The reactivity of [{2-(diphenylphosphino)phenyl}methyl]-3-imidazol-2-ylidenes towards group VIII element precursors

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The reactivity of [{2-(diphenylphosphino)phenyl}methyl]-3-imidazol-2-ylidenes towards

group VIII element precursors

Jens Trampert, Yu Sun, Werner R. Thiel

PII:

S0022-328X(20)30123-6

DOI:

https://doi.org/10.1016/j.jorganchem.2020.121222

JOM 121222

Reference:

To appear in: Journal of Organometallic Chemistry

Received Date: 29 January 2020

Revised Date: 6 March 2020

Accepted Date: 8 March 2020

Please cite this article as: J. Trampert, Y. Sun, W.R. Thiel, The reactivity of [{2-

(diphenylphosphino)phenyl}methyl]-3-imidazol-2-ylidenes towards group VIII element precursors,

Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121222.

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The reactivity of [{2-(diphenylphosphino)phenyl}methyl]-3-imidazol-2-ylidenes

towards group VIII element precursors

Jens Trampert, Yu Sun, Werner R. Thiel*

Dedicated to Professor F. Ekkehardt Hahn on the occasion of his 65^thbirthday

* Technische Universität Kaiserslautern, Fachbereich Chemie, Erwin-Schrödinger-Straße 54,

D-67663 Kaiserslautern, Germany. E-mail: thiel@chemie.uni-kl.de

Abstract

Phosphine functionalized imidazolium precursors were reacted with triiron(0)- resp. triruthe-

nium(0)dodecacarbonyl in order to compare the reactivity of the Group VIII metals. Further-

more, [(η⁶-cymene)RuCl₂]₂was treated in a transmetallation reaction with the silver(I) resp.

copper(I) complexes of the corresponding imidazolylidenes possessing sterically largely

different substituents at the NHC site, in order to evaluate the influence of these

substituents on the catalytic activity of the ruthenium complexes.

Keywords: NHC ligand • phosphine • ruthenium • iron • X-ray structure

Introduction

Transition metal complexes bearing N-Heterocyclic carbene ligands (NHC) have been known

since the pioneering work of Wanzlick and Öfele published in 1968 [1]. However, detailed

investigations on the coordination chemistry of NHC ligands, on the special properties of the

metal-NHC [2] bond and on their use of NHC complexes in catalytic reactions [3] and in

material chemistry [4] started in 1991 with the finding of the first stable NHC complex by

Arduengo et al. [5]. Since imidazolium salts, the most common precursors of NHC ligands,

are readily accessible, it took just a few years to fully develop the principles of the NHC

coordination chemistry [6]. Along with this, combinations of the NHC fragment with other

donor sites in chelating ligands rapidly became established leading to ligands possessing

additional NHC [7], cyclopentadienyl [8], nitrogen [9], oxygen [9a,c,10], sulphur [11], phos-

phorus [12] or olefinic [13] donor centers.

Combinations of NHCs with additional phosphines are of special interest in this context.

Phosphines as well as N-heterocyclic carbenes are good σ-donor and poor π-acceptor

ligands, both stabilizing metal sites in lower oxidation states, which is of special interest for

applications in catalysis. Nevertheless, the more directed lone pair of the NHC donors, makes

them the harder Lewis-acids compared to phosphines, leading to a stronger trans-influence

of the carbene. Connecting a phosphine group and an imidazole as the most common type

of NHC precursor can either be performed directly [14] or via aliphatic [15] or aromatic [16]

carbon based linker units.

A very simple route to a phosphine functionalized NCH precursor was published by Zhou et

al. in 2005. They found that chloromethyl(2-(diphenylphosphino)benzene can be converted

in high yields to the corresponding imidazolium chlorides 1⁺(Cl^-) by reacting it with appro-

priate imidazoles.[17] The required chloromethyl derivative is accessible in a few steps from

N,N-dimethylbenzylamine (Scheme 1).[18]

Scheme 1. i) 1.2 equiv. n-BuLi, Et₂O, r.t., 14 h, ii) 1.2 equiv. Ph₂PCl, Et₂O, -78 °C - r.t., 2 h, iii)

1.3 equiv. ClCO₂Et, benzene, refl., 2 h, iv) 1.0 equiv. N-substituted imidazole, MeCN, refl., 18

h, v) 1.2 equiv. NaBF₄, CH₂Cl₂, 18 h, r.t..

By following this strategy, NHC ligands with a broad variety of substituents R at the imid-

azolium ring can be obtained, which allows playing with the steric and electronic properties

of the NHC ligand. Despite the fact that these ligand precursors are easily accessible, there

are just few reports on their use for coordination to transition metal sites in the literature:

The corresponding palladium(II) complexes bearing an N-aryl NHC [17,19] or an N-methyl

NHC unit [20] turned out to be active catalysts in a series of C-C coupling reactions. Iridium,

rhodium, and palladium complexes of this ligand type were investigated for C-H activation

reactions [21]. An iridium COD complex (COD = cycloocta-1,5-diene) catalyzes the alkylation

of amines with alcohols as the alkylating agents [22]. We recently published the reactivity of

nickelocene towards imidazolium precursors of the type 1⁺[23]. Hereby cationic CpNi(II)

complexes bearing chelating NHC-phosphine ligands are formed. Reacting them with cyanide

leads to cleavage of the Ni-P bond. In ruthenium chemistry, the reactivity of NHC-phosphine

ligands of type 1 towards Ru(CO)₃Cl₂was investigated by Domski et al.. They found that the

N-mesityl derivative gives the complex (NHC)Ru(CO)₂(Cl)₂bearing a bidentate NHC ligand,

while the corresponding N-phenyl ligand undergoes C-H activation in the ortho-position of

the phenyl ring resulting in a tridentate coordination of the ligand to the Ru(CO)₂Cl moiety

[24] Cabeza et al. investigated the reactivity of NHC-phosphine ligands of type 1 with an N-

methyl substituent towards Ru₃(CO)₁₂and published a bridging binding mode of the NHC

ligand as long as the reactants were employed in an equimolar ratio [25]. Heating this

product in tetrahydrofurane resulted in a carbon bridged hydrido cluster, with the hydrido

ligands being delivered from the linking methylene group. Performing the same reaction in a

3:1 ratio (NHC vs. Ru₃(CO)₁₂) gave a mononuclear tricarbonylruthenium(0) complex. When

the tetranuclear hydrido ruthenium carbonyl cluster [1a]⁺[Ru₄(µ-H)₃(CO)₁₂]^-(R = CH₃) was

heated in toluene, the corresponding complex [Ru₄(µ-H)₄(κ₂-1)(CO)₁₀] was formed, wherein

the NHC-phosphine ligand undergoes chelating coordination to one of the ruthenium

centers [26].

Results and discussion

We here first report a series of novel ruthenium and iron compounds bearing NHC ligands of

the type 1, derived from Ru₃(CO)₁₂and Fe₃(CO)₁₂. By treating the methyl substituted

imidazolium salt [1a⁺](Cl^-) (Scheme 1) with Fe₃(CO)₁₂at elevated temperatures in toluene

solution, the corresponding tricarbonyliron(0) complex 2 bearing a chelating NHC-phosphine

ligand is formed readily in almost 50% yield (Scheme 2). Hereby the chloride anion - as the

only base that is present - is essential for the formation of 2. This hypothesis was proven by

treatment of the same iron(0) precursor with the tetrafluoroborate salt [1a⁺](BF₄) (Scheme

-

1). By simply changing the basic chloride anion against the less basic tetrafluoroborate, a

completely different product was formed: The cationic tetracarbonylphosphine iron(0)

complex 3 was formed in 74% yield with tetrafluoroborate as the anion. The action of

chloride as a base to deprotonate the imidazolium salt was also found for the formation of

the nickel(II) complexes cited above [23]. In the absence of the basic anion, the nickelocene

precursor stayed untouched.

Scheme 2. Synthesis of the iron carbonyl complexes 2 and 3: i) X = Cl^-, 0.33 eq. Fe₃(CO)₁₂,

-

toluene, refl., 18 h; ii) X = BF₄, 0.33 eq. Fe₃(CO)₁₂, toluene, refl., 18 h.

Compounds 2 and 3 could be obtained as single crystals by recrystallization from toluene (2,

slow diffusion of pentane) resp. dichloromethane (3, slow diffusion of diethylether), which

allowed to determine their solid state structures (Figure 1).

Figure 1. Molecular structures of compounds 2 (left) and 3 (right) in the solid state. The

ellipsoids are at the 50% level. Selected bond lengths (Å) and angles (°) are listed. 2: Fe1-P1

2.2196(4), Fe1-C1 1.7556(13), Fe1-C2 1.7675(13), Fe1-C3 1.7652(13), Fe1-C4 1.9795(13), O1-

C1 1.1655(18), O2-C2 1.1578(18), O3-C3 1.1481(17), P1-Fe1-C1 91.56(4), P1-Fe1-C2 92.91(4),

P1-Fe1-C3 168.79(4), P1-Fe1-C4 82.16(4), C1-Fe1-C2 120.16(6), C1-Fe1-C3 88.79(6), C1-Fe1-

C4 125.90(5), C2-Fe1-C3 96.61(6), C2-Fe1-C4 113.83(6), C3-Fe1-C4 88.56(5), Fe1-C1-O1

177.83(12), Fe1-C2-O2 176.90(12), Fe1-C3-O3 176.99(12). 3: Fe1-P1 2.2568(6), Fe1-C1

1.792(2), Fe1-C2 1.782(2), Fe1-C3 1.794(2), Fe1-C4 1.793(2), O1-C1 1.138(3), O2-C2 1.148(3),

O3-C3 1.146(3), P1-Fe1-C1 174.41(8), P1-Fe1-C2 86.37(7), P1-Fe1-C3 92.50(7), P1-Fe1-C4

89.19(7), C1-Fe1-C2 88.10(10), C1-Fe1-C3 91.38(10), C1-Fe1-C4 92.87(10), C2-Fe1-C3

125.28(10), C2-Fe1-C4 120.31(10), C3-Fe1-C4 114.37(10), Fe1-C1-O1 177.3(2), Fe1-C2-O2

177.27(19), Fe1-C3-O3 177.90(19), Fe1-C4-O4 178.95(19).

Compound 2 crystallizes in a distorted trigonalbipyramidal coordination geometry, wherein

the NHC donor is oriented in an equatorial and the phosphine donor in an axial position.

Huttner et al. found a slightly longer Fe-C_Carbenedistance (2.007 Å) in the iron(II) compound

(dmi)Fe(CO)₄(dmi = N,N-dimethylimidazolylidene) [27], the difference might be explained by

the chelating situation in compound 2. The molecular structure of the cationic complex 3 is

as expected similar to that of (PPh₃)Fe(CO)₄[28], for which a Fe-P distance of 2.244(1) Å was

reported. Only bulky substituents in the ortho-position of the phenyl substituents of the

PPh₃ligand lead to a pronounced elongation of the Fe-P distance in such complexes [29].

The ³¹P{¹H} NMR resonances of compounds 2 and 3 are almost identical (δ = 67.6 and 66.5

ppm) speaking for similar donor properties of both phosphine sites. According to the solid

state structure of compound 2, the two protons of the methylene unit that connects the

phosphine and the NHC donor sites are structurally inequivalent. However, there is no signal

for these protons in the ¹H NMR spectrum of 2 which can be explained by the fact that there

is a rapid equilibration at room temperature (according to the NMR time scale), thus these

resonances are at the point of coalescence. In addition, a part of the phenyl resonances gives

one broad signal, which supports this interpretation. By the way: the according signals in the

1

¹³C NMR spectrum appear as sharp peaks, due to different time scales of ¹³C and H NMR

spectroscopy. Such a rapid equilibration at room temp. is unexpected for seven-membered

ring systems and can be assigned to a rapid dissociation/association equilibrium of probably

the phosphine site, since Berry-rotation, which is often observed for five-coordinated metal

complexes is not a suitable process to equilibrate the situation of the methylene and phenyl

protons. The C=O stretching vibrations of compound 3 are found at higher energies (2049,

1979, 1921 cm^-1) than of compound 2 (1955, 1881, 1832 cm^-1), which is in agreement with

the strong donating nature of the NHC moiety coordinating the iron site in 2. This influence

weakens the C=O bond by enhancing the π-backbonding to the carbonyl ligands.

In the following we investigated the reactivity of mixed NHC-phosphine ligands towards

carbonyl precursors of the heavier group VIII element ruthenium. Cole et al. had reported

the reactivity of Ru₃(CO)₁₂against monodentate NHC ligands and found that the Ru₃core

often stays untouched.[30] In the presence of a large excess of a series of alkyl substituted

imidazolylidenes, mononuclear complexes of the type (NHC)₂Ru(CO)₃are formed. In

accordance to the results of Cabeza et al.,[25] the reaction of Ru₃(CO)₁₂with aryl substituted

imidazolylidenes in a 1:3 ratio gave complexes of the type (NHC)Ru(CO)₄. However,

treatment of Ru₃(CO)₁₂with the dimesityl substituted imidazolium salt [Mes₂Im⁺]Cl^-in

tetrahydrofurane solution led to the formation of [(Mes₂Im)Ru(CO)₂(µ²-Cl)]₂, a rather unique

dimeric, chloride-bridged ruthenium(I) complex with a Ru-Ru single bond. We therefore

started with treatment of Ru₃(CO)₁₂with [1b⁺](Cl^-) (see: Scheme 1) in a 1:3 ratio and found a

completely different kind of reactivity compared to iron: Instead of deprotonation of the

imidazolium salt by assistance of the chloride anion and the formation of a tetracarbonyl-

NHC complex, the ruthenium precursor reacts under formal oxidative addition of HCl leading

to the ruthenium(II) compound 4 in 73% yield (Scheme 3), which started to precipitate from

the tetrahydrofurane solution shortly after the reaction was started.

Scheme 3. Synthesis of the ruthenium(II) complex 4: i) 0.33 eq. Ru₃(CO)₁₂, tetrahydrofurane,

refl., 18 h.

There are two mechanistic alternatives for the formation of 4. We believe that a five-

coordinate ruthenium(0) intermediate, structurally similar to iron complex 3, should

primarily be formed. This can either be followed by a (ruthenium assisted) deprotonation of

the imidazolium moiety, wherein the chloride anion acts as the base and the NHC formed

this way would then coordinate to the ruthenium(0) site under dissociation of a carbonyl

ligand. Finally the liberated HCl would undergo oxidative addition to ruthenium again under

dissociation of a carbonyl ligand. Alternatively a direct attack of the five-coordinate

ruthenium(0) intermediate to the imidazolium moiety can be postulated. This would lead to

a six-coordinate ruthenium(II) hydrido cation with a chloride counter-anion. To achieve this,

one carbonyl ligand has to be released first, otherwise a seven-coordinate, 20 VE

intermediate would be formed. The ruthenium(II) hydrido intermediate should, due to its

cationic nature rapidly undergo dissociation of a second carbonyl ligand which allows the

coordination of the chlorido ligand.

Single crystals of compound 4 were obtained by slow diffusion of diethyl ether into a

solution in dichloromethane, which allowed to elucidate its molecular structure by means of

an X-ray structure analysis. Figure 2 shows the molecular structure of 4 in the solid state.

Figure 2. Molecular structure of compound 4 in the solid state. The ellipsoids are at the 50%

level. Selected bond lengths (Å) and angles (°) are listed: Ru1-Cl1 2.4552(14), Ru1-P1

2.472(2), Ru1-C1 1.940(8), Ru1-C2 1.851(6), Ru1-C3 2.111(6), Ru1-H1 1.82(8), O1-C1

1.117(11), O2-C2 1.147(8), Cl1-Ru1-P1 94.77(5), Cl1-Ru1-C1 87.5(2), Cl1-Ru1-C2 168.4(2),

Cl1-Ru1-C3 86.67(14), P1-Ru1-C1 96.3(2), P1-Ru1-C2 96.8(2), P1-Ru1-C3 95.23(18), C1-Ru1-

C2 89.8(3), C1-Ru1-C3 167.5(3), C2-Ru1-C3 93.7(2), C2-Ru1-H1 85(2), C3-Ru1-H1 93(2), C1-

Ru1-H1 76(2), Cl1-Ru1-H1 84(2), P1-Ru1-H1 172(2), Ru1-C1-O1 168.6(7), Ru1-C2-O2 175.6(7).

The position of the hydrido ligand could be determined from the structural data. However,

the large standard deviation associated to the Ru-H bond keeps from discussing its length in

more detail. The hydrido ligand is found in trans-orientation to the phosphine site, while the

carbonyl ligands are oriented cis to each other. There is a pronounced difference in the Ru-

C_CObond distances: According to the much stronger trans-influence of the NHC site

compared to the chlorido ligand, the Ru-C1 bond is by about 0.1 Å longer than the Ru-C2

distance. In addition, the carbonyl ligands are bent away from the bulky phosphine moiety

towards the small hydrido ligand. It has to be mentioned at this point, that an attempt to

recrystallize compound 4 from chloroform by slow diffusion of diethyl ether led to formation

of the corresponding ruthenium(II)dichlorido complex. The molecular structure of this

compound is presented in the Electronic Supporting Information. Compound 4 probably

activates chloroform under cleavage of a C-Cl bond.

In addition, NMR and IR spectroscopy proved the molecular structure of compound 4: The

two expected C=O stretching vibrations are observed at 2031 and 1946 cm^-1. Compared to

the iron(0) complexes discussed above, the ³¹P{¹H} NMR resonance of 4 is shifted strongly to

higher field (δ = 11.4 ppm). The presence of the hydrido ligand can be extracted from both,

1

the proton-coupled ¹³P NMR spectrum as well as from the H NMR spectrum, where its

resonance is observed with a chemical shift of -6.63 ppm (Figure 3). The evaluation gave a

large ²J_PHcoupling constant of 129.8 Hz, which is in the typical range for a coupling between

a hydrido and a phosphine ligand being in trans-orientation to each other at a ruthenium(II)

site [31]. In contrast to compound 2, there are two resonances for the protons of the

methylene group (6.45 and 4.66 ppm, ²J_HH= 14.5 Hz) with the typical large coupling constant

of geminal protons. This proves, that the equilibration of these nuclei is slow with respect to

the NMR time scale. Although the coordination number is higher than in compound 2, which

means that steric repulsion should be stronger, the phosphine site does not undergo

dissociation probably caused by a stronger Ru-P bond due to the oxidation state +II of the

ruthenium center.

Figure 3. The de-coupled (top) and coupled ³¹P NMR spectrum (bottom) of compound 4

measured in CD₂Cl₂solution.

In order to obtain a zero valent ruthenium complex for comparison with the iron complex 2,

an alternative synthesis route had to be employed. In-situ generation of the free NHC-

phosphine ligand by deprotonation of [1a⁺](Cl^-) (see: Scheme 1) with KHMDS (potassium

hexamethyldisilazide) as the base in tetrahydrofurane solution followed by the addition of

0.33 equiv. of Ru₃(CO)₁₂to this solution resulted in the formation of the ruthenium(0)

complex 5 in more than 80% yield (Scheme 4).

Scheme 4. Two step synthesis of the ruthenium(0) complex 5 starting from [1a⁺](Cl^-): i) 1

equiv. of KHMDS, 0.33 equiv. of Ru₃(CO)₁₂, tetrahydrofurane, r.t., 20 h.

Recrystallization of compound 5 from toluene by slow diffusion of pentane yielded single

crystals, which allowed to determine its molecular structure in the solid state (Figure 4).

Figure 4. Molecular structure of compound 5 in the solid state. The ellipsoids are at the 50%

level. Selected bond lengths (Å) and angles (°) are listed: Ru1-P1 2.3827(5), Ru1-C1

1.9064(19), Ru1-C2 1.901(2), Ru1-C3 1.8864(19), Ru1-C5 2.1221(18), O1-C1 1.153(2), O2-C2

1.140(2), O3-C3 1.159(3), P1-Ru1-C1 109.66(6), P1-Ru1-C2 87.48(6), P1-Ru1-C3 121.22(6),

P1-Ru1-C5 97.43(5), C1-Ru1-C2 92.30(8), C1-Ru1-C3 129.12(8), C1-Ru1-C5 86.83(7), C2-Ru1-

C3 89.61(8), C2-Ru1-C5 175.03(7), C3-Ru1-C5 87.15(7), Ru1-C1-O1 175.46(19), Ru1-C2-O2

179.09(17), Ru1-C3-O3 177.08(17).

Interestingly, the coordination of the NHC-phosphine ligand in 5 is opposite to the situation

found for compound 2: In the solid state structure of compound 5, the NHC ligand is

oriented in the axial and the phosphine donor is found in the equatorial position. It is hard to

differ the electronic and steric influences on the thermodynamics of the two possible

isomers. Maybe the longer Ru-C_NHCbond allows the NHC site to occupy the axial position,

which might be less favourable for the iron(0) complex 2, since the NHC methyl group and

the methylene unit are pointing towards the metal site. This argument is in agreement with

the fact that the angle C1-Ru1-C3 (129.12(8) °) is much wider than the corresponding angle

C1-Fe1-C2 (120.16(6) °) in compound 2 which reflects the steric impact of the NHC-ligand.

The Ru-P in compound 5 is as expected shorter (2.3827(5) Å) than the Ru-P in compound 4

(Ru1-P1 2.472(2) Å), which is due to the strong trans-influence of the hydrido ligand in 4.

The ³¹P NMR spectrum of compound 5 shows a significant shift of the phosphorous reson-

ance (δ = 39.2 ppm) to lower field compared to the data of ruthenium(II) complex 4 (δ = 11.4

ppm). Compared to the iron(0) complex 2 (δ = 67.6 ppm), the resonance is shifted to higher

field. In 1996 Kaupp published a detailed theoretical study on the contribution of a series of

effects on the ³¹P NMR resonances of pentacarbonyl(phosphine) complexes of Group VI

elements, which allows the interpretation of these differences [32]. In addition to the

structural differences between the iron(0) complex 2 and its ruthenium(0) congener 5

1

discussed above, the dynamic behaviour of these compounds is different, too. The H NMR

spectrum of 5 shows two (broadened) doublets with a coupling constant of 14.7 Hz which

are assigned to two magnetically inequivalent methylene protons, which do not fully

equilibrate at room temperature with respect to the NMR time scale. A part of the reson-

ances of the phenyl groups is broadened too. The energies of the C=O stretching vibrations

(1998, 1900, 1853 cm^-1) are observed at higher energies compared to the data of the struc-

turally closely related iron complex 2 (1955, 1881, 1832 cm^-1) indicating a weaker π-back

donation of the ruthenium centre.

The generation of silver NHC complexes by treatment of imidazolium salts with Ag₂O opens

up

a well-established access to various transition metal NHC complexes by

transmetallation.^[33]Treatment of [1a⁺](Cl^-) with Ag₂O in dichloromethane, resulting in a

dissolution of the black silver oxide and the formation of white AgCl in between 24 h,

followed by addition of [(η⁶-cymene)RuCl₂]₂and KPF₆gives the expected ionic

(cymene)ruthenium(II) NHC complex 6a in 72% yield after work-up (Scheme 5) .

Scheme 5. Synthesis of the ruthenium(II) NCH complexes 6a,b and 7. i) 0.55 equiv. of Ag₂O,

CH₂Cl₂, 18 h, r.t., 0.45 of equiv. [(η⁶-cymene)RuCl₂]₂, 18 h, r.t., 2 equiv. of KPF₆, 24 h, r.t.; ii)

1.0 equiv. of CuMes, tetrahydrofurane, 18 h r.t., 0.50 equiv. of [(η⁶-cymene)RuCl₂]₂, 18 h,

r.t., CH₂Cl₂, sat. aqu. KPF₆.

In an analogous reaction, [1b⁺](Cl^-) was treated with Ag₂O and subsequently with [(η⁶-

cymene)RuCl₂]₂and KPF₆. However, the ligand reacted only partially, even by prolongation of

the reaction time and under reflux conditions. Thus, according to NMR analysis of the raw

product a mixture of the ionic chelate complex 6b and compound 7 is obtained. It was not

possible to obtain 6b in pure form following this procedure. A few single crystals of

compound 7 could be obtained, which allowed to elucidate its molecular structure. The

structure can be found in the Supporting Information, since compound 7 also could not be

obtained in pure form. The largely reduced reactivity of [1b⁺](Cl^-) can be attributed to steric

hindrance by the bulky mesityl substituent. Consequently, treatment of [1b⁺](Cl^-) with well

soluble and highly basic copper(I)mesityl gave the corresponding copper-NHC complex as

transmetallation agent and finally provided access to pure 6b.

Complex 6a was obtained as single crystals by slow diffusion of diethylether into solutions of

the compound in dichloromethane. Figure 5 presents its molecular structures in the solid

state.

Figure 5. Molecular structure of compound 6a in the solid state. Hydrogen atoms and the

-

PF₆anion are omitted for clarity. The ellipsoids are at the 50% level. Selected bond lengths

(Å) and angles (°) are listed. C* denotes the centroid of the cymene ligand. Ru1-Cl1

2.3943(10), Ru1-P1 2.3544(9), Ru1-C1 2.077(4), Ru1-C25 2.299(4), Ru1-C26 2.243(4), Ru1-

C27 2.227(4), Ru1-C28 2.228(4), Ru1-C29 2.210(4), Ru1-C30 2.276(4), Ru1-C* 1.7464(3), Cl1-

Ru1-P1 82.80(3), Cl1-Ru1-C1 84.66(12), Cl1-Ru1-C* 127.80(3), P1-Ru1-C1 94.84(11), P1-Ru1-

C* 129.05(3), C1-Ru1-C* 123.9(1).

Complex 6a occupies a pseudo tetrahedral coordination geometry with an η⁶-coordinating

cymene ligand, a chelating phosphine-NHC donor and a chlorido ligand. In addition, NMR

spectroscopy proves the molecular structures of compounds 6a and 6b. As expected, the

typical resonance at around 10 ppm of the proton bound at the imidazolium carbon atom C1

is not present in the ¹H NMR spectra of 6a and 6b. Since the ruthenium(II) site is a center of

chirality in 6a and 6b, the aromatic protons of the cymene ligands are diastereotopic (four

doublets) as are the methyl groups of the cymene isopropyl substituent (two doublets).

Furthermore, the protons of the methylene group are chemically inequivalent giving two

2

doublets with a large J_HHcoupling constant of about 14.5 Hz for 6a. The according

1

resonances in the H NMR spectrum of 6b are broad, indicating hindered rotation around

the Ru-C* axis (C* = centroid of the cymene ligand due to the bulky mesityl substituent).

1

Additionally, there are three resonances for the mesityl bound methyl groups in the H and

the ¹³C NMR spectrum, which makes clear that the rotation around the N-C_mesbond is

strongly hindered, too. The ³¹P NMR resonances of 6a and 6b are observed at 24.6 resp. 22.4

ppm.

The ruthenium(II) complexes 6a and 6b were investigated for activity in the transfer hydro-

genation of acetophenone with 0.5 mol-% of the catalyst, KOH as the base and isopropanol

as the solvent and hydrogen source. The results are summarized in Table 1.

Table 1. Catalytic transfer hydrogenation of acetophenone.

yield of 1-phenylethanol / % ^a

reaction time/ h

6a

87

6b

0

1

2

96

10

18

29

52

90

3

100

4

8

24

a) 1.0 mmol of acetophenone, 0.5 mol-% of catalyst,

2.5 mol% of KOH, 5.0 mL of isopropanol, 82 °C,

yields by GC analysis with measured response

factors, tetradecane as the internal standard.

Obviously, 6a is much more active than 6b, which can be attributed to the steric influence of

the substituents (methyl vs. mesityl) at the NHC ligand. While the substrate acetophenone is

transferred to 1-phenylethanol with catalyst 6a in about 2-3 h, it takes 24 for 6b to reach

90% yield of the alcohol. Furthermore there is a period of induction of about 1 h for 6b. If

one assumes the reaction to proceed via a so-called “inner-sphere mechanism”, the coordin-

ation of isopropanolate, which is essential for the generation of the central ruthenium hydri-

de intermediate, may be hindered in the case of 6b due to the bulky mesityl substituent.

Conclusion

By reacting phosphine functionalized imidazolium precursors with triiron(0)- resp. triruthe-

nium(0)dodecacarbonyl, a series of new iron and ruthenium carbonyl complexes with

chelating phosphine-NHC ligands could be obtained. While, depending on the imidazolium

counter anion, the iron compound gives tri- resp. tetracarbonyl complexes in the oxidation

state 0, the use of ruthenium leads to an oxidative addition and therefore to a chlorido-

hydridoruthenium(II) derivative. This is a remarkable difference in reactivity between iron

and ruthenium. Only in case the carbene species is pre-formed in solution prior to the

addition of triruthenium(0)dodecacarbonyl, a tricarbonylruthenium(0) complex was

accessible. The free NHCs, generated by treatment of the phosphine functionalized

imidazolium precursors with either Ag₂O or copper(I)mesityl, react with [(η⁶-cymene)RuCl₂]₂

under formation of cationic cymene complexes, which show largely different activities in the

transfer hydrogenation of acetophenone depending on the bulkiness of the NHC substituent.

Experimental section

General Remarks: All reactions were carried out under an argon atmosphere using Schlenk

techniques. The solvents were dried and degassed before use according to standard

techniques. Other reagents were obtained from commercial suppliers and used as received.

¹H, ¹³C and ³¹P NMR spectra were recorded on BRUKER Spectrospin Avance 400 and 600

spectrometers at room temperature (unless otherwise denoted). The chemical shifts are

referenced to internal solvent resonances and the assignment of the resonances refers to

the numbering schemes provided in the Supporting Information to this manuscript. Infra-red

spectra were recorded on a Perkin Elmer FT-ATR-IR spectrometer Spectrum 100 equipped

with a diamond coated ZnSe window. Elemental analyses (C,H,N) were carried out with a

vario MICRO cube elemental analyzer at the Analytical Department of Technische Universität

Kaiserslautern. All commercially available starting materials were purchased from Sigma

Aldrich and used without any further purification. Toluene and dichloromethane were dried

in a MB-SPS solvent dryer. Tetrahydrofurane was dried over potassium/benzophenone,

acetonitrile was dried over CaH₂. 1-Mesityl-1H-imidazol, (2-(chloromethyl)phenyl)diphenyl-

phosphine and copper(I)mesityl were synthesized according to published procedures

[34,35,36].

1-(2-(Diphenylphosphino)benzyl)-3-methyl-1H-imidazol-3-ium chloride [1a⁺](Cl^-): 4.00 g

(12.9 mmol) of (2-(chloromethyl)phenyl)diphenylphosphine and 1.10 g (12.2 mmol) of 1-

methylimidazole were dissolved in 25 mL of acetonitrile and heated for 18 h under reflux.

The solvent was removed and the residue was washed with diethylether and dried under

vacuum. Colourless solid, yield: 4.19

g

(87%). Elemental analysis calcd. for

C₂₃H₂₂ClN₂P∙(H₂O)_0.9: C 67.53, H 5.86, N 6.85, found: C 67.62, H 5.94, N 6.85%. ¹H NMR (400.1

MHz, CDCl₃): δ 10.40 (s, 1H, H2), 7.75 (dd, ³J_HH= 7.5 Hz, J_HP= 4.5 Hz, 1H, H5), 7.46 (td, ³J_HH

=

4

7.5 Hz, J_HH= 1.3 Hz, 1H, H6), 7.40-7.30 (m, 8H, H3, H6, H_Ph), 7.20-7.15 (m, 4H, H_Ph), 7.07 (t,

³J_HH= 1.7 Hz, 1H, H3), 6.99 (ddd, ³J_HH= 7.6 Hz, J_HP= 4.2 Hz, 4J_HH= 1.1 Hz, 1H, H5), 5.78 (s, 2H,

H4), 3.94(s, 3H, H1). ¹³C{¹H} NMR (100.6 MHz, CDCl₃): δ = 138.0, 137.1 (d, J_PC= 25.8 Hz),

136.9 (d, J_PC= 15.3 Hz), 134.7, 134.7 (d, J_PC= 7.9 Hz), 133.8 (d, J_PC= 19.7 Hz), 131.5 (d, J_PC

=

4.6 Hz), 130.5, 130.1, 128.9 (d, J_PC= 7.2 Hz), 123.3, 121.4 (d, J_PC= 4.3 Hz), 51.6 (d, J_PC= 22.6

Hz), 36.5. ³¹P{¹H} NMR (162.0 MHz, CDCl₃): δ -17.32 (s). IR (ATR, cm^-1): ꢀꢁ 3053w, 2938w,

1571w, 1476w, 1434m, 1320w, 1196w, 1160m, 1112w, 1025w, 830w, 744s, 696s, 675m,

658m.

1-(2-(Diphenylphosphino)benzyl)-3-methyl-1H-imidazol-3-ium tetrafluoroborate

-

[1a⁺](BF₄): 2.00 g (5.09 mmol) of compound 1a were dissolved in 25 mL of dichloromethane.

0.62 g (5.60 mmol) of odium tetrafluoroborate were added to the solution. The suspension

was stirred at room temp. for 18 h. The solid residue was filtered off and the solvent was

removed under vacuum. Colourless solid, yield: 2.11 g (93%). Elemental analysis calcd. for

C₂₃H₂₂BF₄N₂P: C 62.19, H 4.99, N 6.31, found: C 62.12, H 5.14, N 6.24%. ¹H NMR (400.1 MHz,

CDCl₃): δ 8.36 (s, 1H, H2), 7.62 (ddd, ³J_HH= 7.5 Hz, J_HP= 4.6 Hz, ⁴J_HH= 1.0 Hz, 1H, H5), 7.47 (td,

³J_HH= 7.5 Hz, ⁴J_HH= 1.3 Hz, 1H, H6), 7.37-7.28 (m, 7H, H6, H_Ph), 7.12 - 7.08 (m, 4H, H_Ph), 7.01-

3

6.98 (m, 2H, H5, 3), 6.94 (t, J_HH= 1.8 Hz, 1H, H3), 5.59 (s, 2H, H4), 3.61 (s, 3H, H1). ¹³C{¹H}

NMR (100.6 MHz, CDCl₃): δ 137.3 (d, J_PC= 15.6 Hz), 136.7 (d, J_PC= 26.1 Hz), 136.7, 135.1,

134.7 (d, J_PC= 7.9 Hz), 133.8 (d, J_PC= 19.7 Hz), 131.9 (d, J_PC= 4.7 Hz), 130.6, 130.4, 129.5,

128.9 (d, J_PC= 7.3 Hz), 123.4, 121.7 (d, J_PC= 2.3 Hz), 52.3 (d, J_PC= 21.7 Hz), 36.2. ³¹P{¹H} NMR

(162.0 MHz, CDCl₃): δ -17.86 (s). ¹⁹F{¹H} NMR (376.5 MHz, CDCl₃): δ 151.6 (s). IR (ATR, cm^-1):

ꢀꢁ 3154w, 1568w, 1478w, 1435m, 1330w, 1206w, 1155m, 1110m 1059s, 1013s, 852m, 783m,

752s, 698s.

1-(2-(Diphenylphosphino)benzyl)-3-mesityl-1H-imidazol-3-ium chloride [1b⁺](Cl^-): 2.50 g

(8.04 mmol) of (2-(chloromethyl)phenyl)diphenylphosphine and 1.50 g (8.05 mmol) of 1-me-

sityl-1H-imidazole were dissolved in 20 mL acetonitrile and heated for 18 h under reflux.

Finally, the solvent was removed under vacuum and the bright yellow residue was purified

by column chromatography (dichloromethane, methanol; product fraction at 7.3 % methan-

ol). Colorless solid, yield: 3.24 g (81%). Elemental analysis calcd. for C₃₁H₃₀ClN₂P∙(H₂O)_0.4: C

73.84, H 6.16, N 5.56, found: C 74.07, H 6.12, N 5.22%. ¹H NMR (400.1 MHz, CDCl₃): δ 11.09

(s, 1H, H4), 8.38 (ddd, ³J_HH= 7.6 Hz, J_HP= 4.5 Hz, ⁴J_HH= 0.9 Hz, 1H, H7), 7.88-7.87 (m, 1H, H5),

3

4

3

7.73 (td, J_HH= 7.6 Hz, J_HH= 1.3 Hz, 1H, H8), 7.67-7.63 (m, 6H, H_Ph), 7.60 (td, J_HH= 7.6 Hz,

⁴J_HH= 1.2 Hz, 1H, H8), 7.56-7.52 (m, 4H, H_Ph), 7.29 - 7.27 (m, 2H, H7, H5), 6.47 (s, 2H, H2),

5.58 (s, 2H, H6), 2.61 (s, 3H, H1), 2.30 (s, 6H, H3). ¹³C{¹H} NMR (100.6 MHz, CDCl₃): δ 141.3,

138.9, 138.2 (d, J_PC= 25.6 Hz), 136.5 (d, J_PC= 14.1 Hz), 135.0 (d, J_PC= 7.5 Hz), 134.4, 134.3,

133.9 (d, J_PC= 19.5 Hz), 132.0 (d, J_PC= 4.6 Hz), 130.9, 130.8, 130.0, 129.9, 129.5, 129.1 (d, J_PC

= 7.2 Hz), 122.6, 122.5 (d, J_PC= 7.9 Hz), 51.1 (d, J_PC= 21.8 Hz), 21.2, 17.7. ³¹P{¹H} NMR (162.0

MHz, CDCl₃): δ -16.05 (s). IR (ATR, cm^-1): ꢀꢁ 2959w, 1561w, 1542m, 1478m, 1449m, 1436m,

1368w, 1280m, 1201m, 1194m, 1153m, 1072m, 1015m, 883m, 849m, 782m, 748s, 742s,

727s, 693s, 669m.

1-(2-(Diphenylphosphino)benzyl)-3-methyl-2H-imidazol-2-ylidene(tricarbonyl)iron(0) (2):

250 mg (0.64 mmol) of compound [1a⁺](Cl^-) and 110 mg (0.22 mmol) of triirondodecarbonyl

were suspended in 25 mL toluene and the reaction mixture was heated to reflux for 18 h.

After this time period it was stirred for additional 4 h at room temp.. During this period, a

brown precipitate formed in the solution, which was removed by filtration. The red filtrate

was concentrated and a red, crystaline solid formed. The product was re-crystallized by slow

diffusion of pentane into a saturated toluene solution. Red solid, yield: 155 mg (48%). Ele-

mental analysis calcd. for C₂₆H₂₁FeN₂O₃P: C 62.92, H 4.27, N 5.64, found: C 62.96, H 4.50, N

5.53. ¹H NMR (400.1 MHz, C₆D₆): δ 7.66 (br, 4H, H_Ph), 7.11-6.96 (m, 7H, H4, H_Ph), 6.91 (t, ³J_HH

= 7.3 Hz, 1H, H5), 6.78 (t, ³J_HH= 7.4 Hz, 1H, H5’), 6.72-6.68 (m, 1H, H4), 6.09 (s, 1H, H2), 5.77

(s, 1H, H2’), 3.29 (s, 3H, H1), the resonance of H3 is missing due to equilibration effects.

¹³C{¹H} NMR (100.6 MHz, C₆D₆): δ 189.1 (d, J_PC= 25.0 Hz),, 140.5 (d, J_PC= 13.8 Hz), 135.4 (d,

J_PC= 24.5 Hz), 129.9, 129.7 (d, J_PC= 6.8 Hz), 129.6 (d, J_PC= 1.5 Hz), 129.5, 129.0, 128.9, 128.6,

127.9, 125.7, 122.9, 120.9, 53.9 (d, J_PC= 11.0 Hz), 38.6. ³¹P{¹H} NMR (162.0 MHz, C₆D₆): δ

67.60 (s). IR (ATR, cm^-1): ꢀꢁ 2920w, 1955s, 1881s, 1832s, 1569m, 1481m, 1431m, 1365m,

1234s, 1186m, 1131m, 1090s, 1027m, 878w, 793w, 747s, 722m, 688s, 667s.

[1-(2-(Diphenylphosphino)benzyl)-3-methyl-1H-imidazol-3-iumtetra(carbonyl)iron(0) tetra-

-

fluoroborate (3): 397 mg (0.90 mmol) of compound [1a⁺](BF₄) and 151 mg (0.30 mmol) of

triirondodecarbonyl were suspended in 50 mL of toluene. The reaction mixture was heated

to reflux for 18 h. While cooling down to room temp. an orange-brown precipitate formed,

which was separated from the reaction mixture by filtration and washed with 20 mL of

dichloromethane. After drying under vacuum the solid was re-crystallized by slow diffusion

of diethylether into a solution in dichloromethane. Orange solid, yield: 331 mg (74%). Ele-

mental analysis calcd. for C₂₇H₂₂BF₄FeN₂O₄P: C 52.98, H 3.62, N 4.58, found: C 52.62, H 3.88,

1

N 4.59%. H NMR (400.1 MHz, CD₃CN): δ 8.10 (s, 1H, H2), 7.68-7.35 (m, 14H, H6, H5, H_Ph),

7.28 (s, 1H, H3), 6.96 (s, 1H, H3’), 5.21 (s, 2H, H4), 3.77 (s, 3H, H1). ¹³C{¹H} NMR (100.6 MHz,

CD₃CN): δ 214.2 (d, J_PC= 18.0 Hz), 137.4, 136.5, 136.4 (d, J_PC= 4.2 Hz), 133.9 (d, J_PC= 10.7 Hz),

133.8, 133.5 (d, J_PC= 3.7 Hz), 133.4 (d, J_PC= 12.3 Hz), 132.7 (d, J_PC= 2.5 Hz), 132.1 (d, J_PC= 7.0

Hz), 130.5, 130.3 (d, J_PC= 10.6 Hz), 125.1, 123.2, 52.1 (d, J_PC= 6.0 Hz), 37.0. ³¹P{¹H} NMR

(162.0 MHz, CD₃CN): δ 66.49 (s).¹⁹F{¹H} NMR (376.5 MHz, CD₃CN): δ -151.46 (s). IR (ATR, cm^-

¹): ꢀꢁ 3159w, 2049s, 1979s, 1921s, 1575w, 1561w, 1435m, 1314w, 1199w, 1158m, 1051s,

1035s, 834m, 821m, 783m, 750s, 695s.

1-(2-(Diphenylphosphino)benzyl)-3-mesityl-2H-imidazol-2-yliden(dicarbonyl)(chlorido)(hy-

drido)ruthenium(II) (4): 233 mg (0.47 mmol) of compound [1b⁺](Cl^-) were suspended in 20

mL of tetrahydrofurane. A solution of 100 mg (0.16 mmol) of trirutheniumdodecacarbonyl in

10 mL of tetrahydrofurane was added. The reaction mixture was heated to reflux for 18 h.

During the cooling process a colourless precipitate formed. Half of the solvent was removed

under vacuum and the resulting solid was isolated by filtration, washed with diethylether

and dried under vacuum. The product was re-crystallized by slow diffusion of diethylether

into a saturated solution in dichloromethane. Colourless solid, yield: 224 mg (7 %). Elemental

analysis calcd. for C₃₃H₃₀ClN₂O₂PRu: C 60.60, H 4.62, N 4.28, found: C 60.40, H 4.81, N 4.29%.

¹H NMR (600.1 MHz, CD₂Cl₂): δ 7.57-7.43 (m, 7H, H7, H_Ph), 7.41-7.34 (m, 5H, H2, H_Ph), 7.17-

7.05 (m, 4H, H7, H6, H4), 6.92 (s, 1H, H4’), 6.86 (d, ⁴J_HH= 1.6 Hz, 1H, H2), 6.45 (d, ²J_HH= 14.6

2

Hz, 1H, H5), 4.66 (d, J_HH= 14.5 Hz, 1H, H5’), 2.35 (s, 3H, H3), 2.12 (s, 3H, H3’), 1.78 (s, 3H,

H1), -6.63 (d, ²J_PH= 129.8 Hz, 1H, RuH). ¹³C{¹H} NMR (150.9 MHz, CD₂Cl₂): δ 199.4 (d, J_PC= 7.9

Hz), 198.9, 139.6, 137.5, 135.6 (d, J_PC= 11.1 Hz), 135.5, 134.2, 133.1 (d, J_PC= 11.6 Hz), 131.6,

131.2 (d, J_PC= 7.1 Hz), 130.9, 130.1, 129.6, 129.5 (d, J_PC= 4.6 Hz), 129.0 (d, J_PC= 20.0 Hz),

128.9 (d, J_PC= 20.5 Hz), 123.1, 122.4, 21.5, 19.3, 18.1. ³¹P{¹H} NMR (242.9 MHz, CD₂Cl₂): δ

11.41 (s). IR (ATR, cm^-1): ꢀꢁ 3130w, 3102w, 2031s, 1946s, 1588w, 1572w, 1481m, 1464m,

1434m, 1389m, 1282m, 1236m, 1186m, 1130m, 1087m, 1037w, 872w, 806m, 791m, 758m,

746s, 695s.

1-(2-(Diphenylphosphino)benzyl)-3-methyl-2H-imidazol-2-ylidentri(carbonyl)ruthenium(0)

(5): 282 mg (0.72 mmol) of [1a⁺](Cl^-) were suspended in 20 mL of tetrahydrofurane. Over a

period of 15 min, a solution of 144 mg (0.72 mmol) of potassium hexamethyldisilazide in 10

mL of tetrahydrofurane was added. During the addition the colour of the reaction mixture

turned slowly to orange. After further stirring the mixture for 90 min at room temp., a

solution of 152 mg (0.24 mmol) of trirutheniumdodecacarbonyl in 15 mL tetrahydrofurane

was added. The resulting suspension was stirred at room temp. for 18 h. The colourless

precipitate formed was removed by filtration and the filtrate was concentrated to half of its

volume. The resulting orange solid was washed with pentane and dried under vacuum. The

raw product was re-crystallized by slow diffusion of pentane into a saturated solution in

toluene. Yellow solid, yield: 320 mg (82%). Elemental analysis calcd. for C₂₆H₂₁N₂O₃PRu: C

57.67, H 3.91, N 5.17, found: C 57.62, H 4.05, N 5.09%. ¹H NMR (400.1 MHz, C₆D₆): δ 7.96 (br,

2H, H_Ph), 7.61 (br, 2H, H_Ph), 7.09-6.99 (m, 7H, H4, H_Ph), 6.89 (t, ³J_HH= 7.3 Hz, 1H, H5), 6.78 (t,

³J_HH= 7.5 Hz, 1H, H5’), 6.67 (ddd, ³J_HH= 7.4 Hz, J_HP= 4.4 Hz, ⁴J_HH= 1.1 Hz, 1H, H4), 6.05 (d, ³J_HH

3

2

= 1.8 Hz, 1H, H2), 5.83 (d, J_HH= 1.9 Hz, 1H, H2), 5.70 (d, J_HH= 14.2 Hz, 1H, H3), 3.51 - 3.46

(m, 4H, H3’, H1). ¹³C{¹H} NMR (100.6 MHz, C₆D₆): δ 214.2 (d, J_PC= 8.3 Hz), 179.2 (d, J_PC= 12.1

Hz), 139.7 (d, J_PC= 15.4 Hz), 138.4 (d, J_PC= 21.2 Hz), 134.8 (d, J_PC= 17.0 Hz), 133.6 (d, J_PC

=

12.2 Hz), 132.1, 129.9 (d, J_PC= 6.1 Hz), 129.6, 129.3, 129.2 (d, J_PC= 4.4 Hz), 128.7 (d, J_PC= 16.6

Hz), 127.9, 122.6, 120.4, 53.5 (d, J_PC= 18.2 Hz), 39.8. ³¹P{¹H} NMR (162.0 MHz, C₆D₆): δ 39.15

(s). IR (ATR, cm^-1): ꢀꢁ 3060w, 1998s, 1900s, 1853s, 1661m, 1586m, 1571m, 1456m, 1434s,

1389m, 1352m, 1228m, 1163m, 1131m, 1088m, 1072m, 1000w, 918w, 746s, 724s, 687s.

Chlorido(η⁶-p-cymene)(1-(2-(diphenylphosphino)benzyl)-3-methyl-2H-imidazol-2-ylidene-

ruthenium(II) hexafluorophosphate (6a): 210 mg (0.53 mmol) of [1a⁺](Cl^-) and 69.0 mg (0.30

mmol) of Ag₂O were suspended in 20 mL of dichloromethane and stirred under exclusion of

light for 18 h at room temp.. After filtration of the mixture, a solution of 150 mg (0.24 mmol)

of [(η⁶-cymene)RuCl₂]₂in 10 mL of dichloromethane was added and the resulting mixture

was stirred again under exclusion of light for further 18 h at room temp.. The precipitated

AgCl was removed by filtration and 200 mg (1.09 mmol) of KPF₆were added to the solution

which was stirred for another 24 h. After removing the solvent under vacuum the resulting

solid was recrystallized by slow diffusion of diethylether into a solution in dichloromethane.

Yellow solid, yield: 295 mg (72%). Elemental analysis calcd. for C₃₃H₃₅ClF₆N₂P₂Ru: C 51.33, H

4.57, N 3.63, found: C 51.24, H 4.69, N 3.61. ¹H NMR (400.1 MHz, CD₂Cl₂): δ 7.62-7.55 (m, 6H,

H_Ph), 7.48 (tt, ⁴J_HH= 1.4 Hz, ³J_HH= 7.6 Hz, 1H, H5), 7.41-7.36 (m, 2H, H2, H4), 7.30-7.23 (m, 3H,

4

3

H5, H_Ph), 7.16-7.10 (m, 3H, H2, H_Ph), 7.03-6.98 (m, 1H, H4’), 5.71 (dd, J_HH= 1.2 Hz, J_HH= 6.6

2

3

Hz, 1H, H_Cym), 5.66 (d, J_HH= 14.1 Hz, 1H, H3), 5.60 (d, J_HH= 6.0 Hz, 1H, H_Cym), 5.40-5.38 (m,

2H, H_Cym), 5.06 (d, ²J_HH= 14.8 Hz, 1H, H3’), 3.96 (s, 3H, H1), 2.32 (hept, ³J_HH= 6.9 Hz, 1H, H7),

3

2.17 (s, 3H, H6), 1.00 (d, J_HH= 7.0 Hz, 3H, H8), 0.90 (d, J_HH= 6.8 Hz, 3H, H8’). ¹³C{¹H} NMR

(100.6 MHz, CD₂Cl₂): δ 167.7 (d, J_PC= 22.5 Hz), 140.5 (d, J_PC= 11.8 Hz), 137.8 (d, J_PC= 44.7 Hz),

135.9, 135.3 (d, J_PC= 9.8 Hz), 134.0, 133.4, 133.0 (d, J_PC= 8.8 Hz), 132.6 (d, J_PC= 2.2 Hz),

131.5, 131.0, 130.6 (d, J_PC= 8.1 Hz), 129.9 (d, J_PC= 7.4 Hz), 129.6 (d, J_PC= 9.5 Hz), 128.8, 128.5

(d, J_PC= 10.6 Hz), 128.3, 126.7, 123.0, 111.6, 106.2, 97.2, 94.5, 91.4, 89.9, 54.6 (d, J_PC= 7.5

Hz), 39.8, 31.1, 23.2, 21.3, 18.3. ³¹P{¹H} NMR (162.0 MHz, CD₂Cl₂): δ = 24.60 (s), -144.39

(hept, ¹J_PF= 710.9 Hz). ¹⁹F{¹H} NMR (376.5 MHz, CD₂Cl₂): δ = -72.83 (hept, ¹J_PF= 711.1 Hz). IR

(ATR, cm^-1): ꢀꢁ 2977w, 1577w, 1475w, 1436m, 1393m, 1324w, 1238m, 1175w, 1088m,

1031w, 829s, 755s, 729s, 696s.

Chlorido(η⁶-p-cymene)(1-(2-(diphenylphosphino)benzyl)-3-mesityl-2H-imidazol-2-ylidene-

ruthenium(II) hexafluorophosphate (6b): 69 mg (0.38 mmol) of copper(I)mesityl dissolved in

further 3 mL tetrahydrofurane were added over a period of 10 min to a suspension of 188

mg (0.38 mmol) of [1b⁺](Cl^-) in 10 mL of tetrahydrofurane. The reaction mixture was stirred

for 18 h at room temp.. The resulting copper(II)-NHC complex was precipitated by addition

of 15 mL of pentane and isolated by filtration to yield 150 mg of a light yellow solid that was

dissolved in 15 mL of dichloromethane followed by the addition of 82 mg (0.14 mmol) of

[(η⁶-cymene)RuCl₂]₂. The resulting mixture was stirred at room temp. for further 20 h. The

solution was reduced to half of its volume and cooled down to -35 °C for 12 h. The

precipitate, which had formed, was filtered off and the filtrate was extracted twice with 5 mL

of a saturated aqueous solution of KPF₆. The organic phase was dried over Na₂SO₄and the

solvent was removed under vacuum. Yellow-brownish solid, yield: 154 mg (65%). Elemental

analysis calcd. for C₄₁H₄₃ClF₆N₂P₂Ru∙(CH₂Cl₂)_0.8: C 55.25, H 4.95, N 3.08, found: C 55.38, H

1

3

5.09, N 3.17. H NMR (400.1 MHz, CDCl₃): δ = 7.96-7.92 (m, 2H, H_Ph), 7.72 (dd, J_HH= 6.9 Hz,

3

J_PH= 4.9 Hz, 1H, H6), 7.64-7.60 (m, 4H, H4, H_Ph), 7.54 (t, J_HH= 7.5 Hz, 1H, H7), 7.30 (t, J_HH

=

7.6 Hz 1H, H7), 7.18-7.12 (m, 2H, H6, H_Ph), 7.02 (td, ³J_HH= 7.8 Hz, ⁴J_HH= 2.5 Hz, 2H, H_Ph), 6.93

4

(d, J_HH= 1.8 Hz, 1H, H4), 6.91 (s, 1H, H2), 6.75 (s, 1H, H2’), 6.55-6.50 (m, 2H, H_Ph), 5.94 (d,

²J_HH= 15.6 Hz, 1H, H5), 5.63 (br, 1H, H_Cym), 5.41 (d, ²J_HH= 15.5 Hz, 2H, H5’, H_Cym), 4.93 (br, 1H,

H_Cym), 4.35 (br, 1H, H_Cym), 2.35-2.27 (m, 4H, H9, H3), 2.02 (s, 3H, H3’), 1.95 (s, 3H, H8), 1.65 (s,

3H, H1), 1.03 (d, ³J_HH= 7.1 Hz, 3H, H10), 0.58 (d, ³J_HH= 6.8 Hz, 3H, H10’). ¹³C{¹H} NMR (100.6

MHz, CDCl₃): δ 140.3 (d, J_PC= 12.7 Hz), 139.1, 137.9, 137.7, 136.6, 135.8, 135.3, 134.8, 132.9

(d, J_PC= 8.7 Hz), 132.3 (d, J_PC= 1.9 Hz), 131.9 (d, J_PC= 8.6 Hz), 131.3 (d, J_PC= 8.8 Hz), 131.1 (d,

J_PC= 2.4 Hz), 129.2 (d, J_PC= 6.5 Hz), 129.1 (d, J_PC= 2.3 Hz), 128.9, 128.8, 128.5, 128.0 (d, J_PC

=

10.7 Hz), 127.7, 124.1, 54.9 (d, J_PC= 7.1 Hz), 31.1, 23.8, 21.0, 20.0, 19.6, 19.6, 18.2. ³¹P{¹H}

1

NMR (162.0 MHz, CDCl₃): δ 22.43 (s), -144.23 (hept, J_PF= 712.8 Hz). ¹⁹F{¹H} NMR (376.5

1

MHz, CDCl₃): δ -72.64 (d, J_FP= 712.9 Hz). IR (ATR, cm^-1): ꢀꢁ 2925w, 1573w, 1482m, 1435m,

1385m, 1269m, 1234m, 1163w, 1090m, 1028w, 929w, 829s, 744s, 692s.

Catalytic transfer hydrogenation: Catalysis was carried out 15 mL in crimp-capped vials

equipped with a magnetic stirring bar. After filling in the catalyst (0.5 mol-% with respect to

acetophenone), the vials were closed with Teflon-coated septa caps and the vials were

flushed with nitrogen gas. Then a solution of 0.05 mmol of KOH in 5 mL of isopropanol was

added via a syringe. Since the amount of KOH is too small to be weighted correctly, 200 mL

of a stock solution were prepared, and aliquots were taken from this solution. After addition

of 1.00 mmol acetophenone by using a syringe, the vial was placed into a block of aluminum

which had been pre tempered to 82 °C. To measure the conversion of the substrate, samples

of about 0.1 mL were taken after 1, 2, 3, 4, 8 und 24 h, filtered over a short bed of silica and

MgSO₄and 50 µL of tetradecane were added as an external standard. Yields were

determined by GC-FID analysis.

X-ray structure analyses: Crystal data and refinement parameters are collected in Table 2.

All structures were solved using direct method with SIR92 [37], completed by subsequent

difference Fourier syntheses, and refined by full-matrix least-squares procedures [38]. Semi-

empirical absorption corrections from equivalents (Multiscan) were carried out for com-

plexes 2-5, analytical numeric absorption corrections was applied on compound 6a [39]. All

non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen

atom H1 in the complex 4, which is bound to Ru1, was located in the difference Fourier

synthesis, and was then refined semi-freely with the help of a distance restraint, while

constraining its U-value to 1.2 times the U(eq) value of Ru1. All other hydrogen atoms were

placed in calculated positions and refined by using a riding model. And the detailed

information has been posted in the final CIF file. CCDC 1980356-1980360 contain the supple-

mentary crystallographic data for this paper. These data can be obtained free of charge from

The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table 2. Crystallographic data, data collection and refinement.

2

3

4

5

6a

empirical formula

formula weight

crystal size [mm]

T [K]

C₂₆H₂₁FeN₂O₃P C₂₇H₂₂BF₄FeN₂O₄P

496.27 612.09

0.37x0.25x0.25 0.534x0.328x0.285 0.30x0.09x0.04

C₃₃H₃₀ClN₂O₂PRu C₂₆H₂₁N₂O₃PRu

C₃₃H₃₅ClF₆N₂P₂Ru

772.09

654.08 541.49

0.470x0.355x0.258 0.390x0.350x0.180

150(2)

0.71073

monoclinic

P2₁/c

0.71073

monoclinic

P2₁/c

0.71073

triclinic

0.71073

monoclinic

P2₁/c

1.54184

monoclinic

C2/c

λ

[Å]

crystal system

space group

a [Å]

ꢂ

P1

15.9395(2)

8.8558(1)

16.2401(2)

90

9.5958(3)

20.7199(5)

14.0479(4)

90

10.7497(9)

12.4462(9)

13.8730(9)

100.072(6)

112.332(7)

110.085(7)

1509.6(3)

2

9.2760(2)

13.8768(4)

18.3020(4)

90

12.9158(1)

14.7534(2)

35.0814(3)

90

b [Å]

c [Å]

α

β

γ

[°]

99.011(1)

90

104.265(3)

90

96.873(2)

90

95.034(1)

90

V [Å³]

2264.11(5)

4

2706.94(14)

4

2338.93(10)

4

6659.04(12)

8

Z

_calcd.[g cm^-3]

[mm^-1]

-range [^o]

1.456

1.502

1.439

1.538

1.540

ρ

0.768

0.681

0.693

0.769

5.986

µ

2.84-32.46

28787

2.941-32.488

33327

2.93-32.49

17728

2.936-32.466

15124

4.560-62.726

19197

θ

refl. coll.

indep. refl.

7553

9092

9795

7664

5291

[R_int= 0.0240]

[R_int= 0.0437]

9092/0/362

[R_int= 0.0650]

9795/0/367

0.0850, 0.2081

[R_int= 0.0212]

7664/0/299

0.0314, 0.0700

[R_int= 0.0316]

5291/141/443

0.0333, 0.0945

data/restr./param. 7553/0/299

final R indices

[I>2

(I)] ^a

R indices (all data) 0.0398, 0.0834 0.0681, 0.1315

0.0340, 0.0808 0.0538, 0.1241

σ

0.1200, 0.2315

1.104

0.0398, 0.0736

1.079

0.0367, 0.1149

1.236

GooF ^b

Δρ_max

_min(e∙Å^-3)

1.101

1.115

/

0.445/-0.355

0.934/-0.781

3.477/-1.724

0.441/-0.621

1.058/-0.879

2 2

2 1/2

2 2

^aR1 = Σ||F_o| - |F_c||/Σ|F_o|, R2 = [Σω(F_o²- F_c) /ΣωF_o] ^bGooF = [Σω(F_o²- F_c) /(n-p)]^1/2

ω . .

Acknowledgement

We gratefully thank the DFG-funded transregional collaborative research center SFB/TRR 88

“Cooperative effects in homo- and heterometallic complexes (3MET)” for the financial

support.

Scheme, Figure and Table captions

Scheme 1. i) 1.2 equiv. n-BuLi, Et₂O, r.t., 14 h, ii) 1.2 equiv. Ph₂PCl, Et2O, -78 °C - r.t., 2 h, iii)