Synthesis and coordination chemistry of 4-azabenzimidazole derivatives

Heteroatom Chemistry

Volume 22, Number 3/4, 2011

Synthesis and Coordination Chemistry

of 4-Azabenzimidazole Derivatives

Mareike C. Jahnke, Dirk Brackemeyer, Tania Pape,

and F. Ekkehardt Hahn

Institut fu¨ r Anorganische und Analytische Chemie der Westfa¨lischen Wilhelms-Universita¨t

Mu¨ nster, Corrensstraße 28/30, D-48149 Mu¨ nster, Germany

Received 15 October 2010; revised 18 December 2010

with chloroacetone gave the corresponding 1-alkyl-

ABSTRACT:

A facile method for the prepa-

3-(2-oxopropyl)-4-azabenzimidazolium chlorides 13–

17 in good yields. The chloride counterion was

exchanged for a tetraﬂuoroborate anion for two

derivatives leading to compounds 18 and 19. Car-

bene complexes were obtained by treatment of 1-

butyl-3-(2-oxopropyl)-4-azabenzimidazolium chloride

15 with silver oxide, followed by transmetalation with

[AuCl(tht)] or [IrCl₂Cp*]₂leading to the gold(I) (20)

and iridium(III) complexes (21) with 1-butyl-3-(2-

oxopropyl)-4-azabenzimidazolin-2-ylidene ligand. Re-

moval of the 2-oxopropyl protecting group by treat-

ment with silica gel proceeded in the case of the

iridium complex to give complex 22, possessing

an NH,NR-stabilized carbene ligand, while it failed

ration of N1- (1a–6a) and N3-alkylated (1b–

6b) 4-azabenzimidazole derivatives is presented.

Both isomers were obtained by alkylation of

4-azabenzimidazole. The isomers were separated, and

the preferred formation of the N1-alkylated derivatives

(1a–6a) was observed. The C2-proton in the alkylated

benzimidazoles can be exchanged for an iodine atom

by treatment with methyl lithium and iodine, lead-

ing to N-alkylated 2-iodo-4-azabenzimidazoles 7a,b

and 8a. 2,2^ꢀ-Bis(4-azabenzimidazole) derivatives 9a,b

and 10a were isolated as by-products of the proton–

halogen exchange. Treatment of the N1- or N3-methyl-

4-azabenzimidazoles 1a,b with methyl lithium and

N-chlorosuccinimide as a chlorine source proceeded

to give 2-chloro-4-azabenzimidazoles 11a,b without

formation of the coupling products, but the yield

of 2-chloro-1-methyl-4-azabenzimidazole 11a is low

(9%) whereas 2-chloro-3-methyl-4-azabenzimidazole

11b has been obtained in reasonable yield (49%).

The reaction of 3-butyl-2-iodo-4-azabenzimidazole

7b with [IrCl₂Cp*]₂gave the iridium complex

12, where 3-butyl-2-iodo-4-azabenzimidazole is co-

ordinating to the metal center via the N1 atom.

The reaction of the N1-alkylated 4-azabenzimidazole

ꢁ

C

for the gold(I) complex.

2011 Wiley Periodicals, Inc.

Heteroatom Chem 22:476–490, 2011; View this article on-

line at wileyonlinelibrary.com. DOI 10.1002/hc.20711

INTRODUCTION

N-Heterocyclic carbenes (NHCs) are established lig-

ands in organometallic chemistry [1], allowing the

synthesis of transition metal complexes with desir-

able catalytic properties [2]. Additional applications

such as the use of polydentate carbene ligands as

building blocks for metalosupramolecular architec-

tures have emerged recently [3].

Dedicated to Professor Kin-ya Akiba on the occasion of his 75th

birthday.

The ubiquitous unsaturated imidazolin-2-

ylidenes normally exist as monomers [1], whereas

their benzannulated congeners, the benzimidazolin-

2-ylidenes A (Fig. 1), tend to dimerize to the

Correspondence to: F. Ekkehardt Hahn; e-mail: fehahn@uni-

muenster .de.

Contract grant sponsor: Deutsche Forschungsgemeinschaft.

Contract grant numbers: IRTG 1444, SFB 858.

2011 Wiley Periodicals, Inc.

c

ꢀ

476

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 477

R

Np

H

N

R

Np

H

R = Np, Ad

A

B

C

D

R

X

N

ML_n

N

H

N

H

X = O, S, NH

E

G

F

FIGURE 1 Selected types of annulated N-heterocyclic carbenes and their complexes.

dibenzotetraazafulvalenes if their N,N^ꢀ-substituents

are of reduced steric demand [4]. Kinetically sta-

stabilized [13a] carbene ligands. This type of alkyla-

tion has enabled access to macrocyclic P₂C^NHC[14]

and P₂C^N₂^HCligands [15], featuring carbene and phos-

phine donors.

bilized monomeric benzimidazolin-2-ylidenes

A

can be obtained with bulky N,N^ꢀ-substituents like

neopentyl or adamantyl [5]. In contrast to the limited

access to the free benzimidazolin-2-ylidenes, several

complexes possessing benzimidazolin-2-ylidene

ligands have been prepared [6].

While complexes of type F, possessing an

NH,NR-stabilized carbene ligand have been ob-

tained by monoalkylation of the complexes of type

E, it is more convenient to prepare this type of

complex by lithiation of a N-alkylbenzimidazole

and subsequent reaction with a suitable metal pre-

cursor followed by protonation of the ring nitro-

gen atom [16]. In selected cases, the access to

complexes of type F with NR,NH-substituted NHC

ligands is possible by base-induced tautomerization

of N-coordinated N-heterocycles [17] or by oxidative

addition of the C2–H bond of 1-alkylbenzimidazole

at a suitable metal center, followed by reductive

elimination under protonation of the unsubstituted

ring nitrogen atom [18]. In addition, 2-chloro azole

derivatives have been oxidatively added to low valent

metal centers which yielded, after addition of a pro-

ton acid, metal complexes with an NH,X-stabilized

carbene ligand (X = O, S, NR) [19]. No reductive

elimination occurred in this case, and the metal re-

mained in the higher oxidation state.

Complexes bearing an NH,NR-stabilized car-

bene ligand are not only easily N-alkylated to give

the “classical” NHC ligands, but the N H function

of the carbene ligand in complexes of type F can act

as a molecular recognition unit via the formation of

hydrogen bonds to selected substrates [11a,16,20].

The close proximity of the NH group to the metal

center is particularly useful for the recognition of

substrates in competitive catalytic reactions [16].

In an effort to introduce a second recognition

site in NHC ligands next to the N H function, we

Heinicke et al. demonstrated that free car-

bene ligands with a modiﬁed annulated ring are

also stable if the nitrogen atoms bear bulky N,N^ꢀ-

substitutents, and the pyrido[b]- (B) [7,8] and

pyrido[c]-annulated (C) carbenes [8] have been de-

scribed recently (Fig. 1).

The carbenic carbon atom in the carbenes of

the type A–C is normally stabilized by two alky-

lated, trigonal-planar nitrogen atoms as it is de-

picted in Fig. 1. Removal of one or both of the

N,N^ꢀ-substituents destabilizes the carbenes and con-

sequently, free and stable NH,NH-functionalized

diaminocarbenes of type D are unknown. Never-

theless, it is possible to stabilize such NH,NH-

functionalized carbene ligands at a variety of metal

centers. Carbene complexes of type E bearing

one or more NH,NH- [9,10] or NH,O-stabilized

[11] carbene ligands have been obtained by the

metal template-controlled cyclization of suitably

β-functionalized phenyl isocyanide ligands [12],

whereas complexes with NH,S-stabilized carbene

ligands were obtained from deprotonated benzo-

thiazoles [13].

Complexes of type E, bearing a carbene ligand

which is not stable in the free state, exhibit a num-

ber of interesting features. They can be alkylated

at the NH-functions, leading to complexes with

NR,NH- or NR,NR- [10], NR,O [11,12a], or NR,S-

Heteroatom Chemistry DOI 10.1002/hc

478 Jahnke et al.

TABLE 1 Yields and Ratio of the Isomers a and b for

Compounds 1–6

studied monoalkylated 4-azabenzimidazoles, which

possess a nitrogen atom in close proximity to the

carbene N H function. After C2 deprotonation, co-

ordination to a metal center, and protonation of the

azole ring nitrogen atom, such ligands would lead to

complexes of type G featuring two recognition units

(NH and N), which are hydrogen bond donors and

acceptors, respectively, and thus might be capable

of selectively recognizing substrates with carboxyl

groups via two N H· · ·O_carboxyland N· · ·H O_carboxyl

hydrogen bonds. Here we describe the preparation of

N1- or N3-alkylated 4-azabenzimidazoles and their

halogenation in C2-position to give the 2-iodo and

2-chloro derivatives. The synthesis of unsymmetri-

cal N1,N3-substituted 4-azabenzimidazolium salts

bearing a removable protecting group at one nitro-

gen atom and of carbene complexes derived from

the 4-azabenzimidazole and 4-azabenzimidazolium

salts is also described.

Yield Isomer a

Yield Isomer b

Compound

(%)

Ratio a:b

1

2

3

4

5

6

48

82

80

62

59

30

10

17

22

12

26

61:39

90:10

83:17

74:26

83:17

70:30

are an excellent probe for the detection of the iso-

mers a and b. In the case of N1-substitution (iso-

mer a), the resonances for atom C3a are detected at

δ ≈ 156 ppm and at δ ≈ 127 ppm for C7a. If the

4-azabenzimidazole is alkylated at N3 (isomer b),

the resonances for C3a are shifted highﬁeld to δ ≈

147 ppm (ꢀδ = 10 ppm) whereas resonances of

atoms C7a are detected downﬁeld (compared to iso-

mer a) at δ ≈ 135 ppm (ꢀδ = 8–10 ppm). It can be

concluded that N-alkylation leads to a highﬁeld shift

of the resonance belonging to the ipso-carbon atom

in α-position of the alkylated nitrogen atom.

Apart from the template-controlled cyclization

of 2-functionalized isocyanide, most complexes

bearing NH,NR-functionalized NHC ligands have

been prepared by an oxidative addition reaction

of C–H or C–X bonds of azoles to transition met-

als, followed by protonation of one ring nitrogen

atom [18,19]. At least for N,N^ꢀ-alkylated azolium

salts, this oxidative addition normally proceeds

faster when 2-halogenated derivatives are used [23].

To facilitate the oxidative addition of N-alkyl-4-

azabenzimidazole derivatives of types 1–6 to tran-

sition metals, we converted selected derivatives into

their 2-halogen derivatives.

RESULTS AND DISCUSSION

Preliminary accounts on the synthesis of

N-alkylated 4-azabenzimidazole derivatives have

been published [21,22], but the yields of the desired

1-alkyl-4-azabenzimidazoles of type 1 (Scheme 1)

were low [21] or a multiple-step synthesis has

to be employed [22]. We found that treatment

of 4-azabenzimidazole with potassium hydroxide

as base and a suitable alkyl halide led to the de-

sired N-alkylated 4-azabenzimidazoles 1a,b–6a,b

(Scheme 1). Since the alkylation could occur at the

N1 or the N3 position, mixtures of two isomers a

and b were obtained for each alkylation reaction.

The isomers were separated by column chromatog-

raphy, and the products were characterized by 2D

NMR spectroscopy.

The yields obtained for the individual isomers

and their ratio are summarized in Table 1. These

data show that the N1-alkylation giving isomers 1a–

6a is preferred under the reaction conditions se-

lected. This observation is in accord with a previous

report by Mu¨ller et al., who found the preferred for-

mation of the N1-alkylated derivatives at an elevated

reaction temperature [21a].

Initial attempts to synthesize 2-iodo-4-

azabenzimidazoles by deprotonation of the N-

alkylated 4-azabenzimidazole derivatives with both

n-butyl lithium at –78^◦C or methyl lithium at −20^◦C

failed and led to the 2-alkylated 4-azabenzimidazole

derivatives [24]. However, treatment of the

4-azabenzimidazole derivatives with methyl lithium

at a lower temperature of –40^◦C and subsequent

addition of iodine to the cold reaction mixture

gave the desired 2-iodo compounds (Scheme 2).

The N-alkylated 2-iodo-4-azabenzimidazoles 7a,8a

(N1-alkylated) and 7b (N3-alkylated) were isolated

from the reaction mixture by column chromatogra-

phy together with the 2,2^ꢀ-bis(4-azabenzimidazole)

derivatives 9a,b and 10a as by-products [13c,25].

Unfortunately, the yields for the 1-alkyl-2-iodo-4-

azabenzimidazoles 7a and 8a are quite low (7a: 8%,

All compounds have been characterized by

NMR spectroscopy, mass spectrometry, and elemen-

tal analyses. A distinction between the two iso-

mers, however, is only possible with the help of

1

NMR spectroscopy. The ¹³C{ H} NMR resonances

of the two quaternary carbon atoms linking the

two cycles (C3a, C7a) are detected mostly down-

ﬁeld. Their chemical shifts depend directly on the

N-substitution pattern, and thus these resonances

Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 479

1: R = Me

2: R = Et

3: R = Pr

4: R = Bu

5: R = CH₂-phenyl

6: R = CH₂-pyridine

R

7

H

N

7a

3a

1

N

RX, KOH

6

5

+

2

THF, Δ

N

4

N

3

R

1a–6w

1b–6b

SCHEME 1 Synthesis of the N-alkylated 4-azabenzimidazole derivatives 1a–6a and 1b–6b.

°C

30 min

R

C to r. t.,

°

2. I₂

4 h

N

+

H

Ι

THF

N

R

4a–5a

4, 7, 9: R = Bu

5, 8, 10: R = CH₂-pheny

7a–8a

9a–10a

°C

30 min

R

2. I₂

4 h

°C to r. t.,

N

H

Ι

+

THF

N

R

N

R

4b

4b, 7b, 9b: R = Bu

7b

9b

°C

60 min

N

2. r. t., 15 min.

or

Cl

1a,b

THF

N

11a

11b

SCHEME 2 Synthesis of the N-alkylated 2-iodo- (top and middle) and 2-chloro-4-azabenzimidazole derivatives (bottom).

8a: 13%), whereas the yield for the 3-butyl-2-iodo-

4-azabenzimidazole 7b (see Scheme 2) is signiﬁ-

cantly higher (36%).

logues. The chlorination was performed in analogy

to the reported synthesis of 2-chlorobenzimidazoles

[26]. N-chlorosuccinimide (NCS) was used as chlo-

rine source, which is more convenient than the use

of chlorine gas. The N-methyl-4-azabenzimidazole

derivatives 1a or 1b were C2-deprotonated with

lithium diisopropylamide (LDA) at –78^◦C, and NCS

was added to give the N-alkylated 2-chloro-4-

azabenzimidazoles 11a and 11b, which were iso-

lated as yellow brown solids (Scheme 2, bottom).

As was observed for the iodo derivatives, the yield of

N1-alkylated 11a (9%) is signiﬁcantly lower than the

yield of N3-alkylated 11b (49%).

The 2-iodo-4-benzimidazole derivatives of types

7 and 8 were characterized by NMR spectroscopy,

EI mass spectrometry, and elemental analyses. The

successful substitution of the C2 proton for iodine

is indicated by the absence of the C2 H resonances

(4a δ = 8.07 ppm; 4b δ = 8.06 ppm; 5a δ = 8.18 ppm)

in the ¹H NMR spectra of 7a,b and 8a. A signiﬁcant

upﬁeld shift of ꢀδ ≈ 37 ppm has been observed for

the C2 I carbon atom in the ¹³C NMR spectra of the

compounds 7a,b and 8a (7a δ = 107.5 ppm; 7b δ

= 106.0 ppm; 8a δ = 108.5 ppm) in comparison to

the 4-azabenzimidazoles 4a,b and 5a (4a δ = 145.0

ppm; 4b δ = 144.2 ppm; 5a δ = 145.3 ppm).

1

In the H NMR spectra of both 11a and 11b,

the previously observed resonance for the C2 pro-

ton of the starting material (1a δ = 8.08 ppm; 1b

δ = 8.05 ppm) is absent. The resonances of the

C2–Cl carbon atom are detected for 11a and 11b

Alternatively, the 4-azabenzimidazole deriva-

tives could also be transformed to 2-chloro ana-

Heteroatom Chemistry DOI 10.1002/hc

480 Jahnke et al.

at δ = 146.5 ppm and δ = 142.8 ppm, respectively,

and thus are not signiﬁcantly shifted when compared

with the chemical shifts observed for the parent

4-azabenzimidazoles 1a,b (1a δ = 145.7 ppm; 1b δ =

144.5 ppm). This behavior contrasts the situation in

the 2-iodo-4-azabenzimidazole derivatives 7a,b and

8a (C2 I: δ = 106–108 ppm), where a signiﬁcant up-

ﬁeld shift was observed upon halogenation of C2.

Apart from the C2 Cl ¹³CNMR resonance, the other

resonances for 11a and 11b resemble those of the

parent 4-azabenzimidazoles.

Ir

Cl1

Cl2

I

N1

C1

N2

N3

All attempts to utilize the 2-halogenated

4-azabenzimidazoles for the preparation of car-

bene complexes by reacting them with Rh^I, Ir^I, or

Ir^IIIcomplexes in the presence of phosphines as

co-ligands have failed so far. The formation of a

complex was only observed in the reaction of 3-butyl-

2-iodo-4-azabenzimidazole 7b with [IrCl₂Cp*]₂. Un-

fortunately, no oxidative addition under forma-

tion of the carbene complex occurred, but instead

complex 12 was obtained with 3-butyl-2-iodo-4-

azabenzimidazole coordinating to iridium via the

N1-nitrogen atom, while the C2–I bond of the lig-

and remained intact (Scheme 3).

FIGURE 2 Molecular structure of the iridium complex 12.

Hydrogen atoms are omitted for clarity. Selected bond lengths

˚

(A) and angles (˚): Ir–Cl1 2.4301(12), Ir–Cl2 2.4182(11), Ir–N1

2.132(4), I–C1 2.081(4), N1–C1 1.328(6), N2–C1 1.368(6);

N1–C1–N2 112.6(4).

complex. The molecular structure of 12 is depicted

in Fig. 2.

The ¹³C NMR spectrum of 12 exhibits no reso-

nances in the range where the chemical shift of a

carbene carbon atom bound to iridium(III) would

be expected (δ ≈ 160–180 ppm). The aromatic car-

bon atoms, which are in close proximity to the Ir-

coordinated nitrogen atom, resonate at δ = 127.9

(C7), 139.9 (C7a), and 147.7 ppm (C3a) in the

¹³C NMR spectrum. The MALDI-TOF mass spec-

trum of compound 12 conﬁrms the formation of

the complex, and the measured isotopic pattern

matches perfectly to the calculated one for the com-

plex fragment [IrClCp*(L)]⁺(L = 3-butyl-2-iodo-4-

azabenzimidazole) after loss of one chloro ligand

[M Cl]⁺.

Up to now, only one molecular structure has

been reported with a 4-azabenzimidazole ligand co-

ordinating via the N1 atom to a metal center (mer-

cury(II)) [21a]. The Ir–N1 bond length in 12 mea-

˚

sures 2.132(4) A and thus falls in the range reported

for Ir–N bond distances in complexes featuring

N-coordinated azoles [27]. The I–C1 bond distance

˚

measures 2.081(4) A, but a comparison with similar

compounds is not possible due to the lack of molec-

ular structure determinations of 2-iodinated azoles.

Since the synthesis of complexes with NH,NR-

functionalized carbene ligands by oxidative addition

of 2-halogenated 4-azabenzimidazoles has failed so

far, we decided to use a 4-azabenzimidazolium salt

with one removable N-substituent. With this type

of starting material, initially a classical NHC com-

plex featuring an NR,NR-substituted NHC ligand

is synthesized and subsequently, one N-substituent

is removed to give the complex with an NH,NR-

substituted NHC ligand. This protocol has been

introduced by Wang et al. [28], who used the

N1-(2-oxopropyl)-N3-butyl-substituted imidazolium

salt for the preparation of an iridium(III) NR,NR-

NHC complex. After “classical” carbene complex

formation, the protecting group (2-oxypropyl) was

removed giving the iridium complex bearing an

NH,NR-functionalized NHC ligand. Recently, Crab-

tree et al. used a similar protocol and a benzoyl pro-

tecting group for the synthesis of NH,NR-NHC com-

plexes [29].

Crystals of 12 suitable for an X-ray diffraction

study have been obtained by slow diffusion of di-

ethyl ether into a dichloromethane solution of the

Cp*

Cl

Ir

N

[IrCl₂Cp*]₂

THF

Cl

I

N

I

N

Bu

7b

12

SCHEME

3

N1-coordination of 3-butyl-2-iodo-4-

azabenzimidazole 7b to iridium(III).

Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 481

Cl

AgBF₄

BF₄

R

O

1a, 13, 18: R = Me

2a, 14: R = Et

4a, 15, 19: R = Bu

5a, 16: R = CH₂-phenyl

6a, 17: R = CH₂-pyridine

Cl

N

+

N

+

CH₂Cl₂

THF, Δ

N

R = Me, Bu

O

1a, 2a, 4a, 5a

13 –17

18–19

SCHEME 4 Synthesis of the N3-(2-oxopropyl)-functionalized 4-azabenzimidazolium salts.

We selected the 2-oxopropyl-protecting group

for the 4-azabenzimidazole derivatives. The N1-

alkyl-4-azabenzimidazole derivatives 1a, 2a, and 4a–

6a were treated with chloroacetone (Scheme 4)

to give the unsymmetrical N,N^ꢀ-substituted 4-

azabenzimidazolium chlorides 13–17. These com-

pounds were obtained as very hygroscopic brown

solids in good yields (90–95%), except for the

N-benzyl-substituted derivative 16, which was only

isolated in 56% yield.

dichloromethane, and silver tetraﬂuoroborate was

added to the solution. After separation of the silver

chloride formed during this reaction, the N1-alkyl-

N3-(2-oxopropyl)-4-azabenzimidazolium tetraﬂuo-

roborates 18 and 19 were isolated in quantitative

yields as beige solids. A comparison of the NMR

spectra of the salts 18 and 19 with those of the

azolium chlorides 13 und 15 showed essentially

no change in the NMR resonances upon anion

exchange.

The preparation of compounds 13–17 was veri-

ﬁed by NMR spectroscopy. Formation of the benz-

imidazolium salts causes a downﬁeld shift (ꢀδ =

0.50–1.25 ppm) for the aromatic protons in

the ¹H NMR spectra, relative to the parent 4-

azabenzimidazole derivatives. This downﬁeld shift

was also observed for the N CH N resonance (ꢀδ =

0.6–1.2 ppm) of compounds 13–17, and the ob-

served resonances fall in the typical range for the

acidic N CH N protons of azolium salts [6,30].

The N1-butyl-N3-(2-oxopropyl)-4-azabenzimid-

azolium chloride 15 was selected as a precursor

for the synthesis of carbene complexes. In analogy

to the reported procedure [28] and following the

Ag₂O method developed by Lin et al. [31], gold(I)

and iridium(III) complexes were prepared. Com-

pound 15 reacted with silver oxide to give the sil-

ver carbene complex [31]. This complex was only

of limited stability, and decomposed quickly af-

ter isolation, which is in marked contrast to the

large number of stable and well characterized sil-

ver carbene complexes [32]. The silver(I) com-

plex bearing the symmetrical N,N^ꢀ-dineopentyl-4-

azabenzimidazolin-2-ylidene ligand, for example,

has been described as stable [8]. As a consequence

of the instability of the silver(I) complex, this com-

plex was not isolated but subsequently reacted

with the transition metal precursors [AuCl(tht)] and

[IrCl₂Cp*]₂, respectively (Scheme 5).

1

The characteristic ¹³C{ H} NMR resonances for the

carbonyl group of the 2-oxopropyl-protecting group

were detected at δ ≈ 199 ppm, conﬁrming the for-

mation of the desired N1-alkyl-N3-(2-oxopropyl)-4-

azabenzimidazolium chlorides 13–17.

Since the 4-azabenzimidazolium chlorides are

quite hygroscopic, the salts 13 and 15 were

converted into the tetraﬂuoroborate salts, which

is

a common procedure for avoiding hygro-

scopic azolium salts and making them easily

storable. Compounds 13 and 15 were dissolved in

The gold carbene complex 20 was obtained

in a reasonable yield of 66%. Its formation was

Cl

Bu

1. Ag₂O

2. [IrCl₂Cp*]₂

1. Ag₂O

2. [AuCl(tht)]

Cp*

Cl

N

+

N

Au Cl

Ir

CH₂Cl₂

N

Cl

O

20

15

21

SCHEME 5 Synthesis of gold(I) and iridium(III) with the asymmetrically substituted 4-azabenzimidazolin-2-ylidene ligand.

Heteroatom Chemistry DOI 10.1002/hc

482 Jahnke et al.

Bu

Cp*

Cl

Cp*

Cl

N

CH₂Cl₂/acetone

SiO₂, 4h

Ir

N

Cl

H

O

21

22

SCHEME 6 Synthesis of complex 22 by removal of the N-(2-oxopropyl)-substituent from 21.

In analogy to the literature reports [28], we

tried next to remove the N-(2-oxopropyl) group from

the coordinated carbene ligand in 20 by treatment

with silica gel but unfortunately, all attempts have

failed so far. We, therefore, prepared the iridium(III)

complex 21 by treatment of the silver complex ob-

tained in situ from Ag₂O and 15 with [IrCl₂Cp*]₂

(Scheme 5). The MALDI mass spectrum of the crude

reaction product exhibits a peak at m/z = 594, which

ﬁts perfectly with the isotopic pattern for the ion [21

O

N3

N2

Au C1

Cl

N1

1

– Cl]⁺. Furthermore, the ¹³C{ H} NMR spectrum of

21 exhibited a resonance at δ = 163.2 ppm, which

can be assigned to the carbene carbon atom and

thus veriﬁes the formation of the iridium carbene

complex 21. The protecting group is still bound to

the carbene ligand, since the ¹³C NMR spectrum of

21 exhibits the characteristic resonance of the car-

bonyl group of the (2-oxopropyl)-substituent at δ =

203.9 ppm.

While the removal of the protecting group failed

in the case of the gold complex 20, it succeeded for

the iridium(III) complex 21. Treatment of 21 with

silica gel led to complex 22, the ﬁrst metal com-

plex with NH,NR-stabilized 4-azabenzimidazolin-2-

ylidene ligand (Scheme 6). The successful removal

of the protecting group with formation of 22 has

been veriﬁed by NMR spectroscopy. The resonances

assigned to the protecting group in complex 21 are

absent in the NMR spectra of complex 22. Instead,

a broad singlet is observed in the ¹H NMR spectrum

at δ = 11.39 ppm belonging to the proton bound to

N3. The chemical shift for the resonance of the N H

proton falls in the typical range for NH,NR-stabilized

carbene ligands (δ = 10–13 ppm) [18,19,28]. Forma-

tion of the complex with the NH,NR-stabilized car-

bene ligand is also veriﬁed by the MALDI mass spec-

trum of 22, which exhibits peaks at m/z= 573 and

538 belonging to [22]⁺and [22-Cl]⁺, respectively.

FIGURE 3 Molecular structure of complex 20. Hydrogen

atoms have been omitted for clarity. Selected bond lengths

(A) and angles ( ): Au–Cl 2.2814(6), Au–C1 1.977(2), C1–N1

1.355(3), C1–N2 1.360(3), Cl–Au–C1 176.61(7), N1–C1–N2

106.5(2).

◦

˚

conﬁrmed by NMR spectroscopy. The resonance for

the carbene carbon atom in 20 was observed at δ =

181.5 ppm in the ¹³C{ H} NMR spectrum, which is a

1

value typical for complexes of the type [AuX(NHC)]

(X = Cl, Br) bearing a benzimidazolin-2-ylidene lig-

and (δ = 175.8–181.2 ppm) [33]. The N-protecting

group is still present and can be detected by the reso-

nance for the carbonyl carbon atom at δ = 198.7 ppm

in the ¹³C{ H} NMR spectrum.

1

Crystals of 20, which were suitable for X-ray

diffraction study, have been obtained by slow dif-

fusion of pentane into a saturated solution of 20

in chloroform. The molecular structure of 20 is

depicted in Fig. 3. The Au C_carbenebond length

˚

(1.977(2) A) in 20 is nearly identical to the Au–C sep-

arations in related gold(I) carbene complexes bear-

ing the benzimidazolin-2-ylidene ligand [33, 34]. The

˚

Au–Cl bond length (2.2814(6) A) also falls in the

range previously reported for complexes of the type

[AuCl(NHC)]. As expected, complex 20 exhibits an

almost linear coordination geometry at the metal

center (angle Cl–Au–C2 176.61(7)^◦). The N1–C1–N2

bond angle (106.5(2)^◦) in 20 falls in the range ob-

served for related carbene ligands coordinating to

transition metals [8]. Since the intermolecular dis-

tance between two gold atoms has been determined

CONCLUSION

We have described the synthesis of several N1-

and N3-alkylated 4-azabenzimidazole derivatives

1a,b–6a,b. Selected N-alkylated 4-azabenzimidazole

derivatives have been converted into the 2-iodo (7a,b

˚

to be 3.438 A, there can be only a weak Au· · ·Au in-

teraction [35] between the gold atoms in the crystal

structure of 20.

Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 483

and 8a) and 2-chloro derivatives (11a,b). During the

alkyl halide (9.2 mmol), and an excess of potassium

hydroxide (1.9 g, 33.6 mmol) were dissolved in

THF (50 mL), and the reaction mixture was heated

under reﬂux for 48 h. Subsequently, the solvent was

removed in vacuo and the residue was dissolved in

water (25 mL). This aqueous solution was extracted

four times with dichloromethane (15 mL each),

and the combined organic layers were dried with

MgSO₄, ﬁltered, and the ﬁltrate was dried in vacuo.

The two isomers were separated by column chro-

matography (SiO₂, CH₂Cl₂:MeOH 10–30:1). The N1-

and N3-alkyl-4-azabenzimidazole derivatives were

obtained as brown oils or beige solids.

iodination reaction, 2,2-bis(4-azabenzimidazole)s

were obtained as by-products and both 2-iodination

and 2-chlorination led to low product yields in case

of the N1-alkylated 4-azabenzimidazoles, whereas

C2-substitution proceeded with better yields for the

N3-alkyl-4-azabenzimidazole derivatives. The ox-

idative addition of the 2-iodo-4-azabenzimidazoles

to rhodium(I) or iridium(I) failed. The reac-

tion of 3-butyl-2-iodo-4-azabenzimidazole 7b with

[IrCl₂Cp*]₂gave the iridium(III) complex 12 with

an N-coordinated 4-azabenzimidazole ligand, leav-

ing the C–I bond unaffected. Conversion of sev-

eral 1-alkyl-4-azabenzimidazoles to the 1-alkyl-3-

(2-oxopropyl)-4-azabenzimidazolium salts 13–17 by

treatment with chloroacetone proceeded with good

yields. Carbene complexes were synthesized with the

N1-butyl-substituted 4-azabenzimidazolium salt 15.

The silver carbene complex is not stable, but acts as

a carbene transfer agent in the preparation of the

gold(I) (20) and iridium(III) complexes (21) bearing

an NHC ligand derived from 4-azabenzimidazole.

Removal of the 2-oxopropyl-protecting group failed

for the gold complex 20, but was possible with

the iridium complex 21 leading to complex 22,

the ﬁrst complex bearing an NH,NR-functionalized

4-azabenzimidazolin-2-ylidene ligand.

1-Methyl-4-azabenzimidazole (1a).Yield: 0.536 g

1

(48%). H NMR (400.0 MHz, CDCl₃): δ 3.87 (s, 3H,

NCH₃), 7.23 (dd, ³J_HH= 8.1, ³J_HH= 4.8 Hz, 1H, H6),

3

4

7.72 (dd, J_HH= 8.1 Hz, J_HH= 1.5 Hz, 1H, H7),

3

4

8.08 (s, 1H, H2), 8.55 (dd, J_HH= 4.8 Hz, J_HH= 1.5

Hz, 1H, H5). ¹³C NMR (100.6 MHz, CDCl₃): δ 31.4

(NCH₃), 117.7 (C6), 118.1 (C7), 126.8 (C7a), 144.8

(C5), 145.7 (C2), 156.0 (C3a). MS (EI): m/z (%) = 133

([1a]⁺, 100]. Calcd for C₇H₇N₃·0.35H₂O: C, 60.29; H,

5.56; N, 30.12%. Found: C, 59.94; H, 5.37; N, 30.09%.

3-Methyl-4-azabenzimidazole (1b).Yield: 0.337 g

1

(30%). H NMR (400.0 MHz, CDCl₃): δ 3.91 (s, 3H,

NCH₃), 7.23 (dd, ³J_HH= 8.0, ³J_HH= 4.8 Hz, 1H, H6),

3

4

8.05 (s, 1H, H2), 8.06 (dd, J_HH= 8.0 Hz, J_HH

=

EXPERIMENTAL

3

4

1.4 Hz, 1H, H7), 8.41 (dd, J_HH= 4.8 Hz, J_HH

1.4 Hz, 1H, H5). ¹³C NMR (100.6 MHz, CDCl₃): δ 29.7

(NCH₃), 118.2 (C6), 127.9 (C7), 135.2 (C7a), 144.4

(C5), 144.5 (C2), 147.7 (C3a). MS (EI): m/z (%) = 133

([1b]⁺, 43). Calcd for C₇H₇N₃·0.35H₂O: C, 60.29; H,

5.56; N, 30.13%. Found: C, 59.96; H, 5.27; N, 29.98%.

Material and Instruments

Solvents were dried with standard methods and

were freshly distilled prior to use. The metal

precursors [AuCl(tht)] [36] and [IrCl₂Cp*]₂[37]

were prepared following the published procedures.

4-Azabenzimidazole and silver oxide were pur-

chased from Acros (Geel, Belgium) and Sigma-

Aldrich (Munich, Germany). NMR spectra were

recorded on Bruker Avance (II) 200 and Bruker

Avance (I) 400 spectrometers with tetramethyl-

silane as the internal standard. MALDI mass

spectra were measured on a MicroTof (Bruker

Daltonics, Bremen), and EI mass spectra were mea-

sured on a Varian MAT 212 instrument. Elemental

analyses (C, H, N) were obtained using a Vario EL

III elemental analyzer at the Westfa¨lische Wilhelms-

Universita¨t Mu¨nster except for the azolium salts

13–19, which were either too hygroscopic or con-

tained too much ﬂuorine thereby preventing correct

elemental analyses. For assignment of NMR reso-

nances, see Scheme 1.

1-Ethyl-4-azabenzimidazole (2a). Yield: 1.008 g

(82%). ¹H NMR (400.0 MHz, CDCl₃): δ 1.40 (t, ³J_HH

=

3

7.3 Hz, 3H, NCH₂CH₃), 4.12 (q, J_HH= 7.3 Hz,

3

2H, NCH₂CH₃), 7.08 (dd, J_HH= 8.1 Hz, J_HH

=

3

4

4.7 Hz, 1H, H6), 7.62 (dd, J_HH= 8.1 Hz, J_HH

3

1.4 Hz, 1H, H7), 8.00 (s, 1H, H2), 8.41 (dd, J_HH

4.7 Hz, ⁴J_HH= 1.4 Hz, 1H, H5). ¹³C NMR (100.6 MHz,

CDCl₃): δ 14.9 (NCH₂CH₃), 40.1 (NCH₂CH₃), 117.6

(C6/C7), 125.6 (C7a) 144.2 (C2), 144.4 (C5), 156.1

(C3a). MS (EI): m/z (%) = 147 ([2a]⁺, 100), 132 ([2a –

Me]⁺, 18). Calcd for C₈H₉N₃·0.35H₂O: C, 62.60; H,

6.37; N, 27.38%. Found: C, 62.33; H, 6.54; N, 27.35%.

3-Ethyl-4-azabenzimidazole (2b). Yield: 0.117 g

(10%). ¹H NMR (400.0 MHz, CDCl₃): δ 1.55 (t, ³J_HH

=

3

7.3 Hz, 3H, NCH₂CH₃), 4.35 (q, J_HH= 7.3 Hz, 2H,

3

General Procedure for the Synthesis of

NCH CH₃), 7.22 (dd, J_HH= 8.2 Hz, J_HH= 4.8 Hz,

2

the

N-Alkyl-4-azabenzimidazoles

1a,b–6a,b. 4-

1H, H6), 8.05 (dd, ³J_HH= 8.2 Hz, ⁴J_HH= 1.3 Hz, 1H,

3

Azabenzimidazole (1.0 g, 8.4 mmol), an appropriate

H7), 8.06 (s, 1H, H2), 8.39 (dd, J_HH= 4.8 Hz,

Heteroatom Chemistry DOI 10.1002/hc

484 Jahnke et al.

⁴J_HH= 1.3 Hz, 1H, H5). ¹³C NMR (100.6 MHz,

CDCl₃): δ 15.5 (NCH₂CH₃), 38.7 (NCH₂CH₃), 118.1

(C6), 127.9 (C7), 135.5 (C7a), 143.4 (C2), 144.1 (C5),

146.9 (C3a). MS (EI): m/z (%) = 147 ([2b]⁺, 100).

Calcd for C₈H₉N₃·0.35H₂O: C, 62.60; H, 6.37; N,

27.38%. Found: C, 62.19; H, 6.35; N, 27.40%.

3-Butyl-4-azabenzimidazole (4b). Yield: 0.325 g

(22%). ¹H NMR (400.0 MHz, CDCl₃): δ 0.94 (t,

³J_HH= 7.4 Hz, 3H, NCH₂CH₂CH₂CH ), 1.37 (sext,

3

³J_HH= 7.4 Hz, 2H, NCH₂CH₂CH CH₃), 1.91 (quint,

2

³J_HH= 7.4 Hz, 2H, NCH₂CH CH₂CH₃), 4.29 (t,

2

³J_HH= 7.4 Hz, 2H, NCH CH₂CH₂CH₃), 7.22 (dd,

2

³J_HH= 7.9 Hz, ³J_HH= 4.8 Hz, 1H, H6), 8.05 (dd, ³J_HH

=

7.9 Hz, ⁴J_HH= 1.4 Hz, 1H, H7), 8.06 (s, 1H, H2), 8.39

(dd, ³J_HH= 4.8 Hz, ⁴J_HH= 1.4 Hz, 1H, H5). ¹³C NMR

(100.6 MHz, CDCl₃): δ 13.5 (NCH₂CH₂CH₂CH₃), 19.9

(NCH₂CH₂CH₂CH₃), 32.0 (NCH₂CH₂CH₂CH₃), 43.5

(NCH₂CH₂CH₂CH₃), 118.2 (C6), 127.8 (C7), 135.3

(C7a), 143.9 (C3a), 144.2 (C2), 147.0 (C5). MS (EI):

m/z (%) = 175 ([4b]⁺, 100), 132 ([4b – Pr]⁺, 31). Calcd

for C₁₀H₁₃N₃·0.1H₂O: C, 67.85; H, 7.52; N, 23.74%.

Found: C, 67.88; H, 7.53; N, 23.56%.

1-Propyl-4-azabenzimidazole (3a). Yield: 1.079 g

(80%). ¹H NMR (400.0 MHz, CDCl₃): δ 0.90 (t, ³J_HH

=

7.2 Hz, 3H, NCH₂CH₂CH ), 1.87 (sext, ³J_HH= 7.2 Hz,

3

2H, NCH₂CH CH₃), 4.11 (t, J_HH= 7.2 Hz, 2H,

2

3

NCH₂CH₂CH₃), 7.17 (dd, J_HH= 8.1 Hz, J_HH

=

3

4

4.7 Hz, 1H, H6), 7.69 (dd, J_HH= 8.1 Hz, J_HH

3

1.4 Hz, 1H, H7), 8.05 (s, 1H, H2), 8.51 (dd, J_HH

4.7 Hz, ⁴J_HH= 1.4 Hz, 1H, H5). ¹³C NMR (100.6 MHz,

CDCl₃): δ 11.2 (NCH₂CH₂CH₃), 23.1 (NCH₂CH₂CH₃),

47.1 (NCH₂CH₂CH₃), 117.7 (C6), 117.8 (C7), 126.0

(C7a), 144.7 (C5), 145.0 (C2), 156.3 (C3a). MS (EI):

m/z (%) = 161 ([3a]⁺, 100), 132 ([(3a – Et]⁺, 62); Calcd

for C₉H₁₁N₃·0.7H₂O: C, 62.19; H, 7.19; N, 24.18%.

Found: C, 62.32; H, 7.07; N, 24.18%.

1-Benzyl-4-azabenzimidazole (5a). Yield: 1.040 g

1

(59%). H NMR (400.0 MHz, CDCl₃): δ 5.30 (s, 2H,

3

NCH₂-Ph), 7.14 (dd, J_HH= 8.1 Hz, J_HH= 4.7 Hz,

1H, H6), 7.19–7.16 (m, 2H, Ph-H), 7.36–7.30 (m, 3H,

3

4

Ph-H), 7.56 (dd, J_HH= 8.1 Hz, J_HH= 1.5 Hz, 1H,

H7), 8.18 (s, 1H, H2), 8.54 (dd, ³J_HH= 4.7 Hz, ⁴J_HH

=

3-Propyl-4-azabenzimidazole (3b). Yield: 0.226 g

1.5 Hz, 1H, H5). ¹³C NMR (100.6 MHz, CDCl₃): δ

49.5 (NCH₂-Ph), 118.2 (C6), 118.4 (C7), 126.2 (C7a),

127.1 (Ph-C), 128.6 (Ph-C), 129.1 (Ph-C), 134.6 (Ph-

C), 145.0 (C5), 145.3 (C2), 156.3 (C3a). MS (EI):

m/z (%) = 209 ([5a]⁺, 100), 91 (CH₂C₆H₅, 28). Calcd

for C₁₃H₁₁N₃·0.2H₂O: C, 73.36; H, 5.40; N, 19.74%.

Found: C, 73.54; H, 4.94; N, 19.45%.

(17%). ¹H NMR (400.0 MHz, CDCl₃): δ 0.96 (t,

3

³J_HH= 7.2 Hz, 3H, NCH₂CH₂CH ), 1.95 (sext, J_HH

3

= 7.2 Hz, 2H, NCH₂CH CH₃), 4.25 (t, ³J_HH= 7.2 Hz,

2

3

2H, NCH CH₂CH₃), 7.21 (dd, J_HH= 8.1 Hz, J_HH

=

2

3

4.8 Hz, 1H, H6), 8.03 (s, 1H, H2), 8.04 (dd, J_HH

4

3

8.1 Hz, J_HH= 1.5 Hz, 1H, H7), 8.38 (dd, J_HH

4.8 Hz, ⁴J_HH= 1.5 Hz, 1H, H5). ¹³C NMR(100.6 MHz,

CDCl₃): δ 11.2 (NCH₂CH₂CH₃), 23.3 (NCH₂CH₂CH₃),

45.4 (NCH₂CH₂CH₃), 118.1 (C6), 127.8 (C7), 135.5

(C7a), 144.1 (C2), 147.1 (C5), 148.3 (C3a). MS (EI):

m/z (%) = 161 ([3b]⁺, 100), 132 ([3b – Et]⁺, 57]. Calcd

for C₉H₁₁N₃·0.6H₂O: C, 62.84; H, 7.14; N, 24.43%.

Found: C, 62.61; H, 7.03; N, 24.14%.

3-Benzyl-4-azabenzimidazole (5b). Yield: 0.214 g

1

(12%). H NMR (400.0 MHz, CDCl₃): δ 5.48 (s, 2H,

3

NCH₂-Ph), 7.26 (dd, J_HH= 8.0 Hz, J_HH= 4.8 Hz,

1H, H6), 7.35–7.20 (m, 5H, Ph-H), 8.04 (s, 1H, H2),

3

4

8.09 (dd, J_HH= 8.0 Hz, J_HH= 1.4 Hz, 1H, H7),

8.43 (dd, J_HH= 4.8 Hz, J_HH= 1.4 Hz, 1H, H5). ¹³C

NMR (100.6 MHz, CDCl₃): δ 47.1 (NCH₂-Ph), 118.4

(C6), 127.8 (C7), 128.0 (Ph-C), 128.3 (Ph-C), 129.0

(Ph-C), 135.2 (C7a), 135.8 (Ph-C), 143.9 (C2), 144.6

(C5), 147.0 (C3a). MS (EI): m/z (%) = 209 ([5b]⁺,

100). Calcd for C₁₃H₁₁N₃·0.2H₂O: C, 73.36; H, 5.40;

N, 19.74%. Found: C, 73.75; H, 5.01; N, 19.47%.

3

4

1-Butyl-4-azabenzimidazole (4a). Yield: 0.916 g

(62%).¹H NMR (200.1 MHz, CDCl₃): δ 0.92 (t,

³J_HH= 7.2 Hz, 3H, NCH₂CH₂CH₂CH ), 1.33 (sext,

3

³J_HH= 7.2 Hz, 2H, NCH₂CH₂CH CH₃), 1.84 (quint,

2

³J_HH= 7.2 Hz, 2H, NCH₂CH CH₂CH₃), 4.16 (t, ³J_HH

=

2

3

7.2 Hz, 2H, NCH CH₂CH₂CH₃), 7.19 (dd, J_HH

2

3

1-Picolyl-4-azabenzimidazole (6a). Yield: 1.045

8.1 Hz, J_HH= 4.8 Hz, 1H, H6), 7.70 (dd, J_HH

1

8.1 Hz, ⁴J_HH= 1.5 Hz, 1H, H7), 8.07 (s, 1H, H2), 8.53

(dd, ³J_HH= 4.8 Hz, ⁴J_HH= 1.5 Hz, 1H, H5). ¹³C NMR

(50.3 MHz, CDCl₃): δ 13.4 (NCH₂CH₂CH₂CH₃), 19.9

(NCH₂CH₂CH₂CH₃), 31.8 (NCH₂CH₂CH₂CH₃), 45.3

(NCH₂CH₂CH₂CH₃), 117.8 (C6), 117.9 (C7), 126.1

(C7a), 144.8 (C5), 145.0 (C2), 156.4 (C3a). MS (EI):

m/z (%) = 175 ([4a]⁺, 100), 132 ([4a – Pr]⁺, 27]. Calcd

for C₁₀H₁₃N₃·0.35H₂O: C, 66.16; H, 7.61; N, 23.15%.

Found: C, 66.35; H, 7.74; N, 22.78%.

g (59%). H NMR (400.0 MHz, CDCl₃): δ 5.46 (s,

3

2H, NCH₂-pyridine), 7.00 (d, J_HH= 7.7 Hz, 1H,

3

pyridine-H), 7.15 (dd, J_HH= 8.2 Hz, J_HH= 4.8 Hz,

3

1H, H6), 7.21 (ddd, J_HH= 7.7 Hz, J_HH= 4.8 Hz,

⁴J_HH= 0.5 Hz, 1H, pyridine-H), 7.61 (td, J_HH

=

3

4

7.7 Hz, J_HH= 1.8 Hz, 1H, pyridine-H), 7.66 (dd,

³J_HH= 8.2 Hz, J_HH= 1.5 Hz, 1H, H7), 8.25 (s, 1H,

4

3

4

H2), 8.51 (dd, J_HH= 4.8 Hz, J_HH= 1.5 Hz, 1H,

H5), 8.56 (d, J_HH= 4.8 Hz, 1H, pyridine-H). ¹³C

3

Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 485

4

³J_HH= 8.1 Hz, J_HH= 1.2 Hz, 1H, H7), 8.49 (dd,

NMR (100.6 MHz, CDCl₃): δ 51.0 (NCH₂-pyridine),

4

³J_HH= 4.5 Hz, J_HH= 1.2 Hz, 1H, H5). ¹³C NMR

118.3 (C6), 118.5 (pyridine-C), 121.3 (pyridine-

C), 123.3 (C7), 129.6 (C7a), 137.3 (pyridine-C),

145.0 (C5), 145.5 (C2), 145.8 (pyridine-C), 149.9

(pyridine-C), 154.6 (C3a). MS (EI): m/z (%) =

210 ([6a]⁺, 100). Calcd for C₁₂H₁₀N₄: C, 68.56; H,

4.79; N, 26.65%. Found: C, 68.15; H, 4.69; N, 26.47%.

(100.6 MHz, CDCl₃): δ 13.7 (NCH₂CH₂CH₂CH₃), 20.0

(NCH₂CH₂CH₂CH₃), 31.8 (NCH₂CH₂CH₂CH₃), 47.6

(NCH₂CH₂CH₂CH₃), 107.5 (C2), 117.2 (C6), 118.0

(C7), 128.2 (C7a), 144.9 (C5), 157.4 (C3a). MS (EI):

m/z (%) = 301 ([7a]⁺, 9), 174 ([7a – I]⁺, 100). Calcd

for C₁₀H₁₂N₃I: C, 39.88; H, 4.02; N, 13.96%. Found:

C, 40.11; H, 3.85; N, 13.57%.

3-Picolyl-4-azabenzimidazole (6b). Yield: 0.450 g

1

(26%). H NMR (400.0 MHz, CDCl₃): δ 5.55 (s, 2H,

3-Butyl-2-iodo-4-azabenzimidazole (7b). Yield:

NCH₂-pyridine), 7.17–7.10 (m, 2H, pyridine-H), 7.19

0.217 g (36%). ¹H NMR (400.0 MHz, CDCl₃): δ

(dd, ³J_HH= 8.1 Hz, ³J_HH= 4.9 Hz, 1H, H6), 7.55 (td,

3

4

0.98 (t, J_HH= 7.5 Hz, 3H, NCH₂CH₂CH₂CH ), 1.42

³J_HH= 7.7 Hz, J_HH= 1.7 Hz, 1H, pyridine-H), 8.03

3

4

(sext, J_HH= 7.5 Hz, 2H, NCH₂CH₂CH CH₃), 1.85

2

(dd, J_HH= 8.1 Hz, J_HH= 1.2 Hz, 1H, H7), 8.20

3

4

(quint, J_HH= 7.5 Hz, 2H, NCH₂CH CH₂CH₃), 4.29

2

(s, 1H, H2), 8.35 (dd, J_HH= 4.9 Hz, J_HH= 1.2 Hz,

3

(t, J_HH= 7.5 Hz, 2H, NCH CH₂CH₂CH₃), 7.17 (dd,

1H, H5), 8.51 (d, ³J_HH= 4.7 Hz, 1H, pyridine-H). ¹³

C

2

3

³J_HH= 8.1 Hz, J_HH= 4.8 Hz, 1H, H6), 7.97 (dd,

NMR (100.6 MHz, CDCl₃): δ 48.4 (NCH₂-pyridine),

118.2 (C6), 121.9 (pyridine-C), 122.9 (pyridine-C),

127.9 (C7), 135.1 (C7a), 137.0 (pyridine-C), 144.3

(C5), 144.4 (C2), 146.9 (pyridine-C), 149.6 (pyridine-

C), 155.4 (C3a). MS (EI): m/z (%) = 210 ([6b]⁺, 100).

Calcd for C₁₂H₁₀N₄: C, 68.56; H, 4.79; N, 26.65%.

Found: C, 68.23; H, 4.67; N, 26.39%.

4

³J_HH= 8.1 Hz, J_HH= 1.4 Hz, 1H, H7), 8.32 (dd,

4

³J_HH= 4.8 Hz, J_HH= 1.4 Hz, 1H, H5). ¹³C NMR

(100.6 MHz, CDCl₃): δ 13.7 (NCH₂CH₂CH₂CH₃), 20.0

(NCH₂CH₂CH₂CH₃), 31.8 (NCH₂CH₂CH₂CH₃), 45.9

(NCH₂CH₂CH₂CH₃), 106.0 (C2), 118.5 (C6), 126.5

(C7), 137.2 (C7a), 144.1 (C5), 147.9 (C3a). MS (EI):

m/z (%) = 174 ([7b – I]⁺, 100). Calcd for C₁₀H₁₂N₃I: C,

39.88; H, 4.02; N, 13.96%. Found: C, 40.24; H, 4.12;

N, 13.76%.

General Procedure for the Synthesis of N-Alkyl-2-

iodo-4-azabenzimidazole Derivatives. An N-alkyl-4-

azabenzimidazole 4a,b or 5a (2.0 mmol) was dis-

solved in THF (20 mL). The solution was cooled to

−40^◦C, and methyl lithium (1.6 M in hexane,

1.63 mL, 2.6 mmol) was added dropwise. The re-

action mixture was stirred for 40 min at –40^◦C. Af-

ter this time, elemental iodine (0.61 g, 2.4 mmol)

was added to the solution and the reaction mixture

was allowed to slowly warm up to ambient tempera-

ture. It was stirred for additional 4 h. The remaining

iodine was removed from the reaction mixture by

washing the THF solution with a saturated aque-

ous solution of sodium thiosulfate (5 mL). Then the

THF solution was washed with brine (10 mL) and the

2-iodo-4-azabenzimidazole derivative was extracted

with dichloromethane. The dichloromethane solu-

tion was dried with MgSO₄, the solvent was re-

moved in vacuo, and the crude reaction product

was puriﬁed by column chromatography (SiO₂, hex-

ane:EtOAc 1:1 or CH₃CN:CH₂Cl₂1:3). The 2-iodo-4-

azabenzimidazoles were obtained as bright yellow

compounds.

1-Benzyl-2-iodo-4-azabenzimidazole (8a). Yield:

0.086 g (13%). H NMR (400.0 MHz, CDCl₃): δ 5.40

(s, 2H, NCH₂-Ph), 7.10 (dd, J_HH= 8.1 Hz, J_HH

1

3

=

4.8 Hz, 1H, H6), 7.16–7.12 (m, 2H, Ph-H), 7.36–7.30

(m, 3H, Ph-H), 7.54 (dd, ³J_HH= 8.1 Hz, ⁴J_HH= 1.5 Hz,

1H, H7), 8.49 (dd, ³J_HH= 4.8 Hz, ⁴J_HH= 1.5 Hz, 1H,

H5). ¹³C NMR (100.6 MHz, CDCl₃): δ 51.3 (NCH₂-

Ph), 108.5 (C2), 118.1 (C6), 118.4 (C7), 126.8 (Ph-C),

127.6 (C7a), 128.5 (Ph-C), 129.2 (Ph-C), 134.5 (Ph-

C), 145.1 (C5), 157.4 (C3a). MS (EI): m/z (%) = 335

([8a]⁺, 100), 208 ([8a – I]⁺, 62), 91 (CH₂C₆H₅, 28).

Calcd for C₁₃H₁₀N₃I: C, 46.58; H, 3.01; N, 12.54%.

Found: C, 46.31; H, 2.72; N, 12.24%.

2,2^ꢀ-Bis(1-butyl-4-azabenzimidazole) (9a). Yield:

0.077 g (11%). ¹H NMR (400.0 MHz, CDCl₃): δ 0.89 (t,

³J_HH= 7.4 Hz, 6H, NCH₂CH₂CH₂CH ), 1.43–1.30 (m,

3

4H, NCH₂CH₂CH CH₃), 1.77 (quint, J_HH= 7.4 Hz,

2

3

4H, NCH₂CH CH₂CH₃), 4.15 (t, J_HH= 7.4 Hz, 4H,

2

3

NCH CH₂CH₂CH₃), 7.32 (dd, J_HH= 8.2 Hz, J_HH

=

2

3

4

4.7 Hz, 2H, H6), 7.84 (dd, J_HH= 8.2 Hz, J_HH

3

4

1.4 Hz, 2H, H7), 8.62 (dd, J_HH= 4.7 Hz, J_HH

1.4 Hz, 2H, H5). MS (EI): m/z (%) = 348 ([(9a]⁺,

100), 291 ([9a-Bu]⁺, 65).

1-Butyl-2-iodo-4-azabenzimidazole (7a). Yield:

1

0.048 g (8%). H NMR (400.0 MHz, CDCl₃): δ 0.98

3

(t, J_HH= 7.4 Hz, 3H, NCH₂CH₂CH₂CH ), 1.40

3

2,2^ꢀ-Bis(3-butyl-4-azabenzimidazole) (9b). Yield:

(sext, J_HH= 7.4 Hz, 2H, NCH₂CH₂CH CH₃), 1.80

2

3

1

(quint, J_HH= 7.4 Hz, 2H, NCH₂CH CH₂CH₃), 4.17

0.035 g (5%). H NMR (200.1 MHz, CDCl₃): δ 0.91

2

3

(t, ³J_HH= 7.4 Hz, 6H, NCH₂CH₂CH₂CH ), 1.36 (sext,

(t, J_HH= 7.4 Hz, 2H, NCH CH₂CH₂CH₃), 7.16 (dd,

2

3

³J_HH= 8.1 Hz, J_HH= 4.5 Hz, 1H, H6), 7.67 (dd,

3

³J_HH= 7.4 Hz, 4H, NCH₂CH₂CH CH₃), 1.87 (quint,

2

Heteroatom Chemistry DOI 10.1002/hc

486 Jahnke et al.

³J_HH= 7.4 Hz, 4H, NCH₂CH CH₂CH₃), 5.04 (t,

0.08 mmol) was dissolved in THF (4 mL). The

metal precursor [IrCl₂Cp*]₂(30 mg, 0.4 mmol) was

added to this solution, and the reaction mixture

was heated under reﬂux for 12 h. The solvent was

then removed in vacuo, and the remaining solid

was recrystallized from THF leaving complex 12

as a yellow solid. Yield: 0.024 g (43%). ¹H NMR

2

³J_HH= 7.4 Hz, 4H, NCH CH₂CH₂CH₃), 7.31 (dd,

2

3

³J_HH= 8.0 Hz, J_HH= 4.8 Hz, 2H, H6), 8.13 (dd,

4

³J_HH= 8.0 Hz, J_HH= 1.4 Hz, 2H, H7), 8.52 (dd,

4

³J_HH= 4.8 Hz, J_HH= 1.4 Hz, 2H, H5). MS (EI): m/z

(%) = 348 ([9b]⁺, 100), 291 ([9b – Bu]⁺, 71).

2,2^ꢀ-Bis(1-benzyl-4-azabenzimidazole)

(10a).

3

(400.0 MHz, CDCl₃): δ 0.97 (t, J_HH= 7.5 Hz, 3H,

Yield: 0.019 g (2%). MS (EI): m/z (%) = 416 ([10a]⁺,

3

NCH₂CH₂CH₂CH ), 1.42 (sept, J_HH= 7.5 Hz, 2H,

3

15), 325 ([10a-Bn]⁺, 100).

NCH₂CH₂CH CH₃), 1.59 (s, 15H, Cp*-CH₃), 1.83

2

3

(quint, J_HH= 7.5 Hz, 2H, NCH₂CH CH₂CH₃), 4.34

2

3

Synthesis of the N-Methyl-2-chloro-4-azabe-

nzimidazoles 11a,b. An N-methyl-4-azabenzimida-

zole derivative (0.185 g, 1.39 mmol) was

(t, J_HH= 7.5 Hz, 2H, NCH CH₂CH₂CH₃), 7.17 (dd,

2

³J_HH= 8.2 Hz, ³J_HH= 4.8 Hz, 1H, H6), 8.27 (s, br, 1H,

H7), 8.31 (dd, ³J_HH= 4.8 Hz, ⁴J_HH= 1.4 Hz, 1H, H5).

¹³C NMR (100.6 MHz, CDCl₃): δ 9.5 (Cp*-CH₃), 13.7

(NCH₂CH₂CH₂CH₃), 20.0 (NCH₂CH₂CH₂CH₃), 31.7

(NCH₂CH₂CH₂CH₃), 46.5 (NCH₂CH₂CH₂CH₃), 86.1

(Cp*-C), 118.5 (C6), 127.9 (C7), 136.9 (C7a), 144.6

(C5), 147.7 (C3a). The resonance of the C2 carbon

atom could not be detected. MS (MALDI-TOF,

DCTB): m/z (%) = 664 ([12 – Cl]⁺, 17].

dissolved in THF (3 mL). LDA (2.0

M in

THF/heptane/ethylbenzene, 1.25 mL, 2.5 mmol)

was added dropwise to this solution at –78^◦C, and

the reaction mixture was then stirred for 1 h. Then

N-chlorosuccinimide (0.37 g, 2.8 mmol) was added

to the cold reaction mixture. The mixture was

allowed to warm up to ambient temperature and

was stirred for an additional 10 min. A saturated

aqueous solution of ammonium chloride was added

to the reaction mixture, and the resulting solution

was extracted four times with dichloromethane

(15 mL each). The combined organic layers were

washed with brine and dried with sodium sul-

fate. The solvent was removed in vacuo, and the

crude reaction product was puriﬁed by column

chromatography (SiO₂, CH₂Cl₂:MeOH 20:1). The

N-methyl-2-chloro-4-azabenzimidazole derivatives

were obtained as orange brown solids.

General Procedure for the Synthesis of the 1-Alkyl-

3-(2-oxopropyl)-4-azabenzimidazolium Chlorides 13–

17. One of the 1-alkyl-4-azabenzimidazole deriva-

tives 1a, 2a, 4a–6a (2.0 mmol) was dissolved in

THF (10 mL). An excess of chloracetone (0.63 mL,

8.0 mmol) was added dropwise to the solution, and

the reaction mixture was heated under reﬂux for

48 h. The solvent was removed in vacuo, and the

residue was dissolved in dichloromethane (3 mL).

This solution was added dropwise to diethyl ether,

and the precipitate obtained was separated from the

solvent by ﬁltration and dried in vacuo. The benzim-

idazolium salts were isolated as bright brown, very

hygroscopic solids.

2-Chloro-1-methyl-4-azabenzimidazole

(11a).

1

Yield: 0.021 g (9%). H NMR (400.0 MHz, CD₃OD):

δ 3.93 (s, 3H, NCH₃), 7.44 (dd, ³J_HH= 8.2 Hz, ³J_HH

=

3

4

4.9 Hz, 1H, H6), 8.08 (dd, J_HH= 8.2 Hz, J_HH

3

4

1.4 Hz, 1H, H7), 8.47 (dd, J_HH= 4.9 Hz, J_HH

1-Methyl-3-(2-oxopropyl)-4-azabenzimidazolium

1.4 Hz, 1H, H5). ¹³C NMR (100.6 MHz, CD₃OD): δ

31.4 (NCH₃), 120.1 (C6), 121.1 (C7), 130.5 (C7a),

145.0 (C5), 146.5 (C2), 153.8 (C3a). MS (EI): m/z

(%) = 167 ([11a]⁺, 100).

1

Chloride (13). Yield: 0.428 g (95%). H NMR (400.0

MHz, CD₃OD): δ 2.49 (s, 3H, C(O)CH₃), 3.39 (s, 2H,

3

NCH₂), 4.17 (s, 3H, NCH₃), 7.88 (dd, J_HH= 8.8 Hz,

³J_HH= 5.6 Hz, 1H, H6), 8.68 (s, 1H, H2), 8.83 (dd,

³J_HH= 8.8 Hz, ⁴J_HH= 0.6 Hz, 1H, H7), 8.92 (dd, ³J_HH

=

2-Chloro-3-methyl-4-azabenzimidazole

(11b).

4

5.6 Hz, J_HH= 0.6 Hz, 1H, H5). ¹³C NMR (100.6

MHz, CD₃OD): δ 27.4 (C(O)CH₃), 33.1 (NCH₃), 49.9

(NCH₂), 119.3 (C6), 129.4 (C7), 132.0 (C7a), 140.4

(C2), 149.2 (C3a), 151.4 (C5), 198.1 (C=O). MS

(MALDI-TOF, DHB): m/z (%) = 190 ([13 – Cl]⁺, 100).

1

Yield: 0.115 g (49%). H NMR (400.0 MHz, CDCl₃):

δ 3.86 (s, 3H, NCH₃), 7.22 (dd, ³J_HH= 8.0 Hz, ⁴J_HH

=

3

4

1.5 Hz, 1H, H6), 7.95 (dd, J_HH= 8.0 Hz, J_HH

3

4

1.5 Hz, 1H, H7), 8.37 (dd, J_HH= 4.9 Hz, J_HH

1.5 Hz, 1H, H5). ¹³C NMR (100.6 MHz, CD₃OD): δ

29.2 (NCH₃), 118.9 (C6), 126.8 (C7), 134.3 (C7a),

142.8 (C2), 144.1 (C5), 147.8 (C3a). MS (EI): m/z

(%) = 167 ([11b]⁺, 100).

1-Ethyl-3-(2-oxopropyl)-4-azabenzimidazolium

1

Chloride (14). Yield: 0.440 g (92%). H NMR (400.0

MHz, CDCl₃): δ 1.61 (t, ³J_HH= 7.3 Hz, 3H, NCH₂CH ),

3

κN1-(3-Butyl-2-iodo-4-azabenzimidazole) (pen-

tamethylcyclopentadienyl)iridium Dichloride (12).

The 3-butyl-2-iodo-4-azabenzimidazole 7b (23 mg,

2.46 (s, 3H, C(O)CH₃), 4.67 (q, J_HH= 7.3 Hz, 2H,

3

NCH CH₃), 6.41 (s, 2H, NCH₂), 7.77 (dd, J_HH

=

2

3

8.2 Hz, J_HH= 6.1 Hz, 1H, H6), 8.78 (s, 1H, H2),

Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 487

9.04 (dd, ³J_HH= 8.2 Hz, ⁴J_HH= 0.5 Hz, 1H, H7), 9.53

General Procedure for the Synthesis of the 1-Alkyl-

3

(d, J_HH= 6.1 Hz, 1H, H5). ¹³C NMR (100.6 MHz,

3-(2-oxopropyl)-4-azabenzimidazolium Tetraﬂuorob-

orates 18–19. One of the 1-alkyl-3-(2-oxopropyl)-4-

azabenzimidazolium chlorides 13 or 15 (0.5 mmol)

was dissolved in dichloromethane (15 mL). Silver

tetraﬂuoroborate (0.117 g, 0.6 mmol) was added to

this solution, and the reaction mixture was stirred

for 12 h at ambient temperature. The reaction mix-

ture was ﬁltrated through Celite, and the ﬁltrate was

dried in vacuo. The reaction products were obtained

as brown solids.

CDCl₃): δ 15.4 (NCH₂CH₃), 27.8 (C(O)CH₃), 42.4

(NCH₂CH₃), 62.1 (NCH₂), 119.0 (C6), 129.6 (C7),

130.5 (C7a), 140.9 (C2), 148.9 (C3a), 149.7 (C5),

198.0 (C=O). MS (MALDI-TOF, DHB): m/z (%) =

204 ([14 – Cl]⁺, 100).

1-Butyl-3-(2-oxopropyl)-4-azabenzimidazolium

Chloride (15). Yield: 0.480 g (90%). ¹H NMR

3

(200.1 MHz, CDCl₃): δ 0.87 (t, J_HH= 7.2 Hz, 3H,

3

NCH₂CH₂CH₂CH ), 1.32 (sext, J_HH= 7.2 Hz, 2H,

3

NCH₂CH₂CH CH₃), 1.86 (quint, J_HH= 7.2 Hz, 2H,

2

1-Methyl-3-(2-oxopropyl)-4-azabenzimidazolium

Tetraﬂuoroborate (18). Yield: 0.137 g (98%). ¹H

NMR (400.0 MHz, CD₃OD): δ 2.47 (s, 3H, C(O)CH₃),

3.39 (s, 3H, NCH₃), 5.99 (s, 2H, NCH₂), 7.93 (dd,

NCH₂CH CH₂CH₃), 2.42 (s, 3H, C(O)CH₃), 4.56 (t,

2

³J_HH= 7.2 Hz, 2H, NCH CH₂CH₂CH₃), 6.29 (s, 2H,

2

3

NCH₂), 7.75 (dd, J_HH= 7.9 Hz, J_HH= 6.4 Hz,

3

1H, H6), 8.75 (s, 1H, H2), 8.97 (d, J_HH= 7.9 Hz,

3

³J_HH= 8.2 Hz, J_HH= 8.2, 6.2 Hz, 1H, H6), 8.67 (d,

1H, H7), 9.45 (d, J_HH= 6.4 Hz, 1H, H5). ¹³C NMR

3

³J_HH= 6.2 Hz, 1H, H7), 8.82 (s, 1H, H2), 8.91 (d, ³J_HH

= 8.2 Hz, 1H, H5). ¹³C NMR (100.6 MHz, CD₃OD):

δ 27.2 (C(O)CH₃), 33.1 (NCH₃), 63.1 (NCH₂), 119.9

(C6), 124.3 (C7), 130.5 (C7a), 140.9 (C2), 150.1

(C3a), 153.2 (C5), 199.4 (C=O). ¹⁹F NMR (376 MHz,

CD₃OD): δ –154.5 (BF₄⁻). MS (MALDI-TOF, DHB):

m/z (%) = 190 ([18 – BF₄]⁺, 100).

(50.3 MHz, DMSO-d₆): δ 13.3 (NCH₂CH₂CH₂CH₃),

19.2 (NCH₂CH₂CH₂CH₃), 27.3 (C(O)CH₃), 31.2

(NCH₂CH₂CH₂CH₃), 45.8 (NCH₂CH₂CH₂CH₃), 61.9

(NCH₂), 118.5 (C6), 129.8 (C7), 130.6 (C7a), 139.8

(C2), 148.5 (C3a), 151.3 (C5), 199.1 (C=O). MS

(MALDI-TOF, DHB): m/z (%) = 232 ([15 – Cl]⁺, 100).

1-Benzyl-3-(2-oxopropyl)-4-azabenzimidazolium

1-Butyl-3-(2-oxopropyl)-4-azabenzimidazolium

1

Chloride (16). Yield: 0.337 g (56%). H NMR (200.1

Tetraﬂuoroborate (19). Yield: 0.158 g (99%). ¹H

MHz, DMSO-d₆): δ 2.40 (s, 3H, C(O)CH₃), 5.83 (s,

NMR (400.0 MHz, CD₃OD): δ 1.03 (t, ³J_HH= 7.3 Hz,

2H, NCH₂-Ph), 6.06 (s, 2H, NCH₂), 7.45–7.31 (m,

3

3H, NCH₂CH₂CH₂CH ), 1.46 (sept, J_HH= 7.3 Hz,

3

3H, Ph-H), 7.55–7.45 (m, 2H, Ph-H), 7.96 (dd, J_HH

3

2H, NCH₂CH₂CH CH₃), 2.00 (quint, J_HH= 7.3 Hz,

3

2

= 8.2 Hz, J_HH= 6.0 Hz, 1H, H6), 8.94 (d, J_HH

=

2H, NCH₂CH CH₂CH₃), 2.47 (s, 3H, C(O)CH₃), 4.56

3

2

6.0 Hz, 1H, H7), 9.05 (d, J_HH= 8.2 Hz, 1H, H5),

9.39 (s, 1H, H2). ¹³C NMR (100.6 MHz, CD₃OD): δ

27.3 (C(O)CH₃), 51.4 (NCH₂-Ph, NCH₂-C(O)), 120.1

(C6), 129.2 (Ph-C), 130.1 (C7a), 130.4 (Ph-C), 130.5

(Ph-C), 130.9 (C7), 135.6 (Ph-C), 141.2 (C2), 150.2

(C3a), 152.6 (C5), 199.5 (C=O). MS (MALDI-TOF,

DHB): m/z (%) = 266 ([16 – Cl]⁺, 100).

3

(t, J_HH= 7.3 Hz, 2H, NCH CH₂CH₂CH₃), 5.99 (s,

2

3

2H, NCH₂), 7.93 (dd, J_HH= 8.0 Hz, J_HH= 6.3 Hz,

3

1H, H6), 8.67 (d, J_HH= 6.3 Hz, 1H, H7), 8.90 (s,

1H, H2), 8.96 (d, J_HH= 8.0 Hz, 1H, H5). ¹³C NMR

3

(100.6 MHz, CD₃OD): δ 13.8 (NCH₂CH₂CH₂CH₃),

20.8 (NCH₂CH₂CH₂CH₃), 27.2 (C(O)CH₃), 32.8

(NCH₂CH₂CH₂CH₃), 47.6 (NCH₂CH₂CH₂CH₃), 63.1

(NCH₂), 119.9 (C6), 124.5 (C7), 130.7 (C7a), 141.0

(C2), 150.2 (C3a), 152.5 (C5), 199.5 (C=O). ¹⁹F

NMR (376 MHz, CD₃OD): δ –154.4 (BF⁻₄). MS

(MALDI-TOF, DHB): m/z (%) = 232 ([19 – BF₄]⁺,

100).

1-Picolyl-3-(2-oxopropyl)-4-azabenzimidazolium

1

Chloride (17). Yield: 0.496 g (82%). H NMR (200.1

MHz, CDCl₃): δ 2.45 (s, 3H, C(O)CH₃), 6.00 (s, 2H,

NCH₂-pyridine), 6.37 (s, 2H, NCH₂-C(O)), 7.24–7.16

(m, 1H, pyridine-H), 7.74–7.57 (m, 2H, pyridine-H),

3

8.43 (d, J_HH= 4.7 Hz, 1H, pyridine-H), 8.54 (dd,

{[1-Butyl-3-(2-oxopropyl)-4-azabenzimidazolin-2-

ylidene]gold(I) Chloride} (20). 4-Azabenzimidazo-

lium chloride 15 (42 mg, 0.156 mmol) was dis-

solved in dichloromethane (4 mL), and silver oxide

(18 mg, 0.078 mmol) was added to this solution.

The reaction mixture was stirred for 1 h at ambient

temperature under exclusion of light. The mixture

was then ﬁltered through Celite, and the ﬁltrate

was treated with [AuCl(tht)] (50 mg, 0.156 mmol).

The reaction mixture was stirred for 30 min under

3

³J_HH= 9.5 Hz, J_HH= 5.6 Hz, 1H, H6), 9.04 (s,

3

1H, H2), 9.14 (d, J_HH= 9.5 Hz, 1H, H7), 9.42

3

(d, J_HH= 5.6 Hz, 1H, H5). ¹³C NMR (50.3 MHz,

CDCl₃): δ 27.7 (C(O)CH₃), 51.8 (NCH₂-pyridine),

62.1 (NCH₂-C(O)), 118.8 (C6), 123.6 (pyridine-C),

123.8 (pyridine-C), 130.6 (C7), 131.1 (C7a), 137.6

(pyridine-C), 144.9 (C2), 145.6 (pyridine-C), 149.8

(pyridine-C), 151.0 (C3a), 152.9 (C5), 198.0 (C=O).

MS (MALDI-TOF, DHB): m/z (%) = 267 ([17 – Cl]⁺,

100).

Heteroatom Chemistry DOI 10.1002/hc

488 Jahnke et al.

exclusion of light and then ﬁltered through Celite.

solution was ﬁltered again through Celite, and the

Removal of the solvent in vacuo gave complex 20

solvent was removed in vacuo to give complex 21.

1

as pale yellow solid. Yield: 0.048 g (66%). H NMR

Yield: 0.033 g (41%). H NMR (400.0 MHz, CDCl₃):

3

(400.0 MHz, CDCl₃): δ 0.97 (t, J_HH= 7.4 Hz, 3H,

δ 1.05 (t, J_HH= 7.4 Hz, 3H, NCH₂CH₂CH₂CH ),

3

NCH₂CH₂CH₂CH ), 1.43 (sept, J_HH= 7.4 Hz, 2H,

1.43–1.34 (m, 2H, NCH₂CH₂CH CH₃), 1.70 (s, 15H,

3

2

3

NCH₂CH₂CH CH₃), 1.94 (quint, J_HH= 7.4 Hz, 2H,

Cp*-H), 1.93–1.84 (m, 2H, NCH₂CH CH₂CH₃),

2

3

NCH₂CH CH₂CH₃), 2.37 (s, 3H, C(O)CH₃), 4.50

2.17 (s, 3H, C(O)CH₃), 4.16 (t, J_HH= 7.4 Hz, 2H,

2

3

(t, J_HH= 7.4 Hz, 2H, NCH CH₂CH₂CH₃), 5.42

NCH CH₂CH₂CH₃), 5.29 (s, 2H, NCH₂), 7.20 (dd,

(s, 2H, NCH₂), 7.39 (dd, J_HH²= 8.2 Hz, J_HH

=

³J_HH= 8.2 Hz, ³J_HH= 4.9 Hz, 1H, H6), 7.74 (dd, ³J_HH

2

3

4

3

4.8 Hz, 1H, H6), 7.83 (dd, J_HH= 8.2 Hz, J_HH

= 8.2 Hz, J_HH= 1.3 Hz, 1H, H7), 8.32 (dd, J_HH=

3

4

1.2 Hz, 1H, H7), 8.45 (dd, J_HH= 4.8 Hz, J_HH

4.9 Hz, ⁴J_HH= 1.3 Hz, 1H, H5). ¹³C NMR (100.6 MHz,

CDCl₃): δ 9.1 (Cp*-CH₃), 13.7 (NCH₂CH₂CH₂CH₃),

20.1 (NCH₂CH₂CH₂CH₃), 27.3 (C(O)CH₃), 29.8

(NCH₂CH₂CH₂CH₃), 44.0 (NCH₂CH₂CH₂CH₃), 51.6

(NCH₂), 90.7 (Cp*-C), 113.1 (C6), 121.4 (C7), 137.0

(C7a), 148.0 (C5), 154.6 (C3a), 163.2 (C2), 203.9

(C=O). MS (MALDI-TOF, DCTB): m/z (%) = 594

([21 – Cl]⁺, 19).

1.2 Hz, 1H, H5). ¹³C NMR (100.6 MHz, CDCl₃): δ

13.7 (NCH₂CH₂CH₂CH₃), 20.0 (NCH₂CH₂CH₂CH₃),

27.4 (C(O)CH₃), 32.0 (NCH₂CH₂CH₂CH₃), 49.7

(NCH₂CH₂CH₂CH₃), 55.6 (NCH₂), 119.6 (C6), 119.9

(C7), 129.0 (C7a), 145.9 (C5), 149.6 (C3a), 181.5

(C2), 198.7 (C = O). MS (EI): m/z (%) = 428 ([20 –

Cl]⁺, 95), 385 ([20 – Cl – Pr]⁺, 100), 328 ([20 – Cl –

Pr – C₂H₅O]⁺, 16).

{(Pentamethylcyclopentadienyl)-(1-butyl-4-azabe-

nzimidazolin-2-ylidene)iridium(III) Dichloride} (22).

Compound 21 was subjected to slow column

chromatography (SiO₂, CH₂Cl₂: acetone 5:1) over a

period of 4 h to remove the protecting group. The

column chromatography was repeated, upon which

complex 22 was isolated as a yellow solid. Yield:

{(Pentamethylcyclopentadienyl)-[1-butyl-3-(2-oxo-

propyl)-4-azabenzimidazolin-2-ylidene]-iridium(III)

Dichloride} (21). 4-Azabenzimidazolium chloride

15 (0.034 g, 0.126 mmol) was dissolved in

dichloromethane (4 mL), and silver oxide (18 mg,

0.078 mmol) was added to this solution. The reaction

mixture was stirred for 1 h at ambient temperature

under exclusion of light and then ﬁltered through

Celite. [IrCl₂Cp*]₂(50 mg, 0.063 mmol) was added

to the ﬁltrate, and the reaction mixture was stirred

for 30 min under exclusion of light. The reaction

1

0.017 g (24%). H NMR (400.0 MHz, CDCl₃): δ 1.02

3

(t, J_HH= 7.3 Hz, 3H, NCH₂CH₂CH₂CH ), 1.57–1.46

3

(m, 2H, NCH₂CH₂CH CH₃), 1.72 (s, 15H, Cp*-CH₃),

2

2.02 (m, br, 2H, NCH₂CH CH₂CH₃), 4.49 (m, br, 2H,

2

3

NCH CH₂CH₂CH₃), 7.23 (dd, J_HH= 7.9 Hz, J_HH

=

2

TABLE 2 Summary of Crystallographic Data for 12 and 20.

12

20

Formula

C₂₀H₂₇Cl₂IIrN₃

C₁₃H₁₇AuClN₃O

M_r

699.45

463.71

Crystal size (mm)

a (A)

b (A)

0.16 × 0.02 × 0.02

0.11 × 0.07 × 0.06

˚

7.3874(5)

11.2920(5)

7.9644(3)

20.2033(15)

˚

c (A)

15.1883(11)

15.9897(7)

α(^◦)

β(^◦)

90

93.7420(10)

90

2262.0(3)

4

90

96.3520(10)

90

1429.19(10)

4

γ(^◦)

3

˚

V(A )

Z

Space group

P2₁/n

2.054

7.514 (Mo Kα)

3.4–60.0

22,543

6,614

P2₁/c

2.155

10.476 (Mo Kα)

3.6–61.0

16,726

4,344

ρ_calcd(g cm⁻³

)

μ (mm⁻¹

)

2θ range (^◦)

Data collected

Unique data

Observed data [I ≥ 2σ(I )]

5,272

3,989

R (all data)

0.0497

0.0781

250

0.0231

0.0501

174

wR (all data)

No. of variables

−3

˚

Peak/hole (e·A

)

2.279, −1.363

1.428, −0.896

Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Coordination Chemistry of 4-Azabenzimidazole Derivatives 489

3

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5.8 Hz, 1H, H6), 7.71 (d, J_HH= 7.9 Hz, 2H, H7),

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(MALDI-TOF, DCTB): m/z (%) = 573 ([22]⁺, 29), 538

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X-Ray Diffraction Studies

Diffraction data for 12 and 20 were collected

with a Bruker AXS APEX CCD diffractometer

equipped with a rotation anode, using graphite-

monochromated Mo Kα radiation (λ = 0.71073

˚

A) at 293(2) K (12) or 153(2) K (20). Data were

collected over the full sphere and were corrected

for absorption. For further crystal data details,

see Table 2. Structures of solutions were found

with the SHELXS-97 [38] package using the heavy-

atom method and were reﬁned with SHELXL-

97 [38] against all F²using ﬁrst isotropic and

later anisotropic thermal parameters for all non-

hydrogen atoms. Hydrogen atoms were added to the

structure models at calculated positions, and their

thermal parameters were ﬁxed to 1.3 U_eqivof the par-

ent atom. Crystallographic data (excluding structure

factors) have been deposited with the Cambridge

Crystallographic Data Centre as supplementary pub-

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797003 (20). Copies of these data can be obtained

free of charge on application to CCDC, 12 Union

Road, Cambridge CB2 1EZ, UK [Fax: +44–1223-

336033, www.ccdc.cam.ac.uk./data/request/cif].

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Heteroatom Chemistry DOI 10.1002/hc

Article Doi

Synthesis and coordination chemistry of 4-azabenzimidazole derivatives

DOI: 10.1002/hc.20711

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