Reaction of octacarbonyldicobalt with the free radicals galvinoxyl and 2,2-diphenyl-1-picrylhydrazyl. Attempts to scavenge the tetracarbonylcobalt radical

Article abstract of DOI:10.1016/S0020-1693(02)00853-8

The addition of galvinoxyl or 2,2-diphenyl-1-picrylhydrazyl to octacarbonyldicobalt in n-octane solution results in the loss of all carbon monoxide ligands and the formation of Co(II)-containing products.

Full text of DOI:10.1016/S0020-1693(02)00853-8

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Inorganica Chimica Acta 334 (2002) 308ꢁ312
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Reaction of octacarbonyldicobalt with the free radicals galvinoxyl
and 2,2-diphenyl-1-picrylhydrazyl. Attempts to scavenge the
tetracarbonylcobalt radical
Ro´bert Tuba a, Ferenc Ungva´ry a,b,
*
a Department of Organic Chemistry, University of Veszpre´m, 8200 Veszpre´m, Hungary
b Research Group for Petrochemistry of the Hungarian Academy of Sciences, University of Veszpre´m, H-8200 Veszpre´m, Hungary
Received 15 October 2001; accepted 6 February 2002
Dedicated to Professor Andrew Wojcicki in recognition of his pioneering work in metal carbonyl chemistry
Abstract
The addition of galvinoxyl or 2,2-diphenyl-1-picrylhydrazyl to octacarbonyldicobalt in n-octane solution results in the loss of all
carbon monoxide ligands and the formation of Co(II)-containing products. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Octacarbonyldicobalt; Tetracarbonylcobalt radical; Galvinoxyl; Hydroxygalvinol; 2,2-Diphenyl-1-picrylhydrazyl; 1,1-Diphenyl-2-
picrylhydrazine
1. Introduction
17.1 J molꢂ1 Kꢂ1 have been calculated [5]. The bond
dissociation energy based on thermochemical calcula-
The 17-electron tetracarbonylcobalt radical was first
observed during the thermal dissociation of Co2(CO)8 in
the gas phase using high vacuum. It has been isolated on
a cold (77 K) finger and its ESR spectrum was recorded
[1]. This species was observed also in the mass spectrum
of pyrolyzed Co2(CO)8 [2]. Using the matrix isolation
tions has been given as D298[(CO)4Coꢀ
/
Co(CO)4]ꢀ
/
839
/
29 kJ molꢂ1 [6].
The generation of the tetracarbonylcobalt radical by
photolysis of Co2(CO)8 in CO matrices has also been
reported [7].
There is ample evidence that the +Co(CO)4 radical
exists not only in the gas phase or under extreme
conditions but also in low concentrations in solutions
of Co2(CO)8 at ambient conditions. Based on high-
pressure infrared spectra the presence of +Co(CO)4
radicals in solutions of Co2(CO)8 in n-hexane has been
technique, +Co(CO)4 [3], and +Co(CO)n (nꢀ
[4] were prepared and characterized by their infrared,
Raman, UVꢁVis and ESR spectra.
/1, 2, 3, 4)
/
Since the thermolysis of the cobaltꢀ/cobalt bond in
Co2(CO)8 changes the magnetic susceptibility of the
+
confirmed in the temperature range of 120ꢁ210 8C [8].
/
solution owing to the production of Co(CO)4 radicals,
The participation of the 17-electron +Co(CO)4 radical
in various reactions of cobalt carbonyls has been
inferred from the observation that the rates of these
reactions were found to be proportional to the square
the extent of this homolysis was determined by measur-
ing the temperature dependence of the volumetric
susceptibility of a gas-phase solution of Co2(CO)8 in
carbon monoxide in the temperature range 120ꢁ
From the temperature dependence of the equilibrium
constant DHꢀ(81.19 123.89
8.5) kJ molꢂ1 and DSꢀ
/
225 8C.
root of Co2(CO)8 concentration [9ꢁ17]. This can be
/
related to the fast equilibrium formation of +Co(CO)4
from Co2(CO)8 which is then involved in the rate-
determining step.
/
/
/
/
K
* Corresponding author. Fax: ꢃ
E-mail address: ungvary@almos.vein.hu (F. Ungva´ry).
/
36-88-427 292.
Co2(CO)8?2+Co(CO)4
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 8 5 3 - 8

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312
309
2.2. Kinetic experiments
[+Co(CO)4]2
[Co2(CO)8]
K ꢀ
and
Kinetic experiments were performed in a thermo-
statted glass-reactor fitted with a silicon-disk-capped
stopcock, and connected to a gas burette through a
reflux condenser (thermostatted to 12.0 8C). The reac-
tions were started by injecting either aliquots of the
Co2(CO)8 stock solution into the solutions of the free
radicals in n-octane or by injecting aliquots of the
freshly prepared stock solutions of the free radicals into
the solutions of Co2(CO)8 in n-octane. The initial rate
was calculated from the gas burette readings collected in
[+Co(CO)4]ꢀK0:5[Co2(CO)8]0:5
The tetracarbonylcobalt radical has a high reactivity.
It dimerizes spontaneously [1], reacts with O2 to form
+O2Co(CO)4 [18,19] and abstracts a hydrogen atom
from transition metal hydrides [5]. According to NMR
line-shape analysis, the degenerate hydrogen-atom
transfer reaction between HCo(CO)4 and +Co(CO)4
proceeds with activation parameters of DH%ꢀ
68.39
2.5 kJ molꢂ1 and DS%ꢀ 4.3 J molꢂ1 Kꢂ1 [5].
/
23.59
/
/
ꢂ
/
/
the first 1ꢁ
monoxide concentration on the rate of gas evolution
was studied by using different total pressures (0.27ꢁ1.0
/
5 min (4ꢁ/20) points). The effect of carbon
It was found recently that Co2(CO)8 in n-pentane
solution under thermal or photochemical conditions
reacts with 2,2,6,6-tetramethylpiperidin-1-oxyl (TMPO)
/
bar) in the gasometric apparatus and in experiments
starting under an argon atmosphere. The rates are
reproducible to within 10%.
to give
a diamagnetic electron-deficient complex
(TMPO)Co(CO)2 in good yields. It was assumed that
the first step of the complex formation is the reaction of
TMPO with +Co(CO)4, followed by the evolution of
two mol of carbon monoxide [20]. Indeed the kinetics of
the gas evolution was found to be half order in
Co2(CO)8 concentration which strongly supports the
idea that cobaltcarbonyl radicals are scavenged by the
organic free radicals [21]. In this paper we describe the
results of the reactions of Co2(CO)8 with two other
organic free radicals, namely of galvinoxyl and with 2,2-
diphenyl-1-picrylhydrazyl. In contrast to the results with
TMPO, in both cases we obtained products of simple
redox reactions.
2.3. Instrumentation
IR spectra were recorded either on a Specord IR-75 or
on a Specord M-80 (Carl Zeiss, Jena) spectrometer.
Electronic spectra were measured on a Shimadzu UV-
160A spectrophotometer. GC analyses of the gas
evolved in the reactions were performed on a HP
5890II gas chromatograph equipped with a thermal
conductivity detector and a 25 m, 0.53 mm Poraplot Q
column. GC MS measurements were recorded on a HP
5890II/5971 GC/MSD at 75 eV. Analyses for C, H, N
were performed using a CHNSO Analysator (Carlo
Erba). Cobalt analyses were performed using established
microanalytical methods.
2. Experimental
2.4. Preparation of the cobalt salt of Gꢂ
2.1. Chemicals
To a magnetically stirred solution of galvinoxyl (1.72
g, 4.08 mmol) in n-octane (70 ml) a solution of
Co2(CO)8 (0.342 g, 1.00 mmol) in n-octane (5 ml) was
added at 25 8C under a CO atmosphere. In 30 min the
gas evolution ceased. A total of 7.88 mmol of CO was
Octacarbonyldicobalt was prepared from cobalt(II)
acetate tetrahydrate, dihydrogen and carbon monoxide
in acetic anhydride solution [22]. The crude product of
this high-pressure synthesis was doubly recrystallized
first from methylene dichloride and then from n-heptane
under carbon monoxide. Carbon monoxide was pre-
pared from formic acid and concentrated sulfuric acid
and was dried on P4O10 and deoxygenated on activated
BTS catalyst R 3-11 (BASF, Germany). Anhydrous n-
collected. The dark bluishꢁblack precipitate was filtered
/
off at 0 8C from the solution, washed with a few
milliliters of cold n-octane, and dried in vacuum. A
black crystalline product (1.78 g, 98.5% yield as Co(G)2
based on Co2(CO)8) with melting point (m.p.)ꢀ
/
205ꢁ
/
207 8C was obtained.
octane 99%ꢃ
/
(Aldrich, Hungary) was distilled over
Anal. Calc. for C58H82O4Co: C, 77.24: H, 9.15; Co,
6.52. Found: C, 77.0; H, 9.2; Co, 6.36%. The visible
spectra in DMSO show a strong absorption at 583 nm
sodium-wire under carbon monoxide. Galvinoxyl free
radical (Aldrich, Hungary) and 2,2-diphenyl-1-picrylhy-
drazyl (hydrate) free radical (Aldrich, Hungary) were
used as received. Stock solutions of Co2(CO)8 (0.20 mol
dmꢂ3), of galvinoxyl (0.06 mol dmꢂ3), and of 2,2-
diphenyl-1-picrylhydrazyl (0.0017 mol dmꢂ3) in n-
octane were freshly prepared under carbon monoxide
before the kinetic runs.
(oꢀ
/
6.6ꢄ
104 cm2 mmolꢂ1).
/
2.5. Reaction of Co(G)2 with acetic acid
Aqueous acetic acid (0.52 mmol) and n-pentane (10
ml) were added to Co(G)2 (0.18 g, 0.20 mmol) at room

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/
temperature (r.t.). After standing overnight, a yellow
upper and a pink lower phase were observed. Separation
of the phases and removing the solvents in vacuum gave
yellow crystals of hydroxygalvinol (HG) (0.154 g, 91.1%
trum is identical with that described in the literature [23].
Evaporation of water from the aqueous phase gave
Co(CH3COO)2×/4H2O in 95% yield. Reaction 1 and 2
summarize the above results.
Co2(CO)8 ꢃ4+G 0 8COꢃ2Co(G)2
yield, m.p.ꢀ
153 8C, MS Mꢃ 422, M-Me 407, M-tBu
365) and pink crystals (42 mg, 84% yield), respectively.
/
(1)
(2)
Co(G)2 ꢃ2CH3COOH 0 Co(CH3COO)2 ꢃ2HG
2.6. Reaction of Co2(CO)8 with 2,2-diphenyl-1-
picrylhydrazyl (+P)
A mixture of 2,2-diphenyl-1-picrylhydrazyl hydrate
(0.177 g, 0.43 mmol) and Co2(CO)8 (34.2 mg, 0.1
mmol) in n-octane (100 ml was stirred under a carbon
monoxide atmosphere at 25 8C. Evolution of 0.728
mmol gas and formation of a brick-red inhomogeneous
precipitate was observed after 24 h. The precipitate was
removed by filtration and digested with five 50 ml
portions of diethyl ether. From the combined ether
In order to obtain information about the mechanism
of reaction 1, the effects of Co2(CO)8, +G, and CO
concentrations on the initial rate of carbon monoxide
evolution (rCO) were studied at 15 8C using n-octane as
the solvent. The results compiled in Table 1 indicate that
the rate of gas evolution is practically first order in
Co2(CO)8 concentration and first order in galvinoxyl
concentration. The concentration of carbon monoxide
does not influence the rate in the studied range.
The addition of 2,2-diphenyl-1-picrylhydrazyl (+P) to
octacarbonyldicobalt in n-octane solution also results in
evolution of carbon monoxide but with the formation of
a brick-red inhomogeneous precipitate. The amount of
gas evolution corresponds to the release of all eight
carbon monoxide ligands from octacarbonyldicobalt
when at least four moles of 2,2-diphenyl-1-picrylhydra-
zyl are present as a reactant. After the complete
solution brownꢁ
drazine (HP) were obtained after cooling at ꢂ
days (0.155 g, 91% yield based on +P. IR in KBr pellets:
n(Nꢀ
H) 3250 and 3280 cmꢂ1). The digested black residue
(23 mg) contained ꢀ50% cobalt by analysis.
/red crystals of 1,1-diphenyl-2-picrylhy-
/
18 8C for 2
/
/
3. Results
The addition of galvinoxyl free radical (+G) to
octacarbonyldicobalt in n-octane or other saturated
hydrocarbon solution results in the evolution of carbon
monoxide and the formation of a dark purple precipi-
tate. By measuring the amount of carbon monoxide at
conversion of octacarbonyldicobalt no Cꢀ/O stretching
absorptions can be seen either in the infrared spectra of
the reaction solution or that of the isolated precipitate.
The solid product of the reaction dissolves partially in
diethyl ether or in methylene dichloride. The remaining
insoluble solid residue is black or brown and contains
various amounts of cobalt depending on the extent of
large (5ꢁ20) molar excess of galvinoxyl we found that
/
Co2(CO)8 releases practically all eight CO ligands
during its conversion. Accordingly at the end of the
reaction with a total conversion of Co2(CO)8 the
reaction solution and the isolated precipitate show in
extraction. The redꢁbrown extract however, contains no
/
the infrared spectra no sign of Cꢀ
/
O stretching vibra-
cobalt and gives red crystals in high yield upon cooling.
The infrared spectra of the crystalline product in KBr
tions. The amounts of carbon monoxide released upon
gradual addition of galvinoxyl to Co2(CO)8 indicated
the consumption of four mol of galvinoxyl for the
complete conversion of one mol Co2(CO)8. By adding
less than four mol of galvinoxyl to one mol of
octacarbonyldicobalt no n(CO) bands other than those
from the unreacted portion of Co2(CO)8 were observed
in the infrared spectra of the reaction solutions. The
dark purple precipitate dissolves in dimethyl sulfoxide to
give a blue solution. The visible spectra of such solutions
show a strong absorption at 583 nm indicating the
presence of Co2ꢃ ion. Treating the solid isolated from
galvinoxyl and Co2(CO)8 with a 2.6-fold molar excess of
aqueous acetic acid at room temperature give a yellow
extract with n-pentane and a pink-colored aqueous
solution. From the yellow extract hydroxygalvinol
(HG) as yellow crystals was isolated in 91% yield
pellets show characteristic n(Nꢀ
/
H) bands at 3250 and
3280 cmꢂ1. Single crystal X-ray structure determination
[24] confirmed that this product is 1,1-diphenyl-2-
picrylhydrazine (HP).
Reaction 3 summarizes the above results.
Co2(CO)8 ꢃ4+P × H2O 0 8COꢃ2Co(OH)2 ꢃ4HP (3)
The effect of Co2(CO)8, +P, and CO concentrations on
the initial rate of carbon monoxide evolution (rCO) at
(m.p.ꢀ
/
153 8C, lit. 150ꢁ155 8C [23]). Its mass spec-
/

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Table 1
Initial rate of carbon monoxide evolution (rCOꢀ8ꢄr, where r is the gross reaction rate) and the observed rate constant (kobsꢀrCO/[Co2(CO)8][+G])
in the reaction of octacarbonyldicobalt with the free radical galvinoxyl (+G) at 15 8C in n-octane solution at various concentrations
Experiment 102ꢄ[Co2(CO)8] (mol dmꢂ3
)
102ꢄ[+G] (mol dmꢂ3
)
102ꢄ[CO] a (mol dmꢂ3
)
105ꢄrCO (mol dmꢂ3 sꢂ1
)
kobs (dm3 molꢂ1 sꢂ1
)
TR135
TR452
TR133
TR451
TR131
TR99
0.033
0.056
0.13
0.23
0.73
1.46
1.46
1.46
0.23
0.23
0.23
0.23
2.00
1.10
2.02
1.00
2.04
0.10
0.12
0.20
0.18
1.00
1.00
1.00
0.981
0.997
0.981
0.995
0.979
0.976
0.986
0.986
0.989
0.536
0.260
0.63
0.66
2.63
2.56
13.87
1.40
1.78
2.61
0.49
2.07
2.17
2.71
0.96
0.97
1.00
1.11
0.93
0.96
1.04
0.91
1.13
0.92
1.03
1.14
TR125
TR124
TR454
TR456
TR457
TR460
b
0
a
Calculated from the PCO and the solubility of CO in n-octane cf. [21].
Experiment started under argon atmosphere.
b
Table 2
Initial rate of carbon monoxide evolution (rCOꢀ8ꢄr, where r is the gross reaction rate) and the observed rate constant (kobs)ꢀrCO
/
[Co2(CO)8]0.5[+P] in the reaction of octacarbonyldicobalt with the free radical 2,2-diphenyl-1-picrylhydrazyl (+P) at 15 8C in n-octane solution at
various concentrations
a
Experiment
102ꢄ[Co2(CO)8]
102ꢄ[+P]
102ꢄ[CO]
105ꢄrCO
kobs
((dm3 molꢂ1 0.5 sꢂ1
)
(mol dmꢂ3
)
(mol dmꢂ3
)
(mol dmꢂ3
)
(mol dmꢂ3 sꢂ1
)
)
TR213
TR208
TR209
TR198
TR199
TR200
TR205
TR469
TR468
TR210
TR211
0.087
0.17
0.43
0.50
0.50
0.50
0.50
0.67
0.68
0.69
1.04
0.15
0.16
0.12
0.024
0.050
0.094
0.13
0.14
0.14
0.15
0.15
0.985
0.985
0.985
0.985
0.985
0.985
0.985
0.49
0.67
0.88
1.99
4.07
7.12
10.1
1.05
1.24
1.23
1.38
0.110
0.100
0.112
0.117
0.115
0.107
0.106
0.092
0.108
0.099
0.091
b
0
b
0
0.985
0.985
a
Calculated from the PCO and the solubility of CO in n-octane cf. [21].
Experiment started under argon atmosphere.
b
15 8C using n-octane as the solvent can be seen in Table
2. The results compiled in Table 2 show that the rate of
gas evolution is practically first order in 2,2-diphenyl-1-
picrylhydrazyl concentration, half order with respect to
the Co2(CO)8 concentration, and independent of the
concentration of carbon monoxide in the concentration
range studied.
the galvinoxyl moiety leading finally to cobalt(II) and
the galvinol anion (Gꢂ).
The kinetics of the reaction of Co2(CO)8 with 2,2-
diphenyl-1-picrylhydrazyl hydrate also show a zero
order dependence with respect to the carbon monoxide
concentration and a first order dependence with respect
to the 2,2-diphenyl-1-picrylhydrazyl hydrate concentra-
tion, but the observed kinetic order of Co2(CO)8 is 0.5.
In this case +Co(CO)4 formed in a fast preequilibrium
from Co2(CO)8 might participate in the rate-determing
one-electron transfer reaction with 2,2-diphenyl-1-pi-
crylhydrazyl. The transfer of the second electron from
the cobalt(I) intermediate to another molecule of 2,2-
diphenyl-1-picrylhydrazyl might happen in a fast con-
secutive step leading to cobalt(II). The isolated organic
product of the reaction is 1,1-diphenyl-picrylhydrazine,
which is probably formed by a fast protonation of the
intermediate anion.
4. Discussion
The first order dependence of the reaction of
Co2(CO)8 with galvinoxyl with respect to both the
Co2(CO)8 and the galvinoxyl concentrations and the
zero order dependence with respect to carbon monoxide
suggests a rate-determining associative step which is
followed by fast electron-transfer steps from cobalt to