemented in TOPAS-R, employing a rigid, idealized model for the crystallographic independent portion of the ligands.22 When viable, the torsion angles around the exocyclic C-C bond were let refine. The final refinements were carried out by the Rietveld method, maintaining the idealized models introduced at the solution stage. The peak shapes were described with the fundamental parameters approach.23 When necessary, peak shape anisotropy was modeled with the aid of spherical harmonics. The background was modeled by a polynomial function. One, refined, isotropic thermal parameter (Biso) was assigned to the metal atoms (BM), lighter atoms being given a Biso = BM + 2.0 Å2 value. The presence of preferred orientation was taken into account by adopting the March-Dollase model24 for 1Cd (along [010]). The final Rietveld refinement plots are collectively supplied in Figures S1-S3 of the Supporting Information. Fractional atomic coordinates are provided in the Supporting Information as CIF files. X-ray crystallographic data in CIF format have been deposited at the Cambridge Crystallographic Data Centre as supplementary publications no.s 1033388-1033396. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-335033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk). Activation of CPs by Supercritical Drying. Supercritical CO2 drying was performed using a Polaron Critical Point Dryer. In a typical experiment, the solvent molecules clathrated in a 50-100 mg sample of as synthesized CP were exchanged with anhydrous ethanol for a period of 1 to 3 days, refreshing the solution every day. The solvent-exchanged sample was separated from the excess ethanol on the glass frit (but not allowed to dry on the filter) and transferred to the Critical Point Dryer. The latter was flushed with CO2 for 1 min, then the venting valve was closed and the chamber was cooled down to 7-9 °C in order to make CO2 condensing inside. After filling the chamber to approximately 60%, the inlet valve was closed and the sample was left to soak into liquid CO2 for 10 h, purging liquid CO2 and refilling the chamber every 2 h. Finally, the drying chamber was heated to 45 °C for 2 h and the supercritical fluid was slowly released. After the supercritical drying, the sample was outgassed at 50 °C in vacuum (0.005 mbar) for 10 h. Gas Adsorption Measurements. Specific surface areas of the eighteen CPs were determined by N2 adsorption at 77 K. Adsorption isotherms were acquired by the volumetric method in liquid nitrogen baths using UHP-grade nitrogen (ALPHAGAZ™ 1 NITROGEN, 99.999% purity), with a Beckman Coulter SA3100 instrument. Ultra-high purity He (ALPHAGAZ™ 1 HELIUM, 99.999% purity) was used for free-space measurements. After supercritical drying, ca. 50 mg of CP were introduced into pre-weighed analysis tubes and outgassed overnight at 50 °C under high vacuum (10-3 mbar) before carrying out the adsorption experiments. Brunauer-Emmett-Teller (BET) specific


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surface areas were determined by fitting the low-pressure zone of the N2 adsorption isotherms by the BET equation and considering the molecular area of N2 at 77 K to be 0.162 nm2.25 Langmuir specific surface areas were determined by the Langmuir equation and considering the molecular area of N2 at 77 K to be 0.162 nm2. Catalytic Tests. Microwave-assisted solvent-free peroxidative oxidation of alcohols. Oxidation reactions of alcohols were carried out in sealed cylindrical Pyrex tubes under focused microwave irradiation (see Materials and Methods Section), by adopting the following procedure: the alcohol (2.5 mmol), the catalyst (1-15 µmol) and a 70% aqueous solution of t-BuOOH (5 mmol) were introduced into the tube. This was then placed into the microwave reactor and the system was left under stirring and irradiation (5 or 20 W), at 80 ºC or 120 ºC, respectively, for 0.08-15 h. After cooling down to room temperature, 150 µL of benzaldehyde, adopted as internal standard, were added together with 2.5 mL of acetonitrile, to extract the substrate and the organic products from the reaction mixture. The obtained mixture was stirred for 10 min; then, an aliquot (1 µL) was taken from the organic phase and analyzed by GC using the internal standard method. Blank experiments, in the absence of any catalyst, were performed under the studied reaction conditions for different alcohols: no significant conversion was observed. Peroxidative oxidation of cyclohexane with aqueous H2O2 in acetonitrile. The peroxidative oxidation of cyclohexane was carried out by adopting the following procedure: the catalyst (0.2-10 µmol) was dissolved in MeCN (3.00 mL) under vigorous stirring; thereafter, cyclohexane (0.54 mL, 5.00 mmol) and 30% H2O2 (1.02 mL, 10.00 mmol) were added. The resulting solution was stirred for 24 h at room temperature and 1 atm. In the experiments carried out in the presence of radical traps, either CBrCl3 (5.00 mmol) or NHPh2 (5.00 mmol) were added to the reaction mixture. The products analysis was performed as follows: 90 µL of cycloheptanone were added as internal standard together with 10.00 mL of diethyl ether, to extract the substrate and the organic products from the reaction mixture, together with an excess of triphenylphosphine, in order to reduce the formed cyclohexyl hydroperoxide to the corresponding alcohol, and hydrogen peroxide to water, following a method developed by Shul’pin.26 The obtained mixture was stirred for 10 min; then an aliquot (1 µL) was taken from the organic phase and analyzed by GC using the internal standard method. Blank experiments were performed with different amounts of H2O2 and other reagents and confirmed that no product of cyclohexane oxidation was obtained unless the CPs were present.


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RESULTS AND DISCUSSION Synthesis of Species 1M-5M. The solvothermal reactions of Co(II), Cu(II), Zn(II), and Cd(II) nitrates or acetates with the bis(pyrazole)-based ligands H2L (Scheme 1) brought about the formation of eighteen CPs of formula [Mx(L)] (x = 2 for 1Cu, 2Cu, and 4Cu; x = 1 in all the other cases – Table 1). Remarkably, the reactions were successful not only when carried out at 200 °C (high temperature synthetic protocol – see the Experimental Section), but also in the milder conditions of 140 °C (low temperature synthetic protocol – see the Experimental Section): regardless the synthetic path adopted, the different batches obtained for a given CP presented analytical data and specific surface areas in very close agreement. All the CPs were isolated in reasonable to high yields in the form of air-stable powders, insoluble in the most common solvents and in water, this evidence suggesting their polymeric nature. As a matter of fact, the absence of N−H stretching bands in their IR spectra indicates the formation of metal-bridging dianions. For some of the copper-containing derivatives (1Cu, 2Cu, and 4Cu), the high-temperature reaction between copper(II) acetate and H2L brought about the reduction of Cu(II) to Cu(I). Indeed, the Cu(II)-containing green suspension appeared at the beginning of the reaction gradually transformed into a Cu(I)-based white precipitate. Reduction of Cu(II)-based pyrazole-containing complexes promoted by heating has been already observed by us1i and other authors27 and attributed to the partial oxidation of pyrazole.28 In the present case, the isolation of 1Cu, 2Cu and 4Cu might have been accompanied by the oxidation of either the bis(pyrazolato)-based ligand or the methanol employed as solvent.29 PXRD Structural Analysis. For all polycrystalline CPs for which a structure determination was feasible, a brief description of the crystal structure is reported in the following.19 Crystal and molecular structure of 1Co. Compound 1Co crystallizes in the tetragonal space group P4122. The asymmetric unit is composed by one Co(II) ion and one L12- ligand, both lying on crystallographic twofold axes. Within a tetrahedral coordination sphere, the Co(II) ions are coordinated by four nitrogen atoms (Figure 1a) and are maintained about 3.5 Å apart, by bridging pyrazolato moieties, along 1-D helical chains (Figure 1b) winding up along the 41 screw axes parallel to the crystallographic axis c. By means of the spacers, each chain is connected, along the crystallographic axes a and b, to four nearby ones. This results into the formation of a bimodal (4,4)-connected 3-D network (Figure 1c) in which the metal centers can be assimilated to tetrahedral tetra-connected nodes, while the ligands can be visualized as square planar tetraconnected nodes. The network possesses a negligible percentage of empty volume (2.5% of the unit ACS Paragon Plus Environment

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cell volume, corresponding to a pore volume of only 0.02 cm3/g).30 This feature is in contrast with the rather high Langmuir specific surface area estimated for this species from the N2 adsorption isotherm (1021 m2/g, see below); we suggest that either framework flexibility, allowed by the flexible spacer, is at work during the adsorption process, or a transition phase takes place when the material is brought at low temperature.31 Crystal and molecular structure of 1Cu. Compound 1Cu crystallizes in the orthorhombic space group Pbca. The asymmetric unit is composed by two metal ions and one L12- spacer, all lying on general positions. Both Cu(I) centers are bi-coordinated and show a slightly bent stereochemistry (Figure 2a). Bridged by pyrazolato moieties, they alternate, at a distance of about 3.1 Å, along 1-D chains (Figure 2b) winding up parallel to the crystallographic axis c. Each chain is bridged to two nearby ones by the ligands, which results into the formation of 2-D corrugated slabs parallel to the ac plane and stacking along b (Figure 2c). Overall, by assimilating the ligands to square planar tetra-connected nodes and disregarding the metal ions, which are bi-connected, 1Cu can be classified as a uninodal 4-connected 2-D layer with sql topology. The methyl groups protrude outside the slabs occupying the inter-slab volume, so that the crystal structure features no empty volume. Again, to explain the observed adsorption performances (see below), either framework flexibility or a low temperature phase transition could be invoked.31 Crystal and molecular structure of 1Cd. Compound 1Cd crystallizes in the orthorhombic space group Pcca. The asymmetric unit is composed by one metal ion and one L12- ligand, both lying on a crystallographic twofold axis. Tetrahedral CdN4 nodes (Figure 3a) are kept about 3.9 Å apart (c/2) by the spacers along 1-D linear chains running parallel to the crystallographic axis c (Figure 3b). Each chain is bridged, along the crystallographic axis a, to two neighboring ones ca. 8.5 Å distant (a/2). As a result, the overall structural motif consists of 2-D corrugated slabs parallel to the ac plane and stacking, eclipsed, along b (Figure 3c). As the spacers are not planar, 1-D rhombic channels parallel to c and hosting the methyl groups are present within the slabs. Overall, the crystal structure does not feature void volume, in agreement with the negligible specific surface area found. As it will be evident below, CPs 3Co, 3Cu and 3Zn possess an analogous structural motif: the “bent” ligand thus acts as a structure directing agent. Overall, by assimilating the ligands to square planar tetra-connected nodes and the metal ions to tetrahedral tetra-connected nodes, 1Cu can be classified as a binodal (4,4)-connected 2-D layer with sql topology. Crystal and molecular structures of 3Co, 3Cu and 3Zn. In spite of the fact that they crystallize in different space groups, the Co(II), Cu(II) and Zn(II) derivatives 3Co, 3Cu and 3Zn possess the same 2-D structural motif. Hence, in the following, their crystal and molecular structures will be ACS Paragon Plus Environment

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described jointly. Compound 3Cu crystallizes in the orthorhombic space group Cmcm. Its asymmetric unit is composed by one metal ion, lying on a crystallographic inversion centre, and one L32- ligand, lying about a crystallographic mirror plane. On the other hand, the isomorphous and isostructural species 3Co and 3Zn crystallize in the monoclinic space group C2/c (a proper subgroup of Cmcm). Their asymmetric unit contains one metal ion and one L32- ligand, both lying about a crystallographic twofold axis. In all of the three species, the metal ions are tetra-coordinated by four nitrogen atoms of four different pyrazolato rings (Figures S4a and 4a, for and 3Co and 3Cu, respectively). Obviously, due to their specific stereochemical requirements, the Co(II) and Zn(II) ions show a tetrahedral geometry, while the Cu(II) ones possess a square planar coordination sphere. With one of its pyrazolato moieties, each ligand bridges metal ions about 3.6 Å (c/2) apart. This leads to the formation of 1-D zig-zag (3Co and 3Zn) or linear (3Cu) chains running along the crystallographic axis c (Figures S4b and 4b). Each ligand employs the second pyrazolato ring to bridge, along the crystallographic axis a, one nearby chain 8.5-8.9 Å (a/2) distant. Overall, each chain is bridged to two neighboring ones, this bringing about the formation of 2-D corrugated slabs parallel to the ac plane (Figures S4c and 4c). The slabs pile, staggered, along b (Figure S4c and 4c), with no evidence of inter-slab non bonding interactions. Again, due to the fact that the spacers are bent, 1-D rhombic channels running along c are present within the slabs (Figures S4c and 4c). On the whole, by considering both the ligands and the metal centers as tetra-connected nodes, 3Co, 3Cu and 3Zn are binodal (4,4)-connected 2-D layers with sql topology. Differently from 1Cu, in which methyl groups protrude within the channels, an empty volume of 6.2, 10.0 and 5.9% of the unit cell volume can be estimated30 in 3Co, 3Cu and 3Zn, respectively, corresponding to a pore volume of 0.04, 0.06 and 0.04 cm3/g. Crystal and molecular structure of 3Cd. Compound 3Cd does not share the same structural motif as 3Co and 3Zn, assembled with the same ligand and with metal centers having the same stereochemical preferences, nor as 1Cd, in which the two pyrazolato rings of the spacer are bismethylated. 3Cd crystallizes in the monoclinic space group P21/a. Its asymmetric unit is composed by two metal ions and two L32- ligands, all lying on general positions. The Cd(II) ions are tetracoordinated by four nitrogen atoms of four different pyrazolato moieties and show, as expected, a tetrahedral coordination sphere (Figures 5a). With one of its pyrazolato rings, each ligand bridges metal centers about 3.7-3.9 Å apart. This leads to the formation of 1-D zig-zag chains running along the crystallographic axis a (Figures 5b). Each ligand employs the second pyrazolato ring to bridge one nearby chain 8.5-9.5 Å distant. At variance with the other 3M species, in the present case each chain is bridged to four neighboring ones, this implying the formation of a 3-D porous network (Figure 5c) possessing 25.1%30 of empty volume per unit cell, corresponding to a pore volume of ACS Paragon Plus Environment

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0.30 cm3/g. Both independent metal centers can be visualized as tetrahedral tetra-connected nodes, while both independent ligands can be visualized as square planar tetra-connected nodes. Hence, 3Cd can be classified as a two-fold interpenetrated (4,4)-connected 3-D network with pts topology. Crystal and molecular structure of 4Co. A DMF solvate of 4Co, labeled MFU-2, was recently published by Tonigold and colleagues.32 Apart from the symmetry decrease imposed by the presence of solvent molecules within the channels of MFU-2, the latter and 4Co share the same structural motif which, for the sake of completeness, will be described hereafter. Compound 4Co crystallizes in the tetragonal space group P42/nnm. The asymmetric unit contains one Co(II) ion and one L42- spacer, both lying on special positions. Tetrahedral CoN4 nodes (Figure 6a) are kept 3.6 Å (c/2) apart by the pyrazolato moieties along linear chains running parallel to c (Figure 6b). The chains are connected along [110] to four neighboring ones within a 3-D porous framework (Figure 6c). 1-D square channels, hosting cylinders 10 Å wide, run along c and account for a void volume as high as 56.6% of the unit cell volume,30 resulting in a pore dimension of 0.66 cm3/g. Apart from the symmetry modifications imparted by either the functional groups on the spacer or the solvent molecules trapped in the channels, 4Co shares the same structural motif as a number of porous CPs obtained, by us and others, with spacers having the same skeleton ([Co(BDP)]33 and [Zn(BDP_X)], H2BDP = 1,4-bis(1H-pyrazol-4-yl)benzene, X = H,34 NH2, NO2, OH35), or with shorter linkers ([Zn(BPZ)],7c H2BPZ = 4,4′-bipyrazole, and [Zn(Me4BPZ)],7d H2Me4BPZ = 3,3′,5,5′-tetramethyl4,4′-bipyrazole). As such, in all these species the metal centers can be visualized as tetrahedral, tetra-connected nodes, while the ligands can be assimilated to square planar tetra-connected nodes. This results into the formation of a (4,4)-connected 3-D network with pts topology. Crystal and molecular structure of 5Cd. Compound 5Cd crystallizes in the orthorhombic space group Pcc2. The asymmetric unit is composed by one metal ion and one L52- ligand, both lying on a twofold axis. The crystal structure features tetrahedral nodes of the kind CdN4 (Figure 7a) bridged, by the L52- spacers, along [001] and [110]. As a result, 1-D chains of collinear metal ions run along the crystallographic axis c (Figure 7b). Each chain is connected by the linkers to four nearby ones, with the consequent formation of a 3-D network featuring ellipsoidal and slit-like channels running parallel to c and hosting the methyl groups and clathrated water molecules.18 In spite of the steric hindrance of the protruding methyl groups, an empty volume of 11.9%30 of the unit cell volume, corresponding to a pore dimension of 0.08 cm3/g, can be estimated. Finally, assimilating the metal centers to tetrahedral tetra-connected nodes, and the ligands to square planar tetra-connected nodes, 5Cd can be classified as a (4,4)-connected 3-D network with pts topology.


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Thermal Behavior. Thermogravimetric analyses (TGA) were performed on CPs 1M−5M in order to investigate their thermal behavior. The TG traces acquired on species 1M and 4M are displayed as examples in Figure 8a and 8b, respectively. The other TG traces are reported in Figures S5-S7 of the Supplementary Information. Overall, the investigated CPs possess a remarkable thermal robustness: the decomposition process of the less stable species begins at 340 °C, while the highest stability materials survive up to 500 °C. More in detail, species 1Co and 1Zn (Figure 8a) are stable up to about 400 °C, temperature at which a progressive decomposition begins.36 Compounds 1Cu and 1Cd (Figure 8a) demonstrate an even higher thermal stability, their degradation beginning only after 450 °C. Also species 2M (Figure S5), 3Co, 3Zn, 3Cd (Figure S6), 4M (Figure 8b) and 5M (Figure S7) display a high thermal robustness: with the exception of 3Cu, the decomposition of 2M, 3M and 5M starts only at about 400 °C. Even more remarkable, compounds 4M begin to decompose at approximately 500 °C. Finally, compound 3Cu is the less stable material: its decomposition begins at 340 °C. Gas Adsorption Properties. The textural properties of compounds 1M−5M were probed by N2 adsorption measurements at 77 K. N2 adsorption isotherms of CPs 1Co, 1Cu, 1Zn (Figure 9a), 2M (Figure S8), 3M (Figure S9a), 4M (Figure 9b) and 5Cd (Figure S9b) can be classified as type I:37 all of these isotherms show a sharp knee at low relative pressures (p/p0 ∼ 0.01), corresponding to the filling of micropore monolayers, followed by a plateau, suggesting that the permanent porosity of these compounds is mainly due to micropores of quite uniform size. The presence, in all of these isotherms, of a hysteresis loop might be indicative of textural mesoporosity arising from interparticle mesoporous voids. On the other hand, the isotherms of species 5Co and 5Zn can be classified as type II37 (Figure S9b), typical of the adsorption on non porous materials or on materials with pores larger than micropores. Both isotherms exhibit a type 237 hysteresis loop, connected with the presence of a wide distribution of pore size radii. Finally, the isotherm of 1Cd can be classified as type III37 (Figure 9a), likely associated to the non-porous crystal structure of the material. Specific surface areas, estimated using Langmuir and BET methods, are collected, together with the amount of N2 adsorbed at STP conditions, in Table 2. As the latter highlights, specific surface areas span a rather vast range, namely 21-939 m2/g (as retrieved with the BET method) or 135-1758 m2/g (as retrieved with the Langmuir method). Worthy of note, CPs 1Co, 2Co, 3Cd and 4M possess Langmuir specific surface areas higher than 1000 m2/g. Catalytic Studies. Various (linear and cyclic) alcohols, mainly 1-phenylethanol, and cyclohexane were used as model substrates to investigate the catalytic properties of CPs 1M and 4M, representative of species with flexible and rigid ligands, respectively. All the catalytic tests ACS Paragon Plus Environment

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consisted in peroxidative oxidations carried out under mild conditions, using hydrogen peroxide (H2O2) or tert-butyl hydroperoxide (t-BuOOH) as oxidants. Microwave-assisted solvent-free peroxidative oxidation of alcohols. The investigation of the catalytic properties of CPs 1M and 4M was undertaken following a procedure developed by us38 for the oxidation of 1-phenylethanol (Scheme 2) as a model substrate, using t-BuOOH (aq. 70%, 2 eq) as the oxidizing agent; the typical conditions adopted consisted of 3 h reaction time in the absence of any added solvent, at 80 ºC and under low-power (5 W) microwave irradiation. The main results of these tests, in terms of yield and of turnover number (TON, moles of product/mol of catalyst), are summarized in Tables 3 and 4. Whatever CP is adopted as catalyst, acetophenone is the only product obtained in the assayed conditions. The high selectivity observed (typically > 98%) was confirmed by mass balance. The best results were obtained with the copper(I)-based MOFs 1Cu and 4Cu (Table 3, entries 2 and 6, respectively). Under the above described conditions, 1Cu and 4Cu heterogeneously catalyze the peroxidative oxidation of 1phenylethanol leading to 29 and 43% of acetophenone and TONs of 465 and 680, respectively, already with a low catalyst loading (catalyst/substrate molar ratio of 0.06%) and in the absence of any additive. The reactions performed under the same conditions but in the presence of the other CPs resulted in lower yields in acetophenone, between 4 and 19% (Table 3). The CPs were easily separated from the final reaction mixture. ICP determination of the Cu content in the filtered solution (0.0023% and 0.0034% for 1Cu and 4Cu, respectively) discarded the possibility of significant lixiviation. However, the Cu-containing solids recovered at the end of the catalytic cycle did not show catalytic activity in a subsequent run, suggesting the formation of new, inactive species. Blank tests were performed under the same reaction conditions but in the absence of CPs: no significant conversion of 1-phenylethanol was observed. Moreover, replacement of the copper(I) CP by an inorganic salt, namely CuI, resulted in a drastic decrease of activity (compare entries 12 or 22 with entries 23 or 24, Table 4). Worthy of note is the fact that, among all the tested CPs, those with the linear and rigid ligand L42- and possessing higher specific surface areas (Table 2) are more active than the corresponding ones with the bent and flexible spacer L12-. Moreover, 4Cu, possessing a Langmuir specific surface area as high as 1760 m2/g, is, together with 1Cu, the best performing CP in both the peroxidative oxidation reactions explored (see Tables 3-6). Nonetheless, by comparing specific surface areas (Table 2) and yields in the peroxidative oxidation of 1-phenylethanol (Table 3), it clearly emerges that the two parameters do not possess the same trend, so that other properties of the title CPs, besides their specific surface areas, must influence their catalytic activity. Given the outstanding


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performances of the copper(I) derivatives, the metal ion, with its specific stereochemical requirements, should possess a prominent role. Based on the above results, we chose the most active CPs, 1Cu and 4Cu, for further studies. The main results, in terms of yield and TON, are gathered in Tables 4 and 5. First of all, the influence of the amount of 4Cu on yield and TON was investigated (entries 12 and 17-19 of Table 4; Figure 10). The increase in the amount of 4Cu from 1 µmol (0.04 mol% vs. substrate) to 15 µmol (0.6 mol% vs. substrate) resulted in an enhancement of the yield from 18% to 75%. As expected, the increase of catalyst amount implied a TON lowering from 1.5×103 to 248 (Table 4, entries 17 and 19, respectively). The favorable effect of MW irradiation was observed even when applying the low power of 5 W, as already reported for other systems.38,39 For example, only 15% of product was obtained under the same conditions (Table 4, entry 12) but using conventional heating (an oil-bath). As a further confirmation of this observation, after 15 h reaction, yields of 89 and 58% were obtained for MW and conventional heating, respectively, in the presence of 4Cu. The use of hydrogen peroxide (aq. 30%) instead of t-BuOOH resulted in a marked lowering of the yield: for example, in the presence of 1Cu, under the same reaction conditions, a reduction from 45% to only 8% was observed when passing form t-BuOOH to H2O2 (Table 4, entries 1 and 2, respectively), in agreement with the expected decomposition of the latter. Performing the reaction with 1Cu in the presence of acetonitrile (other conditions being preserved) results in a yield lowering (from 45% under solvent-free conditions, entry 1 of Table 4, to 37%, entry 3 of Table 4), while the use of water does not change significantly the yield (44% in water, entry 4 of Table 4; Figure 11). By contrast, the use of a basic aqueous solution of K2CO3 1 M (entry 5 of Table 4; Figure 11) results in a significant increase of the yield with respect to the use of water tout court (from 44% in water to 72% in basic solution). As it was shown before,38d,40,41 basic additives facilitate the deprotonation of the alcohol. We also investigated the influence of 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO), a nitroxyl radical that is a renown38e,42,43 promoter in the catalytic oxidation of alcohols. Indeed, a significant yield increase was observed from 45% and 56% in the absence of TEMPO (for 1Cu and 4Cu, respectively, entries 1 and 12 of Table 4) to 78% in its presence (for both 1Cu and 4Cu, entries 6 and 20 of Table 4). The MW-assisted oxidation of the tested alcohols strongly depends on the reaction temperature. In the presence of 4Cu and in the absence of additives, the high yield of 83% was achieved in acetophenone after 30 min at 120 ºC and 20 W (entry 21 of Table 4), which is definitely higher than the yield of 16% obtained for the same reaction time and under MW irradiation but at 80 ºC (entry


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11 of Table 4). When the reaction was carried out in the presence of 4Cu (0.2 mol%) and of TEMPO (2.5 mol%), after 30 min at 120 ºC a yield of 92% in acetophenone was obtained (entry 22, Table 4). With our system, the obtained yields of ketones in the presence of 4Cu are comparable to those of other successful MW-assisted oxidations of secondary alcohols involving a Cu(II) complex with a Schiff base and diethanolamine ligands,38e 2,4-alkoxy-1,3,5- triazapentadienate Cu(II)-containing complexes,39b or bis- and tris-pyridyl amino and imino thioether copper-based complexes.39c In the conditions we adopted, the peroxidative oxidation of 1-phenylethanol is believed to proceed mainly via a radical mechanism which involves either oxygen- or carbon-radicals. As a matter of fact, a strong inhibiting effect on the catalytic activity was observed upon addition, to the reaction mixture, of Ph2NH or CBrCl3, well known44 oxygen- or carbon-radical traps, respectively (entries 7 and 8 of Table 4; Figure 11). Hence, the oxidation mechanism may involve the metalassisted generation of t-BuO● radicals (upon reduction of t-BuOOH by Cu(I) centers)45 which behave as hydrogen atom abstractors from the alcohol.45a,c-e Conceivably, the mechanism is similar to those proposed in other cases, which are also of a radical type.38e,39b,39c The Cu(I) CPs were also tested toward the oxidation of other alcohols, namely cyclohexanol and benzyl alcohol, modeling a secondary and a primary alcohol, respectively. In the presence of a catalytic amount of 1Cu or 4Cu (0.2 mol% vs. substrate) and in the absence of any additive, the two systems yielded 58 and 78% of cyclohexanone after 30 min of reaction at 120 ºC and 20 W (Table 5, entries 2 and 3, for 1Cu and 4Cu, respectively), and 68 and 75% of benzaldehyde under the same reaction conditions (Table 5, entries 9 and 10, for 1Cu and 4Cu, respectively). Under similar reaction conditions, linear aliphatic alcohols, namely 2-hexanol and 3-hexanol, lead to lower yields, as already reported in other cases:38a,d,45c namely, the oxidation of 2-hexanol and 3-hexanol yielded 45 or 59% of 2-hexanone and 31 or 48% of 3-hexanone, for 1Cu or 4Cu, respectively, after 30 min of reaction at 120 ºC (Table 5, entries 4-7). Peroxidative oxidation of cyclohexane. The copper(I)-based CPs 1Cu and 4Cu (mainly the latter) are also active in the peroxidative oxidation of cyclohexane (CyH) carried out at room temperature with aqueous hydrogen peroxide. The primary product of this oxidation is cyclohexyl hydroperoxide (CyOOH), which further evolves to a mixture of cyclohexanol and cyclohexanone (final products, Scheme 3). In contrast, under the same reaction conditions, the corresponding cobalt(II)-based CPs 1Co and 4Co fail to catalyze the reaction. As a matter of fact, after 24 h stirring the reaction mixture (CyH, H2O2 30% aq. and 4Cu in acetonitrile) at room temperature, an 11% overall yield (relative to the alkane) of cyclohexanol and cyclohexanone (0.98 alcohol/ketone ratio) was obtained with an overall TON of 102 moles of products per mole of 4Cu, i.e. 51 moles of


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Page 20 of 46

product per Cu(I) centre (entry 5 of Table 6). Although the reactions were typically run for 24 h, the much lower reaction time of 6 h was sufficient to attain 9% of product yield (Figure 12). The formation of CyOOH (under the conditions of Table 6) was proved by using the method proposed by Shul’pin.26 As observed in other catalytic systems,46 also in the present case the addition of PPh3 prior to GC analysis of the products resulted in a marked increase of the amount of cyclohexanol (due to the reduction of CyOOH by PPh3, with the formation of phosphane oxide) and in a corresponding decrease of cyclohexanone. The obtained yields are considerably lower than those reported for some dinuclear Cu(II) complexes (22-29%).44a Nonetheless, in the case of 4Cu, in spite of the mild reaction conditions used in our case, they are higher than those characterizing the industrial process when a good selectivity is aimed to.13e,47 Moreover, a high selectivity towards the formation of cyclohexanol and cyclohexanone is exhibited by both 1Cu and 4Cu, since no traces of by-products were detected by GC-MS analysis of the final reaction mixtures. Another noteworthy feature of the tests performed on cyclohexane is the fact that even low CP loadings (typically 0.1 mol% vs. substrate) granted high yields of oxygenated products (Table 6, entry 5). The previously recognized promoting effect of an acidic medium26,44d,48 on the peroxidative oxidation of alkanes is not observed for the present systems (Table 6). Indeed, the presence of pyrazine carboxylic acid has a strong inhibiting effect on the catalytic activity of 1Cu and 4Cu (Table 6, compare entries 1 and 2 or 5 and 6). A similar behavior was found for Cu(II) complexes bearing azathia macrocyles49 and also for C-scorpionate Au(III) complexes.50 The effect of the peroxide-to-catalyst molar ratio was also investigated and is depicted in Figure 13. The increase of the peroxide amount up to a n(H2O2)/n(catalyst) molar ratio of 2×103 led to the highest product yield and TON (Table 6, entry 5). Further increase of the oxidant amount resulted in a drastic yield drop eventually due to overoxidation reactions. Indeed, overoxidation products, such as 1,4-cyclohexanedione or 1,2-epoxycyclohexane, were detected by GC-MS. Finally, as already observed for Cu(II)-based and other metal-containing catalytic systems,46,49,50 also in the present case addition of a radical trap (either CBrCl3 or Ph2NH) to the reaction mixture results in a considerable suppression of the catalytic activity. This behavior, along with the formation of cyclohexyl hydroperoxide (typical intermediate product in radical-type reactions) supports the hypothesis that a free-radical mechanism of the type already reported49,51 is at work for the cyclohexane oxidations carried out in this study.


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CONCLUSIONS Solvothermal reactions of five, flexible or rigid bis(pyrazolato)-based tectons with Co(II), Cu(II), Zn(II) and Cd(II) salts led to the isolation of eighteen coordination polymers. Worthy of note, all of them possess a remarkable thermal stability, as their decomposition temperatures lie in the range 340-500 °C. Their Langmuir specific surface areas span the rather vast range of 135-1758 m2/g. Those CPs for which structure determination was feasible, invariably show (porous or non porous) polymeric architectures. Two out of the five families of CPs were employed as catalysts for the peroxidative oxidation of acyclic and cyclic alcohols (with t-BuOOH) and of cyclohexane (with H2O2). Among all the CPs screened, those with the linear and rigid ligand L42-, showing higher specific surface areas, are more active than the corresponding ones with the bent and flexible spacer L12-. Moreover, the copper(I)based systems 1Cu and 4Cu are the most active ones for both oxidation reactions. Noteworthy, using 1-phenylethanol as substrate, a maximum yield of 92% and a TON up to 1.5×103 are achieved. This oxidation occurs rapidly under microwave irradiation: just after 30 min, in the absence of any solvent or additive, ca. 70% of all the substrate is converted to acetophenone. Notably, the use of low-power MW irradiation in a solvent-free system for the oxidation of alcohols is of great significance towards the development of a green catalytic system. Finally, using cyclohexane, a much more inert substrate, 1Cu and 4Cu allowed us to achieve, at room temperature, without MW irradiation and with a high selectivity, yields up to 11%, which are higher than those granted by the present industrial process in order to operate with a good selectivity.

ASSOCIATED CONTENT Supporting Information. Details about the syntheses of the ligands. Final Rietveld refinement plots for 1Co, 1Cu, 1Cd, 2Cd, 3M, 4Co and 5Cd. TGA traces for 2M, 3M and 5M. Representation of the crystal and molecular structure for 3Co. Nitrogen adsorption isotherms for 2M, 3M and 5M. Crystallographic data for 1Co, 1Cu, 1Cd, 2Cd, 3M, 4Co and 5Cd. in CIF Format. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding authors *CP: Tel: +39-0737402234. E-mail: [email protected] *AJLP: +351-218419237. E-mail: [email protected] *SG: Tel. +39-031-2386627. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The Universities of Camerino and Insubria are acknowledged for partial funding. This work has been partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal, and its project UID/QUI/00100/2015. Norberto Masciocchi is acknowledged for helpful discussions. SG acknowledges the precious help of Elisa Lavigna.


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TABLES

Table 1. Formulas and labels used throughout the manuscript for the title CPs.

CP

Label

CP

Label

CP

Label

[Co(L1)]

1Co

[Co(L2)]

2Co

[Co(L3)]

3Co

[Cu2(L1)]

1Cu

[Cu2(L2)]

2Cu

[Cu(L3)]

3Cu

[Zn(L1)]

1Zn

[Cd(L2)]

2Cd

[Zn(L3)]

3Zn

[Cd(L1)]

1Cd

[Cd(L3)]

3Cd

[Co(L4)]

4Co

[Co(L5)]

5Co

[Cu2(L4)]

4Cu

[Zn(L5)]

5Zn

[Zn(L4)]

4Zn

[Cd(L5)]

5Cd

[Cd(L4)]

4Cd


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Table 2. Key parameters retrieved from the N2 adsorption isotherms of 1M−5M.

CP

BET SSA

Langmuir SSA Adsorbed N2

2

(m /g)

(m2/g)

(cm3/g STP)

1Co

730

1021

235

1Cu

531

957

220

1Zn

409

543

125

1Cd

21

135

31

2Co

553

1213

279

2Cu

462

678

156

2Cd

167

426

98

3Co

576

849

195

3Cu

142

351

81

3Zn

203

347

80

3Cd

911

1150

264

4Co

791

1573

362

4Cu

939

1758

404

4Zn

647

1028

236

4Cd

508

1549

356

5Co

376

449

103

5Zn

133

329

76

5Cd

427

510

117


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Table 3. MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone in the presence of CPs 1M and 4M.[a]

Entry CP

Yield TON[c] (%)[b]

1

1Co

13

168

2

1Cu

29

465

3

1Zn

4

71

4

1Cd

5

109

5

4Co

15

204

6

4Cu

43

680

7

4Zn

15

385

8

4Cd

19

470

[a] Reaction conditions: 2.5 mmol of substrate, (0.06% mol CP vs. substrate), 5 mmol of t-BuOOH (aq. 70%), 80 ºC, 3 h, MW irradiation (5 W). [b] Molar yield (%) based on the substrate, i.e. moles of product per 100 moles of substrate, as determined by gas chromatography. [c] Turnover number, i.e. number of moles of product per mol of catalyst (based on the monomeric unit, i.e. dinuclear in the case of copper or mononulear for the other metals).


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Table 4. Oxidation of 1-phenylethanol in the presence of 1Cu or 4Cu.[a]

Entry

CP

Time

[mol% vs. substr]

(h)

Additive [mol% vs. substr]

Yield

TON

[b]

[TOF(h-1)][c]

(%)

1

1Cu [0.2%]

1

-

45

112[112]

2[d]

1Cu [0.2%]

1

-

8

21[21]

3[e]

1Cu [0.2%]

1

-

37

93[92]

4[f]

1Cu [0.2%]

1

-

44

126[126]

5

1Cu [0.2%]

1

K2CO3 [2.5]

72

380[380]

6

1Cu [0.2%]

1

TEMPO [2.5]

78

459[459]

7

1Cu [0.2%]

1

Ph2NH [100]

4

7[7]

8

1Cu [0.2%]

1

CBrCl3 [100]

2

5[5]

9[g]

1Cu [0.2%]

0.5

-

68

331[662]

10

4Cu [0.2%]

0.08

-

11

59[738]

11

4Cu [0.2%]

0.5

-

16

158[316]

12

4Cu [0.2%]

1

-

56

281[281]

13

4Cu [0.2%]

3

-

67

367[122]

14

4Cu [0.2%]

6

-

82

512[85]

15

4Cu [0.2%]

10

-

85

586[59]

16

4Cu [0.2%]

15

-

89

644[43]

17

4Cu [0.04%]

1

-

18 1.5×103[1.5×103]

18

4Cu [0.4%]

1

-

68

374[374]

19

4Cu [0.6%]

1

-

75

248[248]

TEMPO [2.5]

78

525[525]

20

4Cu [0.2%]

1

[g]

4Cu [0.2%]

0.5

-

83

525[1.1×103]

22[g]

4Cu [0.2%]

0.5

TEMPO [2.5]

92

653[1.3×103]

23[h]

CuI [0.4%]

1

-

18

94[94]

24[g,h]

CuI [0.4%]

0.5

TEMPO [2.5]

28

138[276]

21

[a] Reaction conditions (unless stated otherwise): 2.5 mmol of substrate, CPs 1Cu or 4Cu (1-15 µmol, 0.04-0.6 mol% vs. substrate), 5 mmol of t-BuOOH (aq. 70%), 80 ºC, MW irradiation (5 W). [b] Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate, as determined by gas chromatography. [c] Turnover number, i.e. number of moles of product per mol of dinuclear CP; TOF = TON per hour (values in brackets). [d] H2O2 aq. 30% instead of t-BuOOH aq. 70%. [e] In acetonitrile. [f] In H2O. [g] At 120 ºC and 20 W. [h] For comparative purposes.


26

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Table 5. MW-assisted solvent-free oxidation of selected secondary alcohols and of benzyl alcohol in the presence of 1Cu or 4Cu.[a]

Entry

CP

1[d]

1Cu

2

1Cu

3

4Cu

4

1Cu

5

4Cu

6

1Cu

7 8

4Cu [d]

Substrate

Cyclohexanol

2-Hexanol

3-Hexanol

1Cu

9

1Cu

10

4Cu

Benzylalcohol

Time

Yield

TON

(h)

(%)[b]

[TOF(h-1)][c]

1

38

157 [157]

0.5

58

201 [402]

0.5

78

333 [666]

0.5

45

99 [198]

0.5

59

155 [310]

0.5

31

88 [176]

0.5

48

101 [202]

1

34

98 [98]

0.5

68

287 [574]

0.5

75

401 [802]

[a] Reaction conditions (unless stated otherwise): 2.5 mmol of substrate, 5 µmol (0.2 mol% vs. substrate) of 1Cu or 4Cu, 5 mmol of t-BuOOH (aq. 70%), 120 ºC, MW irradiation (20 W). [b] Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate, as determined by gas chromatography. [c] Turnover number, i.e. number of moles of product per mol of dinuclear catalyst; TOF = TON per hour (values in brackets). [d] Reaction performed at 80 ºC and 5 W.


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Table 6. Peroxidative oxidation of cyclohexane[a] with H2O2 catalyzed by 1Cu or 4Cu (selected data).

Yield (%)[b]

TON[c]

n(Cat)/n(CyH)

n(H2O2)/n(Cat.)

×103

×10−3

OL

ONE

Tot[d]

1Cu

1

2

1.3

0.7

2.0

20

1Cu

1

2

0.1

0.1

0.2

2

3

1Cu

0.1

20

0.1

0.3

0.4

35

4

4Cu

2

1

2.8

3.7

6.5

33

5

4Cu

1

2

5.2

5.3

10.5

102

6[e]

4Cu

1

2

0.1

0.1

0.2

2.2

7

4Cu

0.5

4

1.9

0.8

2.7

54

8

4Cu

0.1

20

0.2

0.1

0.3

30

4Cu

1

2

0.5

0.2

0.7

3.4

4Cu

1

2

0.1

-

0.1

1.8

Entry

1 2

[e]

9[f] 10

[g]

CP

[a] Reaction conditions (unless stated otherwise): acetonitrile (3 mL), cyclohexane (5.0 mmol), 0.5-10 µmol of 1Cu or 4Cu, H2O2 (10.0 mmol), 24 h, r.t.; yield and TON determined by gas chromatography upon treatment with PPh3 (see text). [b] Molar yield (%) based on substrate, i.e. moles of products [cyclohexanol (OL) and cyclohexanone (ONE)] per 100 mol of cyclohexane. [c] Turnover number, i.e. moles of product per mol of dinuclear catalyst. [d] Moles of cyclohexanol + cyclohexanone per 100 moles of cyclohexane. [e] Reaction in the presence of pyrazine carboxylic acid (Hpca) [n(Hpca)/n(catalyst) = 25]. [f] Reaction in the presence of CBrCl3 (5.0 mmol). [g] Reaction in the presence of Ph2NH (5.0 mmol).


28

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SCHEMES AND FIGURES

R3 N N H

R3 CH2 R5

R5

N N H

H

N N

N N H2L4

CH3

H3C N N

N N CH3

H

H3C

CH3

H2L1: R3 = R5 = CH3 H2L2: R3 = CH3, R5 = H H2L3: R3 = R5 = H

H

H3C

CH3

H2L5

H

H3C

Scheme 1: The bis(pyrazole)-based ligands adopted in this study.

OH CH3

O CP catalyst

CH3

t-BuOOH, MW

Scheme 2. MW-assisted, solvent-free oxidation of 1-phenylethanol to acetophenone.

OOH

OH

O

1Cu or 4Cu H2O2(aq), MeCN

Scheme 3. Peroxidative oxidation of cyclohexane catalyzed by 1Cu or 4Cu.


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a

Page 30 of 46

b

c

Figure 1: Representation of the crystal structure of 1Co: a) the coordination sphere of the metal centre; b) the 1-D chain of metal ions; c) the 3-D network viewed along c. Horizontal axis, a; vertical axis, b. Carbon, grey; cobalt, yellow; nitrogen, blue. Hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (°): Co-N 1.98(1); Co···Co along c 3.545(1); N-Co-N 99.8(4)-127(2); exocyclic C-C-C 115(1); Co···Co···Co along c 154.


30

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a b

c

Figure 2: Representation of the crystal structure of 1Cu: a) the coordination sphere of the metal centre (one of the two independent Cu(I) ions has been arbitrarily chosen); b) the 1-D chain of metal ions; c) the 3-D network viewed along c. Horizontal axis, a; vertical axis, b. Carbon, grey; copper, yellow; nitrogen, blue. Hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (°) for Cu(1): Cu-N 1.867(8), 2.050(9); N-Cu-N 163.3(4). Main bond distances (Å) and angles (°) for Cu(2): Cu-N 1.936(7), 1.996(7); N-Cu-N 166.6(3); Other bond distances (Å) and angles (°): exocyclic C-C-C 110.3(2); Cu···Cu along c 3.088(8), 3.124(6); Cu···Cu along a 9.092(5), 9.111(2); Cu···Cu···Cu along c 84.6, 134.8.


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b

a

c

Figure 3: Representation of the crystal structure of 1Cd: a) the coordination sphere of the metal centre; b) the 1-D chain of metal ions; c) packing of the 2-D slabs viewed along c. Horizontal axis, a; vertical axis, b. Carbon, grey; cadmium, yellow; nitrogen, blue. The hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (°): Cd-N 2.060(4), 2.16(2); Cd···Cd along c 3.966(3); N-Cd-N 98.6(5)-115(1); exocyclic C-C-C 122.5(9); Cd···Cd···Cd along c 180.

b

a

c

Figure 4: Representation of the crystal structure of 3Cu: a) the coordination sphere of the metal center; b) the 1-D chain of metal ions; c) packing of the 2-D slabs viewed along c. Horizontal axis, a; vertical axis, b. Carbon, grey; copper, yellow; nitrogen, blue. Hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (°): Cu-N 1.99(3); Cu···Cu 3.6398(9) (along c), 8.5065(8) (along b); N-Cu-N 91.55(9), 89.45(9), 180; exocyclic C-C-C 117(2); Cu···Cu···Cu along c 180. ACS Paragon Plus Environment

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b

a

c

Figure 5: Representation of the crystal structure of 3Cd: a) the coordination sphere of the metal centre (one of the two independent Cd(II) ions has been arbitrarily chosen); b) the 1-D chain of metal ions; c) the 3-D porous network viewed along a. Horizontal axis, b; vertical axis, c. Carbon, grey; cadmium, yellow; nitrogen, blue. The hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (°) for Cd(1): Cd-N 2.26(2), 2.29(2), 2.32(1), 2.34(1); N-Cd-N 85(1), 136.6(9). Main bond distances (Å) and angles (°) for Cd(2): Cd-N 2.19(2), 2.20(2), 2.21(2), 2.23(2); N-Cd-N 98.1(6), 124.1(9). Other distances (Å) and angles (°): Cd···Cd along a 3.731(8), 3.923(8); exocyclic C-C-C 113.0(1); Cd···Cd···Cd along a 150.1, 158.7.


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b a

c

Figure 6: Representation of the crystal structure of 4Co: a) the coordination sphere of the metal centre; b) the 1-D linear chain of metal ions; c) the 3-D porous network viewed along c. Horizontal axis, a; vertical axis, b. Carbon, grey; cobalt, yellow; nitrogen, blue. The hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (°): Co-N 2.02(2); Co···Co 3.6372(5) (along [001]), 13.1535(5) (along [110]); N-Co-N 97.0(8), 126(2); Co···Co···Co along c 180.


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a

b

c

Figure 7: Representation of the crystal structure of 5Cd: a) the coordination sphere of the metal centre; b) the 1-D chain of metal ions; c) the 3-D porous network viewed along c. Horizontal axis, a; vertical axis, b. Carbon, grey; cadmium, yellow; nitrogen, blue. The hydrogen atoms of the ligands and oxygen atoms of the clathrated water molecules[21] have been omitted for clarity. Main bond distances (Å) and angles (°): Cd-N 2.12(1), 2.41(1); N-Cd-N 91.3(5)-124.1(5); Cd···Cd 3.8765(1) (along [001]), 17.7101(6) (along [110]); Cd···Cd···Cd 180 (along [001]).


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a 100 90

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Figure 8: TGA curves for a) 1Co (blue), 1Cu (green), 1Zn (black) and 1Cd (red); b) 4Co (blue), 4Cu (green), 4Zn (black) and 4Cd (red).


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a N2 Adsorbed, cm2/g STP

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100

0 0

0,2

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Figure 9: N2 adsorption isotherms at 77 K for 1M (a) and 4M (b). Co-containing species: violet circles; Cu-containing species, cyan circles; Zn-containing species, black circles; Cd-containing species, yellow circles.


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Figure 10. Effect of the amount of 4Cu on the yield (solid line) and on the turnover number (dotted line) in the oxidation of 1-phenylethanol to acetophenone.

80 70 60 50

Yield, %

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40 30 20 10 0 No solvent NCMe

Water

K2CO - 3

TEMPO

Ph2-NH

CBrCl - 3

Figure 11. Influence of different solvents and additives on the yield in acetophenone in the peroxidative oxidation of 1-phenylethanol in the presence of 1Cu.


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Figure 12. Effect of the reaction time on the overall yield (cyclohexanol + cyclohexanone) in the peroxidative oxidation of cyclohexane in the presence of 4Cu.

12 10 8 Yield (%)

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6 4 2 0 0

5

10

15

20

n(ox)/n(cat) x 10-3

Figure 13. Effect of H2O2/catalyst molar ratio on the overall yield (cyclohexanol + cyclohexanone) in the peroxidative oxidation of cyclohaxane in the presence of 4Cu.


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References (1) For reviews on the use of porous CPs as catalysts see e.g. (a) Liu, J.; Chen, L.; Cui, H.; Zhang, L.; Zhang, J.; Su, C.-Y. Chem. Soc. Rev. 2014, 43, 6011-6061. (b) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112, 1196–1231. (c) Corma, A.; Garcia, H.; Llabrés i Xamena, F. X. Chem. Rev. 2010, 110, 4606-4655. (2) (a) Alaerts, L.; Séguin, E.; Poelman, H.; Thibault-Starzyk, F.; Jacobs, P. A.; De Vos, D. E. Chem. Eur. J. 2006, 12, 7353–7363. (b) Dinča, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876–16883. (3) For recent reviews on porous CPs, see e.g. (a) the themed issue by Kitagawa, S.; Zhou, H.C. Eds. Chem. Soc. Rev. 2014, 43, 5415-6172. (b) The themed issue by Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Eds. Chem. Rev. 2012, 112, 673-1268. (4) For recent reviews on porous CPs containing flexible ligands see e.g. (a) Lin, Z.-J.; Lü, J.; Hong, M.; Cao, R. Chem. Soc. Rev. 2014, 43, 5867-5895. (b) Guillerm, V.; Kim, D.; Eubank, J. F.; Luebke, R.; Liu, X.; Adil, K.; Lah, M. S.; Eddaoudi, M. Chem. Soc. Rev. 2014, 43, 6141-6172. (c) Stock, N.; Biswas, S. Chem. Rev. 2012, 112, 933-969. (d) Zhang, J.-P.; Zhang, Y.-B.; Lin, J.-B.; Chen, X.-M. Chem. Rev. 2012, 112, 1001–1033. (5) (a) Wan, S. Y.; Li, Y. Z.; Okamura, T.; Fan, J.; Sun, W. Y.; Ueyama, N. Eur. J. Inorg. Chem. 2003, 3783–3789. (b) Hang, Z. H.; Shen, Z. L.; Okamura, T.; Zhu, H. F.; Sun, W. Y.; Ueyama, N. Cryst. Growth Des. 2005, 5, 1191–1197. (c) Liu, G. X.; Huang, Y. Q.; Chu, Q.; Okamura, T.; Sun, W. Y.; Liang, H.; Ueyama, N. Cryst. Growth Des. 2008, 8, 3233–3245. (6) See e.g. (a) Maspero, A.; Cernuto, G.; Galli, S.; Palmisano, G.; Tollari, S.; Masciocchi, N. Solid State Sci. 2013, 22, 43-49. (b) Maspero, A.; Galli, S.; Masciocchi, N.; Palmisano, G. Chem. Lett. 2008, 37, 956-957. (7) For examples of thermally stable CPs isolated and characterized by our group see the following papers and the references therein: (a) Tăbăcaru, A.; Pettinari, C.; Masciocchi, N.; Galli, S.; Marchetti, F.; Angjellari, M. Inorg. Chem. 2011, 50, 11506–11513. (b) Quartapelle Procopio, E.; Rojas, S.; Padial, N. M.; Galli, S.; Masciocchi, N.; Linares, F.; Miguel, D.; Oltra, J. E.; Navarro, J. A. R.; Barea, E. Chem. Commun. 2011, 47, 11751-11753. (c) Pettinari, C.; Tăbăcaru, A.; Boldog, I.; Domasevitch, K. V.; Galli, S.; Masciocchi, N. Inorg. Chem. 2012, 51, 5235–5245. (d) Tăbăcaru, A.; Pettinari, C.; Timokhin, I.; Marchetti, F.; Carrasco-Marín, F.; Maldonado-Hódar, F. J.; Galli, S.; Masciocchi, N. Cryst. Growth Des. 2013, 13, 3087–3097. (e) Padial, N. M.; Quartapelle Procopio, E.; Montoro, C.; López, E.; Oltra, J. E.; Colombo, V.; Maspero, A.; Masciocchi, N.; Galli, S.; Senkovska, I.; Kaskel, S.; Barea, E.; Navarro, J. A. R. Angew. Chem. Int. Ed. 2013, 52, 8290–8294.


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GRAPHICAL ABSTRACT

Two novel copper(I)-containing CPs lead selectively to a maximum product yield of 92% in the microwave-assisted solvent-free peroxidative oxidation of 1-phenylethanol, and of 11% in the peroxidation of cyclohexane, the latter value being higher than that granted by the current industrial process.


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Novel Coordination Polymers with (Pyrazolato)-Based Tectons

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