Displacement of η5-cyclopentadienyl ligands from half-sandwich: C, C -(NHC-cyanoalkyl)-nickel(II) metallacycles: Further insight into the structure of the resulting Cp-free nickelacycles and a catalytic activity study

Article abstract of DOI:10.1039/c7dt04560c

Four cationic C,C-(NHC-cyanoalkyl)-nickel(ii) metallacyclic complexes, [Ni{Me-NHC-CH₂CH(CN)}(NCMe)](PF₆) (2a), [Ni{Mes-NHC-CH₂CH(CN)}(NCMe)](PF₆) (2b), [Ni{Mes-NHC-(CH₂)₂CH(CN)}(NCMe)](PF₆) (2c) and [Ni{DiPP-NHC-(CH₂)₂CH(CN)}(NCMe)](PF₆) (2d), were prepared by the removal of the Cp ligand under acidic conditions at 0 °C from the corresponding half-sandwich nickelacycles [NiCp{R-NHC-(CH₂)_nCH(CN)}] (1a-1d; Cp = η⁵-C₅H₅; n = 1 or 2; R-NHC-(CH₂)_nCH(CN) = 1-R-3-[(CH₂)_nCH(CN)]-imidazol-2-ylidene). Full characterization of 2a-d by ¹H and ¹³C{¹H} NMR spectroscopy in CD₃CN and pyridine-d₅, ATR-FTIR spectroscopy, mass spectrometry, and CHN microanalyses established the presence of only one acetonitrile ligand per nickel atom in the solid state. A DFT structural study conducted on the cations of the methyl-substituted 5-membered nickelacycle 2a and the mesityl-substituted 6-membered cycle 2c found a small energetic cost (ΔG = 7-12 kcal mol^-1) for the loss of one acetonitrile ligand from the square-planar structures existing in solution, that should be easily amenable upon solvent evaporation (ΔG^? = 14 kcal mol^-1 in the case of 2c). Two structures with one acetonitrile ligand could be optimized in both cases: (i) a truly T-shaped 14-electron structure with an end-on acetonitrile ligand, and (ii) a masked T-shaped structure stabilized by the π-coordination of the dangling CN group of the metallated alkyl chain, the latter being favoured by 2.4 kcal mol^-1 in the case of the flexible 6-membered ring 2c. A comparison of calculated vibrational frequencies with experimental FTIR spectra ruled out π-coordination of the dangling CN group as a ν(CN) band at low frequency was absent. Complexes 2a-d thus probably exist as rare three-coordinate T-shaped 14-electron species in the solid state. Their catalytic activity was studied for the direct arylation of azoles, and 2c proved to be moderately active for the coupling of benzothiazole with aryl iodides. Mechanistic insights suggest that competing processes or a radical process catalysed by nickel particles could follow an initial reduction of 2c by the dimerization of a sacrificial amount of benzothiazole.

Full text of DOI:10.1039/c7dt04560c

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ARTICLE
Displacement of η5-cyclopentadienyl ligands from half-sandwich
C,C-(NHC-cyanoalkyl)–nickel(II) metallacycles: further insight into
the structure of the resulting Cp-free nickelacycles and a catalytic
activity study
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Bernardo de P. Cardoso,a Jean-Marie Bernard-Schaaf,a Saurabh Shahane,a Luis F. Veiros,b Michael
J. Chetcuti*a and Vincent Ritleng*a,c
www.rsc.org/
The four cationic C,C-(NHC-cyanoalkyl)–nickel(II) metallacyclic complexes, [Ni{Me-NHC-CH2CH(CN)}(NCMe)](PF6) (2a),
[Ni{Mes-NHC-CH2CH(CN)}(NCMe)](PF6) (2b), [Ni{Mes-NHC-(CH2)2CH(CN)}(NCMe)](PF6) (2c) and [Ni{DiPP-NHC-
(CH2)2CH(CN)}(NCMe)](PF6) (2d) were prepared by the removal of the Cp ligand under acidic conditions at 0 °C from the
corresponding half-sandwich nickelacycles [NiCp{R-NHC-(CH2)nCH(CN)}] (1a-1d; Cp
= n = 1 or 2; R-NHC-
η5-C5H5;
(CH2)nCH(CN) = 1-R-3-[(CH2)nCH(CN)]-imidazol-2-ylidene). Full characterization of 2a-d by 1H and 13C{1H} NMR spectroscopy
in CD3CN and pyridine-d5, ATR-FTIR spectroscopy, mass spectrometry, and CHN microanalyses established the presence of
only one acetonitrile ligand per nickel atom in the solid state. A DFT structural study conducted on the cations of the
methyl-substituted 5-membered nickelacycle 2a and the mesityl-substituted 6-membered cycle 2c found a small energetic
cost (ΔG = 7–12 kcal.mol-1) for the loss of one acetonitrile ligand from the square-planar structures existing in solution,
that should be easily amenable upon solvent evaporation (ΔG‡ = 14 kcal.mol-1 in the case of 2c). Two structures with one
acetonitrile ligand could be optimized in both cases: (i) a truly T-shaped 14-electron structure with an end-on acetonitrile
ligand, and (ii) a masked T-shaped structure stabilized by the π-coordination of the dangling CN group of the metallated
alkyl chain, the latter being favoured by 2.4 kcal.mol-1 in the case of the flexible 6-membered ring 2c. Comparison of
calculated vibrational frequencies with experimental FTIR spectra ruled out π-coordination of the dangling CN group as a
ν(C≡N) band at low frequency was absent. Complexes 2a-d thus probably exist as rare three-coordinate T-shaped 14-
electron species in the solid state. Their catalytic activity was studied for the direct arylation of azoles, and 2c proved to be
moderately active for the coupling of benzothiazole with aryl iodides. Mechanistic insights suggest that competing
processes or a radical process catalysed by nickel particles could follow an initial reduction of 2c by the dimerization of a
sacrificial amount of benzothiazole.
of truly T-shaped 14-electron nickel(II) species – where no
stabilizing secondary interaction (agostic bond, solvent
molecule or counteranion) is observed at the fourth position of
Introduction
Coordinatively and electronically unsaturated complexes are
often proposed as key intermediates in transition metal-
catalyzed reactions. Thus, ligand dissociation to form
intermediates with open coordination sites is frequently
postulated as the initial step in reactions involving 16-electron
square-planar precatalysts. Resulting T-shaped d8 three-
coordinate complexes remain however a curiosity and well-
characterized examples are rare.1 In particular, only a handful
the square plane – have been described.2 In all but one case,2a
these species bear a bulky 1,3-(2,6-diisopropylphenyl)imidazol-
2-ylidene (IPr) or tricyclohexylphosphine (PCy3) ligand and are
comprised of a mono- or di-anionic C,N- or C,O-nickelacycle
(Scheme 1). Herein we report a series of cationic nickelacycles
bearing a mono-anionic C,C-NHC-cyanoalkyl chelate, for which
elemental analyses, NMR, HRMS, and FTIR spectra, coupled to
DFT calculations suggest they exist as 14-electron T-shaped
three-coordinate species in the solid state.
We recently reported the facile and clean removal of the
η5-cyclopentadienyl (Cp) ligand from 18-electron half-
sandwich nickelacyles
tether under acidic conditions in acetonitrile.3 Neutral 16-
electron square-planar complexes were isolated and fully
characterized after treatment of the resulting cationic species
1 bearing an anionic NHC-cyanoalkyl
3
2, bearing labile acetonitrile ligand(s) with potassium acetyl-
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Scheme 3
Optimized syntheses of the nickelacyles 1a-d and 2a-d.
spectroscopy and by CHN microanalyses. Their spectroscopic
features are similar to those of the previously reported species
1a and 1c and deserve no particular comment (see Fig. S1 to
S8, ESI).
Scheme 1
Well-characterized T-shaped tricoordinate nickel(II) complexes.2
X-ray quality crystals of 1a and 1b were grown from
concentrated solutions in THF and their structures were
established by X-ray diffraction studies. Crystallographic data
and data collection and refinement parameters are grouped
together in Table S1 (see ESI). The structures of 1a and 1b are
depicted in Figure 1 and 2, respectively, and key bond lengths
and bond angles are collected in the figure captions. Both
complexes have structures that are closely related to each
other, with slightly puckered nickelacyclic rings, and to that of
1c that was reported earlier.5 The only notable difference
resides in the CNHC–Ni–Calkyl bite angle that is considerably
narrower in the 5-membered metallacycles 1a and 1b (ca.
84.7°) than in the 6-membered metallacycle 1c (93.9°),5 which
may explain the lower yields observed for the formation of the
5-membered cycles.
Scheme 2
Cp acidolysis in acetonitrile (1 to 2) and subsequent complexation by an
acetylacetonate chelate (2 to 3). Proposed square-planar structure of complexes 2.3
acetonate (Scheme 2). Remarkably, the nickel–alkyl and
nickel–carbene bonds were not ruptured in this
unprecedented Cp acidolysis. This robustness opened up
interesting perspectives for the use of the potentially
unsaturated Cp-free cationic species 2. The latter, however,
proved difficult to recrystallize and were ill-characterized.3
Further work on their characterization was therefore needed
prior to studying their reactivity. Our findings, which show
they are not square-planar with two acetonitrile ligands in the
solid state as initially presumed (Scheme 2), are described
herein. Their activity as precatalysts for the arylation of azoles
is also reported.
Removal of the Cp ligand of 1a-d was then achieved by
reaction with equimolar amounts of HCl (1 M) and KPF6 in
acetonitrile. Adding HCl at 0 °C instead of room temperature
was found to give even cleaner reactions than reported for 2c
(Scheme 2),3 and the nickelacyles 2a-d were obtained in good
to excellent isolated yields‡ after thorough drying under high
vacuum of the resulting moderately air-stable dark yellow
powders (Scheme 3).
Nickelacycles 2a-d are very poorly soluble or insoluble in
most common solvents such as toluene, benzene, n-pentane,
Results and discussion
Synthesis and characterization of a series of cationic species (2)
A series of four species
2 (2a: n = 1, R = Me; 2b: n = 1, R = Mes;
2c: n = 2, R = Mes; 2d: n = 2; R = DiPP, where Mes = mesityl and
DiPP = 2,6-diisopropylphenyl) were prepared via optimized
synthetic routes (Scheme 3).
Treatment of the half-sandwich [Ni(NHC)XCp]4 complexes
(a-d) bearing an alkylnitrile side arm with potassium
bis(trimethylsilyl)amide (KHMDS) in toluene at room
temperature allowed the abstraction of one of the methylene
hydrogen atoms in the α position to the nitrile group. Use of
KHMDS instead of KOtBu, as previously reported,5,6 was found
to give cleaner reactions and the half-sandwich nickelacyles
1a-d were isolated in yields varying from ca. 25 % for the 5-
membered cycles to ca. 65 % for the 6-membered cycles on
multi-hundred mg scales (Scheme 3). The novel complexes 1b
and 1d were fully characterized by 1H and 13C{1H} NMR and IR
Figure 1
Molecular structure of 1a. The only hydrogen atom shown is that of the
CHCN group (as a white isotropic sphere). Ellipsoids are shown at the 50% probability
level, and key atoms are labelled. The centrosymmetric space group P21/c contains
both enantiomers; only the (S) enantiomer is shown. Selected distances (Å) and angles
(°) with esds’s in parenthesis: Ni–C1, 1.8475(11); Ni–C6, 1.9697(11); C6–C7, 1.4491(17);
C7–N3, 1.1491(17); Ni–Cpcent, 1.756; C1–Ni–C6, 84.71(5); C1–Ni–Cpcent, 140.9; C6–Ni–
Cpcent, 134.3; C6–C7–N3, 178.84(14).
2 | J. Name., 2012, 00, 1-3
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indicates an increase in Lewis acid character of the nickel
centre caused by a decrease in the σ-donor ability of the
ancillary ligands (NCMe < acac- < Cp-).7
1
Regarding the acetonitrile ligands, the presence in the H
NMR (CD3CN) spectra of 2a-d of a singlet on the downfield side
of the solvent residual multiplet (at 1.96 ppm) indicates the
presence of free CH3CN resulting from CH3CN–CD3CN
exchange. Integration of this singlet systematically indicated
the presence of three protons relatively to the other signals
(Fig. S9, S13, S17 and S21), which suggested the presence of
only one acetonitrile molecule per “naked” nickelacycle in the
solid state. Similarly, the 1H NMR spectra of 2a-d in pyridine-d5
(see Fig. S10, S14, S18 and S22) all display one singlet
integrating for three protons (relative to the metallacycles’
signals) at 1.83-1.86 ppm, which shows the presence of one
acetonitrile molecule per “naked” nickelacycle. Interestingly in
all cases but 2a, the acetonitrile molecule is observed as free
acetonitrile, as pyridine-d5 has probably displaced the labile
acetonitrile ligand. In contrast for 2a, the chemical shifts of the
acetonitrile molecule (see Fig. S10 and S12) indicate that it is
coordinated to the nickel atom, and suggest that in this case
pyridine-d5 associates as a fourth ligand but does not displace
the MeCN ligand.
Figure 2
Molecular structure of 1b. The only hydrogen atom shown is that of the
CHCN group (as a white isotropic sphere). Ellipsoids are shown at the 50% probability
level, and key atoms are labelled. Two independent but very similar molecules (A and
B) are present in the asymmetric unit. The centrosymmetric space group P-1 contains
both enantiomers of each molecule and only the (S) enantiomer of molecule A is
shown. Selected distances (Å) and angles (°) with esd’s in parenthesis of the two
independent molecules are given here: Ni–C1, 1.842(6), 1.844(5); Ni–C14, 1.980(7),
1.969(6); C14–C15, 1.432(9), 1.444(9); C15–N3, 1.135(8), 1.142(8); Ni–Cpcent, 1.748,
1.754; C1–Ni–C14, 85.2(3), 84.3(2); C1–Ni–Cpcent, 141.8; 142.9 C14–Ni–Cpcent, 132.8,
132.7; C14–C15–N3, 177.8(8), 179.0(7).
chloroform, diethyl ether, tetrahydrofuran, 1,4-dioxane or
water, readily decompose in halogenated solvents such as
dichloromethane and tetrachloroethane, and are extremely
soluble in polar coordinating solvents such as acetone,
dimethylsulfoxide, methanol, nitromethane, acetonitrile or
pyridine – the latter two being the only ones in which they are
soluble and stable. Despite repeated attempts, none of them
could be recrystallized and no X-ray quality crystals could be
grown. Crystals of [Ni(CH3CN)6](PF6)2, resulting from the
decomposition of 2c in acetonitrile, were harvested twice.
Nickelacycles 2a-d were thus characterized by 1H and
13C{1H} NMR spectroscopy in CD3CN and pyridine-d5, ATR-FTIR
spectroscopy, mass spectrometry, and by CHN microanalyses.
In agreement with the NMR data, HRMS spectra of 2a-d
(MeCN solution, electrospray ionization, positive ion mode) all
show the [nickelacycle–NCMe]+ ion as the base peak, and
elemental analyses also confirmed the formulation with only
one acetonitrile ligand per nickel atom. All this excludes the
possibility of complexes
2 being 16-electron square-planar
species with two acetonitrile ligands in the solid state, as
initially presumed.3 The suspected fluxional behaviour
observed in solution (vide supra) may hence originate from an
equilibrium between the saturated square-planar and an
unsaturated species.
Similarly to what had been observed for 2c 3 the 1H and 13C
,
Regarding the nature of this formally unsaturated species,
several hypotheses may be drawn. Complexes
2 may a priori
NMR spectra of 2a,b,d in CD3CN clearly show the presence of
the metallacycles, whose signals’ chemical shifts and
multiplicities are similar to those of their precursors 1a,b,d
exist as: (i) dimers and/or oligomers with bridging acetonitrile
ligands as postulated after similar loss of an acetonitrile ligand
upon drying under vacuum for a related (but with bridging
bromide ligands) C,C-NHC,cycloheptatrienyl-palladacycle,8 (ii)
monomers containing very rare examples of four-electron
donor side-on acetonitrile ligand,9 or (iii) rare three-coordinate
14-electron Ni(II) species with an end-on acetonitrile ligand.2
In the former case, the possible existence of dimers and/or
oligomers in equilibrium with the more likely square-planar
species in solution could explain why crystals of complexes 2a-
1
(see Fig. S9 to S24, ESI). Interestingly, the H NMR spectra of
2b and 2c display two singlets in a 1:1 integrated ratio for the
two meta-hydrogen atoms as well as two singlets in a 3:3
relative integrated ratio for the ortho-methyl groups of the
mesityl substituents, both of which suggest restricted rotation
about the N–mesityl bond on the NMR time scale. Similarly,
two doublets (for the meta aromatic ring protons), two septets
(for the CH protons of the isopropyl groups), and four doublets
(for the methyl protons of the isopropyl groups) are observed
d
could not be grown.8,10 However, bridging acetonitrile
ligands are rare,11,12,13,14 and we are aware of only one
structurally characterized example with a metal of groups 8-
10: a tetranuclear nickel complex, [Ni(PCy3)(NCMe)]4,15 in
which μ2,η1,η2-acetonitrile ligands16 are π-bonded to a nickel
centre and σ-bonded to a second one. Four-electron side-on
acetonitrile ligands are even more rare and have only been
described with group 6 metals.9,17 In addition, in both cases,
the C≡N stretch of the acetonitrile ligands should appear
around 1700 cm-1,9d,e,15,17 which is not the case here (see Fig.
S25 to S28, ESI) and militates against these hypotheses.
1
in the H NMR spectrum of 2d, all of which indicate restricted
rotation about the N–DiPP (2,6-diisopropylphenyl) bond as
well. Furthermore, the 13C NMR spectra of 2a-d all display
relatively broad signals for the carbons that are bound to the
nickel centre, i.e.: for the carbene and CHCN carbons. This may
result from a fluxional process related to the lability of the
acetonitrile ligands (vide infra).§ In addition, it is worth
mentioning that the carbene carbon signals of 2a-d are all high
field shifted (153.8–158.4 ppm in CD3CN) when compared to
1a-d (171.3–175.9 ppm)5,6a and 3a (163.1 ppm)3, which likely
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+
However, it is well known that ν(C≡N) bands may be
weak9d,e,12c,f or sometimes unobserved.9a,b,c,18 Hence, the
absence of a significant band between 1600 and 1800 cm-1 in
the IR spectra of 2a-d (Fig. S25 – S28) should not be considered
definitive, and these hypotheses cannot be totally ruled out on
this sole basis.
2a
2c
Me
H3C
N
C
+
N
N
ν(CN) = 2266 cm-1
Ni
C
N
N
N
2a-NCMe-CN+
ν(CN) = 2263 cm-1
ν(CN) = 2038 cm-1
Ni
C
N
C
H3C
ꢀG = 5 kcal/mol
2c-NCMe+
N
In the case of rare but known three-coordinate 14-electron
Ni(II) species,2 much more frequent end-on acetonitrile ligands
should be present and the acetonitrile C≡N stretch would be
expected to appear between 2200 and 2300 cm-1.19 This in
agreement with the medium intensity peaks observed
between 2233 and 2243 cm-1 in the spectra of 2a-d (Fig. S25 –
ν(CN) = 2194 cm-1
+
ꢀG = 2 kcal/mol
Me
N
N
ν(CN) = 2257 cm-1
+
Ni
C
N
C
H3C
C
N
N
H3C
N
2a-NCMe+
N
S28). However, the C
≡
N stretch of the cyanoalkyl arm is
ν(CN) = 2195 cm-1
Ni
C
ν(CN) = 2275 cm-1
ν(CN) = 2034 cm-1
expected in the same area, and although the observed bands
show a shoulder at ca. 2195-2200 cm-1 and may thus be the
result of an overlap of these two vibrations, we cannot ascribe
them with certainty at this stage.
N
2c-NCMe-CN+
ꢀG = 12 kcal/mol
ꢀG = 7 kcal/mol
Density functional theory (DFT) structural study of complexes (2)
+
+
To try to solve these issues and get further insight to the
Me
N
N
H3C
H3C
C
N
N
structure of complexes 2, we undertook a DFT structural study
C
C
N
N
νs(CN) = 2271 cm-1
as(CN) = 2264 cm-1
νs(CN) = 2273 cm-1
as(CN) = 2269 cm-1
Ni
C
Ni
C
on the cations of both the methyl-substituted 5-membered
nickelacycle 2a and the mesityl-substituted 6-membered cycle
2c. Calculation of the free energy balance of the species with
one and two end-on acetonitrile ligands allowed us to identify
three stable structures in each case: (i) the 16-electron square-
planar structure, (ii) a truly T-shaped 14-electron structure,
and (iii) a T-shaped structure with the dangling CN group of the
metallated alkyl chain π-coordinated to the nickel centre (Fig.
3). Three-coordinate Y-shaped species were found unstable,
ν
ν
N
N
C
H3C
H3C
ν(CN) = 2185 cm-1
N
N
ν(CN) = 2193 cm-1
+
+
2c-(NCMe)2
2a-(NCMe)2
Figure 3
Free energy balances of the structures of 2a and 2c cations with one or
two end-on acetonitrile ligands, and calculated vibrational frequencies of the C≡N
bonds.
which is consistent with the fact that complexes
diamagnetic,20 in solution and in the solid state.§§, §§§
2 are
In the case of the 6-membered ring 2c, the cation with only
one acetonitrile ligand would be stabilized by π-coordination
of the dangling CN group by 2.4 kcal.mol-1. However, this
isomer, 2c-NCMe-CN+, is still about 7.1 kcal.mol-1 higher in
energy than the species with two coordinated acetonitrile
d(Ni-N) = 3.80 Å
7
TS1
6
Int
+
ligands, 2c-(NCMe)2 (Fig. 3-right). The formation of the latter
from 2c-NCMe-CN+ starts from the pair of reagents, Int (Fig. 4),
that is 6 kcal.mol-1 less stable than the separated reagents and
has a 4.76 Å distance between the Ni centre and the N atom of
0
the incoming acetonitrile. The following transition state (TS1
)
d(Ni-N) = 4.76 Å
is a rather early one with a large Ni–N distance (3.80 Å) and a
negligible energy barrier (ΔG# = 1 kcal.mol-1). Overall, the
reaction is exergonic with a free energy balance of ΔG = –7
kcal.mol-1 (Fig. 4).
Importantly, a π-bound nitrile isomer of 2c-NCMe+ could
not be optimized. It is not stable and spontaneously yields 2c-
NCMe+, which allows us to rule out this hypothesis. In
−7
2c-NCMe-CN+
+
2c-(NCMe)2
d(Ni-N) = 1.92 Å
(separated)
addition, the most stable dimer that could be optimized, (2c-
2+
NCMe)2
,
shows one “naked” nickelacycle and one
Figure 4
Free energy profile and optimized structures of the relevant species for
the coordination of a second acetonitrile molecule to 2c-NCMe-Me+. Free energy
nickelacycle bearing an end-on acetonitrile, linked together by
a μ2-η1,η1-bridging acetonitrile ligand (Scheme 4). However,
dimerization is an unfavourable process with a free energy
balance of +11 kcal.mol-1, and thus is not expected.
values (kcal.mol-1) relative to the separated reactants.
4 | J. Name., 2012, 00, 1-3
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2+
! """
$-. /"01"23415"
H3C
C
?@"
+
( "; "<@A>"8#
N
C
N
N
N
ꢀG = + 11 kcal.mol-1
N
Ni
H3C
C
N
Ni
?@"
N
( "; "<<=>"8#
N
N
Ni
C
C
N
C
N
N
! "#$ %& ' #%$ (
674"89:8/"
CH3
2c-NCMe-CN+
2+
(2c-NCMe)2
(
*+, "
(
)
$*+, "
Scheme 4
Free energy balance of the dimerization of 2c-NCMe-CN+
! "#$ %& ' (
674"89:8/"
In the case of the more strained 5-membered nickelacycle
2a, the free energy balance of the cation bearing only one
acetonitrile ligand suggests that the dangling CN group is not
coordinated to the nickel atom; the coordinated isomer, 2a-
NCMe-CN+, being 5 kcal.mol-1 higher in energy than the T-
shaped species, 2a-NCMe+. The latter is 12 kcal.mol-1 less
stable than the square planar species 2a-(NCMe)2+ (Fig. 2-left).
In summary, these DFT calculations demonstrate that
although the square planar isomers are more stable by 7 to 12
kcal.mol-1, loss of one of the two acetonitrile molecules
present in the dissolved species could be easily obtained upon
crystallization (energy barrier of only 14 kcal.mol-1 in the case
of 2c), especially if the latter occurs via solvent evaporation,
which is the case here. Interestingly, the exact structure of the
resulting monomeric species bearing one end-on bound
acetonitrile ligand would depend on the ring size of the
nickelacycle, the dangling CN group of the metallated alkyl
chain being π-coordinated to the nickel centre in the case of
flexible 6-membered cycles. If that were to occur, the IR
signature of the 5-membered species, 2a and 2b, should be
different from those of the 6-membered species, 2c and 2d
(Fig. 3). The IR spectra of 2a-d are however remarkably similar
(see Fig. S27–S30). Moreover, none of them shows a band in
the 2020-2030 cm-1 range that would correspond to the π-
bound CN group of the metallated alkyl chain, and the
experimental spectra of 2c closely resemble the DFT-simulated
spectra of 2c-NCMe+ (Fig. 5). Hence, the complexes 2a-d most
probably exist as truly T-shaped 14-electron species in the
solid state.§§§§
(
*+, "
(
)
$*+, "
! "#)$ %& ' * "
!
674"89:8/"
(
*+, "
(
)
$*+, "
3500
3000
2500
! (cm-1)
2000
1500
Figure 5
NCMe+ and 2c-(NCMe)2
IR (ATR) spectrum of 2c and DFT-simulated spectra of 2c-NCMe-CN+, 2c-
+
.
system,22a,d but with only 5 mol% of nickel instead of 10 mol%,
i.e.: in 1,4-dioxane at 90–140 °C in the presence of 2c (5 mol%)
and LiOtBu (2 equiv.). At 90 °C, no reaction was observed after
16 h (Table 1, entry 1). Increasing the temperature to 140 °C
afforded an encouraging 51 % conversion after 16 h reaction
(entry 2). The conversion, however, dropped to 26 % when the
catalyst loading was reduced to 3 mol%, and did not increase
significantly when it was increased to 10 mol% (entries 3 and
4). No conversion was observed in the absence of LiOtBu
(entry 5) or in the presence of any other base, including the
sodium and potassium analogues of LiOtBu (Table S2, ESI),
demonstrating that the choice of the base is crucial as has
often been reported.22-24 The choice of the solvent also proved
crucial as little or no conversion was observed in any other
solvent (Table S2). Satisfyingly, significant improvements of the
conversion to 69 and 78 % were observed after 18 and 36 h of
reaction, respectively (entries 6 and 7).
Catalytic study of complexes (2)
Over the last 10 years, nickel complexes have emerged as very
promising catalysts for C–H bond functionalization reactions.21
In particular, nickel-catalysed biaryl couplings between
(hetero)arenes and aryl halides,22 phenol derivatives,23 or aryl
esters24 have been demonstrated to be reliable alternatives to
palladium-catalysed cross-couplings. It is noteworthy that in
nearly all reported examples, a Ni(0) or Ni(II) source is
associated to a neutral N,N-22a,b,d or P,P-23,24 bidentate ligand.
The activities of complexes 2a-d were then compared using
the conditions of entry 2 or 6. An equivalent conversion was
obtained with the mesityl-substituted 5-membered
nickelacycle 2a (66 %, Table 1, entry 8), showing that the ring
size has almost no influence on the catalytic activity. In
contrast, the methyl-substituted 5-membered cycle 2b and the
DiPP-substituted 6-membered ring 2d gave lower conversions
of 57 and 8 % (entries 9 and 10), showing the importance of
the steric footprint of the NHC moiety.
Thus, the isolation and characterization of complexes
2
opened up the possibility of observing how a robust anionic
C,C-chelate comprising a NHC moiety would behave in such
couplings.
Finally, the activities of the half-sandwich precursor, 1c
and of the square-planar derivative, 3c, of 2c were evaluated
,
Initial studies focused on the coupling of benzothiazole
with iodobenzene catalysed by 2c under conditions akin to
those established by Itami et al. for their Ni(OAc)2/bipyridine
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Table 1
Optimization of the coupling of benzothiazole with iodobenzene a
iodotoluene, and the electron-poor 4-iodobenzonitrile and 4-
iodobenzotrifluoride (entries 1, 3, 7 and 8). However, it does
not react with bromo- or chloro-arenes (entries 2 and 4). The
electron-rich 4-iodoanisole, and the sterically encumbered 2-
iodo-toluene gave rather low yields (entries 5 and 9). On the
other hand, iodobenzene gave only poor yields with other
azoles, such as 4,5-dimethylthiazole and 5-phenyloxazole
(entries 10 and 13), and complex mixtures of isomers were
obtained with oxazole and thiazole (entries 11 and 12) due to
selectivity issues. All in all, 2c is thus a moderately active
catalyst for the arylation of benzothiazole with aryl iodides.
Regarding the reaction mechanism, it is significant that a
small amount (< 5% based on the maximum possible amount
of the coupling product) of the dimerization product of
benzothiazole was systematically observed. This may suggest
initial reduction of 2c to an active species by nickelation of two
molar equivalents of deprotonated benzothiazole followed by
reductive elimination, as was proposed by Itami et al. for their
Ni(OAc)2/bipyridine system.22d To test this hypothesis, we
conducted a reaction under catalytic conditions but in the
absence of aryl iodide (Scheme 5). 2,2’-Bibenzothiazole was
obtained in 93% yield based on nickel (see ESI). A sacrificial
amount of benzothiazole thus seems indeed necessary for the
generation of the active species.
Entry
Catalyst
(mol%)
2c (5)
2c (5)
2c (3)
2c (10)
2c (5)
2c (5)
2c (5)
2a (5)
2b (5)
2d (5)
1c (5)
3c (5)
Temperature
(° C)
90
Time (h)
Conv. (%)b
1
2
16
16
16
16
16
18
36
18
18
16
16
16
0
140
51
26
56
0
3
140
4
5c
140
140
6
140
69
78
66
57
8
7
140
8
140
9
140
10
11
12
140
140
0
140
24
a
Reaction conditions: benzothiazole (1 equiv.), iodobenzene (1.5 equiv.), LiOtBu
b
(2 equiv.), 1c, 2a-d or 3c (3-10 mol%) in 1,4-dioxane at 90-140 °C for 16-36 h.
Conversion determined by GC; average value of two runs. c No LiOtBu.
Table 2
Arylation of azoles with haloarenes catalysed by 2c a
To get further insight into the mechanism, we also checked
Entry
Azole
Aryl halide
X, Y
X = I
Conv. (%)b
Yield (%)c
whether the biaryl coupling was the result of
a true
1
2
3
4
5
78
0
43
-
homogeneous process by conducting the coupling of
benzothiazole with iodobenzene in the presence of 115 equiv.
of Hg relatively to nickel (see ESI).25 Formation of 2,2’-
bibenzothiazole occurred in the same proportions as in a
typical catalytic run. However, almost complete inhibition of
the heterocoupling was observed (Scheme 6), thus suggesting
that a process catalysed by nickel particles could follow the
initial reduction of the nickel(II) pre-catalyst. Of note, the
generation of such catalytically active nickel particles from 2c
could be compatible with the apparent long induction period
observed with 2c (51 % conv. after 16 h vs. 69 % after 18 h –
Table 1, entries 2 and 6).
X = Br
X = I
56
0
47
-
X = Cl
35
25
6
7
8
9
-
14
45
59
13
49
68
-
10
-
17
n.d.d
11
In addition, an experiment was performed in the presence
of 5 mol% of the radical scavenger, TEMPO (see ESI). As seen in
the run with Hg, formation of 2,2’-bibenzothiazole still
occurred and that of 2-phenylbenzo[d]thiazole was almost
completely suppressed (Scheme 6), thus suggesting that a
radical pathway is also likely. All this draws a complicated
picture with possible competing processes, among which a
radical process catalysed by nickel particles might follow the
11
12
13
Y = S
-
-
-
Y = O
a
Reaction conditions: azole (1 equiv.), aryl halide (1.5 equiv.), LiOtBu (2 equiv.),
2c (5 mol%) in 1,4-dioxane at 140 °C for 36 h. b Conversion to the desired coupling
c
product determined by GC; average value of two runs. Isolated yields; average
value of two runs. d A complex isomeric mixture was obtained.
under the conditions of entry 2. No conversion was observed
with 1c and only 24 % with 3c, showing that labile ligands
and/or an unsaturated nickel centre are required to observe a
satisfactory activity.
Scheme 5
Dimerization of benzothiazole mediated by 2c
With the optimized conditions in hand (Table 1, entry 7),
we then briefly examined the scope of the heteroarene
arylation reaction with 2c (Table 2). Benzothiazole was
arylated with moderate to good yields with iodobenzene, 4-
Scheme 6
Effect of additives
6 | J. Name., 2012, 00, 1-3
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initial reduction of 2c by the dimerization of a sacrificial CNRS 7177, Institut de Chimie, Université de Strasbourg. High
amount of benzothiazole.
resolution mass spectra were recorded on a Bruker micrOTOF-
Q mass spectrometer by the Service the Spectrométrie de
Masse, UMR CNRS 7177, Université de Strasbourg.
Commercially available haloarenes and thiazoles as well as n-
dodecane were distilled and stored under argon before use.
Conclusions
In summary, a series of four cationic C,C-(NHC-cyanoalkyl)-
nickelacyles (2a-d) has been synthesized in high yield via
optimized synthetic procedures involving: (i) the base-assisted
activation of one of the C–H bond in α to the nitrile group of
[NiXCp(NHC)] complexes that bear a cyanoalkyl chain bound to
one of the nitrogen atom of the NHC ring5 and (ii) the
subsequent removal of the Cp ligand from the resulting half-
sandwich C,C-nickelacycles.3 Full characterization of 2a-d
coupled with a DFT structural analysis allowed us to establish
that they bear only one acetonitrile ligand per nickel atom in
the solid state, and suggests they exist as rare T-shaped 14-
electron three-coordinate species.2
[NiICp{Me-NHC-(CH2)2CN}]
),6a and 5-phenyloxazole,26 were prepared according to the
(a
),5 [NiClCp{Mes-NHC-(CH2)3CN}]
(
c
published methods. The optimized syntheses of 1a and 1c are
described hereafter; their spectroscopic data can be found in
the previously published procedures.5,6a The syntheses and
characterizations of 1-(2,6-diisopropylphenyl)-3-butylnitrile-
imidazolium
chloride
and
1-(2,4,6-trimethylphenyl)-3-
propylnitrile-imidazolium bromide27 are described in the
supporting information.
Synthetic procedures
Application of these coordinatively and electronically
unsaturated species bearing a robust anionic C,C-chelate in
direct biaryl couplings between azoles and aryl halides, where
nickel salts associated to classical neutral bidentate ligands
have shown interesting results,22 proved relatively
disappointing, the most active precatalyst, 2c, showing only a
modest activity for the coupling of a couple of azoles with a
limited number of aryl iodides. Interestingly, control
experiments suggest an initial reduction of the nickel(II) pre-
[NiBrCp{Mes-NHC-(CH2)2CN}] (b). Nickelocene (868 mg, 4.60
mmol)
and
1-(2,4,6-trimethylphenyl)-3-propylnitrile-
imidazolium bromide (981 mg, 3.06 mmol) were refluxed in
THF (30 mL) for 4 days. The solution colour progressively
turned from dark green to dark red. The reaction mixture was
cooled to room temperature, and filtered through a Celite pad
that was then rinsed with THF until the washings were
colourless. The solvent was then removed under vacuum and
the residue washed with pentane to remove the excess
nickelocene. The resulting solid was then dried overnight
catalyst by the dimerization of
a sacrificial amount of
benzothiazole that could either be followed by competing
processes or by a radical process catalysed by nickel particles.
under vacuum to afford
b as a pink powder (802 mg, 1.81
mmol, 59 % yield). Anal. Calcd for C20H22N3NiBr: C, 54.22; H,
5.01; N, 9.49. Found: C, 54.48; H, 4.97; N, 9.10. 1H NMR (CDCl3,
3
7.37 (d, J = 1.5, 1H, NCH), 7.09 (s, 2H, m-H),
300.13 MHz):
3
δ
3
Experimental section
6.92 (d, J = 1.5, 1H, NCH), 5.30 (t, J = 6.0, 2H, NCH2), 4.80 (s,
5H, C5H5), 3.44 (t, 3J = 6.0, 2H, CH2CN), 2.43 (s, 3H, p-Me), 2.13
(s br., 6H, o-Me). 13C{1H} NMR (CDCl3, 75.47 MHz):
Materials and methods
δ 166.8
All reactions were carried out using standard Schlenk or
glovebox techniques under an atmosphere of dry argon.
Solvents were distilled from appropriate drying agents under
argon. Acetonitrile was freeze-pump-thaw degassed before
use. Solution NMR spectra were recorded at 298 K on Bruker
Ultra Shield 300, Bruker Spectrospin 400 or Bruker Avance III
HD 500 spectrometers operating at 300.13, 400.14 or 500.14
(NCN), 139.5, 136.6 and 136.0 (o-, ipso- and p-CAr), 129.4 (m-
CAr), 124.6 (NCH), 123.2 (NCH), 117.7 (CN), 92.2 (C5H5), 48.0
(NCH2), 21.3 (p-Me), 20.4 (CH2CN), 18.7 (o-Me). IR [ATR]:
ν
(Csp2-H) 3164 (w), 3131 (m), 3098 (w);
2910 (m), 2855 (w); (C N) 2253 (w).
[NiClCp{DiPP-NHC-(CH2)3CN}] (d). Nickelocene (1.286 g, 6.81
mmol) and 1-(2,6-diisopropylphenyl)-3-butylnitrile-
ν(Csp3-H) 2952 (w),
ν
≡
1
MHz for H and at 75.47, 100.61 or 125.77 MHz for 13C{1H}. 1H
imidazolium chloride (1.508 g, 4.54 mmol) were refluxed in thf
(45 mL) for 19 h. The solution colour progressively turned from
dark green to purple-red. The reaction mixture was cooled to
room temperature, diluted in THF (40 mL), and filtered
through a Celite pad that was then rinsed with THF until the
washings were colourless. The solvent was then removed
under vacuum, and the residue washed with pentane to
remove excess nickelocene. The pink powder was then
dissolved in toluene, and filtered through a silica pad, that was
rinsed with toluene until the washings were colourless. The
resulting dark pink solution was then evaporated and the
resulting solid dried overnight under vacuum, at 50 °C to
2D COSY spectra were obtained for all complexes to help in the
1H signal assignments. The assignments of 13C{1H} NMR spectra
were made with the aid of 1H/13C heteronuclear single
quantum coherence (HSQC) and/or 1H/13C heteronuclear
multiple-bond correlation (HMBC) for all complexes. The
chemical shifts are referenced to the residual deuterated or
13C solvent peaks. Chemical shifts (δ) and coupling constants (J)
are expressed in ppm and Hz respectively. IR spectra were
recorded on a FT-IR Nicolet 380 or Perkin Elmer Spectrum Two
spectrometer equipped with a diamond ATR. Vibrational
frequencies are expressed in cm-1. GC analyses were
performed using n-dodecane as an internal standard with an
Agilent 7820A instrument equipped with a HP-5 column.
Elemental analyses were performed by the Service d’Analyses,
de Mesures Physiques et de Spectroscopie Optique, UMR
afforded
Calcd for C24H30N3NiCl: C, 63.40; H, 6.65; N, 9.24. Found: C,
63.43; H, 6.61; N, 9.24. 1H NMR (CDCl3, 300.13 MHz):
7.57 (t,
d as a pink powder (1.124 g, 2.47 mmol, 54 %). Anal.
δ
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3
3J = 7.7, 1H, p-H), 7.40 (d, J = 7.7, 2H, m-H), 7.21 (d, J = 2.0, on the top of the Celite pad rendered the filtration slow). The
1H, NCH), 6.96 (d, 3J = 2.0, 1H, NCH), 5.20 (t, 3J = 6.8, 2H, collected dark green solution was then evaporated under
NCH2), 4.69 (s, 5H, C5H5), 2.60 (m, 6H, CH2CH2CN and CHMe2), vacuum, and the residue washed with n-pentane (3
1.37 (d, 3J = 6.6, 6H, CHMe2), 1.05 (d, 3J = 6.6, 6H, CHMe2). and dried under vacuum overnight, to afford 1c as a dark
13C{1H} NMR (CDCl3, 75.47 MHz):
165.7 (NCN), 146.8 (o-CAr), green powder (715 mg, 1.90 mmol, 66 %).
x 10 mL)
δ
136.4 (ipso-CAr), 130.5 (p-CAr), 125.8 (NCH), 124.3 (m-CAr), [NiCp{DiPP-NHC-(CH2)2CH(CN)}] (1d). A 0.5 M solution of
122.1 (NCH), 119.1 (CN), 91.9 (C5H5), 50.7 (NCH2), 28.4 KHMDS in toluene (5 mL, 2.50 mmol) was added drop-wise to
(CHMe2), 27.0 (CH2), 26.4 (CHMe2), 22.8 (CHMe2), 15.1 a dark pink solution of
d (1.124 g, 2.47 mmol) in toluene (20
(CH2CN). IR [ATR]:
(m), 2865 (m); (C
ν
(Csp2-H) 3122 (w);
ν
(Csp3-H) 2961 (m), 2926 mL) at room temperature. The resulting dark green mixture
ν
≡N) 2245 (w).
was stirred for 1 h and was filtered through a Celite pad that
[NiCp{Me-NHC-CH2CH(CN)}] (1a).5 A 0.5 M solution of KHMDS was rinsed with toluene until the washings were colourless
in toluene (9.5 mL, 4.75 mmol) was added drop-wise to a dark (note: the dark brown degradation products on the top of the
red solution of
(65 mL) at room temperature. The resulting dark olive mixture green solution was then evaporated under vacuum, and the
was stirred for 2 h and was filtered through an alumina column residue washed with pentane (3 10 mL) and dried under
(5 3 cm) that was eluted with toluene and with THF to vacuum for 3 h to afford 1d as a green powder (665 mg, 1.59
recover dark green solution (note: the dark brown mmol, 64 %). Anal. Calcd. for C24H29N3Ni: C, 68.93; H, 6.99; N,
degradation products on the top of the alumina column 10.05. Found: C, 69.19; H, 7.01; N, 10.02. 1H NMR (CDCl3,
a (1.842 g, 4.77 mmol) suspended in toluene Celite pad rendered the filtration slow). The collected dark
x
x
a
rendered the filtration slow). The collected dark green solution 300.13 MHz):
was then evaporated under vacuum, and the residue washed 1.4, 1H, m-H), 7.27 (dd, 3J = 7.8, 4J = 1.4, 1H, m-H), 7.05 (d, 3J =
δ
7.49 (t, 3J = 7.8, 1H, p-H), 7.34 (dd, 3J = 7.8, 4J =
3
10 mL) and dried under vacuum overnight, 2.1, 1H, NCH), 6.73 (d, J = 2.1, 1H, NCH), 4.67 (s, 5H, C5H5),
with n-pentane (3
x
to afford 1a as a dark green powder (287 mg, 1.11 mmol, 23 4.01 (m, 1H, NCH2), 3.84 (m, 1H, NCH2), 2.78 (sept, 3J = 6.9, 1H,
3
CHMe2), 2.44 (sept, J = 6.9, 1H, CHMe2), 2.15 (t, J = 6.6, 1H,
3
%).
3
[NiCp{Mes-NHC-CH2CH(CN)}] (1b). A 0.5 M solution of KHMDS CHCN), 1.68 (m, 1H, CH2), 1.51 (d, J = 6.9, 3H, CHMe2), 1.43
in toluene (2.4 mL, 1.20 mmol) was added drop-wise to a dark (m, 1H, CH2), 1.34 (d, 3J = 6.9, 3H, CHMe2), 1.07 (d, 3J = 6.9, 3H,
pink solution of
(11 mL) at room temperature. The resulting dark olive mixture 75.47 MHz):
b
(524 mg, 1.18 mmol) suspended in toluene CHMe2), 1.02 (d, 3J = 6.9, 3H, CHMe2). 13C {1H} NMR (CDCl3,
δ
172.6 (NCN), 146.9 and 145.8 (o-CAr), 137.2
was stirred for 1 h and was filtered through a Celite pad that (ipso-CAr), 132.5 (CN), 130.1 (p-CAr), 124.2 (NCH), 123.8 and
was rinsed with toluene until the washings were colourless 123.7 (m-CAr), 121.6 (NCH), 91.2 (C5H5), 50.3 (NCH2), 30.2
(note: the dark brown degradation products on the top of the (CH2), 28.7 and 28.3 (CHMe2), 25.9 and 25.7 (CHMe2), 22.7 and
Celite pad rendered the filtration slow). The collected dark 22.7 (CHMe2), –25.1 (CHCN). IR [ATR]:
olive solution was then evaporated, and the residue was (Csp3-H) 2951 (m), 2924 (w), 2862 (w); (C
extracted with toluene and applied onto an alumina column (7 [Ni{Me-NHC-CH2CH(CN)}(NCCH3)]+PF6 (2a)
5 cm) that was eluted with toluene to give a light blue (37%) diluted in acetonitrile to 1.0 M (1.06 mL, 1.06 mmol) was
ν
ν ≡
(Csp2-H) 3140 (w);
N) 2179 (m).
Aqueous HCl
ν
-
.
x
fraction, and with THF to recover a dark green solution. The added drop-wise to an equimolar amount of a dark green
dark green solution was then evaporated under vacuum, and suspension of 1a (277 mg, 1.07 mmol) and KPF6 (196 mg, 1.06
the residue washed with n-pentane (3 x 3 mL) and dried under mmol) in acetonitrile (16 mL) at 0 °C. The reaction mixture
vacuum overnight, to afford 1b as a green powder (121 mg, rapidly turned ochre yellow and was stirred for 10 min at 0 °C
0.334 mmol, 28 %). Anal. Calcd. for C20H21N3Ni: C, 66.34; H, before being warmed to room temperature and filtered on a
5.85; N, 11.61. Found: C, 66.20; H, 5.89; N, 11.26. 1H NMR Celite pad, which was subsequently rinsed with acetonitrile
(CDCl3, 300.13 MHz):
3
δ 6.99 (s, 1H, m-H), 6.97 (s, 1H, m-H), until the washings were colourless. Volatiles were evaporated
6.96 (d, J = 2.0, 1H, NCH), 6.62 (d, J = 2.0, 1H, NCH), 4.77 (s, under vacuum, and the resulting solid was washed with
3
3
3
5H, C5H5), 3.89 (m, 2H, NCH2), 2.75 (dd, J = 7.8, J = 6.0, 1H, pentane (3
CHCN), 2.35 (s, 3H, p-Me), 2.08 (s, 3H, o-Me), 2.01 (s, 3H, o- room temperature to afford 2a as a dark yellow solid (380 mg,
Me). 13C{1H} NMR (CDCl3, 75.47 MHz):
175.9 (NCN), 139.4, 1.00 mmol, 95%). Anal. Calcd for C9H11F6N4NiP: C, 28.53; H,
x 3 mL), and dried overnight under vacuum at
δ
136.4, 136.1 and 135.5 (p-, ipso-, o-CAr), 131.5 (CN), 129.1 (m- 2.93; N, 14.79. Found: C, 28.78; H, 3.00; N, 12.99. Repeated
CAr), 128.9 (m-CAr), 122.1 (NCH), 117.7 (NCH), 90.0 (C5H5), 54.2 attempts to obtain correct elemental analyses gave low
(NCH2), 21.3 (p-Me), 18.0 (o-Me), 17.8 (o-Me), –11.0 (CHCN). IR nitrogen values. HR-MS (ESI): m/z [M]+ calcd for C9H11N4Ni
[ATR]:
ν
(Csp2-H) 3153 (w), 3127 (m);
ν δ
(Csp3-H) 2952 (w), 2919 233.0332, found 233.0328. 1H NMR (CD3CN, 400.14 MHz):
(m), 2856 (w);
ν(C≡N) 2185 (s).
7.07 (d, 3J = 2.0, 1H, NCH), 6.90 (d, 3J = 2.0, 1H, NCH), 3.86 (dd,
[NiCp{Mes-NHC-(CH2)2CH(CN)}] (1c).6a A 0.5 M solution of 2J = 12.8, J = 7.6, 1H, NCH2), 3.62 (s, 3H, NMe), 3.55 (dd, J =
3
2
3
KHMDS in toluene (5.78 mL, 2.89 mmol) was added drop-wise 12.8, J = 2.8, 1H, NCH2), 2.59 (m, 1H, CHCN), 1.96 (s, 3H, free
to a dark pink solution of
c δ 7.19 (s, 1H, NCH),
(1.193 mg, 2.89 mmol) suspended CH3CN). 1H NMR (C5D5N, 300.13 MHz):
in toluene (24 mL) at room temperature. The resulting dark 6.94 (s, 1H, NCH), 3.81 (m, 2H, NCH2), 2.49 (s, 3H, NMe), 2.42
olive mixture was stirred for 2 h and was filtered through a (br., 1H, CHCN), 1.83 (s, 3H, CH3CN). 13C{1H} NMR (CD3CN,
Celite pad that was rinsed with toluene until the washings 100.61 MHz): δ 156.6 (br., NCN), 126.2 and 119.1 (NCH), 52.4
were colourless (note: the dark brown degradation products (NCH2), 37.7 (NMe), 7.1 (br., CHCN). 13C{1H} NMR (C5D5N,
8 | J. Name., 2012, 00, 1-3
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125.77 MHz):
δ
161.0 (NCN), 128.8 and 128.4 (CH3CN and (d, 3J = n.r., 1H, NCH), 7.1 (d, 3J = 1.8, 1H, NCH), 6.89 (s, 1H, m-
CHCN) 126.3 and 119.1 (NCH), 52.8 (NCH2), 35.4 (NMe), 8.9 H), 6.41 (s, 1H, m-H), 4.41 (m, 1H, NCH2), 4.30 (m, 1H, NCH2),
(br. CHCN), 1.8 (CH3CN). IR [ATR]:
(Csp2–H) 3174 (w), 3149 (w); 2.81 (s, 3H, o-Me), 2.20 (s, 3H, p-Me), 2.01 (dd, 1H, 3J = 8.1, 3J =
(Csp3–H) 2951 (w); (C N) 2239 (m);
[Ni{Mes-NHC-CH2CH(CN)}(NCCH3)]+PF6 (2b)
ν
ν
ν
≡
ν
(P–F) 823 (s).
6.9, CHCN), 1.86 (s, 3H, free CH3CN), 1.82 (m, 1H, CH2), 1.46 (s,
-
.
Aqueous HCl 3H, o-Me), 1.29 (m, 1H, CH2). 13C{1H} NMR (CD3CN, 125.77
(37%) diluted in acetonitrile to 1.0 M (0.13 mL, 0.130 mmol) MHz):
δ 155.4 (br., NCN); 140.4, 136.5, 136.2, 135.7 (ipso-, p-,
was added drop-wise to an equimolar amount of a dark green o-CAr), 130.4 and 130.1 (m-CAr), 125.5 and 123.7 (NCH), 50.5
suspension of 1b (48 mg, 0.133 mmol) and KPF6 (23 mg, 0.125 (NCH2), 29.2 (CH2), 21.1 (p-Me), 19.1 and 18.3 (o-Me), –2.0
mmol) in acetonitrile (2 mL) at 0 °C. The reaction mixture (br., CHCN). 13C{1H} NMR (C5D5N, 125.77 MHz):
δ 161.9 (NCN),
rapidly turned ochre yellow and was stirred for 10 min at 0 °C, 139.6, 134.4, (ipso-, p- or o-CAr), 130.5 and 130.0 (m-CAr), 128.1
before being warmed to room temperature and filtered on a (CHCN), 125.0 (NCH), 118.0 (free CH3CN), 50.9 (NCH2), 30.3
Celite pad that was rinsed with acetonitrile until the washings (CH2), 21.1 (p-Me), 19.8 and 18.4 (o-Me), 1.5 (free CH3CN), 0.9
were colourless. Volatiles were evaporated under vacuum, and (CHCN). IR [ATR]:
ν(Csp2–H) 3174 (w), 3146 (w);
ν(Csp3–H) 2943
the resulting solid was washed with pentane (3 2 mL), and (w), 2922 (w), 2862 (w);
x
ν
(C N) 2236 (m); (P–F) 829 (s).
≡
ν
-
dried overnight under vacuum at room temperature to afford [Ni{DiPP-NHC-(CH2)2CH(CN)}(NCCH3)]+PF6 (2d). Aqueous HCl
2b as a dark yellow solid (52 mg, 0.108 mmol, 86 %). Anal. (37%) diluted in acetonitrile to 1.0 M (0.43 mL, 0.430 mmol)
Calcd for C17H19F6N4NiP: C, 42.27; H, 3.97; N, 11.60. Found: C, was added drop-wise to an equimolar amount of a dark green
41.68; H, 4.07; N, 11.30. HR-MS (ESI): m/z [M]+ calcd for suspension of 1d (207 mg, 0.495 mmol) and KPF6 (79 mg, 0.429
C17H19N4Ni 337.0958, found 337.0985. 1H NMR (CD3CN, 500.15 mmol) in acetonitrile (4 mL) at 0 °C. The reaction mixture
MHz): δ 7.32 (s, 1H, NCH), 7.07 (s, 1H, m-H), 7.02 (s, 1H, m-H), rapidly turned ochre yellow and was stirred for 10 min at 0 °C,
6.89 (d, 1H, NCH), 3.96 (dd, J = 12.5, J = 7.5, 1H, NCH2), 3.67 before being warmed to room temperature and filtered on a
2
3
(m, 1H, NCH2), 2.65 (m, 1H, CHCN), 2.29 (s, 3H, p-Me), 2.20 (br. Celite pad that was rinsed with acetonitrile until the washings
1
s, 3H, o-Me), 2.06 (s, 3H, o-Me), 1.97 (s, 3H, free CH3CN). H were colourless. Volatiles were evaporated under vacuum, and
3
7.58 (d, J = n.r., 1H, NCH), 7.01 the resulting solid was washed with pentane (3
NMR (C5D5N, 300.13 MHz):
δ
x
3 mL), and
(d, 3J = 1.8, 1H, NCH), 6.59 (s, 1H, m-H), 6.31 (s, 1H, m-H), 4.15 dried overnight under vacuum at room temperature to afford
2
3
2
3
(dd, J = 12.6, J = 7.5, 1H, NCH2), 3.92 (dd, J = 12.6, J = 3.0, 2d as a dark yellow solid (200 mg, 0.371 mmol, 86 %). Anal.
1H, NCH2), 2.29 (dd, 3J = 7.5, 3J = 3.0, 1H, CHCN), 2.21 (s, 3H, o- Calcd for C21H27F6N4NiP: C, 46.78; H, 5.05; N, 10.39. Found: C,
Me), 2.06 (s, 3H, p-Me), 1.86 (s, 6H, o-Me and free CH3CN). 46.72; H, 5.33; N, 10.19. HR-MS (ESI): m/z [M]+ calcd for
13C{1H} NMR (CD3CN, 125.77 MHz):
δ
158.6 (br., NCN), 140.6 C21H27N4Ni 393.1584, found 393.1604. 1H NMR (CD3CN, 400.14
3
3
136.5, 136.2, 135.8 (ipso-, p-, o-CAr), 130.0 and 129.8 (m-CAr), MHz):
δ 7.50 (d, J = 5.4, 1H, m-H), 7.50 (d, J = 4.0, 1H, m-H),
125.5 and 120.4 (NCH), 53.1 (NCH2), 21.0 (p-Me), 17.9 and 17.6 7.35 (dd, 3J = 5.4, 3J = 4.0, 1H, p-H), 7.32 (d, J = 1.4, 1H, NCH),
3
(o-Me), 7.8 (CHCN). 13C{1H} NMR (C5D5N, 125.77 MHz):
δ
(NCN), 139.7, 135.2, 134.7, 134.3 (ipso-, p-, o-CAr), 129.8 (m- CHMe2), 2.50 (sept, J = 6.8, 1H, CHMe2), 2.28 (t, J = 6.4, 1H,
162.8 7.11 (d, J = 1.4, 1H, NCH), 4.09 (m, 2H, NCH2), 3.42 (sept, 1H,
3
3
3
3
CAr), 126.6 (CHCN), 125.7 and 120.4 (NCH), 117.9 (free CH3CN), CHCN), 1.96 (s, 3H, free CH3CN), 1.89 (d, J = 6.8, 3H, CHMe2),
3
3
53.2 (NCH2), 20.9 (p-Me), 18.1 and 17.9 (o-Me), 9.7 (CHCN), 1.4 1.75 (m, 1H, CH2), 1.24 (d, J = 6.8, 3H, CHMe2), 1.16 (d, J =
(free CH3CN). IR [ATR]: (Csp2–H) 3169 (w), 3144 (w);
(Csp3–H) 6.8, 3H, CHMe2), 1.15 (m, 1H, CH2), 1.13 (d, 3J = 6.8, 3H,
2941 (w), 2924 (w), 2864 (w); (C N) 2243 (m); (P–F) 829 (s). CHMe2). H NMR (C5D5N, 300.13 MHz): δ 7.62 (d, J = 1.4, 1H,
ν
ν
1
3
ν
≡
ν
[Ni{Mes-NHC-(CH2)2CH(CN)}(NCCH3)]+PF6 (2c). Aqueous HCl NCH), 7.53 (m, 2H, m-H), 7.45 (d, 3J = 1.4, 1H, NCH), 6.97 (dd, 3J
-
(37%) diluted in acetonitrile to 1.0 M (1.30 mL, 1.30 mmol) was = 6.6, 3J n.r., 1H, p-H), 4.54 (m, 2H, NCH2 and CHMe2), 4.40 (m,
3
added drop-wise to an equimolar amount of a dark green 1H, NCH2), 2.08 (d, J = 6.6, 3H, CHMe2), 2.00 (m, 2H, CHMe2
suspension of 1c (536 mg, 1.30 mmol) and KPF6 (239 mg, 1.30 and CHCN), 1.85 (s, 3H, free CH3CN), 1.30 (m, 2H, CH2), 1.04 (d,
mmol) in acetonitrile (10 mL) at 0 °C. The reaction mixture 3J = 6.6, 3H, CHMe2), 0.98 (d, 3J = 6.6, 3H, CHMe2), 0.38 (d, 3J =
rapidly turned ochre yellow and was stirred for 10 min at 0 °C, 6.8, 3H, CHMe2). 13C{1H} NMR (CD3CN, 100.61 MHz):
δ 153.8
before being warmed to room temperature and filtered on a (br., NCN), 146.9 and 146.2 (o-CAr), 136.4 (ipso-CAr), 131.4 (m-
Celite pad that was rinsed with acetonitrile until the washings CAr), 126.9 (NCH), 125.4 and 125.2 (p- and m-CAr), 123.6 (NCH),
were colourless. Volatiles were evaporated under vacuum, and 49.8 (NCH2), 29.7 and 29.1 (CHMe2), 28.1 (CH2), 25.3, 25.1,
the resulting solid was washed with pentane (3 x
dried overnight under vacuum at 50 °C to afford 2c as a dark 125.77 MHz):
10 mL) and 24.4 and 23.5 (CHMe2), –2.7 (br. CHCN). 13C{1H} NMR (C5D5N,
161.8 (NCN), 146.3 and 145.9 (o-CAr), 131.0 (m-
δ
yellow solid (627 mg, 1.26 mmol, 97%). Anal. Calcd for CAr), 127.9 (CHCN), 127.2 (NCH), 125.2 (p- or m-CAr), 123.1
C18H21F6N4NiP: C, 43.50; H, 4.26; N, 11.27. Found: C, 43.49; H, (NCH), 118.0 (free CH3CN), 50.9 (NCH2), 30.3 (CHMe2), 28.4
4.31; N, 10.82. HR-MS (ESI): m/z [M]+ calcd for C18H21N4Ni (CH2), 26.7, 25.6, 24.0 and 22.6 (CHMe2), 1.4 (free CH3CN), 0.8
351.1114, found 351.1126. 1H NMR (CD3CN, ca. 0.1 M,28 (CHCN). IR [ATR]:
500.14 MHz): 7.32 (s, 1H, NCH), 7.30 (br., 1H, m-H), 7.07 (s, 2966 (m), 2922 (m), 2867 (m);
1H, m-H), 7.04 (s, 1H, NCH), 4.13 and 4.03 (2m, 2 1H, NCH2), Typical procedure for the heteroarylation of aryl halides. An
ν(Csp2–H) 3179 (w), 31464 (w);
ν(Csp3–H)
δ
ν
(C N) 2233 (m); (P–F) 834 (s).
≡
ν
x
2.57 (s, 3H, o-Me), 2.42 (br. s, 4H, p-Me and CHCN), 2.03 (s, oven dried Schlenk tube equipped with a magnetic stir bar was
3H, o-Me), 1.96 (s, 3H, free CH3CN), 1.67 (br., 1H, NCH2CH2), charged with 2c (10 mg, 0.0201 mmol), LiOt-Bu (59 mg, 0.737
1
1.05 (br., 1H, NCH2CH2). H NMR (C5D5N, 300.13 MHz):
δ
7.58 mmol, 2.0 equiv.), benzothiazole (40 µL, 0.369 mmol, 1.0
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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equiv.), iodobenzene (60 µL, 0.536 mmol, 1.5 equiv.), profile. The electronic energies (Eb1) obtained at the PBE0/b1
dodecane (15 µL, 0.066 mmol), and 1,4-dioxane (3 mL), and level of theory were converted to free energy at 298.15 K and
sealed. The Schlenk tube was then introduced into an oil bath 1 atm (Gb1) by using zero point energy and thermal energy
that was heated up to a temperature of 140 °C. After 36 h, the corrections based on structural and vibration frequency data
reaction medium was cooled to room temperature, and the calculated at the same level.
volatiles were evaporated under high vacuum. The resulting
brown residue was extracted with diethyl ether and filtered geometries obtained at the PBE0/b1 level using the M06
over a silica pad (2.5 × 1 cm). The collected filtrate was then functional, and
6-311++G(d,p) basis set.37,39 The M06
concentrated under vacuum, and loaded onto a silica column functional is a hybrid meta-GGA functional developed by
(Merck Silica Gel 60 - mesh size 40-60
m; 28 × 3.5 cm) pre- Truhlar and Zhao,40 and it was shown to perform very well for
Single point energy calculations were performed on the
a
ꢁ
wet with petroleum ether (bp. 40-70 °C). Elution with a the kinetics of transition metal molecules, providing a good
petroleum ether/ethyl acetate mixture (benzothiazole description of weak and long range interactions.41 Solvent
derivatives: 98/2; other azoles: 90/10) afforded the coupling effects (MeCN) were accounted for in the M06/6-
product.
311++G(d,p)//PBE0/b1 calculations by means of the
Polarisable Continuum Model (PCM) initially devised by Tomasi
and coworkers42 with radii and non-electrostatic terms of the
SMD solvation model, developed by Truhlar and co-workers.43
The free energy values presented (Gb2soln) were derived
from the electronic energy values obtained at the M06/6-
311++G(d,p)//PBE0/b1 level, including solvent effects
(Eb2soln), according to the following expression: Gb2soln =
Eb2soln + Gb1 – Eb1.
X-ray diffraction studies
Single crystals of 1a and 1b suitable for X-ray diffraction
studies were obtain from slow evaporation of a concentrated
solution in THF at room temperature. Diffraction data were
collected at 173(2) K on a Bruker APEX II DUO Kappa CCD area
detector diffractometer equipped with an Oxford Cryosystem
liquid N2 device using Mo-K
α radiation (λ = 0.71073 Å). A
The calculated frequencies for the CN stretch in all species
were corrected with a scale factor of 0.932 resulting from the
comparison of the calculated and experimental CN frequency
summary of crystal data, data collection parameters and
structure refinements is given in Table S1. The crystal-detector
distance was 38 mm. The cell parameters were determined
(APEX2 software) from reflections taken from three sets of
twelve frames, each at ten seconds exposure. The structure
was solved using direct methods with SHELXS-97 and refined
values observed for 1a. Presented calculated frequency
spectra were drawn with the Chemcraft program44 and a
Lorentzian line broadening of 30 % at peak half-width to
account for thermal line broadening.
against F2 for all reflections using the SHELXL-97 software.29
A
semi-empirical absorption correction was applied using
SADABS in APEX II.30 All non-hydrogen atoms were refined
with anisotropic displacement parameters, using weighted full-
matrix least-squares on F2. Hydrogen atoms were included in
calculated positions and treated as riding atoms using SHELXL
default parameters.
Conflict of interest
There are no conflicts to declare.
Acknowledgements
DFT computational details
We are grateful to the Université de Strasbourg and the CNRS
for their financial help. VR thanks the Agence Nationale de la
Recherche (ANR 2010 JCJC 716 1; SBA-15-NHC-NiCat) for the
post-doctoral fellowship of S. Shahane. The “Investissements
d’avenir” program of the Université de Strasbourg is
acknowledged for the doctoral fellowship of B. de P. Cardoso.
We appreciate the assistance of Corinne Bailly for the
structural determinations of 1a and 1b, and of Dr. Philippe
Bertani for the 13C CP-MAS NMR spectrum of 2c. LFV
acknowledges the Fundação para a Ciência e Tecnologia, FCT
UID/QUI/00100/2013, for funding.
Calculations were performed using the Gaussian 09 software
package31 and the PBE0 functional, without symmetry
constraints. That functional uses a hybrid generalized gradient
approximation (GGA), including 25 % mixture of Hartree-Fock32
exchange with DFT33 exchange-correlation, given by Perdew,
Burke and Ernzerhof functional (PBE).34 The basis set used for
geometry
optimizations
(b1)
consisted
of
the
Stuttgart/Dresden ECP (SDD) basis set35 augmented with a f-
polarization function36 to describe the electrons of Ni, and a
standard 6-31G(d,p) basis set37 for all other atoms. The
transition state optimization was performed with the
Synchronous Transit-Guided Quasi-Newton Method (STQN)
developed by Schlegel et al.,38 following extensive searches of
the Potential Energy Surface. Frequency calculations were
performed to confirm the nature of the stationary points,
yielding one imaginary frequency for the transition state and
none for the minima. The transition state was further
confirmed by following its vibrational mode downhill on both
sides and obtaining the minima presented on the energy
Notes and references
‡ Isolated yields of 2a-d have been calculated for molecules
containing one acetonitrile ligand.
§ We mentioned in our preliminary communication that the H
1
NMR spectrum of 2c is concentration dependent. This is also
observed for 2a,b,d, though to a much lesser extent. A VT NMR
experiment run from +27°C to +75°C on a diluted solution of 2c
in CD3CN did not result in significant changes.
10 | J. Name., 2012, 00, 1-3
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ARTICLE
§§ The solid-state 13C CP-MAS NMR spectrum of 2c displays
broad signals in the expected diamagnetic area, i.e.: at similar
chemical shifts than those of the solution 13C NMR peaks.
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Cp removal from half-sandwich C,C-nickelacycles under mild conditions provides rare three-
coordinate T-shaped 14-electron nickel(II) complexes.