Cavitand Chemistry: Nickel Half-Sandwich Complexes with Imidazolylidene Ligands Bearing One or Two Resorcinarenyl Substituents

Article abstract of DOI:10.1002/ejic.201701143

Two imidazolium salts, one with a single N-μ₄-tetramethylenedioxyresorcin[4]arenyl substituent, the other with two, have been used as pro-ligands (LH) for the synthesis of nickel half-sandwich complexes of the type [NiCpClL] (Cp = cyclopentadienyl). NMR and X-ray structural investigations revealed that in both complexes the NiCp moiety is positioned at the wider entrance of the resorcinarenyl cavity, this location providing high steric encumbrance around the metal centre. Both complexes, once activated with NaBH₄, catalysed the dimerisation of ethylene, the highest activity [555 mol(C₂H₄) mol(Ni)^–1 h^–1] being observed with the complex bearing two cavitand substituents.

Full text of DOI:10.1002/ejic.201701143

A Journal of
Accepted Article
Title: Cavitand chemistry: nickel half-sandwich complexes with
imidazolylidene ligands bearing one or two resorcinarenyl
substituents
Authors: Nallusamy Natarajan, Thierry Chavagnan, David SEMERIL,
Eric Brenner,*, Dominique Matt, Rengan Ramesh, and Loïc
Toupet
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701143
Link to VoR: http://dx.doi.org/10.1002/ejic.201701143

10.1002/ejic.201701143
European Journal of Inorganic Chemistry
1
Cavitand chemistry: nickel half-sandwich complexes with imidazolylidene
ligands bearing one or two resorcinarenyl substituents
Nallusamy Natarajan,[a] Thierry Chavagnan,[a] David Sémeril,*[a] Eric Brenner,*[a] Dominique
Matt,[a] Rengan Ramesh,[b] and Loïc Toupet[c]
Abstract:
Two imidazolium salts, one with a single µ4-tetramethylenedioxy-resorcin[4]arenyl N-
substituent, the other with two, were used as proligands (LH) for the synthesis of nickel half-
sandwich complexes of the type [NiCpClL] (Cp = cyclopentadienyl). NMR and X-ray
structural investigations revealed that in both complexes the NiCp moiety is positioned at the
wider entrance of a resorcinarenyl cavity, this location providing high steric encumbrance
around the metal centre. Both complexes, once activated with NaBH4, catalyse the
dimerisation of ethylene, the highest activity (555 mol(C2H4) mol(Ni)-1 h-1) being observed
with the doubly-substituted complex.
[a]
Université de Strasbourg, Laboratoire de Chimie Inorganique Moléculaire et Catalyse,
UMR 7177 CNRS, 4 rue Blaise Pascal, 67008 Strasbourg, France
E-mail: dsemeril@unistra.fr and eric.brenner@unistra.fr
http://lcimc.u-strasbg.fr
[b]
Centre for Organometallic Chemistry, School of Chemistry, Bharathidasan University,
Tiruchirappalli, 620 024 Tamil Nadu, India
[c]
Institut de Physique, UMR 6251 CNRS, Université de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
2
Introduction
The chemistry of nickel N-heterocyclic carbene complexes (Ni-NHCs) is a rapidly expanding
field, complexes of this class subtly combining the intrinsic properties of nickel[1] (chemical
versatility, high natural abundance, low cost) with those of the particular electronic and steric
features of NHC ligands (electron richness, hemi-ovoidal encumbrance). Thus, valuable
Ni(Cp)-NHC (Cp = cyclopentadienyl) catalysts have recently been developed for applications
in a variety of catalytic reactions, including hydrosilylation,[2] Suzuki-Myiaura,[3] Kumada-
Tamao-Corriu,[3c, d, 4] Buchwald-Hartwig,[5] hydrothiolation,[6] ethylene dimerisation[7] or
polymerisation of styrene,[3c, 8] phenylacetylene[9] and methacrylate.[10]
In several recent studies we have reported the synthesis of imidazolium salts having
one of their nitrogen atoms substituted by a cavitand subunit, namely the (µ4-
tetramethylenedioxy)-2,8,14,20-tetrapentylresorcin[4]aren-5-yl group, and shown their high
efficiency when used as proligands in palladium-[11] and nickel-catalysed[12] cross-couplings
as well as in copper-catalysed allylic substitutions.[13] We further found that in the carbene
complexes obtained from these salts, the metal ion (palladium, copper) is located outside the
cavity of the resorcin[4]arene unit. To rationalise the remarkable performance of the
corresponding catalysts, we proposed that during catalysis two flexible pentyl chains attached
to the smaller rim of the resorcin[4]arene unit may sterically interact with substrates bound to
the exo-oriented metal centre, thereby facilitating the reductive elimination steps. Thus, the
cavity receptacle as such is not involved in the catalytic process in these cases.
As part of a programme aimed at the synthesis of cavitand-based Ni-NHC complexes,
a hitherto unreported class of compounds, we have now found, for the first time, that
resorcinarene-substituted imidazolium salts may give NHC-complexes having their metal
centre positioned at the entrance of the cone delineated by the cavitand. The catalytic impact
of the resulting confinement (Fig. 1) was assessed in ethylene dimerisation.
H11C5
H11
C
5 C5H11
O
O
C5H11
Br
Cl
O
O
N
N
O
Ni
O
Ni
O
O
O
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
H11C5
H11C5
C5H11
C5H11
C5H11
C5H11
H11C5
H11C5
2
1
Figure 1: Half-sandwich nickel complexes investigated in the present study.
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
3
Results and Discussion
The synthesis of complexes 1 and 2 required that of the imidazolium salts 6 and 7,
respectively (Scheme 1). Precursor 6 was conveniently prepared according to a method
reported earlier, viz. by Ullmann coupling between the mono-brominated cavitand 3 and
imidazole, followed by alkylation of the resulting imidazolyl-substituted resorcinarene with n-
propyl bromide. As a similar sequence cannot be applied to the synthesis of bis-aryl-
substituted imidazoles, compound 7 was prepared via a conventional imidazole synthesis.[14]
NH2
O
N3
Br
O
O
Pd/C
H2 (5 bar)
O
O
O
O
O
O
1) nBuLi, -78°C
2) TsN3, -78°C to r.t.
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
CH2Cl2, 24 h, r.t.
(100 %)
THF
(75 %)
H11C5
H11C5
C5H11
C5H11
H11C5
C5H11
H11C5
C5H11
C5H11
H11C5
C5H11
H11C5
5
3
4
1) glyoxal, acetic acid,
70°C, 24 h
2) paraformaldehyde,
TMSCl, 70°C, 18 h
CuI (0.5 equiv.)
EtOAc
(13 %)
HN
(1.5 equiv.)
N
DMEDA (0.5 equiv.)
K2CO3 (2.5 equiv.)
DMF, 120°C, 7 d
(75 %)
H11C5
N
O
O
H11C5
H11C5
N
Cl
O
O
N
O
O
O
O
O
O
O
O
O
1/2
O
O
O
O
N
O
H11C5
O
O
O
C5H11
O
H11C5
C5H11
C5H11
H11C5
C5H11
C5H11
O
O
C5H11
iPrBr
reflux
(95 %)
7
N
Br
N
O
O
O
O
O
O
O
O
H11C5
C5H11
C5H11
H11C5
6
Scheme 1. Synthesis of the NHC precursors 6 and 7
To this end, cavitand 3 was converted into azide 4 via a halogen/lithium exchange
followed by reaction with tosyl azide (Scheme 1). Reduction of 4 with Pd/C under hydrogen
(5 bar) led quantitatively to amine 5. The latter was then reacted with glyoxal in the presence
of acetic acid at 70 °C for 24 hours, and the resulting mixture treated with paraformaldehyde
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
4
and Me3SiCl. Chromatographic separation afforded imidazolium salt 7 in 13 % yield. Despite
several optimisation attempts, the yield of this reaction could not be improved due to rapid
decomposition of the di-imine intermediate.
The new imidazolium salt 7 was characterised by elemental analysis, ESI-TOF MS
1
and H and 13C NMR spectroscopy. Consistent with the presence of two equivalent mono-
1
substituted resorcinarene units, the H NMR spectrum of 7 displays two AB patterns
(intensity 8:8) for the –OCH2O– bridges and two triplets (intensity 4:4) for the methine
protons. The signal of the NCHN proton appears in the expected range, at δ 8.88 ppm.
The two half-sandwich complexes 1 and 2 were obtained following a general method
developed by Cowley,[15] which consisted of reacting the appropriate imidazolium salt with
[NiCp2] in refluxing THF for 12 hours (Scheme 2). The observed yields (20 % for 1 and 35 %
for 2) are relatively low, but such yields are not unusual for Ni(Cp)-NHC complexes obtained
from [NiCp2].[8a] In this study, for comparative purposes, the two cavity-free analogues 8 and
9[15] (Fig. 2) were also prepared (yield: ca. 55% for both complexes). All complexes were
1
characterized by H and 13C{1H} NMR spectroscopy and by elemental analysis (see
experimental part).
X
R
N
R
X
N
Ni
[NiCp2]
N
N
O
O
O
O
O
O
O
O
O
THF, reflux
12 h
O
O
O
O
O
O
O
H11C5
C5H11
C5H11
H11C5
H11C5
C5H11
C5H11
H11C5
1 X = Br; R = iPr
2 X = Cl; R =
O
O
O
O
O
O
O
O
H11C5
C5H11
C5H11
H11C5
Scheme 2: Synthesis of the half-sandwich complexes 1 and 2.
Br
Cl
O
O
Ni
Ni
N
N
N
N
8
9
Figure 2: Reference complexes 8 and 9.
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
5
1
The cyclopentadienyl proton signals in the H NMR spectra of complexes 1 and 2
show a significant upfield shift (δ 3.68 ppm (1); 3.71 ppm (2)) in comparison to those of the
cavity-free complexes 8 and 9 (cf. 4.71 and 4.61 ppm for 8 and 9, respectively) or other
simple [NiCpXL] complexes.[15] These data are indicative of cavity-entrapped Cp rings, the
observed shielding arising from the ring current created by aromatic units of the cavity wall.
The proposed NiCp endo-orientation was confirmed in the case of complex 1 by a single-
1
crystal X-ray diffraction study (vide infra). In line with a confined NiCp fragment, a H-1H
ROESY spectrum for complex 2 (see the Supporting Information) revealed correlations
between the Cp ligand and two aromatic protons of one of its resorcinarene units. Careful
analysis also revealed evidence of the spatial proximity of an H atom of the imidazolyl ring
and two close OCH2O groups. Molecular models unambiguously show that these OCH2O
moieties must belong to the "empty" resorcinarene unit, this cavity having its wider entrance
turned away from the nickel atom (Fig. 3).
Figure 3. View of the proposed anti orientation of the two cavity subunits in 2, as inferred
from NMR experiments (minimisation with SPARTAN[16]).
It is further worth mentioning that while the room temperature 1H NMR spectrum of 1
shows some broad signals indicative of relatively slow conformational changes, those of 2 are
all sharp (see the Supporting Information). Possibly, the presence of the bulky Br atom in 1
slows down the rotation about the C–Ni bond of this complex, the corresponding rotation
occurring much faster in the chloro complex 2 (blocked rotation should result in conformers
with either an exo- or en endo-oriented Ni-X bond). However, attempts to freeze out this
movement by lowering the temperature were unsuccessful.
Complex 1 (Fig. 4) crystallises in the triclinic space group P-1. The asymmetric unit
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
6
contains two inequivalent molecules (denoted A and B hereafter) and a molecule of benzene.
Both complexes have three ligands bound to the Ni, with the Cp rings in the η5-bonding
mode, typical of [NiCpXL] complexes.[8a, 17] The Ni-C(carbene) bond lengths, 1.869(9) Å (A)
and 1.898(9) Å (B), are close to those reported for the related complex 9 (1.917(9) Å).[15] The
observed wider rim diameters (i.e the segments linking the C-2 aromatic carbon atoms of
opposite resorcinol units), 7.53(1)/8.24(1) Å in molecule A and 7.42(1)/8.29(1) Å in molecule
B, reveal that the resorcinol cavities possess an oval cross section, such an observation having
been made for other mono-substituted resorcin[4]arene-cavitands with OCH2O linkers.[18] As
anticipated, both cavities of the unit cell host a NiCp moiety. Short separations can be seen
between two H atoms of the entrapped Cp ring and the centroids of two opposite resorcinolic
rings (2.90(1) Å /2.93(1) Å in A and 2.90(1)/2.89(1) Å in B), this being in line with the
observation of an upfield shift in the Cp protons signal in the 1H NMR spectrum.
Cp
Cl
Figure 4: X-ray structure of complex 1 (left: side view with carbenic C at left; middle: view
from top; right CPK-side view with carbenic C at right). Only one of the two molecules
present in the unit cell is shown. The NiCpCl moiety is in red.
The X-ray structure determination carried out for 1 provided a means of estimating the
buried volume (%VBur) of the corresponding carbene. This calculation was done using the
SambVca tool,[19] which also provided a steric map allowing to visualise the encumbrance of
the ligand. Application of the standard value of R = 3.5 Å to define the first coordination
sphere around the metal led to a %VBur of 37.2 (H atoms included), a value similar to that
reported for IMes (36.5[20]). However, to obtain a realistic steric map taking into account the
presence of a cavity surrounding the nickel atom, we found it better to apply an R value of 4.0
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
7
Å. The results of both calculations are shown in Fig. 5.
O
O
O
O
N
N
z
O
O
O
O
Figure 5 Topographic steric maps of 1 including H atoms. View along the z axis (left:
calculation with a R value of 3.5 Å; right: calculation with R = 4.0 Å). The distances on the
scales indicate the height along the z axis. The z axis points towards the reader and is located
in the midde of the maps.
Finally, it should be emphasised that there is no spectroscopic evidence for the NiCp
moiety of 1 jumping out of the cavity in solution. This makes sense considering the high
degree of confinement of the NiCp unit in 1 found in the solid state (illustrated in Fig. 4, CPK
representation). However, the reason why this unit sits in the cavitand rather than being
located outside the cavity is a matter of discussion. At this stage, it remains unclear whether
the Ni—Cp bond formed inside (after formation of a NiCp2 inclusion complex) or outside and
notably whether its end-position (namely inside the bowl) arises from its thermodynamic
stability.
The nickel complexes 1 and 2 were evaluated as catalysts for ethylene dimerization
using NaBH4 (one equiv. per Ni) as activator.[21] The corresponding tests were performed
under 20 bar of ethylene in toluene at 90 °C for 1 hour (Scheme 3 and Table 1). At the end of
each run, the autoclave was cooled down to 6 °C and depressurised over 8 minutes (to
efficiently limit the loss of butene) (Table 1, entries 9 and 10).
[Ni] / NaBH4
2
toluene, 90°C
1 h
Scheme 3: Nickel-catalysed ethylene dimerization.
Under these conditions, complex 1 produced 1-butene with a turn-over frequency
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
8
(TOF) of 85 mol(C2H4) mol(Ni)-1 h-1 (Table 1, entry 1). The catalyst activity could be
increased up to 350 mol(C2H4) mol(Ni)-1 h-1 by adding one equivalent of PPh3, phosphine
coordination probably resulting in mixed electron-rich phosphine-NHC intermediates that
facilitate the olefin insertion steps (Table 1, entry 2). A significantly higher TOF (555
mol(C2H4) mol(Ni)-1 h-1) was obtained with complex 2 (Table 1, entry 3). Interestingly,
adding PPh3 to the run performed with 2 drastically reduced the formation of 1-butene (Table
1, entry 5). It is likely that in this case, due to the high steric encumbrance of the carbene
ligand (which bears two cavitand subunits), addition of phosphine mainly results in trans-
(carbene,phosphine) intermediates (rather than cis-complexes as for 1), a configuration which
should prevent olefin insertion. Finally, we found that in none of the above runs did
isomerisation of 1-butene occur.
In order to better understand the possible influence of the resorcinarenyl substituent in
1 during catalysis, further runs were carried with the closely related, but cavity-devoid
complex 8. In these experiments the butene production was nearly identical to that obtained
with 1 (in the absence of PPh3), thus suggesting that the carbene ligands of 1 and 8 have
similar steric and electronic properties, and that in fact there is no effective participation of
the cavity. Unlike with 1, the activity of 8 remained nearly unchanged upon addition of PPh3
(Table 1, entries 6 and 7). The latter observation is a clear indication that the presence of the
phosphine, presumably leading to Ni(phosphine)(carbene) intermediates, augments the steric
properties of the carbene ligand of 1 as a result of strong steric interactions between the
phosphine and the resorcinarene moiety. Good activities, which however remained inferior to
those of 2, were also observed for the 1,3-dimesitylimidazol-2-ylidene complex 9 (Fig. 2;
Table 1, entry 8). The activity of 9 dropped upon addition of PPh3, this effect, being, as
expected, weaker than that observed with the bulkier complex 2 (Table 1, entries 5 and 9).
Table 1. Nickel-catalysed ethylene dimerisation.[a]
Entry
Nickel complex
PPh3
no
1-Butene (g) TOF (mol(C2H4) mol(Ni)-1 h-1
1
2
1
1
2
2
2
8
0.012
0.049
0.078
0.108
0.006
0.014
85
350
555
385
40
yes
no
3
4[b]
no
5
yes
no
6
100
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
9
7
8
8
9
9
9
yes
no
0.013
0.050
0.038
0.014
90
355
270
100
9
10[c]
yes
yes
[a]
Conditions: nickel complex (5 µmol), NaBH4 (10 µmol), PPh3 (5 µmol), toluene (15 mL)
[b]
[c]
ethylene (20 bar), 90°C, 1 h, depressurisation over 6 min; 2 h; depressurisation over 2
min.
Conclusions
We have shown that the reaction of resorcinarenyl-substituted imidazolium salts with [NiCp2]
results in half-sandwich [Ni(NHC)CpX] complexes in which the NiCpX moiety lies
embedded in the cavitand part. In all previously reported complexes based on cavitand-
derived NHCs the metal fragments were positioned outside the cavity. The ethylene
dimerisation tests carried out with the reported complexes indicate that the presence of NHCs
bearing resorcinarene N-substituents may positively impact on the catalytic activity, but this
effect only operates if the metal centre is in a sufficiently bulky steric environment. Further
work is aimed at exploiting cavitand endo-orientation of metal centres for performing shape
selective reactions.
Experimental Section
General Remarks: All manipulations were carried out in Schlenk-type flasks under dry
argon. Solvents were dried by conventional methods and were distilled immediately before
use. CDCl3 was passed down a 5 cm-thick alumina column, and stored under nitrogen over
1
molecular sieves (4 Å). Routine H and 13C{1H} spectra were recorded with Bruker FT
1
instruments (AC 300, 400 and 500). H NMR spectra were referenced to residual protiated
solvent peaks (δ = 2.05 for acetone-d6, 7.16 ppm for C6D6 and 7.26 ppm for CDCl3). 13C
NMR chemical shifts are reported relative to deuterated solvent peaks (δ = 29.84 and 206.26
for acetone-d6, 128.00 ppm for C6D6 and 77.16 ppm for CDCl3). Chemical shifts and coupling
constants are reported in ppm and Hz, respectively. Infrared spectra were recorded with a
Bruker FT-IR Alpha-P spectrometer. Elemental analyses were carried out by the Service de
Microanalyse, Institut de Chimie, Université de Strasbourg. The catalytic solutions were
analysed with a Varian 3900 gas chromatograph fitted with a WCOT fused silica column (25
m×0.25
mm,
0.25
µm
film
thickness).
5-bromo-4(24),6(10),12(16),18(22)-
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
10
tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]arene (3),[18a] tosyl azide,[22] 5-N-(3-
propyl-1-imidazolylium)-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-
tetrapentylresorcin[4]arene bromide (6),[11a] chloro-(η5-cyclopentadienyl)-[1-N-3-N-bis-
(mesityl)-imidazol-2-yliden] nickel(II) (9)[15] and 1-(2,6-dimethoxyphenyl)-1H-imidazole[11a]
were prepared by literature procedures.
5-Azido-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]
arene (4): n-Butyllithium (1.6 M in hexane; 2.51 mL, 4.02 mmol) was slowly added to a
solution of bromo-cavitand 3 (3.000 g, 3.35 mmol) in THF (150 mL) at -78 °C. After 1 h, the
resulting anion was quenched with tosyl azide (0.792 g, 4.02 mmol), and the mixture was
stirred at room temperature for 24 h. The reaction mixture was washed three times with brine
(3 x 100 mL). The solvent was evaporated under reduced pressure, and the crude product was
purified by column chromatography (Et2O/petroleum ether, 5:95; Rf = 0.32) to give 4 (2.150
g, 75 %). 1H NMR (500 MHz, CDCl3): δ = 7.10 (s, 3H, arom. CH, resorcinarene), 6.88 (s, 1H,
arom. CH, resorcinarene), 6.52 (s, 2H, arom. CH, resorcinarene), 6.48 (s, 1H, arom. CH,
resorcinarene), 5.78 and 4.43 (AB spin system, 4H, OCH2O, 2J = 7.0 Hz), 5.75 and 4.42 (AB
2
spin system, 4H, OCH2O, J = 7.0 Hz), 4.73 (t, 4H, CHCH2, 3J = 8.0 Hz), 2.24-2.17 (m, 8H,
3
CHCH2), 1.44-1.31 (m, 24H CH2CH2CH2CH3), 0.92 (t, 6H, CH2CH3, J = 7.2 Hz), 0.91 (t,
6H, CH2CH3, 3J = 7.2 Hz) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ = 155.13-115.50 (arom.
C), 100.00 (s, OCH2O), 99.70 (s, OCH2O), 36.74 (s, CHCH2), 36.50 (s, CHCH2), 32.16 (s,
CH2CH2CH3), 32.12 (s, CH2CH2CH3), 29.95 (s, CHCH2), 29.91 (s, CHCH2), 27.69 (s,
CHCH2CH2), 27.67 (s, CHCH2CH2), 22.84 (s, CH2CH3), 14.25 (s, CH2CH3) ppm. IR: ν =
2103 and 1286 (N3) cm-1. Elemental analysis calcd (%) for C52H63N3O8 (858.07): C 72.79, H
7.40, N 4.90; found C 72.58, H 7.51 N 4.64.
5-Amino-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]
arene (5): The hydrogenation experiment was carried out in a glass-lined stainless steel
autoclave (100 mL) containing a magnetic stirrer bar. Palladium-on-carbon (10 % Pd w/w,
0.200 g) was added. The autoclave was closed, flushed twice with hydrogen, then a solution
of azide-cavitand 4 (3.000 g, 3.50 mmol) in CH2Cl2 (15 mL) was added, the autoclave
pressurised to 5 bar and the reaction mixture was stirred at room temperature for 24 h. The
autoclave was depressurised, the solution was passed through a plug of Celite and the solvent
1
was evaporated under reduced pressure to give 5 (2.897 g, 100 %). H NMR (400 MHz,
CDCl3): δ = 7.10 (s, 3H, arom. CH, resorcinarene), 6.51 (s, 1H, arom. CH, resorcinarene),
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
11
6.50 (s, 1H, arom. CH, resorcinarene), 6.47 (s, 2H, arom. CH, resorcinarene), 5.79 and 4.38
(AB spin system, 4H, OCH2O, 2J = 7.0 Hz), 5.74 and 4.47 (AB spin system, 4H, OCH2O, 2J =
7.2 Hz), 4.72 (t, 2H, CHCH2, 3J = 8.4 Hz), 4.69 (t, 2H, CHCH2, 3J = 8.4 Hz), 3.76 (s, 2H
NH2), 2.25-2.17 (m, 8 H, CHCH2CH2), 1.45-1.32 (m, 24H CH2CH2CH2CH3) 0.92 (t, 12H,
CH2CH3, 3J = 7.0 Hz) ppm. 13C{1H} NMR (126 MHz, CDCl3): δ = 154.98-107.39 (arom. C),
99.64 (s, OCH2O), 99.26 (s, OCH2O), 36.75 (s, CHCH2), 36.49 (s, CHCH2), 32.22 (s,
CH2CH2CH3), 32.18 (s, CH2CH2CH3), 29.99 (s, CHCH2), 29.87 (s, CHCH2), 27.78 (s,
CHCH2CH2), 27.72 (s, CHCH2CH2), 22.85 (s, CH2CH3), 14.27 (s, CH2CH3) ppm. MS (ESI-
TOF): m/z = 832.48 [M + H]+, expected isotopic profile. Elemental analysis calcd (%) for
C52H65NO8 (832.08): C 75.06, H 7.87, N 1.68; found C 75.22, H 7.80, N 1.79.
2-N-5-N-Bis-[4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl-
resorcin[4]arene-5-yl]imidazolinium chloride (7): Amino-cavitand 5 (5.000 g, 6.01 mmol)
was dissolved in 100 mL of EtOAc with acetic acid (0.5 mL) and the Schlenk tube was
wrapped in aluminium foil. Glyoxal (40 % weight in water, 0.42 mL, 3.00 mmol) was then
slowly added and the reaction mixture was stirred at 70 °C for 24 h. The yellow mixture was
cooled to room temperature before addition of paraformaldehyde (0.090 g, 3.00 mmol) and
TMSCl (0.38 mL, 3.00 mmol). The resulting solution was stirred at 70 °C for a further 18 h.
After cooling to room temperature, the solvent was evaporated under reduced pressure and the
crude product was purified by column chromatography (MeOH/CH2Cl2, 5:95; Rf = 0.42) to
1
give 7 (0.677 g, 13 %). H NMR (400 MHz, CDCl3): δ = 8.88 (s, 1H, NCHN), 7.59 (s, 2H,
arom. CH, NCH=CHN), 7.38 (s, 2H, arom. CH, resorcinarene), 7.12 (s, 6H, arom. CH,
resorcinarene), 6.56 (s, 4H, arom. CH, resorcinarene), 6.52 (s, 2H, arom. CH, resorcinarene),
2
5.72 and 4.55 (AB spin system, 8H, OCH2O, J = 7.2 Hz), 5.58 and 4.60 (AB spin system,
2
3
8H, OCH2O, J = 7.2 Hz), 4.75 (t, 4 H, CHCH2, J = 7.6 Hz), 4.73 (t, 4 H, CHCH2, 3J = 8.0
Hz), 2.31-2.19 (m, 16 H, CHCH2CH2), 1.47-1.25 (m, 48 H CH2CH2CH2CH3), 0.92 (t, 12H,
CH2CH3, 3J = 6.8 Hz), 0.91 (t, 12H, CH2CH3, 3J = 6.8 Hz) ppm. 13C{1H} NMR (101 MHz,
CDCl3): δ = 155.75-117.28 (arom. C), 138.29 (s, arom. CH, NCHN), 125.44 (s, arom. CH,
NCH=CHN), 100.52 (s, OCH2O), 99.69 (s, OCH2O), 36.80 (s, CHCH2), 36.49 (s, CHCH2),
32.13 (s, CH2CH2CH3), 32.00 (s, CH2CH2CH3), 30.08 (s, CHCH2), 29.84 (s, CHCH2), 27.66
(s, CHCH2CH2), 27.63 (s, CHCH2CH2), 22.78 (s, CH2CH3), 14.18 (s, CH2CH3) ppm. MS
(ESI-TOF): m/z = 1697.93 [M - Cl]+, expected isotopic profile. Elemental analysis calcd (%)
for C107H129N2O16Cl (1734.63): C 74.09, H 7.50, N 1.61; found C 74.53, H 7.42, N 1.84.
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[2-N-(2,6-dimethoxyphenyl)-5-N-propyl]imidazolinium bromide: 1-Aryl imidazole (0.306
g, 1.50 mmol) and propyl bromide (0.27 mL, 3.00 mmol) were heated at reflux for 2 days.
After cooling to room temperature, the solution was evaporated to dryness. The residue was
washed with petroleum ether and dried under vacuum to afford the imidazolium salt (0.455 g,
1
93 %). H NMR (400 MHz, CDCl3): δ = 10.28 (s, 1H, NCHN), 7.52 (br, 1H, NCH=CHN),
7.42 (t, 1H, arom. CH, p-H of C6H3(OMe)2), 3J = 8.6 Hz), 7.23 (br, 1H, NCH=CHN), 6.69 (d,
3
3
2H, arom. CH, m-H of C6H3(OMe)2), J = 8.6 Hz), 4.70 (t, 2H, NCH2, J = 7.2 Hz), 3.86 (s,
3
6H, OCH3), 2.02 (quint, CH2CH3, J = 7.2 Hz), 1.04 (t, 3H, CH2CH3, 3J = 7.2 Hz) ppm. 13C
NMR (101 MHz, CDCl3): δ = 154.58-104.74 (arom. C), 139.46 (s, NCHN), 124.23 (s,
NCH=CHN), 120.84 (s, NCH=CHN), 56.72 (s, OCH3), 51.69 (s, NCH2), 24.03 (s, CH2CH3),
10.65 (s, CH2CH3) ppm. MS (ESI-TOF): m/z: 247.14 [M - Br]+ expected isotopic profile.
Elemental analysis calcd (%) for C14H19N2O8Br (Mr = 327.22): C 51.39, H 5.85, N 8.56;
found: C 51.45, H 5.98, N 8.47.
General procedure for the preparation of the nickel complexes 1, 2 and 8. A
stoichiometric (1:1) mixture of [NiCp2] and the appropriate imidazolium salt in THF was
heated under reflux for 12 h. After cooling to room temperature, the solution was evaporated
to dryness. The residue was purified by column chromatography to give the purple nickel
complexes.
Bromo-(η5-cyclopentadienyl)-[1-N-{4(24),6(10),12(16),18(22)-tetramethylenedioxy-
2,8,14,20-tetrapentylresorcin[4]arene-5-yl}-3-N-propyl-imidazol-2-yliden] nickel(II) (1):
Rf = 0.41 EtOAc/petroleum ether, 20:80. Yield 0.120 g (20 %). 1H NMR (300 MHz, C6D6): δ
= 7.60 (s, 1H, arom. CH, resorcinarene), 7.46 (s, 2H, arom. CH, resorcinarene), 7.40 (s, 1H,
arom. CH, resorcinarene), 6.86-6.50 (br, 2H, arom. CH, resorcinarene), 6.36 (s, 1H, arom.
CH, resorcinarene), 6.34 (d, 1H, NCH=CHN, 3J = 2.5 Hz), 6.28 (d, 1H, NCH=CHN, 3J = 2.5
2
Hz), 5.59 and 4.32 (AB spin system, only 2H are clearly visible, OCH2O, J = 7.2 Hz), 5.35
2
and 4.27 (AB spin system, weakly observed, OCH2O, J = 7.0 Hz), 5.31-5.21 (m, 2H,
CHCH2), 5.13-5.01 (m, 2H, CHCH2), 4.97-4.90 (m, weakly observed, NCH2), 3.68 (s, 5H,
C5H5), 2.45-2.21 (m, 8H, CHCH2), 1.83-1.69 (m, 2H, NCH2CH2), 1.47-1.19 (m, 24H,
CH2CH2CH2CH3), 0.84 (t, 6H, CH2CH3, 3J = 6.9 Hz), 0.81 (t, 6H, CH2CH3, 3J = 7.2 Hz), 0.79
(t, 3H, N(CH2)2CH3, 3J = 7.2 Hz) ppm. 13C NMR (75 MHz, C6D6): δ = 163.91-117.18 (arom.
C), 125.14 (s, NCH=CHN), 122.80 (s, NCH=CHN), 99.96 (s, OCH2O), 91.55 (s, C5H5), 54.75
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(s, NCH2), 37.25 (s, CHCH2), 36.89 (s, CHCH2), 32.30 (s, CH2CH2CH3), 30.55 (s, CHCH2),
28.09 (s, CHCH2CH2), 27.94 (s, CHCH2CH2), 24.43 (s, NCH2CH2), 23.12 (s, CH2CH3), 23.05
(s, CH2CH3), 14.30 (s, CH2CH3), 11.25 (s, N(CH2)2CH3) ppm. MS (ESI-TOF): m/z: 1126.41
[M]+ expected isotopic profile. Elemental analysis calcd (%) for C63H77N2O8NiBr (Mr =
1128.89): C 67.03, H 6.87, N 2.48; found: C 66.89, H 6.82, N 2.41.
Chloro-(η5-cyclopentadienyl)-[1-N-3-N-bis-{4(24),6(10),12(16),18(22)-tetramethylene
dioxy-2,8,14,20-tetrapentylresorcin[4]arene-5-yl}-imidazol-2-yliden] nickel(II) (2): Rf =
0.34 EtOAc/petroleum ether, 20:80. Yield 0.144 g (35 %).1H NMR (300 MHz, C6D6): δ =
7.69 (s, 1H, arom. CH, resorcinarene), 7.65 (s, 1H, arom. CH, resorcinarene), 7.61 (s, 2H,
arom. CH, resorcinarene), 7.52 (s, 1H, arom. CH, resorcinarene), 7.47 (s, 2H, arom. CH,
resorcinarene), 7.41 (s, 1H, arom. CH, resorcinarene), 6.82 (s, 2H, arom. CH, resorcinarene),
6.68 (s, 2H, arom. CH, resorcinarene), 6.49 (s, 1H, NCH=CHN), 6.33 (s, 1H, arom. CH,
resorcinarene), 6.21 (s, 1H, arom. CH, resorcinarene), 6.13 and 5.71 (AB spin system, 4H,
2
2
OCH2O, J = 7.5 Hz), 5.60 and 4.33 (AB spin system, 4H, OCH2O, J = 7.5 Hz), 5.53 and
2
4.07 (AB spin system, 4H, OCH2O, J = 7.0 Hz), 5.37 and 4.33 (AB spin system, 4H,
OCH2O, 2J = 7.0 Hz), 5.31-5.27 (m, 5H, NCH=CHN and CHCH2), 5.17 (t, 2H, CHCH2, 3J =
3
8.0 Hz), 5.06 (t, 2H, CHCH2, J = 8.0 Hz), 3.71 (s, 5H, C5H5), 2.55-2.46 (m, 2H, CHCH2),
2.43-2.30 (m, 10H, CHCH2), 2.27-2.23 (m, 4H, CHCH2), 1.54-1.48 (m, 4H,
CH2CH2CH2CH3), 1.46-1.19 (m, 44H, CH2CH2CH2CH3), 0.87-0.80 (m, 24H, CH2CH3) ppm.
13C NMR (126 MHz, C6D6): δ = 171.57-117.03 (arom. C), 124.17 (s, NCH=CHN), 123.66 (s,
NCH=CHN), 99.93 (s, OCH2O), 99.47 (s, OCH2O), 99.36 (s, OCH2O), 98.95 (s, OCH2O),
92.03 (s, C5H5), 37.47 (s, CHCH2), 37.31 (s, CHCH2), 37.04 (s, CHCH2), 36.88 (s, CHCH2),
32.35 (s, CH2CH2CH3), 32.32 (s, CH2CH2CH3), 32.30 (s, CH2CH2CH3), 32.28 (s,
CH2CH2CH3), 30.56 (s, CHCH2), 30.38 (s, CHCH2), 29.94 (s, CHCH2), 28.10 (s,
CHCH2CH2), 27.99 (s, CHCH2CH2), 27.94 (s, CHCH2CH2), 23.14 (s, CH2CH3), 23.10 (s,
CH2CH3), 23.06 (s, CH2CH3), 23.04 (s, CH2CH3), 14.30 (s, CH2CH3) ppm. MS (ESI-TOF):
m/z: 1819.90 [M - Cl]+ expected isotopic profile. Elemental analysis calcd (%) for
C112H133N2O16NiCl (Mr = 1857.41): C 72.42, H 7.22, N 1.91; found: C 72.11, H 7.15, N 1.53.
Bromo-(η5-cyclopentadienyl)-[1-N-(2,6-dimethoxyphenyl)-3-N-pentyl)-imidazol-2-
yliden] nickel(II) (8): Rf = 0.45 EtOAc/petroleum ether, 20:80. Yield 0.259 g (55 %).1H
3
NMR (500 MHz, acetone-d6): δ = 7.56 (t, 1H, arom. CH, p-H of C6H3(OMe)2), J = 7.7 Hz),
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10.1002/ejic.201701143
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7.39 (s, 1H, NCH=CHN), 7.07 (s, 1H, NCH=CHN), 6.92 (d, 2H, arom. CH, m-H of
3
C6H3(OMe)2), J = 7.7 Hz), 5.11-4.79 (m, 2H, NCH2), 4.71 (s, 5H, C5H5), 3.82 (s, 6H,
OCH3), 2.10 (quint, CH2CH3, 3J = 7.5 Hz), 1.08 (t, 3H, CH2CH3, 3J = 7.5 Hz) ppm. 13C NMR
(101 MHz, acetone-d6): δ = 164.45-105.41 (arom. C), 157.96 (s, C-Ni), 126.07 (s,
NCH=CHN), 122.82 (s, NCH=CHN), 91.71 (s, C5H5), 56.63 (s, OCH3), 54.30 (s, NCH2),
24.91 (s, CH2CH3), 11.55 (s, CH2CH3) ppm. MS (ESI-TOF): m/z: 369.11 [M - Br]+ expected
isotopic profile. Elemental analysis calcd (%) for C19H23N2O2NiBr (Mr = 450.00): C 50.71, H
5.15, N 6.22; found: C 50.55, H 5.06, N 6.14.
General catalytic testing procedure for ethylene dimerisation: The catalytic runs were
carried out in a glass-lined stainless steel autoclave (100 mL) containing a magnetic stirrer
bar. The autoclave was closed, flushed twice with ethylene, then a solution of nickel complex
(5 µmol) in toluene (5 mL) and a solution of NaBH4 (10 µmol) in toluene (10 mL) were
added, the autoclave pressurised to 20 bar and the reaction mixture stirred at 90 °C for 1 h. At
the end of the run, the autoclave was cooled down to 6 °C and depressurised over 8 minutes.
The flask containing the reaction mixture was weighed. This procedure was performed as
quickly as possible to minimise the potential evolution of the butene products. The solution
was analysed by GC. The reaction yield was determined from the final mass of reaction
mixture versus the mass of the control reaction solution. To determine the mass of the control
reaction solution (reaction mixture with no catalyst or co-catalyst), toluene (15 mL) was
added to the autoclave and stirred under 20 bar of ethylene at 90 °C for 1 h. The reactor was
cooled down to 6 °C, vented to ambient pressure over 8 minutes and the flask containing the
reaction mixture was weighed. Each catalytic test was repeated three times.
X-ray crystal structure analysis of nickel complex 1: Single crystals of 1 suitable for X-ray
diffraction were obtained by slow diffusion of hexane into a benzene solution of the complex.
Crystal data: C126H154Br2N4Ni2O16•C6H6, Mr = 2335.88 g mol–1, triclinic, space group P -1, a
= 14.1582(8) Å, b = 15.919(1) Å, c = 27.059(1) Å, α = 106.797(5)°, β = 91.670(4)°, γ =
91.253(5)°, V = 5833.1(5) Å3, Z = 2, D = 1.330 g cm–3, µ = 1.075 mm-1, F(000) = 2468.0, T =
120(2) K. The sample was studied on a Oxford Diffraction Xcalibur Sapphire 3 diffractometer
(graphite monochromated Mo-Kα radiation, λ = 0.71073 Å). The data collection (2θmax
=
27.0°, omega scan frames by using 0.7 ° omega rotation and 30 s per frame, range hkl: h -
17,18 k -20,20 l -33,34) gave 47133 reflections. The structure was solved with SIR- 97,[23]
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10.1002/ejic.201701143
European Journal of Inorganic Chemistry
15
which revealed the non-hydrogen atoms of the molecule. After anisotropic refinement, all of
the hydrogen atoms were found with a Fourier difference map. The structure was refined with
SHELXL97[24] by the full-matrix least-square techniques (use of F square magnitude; x, y, z,
ij for C, Br, N, Ni and O atoms; x, y, z in riding mode for H atoms); 1406 variables and 4978
2
2
observations with I > 2.0 σ(I); calcd. w = 1/[σ2(Fo ) + (0.0P)2 + 0.0P] where P = (Fo +
2Fc2)/3, with the resulting R = 0.0595, RW = 0.1109 and SW = 0.822, Δρ < 0.469 e Å–3. CCDC
entry 876002 contains the supplementary crystallographic data. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif or by e-mailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK.
Acknowledgments
We acknowledge financial support from the Indo-French Centre for the Promotion of
Advanced Research (IFCPAR) (Project No 5005-1).
Keywords: Resorcinarene · Nickel complex · Catalysis · Imidazolium salt · Ethylene
dimerisation
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