Metal specific variable binding modes in Schiff base complexes - Facile synthesis and crystal structure of anionic bis-N-(2-oxyethyl-5-bromosalicylal-diminato) cobalt(III)

Article abstract of DOI:10.1016/S0020-1693(01)00369-3

The Schiff bases N-(2-hydroxyethyl)-X-salicylaldimine (X = H, Br) which show monobasic bidentate binding mode in their copper(II) complexes, in contrast offer dibasic tridentate chelation and almost spontaneous reaction leading to the synthesis of anionic

Full text of DOI:10.1016/S0020-1693(01)00369-3

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Inorganica Chimica Acta 316 (2001) 113–116
Note
Metal specific variable binding modes in Schiff base
complexes — facile synthesis and crystal structure of anionic
bis-N-(2-oxyethyl-5-bromosalicylal-diminato) cobalt(III)
Rajib Lal De a,*, Keka Samanta a, Kamales Maiti a , Egbert Keller b
a Department of Chemistry, Inorganic Chemistry Section, Jada6pur Uni6ersity, Calcutta 700032, India
b Kristallographisches Institut, Albert-Ludwigs-Uni6ersitat Freiburg, Hebelstrasse 25, D-79104 Freiburg, Germany
Received 18 August 2000; accepted 22 December 2000
Dedicated to Professor Dr. H. Vahrenkamp on the occasion of his 60th birthday
Abstract
The Schiff bases N-(2-hydroxyethyl)-X-salicylaldimine (X=H, Br) which show monobasic bidentate binding mode in their
copper(II) complexes, in contrast offer dibasic tridentate chelation and almost spontaneous reaction leading to the synthesis of
anionic bis-tridentate cobalt(III) chelates. On the other hand the Schiff bases N-(2-hydroxyphenyl)-X-salicylaldimine (X=H, Br)
which are in dibasic tridentate binding mode in their copper(II) complexes are involved in monobasic bidentate chelation yielding
neutral bis-bidentate cobalt(II) complexes. Crystal structure analysis of bis-N-(2-oxyethyl-5-bromosalicylal-diminato) cobalt(III)
anion confirms near perfect octahedral geometry around cobalt(III). © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Schiff base; Mono/dibasic; Bi/tridentate chelation; Cobalt complexes; Crystal structures
1. Introduction
with ethanolamine and ortho aminophenol and which
may function as potential tridentate chelating ligands
are available.
Transition metal Schiff base complexes in general
and those derived from salicylaldehyde/substituted sali-
cylaldehyde in particular have been extensively studied
as models for biologically important species [1–4] and
play an important role in living beings [5–7].
Although Schiff bases are one of the most thoroughly
studied class of chelating agents [8–10] only limited
studies [1,11] for the Schiff bases derived from the
condensation of salicylaldehyde/5 bromosalicylaldehyde
As a part of our investigation on the synthesis and
characterisation of N-substituted salicylaldimines
[12,13] and their metallic complexes with a view to
explore the nature of complexation of the above stated
Schiff bases, the title cobalt complexes were prepared
and the crystal structure of one of the isolated
cobalt(III) complexes was determined by a X-ray dif-
fraction method.
2. Experimental
All chemicals were of reagent grade and used as
purchased. Solvents were dried and distilled before use.
Microanalyses of C, H, N were performed on a
Perkin–Elmer-2400 Series II CHN analyser. Cobalt
was estimated complexometrically. Electronic spectra in
* Corresponding author. Tel.: +91-473 4044.
E-mail address: rajiblal@hotmail.com (R.L. De).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00369-3

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R.L. De et al. / Inorganica Chimica Acta 316 (2001) 113–116
methanol solution were recorded on Shimadzu MPS-
2000 UV–Vis spectrophotometer. IR spectra (as KBr
pellets) were recorded on a Perkin–Elmer Paragon 1000
spectrometer. Magnetic susceptibilities were measured
on an EG and G par vibrating sample magnetometer
with Hg[Co(SCN)4] used as standard. Mass spectra
(FAB-MS) and 1H NMR spectra were respectively
recorded on a Jeol SX-102 and Varian EN 360
spectrometers.
The Schiff bases were prepared by the usual method
of condensation of equimolar quantities of salicylalde-
hyde/5 bromosalicylaldehyde with ethanolamine/2-
aminophenol in refluxing ethanol.
2.1.4. [Co(L4)2] (4)
Yield, 63%; dark-brown; m.p. \300°C; veff 3.11 BM
(297 K). Anal. Found: C, 48.55; H, 2.77; N, 4.22; Co,
8.99. Calcd. for C26H18N2O4Br2Co: C, 48.70; H, 2.83;
N, 4.37; Co, 9.19%. FT-IR: w(CꢀN), 1601(vs); l (out-
of-plane C–H (phenyl) deformation), 778(s); w(Co–N),
538(m), 463(m) cm−1. umax: 255, 382, 436, 534 nm.
2.2. Data collection and structure determination of
H[Co(L2)2] (2)
A crystal of size 0.45×0.31×0.12 mm obtained by
recrystallisation of 2 from methanol at 5°C was mea-
sured on a Bruker AXS SMART diffractometer with
CCD detector. About 1300 frames (scan angle=3°)
were measured at r.t. for 10 s each using the SMART [14]
software. Using SAINT software [15] they yielded 17 833
reflections, the position of which led to a rhombohedral
cell. Intensity statistics allowed the assignment of space
2.1. General procedure for the syntheses of the
complexes
Cobalt acetate tetrahydrate (5 mmol) was mixed with
the ethanolic solution of the Schiff bases (15 mmol).
Appreciable reaction took place at room temperature
(r.t.) and to ensure complete reaction, the mixtures were
refluxed for 1 h, cooled to r.t. and filtered off. The
products obtained as orange–red to dark-brown
residues were purified by successively washing with
ethanol and ether and finally dried in vacuo.
(
group R3. The structure was solved by direct methods,
difference Fourier syntheses and least-square refine-
ments using SHELXTL [16]. The final conventional R
value (R1) is 4.26%; residual difference electron densi-
ties are between +0.6 and −0.6 e A−3. Crystal mea-
surement, and refinement data are collected in Table 1.
Selected bond lengths and bond angles are listed in
Table 2. Fig. 1 shows an ORTEP [17] drawing of the
molecule.
2.1.1. H[Co(L1)2] (1)
Yield, 73%; reddish–brown; m.p. 210°C; diamag-
netic. Anal. Found: C, 55.76; H, 4.91; N, 7.42; Co,
14.44. Calc. for C18H19N2O4Co: C, 55.97; H, 4.96; N,
7.25; Co, 14.89%. FT-IR: w(CꢀN), 1647(vs); l ( out-of-
plane C–H (phenyl) deformation), 757(s); w(Co–N),
476(s); w(C–O), 419(m) cm−1. umax: 270, 290, 318, 346,
3. Results and discussion
Both L1 and L2 underwent almost spontaneous reac-
tions to form the anionic cobalt(III) complexes,
H[Co(L1)2] (1) and H[Co(L2)2] (2), although neutral
copper(II) complexes, [Cu(L1)2] and [Cu(L2)2], were
known [8] to form with them. On the contrary, similar
complexation reactions with L3 and L4 under identical
conditions were slow and neutral cobalt(II) complexes,
[Co(L3)2] (3) and [Co(L4)2] (4), were the only isolable
products. Binuclear copper(II) complexes, [Cu(L3)]2 and
[Cu(L4)]2, were, however, known [9] to form with them.
In the FT-IR spectra of the complexes 1 and 2 the
absence of any characteristic band at 3340–3230 cm−1
assignable to w(OH)(ethanolic) suggest tridentate chela-
tion through ethanolic oxygen, phenolic oxygen and
imine nitrogen atoms of the Schiff bases. FT-IR spectra
of the complexes 3 and 4 on the other hand indicated
the presence of strong to medium bands at 3424 cm−1
assignable to w(OH) (phenolic) suggesting the usual
bidentate coordination of the of the Schiff bases. The
mode of coordination (of the stated Schiff bases) oppo-
site to what are known for their copper complexes may
therefore be suggested for their cobalt complexes and is
confirmed by the X-ray crystallographic structure anal-
ysis performed on 2.
1
485, 732 nm. H NMR (CH3OD): l (aromatic), 7.90–
6.50 M (8H); l (CHꢀN), 4.92 D (2H, J=366 Hz); d
(CH2ꢀCH2), 3.36 M ( 8H) ppm; FAB-MS: [M+], 386.
2.1.2. H[Co(L2)2] (2)
Yield, 61%; dark-brown; m.p. 277°C; diamagnetic.
Anal. Found: C, 39.48; H, 3.04; N, 5.40; Co, 10.38.
Calc. for C18H17N2O4Br2Co: C, 39.74; H, 3.15; N, 5.15;
Co, 10.83%. FT-IR: w(CꢀN), 1641(vs); l (out-of-plane
C–H (phenyl) deformation), 792(s); w(Co–N), 478(s);
w(Co–O), 431(m) cm−1. umax: 266, 310, 372, 490, 707
nm. 1H NMR (CH3OD): l (aromatic), 7.85–6.30 M
(6H);
l
(CHꢀN), 4.80 D (2H, J=348 Hz); l
(CH2ꢀCH2), 3.25 M (8H). FAB-MS: [M+], 544.
2.1.3. [Co(L3)2] (3)
Yield, 66%; black; m.p. 262°C; veff 3.15 BM (297 K).
Anal. Found: C, 64.22; H, 4.10; N, 5.70; Co, 11.72.
Calc. for C26H20N2O4Co: C, 64.60; H, 4.17; N,5.80; Co,
12.19%. FT-IR: w(CꢀN), 1605(vs); l (out-of-plane C–H
(phenyl) deformation), 746(s), w(Co–N), 518(m);
w(Co–N), 467(w) cm−1. umax: 240(3.44), 364(2.98),
455(sh), 556(2.72) nm.

R.L. De et al. / Inorganica Chimica Acta 316 (2001) 113–116
115
The complexes 1 and 2 are diamagnetic and their
electronic spectra suggest octahedral coordination
around the cobalt(III) ion and for the paramagnetic
Table 1
Crystal data for complex 2 a
Empirical formula
Formula weight
Temperature (K)
C18H16N2O4Br2Co
543.08
298(2)
,
Wavelength (A)
0.71073
rhombohedral
Crystal system
Space group
(
R3
Unit cell dimensions
,
a (A)
19.065(3)
19.065(3)
27.325(6)
90
90
120
8601(3)
18
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
Fig. 1. An ORTEP drawing and the atom numbering scheme of
complex 2.
,
Volume (A )
Z
Dcalc (mg m−3
)
1.887
5.105
4806
Absorption coefficient (mm−1
F(000)
)
complexes 3 and 4 square planar arrangement of the
Schiff bases are well supported by their electronic spec-
tra (see Section 2).
Crystal size (mm)
q Range for data collection (°)
Index ranges
0.45×0.31×0.12
1.44–28.30
−245h522, −255k525,
−345l536
3.1. X-ray crystallographic structure analysis
Reflections collected
17 833
Independent reflections
Completeness to q =28.30°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices[I\2|(I)]
R indices (all data)
4565 [Rint=0.1085]
95.6%
numerical integration
0.6504 and 0.3462
full-matrix least-squares on F2
4565/0/244
The identity of the diamagnetic molecule 2 which
may also be A formed via bidentate coordination of
three Schiff base ligands with bonding through pheno-
lato oxygen and imino nitrogen only is found to be B
where the cobalt is coordinated by two equal tridentate
ligands involving binding through phenolato oxygen,
imino nitrogen and ethanolato oxygen. The six coordi-
nating atoms (2N, 4O) form a rather ideal octahedron
with bond angles differing maximally by 4.5° from the
ideal ones. The molecular geometry, as found in the
crystal, is C1 but some minor conformational changes
switch it to C2 with the two axis then transforming the
one ligand molecule into the other one. With respect to
bond lengths, the two ligands with few exceptions (see
below), are identical within a few standard deviations.
0.913
R1=0.0426, wR2=0.0959
R1=0.1003, wR2=0.1183
Largest difference peak and hole 0.612 and −0.603
−3
,
(e A
)
a E.s.d. values are in parentheses.
Table 2
Selected bond lengths (A) and angles (°) for complex 2
a
,
Co–O(2)
Co–O(1)
Co–N(1)
Co–N(2)
1.872(3)
1.879(3)
1.884(4)
1.893(4)
1.924(3)
1.928(3)
1.900(4)
1.313(5)
1.260(5)
1.501(6)
1.423(5)
1.903(5)
1.302(5)
1.280(5)
1.463(6)
1.419(8)
1.373(6)
O(2)–Co–O(1)
O(2)–Co–N(1)
O(1)–Co–N(1)
O(2)–Co–N(2)
O(1)–Co–N(2)
N(1)–Co–N(2)
O(2)–Co–O(21)
O(1)–Co–O(21)
N(1)–Co–O(21)
N(2)–Co–O(21)
O(2)–Co–O(11)
O(1)–Co–O(11)
N(1)–Co–O(11)
N(2)–Co–O(11)
O(21)–Co–O(11)
C(1)–N(1)–C(17)
N(2)–C(2)–C(22)
90.86(14)
86.03(14)
94.53(14)
94.96(15)
87.87(14)
177.39(15)
178.37(14)
90.25(14)
92.69(15
86.27(15)
88.71(14)
178.91(13)
86.44(14)
91.17(14)
90.19(13)
122.3(4)
Co–O(21)
Co–O(11)
Br(1)–C(14)
O(1)–C(11)
N(1)–C(1)
C(17)–C(18)
C(18)–O(11)
Br(2)–C(24)
O(2)–C(21)
N(2)–C(2)
N(2)–C(27)
C(27)–C(28)
C(28)–O(21)
124.3(4)
a E.s.d. values are in parentheses.

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R.L. De et al. / Inorganica Chimica Acta 316 (2001) 113–116
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 153865. Copies of this infor-
mation may be obtained free of charge from the Direc-
tor, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: +44-1223-336-033; e-mail: deposit@ccdc.
cam.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgements
The authors thank Central Drug Research Institute,
Lucknow, India for FAB-MS; UGC, New Delhi, India
(for fellowship to K.S.) and DST, New Delhi, India (for
fellowship to K.M. and grant to R.L.D.).
Admittedly, formula B raises some questions with
respect to the oxidation state of the cobalt atom, as
formally the latter would be +4 here, which is highly
improbable. A way out of this dilemma could, at first
sight, be found in the observation that the bonds
,
,
C27–C28 [1.419(8) A] and C28–C21 [1.373(6) A] are
significantly shorter than the corresponding bonds
References
,
,
C17–C18 [1.501(6) A] and C18–O11 [1.423(5) A]. This
could be explained by considering that the formula in
the case of ligand (no. 2) is CH2–CHꢀO instead of
CH2–CH2–O– which would leave the oxidation state
of Co at +3 (but would imply some previous redox
reaction).
[1] S. Gourbatsis, S.P. Perlepes, N. Hadjiliadis, Trans. Met. Chem.
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also be due to the largely anisotropic displacement
parameters of C27 and C28. This view is supported by
the results of attempts to use a disordered model for
these two atoms. Two similar fragments were obtained
with weights 0.7 and 0.3, respectively. In the disordered
model, some of the shortened bond lengths had in-
creased to the expected values. Others, however, espe-
cially the distance C27–C28 in the 0.7 fragment, were
still too short — or too long (therefore only the
‘averaged’ model has been accounted for in the tables
and figure).
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logr., Sect. C 51 (1995) 2344.
Summarising one can state that the shortening of the
bond lengths is probably mainly to be seen in connec-
tion with the anisotropy of the thermal ellipsoids. The
oxidation states of the Co atom is nevertheless +3 and
the complex is supposed to carry one negative charge
which might be compensated by one H atom at any
[13] A.K. Mukherjee, R.L. De, I. Banerjee, C. Samanta, N.P. Nayak,
Acta Crystallogr., Sect. C 55 (1999) 407.
[14] Bruker AXS, SMART Software, Madison, WI, USA, 1998.
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[17] L. Farrugia, ORTEP-III, Version 1.0.2 for Windows 32, University
of Glasgow, UK, 1998.
,
unknown position. The Co–N [1.884(4), 1.893(4) A],
[18] M. Calligaris, G. Nardin, L. Randaccio, Coord. Chem. Rev. 7
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[19] D.C. Ware, W.A. Denny, G.R. Clark, Acta Crystallogr., Sect. C
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,
Co–O (phenolato) [1.872(3), 1.879(3) A] and Co–O
,
(ethanolato) [1.924(3), 1.928(3) A] are consistent with
corresponding values observed in related octahedral
cobalt(III) systems [18–20].
.