Iridium(I) complexes with hemilabile N-heterocyclic carbenes: Efficient and versatile transfer hydrogenation catalysts

ARTICLE

pubs.acs.org/Organometallics

Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes:

Efficient and Versatile Transfer Hydrogenation Catalysts

M. Victoria Jimꢀenez,* Javier Fernꢀandez-Tornos, Jesꢀus J. Pꢀerez-Torrente,* Francisco J. Modrego,

Sonja Winterle, Carmen Cunchillos, Fernando J. Lahoz, and Luis A. Oro

Departamento de Química Inorgꢀanica, Instituto de Síntesis Química y Catꢀalisis Homogꢀenea-ISQCH, Universidad de Zaragoza-CSIC,

50009-Zaragoza, Spain

S Supporting Information

b

ABSTRACT: A series of neutral and cationic rhodium and iridium(I) complexes based on

hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethy-

lamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is

coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]⁺(R =

pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes

[MBr(cod)(MeImR)], [M(NCCH₃)(cod)(MeImR)]⁺(M = Rh, Ir; R = 2-methoxyethyl

and2-methoxybenzyl) and[IrX(cod)(MeImR)] (X=Br,R=pyridin-2-ylmethyl;X=Cl,R=

2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-

methoxybenzyl))] has been determined by X-ray diﬀraction. The iridium complexes are

eﬃcient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH.

A comparative study has shown that cationic complexes are more eﬃcient than the neutral

and also that complexes having O-functionalized NHC ligands provide much more active

systems than the corresponding N-functionalized ligands with TOFs up to 4600 h^ꢀ1. The

complexes [Ir(NCCH₃)(cod)(MeImR)]⁺(R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of

several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by

NMR and MS has allowed the identiﬁcation of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species

[IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species

has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of

the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identiﬁed. This

destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive eﬀect of the methoxy group of the

functionalized NHC ligands on the catalytic activity.

’ INTRODUCTION

compounds with application in homogeneous catalysis.⁶In this

context, a number of complexes containing functionalized NHC

ligands in which a labile donor function (ether, thioether, alkoxy,

pyridine, amine, amido, etc.) is attached to a strongly bonded

carbene moiety have been synthesized. In fact, the hemilabile

character of several O-, N-, or S-functionalized NHCs ligands has

been demonstrated,⁷and in some cases, an active role of

the hemilabile fragment has been identiﬁed in several catalytic

processes.⁸

Transfer hydrogenation is a powerful strategy for the reduc-

tion of unsaturated CdO and CdN compounds, and a range of

alcohols and amines are accessible by transfer hydrogenation

under mild reaction conditions.⁹Transfer hydrogenation is a

valuable and atom-eﬃcient reaction with evident economic and

environmental advantages, as it avoids the use of dihydrogen

pressure or hazardous reducing agents by using nontoxic hydro-

gen donors such as alcohols, formic acid, and hydrazine.¹⁰

N-Heterocyclic carbenes (NHCs) have attracted much inter-

est as ligands for homogeneous catalysis. NHC ligands have

proved to be eﬃcient ancillary ligands because of their strong

coordination ability, which imparts to the metal complexes an

adequate balance between stability and activity, and their tunable

character, which allows for the control of the sterical and

electronic properties on the metal center.¹These ligands are

easily accessible from imidazolium salts,²and a large number of

NHCs precursors—simple, functionalized, bis- or tris-NHC—

with diﬀerent topologies have been prepared that, in combina-

tion with well-established coordination methodologies, has led to

the preparation of a range of transition metal complexes with

diﬀerent geometries and potential catalytic applications.³

Hemilabile ligands can play a dual role in a catalyst since they

can easily enable coordinative sites at the metal center and, at the

same time, protect the coordination sites by a dynamic “on and

oﬀ” chelating eﬀect.⁴Although the most studied hemilabile

ligands are functionalized phosphines containing O and N donor

sets, functionalized NHC ligands can also exhibit hemilability⁵

and represent a good alternative for the preparation of new

Received: August 9, 2011

Published: September 23, 2011

r

2011 American Chemical Society

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Organometallics

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The most active and selective hydrogen transfer catalysts,

particularly in enantioselective reductions, are ruthenium, rho-

dium, and iridium complexes. In particular, it is noteworthy to

mention Noyori’s ruthenium catalysts with chiral tetradentate

ligands and those arene complexes based on chiral β-amino

alcohol or N-tosylethylenediamine ligands operating through a

bifunctional metalꢀligand mechanism.¹¹Eﬃcient rhodium and

iridium hydrogen transfer catalysts are mainly based both on

phosphine and N-donor ligands,¹⁰although NHC ligands have

also been applied to the design of hydrogen transfer catalysts. It

has been found that, in contrast to phosphine-based catalysts,

iridium-NHC complexes are more active than their Rh-NHC

analogues, and accordingly, a number of highly eﬃcient iridium

catalysts have been recently reported.¹²In particular, air-stable

iridium(III) bis-carbene complexes were found to be active

catalysts for transfer hydrogenation of ketones, aldehydes, and

imines at low catalyst loading.¹³

Chart 1. Imidazolium Salt Precursors for Functionalized

NHC Ligands

Chart 2. Neutral Rhodium and Iridium(I) Complexes with

Functionalized NHC Ligands

Our interest is focused on the synthesis and catalytic applica-

tions of transition metal complexes containing heteroditopic

ligands of hemilabile character that incorporate strong electron

donors, such as phosphines and carbenes. We have recently

reported the application of cationic rhodium(I) complexes

containing ﬂexible hemilabile functionalized phosphine ligands

of the type Ph₂P(CH₂)_nZ (Z = OR, NR₂, SR)¹⁴for the

stereoregular polymerization of phenylacetylene^8aand the re-

gioselective anti-Markovnikov oxidative amination of oleﬁns.¹⁵

Mechanistic studies led to the discovery of the active role of the

hemilabile fragment of the ligands in the initiation process of the

polymerization reactions leading to high average molecular

weights.^8a,16In addition, remarkable catalyst eﬃciency in hydro-

amination was observed with those catalytic systems based on the

ligand (3-ethoxypropyl)diphenylphosphine, Ph₂P(CH₂)₃OEt.

It has come to our attention that the catalyst eﬃciency in

transfer hydrogenation of ketones by sterically demanding un-

symmetrically N,N⁰-substituted benzimidazol-2-ylidene iridium-

(I) [IrBr(cod)(NHC)] complexes was improved when using

NHC ligands having a 2-methoxyethyl substituent.¹⁷In the same

way, highly eﬀective iridium(I) hydrogen transfer catalysts based

on unsymmetrical 2-methoxyphenyl donor-functionalized ex-

panded ring NHCs have been recently reported.¹⁸It becomes

evident that the presence of a hemilabile fragment on the catalyst

structure has a considerable impact on hydrogen transfer cata-

lytic activity. Although a possible hemilabile chelating eﬀect at

the metal center for the stabilization of coordinatively unsatu-

rated catalytic intermediates could account for the enhanced

activity, the promotion of directing eﬀects with the substrate or

the stabilization of polar catalytic intermediates through weak

interactions cannot be ruled out.

bromide or chloride (Chart 1). The imidazolium salt precursors

for flexible hemilabile NHC ligands, [MeImH(CH₂)₂OMe]Br

(1)¹⁹and [MeImH(CH₂)_nNHMe₂)]Cl₂(n = 2, 4; n = 3, 5),^6a

were prepared by alkylation with 1-bromo-2-methoxyethane and

the corresponding chloro-alkyl-dimethylamine hydrochloride

derivatives, respectively, following the reported methods. In

the same way, [MeImH(pyridin-2-ylmethyl)]Br (3), a precursor

for a rigid N-functionalized NHC ligand, was prepared using

2-(bromomethyl)pyridine as alkylating reagent.²⁰The new salt

1-(2-methoxybenzyl)-3-methyl-1H-imidazol-3-ium bromide (2),

precursor of a related rigid O-functionalized NHC ligand, was

synthesized by alkylation of 1-methylimidazole with 2-methox-

ybenzyl bromide, prepared by bromination of 2-methoxybenzyl

alcohol,²¹in toluene following the procedure described for the

N-tert-butyl derivative.²²

[MeImH(2-methoxybenzyl)]Br (2) was isolated as a white

In order to investigate further this hemilabile eﬀect, we report

herein on the synthesis and catalytic activity in hydrogen transfer

of a series of rhodium and iridium complexes containing hemi-

labile O- and N-donor-functionalized NHCs ligands. Interest-

ingly, a relationship between the donor function, the ﬂexibility of

the backbone, and the length of the linker in the ligands with the

catalytic activity has been observed.

hygroscopic solid in good yield and characterized by elemental

1

analysis, mass spectrometry (ESI-MS), and H and ¹³C{¹H}

NMR spectroscopy. The characteristic downﬁeld shifted reso-

nances of the NCHN imidazolium fragment were observed in the

¹H and ¹³C{¹H} NMR spectra at δ 10.37 and 137.41 ppm,

respectively.

Synthesis of Neutral Iridium(I) and Rhodium(I) Complexes

with Functionalized NHC Ligands. The attempts to prepare

iridium and rhodium(I) complexes using the free NHC ligands

generated in situ by deprotonation of the imidazolium salts were

unsuccessful, probably due to the acidity of the methylene

protons. However, the direct deprotonation of the methoxo

ligands in the dinuclear complexes [{M(μ-OMe)(cod)}₂] pro-

vides access to neutral mononuclear complexes containing the

’ RESULTS AND DISCUSSION

Synthesis of Precursors for Hemilabile NHC Ligands. The

imidazolium salt precursors for selected O- and N-donor-function-

alized N-heterocyclic carbene ligands were prepared by alkylation of

1-methylimidazole with the corresponding functionalized alkyl

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functionalized NHC ligands. In fact, reaction of the imidazolium

salts with 0.5 equiv of [{M(μ-OMe)(cod)}₂] (M = Rh, Ir) in

tetrahydrofuran gave orange solutions from which the bromo

complexes [MBr(cod)(MeIm(CH₂)₂OMe)] (M = Ir, 6; M = Rh,

8) and [MBr(cod)(MeIm(2-methoxybenzyl))] (M = Ir, 7; M =

Rh, 9) were isolated as orange solids in good yield (Chart 2). This

methodology is also applicable for the synthesis of [IrBr(cod)-

(MeIm(pyridin-2-ylmethyl))] (10). The synthesis of iridium

complexes [IrCl(cod)(MeIm(CH₂)_nNMe₂)] (n = 2, 11; n = 3,

12) containing dimethylamino-functionalized NHC ligands was

carried out in two steps, through the intermediate cyclooctadie-

nedichloroiridate(I), [MeImH(CH₂)_nNMe₂][IrCl₂(cod)], salts,

following the same strategy used for the preparation of analogous

rhodium compounds.^6aReaction of the ammonium-imidazolium

Figure 1. Molecular structure of complex [IrBr(cod)(MeIm(2-meth-

oxybenzyl))] (7) (hydrogen atoms have been omitted for clarity).

Selected bond lengths (Å) and angles (deg): IrꢀBr 2.4904(6), IrꢀC(1)

2.035(4), IrꢀC(13) 2.192(4), IrꢀC(17) 2.105(4), IrꢀC(14) 2.174(4),

IrꢀC(18) 2.098(4), N(1)ꢀC(1) 1.357(5), N(2)ꢀC(1) 1.351(5), N-

(1)ꢀC(2) 1.385(6), N(2)ꢀC(3) 1.390(5), N(1)ꢀC(4) 1.452(5),

N(2)ꢀC(5) 1.463(5), C(13)ꢀC(14) 1.376(6), C(17)ꢀC(18) 1.409(6),

BrꢀIrꢀC(1) 89.03(10), M(1)ꢀIrꢀM(2) 87.26(19), IrꢀC(1)ꢀN(1)

125.4(3), IrꢀC(1)ꢀN(2) 130.4(3), N(1)ꢀC(1)ꢀN(2) 104.2(3).

salts [MeImH(CH₂)_nNMe₂]Cl HCl with [{Ir(μ-OMe)(cod)}₂]

3

resulted in the formation of the amino-imidazolium salts [MeImH-

(CH₂)_nNMe₂][IrCl₂(cod)], which were further deprotonated

by using NaH/H₂O to give the corresponding neutral com-

pounds.

The complexes 6ꢀ12 have been fully characterized by spec-

troscopic means. In addition, their neutral character was con-

ﬁrmed by conductivity measurements in acetone and the

MALDI-TOF mass spectra that showed peaks at m/z corre-

sponding to the molecular ion and the cations resulting from the

loss of the halide ligand.

Chart 3. Cationic Rhodium and Iridium(I) Complexes with

Functionalized NHC Ligands

1

The H NMR spectra of the new iridium/rhodium(I) com-

plexes 6ꢀ12 showed no low-ﬁeld signals attributable to the

acidic NCHN protons, which conﬁrm the deprotonation of the

imidazolium fragment. Noteworthy, the coordination of the

functionalized NHC ligands to the metal center becomes evi-

dent, as low-ﬁeld resonances around δ 180 ppm (doublet for

rhodium complexes 8 and 9, J_RhꢀC≈ 50 Hz) were observed in

the ¹³C{¹H} NMR spectra. Most of the complexes showed two

1

resonances for the vinylic protons of the cod ligand in the H

NMR spectra at δ ≈ 4.5 and 3.0 ppm (complex 8 exhibits four

resonances), the upﬁeld resonance corresponding to the dCH

protons trans to the NHC ligand. This eﬀect has also been

observed in Ir(I)-cod-phosphine complexes,²³although the

chemical shift diﬀerence between the vinylic protons is smaller

than in related Ir(I)/Rh(I)-cod-NHC complexes.^12e,24However,

the ¹³C{¹H} NMR spectra of the complexes featured four

resonances for the oleﬁnic carbons of the cod ligand, which is

indicative of the lack of an eﬀective symmetry plane in the

molecules, probably as a result of the hindered rotation about the

carbeneꢀmetal bond enforced by the presence of the side arm in

the hemilabile NHC ligands cis to large halide and cod ligands. As

a consequence, the methylenic protons of the side arm of NHC

ligands were found to be diastereotopic, and two distinct

resonances for the dCH protons and carbons of the imidazole

fragment were observed in the NMR of the complexes.

In order to get detailed information on the structural para-

meters in these complexes, the X-ray determination of the

molecular structure of [IrBr(cod)(MeIm(2-methoxybenzyl))]

(7) was carried out. The molecular structure of 7 and the more

signiﬁcant bond distances and angles are shown in Figure 1. The

coordination geometry around the iridium center, formed by

the coordination to the metal of the two oleﬁnic bonds of the

cyclooctadiene ligand, the carbon atom of the NHC ligand, and

the bromide atom, is slightly distorted square-planar, as evi-

denced by the trans bond angles BrꢀIrꢀC(17)/C(18)

178.72(12)° and C(1)ꢀIr(1)ꢀC(13)/C(14) 176.11(18)°. The

most remarkable structural feature involves the diﬀerent oleﬁnic

bond distances observed (IrꢀM(1) 2.072(5), C(13)ꢀC(14)

1.376(6) Å versus IrꢀM(2) 1.980(4), C(17)ꢀC(18) 1.409(6) Å,

M(1) and M(2) represent the midpoints of the oleﬁnic bonds

C(13)ꢀC(14) and C(17)ꢀC(18), respectively), most likely as a

clear consequence of the high structural trans eﬀect of the NHC

ligand.

Synthesis of Cationic Rhodium(I) and Iridium(I) Com-

plexes with Functionalized NHC Ligands. The coordination

of the amino/pyridine donor set of the functionalized NHC

ligands can be accomplished, among other methods, by abstrac-

tion of the halide ligand in the neutral complexes.²⁵However, the

coordination of the methoxy fragment by abstraction of the

bromo ligand in the complexes containing O-functionalized

NHC ligands was unsuccessful, and the presence of acetonitrile

is required for the stabilization of the cationic complexes.

Reaction of the bromo complexes [MBr(cod)(MeIm(CH₂)₂-

OMe)] (6, 8) and [MBr(cod)(MeIm(2-methoxybenzyl))] (7, 9)

with AgBF₄in acetone in the presence of 1 equiv of CH₃CN at

room temperature resulted in the precipitation of AgBr and the

formation of yellow solutions of the cationic complexes [M(NC-

CH₃)(cod)(MeIm(CH₂)₂OMe)][BF₄] (M = Ir, 13; M = Rh,

15) and [M(NCCH₃)(cod)(MeIm(2-methoxybenzyl))][BF₄]

(M = Ir, 14; M = Rh, 16), which were isolated as microcrystalline

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Organometallics

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Table 1. Catalytic Hydrogen Transfer from 2-Propanol to

Cyclohexanone with Iridium and Rhodium Complexes Hav-

ing N- and O-Functionalized NHC Ligands^a,b

and KOH (0.5 mol %) were routinely used. Other inorganic

bases such as Cs₂CO₃or Kt-BuO were found to provide less

active catalytic systems with reaction times over 25% higher for

the same level of conversion under the same experimental

conditions. On the other hand, although KN(Si(CH₃)₃)₂re-

duces the reaction time more or less in the same proportion, the

cheaper KOH was selected as base because there are more

reported data in the literature. The reaction times required to

reach conversions over 90% (determined by GC using mesity-

lene as internal standard) and the average turnover frequencies

(TOF) for all the examined catalysts are shown in Table 1. The

temperature has a strong influence on the catalyst performance,

and a sharp decrease in the catalytic activity was observed when

the temperature was lowered from 80 to 40 °C under the

standard conditions using 7 as catalyst precursor (Table 1, entries

2ꢀ4).

entry

catalyst

M = Ir

time (min) conversion (%) TON TOF^c/h^ꢀ1

1

6

110

72

90

98

96

97

90

94

91

90

92

91

93

98

97

98

900

989

961

972

904

941

910

907

924

910

934

50

490

824

180

39

2

7

3

7^d

7^e

320

1500

1730

408

1360

35

4

5

10

11

12

13

14

17

18

31

6

138

40

7

8

1555

4622

248

234

33

9

12

10

11

12

13

14

15

220

240

90

M = Rh 8^f

9^f

15^f

16^f

180

120

210

49

16

50

25

50

14

^aReaction conditions: catalyst/substrate/KOH ratio of 1/1000/5,

[catalyst]_o= 1 ꢁ 10^ꢀ3M in 2-propanol at 80 °C. ^bThe reactions were

monitored by GC using mesitylene as internal standard. ^cAverage

turnover frequency (mol of product/mol of catalyst per hour) deter-

mined at the reaction time. ^dT = 60 °C. ^eT = 40 °C. ^fCatalyst/subtract/

base ratio of 1/100/5.

The neutral iridium complexes with N-functionalized NHCs

10ꢀ12 were found to be moderately active in the transfer

hydrogenation of cyclohexanone to cyclohexanol, with com-

pound [IrCl(cod)(MeIm(CH₂)₂NMe₂)] (11), having a NHC

ligand with a ﬂexible arm, being more active (entries 5ꢀ7). It is

noteworthy that a notable increase in the catalytic activity was

observed with the corresponding cationic complexes 17 and 18,

having NHC ligands k²-C,N coordinated, which exhibited a

TOF of ca. 240 h^ꢀ1(entries 10, 11).

yellow solids in good yield (Chart 3). The more convenient

synthesis of complexes [Ir(cod)(MeIm(pyridin-2-ylmethyl))]-

[BF₄] (17) and [Ir(cod)(MeIm(CH₂)₃NMe₂)][BF₄] (18) re-

lies on the NHC transfer from the silver complexes

[AgCl(NHC)]_x, generated in situ, to the cyclooctadiene solvato

[Ir(cod)(OCMe₂)₂]⁺species containing labile acetone ligands.

The conductivity measurements in acetone solutions were in

agreement with the presence of 1:1 electrolytes, and the MALDI-

TOF mass spectra showed the molecular ion (compounds 17

and 18) or the ion resulting from the loss of the acetonitrile

ligand. The presence of an acetonitrile ligand in complexes

The neutral bromo iridium complexes with O-functionalized

NHCs, 6 and 7, are far more active than the related neutral

N-functionalized precursors (entries 1 and 2), with compound

[IrBr(cod)(MeIm(2-methoxybenzyl))] (7), having a NHC with

a less ﬂexible arm, being more active, with a TOF of 824 h^ꢀ1

.

Interestingly, the corresponding cationic complexes having a

labile acetonitrile ligand and methoxy-functionalized free arms,

[Ir(NCCH₃)(cod)(MeIm(CH₂)₂OMe)]⁺(13) and [Ir(NCCH₃)-

(cod)(MeIm(2-methoxybenzyl))]⁺(14), were highly eﬃcient

catalyst precursors, giving TOFs of 1555 and 4622 h^ꢀ1(entries 8

and 9), respectively. These results point out the superior catalytic

performance of cationic complexes and highlight the positive

inﬂuence of O-functionalized NHC ligands having a donor set

with weak coordination ability. In addition, the outstanding

catalytic activity of 14, with a 2-methoxybenzyl substituent,

suggests that the ﬂexibility of the functionalized arm of the

NHC ligand also inﬂuences the catalytic activity.

The related rhodium complexes having O-functionalized

NHCs were poor catalysts, even when using 1 mol % catalyst

loading, which is in agreement with the general observed trend

that, opposite of the phosphine-based catalysts,¹⁰iridium-

carbene complexes are more active in transfer hydrogenation

than their rhodium analogues.^26,13cIn sharp contrast with the

iridium precursors, the neutral complexes 8 and 9 are slightly

more active than the corresponding cationic complexes 15 and

16 (entries 12ꢀ15), with the ﬂexible ligands giving a somewhat

superior activity.

1

13ꢀ16, which was observed at δ ≈ 2.4 ppm in the H and at

δ ≈ 130 and 3 ppm in the ¹³C{¹H} NMR spectra, suggests an

uncoordinated methoxy moiety in the complexes. In contrast, no

coordinated solvents were observed in the NMR spectra of

complexes 17 and 18, even when the reaction was conducted

in the presence of acetonitrile, from which the coordination of

the pyridine and dimethyl amino moieties was inferred. On the

other hand, the large downﬁeld shift observed for the resonance

of the >CH₂attached to the NHC ring compared to the neutral

precursor in the ¹³C{¹H} NMR spectra (ca. 9 and 14 ppm,

respectively) is a diagnostic for the coordination of the hemilabile

fragment in the complexes.¹⁵

Transfer Hydrogenation Catalysis: Preliminary Screening.

The neutral and cationic rhodium and iridium complexes with N-

and O-functionalized NHCs have been tested as catalysts for

transfer hydrogenation of unsaturated substrates. The reduction

of cyclohexanone to cyclohexanol was chosen as a model reaction

to explore the catalytic performance of the different compounds

(eq 1), and the reaction conditions were optimized using

[IrBr(cod)(MeIm(2-methoxybenzyl))] (7) as catalyst precur-

sor. 2-Propanol was used as hydrogen source because of its

favorable properties, particularly a stable, nontoxic solvent with a

moderate boiling point. A standard catalyst loading of 0.1 mol %

The catalytic activity exhibited by 13 and 14 is signiﬁcantly

higher than that observed for related iridium(I)/unfunctiona-

lized-NHC complexes, as for example [Ir(cod)py(ICy)]⁺(ICy =

1,3-dicyclohexylimidazol-2-ylidene),^12ewhich gave a TOF of

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Table 2. Calculated Rate Constants for the Transfer Hydro-

genation of Cyclohexanone with Catalyst 6, 7, and 14^a,b

catalyst

[Ir]

(mol L^ꢀ1

k

catalyst loading (mol %)

)

(min^ꢀ1

)

R²

6

0.1

1.0 ꢁ 10^ꢀ3

4.0 ꢁ 10^ꢀ4

2.0 ꢁ 10^ꢀ4

2.38 ( 0.06 ꢁ 10^ꢀ20.991

3.31 ( 0.08 ꢁ 10^ꢀ20.994

2.08 ( 0.02 ꢁ 10^ꢀ10.999

7

0.1

14

0.1

0.04

0.02

0.013

8.6 ( 0.2 ꢁ 10^ꢀ2

0.996

3.95 ( 0.08 ꢁ 10^ꢀ20.999

1.33 ꢁ 10^ꢀ42.19 ( 0.06 ꢁ 10^ꢀ20.993

^aReaction conditions: cyclohexanone 5 mmol, catalyst/KOH ratio of

1:5 in 2-propanol at 80 °C. ^bThe reactions were monitored by GC using

mesitylene as internal standard.

Figure 2. Time dependence of the catalytic transfer hydrogenation of

cyclohexanone using 0.1 mol % catalyst (6, 7, and 14) in 2-propanol at

80 °C with KOH as base.

summarized in Table 3. In addition, the catalytic activity of the

neutral bromo complexes 6 and 7 has also been studied in some

cases for comparative purposes.

300 h^ꢀ1for the transfer hydrogenation of cyclohexanone at the

same catalyst loading, benzannulated-NHC iridium(I) complexes

(TOFs from 70 to 250 h^ꢀ1), or [Ir(OSiMe₃)(cod)(IMes)] (TOF =

125 h^ꢀ1) transfer hydrogenation catalysts previously re-

ported.^12,17,26On the other hand, the catalytic activity of both

precursors is even higher than that shown by a number of Ir(I)

complexes having unsymmetrical 2-methoxyphenyl donor func-

tionalized NHC ligands, which exhibited turnover frequencies up

to 400 h^ꢀ1for the transfer hydrogenation of 4-bromoaceto-

phenone.¹⁸

Acetophenone was eﬃciently reduced although at moderate

rates compared to that observed with the smaller and geometry-

constrained substrate cyclohexanone (entries 1ꢀ3). Interest-

ingly, the same activity trend 7 < 13 < 14 was also observed in the

transfer hydrogenation of acetophenone, the catalyst having a

2-methoxybenzyl-functionalized NHC ligand being more active,

with a TOF of 683 h^ꢀ1. 4-Bromacetophenone was reduced to

1-(4-bromophenyl)ethanol by 14 at a similar rate with no

observable dehalogenation due to ArꢀX activation (entry 4).

In general, aldehydes are hard to reduce by hydrogen transfer

catalysis. The reduction of aldehydes to primary alcohols via

transfer hydrogenation can be problematic due to the possible

substrate decarbonylation resulting in deactivation of the cata-

lysts. In addition, the CH α to the carbonyl is susceptible to

deprotonation under the required basic conditions to give aldol

and related products resulting from condensation. In any case,

beyond the activity and chemoselectivity issues the higher redox

potentials of aldehydes compared to ketones shift the transfer

hydrogenation equilibrium toward the formation of the primary

alcohol products. Benzaldehyde was successfully reduced to

phenylmethanol by the neutral complex 7 with no detectable

aldol product in spite of using the strongest basic media (entry

5). The cationic complex 14 showed an outstanding catalytic

activity with a TOF of 2318 h^ꢀ1, calculated at 96% of conversion,

under the same conditions (entry 6). Standard activities for most

transition metal catalysts are around a few hundred turnovers per

hour; however, the remarkable activity of 14 is on the same order

that is exhibited by bis-NHC-Ir(III)^13aor NHC-hydroxo-Os-

(II)²⁸complexes but lower than, for example, [RuCl(CNN)-

(dppb)] (HCNN = 6-(4⁰-methylphenyl)-2-pyridylmethylamine;

dppb = Ph₂P(CH₂)₄PPh₂)²⁹or [Cp*IrCl(μ-Cl)]₂in the pre-

sence of diamines³⁰in aqueous media, which show TOFs up to

The transfer hydrogenation of cyclohexanone with the catalyst

precursors 6, 7, and 14, under standard conditions, was mon-

itored by GC analysis taking 0.1 mL aliquots of the reaction

mixture at intervals of 5 min. As can be observed in the

conversion vs time plots, no induction period was detected, as

cyclohexanone reduction was observed immediately after ther-

mal equilibration of the reactant mixture (Figure 2). Interest-

ingly, quantitative conversion was achieved in less than 15 min

for compound 14, the more eﬃcient catalyst precursor in this

series. A plot of ln[cyclohexanone] vs time gave a linear ﬁt that is

representative of a pseudo-ﬁrst-order kinetic behavior with the

slope of the line corresponding to the observed ﬁrst-order rate

constant k(min^ꢀ1) (Table 2 and Supporting Information). The

TOF₅₀(turnover frequency at 50% of conversion) calculated

from the kinetic data for this catalyst is as high as 8750 h^ꢀ1. The

catalyst reaction order for the transfer hydrogenation of cyclo-

hexanone with 2-propanol at 80 °C has been determined using

catalyst precursor 14. Thus, several catalytic runs were carried

out at diﬀerent catalyst concentrations using substrate/catalyst

ratios of 1000:1, 2500:1, 5000:1, and 7500:1 in order to

determine the corresponding ﬁrst-order rate constants (Table 2).

The representation of ln k vs ln[Ir] gave a straight line with slope

1.10 (R²= 0.993), in agreement with the expected ﬁrst-order

dependence on the catalyst, indicating no loss of catalytic activity

due to catalyst deactivation (see Supporting Information).²⁷

Catalytic Transfer Hydrogenation of Unsaturated Sub-

strates Catalyzed by Iridium Complexes with O-Donor-

Functionalized NHC Ligands. Following these initial studies,

we have examined the performance of the efficient cationic

iridium catalysts containing O-donor-functionalized NHC ligands,

[Ir(NCCH₃)(cod)(MeIm(CH₂)₂OMe]⁺(13) and [Ir(NCCH₃)-

(cod)(MeIm(2-methoxybenzyl))]⁺(14), for the transfer hydro-

genation of several unsaturated substrates, and the results are

10⁵h^ꢀ1

.

Transfer hydrogenation of aliphatic aldehydes by 14 took

place at lower rates. 3-Phenylpropanal, an enolizable aldehyde,

was quantitatively and selectively reduced to 3-phenylpropan-1-ol

in 130 min under the standard conditions with a TOF of 438 h^ꢀ1

(entry 7). Along the same lines, the reduction of nonanal, a long-

chain aliphatic aldehyde, by 14 resulted in selective and quanti-

tative formation of nonan-1-ol in ca. 85 min (entry 8).

Deactivated oleﬁns were also subjected to transfer hydrogena-

tion under the optimized standard conditions. In contrast

with the above-described results for carbonyl compounds, the

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Table 3. Catalytic Transfer Hydrogenation of Unsaturated Substrates Catalyzed by Iridium Complexes with O-Donor-

Functionalized NHC Ligands^a,b

^aReaction conditions: catalyst/substrate/KOH ratio of 1:1000:5, [catalyst]_o= 1 ꢁ 10^ꢀ3M in 2-propanol at 80 °C. ^bThe reactions were monitored by

GC using mesitylene as internal standard. ^cAverage turnover frequency (mol of product/mol of catalyst per hour) determined at the reaction time.

^dCatalyst/substrate/KOH ratio of 1:1000:10. ^eCatalyst/substrate/KOH ratio of 1:100:5.

catalytic activity for the transfer hydrogenation of cyclohexene

follows the order 14 < 7 < 6 (entries 9ꢀ11). Unexpectedly, the

neutral bromo complex [IrBr(cod)(MeIm(CH₂)₂OMe)] (6)

having a functionalized NHC ligand with a ﬂexible 2-methoxy-

ethyl substituent exhibited good activity, giving a 94% conversion

in 120 min with a TOF of 471 h^ꢀ1(entry 9). This result improves

those obtained for the transfer hydrogenation of alkenes with

related NHC-iridium complexes.^12e

The selectivity commonly observed in competitive hydrogen

transfer reduction of mixtures of alkenes and ketones or alde-

hydes often implies the selective reduction of CdO over the

CdC bonds.³¹However, in the case of α,β-unsaturated ketones

the general observed trend in hydrogen transfer catalysis is the

preferential reduction of the activated double bond over the

CdO reduction.¹⁰In agreement with this, mixtures of saturated

ketone^32,33and saturated alcohol^12e,13c,34(major component)

are generally obtained, although a limited number of transition

metal complexes produce the chemoselective reduction to allylic

alcohols.³⁵

Transfer hydrogenation of benzylideneacetone by O-function-

alized NHC iridium complexes produces 4-phenylbutan-2-one as

a result of the selective reduction of the CdC bond (entries

12ꢀ14). The more active catalysts were those containing NHC

ligands with a rigid 2-methoxybenzyl free arm. Interestingly, the

neutral complex 7 is more active than the cationic complex 14, giving

a 90% conversion in 30 min with a TOF of 1817 h^ꢀ1(entry 13).

In order to conﬁrm the selective formation of 4-phenylbutan-2-

1

one, the catalytic reaction was monitored by GC/MS and H

NMR. Thus, the resonances at δ 7.47 and 6.66 ppm of the

benzylideneacetone oleﬁnic protons progressively disappeared

and two new multiplets at δ 2.78 and 2.66 ppm, corresponding to

the methylenic protons of the 4-phenylbutan-2-one, were

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Scheme 1. Hydrogen Transfer Mechanism for the Selective

Reduction of Benzylideneacetone

Scheme 2. Synthesis and Reactivity of Complexes

[Ir(OR)(cod)(MeIm(2-methoxybenzyl))]

observed. In addition, no evidence for the formation of the allylic

alcohol product, 4-phenylbutan-2-ol, was obtained even upon

heating at prolonged reaction times, as it was observed with other

transfer hydrogenation catalysts.^36,37The mechanism for the

selective formation of 4-phenylbutan-2-one probably involves

the 1,4-reduction of benzylideneacetone through a ketoꢀenol

tautomerization (Scheme 1).³⁸

Imines are harder to reduce by transfer hydrogenation catalysis

than aldehydes and ketones, and relatively few examples are

known.³⁹Imine hydrogenation has several important drawbacks

such as the imine nitrogen lone pair coordination to the metal

center, which inhibits the η²-(CdN) coordination required for

the insertion into metalꢀhydride bonds, or the poisoning eﬀect

of the resultant amines on the catalyst. However, recent progress

in the ﬁeld has led to the discovery of new catalytic systems with

high activities for the hydrogenation of imines.⁴⁰

monohydride pathway.⁴⁶In the same way, Crabtree et al. have

observed the specificity of deuterium incorporation in the

hydrogen transfer catalyzed by bis-NHC-Ir(III) complexes

using cyclohexanol-d₁, which also supports a monohydride

mechanism.^13c

The rhodium and iridium catalysts featuring hemilabile O- and

N-donor-functionalized NHC ligands required the presence of a

base for catalytic activity and, according to the above observa-

tions, most likely follow a monohydride pathway with a neutral

hydride/NHC as the catalytically active species. In order to gain

insight into the mechanism of hydrogen transfer using O-func-

tionalized-NHC iridium(I) complexes, we have tried to identify

the key intermediates likely involved in the process.

The relevant neutral alkoxo intermediates have been identiﬁed

by NMR. Thus, when a suspension of [IrBr(cod)(MeIm(2-

methoxybenzyl))] (7) in tetrahydrofuran-d₈was treated with 1

equiv of NaiPrO, a yellow-orange solution of [Ir(OiPr)-

(cod)(MeIm(2-methoxybenzyl))] (19) was immediately formed

(Scheme 2). The most signiﬁcant features in the ¹H NMR of 19

were the i-PrO^ꢀligand, which was observed at δ 3.67 (ꢀCH)

and 0.95 (ꢀCH₃) ppm, the downﬁeld shift of the >CH₂

resonance of the 2-methoxybenzyl fragment, and the high-ﬁeld

shift of the oleﬁn resonances of the cod ligand compared to 7.

The ESI+ mass spectra of solutions of 19 in 2-propanol do not

exhibit the molecular ion, although the species [19 ꢀ OiPr]⁺was

detected at m/z 503.1 (100%). When a 2-propanol solution of 19

was heated at 65 °C for a couple of minutes, the formation of

several unidentiﬁed hydride species was observed by NMR

together with free cyclooctene, also detected by GC/MS, result-

ing from the hydrogen transfer to coordinated 1,5-cycloocta-

diene. Interestingly, when compound 19 was heated in the

presence of cyclohexanone, an external hydride acceptor, cyclo-

hexanol and acetone were identiﬁed in the reaction mixture.

In the same way, the formation of the alkoxo compound

[Ir(OCy)(cod)(MeIm(2-methoxybenzyl))] (20) was observed

in the reaction of 7 with NaOCy in TDF-d₈. Solutions of

20 slowly decomposed at room temperature to give cyclo-

octene (NMR and GC/MS evidence). However, we have

observed that 20 reacts quickly with 2-propanol in TDF-d₈

to give 19 and HOCy in an alkoxide exchange reaction that is

a key step in most of the inner-sphere mechanistic proposals

(Scheme 2).

We have found that cationic complexes 13 and 14 are also

eﬃcient catalysts for transfer hydrogenation of imines, with 14

being considerably more active (entries 15 and 16). Thus, (E)-

N-benzylideneaniline was reduced to N-benzylaniline under

the standard conditions, reaching a 93% conversion in 1 h with

a TOF of 931 h^ꢀ1(entry 16). To the best of our knowledge,

the most active catalytic system described is the bis-NHC-

Ir(III) complex [(NHC)CH₂(NHC)IrI₂(OAc)],^13bhaving

triazole NHCs with neopentyl wingtip groups, with a TOF

of 6000 h^ꢀ1. The activity of 14 is similar to Shvo’s catalyst

(TOF = 800 h^{ꢀ1 41}

and better than triazole-NHC iridium(I)

)

complexes (TOF = 333 h^ꢀ1),⁴²[RhI₂(OAc)(n-BuIm(o-C₆H₄)-

Imn-Bu)] (TOF of 100 h^ꢀ1),^12dand [RuCl₂(PPh₃)(IPr(2,6-

py)IPr] (TOF = 213 h^ꢀ1).⁴³Additionally, the reduction of (E)-

N-benzylidenemethanamine by 14, a harder substrate, took

place at a moderate rate even at 1 mol % catalyst loading with a

TOF of 161 h^ꢀ1(entry 17).

Mechanistic Studies. The currently accepted putative me-

chanism for late-transition-metal-catalyzed hydrogen transfer

involves the participation of metal hydride species. H-transfer

to the unsaturated substrate can take place through an outer-

sphere mechanism with the participation of a ligand in the

catalytic reaction, metalꢀligand bifunctional catalysis,^10,44or

an inner-sphere mechanism when the substrate coordinates to

the metallic center during the catalytic process. In the later case,

different mechanisms involving monohydride or dihydride spe-

cies have been proposed.⁴⁵Studies on the mechanism of metal-

catalyzed hydrogen transfer from alcohols to ketones by mon-

itoring the racemization of (S)-α-deutero-α-phenylethanol have

established that most iridium and rhodium catalysts follow a

The stability of the O-functionalized-NHC iridium(I) catalytic

systems has been established by consecutive hydrogen transfer

reactions. Thus, to the solution obtained from a catalytic run

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base shows exclusively the presence of [7 ꢀ Br]⁺and [19 + Na]⁺

due to the complete transformation of 7 into the catalytic

intermediates.

The detection of the hydride species 21 and the remarkable

stability shown by complex 19, which was generated from 7 and

2-propanol under MALDI conditions, strongly support an inner-

sphere transfer hydrogenation mechanism with a monohydride

complex as the active catalytic species. The proposed mechanism

compatible with the information obtained from the diﬀerent

performed experiments is depicted in Scheme 3. The ﬁrst step is

the β-H elimination from the alkoxo complex [Ir(OiPr)(cod)-

(O-NHC)] (19) to generate the neutral hydride intermediate

[IrH(cod)(O-NHC)] (21) with the concomitant formation of

acetone (step i). The coordination of the ketone substrate to the

unsaturated hydride complex (step ii) and the migratory inser-

tion into the IrꢀH bond (step iii) result in the formation of a new

alkoxo complex. Finally, the protonation of the alkoxo ligand by

2-propanol (solvent) results in the formation of the reduced

substrate, regenerating the starting alkoxo complex in an alkoxide

exchange reaction (step iv).

Figure 3. Iridium species detected in the MALDI-TOF MS (linear

mode) of a 2-propanol solution of (a) [IrBr(cod)(MeIm(2-meth-

oxybenzyl))] (7); (b) 7 plus KOH, 1:5 ratio, registered immediately

at room temperature; (c) 7 plus KOH, 1:5 ratio, registered after 30 min

at room temperature.

In order to investigate the potential participation of iridium-

(III) species in the mechanism, we have synthesized the

cationic hydride complex [IrHBr(NCCH₃)(cod)(MeIm(2-

methoxybenzyl))]⁺(22) by reaction of 7 with 1 equiv of HBF₄

in acetone in the presence of 1 equiv of acetonitrile (eq 2). The

molecular structure of one cation of 22 together with the more

signiﬁcant bond distances and angles are presented in Figure 4.

Although two crystallographically independent molecules were

observed in the crystal structure, both were chemically identical

with minor torsion angle diﬀerences. The whole cationic com-

plexes resemble the structure of 7 where a hydride and an

acetonitrile ligand have been added below and above the metal

coordination plane. Thus, the cationic complexes in 22 could be

described as octahedral with signiﬁcant distortions associated

with the diﬀerent steric ligand requirements and the chelate

behavior of the cod molecule (C(1)ꢀIrꢀM(1) 170.50(11)°,

BrꢀIrꢀM(2) 171.26(8)°, N(3)ꢀIrꢀH 164(2)°, mean values,

M(1) and M(2) represent the midpoints of the oleﬁnic bonds

C(13)ꢀC(14) and C(17)ꢀC(18), respectively). If compared

with 7, the new ligands in the metal environment, together with

the oxidation of the metal center, have slightly longer bonding

distances for all ligands, particularly evidenced in the Irꢀoleﬁnic

bonds (IrꢀM(1) 2.072(5) in 7 vs 2.161(2) Å in 22, or IrꢀM(2)

1.980(4) in 7 vs 2.072(3) Å in 22). This relative elongation is not

particularly signiﬁcant for the NHC ligand (2.035(4) in 7 vs

2.049(3) Å in 22), which maintains identical symmetric bonding

distances within the imidazole moiety. Complex 22 features a

very long IrꢀN(3) bond distance trans to hydride, 2.156(3) Å,

signiﬁcantly longer than values found in other related acetonitrile-

dihydride NHC-Ir(III) complexes (range 2.098ꢀ2.135(5) Å),

where readily dissociable acetonitriles have been observed.^3e

using 14 /cyclohexanone/KOH in 2-propanol under the stan-

dard conditions (>90% conversion was attained in 12 min) was

added subsequently an additional 5 mmol of cyclohexanone.

Monitoring of the reaction by GC showed similar conversion

after the same reaction time, which is an indication of the catalyst

1

stability. Interestingly, the H NMR of the solid residue at the

end of the reaction allowed the identiﬁcation of compound 19 as

the resting state of the iridium catalyst. In addition, there was no

evidence for the loss of imidazolium salt by reductive elimination

in the likely [IrH(cod)(O-NHC)] (21) (O-NHC = MeIm(2-

methoxybenzyl)) intermediate.

We have studied the in situ generation of the active species

under catalytic hydrogen transfer conditions by ESI and MALDI-

TOF mass spectrometry.⁴⁷The ESI mass spectrum of a 1 ꢁ 10^ꢀ3

M solution of complex [IrBr(cod)(O-NHC)] (7) in 2-propanol

under argon atmosphere (diluted with acetonitrile to 1 ꢁ 10^ꢀ6

M, manual injection) showed the molecular ion protonated at

m/z 583.1 (41%) and the peaks at m/z 203.0 (74%) and 705.1

(57%), which correspond to the imidazolium cation [HꢀOꢀ

NHC]⁺and the bis-carbene species [Ir(O-NHC)₂]⁺probably

generated under the operating conditions. The ESI mass spec-

trum after the addition of a solution of KOH (1:5), at room

temperature or after heating at 80 °C, was basically identical,

although a tiny peak at m/z 504.8 (10%) with the right isotopic

distribution for the species [IrH(cod)(O-NHC)] (21) was

observed (see Supporting Information).

The excessive fragmentation observed under ESI conditions

suggests the presence of metastable species. In order to get

further information on the catalytic intermediates, MALDI-TOF

mass spectra were recorded in linear mode⁴⁸(Figure 3). The

mass spectra of a 2-propanol solution of 7 showed the molecular

ion at m/z 582.0 and the ion resulting from the loss of the bromo

ligand [Ir(cod)(O-NHC)]⁺, [7 ꢀ Br]⁺, at m/z 502.9. In

addition, a peak at m/z 585.3 (overlapped with the ﬁrst one),

the isotopic distribution of which corresponds to [Ir-

(OiPr)(cod)(O-NHC)Na]⁺, [19 + Na]⁺, was also observed

(Figure 3a). The mass spectra immediately after the addition of a

solution of KOH (1:5) shows both overlapping ions and a tiny

peak at m/z 526.0 that might correspond to the species

[IrH(cod)(O-NHC)Na]⁺([21 + Na]⁺, Figure 3b). Interest-

ingly, the spectrum recorded after 30 min of the addition of the

The ¹H NMR of 22 in CD₂Cl₂evidenced that the compound

exists in solution as two noninterconvertible isomers, 22a and

22b, in a 28/72 ratio. The major isomer 22b showed a somewhat

broad resonance for the hydride ligand at δ ꢀ14.30 ppm and a

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Scheme 3. Proposed Mechanism for the Catalytic Transfer

Hydrogenation of Ketones

Figure 4. Molecular structure of one independent cation of [IrHBr-

(NCCH₃)(cod)(MeIm(2-methoxybenzyl))][BF₄] complex (22). (Organic

hydrogen atoms have been omitted for clarity.) Selected bond lengths

(Å) and angles (deg) for the two independent cations: IrꢀBr 2.5040(8),

2.5068(8); IrꢀC(1) 2.052(5), 2.046(4); IrꢀN(3) 2.162(4), 2.151(4);

IrꢀH(1) 1.596(10), 1.583(10); IrꢀC(13) 2.261(5), 2.253(5); IrꢀC-

(14) 2.277(5), 2.281(5); IrꢀC(17) 2.183(5), 2.183(5); IrꢀC(18)

2.182(6), 2.192(5); N(1)ꢀC(1) 1.359(6), 1.357(6); N(2)ꢀC(1)

1.366(6), 1.368(6); C(13)ꢀC(14) 1.376(7), 1.384(7); C(17)ꢀC(18)

1.381(7), 1.391(8); BrꢀIrꢀN(3) 85.85(11), 84.35(12); N(3)ꢀIrꢀC-

(1) 94.07(18), 94.16(16); BrꢀIrꢀC(1) 89.81(14), 89.37(14); N-

(3)ꢀIrꢀH(1) 166(3), 162(2); BrꢀIrꢀH(1) 104(3), 79(2);

C(1)ꢀIrꢀH(1) 76(3), 78.1(17); C(1)ꢀN(1)ꢀC(2) 110.8(4),

111.1(4); IrꢀC(1)ꢀN(1) 131.4(4), 131.5(3); IrꢀC(1)ꢀN(2)

124.4(3), 124.8(3); IrꢀN(3)ꢀC(21) 174.1(4), 174.6(4).

labile acetonitrile ligand at δ 2.75 ppm that was found to undergo

exchange with CD₃CN. This dynamic behavior is probably due

to the strong trans eﬀect of the hydride ligand having an

acetonitrile ligand in trans position, and consequently, 22b is

the isomer characterized by X-ray diﬀraction. In contrast, the

minor isomer 22a is not dynamic and displayed sharp resonances

for both the hydride and acetonitrile ligands at δ ꢀ14.51 and 1.24

ppm and probably corresponds to the isomer having the bromo

ligand trans to the hydride ligand.

Interestingly, the formation of 22 is reversible, and the reaction

with 1 equiv of KOH, or even with water, gave the precursor

iridium(I) compound 7. In full agreement with this, both

compounds 7 and 22 exhibited identical catalytic activity for

the transfer hydrogenation of cyclohexanone, which supports the

participation of iridium(I) intermediates.

Influence of the Hemilabile Ligand on the Catalytic Activ-

ity. The hydrogen transfer catalytic activity exhibited by iridium

complexes with donor-functionalized NHC ligands of hemilabile

character evidences that O-donor-functionalized NHC ligands

provide much more active systems that the corresponding

N-functionalized ligands. Although the interplay of several

factors could be responsible for the activity differences, the

chelating effect of the ligands probably plays an important role.

The greater coordination ability of the donor set in the

N-functionalized NHC ligands became evident in the synthesis

of the cationic complexes 17 and 18, where the NHC ligand is k²-

C,N coordinated through the pyridine and dimethylamino

fragments, respectively. In contrast, the methoxy fragment in

the cationic complexes 13 and 14 remained uncoordinated, in

spite of their potential for bidentate coordination, and the

complexes were stabilized by the coordination of an acetonitrile

ligand. As a mechanism through 18-electron pentacoordinated

iridium(I) species is unlikely due to required coordination of the

substrate, the dissociation of the hemilabile fragment of the NHC

ligand could be a requisite for catalytic activity in order to drive

the reaction through square-planar iridium(I) intermediates

(Scheme 3).⁴⁹The easier dissociation of the coordinated acet-

onitrile ligand and the strong coordination ability of the N-

donor-functionalized NHC ligands for the iridium center could

account for the diﬀerent catalytic activity of both types of

complexes. However, the modest catalytic activity of complexes

17 and 18 contrasts with that exhibited by related iridium

complexes containing quinoline-functionalized NHC ligands.⁵⁰

On the other hand, the higher catalytic activity of the cationic

complexes, [Ir(NCCH₃)(cod)(O-NHC)]⁺, compared to the

neutral bromo complexes, [IrBr(cod)(O-NHC)], is probably a

consequence of the easier generation of the alkoxy intermediate

species [Ir(OiPr)(cod)(O-NHC)] from the cationic acetonitrile

complexes.

It became evident that the presence of a methoxy group with

weak coordination ability on the functionalized NHC ligands has

a considerable impact on the catalytic activity. However, this

eﬀect could be more probably associated with weak interactions

in the forward and backward hydrogen transfer from the hydride

intermediate [IrH(cod)(O-NHC)] (21) (hydride migration and

β-H transfer, steps ii and i, respectively, Scheme 3) rather than to

the interaction with the iridium center. The enhanced catalytic

activity of complex 14 compared to 13 should be ascribed to the

diﬀerent ﬂexibility of the O-functionalized NHC ligand. Then,

the rigid 2-methoxybenzyl substituent in 14 probably brings the

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Figure 5. Relative free energies (ΔG, kcal/mol) in the gas phase and geometries of the intermediate species for the β-H elimination in

[Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] (19).

ꢀOMe group close to the external coordination sphere of the

iridium center more eﬀectively than the highly ﬂexible 2-meth-

oxyethyl substituent (compound 13) as a consequence of a

proximity eﬀect. In order to test this hypothesis, we speculate

about the catalytic performance of complexes having more rigid

O-functionalized NHC ligands, as for example a 2-methoxyphe-

nyl-substituted NHC ligand.

The imidazolium salt 1-(2-methoxyphenyl)-3-methyl-1H-imi-

dazol-3-ium iodide (23) was obtained as a white hygroscopic

solid in excellent yield following the general method of synthesis

for arylimidazole derivatives.⁵¹The reaction of 23 with [{Ir(μ-

OMe)(cod)}₂] (0.5 equiv) in tetrahydrofuran gave the iridium-

(I) complex [IrI(cod)(MeIm(2-methoxyphenyl))] (24), which

was isolated as a yellow microcrystalline solid in good yield

(eq 3). Unfortunately, the attempts to prepare the cationic

complex [Ir(NCCH₃)_x(cod)(MeIm(2-methoxyphenyl))]⁺from

23 or using the silver-NHC complex were unsuccessful because

the compound was frequently obtained together with minor

unidentiﬁed hydride species.

(iodo vs bromo) could also contribute to the increase of

the activity. In fact, a catalytic test with the system 24/AgBF₄

(1 equiv), in order to generate the cationic complex in situ, gave

a 92% conversion in 23 min under the standard conditions with

a TOF of 2200 h^ꢀ1, just a half-fold reduction in catalytic activity

compared with 14 under the same conditions.

Role of the O-Donor Hemilabile Fragment: DFT Calcula-

tions. In order to understand the superior catalytic activity of the

complexes having O-donor-functionalized NHC ligands, we

have studied the key steps of the proposed mechanism by DFT

calculations. The unsaturated hydride intermediate [IrH(cod)(O-

NHC)] (21) (O-NHC = MeIm(2-methoxybenzyl)) plays a crucial

role in the catalytic cycle, and thus, its generation from the alkoxo

complex [Ir(OiPr)(cod)(O-NHC)] (19) by β-H elimination

has been investigated in detail (step i, Scheme 3). It is worth

noticing that the reduction of the substrate by this species (steps

ii and iii) results in the formation of an alkoxo intermediate, and

thus it can be considered as the reverse process.

β-H elimination from transition metal alkoxides is believed to

occur via a mechanism analogous to that established for transi-

tion metal alkyls that involved the initial β-H transfer to an

unsaturated metal center followed by elimination of the unsatu-

rated organic compound.⁵²An alternative two-step mechanism

involving σ-bond metathesis followed by oxidative addition has

been proposed in palladium chemistry.⁵³β-H elimination studies

on ruthenium, rhodium, and palladium alkoxide complexes have

shown that the formation of a metal vacant site prior to β-H

transfer is generally required.^49,53,54In the same way, β-H

elimination in the octahedral mer-cis-[Ir(Cl)(H)(OMe)(PMe₃)₃]

compound is promoted by the solvent-assisted dissociation of

the chloro ligand, followed by irreversible β-H transfer and

dissociative substitution of formaldehyde by chloride.⁵⁵

In line with our expectations, compound 24 is an eﬃcient

catalyst precursor for transfer hydrogenation of cyclohexanone

with 2-propanol. A conversion of 92% was attained in 50 min,

under the standard conditions (catalyst:substrate:KOH ratio of

1:1000:5 in 2-propanol at 80 °C), with an average TOF of 1100 h^ꢀ1

.

The catalytic activity of 24 is in the same range as that found for

the related neutral complexes but, interestingly, slightly more

active than 7 (TOF 824 h^ꢀ1). Nevertheless, in addition to the

expected ligand eﬀect, the presence of a better leaving ligand

The β-H elimination in square-planar iridium(I) alkoxides has

been studied recently by Macgregor and Vadivelu⁵⁶with regard

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to the reactions of trans-[Ir(OR)(CO)(PPh₃)₂] observed by

Atwood⁵⁷and the related mechanistic studies by Hartwig and co-

workers.⁵⁸The detailed mechanistic studies by Hartwig evi-

denced the involvement of 14-electron unsaturated species

generated by phosphine dissociation. Calculations by Macgregor

and Vadivelu on this mechanism have shown that the reaction

proceeds via an agostic in-plane intermediate that leads to the

rate-limiting β-elimination process. Theoretical analysis on

the alkoxo complex [Ir(OiPr)(cod)(O-NHC)] (19) has re-

vealed that the β-H elimination process is possible without the

need of ligand dissociation. The analysis has been performed by

DFT using the B3LYP functional and a 6-31G** basis set for

all atoms but Ir, for which the LANL2DZ basis and asso-

ciated pseudopotential was used. The complex was not mod-

elized, and instead its full composition was used in the

calculations.

The geometry of the two isomers of the square-planar alkoxo

intermediate 19 is shown in Figure 5. Both isomers are virtually

of the same energy, with 19b, the isomer having the β-H of

the alkoxo ligand directed toward the NHC ligand, being only

ΔG = +0.8 kcal/mol less stable than isomer 19a, with the β-H

directed to the cod ligand. The possibility of some interaction

between the β-H on the alkoxo ligand and the oxygen atom of the

2-methoxybenzyl substituent on the NHC ligand was analyzed

by approaching this group and fully optimizing the resulting

structure. This leads to a third isomer, 19c, a rotamer in fact,

ligands, as well as the charge of the complexes. Iridium complexes

based on O-donor-functionalized NHC ligands were found to be

much more active than the corresponding N-functionalized

ligands, which is related to the strong coordination ability of

the dimethylamino donor set compared to methoxy, because

the dissociation of the hemilabile fragment is required for the

formation of square-planar alkoxo iridium(I) intermediates. In

fact, the higher catalytic activity of the cationic complexes

compared to the neutral ones is also a consequence of the

easier generation of the alkoxo intermediate species. Investiga-

tion of the reaction mechanism in combination with DFT

calculations have allowed to disclose a potential role of the

hemilabile fragment on the NHC ligands that could be respon-

sible for the observed positive eﬀect on the catalytic activity.

The interaction between the β-H on the alkoxo ligand and the

oxygen atom of the methoxy fragment of the NHC ligand

results in a net destabilization of the alkoxo intermediate that

facilitates the β-H elimination step in route to the key hydrido

intermediate species.

’ EXPERIMENTAL SECTION

Scientific Equipment. C, H, and N analyses were carried out in a

Perkin-Elmer 2400 Series II CHNS/O analyzer. Infrared spectra were

recorded on a FT-Perkin-Elmer Spectrum One spectrophotometer

using Nujol mulls between polyethylene sheets. NMR spectra were

recorded on a Bruker Avance 300 or a Bruker Avance 400 spectrometer.

¹H (300.1276 MHz, 400.1625 MHz) and ¹³C (75.4792 MHz, 100.6127

MHz) NMR chemical shifts are reported in ppm relative to tetramethyl-

silane and referenced to partially deuterated solvent resonances. Cou-

pling constants (J) are given in hertz. Spectral assignments were

which shows a close O H interaction (2.98 Å) and a free

3 3 3

energy of just +1.0 kcal above 19a and 0.2 kcal above 19b.

By moving the H atom closer to the iridium center, in order to

modelize the β-H elimination process, the intermediate 19⁰was

found. This intermediate shows a signiﬁcant agostic interaction

1

13

achieved by combination of Hꢀ H COSY, ¹³C APT, and Hꢀ C

HSQC experiments. MALDI-TOF mass spectra were obtained on a

Bruker MICROFLEX spectrometer using DCTB (trans-2-[3-(4-tert-

butylphenyl)-2-methyl-2-propenylidene]malononitrile) as a matrix.⁵⁹

Electrospray mass spectra (ESI-MS) were recorded on a Bruker Micro-

Tof-Q using sodium formiate as reference. Conductivities were mea-

sured in ca. 5 ꢁ 10^ꢀ4M acetone solutions of the complexes using a

Philips PW 9501/01 conductimeter.

of the β-H with the iridium center with distances of Ir H =

3 3 3

1.86 Å and C H = 1.19 Å. The formation of 19⁰requires the

3 3 3

anticlockwise rotation about the IrꢀO bond in 19c to facilitate

the η²-CH interaction (Ir C = 2.49 Å), which results in a

3 3 3

constrained geometry of the alkoxo ligand (IrꢀOꢀC = 84.7^o).

The intermediate 19⁰has a free energy of +26.6 kcal/mol relative

to 19a, which is comparable to that found (ca. 30 kcal/mol) for

agostic intermediates derived from iridium unsaturated species

generated by ligand dissociation.⁵⁶Finally, the β-elimination

process proceeds, leading to the square-planar hydrido com-

plex [IrH(cod)(O-NHC)] (21), with a IrꢀH bond distance of

1.64 Å, and the prompt elimination of ketone; this process

releases a free energy of ꢀ6.5 kcal/mol relative to 19a.

Organic compounds were identiﬁed by gas chromatographyꢀmass

spectrometry (GC/MS) using an Agilent 6890 GC system with an

Agilent 5973 MS detector, equipped with a polar capillary column HP-

5MS (df = 0.25 μm, 30 m ꢁ 0.25 mm i.d.). The catalytic reactions were

analyzed on an Angilent 4890 D system equipped with an HP-

INNOWax capillary column (df = 0.4 μm, 25 m ꢁ 0.2 mm i.d.) using

mesitylene as internal standard.

Synthesis. All experiments were carried out under an atmosphere of

argon using Schlenk techniques. The solvents were distilled immediately

prior to use from the appropriate drying agents or obtained from a

solvent purification system (Innovative Technologies). Oxygen-free

solvents were employed throughout. CDCl₃and CD₂Cl₂were dried

using activated molecular sieves, and methanol-d₄(<0.02% D₂O) was

purchased from Euriso-top and used as received. MeImH (N-

methylimidazole) was obtained from Sigma-Aldrich and distilled prior

to use. The substrates were obtained from common commercial sources

and used as received or recrystallized or distilled prior to use depending

on their purity. The imidazolium salts [MeImH(CH₂)₂OMe]Br (1),¹⁹

[MeImH(pyridin-2-ylmethyl)]Br (3),²⁰and [MeImH(CH₂)_nNHMe₂)]-

Cl₂(n = 2, 4; n = 3, 5)^6aand the starting materials [{Ir(μ-Cl)(cod)}₂]⁶⁰

and [{Ir(μ-OMe)(cod)}₂]⁶¹were prepared according to literature

procedures.

The described results outline the role of the methoxy fragment

of the substituent in the NHC ligand and its inﬂuence on the

catalytic activity. The interaction between the β-H on the

alkoxo ligand and the oxygen atom of the methoxy fragment

results in a net destabilization of the alkoxo intermediate by a

free energy of 1.0 kcal/mol, probably due to the induced steric

eﬀects. Thus, the destabilization of 19 reduces the activation

barrier leading to the intermediate 19⁰, facilitating the β-H

elimination step in the catalytic process. This fact could explain

the positive eﬀect of the methoxy group of the functionalized

NHC ligands on the catalytic activity.

’ CONCLUSIONS

Iridium(I) complexes having hemilabile O- and N-donor-

functionalized NHC ligands are eﬃcient catalyst precursor for

the transfer hydrogenation of unsaturated compounds in 2-pro-

panol/KOH. The catalytic activity is strongly dependent on the

donor function and ﬂexibility of the backbone in the NHC

Preparation of 1-(2-Methoxybenzyl)-3-methyl-1H-imida-

zol-3-ium Bromide (2). N-Methylimidazole (1.15 g, 14.0 mmol) was

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ARTICLE

added to a solution of 2-methoxybenzyl bromide (14.0 mmol, obtained

in situ from 2-methoxybenzyl alcohol, 1.93 g, and PBr₃, 1.33 g)²¹in

toluene (15 mL), and the mixture refluxed for 12 h. The white solid

formed was separated by decantation, washed with hot n-hexane, and

dried under vacuum. Yield: 97%. Anal. Calcd for C₁₂H₁₅BrN₂O: C,

50.90; H, 5.34; N, 9.89. Found: C, 51.23; H 6.01; N, 9.93. ¹H NMR (298

K, CD₃OD): δ 10.37 (s, 1H, NCHN), 7.59 (dd, J = 7.4, 1.4, 1H, CH Ar),

7.51, 7.35 (m, 1H, 1H, CH Ar), 6.95 (td, J = 7.4, 1.0, 1H, CH Ar), 6.91 (s,

1H, CH Im), 6.88 (s, 1H, CH Im), 5.47 (s, 2H, NCH₂), 4.08 (s, 3H,

OMe), 3.89 (s, 3H, MeIm). ¹³C{¹H} NMR (298 K, CDCl₃): δ 157.45

(C Ar), 137.41 (NCHN), 131.49, 131.48 (CH Ar), 123.33, 122.13 (CH

Im), 121.32 (C Ar), 121.24, 110.97 (CH Ar), 55.79 (OMe), 48.79

(CH₂), 36.75 (MeIm). ESI-MS (CH₃OH): m/z 203.2 [M]⁺.

General Procedure for the Preparation of [MBr(cod)-

(MeImXOMe)] (M = Rh, Ir; X = (CH₂)₂, CH₂C₆H₄). The rhodium

and iridium complexes containing methoxy-functionalized NHC ligands

were synthesized through the following procedure. A mixture of [{M(μ-

OMe)(cod)}₂] (M = Rh, Ir) and [MeImHXOMe)]Br in THF (10 mL)

was stirred overnight at room temperature to give an orange suspension.

The solid was removed by filtration, and the resulting orange solution

was concentrated and evaporated to dryness. Treatment of the yellow

residue with pentane rendered a yellowish solid, which was separated by

decantation, washed with pentane, and dried under vacuum.

DCTB matrix, CH₂Cl₂): m/z 431.2 [M]⁺, 351.8 [M ꢀ Br]⁺. Λ_M

(acetone): 0.6 Ω^ꢀ1cm²mol^ꢀ1

.

[RhBr(cod)(MeIm(2-methoxybenzyl))] (9). [{Rh(μ-OMe)-

(cod)}₂] (150 mg, 0.310 mmol) and [MeImH(2-methoxybenzyl)]Br

(166 mg, 0.620 mmol) were used. Yield: 90%. Anal. Calcd for

C₂₀H₂₆BrN₂ORh: C, 48.70; H, 5.31; N, 5.68. Found: C, 49.16; H,

5.68; N, 5.50. ¹H NMR (298 K, CDCl₃): δ 7.32, 6.94 (m, 2H, 2H, CH

Ar), 6.78, 6.70 (s, 1H, 1H, CH Im), 5.60 (AB system, δ_A= 5.74, δ_B=

5.46, J_AB= 14.7, 2H, NCH₂), 5.12 (m, 2H, CH cod), 4.11 (s, 3H, OMe),

3.88 (s, 3H, MeIm), 3.44 (br, 2H, CH cod), 2.37, 1.91 (m, 4H, 4H, CH₂

cod). ¹³C{¹H} NMR (CDCl₃): δ 182.81 (d, J_RhꢀC= 50.0, NCN),

157.75 (C Ar), 130.78, 129.88 (CH Im), 125.18 (CH Ar), 122.50 (C

Ar), 121.12, 121.05, 110.89 (CH Ar), 97.84 (d, J_RhꢀC= 6.7, CH cod),

97.70 (d, J_RhꢀC= 6.7, CH cod), 69.65 (d, J_RhꢀC= 14.6, CH cod), 69.13

(d, J_RhꢀC= 14.5, CH cod), 55.75 (OMe), 49.41 (NCH₂), 38.06

(MeIm), 33.47, 32.70, 29.75, 29.21 (CH₂cod). MS (MALDI-TOF,

DCTB matrix, CH₂Cl₂) m/z

=

413.5 [M

ꢀ

Br]⁺, 203.5

[MeHIm(CH₂)C₆H₄OMe]⁺. Λ_M(acetone): 1.4 Ω^ꢀ1cm²mol^ꢀ1

.

Preparation of [IrBr(cod)(MeIm(pyridin-2-ylmethyl))] (10).

A mixture of [{Ir(μ-OMe)(cod)}₂] (100 mg, 0.150 mmol) and

[MeImH(pyridin-2-ylmethyl)]Br (76.6 mg, 0.302 mmol) in acetone

(10 mL) was stirred for 4 h at room temperature, giving an orange

suspension. The solid was removed by filtration, and the resulting

solution was evaporated to dryness. The compound was extracted with

diethyl ether, and the solution concentrated to ca. 1 mL and then treated

with n-hexane until a solid was formed. The orange solid was isolated by

decantation, washed with n-hexane (2 ꢁ 2 mL), and dried under

vacuum. Yield: 81%. Anal. Calcd for C₁₈H₂₃BrN₃Ir: C, 39.06; H, 4.19;

N, 7.59. Found: C, 39.21; H, 4.15; N, 8.13. ¹H NMR (298 K, CDCl₃): δ

8.62 (d, J = 5.5, 1H, CH py), 7.64 (ddd, J = 7.9, 7.8, 1.7, 1H, CH py), 7.40

(d, J = 7.8, 1H, CH py), 7.17 (dd, J = 7.9, 5.5, 1H, CH py), 6.86, (s, J =

1.9, 1H, CH Im), 6.75 (d, J = 1.9, 1H, CH Im), 5.53 (AB system, δ_A=

5.73, δ_B= 5.32, J_AB= 14.6, 2H, NCH₂), 4.65, 4.37 (br, 1H, 1H, CH cod),

3.91 (s, 3H, MeIm), 2.82, 2.69 (br, 1H, 1H, CH cod), 2.22, 2.02, 1.61,

1.50 (m, 2H, 2H, 2H, 2H, CH₂cod). ¹³C{¹H} NMR (298 K, CDCl₃): δ

155.59 (C py), 150.74, 136.88, 123.58, 122.89 (CH py), 122.02, 120.38

(CH Im), 101.32, 99.58 (br, CH cod), 56.00 (NCH₂), 52.75, 51.00 (br,

CH cod), 37.65 (MeIm), 32.15, 31.50, 29.67, 28.36 (CH₂cod). MS

(MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z 554.1 [M + H]⁺, 474.0

[IrBr(cod)(MeIm(CH₂)₂OMe)] (6). [{Ir(μ-OMe)(cod)}₂] (270

mg, 0.407 mmol) and [MeImH(CH₂)₂OMe)]Br (180 mg, 0.814 mmol)

were used. Yield: 74%. Anal. Calcd for C₁₅H₂₄BrN₂OIr: C, 34.61; H,

1

4.65; N, 5.38. Found: C, 34.31; H, 4.59; N, 5.36. H NMR (298 K,

CDCl₃): δ 7.01, (d, J = 1.9, 1H, CH Im), 6.77 (d, J = 1.9, 1H, CH Im),

4.71 (ddd, J = 13.9, 4.8, 3.5, 1H, NCH₂), 4.58 (m, 2H, CH cod), 4.36

(ddd, J = 14.1, 7.6, 4.3, 1H, NCH₂), 3.90 (s, 3H, MeIm), 3.76

(m, 2H, CH cod), 3.35 (s, 3H, OMe), 2.95 (ddd, J = 7.0, 7.0, 3.2, 1H,

CH₂O), 2.84 (ddd, J = 7.0, 7.0, 3.2, 1H, CH₂O), 2.17, 1.72, 1.49 (m, 4H,

2H, 2H, CH₂cod). ¹³C{¹H} NMR (298 K, CDCl₃): δ 180.16 (NCN),

121.72, 120.60 (CH Im), 84.49, 83.92 (CH cod), 72.12 (CH₂O), 59.50

(OMe), 52.24, 51.99 (CH cod), 50.03 (NCH₂), 37.34 (MeIm), 33.36,

33.24, 29.93, 29.74 (CH₂cod). MS (MALDI-TOF, DCTB matrix,

CH₂Cl₂) m/z: 522.5 [M + H]⁺, 441.0 [M ꢀ Br]⁺. Λ_M(acetone): 16

Ω

^ꢀ1cm²mol^ꢀ1

.

[IrBr(cod)(MeIm(2-methoxybenzyl))] (7). [{Ir(μ-OMe)(cod)}₂]

[M ꢀ Br]⁺. Λ_M(acetone): 15 Ω^ꢀ1cm²mol^ꢀ1

.

(125 mg, 0.188 mmol) and [MeImH(2-methoxybenzyl)]Br (107 mg, 0.377

mmol) were used. Yield: 73%. Anal. Calcd for C₂₀H₂₆BrN₂OIr: C, 41.23; H,

4.50; N, 4.81. Found: C, 41.38; H, 4.56; N, 4.80. ¹H NMR (298 K, CDCl₃):δ

7.33ꢀ7.18, 6.93ꢀ6.80 (m, 2H, 2H, CH Ar), 6.70 (d, J = 1.6, 1H, CH Im),

Preparation of [IrCl(cod)(MeIm(CH₂)_nNMe₂)] (n = 2, 3).

[MeImH(CH₂)_nNHMe₂)]Cl₂(0.301 mmol) was added to a solution of

[{Ir(μ-OMe)(cod)}₂] (100 mg, 0.150 mmol) in tetrahydrofuran

(10 mL), and the solution was stirred overnight. The resulting suspen-

sion was treated with NaH (7.22 mg, 0.301 mmol) and H₂O (0.1 mL)

and stirred for 10 min. The solid was removed by filtration to give a

yellow solution, which was evaporated until ca. 1 mL. The pale yellow

solids formed after treatment with n-hexane were washed with diethyl

ether, separated by decantation, and dried under vacuum.

6.65 (d, J = 1.6, 1H, CH Im), 5.53 (AB system, δ_A= 5.70, δ_B= 5.42, J_AB

=

14.7, 2H, NCH₂), 4.61 (m, 2H, CH cod), 3.88 (s, 3H, OMe), 3.80 (s, 3H,

MeIm), 2.96 (br, 2H, CH cod), 2.13, 2.01, 1.70, 1.61, 1.48, 1.37 (m, 3H, 1H,

1H, 1H, 1H, 1H, CH₂cod). ¹³C{¹H} NMR (CDCl₃): δ 178.37 (NCN),

155.23 (C Ar), 129.12, 127.22 (CH Im), 124.82 (C Ar), 123.25, 121.12,

120.45, 110.25 (CH Ar), 83.85, 82.95 (CH cod), 55.24 (OMe), 52.51, 52.02

(CH cod), 49.12 (NCH₂), 37.51 (MeIm), 33.54, 32.21, 29.58, 28.12 (CH₂

cod). MS (MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z 582.2 [M]⁺, 503.3

[IrCl(cod)(MeIm(CH₂)₂NMe₂)] (11). Yield: 69%. Anal. Calcd for

C₁₆H₂₇ClN₃Ir: C, 39.29; H, 5.56; N, 8.59. Found: C, 39.11; H, 5.32; N,

8.11. ¹H NMR (298 K, CDCl₃): δ 6.99 (d, J = 2.0, 1H, CH Im), 6.77 (d,

J = 2.0, 1H, CH Im), 4.51 (m, 2H, CH cod and 1H, NCH₂), 4.31 (m, 1H,

NCH₂), 3.88 (s, 3H, MeIm), 2.91, 2.81 (m, 1H, 1H, CH cod), 2.70 (m,

2H, CH₂N), 2.21 (s, 6H, NMe₂), 2.14, 1.72, 1.60, 1.51 (m, 4H, 1H, 2H,

1H, CH₂cod). ¹³C{¹H} NMR (298 K, CDCl₃): δ 180.05 (NCN),

121.15, 120.06 (CH Im), 85.58, 84.32 (CH cod), 60.24 (NCH₂), 53.15,

51.58 (CH cod), 49.78 (CH₂N), 44.85 (NMe₂), 39.78 (MeIm), 35.56,

33.11, 29.10, 27.85 (CH₂cod). MS (MALDI-TOF, DCTB matrix,

CH₂Cl₂): m/z 489.0 [M]⁺, 454.0 [M ꢀ Cl]⁺. Λ_M(acetone): 14 Ω^ꢀ1

[M ꢀ Br]⁺. Λ_M(acetone): 14 Ω^ꢀ1cm²mol^ꢀ1

.

[RhBr(cod)(MeIm(CH₂)₂OMe)] (8). [{Rh(μ-OMe)(cod)}₂] (370

mg, 0.764 mmol) and [MeImH(CH₂)₂OMe)]Br (338 mg, 1.53 mmol)

were used. Yield: 64%. Anal. Calcd for C₁₅H₂₄BrN₂ORh: C, 41.78; H,

1

5.61; N, 6.49. Found: C, 41.42; H, 5.54; N, 6.30. H NMR (298 K,

CDCl₃): δ 6.95, 6.71 (s, 1H, 1H, CH Im), 4.97 (m, 2H, CH cod), 4.81,

4.40 (m, 1H, 1H, >CH₂), 4.11 (s, 3H, OMe), 3.76 (m, 2H, >CH₂), 3.28

(s, 3H, MeIm), 3.21 (br, 2H, CH cod), 2.26, 1.86 (m, 4H, 4H, CH₂cod).

¹³C{¹H} NMR (CDCl₃): δ 181.99 (d, J_RhꢀC= 50.5, NCN), 121.96,

121.77 (CH Im), 98.00 (d, J_RhꢀC= 6.5, CH cod), 97.82 (d, J_RhꢀC= 6.6,

CH cod), 72.13 (CH₂O), 69.06 (d, J_RhꢀC= 11.1, CH cod), 68.91

(d, J_RhꢀC= 11.0, CH cod), 58.97 (OMe), 50.48 (NCH₂), 37.69

(MeIm), 32.77, 32.67, 29.15, 29.06 (CH₂cod). MS (MALDI-TOF,

cm²mol^ꢀ1

.

[IrCl(cod)(MeIm(CH₂)₃NMe₂)] (12). Yield: 72%. Anal. Calcd for

C₁₇H₂₉ClN₃Ir: C, 40.50; H, 5.81; N, 8.55. Found: C, 39.89; H, 5.76;

N, 8.39. ¹H NMR (298 K, CDCl₃): δ 6.81 (d, J = 1.8, 1H, CH Im), 6.73

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(d, J = 1.8, 1H, CH Im), 4.56ꢀ4.44 (m, 2H, CH cod and 1H, NCH₂),

4.19 (ddd, J = 15.1, 8.8, 6.6, 1H, NCH₂), 3.88 (s, 3H, MeIm), 2.91, 2.80

(m, 1H, 1H, CH cod), 2.31 (m, 2H, CH₂), 2.21 (s, 6H, NMe₂), 2.14 (m,

4H, CH₂cod), 1.92 (m, 1H, CH₂), 1.75ꢀ1.42 (m, 4H, 4H, CH₂cod),

0.78 (m, 1H, CH₂). ¹³C{¹H} NMR (298 K, CDCl₃): δ 121.94, 120.61

(CH Im), 84.81, 84.60 (CH cod), 55.66 (NCH₂), 52.30, 51.38 (CH

cod), 47.86 (CH₂N), 45.00 (NMe₂), 37.61 (MeIm), 33.91, 33.17 (CH₂

cod), 32.15 (CH₂), 29.96, 29.11 (CH₂cod). MS (MALDI-TOF, DCTB

matrix, CH₂Cl₂): m/z 504.2 [M + H]⁺, 468.2 [M + H ꢀ Cl]⁺. Λ_M

NCCH₃), 2.26, 1.82 (m, 4H, 4H, CH₂cod). ¹³C{¹H} NMR (298 K,

CDCl₃): δ 176.70 (d, J_RhꢀC= 45.0, NCN), 157.37 (C Ar), 129.75, 129.52

(CH Im), 129.56 (NCCH₃), 123.50 (C Ar), 123.00, 120.78, 111.03, 110.42

(CH Ar), 99.17, 67.51 (CH cod), 55.51 (OMe), 49.85 (NCH₂), 37.67

(MeIm), 32.23, 28.18 (CH₂cod), 3.58 (NCCH₃). MS (MALDI-TOF,

DCTB matrix, CH₂Cl₂): m/z 413.4 [M ꢀ CH₃CN]⁺. Λ_M(acetone):

72 Ω^ꢀ1cm²mol^ꢀ1

.

Preparation of [Ir(cod)(MeIm(pyridin-2-ylmethyl))][BF₄]

(17). The synthesis was carried out in two steps. Step 1: A mixture of

[MeImH(pyridin-2-ylmethyl)]Br (3) (254 mg, 1.00 mmol) and Ag₂O

(116 mg, 0.5 mmol) was refluxed in dichloromethane (20 mL) for

90 min. The excess of Ag₂O was removed by filtration to give a colorless

solution of the NHC-silver complex. Step 2: The solvato complex

[Ir(cod)(OCMe₂)₂]⁺(1 mmol), prepared in situ from AgBF₄(195 mg,

1.00 mmol) and [{Ir(μ-Cl)(cod)}₂] (336 mg, 0.500 mmol) in acetone

(15 mL), was added to concentrated solutions of the NHC-silver

complexes in CH₂Cl₂(1 mL) obtained from step 1. The mixture was

stirred for 3 h at room temperature to give a brownish suspension. The

AgX formed was filtered off to give an orange solution. The solvent was

removed under vacuum and the residue treated with cold pentane

several times to afford the compound as an orange solid, which was

separated by decantation and dried under vacuum. Yield: 77%. Anal.

Calcd for C₁₈H₂₃BF₄N₃Ir: C, 38.58; H, 4.14; N, 7.50. Found: C, 38.57;

H, 4.11; N, 7.51. ¹H NMR (298 K, acetone-d₆): δ 8.78 (d, J = 5.3, 1H,

CH py), 8.09 (td, J = 7.6, 1.5, 1H, CH py), 7.94 (m, 1H, CH py), 7.62

(ddd, J = 7.6, 5.3, 1.5, 1H, CH py), 7.49 (d, J = 1.7, 1H, CH Im), 7.24 (d,

J = 1.7, 1H, CH Im), 5.75 (AB system, δ_A= 5.77, δ_B= 5.63, J_AB= 14.3,

2H, NCH₂), 4.60, 4.46, 4.15, 4.07 (m, 1H, 1H, 1H, 1H, CH cod), 3.87 (s,

3H, MeIm), 2.45, 2.15, 1.77 (m, 2H, 4H, 2H, CH₂cod). ¹³C{¹H} NMR

(acetone-d₆): δ 173.47 (NCN), 152.90 (C py), 151.86, 140.19, 126.53,

125.79 (CH py), 123.34, 121.62 (CH Im), 84.53, 83.70, 65.44 (CH

cod), 65.23 (NCH₂), 58.77 (CH cod), 36.97 (MeIm), 32.76, 32.44,

29.84 (CH₂cod). MS (MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z

(acetone): 10 Ω^ꢀ1cm²mol^ꢀ1

.

General Procedure for the Preparation of [M(NCCH₃)-

(cod)(MeImXOMe)][BF₄] (M = Rh, Ir; X = (CH₂)₂, CH₂C₆H₄).

AgBF₄(1 mmol) was added to a solution of the corresponding complex

[MBr(cod)(MeImXOMe)] (1 mmol) in acetone (15 mL) and CH₃CN

(52 μL, 1 mmol). After 2 h of stirring at room temperature, the AgCl

formed was filtered off to give a yellow solution. The solvent was

pumped off, and the dried oil residue was treated with cold pentane to

give yellowish-orange solids, which were separated by decantation,

washed with pentane (2 ꢁ 2 mL), and dried under vacuum.

[Ir(NCCH₃)(cod)(MeIm(CH₂)₂OMe)][BF₄] (13). [IrBr(cod)(MeIm-

(CH₂)₂OMe)] (6) (100 mg, 0.190 mmol) and AgBF₄(37.4 mg, 0.190

mmol) were used. Yield: 67%. Anal. Calcd for C₁₇H₂₇BF₄N₃OIr: C, 35.92; H,

4.79; N, 7.39. Found: C, 36.03; H, 4.80; N, 6.99. ¹HNMR(acetone-d₆):δ7.37,

7.31 (s, 1H, 1H, CH Im), 4.66 (br, 2H, CH cod), 4.53 (m, 2H, CH₂O), 4.03

(s, 3H, OMe), 3.84 (m, 2H, NCH₂), 3.70 (br, 2H, CH cod), 3.37 (s, 3H,

MeIm), 2.62 (s, 3H, NCCH₃), 2.30, 1.90 (m, 4H, 4H, CH₂cod). ¹³C{¹H}

NMR (acetone-d₆): δ 130.13 (NCCH₃), 123.12, 121.97 (CH Im), 82.73

(CH cod), 71.27 (CH₂O), 58.12 (OMe), 50.07 (NCH₂), 36.85 (MeIm),

32.55, 29.27 (CH₂cod), 2.16 (NCCH₃). MS (MALDI-TOF, DCTB matrix,

CH₂Cl₂): m/z 439.1 [M ꢀ CH₃CN]⁺. Λ_M(acetone): 73 Ω^ꢀ1cm²mol^ꢀ1

.

[Ir(NCCH₃)(cod)(MeIm(2-methoxybenzyl))][BF₄] (14). [IrBr-

(cod)(MeIm(2-methoxybenzyl))] (7) (100 mg, 0.172 mmol) and AgBF₄

(33.4 mg, 0.172 mmol) were used. Yield: 61%. Anal. Calcd for

C₂₂H₂₉BF₄N₃OIr: C, 41.91; H, 4.63; N, 6.66. Found: C, 41.50; H, 4.65;

N, 6.40. ¹H NMR (298 K, acetone-d₆): δ 7.73ꢀ7.22 (m, 3H, CH-Ar), 7.28,

7.09 (s 1H, 1H, CH-Im), 7.03 (dd, J = 7.1, 1H, CH Ar), 5.53 (AB system,

δ_A= 5.55, δ_B= 5.51, J_AB= 14.2, 2H, NCH₂), 4.43 (br, 2H, CH cod), 4.01 (s,

3H, OMe), 3.90 (s, 3H, MeIm), 3.53 (br, 2H, CH cod), 2.44 (s, 3H,

NCCH₃), 2.31, 1.80 (m, 4H, 4H, CH₂cod). ¹³C{¹H} NMR (298 K,

CD₂Cl₂): δ 179.51 (NCN), 157.33 (C Ar), 130.26, 129.91 (CH Im),

129.56 (NCCH₃), 123.20, 122.01, 120.00, 110.42 (CH Ar), 120.68 (C Ar),

83.39, 82.98, 65.98, 63.78 (CH cod), 55.39 (OMe), 49.82 (NCH₂), 37.59

(MeIm), 33.18, 32.23, 29.91, 29.19 (CH₂cod), 3.58 (NCCH₃). MS

(MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z 503.2 [M ꢀ CH₃CN]⁺.

474.2 [M]⁺. Λ_M(acetone): 67 Ω^ꢀ1cm²mol^ꢀ1

.

Preparation of [Ir(cod)(MeIm(CH₂)₃NMe₂)][BF₄] (18). The

compound was prepared from [MeImH(CH₂)₃NHMe₂)]Cl₂(5) (120

mg, 0.500 mmol), Ag₂O (57.9 mg, 0.250 mmol), and [Ir(cod)-

(OCMe₂)₂]BF₄(0.500 mmol) following the two-step procedure de-

scribed above for 17. Yield: 60%. Anal. Calcd for C₁₇H₂₉BF₄N₃Ir: C,

36.83; H, 5.27; N, 7.59. Found: C, 36.98; H, 6.79; N, 7.51. ¹H NMR (298

K, CDCl₃): δ 7.32 (s, 1H, CH Im), 7.01 (s, 1H, CH Im), 4.76 (m, 2H,

CH₂N), 4.40 (m, 1H, NCH₂), 4.25 (br, 2H, CH cod), 4.16 (m, 1H,

NCH₂), 3.93 (s, 3H, MeIm), 3.36 (m, 2H, CH cod), 2.56 (s, 6H, NMe₂),

2.35 (m, 1H, CH₂), 2.30 (m, 4H, CH₂cod), 1.93 (m, 1H, CH₂), 1.75

(m, 4H, CH₂cod). ¹³C{¹H} NMR (298 K, CDCl₃): δ 173.62 (NCN),

123.37, 12183 (CH Im), 85.17, 83.24, 70.70 (CH cod), 66.08

(NCH₂), 65.32 (CH cod), 47.10 (CH₂N), 44.20 (NMe₂), 37.34

(MeIm), 33.45, 32.10 (CH₂cod), 31.23 (CH₂), 29.07, 26.60 (CH₂

cod). MS (MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z 468.2 [M]⁺.

Λ_M(acetone): 69 Ω^ꢀ1cm²mol^ꢀ1

.

[Rh(NCCH₃)(cod)(MeIm(CH₂)₂OMe)][BF₄] (15). [RhBr(cod)-

(MeIm(CH₂)₂OMe)] (8) (100 mg, 0.232 mmol) and AgBF₄(45.2 mg,

0.232 mmol) were used. Yield: 65%. Anal. Calcd for C₁₇H₂₇BF₄N₃ORh:

C, 42.62; H, 5.68; N, 8.77. Found: C, 42.99; H, 5.90; N, 8.55. ¹H NMR

(CD₂Cl₂): δ 7.15, 7.04 (s, 1H, 1H, CH Im), 4.87 (br, 2H, CH cod), 4.64

(m, 2H, CH₂O), 4.04 (s, 3H, OMe), 3.81 (m, 4H, NCH₂, CH cod), 3.38

(s, 3H, MeIm), 2.47 (m, 4H, CH₂cod), 2.31 (s, 3H, NCCH₃), 2.08 (m,

4H, CH₂cod). ¹³C{¹H} NMR (CD₂Cl₂): δ 125.06 (NCCH₃), 123.54,

122.56 (CH Im), 97.78, 77.75 (CH cod), 71.89 (CH₂O), 59.18

(OMe), 50.86 (NCH₂), 37.91 (MeIm), 32.46, 29.32 (CH₂cod), 3.41

(NCCH₃). MS (MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z 351.7

Λ_M(acetone): 78 Ω^ꢀ1cm²mol^ꢀ1

.

Reaction of 7 with Potassium Isopropoxide. [IrBr(cod)-

(MeIm(2-methoxybenzyl))] (7) (20.0 mg, 0.034 mmol) and K[OiPr]

(2.79 mg, 0.034 mmol) were reacted in THF-d₈(0.5 mL, NMR tube) to

give a solution of [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] (19). ¹H

NMR (298 K, THF-d₈): δ 7.43 (d, J = 7.7, 1H, CH Ar), 7.33 (dd, J = 8.2,

7.7, 1H, CH Ar), 7.06 (d, J = 8.2, 1H, CH Ar), 7.04 (d, J = 2.0, 1H, CH

Im), 6.95 (d, J = 7.7, 1H, CH Ar), 6.93 (d, J = 2.0, 1H, CH Im), 5.77 (AB

system, δ_A= 5.91, δ_B= 5.64, J_AB= 14.7, 2H, NCH₂), 4.38 (m, 2H, CH

cod), 4.06 (s, 3H, OMe), 3.95 (s, 3H, MeIm), 3.90 (m, 2H, CH cod),

3.67 (m, 1H, CH(CH₃)₂), 2.40, 2.22, 1.65 (m, 2H, 4H, 2H, CH₂cod),

0.95 (dd, J = 5.6, 4.0, m, 6H, CH(CH₃)₂). ESI-MS (i-PrOH): m/z 503.1

([M ꢀ PrO]⁺, 100%). MS (MALDI-TOF, DCTB matrix, i-PrOH, linear

analysis): m/z 585 [M + Na]⁺, 503.1 [M ꢀ OPr]⁺.

[M ꢀ CH₃CN]⁺. Λ_M(acetone): 81 Ω^ꢀ1cm²mol^ꢀ1

.

[Rh(NCCH₃)(cod)(MeIm(o-methoxybenzyl))][BF₄] (16). [RhBr-

(cod)(MeIm(2-methoxy-benzyl))] (9) (100 mg, 0.200 mmol) and AgBF₄

(38.9 mg, 0.200 mmol) were used. Yield: 85%. Anal. Calcd for C₂₂H₂₉-

BF₄N₃ORh: C, 48.82; H, 5.40; N, 7.76. Found: C, 48.50; H, 5.65; N, 7.40. ¹H

NMR (298 K, CDCl₃): δ 7.34 (m, 2H, CH-Ar), 6.99 (m, 2H, CH-Ar), 6.97,

6.91 (s 1H, 1H, CH-Im), 5.59 (br, 2H, NCH₂), 4.67 (br, 2H, CH cod), 4.14

(s, 3H, OMe), 3.88 (s, 3H, MeIm), 3.49 (br, 2H, CH cod), 2.44 (s, 3H,

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Reaction of 7 with Sodium Cyclohexanoxide. A suspension

of [IrBr(cod)(MeIm(2-methoxybenzyl))] (7) (20 mg, 0.03 mmol) and

NaOCy (4.2 mg, 0.03 mmol) in THF-d₈(0.5 mL, NMR tube) was

stirred at 298 K to give an orange solution containing [Ir(OCy)-

(cod)(MeIm(2-methoxybenzyl))] (20). H NMR (298 K, TDF-d₈)

data for 20: δ 7.39 (d, J = 7.5, 1H, CH Ar), 7.29 (t, J = 7.5, 1H, CH Ar),

7.13 (m, 1H, CH Ar), 7.00 (d, J = 1.8, 1H, CH Im), 6.92 (m, 1H, CH Ar),

Workup as described above for compounds 6ꢀ9 gave the compound as

a yellowish-brown solid. Yield: 69%. Anal. Calcd for C₁₉H₂₄IN₂OIr: C,

37.07; H, 3.93; N, 4.55. Found: C, 37.15; H, 4.41; N, 4.87. ¹H NMR (298

K, C₆D₆): δ 8.86 (dd, J = 7.7, 1.5, 1H, CH), 7.13 (ddd, J = 8.5, 7.7, 1.8,

1H, CH), 6.94 (td, J = 7.7, 1.3, 1H, CH), 6.63 (d, J = 2.0, 1H, CH Im),

6.53 (dd, J = 8.5, 1.3, 1H, CH), 6.15 (d, J = 2.0, 1H, CH Im), 5.29, 5.23

(m, 1H, 1H, CH cod), 3.59 (s, 3H, MeIm), 3.13 (s, 3H, OMe), 3.10, 2.72

(m, 1H, 1H, CH cod), 2.13, 1.88, 1.63, 1.51, 1.34, 1.15 (m, 2H, 1H, 1H,

2H, 1H, 1H, CH₂cod). ¹³C{¹H} NMR (CDCl₃): δ 181.47 (NCN),

153.55 (C Ar), 131.28, 129.57 (CH Ar), 123.30, 121.14 (CH Im),

124.82 (C Ar), 120.53, 110.94 (CH Ar), 83.45, 81.57 (CH cod),

55.07 (CH cod), 53.61 (OMe), 53.45 (CH cod), 37.51 (MeIm),

34.03, 32.29, 30.91, 30.45 (CH₂cod). MS (MALDI-TOF, DCTB

matrix, CH₂Cl₂): m/z 615.1 [M]⁺, 488.2 [M ꢀ Br]⁺. Λ_M(acetone):

1

6.89 (d, J = 1.8, 1H, CH Im), 5.73 (AB system, δ_A= 5.63, δ_B= 5.84, J_AB

=

15.0, 2H, NCH₂), 4.37 (m, 2H, CH-cod), 4.02 (s, 3H, OMe), 3.92 (s,

3H, MeIm), 3.47 (m, 1H, CH OC₆H₁₁), 3.16 (m, 2H, CH cod), 2.19 (m,

4H, CH₂cod), 2.40 (m, 4H, CH₂OC₆H₁₁), 1.59 (m, 2H, CH₂cod),

1.54, 1.24 (m, 4H, CH₂OC₆H₁₁),1.09 (m, 2H, CH₂cod), 0.91(m, 2H,

CH₂OC₆H₁₁).

Preparation of [IrHBr(NCCH₃)(cod)(MeIm(2-methoxybenzyl))]-

[BF₄] (22). HBF₄Et₂O (27.8 mg, 0.172 mmol) was added to a solution

18 Ω^ꢀ1cm²mol^ꢀ1

.

3

of [IrBr(cod)(MeIm(2-methoxybenzyl))] (7) (100 mg, 0.172 mmol) in

acetone (10 mL) and CH₃CN (52 μL, 1 mmol). After 1 h of stirring at

room temperature the colorless solution was concentrated to ca. 1 mL.

Treatment of the pale yellow residue with cold pentane rendered a

yellowish solid, which was separated by decantation, washed with

hexane, and dried in vacuo. Yield: 85.9%. Anal. Calcd for C₂₀H₂₇-

N₂OIrBrBF₄: C, 35.83; H, 4.05; N, 4.17. Found: C, 35.55; H, 4.55; N,

4.29. MS (MALDI-TOF, DCTB matrix, CH₂Cl₂): m/z 623.1 [M ꢀ

H]⁺, 503.1 [M ꢀ H ꢀ Br ꢀ NCCH₃]⁺. Λ_M(acetone): 77 Ω^ꢀ1cm²

mol^ꢀ1. Isomer 22a: ¹H NMR (253 K, CD₂Cl₂): δ 7.39 (t, J = 7.7, 1H,

CH Ar), 7.19 (d, J = 7.0, 1H, CH Ar), 7.05 (m, 2H, CH Ar), 7.03, 7.00 (s,

1H, 1H, CH Im), 5.76 (AB system, δ_A= 6.45, δ_B= 5.07, J_AB= 16.2, 2H,

NCH₂), 4.90, 4.54 (m, 2H, CH-cod), 3.99 (s, 3H, OMe), 3.94 (s, 3H,

MeIm), 3.57 (m, 2H, CH₂cod), 1.24 (s, 3H, NCCH₃), 2.77, 2.15, 2.08

(m, 2H, 2H, 2H, CH₂cod), ꢀ14.51 (s, 1H, IrꢀH). ¹³C{¹H} NMR (253

K, CD₂Cl₂): δ 173.46 (NCN), 156.84, 142.45 (C Ar), 129.75 127.74

(CH Ar), 124.55 (NCCH₃), 120.78, 110.55 (CH Ar), 101.34, 98.75

(CH Im), 77.07, 75.47, 65.41, 62.95 (CH cod), 55.67 (OMe), 51.96

(CH₂), 39.19 (MeIm), 33.28, 29.95, 29.10, 27.87 (CH₂cod), 4.24

(NCCH₃). Isomer 22b: ¹H NMR (298 K, CD₂Cl₂): δ 7.38 (t, J = 7.8,

1H, CH Ar), 7.29 (d, J = 7.2, 1H, CH Ar), 7.19 (d, J = 2.0, 1H, CH Im),

6.90 (m, 1H, CH Ar), 6.81 (d, J = 2.0, 1H, CH Im), 6.69 (d, J = 7.2, 1H,

CH Ar), 5.68 (m, 1H, CH cod), 5.40 (AB system, δ_A= 5.45, δ_B= 5.36,

J_AB= 14.0, 2H, NCH₂), 5.45 (m, 1H, CH cod), 4.45, 4.21 (m, 1H, 1H,

CH cod), 4.15 (s, 3H, OMe), 3.92 (s, 3H, MeIm), 2.75 (s, 3H, NCCH₃),

2.67, 2.21, 1.92, 1.40 (m, 2H, 2H, 2H, 2H, CH₂cod), ꢀ14.30 (s, 1H,

IrꢀH). ¹³C{¹H} NMR (253 K, CD₂Cl₂): δ 172.54 (NCN), 156.17,

143.37 (C Ar), 130.14, 127.14, (CH Ar), 124.39 (NCCH₃), 120.19,

110.10 (CH Ar), 102.96, 100.34 (CH Im), 83.07, 82.64, 75.65, 74.69

(CH cod), 55.58 (OMe), 49.77 (CH₂), 41.57 (MeIm), 35.44, 32.19,

30.71, 25.57 (CH₂cod), 2.60 (NCCH₃).

Preparation of 1-(2-Methoxyphenyl)-3-methyl-1H-imida-

zol-3-ium Iodide (23). Methyl iodide (326 μL, 5.24 mmol) was

added to a solution of 2-methoxyphenyl imidazole (900 mg, 5.24 mmol),

prepared following the general procedure described by Zhang⁵¹for the

preparation of 1-arylimidazoles, and the mixture was refluxed for 12 h.

The yellow solid formed was separated by decantation, washed with hot

diethyl ether, and dried under vacuum. Yield: 85%. Anal. Calcd for

C₁₁H₁₃IN₂O: C, 41.79; H, 4.14; N, 8.86. Found: C, 41.33; H, 4.09; N,

8.89. ¹H NMR (298 K, CD₃OD): δ 9.35 (s, 1H, NCHN), 7.88 (m, 1H,

CH Im), 7.79 (m, 1H, CH Im), 7.62 (d, J = 7.6, 1H, CH Ar), 7.62 (d, J =

7.8, 1H, CH Ar), 7.36 (dd, J = 8.9, 1.2, 1H, CH Ar), 7.20 (td, J = 7.7, 1.2,

1H, CH Ar), 4.08 (s, 3H, OMe), 3.96 (s, 3H, MeIm). ¹³C{¹H} NMR

(298 K, CDCl₃): δ 153.82 (C Ar), 142.43 (NCHN), 133.14, 127.06

(CH Ar), 126.18 (C Ar), 125.02, 124.73 (CH Im), 122.50, 114.19 (CH

Ar), 57.04 (OMe), 37.13 (MeIm). ESI-MS (CH₃OH): m/z 189.3 [M]⁺.

Preparation of [IrBr(cod)(MeIm(2-methoxyphenyl))] (24).

[{Ir(μ-OMe)(cod)}₂] (106 mg, 0.160 mmol) and [MeImH(2-meth-

oxybenzyl)]I (100 mg, 0.319 mmol) were reacted in THF (10 mL).

General Procedure for Transfer Hydrogenation Catalysis.

The catalytic transfer hydrogenation reactions were carried out under an

argon atmosphere in thick glass reaction tubes fitted with a greaseless

high-vacuum stopcock. In a typical experiment, the reactor was charged

with a solution of the substrate (5 mmol) in 2-propanol (4.5 mL),

internal standard (mesitylene, 70 μL, 0.5 mmol), base (104 μL, 0.025

mmol of a KOH solution 0.24 M in 2-propanol), and the catalyst (0.005

mmol, 0.1 mol %). The resulting mixture was stirred at room tempera-

ture until complete dissolution of the catalyst and then placed in a

thermostated oil bath at the required temperature, typically 80 °C.

Conversions were determined by gas chromatography analysis under the

following conditions: column temperature 35 °C (2 min) to 220 at

10 °C/min at a flow rate of 1 mL/min using ultrapure He as carrier gas.

Calculation Details. DFT calculations have been carried out with

Gaussian 09⁶²using the B3LYP functional with the 6-31G** basis set for

all atoms except Ir, for which the LANL2DZ basis set and pseudopo-

tential were used. The molecular structure was treated in full without any

simplification of the ligands. Optimizations have been carried out using

the “verytight” keyword and an “ultrafine” grid for integration. All the

minima have been characterized by frequency calculations, and all the

intermediates, except 19⁰, showed positive frequencies. For the inter-

mediate 19⁰a small negative frequency remains (ꢀ12.89 cm^ꢀ1), which,

by visual inspection, seems to be associated with a rotational movement

on the ligands.

X-ray Structural Determination of Complexes [IrBr(cod)-

(MeIm(o-methoxybenzyl))] (7) and [IrHBr(NCCH₃)(cod)-

(MeIm(2-methoxybenzyl))][BF₄] (22). Suitable crystals for the

X-ray diffraction experiments were obtained at 273 K for complex 7

by slow diffusion of pentane into a concentrated THF solution and for

complex 22 by slow diffusion of diethyl ether into a concentrated

acetone/acetonitrile solution. Intensity data were collected at low

temperature (100(2) K) on a Bruker SMART CCD area detector

diffractometer equipped with graphite-monochromated Mo Kα radia-

tion (λ = 0.71073 Å) using narrow frames (0.3° in ω). Cell parameters

were refined from the observed setting angles and detector positions of

strong reflections (5259 reflns, 2θ < 54.0° for 7, and 6751 reflns, 2θ <

53.96° for 22). Data were corrected for Lorentz and polarization effects,

and a semiempirical absorption correction was applied using the

SADABS program.⁶³The structure was solved by Patterson method

and completed by successive difference Fourier syntheses (SHELXS-

86).⁶⁴Refinement was carried out by full-matrix least-squares on F²with

SHELXL97,⁶⁴including isotropic and subsequent anisotropic displace-

ment parameters for all non-hydrogen atoms. All hydrogen atoms were

included from calculated positions and refined riding on their carbon

atoms. For 22, high values of the Flack parameter suggested the

existence of a partial component of a racemic twin, so this parameter

was included as a free variable in the refinement; a final value of 0.234(5)

was obtained. The hydride atoms in complex 22 were included in the last

cycles of refinement from an electrostatic potential calculation and were

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ARTICLE

refined with a light geometrical restriction affecting the IrꢀH bond

distance. All the highest electronic residuals in both structures (smaller

than 1.41 e/Å³) were observed in close proximity to the Ir metal

and have no chemical sense. Atomic scattering factors, corrected for

anomalous dispersion, were used as implemented in the refinement

programs.⁶⁴

Crystal data for compound 7: C₂₀H₂₆BrIrN₂O, M = 582.54;

yellow block, 0.16 ꢁ 0.15 ꢁ 0.15 mm³; monoclinic, P2₁/n; a =

14.346(2) Å, b = 10.8182(17) Å, c = 14.463(2) Å; β = 119.226(2)°;

Z = 4; V = 1958.9(5) Å³; D_c= 1.975 g/cm³; μ = 8.866 mm^ꢀ1, min. and

max. transmission factors 0.245 and 0.265; 2θ_max= 54.20°; 12 159

reflections collected, 4289 unique [R_int= 0.0301]; number of data/

restraints/parameters 4289/0/228; final GoF 1.039, R₁= 0.0269 [3856

reflections, I > 2σ(I)], wR₂= 0.0651 for all data.

V. P. W.; Herrmann, W. A. J. Organomet. Chem. 2000, 600, 12–22.

(e) Torres, O.; Martín, M.; Sola, E. Organometallics 2009, 28, 863–870.

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Crystal data for compound 22: C₂₂H₃₀BBrF₄IrN₃O, M =

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’ ASSOCIATED CONTENT

S

Supporting Information. Kinetic data for hydrogen

b

transfer catalysis. Full ESI and MALDI-TOF MS. Computational

information: calculated data (B3LYP) for catalytic intermediates.

X-ray crystallographic information ﬁles containing full details of

the structural analysis of complexes 7 and 22 (CIF format). This

material is available free of charge via the Internet at http://pubs.

acs.org.

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(b) Gladiali, S.; Mestroni, G. Transferhydrogenations. In Transition

Metals for Organic Synthesis: Building Blocks and Fine Chemicals; Beller,

M., Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 2;

pp 97ꢀ119.

’ AUTHOR INFORMATION

(11) (a) Noyori, R.; Hashiguchi., S. Acc. Chem. Res. 1997,

30, 97–102. (b) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.;

Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562–7563.

Corresponding Author

*E-mail: vjimenez@unizar.es, perez@unizar.es.

(12) (a) Pontes da Costa, A.; Viciano, M.; Sanaꢀu, M.; Merino, S.;

Tejeda, J.; Peris, E.; Royo, B. Organometallics 2008, 27, 1305–1309. (b)

Hahn, F. E.; Holtgrewe, C.; Pape, T.; Martin, M.; Sola, E.; Oro, L. A.

Organometallics 2005, 24, 2203–2209. (c) Mas-Marzꢀa, E.; Poyatos, M.;

Sanaꢀu, M.; Peris, E. Organometallics 2004, 23, 323–325. (d) Albrecht,

M.; Crabtree, R. H.; Mata, J. A.; Peris, E. Chem. Commun. 2002, 32–33.

(e) Hillier, A. C.; Lee, H. M.; Stevens, E. D.; Nolan, S. P. Organometallics

2001, 20, 4246–4252. (f) Kownacki, I.; Kubicki, M.; Szubert, K.;

Marciniec, B. J. Organomet. Chem. 2008, 693, 321–328.

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2004, 23, 629–631. (b) Miecznikowski, J. R.; Crabtree, R. H. Poly-

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’ ACKNOWLEDGMENT

Financial support from Ministerio de Ciencia e Innovaciꢀon

(MICINN/FEDER) is gratefully acknowledged (Projects:

CTQ2010-15221, CSD2006-0015, and CSD2009-00050). J.F.

T. thanks the Spanish MICINN for a predoctoral fellowship, and

S.W. thanks the Eramus program (Univ. RWTH-Aachen,

Germany). We thank Dr. Jesꢀus Orduna and Dr. María Savirꢀon

(ICMA, Universidad de Zaragoza-CSIC) for helpful discussions.

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Iridium(I) complexes with hemilabile N-heterocyclic carbenes: Efficient and versatile transfer hydrogenation catalysts

DOI: 10.1021/om200747k

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