Bimetallic catalysis involving dipalladium(I) and diruthenium(I) complexes

Article abstract of DOI:10.1002/chem.201001960

Dipalladium(I) and diruthenium(I) compounds bridged by two [{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene (L) ligands have been synthesized. The X-ray structures of [Pd₂L₂][BF ₄]₂ (1) and [Ru₂L₂(CO) ₄][BF₄]₂ (2) reveal dinuclear structures with short metal-metal distances. In both of these structures, naphthyridine bridges the dimetal unit, and the site trans to the metal-metal bond is occupied by weakly coordinating oxygen from the amido fragment. The catalytic utilities of these bimetallic compounds are evaluated. Compound 1 is an excellent catalyst for phosphine-free, Suzuki cross-coupling reactions of aryl bromides with arylboronic acids and provides high yields in short reaction times. Compound 1 is also found to be catalytically active for aryl chlorides, although the corresponding yields are lower. A bimetallic mechanism is proposed, which involves the oxidative addition of aryl bromide across the Pdi￡ Pd bond and the bimetallic reductive elimination of the product. Compound 1 is also an efficient catalyst for the Heck cross-coupling of aryl bromides with styrenes. The mechanism for aldehyde olefination with ethyl diazoacetate (EDA) and PPh₃, catalyzed by 2, has been fully elucidated. It is demonstrated that 2 catalyzes the formation of phosphorane utilizing EDA and PPh₃, which subsequently reacts with aldehyde to produce a new olefin and phosphine oxide. The efficacy of bimetallic complexes in catalytic organic transformations is illustrated in this work. Good couple: Metal-metal singly bonded [Pd^Ii￡Pd^I] and [Ru^Ii￡Ru^I] complexes exhibit bimetallic synergy in the catalytic Ci￡C bond-coupling and aldehyde-olefination reactions, respectively (see figure). Copyright

Full text of DOI:10.1002/chem.201001960

FULL PAPER
DOI: 10.1002/chem.201001960
Bimetallic Catalysis Involving Dipalladium(I) and Diruthenium(I)
Complexes
Raj K. Das, Biswajit Saha, S. M. Wahidur Rahaman, and Jitendra K. Bera*[a]
Dedicated to Professor Pierre H. Dixneuf on the occasion of his 70th birthday
Abstract: Dipalladium(I) and diruthe-
nium(I) compounds bridged by two
[{(5,7-dimethyl-1,8-naphthyridin-2-yl)-
phosphine-free, Suzuki cross-coupling
reactions of aryl bromides with arylbor-
onic acids and provides high yields in
short reaction times. Compound 1 is
also found to be catalytically active for
aryl chlorides, although the corre-
sponding yields are lower. A bimetallic
mechanism is proposed, which involves
the oxidative addition of aryl bromide
uct. Compound 1 is also an efficient
catalyst for the Heck cross-coupling of
aryl bromides with styrenes. The mech-
anism for aldehyde olefination with
ethyl diazoacetate (EDA) and PPh3,
catalyzed by 2, has been fully elucidat-
ed. It is demonstrated that 2 catalyzes
the formation of phosphorane utilizing
EDA and PPh3, which subsequently
reacts with aldehyde to produce a new
olefin and phosphine oxide. The effica-
cy of bimetallic complexes in catalytic
organic transformations is illustrated in
this work.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
clear structures with short metal–metal
distances. In both of these structures,
naphthyridine bridges the dimetal unit,
and the site trans to the metal–metal
bond is occupied by weakly coordinat-
ing oxygen from the amido fragment.
The catalytic utilities of these bimetal-
lic compounds are evaluated. Com-
pound 1 is an excellent catalyst for
ꢀ
across the Pd Pd bond and the bimet-
allic reductive elimination of the prod-
ꢀ
Keywords: catalysis
pling · cooperative effects · palladi-
um · ruthenium
·
C C cou-
[5]
ꢀ
Introduction
formation of the C C bond have been performed at the
axial site of the metal–metal singly bonded ꢀRu2(CO)4ꢁ core.
In a recent report, we have described bimetallic water–gas
shift (WGS) chemistry executed on a diruthenium plat-
form.[6] These stoichiometric reactions have provided valua-
ble insight on fundamental organometallic processes on a di-
metal unit. The present work seeks to evaluate the catalytic
activity of metal–metal bonded dimetal compounds in or-
ganic transformations.
Bimetallic catalysis is based on the premise that sequential
or cooperative participation of two metal components might
lead to an enhanced reaction rate, better selectivity, and in
some cases new types of reactions.[1] Several bimetallic com-
plexes have been exploited as catalysts in organic transfor-
mations. Sequential participation of Pd- and Sn-based re-
ꢀ
agents is demonstrated in the C C bond-forming Stille reac-
tion.[2] The dicopper(I) active site in metallaenzyme tyrosi-
nase exhibits bimetallic cooperativity towards the conver-
sion of catechol to benzoquinone.[3] In our continuing effort
to examine cooperative bimetallic reactivity, we have ex-
The challenging task in designing a bimetallic catalyst is
the selection of ligands that are capable of accommodating
geometrical and electronic reorganization of the dimetal
core during the course of the catalytic cycle. The demon-
strated ability of 1,8-naphthyridine (NP) to stabilize a host
of dimetal cores prompted the utilization of the NP-based
ligand.[7] A donor appendage (such as pyridyl, thiazolyl, py-
rollyl) at the 2 position of the NP unit enables additional
chelate-ring formation and allows axial modulation of the
metal–metal bond.[8] Coordination of 2-(2-thiazolyl)-1,8-
plored organometallic chemistry on a diruthenium(I) plat-
form. Room temperature activation of the C H bond, and
[4]
ꢀ
[a] R. K. Das, B. Saha, S. M. W. Rahaman, Dr. J. K. Bera
Department of Chemistry
Indian Institute of Technology Kanpur
Kanpur 208016 (India)
naphthyridine (tzNP) to
a dimetal unit is shown in
E-mail: jbera@iitk.ac.in
Scheme 1a, as an illustrative example. To access axial sites
Chem. Eur. J. 2010, 16, 14459 – 14468
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14459

Scheme 1. Coordination of a) tzNP and b) L to a dimetal unit.
for substrate coordination, we proposed to incorporate hem-
ilabile amido group.[9] The NP derivative [{(5,7-dimethyl-1,8-
naphthyridin-2-yl)amino}carbonyl]ferrocene (L) bridges the
dimetal core through the N-C-N unit and the oxygen atom
occupies the site trans to the metal–metal axis, as shown in
Scheme 1b. Introduction of ferrocenyl group metal com-
plexes dramatically improves their solubility.
Dipalladium(I) and diruthenium(I) complexes supported
by two L ligands have been synthesized, the catalytic activi-
Figure 1. ORTEP diagram (50% probability thermal ellipsoids) of the
[Pd2L2]2+ unit in compound 1 with important atoms labelled. Carbon
atoms are shown as circles of arbitrary radius. Hydrogen atoms are omit-
I
I
ꢀ
ty of [Pd Pd ] complex towards Suzuki and Heck coupling
ꢀ
ted for the sake of clarity. Selected bond lengths (ꢃ) and angles [8]: Pd1
I
I
ꢀ
reactions has been examined and the [Ru Ru ] complex
has been evaluated as a catalyst for aldehyde olefination re-
actions.
ꢀ
ꢀ
ꢀ
Pd2 2.3952(8), Pd1 N2 2.045(5), Pd1 N4 2.050(5), Pd2 N1 2.052(5),
ꢀ
ꢀ
ꢀ
ꢀ
Pd2 N5 2.039(5), Pd1 O1 2.149(4), Pd2 O2 2.171(4), O1 C21 1.249(7),
ꢀ
ꢀ
O2 C51 1.259(7), N3 C21 1.364(8); N2-Pd1-N4 172.6(2), N5-Pd2-N1
172.9(2), N2-Pd1-Pd2 88.02(14), N5-Pd2-Pd1 87.81(14), N1-Pd2-Pd1
85.15(14), N4-Pd1-Pd2 85.07(15), C21-O1-Pd1 123.4(4), C51-O2-Pd2
121.6(4), N2-Pd1-O1 90.95(18), N5-Pd2-O2 90.23(18), O1-Pd1-Pd2
163.52(12), O2-Pd2-Pd1 159.92(12). Dihedral angles [8]: O1-Pd1-Pd2-O2
15.9(6), N4-Pd1-Pd2-N5 27.3(2), N1-Pd2-Pd1-N2 25.8(2), N2-C20-N3-C21
21.7(10), N5-C50-N6-C51 17.1(10).
Results and Discussion
G
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
ꢀ
in high yield (75%). Analysis of the X-ray structure and
spectroscopic data reveals a dipalladium(I) compound. The
IR absorption of the carbonyl group (n˜ =1684 cmꢀ1) does
not shift by a significant value when compared to the free
ligand (n˜ =1680 cmꢀ1). This indicates a weak coordination of
carbonyl to the metal. In comparison, the corresponding
range (2.039(5)–2.052(5) ꢃ) and the Pd O bond lengths are
ꢀ
comparatively longer (2.149(4) and 2.171(4) ꢃ). The C O
bond lengths (1.249(7), 1.259(7) ꢃ) are marginally elongated
compared to the unbound ligand (1.225(5) ꢃ). The trans ni-
trogens exhibit N-Pd N bond angles 172.9(2) and 172.6(2)8.
The N4-Pd1-Pd2-N5, N1-Pd2-Pd1-N2 torsional angles
27.3(2) and 25.8(2)8 signify distortion of the Pd2(NP)2
framework from planarity.
ꢀ
shift in [CuL2]ACHTUNGTRENNUNG
[ClO4]2 is 37 cmꢀ1 in which L forms a chelate
involving carbonyl oxygen and napththyridine nitrogen.[9]
1
ꢀ
Compound 1 is diamagnetic and analysis of the H NMR
The most remarkable finding of this structure is the Pd
Pd bond length. To the best of our knowledge, compound 1
spectrum indicates the equivalence of two ligands on the
NMR time scale. The naphthyridine protons appear at d=
8.60 (Ha), 8.50 (Hb), and 7.56 ppm (Hc). Two singlets at d=
2.77 and 3.38 ppm are assigned to two methyl groups. Four
protons in the substituted Cp ring appear at d=4.62, 4.68,
5.04, and 5.13 ppm. The second Cp protons resonate at d=
4.35 ppm. The signal at d=9.49 ppm is assigned to the NH
proton. The ESI-MS spectrum of complex 1 reveals a signal
at m/z 492 (z=2) assigned for the molecular ion [Pd2L2]2+
on the basis of the mass and isotopic distribution pattern
(see Figure S5 in the Supporting Information).
ꢀ
exhibits the shortest Pd Pd distance (2.3952(8) ꢃ) among
dipalladium(I) compounds known in literature.[10] A selected
ꢀ
list of dipalladium(I) compounds along with their Pd Pd
ꢀ
distances are collected in Table 1. The Pd Pd distance in un-
supported [Pd
(CH3CN)6]
(BF4)2 is 2.486(1) ꢃ. It increases
when the coordinated acetonitrile molecules are replaced by
stronger ligands, such as halides, isocyanides or phosphines
(Table 1, entries 2–4). Most of the bridged dipalladium(I)
ꢀ
compounds contain phosphine-based ligands. The Pd Pd
separations vary within the range 2.55–2.70 ꢃ.[10e–n] Ortho-
functionalized triazenido ligands have afforded dipalladiu-
ꢀ
X-ray structure of 1: The molecular structure of 1 consists of
a dipalladium unit spanned by two ligands. The ORTEP dia-
gram of the dicationic unit in 1 is shown in Figure 1 and the
relevant parameters are provided in the corresponding cap-
tion. The X-ray structure confirms that two ligands are in a
trans arrangement. The NP unit bridges the dimetal, and the
m(I) compounds in which the Pd Pd bond lengths are com-
paratively smaller (2.416(1)–2.431(1) ꢃ; see entry 15). The
shortest metal–metal distance in 1 is the result of a con-
strained ligand bite and weak axial coordination.
It is evident that the PdII precursor converts to PdI during
the course of the reaction. Reduction of PdII to Pd0 with
phosphine has been postulated in cross-coupling reactions.[11]
Tertiary amines have been implicated for the transformation
ꢀ
site trans to Pd Pd axis is occupied by the O atom of the
ꢀ
amido unit. The Pd N bond lengths vary within a small
14460
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 14459 – 14468

Bimetallic Catalysis
FULL PAPER
ꢀ
Table 1. Comparison of Pd Pd bond lengths in selected dipalladium(I)
indicating the equivalence of two ligands on the NMR time
scale. The aromatic protons appear as two doublets at d=
8.58 (Ha) and 7.43 ppm (Hb) and a singlet at d=7.20 ppm
(Hc). Two methyl groups resonate at d=2.81 and 3.04 ppm.
The protons of the functionalized Cp ring appear at d=4.77
and 5.29 ppm, whereas the second Cp ring protons are ob-
served at d=4.21 ppm. The signal at d=9.36 ppm is attribut-
ed to NH proton.
complexes.[a]
No
Complex
[Pd2A
(MeCN)6]2+
[Pd2A(MeNC)4I2]
[Pd2A(tBuNC)4Cl2]
[Pd2A(tBuNC)4A(PPh3)2]
[Pd2A(dppm)2Br2]
[Pd2A(dppm)2A
(tBuNC)2]2+
[Pd2A(dppm)2A(OCOCF3)2]
[Pd2A(dmpm)2Br2]
[Pd2Cl2A(dpmMe)2]
[Pd2A(dppa)2Cl2]
[Pd2A[PhN{P(OPh)2}2]2Cl2]
[Pd(etp)]2
[Pd2(Ph2PC5H4N)
[Pd2A
{(Ph2P)2py}2]2+
[Pd2(o-MeO2C-C6H4-NNN-p-C6H4-CH3)2]
[Pd2L2]2+
dPdꢀPd [ꢃ]
Ref.
[10a]
[10b]
[10c]
[10d]
[10e]
[10f]
[10g]
[10h]
[10i]
1
2
3
4
5
6
7
8
G
G
2.486(1)
2.533(1)
2.532(2)
2.556(1)
2.699(5)
2.619(1)
2.594(2)
2.603(1)
2.664(1)
2.638(1)
2.620(1)
2.617(1)
2.579(1)
2.552(1)
2.416(1)
2.395(1)
R
E
T
CHTUNGTRENNUNG
R
R
CHTUNGTRENNUNG
G
CHTUNGTRENNUNG
R
X-ray structure of 2: Two independent molecules are ob-
served in the asymmetric unit. The metrical parameters of
these two structures are very similar and we restrict our dis-
cussion to one molecule. Complete data for both molecules
are provided in the Supporting Information. The ORTEP di-
agram of the dicationic unit in 2 is shown in Figure 2 and
9
A
CHTUNGTRENNUNG
[10j]
10
11
12
13
14
15
16
N
CHTUNGTRENNUNG
[10k]
[10l]
C
ACHTUNGTRENNUNG
2+
ACHTUNGTRENNUNG
[10m]
[10n]
[10o]
[b]
2ACHTUNGTNER(NUNG OAc)2]
CHTUNGTRENNUNG
[a] Abbreviations: dppm=bis(diphenylphosphino)methane, dmpm=bis-
(dimethyl-phosphino)methane, dpmMe=bis(diphenylphosphino)ethane,
dppa=bis-(diphenylphosphino)amine,
ethyl)phenylphosphine and L=[{(5,7-dimethyl-1,8-naphthyridin-2-yl)ami-
no}carbonyl]ferrocene. [b] This work.
etp=bis(Diphenylphosphino)-
ACHTUNGTRENNUNG
of PdII to dipalladium(I) compounds.[12,10o] Compound 1 was
obtained in absence of either of these. The presence of
1
redox active ferrocenyl moiety (E = (ox)=0.65
G
2
certainly raises the possibility of ligand-promoted reduction
with concomitant formation of the corresponding ferroceni-
um salt. However, isolation of 1 in yields exceeding 70%,
by employing strictly one equivalent of L, does not support
this premise. Furthermore, the use of excess ligand did not
improve the yield by any significant number, thereby contra-
dicting the role of sacrificial ligand for the reduction. Nota-
bly, the reaction of the solvated dipalladium(I) precursor
Figure 2. ORTEP diagram (50% probability thermal ellipsoids) of the
2 with important atoms labelled.
Carbon atoms are shown as circles of arbitrary radius. Hydrogen atoms
[Ru2L2(CO)4]2+ unit in compound
are omitted for the sake of clarity. Selected bond lengths [ꢃ] and angles
ꢀ
ꢀ
ꢀ
ꢀ
[8]: Ru2 Ru1 2.5825(9), Ru1 N1 2.176(7), Ru1 N5 2.191(5), Ru2 N2
ꢀ
ꢀ
ꢀ
ꢀ
2.174(6), Ru2 N4 2.182(5), Ru1 O6 2.213(5), Ru2 O5 2.181(4) , O5
[Pd2ACHTUNGTRENNUNG(CH3CN)6]ACHTUNGTRENNUNG[BF4]2 with L did not provide 1 in a detecta-
ꢀ
ꢀ
ꢀ
C36 1.250(7), O6 C15 1.241(10), N3 C36 1.366(8), N6 C15 1.316(10),
N1-Ru1-N5 85.4(2), N2-Ru2-N4 85.69(19), C1-Ru1-N5 174.5(3), C2-Ru1-
N1 173.7(3), C4-Ru2-N4 174.2(3), C3-Ru2-N2 174.4(3), N2-Ru2-O5
83.50(19), N5-Ru1-O6 82.5(2), N1-Ru1-O6 86.2(2), O6-Ru1-Ru2
161.89(17), O5-Ru2-Ru1 161.39(13) Dihedral angles [8]: C1-Ru1-Ru2-C4
40.5(5), C2-Ru1-Ru2-C3 39.2(4), N1-Ru1-Ru2-N2 33.0(2), N4-Ru2-Ru1-
N5 33.6(2), N2-C35-N3-C36 38.7(9), N5-C14-N6-C15 22.6(12).
ble amount, as revealed from ESI-MS spectra analysis. It is
likely that the solvent acetonitrile and possibly a trace im-
purity of water in the reaction mixture are responsible for
the reduction of [Pd2]4+ to a [Pd2]2+ core under the influ-
ence of L, although we have no direct experimental evi-
dence to prove this point.
A
E
the relevant parameters are provided in the corresponding
caption. The molecular structure of 2 consists of a dirutheni-
um unit spanned by two cis-coordinating ligands. The coor-
dination motif of the ligand is similar to that observed in 1.
The NP fragment bridges two ruthenium centres, and the
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG[BF4]2 (2) in high yield (82%). ESI-MS spectrum revealed a
signal at a mass-to-charge ratio of 1173, attributed to [{M}2+
ꢀ
+BF4 ]+ in which {M} is [Ru2(CO)4L2]. In addition, signals
corresponding to [M]z+ (z=1 and 2) were observed at m/z
1086 and 543, respectively. The IR spectrum of 2 exhibits
carbonyl absorptions at n˜ =2042, 1998 and 1954 cmꢀ1 and
the aminocarbonyl absorption appears at n˜ =1633 cmꢀ1, a
drop of 47 cmꢀ1 compared to the free ligand indicating its
coordination to the metal. This is further supported by anal-
ysis of the 13C{1H} NMR spectrum, which exhibits a amido
carbonyl resonance at d=176.1 ppm, significantly downfield
site trans to the Ru Ru bond is occupied by the O atom of
the amido unit. Each ruthenium is additionally bonded to
two CO ligands in a cis fashion.
ꢀ
ꢀ
The effects of axial coordination on the Ru Ru bond
length has been examined in a series of [Ru2(CO)
N
compounds.[8] Experimental and computational studies re-
vealed that the variation in metal–metal distances is the
direct consequence of the interaction of different axial
donors with the diruthenium unit. The stronger the axial
1
shifted from the free ligand (d=169.7 ppm). The H NMR
ꢀ
spectrum exhibits three resonances for the aromatic protons,
ligand, the longer the Ru Ru bond length. The shortest
Chem. Eur. J. 2010, 16, 14459 – 14468
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14461

J. K. Bera et al.
ꢀ
Ru Ru bond length 2.6071(9) ꢃ was observed for [Ru
G
though electron-rich arylboronic acids were in general
active (Table 2, entries 10–13), electron-poor arylboronic
acids provided greater yields (Table 2, entries 7–9). Selectivi-
ty of the catalyst towards desired cross-coupled product is
good, although some homo-coupled product was also ob-
served (less than 10%) when the reaction is slow.[14]
MeNP)2(CO)4A(OTf)2] (3-MeNP=3-methyl-1,8-naphthyri-
CHTUNGTRENNUNG
dine) in which triflates are coordinated at the axial sites. It
is rather remarkable that the Ru Ru bond length
(2.5825(9) ꢃ) in 2 is even shorter! The Ru1 O6 and Ru2
O5 bond lengths (2.213(5) and 2.181(4) ꢃ, respectively) are
ꢀ
ꢀ
ꢀ
shorter than the corresponding Ru O bond length
2.267(14) ꢃ in the triflate-
Table 2. Suzuki cross-coupling of aryl bromides with boronic acids catalyzed by 1.[a]
bound complex. The Ru-Ru-O
angles (161.9(1) and 161.4(1)8)
deviate from linearity. The N1-
Ru1-Ru2-N2 and N5-Ru1-Ru2-
N4 torsional angles are 33.0(2)
and 33.6(2)8, respectively, indi-
cating deviation from planarity.
The carbonyl ligands are stag-
Entry
Aryl halide
Coupling partner
Conversion[b] [%]
Yield[c] [%]
1
2
1
2
1
2
ꢀ
ꢀ
ꢀ
ꢀ
Ar Br
Ar B(OH)2
Ar Ar
Ar Ar
1
2
3
4
5
100
96
ꢀ
gered along the Ru Ru axis
100
96
96
90
with C1-Ru1-Ru2-C4 and C2-
Ru1-Ru2-C3 torsion angles of
40.5(5) and 39.2(4)8, respective-
ly.
100
84
100
80
Catalysis using complex 1
6
65
60
Suzuki cross-coupling reactions:
To evaluate the catalytic effica-
cy of dimetal complexes in or-
ganic transformations, we per-
formed Suzuki reactions with
1.[13] Under the optimized reac-
tion conditions (vide infra),
cross coupling of a number of
aryl bromides with arylboronic
acids were investigated and the
results are tabulated in Table 2.
In a typical experiment, 1 mmol
aryl bromide was reacted with
1.2 mmol arylboronic acids in
the presence of 0.5 mol% cata-
7
100
98
88
85
76
70
70
45
94
94
84
80
70
65
65
40
8
9
10
11
12
13
14
lyst
1 and 2 mmolK3PO4 at
1008C for 2 h. Electron-poor
aryl bromides, as well as bromo
benzene, could be efficiently
converted to the desirable
products in high yields (Table 2,
15
16
17
44
42
37
36
40
30
entries 1–4),
yields were observed for elec-
tron-rich aryl bromides
but
moderate
18
68
60
(Table 2, entries 5 and 6). para-
Substituted aryl bromides gen-
erally undergo faster cross-cou-
pling reactions compared to
corresponding ortho-substituted
aryl bromides (Table 2, entries 5
and 6). The substitution effect
on the arylboronic acid was
19
20
0
0
0
0
[a] Reaction conditions: 1 mmol aryl bromide, 1.2 mmol aryl boronic acid, 2 mmol base and 0.5 mol% 1; sol-
vent mixture is CH3CN/H2O (v/v=4:1); T=1008C; t=2 h. [b] Determined by using GC-MS after the reaction.
also explored in our study. Al- [c] Yield of isolated product.
14462
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Chem. Eur. J. 2010, 16, 14459 – 14468

Bimetallic Catalysis
FULL PAPER
Encouraged by Hor and co-workers work[15a] on the cou-
pling of aryl chlorides employing [Pd2ACTHUNTRGENNUG(CH3CN)6]AHCTUNGTREN(NUGN SbF6)2 and
via oxidative pathway to a variety of dimetal unit has been
reported.[16] In particular, dipalladium(I) compounds have
been reported to undergo oxidative addition to a variety of
substrates including dioxygen and RSH.[17] Transmetalation
of the oxidative-addition product [Ar-PdII···PdII-Br] with
ferrocenyl phosphine ligands, we subjected catalyst 1 to
cross-coupling reactions between aryl chlorides and arylbor-
onic acids. Reactions with 1 mol% catalyst loading, and
heating at 1008C for 2 h in CH3CN/H2O solvent mixture, af-
forded nearly 45% conversion for the electron-poor aryl
chlorides (Table 2, entries 14 and 15). However, yields ex-
ceeding 80% could be achieved with 5 mol% catalyst load-
ing and 8 h of prolonged heating at 1008C. Electron-rich
chlorides provided lower yields under identical reaction con-
ditions.
arylACHTNUGRTNEUNG
(Ar’)-boronic acid produces [Ar-PdII···PdII-Ar’], which
undergoes bimetallic reductive elimination to provide the
ꢀ
coupled product Ar Arꢁ with the regeneration of the cata-
lyst to initiate the next catalytic cycle (Scheme 2).
Metal dimers are unique in their mode of bonding to
small molecules and can trigger facile activation.[18] Further-
more, dinuclear elimination of product has been invoked to
rationalize the enhanced reaction rate in homogeneous
The screening of solvents and various bases was per-
formed by using the model coupling reaction between 4-bro-
metal-cluster catalysis.[19] Evidently, the presence of
a
second metal causes the rate enhancement of hydrogen
elimination from a metal hydride, which exhibits sluggish in-
tramolecular elimination otherwise.[20] Nocera has demon-
strated the bimetallic cooperativity for the dihydrogen addi-
tion and elimination on a diiridium core.[21] There are strong
indications to propose that the substrate activation, in addi-
tion to the product elimination steps, are more efficient on a
dimetal platform. The ability of 1 to catalyze the coupling of
aryl chlorides is linked to the superior activity of dipalladiu-
m(I) core compared to mononuclear PdII compounds, which
show poor activity towards aryl chlorides.[22]
To establish the bimetallic reactivity, we performed reac-
tions with bromo pyridines. In general, the presence of a
heteroatom reduces the reactivity, as it binds to the metal
centre, thereby suppressing oxidative addition. We observed
a 68% conversion with 3-bromo pyridine (Table 2, en-
tries 18). Remarkably, no cross-coupled product was detect-
ed for the ortho isomers (Table 2, entries 19, 20) even after
prolonged heating. It is our as-
Table 3. Influence of solvent and base on the coupling reaction.
Entry
Base
Solvent
Conversion[a] [%]
1
2
3
4
5
6
7
8
K2CO3
K2CO3
NaOAc
K3PO4
K2CO3
K2CO3
K3PO4
K3PO4
CH3CN
DMF
THF
51
49
44
68
72
92
88
100
DMF
CH3CN
CH3CN/H2O
DMF/H2O
CH3CN/H2O
[a] Determined by using GC-MS.
moacetophenone and phenyl boronic acid (Table 3). Reac-
tion in acetonitrile gave better yields compared to DMF or
THF (Table 3, entries 1–3). The effect of base was studied
and K3PO4 (2 equiv) turned out to be the best choice as it
provided higher yields. A mixed solvent combination of
CH3CN/H2O (4:1) (Table 3, entries 6 and 8) gave better re-
sults than any other organic solvents we attempted. The pos-
itive effects of water have been attributed to several factors,
including the suppression of the trimerization of aryl boron-
ic acid and the better solubility of inorganic bases.[15]
sertion that the oxidation addi-
tion of 2-bromopyridine leads
to
a bridged cyclometalated
product, as shown in Scheme 3,
which prohibits subsequent re-
actions. Structural precedence
of similar cyclometalated prod-
Scheme 3. Schematic represen-
tation of the proposed
bridged-cyclometalated prod-
uct.
We propose
a
bimetallic mechanism for catalyst
1
ucts
[Pd2Cl2ACHUTNGTRENNUNG
(PPh3)2(m2-C2N-
2A(PPh3)2(m2-
(Scheme 2). The first step is the oxidative addition of aryl
py)2] and [Pd2Cl CHTUNGTRENNUNG
I
I
ꢀ
bromide across the Pd Pd single bond. Substrate addition
C2N-iquinoline)2] gives cre-
dence to this pathway.[23]
Heck coupling reaction: The Heck reaction is a powerful
ꢀ
and efficient method for the C C bond formation in which
haloarenes couple with alkenes in the presence of a Pd cata-
lyst to form a new alkene.[24] Phosphine ligands have been
used extensively along with PdII salts for the in situ genera-
tion of the Pd0 catalysts. The cost and air/moisture sensitivity
of phosphine is a major drawback. We have examined the
utility of 1 as catalyst in phosphine-free Heck-coupling reac-
tions. In a typical reaction, aryl bromide and styrene are re-
acted at 1008C for 2 h with 0.5 mol% catalyst (Table 4). For
electron poor aryl bromides (Table 4, entries 1–5), quantita-
tive conversions were achieved, whereas slightly lower
Scheme 2. Proposed bimetallic catalytic mechanism for a Suzuki cross-
coupling reaction.
Chem. Eur. J. 2010, 16, 14459 – 14468
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14463

J. K. Bera et al.
Table 4. Heck cross coupling of aryl bromides with substituted styrenes catalyzed by 1.
less than 10%. Incorporation of
electron-withdrawing
nitro
groups on the aromatic ring
provided higher yields and E-
selectivity. Quantitative conver-
sion was achieved for 2,4-dini-
trobenzaldehyde at ambient
temperature in 1 h with excel-
lent selectivity (99:1). Similar
Entry
Aryl halide
ꢀ
Ar Br
Coupling partner
Conversion[b]
[%]
E/Z[b]
[%]
Yield[c]
[%]
ꢀ
Ar’ CH=CH2
1
2
100
100
98
96
96
92
90
96
94
94
96/4
96/4
96/4
96/4
96/4
96/4
96/4
96/4
96/4
96/4
96
96
94
90
90
88
86
92
90
90
conversion
and
selectivity
3
values were also observed for
p- and o-nitro benzaldehyde
(Table 5, entries 2 and 3). Intro-
4
duction
of trifluoromethyl,
5
cyano and bromo groups af-
forded marginal improvements
in the yields compared to ben-
zaldehyde (Table 5, entries 4–
6).
6
7
Electron-rich aldehydes ex-
8
hibited
reduced
reactivity.
Lower conversions were ob-
served for o-hydroxy, p-methyl
and p-methoxy benzaldehyde
after 6 h (Table 5, entries 8–10).
p-(Dimethyl)amino benzalde-
hyde was found to be the least
reactive (Table 5, entry 11),
however, after a prolonged re-
9
10
[a] Reaction conditions: 1 mmol aryl bromide, 1.5 mmol substituted styrene and 0.5 mol% 1, temp. 1008C,
time 2 h. [b] Determined by using GC-MS. [c] Combined yields of E and Z isomers after chromatography.
yields were noted for electron-rich counterparts (Table 4,
entries 6,7). Identical trends are noted for different styrene
derivatives. An excellent trans to cis ratio (96:4) is observed
for each case.
action time (10 h) the reaction could be completed.
The reaction is catalytic, as no olefination product was ob-
served without the application of catalyst 2 under identical
reaction conditions. In the absence of catalyst, the azine Ph-
ꢀ
CH=N N=CHCOOEt was observed in the GC-MS spectra
Catalysis using diruthenium(I) complex 2
when the reaction was allowed to continue for 1 day. The
presence of triphenylphosphine was found to be essential
for this reaction and no olefination product was observed in
its absence. We made attempts to optimize the amount of
PPh3 and it was found that 1.2 equivalents with respect to
the catalyst provides maximum conversion. Invariably, the
formation of triphenylphosphine oxide was observed in the
GC-MS spectra.
Aldehyde olefination: Metal–metal singly bonded dirhodiu-
m(II) carboxylate and carboxamide have been used for a
wide range of synthetic transformations, such as cyclopropa-
[26]
nation,[25] C H insertions
and aromatic substitution.[27]
ꢀ
The critical intermediate, which unifies this body of chemis-
try, is an electrophilic dirhodium carbenoid in which a car-
ꢀ
bene occupies the site trans to the Rh Rh axis in a linear
To evaluate the effect of catalyst loading, the olefination
of benzaldehyde was investigated at different catalyst load-
ings. On increasing the catalyst amount from 1% to 5%, a
marginal increase in the conversion was observed with no
significant change in the selectivity.
fashion.[28] Compound 2 is isoelectronic with dirhodium(II)
compounds and offers access to axial sites for substrate co-
ordination. These features prompted us to examine the pros-
pect of 2 as a carbene transfer catalyst. The catalytic poten-
tial was evaluated for aldehyde olefination reaction by using
1.5 mmol ethyl diazoacetate (EDA), 1.2 mmol of PPh3 and
1 mol% catalyst in toluene. Treatment of benzaldehyde with
EDA/PPh3/2 in toluene at 808C provided ethyl cinnamate in
82% conversion after 6 h (Table 5). Purification of the prod-
uct by silica gel column chromatography with petroleum
ether/ethyl acetate (9:1 v/v) afforded 75% isolated yield
(Table 5, entry 7). Ethyl maleate and fumarate were also ob-
served in the GC-MS spectra, as side products resulting
from the dimerization of carbene with a combined yield of
We propose a catalytic cycle as shown in Scheme 4. In the
initial step, ethyl diazoacetate reacts with the diruthenium(I)
catalyst 1 and forms a dimetal-carbene intermediate. To es-
I
I
ꢀ
tablish the intermediacy of the [Ru Ru =CHACTHNUTRGENU(GN COOEt)] spe-
cies, styrene was added to the reaction mixture and the cy-
clopropanation product ethyl-2-phenylcyclopropylcarboxy-
late, which was detected by using GC-MS, was subsequently
isolated and characterized by using NMR spectroscopy. Ger-
hard Maas has noted the formation of axial carbenes in cy-
clopropanation reactions with diazo compounds catalyzed
14464
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Chem. Eur. J. 2010, 16, 14459 – 14468

Bimetallic Catalysis
FULL PAPER
Table 5. Olefination of aldehyde with ethyl diazoacetate catalyzed by 2.[a]
4ACTHNUTRGNEUNG
by [RuI2(CO) (OAc)2].[29] This
is also in line with the mecha-
nism suggested for dirhodi-
um(II) carboxylates.[25] Thus,
there is a strong indication for
Entry
Aldehyde
Product
T
[8C]
t
Conversion
[%][b]
E/Z
Yield
[h]
[%][b] [%][c] the formation of a linear diru-
thenium-carbenoid intermedi-
ate in this reaction. The next
1
2
3
4
5
25
25
25
25
25
1
2
2
2
2
100
98
98
87
86
99/1
99/1
99/1
93/7
94/6
95
92
92
79
78
step is the transfer of carbene
to PPh3 resulting in the phos-
phorane. The ylide reacts with
aldehyde to produce new olefin
and
phosphine
oxide
(Scheme 4). In a controlled ex-
periment, the phosphorane
Ph3P=CHCOOEt was identified
by using 31P NMR spectroscopy
in the absence of aldehyde.[30]
Furthermore, it has been estab-
lished that the stoichiometric
reaction of Ph3P=CHCOOEt
with benzaldehyde produce
ethyl cinnamate in high yield.
Notably, it is the formation of
ylide that is catalyzed and not
the subsequent Wittig reaction.
The pathway proposed here
is akin to the process described
for a FeII-porphyrin catalyst.[31]
An alternate mechanism has
been suggested for the methyl-
6
7
8
80
80
80
6
6
6
84
82
74
92/8
92/8
98/2
77
75
65
9
80
80
6
6
68
61
93/7
91/9
60
55
trioxorhenium
(MTO)-cata-
lyzed aldehyde olefination,
which involves a metallaoxe-
10
tane
intermediate
(see
Scheme S1 in the Supporting
Information).[32] The metallaox-
etane intermediate is formed in
the reaction of an aldehyde
with a metal carbene and frag-
mentation of the metallaoxe-
tane provides the new olefin
and the MTO. Subsequently,
11
80
6
52
91/9
45
[a] Reaction conditions: 1 mmol aldehyde, 1.2 mmol PPh3, 1.5 mmol of EDA, 1 mol% catalyst 2. [b] Deter-
mined by using GC-MS after reaction. [c] Combined yields of E and Z isomers after chromatography.
the PPh3 abstracts oxygen atom from MTO generating a
ReV–dioxo species. The latter reacts with diazo reagent to
produce a metal carbene, which initiates the next catalytic
cycle. To check this possibility in the aldehyde olefination
reaction for catalyst 2, cyclohexene was used instead of PPh3
under the same catalytic conditions and neither the olefin
nor the cyclohexene epoxide is detected in the GC-MS. It is,
therefore, unlikely that the diruthenium(I)-catalyzed alde-
hyde olefination proceeds by means of the metallaoxetane
route. Moreover, the naphthyridines and carbonyls afford a
single site for the catalysis, which is not conducive for metal-
laoxetane formation.
Scheme 4. Mechanism of aldehyde olefination catalyzed by 2.
Chem. Eur. J. 2010, 16, 14459 – 14468
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14465

J. K. Bera et al.
1322(s), 1292(s), 1264(s), nACHTNUGRTNEUNG
(BF4ꢀ) 1069 cmꢀ1 (br); ESI-MS: m/z: 492
Conclusion
[M]2+; elemental analysis (%) calcd for C42H38N6O2F8B2Fe2Pd2: C 43.6, H
3.31, N 7.26; found: C 43.04, H 3.25, N 7.09.
Dipalladium(I) and diruthenium(I) compounds bridged by
two amide-linked naphthyridine-ferrocene hybrid ligands
have been synthesized. Both compounds exhibit short
metal–metal distances aided by a ligand framework. The di-
palladium(I) compound (1) is an excellent catalyst for phos-
phine-free, Suzuki cross-coupling and Heck-coupling reac-
tions. A cooperative bimetallic mechanism is proposed for
the Suzuki reaction in which the substrate addition and
product elimination occur on the dipalladium platform. The
diruthenium(I) compound (2) is shown to be an efficient
catalyst for selective olefination of aldehydes. The mecha-
nism has been fully elucidated. Compound 2 catalyzes the
formation of phosphorane from ethyl diazoaceate and PPh3.
Subsequently the phosphorane reacts with aldehyde to pro-
duce new olefin. The efficacy of bimetallic complexes in cat-
alytic organometallic transformation reactions is demon-
strated in this work. Further work is in progress to extend
this chemistry to other dimetal compounds and to apply
them in more challenging organic reactions.
Synthesis of [RuI2(L)2(CO)4]
(10 mL) of L (44 mg, 0.11 mmol) was added drop wise to a dichlorome-
thane solution (15 mL) of [Ru2(CO)4A(MeCN)6][BF4]2 (40 mg,
ACHTUGNETNR[NUNG BF4]2 (2): A dichloromethane solution
G
ACHTUNGTRENNUNG
0.054 mmol), and the reaction mixture was stirred overnight at room tem-
perature. Solution was concentrated, and diethyl ether (15 mL) was
added with stirring to induce precipitation. The resulting dark-red solid
residue was washed with diethyl ether (3ꢄ10 mL), and dried in a vacuum
to afford 2. Yield: 56 mg (82%). X-ray quality crystals were grown by
layering diethyl ether onto a saturated dichloromethane solution of 2
1
inside an 8 mm o.d. vacuum-sealed glass tube. H NMR (CDCl3): d=9.36
(br. 1H, NH), 8.58 (d, J=9 Hz, 1H, NP-Ha), 7.43 (d, J=9 Hz, 1H, NP-
Hb), 7.2 (s, 1H, NP-Hc), 5.29 (s, 2H, Cp), 4.77 (s, 2H, Cp), 4.21 (s, 5H,
Cp), 3.04 (s, 3H, NP-Me), 2.81 ppm (s, 3H, NP-Me); 13C NMR
(125 MHz, CDCl3, 294 K): d=202.6 (CO), 200.6 (CO), 176.1 (NHCO),
169 (NCNNP), 157 (NCCNP), 154.5 (NCCNP), 150.6 (CCCNP), 141 (CHNP),
125.9 (CHNP), 121.3 (CCCNP), 117.4 (CHNP), 74.9 (CCCCp), 71.1 (CHCp),
70.8 (CHCp), 70 (CHCp), 27.9 (CH3), 18.5 ppm (CH3); IR (KBr): n˜ =
n(CO) 2042 (vs), 1998 (s), 1954 (s), n
1415 (s), 1294 (s), n
(BF4ꢀ) 1083 cmꢀ1 (br). ESI-MS: m/z: 1173 [{M}2+
BF4ꢀ]+, 1086 [M]+, 543 [M]2+
elemental analysis (%) calcd for
ACHTUNGTNER(NUNG CONH) 1633 (m), n(NP) 1610 (m),
G
+
;
C46H38N6O6F8B2Fe2Ru2: C 43.91, H 3.04, N 6.68; found: C 43.48, H 2.91,
N 6.50.
X-ray data collection and refinement: Single-crystal X-ray studies were
completed by using
a CCD Bruker SMART APEX diffractometer
equipped with an Oxford Instruments low-temperature attachment. All
the data were collected at 100(2) K using graphite-monochromated MoKa
radiation (l=0.71073 ꢃ). The frames were indexed, integrated, and
scaled by using the SMART and SAINT software packages,[35] and the
data were corrected for absorption by using the SADABS program.[36]
The structures were solved and refined by using the SHELX suite of pro-
grams.[37] All hydrogen atoms were included in the final stages of the re-
finement and were refined with a typical riding model. Structure solution
Experimental Section
General procedures: All reactions with metal complexes were carried out
under an atmosphere of purified nitrogen by using standard Schlenk-
vessel and vacuum-line techniques. The 1H NMR spectra were obtained
by using JEOL JNM-LA 400 MHz and 500 MHz spectrometers. The
1H NMR chemical shifts were referenced to the residual hydrogen signal
of the deuterated solvents. Elemental analyses were performed by using
a Thermoquest EA1110 CHNS/O analyzer. Recrystallized compounds
were powdered, washed several times with dry ether and dried in
vacuum for at least 48 h prior to elemental analyses. ESI-MS were re-
corded by using a Waters Micromass Quattro Micro triple-quadrupole
mass spectrometer. The ESI-MS of all complexes were recorded in aceto-
nitrile. The Infrared spectra were recorded by using a Bruker Vertex 70
FTIR spectrophotometer in the range n˜ =400 to 4000 cmꢀ1 using KBr
pellets. The GC-MS experiments were performed by using an Agilent
7890 A GC and 5975C MS system.
Table 6. Crystallographic data and pertinent refinement parameters for
compounds 1·2CH2Cl2 and 2.
1·2CH2Cl2
2
empirical formula
formula weight
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
a [8]
b [8]
C44H42B2Cl4F8Fe2N6O2Pd2 C46H35B2F8Fe2N6O6Ru2
1326.76
triclinic
P1
1255.26
tetragonal
¯
¯
P4
10.266(2)
13.121(3)
19.318(4)
82.634(4)
82.136(3)
69.561(3)
2406.4(9)
2
28.309(5)
28.309(5)
14.629(4)
90.00
90.00
90.00
Materials: Solvents were dried by using conventional methods, distilled
under nitrogen and deoxygenated prior to use. RuCl3·xH2O and Pd
sponge were purchased from Arora Matthey, India. Compounds [Ru2-
[33]
[34]
ACHTUNGTRENNUNG(CH3CN)6(CO)4]ACHTUNGTRENNUNG[BF4]2, [PdACHTUNGERTG(NUNN CH3CN)4]ACHTNUGRTNE[NUGN BF4]2 and [{(5,7-dimethyl-1,8-
g [8]
naphthyridin-2-yl)amino}carbonyl]ferrocene (L) were synthesized accord-
V [ꢃ3]
11724(4)
8
ing to the literature procedure.[9]
Z
Synthesis of [PdI2(L)2]
added to an acetonitrile solution (30 mL) of [Pd
ACHTUNGTRENNUNG
1calcd [gcmꢀ3
]
1.831
1.422
U
ACHTUNGTRENNUNG
m [mmꢀ1
(000)
]
1.625
1.061
0.081 mmol), and the mixture was stirred overnight at room temperature.
The solution was concentrated, and diethyl ether (15 mL) was added
with stirring to induce precipitation. The resulting dark-red solid residue
was washed with diethyl ether (3ꢄ10 mL), and dried in a vacuum to
afford 1. Yield: 35 mg (75%). X-ray quality crystals were grown by layer-
ing petroleum ether onto a saturated dichloromethane solution of 1
inside an 8 mm o.d. vacuum-sealed glass tube. 1H NMR (CD3CN): d=
9.49 (br. 1H, NH), 8.60 (d, J=10 Hz, 1H, NP-Ha), 8.50 (d, J=10 Hz, 1H,
NP-Hb), 7.56 (s, 1H, NP-Hc), 4.62 (s, 1H, Cp), 4.68 (s, 1H, Cp), 5.04 (s,
1H, Cp), 5.13 (s, 1H, Cp), 4.35 (s, 5H, Cp) 3.38 (s, 3H, NP-Me),
2.77 ppm (s, 3H, NP-Me); 13C NMR (125 MHz, CD3CN, 294 K): d=169.9
(NHCO), 166.1 (NCNNP), 154.8 (NCCNP), 153.9 (NCCNP), 152.8 (CCCNP),
137.2 (CHNP), 125.8 (CHNP), 124.4 (CCCNP), 120.2 (CHNP), 72.4 (CCCCp),
70.9 (CHCp), 70.2 (CHCp), 69 (CHCp), 25.9 (CH3), 18 ppm (CH3); IR
F
A
1316
4984
reflections
collected
independent
21260
11438
106056
29126
observed [I>2s (I)] 7454
18657
no. of variables
GooF
635
1.055
1303
0.963
Rint
0.0476
0.1063
final R indices
R1=0.0637
wR2=0.1567
R1=0.0605
wR2=0.1142
R1=0.1010
wR2=0.1327
[I>2s(I)][a]
R indices (all data)[a] R1=0.1069
wR2=0.1892
2
2
[a] R1 =S jjFo jꢀjFc jj/SjFo j with Fo2 >2s
ACHTUNGTRENNUNG
2
2
(KBr): n˜ =n
(CONH) 1684 (s), n(NP) 1599 (m), 1504(vs), 1406 (m),
SjFo j ]1/2
.
14466
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 14459 – 14468

Bimetallic Catalysis
FULL PAPER
and refinement details for compounds 1 and 2 are provided in the Sup-
porting Information. The “SQUEEZE” option in PLATON program[38]
was used to remove a disordered solvent molecule from the overall inten-
sity data of compound 2. All non-hydrogen atoms were refined with ani-
sotropic thermal parameters. Pertinent crystallographic data for com-
pounds 1 and 2 are summarized in Table 6. ORTEP-32[39] was used to
produce the diagrams. CCDC-782940 (1) and CCDC-782941 (2) contains
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
This work is financially supported by the Department of Science and
Technology (DST), India and Indo-French Centre for the Promotion of
Advanced Research (IFCPAR). J.K.B. thanks the DST for the Swarna-
jayanti Fellowship and Ramanna Fellowship. R.K.D. and B.S. thank
CSIR and S.M.W.R. thanks UGC, India for fellowships.
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14468
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