2
W.-G. Jia et al. / Journal of Molecular Structure 1217 (2020) 128349
catalytic activity in the reduction reaction of nitro compounds, so as
to achieve high efficiency and environmental protection.
compounds (0.1 mmol, 1.0 equiv) and NaBH4 (10.0 mmol, 10 equiv)
in water (2.0 mL) were charged into a 15 mL pressure tube under air
atmosphere. Subsequently, the resulting mixture was stirred at
room temperature in closed vessel. After completion of the reaction
(monitored by TLC), the crude reaction mixture was extraction with
ether (3 ꢁ 2 mL), and analyzed by GC-MS chromatography using n-
dodecane as an internal standard.
This paper reports the synthesis and characterization of Cu(II)
complexes with bisoxazolines, tridentate pincer pybox and ter-
pyridine ligands, and further examine their catalytic activities for
nitro compounds reduction in the presence of NaBH4. The catalytic
results showed that copper complex with bisoxazoline [Cu(Dmox)
Cl2(m-Cl)2] 1e had promising catalytic activity for nitro compounds
reduction with a broad substrate scope. Solid state structures of the
copper complexes 1a and 1c have been determined by single crystal
X-ray diffraction method.
2.5. X-ray structure determination
Diffraction data of 1a and 1c were collected on a Bruker AXS
SMART APEX diffractometer, equipped with a CCD area detector
2. Experimental
using Mo K
a
radiation (
l
¼ 0.71073 Å). All data were collected at
298K and the structures were solved by direct methods and sub-
sequently refined on F2 by using full-matrix least-squares tech-
niques SHELXL, SADABS absorption corrections were applied to the
data, all non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were located at calculated positions. All calcula-
tion was performed using the Bruker Smart program.
2.1. Materials and measurements
Commercial reagents were analytical grade and used as
received. All the operations were carried out under nitrogen at-
mosphere using standard Schlenk techniques. All solvents were
purified and degassed by standard procedures. The ligands 4,5-
Dihydro-2-(4,5-dihydro-4,4-dimethyloxazol-2-yl)-4,4-
dimethyloxazole (DMOX); 2,6-bis[40,40-dimethyloxazolin-20-yl]
pyridine (Dm-Pybox) and tpy-C6H4-CH]CH-4-tolyl (L) were syn-
thesized according to the procedures described in the literature
[26e28]. 1H and 13C NMR were recorded on a 400 MHz NMR
3. Results and discussions
The mononuclear copper complexes [CuX2L] (L ¼ Dm-Pybox,
X ¼ Cl (1a), L ¼ Dm-Pybox, X ¼ Br (1b), L ¼ tpy-C6H4-CH]CH-4-
tolyl, X ¼ Cl (1c); L ¼ Dmox, X ¼ Br (1d)) and binuclear [Cu(D-
spectrometer at room temperature. Chemical shifts (d) are given in
mox)Cl2(m-Cl)2] (1e) were obtained through the reaction between
ppm relative to internal TMS and are internally referenced to re-
sidual 1H solvent resonances. IR spectra were recorded on a Niclolet
AVATAR-360IR spectrometer. Elemental analyses were performed
on a PerkinElmer 2400 CHN analyzer. Mass spectra were obtained
with MicroTof (Bruker Daltonics, Bremen, Germany) spectrometers.
CuX2 (X ¼ Cl or Br) and one equivalent of ligand in MeOH/DCM (v/
v ¼ 1:1) solvents under room temperature [25] (see Scheme 1).
These copper complexes are stable in air and moisture, soluble in
alcohol and acetonitrile solvents. And the copper complex 1e is
slightly soluble in H2O compare to the other copper complexes.
Single crystals of 1a and 1c suitable for X-ray diffraction studies
were grown by slow diffusion of Et2O into a concentrated solution
of the complexes in MeOH. The crystallographic data for copper
complexes are summarized in Table 1, and selected bond lengths
Complexes 1a and 1e crystallize in the monoclinic system with
P2(1)/c (1a) and P21/n (1c) space group, respectively. The crystal
structures confirm the formation of the complex [CuCl2L]. As
shown in Fig. 1, the crystal structures of 1a and 1c consists of
mononuclear units, and each copper atom is coordinated by three
nitrogen atoms of pincer ligand and two chlorine atoms. As ex-
pected, pyridine-based pincer ligand is coordinated to copper in a
tridentate fashion by the pyridyl nitrogen or oxazoline nitrogen
atoms. The coordination sphere around the Cu(II) atom is midway
between trigonal bipyramidal and square pyramidal. The CueN
2.2. Preparation of copper complex [CuCl2(Dm-Pybox)] (1a)
A 50 mL round-bottomed flask was placed with (Dm-pybox)
(55 mg, 0.2 mmol), CuCl2$2H2O (34 mg, 0.2 mmol), 10 mL MeOH
and 10 mL CH2Cl2 as solvent. The mixture was stirred at room
temperature for 5 h and then the solvent was removed with the
rotary evaporator; the resulting solid was washed with Et2O. The
product was dried under vacuum to give green complex 1a (94 mg,
75%). Anal. Calcd. for C15H19Cl2CuN3O2: C 44.18, H 4.70, N 10.30
Found: C 44.12, H 4.66, N 10.33. IR (KBr cmꢀ1): 3255(w), 1642(s),
1522(m), 1364(m), 1338(m), 1272(m), 1166(m), 1041(m), 912(w),
744(m), 685(m), 439(m).
2.3. Preparation of copper complex [CuCl2(tpy-C6H4-CH]CH-4-
tolyl)] (1c)
A 50 mL Schlenk flask was placed with (CuCl2$2H2O) (42 mg,
0.25 mmol), L (88 mg, 0.2 mmol), 5 mL MeOH and 5 mL CH2Cl2 as
solvent. The mixture was stirred overnight at room temperature
and then the solvent was removed with the rotary evaporator; the
resulting solid was washed with CH2Cl2, MeOH and Et2O. The
product was dried under vacuum to give corresponding green
complex 1c (85 mg, 81%). ESI-MS m/z: calcd for [C30H23CuN3Cl]þ
[M ꢀ Cl]þ 523.0851; found: 523.0871; Calcd for [C30H23CuN3]2þ
[M ꢀ 2Cl]2þ 244.0585; found: 244.0589. IR (KBr cmꢀ1): 3431(m),
1599(s), 1549(m), 1471(s), 1400(s), 1244(m), 1158(w), 1015(s),
781(s), 720(m), 649(w).
2.4. General procedure for the reduction of nitro compounds with
copper complex catalyst
Copper complex (0.001 mmol, 1mol%), appropriate nitro
Scheme 1. The molecular structures of copper complexes (1a-1e).