Tert-butyl-end-capped polyynes: Crystallographic evidence of reduced bond length alternation

Article abstract of DOI:10.1002/anie.200902760

Cumulene or polyyne? The synthesis and X-ray crystallographic analysis of a series of polyynes up to 20 C_sp atoms in length and end-capped with tert-butyl groups has been achieved. The structural data show a distinct reduction in the bondlength alternation as a function of the polyyne length, but this trend appears to saturate before a cumulenic-like structure is achieved.

Full text of DOI:10.1002/anie.200902760

Angewandte
Chemie
DOI: 10.1002/anie.200902760
Carbynes
tert-Butyl-End-Capped Polyynes: Crystallographic Evidence of
Reduced Bond-Length Alternation**
Wesley A. Chalifoux, Robert McDonald, Michael J. Ferguson, and Rik R. Tykwinski*
For over five decades, conjugated polyynes have been
challenging synthetic targets that have captured the interest
of chemists working to develop innovative synthetic methods,
new biologically active molecules, and improved molecular
materials.[1,2] Within the last decade, synthetic efforts toward
polyynes have enjoyed a resurgence, driven in many cases by
the possible applications of such molecules as molecular wires
and new optical materials.[3] Regardless of the projected
application, the study of polyynes is also often motivated by
an infatuation with the structural simplicity of these mole-
cules, which are essentially devoid of both steric and
conformational effects that might alter the electronic and
optical properties. Experimentally, the electronic character-
istics of polyynes are often probed by UV/Vis spectroscopy,
and these analyses document a consistent lowering of the
optical HOMO–LUMO gap as a function of increasing
length.[3d,g,4,5] These analyses also typically show that satura-
tion of this trend has not yet been reached over the length of
polyynes that are currently accessible by modern organic
synthesis.
length by X-ray crystallographic analysis. As polyynes
become increasingly longer, however, the stability decreases
significantly and obtaining single crystals of polyynes beyond
the length of a hexayne for analysis has been a difficult task.[10]
A particular objective in our study of polyynes has been to
employ terminal groups that interact as little as possible with
the sp-hybridized carbon chain so as to minimize the
influence of “end-group effects” on the electronic properties.
Ideally, the use of hydrogen atoms[5a,11] or even methyl[12]
groups would achieve this goal, but the instability of such
molecules often complicates their synthesis, purification, and
analysis. To strike a balance between stabilization and
electronic inertness, trialkylsilyl (1)[3d] and 1-adamantyl
(2)[13] end groups have been employed (Scheme 1). In one
Given that polyynes are essentially 1D conjugated sys-
tems, the changes in the HOMO–LUMO gap versus length
should be intricately dependent on the degree of bond length
alternation (BLA = the difference in the bond length between
the central single and triple bonds) in the polyyne struc-
ture.[6,7] Recent theoretical studies have upheld the prediction
that neither the HOMO–LUMO gap nor the BLA for
polyynes will reach a value of zero,[8] a phenomena commonly
referred to as Peierls distortion.[9] Experimentally, it should be
possible to explore trends in BLA as a function of polyyne
Scheme 1. Triisopropylsilyl (TIPS; 1), 1-adamantyl (2), and tert-butyl
(3) end-capped polyynes.
of these studies, 1-adamantyl polyynes were investigated
experimentally by analysis of the electronic absorption
spectra of radical ions generated by radiolysis.[14] Unfortu-
nately, the molecules proved difficult to study effectively by
computational methods because of the size of the adamantyl
groups. Thus, polyynes bearing the structurally similar, albeit
smaller, tBu end group were considered (3a–k).[15] On the
basis of earlier work, we were convinced that the tBu-
substituted polyynes would be sufficiently stable and chemi-
cally well-behaved for subsequent study. Thus, the Fritsch–
Buttenberg–Wiechell (FBW) method[16] for polyyne synthesis
was applied to the construction of series 3. In addition to
characterization by NMR and UV/Vis spectroscopy, we
report X-ray crystallographic analysis of polyynes 3a–c, 3g,
and 3j; compound 3j is the only decane to be characterized by
X-ray crystallography to date. An analysis of the solid-state
structure of these five polyynes reveals a distinct trend in
reduced BLA as a function of polyyne length, similar to that
predicted by theory.
[*] W. A. Chalifoux, Prof. Dr. R. R. Tykwinski[+]
Department of Chemistry, University of Alberta
Edmonton, Alberta, T6G 2G2 (Canada)
Fax: (+1)780-492-8231
E-mail: rik.tykwinski@ualberta.ca
Dr. R. McDonald, Dr. M. J. Ferguson
X-ray Crystallography Laboratory
Department of Chemistry, University of Alberta
Edmonton, Alberta, T6G 2G2 (Canada)
[+] New address: Institut fꢀr Organische Chemie
Friedrich-Alexander-Universitꢁt, Erlangen-Nꢀrnberg
Henkestrasse 42, 91054 Erlangen (Germany)
[**] This work has been generously supported by the University of
Alberta and the Natural Sciences and Engineering Research Council
of Canada (NSERC) through the Discovery Grant program. W.A.C.
thanks the NSERC (PGS-D) and the Alberta Ingenuity Fund for
scholarship support. We also thank Dr. E. Jahnke for helpful
discussions.
The syntheses of diyne 3a[17] and tetrayne 3c[18] were
accomplished, as previously reported, by oxidative homocou-
pling of tBu-acetylene and tBu-butadiyne, respectively. Triyne
3b was easily assembled by a FBW rearrangement in a
manner reported for other symmetrical triynes.[16f] Pentayne
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902760.
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3d was a bigger synthetic challenge, since oxidative dimeri-
desired product 3j, but also to nonayne 3h.[24] Changing the
catalyst system to that used by Eglinton and Glaser[25]
(Cu(OAc)2·H2O, THF, MeOH, pyridine) circumvented this
problem and gave decayne 3j in a respectable yield of 54%.
Polyynes 3a–e, 3g, and 3j were characterized by EI (3a–e,
3g) and MALDI (3j) MS analysis to confirm their constitu-
tion. Their UV/Vis spectra are consistent with those previ-
ously published by Bohlmann[5b] and Jones et al.,[5c] showing
considerable vibrational fine structure for the HOMO–
LUMO transition, accompanied by a consistent lowering of
the lmax energy as a function of the polyyne length. This trend
culminates in a lmax value for 3j at 362 nm (in hexanes), with a
significant molar absorptivity (e = 736000 Lmolꢀ1 cmꢀ1).[26]
Analysis of the 13C NMR spectra shows consistent trends in
the observed chemical shifts as a function of length, as has
been observed for other homologous series of polyynes.[3d]
The most deshielded acetylenic resonance moves consistently
downfield from d = 86.3 ppm (3a) to d = 89.6 ppm (3j), while
the most upfield resonance of each polyyne is found in a
narrow range of d = 61.7–61.4 ppm (for 3a it is observed at
d = 63.7 ppm).[27] The remaining resonances converge toward
a value of about 63 ppm as the length is increased.
Whereas trends from NMR and UV/Vis spectroscopic
analysis for polyynes have become almost routine in recent
years,[3d,g,4,13,16e,28] there is one particular question that remains
unanswered to date: Do extended polyynes show experimen-
tal evidence of reduced bond length alternation (BLA) as a
function of increasing length? X-ray crystallographic data for
extended polyynes that might offer an answer to this query
are, however, essentially nonexistent.[10]
zation reactions were not particularly applicable.[19] In the
present case, Friedel–Crafts acylation[20] of triyne 5[21] using
the acid chloride derived from 4 gave ketone 6. Since 6 was
not stable to isolation, it was carried on directly to the
dibromoolefination protocol reported by Ramirez et al.,[22]
which gave 7 in 45% yield over the two steps. In the final step,
pentayne 3d was produced in 75% yield from 7 by a FBW
rearrangement (Scheme 2).
Scheme 2. Synthesis of pentayne 3d.
The construction of hexa-, octa-, and decaynes 3e, 3g, and
3j, respectively, followed a comparable sequence of steps
starting from the common precursor, acid 4. Thus, reaction of
4 with thionyl chloride followed by the appropriate a,w-
bis(trimethylsilyl)polyyne gave the corresponding ketone
intermediates 8–10 (Scheme 3). Given their instability, the
For polyyne series 3, useful diffraction
patterns have been obtained for diyne 3a,
triyne 3b, tetrayne 3c, octayne 3g, and
decayne 3j (Figure 1).[29,30] Before examin-
ing structural effects for this series, the
structure of decayne 3j deserves comment,
given that it is the only successful crystallo-
graphic analysis of a polyyne with more
than 16 contiguous sp-hybridized carbon
atoms.[31] Crystals of 3j were grown by slow
evaporation of a CH2Cl2 solution at 48C.
The structure contains two crystallograph-
ically independent molecules in the unit cell
(labeled as molecules 3jA and 3jB in
Figures 1 and 2). The unsymmetrical mole-
Scheme 3. Synthesis of hexayne 3e, octayne 3g, and decayne 3j.
cule 3jA adopts a sort of helical conforma-
ꢀ
tion with C C ꢁ C bond angles that range
crude products (8–10) were used directly in the dibromoole-
fination step after purification through a short column of silica
gel. This gave 11–13 in yields that ranged from 29% (13) to
80% (11) over the two steps. A FBW rearrangement effected
on 11 or 12 gave the expected tri- or tetrayne product,
respectively, which, following aqueous work-up, was sub-
jected directly to Hay oxidative homocoupling (THF, MeOH,
CuCl, N,N,N’,N’-tetramethylethylenediamine (TMEDA),
O2).[23] Under these conditions, the trimethylsilyl protecting
group was removed effectively, and homocoupling afforded
3e or 3g in acceptable yield. Attempts to use this same
procedure with decayne precursor 13 led not only to the
from 174.4(6)8 to 179.3(5)8. The centrosymmetric molecule
3jB adopts a gentle S shape, with a slightly smaller deviation
ꢀ
from linearity and C C ꢁ C bond angles that range from
176.1(6)8 to 179.5(8)8. Perhaps the most impressive aspect of
decayne 3j is its length: the molecule spans an amazing
2.7 nm from end to end (namely, from C2A to C23A or C2B
to C2B’).
A recent review by Szafert and Gladysz estimated a BLA
of about 0.07–0.08 ꢀ for an infinite polyyne system, a
prediction made from the comprehensive analysis of X-ray
crystallographic data for polyyne structures with at least eight
contiguous sp-hybridized carbon atoms.[31] This estimate is
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Table 1: Summary of experimental and theoretical BLA data.
BLA[a]
[ꢂ]
C Cavg
[ꢂ]
CꢁCavg BLAavg
Ref
[b]
ꢀ
Entry Polyyne
[ꢂ]
[ꢂ]
[29]
[29]
[29,30]
[29]
[29]
[29]
[29]
[33]
[3d]
[3d]
[3d]
[3d]
[8a]
[8a]
[8d]
[8d]
[8b]
1
2
3
4
5
6
7
8
3a
3b
3c
3g
3jA
3jB
3jA/Bavg
1a
1c
1d
0.184
0.164
0.151
0.140
0.144
0.134
0.139
0.169
0.157
0.148
0.149
0.153
1.382
1.368
1.365
1.356
1.350
1.354
1.352
1.373
1.366
1.362
1.361
1.361
1.199
1.202
1.204
1.209
1.207
1.206
1.206
1.204
1.209
1.209
1.208
1.200
0.183
0.166
0.161
0.147
0.143
0.148
0.146
0.169
0.157
0.153
0.153
0.161
9
10
11
12
13
14
15
16
17
1e
1g
H (CꢁC)9
H
0.1291[c] 1.36175 1.2254
0.13675
ꢀ
ꢀ
ꢀ
ꢀ
H (CꢁC)1
H
0.1276[c]
0.133[d]
0.131[e]
0.134[f]
–
–
–
–
–
–
–
–
–
–
–
–
ꢀ
ꢀ
H (CꢁC)1
H
H
H
ꢀ
ꢀ
H (CꢁC)1
ꢀ
ꢀ
H (CꢁC)1
ꢀ
[a] See text and Ref. [34]. [b] [C Cavg]ꢀ[CꢁCavg]; note: the calculation
ꢀ
ꢀ
does not include terminal C C bonds (e.g., C2 C3). [c] Calculated at the
CCSD(T)/cc-PVTZ level of theory. [d] Results obtained with the BHHLYP
functional. [e] Results obtained with the CAM-B3LYP functional.
[f] Results obtained with the B3LYP//BH&HLYP functional.
Figure 1. ORTEP drawings (20% probability level) for 3a–c, 3g, 3jA,
and 3jB.
limiting value of BLA = 0.135 ꢀ at the asymptotic limit.[35]
One potential issue with this BLA analysis is however that it
relies on the difference of two specific bonds, where small
errors in either of these bond lengths might be over
emphasized in comparison with other bond lengths. In an
attempt to minimize such an error, the difference between the
average of all single and triple bonds (BLAavg) has also been
examined. Similar to the pure BLA values, the BLAavg values
also decrease consistently as a function of length for 3a–c, 3g,
and 3j, and the onset of saturation is also suggested by the
stage of the decayne.[35] A comparison of the BLA and BLAavg
values for 3g and 3j versus those predicted theoretically for
Figure 2. End-on view of 3jA and 3jB highlighting the nonlinearity of
each molecule (hydrogen atoms are removed for clarity).
potentially complicated by “end-group effects”, however,
since many of the polyynes in this analysis are terminated with
groups that are strongly conjugated with the polyyne core.
Computational chemists have frequently addressed BLA in
polyynes, partly, at least, because of the relative structural
simplicity of these molecules.[8] From these theoretical studies,
it is clear that the choice of basis set and electron correlation
can have a dramatic effect on the results.[8,32] Nevertheless, a
consistent trend has emerged from recent investigations that
predict that BLA values, as defined by the difference in the
length between the central single and triple bonds, converge
to a value of about 0.13 ꢀ (Table 1, entries 13–17).[8a,b,d]
The analysis of the BLA from the X-ray data of 3a–c, 3g,
and 3jA/3jB was conducted in two ways. First, as reported for
computed structures in entries 13–17, the BLA was calculated
as the difference in the bond lengths of the central single and
triple bonds in each structure.[34] This leads to a consistent
reduction in BLA values from a maximum of 0.184 ꢀ for
dimer 3a to a minimum of 0.139 ꢀ (average of 3jA and 3jB).
This analysis also suggests that the reduction in BLA is
approaching saturation by the stage of the decayne, similar to
that predicted by theory;[8] a plot of BLA versus 1/n predicts a
ꢀ
ꢀ
H (CꢁC)9 H (entry 13) shows that the experimental values
are greater by only about 0.01 ꢀ.
The TIPS-end-capped polyynes 1 allow the only substan-
tive comparison of similar experimental BLA and BLAavg
analyses, as shown in entries 8–12. While there is an overall
reduction in both the BLA and BLAavg values as one moves
from diyne 1a to octayne 1g, neither the individual values nor
the overall trend compare well with the data for molecules in
the tBu series. It is particularly interesting that the BLAavg
data for molecules 1 show no real trend. It seems plausible
that this could result from an end-group effect in which the
silane moiety reduces the BLA toward the termini of the
polyyne, which effectively negates any apparent overall BLA
that might be present toward the center of the molecule.
In summary, a series of tBu-end-capped polyynes has been
synthesized to explore the properties of conjugated oligomers
composed of sp-hybridized carbon atoms. The resulting
polyynes exhibit reasonable stability under ambient condi-
tions, which has allowed for X-ray analysis of several
derivatives, including the first crystallographic analysis of a
decayne. Analysis of the bond lengths provides the first
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Communications
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experimental evidence for reduced BLA as a function of
polyyne length.
Received: May 23, 2009
Published online: August 7, 2009
[14] H. Aboulfadl, W. A. Chalifoux, R. R. Tykwinski, T. Bally,
unpublished results.
Keywords: alkynes · carbynes · coupling reactions · polyynes ·
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[29] Data for 3a: C12H18, Mr = 162.26; monoclinic; space group P21/c
(no. 14), a = 10.883 (3), b = 10.917 (4), c = 11.255 (4) ꢀ; b =
118.191 (4)8; V= 1178.6 (7) ꢀ3; Z = 4; 1calcd = 0.914 gcmꢀ3; m =
0.051 mmꢀ1; l = 0.71073 ꢀ; T= ꢀ808C; 2q max = 50.588; total
data collected = 2143; R1 = 0.0846 (1581 observed reflections
2
2
with [Fo ꢂ 2s(Fo )]); wR2 = 0.2756 for 110 variables and 2143
2
2
unique reflections with [Fo
ꢂ
ꢀ3s(Fo )]; residual electron
density = 0.478 and ꢀ0.249 eꢀꢀ3 (CCDC 729156). Data for 3b:
C14H18 Mr = 186.28; orthorhombic; space group Cmca (no. 64),
a = 8.7174 (13), b = 17.826 (3), c = 8.4901 (13) ꢀ; V= 1319.3
[11] a) M. Tsuji, T. Tsuji, S. Kuboyama, S.-H. Yoon, Y. Korai, T.
Tsujimoto, K. Kubo, A. Mori, I. Mochida, Chem. Phys. Lett.
2002, 355, 101 – 108; b) F. Cataldo, Tetrahedron Lett. 2004, 45,
141 – 144.
(3) ꢀ3; Z = 4; 1calcd = 0.938 gcmꢀ3
;
m = 0.052 mmꢀ1
;
l =
7918
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 7915 –7919

Angewandte
Chemie
0.71073 ꢀ; T= ꢀ808C; 2q max = 51.348; total data collected =
deposited with the Cambridge Crystallographic Data Centre.
2
4517; R1 = 0.0787 (560 observed reflections with [Fo ꢂ 2s-
Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
(+44) 1223-336-033; e-mail: deposit@ccdc.comcam.ac.uk).
[30] The crystallographic analysis of tetrayne 3c has previously been
reported in Ref. [3h], but no coordinates were made available to
the CCDC.
[31] S. Szafert, J. A. Gladysz, Chem. Rev. 2006, 106, PR1 – PR33.
[32] For other studies, see a) C. J. Zhang, Z. X. Cao, H. S. Wu, Q. R.
Zhang, Int. J. Quantum Chem. 2004, 98, 299 – 308; b) M. Weimer,
W. Hieringer, F. Della Sala, A. Gꢅrling, Chem. Phys. 2005, 309,
77 – 87; c) A. Scemama, P. Chaquin, M. C. Gazeau, Y. Benilan, J.
Phys. Chem. A 2002, 106, 3828 – 3837; d) L. Horny, N. D. K.
Petraco, C. Pak, H. F. Schaefer, J. Am. Chem. Soc. 2002, 124,
5861 – 5864.
2
(Fo )]); wR2 = 0.2573 for 40 variables and 672 unique reflections
2
2
with [Fo ꢂ ꢀ3s(Fo )]; residual electron density = 0.413 and
ꢀ0.169 eꢀꢀ3 (CCDC 729157). Data for 3c: C16H18, Mr = 210.30;
orthorhombic; space group Pbcn (no. 60), a = 11.0520 (12), b =
11.6678 (13), c = 22.507 (3) ꢀ; V= 2902.4 (6) ꢀ3; Z = 8; 1calcd
=
0.963 gcmꢀ3
;
m = 0.054 mmꢀ1
;
l = 0.71073 ꢀ; T= ꢀ808C; 2q
max = 52.808; total data collected = 21568; R1 = 0.0516 (2275
2
2
observed reflections with [Fo ꢂ ꢀ2s(Fo )]); wR2 = 0.1549 for
2
2
145 variables and 2975 unique reflections with [Fo ꢂ ꢀ3s(Fo )];
residual
electron
density = 0.189
and
ꢀ0.152 eꢀꢀ3
(CCDC 729158). Data for 3g: C24H18, Mr = 306.38; orthorhom-
bic; space group Pnma (no. 62), a = 16.4819 (10), b = 6.8028 (4),
c = 17.4876 (10) ꢀ; V= 1960.8 (2) ꢀ3; Z = 4; 1calcd = 1.038 gcmꢀ3
;
m = 0.059 mmꢀ1; l = 0.71073 ꢀ; T= ꢀ1008C; 2q max = 51.448;
total data collected = 14185; R1 = 0.0412 (1407 observed reflec-
[33] E. C. Constable, D. Gusmeroli, C. E. Housecroft, M. Neuburger,
S. Schaffner, Acta Crystallogr. Sect. C 2006, 62, o505 – o509.
[34] For non-centrosymmetric structures, the BLA was calculated
using the average of positionally equivalent bonds [namely, BLA
2
2
tions with [Fo ꢂ 2s(Fo )]); wR2 = 0.1330 for 139 variables and
2
2
2043 unique reflections with [Fo ꢂ ꢀ3s(Fo )]; residual electron
density = 0.353 and ꢀ0.111 eꢀꢀ3 (CCDC 729159). Data for 3j:
C28H18, Mr = 354.42; monoclinic; space group P21/n (an alternate
setting of P21/c [no. 14]), a = 5.3923 (17), b = 18.777 (6), c =
31.598 (10) ꢀ; b = 94.474 (4)8; V= 3189.6 (17) ꢀ3; Z = 6;
ꢀ
(3a) = (C4 C5)ꢀ[(C3ꢁC4) + (C5 ꢁ C6)]/2}. See the Supporting
Information for further discussion of calculated BLA and
BLAavg values. Given the estimated standard deviation (ESD)
associated with individual bond lengths, it is the overall trend of
BLA values that should be appreciated, rather than the exact
BLA value for each molecule.
1calcd = 1.107 gcmꢀ3
;
m = 0.063 mmꢀ1
;
l = 0.71073 ꢀ.; T=
ꢀ1008C; 2q max = 50.708; total data collected = 22855; R1 =
2
2
0.0896 (4076 observed reflections with [Fo ꢂ 2s(Fo )]); wR2 =
[35] See the Supporting Information for a table of bond lengths for
3a–c, 3g, and 3j (including ESD values), plots of BLA and
BLAavg as a function of n and 1/n (where n is the number of triple
bonds), as well as a graphical depiction of the decrease in BLA
2
0.2304 for 389 variables and 5938 unique reflections with [Fo
ꢂ
ꢀ3s(Fo )]; residual electron density = 0.964 and ꢀ0.235 eꢀꢀ3
(CCDC 729160). Crystallographic data (excluding structure
factors) for the structures reported in this paper have been
2
as a function of length in comparison to BLAavg
.
Angew. Chem. Int. Ed. 2009, 48, 7915 –7919
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7919