Synthesis and structure of tetraarylcumulenes: Characterization of bond-length alternation versus molecule length

Article abstract of DOI:10.1002/anie.201208058

BLA=0? Not so fast! A series of tetraarylcumulenes up to the length of a [9]cumulene has been synthesized and analyzed by X-ray crystallography. The structural data show a distinct reduction in bond-length alternation (BLA) as a function of molecule length, but this trend appears to reach a limit before a cumulenic structure with BLA=0 is achieved. Copyright

Full text of DOI:10.1002/anie.201208058

Angewandte
Chemie
DOI: 10.1002/anie.201208058
Cumulenes
Synthesis and Structure of Tetraarylcumulenes: Characterization of
Bond-Length Alternation versus Molecule Length**
Johanna A. Januszewski, Dominik Wendinger, Christian D. Methfessel, Frank Hampel, and
Rik R. Tykwinski*
Dedicated to Professor FranÅois Diederich on the occasion of his 60th birthday
Chains composed of sp-hybridized carbon atoms have been
explored for decades because of their unique linear structure
and interesting physical properties.[1] More recently, the wire-
like nature of sp-carbon oligomers has inspired a variety of
studies that aim to evaluate these structures as components in
nanometer-sized devices.[2] Particularly interesting is the
reported formation of such wires linking graphene nano-
ribbons, offering the prospect of all-carbon-based devi-
ces.[2d–f,3]
Molecules composed of a skeleton of sp-hybridized
carbon atoms can be constructed from a framework of
either polyynes (alternating single and triple bonds) or
cumulenes (cumulated double bonds). The chemistry of
Figure 1. Structures of [n]cumulenes discussed in this paper.
polyynes has been advanced to systems as long as 44 consec-
utive carbon atoms (22 acetylene units),[4] and studies have
shed considerable light on the physical and optoelectronic
properties of polyynes.[5] However, the study of cumulenes
has lain essentially dormant since early work[6,7] reported by
Kuhn[8,9] and Bohlmann.[10,11] Thus, there remain many
unanswered questions about the physical properties of this
intriguing class of linear molecules. To date, UV/Vis spec-
troscopy has been the most useful method for the character-
ization of cumulenes,[12] and analyses of cumulenes show
a lowering of the lowest-energy electronic absorption (lmax) as
a function of length, such as that for [n]Ph and [n]Cy (n = 3, 5,
7, 9, Figure 1).[9–11] Obviously, changes in lmax versus molec-
ular length are intricately dependent on structure and on the
degree of bond-length alternation (BLA, defined as the bond-
length difference between the two central-most double bonds
of the cumulene chain). Recent theoretical studies predict
that the BLA for cumulenes will rapidly approach zero
(BLA ꢀ 0.01),[13–15] that is, Peierls distortion is essentially
absent.[16] Experimentally, X-ray crystallographic analysis
would provide an opportunity to confirm or refute theoretical
trends in BLA as a function of cumulene length. Unfortu-
nately, few solid-state structures have been reported for
cumulenes, and data for [n]cumulenes with n > 5 are not
available. The results presented herein offer an answer to the
important question of BLA in long cumulenes.
It was clear from the onset of the study that the synthesis
and solid-state analysis of long [n]cumulenes (n > 5) would be
challenging, because available reports emphasized that these
species were not typically stable enough for isolation.[6] In
order to stabilize the cumulene core through steric shielding,
initial efforts targeted formation of the [n]tBuPh series of
cumulenes (Figure 1). It quickly became clear, however, that
the di(tert-butyl)phenyl (R = tBu2C6H3) groups do not afford
a sufficient stabilizing force to easily isolate the [7]- and
[9]tBuPh cumulenes, and our attention then switched to the
[n]Mes series.
Synthesis of [3]tBuPh began with the formation of 1a
through reaction of the Li–acetylide of 2a with the diaryl
ketone (Scheme 1). Reductive elimination of 1a was then
accomplished using SnCl2·2H2O in the presence of hydro-
chloric acid, and pure [3]tBuPh was obtained by column
chromatography in 76% yield as an intensely yellow solid.
Unfortunately, the formation of 1b, and thus [3]Mes, was not
possible through the analogous route, because of the frustrat-
ing inability to form the methylether 2b. Furthermore, all
attempts to form a metal–acetylide directly from alcohol 3b
by reaction with either MgBrEt or nBuLi were unsuccess-
ful.[17]
[*] J. A. Januszewski,[+] D. Wendinger,[+] C. D. Methfessel,
Dr. F. Hampel, Prof. R. R. Tykwinski
Department of Chemistry and Pharmacy & Interdisciplinary Center
of Molecular Materials (ICMM), University of Erlangen-Nuremberg
Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: rik.tykwinski@chemie.uni-erlangen.de
Homepage: http://www.chemie.uni-erlangen.de/tykwinski
[+] These authors contributed equally to this work.
[5]Cumulenes [5]tBuPh and [5]Mes were available
through conventional methods,[6] starting from alcohols 3a
and 3b, respectively (Scheme 2). Formation of diols 4a,b was
achieved by oxidative homocoupling of 3a,b under Hay
conditions.[18] While pure diol 4a could be obtained, diyne 4b
was consistently contaminated with small amounts of by-
[**] Funding from the University of Erlangen-Nuremberg and the
Deutsche Forschungsgemeinschaft (DFG - SFB 953, Project A4
“Synthetic Carbon Allotropes”) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208058.
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Scheme 4. Synthesis of [7]cumulene [7]Mes. PCC=pyridinium chloro-
chromate.
Scheme 1. Synthesis of [3]cumulene [3]tBuPh.
Passing the reaction mixture through a column of basic
alumina under a nitrogen atmosphere resulted in a red
solution of the pure product [7]Mes in Et2O. In solution,
[7]Mes is stable for weeks when kept at circa À208C in
a deoxygenated solvent, and crystals of [7]Mes from CH2Cl2/
hexane are stable for at least a week when stored at circa
À208C in the absence of light and oxygen.
Homocoupling of diynes 9a,b gave tetraynes 10a,b, which
were subjected to reductive elimination with SnCl2
(Scheme 5). After formation of the desired cumulene
Scheme 2. Synthesis of [5]cumulenes [5]tBuPh and [5]Mes. TME-
DA=N,N,N’,N’-tetramethylethylenediamine.
products, and the crude product was thus used directly for the
formation of [5]Mes. Reductive elimination of 4a and 4b gave
[5]tBuPh and [5]Mes.
For the synthesis of [7]tBuPh (Scheme 3), a Fritsch–
Buttenberg–Wiechell (FBW) rearrangement was used to
form the requisite triyne framework of 5a through the
Scheme 5. Synthesis of [9]cumulenes [9]tBuPh and [9]Mes.
[9]tBuPh or [9]Mes was judged to be complete by TLC
analysis, the reaction solution was neutralized by filtration
through basic alumina under
a nitrogen atmosphere.
Attempted isolation of either [9]tBuPh or [9]Mes through
precipitation led to decomposition of the product. [9]tBuPh
and [9]Mes also decomposed rapidly in solution when
exposed to oxygen, but they could be handled for hours
([9]tBuPh) or days ([9]Mes) when kept in a cold (À208C),
deoxygenated solution of Et2O that was shielded from
ambient light. Crystals of [9]Mes grown from a C2D2Cl4
solution at À208C are stable for at least a week when stored
in the absence of light and O2.
X-ray crystallographic analysis of [3]cumulenes is
common (> 20 structures), while data for [5]cumulenes is
rare (three structures), and to our knowledge, no data exists
for longer cumulenes.[22] Herein, we report structural data for
six new cumulenes ([3]tBuPh, [5]tBuPh, [7]tBuPh, [5]Mes,
[7]Mes, [9]Mes), including the first known structures of [7]-
and [9]cumulenes (Figure 2).[23]
Scheme 3. Synthesis of [7]cumulene [7]tBuPh.
reaction of ketone 6a[19] with Colvinꢀs reagent.[20] Subsequent
reduction of 5a gave [7]tBuPh. While attempts to directly
isolate [7]tBuPh through precipitation resulted in decompo-
sition, the pure cumulene could be isolated by slow crystal-
lization from a solution of CH2Cl2/MeOH, and the crystalline
solid was stable indefinitely in the absence of O2.
The FBW route used to form 5a, and ultimately [7]tBuPh,
was not applicable to the synthesis of [7]Mes, because of the
inability to form 6b,[21] thus, an alternative route was
developed (Scheme 4). The synthesis of propargyl alcohol 7
was accomplished using a Cadiot–Chodkiewicz coupling (see
the Supporting Information). Compound 7 was then con-
verted to triyne 5b in two steps, namely oxidation with PCC to
ketone 8 and addition of lithiated mesitylene (Scheme 4).
Reduction of 5b to [7]Mes was accomplished with SnCl2.
While polyynes regularly show bending of the framework
of sp-hybridized carbon atoms in the solid state,[24] cumulenes
appear to maintain a more linear structure. An examination
of the known structures of [3]- and [5]cumulenes, as well as
the cumulenes reported herein, shows that bond angles rarely
vary by more than a few degrees from the ideal of 1808.[25] An
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Figure 2. ORTEP drawings (thermal ellipsoids at 20% probability level) for [3]-, [5]-, [7]tBuPh, and [5]-, [7]-, [9]Mes. For clarity, co-crystallized
solvent molecules and hydrogen atoms are not shown.
exception to this tendency is the structure of [3]tBuPh, which
shows cumulenic angles of 169.478 (C1-C2-C3) and 168.318
(C2-C3-C4). In addition to the likely influence of undefined
crystal-packing forces, the bent shape of the [3]tBuPh might
arise from favorable, intramolecular C-H/p interactions
between hydrogen atoms of one of the tert-butyl groups
with the aromatic ring at the opposite terminus of the
cumulene chain. This premise is supported by short C-H/p
distances of 2.81–3.27 ꢁ,[26] and these interactions are remi-
niscent of a recent study by Grimme and Schreiner high-
lighting dispersive forces in hexaphenylethane derivatives.[27]
Steric congestion prevents coplanarity between the ter-
minal aryl groups and the cumulene core for [n]tBuPh and
[n]Mes cumulenes, although the extent of twisting differs
quite substantially between the two series.[28] Aryl twist angles
of the mesityl groups are circa 508, which limits conjugation
between the aryl ring and the chain of sp-hybridized carbon
atoms (Table 1, Figure 2). On the other hand, aryl twist angles
for the [n]tBuPh cumulenes show an interesting pattern. At
each terminus, the twist angle for one aryl ring is significantly
smaller than that of the other, that is, 14.4–30.98 versus 40.4–
54.98. In each case, therefore, two aryl rings are in a position
to conjugate with the framework of sp-hybridized carbon
atoms of the [n]tBuPh cumulenes.
One would predict that the ability of the aryl rings to
interact with the cumulene core should have an effect on the
bond-length pattern, as depicted in Figure 3 by the two
resonance structures of a [5]cumulene. Indeed, bond lengths
for [5]tBuPh and [7]tBuPh show an accentuated long/short
bonding pattern in comparison to [5]Mes and [7]Mes
(Table 2), although the difference is rather small.
Figure 3. Two canonical forms of a [5]cumulene that highlight the
interconnection between BLA and conjugation of the terminal aryl
groups to the cumulene core.
A key question to be answered based on structural
analysis of cumulene bond lengths is: Do cumulenes show
experimental evidence of reduced BLA as a function of
increasing length? While this question has been answered for
polyynes,[29] to date there has been insufficient crystallo-
graphic data to offer an answer for cumulenes. Computational
chemists have frequently studied BLA in cumulenes. From
these theoretical studies, it seems clear that the basis set can
have a remarkable effect on the results.[15,30] In spite of these
differences, however, theory suggests that BLA values con-
verge quite rapidly to a value of nearly zero (Table 2,
entries 7–13).[13,14,31]
Table 1: Aryl twist angles [8] of aromatic ring relative to cumulenic
framework.[a]
Ring[b] [3]tBuPh [5]tBuPh [7]tBuPh [5]Mes [7]Mes [9]Mes
A
B
C
D
30.9
43.1
26.6
40.4
14.4
47.9
16.6
54.9
20.6
43.6
46.2
51.4
45.4
52.1
48.8
54.4
[a] Aryl twist angles were calculated as the difference between planes
generated from 1) the six carbon atoms of the aryl ring and 2) the carbon
atoms of the cumulene skeleton, along with the four ipso-carbon atoms
of the aryl rings. [b] See Figure 2 for labeling of aryl rings.
Experimental data for cumulenes show that the terminal
À
double bond, C1 C2, is consistently the longest (1.33–1.35 ꢁ;
À
Table 2). The C1 C2 bond lengths do not show a significant
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Table 2: Selected bond lengths (ꢀ) for [n]tBuPh and [n]Mes cumulenes and summary of BLA data.[a]
Entry
1
Cumulene
C1-C2
C2-C3
C3-C4
C4-C5
–
C5-C5’
BLA[b]
0.086
Ref.
[3]tBuPh
1.334(3)
1.336(3)[c]
1.342(2)
1.345(3)
1.347(3)[e]
1.339(2)
1.334(3)
1.330(3)
1.319
1.249(3)
–
2
3
[5]tBuPh
[7]tBuPh
1.255(2)
1.254(3)
1.252(3)[f]
1.255(2)
1.260(3)
1.255(3)
1.274
1.266802
1.274
1.267176
1.309(3)[d]
1.302(3)
1.306(3)[g]
1.303(3)[d]
1.299(3)
1.298(3)
1.289
–
–
–
0.054
0.052
1.252(3)
4
5
[5]Mes
[7]Mes
[9]Mes
[7]H
[7]H
[9]H
[9]H
[11]H
[19]H–[39]H
[29]H
–
–
–
0.048
0.042
0.038
0.014
0.0136
0.010
1.257(4)[h]
1.260(4)
1.275
6
1.298(5)
7[i]
[31]
[13]
[31]
[13]
[14]
[14]
[13]
8[j]
9[i]
1.310342
1.319
1.310424
1.281530
1.289
1.281012
1.267928
1.277
1.268959
1.287
1.279270
10[j]
11[k]
12[k]
13[j]
0.010
0.009[l]
0.006[m]
0.004[n]
[a] See Figure 2 for numbering of atoms. [b] Calculated as difference in bond length between the two central-most bonds. For non-centrosymmetric
structures, BLA was calculated using the average of positionally equivalent bonds. [c] C3-C4. [d] C3-C3’. [e] C7-C8. [f] C6-C7. [g] C5-C6. [h] C4-C4’.
[i] Geometry optimization at B3LYP/6-31G* level. [j] Geometry optimization at PBE1PBE/cc-pVTZ level. [k] Geometry optimization at B3LYP/TZVPP
level. [l] Bond-length values of 1.280 ꢀ and 1.271 ꢀ. [m] Bond-length values of 1.278 ꢀ and 1.272 ꢀ. [n] Estimated value (bond lengths were not given
for this value in Ref. [13]).
dependence on either end-group or cumulene length, sug-
gesting that this bond is mainly dominated by the hybrid-
2
=
ization difference of the carbon atoms, that is, C(sp ) C(sp).
Bond lengths for the internal double bonds fall into a narrow
range of 1.25–1.31 ꢁ. BLAvalues for the [n]tBuPh cumulenes
show a consistent decrease with increasing chain length from
0.086 ꢁ ([3]tBuPh) to 0.052 ꢁ ([7]tBuPh; Table 2, entries 1–
3). Cumulenes end-capped with mesityl groups show a similar
decrease in BLA versus length, from 0.048 ꢁ ([5]Mes) to
0.038 ꢁ ([9]Mes; Table 2, entries 4–6). However, in compar-
ison to [n]tBuPh, BLA values of [n]Mes are distinctively
lower (Table 2). This observation is ascribed to the more
pronounced twist angles of the aromatic rings in the
cumulenes end-capped with mesityl groups.
A plot of cumulene BLA values versus the number of
double bonds n and visual extrapolation to infinite length
n suggests a limiting BLA value of 0.03–0.05 ꢁ (Figure S1 in
the Supporting Information). This prediction is relatively
higher than that predicted by theory (Table 2, entries 7–13)
for the “parent” series [n]H. While computational results can
differ depending on the method of analysis, the trend appears
clear that BLA ꢀ 0.01 ꢁ for [9]H and longer cumulenes.
Similar to the investigation of polyynes, UV/Vis spectros-
copy is an essential tool for the characterization of cumulenes.
For the two series described herein, several tendencies are
identified:
1) In general, cumulenes exhibit two regions of absorption
bands, one at higher and one at lower energy (Figure 4).
Furthermore, fine structure increases with increasing
chain length.
2) As described for other series of cumulenes, such as [n]Ph
and [n]Cy, lmax values shift significantly to lower energy as
a function of chain length, that is, as conjugation is
extended.
Figure 4. UV/Vis spectra of [n]tBuPh and [n]Mes (Et2O); spectra
normalized to the most intense low-energy absorption (inset: lowest-
energy absorption and energy values, lmax and Eg, respectively).
4) End-groups have a dramatic effect on the lmax values for
[5]tBuPh and [5]Mes (lmax = 500 and 460 nm, respec-
tively). This correlates well with the diminished conjuga-
tion between the terminal aryl groups and cumulene
3) With the series of molecules accessible thus far, saturation
has not yet been reached, that is, lmax is further red-shifted
as molecular length n increases.
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Fischer in The Chemistry of Alkenes (Ed.: S. Patai), Wiley, New
York, 1964, pp. 1025 – 1159.
skeleton in [5]Mes, as documented by the increased aryl
twist angle relative to [5]tBuPh (Table 1).
5) Effects of end-groups on lmax values decrease rapidly with
length, that is, the lmax values of [7]tBuPh (564 nm) and
[7]Mes (560 nm) are nearly identical, as are those of
[9]tBuPh (664 nm) and [9]Mes (666 nm).
[7] For two notable exceptions, see: a) Y. Kuwatani, G. Yamamoto,
M. Oda, M. Iyoda, Bull. Chem. Soc. Jpn. 2005, 78, 2188 – 2208;
b) W. Skibar, H. Kopacka, K. Wurst, C. Salzmann, K.-H.
Ongania, F. F. de Biani, P. Zanello, B. Bildstein, Organometallics
2004, 23, 1024 – 1041.
[8] R. Kuhn, H. Krauch, Chem. Ber. 1955, 88, 309 – 315.
[9] [n]Ph: a) R. Kuhn, K. Wallenfels, Ber. Dtsch. Chem. Ges. 1938,
71, 783 – 790; b) R. Kuhn, H. Zahn, Chem. Ber. 1951, 84, 566 –
570.
[10] [n]Ph: F. Bohlmann, K. Kieslich, Abh. Braunschw. Wiss. Ges.
1957, 9, 147 – 160.
In summary, the synthesis of two series of [n]cumulenes
has been accomplished in order to investigate the properties
of these oligomers of sp-hybridized carbon atoms as a function
of length. While the longer [n]cumulenes with n = 7 or 9 are
not particularly stable in solution under ambient conditions,
they show sufficient stability in the solid state so that X-ray
crystallographic analysis could be accomplished for all
derivatives except [9]tBuPh. This includes the first crystallo-
graphic analysis of [7]- and [9]cumulenes to date. Analysis of
bond lengths for the cumulenes provides the first experimen-
tal evidence for reduced BLA as a function of cumulene
length, and experimental values are distinctly higher than
those predicted by computational studies for [n]H cumulenes.
[11] [n]Cy: F. Bohlmann, K. Kieslich, Chem. Ber. 1954, 87, 1363 –
1372.
[12] The electronic structures of [n]cumulenes are fundamentally
different between two classes of molecules, that is, when n is
even (n = 2,4,6…) or odd (n = 3,5,7…). p-Conjugation between
the end-groups through the cumulene framework is only possible
when n = odd. Only odd cumulenes are considered for the
discussion in this manuscript.
[13] F. Innocenti, A. Milani, C. Castiglioni, J. Raman Spectrosc. 2010,
41, 226 – 236.
[14] M. Weimer, W. Hieringer, F. Della Sala, A. Gçrling, Chem. Phys.
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Received: October 6, 2012
[15] U. Mçlder, P. Burk, I. A. Koppel, J. Mol. Struct. THEOCHEM
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[16] R. Hoffmann, Angew. Chem. 1987, 99, 871 – 906; Angew. Chem.
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Keywords: bond-length alternation · carbyne · cumulenes ·
oligomers · polyynes
.
[17] The reaction of 3b with either MgBrEt or nBuLi resulted in
a retro-addition reaction, that is, deprotonation of the alcohol
moiety of 3b, followed by expulsion of the acetylide (M-CCH,
M = MgBr or Li) and formation of the ketone MesC(O)Mes.
[18] A. S. Hay, J. Org. Chem. 1962, 27, 3320 – 3321.
[19] For the synthesis of 6a, see the Supporting Information.
[20] For the use of Colvinꢀs reagent for polyyne synthesis, see: a) P.
Bichler, W. A. Chalifoux, S. Eisler, A. L. K. Shi Shun, E. T.
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[21] Compound 6b could not be formed because of the failure to
synthesize the acetylenic precursor 3b.
[22] Based on a search of the CCDC, 18/09/12 (CSD version 5.33,
Nov. 11) for [n]cumulenes with n = odd and alkyl or aryl end-
groups.
[23] For crystallographic data, see the Supporting Information.
[24] S. Szafert, J. A. Gladysz, Chem. Rev. 2006, 106, PR1 – PR33.
[25] Exceptions include: a) T. Kawase, N. Nishigaki, H. Kurata, M.
Oda, Eur. J. Org. Chem. 2004, 3090 – 3096; b) Y. Kuwatani, G.
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[27] S. Grimme, P. R. Schreiner, Angew. Chem. 2011, 123, 12849 –
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generated from 1) the six carbon atoms of the aryl ring, and
2) the carbon atoms of the cumulene skeleton and the four ipso-
carbon atoms of the aryl rings.
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