DOI:
10.1039/C4RA11743C
(Paper)
RSC Adv., 2014,
4, 54007-54017
An efficient approach to the ammoxidation of alcohols to nitriles and the aerobic oxidation of alcohols to aldehydes in water using Cu(II)/pypzacac complexes as catalysts†
Received
2nd September 2014
, Accepted 10th October 2014
First published on 10th October 2014
Abstract
Reactions of 2-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)acetic acid (pypzacacH) ligand with Cu(OAc)2, Cu(NO3)2, CuSO4, Cu(ClO4)2 or CuCl2 produced four dinuclear Cu(II) complexes [{(MeOH)Cu(OAc)}(μ-κ2:κ1-pypzacac)]2·0.5H2O (1·0.5H2O), [{Cu(pypzacac)}(μ-κ2:κ1-pypzacac)2{Cu(H2O)2}](NO3)·2(MeOH)0.5·6H2O (2·2(MeOH)0.5·6H2O), [{(MeOH)Cu(mpypzacac)}(μ-SO4)]2·2MeOH (3·2MeOH; mpypzacac = methyl 2-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)acetate), [{Cu(mpypzacac)2}(μ-κ2:κ1-pypzacac){Cu(mpypzacac)}](ClO4)3·MeOH (4·MeOH) and one polymeric Cu(II) complex [(CuCl)(μ-κ3:κ1-pypzacac)]n (5), respectively. The mpypzacac ligand in 3 and 4 was in situ generated via the Cu2+-catalyzed dehydrative esterification of acetic acid of the pypzacacH ligand. Complexes 1–5 are characterized by elemental analysis, IR and single-crystal X-ray diffraction. Complex 1 contains two {(MeOH)Cu(OAc)} fragments that are interconnected by two μ-κ2:κ1-pypzacac− ligands, forming a dimeric structure. In 2, {Cu(pypzacac)} and {Cu(H2O)2} units are bridged by a pair of μ-κ2:κ1-pypzacac− ligands. In 3, two {Cu(mpypzacac)} fragments are linked by two μ-κ1:κ1-SO42− ions to form a dinuclear structure. Complex 4 also adopts a dimeric structure in which {Cu(mpypzacac)2} and {Cu(mpypzacac)} units are interconnected by one μ-κ3:κ1-pypzacac− ligand. Complex 5 contains a 1D chain in which (CuCl) fragments are interlinked by μ-κ3:κ1-pypzacac− ligands. Complexes 1–5 exhibited excellent catalytic performance in the ammoxidation of alcohol to nitrile and the aerobic oxidation of alcohol to aldehyde in water. The catalytic aqueous solution was easily separated and could be reused for several cycles without any obvious decay of catalytic efficiency.
Introduction
Nitriles are important building blocks of dyes, natural products, herbicides, agrochemicals, pharmaceuticals, and various fine chemicals.1 These derivatives can be used to construct esters, amides, carboxylic acids, amines,2 and nitrogen-containing heterocycles, etc.3 The approaches to nitriles include Sandmeyer's reaction,4 transition metal-catalyzed cyanation of aryl halides with metal cyanides via a nucleophilic pathway,5 oxidation of amines,6 dehydration of amides and aldoximes,7 transformation of organic molecule to CN− via the C–H functionalization of arenes,8 and oxidative dehydrogenation of benzylic alcohols and ammonia,9 azides10 or methyl arenes.11 These methods require the use of toxic inorganic cyanide salts, high temperature, pressure, and stoichiometric or excess amounts of highly reactive oxidants or expensive transition metal catalysts. Hence, the development of concise, efficient and safe approaches for nitrile synthesis is of general interest. Very recently, the ammoxidation reaction of alcohols using inexpensive copper complexes as catalysts and O2 as the oxidant under ambient conditions has provided an economical and efficient method.12 Huang et al. reported the synthesis of nitriles through the dehydrogenation cascade mediated by CuI/2,2′-bipy/TEMPO (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyloxy) systems in EtOH or MeCN.12a Tao and co-workers prepared nitriles through the reactions of alcohols or aldehydes with aqueous ammonia catalyzed by Cu(NO3)2/TEMPO system in DMSO.12b Cook and Muldoon et al. reported that the Cu(OTf)2/2,2′-bipy in combination with TEMPO could also catalyze synthesis of nitriles from aldehydes or alcohols with aqueous ammonia in aqueous MeCN.12c Although the aforementioned copper catalytic systems exhibited high activity, these reactions were carried out in organic solvents or aqueous organic solvent mixtures. Thus it would be a green and economic approach if the aerobic dehydrogenation cascade reaction could be conducted in pure water. The well-defined water-soluble catalysts might be an effective tool to promote organic transformations in aqueous media. It is noted that water-soluble organic ligands could increase the hydrophilicity of the resulting catalysts.13–16 More importantly, the water-soluble catalyst can be easily separated from the organic products and may be recycled for the repeat use. Recently, we have been interested in the syntheses of water-soluble transition metal coordination complexes and their reusable catalytic properties in water.17 In this manuscript, we deliberately selected a water-soluble organic ligand, 2-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)acetic acid (pypzacacH), to react with Cu(OAc)2, Cu(NO3)2, CuSO4, Cu(ClO4)2 or CuCl2. Four dinuclear Cu(II) complexes [{(MeOH)Cu(OAc)}(μ-pypzacac)]2·0.5H2O (1·0.5H2O), [{Cu(pypzacac)}(μ-κ2:κ1-pypzacac)2{Cu(H2O)2}](NO3)·2(MeOH)0.5·6H2O (2·2(MeOH)0.5·6H2O), [{(MeOH)Cu(mpypzacac)}(μ-SO4)]2·2MeOH (3·2MeOH; mpypzacac = methyl 2-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)acetate), [{Cu(mpypzacac)2}(μ-κ2:κ1-pypzacac){Cu(mpypzacac)}](ClO4)3·MeOH (4·MeOH) and one polymeric complex [(CuCl)(μ-κ3:κ1-pypzacac)]n (5) were isolated therefrom. The mpypzacac ligand in 3 and 4 was in situ formed via the Cu2+-catalyzed esterification of acetic acid of the pypzacacH ligand. Complexes 1–5 are soluble in water and work as efficient catalysts for the ammoxidation of alcohol to nitrile and the aerobic oxidation of alcohol to aldehyde in H2O. The catalytic system modeled by 1 could be also recycled for several times. Their syntheses, crystal structures and catalytic properties are described below.
Experimental section
General
All reagents were used as purchased from commercial sources without further purification. The ligand pypzacacH was prepared according to the published procedure.18 Elemental analyses for C, H and N were performed on a Carlo-Erba CHNO-S microanalyzer. The IR spectra (KBr disc) were recorded on a Nicolet MagNa-IR550 FT-IR spectrometer (4000–400 cm−1). The 1H NMR spectra in CDCl3 were recorded at ambient temperature on a Varian UNITYplus-400 spectrometer and the chemical shifts δ were reported as parts per million (ppm) relative to, respectively, the TMS as the internal standard. Resonance patterns were reported with the notations of s (singlet), d (doublet) and m (multiplet). In addition, coupling constants J were reported in Hertz (Hz).
Synthesis of [{(MeOH)Cu(OAc)}(μ-κ2:κ1-pypzacac)]2·0.5H2O (1·0.5H2O). To the MeOH (3 mL) solution of Cu(OAc)2·H2O (20 mg, 0.1 mmol) was added the solution of pypzacacH (20 mg, 0.1 mmol) in MeOH (3 mL). The mixture was stirred at room temperature for 30 min and then filtered. Et2O (30 mL) was layered onto the filtrate to produce blue crystals of 1·0.5H2O in several days, which were collected by filtration, washed with Et2O and dried in air. Yield: 28 mg (79% based on Cu). Anal. calcd for C26H30Cu2N6O10: C, 43.76; H, 4.24; N, 11.78; found C, 43.54; H, 4.53; N, 12.12. IR (KBr disk, cm−1): 2924 (m), 2853 (m), 1598 (s), 1568 (s), 1409 (s), 1338 (m), 1261 (w), 1099 (w), 1022 (w), 678 (m), 647 (w), 618 (w), 536 (w), 521 (w), 506 (w), 490 (w), 477 (w), 462 (w), 433 (w), 419 (w).
Synthesis of [{Cu(pypzacac)}(μ-κ2:κ1-pypzacac)2{Cu(H2O)2}](NO3)·2(MeOH)0.5·6H2O (2·2(MeOH)0.5·6H2O). Blue crystals of 2·2(MeOH)0.5·6H2O were isolated by a similar manner to that used for the isolation of 1·0.5H2O, using pypzacacH (30 mg, 0.15 mmol) and Cu(NO3)2·2.5H2O (25 mg, 0.1 mmol) as starting materials. Yield: 27 mg (65% based on Cu). Anal. calcd for C30H28Cu2N10O11: C, 43.32; H, 3.39; N, 16.84; found C, 43.57; H, 3.55; N, 16.59. IR (KBr disk, cm−1): 2988 (w), 1633 (s), 1488 (w), 1440 (w), 1384 (s), 1308 (w), 1174 (m), 1100 (w), 1002 (w), 787 (s).
Synthesis of [{(MeOH)Cu(mpypzacac)}(μ-SO4)]2·2MeOH (3·2MeOH). Blue crystals of 3·2MeOH were obtained by a similar manner to that used for the isolation of 1·0.5H2O, using pypzacacH (20 mg, 0.1 mmol) and CuSO4·5H2O (25 mg, 0.1 mmol) as starting materials. Yield: 30 mg, (74% based on Cu). Anal. calcd for C24H30Cu2N6O14S2: C, 35.25; H, 3.70; N, 10.28; found C, 34.81; H, 3.43; N, 9.88. IR (KBr disk, cm−1): 1731 (s), 1616 (s), 1509 (m), 1445 (m), 1426 (w), 1384 (w), 1343 (m), 1305 (w), 1285 (m), 1254 (m), 1173 (s), 1097 (s), 1043 (s), 1003 (m), 959 (w), 782 (m), 621 (m).
Synthesis of [{Cu(mpypzacac)2}(μ-κ2:κ1-pypzacac){Cu(mpypzacac)}](ClO4)3·MeOH (4·MeOH). Blue crystals of 4·MeOH were isolated by a similar manner to that used for the isolation of 1·0.5H2O, using pypzacacH (40 mg, 0.2 mmol) and Cu(ClO4)2·6H2O (37 mg, 0.1 mmol) as starting materials. Yield: 45 mg (70% based on Cu). Anal. calcd for C43H41Cl3Cu2N12O20: C, 40.37; H, 3.23; N, 13.14; found C, 40.03; H, 2.89; N, 12.88. IR (KBr disk, cm−1): 1621 (s), 1508 (w), 1488 (w), 1442 (m), 1396 (w), 1384 (m), 1257 (w), 1121 (s), 1108 (s), 783 (m), 705 (s), 625 (m).
Synthesis of [(CuCl)(μ-κ3:κ1-pypzacac)]n (5). Green crystals of 5 were obtained by a similar manner to that used for the isolation of 1·0.5H2O, using pypzacacH (20 mg, 0.1 mmol) and CuCl (17 mg, 0.1 mmol) as starting materials. Yield: 20 mg (68% based on Cu). Anal. calcd for C10H8ClCuN3O2: C, 39.88; H, 2.68; N, 13.95; found C, 39.59; H, 2.75; N, 13.67. IR (KBr disk, cm−1): 3089 (w), 2988 (w), 1625 (s), 1578 (w), 1488 (w), 1450 (w), 1384 (m), 1311 (w), 1254 (w), 1173 (m), 1002 (w), 787 (m), 619 (w).
Typical procedure for the ammoxidation of alcohol to nitrile and the aerobic oxidation of alcohol to aldehyde in water
A test tube equipped with a magnetic stirring bar was charged with 1 (62 mg, 0.0876 mmol), TEMPO (33 mg, 0.21 mmol), TEAI (72 mg, 0.28 mmol), and K2CO3 (48 mg, 0.35 mmol). The reaction vessel was vacuumed and backfilled with oxygen for three times. Benzyl alcohol (0.378 g, 3.5 mmol), H2O (3 mL), NH3 (aq, 25–28% w/w, 550 μL) (for the synthesis of aldehyde, aqueous ammonia was not added) were added subsequently. The resulting deep blue solution was stirred under an oxygen balloon at 60 °C for one day. Then it was cooled to room temperature, and extracted three times with Et2O (3 × 5 mL). The combined organic layer was washed with brine (20 mL) and dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by flash column chromatography using petroleum ether and ethyl acetate as the eluent. All nitrile and aldehyde products synthesized in this work are known and confirmed by comparing their 1H NMR spectra with those found in the literatures.
X-ray crystallography
Single crystals of 1·0.5H2O, 2·2(MeOH)0.5·6H2O, 3·2MeOH, 4·MeOH, and 5 suitable for X-ray analysis were obtained directly from the above preparations. Each single crystal was mounted on a glass fiber with grease and cooled in a liquid nitrogen stream at 153 K (1·0.5H2O, 2·2(MeOH)0.5·6H2O and 5) or 223 K (3·2MeOH, 4·MeOH). Crystallographic measurements were made on a Bruker APEX-II CCD (1·0.5H2O, 2·2(MeOH)0.5·6H2O and 5), or an Xcalibur Atlas Gemini (3·2MeOH, 4·MeOH) diffractometer by using a graphite-monochromated Mo-Kα (λ = 0.71070 Å) radiation. Diffraction data were collected at a ω mode with a detector distance of 35 mm to the crystal. The collected data were reduced by using the program Bruker APEX2 or CrysAlisPro, Agilent Technologies (CrysAlis171.NET, Version 1.171.36.28) and an absorption correction (multi-scan) was applied. The reflection data were also corrected for Lorentz and polarization effects.
The crystal structures of 1·0.5H2O, 2·2(MeOH)0.5·6H2O, 3·2MeOH, 4·MeOH and 5 were solved by direct methods and refined on F2 by full-matrix least-squares techniques with SHELXTL-97 program.19 For 1·0.5H2O, one O atom of OAc− was refined to be disordered over two positions with an occupancy factor of 0.80/0.20 for O(4)/O(4A). The O(6) atom of uncoordinated H2O molecule was refined to one-fourth-occupancy to give an acceptable thermal parameter. For 2·2(MeOH)0.5·6H2O, three O atoms of three lattice H2O molecules were refined to be disordered over two positions with an occupancy factor of 0.50/0.50 for O(15)/O(15A), O(18)/O(18A) and O(19)/O(19A). For 3·2MeOH, one C atom of the coordinated MeOH molecule and one O and one C atom from the uncoordinated MeOH molecule were refined to be disordered over two positions with an occupancy factor of 0.50/0.50 for C(12)/C(12A), O(8)/O(8A) and C(13)/C(13A). For 4·MeOH, one C atom of one pypzacac ligand and one C atom of the uncoordinated MeOH molecule were refined to be disordered over two positions with an occupancy factor of 0.50/0.50 for C(11)/C(11A) and C(44)/C(44A). One O atom of ClO4− was refined to be disordered over two positions with an occupancy factor of 0.25/0.25 for O(18)/O(18A). Except for one O atom of H2O molecule in 1·0.5H2O, three disordered O atoms of H2O molecules in 2·2(MeOH)0.5·6H2O, one C atom of the coordinated MeOH molecule and one O atom and one C atom from the uncoordinated MeOH molecule in 3·2MeOH, one Cl atom from one ClO4−, six O atoms from two ClO4−, one C atom from the pypzacac ligand, and one C atom from MeOH molecule in 4·MeOH, all other non-hydrogen atoms were refined anisotropically. The hydrogen atoms of O(6) from the H2O molecule in 1·0.5H2O, the hydrogen atoms of O(17), C(32), O(16), C(31), O(15), O(18) and O(19) from the uncoordinated MeOH and H2O molecules in 2·2(MeOH)0.5·6H2O, O(25) and C(44) atoms from MeOH molecule in 4·MeOH were not located. The hydrogen of O(5) atom from the coordinated MeOH molecule in 1·0.5H2O, the hydrogen atoms of O(7), O(8), O(12), O(13) and O(14) atoms from the uncoordinated H2O molecules in 2·2(MeOH)0.5·6H2O, the hydrogen atoms of O(3) and O(8) atoms from the MeOH molecules in 3·2MeOH were firstly located from Fourier maps and subsequently the O–H bond was fixed to be 0.86 Å. All other H atoms introduced at the calculated positions and included in the structure-factor calculations. All the calculations were performed on a Dell workstation using the CrystalStructure crystallographic software package (Rigaku and MSC, Ver.3.60, 2004). Crystal data along with data collection and refinement parameters for 1·0.5H2O, 2·2(MeOH)0.5·6H2O, 3·2MeOH, 4·MeOH and 5 were summarized in Table 1. The selected bond distances and angles of 1·0.5H2O, 2·2(MeOH)0.5·6H2O and 3·2MeOH, are listed in Table 2 and the selected bond distances and angles of 4·MeOH and 5 are listed in Table 3.
Table 1 Crystal data and structure refinement parameters for 1·0.5H2O, 2·2(MeOH)0.5·6H2O, 3·2MeOH, 4·MeOH, and 5
|
1·0.5H2O |
2·2(MeOH)0.5·6H2O |
3·2MeOH |
4·MeOH |
5 |
R1 = Σ||F0| − |Fc||/Σ|F0|. wR2 = {Σw(F02 − Fc2)2/Σw(F02)2}1/2. GOF = {Σw((Fo2 − Fc2)2)/(n − p)}1/2, where n = number of reflections and p = total numbers of parameters refined. |
Emperical formula |
C26H31Cu2N6O10.5 |
C31H44Cu2N10O18 |
C26H38Cu2N6O16S2 |
C44H45Cl3Cu2N12O21 |
C10H8ClCuN3O2 |
Formula mass |
722.65 |
971.84 |
881.82 |
1311.35 |
301.18 |
Crystal system |
Orthorhombic |
Triclinic |
Triclinic |
Triclinic |
Monoclinic |
Space group |
Pbcn |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
P![[1 with combining macron]](https://www.rsc.org/images/entities/char_0031_0304.gif) |
P21/c |
Crystal dimension (mm3) |
0.40 × 0.12 × 0.08 |
0.50 × 0.08 × 0.04 |
0.30 × 0.20 × 0.15 |
0.50 × 0.40 × 0.30 |
0.35 × 0.18 × 0.10 |
a (Å) |
12.5992(6) |
10.9883(4) |
9.9313(5) |
13.2169(3) |
9.1736(5) |
b (Å) |
7.8436(4) |
12.4084(5) |
10.6591(8) |
14.6198(5) |
6.8909(4) |
c (Å) |
30.4801(13) |
16.1803(6) |
10.9968(6) |
15.9990(3) |
16.9201(9) |
α (°) |
|
75.070(2) |
112.268(6) |
75.840(3) |
|
β (°) |
|
85.468(2) |
114.296(6) |
79.165(2) |
103.292(2) |
γ (°) |
|
72.003(2) |
96.811(5) |
78.964(2) |
|
V (Å3) |
3012.1(2) |
2027.37(13) |
928.42(10) |
2766.38(15) |
1040.94(10) |
Z |
4 |
2 |
1 |
2 |
4 |
Dcalc (g cm−3) |
1.594 |
1.592 |
1.577 |
1.574 |
1.922 |
F(000) |
1484 |
1004 |
454 |
1340 |
604 |
μ (mm−1) |
1.479 |
1.137 |
1.334 |
3.044 |
2.345 |
Total no. of reflns |
37 222 |
24 423 |
8269 |
24 860 |
16 640 |
No. of unique reflns |
2761 (Rint = 0.0371) |
7548 (Rint = 0.1058) |
3449 (Rint = 0.0318) |
9869 (Rint = 0.0332) |
2361 (Rint = 0.0416) |
No. of obsd reflns |
2220 (I > 2.00σ(I)) |
5247 (I > 2.00σ(I)) |
2917 (I > 2.00σ(I)) |
8585 (I > 2.00σ(I)) |
2079 (I > 2.00σ(I)) |
No. of variables |
215 |
572 |
240 |
757 |
154 |
Transmission factor |
0.5892–0.8909 |
0.6002–0.9559 |
0.6904–0.8250 |
0.3134–0.4642 |
0.4941/0.7993 |
R1a |
0.0330 |
0.0707 |
0.0373 |
0.0723 |
0.0245 |
wR2b |
0.0772 |
0.1589 |
0.0867 |
0.2191 |
0.0584 |
GOFc |
1.088 |
1.129 |
1.038 |
1.048 |
1.073 |
Table 2 Selected bond lengths (Å) and angles (°) for 1·0.5H2O, 2·2(MeOH)0.5·6H2O and 3·2MeOHa
Symmetry code: (A) −x + 5/2, −y + 1/2, z + 1/2 for 1. (A) −x, 1 − y, −z for 2. |
Compound 1·0.5H2O |
Cu(1)–O(2A) |
1.9507(18) |
Cu(1)–O(3) |
1.9515(19) |
Cu(1)–N(3) |
2.021(2) |
Cu(1)–N(2) |
2.025(2) |
Cu(1)–O(5) |
2.405(2) |
Cu(1)⋯O(4) |
2.642 |
O(2A)–Cu(1)–O(3) |
91.80(8) |
O(2A)Cu(1)–N(3) |
174.67(8) |
O(3)–Cu(1)–N(3) |
93.43(9) |
O(2A)–Cu(1)–N(2) |
94.84(8) |
O(3)–Cu(1)–N(2) |
158.14(9) |
N(3)–Cu(1)–N(2) |
79.91(9) |
O(2A)–Cu(1)–O(5) |
94.30(7) |
O(3)–Cu(1)–O(5) |
96.83(8) |
N(3)–Cu(1)–O(5) |
86.08(8) |
N(2)–Cu(1)–O(5) |
103.40(8) |
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif) |
Compound 2·2(MeOH)0.5·6H2O |
Cu(1)–O(8) |
1.955(5) |
Cu(1)–O(5) |
1.960(4) |
Cu(1)–N(1) |
2.005(5) |
Cu(1)–N(3) |
2.018(5) |
Cu(1)–O(7) |
2.239(5) |
Cu(2)–O(1) |
1.961(4) |
Cu(2)–N(7) |
1.999(5) |
Cu(2)–N(6) |
2.004(5) |
Cu(2)–N(9) |
2.051(5) |
Cu(2)–N(4) |
2.241(5) |
O(8)–Cu(1)–O(5) |
90.29(18) |
O(8)–Cu(1)–N(1) |
170.1(2) |
O(5)–Cu(1)–N(1) |
96.61(18) |
O(8)–Cu(1)–N(3) |
91.2(2) |
O(5)–Cu(1)–N(3) |
166.41(19) |
N(1)–Cu(1)–N(3) |
80.5(2) |
O(8)–Cu(1)–O(7) |
93.60(19) |
O(5)–Cu(1)–O(7) |
95.09(18) |
N(1)–Cu(1)–O(7) |
92.82(19) |
N(3)–Cu(1)–O(7) |
98.29(19) |
O(1)–Cu(2)–N(7) |
97.07(19) |
O(1)–Cu(2)–N(6) |
89.87(19) |
N(7)–Cu(2)–N(6) |
172.7(2) |
O(1)–Cu(2)–N(9) |
159.33(19) |
N(7)–Cu(2)–N(9) |
80.0(2) |
N(6)–Cu(2)–N(9) |
94.2(2) |
O(1)–Cu(2)–N(4) |
106.69(18) |
N(7)–Cu(2)–N(4) |
97.6(2) |
N(6)–Cu(2)–N(4) |
78.2(2) |
N(9)–Cu(2)–N(4) |
93.9(2) |
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif) |
Compound 3·2MeOH |
Cu(1)–O(6A) |
1.930(2) |
Cu(1)–O(4) |
1.9376(19) |
Cu(1)–N(1) |
2.008(2) |
Cu(1)–N(3) |
2.022(2) |
Cu(1)–O(3) |
2.207(2) |
|
|
O(6A)–Cu(1)–O(4) |
93.97(9) |
O(6A)–Cu(1)–N(1) |
91.72(9) |
O(4)–Cu(1)–N(1) |
164.82(8) |
O(6A)–Cu(1)–N(3) |
163.61(9) |
O(4)–Cu(1)–N(3) |
91.20(9) |
N(1)–Cu(1)–N(3) |
79.69(9) |
O(6A)–Cu(1)–O(3) |
95.65(11) |
O(4)–Cu(1)–O(3) |
95.11(9) |
N(1)–Cu(1)–O(3) |
98.31(9) |
N(3)–Cu(1)–O(3) |
99.36(11) |
Table 3 Selected bond lengths (Å) and angles (°) for 4·MeOH and 5a
Symmetry code: (A) 1 − x, y + 1/2, −z + 1/2 for 4. |
Compound 4·MeOH |
Cu(1)–O(5) |
1.971(3) |
Cu(1)–N(6) |
1.999(4) |
Cu(1)–N(3) |
2.003(4) |
Cu(1)–N(1) |
2.089(4) |
Cu(1)–N(4) |
2.178(4) |
Cu(2)–N(10) |
1.929(3) |
Cu(2)–O(6) |
1.993(3) |
Cu(2)–N(9) |
1.994(4) |
Cu(2)–N(12) |
2.069(4) |
Cu(2)–N(7) |
2.210(4) |
O(5)–Cu(1)–N(6) |
90.00(14) |
O(5)–Cu(1)–N(3) |
90.36(14) |
N(6)–Cu(1)–N(3) |
179.40(17) |
O(5)–Cu(1)–N(1) |
143.47(14) |
N(6)–Cu(1)–N(1) |
100.04(16) |
N(3)–Cu(1)–N(1) |
79.39(16) |
O(5)–Cu(1)–N(4) |
121.44(13) |
N(6)–Cu(1)–N(4) |
78.90(16) |
N(3)–Cu(1)–N(4) |
101.32(15) |
N(1)–Cu(1)–N(4) |
95.02(15) |
N(10)–Cu(2)–O(6) |
88.43(13) |
N(10)–Cu(2)–N(9) |
172.81(15) |
O(6)–Cu(2)–N(9) |
96.63(13) |
N(10)–Cu(2)–N(12) |
79.39(15) |
O(6)–Cu(2)–N(12) |
161.89(14) |
N(9)–Cu(2)–N(12) |
94.47(15) |
N(10)–Cu(2)–N(7) |
105.84(15) |
O(6)–Cu(2)–N(7) |
98.53(14) |
N(9)–Cu(2)–N(7) |
78.53(15) |
N(12)–Cu(2)–N(7) |
97.67(15) |
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif) |
Compound 5 |
Cu(1)–N(1) |
1.9355(16) |
Cu(1)–O(1) |
1.9962(14) |
Cu(1)–N(3) |
2.0632(16) |
Cu(1)–O(2A) |
2.2083(15) |
Cu(1)–Cl(1) |
2.2383(5) |
|
|
N(1)–Cu(1)–O(1) |
87.88(6) |
N(1)–Cu(1)–N(3) |
78.51(7) |
O(1)–Cu(1)–N(3) |
160.57(6) |
N(1)–Cu(1)–O(2A) |
102.68(6) |
O(1)–Cu(1)–O(2A) |
101.72(6) |
N(3)–Cu(1)–O(2A) |
94.81(6) |
N(1)–Cu(1)–Cl(1) |
158.76(5) |
O(1)–Cu(1)–Cl(1) |
93.48(4) |
N(3)–Cu(1)–Cl(1) |
94.33(5) |
O(2A)–Cu(1)–Cl(1) |
97.80(5) |
Results and discussion
Synthesis and structural characterization of 1–5
As shown Scheme 1, reactions of Cu(OAc)2·H2O with one equiv. of pypzacacH in MeOH at room temperature, afforded one binuclear copper complex 1·0.5H2O in 79% yield. In this reaction, the acetates in Cu(OAc)2·H2O were partially protonated by pypzacacH to form HOAc, which might be due to the fact that the acidity of pypzacacH is stronger than that of HOAc. The dinuclear Cu(II) complex 2·2(MeOH)0.5·6H2O was obtained by the treatment of pypzacacH with Cu(NO3)2 in MeOH. All the NO3− anions in Cu(NO3)2 were removed and replaced by two N atoms of the pyridyl and pyrazolyl groups and one O atom of the carboxylate group of the pypzacac− ligand. Similar reactions of pypzacacH with CuSO4·5H2O or Cu(ClO4)2·6H2O yielded another two dinuclear Cu(II) complexes 3·2MeOH and 4·MeOH, respectively. However, all (in 3) or partial (in 4) COOH groups of pypzacacH were esterified into COOMe group, thereby forming a new ligand, mpypzacac. It was assumed that the Cu2+ ion may initialize such an esterification, which was similar to the dehydrative condensation of carboxylic acid with alcohol catalyzed by Lewis acids such as HfCl4, Sc(OTf)3, MoO2Cl2, Bi(OTf)3 and FeCl3.20 But the aforementioned esterification was different from the hydrolysis of the ether from ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-yl]acetate (epypzacac) ligand to the parent acetate pypzacac− and ethanol in hydrothermal reaction of [MoO2Cl2(epypzacac)].21 Two SO42− anions in 3 worked as bridging ligands to link two Cu2+ centers, while two ClO4− anions in 4 remained intact. In the case of CuCl2, its reactions with pypzacacH gave one polymeric complex [(CuCl) (μ-κ3:κ1-pypzacac)]n (5). In this reaction, half of the chlorides of CuCl2 was replaced by pypzacac− ligands. In 1–5, the pypzacac− ligand showed three coordination modes: chelating bidentate mode η1(N),η1(N′) (2) and chelating/bridging coordination fashion μ-η1(N),η1(N′)-η1(O) (1 and 2) and μ-η1(N),η1(N′),η1(O)-η1(O′) (4 and 5). Thus, we assumed that the above five anions in copper(II) salts might play an important role in the coordination modes of pypzacac− ligand, and the formation of 1–5 (Scheme 1). Compounds 1–5 were stable toward oxygen and moisture, and readily soluble in H2O, DMF, DMSO, MeOH, EtOH and MeCN, but insoluble in toluene, Et2O, CH2Cl2 and n-hexane. Their elemental analyses were consistent with their chemical formula. In the FT-IR spectra of 1–5, the strong peaks at 1598 cm−1 (1), 1633 cm−1 (2), 1731/1616 cm−1 (3), 1621 cm−1 (4), 1625 cm−1 (5) were assigned as the C
O stretching vibrations of pypzacac− or/and mpypzacac ligand. Their identities were finally confirmed by X-ray crystallography.
 |
| Scheme 1 Syntheses of complexes 1–5. | |
Crystal structure of 1·0.5H2O
Compound 1·0.5H2O crystallizes in the orthorhombic space group Pbcn, and its asymmetric unit contains half a discrete molecule [{(MeOH)Cu(OAc)}(μ-κ2:κ1-pypzacac)]2 and one-fourth of a H2O molecule. Complex 1 has a dimeric structure in which two {(MeOH)Cu(OAc)} moieties are symmetrically bridged by a pair of μ-κ2:κ1-pypzacac− ligands (Fig. 1). A crystallographic inversion center is located on the midpoint of the two Cu centers. Each pypzacac− ligand takes a μ-κ2(N,N′):κ1(O) mode to bind two Cu centers via two Cu–N bonds and one Cu–O bond. Cu(1) or Cu(1A) in 1 can be described as having a square pyramidal coordination geometry in which O(2A), N(2), N(3), and O(5) atoms (or O(2), N(2A), N(3A), and O(5A) atoms) sit on the basal plane while O(3) atom (or O(3A) atom) from the MeOH molecule occupies the apical position. In 1, the mean Cu–O(AcO− and CH2COO−) bond length (1.9515(19) Å) (Table 2) is much shorter than the Cu–O(MeOH) bond length (2.405(2) Å), but is comparable to those observed in [{Cu2(μ-OAc)2}2(μ-dcmpz)2(μ-OAc)2] (1.948(3) Å).22a The Cu(1)–O(4) and Cu(1)–O(5)(MeOH) bond lengths are relatively long (2.642(2) Å, 2.405(2) Å), indicating that the O(4) and O(5) atoms interact weakly with the Cu(1) center. The mean Cu–N bond length (2.023(2) Å) is longer than that observed in [{Cu(μ-dcmpz)}2(μ-OMe)2]3 (1.978(2) Å),22a but close to that in [CuCl2(bppyH2)] (2.009(11) Å; bppyH2 = 2,6-bis(pyrazol-3-yl)pyridine).22b The Cu⋯Cu distance is 6.220 Å, which excludes any metal–metal interaction.
 |
| Fig. 1 View of the molecular structure of 1 with a labeling scheme and 50% thermal ellipsoids. All hydrogen atoms were omitted for clarity. | |
Crystal structure of 2·2(MeOH)0.5·6H2O
Complex 2·MeOH·6H2O crystallizes in the triclinic space group P
, and its asymmetric unit contains one [{Cu(pypzacac)}(μ-κ2:κ1-pypzacac)2{Cu(H2O)2}]+ cation, one NO3− ion, two halves of MeOH and six H2O solvent molecules. In 2, one {Cu(pypzacac)} fragment and one {Cu(H2O)2} fragment are bridged by a pair of μ-κ2:κ1-pypzacac ligands, forming an asymmetric dimeric structure (Fig. 2). Cu(1) or Cu(2) center in 2 also adopts a square pyramidal coordination geometry in which O(5), N(1), N(3), and O(8) atoms (or O(1), N(7), N(9), and N(7) atoms) sit on the basal plane while O(7) atom (or N(4) atom) occupies the apical position. The Cu(1)–O(7) (2.239(5) Å) or Cu(2)–N(4) (2.241(5) Å) distance on the apical line is longer than the Cu–O (1.955(5)–1.961(4) Å) and Cu–N lengths (2.004(5)–2.051(5) Å) on the basal plane (Table 2). The Cu(1)–O(5)(COO) (1.960(4) Å) and Cu(2)–O(1)(COO) (1.961(4) Å) bond lengths are comparable to those of the corresponding ones in 1.
 |
| Fig. 2 View of the [{Cu(pypzacac)}(μ-pypzacac)2{Cu(H2O)2}]+ cation of 2 with a labeling scheme and 50% thermal ellipsoids. All hydrogen atoms and NO3− were omitted for clarity. | |
Crystal structure of 3·2MeOH
Complex 3·2MeOH crystallizes in the triclinic space group P
, and its asymmetric unit contains half a discrete [{(MeOH)Cu(mpypzacac)}(μ-SO4)]2 molecule and one MeOH solvent molecule. In 3, two {(MeOH)Cu(mpypzacac)} moieties are symmetrically bridged by a pair of μ-SO42− ions to form a dimeric structure with a crystallographic center of symmetry lying at the midpoint of Cu(1) and Cu(1A) centers (Fig. 3). Each Cu(II) center is coordinated by two N atoms from mpypzacac, two O atoms from two μ-SO42− anions and one O atom from the coordinated MeOH molecule to form a distorted square pyramidal coordination geometry. The pseudo-equatorial plane is defined by two N atoms and two O atoms of μ-SO42−, while the O(3) atom from the MeOH molecule occupies the apical position. The Cu(1)–N(1)(pz) bond distance of 2.008(2) Å (Table 2) is slightly shorter than the Cu(1)–N(3)(py) bond length (2.022(2) Å) and the Cu(1)–N(2)(pz) bond length (2.025(2) Å) in 1. In 3, the Cu–O(SO42−) bond lengths (1.930(2) Å and 1.9376(19) Å) are much shorter than the Cu–O(3)(MeOH) bond length (2.207(2) Å). This Cu–O(3)(MeOH) bond length is shorter that that of the corresponding one in 1 (2.642(2) Å).
 |
| Fig. 3 View of the molecular structure of 3 with a labeling scheme and 50% thermal ellipsoids. All hydrogen atoms were omitted for clarity. | |
Crystal structure of 4·MeOH
Complex 4·MeOH crystallizes in the triclinic space group P
, and its asymmetric unit contains [{Cu(mpypzacac)2}(μ-κ3:κ1-pypzacac){Cu(mpypzacac)}]3+ trication, three ClO4− anions and one MeOH solvent molecule. The pypzacac ligand in 4 takes a μ-κ3(N,N′,O):κ1(O′) coordination mode to coordinate to two Cu centers via two Cu–N bonds and two Cu–O bonds (Fig. 4). In 4, Cu(1) is coordinated by four N atoms from two mpypzacac ligands and one O atom from the bridging pypzacac− ligand to form a distorted trigonal bipyramidal geometry. The N(4), N(1) and O(5) atoms surrounding Cu(1) sit on the equatorial plane while N(3) and N(6) atoms are located on the equatorial positions. The Cu(1)–N(3)/N(6) bond distances (2.003(4) Å, 1.999(4) Å) on the equatorial plane is shorter than the Cu(1)–N bond lengths (2.089(4) Å, 2.178(4) Å) on the equatorial plane (Table 3). While Cu(2) center displays a square pyramidal coordination geometry in which O(6), N(9), N(10), and N(12) atoms sit on the basal plane while N(7) atom occupies the apical position. The Cu(2)-N(7) bond distance of 2.210(4) is longer than other Cu(2)–N bond lengths (1.929(3) Å, 1.994(4) Å and 2.069(4) Å) on the equatorial plane. The Cu(1)–O(5) bond length of 1.971(3) Å is longer than the Cu(2)–O(6) bond distance (1.993(3) Å), which may be due to the different coordination geometry of Cu centers.
 |
| Fig. 4 View of the [{Cu(mpypzacac)2}(μ-κ2:κ1-pypzacac){Cu(mpypzacac)}]3+ trication of 4 with a labeling scheme and 50% thermal ellipsoids. All hydrogen atoms and three ClO4− ions were omitted for clarity. | |
Crystal structure of 5
Complex 5 crystallizes in the monoclinic space group P21/c, and its asymmetric unit contains one discrete [(CuCl)(μ-κ3:κ1-pypzacac)] molecule. Compound 5 has a 1D a spiral array (extending along the b axis) in which each {(CuCl)(pypzacac)} unit is interlinked by carboxyl groups of pypzacac ligands (Fig. 5). Each pypzacac ligand in 5 also takes a μ-κ3(N,N′,O):κ1(O′) mode to bind two Cu centers via two Cu–N and two Cu–O bonds. Each Cu center is coordinated by two N atoms from one pypzacac and two O atoms from two pypzacac ligand and one Cl atom, forming a distorted square pyramidal coordination geometry. The Cu(1)–Cl(1) bond distance of 2.2383(5) Å (Table 3) is shorter than that found in [CuCl2(bppyH2)] (2.391(10) Å).22b The Cu(1)–N(1)(pz) bond distance of 1.9355(16) Å is shorter than the Cu(1)–N(3)(py) bond length (2.0632(16) Å) and the Cu(1)–N(2) bond distance (2.025(2) Å) in 1. The Cu(1)–O(1) bond length of 1.9962(14) Å is shorter than the Cu(1)–O(2A) bond distance (2.2083(15) Å).
 |
| Fig. 5 View of the [{Cu(mpypzacac)2}(μ-κ2:κ1-pypzacac){Cu(mpypzacac)}]3+ trication of 4 with a labeling scheme and 50% thermal ellipsoids. All hydrogen atoms and three ClO4− ions were omitted for clarity. | |
Ammoxidation of alcohol to nitrile and aerobic oxidation of alcohol into aldehyde catalyzed by 1–5
Complexes 1–5 hold good solubility in water and the Cu(II) centers in 1–5 are coordinatively unsaturated or weakly-coordinated by H2O or MeOH molecule. Thus complexes 1–5 may possess catalytic activity towards some organic transformations. We attempted these compounds as catalysts for the double dehydrogenation reactions of alcohols with aqueous ammonia in water. Firstly, we carried out the reaction of phenylmethanol (6a) (3.5 mmol) with 1 (2.5 mol%), K2CO3 (10 mol%), TEMPO (6 mol%) and 550 μL aqueous ammonia with H2O (3 mL) as a solvent under 1 atm O2 at 60 °C. After 24 hours, the desired product benzonitrile (7a) could be isolated in 71% yield coupled with benzaldehyde (8a) (8% yield) (Table 4, entry 1). This preliminary result implied that 1 might initialize the aerobic dehydrogenation cascade reaction of alcohol into nitrile. For a variety of bases (e.g., K2CO3, K3PO4, KOH and NaOH), K2CO3 is the best one for this reaction in H2O (entry 1). In the presence of benzyltrimethylammonium chloride (TMBAC, 8% mol), the yield of desired product was increased from 71% to 85% (entry 5). Almost complete conversion of 6a into 7a was achieved when tetraethylammonium iodide (TEAI) was added into the reaction (entry 6). In the absence of base, 1 exhibited very low activity at 60 °C (entry 7). The reaction temperature exerted great impact on the ammoxidation reaction of alcohol. When the temperature was raised from 30 °C to 40 °C to 50 °C, the yields of the benzonitrile product could be increased from 23% to 42% and 72%, respectively (entries 8–10). However, the benzonitrile yield was decreased from 99% to 89% when the reaction temperature was increased from 60 °C to 70 °C (entries 8 and 11). The TEMPO loading greatly affected the catalytic activity. The product yield was gradually increased from 59% to 99% when the TEMPO loading was changed from 2% mol to 6% mol at 60 °C (Table 4, entries 6, 12 and 13). The relatively higher TEMPO loading (10% mol) for this reaction seemed not to affect the product yields (entries 14). According to the aforementioned optimization experiments, the optimal reaction conditions were identified as follows: catalyst loading ([Cu] = 5 mol%), 8 mol% TEAI loading, 6 mol% TEMPO loading, 10 mol% K2CO3 loading and H2O (as a solvent) with a reaction temperature of 60 °C. With these optimized conditions, the catalytic performances of 2–5 were also investigated. These complexes exhibited highly catalytic activity for the ammoxidation of phenylmethanol to benzaldehyde (entries 17–20). The catalytic activities of 1, 2 and 4 were higher than those of 3 and 5. Comparative runs with Cu(NO3)2/2,2′-bipyridine (76%) catalytic system under the same reaction conditions indicated that 1–5 exhibited better catalytic performance and higher selectivity (entry 21). The hydrophilic –CO2− group on the pypzacac ligand did play an important role in controlling the active catalytic species in water.
Table 4 Optimizing reaction conditions for the ammoxidation reaction of benzyl alcohol

|
Entrya |
Cat. |
TEMPO (%) |
Base |
Additive |
T (°C) |
Time (h) |
Yieldb (%) 7a |
Yieldb (%) 8a |
3.5 mmol benzyl alcohol and 550 μL aqueous ammonia, 10 mol% base, 5 mol% [Cu] in 3 mL H2O. TMBAC = benzyltrimethylammonium chloride (8 mol%); TEAI = tetraethylammonium iodide (8 mol%). GC yield. 1′ = Cu(NO3)2/2,2′-bipyridine. |
1 |
1 |
6 |
K2CO3 |
— |
60 |
24 |
71 |
8 |
2 |
1 |
6 |
KOH |
— |
60 |
24 |
52 |
7 |
3 |
1 |
6 |
K3PO4 |
— |
60 |
24 |
68 |
Trace |
4 |
1 |
6 |
NaOH |
TMBAC |
60 |
24 |
48 |
Trace |
5 |
1 |
6 |
K2CO3 |
TEAI |
60 |
24 |
85 |
11 |
6 |
1 |
6 |
K2CO3 |
TEAI |
60 |
24 |
99 |
— |
7 |
1 |
6 |
— |
TEAI |
60 |
24 |
30 |
Trace |
8 |
1 |
6 |
K2CO3 |
TEAI |
50 |
24 |
72 |
7 |
9 |
1 |
6 |
K2CO3 |
TEAI |
40 |
24 |
42 |
9 |
10 |
1 |
6 |
K2CO3 |
TEAI |
30 |
24 |
23 |
18 |
11 |
1 |
6 |
K2CO3 |
TEAI |
70 |
24 |
89 |
Trace |
12 |
1 |
2 |
K2CO3 |
TEAI |
60 |
24 |
59 |
35 |
13 |
1 |
4 |
K2CO3 |
TEAI |
60 |
24 |
90 |
2 |
14 |
1 |
10 |
K2CO3 |
TEAI |
60 |
24 |
98 |
— |
15 |
1 |
6 |
K2CO3 |
TEAI |
60 |
24 |
87 |
12 |
16 |
1 |
6 |
K2CO3 |
TEAI |
60 |
24 |
44 |
37 |
17 |
2 |
6 |
K2CO3 |
TEAI |
60 |
24 |
98 |
— |
18 |
3 |
6 |
K2CO3 |
TEAI |
60 |
24 |
86 |
5 |
19 |
4 |
6 |
K2CO3 |
TEAI |
60 |
24 |
99 |
— |
20 |
5 |
6 |
K2CO3 |
TEAI |
60 |
24 |
88 |
4 |
21c |
1′ |
6 |
K2CO3 |
TEAI |
60 |
24 |
76 |
8 |
The aerobic oxidation of alcohols catalyzed by 1
Once the optimization had been performed, we examined the substrate scope of the ammoxidation reaction with 1. As depicted in Table 5, catalyst 1 exhibited a wide applicability towards a broad array of alcohols, affording the corresponding nitriles in high yields. The reaction could tolerate a wide range of aromatic functional groups including electron donating groups such as methoxyl and methyl, aryl chloride, nitro groups and aromatic heterocyclic molecules. It appeared that the electronic nature of substituents on benzyl alcohols had some effect on the catalytic activity towards the transformation from benzylic alcohol to aryl nitrile. Commercially available methyl-, and methoxy-substituted benzyl alcohols gave the respective nitriles in 85–95% yield (Table 5, entries 1–8). For the double dehydrogenation reactions of electron-withdrawing chloride- and nitryl-substituted benzamides, higher reaction temperature (80 °C) was needed for the moderate yields (entries 9 and 10). It seemed that the o-, m-substituted groups on phenyl rings of benzyl alcohols promote rather than hamper the double dehydrogenation reactions. Substrates with substituents at the meta and ortho positions (entries 3, 4, 6 and 7) were more reactive than para-substituted substrates (entries 2 and 5). Notably, heteroatom-containing alcohols including nitrogen and sulfur atoms such as pyridin-3-ylmethanol, thiophen-2-ylmethanol were well tolerated under the reaction conditions and high yields of the desired nitriles were obtained (entries 11 and 12). During the ammoxidation reaction of alcohol, aldehyde was supposed to form initially. Thus, we evaluated the 1/O2/TEMPO catalytic system in the direct transformation of alcohol into aldehyde. The aerobic oxidation of alcohols could afford smoothly the corresponding nitriles in excellent yields even at lower temperature (40 °C) in water. Notably, the presence or absence of an electron-donating group did not significantly affect the yield. Over 90% isolated yields were obtained uniformly. While the result was relatively worse while using 4-nitrobenzyl alcohol as substrates (22%). Elevating the reaction temperature up to 80 °C resulted in the higher yield (70%) of 4-nitrobenzaldehyde. For the heteroaromatic benzylic alcohols such as pyridin-3-ylmethanol and thiophen-2-ylmethanol, the corresponding nitriles were obtained in 85% and 67% isolated yield.
Table 5 Ammoxidation of alcohol to nitrile and aerobic oxidation of alcohol to aldehyde catalyzed by 1
Ar–CH2OH + NH3 → Ar–CN |
Ar–CH2OH → Ar–CHO |
Entry |
Alcohol |
Nitrilea |
Yieldb (%) |
Aldehyded |
Yieldb (%) |
Reaction conditions: alcohol (3.5 mmol), 550 μL aqueous ammonia, 6 mol% TEMPO, 8 mol% TEAI, 10 mol% K2CO3, 1 atm O2, and catalyst loading = 5 mol% [Cu] at 60 °C for 24 hours. Isolated yield. Reaction temperature = 80 °C. Reaction conditions: alcohol (3.5 mmol), 1 atm O2, 5 mol% [Cu], 6 mol% TEMPO, 8 mol% TEAI, 10 mol% K2CO3 in 3 mL H2O at 40 °C for 24 hours. |
1 |
 |
 |
94 |
 |
95 |
2 |
 |
 |
89 |
 |
97 |
3 |
 |
 |
91 |
 |
95 |
4 |
 |
 |
92 |
 |
91 |
5 |
 |
 |
85 |
 |
98 |
6 |
 |
 |
93 |
 |
94 |
7 |
 |
 |
94 |
 |
92 |
8 |
 |
 |
95 |
 |
98 |
9 |
 |
 |
45c |
 |
22/70c |
10 |
 |
 |
70c |
 |
71 |
11 |
 |
 |
85 |
 |
85 |
12 |
 |
 |
41 |
 |
67 |
Reusability of the catalyst
A further advantage of such a water-soluble catalytic system was its reusability. The ammoxidation of phenylmethanol with 550 μL aqueous ammonia, 6 mol% TEMPO, 8 mol% TEAI and 10 mol% K2CO3 was performed using a 2.5 mol% catalyst 1 in order to examine the feasibility of this approach. After 24 h and cooling to room temperature, the organic product, benzonitrile, was readily separated from the aqueous phase by simple extraction with Et2O, and the residual aqueous solution was then subjected to the next reaction run, charged with phenylmethanol, TEMPO and aqueous ammonia. The residual aqueous solution containing 1 still worked and the conversion steadily decreased during five runs, from ca. 95–78% (Table 6). For the aerobic oxidation of alcohol, the catalytic system could also be reused several cycles. No obvious loss of catalytic activity is observed after the catalytic solution is used five times.
Table 6 Reuse of 1-catalyzed the oxidation of phenylmethanol and the aerobic double dehydrogenation
Cycle |
Benzonitrilea |
Benzaldehydeb |
Reaction conditions: alcohol (3.5 mmol), 550 μL aqueous ammonia, 6 mol% TEMPO, 8 mol% TEAI, 10 mol% K2CO3, 1 atm O2, and catalyst loading = 5 mol% [Cu] at 60 °C for 24 hours. Reaction conditions: alcohol (3.5 mmol), 1 atm O2, 5 mol% [Cu], 6 mol% TEMPO, 8 mol% TEAI, 10 mol% K2CO3 in 3 mL H2O at 40 °C for 24 hours. |
1 |
94 |
95 |
2 |
90 |
95 |
3 |
86 |
94 |
4 |
82 |
91 |
5 |
78 |
92 |
Conclusions
In summary, we have demonstrated that reactions of pypzacacH consisting of pyridinyl, pyrazolyl and carboxylate groups with Cu(OAc)2, Cu(NO3)2, CuSO4, Cu(ClO4)2 or CuCl2 produced four dinuclear Cu(II) complexes 1–4, and one polymeric complex 5. The anions (OAc−, NO3−, SO42−, ClO4− and Cl−) are the key parameter for controlling the coordination modes of the pypzacac ligand and the structural variations of the resulting complexes. The mpypzacac ligands in 3 and 4 were in situ produced via the Cu2+-catalyzed dehydrative esterification of acetic acid of the pypzacac ligand. In 1–5, there are three coordination modes for pypzacac, η1(N),η1(N′) chelating bidentate mode (1), μ-η1(N),η1(N′)-η1(O′) (1 and 2) and μ-η1(N),η1(N′),η1(O)-η1(O′) (4 and 5) chelating/bridging coordination fashion (Scheme 2). Complexes 1–5 displayed high catalytic activity towards the ammoxidation of alcohol to nitrile and the aerobic oxidation of alcohol to aldehyde in water. Furthermore, catalyst 1 was easily recovered by extracting the organic products with ether and could be reused several times without loss of its catalytic activity, especially towards the aerobic oxidation of alcohol to aldehyde. It is anticipated that other polar sulfonate and ammonium groups may be employed to functionalize the pyrzaolate ligands to achieve better catalytic performance in H2O. Studies on these respects are currently underway.
 |
| Scheme 2 The coordination modes of pzpypz− ligand in 1–5. | |
Acknowledgements
The authors are grateful for financial supports from the National Natural Science Foundation of China (21171124, 21171125 and 21371126) and State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences (201201006). J. P. Lang also highly appreciates the financial supports from the Qin-Lan Project and the “333” Project of Jiangsu Province, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the “SooChow Scholar” Program of Soochow University.
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