Reaction with dioxygen of a Cu(I) complex of 1-benzyl-[3-(2′-pyridyl) ]pyrazole triggers ethyl acetate hydrolysis: Acetato-/pyrazolato-, dihydroxo- and diacetato-bridged Cu(II) complexes

Article abstract of DOI:10.1039/b512086a

A copper(i) compound [(L²)Cu(MeCN)₂][ClO₄] (1) containing a new bidentate N-donor ligand L², 1-benzyl-[3-(2′-pyridyl)]pyrazole, derived from the condensation of HL¹ [HL¹ = 3-(2-pyridyl)pyra

Full text of DOI:10.1039/b512086a

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Reaction with dioxygen of a Cu(I) complex of
1-benzyl-[3-(2ꢀ-pyridyl)]pyrazole triggers ethyl acetate hydrolysis:
acetato-/pyrazolato-, dihydroxo- and diacetato-bridged Cu(II) complexes†
Jhumpa Mukherjee and Rabindranath Mukherjee*
Received 25th August 2005, Accepted 22nd December 2005
First published as an Advance Article on the web 11th January 2006
DOI: 10.1039/b512086a
A copper(I) compound [(L2)Cu(MeCN)2][ClO4] (1) containing a new bidentate N-donor ligand L2,
1-benzyl-[3-(2ꢀ-pyridyl)]pyrazole, derived from the condensation of HL1 [HL1 = 3-(2-pyridyl)pyrazole]
and benzyl chloride, has been synthesized. Structural analysis reveals that in 1 the copper(I) centre is
coordinated by a pyridine and a pyrazole nitrogen from L2 and two MeCN molecules, providing a
distorted tetrahedral geometry. Reaction of 1 with dioxygen in N,Nꢀ-dimethylformamide (dmf) at 25 ◦C
and subsequent workup with MeCO2Et afforded an acetato-/pyrazolato-bridged polymeric copper(II)
compound [(l-L1)Cu(l-O2CMe)]n (2). Notably, the deprotonated form of HL1 and MeCO2 have
−
originated from debenzylation of L2 and hydrolysis of MeCO2Et, respectively. The structural analysis of
2+
2 reveals a near-planar {Cu2(l-L1)2} core unit in which two adjacent Cu(II) ions are bridged by the
2+
deprotonated N,N-bidentate pyridylpyrazole units of two L1 and each such {Cu2(l-L1)2} unit is
−
˚
bridged by MeCO2 in a monodentate bridging mode [Cu · · · Cu separations (A): 3.9232(4) pyrazolate
bridge; 3.3418(4) acetate bridge], providing a polymeric network. Careful oxygenation of 1 in MeCN
led to the isolation of a dihydroxo-bridged dicopper(II) compound [{(L2)Cu(l-OH)(OClO3)}2] (3).
Interestingly, complex 3 brings about hydrolysis of MeCO2Et under mild conditions (dmf, ca. 60 ◦C),
generating a bis-l-1,3-acetato-bridged dicopper(II) complex,
[{(L2)Cu(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H (4). Compounds 3 and 4 have {Cu2(l-OH)2}
2+
2+
˚
[Cu · · · Cu separation of 2.8474(9) A] and {Cu2(l-O2CMe)2} cores [Cu · · · Cu separation: 3.0988(26)
˚
and 3.0792(29) A (two independent molecules in the asymmetric unit)] in which each Cu(II) centre is
terminally coordinated by L2. A rationale has been provided for the observed debenzylation of L2 and
hydrolysis of MeCO2Et. The intramolecular magnetic coupling between the CuII (S = 1/2) ions was
found to be ferromagnetic (2J = 82 cm−1) in the case of 3, but antiferromagnetic for 2 (2J = −158 cm−1)
and 4 (2J = −96 cm−1). Absorption and EPR spectroscopic properties of the copper(II) compounds
have also been investigated.
products, from the reaction between copper(I) complexes and
Introduction
dioxygen1a,6,7) we have begun to target hydroxo-containing dicop-
per(II) complexes to develop biomimetic systems of hydrolases.8–10
It is well known that a very common reactivity of copper(I)
complexes with dioxygen is the formation of irreversibly oxidized
product, bis-l-hydroxo-bridged dicopper(II) complexes.1a,11–13 Re-
cently we demonstrated the nucleophilic reactivity of copper(II)-
coordinated hydroxide ion, due to reactions of dioxygen with
copper(I) complexes of symmetrical/unsymmetrical tridentate (2-
pyridyl)alkylamine ligands, affording l3-carbonato-bridged tri-
copper(II) complexes.14
Coordination chemistry of copper complexes is a subject of
continuing importance1 in connection with the structures and
the reactivities of copper-containing metalloproteins.2,3 Therefore,
structural modulation of synthetic copper complexes by subtle per-
turbations of various multidentate N-donor ligands to understand
the copper/dioxygen chemistry have attracted much interest over
the past two-decades in relation with the biological systems and
the potential applications in synthetic organic transformations.4,5
As part of our activity in bio-inspired coordination chemistry
of copper (for example, modelling tyrosinase-like activity6 and
catechol oxidase activity,7 and to characterize irreversibly oxidized
Inspired by the recent developments on the use of bidentate
ligands5,15 to investigate the reactivity property of copper(I)
complexes with dioxygen, given literature reports on hydrolysis
of amides/esters by copper(II) coordinated hydroxide ion16,17
and our own experience on nucleophilic reactivity of copper(II)
coordinated hydroxide ion14 our present endeavour is set. Here
we report on new chemistry utilizing a copper(I) complex,
[(L2)Cu(MeCN)2][ClO4] (1) of a bidentate ligand, 1-benzyl-[3-(2ꢀ-
pyridyl)]pyrazole, which led, to the best of our knowledge, to the
discovery of the first nonenzymic hydrolysis of an unactivated
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur
208 016, India. E-mail: rnm@iitk.ac.in; Fax: +91-512-2597437; Tel: +91-
512-2597437
1
† Electronic supplementary information (ESI) available: H NMR spec-
trum of (1) in CD3CN. EPR spectrum (polycrystalline sample) of (2) at
120 K. UV-VIS spectrum of (3) in MeCN. UV-VIS spectrum of (4) in
MeCN. EPR spectrum (polycrystalline sample) of (4) at 300 K. See DOI:
10.1039/b512086a
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ester MeCO2Et by an isolated copper(II)-coordinated hydroxide
ion, as a nucleophile. Thus, reaction of 1 with dioxygen in
N,Nꢀ-dimethylformamide (dmf) at 25 ◦C and subsequent workup
with MeCO2Et afforded an acetato-/pyrazolato-bridged poly-
meric copper(II) compound [(l-L1)Cu(l-O2CMe)]n (2) [HL1 = 3-
(2-pyridyl)pyrazole]. To shed light on the mechanism by which
acetate is generated from the reaction between 1, O2 and
MeCO2Et and to prepare the expected intermediate and a dis-
crete acetato-bridged compound, we synthesized the dihydroxo-
bridged dicopper(II) compound [{(L2)Cu(l-OH)(OClO3)}2] (3)
and [{(L2)Cu(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H (4), re-
spectively. This report presents the structural elucidation of com-
pounds 1–4, along with spectroscopic and detailed temperature-
dependent magnetic behaviour of 2, 3 and 4.
[(l-L1)CuII(l-O2CMe)]n (2).
Method A. To a degassed solution of L2 (0.10 g, 0.43 mmol)
in MeCN (10 cm3) was added solid [Cu(MeCN)4][ClO4] (0.14 g,
0.43 mmol). The resulting bright yellow solution was stirred for
30 min at room temperature. Afterwards all the manipulations
were performed in air, implying the essential participation of
dioxygen in the synthesis of 2. The solvent was then removed, dmf
(3 cm3) was added to the residue, and filtered. Vapour diffusion
of MeCO2Et into the resulting solution afforded, after about a
month, dark blue single crystals, suitable for structural analysis
(yield: 0.03 g, ∼26%). Found: C, 44.99; H, 3.45; N, 16.05. Calc.
for C10H9N3O2Cu: C, 45.02; H, 3.38; N, 15.76%. IR (KBr, cm−1,
selected peaks): 1608 (masym(MeCO2−)), 1378 (msym(MeCO2−)).
UV/VIS (Nujol mull), kmax/nm (e/dm3 mol−1 cm−1): 267, 330,
645.
Method B. Solid [Cu(H2O)6][ClO4]2 (0.512 g, 1.38 mmol) was
added to a stirred solution of HL1 (0.200 g, 1.38 mmol) in MeOH
(10 cm3). The bluish green solution thus obtained was stirred for
30 min and to it a solution of Et3N (0.140 g, 1.386 mmol) in MeOH
(5 cm3) was added dropwise. The solution was filtered to remove
trace amounts of a precipitate and to it solid MeCO2Na (0.114 g,
1.390 mmol) was added, generating a deep blue solution. After
15 min the solid that separated was filtered, washed with MeCN
and Et2O and air-dried (yield: 0.270 g, ∼74%). This product is
identical to that obtained following Method A.
Experimental
General considerations
All reagents were obtained from commercial sources and used
as received, unless stated otherwise. Solvents were dried/purified
as reported previously.6,7,14 3-(2-Pyridyl)pyrazole (HL1)18 and
[Cu(MeCN)4][ClO4]19 were prepared following reported proce-
dures. All manipulations for the synthesis of the copper(I) complex
were performed in an atmosphere of purified dinitrogen using
standard Schlenk and glove box (Mbraun, Germany) techniques.
[{(L2)CuII(l-OH)(OClO3)}2] (3). To a degassed solution of L2
(0.10 g, 0.43 mmol) in MeCN (10 cm3) was added solid
[Cu(MeCN)4][ClO4] (0.14 g, 0.43 mmol). The resulting bright
yellow solution comprising supposedly 1 was stirred for 10 min
at room temperature. Exposure of this solution to dry O2 did not
bring about an immediate noticeable change in colour. However,
on exposure of this solution to O2 for 24 h, a green colour was
generated, the green solution was allowed to slowly evaporate in air
after discarding a minute amount of a greenish yellow solid that
initially separated out. Dark blue crystals which formed within
2–3 days were filtered, washed with a mixture (1 : 3, v/v) of
MeCN/Et2O (4 cm3) and dried in vacuo (yield: 0.075 g, ∼43%).
Found: C, 43.15; H, 3.56; N, 3.56. Calc. for C30H28N6O10Cl2Cu2:
C, 43.37; H, 3.37; N, 10.12%. Molar conductance, KM (MeCN,
∼10−3 mol dm−3, 298 K) = 240 X−1 cm2 mol−1 (expected range20
for 1:2 electrolyte: 220–300 X−1 cm2 mol−1). IR (KBr, cm−1,
selected peaks): 3550 (m(OH−)), 1106 and 623 (m(ClO4−)). UV/VIS
(MeCN), kmax/nm (e/dm3 mol−1 cm−1): 245 (30 700), 295 (25
700), 648 (140).
Synthesis of 1-benzyl-[3-(2ꢀ-pyridyl)]pyrazole (L2). A mixture
of benzyl chloride (0.872 g, 6.90 mmol), HL1 (1 g, 6.90 mmol),
benzene (80 cm3), 40% aqueous NaOH (10 cm3) and 40% aqueous
tetra-n-butylammonium hydroxide (8 drops) was refluxed with
stirring for 8 h and then stirred at room temperature for 12 h.
The organic layer was then separated, washed twice with brine
water, dried over anhydrous Na2SO4 and filtered. Solvent removal
afforded a yellowish white solid (yield: 1.46 g, ∼90%). 1H NMR
(80 MHz; CDCl3): d 5.23 (2 H, s, CH2), 6.87 (1H, d, pz H4), 7.25
(6H, m, py H5, bz H2,6), 7.66 (2H, m, py H3,4), 7.90 (1H, d, pz
H5), 8.56 (1H, d, py H6).
Synthesis of copper complexes. [(L2)CuI(MeCN)2][ClO4] (1).
To a degassed solution of L2 (0.10 g, 0.43 mmol) in MeCN
(10 cm3) was added solid [Cu(MeCN)4][ClO4] (0.14 g, 0.43 mmol).
The resulting bright yellow solution was stirred for 30 min at
room temperature. Then dry degassed Et2O (10 cm3) was added
to the above solution. The yellow solid that precipitated was
filtered, washed with a mixture (1 : 3, v/v) of MeCN/Et2O and
dried in vacuo. Single crystals suitable for structural studies were
obtained by the diffusion of Et2O into a MeCN solution of the
complex under a dinitrogen atmosphere (yield: 0.160 g, ∼78%).
Found: C, 47.76; H, 4.14; N, 14.39. Calc. for C19H19N5O4ClCu:
C, 47.50; H, 3.96; N, 14.58%. Molar conductance, KM (MeCN,
∼10−3 mol dm−3, 298 K) = 130 X−1 cm2 mol−1 (expected range20
for 1 : 1 electrolyte: 120–160 X−1 cm2 mol−1). IR (KBr, cm−1,
selected peaks): 2250 (m(CN) of MeCN; 1089 and 627 (m(ClO4−)).
UV/VIS (MeCN), kmax/nm (e/dm3 mol−1 cm−1): 247 (22 600) and
[{(L2)Cu(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H
(4).
Dark blue crystals of complex 3 were dissolved in dmf (2 cm3).
To the resulting green solution was added dry MeCO2Et (5 cm3),
and warmed on a water bath at ∼60 ◦C. After 4 h the bluish green
solution was kept in a refrigerator. Within 24 h the light bluish
green crystals that formed were collected by filtration and dried in
vacuo. These crystals were found to be suitable for X-ray structural
study (yield: 0.040 g, ∼57%). Found: C, 45.14; H, 4.44; N, 10.98.
Calc. for C44H55N9O16Cl2Cu2: C, 45.40; H, 4.73; N, 10.83%.
Molar conductance, KM (MeCN, ∼10−3 mol dm−3, 298 K): =
260 X−1 cm2 mol−1. IR (KBr, cm−1, selected peaks): 1650 (m(CO)
of DMF)), 1580 (masym(MeCO2−)), 1439 (msym(MeCO2−)), 1106 and
627 (m(ClO4−)). UV/VIS (MeCN), kmax/nm (e/dm3 mol−1 cm−1):
246 (40 150), 291 (31 300), 718 (130), 964 (60).
1
283 (14 000). H NMR (400 MHz, CD3CN): d 8.31 (1H, d, py
6-H), 7.99–7.91 (3 H, m, py 3,4-H and pz 3-H), 7.41 (1 H, t, bz
4-H), 7.22–7.21 (3H, m, bz 3,5-H and py 5-H), 7.11–7.10 (2H, d,
bz 2,6-H), 7.00–6.99 (1H, d, pz 4-H), 5.23 (2H, s, –CH2Ph), 2.18
(6H, s, CH3CN).
1612 | Dalton Trans., 2006, 1611–1621
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Caution: Perchlorate salts of compounds containing organic
ligands are potentially explosive.
a PMX-60 JEOL (60 MHz) NMR spectrometer. Corrections
underlying diamagnetism were applied with use of appropriate
constants.23 Effective magnetic moments were calculated from
leff = 2.828 [vMT]1/2, where vM is the corrected molar susceptibility.
Physical measurements
Elemental analyses were obtained with a Carlo Erba CHNSO
1110 analyzer. Conductivity measurements were performed with
an Elico type CM-82T conductivity bridge (Hyderabad, In-
dia). Spectroscopic measurements were made using the follow-
ing instruments: IR (KBr, 4000–600 cm−1), Bruker Vector 22;
electronic, Perkin Elmer Lambda 2 and Agilent 8453 diode-
array spectrophotometer; X-band EPR, Bruker EMX 1444 EPR
spectrometer operating at 9.455 GHz (fitted with a quartz Dewar
for measurements at 120 K). The EPR spectra were calibrated with
diphenylpicrylhydrazyl, DPPH (g = 2.0037).
Crystal structure determinations
X-Ray data were collected on a Bruker SMART APEX
CCD diffractometer, with graphite-monochromated Mo-Ka (k =
˚
0.71073 A) radiation at 100(2) K. For data reduction the ‘Bruker
Saint Plus’ program was used. Data were corrected for Lorentz and
polarization effects; empirical absorption correction (SADABS)
was applied. Structures were solved by direct methods using
SIR-97 and refined by full-matrix least-squares methods based
on F2 using SHELXL-97, incorporated in the WINGX 1.64
crystallographic collective package.24 For 1–3, all non-hydrogen
atoms were refined anisotropically. For 4, except the chlorine
and the four oxygens of a perchlorate ion, all the non-hydrogen
atoms were refined anisotropically and those four oxygens were
modeled satisfactorily with isotropic displacement parameters,
displaced over two positions with a probability of 60 : 40. All the
perchlorate Cl–O distances were restrained to obtain reasonable
bond distances. Except for 1 (the hydrogen atoms are located from
the difference Fourier map), the positions of the hydrogen atoms
were calculated assuming ideal geometries. A summary of the
data collection and structure refinement information is provided
in Table 1.
Magnetism
Magnetic susceptibility measurements on solid samples of 3
were obtained in the solid state using a Quantum Design
(Model MPMSXL-5) SQUID magnetic susceptometer operating
at magnetic field of 1.0 T. Variable temperature (54–300 K for
2; 60–300 K for 4) magnetic susceptibility measurements on 2
and 4 in the solid state were performed using a locally-built
Faraday balance21 comprising an electromagnet with constant
gradient pole caps (Polytronic Corporation, Mumbai, India), an
ultravacuum Sartorius M25-D/S Balance (Germany), a closed-
cycle refrigerator and a Lake Shore temperature controller (Cryo
Industries, USA). All measurements were made at a fixed main
field strength of ∼0.6 T. Solution-state magnetic susceptibilities
were obtained by the NMR technique of Evans22 in MeCN with
CCDC reference numbers 253143 (1), 253144 (2), 253145 (3)
and 253146 (4).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b512086a
Table 1 Data collection and structure refinement parameters for [(L2)CuI(MeCN)2][ClO4] (1), [(l-L1)CuII(l-O2CMe)]n (2), [{(L2)Cu(l-OH)(OClO3)}2]
(3) and [{(L2)Cu(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H (4)
1
2
3
4
Chemical formula
C19H19N5ClO4Cu
480.38
Yellow, block-like
100(2)
C20H18N6O4Cu2
533.48
Dark blue, block-like
100(2)
C30H28N6Cl2O10Cu2
832.58
Dark blue, block-like
100(2)
C44H52N9Cl2O16Cu2
1160.93
Bluish green, block-like
100(2)
M
Crystal colour, habit
T/K
˚
k/A
0.71073
0.71073
0.71073
0.71073
Crystal
System
Triclinic
Orthorhombic
0.3 × 0.2 × 0.2
Pcab (no. 61)
17.217(5)
6.420(5)
17.644(5)
90.0
90.0
90.0
1950.2(17)
4
1.817
Monoclinic
0.3 × 0.2 × 0.2
Cc (no. 9)
7.571(5)
18.245(5)
23.845(5)
90.0
95.334(5)
90.0
3280(2)
Monoclinic
0.3 × 0.2 × 0.2
P21 (no. 4)
11.234(5)
23.253(5)
19.581(5)
90.0
98.723(5)
90.0
5056(3)
Crystal size/mm × mm × mm
0.2 × 0.2 × 0.1
P-1 (no. 2)
10.823(5)
10.882(5)
11.290(5)
117.244(5)
90.425(5)
115.172(5)
1035.6(8)
2
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
4
4
d
calcd/g cm−3
1.541
1.220
1.686
1.527
1.508
1.023
l/mm−1
2.225
No. reflns. collected
No. indep. reflns.
No. reflns. used [I > 2r(I)]
No. param.
Final R indices [I > 2r(I)] R1, wR2
R indices (all data)
GOF on F2
6951
11553
10814
26671
4924 (Rint = 0.0168)
2358 (Rint = 0.0316)
5357 (Rint = 0.0328)
12404 (Rint = 0.0852)
4474
2101
5026
7788
347
145
451
1277
0.0324 (0.0866)
0.0354 (0.0886)
1.007
0.0322 (0.0892)
0.0366 (0.0922)
1.074
0.0321 (0.0683)
0.0344 (0.0691)
0.971
0.0765 (0.1636)
0.1271 (0.1882)
1.003
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pyridylpyrazole unit of L2 is revealed by the angle between the
two heterocyclic rings (∼3◦). To our knowledge, complex 1 is
the first structurally characterized tetracoordinated bisacetonitrile
copper(I) complex with a bidentate pyrazolyl-pyridine ligand.13
Results and discussion
Synthesis and properties of [(L2)CuI(MeCN)2][ClO4] (1)
The bidentate ligand L2 was synthesized from the reaction between
benzyl chloride and 3-(2-pyridyl)pyrazole (HL1)18a in the presence
of NaOH in benzene. The reaction of [Cu(MeCN)4][ClO4]18
with L2 in MeCN, under anaerobic conditions, affords the yellow
complex [(L2)Cu(MeCN)2][ClO4] (1) in ∼78% yield (Scheme 1).
The presence of coordinated MeCN molecules in 1 is observed by
an IR absorption at 2250 cm−1. The presence of ionic perchlorate
is observed at 1089 and 627 cm−1. The solution-state structure of 1
was judged by 1H NMR spectroscopy (see ESI†). Microanalytical
and solution electrical conductivity data (Experimental) conform
to the above formulation.
Fig. 1 View of [(L2)Cu(MeCN)2]+ in the structure of (1). Hydrogen atoms
are omitted for clarity.
Reactivity of 1 towards dioxygen and concomitant debenzylation
of L2 and hydrolysis of ethyl acetate: synthesis and properties of
[(l-L1)CuII(l-O2CMe)]n (2)
The reaction between 1 and dioxygen was quite slow in MeCN. To
enhance the rate of reaction, the solvent was removed from
solution-generated 1, and the residue was dissolved in dmf.
Vapour diffusion of MeCO2Et into this solution afforded, after
about a month, dark blue single crystals of polymeric (see
below) [(l-L1)Cu(l-O2CMe)]n (2) in ∼26% yield (Scheme 1).
The designed synthesis of compound 2 was achieved in good
yield (∼74%) by direct reaction between [Cu(H2O)6][ClO4]2, HL1,
Et3N and MeCO2Na in MeOH (Scheme 1). Notably, during
the reaction conditions followed for the synthesis of 2 (Method
A), debenzylation of L2 (generating the deprotonated form of
Scheme 1
Crystal structure of [(L2)CuI(MeCN)2][ClO4] (1)
In order to confirm the identity of the copper(I) complex single
crystal X-ray structure determination was carried out on 1. A
perspective view of the cationic part of the complex, with atom-
labeling scheme is shown in Fig. 1. Selected metric parameters
associated with the copper(I) center in 1 are given in Table 2. The
structure of the ligand L2 and its bidentate coordination mode
are confirmed. The structure of 1 shows a distorted tetrahedral
CuN4 geometry around copper(I), provided by the coordination
of the ligand and two MeCN molecules (Fig. 1). In fact, N–Cu–N
angles span the range 79.60(6)–125.96(6)◦, substantially deviating
from the ideal tetrahedral angle. As expected, the dihedral angle
between the N1CuN2 and N4CuN5 planes is 83.533(58)◦, deviating
from the ideal 90◦. There are two types of heterocyclic nitrogen
donor atoms in L2: one pyrazole N(2) and one pyridine N(1).
Interestingly, the Cu–Npz (pz = pyrazole) bond length is shorter
than that of Cu–Npy (py = pyridine). For both a triangular cop-
per(I) complex of tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate25a and
a dimeric copper(I) complex with two bidentate pyrazolyl-pyridine
arms attached to a thiophosphinate head-group,25b similar trends
were observed. The average Cu–NMeCN bond length of 1.9535(17)
HL1) and hydrolysis of MeCO2Et (generating the MeCO2 ion)
−
have occurred (see below). Microanalytical data (Experimental)
conform to the above formulation. X-Ray structural analysis (see
below) authenticated the structure and composition of 2.
The IR bands at 1608 cm−1 and 1378 cm−1 are assigned to
the stretching modes of the monoatomic acetate bridge in 2.26
Owing to the presence of the asymmetric bonding mode of the
monoatomic acetate bridge, a large splitting (D = 230 cm−1) of the –
COO stretching frequencies is observed.26 The electronic spectrum
of polymeric copper(II) compound 2, measured (190–1000 nm) in
the solid state (dispersed in mineral oil mull), displays a broad
absorption at 645 nm, justifying its dark blue colour.
In keeping with the EPR spectral behaviour of weakly exchange-
coupled copper(II) dimers27 solid samples of 2 display at 120 K a
signal at g = 4.23 and a major absorption at g = 2.16, with minor
absorptions on either side of this signal (g = 2.90, 1.86 and 1.76).
The spectral feature is displayed in the ESI†. Minor absorptions
are expected to be due to a lowering of copper(II) site symmetry.
In fact, the EPR signal at g = 4 (due to DMs = 2 transition) for
copper(II) complexes is considered as a signature of the dimeric
varieties.27
˚
A is comparable to that found for [Cu(H2CPZ2)(MeCN)2][ClO4]
[H2CPZ2 = bis(pyrazol-1-yl)methane].13 The planar nature of
1614 | Dalton Trans., 2006, 1611–1621
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◦
2
I
1
II
Table 2 Selected bond distances (A) and angles ( ) of [(L )Cu (MeCN)2][ClO4] (1), [(L )Cu (O2CMe)]n (2), [{(L2)Cu(l-OH)(OClO3)}2] (3) and
˚
[{(L2)CuII(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H (4)
1
Cu–N(1)
Cu–N(2)
Cu–N(4)
Cu–N(5)
2.0716(16)
2.0588(15)
1.9651(17)
1.9418(17)
N(1)–Cu–N(2)
N(1)–Cu–N(4)
N(1)–Cu–N(5)
N(2)–Cu–N(4)
N(2)–Cu–N(5)
N(4)–Cu–N(5)
79.60(6)
111.74(6)
125.96(6)
115.54(7)
113.55(6)
108.23(7)
2
Cu–N(1)
2.0332(19)
1.986(2)
1.9709(19)
1.9625(19)
2.3737(17)
3.3418(4)
3.9232(4)
N(1)–Cu–N(2)
N(1)–Cu–N(3)
N(1)–Cu–O(1)
N(1)–Cu–O(1ꢀ)
N(2)–Cu–N(3)
N(2)–Cu–O(1)
N(2)–Cu–O(1ꢀ)
N(3)–Cu–O(1)
N(3)–Cu–O(1ꢀ)
O(1)–Cu–O(1ꢀ)
Cu–O(1)–Cuꢀ
80.48(8)
169.98(8)
92.04(8)
90.16(7)
97.85(9)
171.95(7)
97.32(8)
90.03(8)
99.85(7)
79.60(7)
100.40(7)
Cu–N(2)
Cu–N(3)
Cu–O(1)
Cu–O(1ꢀ)
Cu–Cuꢀ (acetate-bridged)
Cu–Cuꢀ (pyrazolate-bridged)
3
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(2)–N(5)
Cu(2)–N(6)
Cu(2)–O(1)
Cu(2)–O(2)
Cu(1)–Cu(2)
1.999(3)
2.024(3)
1.915(2)
1.931(2)
2.018(3)
1.992(3)
1.923(2)
1.933(2)
2.8474(9)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(2)
N(2)–Cu(1)–O(1)
N(2)–Cu(1)–O(2)
N(5)–Cu(2)–N(6)
N(5)–Cu(2)–O(1)
N(5)–Cu(2)–O(2)
N(6)–Cu(2)–O(1)
N(6)–Cu(2)–O(2)
O(1)–Cu(1)–O(2)
O(1)–Cu(2)–O(2)
Cu(1)–O(1)–Cu(2)
Cu(1)–O(2)–Cu(2)
80.75(12)
174.24(12)
95.58(11)
99.67(11)
171.77(11)
80.28(13)
100.60(10)
173.62(11)
169.56(12)
95.44(11)
84.73(10)
84.47(10)
95.78(11)
94.95(10)
4
Cu(1)–N(1)
2.037(8)
2.028(11)
2.183(7)
1.939(7)
1.946(8)
2.017(11)
2.010(8)
1.980(7)
2.135(8)
1.941(8)
3.0988(26)
2.001(19)
2.075(16)
2.34(3)
1.919(12)
1.960(8)
2.010(10)
2.018(11)
1.932(9)
2.176(8)
1.946(9)
3.0792(29)
N(1)–Cu(1)–N(2)
80.1(4)
98.7(3)
170.8(3)
92.0(3)
96.9(4)
95.2(4)
169.7(3)
89.7(3)
90.7(3)
91.6(3)
79.9(4)
92.6(3)
96.4(4)
168.7(4)
165.2(4)
100.9(3)
95.7(3)
92.5(3)
89.3(3)
94.6(3)
77.8(8)
93.1(6)
165.4(4)
92.4(5)
98.4(5)
96.3(7)
168.3(7)
101.0(5)
88.5(4)
91.6(4)
92.2(4)
90.4(4)
80.1(4)
93.3(4)
99.0(3)
167.0(4)
171.7(4)
93.8(4)
95.1(4)
92.2(3)
90.4(3)
93.4(3)
Cu(1)–N(2)
N(1)–Cu(1)–O(1)
Cu(1)–O(1)
N(1)–Cu(1)–O(2)
Cu(1)–O(2)
N(1)–Cu(1)–O(6)
Cu(1)–O(6)
N(2)–Cu(1)–O(1)
Cu(2)–N(4)
N(2)–Cu(1)–O(2)
Cu(2)–N(5)
N(2)–Cu(1)–O(6)
Cu(2)–O(3)
O(1)–Cu(1)–O(2)
Cu(2)–O(4)
O(1)–Cu(1)–O(6)
Cu(2)–O(5)
O(2)–Cu(1)–O(6)
Cu(1)–Cu(2)
Cu(1A)–N(1A)
Cu(1A)–N(2A)
Cu(1A)–O(1A)
Cu(1A)–O(2A)
Cu(1A)–O(6A)
Cu(2A)–N(4A)
Cu(2A)–N(5A)
Cu(2A)–O(3A)
Cu(2A)–O(4A)
Cu(2A)–O(5A)
Cu(1A)–Cu(2A)
N(4)–Cu(2)–N(5)
N(4)–Cu(2)–O(3)
N(4)–Cu(2)–O(4)
N(4)–Cu(2)–O(5)
N(5)–Cu(2)–O(3)
N(5)–Cu(2)–O(4)
N(5)–Cu(2)–O(5)
O(3)–Cu(2)–O(4)
O(3)–Cu(2)–O(5)
O(4)–Cu(2)–O(5)
N(1A)–Cu(1A)–N(2A)
N(1A)–Cu(1A)–O(1A)
N(1A)–Cu(1A)–O(2A)
N(1A)–Cu(1A)–O(6A)
N(2A)–Cu(1A)–O(1A)
N(2A)–Cu(1A)–O(2A)
N(2A)–Cu(1A)–O(6A)
O(1A)–Cu(1A)–O(2A)
O(1A)–Cu(1A)–O(6A)
O(2A)–Cu(1A)–O(6A)
O(3A)–Cu(2A)–O(4A)
O(3A)–Cu(2A)–O(5A)
N(4A)–Cu(2A)–N(5A)
N(4A)–Cu(2A)–O(3A)
N(4A)–Cu(2A)–O(4A)
N(4A)–Cu(2A)–O(5A)
N(5A)–Cu(2A)–O(3A)
N(5A)–Cu(2A)–O(4A)
N(5A)–Cu(2A)–O(5A)
O(3A)–Cu(2A)–O(4A)
O(3A)–Cu(2A)–O(5A)
O(4A)–Cu(2A)–O(5A)
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Crystal structure of 2
coupling between the metal centres1a,30a,b,33 and accordingly the
temperature-dependent (54–300 K) magnetic measurements on
powdered samples of 2 were performed. The effective magnetic
moment per copper (leff/Cu) at 300 K was found to be 1.69
lB. A plot of vMT versus T for 2 (Fig. 3a) is typical of a
moderate antiferromagnetically coupled dicopper(II) complex.
The experimentally observed vM values (per dimer) were fitted to
the Bleaney–Bowers equation,34 suitably modified,6c,d,g,12,35 based
A perspective view of the compound, with atom-labeling scheme
is shown in Fig. 2. Table 2 contains the pertinent bond distances
and bond angles. A part of the molecule is related to the other by
a crystallographically imposed inversion centre. X-Ray analysis
revealed three noteworthy features: (i) it is a polymeric copper(II)
compound, (ii) debenzylation of the ligand L2 has occurred, under
our reaction conditions, and the 3-(2-pyridyl)pyrazolate ion (HL1)
thus formed acts as a bridging unit, in its deprotonated form,
holding two metal centres25b,28 and (iii) two kinds of bridge (acetate
and pyrazolate) are present in the binuclear unit. Each Cu(II) ion
is in a slightly distorted square pyramidal geometry (s = 0.033)29
with coordination by two bridging pyrazolate nitrogen atoms N(2)
and N(3), a pyridyl N(1) from a pyridylpyrazole unit, and an
acetate coordination O(1) in the equatorial plane. An additional
acetate, symmetry related to O(1), which is in a monodentate
bridging mode,30 provides axial coordination. It is a dinuclear
ˆ
on the Heisenberg–Dirac–van Vleck, spin Hamiltonian (H =
−2JS1·S2, in which the exchange parameter J is negative for
antiferromagnetic and positive for ferromagnetic interaction),
eqn (1), allowing for the presence of
vM = 2Ng2b2/3k(T − h)[1 + 1/3 exp(−2J/kT)]−1(1 − q)
+ (N g2b2/2kT)q + 2Na
(1)
monomeric copper(II) impurity behaving as a Curie paramagnet,
where q is the mole fraction of such an impurity, where Na is the
temperature-independent paramagnetism. A corrective term (h)
was incorporated for interdimer interactions.35 The singlet–triplet
energy gap is expressed in terms of 2J and other symbols have
their usual meanings. Keeping Na fixed at 60 × 10−6 cm3 mol−1, J,
copper(II) complex in which a near-planar {Cu2(l-L1)2} core
2+
is present, as observed before.25b,31 However, it has additional
acetate bridging as well. Notably, the Cu–O–Cuꢀ bridges are
ꢀ
˚
dissimilar: Cu–O(1) 1.9625(19◦) and Cu–O(1) 2.3737(17) A; the
g, and h parameters were determined by minimizing R = R(vmobs
−
Cu–O(1)–Cuꢀ angle 100.40(7) (Table 2). As in 1 the Cu–Npz
bond distance is shorter than that of the Cu–Npy distance. This
must be due to the stereo-electronic requirement of the copper(II)
vcmalc)2/R(vmobs)2 using eqn (1), which gave good data fits. As shown by
30a,b
˚
ion. The Cu · · · Cu separations are 3.342 A (acetate bridged)
31
˚
and 3.923 A (pyrazolate bridged), comparable to that reported
in the literature. The observed Cu–O(bridging acetate)30a,b and
Cu–N(bridging pyrazolate)31 distances are comparable to that
reported for complexes with similar structural motif. Although
2+
the acetato-bridged {Cu2(l-O2CMe)2} and pyrazolato-bridged
2+
{Cu2(l-C3N2)2} dimeric units are each planar; they make an
angle of 83.055(36)◦ with respect to each other. The Cu–Npy and
Cu–Npz bond lengths are normal.1a,13,32
Fig. 2 View of the structure of [(l-L1)Cu(l-O2CMe)]n (2). Hydrogen
atoms are omitted for clarity. Unlabeled atoms are related to labeled atoms
by an inversion centre. Symmetry operations: (1) −x, −y, −z + 1; (2) −x,
−y + 1, −z + 1.
Magnetism of 2
The magnetic behaviour of compound 2 is clearly of interest owing
to the status of this complex as an example of the presence of both
acetate- and pyrazolate-bridging between the copper(II) centers
with distorted square pyramidal geometry. Such complexes are
well known to exhibit antiferromagnetic/ferromagnetic exchange
Fig. 3 Plot of vMT vs. T (᭹) for powdered samples of (a) [{(L1)Cu(O2-
CMe)}n] (2) and (b) [{(L2)Cu(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H
(4·dmf·0.5MeCO2H). The solid lines represent the best simulated fit using
the equation described in the text.
1616 | Dalton Trans., 2006, 1611–1621
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the trace in Fig. 3a, an excellent simulation (non-linear regression
analyses)6c,d,g of the data could be attained with the following
parameters: J = −79 cm−1, g = 2.15, q = 0.006, h = −0.18 K
and R = 2.74 × 10−7.
Given the reported results on the acetate-bridged (single-atom
bridge through one of the oxygen atoms of an acetate)1a,30a,b
and pyrazolate-bridged1a,33 compounds the magnitude of the
exchange interaction deserves a special attention. It has been
well documented that in pyrazolate-bridged complexes (the pri-
mary pathway for the magnetic interaction must be through
pyrazolate-bridges) the extent of antiferromagnetic coupling is
larger when the coordination of the copper(II) ions is planar or
square pyramidal. On the contrary, for the copper(II) compounds
having acetate bridging mode as in 2 the magnetic exchange
coupling is expected to be antiferromagnetic. In 2, the Cu–
O–Cuꢀ bridge angle of 100.40(7)◦ corresponds to an antifer-
romagnetic interaction (see below). The angle of 83.055(36)◦
Fig. 4 EPR spectrum of [{(L2)Cu(l-OH)(OClO3)}2] (3) (solid) at 120 K.
bipyridine39a and copper(II) cyanoacetate39b analysis and using the
equations of Wasserman et al.40 with the above approximations,
our experimental spectra may be interpreted as follows. The
major band at 7150 G is assigned to the high-field perpendicular
transition. The band at 1600 G results from either the DMs =
2 or the low-field parallel transition. In order to approximate the
value of |D|, we have assumed, following Barnes et al.,39a g⊥ to
equal 2.05 and 2.10 (temperature-dependent magnetic data fitting
analysis of 3 gave a g = 2.12, see below) and then have used these
values of g⊥ with the known value of H⊥, to calculate |D|. We
determine a relatively large zero-field splitting parameter of D =
1.150 (g⊥ = 2.05) or 1.222 (g⊥ = 2.10). The corresponding values
for the bipyridine system were D = 0.761 (g⊥ = 2.05) or 0.813
(g⊥ = 2.10).39a
2+
between acetate-bridged {Cu2(l-O2CMe)2} and pyrazolate-
2+
bridged {Cu2(l-C3N2)2} dimeric units is expected to reduce the
extent of antiferromagnetism.1a,33
Reaction of 1 with dioxygen: synthesis and properties of
[{(L2)Cu(l-OH)(OClO3)}2] (3)
To provide a rationale for the synthesis of 2 we set out for
isolation of (i) a hydroxo-bound copper(II) compound from the
reaction between 1 and dioxygen and (ii) a discrete diacetato-
bridged copper(II) compound. Thus, exposure of a MeCN solution
of 1 to dioxygen generated a green solution. Slow evaporation
of this solution, after discarding an insoluble yellowish green
polymeric material, in the presence of Et2O led to the isolation
of blue crystals of [{(L2)Cu(l-OH)(OClO3)}2] (3), within 2–3 days
in ∼43% yield (Scheme 1). Microanalytical data (Experimental)
conform to the above formulation. X-ray structural analysis (see
below) authenticated the structure and composition of 3.
Crystal structure of 3
A perspective view of 3, with atom-labeling scheme is shown in
Fig. 5. Table 2 contains the essential bond distances and bond
angles. X-Ray crystallography established that it has the binuclear
structure. The geometry at each copper(II) centre is best described
as grossly square planar, with two oxygen atoms of the bridging
hydroxo groups and two nitrogen atoms of the bidentate ligand L2.
The IR spectra of 3 displays a sharp band at 3555 cm−1,
assignable to the stretching vibration of the hydroxo-bridge. For
3 the observed split absorption at 1106 cm−1 is suggestive of
weakly coordinated ClO4− ions36 (see below). The dark blue MeCN
solution of 3 exhibits a band at 648 nm associated with a d–d
transition of a square-based copper(II) centre.37 A strong transition
at 295 nm is also observed, which could be assigned to be due to
OH− → Cu(II) charge-transfer (ESI†).
The EPR spectrum for solid samples of 3 (Fig. 4) is of interest.
This spectrum is typical of a triplet state with an axial zero-
field splitting parameter D larger than the value of the incident
quantum.38,39 If the g and D tensors were coincident, their principal
values could be deduced from the resonant fields by Wasserman’s
equations.40 Notably, the EPR spectrum of 4 is very similar to that
for [{(bpy)CuII(l-OH)}2][SO4]·5H2O (bpy = 2,2ꢀ-bipyridine).39a To
interpret the EPR result we have made two assumptions: (i) the
zero-field splitting parameter E is so small as to be considered
equal to zero and (ii) the feature at about 3000 G resulted from
the monomeric paramagnetic impurity. Although we used very
pure samples but band-shape and temperature-dependent (300 K
and 120 K) studies do not conflict with this assignment. Barnes
et al.39a and Wasson et al.39b were able to support a similar
assignment of a similar band on the basis of the temperature
dependence of the peak intensities. With the aid of the copper(II)
Fig. 5 View of [{(L2)Cu(l-OH)}2]2+ in the structure of (3). Perchlorate
coordinations are not shown for clarity. Hydrogen atoms are omitted for
clarity.
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In addition to the four strong equatorial ligands, each copper(II)
ion is semi-coordinated13,41a to two oxygens of bridging perchlorate
ions [Cu(1)–O(3) 2.7187(31), Cu(1)–O(7) 2.6988(28), Cu(2)–O(5)
˚
2.5882(26), Cu(2)–O(9) 2.6175(27) A]. The axial sites are at an
˚
˚
average distance of 2.7088(30) A at Cu(1) and 2.6029(27) A
at Cu(2), thereby completing a distorted octahedron around
each copper(II) center. The pyridine and pyrazole rings of L2
are almost planar, the angles being 2.46(0.12)◦ and 5.87(0.13)◦
for two ligands. Notably, the Cu–OH distances [Cu(1)–O(1)
1.915(2), Cu(1)–O(2) 1.931(2), Cu(2)–O(1) 1.923(2), Cu(2)–O(2)
˚
1.933(2) A] and Cu–O(H)–Cu angles [Cu(1)–O(1)–Cu(2) 95.78(11)
and Cu(1)–O(2)–Cu(2) 94.95(10)◦] are asymmetric. The distorted
geometry at each copper(II) centre is revealed from the following
considerations as well. The dihedral angles between the planes
N(1)–Cu(1)–N(2) and O(1)–Cu(1)–O(2) and N(5)–Cu(2)–N(6)
and O(1)–Cu(2)–O(2) are 9.83(10) and 11.92(11)◦, respectively.
However, the dihedral angle between the planes O(1)–Cu(1)–
O(2) and O(1)–Cu(2)–O(2) is 2.41(10)◦, implying more or less
planar Cu2O2 core, presumably resulting mainly from the bridging
Fig.
6
Plot of vMT vs.
T
(᭹) for
a
powdered sample of
[{(L2)Cu(l-OH)(OClO3)}2] (3). The solid line represents the best simulated
fit using the equation described in the text.
˚
perchlorate anions. The Cu · · · Cu separation is 2.8474(9) A,
g = 2.12, q = 0.009, TIP (2Na= 120 × 10−6) (fixed), h = −0.20 K,
and R = 1.58 × 10−7. The magnitude of the exchange interaction
is similar to that observed before for similar compounds.38a,41b,d,e
comparable to that reported for a ferromagnetically coupled
system.41b The Cu–Npy and Cu–Npz distances are normal.1a,13,32
In contrast to that observed in the structures of 1 and 2 in the
present complex the average Cu–Npy distance is shorter than that
of the Cu–Npz distance. This is presumably as a consequence of
the ‘plasticity effect’ due to the Jahn–Teller active copper(II) ion.32b
The formation of di-l-hydroxo-bridged dicopper(II) compounds
due to the reaction between copper(I) compounds and dioxygen
is a well known phenomenon.1a,11–13,15 However, compound 3 joins
a family of di-l-hydroxo-bridged dicopper(II) compounds with
terminal bidentate N-coordination,5,15 synthesized following the
reaction conditions used in this work. Compound 3 represents,
to our knowledge, the first compound with such a core structure
having terminal bidentate pyrazole/pyridine coordination.
Reaction of 3 with ethyl acetate: synthesis and properties of
[{(L2)CuII(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H (4)
While an MeCN solution of 3 is blue, in dmf it generates a green
solution. To such dmf solutions addition of MeCO2Et followed
by warming (ca. 60 ◦C) for 4 h generated a bluish green solution.
Cooling (ca. 24 h) of such solutions afforded light bluish green
crystals of [{(L2)Cu(dmf)(l-O2CMe)}2][ClO4]2·dmf·0.5MeCO2H
(4), in ca. 57% yield (Scheme 1). X-Ray structural analysis of
4 (see below) authenticated the structure and composition of this
complex. Microanalytical and solution electrical conductivity data
(Experimental) conform to the above formulation.
The mas(COO) and ms(COO) bands of 4 are at 1580 and
1439 cm−1, respectively; the difference (140 cm−1) is as expected for
this chelated bridging mode of carboxylate coordination.26 Light
bluish green MeCN solutions of 4 displays a broad band at 718 nm
due to a d–d transition with a shoulder at lower energy 964 nm
(ESI†) indicating square-pyramidal geometry around copper(II).37
The EPR spectrum of 4 is essentially unmodified between 120 K
and 300 K. It is characteristic of a noncoupled copper(II) ion in a
rhombic environment with three principal g values of 2.26, 2.13,
and 2.03 (ESI†).35,39
Magnetism of 3
The molecular structure of 3 meets the requirements (average
Cu–O(H)–Cu angle: 95.37◦) of it to exhibit a ferromagnetic
behaviour. According to Hodgson and Hatfield correlation38a
for square-based di-l-hydroxo-bridged dicopper(II) compounds41
when the Cu–O–Cu angle is larger than 97.55◦, the overall
magnetic behavior is antiferromagnetic and for smaller values a
ferromagnetic coupling is observed. At a Cu–O–Cu angle of 97.55◦
the singlet–triplet splitting energy (2J) would amount to zero. The
magnetic susceptibility of the complex 3, measured in the range
5 < T < 300 K, is depicted in Fig. 6.
Crystal structure of 4
The magnetic behaviour of the complex is depicted in Fig. 6
in the form of vMT versus T, and shows a medium ferromagnetic
interaction. At 300 K the vMT starts at a value of 0.95 cm3 mol−1
(lCu = 1.95 lB) and increases to 0.97 cm3 mol−1 (lCu = 1.97 lB)
at about 200 K and to 100 K at 1.03 cm3 mol−1 (lCu = 2.03
lB) and maximizes to 1.11 cm3 mol−1 (lCu = 2.11 lB) at about
30 K. Below this temperature, the vMT values diminish to a value
of 1.08 cm3 mol−1 (lCu = 2.08 lB), which may originate from inter-
molecular antiferromagnetic interactions, or from zero-field split-
ting of the S = 1 state of the dinuclear species.13,14b,35,38a As shown
by the trace in Fig. 6, an excellent simulation of the data could be
attained with eqn (1) with the following parameters: J = 41 cm−1,
In order to investigate whether or not Cu(II)-coordinated hydroxo
groups in 3 are nucleophilic enough to bring about hydrolysis of
MeCO2Et the X-ray structure determination of light bluish green
solid 4 was undertaken. The elucidation of the crystal structure
of 4 confirms that indeed hydrolysis of MeCO2Et has occurred.
The structure of 4 reveals that it is a discrete molecule consisting
of a dinuclear [{(L2)Cu(dmf)(l-O2CMe)}2]2+ cation (Fig. 7), two
well-separated perchlorate anions, a dmf molecule, and half
a molecule of MeCO2H. The asymmetric unit contains two
crystallographically independent molecules. Both dinuclear l-1,3-
acetato-bridged copper(II) complexes have essentially identical
1618 | Dalton Trans., 2006, 1611–1621
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Magnetism of 4
Variable-temperature magnetic susceptibility data were collected
on powdered samples of complex 4 in the temperature range 60–
300 K. The leff/Cu decreases gradually from 1.82 lB at 300 K
to a minimum of 1.08 lB at 60 K. A plot of vMT versus T
for 4 (Fig. 3b) is typical of a moderate antiferromagnetically
coupled dicopper(II) complex. Ignoring h dependence and keeping
g and 2Na fixed at 2.1 [typical value for a tetragonal Cu(II)] and
120 × 10−6 cm3 mol−1, the J and parameters were determined
by minimizing R = R(vombs − vmcalc)2/R(vombs)2 using eqn (1), which
gave good data fits. An excellent simulation of the data could be
attained with the following parameters: J = −48 cm−1 and q =
0.016, R = 6.81 × 10−7 (Fig. 3b).
The synthetic reactions, debenzylation of L2 and ethyl acetate
hydrolysis
Fig. 7 View of [{(L2)Cu(dmf)(l-O2CMe)}2]2+ in the structure of (4).
Hydrogen atoms are omitted for clarity.
Given the successful synthesis of compounds 2, 3 and 4 (Scheme 1)
an attempt has been made here (i) to clarify the synthetic
reactions—mainly their stoichiometry, (ii) to provide a possible
cause of debenzylation of L2 and (iii) to provide a possible
mechanism for the observed ethyl acetate hydrolysis.
coordination geometries, but the corresponding bond lengths and
bond angles are different, to an appreciable extent (Table 2). Each
copper(II) ion is coordinated by the ligand L2 and a dmf molecule,
and two copper(II) centers are bridged by two acetate groups.
The cation represents the first example, to our knowledge, of a
dinuclear copper(II) complex supported terminally by a bidentate
N-donor ligand and bridged solely by two l-1,3-acetate groups.
It is to be noted that the acetate bridging mode present in 2
and 4 is different; 2 has comparatively rare monatomic bridging
mode and 4 has the familiar bidentate g :g :l2 bridging mode.
Notably, out of two bridging acetate groups one is coordinated in
a more asymmetric manner (Table 2). The coordination by dmf
molecules is also asymmetric. As a consequence, the coordination
geometries of the two copper(II) ions and the Cu · · · Cu distances
in each dinuclear unit are different. Based on the structural index
parameter s, the geometry at Cu(1) (s = 0.02)/Cu(1A) (s = 0.06)
and Cu(2) (s = 0.05)/Cu(2A) (s = 0.08) is best described as a
slightly distorted square pyramidal,29 with dmf oxygens O(1)/O(4)
and O(1A)/O(4A) at the apex. In line with the structures of 1 and
2 in the present complex the average Cu–Npz distance is shorter
than that of the Cu–Npy distance.
It is believed that the dioxygen reaction with mononuclear Cu(I)
complexes proceeds generally in steps leading to a 2e− reduced
2+
peroxo intermediate {CuII–(O2)–Cu} [or its isomeric bis-oxo
2+
species {CuIII–(O)2–CuIII} ].4,5 The peroxide disproportionation,
equivalent to the further reaction of {CuII–(O2)–CuII}2+ with Cu(I),
leading to an overall reaction stoichiometry of Cu/O2 = 4 : 1 is a
possibility. Thus, the most common reaction of Cu(I) complexes
with O2 is this 4e− reduction and O–O cleavage reaction to give
oxo “CuII2O” species, which after reaction with H2O eventually
generates bis-hydroxy “CuII2(OH)2” products.11 In the absence
of definitive kinetic/mechanistic information it appears that the
reaction stoichiometry for the formation of 3 follows a similar
behaviour (Scheme 2). It should be noted, however, that the origin
of the hydroxyl proton was not determined (residual water present
in MeCN is a definite possibility).
1
1
The following reaction sequence for the synthesis of 4 from
3 firmly establishes that the degradation of ethyl acetate is due
to Cu(II)-coordinated hydroxide ion and not by any reactive
−
In contrast to 3 having bis-hydroxo bridge between two
Cu(II)-coordinated peroxide species. The loosely bound ClO4
˚
copper(II) centres [Cu · · · Cu distance: 2.8474(9) A], the bis-l-
ions in 3 are replaced easily by coordinating solvent dmf, making
the solvent exchangeable (with substrate) site(s) available. This
condition allows MeCO2Et to bind to the copper(II) centre(s).
Subsequent attack by the OH− ion (nucleophile) bound to Cu(II)
ion causes the hydrolysis of this unactivated ester. This justifies the
reaction stoichiometry for the formation of 4 from 3 (Scheme 2).
Although we do not have any proof for the formation of EtOH
1,3-acetato-bridged dicopper(II) complex induces much longer
˚
Cu · · · Cu distances of 3.0792(29) and 3.0988(26) A in 4. It is to
be noted that during the synthesis of 2 (dmf/MeCO2Et, 25 ◦C)
decomposition of L2 ◦occurred but during the synthesis of 4
(dmf/MeCO2Et, ∼60 C) the ligand L2 remained intact. We are
not in a position to provide an explanation for this behaviour.
Scheme 2
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(Scheme 2), it is reasonable to assume its formation given literature
reports (see below).17b
MeCO2Et is expected to have occurred, as is known from elegant
metalloenzyme hydrolytic model studies,9,10 including copper(II)
complexes.16,17
The synthesis of 2 (Method A, Experimental; Scheme 1) with
concomitant debenzylation of L2 and hydrolysis of ethyl acetate
deserves special attention. Due to the complexity of the reactions
involved and the results available to hand we are not in a position to
provide a stoichiometric reaction for the synthesis of 2 (Scheme 2).
However, the following reaction sequence provides a rationale for
the observed hydrolysis of ethyl acetate. Reaction of complex
1 with O2, the formation/decomposition of copper–dioxygen
intermediate, subsequent formation of bis-hydroxy species and
nucleophilic attack of Cu(II)-coordinated hydroxide ion to ethyl
acetate, generating acetate ion as in the formation of 4 from 3
(vide supra, Scheme 2) is a reasonable hypothesis.
A commonly observed result of decomposition of Cu/O2
complexes of tridentate N-donor ligands (2-pyridylalkylamine11b
or triazacyclononane4h) bearing a benzyl pendant arm is oxidative
N-debenzylation (intramolecular oxidation of C–H bonds that
are “activated” due to their position a to an amine donor and/or
a phenyl ring) to yield a secondary amine and an aldehyde or
ketone. Given the reaction conditions (oxygenation at 25 ◦C,
sluggish nature, low yield of 2; Experimental, Method A) adopted
in the synthesis of 2 we rule out the possibility that a thermally
unstable copper-bound peroxide intermediate, formed due to the
reaction of compound 1 and O2 in air, might have attacked the
benzylic position of L2 effecting debenzylation of L2.4h,11b The
debenzylation of L2 results in liberation of one equivalent of HL1
into the medium per molecule of L2 and deprotonated HL1 acts
as a bridging ligand, as observed in the polymeric compound
2. Solvolysis/hydrolysis reactions of 3-(2-pyridyl)pyrazole-based
polydentate ligands, including reactions of Cu(I) complexes and
O2, have literature precedence.25b,28b,42
Concluding remarks
In conclusion, we have succeeded in preparing a mononuclear
copper(I), a polymeric acetato-/pyrazolato-bridged copper(II)
and two discrete dihydroxo-/diacetatato-bridged copper(II) com-
plexes, of a new bidentate pyrazolyl-pyridine ligand L2, with
a range of structural varieties at our disposal. Under mild
conditions hydrolysis of the unactivated ester MeCO2Et has been
achieved. During the synthesis of polymeric Cu(II) compound,
debenzylation of L2 has also been observed. Complex 3 serves as
the key compound to bring about this unusual reaction and is
proposed to occur by an intramolecular hydrolysis mechanism.
Thus 3 can be regarded as a functional model for the hydrolytic
enzymes, pertinent to the assignment of possible mechanisms
for ester hydrolysis catalyzed by enzymes.8–10 We plan to probe
further the reactivity of 3 toward other substrates (however, it
does not react with phosphate esters) and mechanistic aspects
of the chemistry reported here. The three copper(II) complexes
provide a variable bridge which in turn led to a range of Cu · · · Cu
distances. Temperature-dependent magnetic studies have revealed
the existence of both ferromagnetic and antiferromagnetic ex-
change coupling in this family of dicopper(II) compounds. We
plan to carry out detailed temperature-dependent EPR studies on
the ferromagnetically coupled compound 2.
Acknowledgements
We thank the Council of Scientific & Industrial Research and the
Department of Science and Technology, New Delhi for financial
support. We sincerely thank Dr V. Balamurugan and Mr Akhilesh
K. Singh for their help in the preparation of this manuscript.
Comments of the reviewers were very helpful at the revision stage.
It is worth noting the findings of two reports pertinent to
this work. (i) Using solution-generated OH− bound mononuclear
copper(II) compounds Chin and co-workers reported efficient
hydrolysis of methyl acetate (pH 7.0; 25 ◦C), following kinetic
experiments.16 (ii) Karlin and co-workers reported17b hydrolysis of
MeCO2Et forming acetate ion, which ended up bridging two cop-
per(II) centres, by an alkoxo-bridged dicopper(II) compound with
a terminally coordinated hydroxide ion, suggested (mass spectral
proof) to be present in solution when structurally characterized
tetranuclear singly hydroxo-bridged copper(II) complex (dimer-of-
dimer type) was dissolved in MeCN. During MeCO2Et hydrolysis
the formation of acetic acid16b and other byproduct alcohol17b
were previously observed. Therefore, it is interesting to note the
presence of half a molecule of MeCO2H in the asymmetric unit of
4 (see above). The reactivity of a dihydroxo-bridged dicopper(II)
compound towards hydrolysis of MeCO2Et, to our knowledge,
is unprecedented. Although numerous structurally characterized
bis(l-hydroxo)dicopper(II) complexes terminally coordinated by
bidentate N-donor ligands are known,1a,11–13,15,38a,41 compound 3 is
unique in its reactivity behaviour. The results presented here show
that it is the subtle combination of the ligand electronic/steric
effect and the presence of suitable bridging ligands to hold two
metal ions in correct distance for substrate to get bound and
metal-bound hydroxide ion to act as the nucleophile to bring about
hydrolysis of the unactivated ester MeCO2Et. The intramolecular
attack by a hydroxide ion, coordinated to one copper(II) centre
and serving as the nucleophile, on the carbon atom of the
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