Host-guest interaction of pesticide bifenox with cyclodextrin molecules. an electrochemical study

Article abstract of DOI:10.1135/cccc2009509

The reduction of nitroaromatic compound bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate) was studied in aprotic solvents in the absence or presence of cyclodextrin (CD) molecules of different cavity sizes. βCD and γCD form complexes with bifenox i

Full text of DOI:10.1135/cccc2009509

Host–Guest Interaction of Pesticide Bifenox
1647
HOST–GUEST INTERACTION OF PESTICIDE BIFENOX WITH
CYCLODEXTRIN MOLECULES. AN ELECTROCHEMICAL STUDY
Magdaléna HROMADOVÁa1,*, Romana SOKOLOVÁa2, Lubomír POSPÍŠILa3,*,
b
c
Štěpánka LACHMANOVÁa4, Nicolangelo FANELLI and Stefania GIANNARELLI
a
J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i.,
1
Dolejškova 3, 182 23 Prague 8, Czech Republic; e-mail: hromadom@jh-inst.cas.cz,
2
3
4
romana.sokolova@jh-inst.cas.cz, pospisil@jh-inst.cas.cz, namchal@seznam.cz
b
c
Istituto per i Processi Chimico-Fisici, via Moruzzi 1, 561 24 Pisa, Italy; e-mail: fanelli@ipcf.cnr.it
Dipartimento di Chimica e Chimica Industriale, Universita degli Studi, via Risorgimento 35,
561 26 Pisa, Italy; e-mail: gianna@dcci.unipi.it
Received Septem ber 30, 2009
Accepted Decem ber 11, 2009
Publish ed on lin e February 2, 2010
This work is dedicated to the late Professor Jaroslav Heyrovský for his numerous scientific
achievements in the fields of analytical and physical chemistry. We want to commemorate the
50th anniversary of the Nobel Prize awarded to him for his discovery and development of the
polarographic methods of analysis.
Th e reduction of n itroarom atic com poun d bifen ox (m eth yl 5-(2,4-dich loroph en oxy)-2-n itro-
ben zoate) was studied in aprotic solven ts in th e absen ce or presen ce of cyclodextrin (CD)
m olecules of differen t cavity sizes. βCD an d γCD form com plexes with bifen ox in DMSO
–1
–1
with th e com plex form ation con stan ts (5 ± 2) × 102
M
[βCD–bifen ox] an d (3 ± 1) × 102
M
[γCD–bifen ox], respectively. Bifen ox yields a relatively stable an ion radical in dim eth yl sulf-
oxide, wh ich is furth er reduced at m ore n egative poten tials by an overall addition of th ree
electron s an d four proton s to th e correspon din g ph en ylh ydroxylam in e. In th e presen ce of
βCD th e first reduction wave of bifen ox becom es irreversible, it is sh ifted towards m ore posi-
tive poten tials an d th e uptake of m ore th an on e electron is observed (up to four electron s
durin g th e exh austive electrolysis). Th e first reduction wave of bifen ox is n ot affected by th e
addition of glucose con firm in g th at a sim ple availability of proton s from th e OH groups is
n ot th e m ain factor in furth er tran sform ation of an ion radical in th e presen ce of βCD. Th e
com plex form ation with βCD facilitates th e proton ation an d addition ally protects th e m ole-
cule from disin tegration in to 2,4-dich loroph en ol. A yield of 2,4-dich loroph en ol decreases in
th e order βCD, γCD an d αCD, respectively.
Keyw ord s: Reduction s; Electroreduction ; Nitro an ion radical; Diph en yl eth er h erbicide;
Bifen ox; Cyclodextrin s; Pesticides; Electroch em istry; Polarograph y.
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664
© 2009 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009509

1648
Hromadová et al.:
Bifen ox (m eth yl 5-(2,4-dich loroph en oxy)-2-n itroben zoate), trade n am e
Modown , belon gs to a group of n itro pesticides (Fig. 1) actin g on th e foliar
tissue of broad-leaved weeds an d grasses. Its h erbicidal action is directly
lin ked to th e production of reactive free radicals, wh ich in h ibit th e
ph otosyn th etic process an d cause th e cell dam age.
Electroch em ical properties of bifen ox are prim arily given by th e presen ce
of a n itro group. Th e reduction of arom atic n itro com poun ds was in vesti-
gated for decades an d reduction products in differen t reaction m edia are
well-kn own 1,2. In th e aprotic m edia, it starts with an uptake of on e electron
leadin g to th e form ation of a n itro an ion radical
• –
→
Ar–NO2 + e
Ar–NO2
.
(1)
←
Th e reactivity an d stability of th e electroch em ically-gen erated an ion radi-
cal depen d stron gly on th e presen ce of proton don ors in th e system . Th e
secon d reduction step leadin g to a dian ion of n itro com poun ds is n ot ob-
served at ordin ary experim en tal con dition s an d was detected on ly in liquid
am m on ia3
• –
2–
→
Ar–NO2 + e
Ar–NO2
.
(2)
←
In th e n on -aqueous aprotic solven ts th e an ion radical is furth er reduced
by an overall addition of th ree electron s an d four proton s yieldin g ph en yl-
h ydroxylam in e3–5
• –
Ar–NO2 + 3 e + 4 H+
→
Ar–NHOH + H2O .
(3)
Man y research ers studied differen t aspects of th e form ation an d reactivity
of arom atic n itro an ion radicals6–13. Th is in cluded th e effect of ion -pairin g6,
th e reactan t size11 an d solven t effect12, th e effect of adsorption on th e elec-
tron tran sfer rates7,9,14, an d th e double-layer effect resultin g from th e differ-
en t plan es of th e closest approach for differen t electrolyte cation s15.
Cl
CH3
O
O
Cl
O
NO2
FIG. 1
Ch em ical structure of bifen ox
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1649
Squella et al.16–18 studied th e coupled ch em ical reaction s of th e electro-
ch em ically-gen erated n itro radical an ion by cyclic voltam m etry an d con -
cluded th at th e electrode process follows an EC2 m ech an ism . In buffered
m ixed m edia th e secon d-order kin etics lead to disproportion ation of th e n i-
tro an ion radical
• –
2 Ar–NO2 + 2 H+
→
Ar–NO2 + Ar–NO + H2O
(4)
wh ereas th e dim erization reaction is th e prim ary m ode of th e radical an ion
decay in th e aprotic m edium . Kasten in g19,20 followed th e disappearan ce of
• –
Ar–NO2 an ion radical with EPR an d UV-Vis spectroscopy tech n iques. He
con cluded th at th e decrease in an ion radical con cen tration occurs in two
• –
com petitive processes, on e of wh ich is th e first order in H+ an d Ar–NO2
• –
con cen tration . Th e oth er on e is th e secon d order in Ar–NO2 con cen tra-
tion an d its rate is pH in depen den t.
Th e reduction of bifen ox in aprotic solven ts proceeds in a sequen ce of
redox steps com m on to n itroarom atic com poun ds. Th e reduction is ch arac-
terized by a reversible m on oelectron ic form ation of a reactive an ion radical
at –1.45 V again st ferrocen e/ferrocen ium couple (Fc) accordin g to Eq. (1).
Th e an ion radical is ch aracterized by an EPR spectrum with th e h yperfin e
couplin g con stan ts aN = 9.358 G, aH = 3.31 G an d 2aH = 21.07 G. Th e EPR
spectrum is con sisten t with spectra of radicals of arom atic n itro com -
poun ds. Th e an ion radical is furth er reduced at m ore n egative poten tials
(–1.9 V or m ore again st Fc) in an irreversible th ree-electron process de-
scribed by Eq. (3). Details con cern in g th e reaction m ech an ism , EPR spectra
an d product iden tification were th orough ly described in our previous com -
m un ication 21. Furth er reduction of th e electroch em ically-gen erated bifen ox
an ion radical is stron gly in fluen ced by th e con cen tration an d n ature of th e
cation of th e in differen t electrolyte22. In th is work we focused our atten tion
on th e reactivity of electroch em ically-gen erated bifen ox an ion radical in di-
m eth yl sulfoxide in th e absen ce or presen ce of cyclodextrin (CD) m olecules.
Cyclodextrin s are m acrocyclic glucose oligom ers capable of bin din g a va-
riety of guest m olecules in side th eir torus-sh aped cavities23 alterin g th e im -
m ediate en viron m en t of th e reactin g species. In th is com m un ication we
studied bifen ox reduction in th e presen ce of th ree cyclodextrin m olecules
of differen t cavity size, n am ely, α-cyclodextrin (αCD), β-cyclodextrin
(βCD), an d γ-cyclodextrin (γCD). Th e in tern al diam eter of th eir cavities is in
the range of 0.42–0.53 nm for αCD, 0.56–0.65 nm for βCD and 0.68–0.83 nm
for γCD, respectively.
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1650
Hromadová et al.:
Electroch em ical beh avior of n itroben zen e an d substituted n itroben zen es
in th e presen ce of CD m olecules h as been studied on ly in th e aqueous en vi-
ron m en t24–29. Th e electroch em ical m easurem en ts con firm ed th e com plex
form ation of αCD with p-ch loron itroben zen e an d p-n itroph en olate. Kan o
et al.31 proved th e stabilization of n itroph en olate an ion radical by αCD in
aqueous solution s, sin ce th e an odic reoxidation was barely visible in th e
absen ce an d clearly recogn izable in th e presen ce of CD. EPR spectra pro-
vided furth er eviden ce for th e in corporation of an ion radical in to th e CD
cavity. It was suggested th at th e observed stabilization of th e an ion radical
could h ave resulted from suppression of th e proton ation of th e n itro group
in side th e CD cavity.
We did n ot fin d an y reports dealin g with tran sform ation of th e n itro
an ion radicals in aprotic solven ts in th e presen ce of CD m olecules. Al-
th ough th e stability of com plexes in aprotic solven ts is lower th an in water,
it is still h igh en ough to perm it th e com plex form ation . Main goal of th is
work is to in vestigate th e effect of CD m olecules of differen t cavity size on
th e redox properties of pesticide bifen ox with particular focus on th e fate of
bifen ox an ion radical.
EXPERIMENTAL
Materials
Bifen ox was purch ased as a pesticide referen ce m aterial (with th e 99.0% purity certificate)
from Dr. Eh ren storfer, Gm bH (Augsburg, Germ an y). Nitrofen , pestan al grade, was obtain ed
from Riedel-de-Haën . All th ree cyclodextrin s, glucose (Fluka) an d tetrabutylam m on ium
h exafluoroph osph ate (TBAPF6) from Sigm a were dried before use. 2,4-Dich loroph en ol
(Aldrich ) was used as received. Dim eth yl sulfoxide (DMSO; Sigm a) as received con tain ed
1200 ppm of water, wh ereas th e treatm en t with activated m olecular sieves 3 Å (Lach em a,
Brn o) decreased th e water con ten t to 60 ppm based on th e Karl Fisch er titration .
Tech n iques
Electroch em ical m easurem en ts were don e usin g an electroch em ical system for cyclic voltam -
m etry, ph ase-sen sitive AC polarograph y an d DC polarograph y. It con sisted of a fast rise-tim e
poten tiostat an d a lock-in am plifier (Stan ford Research , m odel SR830). Th e in strum en ts were
in terfaced to a person al com puter via an IEEE-in terface card (PC-Lab, Advan Tech Model
PCL-848) an d a data acquisition card (PCL-818) usin g 12-bit precision . A th ree-electrode
electroch em ical cell was used. Th e referen ce electrode (Ag|AgCl|1 M LiCl) was separated from
th e test solution by a salt bridge with double fritted jun ction . After th e m easurem en t,
ferrocen e was added as an in tern al stan dard, voltam m ogram on th e gold electrode was re-
corded an d all poten tials are reported again st th e poten tial of th e ferrocen e/ferrocen ium
couple (Fc). Th e workin g electrode was eith er a valve-operated static m ercury electrode
(SMDE2, Laboratorn í přístroje, Prague) with an area of 1.03 × 10–2 cm 2 or a droppin g
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1651
m ercury electrode with a flow rate of 1.01 m g s–1, reservoir h eigh t equal to 43 cm an d a
com puter-con trolled drop tim e of 2 s. Th e auxiliary electrode was cylin drical platin um n et.
Oxygen was rem oved from th e solution by passin g a stream of argon .
UV spectra were obtain ed in DMSO usin g a diode-array UV-Vis spectrom eter Agilen t 8453.
Sin ce CDs are well soluble in DMSO, th is solven t was used exclusively in th e experim en ts
th at com pare th e beh avior of bifen ox radical an ion in th e absen ce an d presen ce of cyclo-
dextrin cavities. After exh austive electrolysis of bifen ox th e supportin g electrolyte was pre-
cipitated from th e sam ple by an addition of 4 m l of water to 1 m l of th e sam ple (80% water
v/v). In th is ratio, DMSO an d water form an im m iscible ph ase with dich lorom eth an e, wh ich
was used for th e extraction of th e reduction products. We repeated th e extraction th ree
tim es, each tim e with a volum e of 1 m l. Th e total volum e was adjusted to 5 m l, wh ereas 1 µl
of th e resultin g solution was in jected in to th e GC/MS spectrom eter. Th e ch rom atograph ic
separation was perform ed on a 5% ph en yl–95% m eth ylpolysiloxan e HP-5MS ch em ical-
bon ded fused silica capillary colum n (Hewlett–Packard) of 30 m len gth , 0.25 m m in tern al
diam eter an d 0.25 µm film th ickn ess. A 6890 gas ch rom atograph (Agilen t Tech n ologies)
equipped with a quadrupole m ass spectrom etric detector (at 150 °C), m odel 5973N (electron
im pact 70 eV, ion source 230 °C, in terface tem perature 280 °C), was used for GC/MS an aly-
sis of th e reduction products. Helium of 99.995% purity was used as a carrier gas. Th e tem -
perature profile con sisted of an isoth erm al period of 1 m in , at an in itial tem perature of
35 °C, a tem perature in crease of 10 °C m in –1 up to 300 °C, followed by an isoth erm al period
of 2.5 m in . Th e sam ple was also in jected with out an y previous treatm en t, in wh ich case th e
in itial tem perature was 80 °C, with a tem perature in crease of 10 °C m in –1 up to 300 °C, fol-
lowed by an isoth erm al period of 20 m in . Th e in jector (splitless m ode) was kept at 250 °C.
RESULTS AND DISCUSSION
Complex Formation Equilibria
Th e com plex form ation ability of bifen ox with cyclodextrin m olecules of
differen t cavity size was studied in dim eth yl sulfoxide by UV-Vis spectros-
copy. Th e UV-Vis spectra of bifen ox obtain ed in th e presen ce of differen t
con cen tration s of βCD are sh own in Fig. 2. Th e absorban ce values A de-
crease with th e in creasin g con cen tration of βCD. Th e Ben esi–Hildebran d
an alysis32 of th e absorban ce spectra at th e m axim um decrease of A (λ =
300 n m ) is sh own in th e in set. Lin earity of th e plot poin ts to th e form ation
of a 1:1 com plex between βCD an d bifen ox. Th e Ben esi–Hildebran d equa-
tion for a 1:1 in clusion com plex form ation assum in g th e excess of βCD
con cen tration with respect to bifen ox can be expressed as
1
1
1
1
=
+
(5)
∆A ∆ε[G] ∆ε[G]Ka [H]
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

1652
Hromadová et al.:
FIG. 2
Absorption spectra of 1 × 10–6 M bifen ox in DMSO in th e presen ce of 0 (1), 5 × 10–5 (2), 1 × 10–4
(3), 2 × 10–4 (4), 5 × 10–4 (5) an d 1 × 10–3 M (6) βCD. Ben esi–Hildebran d an alysis of th e absorp-
tion spectra is sh own in th e in set
FIG. 3
Graph of th e relative absorban ce ch an ges A/A0 of 1 × 10–6 M bifen ox as a fun ction of th e αCD
(᭹), βCD (᭺) an d γCD (᭝) con cen tration in DMSO at λ = 300 n m
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1653
wh ere [G] an d [H] are th e con cen tration s of th e guest an d h ost m olecules,
∆A is th e ch an ge in th e absorban ce of th e guest before an d after βCD addi-
tion an d ∆ε is th e differen ce in m olar absorptivities between th e com plexed
an d free guest. Th e com plex form ation con stan t Ka is obtain ed from th e
in tercept/slope ratio.
Ch an ges of th e bifen ox absorban ce expressed as A/A0 at λ = 300 n m as
a fun ction of th e in creasin g con cen tration of αCD (full circles), βCD (᭺)
an d γCD (᭝) are sh own in Fig. 3. Th e absorban ce of bifen ox in th e absen ce
of th e cyclodextrin m olecules is labeled A0. Data dem on strate th at th e
absorban ce sign al of bifen ox was n ot affected by th e presen ce of αCD,
wh ereas th e m ost pron oun ced ch an ges were observed for βCD followed by
γCD. Based on th e Ben esi–Hildebran d equation (5), th e followin g values of
–1
th e com plex form ation con stan ts were obtain ed: (5 ± 2) × 102
M
for th e
–1
[βCD–bifen ox] com plex an d (3 ± 1) × 102
M
for th e [γCD–bifen ox] com -
plex, respectively. αCD does n ot form th e in clusion com plex with bifen ox
in DMSO.
Electrochemical Behavior
Our electroch em ical studies focused on th e effect of CD m olecules of differ-
en t cavity size on th e reduction beh avior of bifen ox in DMSO. Th ese cyclo-
dextrin s can be divided in to two groups: com plexin g (βCD an d γCD) an d
n on -com plexin g (αCD) species. Th e redox beh avior of bifen ox was also in -
vestigated in th e presen ce of D-glucose, wh ich is th e closest an alog to cyclo-
dextrin s lackin g th eir ability to form th e in clusion com plexes. Figures 4 an d
5 com pare th e polarograph ic beh avior of bifen ox in th e presen ce of all four
com poun ds.
In th e presen ce of n on -com plexin g species, th e first polarograph ic wave
rem ain s un ch an ged (data are sh own for D-glucose in Fig. 4a an d for αCD in
Fig. 4b) an d th e first reduction wave is a on e-electron reversible process cor-
respon din g to reaction (1). On ly th e secon d polarograph ic wave sh ifts to-
wards m ore positive poten tials. A sligh t decrease of th e first polarograph ic
wave in th e presen ce of D-glucose is due to th e in creased solution viscosity
sin ce th e con cen tration of D-glucose was purposely h igh er in order to
ach ieve th e con cen tration of h ydroxyl groups sim ilar to th ose of th e CD
m olecules. Th e reversibility of th e first reduction process is retain ed as it is
dem on strated in Fig. 6a (᭹) by th e cyclic voltam m etry of bifen ox in th e
presen ce of D-glucose.
Ch an ges in th e polarograph ic beh avior of bifen ox in th e presen ce of th e
com plexin g species are sh own in Fig. 5a (βCD) an d 5b (γCD). In th e pres-
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

1654
Hromadová et al.:
a
b
FIG. 4
DC polarogram s of 5 × 10–4 M bifen ox in 0.1 M tetrabutylam m on ium h exafluoroph osph ate in
DMSO in th e absen ce (᭺) or presen ce (᭹) of 0.35 M glucose (a) an d 8 × 10–3 M αCD (b)
a
b
FIG. 5
DC polarogram s of 5 × 10–4 M bifen ox in 0.1 M tetrabutylam m on ium h exafluoroph osph ate in
dry DMSO in th e absen ce (᭺) or presen ce (᭹) of 8 × 10–3 M βCD (a) an d 8 × 10–3 M γCD (b)
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1655
en ce of th ese cyclodextrin s, both polarograph ic waves (first an d secon d)
sh ift towards m ore positive poten tials. Th e h eigh t of th e first polarograph ic
wave in creases. Th e log-plot an alysis of th e first polarograph ic wave of
bifen ox (Fig. 5a, ᭺) gives th e slope 60 m V/decade, wh ereas th e h igh er
slopes are obtain ed for data in th e presen ce of th e com plexin g cyclo-
dextrin s. Th e ch em ical irreversibility of th e electron tran sfer process is
clearly visible in th e cyclic voltam m ogram of bifen ox in th e presen ce of
βCD (Fig. 6b, ᭹).
Th e effect of βCD addition s on th e reduction of 5.1 × 10–4 M bifen ox is
sh own in Fig. 7. Th e first reduction wave of bifen ox sh ifts towards m ore
positive poten tials an d is accom pan ied by an in crease of th e lim itin g cur-
ren t. Th e secon d wave is sh ifted in th e sam e direction as th e first on e an d
its h eigh t decreases at th e expen se of th e first on e. Th e sum of th e curren ts
of both waves rem ain s con stan t. Two m ore reduction waves are observed at
m ore n egative poten tials. Th e in set in Fig. 7 sh ows th e graph of th e h alf-
bif
wave poten tial sh ift ∆E1/2 = E1/2 – E1/2 as a fun ction of th e logarith m of
bif
th e βCD con cen tration . E1/2 is th e h alf-wave poten tial of th e first polaro-
graph ic wave of bifen ox. From th e lin ear segm en t of th e depen den ce ∆E1/2
again st log c, th e value of 64 ± 2 m V/decade was obtain ed. Assum in g a sim -
ple precedin g proton ation step, on e estim ates th e value of th e proton ation
–1
rate con stan t as 3 × 106
M
s–1 from th e lim itin g curren ts of th e first reduc-
tion wave33. Th e com plex form ation con stan t Ka was used in stead of th e
a
b
FIG. 6
Cyclic voltam m ogram s of 5.3 × 10–4 M bifen ox in 0.1 M tetrabutylam m on ium h exafluoro-
ph osph ate in dry DMSO in th e absen ce (᭺) or presen ce (᭹) of 0.354 M glucose (a) an d 0.056 M
βCD (b). Scan rate was 0.5 V s–1
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

1656
Hromadová et al.:
acid–base equilibrium an d th e apparen t K′ was taken from th e in tersection
of th e lim itin g curren ts of th e first an d secon d reduction wave as a fun ction
of βCD con cen tration (data n ot sh own ). Th e determ in ation of th e in divid-
ual steps of th e overall m ech an ism is m ore com plicated, sin ce th e com plex
form ation between βCD an d th e electroch em ically-gen erated bifen ox an ion
radical can n ot be excluded34–36
.
It is well kn own th at th e n itro group is reduced in two or m ore separated
on e-electron tran sfer steps on ly in th e absen ce of proton don ors1,2. At suffi-
cien tly low pH an d in protic en viron m en t on ly a sin gle four-electron reduc-
tion wave to th e correspon din g h ydroxylam in e is observed. Th erefore, data
in Fig. 7 give a stron g in dication th at βCD m olecule serves as a proton do-
n or durin g th e bifen ox reduction . More im portan tly, th e data in Figs 4–7
sh ow th at th e presen ce of th e glycosidic OH groups (see data in th e pres-
en ce of D-glucose) is n ot sufficien t for th e an ion radical proton ation an d
th e n on -com plexin g αCD is n ot a sufficien t proton don or eith er. Th ere is
a clear in dication th at th e proton is tran sferred with in th e [CD–bifen ox]
com plex.
FIG. 7
DC polarogram s of 5.1 × 10–4 M bifen ox in 0.1 M tetrabutylam m on ium h exafluoroph osph ate
in dry DMSO in th e presen ce of 0 (1), 0.01 (2), 0.02 (3), 0.03 (4) an d 0.039 M (5) βCD. In set
sh ows th e sh ift of th e h alf-wave poten tial as a fun ction of βCD con cen tration
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1657
An addition of water to DMSO (up to 12% v/v) does n ot affect th e revers-
ibility of th e first reduction wave of bifen ox, on ly th e secon d wave m oves
sligh tly towards m ore positive poten tials (data n ot sh own ). Th erefore, an
in crease in th e curren t an d ch an ge of th e reduction process to an irrevers-
ible on e, as can be seen in Fig. 6b in th e presen ce of βCD can n ot be caused
by an y acciden tal water im purities in th e system . It is a direct con sequen ce
of th e addition of th e com plexin g cyclodextrin . Based on th e Bordwell acid-
ity tables37, water is n ot a good source of proton s in DMSO. Substituted
ph en ols sh ould h ave th is ability. In order to prove th e proton -don atin g
abilities of th e com plex-form in g cyclodextrin s, 2,4-dich loroph en ol was se-
lected as a suitable proton don or. DC polarogram s for reduction of bifen ox
in th e presen ce of in creasin g am oun ts of 2,4-dich loroph en ol are sh own in
Fig. 8. Th ey closely resem ble th ose in Fig. 7 an d th e effect is qualitatively
th e sam e. With an in creasin g am oun t of th e proton don or both th e first
an d secon d wave sh ift towards m ore positive poten tials an d th e first wave
in creases in h eigh t at th e expen se of th e secon d on e. Th is observation con -
firm s our assum ption th at th e com plex-form in g CD m olecules can serve
as proton don ors in th e overall process of bifen ox reduction in th e aprotic
solven ts.
FIG. 8
DC polarogram s of 6.2 × 10–4 M bifen ox in 0.1 M tetrabutylam m on ium h exafluoroph osph ate
in dry DMSO in the absence (1) or presence of 1.2 × 10–3 (2), 4.7 × 10–3 (3) and 16 × 10–3 M (4)
2,4-dich loroph en ol
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1658
Constant Potential Electrolysis
Hromadová et al.:
Furth er in sigh t in to th e tran sform ation of n itro an ion radical in th e ab-
sen ce an d presen ce of CD m olecules was obtain ed by a con stan t poten tial
electrolysis at th e lim itin g curren t of th e first wave (–1.62 V again st Fc). Th e
sam ples were an alyzed at differen t bifen ox con cen tration s from 0.0001 to
0.005 m ol l–1, wh ereas th e cyclodextrin con cen tration varied from 0 to
0.05 m ol l–1. Th e con cen tration ratio of bifen ox an d CD was optim ized for
th e GC/MS an alysis.
If th e an ion radical was ch em ically stable, on e sh ould recover th e origin al
com poun d after th e exposure of th e electrolyzed solution to air due to a
so-called “futile cycle” process
• –
• –
Ar–NO2 + O2
→
Ar–NO2 + O2
.
(6)
A com plete recovery of th e arom atic n itro com poun ds was n ot observed in
th e literature due to th e follow-up ch em ical processes17. In th is work DC
polarogram s are sh own two h ours after th e exposure of th e electrolyzed sam -
ples to th e air. Th ey rem ain ed iden tical after a prolon ged exposure tim e.
Figure 9 sh ows a com parison of th e DC polarogram s of two system s:
bifen ox in th e absen ce (Fig. 9a) an d in th e presen ce (Fig. 9b) of a h un dred-
fold excess of βCD before (᭹) an d after (᭺) th e electrolysis at –1.62 V. Th e
correspon din g cyclic voltam m ogram s for th e electrolyzed sam ples are
sh own in th e in sets of Fig. 9. Up to 1.3 electron s were con sum ed durin g th e
exh austive electrolysis of bifen ox in th e absen ce of CD, wh ereas in th e pres-
en ce of βCD th e total n um ber of electron s varied from 3 to 4 depen din g on
th e βCD con cen tration (data n ot sh own ).
Cyclic voltam m ogram s of th e reduction products of bifen ox (in set in
Fig. 9a) sh ow th ree peaks labeled I, II an d III. Peaks I an d III correspon d to
th e reduction of a n itro group, wh ereas a n ew redox couple II is observed at
–1.87 V at th e poten tial values coin cidin g with th ose reported for th e re-
duction of arom atic azoxy com poun ds21,38,39. Cyclic voltam m ogram of th e
reduction products of bifen ox in th e presen ce of βCD (in set in Fig. 9b)
yields also th ree redox system s. Peaks I an d III correspon d to th e sam e n itro
product as in th e absen ce of βCD. A n ew an odic wave IV is located at th e
poten tial correspon din g to oxidation of th e arylh ydroxylam in e deriva-
tive18,40
.
Th e electrolyzed solution s were subjected to GC/MS an alysis. Gas
ch rom atogram of th e dich lorom eth an e extract of 5 × 10–3 M bifen ox is
sh own in Fig. 10a. Th e sam ple con tain ed bifen ox an d four reduction prod-
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1659
ucts labeled A to D. Th eir m ass spectra are sh own in Fig. 11. Based on th e
an alysis of th e MS fragm en tation pattern s th e m ain reduction products are:
2,4-dich loroph en ol (A), n itrofen , i.e. 2,4-dich loroph en yl-4-n itroph en yl
eth er (B), an d butyl 5-(2,4-dich loroph en oxy)-2-n itroben zoate (D). Th e iden -
tification of com poun ds A an d B was con firm ed also by th e addition of 2,4-
dich loroph en ol an d n itrofen stan dards to th e an alyte. Nitrofen is electro-
active an d produces sim ilar polarogram to bifen ox with two polarograph ic
waves. Th e electroch em ically detected azoxy product (see Fig. 9) was n ot
detected in th e extract.
Th e gas ch rom atogram of th e electrolyzed solution s of bifen ox in th e
presen ce of βCD is sh own in Fig. 10b. Th ese data were obtain ed at less th an
a two-fold excess of βCD an d th ree electron s were con sum ed durin g th e ex-
h austive electrolysis at –1.62 V. Four products (labeled A, C, E an d F) were
detected in th e dich lorom eth an e extract of th is sam ple. From th e an alysis
of th e fragm en tation pattern of th eir m ass spectra (see Fig. 11), th e products
are iden tified as 2,4-dich loroph en ol (A), m eth yl 5-(2,4-dich loroph en oxy)-
2-am in oben zoate (C), 5-(2,4-dich loroph en oxy)-2-n itrosoben zoic acid (E)
an d m eth yl 5-(2,4-dich loroph en oxy)-2-n itrosoben zoate (F). Nitroso prod-
a
b
FIG. 9
DC polarogram s of 5.1 × 10–4 M bifen ox in 0.1 M tetrabutylam m on ium h exafluoroph osph ate
in dry DMSO in th e absen ce (a) or presen ce of 5 × 10–2 M βCD (b) before (᭹) an d after (᭺) ex-
h austive electrolysis at –1.62 V. Drop tim e was 1.5 s. In set sh ows th e correspon din g cyclic
voltam m ogram of electrolyzed solution s at scan rate 0.5 V s–1. Electrolyzed sam ples were ex-
posed to air before th e m easurem en ts
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

1660
Hromadová et al.:
ucts E an d F were n ot detected at h igh er ratios of βCD an d th e correspon d-
in g DC polarogram s of th e electrolyzed solution s sh owed an in crease of th e
an odic polarograph ic wave aroun d –0.8 V (see Fig. 9). Th e GC/MS detection
of th e h ydroxylam in e derivative was n ot possible owin g to th e in stability
of th is species durin g th e an alytical m an ipulation , wh ich ren ders its isola-
tion difficult.
Th e effect of CD on th e reduction of bifen ox was also studied by th e ex-
h austive electrolysis at th e con stan t poten tial of th e secon d reduction wave
(–2.0 V again st Fc). In th ese experim en ts 1 × 10–3 M solution of bifen ox an d
1.6 × 10–2 M αCD, βCD an d γCD in 0.1 M TBAPF6 in DMSO were used. Total
n um ber of electron s con sum ed durin g th e exh austive electrolysis was four
for all th ree CDs an d th e origin ally colorless solution ch an ged color to yel-
low. On ly a sm all con tin uous in crease with respect to th e baselin e curren t
was observed in DC polarograph y in th e poten tial ran ge from –1.1 to –2.6 V.
No well-defin ed cath odic wave was discern ible, but two an odic waves were
observed at E1/2 = –0.88 V an d E1/2 = –0.59 V (data n ot sh own ). Th e first
wave correspon ds to th e oxidation of th e h ydroxylam in e product, th e sec-
on d wave to th e oxidation of an am in o group18,40. Th e GC/MS an alysis
of 2 µl of th e electrolyzed solution s was perform ed with out an y previous
a
b
FIG. 10
Gas ch rom atogram of 5 × 10–3 M bifen ox solution in 0.1 M tetrabutylam m on ium h exafluoro-
ph osph ate in dry DMSO in th e absen ce (a) or presen ce (b) of 8.4 × 10–3 M βCD after an exh aus-
tive electrolysis at –1.62 V vs Fc. Products are labeled by capital letters A to F
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1661
sam ple treatm en t an d yielded up to four ch rom atograph ic peaks in th e gas
ch rom atogram s with th e reten tion tim es 7.3 (A), 10.8 (A′), 18.4 (B) an d
22.3 m in (C), respectively. Th e iden tification of th e products as 2,4-di-
ch loroph en ol (A), butyl 2,4-dich loroph en olate (A′), n itrofen (B) an d m eth yl
5-(2,4-dich loroph en oxy)-2-am in oben zoate (C) was based on th e an alysis of
FIG. 11
Mass spectra of th e m ain reduction products A to F obtain ed after th e gas ch rom atograph ic
separation sh own in Fig. 10
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

1662
Hromadová et al.:
th eir m ass spectra. Table I sum m arizes th e yield of each com poun d in per
cen t of th e peak area to th e total peak area of all com poun ds detected in
th e ch rom atogram .
Th e total percen tage of products origin atin g from th e cleavage of bifen ox
at th e bridgin g eth er bon d (A an d A′) correlates with th e ability of CDs to
form in clusion com plexes with bifen ox. Such correlation is obvious from
Fig. 12, wh ich sh ows th e com parison of th e actual peak area of 2,4-di-
ch loroph en ol in th e GC ch rom atogram s in th e presen ce of differen t CD
m olecules. Th e sm allest am oun t of 2,4-dich loroph en ol was observed after
electrolysis in th e presen ce of βCD, for wh ich th e largest com plex form a-
tion con stan t was obtain ed in DMSO by UV-Vis an alysis.
TABLE I
Sum m ary of th e electrolytic product yields in per cen t of th e peak area to th e total peak area
in th e GC ch rom atogram for bifen ox reduction in th e presen ce of CDs
Com poun d
A, %
A′, %
B, %
C, %
αCD
βCD
γCD
23.4
37.3
76.1
62.8
6.6
37.7
23.9
7.2
25
–
–
–
FIG. 12
Histogram of the peak area of 2,4-dichlorophenol product obtained from GC analysis of 1 × 10–3
M
bifen ox an d 1.6 × 10–2 M CD m olecules in 0.1 M tetrabutylam m on ium h exafluoroph osph ate in
DMSO after exh austive electrolysis at –2.0 V vs Fc
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 11–12, pp. 1647–1664

Host–Guest Interaction of Pesticide Bifenox
1663
Cyclodextrin m olecules h ave been kn own to m im ic activities of th e
h ydrolytic en zym es an d prom ote th e ester h ydrolysis th rough th e in volve-
m en t of th e OH groups at th e rim of th e CD rin g41. Th e ability of CD m ole-
cules to serve as proton don ors in th e electroch em ical reaction sch em es h as
been dem on strated in our previous work on th e reduction of atrazin e pesti-
cide in th e n eutral aqueous en viron m en t. Cyclodextrin s prom ote th e for-
m ation of a proton ated form of atrazin e leadin g to th e electroch em ically-
active form of pesticide42. We dem on strated in th is con tribution th at th e
CD m olecules provide th e n ecessary proton s to com plete th e reduction of
bifen ox accordin g to th e reaction (3) in th e aprotic solven t. Th e com plex
form ation n ot on ly facilitates th e proton tran sfer to a bifen ox m oiety, but
also protects th e m olecule from disin tegration .
This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic
(IAA400400802), Czech Science Foundation (203/08/1157, 203/09/0705 and 203/09/1607) and
Ministry of Education, Youth and Sports of the Czech Republic (LC510 and COST D36 OC140). Joint
cooperation was made possible on the basis of the cooperation agreement between Consiglio Nazionale
delle Ricerche, Rome and the Academy of Sciences of the Czech Republic.
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