Synthesis and characterization of oxygen-functionalised-NHC silver(i) complexes and NHC transmetallation to nickel(ii)

Article abstract of DOI:10.1039/c3dt52773e

The new alcohol- and ether-functionalised-NHC silver(i) complexes bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyethyl)-1H-imidazol-2(3H)-ylidene) silver(i) chloride, [Ag{ImDiPP(C₂OH)}₂]Cl (4), bis(1-(2-hydroxyethyl)-3-mesityl-1H-imidazol-2(3H)-ylidene)silver(i) chloride, [Ag{ImMes(C₂OH)}₂]Cl (5), bis(1-(2-hydroxyethyl)-3-methyl- 1H-imidazol-2(3H)-ylidene)silver(i) chloride, [Ag{ImMe(C₂OH)} ₂]Cl (6), bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyethyl)-1H- imidazol-2(3H)-ylidene)silver(i) tetrafluoroborate, [Ag{ImDiPP(C ₂OH)}₂]BF₄ (9), and bis(1-(2,6- diisopropylphenyl)-3-(2-methoxyethyl)-1H-imidazol-2(3H)-ylidene)silver(i) chloride, [Ag{ImDiPP(C₂OMe)}₂]Cl (13), were synthesized and fully characterized by NMR spectroscopy and single crystal X-ray diffraction. For some complexes, an uncommon heteronuclear coupling ⁴J(^107/109Ag-H) was unveiled. Their ability to transfer the NHC ligand to Ni(ii) was assessed in the presence of different nickel(ii) sources; the bis-NHC Ni(ii) complex bis(1-(2,6-diisopropylphenyl)-3-(2- methoxyethyl)-1H-imidazol-2(3H)-ylidene)nickel(ii) chloride, [NiCl ₂{ImDiPP(C₂OMe)}₂] (15), was obtained from 13 and shown by X-ray diffraction study to have a trans-arrangement of the two NHC ligands. However, in contrast to other Ag(i) NHC complexes the transmetallation reaction failed with the hydroxyl-functionalised silver complexes, possibly due to the acidity of the alcohol OH function, leading overall to reprotonation of the C_NHC and isolation of the corresponding imidazolium salts.

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Synthesis and characterization of oxygen-
functionalised-NHC silver(I) complexes and NHC
transmetallation to nickel(II)†
Cite this: Dalton Trans., 2014, 43,
4700
Sophie Hameury,a Pierre de Frémont,a Pierre-Alain R. Breuil,b
Hélène Olivier-Bourbigoub and Pierre Braunstein*a
The new alcohol- and ether-functionalised-NHC silver(I) complexes bis(1-(2,6-diisopropylphenyl)-3-
(2-hydroxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImDiPP(C2OH)}2]Cl (4), bis(1-(2-hydro-
xyethyl)-3-mesityl-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImMes(C2OH)}2]Cl (5), bis(1-(2-hydro-
xyethyl)-3-methyl-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImMe(C2OH)}2]Cl (6), bis(1-(2,6-
diisopropylphenyl)-3-(2-hydroxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I) tetrafluoroborate, [Ag{ImDiPP-
(C2OH)}2]BF4 (9), and bis(1-(2,6-diisopropylphenyl)-3-(2-methoxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I)
chloride, [Ag{ImDiPP(C2OMe)}2]Cl (13), were synthesized and fully characterized by NMR spectroscopy
and single crystal X-ray diffraction. For some complexes, an uncommon heteronuclear coupling
4J(107/109Ag–H) was unveiled. Their ability to transfer the NHC ligand to Ni(II) was assessed in the presence
of different nickel(II) sources; the bis-NHC Ni(II) complex bis(1-(2,6-diisopropylphenyl)-3-(2-methoxy-
ethyl)-1H-imidazol-2(3H)-ylidene)nickel(II) chloride, [NiCl2{ImDiPP(C2OMe)}2] (15), was obtained from 13
and shown by X-ray diffraction study to have a trans-arrangement of the two NHC ligands. However, in
contrast to other Ag(I) NHC complexes the transmetallation reaction failed with the hydroxyl-functiona-
lised silver complexes, possibly due to the acidity of the alcohol OH function, leading overall to reproto-
nation of the CNHC and isolation of the corresponding imidazolium salts.
Received 4th October 2013,
Accepted 17th January 2014
DOI: 10.1039/c3dt52773e
www.rsc.org/dalton
antimicrobial agents, where they promote a slow release of
silver ions, thus suppressing infections over a long period of
Introduction
Since the isolation of the first stable, crystalline free carbene time, or as potential anti-cancer agents.5
by Arduengo in 1991,1 N-heterocyclic carbenes (NHCs) have
Non-functionalised NHC ligands of the type bis-(alkyl/aryl)-
been widely employed as ligands, in particular to prepare new imidazol(in)-ylidenes form either neutral [AgX(NHC)] or cat-
metal-based catalysts.2 One of the convenient synthetic ionic [Ag(NHC)2]X (X = e.g. halide, BF4, PF6) complexes, which
methods to access NHC transition metal complexes consists of can coexist in solution, in equilibrium, depending on the
the use of the corresponding Ag(I)–NHC complexes as NHC nature of X. The silver(I) cation tends to adopt its preferred
transfer reagents, which avoids the cumbersome synthesis of linear coordination geometry. Whenever X is a halide, the
free carbenes.3 Generally, these silver complexes are readily possible formation of [AgnXm]m–n polynuclear complexes,
synthesized, by reaction of the corresponding imidazolium through capping or bridging coordination modes of X, associ-
salts with silver(I) oxide, without the need to work under an ated with coordination to [Ag(NHC)]+ or [Ag(NHC)2]+ moieties,
inert atmosphere.4 In addition to their transmetallation can give rise to a large variety of silver(I) NHC polynuclear com-
ability, Ag(I)–NHC complexes are frequently used as plexes or coordination clusters which benefit from favorable
d10–d10 attractive interactions between silver(I) cations.4a,6 In
these complexes, the coordination geometry of the silver
aLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS),
Université de Strasbourg, 4 rue Blaise Pascal, CS 90032, 67081 Strasbourg, France.
cations can deviate substantially from linearity to become ulti-
mately trigonal planar or tetrahedral. Furthermore, functiona-
lised NHC ligands with certain pnictogen- and chalcogen-
donor groups can easily provide access to interesting, structu-
rally diverse NHC silver(I) complexes. Indeed, complexes with
triangular [Ag3]-cores can often be obtained when N-functions
such as pyridines, pyrazoles or imines are present.7 [Ag4]-cores
E-mail: braunstein@unistra.fr
bIFP Energies nouvelles, Rond Point de l’Echangeur de Solaize, 69360 Solaize, France
†Electronic supplementary information (ESI) available: All experimental and
spectroscopic details and crystal data of 4–6, 8, 9 and 13–15 with ORTEP views.
CCDC 963275–963282. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt52773e
4700 | Dalton Trans., 2014, 43, 4700–4710
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of rectangular or square geometry are frequently encountered
when the NHC ligand is functionalised with N-heterocyclic
ligands (pyridines, benzimidazoles, pyrazoles), thioethers,
phosphines or is part of a pincer system.7g,8 Linear or open
chain arrays of [Ag∞]-cores are also widespread with hetero-
cyclic functionalities (pyrimidines, triazoles, benzimidazoles,
pyrazoles, pyridines, etc.) or with phosphines.9 Less frequently,
pseudo-cubane [Ag4] substructures with phosphinites or
phenolate functionalisations10 or even higher nuclearity cores,
from [Ag6] to [Ag10], have been observed e.g. with amine or
pyridine functionalisations.11 These diverse structures and
nuclearities result from both the geometric constraints of the
chelating or bridging ligands and the ability of the silver(I)
cations to aggregate through d10–d10 interactions.4a,6
We have recently observed this structural diversity with
phosphinite-10a,c or thioether8c,j-functionalised NHCs, which
led to the formation of cubane- or rectangle-type [Ag4] clusters,
respectively. As an extension of these studies, we decided to
study the reactivity of hydroxy-functionalised NHC ligands
with two main objectives: (i) explore their coordination behav-
iour, including the possibility of an intramolecular coordi-
nation of the weak hydroxy donor function and the stability of
the ensuing silver(I) complexes, and (ii) use these functiona-
lised silver(I) NHC complexes as transmetallating reagents, in
particular toward Ni(II) centres in view of potential appli-
cations in catalytic ethylene oligomerization.12 Indeed, numer-
ous non-functionalised-NHC nickel(II) complexes have already
been synthesized and shown to be active pre-catalysts toward
ethylene oligomerization.12b,c,13 Furthermore, an enolate-func-
tionalised NHC nickel(II) complex synthesized by Waymouth
et al. efficiently oligomerizes ethylene upon activation with
ZnEt2, and alcoholate- or aryloxide-functionalised NHC nickel
(II) complexes catalyze norbornene polymerization upon acti-
vation with MAO.14 Upon activation with NaBPh4, a salicylaldi-
minato-functionalised NHC Ni(II) complex catalyzes the
polymerization of styrene with an activity higher than that of
its analogous non-functionalised-NHC or even of PPh3 Ni(II)
halide complexes.15
Scheme 1 Synthesis of the imidazolium salts 1–3.
are given in the ESI†.) A procedure avoiding the use of a
solvent and thus allowing access to higher reaction tempera-
tures was initially reported for the synthesis of aryl- and bis-
(arylimidazolium)-pyridines7h,17a and applied on many other
occasions, e.g. to the synthesis of precursors to amine-tethered
NHC rhodium complexes.17b The 1H NMR spectra in CD2Cl2 of
1–3 display the characteristic resonances of the NCHN protons
around 9–10 ppm, in agreement with the formation of the imi-
dazolium salts.
A dry methanol solution of the imidazolium salts 1–3 was
reacted with 0.55 equiv. of Ag2O in the presence of molecular
sieves at room temperature under exclusion of light to afford
the complexes bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyethyl)-
1H-imidazol-2(3H)-ylidene)silver(I)
chloride,
[Ag{ImDiPP-
(C2OH)}2]Cl (4), bis(1-(2-hydroxyethyl)-3-mesityl-1H-imidazol-
2(3H)-ylidene)silver(I) chloride, [Ag{ImMes(C2OH)}2]Cl (5), and
bis(1-(2-hydroxyethyl)-3-methyl-1H-imidazol-2(3H)-ylidene)silver(I)
chloride, [Ag{ImMe(C2OH)}2]Cl (6), respectively, in good to
excellent yields (77–93%) (Scheme 2). Complexes 4 and 5 are
stable towards moisture in ambient daylight while 6 is not.
It was found that the use of methanol as the reaction
medium for the synthesis of 4 was beneficial compared to
other commonly used solvents (e.g. THF, MeCN, CH2Cl2 or
DMSO) resulting in shorter reaction times and high conver-
sions. For example, the use of CH2Cl2 as solvent resulted in a
conversion of 80% after 12 h while in methanol complete con-
version was observed within 4 h.
1
The formation of complexes 4–6 was confirmed by H NMR
spectroscopy with the complete disappearance of the charac-
teristic signals of the imidazolium salts. Thus the spectrum of
4 displays two apparent triplets for the imidazole backbone
protons at 7.27 and 6.92 ppm. Since two doublets were initially
expected for these non-equivalents protons, experiments were
carried out to rationalise the appearance of the observed spec-
trum. Since both silver isotopes 107Ag and 109Ag are NMR active
(S = 1/2), a heteronuclear 1H–109Ag HMQC experiment was carried
In this paper, we describe the synthesis and characteriz-
ation of a series of hydroxyethyl-functionalised NHC silver(I)
complexes, attempts to transfer the NHC ligands to different
nickel(II) precursors and the synthesis and characterization of
ether-functionalised NHC silver(I) and nickel(II) complexes.
4
out and revealed a J(H–109Ag) coupling between the backbone
Results and discussion
Alcohol-functionalised NHC complexes
The salt 3-(2-hydroxyethyl)-1-methyl-1H-imidazol-3-ium chlo-
ride ([ImMe(C2OH)]Cl, 3) was prepared by the quaternization
of N-substituted imidazoles following a published procedure.16
This procedure was modified for the synthesis of 1-(2,6-diiso-
propylphenyl)-3-(2-hydroxyethyl)-1H-imidazol-3-ium chloride,
[ImDiPP(C2OH)]Cl (1), and 1-mesityl-3-(2-hydroxyethyl)-1H-imi-
dazol-3-ium chloride, [ImMes(C2OH)]Cl (2), to reduce the reac-
tion times (Scheme 1). (All experimental details for this paper Scheme 2 Synthesis of the Ag(I) complexes 4–6.
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imidazole protons and silver (1.5 Hz). Careful inspection
shows that these apparent triplets are actually doublets of
4
doublets (3J(H–H) = 1.5 Hz and J(107/109Ag–H) = 1.5 Hz). The
4
observation of the J(H–109Ag) coupling highlights the lack of
lability of the C–Ag bond on the NMR time-scale.4a,b In the
13C NMR spectra, the carbene signals appear at 182.6 ppm for
4 and 5 and at 181.2 ppm for 6. For 4 and 5, 1J and 3J couplings
with both 107/109Ag isotopes are observable for the carbene and
the imidazole backbone carbon atoms, respectively. They give
rise to two sets of two doublets having the following coupling
1
1
constants: J(107Ag–13C) = 184 and 182 Hz, J(109Ag–13C) = 212
and 210 Hz and 3J(107/109Ag–13C) = 5.5 and 5.0 Hz, respectively.
For all three complexes, 1H NMR spectroscopy indicates the
presence of both [AgCl(NHC)] and [Ag(NHC)2]+ in solution,7h
which are known to be in equilibrium when the counterion
associated with the latter is [AgCl2]− (its progressive decompo-
sition in Ag(0) or AgCl liberates Cl− anions and affects the
equilibrium).18 Preliminary studies performed in CDCl3 indi-
cated a slow decomposition of the complexes that could be
avoided by filtering the chloroform solution through a plug of
alumina (thus removing traces of HCl). To confirm the
instability of 4–6 toward acidic media, they were dissolved in
dry CD2Cl2 in the presence of a small amount of an ether solu-
tion of HCl. Gratifyingly, the same decomposition pathway was
recorded by NMR as in HCl-contaminated CDCl3. The ESI
mass spectra of complexes 4 and 5 display only two signals for
m/z = [Ag(NHC)2]+ and [(NHC)H]+. For 6, the fragment m/z =
[(NHC)H+] was not detected due to a too small window chosen
for the analysis, but numerous unidentifiable signals are
visible. It is reasonable to assume that 4–6 are unstable under
the mass spectral analysis conditions and mainly reconverted
to some imidazolium salts. There is no signal accounting for
the formation of [AgCl(NHC)] or for any silver-containing
species with m/z higher than [Ag(NHC)2]+. X-Ray quality crys-
tals of 4–6 were grown by slow diffusion or evaporation of solu-
tions of CH2Cl2–pentane, CH2Cl2–hexane, and ether,
respectively (Fig. 1 and 2) (the ORTEP views of all complexes
are given in the ESI†). The formation of complexes of the type
[Ag(NHC)2]X with X = Cl was thus confirmed, ruling out the
formation of higher nuclearity silver complexes.
Complexes 4 and 6 crystallize in the monoclinic system
with the C2/c space group. Complex 5 crystallizes in the tri-
ˉ
clinic system with the P1 space group. The unit cell of 4 con-
tains 4 motifs including slightly disordered CH2Cl2 molecules
around twofold axes passing through chloride anions. The
silver centres are positioned on inversion centres. The unit cell
of 5 contains two motifs. The Ag–C distances fall in the range
between 2.039(4) and 2.086(3) Å, in good agreement with other
reported distances for homoleptic cationic bis-(NHC) silver(I)
complexes.4a,19 The C–Ag–C angles are in the range 174.8(3)
−180.0° and correspond to a nearly linear coordination
environment. No interaction between the silver centres and
the hydroxyl groups is noticeable in the solid state. The unit
cell of 6 contains also 4 motifs without any co-crystallized
solvent. The silver and chloride ions are located on twofold
axes. In contrast to 4 and 5, the structure of 6 reveals a weak
Fig. 1 Ball and stick representation of the structures of [Ag{ImDiPP-
(C2OH)}2]Cl (4) (top) and [Ag{ImMes(C2OH)}2]Cl (5) (bottom). Hydrogen
atoms have been omitted for clarity, except the OH protons (ORTEP
views are provided in Fig. S3 and S4 of the ESI†).
interaction between the silver centre and the chloride anion.
The C–Ag–Cl angle is equal to 93.8(1)°. Similarly to other struc-
tures,18b,20 the Ag–Cl distance of 3.189(2) Å implies a weak
electrostatic interaction, not strong enough to perturb the
linear coordination geometry around the silver centre. Thus
the chloride is considered as a counter-anion for the Ag
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MeOH and led to a deep green solution containing at
least one paramagnetic product. Detailed NMR analyses
were not possible, but confirmed the presence of imidazolium
salts.
Blue crystals were grown from the crude reaction mixture,
and their structure determination by X-ray diffraction estab-
lished that reprotonation of the imidazolylidene moieties
bound to silver has occurred, instead of their transfer to
nickel, with formation of a mixed tetrachloronickelate/chloride
imidazolium salt 8 (Fig. 3). Although transmetallation leading
to 7 would be accompanied by the precipitation of AgCl, for-
mation of stable [NiCl4]2− appears to be kinetically favoured,
thus preventing transmetallation. We were only able to isolate
compound 8 (as hygroscopic crystals) even from carefully dried
solvents. The salt 8 crystallizes in the orthorhombic system
with the Sohncke space group P21221. There are 4 motifs in
the unit cell without any co-crystallized solvent. The nickel
atoms are positioned along twofold axes and the [NiCl4]2− ions
display the expected tetrahedral coordination geometry. The
chloride anions interact with two hydroxyl groups and one
proton from the unsaturated backbone of the imidazole ring.
Fig. 2 Ball and stick representation of the structure of [Ag{ImMe-
(C2OH)}2]Cl (6). Hydrogen atoms have been omitted for clarity, except
the OH protons (an ORTEP view is provided in Fig. S5 of the ESI†).
The azolium protons interact with two chlorides of [NiCl4]2−
Attempts to use complex 6 for transmetallation of the func-
.
+
(NHC)2 moiety. For all three complexes, the cohesion in the
crystalline state is ensured by an extensive network of long dis- tional NHC ligand 3 to Ni(II) were inconclusive because of solu-
1
tance hydrogen bonding interactions. The chloride anions
interact with two hydroxyl groups belonging to two distinct DMSO-d6.
motifs as well as with one proton from the unsaturated imida-
zole ring backbone. There is no noticeable interaction between
bility reasons and only H NMR broad peaks were observed in
To avoid the formation of [NiCl4]2−, the counterion Cl−
present in the silver complex 4 was replaced by the less coordi-
−
the different hydroxyl groups present in the unit cells. Com- nating anion BF4 via a metathesis reaction carried out in
plexes 4–6 are similar to other alcohol-functionalised NHC acetone overnight (Scheme 4). The formation of the new
silver(I) complexes;5d,21 those characterized by X-ray diffraction
complex
bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyethyl)-1H-
did not reveal any interaction between the alcohol and the imidazol-2(3H)-ylidene)silver(I) tetrafluoroborate, [Ag{ImDiPP-
silver centre, ruling out the formation of NHC silver clusters (C2OH)}2]BF4 (9), was supported by the presence of a character-
supported by silver–oxygen bonds.22
istic signal at −152.7 ppm in 19F NMR spectroscopy. As for 4,
The synthesis of the complex bis(1-(2,6-diisopropylphenyl)- the 1H and 13C NMR spectra display 4J and 3J couplings
3-(2-hydroxyethyl)-1H-imidazol-2(3H)-ylidene)nickel(II) chloride, between the 107/109Ag(I) nuclei and the imidazole backbone
hydrogen and carbon atoms, respectively. In the 1H NMR spec-
[NiCl2{ImDiPP(C2OH)}2] (7), was attempted via transmetalla-
tion by reacting overnight 4 with [NiCl2(dme)] (Scheme 3). This trum of 9, the signals of the spacer CH2 protons are shifted
classic method for the synthesis of Ni(II)–NHC complexes9c,23 upfield compared to 4 (by 0.18 and 0.10 ppm for the CH2
was expected to yield a NHC nickel complex bearing a
protons α and β to N, respectively) and one of the imidazole
hydroxyethyl arm. The reaction was carried out in CH2Cl2 or backbone protons is shifted downfield by 0.06 ppm. The
Scheme 3 Attempted synthesis of [NiCl2{ImDiPP(C2OH)}2] (7) via transmetallation.
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Fig. 3 Ball and stick representation of 8. Hydrogen atoms have been omitted for clarity, except the OH and the NCHN protons (an ORTEP view is
provided in Fig. S6 of the ESI†).
Scheme 4 Synthesis of [Ag{ImDiPP(C2OH)}2]BF4 (9).
CHm-Ar is shifted downfield in contrast to the CHp-Ar which is
iPr
shifted upfield (both by 0.06 ppm). The CH3 are shifted
downfield by 0.07 and 0.03 ppm whereas the CHiPr is not
affected. The anion exchange has no significant influence on
the chemical shifts measured on the 13C NMR spectra of both
complexes, except for the appearance in the spectrum of 9 of a
3J coupling (3J = 2.2 Hz) between the N–C carbon atoms of the
alcohol side chain and 107/109Ag(I).
The structure of 9 was confirmed by X-ray diffraction on
single crystals obtained by slow diffusion of pentane into a
saturated CH2Cl2 solution (Fig. 4). This complex crystallizes in
Fig. 4 Ball and stick representation of [Ag{ImDiPP(C2OH)}2]BF4 (9).
the triclinic system with two motifs per unit cell including
slightly disordered diethyl ether molecules. The silver atoms are
positioned on inversion centres and the Ag–C distances are
Hydrogen atoms have been omitted for clarity, except the OH protons
(an ORTEP view is provided in Fig. S7 of the ESI†).
−
equal to 2.090(5) and 2.065(5) Å. The BF4 anions interact with
the two protons from the same imidazole backbones. They also
interact with one hydroxyl group from another molecule, which
interacts itself with another hydroxyl group. The hydrogen
bonding interactions between hydroxyl groups being not recipro-
cal, polymeric zigzag chains are built (see Fig. S1 in the ESI†).
Complex 9 was reacted in dry MeCN or THF with different
nickel precursors generally known to lead to NHC nickel(II)
complexes by transmetallation, such as [NiBr2(PPh3)2],
[NiCl2(dme)], [Ni(NCMe)4](BF4)2 or [NiBr2(dme)]. The for-
mation of [NiX4]2− (X = Cl or Br) was not observed. Due to the
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1
1
paramagnetic nature of the products formed, H NMR signals by H and 13C NMR spectroscopy and ESI-MS. For the sake of
were too broad to provide any valuable information with the comparison, the stability of the non-protected silver complex 4
exception of a broad signal around 9–10 ppm which could was tested under similar reaction conditions (excess of base in
indicate that reprotonation of 9 has occurred. Unfortunately, refluxing dioxane) and was revealed to be stable. During the
X-ray quality crystals could not be grown. While reprotonation formation of the silylether, the HCl released might be trapped
of 9 could be assumed, its pathway is not yet clear and might by HMDS or NEt3 or reacts with 11, thus explaining the for-
be due to the acidic proton of the alcohol which could trap the mation of the imidazolium salt (Scheme 5).
carbene during the transmetallation process.
The synthesis of silylether-functionalised NHC silver(I) com-
plexes being unsuccessful, attempts were made with an ether-
functionalised NHC. The ligand 1-(2,6-diisopropylphenyl)-3-(2-
methoxyethyl)-1H-imidazol-3-ium chloride, [ImDiPP(C2OMe)]-
Cl (12), was prepared in high yield by applying a similar
method to that used for ligands 1 and 2 (Scheme 6). The
corresponding silver complex bis(1-(2,6-diisopropylphenyl)-
3-(2-methoxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I) chloride,
[Ag{ImDiPP(C2OMe)}2]Cl (13), was readily formed by the reaction
of 12 with 0.55 equiv. of Ag2O (Scheme 6).
Silylether- and ether-functionalised NHC complexes
In order to corroborate that reprotonation of the carbene
ligand under transmetallation conditions is due to the OH
proton of the alcohol, protection of the latter function with a
trimethylsilyl group (TMS) was attempted. We first protected
the imidazolium salt 1 before complexation to Ag(I), using tri-
methylsilyl chloride (TMSCl) and hexamethyldisilazane
(HMDS) as a base in refluxing dioxane. The corresponding
The disappearance of the imidazolium NCHN proton was
observed in the 1H NMR spectrum. As observed with 4, a
4J(107/109Ag–H) coupling of 1.5 Hz with the backbone imidazole
hydrogen atoms is visible. The formation of 13 was also estab-
lished by the characteristic downfield shift of the carbenic
carbon at 182.4 ppm, which exhibits coupling with the
107/109Ag isotopes (1J(107/109Ag–13C) = 183 and 212 Hz, respect-
ively). A 3J coupling between the imidazole backbone carbon
atoms and the 107/109Ag isotopes (3J(107/109Ag–13C) = 5.8 and
5.7 Hz) is visible as well as a 3J(107/109Ag–13C) coupling of
2.2 Hz involving the N–C carbon atom from the ether side
chain, as for 9. In contrast to 4–6, the mass spectrum of 13
only displays the characteristic signal at m/z = 681.31 for
salt
1-(2,6-diisopropylphenyl)-3-(2-((trimethylsilyl)oxy)ethyl)-
1H-imidazol-3-ium chloride, [ImDiPP(C2OTMS)]Cl (10), was
isolated in 81% yield. To synthesize the complex bis(1-(2,6-di-
isopropylphenyl)-3-(2-((trimethylsilyl)oxy)ethyl)-1H-imidazol-
2(3H)-ylidene)silver(I) chloride, [Ag{ImDiPP(C2OTMS)}2]Cl (11),
the same procedure as that employed for the synthesis of 4
was applied but resulted in complete deprotection of the
alcohol during metallation and formation of 4 instead. There-
fore, we tried a direct protection of the silver complex 4; the
alcohol was properly protected but a major part (from 60 to
80%) of the carbene ligand was reprotonated during the reac-
tion, even when distilled TMSCl and an excess of base (HMDS
or NEt3) were used. Both compounds 10 and 11 were identified
Scheme 5 Attempted syntheses of [Ag{ImDiPP(C2OTMS)}2]Cl (11) with TMSCl.
Scheme 6 Synthesis [ImDiPP(C2OMe)]Cl (12) and [Ag{ImDiPP(C2OMe)}2]Cl (13).
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[Ag(NHC)2]+,
without
any
trace
of
reprotonation/
decomposition.
Colourless crystals were grown by dissolving 13 in pure
CH2Cl2 or in a mixture of THF and water, followed by slow
diffusion of a layer of pentane. Crystals originating from the
two corresponding batches displayed different cell parameters.
Their structural investigation by X-ray diffraction ruled out any
solvomorphism effect, but rather proved the formation of the
expected complex 13 and of the less anticipated neutral mono-
NHC complex [AgCl{ImDiPP(C2OMe)}] (14)18a (Fig. 5).
Complexes 13 and 14 crystallize in the triclinic system with
ˉ
the P1 space group, and the monoclinic system with the P21/c
space group, respectively, with very similar cell parameters.
The unit cell of 13 contains 2 motifs including slightly dis-
ordered water molecules close to inversion centres. The unit
cell of 14 contains 4 motifs with no solvent or water included.
For both cells, there is no atom in special positions. For 13 the
Ag–C distances are equal to 2.075(4) and 2.077(4) Å and the
C–Ag–C angle is equal to 176.6(1)°. These values are similar to
those found for 4–6. For 14, the Ag–C and Ag–Cl distances of
2.084(5) and 2.323(2) Å, respectively, and the C–Ag–Cl angle of
173.7(1)° are in excellent agreement with those reported in the
literature for similar [AgCl(NHC)] complexes.4a,22c,24 In both
complexes, the ether functions do not interact with the silver
centres. In 13, two chloride anions interact strongly with two
water molecules, forming a diamond arrangement via the 4
protons available (see Fig. S2 in the ESI†). They also participate
in intermolecular interactions with a hydrogen atom from the
imidazole rings. In 14, the ether groups interact slightly with
only one methyl proton per imidazolylidene moiety. The chlor-
ide ligand interacts with a hydrogen atom from the imidazole
ring of an adjacent molecule.
The overnight reaction of 13 with [NiCl2(dme)] afforded an
orange solution and white AgCl precipitate. The solution was
filtered through Celite and the solvent was removed under
reduced pressure. After the resulting orange solid residue was
washed with H2O and redissolved in toluene, the solution was
dried over Na2SO4 and the solvent evaporated to afford 15 in
50% yield (Scheme 7).
The formation of the complex bis(1-(2,6-diisopropylphenyl)-
3-(2-methoxyethyl)-1H-imidazol-2(3H)-ylidene)nickel(II) chlor-
ide ([NiCl2{ImDiPP(C2OMe)}2], 15) was confirmed by 13C NMR
spectroscopy with the displacement of the carbene chemical
shift from 182.4 ppm to 171.7 ppm, a value which is in the
range of those reported for other Ni(II) NHC complexes.25
The structure of 15 was unambiguously established by X-ray
diffraction on single-crystals obtained by slow diffusion of
hexane into a saturated THF solution (Fig. 6).
Fig. 5 Ball and stick representation of [Ag{ImDiPP(C2OMe)}2]Cl (13)
(top) and [AgCl{ImDiPP(C2OMe)}] (14) (bottom). Hydrogen atoms have
been omitted for clarity, except the OH protons (ORTEP views are pro-
vided in Fig. S8 and S9 of the ESI†).
This complex crystallizes in the orthorhombic system with
the Pca21 space group. There are four motifs per unit cell
without any solvent included and no atom in special positions.
The structure exhibits two NHC ligands in trans-position to
each other around a nickel centre in a square-planar coordi-
nation environment. There is no interaction between the ether
groups and the nickel centre. The Ni–C and Ni–Cl distances
are respectively equal to 1.902(2), 1.910(2) Å and 2.1822(8),
2.1828(8) Å. The C–Ni–C and Cl–Ni–Cl angles are equal to
179.1(2) and 179.73(4)°. The metrical data of 15 are in excellent
agreement with those reported for similar square-planar bis-
NHC Ni(II) dichloride complexes.25
Surprisingly, even though the 1H NMR spectrum recorded
when 15 was dissolved in deuterated solvents was consistent
with the structural assignment in the solid state, it also
4706 | Dalton Trans., 2014, 43, 4700–4710
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Scheme 7 Synthesis of [NiCl2{ImDiPP(C2OMe)}2] (15) by transmetallation.
their catalytic potential. Preliminary studies on catalytic ethyl-
ene oligomerization were performed with 15 in the presence of
10 equiv. of ethylaluminium dichloride (EADC) and indicated
a low to moderate activity, with a productivity of 4600 g C2H4
(g Ni · h)−1 and a TOF of 9600 mol C2H4 (mol Ni · h)−1. The
selectivities for C4 olefins and 1-butene (in the C4 cut) were
64 mol%, and 52 mol%, respectively. Note that unsymmetri-
cally substituted NHC ligands bearing a phenoxy group on one
NHC N-atom were found to act as CNHC–O chelates toward Ni
(II) but the resulting complex was inactive toward ethylene.27
Conclusion
We have obtained a series of non-symmetric alcohol-, silyl-
ether- or ether-functionalised imidazolium salts and their
corresponding bis-NHC silver(I) complexes. The cationic bis-
NHC silver(I) complexes 4, 5, 6 and 9 and the neutral halide
complex 13 were structurally characterized and were all mono-
nuclear. No direct interaction was observed between the
oxygen-containing groups and the Ag(I) centre in contrast to
other functionalities mentioned in the introduction. Whereas
this limits the stereoelectronic impact of the functional groups
on the metal centre, access to alcohol-functionalised NHC
silver complexes may have an interesting potential for the
anchoring of moieties on surfaces and interesting biological or
pharmaceutical properties via formation of e.g. ether or ester
bonds. The presence of an OH group on the NHC ligand was
found to be detrimental to the applicability of their silver(I)
complexes in transmetallation to Ni(II) since reprotonation of
the carbene carbon was always observed. Consistent with these
observations, we are not aware of any successful transmetalla-
tion of an alcohol-functionalised NHC ligand to nickel. In con-
trast, the structurally characterized ether-functionalised NHC
nickel(II) complex 15 was readily obtained from 13, emphasiz-
ing the potential of ether-functionalised NHC silver(I) com-
plexes as efficient transmetallating agents. In view of our
continuing interest in catalytic ethylene oligomerization,28 we
are currently using this approach to synthesize a series of
ether-functionalised NHC Ni(II) complexes for this purpose.
Fig. 6 Ball and stick representation of [NiCl2{ImDiPP(C2OMe)}2] (15).
Hydrogen atoms have been omitted for clarity (an ORTEP view is pro-
vided in Fig. S10 of the ESI†).
demonstrated the presence of an additional minor species (ca.
10%), which displayed very similar spectral features: the
signals due to the ligand framework were observable, with the
imid
OMe
CH2
and CH2
signals shifted downfield by 0.69 and
0.34 ppm and the CHiPr and CH3 signals shifted upfield by
0.21 and 0.32 ppm. No significant difference was noticeable in
the corresponding 13C NMR spectra. Attempts to isolate the
minor species by flash chromatography were unsuccessful.
Re-dissolving of the authentic crystals of 15 in deuterated sol-
vents resulted in the reformation of both species in the same
ratio. The infrared spectra of the crystals and the bulk powder
of 15 did not reveal any differences, in agreement with the
existence of only one type of complex in the solid state. Conse-
quently, the establishment of a dynamic equilibrium in solu-
tion involving two species was suspected and could be further
confirmed by a ROESY NMR experiment. Plausible types of
solution equilibria can be envisaged for such NHC nickel(II)
complexes (e.g. cis/trans-isomerization at the metal or the pres-
ence of syn/anti-conformers through the Ni–C(NHC)
rotation).26
iPr
Acknowledgements
Work currently in progress aims at investigating further The Centre National de la Recherche Scientifique (CNRS), the
these equilibria and at extending the family of ether-functiona- Ministère de l’Enseignement Supérieur et de la Recherche
lised NHC nickel(II) complexes in order to thoroughly evaluate (Ph.D. fellowship to S.H.) and the IFP Energies Nouvelles
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(IFPEN) are gratefully acknowledged for support. We are grate-
ful to Mélanie Boucher and Marc Mermillon-Fournier for
experimental support and to Dr A.A. Danopoulos for valuable
suggestions. The NMR service of the Université de Strasbourg
is warmly acknowledged for the 2D correlation experiments.
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