Bi- and trinuclear oxalamidinate complexes of palladium as catalysts in the copper-free Sonogashira reaction and in the Negishi reaction

Journal of Organometallic Chemistry 689 (2004) 3582–3592

www.elsevier.com/locate/jorganchem

Bi- and trinuclear oxalamidinate complexes of palladium as

catalysts in the copper-free Sonogashira reaction and

in the Negishi reaction

*

Sven Rau , Katja Lamm, Helmar Go¨rls, Julia Scho¨ﬀel, Dirk Walther

**

Institut fu¨r Anorganische und Analytische, Chemie der Friedrich-Schiller-Universita¨t Jena, August-Bebel-Straße 2, 07743 Jena, Germany

Received 11 May 2004; accepted 2 July 2004

Available online 15 September 2004

Abstract

The binuclear complex [(acac)Pd(oxam)Pd(acac)] 1 (oxam: tetraphenyl oxalic amidinate) has been prepared from H₂oxam and

Pd(acac)₂in excellent yield. The complex was characterized by elemental analyses, mass spectroscopy, ¹H NMR, ¹³C NMR spectr-

oscopy and in the solid state by X-ray single crystal diﬀraction analyses. 1 consists of a bimetallic centrosymmetric unit in which the

planar oxam ligand acts in a bis-chelating fashion. Each palladium center is in a planar environment.

The complex 1 acts as highly selective pre-catalyst in the copper-free Sonogashira reaction between 4-bromoacetophenone and

phenylacetylene. Its long-time catalytic activity is higher than that of the related binuclear complex 2 (oxam: tetra-p-tolyl oxalic ami-

dinate) or that of the trinuclear compound [(acac)Pd(oxam)Zn(oxam)Pd(acac)] (3), the solid-state structure of which was also deter-

mined by an X-ray structural analysis of single crystals. In addition, 2 is an active and extremely selective pre-catalyst for the Negishi

reaction between 3,5,6,8-tetrabromophenanthroline and R–C„C–ZnCl (R: Ph, (ⁱprop)₃Si) to form tetra-alkyne-substituted

derivatives.

Keywords: Palladium; Sonogashira reaction; Negishi reaction; Binuclear complexes; N-chelate ligands

1. Introduction

chelating ligands which are easily accessible. They bear

two reactive metal centers on the peripheries and there-

fore, both metals may serve as carriers of catalytic activ-

ity. This activity can be tuned by four substituents of the

nitrogen donor atoms of the ligand. Furthermore, the

catalyst metals in these complexes are connected via

oxalamidinate bridges which allow electronic communi-

cation between the peripheral metals. Due to their two-

fold anionic nature the oxalic amidinates are poor

p-acceptor ligands which do not form stable Ni(0) or

Pd(0) complexes. Nevertheless, Ni and Pd complexes

with oxalic amidinate bridges are active in the Kumada

cross coupling or in the Heck reaction, although,

according to the accepted mechanism of these reactions,

Ni(0) or Pd(0) species are postulated to be formed in

these catalytic reactions [6–11].

Recently we have described the syntheses and struc-

tures of oligonuclear nickel(II) and palladium(II) com-

plexes with oxalic amidinates bridging the metal

centers. Some of them have proven to be active catalysts

in a number of C–C-linking reactions [1–5].

From a general point of view these oligonuclear metal

complexes are interesting candidates for homogeneous

catalysts due to the following reasons: most of the com-

plexes are air-stable and contain very simple, cheap bis-

*

Corresponding authors. Tel.: +49 3641 948112.

Tel.: +49 3641 948110.

**

E-mail addresses: sven.rau@uni-jena.de (S. Rau), dirk.Walther@-

uni-jena.de (D. Walther).

doi:10.1016/j.jorganchem.2004.07.061

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

3583

To evaluate the catalytic potential of oligonuclear

oxalamidinate complexes in a range of other C–C-bond

reactions we now report on the copper-free Sonogashira

reaction and show that binuclear complex 1 is most ac-

tive and highly selective. In addition, 2 is an extremely

selective catalyst in the Negishi reaction of tetra-

bromo-phenanthroline.

N,N-chelate ligands for controlling this catalysis as

well as bi- and oligonuclear complexes catalyzing this

reaction are very rare [12,13].

2. Results and discussion

2.1. Synthesis and structures of the complexes 1–4

Scheme 1 shows the molecular formula of the com-

plexes 1–4. Complex 1 can be obtained by treating

tetra(phenyl)oxalamidine (H₂A) with Pd(acac)₂(acac:

acetylacetonate) following a general synthesis for binu-

clear oxalamidinate Ni- and Pd complexes in good yield

[1,2]. Complex 2 was already described using the same

preparative method [2].

˚

Fig. 1. Molecular structure of complex 1; selected bond lengths (A)

and bond angles (ꢁ) Pd–O1 1.9953(16), Pd–O2 2.0066(15), Pd–N1

2.0117(18), Pd–N2 2.0091(18), N1–C1 1.325(3), N2–C1A 1.330(3), C1–

C1A 1.510(4) N1–C1–N2A 131.19(19), O1–Pd–O2 91.54(7), N1–Pd–

N2 80.82(7).

The binuclear complex 1 was characterized by ele-

mental analyses, ¹H NMR, ¹³C NMR and mass spectro-

scopy. The solid-state structure of single crystals of 1

was established by an X-ray study (Fig. 1). As expected,

the two Pd ions are in a square-planar coordination

sphere created by two N atoms of the bridging ligand

and two O atoms of the terminal acac ligand. The dou-

ble chelate ligand moiety containing two CN₂units is

essentially planar, with all C–N bonds being equivalent.

The bond C–N distances in this centrosymmetric struc-

In addition, the aromatic phenyl groups at the four N

atoms are distorted out of the plane of the two CN₂

units (in the average 56ꢁ). Other selected bond distances

are listed in the caption of Fig. 1.

In comparison with the structure of 2 [2] only small

structural diﬀerences in the bond lengths and angles

are found. The Pd–N distances are equal (average

˚

2.0105 A); however, the Pd–O distances diﬀer slightly

˚

with 2.013(3) A for complex 2 and 2.001(2) A for com-

˚

ture are 1.325(3) and 1.330(3) A, indicating a partial

double bond character due to complete electron delocal-

ization over the CN₂units. The C1–C1A bond length in

plex 1 . The C1–C1A distances of the bridging ligand lie

˚

in the same range for both complexes (1.510(4) A (com-

˚

plex 1) and 1.519(9) A (complex 2)), just as the C–N

˚

the bridging ligand is that of a single bond (1.510(4) A).

˚

bonds of the CN₂units do (1.330(3) A (for 1) and

Ar Ar

N

O

Pd

Zn

N

Ar Ar

Ar = tolyl,3

Ar = phenyl,1

= tolyl,2

Ar Ar

N

Pd

Zn

N

Ar Ar

Ar = tolyl,4

Ar Ar

Scheme 1. The complexes 1–4.

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S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

ner Zn ion is coordinated in a distorted tetrahedral

geometry by four N atoms of two bridging oxalic amidi-

nates. The two N–Pd–N bite angles are only slightly

smaller than the two N–Zn–N bite angles. (80.5(2)ꢁ

and 80.96(19)ꢁ compared to 83.2(2)ꢁ and 82.7(2)ꢁ). The

two C–C bond lengths between the CN₂units of the

bridging oxalic amidinates lie in the range of single

bonds, as was already noted for the dinuclear Pd-com-

plexes 1 and 2. Again, the two CN₂units in the bridging

ligand are planar with all C–N bonds being equivalent.

Their bond lengths show a high double bond character

with bond distances ranging from 1.311(8) to

˚

1.341(7) A. The p-tolyl substituents at the N atoms of

the two bridging ligands are distorted out of the plane

of the CN₂units.

˚

Fig. 2. Molecular structure of complex 3; selected bond lengths (A)

and bond angles (ꢁ) Pd1–O1 1.999(5), Pd1–O2 2.010(4), Pd1–N1

2.003(5), Pd1–N3 2.008(6), Pd2–O3 2.012(4), Pd2–O4 2.000(4), Pd2–

N6 1.999(5), Pd2–N8 2.006(5), Zn–N2 1.997(5), Zn–N4 1.990(5), Zn–

N5 2.008(5), Zn–N7 1.993(5) C1–C2 1.518(9), C31–C32 1.524(8), C–N

bond lengths 1.311(8)–1.341(7) O1–Pd1–O2 91.13(19), O3–Pd2–O4

91.53(18), N1–Pd1–N3 80.5(2), N6–Pd2–N8 80.96(19), N2–Zn–N4

83.2(2), N5–Zn–N7 82.7(2), N4–Zn–N5 126.6(2), N4–Zn–N7 130.7(2),

N–C–N angles within the oxalic amidinates 130.8(6)–131.4(6).

The heterotetranuclear compound 4 containing two

peripheral palladium(II) centres and two inner zinc

atoms bridged by three tetra(p-tolyl)oxalic amidinate

ligands (Scheme 1) was synthesized according to

Scheme 2. In the ﬁrst step 2 equiv. of ZnEt₂were reacted

with 3 equiv. of the oxalic amidine H₂B at ꢀ70 ꢁC to

give an instable binuclear oxalamidinate zinc complex

(Scheme 2). The cold solution of this intermediate was

then added to a THF/toluene solution of (allyl)₂Pd at

ꢀ70 ꢁC. After work up 4 was isolated as yellow micro-

crystalline compound in low yield (ca. 25%). Analysis

and mass spectrum conﬁrmed its composition. In the

mass spectrum the tetranuclear complex ion [(allyl-

Pd)₂Zn₂(oxam)₃]⁺(m/z = 1759) was detected as basic

peak.

¹H and ¹³C NMR studies were conducted in CDCl₃

at room temperature and showed all expected shifts

and splitting patterns associated with the structure of

4. The ¹H NMR spectrum of 4 shows the expected

two singlets for the methyl groups at 2.04 and 2.07

ppm which integrated for 36H when compared with

the 48 phenyl protons and the signals of the 10 protons

of the allyl groups. The very simple pattern of the ¹³C

NMR spectrum supports the structure of 4. Only 13 sig-

nals were observed, corresponding to the two diﬀerent

CH₃groups (at 20.5 and 20.6 ppm), two signals for

the g³-bonded allyl groups (at 60.5 and 114.3 ppm),

˚

1.335(6) A (for 2)). The distortion of the aromatic

groups out of the plane of the CN₂units is about 56ꢁ

for complex 1 and 60ꢁ for complex 2.

The heterotrinuclear complex 3 (Scheme 1) was al-

ready obtained in a two-step reaction [2]. In the ﬁrst

step, a bis(oxalamidinato)Zn(II) complex was formed

in situ by treating 1 equiv. of ZnEt₂with a solution of

2 equiv. of the free oxalic amidine H₂B, followed by

the addition of 2 equiv. of Pd(acac)₂. Upon very careful

workup, we succeeded now in isolating single crystals of

complex 3. Its hitherto unknown solid-state structure is

depicted in Fig. 2.

The three metal centres of the heterotrinuclear com-

plex 3 are bridged by two tetra(p-tolyl)oxalic amidi-

nates. The two peripheral Pd ions are each

coordinated in a square-planar geometry created by

two O atoms of the terminal acetylacetonate ligand

(O–Pd–O angle 91.13(19)ꢁ and 91.53(18)ꢁ) and two N

atoms of the tetra(p-tolyl)oxalamidinate ligand. The in-

Scheme 2. Preparation of 4.

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

3585

Scheme 3. Catalytic copper-free coupling of 4-bromoacetophenone and phenylacetylene with the catalysts 1–3.

eight phenyl signals and one signal for the quarternary C

atoms. We assume that the signal at 160.8 ppm belongs

to the latter (see also Section 4).

manner. The most active pre-catalyst is complex 1, cou-

pling the substrates almost quantitatively in 36 h (entry

4). The structurally very similar dinuclear Pd complex 2

is less active than 1 (entries 9–11) and the heterotrinu-

clear Pd/Zn complex 3 in which the palladium atoms

are separated by an additional Zn complex moiety, is

also less active than 1 (entries 12–14). In addition, com-

plex 1 is also the most selective catalyst (100% selectiv-

ity), although the two other complexes are highly

selective catalysts as well.

Although the mechanism of the Sonogashira reaction

is not well established [14], it is generally accepted that

the formation of a Pd(0) species (e.g., by reductive elim-

ination of the product from an Ar–Pd–C„C–Ar⁰inter-

mediate) and the following oxidative addition of Ar–X

to regenerate a Pd(II) species are the key steps in the cat-

alytic cycle. Due to their dianionic nature the oxalamidi-

nates are poor p acceptor ligands which are not able to

stabilize Pd(0). Not surprisingly, all attempts to form

stable palladium(0) complexes with these ligands were

therefore unsuccessful. Nevertheless, 1–3 are active cat-

alysts. We assume therefore that in the catalytic cycle,

for example with 1 or 2 as pre-catalyst, the intermediate

A could be formed as the product of a reductive elimina-

tion reaction (Scheme 4).

2.2. Catalytic behaviour of 1–3 in the Sonogashira

reaction and Negishi reaction

The oligonuclear complexes 1–3 are diﬀerent to those

of commonly used homogeneous Pd-pre-catalysts for

the Sonogashira reaction [12] or for the Negishi reaction

[14,15]. In these catalytic C–C bond forming reactions

mononuclear compounds are used, mostly stabilized

by phosphines or carbenes or as palladacycles. Much

eﬀort has been directed toward the reduction of the

amount of Cu salt in the catalytic system preferently

running the coupling between an aromatic halide and

a terminal alkyne without any such additive. However,

only few copper-free catalyst systems for the Sonogash-

ira reactions were described [14,16–26].

In 1993, several Pd(0) complexes such as Pd(PPh₃)₄

were found to be active in the Sonogashira reaction

without CuI; however, high palladium loading was used

[16]. Recently, lower catalyst concentrations and faster

reactions with (Ph₃P)₂PdCl₂were achieved by develop-

ing an improved methodology [17]. The use of ionic liq-

uids has also been described [18].

Herrmann et al. were successful in developing cata-

lyst systems (such as (dba)₃Pd₂/(t-butyl)₃P) which work

even at room temperature [13]. More recently, a number

of palladacycles have been used as catalysts for the cop-

per free catalysis with aryl bromides and terminal alky-

nes [19,20]. Also, aryl chlorides were successfully

coupled with phenylacetylene by using pincer complexes

containing PCP-type chelate ligands [21]. Astruc and co-

workers [22] synthesized chelating and dendrimeric

phosphines to stabilize Pd(II) in six-membered chelates

for controlling the copper-free Sonogashira reaction at

room temperature. Very interestingly, Buchwald and

Gelman [23] found that the addition of copper cocata-

lysts can even inhibit the high activity of a palladium

catalyst consisting of a steric bulk phosphine and

(CH₃CN)₂PdCl₂.

In an initial experiment we used the substrates

4-bromoacetophenone and phenylacetylene in triethyl-

amine as solvent at 90 ꢁC, yielding 1-(4-acetylphenyl)-

2-(phenyl)acetylene (Scheme 3) to evaluate the catalytic

activity of the oligonuclear palladium complexes 1–3.

Table 1 contains selected results which show, that all

three complexes are catalytically active. No CuI is re-

quired to couple the two substrates in a very selective

A can be represented by two canonical structures.

Where the charges should be located, needs, however,

further investigations. In the next step of the catalytic

cycle this intermediate A may undergo oxidative addi-

tion of the aryl bromide to generate B.

Scheme 4 may also explain the diﬀerences in the cata-

lytic activities of the complexes 1–3 in long-time reac-

tions. The lower p-acceptor property of the bridging

ligand in 2 and 3 compared with that of the ligand in 1

should result in lower thermal stabilities of the interme-

diate A formed from 2 or 3. This leads to the partial

decomposition of 2 and 3 and the observed precipitation

of Pd black during the course of the catalytic reaction. In

contrast, a decomposition to form elemental palladium

was not observed, when 1 was used in the catalytic reac-

tion. This might be the reason for the observed dimin-

ished catalytic activity of 2 and 3 in comparison with 1.

The complexes 1–3 belong to the very rare group of

Pd-catalysts with N,N-chelate ligands which are active

in the copper-free Sonogashira reaction [12]. Interest-

ingly, addition of CuI even lowers the activity of 1 (entry

6). Further advantages of the oligonuclear complexes of

the type 1–3 are their stability toward air and moisture,

the high variability of the steric and electronic properties

of the ligands and their easy access.

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S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

Table 1

Sonogashira- and Negishi reaction with 1–3 as pre-catalysts

Entry

Reaction type

Catalyst

mol% cat

Time (h)

Conversion (%)

Selectivity (%)

TON^e

0

1

2

Sonogashira^a

Sonogashira^b

Sonogashira^a

Negishi^c

–

36

10

22

36

24

47

10

22

36

10

22

36

7

0

36

0

100

52

1

0.2

12

3

1

54

98

4

1

490

295

55

5

1

1/CuI

59

11

6

7

1

1/ZnCl₂

67

70

22

23

8

12

57

98

9

2

0.2

20

13

26

10

11

12

13

14

15

16

17

18

a

2

98

2

47

23

240

3

97

3

40

50

3

97

245

20

9

2

100

47

100

15

Negishi^c

(PPh₃)₄Pd

2

(PPh₃)₄Pd

20

7

Negishi^d

4

100

20

Negishi^d

20

4

Conditions for entries 1–6, 9–14: 5 mmol 4-bromoacetophenone and 6 mmol phenylacetylene as substrates; 0.01 mmol Pd pre-catalyst (in entry 1

without a pre-catalyst; in entry 6 with 0.02 mmol CuI); 15 mL triethylamine; reaction temperature 90 ꢁC; yield of the coupling product: determination

by GC analysis with diethyleneglycol–dibutylether as internal standard.

b

Entries 7–8: 0.46 mmol 3,5,6,8-tetrabromophenanthroline and 2.3 mmol phenylacetylene as substrates, 0.056 mmol pre-catalyst (entry 8: 0.46

mmol ZnCl₂2THF), 12 mL dmf/Et₃N (8/4), reaction temperature 150 ꢁC, yield of the coupling product: determination by ¹H NMR spectroscopy.

*

c

Entries 15–16: 0.4 mmol 3,5,6,8-tetrabromophenanthroline and 2.8 mmol Ph–C„C–ZnCl as subtrates, 0.08 mmol pre-catalyst, 15 mL NMP/

THF (8/7), reaction temperature 150 ꢁC; yield of the coupling product: determination by ¹H NMR spectroscopy.

d

Entries 17–18: 0.4 mmol 3,5,6,8-tetrabromophenanthroline and 2.0 mmol (ⁱprop)₃Si–C„C–ZnCl as subtrates, 0.08 mmol pre-catalyst, 14 mL

toluene/THF (6/8), reaction temperature 90 ꢁC; yield of the coupling product: determination by ¹H NMR spectroscopy.

TON: mol alkine converted to product/mol catalyst.

e

The very high selectivity and good activity of oxam

based palladium complex 1 in the Sonogashira coupling

reaction as well as the moderate reaction conditions

without CuI as essential additive prompted us to deploy

a more challenging pair of substrates. For the terminal

alkyne we used again the phenylacetylene, but for the

aromatic halide we chose 3,5,6,8-tetrabromophenanthr-

oline (Br₄phen). Br₄phen represents an easily accessible

ligand for ruthenium polypyridyl complexes [27]. Its

substitution pattern should allow for the introduction

of a wide variety of alkynylated functional groups with-

out interference with the coordination sphere of the

phenanthroline unit. Alkyne substituents would enlarge

the p-system considerably which has in 3,8-disubstituted

phenanthrolines been proven to be extremely useful [28].

It would therefore be highly desirable to be able to

transform all four bromo functions in Br₄phen.

3,5,6,8-Tetrabromophenanthroline is diﬃcult to acti-

vate due to its extremely low solubility. In addition, it

can function as a potential chelating ligand for the Pd

or a Cu atom of the catalyst system. It has been sug-

gested that this might suppress the catalytic activity

of catalysts in C–C coupling reactions of similar func-

tionalized substrates. Generally, derivatisations of

phenanthroline systems have up to now been hampered

by low yields [29] and extreme reaction conditions

[30].

We used in the catalytic reaction a mixture of dmf

and triethylamine as solvent, the amount of pre-catalyst

was raised to 12 mol% (3 mol% for each C–Br bond)

and the reaction time was prolonged to 47 h (Table 1,

entries 7–8). After this time a mixture of products was

Scheme 4. The possible intermediates A and B in the Sonogashira

coupling with 1 and 2 as the catalysts.

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

3587

Scheme 5. Negishi coupling at Br₄phen catalysed by 2.

detected containing the desired 3,5,6,8-tetra(phenyl-

acetylene)phenanthroline in less than 15% yield together

with species being partially alkynylated.

Due to this low yields by converting 3,5,6,8-tetra-

bromophenanthroline into an acetylenic compound we

employed a diﬀerent type of catalytic reaction that has

proven to couple alkynes and aromatic halides, the

Negishi type coupling (Scheme 5).

Because of its higher solubility compared to complex 1

we used complex 2 as pre-catalyst in a mixture of NMP/

THF as solvent. (Table 1, entry 15). According to mass

spectra and ¹H NMR spectra of the crude reaction mix-

ture after 7 h at 150 ꢁC the substrates were completely

converted into the 3,5,6,8-tetra(phenylacetylene)phen-

anthroline 5 as the result of an extremely selective reac-

tion. No side products, such as partially substituted

species were detected. In contrast, (Ph₃P)₄Pd as catalyst

is much less eﬃcient in this reaction. In this case a mix-

ture of di-, tri- and tetra-alkyne-substituted phenanthro-

lines was isolated (Table 1, entry 16).

mixture (Table 1, entry 17). After the complete work up

(hydrolysis with HCl/water, extraction with CH₂Cl₂and

chromatography on silica gel), the ZnCl₂-free com-

pound 6 could be isolated. For compound 6 we were

also able to obtain a structural motif by X-ray analysis,

shown in Fig. 4.

In contrast to the low eﬃciency of the Negishi cou-

pling to form 5 by using (Ph₃P)₄Pd as precatalyst, this

complex generates the formation of 6 in a very selective

reaction (Table 1, entry 18).

In a preliminary investigation of the coordination

ability of the synthesised alkinylated phenanthrolines

we reacted 6 with [(tbbpy)₂RuCl₂] (tbbpy: bis-4-tert-bu-

tyl-bipyridine).

½ðtbbpyÞ RuCl₂ꢁ þ 6 ! ½ðtbbpyÞ Ruð6ÞꢁðClÞ

2

The reaction carried out under standard conditions

proceeded smoothly and resulted in the formation of

[(tbbpy)₂Ru(6)](PF₆)₂upon addition of NH₄PF₆which

Crystals of the ZnCl₂complex of compound 5 could

be isolated from toluene after hydrolysis of the reaction

mixture, extraction with CH₂Cl₂and washing with tolu-

ene. This complex, being generated during the course of

the Negishi-reaction, seems to be very stable, even in

water and mineral acids (see Section 4). Fig. 3 displays

the molecular structure of single crystals.

In order to evaluate the eﬀect of Zn in such a coordi-

nation sphere on the behavior of Br₄phen as substrate in

the Sonogashira-reaction we performed this catalysis un-

der addition of one equivalent of ZnCl₂2THF (entry 8

*

in Table 1); however it is quite evident from the results of

the catalysis that the coordination of Zn is not suﬃcient

for the improvement of the catalytic performance.

To obtain a 3,5,6,8 alkyne-substituted phenanthro-

line with four terminal C–H groups which might serve

as building block for more extended systems we also re-

acted 3,5,6,8-tetrabromophenanthroline with the tri(iso-

propylsilyl)acetylide–ZnCl complex using 2 as catalyst.

The reaction was carried out by using a mixture of

THF/toluene as solvent and 20 mol% complex 2 as

pre-catalyst. After 4 h heating at reﬂux, all substrate

has been converted into the 3,5,6,8-tetrakis[tri(iso-pro-

pylsilyl)acetylene]phenanthroline 6 according to the

mass spectra and ¹H NMR spectra of the crude reaction

˚

Fig. 3. Molecular structure of 5, selected bond length (A) ZnA–Cl1A

2.1896(15), ZnA–Cl2A 2.2047(15), ZnA–N1A 2.082(4), ZnA–N2A

2.096(4), C13A–C14A 1.192(8), C21A–C22A 1.208(6), angles (ꢁ)

Cl1A–Zn–Cl2A 119.81(6), N1A–Zn–N2A 79.97(15).

3588

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

Fig. 4. Molecular structure of 6 as structural motif.

could be isolated in 80% yield. The complex was charac-

terised with ¹H and ¹³C NMR and ESI-MS. The absorp-

tion maximum in acetonitril is at 439 nm with a distinct

shoulder at 505 nm and the emission maximum at 692

nm. The very red shifted emission wavelength in com-

parison to the parent ruthenium-phenanthroline com-

plex (k_em= 610 nm in acetonitrile [27]) probably

indicates the increased delocalisation within the p* sys-

tem of the coordinated 6.

(Fluka), N-methylpyrrolidone (Fluka) and dmf (Ald-

1

rich) were dried over molecular sieve and distilled. H

and ¹³C NMR spectra were recorded on a Bruker AC

200 F spectrometer. Mass spectra were recorded on a

Finnigan MAT SSQ 710. Values for m/z are for the most

intense peak of isotope envelope. The measured isotopic

pattern for the palladium-containing species are in good

agreement with the calculated isotopic pattern using the

program ^ICIS(version 8.2.1, Finnigan). Elemental anal-

yses were performed with Leco CHNS-932. IR measure-

ments were carried out with a Perkin–Elmer System

2000 FT-IR.

3. Conclusions

The ligands tetra(phenyl)oxalic amidine (H₂A) and

tetra(p-tolyl)oxalic amidine (H₂B) were were prepared

according to described methods [31]; 3,5,6,8-tetra-

bromophenanthroline was prepared as described re-

cently [27]. [(tbbpy)₂RuCl₂] [32] and the complexes 2,

and 3 [2] were synthesized according to literature meth-

ods. Single crystals of 3 crystallizing with ﬁve dioxane

molecules were isolated by recrystallization of the com-

plex from dioxane.

In conclusion, we have shown that binuclear palla-

dium(II) complexes have an interesting catalytic poten-

tial in the copper-free Sonogashira reaction. In this

reaction CuI has an inhibiting eﬀect. The highly selective

Negishi coupling of tetrabromophenanthroline with ter-

minal alkynes results in interesting new ligands for tun-

ing metal-catalyzed reactions and useful for the

formation of oligonuclear organometallics.

4.1.1. Complex 1

4. Experimental section

One equiv. tetra(phenyl)oxalamidine (H₂A (390 mg, 1

mmol) and two equiv. of bis(acetylacetonato)palladi-

um(II) (610 mg, 2 mmol) were suspended in 30 mL tol-

uene and reﬂuxed for 3 h to give a clear, red solution.

After cooling to room temperature red crystals form

which can be ﬁltered oﬀ, washed twice with toluene

and n-hexane and dried in vacuum.

Yield: 0.72 g (90%). Single crystals suitable for X-ray

analysis could be isolated from toluene. Calcd. for

C₃₆H₃₄N₄O₄Pd₂: C 54.08, H 4.30, N 7.00. Found: C

54.06, H 3.89, N 6.83%. MS (APCI) m/z: 800, [MH⁺]

(100%), 700 [M⁺ꢀ acac] (6%), 594 [M⁺ꢀ Pd-acac]

(8%), 389 [M⁺ꢀ 2Pd-2acac] (20%). ¹H NMR (200

MHz; CDCl₃): d 1.44 (s, 12H, CH₃, acac); 4.99 (s, 2H,

CH, acac), 6.78–6.67 (m, 20H, CH-aromat, phenyl).

¹³C NMR (100.6 MHz; CDCl₃): d 25.1 (CH₃, acac),

4.1. General procedures

All manipulations were carried out by using modiﬁed

Schlenk techniques under an atmosphere of argon. Prior

to use, THF, diethyl ether, n-pentane, n-hexane and tol-

uene were dried over potassium hydroxide and distilled

over sodium/benzophenone. Triethylamine (Fluka) was

distilled over sodium/benzophenone. Methyllithium

(1.6 M solution in diethyl ether, Fluka), ZnCl₂(0.5 M

solution in THF, Aldrich), ZnEt₂(1.1 M solution in tol-

uene, Aldrich), bis(acetylacetonato)palladium(II) (Alfa

Aesar), 4-bromoacetophenone (Fluka), sodium acetate

(Aldrich), and tri(iso-propyl)silylacetylene (Aldrich)

were used as received, diethyleneglycol–dibutylether

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

3589

100.3 (CH, acac), 123.8, 126.8, 127.3, (CH-aromat, phe-

nyl), 143.8 (C_quart-aromat, phenyl), 168.9 (C_quart, oxam

bridge), 185.8 (C_quart, acac). IR (nujol): m 1520, 1543

taken after appropriate time intervals, hydrolyzed with 2

mL of distilled water and extracted with diethylether.

The organic phase was dried over sodium sulfate and

stored at 4 ꢁC until GC-analysis for determination of

yield and turnover number could be performed. Results

are shown in Table. Variant b: The catalytic reaction

was run under an argon atmosphere. 0.46 mmol of

3,5,6,8-tetrabromophenanthroline were placed into a

Schlenk tube together with 0.056 mmol (12 mol%) of

complex 1 and a stirring bar and the mixture was de-

gassed and purged with argon. Through a septum 2.3

mmol of phenylacetylene, 4 mL of triethylamine as base

and 8mL dmf as solvent were added. If it was desired a

(CH-aromat), 1581 (C@O) cm^ꢀ1

.

4.1.2. Complex 4

A clear solution of 1.0 mmol bis(g³-allyl)palladium in

THF was prepared by dissolving 366 mg (1.0 mmol)

bis(g³-allyl)di-l₂-chlorodipalladium(II) in dry toluene.

Upon cooling to ꢀ60 ꢁC 1.0 mL of a 2.0-M solution

of allyl-MgCl in THF was added and the mixture was

warmed to ꢀ30 ꢁC to complete the reaction. In a second

Schlenk tube 1.5 mmol tetra(p-tolyl)oxalamidine (H₂B)

(670 mg) were dissolved in 36 mL toluene. To 12 mL

(0.5 mmol) of this solution additional 25 mL of toluene

and 1 mmol of a 1.1-M solution of ZnEt₂(0.91 mL)

were rapidly added under vigorous stirring. After 10

min of stirring the remaining 24 mL (1.0 mmol) of the

tetra(p-tolyl)oxalamidine solution were slowly added at

room temperature. The clear yellow solution was then

added to the previously prepared solution of 1.0 mmol

bis(g³-allyl)palladium in THF at ꢀ70 ꢁC. After warm-

ing to room temperature the mixture was ﬁltered, the

solvents were removed in vacuo and the remaining solid

was extracted three times with 10 mL n-pentane. At

ꢀ18 ꢁC a yellow precipitate formed which was ﬁltered

oﬀ and dried in vacuo. Yield: 0.22 g (25%). Calcd. for

C₉₆H₉₄N₁₂Pd₂Zn₂: C 65.56, H 5.39, N 9.55. Found: C

66.11, H 5.29, N 9.47%. MS (APCI) m/z: 1759 [MH⁺]

solution of 0.46 mmol ZnCl₂2THF (130 mg) in 2 mL

*

dry THF were added as well. The reaction mixture was

heated to 150 ꢁC and held at this temperature for 47 h.

Following this reaction time the mixture was hydrolyzed

with water and halfconcentrated HCl, extracted with

CH₂Cl₂and the organic phase was dried over potassium

carbonate. After removal of all solvents, the product

was analyzed by mass spectroscopy and ¹H NMR

spectroscopy.

4.3. Negishi coupling

4.3.1. Compound 5

The preparation of the Cl–Zn-phenylacetylide was

performed under an argon atmosphere. About 2.8 mmol

of phenylacetylene were dissolved in 1 mL of dry THF

and cooled to ꢀ78 ꢁC. About 2.8 mmol (1.8 mL) of

methyllithium as 1.6 M solution in diethylether were

added and the solution was immediately warmed to room

temperature under formation of methane. The color

changed to orange. About 5.6 mL of a 0.5-M solution

of ZnCl₂(2.8 mmol) in THF were added to this solution

of Li-phenylacetylide and stirred for 1 h at room tem-

perature. 0.4 mmol of 3,5,6,8-tetrabromophenanthroline

were placed into a Schlenk-tube together with 0.08

mmol of complex 2 and the reaction tube was degassed

and purged with argon. Eight mL of NMP (N-methyl-

pyrrolidone) were added to the Schlenk tube and the

mixture was stirred. To this suspension the THF solu-

tion of the Cl–Zn-phenylacetylide was added and the

mixture was stirred in an oil bath for 7 h at 150 ꢁC. After

cooling down to room temperature, the reaction mixture

was hydrolyzed with 20 mL of half concentrated HCl

and extracted with CH₂Cl₂. The organic phase was dried

over potassium carbonate, ﬁltered and the solvent was

removed completely. The residue was washed with tolu-

ene and dried in vacuo. Single crystals of the ZnCl₂com-

plex crystallizing with 1.5 mol toluene suitable for X-ray

analysis could be obtained from toluene. The 3,5,6,8-tet-

ra(phenylacetylene)phenanthroline was recrystallized

from CHCl₃.

1

(100%). H NMR (200 MHz; CDCl₃): 2.04, 2.07 (2s,

36H, CH₃, p-tolyl); 2.50, 2.55–2.70 (2m, 8H, CH₂, allyl),

d 5.30 (m, 2H, CH, allyl), 6.50, 6.20 (2m, 48H, CH-aro-

mat, p-tolyl). ¹³C NMR d (50.3 MHz; CDCl₃): 114.3

(CH, allyl), 60.55 (CH₂, allyl), 20.6, 20.5 (CH₃, p-tolyl);

123.9, 124.2, 127,5, 127.7, 130.5, 131.0. (CH-aromat,

p-tolyl), 149.9, 143.8 (C_quart-aromat, p-tolyl), 160.8

(C_quartoxam bridge). IR (nujol): m c 815 (CH disubst.

aromat), 1505 (C–C-aromat, p-tolyl), 1609 (C–N, oxam

bridge) cm^ꢀ1

.

4.2. Sonogashira reaction

Variant a: The catalytic reaction was run under an ar-

gon atmosphere. About 5.0 mmol of 4-bromoacetophe-

none were placed into a Schlenk tube together with 0.01

mmol of the precatalyst and a stirring bar and the mix-

ture was degassed and purged with argon. Through a

septum 6.0 mmol of phenylacetylene, 0.5 g of the inter-

nal standard and 15 mL of triethylamine as solvent and

base were added. In order to determine the long-term

stability of the catalysts we employed following reaction

procedure. The reaction mixture was heated to 90 ꢁC

and held at this temperature for 7 h, after cooling for

17 h at room temperature heating and cooling was re-

peated for four times (last heating periode 8h) resulting

in a total reaction time of 36 h. Samples of 0.5 mL were

Yield: 0.34 g (60%) C₄₄H₂₄N₂: MS (EI) m/z: 580 [M⁺]

1

(100%), 480 [MH⁺–C„C–C₆H₅] (25%). H NMR (400

3590

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

MHz; CDCl₃): d 7.39 (t), 7.46 (m), 7.66 (d), 7.75 (d)

(20H, CH-aromat, phenyl); 8,90, 9.00 (2s, 4H, CH-aro-

mat, phen). ¹³C NMR (100.6 MHz; CD₂Cl₂): d 83.9,

84.7, 98.7, 104.3 (C_quart, C„C), 123.6, 123.5 (C_quart-aro-

mat, phenyl), 121.4, 121.8, 127.9, 137.4 (C_quart-aromat,

phen), 129.0, 129.2, 130.3, 130.5, 132.7, 132.8 (CH-aro-

mat, phenyl), 139.7 152.6 (CH-aromat, phen). IR

(KBr): m 2210 cm^ꢀ1(C„C), 3057 cm^ꢀ1(CH-aromat),

d, c 912, 756, 688 cm^ꢀ1(CH-phenyl)

min, ventilation). After evaporation of the solvent the

remaining solid was dissolved in 30 mL EtOH. The

product was precipitated by addition of an aqueous

solution of NH₄PF₆, removed by ﬁltration, washed with

Et₂O and dried on air. Yield: 107 mg, 80%. MS (EI) m/z

1

1684.5 (M ꢀ PF₆), 769.2 (M ꢀ 2PF₆/2). H NMR (400

mHz, d₆-DMSO): d 1.02–1.04 (42H), 1.14–1.16 (42H),

1.37 (s, 18H), 1.39 (s, 18H), 7.31 (d, 2H), 7.69 (m,

6H), 7.78 (d, 2H), 8.64 (d, 4H). ¹³C NMR (200 MHz,

d₆-DMSO): d 10.46, 10.63, 18.27, 18.41, 29.92, 35.34,

35.47, 99.20, 101.6, 107.0, 121.8, 123.5, 124.4, 130.0,

136.1, 146.2, 151.0, 152.2, 153.3, 155.8, 156.7, 1561.9,

162.4. UV/Vis (Acetonitrile): k = 439 nm, shoulder at

505 nm; emission (Acetonitrile, k_ex= 439 nm): k = 692

nm.

4.3.2. Compound 6

The preparation of the Cl–Zn-tri(iso-propyl)silyl-

acetylide was performed under an argon atmosphere.

About 2.0 mmol of tri(iso-propyl)silylacetylene were dis-

solved in 2 mL of dry THF and cooled to ꢀ78 ꢁC. 2.0

mmol (1.25 mL) of methyllithium as 1.6 M solution in

diethylether were added and the solution was immedi-

ately warmed to room temperature under formation of

5. Crystal structure determination

methane. 2.0 mmol of ZnCl₂2THF were put in a Sch-

*

The intensity data for the compounds were collected

on a Nonius KappaCCD diﬀractometer, using graphite-

monochromated Mo Ka radiation. Data were corrected

for Lorentz and polarization eﬀects, but not for absorp-

tion [34,35].

The structures were solved by direct methods

(SHELXS [36]) and reﬁned by full-matrix least squares

techniques against F _o²

(SHELXL-97 [37]). The hydrogen

atoms were included at calculated positions with ﬁxed

thermal parameters. Since the quality of the data of

compound 6 is insuﬃcient, we will only publish the con-

formation of the molecule and the crystallographic data.

We will not deposit the data in the Cambridge Crystal-

lographic Data Centre. All non-hydrogen atoms were

reﬁned anisotropically [37]. XP (SIEMENS Analytical

X-ray Instruments, Inc.) was used for structure

representations.

lenk tube under an argon atmosphere and dissolved in 3

mL of dry THF. The ZnCl₂-solution was then added to

the solution of the Li-tri(iso-propyl)silylacetylide and

stirred for 1 h at room temperature. 0.4 mmol of

3,5,6,8-tetrabromophenanthroline were placed in a Sch-

lenk-tube together with 0.08 mmol of complex 2 and the

reaction tube was degassed and purged with argon. 6

mL of toluene were added to the substrates and stirred.

To this suspension the THF solution of the Cl–Zn-tri(i-

so-propyl)silylacetylide was added and the mixture was

heated under reﬂux in an oil bath for 4 h. After cooling

down to room temperature, the clear and dark reaction

mixture was hydrolyzed with 15 mL of halfconcentrated

HCl and extracted with CH₂Cl₂. The organic phase was

dried over potassium carbonate, ﬁltered and the solvents

were removed completely. The residue was then chroma-

tographed on silica gel (toluene/ethylacetate: 99/1). Sin-

gle crystals suitable for X-ray analysis could be obtained

from acetone/water. Yield: 0.22 g (60%) C₅₆H₈₈N₂Si₄:

MS (EI) m/z: 902 [M⁺] (18%), 858 [M⁺ꢀ C₃H₇] (10%),

816 [M⁺ꢀ 2C₃H₇] (40%), 774 [M⁺ꢀ 3C₃H₇] (28%),

731 [M⁺ꢀ 4C₃H₇]. ¹H NMR (400 MHz; CDCl₃): d

1.21 (m, 84H, CH + CH₃Si(ⁱProp)₃), 8.85, 9.11 (2s, 4H,

CH-aromat, phen). ¹³C NMR (100.6 MHz; CDCl₃): d

11.3, 11.4 (CH, Si(ⁱProp)₃), 18.8, 18.7 (CH₃,Si(ⁱProp)₃),

96.9, 101.7, 103.4, 104.9 (C_quart, C„C), 120.6. 123.2,

127.9, 143.7 (C_quart-aromat, phen), 138.1, 153.1 (CH-

aromat, phen). IR (KBr): m 2153 (C„C), 2943, 2865

Crystal Data for 1 [38]: C₃₆H₃₄N₄O₄Pd₂, Mr = 799.47

g mol^ꢀ1, yellow-brown prism, size 0.02 · 0.02 · 0.02

mm³, monoclinic, space group P2₁/c, a = 11.2417(4),

˚

A,

b = 9.0294(2),

c = 15.8736(5)

b = 95.256(2)ꢁ,

3

˚

V = 1604.49(8) A , T = ꢀ90 ꢁC, Z = 2, q_calcd.= 1.655

g cm^ꢀ3, l(Mo Ka) = 11.67 cm^ꢀ1, F(000) = 804, 9475

reﬂections in h(ꢀ14/14), k(ꢀ10/11), l(ꢀ20/17), measured

in the range 2.90ꢁ 6 H 6 27.48ꢁ, completeness H_max

=

99.3%, 3655 independent reﬂections, R_int= 0.025, 3145

reﬂections with F_o> 4r (F_o), 208 parameters, 0 re-

straints, R1_obs= 0.028, wR²_obs¼ 0:073, R1_all= 0.035,

wR²_all¼ 0:078, GOOF = 1.034, largest diﬀerence peak

(CH-aromat) cm^ꢀ1

.

ꢀ3

˚

and hole: 0.556/ꢀ0.902 e A

.

Crystal Data for 3 [38]: C H N O Pd Zn 5C -

*

70 70

8

4

2

4

4.3.3. Preparation of [(tbbpy)₂Ru(6)](PF₆)₂

H₈O₂, Mr = 1806.03 g mol^ꢀ1, yellow-orange prism, size

0.03 · 0.03 · 0.02 mm³, triclinic, space group P1,

a = 15.9614(5), b = 18.6093(7), c = 18.9105(6) A,

ꢀ

˚

The complex was prepared using slightly modiﬁed

standard procedures [33] with [(tbbpy)₂RuCl₂] 52 mg

(0.073 mmol), and 6 66 mg (0.073 mmol) dissolved in

50 mL DMF/H₂O and irradiated in a microwave

(microwave set-up: 30 s, 600 W; 45 min, 200 W, 10

a = 115.719(2)ꢁ, b = 96.270(2)ꢁ, c = 90.307(2)ꢁ, V =

3

5021.5(3) A , T = ꢀ90 ꢁC, Z = 2, q_calcd.= 1.194 g cm^ꢀ3

,

˚

l(Mo Ka) = 6.49 cm^ꢀ1, F(000) = 1880, 34,021 reﬂec-

S. Rau et al. / Journal of Organometallic Chemistry 689 (2004) 3582–3592

3591

[9] E. Negishi, T. Takahashi, K. Akiyoshi, J. Organomet. Chem. 334

(1987) 181.

tions in h(ꢀ20/20), k(ꢀ22/24), l(ꢀ24/22), measured in

the range 1.65ꢁ 6 H 6 27.47ꢁ, completeness H_max

=

[10] C. Amatore, M. Azzabi, A. Jutand, J. Am. Chem. Soc. 113 (1991)

8375.

97.1%, 22,351 independent reﬂections, R_int= 0.045,

14,001 reﬂections with F_o> 4r(F_o), 970 parameters, 1

restraints, R1_obs= 0.082, wR²_obs¼ 0:224, R1_all= 0.136,

wR²_all¼ 0:2665, GOOF = 1.043, largest diﬀerence peak

[11] H. Hartwig, F. Paul, J. Am. Chem. Soc. 117 (1995) 117, 5373.

[12] (a) M.R. Buchmeiser, T. Schareina, R. Kempe, K. Wurst, J.

Organomet. Chem. 634 (2001) 39;

and hole: 2.362/ꢀ1.155 e A^ꢀ3

.

(b) C. Najera, J. Gil-Molto, S. Karlstroem, L.R. Falvello, Org.

Lett. 5 (2003) 1451.

Crystal Data for 5 [38]: C H Cl N Zn 1.5C H ,

*

44 24

2

7

8

[13] V.P.W. Bo¨hm, W.A. Herrmann, Eur. J. Org. Chem. (2000) 3679.

[14] (a) for recent reviews of the Sonogashira reaction, see: K.

Sonogashira, in: F. Diederich, P.J. Stang (Eds.), Metal-Catalyzed

Cross Coupling Reactions, Wiley-VCH, 1998, p. 203;

(b) K. Sonogashira, in: E.-I. Negishi, A. deMeijere (Eds.),

Handbook of Organopalladium Chemistry for Organic Synthesis,

vol. 1, Wiley, Weinheim, 2002, p. 493;

Mr = 855.12 g mol^ꢀ1, colourless prism, size 0.03 ·

0.03 · 0.02 mm³, triclinic, space group P1,

ꢀ

˚

a = 14.410(3), b = 16.890(3), c = 18.871(4) A, a =

85.93(3)ꢁ, b = 89.18(3)ꢁ, c = 77.62(3)ꢁ, V = 4475.0(16)

3

A , T = ꢀ90 ꢁC, Z = 4, q_calcd.= 1.269 g cm^ꢀ3, l(Mo

˚

Ka) = 7.07 cm^ꢀ1, F(000) = 1764, 19,948 reﬂections in

h(0/18), k(ꢀ21/22), l(ꢀ24/23), measured in the range

4.33ꢁH 6 27.84ꢁ, completeness H_max= 93.9%, 19519

independent reﬂections, R_int= 0.040, 12,830 reﬂections

(c) W.A. Herrmann, K. Oefele, D. von Preysing, S.K. Schneider,

J. Organomet. Chem. 687 (2003) 229;

(d) M. Beller, A. Zapf, in: E.-I. Negishi, A. deMeijere (Eds.),

Handbook of Organopalladium Chemistry for Organic Synthesis,

vol. 1, Wiley, Weinheim, 2002, p. 1209;

with F_o> 4r(F_o), 1029 parameters,

0

restraints,

¼

(e) I.J.S. Fairlamb, Annual reports on the progress of chemistry,

Org. Chem. 99 (2003) 104;

R1_obs= 0.080, wR²_obs¼ 0:190, R1_all= 0.136, wR²_all

0:229, GOOF = 1.055, largest diﬀerence peak and hole:

˚

(f) C.E. Tucker, J.G. De Fries, Top. Catal. 19 (2002) 11;

(g) A. Zapf, M. Beller, Top. Catal. 19 (2002) 101.

[15] (a) for recent reviews of the Negishi reaction, see: E. Negishi, Y.

Dumond Handbook of Organopalladium Chemistry for Organic

Synthesis, vol. 1, Wiley, 2002, p. 767;

ꢀ3

1.415/ꢀ0.697 e A

.

Crystal Data for 6: C₅₆H₈₈N₂Si₄, Mr = 901.64

g mol^ꢀ1, colourless prism, size 0.03 · 0.03 · 0.02 mm³,

monoclinic, space group P2₁/n, a = 22.5147(6),

(b) E. Negishi, H.C. Brown Handbook of Organopalladium

Chemistry for Organic Synthesis, vol. 1, Wiley, 2002, p. 229.

[16] M. Alami, F. Ferri, G. Linstrumelle, Tetrahedron Lett. 34 (1993)

6403.

˚

b = 15.4592(4),

c = 33.4707(10)

3

A,

a = 90.00ꢁ,

˚

b = 93.679(1)ꢁ, V = 11625.8(6) A , T = ꢀ90 ꢁC, Z = 8,

q

_calcd.= 1.030 g cm^ꢀ3

,

l(Mo Ka) = 1.36 cm^ꢀ1

,

[17] N.E. Leadbeater, B.J. Tominack, Tetrahedron Lett. 44 (2003)

8653.

F(000) = 3952, 50,570 reﬂections in h(ꢀ29/28), k(ꢀ17/

19), l(ꢀ43/26), measured in the range 1.60ꢁ 6 H 6

27.49ꢁ, completeness H_max= 86.6%.

[18] T. Fukuyama, M. Shinmen, S. Nishitani, S. Satoshi, M. Sato, I.

Ryu, Org. Lett. 4 (2002) 1691.

¨

[19] (a) W.A. Herrmann, C.-P. Reisinger, K. Ofele, C. Broß mer, M.

Beller, H. Fischer, J. Mol. Cat. A: Chem. 108 (1996) 51;

(b) W.A. Herrmann, C.-P. Reisinger, M. Spiegler, J. Organomet.

Chem. 557 (1998) 93;

Acknowledgement

(c) D.S. McGuiness, K.J. Cavell, Organometallics 19 (2000)

741.

Financial support for this work from the Deutsche

Forschungsgemeinschaft and the Fonds der Chemischen

Industrie is gratefully acknowledged.

[20] (a) D.A. Alonso, C. Najera, M.C. Pacheso, Tetrahedron Lett. 43

(2002) 9365, and references therein;

(b) D.A. Alonso, C. Najera, M.C. Pacheso, Adv. Synth. Catal.

345 (2003) 1146.

[21] M.R. Eberhard, Z. Wang, C.M. Jensen, Chem. Commun. (2002)

818.

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Article Doi

Bi- and trinuclear oxalamidinate complexes of palladium as catalysts in the copper-free Sonogashira reaction and in the Negishi reaction

DOI: 10.1016/j.jorganchem.2004.07.061

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