Cobalt(I) Complex
Nucleophile: Catalysis of
Sn2 Reactions with Alkyl
Halides
Kinetics of Bimolecular
Substitution Reactions
1
A Brief Introduction
Half a century ago, it was a common belief that
organocobalt compounds were reactive and
thermodynamically unstable.
In 1964, H. Barker, H. Weissbach and R.D.
Smyth discovered a coenzyme of vitamin B12, 5deoxyadenosyl (5,6-dimethylbenzimidazolyl)cobinamide (I), which was not only naturally
occurring but one of the most stable
organometallic compounds to date.
2
Introduction (cont.)
The stability of the compound
was initially contributed to the
electronic effects of the corrin
ligand.
Since then, chemists have
been trying to find cobalt
complexes other than corrin
that are capable of forming
stable organometallic
derivatives.
5-deoxyadenosyl (5,6-dimethylbenzimidazolyl)
-cobinamide (I
3
Introduction (cont.)
Soon, it was discovered that
bis (dimethyl-glyoximato)
cobalt complexes display
many reactions of the cobalt
atom in the corrins.
The planar compound with
axial bases is also susceptible
to various alkylation reaction at
the axial position (Sn2
mechanism).
B= pyridine
R= Br, alkyl groups
4
Goal of Research



Synthesize a cobalt(III)
complex- bromo (pyridine)
cobaloxime; reduce Co(III) to
Co(I), which is now a
“supernucleophile”
Use Co(I) nucleophile in a
series of Sn2 reactions
involving alkyl halides
Conduct kinetic studies on the
rates of reactions and account
for the rate constants as a
function of the alkyl halide
structures.
Synthesis of Co(I)
nucleophile
EXPERIMENT
Time-resolved
Spectroscopy of
alkylations
Kinetic studies
5
Methods of Experiment
1. Synthesis of Bromo (pyridine) Cobaloxamine
CoII(H2O)6(NO3)2 + NaBr + DMG + pyridine 
2. Reduction to Co(I)- sodium borohydride reduces the Co(III) species into the Co(I)
nucleophile
3. Alkylation: The Co(I) species is a dark blue color. As alkyl halide is added to solution
and reacts with Co(I), the disappearance of the dark blue color is reflective of the
depletion of Co(I) and the progress of reaction. This colorimetric reaction may be
monitor by UV-Vis spec and used to determine the kinetics of the reactions.
6
Br
Alkyl Halides of Interest

Cl
Br
Br
Chlorobutane

Bromobutane

Bromopentane

2-Bromopropane

2-Bromobutane
Br
Br
7
Graphical Analysis of Results
2-Bromobutane
0.94
0.92
0.9
0.88
0.86
0.84
0.82
0.8
0.78
0.76
y = 3E-08x 3 - 1E-05x 2 + 0.0007x + 0.9023
R2 = 0.9756
Absorbance (480nm)
Absorbance (480nm)
2-Bromopropane
1.5
1.45
1.4
y = -9E-09x 3 + 1E-06x 2 - 0.0009x + 1.4803
R2 = 0.9768
1.35
1.3
1.25
1.2
1.15
1.1
0
50
100
150
200
250
300
0
50
Time (seconds)
Co(I) + 2-bromopropane
100
150
200
250
300
Time (seconds)
Co(I) + 2-bromobutane
Time versus Absorbance graphs
8
Graphical Analysis (cont.)
Bromobutane
0.43
0.38
3
2
y = -9E-09x + 3E-06x - 0.0012x + 0.4258
R2 = 0.9933
0.33
0.28
0.23
0.18
Absorbance (690nm)
Absorbance (690nm)
Chlorobutane
0.25
0.2
0.15
0.1
0.13
-10
y = 4E-09x 3 - 3E-06x 2 + 3E-06x + 0.2585
R2 = 0.9908
40
90
140
190
240
290
-30
20
70
Time (seconds)
120
170
220
270
Time (seconds)
Co(I) + chlorobutane
Co(I) + bromobutane
Time versus Absorbance graphs
9
Calculating the Rate Constant
•A third-degree polynomial regression was
calculated for all the graphs
Bromopentane
Absorbance (480nm)
1.05
1
y = -9E-09x 3 + 3E-06x 2 - 0.001x + 1.018
R2 = 0.9083
0.95
0.9
•The 1st derivative of the functions is
representative of the rates of reaction at
each point of the graph
•For example, the regression for
bromopentane is:
0.85
0.8
-30
20
70
120
170
220
270
Time (seconds)
A(t) = -9e-9t3 + 3e-6t2 – 0.001t
Co(I) + bromopentane
Its derivative function is:
Time versus Absorbance graph
dA(T)/dt = -27e-9t2 + 6e-6t - 0.001
10
Calculating the Rate Constant
(cont.)
• Substituting each point in time into the first derivative permits
the calculation of R(t),
the slope of the tangent at each point, which represents the rate of reaction.
• The ratio of the rate at time t and time t+Δ gives the relative rate of a reaction
and presents a consistent relationship between the rates:
R(t)/R(t+Δ) = e-kΔ = r
• The rate constant of a reaction may be obtained from the mean
r over a range of time:
k = (ln rm)/Δ
11
Results: Rate Constants of
Reactions
Alkyl Halide
Br
Br
Cl
Br
Rate Constant
(k, mole/L/sec)
Bromopentane
0.00004246
Bromobutane
0.00005352
Chlorobutane
0.0000009852
2-Bromopropane
-0.004527783
2-Bromobutane
-0.00067206
Br
12
Discussion of Results

Results obey the following chart summarizing the
reactivities of alkyl halides
R-F
R-Cl
R-Br
R-I
-----------------------------
Increasing Reactivity
KChlorobutane=0.0000009852 vs. KBromobutane=0.00005352
kbr /kcl ~54.3
13
Discussion (cont.)

In an Sn2 reaction, the energy of the transition state of a crowded
molecule is higher than that of a less crowded molecule. Hence, it is
expected that the rates of reactions decrease as the molecules are
more sterically hindered:
3° R-X
2° R-X 1° R-X CH3-X
------------------------------------------
Increasing Rate of Sn2
K2-Bromopropane= -0.004527783
K2-Bromobutane= -0.00067206
Sec-alkylcobalt complexes are highly unstable and difficult to isolate
14
Discussion (cont.)

Increasing the length of the alkyl chain by one carbon
decreased the rate constant of the reaction only minimally
KBromopentane = 0.00004246
Br
KBromobutane = 0.00005352
Br
KBromobutane/ KBromopentane = 1: 1.26
15
Conclusions

In an Sn2 mechanistic manner, Co(I) functions as a
supernucleophile in a variety of alkylation reactions.

Lengthening of the alkyl chain of the alkyl halide does
not significantly decrease the rate constant of alkylation
by Co(I)- corroborates Sn2 mechanism.

Attaching alkyl groups at the α-carbon decreases the
rate of reaction by increasing the molecule’s steric
hindrance.
16
Literature Cited
1.
2.
3.
4.
5.
6.
7.
8.
9.
R. Nast and H. Lewinsky, Z. Anorg. Allgem. Chem, 282, 210 (1955).
W. Hieber, O. Vohler, and G. Braun, Z. Naturforsch., 13b, 192 (1958).
J. Chatt and B.L. Shaw, J. Chem. Soc., 285 (1961).
H Barker, H. Weissbach and R.D. Smyth, Proc. Natl. Acad. Sci.U.S., 1093 (1958).
G.N.Schrauzer and J. Kohnle, Chem.Ber., 97, 3056 (1964).
G.N. Schrauzer, E. Deutsch, and R.J. Windgassen, J. Amer. Chem Soc, 90, 2441
(1968).
G.N. Schrauzer; E. Deutsch; Reactions of Cobalt (I) Supernucleophiles. The
Alkylation of Vitamin B12s, Cobaloximes (I) and Related Compounds, December
1968; unpublished experiments with L.P. Lee and J.W. Sibert.
A Laboratory Manual for Advanced Inorganic Chemistry, Roth J.P., The Johns
Hopkins University, Baltimore, Fall 2007.
Organic Chemistry, Fessenden, Ralph; Fessenden., Joan S.; Sixth Edition,
Brooks/Cole Publishing Company, 1998.
17