Characterization of the photolysis products of sec-butylcobaloximes with imidazole and benzimidazole bases

Article abstract of DOI:10.1016/S0022-328X(01)00854-3

The products obtained on the photolysis, under anaerobic conditions, of sec-butylcobaloximes with bound N-donor Imidazole and Benzimidazole bases, and sec-(C₄H₉)Co(DH)₂L are identified and characterized. The results are co

Full text of DOI:10.1016/S0022-328X(01)00854-3

Journal of Organometallic Chemistry 632 (2001) 85–93
www.elsevier.com/locate/jorganchem
Characterization of the photolysis products of
sec-butylcobaloximes with imidazole and benzimidazole bases
Maria Rangel a,*, Eula´lia Pereira b, Celeste Oliveira b, Baltazar de Castro b,*
a Instituto de Cieˆncias Biome´dicas de Abel Salazar, CEQUP, 4099-003 Porto, Portugal
b Departamento de Qu´ımica, Faculdade de Cieˆncias, CEQUP, Porto, Portugal
Received 21 February 2001; accepted 16 April 2001
Abstract
The products obtained on the photolysis, under anaerobic conditions, of sec-butylcobaloximes with bound N-donor Imidazole
and Benzimidazole bases, and sec-(C4H9)Co(DH)2L are identified and characterized. The results are compared with those
obtained for other alkylcobaloximes, mainly in terms of the involvement of the photolysis primary products in further reactions
with the species in solution, namely the ability to form adducts with the base L. Electron paramagnetic resonance (EPR)
spectroscopy was chosen to monitor the photolysis process, to obtain relevant information about the electronic structure of the
cobalt(II) complexes, and to determine the extent of spin density delocalized into the ligands. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Cobaloximes; Photolysis; Cobalt(II) complexes; EPR
1. Introduction
in a series of alkylcobaloximes and Costa-type com-
pounds, with nitrogen [2–4] and phosphorous donor
ligands [2,7]. Most of this work has been performed
with cobalt(III) compounds, and much less information
is available on the structure and reactivity of the
cobalt(II) species formed resulting from the homolysis
of the CoꢀC bond.
Information on the structure and reactions in which
the cobalt(II) species participate has been obtained by
studying the photolysis products of B12 compounds and
the related models using electron paramagnetic reso-
nance (EPR) and XAFS spectroscopies. Recently an in
situ X-ray absorption spectroelectrochemical study on
hydroxocobalamin allowed the structural characteriza-
tion of the cobalt center in the three oxidation states
[19].
The chemistry of cobaloximes, compounds with the
general formula XCoIII(DH)2L where X and L are
neutral mononegative ligands and DH2 the dimethyl-
glyoxime, has been extensively developed owing to its
connection with the B12 chemistry [1–5].
The studies regarding the characterization of the
products of anaerobic photolysis in protic and aprotic
solvents of alkylcobaloximes, compounds with the gen-
eral formula RCoIII(DH)2L where R is an alkyl group,
The structural characterization of B12 systems has
revealed the existence of a cobaltꢀcarbon (CoꢀC) bond,
and the subsequent studies have demonstrated that the
cleavage of this bond is a key step in the enzymatic
mechanisms in which coenzyme B12 and methyl B12 are
involved [1]. Although it has not been possible to find a
class of model compounds that mimic the physical
properties of cobalamines, several groups of com-
pounds have been widely studied [2–11]. Additionally,
decisive information on the development of methods
for understanding the B12 properties, especially the
factors that stabilize/labilize the CoꢀC bond have been
provided. Cleavage of the CoꢀC bond in model com-
pounds may be induced either by photolysis or ther-
molysis, and the study of photolysis products of
cobaloximes has received particular attention [12–18].
The relevance of electronic and steric effects in CoꢀC
bond cleavage mechanisms has been clearly demon-
strated in model compounds by investigations concern-
ing the influence of the ligand trans to the alkyl group
* Corresponding authors.
E-mail address: mcrangel@fc.up.pt (M. Rangel).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00854-3

86
M. Rangel et al. / Journal of Organometallic Chemistry 632 (2001) 85–93
to the predominance of each type of adduct. According
to the results obtained for phosphorus and pyridine
cobaloximes the type of adducts formed is dependent
on the sterical deformations induced by the base on the
equatorial plane.
EPR spectroscopy was used to monitor the photoly-
sis process; to obtain relevant information about the
electronic structure of the cobalt(II) complexes; to de-
termine the extent of spin density delocalized into the
neighboring atoms; and to map the distribution of the
unpaired electron spin density over the molecule (Fig.
1).
L a neutral ligand and DH2 the dimethylglyoxime, with
pyridine [10–16] and phosphorus [17] bases showed
that the nature and reactivity of the species formed
depend on the Lewis base and the alkyl group. Photol-
ysis products of alkylcobaloximes with symmetric phos-
phorus donor bases were shown to be independent of
the alkyl group and photolysis induces, in all cases, the
homolytic cleavage of the CoꢀC bond. The products
obtained are alkyl radicals and cobalt(II) five-coordi-
nate species, and these latter complexes are not in-
volved in the subsequent reactions and are stable in the
temperature range of 77–300 K [18]. For alkylcoba-
loximes with the pyridine-derived bases, it has been
shown that the products observed upon photolysis de-
pend on the alkyl group and also that the photolysis
primary products undergo further reactions in solution
giving rise to secondary products that remain in solu-
tion even after the various thermal cycles [16,17].
2. Experimental
2.1. Reagents
The solvents, obtained from Merck, were dried and
purified according to the standard procedures [20] and
kept over 4A molecular sieves prior to use. All other
chemicals were from Aldrich and used as received.
In order to compare the behavior of pyridinecoba-
loximes with that of the other alkylcobaloximes in
which the nitrogen base is different, we studied the
photolysis of solutions of sec-butylcobaloximes with
imidazole and benzimidazole bases. We also investi-
gated the photolysis of solutions of aqua-(sec-bu-
tyl)bis(dimethylglyoximato)cobalt(III) in the presence
of Lewis bases added in molar ratios of 1:1 and 1:10.
This study allows getting insight on the ability of the
bases to form 1:1 and 1:2 adducts with a common
equatorial moiety and the factors that may contribute
2.2. Instrumentation
The EPR measurements were made on a Varian
E-109 or on a Bruker ESP 300E spectrometer, both
operating at 9 GHz and equipped with variable temper-
ature units (Varian E-257 and Bruker B-VT2000). The
Fig. 1. Structure and formula of the cobaloximes studied.

M. Rangel et al. / Journal of Organometallic Chemistry 632 (2001) 85–93
87
2.5. Photolysis
Sample irradiation was performed under anaerobic
conditions keeping the solutions in EPR quartz tubes
and using a 250 W Philips HP/T Hg lamp as the light
source, umax=560–580 nm. The irradiation times used
are given in Table 1 and were about 15–30 min for
samples kept at 300 K and 60 min for samples at 77 K.
3. Results and discussion
The purpose of this work is to study the products
obtained on the photolysis of sec-butylcobaloximes
with bound N-donor Imidazole and Benzimidazole
bases, under anaerobic conditions. The work involves
the identification and characterization of photolysis
primary products and monitoring the reactivity of the
latter towards species present in solution. Previous
studies on pyridinecobaloximes revealed that the
cobalt(II) primary products are quite reactive and un-
dergo further reactions in solution in contrast with
those obtained from phosphinecobaloximes, for which
the Co(II) primary products are maintained in solution
at room temperature.
The photolysis process is monitored by EPR as
freshly prepared cobaloxime solutions are, as expected
for low-spin Co(III) compounds, EPR silent but be-
come active if the homolytic cleavage of the CoꢀC bond
occurs. Characterization of the photolysis primary
products is achieved by photolysing the solutions in the
frozen matrix and observing the EPR spectra at the
same temperature. The possible involvement of the
primary products in secondary reactions is tested by
allowing the matrix to relax as the temperature is
slowly raised to room temperature and the complete
characterization of the products formed is obtained by
analyzing of the EPR spectra exhibited after re-cooling
at 77 K. These results are compared with those ob-
tained for the photolysis performed on solutions kept at
room temperature.
Fig. 2. (a) Frozen solution EPR spectrum of a toluene–methanol
solution of sec-C4 H9Co(DH)2(2,5,6-Me3Bz) irradiated at 77 K and
recorded at 100 K. (b) Frozen solution EPR spectrum of a toluene–
methanol solution of sec-C4H9Co(DH)2(2-MeBz) irradiated at 77 K
and recorded at 100 K.
spectra were calibrated with diphenylpicrylhydrazyl
(dpph) and the magnetic field was calibrated using
Mn2+ in MgO.
2.3. Synthesis
Alkylcobaloximes, compounds with the general for-
mula RCo(DH)2 L, were prepared by the method out-
lined by Toscano et al. [21] For all the prepared
compound the expected stoichiometry was confirmed
by elemental analysis.
3.1. Photolysis in the frozen matrix, 77 K
Cobaloxime solutions irradiated at 77 K yield EPR
spectra that exhibit a narrow signal at g=2.00, a broad
band at g=2.14 and a signal that is spread over a
wider range of magnetic field values and with substan-
tial g tensor anisotropy (gav=2.15). The latter is as-
signed to a paramagnetic metal center as can be
inferred from the observed hyperfine splitting in the
high-field region owing to the interaction of the un-
paired electron with a cobalt nucleus (59Co, I=7/2).
On warming the solutions to 100 K the band at
g=2.14 vanishes, but the other two remain detectable
(Fig. 2). These two bands are assigned to an organic
2.4. Sample preparation
All manipulations were carried out under nitrogen;
solvents were deoxygenated by reflux/distillation under
nitrogen and all solids degassed under vacuum prior to
use. Samples for photolysis were prepared by making
solutions of the desired compounds, in toluene–
methanol (1:1), or by dissolving the aquacobaloxime
and adding stoichiometric amounts (1:1 and 1:10) of
the Lewis base.

Table 1
Type of Co(II) species formed upon photolysis of cobaloximes
/
sec-C4H9Co(Hdmg)2L
sec-C4H9Co(Hdmg)2H2O+L
1:1
1:1
1:10
hw, 77 K
Coordination number
hw, 77 K
Coordination number hw, room temperature.
Coordination number
Coordination number
Im
2-MeꢀIm
2-EtꢀIm
2-Etꢀ4MeꢀIm
NꢀMeꢀIm
4ꢀMeꢀIm
4,5 Cl2ꢀIm
2-Me-5-NitroꢀIm 60 min
Bz
5,6 Me2ꢀBz
2,5,6 Me3ꢀBz
60 min
60 min
60 min
60 min
60 min
60 min
60 min
Five
Five
Five
Five
Five
Five
Four+five
Four
Five
Im
2-MeꢀIm
2-EtꢀIm
2-Etꢀ4MeꢀIm
NꢀMeꢀIm
4-MeꢀIm
4,5Cl2ꢀIm
2-Me-5-NitroꢀIm 60 min
Bz
5,6 Me2ꢀBz
2,5,6 Me3ꢀBz
60 min
60 min
60 min
60 min
60 min
60 min
60 min
Five
Five
Five
Five
Five
Five
Four+five
Four
Five
15 min
15 min
15 min
15 min
15 min
30 min
30 min
30 min
30 min
30 min
30 min
Five
Five
Five
Five
Six
Five+six
Five+six
Five+six
Six
Five+six
Four+five
Four
Six
Six
Six
Five
Low resolution
Four+five
Four
Five
Five
632
60 min
60 min
60 min
60 min
60 min
60 min
)
Five
Five
Five
Five
Five
8
9

M. Rangel et al. / Journal of Organometallic Chemistry 632 (2001) 85–93
89
III
II
’
RꢀCo complex“[R ]+[Co complex]
radical (R) and to a Co(II) species, and the broad band
that was observed at g=2.14 must be owing to the
strongly exchanged coupled system {[R ]···[Co -
’
’
[R ]+Hsolvent“RH+[solvent ]
II
’
’
’
[R ]“R
complex]} which is kept immobilized by the lattice [15].
This result shows that, for all cobaloximes, photoly-
sis induces the homolytic cleavage of the CoꢀC bond
originating the corresponding alkyl radical and a Co(II)
complex. The results are interpreted as mentioned ear-
lier [16–18] for alkylcobaloximes with pyridine and
phosphorus bases according to the mechanism pro-
posed by Symons [14] which assumes that on light
absorption an electron from the equatorial plane moves
into the Co(d2z) s* orbital. The antibonding character
of this orbital is then relieved by bond fission, following
the proposed sequence:
where the brackets represent cage trapping.
Comparing the EPR spectra recorded at 100 K it is
discernible that they are not identical for all the coba-
loximes studied although all of them exhibit character-
istics of Co(II) spectra. For the compounds with the
Lewis bases Im, 2-MeꢀIm, 2-EtꢀIm, 2-Etꢀ4-MeꢀIm,
N-MeꢀIm, 4-MeꢀIm, Bz, 5,6-Me2ꢀBz, 2,5,6-Me3ꢀBz the
EPR spectrum shows the cobalt hyperfine lines further
split into three of equal intensity (1:1:1) thus indicating
the interaction of the unpaired electron with one axially
bound nitrogen atom (14N, I=1), (Fig. 2a). Coba-
loximes with the bases 4,5-Cl2ꢀMeꢀIm, 2-Meꢀ5-
NO2ꢀIm, exhibit spectra in which cobalt hyperfine lines
are observed but no axial interactions were detected
(Fig. 2b). The spectra recorded for the latter set are
identical with the one obtained when a solution of the
aquacobaloxime is irradiated in the same conditions.
When the samples are further warmed, the radical
signal looses intensity and vanishes near the glass-soft-
ening point and the cobalt(II) signal begins to broaden
and converges into a single line at 300 K. After recool-
ing the solutions to 100 K, three types of EPR spectra
(A, B and C) are observed. Cobaloximes with the bases
Im, 2-MeꢀIm, 2-EtꢀIm, 2-Et-4-MeꢀIm, NꢀMeꢀIm, 4-
MeꢀIm, Bz, 5,6-Me2ꢀBz, and 2,5,6-Me3ꢀBz, exhibit
spectra of type A (Fig. 3), with a large g tensor an-
isotropy and in which three g features may be distin-
guished (type A). In the high-field region the hyperfine
coupling of the unpaired electron with the cobalt (59Co,
I=7/2) and nitrogen (14N, I=1) atoms is clearly de-
tected. Each of the eight lines arising from coupling to
cobalt is further split into three with relative intensities
1:1:1, resulting from the superhyperfine coupling with
one axially bound nitrogen atom. This spectrum is
assigned to a low-spin cobalt(II) compound with one
axially bound imidazol or substituted imidazol base,
[Co(DH)2L]. On the other hand cobaloximes with the
base 2-Me-5-NO2ꢀIm show spectra (Fig. 4) with a large
g tensor anisotropy and in which three g features are
also discernible, exhibiting hyperfine coupling of the
unpaired electron with the cobalt (59Co, I=7/2) atom
(type B) in the high-field region. The eight lines arising
from coupling to cobalt are not split further, thus
indicating that there is no axially bound nitrogen base.
This spectrum is assigned to a low-spin cobalt(II) com-
pound, [Co(DH)2]. Finally, the cobaloxime with the
base 4,5-Cl2ꢀMeꢀIm exhibits a spectrum of type C (Fig.
5), which is a superposition of spectra of types A and B
thus implying the presence of two cobalt species in
solution, [Co(DH)2L] and [Co(DH)2] (type C).
Fig. 3. Frozen solution EPR spectrum of a toluene–methanol solu-
tion of sec-C4 H9Co(DH)2(2,5,6-Me3Bz), irradiated at 77 K and
recorded at 100 K, after being warmed to room temperature and
frozen to 100 K. Spectrum of type A.
Fig. 4. Frozen solution EPR spectrum of a toluene–methanol solu-
tion of sec-C4 H9Co(DH)2(2-Me-5-NO2Im), irradiated at 77 K and
recorded at 100 K, after being warmed to room temperature and
frozen to 100 K. Spectrum of type B.

90
M. Rangel et al. / Journal of Organometallic Chemistry 632 (2001) 85–93
could assume that base replacement did not occur and
so the result is coincident with that of the aquacoba-
loxime itself. However, as it is observed for solutions of
cobaloximes that have been isolated as powders it must
be admitted that in methanol–toluene solutions the
Lewis base is displaced by the solvent. In both cases the
base has a substituent in a position, which is adjacent
to the coordinating nitrogen atom, and as it has been
shown for a set of ortho-pyridine bases [16] this is
enough reason to prevent the coordination or induce
the displacement of the base. It is well known that
bases trans to the alkyl groups are labile and also that
five-coordinate compounds have been identified for
other B12 model compounds such as RCoIII(bae) and
RCoIII(salen) [22,23].
Fig. 5. Frozen solution EPR spectrum of a toluene–methanol solu-
The observation of a type C spectrum for the 4,5-
Cl2ꢀMeꢀIm cobaloxime is interesting because it shows
the coexistence of the species [CoII(DH)2L] and
[CoII(DH)2] in solution.
tion of sec-C4H9Co(DH)2(4,5-Cl2Im), irradiated at 77
K and
recorded at 100 K. Spectrum of type C.
The results obtained for the photolysis of coba-
loximes are summarized in Table 1 together with those
obtained for the photolysis of solutions of the aqua-
cobaloxime to which the imidazole bases have been
added in a 1:1 ratio. The results are identical for a given
L base and the same primary and secondary photolysis
products are observed thus confirming that for the
imidazole bases also the solutions are equivalent [16].
The results indicate that, for all cobaloximes, the
homolytic cleavage of the CoꢀC bond is induced on
photolysis and the cobalt primary products, complexes
of type [CoII(DH)2L] or [CoII(DH)2], remain in solution
after a thermal cycle from 100 to 300 K and back to
100 K. The result is identical with the one observed for
cobaloximes with symmetric phosphorus bases [18] and
contrasts with the one observed for pyridinecoba-
loximes [16] for which the photolysis primary product
[CoII(DH)2L] is never observed after the thermal cycle
which means that the species is involved in further
reactions in solution. For cobaloximes with pyridine
bases a six-coordinate complex [CoII(DH)2L2] is always
present after the thermal cycle even in solutions with no
free base present. A possible interpretation is the ab-
straction of base molecules from other cobaloximes in
solution [16] and studies to clarify this mechanism using
other spectroscopic and magnetic methods are in
progress.
3.2. Photolysis at 300 K
Cobaloxime solutions were irradiated at room tem-
perature and their EPR spectra recorded at 300 and 100
K. Spectra at 300 K comprises a single broad band and
spectra recorded after cooling at 100 K are identical to
those obtained for samples irradiated at 77 K and
warmed at 300 K. The spectra are characteristic of
low-spin Co(II) species with coordination numbers five
or four, [CoII(DH)2L] or [CoII(DH)2], and the results
obtained for each base are summarized in Table 1.
3.3. Photolysis in the presence of an excess of base
The results of photolysis of aqua-sec-butylcoba-
loxime solutions to which the L base was added in a
1:10 molar ratio are summarized in Table 1.
In the above conditions, solutions of cobaloximes
with bases Im, NꢀMeꢀIm, Bz, 5,6-Me2ꢀBz and 2,5,6-
Me3ꢀBz exhibit spectra that show two well-separated g
features and lines owing to the hyperfine coupling of
the unpaired electron with the cobalt atom (59Co, I=7/
2) and with two nitrogen atoms (14N, I=1) in the
high-field region. Each of the eight lines arising from
the hyperfine coupling to cobalt is split into five with
relative intensities 1:2:3:2:1, resulting from the superhy-
perfine coupling with two equivalent nitrogen atoms
(Fig. 6). This spectrum is assigned to a low-spin
cobalt(II) species with two axially bound base
molecules, [Co(DH)2L2]. Solutions of cobaloxime with
2-Me-5-NO2ꢀIm yield as before an EPR spectrum char-
acteristic of the four-coordinate species and for the
other bases a mixture of the five-and six-coordinate
species is observed.
A new result that emerged with the imidazole bases is
the observation of the species [CoII(DH)2] as a primary
product of photolysis of a cobaloxime solution, as is
evident from the recording of type B spectra for com-
pounds with the base 2-Me-5-NO2ꢀIm. We believe that
this result is a consequence of the non-stability of the
cobaloxime sec-C4H9Co(DH)2(2-Me-5-NO2ꢀIm) in
methanolic solutions.
If the result had been observed only for a solution of
aquacobaloxime to which the base had been added, one

M. Rangel et al. / Journal of Organometallic Chemistry 632 (2001) 85–93
91
try at the cobalt center and the non-coincidence of the
g and A(Co) tensor axes in the xy plane. As no
interaction of the unpaired electron with the nitrogen
atoms of the equatorial plane was observed, the ligand
term includes only the nitrogen axial interaction and it
was assumed that the nitrogen tensor has the same
principal axis as that of the g tensor. In simulating the
experimental spectra, the best fit was obtained for
collinear g and A tensors in the x and y plane (rhombic
symmetry) with the absence of quadrupolar contribu-
tion to the spin-Hamiltonian.
3.4.2. Cobalt spin densities
The analysis of the g and cobalt hyperfine tensors
allows the calculation of the unpaired spin density in
the 3d and 4s orbitals of the metal. The procedures for
these calculations have been described in Ref. [27]
based on those by McGarvey [24] and the results
obtained are shown in Table 2.
Fig. 6. Frozen solution EPR spectrum of a toluene–dichloromethane
solution of of sec-C4H9Co(DH)2(H2O) and the base (2,5,6-Me3Bz)
added in a ten-fold excess, irradiated at 77 K and recorded at 100 K,
after being warmed to room temperature and frozen to 100 K.
3.4. EPR spectra analysis
3.4.3. Ligand spin density
The components of the spin-Hamiltonian parameters
(Table 2) were obtained by the computer simulation of
the experimental spectra and the results confirm that
the spectra must be interpreted in terms of three differ-
ent g values. The analysis was based on the assumption
No hyperfine coupling was observed with the equato-
rial nitrogen atoms, but from the observed hyperfine
coupling to the axial donor nitrogen atoms, spin den-
sity on base nitrogen 2s and 2p orbitals could be
determined. The procedures for this have been de-
scribed in Ref. [18,27] and the results are shown in
Table 2.
2
of a A1 ground state for the cobalt(II) atom, compris-
ing a mixture of the d 2 and d
metal orbitals of the
2
2
x
z
–y
form b=ad 2+bd
(taking the point-group sym-
2
2
x
z
–y
The EPR parameters are similar to those obtained
for low-spin Co(II) complexes with the same number
and type of axial ligands. The values obtained are
metry to be C26) [24].
2
consistent with a A1 ground state and the values of a2
3.4.1. Computer simulations
and b2 show that it is essentially a d2z state thus justify-
ing the absence of hyperfine coupling with the nitrogen
atoms of the coequatorial ligand and strong coupling
with the axial ligands. The geometry is confirmed as
rhombic and it is evident that the degree of anisotropy,
The EPR spectra of cobalt(II) compounds were simu-
lated using a program based on Pilbrow’s formalism
[25,26] which uses a spin-Hamiltonian of the type H=
H(Zeeman)+H(hyperfine)+H(ligand). The hyperfine
term was deduced assuming a C26 point-group symme-
Table 2
EPR parameters of the Co(II) species formed upon photolysis of cobaloximes. The units of Ax, Ay, Az, −kP and P are 10−4 cm−1
gx
gy
gz
Ax
Ay
Az
a2
b2
(−kP)
P
z 3d
z 4s
z Co
z Total
Fi6e coordinate
Im
2-MeꢀIm
2-EtꢀIm
2-Etꢀ4MeꢀIm
NꢀMeꢀIm
Bz
5,6 Me2ꢀBz
2,5,6 Me3ꢀBz
2.246
2.370
2.265
2.242
2.255
2.190
2.230
2.220
2.239
2.255
2.255
2.196
2.208
2.177
2.173
2.191
2.032
2.026
2.056
2.009
2.030
2.007
2.006
2.006
65
40
84
65
65
90
66
86
8
10
5
18
18
14
18
18
88
92
95
91
96
91
90
92
0.9472
0.9990
0.9238
0.9566
0.9680
0.8620
0.9523
0.8851
0.0528
0.0010
0.0762
0.0434
0.0320
0.1380
0.0477
0.1149
0.0037
0.0021
0.0044
0.0043
0.0045
0.0052
0.0045
0.0052
128
156
150
107
121
111
101
107
0.50
0.61
0.59
0.42
0.48
0.44
0.40
0.42
0.06
0.06
0.08
0.06
0.07
0.07
0.06
0.07
0.57
0.67
0.67
0.48
0.55
0.51
0.46
0.49
0.57
0.68
0.67
0.49
0.55
0.51
0.47
0.50
Six coordinate
Im
NꢀMeꢀIm
Bz
5,6 Me2ꢀBz
2,5,6 Me3ꢀBz
2.238
2.195
2.185
2.220
2.185
2.218
2.180
2.169
2.172
2.170
2.018
2.012
2.011
2.011
2.012
25
25
25
25
25
25
35
35
35
35
70
80
87
83
85
0.9999
0.9958
0.9965
0.9942
0.9963
0.0001
0.0042
0.0035
0.0058
0.0037
0.0029
0.0037
0.0039
0.0036
0.0038
86
87
97
94
94
0.34
0.34
0.38
0.37
0.37
0.05
0.05
0.06
0.05
0.06
0.39
0.40
0.44
0.42
0.43
0.77
0.80
0.88
0.85
0.86

92
M. Rangel et al. / Journal of Organometallic Chemistry 632 (2001) 85–93
measured as Dxy=gx−gy, is higher for the five-coordi-
nate compound. The largest values of hyperfine cou-
pling to cobalt are those of the Az component and
according to the results obtained as a solution for
McGarvey’s equations the signs of the Ax and Ay
components were both taken as positive. The values of
Fermi contact, −kP, term suggest a significant contri-
presence of the six-coordinate complex [CoII(DH)2L2] is
detected in solutions of cobaloximes with the non-sub-
stituted bases Im and Bz or bases with substituents
having electron-donating properties, NꢀMeꢀIm, 5,6-
Me2ꢀBz and 2,5,6-Me3ꢀBz added in a ten-fold excess.
The influence of steric factors seems to have some
relevance in preventing the predominance of the six-co-
ordinate complex in the case of imidazole bases with
substituents in positions 2 or 4 as a pure spectrum of
the six-coordinated species, [Co(DH)2L2], is not ob-
served but always a mixture of five- and six-coordinate
compounds even in the presence of the base in a
ten-fold excess.
In these conditions the results for the remaining
bases imply that: (a) 2-Me-5-NO2ꢀIm is never bound to
cobalt in methanol–toluene solutions as EPR spectra of
the four-coordinate complex, [Co(DH)2], are always
registered; (b) for the base 4,5-Cl2ꢀMeꢀIm the species
[Co(DH)2L] and [Co(DH)2] exist in equilibrium in all
the conditions studied; (c) for imidazole bases with
substituents in positions 2 or 4 a pure spectrum of the
six-coordinated species, [Co(DH)2L2], is not observed.
2
bution of the 4s orbital to the A1 ground state and the
values of anisotropic spin density, P, lie within the
range expected for this type of complexes, with the
largest values being observed for the five-coordinate
species.
Taking the values of the hyperfine coupling tensor
with the axial ligands, spin densities on the 2s and 2p
orbitals have been calculated according to the usual
procedures giving values of ca. 6% for the 2p orbital
and 2% in the 2s orbital.
4. Conclusions
For all the cobaloximes studied photolysis resulted in
the homolytic cleavage of the CoꢀC bond and the
cobalt primary products, complexes of type
[CoII(DH)2L] or [CoII(DH)2], remain in solution after a
thermal cycle 100–300–100 K. This result is similar to
the one observed for cobaloximes with symmetric phos-
phorus bases [18], and contrasts with the one observed
for pyridinecobaloximes [16,17] for which the photoly-
sis primary product [CoII(DH)2L] is never observed
after the thermal cycle always giving rise to the com-
pound [CoII(DH)2L2]. For imidazole and benzimidazole
bases the formation of the 1:2 adduct is observed only
in the presence a 1:10 excess and for some of the bases
the 1:1 adduct is still detected in solution. An important
conclusion of this work is that the ability exhibited by
pyridines to form 1:2 adducts in the present non-excess
of base is not observed for all nitrogen bases and must
be related to the properties of the pyridine compounds.
From X-ray data of alkylcobaloximes [2,3] with the
various types of bases it is known that the phosphorus
bases induce significant deformations on the equatorial
moiety and thus the five-coordinate species is favored;
while for the pyridine bases the absence of such stereo-
chemical factors favor the formation of the six-coordi-
nate compound. The results obtained with the
imidazole bases, suggest that the electronic effects are in
this case important, and in fact it is observed that the
presence of electron-withdrawing substituents favors
the presence of the species with non-bound bases. It
was observed that the base 2-Me-5-NO2ꢀIm is never
bound to cobalt in methanol–toluene solutions as EPR
spectra of the four-coordinate complex [Co(DH)2] are
always registered and for the base 4,5-Cl2ꢀMeꢀIm the
species [Co(DH)2] is in equilibrium with the five-coordi-
nate one [Co(DH)2L] in all the conditions studied. The
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