Silver(I) and Nickel(II) Complexes with Oxygen- or Nitrogen-Functionalized NHC Ditopic Ligands and Catalytic Ethylene Oligomerization

A Journal of

Accepted Article

Title: Silver(I) and Nickel(II) Complexes with Oxygen- or Nitrogen-

Functionalized NHC Ditopic Ligands and Catalytic Ethylene

Oligomerisation

Authors: Xiaoyu Ren, Marcel Wesolek, Corinne Bailly, Lydia Karmazin,

and Pierre Braunstein

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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901064

Link to VoR: http://dx.doi.org/10.1002/ejic.201901064

10.1002/ejic.201901064

European Journal of Inorganic Chemistry

Silver(I) and Nickel(II) Complexes with Oxygen- or Nitrogen-Functionalized

NHC Ditopic Ligands and Catalytic Ethylene Oligomerisation

Xiaoyu Ren,^[a]Marcel Wesolek^*,[a]Corinne Bailly,^[b]Lydia Karmazin,^[b]and Pierre Braunstein^*,[a]

Dedicated to Professor Charles U. Pittmann, Jr., Mississippi State University (USA), on the occasion of his 80^th

birthday, and to Professor F. Ekkehardt Hahn, University of Münster (Germany), on the occasion of his 65^th

birthday, with our warmest congratulations and best wishes.

Abstract. Potentially bidentate ditopic ligands containing a N-heterocyclic carbene (NHC) donor

associated with an ether or an amine have been prepared and coordinated to Ni(II) centres. The

influence of the length of the alkyl chain, -(CH₂)₂- or –(CH₂)₃- connecting the ether or the amine

group to the heterocycle was examined. In the analogous Ag(I) complexes

[Ag{Im(Dipp)(C₃OMe)-k¹C^NHC}₂]Cl (8), 9 and 10, in the neutral Ni(II) complexes with a C₃

spacer trans-[NiCl₂{Im(Dipp)(C₃OMe)-k¹C^NHC}₂] (5a), 6, and 7, and in the cationic cis-

[Ni{Im(Dipp)(C₃OMe)-k¹C^NHC}₂(NCMe)₂](PF₆)₂

k¹C^NHC}₂(NCMe)₂](PF₆)₂

(16), the

(15)

and

cis-[Ni{Im(Mes)(C₃OMe)-

ligand

is monodentate.

[(ImH)(Dipp)(C₃OMe)][NiX₃{Im(Dipp)(C₃OMe)-k¹C^NHC}] (14a, X = Cl) and (14b, X = Br) are

rare examples of complexes of the type [NiX₃(NHC)]^-. For comparison,

[NiBr₂{(Im)(Dipp)(C₂NMe₂)-k²,C^NHC,N^amine}] (24)

and [NiBr₂{Im)(Dipp)(C₃NMe₂)-

k²,C^NHC,N^amine}] (25) contain a six- or a seven-membered k²C^NHC,N^aminechelate, respectively.

Various Ni(II) complexes were evaluated as precatalysts for ethylene oligomerisation. The

structures of the carbene (Dipp)(C₃OMe)imidazole-ylidene (4) and of the complexes 5a, 7, 8,

[(ImH)(Dipp)(C₃OMe)]₂[NiCl₄]

(11a),

[(ImH)(Dipp)(C₃OMe)]₂[NiBr₄]

(11b),

[(ImH)(Me)(C₃OMe)]₂[NiCl₄] (13), 14b, 16·NCMe, [Im(H){C(Me)(=NDipp)}(C₃OMe)]₂[NiCl₄]

(18), [AgCl{Im[C(Me)=NDipp](C₃OMe)}-k¹C^NHC] (19), [AgCl{Im(Dipp)(C₃NMe₂)-k¹C^NHC}]

(23), 24, 25 and trans-[NiCl₂{Dipp[CH₂CH₂C(O)OEt]}₂] were analysed by X-ray diffraction.

[a]

[b]

X. Ren, Dr. M. Wesolek, Prof. Dr. P. Braunstein

Université de Strasbourg, CNRS CHIMIE UMR 7177, Laboratoire de

Chimie de Coordination, 4 rue Blaise Pascal, 67081 Strasbourg

Cedex, France

E-mail: wesolek@unistra.fr; braunstein@unisra.fr

https://lcc.chimie.unistra.fr/contact/

C. Bailly, Dr. L. Karmazin

Université de Strasbourg, Service de Radiocristallographie

Fédération de Chimie « Le Bel » FR2010

1 rue Blaise Pascal, 67008 Strasbourg Cedex, France

Supporting information for this article is given via a link at the end of

the document.

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Introduction

Following the report of the first complexes containing N-heterocyclic carbene (NHC) ligands¹and

the isolation of the first stable NHC ligands,²a fast growing interest developed for this class of

ligands and their metal complexes, largely triggered by their often unique properties and wide

range of applications.³Hundreds of ligands have been synthesized in which the NHC functionality

is part of multidentate, homofunctional ligands⁴or of hybrid ligands, which associate at least two

different donor groups (e.g. NHC with nitrogen, phosphorus, oxygen or sulphur) within the same

molecule.⁵Combining one (or more) NHC(s) with a neutral or an anionic donor group has been

shown to be beneficial for stabilizing reactive species, providing structural diversity and leading

to the formation of novel polynuclear motifs.⁶One such family of hybrid ligands, which is

investigated in this paper, combines NHC and oxygen-donor groups and, depending on their

stereoelectronic properties and on the type and length of the spacer connecting a NHC nitrogen

atom to the oxygen-donor, monodentate, chelating and bridging behaviour have been observed in

their metal complexes, as documented in a recent review article (Scheme 1).⁷

Hybrid C^NHC,O ligand

C^NHCdonor Oxygen donor

monodentate κ¹C^NHCbehaviour

M

chelating κ²C^NHC,O behaviour

M

bridging µ-κ²C^NHC,O behaviour

M

Scheme 1. Bonding modes of a C^NHC,O ditopic ligand

In the course of our studies on the synthesis of bis(ether-functionalized NHC) Ni(II) complexes

aimed at examining the impact of the presence of an oxygen donor group on the structure and

properties of the corresponding NHC metal complexes, ⁸we found that only monodentate

behaviour through the NHC function (k¹C^NHC) was observed in the case of the C^NHC,O^etherditopic

ligands studied. In contrast, alcoholate-functionalized NHC ligands with a similar –CH₂CH₂– (C₂)

spacer connecting a NHC nitrogen atom with the oxygen-donor led to mononuclear, chelated

complexes (k²C^NHC,O bonding type) or to oxygen-bridged dinuclear NHC complexes (µ-

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k²C^NHC,O bonding type).⁹Interestingly, the chelating k²C^NHC,O bonding mode of an anionic

ligand, where the oxygen donor belongs e.g. to an alcoholate or an enolate function, is reminiscent

of the k²P,O bonding mode of functional phosphine ligands endowed with a very rich chemistry,¹⁰

in particular in relation to SHOP-type ethylene oligomerization catalysts,¹¹and this has certainly

provided an incentive for their study.^9,12We thus wondered whether in C^NHC,O^ethersystems,

formation of (k²C^NHC,O^ether) chelates could be favoured by an increase in the length of the spacer

from C₂to C₃. For comparison, C^NHC,N^aminehybrid ligands containing an amino group and a C₃

spacer are also reported here.

Furthermore, we recently showed that binding of the amine group present in N^imine,C^NHC,N^amine

pincer-type ligands depends on a number of parameters, in addition to the length of the spacer

connecting a NHC nitrogen atom with the amine.¹³Considering the impressive developments of

the research on transition metal complexes with pincer ligands¹⁴and capitalizing on our own

recent studies on N,C^NHC,N-based pincer ligands,^13,15we investigated for comparison the bonding

behaviour of potentially tridentate N^imine,C^NHC,O^etherpincer ligands to nickel in order to examine

whether the desired chelation of the O^etherdonor could be promoted by the binding of the better

N^iminedonor. Interestingly, palladium complexes stabilized by N^pyr,C^NHC,O^aryloxidepincer ligands

were recently found to display better catalytic activities than the corresponding bidentate

bis(aryloxide-NHC) palladium complexes in norbornene polymerization.¹⁶In addition to our

interest for NHC-type hybrid ligands and the selectivity of the interactions between their donor

groups and metal centres in their complexes, we also evaluated the catalytic properties of the new

nickel complexes obtained in ethylene oligomerization. Triggered by industrial market

requirements, in particular the rapid growth of the demand for short-chain a-olefins, catalytic

ethylene oligomerization remains of considerable academic and industrial interest,¹⁷and nickel

complexes have a long and very successful history in this field.

Results and Discussion

Syntheses and Structures.

Extending a protocol previously established to access C^NHC,O^etherditopic ligands containing a –

CH₂CH₂– (C₂) spacer between a NHC nitrogen atom and the ether oxygen⁸to analogous ligands

containing a –(CH₂)₃– (C₃) spacer, we prepared 1-(2,6-diisopropylphenyl)-3-(3-methoxypropyl)-

1H-imidazol-3-ium chloride, [(ImH)(Dipp)(C₃OMe)]Cl (1a), 1-mesityl-3-(3-methoxypropyl)-1H-

imidazol-3-ium chloride, [(ImH)(Mes)(C₃OMe)]Cl (2), and 1-methyl-3-(3-methoxypropyl)-1H-

imidazol-3-ium chloride, [(ImH)(Me)(C₃OMe)]Cl (3) in good yields according to the reactions of

eq. 1.

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neat

Cl

N

R

N

(1)

Cl

O

N

O

R

Δ, 1.5 d

1a R = Dipp

2 R = Mes

3 R = Me

As detailed below, the corresponding bis(ether-functionalized NHC) nickel complexes were

typically obtained by deprotonation of these imidazolium salts followed by direct addition of the

resulting free carbene to [NiCl₂(dme)] (dme = 1,2-dimethoxyethane), by ligand transmetallation

from a NHC silver complex, or by an indirect pathway consisting firstly in the formation of a bis-

imidazolium salt of [NiX₄]^2-followed by its deprotonation (Scheme 2). In the latter case, using a

slight excess of NaOtBu (2.2 equiv.) as a base led to improved yields.

O

N

R

2.2 equiv. NaOtBu

1 equiv. NiX₂(dme)

THF

X

N

O

2

R

2

N

Cl Ni Cl

R

O

R

- 2 tBuOH, - 2 NaX

O

N

-78 °C, THF

1a R = Dipp, X = Cl

1b R = Dipp, X = Br

2 R = Mes, X = Cl

3 R = Me, X = Cl

4 R = Dipp

5a R = Dipp, X = Cl

5b R = Dipp, X = Br

6 R = Mes, X = Cl

7 R = Me, X = Cl

N

O

R

1.1 equiv. Ag₂O

1 equiv. NiCl₂(dme)

- AgCl, - dme, THF

Ag

Cl

R

- H₂O

THF, MS 4 Å

O

R

8 R = Dipp

9 R = Mes

10 R = Me

[NiX₄]²

2.2 equiv. NaOtBu

1 equiv. NiX₂(dme)

- dme, THF

N

O

- 2 tBuOH, - 2 NaX

2

THF

11a R = Dipp, X = Cl

11b R = Dipp, X = Br

12 R = Mes, X = Cl

13 R = Me, X = Cl

Scheme 2. Various synthetic approaches to trans-coordinated bis-ether-functionalized NHC Ni(II)

complexes.

The free carbene 1-(2,6-diisopropylphenyl)-3-(3-methoxypropyl)-1H-imidazol-2(3H)-

ylidene (4) was prepared by dropwise addition of a THF solution of NaOtBu to a THF suspension

of the corresponding imidazolium salt at –78 °C and colourless crystals were isolated. An X-ray

diffraction analysis, performed on single crystals grown from a saturated Et₂O solution at –30 °C

(Scheme 2, Figure 1), confirmed the nature of the ligand. Compound 4 crystallizes in the

monoclinic system (space group P2₁/c, Table S1). The lengths of the C1-N1 and C1-N2 bonds of

1.367(2) and 1.363(2) Å, respectively, are perfectly consistent with their partial double bond

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character and with literature values.²

Figure 1. View of the structure of the C^NHC,O^etherfree carbene 4 with H atoms omitted for clarity. Thermal

ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): C1−N1 1.367(2),

C1−N2 1.363(2), N1−C1−N2 101.7(1).

The Ni(II) complexes dichlorobis[1-(2,6-diisopropylphenyl)-3-(3-methoxypropyl)-1H-

imidazol-2(3H)-ylidene]nickel, [NiCl₂{Im(Dipp)(C₃OMe)-k¹C^NHC}₂] (5a), dichlorobis[1-

mesityl-3-(3-methoxypropyl)-1H-imidazol-2(3H)-ylidene]nickel,

[NiCl₂{Im(Mes)(C₃OMe)-k¹C^NHC}₂] (6), and dichlorobis[1-methyl-3-(3-methoxypropyl)-1H-

imidazol-2(3H)-ylidene]nickel, [NiCl₂{Im(Me)(C₃OMe)-k¹C^NHC}₂] (7), were obtained as orange,

diamagnetic, air-stable powders in good yield by reaction of the free carbene with [NiCl₂(dme)]

(Scheme 2). Complexes 5a and 7 were characterized by X-ray diffraction (Figure 2), they

crystallize in the monoclinic system (space group P2₁/c, Table S1). The nickel centre displays a

square-planar coordination environment with two trans chlorides and two trans NHC ligands. The

Ni1−C1 distances of 1.918(3) and 1.923(2) Å, respectively, are typical for Ni-C^NHCbonds in such

complexes.^3n,8The ether groups remain dangling, as previously observed with the shorter spacer

–CH₂CH₂–between the N and O atoms, which demonstrates that ether chelation was not hampered

by steric considerations.⁸Moreover, a minor isomer present in solution, as indicated by ¹H NMR

spectroscopy, could not be unambiguously identified, but we suggest that it corresponds to mutual

syn- and anti-orientations of the NHC ligands in complex 5a (see Experimental section).⁸

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Figure 2. Views of the structures of the trans-Ni(II) complexes 5a (top) and 7 (bottom) with H atoms

omitted for clarity. Thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles

(deg): in 5a Ni1−C1 1.918(3), Ni1−Cl1 2.190(7), C1−N1 1.355(3), C1−N2 1.352(3), C1−Ni1−Cl1 87.5(8),

C1−Ni1−Cl1’ 92.5(8), N1−C1−Ni1 131.1(2), N2−C1−Ni1 124.4(2), N1−C1−N2 104.3(2); in 7 Ni1−C1

1.923(2), Ni1−Cl1 2.199(4), C1−N1 1.350(2), C1−N2 1.351(2), C1−Ni1−Cl1 90.0(5), C1−Ni1−Cl1’

90.0(5), N1−C1−Ni1 127.8(1), N2−C1−Ni1 127.2(1), N1−C1−N2 104.8(1).

To examine the possibility to synthesize these Ni(II) complexes by transmetallation reaction,

the corresponding Ag(I) complexes 8-10 were prepared according to Scheme 2. X-ray quality

crystals of the complex [Ag{ImDipp(C₃OMe)-k¹C^NHC}₂]Cl (8) were obtained by slow diffusion

of pentane into a THF solution of the complex, which crystallizes in the monoclinic system (C2/c

space group, Table S1) (Figure 3). As expected, the coordination geometry about the silver cation

is nearly linear (C^NHC−Ag−C^NHCangle = 174.4(7)°) and the mean planes of the two NHC ligands

make an angle of 40.51(4)°, most likely for steric reasons. The Ag−C^NHCdistance of 2.094(1) Å is

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in good agreement with data reported for related homoleptic cationic bis-(NHC) Ag(I)

complexes.^8a,18In the crystal, the Ag−Cl separation of 2.8777(6) Å is similar to that observed in

related complexes,^{8a, 19}but is much longer than in dicoordinated complexes of the type

[AgCl(NHC)]^13c(see below the Ag-Cl distances of 2.332(4) Å in 19 and 2.314(1) Å in 23, ESI).

Consistent with an almost linear C1−Ag1−C1’ array (angle =174.4(7)°), this Ag−Cl separation

reflects the electrostatic interaction between the silver cation and the halide anion.

Figure 3. Structure of the Ag(I) complex 8 with H atoms omitted for clarity. Thermal ellipsoids at the 30%

probability level. Selected bond lengths (Å) and angles (deg): Ag1−C1 2.094(1), Ag1−Cl1 2.877(6),

C1−N1 1.358(2), C1−N2 1.352(2), C1−Ag1−Cl1 92.8(3), C1−Ag1−C1’ 174.4(7), N1−C1−Ag1 128.9(9),

N2−C1−Ag1 127.0(9), N1−C1−N2 103.8(1). The electrostatic Ag···Cl interaction is materialized in order

to better visualize the coordination geometry around the metal centre.

A third synthetic method was explored to access the bis-NHCs nickel complexes 5a-7

(Scheme 2), which involved first formation of the bis-imidazolium salts of [NiCl₄]^2-followed by

their deprotonation with a slight excess of NaOtBu. This method presented the clear advantages

of requiring shorter reaction times and of not involving air-sensitive, free carbenes. The salts

[(ImH)(Dipp)(C₃OMe)]₂[NiCl₄]

(11a),

[(ImH)(Dipp)(C₃OMe)]₂[NiBr₄]

(11b),

[(ImH)(Mes)(C₃OMe)]₂[NiCl₄] (12) and [(ImH)(Me)(C₃OMe)]₂[NiCl₄] (13) were obtained in high

yield by reaction of 2 equiv. of the corresponding imidazolium chloride with 1 equiv. of

[NiX₂(dme)]. Blue (X = Cl) or purple (X = Br) powders were isolated which are soluble in MeCN

or CH₂Cl₂, but insoluble in THF. Crystallographic analyses of 11a and 13 confirmed the presence

of [NiCl₄]^2-dianions associated with two imidazolium cations (Figure 4, Table S1). The nickel

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centre is in a tetrahedral environment constituted by four chlorides at a typical distance of 2.245(1)

and 2.250(1) Å, respectively.²⁰Complex 11a crystallizes in the orthorhombic system (space group

P2₁2₁2₁), and the H1···Cl2 and H1···Cl4 distances of 2.673 Å and 2.952 Å, respectively, reveal

weak hydrogen-bonding interactions. Complex 13 crystallizes in the monoclinic system (space

group C2/c) and the asymmetric unit contains one organic cation and half a [NiCl₄]^2-dianion, with

the nickel atom occupying a special position (population 50%).

Figure 4. Structures of the complexes 11a (top) and 13 (bottom) with H atoms omitted for clarity, except

the NCHN hydrogen atoms. Thermal ellipsoids at the 40% (11a) and 30% (13) probability level. Selected

bond lengths (Å) and angles (deg): in 11a Ni1−Cl1 2.245(1), Ni1−Cl2 2.282(9), Ni1−Cl3 2.275(9),

Ni1−Cl4 2.266(9), C1−N1 1.331(4), C1−N2 1.331(4), C20−N3 1.334(4), C20−N4 1.317(4); N1−C1−N2

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109.2(3), N3−C20−N4 109.0(3); in 13 Ni1−Cl1 2.250(1), Ni1−Cl2 2.277(1), C1−N1 1.332(4), C1−N2

1.322(5); N1−C1−N2 108.8(3).

When the salt 11a was used for the synthesis of the corresponding bis(NHC) Ni(II) complex,

few dark blue crystals of the Ni(II) complex [(ImH)(Dipp)(C₃OMe)][NiCl₃{Im(Dipp)(C₃OMe)-

k¹C^NHC}] (14a) were unexpectedly obtained. Similar blue crystals, previously isolated in a

irreproducible manner, were shown by X-ray diffraction analysis to contain the first example of

anionic Ni(II) complex where three chlorides and one NHC ligand coordinate the metal, associated

with an imidazolium cation.^8bThe isolation of the mono-NHC nickel complex 14a suggested that

it may be more rationally prepared by decreasing the quantity of base from 2 to 1 equiv. (eq. 2).

[NiX₄]²

N

1 equiv. NaOtBu

O

Dipp

N

O

N

O

(2)

Dipp

Ni

X

- tBuOH, - NaX

2

X

THF

11a X = Cl

11b X = Br

14a X = Cl

14b X = Br

Addition of diethyl ether to a toluene solution of the reaction mixture afforded dark blue,

paramagnetic, air-stable crystals, but these were not suitable for crystallographic analysis. We then

turned

to

the

bromide

analogue

and

isolated

in

good

yield

[(ImH)(Dipp)(C₃OMe)][NiBr₃{Im(Dipp)(C₃OMe)-k¹C^NHC}] (14b) from the imidazolium salt

[(ImH)(Dipp)(C₃OMe)]₂[NiBr₄] (11b) under similar reaction conditions. Single crystals of

complexes 11b (monoclinic system, space group Cc) and 14b (monoclinic system, space group

P2₁/c) were analysed by X-ray diffraction (Figure 5, Table S1). These complexes are similar to

their chloride analogues^8band the structure of 11b displays weak hydrogen-bonding interactions

(H20···Br1 = 2.943 Å and H20···Br2 = 2.876 Å). In complex 14b, the nickel centre is surrounded

by three bromides and one NHC ligand which form a tetrahedral coordination environment. To the

best of our knowledge, such an arrangement has only been characterized once before by X-ray

diffraction.²¹The first structurally characterized example of a [NiX₃(NHC)]^-anion was reported

only recently.^8bThe Ni−Br bond lengths, in the range 2.376(9) - 2.404(8) Å and 2.402(5) - 2.424(5)

Å, in 11b and 14b, respectively, compare well with literature values.²²The distances H20···Br1

(2.738 Å), H20···Br2 (3.043 Å), H16B···Br2 (2.914 Å) and H35A···Br2 (3.128 Å) are indicative

of weak hydrogen-bonding interactions in 14b.

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Figure 5. Views of the structures of complexes in 11b (top) and 14b (bottom), with H atoms omitted for

clarity, except those involved in H-bonding interactions. Thermal ellipsoids at the 30% probability level.

Selected bond lengths (Å) and angles (deg): in 11b Ni1−Br1 2.394(8), Ni1−Br2 2.401(7), Ni1−Br3 2.404(8),

Ni1−Br4 2.376(9), C1−N1 1.333(6), C1−N2 1.323(6), C20−N3 1.315(6), C20−N4 1.317(6); N1−C1−N2

108.6(4), N3−C20−N4 109.8(4); in 14b Ni1−Br1 2.402(5), Ni1−Br2 2.424(5), Ni1−Br3 2.405(5), Ni1−C1

1.985(3), C1−N1 1.370(4), C1−N2 1.357(3), C20−N3 1.329(4), C20−N4 1.327(4); C1−Ni1−Br1 120.7(8),

C1−Ni1−Br2 103.2(8), C1−Ni1−Br3 104.1(8), Br1−Ni1−Br2 109.0(2), Br1−Ni1−Br3 107.4(2),

Br2−Ni1−Br3 112.5(2), N1−C1−Ni1 129.0(2), N2−C1−Ni1 127.0(2), N1−C1−N2 104.0(2), N3−C20−N4

108.6(2).

As shown in Figure 6, the paramagnetic nickel complex 14a displays ¹H NMR peaks between

92 and -32 ppm in THF-d₈.

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Figure 6. Paramagnetic ¹H NMR spectrum of 14a in THF-d₈

Because of the presence of an ether-functionalized heterocycle in both the cationic and anionic

moieties of this complex, further ¹H NMR investigations were performed in order to assign their

signals. In Figure 7, the top spectrum (blue) is that of 14a (a), and the three spectra below illustrate

the shifts resulting from the addition of increasing amounts of imidazolium salt (1a = b⁺Cl^-) to

14a. While some peaks (a^-) remained unchanged, other (a⁺) were significantly shifted. Thus, the

a^-peaks belong to the anionic part of the complex with the carbene ligand bound to the nickel

centre whereas the a⁺peaks belong to the imidazolium cation that interacts by hydrogen bonding

with the paramagnetic nickel complex. In solution, the signals for a⁺and b⁺cannot be

distinguished.

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Cl

N

O

Dipp

N

O

n

Dipp

Ni

Cl

b

a

b

a

a+/-/+ a+/+/+

a-/-

a-

a+

a-

Figure 7. Changes in the ¹H NMR spectrum of 14 in THF-d₈upon addition of increasing amounts of the

corresponding imidazolium salt, [a+] represents the peaks assigned to [(ImH)(Dipp)(C₃OMe)]⁺and [a-]

those assigned to the anion [NiCl₃{Im(Dipp)(C₃OMe)-k¹C^NHC}]^-.

1

For comparison of the H NMR spectrum of 14a with those of relevant complexes, we

searched for other Ni(II) complexes possessing a [NiX₃]^-moiety. In 2014, Whittlesey and co-

workers obtained a complex containing the Ni(II) anion [NiBr₃(PPh₃)]^-as a byproduct in the

formation of a Ni(II) NHC complex by deprotonation of a N-aryl-substituted six-membered

1

imidazolium ring followed by the addition of [NiBr₂(PPh₃)₂] (Figure 8). ²³The H NMR

resonances of the associated imidazolium cation were observed between 19.55 and -4.85 ppm. In

2017, Fout and co-workers²¹isolated a [NiBr₃(NHC)]^-complex from the chemical oxidation of a

1

bidentate NHC-η²-iminoacyl Ni(II) complex by [Ph₃CBr] and its H NMR signals ranged from

87.29 to -22.21 ppm (Figure 8). The latter range is more similar to that observed for 14a, which is

consistent with the structural similarity of the complexes.

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^tBu

N

Dipp

Br

PPh₃

Ni

N

Br

N

Br

Whittlesey

Fout

Figure 8. Ni(II) complexes containing a [NiBr₃]^-moiety (see text)

In an attempt to promote chelation of the ether group, halogen abstraction was performed on

complex 6, using AgPF₆or TlPF₆, the latter in order to prevent redox side reactions. With THF as

a solvent, a yellow compound was formed that could not be fully identified. We then carried out

the same reaction in the presence of very small amounts of water since previous work with a related

Ni(II) complex has evidenced the stabilizing role of coordinated water molecules.^8bHowever, we

could not characterize such a species, but after dissolution of the solid in acetonitrile, the solvento

complex cis-[Ni{Im(Mes)(C₃OMe)-k¹C^NHC}₂(NCMe)₂](PF₆)₂(16) was obtained. Pale yellow

single crystals of 16·NCMe were grown by slow diffusion of ether into an acetonitrile solution of

the complex and characterized by X-ray diffraction (Figure 9). The complex crystallises in the

triclinic system (space group P-1, Table S1). The structural analysis established that the nickel

centre adopts a square planar coordination geometry, defined by two cis-NHC ligands and two cis-

acetonitrile molecules. The trans to cis isomerisation observed on going from 6 to 16 is favoured

by the solvent polarity. The Ni−C^NHCbond lengths of 1.884(2) Å are slightly shorter than in the

trans complexes 5a and 7 (see above), most likely because of the weaker trans-influence of

acetonitrile compared with the NHC ligand 4. The planes of the two carbene ligands in 16 are

1

almost orthogonal to each other (88.74°) in order to minimize steric repulsions. The H NMR

spectra indicated that the gem-protons on the ether arms -NCH₂CH₂CH₂OMe are diastereotopic

because of the cis-coordination geometry of the two NHC ligands (Supporting information,

Figures S1 and S2). When the crude product obtained from the halide abstraction reaction

performed in dry THF on 6 was treated with acetonitrile, formation of the acetonitrile-coordinated

complexes was not observed by NMR spectroscopy. However, when halide abstraction was

directly carried out in dry acetonitrile, complex 16 was readily isolated (eq. 3). These reactions

confirm the difficulty to coordinate the ether arm of the NHC ligand 4 to a Ni(II) centre. Clearly,

acetonitrile is a better donor than an ether group.

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O

(PF₆)₂

N

O

N

Mes

2 equiv. AgPF₆

MeCN

N

Ni

N

(3)

Cl

Mes

Cl Ni

N

O

N

6

16

Figure 9. Structure of the Ni(II) complex 16 with H atoms and PF₆anions omitted for clarity. Thermal

ellipsoids at the 40% probability level. Selected bond lengths (Å) and angles (deg): Ni1−C1 1.884(2),

Ni1−C17 1.884(2), Ni1−N5 1.898(2), Ni1−N6 1.906(2), C1−N1 1.364(3), C1−N2 1.349(3), C17−N3

1.359(3), C17−N4 1.348(3); C1−Ni1−C17 89.8(9), C1−Ni1−N6 92.8(8), C17−Ni1−N5 91.0(8),

N5−Ni1−N6 87.4(8), C1−Ni1−N5 172.4(9), C17−Ni1−N6 172.7(9), N1−C1−N2 105.2(2), N3−C17−N4

105.5(2).

Pincer-type Ligands

Since potentially C^NHC,O^etherbidentate ligands with a C2⁸or a C3 spacer (this work)

connecting the heterocycle to the ether group were observed to only bind to Ni(II) in a monodentate

k¹C^NHCmode, the oxygen-function remaining dangling, we considered introducing an additional

N^iminedonor in the ligand system that could provide anchimeric assistance for ether coordination

in a potentially N^imine,C^NHC,O^ethertridentate, pincer-type ligand. The imidazolium salt

[(ImH){C(Me)=NDipp}(C₃OMe)]Cl (17) (eq. 4) was isolated as a white powder but its

deprotonation followed by the addition of [NiCl₂(dme)] afforded an intractable mixture. However,

the indirect pathway mentioned above consisting in the formation of the bis-imidazolium salt

[(ImH){C(Me)=NDipp}(C₃OMe)]₂[NiCl₄] (18) (eq. 5), followed by deprotonation with 1 or 2

equiv. of base, led to better results.

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Cl

Toluene

r.t.

N

O

(4)

(5)

N

O

N

Dipp

N

Dipp

17

[NiCl₄] ²

1 equiv. NiCl₂(dme)

THF

N

O

2

17

Dipp

2

18

The salt 18 crystallizes in the monoclinic system (C2/c space group, Table S1) and the

asymmetric unit contains one organic cation and half a dianion [NiCl₄]^2-, with the nickel atom

occupying a special position (population 50%), as in compound 13 (see above) (Figure 10). A

green, paramagnetic complex was obtained by deprotonation of 18 with 1 equiv. of NaOtBu, but

1

no crystals formed. From the H NMR analysis, formation of a compound of the type

[NiCl₃(NHC)]^-could not be evidenced.

Figure 10. View of the structure of complex 18 with H atoms omitted for clarity, including the

NCHN hydrogen atoms. Thermal ellipsoids at the 40% probability level. Selected bond lengths

(Å) and angles (deg): Ni1−Cl1 2.263(4), Ni1−Cl2 2.271(4), C1−N1 1.340(2), C1−N2 1.324(2),

N1−C1−N2 108.3(1).

Considering the wide use of silver(I) complexes as transmetalating agents in the synthesis of

transition metal NHC complexes,²⁴the silver complex [AgCl{Im[C(Me)=NDipp](C₃OMe)}-

k¹C^NHC] (19) was prepared (eq. 6) and used for transmetallation reactions to Ni(II). Its

crystallographic analysis (monoclinic system, space group P2₁/c, Table S1) (Figure 11) confirmed

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the expected nearly linear coordination geometry around the metal (C1−Ag1−Cl1 angle of 177.1(4)

Å), and the presence of a monodentate NHC ligand with pendent N^imineand O^ethergroups. The Ag-

C^NHCbond length of 2.080(1) Å is slightly shorter than that in 8 (2.094(1) Å, see above) or in

comparable complexes.^13cThe N and O atoms are directed toward the Ag atom although the

Ag1×××N3 (2.801 Å) and Ag1×××O1 (3.334 Å) separations are too long to represent significant

bonding interactions. Similar features were recently encountered with a potentially

N^imine,C^NHC,N^aminetridentate ligand.^13c

Cl

0.55 equiv. Ag₂O

THF, MS 4 Å

N

O

(6)

N

Ag

Dipp

17

Cl

19

Figure 11. View of the structure of the Ag(I) complex 19 with H atoms omitted for clarity. Thermal

ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ag1−C1

2.080(1), Ag1−Cl1 2.332(4), C1−N1 1.366(2), C1−N2 1.346(2); C1−Ag1−Cl1 177.1(4),

N1−C1−Ag1 125.1(1), N2−C1−Ag1 130.6(1), N1−C1−N2 104.3(1).

When [NiCl₂(dme)] was used for a transmetallation reaction from 19 in THF, a green oil was

obtained that could not be analysed owing to its paramagnetic nature. The low solubility of

[NiCl₂(dme)] that could favour formation of a bis-NHC complex prompted us to use a more

soluble precursor. Therefore, [Ni(NCMe)₆](BF₄)₂was employed in a Ni/Ag-NHC ratio of 1:1 or

1

2:1 but similar results were obtained in both cases, according to H NMR spectroscopy. The

product could not be crystallized. The additional imine donor introduced in the hybrid ligand has

therefore not allowed the characterization of Ag(I) or Ni(II) complexes that would display

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coordination of the ether oxygen.

Introduction of an Amine Donor.

The results reported above and in the literature^7,8clearly indicate that chelation of the ether

group in C^NHC,O^etheror in N^imine,C^NHC,O^ethersystems is not favourable, whether the spacer

connecting an imidazole nitrogen atom to the ether group is a -(CH₂)₂- or a (-CH₂)₃- unit. In

contrast, chelation of the stronger alcoholate donor group to Ni(II) has been reported.^7,9In view of

this and for comparison, we introduced a stronger amine donor in place of the ether to investigate

the possibility of forming a C^NHC,N^aminechelate with either a C₂or a C₃spacer. The ammonium

salts [(ImH)(Dipp)(C₂NHMe₂)]Cl₂(20) and [(ImH)(Dipp)(C₃NHMe₂)]Cl₂(21) were prepared

according to eq. 7 and reacted with 2 equiv. of base and then with [NiBr₂(dme)].

2 Cl

MeCN

Cl

N

(7)

N

Dipp

n

Δ, 24 h

H

20 n = 2

21 n = 3

The purple Ni(II) complexes 24 and 25 thus isolated (Scheme 3) were also obtained by

transmetallation from the Ag(I) complexes [AgCl{(Im)(Dipp)(C₂NMe₂)-k¹C^NHC}] (22) and

[AgCl{(Im)(Dipp)(C₃NMe₂)-k¹C^NHC}] (23), respectively. The latter complex crystallizes in the

monoclinic system (P2₁/c space group, Table S1). As expected, its structure is similar to that of 19

(Figure 11), with a NMe₂instead of OMe pending donor group and is shown in the Supporting

Information (Figure S3). However, ligand chelation in the Ni(II) complexes was unambiguously

established by an X-ray diffraction analysis of [NiBr₂{(Im)(Dipp)(C₂NMe₂)-k²N^amine,C^NHC}] (24)

(Figure 12). This complex crystallizes in the monoclinic system (P2₁/c space group, Table S1).

The nickel centre is in a tetrahedral coordination environment consisting of two bromides, the

NHC and the amine donor groups. Formation of a C^NHC,N^aminechelated Ni complex is consistent

with the stronger amine group coordinating more readily to Ni(II) than an ether. The purple Ni(II)

complex [NiBr₂{(Im)Dipp(C₃NMe₂)-k²C^NHC,N^amine}] (25) has a similar structure (monoclinic

system, space group P2₁/c, Table S1) but contains a seven-membered ring C^NHC,N^aminechelate. Its

bond distances and angles are similar to those in 24, except for the much more open C1−Ni1−N3

angle (104.7(3)° in 25, 94.8(2)° in 24). In both complexes, the length of the Ni-N^aminedative bond,

2.042(5) and 2.056(8) Å, in 24 and 25, respectively, compares well with those in recently reported

Ni(II) complexes with k²C^NHC,N^aminechelates^13b,cand are typical for Ni(II)-tertiary amine bonds

in tetracoordinated complexes (aver. 1.957 Å).²⁵

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N

2.2 equiv. NaO^tBu

r.t., THF

Dipp

Br

1 equiv. NiBr₂(dme)

THF

n

20 n = 2

21 n = 3

Ni

N

Br

24 n = 2

25 n = 3

N

n

N

Dipp

1.1 equiv. Ag₂O

THF, MS 4 Å

1 equiv. NiBr₂(dme)

THF

Ag

Cl

22 n = 2

23 n = 3

Scheme 3. Synthesis of the amine-functionalized NHC Ag(I) and Ni(II) complexes 22-25

Figure 12. Views of the structures of complexes 24 (top) and 25 (bottom) with H atoms omitted

for clarity. Thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles

(deg): in 24 Ni1−Br1 2.342(1), Ni1−Br2 2.397(1), Ni1−C1 1.970(6), Ni1−N3 2.042(5), C1−N1

1.358(8), C1−N2 1.343(8); C1−Ni1−Br1 124.3(2), C1−Ni1−Br2 103.4(2), C1−Ni1−N3 94.8(2),

Br1−Ni1−Br2 122.2(5), Br1−Ni1−N3 102.4(2), Br2−Ni1−N3 104.2(2), N1−C1−N2 104.5(5),

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N1−C1−Ni1 130.9(5), N2−C1−Ni1 124.6(5); in 25 Ni1−Br1 2.353(2), Ni1−Br2 2.348(2), Ni1−C1

1.989(9), Ni1−N3 2.056(8), C1−N1 1.352(1), C1−N2 1.362(1); C1−Ni1−Br1 118.1(2),

C1−Ni1−Br2 101.2(2), C1−Ni1−N3 104.7(3), Br1−Ni1−Br2 123.6(8), Br1−Ni1−N3 104.6(2),

Br2−Ni1−N3 102.2(2), N1−C1−N2 104.5(7), N1−C1−Ni1 129.2(6), N2−C1−Ni1 125.3(6).

These results confirm that both a C2 and a C3 spacer have a suitable length for allowing chelation

of a functional NHC ligand to Ni(II) provided the affinity of the donor group associated with the

C^NHCis sufficient.

Catalytic Ethylene Oligomerization

In view of the continuing interest in the catalytic ethylene oligomerization, in particular for the

production of short-chain a-olefins,¹⁷and in continuation of previous investigations in our group

on the use of Ni NHC complexes for this reaction, we examined the catalytic properties of the

neutral complexes 5a, 6, 24 and 25, of the monoanionic complex 14a and of the dianionic

complexes 11a and 13 in toluene or in a mixture of toluene and chlorobenzene, in the presence of

10 equiv. ethylaluminum dichloride (EADC) as co-catalyst (Table 1). This diversity of precatalysts

could allow an evaluation of the influence of the N-substituents (R = Dipp, Mes, Me), of the N’-

bound functionality (C₃OMe, C₂NMe₂and C₃NMe₂), and of the different cations present in the

salts containing a [NiCl₄]^2-dianion. A comparison will also be made with the data previously

obtained with [NiCl₂{ImDipp(C₂OMe)}₂], a complex analogous to 5a but containing the shorter

C₂OMe spacer.^8b

The neutral bis-NHC Ni(II) complexes 5a and 6 with the C₃OMe N-functionality displayed

similar activities with R = Dipp or Mes (productivity: 3500 g C₂H₄/(g Ni·h) for 5a and 3100 g

C₂H₄/(g Ni·h) for 6, Table 1, entries 2, 3). These values are slightly lower than that found under

similar conditions with [NiCl₂{ImDipp(C₂OMe)}₂], which contains the C₂OMe substituent in

place of C₃OMe (4600 g C₂H₄/(g Ni·h) entry 1).^8bHowever, with the neutral Ni(II) complexes

bearing the C₂NMe₂(24) or C₃NMe₂(25) N-functionality, productivities under similar catalytic

conditions were ca. 4 times higher than with 5a (15600 g C₂H₄/(g Ni·h) for 24 and 13300 g C₂H₄/(g

Ni·h) for 25, Table 1, entries 4, 5). There is clearly a beneficial effect of amine chelation, possibly

by stabilizing the active species and increasing its lifetime. To evaluate the influence of the

quantity of cocatalyst on the catalytic activity of 25, a lower amount (4 equiv.) of EADC was

employed but this significantly reduced ethylene consumption and activity (productivity: 2000 g

C₂H₄/(g Ni·h), entry 6). Butenes were the major products, with a selectivity ranging from 66% to

81%, and the formation of 1-butene being, as expected, more selective when the catalytic

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productivity is lower since re-insertion of the primary reaction product 1-butene, giving branched

products, is less likely.

We used the same catalytic conditions with the monoanionic NHC Ni(II) complex

[(ImH)Dipp(C₃OMe)][NiBr₃{ImDipp(C₃OMe)-k¹C^NHC}] (14b). Its productivity (12300 g

C₂H₄/(g Ni·h), entry 7) was more than 3 times that of the neutral bis-NHC Ni(II) complexes (5a

and 6, entries 2, 3) but slightly lower than that of the neutral, k²C^NHC,N^aminemono-NHC Ni(II)

complexes (24 and 25, entries 4, 5). The selectivity of 14b for the formation of butenes (80%) and

hexenes (19%) is similar to that of the previous pre-catalysts (butenes: 66-81%, hexenes: 17-28%).

After examining the neutral and monoanionic complexes, the dianionic Ni(II) complexes were

found to exhibit higher ethylene consumption and productivities (productivity: 16900 g C₂H₄/(g

Ni·h) of 11a and 18000 g C₂H₄/(g Ni·h) of 13, entries 8, 9). For comparison, the well-known

dianionic complex [NEt₄]₂[NiBr₄] was tested under similar conditions and it showed the best

productivity (19900 g C₂H₄/(g Ni·h), entry 10) among the dianionic nickel complexes. Butenes

were the major products, as for the other nickel complexes discussed above. These results indicate

that the presence of the imidazolium cation in 11a and 13 has no positive impact on the catalytic

properties of the metal complex.

Table 1. Catalytic performances of the Ni(II) complexes in ethylene oligomerization

selectivity^c

ethylene

consumed^a

1-

butene^d

1-

hexene^d

linear

C₆

entry

complex

Productivity^bC₄

C₆

C₈+

e

1^8b

[NiCl₂{ImDipp(C₂OMe)}₂]^8b

[NiCl₂{ImDipp(C₃OMe)}₂]

[NiCl₂{ImMes(C₃OMe)}₂]

[NiBr₂{ImDipp(C₂NMe₂)}]

[NiBr₂{ImDipp(C₃NMe₂)}]

6.3

4.8

4600

3500

64 32

73 23

81 17

71 26

74 24

66 28

80 19

68 29

4

3

2

3

2

6

1

3

52

51

48

9

3

5

6

1

2

1

17

2

5a

6

3

4.2

3100

4

24

21.3

18.2

2.8

15600

13300

2000

5

25

8

6

25 ^f

14b

11a

32

9

[(ImH)Dipp(C₃OMe)]

[NiBr₃{ImDipp(C₃OMe)}]

7

15.3

23.2

12300

16900

[(ImH)Dipp(C₃OMe)]₂

[NiCl₄]

8

5

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9

13

[(ImH)Me(C₃OMe)]₂[NiCl₄]

[NEt₄]₂[NiBr₄]

24.7

27.2

18000

19900

72 26

67 30

2

3

7

1

10

Conditions: amount of catalyst: 4×10^-5mol, amount of cocatalyst (EADC): 4×10^-4mol (10 equiv.), T = 30–35 °C,

solvent: toluene (except for 11a, 13 and [NiCl₂{(Im)(Dipp)(C₂OMe)}₂] in chlorobenzene for solubility issues), total

volume: 15 mL, 10 bar C₂H₄, reaction time 35 min., every test was repeated at least twice.

^aequal to the quantity of ethylene introduced minus the unreacted ethylene collected at the end of the reaction (see

experimental), expressed in g; ^bexpressed in g C₂H₄/(g Ni·h);^cexpressed in %, calculated by GC analysis; ^dselectivity

for a within the C₄or C₆fraction, expressed in % mol; ^eselectivity for linear olefins within the C₆fraction; ^famount

of cocatalyst (EADC): 1.6×10^-4mol (4 equiv.).

Conclusions

A series of N-heterocyclic carbene-based hybrid ligands of the C^NHC,O^ethertype with different N-

substituents (R = Dipp, Mes, Me) has been investigated and revealed a monodentate k¹C^NHC

coordination behaviour in both their silver and nickel complexes. Whereas the bis-NHC Ni(II)

complex 5a could be obtained from the isolated, free NHC ligand 4, complexes 5a–7 were

prepared by transmetallation reaction from the bis-NHC Ag(I) complexes 8-10 or via an indirect

synthetic method (Scheme 1). The latter proceeds in two steps: formation of bis-imidazolium salts

with a [NiCl₄]^2-dianion, followed by deprotonation of this salt with a base. This procedure avoids

the isolation of a sensitive free carbene and it is noteworthy that the formation of mono- or bis-

NHC Ni(II) complexes 14 and 5a – 7, respectively, could be simply controlled by the quantity of

base used. Interestingly, the rare examples of anionic, mono-NHC nickel complexes of the type

[NiX₃(NHC)]^-, present in 14, exhibit a paramagnetic ¹H NMR spectrum with well-defined peaks

ranging from 92 to -32 ppm, probably associated with hydrogen bonding interactions between the

imidazolium cation of the salt [(ImH)(Dipp)(C₃OMe)]⁺and the anionic moiety. Such interactions

have been evidenced in the solid-state by an X-ray diffraction analysis of 14b. Detailed ¹H NMR

spectroscopic analyses allowed the assignment of the resonances of both the cationic and anionic

fragments, by addition of increasing amounts of the corresponding imidazolium salt

[(ImH)(Dipp)(C₃OMe)]Cl

(1a)

to

the

nickel

complex

[(ImH)(Dipp)(C₃OMe)][NiCl₃{Im(Dipp)(C₃OMe)-k¹C^NHC}] (14b).

With either a C₂or C₃spacer between the heterocycle and the ether donor group, only k¹C^NHC

bonding of the C^NHC,O^etherditopic ligands was observed in the metal complexes reported in this

work, which is fully consistent with previous findings,^7,8,26For comparison, related C^NHC,N^amine

hybrid ligands with a C₂or a C₃spacer were evaluated and found to chelate the tetrahedral Ni(II)

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centre, demonstrating the ability of both spacers to allow chelation, provided the end donor group

is strong enough. With the hope to promote anchimeric assistance to ether chelation, an additional

imine donor function was introduced in the ligand to generate a new potential pincer system of the

type N^imine,C^NHC,O^ether. Not surprisingly, its Ag(I) complex 19 only displayed k¹C^NHCbehaviour

but we could not characterize the ligand bonding mode in its Ni(II) complexes owing to their

paramagnetic nature.

Catalytic studies in ethylene oligomerization have shown an increased productivity for the

neutral precatalysts with the chelating k²C^NHC,N^amineligand containing a C₂spacer and with the

mono- or dianionic Ni(II) complexes. However, in the case of the dianionic complexes, the

functional imidazolium counter cation did not bring about any improvement of the catalytic

performances, except a slight increase in selectivity for 1-butene.

This study has further demonstrated that with potentially bi- or tri-dentate hybrid ligands

containing a NHC donor, coordination of the additional donor function(s) mostly depends on its

chemical nature, as demonstrated by comparing ether and tertiary amine donors. A change in the

length of the spacer used to connect it to a N atom of the heterocycle, from C2 to C3, was found

not to be decisive. These results answer some key questions asked at the onset of our investigations.

Identifying the key parameters allowing a better control of the coordination chemistry of polytopic

ligands and the reactivity and catalytic properties of their metal complexes remains a topic of major

importance.

Experimental Section

General methods

All manipulations involving organometallics were performed under nitrogen or argon in a Braun

glovebox or using standard Schlenk techniques. Solvents were dried using standard methods and

distilled under nitrogen prior use or passed through columns of activated alumina and subsequently

purged with nitrogen or argon. The compounds 1-(2,6-diisopropylphenyl)-1H-imidazole, 1-

mesityl-1H-imidazole, 1-metyl-1H-imidazole and [NiCl₂(dme)] were synthesized according to

published procedures.²⁷NMR spectra were recorded on Bruker spectrometers (AVANCE I − 300

MHz, AVANCE III − 400 MHz or AVANCE I − 500 MHz equipped with a cryogenic probe). IR

spectra were recorded in the region 4000-200 cm^-1on a Nicolet 6700 FT-IR spectrometer (ATR

mode, diamond crystal). Elemental analyses were performed by the “Service de microanalyses”,

Université de Strasbourg. For the X-ray diffraction studies, suitable crystals were obtained as

described below. Summary of the crystal data, data collection and refinement for compounds are

given in Table S1 (see Supporting Information). The crystals were mounted on a glass fibre with

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grease, from Fomblin vacuum oil. For compounds 4, 5a, 8, 11a, 11b, 14b, 16·MeCN, 18, 19, 23

and 25, data sets were collected at 173(2) K on a Bruker APEX-II CCD Duo diffractometer using

Mo-Ka (l = 0.71073 Å) or Cu-Ka radiation (l = 1.54178 Å). The cell parameters were determined

(APEX2 software)^28afrom reflections taken from three sets of 12 frames, each at 10 s exposure.

For compounds 7, 13 and 24, data sets were collected at 173(2) K on a Nonius Kappa-CCD

diffractometer using Mo-Kα radiation (λ = 0.71073 Å). The cell parameters were determined

(Denzo software)^28bfrom reflections taken from one set of 10 frames (1.0° steps in phi angle),

each at 20s exposure. The structures were solved by direct methods using the program SHELXS-

2013.^29aThe refinement and all further calculations were carried out using SHELXL-2013^29bor

SHELXL-2014.^29cThe H-atoms were introduced into the geometrically calculated positions and

refined riding on the corresponding parent atoms. The non-H atoms were refined anisotropically,

using weighted full-matrix least-squares on F². Crystallographic information files (CIF) for the

compounds 4, 5a, 7, 8, 11a, 11b, 13, 14b, 16·MeCN, 18, 19, 23, 24 and 25 have been deposited

with the CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K., and can be obtained on request free

of charge, by quoting the publication citation and deposition numbers CCDC 1955230-1955243

and 1956078. The catalytic reactions were performed in a magnetically stirred (1200 rpm) 145 mL

stainless steel autoclave. A 125 mL glass container was used to avoid corrosion of the autoclave

walls. The procedures used have been previously detailed.^8b,13c

Preparation of [(ImH)(Dipp)(C₃OMe)]Cl (1a).

A mixture of 1-(2,6-diisopropylphenyl)-1H-imidazole (5.000 g, 21.9 mmol) and 1-chloro-3-

methoxypropane (12 mL, 109.5 mmol) was refluxed for 1.5 day. The unreacted 1-chloro-3-

methoxypropane was removed under reduced pressure. The resulting sticky solid was triturated

twice with Et₂O. It was then stirred in Et₂O for 16 h and the white solid 1a was collected by

filtration (7.260 g, 98%). ¹H NMR (500 MHz, CD₂Cl₂) δ: 10.32 (br, 1H, CH^imidazole), 7.95 (br, 1H,

3

CH^imidazole), 7.57 (t, J = 7.8 Hz, 1H, CH^p-Ar), 7.34 (d, J = 7.8 Hz, 2H, CH^m-Ar), 7.21 (br, 1H,

3

CH^imidazole), 4.79 (t, J = 6.7 Hz, 2H, CH₂CH₂CH₂OMe), 3.51 (t, J = 5.6 Hz, 2H,

CH₂CH₂CH₂OMe), 3.28 (s, 3H, CH₂CH₂CH₂OMe), 2.33-2.26 (m, 4H, CH₂CH₂CH₂OMe

3

overlapped with CH^iPr), 1.22 (d, J = 6.7 Hz, 6H, CH₃^iPr), 1.16 (d, J = 6.7 Hz, 6H, CH₃^iPr).

¹³C{¹H} NMR (126 MHz, CD₂Cl₂) δ: 145.8 (C^o-Ar), 138.7( C^imidazole), 132.1(C^p-Ar), 130.6 (C^ipso-Ar),

125.0 (C^m-Ar), 124.2 (C^imidazole), 123.9 (C^imidazole), 69.5 (CH₂CH₂CH₂OMe), 59.0

(CH₂CH₂CH₂OMe), 48.6 (CH₂CH₂CH₂OMe), 30.7 (CH₂CH₂CH₂OMe), 29.0 (CH^iPr), 24.5

(CH₃^iPr), 24.3 (CH₃^iPr). Anal. calcd for C₁₉H₂₉ClN₂O: C, 67.74; H, 8.68; N, 8.32%. Found: C, 67.35;

H, 9.05; N, 7.95%.

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Preparation of [(ImH)(Mes)(C₃OMe)]Cl (2).

A procedure similar to that used for compound 1a afforded 2 as a white solid (7.740 g, 98%). ¹H

NMR (500 MHz, CD₂Cl₂) δ: 10.79 (br, 1H, CH^imidazole), 7.79 (br, 1H, CH^imidazole), 7.20 (br, 1H,

CH^imidazole), 7.04 (s, 2H, CH^m-Ar), 4.70 (t, ³J = 6.7 Hz, 2H, CH₂CH₂CH₂OMe), 3.48 (t, ³J = 5.6 Hz,

2H, CH₂CH₂CH₂OMe), 3.27 (s, 3H, CH₂CH₂CH₂OMe), 2.34 (s, 3H, Me^para), 2.08 (s, 6H, Me^ortho).

¹³C{¹H} NMR (126 MHz, CD₂Cl₂) δ: 141.5 (C^p-Ar), 139.4 (C^imidazole), 134.7 (C^o-Ar), 131.3 (C^ipso-

^Ar), 130.0 (C^m-Ar), 123.4 (C^imidazole), 123.1 (C^imidazole), 69.5 (CH₂CH₂CH₂OMe), 58.9

(CH₂CH₂CH₂OMe), 48.3 (CH₂CH₂CH₂OMe), 30.6 (CH₂CH₂CH₂OMe), 21.2 (Me ^para-Mes), 17.7

(Me ^ortho-Mes). Anal. calcd for C₁₆H₂₃ClN₂O: C, 65.18; H, 7.86; N, 9.50%. Found: C, 64.85; H, 8.10;

N, 9.20%.

Preparation of [(ImH)(Me)(C₃OMe)]Cl (3)

A procedure similar to that used for compound 1a afforded 3 as a white solid (11.145 g, 96%). ¹H

NMR (500 MHz, CDCl₃) δ: 10.76 (br, 1H, CH^imidazole), 7.57 (t, ³J = 1.7 Hz, 1H, CH^imidazole), 7.43

(t, ³J = 1.7 Hz, 1H, CH^imidazole), 4.37 (t, ³J = 7.0 Hz, 2H, CH₂CH₂CH₂OMe), 4.04 (s, 3H, Me^N-Me),

3

3.35 (t, J = 5.8 Hz, 2H, CH₂CH₂CH₂OMe), 3.24 (s, 3H, CH₂CH₂CH₂OMe), 2.17-2.12 (m, 2H,

CH₂CH₂CH₂OMe). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂) δ: 138.6 (C^imidazole), 123.(C^imidazole), 122.6

(C^imidazole), 68.6 (CH₂CH₂CH₂OMe), 58.8 (Me^N-Me), 47.4 (CH₂CH₂CH₂OMe), 36.6

(CH₂CH₂CH₂OMe), 30.42 (CH₂CH₂CH₂OMe).

Preparation of 1-(2,6-diisopropylphenyl)-3-(3-methoxypropyl)-1H-imidazol-2(3H)-ylidene

(4)

To a suspension of the imidazolium salt [(ImH)(Dipp)(C₃OMe)]Cl (1a)

N

O

N

DiPP

o

(0.250 g, 0.74 mmol) in THF (5 ml) at -40 C was added dropwise a

solution of sodium hexamethyldisilazide (0.177 g, 0.96 mmol) in THF (8 ml). The reaction mixture

was allowed to warm to room temperature and stirred for 30 min. Evaporation of the solvent under

reduced pressure, extraction of the residue into pentane (3×20 ml), evaporation of the extracts to

o

dryness and washing of the solid residue with pentane (5 ml) at -30 C gave 4 as a colourless

powder (0.102 g, 46%). X-ray quality crystals were obtained by slow cooling (-30 ^oC) of a dilute

pentane solution for two days. ¹H NMR (300 MHz, C₆D₆) δ: 7.30-7.25 (m, 1H, CH^p-Ar), 7.17-7.15

(m, overlapped with C₆D₆, CH^m-Ar), 6.56 (AB spin system, J_AB= 1.5 Hz, 1H, CH^imidazole), 6.54 (AB

spin system, J_AB= 1.5 Hz, 1H, CH^imidazole), 4.08 (t, ³J = 6.8 Hz, 2H, CH₂CH₂CH₂OMe), 3.21 (t, ³J

3

= 6.2 Hz, 2H, CH₂CH₂CH₂OMe), 3.09 (s, 3H, CH₂CH₂CH₂OMe), 2.88 (sept, J = 6.9 Hz, 2H,

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European Journal of Inorganic Chemistry

CH^iPr), 1.98 (apparent quint, ³J = 6.5 Hz, 2H, CH₂CH₂CH₂OMe), 1.24 (d, ³J = 6.9 Hz, 6H, CH₃^iPr),

1.13 (d, ³J = 6.9 Hz, 6H, CH₃^iPr). Anal. calcd for C₁₉H₂₈N₂O: C, 75.96; H, 9.39; N, 9.32%. Found:

C, 75.65; H, 9.75; N, 9.05%.

k¹

Preparation of [NiCl₂{(Im)(Dipp)(C₃OMe)- C^NHC}₂] (5a).

Method A: To solution of the imidazolium salt

a

O

N

Dipp

Cl Ni Cl

Dipp

[(ImH)Dipp(C₃OMe)]Cl (1a) (0.200 g, 0.59 mmol) in THF (8 mL) was

added a solution of NaOtBu (0.418 g, 0.60 mmol) in THF (5 mL). The

reaction mixture was stirred for 1 h and to this solution was then added

O

N

[NiCl₂(dme)] (0.065 g, 0.30 mmol). The reaction mixture was stirred for 3 h. The orange solution

was then filtered through a Celite pad and the filtrate was evaporated under reduced pressure. The

complex was precipitated from THF/Et₂O and washed with Et₂O to afford an orange powder.

(0.164 g, 75%). X-ray quality crystals were obtained by slow diffusion of Et₂O into a THF solution

of the complex.

Method B: To a solution of [(ImH)(Dipp)(C₃OMe)]₂[NiCl₄] (11a) (0.200 g, 0.25 mmol) in THF (8

mL) was added a solution of NaOtBu (0.055 g, 0.57 mmol) in THF (5 mL). The reaction mixture

was stirred for 3 h. The orange solution was then filtered through a Celite pad and the filtrate was

evaporated under reduced pressure. The complex was precipitated from THF/Et₂O and washed

with Et₂O to afford an orange powder (0.145 g, 80%). X-ray quality crystals were obtained by

slow diffusion of Et₂O into a THF solution of the complex. Anti-form (82%): ¹H NMR (500 MHz,

CDCl₃): δ 7.56 (t, ³J = 7.8 Hz, 1H, CH^p-Ar), 7.40 (d, ³J = 7.8 Hz, 2H, CH^m-Ar), 6.78 (d, ³J = 1.5 Hz,

1H, CH^imidazole), 6.62 (d, ³J = 1.5 Hz, 1H, CH^imidazole), 4.80 (t, ³J = 7.2 Hz, 2H, CH₂CH₂CH₂OMe),

3.32 (s, 3H, CH₂CH₂CH₂OMe), 3.17 (t, ³J = 5.8 Hz, 2H, CH₂CH₂CH₂OMe), 2.71 (sept, ³J = 6.9

Hz, 2H, CH^iPr), 2.29 (apparent quint.,³J = 7.2-5.8 Hz, 2H, CH₂CH₂CH₂OMe), 1.30 (d, ³J = 6.9 Hz,

3

6H, CH₃^iPr), 0.91 (d, J = 6.9 Hz, 6H, CH₃^iPr).¹³C NMR (126 MHz, CDCl₃) δ: 168.9 (C^carbene),

148.0 (C^o-Ar), 136.0 (C^ipso-Ar), 129.9 (C^imidazole), 123.9 (C^p-Ar), 123.7 (C^m-Ar), 120.8 (C^imidazole), 69.5

(CH₂CH₂CH₂OMe), 58.7 (CH₂CH₂CH₂OMe), 47.8 (CH₂CH₂CH₂OMe), 30.3 (CH₂CH₂CH₂OMe),

1

28.4 (CH^iPr), 26.7 (CH₃^iPr), 22.8 (CH₃^iPr). Syn-form (18%): H NMR (500 MHz, CDCl₃) δ: 7.41

3

(overlapped with the anti-form) (t, J = 7.8 Hz, 1H, CH^p-Ar), 7.14 (d, J = 7.8 Hz, 2H, CH^m-Ar),

6.87 (d, ³J = 1.8 Hz, 1H, CH^imidazole), 6.54 (d, ³J = 1.8 Hz, 1H, CH^imidazole), 5.54 (t, ³J = 7.2 Hz, 2H,

CH₂CH₂CH₂OMe), 3.62 (t, ³J = 5.8 Hz, 2H, CH₂CH₂CH₂OMe), 3.44 (s, 3H, CH₂CH₂CH₂OMe),

3

2.65 (partly overlapped with the anti-form) (apparent quint, J = 7.2-5.8 Hz, 2H,

CH₂CH₂CH₂OMe), 2.46 (sept, ³J = 6.9 Hz, 2H, CH^iPr), 0.92 (partly overlapped with the signals of

the anti-form) (d, ³J = 6.9 Hz, 6H, CH₃^iPr), 0.84 (d, ³J = 6.9 Hz, 6H, CH₃^iPr). ¹³C NMR (126 MHz,

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European Journal of Inorganic Chemistry

CDCl₃) δ: not seen (C^carbene), 146.9 (C^o-Ar), 135.6 (C^ipso-Ar), 129.4 (C^imidazole), 124.4 (C^p-Ar), 123.9

(C^m-Ar), 120.2 (C^imidazole), 69.9 (CH₂CH₂CH₂OMe), 58.9 (CH₂CH₂CH₂OMe), 48.2

(CH₂CH₂CH₂OMe), 31.2 (CH₂CH₂CH₂OMe), 28.1 (CH^iPr), 26.4 (CH₃^iPr), overlapped with the

anti-form (CH₃^iPr). Anal. calcd. for C₃₈H₅₆Cl₂N₄NiO₂: C, 62.48; H, 7.73; N, 7.67%. Found: C,

62.16; H, 7.67; N, 7.71%.

-k¹

C

^NHC}₂] (5b).

Preparation of [NiBr₂{(Im)(Dipp)(C₃OMe)

To a solution of the imidazolium bromide [(ImH)(Dipp)(C₃OMe)]Br

(1b) (0.570 g, 1.49 mmol) in THF (8 mL) was added a solution of

NaOtBu (0.157 g, 1.64 mmol) in THF (5 mL). The reaction mixture

was stirred for 1 h and [NiBr₂(dme)] (0.230 g, 0.75 mmol) was then

O

N

Dipp

Br Ni Br

Dipp

O

N

added to this solution. The reaction mixture was stirred for 3 h. The orange solution was then

filtered through a Celite pad and the filtrate was evaporated under reduced pressure. The complex

was precipitated from THF/ether and washed with Et₂O to afford a red powder (0.471 g, 76%). A

crystalline product was obtained by slow diffusion of Et₂O into a THF solution. FTIR:

ν_max(solid)/cm^-1: 3127w, 2926m, 2867m, 2819w, 1460m, 1417m, 1389m, 1382m, 1361m, 1321w,

1302w, 1285w, 1267w, 1223w, 1192w, 1118m, 1087w, 1068w, 1045w, 954m, 930m, 901w, 886w,

855w, 843w, 806m, 787w, 766m, 743m, 721w, 711m, 582w, 571m, 530m, 475m, 420w, 386w,

373w, 362w, 344w, 327w, 303w, 289w, 282w, 225w, 222w. Anti-form (87 %): ¹H NMR (500 MHz,

CDCl₃): δ 7.54 (t, ³J = 7.8 Hz, 1H, CH^p-Ar), 7.38 (d, ³J = 7.8 Hz, 2H, CH^m-Ar), 6.80 (d, ³J = 1.4 Hz,

1H, CH^imidazole), 6.66 (d, ³J = 1.4 Hz, 1H, CH^imidazole), 4.80 (t, ³J = 7.2 Hz, 2H, CH₂CH₂CH₂OMe),

3.33 (s, 3H, CH₂CH₂CH₂OMe), 3.18 (t, ³J = 5.8 Hz, 2H, CH₂CH₂CH₂OMe), 2.87 (sept, ³J = 6.9

Hz, 2H, CH^iPr), 2.27 (apparent quint, ³J = 7.2-5.8 Hz, 2H, CH₂CH₂CH₂OMe), 1.32 (d, ³J = 6.9 Hz,

6H, CH₃^iPr), 0.91 (d, ³J = 6.9 Hz, 6H, CH₃^iPr). ¹³C{¹H} NMR (CDCl₃, 126 MHz) δ: 170.5 (C^carbene),

148.0 (C^o-Ar), 135.9 (C^ipso-Ar), 130.0 (C^imidazole), 124.8 (C^p-Ar), 123.8 (C^m-Ar), 121.0 (C^imidazole), 69.5

(CH₂CH₂CH₂OMe), 58.7 (CH₂CH₂CH₂OMe), 48.2 (CH₂CH₂CH₂OMe), 29.9 (CH₂CH₂CH₂OMe),

1

28.7 (CH^iPr), 26.8 (CH₃^iPr), 23.0 (CH₃^iPr). Syn-form (13 %): H NMR (500 MHz, CDCl₃) δ: 7.40

(t, ³J = 7.8 Hz, 1H, CH^p-Ar), 7.12 (d, ³J = 7.8 Hz, 2H, CH^m-Ar), 6.89 (d, ³J = 1.4 Hz, 1H, CH^imidazole),

6.58 (d, ³J = 1.4 Hz, 1H, CH^imidazole), 5.50 (t, ³J = 7.2 Hz, 2H, CH₂CH₂CH₂OMe), 3.63 (t, ³J = 5.8

Hz, 2H, CH₂CH₂CH₂OMe), 3.44 (s, 3H, CH₂CH₂CH₂OMe), 2.64 (apparent quit, ³J = 7.2-5.8 Hz,

2H, CH₂CH₂CH₂OMe), 2.58 (sept, ³J = 6.9 Hz, 2H, CH^iPr), 0.94 (d, ³J = 6.9 Hz, 6H, CH₃^iPr), 0.83

3

13

(d, J = 6.9 Hz, 6H, CH₃^iPr). C{¹H} NMR (CDCl₃, 126 MHz) δ: 169.5 (C^carbene), 146.9 (C^o-Ar),

135.6 (C^ipso-Ar), 129.5 (C^imidazole), 125.1 (C^p-Ar), 124.0 (C^m-Ar), 120.5 (C^imidazole), 69.9

(CH₂CH₂CH₂OMe), 58.9 (CH₂CH₂CH₂OMe), 48.6 (CH₂CH₂CH₂OMe), 30.7 (CH₂CH₂CH₂OMe),

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28.2 (CH^iPr), 26.5 (CH₃^iPr), 22.9 (CH₃^iPr). Anal. calcd. for C₃₈H₅₆Br₂N₄NiO₂: C, 55.70; H, 6.89; N,

6.84%. Found: C, 54.85; H, 6.55; N, 6.65%. Despite several attempts, no better elemental analysis

data could be obtained.

k¹

C

^NHC}₂] (6).

Preparation of [NiCl₂{(Im)(Mes)(C₃OMe)-

To a solution of the [(ImH)(Mes)(C₃OMe)]₂[NiCl₄] (12) (0.200 g, 0.28

mmol) in THF (8 mL) was added a solution of NaOtBu (0.059 g, 0.61

mmol) in THF (5 mL). The reaction mixture was stirred for 3 h. The

orange solution was then filtered through a Celite pad and the filtrate

O

N

Mes

Cl Ni Cl

Mes

O

N

was evaporated under reduced pressure. The complex was precipitated from THF/Et₂O and

washed with Et₂O to afford an orange powder. (0.145 g, 80%). Crystals were obtained by slow

diffusion of Et₂O into a THF solution of the complex. Anal. Calcd for C₃₂H₄₄Cl₂N₄NiO₂: C, 59.47;

H, 6.86; N, 8.67%. Found: C, 59.25; H, 6.75; N, 8.45%.

k¹

Preparation of [NiCl₂{(Im)(Me)(C₃OMe)- C^NHC}₂] (7).

To a solution of [(ImH)Me(C₃OMe)]₂[NiCl₄] (13) (0.102 g, 0.20 mmol)

O

N

Me

Cl Ni Cl

Me

in THF (8 mL) was added a solution of NaOtBu (0.042 g, 0.44 mmol)

in THF (5 mL). The reaction mixture was stirred for 3 h. The orange

solution was then filtered through a Celite pad and the filtrate was

O

N

evaporated under reduced pressure. The complex was precipitated from THF/ether and washed

with Et₂O to afford an orange powder (0.055 g, 63%). X-ray quality crystals were obtained by

slow diffusion of Et₂O into a THF solution of the complex. Anal. calcd. for C₁₆H₂₈Cl₂N₄NiO₂: C,

43.87; H, 6.44; N, 12.79%. Found: C, 43.65; H, 6.30; N, 12.55%.

k¹

Preparation of [Ag{(Im)(Dipp)(C₃OMe)- C^NHC}₂]Cl (8).

A mixture of the imidazolium salt [(ImH)(Dipp)(C₃OMe)]Cl (1a)

(0.420 g, 1.25 mmol), Ag₂O (0.159 g, 0.69 mmol) and molecular

sieves in THF (10 mL) was stirred overnight under exclusion of light.

The beige reaction mixture was then filtered through Celite and the

N

O

Dipp

Ag

Cl

Dipp

O

N

filtrate was dried under reduced pressure. The resulting solid was washed with Et₂O to give a beige

powder (0.359 g, 76%). X-ray quality crystals were obtained by slow diffusion of pentane into a

THF solution of the complex. ¹H NMR (CD₂Cl₂, 500 MHz): δ 7.47 (t, ³J = 7.8 Hz, 1H, CH^p-Ar),

7.44 (t, ³J(H-H) = 1.5 Hz, ⁴J(H-Ag) = 1.5 Hz, 1H, CH^imidazole), 7.23 (d, ³J = 7.8 Hz, 1H, CH^m-Ar),

3

4

3

7.00 (t, J(H-H) = 1.5 Hz, J(H-Ag) = 1.5 Hz, 1H, CH^imidazole), 4.01 (t, J = 7.0 Hz, 2H,

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CH₂CH₂CH₂OMe), 3.26 (s, 3H, CH₂CH₂CH₂OMe), 3.21 (t, ³J = 5.8 Hz, 2H, CH₂CH₂CH₂OMe),

2.25 (sept, ³J = 6.9 Hz, 2H, CH^iPr), 1.85 (apparent quintet, ³J = 7.0-5.8 Hz, 2H, CH₂CH₂CH₂OMe),

3

13

1.08 (d, J = 6.9 Hz, 6H, CH₃^iPr), 0.98 (d, J = 6.9 Hz, 6H, CH₃^iPr). C{¹H} NMR (126 MHz,

CD₂Cl₂) δ: 181.2 (d, ¹J(¹³C-¹⁰⁷Ag) = 182 Hz, ¹J(¹³C-¹⁰⁹Ag) = 210 Hz, C^carbene), 145.9 (C^o-Ar), 134.9

(C^ipso-Ar), 130.5 (C^p-Ar), 124.1 (C^m-Ar), 123.8 (³J(C-Ag) = 5.6 Hz, C^imidazole), 122.4 (³J(C-Ag) = 5.6

Hz, C^imidazole), 68.5 (CH₂CH₂CH₂OMe), 58.5 (CH₂CH₂CH₂OMe), 48.7 (CH₂CH₂CH₂OMe), 31.4

(CH₂CH₂CH₂OMe), 28.2 (CH^iPr), 24.4 (CH₃^iPr), 23.9 (CH₃^iPr). Anal. calcd. for C₃₈H₅₆N₄O₂AgCl:

C, 61.33; H, 7.58; N, 7.53%. Found: C, 60.87; H, 7.67; N, 7.08%. Despite numerous attempts, no

better elemental analysis data could be obtained.

k¹

Preparation of [Ag{(Im)(Mes)(C₃OMe)- C^NHC}₂]Cl (9).

A mixture of the imidazolium salt [(ImH)(Mes)(C₃OMe)]Cl (2) (0.500

g, 1.70 mmol), Ag₂O (0.216 g, 0.94 mmol) and molecular sieves in

THF (10 mL) was stirred overnight under the exclusion of light. The

beige reaction mixture was then filtered through Celite and the filtrate

N

O

Mes

Ag

Cl

Mes

O

N

was evaporated to dryness under reduced pressure. The resulting solid was washed with Et₂O to

give a beige powder (0.454 g, 81%). Crystals were obtained by slow diffusion of pentane into a

THF solution of the complex. ¹H NMR (500 MHz, CD₂Cl₂): δ 7.21 (d, ³J = 1.8 Hz, 1H, CH^imidazole),

3

7.02 (br s, 2H, CH^m-Ar), 6.98 (d, J = 1.8 Hz, 1H, CH^imidazole), 4.32 (t, J = 6.8 Hz, 2H,

CH₂CH₂CH₂OMe), 3.36 (t, ³J = 5.8 Hz, 2H, CH₂CH₂CH₂OMe), 3.33 (s, 3H, CH₂CH₂CH₂OMe),

2.35 (s, 3H, CH₃^p-Ar), 2.12 (apparent quintet, ³J = 6.8-5.8 Hz, 2H, CH₂CH₂CH₂OMe), 1.99 (s, 6H,

CH₃^o-Ar). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz) δ: 181.1 (d, ¹J(¹³C-¹⁰⁷Ag) = 182 Hz, ¹J(¹³C-¹⁰⁹Ag) =

210 Hz, C^carbene), 145.9 (C^o-Ar), 134.9 (C^ipso-Ar), 130.5 (C^p-Ar), 124.1 (C^m-Ar), 123.8 (³J(C-Ag) = 5.6

Hz, C^imidazole), 122.4 (³J(C-Ag) = 5.6 Hz, C^imidazole), 68.5 (CH₂CH₂CH₂OMe), 58.5

(CH₂CH₂CH₂OMe), 48.7 (CH₂CH₂CH₂OMe), 31.4 (CH₂CH₂CH₂OMe), 28.2 (CH^iPr), 24.4

(CH₃^iPr), 23.9 (CH₃^iPr). Anal. calcd. for C₃₂H₄₄AgClN₄O₂: C, 58.23; H, 6.72; N, 8.49%. Found :

C, 57.9; H, 6.80; N, 8.20%.

k¹

Preparation of [Ag{(Im)(Me)(C₃OMe)- C^NHC}₂]Cl (10).

A mixture of the imidazolium salt [(ImH)(Me)(C₃OMe)]Cl (3) (0.500

g, 2.62 mmol), Ag₂O (0.334 g, 1.44 mmol) and molecular sieves in

THF (10 mL) was stirred overnight under exclusion of light. The beige

reaction mixture was then filtered through Celite and the filtrate was

N

O

Me

Ag

Cl

Me

O

N

evaporated to dryness under reduced pressure. The resulting solid was washed with Et₂O to give

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a beige powder (0.462 g, 78%). The complex can be recrystallised by slow diffusion of pentane

into a THF solution.

Preparation of [(ImH)(Dipp)(C₃OMe)]₂[NiCl₄] (11a).

Amixture of the imidazolium salt [(ImH)(Dipp)(C₃OMe)]Cl (1a)

(1.000 g, 2.97 mmol) and [NiCl₂(dme)] (0.327 g, 1.49 mmol) in

THF was stirred overnight. The solvent was evaporated under

[NiCl₄]²

N

O

Dipp

2

reduced pressure. The blue slurry was then triturated with Et₂O until a blue powder was obtained

(1.027 g, 86%). X-ray quality crystals were obtained by slow diffusion of Et₂O into a MeCN

solution of the complex. ¹H NMR (300 MHz, CD₂Cl₂): δ 7.58 (t, ³J = 7.9 Hz, 1H, CH^p-Ar), 7.35 (d,

³J = 7.9 Hz, 2H, CH^m-Ar), 7.08 (br s, 1H, CH^imidazole), 6.74 (br s, 1H, CH^imidazole), 4.32 (br s, 2H,

CH₂CH₂CH₂OMe), 4.12 (br s, 2H, CH₂CH₂CH₂OMe), 3.62 (br s, 3H, CH₂CH₂CH₂OMe), 2.67 (br

3

s, 2H, CH₂CH₂CH₂OMe), 2.09 (br sept, J = 6.4 Hz, 2H, CH^i-Pr), 1.94 (br d, J = 6.4 Hz, 6H,

CH₃^iPr), 1.24 (br d, ³J = 6.4 Hz, 6H, CH₃^iPr) (no clear resonance for the NCHN proton was observed

in the range δ =10-16).

Preparation of [(ImH)(Dipp)(C₃OMe)]₂[NiBr₄] (11b).

To

a

solution

of

the

imidazolium

bromide

[NiBr₄]²

N

O

[(ImH)(Dipp)(C₃OMe)]Br (1b) (1.000 g, 2.62 mmol) in THF was

Dipp

2

added [NiBr₂(dme)] (0.405 g, 1.31 mmol). After the reaction

mixture was stirred overnight, the solvent was evaporated under reduced pressure. The blue slurry

was then triturated with Et₂O until a purple powder was obtained (1.073 g, 83%). X-ray quality

crystals were obtained by slow diffusion of Et₂O into a MeCN solution of the complex.

Preparation of [(ImH)(Mes)(C₃OMe)]₂[NiCl₄] (12).

A mixture of the imidazolium salt [(ImH)(Mes)(C₃OMe)]Cl (2)

(0.200 g, 0.68 mmol) and [NiCl₂(dme)] (0.075 g, 0.34 mmol) in

THF was stirred overnight. The solvent was evaporated under

[NiCl₄]²

N

O

Mes

2

reduced pressure. The blue slurry was then triturated with Et₂O until a blue powder was obtained

(0.208 g, 85%). The complex can be recrystallised by slow diffusion of Et₂O into a MeCN solution.

Preparation of [(ImH)(Me)(C₃OMe)]₂[NiCl₄] (13).

29

Article Doi

Silver(I) and Nickel(II) Complexes with Oxygen- or Nitrogen-Functionalized NHC Ditopic Ligands and Catalytic Ethylene Oligomerization

DOI: 10.1002/ejic.201901064

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Article abstract of DOI:10.1002/ejic.201901064

Full text of DOI:10.1002/ejic.201901064

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