Water-assisted intramolecular electron transfer from the ring to the side chain in N,N,N′,N′-tetraalkyl-para-phenylenediamine radicals - The reverse of side chain deprotonation of radical cations

Article abstract of DOI:10.1002/1521-3773(20010202)40:3<571::AID-ANIE571>3.0.CO;2-8

In aqueous solution, the neutral radical formed by OH from one of the side-chain methyl groups of N,N,N′, N′,-tetramethyl-p-phenylenediamine (TMPD) is protonated by water to give the aromatic radical cation, TMDP⁺(see scheme). This reaction inv

Full text of DOI:10.1002/1521-3773(20010202)40:3<571::AID-ANIE571>3.0.CO;2-8

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Water-Assisted Intramolecular Electron
Transfer from the Ring to the Side Chain in
N,N,N',N'-Tetraalkyl-para-phenylenediamine
RadicalsÐThe Reverse of Side Chain
Deprotonation of Radical Cations
Steen Steenken* and Abel J. S. C. Vieira
For arenes containing (substituted) alkyl groups as sub-
stituents, ªactivationº of the side chain is often possible by
changing the ªoxidation stateº of the arene, for example, by
[
1±7]
one-electron reduction,
dation.
or more often, one-electron oxi-
[
8±13]
A very characteristic process for the side chain
reactivity of radical cations is deprotonation from carbon
which leads typically to (neutral) benzyl-type radicals. The
reciprocal reaction, protonation on carbon, has also been
.
�
Figure 1. Absorption spectra recorded after reaction of O with 0.4 mm
TMPD in N
2
O-saturated water ([KOH] ˆ 0.4m, 208C. * ˆ spectrum of
.
‡
.
TMPD
>
,
* ˆ spectrum of >NCH
2
). Insets a and b show the decay of
[
14±20]
observed (with radical anions),
but it typically takes place
2.
.
‡
NCH (350 nm) and the build up of TMPD (565 nm) at 0.4 m KOH and
not on the side chain but on the ring. We have now found
evidence for the protonation of a side chain carbon atom (a
reaction which involves a neutral radical), electron transfer
from the ring to the side chain providing the (high) electron
density necessary for the protonation on carbon to occur.
pH 7.4, respectively. Inset c shows the increase in conductance G at pH 10,
which primarily reflects the formation of OH .
�
equal (within 10%) with those optically measured (compare
with inset b).
.
.�
.
N O-saturated aqueous solutions at pH 7 ± 12 containing
2
On replacement of OH by O , the conjugate base of OH
.
N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD; used as
the dihydrochloride) were irradiated with the 100 ns pulses
from a 3 MeV van de Graaff electron accelerator, and the
time-resolved changes in the transmission or of the conduc-
tance of the solution monitored. Irradiation of water leads to
(
pK ( OH) ˆ 11.9), the height of the initial step at 565 nm (see
a
[27]
insets) was found to be much smaller than at pH 7 ± 8.
[28]
Under these conditions, the first process contributed only
5% of the total formation of TMPD , and the main reaction
.
‡
1
.
‡
4
� 1
was the slow formation of TMPD with k ˆ 6.0  10 s , that
.
.
�
the radicals OH (45%), H (10%), and e (45%). The latter
aq
is with essentially the same rate constant as for the reaction at
.
.
reacts with N O within 20 ns to yield another OH radical. In
the presence of 0.2 ± 0.5 mm TMPD, its radical cation TMPD
2
pH 7 ± 12, where, however, OH is the reactant.
Based on the fact that O reacts with arenes typically by
H-abstraction from the side chain, the side-chain-derived
radical >NCH2 is expected to be formed in its reaction with
.
‡
.
�
is produced, whose l
values are at 330, 565, and 610 nm
[29]
max
.‡
[
21, 22]
.
(Figure 1).
TMPD is formed by two routes, a direct and
an indirect one. The direct (ªfastº) reaction accounts for
TMPD (see Scheme 1). Similar to other a-aminoalkyl-type
.
‡
radicals,[ >NCH2 should be a good one-electron reducing
30]
.
(
55 Æ 5)% of the total formation of TMPD (Figure 1,
inset b). The rate, but not the yield, of this reaction depends
agent, and this was in fact found to be the case since this
[
23]
on the concentration [TMPD],
second, slower reaction (k ˆ (6.6 Æ 1)  10 s at 208C
turned out to be independent of [TMPD] (0.1 ± 1 mm) and pH
7 ± 12), but dependent on temperature (between 0 and 508C),
whereas the rate of the
radical could be scavenged by the mild oxidant methylviol-
4
� 1
[24]
.
)
2‡
‡
9
� 1 � 1
ogen (MV ) to yield MV (k ˆ 2.8  10 m s ). The yield of
.
[31]
>
NCH , determined at pH 9 with this method, is 53%
relative to the initial OH present. This value is in agreement
with the numbers derived from the optical and conductance
2
.
(
the resulting Eyring parameters for the electron transfer and
=
side chain protonation (see Scheme 2) being DH ˆ (4.9 Æ
step heights (Figure 1, insets b and c).
�
1
=
� 1 � 1
.
0
.5) kcalmol and DS ˆ (� 19 Æ 1.6) calmol K . Further-
The reaction of OD with TMPD in heavy water (D O) also
2
.
.
‡
more at pH 8 ± 11 the reaction of OH with TMPD leads to an
increase in conductance of the solution (Figure 1, inset c)
led to the formation of TMPD in two steps. However, the
rate constant for the second, slow process, which is assigned to
the conversion of >NCH2 to TMPD (see Scheme 2), was
[
25]
.
.
‡
assignable to the formation of an organic (radical) cation
�
and the OH ion, again in a two-step process, as observed with
smaller than that for the reaction in H O by the factor 3.0;
2
optical detection. The rate of the slow process and the step
height of the change in conductance were independent of the
pH value (8 ± 11) and of the [TMPD] (0.1 ± 1 mm), and were
thus, the solvent kinetic isotope effect (KIE) is 3.0, a value
that is consistent with the (heterolytic) rupture of an O H
bond in the transition state. The activation entropy of
�
[
26]
�
1
� 1
�
19 calmol
K
is in agreement with this concept, since
*] Prof. S. Steenken, Prof. A. J. S. C. Vieira[‡]
motional degrees of freedom are lost by participation of a
water molecule in the transition state of the intramolecular
electron transfer from the ring to the side chain.
[
Max-Planck-Institut für Strahlenchemie
45413 Mülheim (Germany)
Fax : (‡49)208-306-3951
To study the effect of the alkyl substituent on the amino
nitrogen atom of the para-phenylenediamine moiety, the OH
radical was also allowed to react with the N,N,N',N'-tetraethyl
and n-propyl derivatives. As with TMPD, the corresponding
radical cations were formed in two steps; the yields for the
E-mail: steenken@mpi-muelheim.mpg.de
‡
[
] Current address:
Departamento de Química
Instituto Superior Tecnico
1096 Lisboa (Portugal)
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1433-7851/01/4003-0571 $ 17.50+.50/0
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ªfastº (bimolecular) reaction were 70 ± 80%.[32] The subse-
radical; a water molecule acts as the protonating agent[38, 39]
(and possibly as an electron transfer mediator) in the
transition state of the electron transfer. Evidence for this
comes from the kinetic isotope effect of 3 and the negative
activation entropy. The anion complementary to the radical
4
quent unimolecular reaction revealed k values of 1.2 Â 10 and
3
� 1
[33]
7
.1 Â 10 s for the ethyl
and n-propyl derivatives, respec-
tively. Figure 2 shows a Taft plot (s* values) of the rate
�
cation is OH , and this was detected directly (as a hydrated
ion) by time-resolved measurement of the conductance (see
Figure 1, inset c).
To summarize, evidence has been presented for a water-
assisted intramolecular transfer of an electron from the ring as
the electron donor to a carbon-centered radical located on the
side chain at a position b to the ring as the (weak!) electron
acceptor. Formally, the result of this electron transfer would
be a carbanion on the side chain. However, as concluded from
the solvent kinetic isotope effect which indicates the breakage
of an O�H bond in the transition state of the electron transfer,
.
Figure 2. Taft plot of the transformation of >NCH R (R ˆ H, Me, Et) into
protonation by water of this carbon atom is concerted with the
electron transfer. The (new) mechanism presented in
Scheme 2, which corresponds to the reverse of side chain
the corresponding radical cation. The Taft parameter was determined to be
1
* ˆ 1.6 from the slope.
deprotonation[ of radical cations, is an example of the
importance of proton transfer in electron transfer processes in
aqueous solution.
40]
[41]
constants for the transformations of the a-aminoalkyl-type
radicals into the corresponding phenylenediamine radical
cations. The deceleration of the reaction on going from Me to
nPr, reflected in the positive apparent 1* value of 1.6, may be
due to an increase of electron density at the carbon carrying
Experimental Section
[
34]
The pulse radiolysis experiments were performed by using a 3 MeV
van de Graaff accelerator which supplied 100 ns pulses with doses such that
radicals were produced in concentrations of 0.5 ± 3 mm. A thermostatable
quartz flow cell, typically set at 208C, was employed in all experiments.
the unpaired electron or to steric effects.
On the basis of the experimental data presented, a
mechanism is suggested which involves the formation of two
.
different radicals from the reaction of OH with TMPD:
2
Dosimetry was done with N O-saturated 10 mm KSCN solutions taking
.
� 7
� 1
.2�
� 1
� 1
[42]
.
‡
G( OH) ˆ 6.0  10 molJ and e[(SCN) ] ˆ 7600m cm at 480 nm.
The solutions were prepared by dissolving the tetraalkyl-p-phenylenedi-
amine dihydrochlorides in water deoxygenated with and then
HPO . The radiation-
a) the radical cation, TMPD , generated by an addition/
[
35]
elimination sequence
unoccupied?) ring position,
and b) the carbon-centered rad-
ical >NCH , produced by
H-abstraction from the a-C
atom of the alkyl substituent
on the phenylenediamine nitro-
gen (Scheme 1). This mecha-
nism is supported by the finding
that the relative yield of the
at an
2
N O
(
adjusting the pH to ꢀ7 by addition of KOH or Na
2
4
induced changes of the solution were digitized and analyzed by an on-line
.
‡
DEC-PDP 11/73 computer system which also controlled the apparatus
2
and pre-analyzed the data. For the time-resolved conductance changes, an
AC system with 1 ms time resolution was used.
Received: July 7, 2000 [Z15406]
[1] P. Neta, D. Behar, J. Am. Chem. Soc. 1980, 102, 4798 ± 4802.
hydrogen abstraction product,
[2] P. Neta, D. Behar, J. Am. Chem. Soc. 1981, 103, 103 ± 106.
.
[
[
[
3] D. Behar, P. Neta, J. Am. Chem. Soc. 1981, 103, 2280 ± 2283.
4] D. Behar, P. Neta, J. Phys. Chem. 1981, 85, 690 ± 693.
5] P. Maslak, J. N. Narvaez, J. Chem. Soc. Chem. Commun. 1989, 138 ±
>
NCH , increases on going
2
from the electron-transfer (via
[
36]
addition/elimination)
agent
1
39.
.
[29]
.�
Scheme 1.
OH to the H-abstractor
cf. insets b and a of Figure 1).[
O
[6] J. M. Saveant, Acc. Chem. Res. 1993, 26, 455 ± 461.
37]
(
[7] J. M. Saveant in Advances in Electron Transfer Chemistry, Vol. 4, (Ed.:
P. S. Mariano), Jai, Greenwich, CO, 1994, pp. 53 ± 116.
[8] A. M. D. Nicholas, D. R. Arnold, Can. J. Chem. 1982, 60, 2165 ± 2179.
Clearly, for the ªunimolecu-
.
.‡
larº transformation of >NCH2 to TMPD a proton donor is
necessary, and it is proposed that a water molecule serves this
purpose. The mechanism suggested (Scheme 2) involves the
transfer of an electron from the ring to the N-substituted alkyl
[
9] S. Steenken, R. A. McClelland, J. Am. Chem. Soc. 1989, 111, 4967 ±
973.
[10] S. Steenken, C. J. Warren, B. C. Gilbert, J. Chem. Soc. Perkin Trans. 2
990, 335 ± 342.
4
1
[
[
[
11] P. Maslak, J. N. Narvaez, Angew. Chem. 1990, 102, 302 ± 304; Angew.
Chem. Int. Ed. Engl. 1990, 29, 283 ± 285.
12] M. Schmittel, A. Burghart, Angew. Chem. 1997, 109, 2658 ± 2699;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2550 ± 2589.
13] a) E. Baciocchi, M. Bietti, L. Manduchi, S. Steenken, J. Am. Chem.
Soc. 1999, 121, 6624 ± 6629; b) M. Bietti, S. Steenken in Handbook of
Electron Transfer in Chemistry (Ed.: V. Balzani), Wiley-VCH,
Weinheim, 2001, in press.
[14] R. W. Fessenden, O. P. Chawla, J. Am. Chem. Soc. 1974, 96, 2262 ±
Scheme 2.
2263.
572
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1433-7851/01/4003-0572 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 3

COMMUNICATIONS
[
15] R. H. Schuler, P. Neta, H. Zemel, R. W. Fessenden, J. Am. Chem. Soc.
976, 98, 3825 ± 3831.
16] D. J. Deeble, S. Das, C. von Sonntag, J. Phys. Chem. 1985, 89, 5784 ±
788.
Liquid-Phase Synthesis of Colloids and
1
Redispersible Powders of Strongly Luminescing
[
5
LaPO :Ce,Tb Nanocrystals**
4
[
[
17] H. M. Novais, S. Steenken, J. Am. Chem. Soc. 1986, 108, 1 ± 6.
18] S. Steenken, J. P. Telo, H. M. Novais, L. P. Candeias, J. Am. Chem. Soc.
Karsten Riwotzki, Heike Meyssamy,
Heimo Schnablegger, Andreas Kornowski, and
Markus Haase*
1
992, 114, 4701 ± 4709.
19] L. P. Candeias, P. Wolf, P. OꢁNeill, S. Steenken, J. Phys. Chem. 1992, 96,
0302 ± 10307.
[
1
[
[
[
20] L. P. Candeias, S. Steenken, J. Phys. Chem. 1992, 96, 937 ± 944.
An increasing number of nanocrystalline materials have
been synthesized in high-boiling coordinating solvents, since
binding of the solvent molecules to the particle surface leads
21] S. Fujita, S. Steenken, J. Am. Chem. Soc. 1981, 103, 2540 ± 2045.
.
‡
22] The total yield of TMPD was 100% as calibrated by the reaction of
.
�
TMPD with (SCN)
2
, whose yield (in 10 mm KSCN) is equal to that
.
of OH, see: R. H. Schuler, L. K. Patterson, E. Janata, J. Phys. Chem.
[1]
to colloidal solutions of well-separated particles. If process
1980, 84, 2088.
parameters such as concentration and temperature are
properly adjusted[ and, most importantly, a suitable solvent
for the reaction is found, colloidal solutions of highly
crystalline nanoparticles with very narrow particle size
distributions are the result. Examples for this method are
the synthesis of high-quality cadmium chalcogenide nano-
.
‡
[
23] From the dependence on [TMPD] of the rate of formation of TMPD
2]
in the initial direct, ªfastº reaction the rate constant for its formation
.
10 � 1 � 1
by OH is 1.0 Â 10
m
s
. The same value was obtained from an
.
experiment where 2-propanol competed with TMPD for OH (with
.
9
� 1 � 1
k( OH ‡ iPrOH) ˆ 2.2  10 m
s ).
[
[
24] All rate constants given in the paper refer to (20 Æ 0.2)8C.
25] The time-resolved AC method was used. Dosimetry was performed
clusters[ and TiO nanoclusters in trioctylphosphine oxide
1a]
[1b]
.
with the reaction OH and N,N-dimethylaniline which yielded N,N-
2
.
‡
�
[1c,d]
dimethylaniline and OH : J. Holcman, K. Sehested, J. Phys. Chem.
977, 81, 1963.
26] This also shows that TMPD does not react with OH , as also
(TOPO) and the preparation of InP
and InAs nano-
1
[1e]
[1f]
clusters
in trioctylphosphane (TOP). Similarly, ZnSe,
.
‡
�
[
[
[1g]
Fe O , Mn O , and Cu O nanoclusters have been prepared
.
‡
2
3
3
4
2
concluded from the independence of the lifetime of TMPD on
�
in long-chain alkylamines. Our synthesis of LaPO :Eu and
4
[
OH ].
[1h]
27] In contrast, the absorption at 350 nm was much higher than that at
pH 7 ± 8 (cf. insets a, b in Figure 1).
28] Using 0.2 ± 0.5 mm TMPD and 0.4 ± 0.5m KOH.
29] P. Neta, R. H. Schuler, Radiat. Res. 1975, 64, 233.
30] See, for example: S. Steenken and V. Jagannadham, J. Am. Chem. Soc.
CePO :Tb in tris-ethylhexylphosphate
4
shows that the
method is also applicable to doped nanoparticles.
[
[
[
Oxide materials form the active material of most solid-state
lasers and are technologically important as phosphors in
cathode ray tubes, X-ray detectors, and in lighting applica-
tions.[ The latter group includes the mixed phosphate
1985, 107, 6818, and references therein.
3]
[
31] For calibration an N
2
O-saturated solution of 10 mm sodium formate
and 0.5 mm MV2 was used (yield of MV ꢁ 100%).
‡
.‡
La0.40Ce0.45Tb0.15PO , which is used in luminescent lamps as
4
.
�
[
32] As calibrated by reaction with (SCN)
2
. However, the total yield of
[4]
highly efficient emitter of green light. The material is
radical cation was less than 100%, indicating H-abstraction from
terminal carbon atoms in the alkyl groups.
chemically very stable, even in the presence of the mercury
plasma discharge inside the lamp, and has an overall
luminescence quantum yield of 93%.[ Redispersible nano-
[
33] In the case of tetraethyl-p-phenylenediamine, the activation param-
eters were measured for the intramolecular formation of radical
5]
=
� 1
=
� 1 � 1
cation: DH ˆ 6.2 kcalmol , DS ˆ � 19 calmol
K .
particles of La0.40Ce0.45Tb0.15PO may therefore form a stable
4
[
34] Alkyl substitution at C leads to stabilization of the radical and thus
a
and efficient substitute for organic laser dyes in various
applications. In fact, semiconductor nanoparticles such as
CdSe have successfully been applied, for instance, as lumi-
disfavors the transformation to the aromatic radical cation.
6
� 1
[
[
[
35] With kelimination ꢀ 3 Â 10 s
.
36] S. Steenken, J. Chem. Soc. Faraday Trans. 1 1987, 83, 113 ± 124.
37] The conceivable alternative that the radical which undergoes the
[6]
nescing labels for biomolecules and as light-emitting mo-
.
‡
transformation reaction to TMPD is the ipso-OH adduct, is thereby
excluded, since the yield of the ipso-OH adduct would decrease on
[7]
lecular substances in electroluminescent devices.
.
.�
Herein we present an improved synthesis that yields more
going from OH to O
.
.
‡
[
38] The thermodynamic acidity of TMPD is not known. However, on
than 10 grams of the ternary system La0.40Ce0.45Tb0.15PO in a
4
.
‡
the basis of the observation that even in 1m KOH, TMPD persists, its
simple one-pot reaction which does not involve rapid mixing
of dissolved compounds. Rapid mixing at elevated temper-
atures is often applied to separate nucleation and growth of
the nanoparticles,[ but becomes increasingly difficult with
larger amounts of reactants. Nevertheless, our procedure
yields a remarkably narrow particle size distribution even on
this preparative scale. This is shown by transmission electron
pK
a
value is estimated to be ꢂ14 which is in line with the conjugate
.
base, >NCH
2
, being protonatable by H
2
O.
.
.‡
[
39] The conversion of >NCH
2
into TMPD can be accelerated by
8]
6
� 1 � 1
phosphate: For this reaction at pH 8, k ˆ 2  10 m
from the linear dependence of kobs on [phosphate]).
40] E. Baciocchi, M. Bietti, O. Lanzalunga, Acc. Chem. Res. 2000, 33,
43 ± 251.
s
(determined
[
[
2
41] The basis for the electron transfer from the ring to the side chain lies in
the very high electron density at the ring which is enhanced through
the second dialkylamine function. In line with this, the reaction
corresponding to that in Scheme 2 does not take place for the
analogous a-aminoalkyl radical of N,N-dimethylaniline.
[*] Dr. M. Haase, K. Riwotzki, H. Meyssamy, Dr. H. Schnablegger,
A. Kornowski
Institut für Physikalische Chemie
Universität Hamburg
[
42] R. H. Schuler, A. L. Hartzell, B. Behar, J. Phys. Chem. 1981, 85, 192 ±
Bundesstrasse 45, 20146 Hamburg (Germany)
Fax : (‡49)40-42838-3452
199.
E-mail: haase@chemie.uni-hamburg.de.
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(
DFG). We thank S. Bartholdi-Nawrath for help with the electron
micrographs and J. Kolny for measuring the powder X-ray diffraction
data.
Angew. Chem. Int. Ed. 2001, 40, No. 3
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1433-7851/01/4003-0573 $ 17.50+.50/0
573