Reaction of aromatic radical cations with RuCl3·3H2O

Article abstract of DOI:10.1135/cccc20000924

Interactions of RuCl₃·3H₂O with radical cations of aromatic ethers, 1,4-di-tert-butyl-2,5-dimethoxybenzene, 3,5-di-tert-butyl-1,2-dimethoxybenzene and 2-tert-butyl-1,4-dimethoxybenzene, and with aromatic amines, 2,4,6-tri-tert-butylaniline and N,N,N′,N′-tetramethyl-1,4-phenylenediamine, were observed by voltammetry. ESR and UV spectroscopies were used for the study of the the first two ethers. The effect of RuCl₃·3H₂O was also examined by controlled potential electrolysis.

Full text of DOI:10.1135/cccc20000924

924
Michman, Oron, Schaefer:
REACTION OF AROMATIC RADICAL CATIONS WITH Ru Cl3·3H2O
a
b
Michael MICHMANa1,*, Miriam ORON and Hans J. SCHAEFER
a
Department of Organic Chemistry, The Hebrew University of Jerusalem, 91904Jerusalem, Israel;
1
e-mail: michman@cc.huji.ac.il
b
Department of Organic Chemistry, Organisch-Chemisches Institut der Westfälischen Wilhelms
Universität, Correnstrasse 40, D-48148, Münster, Germany; e-mail: schafeh@uni-muenster.de
Received Novem ber 18, 1999
Accepted March 14, 2000
Presented at the 32nd Heyrovský Discussion on Organic Electrochemistry, Třeš7, June 6–10, 1999.
(This work is dedicated to Professor Herbert Schumann from TU Berlin on the occasion of his 65th
birthday).
In teraction s of RuCl3·3H2O with radical cation s of arom atic eth ers, 1,4-di-tert-butyl-
2,5-dim eth oxyben zen e, 3,5-di-tert-butyl-1,2-dim eth oxyben zen e an d 2-tert-butyl-1,4-di-
m eth oxyben zen e, an d with arom atic am in es, 2,4,6-tri-tert-butylan ilin e an d N,N,N′,N′-tetra-
m eth yl-1,4-ph en ylen ediam in e, were observed by voltam m etry. ESR an d UV spectroscopies
were used for th e study of th e th e first two eth ers. Th e effect of RuCl3·3H2O was also exam -
in ed by con trolled poten tial electrolysis.
Key w ord s: Electrooxidation ; Electrocatalysis; Oxidation s; Radical cation s; Ruth en ium ch lo-
ride; Ben zoquin on es; Eth ers; An ilin es.
Th e first step in electrooxidation of m an y organ ic arom atic com poun ds is
th e form ation of radical cation s by sin gle electron tran sfer1 (ET). Sin ce
m an y com petin g path ways are available for furth er reaction s of radical cat-
ion s2,3, seekin g possibilities of con trollin g selectivity with catalysts is an im -
portan t objective. With very reactive radical cation s an d fast, con secutive
reaction s, th e first step m ay becom e in distin guish able from furth er irrevers-
ible ET or ch em ical steps4. Arom atic com poun ds th at can be oxidized to rel-
atively stable radical cation s, provide a useful n ich e for m ech an istic an d
syn th etic in form ation on catalysis by th e study of distin ct oxidation steps.
Ruth en ium ch loride trih ydrate, RuCl3·3H2O, h as been described in previ-
ous publication s as a possible catalyst for electrooxidation of water5. It was
also used for electrooxygen ation of arom atic h ydrocarbon s such as n aph -
th alen e an d 2-m eth yln aph th alen e to 1,4-n aph th oquin on e an d to
2-m eth yl-1,4-n aph th oquin on e6. Water is th e obvious source of oxygen an d
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Aromatic Radical Cations
925
RuCl3·3H2O en h an ces th e selectivity an d rate of quin on e form ation . Lin ear
sweep voltam m etry sh ows two irreversible oxidation steps for n aph th alen e
an d 2-m eth yln aph th alen e6. In th e presen ce of RuCl3·3H2O, th e secon d oxi-
dation step is sh ifted towards a lower oxidation poten tial by 120 ± 20 m V,
with a sm all in crease in th e peak curren t wh ereas th e first oxidation poten -
tial rem ain s con spicuously un affected6. It was proposed th at th is pattern in
voltam m etry reflects form ation of an in term ediate com plex of th e arom atic
radical cation with RuCl3·3H2O an d th at such com plex provides a clue to
th e lin kage between organ ic oxidation an d oxygen ation with water7. How-
ever, in th e absen ce of furth er exam ples, oth er explan ation s such as possi-
ble effects on overpoten tial of th e water un its coordin ated in RuCl3·3H2O,
could n ot be ruled out. Both oxidation steps of n aph th alen e an d 2-m eth yl-
n aph th alen e are irreversible an d th eir radical cation s react fast with water.
Wh en water con cen tration in aceton itrile in creases, say to 0.1 m ol l–1, th e
two oxidation waves ten d to con verge. All th is lim its th e value of com par-
in g m icroscale curren t–poten tial data obtain ed in dry aceton itrile with
m acroscale electrolysis in wet aceton itrile. Th e presen t study h as been ex-
ten ded to in clude com poun ds th at yield stable radical cation s displayin g a
reversible redox beh aviour in dicated by voltam m etry an d a separate secon d
oxidation step. The com pounds studied were: 1,4-di-tert-butyl-2,5-dimethoxy-
ben zen e (1), 3,5-di-tert-butyl-1,2-dim eth oxyben zen e (2), 2-tert-butyl-1,4-di-
methoxybenzene (3), 2,4,6-tri-tert-butylaniline (4), N,N,N′,N′-tetram ethyl-1,4-
phenylenediam ine (5). Th e role of RuCl3·3H2O was tested by voltam m etry
with com poun ds 1–5, by sim ultan eous electroch em ical ESR (SEESR) an d
UV-VIS spectroscopy with 1 an d 2. Th e relevan ce of th ese observation s to
preparative electrolysis h as been exam in ed.
EXPERIMENTAL
Materials
Aceton itrile (Mallin ckrodt an alytical, m axim um 0.03% H2O, 16.7 · 10–3 m ol l–1) was used as
supplied. Tetrabutylam m on ium perch lorate (TBAP), tetrabutylam m on ium h exafluoroborate
(TBAH, Fluka), silver perfluoroborate (Fluka), sodium h ydride, m eth yl iodide, ruth en ium
ch loride trih ydrate (Aldrich , Joh n son Matth ey), 2,4,6-tri-tert-butylan ilin e (4), N,N,N′,N′-tetra-
m eth yl-1,4-ph en ylen ediam in e (5), 3,5-di-tert-butylpyrocatech ol, 3,5-di-tert-butyl-1,2-ben zo-
quin on e (7) an d 2-tert-butylh ydroquin on e (10), were used as received from Aldrich , BDH, or
Fluka. Th e preparation of 1 an d 6 h as been described8.
3,5-Di-tert-butyl-1,2-dimethoxybenzene (2) was prepared by m eth ylation of 3,5-di-tert-butyl-
pyrocatech ol with sodium h ydride followed by m eth yl iodide accordin g to Matsuura et. al.9.
Th e crude yellow oil obtain ed was purified by ch rom atograph y over silica gel with 40%
ch loroform in petroleum eth er an d crystallized from dich lorom eth an e. Th e fin al product is
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

926
Michman, Oron, Schaefer:
colourless with m .p. 53 °C (yield ≈50%). GC-MS, m/z (%): 250 (40), 236 (18), 235 (100), 164
(11), 57 (14). 1H NMR, δ (ppm ): 1.31 (9 s); 1.39 (9 s); 3.87 (6 s); 6.83 (1 d, J = 2.4 Hz); 6.91
(1 d, J = 2.4 Hz).
2-tert-Butyl-1,4-dimethoxybenzene (3) was prepared by th e reaction of 2-tert-butyl-
h ydroquin on e with dim eth yl sulfate in accord with Kh arasch et. al.10. Altern atively,
2-tert-butylh ydroquin on e (2 g) in dry dim eth yl sulfoxide (25 m l) was added over 5 m in to
NaH (1 g) in dry dim eth yl sulfoxide (50 m l) an d stirred for 30 m in to yield a brown slurry.
Meth yl iodide (2 g) in dim eth yl sulfoxide (50 m l) was slowly added. Products, a brown solu-
tion an d a wh ite precipitate, were cooled, h ydrolyzed an d extracted with eth er. Th e eth er
extract was wash ed an d dried over MgSO4. Evaporation an d ch rom atograph y as above or
evaporation an d distillation yields 3 as an oil, b.p. 240 °C/50 m m , yield ≈60%. GC-MS, m/z (%):
195 (4), 194 (60), 180 (15), 179 (100), 164 (45), 151 (35), 149 (15), 121 (15), 91 (16), 77
(14). 1H NMR, δ (ppm ): 1.43 (9 s); 3.84 (3 s); 3.83 (3 s); 6.75 (1, 2 × d), 6.85 (1, d); 6.97 (1 d,
J(ab) = 0.03, J(ac) = 0.01). (Ha, Hb, an d Hc proton s at position s 6, 5 an d 3, respectively).
N-(3-tert-butyl-4,5-dimethoxyphenyl)acetamide (8) was foun d am on g products of electrolysis
of 2 at 2.2 V. It was separated by ch rom atograph y on silica gel 60 with CHCl3–petroleum
eth er an d was iden tified by GC-MS on ly. GC-MS, m/z (%): 251 (5), 226 (14), 225 (100), 224
(10), 223 (58), 210 (6), 133 (3).
2-tert-Butyl-1,4-benzoquinone (9) was prepared by reaction of 2-tert-butylh ydroquin on e (3 g),
in ice cool con cen trated H2SO4 (10 m l) with an aqueous solution of K2Cr2O7 (8 g), th at was
added dropwise. Th e m ixture was allowed to react for 15 m in an d term in ated with 5 m l of
m eth an ol; th en water was added an d th e product extracted with eth er (yield 80%). GC-MS,
m/z (%): 166 (15), 164 (60), 151 (39), 149 (100), 121 (63), 93 (35), 77 (42). 1H NMR, δ (ppm ):
1.28 (9 s); 6.59 (1 s); 6.66 (2 s).
Voltammetry was perform ed un der Ar usin g EG&G Prin ceton Applied Research Versastat in
a th ree-electrode cell with Pt tip (2 m m 2) as a workin g electrode, Pt plate (1 cm 2) as a coun -
ter electrode an d Ag|AgCl|3 M KCl as a referen ce (0.1 m m Ag foil, Sigm a). Pt electrodes were
clean ed with con cen trated HNO3, wash ed successively with triple-distilled water, H2SO4 an d
fin ally with water again . Backgroun d curren ts were ch ecked after each wash in g. Con cen tra-
tion of tetrabutylam m on ium perch lorate (TBAP) was 0.1 m ol l–1 in aceton itrile an d th at of
ruth en ium salt was 3 m m ol l–1. Tests were con ducted in 50 m l of CH3CN solution s (<16.7
m M H2O), usin g scan rates from 50 to 1 000 m V s–1. Th e in fluen ce of water was exam in ed
for com poun ds 2 an d 3 at 10–3 M con cen tration s in 20 m l of CH3CN solution s (<16.7 m M
H2O). On e series was with out ruth en ium salt, an oth er series with 10–4 M RuCl3·3H2O.
Con trol Tests
Th e reproducibility was ch ecked by repeated voltam m etric m easurem en ts with m aterials,
electrolytes an d solven ts from various sources. Electrodes were ch ecked for possible effects of
in visible coatin g by voltam m etric test of pairs of an odes A an d B. An odes in group A were
fresh ly clean ed between voltam m etric m easurem en ts wh ereas th ose in group B were used
con tin uously with out an y treatm en t. Electrolysis was th en perform ed with th e two sets of
electrodes, in th e presen ce or absen ce of RuCl3, at 1.0, 1.5, 1.8 an d 2.0 V, for th e duration
of 5 m in each , a tim e lon g en ough to cover an y voltam m etric test. Subsequen tly, CV scan s
were perform ed at 50 m V s–1 from 0.0 to 2.8 V. From 0.0 to 2.45 V, n o ch an ges or differ-
en ces were detected between electrodes of groups A an d B. No surface effects h ave been de-
tected in tests wh ere com poun ds 1 an d 2 were presen t. Th e build-up of an an odic wave was
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Aromatic Radical Cations
927
observed at 2.49 V with all electrodes. (Th e poten tial ran ge relevan t in th is work is <2.2 V).
With 3, visible coatin g appeared on th e surface an d two broad irreversible reduction waves
at 0.8 an d 0.3 V were recorded.
Controlled potential electrolysis (CPE) was run on an EG&G Prin ceton Applied Research
Poten tiostat m odel 362, un der Ar, at 26 ± 1 °C, in a th ree-electrode cell with Pt plates (300
m m 2) as a workin g an d coun ter electrodes. Th e volum e of solution was 50 m l. Th e referen ce
electrode was a silver foil dipped in 0.1
M AgBF4 in CH3CN, in a glass tube with a
sin tered-glass tip. Th e referen ce electrode was fresh ly prepared an d calibrated again st
Ag|AgCl. Its poten tial was +400 m V relative to Ag|AgCl an d +600 m V relative to stan dard
h ydrogen electrode (SHE). Products were isolated by addition of 50 m l water an d extraction
with CHCl3. After dryin g, ch rom atograph y was carried out on Dowex 50W X2-200 with 50%
CH3CN–H2O as an eluen t wh ich rem oves electrolyte residues. Th e extract was an alyzed by
GC-MS an d TLC. Ch rom atograph y with CHCl3 over alum in a was used to isolate m aterial for
NMR an alysis. Th e Dowex 50W X2-200 colum n was prepared by wash in g in triple-distilled
water (TDW). Th e resin was regen erated by wash in g with 10% HClO4.
Con trolled Poten tial Electrolysis of 2
From stock solutions of 2, 50 m l sam ples were prepared in pairs, one of them containing 0.1 m M
RuCl3·3H2O. Sam ples were electrolyzed at th e sam e tem perature. Sam ples were prepared in
CH3CN con tain in g <16.7 m M or 11.1 M H2O with 0.1 M TBAP an d con cen tration of 2 1.0
m m ol l–1. Electrolysis was perform ed at th ree con stan t poten tials 1.1, 1.5 an d 1.9 V vs 0.1
M
Ag|0.1 M AgBF4, i.e. at 1.4, 1.8, 2.2 V vs Ag|AgCl (cf. CV tests in Fig. 9). Electrolysis in pres-
en ce of RuCl3·3H2O was run for 2 h an average curren t den sity of ≈4 µA m m –2 (12 · 10–9
F s–1). Aliquots of 50 µl were an alyzed in 10 m in in tervals by HPLC equipped with a UV de-
tector (245 n m ) an d a reverse-ph ase C18 colum n with aceton itrile–water or m eth an ol–water
as solven ts. HPLC in jection s were of pre-set to 20 µl. In itial reaction rates (for th e first ten
coulom bs) of 2 an d of form ation of 7 are given in Fig. 9.
Con trolled Poten tial Electrolysis of 3
Electrolysis was perform ed at 1.75 V (Ag|AgCl) (Fig. 3b) in 50 m l of CH3CN solution s (16.7 m M
H2O) con tain in g 30 m M 3, 0.1 M TBAP an d in CH3CN–H2O (11.1 m ol l–1) solution s with an d
with out 5 m M RuCl3·3H2O un til a ch arge of 600–800 °C passed. In CH3CN with water con -
cen tration less th an 16.7 m ol l–1, som e 2-tert-butyl-1,4-ben zoquin on e (9) an d 2-tert-butyl-
h ydroquin on e (10) form ed but reaction was very slow. Wh en RuCl3 was presen t, on ly two
isom eric ch lorin ation products, two isom ers of 2-tert-butyl-1,4-dim eth oxych loroben zen e, 11
an d 12 were form ed11. In a CH3CN–H2O m ixture (11.1 M H2O), form ation of 9 an d 10 was
predom in an t, an d th e rate of form ation of 9 is doubled wh en RuCl3 is presen t. In som e
cases, black coatin g was form ed on th e electrodes un der prolon ged CPE.
Sim ultan eous Electroch em ical ESR (SEESR)
In th e SEESR m eth od electroch em ical an alysis is perform ed in a cell m oun ted in side th e cav-
ity of an ESR spectrom eter. Th e SEESR m eth od an d in strum en t used h ere h as been already
described in ref.12. Con dition s of specific SEESR tests are described in th e caption s to Figs
5–7.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

928
Michman, Oron, Schaefer:
UV-VIS Measurem en t of Radical Cation 1+•
A 50 m l solution of 1.0 M 1 in CH3CN with 0.1 M TBAP was electrolyzed for 10 m in at 2.9 m A
(a ch arge of 1.8 C). Th e m axim um con cen tration of [1]+• is th erefore 1.8 · 10–5 m ol l–1. A
green colour of th e radical cation appears in solution . An aliquot of 2 m l was tran sferred to
quartz cuvettes con tain in g 0.1 m l CH3CN or 0.1 m l CH3CN with 0.25 m M RuCl3·3H2O an d
exam in ed usin g a Kon tron UV 930 spectroph otom eter. Th e sam ple tran sfer required less
th an 2 m in . Th e absorban ce was recorded at 463 n m for [1]+• to obtain th e decay curve.
RESULTS AND DISCUSSION
Com poun ds 1–5 were studied by voltam m etry in com m ercial dry aceto-
n itrile (0.3 % H2O). In all figures, poten tial sweep started from zero to posi-
tive poten tial (oxidation ) an d back. All com poun ds sh ow a sequen ce of two
oxidation steps. Th e first is a sin gle-electron tran sfer (ET) form in g a radical
cation , reversible at scan rates of 30–1 000 m V s–1. Th e secon d step repre-
sen ts furth er irreversible oxidation for com poun ds 1–4 as sh own in Figs 1a
(ref.8), 2a an d 3. An exception was foun d for com poun d 5 wh ere th e secon d
step is reversible as well13 (Fig. 4). Th e relation of th e first reversible step to
th e form ation of radical cation s is in dicated for 1 an d 2 by appearan ce of
an ESR sign al (Figs 5 an d 6) wh ile th e secon d tran sition is ESR-silen t. A sim -
ilar status h as been establish ed elsewh ere for com poun ds 4 (ref.14) an d 5
(ref.13).
Com poun ds 1–4 sh ow a sh ift of th e secon d tran sition in presen ce of
RuCl3·3H2O. Th e first CV is n ot affected by RuCl3·3H2O as it is sh own in
Figs 1–3. Sim ilar beh aviour was foun d for n aph th alen e an d 2-m eth yl-
n aph th alen e h avin g irreversible CV. Except for 4 an d 5, th e poten tial sh ift
is between 100 an d 140 m V towards less positive poten tials. In th e case of
com poun d 4, th e sh ift is to m ore positive poten tials wh ereas with 5, th e
secon d peak poten tial is n ot sh ifted at all. Its peak curren t decreases wh ile
both tran sition s rem ain reversible (Fig. 4). Clearly, in all th e cases, th e ef-
fect can be attributed to th e secon d oxidation stage rath er th an to th e start-
in g m aterial. For 1 an d 2, it is clearly sh own by CV, SEESR an d by UV
spectrum of 1 th at a radical cation form s in th e first oxidation step (Figs 5
an d 6). Wh en poten tial steps were applied in th e SEESR experim en t of val-
ues of th e first oxidation step (1.2 V for 1 an d 1.4 V for 2), h alf-life tim es
were m easured from th e in ten sity of th e ESR sign al. Values of τ1/2 in dry
aceton itrile are 1 800 s for 1 an d 300 s for 2. Both values are reduced in th e
presen ce of RuCl3·3H2O by a factor of 3–4. Th e decay of 2 sh own in Fig. 7 is
3.7 tim es faster in th e presen ce of ruth en ium ch loride. In cases of 1 an d 2,
th e presen ce of RuCl3·3H2O does n ot alter th e sh ape of th e ESR sign al of
cation radicals an d th ere is n o n ew sign al detected between 0.0–2.2 V. Th is
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Aromatic Radical Cations
929
im plies ch arge tran sfer between [ArH]+• an d RuCl3·3H2O. Th is differs from
th e reported beh aviour of [4]+• wh ere an added base produces th e
deproton ated radical with con sequen t display of a differen t ESR sign al14 but
th is is expected sin ce 4 h as available proton s.
In CV an d SEESR experim en ts RuCl3·3H2O is added at th e begin n in g an d
th erefore it is already partly oxidized at poten tials above 1.4 V as con cluded
from th e voltam m etry of RuCl3·3H2O. In order to test th e effect of
n on -oxidized RuCl3·3H2O, th e ruth en ium salt was added to prepared solu-
200
a
I, µA
4
150
3
100
2
50
0
1
–50
0.6
1.2
1.8
E, V
200
150
100
50
I, µA
b
4
3
2
1
0
–50
0.6
1.2
1.8
E, V
FIG. 1
CV of 1.0 m M 1,4-di-tert-butyl-2,5-dim eth oxyben zen e (1) in CH3CN with 0.1 M TBAP; v = 50
(1), 200 (2), 500 (3), 1 000 (4) m V s–1; at 28(±2) °C; Pt electrodes; referen ce Ag|AgCl|KCl
(sat). a: In absen ce of RuCl3·3H2O, b: in presen ce of 1.8 m M RuCl3·3H2O
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

930
Michman, Oron, Schaefer:
tion s of radical cation . Th e particularly stable [1]+• (ref.15) sh ows absorption
at λm ax = 463 n m . It was electrogen erated from solution s of com poun d 1,
aliquots of th e oxidized solution were tran sferred to quartz cuvettes an d th e
absorption spectra m on itored. If required, RuCl3·3H2O was added on ly at
th at stage. Th e value of τ1/2 for 1 was 1 243 s. As 1 is n ot produced in situ,
th e estim ated τ1/2 from UV-VIS absorption experim en ts is less accurate th an
th e value of 1 800 s obtain ed from ESR. In a represen tative UV-VIS test, th e
40
a
1
I, µA
2
30
20
10
0
–10
–0.5
0
0.5
1.0
1.5
2.0
2.5
E, V
70
3
I, µA
b
50
2
1
30
10
–10
0
0.5
1.0
1.5
2.0
E, V
FIG. 2
CV of 10–3 M 3,5-di-tert-butyl-1,2-dim eth oxyben zen e (2) in CH3CN with 1.0 M TBAP, at
28(±2) °C, Pt electrodes, referen ce Ag|AgCl|KCl (sat) in solution s. a CH3CN con tain s 0.03%
H2O (16.7 m M H2O), v = 50 m V s–1. In absen ce of RuCl3·3H2O (1), in presen ce of 1.8 m M
RuCl3·3H2O (2). b 20 m l CH3CN with 0.1 M TBAP, v = 100 m V s–1, 22(±2) °C, in absen ce of
RuCl3·3H2O, at various con cen tration s of water: 22 (1), 122.7 (2), 176.7 (3) m M H2O
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Aromatic Radical Cations
931
decay of [1]+• was 3.5 faster in th e presen ce of 12.5 · 10–6 M RuCl3·3H2O
(Fig. 8). Th e in itial con cen tration of [1]+• does n ot exceed 1.8 · 10–5 m ol l–1.
Th e value of τ1/2 = 1 243 ± 10 with a first-order rate con stan t for th e decay,
k = 6 · 10–3 s–1, was reduced to τ1/2 = 347 s, k = 19 · 10–3 s–1 in presen ce of
RuCl3·3H2O.
Th e effect of water on th e redox beh aviour was tested for two reason s.
First, to determ in e th e in fluen ce of water con ten t in RuCl3·3H2O on
a
200
100
I, µA
6
5
4
3
2
1
0
–100
600
1 000
1 400
1 800
E, mV
120
I, µA
b
2
1
100
80
60
40
20
0
–20
1.0
1.2
1.4
1.6
1.8
2.0
2.2
E, mV
2.4
FIG. 3
CV of 10–3 M 2-tert-butyl-1,4-dim eth oxyben zen e (3) in CH3CN with 0.1 M TBAP, at 28(±2) °C,
Pt electrodes, referen ce Ag|AgCl|KCl (sat). a First oxidation , v = 50 (1), 100 (2), 200 (3), 300
(4), 500 (5), 1 000 (6) m V s–1; b full oxidation ran ge at 500 m V s–1: in absen ce of
RuCl3·3H2O (1), in presen ce of 1.8 m M RuCl3·3H2O (2)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

932
Michman, Oron, Schaefer:
voltam m etry an d, secon d, to judge th e relevan ce of com parin g voltam -
m etry with electrolysis. Aceton itrile is th e preferred m edium for
voltam m etry wh ereas effective electrolysis often requires th e presen ce of
water. Th e effect of water on voltam m etry of 1–3 was in vestigated for water
con cen tration s ran gin g from 0.017 m m ol l–1 (in com m ercial-grade dry
aceton itrile) to 1.1 m ol l–1 (Fig. 2b). Con cen tration s of water up to 0.5
m ol l–1 do n ot affect th e first reversible tran sition . In th e ran ge of 0.5–1.0
m ol l–1, water causes gradual loss of reversibility of th e first oxidation step.
As far as th e secon d oxidation step is con cern ed, th e effect of water differs
from th at caused by th e ruth en ium salt in th ree ways: Th e poten tial sh ift is
sign ifican tly sm aller. Approxim ately 240 m M water is required to cause a
sh ift of th e sam e m agn itude as caused by 1 m M RuCl3·3H2O (i.e. 3 m M wa-
ter in RuCl3·3H2O). Th e effect of th e ruth en ium com poun d is 80 tim es
larger. Th e CV curve of th e secon d tran sition ch an ges sh ape. Th e Ipox of th e
secon d step, n ot detected above 170 · 10–3 M water, m erges with th e back-
groun d at th e win dow lim it. In con trast, RuCl3·3H2O causes a 100 m V sh ift
with out loss of distin ction of th e secon d oxidation peak, over scan rates of
50–500 m V s–1 (Figs 1–3). For th e con cen tration of arom atic com poun d
about 0.1 m ol l–1, th e poten tial sh ift caused by RuCl3·3H2O reach es its m ax-
im um at con cen tration s below 3 m m ol l–1 wh ereas th e effect of water is
con tin uous over a wider ran ge. Obviously, th e in fluen ce of RuCl3·3H2O is
of a differen t n ature th an th at of water.
450
I, µA
1
350
250
150
50
2
2
1
2
1
–50
–150
–200
200
600
1 000
1 400
E, mV
FIG. 4
CV of 10–3 M N,N,N′,N′-tetram eth yl-1,4-ph en ylen ediam in e (5) in CH3CN with 0.1 M TBAP, v =
200 m V s–1
at 28(±2) °C, Pt electrodes, referen ce Ag|AgCl|KCl (sat). In absen ce of
,
RuCl3·3H2O (1), in presen ce of 1.8 m M RuCl3·3H2O (2). Voltam m ogram of th e secon d oxida-
tion step in presen ce of ruth en ium salts is m arked by sh adin g
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Aromatic Radical Cations
933
Th e effect of water in th e presen ce of 1 m M RuCl3·3H2O is sim ilar up to a
poin t. Th e added water (150 m m ol l–1) sh ifts th e poten tial of th e secon d ox-
idation step. Th e first oxidation curves of 2 an d 3 rem ain reversible or
347.0
347.5
348.0
348.5
349.0
349.5
350.0
Field, mT
FIG. 5
SEESR; first-derivative ESR spectrum of th e radical cation of 5 · 10–3
M
1,4-di-tert-butyl-2,5-
dim eth oxyben zen e in CH3CN with 0.1 M TBAH, [1]+• gen erated by electrolysis in situ at a Pt
electrode; g = 2.0031, a(1H) = 0.322 m T (6 H), a(1H) = 0.100 m T (2 H), solid lin e experim en -
tal, dotted lin e sim ulated
1
2
347
348
349
350
Field, mT
FIG. 6
SEESR; first-derivative of in situ ESR spectrum obtain ed durin g electrolysis at 20(±1) °C (Pt
electrode) in CH3CN con tain in g 0.1 M TBAP an d 3,5-di-tert-butyl-1,2-dim eth oxyben zen e (2)
of con cen tration s: (1) 5, (2) 30 m m ol l–1. Th e m odulation am plitude is 0.02 m T an d m odu-
lation frequen cy 50 kHz; g = 2.0032(±0.0001)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

934
Michman, Oron, Schaefer:
sligh tly distorted up to 1.4 M water. Wh en th e water con cen tration ex-
ceeded 1.4 m ol l–1, both tran sition s gradually con verged; th is was observed
for com poun ds 2 an d 3 on ly. Com poun d 1, on th e oth er h an d, is catalyti-
cally oxidized in presen ce of both water an d ruth en ium salt at water con -
cen tration above 0.2 m ol l–1. Here a com plicated m ultistep oxidation takes
place an d 1 is oxidized to 6 (ref.8).
a
3,5-di-tert-butyl-1,2-dimethoxybenzene
1
Intensity,
a.u.
3,5-di-tert-butyl-1,2-dimethoxybenzene + RuCl ·3H O
3
2
2
Potentiostat open-circuited
400 600 800
0
200
1 000
1 200
t, s
0.0
b
ln (C /C )
x
0
3,5-di-tert-butyl-1,2-dimethoxybenzene
y = –0.0050x – 0.0073
1
–0.5
–1.0
2
–1.5
–2.0
y = –0.0180x – 0.0455
–2.5
3,5-di-tert-butyl-1,2-dimethoxybenzene + RuCl ·3H O
3
2
0
100
200
300
400
500
t, s
FIG. 7
Tim e developm en t of th e ESR sign al at open circuit con dition s followin g th e oxidative elec-
trolysis of 2 (30 m m ol l–1) with 0.1 M TBAP, at 20 °C. In absen ce of RuCl3·3H2O (1), in pres-
en ce of 5 m M RuCl3·3H2O (2), m odulation am plitude 1 m T (a). Th e relation of ln (Cx/C0)
(wh ere C0 is th e ESR sign al in ten sity at t = 0 an d Cx is th e sign al in ten sity at tim e t) plotted
vs t (s) (b). Data from Fig. 7a
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Aromatic Radical Cations
935
Th e possibility th at poten tial sh ifts in voltam m etric curves can be caused
by adsorption 16, h ere by [ArH]a+d•s , h as to be con sidered. In presen t experi-
m en ts, adsorption can n ot be detected eith er visually or by voltam m etry. In
a series of voltam m etric an d electrolysis experim en ts, clean electrodes were
com pared with electrodes used repeatedly with out clean in g. Th ere were n o
eviden ce of coatin g or an y differen ces in voltam m ogram s m easured both in
aceton itrile or aceton itrile–water solution s. Furth erm ore, th e peak curren t
at th e poten tial sh ifted by an in fluen ce of RuCl3·3H2O fits with diffusion
con trol criteria, i.e., it is lin ear with v1/2 an d sh ows n o saturation level even
with sm all (1 m m 2) electrodes. At h igh scan rates, it sh ows th e sam e IR po-
ten tial drop as th e un sh ifted wave. Th e exception was observed wh en solu-
tion s of com poun d 3 were electrolyzed. Electrode coatin g appears durin g
electrolysis in th e presen ce of RuCl3·3H2O an d seem s to be associated with
ch lorin ation . With com poun d 3, ch lorin ation is a very effective reaction at
low con cen tration s of ch loride11. Th is com plication was n ot observed with
1 an d 2.
A sim ilar sh ift of th e secon d oxidation wave of com poun ds 2 an d 3 was
observed in th e presen ce of Ru(CH3CN)3Cl3 (ref.17). On th e oth er h an d, n o
sh ift was foun d in oxidation of com poun ds 1–3 in th e presen ce of
tris(acetylaceton ato)ruth en ium , Ru(acac)3.
Th e in itial oxidation rates in CPE at th e sh ifted poten tial were con sidered
rath er th an optim um yields of specific products. In accord with th e CV
0.40
0.30
1
0.20
2
0.10
0.00
0
4
8
12
16
t, min
FIG. 8
Tim e depen den ce of th e absorban ce at 463 n m for [1]+• in CH3CN with 1.0 M TBAP, at 20 °C.
In absen ce of RuCl3·3H2O (1), in presen ce of 1.8 m M RuCl3·3H2O (2)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

936
Michman, Oron, Schaefer:
study, CPE of 2 was perform ed at th ree predeterm in ed poten tials, 1.4, 1.8
an d 2.2 V (Ag|AgCl|KCl as a referen ce, cf. Fig. 2). Experim en ts in th e pres-
en ce an d absen ce of RuCl3·3H2O were com pared un der iden tical con dition s
an d products 3,5-di-tert-butyl-1,2-ben zoquin on e (7) an d N-(3-tert-butyl-
4,5-dim eth oxyph en yl)acetam ide (8) were iden tified (Sch em e 1). Figures 9a
an d 9b display reaction rates of 2 an d form ation of 7 at differen t poten tials
in dry aceton itrile (16.7 m M H2O). In th e n on -catalyzed process, con sum p-
tion of 2 in creases gradually an d th e rate of form ation of 7 decreases regu-
larly with in creasin g poten tial. In th e presen ce of RuCl3·3H2O,
con sum ption of 2 in creases sim ilarly, but th e rate of form ation of 7 is par-
ticularly accelerated with a m axim um value aroun d 1.8 V (Fig. 9). Th e cur-
ren t efficien cy for 7 is also exception ally h igh at 1.8 V: 20% with out
RuCl3·3H2O an d 50% in its presen ce. Th e production of 7 in duced by
RuCl3·3H2O at 1.8 V is outstan din g. Th e rate in th is case is on ly doubled
but th e effect depen ds on th e exact value of poten tial selected for electroly-
sis (cf. Fig. 2a). Voltam m etry in dicates th at a larger effect is possible. Form a-
tion of 7 slows down again at m ore positive poten tials startin g from 2.2 V
wh ere com poun d 8 becom es an im portan t product.
To form quin on es, th e position s at th e m eth oxy groups are attacked an d
th e tert-butyl groups rem ain in tact. At m ore positive poten tials th e reaction
with aceton itrile becom es sign ifican t. Th is is altogeth er a differen t reaction
wh ereby th e m eth oxy groups rem ain in tact an d a tert-butyl group is re-
5
4
3
30
a
b
2
1
20
10
1
2
2
1
0
0
0.0
1.5
2.0
2.5
0.0
1.5
2.0
2.5
E, V
E, V
FIG. 9
In itial rates of oxidation of com poun d 2 (a); in itial rates of form ation of quin on e 7 at 1.4,
1.8 an d 2.2 V vs Ag|AgCl, at 28(±2) °C, Pt electrodes (b). In absen ce of RuCl3·3H2O (1), in
presen ce of RuCl3·3H2O (2)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Aromatic Radical Cations
937
placed. Also th e arom atic m olecule is oxidized beyon d th e level of radical
cation . Th e com petition between water an d aceton itrile was n oted in oth er
cases depen din g eith er on th e applied poten tial18 or on th e electrolyte19
No detailed study of th is problem is available.
.
Applicability of con dition s foun d for m icroscale voltam m etry to m acro-
scale electrolysis h as lim itation s an d sign ifican t differen ces prevail20. Elec-
trolysis of several arom atic eth ers in a very dry aceton itrile causes polym er
deposition on th e electrodes21,22. Our experim en ts were perform ed in aceto-
n itrile with 0.03% (16.7 m m ol l–1) water con ten t an d in aceton itrile– water.
In th ese solven ts, polym er deposition is suppressed. At water con cen tra-
OCH3
O
OCH3
H2O
1.2 V catalyzed
1.2 V
- e
- e
- 2 CH3OH
OCH3
O
OCH3
1
6
H2O
O
1.8 V catalyzed
O
- e
- 2 CH3OH
OCH3
OCH3
OCH3
1.4 V
- e
7
H3CO
- e
CH3CN
2.1 V
OCH3
OCH3
2
- [(CH3)3C-]
CH3CONH
8
OCH3
OCH3
O
OCH3
OH
H2O
1.8 V catalyzed
1.35 V
- e
+
- e
- 2 CH3OH
OCH3
O
OCH3
OH
9
3
10
OCH3
Cl
Cl- at <1.4 V
- e
Cl
OCH3
OCH3
11
12
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

938
Michman, Oron, Schaefer:
tion s 0.017–0.2 m ol l–1, oxidation steps are well resolved in voltam m etry
an d, in th is respect, th e results of electrolysis sh ould reason ably reflect th e
observed voltam m etry. Still, voltam m etry does n ot reveal effects wh ich are
due to th e larger scale an d exten ded tim e required by electrolysis. Th ese ef-
fects are specific in each case (Sch em e 1).
As already m en tion ed, reaction s of 1 becom e autocatalyzed in th e pres-
en ce of both water an d ruth en ium com poun ds as observed by voltam-
m etry8. Electrolysis of 1 is very differen t from th at of 2 or 3.
Com poun d 3 sh ows th e sam e tren d as com poun d 2 – a faster reaction
with water at th e sh ifted poten tial in th e presen ce of RuCl3·3H2O wh ere
tert-butyl-1,4-ben zoquin on e (9) an d th e h ydroquin on e (10) are produced.
In th e case of 3, th e situation is com plicated in th e presen ce of ch loride due
to in terferen ce of ch lorin ation 11 (n ot observed with 1 an d 2), wh ich is very
fast already at 1.0 V, an d obstructs detailed rate m easurem en ts. Ch lorin a-
tion is predom in an t in dry aceton itrile. At h igh water con cen tration s th e
quin on e 9 an d h ydroquin on e (10) are form ed. Th eir form ation is acceler-
ated by RuCl3·3H2O at 1.75 V (Fig. 3).
CONCLUSIONS
Com poun ds 1–5 form in th e first oxidation step a radical cation wh ich is
stable with in th e tim e scale of voltam m etric m easurem en ts, even in th e
presen ce of water in con cen tration s exceedin g 0.2 m ol l–1. A secon d step is
an irreversible (reversible for 5) oxidation of th e radical cation at poten tials
by about 400 m V m ore positive. Its poten tial is sh ifted in th e presen ce of
RuCl3·3H2O n egatively for 1–3, positively for 4. No sh ift was observed for
com poun d 5. RuCl3·3H2O h as n o effect on th e first oxidation step. On th e
basis of th e results, th e secon d oxidation in th e sh ifted position reflects th e
oxidation of a differen t species, n ot th e radical cation itself but presum ably
eith er som e tran sien t com plex with RuCl3·3H2O or a product of reaction
between th e radical cation an d RuCl3·3H2O. In teraction between th e radical
cation an d RuCl3·3H2O was observed directly by SEESR an d UV-VIS spec-
troscopy. Such reaction in volves electron tran sfer between [ArH]+• an d
RuCl3·3H2O causin g its furth er oxidation or couplin g with a h igh -spin posi-
tion on RuCl3·3H2O, as eviden t from th e loss of ESR sign al. Th e tran sfer is
apparen tly from th e radical cation to RuCl3·3H2O. Th e oth er direction rep-
resen ted by recovery of ArH from [ArH]+• would be in dicated by a catalytic
wave in th e first oxidation step. Such situation h as in deed been observed
with 1 in th e presen ce of RuCl3·3H2O or Ru(acac)3 an d water8 but n ot with
th e oth er com poun ds.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Aromatic Radical Cations
939
RuCl3·3H2O provides faster reaction s at lower poten tials applied. Th is im -
proves th e cell perform an ce an d facilitates th e use of un divided cells. It also
en ables m ore efficien t separation of reaction s th at require differen t
overpoten tials (Sch em e 1). Th e presen ce of RuCl3·3H2O m ay also reduce th e
con cen tration of radical cation s at th e electrode wh ich would be oth erwise
available for side reaction s such as couplin g. Th is h as been clarified particu-
larly in th e case of n aph th alen e an d 2-m eth yln aph th alen e, wh ere catalysis
distin ctly im proved con version to quin on e over dim erization 6.
Th e results sh ow th at RuCl3·3H2O affects several types of arom atic com -
poun ds but th e activity of RuCl3·3H2O rem ain s a very com plex issue. In th e
“ch em ical” catalysis23 like in recen t reviews dealin g m ostly with
reduction24,25, the first step commonly involves ET of a catalytic pair in the
context of indirect electrolysis. Here, RuCl3·3H2O appears to act as a “chemical”
catalyst but in a differen t m an n er. Th e substrate ArH reacts directly to
[ArH]+• in a reversible step (reaction (4), irreversible with n aph th alen e). Th e
additive, RuCl3·3H2O, acts in th e secon dary oxidation (reaction (5)) fol-
lowed by th e reaction with water at a lower overpoten tial (Sch em e 2).
Indirect electrolysis:
[Cat]+
[ArH-Cat]+
e
(1)
(2)
(3)
-
Cat
E
C
E
[Cat]+
ArH
+
[ArH-Cat]+
e
-
Products
+
Nu
This work:
e
[ArH]
[ArH-Cat]+
(4)
(5)
(6)
-
ArH
E
C
E
[ArH]
Cat
+
[ArH-Cat]+
e
-
Products
+
Nu
SCHEME 2
This work was supported by a research grant from the German–Israeli Fund (GIF). We thank Dr Y.
Norbert for technical help. The authors also thank Dr D. A. Fiedler and Dr R. D. Webster for carrying
out SEESR studies with the facilities of La Trobe University, Bundoora, Australia, and Prof. A. Bond
for encouraging this cooperation.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

940
Michman, Oron, Schaefer:
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