Diverse copper(II) complexes with simple nitrogen ligands: Structural characterization and applications in aerobic alcohol oxidations in water

Article abstract of DOI:10.1016/j.poly.2015.01.026

Four copper(II) complexes based on simple mono- or bi-dentate nitrogen ligands 2-5 were synthesized and structurally characterized by X-ray crystallography. Structural analysis revealed that diverse solid state structures of these Cu(II) complexes were fo

Full text of DOI:10.1016/j.poly.2015.01.026

Polyhedron xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Diverse copper(II) complexes with simple nitrogen ligands: Structural
characterization and applications in aerobic alcohol oxidations in water
Guoqi Zhang a, , Yuan Zhuo Zhang , Wen-Feng Lo , Jianfeng Jiang , James A. Golen ,
⇑
a
a,b
b
c
c
Arnold L. Rheingold
a
Department of Sciences, John Jay College and The Graduate Center, The City University of New York, New York, NY 10019, USA
Department of Chemistry, Yeshiva University, New York, NY 10033, USA
Department of Chemistry, University of California San Diego, La Jolla, CA 92093, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Four copper(II) complexes based on simple mono- or bi-dentate nitrogen ligands 2–5 were synthesized
and structurally characterized by X-ray crystallography. Structural analysis revealed that diverse solid
state structures of these Cu(II) complexes were formed, depending on the ligands used. The new cop-
per(II) assembles include dinuclear tetra-ligand complex (6), mononuclear bis-ligand complex (7), one-
dimensional (1-D) polymeric chain (8) and two-dimensional (2-D) hydrogen-bonded network composed
of mononuclear bis-ligand complexes (9). In continuation with our previous findings of an efficient Cu/
DMAP catalysis system for aerobic alcohol oxidation in water (DMAP = 4-dimethylaminopyridine), we
tested the catalytic activity of newly synthesized copper(II) complexes 6–9 under similar conditions. Both
the isolated complexes and in-situ combinations of ligands with copper(I) or copper(II) salts were exam-
ined as catalysts toward aerobic oxidations of both primary and secondary alcohols in water in the pres-
ence of nitroxyl radicals (TEMPO or ABNO) and the results confirmed that complex 6 showed high
catalytic activity, similar to the previously reported Cu/DMAP catalyst, while other complexes were less
efficient catalysts. The remarkable differences on catalytic activity were tentatively attributed to their
diverse molecular structures, in comparison with previous results in relevant systems.
Received 25 December 2014
Accepted 25 January 2015
Available online xxxx
This is dedicated to my mentor Professor
Catherine E. Housecroft on the occasion of
her 65th birthday.
Keywords:
Nitrogen ligand
Copper(II)
Crystal structure
Aerobic oxidation
Catalysis
Ó 2015 Elsevier Ltd. All rights reserved.
1
. Introduction
Catalytic selective oxidation of alcohols to aldehydes or ketones
necessary [8d]. In contrast, there will be of great advantages if a
reaction could be performed in water other than organic solvents.
For example, the oxidations in water could be much cheaper and
safer than those performed in organic solvents, and products can
be isolated by simple procedures, enabling catalysts to be readily
recovered and reused [9,10]. However, the catalytic aerobic alcohol
oxidation in water was only scarcely explored. In 2000, the Sheldon
group reported the first water-soluble palladium(II) complex for
the catalytic oxidation of primary and secondary alcohols in water.
In this protocol, high temperature (100 °C) and air pressure (30-
bar) were required, as well as the use of Pd metal [11]. More
recently, a b-cyclodextrin derived bidentate ligand in combination
with copper(II) acetate was found to catalyse the aerobic oxida-
tions of primary aromatic and allylic alcohols in water at 100 °C,
in the presence of an extra base [12].
is an important organic transformation in modern synthetic chem-
istry and in industry [1,2]. Copper-catalysed, nitroxyl radical-med-
iated aerobic oxidation appears to be the most efficient method for
alcohol oxidation in recent years [3], since the pioneering work by
Semmelhack and coworkers, who observed the CuCl/TEMPO/DMF
system (DMF = N,N-dimethylformamide, TEMPO = 2,2,6,6-tetram-
ethylpiperidinyl-1-oxyl) for the effective oxidation of primary
alcohols to aldehydes [4]. Thereafter, there were numerous reports
on this type of reactions using Cu/TEMPO catalyst systems in
combination with organic nitrogen ligands, typically 2,2 -bipyri-
dine(bpy)-type compounds, mostly developed by the Sheldon [5],
Koskinen [6] and Stahl groups [7,8]. A common feature for these
known catalyst systems is that an additional base such as KO Bu,
DBU (1,8-diazabicycloundec-7-ene) or NMI (N-methylimidazole)
0
t
We have been interested in exploring various organic ligands
for copper-catalysed aerobic alcohol oxidations using nitroxyl rad-
icals [13,14], and recently observed a simple, inexpensive copper/
3
was required and organic solvents (CH CN or DMSO) were
4
-dimethylaminopyridine (DMAP) system that catalysed aerobic
⇑
Corresponding author.
E-mail address: guzhang@jjay.cuny.edu (G. Zhang).
oxidations of a range of primary, secondary and allylic alcohols
in water under mild conditions, without the addition of an
http://dx.doi.org/10.1016/j.poly.2015.01.026
277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
0
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2
G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
Fig. 1. (a) The ORTEP representation of Cu
2
(2)
4
Cl
4
(6) with ellipsoids plotted at the 50% probability level, crystallized solvent molecules are omitted for clarity. Selected bond
i
i
parameters: Cu1–N1 = 1.970(2), Cu1–N3 = 1.974(2), Cu1–Cl1 = 2.3558(8), Cu1–Cl2 = 2.3152(8), Cu1–Cl1A = 2.6680(9), Cu1A –Cl1 = 2.6681(8) Å; N1–Cu1–N3 = 174.73(11),
i
N1–Cu1–Cl2 = 89.46(7), N3–Cu1–Cl2 = 90.56(7), N1–Cu1–Cl1 = 91.77(7), N3–Cu1–Cl1 = 91.45(8), Cl2–Cu1–Cl1 = 142.12(3), N1–Cu1–Cl1A = 87.51(7), N3–Cu1–
i
i
o
Cl1A = 88.10(8), Cl2–Cu1–Cl1A = 123.79(3) . Symmetry code: i = ꢀx + 3/2, ꢀy + 1/2, ꢀz + 2. (b) The packing mode in 6 showing the intermolecular
p-stacking interactions.
extra base [14]. Interestingly, a catalytically active, tetranuclear
Cu -O)(DMAP) cluster complex was isolated and structurally
characterized, which was likely to act as a reactive intermediate
in the catalytic cycle [14]. Based on this result and our earlier
findings on active copper(II) cluster catalysts [13], we assumed
that the structures of copper complexes resulting from various
organic ligand would be highly related to their catalytic activity
I
II
4
(l
4
4
for aerobic alcohol oxidations. Although Cu and Cu -containing
complexes with pyridine-containing ligands were extensively
studied as catalysts for aerobic alcohol oxidations [4–8], reports
on the structural aspects of catalytically active copper complexes
were still limited [15,16,9c,17].
Therefore, in this work we aim to extend the nitrogen ligand
sets to other structurally related ligands 2–5 (Scheme 1) and report
on the X-ray structures of four diverse copper(II) complexes (6–9)
based on these ligands. The catalytic activity of these new
complexes has been studied for nitroxyl radical-mediated aerobic
alcohol oxidations in water, in order to better understand the struc-
ture–catalytic activity relationship in copper(II)-based catalyst
systems.
N
N
N
N
O
N
N
N
N
NH
2
NH
N
1
O
2
5
2
3
4
Scheme 1. The structures of simple nitrogen ligands 1–5 studied in this work.
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G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
3
2
. Experimental section
1377s, 1315m, 1234w, 1215m, 1168w, 1138w, 1110m, 1084s,
031w, 901w, 830s, 777s, 727m, 637m, 611m, 535w, 513m.
1
2.1. General
Found: C, 46.85; H, 4.12; N, 12.63; C18
requires C, 47.12; H, 4.39; N, 12.21%.
2 4 2
H20Cl CuN O (458.82)
Solvents, reagents and ligands 1–5 were purchased from Fisher
Scientific or Sigma–Aldrich in the US. All reactions were performed
under ambient conditions (no inert atmosphere). Solution elec-
tronic absorption spectra were recorded on a Shimadzu UV-1800
spectrophotometer, and FT-IR spectra using a Shimadzu 8400S
instrument with solid samples using a Golden Gate ATR accessory.
GC–MS analysis was carried out on a Shimadzu GCMS-QP2010S
gas chromatograph mass spectrometer. Elemental Analyses were
performed by Midwest Microlab LLC in Indianapolis.
2.6. General procedure for catalytic aerobic oxidation in water
Under typical conditions, the reactions were performed in flasks
at room temperature open to the air. 1.0 mmol of alcohol substrate,
complexes 6–9 (2 mol% based on Cu) and TEMPO or ABNO
(0.050 mmol, 5 mol%) were placed in the flask, to which 5 cm3 of
deionized water was sequentially added. The reaction was allowed
to stir rigorously upon exposure to the air at room temperature for
3
2.2. Cu
(2) Cl
2 4 4
(6)
indicated times, after which ethyl acetate (8 cm ) was added to
extract the organic components. The products were diluted with
dichloromethane and analysed by GC–MS to give the indicated
conversions.
To a stirred solution of 2 (0.066 g, 0.500 mmol) in CH
5 cm ) was added dropwise a methanolic solution (5 cm ) of
2
Cl
2
3
3
(
CuCl O (0.085 g, 0.500 mmol) at room temperature, the result-
2
ꢁ2H
2
ing green solution was stirred for 10 min, then filtered, and the fil-
trate was allowed to slowly evaporate at room temperature for
2.7. X-ray structure determinations
5
days, during which time green block-like crystals of 6 had formed
and were collected by decanting the solvent, washed with metha-
Suitable crystals of 6–9 were mounted on Cryoloops with Par-
nol and dried in air. Yield: 0.082 g (82.6% based on 2). FT-IR (solid,
cm ): 3114w, 1523s, 1487w, 1462s, 1425m, 1371m, 1343m,
atone-N oil. Data were collected at 100 K with a Bruker APEX I or
II CCD using Mo-Ka radiation and corrected for absorption with
ꢀ1
1
6
CH
298w, 1254s, 1188s, 1138w, 1074m, 1028m, 927m, 888w, 752s,
SADABS and structures solved by direct methods. All non-hydro-
ꢀ5
ꢀ3
22m, 536w, 508s. UV–Vis
k
max/nm (2.89 ꢂ 10 mol dm
,
gen atoms were refined anisotropically by full-matrix least squares
3
3
ꢀ1
ꢀ1
2
Cl
2
)
253
(
e
/10 dm mol cm
52.8), 279 (49.1),
on F . For compound 8 the nitrogen hydrogen atoms were found
2
2
C
95sh (12.6), 616 (1.17). Found: C, 46.65; H, 4.12; N, 13.42;
from a Fourier difference map and were refined isotropically with
N–H distance of 0.85(0.01) angstroms and 1.50 Ueq of parent atom.
All other Hydrogen atoms were placed in calculated positions with
appropriate riding parameters. For crystals of 9, the ligand mole-
cule exhibited flip-type disordered over itself (67.38/32.62) and
was refined with rings constrained as pseudo-naphthalenes and
with EADP constraints for carbon atoms. ORTEP and molecular
packing figures were drawn with the programme MERCURY v. 2.4
[18,19].
32
H32Cl
4
Cu OHꢁH O (847.61) requires C, 46.76; H, 4.52;
2
N
8
ꢁCH
3
2
N, 13.22%.
.3. Cu(3)
The procedure was similar to that for 6, except the replacement
2
2 2
Cl (7)
of ligand 2 with 3 (0.102 g, 0.500 mmol). Blue-green blocks of 7
suitable for X-ray diffraction analysis were obtained by slow evap-
oration of the reaction mixture after 5 days. Yield: 0.104 g (76.9%
6: C33H36Cl Cu N O, M = 829.58, green block, monoclinic, space
4
2
8
ꢀ1
based on 3). FT-IR (solid, cm ): 3365br, 1742m, 1697m, 1641s,
group C2/c, a = 20.7180(18), b = 7.9948(7), c = 20.844(2) Å,
3
ꢀ3
1
9
457s, 1443s, 1414s, 1273s, 1170s, 1096w, 1061m, 994s, 949m,
00s, 854w, 776s, 731s, 699s, 672w, 641s, 542s. UV–Vis kmax/nm
b = 92.275(3)°, U = 3449.9(6)Å , Z = 4, D = 1.597 Mg m
,
l(Mo-
c
ꢀ1
Ka
) = 1.584 mm
unique, Rint = 0.0565. Refinement of 2620 reflections (210 parame-
ters) with I > 2 (I) converged at final R1 = 0.0391 (R1 all
, T = 100(2) K. Total 5156 reflections, 3523
ꢀ5
ꢀ3
3
3
ꢀ1
ꢀ1
(
5
(
2.54 ꢂ 10 mol dm , CH
63 (0.98). Found: C, 48.35; H, 4.29; N, 10.61; C22
542.89) requires C, 48.67; H, 4.46; N, 10.32%.
2
Cl
2
) 279 (
e
/10 dm mol cm 42.5),
H24Cl
2 4
CuN O
4
r
data = 0.0637), wR = 0.0736 (wR all data = 0.0806), gof = 1.040.
2
2
2.4. [Cu(4)Cl
2 n
] (8)
To a test tube was added a solution of 4 (0.095 g, 1.00 mmol) in
3
3
CH
(
2
Cl
2
(15 cm ), then a methanolic solution (5 cm ) of CuCl
2
ꢁ2H
2
O
0.170 g, 1.00 mmol) was carefully layered on the top of the CH
2
Cl
2
layer, the tube was sealed and stood at room temperature for a
week. Green block-like crystals of 8 were observed on the wall of
the tube, and collected by decanting the solvent, washed with
methanol and dried in air. Yield: 0.198 g (86.6%). FT-IR (solid,
ꢀ1
cm ): 3375m, 3320m, 3247m, 3215m, 3101w, 1650s, 1578s,
1
6
505s, 1358s, 1203s, 1114w, 1051w, 983w, 864w, 808m, 808s,
70s, 519s. Found: C, 21.02; H, 2.13; N, 18.13; C Cl CuN
3
4
H
5
2
(
228.54) requires C, 20.93; H, 2.20; N, 18.31%.
ꢀ
2
.5. [Cu(5)
2
(Cl)(H
2
O)]ꢁ(Cl )ꢁH
2
O (9)
The procedure was similar to that for 8, except the replacement
Fig. 2. The ORTEP structure of Cu(3)
2
Cl
2
(meso-(S,R)-7) with ellipsoids plotted at the
50% probability level. Selected bond parameters: Cu1–N1 = 1.9695(15), Cu1–
Cl1 = 2.2802(5), C1–O1 = 1.342(2), C3–O1 = 1.456(2), C2–O2 = 1.213(2), N1–C1 =
of ligand 4 with 5 (0.144 g, 0.100 mmol). Blue blocks of 9 suitable
for X-ray diffraction analysis were obtained by the layering tech-
nique after one week. Yield: 0.169 g (73.7% based on 5). FT-IR
i
1
.335(2), N1–C2 = 1.380(2) Å; N1–Cu1–N1A = 180.00(7), N1–Cu1–Cl1 = 87.97(5),
i
i
i
i
o
N1A –Cu1–Cl = 92.03(5), N1A –Cu1–Cl1A = 87.97(5), Cl1–Cu1–Cl1A = 180.0 . Sym-
(
solid,: 3217m, 3157m, 3103m, 1589s, 1561m, 1506s, 1472s,
metry code: i = ꢀx + 1, ꢀy, ꢀz + 1.
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4
G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
Fig. 3. (a) The ORTEP structure of [Cu(4)Cl
2
]
n
(8) with ellipsoids plotted at the 50% probability level. Selected bond parameters: Cu1–N1 = 2.084(2), Cu1–Cl1 = 2.2586(8),
i
i
i
N1–C2 = 1.342(4), N1–C1 = 1.365(3), N2–C1 = 1.328(5) Å; N1–Cu1–N1A = 179.999(1), N1A –Cu1–Cl1 = 88.53(7), N1–Cu1–Cl1 = 91.47(7), N1–Cu1–Cl1A = 88.53(7), Cl1–
i
o
Cu1–Cl1A = 180.0 . Symmetry code: i = ꢀx + 1, ꢀy + 1, ꢀz + 1. (b) The 1-D coordination polymeric chain running along the crystallographic b axis observed in 8.
(
c) A hydrogen-bonded 2-D sheet along the crystallographic a axis found in the molecular packing mode.
7
: C22
H24Cl
2
CuN O
4 4
, M = 542.89, blue block, monoclinic, space
9: C18
2 4 2
H20Cl CuN O , M = 458.82, blue block, orthorhombic,
group P2(1)/c, a = 9.2508(11), b = 9.3298(14), c = 14.117(2) Å,
space group Pbca, a = 8.9648(17), b = 12.972(3), c = 32.548(6) Å,
3
ꢀ3
3
ꢀ3
ꢀ1
b = 104.507(6)°, U = 1179.5(3)Å , Z = 2, D
c
= 1.529 Mg m
) = 1.189 mm , T = 100(2) K. Total 12820 reflections, 6451
unique, int = 0.0321. Refinement of 2414 reflections (153
parameters) with I > 2 (I) converged at final R = 0.0271 (R all
data = 0.0321), wR = 0.0691 (wR all data = 0.0720), gof = 1.041.
: C Cl CuN , M = 228.54, green block, monoclinic, space
group P2(1)/m, a = 3.8773(8), b = 12.384(2), c = 7.0277(16) Å,
,
l
(Mo-
U = 3785.0(13)Å , Z = 8, D
T = 100(2) K. Total 21912 reflections, 4469 unique, Rint = 0.0825.
Refinement of 2167 reflections (209 parameters) with I > 2 (I)
converged at final R = 0.0954 (R all data = 0.1352), wR = 0.2512
(wR all data = 0.2713), gof = 1.091.
c
= 1.610 Mg m , l(Mo-Ka) = 1.458 mm ,
ꢀ1
Ka
R
r
r
1
1
1
1
2
2
2
2
8
4
H
4
2
3
3. Results and discussion
3
ꢀ3
b = 90.270(9)°, U = 337.45(12)Å , Z = 2, D
c
= 2.249 Mg m
T = 100(2) K. Total 1769 reflections, 1311
unique, Rint = 0.0240. Refinement of 714 reflections (55 parame-
ters) with I > 2 (I) converged at final = 0.0254 (R all
data = 0.0265), wR = 0.0662 (wR all data = 0.0668), gof = 1.059.
, l(Mo-
ꢀ1
II
Ka) = 3.937 mm
,
3.1. Structures of Cu complexes 6–9
r
R
1
1
The stoichiometric reaction of monodentate ligand 2 with cop-
2
2
per(II) chloride in a CH Cl –MeOH solution gave a green solution,
2
2
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G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
5
Fig. 4. (a) The ORTEP structure of [Cu(5)(Cl)(H
N2 = 1.991(10), Cu1–N1 = 2.014(10), Cu1–N4 = 2.033(10), Cu1–Cl1 = 2.841(2), Cu1–O2 = 2.566(7) Å; N2–Cu1–N3 = 97.2(4), N1–Cu1–N3 = 175.6(4), N1–Cu1–N2 = 83.8(4),
N3–Cu1–N4 = 84.1(4), N2–Cu1–N4 = 178.6(4), N1–Cu1–N4 = 94.9(4) . (b) A 2-D hydrogen bonding network mediated by crystalline H O and Cl anions.
2
2
O)Cl]ꢁH
2
O (9) with ellipsoids plotted at the 40% probability level. Selected bond parameters: Cu1–N3 = 1.985(9), Cu1–
o
ꢀ
from which green block-like crystals of Cu
2
(2)
4
Cl
4
(6) formed by
largest angles in the coordination sphere, and
perfect square pyramid and = 1 means a perfect trigonal bipyra-
mid. Therefore, value for copper(II) centre in 6 is calculated to be
0.54, indicating a geometry between the square pyramid and trigo-
nal bipyramid. The Cu–N distances are 1.970(2) and 1.974(2) Å,
respectively, while Cu–Cl bond lengths vary from 2.3152(8) to
2.6681(8) Å, with Cu1–Cl1A being the longest bond, indicating
the bridging Cl atoms are labile. The N–Cu–N arrangement is
almost linear with an angle of 174.73(11)°. Molecules of 6 pack
along the crystallographic a-axis through significant intermolecu-
s = 0 represents a
slow evaporation of the reaction solution within 5 days. The bulk
sample of 6 was characterized by UV–Vis and IR spectroscopies,
elemental analysis, and structurally determined by X-ray diffrac-
tion analysis. 6 crystallised in a monoclinic space group C2/c, as a
dinuclear tetra-ligand complex, which is consistent with the
molecular component revealed by elemental analysis. An ORTEP
structure is presented in Fig. 1a and relevant bond parameters
given in the caption are unexceptional. The dinuclear structure is
centrosymmetric and only half a complex is contained in the asym-
metric unit, and the second half is generated by symmetry opera-
s
s
lar
p-stacking interactions between the aromatic domains (see
II
tion (symmetry code: ꢀx + 3/2, ꢀy + 1/2, ꢀz + 2). Each Cu centre is
Fig. 1b). There are large void spaces observed in the 3-D packing
mode, which are occupied by disordered solvent molecules that
could not be appropriately modelled, and so the data were treated
using the programme SQUEEZE [21].
five-coordinate with two imine nitrogen atoms of the ligands and
three chloride ions, of which Cl1 and the centrosymmetric Cl1A
II
ions serve to bridge the two Cu centres. To determine the coordi-
II
nation geometry around the Cu centres, the distortion parameter
Similarly, reacting the monodentate ligand 3 with copper(II)
s
value was applied. According to Addison’s approach [20],
s
was
2 2
chloride in a CH Cl –MeOH solution at room temperature resulted
defined by the equation, = (h1–h2)/60, where h1 and h2 are the
s
in the isolation of blue-green blocks of 7 in good yield. The crystals
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6
G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
were well suitable for X-ray structural analysis. Unlike the dinucle-
ar structure found in 6, 7 was found to exist as mononuclear com-
plex in the crystals. An ORTEP representation in Fig. 2 revealed a
centrosymmetrical bis-ligand structure. 7 crystallises in the mono-
observed that the Cu. . .Cl and Cu. . .OH2O distances are significantly
longer than those found in regular coordination bonds, indicating
only weak binding interactions. Further inspection of the crystal
packing structure revealed interesting hydrogen bonding interac-
tions between the two chloride anions, two water molecules and
two coordinated amine groups of the ligands. The multiple hydro-
gen bonds contributed to the formation of a 2-D network as shown
in Fig. 4b.
1
clinic space group P2 /c and the asymmetric unit contains only half
a complex, and the second half is generated by symmetry opera-
tion (symmetry code: i = ꢀx + 1, ꢀy, ꢀz + 1). Interestingly, although
racemic 3 with both R and S configurations were used in the reac-
tion as starting materials, the complex was formed selectively, con-
taining both R-3 and S-3 ligands in a single complex molecule, and
thus (S,R)-7 can be used to represent the stereochemistry of the
compound. However, the molecule is overall achiral because of
the centrosymmetry of the complex, which resulted in the forma-
3.2. Catalytic studies
Having revealed that diverse solid state structures could be
II
derived from the coordination of various ligands with copper(II)
chloride, we were interested to examine the catalytic activity of
the isolated complexes 6–9 that were structurally characterized
towards the aerobic oxidations of both primary and secondary
alcohols in water, in order to give insights into the structure–
catalytic activity relationship. We first examined the oxidation of
benzylic alcohol by using the combination of complexes 6–9
tion of a meso-coordination compound (meso-(S,R)-7). The Cu cen-
tre is tetra-coordinate with two ligands and two chloride anions,
being in a perfect square-planar environment. Relevant bond
parameters are listed in the caption of Fig. 2. The large difference
in structures of 6 and 7 is probably attributed to the presence of
both the carbonyl group and bulky dimethylamino substituents
around the nitrogen centre in ligand 3, which prohibits the forma-
tion of higher-nuclearity structures.
The similar reaction conditions were employed to the reaction
2 2
of ligand 4 with copper(II) chloride in a CH Cl –MeOH solution,
however, green solid that is insoluble in common organic solvents
immediately precipitated. Instead, single crystals suitable for X-ray
crystallography were obtained by a layering technique. Thus, care-
(
2 mol% based on Cu) and TEMPO without the presence of an addi-
tional base in water and at room temperature, upon exposure to
the air. The results are summarized in Table 1. Different catalytic
efficiencies were observed for four copper(II) complexes and com-
plex 6 appeared to be the most active catalyst, giving the product
benzaldehyde in good conversion as detected by GC–MS analysis
(
(
entries 1–4, Table 1). Interestingly, although the conversion
71%) was lower than that from Cu/DMAP (5/10 mol% loading) cat-
fully layering a methanolic solution of CuCl
solution of 4 in dichloromethane resulted in the formation of green
crystals of 8 in high yield over a period of one week. Elemental
2
ꢁ2H
2
O onto the top of a
alysed reaction [14], it is remarkable that only 1 mol% of isolated
2
analysis revealed an empirical formula of Cu(4)Cl , whose struc-
ture was determined by X-ray diffraction analysis. 8 crystallises
in the monoclinic space group P2(1)/m and an infinite polymeric
chain is observed in the crystal (Fig. 3). Each Cu centre binds to
Table 1
Copper(II) complexes for catalysed aerobic oxidation of primary and secondary
alcohols in water.
II
a
2
two C -symmetry ligands with the Nsp2 atoms and two chloride
II
anions to form a centrosymmetric structure, with Cu being in a
square-planar environment. The divergent di-imine ligands
expand the repeating structural units to form a 1-D coordination
polymer along the crystallographic b axis (Fig. 3b). The coordina-
tion environment of Cu in 8 is quite close to that in 7, except that
a slightly longer Cu–N bond is observed in 8 (2.084(2) vs.
II
Entry
Catalyst
Substrate
Nitroxyl radical
Conv.b (%)
1
2
3
4
6
7
8
9
6
6
6
7
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
Benzylic alcohol
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
ABNO
ABNO
ABNO
ABNO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
None
71
15
2
1
2
.9695(15) Å). The amine NH group remained non-coordinated,
however, which contributed to significant interchain interactions
18
90
>99
>99
54
18
25
>99
>99
6
i
resulting from effective N–H. . .Cl hydrogen bonds (N2. . .C1A =
5
6
7
8
9
1
11
12
1
1
1
16
17
1
19
2
2
c
i
d
3
.176(2) Å, Cl1A . . .H2B–N2 = 159(7)°, i = x ꢀ 1, 1.5 ꢀ y, z). The
hydrogen-bonding interactions, along with strong
p. . .p stacking
between the pyrimidine rings of adjacent polymeric chains expand
the 1-D chains into a 2-D network (Fig. 3c).
8
9
0
e
e
e
e
e
f
The same layering protocol was then used to gain single crystals
from the reaction between ligand 5 and copper(II) chloride, as an
insoluble precipitate was also found upon mixing the reactants.
Thus, blue crystals of 9 was obtained and structurally determined
by X-ray crystallography. An ORTEP structure of 9 is shown in
Fig. 4a. 9 crystallises in the orthorhombic space group Pbca and
one independent molecule was found in the asymmetric cell. The
1/CuCl2
2/CuCl
3/CuCl
4/CuCl
5/CuCl
1/CuCl
2/CuCl
3/CuCl
4/CuCl
5/CuCl
None
2
2
2
2
3
4
5
0
5
>99
93
39
7
21
2
f
f
8
f
II
Cu centre coordinates to two ligands with both the imine- and
f
0
1
amine-N atoms to form a mononuclear bis-ligand complex, with
the ligand being bidentately coordinated. In addition, the axial
positions are occupied with one chloride anion and a water mole-
cule with relatively weak Cu–Cl and Cu–O bonds (Fig. 4). Another
chloride counteranion and an extra water molecule were found
to co-crystallise with the complex in the asymmetric unit. Refine-
ment of structure indicated that the molecule was disordered over
itself (67.38/32.62). Important bond parameters that are unexcep-
tional are summarized in the caption of Fig. 4. Although the coor-
22
6
0
a
Condition: 1.0 mmol of the substrate, copper complexes 6–9 (2 mol% based on
Cu) and 0.050 mmol (5 mol%) of TEMPO or ABNO in 5 mL H
temperature, 10 h.
2
O, 1 atm air, room
b
Conversions based on GC–MS analysis.
c
5
5
0
mol% of 2 was added.
mol% of 6 was used.
.050 mmol (5 mol%) of copper(II) dichloride, 0.100 mmol (10 mol%) of ligands
O.
0.050 mmol (5 mol%) of copper(I) chloride, 0.100 mmol (10 mol%) of ligands
and 0.050 mmol (5 mol%) of TEMPO in 5 mL H O.
d
e
and 0.050 mmol (5 mol%) of TEMPO in 5 mL H
2
II
f
dination environment of Cu in 9 is remarkably distinct from those
found in 6-8, their Cu–N bond distances are comparable. It was also
2
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G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
7
catalyst 6 was used without the addition of an additional base. The
conversion was further improved to 90% when extra ligand 2
terized, and their structural diversity was observed. The results
from catalytic aerobic alcohol oxidations revealed that the molec-
ular structures of copper complexes were well related to their cat-
alytic performance, and that both the multinuclear nature and the
presence of an effective internal base in Cu(II) complexes might be
responsible for the higher catalytic activity. Not only does this
work further confirm the utilization of simple, monodentate nitro-
gen ligands for promoting efficient aerobic oxidations of alcohols
in water, but also it opens new opportunities for understanding
the structure–catalytic activity relationship for this important cat-
alytic transformation.
(
5 mol%) was added to the 6 (1 mol%)-catalysed reaction (entry 5,
Table 1). Furthermore, complete conversion was found while
increasing the loading amount of catalyst 6 from 1 mol% to
5
mol% (entry 6, Table 1). Since this catalyst combination was
poorly reactive for secondary alcohols, we also tested the oxidation
of 1-phenylethanol with the isolated catalysts 6–9 in water by
using ABNO (9-azabicylco[3.3.3]nonane N-oxyl) as the radical, in
replacement of TEMPO radical, as it was known that the less steri-
cally hindered ABNO could promote the oxidation of secondary
alcohols [8e,14]. It turned out that 6 was still the most active cat-
alyst, quantitatively converting 1-phenylethanol to acetophenone,
while other catalysts were only moderately active for this reaction
Acknowledgements
(
entries 7–10, Table 1).
In order to compare with our previous results regarding in-situ
We gratefully thank the American Chemical Society Petroleum
Research Fund for a New Investigator Award (#54247-UNI3), a
PSC-CUNY award (Nr. 67312-0045) from the Research Foundation
of the City University of New York and a CUNY Collaborative
Research Grant (CIRG #80209-06) for support.
Cu/DMAP catalysed aerobic oxidation of alcohols [14], we also
tested the reactions catalysed by in-situ formed copper complexes
between copper(II) dichloride and ligands 1–5. It was observed
2 2
that only the combinations of CuCl /1 (DMAP) and CuCl /2 showed
high catalytic efficiency (>99% conversions) under the conditions
identical to those reported [14], while ligands 3–5 were essentially
inactive for the oxidation of benzylic alcohol (entries 11–15,
Table 1). The trend is consistent with reactivity difference of iso-
lated complexes 6–9 described above. CuCl was also tested as the
starting copper source for the in-situ catalytic oxidation of benzylic
Appendix A. Supplementary data
CCDC 1028210–1028213 contain the supplementary crystallo-
graphic data for compounds 6–9, respectively. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
I
alcohol, as it was reported that Cu often behaves as a more active
II
catalyst precursor than Cu [3,8]. Our findings from the reactivity
tests of CuCl/ligands (1–5) catalyst combinations revealed that
CuCl/2 was slightly less active than CuCl /2, although CuCl per-
2
0
33; or e-mail: deposit@ccdc.cam.ac.uk.
formed a little better in combination with ligands 3–5 for the same
reaction (entries 16–20, Table 1). At this end, controlled experi-
ments with no addition of either copper catalyst or the nitroxyl
radical revealed almost no reactivity toward the oxidation of ben-
zylic alcohol (entries 21 and 22, Table 1), indicating the importance
of both the copper catalysts and nitroxyl radicals.
References
[1] M. Hudlick, Oxidations in Organic Chemistry, American Chemical Society,
Washington, DC, 1990.
[
2] I.W.C.A. Arends, R.A. Sheldon, Modern Oxidation Methods, in: J.-E. Bäckvall
Ed.), Wiley, Weinheim, 2004, p. 83.
(
The observation of high catalytic activity for isolated dinuclear
copper(II) assemble 6 without using extra bases is interesting, as
it was previously documented that an extra strong base is required
for the copper-catalysed aerobic oxidation in almost all cases
[3] (a) For recent publications on copper-catalysed aerobic alcohol oxidations:
B.L. Ryland, S.S. Stahl, Angew. Chem., Int. Ed. 53 (2014) 8824. and references
therein;
(
b) Q. Cao, L.M. Dornan, L. Rogan, N.L. Hughes, M.J. Muldoon, Chem. Commun.
50 (2014) 4524;
[
6–8]. Our previous work on the Cu/DMAP catalysed reaction indi-
(c) Y. Seki, K. Oisaki, M. Kanai, Tetrahedron Lett. 55 (2014) 3738;
(
d) X. Zhang, W. Dong, Y. Luan, M. Yang, L. Tan, Y. Guo, H. Gao, Y. Tang, R. Dang,
cated that a tetranuclear Cu -O)(DMAP) species could be active
4
(
l
4
4
J. Li, G. Wang, J. Mater. Chem.
C4TA06046F;
A (2015), http://dx.doi.org/10.1039/
intermediate in catalytic alcohol oxidations [14]. In that work, we
also attributed its high reactivity to the unique structure contain-
ing both four copper centres and internal tertiary amine as a base
(e) R.A. Molla, K. Ghosh, K. Tuhina, S. ManirulIslam, New J. Chem. (2015),
http://dx.doi.org/10.1039/C4NJ01457J;
(
3
f) Z. Alaji, E. Safaei, L. Chiang, R.M. Clarke, C. Mu, T. Storr, Eur. J. Inorg. Chem.
5 (2014) 6066;
[
11]. In addition, we also reported that tetranuclear copper cluster
performed better than its dinuclear analogue in a related catalysis
(g) L. Rogan, N.L. Hughes, Q. Cao, L.M. Dornan, M.J. Muldoon, Catal. Sci.
Technol. 4 (2014) 1720;
study [13a]. Recently, Waymouth and coworkers suggested that a
(
(
h) Y. Liu, A. Xie, J. Li, X. Xu, W. Dong, B. Wan, Tetrahedron 70 (2014) 9791;
i) Y. Qi, Y. Luan, J. Yu, X. Peng, G. Wang, Chem. Eur. J. 4 (2015) 1589.
II
trinuclear Pd
3
O
2
cluster complex be the real intermediate for Pd -
catalysed aerobic oxidation of alcohols [22]. The observed high
reactivity for dinuclear 6 that contains internal bases in this work
is, therefore, not surprising. It is also worth noting that although
complex 7 contains an internal base (the dimethylamino group)
similar to 6, its low catalytic activity towards the TEMPO-mediated
oxidation of benzylic alcohol could be owing to the presence of a
[4] M.F. Semmelhack, C.R. Schmid, D.A. Cortes, C.S. Chou, J. Am. Chem. Soc. 106
1984) 3374.
5] P. Gamez, I.W.C.E. Arends, R.A. Sheldon, J. Reedijk, Adv. Synth. Catal. 346 (2004)
05.
[6] E.T.T. Kumpulainen, A.M.P. Koskinen, Chem. Eur. J. 15 (2009) 10901.
(
[
8
[
[
[
7] (a) Pd-catalysed aerobic alcohol oxidations, see: B.A. Steinhoff, I.A. Guzei, S.S.
Stahl, J. Am. Chem. Soc. 126 (2004) 11268;
(
b) B.A. Steinhoff, S.S. Stahl, J. Am. Chem. Soc. 128 (2006) 4348;
(c) X. Ye, M.D. Johnson, T. Diao, M.H. Yates, S.S. Stahl, Green Chem. 12 (2010)
180.
8] (a) J.M. Hoover, B.L. Ryland, S.S. Stahl, J. Am. Chem. Soc. 135 (2013) 2357;
b) J.M. Hoover, S.S. Stahl, J. Am. Chem. Soc. 133 (2011) 16901;
(c) J.M. Hoover, J.E. Steves, S.S. Stahl, Nat. Protoc. 7 (2012) 1161;
II
sterically hindered Cu centre, which decreases its reactivity
1
towards substrates. In contrast, the poorer catalytic activity of
complexes 8 and 9 could be attributed to both their robust network
structures and the lack of free internal bases.
(
(
(
d) J.M. Hoover, B.L. Ryland, S.S. Stahl, ACS Catal. 3 (2013) 2599;
e) J.E. Steves, S.S. Stahl, J. Am. Chem. Soc. 135 (2013) 15742.
4
. Conclusions
9] (a) I.E. Marko, P.R. Giles, M. Tsukazaki, S.M. Brown, C.J. Urch, Science 274
1996) 2044;
(b) S. Striegler, Tetrahedron 62 (2006) 9109;
c) I. Gamba, I. Mukikainen, E. Bouwman, J. Reedijk, S. Bonnet, Eur. J. Inorg.
Chem. (2013) 115.
(
To continuingly investigate on green, mild copper-catalysed
(
aerobic oxidations of alcohols, we extended our studies to several
new ligand candidates and explored their structures and catalytic
activity. New copper(II) complexes 6–9 were isolated and charac-
[
10] T. Hara, M. Ishikawa, J. Sawada, N. Ichikuni, S. Shimazu, Green Chem. 11 (2009)
2034.
Please cite this article in press as: G. Zhang et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.01.026

8
G. Zhang et al. / Polyhedron xxx (2015) xxx–xxx
[
[
11] G.J. ten Brin, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636.
12] G. Zhang, X. Han, Y. Luan, Y. Wang, X. Wen, L. Xu, C. Ding, J. Gao, RSC Adv. 3
2013) 19255.
[17] R. Nasani, M. Saha, S.M. Mobin, L.M.D.R.S. Martins, A.J.L. Pombeiro, A.M.
Kirillov, S. Mukhopadhyay, Dalton Trans. 43 (2014) 9944. and references
therein.
(
[
13] (a) G. Zhang, G. Proni, S. Zhao, E.C. Constable, C.E. Housecroft, M. Neuburger,
J.A. Zampese, Dalton Trans. 43 (2014) 14313;
[18] I.J. Bruno, J.C. Cole, P.R. Edgington, M.K. Kessler, C.F. Macrae, P. McCabe, J.
Pearson, R. Taylor, Acta Crystallogr., Sect. B 58 (2002) 389.
[19] C.F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. McCabe, E. Pidcock, L.
Rodriguez-Monge, R. Taylor, J. van de Streek, P.A. Wood, J. Appl. Crystallogr. 41
(2008) 466.
[20] A.W. Addison, T.N. Rao, J. Reedijk, J. Van Rijn, G.C. Verschoor, J. Chem. Soc.,
Dalton Trans. (1984) 1349.
[21] A.L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr. 65 (2009) 148.
[22] A.J. Ingram, D. Solis-Ibarra, R.N. Zare, R.M. Waymouth, Angew. Chem., Int. Ed.
53 (2014) 5648.
(
b) G. Zhang, C. Yang, L. Li, E. Liu, J.A. Golen, A.L. Rheingold, Inorg. Chem.
Commun. 56 (2014) 243.
[
[
[
14] G. Zhang, C. Yang, L. Li, E. Liu, J.A. Golen, A.L. Rheingold, RSC Adv. 4 (2014)
6
1907.
15] Z. Ma, L. Wei, E.C.B.A. Alegria, L.M.D.R.S. Martins, M.F.C.G. da Silva, A.J.L.
Pombeiro, Dalton Trans. 43 (2014) 4048.
16] M.N. Kopylovich, K.T. Mahmudov, M. Haukka, P.J. Figiel, A. Mizar, J.A.L. da Silva,
A.J.L. Pombeiro, Eur. J. Inorg. Chem. (2011) 4175.
Please cite this article in press as: G. Zhang et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.01.026