Synthesis of Polyynes Using Dicobalt Masking Groups

Article abstract of DOI:10.1021/acs.joc.7b03015

Extended triisopropylsilyl end-capped polyynes have been prepared from the corresponding tetracobalt complexes by removing the complexed dicobalt tetracarbonyldiphenylphosphinomethane (Co₂(CO)₄dppm) moieties. Unmasking of this "masked alkyne equivalent" was achieved under mild conditions with elemental iodine at room temperature, making it possible to obtain fragile polyynes with up to 20 contiguous sp-hybridized carbon atoms. The Co₂(CO)₄dppm moiety has a strong geometric and steric effect on the polyyne but does not have a marked electronic effect on the terminal alkyne, as indicated by NMR and IR spectroscopy, density functional theory calculations, and X-ray crystallography. An unusual "alkyne hopping" migration of the dicobalt group was noticed as a minor side reaction during copper-catalyzed Eglinton coupling.

Full text of DOI:10.1021/acs.joc.7b03015

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Article
Synthesis of Polyynes Using Dicobalt Masking Groups
Daniel R. Kohn, Przemyslaw Gawel, Yaoyao Xiong, Kirsten E. Christensen, and Harry L. Anderson
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b03015 • Publication Date (Web): 23 Jan 2018
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The Journal of Organic Chemistry
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Synthesis of Polyynes Using Dicobalt Masking Groups
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Daniel R. Kohn, Przemyslaw Gawel, Yaoyao Xiong, Kirsten E. Christensen, and Harry L. Anderson*
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UniversityꢀofꢀOxford,ꢀDepartmentꢀofꢀChemistry,ꢀChemistryꢀResearchꢀLaboratory,ꢀOxford,ꢀOX1ꢀ3TA,ꢀUnitedꢀKingdomꢀꢀꢀ
PPh2
Ph2P
ABSTRACT: Extended triisopropylsilyl end-capped
polyynes have been prepared from the corresponding
tetracobalt complexes by removing the complexed dicobalt
I2, THF
Ph2P
OC)2Co
Co(CO)2
(OC)2Co
PPh2
Co(CO)2
TIPS
TIPS
m
(
25 °C, 3 h
m
m = 1 (55%)
tetracarbonyldiphenylphosphinomethane (Co
moieties. Unmasking of this ‘masked alkyne equivalent’ was
2
(CO)
4
dppm)
m = 3 (45%)
m = 5 (39%)
TIPS
TIPS
achieved under mild conditions with elemental iodine at room temperature, making it possible to obtain fragile polyynes with up to
0 contiguous sp-hybridized carbon atoms. The Co (CO) dppm moiety has a strong geometric and steric effect on the polyyne, but
2
2
4
does not have a marked electronic effect on the terminal alkyne, as indicated by NMR and IR spectroscopy, density functional
theory calculations and X-ray crystallography. An unusual ‘alkyne hopping’ migration of the dicobalt group was noticed, as a minor
side reaction during copper-catalyzed Eglinton coupling.
studies failed to unmask the desired curved π-systems from the
corresponding cobalt complexes. Haley et al. successfully
Introductionꢀ
Polyynes are fragile molecules, and they readily undergo removed the dicobalt carbonyl moieties with excess I
cross-linking reactions, leading to decomposition and sometimes model compounds, but this could not be reproduced on the
octacobalt cyclophane complex. We believe the origin of these
2
in simple
even to explosions.1 One method of circumventing this
undesired reactivity is to conceal the alkyne as a ‘masked alkyne failures is the inherent reactivity of the desired curved polyynes,
2
rather than the unmasking of the dicobalt group itself.
equivalent’ (MAE). The alkyne-dicobalt carbonyl moiety is a
potential MAE, as it can reversibly complex a C≡C bond
(
Ph
Ph
Figure 1a).3-5 The affinity of cobalt(0) carbonyl complexes for
Ph
P
Ph
OC Co Co CO
OC
CO
alkynes is utilized in many important reactions such as the
a)
b)
R
R
6
7,8
Nicholas reaction and the Pausson-Khand reaction. Cobalt
oxidation or
dissociation
clusters have also been used as electrochemical probes in
acetylenic molecular wires.9-12
Co2(CO)8
(
CO) Co Co(CO)
CO
OC CO
3
OC
Ph
Ph
The demand for new organic semiconductors has led to
increasing interest in carbon-rich materials13 and alkynes play
an essential role in this field.13-19 The dicobalt carbonyl MAE
Ph
Ph
Co
Co
P
Co
Co
OC
CO
CO
R
R
P
OC
Ph Ph
Ph Ph
group is particularly interesting for synthesizing acetylene-based
structures because it can stabilize conjugated alkynes in two
ways. Firstly, its steric bulk prevents close contacts between sp-
chains, blocking cross-linking reactions.1a Secondly, it breaks the
conjugation of the polyyne by changing the hybridization of the
cobalt complexation
C18 hexacobalt complex
c)
d)
PPh
Co(CO)
2
Ph
2
P
Ph
2
P
2
(OC) Co
2
PPh2
(OC) Co
2
Co(CO)2
PPh
2
Ph P
2
3
carbon atoms from sp to sp . Additionally, it induces significant
bending between the attached groups, aiding formation of
Ph
2
P
Co(CO)
2
(OC) Co
PPh2
2
(
OC)
2
Co
Co(CO)2
m
curved structures.20,21 The pristine Co
(CO) group can be
2 6
removed by oxidation,22 alkyne-ligand exchange23 or flash
TIPS
TIPS
vacuum pyrolysis.24 However, it is more useful when modified to
incorporate a bis(diphosphinomethane) ligand, which improves
the stability of the complexes but makes the MAE more difficult
to unmask.25,26 Lewis et al. prepared tetracobalt complexes of
(OC)
2
Co
Co(CO)
PPh2
2
m = 1, 3, 5
tetracobalt masked polyynes
Ph P
2
Co(CO)2 (OC) Co
2
PPh
2
2
Ph P
cyclophane octacobalt complex
2 4
tetraynes with the (Co (CO) dppm) group and used ferric nitrate
to unmask this moiety for the first time.27 The synthesis of two
Figure 1. a) Dicobalt complexation of an alkyne, b) C18 hexacobalt
2
0
2
1
d
)
c
o
m
p
l
e
x
,
c
)
[
8
,
8
]
p
a
r
a
c
y
c
l
o
p
h
a
n
e
o
c
t
a
y
n
e
o
c
t
a
c
o
b
a
l
t
c
o
m
p
l
e
x
,
carbon-rich compounds with curved acetylenic π-systems has
been attempted via their masked cobalt complexes. In the first
example, Diederich and coworkers used a cobalt-masked triyne
to assemble a hexacobalt masked cyclo[18]carbon macrocycle
tetracobalt masked polyynes (this work).
Polyyne synthesis via masked alkyne equivalent (MAE)
groups, such as dibromoolefins, dialkynyl-3-cyclobutene-1,2-
diones and dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-triene,
offers high yields in relatively short systems.28-30 Further
(
Figure 1b).20 Haley et al. prepared an octacobalt complex of a
cyclophane with a curved tetrayne bridges (Figure 1c).21 Both
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development of masking groups would be useful for the again to furnish 6 in 84% yield and the following deprotection
synthesis of longer polyynes. Several other MAEs have been gave terminal triyne 7 (Scheme 2). Complex 7 must be handled
developed and used in efforts to unmask cyclo[n]carbons from with care, as it is relatively unstable when dry; it decomposes
the corresponding precursors. These MAEs require relatively over a period of a few hours at 20 °C and a few weeks at –20
harsh unmasking conditions, including high temperatures,31 °C. However, it is more stable than the corresponding
flash vacuum pyrolysis,32 n-BuLi28,33 or intense UV light,34-36 unmasked terminal pentayne.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
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2
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4
5
5
5
5
5
5
5
5
5
5
6
which limits their application for long polyynes and other fragile
alkynes.
Scheme 2. Cobalt oligoyne synthesis.
The aim of this work was to prepare cobalt-masked
acetylenic building blocks and cobalt-masked long polyynes
Ph2P
OC) Co Co(CO)2
PPh2
Ph2P
(OC) Co Co(CO)2
PPh2
TMS-acetylene
CuCl, TMEDA
CH2Cl2, 25 °C, 2 h
K2CO3
THF/MeOH
5 °C, 30 min
(
2
(
Figure 1d), as part of an effort towards carbon-rich
0
1
2
3
4
5
6
7
8
9
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1
2
3
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0
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3
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9
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2
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5
6
7
8
9
0
82%
supramolecular structures.37,38 Here we show that the dicobalt
group is removed from tetracobalt complexes of long polyynes
in moderate to good yields. The successful unmasking of
polyynes from their corresponding tetracobalt complexes shows
the great potential of this MAE. Further understanding of the
effect of the dicobalt moiety on conjugated polyynes is obtained
from analysis of NMR, IR, and UV-vis spectroscopy, density
functional theory (DFT) and X-ray crystallography. We have
also discovered a surprising rearrangement in the synthesis of
the tetracobalt-masked polyynes during copper-catalyzed
homocoupling.
TMS
96%
H
TIPS
TIPS
2
a
3
Ph2P
OC) Co Co(CO)2
PPh2
K CO
Ph P
PPh2
TMS-acetylene
CuCl, TMEDA
2
3
2
(
(OC) Co Co(CO)2
THF/MeOH
5 °C, 30 min
2
CH Cl , 25 °C, 40 min
2
2
1
00%
82%
TIPS
TIPS
4
5
TMS
H
Ph2P
PPh2
Ph2P
PPh2
K2CO3
THF/MeOH
5 °C, 30 min
(OC)
Co Co(CO)2
(OC)
Co Co(CO)2
2
9
0%
TIPS
TIPS
6
7
RESULTSꢀANDꢀDISCUSSIONꢀ
Synthesisꢀ
TMS
H
Three dicobalt-masked building blocks containing terminal
oligoynes of different length were prepared in order to access a
series of tetracobalt-masked polyynes. TIPS,TMS-protected
triyne 1 was used because the TMS group can be selectively
removed, while leaving the TIPS group untouched, using
The oxidative homocoupling of
3
using Eglinton
conditions,42 Cu(OAc)
2
in pyridine, gives two separable
products. The expected isomer 8a is the major product, in 80%
yield, but a regioisomer 8b can also be isolated in 7% yield
Scheme 3). The structures of these compounds were confirmed
(
in THF/MeOH at room temperature.39,40 We
K
2
CO
3
by analysis of single crystals, vide infra. Diederich and coworkers
prepared a similar complex containing a butadiyne bridge,
without reporting this type of isomerization.20,43 Repeating the
reaction using different conditions, including Glaser-Hay44
coupling, shorter reaction times and heating (Scheme S3, S4)
did not change the outcome, and, a small amount of rearranged
product 8b was always formed. Surprisingly, formation of the
partially rearranged hybrid of 8a and 8b was never detected.
Despite investigation of this rearrangement we were unable to
ascertain the reason for this unexpected behavior.
anticipated that dicobalt complex formation with TIPS,TMS-
triyne 1 would occur at the central alkyne to give 2a
2
0
(analogously to the reported bis-TIPS complex ) and that the
regioisomer 2b would be a minor byproduct. However, we
found that the major product is the undesired product 2b, with
the dicobalt group adjacent to the less bulky TMS group (45%
yield). The desired product 2a was isolated in 27% yield
(
Scheme 1). The identities were confirmed by NMR
spectroscopy and X-ray crystallography, as discussed below.
Despite the large amount of undesired regioisomer formed, this
is a more efficient synthetic route than statistical deprotection of
a bis-TIPS complex (Scheme S1, SI).
The analogous longer octatetrayne-linked dimer complex 9
was prepared via Eglinton coupling. Once again, two products
were observed by TLC; however, the compound with the higher
Scheme 1. Cobalt complexation.
f
R decomposed during column chromatography on silica. By
a) Co2(CO)8
hexane
25 °C, 12 h
analogy to 8b, we assume that the unexpected byproduct
contains a hexayne. The stable product was identified as 9 by
13C NMR spectroscopy, through observation of the
characteristic signals from acetylene carbons adjacent to the
TIPS group. The analogous complex 10 was also formed from
7 using Eglinton coupling conditions (Scheme 3). The loss of
Ph2P
OC)2Co Co(CO)2
PPh2
Ph2P
PPh2
(
(OC) Co Co(CO)2
+
2
TIPS
TMS
b) dppm
toluene
10 °C
0 min
TMS
1
TIPS
TMS
TIPS
1
3
2a (27%)
2b (45%)
terminal acetylene peaks in the H and 13C NMR spectra, the
1
2 3
Deprotection of 2a with K CO
in MeOH/THF yielded increased symmetrization of the alkyl diphosphinomethane
the desired cobalt complex 3 quantitatively (Scheme 2). The signal and the presence of the expected 13C acetylene signals for
acetylenic chain in this cobalt complex can be readily extended the tetracobalt complex confirmed the successful formation of
via a Glaser coupling with TMS-acetylene.41 Use of an excess of
tetracobalt complex 10.
TMS-acetylene, CuCl and TMEDA gave the desired butadiyne
4
in 82% yield (with bis-TMS-butadiyne as a side product,
Scheme 2). A large excess of reagents is required to avoid
homocoupling of 3. The structure of 4 was confirmed by single
crystal X-ray diffraction, vide infra. Deprotection of 4 using
K
2
CO
3
in THF/MeOH yielded the desired 5 in a quantitative
yield. The acetylene chain extension methodology was used
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The Journal of Organic Chemistry
Scheme 3. Synthesis of tetracobalt complexes 8a, 8b, Scheme 5. Synthesis of polyynes 11–13.
9, 10.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
PPh
2
Ph
2
P
PPh2
2
Ph P
Ph
2
P
Co(CO)
2
(OC) Co
2
PPh
2
Ph P
Co(CO)2
(OC) Co
2
2
PPh
Co(CO)
2
2
8
0%
(
OC)
2
Co
Co(CO)2
(OC) Co
Ph
OC)
2
P
PPh2
2
m
(
2
Co Co(CO)
Cu(OAc)
Py, 25 °C
TIPS
TIPS
2
2
I2, THF
25 °C, 3 h
m
8a
TIPS
TIPS
TIPS
TIPS
TIPS
3
H
m = 1 11 (55%)
m = 3 12 (45%)
m = 5 13 (39%)
m = 1 8a
m = 3 9
m = 5 10
7
%
PPh
2
Ph P
2
+
Ph
2
P
Co(CO)
2
(OC) Co
2
PPh
Co(CO)2
2
(
OC)
2
Co
8b
TIPS
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
TIPS
The yields for polyyne unmasking (Scheme 5) are
significantly higher than those for the Fritsch-Buttenberg-
Wiechell rearrangements reported by Tykwinski and coworkers
for the synthesis of the same compounds.33 The octayne 7 is
obtained in 45% yield from a tetracobalt complex. The same
octayne was isolated in 10% yield from a dibromoolefin-masked
precursor.33 It was not previously possible to directly unmask a
decayne from a masked precursor; Tykwinski and coworkers
reported that this reaction led only to decomposition. In
contrast, decayne 13 is isolated in 39% yield after simple
purification from tetracobalt complex 10. The lower yields of
the longer polyynes reflect their lower stability, and may be
attributed to decomposition under the reaction conditions.
These results demonstrate that unmasking of the
Ph
2
P
PPh2
PPh
2
Ph
2
P
(OC)
2
Co Co(CO)
Cu(OAc)
2
2
2
Ph P
Co(CO)
2
(OC) Co
2
PPh
Co(CO)2
2
Py, 25 °C
(
OC)
2
Co
77%
TIPS
5
9
H
TIPS
TIPS
Ph
2
P
PPh2
PPh
2
Ph
2
P
(OC)
2
Co Co(CO)
Cu(OAc)
2
2
2
Ph P
Co(CO)
2
(OC) Co
2
PPh
2
Py, 25 °C (
OC)
2
Co
Co(CO)2
7
5%
TIPS
7
1
0
TIPS
TIPS
H
2 4
The removal of the Co (CO) dppm masking group was first Co
2 4
(CO) dppm MAE is a promising methodology in polyyne
tested on simple cobalt complexes. Both 2a and 2b were used as synthesis and offers new opportunities in carbon-rich chemistry.
model systems for optimizing the reaction conditions. We found
that both 2a and 2b undergo decomplexation of the dicobalt
group in good yield by oxidation with I in THF, the conditions
2
reported previously by Haley et al. (Scheme 4).21 The successful
removal of the masking group in simple systems encouraged us
to investigate extended polyynes.
NMRꢀspectroscopyꢀ
1
The H NMR spectra of the regioisomers 2a and 2b are
similar, as the variance between them is remote from any
protons (Figure 2). The two geminal protons of the diphosphine
bridge (δ ≈ꢀ 3.5 ppm) are inequivalent, as observed in both
H
spectra. There is a greater splitting of these signals in the 2b
regioisomer, as the environments of the protons are more
different. The chemical shift of the trimethylsilyl group supports
this assignment, as the signal comes at an unusual chemical shift
of 0.40 ppm in 2b. The TMS signal in 2a has a chemical shift
of 0.25 ppm, which is in the usual range for a TMS group
Scheme 4. Removal of a dicobalt group; unmasking
triyne 1.
Ph2P
PPh2
Co(CO)2
6
5%
attached to an alkyne. The carbon atoms directly bonded to
cobalt are observed as two triplets (due to coupling to 31P) at
TIPS
2a
TMS
I2, THF, 25 °C, 3 h
TIPS
TMS
75.0 and 91.5 ppm in 2b, and as two broad signals at 68.9 and
71.5 ppm in 2a (Figure 3). The chemical shifts are as expected
for each isomer; two acetylenic carbons with satellites are
observed in 2a, in contrast to 2b. The structures of 2a and 2b
were confirmed by single crystal X-ray diffraction analysis, vide
infra.
Ph2P
PPh2
1
(
OC)2Co Co(CO)2
7
4%
TMS
2
b
TIPS
The tetracobalt complexes 8a, 9 and 10 were unmasked
using the conditions described above to give the corresponding
polyynes in 39–55% yields (Scheme 5). The unmasked polyynes
were purified easily by chromatography on silica gel with
petroleum ether as the eluent to separate the desired non-polar
TIPS polyyne from the polar cobalt species. As expected, the
longest polyyne was isolated in a slightly lower yield, reflecting
its lower stability. UV-vis spectroscopy was invaluable in
identifying the desired polyynes, due to the characteristic
vibronically coupled absorption bands (Figure S33).33 UV-vis
spectroscopy is very sensitive to the presence of an incomplete
reaction or other polyynic products, as both outcomes result in
the presence of other chromophores with absorption bands that
overlap with those of the desired polyynes. The products were
conclusively identified by 13C NMR spectroscopy. This is the
first time, to the best of our knowledge, that the dicobalt
masking moiety has been used in the synthesis of polyynes.
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frequency with increased number of acetylenes in the dicobalt
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
oligoynes is observed (Table 1), which indicates a slightly weaker
C-H bond in 7 than in 3. The same overall trend is repeated in
both series of compounds (with the exception of Tr*-(C≡C) -H).
2
Table 1. Comparison of H and 13C chemical shifts and
1
acetylenic C-H stretch frequencies in cobalt and
supertrityl oligoynes.15
t-Bu
t-Bu
t-Bu
Ph2P
PPh2
(OC)2Co Co(CO)2
= Tr*
t-Bu
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
t-Bu
t-Bu
TIPS
n H
Compound
C≡C-H
C≡C-H 13
C
C≡C-H stretch
1H δ
δ
C
/ ppm(a)
–1
H
/
frequency /cm
ppm(a)
(b)
1
Figure 2. Partial H NMR spectra of isomers 2a and 2b (500
MHz, CDCl ).
3
Tr*-C≡C-H
2.65
2.10
2.07
3.63
2.81
2.43
72.0
66.7
61.3
83.8
73.0
69.8
3306.0
3315.1(c)
3282.2
3306.4
3305.8
3296.6
Tr*-(C≡C)
2
3
-H
-H
Tr*-(C≡C)
3
5
7
(a) 1H and 13C NMR spectra were recorded in CDCl
00 MHz respectively. (b) IR spectra recorded on thin films. (c) The
stretch frequency for Tr*-(C≡C) -H is from ref. 15.
at 400 and
3
1
2
We also performed computational studies on both types of
terminal oligoynes. The electrostatic potential maps (ESP) on
total density surfaces show significantly higher charge
accumulation over the oligoyne chain in dicobalt complexes
compared to supertrityl end-capped oligoynes (–0.04 vs. –0.02
a.u., respectively; Figures S37 and S38, SI). However, the
positive charge on the terminal hydrogen is similar in both
cases, suggesting similar reactivity. In order to get detailed
charge distribution along the oligoyne chains, we calculated
atomic charges using three different methods: Mulliken,
Figure 3. Partial 13C NMR spectra of isomers 2a and 2b (125
MHz, CDCl
3
) with proposed assignments.
Reactivityꢀofꢀterminalꢀacetyleneꢀ
In the context of our research towards carbon rich Hirshfield, and Natural Population Analysis (Table S1, Table
supramolecular structures, we planned to form polyyne S2, SI). In all three methods, the positive charge on the terminal
rotaxanes from cobalt complexes 3, 5 and 7, via active-metal hydrogen atom increases with increasing acetylenic chain length
template synthesis.37,45-47 However, experiments in this direction
(Figure S39, SI). In contrast, the charge difference between
have not yet been successful. Hence, we sought to understand terminal carbon and hydrogen exhibits the reverse trend (Figure
the differences in reactivity between dicobalt oligoynes 3, 5, 7 S40, SI). Nevertheless, the length-dependent changes, as well as
and their tris(3,5-di-t-butylphenyl)methyl (supertrityl, Tr*) end- the range of values, are very similar in both types of oligoynes
capped oligoyne counterparts, which were previously used to suggesting similar reactivity.
prepare polyyne rotaxanes.37,38,47
In general, the multiple similarities between dicobalt and
The most striking property of the dicobalt acetylenes 3, 5, 7 supertrityl oligoynes imply that incompatibility with the active
is the unusually high chemical shift of the terminal acetylene metal template rotaxane formation is a result of incompatibility
protons (Table 1). In 3, this resonance appears at 3.63 ppm, with oxidative coupling conditions, rather than reactivity
which is 0.98 ppm greater than that of the supertrityl monoyne differences of the terminal alkyne. The successful formation of
counterpart. This difference diminishes as the distance from the tetracobalt complexes via homocoupling supports this
end-capping group increases in the corresponding diyne and conclusion (Scheme 3).
triyne. The same trend is reflected in the 13C NMR spectra. The
Electronicꢀabsorptionꢀspectraꢀandꢀmolecularꢀorbitalsꢀ
chemical shift of the terminal carbon in 3 is 11.8 ppm larger
UV-vis absorption spectra of all of the novel cobalt
than that for the analogous supertrityl compound. In the longest
2 2
complexes were recorded (CH Cl , 298 K). The change in
analogues, this difference is reduced to 8.5 ppm (7 vs. Tr*-
C≡C) -H). This increase in chemical shift can be attributed to
the electron-withdrawing effect of the dicobaltcarbonyl group or
to deshielding by the phenyl substituents of the phosphine.
absorption upon cobalt complexation of triyne 1 is pronounced
Figure 4). After complexation, the fine vibronic band (280–330
(
3
(
nm) is dwarfed by a new broad absorption with a maximum at
290 nm and high molar absorption coefficient (25000 M–1 cm–1).
There is also a low-intensity broad feature at lower energy,
which results from several weak transitions (Figures S42 and
IR spectroscopy is a useful tool for providing insight into
bond strengths in acetylenes. A decrease in the C-H stretch
4
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S43, SI). Solutions of 2a–7 in CH
The Journal of Organic Chemistry
2
Cl
2
have an intense red color. changing the solvent from hexane to acetonitrile (Figures S34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The very broad signals of the other TMS-protected dicobalt and S35, SI).
complexes (4 and 6) did not reveal any particularly unexpected
behavior (Figure S32, SI).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. The effect of dicobalt complexation on electronic
absorption spectra (CH Cl ).
2 2
Figure 6. Calculated frontier molecular orbitals of E isomers of
a, 9 and 10 (DFT/B3LYP/STO-3G).
8
The absorption spectra of tetracobalt polyyne complexes are
significantly red-shifted compared to those of the dicobalt
oligoynes (Figure 5), and there is a bathochromic shift with
increasing length of polyyne. They are considerably more red-
shifted than those of the unmasked polyynes (Figure S33, SI).
CrystallographicꢀAnalysisꢀ
Single crystal X-ray diffraction studies provide insight into
the geometries of cobalt-polyyne complexes, and acted as an
essential tool for confirming the structure of these unusual
compounds. The crystal structures of polyynes can reveal the
bond length alternation (BLA) and the degree of deviation from
linearity.50 BLA is the difference between the carbon-carbon
bond length of adjacent single and triple bonds. In polyynes, the
BLA is defined as the difference in length between the central
single and triple bonds.51,52
Here, seven crystal structures of cobalt-polyyne complexes
are presented: 2a, 4, 6, 2b, 8a and 8b and bis-TIPS complex
S6. All crystals were grown by layered addition of MeOH to a
solution in CHCl . Diffraction data for 8a and 8b were collected
3
at 100 K using synchrotron radiation on beamline I19-1 at
Diamond Light Source. The structures were solved using charge
flipping53,54 with SuperFlip55 and refined using the full-matrix
least-squares method within the CRYSTALS software package
(
Section S5).56,57 Crystallography proved to be an essential tool
when distinguishing between the regioisomers 2a and 2b
Figure 7). The bond angles are broadly similar in the two
regioisomers. However, the dppm ligand is directed towards
Figure 5. The effect of polyyne length on absorption in tetracobalt TMS group in 2a and towards TIPS group in 2b, which may
(
complexes 8a, 9, 10 (UV-vis spectra, CH
2
Cl
2
).
be a result of crystal packing. The variation in the solid-state
structures suggests that the conformation of the cobalt complex
is dynamic in solution, as concluded from previous NMR
spectroscopy studies.9,43
Frontier molecular orbitals were calculated for 8a, 9 and 10
at the B3LYP/STO-3G level of theory (Figure 6), using the
crystal structures of complexes 2a, 4 and 6 as starting points for
geometry optimization (Section S4, SI). In all cases, the HOMO
is localized mainly on the polyyne core whereas the LUMO
delocalizes into the cobalt atoms and carbonyl ligands. This
implies a HOMO-LUMO transition that possesses significant
charge transfer character, shifting electron density from the
conjugated π-system to the metal carbonyl. The intense red
color of all of the cobalt complexes could be attributed to a
charge-transfer band, and this hypothesis is supported by the
observation of modest solvatochromism for the cobalt complex
2
a;48,49 its absorption maximum shifts to the red by 358 cm–1 on
5
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ꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7. Displacement ellipsoid plots of 2a and 2b (H atoms and
solvent omitted for clarity, thermal ellipsoids drawn at 50%
probability).
Single crystal X-ray diffraction led to the serendipitous
discovery of an unexpected product. As discussed above, when 3
was subjected to Eglinton coupling conditions, two products
were observed (8a and 8b, Scheme 3 and Figure 8). The end
groups in 8a are oriented in (E) fashion, with the TIPS groups
perfectly anti-periplanar because the compound crystallizes with
a center of symmetry. The diyne core is highly linear, while 8b
has a far more curved polyyne chain and a slightly twisted syn-
periplanar geometry of the TIPS groups.
ꢀ
Figure 9. Displacement ellipsoid plots of 2a, 4 and 6 (H atoms
omitted for clarity, thermal ellipsoids drawn at 50% probability)
Systematic analysis of the set of complexes with increasing
length of sp-chain was performed based on the crystal structures
of 2a, 4 and 6 (Figure 9, Table 2). The length of the Csp3–Csp
bond decreases as the polyyne gets longer. In 2a, this bond is
markedly longer than in 6 (0.02 Å). In contrast, there appears to
be no relationship between the length of the Co-C bond and the
polyyne length. The average Co-C bond length is 1.95 Å in all
of the compounds. This implies that the complexation of the
dicobalt group is of very similar strength in all cases. The BLA is
smaller in the longer masked polyyne 6 than in 4. One of the
features that attracted us and others to the dicobalt alkyne
masking group is the angle that it infers on the polyyne system
while masking it. As the chain lengthens, the angle θ
and θ decreases (Table 2).
2
increases
1
Figure 8. Displacement ellipsoid plots of regioisomers 8a and 8b
(
H atoms and disorder omitted for clarity, thermal ellipsoids drawn
at 50% probability)
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The Journal of Organic Chemistry
Table 2. Bond lengths and angles in crystal structures of
compounds 2a, 4 and 6.
EXPERIMENTALꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
General. Reagents (Acros, Aldrich, Fluorochem, and TCI)
were purchased as reagent grade and used without further
purification. Solvents for extraction or column chromatography
were used in HPLC grade. Dry solvents (THF, CH
for reactions were purified by the solvent drying system MBraun
MB-SPS-5-Bench Top under nitrogen atmosphere (H
content < 20 ppm as determined by Karl-Fischer titration). All
other solvents were purchased in p.a. quality. Reactions in the
absence of air and moisture were performed in oven-dried
Ph2P
OC
PPh2
CO
Co
OC
CO
2 2 2
Cl , Et O)
Co-C (1,2)
Co-C (3,4)
Csp3-Csp
θ
1
θ
2
2
O
n
TIPS
TMS
Complex
2
4
6
glassware under Ar or
chromatography was performed using SiO
N
2
atmosphere. Flash column
(60 Å, 230–400
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Co–C (1)/ Å
Co–C (2)/ Å
Co-C (3)/ Å
Co–C (4)/ Å
1.946(2)
1.949(2)
1.956(2)
1.963(2)
1.409(3)
1.211(3)
N/A
1.947(2)
1.947(3)
1.952(2)
1.954(2)
1.391(3)
1.207(3)
1.373(4)
1.207(4)
N/A
1.944(2)
1.953(2)
1.958(2)
1.958(2)
1.381(2)
1.215(2)
1.358(2)
1.214(2)
1.365(2)
1.208(2)
1.852(2)
0.156(4)
1.953(2)
2
mesh, particle size 0.040–0.063 mm, Merck) at 25 °C with a
head pressure of 0.0–0.5 bar. The used solvent compositions are
reported individually in parentheses. Analytical thin layer
chromatography (TLC) was performed on aluminum sheets
coated with silica gel 60 F254 (Merck). Visualization was
achieved using UV light (254 or 366 nm) or by staining with
C
sp–Csp3 / Å
C≡C / Å
C–C / Å
C≡C / Å
C–C / Å
C≡C / Å
C–Si / Å
iodine adsorbed on SiO
2
. Evaporation in vacuo was performed at
2
5–60 °C and 900–10 mbar. Reported yields refer to
N/A
spectroscopically and chromatographically pure compounds
that were dried under high vacuum (0.1–0.05 mbar) before
analytical characterization. 1H and 13C nuclear magnetic
resonance (NMR) spectra were recorded on Bruker AVIII HD
N/A
N/A
N/A
1.839(3)
N/A
1.842(3)
0.175(7)
1.950(2)
4
00 and AVIII HD 500 spectrometers at 400 MHz, 500 MHz
1
13
Avg. BLA / Å
Avg. Co-C / Å
( H) and 101 MHz, 125 MHz ( C), respectively. Temperatures
of measurements are indicated in the procedures and on the
spectra. Chemical shifts δ are reported in ppm downfield from
tetramethylsilane using the residual deuterated solvent signals as
an internal reference (CDCl : δ = 7.26 ppm, δ = 77.0 ppm).
1.953(2)
θ
θ
1
2
/ °
/ °
146.3(2)
136.8(2)
178.9(2)
N/A
137.5(2)
139.1(2)
174.9(2)
175.6(2)
177.3(3)
N/A
138.0(2)
143.6(2)
174.9(2)
178.1(2)
179.2(2)
177.4(2)
177.2(2)
177.4(2)
3
H
C
Melting points are uncorrected. MALDI-TOF mass
spectrometry was conducted using a Micromass MALDI micro
MX spectrometer in positive reflectron mode. Dithranol was
used as the matrix unless stated otherwise. UV-vis spectra were
recorded on a Perkin Elmer Lambda 20 at 298 K (240–700 nm)
or a Perkin Elmer Lambda 1050 (240–700 nm). Elemental
analysis was performed at London Metropolitan University with
a Thermo (Carlo Erba) Flash 2000 Elemental Analyzer,
configured for %CHN.
∠
CCo - C≡C / °
C≡C-C / °
C-C≡C / °
C≡C-C / °
C-C≡C / °
N/A
N/A
N/A
N/A
Avg. chain /°
178.9(2)
175.9(2)
2
Tetracarbonyl[μ -(3,4-η:3,4-η)-triisopropyl((trimethylsilyl)hexa-
1
,3,5-triyn-1-yl)silane][μ-methylenebis(diphenylphosphine)-P:P’]dicobalt
a. A solution of triyne 1 (1.22 g, 4.03 mmol) and Co (CO)
(1.52 g, 4.44 mmol) in O -free hexane (60 mL) was stirred at 20
°C under N for 12 h. The hexane was evaporated and the
residue dissolved in dry toluene (25 mL). Bis-
2
2
8
CONCLUSIONSꢀ
2
A series of cobalt complexes with masked alkyne π-systems
have been synthesized. A synthetic route that yielded a series of
dicobalt complexes with an exposed arm of one, two and three
ethynyl units was followed. This series provided substrates for
forming long masked polyynes and allowed a systematic
investigation into optical and solid state properties of these
structures. The absorption spectra of these complexes have been
investigated. A charge transfer band between the polyyne and
the dicobalt moiety is supported by DFT calculations and
solvatochromism experiments.
2
diphenylphosphinomethane (1.60 g, 4.16 mmol) was added and
the solution heated to reflux for 30 min. The crude reaction
mixture contained two products. Evaporation and column
chromatography (hexane/CH
intensely dark red crystals of 2a (0.98 g, 27%); R
hexane), mp 181–185 °C after recrystallization from
CH Cl /MeOH
1H NMR (400 MHz, CDCl
H, i-Pr-H), 3.34 (q, 2JH-P = 10.9 Hz, 1H, PCHP), 3.51 (q, 2JH-P
2
Cl
2
9:1 and 8:2) afforded
f
= 0.35
(
2
2
3
) δ 0.26 (s, 9H), 1.12–1.16 (m, 21
Further chemistry with these cobalt complexes led to
preparation, by Eglinton couplings, of tetracobalt masked TIPS-
endcapped polyynes of lengths up to the decayne. An
unexpected isomerization during Eglinton coupling was
discovered by crystallographic analysis. These structures could
be readily unmasked with elemental iodine, offering the most
facile method of unmasking long polyynes yet reported. This
method offers promise for mild unmasking of supramolecular
carbon-rich structures and other extended π-systems.
=
1
8
0.9 Hz, 1H, PCH’P), 7.15–7.20 (m, 4H, H–Ar), 7.21–7.31 (m,
H, H–Ar), 7.32–7.41 ppm (m, 8H, H–Ar). 13C NMR (100 MHz,
) δ 0.5, 11.7, 19.0, 35.1 (t, 1JC-P = 22.1 Hz), 68.9 (br), 71.5
CDCl
3
(br), 99.6, 102.3, 107.6, 109.6, 128.4–128.6 (m), 129.8 (s) 130.0
s), 131.8 (t, 3JC-P = 4.7 Hz), 132.4 (t, 2JC-P = 5.1 Hz), 135.1 (t,
C-P = 19.1 Hz), 136.5 (t, JC-P = 19.8 Hz), 202.3 (br), 203.8 ppm
br). IR (ATR) 2105.1 (C≡C), 2026.5 (C=O), 1996.2 (C=O),
(
J
(
–
1
1
973.2 (C=O), 1956.0 (C=O) cm . MALDI MS 804.17 (Calc (–
7
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Page 8 of 10
4
C
CO). C43
H
52Co
2
P
2
Si
: C, 61.57; H, 5.72 Found: C, 61.56; H, 5.75. through a SiO
, 25 °C) λmax (logε) 290 nm (4.40), 350 nm (3.99).
Si = 916.91 (Z 4).
; a = 14.1381(1) Å, b = 15.8779(1) Å, c =
2
)), 804.16 (–4 CO). Anal. Calcd for organic phase was concentrated in vacuo and then passed
plug to yield 4 as a red solid (42 mg, 82%)
) δ 0.25 (s, 9H), 1.08–1.12 (m, 21
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
H
52Co
O
P
Si
2
47
2
4
2
2
UV-vis: (CH
Crystal Data:
Orthorhombic P2
0.5634(2) Å, V = 4616.14(6) Å , R
2σ(I)).
Additionally, a regioisomer tetracarbonyl[μ -(5,6-η:5,6-η)-
triisopropyl((trimethylsilyl)hexa-1,3,5-triyn-1-yl)silane][μ-
2
Cl
2
1H NMR (400 MHz, CDCl
H, i-Pr-H), 3.36 (q, 2JH-P = 10.5 Hz, 1H, PCHP), 3.46 (q, 2JH-P
3
C
47
H
52Co
2
O
4
P
2
2
,
M
r
=
=
1 1 1
2 2
10.9 Hz, 1H, PCH’P), 7.09–7.12 (m, 4H, H–Ar), 7.20–7.33 (m,
12H, H–Ar), 7.37–7.43 ppm (m, 4H, H–Ar). 13C NMR (100
3
2
>
1 2
= 0.0296, wR = 0.0641 (I
) δ 0.2, 11.8, 19.1, 35.8 (t, 1JC-P = 22.4 Hz), 67.9
MHz, CDCl
(br), 70.6 (br), 80.9, 81.2, 86.3, 88.3, 100.5, 108.8, 128.6 (t,
= 4.8 Hz), 128.8 (t, 3JC-P = 4.7 Hz), 130.0 (s) 130.2 (s), 131.6 (t,
3
2
3
JC-P
methylenebis(diphenylphosphine)-P:P’]dicobalt 2b was isolated 2JC-P = 5.4 Hz), 132.6 (t, 2JC-P = 5.6 Hz), 134.0 (t, 1JC-P = 21.8
from the same reaction and was characterized as dark red Hz), 136.9 (t, 1JC-P = 22.7 Hz), 201.6 (br), 204.0 ppm (br). IR
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
crystals (1.63 g, 45%), R
= 0.51 (hexane); mp 181–183 °C. The (ATR): 2156.0 (C≡C), 2105.8 (C≡C), 2038.3 (C=O), 2018.5
structure of this regioisomer 2b was confirmed by X-ray (C=O), 1986.5 (C=O), 1960.7 cm–1 (C=O). MALDI MS 828.17
f
crystallographic analysis.
(Calc. (–4 CO) C45
CH Cl , 25 °C) λmax (logε) 304 nm (4.36), 372 nm (3.90). Crystal
Data: C49 Si , M = 940.93 (Z = 2). Triclinic P ¯1 ; a =
2 2 2
H52Co P Si )), 828.17 (–4 CO). UV-vis:
(
2
2
1H NMR (400 MHz, CDCl
H, i-Pr-H), 3.27–3.32 (m, PCHP), 3.94–4.03 (m, 1H, PCH’P),
.12–7.16 (m, 4H, H–Ar), 7.21–7.24 (m, 2H, H–Ar), 7.27–7.34
m, 10H, H–Ar), 7.39–7.49 ppm (m, 4H, H–Ar). 13C NMR (100
) δ 0.37 (s, 9H), 1.16–1.21 (m, 21
3
H
52Co
2
O
4
P
2
2
r
11.5356(2) Å,
b
β
=
=
12.1471(2) Å,
81.4150(7)°,
c
=
=
19.7928(3) Å, α=
64.0407(6)°,
7
(
76.0952(6)°,
γ
V =
1
2417.46(7) Å
3
1 2
, R = 0.0382, wR = 0.0755 (I > 2σ(I)).
MHz, CDCl
3
) δ 0.9, 11.8, 19.1, 38.6 (t, JC-P = 19.5 Hz), 75.0 (t,
2JC-P = 11.6 Hz), 80.3 (t, JC-P = 4.1 Hz), 81.7, 88.5 (satellite: d,
3
2
Tetracarbonyl[μ -(3,4-η:3,4-η)-triisopropyl(octa-1,3,5,7-tetrayn-1-
1JC-Si = 77.1 Hz, 91.5 (t, 2JC-P = 7.7 Hz), 92.0, 128.5 (t, 3JC-P
=
yl)silane][μ-methylenebis(diphenylphosphine)-P:P’]dicobalt 5. A solution
of cobalt complex 4 (30 mg, 0.039 mmol) was dissolved in 1:1
4
.8 Hz), 128.7 (t, 3JC-P = 4.7 Hz), 129.8 (s) 130.1 (s), 131.5 (t, 2JC-
= 6.4 Hz), 132.7 (t, 2JC-P = 6.1 Hz), 135.2 (t, 1JC-P = 17.8 Hz), MeOH/THF mixture (6 mL). K CO (52.5 mg, 0.38 mmol)
P
2
3
1
2
(
8
37.1 (t, 1JC-P = 22.6 Hz), 201.4 (br), 207.2 ppm (br). IR (ATR):
159.5 (C≡C), 2024.5 (C=O), 2001.2 (C=O), 1973.2 cm–1 monitored via TLC, completion occurred after 30 min. The
Si )), reaction mixture was evaporated and then dissolved in
Cl CH Cl /petroleum ether (1:1) and poured over a silica plug.
was added in one portion. The reaction was stirred at 25 °C and
C=O). MALDI MS 804.17 (Calc. (–4 CO) C43
04.16 (–4 CO). Anal. Calcd for C47
H
52Co
2
P
2
2
H
52Co
2
O
4
P
2
Si
2
(CH
2
2
)
0.5
:
2
2
C, 59.47; H, 5.57 Found: C, 59.15; H, 5.79 Crystal Data: Complex 5 was isolated as red crystals (26 mg, 100%); mp 85
Co Si , M
= 1918.76 (Z = 2). Monoclinic °C (decomposition).
/n; a = 17.1505(2) Å, b = 12.7684(2) Å, c = 22.9778(3) Å, β
C
95
H
106Cl
2
4
O
8
P
4
4
r
P2
106.2309(5)°, V = 4831.23(11) Å , R
(I > 2σ(I)).
1
1H NMR (400 MHz, CDCl
3
) δ 1.10–1.14 (m, 21 H, i-Pr-H),
.80 (s, 1H, C≡C–H), 3.41 (m, 2H, PCH P), 7.13–7.17 (m, 4H,
H–Ar), 7.20–7.33 (m, 12H, H–Ar), 7.38–7.43 ppm (m, 4H, H–
3
=
1 2
= 0.0348, wR = 0.0748
2
2
2
Ar). 13C NMR (100 MHz, CDCl ) δ 11.8, 19.1, 36.1 (t,
22.4 Hz), 70.7, 72.9, 79.6, 80.1, 100.6, 108.8, 128.6 (t, 3JC-P
1
JC-P
=
=
Tetracarbonyl[μ -(3,4-η:3,4-η)-triisopropyl(hexa-1,3,5-triyn-1-
yl)silane][μ-methylenebis(diphenylphosphine)-P:P’]dicobalt 3. A solution
of cobalt complex 2a (67 mg, 0.073 mmol) was dissolved in 1:1 4.8 Hz), 128.8 (t, 3JC-P = 4.7 Hz), 130.1 (s) 130.3 (s), 131.6 (t, 2JC-
3
= 5.4 Hz), 132.6 (t, 2JC-P = 5.6 Hz), 134.1 (br), 136.9 (br), 201.9
MeOH/THF mixture (6 mL). K
2
CO
3
(100 mg, 0.73 mmol) was
P
added in one portion. The reaction was stirred at 25 °C and (br), 203.6 ppm (br). IR (ATR): 3305.1 (C≡C-H), 2161.3 (C≡C),
monitored via TLC, completion occurred after 30 min. The 2102.0 (C≡C), 2034.8 (C=O), 2010.0 (C=O), 1985.0 cm–1
reaction mixture was evaporated and then dissolved in (C=O).
CH
Compound 3 was isolated as red crystals (57 mg, 96%).
) δ 1.09–1.15 (m, 21 H, i-Pr-H),
P), 3.71 (s, 1H, C≡C–H), 7.11–7.18 (m, acetylene (0.580 g, 0.813 mL, 5.90 mmol), CuCl (1.17 g, 11.8
Cl (400 mL) and stirred vigorously
2
Cl
2
/petroleum ether (1:1) and filtered through SiO
2
plug.
2
Tetracarbonyl[μ -(3,4-η:3,4-η)-triisopropyl((trimethylsilyl)deca-
1
,3,5,7,9-pentayn-1-yl)silane][μ-methylenebis(diphenylphosphine)-
1H NMR (400 MHz, CDCl
P:P’]dicobalt 6. Cobalt complex 5 (100 mg, 0.12 mmol), TMS
3
3
4
.34–3.50 (m, 2H, PCH
2
H, H–Ar), 7.21–7.33 (m, 8H, H–Ar), 7.38–7.45 ppm (m, 8 H, mmol) were dissolved in CH
2
2
H–Ar). 13C NMR (100 MHz, CDCl
) δ 11.8, 19.1, 35.5 (t, 1JC-P
under dry air. TMEDA (1.68 mL, 11.7 mmol) was added and
the reaction mixture was stirred for 40 mins. The reaction was
quenched with water (200 mL), organic phase separated and
washed again with water (150 mL) to remove Cu salts. The
organic phase was reduced in vacuo and then passed through a
3
=
2
1.9 Hz), 69.8 (br), 70.4 (br), 83.8, 86.4, 99.6, 109.0, 128.4 (t, 3JC-
= 5.0 Hz), 128.7 (t, 3JC-P = 5.0 Hz), 130.0 (s) 130.2 (s), 131.7 (t,
P
2JC-P = 5.7 Hz), 132.5 (t, 2JC-P = 5.1 Hz), 134.6 (t, 1JC-P = 21.8
Hz), 136.8 (t, 1JC-P = 23.1 Hz),), 202.2 (br), 204.0 ppm (br). IR
(ATR): 3306.4 (C≡C-H), 2105.7 (C≡C), 2070.1 (C≡C), 2033.1 SiO
(decomposition).
H NMR (400 MHz, CDCl ) δ 0.24 (s, 9H), 1.08–1.13 (m, 21
2
plug to yield 6 as a red solid (84 mg, 82%); mp 150 °C
(C=O), 2008.6 (C=O), 1982.9 cm–1 (C=O). MALDI MS 732.13
(
Calc. C40
H
44Co
2
P
2
Si), 732.90 (Found –4 CO).
1
3
2
H, i-Pr-H), 3.35–3.42, m, 2H, PCH
Ar), 7.22–7.33 (m, 12H, H–Ar), 7.37–7.43 ppm (m, 4H, H–Ar).
P:P’]dicobalt 4. Cobalt complex 3 (50 mg, 0.059 mmol), TMS 13C NMR (100 MHz, CDCl ) δ 0.0, 11.8, 19.1, 36.3 (t, 1JC-P
2
P), 7.11–7.129 (m, 4H, H–
Tetracarbonyl[μ -(3,4-η:3,4-η)-triisopropyl((trimethylsilyl)octa-
1,3,5,7-tetrayn-1-yl)silane][μ-methylenebis(diphenylphosphine)-
3
=
2
1
1
0.5 Hz), 64.4, 66.1 (br), 68.9, 72.3 (br), 81.1, 82.1, 89.7, 89.8,
01.2, 108.9, 128.7 (t, 3JC-P = 4.7 Hz), 128.8 (t, 3JC-P = 4.9 Hz),
30.2 (s) 130.4 (s), 131.7 (t, 2JC-P = 6.0 Hz), 132.5 (t, 2JC-P = 5.7
acetylene (0.29 g, 0.406 mL, 2.95 mmol), CuCl (3.01 g, 30.4
mmol) were dissolved in CH Cl (400 mL) and stirred vigorously
under dry air. TMEDA (3.22 g, 4.25 mL, 27.6 mmol) was
added and the reaction was stirred vigorously for 2 h. The
2
2
Hz), 134.1 (t, 1JC-P = 20.8 Hz), 136.9 (t, 1JC-P = 21.5 Hz), 201.8
reaction was quenched with water (200 mL), organic phase (br), 203.3 ppm (br). IR (ATR): 2126.4 (C≡C), 2054.6 (C=O),
separated and washed again with water to remove Cu salts. The 2035.9 (C=O), 2014.9 (C=O), 1991.5 cm–1 (C=O). MALDI MS
852.17 (Calc.
C
47
H
52Co
2
P
2
Si
2
), 853.95 (–4 CO). UV-vis:
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The Journal of Organic Chemistry
4
(
CH
Data: C51
2.9206(2) Å, b = 13.1057(2) Å, c = 17.0055(3) Å, αꢀ =
104.4641(7)°, β = 100.3881(8)°, γ = 111.9056(9)°, V =
2
Cl
2
, 25 °C) λmax (logε) 342 nm (4.20), 417 nm (3.55). Crystal
Si , M
= 964.96 (Z = 2). Triclinic P ¯1 ; a = η:3,4-η;13,14-η:13,14-η)-1,16-bis(triisopropylsilyl)-
1,3,5,7,9,11,13,15-hexadecaoctayne]tetracobalt 9.
butadiyne Co complex 5 (30 mg, 47.3 μmol) was dissolved in
dry pyridine (3 mL). Cu(OAc) (75 mg, 473 μmol) was added
under N and stirred overnight at 25 °C. The pyridine was
removed in vacuo. This yielded two products which were
separated by column chromatography (CH Cl /petroleum
ether 9:1). The lower R product 9 was isolated as a brown solid
23 mg, 77%); mp 110 °C (decomposition).
Octacarbonylbis[μ-methylenebis(diphenylphosphine)-P:P’][μ -(3,4-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
H
52Co
O
P
2
4
2
2
r
Mono-TIPS
1
3
2
2464.22(7) Å , R
1
= 0.0357, wR
2
= 0.0837 (I > 2σ(I)).
Tetracarbonyl[μ -(3,4-η:3,4-η)-triisopropyl(deca-1,3,5,7,9-pentayn-
1-yl)silane][μ-methylenebis(diphenylphosphine)-P:P’]dicobalt 7.
solution of cobalt complex 6 (50 mg, 0.052 mmol) was dissolved
in 1:1 MeOH/THF mixture (6 mL). K CO (65 mg, 0.47
mmol) was added in one portion. The reaction was stirred at
5 °C and monitored via TLC, completion occurred after 30
mins. The solvents were evaporated and the residue dissolved in
CH Cl /petroleum ether (1:1) and filtered through a SiO plug.
Compound 7 was isolated as red crystals (40 mg, 90%).
1H NMR (400 MHz, CDCl
) δ 1.07–1.13 (m, 21 H, i-Pr-H),
.43 (s, 1H, C≡C–H), 3.41 (q, 2JH-P = 10.9 Hz, 2H, PCH
P),
.11–7.17 (m, 4H, H–Ar), 7.22–7.33 (m, 12H, H–Ar), 7.37–
.43 ppm (m, 4H, H–Ar). 13C NMR (100 MHz, CDCl
) δ 11.7,
2
2
A
2
2
f
2
3
(
1H NMR (400 MHz, CDCl
) δ 1.09–1.14 (m, 42H, i-Pr-H),
.36–3.43 (m, 4H, PCH P), 7.13–7.18 (m, 8H, H–Ar), 7.23–
.36 (m, 24H, H–Ar), 7.38–7.42 (m, 8H, H–Ar) ppm. 13C NMR
) δ 11.9, 19.1, 36.3 (t, 1JC-P = 21.9 Hz), 67.1,
70.8, 82.4, 83.3, 101.3, 108.7, 128.5–128.8 (m), 130.0 (s) 130.2
(s), 131.6 (t, 2JC-P = 5.7 Hz), 132.3 (t, 2JC-P = 5.1 Hz), 134.1 (t,
1JC-P = 21.8 Hz), 136.3 (t, 1JC-P = 23.1 Hz), 201.7 (br), 202.9 (br)
ppm. IR (ATR): 2148.8 (C≡C), 2079.5, 2034.5 (C=O), 2013.8
2
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
7
(
2
2
2
2
100 MHz, CDCl
3
3
2
7
7
1
1
2
3
(
2
C=O), 1987.8 cm–1 (C=O). UV-vis: (CH
85 nm (4.64), 385 nm (4.61), 409 nm (4.62).
2
Cl
2
, 25 °C) λmax (logε)
9.0, 36.4 (t, 1JC-P = 20.5 Hz), 62.9, 68.2, 69.7, 70.0, 80.6, 81.2,
01.1, 108.6, 128.5 (t, 3JC-P = 4.7 Hz), 128.7 (t, 3JC-P = 4.9 Hz),
4
130.0 (s) 130.3 (s), 131.5 (t, 2JC-P = 6.0 Hz), 132.4 (t, 2JC-P = 5.7
Hz), 133.9 (br), 136.7 (br), 201.8 (br), 203.3 ppm (br). IR (ATR):
Octacarbonylbis[μ-methylenebis(diphenylphosphine)-P:P’][μ -(3,4-
η:3,4-η;17,18-η:17,18-η)-(1,20-bis(triisopropylsilyl)-
,3,5,7,9,11,13,15,17,19-einacosadecayne)]tetracobalt 10. Mono-
TIPS hexatriyne Co complex 7 (100 mg, 157 μmol) was
dissolved in dry pyridine (12 mL). Cu(OAc) (225 mg, 0.142
mmol) was added under N and stirred overnight at 25 °C. The
pyridine was removed in vacuo. This yielded two products, which
1
3
1
296.6 (C≡C-H), 2131.8 (C≡C), 2033.0 (C=O), 2003.6 (C=O),
988.9 (C=O), 1963.9 cm–1 (C=O).
2
4
Octacarbonylbis[μ-methylenebis(diphenylphosphine)-P:P’][μ -(3,4-
2
η:3,4-η;9,10-η:9,10-η)-(1,12-bis(triisopropylsilyl)-1,3,5,7,9,11-
dodecahexayne)]tetracobalt 8a. Mono-TIPS acetylene Co complex 3
(
Cu(OAc) (86 mg, 473 μmol) was added under N and stirred
overnight at 25 °C. Pyridine was removed in vacuo. This yielded
two products, which were separated via column
were
(CH
isolated as a brown solid (74 mg, 75%).
) δ 1.09–1.12 (m, 42H, i-Pr-H),
3.40 (t, 2JH-P = 10.8 Hz, 4H, PCH
P), 7.10–7.40 (m, 40H, H–
Ar) ppm. 13C NMR (100 MHz, CDCl
) δ 11.6, 19.0, 36.4–36.6
m), 65.0, 65.6, 66.7, 69.9, 81.3, 83.3, 101.6, 108.5, 128.4–128.8
m), 130.0 (s) 130.3 (s), 131.5 (t, 2JC-P = 5.7 Hz), 132.2 (t, 2JC-P
separated
via
column
chromatography
40 mg, 47.3 μmol) was dissolved in dry pyridine (3 mL).
2
Cl
2
/petroleum ether 9:1). The lower R
f
product 10 was
2
2
1H NMR (400 MHz, CDCl
3
chromatography (CH
product 8a was isolated as a brown solid (32 mg, 80%).
1H NMR (400 MHz, CDCl
) δ 1.09–1.13 (m, 42H, i-Pr-H),
.40 (t, 2JH-P = 10.3 Hz, 2H, PCH
P), 7.11–7.18 (m, 8H, H–Ar),
2
2 f
Cl /petroleum ether 9:1). The higher R
2
3
(
(
5
3
=
3
7
2
.1 Hz), 133.6–134.2 (m), 136.2–136.3 (m), 200.5 (br), 203.1 (br)
.21–7.33 (m, 24H, H–Ar), 7.38–7.45 ppm (m, 8H, H–Ar). 13
C
ppm. IR (ATR): 2092.4 (C≡C), 2008.6 (C=O), 1991.9 (C=O),
970.9 cm–1 (C=O). UV-vis: (CH
Cl , 25 °C) λmax (logε) 285 nm
4.64), 385 nm (4.61), 409 nm (4.62).
1,12-Bis(triisopropylsilyl)-1,3,5,7,9,11-dodecahexayne
Tetracobalt masked hexayne 8a (50 mg, 29 μmol) was dissolved
in THF (3 mL), I (37.0 mg, 144 μmol) was added and the
reaction mixture was stirred for 3 h at 25 °C. After completion
was observed by TLC chromatography, the reaction mixture
2
was filtered through a SiO plug (petroleum ether). This yielded
NMR (100 MHz, CDCl
68.8 (br), 71.4 (br), 84.4, 87.6, 100.5, 108.2, 128.4–128.6 (m),
3
) δ 11.8, 19.1, 35.5 (t, 1JC-P = 21.9 Hz),
1
(
2
2
1
29.9 (s) 130.2 (s), 131.7 (t, 2JC-P = 5.7 Hz), 132.3 (t, 2JC-P = 5.1
Hz), 134.3 (t, 1JC-P = 21.8 Hz), 136.6 (t, 1JC-P = 23.1 Hz), 203.1
11.
ppm (br). IR (ATR): 2169.9 (C≡C), 2079.5 (C≡C), 2034.5
(
C=O), 2013.8 (C=O), 1989.5 cm–1 (C=O). Anal. Calcd for
2
C
5
(
88
H
86Co
4
O
8
P
4
Si
.31. UV-Vis: λmax (logε) 270 nm (4.65), 361 nm (4.52), 378 nm
4.53). Crystal Data: C88 Si , M = 1687.44 (Z = 1).
Triclinic P ¯1 ; a = 11.5368(11) Å, b = 12.2121(10) Å, c =
2
: C, 62.64; H, 5.14. Found: C, 62.57; H,
H
86Co
4
O
8
P
4
2
r
a pale yellow solution, which was confirmed to contain the
desired hexayne using UV-vis spectroscopy. The hexayne 11
was isolated as a yellow solid (7.3 mg, 55%). As in lit.33
1
=
7.3928(18) Å, αꢀ= 82.618(7)°, β = 84.683(8)°, γ = 81.577(7)°, V
3
2397.1(4) Å , R
1
= 0.0542, wR
2
= 0.1031 (I > 2σ(I)).
1H NMR (500 MHz, CDCl
) δ 1.09 (s, 42H) 13C NMR (125
) δ 11.8, 18.7, 61.2, 62.5, 62.8, 63.0, 87.9, 89.5
3
The lower
R
f
product
octacarbonylbis[μ-
MHz, CDCl
ppm.
3
4
methylenebis(diphenylphosphine)-P:P’][μ -1,2-η:1,2-η;11,12-
η:11,12-η)-(1,12-bis(triisopropylsilyl)-1,3,5,7,9,11-
dodecahexayne)]tetracobalt 8b was isolated as a red-brown
solid (3 mg, 7%)
1,16-Bis(triisopropylsilyl)-1,3,5,7,9,11,13,15-hexadecaoctayne 12.
Tetracobalt masked octayne 9 (25 mg, 14 μmol) was dissolved in
2
THF (10 mL), I (18 mg, 70 μmol) was added and the reaction
mixture was stirred for 3 h at 25 °C. After completion was
observed by TLC chromatography, the reaction mixture was
filtered through a SiO plug (petroleum ether). This yielded a
2
yellow solution, which was confirmed to contain the desired
1H NMR (400 MHz, CDCl
3.40 (t, 2JH-P = 10.3 Hz, 4H, PCH
7.21–7.33 (m, 24H, H–Ar), 7.38–7.45 ppm (m, 8H, H–Ar).
) δ 1.09–1.13 (m, 42H, i-Pr-H),
P), 7.11–7.18 (m, 8H, H–Ar),
3
2
Crystal Data:
88 4 8 4 2 r
C H87Co O P Si , M = 1688.45 (Z = 4).
Monoclinic P2
1
/n; a = 14.4102(5) Å, b = 16.2137(7) Å, c =
octayne using UV-vis spectroscopy. The octayne 12 was isolated
as a yellow solid (3.2 mg, 45%). As in lit.33
3
3
7.474(2) Å, β = 98.528(5)°, V = 8658.7(7) Å , R
1
= 0.0348, wR
2
=
0.0748 (I > 2σ(I)).
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The Journal of Organic Chemistry
Page 10 of 10
1H NMR (500 MHz, CDCl
) δ 1.08 (s, 42H). 13C NMR (125
3
MHz, CDCl ) δ 11.7, 18.7, 61.1, 62.3, 62.6, 63.1, 63.4, 63.4,
(12)
Bruce, M. I.; Smith, M. E.; Zaitseva, N. N.; Skelton, B. W.; White, A.
H. J. Organomet. Chem. 2003, 670, 170.
13) Diederich, F.; Rubin, Y. Angew. Chem. Int. Ed. Engl. 1992, 31, 1101.
14) Neenan, T. X.; Whitesides, G. M. J. Org. Chem. 1988, 53, 2489.
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
(
88.5, 89.4 ppm.
1,20-Bis(triisopropylsilyl)-1,3,5,7,9,11,13,15,17,19-
einacosadecayne 13. Tetracobalt masked decayne 10 (50 mg, (16) Diederich, F.; Kivala, M. Adv. Mater. 2010, 22, 803.
28.0 μmol) was dissolved in THF (10 mL), I (10 mg) was added
2
and the reaction mixture was stirred for 3 h at 25 °C. After
completion was observed by TLC chromatography, the reaction
(15) Chalifoux, W. A.; Tykwinski, R. R. Nat. Chem. 2010, 2, 967.
(
17) Schrettl, S.; Stefaniu, C.; Schwieger, C.; Pasche, G.; Oveisi, E.;
Fontana, Y.; i Morral, A. F.; Reguera, J.; Petraglia, R.; Corminboeuf,
C.; Brezesinski, G.; Frauenrath, H. Nat. Chem. 2014, 6, 468.
(18) Jevic, M.; Nielsen, M. B. Asian J. Org. Chem. 2015, 4, 286.
2
mixture was filtered through a SiO plug (petroleum ether). This (19) Matsuoka, R.; Sakamoto, R.; Hoshiko, K.; Sasaki, S.; Masunaga, H.;
yielded an orange solution, which was confirmed to contain the
desired decayne using UV-vis spectroscopy. The decayne 13
was isolated as an orange solid (6.0 mg, 39%). As in lit.33
Nagashio, K.; Nishihara, H. J. Am. Chem. Soc. 2017, 139, 3145.
20) Rubin, Y.; Knobler, C. B.; Diederich, F. J. Am. Chem. Soc. 1990, 12,
4966.
(
(
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
21) Haley, M. M.; Langsdorf, B. L. Chem. Commun. 1997, 1121.
1H NMR (500 MHz, CDCl
) δ 1.08 (s, 42H). 13C NMR (125 (22) Shvo, Y.; Hazum, E. J. Chem. Soc., Chem. Commun. 1974, 336.
3
MHz, CDCl
8.8, 89.3 ppm.
3
) δ 11.7, 18.7, 61.0, 62.2, 62.9, 63.2, 63.5, 63.8, (23) Cetini, G.; Gambino, O.; Rossetti, R.; Sappa, E. J. Organomet. Chem.
1967, 8, 149.
8
(
(
24) Allison, N. T.; Fritch, J. R.; Vollhardt, K. P. C.; Walborsky, E. C. J.
Am. Chem. Soc. 1983, 105, 1384.
25) Meyer, A.; Gorgues, A.; Le Floc’h, Y.; Pineau, Y.; Guillevic, J.; Le
Marouille, J. Y. Tetrahedron Lett. 1981, 22, 5181.
ASSOCIATED CONTENT
Supporting Information
(26) Hanson, B. E.; Mancini, J. S. Organometallics 1983, 2, 126.
The Supporting Information is available free of charge on the ACS (27) Lewis, J.; Lin, B.; Khan, M. S.; Al-Mandhary, M. R. A.; Raithby, P.
Publications website at DOI:
R. J. Organomet. Chem. 1994, 484, 161.
Supplementary NMR figures, electronic structure calculations, (28) Eisler, S.; Tykwinski, R. R. J. Am. Chem. Soc. 2000, 122, 10736.
(29) Luu, T.; Morisaki, Y.; Cunningham, N.; Tykwinski, R. R. J. Org.
additional
experiments,
characterization
spectra,
and
Chem. 2007, 72, 9622.
crystallography data (PDF)
DFT Cartesian coordinates (ZIP)
X-ray crystallographic data for compounds 2a, 2b, 4, 6, 8a, 8b, S6
(
30) Luu, T.; Elliott, E.; Slepkov, A. D.; Eisler, S.; McDonald, R.; Heg-
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AUTHOR INFORMATION
Corresponding Author
(
33) Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.;
Hegmann, F. A.; Tykwinski, R. R. J. Am. Chem. Soc. 2005, 127, 2666.
34) Tobe, Y.; Fujii, T.; Naemura, K. J. Org. Chem. 1994, 59, 1236.
35) Tobe, Y.; Fujii, T.; Matsumoto, H.; Naemura, K.; Achiba, Y.;
Wakabayashi, T. J. Am. Chem. Soc. 1996, 118, 2758.
* Email: harry.anderson@chem.ox.ac.uk
(
(
ACKNOWLEDGMENT
We thank the EPSRC and the European Research Council (Grant (36) Tobe, Y.; Umeda, R.; Iwasa, N.; Sonoda, M. Chem. Eur. J 2003, 9,
no. 320969) for funding, the University of Oxford Advanced
Research Computing Service (ARC) for support
DOI:10.5281/zenodo.22558) and Diamond Light Source for
beamtime (MT13629). P.G. acknowledges the receipt of an Early
Postdoc.Mobility fellowship from the Swiss National Science
Foundation. We thank Dr. A. L. Thompson for valuable discussion.
5549.
(
37) Movsisyan, L. D.; Franz, M.; Hampel, F.; Thompson, A. L.;
Tykwinski, R. R.; Anderson, H. L. J. Am. Chem. Soc. 2016, 138, 1366.
(
(38) Kohn, D. R.; Movsisyan, L. D.; Thompson, A. L.; Anderson, H. L.
Org. Lett. 2017, 19, 348.
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39) Zhao, Y.; Slepkov, A. D.; Akoto, C. O.; McDonald, R.; Hegmann, F.
A.; Tykwinski, R. R. Chem. Eur. J. 2005, 11, 321.
(40) Platonov, A. Y.; Evdokimov, A. N.; Kurzin, A. V.; Maiyorova, H. D.
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