Bimolecular homolytic reactions of alkylcobalt complexes

Article abstract of DOI:10.1016/j.poly.2007.01.032

The reactions of RCoL¹(H₂O)²⁺ (R = CH₃, C₂H₅ and n-C₃H₇; L¹ = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) and CH₃CoL

Full text of DOI:10.1016/j.poly.2007.01.032

Polyhedron 26 (2007) 2901–2905
www.elsevier.com/locate/poly
Bimolecular homolytic reactions of alkylcobalt complexes
*
Shaoyung Lee , Tsung Yao Ku
Department of Chemical Engineering, I-Shou University, Kaohsiung County 84008, Taiwan, ROC
Received 10 August 2006; accepted 24 January 2007
Available online 3 February 2007
Abstract
The reactions of RCoL1(H2O)2+ (R = CH3, C2H5 and n-C3H7; L1 = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-
diene) and CH3CoL2(H2O)2+ (L2 = 1,4,8,11-tetraazacyclotetradecane) with Cr2+ in aqueous solution were studied. The reaction prod-
2þ
2þ
ucts were CoL1ðH2OÞ (or CoL2ðH2OÞ ) and RCr2+, following which the related alkanes were obtained via acidolysis. The reaction
2
2
mechanism appears to consist of an SH2 displacement at the saturated carbon. The reactivity is clearly dependent on the size of the alkyl
group, the ligand and the configuration of the complex. The kinetics for this reaction were also studied at various temperatures. The
reaction of RCoL1(H2O)2+ with V2+ was investigated as well in this study.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Kinetics; Bimolecular homolysis; Alkylcobalt complexes
1. Introduction
CH3CoL1(H2O)2+ also shows a remarkable steric effect
with the following reactivity order: sec,rac- ꢁ meso-
ꢂpri,rac- [13].
Coenzyme B12(cobalamin) has attracted attention since
the structure determination of a cobalt–carbon bond in
the early 1960s [1,2]. Transition metal-alkyl bond dissoci-
ation energies are of importance in the context of a vari-
ety of organometallic and catalytic systems [3]. It is
known that the steric hindrance around the metal center
affects this metal–carbon bond [4–7]. Among the various
alkylcobalt macrocyclic complexes prepared recently
[8,9], the complex CH3CoL1(H2O)2+ (L1 = 5,7,7,12,14,
14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)
had three configurational isomers, as shown in the follow-
ing diagrams [10,11]. These complexes provide a good
opportunity to examine the dependence of configuration-
ally steric hindrance on their reactivity. The isomerization
of these isomers reveals that pri,rac- is the most stable
configuration and the isomerization rate is truly depen-
dent on the alkyl group at the cobalt centre [12]. The sub-
stitution of SCNꢀ with H2O for these three isomers of
H
H
H
CH3
Co
CH3
Co
OH
Co
2 N
N
N
N
N
N
N
N
N
N
N
N
CH3
H
OH2
OH2
H
H
trans-primary, rac
trans-secondary, rac
trans, meso
Although the dissociation energy of the cobalt–carbon
bond was well determined through a kinetic study of the
unimolecular
homolysis
of
meso-RCoL1(H2O)2+
(R = C2H5, n-C3H7), there is no information about the
bond dissociation energy of the other configurational
isomers of RCoL1(H2O)2+. Both unimolecular and bimo-
lecular homolysis were observed in the reaction of meso-
RCoL1(H2O)2+ with Cr2+ (R = C2H5, n-C3H7) [8]. The
*
Corresponding author.
E-mail address: sylee@isu.edu.tw (S. Lee).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.01.032

2902
S. Lee, T.Y. Ku / Polyhedron 26 (2007) 2901–2905
bimolecular homolytic reaction is interesting since homo-
lytic metal–carbon bond cleavage and atom transfer is
involved. In the present study, the reactions of alkylcobalt
complexes with different ligands and various configurations
with the metal ions Cr2+ and V2+ were investigated.
The kinetics and the activation parameters for the reaction
of sec,rac-CH3CoL1(H2O)2+, pri,rac-RCoL1(H2O)2+ (R =
CH3, C2H5 and n-C3H7) and CH3CoL2(H2O)2+ (L2 = 1,4,
8,11-tetraazacyclotetradecane) with Cr2+ in aqueous solu-
tion are reported.
2. Experimental
Fig. 1. Visible spectrum for the reaction of pri,rac-C2H5CoL1(H2O)2+
with Cr2+ The time interval is 60 s. [pri,rac-C2H5CoL1
(H2O)2+] = 2.5 · 10ꢀ4 M, [Cr2+] = 0.02 M (l = 0.5 M, T = 25 ꢁC).
2.1. Reagents
.
The macrocycle Me6[14]4,11-dieneN4 (L1) was prepared
by the condensation of 1,2-diaminoethane and acetone
[14]. [14]aneN4 (L2) was purchased from Aldrich and used
directly. meso-CoL1(H2O)2(ClO4)2, sec,rac-CH3CoL1-
(H2O)(ClO4)2, meso-RCoL1(H2O)(ClO4)2, pri,rac-RCoL1-
(H2O)(ClO4)2 (R = CH3, C2H5, n-C3H7) and an aqueous
solution of CH3CoL2(H2O)2+ were prepared according to
a published procedure [6,8,12,15]. (Caution: Perchlorate
salts of metal complexes with organic ligands are poten-
tially explosive.) The products of the alkylcobalt macrocy-
2þ
sponds to the formation of CoL1ðH2OÞ and C2H5Cr2+
,
2
respectively. However, the subsequent decrease in absorp-
tion at k 390 nm is caused by the slow acidolysis of
C2H5Cr2+ [20,21]. The related alkanes were obtained as
the gas products for the eventual reactions of Cr2+ with
various RCoL1(H2O)2+ complexes. The reactions involved
are shown in:
2þ
2þ
RCoL1ðH2OÞ þ Cr2þ ! RCr2þ þ CoL1ðH2OÞ
ð1Þ
ð2Þ
1
2
cles were characterized by UV–Vis and H NMR spectra
RCr2þ þ Hþ ! RH þ Cr3þ
[13,16]. t-Butyl hydroperoxide was commercially available
and used directly. The other alkyl hydroperoxides were
prepared from the appropriate alcohol and hydrogen per-
oxide [17,18]. The concentration of the V2+ solution was
determined from the absorbance at a wavelength (k) of
850 nm [19]. All the other chemicals purchased were of
reagent grade and were used directly. Water was purified
using a Millipore Milli-Q SP system and was used for all
the kinetic measurements.
The kinetics of these reactions were studied under pseudo-
first-order conditions with Cr2+ in excess and monitored by
measuring the increase in absorption at k = 330 nm as a
function of time. The plots of ln(A1 ꢀ At) versus time
showed a good linear behavior for at least three to four
half-lives to establish the first order dependence on
[RCoL1(H2O)2+]. The observed rate constants depend lin-
early on the concentration of the excess chromous ions,
as shown in Figs. 2 and 3. The plots for the reaction of
meso-C2H5CoL1(H2O)2+ and meso-C3H7CoL1(H2O)2+ are
2.2. Spectroscopy and kinetics
The UV–Vis spectra and single-wavelength absorbance
versus time data were acquired by using the Hitachi 3000
or HP 8453 spectrophotometer. Temperature control of
0.5 ꢁC was maintained by circulating water from a con-
stant-temperature bath through the jacket of a cell holder.
All the kinetic measurements were made using sodium per-
chlorate and perchloric acid adjusted to l = 0.5 M in most
1
cases. The H NMR spectra were studied using a Varian
GEMINI-200 spectrometer. The NMR experiments were
carried out in D2O with HOD as the reference. The gas
products of the hydrolysis of the organochromium com-
plexes were determined by GC (Perkin–Elmer) and a col-
umn of GS-GASPRO.
3. Results and discussion
103 × [Cr2+], M
The typical visible spectral changes observed on mixing
pri,rac-C2H5CoL1(H2O)2+ with Cr2+ are shown in Fig. 1.
The increase in absorption at k 330 and 390 nm corre-
Fig. 2. Dependence of kobs on [Cr2+] for the reaction of CH3CoL1(H2O)2+
with Cr2+ at 25 ꢁC and l = 0.5 M. r, pri,rac-CH3CoL1(H2O)2+; n,
sec,rac-CH3CoL1(H2O)2+
.

S. Lee, T.Y. Ku / Polyhedron 26 (2007) 2901–2905
2903
RCo(dmgBF2)2 with Cr2+ [22]. Among the three different
configurational isomers of CH3CoL1(H2O)2+, the pri,rac-
form has the least steric hindrance and shows the highest
reactivity with Cr2+. However, the difference between the
rate constants of meso- and sec,rac- is small. The larger hin-
drance of the sec,rac-configuration would certainly prevent
the attack by Cr2+ and reduce the reaction rate. On the
other hand, it would also decrease the cobalt–carbon bond
energy and accelerate the reaction. The experimental re-
sults show a combination of these two factors and therefore
the difference is not large. The kinetics of the reactions of
CH3CoL1(H2O)2+ with Cr2+ were studied at different tem-
peratures. The second-order rate constants are listed in Ta-
ble 2. The data were fit to the Eyring equation with plots of
ln(k/T) against 1/T. The values of DHà and DSà are listed
in Table 3.
102 × [Cr2+], M
Fig. 3. Dependence of kobs on [Cr2+
for the reaction of pri,rac-
]
RCoL1(H2O)2+ with Cr2+ at 25 ꢁC. r, R = C2H5 (l = 0.5 M); n, R = n-
C3H7 (l = 1.0 M).
The reaction of CH3CoL2(H2O)2+ with Cr2+ was also
studied under pseudo-first-order kinetic conditions with
2þ
Cr2+ in excess. Since the absorption of CoL2ðH2OÞ in
2
linear but with a significant intercept that corresponds to
unimolecular homolysis, as shown in Eq. (3). The value
of the unimolecular homolysis rate constant kh for
the visible region is small, the reaction was monitored at
k 390 nm, which was the maximum absorption of
CH3Cr2+. A typical biphase kinetic trace was observed;
the increase in absorption was followed by a decrease due
to acidolysis (Eq. (2)). The slow acidolysis was caused by
the first-order dependence on [H+], and it gave a second-
order rate constant of 5 · 10ꢀ3 Mꢀ1 sꢀ1, which was in good
agreement with the literature value of 4.94 · 10ꢀ3 Mꢀ1 sꢀ1
[21]. The consecutive reaction analysis of the fast increase
2þ
2þ
meso-RCoL1ðH2OÞ !ÅR þ CoL1ðH2OÞ
ðR ¼ C2H5 or n-C3H7Þ
2
ð3Þ
meso-C2H5CoL1(H2O)2+ calculated in this study
(9.1 · 10ꢀ4 sꢀ1, l = 1.0 M) is consistent with the value ob-
tained in a previous report (3.5 · 104 sꢀ1, l = 0.14 M) [8].
These results suggest a general rate law for the reactions
of Cr2+ with these alkylcobalt complexes:
absorption established
a
first-order dependence on
[CH3CoL2(H2O)2+]. The plot of kobs versus [Cr2+] was lin-
ear with a slope equal to 0.268 0.005 Mꢀ1 sꢀ1, as shown
in Fig. 4. This reactivity is much slower than that of
CH3CoL1(H2O)2+. This reactivity results from the stronger
cobalt–carbon bond in RCoL2(H2O)2+ [6,8]. No attempt to
study the kinetic rate of the reaction of Cr2+ with
ꢀ d½RCoL1ðH2OÞ ꢃ=dt ¼ ðkh þ k½Cr2þꢃÞ½RCoL1ðH2OÞ2þꢃ
2þ
kobs ¼ kh þ k½Cr2þꢃ
The unimolecular homolysis rate constant kh does not exist
for these alkylcobalt complexes, except for meso-
C2H5CoL1(H2O)2+ and meso-C3H7CoL1(H2O)2+. These
results suggest that the configuration of the complex affects
the bond energy of the cobalt–carbon bond in these alkyl
cobalt complexes. sec,rac-C2H5CoL1(H2O)2+ could not be
prepared successfully probably due to the unstable co-
balt–carbon bond. The values of the second-order rate con-
stants k measured at 25 ꢁC are listed in Table 1. The
reactivity of pri,rac-RCoL1(H2O)2+ with Cr2+ is clearly
dependent on the size of the alkyl group R with the order
CH3 > C2H5 > n-C3H7. A larger alkyl group would prevent
the attack by Cr2+. This is consistent with the reactivity of
Table 2
Dependence of temperature for the reaction of CH3CoL1(H2O)2+ with
Cr2+a
Temperature (ꢁC)
k (Mꢀ1 sꢀ1
)
meso-
pri,rac-
sec,rac-
15
20
25
30
35
a
3.7 0.2
4.3 0.1
7.0 0.1
9.5 0.2
11.6 0.5
26.0 0.8
39.8 0.4
5.0 0.1
7.5 0.1
7.86 0.01
10.1 0.2
12.5 0.1
46
59
87
2
2
2
T = 25 ꢁC, l = 0.5 M, k = 330 nm.
Table 1
Kinetic data for the reaction of RCoL1(H2O)2+ with Cr2+a
k (Mꢀ1 sꢀ1
)
Table 3
R
Thermodynamic activation parameters for the reaction of
CH3CoL1(H2O)2+ with Cr2+
meso-
pri,rac-
sec,rac-
CH3
C2H5
n-C3H7
a
7.0 0.1
46
2
7.86 0.01
Configuration
DHà (kJ/mol)
DSà (J/K mol)
(3.4 0.7) · 10ꢀ2
(2.12 0.04) · 10ꢀ1
(1.1 0.1) · 10ꢀ3 b
(4.8 0.1) · 10ꢀ3 b
meso-
pri,rac-
sec,rac-
43
39
29
4
4
4
ꢀ85 10
ꢀ82
9
T = 25 ꢁC, l = 0.5 M, k = 330 nm; unless otherwise noted.
T = 25 ꢁC, l = 1.0 M, k = 330 nm.
ꢀ130 25
b

2904
S. Lee, T.Y. Ku / Polyhedron 26 (2007) 2901–2905
C2H5CoL1(H2O)2+ was still effective under these condi-
tions. This is due to V2+ which is not an effective scavenger
for ethyl radicals. The slow rate of exchange with solvent
2þ
results in the lower reactivity of VðH2OÞ6 in electron
transfer reactions [19]. The reactions involved were proba-
bly the following:
2þ
2þ
meso-C2H5CoL1ðH2OÞ ! ÅC2H5 þ CoL1ðH2OÞ
ð4Þ
2
2þ
meso-C2H5CoL1ðH2OÞ þ ÅC2H5 ! C2H6
2þ
þ C2H4 þ CoL1ðH2OÞ
ð5Þ
ð6Þ
2
102 × [Cr2+], M
V2þ þ ÅC2H5 þ Hþ ! C2H6 þ V3þ
Fig. 4. Dependence of kobs on [Cr2+] for the reaction of CH3CoL2(H2O)2+
with Cr2+ at 25 ꢁC and l = 1.0 M.
The kinetics study was therefore repeated with a high con-
centration of V2+ ([V2+] = 0.08–0.31 M). In this case, C2H6
was the sole gas product. The plot of kobs versus [V2+] is
linear with an intercept of 4.37 · 10ꢀ4 Mꢀ1 sꢀ1, which is
consistent with the value of the homolysis rate constant
of meso-C2H5CoL1(H2O)2+. The slope of the line is
4.04 · 10ꢀ4 Mꢀ1 sꢀ1. Due to the poor linearity and small
value of the slope for the plot of kobs versus [V2+] and no
change in absorption of the reaction of CH3CoL1(H2O)2+
with V2+ under similar conditions, a direct reaction be-
tween meso-C2H5CoL1(H2O)2+ and V2+ was probably
non-existent.
Table 4
The second-order rate constants for the bimolecular reaction of
CH3CoL(H2O)2+ with Cr2+
L
k (Mꢀ1 sꢀ1
)
[14]aneN4
0.27a
7a
meso-Me6[14]4, 11-dieneN4
pri,rac-Me6[14]4, 11-dieneN4
sec,rac-Me6[14]4, 11-dieneN4
Me6[14]aneN4
46a
7.86a
1.64b
50.8c
(dmgBF2)2
a
This study.
Ref. [8].
Ref. [22].
b
c
4. Conclusion
C2H5CoL2(H2O)2+ was made since the reaction was
expected to be even slower.
The kinetic studies clearly show that there is a bimolec-
ular reaction between RCoL1(H2O)2+ and Cr2+. The reac-
tion rates show a strong dependence on steric effects of the
alkyl group, consistent with an SH2 mechanism. The reac-
tivity is also clearly dependent on the configuration of the
complex. However, the difference between the rate con-
stants of meso- and sec,rac- is small.
The present kinetic studies clearly indicate that there is a
bimolecular reaction between RCoL(H2O)2+ and Cr2+
.
The reaction mechanism appears to consist of an SH2 dis-
placement at the saturated carbon [23]. The second-order
rate constants for the methyl cobalt macrocyclic complexes
with Cr2+ are summarized in Table 4. The reactivity shows
a clear dependence on the structure of the ligand. It is note-
worthy that the rate constant for the diene ligand (L1) is
higher than that for the saturated macrocycle
Me6[14]aneN4(5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraaz-
atetradecane). This probably indicates that the former is
less sterically crowded owing to the double bonds that
cause the ring configuration to become more planar.
The reaction of RCoL1(H2O)2+ with V2+ was also stud-
ied under pseudo-first-order kinetic conditions with V2+ in
excess. A significant absorption change was observed only
for the reactions with meso-C2H5CoL1(H2O)2+ and meso-
C3H7CoL1(H2O)2+. The first-order kinetic plots of lnDA
versus time were linear for the reactions with both meso-
C2H5CoL1(H2O)2+ and meso-C3H7CoL1(H2O)2+. How-
ever, the maximum value of the pseudo-first-order rate
constants measured in these reactions ([V2+] = 5.5–
27.4 mM) were not even close to the values of the unimo-
lecular homolysis rate constant, kh, for these two
complexes. The major gas products in the reaction with
meso-C2H5CoL1(H2O)2+ were ethylene and ethane. This
shows that the reaction of ethyl radicals with meso-
Acknowledgements
We thank the ISU for supporting this work through
Grant ISU-94-02-13. A part of this work was supported
through Grant NSC-90-2113-M-214-002.
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