Electrochemically controlled hydrogen bonding. Electrolyte effects in an oxidation-based arylurea-amide system

Article abstract of DOI:10.1021/ja803453e

Oxidation of a dimethylaminophenyl-substituted urea leads to a > 2000-fold increase in binding strength between the urea and a diamide guest in 0.1 M NBu₄B(C₆F₅)₄/CH₂Cl₂. The strength of th

Full text of DOI:10.1021/ja803453e

Published on Web 07/10/2008
Electrochemically Controlled Hydrogen Bonding. Electrolyte Effects in an
Oxidation-Based Arylurea-Amide System
Jessica E. Woods, Yu Ge, and Diane K. Smith*
Department of Chemistry and Biochemistry, San Diego State UniVersity, San Diego, Califorinia 92182-1030
Received May 9, 2008; E-mail: dsmith@sciences.sdsu.edu
Over the past decade, it has been shown that it is possible to
selectively and significantly perturb the strength of H-bonding
between organic molecules using electrochemistry. However,
although both reduction1 and oxidation2 reactions have been used,
reductions have generally proven more successful.3 In this paper,
we show that the difficulty with oxidations can be due to an
electrolyte effect resulting from competition between the guest and
the electrolyte anion for H-bonding to the oxidized host. Further-
more, we show that, by switching to an electrolyte with a very
large anion, significant changes in binding strength are observed
upon oxidation of an electroactive urea in the presence of a diamide
guest.
The basic idea behind redox-dependent hydrogen bonding is to
use the change in charge accompanying e-transfer to perturb the
strength of H-bonds between host and guest. One possibility is to
Figure 1. CVs (100 mV/s) of 1 mM U in CH2Cl2 with (a) 0.1 M NBu4ClO4,
use a reduction reaction to increase negative charge on H-accepting
atoms in a host. Another possibility, which is explored in this study,
is to use an oxidation reaction to increase the positive charge on
H-donating groups in the host. H-accepting guests will then bind
more strongly to the oxidized form, making it easier to oxidize the
host. This results in a negative shift in the half-wave potential,
E1/2, of the host in the presence of the guest.
(b) 0.1 M NBu4PF6, and (c) 0.1 M NBu4B(C6F5)4.
Scheme 1. Mechanism for U Oxidation with Guest, G, Present
Since the perturbation of H-bond strength relies on a change in
charge, ion-ion interactions may also play a role in these systems.
They can be used to advantage when the guest itself is ionic,4 but
with neutral guests, they may pose a problem. This is particularly
an issue in electrochemical studies, where an “inert” electrolyte
will be present in excess. The most common electrolytes used in
these studies are NBu4ClO4 or NBu4PF6.2 Use of NBu4+ is a good
choice for reduction-based systems because it is a very large ion
and is unlikely to significantly ion pair in the solvents commonly
used for these studies.5 However, the same cannot be said about
electrolyte. Clearly, the electrolyte anion has a big effect on the
electrochemistry. With ClO4-, reversible oxidation to the radical
cation, U+, is observed at 1.47 V, with PF6-, the same process
occurs at 1.51 V, and with B(C6F5)4-, it is at 1.60 V. In addition,
-
ClO4 or PF6-. This has been particularly noted by Geiger in
-
the U0/+ wave is considerably broader with B(C6F5)4- than ClO4
electrochemical studies of organometallic cations.6
or PF6-. Close inspection of the U0/+ wave in Figure 1c suggests
that it actually represents two strongly overlapping waves of equal
height. Interestingly, when the same set of CVs is run with UMe,
the E1/2 values are much closer (within 0.03 V) and there is no
significant difference in wave shape between the three electrolytes.7
This strongly suggests that it is an ionic H-bonding interaction
between the anions and the urea NH’s that is mainly responsible
for the differences between electrolytes seen in Figure 1.
A mechanism capable of explaining the differences observed in
Figure 1 is given in Scheme 1. If a guest, G, is present that binds
more strongly to U+ than U, the observed E1/2 of U0/+ will shift
negative in the presence of G. This is what appears to happen in
Figure 1a and b, with ClO4- or PF6- playing the role of G through
strong interaction with the urea NH’s. In contrast, if no G is present,
as with B(C6F5)4-, then U+ could interact with U to form the UU+
dimer shown below. Since U would bind more strongly to U+ than
another U, the E1/2 shifts negative as UU+ is being formed. This
continues until half of the U is oxidized to U+. Oxidation of the
-
-
In this work, we show that interaction with ClO4 or PF6 can
also be an issue for oxidation-based, redox-dependent H-bonding.
The system under investigation is based on the urea, U. This
compound was chosen because diarylureas are known to be very
good H-donors, and oxidation of the NMe2-phenyl ring should lead
to a significant increase in positive charge on one of the urea NH’s
(eq 1). For comparison purposes, we also synthesized the methylated
derivative, UMe.
Figure 1 shows cyclic voltammograms (CVs) of U in CH2Cl2
with (a) NBu4ClO4, (b) NBu4PF6, and (c) NBu4B(C6F5)4 as the
9
10070 J. AM. CHEM. SOC. 2008, 130, 10070–10071
10.1021/ja803453e CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
successfully with U dimerization. By 5 mM PZD, the wave has
reached full height, and by 100 mM, the system appears to be
closing in on a maximum shift of about -200 mV. This corresponds
to a >2000-fold increase in binding strength upon oxidation. To
the best of our knowledge, this is by far the largest binding
enhancement reported to date for an oxidation-based, redox-
dependent H-bonding system with a neutral molecular guest. From
the simulated CVs, we obtain reliable estimates of the actual binding
constants in the two oxidation states: KUG ) 62 M-1 and KUG+
)
1.6 × 105 M-1
.
In conclusion, we have demonstrated a simple redox-dependent
H-bonding system in which oxidation leads to a substantial increase
in binding strength to a molecular guest. Indeed, the magnitude of
the effect is comparable to that observed for the better reduction-
based systems. However, it is doubtful that this system is unique.
The difference is the electrolyte. Binding to the commonly used
electrolyte anions ClO4- and PF6- is strong enough to obscure the
strength of binding to the guest. It is likely that other oxidation-
based systems would also reveal much stronger binding if re-
examined using less competitive electrolytes.
Figure 2. Experimental (solid lines) and simulated (dots) CVs (500 mV/
s) of 1 mM U in 0.1 M NBu4B(C6F5)4/CH2Cl2 + PZD: (a) 0 mM, (b) 0.5
mM, (c) 1 mM, (d) 5 mM, (e) 100 mM.
rest of U then occurs upon dissociation of UU+. This would occur
at a slightly more positive potential, producing the broad wave
-
observed for U0/+ with B(C6F5)4
.
Acknowledgment. Acknowledgment is made to the Donors of
the American Chemical Society Petroleum Research Fund for
support of this research.
Supporting Information Available: Additional CVs, NMR titration
data and analysis, general voltammetry and CV simulation procedures,
synthesis and structural data for UMe. This material is available free
of charge via the Internet at http://pubs.acs.org.
The occurrence of U dimerization in CH2Cl2 is supported by 1H
NMR data for U in CD2Cl2 that show the NH chemical shifts are
concentration-dependent. Analysis of these data gives KUU ) 22
M-1. Using this value, CVs of 0.5 and 1 mM U in NBu4B(C6F5)4/
CH2Cl2 were fit to a slightly modified version7 of the mechanism
References
(1) (a) Kajiki, T.; Moriya, H.; Hoshino, K.; Kuroi, T.; Kondo, S. i.; Nabeshima,
T.; Yano, Y. J. Org. Chem. 1999, 64, 9679–9689. (b) Legrand, Y.-M.; Gray,
M.; Cooke, G.; Rotello, V. M. J. Am. Chem. Soc. 2003, 125, 15789–15795.
(c) Bu, J.; Lilienthal, N. D.; Woods, J. E.; Nohrden, C. E.; Hoang, K. T.;
Truong, D.; Smith, D. K. J. Am. Chem. Soc. 2005, 127, 6423–6429. (d) Ge,
Y.; Miller, L.; Ouimet, T.; Smith, D. K. J. Org. Chem. 2000, 65, 8831–
8838. (e) Gray, M.; Cuello, A. O.; Cooke, G.; Rotello, V. M. J. Am. Chem.
Soc. 2003, 125, 7882–7888. (f) Jordan, B. J.; Pollier, M. A.; Miller, L. A.;
Tiernan, C.; Clavier, G.; Audebert, P.; Rotello, V. M. Org. Lett. 2007, 9,
2835–2838. (g) Kato, S. i.; Matsumoto, T.; Ideta, K.; Shimasaki, T.; Goto,
K.; Shinmyozu, T. J. Org. Chem. 2006, 71, 4723–4733.
(2) (a) Boyd, A. S. F.; Cooke, G.; Duclairoir, F. M. A.; Rotello, V. M.
Tetrahedron Lett. 2003, 44, 303–306. (b) Cooke, G.; de Cremiers, H. A.;
Duclairoir, F. M. A.; Leonardi, J.; Rosair, G.; Rotello, V. M. Tetrahedron
2003, 59, 3341–3347. (c) Goldenberg, L. M.; Neilands, O. J. Electroanal.
Chem. 1999, 463, 212–217. (d) Saweczko, P.; Enright, G. D.; Kraatz, H.-B.
Inorg. Chem. 2001, 40, 4409–4419. (e) Westwood, J.; Coles, S. J.; Collinson,
S. R.; Gasser, G.; Green, S. J.; Hursthouse, M. B.; Light, M. E.; Tucker,
J. H. R. Organometallics 2004, 23, 946–951. (f) Carr, J. D.; Coles, S. J.;
Hursthouse, M. B.; Light, M. E.; Tucker, J. H. R.; Westwood, J. Angew.
Chem., Int. Ed. 2000, 39, 3296–3299.
(3) There are a number of examples (see ref 1) where perturbation of H bonds
through reduction results in a >10-fold change in binding strength between
an electroactive host and a neutral guest. In contrast, to our knowledge, the
ferrocene diamides studied by Green, Tucker, and colleagues (ref 2e) remain,
until now, the only reported examples where oxidation does the same.
(4) Much greater success has been achieved with oxidation-based H-bonding
receptors for anions. For a recent example, see Willener, Y.; Joly, K. M.;
Moody, C. J.; Tucker, J. H. R. J. Org. Chem. 2008, 73, 1225–1233.
(5) Macias-Ruvalcaba, N. A.; Evans, D. H. J. Phys. Chem. B 2005, 109, 14642–
14647.
shown in Scheme 1, giving KUU+ ) 4600 M-1
.
The fact that U+ binds so strongly to another U suggests there
must be other molecular guests that will strongly bind to U+ and
produce large shifts in the observed E1/2 of U0/+. Past experience1c,d
suggests that this requires a guest with two strong H-acceptor atoms
preorganized to H-bond with both urea NH’s. 1,4-Dimethylpiper-
izine-2,3-dione, abbreviated PZD, nicely satisfies these criteria as
shown above.
It turns out that PZD is a good enough guest for U that large
E1/2 shifts are observed in all electrolytes. The difference is how
much PZD needs to be added. With ClO4- or PF6-, addition of 1
mM PZD to 1 mM U in CH2Cl2 produces a -2 or -16 mV shift,
respectively, in the E1/2 of U0/+, with little change in the shape or
size of the CV wave.7 As more PZD is added, the wave shifts
further negative. By 400 mM PZD, the shift is significant, -60
mV with ClO4- and -114 mV with PF6-, but there is no evidence
that the maximum shift is nearing.
Figure 2 shows experimental and simulated CVs for the same
type of experiment done with NBu4B(C6F5)4. Now addition of 1
mM PZD causes a -109 mV shift, almost as large as that observed
with 400 mM in PF6-. Along with the shift, there is a substantial
sharpening of the wave, indicating that PZD binding is competing
(6) (a) LeSuer, R. J.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 39, 248–250.
(b) Barriere, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980–3989.
(7) See Supporting Information.
JA803453E
9
J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10071