Copper fixation to guaiacyl lignin units by nitrogen donor ligands

Article abstract of DOI:10.1515/znb-2008-0501

Several new copper(II) complexes with guaiacyl lignin models vanillin (HL1) [Cu(L1)₂(nia)₂] (1) (nia = nicotinamide) or vanillic acid (HL2) [Cu(L2)₂(nia)₂] (2) [Cu₂(μ-L2) ₄(nia)₂] (3), and [Cu(L2)₂(Hetam)₂] (4) (Hetam = ethanolamine) were isolated and characterized. The molecular structure of complex 1 reveals bidentate vanillin (HL1) coordination via the methoxy and the deprotonated hydroxy groups. On the other hand, the vanillic acid (HL2) complexes 2-4 show a deprotonated carboxylate group with chelating coordination mode in 2, bridging in 3 and monodentate coordination in 4. The mononuclear complexes 1, 2 and 4 show a distorted trans octahedral coordination sphere with pairs of monodentate and chelating ligands. A replacement of the monodentate nicotinamide ligand in 2 with the bidentate ethanolamine ligand in 4 changes the coordination mode of the vanillic acid anion from bidentate (complex 2) to monodentate (complex 4). This shift inside the coordination sphere reveals different Q-Cu-O Jahn-Teller axes by the vanillic acid anion in 2 and ethanolamine in 4. Empty channels are present in the crystal structure of the dinuclear complex 3, stabilized by hydrogen bonds and π-π stacking.

Full text of DOI:10.1515/znb-2008-0501

Copper Fixation to Guaiacyl Lignin Units by Nitrogen Donor Ligands
Bojan Kozlevcˇar, Polonca Basˇkovicˇ, Aleksej Arko, Amalija Golobicˇ, Nives Kitanovski,
ˇ
and Primozˇ Segedin
Faculty of Chemistry and Chemical Technology, University of Ljubljana, 1001 Ljubljana, Slovenia
Reprint requests to Bojan Kozlevcˇar. E-mail: bojan.kozlevcar@fkkt.uni-lj.si
Z. Naturforsch. 2008, 63b, 481 – 488; received January 7, 2008
Several new copper(II) complexes with guaiacyl lignin models vanillin (HL1) [Cu(L1)2(nia)2]
(1) (nia = nicotinamide) or vanillic acid (HL2) [Cu(L2)2(nia)2] (2) [Cu2(µ-L2)4(nia)2] (3), and
[Cu(L2)2(Hetam)2] (4) (Hetam = ethanolamine) were isolated and characterized. The molecular
structure of complex 1 reveals bidentate vanillin (HL1) coordination via the methoxy and the de-
protonated hydroxy groups. On the other hand, the vanillic acid (HL2) complexes 2 – 4 show a de-
protonated carboxylate group with chelating coordination mode in 2, bridging in 3 and monodentate
coordination in 4. The mononuclear complexes 1, 2 and 4 show a distorted trans octahedral coordi-
nation sphere with pairs of monodentate and chelating ligands. A replacement of the monodentate
nicotinamide ligand in 2 with the bidentate ethanolamine ligand in 4 changes the coordination mode
of the vanillic acid anion from bidentate (complex 2) to monodentate (complex 4). This shift inside
the coordination sphere reveals different O–Cu–O Jahn-Teller axes by the vanillic acid anion in 2 and
ethanolamine in 4. Empty channels are present in the crystal structure of the dinuclear complex 3,
stabilized by hydrogen bonds and π-π stacking.
Key words: Copper, Guaiacyl, Lignin, Vanillin, Vanillic Acid, N-Donor Ligands
Introduction
Copper-based wood preservatives are widely used
due to the significant fungicidal activity of copper
and its environmental acceptability [1 – 3]. Presum-
ably, these preservatives will be used for wood preser-
vation also in the following decades as there is no
economically acceptable alternative available as yet.
The interactions of copper with wood are one of
the elements determining its role in the wood pro-
tection process. In particular, such a copper formu-
lation has to satisfy several requirements concerning
its penetration into wood, a fixation there and pre-
vention of water leaching [4]. All these details are
key elements especially for an efficient outdoor wood
protection. It was suggested that the main copper-to-
wood interaction takes place via lignin and hemicel-
lulose [5, 6]. The specific coordination sites of the
naturally occurring macromolecular lignin are sim-
ulated by different phenyl types of model ligands.
Syringyl (4-hydroxy-3,5-dimethoxyphenyl), guaiacyl
(4-hydroxy-3-methoxyphenyl) and p-hydroxyphenyl
derivatives all comprise methoxy and hydroxy oxygen
coordination sites (Scheme 1). The softwood lignin
is mainly composed of guaiacyl units from the pre-
Scheme 1. Two families of lignin model compounds: guaia-
cyl (a) and syringyl (b).
dominant precursor ferulic acid (Scheme 2d) [7]. Be-
side one cobalt and one nickel complex [8], only cop-
per complexes with vanillin or vanillic acid as lig-
ands (Scheme 2) have been reported for this family
in the literature [8 – 17]. All examples reveal depro-
tonation of vanillin and vanillic acid and their biden-
tate coordination via hydroxy and methoxy groups
in the case of vanillin, while vanillic acid coordi-
nates through its carboxylate group. Water is the most
convenient solvent for an application of copper-based
wood protection formulations, though organic sol-
vents should not be excluded. To prepare a suitable
wood protecting formulation, the fixation agent and
some other additives are needed next to cooper and
c
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B. Kozlevcˇar et al. · Nitrogen Donor Ligands
Experimental Section
Synthesis
[Cu(L1)2(H2O)2] (HL1 = vanillin) was prepared via
trans-[Cu(L1)2(H2O)2] · 2H2O, which decomposes in air, as
described previously [16]. All other chemicals were readily
available from commercial sources and were used as received
without further purification.
[Cu(L1)2(nia)2] (1)
Nicotinamide (0.35 g, 2.9 mmol) was dissolved in
20 mL of methanol. This solution was added to solid
[Cu(L1)2(H2O)2] (HL1 = vanillin) (0.32 g, 0.80 mmol). Af-
ter one day, gold-brown crystals of 1 started to precipitate
beside undissolved [Cu(L1)2(H2O)2]. The solid mixture was
stirred with a glass rod, and after 24 h pure 1 was filtered
off and dried over KOH for one day. Yield 65 %. – Anal.
for C28H26CuN4O8 (610.1): calcd. C 55.13, H 4.30, N 9.18,
Cu 10.42; found C 54.9, H 4.41, N 8.78, Cu 10.80. – IR:
ν = 3355, 3175 (N–H), 1695 (C=O)nia, 1653 (C=O)L1, 1582,
1557, 1490, 1470, 1435 (CH3), 1385, 1305 cm−1. – UV/Vis:
Scheme 2. The guaiacyl lignin model compounds: (a) 4-
hydroxy-3-methoxybenzaldehyde (vanillin), (b) 4-hydroxy-
3-methoxybenzoic acid (vanillic acid), (c) 4-hydroxy-3-
methoxybenzyl alcohol, and (d) 3-(4-hydroxy-3-methoxy-
phenyl)prop-2-enoic acid (ferulic acid).
λ
ꢀ
max = 260, 330, 470(sh) 620 nm. – EPR (298 K): g = 2.07,
g = 2.34, A = 10.9 mT. – χ (298 K): µeff = 1.95 µ⊥B.
ꢀ
[Cu(L2)2(nia)2] (2)
Vanillic acid (HL2; 0.25 g, 1.5 mmol) was added to
a water solution (20 mL) of Cu(NO3)2 · 3H2O (0.19 g,
0.79 mmol). Nicotinamide (1.4 g, 11.5 mmol) was simul-
taneously dissolved in 20 mL of water and then added to
vanillic acid in the copper nitrate solution. Violet crystals
of 2 soon started to precipitate. They were filtered off af-
ter 2 d and dried over KOH for a day. Yield 65 %. – Anal. for
C28H26CuN4O10 (642.1): calcd. C 52.38, H 4.08, H 8.73,
Cu 9.90; found C 51.90, H 4.15, N 8.52, Cu 10.20. – IR: ν =
3385, 3260, 3140 (O–H), (N–H), 1690 (C=O)nia, 1625, 1604,
1594, 1536 (O2C)as, 1511, 1465, 1440 (CH3), 1394 (O2C)s,
1354 cm−1. – UV/Vis: λmax = 220, 270, 290, 400(sh), 510,
560(sh) nm. – EPR (298 K): g = 2.06, g = 2.34. – χ
the appropriate solvent. During the last two decades,
ammonia- and amine-based fixation agents [5, 6, 18]
have replaced highly toxic chromium(VI) compounds
used in the old formulations [19, 20]. Ethanolamine
(Hetam) seems to be the most appropriate replace-
ment for chromium [18, 21]. Copper(II) ions easily
form cationic Cu-Hetam species [22] helping the cop-
per penetration into wood, but can be washed out of
it as well. Efforts to prevent such leaching are cur-
rently in progress [4]. On the other hand, the addition
of nicotinamide showed enhanced fungicidal activity
of the binuclear fatty acid copper(II) carboxylates [23].
This is important, because copper(II) itself cannot en-
sure sufficient protection against wood destroying or-
ganisms, which is the reason why in practice other co-
biocides are combined with copper(II) [24].
Herein, we present four new copper(II) complexes
with two lignin model ligands, viz. vanillin (HL1) and
vanillic acid (HL2): [Cu(L1)2(nia)2] 1 (nia = nicoti-
namide), [Cu(L2)2(nia)2] 2, [Cu2(µ-L2)4(nia)2] 3 and
[Cu(L2)2(Hetam)2] 4 (Hetam = ethanolamine). In ad-
dition to the lignin model ligands, the nitrogen donor
ligand nicotinamide is additionally coordinated in all
cases. The particular combination of both types of lig-
ands is crucial for the coordination mode realized in
these complexes.
⊥
ꢀ
(298 K): µeff = 1.93 µB.
[Cu2(µ-L2)4(nia)2] (3)
Solid Cu2O (20 mg, 0.14 mmol), vanillic acid (80 mg,
0.48 mmol) and nicotinamide (80 mg, 0.66 mmol) were ho-
mogenized. 20 mL of acetonitrile was added to the solid
mixture, and the obtained suspension was stirred and then
left standing for 24 h. Green needle-like crystals of 3 were
filtered off, washed with acetonitrile, and dried over KOH
for 24 h. Yield 35 %. – Anal. for C44H40Cu2N4O18 (1039.9):
calcd. C 50.82, H 3.88, N 5.39, Cu 12.22; found C 50.19,
H 3.89, N 5.69, Cu 11.70. – IR: ν = 3400, 3160 (O–H),
(N–H), 1685 (C=O)nia, 1620, 1600, 1570 (O2C)as, 1515,
1455, 1427 (CH3), 1390 (O2C)s cm−1. – UV/Vis: λmax
=
=
270, 300, 400(sh), 580(sh), 720 nm. – EPR (298 K): g
⊥
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B. Kozlevcˇar et al. · Nitrogen Donor Ligands
483
Table 1. Relevant crystal data and data collection summary for 1 – 4.
1
2
3
4
Formula
C28H26CuN4O8
610.08
C28H26CuN4O10 C44H40Cu2N4O18
C20H28CuN2O10
519.99
Formula weight
Crystal color
Crystal shape
Crystal size, mm3
Crystal system
Space group
642.08
violet
plate
1039.90
green
needle
brown
prism
blue
prism
0.40×0.2×0.14
monoclinic
C2/c
20.0558(3)
9.4856(2)
16.1960(3)
115.5853(8)
2779.02(9)
4
0.40×0.20×0.05 0.55×0.09×0.02
0.12×0.10×0.10
monoclinic
P21/n
10.2017(2)
6.4205(1)
17.1375(5)
95.701(1)
1116.95(4)
2
monoclinic
P21/n
monoclinic
P21/c
˚
a, A
9.0111(2)
17.1988(4)
9.2702(2)
101.6614(12)
1407.04(5)
2
1.515
0.842
293
11.0012(2)
19.9877(3)
11.0368(2)
96.1050(10)
2413.10(7)
2
1.434
0.958
293
˚
b, A
˚
c, A
β, deg
3
˚
V, A
Z
Dx, g cm−3
µ(MoKα ), mm−1
T, K
1.458
0.843
293
1.546
1.037
293
θ max, deg
hkl range
Total data
Independent data
Rint
27.80
27.47
11, 22, 12
19100
3199
0.031
2602
196
0.041
0.029
25.34
27.48
−25/+26, −11/+12, 21
−12/+13, −23/+24, 13 −12/+13, −7/+8, 22
20664
29799
4377
0.053
3526
15443
2559
0.034
2123
3280
0.037
Observed data [F2 ≥ 2σ(F2)] 2813
Refined parameters
Ra (observed)
Rwb
235
0.033
0.028
−0.79/0.41
307
195
0.041
0.029
−1.07/0.75
0.038
0.028
−1.23/0.44
−3
˚
∆ρmin/max, e A
−1.03/0.66
a
b
R = Σ(ꢀFo|−|Fcꢀ)/Σ|Fo|; Rw = Σ(w(|Fo|−|Fc|))/Σ(w|Fo|).
2.06, g = 2.35, D = 0.322 cm−1, −2J = 238 cm−1. – χ out electrogravimetrically with Pt electrodes. Infrared spec-
ꢀ
(294 K): µeff = 1.43 µB.
[Cu(L2)2(Hetam)2], (4)
tra were recorded on a Perkin-Elmer Spectrum 100 FT-IR
spectrometer, equipped with a Specac Golden Gate Diamond
ATR as a sample support. Electronic spectra were recorded
as nujol mulls with a Perkin-Elmer UV/Vis/NIR spectrome-
ter Lambda 19. X-Band EPR spectra of the powdered sam-
ples were recorded at r. t. using a Bruker ESP-300 spectrome-
ter. The magnetic susceptibility was recorded at r. t. for pow-
dered samples with a Sherwood Scientific MSB-1 balance.
Diamagnetic corrections were estimated from Pascal’s con-
stants [26]. Powder XRD data were obtained with a Guinier
Enraf Nonius camera for 1 and 2 and with a PANalytical
X’Pert PRO MPD diffractometer for 3 and 4, both operating
with CuKα radiation.
Solid CuI (19 mg, 0.10 mmol) and vanillic acid (50 mg,
0.30 mmol) were homogenized, and 30 mL of acetonitrile
was added. The reactants were dissolved by stirring, and
a drop of ethanolamine (Hetam) was added after 3 min.
Blue crystalline aggregates were filtered off and dried over
KOH for one day. Yield 70 %. – Anal. for C20H28CuN2O10
(520.0): calcd. C 46.20, H 5.43, N 5.39, Cu 12.22; found
C 46.31, H 5.12, N 5.80, Cu 11.90. – UV/Vis: λmax = 270,
300, 400(sh), 570 nm. – IR: ν = 3305, 3245, 3180 (O–H),
(N–H), 1596, 1538 (O2C)as, 1505, 1450 (CH3), 1398, 1376
(O2C)s cm−1. – EPR (298 K): g = 2.06, g = 2.28. – χ
⊥
ꢀ
Crystal structure analysis
(294 K): µeff = 1.89 µB.
The identity of the compounds was confirmed by powder
XRD analysis [25].
Single crystals of compounds 1 – 4, suitable for X-ray
analysis, were obtained by the experimental procedures de-
scribed above, but using lower concentrations of the reac-
tants.
The diffraction data for complexes 1 – 4 were collected
on a Nonius Kappa CCD diffractometer with graphite-
monochromated MoKα radiation. The data were processed
using the programs HKL DENZO, SCALEPACK [27]. All four
structures were solved by Direct Methods using SIR97 [28]
and refined by full-matrix least-squares procedures based
on |F| using XTAL3.6 [29]. All non-hydrogen atoms were
refined with anisotropic displacement parameters. Most of
Physical measurements
C, H, N analyses were performed with a Perkin-Elmer, the positions of the hydrogen atoms (including all hydrogen
Elemental Analyzer 2400 CHN. Metal analysis was carried atoms involved in hydrogen bonding as described in Table 2)
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484
B. Kozlevcˇar et al. · Nitrogen Donor Ligands
a
˚
˚
Table 2. Bond lengths (A), selected angles (deg) and hydrogen bond parameters [distances (A) and angles (deg)] in 1 – 4.
1
Cu–O2
2.3925(13)
76.12(5)
103.88(5)
D···A
Cu–O3
1.9227(11)
93.69(5)
86.31(5)
Cu–N1
2.0715(13)
89.52(5)
90.48(5)
D–H···A
157(2)
O2–Cu–O3
O2–Cu–O3i
D–H···A
O2–Cu–N1
O2–Cu–N1i
D–H···A
162(3)
O3–Cu–N1
O3–Cu–N1i
D···A
D–H···A
N2–H22···O1ii
2.882(3)
N2–H21···O3iii
2.916(2)
2
Cu–O1
1.9722(11)
55.95(4)
Cu–O2
2.5884(12)
89.48(5)
Cu–N1
2.0053(13)
87.06(4)
90.52(5)
D–H···A
162
O1–Cu–O2
O1–Cu–O2i
D–H···A
O1–Cu–N1
O2–Cu–N1i
D–H···A
153
O2–Cu–N1
O1–Cu–N1i
D···A
124.05(4)
D···A
92.94(4)
D–H···A
O4–H4···O2iv
O4–H4···O3
2.7329(17)
2.6834(17)
N2–H22··· O2v
3.0386(19)
2.870(2)
110
N2–H21···O5vi
171
3
4
Cu–O1
1.9596(17)
2.0078(19)
168.92(8)
90.67(9)
92.32(8)
D···A
Cu–O5
Cu–N1
1.991(2)
Cu–O2vii
O1–Cu–O5
O1–Cu–N1
O5–Cu–N1
O6vii–Cu–N1
D···A
1.9565(18)
86.86(9)
96.60(8)
96.15(9)
95.00(8)
D–H···A
136
Cu–O6vii
2.159(2)
88.03(8)
168.19(8)
94.40(8)
D–H···A
O8–H8···O7
N2–H201···O9ix
2.5697(12)
91.31(5)
O1–Cu–O2vii
O5–Cu–O2vii
O2vii–Cu–O6vii
D–H···A
O1–Cu–O6vii
O5–Cu–O6vii
O2vii–Cu–N1
D–H···A
117
O4–H41···O3
2.640(3)
2.984(3)
2.701(3)
2.895(4)
O4–H41···O6viii
145
173
Cu–O1
2.0081(9)
90.21(4)
89.79(4)
D···A
Cu–O5
Cu–N1
1.9968(13)
75.04(5)
104.96(5)
D–H···A
162(3)
O1–Cu–O5
O1–Cu–O5i
D–H···A
O4–H4···O5x
O5–H51···O2
i
O1–Cu–N1
O1–Cu–N1i
D–H···A
179(4)
O5–Cu–N1
O5–Cu–N1i
D···A
88.70(5)
D–H···A
2.6584(17)
2.5834(15)
N1–H11···O2xi
2.9164(16)
171(3)
Symmetry operations: 1− x, 1− y, 1− z; ii 1/2− x, 3/2− y, 1− z; iii x, 1+ y, z; iv 1/2+ x, 1/2− y, 1/2+ z; v 1− x, 1− y, 2− z; vi −x,
a
1−y, 2−z; vii −x, 1−y, 1−z; viii x, y, −1+z; ix 1−x, 1−y, −z; x −1+x, y, z; xi x, −1+y, z.
were located using difference Fourier maps. Only the posi-
tions of H83 (bonded to C8) in 1, H2 (bonded to C2) and H14
(bonded to C14) in 3, and H81 and H83 (both bonded to C8)
in 4 were calculated, because those obtained from difference
Fourier maps resulted in unacceptable geometries. The pa-
rameters of the hydrogen atoms in compounds 2 and 3 were
not refined, nor were those of H83 in 1, and of H81, H82
and H83 in 4. The positional and isotropic displacement pa-
rameters of the remaining hydrogen atoms in 1 and 4 were
refined. Crystal data and numbers pertinent to data collection
and refinement are listed in Table 1. Selected bond lengths
and angles for 1 – 4 are summarized in Table 2. The structure
plots were drawn with PLATON [30].
CCDC 671024-671027 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Fig. 1. Molecular structure of [Cu(L1)2(nia)2] (1) in the crys-
Centre via www.ccdc.cam.ac.uk/data request/cif.
tal showing a typical bidentate vanillin coordination via hy-
droxy and methoxy moieties.
Results and Discussion
is coordinated via the ring nitrogen atom [Cu–N1
Crystal structures
˚
2.072(2) A], while vanillin is attached through the de-
˚
protonated hydroxyl oxygen atom [Cu–O3 1.923(2) A]
A centrosymmetric molecule of [Cu(L1)2(nia)2]
(1) (HL1 = vanillin, nia = nicotinamide) is formed
by two trans oriented nicotinamide molecules and
two bidentate vanillin anions (Fig. 1). Nicotinamide
˚
and the methoxy oxygen atom [Cu–O2 2.393(2) A],
the latter forming the elongated Jahn-Teller axis (Ta-
ble 2). The molecular structure of 1 is similar to
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B. Kozlevcˇar et al. · Nitrogen Donor Ligands
485
Fig. 2. The molecular structure of [Cu(L2)2(nia)2] (2) with
bidentate chelating carboxylate coordination.
that of other reported vanillin complexes, showing a
hydroxy/methoxy bidentate vanillin coordination and
an additional ligand in a mononuclear structure [10 –
14, 16, 17]. Some variation is found in the binuclear
vanillin-ethanolamine complex [Cu2(µ-etam)2(L1)2],
where the ethanolamine anion serves as a chelat-
ing ligand as well as a monoatomic bridge be-
tween the adjacent copper atoms [10]. The nicoti-
namide amide group in 1 is forming two intermolec-
ular hydrogen bonds to the hydroxy [N2–H21···O3
Fig. 3. The dimeric molecular structure of [Cu2(µ-
L2)4(nia)2] (3). Hydrogen atoms are omitted for clarity.
boring weakly coordinated carboxylate oxygen atom.
Beside them, two nia hydrogen bonds [N2–H21···O5
◦
◦
˚
˚
2.869(2) A, 171 ; N2–H22···O2 3.039(2) A, 162 ]
are found, similar as in the structure of 1. A weak π-
π stacking of nia rings [centroid···centroid distance
◦
˚
˚
˚
2.916(2) A, 157(2) ] and the aldehyde oxygen atoms
4.020(1) A, mean inter-planar separation 3.541 A] is
additionally stabilizing the crystal structure.
◦
˚
[N2–H22···O1 2.882(3) A, 162(3) ]. Apparently, they
are the driving force for the crystal packing together
with the π-π stacking of the pyridine (nia) rings
The binuclear complex [Cu2(µ-L2)4(nia)2] (3) is
of the paddle-wheel type. Four vanillic acid an-
ions are defining four triatomic carboxylate bridges
˚
[centroid···centroid distance 3.860(1) A, mean inter-
˚
˚
planar separation 3.595 A].
[Cu–O 1.957(2) – 2.008(2) A], and two nicotin-
The other lignin model vanillic acid (HL2) in the
complexes 2 – 4 is coordinated to the central copper(II)
atom by the carboxylate group through three different
coordination modes [chelating (2), bridging (3), and
monodentate (4), respectively].
amide molecules are occupying the apical posi-
tions [Cu–N1 2.159(2) A] of the square-pyramidal
CuO4N coordination geometries via their ring nitro-
gen atoms (Fig. 3). The amide (nia) and the hy-
droxy (L2) moieties are determining the hydrogen
˚
The mononuclear complex [Cu(L2)2(nia)2] (2) is bonding network. Besides the expected intramolecu-
built up by two bidentate vanillic acid anions [Cu–O1 lar methoxy/hydroxy hydrogen bonds [O4–H41···O3
◦
◦
˚
˚
˚
˚
1.972(2) A, Cu–O2 2.589(2) A] and two nicoti- 2.640(3) A, 117 ; O8–H8··· O7 2.701(3) A, 136 ],
˚
namide molecules [Cu–N1 2.005(2) A] in trans ori- one of the vanillic acid hydroxy groups is addition-
entation (Fig. 2). The elongated octahedral coordina- ally intermolecularly hydrogen-bonded to a carboxy-
◦
˚
tion sphere CuO2N2O2 with a chelating lignin model late oxygen atom [O4–H41··· O6 2.984(3) A, 145 ].
L2− is similar as in the vanillin complex 1, al- Each nicotinamide molecule is forming two symmetry-
though different coordinating groups are involved, equivalent intermolecular hydrogen bonds with the
namely carboxylate in 2, and hydroxy and methoxy nicotinamide molecule of the adj◦acent binuclear unit
˚
oxygen atoms in 1. This makes an important dif- [N2–H201··· O9 2.895(4) A, 173 ]. In addition to ex-
ference in the hydrogen bonding network due to a tensive hydrogen bonding, efficient π-π stacking [nia
˚
hydroxy group acting as hydrogen bond donor in 2 pyridine rings centroid···centroid distance 3.555(2) A,
˚
(-O4–H4). It participates in an intramolecu◦lar hy- mean inter-planar separation 3.424 A] stabilizes the
˚
drogen bond O4–H4···O3 [2.683(2) A, 110 ] with structure and leads to empty channels, as can be seen in
the methoxy group and in an intermolecular con- the crystal structure diagram (Fig. 4). These channels
◦
˚
tact O4–H4···O2 [2.733(2) A, 153 ] with a neigh- are not destroyed after filtration of the solid compound
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486
B. Kozlevcˇar et al. · Nitrogen Donor Ligands
˚
bonded to ethanolamine [N1–H11··· O2 2.916(2) A,
162(3)◦] and the hydroxy oxygen atom [O4–H4···O5,
◦
˚
2.658(2) A, 179(4) ]. The latter hydrogen bond (O4–
H4···O5) is crucial for the orientation of the hydrogen
atom H4 away from the methoxy oxygen atom O3 of
the same vanillic acid anion, which is in contrast to
the situation in compounds 2, 3, [Cu2(µ-L2)4(H2O)2]
[8, 15], and [Cu2(µ-L2)2(µ-O2CCH3)2(CH3OH)2]
[9], where intramolecular methoxy/hydroxy hydrogen
bonds are formed (Table 2). Besides the extensive
hydrogen bonding network, π-π stacking is noticeable
Fig. 4. Empty channels in the crystal structure of [Cu2(µ-
L2)4(nia)2] (3) as shown in the space-filling structure repre-
sentation. Hydrogen atoms are omitted for clarity.
˚
[centroid···centroid distance 3.574(1) A, mean inter-
˚
planar separation 3.419 A] in 4. Due to the hydrogen
bonding network and the π-π stacking, a competition
of two candidates for the bidentate coordination mode,
namely L2 and Hetam, results in favor of chelating
Hetam and monodentate L2 in 4, as was discussed
above.
Spectroscopy
The electronic spectrum of the vanillin complex 1
shows a dominant UV band at 330 nm due to π-
π and LMCT transitions, accompanied by a shoulder
at 470 nm. In the visible light region, a weak intensity
d-d band at 620 nm appears. The absence of a domi-
nant band in the 600– 800 nm region and the presence
of a stronger intensity shoulder at 400 – 500 nm as ob-
served in the spectrum of the golden-yellow colored
Fig. 5. The molecular structure of [Cu(L2)2(Hetam)2]) (4) in
the crystal, showing an extensive inter- and intra molecular
hydrogen bonding network.
and thus its separation from the solvent acetonitrile, as
was shown by powder diffraction.
Compound [Cu(L2)2(Hetam)2] (4) (Hetam
=
ethanolamine) differs from the other three complexes compound 1 are also reported for the yellow-green and
reported herein due to Hetam as a N-donor ligand orange vanillin complexes [Cu(L1)2(H2O)2] [16, 17].
instead of nia which is present in 1 – 3. Nicotinamide The mononuclear vanillic acid complexes 2 and 4
is preferably acting as a monodentate ligand, and only show a low intensity d-d band at 500– 600 nm and
a few examples of a bridging coordination mode have a strong 300 nm signal. Both compounds are blue-
been reported [31– 33]. On the other hand, in 4 neutral purple colored as often observed for octahedral Cu(II)
ethanolamine acts as a bidentate ligand via the N,O complexes. The spectrum of the green binuclear com-
˚
˚
donor set [Cu–N1 1.997(2) A, Cu–O5 2.570(2) A]. plex 3 shows signals at 270, 300, 400(sh), 580(sh), and
This may be the key reason that the vanillic acid 720 nm (d-d). Compared to the spectra of the mononu-
carboxylate coordinates via one oxygen atom only clear compounds 1, 2 and 4, the spectrum of 3 shows
˚
[Cu–O1 2.008(1) A] (Fig. 5), and not as a chelate as a significantly stronger d-d band, though not as inten-
in the related mononuclear complex 2. A priority of a sive as in the UV region. The 400(sh) nm band can not
chelating coordination of a neutral Hetam ligand over a be classified as typical for a binuclear complex, since
carboxylate anion within the same complex is noticed a similar shoulder band appears also in the spectra of
also in a related ethanolamine complex [34]. The weak mononuclear 1, 2 and 4.
coordination of Hetam via the oxygen atom O5, giving
IR spectra of the complexes 1 – 4 show signals of
the elongated Jahn-Teller axis, is certainly related the NH2 (nia, Hetam) and OH (L2, Hetam) groups
to the presence of a strong intramolecular hydrogen in the region above 3000 cm−1. A dominant amide
◦
bond O5–H51···O2 [2.583(2) A, 171(3) ]. Addition- C=O signal at 1690 cm−1 is noticed for the nia com-
ally, both oxygen atoms involved in this hydrogen plexes 1 – 3, and an additional one for 1 at 1650 cm−1
bond (O5, O2) are also intermolecularly hydrogen- due to vanillin. The signal for the asymmetric stretch-
˚
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B. Kozlevcˇar et al. · Nitrogen Donor Ligands
487
ing of the carboxylate group (vanillic acid anion) in 3 is of ligands (the lignin model and the nitrogen donor
found as expected at 1570 cm−1, whereas it appears for molecule) show a variety of coordination geome-
the mononuclear complexes 2 and 4 at unusually low tries. Two lignin models, vanillin (L1) and vanillic
energy (1540 cm−1). Similar low values for νas(O2C) acid anions (L2), prefer to coordinate as bidentate
were reported for the related Hetam complex [34] and ligands via methoxy/hydroxy (vanillin) or carboxy-
attributed to strong hydrogen bonds of the carboxylate late (vanillic acid) groups. A large group of the
moiety.
previously reported lignin model complexes reveal
The EPR spectrum of the paddle-wheel-type com- an additional nitrogen ligand in the coordination
plex 3 shows four spin S = 1 signals (Hz1 9.3, Hx2 44.6, sphere [10 – 14], though oxygen donor ligands may
Hy2 478, Hz2 576 mT), characteristic of antiferro- be suitable as well [8, 9, 15 – 17]. In some of the
magnetically coupled binuclear Cu(II) species (−2J = latter complexes, e. g. cis-, trans-[Cu(L1)2(H2O)2]
238 cm−1), while signals at 300 mT can be assigned and [Cu2(L2)4(H2O)2], water is completing the
1
to mononuclear impurities with spin S = /2. A clear Cu-lignin model coordination sphere, suggesting easy
splitting of the H signal into two separate signals water binding and thus a disturbance of a copper
⊥2
H
y2 and Hz2 is due to the non-negligible rhombic sym- protected wood by outdoor washing. On the other
metry zero-field splitting parameter E. The elongated hand, the presence of a strongly bonded N-donor
axial symmetry signals (g , g ) are found in the spec- ligand allows versatile coordination possibilities even
⊥
ꢀ
tra of all three mononuclear species 1, 2 and 4. Poorly with relatively small molecules such as nicotinamide
resolved hyperfine splitting A (10.9 mT) is noticed for and ethanolamine. The formation of the copper-to-
ꢀ
the complex 1.
lignin model complexes is additionally stabilized by
The room temperature magnetic susceptibility mea-
hydrogen bonding and π-π stacking interactions. An
surements give µeff(µB) values of 1.95 (1), 1.93 (2), application of the suitable ligand may support the
1.43 (3), and 1.89 (4) that are in agreement with the
copper fixation to the natural lignin, thus improving
observed EPR spectra and molecular structures for all the copper-based wood-protecting formulations.
four complexes 1 – 4.
Acknowledgements
We thank Dr. Miha Humar, Biotechnical Faculty, Uni-
versity of Ljubljana for helpful discussions and Dr. Marjeta
Sentjurc, EPR Center, ‘Jozˇef Stefan’ Institute, Ljubljana, for
the EPR data. The financial support of MVZT L4-6209, P1-
0175-103, Republic of Slovenia, is gratefully acknowledged.
Conclusions
ˇ
The copper complexes [Cu(L1)2(nia)2] (1),
[Cu(L2)2(nia)2] (2), [Cu2(µ-L2)4(nia)2] (3) and
[Cu(L2)2(Hetam)2]) (4), with two different types
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