Dinuclear CuII-CuII and CuI-Cu II complexes of a compartmental ligand - Syntheses, structures, magnetic, and catalytic studies

Article abstract of DOI:10.1002/ejic.201300576

Dinuclear Cu^II-Cu^II (1) and Cu^I-Cu ^II (2) complexes were derived from a new N₄O₂ donor compartmental ligand (H₂L) by changing the nature of the Cu precursors used. Single-crystal X-ray diffraction studies revealed that the Cu1 site in 1 has octahedral geometry and is in the inner compartment of the ligand axially coordinated by two ClO₄^- anions, whereas the outer Cu2 ion has square-pyramidal geometry. In 2, there are two copper dinuclear complexes (A and B) in the asymmetric unit; the inner core is occupied by Cu1/Cu3 in the +2 oxidation state in a square-planar geometry. The Cu2/Cu4 ion occupies the outer sites and has distorted tetrahedral geometry with a +1 oxidation state. Complexes 1 and 2 were obtained simply by changing Cu(ClO ₄)₂ in 1 to Cu(ClO₄)₂ and Cu(bipy)(NO₃)₂ in 2; the bipyridyl (bipy) ligand induces the reduction of Cu^II to Cu^I, which is trapped in the Cu^I-Cu^II dinuclear product. The oxidation states of the metal ions were ascertained from charge-balance considerations as well as from bond valence sum (BVS) calculations. No signature of intervalence charge transfer (IVCT) was observed by spectroscopy (UV/Vis/NIR) as well as from the low-temperature magnetic studies. This might be because of the presence of two copper centers in two different geometries with a wide separation between them (ca. 3.27 A). For 1, the best fit of the χT vs. T data to the dimer model gives J/k_B = -262(1) K and g_av = 2.05(5), which indicates that there is a strong antiferromagnetic coupling between the two Cu atoms. In 2, the Cu^I center is diamagnetic and, thus, the remaining S = 1/2 Cu^II magnetic center follows a Curie law with g = 2.06(5). Under homogeneous conditions, both complexes showed catalytic epoxidation of cyclooctene, styrene, and norbornene to the corresponding epoxides with high selectivities and turnover numbers (TONs), which seem to be slightly better than the reported values for Cu^II Schiff base complexes. Dinuclear Cu^II-Cu^II (1) and Cu^I-Cu^II (2) complexes derived from a new N₄O₂ donor ligand (H ₂L) are synthesized and characterized by single-crystal X-ray diffraction, UV/Vis spectroscopy, and magnetic measurements. In 1, there is strong antiferromagnetic coupling between the Cu atoms. In 2, the Cu^I ion is diamagnetic and the remaining S = 1/2 Cu^II magnetic center follows a Curie law with g = 2.06(5). Copyright

Full text of DOI:10.1002/ejic.201300576

Job/Unit: I30576
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DOI:10.1002/ejic.201300576
II
II
I
II
Dinuclear Cu –Cu and Cu –Cu Complexes of a
Compartmental Ligand – Syntheses, Structures, Magnetic,
and Catalytic Studies
[
a]
[a]
[a]
Surajit Biswas, Arpan Dutta, Malay Dolai,
[b,c]
[b,c]
[a]
Hon Man Lee,[d]
Indrani Bhowmick,
Mathieu Rouzières,
Rodolphe Clérac,[ and Mahammad Ali*
b,c]
Keywords: Schiff bases / Copper / Structure elucidation / Magnetic properties / Epoxidation
II
II
I
II
Dinuclear Cu –Cu (1) and Cu –Cu (2) complexes were de-
rived from a new N donor compartmental ligand (H L) by
from bond valence sum (BVS) calculations. No signature of
intervalence charge transfer (IVCT) was observed by spec-
troscopy (UV/Vis/NIR) as well as from the low-temperature
magnetic studies. This might be because of the presence of
two copper centers in two different geometries with a wide
separation between them (ca. 3.27 Å). For 1, the best fit of
4
O
2
2
changing the nature of the Cu precursors used. Single-crystal
X-ray diffraction studies revealed that the Cu1 site in 1 has
octahedral geometry and is in the inner compartment of the
ligand axially coordinated by two ClO
–
4
anions, whereas the
outer Cu2 ion has square-pyramidal geometry. In 2, there are
B
the χT vs. T data to the dimer model gives J/k = –262(1) K
two copper dinuclear complexes (A and B) in the asymmetric
and gav = 2.05(5), which indicates that there is a strong anti-
unit; the inner core is occupied by Cu1/Cu3 in the +2 oxi- ferromagnetic coupling between the two Cu atoms. In 2, the
I
dation state in a square-planar geometry. The Cu2/Cu4 ion
occupies the outer sites and has distorted tetrahedral geome-
try with a +1 oxidation state. Complexes 1 and 2 were ob-
Cu center is diamagnetic and, thus, the remaining S = 1/2
II
Cu magnetic center follows a Curie law with g = 2.06(5).
Under homogeneous conditions, both complexes showed
catalytic epoxidation of cyclooctene, styrene, and norbor-
nene to the corresponding epoxides with high selectivities
and turnover numbers (TONs), which seem to be slightly bet-
tained simply by changing Cu(ClO
4 2 4 2
) in 1 to Cu(ClO ) and
Cu(bipy)(NO in 2; the bipyridyl (bipy) ligand induces the
3 2
)
II
I
I
II
reduction of Cu to Cu , which is trapped in the Cu –Cu di-
nuclear product. The oxidation states of the metal ions were
ascertained from charge-balance considerations as well as
II
ter than the reported values for Cu Schiff base complexes.
Introduction
are of special interest because of the possible electron trans-
[
7]
fer between these states; this is the basic activity mecha-
nism of many important chemical and biochemical catalytic
The synthesis and investigation of dinuclear complexes
have attracted much attention because of their widespread
application in the design of molecular materials (e.g., mole-
cule-based magnets)[ and as models for the active sites of
[
8]
reactions. The most commonly studied mixed-valence
I
II
transition-metal system incorporates Cu /Cu pairs because
copper can adopt different coordination numbers and
stereochemistry in its different oxidation states, which are
1]
[2]
many metalloproteins. Many metalloenzymes such as
hemocyanin (2Cu),[
3]
hemerythrin (2Fe),
and methane monooxygenase (2Fe)
[4]
tyrosinase
[
9,10]
both highly labile and stereochemically flexible
unlike
[
5]
[6]
(2Cu),
possess
other couples such as iron, cobalt, and ruthenium. Al-
though a pronounced geometrical change is usually ob-
active sites with homodimetallic cores and involve either
activation or transport of dioxygen in biological systems.
Compounds containing metal atoms in two valence states
I
II
served during the Cu /Cu interconversion, allosteric effects
[
11]
can also be observed.
Moreover, the control of the geo-
metric environment around the copper centers allows the
[
a] Department of Chemistry, Jadavpur University,
I
II
Kolkata 700 032, India
redox potential of the Cu /Cu couple to be tuned; this is
the basis of the redox properties of type 1 copper pro-
E-mail: m_ali2062@yahoo.com
Homepage: http://www.jaduniv.edu.in/profile.php?uid=30
b] CNRS, CRPP, UPR 8641,
[
12]
[13,14]
teins and many synthetic analogues.
Nevertheless, it
[
[
[
3
3600 Pessac, France
c] Univ. Bordeaux, CRPP, UPR 8641,
3600 Pessac, France
is very difficult to control the final product with two dif-
ferent oxidation states, which is reflected by the very limited
number of reports on mixed-valence copper coordination
3
d] National Changhua University of Education, Department of
Chemistry,
[
15–17]
compounds.
Changhua 50058, Taiwan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300576.
On the other hand, compartmental ligands of the “end-
off” type with a phenolic or alcoholic oxygen atom as an
Eur. J. Inorg. Chem. 0000, 0–0
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Scheme 1.
endogenous bridge are popular and often used to model ions are coordinated by a N O donor set of atoms from
4
2
dimetallic biosites. Indeed, they provide μ-phenoxido-μ- an L2– ligand along with one water molecule (O10) and two
carboxylato and μ-phenoxido-di(μ-carboxylato) dinuclear perchlorate anions. The Cu1 ion is coordinated by two N
cores that mimic the active sites of some dimetallic enzymes. atoms (N1 and N2) and two bridging phenoxido O atoms
Symmetric or asymmetric, cyclic or acyclic Schiff base li- (O8 and O9) to form the equatorial plane; O1 and O7 from
gands or their reduced analogues have generally been em- two perchlorate anions are coordinated in axial positions,
ployed in these studies. The symmetric ligands are quite and this leads roughly to an overall octahedral geometry
often prepared by one-step condensation of appropriately around the Cu1 metal ion. The Cu1–O7 bond [2.852(2) Å]
designed formyl or diformyl precursors. Moreover, epoxid- is significantly longer than the Cu1–O1 bond [2.504(2) Å]
ation of olefins catalyzed by metal complexes has become and, therefore, O7 can be considered as only semibonded
[
35]
an important research area in both organic synthesis and to Cu1.
bioinorganic modeling of oxygen-transfer metalloen- tween the Cu1 and Cu2 ions. The two nitrogen atoms (N3
zymes.
The two phenoxido O atoms are μ-bridges be-
[
18–24]
For a variety of olefin substrates and oxygen- and N5) from the two appended pyrazole moieties and the
atom sources, good overall epoxidation is obtained with two phenoxido oxygen atoms form the equatorial plane of
chiral Schiff base complexes of manganese(III) as cata- the Cu2 site. Only one axial position is occupied by a water
[
18,19,25]
lysts,
which lead to significant enantiomeric excesses, molecule (O10) to give a square-pyramidal geometry
more pronounced than those with the analogous manga- around the Cu2 ion. Hence, the geometries around the two
[
25]
[26,27]
nese(II) chromium(III),
plexes.
and ruthenium(II/III) com- Cu sites in the same asymmetric unit are different (Fig-
[
19,28]
In general, the oxygen-transfer processes medi- ure 1). The Cu1–Nx bond lengths (x = 1 and 2) are
ated by the above-quoted complexes, and others such as ca. 1.91 Å, whereas the Cu2–Nx (x = 3 and 4) bond lengths
iron(III), manganese(III), and chromium(III) por- are ca. 1.97 Å (Table 1). The Cu1···Cu2 separation within
[
20]
phyrins,
occur through the formation of high-valent the dinuclear unit is 3.002 Å. The μ-phenoxido bridges are
[29,30]
metal oxido intermediates.
Although copper(II) Schiff not symmetrical and have different Cu–O bond lengths: the
base complexes have been known for more than 100 years, Cu1–Ophenoxido and Cu2–Ophenoxido bond lengths are
studies of their catalytic activities towards olefin epoxid-
ation are rarely described.[
31–33]
We have already established
II
that the H L ligand can form mononuclear LCu com-
Herein, we report the synthesis and
characterization of the dinuclear species LCu (1) and
LCu Cu (2) through single-crystal X-ray diffraction stud-
ies, magnetic-susceptibility measurements. Moreover, the
catalytic activities towards the epoxidation of olefins,
namely, cyclohexene, cyclooctene, and norbornene, by tert-
butyl hydroperoxide (TBHP) were explored.
2
[
34]
plexes (Scheme 1).
II
2
I
II
Results and Discussion
Structural Description
Single-crystal X-ray diffraction studies show that 1 crys-
II
Figure 1. Molecular view of 1. Color codes: green Cu , gray C, red
II
tallizes in the monoclinic P2 /c space group. In 1, two Cu
O, blue N, yellow Cl. All H atoms are omitted for clarity.
1
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I
II
1
.936(1)/1.917(1) and 2.016(1)/2.055(1) Å, respectively. The plex LCu Cu (2) was formed in quantitative yield under
slightly longer Cu–Ox (x = 8 and 9) bonds for Cu2 are reflux conditions.
likely the consequence of an imperfect match of the Cu2 There is a report on the kinetic and spectral studies on
ion size within the outer cavity. The displacement of Cu2 the autoreduction of Cu complexes of bipyridine and its
II
[
36]
from the square plane constructed by O8, O9, N3 and N5 derivatives under thermal conditions.
towards the axially coordinated water molecule by 0.194 Å 2,2Ј-bipyridine (dmbp) Cu complex was automatically re-
The 6,6Ј-dialkyl-
II
induces the distorted square-pyramidal geometry. An in- duced in ethanol, whereas the bipy complex requires a re-
[
37,38]
II
spection of supramolecular interactions present in the ducing agent. Ultraviolet light
also reduces the Cu
structure reveals that there are two strong H-bonding inter- dmp complex in methanol. We have found that the re-
II
actions between the coordinated water molecule and free duction of Cu to form 2 in MeOH proceeds thermally
perchlorate ions of the neighboring asymmetric unit and without a reducing agent. This type of autoreduction of
II
II
vice versa to give a H-bonded [O10–H1···O5 and O10– Cu complexes has also been found for a Cu complex of
H1···O7] pair of dinuclear units (Figure 2).
[
39,40]
8-dimethylarsinoquinoline.
Single-crystal X-ray diffraction shows that 2 crystallizes
in the triclinic P1 space group. In 2, the Cu and Cu ions
Table 1. Selected bond lengths [Å] of 1 and 2.
II
I
¯
Bond lengths of 1
Bond lengths of 2
are coordinated by N O donor atoms from the deproton-
4
2
ated L2– ligand. In the asymmetric unit, two crystallograph-
ically independent dinuclear units A and B are present.
Both complexes display a very similar structure with subtle
differences in bond lengths (Table 1) and angles (Table 2).
Cu1 and Cu3 are coordinated by two N atoms (namely, N1
and N2 in A and N7 and N8 in B) of the ethylene diamine
part of the ligand and two O atoms (O3 and O4 in A and
O1 and O2 in B) from μ-phenoxido oxygen atoms, which
also coordinate to the Cu2 and Cu4 metal ions, respectively,
in the outer cavity. The geometry around the Cu1 and Cu3
sites is square planar. These Cu ions fit well within the li-
gand cavity with a very small displacement from the mean-
square plane formed by N1, N2, O3, and O4 in A and N7,
N8, O1, and O2 in B [0.030 Å for A and 0.028 Å for B]. On
Cu1–O1
Cu1–O7
Cu1–O8
Cu1–O9
Cu1–N1
Cu1–N2
Cu2–O8
Cu2–O9
Cu2–O10
Cu2–N3
Cu2–N5
Cu1···Cu2
2.5042(17)
2.8523(16)
1.9364(14)
1.9171(14)
1.9150(18)
1.9122(18)
2.0159(14)
2.0548(14)
2.2578(16)
1.9740(18)
1.9697(18)
3.002
Cu3–O1
Cu3–O2
Cu3–N7
Cu3–N8
Cu4–O1
Cu4–O2
Cu4–N9
Cu4–N12
Cu1–N2
Cu1–N1
Cu1–O3
Cu1–O4
Cu2–O3
Cu2–O4
Cu2–N3
Cu2–N6
Cu1···Cu2
Cu3···Cu4
1.906(5)
1.920(5)
1.932(7)
1.938(6)
2.336(5)
2.371(5)
1.906(7)
1.910(6)
1.935(8)
1.926(7)
1.908(5)
1.921(5)
2.373(5)
2.368(5)
1.899(6)
1.917(7)
3.274
3.261
Table 2. Selected bond angles [°] of 1 and 2.
Bond angles of 1
Bond angles of 2
O1–Cu1–O7
O1–Cu1–O8
O1–Cu1–O9
O1–Cu1–N1
O1–Cu1–N2
O7–Cu1–O8
O7–Cu1–O9
O7–Cu1–N1
O7–Cu1–N2
O8–Cu1–O9
O8–Cu1–N1
O8–Cu1–N2
O9–Cu1–N1
O9–Cu1–N2
N1–Cu1–N2
O8–Cu2–O9
172.17(5)
98.26(6)
91.79(6)
96.27(7)
82.13(7)
88.59(5)
92.67(5)
76.88(7)
93.42(6)
84.22(6)
165.46(7)
96.16(7)
95.82(7)
173.90(7)
85.35(8)
78.80(6)
85.13(6)
171.54(7)
95.36(7)
87.01(6)
99.27(7)
149.09(7)
86.55(7)
123.04(7)
90.44(7)
98.80(6)
98.11(6)
O1–Cu3–O2
O1–Cu3–N7
O1–Cu3–N8
O2–Cu3–N7
O2–Cu3–N8
N7–Cu3–N8
O1–Cu4–O2
O1–Cu4–N9
O1–Cu4–N12
O2–Cu4–N9
O2–Cu4–N12
N9–Cu4–N12
N1–Cu1–N2
O3–Cu1–N2
O3–Cu1–O4
O3–Cu1–N1
O4–Cu1–N1
O4–Cu1–N2
O3–Cu2–O4
O3–Cu2–N6
O4–Cu2–N3
O3–Cu2–N3
N3–Cu2–N6
O4–Cu2–N6
Cu3–O1–Cu4
Cu3–O2–Cu4
Cu1–O3 –Cu2
Cu1–O4–Cu2
90.6(2)
175.2(2)
93.0(2)
92.5(2)
171.3(3)
84.5(3)
70.60(16)
88.0(2)
113.4(2)
117.3(3)
87.3(2)
152.4(3)
84.2(3)
171.5(3)
91.2(2)
92.4(3)
175.3(3)
92.6(3)
70.50(16)
89.0(2)
88.1(2)
115.10(19)
150.9(3)
116.6(2)
100.0(2)
98.4(2)
99.2(2)
99.0(2)
Figure 2. H-bonded supramolecular pairs of 1. All hydrogen atoms
are omitted for clarity [except those from the axially coordinated
water molecules (O10), which are shown as pink spheres]. Bond
lengths [Å] and angles [°] (D = donor, A = acceptor): D–H 0.690,
H···A 2.260, D···A 2.937(2), ЄD–H···A 165.00 for O10–H1···O7; O8–Cu2–O10
D–H 0.820, H···A 1.980, D···A 2.795(2), ЄD–H···A 174.00 for O8–Cu2–N3
O10–H1···O5.
O8–Cu2–N5
O9–Cu2–O10
O9–Cu2–N3
O9–Cu2–N5
O10–Cu2–N3
O10–Cu2–N5
N3–Cu2–N5
II
Complex 2 was obtained by reacting H L with Cu -
2
II
(
ClO ) ·6H O in a 1:1 molar ratio and then with [Cu -
4
2
2
(
bipy)(NO ) ] (bipy = 2,2Ј-bipyridine) in a 1:2 molar ratio
3
2
in the presence of 2 equiv. triethylamine (TEA) with the
motivation to synthesize
a
trinuclear complex Cu1–O8–Cu2
II
2+
Cu1–O9–Cu2
[
[
LCu {Cu(bipy)(NO )} ] .
However,
instead
of
3
2
II
2+
LCu {Cu(bipy)(NO )} ] , a mixed-valent dinuclear com-
3
2
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the other hand, Cu2 and Cu4 are coordinated by two N
atoms (namely, N6 and N3 in A and N12 and N9 in B)
from the two pyrazole units of the ligand and two O atoms
(O4 and O3 in A and O1 and O2 in B) from the phenolate
oxygen atoms, which leads to a distorted tetrahedral geome-
try around the Cu2 and Cu4 centers (Figure 3). Their oxi-
dation state is +I on the basis of charge balance, coordina-
tion geometry, and bond valence sum (BVS) calculations
[
41]
(Table S1).
Two perchlorate anions remain uncoordi-
nated to satisfy the +2 charge of the complex in the asym-
metric unit. The Cu···Cu separation within the dinuclear
complexes is 3.274 Å in A and 3.261 Å in B.
Figure 4. UV/Vis spectra of 1 and 2 in MeCN. [c] = 1.0ϫ10–
3
m
m
–
5
for d–d bands at wavelengths above 450 nm and [c] = 1.0ϫ10
for MLCT bands at wavelengths below 400 nm.
hole in d
x2–y2
orbital, which is influenced by the solvent
strength towards axial coordination. The visible spectral
changes of these complexes in some selected solvents are
illustrated in Figure 5. For 2, no such solvatochromic be-
II
havior is apparent (inset Figure 5), maybe because the Cu
ion is in a square-planar geometry and is quite reluctant to
accommodate any solvent molecule axially.
Figure 3. Molecular view of 2. All H atoms are omitted for clarity.
II
I
Color codes: green Cu , light violet Cu , gray C, red O, blue N,
yellow Cl.
UV/Vis Spectra
The electronic spectra of 1 and 2 were recorded in MeCN
(Figure 4). The electronic transition bands for 1 appear at
5
8
18 and 348 nm with molar extinction coefficients (ε) of
11.3 and 9.7ϫ10 dm mol cm , respectively. The corre-
4
3
–1
–1
sponding bands for 2 appear at 586 and 368 nm with ε val-
5
3
–1
–1
ues of 367.1 and 1.9ϫ10 dm mol cm , respectively. The
bands at longer wavelengths may be attributed to the d–d
transitions, whereas bands at ca. 348 and 368 nm are likely
due to metal-to-ligand charge-transfer transitions. UV/Vis/
NIR studies did not provide any indication of intervalence
charge transfer (IVCT), which indicates that the charges are
localized on the two Cu centers. This is also supported by
the low-temperature magnetic studies (vide infra).
Figure 5. Absorption spectra of 1 in selected solvents. Inset is the
absorption spectra of 2.
Catalytic Activities
Solvatochromism arises mainly because of the influence
of the solvent on the electronic absorption and emission
spectra of molecules. Among the solvatochromic metal sition-metal-based catalysts under homogeneous
complexes, the mixed-chelate copper(II) complexes have re- heterogeneous conditions
Olefin epoxidation reactions catalyzed by different tran-
[
43]
and
[
44–49]
are well documented. The
ceived great attention because the presence of a strong epoxidation of styrene by copper phthalocyanine immobi-
Jahn–Teller effect results in simple and regular changes in lized on NaY under heterogeneous conditions showed over
their electronic spectra according to the strength of interac- 95% conversion with epoxide selectivity of only ca. 24%.[
44]
tions with solvent molecules at the axial sites.[
42]
MCM-41-anchored copper phthalocyanine has increased
Complex 1 is soluble in a wide range of organic solvents epoxide selectivity of ca. 53%; however, the conversion re-
[
45]
and demonstrates solvatochromic properties. As the elec- mains at ca. 47%.
tronic configuration of the copper(II) ion is d , the broad
9
The epoxide selectivity of styrene epoxidation with
structureless absorption band is associated with the transi- tBuOOH as oxidant rarely goes above 40% under hetero-
tion of the electron from the lower-energy orbitals to the geneous conditions with copper/copper complexes immobi-
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lized on zeolite or with molecular sieves.[45] Very recently, for different alkenes are given in Figures 6 and S1 for 1 and
an MCM-41-anchored copper(II) Schiff base catalyst im- 2, respectively.
[
31c]
proved the styrene conversion to epoxide to ca. 86%.
Table 3. Homogeneous catalytic oxidation of olefins by TBHP to
epoxides catalyzed by 1 and 2 (catalyst concentration: 0.03 mmol).
Rayati et al. studied the styrene epoxidation over two cop-
per(II) Schiff base complexes under homogeneous condi-
tions with tBuOOH, and over 95% conversion with an ep- Cat. Substrate
Reaction time Conv. % Yield of products TON
oxide selectivity of only ca. 25% was obtained.[
31b]
[h]
[%]
Epoxide
Others
Similar
styrene epoxidation under homogeneous conditions with
1
2
norbornene 24
cyclooctene 24
87
78
70
85
74
68
85
60
53
81
57
52
02
18
17
04
17
16
348
312
280
340
296
272
tBuOOH results in only 39–54% yield with selectivity of
[
31c,31d]
styrene
24
39–72%.
Thus, although aromatic and aliphatic alk-
norbornene 24
cyclooctene 24
enes react with tBuOOH to produce the corresponding ep-
oxides with only low-to-moderate selectivity when catalyzed
styrene
24
by the Cu Schiff base complexes,[
44–50]
these complexes have
been scarcely used as homogeneous catalysts in olefin oxi-
[
50,51]
dation reactions.
In the present report, we have ex- Table 4. Comparison of the catalytic efficiency of 1 and 2 with
other reported copper(II) catalysts for the cyclohexene oxidation
plored the catalytic epoxidation of three olefins, namely,
with tBuOOH in MeCN.
styrene, cyclooctene, and norbornene, by tBuOOH in the
presence of a catalytic amount of Schiff base complexes 1 Catalyst[a]
and 2 in MeCN (Scheme 2). The results of the catalytic oxi-
dation of different substrates are given in Table 3. Under [Cu(L )(H O)](ClO )
homogeneous conditions, the oxidation of styrene with
tBuOOH gives styrene epoxide in ca. 52% yield (selectivity
6%) with a turnover number (TON) of ca. 270 (Table 4)
along with a moderate amount of benzaldehyde (ca. 16%). LCuII (1)
Conv. [%] Epoxide selec- Ref.
tivity [%]
1
[31c]
2
4
86
75
99
62
52
70
68
42
54
39
64
52
53
52
2
[31c]
[31c]
[52]
[
[
[
{
Cu(L )]
3
Cu(L )]
4
Cu(HL )(NO
[Mg(H O) ][Cu(pydca)·2H O]}
3
)]
7
[52]
2
6
2 n
2
this work
this work
I
II
Both complexes show almost similar selectivity and TON LCu Cu (2)
in the production of styrene epoxide. Das et al. have ob-
tained a moderate yield of olefin epoxide by using a cop-
per(II) Schiff base complex as a catalyst in the presence of
1
[
a] HL = 1-(N-ortho-hydroxyacetophenimine)-2-methylpyridine;
2
H
H
H
2
2
2
L
L
=
N,NЈ-(2-hydroxypropane-1,3-diyl)bis(salicylideneimine);
= N,NЈ-(2,2-dimethylpropan-1,3-diyl)bis(salicylideneimine);
pydca =
3
4
L = 1-(N-ortho-hydroxyacetophenimine)ethan-2-ol; H
2
2
-methylpropanal and molecular oxygen under homogen-
2,5-pyridine dicarboxylic acid.
[
50a]
eous conditions,
and a maximum yield of ca. 15% of
styrene oxide has been achieved with iodosylbenzene and
[50b]
the same Schiff base copper complexes.
The oxidation
of cyclooctene by tBuOOH (Table 4) catalyzed by 1 and 2
proceeds smoothly and shows excellent conversion of 74–
78%. Nevertheless, the desired product, cyclooctene oxide,
was not the sole product of this reaction and was produced
with moderate selectivity (ca. 60%). Apart from cyclo-
octene oxide, cyclooctane-1,2-diol (18%) was also generated
owing to allylic C–H oxidation. The bulkier cycloalkene
norbornene has been effectively converted to exo-epoxynor-
bornane (conversion 85–87%, selectivity 98%). Graphical
representations of the relative efficiencies of the catalysts
Figure 6. Conversions [%] of styrene (green), cyclooctene (red), and
norbornene (black) at different reaction times in the liquid phase
catalyzed by 1.
The efficacy of the different catalysts in MeCN with
tBuOOH as oxidant is summarized in Table 4. Interestingly,
our dinuclear complexes lead to almost comparable conver-
sions but with comparable/better selectivity under identical
reaction conditions. Both complexes exhibit almost equal
catalytic activities towards olefin epoxidation irrespective of
the Cu oxidation state and complex geometry. More study
is needed on this aspect in related systems before further
speculation.
Scheme 2.
Eur. J. Inorg. Chem. 0000, 0–0
5
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
Magnetic properties
As shown in Figure 7, the χT product of 2 at 290 K is
3
–1
0
.40 cm Kmol . This value corresponds to the theoretical
The temperature-dependent magnetic susceptibilities for
II
one expected for one isolated S = 1/2 Cu center with g =
2
1
and 2 were measured at 1000 Oe direct current (dc) field
I
.06(5) associated with one diamagnetic Cu metal ion. As
between 1.8 and 300 K and are shown as χT vs. T plots
the temperature decreases, the χT product remains un-
changed in magnitude until 1.8 K. Such thermal Curie-type
paramagnetic behavior implies that the magnetic interac-
(
0
Figure 7). At 300 K, the χT product of
1
is
3
–1
.72 cm Kmol , which is lower than the theoretical value
3
–1
of 0.75 cm Kmol expected for two isolated paramagnetic
II
I
tions between the S = 1/2 Cu /Cu mixed valence com-
plexes are negligible above 1.8 K. Furthermore, the field de-
pendence of the magnetization was measured at 1.83–8 K.
II
9
Cu ions (d , S = 1/2, g = 2). This result suggests the pres-
ence of antiferromagnetic interactions between the CuII
ions within the dinuclear complex. Confirming this hypoth-
esis, the χT product continuously decreases as the tempera-
ture decreases to reach a value close to zero below 50 K.
As shown in the crystal structure analysis (vide supra),
complex 1 can be viewed as a dinuclear S = 1/2 CuII unit.
Thus, the magnetic data have been modeled by using the
Heisenberg spin dimer Hamiltonian of isotropic S = 1/2
spins given by Equation (1).
At 1.83 K, the magnetization is almost saturated at 7 T to
II
a value of 0.99 μ (Figure S2). As the Cu spin carriers in
B
2
are magnetically isolated, the M vs. H/T data at 1.83 K
fit perfectly to an S = 1/2 Brillouin function (Figure S2)
with a Landé factor of 2.05(3) in excellent agreement with
the estimated value from the χT vs. T data (vide supra).
H = –2JSCu1S
Cu2
(1) Conclusions
J is the magnetic interaction within the dinuclear CuII unit
II
We have synthesized two new dinuclear complexes LCu
2
I
II
II
and S are the spin operators for each S = 1/2 center (Cu1 (1) and LCu Cu (2) from a mononuclear LCu species of
i
and Cu2).
a new compartmental ligand H L. Both complexes have
2
been characterized by spectroscopic techniques and single-
crystal X-ray diffraction. In 1, the Cu1 site is in an octahe-
dral geometry positioned in the inner compartment of the
2–
L
ligand, whereas Cu2 occupies the outer compartment
in a square-pyramidal geometry. In 2, the inner core is oc-
II
cupied by a Cu metal ion in a square-planar geometry,
I
whereas the outer site is filled by a Cu center in a distorted
tetrahedral geometry. The modeling of the magnetic prop-
erties of 1 by a Heisenberg S = 1/2 dimer model allows us
to quantify the large intracomplex antiferromagnetic inter-
action [–262(1) K] that is usually observed in this type of
II
I
dinuclear Cu complexes. In 2, the Cu ion is diamagnetic
and, thus, its magnetic properties are dominated by the iso-
II
lated Cu S = 1/2 spin center, which follows a simple Curie
law. Under homogeneous conditions, both complexes
lar magnetic susceptibility that equals M/H per complex) recorded showed catalytic epoxidation of cyclooctene, styrene, and
in an applied dc magnetic field of 1000 Oe for 1 (black) and 2 norbornene to the corresponding epoxides with high selec-
Figure 7. Temperature dependence of the χT product (χ is the mo-
(blue). The black circles and blue squares correspond to the experi-
tivities and TONs, which appear to be higher than the re-
mental data for 1 and 2, and the solid lines are the best fits obtained
from a Heisenberg S = 1/2 spin dimer model and the Curie law,
respectively (see text).
II
ported values for other Cu Schiff base complexes.
[
53]
The application of the van Vleck equation
to the
Experimental Section
Kambe vector coupling scheme[ allows determination of
54]
the low-field analytical expression of the magnetic suscep- Reagents: 2-(Chloromethyl)-6-formyl-4-methylphenol, 3,5-dimeth-
tibility [Equation (2)].[
55]
ylpyrazole, and [Cu(2,2Ј-bipy)(H O) ](NO ) were prepared by a
2
2
3 2
reported method.[
57–59]
Cu(ClO
)
2
·6H
O (Aldrich), ethylenedi-
4
2
2
Ng2μ 2
B
1
amine (Merck, India), triethylamine (Merck, India), and TBHP
Aldrich) were of reagent grade and used as received. Solvents such
as MeCN (Merck India), methanol, ethanol, and others are of rea-
χT =
(2)
(
k
B
B
3 + exp(–2J/k T)
The best fit of the χT vs. T data (solid line in Figure 7) gent grade and were dried by using standard methods before use.
is obtained with J/k = –262(1) K and g = 2.05(5). This
B
av
Physical Measurements: Elemental analyses were performed with a
Perkin–Elmer 240 elemental analyzer. H NMR spectra were re-
simple magnetic model is able to reproduce well the experi-
mental χT vs. T data from 300 to 1.8 K. The sign of the
magnetic interactions implies that this dinuclear Cu com-
1
3
corded with samples in CDCl with a Bruker 300 MHz NMR spec-
trophotometer and tetramethylsilane (δ = 0 ppm) as an internal
plex possess an ST = 0 spin ground state as expected for standard. Electronic spectra were recorded with an Agilent-8453
II
[1,56]
similar di-alkoxido-bridged dinuclear Cu complexes.
diode array UV/Vis spectrophotometer. Infrared spectra (400–
Eur. J. Inorg. Chem. 0000, 0–0
6 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I30576
/KAP1
Date: 30-07-13 17:12:20
Pages: 10
www.eurjic.org
FULL PAPER
–
1
4
000 cm ) were recorded from KBr pellets with a Nicolet Magna
formed with 1 and 2 as catalysts and TBHP as oxidant. The overall
reaction was performed in a two-necked round-bottomed flask fit-
ted with a condenser and heated with a temperature-controlled oil
bath on a magnetic stirrer. In a typical procedure, the substrate
IR 750 series-II FTIR spectrophotometer.
Single-Crystal X-ray Diffraction: The single-crystal X-ray diffrac-
tion data of 1 and 2 were collected with a Bruker SMART APEX-
II CCD diffractometer with graphite-monochromated Mo-K radi-
α
(
0.5 mg) was dissolved with the same molar ratio of TBHP in
acetonitrile (10 mL) containing the catalyst (0.02 mmol). The tem-
perature of the reaction was maintained at 60 °C, and the products
were collected at regular intervals. The progress of the reactions
was monitored by an Agilent 7890D gas chromatograph (FID de-
tector) fitted with a capillary column. The products were identified
by comparison with known standards and confirmed by GC–MS
with a Shimadzu-QP5050A GC–MS-E1 instrument.
ation (λ = 0.71073 Å). Data collection, reduction, structure solu-
tion, and refinement were performed by using the Bruker Apex-
II suite (v2.0–2) software. All available reflections to 2θmax were
harvested and corrected for Lorentz and polarization factors with
Bruker SAINT plus. The reflections were then corrected for ab-
sorption, interframe scaling, and other systematic errors with
[
60]
SADABS. The structures were solved by direct methods and re-
2
fined by the full-matrix least-square technique based on F with
2-Formyl-4-methyl-6-(3,5-dimethylpyrazole)phenol (HL): 2-Chloro-
methyl-6-formyl-4-methylphenol (1.515 g, 8.2 mmol) was dissolved
in dry tetrahydrofuran (THF, 15 mL) in a round-bottomed flask.
3,5-Dimethylpyrazole (0.7883 g, 8.2 mmol) and triethylamine
the SHELX-97 software package.[ All non-hydrogen atoms were
refined with anisotropic thermal parameters. All hydrogen atoms
bound to carbon and nitrogen atoms were placed in their geometri-
cally idealized positions, and hydrogen atoms bound to oxygen
atoms of coordinated water molecules were found on the difference
Fourier map; all of them were constrained to ride on their parent
61]
(Et N, 1.659 g, 16.4 mmol) were dissolved in dry THF (10 mL),
3
and this mixture was added dropwise to the 2-chloromethyl-6-
formyl-4-methylphenol solution. The instant precipitation of
atoms. Drawings of molecules were generated with DIAMOND 3.0 Et NHCl was observed and the solution turned bright yellow
3
and PLATON v1.6 software. The crystallographic data for 1 and 2
(Scheme 3). The solution was stirred for 24 h, and the precipitated
are listed in Table 5.
3
Et NHCl was removed by filtration. The solvent (THF) was re-
moved under reduced pressure to afford an oily product, which
yielded a light yellow crystalline solid after a couple of days in a
Table 5. Crystal data and Refinement parameters.
1
2
refrigerator. The solid product was then collected by filtration and
1
washed with cold ether, yield 72%. H NMR (CDCl
3
, 300 MHz):
Formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C
30
H
36
N
6
O
11Cl
2
Cu
2
C
30 34 6 6 2
H N O ClCu
δ = 2.23–2.28 (m, 9 H, ArCH
3
), 5.27 (s, 2 H, CH
2
), 7.09 (s, 1 H,
854.65
monoclinic
P2 /c (No. 14)
10.7666(2)
14.6661(4)
22.1135(5)
90
92.202(2)
90
3489.23(14)
4
737.18
triclinic
ArH), 7.27 (s, 1 H, ArH), 7.28 (s, 1 H, ArH), 9.88 (s, 1 H, CHO),
11.23 (br s,1 H, ArOH) ppm.
¯
1
P1 (No. 2)
8.351(2)
15.650(4)
23.231(5)
89.482(5)
88.128(4)
87.402(5)
3031.3(12)
4
γ [°]
V [Å ]
3
Z
3
D(calcd.) [Mg/m ]
μ(Mo-K ) [mm]
F(000)
1.627
1.615
α
1.440
1752
1.546
1516
Scheme 3.
Crystal size [mm]
Temperature [K]
0.20ϫ0.25ϫ0.30
293
0.71073
0.25ϫ0.28ϫ0.30
100
0.71073
[CuII
azole)phenol (0.244 mg, 1.00 mmol) and ethylenediamine (0.03 mg,
.50 mmol) in methanol (30 mL) were heated to reflux for 40 min
Scheme 4). After the solution had cooled to room temperature,
copper perchlorate hexahydrate (0.370 mg, 1.00 mmol) was added,
and the mixture was heated to reflux for a further 60 min, where-
upon the yellow solution turned green. The solution was filtered,
and slow evaporation of the methanol yielded rod-shaped green
2
2 4 2
(L)(H O)(ClO ) ] (1): 2-Formyl-4-methyl-6-(3,5-dimethylpyr-
Mo-K
α
wavelength [Å]
θ min, max [°]
Data set
Total, unique data, R(int) 16293, 7621, 0.023
Observed data [IϾ2σ(I)] 5474
2.9, 27.00
1.3, 24.9
0
(
–13:13; –18:18; –28:28 –9:9; –18:18; –27:27
25049, 10518, 0.064
7418
10518, 823
Nref, Npar
7621, 467
0.0277, 0.0831, 0.63
2
R, wR , Goof
0.0769, 0.2025, 1.08
crystals suitable for X-ray studies, yield 78%. C30
H
36Cl
2
Cu
2 6 11
N O
CCDC-914813 (for 1) and -937186 (for 2) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(
9
854.65): calcd. C 42.12, H 4.21, N 9.83; found C 42.35, H 4.28, N
–
4 2
.38. IR: ν˜ = 1091 (ClO , coordinated), 1650 (C=N), 3429 (H O)
–1
cm .
I
II
[
Cu Cu (L)](ClO
4
)
(2): 2-Formyl-4-methyl-6-(3,5-dimethylpyr-
Magnetic Measurements: The magnetic susceptibility measure-
ments were obtained with the use of a Quantum Design supercon-
ducting quantum interference device (SQUID) magnetometer
MPMS-XL. This magnetometer works between 1.8 and 300 K for
dc applied fields of –7 to 7 T. Measurements were performed on
polycrystalline samples of 11.63 mg for 1 and 25.47 mg for 2. The
magnetic data were corrected for the sample holder and diamag-
netic contributions.
azole)phenol (0.244 mg, 1.00 mmol) and ethylenediamine (0.03 mg,
.50 mmol) in methanol (30 mL) were heated to reflux for 40 min.
After the solution had cooled to room temperature, copper per-
chlorate hexahydrate (0.185 mg, 0.50 mmol) was added, and the
0
mixture was heated to reflux for 1 hour. [Cu(2,2Ј-bipy)(H
2 2
O) ]-
(
NO (0.382 mg, 1.0 mmol) was then added, and the mixture was
3 2
)
heated to reflux for another hour. The yellow solution turned
green. The solution was filtered, and slow evaporation of the meth-
anol yielded rod-shaped green crystals suitable for X-ray studies,
Experimental Set-up for Catalytic Oxidation: Liquid-phase oxi-
dation reactions of styrene, cyclooctene, and norbornene were per-
yield 65%. C30
2 6 6
H34ClCu N O (737.18): calcd. C 48.88, H 4.65, N
Eur. J. Inorg. Chem. 0000, 0–0
7 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I30576
/KAP1
Date: 30-07-13 17:12:20
Pages: 10
www.eurjic.org
FULL PAPER
Scheme 4.
1
1
1.40; found C 48.35, H 4.78, N 11.38. IR: ν˜ = 1096 (ClO
630 (C=N) cm .
4
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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/KAP1
Date: 30-07-13 17:12:20
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Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I30576
/KAP1
Date: 30-07-13 17:12:20
Pages: 10
www.eurjic.org
FULL PAPER
Dinuclear Complexes
S. Biswas, A. Dutta, M. Dolai,
I. Bhowmick, M. Rouzières, H. M. Lee,
R. Clérac, M. Ali* .......................... 1–10
II
II
I
II
Dinuclear Cu –Cu and Cu –Cu Com-
plexes of a Compartmental Ligand – Syn-
theses, Structures, Magnetic, and Catalytic
Studies
Keywords: Schiff bases / Copper / Structure
elucidation / Magnetic properties / Epoxid-
ation
II
II
I
II
Dinuclear Cu –Cu (1) and Cu –Cu (2)
complexes derived from a new N donor
ligand (H L) are synthesized and charac-
measurements. In 1, there is strong anti-
4
O
2
ferromagnetic coupling between the Cu
I
2
atoms. In 2, the Cu ion is diamagnetic and
II
terized by single-crystal X-ray diffraction,
UV/Vis spectroscopy, and magnetic
the remaining S = 1/2 Cu magnetic center
follows a Curie law with g = 2.06(5).
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim