Facile tandem Suzuki coupling/transfer hydrogenation reaction with a bis-heteroscorpionate Pd-Ru complex

Article abstract of DOI:10.1039/c4dt02465f

Design and synthesis of the bis(pyrazol-1-yl)methane based bis-heteroscorpionate Pd-Ru complex results in efficient tandem Suzuki coupling/transfer hydrogenation reaction with a broad range of substrate reactivity.

Full text of DOI:10.1039/c4dt02465f

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Facile tandem Suzuki coupling/transfer
hydrogenation reaction with a
Cite this: DOI: 10.1039/c4dt02465f
bis-heteroscorpionate Pd–Ru complex†
Received 13th August 2014,
Accepted 16th September 2014
Niranjan Dehury, Suman Kumar Tripathy, Anupam Sahoo, Niladri Maity* and
Srikanta Patra*
DOI: 10.1039/c4dt02465f
www.rsc.org/dalton
Design and synthesis of the bis(pyrazol-1-yl)methane based developed with such ligands and employed for designing
bis-heteroscorpionate Pd–Ru complex results in efficient tandem models for enzymes, supramolecular architecture, precatalysts
Suzuki coupling/transfer hydrogenation reaction with a broad or catalysts for numerous homogeneous catalytic processes,
range of substrate reactivity.
etc.7 In contrast, the use of a multidentate bis-heteroscorpio-
nate ligand is relatively rare.8 Recently, we have reported the
preparation and application of dinuclear [(p-cym)RuX] (X = Cl
or H2O), incorporating an α,α,α′,α′-tetra(pyrazol-1-yl)-2,6-luti-
dine (pz4lut) bis-heteroscorpionate ligand.8d However, to the
best of our knowledge, development of a heterodimetallic
complex of bis-heteroscorpionate ligands is yet to be explored.
Herein, we extend the use of the pz4lut ligand towards syn-
thesis of heterodimetallic [{(p-cym)RuCl}(µ-pz4lut)PdCl2]ClO4 (1)
and homodimetallic complexes {[(PdCl2)2(µ-pz4lut)] (2) and
[{(p-cym)RuCl}2(µ-pz4lut)]Cl2 (3)8d} (Fig. 1). The complexes
were characterised via arrays of analytical techniques, and
were tested for tandem Suzuki coupling/transfer hydrogen-
ation reaction. The advantage of heterometallic 1 over its
homodimetallic analogues 2 and 3, and non-ligated precursors
PdCl2 (4) and [(p-cym)RuCl(μ-Cl)]2 (5) towards the aforemen-
tioned reaction was also investigated.
With the increasing demand for environmentally benign and
economically viable synthetic processes one-pot multi-step
reactions have emerged as a fascinating area of current
research.1 Introduction of multiple catalytic units within a
single ligand framework can promote cooperativity between
the reaction centers, thereby improving the activity and/or
selectivitiy.1c,2 Furthermore, heterometallic systems are favored
over homometallic analogues not only due to their crucial
synergistic effect but also due to that they increase the number
of possible combinations of reactions that are mechanistically
different in nature, known as tandem-reactions.3,4a Thus,
development of a new heterometallic catalytic system exhibit-
ing one-pot multi-step reaction is highly challenging. In this
context, Peris and co-workers have done extensive work on the
development of multidentate N-heterocyclic carbene (NHC)
and triazolyl-diylidene bridged heterodimetallic complexes of
Pd, Pt, Rh, Ir, Au and Ru for efficient multiple tandem reac-
tions, such as dehalogenation/transfer hydrogenation, Suzuki
coupling/transfer hydrogenation, Suzuki coupling/α-alkylation
reaction, etc.4 In addition, Djakovitch et al. and Chen et al.
have independently developed the heterometallic Pd–Cu
system for one-pot indole synthesis5 and tandem click/Sonoga-
shira coupling reactions, respectively.6
Heterodimetallic 1 is obtained by reacting the pz4lut ligand
with [(p-cym)RuCl(μ-Cl)]2 and in situ generated Li2[PdCl4], suc-
cessively, and isolated as ClO4 salt (Experimental section,
Scheme S1, ESI†). The homodimetallic 2 is synthesised by
reacting the pz4lut ligand with in situ generated Li2[PdCl4], in
Tridentate N,N,X donor based heteroscorpionate ligands
are well known for their spectator nature and tunable steric
and electronic properties.7 A large number of metal complexes
of the main group, early and late transition metals have been
School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar,
Orissa-751007, India. E-mail: niladriiitb@gmail.com, srikanta@iitbbs.ac.in
†Electronic supplementary information (ESI) available: Experimental details,
spectroscopic characterization, catalytic results and crystallographic data. CCDC
1017846 for 1. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt02465f
Fig. 1 Structures of heterodimetallic Pd–Ru (1), and homodimetallic
Pd–Pd (2) and Ru–Ru (3) complexes of the bis-heteroscorpionate
ligand.
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1 : 2 molar ratio and 3 has been prepared and characterized by based reductions are observed at negative potentials in cyclic
following the reported procedure.8d
voltammetry. Several attempts were made to record the cyclic
Complex 1 behaves as a 1 : 1 electrolyte in acetonitrile voltammogram of homodinuclear 2, however no representable
whereas complex 2 is neutral. Both 1 and 2 gave satisfactory cyclic voltammogram was obtained. This may be due to the
microanalytical data. The presence of the ClO4 counter anion poor solubility of the complex 2 in most of the solvents includ-
in 1 was confirmed by the characteristic IR vibrational fre- ing acetonitrile.
quency at 1100 cm−1 and 690 cm−1
.
Mononuclear complexes of palladium and ruthenium with
Molecular identities of the complexes 1 and 2 are con- a heteroscorpionate ligand are known to show activity towards
firmed from positive ion electrospray ionization mass spectra Suzuki coupling and transfer hydrogenation reaction, respect-
(Fig. S1, ESI†). The molecular ion peaks centred at 931.8 (calcd ively.11,12 Complex 1 constituting two catalytically independent
931.69) for [1–ClO4]+ and at 803.9 (calcd 803.9) for [2–Cl]+ moieties [PdCl2 and {(p-cym)RuCl}] in the pz4lut ligand frame-
confirm the existence of the entire molecular framework of work is a point of interest whether it can successfully combine
1 and 2, respectively, in solution. Furthermore, the 1H NMR mechanistically two different reactions. To prove our concept,
spectra of 1 and 2 in DMSO-d6 exhibiting 13 and 9 protons, 4-bromoacetophenone and phenylboronic acid were taken as
respectively, in the region 4.5–10.5 ppm support the presence substrates. Reaction conditions were screened for 1 by varying
of the pz4lut ligand and [(p-cym)Ru] fragment (Fig. S2, ESI†). the base and solvent at 80 °C (Table 1). It appears that
Intense singlets and doublets at 0–2.0 ppm corresponding to although moderate C–C coupling reaction (product A) pro-
CH3 and CH(CH3)2 protons indicate the existence of the ceeds using carbonate bases, transfer hydrogenation (product B)
[(p-cym)Ru] fragment in complex 1.
does not occur until KOtBu is introduced into the reaction
The formation of 1 is further authenticated by single crystal mixture (entries 1–4). However, under the identical reaction
X-ray analysis (Fig. 2). Important crystallographic parameters conditions none of A and B were identified in the absence of
and selected bond lengths and bond angles are presented in 1 (entry 5). Moreover, among the alcohols an aliphatic second-
tables (Table T1 and T2, ESI†). The pyrazolyl units of the ary alcohol (iPrOH) brings more activity than its primary ana-
pz4lut ligand form a six membered metallacycle adopting a logue nPrOH (entries
4 and 6). An aromatic alcohol
boat conformation with metal centres. The ruthenium centre (PhCH2OH) is even unable to induce the transfer hydrogen-
shows expected pianostool geometry and palladium shows dis- ation process (entry 7).
torted square planar geometry with N3–Ru1–N5 and N7–Pd1–
At 80 °C, no difference in activity for tandem Suzuki coup-
N9 bite angles 86.99(11)° and 86.57(12)°, respectively. The Ru– ling/transfer hydrogenation reaction between heterodimetallic
Cl (2.4020(10) Å), Ru–Ccentroid (1.700 Å) and Pd–Cl (2.28(11) Å) 1 and the equimolar mixture of homodimetallic 2 and 3 was
distances are in well agreement with the values reported ear- observed. Thus, the tandem reaction was monitored at a lower
lier.8d,9 The intermetallic Ru–Pd distance is 8.01 Å.
temperature (30 °C) to follow the reaction profile with time
The heterodimetallic Pd–Ru (1) in dichloromethane shows
an MLCT band at 440 nm similar to homodinuclear 3 (Fig. S3,
ESI†).8d,10 However, a blue shifted MLCT band at 384 nm was
observed for homodimetallic Pd–Pd (2) (Fig. S3†). In addition,
both 1 and 2 exhibit high energy intraligand charge transfer
transitions (π → π*) below 300 nm.
Table 1 Tandem Suzuki coupling/transfer hydrogenation of 4-bromo-
acetophenonea
Complex 1 in acetonitrile shows two successive irreversible
oxidation peaks Epa at 0.81 V and 1.44 V which could be
assigned as oxidation of RuII/RuIII and RuIII/RuIV, respectively
(Fig. S4, ESI†). The peak potentials are almost similar to pre-
viously reported [(p-cym)Ru] complexes.4g,8d,10 The ligand
Yieldb (%)
Entry
Cat.
ROH
Base
Temp. (°C)
A
B
1
2
3
4
5
6
7
1
1
1
1
—
1
1
1
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
nPrOH
PhCH2OH
iPrOH
iPrOH
iPrOH
Na2CO3
K2CO3
Cs2CO3
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
80c
80c
80c
80c
80c
80c
80c
30d
30d
30d
47
43
32
0
0
0
0
97
0
30
0
67
53
25
0
14
14
16
24
14
8
9e
10f
2 + 3
4 + 5
a Reaction conditions: catalyst/4-bromoacetophenone/PhB(OH)2/base =
1/800/1000/3000 (molar ratio), catalyst (0.125 mol%), 4-bromo-
acetophenone (0.5 mmol), PhB(OH)2 (0.6 mmol), base (1.8 mmol),
ROH (1 mL). b Isolated yields after preparative thin layer
chromatography. c Reaction time = 2.5 h. d Reaction time = 48 h. e 2 : 3 =
1 : 1 (molar ratio). f 4 : 5 = 1 : 0.5 (molar ratio).
Fig. 2 ORTEP diagram of complex 1. Ellipsoids are drawn at the 30%
probability level. Hydrogen atoms and ClO4 anion have been removed
for clarity.
−
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indicate the consecutive nature of the two mechanistically
independent reactions, where the C–C coupling process leads
the reaction in step I with subsequent hydrogenation of the
keto group of the coupling product in step II (Scheme S2,
ESI†). At low temperatures, the Pd–Ru heterodimetallic 1 pro-
vides the catalytic result (entry 8, Table 1) with the maximum
yield of B. Although a mixture of 2 and 3 (entry 9) affords a
comparable overall yield (A + B), the reaction profile (Fig. 3b)
clearly shows that the rate of reaction is considerably slower as
compared to 1 (Fig. 3a). The mixture of precursors 4 and 5
with much inferior catalytic outcomes (entry 10) indicates the
importance of the pz4lut ligand framework to provide a well-
defined active heterodimetallic tandem catalyst.
Fig. 3 Time-dependent transformation of 4-bromoacetophenone
towards tandem Suzuki coupling/transfer hydrogenation reaction by (a)
1 and (b) 1 : 1 mixture of 2 and 3. Reaction conditions: 4-bromoaceto-
phenone (0.5 mmol), phenylboronic acid (0.6 mmol), KOtBu
(1.8 mmol), iPrOH (1 mL), 1 or 2 + 3 (0.125 mol%), at 30 °C.
=
In order to check the generality of the trend, reaction pro-
files were also monitored by varying the substituents of phe-
and to compare the activity between the hetero- (1) and homo- nylboronic acid. It appeared that the trends are almost similar,
dimetallic (2 + 3) catalytic systems (Fig. 3a and 3b). It is however the rate of reactions are slower as compared to the
observed that though C–C coupling and transfer hydrogen- combination of 4-bromoacetophenone and phenylboronic acid
ation reactions occur simultaneously, the initial rate of coup- (Fig. S5, ESI†).
ling reaction is much faster than the later in both cases. In
Furthermore, the tandem reaction is extended to varieties
fact, transfer hydrogenation follows an induction period of of bromoarylketones and arylboronic acids (Table 2, entries
3 h, and accelerates at a point with the maximum yield of A 1–7). Notably, under moderate heating conditions (80 °C)
(8 h for 1 and 12 h for 2 + 3), suggesting that the hydrogen- complex 1 is active towards all the substrates with good to
ation of A is more facile than 4-bromoacetophenone. This excellent yields of the corresponding tandem products.
trend is in agreement with the observation reported by Peris
Finally, to check the homogeneous nature of the present
et al. for tandem dehalogenation/transfer hydrogenation reac- catalytic systems (1 and 2 + 3), the mercury drop test was con-
tion of 4-bromoacetophenone on the Pd–Ir heterodimetallic ducted.13 Metallic mercury was added to the reaction mixture
catalyst.4a Indeed, no hydrogenation product of 4-bromoaceto- after ∼50% completion of the reaction. No significant change
phenone was detected in the reaction mixture. These results in the yield of the final product (B) was observed (Table T3†)
Table 2 Substrate scope of tandem Suzuki coupling/transfer hydrogenation on catalyst 1a
Entry
1
ArBr
Ar/B(OH)2
Product
Yieldb (%)
97c
2
3
4
82d
92d
86e
5
6
87e
90f
a Reaction conditions: 1/ArBr/Ar/B(OH)2/KOtBu = 1/800/1000/3000 (molar ratio), 1 (0.125 mol%), ArBr (0.5 mmol), Ar/B(OH)2 (0.6 mmol), KOtBu
(1.8 mmol), iPrOH (1 mL), 80 °C. b Isolated yields after preparative thin layer chromatography. c Reaction time = 2.5 h. d Reaction time = 3.5 h.
e Reaction time = 4.5 h. f Reaction time = 8.5 h.
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which supports the homogeneous nature of both the catalytic
systems 1 and 2 + 3.
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In conclusion, we have developed heterodimetallic 1 and its
homodimetallic analogues 2 and 38d using the bis(pyrazol-1-yl)-
methane based bis-heteroscorpionate ligand (pz4lut). The
complexes were characterised using various analytical tech-
niques. The structure of 1 was authenticated by single crystal
X-ray diffraction which shows the expected pianostool geome-
try for Ru and distorted square planar geometry for the Pd
centre. All the complexes were tested for tandem Suzuki coup-
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important in terms of atom economy, saving chemicals, and
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This work was supported by DST (SR/S1/IC-20/2010), New
Delhi, India. ND, SKT and AS are grateful to the University
Grant Commission (UGC) and the Council of Scientific and
Industrial Research (CSIR), DST Inspire programme, New
Delhi, India, respectively, for their fellowship. The authors
gratefully acknowledge Professor S. Chatterjee, National Insti-
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1
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mass spectra of the complexes. The authors would also like to
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