Mononuclear transition metal complexes with sterically hindered carboxylate ligands: Synthesis, structural and spectral properties

Article abstract of DOI:10.1016/j.poly.2010.10.019

The metal coordination geometry in the active site of metalloproteins are very different from the one of small inorganic complexes, due to the inflexibility of the ligand set from amino acid side chains different from freely moving ligand set in synthesis

Full text of DOI:10.1016/j.poly.2010.10.019

Polyhedron 30 (2011) 340–346
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Mononuclear transition metal complexes with sterically hindered carboxylate
ligands: Synthesis, structural and spectral properties
Sethuraman Kannan a, Galmari Venkatachalam a, Ha-Jin Lee b, Byoung Koun Min c, Woong Kim d,
Eunhae Koo e, Young Rag Do a, Sungho Yoon a,
⇑
a Department of Chemistry, Kookmin University, 861-1 Jeoungnung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea
b Jeonju Center, Korea Basic Science Institute, 664-14 Dukjindong-1-ga, Dukjin-gu, Jeonju 561-756, Republic of Korea
c Department of Clean Energy Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
d Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea
e Bio-IT Convergence Center, Korea Institute of Ceramic Engineering and Technology, Seoul, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The metal coordination geometry in the active site of metalloproteins are very different from the one of
small inorganic complexes, due to the inflexibility of the ligand set from amino acid side chains different
from freely moving ligand set in synthesis. Using the sterically hindered 2,6-di-(p-fluorophenyl)benzo-
Received 28 May 2010
Accepted 28 October 2010
Available online 5 November 2010
ate(L) ligand,
a series of mononuclear Co(II), Ni(II) and Cu(II) complexes of general formula
[M(L)2(Hdmpz)2] (where, Hdmpz = 3,5-dimethyl pyrazole) have been synthesized and characterized by
the variety of spectroscopic methods. A distorted octahedral geometry in case of nickel, tetrahedral
geometry for cobalt and square pyramidal in copper was observed in the X-ray studies, which also
revealed that the uncoordinated oxygen atom of the carboxylate group forms intramolecular hydrogen
bonding with the N–H group of the coordinated 3,5-dimethylpyrazole in case of cobalt and copper.
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords:
Mononuclear metal complexes
Sterically hindered carboxylate ligand
3,5-Dimethylpyrazole
Crystal structures
1. Introduction
biological functions of some selected metal ions like copper, iron,
cobalt manganese and nickel have undergone an extensive re-
The research on the metal carboxylates has always been
intriguing in that they play important roles not only in synthetic
chemistry with the essence of labile coordination modes of carbox-
ylate group, such as architecture of open and porous framework
[1,2], but also in biologic activities [3,4] and physiological effects
[5,6]. A versatile carboxylate anion can adopt a wide range of bond-
ing modes, including monodentate, symmetric and asymmetric
chelating, and bidentate and monodentate bridging [7]. It is gener-
ally known that the carboxylate complexes of the 3d elements in
active sites of enzymes play an important role in redox chemistry.
For example, metal active sites catalyze fundamental transforma-
tions such as the conversions of oxygen to water, water to oxygen,
nitrogen to ammonia and methane to methanol. Because the
behavior of metal ions in proteins cannot be divorced from the
fundamental chemistry of the particular metal, the study of small
molecule, active-site synthetic analogs is useful [8,9]. Synthetic
models with carboxylates, imines and thiolates as the ligands are
exploited in order to sharpen or focus certain questions related
to the role of metals in metalloenzymes [10–12]. The goal is to elu-
cidate fundamental aspects of structure, spectroscopy, magnetic
and electronic structure, reactivity and chemical mechanism. The
search but the quest for more advancement still exists. We have fo-
cused on cobalt, nickel and copper mononuclear complexes of the
carboxylate ligand with a potential to expand the understanding of
the coordination chemistry and substrate binding in enzymes with
these metal active sites.
A common structural feature found in most of the metallopro-
teins and enzymes is the presence of one or more carboxylate
groups derived from aspartate or glutamate side chains of the pro-
tein. The carboxylate group of glutamate and aspartate works as
supporting ligand for the metal center in various metalloproteins
and behave as monodentate or bidentate depending upon the
requirement of active site. Besides vitamin B12 which catalyses
the trans-alkylation and isomerization reactions via Co(III)-alkyl
intermediate [13], there are several other cobalt containing pro-
teins and enzymes such as ribonucleotide reductase (RR), MAPs
(methionine aminopeptidases), glucose isomerases, cobalt
transporters, bromoperoxidase, etc. where cobalt plays directly or
indirectly an important role [14]. Similarly, copper-containing oxi-
dases and oxygenases comprise a large class of enzymes containing
varying numbers of copper ions having diverse structures. The or-
ganic cofactors in copper-containing amine oxidases (AO) and lysyl
oxidase (LO), dopamine b-monooxygenase (DbM) and peptidylgly-
cine
a-hydroxylating monooxygenase (PHM) all involves mononu-
⇑
Corresponding author. Tel.: +82 2 910 4763; fax: +82 2 910 4415.
E-mail address: yoona@kookmin.ac.kr (S. Yoon).
clear copper active-oxygen species [15–17].
0277-5387/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.10.019

S. Kannan et al. / Polyhedron 30 (2011) 340–346
341
F
F
NaO
O
Cu(OAc)2.H2O
Co(NO3).6H2O
Hdmpz / THF
Ni(NO3).6H2O
Hdmpz / THF
CH3OH / THF
Hdmpz
F
F
F
NH
F
F
NH
F
N
N
N
NH
O
O
O
O
F
O
Co
O
H
N
O
Cu
O
O
Ni
O
N
O
N
NH
O
N
F
F
NH
F
F
F
1
3
2
Scheme 1. General reaction scheme for synthesis of metal complexes.
The literature also show that dinuclear nickel complexes with
bridging carboxylates [18,19] are interesting for their magnetic
interactions, whereas the mononuclear complexes play an impor-
tant role in modeling the active site of metalloproteins [2,20–22].
With nickel(II) as its native metal ion, urease catalyses the hydro-
lysis of urea to ammonia and carbamate and in turn spontaneous
decomposition of CO2 and a second molecule of ammonia [23].
However structurally characterized nickel carboxylate complexes
are limited in literature [24,25].
2.2. Preparation of NaO2CAr4-FPh
The m-terphenyl carboxylate ligand 2,6-di-(p-fluorophenyl)-
benzoate) (HO2CAr4-FPh) was prepared according to literature pro-
cedures [33–35]. The sodium salt of the carboxylic acid (NaO2CAr4-
FPh) was prepared by treating a MeOH solution of the free acid with
1 equiv. of NaOH and removing the volatile fractions under re-
duced pressure.
Ever since, the simultaneous recognition of Lippard and Tolman
that sterically hindered m-terphenyl carboxylates were ideal li-
gands for the preparation of model compounds of the enzyme ac-
tive site of non-heme diiron enzymes [26,27], several reports on
the use of this class of sterically hindered m-terphenyl ligands
for diiron enzymes as well as copper and manganese enzymes have
appeared in the literature which involves the dinuclear metal car-
boxylates [28,29]. Lee et al. also reported the carboxylate bridged
dinuclear cobalt and nickel complexes of sterically hindered 2,6-
di(p-tolyl)benzoic acid as a model of metallohydrase active sites
[30]. However, mononuclear copper(II), cobalt(II) and nickel(II)
complexes of this class of ligands are rarely reported [31,32]. Here-
in, we report the synthesis and characterization of mononuclear
cobalt, nickel and copper complexes incorporating the sterically
hindered terphenyl carboxylate (ꢀO2CAr4-FPh) and 3,5-dimethyl-
pyrazole (Hdmpz) as a co-ligand (Scheme 1).
2.3. Synthesis of complexes
2.3.1. Preparation of [Co(O2CAr4-FPh)2(Hdmpz)2] (1)
To a 20 mL THF solution of Co(NO3)2ꢁ6H2O (0.25 mmol; 0.072 g)
was added (0.42 mmol; 0.17 g) of NaO2CAr4-FPh and the reaction
mixture was stirred for 5 h under N2. To this reaction mixture,
3,5-dimethylpyrazole (0.50 mmol; 0.048 g) abbreviated as Hdmpz
was added and stirring was continued for overnight. The volatile
fractions were removed under reduced pressure and extracted
with CH2Cl2 and filtered to remove any undissolved solid. Violet
blocks of 1 suitable for X-ray diffraction were obtained by vapor
diffusion of pentane into the dichloromethane solution. Yield:
0.050 g (46% based on Co). FT-IR (NaClr, cmꢀ1): 2961(w),
2927(w), 2341(s), 2359(m), 1606(s), 1562(s), 1510(s), 1456(m),
1409(m), 1367(s), 1288(m), 1222(s), 1159(s), 1048(s), 1015(w),
843(m), 809(m), 793(w), 738(m), 705(m), 556(m), 534(m). UV–
Vis (CH2Cl2) (kmax, nm (e
, Mꢀ1 cmꢀ1)): 533 (342), 568 (441), 592
(410). Anal. Calc. for C48H38F4N4CoO4: C, 66.28; H, 4.40; N, 6.44.
Found: C, 66.40; H, 4.88; N, 6.60%.
2. Experimental
2.1. General considerations
2.3.2. Preparation of [Ni(O2CAr4-FPh)2(Hdmpz)2] (2)
All chemicals purchased were of analytic grade and were used
without further purification. All reactions were carried out under
N2 atmosphere by using Schlenk techniques. FT-IR spectra were re-
corded using Varian 640-IR FT-IR spectrometer. UV–Vis spectra
Compound 2 was synthesized in a manner similar to that of 1
with Ni(NO3)2ꢁ6H2O as the starting metal salt. Light green single
crystals suitable for X-ray diffraction were obtained by vapor diffu-
sion of pentane into the dichloromethane solution. Yield: 0.110 g
(50% based on Ni). FT-IR (NaCl, cmꢀ1): 3397(s), 2925(m),
2856(w), 2359(m), 2341(w), 1652(w), 1596(m), 1558(s), 1540(s),
1511(s), 1456(s), 1410(m), 1380(s), 1281(w), 1223(s), 1159(s),
1039(w), 838(s), 811(s), 792(s), 731(m), 704(m), 582(w), 555(m),
were recorded on
a OPTIZEN 2120UV spectrophotometer.
Electrochemical studies were performed using a Zahner Elektrik
IM 6 model potentiostat with 0.5 M dichloromethane solutions of
[(n-C4H9)4N]PF6 as supporting electrolyte under a nitrogen atmo-
sphere. A three-electrode configuration consisting of a platinum
working electrode, a platinum wire auxiliary electrode and an
Ag/AgCl reference electrode.
530(m), 473(w), 461(m). UV–Vis (CH2Cl2) (kmax, nm (e
, Mꢀ1 cmꢀ1)):
419 (51), 685 (21), 785 (13). Anal. Calc. for C48H38F4N4NiO4: C,
66.30; H, 4.40; N, 6.64. Found: C, 66.70; H, 5.00; N, 6.60%.

342
S. Kannan et al. / Polyhedron 30 (2011) 340–346
2.3.3. Preparation of [Cu(O2CAr4-FPh)2(Hdmpz)2] (3)
Table 2
To a 10 mL solution of of HO2CAr4-FPh (0.50 mmol; 0.077 g) in
methanol, was added Cu(II) acetate monohydrate (0.25 mmol;
0.050 g) and the reaction mixture was stirred for 15 min. To this
reaction mixture, Hdmpz (0.50 mmol; 0.048 g) was added. The
color of the solution changed to green and THF (10 mL) was added
to ensure complete dissolution. Stirring was continued for another
3 h. Green crystals of 3 suitable for X-ray diffraction were obtained
by slow evaporation. Yield: 0.095 g (44% based on Cu). FT-IR (NaCl,
cmꢀ1): 3367(br,w), 3226(br,w), 3057, 3042, 2927, 2605, 2513,
1606, 1552, 1511, 1454, 1407, 1379, 1298, 1222, 1159, 1095(w),
1047(w), 838, 809, 792, 769, 730, 554, 531, 490, 475, 461. UV–
Selected inter-atomic distances (Å) and angles (°) for
[Co(O2CAr4-FPh)2(Hdmpz)2] (1)a.
Inter-atomic distance
Co(1)–O(1)
Co(1)–O(3)
Co(1)–N(1)
Co(1)–N(3)
1.943(3)
1.968(3)
2.037(4)
2.009(4)
2.959(3)
3.076(4)
2.778(4)
2.698(4)
Co(1)ꢁ ꢁ ꢁO(2)
Co(1)ꢁ ꢁ ꢁO(4)
O(4)ꢁ ꢁ ꢁN(4)
O(2)ꢁ ꢁ ꢁN(2)
Inter-atomic angles
O(1)–Co(1)–O(3)
O(1)–Co(1)–N(3)
O(3)–Co(1)–N(3)
O(1)–Co(1)–N(1)
O(3)–Co(1)–N(1)
N(3)–Co(1)–N(1)
112.08(14)
108.25(14)
108.88(14)
114.59(15)
101.84(14)
111.02(17)
Vis (CH2Cl2) (kmax, nm (e
, Mꢀ1 cmꢀ1)): 755 (73). Anal. Calc. for
C48H38F4N4CuO4: C, 65.93; H, 4.38; N, 6.41. Found: C, 66.82; H,
4.82; N, 6.42%.
a
Numbers in parentheses are estimated standard
2.4. X-ray crystallographic studies
deviations of the last significant figures.
Single crystals were mounted at room temperature on the tips
of quartz fibers, coated with Partone-N oil, and cooled under a
stream of cold nitrogen. Intensity data were collected on a Bruker
experimental details for the complex are summarized in Table 1
and relevant interatomic distances and angles are listed in Table 2.
CCD area diffractometer running the SMART software package, with
0
Mo K
a radiation (k = 0.71073 ÅA). The structures were solved by
direct methods and refined on F2 by using the SHELXTL software
package [36]. Empirical absorption corrections were applied with
SADABS [37], part of the SHELXTL program package, and the structures
were checked for higher symmetry by the program PLATON [38]. All
non-hydrogen atoms were refined anisotropically. In general,
hydrogen atoms were assigned idealized position and given ther-
mal parameters equivalent to 1.2 times the thermal parameter of
the carbon atom to which they are attached. Data collection and
3. Results and discussion
3.1. Rationale for the synthetic approach
The use of pyrazole or pyrazole derivatives has drawn strong
interest in modeling biological systems. Trofimenko has used
pyrazole in polypyrazolyborates to stabilize a variety of organome-
tallic and coordination complexes [39] which are valuable in bio-
mimetic coordination chemistry of numerous metalloproteins
since these monoanionic, facially coordinating ligands have histi-
dine-like donors which can hold three cis sites fixed while leaving
other coordination sites open. Thus, a variety of complexes with
3,5-dimethylpyrazoles have been synthesized and employed in
coordination and organometallic chemistry [40–42]. To the best
of our knowledge direct synthesis of mononuclear transition metal
complexes of 2,6-di(4-fluorophenyl)benzoate) (HO2CAr4-FPh) con-
taining 3,5-dimethylpyrazole as co-ligand have not been reported.
The synthesis of the mononuclear complexes were achieved via the
reaction of metal salts, carboxylate ligand and 3,5-dimethylpyra-
zole in 1:2:2 M ratio. The complexes were characterized by IR,
UV–Vis spectral techniques. The coordination modes are different
in all three cases as evidenced by single crystal diffraction studies.
The characteristic bands of carboxylate groups are shown in the
range of 1540–1606 cmꢀ1 for asymmetric stretching and 1407–
1456 cmꢀ1 symmetric stretching [43], respectively. The absence
of any bands in the area of 1710 cmꢀ1 indicates full deprotonation
of all carboxylate groups in 1–3 [44], which is consistent with the
results of the X-ray analysis.
Table 1
Summary of X-ray crystallographic data for [Co(O2CAr4-FPh)2(Hdmpz)2](1), [Ni(O2-
CAr4-FPh)2(Hdmpz)2](2), and [Cu(O2CAr4-FPh)2(Hdmpz)2](3).
1
2
(3)4ꢁCH3OH
Empirical
formula
Formula
weight
Space group
a (Å)
C
48H36CoF4N4O4 C48H38F4N4NiO4 C193H156Cu4F16N16O17
867.74
869.53
3529.50
ꢀ
C2/c
P21/n
P1
11.1281(12)
13.7586(13)
15.5806(16)
68.788(2)
88.767(2)
77.525(3)
2167.2(4)
2
26.839(5)
8.2580(16)
20.067(4)
18.4779(3)
10.8361(2)
21.7164(4)
b (Å)
c (Å)
a
(°)
b (°)
110.809(3)
101.5940(10)
c
(°)
V (Å3)
4157.4(14)
4
4259.52(13)
1
Z
Dcalc, (g/cm3)
T (K)
1.330
296(2)
0.461
1.389
173(2)
0.535
1.376
296(2)
0.581
Absorption
coefficient
(mmꢀ1
)
Total number
of data
Number of
unique data
points
16286
10601
17270
4824
22710
4422
3.2. Synthesis and characterization of [Co(O2CAr4-FPh)2(Hdmpz)2] (1)
The reaction of the terphenyl carboxylate ligand with cobalt(II)
nitrate hexahydrate in THF with 3,5-dimethylpyrazole, gives com-
plex 1 with a square planar geometry around Co(II) ion (Fig. 1). The
selected bond lengths and bond angles are given in Table 2. Both
the carboxylic acid groups are deprotonated, necessitating a 1:2
metal:ligand ratio in the neutral compound. The two pyrazoles
are present in trans conformation across a crystallographic mirror
plane. The deprotonated pocket carboxylates are also trans and
bound to the cobalt ions through a single oxygen, giving the metal
tetrahedral geometry with O–Co–N angles of 108.34 and 114.77.
The Co1–O1 and Co1–O3 bond distances are 1.943(3) Å and
Number of
parameters
R1 (%)a
554
282
582
7.29
12.67
0.464 and
ꢀ0.536
3.45
9.31
0.394 and
ꢀ0.342
3.71
6.24
wR2 (%)b
Largest
0.238 and ꢀ0.352
difference
in peak and
hole (e Å-3
)
a
R1
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
b
2
wR2 = {
R
[w(Fo ꢀ Fc2)2]/
R .
[w(Fo2)2]}1/2

S. Kannan et al. / Polyhedron 30 (2011) 340–346
343
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
900
Wavelength (nm)
Fig. 2. UV–Vis spectra for (a) [Co(O2CAr4-FPh)2(Hdmpz)2] (1) (solid line); (b)
[Ni(O2CAr4-FPh)2(Hdmpz)2] (2) (dashed line) and (c) [Cu(O2CAr4-FPh)2(Hdmpz)2] (3)
(dotted line) in CH2Cl2. Inset shows the UV–Vis spectrum of the complex 2.
the atom numbering scheme is illustrated in Fig. 3, and selected
bond lengths and bond angles are given in Table 3. One coordina-
tion environment present in the asymmetric unit involves the
coordination of Ni(II) ions by four oxygen atoms from two ter-
phenyl carboxylate ligands in chelating bidentate modes and two
nitrogen atoms from Hdmpz groups that are oriented cis to each
another. The O2–Ni–O4 bite angle of the carboxylate in complex
2 is 104.77. One of the Ni–O1 bond distances is 2.193 Å whereas
the other Ni–O2 is 2.040 Å reflecting the asymmetric binding of
the carboxylate ligands. The difference of at least 0.018 Å for the
M–O bond and 0.027 Å for the M–N bond may be due to the in-
creased steric hindrance around the six-coordinate nickel com-
pared to the four-coordinate cobalt in 1. Thus, the complex 2 has
axially elongated octahedral geometry around the Ni(II) cation.
Compared with complex 1, the oxygen atoms of the carboxylate li-
gand did not form intramolecular hydrogen bonding with N–H
group of Hdmpz.
Fig. 1. Ortep drawing of [Co(O2CAr4-FPh)2(Hdmpz)2] (1) showing 50% probability
thermal ellipsoids. The hydrogen atoms and the phenyl rings of Ar4-FPhCOOꢀ ligand
are omitted for clarity.
This asymmetric binding is small when compared to iron mono-
nuclear complex incorporating a sterically hindered terphenyl car-
boxylate ligand reported by Tolman et al. [47] and some other
reports on similar binding of zinc and copper mononuclear com-
plexes containing carboxylate ligands [48]. Complex 2 showed
1.968(3) Å respectively, whereas Co1–O2 and Co1–O4 bond
distances are 2.959(3) Å and 3.076(4) Å, respectively. The long
distances for Co1–O2 and Co1–O4 indicated that these oxygens
were not involved in bond formation with metal center. Two
hydrogen-bonding interactions are observed between the carbox-
ylate and Hdmpz ligands, the Oꢁ ꢁ ꢁH–N distances being 2.778 and
2.698 Å. The electronic spectrum of cobalt complex (Fig. 2a) in
dichloromethane shows a multicomponent absorptions with a
two peaks (Fig. 2b) at 419 nm (
= 21 Mꢀ1 cmꢀ1) caused by 3A2g ? 3T1g(P) and 3A2g ? 3T1g(F)
transitions respectively and a weak absorption band at 785 nm
= 13 Mꢀ1 cmꢀ1) is due to the spin-forbidden 3A2g ? 1E1g transi-
e
= 51 Mꢀ1 cmꢀ1) and 685 nm
(e
(e
tion in the electronic spectrum recorded in dichloromethane. This
pattern of UV–Vis spectra of nickel complex further supports the
octahedral geometry around Ni(II) ion [8,9].
resolved peak at 568 nm (
e
= 342 Mꢀ1 cmꢀ1) and two shoulders
= 410 Mꢀ1 cmꢀ1
)
at 533 nm (
e
= 441 Mꢀ1 cmꢀ1) and 592 nm (
e
which definitely resulted from 4A2 ? 4T1 characteristic of Co(II)
complexes [45]. Such weak absorption bands in the range of
550–700 nm, has been reported for d7 Co(II) ions in methionine
aminopeptidases (MAPs) [46].
3.4. Synthesis and characterization of [Cu(O2CAr4-FPh)2(Hdmpz)2] (3)
Treatment of the copper(II) acetate monohydrate and the ter-
phenyl carboxylate ligand in methanol followed by 3,5-dimethyl-
pyrazole (Hdmpz) leads to the formation of five coordinated
mononuclear complex 3 in which one carboxylate is monodentate,
the other is in a chelating bidentate mode, and the pyrazoles are
cis-coordinated. The unsymmetric bidentate chelating mode of
one of the carboxylate is supported by the difference in Cu1–O3
and Cu1–O4 bond distances. The ortep view of the crystal structure
is shown in Fig. 4, and selected bond lengths and bond angles are
given in Table 4. The stereochemistry of the copper center can be
3.3. Synthesis and characterization of [Ni(O2CAr4-FPh)2(Hdmpz)2] (2)
Compound 2 was synthesized by reaction of Ni(NO3)2ꢁ6H2O
with NaO2CAr4-FPh in THF solution followed by treatment with
3,5-dimethylpyrazole in a 1:2:2 M ratio although analogous to
preparation of complex 1, affording a six coordinated mononuclear
nickel complex 2. The molecular structure of the complex 2 with

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S. Kannan et al. / Polyhedron 30 (2011) 340–346
Table 3
Selected inter-atomic distances (Å) and angles (°) for
[Ni(O2CAr4-FPh)2(Hdmpz)2] (2)a.
Inter-atomic distance
Ni(1)–O(1)
Ni(1)–O(2)
Ni(1)–N(1)
O(2)ꢁ ꢁ ꢁN(2A)
2.1929(10)
2.0403(10)
2.0194(12)
3.0425(13)
Inter-atomic angles
O(1)–Ni(1)–N(1)
N(1)–Ni(1)–N(1A)
O(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(1A)
O(2)–Ni(1)–O(1)
152.66(4)
100.87(7)
92.77(4)
97.37(4)
104.77(4)
a
Numbers in parentheses are estimated standard
deviations of the last significant figures.
Fig. 3. Ortep drawing of [Ni(O2CAr4-FPh)2(Hdmpz)2] (2) showing 50% probability
thermal ellipsoids. The hydrogen atoms and the phenyl rings of Ar4-FPhCOOꢀ ligand
are omitted for clarity.
determined by the Addison
distortion from regular square pyramidal or trigonal bipyramidal
[49]. In a regular square pyramidal, = 0, and for a perfect trigonal
bipyramidal = 1. The coordination environment of the copper is a
perfect square pyramidal with factor of 0.04. The two oxygen
s factor, a parameter that describes the
Fig. 4. Ortep drawing of [Cu(O2CAr4-FPh)2(Hdmpz)2] (3): left; showing 50% prob-
ability thermal ellipsoids. Structure with the phenyl rings of Ar4-FPhCOOꢀ ligand are
omitted for clarity: right. The hydrogen atoms in the N–H group of Hdmpz ligands
are added to show the hydrogen bonding interaction.
s
s
s
atoms of the bidentate carboxylate (O3 and O4) with the bite angle
of 64.43° and the two coordinated nitrogen atoms (N1 and N3) of
the pyrazole constitute the basal plane of the square pyramidal
structure and the apical position is fulfilled by the oxygen atom
(O1) of the monodentate carboxylate ligand. The apical Cu–O1 dis-
tance 2.109(6) Å is distinctly longer than the other coordinated
oxygens of the carboxylate group comprising the basal plane as

S. Kannan et al. / Polyhedron 30 (2011) 340–346
345
expected for the square pyramidal geometry of the copper chelate.
Similar, to the cobalt complex 1 the Cu1–O1, Cu1–O3 and Cu1–O4
bond distances are 2.109(6) Å, 2.061(10) Å and 2.030(10) Å respec-
tively are shorter than Cu1–O2 bond distance of 3.449(9) Å, which
confirms the non-bonding of O2 with the metal center. This unco-
ordinated oxygen O2 exhibits intramolecular hydrogen bonding
with the N–H group of the pyrazole moiety, with O2ꢁ ꢁ ꢁN2 and
O2ꢁ ꢁ ꢁN4 interatomic distances of 2.754 and 2.821 Å, respectively.
The electronic spectra of the copper complex 3 (Fig. 2c) recorded
in dichloromethane solution, displayed a broad band centered at
The cyclic voltammograms for the 1 and 2 complexes in CH2Cl2
between 1.5 and ꢀ1.5 V shows one irreversible reduction peak at
ꢀ1.166 V and ꢀ0.319 V versus Ag/AgCl for cobalt and nickel,
respectively. The representative cyclic voltammograms of cobalt
and nickel are given in Fig. S1 in the supporting information. Re-
peated scans between 0.0 and ꢀ1.5 V showed a gradual decrease
in current both in case of complex 1 and 2 demonstrating the
irreversibility. The reason for the irreversibility observed for the
reductive response of the complexes may be due to a short lived
reduced state of the metal ion or due to oxidative degradation of
the ligands. As compared with the nickel(II) complex, the cobalt(II)
complex has more negative metal centered reduction, that means
the metal centered reduction is more difficult to achieve in case
of cobalt than nickel. The electrochemical processes in both nickel
and cobalt are metal centered as the ligand shows no peak in the
potential window scanned for the complexes. Copper(II) mononu-
clear complex 3 exhibit an electrochemically quasi-reversible
oxidation (CuII/CuIII) at 0.317 V versus Cp2Fe+/Cp2Fe with
749 nm
(e )
= 73 Mꢀ1 cmꢀ1 arising from 2Eg ? 2T2g transition,
showing the characteristic feature of Cu(II) ions.
Because mononuclear iron(II) complexes with the sterically
bulky carboxylates also been reported [50], the geometrical prefer-
ence in a series of 3rd transition metal ions with sterically hindered
coordination sphere was induced based on the structures in
Scheme 2. The combined influence of the sterically hindered car-
boxylate ligands and the intramolecular hydrogen-bonding inter-
actions changes the binding mode of the carboxylate ligands
from monodentate to bidentate and stabilizes Fe(II) and Co(II)
complexes, [Fe(O2CAr4-FPh)2(Hdmpz)2] and 1, with the low coordi-
nation number of 4.
D
Ep = 103 mV with scan rate of 100 mV/s. Even with a scan rate
of 250 mV/s the redox couple remained quasi-reversible. Upon
the positive scan towards the cathodic potential the copper com-
plex shows an oxidation peak and only a small reduction peak
(Fig. 5). In case of copper chelate also we notice a decrease in the
anodic and cathodic current during the second cycle. Further scan-
ning toward either more positive or more negative potential within
the solvent potential window indicated the absence of redox
activity.
3.5. Electrochemical properties of the complexes
Out of curiosity to determine the redox property of the
complexes, the electrochemical experiments were studied in
dichloromethane solution by cyclic voltammetry under N2 atmo-
sphere using
a platinum working electrode and the redox
potentials are expressed with reference to Ag/AgCl.
4.0
Table 4
369 mV
Selected inter-atomic distances (Å) and angles (°) for
2.0
[Cu(O2CAr4-FPh)2(Hdmpz)2] (3)a.
Interatomic distance
0.0
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–O(3)
Cu(1)–O(4)
Cu(1)ꢁ ꢁ ꢁO(2)
O(2)ꢁ ꢁ ꢁN(2)
O(2)ꢁ ꢁ ꢁN(4)
Interatomic angles
2.109(6)
1.980(10)
1.964(10)
2.061(10)
2.030(10)
3.449(9)
266 mV
-2.0
-4.0
-6.0
-8.0
2.754(10)
2.821(10)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
O(1)–Cu(1)–O(3)
O(1)–Cu(1)–O(4)
O(3)–Cu(1)–O(4)
99.47(4)
98.16(4)
98.33(5)
96.97(5)
96.00(1)
64.43(11)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2 -0.4
Potential (V)
a
Fig. 5. Cyclic voltammogram for [Cu(O2CAr4-FPh)2(Hdmpz)2] (3) in CH2Cl2 with
0.5 M (Bu4N)PF6 as supporting electrolyte and a scan rate of 100 mV/s.
Numbers in parentheses are estimated standard
deviations of the last significant figures.
R
O
R
H
N
N
H
H
N
O
N
N
O
O
O
N
R
N
N
M
N
Cu
H
Ni
O
O
O
O
O
H
N
N
N
O
H
R
O
R
R
M = Fe and Co
2
3
Scheme 2. Variation of coordination geometries in the complexes.

346
S. Kannan et al. / Polyhedron 30 (2011) 340–346
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Center Program through the National Research Foundation of Kor-
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(MEST). This work was also supported in part by KIST (Korea Insti-
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gram’, the faculty research program 2009 of Kookmin University
in Korea and the Korea Science and Engineering Foundation
(KOSEF) grant funded by the Korea government (MEST) (No. R11-
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