Mechanistic behaviour of alkylcobaloximes and imino-oxime complexes related to vitamin B12

Article abstract of DOI:10.1039/b816511d

The ligand substitution reactions of complexes of the type trans-[(R)Co(Chel)S]^+/0 with L, where chel = (DO)(DOH)pn = 2,2′-(1,3-diaminopropanebis(2-methyl-3-butanone)oxime), R = CH ₃, L = imidazole, pyrazole, 1,2,4-triazole and 1-met

Full text of DOI:10.1039/b816511d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Mechanistic behaviour of alkylcobaloximes and imino-oxime complexes
related to vitamin B12†
Basam M. Alzoubi,a,b Fanny Vidali,a Ralph Puchta,a,c Carlos Du¨cker-Benfer,a Alessandro Felluga,d
Lucio Randaccio,d Giovanni Tauzherd and Rudi van Eldik*a
Received 22nd September 2008, Accepted 18th December 2008
First published as an Advance Article on the web 10th February 2009
DOI: 10.1039/b816511d
The ligand substitution reactions of complexes of the type trans-[(R)Co(Chel)S]+/0 with L, where chel =
(DO)(DOH)pn = 2,2¢-(1,3-diaminopropanebis(2-methyl-3-butanone)oxime), R = CH3, L = imidazole,
pyrazole, 1,2,4-triazole and 1-methylimidazole, and S = water and MeOH, and chel = (Hdmg)2 =
bis(dimethylglyoximate), R = CH2Cl, CH2Br, and CH2I, L = thiourea and pyridine, and S = water,
were studied in detail as a function of temperature and pressure. The reported activation parameters
(DH‡, DS‡ and DV‡) support the operation of a dissociative interchange (Id) mechanism. Complexes of
the type trans-[RCo(Hdmg)2L] (R = CH2Cl, CH2Br, and CH2I; L = H2O and Py) were fully optimized
at the B3LYP/LANL2DZp level, and the structural data support the mechanistic assignment based on
the reported activation parameters. For the reaction of trans-[(CH3)2Co((DO)(DOH)pn)] with acid, the
activation parameters (DH‡, DS‡ and DV‡) were found to be 37 1 kJ mol-1, -86 3 J mol-1 K-1 and
-18.9 0.7 cm3 mol-1, respectively, and support a protonation mechanism.
two series of complexes exhibit considerable differences in their re-
Introduction
activity. In particular, the reaction of trans-[(CH2Br)Co(Hdmg)2L]
Since alkylcobaloximes have been proposed as model compounds1
for vitamin B12, a number of other cobalt complexes containing
an equatorial chelating ring have been synthesised. Several reviews
in this area have appeared.2–7 Closely related complexes to the
alkylcobaloximes are the imino-oxime8–12 derivatives of the type
trans-[(R)Co((DO)(DOH)pn)L]+ (where R = alkyl group and L =
neutral base) as shown in Fig. 1, which are formally derived
from cobaloximes by displacement of an O–H–O- fragment by
a propylene chain. Despite the close resemblance in structure, the
with CH3O- gives the methoxymethyl derivative (eqn (1)) as
follows:
CoCH2Br + CH3O- → CoCH2OCH3 + Br-
(1)
The reaction may be interpreted as a nucleophilic addition of the
CH3O- fragment to the axial carbon.13 In the attempt to reproduce
the same reaction for trans-[(CH2X)Co((DO)(DOH)pn)L]+ (X =
Cl, Br, I) derivatives, quite a different type of compound was
isolated (in almost quantitative yield), which is characterized
by the presence of a three-membered Co–N–C ring resulting
from an interligand nucleophilic addition of an equatorial N
donor to the ligated axial carbon.14 In addition, the cobalt
complexes of the type trans-[(R)2Co((DO)(DOH)pn)] or trans-
[(R¢)(R¢¢)Co((DO)(DOH)pn)]10,15–17 are well known. However, the
synthesis of the corresponding trans-dialkylcobaloximes has not
been reported so far.
It is of further interest to compare the kinetic properties of the
above-mentioned complexes, since the availability of two series
of complexes with an identical axial fragment R–Co–H2O and
different equatorial chelates, offers the opportunity to evaluate
the relative effect of the equatorial moiety on the axial positions
in terms of kinetic parameters for the substitution of coordinated
water by different nucleophiles.
Fig. 1 Structures of trans-[(R)Co((DO)(DOH)pn)L]+ and trans-[(R)-
Co(Hdmg)2L].
aInorganic Chemistry, Department of Chemistry and Pharmacy, University
of Erlangen-Nu¨rnberg, Egerlandstr. 1, 91058 Erlangen, Germany
Two fundamental aspects emerge from these studies, i.e., the
strong trans effect due to the alkyl R group, as compared to
bZarka University College, Al-Balqa Applied University, P.O. Box 313,
Zarka, Jordan
-
inorganic groups (viz. NO2 ), and the dissociative nature of the
cComputer Chemistry Center, Department of Chemistry and Pharmacy,
University of Erlangen-Nu¨rnberg, Na¨gelsbachstr., 25, 91052 Erlangen,
Germany
process. Based on activation volume data, we have recently shown
that the activation mode is strongly affected by the R group. For the
displacement of water by cysteine in a series of cobaloximes with
R = CF3CH2, Ph, Me, Bz, Et and cyclo-C5H9, the corresponding
DV‡ were found to be: 0 1, +4.5 0.5, +5.5 0.5, +5.9
dDipartimento di Scienze Chimiche, Universita` degli Studi di Trieste, 34127
Trieste, Italy
† Electronic supplementary information (ESI) available: Additional kinetic
and structural data. See DOI: 10.1039/b816511d
0.4, +6.6
0.4 and +8.9
0.7 cm3 mol-1, suggesting that a
2392 | Dalton Trans., 2009, 2392–2399
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gradual mechanistic change over from I to D along the series
of R groups from CF3CH2 to cyclo-C5H9 occurs.18 In addition,
we found that the mechanism is also affected by the nature of the
entering ligand. This results from a study of the reactions of trans-
[(R)Co(Hdmg)2S] (where R = PhCH2 or CF3CH2 and S = H2O or
MeOH) with 4-NH2Py, Py and 4-CNPy.19 By way of example for
the reaction of the PhCH2 derivative in methanol, the activation
tion is of particular interest since it represents a rare case of
heterolytic cleavage of the cobalt–carbon bond.29–31
Experimental
Materials
All chemicals were of p.a. grade and used as received. Imidazole,
pyrazole, 1,2,4-triazole and perchloric acid were purchased from
Fluka, 1-methylimidazole from Acros, and taps buffer from
Sigma. Ultra pure water was used in the preparation of all solu-
tions for kinetic measurements. Trans-[CH3Co((DO)(DOH)pn)-
H2O]ClO4,8 trans-[(CH3)2Co((DO)(DOH)pn)],10 and trans-[RCo-
(Hdmg)2H2O] (R = CH2Cl, CH2Br, and CH2I)32 were prepared
as previously reported, and were characterized by elemental
analysis, UV-vis and NMR spectroscopy. The results were in
agreement with literature data. The preparation of solutions
and all measurements were carried out in the dark because the
investigated complexes are light sensitive.
volumes +9.8
0.1, +10.3
0.3 and +13.0
0.3 cm3 mol-1
were found for the entering ligands 4-NH2Py, Py and 4-CNPy,
respectively. The same trend was also found for the reactions in
water and for the reactions of the CF3CH2 derivative in both
methanol and water.
Kinetic studies pertinent to trans-[RCo((DO)(DOH)pn)H2O]+
are less in number.20,21 However, specific studies aiming at a
comparison of the kinetic properties of alkylcobalt imino-oxime
complexes with those of alkylcobaloximes, have been reported.22
Reactions studied are of the type shown in eqn (2):
trans-[(R)Co(chel)Py]n + P→trans-[(R)Co(chel)P]n + Py
(2)
where chel represents (DO)(DOH)pn (= 2,2¢-(1,3-diamino-
propanebis(2-methyl-3-butanone)oxime) with n = +1 or (Hdmg)2
with n = 0, R represents nine different alkyl groups with varying
electronic properties, and P represents tri-n-butylphosphine or
trimethylphosphite. In the presence of an excess of the entering
ligand, the observed rate constants were found to be independent
of the nucleophile concentration, suggesting that eqn (2) follows
a limiting dissociative (D) mechanism presented by eqn (3) and
eqn (4).
Instrumentation
The pH of the solution was measured using a Mettler Delta
350 pH meter. The pH meter was calibrated with standard buffer
solutions at pH 4.0, 7.0 and 10.0. UV-vis spectra were recorded on
a Shimadzu UV-2101 spectrophotometer. Kinetic measurements
were carried out on an Applied Photophysics SX 18MV stopped-
flow instrument coupled to an online data acquisition system.
At least eight kinetic runs were recorded under all conditions,
and the reported rate constants represent the mean values. All
kinetic measurements were carried out under pseudo-first order
conditions, i.e. the ligand concentration was in at least a ten-fold
excess. Measurements at high pressure were carried out using a
home-made high pressure stopped-flow unit.33 Kinetic data were
analyzed with the OLIS KINFIT set of programs. All instruments
used were thermostated to the desired temperature ( 0.1 ◦C).
n
k
n
1
æææÆ
¨ææ
È
˘
È
˘
trans- R Co chel Py
R Co chel P + Py
(3)
( )
(
)
( )
(
)
Î
˚
Î
˚
k
-1
n
n
k
2
(4)
È
˘
È
˘
R Co chel + P ææÆtrans- R Co chel P
( )
(
)
( )
(
)
Î
˚
Î
˚
The dissociation rate constants k1 (s-1) could be obtained from
the kinetic data and turned out to be virtually identical for the two
series. Also the relative dependence on R was found to be quite
similar for the two series of complexes.
Quantum chemical methods
In order to gain further understanding on this aspect,
we report here kinetic parameters for the reaction of trans-
[(CH3)Co((DO)(DOH)pn)S]+ (where S = H2O or MeOH) with
various azoles, viz. imidazole, pyrazole, 1,2,4-triazole and 1-
methylimidazole. These data together with that previously re-
ported for methylcobaloxime23 and methylcobalt bisethylendi-
amine complexes,24 enable a comparative analysis of three systems
having different chelating rings, but the same CH3–Co–H2O axial
fragment.
Furthermore, azoles as entering nucleophiles are interesting
from a bioinorganic chemistry point of view. Recent structural re-
sults have shown that in some enzymes utilizing coenzyme B12, the
benzimidazole residue coordinated to the cobalt atom is replaced
by the imidazole moiety of a histidine of the protein chain.25–28
In the present investigation we measured rate and activation pa-
rameters for substitution reactions of trans-[(R)Co(Hdmg)2H2O]
(R = CH2Cl, CH2Br and CH2I) with thiourea and pyridine, and
supplemented it with structural data obtained from hybrid density
functional theoretical calculations. In addition, we examined a
further aspect related to the kinetic properties of imino-oxime
complexes and in particular the demethylation of the complex
trans-[(CH3)2Co((DO)(DOH)pn)] promoted by acids. This reac-
Trans-[(R)Co(Hdmg)2H2O] (R = CH2Cl, CH2Br and CH2I)
complexes were optimized at the B3LYP/LANL2DZp34–36 hy-
brid functional with pseudo-potentials on the heavy elements
and valence basis set augmented with polarisation functions.
During the optimization of the structure no constrains other
than symmetry were applied. In addition, the resulting structures
were characterised as minima, transition structures, etc., by
computation of their vibrational frequencies. The relative energies
were corrected for zero-point vibrational energies (ZPE). The
Gaussian 03 program was used for all calculations.37
Results
The kinetics of the ligand substitution eqn (5) of trans-
[(R)Co(Chel)S]+/0 were studied for different nucleophiles L to form
trans-[RCo(Chel)L]+/0, where chel = (DO)(DOH)pn, R = CH3,
L = imidazole, pyrazole, 1,2,4-triazole, and 1-methylimidazole,
and S = H2O or MeOH, and chel = (Hdmg)2, R = CH2Cl,
CH2Br, and CH2I, L = thiourea, and S = H2O. Fig. 2 and 3,
and Fig. S1, S2, S3 and S4 (ESI)† report plots of the pseudo-first-
order rate constant kobs vs. [L] for the reactions with imidazole,
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Fig. 4 Plots of kobs vs. [pyridine] for the reaction with 1 ¥ 10-3
M
trans-[(CH2Cl)Co(Hdmg)2H2O] as a function of temperature. Experimen-
tal conditions: pH = 8, I = 0.1 M, and temperature 20 (A), 25 (B), 30 (C),
35 (D) and 40 (E) ◦C.
Fig. 2 Plots of kobs vs. [pyrazole] (A), [imidazole] (B), and [1,2,4-tri-
azole] (C) for the reaction with 1 ¥ 10-4 M trans-[(CH3)Co((DO)-
(DOH)pn)H2O]+at 25.0 ◦C, pH 9 and I = 0.1 M NaClO4.
constant for precursor formation, and k the interchange rate
constant.
æKæÆ
È
Î
˘
È
˘
2
(7)
R Co Hdmg H O + L
R Co Hdmg H O .L
( )
( )
(
)
(
)
¨ææ
{
È
}
2
2
˚
Î
2
˚
k
È
( )
˘
˘
(8)
(9)
R Co Hdmg H O .L ææÆ R Co Hdmg L + H2O
(
)
( )
(
)
{
}
2
Î
2
˚
Î
2
˚
kobs = kK[L]/(1+ K[L])
The reaction of trans-[(CH3)2Co((DO)(DOH)pn)] with acid was
studied in a solution of MeOH and H2O (1 : 1) to form trans-
[(CH3)Co((DO)(DOH)pn)H2O]+. Fig. 5 shows some curvature
at high acid concentration and a zero intercept, which can be
interpreted in terms of the protonation mechanism outlined in
eqn (10) and (11), for which the [H+] dependence of kobs can
also be expressed by eqn (9), where [L] = [H+]. The protonated
intermediate formed in eqn (10) is formulated in a general way
since it is uncertain on which site of the ligands the proton is
located. The subsequent reaction step leads to the displacement
of a methyl group accompanied by the release of methane.
Fig. 3 Plots of kobs vs. [1-methylimidazole] for the reaction with 1 ¥
10-4 M trans-[(CH3)Co((DO)(DOH)pn)H2O]+ in H2O (A) and MeOH
(B) at 25.0 ◦C, pH 9 and I = 0.1 M NaClO4 in aqueous solution.
pyrazole, 1,2,4-triazole, and 1-methylimidazole in H2O and MeOH
as solvents, and also thiourea in H2O at 25 ◦C. The linear plots have
negligible intercepts, indicating that a reverse or parallel reaction
does not contribute significantly under the selected experimental
conditions. This behaviour can be expressed by the dependence
given in (6), where ka represents the second order rate constant for
the reaction.
+
+
k
0
0
a
È
˘
È
˘
trans- R Co chel S
+ L ææÆtrans- R Co chel L
+ S
(5)
(6)
( )
(
)
( )
(
)
Î
˚
Î
˚
kobs = ka[L]
The reaction between trans-[RCo(Hdmg)2H2O] (R = CH2Cl,
CH2Br, and CH2I) and pyridine was studied at different tempera-
tures for which the results, presented in Fig. 4, and Fig. S5 and S6
(ESI),† show a significant deviation from linearity at high pyridine
concentration. This behaviour can be explained in terms of an
interchange mechanism presented in eqn (7) and (8), for which the
rate law is given in eqn (9), where K represents the equilibrium
Fig.
5
Plot of kobs vs. [H+] for the reaction with 10-5
3 ¥ M
trans-[(CH3)2Co((DO)(DOH)pn)] in H2O and MeOH (1 : 1) at 25.0 ◦C
and I = 0.2 M NaClO4.
2394 | Dalton Trans., 2009, 2392–2399
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Table 1 Kinetic data and activation parameters for the reaction of trans-[(R)Co(Chel)S]+/0 with various ligands
trans-[(CH3)Co((DO)(DOH)pn)S]+
Ligand
Imidazole
Pyrazole
1,2,4-Triazole
1-Methylimidazole
Solvent
H2Oa
94.6 1.4
69
+23
MeOHb
26.0 0.2
H2Oa
105.6 1.8
71
+33
MeOHb
21.0 0.1
H2Oa
74.0 0.6
67
+17
MeOHb
20.2 0.1
H2Oa
68.0 0.4
70
+27
MeOHb
24.3 0.2
ka/M-1 s-1c
DH‡/kJ mol-1d
DS‡/J K-1 mol-1d
DV‡/cm3 mol-1cd
2
6
70
+16
1
4
2
6
72
+21
2
5
2
7
70
+16
2
5
2
5
67
+7
2
8
+2.7 0.1
+3.4 0.1
+3.7 0.1
+4.1 0.1
+3.4 0.1
+4.4 0.1
+2.9 0.1
+4.3 0.1
trans-[(R)Co(Hdmg)2H2O]
R
CH2Cl
CH2Br
CH2I
Ligand
Thiourea
17.8 0.3
72 1d
Pyridine
Thiourea
7.3 0.2
75 1d
Pyridine
Thiourea
6.6 0.1
73 3d
Pyridine
4.9 0.6f
78 2d
ka/M-1 s-1e
13.7 1.1f
5.9 0.8f
76 2d
DH‡/kJ mol-1
DS‡/J K-1 mol-1
DV‡/cm3 mol-1h
71 1d
+14 3d
+19 3d
+6.1 0.4d
+23 4d
+10.4 0.5d
+25 5d
+6.0 0.2g
+16 10d
+6.4 0.2d
+31 6d
+4.9 0.3g
+5.2 0.2g
a [CoIII] = 1 ¥ 10-4 M, pH = 9, I = 0.1 M. b [CoIII] = 1 ¥ 10-4 M. c T = 25 ◦C. d The activation parameters were calculated for the second-order rate
constants. e [CoIII] = 1 ¥ 10-3 M, pH = 8, I = 0.1 M, T = 25 ◦C. f This value represents the product kK. g The activation volumes were calculated for the
limiting rate constant k reached at high nucleophile concentration, see Discussion. h [CoIII] = 1 ¥ 10-3 M, pH = 8, I = 0.1 M, T = 25 ◦C, [TU] = 0.8 M,
[Py] = 0.64 M.
linear relationships for these reactions as shown in Fig. S7, S8,
K
+
ææÆ
¨ææ
È
˘
trans- CH Co DO DOH pn + H
(
)
(
)(
)
(
)
3
Î
2
˚
S9, S10 and S11 (ESI).† Kinetic data for the reaction of trans-
[(CH3)2Co((DO)(DOH)pn)] with acid was studied in a mixture of
MeOH and H2O (1 : 1) and are summarized in Table 2. Fig. S12
(ESI)† shows the relationship between lnkobs and pressure for the
reaction of trans-[(CH3)2Co((DO)(DOH)pn)] with acid.
The B3LYP/LANL2DZp computed structures of trans-
[(R)Co(Hdmg)2H2O], where R = CH2Cl, CH2Br and CH2I, are
shown in Fig. 6 and Fig. S13 (ESI),† and the bond lengths of trans-
[(R)Co(Hdmg)2H2O] with different alkyl groups are summarized
in Table 3.
(10)
(11)
trans- CH Co DO DOH pn . H+
È
˘
)
(
)
(
)(
)
(
)
(
3
2
Î
˚
k
trans- CH Co DO DOH pn . H+ + H2O ææÆ
È
˘
(
)
(
)(
)
(
)
(
)
3
2
Î
˚
È
˘
trans- CH Co DO DOH pn H O + CH4
(
)
(
)(
)
(
)
3
2
Î
2
˚
Tables 1 and 2, and Tables S1, S2, S3 and S4 (ESI)† summarize
the kinetic data obtained as a function of temperature and pressure
for the reaction of trans-[(R)Co(Chel)S]+/0 with L, where chel =
(DO)(DOH)pn, R = CH3, L = imidazole, pyrazole, 1,2,4-triazole
and 1-methylimidazole, and S = H2O and MeOH, and where
chel = (Hdmg)2, R = CH2Cl, CH2Br, and CH2I, L = thiourea
and pyridine, and S = H2O. Plots of lnkobs vs. pressure gave good
˚
Table 3 Bond lengths (A) in trans-[(R)Co(Hdmg)2H2O] and trans-
[(R)Co(Hdmg)2Py] complexes calculated by B3LYP/LANL2DZp
trans-[(R)Co(Hdmg)2H2O]
Table 2 Kinetic data for the reaction of trans-[(CH3)2Co-
((DO)(DOH)pn)] with acid as function of temperature and pressure
˚
˚
R
Co–R/A
Co–OH2/A
T/◦C
kK/M-1 s-1ab
CI3
CF2H
CF3
CCl3
CBr3
OH2
2.09
1.94
1.95
2.03
2.05
—
1.96
1.94
1.96
1.96
1.96
2.11
2.15
2.11
2.11
2.11
1.96
2.15
2.16
2.14
2.14
2.13
5
10
15
20
25
20.1 1.2
26.3 0.8
35.1 1.2
46.6 1.2
63.4 1.6
CH3
CH2F
CH2Cl
CH2Br
CH2I
Pressure/MPa
10
50
90
130
74
95
134
181
37
-86
-18.9 0.7
2
6
4
8
1
3
trans-[(R)Co(Hdmg)2Py]
˚
˚
DH‡/kJ mol-1c
DS‡/J mol-1 K-1c
DV‡/cm3 mol-1de
R
Co–R/A
Co–Py/A
CH3
1.98
1.96
1.98
1.99
1.99
2.13
2.14
2.12
2.11
2.11
CH2F
CH2Cl
CH2Br
CH2I
a Overall second-order rate constant for eqn (10) and (11). b Solvent is H2O
and MeOH (1 : 1), I = 0.2 M. c [H+] = 2.5 ¥ 10-2 M. d [H+] = 1.25 ¥ 10-2
M. e T = 25 ◦C.
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an interchange of ligands controlled by the breakage of the
Co–H2O bond via an Id mechanism. The second-order rate con-
stants for the substitution of H2O in trans-[(R)Co(Hdmg)2H2O]
by pyridine follow the sequence 1.49 ¥ 104 (isopropyl), 1.22 ¥
103 (phenylethyl), 1.17 ¥ 103 (ethyl), 1.65 ¥ 102 (2-methoxyethyl),
1.64 ¥ 102 (3-cyanopropyl), 1.16 ¥ 102 (methyl), 23.4 (1-propyl),45
13.7 (chloromethyl), 5.9 (bromomethyl), 4.9 (iodomethyl), 3.17 ¥
10-1 (2,2,2-trifluoroethyl)19 and 5.59 ¥ 10-2 M-1 s-1 (cyanomethyl)45
at 25 ◦C. This sequence suggests that the trans effect of the R
group decreases in the order isopropyl > phenylethyl > ethyl
> 2-methoxyethyl > 3-cyanopropyl > methyl > 1-propyl >
chloromethyl > bromomethyl > iodomethyl > trifluoroethyl >
cyanomethyl.
The temperature and pressure dependence of the substitution
of trans-[(R)Co(Hdmg)2(H2O)] (R = CH2Cl, CH2Br and CH2I)
by thiourea and pyridine for which the data are summarized
in Table 1, and Tables S3 and S4 (ESI),† indicate that the
second-order rate constants for the reaction with thiourea are
characterized by small positive activation entropy and volume
data. These support the operation of a dissociative interchange
(Id) substitution mechanism. For the reactions with pyridine, the
precursor formation constants K for eqn (7) (see Tables S4 and
S5†) are relatively small, approx. 1 M-1, and the thermodynamic
parameters indicate that in all three cases precursor formation is an
exothermic process, i.e. K decreases with increasing temperature.
The values of DS◦ are significantly negative, especially for R =
CH2Cl and CH2I, indicating the higher order of the intermediate,
which is ascribed to weak p-interaction of the pyridine nucleophile
with the ligand sphere prior to substitution of water by pyridine.
The pressure dependence of this reaction measured at a high
pyridine concentration, i.e. where eqn (9) simplifies to kobs = k,
the rate constant for the interchange process (eqn (8)), reveals
activation volumes between 5 and 6 cm3 mol-1, typical for the
operation of an Id mechanism.
The solvent has a significant effect on the second-order
rate constant of the ligand substitution reaction. The sec-
ond order rate constant for substitution of the solvent in
trans-[(CH3)Co((DO)(DOH)pn)S]+ by imidazole, pyrazole, 1,2,4-
triazole, and 1-methylimidazole were found to fall in the range
68–106 and 20–26 M-1 s-1 at 25 ◦C for S = H2O and MeOH,
respectively. The slower reaction in methanol could be due to
the partial protonation of an imidazole nitrogen in methanol as
solvent,19,23 and the fact that we are dealing with a totally different
leaving group. The values of the second-order rate constants
in MeOH are all independent of the nature of the entering
nucleophile.
Fig.
6
DFT calculated B3LYP/LANL2DZp structure of trans-
[(CH2Cl)Co(Hdmg)2H2O].
Discussion
Second order rate constants for substitution of trans-
[(CH3)Co((DO)(DOH)pn)H2O]+ by the series of entering ligands
in Table 1 at 25 ◦C do not depend strongly on the nature of
the entering ligand, although the pKBH+ values for imidazole,
pyrazole, 1,2,4-triazole, and 1-methylimidazole are 7.2, 2.5, 2.3
and 7.2, respectively.38–40 This is typical for a process that is
dissociatively activated, either via a dissociative interchange (Id)
or limiting dissociative (D) mechanism. Thus, bond breakage in
these reactions seems to largely control the rate and nature of
the substitution mechanism. Moreover, it can be seen from the
rather similar kinetic data for the reactions with imidazole and 1-
methylimidazole that tautomerism present in the imidazole ligand
but not in 1-methylimidazole, does not affect the coordination
process. The activation volume data support in each case the
operation of an Id mechanism.
The kinetic data in Table 1 for substitution of trans-
[(R)Co(Hdmg)2(H2O)] (R = CH2Cl, CH2Br and CH2I) by thiourea
and pyridine show that the overall second order rate constants do
not vary drastically on changing the nature of the halomethyl
derivatives. This observation is opposite to that expected on
the basis of the electronegativity of the halomethyl group. A
comparison of other alkyl groups with the halomethyl group
shows that the trend in the second order rate constants depends
on the trans effect and steric influence as seen for the reactions
of trans-[(R)Co(Hdmg)2(H2O)] (R = C2H5, CH2C6H5, CH3, CH2I
and CH2CF3) for which the second rate constants are 2042, 1445,
132, 8.9 and 0.9 M-1 s-1, respectively.41,42 The second-order rate
constant for the reaction of trans-[(CH2Cl)Rh(Hdmg)2(H2O)] with
thiourea is higher than in ◦the case of cobaloxime complexes, viz.
312 and 10.4 M-1 s-1 at 20 C, respectively.43
In terms of rate constants and mechanism, the reaction of
trans-[(CH3)Co(Hdmg)2S] with imidazole (S = H2O and MeOH),
pyrazole and 1,2,4-tiazole (S = H2O),23 exhibits a well correlated
behaviour with the data for the corresponding reactions with trans-
[(CH3)Co((DO)(DOH)pn)S]+ as summarized in Table 4. For both
trans-[(CH3)Co(en)2H2O]2+ and aqua-cobalamin, the reactions
with certain azoles are more complicated, viz. the reaction with
1,2,4-triazole occurs in a biphasic process, where the second slower
step was found to be independent of the triazole concentration.
It was suggested that in both cases 1,2,4-triazole coordinates
to the Co centre through N-1 or N-2 donors in a fast step,
followed by a linkage isomerization to bind through N-4.24,46
In addition, kinetic data for N-acetylimidazole binding in both
The non-linear dependence of kobs on the entering ligand
concentration has before been reported for the reaction of trans-
[(R)Co(Hdmg)2(H2O)]19 and aqua-cobalamin44 with substituted
pyridines. The observed saturation kinetics was ascribed to the
formation of a precursor adduct that involves both the attacking
nucleophile and cobaloxime or cobalamin, in line with a disso-
ciative interchange mechanism. The relatively strong precursor
formation was ascribed to a p-interaction of the pyridine ligand
with the coordination sphere of the metal complex. This results
in the effective binding of the entering nucleophile close to the
metal centre during precursor formation, which is followed by
2396 | Dalton Trans., 2009, 2392–2399
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Table 4 Kinetic data and activation parameters for the reaction of trans-
[(CH3)Co(Hdmg)2S] with imidazole, pyrazole, and 1,2,4-triazole for S =
H2O and MeOHa
Table 5 Comparison of structural, pKa and E1/2 data for trans-
[(CH3)Co((DO)(DOH)pn)H2O]+ and trans-[(CH3)Co(Hdmg)2H2O]
trans-[(CH3)Co((DO)-
trans-[(CH3)Co-
(Hdmg)2H2O]
Ligand
Imidazoleb Pyrazoleb 1,2,4-Triazoleb Imidazolec
(DOH)pn)H2O]+
ka/M-1 s-1
70
74
+40
1
2
8
91
79
+56
2
2
7
71
72
+31
1
3
9
23.6 0.2
a
a
˚
Co–C/A
1.977(4)
2.103(3)
+2.0
1.990(5)
2.058(3)
-4.0
DH‡/kJ mol-1
DS‡/J mol-1 K-1
DV‡/cm3 mol-1
74
+29
2
5
˚
Co–O/A
a/◦
ab
+3.7 0.2 +4.5 0.4 +4.0 0.1
+7.3 0.1
ab
˚
d/A
0.01
0.00
pKa
12.31c
-1.30
12.68d
-2.2
a Data taken from ref. 23. b S = H2O. c S = MeOH.
E1/2 vs.
SCEe
trans-[(CH3)Co(en)2H2O]2+ and aqua-cobalamin are consistent
with the presence of precursor formation preceding the slow
coordination step. In trans-[(CH3)Co(en)2H2O]2+, the inter-
mediate is suggested to involves hydrogen bonding between
N-acetylimidazole and equatorial ethylenediamine. In aqua-
cobalamin, hydrogen bonding involves the amide side-chain.
To gain further information on the cis effect of the equatorial
ligand, we examined the axial substitution rates. The cis effect
is well documented in the case of non-organometallic complexes
and generally correlated with the extent of unsaturation of the
equatorial ring. For instance, it has been found that the rate
constants for the hydrolysis of CoIII(N4) complexes increase as
the extent of unsaturation of the equatorial ligand is increased
along the series 1,4,8,11-tetraazacyclotetradecane < 1,4,8,11-
tetraazacyclotetradecane-1,7-diene < cobaloxime.47 The substitu-
tion reaction of water in trans-[CoIII(salen)(H2O)2]+ represents a
case of extreme labilisation of the non-organometallic complexes.
The second order rate constants for water substitution in trans-
a Taken from ref. 61. b a is the bending angle between the two equatorial
halves and d the displacement of the Co out of the four N-donor plane;
positive values for a and d indicate bending towards methyl group and
displacement towards H2O, respectively. c From ref. 21. d From ref. 62.
e Values for the half-wave potential vs. SCE for the Co(III)–Co(II) reduction
at -26 ◦C, solvent CH3CN, data taken from ref. 63.
are of much benefit to interpret the kinetic behaviour. This is due
to the different formal charges of the two complexes. Thus, the
lower pKa of the imino-oxime mono-cation with respect to the
neutral cobaloxime complex may be due to electrostatic repulsion
that is larger in the former complex, and consequently these values
cannot be taken as a measure of the cobalt-water bond strength.
Moreover, it has been shown that the Co(III/II) half-wave potential
is strongly affected by the formal charge of the complex.50
The bond lengths depend on the nature of the alkyl group
and the equatorial ligands, the hybridization of the carbon atom
attached to the cobalt centre, steric interactions between the
axial alkyl and the equatorial ligands (steric cis influence), and
the nature of the equatorial ligands.3,51 The X-ray structure data
[CoIII(salen)(H2O)2]+ by various nucleophiles,48 viz. HSO3 , S2O3
,
-
2-
Py, NCO- and thiourea, all fall in the range (5–32) ¥ 103 M-1 s-1,
that is one order of magnitude faster than those of aquacobalamin.
This enhancement in the rate constants was attributed not only to
the extensive unsaturation of the equatorial ring causing cobalt to
lose its d6 character, but also to a possible deviation of the planarity
of the equatorial ring, which is a characteristic feature of the Co-
salen complexes.49 The idea is also accepted that a co-operative
effect due to the simultaneous presence of an unsaturated chelating
system and a strong activating axial ligand may lead to five-
coordinate species as in the case of alkyl cobalt salen and acacen
(bis(acetylacetone)ethylenediimine) complexes.50 These complexes
in the presence of strong nucleophiles produce six-coordinate
species, probably formed in a diffusion-controlled reaction.
Data from the present study permit a comparison of the
cis effect for three hexa-coordinate aqua complexes with dif-
ferent equatorial chelates and the same strong trans acti-
vating group, viz. trans-[(CH3)Co((DO)(DOH)pn)H2O]+, trans-
[(CH3)Co(Hdmg)2H2O] and trans-[(CH3)Co(en)2H2O]2+. Second
order rate constants for the substitution of water by azoles
in trans-[(CH3)Co(Hdmg)2H2O] were found to be 91, 71 and
70 M-1 s-1 at 25 ◦C for pyrazole, 1,2,4-triazole and imidazole,
respectively.24 These values are very close to those found for
the reactions of the corresponding nucleophiles with trans-
[(CH3)Co((DO)(DOH)pn)H2O]+, viz. 106, 75 and 95 M-1 s-1,
respectively. These values indicate a similar contribution of the
two different equatorial ligands in assisting the axial substitution
and are in agreement with earlier observations.22 Available ther-
modynamic and structural data are collected in Table 5. In this
case, neither the pKa values nor the Co(III/II) half-wave potentials
show that the bond lengths of Co–OH2 are 1.955(11),3 2.058(3),52
23
2.14(2)53 and 2.153(6) A for trans-[(Br)Co(Hdmg)2H2O], trans-
˚
[(CH3)Co(Hdmg)2H2O], trans-[(CH3)Co((DO)(DOH)pn)H2O]+,
and trans-[(CH3)Co(en)2H2O]2+, respectively. The structural data
suggest a greater flexibility of the ((DO)(DOH)pn) moiety with
respect to (Hdmg)2. However, the expected enhancement in the
rate constants of the imino-oxime complexes over the cobaloximes
was not experimentally confirmed. The only difference in the
two series of complexes lies in their activation volumes. The
values for the reactions of pyrazole, 1,2,4-triazole and im-
idazole with trans-[(CH3)Co((DO)(DOH)pn)H2O]+ and trans-
[(CH3)Co(Hdmg)2H2O] (in brackets) were found to be +3.7 0.1
(+4.5 0.4), +3.4 0.1 (+4.0 0.1) and +2.7 0.1 (+3.1 0.2) cm3
mol-1, respectively. Although not large, these differences suggest
that an enhanced dissociative character of the process operates in
the cobaloximes with respect to the imino-oxime complexes. The
activation volumes for the reaction of trans-[(R)Co(Hdmg)2H2O]
(R = CH2Cl, CH2Br and CH2I) with pyridine and thiourea
clearly support the operation of a dissociative interchange (Id)
mechanism.
The DFT calculated bond lengths in Table 3 and the avail-
able structural data are in reasonable agreement for trans-
[(CH3)Co(Hdmg)2H2O] and trans-[(CH3)Co(Hdmg)2Py], viz. Co–
OH2 = 2.15 and 2.05852 A, respectively, and Co-N(Py) = 2.13
˚
and 2.06854 A, respectively. More recent structural data for closely
˚
related complexes55 report Co–OH2 and Co–N(Py) bond lengths
that are in almost perfect agreement with the calculated bond
lengths reported in Table 3. The calculated bond lengths for
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Co–O in trans-[(R)Co(Hdmg)2H2O], where R = CH2Cl, CH2Br
and CH2I, have very similar values which account for the similarity
in the second order rate constants. Therefore, the lability of
coordinated water in these complexes is the same, which further
supports the dissociative character of the reaction mechanism.
Apparently, the difference in the electronegativity and s-donor
property of CH2X (X = Cl, Br and I) is not large enough to affect
the rate of the ligand substitution reactions of these complexes.
For the substitution of water by imidazole in trans-
[(CH3)Co(en)2H2O]2+, the following kinetic parameters were
effects play an important role. This is coherent with a bimolecular
process, where the transition state may be seen as a mono-cationic
species since it forms through the interaction of the neutral
complex with a proton.
A question that arises from the fact that Co((DO)(DOH)pn)
complexes are capable to generate dialkyl complexes, but not
so in the case of the corresponding cobaloximes. This is rather
surprising since the data reported in the first part of the paper
concerning axial ligand substitution clearly indicate that the
transmission of electronic and steric effects exerted by the methyl
group in the trans position occur to about the same extent in
the two types of complexes. In order to justify this apparent
incongruence, an attractive hypothesis can be made on the basis
of kinetic arguments. It can be imagined that cobaloximes are
also capable to generate (short-lived) dialkyl derivatives. The
reaction of these mono-anionic species with protons could involve
neutral transition states. Such a process must to be characterized
by positive activation entropies, which are associated with fast
reactions rendering the detection of the dimethyl-cobalt complex
to be problematic. Similarly to that observed for cobaloximes,
there is no evidence for the occurrence of dialkyl corrinoids.56–58
However, it has been observed that the reaction of the Co(I)
form B12s with methyl iodide produces in addition to methyl-
cobalamin (the so called methylcobalamin b) a small amount of
methylcobalamin a, a distereoisomer in which the methyl group
replaces the 3,5-dimethyl benzimidazole in the “lower” position
and water occupies the “upper” axial position. Later,59,60 several
other alkylcobalamins with more complicated alkyl groups have
been characterized both in the a and b form, and facile a/b
diastereomerism has been demonstrated to occur in several cases.
Although, evidence for dialkyl cobalamines has not been reported,
it is reasonable to hypothesize the existence of these complexes as
unstable intermediates.
◦
reported:24 k = 198 13 M-1 s-1 at 25 C, DH‡ = 53 2 kJ
mol-1, DS‡ = -22 7 J K-1 mol-1 and DV‡ = +4.7 0.1 cm3
mol-1. The bond length of Co–OH2 in trans-[(CH3)Co(en)2H2O]2+
is longer than that in trans-[(CH3)Co((DO)(DOH)pn)H2O]+ and
trans-[(CH3)Co(Hdmg)2H2O]. The stoichiometric mechanism that
is more complicated in the substitution reaction of trans-
[(CH3)Co(en)2H2O]2+ with respect to the other two complexes,
makes it difficult to compare the kinetic parameters for these
complexes. Nevertheless, there is little doubt that the same Id
mechanism is operative in all the examined cases. On the basis
of the available, it can be concluded that the cis effect exerted by
the three equatorial systems is of comparable entity and that the
main factor governing the axial substitution reactions is the trans
effect of the strong electron donating methyl group.
Another aspect of the chemistry of imino-oxime com-
plexes that has been considered, concerns the reaction
of trans-[(CH3)2Co((DO)(DOH)pn)] with protons. It is well
known that mono-alkylated complexes such as trans-
[(R)Co((DO)(DOH)pn)H2O]+ and trans-[(R)Co(Hdmg)2H2O] do
not undergo dealkylation in acidic media. However, dialkyl imino-
oxime complex in dilute acid generates the mono alkylaqua
complex and methane according to eqn (10) and (11). The
products of the reaction suggest that a heterolytic cleavage of
the cobalt–carbon bond takes place. In order to elucidate the
reaction mechanism, we studied the reaction as function of the
acid concentration, temperature and pressure. The reaction was
studied under nitrogen atmosphere and in absence of light, since
it is well known that cobalt-alkyl complexes are light sensitive.
The kinetic data reported for the protonation reaction sequence,
(eqn (10) and (11)), presented in Fig. 5 exhibits some curvature
from which K = 0.56 0.06 M-1 and k = 120 12 s-1 were esti-
mated. The overall second-order rate constant, kK, for protona-
tion and release of methane in trans-[(CH3)2Co((DO)(DOH)pn)]
is 68 M-1 s-1 (see Table 2), in satisfactory agreement with the
previously reported value of 40 M-1 s-1 at 25 ◦C and 0.10 M
(LiClO4) ionic strength.31 The curvature in Fig. 5 is too small to
allow a detailed study of the temperature and pressure dependence
of both K and k. We, therefore, selected a low [H+] (≤ 0.025 M)
where eqn (9) reduces to kobs = kK[H+], to study the temperature
and pressure dependence of the overall second-order rate constant
(see Table 2). The low value of DH‡ (37 1 kJ mol-1) suggests
that little energy is required for the proton-assisted elongation of
the cobalt–carbon bond on going from the ground state to the
transition state. This is expected on the basis of the strong trans
effect exerted by the non-labile methyl group on the outgoing
methyl group. However, the reaction is slowed down because of
the unfavourable (negative) value of DS‡ (-86 3 J mol-1 K-1).
The negative values of both DS‡ and DV‡ (-18.9 0.7 cm3 mol-1)
are consistent with a polar transition state, where electrostriction
Acknowledgements
The authors gratefully acknowledge financial support from CNR
(Rome), MIUR (Rome), PRIN N◦ MMO3185591 and the
Deutsche Forschungsgemeinschaft within SFB 583. Prof. Tim
Clark, Computer Chemistry Center, is kindly acknowledged for
his support. BMA thanks the Higher Council for Science and
Technology in Jordan, and Al-Balqa Applied University for their
support.
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