Synthesis and application of Co(salen) complexes containing proximal imidazolium ionic liquid cores

Article abstract of DOI:10.1139/v11-106

Ionic ligands derived from a salen ligand containing two proximal 1,3-disubstituted imidazolium ionic liquid cores form cobalt(III) complexes capable of selectively oxidizing veratryl alcohol, a lignin model compound, to veratraldehyde using air as the source of oxygen. These complexes are easy to prepare, inexpensive, water stable, and soluble in ionic liquids, making them viable candidates for use as oxidation catalysts.

Full text of DOI:10.1139/v11-106

60
Synthesis and application of Co(salen) complexes
containing proximal imidazolium ionic liquid cores
Swapnil Sonar, Kenson Ambrose, Arthur D. Hendsbee, Jason D. Masuda, and
Robert D. Singer
Abstract: Ionic ligands derived from a salen ligand containing two proximal 1,3-disubstituted imidazolium ionic liquid
cores form cobalt(III) complexes capable of selectively oxidizing veratryl alcohol, a lignin model compound, to veratralde-
hyde using air as the source of oxygen. These complexes are easy to prepare, inexpensive, water stable, and soluble in ionic
liquids, making them viable candidates for use as oxidation catalysts.
Key words: task-specific ionic liquid (TSIL), oxidation, IL–salen–Co complex, catalysis, veratryl alcohol.
Résumé : Les ligands ioniques dérivés d’un ligand salène comportant deux noyaux proximaux de liquides ioniques d’imida-
zolium 1,3-disubstitué forment des complexes avec le cobalt(III) qui peuvent oxyder d’une façon sélective le vératrol, un
composé modèle de la lignine, en vératraldéhyde en utilisant de l’air comme source d’oxygène. Ces complexes, qui peuvent
être préparés facilement et qui ne sont pas coûteux, sont stables dans l’eau et sont solubles dans les liquides ioniques ce qui
en fait des candidats intéressants comme catalyseurs de réactions d’oxydation.
Mots‐clés : liquide ionique (ILTS) à tâche spécifique, oxydation, complexe IL–salène–Co, catalyse, vératrol.
[Traduit par la Rédaction]
applications in addition to being used merely as reaction me-
dia. These include applications such as lubricants,5 extrac-
tants,6 electrolytes,7 surfactants,8 and biomass processing.9
These applications arose because of the unique set of proper-
ties attributed to ILs that make them interesting not only
from a green chemistry point of view but also from a more
purely chemical point of view. Such properties include a gen-
eral lack of volatility and flammability, polarity and (or) ionic
nature, and the ability to dissolve a wide array of organic and
inorganic materials. Pertinent to this report is the ability to
tune these properties through careful manipulation of the
structural features of the IL under investigation. The coordi-
nating ability, viscosity, thermal stability, and biodegradabil-
ity can all be easily tuned by adopting a modular approach
to the synthesis of ILs. Simple variation of the cationic core,
the counterion, and substituents on either, or both, of these
can have profound effects on these properties. The incorpora-
tion of functionality into an IL to control its properties and to
impart the ability to perform a predetermined function gives
rise to task-specific ionic liquids (TSILs).10
Introduction
As one of the 12 principles of green chemistry,1 the use of
alternative solvents in chemical applications has become an
increasingly important and highly studied area of interest
over the last decade. The advent of green chemistry in the
1990s was due in no small part to the earlier work of chem-
ists such as Trost who coined the term, “Atom Economy”,2
and Chan who reported pioneering work in which organome-
tallic reagents were used in aqueous systems.3 Although
water is the most obvious solvent of choice in terms of its
environmental friendliness and its ubiquitous nature and ac-
ceptance as the “universal solvent” as described in many
undergraduate textbooks, there are a number of other poten-
tial alternative solvents that warrant investigation. Hence, the
work of Chan has provided inspiration for the study of a
number of materials that have emerged as viable alternatives
to conventional solvents. These include supercritical fluids,
polymeric liquids, fluorous solvents, and ionic liquids (ILs).
ILs are now well-established in the chemical literature and
are now enjoying growing utility in industrial applications.
Among the current industrial applications are ILs for acid
scavenging in the BASIL process developed by BASF, per-
formance additives in paints developed by Degussa, cleaning
fluids by Iolotec–Wandres, as well as a number of other
processes at the pilot scale.4 Hence, it is obvious that these
liquids are now being investigated for their utility in several
We recently reported the preparation of TSILs 1a–1d, ca-
pable of forming octahedral complexes with copper, cobalt,
and nickel (Scheme 1).11 Replacement of a methyl group on
these complexes with an n-decyl group imparted sufficient
hydrophobicity on the complex to enable it to spontaneously
separate from aqueous solution upon its formation. These
TSILs were shown to be useful in the removal of copper
Received 18 April 2011. Accepted 28 June 2011. Published at www.nrcresearchpress.com/cjc on 24 October 2011.
S. Sonar, K. Ambrose, A.D. Hendsbee, J.D. Masuda, and R.D. Singer. Maritimes Centre for Green Chemistry, Department of
Chemistry, Saint Mary’s University, Halifax, NS B3H 3C3, Canada.
Corresponding author: Robert D. Singer (e-mail: Robert.Singer@SMU.ca).
This article is part of a Special Issue dedicated to Professor Tak-Hang Chan.
Can. J. Chem. 90: 60–70 (2012)
doi:10.1139/V11-106
Published by NRC Research Press

Sonar et al.
61
Scheme 1. Copper chelation using a task-specific ionic liquid (TSIL).11
R
N
O
N
O
M
O
(i) CF3COOH/CH2Cl2
24 h/RT
a R = n-C4H9
b R = n-C6H13
c R = n-C8H17
d R = n-C10H21
O
-
t
CO2 Bu
PF6
N
O
O
N
N
N
N
t
CO2 Bu
(ii) MCl2 (aq)
(iii) NH4OH
R
O
1
N
O
2
N R
M = Cu2+, Co2+, Ni2+
from an aqueous solution indicating potential use in hydro-
metallurgical applications.
time.14 Of particular significance is the Co–salen-catalyzed
oxidation of veratryl alcohol (3,4-dimethoxybenzyl alcohol)
a lignin model compound, to veratraldehyde (3,4-dimethoxy-
benzaldehyde). The ability to selectively perform such a reac-
tion without further oxidation to the carboxylic acid (3,4-
dimethoxybenzoic acid) provides a potential commercially
viable route to value-added products based on lignin degrada-
tion products such as veratryl alcohol.
It became clear that if the ligand contained in the IL–
salen–CuII (6) complex could be isolated in the absence of a
chelated metal, the opportunity to coordinate it to a wider va-
riety of metals would be presented. The catalytic ability of
such IL–salen–metal complexes and the ability to entrain
them in other ILs could then be investigated. Hence, we re-
port herein our investigations in which the IL–salen ligand
used in 6 was prepared and characterized. We also report the
chelation of cobalt(III) using this ligand to afford an octahe-
dral complex containing two axial ammine ligands as well as
the catalytic activity of this complex in the oxidation of ve-
ratryl alcohol using air as the source of oxygen.
A subsequent study reported by our research group dem-
onstrated the modular approach to preparing TSILs capable
of metal chelation. Incorporation of a salicylaldehyde moiety
into an imidazolium-based IL allowed the chelating moiety in
the resulting TSILs to be easily altered (Scheme 2).12 A com-
mon precursor to a series of TSILs is easily prepared through
the chloromethylation of salicylaldehyde followed by quater-
nization of 1-methylimidazole to afford compound 3a after
metathesis of the chloride salt to the hexafluorophosphate
analogue. Reaction of compound 3a with hydroxylamine hy-
drochloride resulted in the formation of an imidazolium salt
containing a salicylaldoxime moiety, 4. The salicylaldoxime
moiety is a well-established chelator of copper and this
TSIL, not surprisingly, easily formed an isolable complex
with copper (5). The same precursor (3a) was also reacted
with copper(II) acetate and 1,2-diaminoethane in situ to af-
ford an IL–salen–CuII complex (6). Incorporation of a man-
ganese(III) atom into a complex using this same ligand
resulted in a complex capable of catalyzing the epoxidation
of alkenes. Hence, this study demonstrated the ability of
such TSILs to be used both in metal separations and in the
preparation of ionic complexes for use in catalysis.
Results and discussion
The key precursors to the imidazolium-tagged ligands con-
taining the salen-chelating ligand were prepared according to
a previous procedure used in our laboratory starting with the
reaction of salicylaldehyde with formaldehyde in the presence
of HCl at room temperature over 1 day to afford 5-chloro-
methyl-2-hydroxybenzaldehyde.12 Quaternization of 1-meth-
ylmidazole using this chloromethylated salicylaldehyde as an
alkylating agent afforded the chloride salt, which was then
metathesized to either the hexafluorophosphate salt (3a) or
the tetrafluoroborate salt (3b) using aqueous hexafluorophos-
phoric acid (HPF6(aq)) or sodium tetrafluoroborate as appro-
priate. The salicylaldehyde-functionalized imidazolium salts
3a and 3b could then be reacted with ethylene diamine to af-
ford the IL–salen ligands 8a and 8b (Scheme 4 and Fig. 1).
Recrystallization of 8a from the slow evaporation of an ace-
tone solution afforded a 64% yield of crystals appropriate for
X-ray diffraction. This salen ligand is a dication owing to the
presence of two proximal 1,3-disubstituted imidazolium sub-
stituents at either end of the salen moiety. Two poorly coordi-
nating hexafluorophosphate ions act as the counterions as
demonstrated by their remote locations relative to any posi-
tive charges or lone pairs of electrons on the complex as
seen in its crystal structure (Fig. 1).
It has been demonstrated that catalytic species derived
from TSILs, or ion-tagged ligands, provide the ability to
more efficiently entrain the resulting catalytic ionic complex
in an IL of similar structure.13 If the IL used to entrain the
TSIL-derived catalyst is water immiscible then the IL/catalyst
phase may be more easily separated from aqueous- or organic-
phase reaction mixtures containing substrates and products.
Hence, not only is product isolation facilitated but catalyst
recycling becomes possible. Furthermore, the advantages of
both a heterogeneous catalyst and a homogenous catalyst
may be exploited.
The catalytic abilities of a number of salen–Co complexes
have been reported in the literature.14 Among these are the
Co–salen complexes (7a and 7b, Scheme 3), which have
been reported to catalyze the oxidation of benzylic alcohols
to aldehydes without over oxidation to the corresponding car-
boxylic acids. These reactions have been reported to proceed
via a superoxo- or a bridging m-peroxo complex formed from
the Co–salen complex reacting with dioxygen.15 The nature
of the actual catalytic species present in solution has been
shown to be dependent upon reaction conditions such as tem-
perature, pH, the nature of added base, and the reaction
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Can. J. Chem. Vol. 90, 2012
Scheme 2. Synthesis of ionic liquid (IL) – salen – Cu (6) and bis(salicylaldoxime)Cu (5) complexes from task-specific ionic liquids (TSILs).12
The cobalt chelate complex 9a was easily prepared through
reaction of the ligand 8a and cobalt(II) acetate under basic
conditions and reflux for 10 h, after which the complex pre-
cipitated out of solution. Recrystallization from a 9:1 v/v
mixture of acetone and methanol afforded a 58% yield of
crystals appropriate for X-ray diffraction.
The structure of 9a shows an octahedral geometry about
the cobalt atom with two axial ammine ligands. These li-
gands originate from the ammonium hydroxide solution used
to basify the solution to ensure the phenoxide oxygen atoms
were deprotonated and hence be available for coordination to
the metal centre. Interestingly, the structure shows the pres-
ence of three hexafluophosphate ions in the asymmetric unit
indicating a cobalt(III) metal centre. It appears the cobalt(II)
metal centre contained in the cobalt(II) acetate reactant was
oxidized under the aerobic conditions under which complex
formation occurred. Hence, three hexafluorophosphate ions
are required to balance the charge on the ionic complex. The
stoichiometry of the reaction, in which ligand 8a is the limit-
ing reactant, then dictates that one-third of the IL–salen li-
gand will be uncomplexed and is removed during the
ethanol wash of the product or during recrystallization. The
two tethered, proximal 3-methylimidazolium cores of this
complex provide for the potential exploitation of enhanced
solubility in typical ILs such as 1-butyl-3-methylimidazolium
hexafluorophosphate ([bmim]PF6). The use of such enhanced
solubility is a topic of a subsequent paper focusing on the use
of entrained catalysts of this type in ILs. The present study
Published by NRC Research Press

Sonar et al.
63
Scheme 3. Oxidation of veratryl alcohol with salen–Co catalysts.14
Scheme 4. Synthesis of ionic liquid (IL) – salen ligands 8a and 8b.
Fig. 1. Crystal structure of ionic liquid (IL) – salen task-specific ionic liquid (TSIL) 8a. Thermal ellipsoids are drawn at 30% probability.
has focused on establishing the presence of any catalytic ac-
tivity of this complex in oxidations in aqueous systems.
An isostructural tetrafluoroborate complex (9b) was iso-
lated in an identical fashion through the reaction of the IL–
salen ligand 8b, again under basic conditions using NH4OH
(aq) as the base. The octahedral cobalt centre in this case
also displays two axial ammine ligands and three tetrafluoro-
borate counterions. Once again there is a cobalt(III) metal
centre required to account for the overall neutral charge on
the complex and the presence of three tetrafluoroborate
anions in the asymmetric unit.
With the IL–salen–Co complexes 9a and 9b in hand, we
sought to determine whether they possessed any catalytic ac-
tivity in the oxidation of veratryl alcohol to veratraldehyde.
Veratryl alcohol was chosen as the substrate for this study,
since, as alluded to above, it is a typical lignin model
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64
Can. J. Chem. Vol. 90, 2012
compound and is often used in studies of enzymatic lignin
degradation.16 It had been shown in a previous study by
Repo and co-workers14 that the optimum pH for these reac-
tions was ca. pH 12.5 and that the optimum temperature was
above 60 °C. These reported reactions were typically run at
80 °C; hence, we used these reaction conditions to conduct
our oxidation reactions of veratryl alcohol catalyzed by IL–
salen–Co complexes 9a and 9b.
Initial reactions in which IL–salen–Co complex 9a was
used as the catalyst utilized pure oxygen as the oxidant.
Whereas these reactions generally gave very high yields of
veratraldehyde, as determined by gas chromatographic analy-
sis and were consistent with other catalysts that have been
used using oxygen as the oxidant, we opted to use air as our
source of oxygen. We regarded this as a much safer, cheaper,
and therefore a green advantage of our approach.
A series of reactions using IL–salen–Co complex 9a as the
catalyst in which the base used to adjust the pH to ca. 12.5
was varied, gave moderate yields of veratraldehyde. Essen-
tially no difference in the yield of the aldehyde product was
observed when the base was changed from potassium hy-
droxide to sodium hydroxide (Table 1, entries 1 and 2). A
noticeable decrease in yield occurred when NH4OH(aq) was
used. However, use of sodium carbonate resulted in an in-
crease in yield even in the absence of pyridine (Table 1, en-
tries 3–5). Switching the catalyst to IL–salen–Co complex 9b
with a tetrafluoroborate counterion gave slightly lower yield
of the aldehyde product (Table 1, entry 7). IL–salen–Co
complex 9a could be dissolved in the water immiscible IL
[bmim]PF6, giving a biphasic catalytic system when used
with water. Using air as the source of oxygen, this system af-
forded a comparable, yet moderate, yield of veratraldehyde
(Table 1, entry 6). This biphasic system facilitated separation
of the catalytic layer from the product and unreacted starting
material. Further investigation into the utility of this, and
other similar biphasic systems in which catalysts are en-
trained in IL phases, are the topics of current studies ongoing
in our laboratory.
the oxygen source are generally lower than those when pure
oxygen is used, the method does offer a safer and less expen-
sive methodology. Future studies in which these and other re-
lated catalysts useful for the oxidation of lignin model
compounds are entrained in other ILs are ongoing and will
be the topic of future publications. Such immobilization of
the catalysts in these systems may enable ready recycling of
the catalyst while also facilitating product isolation and yield,
hence enhancing the overall greenness of this methodology.
Experimental
General procedures
Veratryl alcohol (96%) and all other commercial reagents
were purchased from Sigma-Aldrich and used without further
purification. Thin layer chromatography (TLC) was per-
formed on silica gel plates and visualization was done by
UV light. Column chromatography was performed using
230–400 mesh silica gel. IR spectra were acquired on a
Bruker Vertex-70 IR using a KBr disc. 1H, 13C, and 31P
NMR spectra were acquired on a Bruker AV Sspectrometer
(1H, 500 MHz; 13C, 126 MHz; and 31P, 202 MHz) or on a
Bruker/Tecmag AC spectrometer (1H, 250 MHz; 13C,
62 MHz); 19F NMR spectra (235 MHz) were also acquired
using the Bruker/Tecmag spectrometer. When DMSO-d6 was
used as a solvent, the chemical shifts are reported in ppm rel-
1
ative to TMS as an internal standard (0.00 ppm for H and
1
13C) or DMSO-d6 (2.50 ppm for H and 39.52 ppm for 13C).
The chemical shifts in 31P NMR spectra are reported in ppm
with respect to aq 85% H3PO3 as the external standard
(0.00 ppm for 31P) and in 19F NMR spectra with respect to
0.5% CF3C6H5 in CDCl3 as the external standard
(63.72 ppm for 19F). Multiplicities are indicated by singlet
(s), doublet (d), and triplet (t). Coupling constants (J) are re-
ported in Hz. ESI-MS data were acquired in positive and
negative modes on an Agilent 1100 LC equipped with an
MSD trap. High-resolution mass spectra were acquired in
positive mode on a Bruker microTof Focus orthogonal ESI-
TOF.
Noteworthy is the experiment in which no IL–salen–Co
catalysts 9a or 9b were added. Not surprisingly, only a trace
amount of aldehyde product was detected in this reaction
where all other reaction conditions were maintained except
for the exclusion of the catalytic complex (Table 1, entry 8).
This confirms the role of the cobalt complexes 9a and 9b as
active catalysts for the selective oxidation of veratryl alcohol
to veratraldehyde.
Single crystals were placed in Paratone-N oil, mounted in
a nylon loop, and cooled to 150 K (8a), to 100 K (9a), or to
130 K (9b) using an Oxford Cryostream cooling system. The
data were collected using the Bruker APEX2 software pack-
age17,18 on a Siemens diffractometer equipped with an
APEXII CCD detector, a graphite monochromator, and
Mo Κa radiation (l = 0.71073 Å). A hemisphere of data
was collected in 1664 frames. Data processing and absorp-
tion corrections were applied using the APEX2 software
package.17,18 The structure was solved (direct methods) and
all non-H atoms were refined anisotropically. Hydrogen
atoms were placed in calculated positions using an appropri-
ate riding model and coupled isotropic temperature factors.
ORTEP-319 renderings are provided in Figs. 1–3. Selected
crystallographic data are included in Tables 2–4 and are also
included in the Supplementary data for this manuscript.
All spectra of known compounds agreed with those re-
ported in the literature. For new compounds (i.e., 8a and 8b
Conclusions
The synthesis and isolation of two new IL–salen ligands
(8a and 8b) containing proximal 1,3-disubstituted imidazo-
lium IL moieties, from cheap and readily available starting
materials, further demonstrates the effectiveness of the modu-
lar approach we have developed for the preparation of TSILs
capable of chelating metals. Theses complexes are easy to
prepare, water stable, and soluble in ILs. The potential of the
ionic metal complexes described herein as catalysts has been
established through the selective oxidation of veratryl alco-
hol, using air as the source of oxygen, to afford moderate
yields of veratraldehyde. No evidence of further oxidation to
the carboxylic acid was detected. Although yields using air as
1
and 9a and 9b), the H and 13C NMR of the purified com-
pounds and the HRMS (in addition to other spectral data)
are provided in lieu of combustion analysis, since these com-
pounds are characteristically resistant to combustion.
Published by NRC Research Press

Sonar et al.
65
Table 1. Oxidations of veratryl alcohol catalyzed by ionic liquid (IL) –
salen – Co complexes 9a and 9b.
Entry
Catalyst
9a
9a
9a
9a
9a
9a
9b
None
Base
Solvent
H2O
H2O
H2O
H2O
H2O
H2O, [bmim]PF6
H2O
Yield (%)a
40
38
24
1
2
3
4
5
6
7
8
NaOH(aq)
KOH(aq)
NH4OH(aq)
K2CO3
K2CO3
K2CO3
56
52b
41
K2CO3
K2CO3
46
Trace
H2O
aIsolated yield after chromatography.
bNo pyridine, as axial ligand, added.
Fig. 2. Crystal structure of ionic liquid (IL) – salen – Co PF6 complex 9a. Thermal ellipsoids are drawn at 30% probability.
Preparation of 5-chloromethyl-2-hydroxybenzaldehyde
To a well stirred mixture of concd hydrochloric acid
(150 mL) and formaldehyde (10.8 mL; 37 wt% in H2O) was
added salicylaldehyde (15.0 mL, 141.4 mmol). The reaction
was stirred at room temperature for 24 h. The solid that sepa-
rated out was filtered, dissolved in diethyl ether, and reisolated
through evaporation of the solvent. Recrystallization from n-
hexane afforded a white solid. Yield: 51.5%; mp 84–85 °C.
140.3 mmol) in toluene (30 mL) at room temperature was
added 5-chloromethyl-2-hydroxybenzaldehyde (20.0 g,
117 mmol) under a nitrogen atmosphere. The resulting solu-
tion was stirred at room temperature for 3 h. The solid that
separated out was washed with ether (3 × 20 mL) and dried
to give a pale yellow solid. Yield: 97.0%; mp 185–187 °C.
IR (KBr, cm–1): 1562 (C=C), 1611 (C=N), 1675 (C=O),
1
3459 (O–H). H NMR (500 MHz, DMSO-d6) d: 3.86 (s, 3H,
1H NMR (250 MHz, DMSO-d6) d: 4.74 (s, 2H, Cl–CH2), 7.00
(d, 1H, J = 8.5 Hz, Ar–H), 7.57 (dd, 1H, J = 2.3, 6.3 Hz, Ar–
H), 7.70 (d, 1H, J = 2 Hz, Ar–H), 10.26 (s, 1H, CHO), 10.92
(s, 1H, OH). 13C NMR (62 MHz, DMSO-d6) d: 45.71, 117.73,
122.15, 128.78, 129.08, 136.97, 160.74, 190.66.
N–CH3), 5.39 (s, 2H, +N–CH2), 7.21 (d, 1H, J = 8.5 Hz,
Ar–H), 7.63 (dd, 1H, J = 2.5, 6.0 Hz, Ar–H), 7.73 (t, 1H,
J = 1.5, 2.0 Hz, Ar–H), 7.76 (d, 1H, J = 2.5 Hz, Ar–H),
7.83 (t, 1H, J = 1.5 Hz, Ar–H), 9.33 (s, 1H, Ar–H), 10.31
(s, 1H, CHO), 11.31 (s, 1H, OH). 13C NMR (126 MHz,
DMSO-d6) d: 35.85, 51.03, 118.20, 122.14, 122.54, 123.96,
125.59, 128.47, 136.50, 136.53, 161.43, 190.06. MS ESI(+):
217.1, 135.0, 120.8, 99.4, 83.1.
Preparation of 1-(3-formyl-4-hydroxybenzyl)-3-
methylimidazolium chloride
To
a
solution of 1-methylimidazole (14.0 mL,
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66
Can. J. Chem. Vol. 90, 2012
Fig. 3. Crystal structure of ionic liquid (IL) – salen – Co BF4 complex 9b. Thermal ellipsoids are drawn at 30% probability.
Table 2. Crystal data and structure refinement for task-specific ionic liquid (TSIL)
8a.
Parameter
Crystal data
Empirical formula
Formula mass
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C26H30F12N6O2P2
748.50
150(2)
0.71073
Triclinic
P-1
Unit cell dimensions
a = 9.851(5) Å, b = 16.967(8) Å,
c = 20.443(9) Å
a = 69.197(5)°, b = 81.843(5)°, g =
89.738(5)°
3158(2)
Volume (Å3)
Z
4
Density (calculated, Mg/m3)
Absorption coefficient (mm–1)
F(000)
1.574
0.246
1528
Crystal dimensions
q range for data collection (°)
Index ranges
0.387×0.139×0.101 mm3
1.95–25.00
–11≤ h≤11, –20≤ k≤20, –24≤l≤24
31 304
Reflections collected
Independent reflections
Completeness to q = 25.00° (%)
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I > 2s(I))
R indices (all data)
11 107 (R(int) = 0.0395)
99.7
Full-matrix least-squares on F2
11 107 / 2 / 873
1.033
R1 = 0.0526, wR2 = 0.1278
R1 = 0.0910, wR2 = 0.1496
0.529 and –0.424
Largest diff. peak and hole (e Å–3)
1-(3-Formyl-4-hydroxybenzyl)-3-methylimidazolium
hexafluorophosphate (3a)
To a solution of 1-(3-formyl-4-hydroxybenzyl)-3-methyl-
imidazolium chloride (14.80 g, 58.70 mmol) in water
(100 mL) at room temperature was added an aqueous solu-
tion of hexafluorophosphate (12.8 mL, 88.1 mmol) while
cooling in an ice bath over 1 h. The resulting solution was
stirred at room temperature for 3 h. The solid that separated
out was washed with water and then with diethyl ether (3 ×
20 mL) and dried to give a solid white product. Yield:
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Sonar et al.
67
Table 3. Crystal data and structure refinement for ionic liquid (IL) – salen – Co com-
plex 9a.
Parameter
Crystal data
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C28H39CoF18N9O2P3
1027.52
100(2)
0.71073
Monoclinic
P2(1)/c
Unit cell dimensions
a = 18.028(3) Å, b = 13.648(2) Å, c =
17.958(3) Å
a = 90°, b = 115.856(2)°, g = 90°
3976.1(10)
Volume (Å3)
Z
4
Density (calculated, Mg/m3)
Absorption coefficient (mm–1)
F(000)
1.716
0.680
2080
Crystal dimensions (mm3)
q range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Completeness to q = 25.00° (%)
Absorption correction
Max and min transmission
Refinement method
0.3021×0.2128×0.1126
1.95–25.00
–21≤h≤21, –16≤k≤16, –21≤l≤21
38 333
6996 (R(int) = 0.0748)
100.0
Semiempirical from equivalents
0.7457 and 0.6957
Full-matrix least-squares on F2
6996/3/527
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I > 2s(I))
R indices (all data)
1.050
R1 = 0.0480, wR2 = 0.1106
R1 = 0.0757, wR2 = 0.1200
0.693 and –0.581
Largest diff. peak and hole (e Å–3)
97.7%; mp 132–134 °C. IR (KBr, cm–1): 1573 (C=C), 1627
(C=N), 1663 (C=O), 3443 (O–H). 1H NMR (500 MHz,
DMSO-d6) d: 3.85 (s, 3H, N–CH3), 5.36 (s, 2H, +N–CH2),
7.05 (d, 1H, J = 8.5 Hz, Ar–H), 7.60 (dd, 1H, J = 2.5,
6.0 Hz, Ar–H), 7.69 (t, 1H, J = 1.5, 2.0 Hz, Ar–H), 7.77 (t,
2H, J = 2.0 Hz, Ar–H), 9.16 (s, 1H, Ar–H), 10.30 (s, 1H,
CHO), 10.98 (s, 1H, OH). 13C NMR (126 MHz, DMSO-d6)
d: 35.86, 51.11, 118.06, 122.18, 122.50, 124.02, 125.74,
128.93, 136.50, 136.62, 161.13, 190.39. 19F NMR
(235 MHz, DMSO-d6) d: –66.3 (d, J = 705 Hz). 31P NMR
(202 MHz, DMSO-d6) d: –44.19 (septet, J = 709.4 Hz). MS
ESI(+) m/z: 217.0; ESI(–) m/z: 144.7.
9.14 (s, 1H, Ar–H), 10.29 (s, 1H, CHO), 10.97 (s, 1H, OH).
13C NMR (126MHz, DMSO-d6) d: 35.88, 51.14, 118.09,
122.18, 122.50, 124.04, 125.77, 129.06, 136.51, 136.67,
161.14, 190.53. 19F NMR (235 MHz, DMSO-d6) d: –144.2.
MS ESI(+) m/z: 217.1.
Preparation of 1–4-hydroxy-3-[(2-[((E)-1–2-hydroxy-5-[(3-
methyl-1H-imidazol-3-ium-1-yl)methyl]
phenylmethylidene)amino]ethylimino)methyl]phenyl-3-
methyl-1H-imidazol-3-ium hexafluorophosphate (8a)
To a solution of 1-(3-formyl-4-hydroxybenzyl)-3-methyl-
imidazolium hexafluorophosphate (5.00 g, 13.80 mmol, 3a)
in dry ethanol (40 mL) was added ethylenediamine
(1.12 mL, 6.86 mmol) slowly at room temperature. The re-
sulting solution was stirred at reflux temperature for 6 h.
The solution was allowed to cool to room temperature after
which a solid separated out. The solid was washed with etha-
nol (3 × 5 mL) and dried in vacuo to give a yellow solid.
Yield: 64.7%; mp 167–169 °C. IR (KBr, cm–1): 1593
Preparation of 1-(3-formyl-4-hydroxybenzyl)-3-
methylimidazolium tetrafluoroborate (3b)
To a stirred solution of 3-(3-formyl-4-hydroxybenzyl)-3-
methylimidazolium chloride (10.00 g, 39.57 mmol) in water
(100 mL) at room temperature was added sodium tetrafluoro-
borate (5.21 g, 47.49 mmol). The resulting solution was
stirred at room temperature for 3 h. The solution was concen-
trated in vacuo and filtered off to obtain a light pink solid
product. This was washed with water to remove NaCl and
unreacted sodium tetrafluoroborate. Pink solid. Yield:
95.30%; mp 102–104 °C. IR (KBr, cm–1): 1575 (C=C),
1
(C=C), 1637 (C=N), 3459 (O–H). H NMR (500 MHz, ace-
tone-d6) d: 4.0 (s, 10H, 2 × N–CH3, 2 × =N–CH2), 5.51 (s,
+
4H, 2 × N–CH2), 6.94 (d, 2H, J = 8.5 Hz, Ar–H), 7.51 (dd,
2H, J = 2.5, 6.0 Hz, Ar–H), 7.62 (d, 2H, J = 2.0 Hz, Ar–H),
7.70 (t, 2H, J = 1.5, 2.0 Hz, Ar–H), 7.73 (t, 2H, J = 2.0 Hz,
Ar–H), 8.59 (s, 2H, N=C–H), 9.01 (s, 2H, Ar–H), 13.50
(s, 2H, OH). 13C NMR (126 MHz, acetone-d6) d: 36.76,
53.15, 60.38, 118.35, 120.03, 123.26, 125.13, 125.23,
133.53, 133.86, 137.38, 162.33, 167.39. 19F NMR
1
1622 (C=N), 1660 (C=O). H NMR (500 MHz, DMSO-d6)
+
d: 3.85 (s, 3H, N–CH3), 5.36 (s, 2H, N–CH2), 7.05 (d, 1H,
J = 8.5 Hz, Ar–H), 7.12 (dd, 1H, J = 2.5, 6.0 Hz, Ar–H),
7.68 (t, 1H, J = 1.5 Hz, Ar–H), 7.76–7.78 (m, 2H, Ar–H),
Published by NRC Research Press

68
Can. J. Chem. Vol. 90, 2012
Table 4. Crystal data and structure refinement for ionic liquid (IL) – salen – Co
complex 9b.
Parameter
Crystal data
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C26H35.2B3CoF12N8O2.6
820.76
130(2)
0.71073
Monoclinic
P2(1)/c
Unit cell dimensions
a = 16.944(3) Å, b = 13.424(2) Å, c =
16.654(3) Å
a = 90°, b = 113.072(2)°, g = 90°
3485.0(10)
Volume (Å3)
Z
4
Density (calculated, Mg/m3)
Absorption coefficient (mm–1)
F(000)
1.426
0.587
1457
Crystal dimensions (mm3)
q range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Completeness to q = 25.00° (%)
Refinement method
0.239×0.177×0.121
2.0225.00
–20≤h≤20, –15≤k≤15, –19≤l≤19
33 387
6126 (R(int) = 0.0704)
100.0
Full-matrix least-squares on F2
6126/332/526
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I > 2s(I))
R indices (all data)
1.065
R1 = 0.0417, wR2 = 0.1171
R1 = 0.0601, wR2 = 0.1252
0.691 and –0.374
Largest diff. peak and hole (e Å–3)
(235 MHz, acetone-d6) d: –68.0 (d, J = 705 Hz). 31P NMR
(202 MHz, acetone-d6) d: –144.2 (septet, J = 707.0 Hz).
MS (ESI+) m/z: 603.2; ESI(–) m/z: 144.6. HRMS (ESI)
ESI+ calcd for C26H30F6N6O2P+: 603.2067; found:
603.2064.
87.0. HRMS (ESI) ESI+ calcd for C26H30BF4N6O2 :
545.2454; found: 545.2455.
+
Preparation of IL–salen–Co hexafluorophosphate
complex (9a)
To a solution of hexafluorophosphate ligand (3.30 g,
4.41 mmol, 8a) in dry ethanol (30 mL) was added ammo-
nium hydroxide to bring the pH to 8–9. This solution was
stirred for 15 min after which cobalt(II) acetate (1.21 g,
4.85 mmol) was added at room temperature. The resulting
solution was stirred at reflux temperature for 10 h after which
a solid separated out. This solid was washed with ethanol
(3 × 5 mL) and dried to give a brown solid product. The
product was recrystallized from an acetone–methanol mixture
(9:1 v/v) to give crystals suitable for X-ray crystallographic
analysis. Yield: 2.53 g (58% based upon the ligand 8a). IR
(KBr, cm–1): 1574 (C=C), 1620 (C=N), 3595 (N–H). 1H
NMR (500 MHz, acetone-d6) d: 2.88 (s, 6H, 2 × NH3), 4.06
(s, 6H, 2 × N–CH3), 4.20 (s, 4H, 2 × =N–CH2), 5.45 (s, 4H,
Preparation of 1–4-hydroxy-3-[(2-[((E)-1–2-hydroxy-5-[(3-
methyl-1H-imidazol-3-ium-1-yl)methyl]
phenylmethylidene)amino]ethylimino)methyl]phenyl-3-
methyl-1H-imidazol-3-ium tetrafluoroborate (8b)
To a stirred solution of 1-(3-formyl-4-hydroxybenzyl)-3-
methylimidazolium tetrafluoroborate (5.00 g, 16.44 mmol,
3b) in dry ethanol (40 mL) was added ethylenediamine
(0.55 mL, 8.15 mmol). The resulting solution was refluxed
for 6 h with constant stirring. The solution was allowed to
cool to room temperature after which a solid separated out.
The solid was washed with ethanol (3 × 5 mL) and dried in
vacuo to give a yellow solid product. Yield: 47.33%; mp
162–164 °C. IR (KBr, cm–1): 1580 (C=C), 1636 (C=N),
+
2 × N–CH2), 7.07 (d, 2H, J = 9.0 Hz, Ar–H), 7.37 (dd, 2H,
1
3405 (O–H). H NMR (500 MHz, DMSO-d6) d: 3.84 (s, 6H,
J = 2.5, 6.5 Hz, Ar–H), 7.55 (d, 2H, J = 2.0 Hz, Ar–H),
7.70 (d, 2H, J = 1.5 Hz, Ar–H), 7.72 (d, 2H, J = 1.5 Hz,
Ar–H), 8.22 (s, 2H, N=C–H), 9.00 (s, 2H, Ar–H), (s, 2H,
OH). 13C NMR (126 MHz, acetone-d6) d: 36.78, 53.17,
59.40, 119.35, 120.86, 123.11, 124.61, 125.24, 135.82,
136.46, 137.15, 167.75, 169.46. 19F NMR (235 MHz, ace-
tone-d6) d: –68.0 (d, J = 705 Hz). 31P NMR (202 MHz, ace-
tone-d6) d: –144.2 (septet, J = 707.0 Hz). MS (ESI+) m/z:
804.9; ESI(–) m/z: 144.6. HRMS (ESI) ESI+ calcd for
2 × N–CH3), 3.96 (s, 4H, 2 × =N–CH2), 5.34 (s, 4H, 2 ×
+N–CH2), 6.92 (d, 2H, J = 8.5 Hz, Ar–H), 7.42 (dd, 2H,
J = 2.0, 6.5 Hz, Ar–H), 7.54 (d, 2H, J = 2.5 Hz, Ar–H),
7.69 (d, 2H, J = 1.5 Hz, Ar–H), 7.73 (t, 2H, J = 1.5 Hz,
Ar–H), 8.59 (s, 2H, N=C–H), 9.13 (s, 2H, Ar–H), 13.59 (s,
2H, OH). 13C NMR (126 MHz, DMSO-d6) d: 35.84, 51.30,
58.54, 117.40, 118.44, 123.13, 123.96, 124.66, 132.03,
132.93, 136.42, 161.26, 166.33. 19F NMR (235 MHz,
DMSO-d6) d: –144.2. MS (ESI+) m/z: 545.0; ESI(–) m/z:
+
C26H34CoF12N8O2P2 : 839.1420; found: 839.1438.
Published by NRC Research Press

Sonar et al.
69
Preparation of IL–salen–Co tetrafluoroborate complex
(9b)
Acknowledgements
We are thankful to the Natural Sciences and Engineering
Research Council of Canada (NSERC, Discovery and
LIGNOWORKS, The NSERC Biomaterials and Chemicals
Network) and Saint Mary’s University (SMU–FGSR) for
funding this research. J.D.M. wishes to thank the Canadian
Foundation for Innovation and the Nova Scotia Research and
Innovation Trust for upgrades to the X-ray diffractometer.
To
a solution of tetrafluoroborate ligand (3.00 g,
4.75 mmol, 8b) in dry ethanol (30 mL) was added ammo-
nium hydroxide to bring the pH to 8–9. This solution was
stirred for 15 min after which cobalt(II) acetate (1.30 g,
5.22 mmol) was added at room temperature. The resulting
solution was stirred at reflux temperature for 10 hafter which
a solid separated out. This solid was washed with ethanol
(3 × 5 mL) and dried to give a brown solid product. The
product was recrystallized from an acetone–methanol mixture
(9:1 v/v) to give crystals suitable for X-ray crystallographic
analysis. Yield: 2.58 g (67% based upon the ligand, 8b). IR
(KBr, cm–1): 1573 (C=C), 1646 (C=N), 3618 (N–H). 1H
NMR (500 MHz, DMSO-d6) d: 2.76 (s, 6H, 2 × N–CH3),
3.86 (s, 6H, 2 × N–CH3), 3.88 (s, 4H, 2 × =N–CH2), 5.27
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d: –144.3. MS (ESI+) m/z: 723.0; ESI(–) m/z: 86.9. HRMS
(ESI) ESI+ calcd for C26H34B2CoF8N8O2 : 723.2195; found:
+
723.2176.
Typical procedure for the oxidation of veratryl alcohol
using IL–salen–Co catalysts 9a and 9b
In a 50 mL two-necked flask IL–salen–Co complex (9a or
9b, 10 mol%) and pyridine (10 mol%) were added to 10 mL
of distilled water and stirred for about 15 min. Base, as indi-
cated in Table 1, was then added to adjust the pH to about
12.5, followed by the slow addition of veratryl alcohol. At
this point, the pH of the solution was monitored to ensure a
basic pH. The reaction mixture was heated at 80 °C with
constant air bubbling. The reaction was allowed to proceed
for the desired time. The reaction was stopped by cooling to
room temperature. The pH of the reaction mixture was ad-
justed to 6–7 with 2 mol/L HCl. The product (veratralde-
hyde) was extracted by dichloromethane, and the combined
organic extracts were dried over magnesium sulfate followed
by solvent removal in vacuo to afford a white solid; mp 41–
1
43 °C. H NMR (250 MHz, DMSO-d6) d: 3.8 (s, 3H, O–
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Supplementary data
Supplementary data are available with the article through
the journal Web site (http://nrcresearchpress.com/doi/suppl/
10.1139/v11-106). CCDC 822063–822065 contain the X-ray
data in CIF format (for compounds 8a, 9a, and 9b) for this
manuscript. These data can be obtained, free of charge, via
http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi (Or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1Ez, UK; fax +44 1223 336033; or deposit
@ccdc.cam.ac.uk).
Published by NRC Research Press

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