Synthesis and study of differentially substituted dibenzotetraazafulvalenes

Article abstract of DOI:10.1021/ol702230r

A series of functionalized dibenzotetraazafulvalenes have been synthesized and characterized using X-ray crystallography, UV-vis spectroscopy, and cyclic voltammetry. The solid-state structures, electronic properties, and redox potentials of these compoun

Full text of DOI:10.1021/ol702230r

ORGANIC
LETTERS
2007
Vol. 9, No. 26
5401-5404
Synthesis and Study of Differentially
Substituted Dibenzotetraazafulvalenes
Justin W. Kamplain, Vincent M. Lynch, and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, UniVersity of Texas at Austin,
Austin, Texas 78712
bielawski@cm.utexas.edu
Received September 21, 2007
ABSTRACT
A series of functionalized dibenzotetraazafulvalenes have been synthesized and characterized using X-ray crystallography, UV−vis spectroscopy,
and cyclic voltammetry. The solid-state structures, electronic properties, and redox potentials of these compounds varied in accordance with
the nature of the pendant arene substituent.
The design and development of conjugated organic polymers
is a rapidly expanding area of research, holding considerable
promise in applications ranging from lightweight batteries
to solar cells.1 Common conjugated polymers such as
polyacetylene, polythiophene, and polyaniline are structurally
robust materials prepared using kinetically controlled (i.e.,
nonreversible) polymerization methods. With applications in
self-healing electronics and reconfigurable systems in mind,
we are interested in developing structurally reversible,
conjugated polymers that respond to changes in externally
applied stimuli.2
benes), we reported4 a series of polyenetetraamines derived
from a Janus-type bis(carbene)5 building block. While this
thermally reversible polymerization formally conserved
chemical unsaturation as monomer was converted to polymer,
only modest bathochromic shifts in the respective UV-vis
absorption spectra were observed (∆λmax ) 80 nm). This
result indicated that the material was only moderately
electronically delocalized, restricting utility in the aforemen-
tioned applications.
One possible source of the electronic impedance was the
tetraaminoethylene groups within the dibenzotetraazaful-
valene6 (DBTF) repeat unit of the polymer backbone (see
Scheme 1). While DBTFs have found utilities in many facets
of chemistry, including use in electrically conductive charge-
transfer complexes,7 in the formation of organometallic
complexes,8 in the synthesis of benzimidazole derivatives,9
Capitalizing on the Wanzlick equilibrium3 (i.e., the revers-
ible dimerization of certain classes of N-heterocyclic car-
(1) (a) Shirakawa, H. Angew. Chem., Int. Ed. 2001, 40, 2574. (b)
MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581. (c) Heeger, A.
J. Angew. Chem., Int. Ed. 2001, 40, 2591. (d) McCullough, R. D. AdV.
Mater. 1998, 10, 93. (e) Conjugated Polymers: Theory, Synthesis, and
Characterization. In Handbook of Conducting Polymers, 3rd ed.; Skotheim,
T. A., Reynolds, J. R., Eds.; CRC Press: Boca Raton, FL, 2006.
(2) (a) Williams, K. A.; Boydston, A. J.; Bielawski, C. W. J. R. Soc.
Interface 2007, 4, 359. (b) Boydston, A. J.; Williams, K. A.; Bielawski, C.
W. J. Am. Chem. Soc. 2005, 127, 12496.
(3) (a) Bo¨hm, V. P. W.; Herrmann, W. A. Angew. Chem., Int. Ed. 2000,
39, 4036. (b) Hahn, F. E.; Wittenbecher, L.; Le Van, D.; Fro¨hlich, R. Angew.
Chem., Int. Ed. 2000, 39, 541. (c) Liu, Y. F.; Lemal, D. M. Tetrahedron
Lett. 2000, 41, 599. (d) Liu, Y.; Lindner, P. E.; Lemal, D. M. J. Am. Chem.
Soc. 1999, 121, 10626. (e) Wanzlick, H. W.; Ahrens, H. Chem. Ber. 1964,
97, 2447.
(4) Kamplain, J. W.; Bielawski, C. W. Chem. Commun. 2006, 1727.
(5) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem.,
Int. Ed. 2006, 45, 6186.
(6) Although compounds 1-7 are formally 1,1′,3,3′-tetraalkylbibenz-
imidazolinylidenes, they are also known as DBTFs by analogy to tetrathi-
afulvalenes; see: (a) Hahn, F. E.; Wittenbecher, L.; Ku¨hn, M.; Lu¨gger, T.;
Fro¨lich, R. J. Organomet. Chem. 2001, 617-618, 629. (b) Shi, Z.; Goulle,
V.; Thummel, R. P. Tetrahedron Lett. 1996, 37, 2357.
(7) Wheland, R. C.; Gillson, J. L. J. Am. Chem. Soc. 1976, 98, 3917.
(8) Hitchcock, P. B.; Lappert, M. F.; Terroros, P.; Wainwright, K. P. J.
Chem. Soc., Chem. Commun. 1980, 1180.
10.1021/ol702230r CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/30/2007

metrically functionalized DBTFs were also prepared. The
structural, electronic, and redox properties of these com-
pounds were evaluated using X-ray crystallography, UV-
vis spectroscopy, and cyclic voltammetry, respectively.
Scheme 1. The Repeating Unit of the Polyenetetraamine
Shown Is Effectively a Derivative of Dibenzotetraazafulvalene
Various symmetrically and asymmetrically functionalized
DBTFs bearing electron-donating and -withdrawing groups13
are shown in Table 1. Symmetric DBTFs 1-4 were
synthesized using a two-step procedure. First, their respective
1-methylbenzimidazoles were alkylated with CH3I in CH3-
CN to afford the corresponding 1,3-dimethylbenzimidazo-
lium salts. These salts were then deprotonated with NaH,14
facilitated with a catalytic amount of KOtBu, in benzene to
generate the respective benzimidazolinylidenes in situ, which
ultimately dimerized to afford the desired DBTFs.15,16
To prepare an asymmetrically functionalized DBTF (i.e.,
a donor-acceptor analogue), a linker was required to ensure
proper registry. DBTF 5 was synthesized by first alkylating
5,6-dimethoxy-1-methyl-benzimidazole with excess 1,4-
dibromobutane in CH3CN. The resulting salt was then treated
with 4,5-dibromo-1-methylbenzimidazole to obtain the re-
spective bisbenzimidazolium salt. Subsequent deprotonation
using the protocol described above afforded the desired
asymmetric DBTF. To account for effects caused by the four-
carbon linker, tethered analogues 6 and 7 were synthesized
in a similar fashion17 and compared with their parent
compounds 2 and 3.
and as “super electron donor” reagents,10 less is known about
their structural and electronic properties.11 For example,
DBTF 1 (see Table 1) was reported12 to exhibit typical C-N
Table 1. Selected Structural Data of DBTFsa
no.
R
X
Y
CdCb (Å) CNCc (deg) τC)Cd (deg)
Crystals suitable for X-ray diffraction were obtained for
2, 3, and 7 by slow diffusion of ether into saturated THF
solutions;18 ORTEP diagrams for 2 and 3 are shown in parts
A and B, respectively, of Figure 1.19 While the lengths of
the carbon-carbon double bonds were similar for each of
these compounds (see Table 1), comparison of 2 and 3
revealed that the geometries about the enetetraamine units
were strongly influenced by the pendant functional groups.
For example, the methoxy substituents in 2 appeared to
enhance N-pyramidalization (avg C-N-C angle ) 113°).
This structural arrangement effectively minimized steric
interactions between opposing N-methyl groups and afforded
a nearly planar enetetraamine moiety (abs torsion ) 7°).20
1
2
3
4
5
6
7
2H
H
H
1.344(4)
1.338(16)
1.348(8)
113
113
116
nd
nd
nd
14
7
2H
2H
2H
OMe OMe
Br
Cl
Br
Cl
27
nd
nd
nd
18
nd
nd
(CH2)2 OMe OMe nd
(CH2)2 Br Br 1.343(10)
(CH2)2 OMe Br
117
a See text for synthesis. b Average length of the carbon-carbon double
bond. c Average angle of the contiguous C-N-C systems. d Absolute
torsion angle about the enetetraamine. nd ) not determined.
lengths (∼1.43 Å) and a 14° torsion angle about the
enetetraamine unit; hence, electronic delocalization between
the arene rings appeared to be minimal in this compound.
The primary purpose of this study was to determine the
extent to which electronic communication occurs through
the enetetraamine linkage. In particular, the synthesis and
study of a donor-acceptor DBTF, featuring electron-
donating groups on one arene ring and electron-withdrawing
groups on the other, is described. For comparison, sym-
(13) 5-Nitro-1,3-dimethylbenzimidazolium bromide was found to de-
compose upon treatment with NaH/KOtBu.
(14) (a) Bourson, J. Bull. Soc. Chim. Fr. 1971, 3541. (b) Hu¨nig, S.;
Scheutzow, D.; Schlaf, H.; Quast, H. Liebigs Ann. Chem. 1972, 765, 110.
(15) Due to the extreme sensitivities of DBTFs toward O2 and/or H2O,
reaction mixtures were typically filtered through PTFE membranes to
remove residual inorganic salts and analyzed directly.
(16) Small amounts of free benzimidazolinylidenes were observed for
each of the DBTFs shown in Table 1. For an excellent, comprehensive
analysis of diaminocarbene dimerizations, see: Alder, R. W.; Blake, M.
E.; Chaker, L.; Harvey, J. N.; Paolini, F.; Schu¨tz, J. Angew. Chem., Int.
Ed. 2004, 43, 5896.
(17) For related syntheses, see: (a) Ames, J. R.; Houghtaling, M. A.;
Terrian, D. L.; Mitchell, T. A. Can. J. Chem. 1997, 75, 28. (b) Taton, T.
A.; Chen, P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1011. (c) Shi, Z.;
Thummel, R. P. Tetrahedron Lett. 1995, 16, 2741. (d) Thummel, R. P.;
Goulle, V.; Chen, B. J. Org. Chem. 1989, 54, 3057. (e) Hu¨nig, S.; Sheutzov,
D.; Schlaf, H. Justus Liebigs Ann. Chem. 1972, 765, 126.
(9) (a) Ku¨c¸u¨kbay, H.; C¸ etinkaya, E.; Cetinkaya, B.; Lappert, M. F. Synth.
Commun. 1997, 27, 4059. (b) Shi, Z.; Thummel, R. P. J. Org. Chem. 1995,
60, 5935.
(10) (a) Schoenebeck, F.; Murphy, J. A.; Zhou, S.-Z.; Uenoyama, Y.;
Miclo, Y.; Tuttle, T. J. Am. Chem. Soc. 2007, 129, 13368. (b) Murphy, J.
A.; Zhou, S.-Z.; Thomson, D. W.; Schoenebeck, F.; Mohan, M.; Park, S.
R.; Tuttle, T.; Berlouis, L. E. Angew. Chem., Int. Ed. 2007, 46, 5178. (c)
Murphy, J. A.; Khan, T. A.; Zhou, S.-Z.; Thomson, D. W.; Mahesh, M.
Angew. Chem., Int. Ed. 2005, 44, 1356.
(11) This is primarily due to their extreme sensitivities toward air and
water. As a result, very few X-ray structures of DTBFs are known.3b,5,12
(12) C¸ etinkaya, E.; Hitchcock, P. B.; Ku¨c¸u¨kbay, H.; Lappert, M. F.; Al-
Juaid, S. J. Organomet. Chem. 1994, 481, 89.
(18) All attempts to grow quality crystals for 4-6 were unsuccessful.
(19) The structure of 7 was found to be superficially similar to 3; see
the Supporting Information.
(20) Low torsion (15°) and avg C-N-C (116°) angles were observed5
in a bibenzobisimidazolinylidene, another electron-rich DBTF.
5402
Org. Lett., Vol. 9, No. 26, 2007

DBTFs 3 (444 nm) and 4 (441 nm). Although similar
absorption characteristics were observed for bridged DBTFs
6 and 7, their respective signals were bathochromically
shifted by ca. 20 nm relative to their parent analogues,
presumably due to linker-induced planarization. The donor-
acceptor DBTF 5 exhibited a λmax ) 441 nm which was
intermediate of its symmetric dimethoxy (425 nm) and
dibromo (464 nm) analogues. Collectively, these results
suggested that electronic delocalization through the enetet-
raamine moiety in the DBTFs analyzed was relatively
minimal.
Finally, the redox potentials (E1/2) of 1-7 were evaluated
using cyclic voltammetry.22 As summarized in Table 2, not
only was a relatively broad range of E1/2 observed, but these
values correlated with the electron-donating or -withdrawing
nature of peripheral substituents on the respective DBTFs.23
For example, 2, which possessed pendant methoxy groups,
exhibited the highest E1/2 at -1.07 V versus SCE whereas
chloro bearing DBTF 4 exhibited the lowest at -0.69 V.
The E1/2 of parent hydrido DBTF 1 was intermediate of these
values (-0.90 V) as was the donor-acceptor DBTF 5 (-0.93
V). The four-carbon linker in 6 (-1.07 V) and 7 (-0.76 V)
appeared to have only a minor effect on E1/2 when compared
to their parent analogues (2 and 3, respectively). Collectively,
the following trend in E1/2 was observed (pendant substituents
are shown in parentheses): 2 (OMe) ∼ 6 (OMe) > 5 (OMe,
Br) ∼ 1 (H) > 7 (Br) ∼ 3 (Br) > 4 (Cl).
Figure 1. Two ORTEP views of 2 (A) and 3 (B). Hydrogen atoms
have been removed for clarity.
In contrast, the halogen atoms in 3 appeared to decrease
N-pyramidalization (avg C-N-C angle ) 116°), which
caused significant twisting about the enetetraamine (abs
torsion ) 27°) to avoid unfavorable steric interactions
between opposing N-methyl groups. Although superficially
similar to 3, the enetetraamine moiety in 7 (not shown) was
slightly less twisted, indicating that the four-membered linker
induced partial planarization of the enetetraamine core (abs
torsion angle ) 18°, avg C-N-C angle ) 117°).21
The observed differences in electron densities manifested
in unique chemical reactivities. Although enetetraamines are
known24 to instantly oxidize to their respective cyclic ureas
upon exposure to ambient oxygen, a much slower reaction
was observed with DBTF 4,25 which features pendant
chlorine groups and exhibits a relatively low oxidation
As summarized in Table 2, the pendant functional groups
Scheme 2. Exposure of DBTF 4 to Air Afforded Urea 8 and
Hydrolysis Product 9 as a 4:1 Mixture
Table 2. Selected Spectroscopic Properties and Redox
Potentials of Dibenzotetraazafulvalenes
no.
λmaxa (nm)
log (ꢀ)a
E1/2b (V)
E1/2c (V)
1
2
3
4
5
6
7
397
404
444
441
441
425
464
3.92
4.06
3.98
4.00
4.05
4.00
3.97
-0.90
-1.07
-0.73
-0.69
-0.93
-1.07
-0.76
-0.64
-0.82
-0.59
a Determined in C6D6 at 23 °C. b Values reported are relative to SCE.
Redox potentials were determined in CH3CN at 23 °C using 0.1 M
tetrabuylammonium hexafluorophosphate as electrolyte versus a Pt-poly-
pyrrole electrode externally referenced to ferrocene/ferrocenium at a scan
rate ) 100 mV/s. c For compounds 3, 5, and 7, a second redox process
was observed at the potential indicated.
potential. As shown in Scheme 2, exposure of a benzene
solution of 4 to ambient atmosphere afforded urea 8 as well
(21) The solid-state structures of DBTFs 1-3 and 7 may also be
influenced by crystal packing forces.
(22) Although the redox processes of DBTFs 1-7 appeared to be
reversible, solutions of these compounds were found to decompose in the
presence of electrolyte. The decomposition process may occur via their free
NHCs;16 see: Ramnial, T.; McKenzie, I.; Gorodetsky, B.; Tsang, E. M.
W.; Clyburne, J. A. C. Chem. Commun. 2004, 1054.
(23) For DBTFs 3, 5, and 7, two E1/2 were observed (see Table 2).
(24) Winberg, H. E.; Downing, J. R.; Coffman, D. D. J. Am. Chem. Soc.
1965, 87, 2054.
in DBTFs also resulted in changes in their respective UV-
vis spectroscopic properties. Relative to 1 (397 nm), the λmax
for dimethoxy analogue 2 was bathochromically shifted to
404 nm; additional redshifting was observed for dihalo
Org. Lett., Vol. 9, No. 26, 2007
5403

as a novel hydrolysis product (9) as a 4:1 mixture of over
the course of 24 h (73% isolated yield). The ORTEP diagram
shown in Figure 2 reveals that the solid-state structure of 9
electron-deficient enetetraamines are more robust toward O2
than their electron-rich analogues.
In summary, a series of dibenzotetraazafulvalenes pos-
sessing a range of electron-donating and electron-withdraw-
ing substituents were synthesized. Although X-ray crystal-
lography and UV-vis spectroscopy suggested that electronic
communication between these peripheral groups was rela-
tively minimal, cyclic voltammetry revealed that the redox
potentials of these compounds were tunable. Collectively,
these results should help guide the development of new
enetetraamine-based materials for electronic applications as
well as new reagents for organic and organometallic syn-
thesis.
Acknowledgment. We are grateful to the U.S. Army
Research Office (W911NF-05-1-0430 and W911NF-06-1-
0147), the National Science Foundation (CHE-0645563), the
Petroleum Research Fund as administered by the American
Chemical Society (44077-G1), the Robert A. Welch Founda-
tion (F-1621), the Arnold and Mabel Beckman Foundation,
the Research Corporation and UTsAustin for generous
financial support of these studies.
Figure 2. ORTEP view of 9. Hydrogen atoms and solvent
molecules have been removed for clarity.
adopted an s-trans conformation about the amide linkage.
This observation may be rationalized by the intramolecular
distance of 3.98 Å found between the offset centroids of the
phenyl groups, indicative of a π-π interaction.26 Regardless,
the relatively slow rate of this reaction suggested that
Supporting Information Available: Experimental pro-
cedures and spectroscopic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(25) Similar results were observed with DBTF 3. Chemiluminescense
was not visually observed in these reactions.
(26) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525.
OL702230R
5404
Org. Lett., Vol. 9, No. 26, 2007