Isolation and low-temperature X-ray analysis of intramolecular triarylmethane-triarylmethylium complex: Preference for a C-H-bridged unsymmetric structure exhibiting a facile 1,5-hydride shift and charge-transfer interaction

Article abstract of DOI:10.1021/ja053161h

The intramolecular triarylmethane-methylium complex with an averaged C_2v-symmetry was successfully generated by N-methylation of the acridine-acridan hybrid. Theoretical calculation and VT-NMR analyses in solution indicate the unsymmetric geometry for the three-center bond, which was finally confirmed crystallographically. Significant degree of CT interaction is induced through the very short C-H...C⁺ contact. Copyright

Full text of DOI:10.1021/ja053161h

Published on Web 08/10/2005
Isolation and Low-Temperature X-ray Analysis of Intramolecular
Triarylmethane-Triarylmethylium Complex: Preference for a C-H-Bridged
Unsymmetric Structure Exhibiting a Facile 1,5-Hydride Shift and
Charge-Transfer Interaction
Hidetoshi Kawai, Takashi Takeda, Kenshu Fujiwara, and Takanori Suzuki*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan
Received May 14, 2005; E-mail: tak@sci.hokudai.ac.jp
A carbocation with a short C-H‚‚‚C+ contact has long attracted
Scheme 1
considerable attention since the first observation of the delocalized
three-center two-electron (3c-2e) bond of (C‚‚‚H‚‚‚C)+ by So-
rensen and McMurry in superacidic media.1 From both experimen-
tal2 and theoretical3 perspectives, the most interesting point in
studying these cations is to clarify what factors affect the preference
of the localized or delocalized position of the bridging hydrogen,
which should provide the key information for understanding the
nature of the nontriangular 3c bond between carbon and hydrogen.
Recently, the existence of a higher analogue of (C‚‚‚H‚‚‚C‚‚‚H‚‚‚
C)+ was theoretically predicted,3a and such bonding would not be
stabilized in the alicyclic-bridged propellane studied before, but
rather in a novel motif, triarylmethane-triarylmethylium complex,4
where the interacting parts are geometrically constrained by being
located at the peri positions of the rigid arylene spacer.
We envisaged that the isolable member of such triarylmethane-
triarylmethylium complexes4 would provide invaluable information
on the less-explored 3c bonds in organic chemistry5 when analyzed
not only in solution but also crystallographically. We designed
nances became broad (Figure S1).10 A sharp Cs-symmetric spectrum
molecules with the general formula A (localized) or B (delocalized)
appeared with two distinct N-methyl resonances at 4.81 (acridinium)
as the first isolable examples of C-H-bridged carbocations with
and 3.13 (acridan) ppm (-90 °C), showing that the bridging hydro-
gen is localized on one of the C9 carbons in solution and fluctuates
between the two equivalent sites. The chemical shift of this hydro-
gen is 3.90 and 4.26 ppm at 0 and -90 °C, respectively, which is
(pseudo) C2V-symmetry. Since bis(N-methylacridinium) 2a2+ with
a rigid naphthalene-1,8-diyl framework6 has the shortest C+‚‚‚C+
nonbonded contacts [2.927(9) Å]7 among arylenebis(triarylmethy-
lium)s,8 hydride insertion between the C9 carbon of acridinium units
close to that of precursor 3b (4.36 ppm) but far different from the
as in 1a+ should provide a unique chance9 to study an intramolecular
very high-field value characteristic of the delocalized 3c-2e bond-
triarylmethane-methylium complex with a very short C‚‚‚H
contact. Acridinium is also suitable as the Ar2C+ unit in terms of
cation stability and planar geometry. We report here the successful
generation and isolation of 1a+ as well as its acenaphthene-5,6-
diyl derivative 1b+ and describe their detailed structures and
dynamic properties based on VT-NMR analyses as well as low-
temperature crystallographic data.10
ing hydrogen.2,3b The 13C NMR spectrum at -90 °C is also consist-
ent with Cs-symmetric Form A and exhibits two distinct resonances
for C9: 163.63 and 42.33 ppm. The 13C-1H coupling constant (126
Hz) is comparable to that of precursor 3b (128 Hz), indicating full
bonding character between the bridging hydrogen and one of the
2
C9 carbons. By measuring the H NMR spectrum of 1b+-dOTf-,
the ∆δ test (1H, 2H) of Altman and Forse´n was conducted, and the
very small value (-0.02 ppm at -10 to -60 °C) clearly shows a
deep double-minimum potential for the bridging hydrogen.2,12
The facile 1,5-hydride shift is the most characteristic process in
1+, the energy barrier (∆Gq) of which was determined to be 9.6
kcal mol-1 (at Tc ) -55 °C for N-methyl protons) for 1b+ in CD2-
Cl2 by VT-NMR analyses (300 MHz). This value is about 3-fold
larger than that for the 1,5-shift in 3,7-dimethylbicyclo[3.3.1]nonan-
3-yl cation.2b In the more polar and Lewis basic acetone-d6, an even
larger value of 10.1 kcal mol-1 was obtained (Tc ) -43 °C),
suggesting that the transition state is less polar than 1+ due to charge
delocalization over the two Ar2C units. The values for 1b+-d (10.4
kcal mol-1 in CD2Cl2; 10.7 kcal mol-1 in acetone-d6) correspond
to the marginal primary isotope effect (kH/kD ) ca. 2.5), indicating
that the tunneling effects are not important for the hydride shift in
1b+. The 1H NMR spectrum of the naphthalene derivative 1a+OTf-
exhibits essentially the same features and temperature dependence
Half-reduction of dications 2a,b2+ seems to be the most direct
way toward the desired cations but was found to be unsuitable for
their formation.11 At last, by careful N-methylation of the acridine-
acridan hybrids 3a,b,10 we were able to isolate 1a,b+ as OTf- salts10
in respective yields of 78 and 68% (Scheme 1). Deuterium-bridged
derivatives 1a,b+-d were similarly obtained from 3a,b-d. They were
stable dark-orange crystalline materials and can be kept for months
at room temperature under air. The striking stability of 1a,b+
compared with the C-F‚‚‚C+-bridged cation8b reported recently
may be related to the higher stabilizing effect by the hydrogen
bridge as well as the different pKR+ value of the Ar2C+ unit.
The 1H NMR spectrum of 1b+OTf- in CD2Cl2 is C2V-symmetric
at room temperature, with only one N-methyl resonance at 3.94
ppm, which can be accounted for by assuming a rapid degenerated
1,5-hydride shift in Form A or delocalized 3c-2e bond formation
as in Form B. With a decrease in temperature, most of the reso-
9
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10.1021/ja053161h CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
hydrogen bridge, although a Coulomb effect in the dications 2a,b2+
could not be completely ruled out.
These results demonstrate that the intramolecular complex of
triarylmethane-methylium complexes 1 with the naphthalenediyl
or acenaphthenediyl skeleton can best be described as a localized
C-H-bridged cation (Form A),14 with a facile 1,5-hydride shift and
a significant degree of CT interaction. Adequate electron-donating
properties of the Ar2CH unit in 1 must play a determining role in
selecting the CT-type stabilizing mode over the delocalized one in
organic 3e bonds. This work suggests that studies on isolable
triarylmethane-methylium complexes should keep giving deep
insights into 3c bonds between carbon and hydrogen, thanks to the
finely tunable electronic properties of Ar2C+ and Ar2CH units, as
well as the definable rigid geometry. Studies in this vein are now
in progress.
Figure 1. (a) ORTEP drawing of 1b+ in OTf- salt determined by X-ray
analysis at -170 °C; (b) HOMO (-7.2908 eV) and (c) LUMO (-5.9265
eV) of 1b+ (B3LYP/6-31G*).
as that of 1b+OTf-, but the much lower Tc corresponds to the
smaller ∆Gq value (e.g., 8.4 kcal mol-1 for 1a+-d in acetone-d6, Tc
) -80 °C). The larger energy barrier for the hydride shift in the
less flexible acenaphthene derivative 1b+ must be due to the skeletal
constraint by the ethano bridge, which prevents the two Ar2C units
from coming closer to attain the low-energy transition-state
geometry with a narrower C‚‚‚C+ separation. It is highly probable
that the transition state for the degenerated 1,5-hydride shift of 1+
(Form A) adopts the geometry close to the charge-delocalized Form
B with a shorter C‚‚‚C+ distance.
Acknowledgment. Dedicated to Prof. Takashi Tsuji on the
occasion of his 65th birthday.
Supporting Information Available: Synthetic procedures and
characterization data. 1H VT-NMR spectra of 1b+OTf-. ORTEP
drawings of 1a+OTf-, 1b+OTf-, and 3a. Calculated HOMO and LUMO
in 1a,b+. UV-vis spectra of 1a,b+. Crystallographic information files
of 1a+OTf-, 1b+OTf-, and 3a. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
To investigate the detailed geometry around the bridging atoms,
low-temperature X-ray analyses were carried out for both crystals
of 1a,b+OTf-.10 Regardless of their different packing arrangements,
one of the Ar2C units is the planar acridinium with an sp2 carbon
for C9 [sum of C-C-C bond angles: 359.9(6)° for 1a+ and 360.0-
(2)° for 1b+], whereas the other is the butterfly-shaped acridan unit
with sp3-hybridized C9 [342.9(6)° for 1a+ and 343.5(2)° for 1b+]
(Figure 1a). The latter values are close to that found for the acridan
unit in the precursor 3a [343.0(2)°]. The geometries for the C-H‚
‚‚C+ atomic array clearly show that the bridging hydrogen is
localized on one of the C9 carbons as in Form A [the distances for
C-H, H‚‚‚C+, and C‚‚‚C+ and the angle for C-H‚‚‚C+ are 0.98-
(6), 2.14(6), 2.960(9) Å and 139(4)° for 1a+; and 1.03(3), 2.13(3),
3.004(4) Å and 140(2)° for 1b+, respectively]. The above geom-
etries are the first experimental data4a for the interconvertible/
delocalized C2V-symmetric cations with (C-H‚‚‚C+)/(C‚‚‚H‚‚‚C)+
contacts.
The theoretical calculation for 1a,b+ (B3LYP/6-31G*)13 nicely
reproduced the observed solid-state structure [1.102, 2.047, 2.999
Å and 142.7° for 1a+; 1.102, 2.148, 3.008 Å and 141.6° for 1b+]
with two distinct Ar2C units [sum of C-C-C bond angles around
C9: 359.3 and 339.1° for 1a+; 359.7 and 339.1° for 1b+]. The
coefficients in LUMO are mainly localized on the acridinium unit,
whereas those in HOMO are on the acridan moiety (Figure 1b,c).
A considerable portion of the coefficient is located on the bridging
hydrogen. Although the electrochemical amphotericity of 1a,b+ is
not so high [Esum ) Eox - Ered ) ca. +1.4 V; +0.90 V (irrev.) -
-0.49 V (rev.) for 1a+ and +0.81 V (irrev.) - -0.55 V (rev.) for
1b+],10 the close proximity induces intramolecular charge-transfer
(CT) interaction6a through the bridge. Weak, but significant,
absorptions in the long-wavelength region up to 750 nm (Figures
S7 and S8; log ꢀ ) ca. 3) are assigned to the CT absorption bands
of 1a,b+, and the optically determined HOMO-LUMO gap (1.65
eV) well corresponds to those obtained by electrochemistry (ca.
1.4 eV) and by calculation (1.44 for 1a+ and 1.36 eV for 1b+).
Electrochemical oxidation of the bridged cations to the dications
2a,b2+ occurs at a much higher potential than the corresponding
arylacridans without the acridinium unit [ca. +0.78 V (irrev.) for
3a,b], which must be the result of CT interaction through the
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