1. Introduction
Methods and MaterialsIntroduction
Halogen-containing volatile organic compounds are important carriers of fluorine, chlorine,
bromine, and iodine within the planet’s biogeochemical cycling for the Group VII elements.
Small, naturally occurring haloorganic compounds, mainly containing chlorine and bromine are
either produced by living organisms or formed during processes such as volcanic eruptions,
forest fires, and geothermal transformations (Gribble 2003). These compounds are usually
benign, having relatively short lifetimes, being transformed either by hydrolysis, consumed as
co-factors in microbial metabolism or by photochemical decomposition in the lower
troposphere (Mohn et. Al. 1992). However, commercial use of anthropogenic chloro-
fluoroanated saturated alkanes (CFCs) in cleaning solvents, aerosols, flame-retardants,
refrigerants etc. has greatly affected these natural cycles. The carbon halide bond is strong,
making the molecules very stable, but when haloorganics reach the Stratosphere, photolysis of
the carbon-halide bond occurs due to the strong ultraviolet radiation. The X− halogen anion
destroys ozone molecules in the atmosphere by reacting in catalytic cycles in which one
chlorine or bromine atom can destroy thousands of ozone molecules before its converted into
a form that is harmless to ozone, all the while acting as greenhouse gases, leading to global
warming. While current production and consumption controls have decreased the
atmospheric concentration of synthetic haloorganics, and ozone recovery has been observed,
there is still a negative effect on global warming.
Studies have been conducted to determine the fate of halocarbons in the marine
environment by both microorganisms (Häggblom 2003) and abiotic processes (Kriegman-King,
1994), where abiotic processes are considered slow in comparison with marine biotic microbial
pathways. Transformations are catalyzed by reduction and oxidation by cytochrome P450s.
(Mohn et. al. 1992, Häggblom et. al. 2006). Cytochrome P450s are a class of iron-centered
hemeproteins that are an integral part of the respiratory sequence of all aerobic and facultative
marine organisms. Despite their importance and ubiquitous nature, the redox chemistry of
these units is not well defined within the protein matrix. Therefore, a better understanding of
the processes that are responsible for the removal of halocarbons from the oceans before
returning to the atmosphere is required.
Abstract Results
Biomimetic CFCs Degradation:
an Insight into Biotic Halogen Cycling
Mitchell Crick & Stephen K. O’Shea
Department of Chemistry, Roger Williams University, Bristol, RI
Understanding the marine microbial metabolic degradation pathways of natural halo-
carbons is important not only from a climate perspective but also for what it reveals about the
overall balance of biotic halo-carbons cycling and their release into the ecosystem, of which
there is currently only limited knowledge. This research aims to discern the scope and
mechanism for the oxidation and reduction of iron porphyrins by volatile halo-carbons as
biomimetic models of marine heme systems. Furthermore, understanding the roles of marine
iron porphyrin reductive dehalogenation in the transformation of halo-carbons gives insight
into potential routes of anthropogenic chlorofluorocarbons marine decomposition and their
metabolites potential impact on the halo-carbons global budget. Understanding the
mechanism of reaction of reductive dehalogenation of these halo-carbons is important
because eventually this chemistry could be used to rid the environment of these potentially
harmful and robust compounds by converting them into simpler, less harmful ones. By
understanding the redox potential of these porphyrins, it may be possible to use other similar
pentadentate ligand complexes the achieve the same results. Rates of reaction were
determined by monitoring the change in the UV-Vis absorption spectra. Products were
determined in situ and from head-space analysis of the reaction mixtures by GC-MS
compared to authentic standards. It was determined that the redox of the carbon-halo bond
regulates the rate of reaction, where the iodo reactions were found to have higher kinetic
rates in comparison to those of the fluoro-organics.
Schemes
References
Reaction of a haloorganic with an Iron complex. In the first step, a radical results, the iron
complex is oxidized, and the halogen is removed. In the second step, under acidic
conditions the radical reacts with another iron(II) complex. The radical is protonated, and
the iron center is oxidized
Mechanisms of dehalogenation of a general haloorganic as well as for iodoethane
and iodoform. Formation of the alkene occurs naturally in soil. Methane and ethylene
products were successfully produced from the iron complexes shown below.
Gribble G.W., 2003 Chemosphere 52, (2), pp 289-297
Mohn W.W. and Tiedje J.M. Microbiological Reviews, 1992, 56, (3), pp 482-507.
Häggblom M.M., Fennell D.E., Ahn Y, Ravit B. and Kerkhof L.J. 2006 Twardowska (eds.), Soil and Water Pollution Monitoring, Protection and Remediation, pp 3-23. Springer.
Kriegman-King M.R. and Reinhard M. 1994 Environmental Science and Technology., 28, (4), pp 692-700
Fahey, D.W., and M.I. Hegglin. “The Ozone Hole-Ozone Destruction.” The Ozone Hole-Ozone Destruction. Web. 09 Apr. 2015
Murphy B.L. and Morrison R.D. 2007 Introduction to Environmental Forensics 2nd edition Academic Press.
Matsumoto, K.; Gruber, N. (2005). “How accurate is the estimation of anthropogenic carbon in the ocean? An evaluation of the DC*method”. Global Biogeochem. Cycles
Acknowledgements
Structure of simple iodinated substrates of interest.
It is observed that iodo reactions occurred fastest,
followed by chloro and then bromo. Reactions with
a vicinical dihalide produces an alkene. Iodoethane
and iodoform were reacted with Iron(II) complexes
in acidic conditions. Headspace was analyzed using
Gas Chromatography.
RWU Provost Foundation for Undergraduate Research and Roger Williams University Student Senate for travel support. This material is
based upon work supported in part by the National Science Foundation EPSCoR Cooperative Agreement #EPS-1004057. Funding was
received from the RWU Foundation for faculty sponsored undergraduate research. Thanks to Jim Lemire for poster printing.
Ligand Structure
E1/2
(mV)
λ
(nm)
Octaethyl Porphyrin -241 540
Phthalocyanine +208 658
Cyclam -57 360
White-Chen Catalyst a 386
Triflate +72 330
Acetate +75 332
0
0.5
1
1.5
2
2.5
3
250 300 350 400 450 500
Absorbance(AU)
Wavelength (nm)
Fe (II) acetate
Fe (III) acetate
0
0.2
0.4
0.6
0.8
1
1.2
300 400 500 600 700 800 900
Absorbance(AU)
Wavelength (nm)
Fe (III) phthalocyanine
Fe (II) phthalocyanine
0
0.5
1
1.5
2
2.5
3
250 300 350 400 450 500 550
Absorbance(AU)
Wavelength (nm)
Fe (II) cyclam
Fe (III) cyclam
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
250 300 350 400 450 500 550 600 650 700
Absorbance(AU)
Wavelength (nm)
Fe (II) White-Chen
Fe (III) White-Chen
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
200 250 300 350 400 450 500 550 600 650 700
Absorbance(AU)
Wavelength (nm)
Fe (II) triflate
Fe (III) triflate
RX R'CH
-HX
CH₂
RH
Fe (II) OEP
Fe (III) OEP
CHI₃ CH₄
-3I
H⁺
H₃C CH₂I
-HI
H₂C CH₂
C I
H
H
H C I
I
H
H
C I
I
H
I C C
H
H
H I
H
H
UV-Vis spectra of Iron(II) and iron(III) complexes. Oxidation of the complexes were monitored at the specific
wavelengths indicated by blue arrows and the table to the right. The complexes were dissolved in 50:50
DMF/acetic acid.
Gas chromatograms of headspace taken from reaction vessel of iron(II) phthalocyanine and iodinated
substrates. Note the elution of the products methane and ethylene that demonstrates the dehalogenation
Cyclic voltammogram of OEP Fe(III)/(II) in NMP
Glassy carbon electrode Ag/AgCl reference
electrode and Pt auxiliary
Iron complexes were reacted with iodinated substrates in acidic conditions with 50:50 DMF/Acetic acid as
solvents. All Iron complexes were purchased, except for iron cyclam which was prepared in situ by the
addition of cyclam to a boiling acetonitrile solution of iron trifluoromethanesulfonate. The solution was
refluxed for 30 minutes, and then a hot ethanolic solution containing NH4PF6 was added. Precipitation of
crude product was brought about by azeotropic distillation of the acetonitrile with ethanol. Product was
collected by filtration and recrystallized from acetonitrile and ethanol. UV-vis spectroscopy was used to
monitor the change from iron (II) to iron (III). Monitored wavelengths are shown in the table below, as well as
reduction potentials and the structures of the complexes of interest.
O2 , CH4
CH3COOH
DMF
CHI3
O2 ,CH2CH2
CH3CH2I
DMF
CH3CH2I
• The reactions between iron and the iodo molecules are fully completed within two hours
• The reaction products were characterized by GC-MS, eliciting the step-wise dehalogenation
• Wavelength of the reaction kinetics were determined, the relationship to the redox potential can be investigated
a To be determined. Due to ligand exchange