Complexes with lignin model compound vanillic acid. Two different carboxylate ligands in the same dinuclear tetracarboxylate complex [Cu 2(C8H7O4)2(O 2CCH3)2(CH3OH)2]

Article abstract of DOI:10.1016/j.poly.2005.08.031

Two copper(II) coordination compounds with vanillic acid C ₈H₈O₄ (1), namely [Cu₂(C ₈H₇O₄)₂(O₂CCH ₃)₂(CH₃OH)₂] (2) and [Cu ₂(C₈H₇O₄)₄(H ₂O)₂] (3), were synthesized and characterized. Single crystals of 1-3 were obtained and their crystal structures determined. The structure of 2 shows dinuclear cage structure of copper acetate hydrate type, however with two different carboxylates, acetates and vanillic acid anions, respectively. Both bridging anions are in pairs in trans orientation. Methanol molecules are apically coordinated (Cu-O7 2.160(2) ?), fulfilling square-pyramidal coordination sphere around both copper ions. The compound 2 decomposes outside mother-liquid (yielding [Cu₂(C₈H ₇O₄)₂(O₂CCH₃) ₂(H₂O)₂] (2a)) with the removal of methanol, but without significant change of the dicopper tetracarboxylate cage structure, as noticed by μ_eff 1.48 BM for 2a. Similar was found also in the X-band EPR spectra with three signals H_z1, H_⊥2 and H_z2 in the region from 0 to 600 mT. The structure of free vanillic acid 1 is composed of dimeric units of two molecules, connected by two parallel hydrogen bonds between carboxylate group of each other (O1-H?O2 2.642(3) ?), while the structure of 3 is of [Cu₂(O₂CCH ₃)₄(H₂O)₂] type. Interestingly, an additional signal in the EPR spectra of 3 is found at 80 mT (H _⊥1) at 298 and at 116 K, next to three signals H_z1, H_⊥2 and H_z2.

Full text of DOI:10.1016/j.poly.2005.08.031

Polyhedron 25 (2006) 1161–1166
www.elsevier.com/locate/poly
Complexes with lignin model compound vanillic acid. Two
different carboxylate ligands in the same dinuclear
tetracarboxylate complex [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2]
a,
a
a
a
Bojan Kozlevcˇar *, Darja Odlazek , Amalija Golobicˇ , Andrej Pevec ,
b
a
ˇ
Peter Strauch , Primozˇ Segedin
a
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Asˇkercˇeva 5, P.O. Box 537, SI-1001 Ljubljana, Slovenia
b
Institute of Chemistry, University of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany
Received 14 July 2005; accepted 19 August 2005
Available online 3 October 2005
Abstract
Two copper(II) coordination compounds with vanillic acid C8H8O4 (1), namely [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2] (2) and
[Cu2(C8H7O4)4(H2O)2] (3), were synthesized and characterized. Single crystals of 1–3 were obtained and their crystal structures deter-
mined. The structure of 2 shows dinuclear cage structure of copper acetate hydrate type, however with two different carboxylates, ace-
tates and vanillic acid anions, respectively. Both bridging anions are in pairs in trans orientation. Methanol molecules are apically
˚
coordinated (Cu–O7 2.160(2) A), fulfilling square-pyramidal coordination sphere around both copper ions. The compound 2 decom-
poses outside mother-liquid (yielding [Cu2(C8H7O4)2(O2CCH3)2(H2O)2] (2a)) with the removal of methanol, but without significant
change of the dicopper tetracarboxylate cage structure, as noticed by leff 1.48 BM for 2a. Similar was found also in the X-band EPR
spectra with three signals Hz1, H^2 and Hz2 in the region from 0 to 600 mT. The structure of free vanillic acid 1 is composed of dimeric
˚
units of two molecules, connected by two parallel hydrogen bonds between carboxylate group of each other (O1–Hꢀ ꢀ ꢀO2 2.642(3) A),
while the structure of 3 is of [Cu2(O2CCH3)4(H2O)2] type. Interestingly, an additional signal in the EPR spectra of 3 is found at
80 mT (H^1) at 298 and at 116 K, next to three signals Hz1, H^2 and Hz2.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Copper; Vanillic acid; Dinuclear; Heterocarboxylate; H^1; EPR
1. Introduction
tonated hydroxy and methoxy group) coordination of
vanillin, yielding trans or cis mononuclear species. The
didentate coordination was observed also in the copper
complex with very similar lignin model compound vanil-
lic acid [Cu2(C8H7O4)4(H2O)2] [8], however the structure
is dinuclear and the carboxylate group serves as a bridge
as observed in [Cu2(O2CCH3)4(H2O)2] type of complexes.
Vast majority of the described dinuclear paddle-wheel
complexes is with four identical carboxylates for all four
bridges in a dinuclear unit, and only rare examples of
different carboxylate anions in one dinuclear unit are
known [9–12]. A more detailed description of copper–
vanillic acid system of compounds might help in better
evaluation of the coordination role of the oxygen groups
Copper fungicides play a very important role in the
wood protection [1]. To investigate copper to wood inter-
actions, copper complexes with lignin model compounds
(Scheme 1) were prepared. So far, several vanillin com-
plexes [2–7] and one vanillic acid coordination compound
[8] have been presented. It appeared that Cu–vanillin
compounds might be obtained either with or without
the presence of a nitrogen donor ligand. The main anal-
ogy in Cu–vanillin complexes is in the didentate (depro-
*
Corresponding author. Tel.: +386 1 241 91 27; fax: +386 1 241 92 20.
E-mail address: bojan.kozlevcar@fkkt.uni-lj.si (B. Kozlevcˇar).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.08.031

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B. Kozlevcˇar et al. / Polyhedron 25 (2006) 1161–1166
10 mL of hot water. The solution was added to 0.244 g
(1.45 mmol) of vanillic acid. Green crystallites were filtered
off next day and then dried one day in a desiccator. Yield
80%. Anal. Calc. for C32H32Cu2O18 (3): C, 46.2; H, 3.88;
Cu, 15.3. Found: C, 46.0; H, 4.18; Cu, 15.2. UV–Vis–
NIR (DMSO) kmax, 260, 290, 450(sh), 720 nm; (mull) kmax
,
230, 265, 300, 380(sh), 690 nm. IR, 3456 m(O–H), 1615,
1596, 1570 mas(CO2), 1518, 1455, 1426 ms(CO2), 1394
ms(CO2), 774 cmꢁ1. EPR (298 K), gx = 2.05, gy = 2.06,
gz = 2.34,
241 cmꢁ1; (116 K), gx = 2.05, gy = 2.06, gz = 2.35, Az1
7.0 mT, D = 0.303 cmꢁ1, E = 0.0030 cmꢁ1, 2J = 231 cmꢁ1
D = 0.306 cmꢁ1
,
E = 0.0025 cmꢁ1
,
2J =
=
.
Scheme 1. Two lignin model compounds vanillin (C8H8O3) and vanillic
acid (C8H8O4) (1).
leff (298 K), 1.56 BM. The single crystals of 3, suitable
for X-ray analysis, were obtained by a similar procedure,
but at lower concentration of the starting compounds.
on benzene ring, present in simple lignin model com-
pounds (Scheme 1).
2.2. X-ray structure analysis
A dinuclear heterocarboxylate copper paddle-wheel
compound with vanillic and acetic acid anions [Cu2-
(C8-H7O4)2(O2CCH3)2(CH3OH)2] was synthesized and
characterized. It was compared to a related homocarboxy-
late complex [Cu2(C8H7O4)4(H2O)2].
Details of the crystal data, data collection and refine-
ment parameters for 1, 2 and 3 are listed in Table 1. The
selected distances and angles are presented in Table 2. A
colorless plate of compound 1 was glued, while green
prisms of compounds 2 and 3 were greased on a glass
thread. The diffraction data for 1–3 were collected on a
Nonius Kappa CCD diffractometer with graphite mono-
chromated Mo Ka radiation. A Cryostream Cooler
(Oxford Cryosystems) was used for cooling the samples of
2 and 3. The data were processed using DENZO [13].
Structures of 1 and 2 were solved by direct methods using
SIR97 [14] and refined using XTAL 3.6 [15] by a full-matrix
least-squares procedure based on F. Structure of 3 was
solved by direct methods implemented in SHELXS-97 [16]
and refined by a full-matrix least-squares procedure based
on F2 (SHELXL-97) [17]. All of the non-hydrogen atoms were
refined anisotropically. The positions of hydrogen atoms in
1 and 2 were obtained from the difference Fourier maps.
Positional parameters of H atoms for 1, while positional
and isotropic displacement parameters of H atoms for 2,
were refined. The water hydrogen atoms in 3 were visible
in the last stages of the refinement and were refined freely,
while the other hydrogen atoms were included in the model
at geometrically calculated positions and refined using a
riding model.
2. Experimental
2.1. Synthesis
All starting compounds and solvents were used as pur-
chased, without any further purification. The single crystals
of vanillic acid (C8H8O4) (1), suitable for X-ray analysis,
were obtained by a recrystallization from ethylacetate.
[Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2] (2), [Cu2(C8H7O4)2
(O2CCH3)2(H2O)2] (2a): Vanillic acid (0.061 g; 0.36 mmol)
was added to a hot methanol solution (10.0 mL) of [Cu2-
(O2CCH3)4(H2O)2] (0.040 g; 0.10 mmol). Next day, a dark
green precipitate of 2 was filtered off, washed three times
with methanol and then left for a day on air, yielding 2a.
Yield 80%. Anal. Calc. for C20H24Cu2O14 (2a): C, 39.0;
H, 3.93; Cu, 20.7. Found: C, 38.9; H, 4.17; Cu, 20.7%.
UV–Vis–NIR (DMSO) kmax, 260, 290, 450(sh), 720 nm;
(mull) kmax, 230, 265, 290, 360(sh), 695 nm. IR, 3420
m(O–H), 1623 mas(CO2), 1602, 1591 mas(CO2), 1518, 1417
ms(CO2), 1398 ms(CO2), 780 cmꢁ1. EPR (298 K), g^ = 2.09,
gi = 2.36, D = 0.329 cmꢁ1, 2J = 278 cmꢁ1; (116 K), g^ =
2.07, gi = 2.36, Az1 = 5.6 mT, D = 0.326 cmꢁ1, 2J = 237
cmꢁ1. leff (298 K), 1.48 BM. The single crystals of 2, suit-
able for X-ray analysis, were obtained by a similar proce-
dure, but at lower concentration of the starting
compounds. The crystals were frozen immediately after
removal from mother-liquid, to prevent their decomposi-
tion due to evaporation of methanol and its probable
replacement with water molecules (elemental analysis, IR).
[Cu2(C8H7O4)4(H2O)2] (3): The starting compounds and
a procedure was different as already described [8]. 0.160 g
(0.40 mmol) of [Cu2(O2CCH3)4(H2O)2] was dissolved in
2.3. Physical measurements
Metal analysis was carried out electrogravimetrically
with Pt electrodes. C,H analysis was performed with a Per-
kin–Elmer, Elemental Analyzer 2400 CHN. The magnetic
susceptibility of the substances was determined at room
temperature by powdered samples with a Sherwood Scien-
tific MSB-1 balance. Diamagnetic corrections were esti-
mated from PascalÕs constants [18]. Infrared spectra were
measured on mineral mulls, using a Perkin–Elmer FT-IR
1720X spectrometer. Electronic spectra were recorded as
nujol mulls and DMSO solutions with a Perkin–Elmer

B. Kozlevcˇar et al. / Polyhedron 25 (2006) 1161–1166
1163
Table 1
Relevant crystal data and data collection summary
1
2
3
Formula
C8H8O4
168.15
monoclinic
P21/c
3.9101(1)
17.3900(6)
11.3267(5)
90
95.313(1)
90
766.87(5)
4
C22H28Cu2O14
643.54
triclinic
C32H32Cu2O18
831.66
monoclinic
P21/n
8.2747(4)
19.5282(11)
10.0006(5)
90
102.725(3)
90
1576.31(14)
2
Formula weight
Crystal system
Space group
ꢀ
P1
˚
a (A)
8.3217(2)
8.9495(2)
9.7552(3)
69.633(1)
76.824(1)
64.165(1)
610.52(3)
1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g/cm3)
1.456
0.118
293
1.750
1.812
150
1.752
1.438
150
l (mmꢁ1
T (K)
)
Crystal color
colorless
plate
0.25 · 0.25 · 0.10
27.5
green
prism
0.15 · 0.10 · 0.09
25.0
0.024
green
prism
0.23 · 0.10 · 0.10
27.5
0.035
Crystal shape
Crystal size (mm)
hmax (ꢁ)
Rint
0.049
Refined parameters
Total data
133
10029
172
7267
248
5813
Independent data
Observed data [F2 > 2r(F2)]
Ra (observed)
1715
981
0.052
0.056b
ꢁ0.408, 0.281
2130
1985
0.031
0.026b
3420
2869
0.067
0.135c
c
Rwb, wR2 (observed)
ꢁ3
˚
Dqmin,max (e A
)
ꢁ0.807, 0.555
ꢁ0.520, 1.294
P
P
a
R = (||Fo| ꢁ |Fc||)/ |Fo|.
P
P
b
c
R = (w(|Fo| ꢁ |Fc|))/ (w|Fo|).
P
P
2
2 1=2
wR2 ¼ ð ½wðF 2o ꢁ F c2Þ ꢂ= ðwF 2oÞ Þ
.
Table 2
˚
Selected bond distances (A) and angles (ꢁ) in the structures 2 and 3
2
3
Cuꢀ ꢀ ꢀCu
Cu–O1
Cu–O2
Cu–O5
Cu–O6
Cu–O7
2.608(1)
1.960(1)
1.951(1)
1.965(2)
1.984(2)
2.160(2)
O1–Cu–O2
O1–Cu–O5
O1–Cu–O6
O1–Cu–O7
O2–Cu–O5
O2–Cu–O6
O2–Cu–O7
O5–Cu–O6
O5–Cu–O7
O6–Cu–O7
169.31(5)
89.99(6)
90.22(6)
97.85(5)
89.02(6)
88.76(7)
92.83(5)
169.13(5)
96.87(6)
93.87(6)
Cuꢀ ꢀ ꢀCu
Cu–O3
Cu–O2
Cu–O5
Cu–O4
Cu–O6
2.611(1)
1.956(3)
1.958(3)
1.977(3)
1.981(3)
2.170(4)
O3–Cu–O2
O3–Cu–O5
O2–Cu–O5
O3–Cu–O4
O2–Cu–O4
O5–Cu–O4
O3–Cu–O6
O2–Cu–O6
O5–Cu–O6
O4–Cu–O6
169.51(16)
87.06(15)
90.37(15)
86.93(15)
93.61(15)
167.67(15)
97.29(16)
93.11(16)
96.76(17)
94.68(17)
Hydrogen-bonding geometry
O7–H7ꢀ ꢀ ꢀO4 2.735(2)
169(5)i
O6–H1ꢀ ꢀ ꢀO10 2.877(6)
O6–H2ꢀ ꢀ ꢀO9 2.959(6)
O10–H10ꢀ ꢀ ꢀO5 2.873(5)
167(8)a
159(7)b
O4–H4ꢀ ꢀ ꢀO6 2.751(2)
O4–H4ꢀ ꢀ ꢀO3 2.707(2)
157(3)ii
102(2)
155c
O9–H9ꢀ ꢀ ꢀO7 2.679(6)
114
111
O10–H10ꢀ ꢀ ꢀO8 2.731(6)
Symmetry codes: (i) 2 ꢁ x, 1 ꢁ y, 1 ꢁ z; (ii) x, 1 + y, ꢁ1 + z.
(a) 1/2 + x, 1/2 ꢁ y, ꢁ1/2 + z; (b) ꢁx, ꢁy, 1 ꢁ z; (c) 1/2 ꢁ x, 1/2 + y, 5/2 ꢁ z.
UV/Vis/NIR spectrometer Lambda 19. EPR spectra of the
powdered samples were recorded by a Bruker ESP-300
spectrometer, operating at X-band (9.59 GHz) at room
temperature and at 116 K. The values of parameters g, A,
D, E and 2J were calculated directly from the signal posi-
tions in the spectra [19–21].
3. Results and discussion
3.1. The structure of free vanillic acid C8H8O4 (1)
Two molecules of free vanillic acid are strongly H-
bonded by two mutual H-bonds between their carboxylate

1164
B. Kozlevcˇar et al. / Polyhedron 25 (2006) 1161–1166
0
˚
groups (O1–Hꢀ ꢀ ꢀO2 2.642(3) A), thus forming a centro-
symmetrical dimeric unit (Fig. 1), as often found at carbox-
ylic acids. Another H-bond is intramolecular, formed by
the hydroxy group and the methoxy group (O4–Hꢀ ꢀ ꢀO3
is of [Cu2(O2CCH3)4(H2O)2] type with four identical car-
boxylate ligands. Similar pattern, as in 2, is found in the
vanillic acid anion, with the phenolate group involved in
all H-bonds and an intra-molecular H-bond between hy-
droxy and methoxy groups. Each hydroxy group is H-
bonded also inter-molecularly to axially coordinated water
˚
2.631(3) A). A comparison to the structure of related lignin
model compound vanillin (Scheme 1) [22] reveals signifi-
cantly different intermolecular H-bonding, due to the alde-
hyde group instead of the carboxylic group. The vanillin
molecules are packed in chains, where strong H-bonds con-
nect aldehyde and phenolato groups on positions 1 and 4
˚
molecules (O6–H1ꢀ ꢀ ꢀO10 2.876(6) A, O6–H2ꢀ ꢀ ꢀO9 2.958(6)
˚
A), giving two H-bonds with each water molecule.
Two H-bonds (3) to the axial ligand instead of one (2)
and better volatility of methanol (2) than water (3) may
be the main reasons for better stability of 3 than 2 outside
mother-liquid. Interestingly, only two vanillic acid anions
in 3 are forming another H-bond to carboxylate oxygen
˚
of the benzene ring (O–Hꢀ ꢀ ꢀO 2.712(2)–2.743(3) A).
3.2. The structure of bis(l-(4-hydroxy-3-methoxybenzoato-
O:O0))bis(l-(acetato-O:O0))bis(methanol) dicopper(II)
(2)
˚
atom (O10–H10ꢀ ꢀ ꢀO5 2.873(5) A), while for another two,
no such connections were found, probable due to steric
reasons. This is very probable reason that only two vanillic
acid anions are replaced with acetates in 2. Small acetates
enable stronger (shorter) inter-molecular H-bonds, possi-
bly due to less steric hindrance (see Table 2). Additionally,
stronger stabilization in 3 than in 2 is found also in
p-stacking interaction (mean inter-planar separation
The centrosymmetric paddle-wheel central core [Cu2
(C8H7O4)2(O2CCH3)2(CH3OH)2] (2) is of copper(II) ace-
tate hydrate type, however with two different carboxylates
(two acetates and two vanillic acid anions) in trans orienta-
tion, while methanol molecules occupy the apical positions
(Fig. 2). The phenolate group is involved in all three hydro-
gen bonds, either intra-molecularly to methoxy oxygen
˚
˚
˚
3.30 A (3.37 A) for 3 (2), centroidꢀ ꢀ ꢀcentroid distance
˚
3.94 A (4.22 A) for 3 (2)). It seems that the decisive factors
for the competition between the homocarboxylate (3) and
heterocarboxylate (2) are inter-molecular interactions.
Either weaker H-bonds, stronger p-stacking and two H-
bonds to the axial ligand (3), or stronger H-bonds, weaker
p-stacking and one H-bond to the axial ligand (2).
˚
atom (O4–Hꢀ ꢀ ꢀO3 2.707(2) A), or inter-molecularly to
˚
methanol molecule (O7–Hꢀ ꢀ ꢀO4 2.735(2) A) and acetate
˚
group (O4–Hꢀ ꢀ ꢀO6 2.751(2) A).
3.3. The structure of tetrakis(l-(4-hydroxy-3-methoxy-
benzoato-O:O0))bis(aqua) dicopper(II) (3)
3.4. Spectroscopic and magnetic analysis
The structure of complex [Cu2(C8H7O4)4(H2O)2] (3) was
already reported [8], but was redetermined due to unavail-
ability of the atom positions from the CSD [25]. Complex 3
Although the compound 2 is not stable outside mother-
liquid, yielding 2a, the leff 1.48 BM (2a) suggests only
partial decomposition of 2 and a retainment of the dinu-
clear tetracarboxylate central core in the compound 2a
(1.55 BM for 3). This is in agreement with almost identical
solution or mull UV–Vis–NIR spectra for 2a and 3, respec-
tively, and with EPR spectra, where typical signals (Hz1,
H^2, Hz2 [19], Fig. 3) for the triplet state S = 1 of
[Cu2(O2CCH3)4(H2O)2] type of compounds may be ob-
served at room temperature and at 116 K (see Section 2).
In the low temperature EPR spectra, the splitting of a
Fig. 1. The dimeric structure of free vanillic acid with two hydrogen-
bonds between the carboxylic groups of the vanillic acid molecules [23,24].
H
z1 signal to septet, H^2 to Hx2 and Hy2 and an appearance
of the signal of mononuclear species Hmono at 330 mT were
noticed. Such phenomena are often found in the low tem-
perature spectra of the dinuclear copper(II) tetracarboxy-
lates. Differences in the Hz1 splitting, the hyperfine
constant Az1 (5.6 mT; 7.0 mT), and the H^2 separation
for the spectra of 2a and 3 may be related to a different role
of the bridging anions present in both complexes, and/or
replacement of the apical ligand (2 ! 2a) that might not
enable full equivalency of both Cu ions in the dinuclear
unit after rearrangement. Close to the Hz1 signal in the
spectra of 3, an additional distinctive signal may be ob-
served at 80 mT (Fig. 3). This signal is assigned as H^1
(Hx1, Hy1) that is observable, since D < hm, and may be
described by the equations [19–21]:
Fig. 2. Two types of carboxylate anions in a dinuclear copper(II)
tetracarboxylate complex [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2] (2)
[23,24]. The intermolecular H-bonds are connecting the phenolate group
(O4) with the acetate anion (O6) and methanol molecules (O7).

B. Kozlevcˇar et al. / Polyhedron 25 (2006) 1161–1166
1165
Fig. 4. Torsion in the O–Cuꢀ ꢀ ꢀCu–O angles in 3 [23–25]. The second
central copper ion is positioned behind the first one. All hydrogen atoms
and aromatic rings are omitted for clarity.
Fig. 3. Three signals Hz1, H^2 (Hx2, Hy2) and Hz2, characteristic for
[Cu2(O2CCH3)4(H2O)2] type of compounds for 2a and 3, and an
additional signal H^1 (Hx1, Hy1) at 80 mT, in the spectra of 3 at room
temperature (RT) and at 116 K.
of mother solution) or from the frozen solution spectra
could be obtained.
A broad band at 3600–3300 cmꢁ1 in the IR spectrum of
2a agrees with the presence of water instead of methanol (a
narrow band for O–H bond would be expected for 2). One
strong band in the region 1650–1550 cmꢁ1 found for 3
(1570 cmꢁ1) and two strong bands in the spectrum of 2a
(1623, 1591 cmꢁ1) are probably due to the asymmetric
stretching vibration [30] of one (vanillic acid anion) and
two (acetate, vanillic acid anion) types of carboxylate an-
ions in the compounds 2a and 3, respectively. Since in
[Cu2(O2CCH3)4(H2O)2], an acetate analogue of 3, mas(CO2)
was observed at 1605 cmꢁ1 [31], the higher energy band at
1623 cmꢁ1 in the spectrum of 2a is assigned to the acetate,
while the 1591 cmꢁ1 band to the vanillic acid anion.
ꢀ ꢁ
H2x1
H2y1
¼
¼
2ðH0 ꢁ D0 þ E0ÞðH0 þ 2E0Þ;
ð1Þ
ð2Þ
ge
gx
!
2
ge
gy
ðH0 ꢁ D0 ꢁ E0ÞðH0 ꢁ 2E0Þ;
H0 ¼ hm=geb; D0 ¼ D=geb; E0 ¼ E=geb.
Such signal and its assignment was, up to our knowledge,
so far not reported for dicopper(II) tetracarboxylates, but
only for [Cu2(O2CCH3)2L2(dmf)2], with two O–C–O and
two N–C–N bridges in Cu tetracarboxylate analogue [26].
Similar signal was noticed also in the spectra of copper(II)
dinuclear complex with four O–C–N bridges in [Cu2(chp)4]
[27]. The partial coordination sphere in these two
complexes is distorted square-planar CuO2N2 (only coordi-
nation atoms from bridges and without the axial ligand).
The presence of the H^1 signal for these two tetracarboxy-
late analogues is in agreement with low D values (0.265;
0.275 cmꢁ1 ) D < hm). Structurally, the complex 3 is re-
lated to these two complexes by a relatively large torsion
angle 6.0ꢁ for two trans O–Cuꢀ ꢀ ꢀCu–O moieties (both O
atoms are from the same carboxylate group; Fig. 4). For
[Cu2(chp)4], the O–Cuꢀ ꢀ ꢀCu–N torsion angle is 5.1–8.6ꢁ,
while for [Cu2(O2CCH3)2L2(dmf)2], O–Cuꢀ ꢀ ꢀCu–O 3.4ꢁ
and N–Cuꢀ ꢀ ꢀCu–N 0.9ꢁ, respectively. Comparable angles
for 2 and for [Cu2(O2CCH3)4(H2O)2] are significantly smal-
ler (<1.2ꢁ, <2ꢁ) [28]. The Cuꢀ ꢀ ꢀCu distances for both com-
4. Concluding remarks
The compounds [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2]
(2) and [Cu2(C8H7O4)4(H2O)2] (3) precipitate from the
solution of the same starting compounds [Cu2(O2CCH3)4
(H2O)2] and vanillic acid, however in different polar solvent
methanol or water for 2 or 3, respectively. Heterocarboxy-
late compound [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2] (2) is
unstable outside mother-liquid. Its decomposition is par-
tial, since the central carboxylate cage remains, however
water molecules replace the apical methanol molecules.
On the other hand, the homocarboxylate compound [Cu2-
(C8H7O4)4(H2O)2] (3) is stable also in air, probably due to
two H-bonds to the axial ligand instead of one (2) and less
volatile water (3) than methanol (2). Significant differences
were found in the EPR spectra, resulting in an unexpected
signal at 80 mT in the spectra of 3, next to Hz1, H^2 and
Hz2, typical for S = 1 spin state. This signal is assigned as
H^1, due to low D axial zero field splitting parameter
value (D < hm). The appearance of this signal is probably
related to a relatively large torsion angle O–Cuꢀ ꢀ ꢀCu–O,
found in compound 3, and not with the Cuꢀ ꢀ ꢀCu distance
˚
plexes 2 and 3 (2.608(1), 2.611(1) A) are similar and typical
for dicopper tetracarboxylates [29], therefore this distance
can not be directly correlated to the appearance of the
H^1 signal in the EPR spectra of 3.
Since herein, the complex 3 is structurally characterized
with 2, and with 2a by EPR, this correlation may not be
absolutely exact. Maybe some additional information from
the EPR spectra of the undecayed 2 (powder inside traces

1166
B. Kozlevcˇar et al. / Polyhedron 25 (2006) 1161–1166
[6] T.J. Greenhough, M.F.C. Ladd, Acta Crystallogr., Sect. B 34 (1978)
2744.
inside a dinuclear molecule. Coordination of the acetates,
together with vanillic acid anions, in 2 and 2a, yields stron-
ger H-bonds and much less torsion in dicopper tetracarb-
oxylate central core than in 3, and seems to be an
appropriate alternative to more extensive H-bonding with
the axial ligand and more effective p-stacking in homocarb-
oxylate complex 3.
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[12] M.A. Banares, L. Dauphin, X. Lei, W. Cen, M. Shang, E.E. Wolf,
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The financial support of the Ministry of Education, Sci-
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MSZS P-6209-124, P1-0175-103 and X-2000, is gratefully
ˇ
acknowledged. We thank Dr. M. Sentjurc, EPR center,
ÔJozˇef StefanÕ Institute, Ljubljana, for EPR spectra.
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Appendix A. Supplementary data
The final atomic and geometrical parameters, crystal
data and details concerning data collection and refinement
for all three compounds have also been deposited with the
Cambridge Crystallographic Data Center as supplemen-
tary material with the deposition numbers 277423 (1),
277424 (2) and 277425 (3), respectively. These data can
be obtained, free of charge via http://www.ccdc.cam.
ac.uk/const/retrieving.html. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.poly.2005.08.031.
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