Two-electron oxidation of N,N,N′,N′-tetramethylphenylenediamine with a chromium(v) salen complex

Article abstract of DOI:10.1039/b811140e

The oxidation of tetramethylphenylenediamine (TMPD) with (salen)Cr ^VO⁺ generates initially the 2-electron product TMPD ²⁺, followed by the reaction with excess TMPD to yield the radical cation, TMPD⁺. The kineti

Full text of DOI:10.1039/b811140e

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www.rsc.org/dalton | Dalton Transactions
Two-electron oxidation of N,N,N¢,N¢-tetramethylphenylenediamine with a
chromium(V) salen complex
Gezahegn Chaka† and Andreja Bakac*
Received 1st July 2008, Accepted 15th September 2008
First published as an Advance Article on the web 5th November 2008
DOI: 10.1039/b811140e
V
+
The oxidation of tetramethylphenylenediamine (TMPD) with (salen)Cr O generates initially the
2
+
2
-electron product TMPD , followed by the reaction with excess TMPD to yield the radical cation,
∑
+
V
+
2+
TMPD . The kinetics of both TMPD/(salen)Cr O and TMPD/TMPD reactions are
acid-dependent, with TMPD being the most reactive form, and the doubly protonated TMPDH
exhibiting no discernible reactivity toward either (salen)Cr O or TMPD . The specific rate constants
2
+
2
V
+
2+
V
+
9
-1 -1
for the individual reactions are: TMPD/(salen)Cr O , k = (6.28 ± 0.50) ¥ 10 M s ,
+
V
+
5
2+
8
TMPDH /(salen)Cr O , (3.89 ± 0.31) ¥ 10 , TMPD/TMPD , (7.61 ± 0.42) ¥ 10 , and
+
2+
4
TMPDH /TMPD , (3.46 ± 0.22) ¥ 10 .
Introduction
assignments are incorrect, the results reported here demonstrate
2
+
how a 2-e oxidation followed by TMPD /TMPD compropor-
tionation could be easily disguised as 1-e chemistry unless specific
experiments are designed to search for the 2-e pathway.
Tetramethylphenylenediamine (TMPD) and related amines have
been the focus of a large number of mechanistic studies. TMPD
itself is water-soluble and easily oxidized to the intense blue radical
V
+
The oxidant employed in this work, (salen)Cr O ,
∑
+
1
cation TMPD (E = 0.12 V vs SCE). In mechanistic work,
V
IV
23
(
E(Cr /Cr ) = 0.47 V vs SCE) has ample potential to oxidize
the role of TMPD has been that of an electron transfer reagent
tmpd to the radical cation by single electron transfer. Such a path
might appear preferable mechanistically, but 2-electron oxidations
by (salen)Cr O , typically by oxygen atom transfer,
1–4
5,6
7
in thermal
and photochemical reactions, kinetic probe,
8,9
mediator for generation of non-thermodynamic products, and
hole acceptor in quantum dots. In zeolites, TMPD was shown to
undergo spontaneous charge separation, while photoexcitation
in solutions leads to electron transfer to solvent.
V
+
23,24
are also
10
common. An overall 2-electron oxidation of TMPD should
be therefore a realistic possibility, either because the chemistry
might involve a genuine 2-e step, or the products of 1-e reaction,
11
12
∑
+
IV
{
TMPD + (salen)Cr }, could engage in second electron transfer
within the solvent cage prior to diffusing into bulk solution. In
both cases, the initial product of TMPD oxidation would be the
rarely observed TMPD . The results are presented herein.
2
+
Reactions with organic and inorganic radicals and with some
metal complexes are clearly 1-e processes, either taking place
by outer-sphere electron transfer or requiring preassociation.
Experimental
7,13
V
The complex salt [(salen)Cr (O)](CF
3
SO
with iodosobenzene following
Commercial (Aldrich) N,N,N¢,N¢-
3
) was prepared by oxidiz-
Reactions with potential 2-e oxidants, on the other hand, may
proceed in single 2-e steps or in a sequences of 1-e steps. The latter
would appear more likely in view of the pronounced preference
III
25
ing [(salen)Cr (H
2
O)](CF
3
3,26
SO )
3
2
a literature procedure.
tetramethylphenylenediamine (TMPD) was used as received.
The concentrations of (salen)Cr(O) in aqueous or acetonitrile
14–18
of TMPD for 1-e chemistry. Among numerous studies
on
+
electrochemistry of TMPD, we found no reports on a successful
3
-1
-1 26
∑+
15
solutions (e600 = 2 ¥ 10 M cm ), and of TMPD (e600
=
one-step 2-e oxidation. It appears that in all of the solvents and
under all of the conditions, the oxidation took place in distinct
-
1
-1 13
8
600 M cm ) were determined spectrophotometrically.
+
The kinetics of the (salen)Cr(O) /TMPD reaction in homoge-
1
2
-e steps. We found only one reliable example of a single-step
neous aqueous solution were measured with an Olis RSM-1000
rapid-scan stopped-flow instrument (2 cm pathlength). The data
were collected under pseudo-first order conditions using either of
the two reagents in large excess, and fitted to an expression for
first-order kinetics,
-e chemical oxidation, which was observed in the reaction with
19
∑+
I
2
.
The final product, TMPD , was generated in the follow-up
2
+
reduction of TMPD with unreacted TMPD.
20–22
In other cases of overall 2-e oxidation,
it either has been
shown or can be reasonably assumed that the reactions proceeded
∑
+
Abs
•
)e-
•
k
t
obs
in two 1-e steps involving TMPD as an intermediate. Although
we have no evidence that any of the reported mechanistic
t
= Abs
+ (Abs
0
- Abs
where Abs
t
, Abs , and Abs
•
0
stand for absorbance at times t, zero,
and infinity, respectively. To ensure that the acid/base equilibria of
Ames Laboratory, Iowa State University, Ames, IA 50011, USA. E-mail:
bakac@ameslab.gov
+
organic amines do not alter the concentration of [H ] in the course
+
of the kinetic experiments, the concentration of H was always at
†
Current address: Department of Chemistry, Louisiana State University
at Alexandria, Alexandria, LA 71302, USA.
least 50 times greater than the concentration of the amines. The
3
18 | Dalton Trans., 2009, 318–321
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ionic strength was kept constant at 0.10 M (HClO
All of the kinetic data were obtained at 25 ± 0.02 C.
All the UV-Vis spectral data were acquired with a Shimadzu
4
+ NaClO
4
).
Clearly, different reactions are observed under the two sets of
conditions. A reasonable explanation for the delayed formation
◦
·
+
of TMPD is shown in eqn (2–3), according to which the initial
III
+
3
101 PC instrument. In-house distilled and ion-exchanged water
step is a two-electron reaction generating (salen)Cr (H
2
O) and
was further purified by passage through a Millipore Milli-Q water
purification system. The fitting of the kinetic data was done with
the program Kaleidagraph 3.6.
the doubly oxidized amine. No further reaction occurs when
V
+
(salen)Cr O is in excess, but under the reversed conditions,
2
+
∑+
the initially generated TMPD is reduced to TMPD in the
comproportionation step of eqn (3). It is this step that is
experimentally observed with TMPD as excess reagent.
Results
V
+
+
III
+
(
salen)Cr O + TMPD (+ 2 H ) → (salen)Cr (H
2
O)
V
+
2+
The reaction between (salen)Cr O and TMPD generated the
+ TMPD
(2)
(3)
∑
+
blue radical cation, TMPD , which was easily recognized by its
characteristic and intense spectrum in the 550–600-nm region.
The rising absorbance in the stopped-flow kinetic traces obtained
in the presence of a large excess of TMPD at 600–610 nm were
exponential, Fig. 1, and yielded pseudo-first order rate constants
2+
∑
+
TMPD + TMPD → 2TMPD
According to this scheme, it should be possible to detect
reaction 2 even in experiments with excess TMPD, although the
+
large absorbance increase in the follow-up reaction 3 (De610
=
that increased with [TMPD] at constant [H ]. The reaction
-
1
-1
∑
+
V
+
8600 M cm ) may overshadow the small absorbance decrease
caused by reaction 2. This absorbance decrease is made even
smaller by the stoichiometric factor of 2 in eqn (1). None the
less, convincing evidence for reaction 2 was observed in the early
stages of an experiment with a 7-fold excess of TMPD, Fig. 3.
produced two equivalents of TMPD per mole of (salen)Cr O ,
eqn (1).
V
+
+
III
+
(
salen)Cr O + 2TMPD (+ 2 H ) → (salen)Cr (H
2TMPD
2
O)
∑
+
+
(1)
V
+
The reasonably large concentration of (salen)Cr O (24 mM) was
necessary to increase the signal to noise ratio in the crucial initial
+
stages (shown), and the high [H ] (0.0604 M) slowed the reaction
sufficiently to bring it into a measurable range at the high [TMPD]
used. The initial absorbance decrease in Fig. 3 was small, but
∑
+
clearly detectable before the onset of the TMPD build-up.
V
+
Fig. 1 Kinetic plot at 610 nm for the reaction of (salen)Cr O (4 mM)
with excess TMPD (2.1 mM) in 0.0242 M HClO
4
.
V
+
When the conditions were reversed, and (salen)Cr O was used
in large excess over TMPD, the absorbance at 600 nm decreased
+
signaling the loss of (salen)CrO without the accompanying
V
+
Fig. 3 Initial stages of a reaction between (salen)Cr O (24 mM) and
excess TMPD (0.17 mM) in 0.0604 M HClO
∑
+
formation of TMPD , Fig. 2. Kinetic traces were exponential, but
the observed rate constants were about ten times larger than those
measured with excess TMPD at comparable acid concentrations.
4
.
V
+
Kinetics of (salen)Cr O /TMPD reaction
At a constant acid concentration, the reaction is first-order in
TMPD, as evidenced by excellent fits of the kinetic traces to
the first-order rate equation. The reaction is also first-order
V
+
in (salen)Cr O . The plots of kobs against the concentration of
V
+
(
salen)Cr O are linear with zero intercept, and yield second-order
+
+
rate constants at a given [H ]. The dependence on [H ], studied in
the range 0.005 M < [H ] < 0.10 M, is displayed in Fig. 4.
+
The data were fitted to eqn (4), which takes into account the
two acid/base equilibria for TMPD and different reactivities for
the three protonation levels of TMPD, eqn (5–9). The acidity
2
-3
constants were fixed at their known values, Ka1 = 6.3 ¥ 10 M,
-
7
Fig. 2 Kinetic plot at 600 nm for the reaction of TMPD (6.5 mM) with
and Ka2 = 4.7 ¥ 10 M. The fit yielded the rate constants
V
+
9
-1 -1
5
excess (salen)Cr O (58 mM) in 0.0181 M HClO
4
.
k
1
= (6.28 ± 0.74) ¥ 10 M s , k
2
= (3.88 ± 0.58) ¥ 10 , and
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Dalton Trans., 2009, 318–321 | 319

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Fig. 5 Acid-dependence of the second-order rate constant for the reaction
2
+
Fig. 4 Acid-dependence of the second-order rate constant for the
oxidation of TMPD (3.5–7 mM) with excess (salen)Cr O (33–53 mM).
of TMPD with excess TMPD (0.4–2.6 mM).
V
+
material was quickly loaded into a stopped flow and mixed with
TMPD. The formation of TMPD followed the same kinetic
2
+
∑+
k
3
= 89 ± 11000. Obviously, the reactivity of TMPDH
and subject to large error. Fixing k at zero had almost no effect
on the calculated values of k , (6.28 ± 0.50) ¥ 10 M s , and
2
is neligible
3
profile as that shown in Fig. 5, and yielded k = (3.77 ± 0.21) ¥
1
9
-1 -1
8
-1 -1
4
-1 -1
1
10 M
s
and k = (0.80 ± 0.06) ¥ 10 M s . These values are
2
5
-1 -1
V
+
k
2
, (3.89 ± 0.31) ¥ 10 M s , and even the standard deviations
somewhat lower than those derived from (salen)Cr O reactions,
but the overall picture strongly supports our reaction scheme for
the TMPD/(salen)Cr O reaction in eqn (2–3).
decreased only moderately compared to those obtained in the
three-parameter fitting.
V
+
29
+
+ 2
k K K + k K [H ] + k [H ]
1
a1 a2
2
a1
3
kobs
=
(4)
Discussion
+
2
+
[
H ] + K [H ] + K K
a1
a1 a2
The combination of the reduction potentials (E
1
= 0.12 V vs
2+
∑+
2
+
+
+
H
2
TMPD ꢀ H + HTMPD Ka1
(5)
(6)
(7)
(8)
(9)
∑
+
1
SCE for TMPD /TMPD, E
2
= 0.72 V for TMPD /TMPD ,
30
2
i. e. 0.6 V greater than E
1
)
and the acidity constants for
+
+
HTMPD ꢀ H + TMPD Ka2
2+
+
TMPDH
2
and TMPDH yields the equilibrium constant for
∑
+
disproportionation of TMPD , log K10 = -10. The reaction
V
+
TMPD + (salen)Cr O → Products k
1
2+
2+
yielding TMPDH
2
and TMPD , eqn (11), has log K11 = -1.7.
Disproportionation is thus unfavorable in both acidic and neutral
solutions, which makes the radical the thermodynamic product,
as experimentally observed.
+
V
+
HTMPD + (salen)Cr O → Products k
2
2
+
V
+
H
2
TMPD + (salen)Cr O → Products k
3
∑
+
2+
2
TMPD ꢀ TMPD + TMPD log K = -10
(10)
2
+
Kinetics of the TMPD /TMPD reaction
∑+
+
2+
2+
2
TMPD + 2H ꢀ TMPDH
2
+ TMPD log K = -1.7 (11)
These experiments required an excess of [TMPD] over
Based on this analysis, one might have expected the reaction
V
+
[(salen)Cr O ], as described in the previous section. Since both
to adopt a direct 1-e path, but the 2-e reduction potential for
reactions 2 and 3 are first order in [TMPD], the formation of
V
+
2+
(
salen)Cr O is obviously large enough to generate TMPD .
2
+
TMPD in reaction 2 was about 10 times faster than reaction
under all of the conditions, so that there was no interference
This may be accomplished in a single 2-e step (by initial electron
or oxygen atom transfer), although the large structural changes
3
from reaction 2 in kinetic measurements. The reaction was
first order in the limiting reagent and in [TMPD]. The form
2+
accompanying the oxidation of TMPD to TMPD may favor a
rebound mechanism as in eqn (12), or formation of 1-e products,
+
of the [H ] dependence, shown in Fig. 5, closely mimics that
IV
+
∑+
(
salen)Cr O and TMPD within the solvent cage followed by
V
+
observed in the (salen)Cr O /TMPD reaction, but values of the
derived rate constants are smaller, i. e. k
rapid second electron transfer. In a related reaction between a high-
valent oxo-iron porphyrin complex and dimethylaniline, oxygen
atom transfer has been demonstrated recently in the gas phase,
although this path seems to be significant only when formation of
1
¢ = (6.80 ± 0.60) ¥
8
-1
-1
2+
1
1
(
0
0 M
s
s
(TMPD/TMPD reaction), k
2
¢ = (4.08 ± 0.40) ¥
4
-1 -1
+
2+
-1 -1
0 M
(HTMPD /TMPD ), and k
3
¢ = -1110 ± 60 M
s
2
+
2+
H
2
TMPD /TMPD ). Setting k
3
¢ = 0, yielded k ¢ = (7.61 ±
1
31
the radical becomes endothermic.
8
-1 -1
4
-1 -1
.42) ¥ 10 M
s
and k
2
¢ = (3.46 ± 0.22) ¥ 10 M s .
∑
V
+
IV
+
+
(
salen)Cr O + TMPD → [(salen)Cr O , TMPD ]
2
+
To confirm that the TMPD/TMPD reaction was indeed the
source of the absorbance increase at 600 nm, we looked for
III
+
2+
→
(salen)Cr (OH
2
) + TMPD
(12)
2
+
27,28
III
+
2+
alternative ways to generate TMPD in solution.
The oxi-
In any case, {(salen)Cr (OH
2
)
+ TMPD } are generated.
dation of TMPD with a two-fold excess of Ce(IV) was successful.
Upon mixing, the solution developed a dark blue color typical
These products are thermodynamically greatly disfavored (by
30
III
+
about 0.6 V) over the alternative, {(salen)Cr (OH
2
)
+ 2
∑
+
∑+
∑+
for TMPD , followed by the fading of the color as TMPD
TMPD }, into which they are transformed in a separate, follow-
2
+
2+
∑+
was further oxidized by Ce(IV) to the colorless TMPD . This
up step. The extra energy stored in TMPD relative to TMPD
3
20 | Dalton Trans., 2009, 318–321
This journal is © The Royal Society of Chemistry 2009

View Article Online
+
decreases at higher [H ] as TMPD is protonated, eqn (10–11), but
11 S. Marquis, A. Moissette, C. Bremard and H. Vezin, J. Mol. Struct.,
2003, 651–653, 305–314.
12 L. Boilet, G. Buntinx, C. Lefumeux and O. Poizat, J. Photochem.
Photobiol., A, 2004, 163, 529–536.
13 P. S. Rao and E. Hayon, J. Phys. Chem., 1975, 79, 1063–1066.
14 J. -M. Lue, X. -L. Wen, L. -M. Wu, Y. -C. Liu and Z. -L. Liu, J. Phys.
Chem. A, 2001, 105, 6971–6975.
+
even in 1 M H , reaction 11 still generates products that are 0.1 V
∑
+
uphill of 2 TMPD .
In summary, a complementary 2-e reaction between (salen)-
V
+
Cr O + TMPD generates products that are significantly higher
in energy than the thermodynamically more favorable alterna-
tive. Quantitative formation of the kinetic products, {(salen)-
1
1
5 N. Leventis and X. Gao, J. Electroanal. Chem., 2001, 500, 78–94.
6 M. Jonsson, A. Houmam, G. Jocys and D. D. M. Wayner, J. Chem.
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III
+
∑+
Cr (OH
2
) + 2TMPD }, requires either that the initial reaction
1
1
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8 A. B. Bocarsly, E. G. Walton, M. G. Bradley and M. S. Wrighton,
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one-electron steps within the solvent cage be extremely rapid so as
to prevent any cage escape.
1
2
2
9 U. Nickel, M. Borchardt, M. R. Bapat and W. Jaenicke, Ber. Bunsen-
Ges., 1979, 83, 877–884.
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Acknowledgements
This manuscript has been authored under Contract No. DE-
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2
2
2
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Dalton Trans., 2009, 318–321 | 321