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
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
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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
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J. AM. CHEM. SOC. VOL. 130, NO. 31, 2008 10071