Steric effect on construction of Cu(II) complexes with pyridine carboxamide ligands

Article abstract of DOI:10.1016/j.molstruc.2007.10.006

The structures of three new Cu(II) complexes with pyridine carboxamide ligands (Me₂bpb, 6-Me₂-Mebpb, and 6-Me₂-Me₂bpb) have been determined. 6-Methyl-substituted pyridyl bpb ligands produced dimeric compounds wi

Full text of DOI:10.1016/j.molstruc.2007.10.006

Available online at www.sciencedirect.com
Journal of Molecular Structure 885 (2008) 56–63
www.elsevier.com/locate/molstruc
Steric effect on construction of Cu(II) complexes
with pyridine carboxamide ligands
a
b
b
b
b
Do Nam Lee , Ji Young Ryu , Han Kwak , Young Min Lee , Sun Hwa Lee ,
b
c
c
b,
*
Jung In Poong , Junghyun Lee , Woonsup Shin , Cheal Kim
,
d
d,
*
Sung-Jin Kim , Youngmee Kim
a
Department of Chemistry, Kwangwoon University, Seoul 139-701, Republic of Korea
Department of Fine Chemistry, Eco-Product and Materials Education Center, Seoul National University of Technology, Seoul 139-743, Republic of Korea
b
c
Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea
d
Division of Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea
Received 23 August 2007; received in revised form 9 October 2007; accepted 9 October 2007
Available online 16 October 2007
Abstract
The structures of three new Cu(II) complexes with pyridine carboxamide ligands (Me2bpb, 6-Me2-Mebpb, and 6-Me2-Me2bpb) have
been determined. 6-Methyl-substituted pyridyl bpb ligands produced dimeric compounds with Cu(II) ions, and weak interactions
between dimers can make even polymeric compounds, while bpb ligands without 6-methyl substitution produced monomeric Cu(II)
complexes. The large distortion effects of 6-methyl-substitution are shown in Cu(II) complexes with 6-methyl-substituted pyridyl bpb
ligands. This result suggests that the steric effect of 6-methyl-substitution plays important role for distortion of the structure, and 6-
methyl-substitution can also influence to make polymeric compounds with interactions between Cu(II) ions and neighbor carbonyl oxy-
gen atoms. In addition, the voltammetric behaviors of the Cu complexes were examined and classified into two groups, with/without 6-
methyl group. The complexes without 6-methyl group show reversible redox waves at ꢀꢁ1.6 V, and the complexes with 6-methyl group
do irreversible redox ones at ꢀꢁ1.3 V, indicating that the presence of the methyl group of 6-position of the complex makes the reduction
of the complexes easier.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Pyridine carboxamide; Copper(II) complex; Crystal structure; Steric effect
1. Introduction
act as a tetradentate ligand to chelate a metal ion. Impor-
tantly, metal complexes containing pyridine carboxamide
ligands can show excellent charateristics in hydroxylation,
epoxidation and asymmetric catalysts [2], NO deliver [3],
molecular receptor [4], dendrimer synthesis [5], and spin
ground state control [6] as well as magnetic interactions
[7]. Metal complexes containing deprotonated pyridine
carboxamide ligands have shown tendency for planar N4-
tetradentate coordination. In this planar geometry, hydro-
gen atoms in the pyridyl 60-position have, structurally,
close intramolecular contacts. If bulky substituents have
been replaced on the pyridyl 60-position, they can influence
highly distorted geometries. The copper complex 4 is the
case, which is an example to show this steric effect so far
Pyridine carboxamide ligands containing ubiquitous
carboxamide [–C(O)NH–] group in the primary structure
of proteins have been much attention, and they are also
important for construction of metal complexes [1]. The
pyridine carboxamide can be obtained from the condensa-
tion of pyridine carboxylic acids and amines, and deproto-
nation of the carboxamide nitrogen atom makes the ligand
*
Corresponding authors. Tel.: +82 2 970 6693; fax: +82 2 973 9149 (C.
Kim); tel.: +82 2 3277 3589; fax: +82 2 3277 2384 (Y. Kim).
E-mail addresses: chealkim@snut.ac.kr (C. Kim), ymeekim@
ewha.ac.kr (Y. Kim).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.10.006

D.N. Lee et al. / Journal of Molecular Structure 885 (2008) 56–63
57
[8]. In order to systematically investigate such a steric effect
on construction of Cu(II) complexes, therefore, two new
ligands containing –CH3 on the pyridyl 60-position (6-
Me2-Mebpb L5, and 6-Me2-Me2bpb L6) were synthesized.
Herein we report syntheses and crystal structures of three
new Cu(II) complexes with pyridine carboxamide ligands
(Me2bpb L3, 6-Me2-Mebpb L5, and 6-Me2-Me2bpb L6),
and also, discuss the steric effect of 6-methyl-substitution
for six copper complexes with/without 6-methyl group.
300 MHz): d 2.44 (s, 3H, CH3), 2.46 (s, 3H, CH3), 2.52
(s, 3H, CH3), 7.10–8.25 (m, 9H, aromatic-H), 10.23 (s,
1H, N–H), 10.41 (s, 1H, N–H). IR (KBr): (N–H) 3318
and 3337 cmꢁ1and (C@O) 1681 cmꢁ1. Anal. Calcd for
C21H20N4O2 (360.41): C, 69.98; H, 5.60; N, 15.55. Found:
C, 69.70; H, 5.57; N, 15.85.
2.4. Syntheses of 3, 5 and 6
Complexes 3, 5, and 6 were obtained from the reaction
of each ligand (0.5 mmol) with Cu(ClO4)2Æ6H2O (0.185 g,
0.5 mmol) in CH3OH (20 mL) in good yields. Crystals of
the complexes 3, 5, and 6 were grown by slowly evaporat-
ing their solutions in ether/pyridine for 3, DMSO for 5,
and DMSO for 6, respectively. The yields were 0.13 g
(65.0%), 0.10 g (50.0%), and 0.11 g (48.0%), respectively.
For 3, IR (KBr): (C@O) 1641 cmꢁ1. Anal. Calcd for
C20H16CuN4O2: C, 58.89; H, 3.96; N, 13.74. Found: C,
58.73; H, 3.86; N, 13.72. For 5, IR (KBr): (C@O)
1623 cmꢁ1. Anal. Calcd for C21H18CuN4O2: C, 59.78; H,
4.31; N, 13.28. Found: C, 60.01; H, 4.62; N, 12.85. For
2. Experimental
2.1. Materials
Diamines, 6-methyl-2-picolinic acid, picolinic acid, pyr-
idine, ethanol, diethyl ether, chloroform, Cu(ClO4)2Æ6H2O,
and triphenylphosphite were purchased from Aldrich and
were used as received. H2bpb [9], H2Mebpb [10],
H2Me2bpb [11], H26-Me2-bpb [12], H26-Me2-Me2bpb
[13], [Cu(bpb)(H2O)] [14], [Cu(Mebpb)(H2O)] [10], and
[Cu(6-Me2-bpb)(H2O)] [8], were synthesized by previous
methods.
6, IR (KBr): (C@O) 1620 cmꢁ1
. Anal. Calcd for
C22H20CuN4O2: 60.61; H, 4.63; N, 12.85. Found: C,
2.2. Instruments
60.78; H, 4.30; N, 13.11.
Elemental analysis for carbon, nitrogen, and hydrogen
was carried out by using an EA1108 (Carlo Erba Instru-
ment, Italy) in the Organic Chemistry Research Center of
Sogang University, Korea. 1H NMR spectra were recorded
on Oxford Unity 300 spectrometer at ambient temperature.
IR spectra were measured on a BIO RAD FTS 135 spec-
trometer as KBr pellets. Electrochemical measurements
were performed with CH Instruments Model 910B poten-
tiostat and were carried out in DMSO containing copper
complexes (1 mM) and tetrabutylammonium hexafluoro-
phosphate (TBABF4, 0.1 M) as a supporting electrolyte
in one chamber. The working and auxiliary counter elec-
trodes were platinum. The potential was measured by using
an Ag/Ag+ (0.01 M) reference electrode and reported ver-
sus a Fc+/Fc (Fc = ferrocene) couple. The cyclic voltam-
2.5. Crystallography
The diffraction data for compounds 3, 5 and 6 were col-
lected on a Nonius Kappa-CCD diffractometer using Mo
˚
Ka (k = 0.71073 A) [15]. The crystals were mounted on
glass fibers under epoxy. The CCD data were integrated
and scaled using the DENZO-SMN software package
[16], and the structures were solved and refined by using
SHELXTL V5.1 [17]. All hydrogen atoms were located in
the calculated positions. The crystallographic data for
compounds 3, 5, and 6 are listed in Table 1.
CCDC Reference Nos. 658073, 658074 and 658075 for
3, 5 and 6, respectively.
3. Results and discussion
mograms were run at a scan rate of 50 mV sꢁ1
.
3.1. Syntheses
2.3. Synthesis of ligand H26-Me2-Mebpb L5
Ligands L1–L6 were prepared in moderate yields (40–
60%) by the reactions between appropriate diamines with
6-methyl-2-picolinic acid or 2-picolinic acid in pyridine in
the presence of triphenylphosphite (Scheme 1) according
to the literature method [14]. The complexes 3, 5, and 6
were obtained from the reaction of each ligand with
copper(II) perchlorate hexahydrate in CH3OH in good
yields (50–70%). The complexes 1–3 (without 6-methyl)
were isolated as brown solids and 4–6 (with 6-methyl) as
green solids, which are much soluble in DMSO and
DMF. Crystals of the complexes 3, 5, and 6 were grown
by slowly evaporating their solutions in appropriate
solvent systems as explained in Section 2. All complexes
are stable in the air.
For the preparation of the 1,2-bis(6-methylpyridine-2-
carboxamido)-4-methylbenzene (H26-Me2-Mebpb),
a
modified method by Lee et al. was used [13]. To a stirred
solution of 6-methylpicolinic acid (1.37 g, 10 mmol) in
pyridine (10 mL), a solution of 4-methyl-1,2-phenylenedi-
amine (0.610 g, 5 mmol) in pyridine (5 mL) was added drop
by drop. The solution was stirred for 15 min and triphenyl
phosphite (2.62 mL, 10 mmol) was slowly added. The reac-
tion mixture was warmed up to 100 ꢁC, and the mixture
was stirred for 4 h. The volume of solution was then
reduced to 2 ml and kept in air. Crystallization from water
afforded pale-yellow powder, which was washed with etha-
nol. The yield was 0.76 g (42%). 1H NMR (CDCl3,

58
D.N. Lee et al. / Journal of Molecular Structure 885 (2008) 56–63
Table 1
The crystallographic data for 3, 5 and 6
Cu(Me2bpb), 3
20H16CuN4O2
Cu(6-Me2-Mebpb), 5
Cu(6-Me2-Me2bpb), 6
Empirical formula
Formula weight
C
C21H18CuN4 O2
421.93
C22H20CuN4O2
435.96
407.91
Temperature
Wavelength
Crystal system
Space group
150(1) K
150(1) K
0.71073 A
Monoclinic
C2/c
150(1) K
0.71073 A
Monoclinic
C2/c
˚
˚
˚
0.71073 A
Orthorhombic
Pbcn
˚
˚
˚
a
18.3150(2) A
10.9224(5) A
11.0200(3) A
˚
˚
˚
b
13.1010(4) A
20.9404(12) A
21.6170(8) A
˚
˚
˚
c
6.8570(5) A
16.0231(6) A
15.9040(5) A
a
b
c
90.000ꢁ
90.000ꢁ
90.000ꢁ
90.000ꢁ
100.789(3)ꢁ
90.000ꢁ
90.000ꢁ
101.2390(18)ꢁ
90.000ꢁ
3
3
˚
3
˚
˚
Volume
1645.30(13) A
3600.0(3) A
3716.0(2) A
Z
4
8
8
Density (calculated)
Absorption coefficient
Crystal size
Reflections collected
Independent reflections
Data/restraints/parameter
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
Largest diff. peak and hole
1.647 mg/m3
1.557 mg/m3
1.559 mg/m3
1.353 mmꢁ1
0.30 · 0.10 · 0.08 mm3
8083
1888 [R(int) = 0.0653]
1888/0/125
1.044
R1 = 0.0377, wR2 = 0.0966
R1 = 0.0483, wR2 = 0.1048
0.823 and ꢁ0.764 e A
1.239 mmꢁ1
0.24 · 0.10 · 0.10 mm3
16261
4124 [R(int) = 0.0574]
4124/1/263
1.042
R1 = 0.0451, wR2 = 0.1064
R1 = 0.0783, wR2 = 0.1213
0.474 and ꢁ0.530 e A
1.203 mmꢁ1
0.30 · 0.20 · 0.04 mm3
12,582
4258 [R(int) = 0.0658]
4258/0/267
1.051
R1 = 0.0487, wR2 = 0.1174
R1 = 0.0646, wR2 = 0.1264
0.420 and ꢁ0.747 e A
ꢁ3
ꢁ3
ꢁ3
˚
˚
˚
Scheme 1. Syntheses of ligands and their Cu complexes.
3.2. Structure description
bpb ligands are coordinated to Cu(II) ions to form dimeric
and polymeric compounds as well as monomeric com-
plexes. IR spectroscopy confirmed deprotonation as mNH
bands were disappeared while the mCO bands (1620,
The structures of three new Cu complexes 3, 5 and 6
have been determined by X-ray crystallography. R1R2R3-
Table 2
˚
The selected bond distances (A) and angles (ꢁ)
Cu(bpb)(H2O)a
Cu(Mebpb)b
Cu(Me2bpb)
Cu(6-Me2-bpb)c
Cu(6-Me2-Mebpb)
Cu(6-Me2-Me2bpb)
1
2
3
4
5
6
Cu–N(amide)
Cu–N(pyridyl)
1.933(2)
1.930(3)
2.023(3)
6.67(4)
1.9123(2)
2.018(2)
ꢁ5.77(1)ꢁ
1.934(7)
2.079(7)
ꢁ19.6(2), ꢁ23.4(2)
1.918(2), 1.928(3)
2.041(2), 2.057(2)
23.2(1), 30.58(1)ꢁ
1.923(2), 1.926(2)
2.0550(2), 2.074(2)
23.01(1), 31.66(1)
2.02(2), 2.25(2)
ꢁ11.01(4), 17.02(4)
Torsion angle between two
pyridyl rings (C–N–Cu–N)
Cu ꢂ ꢂ ꢂ O(neighbor)
C@O
N(amide)–Cu–N(amide)
N(pyridyl)–Cu–N(pyridyl)
Space group
–
3.855(3)
1.238(4)
82.9(1)
111.6(1)
Pbcn
3.883(1)
1.238(2)
82.9(1)
118.9(1)
Pbcn
2.693(6), 2.798(6)
1.25(9), 1.23(7)
81.3(2)
117.3(2)
P21/c
2.681(1), 3.185(1)
1.238(3), 1.255(4)
82.66(9)
116.1(1)
C2/c
2.645(1), 3.223(1)
1.247(3), 1.241(4)
82.9(1)
117.1(1)
C2/c
1.239(3), 1.241(3)
82.8
110.7
P1
a
Ref. [14].
Ref. [10].
Ref. [8].
b
c

D.N. Lee et al. / Journal of Molecular Structure 885 (2008) 56–63
59
Fig. 1. (a) Structure of compound 3 with 50% ellipsoid. (b) Packing diagram of 3.

60
D.N. Lee et al. / Journal of Molecular Structure 885 (2008) 56–63
1623 cmꢁ1) were moved to lower wave-number from those
in H2R1R2R3-bpb. The selected bond distances and angles
are listed in Table 2.
torted square planar. These monomeric compounds are
stacked where the Cu(II) ion is towards the carbonyl oxy-
gen atom of the neighbor molecule as shown in Fig. 1b.
3.2.1. Crystal structure of monomeric compound
[Cu(Me2bpb)] 3
3.2.2. Crystal structures of dimeric compounds [Cu(6-Me2-
Mebpb)] 5 and [Cu(6-Me2-Me2bpb)] 6
There is a 2-fold axis through a Cu(II) atom as shown in
Fig. 1a. Four N atoms of the Me2bpb2ꢁ ligand are coordi-
nated to the Cu(II) ion. The Cu–N(amide) distance,
Four N atoms of both ligands (6-Me2-Mebpb2ꢁ and 6-
Me2-Me2bpb2ꢁ) are coordinated to the Cu(II) ions (Figs.
2 and 3a). The Cu–N(amide) distances of 5 and 6 are also
shorter than the Cu–N(pyridyl) distances (Table 2). The
distorting effects of methyl-substitution are obvious in both
compounds. Two pyridyl rings are tilted a lot with the dihe-
dral angle of 43.12(7)ꢁ for 5 and 43.95(8)ꢁ for 6. The coor-
dination of Cu(II) ion for both compounds has large
˚
1.91230(17) A, is shorter than the Cu–N(pyridyl) distance,
2ꢁ
˚
2.0183(17) A (Table 2). The C–O distance of the Me2bpb
˚
ligand is 1.238(2) A. Two pyridyl rings are a little tilted
with the dihedral angle between the two symmetry related
pyridyl rings of 9.48(9). The geometry of Cu(II) ion is dis-
Fig. 2. (a) Structure of compound 5 with 50% ellipsoid. (b) Polymeric structure containing dimeric units with Cu ꢂ ꢂ ꢂ O(neighbor) interactions (dotted line)
and weak interactions between dimeric units (green dotted line). All hydrogen atoms were omitted for clarity.

D.N. Lee et al. / Journal of Molecular Structure 885 (2008) 56–63
61
Fig. 3. (a) Structure of compound 6 with 50% ellipsoid. (b) Polymeric structure containing dimeric units with Cu ꢂ ꢂ ꢂ O(neighbor) interactions (dotted line)
and weak interactions between dimeric units (green dotted line). All hydrogen atoms were omitted for clarity.
deviation from the N4 plane towards the tetrahedral geom-
etry. There are significant interactions between Cu(II) ions
and carbonyl oxygen atoms of the neighbor molecule to
form a dimeric compound with Cu ꢂ ꢂ ꢂ O(neighbor) dis-
carbonyl oxygen atom of the neighbor molecule produced
a polymeric compound [14]. Monomethyl and dimethyl
bpb ligands with Cu(II) ions produced monomeric com-
plexes, 2 [10] and 3, and the geometry of Cu(II) ion is dis-
torted square planar with a little deviation from D4h
symmetry. The Cu(II) ion is towards a carbonyl oxygen
atom of the neighbor molecule without interactions
˚
˚
tances of 2.681(1) A and 2.645(1) A for compound 5 and
6, respectively (Figs. 2 and 3b). There are also weak inter-
actions between this dimeric units through Cu(II) ions and
carbonyl oxygen atoms with Cu(dimeric) ꢂ ꢂ ꢂ O(neighbor)
˚
between them (Cu ꢂ ꢂ ꢂ O(neighbor) 3.855 A). 6-Methyl-
˚
˚
distances of 3.185(1) A and 3.223(1) A for 5 and 6, respec-
tively. Considering both interactions, the geometry of
Cu(II) ion is elongated distorted octahedral.
substituted pyridyl bpb ligands produced dimeric com-
pounds with Cu(II) ions, 4 [8], 5, and 6, and weak interac-
tions between dimers can make even polymeric
compounds. The large distortion effects of 6-methyl-sub-
stitution are shown in compounds 4, 5, and 6. This result
suggests that the steric effect of 6-methyl-substitution
plays important role for distortion of the structure, and
6-methyl-substitution can also influence to make poly-
meric compounds with interactions between Cu(II) ions
and neighbor carbonyl oxygen atoms.
Picolinic acid and various diamines produced several
tetradentate bis-amide ligands R1R2R3bpb. Upon depro-
tonation of the carboxamide nitrogen atom in
R1R2R3bpb, ligands chelate to Cu(II) ions. N,N0-bis(20-
pyridinecarboxamide)-1,2-benzene (H2bpb) produced a
five-coordinated Cu(II) complex, 1 and the hydrogen bond
between the water ligand in the apical position and the

62
D.N. Lee et al. / Journal of Molecular Structure 885 (2008) 56–63
Acknowledgements
Financial support from the Korean Science & Engineer-
ing Foundation (R01-2005-000-10490-0(2005)), the Korea
Research Foundation (KRF-2004-005-C00093, 2005-070-
C00084, 2006-312-C00569 and 2007-314-C00159), and
Seoul R&BD Program is gratefully acknowledged.
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Table 3
Redox potential data for Cu complexes in 0.1 M TBABF4/DMSO (V vs.
Fc/Fc+)
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Compound
Cu(II/I) Epc (V)
Cu(II/I) Epa (V)
Cu(II/I) Eo (V)
1
2
3
4
5
6
ꢁ1.59
ꢁ1.60
ꢁ1.59
ꢁ1.36
ꢁ1.37
ꢁ1.37
ꢁ1.51
ꢁ1.55
(b) Z.-H. Ni, H.-Z. Kou, L. Zheng, Y.-H. Zhao, L.-F. Zhang, R.-J.
Wang, A.-L. Cui, O. Sato, Inorg. Chem. 44 (2005) 4728;
(c) E. Colacio, J.-P. Costes, J.M. Domınguez-Vera, I.B. Maimoum, J.
ꢁ1.52
ꢁ1.56
ꢁ1.52
ꢁ1.55
´
–
–
–
–
–
–
´
Suarez-Varela, Chem. Commun. (2005) 534;
(d) R. Lescoue¨zec, J. Vaissermann, L.M. Toma, R. Carrasco, F.
Lloret, M. Julve, Inorg. Chem. 43 (2004) 2243.

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