Intramolecular Lewis acid-base pairs based on 4-ethynyl-2,6-lutidine

Article abstract of DOI:10.1039/c2dt30719g

The reaction of 4-ethynyl-2,6-lutidine, (2,6-Me₂)(4-HCC)C ₅H₂N (2), with B(C₆F₅)₃ afforded the zwitterion [(2,6-Me₂)(4-(C₆F ₅)₃BCC)C₅H

Full text of DOI:10.1039/c2dt30719g

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issue entitled:
Frustrated Lewis Pairs
Guest Editor: Douglas Stephan
Published in issue 30, 2012 of Dalton Transactions
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Communication
Hydrogen activation by 2-boryl-N,N-dialkylanilines: a revision of Piers’ ansa-aminoborane
Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo
Paper
Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene
derivatives
Philipp Feldhaus, Birgitta Schirmer, Birgit Wibbeling, Constantin G. Daniliuc, Roland Fröhlich,
Stefan Grimme, Gerald Kehr and Gerhard Erker
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Fixation of carbon dioxide and related small molecules by a bifunctional frustrated pyrazolylborane
Lewis pair
Eileen Theuergarten, Janin Schlösser, Danny Schlüns, Matthias Freytag, Constantin G. Daniliuc,
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PAPER
Intramolecular Lewis acid–base pairs based on 4-ethynyl-2,6-lutidine†
Daniel Winkelhaus, Beate Neumann, Hans-Georg Stammler and Norbert W. Mitzel*
Received 30th March 2012, Accepted 18th May 2012
DOI: 10.1039/c2dt30719g
The reaction of 4-ethynyl-2,6-lutidine, (2,6-Me2)(4-HCuC)C5H2N (2), with B(C6F5)3 afforded the
zwitterion [(2,6-Me2)(4-(C6F5)3BCuC)C5H2NH] (3) via a deprotonation pathway. By treatment of 2 with
the group 13 trialkyls AlMe3, AlEt3, GaMe3, GaEt3 and InMe3, metallation of the ethynyl group afforded
compounds 4–8 under extrusion of the corresponding alkane. The resulting products were characterised
by elemental analyses and NMR spectroscopy. Compounds 4 and 8 were crystallized from THF and were
yielded as monomers with coordinated THF molecules. The gallium compound 7 could be crystallised
from benzene and was afforded as coordination polymer. The structures of these three compounds
(4·THF, 7 and 8·2THF) were determined by single-crystal X-ray diffraction experiments. The aluminium
compounds 4 and 5 show redistribution reaction of their substituents.
Introduction
The field of Frustrated Lewis Pairs (FLP) has developed tremen-
dously over the last years.1,2 Based on the steric overcrowding of
the Lewis acid and base sites precluding the formation of a clas-
sical Lewis acid–base adduct the concept aims at conserving an
unquenched reaction potential. FLP systems have been demon-
strated to be capable of activating dihydrogen3–7 and carbon
dioxide8–10 and other small molecules.11–13
Scheme 1 Reaction of FLPs with alkynes (Z = Lewis base phosphorus
The first systems under focus during development of this
chemistry have been B(C6F5)3 as Lewis acid and bulky phos-
phines as Lewis bases, rapidly followed by sterically encumbered
amines. Inspired by the early work of Brown et al.14 who
reported 2,6-dimethylpyridine (lutidine) to be unable to form a
Lewis adduct with BMe3, Stephan and co-workers discovered
the ability of a mixture of lutidine and B(C6F5)3 to activate H2.15
Recently this work has been extended to several substituted pyri-
dine derivatives.16
The capability of heterolytic activation of H2 by FLPs has
been applied for metal-free hydrogenation catalysis of imines,
nitriles and enamines,17–22 THF,23 terminal olefins24,25 and
dienes.26 These reactivity studies were expanded to further
systems including alkynes. It has been shown that depending on
the constitution of the reactants two different reaction pathways
are possible: either a deprotonation of the terminal C–H bond
or nitrogen).
who have investigated the metallation of terminal alkynes
mainly by aluminium and gallium hydrides. These investigations
have provided detailed insight into the bonding situation and the
different bridging modes of the alkynyl group.29–31 We have
recently contributed to this field with dialkyl-aluminium,
-gallium and -indium compounds containing rigid 1,8-diethynyl-
anthracene backbones which behave as poly-Lewis acids.32
Herein we present the reactions of an alkyne substituted luti-
dine tris(pentafluorophenyl)borane and Group 13 alkyls of alu-
minium, gallium and indium. We demonstrate that in all cases
deprotonation of the terminal C–H bonds and subsequent metal-
lation occurs.
(product A) or
a
1,2-addition reaction (product B)
Results and discussion
(Scheme 1).27,28
The direct deprotonation of alkynes by main group organo-
metallics has seen a rapid progress over the last few years.
Important contributions to this chemistry stem from Uhl et al.
Synthesis of ethynyl substituted lutidine
The lutidine derivative (2,6-Me2)(4-Me3SiCuC)C5H2N (1) was
synthesized in a Sonogashira coupling reaction33–35 by the reac-
tion of the iodine substituted lutidine [(2,6-Me2)(4-I)C5H2N]36
with trimethylsilylacetylene in triethylamine at 75 °C in the pres-
ence of a palladium catalyst (Scheme 2). The reaction was
finished after three hours and the product could be obtained after
Universität Bielefeld, Fakultät für Chemie, Anorganische Chemie und
Strukturchemie, Universitätsstraße 25, 33615 Bielefeld, Germany.
E-mail: mitzel@uni-bielefeld.de; Fax: (+49) (0)521 106-6182
†CCDC 869559, 869560, 869561, 869562. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt30719g
1
workup as an orange oil in 93% yield. The H NMR spectrum
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9143–9150 | 9143

Scheme 2 Synthesis of (2,6-Me2)(4-Me3SiCuC)C5H2N (1) and (2,6-
Me2)(4-HCuC)C5H2N (2); (a) (PPh3)PdCl2, CuI, NEt3, 75 °C (b)
K2CO3, MeOH.
Scheme 3 Reaction of (2,6-Me2)(4-HCuC)C5H2N (2) with B(C6F5)3.
The C(1)–C(2) triple bond of the ethynyl function has a
length of 1.189(2) Å. This is nearly the same as in 4-ethynylpyr-
idine with a value of 1.188 Å38 and slightly shorter than the stan-
dard CuC triple bond length with 1.20 Å.39 The C(2)–C(3)
single bond at 1.439(2) Å is slightly longer this bond length in
4-ethynylpyridine with a value of 1.428 Å.40
The C(6)–N(1)–C(5) angle within in the aromatic ring has a
value of 118.9(1)°. Due to the higher electron density in the aro-
matic system by the substitution of the methyl group this angle
is larger than in pyridine (116.6°)40 and 4-ethynylpyridine
(116.9°). Consequently, the N(1)–C(5) and N(1)–C(6) bond
length are slightly longer [both at 1.346(2) Å] than in pyridine
(1.338 Å) and 4-ethynylpyridine (1.325 Å).
Combination of 2 with B(C6F5)3
By heating a mixture of (2,6-Me2)(4-HCuC)C5H2N (2) and
B(C6F5)3 to 60 °C overnight the ethynyl function was deproto-
nated and the zwitterion [(2,6-Me2)(4-(C6F5)3BCuC)C5H2NH]
(3) was obtained in 50% yield as colourless solid (Scheme 3).
Compound 3 was characterized by elemental analysis and by
1H, 11B, 13C and 19F NMR spectroscopy. The 1H NMR spectrum
shows a broad signal at δ = 13.49 ppm which can be assigned to
the NH fragment. The product 3 gives rise to an 11B NMR
signal at δ = −22.8 ppm. In the 19F NMR spectrum the fluorine
signals are observed at δ = −134.4 (o-), −165.9 (p-) and −169.8
(m-C6F5). The 11B and 19F NMR data indicate the presence of a
tetra-coordinate boron atom.
This reaction has shown that deprotonation of the terminal
alkyne C–H bond by the Lewis base (lutidine fragment) and
B(C6F5)3 affords the zwitterion [(2,6-Me2)(4-(C6F5)3BCuC)
C5H2NH] (3). The alternative 1,2-addition of the boron and the
nitrogen components to the alkyne function to a third molecule
was not observed.
Fig. 1 Molecular structure of 2 in the crystal. Hydrogen atoms are
omitted for clarity. Bond lengths [Å] and angles [°]: N(1)–C(6) 1.346
(2), N(1)–C(5) 1.346(2), C(1)–C(2) 1.189(2), C(2)–C(3) 1.439(2), C(3)–
C(4) 1.395(2), C(3)–C(7) 1.398(2), C(4)–C(5) 1.394(2), C(5)–C(8)
1.502(2), C(6)–C(7) 1.393(2), C(6)–C(9) 1.508(2), C(6)–N(1)–C(5)
118.9(1), C(1)–C(2)–C(3) 179.7(7), C(4)–C(3)–C(7) 118.0(1), C(4)–
C(3)–C(2) 120.9(1), C(7)–C(3)–C(2) 121.0(1), C(5)–C(4)–C(3)
119.5(1), N(1)–C(5)–C(4) 122.1(1), N(1)–C(5)–C(8) 116.8(1), C(4)–
C(5)–C(8) 121.2(1), N(1)–C(6)–C(7) 122.3(1), N(1)–C(6)–C(9)
116.3(1), C(7)–C(6)–C(9) 121.5(1), C(6)–C(7)–C(3) 119.3(1).
of compound 1 shows singlets at 6.84, 2.30 and 0.23 ppm
attributable to the CH fragment, the methyl group of the lutidine
unit and the trimethylsilyl group, respectively.
The de-protected literature known (2,6-Me2)(4-HCuC)
C5H2N (2)37 was synthesized using an alternative reaction
pathway. The trimethylsilyl protecting group of compound 1 was
cleaved in methanol over two hours with potassium carbonate
(Scheme 2). Removal of the solvent resulted in the crude
product of the ethynyl substituted lutidine (2). Subsequent subli-
mation under vacuum at 60 °C afforded colourless crystals of
compound 2 in 82% yield.
Reactions of 2 with trialkyl-aluminium, -gallium and -indium
The reactions of (2,6-Me2)(4-HCuC)C5H2N (2) with the trialk-
yls of aluminium, gallium and indium in toluene or benzene led
to metallation of the ethynyl function under extrusion of the cor-
responding alkane (methane or ethane). Depending on the
elements the initiation of the reaction required different con-
ditions from ambient temperatures (Al) to heating to 75 °C (In).
Products 4–8 were obtained in 61–93% yield as colourless
powders (Scheme 4).
Compound 2 was characterized by elemental analysis and
1
NMR spectroscopy (1H and 13C). The H NMR spectrum is in
agreement with the literature. In the 13C NMR spectrum reson-
ances of the carbon atoms of the ethynyl fragment are observed
at δ = 82.1 and 80.6 ppm, respectively.
For comparison with the metallated compounds we deter-
mined the crystal structure of 2. Compound 2 crystallizes in the
monoclinic space group P21/c and its structure is displayed in
Fig. 1; selected structural parameters are listed in the caption.
The identities of the metallated compounds were proven by
1
elemental analyses, H and 13C NMR spectroscopy and single
9144 | Dalton Trans., 2012, 41, 9143–9150
This journal is © The Royal Society of Chemistry 2012

Scheme 4 General procedures for the metallation of (2,6-Me2)-
(4-HCuC)C5H2N (2).
Fig. 2 Molecular structure of 4·THF in the crystal. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and angles [°]: Al(1)–
O(1) 1.893(1), Al(1)–C(10) 1.965(2), Al(1)–C(11) 1.966(2), Al(1)–C(1)
1.966(2), C(1)–C(2) 1.213(2), C(2)–C(3) 1.442(2), N(1)–C(5) 1.348(2),
N(1)–C(6) 1.349(2), O(1)–Al(1)–C(10) 105.9(1), O(1)–Al(1)–C(11)
104.4(1), C(10)–Al(1)–C(11) 116.8(1), O(1)–Al(1)–C(1) 99.7(1),
C(10)–Al(1)–C(1) 113.1(1), C(11)–Al(1)–C(1) 114.5(1), C(5)–N(1)–
C(6) 117.7(1), C(2)–C(1)–Al(1) 178.4(1), C(1)–C(2)–C(3) 178.6(2),
C(7)–C(3)–C(4) 117.4(1), C(7)–C(3)–C(2) 121.8(1), C(4)–C(3)–C(2)
120.8(1).
Scheme 5 Redistribution reaction of compound 4.
crystal X-ray diffraction (for 4, 7 and 8). Due to the fact that
most compounds are virtually insoluble in non-polar solvents,
NMR spectroscopy experiments had to be carried out in THF-d8
solutions.
The NMR spectra for all compounds, 4–8, show similar fea-
tures. In the H NMR spectra the signals for the CH fragments
Molecular structures
1
of the lutidine system of compound 4–8 are observed between δ
= 6.91 and 6.86 ppm with the AlMe2 compound (4) defining the
lower frequency and the InMe2 compound (8) the higher fre-
quency extreme. The lutidine methyl resonances are detected at
δ = 2.39 (4), 2.35 (5), 2.36 (6), 2.37 (7) and 2.35 ppm (8),
respectively. Signals for the metal bound methyl and ethyl sub-
stituents are observed at δ = −0.81 (4), 1.03 and −0.22 (5),
−0.34 (6), 1.14 and 0.38 (7) and −0.35 ppm (8), respectively.
Due to solubility issues, compounds 4 and 8 could only be crys-
tallized from concentrated THF solutions in the form of adducts
4·THF and 8·2THF, whereas crystals of 7 were obtained from
concentrated solutions in the non-polar solvent benzene. The
molecular structures of compound 4·THF, 7 and 8·2THF were
determined by single-crystal X-ray crystallography. They are dis-
played in Fig. 2–4; selected structural parameters are listed in the
captions.
The structures of the adducts 4·THF and 8·2THF contain four-
and five-coordinate Al and In atoms with one and two molecules
THF coordinated to the Lewis acidic earth metal atom site,
respectively. Due to the larger covalent radius the indium atom is
capable of coordinating two THF molecules. By contrast, in the
absence of a donor solvent the electron deficient gallium atoms
in compound (7)∞ are saturated by coordination to the nitrogen
atom of another molecule. This leads to formation of a coordi-
nation polymer.
In compound 4·THF the aluminium atom exhibits a distorted
tetrahedral coordination sphere with bond angles between 99.7
(1) and 116.8(1)°. The Al(1)–O(1) bond [1.893(1) Å] is signifi-
cantly shorter than in the comparable four-coordinate aluminium
THF adduct (Me3Si)3CAlMe2·THF [1.969(2) Å],41 and also
shorter than in Me3Al–OMe2 [2.014(25) Å]42 and Me3Al–O
(CH2)3 [2.03(4) Å],43 both determined for free molecules in the
gas phase, for which longer values are typically expected.
In compound (7)∞ the distortion of the tetrahedral coordi-
nation sphere of the metal atom [Ga(1)] is more pronounced than
1
Additional signals to the expected were observed in the H
NMR spectrum of the methyl substituted aluminium compound
4 at δ = 6.92, 2.40 ppm and −0.80 ppm with a relative ratio of
0.8 : 1.0 to the signals of 4. This matches the constitution of a
species 4b, which is most likely the result of a redistribution
reaction of the ethynyl and methyl functionalities at the alu-
minium atom (Scheme 5). The observation of a singlet at δ =
−0.93 ppm assignable to trimethylaluminium confirms this
assignment.
The 13C NMR spectrum of the AlMe compound also reveals
three sets of signals attributable to the species 4, 4b and AlMe3.
Due to the broad resonances in the 27Al NMR spectrum, individ-
ual signals for these species cannot be resolved.
Analogous ligand scrambling is also observed for the ethyl
substituted aluminium compound 5. However, in this case the
signals of both species overlap in the spectrum which prevents
quantifying the relative abundances. These redistribution reac-
tions distinguish the aluminium compounds 4 and 5 from the
gallium and indium compounds 6–8.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9143–9150 | 9145

Fig. 3 Structure of a part of the coordination polymer of (7)∞ in the
crystal. Hydrogen atoms are omitted for clarity. Selected bond lengths
[Å] and angles [°]: Ga(1)–C(1) 1.992(3), Ga(1)–C(10) 1.995(3), Ga(1)–
C(12) 1.988(3), Ga(1)–N(2) 2.137(2), Ga(2)–C(23) 1.977(3), Ga(2)–
C(25) 1.978(3), Ga(2)–C(14) 1.986(3), Ga(2)–N(1) 2.153(2); N(1)–C(5)
1.367(4), N(1)–C(6) 1.368(4), C(1)–C(2) 1.220(4), C(2)–C(3) 1.433(4),
C(14)–C(15) 1.208(4), C(15)–C(16) 1.442(4), C(12)–Ga(1)–C(1) 99.7
(1), C(12)–Ga(1)–C(10) 124.2(1), C(1)–Ga(1)–C(10) 113.3(1), C(12)–
Ga(1)–N(2) 112.5(1), C(1)–Ga(1)–N(2) 105.3(1), C(10)–Ga(1)–N(2)
100.9(1), C(5)–N(1)–C(6) 117.1(3), C(19)–N(2)–Ga(1) 120.8(2),
C(18)–N(2)–Ga(1) 121.3(2), C(2)–C(1)–Ga(1) 172.6(3), C(1)–C(2)–
C(3) 175.1(3), C(15)–C(14)–Ga(2) 174.1(3), C(14)–C(15)–C(16)
176.2(3).
Fig. 4 Molecular structure of 8·2THF in the crystal. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and angles [°]: In(1)–
C(11) 2.152(2), In(1)–C(10) 2.154(2), In(1)–C(1) 2.166(2), In(1)–O(1)
2.456(1), In(1)–O(2) 2.459(1), C(1)–C(2) 1.207(3), C(2)–C(3) 1.442(3),
C(11)–In(1)–C(10) 129.8(1), C(11)–In(1)–C(1) 112.4(1), C(10)–In(1)–
C(1) 117.7(1), C(11)–In(1)–O(1) 91.1(1), C(10)–In(1)–O(1) 88.3(1),
C(1)–In(1)–O(1) 89.2(1), C(11)–In(1)–O(2) 91.6(1), C(10)–In(1)–O(2)
91.5(1), C(1)–In(1)–O(2) 87.9(1), O(1)–In(1)–O(2) 176.6(1), C(6)–
N(1)–C(5) 117.9(2), C(2)–C(1)–In(1) 176.3(2), C(1)–C(2)–C(3)
179.3(2).
involving the ethynyl group, while the one between the two
methyl groups is larger [129.8(1)°]. The axial angle O(1)–In(1)–
O(2) is almost linear [176.6(1)°]. The bond In(1)–C(1) to the
ethynyl group [2.166(2) Å] is slightly longer than the two other
In–C bonds [In(1)–C(11) 2.152(2), In(1)–C(10) 2.154(2) Å]. By
comparison, the In–C bond lengths in the three polymorphs of
InMe3 range from 2.121–2.179 Å.49 The axial In–O bonds in
8·2THF at 2.456(1) [In(1)–O(1)] and 2.459(1) Å [In(1)–O(2)]
are longer than the equatorial indium carbon distances and
remarkably longer than in the related compound In
(CuCCF3)3·2THF [2.300(8) Å].50
Both, the angles CuC–M and C–CuC, are almost linear in
compounds 4·THF, (7)∞ and 8·2THF. In the structure of 4·THF,
these angles adopt values of 178.4(1) and 178.6(2)°, respect-
ively. In compound (7)∞ the deviation from the ideal 180° is
slightly larger with angles of 172.6(3) [174.1(3)]° and 175.1(3)
[176.2(3)]°, respectively (values for the second molecule in
squared brackets). In the indium compound 8·2THF the angles
CuC–M and C–CuC are 176.3(2)° and 179.3(2)°, i.e. there are
only slight deviations from linearity.
Due to the similar covalent radii of Al and Ga the bond
lengths Al(1)–C(1), Ga(1)–C(1) and Ga(2)–C(14) in compounds
4·THF and (7)∞ [1.966(2) and 1.992(3)/1.986(3) Å],
show nearly the same values. By contrast, the In(1)–C(1) bond
[2.166(2) Å] to the heavier homologue in 8·2THF is distinctly
larger than the latter.
The CuC triple bond lengths are 1.213(2) in 4·THF, 1.220(4)
and 1.208(4) in (7)∞, and 1.207(3) Å in 8·2THF. This is in each
case longer than in the non-metallated species 2 [1.189(2) Å].
in 4·THF. The bond angles at Ga(1) in (7)∞ lie between 99.7(1)°
for C(12)–Ga(1)–C(1) and 124.2(1)° for C(12)–Ga(1)–C(10).
Both, the Ga(1)–N(2) bond at 2.137(2) and the Ga(2)–N(1) bond
at 2.153(2) Å are slightly longer than the gallium nitrogen bond
in the adduct triethylgallane–diethylgallium-bis(2-pyridyl-
methyl)amide [2.09(1) Å]44 and distinctly longer than in 2,6-
dimesityl pyridine·GaCl3 [2.043(2) Å].45 However, the Ga–N
bond is 0.063 Å shorter than in the gas phase structure of the
adduct Me3Ga–NMe3 [2.20(3) Å].46 So far adduct formation of
lutidine with gallium alkyls have not been reported in the litera-
ture, and as stated in the introduction this was also not observed
between lutidine and BMe3 in Brown’s classical experiments.
The two Ga–C bonds of the GaEt2 fragment in (7)∞ have
values of 1.995(3) and 1.988(3) Å for the Ga(1)–C(10) and
Ga(1)–C(12) bond, respectively. The values for the second
molecule are 1.977(3) and 1.978(3) Å for Ga(2)–C(23) and
Ga(2)–C(25), respectively. These values are all in the range of
the Ga–C bond length in GaEt3 [1.966(3)–1.996(3) Å].47
As mentioned above, the indium atom in compound 8·2THF
is coordinated by two THF molecules and exhibits a trigonal
bipyramidal coordination sphere. Three carbon atoms are in the
equatorial positions, while the THF oxygen atoms are in the
axial positions. A related system containing an indium atom
coordinated by three carbon and two oxygen atoms is found in
In(CuCCF3)3·2THF.48 There are two smaller equatorial C–In–C
bond angles in 8·2THF with values of 112.4(1) and 117.7(1)°
9146 | Dalton Trans., 2012, 41, 9143–9150
This journal is © The Royal Society of Chemistry 2012

By contrast, the metallation has a little effect on the length of the
single bond C–C(uC) to the ethynyl fragment; the values of
1.442(2) Å in 4·THF, 1.433(4) and 1.442(4) Å in (7)∞ and 1.442
(3) Å in 8·2THF are very close to the value in 2 [1.439(2) Å].
Metallation has also little impact on the structure of the lutidine
system. For instance, the C(5)–N(1)–C(6) angle along the series
4·THF, (7)∞ and 8·2THF is narrowed by 1.2° [117.7(1)° in
4·THF], 1.8° [117.1(3)° in (7)∞] and 1.0° [117.9(2)° in 8·2THF]
in comparison with compound 2 [118.9(1)°].
CH3), 0.23 ppm (s, 9H, Si(CH3)3); 13C{1H} NMR (125.7 MHz,
C6D6, 298 K): δ = 158.3 (CCH3), 131.4 (CCCSi(CH3)3), 122.3
(CH), 103.9 (CCSi(CH3)3), 97.7 (CSi(CH3)3), 24.4 (CCH3),
−0.15 ppm (Si(CH3)3).
(2,6-Me2)(4-HCuC)C5H2N (2). To a solution of (2,6-Me2)-
(4-Me3SiCuC)C5H2N (1) (2.76 g, 13.6 mmol) in 130 mL
methanol was added K2CO3 (14.4 g, 104 mmol, 7.7 eq). The
reaction mixture stirred for 2 h. After 100 mL water and 100 mL
pentane was added the layers were separated and the organic
layer was extracted with pentane (four times 100 mL). The com-
bined organic extracts were dried over MgSO4, filtered and the
solvent was removed in vacuum to yield a light grey solid. Subli-
mation in vacuum (10−2 mbar) at an oil bath temperature of
60 °C produced the product as a colourless, crystalline solid.
Conclusions
The reaction of the alkyne substituted lutidine (2,6-Me2)-
(4-HCuC)C5H2N (2) with the strong Lewis acid B(C6F5)3 led
to the deprotonation of the alkyne C–H bond and afforded the
zwitterion [(2,6-Me2)(4-(C6F5)3BCuC)C5H2NH] (3). Likewise,
deprotonation of the alkyne fragment was observed by treatment
of 2 with Group 13 alkyls of aluminium, gallium and indium.
Subsequent metallation under alkane elimination afforded the
products 4–8. The aluminium compounds 4 and 5 undergo
ligand scrambling reactions and thus cannot be prevented from
dismutation in solution.
The Lewis acidic compounds are monomeric in THF solution
and crystallize as THF adducts from this solvent; they are likely
to form coordination polymers in the absence of donor solvents,
as was demonstrated for the gallium compound 7. The adduct
formation of lutidine to an organogallium unit is surprising in
the light of Brown’s classical experiments, which is interpreted
as BMe3 being sterically too crowded to form an adduct with
lutidine.
1
Yield: 1.47 g (82%). H NMR (500.1 MHz, C6D6, 298 K): δ =
6.77 (s, 2H, CH), 2.73 (s, 1H, CuCH), 2.27 ppm (s, 6H, CH3);
13C{1H} NMR (125.7 MHz, C6D6, 298 K): δ = 158.4 (CCH3),
130.5 (CCuCH), 122.5 (CH), 82.1 (CuCH), 80.6 (CuCH),
24.3 ppm (CH3). C9H9N (131.17): calcd C 82.41, H 6.92,
N 10.68; found C 82.43, H 7.05, N 10.74.
[(2,6-Me2)(4-(C6F5)3BCuC)C5H2NH] (3). To a solution of
(2,6-Me2)(4-HCuC)C5H2N (2) (20 mg, 0.15 mmol) in 3 mL
toluene was added B(C6F5)3 (79 mg, 0.15 mmol). The colourless
solution was stirred at 60 °C for 12 h and the solvent was
removed in vacuum. The solid was washed with hexane and
dried in vacuum obtaining the product as a colourless solid.
1
Yield: 49 mg (50%). H NMR (500.1 MHz, THF-d8, 298 K):
δ = 13.49 (br, 1H, NH), 7.46 (s, 2H, CH), 2.62 ppm (s, 6H,
CH3); 11B{1H} NMR (160.4 MHz, THF-d8, 298 K): δ =
−22.8 ppm (ν1/2 = 20 Hz); 13C{1H} NMR (125.7 MHz, THF-d8,
1
298 K): δ = 153.0 (CCH3), 149.4 (dm, JCF = 241.5 Hz, C6F5),
1
Experimental part
146.9(CuCB), 139.6 (dm, JCF = 245.5 Hz, C6F5), 137.7 (dm,
1JCF = 247.8 Hz, C6F5), 127.1 (CH), 93.1 (CuCB), 19.2 ppm
(CCH3), Quaternary carbons of C6F5 and ethynyl moiety
directed bonded to the boron atom were not observed; 19F NMR
General methods
All manipulations, except for (2,6-Me2)(4-HCuC)C5H2N (2),
were performed under a rigorously dry inert atmosphere of argon
using standard Schlenk and glove-box techniques. Benzene and
toluene were dried with potassium, hexane with LiAlH4 before
being employed in reactions. C6D6 and THF-d8 were dried with
Na/K alloy and degassed. (2,6-Me2)(4-SnMe3)C5H2N and (2,6-
Me2)(4-I)C5H2N were synthesized according to literature pro-
cedures. NMR measurements were undertaken with Bruker DRX
500 and Bruker Avance 500 instruments. NMR chemical shifts
were referenced to the residual peaks of the protons of the used
solvents (1H, 13C) or externally (11B, BF3·OEt2; 19F, CFCl3;
27Al, Al(NO3)3 in H2O). Elemental analyses were performed by
using a CHNS elemental analyzer HEKAtech EURO EA.
3
(470.5 MHz, THF-d8, 298 K): δ = −169.8 (tm, JFF = 20.0 Hz,
3
4F, m-C6F5), −165.9 (t, JFF
= 20.1 Hz, 2F, p-C6F5),
−134.4 ppm (m, 4F, o-C6F5). C27H9BF15N (643.15): calcd
C 50.42, H 1.41, N 2.18; found C 51.31, H 2.50, N 1.98.
[(2,6-Dimethylpyridin-4-yl)ethynyl]dimethylaluminium·THF
(4) and bis[(2,6-dimethylpyridin-4-yl)ethynyl]methylaluminum
(4b). To a solution of (2,6-Me2)(4-HCuC)C5H2N (2) (50 mg,
0.38 mmol) in 2 mL benzene was added trimethylaluminium
(27 mg, 0.38 mmol). The reaction was stirred at ambient temp-
erature over night. After the solvent was removed in vacuum the
product was obtained as a colourless powder. Crystals suitable
for X-ray analysis were obtained from a THF solution at −35 °C
1
(2,6-Me2)(4-Me3SiCuC)C5H2N (1). To a solution of (2,6-
Me2)(4-I)C5H2N (3.42 g, 14.6 mmol) in triethylamine was
added copper(I) iodide (34.4 mg, 0.18 mmol, 0.012 eq), bis(tri-
phenylphosphine)palladium(II) dichloride (181 mg, 0.25 mmol,
0.017 eq) and trimethylsilylacetylene (2.01 g, 20.4 mmol, 1.4
eq). The suspension was warmed to 75 °C for 3 h while it turned
black. After the reaction mixture was cooled to room temperature
the solid residue was filtered off and the solvent was removed in
after several days. Yield: 85 mg (86%). H NMR (500.1 MHz,
THF-d8, 298 K): species 4 (38%): δ = 6.91 (s, 2H, CH), 2.39 (s,
6H, CH3), −0.81 ppm (s, 6H, Al(CH3)2); species 4b (31%): δ =
6.92 (s, 2H, CH), 2.40 (s, 6H, CH3), −0.80 ppm (s, 6H, AlCH)3;
residual peaks: δ = 3.65 (m, 14H, O(CH2CH2)2), 1.81 (m, 14H,
O(CH2CH2)2), −0.93 ppm (s, 9H, Al(CH3)3) (31%); 13C{1H}
NMR (125.7 MHz, THF-d8, 298 K): δ = 158.3 and 158.2
(CCH3), 134.9 and 134.7 (CuCAl), 122.7 and 122.6 (CH),
113.2 (br, CuCAl), 104.7 and 103.2(CCuC), 68.4
(O(CH2CH2)2), 26.5 (O(CH2CH2)2), 24.5 (two signals,
1
vacuum to give an orange oil. Yield: 2.76 g (93%). H NMR
(500.1 MHz, C6D6, 298 K): δ = 6.84 (s, 2H, CH), 2.30 (s, 6H,
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9143–9150 | 9147

2× CCH3), −9.4 ppm (br, AlCH3); 27Al NMR (130.3 MHz,
THF-d8, 298 K): δ = 182.5 (br), 151.7 ppm (br); 1H NMR
(500.1 MHz, C6D6, 298 K): species 4 (60%): δ = 7.05 (s, 2H,
CH), 2.37 (s, 6H, CH3), −0.29 ppm (s, 6H, Al(CH3)2); species
4b (20%): δ = 7.04 (s, 2H, CH), 2.36 (s, 6H, CH3), −0.16 ppm
(s, 6H, AlCH)3; residual peaks: δ = 3.49 (m, 14H, O
(CH2CH2)2), 0.99 (m, 14H, O(CH2CH2)2), −0.39 ppm (s, 9H,
Al(CH3)3) (20%); 13C{1H} NMR (125.7 MHz, C6D6, 298 K):
δ = 158.2 and 158.1 (2× CCH3), 134.1 and 133.4 (2× CuCAl),
122.7 and 122.6 (2× CH), 112.3 and 108.8 (br, 2× CuCAl),
105.5 and 105.3 (2× CCuC), 70.9(O(CH2CH2)2), 24.9
(O(CH2CH2)2), 24.4 (two signals, 2× CCH3), −8.4 to −9.6 ppm
(br m, AlCH3); 27Al NMR (130.3 MHz, C6D6, 298 K): δ =
187.2(br), 162.0 ppm (br). C11H14AlN·C4H8O (259.32): calcd C
69.47, H 8.55, N 5.40; found C 69.01, H 8.69, N 5.31.
298 K): δ = 177.1 (br), 156.7 ppm (br). C13H18AlN (215.27):
calcd C 72.53, H 8.43, N 6.51; found C 70.09, H 8.73, N 5.82.
[(2,6-Dimethylpyridin-4-yl)ethynyl]dimethylgallium (6). To a
solution of (2,6-Me2)(4-HCuC)C5H2N (2) (50 mg, 0.38 mmol)
in
2 mL benzene was added trimethylgallium (43 mg,
0.38 mmol). The reaction was warmed to 75 °C for 3 d. After
the solvent was removed in vacuum the product was obtained as
a colourless powder. Yield: 59 mg (68%). 1H NMR (500.1 MHz,
THF-d8, 298 K): δ = 6.89 (s, 2H, CH), 2.36 (s, 6H, CH3),
−0.34 ppm (s, 6H, Ga(CH3)2); 13C{1H} NMR (125.7 MHz,
THF-d8, 298 K): δ = 158.3 (CCH3), 134.5 (CuCGa), 122.6
(CH), 112.6 (CuCGa), 104.6 (CCuC), 24.5 (CCH3),
−6.02 ppm (Ga(CH3)2. C11H14GaN (229.96): calcd C 57.45, H
6.14, N 6.09; found C 53.80, H 6.06, N 5.50.
[(2,6-Dimethylpyridin-4-yl)ethynyl]diethylgallium (7). To
a
[(2,6-Dimethylpyridin-4-yl)ethynyl]diethylaluminum (5). To a
solution of (2,6-Me2)(4-HCuC)C5H2N (2) (50 mg, 0.38 mmol)
in 2 mL benzene was added triethylaluminium (43 mg,
0.38 mmol). The reaction was stirred at ambient temperature
over night. After the solvent was removed in vacuum the product
solution of (2,6-Me2)(4-HCuC)C5H2N (2) (50 mg, 0.38 mmol)
in 2 mL benzene was added triethylgallium (59 mg, 0.38 mmol).
The reaction was warmed to 75 °C for 3 d. After the solvent was
removed in vacuum the product was obtained as a colourless
powder. Crystals suitable for X-ray analysis were obtained from
a concentrated benzene solution after several days. Yield: 85 mg
1
was obtained as a brown solid. Yield: 50 mg (61%). H NMR
1
(500.1 MHz, THF-d8, 298 K): δ = 6.89 (s, 2H, CH), 2.36 (s, 6H,
CH3), 1.04 (t, 2JHH = 8.14 Hz, CH2), −0.14 ppm (q, 2JHH = 8.14
Hz, CH2CH3); 13C{1H} NMR (125.7 MHz, THF-d8, 298 K):
(87%). H NMR (500.1 MHz, THF-d8, 298 K): δ = 6.89 (s, 2H,
2
CH), 2.37 (s, 6H, CH3), 1.14 (t, JHH = 8.06 Hz, CH2),
2
0.37 ppm (q, JHH = 8.06 Hz, CH2CH3); 13C{1H} NMR
δ
=
158.3 (CCH3), 134.6(CuCAl), 122.6 (CH), 111.2
(125.7 MHz, THF-d8, 298 K): δ = 158.3 (CCH3), 134.5
(CuCGa), 122.7 (CH), 110.9 (CuCGa), 106.0 (CCuC), 24.5
(CCH3),10.7 (GaCH2), 4.51 ppm (Ga(CH2CH3)2. C13H18GaN
(CuCAl), 105.7 (CCuC), 24.4 (CCH3), 9.7 (Al(CH2CH3)2),
−0.2 ppm (br, Al(CH2CH3)2; 27Al NMR (130.3 MHz, C6D6,
Table 1 Crystallographic data for compounds 2, 4·THF, (7)∞ and 8·2THF
2
4·THF
(7)∞
8·2THF
Formula
Mr
T [K]
C9H9N
131.17
100(2)
C11H14AlN·C4H8O
259.32
100(2)
C16H21GaN
297.06
100(2)
C11H14InN·C8H16O2
419.26
100(2)
Crystal size [mm]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
0.30 × 0.30 × 0.16
Monoclinic
P21/c
8.7308(5)
12.8695(8)
7.3121(3)
90
113.945(3)
90
750.88(7)
4
0.30 × 0.29 × 0.20
Triclinic
P1
0.14 × 0.12 × 0.06
Monoclinic
C2/c
24.6455(10)
15.5940(7)
17.8708(8)
90
119.238(2)
90
5993.1(5)
16
0.30 × 0.28 × 0.18
Monoclinic
P21/c
8.6698(2)
9.1564(2)
25.2090(7)
90
93.3099(13)
90
1997.86(8)
4
ˉ
7.9330(5)
8.1336(5)
12.4228(8)
72.661(4)
84.982(3)
83.054(4)
758.41(8)
2
α [°]
β [°]
γ [°]
V [Å3]
Z
ρcalcd [g cm−3
]
1.16
0.069
280
1.136
0.123
280
1.317
1.820
2480
1.1394
1.192
864
μ [mm−1
F(000)
]
Θ Range [°]
Index range
3.0 to 27.5
−11 ≤ h ≤ 11
−14 ≤ k ≤ 16
−9 ≤ l ≤ 9
6299
1722
1354
3.0 to 27.5
−10 ≤ h ≤ 10
−10 ≤ k ≤ 10
−16 ≤ l ≤ 16
19 843
3462
2814
2.9 to 27.5
−31 ≤ h ≤ 31
−20 ≤ k ≤ 20
−23 ≤ l ≤ 23
13 304
6848
4270
2.9 to 30.0
−12 ≤ h ≤ 12
−12 ≤ k ≤ 12
−35 ≤ l ≤ 35
43 153
5816
4806
Reflns collected
Unique reflns
Observd refl (2σ)
Rint
0.036
0.045
0.0534
0.057
Data/restr./param
1722/0/93
1.098
3462/0/167
1.042
6848/0/333
1.010
5816/0/212
1.044
GoF (F2)
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0530, 0.1499
0.0677, 0.1595
0.339/−0.328
869559
0.0411, 0.1034
0.0542, 0.1103
0.261/−0.304
869560
0.0433, 0.0853
0.0888, 0.0995
0.705/−0.336
869561
0.0273, 0.0634
0.0393, 0.0683
0.826/−0.732
869562
Δρ(max/min) [e Å−3
]
CCDC-no.
9148 | Dalton Trans., 2012, 41, 9143–9150
This journal is © The Royal Society of Chemistry 2012

10 A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132,
10660–10661.
11 E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem. Soc., 2009, 131,
9918–9919.
12 R. C. Neu, E. Otten, A. Lough and D. W. Stephan, Chem. Sci., 2011, 2,
170–176.
(258.01): calcd C 60.52, H 7.03, N 5.43; found C 60.57, H 7.11,
N 5.41.
[(2,6-Dimethylpyridin-4-yl)ethynyl]dimethylindium (8). To a
solution of (2,6-Me2)(4-HCuC)C5H2N (2) (25 mg, 0.19 mmol)
13 R. C. Neu, E. Otten and D. W. Stephan, Angew. Chem., 2009, 121, 9889–
9892, (Angew. Chem., Int. Ed., 2009, 48, 9709–9712).
14 H. C. Brown, H. I. Schlesinger and S. Z. Cardon, J. Am. Chem. Soc.,
1942, 64, 325–329.
15 S. J. Geier and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 3476–
3477.
16 C. B. Caputo, S. J. Geier, D. Winkelhaus, N. W. Mitzel, V. N. Vukotic,
S. J. Loeb and D. W. Stephan, Dalton Trans., 2012, 41, 2131–2139.
17 P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fröhlich and G. Erker,
Angew. Chem., 2008, 120, 7654–7657, (Angew. Chem., Int. Ed., 2008,
47, 7543–7546).
18 V. Sumerin, F. Schulz, M. Nieger, M. Leskelä, T. Repo and B. Rieger,
Angew. Chem., 2008, 120, 6090–6092, (Angew. Chem., Int. Ed., 2008,
47, 5861).
in
2 mL toluene was added trimethylindium (30 mg,
0.19 mmol). The reaction was warmed to 90 °C for 3 d. After
the solvent was removed in vacuum the product was obtained as
a dark red powder. Crystals suitable for X-ray analysis were
obtained from a THF solution at −35 °C after several days.
1
Yield: 48 mg (93%). H NMR (500.1 MHz, THF-d8, 298 K):
δ = 6.85 (s, 2H, CH), 2.35 (s, 6H, CH3), −0.36 ppm (s, 6H, In
(CH3)2); 13C{1H} NMR (125.7 MHz, THF-d8, 298 K): δ =
158.2 (CCH3), 135.3 (CuCIn), 122.6 (CH), 120.7 (CuCIn),
106.4 (CCuC), 24.5 (CCH3), −7.8 ppm (In(CH3)2. C11H14InN
(275.05): calcd C 48.03, H 5.13, N 5.09; found C 49.71, H 5.40,
N 4.89.
19 V. Sumerin, F. Schulz, M. Nieger, M. Atsumi, C. Wang, M. Leskelä,
P. Pyykkö, T. Repo and B. Rieger, J. Organomet. Chem., 2009, 694,
2654–2660.
20 P. A. Chase, T. Jurca and D. W. Stephan, Chem. Commun., 2008, 1701–
1703.
Crystallographic structure determinations
21 V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger, M. Leskelä,
T. Repo, P. Pyykkö and B. Rieger, J. Am. Chem. Soc., 2008, 130, 14117–
14119.
Single crystals suitable for X-ray diffraction measurement were
picked inside a glove-box, suspended in a paratone-N/paraffin oil
mixture, mounted on a glass fibre and transferred onto the gonio-
meter of the diffractometer. The measurements were carried out
with Mo-Kα radiation (λ = 0.71073 Å). The structures were
solved by direct methods and refined by full-matrix least-squares
cycles (program SHELX-9751). Crystallographic data (excluding
structure factors) for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publications. The deposition numbers are pro-
vided in Table 1.
22 D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie,
S. J. Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden, G. C. Welch and
M. Ullrich, Inorg. Chem., 2011, 50, 12338–12348.
23 G. C. Welch, L. Cabrera, P. A. Chase, E. Hollink, J. D. Masuda, P. Wei
and D. W. Stephan, Dalton Trans., 2007, 3407–3414.
24 J. S. J. McCahill, G. C. Welch and D. W. Stephan, Angew. Chem., 2007,
119, 5056–5059, (Angew. Chem., Int. Ed., 2007, 46, 4968–4971).
25 T. Voss, C. Chen, G. Kehr, E. Nauha, G. Erker and D. W. Stephan,
Chem.–Eur. J., 2010, 16, 3005–3008.
26 M. Ullrich, K. S. H. Seto, A. J. Lough and D. W. Stephan, Chem.
Commun., 2009, 2335–2337.
27 M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396–
8397.
28 C. Jiang, O. Blacque and H. Berke, Organometallics, 2010, 29, 125–133.
29 W. Uhl, F. Breher, S. Haddadpour, R. Koch and M. Matar, Z. Anorg. Allg.
Chem., 2004, 630, 1839–1845.
Acknowledgements
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1017.
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4467–4470.
We thank Prof. Douglas W. Stephan and his co-workers for
helpful discussions and collaboration. We are grateful to Klaus-
Peter Mester and Gerd Lipinski for recording the NMR spectra
and Brigitte Michel for performing the elemental analyses. We
gratefully acknowledge the financial support of the Fonds der
Chemischen Industrie (stipend for D. W.) and the DFG within
the priority program SPP1178.
34 S. Kenkichi, J. Organomet. Chem., 2002, 653, 46–49.
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9150 | Dalton Trans., 2012, 41, 9143–9150
This journal is © The Royal Society of Chemistry 2012