Carbon-nitrogen bond cleavage by copper(II) complexes

Article abstract of DOI:10.1016/j.poly.2006.02.019

N-benzoylimidazole reacts with copper(II) nitrate in the presence of different alcohols to give the benzoate of the corresponding alcohol and tetra(imidazole)copper(II) nitrate. The hydrolytic cleavage of N-benzoylimidazole in aqueous ethyleneglycol leads to the formation of imidazoliumbenzoate. Copper(II)nitrate trihydrate hydrolytically degrades N-benzoyl 3,5-dimethylpyrazole to form a copper complex in the form of an adduct having the composition cis-(pz)₂Cu(OBz)₂ · trans-(pz)₂Cu(OBz)₂H₂O (where pz = 3,5-dimethylpyrazole and OBz = benzoate anion).

Full text of DOI:10.1016/j.poly.2006.02.019

Polyhedron 25 (2006) 2525–2529
www.elsevier.com/locate/poly
Carbon–nitrogen bond cleavage by copper(II) complexes
*
Kaustavmoni Deka, Moitree Laskar, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India
Received 20 January 2006; accepted 25 February 2006
Available online 22 March 2006
Abstract
N-benzoylimidazole reacts with copper(II) nitrate in the presence of different alcohols to give the benzoate of the corresponding alco-
hol and tetra(imidazole)copper(II) nitrate. The hydrolytic cleavage of N-benzoylimidazole in aqueous ethyleneglycol leads to the forma-
tion of imidazoliumbenzoate. Copper(II)nitrate trihydrate hydrolytically degrades N-benzoyl 3,5-dimethylpyrazole to form a copper
complex in the form of an adduct having the composition cis-(pz)2Cu(OBz)2 Æ trans-(pz)2Cu(OBz)2H2O (where pz = 3,5-dimethyl-
pyrazole and OBz = benzoate anion).
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: N-benzoylimidazole; N-benzoyl 3,5-dimethylpyrazole; Copper(II)nitrate; Benzoylation; Catalysis; Copper(II)benzoate complex
1. Introduction
degrade an acid sensitive functional group. Thus, liberated
acid in such a reaction needs to be scavenged by adding an
amine [22–26]. Generally, tertiary amines are used for this
purpose. Use of tertiary amine prevents formation of a
stable N-benzoylated amine derivative, that can be formed
as a side product if primary or secondary amines are used
as the acid scavanger. Imidazole is observed to be an acid
scavenger as well as a catalyst in esterification reactions
[26,27]. Although, imidazole itself can react with benzoyl-
chloride to form N-benzoylimidazole [28], catalytic reac-
tions based on this as a reagent have not been studied in
detail [26,27]. We discuss here the reactions of N-benzoy-
limidazole and N-benzoyl 3,5-dimethylpyrazole with cop-
per(II) nitrate, and catalytic C–N bond cleavage reactions.
Protection of functional groups such as alcohol, amine
and thiol is important in organic chemistry [1–3]. Various
transition metal complexes catalyze such reactions [3]. Pro-
tection of a functional group may be done with an objec-
tive to facilitate further reactions on the parent substrate
in the presence or absence of a transition metal catalyst.
Thus, the stability of a functional group towards transition
metal complexes is an important factor in the success of
using them as a protected group. Hence, the development
of new reagents for chemoselective protection and depro-
tection is a challenge [4–14]. Among such protection reac-
tions, benzoylation is a fundamental reaction which can
be performed under mild conditions [7,15–21]. Benzoyl-
chloride is the most commonly used benzoylating reagent
[7,15–21]. But, benzoylchloride is toxic and extra care is
needed for performing benzoylation reactions with benzoyl-
chloride. In addition to this, benzoylation of an alcohol by
benzoylchloride leads to the formation of hydrochloric acid
which can affect the benzoylation process and also can
2. Experimental
2.1. Synthesis of N-benzoylimidazole (I)
To a solution of benzoylchloride (700 mg, 5 mmol) in
dichloromethane (5 ml), triethylamine (606 mg, 6 mmol)
was added. To this solution another solution of imidazole
(340 mg) in dichloromethane (3 ml) was added dropwise
at room temperature. The solution was stirred for 1.5 h.
Dichloromethane (20 ml) was added to the reaction mixture
*
Corresponding author. Tel.: +91 361 2582301; fax: +91 361 2690762.
E-mail address: Juba@iitg.ernet.in (J.B. Baruah).
0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.02.019

2526
K. Deka et al. / Polyhedron 25 (2006) 2525–2529
followed by water (40 ml). The dichloromethane layer was
separated and dried over anhydrous sodium sulfate. Distil-
lation of dichloromethane extract gave N-benzoylimidazole
stirred to obtain a homogeneous solution. To this solution
water (1 ml) was added. A blue crystalline precipitate
appeared after 7 days (30 mg) on standing. The crystals
were picked out and the structure was determined by X-
ray diffraction. Elemental Anal. Calc. for C48H54N8O9Cu2:
C, 56.83; H, 5.37; N, 11.05. Found: C, 57.05; H, 5.56; N,
10.95; IR(KBr, cmꢀ1): 3206(m), 3145(m), 3047(w), 1628(s),
1582(m), 1531(s), 1403(s), 1296(m), 1183(w), 1070(w),
860(w), 804(w), 717(s). Crystal parameters of B: Mono-
clinic, space group C2/c, a = 39.461(5), b = 16.2272(17),
(yield 90%) a colorless pasty liquid. IR (neat, cmꢀ1
)
3150(s), 2843(s), 1711(s), 1603(s), 1552(s), 1383(s),
1327(m), 1327(m), 1276(s), 1178(m), 1071(s), 835(s),
1
717(s); H NMR (CDCl3) 7.84(s, 1H) 7.76–7.69(m, 1H),
7.56–7.53(m,2H), 7.46–7.42(m, 1H) 7.33–7.29(m, 2H),
6.918(s, 1H); 13C NMR (CDCl3) 137.48, 132.86, 131.12,
130.95, 129.06, 128.26, 127.65, 126.92; GC mass (m/e)
344 (for dimer), 277, 176, 116, 106, 78, 49, 38.
˚
c = 11.6867(12) A, a = 90.00°, b = 94.998(7)°, c = 90.00°,
3
˚
V = 7455.1(15) A , Z = 4, T = 296 K.
2.2. Synthesis of N-benzoyl 3,5-dimethylpyrazole (II)
2.5. Copper(II) catalyzed benzoylation of alcohols by
N-benzoylimidazole
To a solution of benzoylchloride (700 mg, 5 mmol) in
dichloromethane (5 ml), triethylamine (606 mg, 6 mmol)
was added. To this solution another solution of 3,5-dim-
ethylpyrazole (480 mg, 5 mmol) in dichloromethane
(6 ml) was added dropwise at room temperature. The solu-
tion was stirred for 3.5 h and to the reaction mixture
dichloromethane (20 ml) was added, followed by addition
of water (40 ml). The dichloromethane layer was separated
by a separating funnel and dried over anhydrous sodium
sulfate. Distillation of the dichloromethane extract gave
the crude product. The crude product was washed with
hexane (10 ml) to remove any hexane soluble impurity.
The residue was filtered and the residue was found to be
N-benzoyl 3,5-dimethylpyrazole (yield 92%). IR (neat,
In a typical experiment N-benzolylimidazole (115 mg,
1 mmol), copper(II) nitrate trihydrate (12 mg, 0.05 mmol)
and nitric acid (0.01 M, 0.1 ml) were mixed together in
methanol (2 ml) and stirred at room temperature. After
2 h of stirring, water (20 ml) was added to the reaction mix-
ture and the organic products were extracted with dichloro-
methane (40 ml). The dichloromethane layer was separated
using a separating funnel and dried over anhydrous sodium
sulfate. Removal of dichloromethane from this extract
under reduced pressure gave methylbenzoate as the only
product. The compound was characterized by recording
1
its H NMR and IR spectra and comparing them with an
cmꢀ1
)
3068(w), 2981(m), 2935(m), 1700(s), 1598(s),
authentic sample. Products from similar reactions with
other alcohols were also confirmed by recording 1H
NMR and GC–MS spectra. For benzoylation of solid alco-
hols dry acetonitrile was used as the solvent. From a simi-
lar reaction of ethyleneglycol with N-benzoylimidazole
under catalytic conditions only the mono-benzoylated ester
was obtained. Spectral details of C6H5COOCH2CH2OH:
IR (neat, cmꢀ1) 3416(bs), 1716(s), 1603(m), 1347(s),
1265(s), 1117(s), 707(s). 1H NMR (CDCl3): 8(m, 2H),
7.55(m, 1H), 7.4(m, 2H), 4.4(m, 2H), 3.9(m, 2H), 2.4(bs,
exchangeable, 1H). GC–MS: (m/e), 167, 148, 135, 105,
77, 51.
1459(s), 1413(s), 1342(s), 1280(s), 1193(m), 1127(m),
1
1039(m), 978(s), 922(s), 804(m), 712(s). H NMR (CDCl3)
7.96–7.94(m, 2H), 7.58–7.40(m, 1H), 7.39–7.37(m, 2H),
6.0(s, 1H), 2.59(s, 3H), 2.20(s, 3H); 13C NMR (CDCl3):
167.82, 151.59, 144.59, 132.96, 131.96, 130.95, 127.40,
110.72, 14.17, 13.69. GC mass (m/e) 202, 172, 115, 105,
77, 59, 39.
2.3. Synthesis of imidazolium benzoate (III)
N-benzoylimidazole (176 mg 1 mmol) was dissolved in
ethyleneglycol (1 ml). To this solution water (0.2 ml) was
added. Standing for two days gave white crystals of imi-
dazoliumbenzoate (60 mg). IR(KBr, cmꢀ1): 3401(bs),
3139(s), 2842(s), 1710(s), 1598(s), 1547(s), 1465(s),
1383(s), 1306(s), 1255(s), 1183(s), 1070(s), 1024(s), 896(s),
717(s). Elemental Anal. Calc. for C10H10N2O2: C, 63.13;
H, 5.30; N, 14.73. Found: C, 63.58; H, 5.28; N, 14.26%.
1H NMR (CDCl3): 13.5(s,1H), 8.00–7.96(m, 1H), 7.67–
7.65(m, 2H), 7.45–7.34(m,1H), 7.30–7.28(m, 2H), 7.06–
7.01(m, 2H).
3. Results and discussions
N-benzoylimidazole reacts with copper(II) nitrate tri-
hydrate in methanol to form methylbenzoate and tetra-
(imidazole)copper(II) nitrate (A) (Eq. (1)) under ambient
conditions. Complex A has been characterized by deter-
mining its crystal structure by single crystal X-ray diffrac-
tion. The structure of this complex tallies with the one
reported in the literature [28]. This reaction is a stoichiom-
etric reaction. From this reaction, we could prepare the
corresponding ester of methanol by using 1/4 molar ratio
of copper(II) nitrate with respect to the N-benzoylimida-
zole. However, we had an important observation at this
stage, if a drop of dilute nitric acid is added to the reaction
mixture the esterification reaction worked catalytically.
Inspired by this observation, we carried out a series of
2.4. Synthesis of cis-(pz)2Cu(OBz)2 Æ trans-
(pz)2Cu(OBz)2H2O (B)
To a solution of copper(II) nitrate trihydrate (242 mg,
1 mmol) in acetonitrile (3 ml) N-benzoyl 3,5-dimethyl-
pyrazole (404 mg, 2 mmol) was added. The mixture was

K. Deka et al. / Polyhedron 25 (2006) 2525–2529
2527
benzoylation reactions by N-benzoylimidazole with differ-
ent alcohols (Eq. (2)). The results showing the reaction time
and yield obtained from these esterification reactions cata-
lyzed by copper(II) under ambient conditions are listed in
Table 1. In order to ascertain that the copper(II) nitrate,
along with nitric acid, acts as a catalyst, we have carried
out reactions under identical conditions without the cata-
lyst and observed no significant reaction in this case. It is
worth mentioning that the reaction of N-benzoylimidazole
with nitric acid without copper(II) nitrate led to the forma-
tion of benzoic acid (1 ml, 0.1 M HNO3, approximately 2 h
for conversion of 1 mmol of N-benzoylimidazole). How-
ever, we used a lesser amount of the acid in our reactions;
namely 0.1 ml of 0.01 M nitric acid for conversion of
1 mmol of N-benzoylimidazole to ester. These results
clearly indicate that the catalytic ability of the copper(II)
nitrate trihydrate is enhanced by the presence of the nitric
acid. This probably happens because of protonation of the
imidazole that is formed in the reaction by C–N bond
cleavage. Imidazole in the absence of an acid can otherwise
coordinate strongly to the copper(II) ion. This is further
confirmed by adding nitric acid to tetra(imidazole)cop-
per(II) nitrate in an independent experiment, which led to
decomposition of this complex.
alcohols such as tert-butanol. We also examined the reac-
tion of N-benzoylimidazole with methanol in the presence
of tert-butanol under catalytic conditions to ascertain if
tert-butanol has a retarding effect on the rate. In this
O
H
cat. Cu(NO3)2/ HNO3
RT
OR
N
+ ROH
+
N
N
O
N
R = Methyl, Ethyl, iso-propyl, benzyl, allyl, crotyl, proporgyl etc.
ð2Þ
reaction only methylbenzoate was formed. It showed that
tert-butanol does not hinder the benzolyation and also it
does not itself give the corresponding ester under ambient
conditions. Different allylic esters from allylic alcohols
were also prepared by this copper(II) catalyzed reaction.
However, in the case of decanol the reaction had to be car-
ried out by suspending copper(II) nitrate trihydrate under
heterogeneous conditions, as the later is not soluble in dec-
anol. This solubility problem could be overcome by using
complex A with very dilute nitric acid as a catalyst in ace-
tonitrile as the solvent. Thus, the use of tetra(imidaz-
ole)copper(II) nitrate (A) as
a catalyst instead of
copper(II) nitrate trihydrate has an advantage as it mini-
mizes the solubility problem in the case of long chain alco-
hols. Complex A is soluble in common solvents such as
methanol, ethanol, acetonitrile; so the benzoylation reac-
tions using complex A as a catalyst could be carried out
in solvents other than the alcohols. The copper complex
A does not have any water of crystallization, so the water
content in the reaction is minimized and also the side reac-
tion to give benzoic acid. Two illustrative examples using
complex A as a catalyst are given in Table 1 (last two rows).
Since our interest is on an alternative reagent for benzoyl-
chloride, it is essential to know the stability of N-benzoyl-
imidazole. N-benzoylimidazole itself gets hydrolytically
cleaved in methanol/water mixture (1:1) in 24 h to give
methylbenzoate and benzoic acid. However, it undergoes
slow hydrolysis in ethyleneglycol. When N-benzoylimidaz-
ole was dissolved in ethyleneglycol containing traces of
water; the water present in the ethyleneglycol reacted with
it. After two days of reaction, we obtained white crystals of
imidazoliumbenzoate from the reaction mixture. The role
of ethyleneglycol in this salt formation reaction is presum-
ably to provide a medium for the reaction to control the
crystallization process. This fact is supported by the obser-
vation that an aqueous methanolic solution of N-benzoyl-
imidazole on standing under ambient conditions gave
benzoic acid as a crystalline product after five days. The
reaction of ethyleneglycol with N-benzoylimidazole in the
presence of a catalytic amount of copper(II) nitrate trihy-
drate and nitric acid gave the mono-benzolylated derivative
of ethyleneglycol. It was earlier reported [18] that 1-
(benzoyl)benzotriazole can be used for benzoylation of
different diols in methylenechloride to give mono-benzoy-
lated derivatives. These reactions were carried out in basic
N
N
Cu(NO3)2.3H2O
+
4
O
2+
H
N
ð1Þ
MeO
HN
O
N
N
2(NO3)-
Cu
MeOH
N
4
+
NH
N
N
H
In addition to the primary alcohols, secondary alcohols
such as isopropyl alcohol reacted very well to give the cor-
responding esters. The reaction does not work with tertiary
Table 1
The results on copper(II) nitrate catalyzed benzoylation
Alcohol
Product
Time, yield (%)
CH3OH
CH3CH2OH
C6H5COOCH3
C6H5COOCH2CH3
2 h, 95
1.5 h, 86
2 h, 68
2 h, 94
2 h, 90
3 h, 56
2 h, 90
2 h, 86
CH3(CH2)3OH
(CH3)2CHOH
C6H5CH2OH
CH2@CHCH2OH
CH3CH@CHCH2OH
CHCCH2OH
C6H5COO(CH2)3CH3
C6H5COOCH(CH3)2
C6H5COOCH2C6H5
C6H5COOCH2CH@CH2
C6H5COOCH2CH@CHCH3
C6H5COOCH2CCH
C6H5CH@CHCH2OH C6H5COOCH2CH@CHC6H5 2 h, 76
Cyclo-C6H11OH
CH2OHCH2OH
CH3(CH2)7OH
CH3OHa
CH3CH2OHa
a
C6H5COO(cyclo-C6H11
C6H5COOCH2CH2OH
C6H5COO(CH2)7CH3
C6H5COOCH3
)
2 h, trace
1 h, 92
3 h, trace
1 h, 95
1 h, 97
C6H5COOCH2CH3
With a catalytic amount of tetra(imidazole)copper(II) nitrate.

2528
K. Deka et al. / Polyhedron 25 (2006) 2525–2529
medium in the presence of triethylamine. Our copper cata-
lyzed benzoylation reactions of N-benzoylimidazole with
diols also gave only the mono-benzoylated product, but
the difference is that the copper(II) catalyzed reactions were
carried out under acidic conditions.
N-benzoyl 3,5-dimethylpyrazole on reaction with cop-
per(II) nitrate trihydrate (2:1 molar ratio) in aqueous meth-
anol at 25 °C for three days gave an interesting mixed
copper(II) benzoate complex having the composition cis-
(pz)2Cu(OBz)2 Æ trans-(pz)2Cu(OBz)2H2O (B) (where pz =
3,5-dimethylpyrazole and OBz = benzoate anion). The
compound is structurally important (Fig. 1) as it has the
distinction of having a unique lattice in which both cis
and trans coordination of 3,5-dimethylpyrazole are present
around two independent copper(II) centers. From this struc-
ture, the process can be understood to be co-crystallization
of two inorganic neutral molecules in the form of an
adduct. It is interesting to note that each of the two inde-
pendent units, namely cis-(pz)2Cu(OBz)2 and trans-(pz)2-
Cu(OBz)2H2O, is not held by any common weak interactions
such as hydrogen-bonding, p-interactions or C–Hꢁ ꢁ ꢁO inter-
actions in the lattice but probably they are held by dipolar
interactions due to a difference in the dipole moment of two
the units. Such a difference in dipole moments is obvious
from the orientation of the ligand as well as the coordina-
tion number of the two counterparts. The adduct (B) can
also easily be prepared by the reaction of copper(II) acetate
with benzoic acid in the presence of 3,5-dimethylpyrazole
in methanol. However, reactions of copper(II) acetate
with o-chlorobenzoic acid or o-nitrobenzoic acid and 3,
5-dimethylpyrazole gives monomeric bis-3,5-dimethyl-
pyrazole copper(II) o-chlorobenzoate or bis-3,5-dimethyl-
pyrazole copper(II) o-nitrobenzoate respectively. Complex
B has an interesting packing pattern, as shown in Fig. 2.
It forms a parallel layered structure and the layers are
formed by H-bonding interactions among the cis-
(pz)2Cu(OBz)2 and trans-(pz)2Cu(OBz)2H2O, indepen-
dently. The layers are held by H-bonding interactions
between the NH groups of the 3,5-dimethylpyrazole and
the carboxylate oxygen atoms of each independent cis-
(pz)2Cu(OBz)2 unit. In the trans counterpart, the H-bond-
ing interactions occurs through the carboxyl oxygens with
coordinated water and also through N–Hꢁ ꢁ ꢁO interactions
among carboxyl groups and N–H protons of the 2,3-dim-
ethylpyrazole ligands.
All these results show that the formation of complex A is
one of the driving forces for catalysis of N-benzoylimidaz-
ole, but in the case of N-benzoyl 3,5-dimethylpyrazole,
such reactions gave a catalytically inactive complex B.
In conclusion we have shown that C–N bond cleavage
by copper(II) complexes are not only useful for catalytic
Fig. 1. Crystal structure of the adduct cis-(pz)2Cu(OBz)2 Æ trans-
(pz)2Cu(OBz)2H2O (B).
Fig. 2. The crystal packing in the adduct B viewed along crystallographic c-axis.

K. Deka et al. / Polyhedron 25 (2006) 2525–2529
2529
[6] H.A. Prescott, M. Wloka, E. Kemnitz, J. Mol. Catal. A 223 (2004) 67.
[7] H. Nakamura, K. Arata, Bull. Chem. Soc. Japan 77 (2004) 1893.
[8] U.B. Gangadharmath, A.V. Demchenko, Synlet (2004) 2191.
[9] H.S. Wang, J. She, L.H. Zhang, X.S. Ye, J. Org. Chem. 69 (2004)
5774.
benzoylation reactions but also provide a method for the
preparation of metal complexes.
Acknowledgments
[10] S.M. Landge, M. Chidambaram, A.P. Singh, J. Mol. Catal. A 213
(2004) 257.
[11] P. Ciuffreda, L. Alessandrini, G. Terraneo, E. Santaniello, Molecules
(2003) 374.
[12] Y. Matsumura, T. Maki, S. Murakami, O. Onomura, J. Am. Chem.
Soc. 125 (2003) 2052.
The authors thank DST (New Delhi, India) for funding
and the X-ray diffractometer facility. We thank Mr. R.J.
Sarma for help with the crystallography.
[13] T. Iwata, Y. Miyake, Y. Nishibayashi, S. Uemura, J. Chem. Soc.,
Perkin Trans. 1 (2002) 1548.
Appendix A. Supplementary material
[14] I. Mohammadpoor-Baltork, H. Aliyan, A.R. Khosropour, Tetrahe-
dron 57 (2001) 5851.
[15] R.R. Mukti, H. Nur, S. Endud, H. Hamdan, in: E. Van Steen, L.H.
Callanan, M. Claeys (Eds.), Studies in Surface Science and Catalysis,
vol. 154, Elsevier, Amsterdam, 2004, p. 2767.
[16] E. Santaniello, S. Casati, P. Ciuffreda, L. Gamberoni, Tetrahedron
Asym. 15 (2004) 3177.
[17] G.L. Rebeiro, B.M. Khadilkar, Syn. Commun. 30 (2000) 1605.
[18] T. Wang, Z.X. Zhang, N.A. Meanwell, Tetrahedron Lett. 40 (1999)
6745.
Crystallographic data for the structure have been depos-
ited with the Cambridge Crystallographic Data Centre,
CCDC No. 281721. Copies of this information can be
obtained from the Director, CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK, e-mail: deposit@ccdc.cam.ac.uk.
The GC mass of the N-benzoylimidazole and N-benzoyl
3,5-dimethylpyrazole are also available. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.poly.2006.02.019
[19] B. Bhat, Y.S. Sanghvi, Abstract of papers of the Am. Chem. Soc. 213,
105-carb part 1, April 13, 1997.
[20] A.J. Moczulska, Carbohydrate Chem. 13 (1994) 1179.
[21] S. Kim, H. Chang, W. Kim, J. Org. Chem. 50 (1985) 1751.
[22] S. Paul, P. Nanda, R. Gupta, Tetrahedron Asym. 14 (2003) 3197.
[23] T. Sano, K. Ohashi, T. Oriyama, Synthesis (1999) 1141.
[24] M.S. Wolfe, Syn. Commun. 27 (2004) 2975.
[25] H.J. Koh, K.L. Han, I. Lee, J. Org. Chem. 64 (1999) 4783.
[26] O.A. El Seoud, M. Ferreira, W.A. Rodrigues, M.F. Ruasse, J. Phys.
Org. Chem. 18 (2005) 173.
References
[1] P.J. Kocienski, Protecting Groups, Theime, Stuttgart, 1994.
[2] T.W. Greene, P.G.M. Wutz, Protective Groups in Organic Synthesis,
third ed., Wiley, New York, 1999.
[3] J. Otera, Esterification: Methods, Reactions and Applications, Wiley-
VCH, Weinheim, 2003.
[4] A.K. Prasad, V. Kumar, J. Maity, Z.W. Wang, V.T. Ravikumar, Y.S.
Sanghvi, V.S. Parmar, Syn. Commun. 35 (2005) 935.
[5] J.K. Murthy, U. Gross, S. Rudiger, E. Kemnitz, Appl. Catal. A 278
(2004) 133.
[27] O.A. El Seoud, M.F. Ruasse, W.A. Rodrigues, J. Chem. Soc., Perkin
Trans. 2 (2002) 1053.
[28] D.L. McFadden, A.T. McPhail, C.D. Garner, F.E. Mabbs, J. Chem.
Soc., Dalton Trans. (1976) 47.