Aqueous reductive dechlorination of chlorinated ethylenes with tetrakis(4-carboxyphenyl)porphyrin cobalt

Article abstract of DOI:10.1021/ic0504339

The catalytic dechlorination of chlorinated ethylenes by 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt ((TCPP)-Co), a cobalt complex structurally similar to vitamin B₁₂, was studied. It was found to have superior aqueous-phase dechlorination activity on chlorinated ethylenes (CEs) relative to vitamin B₁₂. Bimolecular rate constants for the degradation of CEs by (TCPP)Co of 250, 24, 0.24, and 1.5 M^-1 s^-1 were found for perchloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cDCE), and trans-dichloroethylene (tDCE), respectively. Through kinetic analysis, the rate laws for PCE and TCE were determined to be first order in substrate and catalyst and PCE degradation was shown to be sensitive to the concentration of the titanium citrate bulk reductant and pH. The importance of the Co^l oxidation state on dehalogenation was studied with UV-vis absorbance spectroscopy, a variety of reducing agents, and cyclic voltammetry. Evidence of chlorovinyl complexes as potential catalytic cycle intermediates was obtained through the preparation of (TPP)Co(trans-C ₂H₂Cl) and the observation of (TPP)Co-(C ₂HCl₂) and (TCPP)Co(C₂HCl₂) by mass spectrometry. The X-ray crystal structure of (TPP)Co(trans-C₂H ₂Cl) is reported.

Full text of DOI:10.1021/ic0504339

Inorg. Chem. 2005, 44, 4852−4861
Aqueous Reductive Dechlorination of Chlorinated Ethylenes with
Tetrakis(4-carboxyphenyl)porphyrin Cobalt
Joseph M. Fritsch and Kristopher McNeill*
Department of Chemistry, UniVersity of Minnesota, 225 Pleasant Street SE,
Minneapolis, Minnesota 55455
Received March 23, 2005
The catalytic dechlorination of chlorinated ethylenes by 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt ((TCPP)-
Co), a cobalt complex structurally similar to vitamin B12, was studied. It was found to have superior aqueous-phase
dechlorination activity on chlorinated ethylenes (CEs) relative to vitamin B12. Bimolecular rate constants for the
1
1
degradation of CEs by (TCPP)Co of 250, 24, 0.24, and 1.5 M- s- were found for perchloroethylene (PCE),
trichloroethylene (TCE), cis-dichloroethylene (cDCE), and trans-dichloroethylene (tDCE), respectively. Through kinetic
analysis, the rate laws for PCE and TCE were determined to be first order in substrate and catalyst, and PCE
degradation was shown to be sensitive to the concentration of the titanium citrate bulk reductant and pH. The
importance of the CoI oxidation state on dehalogenation was studied with UV
−vis absorbance spectroscopy, a
variety of reducing agents, and cyclic voltammetry. Evidence of chlorovinyl complexes as potential catalytic cycle
intermediates was obtained through the preparation of (TPP)Co(trans-C2H2Cl) and the observation of (TPP)Co-
(C2HCl2) and (TCPP)Co(C2HCl2) by mass spectrometry. The X-ray crystal structure of (TPP)Co(trans-C2H2Cl) is
reported.
Introduction
of its involvement in natural systems, potential in remedia-
tion, and diverse mechanisms utilized for degradation of
different CE substrates (including outer-sphere electron
transfer, inner-sphere electron transfer, and cobalt-carbon
bond formation) (Scheme 1).13-23
Chlorinated ethylenes (CEs), specifically perchloroethylene
(PCE) and trichloroethylene (TCE), are widespread ground-
water pollutants listed as United States Environmental
Protection Agency priority pollutants.1-3 Many studies have
been undertaken aimed at the dechlorination of these
compounds by zero-valent metals,4-6 electrochemical
processes,7-10 and microbial approaches.11,12 Dechlorination
catalysis involving vitamin B12 has attracted interest because
(12) Middeldorp, P. J. M.; Luijten, M. L. G. C.; Van De Pas, B. A.; Van
Eekert, M. H. A.; Kengen, S. W. M.; Schraa, G.; Stams, A. J. M.
Biorem. J. 1999, 3, 151-169.
(13) Gantzer, C. J.; Wackett, L. P. EnViron. Sci. Technol. 1991, 25, 715-
722.
(14) Glod, G.; Angst, W.; Holliger, C.; Schwarzenbach, R. P. EnViron.
* To whom correspondence should be addressed. E-mail: mcneill@
chem.umn.edu.
Sci. Technol. 1997, 31, 253-260.
(15) Glod, G.; Brodmann, U.; Angst, W.; Holliger, C.; Schwarzenbach, R.
P. EnViron. Sci. Technol. 1997, 31, 3154-3160.
(16) Burris, D. R.; Delcomyn, C. A.; Deng, B.; Buck, L. E.; Hatfield, K.
EnViron. Toxicol. Chem. 1998, 17, 1681-1688.
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Sci. Technol. 1996, 30, 3047-3052.
(1) Doherty, R. E. EnViron. Forensics 2000, 1, 83-93.
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R. M.; Zogorski, J. S. EnViron. Sci. Technol. 1999, 33, 4176-4187.
(4) Roberts, A. L.; Totten, L. A.; Arnold, W. A.; Burris, D. R.; Campbell,
T. J. EnViron. Sci. Technol. 1996, 30, 2654-2659.
(5) Arnold, W. A.; Roberts, A. L. EnViron. Sci. Technol. 1998, 32, 3017-
3025.
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J. Chimia 1997, 51, 908-914.
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32, 1207-1213.
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1805.
(20) Shey, J.; van der Donk, W. A. J. Am. Chem. Soc. 2000, 122, 12403-
12404.
(7) Li, T.; Farrell, J. EnViron. Sci. Technol. 2001, 35, 3560-3565.
(8) Li, T.; Farrell, J. EnViron. Sci. Technol. 2000, 34, 173-179.
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(21) McCauley, K. M.; Wilson, S. R.; van der Donk, W. A. Inorg. Chem.
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4852 Inorganic Chemistry, Vol. 44, No. 13, 2005
10.1021/ic0504339 CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/27/2005

(TCPP)Co Dechlorination Catalyst
Scheme 1. Partial Perchloroethylene (PCE) Dechlorination Pathway
Mediated by Vitamin B12
C6, 1000 ppm in He) were purchased from Matheson TriGas via
Alltech Associates, Inc. (Deerfield, IL). Acetylene (1.06% in He)
was purchased from Scott Specialty Gases via Alltech Associates,
Inc. Trisodium citrate monohydrate was purchased from Mallinck-
rodt. Both cis- and trans-DCE were purchased from TCI America.
Titanium(III) citrate was prepared in a manner similar to a
previously published method.32 Chromous ions were prepared by
reduction of CrCl3(aq) with Zn amalgam.33 Sodium benzophenone
ketyl was prepared by the addition of 0.9 equiv of sodium metal to
benzophenone in THF.
General Methods. 1H NMR and 13C NMR spectra were
determined at 300 and 75 MHz, respectively, on a Varian Inova
instrument. ESI-TOF mass spectra were collected on a Bruker
BioTOF II instrument. UV-vis absorbance spectra were collected
with a Jasco V530 spectrophotometer (300-700 nm) with a 4 mL
septum-sealed quartz cuvette. Cyclic voltammograms were collected
with a BAS 100B electrochemical analyzer with a normal three-
electrode configuration consisting of a highly polished glassy carbon
working electrode (A ) 0.07 cm2), a Pt auxiliary electrode, and a
Ag/AgCl reference electrode containing 1.0 M KCl.
Kinetic Time Courses. The following instrumentation was used
for studies of CE substrate, catalyst, pH, and titanium dependence.
Headspace samples (100 µL) were withdrawn and analyzed by
splitless injection with a Hewlett-Packard 5890 gas chromatograph,
a VOCOL column (30 m × 0.53 mm i.d., 3 µm film thickness,
Supelco), and a flame ionization detector. Isothermal temperature
programs (PCE 65 °C, TCE 55 °C, and DCEs 35 °C) were used
with helium carrier gas (flow ) 1 mL/min). Individual CE
congeners were identified and quantified by comparison of retention
time and peak areas to those of standard samples.
Mass Balance Determination. The following instrumentation
was used for determination and quantitation of PCE and TCE
degradation products. Headspace samples (100 µL) were analyzed
by splitless injection with a ThermoQuest Trace GC, a DB-1 column
(30 m × 0.32 mm i.d., 5 µm film thickness, J+W Scientific), and
a flame ionization detector. The system was computer controlled
with ChromeQuest software as the controller and data storage. The
temperature program used was 60 (5 min) f 240 (3.5 min) °C at
a rate of 20 °C/min with helium carrier gas (flow ) 1 mL/min).
Dechlorination products were identified and quantified by com-
parison of retention time and peak area to those of standard samples.
Authentic gas samples were used to quantify the amount of vinyl
chloride, acetylene, ethylene, and ethane produced.
Following a long tradition of modeling B12 with simpler
cobalt complexes, there have been studies using structurally
related cobalt complexes to model intermediates21-24 and
function as catalysts.25-27 Dror and Schlautmann have
recently reported on the PCE dechlorination activity of a
broad survey of metalloporphyrins (M ) Fe, Co, Ni).25
Among the findings, catalytic activity was linked to water-
soluble porphyrin complexes, including a catalyst precursor
tetrakis(4-carboxyphenyl)porphyrin cobalt(II) ((TCPP)Co)
that was found to have significant dehalogenation activity
against PCE. Vitamin B12 and (TCPP)Co have several
structural and chemical similarities, including planar tet-
rapyrrole coordination of cobalt and readily supported
common oxidation states of cobalt.28-31 Because of our
interest in the development of synthetic aqueous dehaloge-
nation catalysts, we sought to study the dechlorination
chemistry of (TCPP)Co in detail. Here, we report the results
of studies designed to establish the kinetic behavior and
substrate selectivity of (TCPP)Co with an emphasis on the
comparison to structurally similar vitamin B12 for which a
large amount of mechanistic work has already been per-
formed. This study also features the use of tetraphenyl-
porphyrin cobalt ((TPP)Co) and 5-(4-carboxyphenyl)-10,-
15,20-tri(phenyl)porphyrin cobalt ((MCPP)Co), which are
useful as synthetic models and in probing the importance of
the peripheral carboxylates of (TCPP)Co. Through reaction
kinetics, spectroscopic and crystallographic studies of inter-
mediates, substrate scope, and product selectivity, we
establish that (TCPP)Co is a superb synthetic mimic of B12
with superior dechlorination rates.
Experimental Section
Materials. 5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin co-
balt(II) ((TCPP)Co) and 5-(4-carboxyphenyl)-10,15,20-tri(phenyl)-
porphyrin cobalt(II) ((MCPP)Co) were purchased from Porphyrin
Systems. PCE, TCE, tetraphenylporphyrin cobalt(II) ((TPP)Co),
lithium aluminum hydride, chromium(III) chloride, benzophenone,
and tris(hydroxymethyl)aminomethane (Tris) were purchased from
Aldrich. Cyanocobalamin (vitamin B12) and titanium(III) chloride
(30 wt % in 2 N HCl) were purchased from Acros. Tetrabutyl-
ammonium hexafluorophosphate (TBA+PF6-) was purchased from
Fluka. Vinyl chloride (1000 ppm in He) and alkene standards (C2-
X-ray Structure Analysis. Diffraction data on (TPP)Co(trans-
C2H2Cl) mounted on a thin glass capillary with oil were collected
on a Siemens SMART Platform CCD diffractometer with Mo KR
radiation (graphite monochromator) at 173(2) K. The structure was
solved with direct methods using SHELXS-97 and refined using
SHELXL-97.34 Crystallographic data for (TPP)Co(trans-C2H2Cl)
are given in Table 1. Additional crystallographic data, including
tables of bond lengths and angles, are presented in the Supporting
Information.
Electrochemistry. Potentials are reported vs aqueous Ag/AgCl
and are not corrected for the junction potential. The E°′ values for
the ferrocenium/ferrocene couple for concentrations similar to those
used in this study were +0.44 V for DMF solutions at a glassy
carbon electrode. Cyclic voltammetry (CV) experiments were
performed at room temperature (23-24 °C). The 2 mL working
compartment was separated from the reference compartment by a
(24) Rich, A. E.; DeGreeff, A. D.; McNeill, K. Chem. Commun. 2002,
234-235.
(25) Dror, I.; Schlautman, M. A. EnViron. Toxicol. Chem. 2004, 23, 252-
257.
(26) Marks, T. S.; Allpress, J. D.; Maule, A. Appl. EnViron. Microbiol.
1989, 55, 1258-1261.
(27) Lewis, T. A.; Morra, M. J.; Habdas, J.; Czuchajowski, L.; Brown, P.
D. J. EnViron. Qual. 1995, 24, 56-61.
(28) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976,
98, 3484-3489.
(29) Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2260-2267.
(30) Lexa, D.; Saveant, J. M.; Soufflet, J. P. J. Electroanal. Chem.
Interfacial Electrochem. 1979, 100, 159-172.
(32) Smith, M. H.; Woods, S. L. Appl. EnViron. Microbiol. 1994, 60, 4107-
4110.
(33) Dunne, T. G.; Hurst, J. K. Inorg. Chem. 1980, 19, 1152-1157.
(34) SHELXTL, version 6.1; Bruker-AXS: Madison, WI, 2000.
(31) Lexa, D.; Lhoste, J. M. Experientia, Suppl. 1971, No. 18, 395-405.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4853

Fritsch and McNeill
Table 1. Crystal Data and Details of the Structural Refinement for
(TPP)Co(trans-C2H2Cl)
concentration was varied from 1.9 to 19.2 mM. Time points were
collected at regular time intervals for the degradation of the first
10% of the PCE to observe the initial rate of dehalogenation. The
initial reaction rates of PCE degradation were plotted against the
titanium concentration.
PCE Degradation. pH Dependence. Kinetic profiles of de-
halogenation reactions were prepared as above but in DI H2O. The
pH was adjusted from pH 5 to 9 with 1-150 µL spike additions of
5 M NaOH. The pH was measured before and after reductive
dehalogenation by sacrificing vials.
Pairwise CE Competition Studies. PCE/cDCE degradation
competition experiments were prepared as above with (TCPP)Co
(9.44 µM) and initial PCE and cDCE concentrations of 54 and 65
µM, respectively. TCE/cDCE competition experiments were pre-
pared with (TCPP)Co (9.44 µM) and initial TCE and cDCE
concentrations of 81 and 84 µM, respectively.
formula
fw (g/mol)
space group
T (K)
C46H30ClCoN4
733.12
P21/n
173(2)
λ (Å)
0.71073
10.0398(16)
16.148(3)
21.493(3)
90
94.380(3)
90
1.402
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
F
calcd (g cm-1
Z
)
4
vol (Å3)
3474.4(10)
6.12
0.0482, 0.1103
0.0770, 0.1228
µ (cm-1
)
R1,a R2b (%) [I > 2σ(I)]
R1,a R2b (%) all data
Generation of CoI Porphyrin Complexes. A variety of reducing
agents were used to generate the CoI oxidation state. The existence
of the CoI oxidation state was monitored with UV-vis absorbance
spectroscopy at 364 and 411 nm for (TCPP)Co, (TPP)Co, and
(MCPP)Co. Inner-sphere reducing agents, TiIII and CrII, were
used to generate the CoI oxidation state ([cobalt porphyrin] ) 7.5
µM, [TiIII] ) 0.8 mM, [CrII] ) 2.3 mM) in H2O, pH 8 (adjusted
with aliquots of NaOH) and 2:1 nPrOH/H2O (v/v). Additional
reducing agents were also used in dried, degassed THF ([cobalt
porphyrin] ) 7.5 µM, [LiAlH4(s)] ) saturated in THF, [sodium
benzophenone ketyl] ) 0.43 mM). The CoI porphyrin complex was
reacted with benzyl bromide (50 µM) as a trap yielding CoIII
complexes.
(Dichlorovinyl)(tetrakis(4-carboxyphenyl)porphyrin) Cobalt-
(III), (TCPP)Co(C2HCl2), 1. (TCPP)Co (2.1 mg, 2.5 µmol)
dissolved in THF was reduced with sodium benzophenone ketyl
(2 mL of 69 mM solution) dissolved in THF. The ketyl was
prepared with 0.9 equiv of sodium metal (15.1 mg, 0.66 mmol) to
favor the presence of the ketyl monoanion. The red porphyrin
solution turned dark green upon treatment with the ketyl, and then
TCE (0.5 mL, 5.62 mmol) was added. (-)-ESI-MS TOF (MeOH):
m/z M2- ) 470.0 [M]- ) 940.0.
(Dichlorovinyl)(tetraphenylporphyrin) Cobalt(III), (TPP)Co-
(C2HCl2), 2. A reaction sequence similar to the one above was
followed. (TPP)Co (2.2 mg, 3.3 µmol) dissolved in THF was
reduced with sodium benzophenone ketyl (2 mL of 69 mM solution)
dissolved in THF. The red porphyrin solution turned dark green
upon treatment with ketyl, and then TCE (0.5 mL, 5.62 mmol) was
added. (+)-ESI-MS TOF (MeOH): m/z M+ ) 766.11, [M + Na]+
) 789.10.
(trans-2-Chlorovinyl)(tetraphenylporphyrin) Cobalt(III), (TP-
P)Co(trans-C2H2Cl), 3. The methods of Sakurai and Setsune were
followed for this preparation.35,36 (TPP)CoCl (300 mg, 0.5 mmol)
was prepared using the method of Sakurai with overnight stirring,
and it was isolated in 72% yield. (TPP)Co(C2H2Cl) was prepared
using Setsune’s method. (TPP)CoCl (50 mg, 0.07 mmol) was
dissolved in 20 mL of CH2Cl2 in a sealed flask and degassed by
freeze-pump-thaw cycles. Acetylene (200 mL, 1 atm, 8.2 mmol)
was added via vacuum transfer. The flask was sealed, thawed, and
stirred overnight. Purification was performed by column chroma-
tography (SiO2, CH2Cl2) with a yield of 84%. X-ray quality crystals
were obtained by slow evaporation from a hexane/CH2Cl2 (3:1)
mixture. UV-vis (CH2Cl2): λmax 409, 527 nm. 1H NMR (CD2Cl2)
a R1 ) ∑||Fo| - |Fc||/∑Fo. b R2 ) [∑[w(Fo - Fc )2]/∑w(Fo )2]1/2
.
2
2
2
modified Luggin capillary. All three compartments were filled with
a 0.1 M solution of the supporting electrolyte, TBAPF6. The DMF
solvent was dried over 4 Å sieves, and the electrolyte and analyte
solutions (0.5 mM cobalt porphyrin) were degassed by two freeze-
pump-thaw cycles prior to use. The working compartment of the
cell was bubbled with argon to deaerate the solution. The electrolyte
solution background cyclic voltammograms were collected prior
to the analysis of the porphyrin solutions. The electrode was
removed from the solution and cleaned to remove any electrode
surface aggregates that formed during the electrochemical experi-
ments. The E°′ value for the ferrocenium/ferrocene couple was
determined for solutions and concentrations similar to those used
in the study of the porphyrin complexes to allow for the correlation
of the Ep,a and Ep,c values to past and future studies. As the peak
potentials, in general, depend on the concentration and scan rate,
the CV’s for the complexes were recorded at similar concentrations
(0.5 mM) and scan rates (100 mV/s).
CE Substrate Degradation Kinetic Profiles. The reductive
dehalogenation reactions were conducted in 42 mL septum-sealed
vials. A stock solution of (TCPP)Co was prepared in DI H2O with
NaOH. The total aqueous reaction volume in the vial was 10 mL.
Tris buffer (9 mL of a 110 mM stock solution, pH 8, diluted to 10
mL to give a final concentration of 100 mM) and catalyst
(concentrations varied by substrate, for PCE mass balance the
catalyst addition was as follows: 250 µL of a 30 µM stock solution
diluted to 10 mL to give a final concentration of 750 nM) were
added to the vial and deoxygenated by sparging with N2 for 20
min. The CE stock solution (500 µL of a 1.6 mM PCE and 0.34
mM toluene stock solution diluted to 10 mL to give a final
concentration of 80 µM PCE and 17 µM toluene) in methanol was
added to the vial. Vials were placed in a 35 °C water bath for 20
min. A time zero data point was collected, and titanium(III) citrate
(250 µL of a 640 mM stock solution diluted to 10 mL, final
concentration 16 mM) was added to initiate the reaction. The
dehalogenation reaction was monitored at regular intervals. Peak
areas from CE substrate in the standard and samples were compared
to determine the remaining CE concentration. Observed rate
constants were determined. The reactions of TCE, cDCE, and tDCE
were studied in a similar manner.
Catalyst Dependence Studies. Kinetic time courses were
prepared as above with the [(TCPP)Co] varied from 80 to 800 nM
for PCE degradation and from 180 to 1400 nM for TCE.
PCE Degradation. Titanium(III) Citrate Dependence Studies.
Reaction vials were prepared with [(TCPP)Co] ) 400 nM, and the
reaction was initiated as described above. The titanium(III) citrate
(35) Setsune, J.; Saito, Y.; Ishimaru, Y.; Ikeda, M.; Kitao, T. Bull. Chem.
Soc. Jpn. 1992, 65, 639-648.
(36) Sakurai, T.; Yamamoto, K.; Naito, H.; Nakamoto, N. Bull. Chem. Soc.
Jpn. 1976, 49, 3042-3046.
4854 Inorganic Chemistry, Vol. 44, No. 13, 2005

(TCPP)Co Dechlorination Catalyst
Table 2. Rate Constants of Chlorinated Ethylene Dehalogenation by (TCPP)Co
catalyst
concn (µM)
kobs
k
products
(yield)
substrate one e-
substrate
PCE
(×10-4 s-1
)
(M-1 s-1
)
redox potential (E°, mV)a
0.68
1.69 ( 7
250
24
TCE (87%)
cDCE (5%)
cDCE (84%)
tDCE (4%)
C2H2 (4%)
C2H3Clb,c
-0.598
-0.674
TCE
0.91
0.22 ( 3
cDCE
tDCE
28
16
0.065 ( 0.4
0.235 ( 0.1
0.24
1.5
-0.955
-1.012
C2H3Clb,d
a Calculated redox potentials vs SHE.37 b The C2H3Cl yield was not determined, but its identity was verified with an authentic standard. c Isomerization
to tDCE accounting for 33% of the cDCE loss was also observed. d Isomerization to cDCE accounting for 17% of the cDCE loss was also observed.
Figure 1. PCE degradation by 765 nM (TCPP)Co with the concurrent
appearance of TCE and cDCE at 35 °C. [TiIII] ) 16 mM. PCE b, TCE 9,
cDCE 1, mass balance O. Curves are nonlinear first-order decay and growth
fits to the data. No tDCE was observed.
Figure 2. TCE degradation by 3.8 µM (TCPP)Co with the concurrent
appearance of cDCE and other CEs at 35 °C. [TiIII] ) 16 mM. TCE 9,
cDCE 1, tDCE 2, mass balance O. Trace amounts of 1,1-DCE, vinyl
chloride, acetylene, and ethylene were observed but not shown above.
Curves are nonlinear first-order decay and growth fits yielding the observed
rate constants for degradation and formation.
δ 8.91 (s, 8H), 8.12 (s, 8H), 7.75 (s, 12H), -0.40 (d, J ) 11.7 Hz,
1H), -0.92 (d, J ) 11.7 Hz, 1H). 13C NMR (CD2Cl2) δ 145.4,
141.3, 133.6, 133.1, 128.0, 127.0, 122.1, 99.7. ESI-MS TOF
(MeOH): m/z M+ ) 732.15, [M + Na]+ ) 755.13.
TCE degradation was found to occur an order of magni-
tude slower than the PCE degradation rate (2.2 × 10-5 s-1,
910 nM catalyst, 35 °C). The major degradation product was
cDCE (84%), and other observed products included tDCE
(4%), 1,1-dichloroethylene (<1%), vinyl chloride (<1%),
acetylene (4%), and ethylene (<1%) (Table 2). During
dechlorination, a product selectivity of 21:1 for cDCE over
tDCE was observed. The mass balance for TCE degradation
was determined from the concentrations of TCE, cDCE,
tDCE, and acetylene at each time point, and approximately
90% of the C2 mass could be accounted for (Figure 2).
Both cDCE and tDCE are degraded considerably slower
than PCE and TCE with cDCE being the most difficult to
dechlorinate (cDCE, 6.5 × 10-6 s-1, 28 µM catalyst, 35 °C;
tDCE, 2.3 × 10-5 s-1, 16 µM catalyst, 35 °C). In each case,
the major degradation product was vinyl chloride. Isomer-
izations of cDCE and tDCE were observed during the
dechlorination experiments. During cDCE degradation, the
rate of isomerization of cDCE to tDCE was 33% of the
overall cDCE degradation rate, determined by comparing the
initial rates of degradation and formation. The rate of
isomerization of tDCE to cDCE was found, in a similar
manner, to be 17% of the overall tDCE degradation rate.
No experiments were conducted to elucidate the isomeriza-
tion mechanism.
Results
CE Degradation Kinetics. The kinetics of the aqueous
reductive dechlorination of PCE, TCE, cDCE, and tDCE
catalyzed by (TCPP)Co were studied under pseudo-first-order
conditions at pH 8 with titanium citrate as the bulk reducing
agent. PCE and TCE were degraded under the reaction
conditions, and the decay curves were well-fit to a single
exponential over three half-lives indicating that the degrada-
tion of the chlorinated ethylene substrate was first order
(Table 2). Because of the much slower dechlorination rates,
the kinetic decays of cDCE and tDCE degradations were
followed only for one to two half-lives (see Supporting
Information). Because of this, the kinetic order is less certain
for these substrates, but they also appear to follow first-order
kinetics on the basis of fits to single exponential decays.
During the control experiments, no dehalogenation was
observed for any of the CE substrates in the absence of
catalyst or bulk reductant.
PCE was rapidly degraded under the reaction conditions
(1.69 × 10-4 s-1, 680 nM catalyst, 35 °C). During PCE
degradation, TCE was the major product (87%) with a small
amount of cDCE (5%) also observed (Table 2). The mass
balance for PCE degradation was determined from the
concentrations of PCE, TCE, and cDCE (no tDCE detected)
at each time point, and approximately 90% of the C2 mass
could be accounted for (Figure 1).
Pairwise competition experiments (PCE/cDCE and TCE/
cDCE) were carried out to further establish the faster
degradation of the more chlorinated congener in the presence
of its dehalogenation products and as a test for catalyst
poisoning by product inhibition through the formation of an
Inorganic Chemistry, Vol. 44, No. 13, 2005 4855

Fritsch and McNeill
Figure 3. PCE degradation rate constant (kobs) vs [(TCPP)Co]. The
reactions were performed in pH 8 water at 35 °C. [TiIII] ) 16 mM. Error
bars represent the standard error of the nonlinear fits to the data.
Figure 5. Initial rate of PCE degradation by (TCPP)Co (400 nM) during
kinetic time courses in which the concentration of titanium(III) citrate was
varied. Error bars represent the error determined from nonlinear fitting of
the data.
Figure 4. TCE degradation rate constant (kobs) vs [(TCPP)Co]. The
reactions were performed in pH 8 water at 35 °C. [TiIII] ) 16 mM. Error
bars represent the standard error of the nonlinear fits to the data.
Figure 6. PCE degradation in DI water. [(TCPP)Co] ) 770 nM and [TiIII]
) 16 mM, at 35 °C. The pH was adjusted with 1-150 µL additions of 5
M NaOH. The pH was measured before and after each reductive dehalo-
genation experiment. Error bars represent the standard error of the nonlinear
fits to the data.
intermediate of low reactivity. During PCE degradation, no
changes to the observed rate constants were observed in the
presence of cDCE. For TCE, the presence of cDCE did not
change the kinetics of TCE dehalogenation. Importantly, the
presence of the slower reacting substrate did not inhibit the
degradation of the faster one through catalyst poisoning.
The catalyst dependence of PCE and TCE degradation was
determined by varying the catalyst concentration during
degradation. For PCE, the concentration of (TCPP)Co was
varied over the range of 80-800 nM. The observed rate
constant varied linearly in response to (TCPP)Co concentra-
tion indicating a first-order dependence on the catalyst
(Figure 3).
For TCE, the concentration of (TCPP)Co was varied over
the range of 200-1400 nM. The observed rate constant
varied linearly in response to (TCPP)Co concentration
indicating a first-order dependence on catalyst (Figure 4).
We believe that the non-zero intercept may be the result
of inhibition of the catalyst by an impurity. The nature of
this impurity was pursued, and a TCE impurity, 1,1,2-
trichloroethane, TCA, was identified with GC-MS. It was
found to be present at 0.004% in TCE. While this concentra-
tion is too low to inhibit the (TCPP)Co-mediated TCE
degradation at the level observed through the formation of
a kinetically stable intermediate, experiments performed with
both TCE (100 µM) and varying amounts of TCA (0, 7.0,
and 70 µM) present showed that the TCE dehalogenation
was inhibited by TCA (see Supporting Information).
Titanium(III) Citrate Dependence. The concentration of
titanium(III) citrate was varied (1.9-19.2 mM), and the
initial rates of PCE dechlorination were determined with 400
nM (TCPP)Co (Figure 5). At low concentrations, the rate
increased linearly with bulk reductant concentration, but at
high concentrations, saturation was observed. The trend line
illustrates two zones of titanium dependence in the PCE
degradation rate law: a linear first-order region and a
saturation zero-order region.
pH Dependence. As the pH was varied from 5 to 9, the
observed rate constant of PCE dechlorination (770 nM
(TCPP)Co) changed significantly. No dechlorination activity
was observed below pH 7.0, but the observed rate constant
increased linearly between pH 7 and 9 (Figure 6).
Generation and UV-Vis Characterization of CoI Por-
phyrin Complexes. UV-vis absorbance spectroscopy was
used to observe the cobalt oxidation state resulting from the
interaction between (TCPP)CoII and TiIII. Cobalt porphyrin
complexes have characteristic Soret band absorbance maxima
for each oxidation state. The CoI oxidation state is character-
ized by two bands in the Soret region with maxima at 364
and 411 nm and a reduced molar absorptivity coefficient
relative to the CoII and CoIII oxidation states.38-40 In addition
(37) Totten, L. A.; Roberts, A. L. Crit. ReV. EnViron. Sci. Technol. 2001,
31, 175-221.
(38) Kobayashi, H.; Hara, T.; Kaizu, Y. Bull. Chem. Soc. Jpn. 1972, 45,
2148-2155.
(39) Momenteau, M.; Fournier, M.; Rougee, M. J. Chim. Phys. Phys.-
Chim. Biol. 1970, 67, 926-933.
4856 Inorganic Chemistry, Vol. 44, No. 13, 2005

(TCPP)Co Dechlorination Catalyst
Table 3. Reducing Agents and Cobalt(I) Absorbance Spectrum
Observed
Table 4. Cyclic Voltammetry of the Cobalt Porphyrins Collected in 0.1
M [nBu4N][PF6] DMF with a Ag/AgCl Reference Electrode
generation of CoI c
CoI/II redox
CoI/II redox
reducing
metal complex
couple (mV)a
couple (mV)b
ref
agenta
solventb
H2O, pH 8
2:1 nPrOH/H2O, pH 8
(H2O)6 CrII H2O, pH 8
2:1 nPrOH/H2O, pH 8
THF
Na+Ph2CO- THF
(TPP)Co (TCPP)Co (MCPP)Co
(TCPP)Co
(TPP)Co
-700
-695
-745
-740
-870c
-800d
-725
this work
this work
29
TiIII citrate
d
no
d
no
yes
yes
yes
yes
yes
yes
yes
yes
d
no
d
no
yes
yes
45
(MCPP)Co
-680
this work
LiAlH4
a Referenced to the Ag/AgCl electrode. b Referenced to the SCE
electrode. c DMF with TBAClO4. d DMF with TEAClO4.
a [TiIII citrate] ) 0.8 mM; [(H2O)6 CrII] ) 2.3 mM; [LiAlH4] ) saturated
in THF; [Na+Ph2CO-] ) 0.43 mM. b All porphyrin complexes soluble at
7.5 µM unless otherwise indicated. c Based on appearance of bands at 364
and 411 nm. d Porphyrin complex not soluble.
(TCPP)Co, (MCPP)Co, and (TPP)Co with 0.5 mM cobalt
complex in 0.1 M TBAPF6 in DMF with a Ag/AgCl
reference electrode. Cyclic voltammetry was used to deter-
mine the Ep,c and Ep,a for each complex (Table 4).
Chlorovinyl Cobalt Porphyrin Complexes. Mass Spec-
trometric Observation of Dichlorovinyl Cobalt Complexes
(TCPP)Co(C2HCl2) and (TPP)Co(C2HCl2). A dichloro-
vinyl cobalt porphyrin complex was observed in the reaction
of in situ generated (TCPP)CoI with TCE. The (TCPP)CoII
complex was dissolved in THF and reduced with sodium
benzophenone ketyl, and the intensity of the color dimin-
ished. Then, 3 drops of pyridine were added, followed by
500 µL of neat TCE (eq 2). An immediate color change, to
a brown-green solution, upon addition of TCE was observed.
Figure 7. (TCPP)Co (7.5 µM) in H2O at pH 8 treated with CrII and
(aq)
then benzyl bromide. Dashed line (TCPP)CoII spectrum, solid line (TCPP)-
CoI spectrum (x2 absorbance values), and dotted line after addition of benzyl
bromide.
+
-
Na Ph2CO
TCE
(TCPP)CoII
8 (TCPP)CoI THF, py8
(TCPP)CoIII(C2HCl2) (2)
THF
to (TCPP)Co, the interactions between (TPP)Co and (MCP-
P)Co and various reducing agents were compared for their
ability to yield CoI spectra for the three different cobalt
porphyrin complexes.
The analysis of the reaction products with negative mode
electrospray ionization ((-)-ESI-TOF) mass spectrometry
indicated masses for M- and M2- (940.0 and 470.0 m/z,
respectively) for a (TCPP)Co(C2HCl2) complex, 1, and the
signals exhibited an isotope pattern indicative of the incor-
poration of two chlorine atoms (high-resolution mass spec-
trometry (HRMS) was unsuccessful).
In a similar manner, (TPP)Co was treated with the same
reaction conditions, and the products were analyzed with (+)-
ESI-TOF. The observed masses for M+ and [M + Na]+
(766.11 and 789.10 m/z, respectively) were consistent with
those of (TPP)Co(C2HCl2), 2. The isotope patterns were
consistent with two chlorine atoms being incorporated into
the products. The composition of (TPP)Co(C2HCl2) was
confirmed with ESI-TOF HRMS by comparison to a pro-
pylene glycol 1000 (1000 PPG) standard.
Cobalt porphyrin solutions were prepared with 7.5 µM
cobalt porphyrin in several solvents (H2O pH 8, 2:1 nPrOH/
H2O (v/v) pH 8, and THF). (TPP)Co and (MCPP)Co were
not soluble in water, but all of the cobalt porphyrin
complexes were soluble in THF and 2:1 nPrOH/H2O. A
variety of reducing agents (titanium(III) citrate, aqueous
chromous chloride, lithium aluminum hydride, and sodium
benzophenone ketyl) were used to generate CoI (Table 3).
Solutions of the complexes were treated with the reducing
agents, and the changes in the absorbance spectra were
monitored (Figure 7). In the experiments that resulted in the
appearance of the characteristic CoI spectrum, the presence
of CoI was further verified through reaction with benzyl
bromide (eq 1).30,41-44 The change of the oxidation state to
CoIII was monitored by changes in the absorbance spectrum.
Preparation of (TPP)Co(trans-C2H2Cl). The (TPP)CoCl
complex was prepared by the method of Sakurai,36 and
(TPP)Co(trans-C2H2Cl), 3, was prepared from the chloro
complex following the method of Setsune35 (eq 3). It was
isolated in a 60% overall yield.
benzyl
(P)CoII reducing8 (P)CoI bromide8 (P)CoIII-benzyl
(1)
agent
Electrochemistry. Cyclic voltammetry was used to de-
termine the CoI/CoII potential for the redox couples of
C2H2
HCl concd
(TPP)CoII MeOH, rt 8 (TPP)CoIIICl CH Cl , rt8
2
2
(40) Whitlock, H. W., Jr.; Bower, B. K. Tetrahedron Lett. 1965, 4827-
4831.
(41) Fukuzumi, S.; Maruta, J. Inorg. Chim. Acta 1994, 226, 145-150.
(42) Maiya, G. B.; Han, B. C.; Kadish, K. M. Langmuir 1989, 5, 645-
650.
(TPP)CoIII(C2H2Cl) (3)
(44) Zhou, D. L.; Walder, P.; Scheffold, R.; Walder, L. HelV. Chim. Acta
1992, 75, 995-1011.
(43) Zeng, Z.; Jewsbury, R. A. Analyst 1998, 123, 2845-2850.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4857

Fritsch and McNeill
Table 5. Comparison of Vitamin B12 and (TCPP)Co Bimolecular Rate
Constants
a
substrate B12 k (M-1 s-1
)
(TCPP)Co k (M-1 s-1
)
k((TCPP)Co)/k(B12)
PCE
TCE
44.1
1.69
250
24
5.6
14
cDCE
tDCE
0.01
0.03
0.24
1.5
24
50
a Reaction conditions: 46 µM vitamin B12, 27 mM titanium(III) citrate,
2.2 mM CE substrate, pH 8.2, 22 °C.13
genation.13-15,20,22,23 From the work of Pasternack and the
pH during dehalogenation, we propose that (TCPP)CoII
should be coordinated with a hydroxo and an aquo ligand
prior to reduction.46 As established by the UV-vis experi-
ments, the reduction of (TCPP)CoII by TiIII citrate yields
(TCPP)CoI, which is critical to CE dehalogenation because
no degradation was observed in controls without TiIII citrate.
On the basis of solution studies and the solid-state structure
of (TPP)CoI, the cobalt(I) oxidation state should be free of
axial ligands.29,38,47,48 The mass spectrometry results from the
reaction of (TCPP)CoI or (TPP)CoI with TCE and the
isolation and single-crystal structure of (TPP)Co(trans-C2H2-
Cl) give evidence for the intermediacy of chlorovinyl cobalt
complexes.
Figure 8. X-ray structure of (TPP)Co(trans-C2H2Cl) with thermal
ellipsoids drawn at the 50% probability level.
Scheme 2. Simple Proposed Catalytic Cycle for the
(TCPP)Co-Mediated Reductive Dechlorination of Chlorinated Ethylenes,
Exemplified by the Conversion of TCE to cDCE
The B12 and (TCPP)Co systems share a number of
similarities that suggest they may operate by the same
mechanisms. Most notably, the two systems degrade PCE
and TCE through the sequential replacement of chlorine
atoms by hydrogen atoms with the same empirical rate laws.
Both systems display higher degradation rate constants for
the more chlorinated congeners. In addition, the importance
14,20
of the CoI oxidation state to the vitamin B12
and
(TCPP)Co systems and the preparation of chlorovinyl cobalt
complexes are further indications of shared mechanisms.
(TCPP)Co is a highly active aqueous-phase dechlorination
catalyst which works against CEs. Degradation rate laws for
PCE (eq 4) and TCE (eq 5), based on the kinetic data, are
first order in both catalyst and CE substrate.
An absorbance spectrum was collected in CH2Cl2 of the
purified material with λmax values of 409 and 527 nm, which
agrees with the reported data.35 The 1H NMR chemical shift
of the vinyl protons is notable because they are significantly
shielded upfield to δ -0.40 and -0.92 and have a coupling
constant of 11.7 Hz, which is slightly smaller than the normal
12-18 Hz range for trans-alkenyl protons, but consistent
with the reported spectrum.35 The composition was confirmed
with (+)-ESI-TOF HRMS by comparison to the standard.
X-ray quality crystals were obtained by slow evaporation
from a 3:1 mixture of hexanes and CH2Cl2. The structure is
shown in Figure 8 and confirms the trans-chlorovinyl
confirmation. (See Supporting Information for crystal-
lographic data.) The cobalt atom is 0.123 Å above the N4
porphyrin plane, showing little distortion from planarity. The
Co-N distances are equal and are consistent with other Co
porphyrin structures. The Co-C bond distance (1.926 Å) is
slightly shorter than those of other chlorovinyl cobalt
complexes.21-24 (See Supporting Information.)
-d[PCE]
) k[PCE][(TCPP)Co]
) k[TCE][(TCPP)Co]
(4)
(5)
dt
-d[TCE]
dt
Comparison of the dechlorination bimolecular rate con-
stants of B12 to our (TCPP)Co results yields a dechlorination
enhancement of 5.6 and 14 times for PCE and TCE,
respectively, over B12 (Table 5).
During TCE degradation, both vitamin B12 and (TCPP)-
Co yielded a decisive product selectivity for cDCE over
tDCE. For (TCPP)Co, a 21:1 selectivity of cDCE over the
tDCE isomer was observed, while for B12, there was a 23:1
selectivity for the cis isomer. For B12, this selectivity has
Discussion
(45) Giraudeau, A.; Callot, H. J.; Jordan, J.; Ezhar, I.; Gross, M. J. Am.
Chem. Soc. 1979, 101, 3857-3862.
Simplified Catalytic Cycle. We propose the simplified
catalytic cycle for (TCPP)Co degradation of CEs shown in
Scheme 2 on the basis of these experimental results and the
catalytic cycle for vitamin B12-mediated reductive dehalo-
(46) Pasternack, R. F.; Parr, G. R. Inorg. Chem. 1976, 15, 3087-3093.
(47) Doppelt, P.; Fischer, J.; Weiss, R. Inorg. Chem. 1984, 23, 2958-
2962.
(48) Kellett, R. M.; Spiro, T. G. Inorg. Chem. 1985, 24, 2373-2377.
4858 Inorganic Chemistry, Vol. 44, No. 13, 2005

(TCPP)Co Dechlorination Catalyst
been explored with CEs13,14,49 and CE analogues20 and has
been regularly observed. This selectivity appears to be the
result of the formation of (E)-dichlorovinyl cobalt com-
plexes23,24 as catalytic cycle intermediates, and this is a likely
cause in the (TCPP)Co system as well.
TiIII citrate), and even at this ratio, the calculated conversion
of (TCPP)CoII to (TCPP)CoI is only 70% (see Supporting
Information). The saturation behavior in the TiIII citrate
dependence is consistent with competitive destruction of
(TCPP)CoI by reaction with PCE. Application of the steady-
state approximation for (TCPP)CoI (eq 8) to the mechanism
outlined in eqs 6 and 7 gives the rate law shown in eq 9
where [Co]T ≈ [CoI] + [CoII].
An interesting finding for TCE degradation was the
observation that TCA inhibits the degradation. This fact
combined with the non-zero intercept of the dependence of
the TCE dechlorination rate on catalyst concentration (Figure
4) suggested that TCA contamination might be responsible
for the apparent inhibition at catalyst concentrations below
200 nM. However, the TCA impurity concentration is too
low (4 nM TCA in a 100 µM TCE solution) to fully account
for the inhibition observed, and TCA does not appear to be
a potent inhibitor. The apparent lack of catalyst activity at
concentrations below 200 nM may be a result of inhibition
by an as yet unidentified species. Nevertheless, the fact that
TCA can act as an inhibitor at higher concentrations raises
concerns about the use of (TCPP)Co for remediation of sites
with a mixture of chlorinated ethenes and ethanes.
k1[CoII][TiIII]
k-1[TiIV] + k2[PCE]
[CoI] )
(8)
[TiIII]
k1
k2
[Co]T [PCE]
IV
k-1 [Ti ]
d[PCE]
) -
(9)
[TiIII]
dt
k1
k2
1 +
+
[PCE]
IV
k-1 [Ti ] k-1
Under our experimental conditions ([PCE] e 100 µM),
k2/k-1[PCE] , 1 which allows us to simplify the rate law
(eq 10).
The slow degradation of cDCE by (TCPP)Co could be
caused by either the slow initial reaction between (TCPP)-
CoI and cDCE or the slow regeneration of the active catalyst
species through the formation of stable intermediates that
deactivate a portion of the catalyst. By stable intermediates,
we mean species formed that are effectively unreactive over
the course of hours (the time scale of our experiments) under
the catalytic conditions employed and, thus, represent
catalytically inactive cobalt porphyrin species. To test this,
PCE degradation was performed in the presence of cDCE
and compared to control experiments without cDCE. Simi-
larly, TCE degradation was performed in the presence of
cDCE. Inhibition was not observed in either case, indicating
that the slow catalytic degradation of cDCE likely results
from the slow reaction between (TCPP)CoI and cDCE and
not the formation of a kinetically stable porphyrin complex
that slows or prevents catalyst turnover.
Titanium Dependence. Varying the titanium(III) citrate
concentration had a strong effect on the initial rates of PCE
dehalogenation. At a low reductant concentration, a linear
relationship between increasing [TiIII] and rate was observed
suggesting a first-order dependence of the rate-limiting step
upon the bulk reductant. However, at higher TiIII concentra-
tions, saturation was reached indicating no bulk reductant
dependence. The degradation of PCE can be represented as
the sum of two processes: catalyst activation (eq 6) and
reaction with CE substrate (eq 7).
[TiIII]
k1
k2
[Co]T[PCE]
IV
k-1 [Ti ]
d[PCE]
) -
) kobs[PCE] (10)
[TiIII]
dt
k1
1 +
IV
k-1 [Ti ]
The low and high [TiIII] limits are given in eqs 11 and 12.
[TiIII]
[TiIII]
k1
k-1 [Ti ]
k1
kobs ) k2
[Co]T;
, 1
(11)
(12)
IV
IV
k-1 [Ti ]
[TiIII]
k1
kobs ) k2[Co]T;
. 1
IV
k-1 [Ti ]
We believe that the simple kinetic model satisfactorily
explains the TiIII dependence on the reaction kinetics and
indicates that the most favorable condition for PCE degrada-
tions is when the Ti/Co ratio is high enough to be near the
saturated rate limit (>20 000:1).
pH Dependence. The pH of the reaction conditions has a
significant effect on the degradation kinetics and on the
chemical properties of the constituents of this system (i.e.,
the titanium(III) citrate reduction potential, (TCPP)Co aque-
ous solubility, and the protonation state of the catalyst). The
titanium reduction potential varies with pH; at pH 7 the
reduction potential is -480 mV, and it varies linearly (-60
mV/pH) from pH 7 to 9.15,50 As a result, the pH data can be
interpreted with an electrochemical analysis based on the
redox couples of titanium citrate and (TCPP)Co and their
concentrations (Figure 9 and see Supporting Information).
However, the pH versus degradation data may reflect a
preference for a specific protonation state of the catalyst
during the reduction. On the basis of the pH of the
dechlorination reaction conditions, the (TCPP)Co protonation
state in Scheme 2 is suggested by the following data. (TCPP)-
Co has sparing aqueous solubility in strongly acidic solutions
(TCPP)CoII + TiIII yk1z (TCPP)CoI + TiIV
(6)
k-1
k2
9
(TCPP)CoI + PCE
8 (TCPP)CoII + products
(7)
Reduction of (TCPP)CoII to (TCPP)CoI occurs, despite the
fact that (TCPP)CoI is a stronger reductant than Ti(III) citrate,
by virtue of the large excess of TiIII. In a typical experiment,
the Co/Ti ratio is 1:40 000 (400 nM (TCPP)CoII and 16 mM
(49) Follett, A. D.; McNeill, K. J. Am. Chem. Soc. 2005, 127, 844-845.
(50) Zehnder, A. J. B.; Wuhrmann, K. Science 1976, 194, 1165-1166.
Inorganic Chemistry, Vol. 44, No. 13, 2005 4859

Fritsch and McNeill
of the degradation conditions. (TPP)Co and (MCPP)Co were
used because of the differences between their functional
groups and those of (TCPP)Co. In aqueous solvent mixtures,
(TCPP)Co was more amenable to reduction to CoI with TiIII
or CrII than either (TPP)Co or (MCPP)Co. Yet in THF with
powerful reducing agents, LiAlH4 and NaPh2CO, spectro-
scopic evidence of CoI generation was obtained for all cobalt
porphyrins. The differences in reducibility between (TCPP)-
Co and the other cobalt porphyrin complexes is not a result
of differences in redox potential. As has been reported, meso
substitution of cobalt porphyrins has a negligible effect on
the redox couple of the metal center.28,58 This observation is
also supported by our electrochemical measurements. Even
though there are essentially no electrochemical differences
among (TPP)Co, (MCPP)Co, and (TCPP)Co, the latter is
more readily reduced, indicating that the carboxylic acid
groups do have some as yet undetermined effect on facilitat-
ing reduction.
Chlorovinyl Cobalt Porphyrin Complexes. There is a
growing body of evidence that implicates chlorovinyl
complexes as intermediates in the catalyzed dechlorination
of CEs by B12 through their observation and synthesis for
the native catalyst and with model complexes.14,22-24,63 We
have obtained evidence for such complexes as intermediates
in this system through the reaction of in situ generated CoI
and TCE yielding 1 and 2 and the preparation of 3. The
preparation of 3 and its stability indicate that isolation and
characterization of the dichlorovinyl cobalt complexes (1 and
2) may be possible, and such work is ongoing.
Figure 9. pH dependent active catalyst concentration vs PCE degradation,
obs ([(TCPP)Co]tot ) 770 nM, [TiIII] ) 16 mM, 35 °C), as determined by
electrochemical data and species concentrations. Data points represent
individual kinetic profiles at different pH values, and error bars are the
error in the least-squares fitting of the degradation data.
k
but is readily soluble in alkaline solutions (see Supporting
Information). This effect is likely due to the pKa values of
(TCPP)Co, which can be estimated or are known. If the
carboxyphenyl moieties are similar to the substituted benzoic
acids, the pKa values would be ∼4.2. Two cobalt-coordinated
water molecules have pKa values of 7.5 and 9.0.46 Thus,
between pH 4 and 7, predominantly peripheral deprotonation
of the carboxylic acids takes place. Yet, under those
conditions, no dehalogenation activity was observed. The
fractional composition of a complex with at least one
deprotonated coordinating waters increases above approxi-
mately pH 7.0 and dehalogenation is observed (see
Supporting Information).
With (TCPP)Co in aqueous solution at pH 8, two types
of binding are available for coordinating a reduced metal
center (e.g., TiIII or CrII) through the peripheral carboxylates
and the metal-bound hydroxo/aquo ligands. Arguments for51
and against52 the involvement of the peripheral moieties, such
as the carboxylates of TCPP complexes, in electron-transfer
reactions have been put forth. There are significant precedents
for OH and other groups to act as bridging ligands for
electron transfer with TiIII and CrII as the reducing agents.51-56
We, currently, do not favor an inner-sphere pathway involv-
ing the peripheral carboxylates. Evidence against a peripheral
inner-sphere mechanism comes from the pH dependence data
(where no dehalogenation was observed despite the presence
of free carboxylates) and dehalogenation experiments using
CrII as the bulk electron source. With a peripheral pathway,
we expected, because of the formation of kinetically inert
CrIII carboxylate complexes, that at most four turnovers
would be possible in this system; more than 10 turnovers
were observed (see Supporting Information).
Conclusions
A highly active dechlorination catalyst has been identified,
and its kinetic activity has been characterized under a variety
of conditions. While similar in its dehalogenation behavior,
(TCPP)Co is kinetically superior to vitamin B12. The
dechlorination is sensitive to pH and shows heightened
activity in response to increasing concentrations of titanium
citrate. Mass spectrometric evidence for chlorovinyl cobalt
porphyrin complexes as catalytic cycle intermediates has
been obtained, and a chlorovinyl model complex of (TPP)-
Co has been prepared and structurally characterized.
Acknowledgment. This work was funded in part by the
EPA STAR Fellowship program (Grant U-91610701-1) and
the National Science Foundation (Grant CHE-0239461). We
thank Victor G. Young, Jr., and the X-ray Crystallographic
Laboratory for help in solving the (TPP)Co(trans-C2H2Cl)
structure.
Co(I) as the Active Oxidation State. In an effort to
elucidate the active dehalogenation oxidation state and the
preferred cobalt reduction pathway, we subjected (TCPP)-
Co, (TPP)Co, and (MCPP)Co to the reducing environment
(57) Zheng, G.; Stradiotto, M.; Li, L. J. Electroanal. Chem. 1998, 453,
79-88.
(58) Kadish, K. M.; Morrison, M. M. Bioinorg. Chem. 1977, 7, 107-115.
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480-490.
(51) Fleischer, E. B.; Cheung, S. K. J. Am. Chem. Soc. 1976, 98, 8381-
8387.
(52) Balahura, R. J.; Trivedi, C. P. Inorg. Chem. 1978, 17, 3130-3132.
(53) Pasternack, R. F.; Sutin, N. Inorg. Chem. 1974, 13, 1956-1960.
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4860 Inorganic Chemistry, Vol. 44, No. 13, 2005

(TCPP)Co Dechlorination Catalyst
Supporting Information Available: cDCE and tDCE degrada-
tion kinetic profiles, calculated active catalyst concentration as a
function of pH, electrochemical data, and (TPP)Co(trans-C2H2Cl)
crystallographic information. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0504339
Inorganic Chemistry, Vol. 44, No. 13, 2005 4861