Nickelacyclic carboxylates derived from 3-hexyne and CO2 and their application in the synthesis of a new muconic acid derivative

Article abstract of DOI:10.1016/j.poly.2011.05.047

The mononuclear unsaturated nickelalactones [Ni{C(Et)C(Et)-COO}(DBU) ₂] (1) and [Ni{C(Et)C(Et)-COO}(dcpe)] (2) were synthesized by oxidative coupling of CO₂ and 3-hexyne at zero-valent nickel in presence of DBU or dpce, respectively.

Full text of DOI:10.1016/j.poly.2011.05.047

Polyhedron 32 (2012) 60–67
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nickelacyclic carboxylates derived from 3-hexyne and CO2 and their application
in the synthesis of a new muconic acid derivative
⇑
Jens Langer , Helmar Görls, Dirk Walther
Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University, Am Steiger 3 Haus 4, D-07743 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 21 June 2011
The mononuclear unsaturated nickelalactones [Ni{C(Et)@C(Et)–COO}(DBU)2] (1) and [Ni{C(Et)@C(Et)–
COO}(dcpe)] (2) were synthesized by oxidative coupling of CO2 and 3-hexyne at zero-valent nickel in
presence of DBU or dpce, respectively. Attempts to use Cy3P or dppe as ligands in this reaction gave the
zero-valent complex [Ni(cy3P)2(g
2-3-hexyne)] (3) and the well known [Ni(dppe)2] instead. However,
Keywords:
Carbon dioxide
Nickelalactones
Oxidative coupling
C–C bond formation
Muconic acid derivative
the dppe stabilized unsaturated nickelalactone [Ni{C(Et)@C(Et)–COO}(dppe)] (4) was synthesized from
the corresponding 2,20-bipyridine complex by ligand displacement with dppe. Complex 1 is a useful syn-
thon for the synthesis of carboxylic acid derivatives, as shown by its reaction with NBS leading to 3-bromo-
2-ethyl-pent-2-enoic acid in good yield. On the contrary, the reaction between [Ni{C(Et)@C(Et)–COO}
(bipy)] and NBS gave the hitherto unknown 2-(2,3,4-triethyl-5-oxo-2,5-dihydrofuran-2-yl)butyric acid
(5) as major product. Compounds 1–5 were structurally investigated by NMR and IR spectroscopy and
in the solid state by X-ray diffraction analysis of single crystals.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
stoichiometric synthesis of carboxylic acids or their derivatives [13–
15]. In light of their importance it is surprising that only a few deriv-
The use of non-toxic carbon dioxide as building block in organic
synthesis is highly attractive due to its low cost and abundance. In
this context, efficient, atom economic catalytic transformations
involving C–C bond formation between CO2 and organic substrates
are of particular interest. A reaction that fulfills all those criteria is
the well-known nickel catalyzed co-oligomerisation of CO2 and
alkynes leading to 2H-pyran-2-one derivatives (see Scheme 1). This
transformation, which was initially reported by Inoue and co-
workers over 30 years ago [1,2], was later on optimized by Walther
and Dinjus [3], who used monodentate phosphane ligands to
achieve high selectivity towards pyranone formation, and Tsuda
and coworkers who extended the substrate scope [4] and used this
reaction e.g. in polymer synthesis [5].
atives have been isolated and studied thoroughly yet [3,14,16–19].
While complexes with a variety of different ligands, ranging
from phosphanes [20–23], N-heterocyclic carbenes (NHC’s) [24],
amines [16,25], amidines [26,27], imine [21] and pyridine deriva-
tives [16,19,21,22,25] to even inorganic ligands [28] were isolated
and structurally studied in case of their saturated analogues, only
four derivatives of the unsaturated type have been characterized
by X-ray diffraction experiments yet. Three of them contain nitro-
gen donor ligands [14,17,19], the fourth was stabilized by an NHC
ligand. Interestingly, this NHC stabilized derivative was found to be
a dinuclear complex [24].
However, little is known about such nickelacyclic carboxylates
with other ligand classes. Since DBU (1,8-diazabicyclo[5.4.0] un-
dec-7-ene) was successfully applied as ligand in the nickel cata-
lyzed arylative and alkylative carboxylation of alkynes [10–12]
and phosphanes are known to be suitable ligands for co-oligomer-
isation of alkynes and CO2 by nickel [1–4], we investigated the
coordination behavior of these ligands towards unsaturated nick-
elacyclic carboxylates.
A recent development is the use of N-heterocyclic carbenes as
ligands for nickel catalysts, which allow much milder reaction con-
ditions (60 °C, 1 atm CO2) [6–8].
Theinitial step of this catalysisis supposedto betheoxidative cou-
pling of an alkyne and CO2 at the zero-valent nickel center leading to
unsaturated nickelalactones. The first derivative of this type, namely
[Ni{C(Me)@C(Me)–COO}(tmeda)] was reported by Burkhart and
Hoberg in 1982 [9]. These compounds are not only proposed interme-
diates in the catalysis mentioned above, but also in the
nickel-catalyzed arylative and alkylative carboxylation of alkynes
[10–12]. Additionally, they are also interesting synthons for the
2. Experimental
2.1. General procedures
⇑
1H NMR and 13C NMR spectra were recorded at ambient tem-
perature with a Bruker AC 200 or AC 400 MHz spectrometer. All
Corresponding author. Tel.: +49 3641 948127; fax: +49 3641 948102.
E-mail address: j.langer@uni-jena.de (J. Langer).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.05.047

J. Langer et al. / Polyhedron 32 (2012) 60–67
61
54.26 (CH2 DBU), 54.28 (CH2 DBU), 135.3 (@C), 135.4 (@C), 158.4
(C@), 158.6 (C@), 159.8 (2x C@N DBU⁄), 164.5 (C@N), 164.7
(C@N), 175.95 (COO), 175.99 (COO) (Signals of not coordinated
DBU are marked with an asterisk). IR (nujol, cmꢀ1): 1604vs
(C@O). Suitable crystals of 1 for X-ray diffraction experiments were
obtained by recrystallization in THF (room temperature to ꢀ20 °C).
Recrystallization from toluene affords crystals of 1ꢁ0.5toluene.
2.3. Synthesis of [Ni{C(Et)@C(Et)–COO}(dcpe)] (2)
Dcpe (753 mg, 1.78 mmol) was added to a stirred solution of
[Ni(cod)2] (466 mg, 1.69 mmol) in THF (15 mL). The reaction mix-
ture was cooled to ꢀ20 °C and 3-hexyne (280 mg, 3.41 mmol was
added to the pale yellow suspension obtained. Afterwards, the
mixture was saturated with CO2 at this temperature. Then it was
allowed to slowly warm up to room temperature and stirred for
20 h. The resulting red solution was evaporated to dryness and
the remaining oil was taken up in diethyl ether (40 mL) with rapid
stirring, resulting in precipitation of the yellow product. The pre-
cipitate was collected by filtration, washed with diethyl ether
(10 mL) and dried in a vacuum. Yield: 540 mg (0.88 mmol, 52%).
Suitable crystals of 2ꢁ2THF for X-ray diffraction experiments were
obtained by recrystallization in THF (room temperature to 4 °C).
Recrystallization from a two to one mixture of toluene and THF
(room temperature to 4 °C) affords crystals of 2ꢁtoluene. Anal. Calc.
for C33H58NiO2P2ꢁtoluene (699.61 g/mol): C, 68.67; H, 9.51. Found:
C, 68.38; H, 9.42%. 13C{1H} NMR (150 MHz, d8-THF): d 15.4 (s, CH3),
15.5 (s, CH3), 17.5 (dd, JPC = 21.2 Hz, JPC = 9.7 Hz, P–CH2), 22.8 (br,
CH2–CH3), 23.2 (dd, JPC = 27.8 Hz, JPC = 21.8 Hz, P–CH2), 27.0 (s, 2x
CH2), 27.1 (s, 2x CH2), 27.95–28.25 (m, 6x CH2), 28.5 (d,
JPC = 14.0 Hz, 2x CH2), 29.7 (s, 2x CH2), 30.20 (s, 2x CH2), 30.21 (d,
JPC = 9.3 Hz, 2x CH2), 32.2 (pseudo-t, JPC = 6.5 Hz, CH2–CH3), 33.3
(d, JPC = 5.7 Hz, 2x CH2), 34.5 (d, JPC = 16.2 Hz, 2x CH), 36.9 (d,
JPC = 20.3 Hz, 2x CH), 144.9 (s, @C), 166.8 (dd, JPC = 83.2 Hz,
JPC = 23.1 Hz, Ni–C@), 182.7 (dd, JPC = 17.3 Hz, JPC = 2.3 Hz, COO).
Scheme 1. Proposed mechanism of the nickel catalyzed co-oligomerisation of
alkynes and CO2.
spectra were referenced to TMS or the deuterated solvent as an
internal standard. In the 1H NMR spectra of compounds 1–3, ill de-
fined overlapping signals over a large range were observed due to
the high number of only slightly different CH2 groups. 1H NMR data
of these compounds is rather uninformative and therefore not
given.
FAB-mass spectra were obtained using a Finnigan MAT SSQ 710
system (2,4-dimethoxybenzylalcohol as matrix), IR measurements
were performed on a Perkin–Elmer System 2000 FT-IR. All manip-
ulations were carried out by using Schlenk techniques under an
atmosphere of argon. Prior to use, tetrahydrofuran and diethyl
ether were dried over potassium hydroxide and distilled over
Na/benzophenone. The starting complexes [Ni(cod)2] and
[(Ni{C(Et)@C(Et)–COO}(bipy)] were prepared according to known
methods [3,29].
2
31P{1H} NMR (81 MHz, d8-THF): d 67.2 (d, JPP = 19.8 Hz), 74.0 (d,
2JPP = 19.8 Hz). IR (nujol, cmꢀ1): 1621vs (C@O).
2.4. Synthesis of [Ni(Cy3P)2(3,4-g
2-hex-3-yne)] (3)
Cy3P (3.02 g, 10.76 mmol) was added to a stirred suspension of
yellow [Ni(cod)2] (1.48 g, 5.38 mmol) in THF (25 mL). While the
starting materials dissolved, a color change to orange was ob-
served. 3-Hexyne (0.53 g, 6.45 mmol) were added and the formed
red solution was stirred for 12 h at room temperature. Afterwards
the solution was kept at ꢀ20 °C for four days, resulting in precipi-
tation of a pale yellow solid, which was isolated by filtration and
dried in a vacuum. A second crop of the product was obtained from
the mother liquor after evaporation to dryness in vacuo and uptake
of the residue in diethyl ether (15 mL). Combined yield: 2.6 g (69%)
pale yellow 3. Anal. Calc. for C42H76P2Ni (701.711 g/mol): C, 71.89;
H, 10.92. Found: C, 70.86; H, 10.30%. 13C{1H} NMR (50 MHz, d8-
2.2. Synthesis of [Ni{C(Et)@C(Et)–COO}(DBU)2] (1)
DBU (490 mg, 3.22 mmol) was added to [Ni(cod)2] (435 mg,
1.58 mmol) in THF (15 mL). Then 3-hexyne (148 mg, 1.80 mmol)
was added to the mixture via syringe resulting in a color change
of from yellow to red. Afterwards the formed solution was satu-
rated with CO2 at ꢀ10 °C and stirred for 2 h at room temperature.
Storage of the reaction mixture at ꢀ40 °C over night resulted in
precipitation of yellow needles (the solution tends to oversaturate
and in some cases it was necessary to shake the cooled solution to
initiate crystallization). The precipitate was collected on a cooled
Schlenk frit, washed with precooled diethyl ether (15 mL) and
dried in a vacuum. Yield: 495 mg (64%) yellow 1. Anal. Calc. for
3
THF): d 15.9 (s, 2x CH3 hexyne), 23.3 (t, JCP = 6.8 Hz, 2x CH2 hex-
3
yne), 27.6 (s, 6x CH2 Cy3P), 28.9 (pt, JCP = 4.1 Hz, 12x CH2 Cy3P),
31.0 (s, 12x CH2 Cy3P), 36.7 (br, 6x CH Cy3P), 130.2 (dd,
2
2JCP = 18.9 Hz, JCP = 49.2 Hz, 2x C hexyne). 31P{1H} NMR (81 MHz,
d8-THF): d 58.6 (s).
C
25H42N4O2Ni (489.33 g/mol): C, 61.36; H, 8.65; N, 11.45. Found:
2.5. Synthesis of [Ni{C(Et)@C(Et)–COO}(dppe)] (4)
C, 61.12; H, 8.28; N, 11.31%. 13C{1H} NMR (100 MHz, d8-THF): d
14.4 (2x CH3), 15.0 (2x CH3), 20.8 (2x CH2–CH3), 22.7 (2x CH2
DBU), 23.9 (2x CH2 DBU⁄), 24.9 (2x CH2 DBU), 26.3 (2x CH2–CH3),
27.2 (2x CH2 DBU⁄), 28.5 (2x CH2 DBU), 29.6 (2x CH2 DBU⁄), 30.4
(2x CH2 DBU), 30.7 (2x CH2 DBU⁄), 37.9 (2x CH2 DBU⁄), 41.15
(CH2 DBU), 41.2 (CH2 DBU), 45.2 (2x CH2 DBU⁄), 47.5 (2x CH2
DBU), 48.2 (2x CH2 DBU), 49.0 (2x CH2 DBU⁄), 53.2 (2x CH2 DBU⁄),
Solid dppe (0.76 g, 1.91 mmol) was added to a stirred suspen-
sion of orange red [Ni{C(Et)@C(Et)–COO}(bipy)] in THF (20 mL).
While the starting materials dissolved, the precipitation of a pale
yellow solid occurred. The stirring was continued for additional
30 min. Afterwards the precipitate was isolated by filtration,
washed with diethyl ether (10 mL) and dried in a vacuum. Yield:

62
J. Langer et al. / Polyhedron 32 (2012) 60–67
0.98 g (92%) pale yellow 4. 1H NMR (d6-DMSO): d ꢀ0.05 (3H, t,
elemental analysis were obtained by slow evaporation of a solution
of 5 in a heptane/THF mixture at room temperature.
3
3JHH = 7.3 Hz, Ni–C–CH2–CH3), 0.83 (3H, t, JHH = 7.3 Hz, CH3), 1.60
3
(2H, m, Ni–C–CH2–CH3), 1.90 (2H, q, JHH = 7.3 Hz, CH2), 2.08 (2H,
Anal. Calc. for C14H22O4 (254.33 g/mol): C, 66.12; H, 8.72. Found:
C, 66.06; H, 8.45%. 1H NMR (400 MHz, CDCl3): major pair of enan-
tiomers: d 0.69 (3H, t, 3JHH = 7.4 Hz, CH3), 0.92 (3H, t, 3JHH = 7.6 Hz,
CH3), 1.03 (3H, t, 3JHH = 7.5 Hz, CH3), 1.14 (3H, t, 3JHH = 7.6 Hz, CH3),
m, CH2 dppe), 2.35 (2H, m, CH2 dppe), 7.45–7.57 (12H, m, m-
CH + p-CH dppe), 7.88 (8H, m, o-CH dppe). 13C{1H} NMR
(100⁄MHz, d6-DMSO): d 13.4 (CH3),⁄ 14.8 (CH3), 21.8 (CH2), 21.8
(CH2 dppe), 30.5 (CH2⁄), 31.0 (CH2 dppe), 128.4 (2x i-C), 128.9
3
1.50–1.80 (3H, m, CHH0 + CHH0), 2.14 (1H, m, JHH = 7.2 Hz, CHH0),
3
3
3
3
(d, JPC = 9.5 Hz, 4x m-CH dppe), 129.0 (d, JPC = 10.2 Hz, 4x m-CH
dppe), 130.5 (2x i-C), 130.8 (2x p-CH dppe), 131.5 (2x p-CH dppe),
2.20–2.40 (4H, m, CH2), 2.64 (1H, dd, JHH = 11.4 Hz, JHH = 3.6 Hz,
CH), 7.3–8.5 (1H, br, COOH). minor pair of enantiomers: d 0.63
2
2
3
3
132.7 (d, JPC = 11.2 Hz, 4x o-CH dppe), 133.4 (d, JPC = 11.4 Hz, 4x
o-CH dppe), 143.1 (C@⁄), 167.7 (C@⁄), 182.3 (COO⁄). The signals
marked with an asterisk were observed via HMBC and HSQC exper-
iments. 31P{1H} NMR (d6-DMSO): d 43.7 (d, 2JPP = 15.6 Hz), 62.3 (d,
2JPP = 15.6 Hz). IR (nujol, cmꢀ1): 1620vs (C@O). Suitable crystals of
4ꢁCH2Cl2 for X-ray diffraction experiments were obtained by
recrystallization in CH2Cl2 (room temperature to ꢀ20 °C).
(3H, t, JHH = 7.4 Hz, CH3), 0.87 (3H, t, JHH = 7.2 Hz, CH3), 1.08
3
3
(3H, t, JHH = 7.4 Hz, CH3), 1.14 (3H, t, JHH = 7.6 Hz, CH3), 1.10–
3
1.25 (1H, m, JHH = 2.8 Hz, CHH0), 1.63 (1H, m, CHH0), 1.82 (1H, m,
3JHH = 7.2 Hz, CHH0), 1.99 (1H, m, JHH = 7.2 Hz, CHH0), 2.15–2.35
3
3
3
(4H, m, CH2), 2.57 (1H, dd, JHH = 11.8 Hz, JHH = 3.0 Hz, CH), 8.2–
.3 (1H, br, COOH). 13C{1H} NMR (100 MHz, CDCl3): major pair of
enantiomers: d 6.9 (CH3), 12.0 (CH3), 12.2 (CH3), 12.7 (CH3), 17.1
(CH2), 19.7 (CH2), 20.2 (CH2), 26.5 (CH2), 53.8 (CH), 89.4 (C),
131.9 (>C@), 163.2 (@C<), 173.4 (COO), 177.4 (COO). Minor pair
of enantiomers: d 6.9 (CH3), 11.7 (CH3), 12.0 (CH3), 12.9 (CH3),
17.3 (CH2), 19.2 (CH2), 19.6 (CH2), 27.1 (CH2), 53.7 (CH), 89.4 (C),
132.1 (>C@), 162.4 (@C<), 173.2 (COO), 177.1 (COO). IR (as methyl
ester) (gas phase, cmꢀ1): major pair of enantiomers: 2979m,
2951m, 1782vs (C@O), 1746m (C@O), 1464w, 1352w, 1169m,
1080w. Minor pair of enantiomers: 2979m, 2951m, 1782vs
(C@O), 1745m, (C@O), 1464w, 1351w, 1240w, 1160w, 1083w.
MS (DEI): m/z (%) = 255 [M+1]+ (94), 167 [MꢀC4H7O2]+ (100), 139
(20), 111 (18), 74 (23).
2.6. Reaction of 1 with N-bromosuccinimide
Solid N-bromosuccinimide (260 mg, 1.46 mmol) was added to a
stirred orange colored solution of [Ni{C(Et)@C(Et)–COO}(DBU)2]
(690 mg, 1.40 mmol) in THF (30 mL). While the added starting
material dissolved, a color change to yellowish brown was ob-
served. The reaction mixture was stirred over night and reduced
to dryness afterwards. Thereafter, the residue was hydrolyzed with
diluted hydrochloric acid (2 M, 15 mL) (further workup did not re-
quire inert handling). The aqueous phase was extracted twice with
chloroform (15 mL). The organic phases were extracted with satu-
rated, aqueous sodium carbonate solution (2xꢁ20 mL). Then the
combined aqueous phases were acidified (HCl) and extracted with
chloroform (3x 20 mL). These organic layers were dried with anhy-
drous sodium sulfate and the solvent was removed in vacuum to
give the crude product as pale yellow oil (257 mg). This crude
product consists of a seven to one mixture of (Z)-3-bromo-2-ethyl-
pent-2-enoic acid and succinimide besides minor amounts of 5.
Analytical data of (Z)-3-bromo-2-ethylpent-2-enoic acid: 1H NMR
2.8. Structure determinations
The intensity data for the compounds were collected on a Non-
ius KappaCCD diffractometer using graphite-monochromated Mo
Ka radiation. Data were corrected for Lorentz and polarization ef-
fects but not for absorption effects [30,31].
The structures were solved by direct methods (SHELXS [32])
and refined by full-matrix least squares techniques against Fo2
3
(SHELXL-97 [32]). The hydrogen atoms of the carboxylic acid groups
(200 MHz, CDCl3): d 1.07 (3H, t, JHH = 7.6 Hz, CH3), 1.14 (3H, t,
3
3JHH = 7.4 Hz, CH3), 2.36 (2H, q, JHH = 7.5 Hz, CH2), 2.55 (2H, q,
from the two independent molecules of 5 were located by differ-
ence Fourier synthesis and refined isotropically. All other hydrogen
atoms were included at calculated positions with fixed thermal
parameters. All non-hydrogen atoms were refined anisotropically
[32]. Crystallographic data as well as structure solution and refine-
ment details are summarized in Table 1. XP (SIEMENS Analytical
X-ray Instruments, Inc.) was used for structure representations.
3JHH = 7.4 Hz, CH2), 10.98 (1H, br, COOH). 13C{1H} NMR (50 MHz,
CDCl3): d 12.7 (CH3), 12.9 (CH3), 24.2 (CH2), 30.8 (CH2), 130.0
(Br–C@), 133.8 (@C), 173.4 (COO).
2.7. Synthesis of 2-(2,3,4-triethyl-5-oxo-2,5-dihydrofuran-2-yl)butyric
acid (5)
Solid N-bromosuccinimide (0.39 g, 2.23 mmol) was added to a
stirred red solution of [Ni{C(Et)@C(Et)–COO}(bipy)] (0.76 g,
2.23 mmol) in DMF (35 mL). A color change to yellow brown was
observed while the starting material dissolved. The resulting solu-
tion was stirred over night. Afterwards the now green solution was
reduced to dryness at 40 °C under reduced pressure and the
remaining sticky residue was dried at that temperature in a vac-
uum. Thereafter, the residue was hydrolyzed with diluted hydro-
chloric acid (2 M, 15 mL) (further workup did not require inert
handling). The aqueous phase was extracted twice with chloroform
(15 mL). The organic phases were extracted with saturated, aque-
ous sodium carbonate solution (2xꢁ20 mL). Then the combined
aqueous phases were acidified (HCl) and extracted with chloro-
form (3x 20 mL). These organic layers were dried with anhydrous
sodium sulfate and the solvent was removed in vacuum to give
the product as colorless oil which transforms upon storage at
4 °C into a waxy solid. Yield: 210 mg crude product, containing 5
(90% purity; mixture of diastereomers, ratio of enantiomeric pairs:
ca. 2:1) besides (Z)-3-bromo-2-ethylpent-2-enoic acid as major
impurity. Suitable crystals for X-ray diffraction experiments and
3. Results and discussion
3.1. Synthesis and characterization of the nickel complexes
In principle, two ways are known to prepare this type of unsat-
urated five-membered nickelalactones. Besides ligand exchange
reactions, the straightforward synthesis by oxidative coupling of
an alkyne and CO2 in presence of a suitable zero-valent nickel com-
pound succeeds, if strongly basic ligands are used. It is known that
nitrogen donor ligands like 2,20-bipyridine (bipy) [3], N,N,N0,N0-
tetramethylethylendiamine (tmeda) [9] or amidine ligands
[13,33] are applicable for this reaction. One of the most efficient
and therefore frequently used ligands is 1,8-diazabicyclo [5.4.0]un-
dec-7-ene (DBU). Although the formation of unsaturated nickel-
alactones in presence of DBU was investigated by quantum
chemical calculations, these complexes have not been isolated
yet [34,35]. In this investigation, 3-hexyne was chosen as a model
substrate to ensure comparability to the known derivatives.
Initial attempts to synthesize a well-defined DBU stabilized
nickel alkyne precursor, starting from [Ni(cod)2], DBU and 3-hex-

J. Langer et al. / Polyhedron 32 (2012) 60–67
63
Table 1
Crystal data and refinement details for the X-ray crystal structure determinations of the compounds 1–5.
Compound
1
1ꢁ0.5toluene
2ꢁ2THF
2ꢁtoluene
3
4
5
Formula
C
25H42N4NiO2 C25H42N4NiO2 0.5C7H8 C33H58NiO2P2 2C4H8O C33H58NiO2P2 C7H8 C42H76NiP2
C33H34NiO2P2 CH2Cl2 C14H22O4
Formula weight (g molꢀ1
)
489.34
ꢀ90(2)
monoclinic
P21/c
10.1391(3)
12.7123(4)
19.4762(5)
90
535.40
ꢀ90(2)
monoclinic
P21/n
751.65
ꢀ140(2)
monoclinic
P21/c
699.58
ꢀ140(2)
monoclinic
P21/c
701.68
668.18
ꢀ90(2)
254.32
ꢀ90(2)
monoclinic
P21/c
T (°C)
ꢀ90(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic orthorhombic
Pbca
Pna21
11.5645(7)
15.3234(8)
16.2987(9)
90
11.5442(3)
17.9407(3)
20.3550(5)
90
11.4715(2)
14.0123(2)
23.3296(3)
90
18.7832(2)
10.0580(1)
43.5820(5)
90
28.0005(7)
11.5745(2)
9.6567(2)
90
14.6232(3)
7.1437(2)
26.5888(8)
90
a
(°)
b (°)
99.560(2)
90
2475.45(12)
4
1.313
8.12
106.614(2)
90
2767.7(3)
4
1.285
7.33
103.521(1)
90
4098.90(16)
4
1.218
5.89
95.889(1)
90
3730.26(10)
4
1.246
6.39
90
90
90
90
91.636(1)
90
2776.43(13)
8
1.217
.88
c
(°)
V (Å3)
8233.57(15)
8
1.132
3129.66(12)
4
1.418
Z
q
l
(gꢁcmꢀ3
)
(mmꢀ1
)
5.75
9.23
Measured data
Data with I > 2
Unique data (Rint
wR2 (all data, on F2)a
9956
4168
5650/0.0356
0.0944
0.0371
1.011
10067
3582
6083/0.0619
0.1727
0.0717
1.030
24962
7842
9359/0.0449
0.1237
0.0505
21058
7361
8538/0.0368
0.0943
0.0405
34531
6326
9346/0.1129
0.1449
0.0531
1.025
6767
4942
6767/0.0000
0.1188
0.0538
1.012
15609
3994
6321/0.0469
0.1324
0.0519
0.997
r
(I)
)
R1 (I > 2
r
(I))a
Sb
1.118
1.046
Residual density (e Åꢀ3
Absorption method
CCDC No.
)
0.335/ꢀ0.358 0.436/ꢀ0.320
0.395/ꢀ0.535
0.416/ꢀ0.323
0.834/ꢀ0.733 0.751/ꢀ0.679
0.202/ꢀ0.218
none
none
818502
none
none
none
none
818506
none
818501
818503
818504
818505
818507
[w(Fo ꢀ Fc2)2]/
R
[w(Fo2)2]}1/2 with wꢀ1 2(Fo2) + (aP)2 + bP; P = [2Fc2 + Max(Fo2]/3.
= r
a
2
Definition of the R indices: R1 = (
R
.
||Fo| ꢀ |Fc||
R
|Fo|; wR2 = {
R
b
2
s = {
R
[w(Fo ꢀ Fc2)2]/(No ꢀ Np)}1/2
Table 2
Selected structural data of unsaturated nickelacyclic carboxylates [Ni{C(Et)@C(Et)–
COO}(L)n]m (bond lengths [Å]).
Ligand L
dppe
n
m
Ni–C
Ni–O
1.886
Ni–L
C1–C2
1.337
References
–
1
1
1.963
2.135 (P1)
2.235 (P2)
2.150 (P1)
2.210 (P2)
1.987 (N1)
2.010 (N2)
1.903 (N1)
1.953 (N2)
1.895 (N1)
1.997 (N2)
1.912 (N1)
1.994 (N3)
1.859 (C4)
1.960 (O3)
dcpe
tmeda
bipy
py
1
1
1
2
2
1
1
1
1
1
1
2
1.961
1.944
1.926
1.909
1.900
1.890
1.892
1.855
1.848
1.864
1.874
1.890
1.346
1.341
1.346
1.344
1.352
1.354
–
[17]
[14]
[19]
–
DBU
ItBu
[24]
Recystallization of 1 in either THF or toluene led to crystals of 1
(or 1ꢁ0.5toluene) suitable for X-ray diffraction experiments (see
Section 2). The molecular structures of both compounds are rather
similar, however, a different conformation of the seven-membered
ring of the DBU ligand in trans position to the Ni–C bond was ob-
served. The molecular structure of 1 is shown in Fig. 1.
For the following structure discussion the average values from
both structure determinations are used. As expected, compound
1 contains the nickel atom in a square planar environment. Since
the use of strongly basic DBU as ligand leads to an electron-rich
nickel centre, a rather short Ni–C1 bond of 1.900 Å and a slightly
elongated C1–C2 bond of 1.352 Å were observed. This indicates en-
hanced backbonding of electrons of the nickel atom into the empty
p⁄-orbital of the C1–C2 double bond when compared to related
complexes with N-donor ligands (see Table 2). The Ni–N bond
lengths of 1.912 Å (Ni–N1) and 1.994 Å (Ni–N3) vary due to the dif-
fering trans influence of C1 and O1. Compared to the calculated
structure of [Ni{C(Me)@CH–COO}(DBU)2] optimized at the B3LYP
level of density functional theory [40], significant deviations
around the nickel center were observed. Especially the observed
Ni–N3 bond length of 1.994 Å is much shorter than the calculated
Fig. 1. Molecular structure of complex 1 (H-atoms are omitted for clarity. Values in
square brackets correspond to 1ꢁ0.5toluene). The ellipsoids represent a probability
of 40%. Selected bond distances (Å) and bond angles (°): Ni–O1 1.8681(13)
[1.881(3)], Ni–C1 1.900(2) [1.899(4)], Ni–N1 1.9128(16) [1.911(4)], Ni–N3
1.9907(16) [1.997(4)], C1–C2 1.345(3) [1.360(7)], C2–C3 1.482(3) [1.481(7)], C3–
O1 1.303(2) [1.298(5)], C3–O2 1.238(2) [1.240(6)], C1–Ni–O1 85.37(8) [84.96(17)],
N1–Ni–N3 87.95(7) [87.27(15)], N1–Ni–C1 97.62(8) [97.02(18)], N3–Ni–O1
89.11(6) [91.15(14)].
yne, failed since the equilibrium of this reaction is on the side of
starting materials. Therefore, the in situ system was exposed to a
CO2 atmosphere at room temperature. Under these conditions
the formation of the desired nickelalactone [Ni{C(Et)@C(Et)–
COO}(DBU)2] (1) proceeds smoothly and the product was crystal-
lized at ꢀ40 °C and isolated by filtration in good yield.

64
J. Langer et al. / Polyhedron 32 (2012) 60–67
Table 3
13C NMR data of the nickelacyclic moiety in 1, 2, 4 and related complexes.
Compound
Solvent
d Ni–C@
d C@C–COO
d COO
References
[Ni{C(Et)@C(Et)–COO}(dppe)]
[Ni{C(Et)@C(Et)–COO}(dcpe)]
[Ni{C(Et)@C(Et)–COO}(py)2]
[Ni{C(Et)@C(Et)–COO}(DBU)2]
[Ni{C(Et)@C(Et)–COO}(IMes)]2
[Ni(CH@CH–COO)(dcpe)]
d6-DMSO
d8-THF
d6-DMSO
d8-THF
d7-DMF
CDCl3
167.7
166.8
159.0
158.5
158.6
161.2
143.1
144.9
139.0
135.4
141.5
139.0
183.3
182.7
178.9
176.0
178.5
186.3
–
–
[19]
–
[24]
[16]
tive coupling of imines and CO2 also imine ligands trigger such an
oligomerisation [36]. In our case, there have to be two slightly dif-
ferent oligomers or at least two stable conformations, since a dou-
bled signal set for the carbon atoms of the nickelacyclic moiety at d
158.6 and 158.7 (Ni–C@), 135.47 and 135.54 (@C–) as well as
175.95 and 175.99 (COO) was observed. Additionally, two of the
eight CH2 groups of the coordinated DBU show two slightly differ-
ent singlets each, while the others only gave one singlet per CH2
group.
Besides the major signals, another incomplete signal set was
observed via NMR measurement which might corresponds to the
species observed in the solid state. Table 3 summarizes selected
NMR data of 1 and related complexes.
In order to gain further inside into the solution behavior and so-
lid state structures of such unsaturated nickelalactones, the syn-
thesis of phosphane stabilized complexes was attempted. It is
known that the use of monodentate phosphanes as ligands results
in active catalysts for the co-oligomerisation of CO2 and alkynes
[3], although more forcing conditions are required to achieve satis-
fying yields. Additionally, related saturated nickelalactones are
known to form different types of oligomers [20].
In contrast, the use of chelating diphosphane ligands give sys-
tems of poor catalytic activity [2], although some of them mediate
the oxidative coupling of alkynes and CO2 at the nickel center un-
der mild conditions [16,18].1,2-Bis(dicyclohexylphosphino)ethane
(dcpe), 1,2-bis(diphenylphosphino)ethane (dppe) and tricy-
clohexylphosphane (cy3P) were chosen as representative ligands
and their reactions with zero-valent nickel were carried out in
analogy to the synthesis of 1. While dcpe promotes the oxidative
coupling of 3-hexyne and CO2 to the desired nickelalactone 2 un-
der these conditions as well, dppe or cy3P did not lead to the envi-
sioned products, even when prolonged reaction times were used.
Fig. 2. Molecular structure of complex 2ꢁ2THF (H-atoms and co-crystallized THF are
omitted for clarity). The ellipsoids represent a probability of 40%.Values in square
brackets correspond to 2ꢁtoluene. Selected bond distances (Å) and bond angles (°):
Ni–O1 1.8943(17) [1.8904(13)], Ni–C1 1.965(2) [1.9561(19)], Ni–P1 2.1485(7)
[2.1525(5)], Ni–P2 2.2145(6) [2.2054(5)], C1–C2 1.342(3) [1.349(3)], C2–C3
1.499(3) [1.495(3)], C3–O1 1.307(3) [1.308(2)], C3–O2 1.227(3) [1.231(2)], C1–
Ni–O1 85.78(9) [86.04(7)], P1–Ni–P2 87.61(2) [87.49(2)], P1–Ni–C1 98.70(7)
[99.44(6)], P2–Ni–O1 87.70(5) [87.11(4)].
value of 2.070 Å. Similar, but less pronounced deviations occurred
between the calculated und observed molecular structures of the
related bipyridine complexes [14,35].
Instead, the zero-valent nickel complexes [Ni(cy3P)2(g
2-3-hex-
yne)] 3 (see Fig. 3) and [Ni(dppe)2] [37,38] were isolated yield from
The bonding situation in the nickelalactone subunit of 1 is quite
similar to the related NHC complex [Ni{C(Et)@C(Et)–COO}(IMes)]2
[24]; however, 1 is a mononuclear compound. Those similarities
are remarkable in light of the differing reactivity of these com-
pounds. DBU stabilized nickel complexes are not known to catalyze
the co-oligomerisation of CO2 and alkynes (although they effi-
ciently promote the initial oxidative coupling of alkynes with
CO2), in contrast to the combination NHC ligand/[Ni(cod)2], which
is so far the most active catalytic system for this reaction [6,7].
While the solid state structure of 1 is in agreement with other
nitrogen donor substituted nickelalactones [14,17,19], the NMR
spectroscopic investigation of 1 in d8-THF at 25 °C strongly indi-
cates, that this structure is not maintained in solution. Dissolution
of a crystalline sample results in the liberation of approximately
half of the DBU, and the well known signals of the non coordinated
DBU was observed in the 1H and 13C NMR spectra. However, the
nickelalactone subunit remains intact. Therefore this behavior
can either be interpreted as displacement of DBU by THF or more
likely as an oligomer formation. Oligomeric nickelalactones are
so far only known for NHC and phosphane stabilized systems
[20,24], but in case of related nickel compounds derived by oxida-
the reaction mixtures (see Scheme 2).
Recrystallization of 2 in THF or toluene affords suitable crystals
for X-ray diffraction experiments of the composition 2ꢁ(THF)2 and
2ꢁtoluene, respectively. The molecular structures determined did
not differ significantly from each other (see Fig. 2). The average val-
ues of bond lengths and angles are given in Table 2. A comparison
to related complexes shows the influence of the chelating diphos-
phane ligand on the overall structure of such nickelalactones. Due
to the slightly altered p-backbonding capacity of this ligand class, a
diminished backbonding into the nickelalactone itself was ob-
served leading to a rather long Ni–C bond of 1.961 Å in 2 (1: Ni–
C: 1.900 Å) together with an insignificant shortening of the C@C
bond.
The solid state structure is maintained in solution as indicated
by the sharp doublets observed at d 43.7 and 62.3 for the complex
in the 31P NMR spectra.
Ligand exchange reactions offer an alternative strategy to obtain
the desired nickelalactones. Complex 2 was independently pre-
pared by addition of dcpe to a solution of 1 in THF; however, the ob-
served product contained DBU as impurity after workup. In case of
dppe, the easily accessible complex [Ni{C(Et)@C(Et)–COO}(bipy)]

J. Langer et al. / Polyhedron 32 (2012) 60–67
65
Scheme 2. Synthesis of 1–3.
of 4, determined by X-ray diffraction experiments. Bond lengths
and angles are comparable to the values observed for 2 and there-
fore do not need further discussion.
3.2. Reactivity towards N-bromosuccinimide
Unsaturated nickelalactones like 1 are not only of interest as
proposed intermediates in catalysis but are also valuable starting
materials for the synthesis of organic fine chemicals.
Not surprising, the reaction of 1 with N-bromosuccinimide in
THF resulted in the formation of 3-bromo-2-ethyl-pent-2-enoic
acid as major product after hydrolysis. Similar procedures using re-
lated titanium or zirconium complexes are known. However, the
addition of CuI in case of the zirconium derivatives was necessary
[40–42]. Interestingly, the reaction route dramatically changes if
the related 2,20-bipyridine stabilized complex [Ni{C(Et)@C(Et)–
COO}(bipy)] was used as starting material. The initial orange red
suspension of this adduct in THF changed to beige upon addition
of NBS. Due to the lower solubility of the bipyridine complexes, a
clear solution was never observed during the reaction. After hydro-
lytic workup the hitherto unknown 2-(2,3,4-triethyl-5-oxo-2,5-
dihydrofuran-2-yl)butyric acid (5) was isolated as the main prod-
uct from the reaction mixture beside minor amounts of 3-bromo-
2-ethyl-pent-2-enoic acid (see Scheme 3).
In order to elucidate the influence of the biphasic reaction con-
ditions in case of [Ni{C(Et)@C(Et)–COO}(bipy)] on the outcome of
the reaction, the synthesis of 5 was repeated in DMF, in which
the starting materials are well soluble. Again, 5 was isolated as ma-
jor product upon workup. The constitution of 5 was undoubtedly
confirmed by X-ray diffraction experiments. The molecular struc-
ture determined is shown in Fig. 5.
Fig. 3. Molecular structure of complex 3 (H-atoms are omitted for clarity). The
ellipsoids represent a probability of 40%. Selected bond distances (Å) and bond
angles (°): Ni–C1 1.902(3), Ni–C2 1.895(3), Ni–P1 2.1934(7), Ni–P2 2.1848(7), C1–
C2 1.269(4), C2–C3 1.502(4), C1–C5 1.498(4), P1–Ni–P2 115.26(3), P1–Ni–C1
101.66(9), P2–Ni–C2 104.38(8), C1–Ni–C2 39.04(11), C1–C2–C3 139.8(3), C2–C1–
C5 141.3(3).
was used as starting material, in analogy to known procedures [39]
resulting in [Ni{C(Et)@C(Et)–COO}(dppe)], isolated as pale yellow
solid in excellent yield (92%). Fig. 4 shows the molecular structure
Compound 5 was obtained as mixture of diasteriomers, with a
ratio between the enantiomeric pairs being approximately two to

66
J. Langer et al. / Polyhedron 32 (2012) 60–67
Scheme 3. Ligand influence on the outcome of the reaction with NBS.
points toward a radical pathway leading to a nickel coordinated
cis,cis-tetraethylhexa-2,4-diendioate anion. Hydrolysis would re-
sult in the corresponding cis,cis-tetraethylhexa-2,4-diendioic acid
which is supposed to form 5 by cyclization under acidic conditions.
Similar behavior was reported for related hexa-2,4-diendioic acids.
Depending on the substitution pattern of the muconic acid deriva-
tive, more or less harsh conditions were required to enforce lacton-
ization [43,44].
4. Conclusion
The new unsaturated nickelalactons 1 and 2 are easily accessible
by oxidative coupling of CO2 and 3-hexyne at the zero-valent nickel
center. While their solid state structures are similar, 1 partially lib-
erates DBU in solution and most likely forms oligomeric aggregates.
The nickelalactones 1 and [Ni{C(Et)@C(Et)–COO}(bipy)] are useful
starting materials in the synthesis of highly substituted carboxylic
acids. The predominant formation of either 3-bromo-2-ethyl-pent-
2-enoic acid or 2-(2,3,4-triethyl-5-oxo-2,5-dihydrofuran-2-yl)buty-
ric acid (5) was observed in the reaction with NBS, depending on the
neutral ligand present. Hence, this nickel mediated reaction
sequence offers a simple strategy towards new muconic acid deriv-
atives from the corresponding alkynes and CO2. Scope and limita-
tions of this reaction are subjects of ongoing studies.
Fig. 4. Molecular structure of complex 4ꢁCH2Cl2 (H-atoms and co-crystallized
CH2Cl2 are omitted for clarity). The ellipsoids represent a probability of 40%.
Selected bond distances (Å) and bond angles (°): Ni–O1 1.886(3), Ni–C1 1.963(4),
Ni–P1 2.1350(12), Ni–P2 2.2348(12), C1–C2 1.337(6), C2–C3 1.487(6), C3–O1
1.299(5), C3–O2 1.228(5), C1–Ni–O1 85.46(16), P1–Ni–P2 86.58(5), P1–Ni–C1
97.31(13), P2–Ni–O1 90.63(10).
Acknowledgments
This research was supported by a grant to J.L. from the Deutsche
Bundesstiftung Umwelt and by the Deutsche Forschungsgemeins-
chaft (SFB 436).
Appendix A. Supplementary data
CCDC 818501, 818502, 818503, 818504, 818505, 818506,
818507 contains the supplementary crystallographic data for 1,
1ꢁ0.5toluene, 2ꢁ2THF, 2ꢁtoluene, 3, 4, 5. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
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