Synthesis and characterization of binuclear Co(II) complexes with bis(salen-type) ligands

Article abstract of DOI:10.1016/j.ica.2012.08.007

Two new, bridged bis(salen-type) ligand precursors, 1,1,3,3- tetrakis(salicylidene-3-iminopropyl)butylenediamine (I) and 1,1,3,3- tetra(salicylideneiminomethyl)propane (IV), were prepared by Schiff base condensation of salicylaldehyde with appropriate tet

Full text of DOI:10.1016/j.ica.2012.08.007

Inorganica Chimica Acta 394 (2013) 203–209
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Inorganica Chimica Acta
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Synthesis and characterization of binuclear Co(II) complexes
with bis(salen-type) ligands
a,
⇑
a
a
b
a
Minna T. Räisänen , Heikki Korpi , Markku R. Sundberg , Alexander Savin , Markku Leskelä ,
Timo Repo
a
a,
⇑
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
Low Temperature Laboratory, Aalto University, P.O. Box 15100, FI-00076 Aalto, Finland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 May 2012
Received in revised form 31 July 2012
Accepted 5 August 2012
Available online 28 August 2012
Two new, bridged bis(salen-type) ligand precursors, 1,1,3,3-tetrakis(salicylidene-3-iminopropyl)butylen-
ediamine (I) and 1,1,3,3-tetra(salicylideneiminomethyl)propane (IV), were prepared by Schiff base con-
densation of salicylaldehyde with appropriate tetraamines. Corresponding binuclear Co(II) complexes 1
and 2 were obtained with moderate yields and the complexes were characterized in detail. Structural
⁄
optimizations of the complexes were carried out at the B3LYP/6-311G level of theory. Dioxygen coordi-
nation abilities of 1 and 2 were studied experimentally by UV–Vis spectroscopy and compared with tra-
ditional N,N -ethylenebis(salicylideniminato)Co(II), Co(salen) (3). In addition, catalytic activities of 1 and
Keywords:
0
Binuclear Co(II) Schiff bases
Molecular modeling
O–O Activation
2
in comparison with various mononuclear salen-type Co complexes (3–7) were studied in the oxidation
of 3,4-dimethoxybenzyl alcohol (veratryl alcohol) in alkaline aqueous solutions.
Ó 2012 Elsevier B.V. All rights reserved.
Alcohol oxidation
1
. Introduction
behavior of the complexes in oxidation of 3,4-dimethoxybenzyl
alcohol (veratryl alcohol) is studied as well.
Ability of synthesized transition metal complexes to activate
dioxygen was initially observed by Tsumaki et al. in 1930’s with
0
N,N -ethylenebis(salicylideniminato)Co(II) (Co(salen)) [1] and since
2. Experimental section
then reversible activation of dioxygen has been studied with
numerous metal complexes [2]. Salen-type complexes have been
found to catalyze a variety of oxidation reactions, such as oxidation
of phenols, sulfides, alcohols and lignin model compounds [2c,3].
The oxidation capability of the complexes can be further improved
by altering the ligand framework by different substituents or by
bridging metal centers to form a multinuclear complex. In fact, a
significant amount of recent research has been focused on towards
multinuclear transition metal complexes where closely adjacent
metal centers may function co-operatively resulting in increased
reaction rates in comparison to analogous mononuclear complexes
2.1. General methods and instrumentation
All reagents were purchased from Aldrich and used as received.
The complexes were prepared and handled under argon atmo-
sphere using standard Schlenk techniques. The products were
characterized (when possible) with a Varian Gemini 200 spectrom-
eter (200 MHz). Electron ionization (EI) mass spectra were run
with a JEOL JMS-SX 102 mass spectrometer (ionizing voltage
70 eV) from solid samples and high resolution (HR) electrospray
ionization time-of-flight (ESI-TOF) mass spectra were run with a
Bruker micrOTOF mass spectrometer from methanol–acetonitrile
solutions. Fast atom bombardment (FAB) mass spectrometry mea-
surements were carried out with a Finnigan MAT TSQ-7000 instru-
ment at the University of Ulm, Germany. FT-IR spectra were
recorded from neat samples pressed against a diamond window
using a Perkin Elmer Spectrum One spectrometer. For the far-IR
measurements with a Perkin Elmer Spectrum GX spectrometer,
polyethylene pellets of the compounds were prepared. UV–Vis
spectra were recorded with a Hewlett Packard 8453 spectropho-
tometer from methanol (I and 1) or chloroform (IV and 2) solu-
tions. Magnetic measurements were performed for powdered
samples mixed with stycast epoxy using a Quantum Design
[
3a,4]. For example, when the two metal centers are coordinated at
a suitable predetermined distance, they may have a tendency to
coordinate and activate small molecules, such as dioxygen,
between them [4c,f,5].
As an augmentation to our previous studies on Schiff base oxi-
dation catalysts [6], we report herein the synthesis and character-
ization of two new [7] bimetallic Co(II) complexes (Fig. 1). Catalytic
⇑
Corresponding authors. Fax: +358 9 191 50198.
E-mail addresses: minna.t.raisanen@helsinki.fi (M.T. Räisänen), timo.repo@
helsinki.fi (T. Repo).
0
020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.08.007

2
04
M.T. Räisänen et al. / Inorganica Chimica Acta 394 (2013) 203–209
Table 1
The main bands (cmꢀ1) in the IR spectra of ligand precursors I and IV and complexes 1
and 2.
I
1
IV
2
m
m
(C@N), s
(C@C, arom.), m
), m
(C–C), s
1630
1579
1497
–
1284
1147
1113
751
–
1630
1599
1466
1446
1307
1152
1126
757
1629
1578
1500
–
1276
–
–
754
–
1625
1571
1471
1448
1300
–
d(CH
2
m
m
m
m
m
(C–O, phenol), s
(C–N, tert. amine), m
(Ar–H), s
–
(Co–N)
752
471
–
474
444
–
–
m(Co–O)
–
–
346
292
–
–
324
279
s = strong, m = medium.
Table 2
Oxidation studies of veratryl alcohol with Co(II) salen-type
complexes 1–7 with different N,N’-bridge structures.
Complex
Aldehyde conversion (%)
1
2
3
4
5
6
7
4
13
93
7
0
77
46
Fig. 1. Synthesized binuclear Co(salen) complexes 1 and 2.
MPMS-XL SQUID magnetometer. The SQUID outputs were cor-
rected for sample holder and stycast magnetizations. Diamagnetic
corrections were carried out using Pascal’s constants. The magnetic
The reaction conditions: veratryl alcohol (0.2 mL,
.377 mmol), 0.05 M NaOH solution (pH 12.6), 80 °C, O
pressure 10 bar, 1:40 M ratio of metal and veratryl alcohol,
1
2
-
½
moments were calculated using the equation
l
eff = 2.828(
v
M
T) .
reaction time 20 h.
Oxidation products were analyzed with an Agilent Technologies
6
890 N gas chromatograph.
2
.4.1. 1,1,3,3-Tetrakis(salicylidene-3-iminopropyl)butylenediamine
1
2
.2. Computational details
4
(I), H L
Compound I was prepared by a condensation reaction of com-
mercial DAB-Am-4 (polypropylenimine tetraamine dendrimer)
(4.00 g, 12.6 mmol) and salicylaldehyde (6.17 g, 50.5 mmol). The
starting materials were mixed (1:4 M ratio) in EtOH (20 mL) and
stirred at 50 °C for 2 h. The mixture was cooled and two yellow
The initial coordinates for the complexes 1 and 2 were obtained
by Spartan [8] optimizations at semiempirical level (PM3). The
output coordinates were then used as input for the optimizations
carried out by GAUSSIAN03 program suite [9] at the B3LYP/6-311G
⁄
level. Since the experimental magnetic measurements clearly indi-
cated that there are three unpaired electrons at each cobalt atom,
only the spin multiplicity of seven was utilized in the subsequent
optimizations.
phases were separated. Lower oily phase was dissolved in CHCl
EtOH (1:1) in which the product (8.00 g, 87%) crystallized over-
night at ꢀ10 °C. Mp. 54–56 °C. Anal. Calc. for C44 : C,
72.10; H, 7.70; N, 11.47. Found: C, 72.45; H, 7.89; N, 11.71%. EI-
3
–
56 6 4
H N O
+
1
MS m/z 732, M . H NMR (200 MHz, CDCl
3
, TMS, 25 °C): d = 1.39
HH = 6.8 Hz, 8H), 2.39 (s, CH , 4H),
HH = 6.8 Hz, 8H),
6.79–6.96 (m, H–Ar, 8H), 7.18–7.32 (m, H–Ar, 8H), 8.31 (s, CH@N,
3
2.3. Oxidation experiments
(s, CH
2
, 4H), 1.80 (qv, CH
2
,
J
2
3
3
2
.49 (t, CH , JHH = 7.4 Hz, 8H), 3.60 (t, CH , J
2
2
For the oxidation experiments, 10 mL of 0.05 M NaOH solution
1
3
(
(
pH 12.6), veratryl alcohol (0.2 mL, 1.377 mmol) and the catalyst
1:40 M ratio of metal and veratryl alcohol, 17.2 mol of binuclear
mol of mononuclear complexes) were mixed in a glass
vessel fitted inside a steel autoclave. O -pressure of 10 bar was
then applied and the autoclave was set in an oil bath (80 °C) for
0 h. After cooling to ambient temperature, the solution was ex-
4H) ppm. C NMR (50 MHz, CDCl
(4CH ), 51.59 (4CH , –N–C–), 54.15 (2CH
6 4
@N–C), 117.18, 118.61, 118.97, 131.31, 132.24, 161.49 (C H ),
3
, TMS): d = 25.35 (2CH
2
), 28.68
l
2
2
2
, –N–C–), 57.60 (CH ,
2
and 34.4
l
165.04 (4CH, C@N) ppm. HR ESI-TOF MS m/z (%): 394 (69); 733
2
+
(24), [M+H] (calc. 733.4436; found 733.4417; error 2.54 ppm);
+
2
755 (100), [M+Na] . For IR and UV–Vis data, see Tables 1 and 2.
tracted with ethyl acetate and analyzed with gas chromatography.
Activity of the catalysts is expressed as a percentage of formed
veratraldehyde, which was the only product observed. Each oxida-
tion experiment was done twice and the reported values refer to
the average values.
2.4.2. 1,1,3,3-Tetra(methylamine)propane (III)
Compound III was prepared by reduction of 1,1,3,3-tetracarbox-
amidopropane (II). Fresh BH THF (1 M, 200 mL) was slowly added
3ꢁ
over 1 h to a THF (40 mL) suspension of II (5.0 g, 23.1 mmol). Reac-
tion vessel was kept in an ice bath during the addition and once the
addition was complete, the mixture was refluxed overnight. The
reaction mixture was cooled to ambient temperature and the ex-
2.4. Synthesis
Complex 3 was purchased from Aldrich and used without fur-
2
cess of borane was destroyed by cautious addition of H O
ther purification, whereas 1,1,3,3-tetracarboxamidopropane (II)
and complexes 4–7 were prepared according to literature methods
(20 mL). The solution was evaporated to dryness and slowly trea-
ted with 5 M HCl (100 mL). Resulting mixture was refluxed for
1 h and then taken to dryness. Obtained white residue was
[
10].

M.T. Räisänen et al. / Inorganica Chimica Acta 394 (2013) 203–209
205
2
dissolved in H O (50 mL) and 5 M NaOH (40 mL) was added. The
2.4.4. Co(II) complexes 1 and 2
solution was stirred for 30 min and evaporated to dryness. The ob-
tained white product contained residues from borane, HCl and
NaOH but it could be used for the further reactions without purifi-
Complexation reactions of 1 and 2 were carried out in a similar
way. Ligand was dissolved in absolute EtOH (20 mL) and two
equivalents of Co(II) acetate were added. Color of the reaction mix-
ture changed immediately and the complex started to precipitate.
The reaction mixture was stirred at 70 °C for 20 h and filtered
cation. The product is water-soluble but only moderately soluble in
1
DMSO, MeOH and EtOH. H NMR (200 MHz, CD
3
OD, 25 °C):
3
d = 1.23 (t, CH
2
, JHH = 6.8 Hz, 2H), 1.50–1.62 (m, 2H, CH), 2.67 (d,
before cooling to ambient temperature.
3
1
2
CH , JHH = 5.8 Hz, 8H) ppm.
2
Co(II) L
(1): Reaction of ligand I (1.00 g, 1.36 mmol) and
ꢁ4H O (0.68 g, 2.72 mmol) gave a brown product,
which was filtered (0.69 g, 56%). Anal. Calc. for C44 Co
ꢁ1.5
O: C, 60.48; H, 6.34; N, 9.62; O, 10.07. Found: C, 60.98; H,
Co(OOCCH
3
)
2
2
2
H
52
N
O
6 4
2
4
2.4.3. 1,1,3,3-Tetra(salicylideneiminomethyl)propane (IV), H L
H
2
When salicylaldehyde was added dropwise in EtOH suspension
of III, the solution turned immediately yellow. The reaction mix-
ture was stirred at 60 °C for 2 h and filtered. Liquid phase was
+
6
.16; N, 9.40; O, 10.42%. FAB-MS m/z 846, M . HR ESI-TOF MS
+
m/z (%): 846,
M
(calc. 846.2709, found 846.2675, error
3
.95 ppm). For IR and UV–Vis data, see Tables 1 and 2.
evaporated to dryness and residue was treated with CHCl
1:1, 40 mL). Water phase was washed with CHCl (20 mL) and ex-
tracts were added to the previous organic phase. Combined CHCl
phase was dried with Na SO , filtered, and the solvent was evapo-
rated. Crystallization of the product from EtOH–CHCl (1:1) gave
yellow needles. Mp. 166–168 °C. Anal. Calc. for C35 ꢁH O:
C, 70.69; H, 6.44; N, 9.42; O, 13.45. Found: C, 70.64; H, 6.04; N,
3 2
–H O
2
Co(II)
Co(OOCCH
was washed with CHCl
2
L
(2): Reaction of ligand IV (1.00 g, 1.73 mmol) and
ꢁ4H O(0.86 g, 3.46 mmol)gaveabrownproductwhich
after filtration (0.50 g, 42%). Anal. Calc. for
O: C, 56.46; H, 5.14; N, 7.52; O, 15.04. Found: C,
(
3
3
)
2
2
3
3
2
4
C
35
32
H N
4
O
4
Co
2
ꢁ3H
2
3
+
5
6.51; H, 5.01; N, 7.60; O, 15.16%. EI-MS m/z 691, M . HR ESI-TOF
36
H N
4
O
4
2
+
MS m/z (%): 690, M (calc. 690.1082; found 690.1065; error
+
1
2.50 ppm). For IR and UV–Vis data, see Tables 1 and 2.
9
3
.37; O, 13.29%. EI-MS m/z 576, M . H NMR (200 MHz, CDCl ,
3
3
TMS, 25 °C): d = 1.57 (t, JHH = 7.0 Hz, 2H, CH
Hz, 2H, CH), 3.69 (d, JHH = 5.6 Hz, 8H, CH
Ar), 7.18–7.35 (m, 8H, H–Ar), 8.35 (s, 4H, CH@N) ppm. C NMR
2
), 2.34 (qv, JHH = 6.1
), 6.81–6.96 (m, 8H, H–
3
2
3. Results and discussion
1
3
(
50 MHz, CDCl
3
, TMS, 25 °C): d = 31.38 (CH
2
), 37.84 (2CH), 61.14
),
3.1. Syntheses
(
4CH ), 116.90, 118.65, 118.71, 131.46, 132.38, 161.00 (C H
2
6 4
1
(
66.27 (4CH, C@N) ppm. HR ESI-TOF MS m/z (%): 496 (68); 577
Ligand precursor I was prepared from commercially available
starting materials by a one-step Schiff base condensation reaction.
Ligand precursor IV was synthesized with a three-step synthesis
90), [M+H]+ (calc. 577.2809; found 577.2796; error 2.23 ppm);
+
5
99 (100), [M+Na] . For IR and UV–Vis data, see Tables 1 and 2.
O
NH2
(
a) H2N
H
N
+
N
OH
NH2
H2N
N
N
OH
OH
OH
OH
N
N
N
N
I
O
O
O
O
(
b)
O
NH2
NH2
H2N
H2N
H2N
H2N
Na
+
N
O
O
II
OH
OH
N
N
N
OH
OH
.
H N
NH2
NH2
salicylaldehyde
BH3 THF
2
N
H2N
III
IV
Scheme 1. Synthesis strategies for (a) 1,1,3,3-tetrakis(salicylidene-3-iminopropyl)-butylenediamine (I) and (b) 1,1,3,3-tetra(salicylideneiminomethyl)propane (IV).

2
06
M.T. Räisänen et al. / Inorganica Chimica Acta 394 (2013) 203–209
where the reduction of tetracarboxamidopropane (II) to corre-
sponding tetra(methylamine)propane (III) was problematic and
thus decreased the total yield (Scheme 1). In order to improve
the yields of bimetallic complexes 1 and 2 and avoid formation
of mononuclear complex instead, reaction mixtures were heated
overnight after which soluble impurities were separated by filtra-
tion from the hot reaction mixture. Complexes 1 and 2 were iso-
lated in moderate yields as brown powders.
3.2. Characterization
Complexes 1 and 2 were characterized by elemental analysis
and different spectroscopic and mass spectrometric techniques.
Complexes 1 and 2 are paramagnetic at 300 K with eff values of
.35 and 6.08 B.M., respectively. These values are significantly lar-
ger than the spin only value of two Co(II) ions with S = 3/2
5.48 B.M. with g = 2) but in the range expected for high-spin binu-
l
6
(
clear complexes [11].
In the IR spectra of salen ligand precursors I and IV (Table 1),
C@N, phenolic C–O and aromatic C@C stretching vibrations are on
the regions usually observed in Schiff bases [12]. The aromatic C–
ꢀ1
H vibration is observed in the region of 750 cm , which is assigned
for aromatic rings with four adjacent hydrogen atoms. Upon com-
plex formation, the C@N, C–O and C@C vibrations are shifted and
their intensities are changed. For example, the C–C vibrations of
the complexes are notably stronger than those of the ligands. In
complex 1, the tertiary amine in the bridge coordinates to cobalt
ꢀ1
ion causing the C–N vibration to shift in 1100–1150 cm region.
The metal–ligand vibrations are observed in the region of
ꢀ
1
<
700 cm . According to far-IR data of Schiff base complexes, M–O
ꢀ1
and M–N vibrations appear in 270–330 and 410–450 cm regions,
respectively [12b]. Aromatic ring vibrations observed in 450 cm
region in the ligand spectra fade upon the complexation and new
ꢀ1
Fig. 2. UV–Vis spectra of complexes (a) 1 and (b) 3 in DMF (pyridine as an axial
base) in the range of 300–500 nm. Spectra (1) have been recorded from a cooled
solution that was heated at 100 °C for 15 min with a continuous argon flow, spectra
ꢀ1
bands of the chelate rings are observed in 550–600 cm region.
⁄
⁄
In the electronic spectra of I and IV, the
p ? p and n ? p tran-
(
2) after an overnight exposure to air, spectra (3) after a second argon treatment
sitions are at 214–315 nm and ca. 405 nm regions, respectively
and spectra (4) after another overnight exposure to air. In the inset of (a) is shown
the 350–1050 nm region of spectra (1) (solid line) and (4).
⁄
[
12c,13]. Upon formation of complexes 1 and 2, the
p
?
p
transi-
tions appear at 213–314 nm region in the spectra recorded under
⁄
protective gas. In the spectra of 1 and 2 the n ?
p
and d ? d tran-
sitions appear at ca. 405 nm and at 624–660 nm regions, respec-
tively, with weak intensities. The d ? d transitions suggest that
the geometries around Co(II) centers of the complexes in solution
are the same as in their solid forms.
different ligand structures in nonaqueous solution see the study of
Pui [16].
The bands at about 292, 343, 406 and 489 nm (shoulder) in the
spectrum of 3 recorded under protective gas can be assigned to
Abilities of binuclear complexes 1 and 2, and mononuclear
Co(salen) complex 3 as a comparison, to coordinate O
ied experimentally by UV–Vis spectroscopy from 0.1 mM DMF
solutions using pyridine as a base. Both 1 and 3 coordinated O
reversibly at room temperature but the former considerably slower
than the latter. Whereas 3 coordinates O within minutes when air
is bubbled through the solution, 1 must be in air contact overnight
before similar changes can be seen in the spectrum (Fig. 2). The O
2
were stud-
2
Co
Co2
2
2
coordination and subsequent desorption by argon gas were done
twice. As expected, raise of the reaction temperature to 80 °C
seems to be unfavorable for the coordination of O
was not observed to coordinate O at that temperature during the
period of 18 h. In similar experiments with 2 at ambient tempera-
ture the O coordination was found to be irreversible. This behavior
2
[14] as complex
1
2
Co1
2
has been previously observed for example with the mononuclear
Co
analog of complex 1 [15]. After one day exposure to air, the
⁄
p
?
p
transition of 2 is red shifted from 272 nm to 287 nm and
⁄
a new absorption band at 399 nm assignable to d ?
appears. Transitions n ?
The increased intensity of the d ?
p
transition
⁄
p
and d ? d are too weak to be observed.
⁄
p
transition upon oxygen
⁄
Fig. 3. Optimized structures of complexes (a) 1 and (b) 2 at the B3LYP/6-311G
adduct formation has been previously reported in the literature
level of theory. In 1 the Co1–H92 interaction is indicated by a dashed line. Color
code: O, red; N, blue. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
0
for N,N -(o-phenylene)bis(salicylideneiminato)Co(II) (7) [13b]. For
2
more details about O coordination abilities of Co Schiff bases with

M.T. Räisänen et al. / Inorganica Chimica Acta 394 (2013) 203–209
207
0
Fig. 4. Mononuclear Co(II) salen-type complexes 3–7 with different N,N -bridge structures for oxidation studies of veratryl alcohol.
⁄
⁄
⁄
p
[
d ?
?
p
,
n ?
p
,
d ?
p
and d ? d transitions, respectively
cally longer than the Co–O bonds and the axial Co–N bond is short-
er than the equatorial Co–N bonds. Although formally similar, the
two chromophores are different. This is clearly seen in the orienta-
tion of the aromatic rings. Related with this, there is hydrogen
atom H92 pointing towards Co1, whereas there is no similar
hydrogen in the vicinity of Co2. The Co1–H92 distance is 2.778 Å,
which excludes existence of normal covalent bond. However,
QTAIM analysis found a bond critical path between Co1 and H98.
Therefore, we may also describe the coordination polyhedron as
distorted octahedron. A similar bond path was not found for Co2.
Also in complex 2 both coordination spheres display the same
type coordination polyhedron, namely tetrahedron. This is in sharp
contrast with the previous case, where the Co(II) cations are five-
coordinated. The difference is due to the different number of avail-
able ligand atoms. Again, the Co–N bond lengths are longer than
the Co–O bond lengths in the two chromophores. The coordination
polyhedra are distorted, which can be seen in the bond angles. It
should be noted here, that square-planar Co(II) complexes should
have magnetic moments in the range of 2.2–2.7 B.M. per metal
center [22]. Our experimental value of 4.3 B.M./Co(II) is clearly big-
ger and is in accordance with the values observed in the literature
for tetrahedral Co(II) complexes [17b,23]. Thus the experimental
results support the optimized geometry of distorted tetrahedron.
⁄
13,15,17]. In the respective spectrum of 1
p
?
p
(286 nm) and
⁄
p
(349 nm) transitions are blue shifted compared to those
⁄
of 3 whereas n ?
served. The shifts are probably due to the –(CH
between the two imine groups which donates more electron den-
sity to the Co ion than the, respective ethylene bridge of 3 [13a].
Upon exposure to air, the
p
and d ? d transitions are too weak to be ob-
2
)
3
N(CH – bridge
2 3
)
2
+
⁄
p
?
p
transitions of 1 and 3 have not
shifted but their intensities have changed. In the case of 1,
⁄
d ?
p
transition is splitted into two bands at 349 and 367 nm
3+
which suggests that the Co centers are monobridged by O
-peroxo complex is formed [14a,18; for crystal structure of
oxo complex formed from two mononuclear species similar to
complex 1 see Ref. 19]. The oxygenated form of 1 shows new weak
2
, i.e.
-per-
l
l
bands at 474 nm (shoulder) and 665 nm which are also likely to be
⁄
d ?
p
transitions [14a]. A new band at 974 nm is assignable to
d ? d transition and implies octahedral geometry around the me-
tal centers [20]. In the case of 3, n ?
blue shifted upon exposure to air.
⁄
⁄
p and d ? p transitions are
3
.3. Structural modeling
Despite of our efforts, single crystals suitable for X-ray crystal
structure determination were not obtained for 1 and 2. Therefore,
⁄
the structures were optimized at the B3LYP/6-311G level of
3.4. Oxidation experiments
theory (Fig. 3). In the dinuclear complex 1 the cobalt atoms are
seemingly five-coordinated. The coordination polyhedra resemble
distorted square pyramids with a nitrogen atom in the axial posi-
Due to the inherent complexity of lignin, small and structurally
define model compounds, such as veratryl alcohol, are needed for
efficient evaluation of catalyst candidates. The ability of binuclear
complexes 1 and 2 to catalyze the oxidation of veratryl alcohol
[24] to veratraldehyde under alkaline (pH 12.6) water solutions
tion. The
s value for Co1 and Co2 coordination polyhedra is 0.03,
indicating that the distortion is almost negligible, since the value
for ideal square pyramid is 0 [21]. The Co–N bonds are systemati-

2
08
M.T. Räisänen et al. / Inorganica Chimica Acta 394 (2013) 203–209
1
at 80 °C [25] was studied and their catalytic properties were com-
pared with a series of mononuclear complexes 3–7 (Fig. 4). The
reaction conditions, under which the studied complexes were mod-
erately soluble, were selected on the basis of our earlier experiments
wherein the reaction conditions were optimized for 3 [6a,b]. The
reaction mechanism is expected to be similar to the previously
dination. Despite of their O
showed low catalytic activities in the oxidation of veratryl alcohol
in basic aqueous solutions with O as oxidant.
2
2
coordination ability, the complexes
Acknowledgments
reported one: The oxidation cycle begins with a formation of
l-hy-
The financial support of the Academy of Finland is gratefully
acknowledged by MTR (251531), HK, ML and TR (209739). MRS
acknowledges the generous computational resources provided by
CSC (CSC — IT Center for Science Ltd).
droxy bridges between two Co(II) centers followed by deprotonation
of veratryl alcohol and formation of Co(II)- alkoxo intermediate. In
the presence of oxygen, the metal centers of the intermediates are
oxidized to Co(III) and
l-peroxo bridges are formed between them.
In the final stage of the catalytic cycle, veratraldehyde molecules
are formed and metal centers are reduced back to hydroxy-bridged
Co(II) [26].
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.08.007.
2
Even though 1 and 2 are capable to coordinate O , as indicated
here by the UV–Vis measurements, they gave low aldehyde con-
versions, 4% and 13%, respectively (Table 2). However, these con-
versions are in the same range as achieved with mononuclear
complexes 4 and 5 (0% and 7%, respectively). Since other mononu-
clear complexes (3, 6 and 7) yielded significantly higher conver-
sions (93%, 77%, and 46%, respectively), the reason for different
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