Angewandte
Chemie
DOI: 10.1002/anie.201411234
Matrix Isolation Very Important Paper
The Fluorenyl Cation**
Paolo Costa, Iris Trosien, Miguel Fernandez-Oliva, Elsa Sanchez-Garcia,* and Wolfram Sander*
In memory of Reiner Sustmann
Abstract: The fluorenyl cation is a textbook example for a 4p
antiaromatic cation. However, contrasting results have been
published on how the annelated benzene rings compensate the
destabilizing effect of the 4p antiaromatic five-membered ring
in its core. Whereas previous attempts to synthesize this cation
in superacidic media resulted in undefined polymeric material
only, we herein report that it can be generated and isolated in
amorphous water ice at temperatures below 30 K by photolysis
of diazofluorene. Under these conditions, the fluorenylidene is
protonated by water to give the fluorenyl cation, which could
be characterized spectroscopically. Its absorption in the visible-
light range matches that previously obtained by ultrafast
absorption spectroscopy, and furthermore, its IR spectrum
could be recorded. The IR bands in amorphous ice very nicely
match predictions from DFT and DFT/MM calculations,
suggesting the absence of strong interactions between the
cation and surrounding water molecules.
plane, the higher stability of 2 compared to 1 might reflect
a kinetic stabilization of 2 and not the antiaromatic destabi-
lization of 1. Amyes, Richard, and Novak determined the pKR
values of 1 and 2, which compare the stabilities of the cations
to their respective alcohols, and concluded that “the descrip-
tion of 1 as antiaromatic is misleading because it implies an
exceptionally unstable carbocation, and this is not supported
by the data”.[1] Schleyer et al. concluded on the basis of
calculations of the nucleus-independent chemical shifts
(NICS) that the fluorenyl cation 1 is non-aromatic, whereas
the fluorenyl anion bearing two more electrons is clearly
aromatic.[3]
T
here has been some debate in the literature whether the
An obvious problem when defining the (anti)aromaticity
of a molecule is the choice of an appropriate reference
system. More OꢀFerrall and co-workers selected the planar
anthracenyl cation 3 as reference system for 1, and in very
careful experiments, they determined the pKR value of 1 to be
À15.9, that of 2 to be À11.7, and that of 3 to be À5.1.[8] They
concluded that 1 is destabilized compared to 3 by 12–
13 kcalmolÀ1. Therefore, the fluorenyl cation 1 is approxi-
mately as much destabilized by antiaromaticity as the
fluorenyl anion is stabilized by aromaticity. This was con-
firmed in a later study of Herndon and Mills by calculating the
aromatic stabilization energy (ASE) of 1 using various
reference systems to be 16.3 Æ 1.6 kcalmolÀ1.[4]
fluorenyl cation 1 with a formal 4p electron five-membered
ring system in its core should be regarded as a prototypical
antiaromatic cation, or if the two annelated benzene rings
compensate the antiaromaticity, and the molecule is non-
aromatic at best or even aromatic.[1–6] In 1980, Olah and
Schleyer et al. described unsuccessful attempts to isolate 1 in
superacidic media: “We were not able to generate the parent
9-fluorenyl cation 1-H by slowly adding SO2 or SO2ClF
solutions of possible precursors… at À1208C. The resulting
solutions immediately became dark and unidentifiable poly-
meric materials were formed”.[7] In contrast, the benzhydryl
cation 2 was readily obtained under similar conditions.
However, as in 2 the two phenyl groups are rotated out of
The only available spectroscopic data for 1 are transient
absorptions obtained by ultrafast spectroscopy. Mecklenburg
and Hilinski observed that the photolysis of fluorenol 4 in
H2O/CH3OH mixtures produced an absorption at 515 nm
with a lifetime of less than 20 ps, which was assigned to 1.[9]
This was later confirmed by McClelland et al.[10] More
recently, Platz et al. studied the protonation of fluorenylidene
6 in methanol by picosecond spectroscopy using 9-diazofluor-
ene (5) as the precursor (Scheme 1).[11] The same 515 nm
transient absorption peak as for 1 was observed, and its
lifetime was determined to be only 5 ps. Under similar
conditions, the lifetime of benzhydryl cation 2 is 30 ps,[12]
which demonstrates the higher reactivity of 1 compared to
2. In aprotic solvents, laser photolysis of 5 produces singlet
fluorenylidene S-6 (lmax = 420 nm), triplet fluorenylidene T-6
(lmax = 470 nm), and the fluorenyl radical 7 (lmax = 470,
497 nm).[11]
[*] P. Costa, I. Trosien, Prof. Dr. W. Sander
Lehrstuhl fꢀr Organische Chemie II
Ruhr-Universitꢁt Bochum
44780 Bochum (Germany)
E-mail: wolfram.sander@rub.de
M. Fernandez-Oliva, Dr. E. Sanchez-Garcia
Department of Theory
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim an der Ruhr (Germany)
E-mail: esanchez@kofo.mpg.de
[**] This work was supported by the Cluster of Excellence RESOLV (EXC
1069) funded by the Deutsche Forschungsgemeinschaft (DFG).
E.S.-G. acknowledges a Liebig Stipend of the Fonds der Chemischen
Industrie and support from the Collaborative Research Center
SFB 1093 funded by the DFG.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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