Synthesis and characterization of chlorinated alkenylcobaloximes to probe the mechanism of vitamin B12-catalyzed dechlorination of priority pollutants

Article abstract of DOI:10.1021/ic011023x

Vitamin B₁₂ catalyzes the reductive dechlorination of several ubiquitous pollutants such as perchloroethylene (PCE) and trichloroethylene (TCE). Several mechanisms have been proposed for these transformations, some of which involve the intermediacy of chlorinated vinylcobalamins. To evaluate the currently unknown chemical and physical properties of such species, various chlorinated vinylcobaloxime complexes [cobaloxime = bis(dimethylglyoximato)-(pyridine)cobalt(III)] were prepared and characterized. X-ray structures are reported for (cis-1,2-dichloroethenyl)-cobaloxime (4), (cis-monochloroethenyl)cobaloxime (5), (α-chloroethenyl)cobaloxime (6), and vinylcobaloxime (7), and the reactivities of these isolated complexes were investigated. They were stable in the presence of ethanolic NaBH₄ unless external cobaloximes were added. The cob(I)aloxime formed under the latter conditions promoted the conversion of 4 to 5 and 6, and of 5 and 6 into 7. Mechanistic studies of these transformations are consistent with a pathway in which the conversion of 4 into 5 and 6 takes place via chloroacetylene as an intermediate, and the conversion of 6 to 7 involves vinyl chloride as an intermediate. Cyclic voltammetry on the chlorinated vinylcobaloximes resulted in irreversible reduction waves, with 4 displaying the least negative and 7 the most negative peak potential: These results are discussed in the context of the B₁₂-catalyzed reductive dechlorination of PCE and TCE.

Full text of DOI:10.1021/ic011023x

Inorg. Chem. 2002, 41, 393−404
Synthesis and Characterization of Chlorinated Alkenylcobaloximes To
Probe the Mechanism of Vitamin B12-Catalyzed Dechlorination of Priority
Pollutants
Kevin M. McCauley, Scott R. Wilson,† and Wilfred A. van der Donk*
Department of Chemistry, UniVersity of Illinois at Urbana-Champaign,
600 South Mathews AVenue, Urbana, Illinois 61801
Received October 2, 2001
Vitamin B catalyzes the reductive dechlorination of several ubiquitous pollutants such as perchloroethylene (PCE)
12
and trichloroethylene (TCE). Several mechanisms have been proposed for these transformations, some of which
involve the intermediacy of chlorinated vinylcobalamins. To evaluate the currently unknown chemical and physical
properties of such species, various chlorinated vinylcobaloxime complexes [cobaloxime ) bis(dimethylglyoximato)-
(pyridine)cobalt(III)] were prepared and characterized. X-ray structures are reported for (cis-1,2-dichloroethenyl)-
cobaloxime (4), (cis-monochloroethenyl)cobaloxime (5), (R-chloroethenyl)cobaloxime (6), and vinylcobaloxime (7),
and the reactivities of these isolated complexes were investigated. They were stable in the presence of ethanolic
NaBH unless external cobaloximes were added. The cob(I)aloxime formed under the latter conditions promoted
4
the conversion of 4 to 5 and 6, and of 5 and 6 into 7. Mechanistic studies of these transformations are consistent
with a pathway in which the conversion of 4 into 5 and 6 takes place via chloroacetylene as an intermediate, and
the conversion of 6 to 7 involves vinyl chloride as an intermediate. Cyclic voltammetry on the chlorinated
vinylcobaloximes resulted in irreversible reduction waves, with 4 displaying the least negative and 7 the most
negative peak potential. These results are discussed in the context of the B -catalyzed reductive dechlorination of
12
PCE and TCE.
Introduction
predominantly (∼95%) cis-dichloroethylene (DCE),10 which
is further reduced over time to ethene and ethane.10-12 At
present, the mechanistic details of these transformations are
unclear. The strong reductants that are required for turnover
suggest that the CoI oxidation state of B12 (cob(I)alamin) is
essential for catalysis. Assuming that cob(I)alamin indeed
initiates the dechlorination process, at least five different
mechanisms can be written on the basis of the vast literature
Perchloroethylene (PCE) is an abundant pollutant that
ranks high on the priority list of the U.S. Environmental
Protection Agency, as its discharge into subsurface environ-
ments has extensively contaminated soils and aquifers.1,2 PCE
causes liver and kidney tumors in animal studies3,4 and has
been classified as a probable human carcinogen. Several
recent reports have indicated that vitamin B12 can function
as a catalyst for the reductive dechlorination of PCE (eq 1),
suggesting B12 may be used for the decontamination of
polluted environments.5-9 Initially, this process produces
(5) Gantzer, C. J.; Wackett, L. P. EnViron. Sci. Technol. 1991, 25, 715-
722.
(6) Lesage, S.; Brown, S.; Millar, K. Ground Water Monit. Rem. 1996,
16, 76-85.
(7) Glod, G.; Angst, W.; Holliger, C.; Schwarzenbach, R. P. EnViron.
Sci. Technol. 1997, 31, 253-260.
* To whom correspondence should be addressed. Tel: (217) 244 5360.
Fax: (217) 244 8024. E-mail: vddonk@uiuc.edu.
(8) Burris, D. R.; Delcomyn, C. A.; Smith, M. H.; Roberts, A. L. EnViron.
Sci. Technol. 1996, 30, 3047-3052.
† To whom inquiries regarding X-ray should be addressed. E-mail:
srwilson@uiuc.edu.
(9) Burris, D. R.; Delcomyn, C. A.; Deng, B. L.; Buck, L. E.; Hatfield,
K. EnViron. Toxicol. Chem. 1998, 17, 1681-1688.
(10) Glod, G.; Brodmann, U.; Angst, W.; Holliger, C.; Schwarzenbach, R.
P. EnViron. Sci. Technol. 1997, 31, 3154-3160.
(11) Semadeni, M.; Chiu, P. C.; Reinhard, M. EnViron. Sci. Technol. 1998,
32, 1207-1213.
(1) Squillace, P. J.; Moran, M. J.; Lapham, W. W.; Price, C. V.; Clawges,
R. M.; Zogorski, J. S. EnViron. Sci. Technol. 1999, 33, 4176-4187.
(2) Doherty, R. E. EnViron. Forensics 2000, 1, 69-81.
(3) National Cancer Institute Bioassay of Tetrachloroethylene for Possible
Carcinogenicity; NCI: Bethesda, MD, 1977.
(4) National Toxicology Program Toxicology and Carcinogenesis Studies
of Tetrachloroethylene; NTP: Research Triangle Park, NC, 1986.
(12) Lesage, S.; Brown, S.; Millar, K. EnViron. Sci. Technol. 1998, 32,
2264-2272.
10.1021/ic011023x CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/04/2002
Inorganic Chemistry, Vol. 41, No. 2, 2002 393

McCauley et al.
Scheme 1
on the chemistry of vitamin B12 (Scheme 1).13 Four of these
(pathways A-D) involve the intermediacy of chlorinated
alkyl or alkenylcobalamins formed by attack of the strongly
nucleophilic14,15 cob(I)alamin on the electron deficient alkene,
possibly after initial π-complexation.16 Mechanisms A-D
differ in the manner in which chloride is eliminated and the
Co-C bond is broken. In pathway A, this occurs via a
straightforward â-elimination from tetrachloroethylcobalamin
(1). Much precedent exists in the literature for elimination
of good nucleofuges from the â-carbon of alkylcobal-
amins.17,18 Alternatively, R-elimination of chloride from the
carbenoid 1 could generate a carbene that can undergo
C-H19 or C-Cl20 bond insertion to provide the product
(mechanism B). Carbene species have been proposed previ-
ously in rearrangements of putative R,R-dichloroorganocobalt
intermediates.21-23 A third mechanism involves dehydro-
chlorination of intermediate 1 providing (trichloroethenyl)-
cobalamin (2). Either homolytic cleavage of the Co-C bond
followed by reduction of the resulting vinyl radical (route
C) or reductive dechlorination of 2 by electron transfer from
one of the strong reductants in the reaction mixture (pathways
D) could produce the observed product.
energy electron scattering studies have provided vertical
electron attachment energies for PCE.24 These measurements
indicate that the radical anion is unstable and has an energy
that lies above the ground state of the neutral alkene. The
electron initially attaches to the π* orbital, followed by
molecular distortions that lead to occupation of the σ* orbital
of the C-Cl bonds resulting in chloride anion elimination.25-29
The trichlorovinyl radical so produced may be converted to
TCE by either hydrogen atom abstraction from a suitable
source or reduction to the anion followed by protonation.30
We31 and others7 have recently provided support for
mechanism E in the first step of dechlorination of PCE with
cob(I)alamin. Chlorinated ethenylcobalamins, however, are
formed in dechlorination reactions as evidenced by their
detection by mass spectrometry.12 Furthermore, Schwarzen-
bach and co-workers have postulated their intermediacy in
the B12-catalyzed dechlorination of cis- and trans-dichloro-
ethylenes (DCEs).10 What the roles are of these complexes,
whether they are catalytic intermediates or dead-end products,
is not known. If they are catalytic intermediates, the question
arises as to how the Co-C bond is cleaved to release the
products and regenerate the catalyst. In an initial study to
address some of these issues, we report here the synthesis
of several chlorinated alkenylcobaloximes and their spec-
(20) Patterson, E. V.; Cramer, C. J.; Truhlar, D. G. J. Am. Chem. Soc.
2001, 123, 2025-2031.
(21) Wood, J. M.; Kennedy, F. S.; Wolfe, R. S. Biochemistry 1968, 7,
1707-1719.
(22) Zanette, D.; Nome, F. J. Org. Chem. 1979, 44, 2308-2309.
(23) Nome, F.; Rezende, M. C.; de Souza, N. S. J. Org. Chem. 1983, 48,
5357-5359.
In contrast to mechanisms A-D, which all involve alkyl-
or alkenylcobalamins, mechanism E features a one-electron
transfer from cob(I)alamin to PCE. In the gas phase, low-
(24) Burrow, P. D.; Modelli, A.; Chiu, N. S.; Jordan, K. D. Chem. Phys.
Lett. 1981, 82, 270-6.
(25) Burrow, P. D.; Aflatooni, K.; Gallup, G. A. EnViron. Sci. Technol.
2000, 34, 3368-3371.
(13) Dolphin, D. B12; John Wiley & Sons: New York, 1982.
(14) Schrauzer, G. N.; Deutsch, E.; Windgassen, R. J. J. Am. Chem. Soc.
1968, 90, 2441-2442.
(26) Johnson, J. P.; Christophorou, L. G.; Carter, J. G. J. Chem. Phys. 1977,
67, 2196-215.
(15) Schrauzer, G. N.; Deutsch, E. J. Am. Chem. Soc. 1969, 91, 3341-
(27) Alajajian, S. H.; Chutjian, A. J. Phys. B: At. Mol. Opt. Phys. 1987,
20, 2117-30.
3350.
(16) Brown, K. L.; Ramamurthy, S. Organometallics 1982, 1, 413-15 and
references therein.
(17) Hogenkamp, H. P. C. In B12; Dolphin, D., Ed.; John Wiley: New
York, 1982; Vol. 1.
(28) Marawar, R. W.; Walter, C. W.; Smith, K. A.; Dunning, F. B. J. Chem.
Phys. 1988, 88, 176-80.
(29) Walter, C. W.; Smith, K. A.; Dunning, F. B. J. Chem. Phys. 1989,
90, 1652-6.
(18) Fischli, A. HelV. Chim. Acta 1982, 65, 1167-1190 and references
(30) Nonnenberg, C.; van der Donk, W. A.; Zipse, H. Submitted for
publication.
therein.
(19) Shustov, G. V.; Liu, M. T. H.; Rauk, A. J. Phys. Chem. 1997, 101,
2509-2513.
(31) Shey, J.; van der Donk, W. A. J. Am. Chem. Soc. 2000, 122, 12403-
12404.
394 Inorganic Chemistry, Vol. 41, No. 2, 2002

Chlorinated Alkenylcobaloximes
Table 1. Crystal Data and Details of the Structural Refinement for Complexes 4-7
4
5
6
7
formula
fw
C16H22C14Co1N5O4
549.12
C15H21C11Co1N5O4
429.75
C15H21C1Co1N5O4‚1/3(CH2Cl2)
458.06
C15H22Co1N5O4
395.31
space group
T (K)
λ (Å)
a (Å)
C2/c
223 ( 2
0.710 73
12.0964(11)
13.3142(12)
14.9277(14)
90
109.721(2)
90
1.612
P1h
P21/c
P21/n
193 ( 2
193 ( 2
0.710 73
8.8489(8)
39.436(4)
16.9814(16)
90
101.6100(10)
90
1.572
12
5804.6(9)
11.49
R1 ) 0.0578
wR2 ) 0.0639
R1 ) 0.1786
wR2 ) 0.0829
183 ( 2
0.710 73
8.8598(6)
14.3906(9)
14.0123(9)
90
95.117(2)
90
1.476
0.710 73
15.874(2)
17.136(3)
17.314(3)
60.419(2)
87.617(3)
64.356(2)
1.585
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
F
cald (g cm-1
Z
)
4
8
4
V (Å3)
µ (cm-1
2263.2(4)
12.63
R1 ) 0.0358
wR2 ) 0.0937
R1 ) 0.0557
wR2 ) 0.0991
3602.2(9)
11.33
R1 ) 0.0468
wR2 ) 0.0939
R1 ) 0.1188
wR2 ) 0.1133
1779.4(4)
9.95
R1 ) 0.0343
wR2 ) 0.0844
R1 ) 0.0539
wR2 ) 0.0910
)
R [I > 2σ(I)]a
R (all data)a
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for
Complexes 4-7 with Esd’s in Parentheses
troscopic, structural, and electrochemical characterization.
The implications of these studies for the overall process of
B12-catalyzed dechlorination of chlorinated alkenes are
discussed.
4
5a
6a
7
Co(1)-C(14)
C(14)dC(15)
Co(1)-N(5)
1.945(5) 1.946(2) 1.947(4) 1.953(3)
1.947(3) 1.951(4) 1.958(8)
1.945(2) 1.952(4)
Experimental Section
1.949(2)
1.32(3)
1.314(3) 1.341(4) 1.292(4)
1.312(3) 1.342(4) 1.291(9)
1.312(3) 1.339(4)
General Information. All NMR spectra were recorded on Varian
U400 and U500 spectrometers. 1H spectra were referenced to CHCl3
at 7.26 ppm, and 13C spectra were referenced to CDCl3 at 77.00
ppm. All spectra were taken in CDCl3. Mass spectrometry (MS)
experiments were carried out by the Mass Spectrometry Laboratory
at the University of Illinois at Urbana-Champaign (UIUC). Infrared
(IR) spectra were taken on a Mattson Galaxy Series FTIR 5000.
Thin-layer chromatography (TLC) was carried out on Merck silica
gel 60 F254 plates. Compounds and solvents were obtained from
Fisher, Aldrich, Acros, Baker, Fluka, and Mallenkrodt. All ma-
nipulations were performed using Schlenk and inert-glovebox
techniques.
1.311(3)
2.030(2) 2.047(2) 2.035(3) 2.0518(15)
2.046(2) 2.035(3)
2.046(2) 2.033(3)
2.046(2)
Co(1)-C(14)dC(15) 121(2)
140.2(3) 126.7(3) 127.8(3)
139.5(3) 127.0(4) 122.4(9)
138.6(3) 126.2(4)
139.1(3)
a For compounds 5 and 6 multiple crystallographically independent
molecules were present with slight differences in the bond distances and
angles listed.
X-ray Structure Analysis. Diffraction data on complexes 4-7,
mounted on thin glass fibers with oil (Paratone-N, Exxon), were
collected using a Siemens Platform/CCD diffractometer with
graphite-monochromated Mo KR radiation. All structures were
solved by direct methods and refined by full-matrix least-squares
methods against F2 (SHELX-97-2).32,33 Crystallographic data for
complexes 4-7 are given in Tables 1 and 2. Further details
regarding the crystallographic data as well as full tables of bond
lengths and angles for each complex reported in this paper are
presented in CIF format in the Supporting Information and have
been deposited in the Cambridge Crystallographic Data Center.
Electrochemistry. Cyclic voltammetry measurements were
carried out using a CH Instruments Electrochemical Analyzer 617A
with a glassy carbon working electrode, Ag+/AgCl reference
electrode (3 M KCl, 0.213 V vs NHE), and a platinum wire counter
electrode. All measurements were performed in 0.1 M Bu4NClO4
in anhydrous DMF/t-BuOH (1:1), at a concentration of 1 mM of
each complex. The peak potentials given in the text are referenced
to the Ag+/AgCl electrode. In addition, in a separate series of
experiments, an internal reference system (ferrocene/ferrocenium
ion) was used, which has been shown to remain constant regardless
of media,34,35 thereby allowing better comparison with other studies
by eliminating problems of variable liquid junction potentials.36-39
Under the conditions used, the reversible Fc/Fc+ potential occurred
at 0.53 V vs the Ag+/AgCl electrode [(Ep + Ec)/2; for an example,
see Figure S3 in the Supporting Information]. When using this as
an internal reference system and taking the Fc+/Fc couple as 0.40
V vs NHE (Koepp, H. M.; Wendt, H.; Strehlow, H. Z. Z.
Elektrochem. 1960, 64, 483-491), the following peak potentials
can be derived vs NHE: -1.26 V for 4; -1.43 V for 5; -1.36 V
for 6; -1.58 V for 7; -1.15 V for 8; -1.39 V for 13.
Synthesis of [Co(dmgH)2(py)(CCldCHCl)] (4). A 25 mL
Schlenk flask was charged with CoCl2‚6H2O (0.64 g, 2.7 mmol),
10 mL of degassed MeOH, and a stir bar. After the CoCl2‚6H2O
dissolved, dimethylglyoxime (0.62 g, 5.4 mmol) was added followed
(34) Gutmann, V.; Peychal-Heiling, G.; Michlmayr, M. Inorg. Nucl. Chem.
Lett. 1967, 3, 501-5.
(35) Gutmann, V. Chimia 1969, 23, 285-92.
(36) Diggle, J. W.; Parker, A. J. Electrochim. Acta 1973, 18, 975-9.
(37) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854-5.
(32) Sheldrick, G. M. SHELX-97-2. Program for Crystal Structure Solution
and Refinement; Institute fur Anorg Chemie: Go¨ttingen, Germany,
1998.
(38) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1982, 54, 1527-32.
(39) IUPAC Pure Appl. Chem. 1984, 56, 461-6.
(33) Bruker AXS Inc., Madison, WI.
Inorganic Chemistry, Vol. 41, No. 2, 2002 395

McCauley et al.
by NaOH (0.22 g, 5.4 mmol). After the reaction mixture was stirred
for 5 min, pyridine (0.21 g, 2.7 mmol) was added, and after an
additional 20 min, TCE (0.39 g, 5.4 mmol) was added. The
atmosphere over the mixture was replaced with H2, and the mixture
remained under H2 for 24 h. The volume of the mixture was doubled
with H2O, which precipitated an orange solid. The solid was
dissolved in acetone, and the solution was passed through a silica
plug eluting with acetone. The orange fractions were collected, the
solvent was removed under reduced pressure, and the residue was
further purified by silica gel chromatography (5:1 hexane/acetone,
Rf ) 0.13) to yield the desired product as an orange powder. X-ray-
quality crystals were obtained by crystallization from CH2Cl2/
cyclohexane (0.23 g, 18%): FT-IR (KBr) 2767, 1563, 1452, 1368,
1237, 1092 cm-1; 1H NMR (400 MHz, CDCl3) δ ppm 2.20 (12 H,
s, CH3), 5.80 (1 H, s, CdCHCl), 7.34 (2 H, m, pyr), 7.76 (1 H, m,
pyr), 8.57 (2 H, m, pyr), 18.00 (2 H, bs, OH); 13C NMR (APT,
125 MHz) δ ppm 12.5 (CH3), 107.3 (CH), 125.5 (Ar CH), 138.3
(Ar CH), 150.1 (Ar CH), 151.2 (CdN, Cq); LRMS-FAB (m/z) 464
(M + 1).
(12 H, s, CH3), 5.05 (1H, d, J ) 2.07 Hz, dCH2), 5.24 (1 H, d, J
) 2.07 Hz, dCH2), 7.32 (2 H, m, Ar H), 7.73 (1 H, m, Ar H),
8.58 (2 H, m, Ar H) 18.03 (2 H, bs, OH); 13C NMR (APT, 125
MHz) δ ppm 12.4 (CH3), 119.3 (dCH2), 125.4 (Ar CH), 138.0
(Ar CH), 150.1 (Ar CH), 150.6 (CdN, Cq); LRMS-FAB (m/z) 430
(M + 1).
Synthesis of [Co(dmgH)2(py)(CHdCH2)] (7). Two different
methods were used to synthesize complex 7. Under an argon
atmosphere, a 100 mL round-bottom flask was charged with
Co(dmgH)2(py)(CCldCHCl) (4) (0.10 g, 0.22 mmol), cobaloxime
dimer 8 (0.081 g, 0.11 mmol), and 50 mL of MeOH. A solution of
NaBH4 (0.24 g, 6.3 mmol) and NaOH (0.21 g, 5.3 mmol) in 1.1
mL of H2O was added, and the reaction was stirred for 2 h. MeOH
and H2O were removed under reduced pressure, and the crude
product was passed through a silica plug eluting with acetone
followed by purification using silica gel chromatography (5:1
hexane/acetone, Rf ) 0.13) to yield an orange-yellow powder.
X-ray-quality crystals were obtained by crystallization from CH2Cl2/
cyclohexane (2 mg, 2%). Although complete dechlorination of
complex 4 resulted in the formation of complex 7, a more practical
synthesis was used to prepare complex 7 in higher yields. Using
similar reaction conditions as described for complex 5, vinyl
bromide in THF (1 M) was substituted for chloroacetylene (yield
of 7, 0.54 g, 51%): FT-IR (KBr) 2960, 2925, 1571, 1450, 1236,
Synthesis of cis-[Co(dmgH)2(py)(CHdCHCl)] (5). A 25 mL
Schlenk flask was charged with CoCl2‚6H2O (0.64 g, 2.7 mmol),
10 mL of degassed MeOH, and a stir bar. After the CoCl2‚6H2O
dissolved, dimethylglyoxime (0.62 g, 5.4 mmol) was added followed
by NaOH (0.32 g, 8.1 mmol). After the reaction mixture was stirred
for 5 min, pyridine (0.21 g, 2.7 mmol) was added, and after an
additional 20 min, the cobaloxime mixture was reduced with a
solution of NaBH4 (0.24 g, 6.3 mmol) and NaOH (0.21 g, 5.3 mmol)
in 1.1 mL of H2O. Chloroacetylene (8.40 g, 13.9 mmol) was
distilled into the reaction mixture over 2 h as described previously.40
(Caution! Although no problems haVe been encountered in this
study, chloroacetylene is potentially explosiVe.) The volume of the
mixture was doubled with H2O, which precipitated an orange solid.
The solid was passed through a silica plug eluting with acetone.
The orange fractions were collected, the solvent was removed under
reduced pressure, and the residue was further purified by silica gel
chromatography (5:1 hexane/acetone, Rf ) 0.13) to yield the desired
product as an orange powder. X-ray-quality crystals were obtained
from CH2Cl2/cyclohexane (0.023 g, 2%). A higher yield (11%) can
be obtained when less NaOH is added initially (0.22 g, 5.4 mmol)
producing a mixture of 5 and 6. The ratio of 5 to 6 was dependent
on the pH of the reaction mixture: 5:6 was 95:5 at pH 11, 7:3 at
pH 10, and 1:3 at pH 9. FT-IR (KBr): 2949, 1558, 1448, 1240,
1
1089, cm-1; H NMR (400 MHz, CDCl3) δ ppm 2.12 (12 H, s,
CH3), 4.67 (1H, dd, J ) 6.86, 1.64 Hz, dCH2), 4.81 (1 H, dd, J )
15.14, 1.64 Hz, dCH2), 6.41 (1 H, dd, J ) 15.14, 6.81 Hz,
CoCH)), 7.33 (2 H, m, Ar H), 7.74 (1 H, m, Ar H), 8.63 (2 H, m,
Ar H), 18.32 (2H, bs, OH); 13C NMR (APT, 125 MHz) δ 12.1
(CH3), 120.5 (dCH2), 125.3 (Ar CH), 137.7 (Ar CH), 149.4 (CdN,
Cq), 150.1 (Ar CH); LRMS-FAB (m/z) 396 (M + 1).
Synthesis of trans-[Co(dmgH)2(py)(CHdCHCl)] (13). A 25
mL Schlenk flask was charged with CoCl2‚6H2O (0.35 g, 1.5
mmol), 10 mL of degassed MeOH, and a stir bar. After the CoCl2‚
6H2O dissolved, dimethylglyoxime (0.63 g, 5.4 mmol) was added
followed by NaOH (0.22 g, 5.4 mmol). After the reaction mixture
was stirred for 5 min, pyridine (0.23 g, 3.0 mmol) was added, and
after an additional 25 min, trans-1,2-DCE (0.52 g, 5.4 mmol) was
added. The atmosphere over the mixture was replaced with H2,
and the mixture remained under H2 for 2.5 h. An orange solid
precipitated from the solution, and it was filtered off using a medium
frit. The volume of the filtrate was doubled with H2O and cooled
to -20 °C, which precipitated an orange solid. The solid was
dissolved in acetone, and the solution was passed through a silica
plug eluting with acetone. The orange fractions were collected, the
solvent was removed under reduced pressure, and the residue was
further purified by silica gel chromatography (5:1 hexane/acetone,
Rf ) 0.13) to yield the desired product as a yellow powder (0.007
1
1090 cm-1. H NMR (400 MHz, CDCl3): δ ppm 2.16 (12 H, s,
CH3), 5.85 (1 H, d, J ) 5.69 Hz, dCHCl), 6.01 (1 H, d, J ) 5.69
Hz, CoCH)) 7.36 (2 H, m, pyr), 7.77 (1 H, m, pyr), 8.62 (2 H, m,
pyr) 18.24 (2 H, bs, OH). 13C NMR (APT, 125 MHz): δ ppm
12.1 (CH3), 125.4 (Ar CH), 126.6 (CH), 137.9 (Ar CH), 149.8 (Ar
CH), 150.6 (CdN, Cq). LRMS-FAB (m/z) 430 (M + 1).
Synthesis of [Co(dmgH)2(py)(CCldCH2)] (6). Complex 6 was
obtained from 1,1-dichloroethylene using similar experimental
conditions as described for complex 4 except the reaction mixture
was kept under an H2 atmosphere for 12 h. The volume of the
mixture was doubled with H2O, which precipitated a yellow solid.
The solid was dissolved in acetone, and the solution was passed
through a silica plug eluting with acetone. The orange fractions
were collected, the solvent was removed under reduced pressure,
and the residue was further purified by silica gel chromatography
(5:1 hexane/acetone, Rf ) 0.13) to yield the desired product as a
yellow powder. X-ray-quality crystals were obtained from CH2Cl2/
cyclohexane (0.27 g, 23%): FT-IR (KBr) 3118, 3051, 2925, 1565,
1450, 1238, 1091 cm-1; 1H NMR (400 MHz, CDCl3) δ ppm 2.18
g, 1%): FT-IR (KBr) 3040, 2849, 1560, 1452, 1222, 1092 cm-1
;
1H NMR (400 MHz, CDCl3) δ ppm 2.14 (12 H, s, CH3), 5.29 (1H,
d, J ) 12.57 Hz, dCHCl), 6.22 (1H, d, J ) 12.57 Hz, CoCH)),
7.33 (2 H, m, pyr), 7.74 (1 H, m, pyr), 8.57 (2 H, m, pyr), 18.15
(2 H, bs, OH); 13C NMR (APT, 125 MHz) δ ppm 12.2 (CH3),
109.6 (CH), 125.4 (Ar CH), 138.0 (Ar CH), 150.0 (Ar CH), 150.2
(CdN, Cq); HRMS-FAB (m/z) calcd for C15H22N5O4ClCo (M +
1) 430.0692, found 430.0690.
Results and Discussion
Alkylbis(dimethylglyoximato)(pyridine)cobalt(III) com-
plexes (RCo(dmgH)2py), designated henceforth in this report
as alkylcobaloximes, have been extensively used as structural
(40) Beit-Yannai, M.; Rappoport, Z.; Shainyan, B. A.; Danilevich, Y. S.
J. Org. Chem. 1997, 62, 8049-8057.
396 Inorganic Chemistry, Vol. 41, No. 2, 2002

Chlorinated Alkenylcobaloximes
and functional mimics for vitamin B12.41-43 Although these
compounds do not accurately reproduce all chemical proper-
ties of B12,44 the wealth of available information on their
reactivities in comparison with B12 derivatives renders
cobaloximes good starting points for understanding the
properties of chlorinated ethenylcobalamins. Of particular
interest with respect to the proposed mechanisms of B12-
catalyzed dechlorination are the thermal stability, redox
properties, and mechanisms of formation and reduction of
these complexes.
Synthesis and Structural Characterization of Chloro-
ethenyl Cobaloximes. Purified homogeneous compounds
were required for the investigation of the chemical and
physical properties of various chlorinated ethenylcobaloximes.
We evaluated several synthetic routes to these products by
reaction of cob(I)aloximes (CoI(dmgH)2) with PCE or TCE.
In these experiments, cob(I)aloxime was formed either by
reduction of in situ generated CoII(dmgH)2py (3) with NaBH4
or by stirring 3 under an atmosphere of H2. With PCE both
methods resulted in the isolation of chlorinated vinyl-
cobaloximes in low yields45 (10-25%) producing hetero-
geneous mixtures, the composition of which was difficult
to control (eq 2). The reaction product obtained from the
reaction under H2 with 2 equiv of TCE on the other hand
consisted only of the cis-dichlorinated complex 4 in 18%
yield, which was further purified by silica gel flash chro-
matography.46 The identity of the compound was confirmed
Figure 1. ORTEP diagrams of complexes 4 (panel A), 5 (B), 6 (C), and
7 (D). For compounds containing more than one molecule in the unit cell
with slightly varying geometries, only one molecule is depicted here. Full
details for all structures are provided in the Supporting Information.
Hydrogens are omitted for clarity except for the vinyl hydrogens, and atoms
are drawn as 35% thermal ellipsoids. Complexes 4, 6, and 7 have the vinyl
ligand situated above the oxime moieties of the equatorial ligand, whereas
the vinyl ligand in the cis-complex 5 is located above the backbone of the
equatorial ligand.
1
by FAB mass spectrometry and H NMR spectroscopy,
which showed a singlet at 5.80 ppm assigned to the vinylic
proton. The stereochemistry of the double bond was unam-
biguously established by X-ray crystallography as depicted
in Figure 1A. The vinyl ligand in this structure is situated
over the protons that bridge between two oxygens of each
dimethylglyoximato ligand.47 The Co-C bond length of
1.945(5) Å is close to those reported for other alkenyl-
cobaloximes (e.g. 1.966(6) Å in vinylCo[dmgH]2py,48
1.971(13) Å in [{p-ClPh}2CdCCl]Co[dmgH]2py,49 and
1.958(3) Å in [ClHCdCCl]Co[dmgH]2py47) and shorter than
found for alkylcobaloximes42 as expected for an sp2-
hybridized ligand.
(41) Schrauzer, G. N. Acc. Chem. Res. 1968, 1, 97-103.
(42) Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.; Randaccio, L.;
Summers, M. F.; Toscano, P. J. Coord. Chem. ReV. 1985, 63, 1-125.
(43) Randaccio, L. Comments Inorg. Chem. 1999, 21, 327-376.
(44) For instance, the Co(I)/(II) and Co(II)/(III) redox potentials for
cobalamins and cobaloximes differ with the latter being about 0.4 V
more negative. See: (a) Elliott, C. M.; Hershenhart, E.; Finke, R. G.;
Smith, B. L. J. Am. Chem. Soc. 1981, 103, 5558-66. (b) Finke, R.
G.; Smith, B. L.; Droege, M. W.; Elliott, C. M.; Hershenhart, E. J.
Organomet. Chem. 1980, 202, C25-C30. Furthermore, Co-C bond
dissociation energies have been reported to be generally lower for
cobalamins than for cobaloximes (Halpern, J.; Ng, F. T. T.; Rempel,
G. L. J. Am. Chem. Soc. 1979, 101, 7124-7126). E.g. for benzyl-
cobalamin and -cobaloxime, compare (i) and (ii) with (iii): (i) Brown,
K. L.; Brooks, H. B. Inorg. Chem. 1991, 30, 3420-3430. (ii)
Schrauzer, G. N.; Grate, J. H. J. Am. Chem. Soc. 1981, 103, 5541-
5546. (iii) Toscano, P. J.; Seligson, A. L.; Curran, M. T.; Skrobutt,
A. T.; Sonnenberger, D. C. Inorg. Chem. 1989, 28, 166-168.
(45) These low yields of formation of chlorinated ethenylcobaloximes are
comparable to previously reported reactions of chlorinated styrenes
with cob(I)aloximes that provided ethenylcobaloximes in low yield
(10%). See for instance ref 64.
(46) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-
2925.
(47) The structure is generally in agreement with that reported by Jones
and co-workers (Jones, P. G.; Yang, L.; Steinborn, D. Acta Crystal-
logr., Sect. C 1996, C52, 2399-2402) although the current structure
was solved in the C2/c space group instead of the P1 space group.
(48) Bresciani-Pahor, N.; Calligaris, M.; Randaccio, L. J. Organomet. Chem.
1980, 184, C53-C56.
As indicated in eq 2, two isomeric monochlorinated
ethenylcobaloximes 5 and 6 were also observed in the
(49) Stotter, D. A.; Sheldrick, G. M.; Taylor, R. J. Chem. Soc., Dalton
Trans. 1975, 2124-2128.
Inorganic Chemistry, Vol. 41, No. 2, 2002 397

McCauley et al.
reaction of cob(I)aloxime with PCE. Unfortunately, despite
much effort, chromatographic purification of significant
amounts of complex 5 did not prove possible from this
reaction due to coelution with 4 and 6. However, as described
below, the complex could be obtained in larger quantities
by reaction of CoI(dmgH)2 with chloroacetylene. Its 1H NMR
spectrum showed doublet resonances at 5.85 ppm (3J ) 5.69
Hz) and 6.01 ppm (3J ) 5.69 Hz), consistent with a cis-
substituted olefin. This assignment was subsequently cor-
roborated by X-ray diffraction (Figure 1B) indicating an
unusually large Co-CdC angle, 139.4°, in comparison to
the angles in known vinylcobalt complexes.50-55 A similarly
large Co-CdC angle has been previously reported for
another cis-substituted ethenyl cobaloxime, ([p-ClPh]2Cd
CCl)Co[dmgH]2py (133.0°).49
N
py bond length increases slightly in the order 4 < 6 < 5 <
7, which reflects the decreased electron-withdrawing strength
of the vinyl group in this series (Table 2). A similar effect
has been observed previously in substituted methyl-
cobaloximes.42 The Co-C bond lengths show a similar albeit
smaller trend (close to experimental error, 4 < 6 < 7), in
line with reported shortening of the Co-C bond in a series
of progressively higher fluorinated alkylcobaloximes.43 The
Co-C bond in the cis-substituted complex 5 does not clearly
fit this order, probably because both steric and electronic
effects are involved as indicated by the large Co-CdC bond
angle.
The R-carbon of the axial ligands in complexes 4-7 is
sp2-hybridized predicting a significantly stronger Co-C bond
than in alkyl cobaloximes. Qualitatively all four organo-
cobaloximes indeed proved quite stable at room temperature
in air. Furthermore, although all preparations were conducted
in the dark, isolated complexes 4, 6, and 7 could be handled
in visible light without special precautions. These observa-
tions suggest that once formed, these complexes are unlikely
to reenter a catalytic process by thermal homolytic scission
of the Co-C bond to generate vinyl radicals and Co(II).
Cobaloxime 5 was more reactive toward photodecomposition
and required handling in the dark. This may perhaps be due
in part to strain imposed by the large Co-CdC bond angle
compared to the considerably smaller angles for the other
complexes in Figure 1.
Reductive Dechlorination of (Chloroethenyl)cobaloximes.
With purified cobaloxime 4 in hand, we tested the possibility
of converting dichlorinated into monochlorinated ethenyl
complexes as proposed in the literature for vitamin B12
derivatives.12 Complex 4 proved inert when treated with
NaBH4 or H2 alone, but addition of catalytic amounts of
[CoII(dmgH)2py]2 (8) resulted in the disappearance of 4 and
the generation of 5 and 6. The transformation was consider-
ably accelerated when a stoichiometric amount of 8 was
added. With time the only ethenylcobaloxime remaining in
the reaction mixture was complex 7. These conversions were
generally low yielding with NaBH4 (5-10%); using H2 as
the reductant usually resulted in a slower reaction from which
higher yields of dechlorinated cobaloximes (25%) could be
recovered. Presumably, these low yields are due to the
formation of volatile products derived from the axial alkyl
ligand or possibly because of base-catalyzed degradation as
has been reported for other cobaloximes.57-59
Conversion of (Dichloroethenyl)cobaloxime 4 to (Mono-
chloroethenyl)cobaloximes 5 and 6 and Vinylcobaloxime
7. At least three different mechanisms can be proposed for
the reductive conversion of 4 to 5. Homolytic or heterolytic
scission of the Co-C bond followed by hydrogen atom or
proton transfer, respectively, would produce cis-dichloro-
ethylene (pathway F, Scheme 2). Subsequent reaction of cis-
DCE with cob(I)aloxime via addition-elimination (AE) with
retention of configuration would provide complex 5. The
mechanisms of such vinylic nucleophilic substitutions have
The geminal complex 6 was obtained in pure form by
reaction of CoII(dmgH)2py with 1,1-dichloroethene (1,1-DCE,
1
2 equiv) under an atmosphere of H2 (eq 3). The H NMR
spectrum of the yellow solid showed doublet resonances for
the vinyl protons at 5.05 and 5.24 ppm (3J ) 2.07 Hz), and
the compound was unambiguously identified as the R-
chlorovinyl complex 6 by X-ray diffraction (Figure 1C).
Vinylcobaloxime 7 was obtained by treating a mixture of
complexes 4-6 with [CoII(dmgH)2py]2 (8) and NaBH4. FAB
mass spectrometry and 1H NMR spectroscopy of 7 confirmed
its identity. An X-ray structure was obtained (Figure 1D)
that is in agreement with a previously reported structure.48
However, the C-C bond length of the vinyl ligand in the
current structure is 1.292 Å compared to the previously
reported bond length of 1.149 Å.56
The four structures in Figure 1 provide the opportunity to
extract trends for this series of vinylcobaloximes. The Co-
(50) Brueckner, S.; Calligaris, M.; Nardin, G.; Randaccio, L. Inorg. Chim.
Acta 1968, 2, 416-20.
(51) Smalley, T. L., Jr.; Wright, M. W.; Garmon, S. A.; Welker, M. E.;
Rheingold, A. L. Organometallics 1993, 12, 998-1000.
(52) Wright, M. W.; Smalley, T. L., Jr.; Welker, M. E.; Rheingold, A. L.
J. Am. Chem. Soc. 1994, 116, 6777-91.
(53) Adams, T. A.; Welker, M. E.; Liable-Sands, L. M.; Rheingold, A. L.
Organometallics 1997, 16, 1300-1307.
(54) Adams, T. A.; Welker, M. E.; Day, C. S. J. Org. Chem. 1998, 63,
3683-3686.
(55) Dreos, R.; Geremia, S.; Nardin, G.; Randaccio, L.; Tauzher, G.; Vuano,
S. Inorg. Chim. Acta 1998, 272, 74-79.
(56) In both structures, the vinyl ligand occupies two positions above the
dimethylglyoximato ligands. The previous report treated the position
of the R-carbon in both vinyl groups as identical during the analysis
of the diffraction data, which may have led to the short bond length
between the vinyl carbons. On the other hand, current technology
allowed the structure reported here to be refined by treating each vinyl
ligand as an individual moiety leading to a C-C distance that is more
in line with other reported organocobalt compounds containing a vinyl
ligand. See refs 50-55.
(57) Brown, K. L. Inorg. Chim. Acta 1978, 31, L401-L403.
(58) Brown, K. L. J. Am. Chem. Soc. 1979, 101, 6600-6.
(59) Brown, K. L.; Hessley, R. K. Inorg. Chem. 1980, 19, 2410-14.
398 Inorganic Chemistry, Vol. 41, No. 2, 2002

Chlorinated Alkenylcobaloximes
Scheme 2
ligand exchange was observed leading to an uninformative
mixture of complexes containing a mixed ethyl/methyl-
glyoximato equatorial ligand set (e.g. 11a-c, eq 5). Regard-
less, the results in eq 4 strongly suggest that the axial vinyl
ligand is removed from the cobalt during its dechlorination.
been studied extensively for numerous reactions60,61 including
substitutions involving cob(I)aloximes and halogenated
styrenes62-64 or alkenes.65 In these investigations, substitution
occurred with retention of configuration consistent with
pathway F. However, despite the fact that halogenated
styrenes as well as vinyl chloride66 readily react with cob(I)-
aloximes to give vinylcobaloximes, we did not succeed to
produce any detectable monochlorinated ethenylcobaloximes
by treating cis-1,2-DCE with cob(I)aloxime under a variety
of reaction conditions. Thus, pathway F is not feasible for
the observed production of 5 from 4.
The inability to provide support for pathway F led us to
explore whether the dechlorination might occur without
breaking the cobalt-carbon bond. Such a pathway has been
proposed for the B12-catalyzed dechlorination with Ti(III)
citrate as the reductant,12 although no details regarding the
possible mechanisms of the transformation were suggested.
We employed a crossover strategy using the (1,2-dichloro-
ethenyl)bis(diethylglyoximato)(pyridine)cobalt(III) complex
9 (eq 4). Reaction of 9 with 1 equiv of the bis(dimethyl-
glyoximato) dimer 8 under an atmosphere of hydrogen
produced (cis-chloroethenyl)dimethylcobaloxime 5.67 No
corresponding monochlorinated diethylcobaloximes were
detected, and no ligand exchange was detected in recovered
complex 9. The observed complete crossover is consistent
with cleavage of the Co-C bond during the transformation.
We also attempted the reverse experiment starting with 4
and the bis(diethylglyoximato)(pyridine)cobalt(II) dimer 10
(eq 5). Unfortunately, the Co(I) form of 10 is much less
reactive than the analogous methyl-substituted cob(I)aloxime,
and in the time frame of the reaction of 10 with complex 4,
A third mechanism for the conversion of 4 to 5 involves
an elimination-addition with chloroacetylene as an inter-
mediate (pathway G, Scheme 2). For this pathway to be
viable for generation of the cis-complex 5, the addition must
occur with regio- and antistereoselectivity. Gaudemer and
Johnson have shown in an elegant set of studies that cob(I)-
aloximes can add either stereospecifically syn or anti to
alkynes, depending on the reaction conditions.63,72 Below the
pKa of the protonated form of cob(I)aloxime (i.e. the pKa of
the cobalt-bound proton in “hydridocobaloxime” estimated
to be ∼10.5),68 the metal hydride adds selectively syn to
phenylacetylene, probably via a stepwise process of hydrogen
atom addition followed by trapping of the resulting vinyl
radical with cob(II)aloxime.69-71 Above the pKa, addition of
“free” cob(I)aloxime followed by protonation by solvent
occurs in anti fashion (Scheme 3).72 Furthermore, the
regioselectivity was reversed giving (R-phenylethenyl)-
cobaloxime below pH ∼ 8 and (cis-â-phenylethenyl)-
cobaloxime at alkaline pH. We evaluated the feasibility of
pathway G by reaction of cob(I)aloxime with chloro-
acetylene, generated in small scale by gently heating a
biphasic mixture of 1,2-dichloroethylene in diethyl ether and
aqueous sodium hydroxide in the presence of a phase transfer
catalyst.40 The gaseous chloroacetylene produced was led
directly into a reaction vessel containing cob(I)aloxime,
which rapidly changed color from the characteristic purple
(60) Modena, G. Acc. Chem. Res. 1971, 4, 73-80.
(61) Rappoport, Z. Recl. TraV. Chim. Pays-Bas 1985, 104, 309-349.
(62) Duong, K. N. V.; Gaudemer, A. J. Organomet. Chem. 1970, 22, 473-
478.
(63) Johnson, M. D.; Meeks, B. S. J. Chem. Soc. B 1971, 185-189.
(64) Dodd, D.; Johnson, M. D.; Meeks, B. S.; Titchmarsh, D. M.; Duong,
K. N. V.; Gaudemer, A. J. Chem. Soc., Perkin Trans. 2 1976, 1261-
1267.
(68) Schrauzer, G. N.; Holland, R. J. J. Am. Chem. Soc. 1971, 93, 1505-
1506.
(69) Baldwin, D. A.; Betterton, E. A.; Chemaly, S. M.; Pratt, J. M. J. Chem.
Soc., Dalton Trans. 1985, 1613-1618.
(65) Tada, M.; Kubota, M.; Shinozaki, H. Bull. Chem. Soc. Jpn. 1976, 49,
1097-1100.
(70) Garr, C. D.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 10440-10445.
(71) Ng, F. T. T.; Rempel, G. L.; Mancuso, C.; Halpern, J. Organometallics
1990, 9, 2762-2772.
(66) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1967, 89,
1999-2007.
(67) Control experiments showed that chlorinated ethenylcob(III)aloximes
do not exchange the equatorial ligands under these conditions.
(72) Naumberg, M.; Duong, K. N. V.; Gaudemer, A. J. Organomet. Chem.
1970, 25, 231-242.
Inorganic Chemistry, Vol. 41, No. 2, 2002 399

McCauley et al.
Scheme 3
Scheme 4
of Co(I) to the yellow-orange of organocobaloximes. The
cis-chloroethenyl complex 5 was the major product formed
at pH 11 (5:6 ) ∼95:5). The R-chlorinated ethenyl-
cobaloxime 6 was formed along with 5 when the reaction
was performed at pH 10 (5:6 ) 7:3), and 6 was formed as
the major product at pH 9 (5:6 ) 1:3), although the reaction
was less clean under these latter conditions. The formation
of ethynylcobaloxime was not observed in these experiments,
similarly to the absence of ethynylcobalamin formation in
B12-catalyzed dechlorinations of PCE and TCE.12 This
contrasts with a previous study that reported the formation
of ethynylcobalamin as the major product and bromo-
ethenylcobalamin as a minor product in the reaction between
cob(I)alamin and bromoacetylene.73 This difference may be
attributed to the better leaving group ability of bromide
compared with chloride, resulting in products that are
presumably formed by addition-elimination for the former
and via addition and protonation for the latter. In summary,
pathway G, Scheme 2, is the most likely route to convert 4
to 5, and chloroacetylene can also account for the formation
of 6 (pathway H, Scheme 2).
With regards to the formation of 7, submitting either 5 or
6 to the reductive conditions induced slow consumption of
the starting complexes and formation of a substoichiometric
quantity (10-20%) of vinylcobaloxime 7. These findings
can be accounted for by the formation of volatile products
in the reductive dechlorination of 5 and 6 that are trapped
to some extent to provide 7. These volatile products could
be acetylene (sublimation point -81 °C) or vinyl chloride
(VC, bp -13 °C). Both compounds are known to form
vinylcobaloxime when reacted with cob(I)aloxime (Scheme
4).66,74
one-electron transfer followed by cleavage of the Co-C
bond. Alkylcobaloximes with a pyridine axial base generally
exhibit one electron redox potentials that are significantly
more negative than the Co(I)/Co(II) redox potential of
cob(I)aloxime, questioning whether the latter is a sufficiently
strong reductant to transfer an electron to complexes such
as 4-7. Several studies have shown, however, that the one-
electron reduction potentials for alkylcobaloximes show a
trend to less negative values when more electron withdrawing
axial alkyl ligands are present.75-79 Scheffold and co-workers
reported that alkylcobalamins (RCbl) containing alkyl ligands
with a pKa less than ∼30 for R-H have reduction potentials
less negative than that of the Co(I)/Co(II) couple of B12.79
For instance, in protic solvents, cob(I)alamin was found to
be a sufficiently strong electron donor to reduce (methoxy-
carbonyl)methylcobalamin (MeO2CCH2Cbl).78 The pKa of
vinyl chloride has been reported as 31,80 and that of cis-
DCE has been estimated to be ∼34 (in MeOH),81 whereas a
value of 30-31 has been determined for methyl acetate (in
DMSO).82 These pKa’s suggest reductive cleavage of the
Co-C bond in the cobalamin analogues of 4-6 may in fact
be feasible. To gain insights into the redox properties of this
type of complexes cyclic voltammetry was applied to 4-7.
The cyclic voltammogram of purified (cis-chloroethenyl)-
cobaloxime 5 is shown in Figure 2 along with the voltam-
mogram of dimer 8. Whereas the latter displayed an
electrochemically reversible wave with a cathodic peak
potential of -1.02 V vs Ag+/AgCl (ipc ) ipa, ∆Ep ) 75
mV),83 the wave for 5 was completely irreversible at scan
Electrochemical Properties of (Chloroethenyl)cobal-
oximes. An important unanswered question that arises from
the studies presented above is how the Co-C bonds are
cleaved in the chlorinated vinylcobaloximes. As mentioned,
these complexes appear to be thermally quite stable and do
not appreciably eliminate chloride on the time scale of the
dechlorination process when cob(I)aloxime is not present.
The conversion of 4 into 5-7 only after addition of cob(I)-
aloxime suggests that the latter is responsible for promoting
these transformations. One conceivable mechanism involves
(75) Costa, G. Pure Appl. Chem. 1972, 30, 335-352.
(76) Costa, G. Coord. Chem. ReV. 1972, 8, 63-75.
(77) Costa, G.; Puxeddu, A.; Tavagnacco, C. J. Organomet. Chem. 1985,
296, 161-172.
(78) Tinembart, O.; Walder, L.; Scheffold, R. Ber. Bunsen-Ges. Phys. Chem.
1988, 92, 1225-1231.
(79) Zhou, D.-L.; Tinembart, O.; Scheffold, R.; Walder, L. HelV. Chim.
Acta 1990, 73, 2225-2241.
(80) Butin, K. P.; Beletskaya, I. P.; Kashin, A. N.; Reutov, O. A. J.
Organomet. Chem. 1967, 10, 197-210.
(73) Johnson, A. W.; Mervyn, L.; Shaw, N.; Smith, E. L. J. Chem. Soc.
1963, 4146-56.
(74) Schrauzer, G. N.; Kohnle, J. Chem. Ber. 1964, 97, 3056-3064.
(81) Miller, S. I.; Lee, W. G. J. Am. Chem. Soc. 1959, 81, 6313.
(82) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-63.
(83) See Experimental Section for peak potentials vs NHE.
400 Inorganic Chemistry, Vol. 41, No. 2, 2002

Chlorinated Alkenylcobaloximes
Figure 2. Cyclic voltammogram of complex 5 (1 mM) in DMF/tert-butyl
alcohol (1:1). Dashed line: CV of dimer 8. Potentials are vs Ag+/AgCl,83
with sweep rate 100 mV s-1, glassy carbon working electrode, and 0.1 M
Bu4NClO4.
Figure 3. Dependence of the peak positions in the cyclic voltammogram
of complex 5 on the scan rate. Potentials are vs Ag+/AgCl,83 with sweep
rate 100 mV s-1, glassy carbon working electrode, 0.1 M Bu4NClO4, and
solvent DMF.
rates up to 1 V/s with a peak potential of -1.30 V at 100
mV/s. An anodic return wave was observed at -0.96 V, sug-
gestive of the oxidation of cob(I)aloxime to cob(II)aloxime.
Indeed when multiple scans were performed, a cathodic peak
corresponding to the reduction of cob(II)aloxime to cob(I)-
aloxime was visible in the second scan (Figure 2). Thus,
during the reduction of complex 5, cob(I)aloxime is formed,
although not in stoichiometric quantitities as the anodic peak
current (ipa) for the Co(I) f Co(II) conversion is only about
one-third that of the cathodic peak current (ipc) for the
reduction of 5. On the basis of these observations, it is likely
that the initial reversible electrochemical reduction is fol-
lowed by one or more fast irreversible chemical step(s) as
has been observed previously for other alkylcobaloximes.84,85
In the initial chemical step, the Co-C bond is cleaved and
either Co(I) and an alkyl radical or Co(II) and an alkyl anion
are formed. These species can undergo various follow-up
reactions including migration of the alkyl radical to the
ligand86 and/or formation of bisalkylated cobaloximes.85,87-91
As expected for such EC type mechanisms,92 the peak
potential shifted to more negative potentials with faster scan
rates (Figure 3). Since reversibility was never observed,93
the peak potential for 5 in Figure 2 is probably less negative
than the true reduction potential.
Figure 4. Cyclic voltammogram of complex 4 (1 mM). Experimental
conditions as in Figure 2.
The cathodic scan of dichlorinated complex 4 produced a
broad wave with a peak potential of -1.13 ( 0.03 V vs
Ag+/AgCl (Figure 4). As observed for complex 5, the
cathodic peak potential shifted to more negative values upon
increasing the scan rate (data not shown). The 1,1-disubsti-
tuted complex 6 gave a voltammetric profile exhibiting two
cathodic peaks (Figure 5). The anodic return scan also
exhibits two anodic waves, suggesting at first glance that
the two redox processes may be partially reversible. The
anodic peak at -1.18 V, however, is not due to the
reoxidation of a species formed in the first reduction wave
(at -1.23 V), because when the switching potential was set
at -1.35 V, no anodic peak was observed at -1.18 V (see
Figure S1 in the Supporting Information). Interestingly, the
second cathodic peak as well as both anodic peaks are also
present in the CV of vinylcobaloxime 7 (dashed line). This
observation suggests that in the electrochemical reduction
of 6 dechlorination occurs to give 7, followed by reduction
of 7 itself. From the voltammogram of the latter species
(dashed line, Figure 5), it appears that its reduction is partially
reversible. Indeed at higher scan rates (>1 V/s), the reduction
(84) Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1966, 88,
3738-3743.
(85) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J. Am. Chem.
Soc. 1981, 103, 5558-66.
(86) Le Hoang, M. D.; Robin, Y.; Devynck, J.; Bied-Charreton, C.;
Gaudemer, A. J. Organomet. Chem. 1981, 222, 311-321.
(87) Costa, G.; Puxeddu, A.; Reisenhofer, E. Experientia, Suppl. 1971, No.
18, 235-45.
(88) Costa, G.; Puxeddu, A.; Reisenhofer, E. J. Chem. Soc. D 1971, 993-
4.
(89) Costa, G.; Puxeddu, A.; Reisenhofer, E. J. Chem. Soc., Dalton Trans.
1972, 1519-23.
(90) Seeber, R.; Marassi, R.; Parker, W. O., Jr.; Kelly, G. Inorg. Chim.
Acta 1990, 168, 127-38.
(91) At present, the identity of the products of follow-up reactions is unclear.
To investigate whether they consist of bisalkylated products as reported
for Costa type complexes in refs 85 and 90, controlled potential bulk
electrolysis and optically transparent thin-layer electrode spectroscopic
studies are ongoing.
(92) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706-23.
(93) In contrast to Costa-type complexes,85 addition of excess axial ligands
such as N-methylimidazole or pyridine did not lead to reversible
electrochemical behavior.
of 7 appears chemically reversible (∆Ep ) 72 mV, ipc
)
ipa).95
Inorganic Chemistry, Vol. 41, No. 2, 2002 401

McCauley et al.
Figure 6. Comparison of the cathodic peaks of dimer 8 (- - -) and
complexes 4 (closed circles), 5 (closed squares), 6 (open circles), and 7
(open squares). Potentials are vs Ag+/AgCl,83 with sweep rate 100 mV s-1
and glassy carbon working electrode.
Figure 5. Cyclic voltammogram of a 1 mM solution of complex 6 (solid
line). Dashed line: cyclic voltammogram of vinylcobaloxime 7. Instrument
settings and experimental conditions as in Figure 2.
Comparison of the voltammograms of all species shows
that complex 4 containing two chlorides displays a less
negative peak potential (-1.13 ( 0.03 V) than the mono-
chlorinated complexes 5 (-1.30 ( 0.01 V) and 6 (-1.23 (
0.01 V), whereas vinylcobaloxime 7 has the most negative
peak potential (-1.45 ( 0.01 V) in this series (Figure 6).96
This trend is consistent with the expected acidity of the vinyl
ligands in these complexes and parallels the trends in Co-
Npyr and Co-C bond lengths. Although reductive cleavage
of the Co-C bond in 4-6 accounts qualitatively for the
observed dechlorination in eq 2, the peak potentials of dimer
8 and the chlorinated complexes are still at least ∼130 mV
apart. Assuming that the difference in peak potentials
provides a rough estimate of driving force, the equilibrium
constant for electron transfer from cob(I)aloxime to 4 would
be ∼ 1.4 × 10-2 or smaller.97 This unfavorable equilibrium
for an outer sphere electron-transfer process may be driven
toward the reduced vinylcobaloximes by coupling the
electron-transfer event to a fast, irreversible chemical follow-
up reaction. Martin and Finke have shown that one electron
reduction of methylcobalamin reduces the Co-C bond
strength by 21 ( 4 kcal/mol resulting in a g1015 enhance-
ment of the rate of bond breakage.98,99 A similar large rate
acceleration induced by electron transfer to the complexes
discussed herein may compensate to some extent for the
unfavorable preequilibrium. An alternative or additional
increase in the rate of reductive Co-C bond cleavage could
potentially be achieved if the reduction took place via an
inner-sphere electron-transfer process. Further experiments
that are beyond the scope of the current investigation are in
progress to address these issues for the complexes reported
here.
The Elusive (Trichloroethenyl)cobaloxime and -cobal-
amin. It is noteworthy that (trichloroethenyl)cobaloximes
were never detected in the studies described above, similar
to the absence of (trichloroethenyl)cobalamins in mass
spectrometric analysis of reaction mixtures containing B12,
Ti(III) citrate, and PCE.12 In previous work, we31 and others7
have provided support for the first dechlorination (PCE to
TCE) to take place via an electron-transfer mechanism
producing Co(II) and, presumably, a trichlorovinyl radical.
This is consistent with the lack of detectable (trichloro-
ethenyl)cobalt(III) complexes. However, it is not clear why
cob(II)alamin, observed during stopped-flow analysis of the
dechlorination of PCE,31 would not rapidly combine with
the trichlorovinyl radical to form (trichloroethenyl)cobalamin.
The rates of combination of alkyl radicals and cob(II)alamin
is at the diffusion controlled limit,100-103 and combination
of Co(II) with the trichlorovinyl radical could be even more
favorable if electron transfer were to take place in a solvent
(94) This transformation occurred in low yield (∼1-2%), but sufficient
1
quantities could be obtained for full characterization by H and 13C
NMR spectroscopy as well as high-resolution mass spectrometry. The
1H NMR spectrum of 13 displayed doublets at 6.22 and 5.29 ppm
with a coupling constant of 12.6 Hz, which forms the basis for the
assignment of trans-geometry of the metal and chlorine substituents.
Thus, as reported for halogenated styrenes,62-64 cob(I)aloxime reacts
with trans-DCE with retention of configuration.
(95) At high scan rates, when the reduction of 7 appears reversible, the
anodic peak at -1.18 V is still present. Furthermore, this peak is quasi-
reversible as an associated cathodic peak appears in multiple scan
experiments (see Figure S2 in the Supporting Information). Two
possible explanations can been envisioned. One mechanism involves
the formation of a new species upon reduction of 7: 7 + e- f X.
This new species is then oxidized on the return wave at -1.37 V to
give compound Y, which is further oxidized at -1.18 V. A different
mechanism produces two species upon reduction of 7: 7 + e- f X
+ Y, which are oxidized in the return wave giving rise to the two
observed anodic peaks. To differentiate between these possibilities
and to determine the identity of the reduction products of 7, controlled-
potential bulk electrolysis and OTTLE experiments are ongoing.
(96) Because the peak potentials in Figure 6 are associated with irreversible
processes, the rate constants for the follow-up reactions affect the
observed position of the peak. Thus, the order of the true one-electron
reduction potentials could change if these rate constants are widely
different for the individual complexes.
(98) Martin, B. D.; Finke, R. G. J. Am. Chem. Soc. 1990, 112, 2419-
2420.
(99) Martin, B. D.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 585-592.
(100) Endicott, J. F.; Ferraudi, G. J. Am. Chem. Soc. 1977, 99, 243-
245.
(101) Endicott, J. F.; Netzel, T. L. J. Am. Chem. Soc. 1979, 101, 4000-
4002.
(102) Lott, W. B.; Chagovetz, A. M.; Grissom, C. B. J. Am. Chem. Soc.
(97) Using the same assumptions, the equilibrium constants for electron
transfer to the other complexes can be estimated as ∼2 × 10-5 for 5
and ∼3 × 10-4 for 6.
1995, 117, 12194-12201.
(103) Daikh, B. E.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 2938-
2943.
402 Inorganic Chemistry, Vol. 41, No. 2, 2002

Chlorinated Alkenylcobaloximes
Scheme 5
Table 3. Calculated LUMO Energies and Vertical Attachment Energies
(VAE) for Various Chlorinated Alkenes
alkene
PCE
VAE (eV)24
LUMO (eV)106
Ema (V)107
0.3
-1.689
-1.435
-1.140
-1.200
-1.200
-0.761
-0.60b
-0.67c
-0.80d
-0.96e
-1.01f
-1.14g
TCE
0.59
0.76
0.80
1.11
1.28
1,1-DCE
trans-1,2-DCE
cis-1,2-DCE
VC
a Estimated one electron redox potentials vs NHE from thermodynamic
data.107 b For the reaction PCE + e- a TCE• + Cl-. c For TCE + e-
a
1,2-trans-DCE• + Cl-. For going from TCE to Cl2CdCH•, a value of -1.0
V was estimated. d For 1,1-DCE + e- a •CCldCH2 + Cl-. e For trans-
1,2-DCE + e- a ClHCdCH• + Cl-. f For cis-1,2-DCE + e-
ClHCdCH• + Cl-. g For VC + e- a CH2dCH• + Cl-.
a
cage.103-105 Rapid reduction of (trichloroethenyl)cobalt com-
plexes under the reaction conditions might present a possible
explanation for their absence. As discussed in the previous
section, the reduction potentials of alkylcobalamins are
shifted to less negative values for axial alkyl ligands (-R)
that have a low pKa (R-H). A value of 18 has been reported
for the pKa of TCE from polarographic measurements.80
Based on this value, reductive Co-C bond cleavage of a
putative (trichlorovinyl)cobalt complexes could be quite
facile. Such reduction could lead to a partitioning resulting
in formation of trichlorovinyl radical as well as the corre-
sponding anion that is subsequently protonated (Scheme 5).
This scenario is consistent with the results of a labeling study
in which B12-catalyzed dechlorination of PCE was carried
out in a H2O/2-d1-iPrOH solvent mixture. The observed
distribution of deuterium in the TCE product suggested the
simultaneous involvement of both homolytic and heterolytic
mechanisms of product formation (Scheme 5).7
The reduction potentials of (trichloroethenyl)cobaloxime
and -cobalamin are not available, but the data in Figure 2
can be used to estimate the former. Comparing vinyl-
cobaloxime to the cis-chlorovinyl complex 5, the peak
potential is shifted by approximately 150 mV to less negative
potentials for the latter. Substituting the other two hydrogens
with chlorines resulted in a shift of 320 mV in going from
7 to 4. If the changes in peak potential induced by these
substitutions are additive, then a tentative shift in peak
potential of 470 ( 40 mV (320 + 150) might be expected
when replacing all three hydrogens of 7 with chlorines,
predicting a peak potential of ∼ -1.0 V for a putative
(trichloroethenyl)cobaloxime. This value is in fact close to
that for the Co(I)/Co(II) couple of dimer 8 (Epc ) -1.02 V)
and suggests that reduction of such a species by cob(I)-
aloxime could indeed be thermodynamically favorable. To
evaluate the additivity hypothesis, (trans-chloroethenyl)-
cobaloxime 13 was prepared by reaction of trans-DCE with
cob(I)aloxime.94 With compound 13 in hand, additivity of
peak potential shifts upon replacement of hydrogens in 7
with chlorines was experimentally tested. The Ep of 13 was
determined to be -1.26 V vs Ag+/AgCl, 190 mV less
negative than 7. Therefore, using the potentials for 6, 7, and
13, one arrives at a cumulative shift of 410 mV for
substituting two protons for chlorines and a predicted peak
potential of -1.04 V for 4, somewhat less negative than the
experimentally observed value of -1.13 V. Thus, the change
in redox potential upon replacement of a hydrogen with a
chlorine is not strictly additive in a quantitative sense, and
the putative (trichloroethenyl)cobaloxime would likely have
a peak potential between that of (dichloroethenyl)cobaloxime
4 (-1.13 V) and that calculated for a purely additive process
(-1.0 V). Therefore, although we cannot accurately predict
its reduction potential, the reduction potential of a putative
trichlorinated vinylcobaloxime is likely to be in the vicinity
of the cob(I)/(II)aloxime potential supporting its rapid
reduction under the conditions of eq 2.
Implications for B12-Catalyzed Reductive Dechlorina-
tions. It is worthwhile to compare the findings in this work
on cobaloxime models with the vitamin B12-catalyzed
dechlorination of PCE. In both processes the first step does
not lead to detectable trichloroethenyl complexes, but vinyl-
cobalt complexes containing one or two chlorines are
detected along with the parent vinyl compounds. Available
thermodynamic data may explain the change from a one-
electron reductive process to the formation of these vinyl-
cobalamins. The substrate for each subsequent dechlorination
is less electron deficient, and thus electron-transfer pathways
become less favorable in the series PCE, TCE, DCE, and
vinyl chloride (VC). Although experimentally determined one
electron reduction potentials are not available in solution,
the reduced electron affinity is supported by gas-phase
vertical attachment energies determined by low-energy
electron scattering measurements,106 as well as by computed
energies of the LUMO’s of these compounds (Table 3).106
Also listed in Table 3 are the estimated reduction potentials
for dissociative electron transfer calculated from thermody-
namic data.107 As a result of the progressively less favorable
energetics for electron transfer, chlorinated ethenylcobal-
amins are formed via either vinylic nucleophilic substitution
by cob(I)alamin or by addition to chloroacetylene. Our results
with cobaloximes suggest that the observed monochlorinated
vinylcobalamins in B12-catalyzed dechlorinations are pro-
duced by the reaction of cob(I)alamin with chloroacetylene
rather than DCE. Chloroacetylene formation as an intermedi-
ate during TCE dechlorination is supported by its direct
(104) Koenig, T. W.; Hay, B. P.; Finke, R. G. Polyhedron 1988, 7, 1499-
(106) Scherer, M. M.; Balko, B. A.; Gallagher, D. A.; Tratnyek, P. G.
EnViron. Sci. Technol. 1998, 32, 3026-3033.
(107) Totten, L. A.; Roberts, A. L. Crit. ReV. EnViron. Sci. Technol. 2001,
31, 175-221.
1516.
(105) Garr, C. D.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 10440-
10445.
Inorganic Chemistry, Vol. 41, No. 2, 2002 403

McCauley et al.
detection by GC-MS,8,11 and we have recently verified that
cob(I)alamin reacts readily with chloroacetylene to provide
(cis-chloroethenyl)cobalamin.108
progress to probe if the conclusions reached in this work
also hold for B12 derivatives.
Acknowledgment. This work was supported in part by
the Camille and Henry Dreyfus Foundation (Grant NF-97-
078), the Petroleum Research Fund administered by the
American Chemical Society (Grant 33490-G), and the
Beckman Foundation (BYI award to W.A.v.d.D.). We also
acknowledge funds from the Roy J. Carver Charitable Trust
for part of the work reported here. The Materials Chemistry
Laboratory at the University of Illinois in which the X-ray
structures were determined was supported in part by the NSF
(Grant CHE 95-03145). NMR data were obtained in the
Varian Oxford Instrument Center for Excellence, funded in
part by the W.M. Keck Foundation, NIH (Grant PHS 1 S10
RR10444), and NSF (Grant CHE 96-10502). Mass spectra
were recorded on a ZAB-SE mass spectrometer funded in
part by the Division of Research Resources, National
Institutes of Health (Grant RR 01575), the National Science
Foundation (Grant PCM 8121494), and the National Institute
of General Medical Sciences (Grant GM 27029).
Summary
The results detailed in this work support the following
conclusions. Chlorinated ethenylcobaloximes are stable
compounds that are unlikely to undergo homolytic Co-C
bond cleavage at a sufficient rate to support dechlorination
of chlorinated alkenes. These compounds can, however, be
dechlorinated via a reductive process that is promoted by
cob(I)aloxime. (Trichloroethenyl)cobaloxime has not been
detected, possibly because its reduction may occur under the
conditions used. Direct support for such a process, however,
is still lacking. The monochlorinated cobaloximes 5 and 6
are formed by reaction of cob(I)aloxime with chloroacetylene
rather than with cis- or trans-DCE, and no ethynylcobaloxime
was formed under the experimental conditions used. Com-
pounds 4-6 all display irreversible electrochemistry with
the observed peak potentials reflecting the relative electron-
withdrawing nature of the vinyl ligand. This trend is also
reflected in the Co-Npy bond distances in the X-ray
structures. Vinylcobaloxime 7 appears to display a reversible
wave at scan rates > 1 V/s. Investigations with vitamin B12
along the same lines as reported here for cobaloximes are in
Supporting Information Available: Listings of crystallographic
data as well as bond lengths and angles for complexes 4-7 in CIF
format and tables and figures for the X-ray structures. This material
is available free of charge via the Internet at http://pubs.acs.org.
(108) McCauley, K. M.; van der Donk, W. A. Unpublished results.
IC011023X
404 Inorganic Chemistry, Vol. 41, No. 2, 2002