Triple bridged μ-phenoxo-bis(μ-carboxylate) and double bridged μ-phenoxo-μ1,1-azide/μ-methoxide dicopper(II) complexes: Syntheses, structures, magnetochemistry, spectroscopy and catecholase activity

Article abstract of DOI:10.1016/j.poly.2012.10.050

The work in this paper presents syntheses, characterization, crystal structures, catecholase activity, electrospray ionization mass spectroscopy (ESI-MS positive), and magnetic properties of seven triple bridged μ-phenoxo-bis(μ-carboxylate) dicopper(II) complexes [Cu^II ₂L(μ-HCOO)₂](ClO₄)·CH₃OH (1), [Cu^II₂L(μ-CH₃COO)₂](ClO ₄) (2), [Cu^II₂L(μ-CCl₃COO) ₂(H₂O)](ClO₄) (3), [Cu^II ₂L(μ-CH₃CH₂COO)₂](ClO ₄) (4), [Cu^II₂L(μ-CH₃CH ₂CH₂COO)₂](ClO₄) (5), [Cu ^II₂L(μ-PhCOO)₂](ClO₄) ·CH₃CN (6) and [Cu^II₂L(μ-o-ClPhCOO) ₂](ClO₄) (7), one double bridged μ-phenoxo-μ _1,1-azide system [Cu^II₂L(μ_1,1- N₃)(N₃)₂] (8) and one double bridged μ-phenoxo-μ-methoxide system [Cu^II₂L(μ-OCH ₃)(NCO)₂] (9), derived from a new dinucleating ligand HL, which is the 1:2 condensation product of 2,6-diformyl-4-ethylphenol and 3-dimethylaminopropylamine. The variable-temperature (300-2 K) magnetic studies reveal moderate or strong antiferromagnetic interaction in 1-9 with J values of -81.5, -64.8, -136.5, -66.9, -61.4, -49.1, -61.5, -186.3 and >-350 cm- ¹ for 1-9, respectively. The seven μ-phenoxo-bis(μ-carboxylate) dicopper(II) complexes 1-7 show catecholase activity with turnover (K _cat) numbers lying in the range 18.8-89.1 h^-1. The positive ions observed in the ESI-MS spectra of 1-8 are heterobridged dinuclear species having oxidized Cu^IICu^II or reduced Cu ^ICu^I or mixed-valence Cu^IICu^I metal centers having double/triple bridging moieties. A complex-substrate aggregate [Cu^II₂L(μ-3,5-DTBC^2-)]⁺ has been identified in the ESI-MS positive spectrum of the mixture of complex 4 and 3,5-DTBCH₂.

Full text of DOI:10.1016/j.poly.2012.10.050

Polyhedron 50 (2013) 270–282
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Triple bridged
l-phenoxo-bis(
l-carboxylate) and double bridged
l
-phenoxo- 1,1-azide/l
l
-methoxide dicopper(II) complexes: Syntheses, structures,
magnetochemistry, spectroscopy and catecholase activity
Sohini Sarkar a, Samit Majumder a, Sujit Sasmal a, Luca Carrella b, Eva Rentschler b,
,
⇑
Sasankasekhar Mohanta a,
⇑
a Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata 700 009, India
b Institut für Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg, Universität Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The work in this paper presents syntheses, characterization, crystal structures, catecholase activity, elec-
trospray ionization mass spectroscopy (ESI-MS positive), and magnetic properties of seven triple bridged
Received 10 August 2012
Accepted 23 October 2012
Available online 16 November 2012
l
-phenoxo-bis(
COO)2](ClO4) (2), [CuII2L(
CH2CH2COO)2](ClO4) (5), [CuII2L(
double bridged -phenoxo-
oxo-
-methoxide system [CuII2L(
l
-carboxylate) dicopper(II) complexes [CuII2L(
l
l
-HCOO)2](ClO4)ÁCH3OH (1), [CuII2L(
l
l
-CH3-
-CH3-
l
-CCl3COO)2(H2O)](ClO4) (3), [CuII2L(
-CH3CH2COO)2](ClO4) (4), [CuII2L(
l
-PhCOO)2](ClO4)ÁCH3CN (6) and [CuII2L(
l
-o-ClPhCOO)2](ClO4) (7), one
l
l l1,1-N3)(N3)2] (8) and one double bridged l-phen-
1,1-azide system [CuII2L(
Keywords:
Magnetic properties
Catecholase activity
ESI-MS positive spectra
Crystal structures
Heterobridged
l
l
-OCH3)(NCO)2] (9), derived from a new dinucleating ligand HL, which
is the 1:2 condensation product of 2,6-diformyl-4-ethylphenol and 3-dimethylaminopropylamine. The
variable-temperature (300–2 K) magnetic studies reveal moderate or strong antiferromagnetic interac-
tion in 1–9 with
J
values of À81.5, À64.8, À136.5, À66.9, À61.4, À49.1, À61.5, À186.3 and
1
>À350 cmÀ for 1–9, respectively. The seven
l-phenoxo-bis(l-carboxylate) dicopper(II) complexes 1–
7 show catecholase activity with turnover (Kcat) numbers lying in the range 18.8–89.1 hÀ1. The positive
ions observed in the ESI-MS spectra of 1–8 are heterobridged dinuclear species having oxidized CuIICuII or
reduced CuICuI or mixed-valence CuIICuI metal centers having double/triple bridging moieties. A com-
plex-substrate aggregate [CuII2L(
l
-3,5-DTBC2À)]+ has been identified in the ESI-MS positive spectrum
of the mixture of complex 4 and 3,5-DTBCH2.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
tarity or enhanced antiferromagnetic interaction due to orbital
complementarity [1,4–19]. Exceptional cases of showing ferromag-
netic interaction in spite of orbital countercomplementarity have
also been reported [5]. In contrast to the extensive studies on the
Dinuclear compounds having heterobridged moiety, such as l-
phenoxo/alkoxo/hydroxo-l-X moiety, are interesting in magneto-
chemistry because of the effect of one type of bridge on the mag-
netic exchange propagated through the second type of bridge
(X = carboxylate, azide, pyrazolate, nitrite, 7-azaindolate, etc.) [1-
19]. In such heterobridged dinuclear compounds of copper(II),
the symmetry of the HOMO of the second bridging ligand, X, and
the associated complementarity or countercomplementarity ef-
fects are the crucial factors to govern the nature and magnitude
of magnetic exchange [1,4–19]. Starting from the 80s, magnetic
studies have been carried out for a number of heterobridged
dicopper(II) compounds which exhibit reduced antiferromag-
netic/ferromagnetic interaction due to orbital countercomplemen-
double bridged
per(II) systems [1,6–13], magnetic properties of the triple bridged
-phenoxo/alkoxo/hydroxo-bis( -carboxylate) dicopper(II) sys-
tems have been much less investigated and the exploration of such
systems deserves more importance [14–19].
Catechol oxidase is an enzyme with the type-3 active site that
catalyzes the oxidation of a wide range of o-diphenols (catechols)
to the corresponding o-quinones coupled with 2e/2H+ reduction
of O2 to H2O, in a process known as catechol oxidase or catecholase
activity [20–40]. While the metallo-enzyme contains hydroxo-
bridged dicopper(II) moiety [20], activity has been observed for
dicopper(II) systems having hydroxo [20–22,28] or various other
bridging moieties [11,12,20–22,27,29,30,33–40]. Few monocop-
per(II) complexes are also known to exhibit the activity [20–26].
Recently, few copper(II) clusters and polymers [27] have also
been observed to show catecholase activity. Moreover, few
l-phenoxo/alkoxo/hydroxo-l-carboxylate dicop-
l
l
⇑
Corresponding authors. Tel.: +91 33 23512941; fax: +91 33 23519755 (S.
Mohanta).
E-mail addresses: rentschl@uni-mainz.de (E. Rentschler), sm_cu_chem@yahoo.
co.in (S. Mohanta).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.10.050

S. Sarkar et al. / Polyhedron 50 (2013) 270–282
271
diformyl-4-methylphenol [43]. Elemental (C, H, and N) analyses
were performed on a Perkin–Elmer 2400 II analyzer. IR spectra
were recorded in the region 400–4000 cmÀ1 on Bruker-Optics Al-
pha-T spectrophotometer with samples as KBr disks. Variable-
temperature (2–300 K) magnetic susceptibility measurements
were carried out at 1 T magnetic field with a Quantum Design
MPMS-XL 7 SQUID magnetometer. Diamagnetic corrections were
estimated from the Pascal constants. The diamagnetic contribution
from the holder was determined experimentally and substituted
from the data. Electronic spectroscopic measurements and Cat-
echolase activities were performed with a Shimadzu UV-3600
spectrophotometer. The electrospray ionization mass spectra of
1–8 were recorded on a Micromass Qtof YA 263 mass spectrome-
ter. Molar conductivity (KM) of 1 mM solution in DMF was mea-
sured at 25 °C with a Systronics conductivity bridge.
N
N
OH
N
N
2.2. Syntheses
Scheme 1. Chemical structure of HL.
2.2.1. Synthesis of 2,6-bis[3-(dimethylamino)propyliminomethyl]-4-
ethyl-phenol (HL) solution in methanol
3-Dimethylaminopropylamine (2.04 g, 20 mmol) was dissolved
in 20 mL methanol and was added dropwise into a 30 mL metha-
nolic suspension of 2,6-diformyl-4-ethylphenol (1.78 g, 10 mmol)
under warming condition. Then the total reaction mixture was re-
fluxed for 2–3 h. After cooling, the volume of the solution was ad-
justed to 100 mL. This ‘solution of HL’ was considered to contain
10 mmol of the ligand HL and was used for subsequent reactions
without further purification.
structure–property correlations that have been determined are not
sufficiently wide but only applicable for a set of few compounds
[12,20–32,34–37]. All in all, the problem of modeling the catecho-
lase activity remains still open and therefore catecholase activity of
new systems should be explored.
Few double bridged
l-phenoxo-l-carboxylate dicopper(II)
[11,12,38–40] and very few
l
-phenoxo-bis(
l
-carboxylate) dicop-
per(II)[30,33] compounds have been reported to show catecholase
activity. As already mentioned, such systems are important to ex-
plore their magnetic properties also. To explore the magnetic prop-
erties and catecholase activity of the heterobridged dicopper(II)
systems having both phenoxo bridge and carboxylate bridge (s),
2.2.2. [CuII2L(
l-HCOO)2](ClO4)ÁCH3OH (1)
Five milliliters of ‘solution of HL’ containing 0.5 mmol ligand
was taken and diluted to 20 mL. Ten milliliters methanolic solution
of Cu(ClO4)2Á6H2O (0.371 g, 1 mmol) was added to it and the
resulting green solution was stirred for 1 h. Then an aqueous solu-
tion of HCOONa (0.068 g, 1 mmol) was added dropwise. Immedi-
ately after addition the green colored solution became dark. After
half an hour a green colored shiny precipitate started to deposit.
After 2 h stirring the reaction mixture was heated gently on water
bath to dissolve the precipitate, the solution was cooled, filtered to
remove any suspended particles, and kept at room temperature for
slow evaporation. After one day a green colored crystalline com-
pound containing diffraction quality single crystals deposited
which were collected by filtration and washed with 1:1 metha-
we have synthesized
a new dinucleating acyclic ligand HL
(Scheme 1), which is the 1:2 condensation product of 2,6-difor-
myl-4-ethylphenol and 3-dimethylaminopropylamine, and
utilized this ligand to derive the following seven triple bridged
l
-phenoxo-bis(
a number of carboxylates: [CuII2L(
[CuII2L( -CH3COO)2](ClO4) (2), [CuII2L(
(3), [CuII2L(
COO)2](ClO4) (5), [CuII2L(
L(
l
-carboxylate) dicopper(II) complexes containing
-HCOO)2](ClO4)ÁCH3OH (1),
-CCl3COO)2(H2O)](ClO4)
l
l
l
l
-CH3CH2COO)2](ClO4) (4), [CuII2L(
l-CH3CH2CH2-
l
-PhCOO)2](ClO4)ÁCH3CN (6) and [CuII
2-
l
-o-ClPhCOO)2](ClO4) (7). Further, with the aims to enlighten
on the magnetic properties and to explore the possibility of cat-
echolase activity of heterobridged dicopper(II) systems containing
both the phenoxo and azide/cyanate bridges [12,27,41,42], we
have used azide/cyanate instead of carboxylates and obtained
nol–water mixture. Yield: 0.27 g (80%).
(689.07): Anal. Calc. C, 40.09; H, 4.97; N, 8.13. Found: C, 40.30; H,
5.10; N, 8.25%. FTIR (KBr, cmÀ1): 1640s [(
C@N)], 1565 m
[( asCO2)], 1412 m [( sCO2)], 1090vs, 624w [( ClO4)].
C23H34N4O10ClCu2
m
one
one
l
-phenoxo-
l
1,1-azide system [CuII2L(
-methoxide system [CuII2L(
l
l
1,1-N3)(N3)2] (8) and
-OCH3)(NCO)2] (9).
m
m
m
l-phenoxo-
l
2.2.3. [CuII2L(
l-CH3COO)2](ClO4) (2)
Herein, we report the syntheses, crystal structures, catecholase
activity, electrospray ionization mass (ESI-MS positive) spectral
study and magnetic properties of these nine dicopper(II) com-
plexes 1–9.
The compound was synthesized by the similar procedure as
used for 1, except adding CH3COONa instead of HCOONa. Color:
green. Yield: 0.28 g (82%). C24H39N4O9ClCu2 (690.12): Anal. Calc.
C, 41.77; H, 5.70; N, 8.12. Found: C, 41.89; H, 5.55; N, 8.25%. FTIR
(KBr, cmÀ1): 1642s [(
1097vs, 622w [( ClO4)].
mC@N)], 1563 m [(masCO2)], 1408 m [(msCO2)],
2. Experimental
m
Caution! Perchlorate and azide complexes of metal ions are
potentially explosive. Only a small amount of material should be
prepared, and it should be handled with caution.
2.2.4. [CuII2L(
l-CCl3COO)2(H2O)](ClO4) (3)
The compound was synthesized by the similar procedure as
used for 1, except adding 1:1 mixture of CCl3COOH and triethyl
amine instead of HCOONa. Color: green. Yield: 0.36 g (80%).
2.1. Materials and physical methods
C
24H33N4O10Cl7Cu2 (912.77) Anal. Calc. C, 31.58; H, 3.64; N, 6.14.
Found: C, 31.37; H, 3.45; N, 6.30%. FTIR (KBr, cmÀ1): 3443w
[( H2O)], 1639s [( C@N)], 1679s, [( asCO2)], 1450 m [( sCO2)],
1093vs, 623w [( ClO4)].
All the reagents and solvents were purchased from commercial
sources and used as received. 2,6-Diformyl-4-ethylphenol was pre-
pared according to the procedure reported for the synthesis of 2,6-
m
m
m
m
m

Table 1
Crystallographic data for 1–9.
1
2
3
4
5
6
7
8
9
Empirical Formula
C
23H34N4O10ClCu2
C24H39N4O9ClCu2
690.12
triclinic
C24H33N4O10Cl7Cu2
912.77
monoclinic
C2/c
28.129(7)
11.269(3)
24.007(6)
103.777(3)
7391(3)
C26H43N4O9 ClCu2
718.17
triclinic
C28H47N4O9ClCu2
746.23
triclinic
C36H46N5O9ClCu2
855.31
triclinic
C34H41N4O9Cl3Cu2
883.14
monoclinic
P21/c
11.058(2)
24.171(5)
14.556(3)
98.889(6)
3843.8(13)
4
C20H33N13OCu2
598.67
monoclinic
P21/c
12.021(2)
13.624(2)
16.712(3)
106.566(7)
2623.4(7)
4
C23H33N6O4Cu2
584.63
monoclinic
P21/m
8.1964(4)
17.9608(9)
9.1805(5)
98.338(2)
1337.21(12)
2
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
689.07
monoclinic
P21/c
7.6187(5)
24.0817(15)
16.2327(10)
91.622(2)
2977.0(3)
4
1.537
1.576
296(2)
1420
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
10.9243(4)
11.6010(4)
12.8277(4)
75.8560(10)
1533.27(9)
2
1.495
1.527
296(2)
716
10.6530(4)
12.4065(4)
12.6596(4)
80.2270(10)
1628.83(10)
2
1.464
1.441
296(2)
748
10.7597(19)
12.339(2)
13.936(3)
75.910(2)
1766.8(5)
2
1.403
1.331
296(2)
780
10.754(2)
11.760(3)
16.691(4)
86.096(3)
2004.5(7)
2
1.417
1.184
296(2)
888
b (°)
V (Å3)
Z
8
Dcalc (g cm À 3
)
1.641
1.711
296(2)
3696
1.526
1.372
296(2)
1816
1.516
1.662
296(2)
1240
1.452
1.629
296(2)
606
Absorption coefficient (mmÀ1
Temperature (K)
F(000)
)
2h (°)
Absorption correction
Index ranges
3.02–53.44
multi-scan
À9 6 h 6 9
À28 6 k 6 30
À20 6 l 6 20
37467
3.34–58.00
multi-scan
À14 6 h 6 14
À14 6 k 6 15
À17 6 l 6 16
20643
2.98–49.32
multi-scan
À32 6 h 6 32
À13 6 k 6 12
À28 6 l 6 28
24018
3.28–58.00
multi-scan
À12 6 h 6 14
À16 6 k 6 15
À17 6 l 6 17
26777
3.02–49.50
multi-scan
À11 6 h 6 12
À14 6 k 6 13
À16 6 l 6 16
12188
2.54–51.94
multi-scan
À12 6 h 6 13
À14 6 k 6 14
À18 6 l 6 20
14691
3.30–54.04
multi-scan
À14 6 h 6 13
À30 6 k 6 28
À18 6 l 6 18
48762
3.54–70.00
multi-scan
À19 6 h 6 19
À21 6 k 6 21
À23 6 l 6 26
42898
4.48–60.94
multi-scan
À10 6 h 6 11
À22 6 k 6 25
À13 6 l 6 13
22957
Reflections collected
Independent reflections (Rint
)
6272/0.0353
0.0516/0.1801
0.0645/0.1944
7943/0.0269
0.0451/0.1417
0.0678/0.1602
6195/0.0933
0.0690/0.1784
0.1177/0.2017
8460/0.0238
0.0403/0.1435
0.0512/0.1541
5964/0.0288
0.0512/0.1530
0.0767/0.1719
7478/0.0311
0.0475/0.1340
0.0752/0.1564
8341/0.0368
0.0479/0.1717
0.0588/0.1834
11240/.0288
0.0361/0.1180
0.0620/0.1425
4118/0.0298
0.0385/0.1308
0.0554/0.1435
b
R1a/wR2 (for I > 2
r
(I))
b
2
R1a/wR2 (for all F0
)
P
P
a
R1 = [ ||F0| À |Fc||/ |F0|].
P
P
b
2
wR2 = [ w(F0 À Fc2)2/ w(F02)2]1/2
.

S. Sarkar et al. / Polyhedron 50 (2013) 270–282
273
2.2.5. [CuII2L(
l-CH3CH2COO)2](ClO4) (4)
Yield: 0.24 g (82%). C20H33N13OCu2 (598.67) Anal. Calc. C, 40.13; H,
Five milliliters of ‘solution of HL’ containing 0.5 mmol ligand
was taken and diluted to 20 mL. Ten milliliters methanolic solution
of Cu(ClO4)2Á6H2O (0.371 g, 1 mmol) was added to it and the
resulting green solution was stirred for 1 h. Then an aqueous solu-
tion of CH3CH2COONa (0.096 g, 1 mmol) was added dropwise.
Immediately after addition, a green colored shiny precipitate began
to deposit. The stirring was continued for 2 h. Then with careful
addition of requisite amount of acetonitrile the reaction mixture
was heated gently on water bath to dissolve the precipitate. After
cooling, the green colored solution was filtered to remove any sus-
pended particles and kept at room temperature for slow evapora-
tion. After a few hours a green colored crystalline compound
containing diffraction quality single crystals began to deposit,
which was collected by filtration and washed with 1:1 metha-
nol–water mixture. Yield: 0.29 g (80%). C26H43N4O9ClCu2 (718.17)
Anal. Calc. C, 43.48; H, 6.03; N, 7.80. Found: C, 43.30; H, 6.18; N,
5.55; N, 30.42. Found: C, 40.30; H, 5.30; N, 30.56%. FTIR (KBr,
cmÀ1): 1643s [(
m
C@N)], 2033vs, 2090s [(
mN3)].
2.2.10. [CuII2L(
l-OCH3)(NCO)2] (9)
The compound was synthesized by the similar procedure as
used for 1, except adding Cu(NO3)2Á6H2O and NaOCN instead of
Cu(ClO4)2Á6H2O and HCOONa. Color: green. Yield: 0.24 g (82%). C23-
H33N6O4Cu2 (584.63) Anal. Calc. C, 47.25; H, 5.69; N, 14.37. Found:
C, 47.07; H, 5.48; N, 14.50%; FTIR (KBr, cmÀ1): 1643s [(
mC@N)],
2222vs [(
mNCO)].
2.3. Crystal structure determination of 1–9
The crystallographic data for the nine compounds are summa-
rized in Table 1. Diffraction data of 1–9 were collected on a
Bruker-APEX II SMART CCD diffractometer at 296 K using graph-
7.94%. FTIR (KBr, cmÀ1): 1642s [(
1414 m [( sCO2)], 1097vs, 622w [( ClO4)].
mC@N)], 1566 m [(masCO2)],
ite-monochromated Mo K
processing and absorption correction the packages SAINT [44] and
ADABS [45] were used. The structures were solved by direct and
a radiation (k = 0.71073 Å). For data
m
m
S
2.2.6. [CuII2L(
l-CH3CH2CH2COO)2](ClO4) (5)
Fourier methods and refined by full-matrix least-squares based
Five milliliters of ‘solution of HL’ containing 0.5 mmol ligand was
taken and diluted to 20 mL. Ten milliliters methanolic solution of
Cu(ClO4)2Á6H2O (0.371 g, 1 mmol) was added to it under warming
condition and the resulting green colored solution was refluxed
for 1 h. Then an aqueous solution of CH3CH2CH2COONa (0.220 g,
2 mmol) was added dropwise into it and the overall reaction mix-
ture was refluxed for another 2 h. After cooling, the solution was
concentrated on rotary evaporator and a green colored amorphous
solid was obtained. Recrystallization was performed by slow diffu-
sion of diethyl ether into the 1:1 methanol–acetonitrile solution to
yield deep green crystals, which were collected by filtration and
washed with 1:1 methanol–water mixture. Yield: 0.28 g (74%).
on F2 using SHELXTL [46] and SHELXL-97 packages [47].
One ethyl carbon atom (C8) of compound 1 was found as disor-
dered over two sites. The disorder was modeled by refining these
two positions (C8A and C8B) freely and setting sum of their occu-
pancies to be equal to one; the final linked occupancy parameters
were 0.70 and 0.30 for C8A and C8B, respectively. In case of com-
pound 2, The C–C bond length involving C16, C17 and C18 varied
significantly and therefore had to be constrained to get standard
and consistent values. Two hydrogen atoms of the water molecule
(O10) in compound 3 and three methyl hydrogen atoms at C14 of
the methoxy bridging moiety in compound 9 could not be located.
On the other hand, the two hydrogen atoms at the ethyl carbon
atom C6 in compound 9 were located from difference Fourier
map. All the other hydrogen atoms in 1–9 were inserted on geo-
metrical calculated positions with fixed thermal parameters. All
the hydrogen atoms in these nine structures, either located or in-
serted were refined isotropically, while the non hydrogen atoms
were refined anisotropically. The final least-squares refinements
C
28H47N4O9ClCu2 (746.23) Anal. Calc. C, 45.07; H, 6.35; N, 7.51.
Found: C, 45.20; H, 6.15; N, 7.69%. FTIR (KBr, cmÀ1): 1643s [(
C@N)],
1567 m [( asCO2)], 1410 m [( sCO2)], 1094vs, 622w [( ClO4)].
m
m
m
m
2.2.7. [CuII2L(
l-PhCOO)2](ClO4)ÁCH3CN (6)
The compound was synthesized by the similar procedure as
used for 4, except adding PhCOONa (0.144 g, 1 mmol) instead of
CH3CH2COONa. Color: green. Yield: 0.35 g (83%). C36H46N5O9ClCu2
(855.31) Anal. Calc. C, 50.55; H, 5.42; N, 8.19. Found: C, 50.68; H,
(R1) based on I > 2r(I) converged to 0.052, 0.045, 0.069, 0.040,
0.051, 0.047, 0.048, 0.036 and 0.038, respectively for 1–9.
5.34; N, 8.30%. FTIR (KBr, cmÀ1): 1643s [(
mC@N)], 1606 m
[(
m
asCO2)], 1447 m [(
m
sCO2)], 1093vs, 624w [(
m
ClO4)].
3. Results and discussion
2.2.8. [CuII2L(
l-o-ClPhCOO)2](ClO4) (7)
3.1. Syntheses and characterizations
The compound was synthesized by the similar procedure as used
for 5, except adding 1:1 mixture of o-ClPhCOOH and triethylamine
instead of CH3CH2CH2COONa. Color: green. Yield: 0.33 g (75%).
C34H41N4O9Cl3Cu2 (883.14) Anal. Calc. C, 46.24; H, 4.68; N, 6.34.
The nine compounds 1–9 were readily prepared in moderate to
good yields on reacting Cu(ClO4)2Á6H2O (for 1–7) or Cu(CH3COO)2-
ÁH2O (for 8) or Cu(NO3)2Á6H2O (for 9) with the solution of HL in
presence of sodium formate (for 1)/sodium acetate (for 2)/a 1:1
mixture of trichloroacetic acid and triethylamine (for 3)/sodium
propionate (for 4)/sodium butyrate (for 5)/sodium benzoate (for
6)/a 1:1 mixture of o-chloro benzoic acid and triethylamine (for
7)/sodium azide (for 8)/sodium cyanate (for 9).
Found: C, 46.11; H, 4.80; N, 6.51%. FTIR (KBr, cmÀ1): 1642s [(
mC@N)],
1606s [(
m
asCO2)], 1434 m [(
msCO2)], 1095vs, 622w [(
mClO4)].
2.2.9. [CuII2L(
l1,1-N3)(N3)2] (8)
Five milliliters of ‘solution of HL’ containing 0.5 mmol ligand
was taken and diluted to 20 mL. Ten milliliters aqueous solution
of Cu(CH3COO)2ÁH2O (0.199 g, 1 mmol) was added into it under
warming condition and the resulting green colored solution was
refluxed for 1 h. Then an aqueous solution of NaN3 (0.130 g,
2 mmol) was added dropwise to it. Immediately after addition,
the color changed from green to reddish brown. The overall reac-
tion mixture was refluxed for another 2 h. Then the solution was
cooled and filtered to remove any suspended particles. After a
few hours a red colored crystalline compound containing single
crystals suitable for X-ray diffraction deposited which were col-
lected by filtration and washed with 1:1 methanol–water mixture.
All these compounds were characterized by FT-IR spectroscopy
and elemental analysis. The C@N stretching frequency appears in
the range 1639–1643 cmÀ1 as a strong band. Carboxylate stretch-
ing frequencies due to the asymmetric and symmetric vibrational
modes of different carboxylate ligands present in compounds 1,
2, 4, and 5 arise at 1563–1567 cmÀ1 and 1408–1414 cmÀ1, respec-
tively, giving
nature of carboxylates in bidentate (
tic asymmetric and symmetric stretching bands of carboxylate
groups are observed at 1679 and 1450 cmÀ1 = 229 cmÀ1) for
3, 1606 and 1447 cmÀ1 = 159 cmÀ1) for 6, and 1606 and
D
m
values 152–157 cmÀ1 which indicate bridging
1,3-) fashion. The characteris-
l
(Dm
(Dm

274
S. Sarkar et al. / Polyhedron 50 (2013) 270–282
1434 cmÀ1
(D
m
= 172 cmÀ1) for 7, where the cause of shifting of
The bond lengths and angles in the coordination environment of
the metal ions of the seven compounds 1–7 are listed in Table S1,
while several structural parameters including bond lengths and an-
asymmetric stretching vibration band to higher frequency region
and the corresponding higher values for these three compounds
D
m
are due to the presence of strongly electron withdrawing substitu-
ent, three chloro (in 3), benzoate (in 6), and o-chlorobenzoate (in 7)
[48]. Presence of perchlorate moiety in 1–7 is evident from the
observation of a very strong band at 1090–1097 cmÀ1 and a weak
band at 622–624 cmÀ1. Two strong bands at 2090 and 2033 cmÀ1
in the spectrum of 8 are indicative of the presence of two types
of azide in this complex. On the other hand, a strong band at
2222 cmÀ1 in the spectrum of 9 reflects one type of cyanate ligand.
Presence of a water molecule in compound 3 is evident by the
appearance of a band of weak intensity at 3443 cmÀ1
.
All the compounds 1–9 are practically insoluble in methanol. In
acetonitrile, some are soluble enough but some others are poorly
soluble. On the other hand, all are sufficiently soluble in DMF.
Therefore, to keep consistency, solution studies like for conduc-
tance measurements in solution and for studies of catecholase
activity (vide infra) were carried out with DMF solution of the com-
plexes. The molar conductivity values of 10À3 M solution in DMF
are 65–75
X X
À1 MÀ1 cmÀ1 L for 1–7 and 19–21 À1 MÀ1 cmÀ1 L for
8 and 9, respectively, indicating behavior that is attributable to
1:1 electrolytes for 1–7 and nonelectrolytes for 8 and 9 [49]. Evi-
dently, the electrolytic nature and basic composition of all the
complexes 1–9 are same in solid state and in solution.
3.2. Description of the structures of 1–9
All these nine compounds 1–9 are dicopper(II) systems, each of
which contains one dinucleating ligand LÀ. The ligand LÀ has two
N2O compartments, each of which has one imine and one tertiary
amine nitrogen atom and a common phenoxo oxygen atom.
Clearly, the phenoxo oxygen atom in these complexes provides
an endogeneous bridge for two copper(II) centers and thus LÀ sat-
isfies three coordination positions of each of the two metal ions.
The crystal structures of compounds 1–9 are shown in Figs. 1
(for 1), 2 (for 3), 3 (for 7), 4 (for 8), and S1–S5 (for 2, 4, 5, 6, and 9).
Fig. 2. Crystal structure of [CuII2L(
anion and all the hydrogen atoms are omitted for clarity.
l-CCl3COO)2(H2O)](ClO4) (3). The perchlorate
Fig. 1. Crystal structure of [CuII2L(
anion, the methanol molecule and all the hydrogen atoms are omitted for clarity.
Between two disordered positions of one ethyl carbon atom one is shown.
l-HCOO)2](ClO4)ÁCH3OH (1). The perchlorate
Fig. 3. Crystal structure of [CuII2L(
and all the hydrogen atoms are omitted for clarity.
l-o-ClPhCOO)2](ClO4) (7). The perchlorate anion

S. Sarkar et al. / Polyhedron 50 (2013) 270–282
275
Fig. 4. Crystal structure of [CuII2L(
l1,1-N3)(N3)2] (8). All the hydrogen atoms are
omitted for clarity.
gles as sets are compared in Tables 2 and 3. The structures reveal
that all these seven compounds have similar structures in which
the two copper(II) centers are triply bridged by the phenoxo oxygen
atom and two carboxylate ligands in 1,3-bridging mode (through
two oxygen atoms), i.e., these compounds have
l-phenoxo-
bis( 1,3-carboxylate) dicopper(II) core structures. The carboxylate
l
ligands are formate for 1, acetate for 2, trichloroacetate for 3, propi-
onate for 4, butyrate for 5, benzoate for 6, and o-chlorobenzoate for
7. One (Cu1 in 3) but all metal centers in these seven compounds
are pentacoordinated by the three nitrogen/oxygen donor centers
of LÀ and two oxygen atoms of two bridging carboxylate ligands,
while the Cu1 center in 3 is hexacoordinated having the additional
coordination with a water oxygen atom. The
s values [50] for the
pentacoordinated coordination environment in 1–7 lie in the range
0.001–0.377 (Table 3), indicating that coordination geometry for
these metal ions is square pyramidal with different extent of distor-
tion; for example, that with
s = 0.001 is only slightly distorted but
that with = 0.377 is largely distorted. On the other hand, the coor-
s
dination environment of the only hexacoordinated metal center
(Cu1 in 3) is tetragonally distorted octahedral. One apical position
for Cu1 in 3 or the apical position in the square pyramidal coordina-
tion environment for each of others is occupied by a carboxylate
oxygen atom (O2 for Cu1 and O5 for Cu2) while the basal plane is
formed by the second carboxylate atom (O4 for Cu1 and O3 for
Cu2) for that metal ion, the phenoxo oxygen atom (O1), and the
two nitrogen atoms (N1 and N2). Clearly, of the two oxygen atoms
of each of the two bridging carboxylates, one oxygen atom (O2 for
Cu1/O5 for Cu2) occupies the apical position of one copper(II) cen-
ter, while the second oxygen atom (O3 for Cu2/O4 for Cu1) occupies
a basal position of the second copper(II) center, i.e., each of the two
bridging carboxylates bridges the two metal ions in axial–
equatorial fashion. The second apical coordination position of Cu1
in 3 is occupied by the water oxygen atom O10.
The crystal structures of compounds 8 and 9 are shown in Fig. 4
and S5, respectively. The selected bond lengths and angles in the

276
S. Sarkar et al. / Polyhedron 50 (2013) 270–282
Table 3
tively. On the other hand, the overall ranges of the cisoid angles
are wider for the two metal ions in the trichloroacetate compound
3 (75.9(3)–108.8(2)° and 87.4(2)–115.4(2)°), azide compound 8
(76.47(6)–122.75(5)° and 76.31(6)–106.86(5)°), and methoxide/
cyanate compound 9 (76.49(7)–113.44(8)°). The transoid angles
are also much deviated in 1–9. However, the extent of deviation
Comparison of the magnetic and governing structural parameters of the triply bridged
l
-phenoxo/alkoxo/hydroxo-bis(
l-carboxylate) dicopper(II) systems 1–7.
Compound Geometry
Bridging
h (°)b d (°)c
s
J (cmÀ1
)
mode of a
carboxylate
1
2
3
4
5
6
7
Sq Py–Sq Pya ax–eq
113.9 90.4 0.13, 0.20
112.7 91.4 0.02, 0.13
118.4 90.0 0.38
112.3 87.8 0.33, 0.26
112.1 91.6 0.19, 0.34
112.1 93.8 0.00, 0.32
112.9 87.1 0.18, 0.16
À81.5
À64.8
À136.5
À66.9
À61.4
À49.1
À61.5
is larger in
8 (140.76(6)–166.99(6)°) and 9 (148.45(11)–
Sq Py–Sq Pya ax–eq
166.96(8)°) than in 1–7 (overall range: 152.8(2)–175.4(2)°).
From the corresponding least-squares basal plane, the hexaco-
ordinated copper(II) center in 3 is displaced by relatively smaller
extent, 0.12 Å. On the other hand, this displacement (dM) value
for all the square pyramidal metal centers is larger, lying in the
range 0.225–0.351 Å. The average deviation (dN/O) of the basal
atoms from the least-squares basal plane ranges between small va-
lue like 0.004 Å (for Cu1 in 6) and significantly large value like
0.317 Å (for Cu1 in 8). As listed in Table 2, dM is not parallel with
Sq Py–Octa
ax–eq
Sq Py–Sq Pya ax–eq
Sq Py–Sq Pya ax–eq
Sq Py–Sq Pya ax–eq
Sq Py–Sq Pya ax–eq
a
All geometries are distorted; Sq Py means square pyramidal; Oct means
octahedral.
b
Cu–O(phenoxo/alkoxo/hydroxo)–Cu bridge angle.
Dihedral angle between two basal planes.
c
dN/O but the trend of the later is same as that of s.
coordination environment of the metal ions are listed in Tables S2
and S3, while several structural parameters including bond lengths
and angles as sets are compared in Table 2. The structures reveal
As listed in Table 3, the copper–phenoxo–copper bridge angle in
the six (1, 2, 4–7) of the seven carboxylate compounds is not very
different, lying in the range 112.12(12)–113.89(11)°. In the
remaining carboxylate compound, 3, this angle (118.4(2)°) is larger
by ca. 5°. On the other hand, the phenoxo bridge angle is signifi-
that compounds 8 and 9 are, respectively, heterobridged
l-phen-
oxo- 1,1-azide dicopper(II) and -phenoxo- -methoxide dicop-
l
l
l
per(II) systems. Both the metal ions in both the compounds are
pentacoordinated; a nitrogen atom of a monodentate azide ligand
(in 8) or a monodentate cyanate ligand (in 9) occupies the fifth
cantly smaller in the
l-phenoxo-l1,1-azide (8; 101.40(5)°) and l-
phenoxo- -methoxide (9; 99.74(9)°) systems. The copper–
l
N(azide)–copper and copper–O(methoxide)–copper bridge angles
in 8 and 9, respectively, are not very different, 105.65(7)° and
coordination position. The
s values for both the metal centers in
9 and one metal center, Cu2, in 8 lie in the range 0.308–0.341, indi-
cating that the geometry in these cases is largely distorted square
pyramidal. On the other hand, the s value (0.42) for the second me-
tal center, Cu1, in 8 is greater than 0.4, indicating that this geom-
106.28(10)°. The
l-phenoxo-bis(l1,3-carboxylate) dicopper(II)
bridging cores in 1–7 are significantly distorted as evidenced by
the dihedral angles (d), 87.1–93.8°, between the basal planes of
the two metal centers. On the other hand, the bridging moieties
etry is intermediate between square pyramidal and trigonal
in the
l-phenoxo-l1,1-azide (in 8) and l-phenoxo-l-methoxide
bipyramidal. However, as the
s value is only slightly greater than
(in 9) compounds are less distorted as evidenced by the d values,
28.7° and 29.1°, respectively. The copperÁ Á Ácopper distance in the
dinuclear core is largest (3.39 Å) in the trichloroacetate compound
3, second largest in the formate compound 1 (3.33 Å), smallest
(3.07 Å) in the methoxide/cyanate compound 9, second smallest
(3.14 Å) in the azide compound 8, while this distance is intermedi-
ate and not very different (3.27–3.30 Å) for the other five carboxyl-
ate compounds, 2 and 5–7.
0.4, the geometry of Cu1 in 8 can also be approximately considered
as significantly distorted square pyramidal. The apical position in
the square pyramidal environment of both the metal ions in both
the compounds 8 and 9 is occupied by an amine nitrogen atom
(N2 for Cu1 and N3 for Cu2 in 8; N2 for Cu1 and N2A for Cu1A
in 9). Clearly, both the bridging atoms, phenoxo oxygen atom
(O1) and
l1,1-azide nitrogen atom (N5) in 8 and phenoxo (O1)
and methoxide (O2) oxygen atom atoms in 9, occupy two basal
positions for both the metal ions in 8 and 9.
3.3. Catecholase activity
In the l-phenoxo-bis(l1,3-carboxylate) dicopper(II) compounds
1–7, the copper–amine bond distance (overall range for all the se-
ven compounds: 2.030(6)–2.091(4) Å) is longer than other three
bond distances (overall range for all the seven compounds:
1.946(2)–2.004(2) Å) in the equatorial plane of the corresponding
complex. As usual, the apical bond distances are longer than the
bond distances involving the basal atoms in the coordination envi-
ronment of the copper(II) centers in 1–7. However, except for one
(Cu1) metal center in 6 and two metal centers in 3, the apical bond
length (overall range: 2.125(2)–2.164(3) Å) is not much longer
than the longest equatorial bond distance (overall range:
2.048(3)–2.091(4) Å) for the corresponding metal ion. On the other
hand, the Cu1–O(carboxylate, apical; 2.204(3) Å) bond distance in
6 and Cu1/Cu2–O(carboxylate, apical; 2.211(5) and 2.329(5) Å in
3) bond distances are relatively much longer, while the Cu1–
O(water) apical distance in 3 is significantly longer, 2.736(14) Å,
The possibility of showing catecholase activity of complexes 1–
9 was checked by recording time-dependent electronic spectra
near 400 nm of a 0.25 Â 10À2 M solution of 3,5-di-tert-butylcate-
chol (3,5-DTBCH2) in presence of a 1/100th fold solution of a com-
plex among 1–9 in DMF. The spectra of the mixture of a complex
and 3,5-DTBCH2 were recorded after every 5 min up to a total time
of 90 min. In the cases of the carboxylate compounds 1–7, the ori-
ginal charge transfer band of the complexes at 372–379 nm be-
comes red shifted to 392–398 nm with gradual increase of
intensity, indicating more and more formation of 3,5-di-tert-butyl-
quinone (3,5-DTBQ), which, in turn, indicate that complexes 1–7 in
DMF show catechol oxidase activity. The spectral profiles in the
cases of the propionate and o-chlorobenzoate complexes, 4 and
7, are shown in Figs. 5 and 6, while the profiles of other five carbox-
ylate complexes 1–3, 5, and 6 are shown in Figs. S6–S10. The obser-
vation indicates that the compounds 1–7, like several other
reported systems, can be considered as biomimetic models of cat-
echol oxidase. In contrast to what is observed for the carboxylate
compounds 1–7, no generation of quinone band takes place for
either of the azide compound 8 or the methoxide/cyanate com-
pound 9, indicating that these two later compounds are not cata-
lysts of catecholase activity. At this point, it is relevant to
mention that blank experiment without complex showed no for-
mation of quinone up to 3 h.
indicating that water is semicoordinated. In the
l
-phenoxo-l1,1
-
azide dicopper(II) compound and -phenoxo-l
8
l
-methoxide
dicopper(II) compound 9, the copper–N(amine/immine apical)
bond distances (2.2708(14) and 2.2784(14) Å in 8; 2.3429(19) Å
in 9) are much longer than the equatorial bond distances
(1.9490(13)–2.0361(10) Å in 8; 1.9216(13)–2.0109(13) Å in 9).
The cisoid angles in 1–9 are much distorted from the ideal val-
ues. For 1, 2, 4–7, the smallest and largest cisoid angles lie in the
ranges 85.06(11)–86.32(8)° and 101.90(10)–109.98(8)°, respec-

S. Sarkar et al. / Polyhedron 50 (2013) 270–282
277
1.8
1.5
1.2
0.9
0.6
0.3
0.0
3,5-DTBQ
Complex 4
3,5-DTBCH2
Fig. 7. Initial rates vs. substrate concentration for the 3,5-DTBCH2 ? 3,5-DTBQ
oxidation reaction catalyzed by complex [CuII2L(
l-CH3CH2COO)2](ClO4) (4) in DMF.
Inset shows Lineweaver–Burk plot. Symbols and solid lines represent the observed
and simulated profiles, respectively.
300
350
400
450
500
Wavelength (nm)
Fig. 5. The spectral profile showing the increase of quinone band at 398 nm after
the addition of 100-fold concentrated solution of 3,5-DTBCH2 to a 0.25 Â 10À4
M
solution of [CuII2L(
after each 5 min.
l-CH3CH2COO)2](ClO4) (4) in DMF. The spectra were recorded
0.6
0.5
0.4
0.3
0.2
0.1
0.0
3,5-DTBQ
Complex 7
Fig. 8. Initial rates vs. substrate concentration for the 3,5-DTBCH2 ? 3,5-DTBQ
oxidation reaction catalyzed by complex [CuII2L(
l-o-ClPhCOO)2](ClO4) (7) in DMF.
3,5-DTBCH2
Inset shows Lineweaver–Burk plot. Symbols and solid lines represent the observed
and simulated profiles, respectively.
Table 4
Kinetic parameters for the catecholase activity of the complexes 1–7.
300
350
400
450
500
Complex Vmax
(M minÀ1
Std. error
KM (M)
Std. error
Kcat
Wavelength (nm)
)
(hÀ1
)
1
2
3
4
5
6
7
1.178 Â 10À5 6.295 Â 10À7 0.001958
0.0001994
28.272
32.616
Fig. 6. The spectral profile showing the increase of quinone band at 392 nm after
1.359 Â 10À5 6.034 Â 10À7 0.0009902 0.0001081
the addition of 100-fold concentrated solution of 3,5-DTBCH2 to a 0.25 Â 10À4
M
9.200 Â 10À6 1.592 Â 10À7 0.0001993 1.814 Â 10À5 22.080
solution of [CuII2L(
after each 5 min.
l-o-ClPhCOO)2](ClO4) (7) in DMF. The spectra were recorded
3.711 Â 10À5 3.746 Â 10À6 0.003452
0.0005304
89.064
1.368 Â 10À5 2.667 Â 10À7 0.0004703 2.912 Â 10À5 32.832
9.897 Â 10À6 5.323 Â 10À7 0.001512
7.836 Â 10À6 2.732 Â 10À6 0.003665
0.0001606
0.001955
23.753
18.801
To understand the kinetic aspects of the catalysis, a set of time-
scans (up to 90 min) at the band maximum of quinone was per-
formed for each complex among 1–7 at a fixed temperature,
25 °C, on keeping the complex concentration at 0.25 Â 10À4
M
ence the catalytic activity, metalÁ Á Ámetal distance, steric bulk pro-
vided by the carboxylate and distortion value) in the
coordination environment [27–29,34]. The activity should be en-
but varying the 3,5-DTBCH2 concentration in between
0.25 Â 10À3 M and 0.25 Â 10À2 M. The rate constant for a catalyst
complex was determined by initial rate method. The rate constant
versus substrate concentration data were then analyzed on the ba-
sis of the Michaelis–Menten approach of enzymatic kinetics to get
the Lineweaver–Burk plot as well as the values of the parameters
Vmax, KM, and Kcat. Observed and simulated rate constant versus
[substrate] plots and the Lineweaver–Burk plots for the propionate
and o-chlorobenzoate complexes, 4 and 7, are shown in Figs. 7 and
8, while the similar plots of other five carboxylate complexes 1–3,
5, and 6 are shown in Figs. S11–S15, respectively. As listed in
Table 4, turnover number (Kcat) for the complexes 1–7 lie in the
range 18.8–89.1 hÀ1. In the present series, three factors may influ-
(
s
hanced for smaller metalÁ Á Ámetal distance, less steric bulk and
more distortion (greater
s
value). Although metalÁ Á Ámetal distance
of 4 (3.28 Å), 5 and 6 (3.27 Å) are smaller, steric bulk in 5 and 6 are
greater than that in 4 and also the values are greater in 4 than
those in 5 and 6, resulting in greater Kcat value in 4 than those in
5 and 6. It may be mentioned that the distortion of the coordina-
s
tion environment of the metal centers in 3 (s = 0.38) is greater than
that in 4, so apparently it may be expected that 3 should show bet-
ter activity than 4. However, Kcat value of 3 (22.080 hÀ1) is less than
that in 4 (89.064 hÀ1) because of longer metalÁÁÁmetal distance
(3.39 Å) and large steric bulk of the trichloacetate moiety in 3.

278
S. Sarkar et al. / Polyhedron 50 (2013) 270–282
At this juncture, it is relevant to mention that catecholase activity
of five compounds among the previously reported -phenoxo/al-
koxo/hydroxo-bis( -carboxylate) dicopper(II) systems has been
studied [30,33]. All these five compounds are -phenoxo-bis( -aca-
is five for 1 six for 2–4, 6–8 and seven for 5. The peak positions
l
and the isotopic distributions have been well assigned, as listed
in Table 4 and demonstrated in Figs. 9 (for 2), 10 (for 4), and
S16–S21 (for 1, 3, 5–8).
l
l
l
tate) dicopper(II) systems with eq–eq combination for both the car-
boxylates, i.e., these five systems are closely similar to 1–7. The Kcat
values for these five systems lie in the range 10.7–155 hÀ1. So, the
Kcat values of 1–7 lie within the range of previous observation. It
The peaks of 1–8 are well assigned to 13 positive ions, A–M (Ta-
ble 5). Eleven (B–J, L, and M) of these ions are monocationic; the
line to line separation for the corresponding peaks is 1.0. The
remaining species (A) and (K) are dicationic; the line to line sepa-
ration for the corresponding peak is 0.5. All the 13 positive ions are
heterobridged (double bridged or triple bridged) dicopper species
in the environment of the original ligand LÀ or the partially decom-
posed ligand [L1]À, where HL1 is the 1:1 condensation product of
2,6-diformyl-4-ethylphenol and 3-dimethylaminopropylamine
(Scheme S1). Obviously, the common bridge between the two cop-
per centers in all the positive ions is the phenoxo moiety provided
by either LÀ or [L1]À. The another bridge for the double bridged
species and the other two bridges for the triple bridged species
may also be mentioned that catecholase activity of few
oxo/alkoxo- -carboxylate dicopper(II) systems has also been stud-
ied, for which the Kcat values lie in the range 1.1–28 hÀ1 [12,38–
40]. The lack of catecholase activity of the -phenoxo- 1,1-azide/
methoxide systems (8 and 9) indicates that 1,1-azide/ -methoxide
l-phen-
l
l
l
l
l
are poor leaving groups in these systems. At this point, it may be
mentioned that some such previously reported compounds show
activity [27,29,30] while some others are inactive [12,29].
3.4. Electrospray ionization mass spectral study
in the positive ions are among hydroxo, oxo, l1,3-carboxylates,
and l1,1-azide. The combinations of oxidation states of the copper
ESI-MS (positive) spectrum of the acetonitrile solutions of the
complexes 1–8 have been studied. The number of peaks observed
centers in the 13 positive ions are among CuIICuII, CuICuI, and
CuIICuI.
404.43
589.06
235.23
488.6
244.80
589.06
589.06
636.67
100
Simulated
Observed
636.67
404.43
244.80
488.6
235.23
%
50
488.6
235.23
636.67
244.80
300
404.43
0
m /z
900
800
600
200
400
500
50
Fig. 9. Electrospray mass spectrum (ESI-MS positive) of [CuII2L(
l-CH3COO)2](ClO4) (2) in acetonitrile showing observed and simulated isotopic distribution pattern.
488.60
618.74
589.06
488.60
488.60
244.80
244.80
404.43
100
562.68
Simulated
Observed
562.68
404.43
618.74
589.06
618.74
% 50
244.80
589.06
562.68
404.43
m/z
0
100
200
400
500
600
800
Fig. 10. Electrospray mass spectrum (ESI-MS positive) of [CuII2L(
l-CH3CH2COO)2](ClO4) (4) in acetonitrile showing observed and simulated isotopic distribution pattern.

S. Sarkar et al. / Polyhedron 50 (2013) 270–282
279
Table 5
The species and their peak positions in the ESI-MS positive spectra of 1–8.
Species Composition of species
type
1
2
3
4
5
6
7
8
A
B
C
D
E
F
l
l
l
l
l
l
l
l
-phenoxo-
-phenoxo-
-phenoxo-
-phenoxo-
-phenoxo-
-phenoxo-
-phenoxo-
-phenoxo-
l
l
l
l
l
l
l
l
-hydroxo CuIICuII [CuII2L(
l
-OH)]2+
m/z = 245 m/z = 245 m/z = 245 m/z = 245
10% 22% 15% 28%
m/z = 245 m/z = 245 m/z = 245 m/z = 245
10%
m/z = 589 m/z = 589 m/z = 589
28% 12% 7%
m/z = 489 m/z = 489 m/z = 489 m/z = 489
10%
5%
5%
-hydroxo CuIICuII [CuII2L(
l
-OH)(ClO4)]+ m/z = 589 m/z = 589 m/z = 589 m/z = 589
–
68% 100% 38% 23%
m/z = 489 m/z = 489 m/z = 489 m/z = 489
-oxo CuIICuII [CuII2L( -O)]+
l
100%
m/z = 404
27%
50%
m/z = 404
20%
–
42%
m/z = 404
15%
–
100%
m/z = 404
15%
100%
m/z = 404
10%
m/z = 458
12%
100%
m/z = 404
15%
42%
m/z = 404
7%
–
100%
m/z = 404
6%
m/z = 458
18%
-oxo CuIICuII [CuII2L1(
-oxo CuIICuII [CuII2L1(
l
-O)]+
l
-O)(H2O)3]+
–
–
–
-oxo CuIICuII [{CuII2L1(
l
-O)(N3)}Na]+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
m/z = 469
18%
–
G
H
I
-oxo CuIICuII [{CuII2L(
-carboxylate CuIICuII
l
-O) (H2O)2}]+
m/z = 524
22%
–
–
m/z = 575
38%
–
m/z = 643
100%
–
–
–
–
–
–
[{CuII2L1(
-phenoxo-
[{CuII2L(
-OH)(
-phenoxo-bis(
carboxylate)2]+
-phenoxo-
-acetate CuIICuI [{CuIICuIL1(
acetate)Na]2+
-phenoxo-bis(
acetate)2}Na2]+
-phenoxo-
1,1-azide CuICuI [{CuI2L(l1,1
N3)(N3)}Na2]+
l
-carboxylate)(ClO4)]+
l
l
-hydroxo-
-carboxylate)]+
-carboxylate) CuIICuII [{CuII2L(
l
-carboxylate CuIICuII
m/z = 534
40%
–
m/z = 652 m/z = 561
100%
–
m/z = 610
98%
l
l
l
25%
m/z = 618
J
l
l
-
m/z = 646 m/z = 714 m/z = 783
98%
–
40%
–
60%
–
20%
–
K
L
l
l
l
-
–
–
–
m/z = 235
28%
m/z = 637
23%
–
–
–
l
l
-acetate) CuICuI [{CuI2L(
l-
–
–
–
–
–
–
–
–
M
l
l
-
–
m/z = 603
8%
Three peaks are common in the spectra of all the complexes 1–
8, at m/z = 245, 404, and 489. The peak at m/z = 245 corresponds to
ing the original bridging moiety but having both the metal centers
in reduced state has been identified at m/z = 603.
l
-phenoxo-
the peaks at m/z = 489 and 404 correspond to the two monocation-
ic -phenoxo-
-oxo CuIICuII species [CuII2L( -O)]+ (C) and [CuII2L1(-
-O)]+ (D) respectively. In addition to [CuII2L( -OH)]2+ (A), another
-phenoxo-
-hydroxo CuIICuII species [CuII2L( -OH)(ClO4)]+ (B),
l
-hydroxo CuIICuII species [CuII2L(
l
-OH)]2+ (A), while
To get an insight on the nature of possible complex-substrate
intermediate, ESI-MS positive spectrum of a 1:100 mixture of the
l
l
l
propionate complex 4, which has the highest Kcat value (89 hÀ1
)
l
l
l
among 1–7, and 3,5-DTBCH2 was recorded after 5 min of mixing.
As shown in Fig. 11 (both observed and simulated), the mixture
exhibits four peaks at m/z = 243 (100%, line to line separation
1.0), 463 (55%, line to line separation 1.0), 489 (20%, line to line
separation 1.0), and 693 (10%, line to line separation 1.0). While
l
l
corresponding to the peak at m/z = 589, is identified in the spectra
of 1–7. Compound 8 does not exhibit the signal due to B, obviously
because of the lack of perchlorate in this molecule. While com-
pounds 1, 2, 4, 6 and 7 exhibit only two
species [CuII2L( -O)]+ (C) and [CuII2L1( -O)]+ (D), compounds 3 and
5 exhibit three and 8 exhibits four -phenoxo-
-oxo CuIICuII spe-
cies respectively and; the additional analog is [CuII2L1(
O)(H2O)3]+ (E; m/z = 458) for 5, [CuII2L(
l
-phenoxo-
l
-oxo CuIICuII
the peak at m/z = 489 due to the
l-phenoxo-l
-oxo CuIICuII species
l
l
[CuII2L( -O)]+ appears in the spectrum of the complex 4 itself, the
l
l
l
peaks at m/z = 243 and 463 correspond, respectively, to the qui-
none-sodium aggregates [(3,5-DTBQ)Na]+ (C14H20O2Na) and [(3,5-
DTBQ)2Na]+ (C28H40O4Na). The remaining peak at m/z = 693 is quite
interesting because the peak position and matching of the isotopic
distribution of the observed and the simulated patterns (Fig. 11)
clearly indicate that this peak arises due to 1:1 complex-substrate
l
-
-
l
-O)(H2O)2]+ (G; m/
z = 524) for 3 and the two additional analogs for 8 are [CuII2L1(
l
O)(H2O)3]+ (E; m/z = 458) and [{CuII2L1(
z = 469).
l
-O)(N3)}Na]+ (F; m/
As already discussed, complexes 1–7 contains a
l
-phenoxo-
aggregate [CuII2L( -3,5-DTBC2À)]+ (N; C34H53N4O3Cu2); we con-
l
bis(l
-carboxylate) CuIICuII moiety. A positive ion [{CuII2L(
l
-car-
sider here the bridging mode of 3,5-DTBC2À because one or more
exogeneous bridging ligand can potentially coordinate to the two
metal ion in such systems (as in 1–9). Although the ESI-MS positive
species may not be considered as true intermediate, it is worth
mentioning that a 1:1 complex-substrate aggregate with bidentate
bridging catechol is involved in the catalytic cycle proposed by Sol-
omon [23]. It is relevant also at this point to mention the previ-
ously reported complex-substrate aggregates. Using 3,5-DTBCH2
or differently substituted catechols, 1:1 or 1:2 dicopper(II)-sub-
strate aggregates having dideprotonated dinegative catechol in
either bidentate chelating or bridging modes have been identified
crystallographically or from UV–Vis and ESI-MS spectral studies
[27,51–54]. We have reported an aggregate containing both bridg-
ing 3,5-DTBC2À and monodentate 3,5-DTBCHÀ[27]. It is relevant to
mention at this context that all the catalysts 1–7 are similar triply
boxylate)2]+ (J) having such a triple bridged dicopper(II) moiety
has been identified in the spectra of 4, 5, 6, and 7 at m/z = 618,
646, 714, and 783, respectively. Interestingly, a positive ion [{CuI2-
L(
l
-acetate)2}Na2]+ (L; m/z = 637) having the same triple bridging
l-phenoxo-bis(l-carboxylate) moiety but having both the metal
ions in reduced state has been identified in the spectrum of 2.
Some other species having one carboxylate moiety have been iden-
tified in the spectra of some systems. These species are the double
bridged
l-phenoxo-l l-car-
-carboxylate CuIICuII moiety [{CuII2L1(
boxylate)(ClO4)]+ (H) for 5 (m/z = 575) and 7 (m/z = 643), triple
bridged
[{CuII2L(
l
-OH)(
-phenoxo-
l-hydroxo-l
-carboxylate CuIICuII moiety
l
l
-carboxylate)]+ (I) for
(m/z = 534), (m/
1
3
z = 652), 4 (m/z = 561), and 6 (m/z = 610), and an interesting mixed
valence system [{CuIICuIL1( -acetate)Na]2+ (K; m/z = 235) for 2.
Although compound 8 contains a -phenoxo-
1,1-azide CuIICuII
moiety, no such species has been observed in its spectrum. How-
ever, an interesting positive ion [{CuI2L( 1,1-N3)(N3)}Na2]+ (M) hav-
l
l
l
bridged l-phenoxo-bis(l-carboxylate) dicopper(II) systems and,
moreover, the spectral profiles during catalyzed conversion of
3,5-DTBCH2 to 3,5-DTBQ are similar for all complexes 1–7.
l

280
S. Sarkar et al. / Polyhedron 50 (2013) 270–282
488.6 692.9
463.62
243.30
243.30
100
Simulated
Observed
463.62
463.62
488.6
692.9
600
243.30
%
50
0
488.6
692.9
m/z
50
100
200
300
400
500
700
800
Fig. 11. Electrospray mass spectrum (ESI-MS positive) of a 1:100 mixture of [CuII2L(
l
-CH3CH2COO)2](ClO4) (4) and 3,5-DTBCH2 in acetonitrile, recorded after 10 min of
mixing. Both the observed and simulated isotopic distribution patterns are shown.
Therefore, it is logical to assume that the intermediate with 3,5-
pound 9 is very small, less than 0.1 cm3 molÀ1 K, throughout the
temperature range 300–2 K, indicating very strong interaction.
DTBCH2 for all these seven compounds are same, i.e., [CuII2L(
3,5-DTBC2À)]+, having a bidentate bridging catecholate moiety.
l
-
Although
a
good quality simulation was not possible,
J > À350 cmÀ1 was understood.
The magnetic data were simulated with the Bleaney-Bower
equation (H = –2JS1S2). A good quality fitting has been obtained
in each case of 1–8 with the following parameters: J = –81.5 cmÀ1
and g = 2.146 for 1; J = –64.8 cmÀ1 and g = 2.156 for 2; J = –
136.5 cmÀ1 and g = 2.030 for 3; J = –66.9 cmÀ1 and g = 2.175 for
4; J = –61.4 cmÀ1 and g = 2.089 for 5; J = –49.1 cmÀ1 and g = 2.072
for 6; J = –61.5 cmÀ1 and g = 2.168 for 7; J = –186.3 cmÀ1 and
g = 2.213 for 8. It may be mentioned that smaller room tempera-
3.5. Magnetic properties of 1–9
The
vMT versus T plots of the trichloroacetate (3), benzoate (6),
and azide (8) compounds are shown in Figs. 12–14, while the plots
of the formate (1), acetate (2), propionate (4), butyrate (5), o-chlo-
robenzoate (7), and methoxide/cyanate (9) compounds are shown
in Figs. S22–S27. Except for the trichloroacetate compound 3, the
v
MT value of the other six carboxylate compounds 1, 2, and 4–7
ture
v
MT values of 3 (0.48 cm3 molÀ1 K), 8 (0.42 cm3 molÀ1 K)
at 300 K is ca. 0.7 cm3 molÀ1 K, while that for the trichloroacetate
(3) and also for the azide (8) compounds at 300 K is relatively
smaller, 0.48 and 0.42 cm3 molÀ1 K, respectively. On lowering of
and 9 (0.1 cm3 molÀ1 K) compared to other compounds 1, 2, 4–7
(ca. 0.7 cm3 molÀ1 K) arise due to stronger antiferromagnetic inter-
action in the former three compounds (J > –350 cmÀ1 for 9, J = –
136.5 cmÀ1 for 3, J = –186.3 cmÀ1 for 8) compared to weaker inter-
action in the later six compounds (J lies between –49.1 cmÀ1 and –
81.5 cmÀ1 for 1, 2, 4–7).
temperature,
vMT for all these compounds gradually decreases to
reach almost diamagnetic state at 80 K for 8, 60 K for 3, 40 K for
1, 30 K for 2, 4, 5, and 7, and 20 K for 6. The profiles indicate anti-
ferromagnetic interaction between the two metal centers in all
these eight compounds, with relatively stronger interaction in
the azide compound 8 and the trichloroacetate compound 3. On
The
l-phenoxo-bis(l-carboxylate) dicopper(II) compounds 1–7
show moderate antiferromagnetic interaction with J values ranging
the other hand, the
vMT value for the methoxide/cyanate com-
Fig. 12. Observed (symbols) and simulated (solid lines)
-CCl3COO)2](ClO4) (3).
v
MT vs. T plot of [CuII2L
Fig. 13. Observed (symbols) and simulated (solid lines)
-PhCOO)2](ClO4)ÁCH3CN (6).
v
MT vs. T plot of [CuII2L
(l
(l

S. Sarkar et al. / Polyhedron 50 (2013) 270–282
281
As in the azide compound 8, both the bridging ligands (phenoxo
and methoxide) in the -phenoxo-
-methoxide system [CuII2L(
l
l
l-
OCH3)(NCO)2] (9) coordinate in eq–eq mode. The phenoxo and
methoxide bridge angles here are 99.74° and 106.28°, respectively.
In terms of the average bridge angle, 103.01°, the strong antiferro-
magnetic interaction (with J > À350 cmÀ1) is in line with the re-
ported similar compounds [56].
4. Conclusions
The
are among a few examples of the family of dicopper(II) compounds
containing -phenoxo/alkoxo/hydroxo-bis( -carboxylate) bridg-
ing moiety. While acetate or benzoate has been the secondary li-
gand in all previous examples of -phenoxo/alkoxo/hydroxo-
bis( -carboxylate) dicopper(II) systems, various carboxylates (for-
l-phenoxo-bis(l-carboxylate) dicopper(II) compounds 1–7
l
l
l
l
mate, acetate, trichloroacetate, propionate, butyrate, benzoate, and
o-chlorobenzoate) have been introduced in 1–7 with the aim to
monitor the structures and properties. The composition of 1–7
indicates that the basic bridging moiety here is not dependent on
the carboxylates but is governed mainly by the primary ligand.
The magnetic exchange interaction of the systems 1–7 are moder-
ately antiferromagnetic, which is in line with the countercomple-
mentarity effect of carboxylates. An interesting correlation of the
extent of antiferromagnetic interaction with the electron donat-
ing/withdrawing effect has been observed.
Fig. 14. Observed (symbols) and simulated (solid lines)
1,1-N3)(N3)2] (8).
v
MT vs. T plot of [CuII2L
(l
between À49.2 cmÀ1 and À136.6 cmÀ1. It is relevant to mention
The triply bridged
l-phenoxo-bis(l-carboxylate) dicopper(II)
that the two copper(II) centers in previously published
l-phen-
compounds 1–7 have been found as functional model of catechol
oxidase with Kcat values lying in the range 18.8–89.1 hÀ1. The rea-
son of best catalytic activity in the propionate compound 4 has
been understood in terms of copperII)Á Á Ácopper(II) distance, steric
bulk provided by the carboxylate moiety and distortion of the
coordination environment of the metal centers.
oxo/alkoxo/hydroxo-bis(l-carboxylate) dicopper(II) compounds
are coupled either by weak/moderate antiferromagnetic, with J
values ragning between À2.1 and À46.5 cmÀ1, or by weak ferro-
magnetic interaction with J values 7.8 and 22.5 cmÀ1 [14–19]. It
may be noted that the origin of such weak/moderate antiferromag-
netic or weak ferromagnetic interactions is the countercomple-
mentarity effect of the bridging carboxylates [1,6–19]. Magnetic
exchange integral and governing structural parameters of 1–7 are
compared in Table 3. While it has not been possible to find a sta-
rightforward correlation of magnetic properties on the structural
parameters, the J values of the compounds 1–7 can be correlated
with the nature of the carboxylate ligand. Electron donating effect
of acetate, propionate and butyrate is greater than formate and the
The nature of interaction in the
l-phenoxo-l-azide (8) and l-
phenoxo- -methoxide (9) compounds are strong or very strong
l
antiferromagnetic, which are in line with the previous related sys-
tems. These two compounds have been found as inactive towards
catechol oxidase activity.
The ESI-MS positive spectral studies of 1–8 and the mixture of 4
and 3,5-DTBCH2 have been appeared interesting to us because of
the observations/aspects including the following: (i) The original
antiferromagnetic interaction of the formate compound
1
triple bridged
l-phenoxo-bis(l-carboxylate) dicopper(II) moiety
(À81.5 cmÀ1) is greater than that of the acetate compound 2
(À64.8 cmÀ1), propionate compound 4 (À66.9 cmÀ1) and butyrate
compound 5 (À61.4 cmÀ1), while electron withdrawing effect of
trichloroacetate is greater than formate and the antiferromagnetic
interaction in the formate compound 1 (À81.5 cmÀ1) is less than
that in the trichloroacetate compound 3 (À136.5 cmÀ1). Thus, it
seems that electron withdwaring effect of the carboxylate in the
present series increases antiferromagnetic interaction and vice
versa. The situation is similar for the two compounds in the pres-
ent series having aromatic carboxylates; electron withdrawing ef-
fect of o-chlorobenzoate is greater than benzoate and the
antiferromagnetic interaction in the o-chlorobenzoate compound
(in 4–7); (ii) A partially reduced mixed-valence CuIICuI species
(in 2); (iii) The fully reduced CuICuI moieties (in 2 and 8); (iv) A
complex-substrate aggregate [CuII2L( -3,5-DTBC2À)]+ (such a moi-
l
ety is involved in Solomon’s mechanism).
Acknowledgements
Financial support has been received from the Government of In-
dia through Department of Science and Technology, the Govern-
ment of India (SR/S1/IC-42/2011). S. Sarkar and S. Sasmal are
thankful to University Grants Commission and S. Majumder is
thankful to Department of Science and Technology, Government
of India, for fellowships. Crystallography was performed at the
DST-FIST, India-funded Single Crystal Diffractometer Facility at
Department of Chemistry, University of Calcutta. Indian Associa-
tion for the Cultivation of Science is gratefully acknowledged for
providing the facility of ESI-MS studies.
7
(À61.5 cmÀ1
)
is greater than the benzoate compound
6
(À49.1 cmÀ1).
Both the phenoxo and end-on azide ligands in the
l-phenoxo-
l
1,1-azide system [CuII2L(
l1,1-N3)(N3)2] (8) coordinate in eq–eq
mode. The phenoxo bridge angle (101.4°) is greater than 97.5°,
indicating that this route is antiferromagnetic [1]. On the other
hand, the azide bridge angle (105.65°) is less than 108° [55], indi-
cating that this route is ferromagnetic. The J value of À186.3 cmÀ1
of 8 clearly indicates that the phenoxo route dominates over the
azide route. The J value of this compound is in the range of J values
Appendix A. Supplementary data
CCDC 886272–886280 contain the supplementary crystallo-
graphic data for 1–9, respectively. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
of the reported
[12,41,42].
l-phenoxo-l1,1-azide dicopper(II) complexes

282
S. Sarkar et al. / Polyhedron 50 (2013) 270–282
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