If we assume that diamagnetic anisotropy cone as shown
in Scheme 2C is present in the molecule, then the observed
chemical shifts are better explained by structure B
2þ
Scheme 2. Possible Structures of TAA
(Scheme 2). Olah and co-workers reported few dicationic
aromatics where the ring carbon attached to OCH
3
10
1
3
showed C chemical shifts in the 170-184 ppm range.
1
They, however, did not report any shielding of the H and
C signals of the methoxy group. If the C-O bond
remains as a single bond as in the case for A (Scheme 2),
1
3
1
3
shielding would shift the C signal to <150 ppm. On the
other hand if this bond is present as CdO, which is
normally observed at ∼210 ppm, then the shielding as
that in C (Scheme 2) could shift the signal to the observed
value of 186 ppm. Olah et al. have suggested that wher-
ever possible, the contribution from quinoidal structures
of one of the methoxy groups. The other two methoxy
1
groups exhibited a small downfield shift. In the C NMR
3
10
will be substantial in dicationic systems. Scheme 2C
shows that ring protons ortho to the methoxy group
these methoxy groups exhibited slightly different δ values.
In the H NMR the methoxy peak corresponding to six
1
1
are shielded. Assignments of the protons in the
NMR (SI) in fact show that the H protons in TAA
H
protons was somewhat broad at 298 K (see SI) but became
narrower at lower temperatures, indicating that these
methoxy groups in fact have slightly different δ values.
These results show thatthe effect of the two positive charges
is confined to one of the three aryl rings and the other two,
although they experience the effect of the overall positive
charge of the molecule, are largely unaffected structurally.
2þ
d
are shielded. We observed that the IR spectrum of the
-1
dicationic species exhibited a peak at 1645 cm , (see SI)
which further supported our assignment of the structure
2þ
of TAA as shown in Scheme 2B. It is to be mentioned
1
13
here that the H and C signals in Figure 4a, b are not
due to any products arising from decomposition of
1
3
Olah and co-workers have reported C NMR spectra of
2þ
TAA . In fact, more than 90% of TAA can be recov-
ered unchanged by workup of the solutions used for
9,10
several dicationic molecules. In light of these reports two
possible structures can be considered (A and B in Scheme 2)
13
recording C NMR spectra.
2þ
as candidates for the structure of TAA .
In conclusion we have shown that TAA can react with
2þ •þ
Both of these structures cannot satisfactorily explain
the shielding observed for the protons and carbon of one
of the methoxy groups. To explain the observed shielding
it is necessary to invoke a diamagnetic anisotropy which
leads to shielding of the methoxy group. Normally dia-
magnetic anisotropy cones of C;O and CdO have shield-
ing regions perpendicular to the bond axes and deshielding
cones along the bond axes. If we invoke the same here, the
methoxy group would be in the deshielding cone which
1
with 2 equiv Cu gave TAA , both in relatively stable
equiv of Cu to generate TAA , whereas a reaction
2þ
2þ
•þ
conditions. TAA was characterized by absorption and
EPR spectra. The dicationic species was identified by its
absorption spectrum, and detailed characterization of its
1
13
structure was attempted based on its H and C NMR
spectra. In order to explain the shielding observed in the
1 13
H and C signals of one of the three methoxy groups,
a reversal in the sign of the diamagnetic anisotropy cone of
the CdO group is proposed.
1
13
would shift the H and C signals downfield. Since the
1
13
observed H and C chemicalshiftsare upfield we propose
a reversal in the sign of the anisotropy of the diamagnetic
susceptibility, which leads to shielding of atoms inside the
cone (þ sign) and deshielding outside the cone (- sign), as
shown in Scheme 2C. The reason for the sign reversal is not
understood, but the positive charges on the molecule could
be a possible reason. It may be noted that a reversal in the
sign of the diamagnetic susceptibility has been reported
Acknowledgment. The authors thank the CSIR (NWP
6) and the DST (grant No. SR/S5/OC-15/2003), Govern-
2
ment of India, for financial support. K. S. thanks CSIR
for a research fellowship. This is contribution number
NIIST-PPG-293 from the Photosciences and Photonics
section. We also thank Dr. A. B. Mandal, Director,
Central Leather Research Institute, Chennai and
Dr. J. Subramanian for EPR measurements.
1
1
previously for several systems.
(
9) (a) Olah, G. A. J. Org. Chem. 2001, 66, 5943–5957. (b) Stadler, D.;
Goeppert, A.; Rasul, G.; Olah, G. A.; Prakash, G. K. S.; Bach, T. J. Org.
Chem. 2009, 74, 312–318. (c) Olah, G. A.; Reddy, V. P.; Rasul, G.;
Prakash, G. K. S. J. Am. Chem. Soc. 1999, 121, 9994–9998. (d) Olah,
G. A.; Shamma, T.; Burrichter, A.; Rasul, G.; Prakash, G. K. S. J. Am.
Chem. Soc. 1997, 119, 3407–3408.
Supporting Information Available. General experimen-
tal procedures, photophysical measurements, and spec-
troscopic characterization data for TAA and its dication
are described. This material is available free of charge via
the Internet at http://pubs.acs.org.
(
10) (a) Olah, G. A.; Shamma, T.; Burrichter, A.; Rasul, G.; Prakash,
G. K. S. J. Am. Chem. Soc. 1997, 119, 12923–12928. (b) Cupas, C. A.;
Comisarow, M. B.; Olah, G. A. J. Am. Chem. Soc. 1966, 88, 361–362. (c)
Olah, G. A.; Grant, J. L.; Spear, R. J.; Bollinger, J. M.; Serianz, A.; Sipos,
G. J. Am. Chem. Soc. 1976, 98, 2501–2507. (d) Prakash, G. K. S.;Rawdah,
T. N.; Olah, G. A. Angew. Chem., Int. Ed. Engl. 1983, 22, 390–401.
(11) (a) Lambert, J. B.; Goldstein, J. E. J. Am. Chem. Soc. 1977, 99,
5689–5693. (b) Khan, S. A.; Lambert, J. B.; Hernandez, O.; Carey, F. A.
J. Am. Chem. Soc. 1975, 97, 1468–1473.
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