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Application of Transition-Metal Complexes in Cyclohexane Oxidation: Synthesis, Structure, and Properties of Copper and Nickel Complexes with a Pincer N-Heterocyclic Ligand [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A series of coordination complexes, [CuCl2 (H2 L)] (1), [Ni(HL)2]·4CH3 CH2 OH (2), [Cu(H2 L)(dipic)]·0.5 HOCH2 CH2 OH·2H2 O (3), and [Ni(H2 L)(dipic)]·0.5 HOCH2 CH2 OH.2H2 O (4) (H2 L= 2,6-di(5-methyl-1H-pyrazol-3-yl)pyridine, H2 dipic=2,6-pyridinedicarboxylic acid) are synthesized from bipyrazolyl derivative ligands. All of the complexes have been characterized by elemental analysis, IR and UV/Vis spectroscopy, powder X-ray diffraction, and single-crystal X-ray diffraction. Structural analyses reveal that the H2 L ligand in complexes 1-4 connects with one metal center in a tridentate manner. In addition, these complexes are potential catalysts in cyclohexane (Cy) oxidation with an acidic mixture of HCl and lactic acid. A putative mechanism for Cy oxidation promoted by complex 2 is also proposed. Keywords : acidity · cyclohexane · homogeneous catalysis · oxidation · transition metals Introduction Considerable attention has been paid to pyrazoles, pyrimi- dines, and related N-containing heterocyclic derivatives in pharmacy,[1-4] agrochemistry,[5] pesticides,[6] molecular prepara- tions,[7] anion sensing,[8] and hydrometallurgy.[9] As a member of N-containing heterocyclic families, derivatives of 2,6-di(pyra- zol-1-yl)pyridine are commercially available or straightforwardly synthesized, especially those substituted at the 3,4,5-position on the pyrazolyl moiety, and the introduction of pyrazole rings onto the 2- and 6-positions of pyridine is a common strategy to synthesize tridentate ligands that are analogous to substi- tuted derivatives of 2,6-di(pyrazol-1-yl)pyridine.[10-12] In addi- tion, the 2,6-di(pyrazol-1-yl)pyridine ligand can bind to various valence-state metal ions in a tridentate manner,[13-15] which is widely studied in coordination chemistry.[16-20] Furthermore, transition-metal complexes with 2,6-bis(pyra- zol-1-yl)pyridine and their derivatives are of great interest owing to their biological activity[21, 22] and rich applications in materials science,[23] semiconductors,[24] photolumines- cence,[25-27] magnetism,[28, 29] and especially in homo- and heter- ogeneous catalytic oxidation reactions.[10, 30-35] Lopes et al. syn- thesized a molybdenum complex with a 2,6-bis(pyrazol-1-yl)- pyridine ligand and found that it showed good catalytic activi- ties in olefin epoxidation ;[30] Li and co-workers reported that the yield of poly(methyl methacrylate) (PMMA) catalyzed by the complex [Cu(bmppy)(m-1)]2 (bmppy = 2,6-bis(1-methyl-1H- pyrazol-3-yl)pyridine) could reach 99 %;[31] Thiel et al. synthe- sized a series of ruthenium complexes with ligands that were derivatives of 2,6-bis(pyrazol-1-yl)pyridine and studied their catalytic activities in the transfer hydrogenation of aryl ketones ;[33] Wan and co-workers obtained a series of zinc com- plexes with H2L1 or H2L2 (H2L1 = 2,6-di-(5-phenyl-1H-pyrazol-3- yl)pyridine; H2L2 = 2,6-di-(5-methyl-1H-pyrazol-3-yl)pyridine) li- gands, and found that these complexes had potential catalytic activity for cyclohexane (Cy) oxidation.[10] However, reports fo- cusing on the catalytic performance of transition-metal com- plexes with derivatives of 2,6-bis(pyrazol-1-yl)pyridine ligands as homogeneous catalysts in Cy oxidation are still limited. Nevertheless, HNO3 was often used as an additive in a tradi- tional system of Cy oxidation, which had the main disadvant- age that significant amounts of environmental harmful NOx ef- fluents (NO, NO2, and N2O) were produced.[36] Therefore, it is highly desirable to develop an alternative green route with en- vironmentally friendly additives and efficient processes.[37, 38] One potential environmentally friendly additive is lactic acid, which is an important naturally occurring organic acid. Howev- er, no reports have focused on the performance of a lactic acid additive in Cy oxidation. Herein, we designed and synthesized four novel transition- metal complexes with the 2,6-di-(5-methyl-1H-pyrazol-3-yl)pyri- dine ligand: [CuCl2(H2L)] (1), [Ni(HL)2]·4CH3CH2OH (2), [Cu(H2L)- (dipic)]·0.5 HOCH2CH2OH ·2H2O(3), and [Ni(H2L)(dipic)]·0.5 HOCH2CH2OH ·2H2O(4)(H2L = 2,6-di-(5-methyl-1H-pyrazol-3-yl)- pyridine, H2dipic = 2,6-pyridinedicarboxylic acid). Their catalytic activities in Cy oxidation with H2O2 as the oxidation have been studied in detail. The most effective acid additives (HCl and lactic acid) are also discussed. Results and Discussion Synthesis Complexes 1-4 were successfully synthesized, for the first time, through the process shown in Scheme 1. The starting materials are essential factors for the preparation of complexes 1 (CuCl2·2H2O) and 3 (Cu(NO3)2·3H2O, cannot be CuAc2·H2O), as well as the correct solvents : methanol for complexes 1 and 3, ethanol for 2, and ethanol/water for 4. The temperatures for growing complexes 2-4 were also different: 100 8C for 2 and 3, and 120 8C for 4. The pH value was adjusted to 5-6 for all complexes. Notably, although the structure of complexes 3 and 4 were similar, the order of adding the materials for com- plex 4 was different from that for 3 :H2L was added at the be- ginning of the synthesis for 3, whereas it was added after stir- ring of the metal salt and dipic for 4 h for 4. IR spectra The IR spectra of complexes 1-4 are shown in Figures S1-S4 in the Supporting Information and selected data are listed in Table 1. The broad absorption band appearing at ñ^ 3400 cm^1 indicates the presence of water molecules. The absorption peaks at ñ=3000- 3100 cm^1 are attributed to the C^H/N^H stretching vibration in the pyridine/pyrazolyl rings. Bands at ñ= 2800-2900 cm^1 are features of the C^H stretching vibration in the ^CH3/^CH2^ groups. The characteristic bands between ñ= 1000 and 1500 cm^1 are assigned to the characteristic stretching vibra- tions of the pyrazolyl and pyri- dine rings, such as ñ= 1572, 1456, 1384, 1293 and 1030 cm^1 for complex 1. The wide and strong bands at ñ^ 1600 and 1300 cm^1 for complexes 3 and 4 are attributed to the absorp- tion of the asymmetric and sym- metric stretching vibration of the C=O group. Peaks at ñ^ 500 cm^1 are due to the Cu^N stretching vibration: ñ= 526 cm^1 for 2, ñ= 517 cm^1 for 3, ñ= 537 cm^1 for 4, and none for 1. The IR spectra of com- plexes 1-4 are consistent with their structural characterization. UV/Vis absorption spectra The UV/Vis absorption spectra of complexes 1-4 (Figures S5- S8 in the Supporting Information) were recorded at room tem- perature in the form of the solid sample. The shape of the UV/ Vis spectra of the four complexes is similar. The weak absorp- tions observed at l = 206-208 and 258-264 nm for complexes 1-4 are attributed to p !p* transitions of the H2L and dipic li- gands, respectively.Weak absorptions at l=328nm for 1, l= 322 nm for 2, l= 342 nm for 3, and l= 336 nm for 4 are as- signed to the metal-to-ligand charge-transfer (LMCT) process. In the visible range, complexes 1 and 3 exhibit one absorption peak at l=688 and 700 nm, respectively, which are assigned to d-d transitions of the central copper metal. Complexes 2 and 4 exhibit two sets of absorption peaks at l =556, 830 nm andl=576, 840nm,respectively,whichareattributed tod-d transitions of the central nickel metal. Detailed absorption data and a comparison of UV/Vis spectra for complexes 1-4 are given in Table 2. XRD analysis The powder X-ray diffraction (PXRD) patterns of complexes 1- 4 were recorded at 2q values between 5 and 50 8 and com- pared with the corresponding simulated single-crystal diffrac- tion data (Figures S9-S12 in the Supporting Information). The phase of the corresponding complexes was considered to be pure owing to agreement of the peak positions. The different intensities may be due to the preferred orientation of the powder samples. Structural description of complexes 1-4 The molecular structures of complexes 1-4 are depicted in Fig- ures 1 a, 2, and 3 a and Figure S13 a in the Supporting Informa- tion, respectively. The principal bond lengths and angles of complexes 1-4 are listed in Table 3. The relevant hydrogen- bond parameters in complexes 1, 3,and4 are listed in Table 4. Single-crystal X-ray studies show that complex 1 crystallizes in the triclinic system with the P1 space group. The asymmetric unit contains one copper atom, one H2L ligand, and two chlor- ine coordination atoms. The copper atom is coordinated by three nitrogen atoms (N2, N3, N4) from the H2L ligand and two chlorine atoms (Cl1, Cl2). The bond lengths of Cu^N are in the range of 2.001(3)-2.031(2) ^. The distances of the Cu^Cl bonds are 2.2694(10) and 2.4426(10) ^, respectively. The angles of N- Cu-N and N-Cu-Cl are in the range of 78.07(10)-154.75(11) and 93.70(8)-145.87(8)8, respectively. The angle of Cl2-Cu-Cl1 is 107.00(4)8. In addition, there are two types of hydrogen bonds in complex 1: 1) a hydrogen bond between the chlorine atoms and N^H of the H2L ligand, such as N5^H5···Cl2#1 (3.1727 ^, Symmetry code : #1: ^x, ^y, ^z) and N1^H1···Cl1#2 (3.1663 ^, Symmetry code : #2: ^x,1^y, ^z); 2) C^H···Cl from carbon atoms of H2L and chlorine atoms of CuCl2·2H2O: C8^H8A···Cl1#3 (3.6715 ^, Symmetry code : #3 : 1^x,1^y,1^z). The molecules are connected in a 1D chain through a hydrogen-bonded supramolecular structure (shown in Figure 1 b). Single-crystal X-ray studies show that complex 2 (shown in Figure 2) is crystallized in the tetragonal system with the P4(2)/ n space group. The asymmetric unit contains one nickel atom, two HL anions, and one lattice ethanol molecule. The central nickel atom is six coordinated by six nitrogen atoms from two HL anions to form a distorted octahedral geometry. The bond lengths of Ni^N are in the range of 2.024(8)-2.117(4) ^. The angles of the N^Ni^N are in the range of 76.45(16)-1808. Although the central metals in complexes 3 and 4 are differ- ent, their coordination environments are similar. Take complex 3 as an example, Single-crystal X-ray studies show that complex 3 crystallizes in the triclinic system with the P1 space group (shown in Figure 3 a). The asym- metric unit contains one copper atom, one H2L ligand, one dipic ligand, one lattice glycol mole- cule, and two lattice water mole- cules. The copper atom is six co- ordinated by three nitrogen atoms (N2, N3, and N4) from one H2L ligand, two oxygen atoms (O2, O3), and another ni- trogen atom from the dipic ligand to form a distorted octa- hedral geometry. The deviations of N2, N4, O2, and O3 atoms that composed the least-squares plane are ^0.4716(18), ^0.4905(19), 0.4877(18), and 0.4744(17) ^, respectively ; this shows that these atoms are almost on one plane. Cu, N3, and N6 from the axial position lie ^0.0656(16), ^2.0199(36), and 1.9153(36) ^ out of the equatorial plane, respectively. The bond lengths of Cu^N and Cu^O are in the range of 1.966(3)-2.074(4) and 2.264(3)- 2.348(3) ^, respectively. The angles of N-C-N and N-Cu-O are in the range of 78.11(13)-174.97(14) and 75.32(12)-109.52(12)8, respectively. The angle of O2-Cu-O3 is 152.52(10)8. There are relevant hydrogen bonds in complexes 3 and 4. All hydrogen-bond types in complexes 3 and 4 are similar (shown in Figure 3 b and c, and Figure S13 b and c in the Sup- porting Information). Thus, we take complex 3 as an example to analyze the hydrogen-bond types and linking manner in detail. All hydrogen bonds can be divided into three types : N^ H-O, C-H-O, and O-H-O. The N-H-O hydrogen bond is from the nitrogen atoms of H2L and oxygen atoms of the dipic ligand or lattice glycol molecule, such as N1- H1D-O4#1 (2.7691 ^, Symmetry code: #1: 1-x,2-y,1-z) and N5- H5A-O9#3 (2.7229 Â, Symmetry code: #3: -x, 1-y, 1-z); the hydrogen bond between the oxygen atoms of the dipic ligand and C-H of the H2L ligand is C3-H3-O2#2 (3.2081 Â, Symmetry code: #2: -x, 1-y, -z); the hydrogen bond O-H-O comes from the lattice glycol molecule and the lattice water molecule, such as O9-H9-O1W. The molecules are connected to a dimmer by the hydrogen bond of N1- H1D-O4#1. A supra- molecular chain is formed through bridging the dimeric mole- cules as a unit by the interactions of C3-H3-O2#2 along the b axis (shown in Figure 3 b). The N5-H5A-O9#3 hydrogen bond further links the 1D chains with hydrogen bonds to generate a 2D supramolecular network (shown in Figure 3c). From the structural analysis outlined above, the lengths of Cu-N(pyridine) or Ni-N(pyridine) in complexes 1 or 2 are longer than those in complexes 3 or 4, whereas the distances of Cu- N(pyrazole) or Ni-N(pyrazole) are similar. This may be caused by the different coordination modes. The lengths of Cu-N(pyrazole),Cu- N(pyridine), and Cu-O in complex 3 are shorter, similar, and longer than those of Ni-N (pyrazole),Ni-N(pyridine), and Ni-Oin complex 4, respectively; this may be due to different central metals. Catalytic activity When the nickel complex (complex 2) takes part as a catalyst, only trace amounts of cyclohexanone (Cyone) and cyclohexa- nol (Cyol) can be detected. Regardless of the molar ratio of H2O2/Cy (from 0 to 1 ^ 106:1) and HNO3/Cy (from 0 to 4000:1) or changes to the reaction times, the maximum turnover num- bers (TONs) for Cyone and Cyol were less than 20 (as shown in Figure 4), which is meaningless for the application. Meanwhile, no Cyone and Cyol could be detected if MeCN was changed to MeOH, EtOH, CHCl3, or petroleum ether fraction (PE) ; this indi- cates that MeCN is an important solvent for Cy oxidation cata- lyzed by the nickel complex. The TONs can be noticeably in- creased by higher temperature, as shown in Figure 5, which for Cyone and Cyol can reach 77 and 115, respectively, at 80 8C. However, such characteristics do not satisfy the economical concentration. Finally, HNO3 changed to HCl, H2SO4,CH3COOH, HClO4, lactic acid, and CHCl3, as shown in Figure 6. Unfortu- nately, the catalyst has no reactivity in CH3COOH. The TONs in lactic acid, HClO4, and CHCl3 were less than that in HNO3. Al- though the TONs in HCl and H2SO4 were higher than that in HNO3, the maximum values were less than 20. To improve the activity of the nickel complex in Cy oxidation under mild conditions (40 8C), two different kinds of acids were mixed as additives, as shown in Table 5. Unfortunately, the TON value cannot be enhanced by mixing HNO3 or H2SO4 with any other kinds of acid discussed herein. Interestingly, it can be promoted by mixing HCl with any other kinds of acid, except CH3COOH. For example, the TONs were 57, 62, 68, 78, and 90 if HCl was mixed with H2SO4,HNO3, CCl3COOH, HClO4, and lactic acid, respectively. To find out if mixtures of acid with HCl acted themselves or in cooperation with the catalyst, the TON values were detected with and without complex 2,as shown in Figure 7. The high TONs obtained in a mixture of HCl with H2SO4 or CCl3COOH may be attributed to the action of the acid additives themselves, rather than the catalyst, because the TON promoted by the catalyst is nearly the same as that without catalyst. For the mixture of HCl with HNO3 or HClO4, although the TONs without catalyst are not as high as those with catalyst, they are still high enough to cover the role of the complex. Interestingly, the TON was 35 if HCl was mixed with lactic acid without complex 2 ; this could be increased to 110 after addition of the complex. This indicates that HCl and lactic acid was the best additive mixture in Cy oxidation pro- moted by complex 2. Thus, the optimal amounts of HCl and lactic acid were investigated, as shown in Figure 8. The in- crease in the nacid/ncatalyst ratio results in an increase of the TON values for both Cyone and Cyol, regardless of HCl or lactic acid. However, the maximum TON values were 3000 and 6000 for HCl and lactic acid, respectively. Therefore, the best ratio of nHCl/nlacticacid was 1:2. Finally, we explored the catalytic activities of complexes 1, 3, and 4 for Cy oxidation by keeping the amounts of Cy (0.7200 g, 9.170 mmol), H2O2 (0.3800 g, 11.17 mmol), HCl (0.0608 g, 1.668 mmol), lactic (0.3005 g, 3.336 mmol), and cata- lyst (0.0004 g) in acetonitrile (3 mL) at 40 8C for 3 h fixed; the total TONs were 18, 45, and 29, respectively. Considering that the total TON of complex 2 was 110 under the same condition (Figure 7), we suggested that nickel complexes (complexes 2 and 4) exhibited better catalytic activity for Cy oxidation com- pared with copper complexes (complexes 1 and 3). Although understanding the mechanism of these processes should be achieved through further experiments, a mechanism for Cy oxidation with complex 2 was proposed based on the results discussed above (Scheme 2) ; this includes two process- es of catalytic cycles in HNO3 media only (circle 1) and in a mix- ture of HCl and lactic (circle 2). In circle 1 (Scheme 2), the pro- cess is represented by steps a)!b)!c)!d)!e)!a) ; whereas in circle 2 (Scheme 2), the process is illustrated by steps a)! f)!g)!h)!a). The HO* radical released from circle 1 c and the HOO* radical released from circle 2 f were active intermediate species that could react with Cy to form the targeted products Cyone and Cyol. However, the activity of HO* radical was weaker than that of the HOO* radical in Cy oxidation, which di- rectly led to the lower TON value in HNO3 media than that in the mixture of HCl and lactic acid. This is in accordance with the large amount of cyclohexyl- hydroperoxide (CyOOH ; TON = 109) that could be produced in the acid mixture, whereas none could be found in HNO3 media. Based on the above analyses, we think that the HOO* and HO* radicals play an important role in Cy oxidation catalyzed by com- plexes. This was also supported by recently published papers : the groups of Pombeiro[39] and Shul'pin[38] examined the reactivity of peroxovanadium com- plexes toward alkanes and proposed the formation of HOO* and HO* radicals as reactive species toward organic substrates. Kirillova et al. found that the formation of both HOO* and HO* radicals was more favorable along the cycle based on com- plexes containing only one peroxo fragment, V(OO), than in the cycle based on diperoxo species containing V(OO)2 frag- ments.[40] In the meantime, in our research into Cy oxidation promoted by molybdenum complexes,[41] we also discovered that HOO* and HO* radicals were the active species that deter- mined the conversion of targeted products Cyol and Cyone. Conclusion Four new complexes with 2,6-bis(3-pyrazolyl)pyridine-derived ligands were successfully synthesized, for the first time, by changing the starting material or solvent and controlling the pH value or reaction temperature. X-ray analyses revealed that the central metals in complexes 2-4 were six coordinate, whereas the copper atom in complex 1 was five coordinate. The molecules of complex 1 formed a 1D chain through intra- or intermolecular hydrogen bonds, but those of complexes 3 and 4 were linked to form a 2D supramolecular network struc- ture by hydrogen bonds. In the catalytic study, all TON values of Cyone or Cyol were less than 20 by changing the H2O2 and HNO3 concentrations, solvents, or reaction time, and no reac- tive intermediate species of CyOOH were detected in the pres- ence of complex 2. However, by adding HCl and lactic acid as additives together, a high total TON value of up to 110 and a large amount of CyOOH could be found (TONCyOOH = 109). This indicated that the mixed acid additives could help the for- mation of CyOOH, and therefore, enhance the catalytic activity of the complex. Finally, a primary catalytic mechanism of com- plex 2 for Cy oxidation was proposed. Comparing the total TONs of complexes 1-4, we suggested that nickel complexes exhibited better catalytic activity in Cy oxidation than those of copper complexes. Experimental Section Reagents and instruments Elemental analyses (C, H, and N) were performed on a PerkinElmer 240C automatic analyzer. IR spectra were recorded on a JASCO FT/IR- 480 PLUS Fourier Transform spec- trometer (ñ= 200-4000 cm^1,as pressed KBr pellets). UV/Vis spectra were determined by using a JASCO V-570 UV/Vis spectrome- ter (l = 200-1500 nm, as solids). PXRD patterns were obtained on a Bruker Advance-D8 instrument equipped with CuKa radiation, in the range of 58< 2q < 508, with a step size of 0.02 8 (2 q ) and a count time of 2 s per step. All chemicals used were of analytical grade and without purification. H2L was synthesized according to a modified method reported in the literature.[42-44] A 30 % aqueous solution of hydrogen peroxide was used as the primary oxidant in the oxidation reactions. Synthesis of complexes Synthesis of [CuCl2(H2L)] (1): CuCl2·2H2O (0.1 mmol, 0.0170 g) and H2L (0.1 mmol, 0.0239 g) in MeOH (10 mL) were stirred for 3 h at room temperature, which gave a blue solution. The solution was left at room temperature for a few days before bluish green crys- tals were obtained. Yield (based on Cu): 0.014 g, 37 %; IR data (KBr): ñ=3430, 3193, 3125, 3071, 2964, 2926, 2855, 2364, 2339, 1614, 1572, 1521, 1456, 1384, 1294, 1239, 1162, 1124, 1077, 1030, 984, 811,673,312cm^1;UV/Vis(lmax):208,258,328,688nm; elemental analysis calcd (%) for C13H13N5Cl2Cu : C 41.74, H 3.48, N 18.73 ; found: C 41.69, H 3.52, N 18.76. Synthesis of [Ni(HL)2]·4CH3CH2OH (2): NiCl2·6H2O (0.1 mmol, 0.0237 g) and H2L (0.1 mmol, 0.0239 g) in EtOH (10 mL) were stirred for 4 h in a 25 mL beaker, with the addition of 3-4 drops of glycol. The final reaction mixture was sealed in a 25 mL Teflon-lined stain- less-steel vessel under autogenous pressure and heated at 100 8C for 3 days, followed by cooling to room temperature slowly. The blue mother liquor was obtained after filtration and purple crystals suitable for X-ray diffraction were collected after several days at room temperature. Yield (based on Ni): 0.072 g, 46 %; IR data (KBr): ñ=3612, 3539, 3431, 3368, 3187, 3142, 3079, 2971, 2926, 2863, 1627, 1581, 1518, 1455, 1383, 1293, 1248, 1193, 1085, 1058, 1013, 887, 806, 751, 670, 526, 454cm^1; UV/Vis (lmax): 208, 260, 322, 556, 830 nm ; elemental analysis calcd (%) for C34H48N10O4Ni : C 56.70, H 6.67, N 19.46; found: C 56.63, H 6.61, N 19.38. Synthesis of [Cu(H2L)(dipic)]·0.5 glycol·2 H2O (3): A mixture of Cu- (NO3)2·3H2O (0.1mmol, 0.0242 g), H2L (0.1 mmol, 0.0239 g), and H2dipic (0.1 mmol, 0.0167 g) in MeOH (10 mL) at pH 5-6, adjusted with tert-butylamine, was stirred for 4 h before being sealed in a 25 mL Teflon-lined stainless-steel vessel under autogenous pres- sure and heated at 100 8C for 3 days. After cooling to room tem- perature slowly, some blue crystals suitable for X-ray diffraction were obtained after leaving the dark-green mother solution (filter- ing off the solid impurity) at room temperature for several days. Yield (based on Cu): 0.036 g, 67%; IR data (KBr,): ñ=3503, 3449, 3178, 3115, 3070, 2935, 2863, 2818, 1627, 1581, 1527, 1464, 1374, 1284, 1212, 1166, 1130, 1094, 1058, 1022, 977, 869, 824, 769, 733, 688, 616, 517 cm^1; UV/Vis (lmax): 206, 264, 342, 700 nm ; elemental analy- sis calcd (%) for C21H23N6O7Cu: C 47.10, H 4.30, N 15.70 ; found : C 47.23, H 4.24, N 15.68. Synthesis of [Ni(H2L)(dipic)]· 0.5 glycol·2 H2O (4): NiCl2·6H2O (0.1 mmol, 0.0240 g) and H2dipic (0.1 mmol, 0.0162 g) were dis- solved in a mixture of water/etha- nol (13 mL), stirred at room tem- perature for 4 h, and then H2L (0.1 mmol, 0.0240 g) and glycol (1 mL) were added with continu- ous stirring for 1 h to give a light- yellow, transparent solution. Some dark-purple crystals suitable for X- ray diffraction analysis were ob- tained after placing the solution at room temperature for several days. Yield (based on Ni): 0.022 g, 41 %; IR data (KBr,): ñ=3442, 3190, 3127, 3072, 2937, 2865, 1620, 1521, 1448, 1376, 1286, 1205, 1169, 1124, 1087, 1060, 1024, 916, 862, 826, 772, 745, 690, 618, 537, 474 cm^1; UV/Vis (lmax): 208, 260, 336, 576, 840 nm ; elemental analysis calcd (%) for C21H23N6O7Ni : C 47.53, H 4.34, N 15.84 ; found : C 47.41, H 4.39, N 15.79. X-ray crystallographic determination Suitable single crystals of the four complexes were mounted on glass fibers for X-ray measurements. Reflection data were collected at room temperature on a Bruker AXS SMART APEX II CCD diffrac- tometer with graphite-monochromated MoKa radiation (l = 0.71073 ^) and w scanning mode. All measured independent re- flections (I > 2s(I)) were used in the structural analyses, and semi- empirical absorption corrections were applied by using SADABS.[45] All hydrogen atoms were positioned geometrically and refined by using a riding model. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms from coordi- nated water molecules were found in the Fourier difference map. The crystallographic data and experimental details for data collec- tion, as well as structure refinements, are given in Table 6. The drawings were made with the Diamond program, and all calcula- tions were performed by using SHELX-97 program.[46] CCDC 958819 (1), 958820 (2), 958821 (3), and 958822 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallo- graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Experimental procedure for catalytic oxidation Generally, Cy (0.72 g) and catalyst (0.0004 g) dissolved in the de- sired solvent (3 mL) were added to a glass reactor with a two- necked glass flask. The oxidation reactions were performed at at- mospheric pressure and the required amounts of H2O2 and HNO3 were added sequentially. The mixture was heated to 40 8Cin a water bath and stirred with a magnetic stirring bar for 5 h. The products were analyzed by means of a 9790 GC instrument. Methylbenzene (0.03 g ; internal standard) and diethyl ether (1.5 mL; to extract the substrate and the organic products from the reaction mixture) were added to these samples with stirring for 10 min. Then a sample (0.2 mL) was removed from the mixture and analyzed by GC with the internal standard method. Acknowledgements We are thankful for grants from the National Natural Science Foundation of China (no. 21071071, 21371086, and 21306073), and the Science and Technology Fund for Outstanding Young Tal- ents in Dalian (no 2012J21DW007) for financial assistance. [1] K. Sakai, Y. Tomita, T. Ue, K. Goshima, M. Ohminato, M. Tsuboruma, K. Matsumoto, K. Ohmura, K. Kawakami, Inorg. Chim. Acta 2000, 297,64- 71. [2] G. B. Onoa, V. Moreno, M. Font-Bard^a, X. Solans, J. M. P^rez, C. Alonso, J. Inorg. Biochem. 1999, 75, 205 - 212. [3] J. A. Broomhead, M. J. Lynch, Inorg. Chim. Acta 1995, 240, 13 - 17. [4] J. A. Broomhead, L. M. Rendina, L. K. Webster, J. Inorg. Biochem. 1993, 49, 221- 224. [5] J. 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Huang, X. Tang, Chem. Res. Appl. 2008 , 20, 299-303. [45] G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction for Area Detector Data, University of Gçttingen, Gçttingen (Germany), 1996. [46] G. M. Sheldrick, Program for Crystal Structure Refinement, University of Gçttingen, Gçttingen (Germany), 1997. Received : March 5, 2014 Published online on June 10, 2014 Na Xing, Li Ting Xu, Xin Liu, Qiong Wu, Xi Tong Ma, and Yong Heng Xing*[a] [a] Dr. N. Xing, L. T. Xu, X. Liu, Q. Wu, X. T. Ma, Prof. Y. H. Xing College of Chemistry and Chemical Engineering Liaoning Normal University Dalian 116029 (P. R. China) E-mail : [email protected] * Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402051. (c) 2014 Blackwell Publishing Ltd. |