Synthesis of cobalt(II) complexes of derivatised salicylaldoxime ligands; X-ray crystal structures of DMSO adducts of bis(3-nitro-5 -methylsalicylaldoximato)cobalt(II) and bis(3-nitro-5-phenylsalicylaldoximato)cobalt(II)

Article abstract of DOI:10.1016/S0277-5387(01)00947-0

A total of five salicylaldoxime ligands, (HL^1-5), analogues of commercial metal extractants were synthesised and characterised. The cobalt(II) complexes [CoL₂] (L = L^1-5) of these ligands were synthesised and characterised by microanalysis, atomic absorption, infra and mass spectroscopy. X-ray structure analysis was carried out on the adducts [Co(L)₂(dmso)₂] (L = L^4,5). These are the only structurally characterised cobalt salicylaldoxime complexes apart from three five-coordinate nitroso complexes [CoL₂(NO)] reported in the 1980s (J. Crystallogr. Spectrosc. Res. 13 (1983) 413; J. Crystallogr. Spectrosc. Res. 16 (1986) 6). The structures of the two adducts characterised show octahedral coordination with two dmso molecules occupying axial positions and two salicylaldoxime units in each molecule occupying equatorial positions. The introduction of nitro groups on the two ligands has been found to stabilise oxidation (II) in preference to (III) in both cobalt complexes. These groups were found to have what appears to be a significant contribution to intra-ligand hydrogen bonding resulting in three-centre H bonds on either side of the molecules, which in turn adopt unusual chair conformations.

Full text of DOI:10.1016/S0277-5387(01)00947-0

Polyhedron 20 (2001) 3239–3247
www.elsevier.com/locate/poly
Synthesis of cobalt(II) complexes of derivatised salicylaldoxime
ligands; X-ray crystal structures of DMSO adducts of
bis(3-nitro-5-methylsalicylaldoximato)cobalt(II) and
bis(3-nitro-5-phenylsalicylaldoximato)cobalt(II)
Domenico Cupertino a, Mary McPartlin b, Andreas M. Zissimos b,*
a AVECIA, Hexagon House, Blackley, Manchester M9 8ZS, UK
b School of Biological and Applied Sciences, Uni6ersity of North London, 166-220 Holloway Road, London N7 8DP, UK
Received 11 July 2001; accepted 24 September 2001
Abstract
A total of five salicylaldoxime ligands, (HL1–5), analogues of commercial metal extractants were synthesised and characterised.
The cobalt(II) complexes [CoL2] (L=L1–5) of these ligands were synthesised and characterised by microanalysis, atomic
absorption, infra and mass spectroscopy. X-ray structure analysis was carried out on the adducts [Co(L)2(dmso)2] (L=L4,5).
These are the only structurally characterised cobalt salicylaldoxime complexes apart from three five-coordinate nitroso complexes
[CoL2(NO)] reported in the 1980s (J. Crystallogr. Spectrosc. Res. 13 (1983) 413; J. Crystallogr. Spectrosc. Res. 16 (1986) 6). The
structures of the two adducts characterised show octahedral coordination with two dmso molecules occupying axial positions and
two salicylaldoxime units in each molecule occupying equatorial positions. The introduction of nitro groups on the two ligands
has been found to stabilise oxidation (II) in preference to (III) in both cobalt complexes. These groups were found to have what
appears to be a significant contribution to intra-ligand hydrogen bonding resulting in three-centre H bonds on either side of the
molecules, which in turn adopt unusual chair conformations. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Salicylaldoxime ligands; Cobalt(II) complexes; X-ray crystal structures; DMSO adducts
1. Introduction
When cobalt was extracted from solutions at pH 7–9,
by salicylaldoxime-based extractants, it was found to be
very difficult to strip (i.e. to be separated from the
ligand used in the extraction process by acidification of
the complex); even concentrated acids had little effect
on the extracted complex. This was generally assumed
to be the result of oxidation of Co(II) to Co(III), with
the formation of very stable Co(III) salicylaldoxime
complexes in the solvent phase.
The absence of any structural data on the square
planar bis chelate cobalt(II) complexes of salicylal-
doxime ligands, apart from 3 five coordinate nitroso
complexes [CoL2(NO)] reported in the 1980’s [1,2], and
the need to develop new derivatives of these ligands
that might stabilise cobalt in oxidation state (II) upon
complexation has motivated the present work. It was
thought, that steric hindrance introduced on the ligands
in the form of bulky substituents R1 and R2, would
Early work on the extractive properties of salicylal-
doximes had established them as very good extractants
for copper [3–5] and various extractants were devel-
oped based on these ligands [6]. Attempts, however, to
use these compounds in the extraction of cobalt [7]
failed for a number of reasons, but much of this work
has been carried out in industrial laboratories and has
not been well documented in the general scientific liter-
ature. In 1975, Ashbrook [8], in a review on chelating
reagents in solvent extraction, discussed the problem
involved in the extraction of cobalt using these ligands.
* Corresponding author. Present address: Chemistry Department,
University College London, 20 Gordon Street, London WC1H 0AJ,
UK. Tel.: +44-207-679-4639; fax: +44-207-679-4603.
E-mail address: a.zissimos@ucl.ac.uk (A.M. Zissimos).
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00947-0

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D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
Table 3
Table 1
Room temperature effective magnetic moments of cobalt(II) com-
plexes
Cobalt complexes
Complex
veff (BM)
[Co(L1)2]
[Co(L2)2]
[Co(L3)2]
[Co(L4)2]
[Co(L5)2]
2.17
2.29
1.91
4.11
3.21
HL
R1
R2
Cobalt complex
HL1
HL2
HL3
HL4
HL5
But
H
[Co(L1)2]
[Co(L2)2]
[Co(L3)2]
[Co(L4)2]
[Co(L5)2]
But
But
H
Busec
NO2
NO2
sponding to the CꢀC and CꢀN groups are encountered
[11]. The band observed around 3400 cm−1 is common
to all complexes throughout, and is observed as a broad
band, which supports the existence of a hydrogen
bridge as a common feature of all the transition metal
complexes of the salicylaldoxime ligands. The first band
(highest wave number) observed in the 1700–1500
cm−1 region was assigned to the bending (or deforma-
tion) vibration of the OꢁH group. These bands are of
weak to medium intensities; their shifts, relative to
those in the free ligands, do not show a consistent trend
but vary in direction and magnitude for different com-
plexes. The next highest wave number band in this
region was assigned to the CꢀC groups present in the
aromatic rings in all the compounds. In the current
work, it was found that the CꢀN bands are shifted to
lower frequencies by approximately 30 cm−1 compared
to the free ligands [12] and this was interpreted as
evidence of coordination through the nitrogen atom of
the oximic group in all the complexes studied in the
present work. Finally, the bands observed in the 1300–
800 cm−1 region have been assigned to the NꢁO and
CꢁO stretching vibrations.
The results of the magnetic measurements may be
divided into two sets, one with veff in the range 1.91–
2.71 BM and the second with veff in the higher range
3.21–4.11 BM (Table 3). The first set of veff values were
obtained from the complexes, [Co(L1)2], [Co(L2)2] and
[Co(L3)2], of the organo-derivatised salicylaldoxime lig-
ands. The veff values, 1.91–2.71 BM, are consistent
with the presence of low spin d7 ions (with one un-
paired electron in their dxy orbitals) and indicate square
planar coordination. The second set of results is that
CH3
Ph
help to distort their cobalt(II) complexes from planarity
and possibly favour an oxidation state of (II) for
cobalt. A series of ligands were synthesised which had
substituents R1 and R2 in the form of tertiary or
secondary butyl or nitro groups and the parent ligand
salicylaldoxime (R1=R2=H), shown in Table 1.
2. Results and discussion
Several analytical techniques were used to character-
ise the complexes including microanalysis (CHN), in-
frared, mass and atomic absorption spectroscopy.
Magnetic measurements were also carried out on the
complexes.
The infrared spectra of all the cobalt complexes were
obtained as KBr discs in the range 400–4000 cm−1
.
Selective bands from the spectra were tentatively as-
signed by comparison with previous studies [9,10] of
bissalicylaldoximato metal(II) complexes. The assign-
ments are all tabulated in Table 2. Infrared spec-
troscopy has proved to be a very useful tool in the
characterisation, identification and assessment of the
purity of the complexes, and provided several useful
indicators in the gathering of structural information.
The regions proved to be of greater significance for
structural analysis are the 3500–2500 cm−1 region
where the hydroxyl stretching mode is usually found
and the 1700–1500 cm−1 region where the bands corre-
Table 2
Infrared bands observed for cobalt(II) complexes of salicylaldoxime ligands
Complexes
w(OH)
l(OꢁH)
w(CꢀC)
w(CꢀN)
w(NꢁO)
w(CꢁO)
[Co(L1)2]
[Co(L2)2]
[Co(L3)2]
[Co(L4)2]
[Co(L5)2]
3398
3428
3391
3389
3410
1655
1636
1636
1627
1635
1612
1603
1593
1610
1612
1551
1555
1565
1547
1544
1188, 847
1178, 841
1216, 958
1223, 846
1250, 897
1297, 1018
1276, 1020
1290, 1017
1308, 944
1278, 984

D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
3241
obtained from complexes of the nitro-derivatised lig-
ands HL4 and HL5. The higher values of veff for these
compounds (4.11 and 3.21 BM, respectively) indicate
three unpaired electrons, which may be consistent with
high spin square planar, tetrahedral or low spin octahe-
dral coordination. High spin square planar coordina-
tion for cobalt(II) is virtually unknown, and low spin
octahedral coordination would require a different for-
mulation from [Co(L)2]. Tetrahedral coordination
which was the target of this work appeared possible,
but as the analytical figures for [Co(L)2] were signifi-
cantly low in carbon and nitrogen the possibility of an
octahedral adduct with water being present, could not
be ruled out.
hedral structures anticipated originally for the parent
molecules. The central cobalt atom in both complexes
has been stabilised in an oxidation state of (II) which
was the principal aim of this work. In each complex the
two salicylaldoxime ligands each bear a formal charge
of −1, where a phenolic proton has been lost upon
complexation to the metal. The possibility that one of
the oximic groups had also been deprotonated was
ruled out by the direct location of the hydrogen atom
H(1) on both oximic oxygen atoms. Although, in X-ray
crystallography, directly located hydrogen atoms are
not entirely reliable, further evidence of their presence
may be deduced by the close proximity of the oximic
oxygen O(2) atom to both the phenolic oxygen O(1A)
and the nitro oxygen atom O(4A) of the opposite ligand
indicating inter-ligand hydrogen bonding involving
H(1). The dimethylsulfoxide (DMSO) axial ligands are
neutral with the expected SꢀO bonding distances,
S(1)ꢁO(5), 1.518(3) in [Co(L4)2(dmso)2] and S(1)ꢁO(5),
1.524(4) in [Co(L5)2(dmso)2] (Table 4). Supporting evi-
dence of the oxidation state of the cobalt atom in the
complexes is difficult to obtain from the coordination
3. The structures of the cobalt(II) complexes of HL4
and HL5
Both compounds proved to be dmso adducts (struc-
tures illustrated in Figs. 1 and 2), with octahedral
coordination, as opposed to the square planar or tetra-
Fig. 1. The molecular centrosymmetric structure of [Co(L4)2(dmso)2].
Fig. 2. The molecular centrosymmetric structure of [Co(L5)2(dmso)2].

3242
D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
Table 4
,
1.871 A, respectively, which may be attributed to the
,
,
Selected bond lengths (A), bond distances (A) and bond angles (°) for
difference in the coordination number. No other salicy-
laldoxime compounds of cobalt have been reported.
The X-ray results obtained for [Co(L4)2(dmso)2] and
[Co(L5)2(dmso)2] show that the role of the nitro sub-
stituents has not been what was originally anticipated
as far as increasing steric hindrance to planarity and
thus facilitating distortion. It is evident that the salicy-
laldoxime ligands, despite the added bulkiness, have
been accommodated in perfectly planar arrangements
of the equatorial planes in the centrosymmetric
cobalt(II) complexes. It seems that the reasons behind
stabilisation of oxidation (II) for cobalt lie elsewhere.
Significantly, the nitro substituents in both cases are
making a contribution to the intra-ligand hydrogen
bonding resulting in a three-centre bond which would
contribute to stability (Fig. 3(a) and (b)) [15]. All atoms
involved in the hydrogen bonding are oxygen atoms
attached to different groups. The van der Waals radius
[Co(L4)2(dmso)2] and [Co(L5)2(dmso)2]
[Co(L4)2(dmso)2]
[Co(L5)2(dmso)2]
Bond lengths
Co(1)ꢁO(1)
Co(1)ꢁN(1)
Co(1)ꢁO(5)
S(1)ꢁO(5)
O(2)ꢁN(1)
O(3)ꢁN(2)
N(2)ꢁC(3)
N(1)ꢁC(7)
O(4)ꢁN(2)
H(1)ꢁO(2)
1.999(3)
2.038(4)
2.153(3)
1.518(3)
1.409(5)
1.208(6)
1.449(6)
1.281(7)
1.224(6)
0.928
Co(1)ꢁO(1)
Co(1)ꢁN(1)
Co(1)ꢁO(5)
S(2)ꢁO(5)
O(2)ꢁN(1)
O(3)ꢁN(2)
N(2)ꢁC(3)
N(1)ꢁC(7)
O(4)ꢁN(2)
H(1)ꢁO(2)
1.997(4)
2.042(4)
2.155(4)
1.524(4)
1.391(5)
1.215(7)
1.453(7)
1.288(7)
1.222(6)
0.928
Bond distances
H(1)···O(1A)
H(1)···O(4A)
O(2)···O(1A)
O(2)···O(4A)
2.096
2.222
2.881
2.970
H(1)···O(1A)
H(1)···O(4A)
O(2)···O(1A)
O(2)···O(4A)
2.076
2.311
2.823
3.140
,
Bond angles
used for oxygen (1.50 A) is based on values given by
O(1)ꢁCo(1)ꢁN(1)
O(1)ꢁCo(1)ꢁO(5)
N(1)ꢁCo(1)ꢁO(5)
O(1A)ꢁCo(1)ꢁN(1) 90.97(15) O(1)ꢁCo(1)ꢁN(1A)
O(1A)ꢁCo(1)ꢁO(5) 92.74(14) N(1A)ꢁCo(1)ꢁO(5)
N(1A)ꢁCo(1)ꢁO(5) 91.24(14) N(1)ꢁCo(1)ꢁO(5)
89.03(15) O(1)ꢁCo(1)ꢁN(1)
87.26(14) O(1)ꢁCo(1)ꢁO(5)
88.76(14) O(1A)ꢁCo(1)ꢁO(5)
89.9(2)
90.4(2)
89.6(2)
90.1(2)
87.7(2)
92.3(2)
114.0(4)
118.6(3)
Bondi [16] and Kitaigorodsky [17] and has been also
used in recent works [18]. Both structures show some
features, which might be relevant to the existence of a
three-centre hydrogen bond. A recent study of amino
acid structures suggested that three-centre bonds tend
to occur in ‘proton deficient’ structures, i.e. structures
that lack active protons (OꢁH, NꢁH) to satisfy the
normal hydrogen bonding requirements of acceptor
atoms. Such acceptor atoms are present in both com-
pounds in the form of the phenolic oxygen atoms, and
the oxygen atoms of the nitro group which are also
potential hydrogen bond acceptors. According to statis-
tical surveys [18], the hydrogen atom involved in a
three-centre bond does not lie in the plane of the
acceptor atoms, and in complexes [Co(L4)2(dmso)2] and
[Co(L5)2(dmso)2] the hydrogen atoms H(1) are 0.22 and
S(1)ꢁO(5)ꢁCo(1)
C(7)ꢁN(1)ꢁCo(1)
117.8(2)
127.3(3)
C(7)ꢁN(1)ꢁO(2)
O(2)ꢁN(1)ꢁCo(1)
sphere bond lengths because of the lack of similar
cobalt(II) compounds. The salicylaldoxime ligands in
both complexes coordinate to the central cobalt atom
via the oximic nitrogen atoms N(1) and the phenolic
oxygen atoms O(1). Although there are no closely
related six-coordinate cobalt(II) compounds the mean
,
CoꢁN bond length of 2.040(4) A is very similar to the
mean CoꢁN distance of 2.072(4) A for the oxime
,
,
0.23 A, respectively, above the triangle formed by the
nitrogen donor in the binuclear cobalt(II) complex of
2,6-diformyl-4-methylphenoldioxime [13], the mean
CoꢁO distance in this dinuclear compound of 2.055(4)
three oxygen atoms involved in the hydrogen bonding.
Within the salicylaldoxime ligand in each molecule,
the phenyl ring and all the atoms directly bonded to it
,
,
A is longer than the mean value of 1.998(4) A in the
present study but the longer CoꢁO bond may be at-
tributed to the fact that the oxygen donor is bridging
two cobalt(II) atoms. The mean CoꢁN and CoꢁO
lengths in the six-coordinate Co(II) bischelate com-
,
are nearly coplanar (maximum deviation N(2) 0.028 A
in [Co(L4)2(dmso)2] and C(7) 0.065
A
in
,
[Co(L5)2(dmso)2]); a slight rotation about the C(1)ꢁC(7)
bond results in N(1) and O(2) of the oximic group lying
,
0.277 and 0.281 A, respectively, out of the plane in
4
5
[Co(L4)2(dmso)2] and N(1) and O(2) atoms of the ox-
,
pounds of L and L (2.040(4) and 1.998 A, respec-
tively) are considerably longer than the corresponding
lengths of 1.888 and 1.927 A observed in the octahedral
,
imic group lying 0.223 and 0.321 A, respectively, out of
the plane of the ligand in [Co(L5)2(dmso)2]. Conse-
quently, there is a dihedral angle of 26.0 and 9.1°
between the mean plane of the ligand and the coordina-
tion plane of the metal in the respective complexes. The
inversion symmetry at the cobalt atom dictates that the
two trans ligands are parallel to each other and results
in an overall ‘chair’ conformation for both molecules.
A study of models of the molecules indicated that the
chair conformation brings closer together oxygens O(2),
,
cobalt(III) complex, tris-{3-phenyl-3-hydroxypropan-2-
oximato}cobalt(III) [14], which is consistent with the
difference in oxidation state. In the six-coordinate
Co(II) complexes of L4 and L5 the mean CoꢁN and
CoꢁO lengths are also considerably longer (2.040(4)
,
and 1.998 A, respectively) than those in the five-coordi-
nate cobalt(II) complex [Co(L)2(NO)]1,2 (where LH is
the unsubstituted salicylaldoxime ligand) of 1.863 and

D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
3243
O(1A) and O(4A) in contrast to more planar arrange-
ments and thus facilitating the three-centre hydrogen
bonding. Significantly, the more folded molecule has
slightly stronger hydrogen bonding as judged by the
because crystallisation methods used to provide suitable
crystals were not successful. Crystals suitable for X-ray
analysis, however, were successfully prepared from so-
lutions of [Co(L4)2] and [Co(L5)2] in DMSO. The struc-
tures studied were found to be those of the respective
adducts of the complexes with DMSO ligands coordi-
nated at axial sites of octahedral cobalt(II) structures.
The addition of nitro groups on the ligands resulted in
molecules adopting chair conformations with three-cen-
tre hydrogen bonds at either side of the molecules. It is
difficult to attribute stability to bulky ligands inducing
non-square planar coordination because the two lig-
ands in the stable [CoL2] octahedral compounds give
exactly planar CoN2O2 coordination planes. The stabil-
ity observed for the two nitro-derivatised complexes,
therefore, appears to arise from electronic factors.
,
mean O(H)···O(1A, 4A) distances of 2.925 A when the
,
dihedral angle is 26.0° and 2.981 A when the dihedral
angle is 9.1°. The factors controlling the degree to
which the molecules fold are not easy to determine but
appear to be related to the optimisation of inter-molec-
ular forces.
4. Concluding remarks
A series of cobalt(II) complexes of a range of salicy-
laldoxime ligands of the general formula [Co(L)2] have
been successfully synthesised and characterised using
various analytical and spectroscopic techniques. Char-
acterisation of the complexes using the above men-
tioned techniques indicated that complexes [Co(L1–3)2]
were square planar with strong hydrogen bonds. Unfor-
tunately, X-ray structural studies have not been possi-
ble for complexes [Co(L1)2], [Co(L2)2] and [Co(L3)2]
5. Experimental
The literature procedures were followed to prepare
ligands HL2, HL3 and HL4 [19–21]. Ligands HL1 and
Fig. 3. Views of the equatorial planes of: (a) [Co(L4)2(dmso)2]; and (b) [Co(L5)2(dmso)2] showing the three-centre hydrogen bonding.

3244
D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
HL5 are reported here for the first time using modifica-
tion of methods reported in literature [20,22].
tion for a carbonyl to oxime transformation [24] em-
ploying the reagent NH2OH·HCl.
Cobalt(II) complexes of ligands HL1, HL2, HL3, HL4
and HL5 (Table 1) were prepared using hydrated cobalt
acetate salts. All ligands were prepared and purified
prior to use. The methods for synthesising the cobalt
complexes were developed from known reaction routes
for cobalt complexes, proposed by Bailes and Calvin
[23]. Modifications on the methods were mainly the use
of Schlenk techniques and argon atmospheres. Com-
plexes [Co(L1)2], [Co(L2)2] and [Co(L3)2] were synthe-
sised under these conditions because during preliminary
attempts they proved to be sensitive even in the slight-
est presence of the solvent; that is the complexes would
oxidise even when only slightly wet. For the synthesis
of these complexes the vigorous exclusion of oxygen
from any solvents (methanol or ethanol) used, proved
to be very significant. Deoxygenation of the solvents
was carried out using the freeze–thaw technique. The
method involved application of a vacuum and simulta-
neous immersion of the solvent in liquid nitrogen. Once
the solvent was frozen, the nitrogen was removed and
argon was admitted until the solvent was fully thawed.
Repetition of the process twice ensured full removal of
any oxygen present. The technique proved to be of
great efficiency in all experiments that required vigor-
ous degassing. The cobalt acetate used during the ex-
periments proved to be hydroscopic and, therefore,
storage in dessicators was necessary. Complexes
[Co(L4)2] and [Co(L5)2] were made under normal condi-
tions because the solids obtained proved to be stable.
All cobalt complexes were of red–brown colours and
were dried in mild temperatures of 50–55 °C. Com-
pounds [Co(L1)2], [Co(L2)2] and [Co(L3)2] were soluble
in acetone but upon dissolving them they turned black
immediately showing evidence of oxidation. Com-
pounds [Co(L4)2] and [Co(L5)2] were found to be solu-
ble only in DMSO from which crystals suitable for
X-ray analysis were obtained.
Yield 4.33 g, 86.6%. Found: C, 73.9; H, 7.2. Calc. for
C11H14O2: C, 74.1; H, 7.2%. H NMR (CDCl3, ppm): l
11.33 (s, 1H, CHO), 9.85 (s, 1H, OH), 7.05 (m, 3H,
1
ArꢁH), 2.51 (sextet, 1H, CH), 1.59 (q, 2H, CH2), 1.22
3
(d, 3H CH3, J6,7=7.08 Hz), 0.85 (t, 3H, CH3).
5.2. Synthesis of 3-nitro-5-phenylsalicylaldoxime (HL5)
3-Nitro-5-phenylsalicylaldoxime was also synthesised
using the Levin route [20]. 4-Hydroxybiphenyl (17.02 g,
0.10 mol) was dissolved in toluene (15 g). Magnesium
methoxide (61 cm3 containing 4.31 g, 7 wt.%) was
added to the reaction dropwise while stirring over 1.5 h
until all the phenol was dissolved and the reaction
turned brown. Volatiles were removed by distillation till
the temperature reached 95 °C. On removal of the
toluene–methanol azeotrope a white creamy solution
was obtained. Toluene (10 g) was added to the reaction
in order to maintain fluidity. Paraformaldehyde (9.4 g,
0.31 mol) as a slurry in toluene was added over 1 h and
the reaction was stirred at 100 °C overnight. It was
followed by gas chromatography and upon satisfactory
completion was turned off. Some starting material,
however, remained. The reaction mixture was worked
up by partitioning with ethyl acetate–water. The or-
ganic layer was separated and after it was washed with
a saturated solution of sodium chloride it was dried
over MgSO4. The solvent was removed and a mixture
of the desired aldehyde and phenol was obtained (13.6
g). The product was further purified by column chro-
matography using ethylacetate–hexane (3:1) as an elu-
ent. Nitration of the aldehyde was carried out by using
a mixture of glacial acetic acid and nitric acid. Oxima-
tion of the aldehyde was carried out by reacting it with
hydroxylamine hydrochloride. The final product was
recrystallised from methanol.
Yield 5.6 g, 35.6%. Found: C, 59.9; H, 3.6; N, 10.7.
Calc. for C13H10N2O4: C, 60.5; H, 3.9; N, 10.8%.
5.1. Synthesis of 3-t-butylsalicylaldoxime (HL1)
5.3. Synthesis of
2-tert-Butylphenol (5 g, 0.033 mol) was dissolved in
dry toluene (20 cm3) and stannic chloride (0.40 ml,
0.033 mol) was added dropwise. Lutidine (1.5 g, 0.014
mol) was also added dropwise. Paraformaldehyde (1.5
g, 0.05 mol) was added portionwise as a solution in dry
toluene (10 cm3). After 1 h under nitrogen reflux the
solution turned white and reflux was continued for 6 h.
The reaction mixture was then added to 200 ml of
water, acidified using HCl 2 M to pH 2 and extracted
with ether. The ether extract was then treated with a
saturated sodium chloride solution, separated, dried
over magnesium sulfate and concentrated to give the
crude salicylaldehyde. The preparation of the oxime
was relatively uncomplicated utilising the familiar reac-
bis(3-tert-butylsalicylaldoximato)cobalt(II) [Co(L1)2]
A solution of cobalt(II) acetate tetrahydrate (0.06 g,
0.23 mmol) in methanol (5 ml) was added dropwise to
a methanolic solution (10 ml) of the salicylaldoxime
(0.1 g, 0.5 mmol). The solution turned brown–red upon
addition and a fine precipitate was formed. Stirring was
continued for further 2 h and the red–brown precipi-
tate was filtered off under an inert atmosphere of
argon. The solid collected was washed with methanol (2
ml) and dried under vacuum. The product obtained was
stable when solid but in solution turned black straight
away. Product was soluble in acetone. Yield: 0.08 g,
80%, appearance brown powder. Found: C, 51.7; H,

D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
3245
6.0; Co, 11.8; N, 4.6. Calc. for C22H28N2O4Co: C, 59.6;
H, 6.3; Co, 13.3; N, 6.3%. LSI Mass Spectrum:
443(42.3) [CoL2]+, 634(8.4) [CoL3]+. M.p. 209.3–
211.2 °C (dec.).
Yield 0.31 g, 86%, appearance red powder. Found: C,
37.6; H, 3.3; Co, 12.9; N, 10.2. Calc. for
C16H14N4O8Co: C, 42.8; H, 3.1; Co, 13.1; N, 12.5%.
LSI Mass Spectrum: 703(10.6) [CoL3]+, 898(1.2)
[Co2L4]+. M.p. 205.2–207.1 °C.
5.4. Synthesis of bis(3,5-di-tert-butylsalicyl-
aldoximato)cobalt(II) [Co(L2)2]
5.7. Synthesis of bis(3-nitro-5-phenylsalicylal-
doximato)cobalt(II) [Co(L5)2]
A solution of cobalt(II) acetate tetrahydrate (0.15 g,
0.5 mmol) in methanol (5 ml) was added dropwise to a
methanolic solution (10 ml) of the salicylaldoxime(0.25
g, 1 mmol). The solution turned brown–red upon
addition and a fine precipitate was formed. Stirring was
continued for further 2 h and the red–brown precipi-
tate was filtered off under an inert atmosphere of
argon. The solid collected was washed with methanol (2
ml) and dried under vacuum. The product obtained was
stable when solid but in solution turned black straight
away. Product was soluble in acetone. Yield 0.18 g,
65%, appearance brown powder. Found: C, 58.7; H,
7.3; Co, 9.5; N, 3.7. Calc. for C30H44N2O4Co: C, 58.7;
H, 8.9; Co, 10.6; N, 3.9%. LSI Mass Spectrum:
555(27.3) [CoL2]+, 803(2.3) [CoL3]+. M.p. 184.1–
185 °C (dec.).
The reaction of 3-nitro-5-phenylsalicylaldoxime with
cobalt(II) acetate was also carried out in the presence of
atmospheric oxygen. A warm methanolic solution of
Co(CH3CO2)2·4H2O (0.4 g, 0.0017 mol) was added
dropwise at 55 °C to a warm methanolic solution of
the ligand (0.88 g, 0.0034 mol). Upon addition a fine
red solid appeared and the reaction was stirred for 1 h.
The solvent was allowed to evaporate overnight and the
fine red solid was collected (0.93 g) and washed with
plenty of methanol. It was then dried at 55 °C. The red
solid was found to be insoluble in methanol, acetone
and dichloromethane. Yield 0.93 g, 95%, appearance
red powder. Found: C, 52.5; H, 3.3; Co, 10.1; N, 9.0.
Calc. for C26H20N4O8Co: C, 54.3; H, 3.5; Co, 10.3; N,
9.7%. M.p. 191–192.6 °C.
5.5. Synthesis of bis(3-sec-butylsalicylaldoximato)-
cobalt(II) [Co(L3)2]
5.8. Single crystal X-ray structure determination of
[Co(L)2] complexes
The ligand (0.4 g, 2 mmol) was dissolved in deoxy-
genated methanol (5 ml) and a solution of cobalt
acetate pentahydrate (0.12 g, 1 mmol) in deoxygenated
methanol (5 ml) was added dropwise to it under an
inert atmosphere. A brown precipitate was formed
straight away upon addition, which was filtered off and
washed with 2×5 ml of deoxygenated solvent. The
product was found to be soluble in acetone and in
solution oxidised immediately. Yield 0.24 g, 70.6%,
appearance brown powder. Found: C, 59.7; H, 6.4; Co,
5.6; N, 6.2. Calc. for C22H28N2O4Co: C, 59.6; H, 6.3;
Co, 13.3; N, 6.3%. LSI Mass Spectrum: 443(59.6)
[CoL2]+, 635(2.1) [CoL3]+. M.p. 189–192 °C.
Three methods were used in attempts to obtain crys-
tals from the cobalt complexes synthesised. These in-
cluded evaporation, liquid–liquid diffusion, and vapour
diffusion. For complexes [Co(L1)2], [Co(L2)2] and
[Co(L3)2] (Table 1) all three methods were employed
because of the good solubility of complexes in
methanol, ethanol and acetone. However, they were
found to be unstable in solution and an immediate
change of colour from red to black occurred on dissolu-
tion which was thought to indicate oxidation of
cobalt(II) to cobalt(III). The resulting black solutions
did not afford any crystals suitable for X-ray studies. In
contrast, good crystals were obtained from complexes
[Co(L4)2] and [Co(L5)2] using DMSO in which the
compounds dissolved readily. Crystals obtained from
the two adducts were found to be good diffractors of
X-rays and, therefore, suitable for crystallographic
studies.
5.6. Synthesis of bis(3-nitro-5-methylsalicylal-
doximato)cobalt(II) [Co(L4)2]
The reaction was carried out on the bench in the
presence of atmospheric oxygen. To a warm solution of
the ligand (0.24 g, 0.0016 mol) in ethyl acetate a warm
solution of Co(CH3CO2)2·4H2O (0.20 g, 0.0008 mol)
was added dropwise as a solution in methanol. On
addition, a bright red solid precipitated which was
filtered and separated from the filtrate. The solid was
then washed with methanol and dried at 50 °C. The
red solid was very stable and did not change colour
when left standing in open air. It was found to be
insoluble in methanol, acetone and dichloromethane.
5.9. X-ray structure determination of [Co(L4)2(dmso)2]
and [Co(L5)2(dmso)2]
X-ray intensity data for the two complexes were
collected with graphite-monochromated radiation on a
Siemens P4 four-circle diffractometer. Details of data
collection, refinement and crystal data are listed below.
Lorentz–polarisation and semi-empirical absorption
corrections („-scans) were applied to the data of both

3246
D. Cupertino et al. / Polyhedron 20 (2001) 3239–3247
Table 5
versity of North London using a Carlo Erba 1106
microanalyser. IR spectra were recorded on a Digilab
FTS-40 Infrared Spectrometer as KBr microdiscs. The
solid state susceptibilities of all cobalt complexes were
measured using a Johnson–Mathey balance at room
temperature. LSI mass spectra were obtained using a
Vacuum Generator (VG) Kratos ‘Profile’ Double Fo-
cusing mass spectrometer, equipped with Cs fast ion
gun operating 10 kV.
Crystal data and structure refinement parameters for [Co(L4)2-
(dmso)2] and [Co(L5)2(dmso)2]
[Co(L4)2(dmso)2]
[Co(L5)2(dmso)2]
Empirical formula
C
20H26CoN4O10S2 C30H32CoN4O10S2
Formula weight (g mol−1
)
605.50
729.63
,
Radiation, wavelength (A) Mo Ka, 0.71073
Crystal system
Space group
Mo Ka, 0.71073
triclinic
monoclinic
P21/n (Alt. No.
14)
(
P1 (No. 2)
Unit cell dimensions
,
a (A)
10.587(3)
8.328(2)
15.296(5)
90
104.10(4)
90
1307.9(7)
2
1.537
0.875
626
0.22×0.30×0.40
2.12–23.99
2052
8.194(5)
8.483(5)
12.578(7)
100.21(5)
92.31(5)
108.53(5)
2834.3(2)
1
1.493
0.720
377
0.20×0.30×0.50
2.58–24.00
2547
6. Supplementary material
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 159101 and 159102 for com-
pounds [Co(L4)2(dmso)2] and [Co(L5)2(dmso)2], respec-
tively. Copies of this information may be obtained free
of charge from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
3
,
Z
Dcalc (Mg m−3
)
v (mm−1
F(000)
)
Crystal size (mm)
q Range (°)
Measured reflections
Independent reflections
1435
1739
[Rint=0.0196]
0.52316, 0.50442
R1=0.0484
0.1373
[Rint=0.0289]
1.0000, 0.8352
R1=0.0661
0.1910
Max/min transmission
R(F) [I\2|(I)]
wR(F2)
Acknowledgements
S on F2
0.921
1.029
The authors acknowledge the EPSRC (A.Z.,
M.McP.) and Zeneca Specialties for financial support,
and EPSRC for the use of the Chemical Database
Service at Daresbury.
metal complexes. The structures were solved by the
direct methods routine of SHELXTL [25] in the space
(
groups P21/n and P1. All non-hydrogen atoms were
assigned anisotropic displacement parameters, and all
parameters were refined using a full-matrix least-
squares procedure on F2. A difference-Fourier map
using low angle data was calculated in each case and
revealed the positions of several hydrogen atoms, in-
cluding those of the oximic group H(1) and H(7) in
both complexes. The oximic hydrogen atoms were in-
cluded in structure factor calculations but their parame-
ters were not refined. For consistency, all the
carbon-bonded hydrogen atoms were included in calcu-
lated positions, and these were adjusted after each
refinement cycle. Refinement converged after four fur-
ther cycles of full-matrix least-squares refinement and
of all parameters and the weighting scheme the final R1
value was 0.0484 and 0.0661 with goodness-of-fit
parameters of 1.031 and 0.958, respectively. In the final
difference-Fourier synthesis there were no residual
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