Polyhedron 20 (2001) 3239 – 3247
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 M98ZS, UK
b
School of Biological and Applied Sciences, Uniersity of North London, 166 -220 Holloway Road, London N78DP, UK
Received 11 July 2001; accepted 24 September 2001
Abstract
A total of five salicylaldoxime ligands, (HL
1–5
), analogues of commercial metal extractants were synthesised and characterised.
The cobalt(II) complexes [CoL
2
] (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
[CoL
2
(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
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1. Introduction
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.
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 [CoL
2
(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 R
1
and R
2
, would
* 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