Organocobaloximes: Synthesis, oxygen insertion and kinetics

Article abstract of DOI:10.1016/s0022-328x(98)00809-2

A total of 27 organocobaloximes have been synthesised and characterized with three different dioxime ligands, dmgH, chgH and dpgH. Many of the chgH and all the dpgH complexes have been synthesised for the first time. The insertion of molecular oxygen into these organocobaloximes, RCo^III(L₂)B [L=dmgH, chgH and dpgH) under thermal and photochemical conditions have been studied. Kinetic studies at ambient temperature under irradiation show that the rate of insertion depends upon the nature of R, L, B and the solvent. The rate follows the order dpgH>chgH>dmgH; Naphthyl>heteroaromaticmethyl>benzyl; piperidine>morpholine>γ-picoline>pyridine>2- bromopyridine>2-acetylpyridine; and the rates are faster in acetonitrile than in acetone and chloroform. In methanol, the benzylic cobaloximes exist as an equilibrium mixture of solvated penta-coordinated and hexa-coordinated species and in chloroform these exist as hexa-coordinated species.

Full text of DOI:10.1016/s0022-328x(98)00809-2

Journal of Organometallic Chemistry 570 (1998) 1–7
Organocobaloximes: synthesis, oxygen insertion and kinetics
B.D. Gupta *, V.Vijai Kanth, Veena Singh
Department of Chemistry, IIT Kanpur, Kanpur-208016, India
Received 12 March 1998
Abstract
A total of 27 organocobaloximes have been synthesised and characterized with three different dioxime ligands, dmgH, chgH
and dpgH. Many of the chgH and all the dpgH complexes have been synthesised for the first time. The insertion of molecular
oxygen into these organocobaloximes, RCoIII(L2)B [L=dmgH, chgH and dpgH) under thermal and photochemical conditions
have been studied. Kinetic studies at ambient temperature under irradiation show that the rate of insertion depends upon the
nature of R, L, B and the solvent. The rate follows the order dpgH\chgH\dmgH; Naphthyl\heteroaromaticmethyl\benzyl;
piperidine\morpholine\k-picoline\pyridine\2-bromopyridine\2-acetylpyridine; and the rates are faster in acetonitrile than
in acetone and chloroform. In methanol, the benzylic cobaloximes exist as an equilibrium mixture of solvated penta-coordinated
and hexa-coordinated species and in chloroform these exist as hexa-coordinated species. © 1998 Elsevier Science S.A. All rights
reserved.
Keywords: Organocobaloximes; Dioxygen insertion reactions; Kinetic studies
other than dmgH has recently been perceived in many
reactions in organic synthesis [7,8].
1. Introduction
Organocobaloximes, RCoIII(dmgH)2B (where R is an
organic group, B is the coordinating base) have been
extensively studied [1], ever since Schrauzer first high-
lighted their importance as models of coenzyme B12 [2].
Since then it has outgrown its initial relevance and has
acquired an independent research field because of its
rich chemistry [3]. The key feature of these complexes is
the weak Co–C bond ([4]a,b) and this bond in alkyl
cobaloximes has been reported to be homolytically
cleaved by irradiation with visible light in a manner
similar to the activation of vitamin B12 by apoenzyme
([4]c,d,e). These compounds find a great use in organic
synthesis [5] and in catalysis [6]. Since small structural
changes in the molecule have a profound effect on its
reactivity, the synthesis of new organocobaloximes con-
tinues to attract the attention of the chemists [1,3,7].
The importance of organocobaloximes with ligands
We are, in particular, interested in the reaction of
organocobaloximes with molecular oxygen. Reactions
involving fixation of O2 are important not only for a
biological system but also for some synthetic processes
([9]a–e). Some reactions of alkylcobaloximes have been
reported, e.g. O2 has been shown to insert into Co–C
bond to give a stable 1:1 dioxy adduct ([9]f,g,h).
RCoIII(dmgH)2PY+O2“ROOCoIII(dmgH)2PY
Both thermal and photochemical activation of these
reactions have been observed. The reaction conditions
vary over a wide range depending on the nature of R
group, for example, when R is benzyl or allyl the
reaction proceeds both under thermal as well as photo-
chemical conditions but when R is alkyl, the photo-
chemical conditions are essential [10]. Stereochemical
studies have confirmed that such reactions occur with
racemisation at the h carbon [11]. The last review on
the insertion of oxygen in to M–C bond giving peroxo
complexes was published by Wojcocki [12].
* Corresponding author. Fax: +91 512 597436; e-mail:
bdg@iitk.ernet.in
0022-328X/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00809-2

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B.D. Gupta et al. / Journal of Organometallic Chemistry 570 (1998) 1–7
Most of the insertion studies have been confined to
both in chloroform as well as in methanol at room
temperature. Our results show that in methanol the
benzylic cobaloximes behave similarly to alkyl coba-
loximes and exist as an equilibrium mixture of five and
six co-ordinated species, with equilibrium largely to the
right.
organocobaloximes with dmgH as the equatorial lig-
and. The photochemical insertion of O2 into Co–C
bond in [PhCH2Co(CN)5]3− and [RCo(tpp)] (R=
alkyl; TPP=tetraphenylporphyrinato) have also been
reported [13]. A number of mechanisms have been
proposed including the radical chain reaction, radical
cage mechanism, insertion via five co-ordinate species
etc. [14]. No one general unified mechanism can be
written for these reactions. One possible reason is the
lack of organocobaloximes with ligands other than
dmgH and the other is the paucity of the precise rate
data on such organocobaloximes [9].
Keeping the above in view we report here (a) the
synthesis of many new organocobaloximes with chgH
and dpgH as the equatorial ligands; and (b) the inser-
tion reaction with molecular oxygen and their rate
studies
RCH2Co–(L2)PyURCH2Co(L2){solvated}+Py
The evidence for this equilibrium is shown in Fig. 1
which depicts the changes in the appearance of the CT
band on addition of increasing quantities of pyridine to
a solution of PhCH2Co(dmgH)2Py in methanol. Similar
changes have been observed earlier by Brown et al. in
aqueous solutions [16]. The shape of the CT band (Fig.
1 saturation curve) is similar to the one in chloroform.
Furthermore the band in chloroform does not change
in shape on the addition of a large excess of pyridine.
In another experiment when the insertion is carried out
in the presence of 2-acetyl pyridine (3–4-fold) in chlo-
roform, the insertion product does not contain any
2-acetylpyridine. This further confirms that the axial
base pyridine does not come off at any stage during the
insertion.
2. Results and discussion
The reaction of oxygen gas with organocobaloximes
(1–27) in dichloromethane at 0°C and under irradiation
proceed smoothly and are complete within 1 h (as
judged from TLC). The corresponding dioxy product
(1a–27a) are formed in nearly quantitative yield. Simi-
lar results are obtained in the reaction of 2-thienyl-
methyl-bis(diphenylglyoximato)base cobalt(III) com-
plexes containing various bases like 2-acetylpyridine,
2-bromopyridine, k-picoline, morpholine, piperidine,
([24]b–f). The characteristics of the organocobaloximes
and their inserted products are given in Table 1.
These results clearly show that the species exists as a
hexa-coordinated benzyl (pyridine) cobaloxime in chlo-
roform. ChgH and dpgH complexes behave similarly
1
(Fig. 1). A similar observation is made in the H-NMR
study. The species exist as six coordinated
RCH2Co(L2)Py in CDCl3 (the solvent used for NMR)
as the cobalt bound methylene as well as the dmgH–
Me proton resonances did not shift on the addition of
a large excess of pyridine directly to the NMR sample.
The detailed kinetic study of the molecular insertions
have been carried out in chloroform using UV-vis
spectrophotometer. Spectrophotometery has shown
that for every molecule of oxygen consumed, one
molecule of the complex is transformed ([14]a). Thus
the intensity of the characteristic band at around 420
nm decreases proportionally with the quantity of oxy-
gen consumed. The absorbance (At) is noted down at
different intervals and the concentration of the product
(Ct) at any time (t) is calculated using the equation
Ct=A0−At/m−mA where A0 and At are absorbances at
time t=0 and at time t and m and mA are molar
extinction coeffcients of the initial and final complex.
The following findings are noteworthy from the overall
insertion studies. Some of these findings are similar to
earlier studies ([9]f, [14]).
hv/O
“
2
RCH2CoIII(L2)Py
RCH2OOCoIII(L2)Py
1−27
0°C
1a−27a
R=Ph
R=4 Cl–C6H4
R=4 CN–C6H4
R=4 OMe–C6H4
R=3 OMe–C6H4
R=2-thienyl
R=3-thienyl
R=furfuryl
(A) The reactions do not proceed in the dark at 0°C;
(B) the reactions stop as soon as the irradiation is
stopped; (C) the best temperature for insertion is 0°C,
though the reaction proceeds at ambient temperature
also but the time taken is more than the reaction at 0°C
(TLC inference only). (D) A plot of log (C0−Ct)
against t is linear indicating the first order kinetics. The
results point to the overall order of reaction as 2, i.e.
first order with respect to both the complex and oxy-
R=1 naphthyl
and (1–9), L=dmgH; (10–18), L=chgH; (19–27),
L=dpgH
In view of our recent observations ([7]b, [15]) that the
alkyl cobaloximes exist as an equilibrium mixture of
solvated penta-coordinated and hexa-coordinated spe-
cies, we have undertaken the preliminary kinetic study

B.D. Gupta et al. / Journal of Organometallic Chemistry 570 (1998) 1–7
3
Table 1
The yield, spectroscopic and Rf data for RCH2CoIIII(L2)Py and RCH2OOCoIII(L2)Pya
Composition yield
Yield (%)b
1H-NMR chemical shifts, l (ppm), CDCl3
umax (nm)c
Rfd
Aromatic
CH2Co (s)
L
Pyridine
OH···H
h
k
i
2a
3a
4a
5a
9a
10
10a
11
11a
12
12a
13
13a
14
14a
18
18a
19
19a
20
20a
21
21a
22
22a
23
6.87–7.21
6.87
4.31
4.31
4.31
4.31
4.86
2.90
4.35
2.80
4.31
2.76
4.37
2.91
4.38
2.87
4.38
3.40
5.00
3.55
4.71
3.36
4.55
3.35
4.57
3.41
4.63
3.41
4.64
3.68
4.84
3.55
4.60
3.32
4.54
4.03
5.03
2.21,2.31
2.21, 2.31
2.15, 2.25
2.28, 2.38
2.21
1.46 2.48,
1.57, 2.80
1.51, 2.50
1.60, 2.82
1.52, 2.51
1.59, 2 71
1.47, 2.49
1.56, 2.88
1.47, 2.49
2.88
1.26, 2.29
8.38
8.34
8.35
8.41
8.48
8.60
8.39
8.52
8.38
8.62
8.35
8.53
8.12
8.56
8.12
8.57
9.06
8.97
8.84
8.87
8.72
8.77
8.67
8.87
8.67
8.81
8.67
8.97
8.77
8.97
8.69
8.85
8.68
8.92
8.74
7.62
7.62
7.59
7.54
8.08
7.62
7.65
7.69
7.71
7.66
*
7.53
7.81
7.71
7.81
*
7.28
7.28
*
*
*
7.38
*
7.25
*
*
*
7.28
*
7.29
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
17.81
17.81
18.66
18.00
17.81
17.81
—
0.18
0.17
0.17
0.16
0.11
0.62
0.18
0.59
0.18
0.58
0.18
0.59
0.18
0.63
0.20
0.60
0.18
0.34
0.14
0.34
0.14
0.30
0.10
0.30
0.11
0.30
0.10
0.36
0.16
0.36
0.16
0.31
0.11
0.30
0.10
6.65–7.20
6.73–7.36
7.00–7.97
6.67–7.12
6.88–7.50
6.59–7.20
6.88–7.38
6.88–7.50
6.88–8.12
6.60–699
6.56–7.50
6.25–7.06
6.56–7.52
6.88–8.12
6.80–8.75
6.71–7.68
6.84–7.55
6.64–7.48
7.19–7.27
6.58–7.55
7.16–7.24
7.00–7.81
7.16–7.81
6.76–7.50
7.60–7.81
6.71–7.61
6.77–7.50
6.64–7.68
7.17–7.25
6.77–7.74
7.21–7.42
6.66–8.00
6.84–7.81
74
71
76
62
64
72
70
75
72
71
70
78
76
78
70
245, 466.7
244, 464.8
231, 461.6
240, 468.0
243.5, 466.7
231, 467.2
244, 472.9
236.5, 471.9
229, 472.5
261.5, 472.6
245, 472.6
245
17.69
17.69
17.19
17.94
1.62, 2.81
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
7.87
7.87
7.74
7.42
7.81
7.76
7.81
7.74
7.72
7.61
7.81
7.80
8.01
7.75
7.87
7.75
8.02
7.87
18.68
18.90
18.68
18.75
18.68
18.72
18.74
18.76
18.74
18.76
18.62
18.88
18.75
18.68
18.75
18.75
18.38
18.65
23a
24
24a
25
25a
26
26a
27
242.5, 472.4
244.5
226sh, 472.5
27a
a All the complexes give the satisfactory elemental analysis.
b Yields for the inserted products are almost quantitative in each case. The known compounds are not listed, ([9]f, [30]).
c The first value refers to MLCT and the second one to Co–C CT band.
d Ethylacetate is the eluent for the compounds 1–18 and 1a–18a and a mixture of EtOAc:CCl4 (1:9) is used for the compounds 19–27 and 19a–27a.
e Merge with aromatic.
gen. (E) The rate of reaction is slowed down in the
presence of radical inhibitor like galvinoxyl; (F) the
rate of insertion changes as the equatorial ligand is
changed and it follows the order dpgH\chgH\dmgH
in all cases. (G) The solvent affects the rate and it
follows the order acetonitrile\acetone\chloroform;
(H) the reaction proceeds faster as the axial base
strength increases; 2-acetylpyridineB2-bromopyri-
dineBpyridineBk-picolineBmorpholineBpiperidine;
(I) the rate changes with the change in R group and it
follows the order naphthyl\heteroaromaticmethyl\
benzyl for the three series of complexes. Among the
benzyls, p-methoxybenzyl has the highest rate and
among the heteroaromaticmethyl groups it follows the
order 2-thienylmethyl\3-thienylmethyl\furfuryl for
all the three series of complexes. It seems clear that the
rate of insertion depends upon various factors including
the nature of R, L, B and the solvent.
Oxygen insertion into M–C bond is a reaction which
occurs with remarkably a wide range of organometallic
compounds. Both thermal as well as photochemical
activation of these insertions have been observed in the
Co–C bond of organocobalt complexes. The results in
the photochemical reactions are much more difficult to

4
B.D. Gupta et al. / Journal of Organometallic Chemistry 570 (1998) 1–7
interpret owing to the greater complexity of this reac-
tion compared to thermal reactions; the exact role of
light in these reactions is not known. Many suggestions
have been made by various workers, e.g. Schrauzer has
proposed, based in part on MO calculations, that
photolysis of alkyl cobaloximes leads to excitation
and subsequent homolytic cleavage of the Co–C bond
[17]. Giannotti and coworkers have suggested that
photolysis with light in the charge transfer region
(about 450 nm) leads to the expulsion of the axial base
ligand as the primary photochemical process [18]. The
Co–B bond is reformed after the insertion has taken
place.
Several mechanisms including a radical chain mecha-
nism can be envisaged for the insertion of oxygen
into Co–C bond in organocobaloximes. Such radical
chain reactions generally require induction period
and once these start they proceed to completion
without the external stimulation by light. However,
in view of the experimental observations in the present
study that (a) there is no induction period; (b) the
reaction stops as soon as the light stimulation is
stopped; (c) the organocobaloximes in the absence of
oxygen are stable at 30°C, i.e. at a temperature where
the insertion occurs readily as shown by kinetics,
we rule out the participation of such a chain mecha-
nism.
the decrease in base strength, however a reverse order
has been observed in the present study.
Among the three other possibilities A, B and C, the
basic difference in these mechanisms is that in (A), the
free radicals R and (Co)Py are involved. R picks up O2
and the adduct combines with (Co)Py to give the
insertion product. In this the stability of R plays an
important role and should affect the rate. Mechanism
(C) on the other hand does not involve a complete
rupture of Co–C bond, the resulting radical pair
(R…Co) within the solvent cage on reaction with O2
gives the final inserted product. In this case the nature
of solvent should affect the rate. Such a mechanism has
been proposed by many workers [19–21].
It seems difficult to distinguish between these possi-
bilities without doing a more detailed studies but since
the solvent and the R group do affect the rates in these
reactions, it looks certain that the mechanism (A) and
(C) are operative. It is, however, very difficult to know
their relative contribution to the overall rate. The for-
mation of cross insertion products further suggest that
a complete rupture of bond takes place, i.e. the mecha-
nism (A) is operating.
We have not considered any mechanism involving
thermal conditions since no insertion has been observed
in the dark under thermal conditions,. Similarly, any
mechanism which involves the loss of pyridine at any
stage in the photochemical process has also been ig-
nored. Even if such a base off process was operating
then the rate of insertion should have increased with

B.D. Gupta et al. / Journal of Organometallic Chemistry 570 (1998) 1–7
5
3. Experimental
Chlorocobaloxime, ClCoIII(L2)Py (L=dmgH, chgH
and dpgH) [15]. 2-Chloromethylthiophene, 3-bro-
momethylthiophene, furfurylbromide, 4-cyanobenzyl-
bromide, 4-methoxybenzylbromide, 3-methoxybenzyl-
bromide were synthesised according to the published
procedures [23–28]. Diphenylglyoxime, 1,2-cyclohex-
anedionedioxime and all the other halides were ob-
tained from Aldrich and were used as such.
The organocobaloximes were synthesised by a gen-
eral procedure detailed earlier in many papers ([7]b,
[15]) and involves the reaction of cobaloxime(I), with
the corresponding benzylic halide. Cobaloxime(I) was
prepared in situ by the NaBH4 reduction of chloro-
cobaloximes in aqueous methanol under strict oxygen
free atmosphere. The work-up procedure was similar to
the ones described earlier ([7]b, [15]). The crude prod-
ucts were purified by column chromatography as de-
scribed earlier for alkyl cobaloximes [15].
2-ThienylmethylCoIII(dpgH)2B [B=2-acetylpyridine,
2-bromopyridine, picoline, morpholine, piperidine] were
prepared by replacing the aqua group from 2-
thienylmethylCoIII(dpgH)2H2O. In a typical experiment
morpholine (0.029, 0.23 mmol) in acetone (1 ml) was
added dropwise to a solution of 2-thienylmethy-
l(aquo)cobaloxime (0.129, 0.18 mmol) in acetone (3
ml). The reaction mixture was stirred in dark in an inert
atmosphere. The completion of the reaction was
checked with TLC on silica gel using ethyl ac-
etate:CCL4 in 1:9 ratio. The wine red coloured solution
fumed orange within minutes and the orange precipi-
tate which formed were filtered, washed with acetone
(2×2 ml) and dried in vacuum over P2O5. Yield 68%
The aqua complex was prepared from the corre-
sponding pyridine complex following the literature pro-
Fig. 1. Effect of the addition of pyridine (— 0.0 M, ---- 10−3 M, ···
10−2 M saturation curve) to a 10−5 M solution of benzyl(L2)Py in
methanol (L=dmgH (1), chgH (10); for the complex L=dpgH (19),
the plot is only qualitative.
cedure [29]. In
a typical experiment, 2-thienyl-
methylCoIII(dpgH)2Py (214 mg 3 mmol) was dissolved
in acetone (30 ml). To this was added 5 ml of water and
248 mg of Dowex 50w-x ion exchange resin (H+ form)
and the mixture was stirred for 16h in the dark. The
mixture was filtered and the filtrate was evaporated to
510 ml, cooled in ice and filtered to produce orange
powder which was washed with cold water and dried in
vacuo over P2O5. Yield 80%
Finally, the change in the insertion rate for a given
compound having the same R and B but different
equatorial ligand L is hard to explain. One of the
contributing factors may be the cis influence exerted by
the bulkier equatorial ligand on the Co–C bond. We
find that the magnitude of Didp is significantly larger
[0.56–0.92] ppm than Dich [0.00–0.34] ppm in all cases
3.1. Physical measurements and instruments
where Didp=lidpgH−lidmgH and Dich=lichgH
−
lidmgH and li is the chemical shift. If this is taken to be
as the extent of cis influence, as described in earlier
papers ([7]b, [16,22]), then the order of cis influence
follows the order dpgH\chgH$dmgH. The extent of
cis influence is felt most on the Co–CH2 followed by
Ph.
The 1H-NMR spectra were recorded on a Bruker
WP-80 FT NMR machine. UV-vis absorption spectra
were recorded on Shimadzu UV-190 spectrometer at
ambient temperature. The elemental analysis were un-
dertaken at the Regional Sophisticated Instruments
Centre, Lucknow.

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B.D. Gupta et al. / Journal of Organometallic Chemistry 570 (1998) 1–7
3.2. General procedure for oxygen insertion under
photochemical conditions
200 W tungsten lamp at 30°C. The progress of the
reaction was monitored by measuring the absorbance at
420 nm at intervals.
In a typical experiment a solution of organocoba-
loxime (0.2 mmol in 100 ml dichloromethane) at 0°C
was irradiated with 2×200 W tungsten lamps kept at a
distance of ca. 10 cm. and pure oxygen was bubbled
into this solution. The progress of the reaction was
monitored by TLC on silica gel using ethylacetate
(dmgH and chgH) or 1:9 ethylacetate/CCl4 mixture
(dpgH) as the eluent. The reaction was complete within
2 h in all cases. There was a distinct colour change from
yellow-orange to dark brown at this stage. At the end
of reaction, the solvent was stripped off and the crude
product was purified on silica gel column using ethyl
acetate as the eluent. The yields were almost quantita-
tive in all cases.
Acknowledgements
The authors thank DST, New Delhi for funding this
project. Thanks are also due to RSIC, Lucknow for the
elemental analysis.
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The same general procedure as above was followed
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added to the reaction mixture. After the completion of
the reaction, the solvent was completely evaporated and
the solid was washed three to four times with di-
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1
ethylether. The H-NMR on the residue showed that it
had no 2-acetylpyridine. The ether on evaporation gave
back all the 2-acetylpyridine.
3.4. Cross insertion reaction between
2-thienylmethyldpgH complex (15) and
4-methoxybenzyldmgH complex (4) in dichloromethane
at 0°C
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a typical experiment, a solution of two organocoba-
loximes 2-thienylmethyl dpgH complex (15, 0.424g,
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