Copper(II) complexes of two TEMPO-functionalized polypyridyl ligands: structure and catalytic activity in alcohol oxidation

Article abstract of DOI:10.1080/00958972.2016.1272675

The reaction of copper(II) salts with Bpy-TEMPO and Tpy-TEMPO (Bpy-TEMPO?=?[2,2′]Bipyridinyl-5,5′-dicarboxylic acid bis-[(2,2,6,6-tetramethyl-1-oxy-piperidin-4-yl)-amide]; Tpy-TEMPO?=?2,2,6,6-tetramethyl-4-(2,2′:6′,2″-terpyridin-4′-yloxy)piperidin-1-oxyl) gave dinuclear Bpy-TEMPO-Cu₂ (1) and mononuclear Tpy-TEMPO-Cu (2), respectively. The Cu(II) complexes were characterized by single crystal X-ray analysis. In 1, Cu(II) has a distorted square pyramidal coordination geometry, with a bridging chloride as the axial ligand. The Cu(II) core in 2 also exhibited a distorted square pyramidal coordination geometry, with one chloride as an axial ligand. Weak interactions such as π-interactions and hydrogen bonds are observed in both complexes. When applied as catalysts for the oxidation of benzyl alcohol to benzaldehyde in air, 1 exhibited higher activity than 2 for reactions in o-xylene at 60°C with DBU as a base. High yield (67%) of benzaldehyde was observed when using 1 as a catalyst in a solution of o-xylene with DBU at 60°C.

Full text of DOI:10.1080/00958972.2016.1272675

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Copper(II) complexes of two TEMPO-functionalized
polypyridyl ligands: structure and catalytic activity
in alcohol oxidation
Xiaoshuang Lu, Nini Wu, Bingguang Zhang & Kejian Deng
To cite this article: Xiaoshuang Lu, Nini Wu, Bingguang Zhang & Kejian Deng (2017): Copper(II)
complexes of two TEMPO-functionalized polypyridyl ligands: structure and catalytic activity in
alcohol oxidation, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2016.1272675
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1272675
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Date: 04 January 2017, At: 00:49

Journal of Coordination Chemistry, 2017
http://dx.doi.org/10.1080/00958972.2016.1272675
Copper(II) complexes of two TEMPO-functionalized
polypyridyl ligands: structure and catalytic activity in alcohol
oxidation
Xiaoshuang Lu, Nini Wu, Bingguang Zhang and Kejian Deng
Key laboratory of Catalysis and materials sciences of the state ethnic affairs Commission & ministry of
education, CollegeofChemistryandmaterialscience, south-Centraluniversityfornationalities, Wuhan, China
ABSTRACT
ARTICLE HISTORY
received 15 october 2015
accepted 25 october 2016
The reaction of copper(II) salts with Bpy-TEMPO and Tpy-TEMPO
(Bpy-TEMPO = [2,2′]Bipyridinyl-5,5′-dicarboxylic acid bis-[(2,2,6,6-
tetramethyl-1-oxy-piperidin-4-yl)-amide];Tpy-TEMPO = 2,2,6,6-tetrame
thyl-4-(2,2′:6′,2″-terpyridin-4′-yloxy)piperidin-1-oxyl) gave dinuclear
Bpy-TEMPO-Cu2 (1) and mononuclearTpy-TEMPO-Cu (2), respectively.
The Cu(II) complexes were characterized by single crystal X-ray analysis.
In 1, Cu(II) has a distorted square pyramidal coordination geometry,
with a bridging chloride as the axial ligand. The Cu(II) core in 2 also
exhibited a distorted square pyramidal coordination geometry, with
one chloride as an axial ligand. Weak interactions such as π-interactions
and hydrogen bonds are observed in both complexes. When applied
as catalysts for the oxidation of benzyl alcohol to benzaldehyde in air,
1 exhibited higher activity than 2 for reactions in o-xylene at 60°C with
DBU as a base. High yield (67%) of benzaldehyde was observed when
using 1 as a catalyst in a solution of o-xylene with DBU at 60°C.
KEYWORDS
temPo; copper(ii) complex;
crystal structure; catalysis;
alcohol oxidation
1. Introduction
Oxidation of alcohols is an important reaction because the resulting aldehydes or ketones,
are widely used in the synthesis of pharmaceuticals, agrochemicals, and fine chemicals
CONTACT Bingguang Zhang
ralph@mail.scuec.edu.cn,
zhangbg68@yahoo.com
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2016.1272675.
© 2017 informa uK limited, trading as taylor & francis Group

2
X. LU ET AL.
[1]. Since the first example of copper-catalyzed aerobic oxidation of an alcohol mediated
by 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) was reported as an effective aerobic
oxidant of benzylic and allylic alcohols [2], various catalysts containing copper as a
redox-active center have been developed for the aerobic oxidation of alcohols, such as
copper/2,2′-bipyridine/TEMPO systems (Bpy = 2,2′-bipyridine) [3–6], copper/2,2′:6′,2′-ter-
pyridine/TEMPO systems (Tpy = 2,2′:6′,2′-terpyridine) [7, 8], etc. [9]. Recently, Kitagawa’s
group reported DPIO(4,7-bis(4-pyridyl)-1,1,3,3-tetramethyl isoindolin-2-yloxyl) as an
organic linker to synthesize a PCP (porous coordination polymer) decorated with nitroxyl
radicals, and this PCP was an efficient, recyclable, and widely applicable catalyst for
selective oxidation of various alcohols to their corresponding aldehydes or ketones [10].
The use of TEMPO is the key in the aerobic oxidation of alcohols because TEMPO is a
precursor in the reaction system. In 2015, Maayan’s group introduced a peptoid backbone
into Cu-TEMPO-based catalytic oxidation systems for aerobic oxidation of primary alco-
hols, operating in a loading of 0.1 mol% with a high TON [11]. However, there are few
reports on TEMPO derivatives, especially TEMPO–polypyridine systems. Herein, we report
two ligands, Bpy-TEMPO and Tpy-TEMPO, synthesized from TEMPO and polypyridine lig-
ands, the crystal structures of their copper complexes, dinuclear Bpy-TEMPO-Cu2 (1) and
mononuclear Tpy-TEMPO-Cu (2), and their application for aerobic oxidation of
alcohols.
2. Experimental
2.1. Materials
Commercially available solvents and chemicals (analytical grade) were used without purification
as received from the following sources: 4′-chloro-2,2′:6′,2″-terpyridine, DBU (1,8-diazabicy-
clo[5.4.0]undec-7-ene), 2,2′-bipyridine-5,5′-dicarboxylic acid, 4-hydroxy-2,2,6,6-tetramethylpip-
eridinooxy free radical (TEMPO-OH), 4-amino-2,2,6,6-tetramethylpiperidinooxy free radical
(TEMPO-NH2), o-xylene, CH3CN, THF and benzylic alcohol (J & K Chemical Technology).
TEA(N(CH2CH3)3), DMF, thionylchloride and K2CO3 (Sinopharm Chemical Reagent Co., Ltd.).
2.2. Physical measurements
Elemental analyses (C, H and N) were performed on a Vario Micro cube C, H, N, S elemental
analyzer. MALDI-TOF MS was performed on a VOYAGER DE STR Mass Spectrometer (Bpy-
TEMPO and Tpy-TEMPO). ESI-MS was performed on a Bruker APEX IV Fourier Transform Ion
Cyclotron Resonance Mass Spectrometer (for 1 and 2). Infrared (IR) spectra were recorded
from 4000 to 400 cm−1 on a Tensor 27 OPUS (Bruker) FT-IR spectrometer as KBr pellets.
Electronic absorption spectra were obtained in DMF solution with a Shimadzu UV-2600
spectrophotometer. Cyclic voltammetry (CV) was performed using a Model CHI600 instru-
ment (CH Instruments) in a three-electrode cell with pure argon (Ar) gas inlet and outlet.
The working and counter electrodes were platinum (Pt) and the reference electrode was a
AgCl/Ag electrode. The experiments were carried out in DMF solution with Bu4NClO4 (0.1 M)
and a scan rate of 100 mV·s−1.

JOURNAL OF COORDINATION CHEMISTRY
3
2.3. Synthesis
2.3.1. Preparation of 2,2,6,6-tetramethyl-4-(2,2′:6′,2″-terpyridin-4′-yloxy) piperidin-1-
oxyl (Tpy-TEMPO)
Tpy-TEMPO was prepared following the procedure reported by Halcrow et al. [12].
2.3.2. Preparation of [2,2′]bipyridinyl-5,5′-dicarboxylic acid bis-[(2,2,6,6-tetramethyl-
1-oxy-piperidin-4-yl)-amide] (Bpy-TEMPO)
2,2′-Bipyridine-5,5′-dicarboxylic acid (1.22 g, 5 mmol) was suspended in 30 mL of thionyl
chloride with refluxing for 48 h. After the solvent was removed under reduced pressure, the
residue, as a yellow solid, was dissolved in 50 mL of dry THF at room temperature under an
argon atmosphere. To this solution, a solution of TEMPO-NH2 (4.28 g, 25 mmol) and TEA
(4.0 mL, 30 mmol) in dry THF was added dropwise. After the reaction mixture was stirred for
24 h, the precipitate was collected by filtration, washed with THF and H2O, and dried under
reduced pressure to afford the product (2.01 g, yield 73%). Anal. Calcd for C30H42N6O4: C,
65.43; H, 7.69; N, 15.26%. Found: C, 65.05; H, 7.20; N, 15.00%. UV/vis spectrum (DMF)
λ
max = 302 nm (logε = 4.49). IR (KBr pellets, cm−1): ν = 3378 (NH), 2977 (CH), 1666 (C=O), 1542,
1465 (Py), 1326 (N → O). MALDI-TOF MS: 552.3 {[Bpy-TEMPO + H]+} (Calculated: 552.0).
2.3.3. Synthesis of 1
Bis{[2,2′]Bipyridinyl-5,5′-dicarboxylic acid bis-[(2,2,6,6-tetramethyl-1-oxy-piperidin-4-yl)
amide]}di-μ-chlorodichlorodicopper(II) acetonitrile tetrasolvate (Cu2(BpyTEMPO)2Cl4·4CH3CN)
Bpy-TEMPO (280 mg, 0.5 mmol) with one equivalent of CuCl2·2H2O (100 mg, 0.5 mmol)
in 30 mL of CH3CN afforded a dark yellow mixture, which was kept at 80°C until the solid
dissolved completely to obtain a yellowish-green solution which was then cooled to room
temperature; after four days, green single crystals were obtained in good yield (290 mg,
80%). Anal. Calcd for C60H84Cl4Cu2N12O8·4CH3CN: C, 53.22; H, 6.31; N, 14.60%. Found: C, 53.05;
H, 6.47; N, 13.00%. UV/Vis spectrum (DMF) λmax = 306 nm (logε = 4.41), 667 nm (logε = 2.11).
IR (KBr pellets, cm−1): 3279 (NH), 2977, 2978 (CH), 1650 (C=O), 1542, 1465 (Py), 1362 (N → O).
ESI-MS:1200{[(Bpy-TEMPO)2CuCl]+} (when Bpy-TEMPO-Cu2 was under ESI-MS conditions,
rearrangement occurred).
2.3.4. Synthesis of 2
Dichloro[2,2,6,6,-tetramethyl-4-(2,29:69,20-terpyridin-49-yloxy)piperidin-1-oxyl]copper(II)
N,N-dimethylformamide bis(dimethylformamide) (Cu(Tpy-TEMPO)Cl2·2DMF)
Tpy-TEMPO (200 mg, 0.5 mmol) was dissolved in 30 mL of CH3CN and stirred at room
temperature for 10 min, following which one equivalent of CuCl2·2H2O (10 mg, 0.5 mmol)
in DMF was added. The mixture was kept at 150°C until a blue–green solution was obtained.
The resulting mixture was cooled to room temperature; after four days green needle-like
crystals were obtained in good yield (280 mg, 83%). Anal. Calcd for C24H27Cl2CuN4O2·2DMF:
C, 52.67; H, 6.04; N, 12.28%. Found: C, 52.49; H, 6.12; N, 12.00%. UV/vis spectrum (DMF)
λ
max = 277 nm (logε = 4.61), 769 nm (logε = 2.17). IR (KBr pellets, cm−1): ν = 2977 (CH), 1558,
1465 (Py), 1357 (N → O), 1203 (C–O–C). ESI-MS: 501,503{[(Tpy-TEMPO)CuCl]+} (Calculated:
501, 503).

4
X. LU ET AL.
2.4. X-ray crystallography
Single crystals of 1 and 2 with appropriate dimensions were selected from the reaction
products for X-ray crystal diffraction. Crystallographic data for the compounds were collected
with a Bruker SMART CCD diffractometer equipped with a SMART CCD system [13] using
graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 173(2) K. Data reductions and
absorption corrections were performed with SAINT and SADABS software packages, respec-
tively [13, 14]. The structures of the compounds were solved by direct methods with
SHELXS-97 [15, 16] and refined by full-matrix least-squares on F2 with SHELXL-97 [15, 16]. All
non-hydrogen atoms were refined anisotropically using reflections I > 2σ (I). The C-bound
hydrogens were included in calculated positions and refined as riding. The N-bound hydro-
gens were located from difference Fourier maps and then included in refinement with bond-
length restraints of N–H = 0.88 (1) Å with Uiso = 1.2Ueq. The solvent molecules are highly
disordered, and attempts to locate and refine the solvent peaks were unsuccessful. These
solvent molecules were removed using the SQUEEZE routine of PLATON (92 electrons per
cell were squeezed, corresponding to diffraction contribution of two normal acetonitrile
molecules and two disordered acetonitrile molecules) and the structure was then refined
again using the data generated [17].
2.5. Experimental procedure for oxidation of benzyl alcohol
Oxidation experiments were carried out at 60°C. A solid catalyst of 1 or 2 (3.5 mg, 5 mol%)
was added to a 10 mL solution containing benzylic alcohol (10 μL, 0.1 mmol) with equivalent
base in a 25 mL open vial. The reaction mixture was stirred for 12 h in open air. Then the
products of catalytic experiments were identified by comparing with the standard samples
(purchased from J & K Chemical Technology). Yield and selectivity were calculated by GC
analysis via the internal method (bromobenzene was used as an internal standard).
3. Results and discussion
3.1. Synthesis
The synthesis routes of these two ligands and their copper(II) complexes are depicted in
scheme 1. TPy-TEMPO has been reported by Halcrow et al. [12]. Bpy-TEMPO was synthesized
from 2,2′-bipyridine-5,5′-dicarboxylic acid, thionylchloride and TEMPO-NH2 in high yield.
The complexes were prepared from the ligands with an equivalent of CuCl2·2H2O in CH3CN
in high yield.
3.2. Description of structures
The structures of 1 and 2 were determined by X-ray diffraction analysis and the crystal data
and refinement details are given in table 1. The crystal structure of 1 is presented in figure
1(a) and the unit cell structure including solvent molecules is shown in figure S1. Relevant
bond parameters are listed in table 2. Compound 1 crystallized in the triclinic space group
P-1 and contains two copper centers, two Bpy-TEMPO ligands, four coordinated chlorides,
and four acetonitrile molecules. The Cu1 center has a distorted square-pyramid geometry
with two nitrogen donors (N1 and N4) and two chlorides (Cl1 and Cl2) in the basal plane and

JOURNAL OF COORDINATION CHEMISTRY
5
Scheme 1. syntheses of Bpy-temPo, tpy-temPo and their copper(ii) complexes 1 and 2.
Table 1. Crystal data and refinement parameters of 1 and 2.
1
2
formula
Mw
Crystal system
space group
a(Å)
b(Å)
c(Å)
α(°)
β(°)
C
68h96Cl4Cu2n16o8
1534.49
triclinic
P-1
11.7014(12)
12.6663(13)
14.9410(15)
67.487(2)
75.135(2)
87.811(2)
1972.8(3)
1
C30h41Cl2Cun6o4
684.13
triclinic
P-1
8.0645(9)
12.3568(13)
17.4638(19)
77.199(2)
89.144(2)
74.932(2)
1637.0(3)
2
γ(°)
V(Å−3
)
Z
Μ(mo-Kα) (mm−1
T (K)
)
0.71073
173(2)
0.71073
173(2)
density (calculated)
absorption coefficient
F(0 0 0)
unique reflns.
reflns. obsd. [(I > 2σ(I)]
R1 [(I > 2σ(I)]
wR2 [(I > 2σ(I)]
Rint
1.292
0.734
806
7625
4210
0.0543
0.1089
0.0498
1.388
0.874
716
6579
3940
0.0529
0.0874
0.0660
0.1163
1.018
Rsigma
Goof
0.1142
0.918

6
X. LU ET AL.
Figure 1. (a) orteP structure of 1 with selected atomic-labeling scheme showing the local coordination
environment of Cu(ii). thermal ellipsoids are drawn at 50% probability. hydrogens are omitted for clarity.
(b) the n–h⋯o and n–h⋯Cl hydrogen bonds link dimers into one infinite chain along the a-axis.
Table 2. selected bond lengths (Å) and angles (°) for 1.
Bond distances
Bond angles
80.01(12)
Cu1–n1
Cu1–n4
Cu1–Cl2i
Cu1–Cl1
Cu1–Cl2
n6–o4
2.022(3)
2.019(3)
2.6684(11)
2.2660(11)
2.2721(10)
1.281(4)
n1–Cu1–n4
n1–Cu1–Cl1
n4–Cu1–Cl1
n1–Cu1–Cl2
n4–Cu1–Cl2
Cl1–Cu1–Cl2
Cl1–Cu1–Cl2i
Cl2–Cu1–Cl2i
n1–Cu1–Cl2i
n4–Cu1–Cl2i
93.15(9)
167.42(8)
170.37(9)
93.40(9)
91.97(4)
101.45(4)
92.49(4)
94.48(9)
89.68(9)
n3–o2
1.284(4)
symmetry code: a and (i) 1−x, 1−y, −z.

JOURNAL OF COORDINATION CHEMISTRY
7
Cl2A in the apical position. The mean deviation of N1, N4, Cl1, and Cl2 from the plane is 0.03 Å.
The two copper centers are bridged by two chlorides (Cl(2) and Cl(2a)), forming a dinuclear
unit. The Cu–N bond distances are 2.019(3) Å and 2.022(3) Å. The Cu1–Cl bond lengths are
2.2660(11)–2.6684(11) Å, comparable to those reported for related Cu(II) complexes [18]. The
bond angles around CuII center are 80.01(12)–170.37(9)°, which are all unexceptional. In 1,
hydrogen bonds are observed among solvent molecules and the dinuclear units (table S1).
Parallel to the a axis, the dimers are linked into infinite chains by N–H⋯O and N–H⋯Cl hydro-
gen bonds (figure 1(b)), which contribute to stabilization of the neighboring dinuclear units.
The structure of 2 is shown in figure 2(a) and selected bond parameters are listed in
table 3. Compound 2 crystallized in the triclinic space group P-1 and contains one copper,
one Tpy-TEMPO, two coordinated chlorides, and two DMF molecules. The Cu(II) center coor-
dinates to three chelating nitrogens of oneTpy-TEMPO ligand and two chlorides that saturate
the coordination sphere. Thus, Cu(II) has a distorted square-pyramidal environment. The
Cu–N bond lengths are 1.948(3)–2.042(3) Å. The Cu–Cl bond distances are 2.2315(10) Å and
2.5020(11) Å. The bond angles around Cu(II) are 78.82(12)–161.41(9)°. These are all normal
compared with similar Cu(II) compounds [18, 19]. Like 1, weak π⋯π interactions are also
observed in 2. The centroid–centroid distance between pyridyl rings in different Tpy-TEMPO
ligands is ca. 3.97 Å and the shortest distances between the atoms are 3.31 Å for C–C and
3.40 Å for C–N (figure 2(b)). The DMF molecules are located on the side of the mononuclear
Cu(II) unit and hydrogen bonds exist between DMF molecules. Weak interactions between
DMF molecules and the mononuclear unit are not discussed here. Weak C–H⋯Cl and C–H⋯O
hydrogen bonds exist in the packing structure of 2.
3.3. Characterizations
Due to the presence of 2,2,6,6′-tetramethylpiperidin-1-oxyl groups in these ligands [12], the
two compounds were further characterized by ESI-MS, FTIR, UV–vis, CV (cyclic voltammetry),
and elemental analysis.
3.3.1. Mass spectral analysis
The ESI-MS spectrum of 2 exhibited one strong peak at 501, which was assigned to [(Tpy-
TEMPO)CuCl]+. A peak at 1200 from the ESI-MS spectrum of 1 could not be assigned as its
molecular ion peak, but rather [(Bpy-TEMPO)2CuCl]+ due to rearrangement of Bpy-TEMPO
and Cl- under the experimental conditions. The ESI-MS spectra of the complexes are shown
in figures S2 and S3.
3.3.2. IR spectra and electronic absorption spectra
Synthesis of 1 and 2 was also shown by FTIR spectra, which indicated the existence of char-
acteristic groups (the stretching frequencies of N → O at around 1362 cm−1, C=C and C=N at
around 1542 and 1465 cm−1 in py) of Bpy-TEMPO and Tpy-TEMPO (see figures S4–S7).
The electronic absorption spectral data of the complexes are presented in table 4. The
complexes exhibited intense absorptions at 305 and 278 nm due to spin-allowed, ligand
centered (1LC) π–π* transitions of the ligand framework of Bpy-TEMPO and Tpy-TEMPO,
respectively. The broad band at 667 nm of 1 and 769 nm of 2 were assigned to the ligand
field transition. The ligand field transition of 2 was bathochromically shifted with respect to
its extended structural framework of Tpy-TEMPO [20].

8
X. LU ET AL.
Figure 2. (a) orteP structure of 2 with selected atomic-labeling scheme showing the local coordination
environment of Cu(ii). thermal ellipsoids are drawn at 50% probability. hydrogens are omitted for clarity.
(b) intermolecular π–π stacking interactions of 2.
3.3.3. Electrochemistry studies
Voltammetric data for the ligands and complexes in MeCN–0.1 mol/L NBun PF6 at 293 K are
4
summarized in table 4. The cyclic voltammogram of Bpy-TEMPO exhibited a reversible oxi-
dation at E1/2 = + 0.90 V versus the AgCl/Ag couple, corresponding to one-electron oxidation
of the aminoxyl moiety to an oxoammonium center at E1/2 = + 0.70 V in Tpy-TEMPO. All

JOURNAL OF COORDINATION CHEMISTRY
Bond angles
9
Table 3. selected bond lengths (Å) and angles (°) for 2.
Bond distances
Cu1–n1
Cu1–n2
Cu1–n4
Cu1–Cl1
Cu1–Cl2
n3–o2
1.948(3)
2.033(3)
2.042(3)
2.5020(11)
2.2315(10)
1.285(4)
n1–Cu1–n2
n1–Cu1–n4
n2–Cu1–n4
n1–Cu1–Cl2
n2–Cu1–Cl2
n4–Cu1–Cl2
n1–Cu1–Cl1
n2–Cu1–Cl1
n4–Cu1–Cl1
Cl1–Cu1–Cl2
78.91(12)
78.82(12)
154.78(12)
161.41(9)
98.99(9)
98.33(9)
94.07(9)
93.11(9)
100.13(9)
104.50(4)
symmetry code: a −x, −y, 2−z.
Table 4. Photophysical and electrochemical data of the ligands and complexes.
Absorption λmax (nm)
(logε, dm3 mol−1 cm−1
E1/2(V)
Compound
)
Metal centered oxidation
Ligand centered reductions
Bpy-temPo
Complex 1
tpy-temPo
Complex 2
302 (4.49)
305 (4.41), 667 (2.11)
278 (4.41)
0.90, −0.89
0.62, −1.04
0.69, −1.05
0.89, −0.82
0.22
277 (4.61), 769 (2.17)
−0.19
ligands and complexes in the study exhibited an irreversible reduction from −0.8 to −1.10 V,
assigned to a one-electron reduction in the ligand π system (see figures S8–S11). The met-
al-centered electrochemical behavior of 1 occurred at 0.22 V due to oxidation of Cu(II)/Cu(I),
and this couple in 2 was observed at −0.19 V [21, 22].
3.4. Catalytic oxidation of alcohols
Both CuII complexes have been examined for catalytic aerobic oxidation of benzyl alcohol,
and the results are summarized in table 5. Our investigation started with an Eppendorf vial
of benzyl alcohol catalyzed by 1 and 2 under an air atmosphere at 60°C. We studied the
details for different catalytic activities.
For 1, the effects of solvents on catalytic activities were investigated (entries 1, 2 and 3 in
table 5 and figure S12). The results showed that the complex exhibited more activity in a
low-polarity solvent (o-xylene) than in high-polarity solvents (CH3CN, THF). In the catalytic
oxidation of benzyl alcohol, the oxygen from TEMPO acted as a proton acceptor [23], how-
ever, in high-polarity solvents, electronegativity atoms (N, O) from the solvents compete for
the proton transfer and decrease the efficiency of TEMPO [24]. As TEMPO oxidation of alco-
hols is often accelerated under basic conditions, three bases were screened. To obtain opti-
mum conditions for catalytic oxidation, the effects of different bases including K2CO3, DBU,
and TEA on the catalytic activity of the complex were investigated (entries 3, 4 and 7 in table
5 and figure S13). As previously reported [25], the activity of the catalyst exhibited its highest
value with DBU as the base. Temperature affects the catalytic efficiency of catalyst. The con-
version rate of benzyl alcohol was 0% at room temperature and increased with increasing
reaction temperature. However, at 80°C, the color of 1 changed from green to yellow, and
the conversion rate of benzyl alcohol decreased to 21%, indicating deactivation of the
catalyst.

10
X. LU ET AL.
Table 5. the reaction optimization for copper complexes catalysted aerobic oxidation of benzyl
alcohol[a].
OH
CHO
Catalyst / Base
Solvent, Air
Entry
Catalyst
Base
Solvent
Conv. (%)
1[a]
1
K2Co3
K2Co3
K2Co3
tea
Ch3Cn
thf
1
5
25
18
9
26
37
39
40
15
12
21
67
71
52
1
2[a]
1
3[a]
1
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
o-xylene
4[a]
1
5[b]
1
–
6[c]
1
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
dBu
–
7[a]
1
8[d]
1
9[e]
1
10[f]
11[g]
12[a, h]
13[a, i]
14[a, j]
15[a, i]
16[i, k]
17[i, l]
18[i, l]
19[i, m]
20[i, m]
21[b]
1
1
1
1
1
2
–
1
2
0
0
3
2
Bpy-temPo
tpy-temPo
2
6
note: dBu denotes 1,8-diazabicyclo[5.4.0]undec-7-ene.
the basic reaction condition: 1.0 mmol of the substrate, the catalyst and the base in 10 ml of solvent, in open air at 60°C for
12 h. the conversions based on GC analysis, selectivity n99% in all cases.
areaction run with 0.05 mmol (5.0 mol%) of copper complex and 1.0 mmol(100 mol%) of base.
breaction run with 0.05 mmol (5.0 mol%) of copper complex and without any base.
creaction run with 0.05 mmol (5.0 mol%) of copper complex and 0.50 mmol(50 mol%) of dBu.
dreaction run with 0.05 mmol (5.0 mol%) of copper complex and 2.0 mmol(200 mol%) of dBu.
ereaction run with 0.05 mmol (5.0 mol%) of copper complex and 3.0 mmol(300 mol%) of dBu.
freaction run with 0.02 mmol (2.0 mol%) of copper complex and 1.0 mmol(100 mol%) of dBu.
greaction run with 0.10 mmol (10 mol%) of copper complex and 1.0 mmol(100 mol%) of dBu.
hreaction run for 6 h.
ireaction run for 18 h.
jreaction run for 24 h.
kreaction run without catalyst and with 1.0 mmol (100 mol%) of dBu.
lreaction run in a sealed vessel under n2 atmosphere.
mreaction run with 0.05 mmol (5.0 mol%) of the ligand and 1.0 mmol (100 mol%) of dBu.
With o-xylene as the solvent, DBU as the base and the molar ratio of 1 to substrate 1:20,
the effects of molar ratio of base to substrate were investigated (entries 5, 6, 7, 8, and 9 in
table 5 and figure S14). Conversion rate increases with increasing mole ratio of substrate to
base, and with the mole ratio above 1:1, the conversion rate reaches 40%. The study of the
effect of molar ratio of catalyst:substrate on the conversion rate of substrate indicated that
the optimal molar ratio of catalyst:substrate is 5:100 (entries 5, 10. and 11 in table 5 and
figure S15). The effects of reaction time on the conversion rate of benzyl alcohol exhibited
that the conversion rate increased up to 18 h (entries 7, 12, 13. and 14 in table 5 and figure
S15). The optimum conditions for catalytic oxidation occur at 60°C, o-xylene as solvent, DBU
as base and substrate:BU:complex 1 of 100:100:5; the conversion rate of benzyl alcohol
reached 67% after 18 h. However, under the same conditions, with 2 as the catalyst, the

JOURNAL OF COORDINATION CHEMISTRY
11
conversion rate of benzyl alcohol was 53% (entry 15); the additional TEMPO groups in 1 are
responsible for improved catalytic efficiency.
Under the same conditions, without a catalyst, or with Bpy-TEMPO replacing 1 or with
Tpy-TEMPO replacing 2, the conversion rate of benzyl alcohol is 1, 2. and 3%, respectively
(entries 16, 19 and 20). It can be concluded that catalyst was essential for the oxidation of
benzyl alcohol, and that copper or other metal ions are the catalytic center, while TEMPO
participates in the proton transfer process [26–30]. Without oxidants, oxidation of benzyl
alcohol to benzaldehyde is not observed (entries 17 and 18). Other oxidants, such as hydro-
gen peroxide, t-butylhydroperoxide, nitrites, etc. [31–34], can oxidize benzyl alcohol, how-
ever, oxygen from air is a green choice.
For oxidation of benzyl alcohol to benzaldehyde in air, 1 was more effective than 2 for
reaction in o-xylene at 60°C with DBU as an extra base. A high yield (67%) of benzaldehyde
was observed and negligible over-oxidation to the corresponding carboxylic acid when
using 1 as a catalyst in these reaction conditions.
4. Conclusion
We have reported the synthesis and characterization of two TEMPO-functionalized
polypyridyl ligands and their copper(II) complexes 1 and 2. Single-crystal X-ray diffraction
demonstrates that 1 contains a dinuclear copper(II) unit, while 2 is a mononuclear copper(II)
structure. Cu(II) ions in both complexes are five-coordinate, exhibiting distorted square
pyramidal geometry. In their packing modes, various hydrogen bonds and weak π interac-
tions stabilize 1 and 2. The complexes have been used as catalysts for benzyl alcohol oxida-
tion in the presence of air and 1 is more effective than 2.
Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication. CCDC 1509883 and 1401553 contain the
supplementary Crystallographic data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Center,
12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223–336-033; or E-mail: deposit@ccdc.cam.ac.uk).
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the Natural Science Foundation of China [grant number 21272281], [grant
number 21271189].
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