Copper(II)-mediated in-situ hydrolyses of pyrroline N-oxide and benzonitrile leading to a mixed ligand complex

Article abstract of DOI:10.1016/j.inoche.2012.01.012

Copper(II)-mediated in-situ hydrolyses of 5,5-dimethyl-1-pyrroline N-oxide into 5,5-dimethyl-2-oxo-pyrrolidine-1-olate (L¹), and of benzonitrile into benzamide, are observed in the formation of complex [Cu(L ¹)₂(C₆H₅C( O)NH₂)] (1), which has been characterized by IR, elemental analysis and X-ray diffraction crystallography.

Full text of DOI:10.1016/j.inoche.2012.01.012

Inorganic Chemistry Communications 18 (2012) 69–72
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Copper(II)-mediated in-situ hydrolyses of pyrroline N-oxide and benzonitrile leading
to a mixed ligand complex
a
a,b,
a,
Jamal Lasri a, , Samik Gupta , M. Fátima C. Guedes da Silva
⁎
⁎
⁎
, Armando J.L. Pombeiro
a
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
Universidade Lusófona de Humanidades e Tecnologias, Av. Campo Grande 376, ULHT Lisbon, 1749-024, Lisbon, Portugal
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper(II)-mediated in-situ hydrolyses of 5,5-dimethyl-1-pyrroline N-oxide into 5,5-dimethyl-2-oxo-
pyrrolidine-1-olate (L1), and of benzonitrile into benzamide, are observed in the formation of complex
[Cu(L1)2(C6H5C( O)NH2)] (1), which has been characterized by IR, elemental analysis and X-ray diffrac-
tion crystallography.
Received 6 December 2011
Accepted 17 January 2012
Available online 25 January 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Copper(II)-mediated hydrolysis
Nitrile
Pyrroline N-oxide
One-pot synthesis
Mixed ligand complex
Nitriles and isonitriles are versatile reagents in coordination
chemistry [1], being reactive towards a number of different nucleo-
philes [2, 3] and 1,3-dipole reagents [4]. Palladium and platinum
have been found to be within the most effective metals for the acti-
vation of nitriles through coordination [1a]. In recent years, our
group has been involved in studying the [2+3] cycloaddition reac-
tions of cyclic and acyclic nitrones with organonitriles [5]. Cyclic
nitrones were found to be more reactive than acyclic ones in several
metal-mediated cycloadditions [6]. We have also observed that ac-
tivation of nitriles by coordination with first row transition metal
ions, such as zinc [7], has led to hydrolysis of the nitriles into car-
boxamides. It has also been found [8] that a Co(II)/ketoxime system
promotes a facile conversion of an alkyl nitrile RCN to the appropri-
ate amidine RC( NH)NH2 and carboxylic acid RC( O)OH. Moreover,
when the nitrile was diluted with water, the reaction went in an-
other direction giving carboxamides RC( O)NH2 [8]. Hydrolysis of
nitriles to carboxamides is a chemical challenge as most of the
base catalyzed hydrolytic processes lead to the final hydrolysis
products, i.e. carboxylic acids [9].
cycloaddition of nitrones to other activated species have been
revealed, and very recently, the 1,3-dipolar cycloaddition of an acyclic
nitrone to an alkenoyl pyridine N-oxide catalyzed by a chiral bis(oxa-
zoline) copper(II) system was reported [11, 12].
Keeping these reports in mind, we have investigated the reaction
of a cyclic nitrone (5,5-dimethyl-1-pyrroline N-oxide) with benzoni-
trile in the presence of copper(II) ion. Benzonitrile is a better choice
than an alkyl nitrile due to the electron-withdrawing character of
the phenyl ring, promoting the electrophilicity of the cyano group.
Further activation upon coordination to copper(II) ion should addi-
tionally facilitate the reaction.
We were interested to examine the role that the copper ion plays
in determining whether the reaction proceeds towards cycloaddition
and/or hydrolysis of benzonitrile (Scheme 1). The mononuclear com-
plex 1 which emerged from the reaction mixture is the result of
copper-promoted hydrolyses of both benzonitrile and nitrone, form-
ing metal ligated benzamide and 5,5-dimethyl-2-oxo-pyrrolidine-
1-olate, respectively, in the mixed ligand complex [Cu
{ONC( O)(CH2)2C(CH3)2}2{C6H5C( O)NH2}] (1). Prior to this one, a
few reports of copper(II) complexes of nitrone based ligands have
appeared [13], but mostly containing acyclic nitrone ligands and
only some of them have been structurally characterized [14]. The
complex 1 has been characterized by IR, elemental analysis and
X-ray diffraction structural analysis.
Copper-mediated hydrolysis of nitriles in macrocyclic ligands with
pendant nitrile groups is known [10], and copper-catalyzed cycload-
ditions of nitrones to activated dipolarophiles other than nitriles are
also being studied [11, 12]. Catalytic effects of copper(II) in the
The reaction of copper(II) acetate, benzonitrile and 5,5-dimethyl-
1-pyrroline N-oxide, under heating at 100 °C for 48 h, affords the
complex 1 in moderate isolated yield, ca. 50% (Scheme 1). During
the heating, the in-situ copper(II)-mediated hydrolyses of benzoni-
trile and nitrone took place, and upon removal of the excess of the
benzonitrile, complex 1 separated out [15].
⁎
Corresponding authors.
E-mail addresses: jamal.lasri@ist.utl.pt (J. Lasri), fatima.guedes@ist.utl.pt
(M.F.C.G. da Silva), pombeiro@ist.utl.pt (A.J.L. Pombeiro).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.01.012

70
J. Lasri et al. / Inorganic Chemistry Communications 18 (2012) 69–72
Scheme 1. The IR spectrum of complex 1 shows a broad peak at 3402 cm−1 assigned [16] to N―H stretching. A sharp peak at 1633 cm−1 is due [17] to C O stretching of the amide
functionality in the ligated benzamide.
In the X-ray structure of 1 [18, 19] (Fig. 1) the copper cation is in an
all oxygen penta-coordinate environment; four of the oxygen atoms
pertain to the two uni-negative 5,5-dimethyl-2-oxo-pyrrolidine-1-
olate ligands and occupy the equatorial positions, whereas the fifth is
from a benzamide unit and seats on the apical location. According to Ad-
dison et al. [20], who introduced a parameter (τ5) to describe the geom-
etry of a five-coordinate metal system, which is determined by the
simple equation τ5=(β−α)/60° (β and α are the largest angles involv-
ing the metal), perfect square pyramid or trigonal bipyramid geome-
tries should have τ5 values of 0 or 1, respectively. For our complex,
such a parameter assumes the value of 0.08, thus showing that the
structure is square pyramid. As expected, and due to ring constraints,
the endo OCuO angles (85.75(9) and 86.05(9)°, see Fig. 1 legend) are
shorter than the exo ones (93.24(9) and 93.46(9)°). The basal CuO
bond length lies within the 1.990(2)–1.933(2)Å range, being compara-
ble to other square pyramidal copper complexes [21]. The apical CuO
bond length, 2.437(2)Å, is naturally longer.
The Cu1―On―Nn―Cn1―On1 metallacycles (n=1 or 2) are rough-
ly planar with the deviations ranging from 0.006 to 0.038 Å (for
n=1) or from 0.023 to 0.055 Å (for n=2). The copper cation is
0.109 Å above the basal plane defined by O1, O2, O11 and O21, in
the direction of the apical benzamide O-atom.
The complex also features twofold linkages of neighboring units
(Fig. 2) giving rise to the formation of dimeric copper aggregates via
strong intermolecular contacts involving, as donor atoms, one of the
benzamide NH2 hydrogen atoms and one of the CH groups of the ar-
omatic ring; the metallacycle O1 and O21 oxygen atoms act as accep-
tors. In this respect, it is worth to stress the intensity of the
C7―H7∙∙∙O1 contact which falls in the range of a classical hydrogen
interaction and is, therefore, a strong one [22]. The remaining benza-
mide NH2 hydrogen is involved in a strong S(6) interaction [23], thus
generating an H-containing highly twisted metallacycle. Probably on
account of this interaction, the amido CONH2 group is significantly ro-
tated from coplanarity with the aromatic ring to which it is attached
[acute angle of 30.10°].
Fig. 1. Crystal structure of 1 with atomic numbering scheme. Atoms drawn at 50% prob-
ability. Selected bond lengths (Å) and angles (°): O1―Cu1 1.933(2), O2―Cu1 1.912(2),
O3―Cu1 2.437(2), O11―Cu1 1.976(2), O21―Cu1 1.990(2), N1―O1 1.367(3),
C11―O11 1.265(4), N2―O2 1.365(3), O21―C22 1.281(4), C11―N1 1.313(4),
C22―N2 1.296(4); O2―Cu1―O1 167.81(9), O2―Cu1―O11 93.46(9), O1―Cu1―O11
85.75(9), O2―Cu1―O21 86.05(9), O1―Cu1―O21 93.24(9), O11―Cu1―O21
172.89(9), O2―Cu1―O3 97.68(9), O1―Cu1―O3 94.47(8), O11―Cu1―O3 97.85(9),
O21―Cu1―O3 89.24(8), N1―O1―Cu1 105.28(16), N2―O2―Cu1 105.22(16),
C1―O3―Cu1 121.9(2), C11―O11―Cu1 107.2(2).
In this work, we reported the first example of in-situ copper(II)-
mediated hydrolyses of both benzonitrile and a cyclic nitrone into li-
gated benzamide and a 2-oxo-pyrrolidine-1-olate, respectively, under
relatively mild condition.

J. Lasri et al. / Inorganic Chemistry Communications 18 (2012) 69–72
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This work has been partially supported by the Fundação para a
Ciência
e a Tecnologia (FCT), Portugal, and its PTDC/QUI-QUI/
102150/2008 and PEst-OE/QUI/UI0100/2011 projects. J.L. expresses
gratitude to the FCT and the Instituto Superior Técnico (IST) for his re-
search contract (CIÊNCIA 2007 program), and S.G. also thanks FCT for
a fellowship (grant SFRH/BPD/69415/2010).
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Crystal data for complex 1 have been deposited at the Cambridge
Crystallographic Data Centre (CCDC) with deposition no. 856662.
Copies of this information may be obtained free of charge from
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk. Sup-
plementary materials related to this article can be found online at
doi:10.1016/j.inoche.2012.01.012.
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