Electronic structure and catalytic aspects of [(trpy)(Cl)Ru(L)]n incorporating potential non-innocent ligands, L-: 9-Oxidophenalenone and trpy: 2,2′:6′,2″-terpyridine

Article abstract of DOI:10.1016/j.poly.2012.06.057

The title complex [(trpy)(Cl)Ru^II(L)] (1) incorporating potential redox non-innocent ligands, L^- = 9-oxidophenalenone and trpy = 2,2′:6′,2″-terpyridine has been structurally characterized. The crystal structure of 1 establishes the distorted octahedral arrangement, meridional coordinating mode of trpy and delocalized C-O bond distances of coordinated L^-. Compound 1 displays two one-electron oxidations at E2980, 0.12 V (Ox1) and 1.32 V (Ox2) and one reduction, -1.58 V versus SCE. Predominantly ruthenium based first oxidation (Ox1) and L based second oxidation (Ox2) lead to the valence configurations of [(trpy)(Cl)Ru ^III(L^-)]⁺ (1⁺) and [(trpy)(Cl)Ru^III(L)]²⁺ (1²⁺), respectively. The antiferromagnetic coupling of spins on Ru(III) (low-spin, t_2g ⁵) and L develops a singlet (S = 0) ground state in 1²⁺. The reduction, however, occurs at the trpy site. The electronic transitions in 1 and 1⁺ could be assigned based on the TD-DFT calculations. Interestingly, 1 has been established to be an efficient pre-catalyst for the oxidative cleavage of alkenes to carbonyl derivatives.

Full text of DOI:10.1016/j.poly.2012.06.057

Polyhedron 52 (2013) 1130–1137
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Electronic structure and catalyticꢀaspects of [(trpy)(Cl)Ru(L)]n incorporating
potential non-innocent ligands, L : 9-Oxidophenalenone and trpy: 2,20:60,200-
terpyridine
⇑
Amit Das, Tamal Kanti Ghosh, Abhishek Dutta Chowdhury, Shaikh M. Mobin, Goutam Kumar Lahiri
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
The title complex [(trpy)(Cl)RuII(L)] (1) incorporating potential redox non-innocent ligands, Lꢀ = 9-oxid-
ophenalenone and trpy = 2,20:60,200-terpyridine has been structurally characterized. The crystal structure
of 1 establishes the distorted octahedral arrangement, meridional coordinating mode of trpy and delocal-
Article history:
Available online 1 July 2012
ized C–O bond distances of coordinated Lꢀ. Compound 1 displays two one-electron oxidations at E0298
,
Keywords:
Ruthenium
9-Oxidophenalenone ligand
Structure
Spectro-electrochemistry
Catalysis
0.12 V (Ox1) and 1.32 V (Ox2) and one reduction, ꢀ1.58 V versus SCE. Predominantly ruthenium based
first oxidation (Ox1) and L based second oxidation (Ox2) lead to the valence configurations of
[(trpy)(Cl)RuIII(Lꢀ)]+ (1+) and [(trpy)(Cl)RuIII(LÅ)]2+ (12+), respectively. The antiferromagnetic coupling of
spins on Ru(III) (low-spin, t2g5) and LÅ develops a singlet (S = 0) ground state in 12+. The reduction, how-
ever, occurs at the trpy site. The electronic transitions in 1 and 1+ could be assigned based on the TD-DFT
calculations. Interestingly, 1 has been established to be an efficient pre-catalyst for the oxidative cleavage
of alkenes to carbonyl derivatives.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Hence, as a part of our ongoing program of establishing
electronic structural aspects of ruthenium complexes of redox
The mostly redox innocent 2,4-pentanedionate (= acetylaceto-
nate) derivatives have been utilized since long as chelating ligands
with most of the metal ions from the broader perspectives of coor-
dination chemistry [1]. Furthermore, metal complexes of b-diketo-
nates have been established to exhibit applications for MOCVD and
related processes [2]. However, it has recently been demonstrated
that unlike 2,4-pentanedionate derivatives, the analogous 1,9-
substituted b-diketonate form, 9-oxidophenalenone (Lꢀ) can un-
dergo stepwise two-step reductions, Lꢀ ? LÅ2ꢀ ? L3ꢀ in
active non-innocent ligands [5], the present report is directed to-
wards the probing of non-innocent tendencies of Lꢀ in the newer
ruthenium molecular form of [(trpy)(Cl)Ru(Lꢀ)]n (1) (trpy = 2,20:
60,200-terpyridine).
combination with Lewis-acids: BIII, AlIII, SiIV and GeIV [3], as well as
one-step oxidation to its radical state (LÅ) in selective ruthenium
framework of [(bpy)Ru(L)2]n (bpy = 2,20-bipyridine) [4] due to the
Herein we describe the synthesis, characterization and single
crystal X-ray structure of 1. The electronic structural aspects of 1
in native and accessible redox states are investigated by experimen-
tal and DFT calculations. In addition, the catalytic potential of 1 to-
wards the selective oxidative cleavage of alkenes to the
corresponding carbonyl compounds has been explored as this sort
of metal complex frameworks have recently been reported as
presence of underlying
p orbitals.
⇑
Corresponding author.
E-mail address: lahiri@chem.iitb.ac.in (G.K. Lahiri).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.057

A. Das et al. / Polyhedron 52 (2013) 1130–1137
1131
Table 1
efficient pre-catalysts for oxidation of alkenes and single-site water-
oxidation process [6,7].
Selected crystallographic parameters for 1.
It should be noted that the metal complexes of L have been pri-
marily restricted to main group elements such as BIII, AlIII, SiIV and
GeIV [3], where B, Si and Al complexes show radical-based molecu-
lar conducting and catalytic properties, respectively. However, only
one article involving the transition metal complexes of L: [(bpy)2R-
u(L)]n, [(bpy)Ru(L)2]n, [Ru(L)3]n has been appeared just recently [4].
Formula
C28H18ClN3O2Ru
564.97
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
9.7270(12)
16.191(3)
29.494(3)
90
a
(°)
b (°)
90
2. Experimental
c
(°)
90
V (Å3)
4645.2(11)
8
0.822
Z
l
2.1. Materials and instrumentation
(mmꢀ1
)
T (K)
293(2)
1.616
2272
3.26–25.00
4080/0/316
0.0475, 0.0602
0.1692, 0.0726
0.651
The precursor complexes Ru(trpy)Cl3 [8] (trpy = 2,20:60,200-ter-
pyridine) and the ligand (HL = 9-hydroxy-1H-phenalen-1-one) [9]
were prepared according to the literature procedures. Other re-
agents and chemicals were obtained from Aldrich and used as re-
ceived. Solvents were dried by following the standard procedures
[10], distilled under nitrogen, and used immediately. For spectro-
scopic and electrochemical studies, HPLC-grade solvents were
used. 1H NMR spectrum was recorded on a Bruker Avance III 400
spectrometer. IR and UV–Vis spectra were recorded using Thermo
Nicolet 320 and Perkin–Elmer Lambda 950 spectrophotometers,
respectively. ESI-mass spectrum was recorded using a micromass
Q-TOF mass spectrometer. The EPR measurement was made in a
two-electrode capillary tube with an X-band Bruker system
ESP300, equipped with a Bruker ER035M gaussmeter and a HP
5350B microwave counter [11]. Cyclic voltammetric studies were
carried out using a PAR model 273A electrochemistry system. Plat-
inum wire working and auxiliary electrodes and an aqueous satu-
rated calomel electrode (SCE) were used in a three-electrode
configuration. The supporting electrolyte was 0.1 M [NEt4][ClO4]
and the solute concentration was ꢁ10ꢀ3 M. The half-wave poten-
tial E0298 was set equal to 0.5(Epa + Epc), where Epa and Epc are the
anodic and cathodic cyclic voltammetric peak potentials, respec-
tively. Elemental analyses were carried out on a Perkin–Elmer
240C elemental analyzer.
Dcalc (mg mꢀ3
F(000)
)
h range (°)
Data/restraints/parameters
R1, wR2 [I > 2 (I)]
R1, wR2 (all data)
r
Goodness-of-fit (GOF)on F2
Largest difference in peakand hole (e Åꢀ3
)
0.631 and ꢀ0.430
an X-CALIBUR-S CCD diffractometer at 293 K. All data were cor-
rected for Lorentz polarization and absorption effects. The struc-
ture was solved and refined using full matrix least-squares
techniques on F2 using the SHELX-97 program [12]. Hydrogen atoms
were included in the refinement process as per the riding model.
Selected crystallographic parameters are given in Table 1.
2.4. UV–Vis spectro-electrochemical measurements
UV–Vis spectra were recorded using a Perkin–Elmer Lambda
950 spectrophotometer at room temperature in acetonitrile. The
cerium ammonium nitrate (CAN) titration experiments were done
as follows: to a 1 ꢂ 10ꢀ4 M acetonitrile solution of complex 1 in a
cuvette with a 1 cm light path length was gradually added
ꢁ2 ꢂ 10ꢀ3 M solution of CAN in acetonitrile. The absorption spec-
tral changes were monitored after each addition. Each absorption
spectrum was plotted with consideration of the volume change
when the CAN solution was added.
2.2. Preparation of [(trpy)(Cl)Ru(L)] (1)
A
mixture of 100 mg (0.23 mmol) Ru(trpy)Cl3, 45 mg
(0.23 mmol) HL and 23 mg (0.23 mmol) NEt3 were taken in 30 mL
of ethanol and the mixture was refluxed under N2 atmosphere for
8 h. The initial reddish brown solution was gradually changed to
blue. This mixture was evaporated to dryness and purified by using
a neutral alumina column. The product was eluted by 20:1 dichlo-
romethane–methanol. On evaporation of the solvent under reduced
pressure yielded the pure complex 1 in the solid form. Yield: 80 mg
(63%). Anal. Calc. for C28H18ClN3O2Ru: C, 59.47; H, 3.21; N, 7.44.
Found: C, 59.42; H, 3.25; N, 7.39%. ESI MS (in dichloromethane):
m/z = 564.94 corresponding to 1+ (Calc. 1+: 565.01) and m/
z = 528.97 corresponding to [1-Cl]+ (Calc. [1-Cl]+: 529.54. 1H NMR
in (CD3)2SO [d/ppm(J/Hz)]: 8.69 (d, 5, 2H), 8.62 (d, 8.1, 2H), 8.52
(d, 8, 2H), 8.15 (d, 9.4, 1H), 8.08 (d, 6.68, 1H), 7.99 (d, 8.1, 2H),
7.89 (d, 7.6, 1H), 7.69 (t, 8, 1H), 7.61–7.53 (m, 4H), 7.33 (t, 7.6,
2.5. Computational details
Full geometry optimizations were carried out using the density
functional theory method at the (R)B3LYP level for 1, 12+ and
(U)B3LYP for 1+ [13]. All elements except ruthenium were assigned
the 6-31G⁄ basis set. The LANL2DZ basis set with effective core po-
tential was employed for the ruthenium atoms [14]. All calcula-
tions were performed with GAUSSIAN03 program package [15]. No
symmetry constraints were imposed during structural optimiza-
tions. The calculated structures were visualized with ChemCraft
[16]. Chemissian 1.7 was used to calculate the fractional contribu-
tions of various groups to each molecular orbital [17].
2.6. General methods for catalytic reaction
1H), 6.10 (d, 9.3, 1H). IR KBr disk:
m
CO: 1624 cmꢀ1. UV–Vis in CH3CN:
(k[nm] (
e
[Mꢀ1 cmꢀ1])) 1: 600 (23750); 510 (sh, 11900); 400
In a typical reaction, the catalyst (0.01 mmol) and 2 equivalent
of PhI(OAc)2 in 3 mL of dichloromethane were placed in a 25 mL
Schlenk tube and the mixture was stirred for 10 min. The solution
immediately turned to reddish brown. The corresponding alkenes
and dodecane (1.5 mmol each) were then added in additional
1 mL of solvent and then stirred at room temperature for 16 h at
298 K (353 K for aliphatic substrates). Then the reaction mixture
was filtered through a short silica gel column using dichlorometh-
ane as eluent prior to GC analysis. The percent conversion and
(23640); 375 (sh, 17150); 325 (53580); 310 (sh, 50100); 280
(58880). 1+: 720 (3900); 440 (23600); 400 (sh, 21100); 350 (sh,
26000); 320 (33400); 275 (sh, 39400).
2.3. Crystal structure determination
Single crystals of 1 were grown by slow evaporation of its 1:1
dichloromethane-hexane solution. X-ray data were collected using

1132
A. Das et al. / Polyhedron 52 (2013) 1130–1137
percent selectivity were determined either by GC against authenti-
cated sample with respect to dodecane as an internal standard or
by 1H NMR.
distances involving the terminal pyridine rings of trpy, RuII–
N1(trpy), 2.050(5) Å and RuII–N3(trpy), 2.059(5) Å. The RuII–O(L)
bond distances of 2.046(4) and 2.047(4) Å in 1 are close to the
RuII–O(L) distances reported in [(bpy)2RuII(L)]ClO4 (Ru–O dis-
tances: 2.0337(16) and 2.0373(16) Å) but appreciably longer with
respect to the RuIII–O(L) distances in [(bpy)RuIII(L)2]ClO4 (average
Ru–O distance = 1.994(2) Å) [4]. The C–O bond distances of coordi-
nated Lꢀ of C1–O1, 1.248(7) Å and C11–O2, 1.268(7) Å in 1 exist in
between the standard C–O single bond (1.34 Å) and C@O double
bond (1.22 Å) distances. This in turn implies a delocalized situation
as generally observed in b-diketonato complexes (average C–
O(acac) distances: Ru(acac)3: 1.282 Å [18], Co(acac)2: 1.270 Å
[19], Cu(acac)2: 1.273 Å [20]. The average C–O distances of coordi-
nated L in other two structurally characterized ruthenium com-
plexes, [(bpy)2RuII(L)]ClO4 and [(bpy)RuIII(L)2]ClO4 are 1.287(3) Å
and 1.288(4) Å, respectively [4]. The other bond parameters involv-
ing coordinated L are similar to those reported for the structurally
characterized GeIV, SiIV, AlIII, RuII and RuIII complexes of L [3b,c,4].
The bond parameters of DFT (B3LYP/LANL2DZ/6-31G⁄) opti-
mized 1 (Fig. S2) are in fairly good agreement with the experimen-
tally obtained values (Table 2).
3. Results and discussion
3.1. Synthesis and characterization
The complex [(trpy)(Cl)Ru(Lꢀ)] (1) (trpy = 2,20:60,200-terpyridine,
Lꢀ = 9-oxidophenalenone) has been prepared via the reaction of
the protonated free ligand, HL and the metal precursor Ru(trpy)Cl3,
under dinitrogen atmosphere and in presence of triethylamine as a
base. The crude product has been purified by using a neutral alu-
mina column and CH2Cl2–CH3OH mixture as eluent (see Section 2).
The ESI mass spectrometry shows peaks at 564.94 and 528.97, cor-
responding to 1+ (Calc. mass: 565.01) and [1-Cl]+ (Calc. mass:
529.54), respectively, with appropriate isotopic distributions
(Fig. S1). The electrically neutral complex 1 gives satisfactory
microanalytical data (see Section 2).
1H NMR spectrum of the diamagnetic complex 1 exhibits par-
tially overlapping calculated 18 aromatic proton resonances within
the chemical shift range of 6–9 ppm (Fig. 1, see Section 2). It is
rather difficult to make individual assignments of all the protons
due to overlapping of signals with similar chemical shift values.
3.3. Electrochemistry, EPR and DFT calculations
Complex 1 exhibits two one-electron oxidations and one one-
electron reduction in CH3CN within the experimental potential
range of 2.0 V versus SCE (Fig. 3). The first oxidation (Ox1) at
3.2. Structural aspects
E0298, V(
DEp, mV), 0.12 (80) may occur either at the metal or at
Crystal structure of [(trpy)(Cl)Ru(Lꢀ)] (1) is shown in Fig. 2 and
selective crystallographic data and bond distances, bond angles are
listed in Tables 1 and 2, respectively. The central ruthenium atom
forms two five-membered chelate rings with the tridentate trpy li-
gand. The bidentate Lꢀ is bonded to the Ru(II) atom via the oxygen
donors forming a six-membered chelate ring. The trans angles
involving trpy, Lꢀ and Cl ligands, N(2)–Ru(1)–O(2), O(1)–Ru(1)–
Cl(1) and N(1)–Ru(1)–N(3) are 178.7(2)°, 175.26(14)° and
161.1(3)°, respectively, indicating a distorted octahedral environ-
ment around the Ru atom. The geometrical constraints due to
the meridional mode of trpy has been reflected in the appreciably
smaller trans angle involving the trpy ligand, N(1)–Ru(1)–N(3) at
161.1(3)° as has been reported for the analogous ruthenium–ter-
pyridine complexes [6a]. The central RuII–N2(trpy) bond length
of 1.923(5) Å is significantly shorter than the corresponding
the potentially redox active ligand (L) site to yield the valence for-
mulation of [(trpy)(Cl)RuIII(Lꢀ)]+ or [(trpy)(Cl)RuII(LÅ)]+, respec-
tively, for 1+. DFT calculations on the optimized structure (singlet
(S = 0) state) of 1 (Fig. S2) reveal that the HOMO is primarily com-
posed of ruthenium based orbitals with appreciable contributions
from L and Cl (Ru, 54%; L, 22%; Cl, 17%; trpy, 7%, Table S1). This
essentially suggests that the valence form of 1+ can be best de-
scribed as a mixed situation of: [(trpy)(Cl)RuIII(Lꢀ)]+ (major compo-
nent)/[(trpy)(Cl)RuII(LÅ)]+ (minor component) as has been
frequently considered for the ruthenium-quinonoid moieties
[5h–o]. However, the spin density plot of 1+ shows that ruthenium
is the primary spin-bearing center with minor contributions from
other constituents (Mulliken spin distribution: Ru, 0.727; Cl,
0.112; trpy, 0.069; L, 0.05) (Fig. 4). This has further been
experimentally authenticated by the observed Ru(III)-based broad
Fig. 1. 1H NMR spectrum of 1 in (CD3)2SO.

A. Das et al. / Polyhedron 52 (2013) 1130–1137
1133
Fig. 2. ORTEP diagram of 1. Ellipsoids are drawn with 50% probability. Hydrogen atoms are omitted for clarity.
Table 2
Selected bond distances and bond angles in 1.
Bond length (Å)
X-ray
DFT
Bond angle (°)
X-ray
DFT
91.03
Ru(1)–N(1)
Ru(1)–N(2)
Ru(1)–N(3)
Ru(1)–O(1)
Ru(1)–O(2)
Ru(1)–Cl(1)
C(1)–C(2)
C(1)–C(12)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(4)–C(13)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
2.050(5)
1.923(5)
2.059(5)
2.046(4)
2.047(4)
2.3761(19)
1.431(9)
1.440(9)
1.337(8)
1.385(10)
1.399(11)
1.409(10)
1.345(14)
1.439(11)
1.385(10)
1.404(10)
1.414(9)
1.343(9)
1.441(9)
1.454(9)
1.248(7)
1.268(7)
1.455(8)
2.091
1.950
2.090
2.082
2.104
2.437
1.449
1.444
1.358
1.433
1.405
1.429
1.392
1.394
1.403
1.435
1.430
1.357
1.449
1.438
1.282
1.276
1.451
N(2)–Ru(1)–O(1)
N(2)–Ru(1)–O(2)
O(1)–Ru(1)–O(2)
N(2)–Ru(1)–N(1)
O(1)–Ru(1)–N(1)
O(2)–Ru(1)–N(1)
N(2)–Ru(1)–N(3)
O(1)–Ru(1)–N(3)
O(2)–Ru(1)–N(3)
N(1)–Ru(1)–N(3)
N(2)–Ru(1)–Cl(1)
O(1)–Ru(1)–Cl(1)
O(2)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(1)
N(3)–Ru(1)–Cl(1)
91.3(2)
178.7(2)
88.81(18)
80.6(3)
88.61(18)
100.7(3)
80.5(2)
177.70
86.67
79.87
91.10
100.20
79.88
90.87
100.09
159.69
91.92
177.05
90.38
89.48
89.59
92.4(2)
98.2(2)
161.1(3)
91.97(15)
175.26(14)
87.95(14)
88.59(14)
91.52(17)
C(8)–C(9)
C(8)–C(13)
C(9)–C(10)
C(11)–C(12)
C(12)–C(13)
O(1)–C(1)
O(2)–C(11)
C(10)–C(11)
anisotropic EPR signal of 1+ in CH3CN with (g1 = 2.379, g2 = 2.118,
[(trpy)(Cl)RuIII(LÅ)]2+ where unpaired spins on RuIII and (LÅ) are
g3 = 1.989,
D
g = 0.39
(
D
g = g1–g3) and hgi = 2.418 (hgi ¼
antiferromagnetically coupled leading to a singlet (S = 0) ground
ð1=3ðg21 þ g22 þ g23ÞÞ1=2) [5a]. The highly rhombic EPR spectrum of
1+ [5f] indeed corroborates the structurally characterized severely
distorted octahedral geometry of 1 (Fig. 2 and Table 2).
state [
D
E(S=1)–(S=0) = 0.339 a.u]. However, sizable contribution of
ruthenium based orbitals (22%) in the
a
-SOMO of 1+ implies the
minor contribution of the alternate valence configuration of
[(trpy)(Cl)RuIV(Lꢀ)]2+ for the doubly oxidized 12+. This has further
been supported by the LUMO composition of the optimized 12+
in singlet (S = 0) ground state (LUMO of 12+: Ru, 52%; L, 30%; trpy,
6%; Cl, 12%) (Table S3). It should be noted that the oxidation of
coordinated Lꢀ ? LÅ has also been established recently in the
molecular framework of [(bpy)RuIV(Lꢀ)(LÅ)]3+ [4].
Since RuIV state can also be accessible in 1 having electron-
donating Lꢀ and Clꢀ, as has been reported earlier for the analogous
molecular framework [21], the second oxidation (Ox2, E0298, V(
DEp,
mV), 1.32(100)) may therefore be taken place either at the ruthe-
nium center (RuIII ? RuIV) or at the ligand-site (Lꢀ ? LÅ). The DFT
calculations on the optimized 1+ in doublet (S = 1/2) ground state
predict that the
a
-SOMO is dominated by the L based orbitals
On the other hand, LUMO and LUMO+1 of 1 is composed of
trpy-based orbitals (82% and 89% trpy in LUMO and LUMO+1,
respectively, Table S1) which suggests the involvement of trpy
(66%) with reasonable contribution from Ru (22%) (Table S2). This
indeed suggests that the second oxidation predominantly occurs at
the ligand site to result in the valence configuration of 12+ as
based redox orbitals for the observed reduction, E0298, V(
DEp, mV),

1134
A. Das et al. / Polyhedron 52 (2013) 1130–1137
3.0
2.5
2.0
1.5
1.0
0.5
0.0
400
600
800 1000 1200
λ / nm
Fig. 5. UV–Vis-NIR spectro-electrochemical plots for the conversion of 1 ? 1+ in
acetonitrile on addition of ꢁ2 ꢂ 10ꢀ3
M (NH4)2Ce(NO3)6 (CAN) solution in
Fig. 3. Cyclic voltammogram (—) and differential pulse (---) voltammograms of 1
acetonitrile.
in acetonitrile/0.1 M Et4NClO4 (scan rate: 100 mV Sꢀ1).
on the TD-DFT calculations as RuII(d
RuII(d ) ? trpy(p⁄) MLCT (metal-to-ligand charge-transfer) transi-
tions, respectively [5g]. Multiple absorptions in the UV-region are
originated from the intraligand
p⁄ transitions involving coordi-
nated trpy and L moieties [4,21].
p ) and
) ? trpy(p⁄)/L(p⁄
p
p
?
On one-electron oxidation of 1 to 1+ the charge-transfer bands
at 600 and 400 nm are red shifted to 720 and 440 nm, respectively
(Fig. 5 and Table 3). The TD-DFT calculations based on the opti-
mized structure of 1+ predict that the lowest energy band at
720 nm has been developed through the L(
metal charge-transfer (LMCT)) transition. The higher energy band
at 440 nm has been originated via Ru(d )/L(
) ? trpy(p⁄) (me-
p) ? Ru(dp) (ligand to
p
p
tal–ligand to ligand charge-transfer (MLLCT)) transition (Table 3,
see Section 2).
Fig. 4. Spin density plot of 1+ in doublet (S = 1/2) ground state.
3.5. Catalytic activity of 1 towards the oxidative cleavage of alkenes
ꢀ1.58(70) [6a,21]. It should be noted that the reduction of the
coordinated Lꢀ (Lꢀ ? LÅ2ꢀ) has been reported in the tris-complexes
of Lꢀ with Si, Ge and Ru [3,4]. The LUMO+2 of 1 is however, mostly
composed of L based orbitals (83% L and 12% trpy, Table S1) but no
such second reduction has been detected within the experimental
potential limit of ꢀ2.0 V versus SCE in CH3CN.
Oxidation of olefins is an important process for the production of
fine chemicals. The oxidation of olefin can be divided into three cat-
egories: (i) the cleavage of C@C bond of alkenes to form carbonyl
compounds [22], (ii) the ozonolysis of olefins to ozonides and the
subsequent conversion to aldehydes or ketones in reductive workup
conditions [22a,23] and (iii) oxidation of olefins by hydrogen perox-
ide or other oxidizing agents [24] to yield carbonyl compounds or
epoxides or diols. Furthermore, such oxidative cleavage of alkene
is a frequently utilized method in synthetic organic chemistry for
the introduction of oxygen functionalities, selective splitting of large
molecules and specifically for removal of protecting groups. In this
regard reductive ozonolysis is considered to be the ‘‘cleanest’’
3.4. Electronic spectra and TD-DFT calculations
In acetonitrile 1 exhibits two intense bands in the visible region
at 600 nm (e e )
= 23750 Mꢀ1 cmꢀ1) and 400 nm ( = 23640 Mꢀ1 cmꢀ1
(Fig. 5 and Table 3, see Section 2) which are assigned based
Table 3
TD-DFT results for 1n (n = +, 0).
Energy (eV)
k (nm) (DFT)
k (nm) (expt.)
osc. strength (f)
e
(Mꢀ1 cmꢀ1
)
Key transitions
Character
[(trpy)(Cl)RuIII(L)]+ (1+) (S = 1/2)
1.7195
3.0531
721
406
720
440
0.0975
0.1010
3900
HOMOꢀ2(b) ? LUMO(b) (0.72)
HOMOꢀ1(b) ? LUMO+1(b) (0.39)
L(
L(
Ru(d
L(
Cl(p
p
) ? Ru(dp)
23600
sh
p
) ? trpy(p⁄
)
HOMOꢀ1(
HOMOꢀ3(
a
) ? LUMO+1(
a) (0.46)
p
) ? trpy(p⁄
)
3.6424
340
345
600
0.0444
0.0252
a) ? LUMO(a) (0.56)
p
) ? trpy(p⁄)/Ru(d
) ? trpy(p⁄
p)
HOMOꢀ10(b) ? LUMO(b) (0.37)
p
)
[(trpy)(Cl)RuII(L)] (1) (S = 0)
2.1867
567
23 750
HOMO ? LUMO+3 (0.48)
HOMO ? LUMO (0.38)
Ru(d
p
) ? trpy(p⁄)/L(p⁄
)
HOMOꢀ1 ? LUMO+1 (0.34)
HOMO ? LUMO+3 (0.70)
HOMOꢀ4 ? LUMO (0.60)
HOMOꢀ3 ? LUMO+2 (0.41)
HOMOꢀ1 ? LUMO+5 (0.20)
3.1987
3.7858
4.5624
388
327
271
400
325
280
0.047
0.1129
0.2997
23 640
53 580
58 880
Ru(d
p
) ? trpy(p⁄
)
L(
p
) ? trpy(p⁄
)
Ru(d
p
)/Cl(pp )
) ? L(p⁄

A. Das et al. / Polyhedron 52 (2013) 1130–1137
1135
Table 5
Catalytic (Scheme 1) results.a
Entry
Substrate
Conversion (selectivity)b
Scheme 1. General catalytic reaction.
1
98(84)
Table 4
Optimization of solvent for the catalytic reaction in Scheme 1.a
Solvent
Conversion (%)
Selectivity (%)b
CH2Cl2
CHCl3
Toluene
Xylene
EtOH
98
96
76
68
12
86
88
74
78
83
2
3
16(100)
100(100)
a
Detailed reaction conditions are given in Section 2 with styrene as a model
substrate. Products are characterized by GC with respect to dodecane as an external
standard.
b
Selectivity in terms of benzaldehyde formation.
100
4
96(100)
% conversion
80
% benzalaldehyde
formation
60
40
20
0
5
6
100(100)
100(92)
0
4
8
12
16
time (h)
7
8
12(100)
Fig. 6. Conversion (%) and benzaldehyde formation (%) as a function of time with
complex 1 as a pre-catalyst under experimental catalytic conditions taking styrene
as a model substrate.
option. However, there are several practical limitations of this pro-
cess such as the requirement of specially designed equipment
(ozonizer), low reaction temperature (generally ꢀ78 °C), use of
additional stoichiometric reducing reagent for reductive workup
(e.g. dimethyl sulfide, zinc, hydrogen, phosphines) and accidental
issues. Due to these inherent problems of ozonolysis, metal cata-
lyzed chemoselective oxidative cleavage of alkene functionalities
has been considered to be an attractive approach. Metal complex
derived pre-catalysts such as FeIII(Salen)Cl (Salen = 2,20-(1E,10
E)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(methan-
1-yl-1-ylidene)diphenolate) and [cis-RuII(dmp)2(H2O)2]2+ (dmp =
2,9-dimethylphenanthroline) have been effectively utilized for the
transformation of olefins to carbonyl compounds via the selective
cleavage of C@C bond [25]. Alternatively, He and co-workers have
employed PdCl2 as a catalyst for the oxidation of styrene in scCO2/
PEG biphasic catalytic system which however shows moderate
selectivity towards the formation of benzaldehyde [26]. It should
also be noted that such oxidative alkene cleavage has been reported
to take place as a minor or undesired side product catalyzed by per-
oxidases [27].
100(100)
9
100(100)
Br
10
68(100)
Cl
(continued on next page)

1136
A. Das et al. / Polyhedron 52 (2013) 1130–1137
to activate the desired hydrogen peroxide or molecular oxygen.
Table 5 (continued)
In general the chlorinated solvents, CH2Cl2 and CHCl3 have shown
the maximum impact on the desired chemoselectivity of the cata-
lytic process (Table 4). Effectivity of the chlorinated solvents could
be attributed to their non-coordinating nature as well as their
inherent tendency to stabilize the intermediates during the oxida-
tion process [28]. Thus, dichloromethane has been selectively cho-
sen for rest of the catalytic processes (Table 4). Furthermore, most
of these catalytic reactions proceed well at room temperature ex-
cept for aliphatic substrates where heating of about 80 °C is neces-
sary to maximize the conversion. The time monitored reaction
profile shows that chemoselectivity enhances with the increase
in conversion up to 12 h and then it levels off with slight increase
in conversion (Fig. 6). As stated above the present catalytic proto-
col is effective to oxidize the terminal alkene functionalities to the
corresponding carbonyl compounds via the oxidative C@C bond
cleavage (Table 5). Better conversion and selectivity have been
achieved with the substrates comprising of electron donating
group whereas poor conversion is obtained for the substrates hav-
ing electron withdrawing group. Interestingly, the observed very
poor conversion for the electron rich 2-methoxystyrene could be
attributed to the catalytic site inhibition via possible co-ordination
of the o-methoxy group to the active ruthenium center. The advan-
tage of using the preformed catalyst (1) for the said process is
clearly evident from the formation of only 8% benzaldehyde by
the in situ generated catalyst (RuCl3 + L + trpy + oxidant) with sty-
rene as a model substrate [6a]. In essence, 1 can function as an
effective pre-catalyst for the oxidation of terminal alkene function-
alities to the corresponding carbonyl compounds under mild opti-
mized reaction conditions.
Entry
Substrate
Conversion (selectivity)b
20(100)
11
12
13
60(100)
c
27(100)
14
15
16
97(100)
88(100)
100(100)
4. Conclusion
Monomeric ruthenium-terpyridine complex, [(trpy)(Cl)RuII(Lꢀ)]
(1) incorporating potential non-innocent ligand Lꢀ [4] has been
synthesized and structurally characterized. Considerable mixing
of Ru and L based frontier orbitals in the MOs (HOMO/SOMO) have
led to the complex valence situations at the oxidized states, 1+ and
17
18
100(100)c
80(100)
1
2+. Compound 1 has been established to be an efficient catalyst for
the oxidative cleavage of alkenes to the corresponding carbonyl
compounds under mild reaction conditions.
Acknowledgments
19
78(100)c
84(100)c
The financial support received from Department of Science and
Technology (DST), and Council of Scientific and Industrial Research
(CSIR), (fellowship to A.D. and A.D.C.) New Delhi, India is gratefully
acknowledged. X-ray structural study for 1 was carried out at the
National Single Crystal Diffractometer Facility, Indian Institute of
Technology, Bombay. We thank Mr. Thomas Scherer, Institut für
Anorganische Chemie, Universität Stuttgart for EPR measurement.
20
a
Detailed reaction conditions are given in Section 2. Products are characterized
by GC with respect to dodecane as an external standard and 1H NMR.
b
Selectivity in terms of aldehyde formation.
NMR yield.
c
Appendix A. Supplementary data
CCDC 878994 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.06.057.
In this regard complex 1 has been explored towards the oxida-
tion of a wide variety of alkenes and it shows selective oxidative
cleavage of the C@C bond to yield the carbonyl compounds in pres-
ence of suitable oxidizing agent, iodobenzene diacetate in dichlo-
romethane solution (Scheme 1).
Initial optimization for the appropriate catalytic conditions with
styrene as a model substrate establishes that the combination of
0.75 mol % of the catalyst and 2 equivalent of oxidant (iodobenzene
diacetate) yields the best results. Among the various oxidants
tested, iodobenzene diacetate has been found to be the most
effective one. Unfortunately, the present catalytic protocol failed
References
[1] R.C. Mehrota, R. Bohra, D.P. Gaur, Metal b-Diketonates and Allied Derioatioes,
Academic Press, New York, 1978.

A. Das et al. / Polyhedron 52 (2013) 1130–1137
1137
[2] (a) J.P. Fackler Jr., Prog. Inorg. Chem. 7 (1966) 361;
(b) J.J. Fortman, Coord. Chem. Rev. 6 (1971) 331;
[7] (a) J.T. Muckerman, D.E. Polyansky, T. Wada, K. Tanaka, E. Fujita, Inorg. Chem.
47 (2008) 1787;
(c) R.C. Fay, Coord. Chem. Rev. 154 (1996) 99;
(d) W.S. Rees Jr., Coord. Chem. Rev. 210 (2000) 279;
(e) D.R. Eaton, J. Am. Chem. Soc. 87 (1965) 3097;
(b) J.K. Hurst, J.L. Cape, A.E. Clark, S. Das, C. Qin, Inorg. Chem. 47 (2008) 1753;
(c) Z.P. Deng, H.W. Tseng, R.F. Zong, D. Wang, R. Thummel, Inorg. Chem. 47
(2008) 1835;
(f) G.J. Bullen, R. Mason, P. Pauling, Nature 189 (1961) 291;
(g) G.G. Condorelli, G. Malandrino, I.L. Fragalà, Coord. Chem. Rev. 251 (2007)
1931;
(h) Q. Meng, R.J. Witte, P.S. May, M.T. Berry, Chem. Mater. 21 (2009) 5801;
(i) G. Malandrino, L.M.S. Perdicaro, G. Condorelli, I.L. Fragalà, P. Rossi, P.
Dapporto, Dalton Trans. (2006) 1101;
(d) F. Liu, J.J. Concepcion, J.W. Jurss, T. Cardolaccia, J.L. Templeton, T.J. Meyer,
Inorg. Chem. 47 (2008) 1727;
(e) J. Mola, E. Mas-Marza, X. Sala, I. Romero, M. Rodriguez, C. Vinas, T. Parella,
A. Llobet, Angew. Chem., Int. Ed. 47 (2008) 5830;
(f) J.J. Concepcion, J.W. Jurss, M.R. Norris, Z. Chen, J.L. Templeton, T.J. Meyer,
Inorg. Chem. 49 (2010) 1277;
(d) M. Andrieux, C. Gasquères, C. Legros, I. Gallet, M. Herbst-Ghysel, M. Condat,
V.G. Kessler, G.A. Seisenbaeva, O. Heintz, S. Poissonnet, Appl. Surf. Sci. 253
(2007) 9091.
(g) L.L. Duan, A. Fischer, Y.H. Xu, L.C. Sun, J. Am. Chem. Soc. 131 (2009) 10397;
(i) D.J. Wasylenko, C. Ganesamoorthy, M.A. Henderson, B.D. Koivisto, H.D.
Osthoff, C.P. Berlinguette, J. Am. Chem. Soc. 132 (2010) 16094;
(j) J.J. Concepcion, J.W. Jurss, J.L. Templeton, T.J. Meyer, Proc. Natl. Acad. Sci.
USA 105 (2008) 17632;
[3] (a) S.K. Mandal, S. Samanta, M.E. Itkis, D.W. Jensen, R.W. Reed, R.T. Oakley, F.S.
Tham, B. Donnadieu, R.C. Haddon, J. Am. Chem. Soc. 128 (128) (2006) 1982;
(b) S.K. Pal, F.S. Tham, R.W. Reed, R.T. Oakley, R.C. Haddon, Polyhedron 24
(2005) 2076;
(k) J.J. Concepcion, J.W. Jurss, J.L. Templeton, T.J. Meyer, J. Am. Chem. Soc. 130
(2008) 16462;
(c) T.K. Sen, A. Mukherjee, A. Modak, P.K. Ghorai, D. Kratzert, M. Granitzka, D.
Stalke, S.K. Mandal, Chem. Eur. J. 18 (2012) 54.
[4] A. Das, T.M. Scherer, S.M. Mobin, W. Kaim, G.K. Lahiri, Inorg. Chem. 51 (2012)
4390.
[5] (a) S. Patra, B. Sarkar, S.M. Mobin, W. Kaim, G.K. Lahiri, Inorg. Chem. 42 (2003)
6469;
(l) S.W. Gersten, G.J. Samuels, T.J. Meyer, J. Am. Chem. Soc. 104 (1982) 4029.
[8] B.P. Sullivan, J.M. Calvert, T.J. Meyer, Inorg. Chem. 19 (1980) 1404.
[9] R.C. Haddon, R. Rayford, A.M. Hirani, J. Org. Chem. 46 (1981) 4587.
[10] D.D. Perrin, W.L.F. Armarego, Purification of Laboratory Chemicals, second ed.,
Pergamon Press, Oxford, 1980.
[11] W. Kaim, S. Ernst, V. Kasack, J. Am. Chem. Soc. 112 (1990) 173.
[12] G.M. Sheldrick, SHELX-97, Program for Crystal Structure Solution and
Refinement, University of Göttingen, Germany, 1997.
(b) A. Das, T.M. Scherer, A. Dutta Chowdhury, S.M. Mobin, W. Kaim, G.K. Lahiri,
Inorg. Chem. 51 (2012) 1675;
(c) S. Maji, B. Sarkar, S. Patra, J. Fiedler, S.M. Mobin, V.G. Puranik, W. Kaim, G.K.
Lahiri, Inorg. Chem. 45 (2006) 1316;
(d) S. Maji, B. Sarkar, S.M. Mobin, J. Fiedler, F.A. Urbanos, R. Jimenez-Aparicio,
W. Kaim, G.K. Lahiri, Inorg. Chem. 47 (2008) 5204;
[13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[14] (a) T.H. Dunning Jr., P.J. Hay, in: H.F. Schaefer III (Ed.), Modern Theoretical
Chemistry, Plenum, New York, 1976, p. 1;
(b) P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299.
(e) S. Ghumaan, B. Sarkar, S. Maji, V.G. Puranik, J. Fiedler, F.A. Urbanos, R.
Jimenez-Aparicio, W. Kaim, G.K. Lahiri, Chem. Eur. J. 14 (2008) 10816;
(f) D. Kumbhakar, B. Sarkar, S. Maji, S.M. Mobin, J. Fiedler, F.A. Urbanos, R.
Jimenez-Aparicio, W. Kaim, G.K. Lahiri, J. Am. Chem. Soc. 130 (2008) 17575;
(g) S. Kar, B. Sarkar, S. Ghumaan, D. Janardan, J. van Slageren, J. Fiedler, V.G.
Puranik, R.B. Sunoj, W. Kaim, G.K. Lahiri, Chem. Eur. J. 11 (2005) 4901;
(h) D. Das, A.K. Das, B. Sarkar, T.K. Mondal, S.M. Mobin, J. Fiedler, S. Za´lis, F.A.
[15] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann,
O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT, 2004.
[16] D.A. Zhurko, G.A. Zhurko, ChemCraft 1.6, Plimus, San Diego, CA. Available at
<http://www.chemcraftprog.com>.
´
Urbanos, R. Jimenez-Aparicio, W. Kaim, G.K. Lahiri, Inorg. Chem. 48 (2009)
11853;
(i) D. Das, T.K. Mondal, S.M. Mobin, G.K. Lahiri, Inorg. Chem. 48 (2009) 9800;
(j) D. Das, B. Sarkar, T.K. Mondal, S.M. Mobin, J. Fiedler, W. Kaim, G.K. Lahiri,
Inorg. Chem. 50 (2011) 7090;
(k) D. Das, T.K. Mondal, A. Dutta Chowdhury, F. Weisser, D. Schweinfurth, B.
Sarkar, S.M. Mobin, F.A. Urbanos, R. Jiménez-Aparicio, G.K. Lahiri, Dalton Trans.
40 (2011) 8377;
(l) D. Das, B. Sarkar, D. Kumbhakar, T.K. Mondal, S.M. Mobin, J. Fiedler, F.A.
Urbanos, R. Jiménez-Aparicio, W. Kaim, G.K. Lahiri, Chem. Eur. J. 17 (2011)
11030;
[17] S.
Leonid,
Chemissian
1.7,
2005–2010.
Available
at
<http://
www.chemissian.com>.
(m) T. Kundu, B. Sarkar, T.K. Mondal, S.M. Mobin, F.A. Urbanos, J. Fiedler, R.
[18] H. Matsuzawa, Y. Ohashi, Y. Kaizu, H. Kobayashi, Inorg. Chem. 27 (1988) 2981.
[19] J. Burgess, J. Fawcett, D.R. Russell, S.R. Gilani, Acta Crystallogr., Sect. C 56
(2000) 649.
[20] P.C. Lebrun, W.D. Lyon, H.A. Kuska, J. Crystallogr. Spectrosc. Res. 16 (1986) 889.
[21] B. Mondal, S. Chakraborty, P. Munshi, M.G. Walawalkar, G.K. Lahiri, J. Chem.
Soc., Dalton Trans. (2000) 2327.
´
Jimenez-Aparicio, W. Kaim, G.K. Lahiri, Inorg. Chem. 50 (2011) 4753;
(n) H. Agarwala, D. Das, S.M. Mobin, T.K. Mondal, G.K. Lahiri, Inorg. Chim. Acta
374 (2011) 216;
(o) D. Kumbhakar, B. Sarkar, A. Das, A.K. Das, S.M. Mobin, J. Fiedler, W. Kaim,
G.K. Lahiri, Dalton Trans. (2009) 9645;
(p) G.K. Lahiri, W. Kaim, Dalton Trans. 39 (2010) 4471;
(q) P. De, S. Maji, A. Dutta Chowdhury, S.M. Mobin, T.K. Mondal, A. Paretzki,
G.K. Lahiri, Dalton Trans. 40 (2011) 12527.
[22] (a) R. Pappo, D.S. Allen Jr., R.U. Lemieux, W.S. Johnson, J. Org. Chem. 21 (1956)
478;
(b) P.H.J. Carlsen, T. Katsuki, V.S. Martin, K.B. Sharpless, J. Org. Chem. 46 (1981)
3936;
(c) B.S. Michael, Organic Synthesis, second ed., McGraw-Hill, New York, 2002.
[23] P.S. Bailey, Chem. Rev. 58 (1958) 925.
[6] (a) A. Dutta Chowdhury, A. Das, K. Irshad, S.M. Mobin, G.K. Lahiri, Inorg. Chem.
50 (2011) 1775;
(b) D. Chatterjee, Inorg. Chim. Acta 361 (2008) 2177;
(c) M. Klawonn, M.K. Tse, S. Bhor, C. Döbler, M. Beller, J. Mol. Catal. A 218
(2004) 13;
[24] (a) R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic
Compounds, Academic Press, New York, 1981;
(d) S. Bhor, M.K. Tse, M. Klawonn, C. Döbler, M. Beller, W. Mägerlein, Adv.
Synth. Catal. 346 (2004) 263;
(e) M.K. Tse, C. Döbler, S. Bhor, M. Klawonn, W. Mägerlein, H. Hugl, M. Beller,
Angew. Chem., Int. Ed. 43 (2004) 5255;
(f) M.K. Tse, S. Bhor, M. Klawonn, G. Anilkumar, H. Jiao, S. Spannenberg, C.
Döbler, W. Mägerlein, H. Hugl, M. Beller, Chem. Eur. J. 12 (2006) 1875;
(g) M.K. Tse, S. Bhor, M. Klawonn, S. Bhor, C. Döbler, G. Anilkumar, H. Hugl, W.
Mägerlein, M. Beller, Org. Lett. 7 (7) (2005) 987;
(b) D.T. Sawyer, Oxygen Chemistry, Oxford University Press, New York, 1991.
[25] (a) A. Dhakshinamoorthy, K. Pitchumani, Tetrahedron 62 (2006) 9911;
(b) V. Kogan, M.M. Quintal, R. Neumann, Org. Lett. 7 (2005) 5039.
[26] J.Q. Wang, F. Cai, E. Wang, L.N. He, Green Chem. 9 (2007) 882.
[27] (a) A. Tuynman, J.L. Spelberg, I.M. Kooter, H.E. Schoemaker, R. Wever, J. Biol.
Chem. 275 (2000) 3025;
(b) H. Mang, J. Gross, M. Lara, C. Goessler, H.E. Schoemaker, G.M. Guebitz, W.
Kroutil, Angew. Chem., Int. Ed. 45 (2006) 5201.
(g) K. Schröder, S. Enthaler, B. Join, K. Junge, M. Beller, Adv. Synth. Catal. 352
(2010) 1771.
[28] A. Dutta Chowdhury, R. Ray, G.K. Lahiri, Chem. Commun. 48 (2012) 5497.