Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H 2O)]n, tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine

Article abstract of DOI:10.1039/c2dt32402d

The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO₄ ([1]ClO ₄) (tpm = tris(1-pyrazolyl)methane, bqdi = o-benzoquinonediimine) and [Ru(tpm)(bqdi)(H₂O)](ClO₄)₂ ([2](ClO ₄)₂) have been synthesized. The valence state-sensitive bond distances of coordinated bqdi [C-N: 1.311(5)/1.322(5) A in [1]ClO₄; 1.316(7)/1.314(7) A in molecule A and 1.315(6)/1.299(7) A in molecule B of [2](ClO₄)₂] imply its fully oxidised quinonediimine (bqdi⁰) character. DFT calculations of 1⁺ confirm the {Ru^II-bqdi⁰} versus the antiferromagnetically coupled {Ru^III-bqdi^-} alternative. The ¹H NMR spectra of [1]ClO₄ in different solvents show variations in chemical shift positions of the NH (bqdi) and CH (tpm) proton resonances due to their different degrees of acidity in different solvents. In CH₃CN/0.1 mol dm^-3 Et₄NClO ₄, [1]ClO₄ undergoes one reversible Ru^II ? Ru^III oxidation and two reductions, the reversible first electron uptake being bqdi based (bqdi⁰/bqdi^-). The electrogenerated paramagnetic species {Ru^III-bqdi⁰}(1 ²⁺) and {Ru^II-Q^-}(1) exhibit Ru ^III-type (1²⁺: = 2.211/Δg = 0.580) and radical-type (1: g = 1.988) EPR signals, respectively, as is confirmed by calculated spin densities (Ru: 0.767 in 1²⁺, bqdi: 0.857 in 1). The aqua complex [2](ClO₄)₂ exhibits two one-electron oxidations at pH = 7, suggesting the formation of {Ru^IVO} species. The electronic spectral features of 1ⁿ (n = charge associated with the different redox states of the chloro complex: 2+, 1+, 0) in CH₃CN and of 2²⁺ in H₂O have been interpreted based on the TD-DFT calculations. The application potential of the aqua complex 2 ²⁺ as a pre-catalyst towards the epoxidation of olefins has been explored in the presence of the sacrificial oxidant PhI(OAc)₂ in CH₂Cl₂ at 298 K, showing the desired selectivity with a wide variety of alkenes. DFT calculations based on styrene as the model substrate predict that the epoxidation reaction proceeds through a concerted transition state pathway.

Full text of DOI:10.1039/c2dt32402d

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TOC
II
0
n+
III
0
2+
II
•−
0
Redox series including {Ru (bqdi )Cl/H O} , {Ru (bqdi )Cl} , {Ru (bqdi )Cl} ,
2
III
0
2+
IV
0
2+
{
Ru (bqdi )OH} , {Ru (bqdi )O} and the application of the aqua complex for
selective epoxidation process are reported.
n=2
n=0
X=Cl
X=H O
2

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n
Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H O)] ,
2
tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine
a
b
a
a
Hemlata Agarwala, Fabian Ehret, Abhishek Dutta Chowdhury, Somnath Maji, Shaikh
a
*b
*a
M. Mobin, Wolfgang Kaim, and Goutam Kumar Lahiri
a
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbaiꢀ
4
00076, India. Eꢀmail: lahiri@chem.iitb.ac.in
b
Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, Dꢀ70550
Stuttgart, Germany. Eꢀmail: kaim@iac.uniꢀstuttgart.de
1

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Abstract
The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO4 ([1]ClO ) (tpm = tris(1ꢀ
4
pyrazolyl)methane, bqdi = oꢀbenzoquinonediimine) and [Ru(tpm)(bqdi)(H O)](ClO )
2
4 2
(
[2](ClO ) ) have been synthesized. The valence stateꢀsensitive bond distances of
4 2
coordinated bqdi [CꢀN: 1.311(5)/1.322(5) Å in [1]ClO ; 1.316(7)/1.314(7) Å in molecule
4
A and 1.315(6)/1.299(7) Å in molecule B of [2](ClO ) ] imply its fully oxidised
4
2
0
+
II
0
quinonediimine (bqdi ) character. DFT calculations of 1 confirm the {Ru ꢀbqdi } versus
III
•−
1
the antiferromagnetically coupled {Ru ꢀbqdi } alternative. The H NMR spectra of
[
1]ClO in different solvents show variations in chemical shift positions of the NH (bqdi)
4
and CH(tpm) proton resonances due to their different degrees of acidity in different
solvents. In CH CN/0.1 mol dm− Et NClO , [1]ClO undergoes one reversible
3
3
4
4
4
II
III
Ru ⇌Ru oxidation and two reductions, the reversible first electron uptake being bqdi
0
•−
III
0
2+
based (bqdi /bqdi ). The electrogenerated paramagnetic species {Ru ꢀbqdi }(1 ) and
II
•−
III
2+
{
Ru ꢀQ }(1) exhibit Ru ꢀtype (1 : <g>=2.211/ꢁg=0.580) and radicalꢀtype (1: g=1.988)
2
+
EPR signals, respectively, as is confirmed by calculated spin densities (Ru: 0.767 in 1 ,
bqdi: 0.857 in 1). The aqua complex [2](ClO ) exhibits two oneꢀelectron oxidations at
4
2
IV
pH = 7, suggesting the formation of {Ru =O} species. The electronic spectral features of
n
1
(n = charge associated with the different redox states of the chloro complex: 2+, 1+, 0)
2+
in CH CN and of 2 in H O have been interpreted based on the TDꢀDFT calculations.
3
2
2+
The application potential of the aqua complex 2 as a preꢀcatalyst towards the
epoxidation of olefins has been explored in the presence of the sacrificial oxidant
PhI(OAc) in CH Cl at 298 K, showing the desired selectivity with a wide variety of
2
2
2
2

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alkenes. DFT calculations based on styrene as the model substrate predict that the
epoxidation reaction proceeds through a concerted transition state pathway.
3

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Introduction
The well recognized mixing of ruthenium dπ orbitals and the π orbitals of redox nonꢀ
innocent quinonoid moieties introduces several manifestations with respect to the valence
1
distribution at the metalꢀquinonoid interface, as depicted in Scheme 1.
−
−
X
Y
X
−
Y
X
Y
n+
n+1
n+2
.
Ru
Ru
Ru
n o
n
+1
.
−
n
+
2 2
[Ru Q
−
[
Ru Q ]
[ Ru
Q
]
]
X = Y = NH
X = Y = O
X = O, Y = NH
X = O, Y = NR (R = alkyl, aryl)
X = S, Y = NH
Scheme 1. Alternative valence configurations of the rutheniumꢀoꢀquinonoid moiety.
This makes the electronic structure of such complexes sensitive to the molecular
frameworks, and in many occasions the experimental results fit well with an intermediate
description i.e. the superposition of more than one forms of Scheme 1 instead of any
2
3
4
single specific form. Thus, a wide variety of mononuclear and polynuclear rutheniumꢀ
quinone complexes have been designed over the past years in combination with coꢀ
ligands having different electronic properties. The primary impetus was to develop a
fundamental understanding of the bonding and electronic features, however, the catalytic
potential of rutheniumꢀquinonoid complexes including the water oxidation process has
5
also been demonstrated recently.
4

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In this regard the electronic structure aspects of mononuclear ruthenium complexes of
oꢀbenzoquinonediimine (bqdi) (X=Y=NH, Scheme 1) have been studied in combination
/
6
/
/
//
with different coꢀligands such as πꢀaccepting 2,2 ꢀbipyridine (bpy), 2,2 :6 ,2 ꢀterpyridine
7
aꢀd
7e
8
(trpy),
2,3,5,6ꢀtetrakis(2ꢀpyridyl)pyrazine (tppz), 1,4,7ꢀtrithiacyclononane ([9]aneS3),
9
1
,4,7,10ꢀtetrathiacyclododecane
([12]aneS4),
1,4,8,11ꢀtetrazacyclotetradecane
1
0
11
11
12
12,13
(
cyclam), pꢀcymene (pꢀcym), hexamethylbenzene (hmb), PPh , CO,
as well as
3
donating acetylacetonate (acac−),14 NH3,6a,f,g,14b,15 and Cl−.6c,e,7a,12a,14a,b,15b,16 The detailed
experimental and theoretical investigations with such varying molecular frameworks
revealed an appreciable contribution of the coꢀligands in tuning the valence distribution
process, including the degree of covalency of the metalꢀquinone moiety.
The present article deals with the structurally characterised new class of rutheniumꢀbqdi
II
II
complexes, [Ru (tpm)(bqdi)(Cl)]ClO ([1]ClO ) and [Ru (tpm)(bqdi)(H O)](ClO )
4 2
4
4
2
(
[2](ClO ) ), (CAUTION! Perchlorate salts of metal complexes are generally
4 2
explosive. Care must be taken while handling such complexes), incorporating the
1
7
facially coordinating scorpionate coꢀligand, tris(1ꢀpyrazolyl)methane (tpm) (Scheme 2).
n
The electronic structural features in accessible redox states of 1 , n = 2+, 1+, 0, have been
scrutinised via experimental and theoretical analyses.
5

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δ
(ClO4)
n
H
C
γ /
γ /
γ
N
N
N
N
N
β /
β /
β
N
α /
α /
α
Ru
H
b
N
X
a
N
b
H
a
X = Cl, n = 1: [1]ClO4
X = H O, n = 2: [2](ClO )
2
4 2
Scheme 2. Representation of complexes [1]ClO and [2](ClO ) along with atom
4
4 2
labelling.
2
+
Furthermore, the potential application of the corresponding aquaꢀderivative (2 ) as a
preꢀcatalyst towards the catalytic epoxidation of alkenes has been explored. While the
traditional method for epoxidation of alkenes involves the use of organic peracids with
the disadvantages of (i) the formation of organic waste, (ii) a narrow substrate scope, and
18
(
iii) the inconvenience in separating the products, other, better oxidants have been
1
9
employed as efficient and convenient reagents. For instance, rutheniumꢀcomplex
catalysed epoxidation of olefins using TBHP (tertꢀbutylhydroperoxide) as oxidant was
2
0
first reported by Nishiyama et al. in 1997. Consequently, a variety of ruthenium
complexes have been successfully utilised for epoxidation reactions in the presence of
20,21
such suitable oxidants.
6

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Results and discussion
Synthesis and characterisation
II
The chloro complex [Ru (tpm)(bqdi)(Cl)]ClO
([1]ClO4) (tpm
=
tris(1ꢀ
4
pyrazolyl)methane, bqdi = oꢀbenzoquinonediimine) has been synthesised from 1,2ꢀ
III
17c
diaminobenzene and the precursor complex Ru (tpm)Cl
•
1.5H O in the presence of
3
2
II
NEt as a base in absolute ethanol. The aqua derivative [Ru (tpm)(bqdi)(H O)](ClO )
3
2
4 2
(
[2](ClO ) ) was prepared from [1]ClO via a Cl/H O exchange reaction using AgNO in
4 2 4 2 3
1
:4 acetoneꢀwater mixture. The diamagnetic [1]ClO and [2](ClO ) exhibit 1:1 and 1:2
4 4 2
conductivities, respectively, and give satisfactory microanalytical data. The mass spectra
ESIꢀMS(+)) of complexes [1]ClO and [2](ClO ) show molecular ion peaks at 457.02
(
4
4 2
+
+
and 520.92 corresponding to [[1]ClO ꢀClO ] and [[2](ClO ) ꢀClO ꢀH O] , respectively,
4
4
4 2
4
2
(Fig. S1
†
and Experimental). The complexes show characteristic
ν
(ClO ) vibrations in
4
−
1
the IR spectra near 1100/625 cm .
The identities of [1]ClO and [2](ClO ) have been authenticated by their single crystal
4
4 2
Xꢀray structures (Figs. 1ꢀ2, Tables 1, 2 and S1†). The asymmetric unit of [2](ClO4)2
contains two crystallographically independent molecules (Molecule A and Molecule B)
with only slight difference in bond parameters.
The tridentate tpm ligand is coordinated to the metal ion in the usual facial mode as an
N,N′,N′′ donor, forming two shared sixꢀmembered chelate rings around the ruthenium
ion. The bidentate bqdi is bonded to the ruthenium ion as an NH,NH′ donor, resulting in
a fiveꢀmembered chelate ring. The redox nonꢀinnocent feature of the quinonoid moiety,
0
•−
bqdi (fully oxidised quinone state (bqdi ), intermediate semiquinone radical form (bqdi )
7

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2
− 3,14
and fully reduced form (bqdi ), and the facile access of various oxidation states of the
II
III
IV
ruthenium ion (Ru , Ru , Ru ) extend the feasibility of three valence configurations for
II
0
III
•−
IV
2−
[
1]ClO or [2](ClO ) : {Ru ꢀbqdi }, {Ru ꢀbqdi }, or {Ru ꢀbqdi }, as shown in
4 4 2
Scheme 1. Since the coordinated quinonoid moieties are known to exhibit distinctive
0
bond parameters depending on the redox state (e.g. CꢀNH distances, bqdi : 1.30 Å,
•
−
2−
22
bqdi : 1.35 Å, bqdi : 1.38 Å), an analysis of the relevant bond parameters of the
complexes can therefore provide important information regarding the valence distribution
at the metalꢀquinonoid interface. The close to double bond distances of HN1ꢀC1, HN2ꢀ
C6 (average, 1.318(5) Å; DFT: 1.318 Å), the short intra ring distances, C2ꢀC3/C4ꢀC5
(average, 1.349(5) Å; DFT: 1.364 Å), and the single bond distances for C1ꢀC2, C3ꢀC4,
C5ꢀC6, C6ꢀC1 (average, 1.436 Å; DFT: 1.444 Å) of coordinated bqdi in [1]ClO (Table
4
II
0
2
) strongly suggest the {Ru ꢀbqdi } configuration. Similarly, the corresponding sensitive
bond parameters of coordinated bqdi in the analogous aqua derivative, [2](ClO ) (Table
4
2
II
0
2
) also establish the {Ru ꢀbqdi } configuration. The structure data rule out the possibility
III
•−
IV
2−
of other valence tautomeric forms, i.e. {Ru ꢀbqdi } or {Ru ꢀbqdi }.
II
3g,17d,7a,14d
II
17d,21,4
II
23
The Ru ꢀN(tpm/bqdi),
Ru ꢀCl
and Ru ꢀO(H O) bond distances are in
2
good agreement with those of related reported ruthenium(II) complexes. The bite angle of
the fiveꢀmembered chelate ring involving the bqdi ligand, N1ꢀRu1ꢀN2 or N9ꢀRu2ꢀN10,
~
77° is slightly smaller than that of the sixꢀmembered chelate rings involving the tpm
ligand, N3ꢀRu1ꢀN5/N5ꢀRu1ꢀN7/N3ꢀRu1ꢀN7 or N11ꢀRu2ꢀN15/ N13ꢀRu2ꢀN15/ N11ꢀRu2ꢀ
N13, ~82ꢀ88° (Table S1 ).
†
8

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The DFT calculated bond distances and angles (Tables 2, S1†) based on the optimised
+
2+
structures of 1 and 2 (Molecule A) (Fig. S2
†
) are in fairly good agreement with the Xꢀ
+
ray data. An openꢀshell singlet optimisation of 1 at the B3LYP level gives rise to the
same energy as that obtained from the closedꢀshell singlet calculations which rules out a
III
•−
major contribution from the antiferromagnetically coupled {Ru ꢀbqdi } form, in
agreement with the experimental information.
1
H NMR spectra of [1]ClO in different solvents such as CDCl , (CD ) SO, CD CN and
4
3
3 2
3
(
CD ) CO exhibit partially overlapping resonances, corresponding to 16 protons, within
3 2
the chemical shift range of 14ꢀ5.5 ppm (Fig. S3(a)†). Though the solvent dependence of
chemical shift values for the aromatic protons of tpm or bqdi is insignificant, some
variations for the methine (CH) proton of tpm (9.11ꢀ10.02 ppm) and appreciable
variations for the NH proton resonances of bqdi (11.48ꢀ13.64 ppm) have been observed
due to the acidity of these protons (Fig. S3(a)
†). The proton resonances of the
corresponding aqua complex [2](ClO ) in (CD ) CO are slightly upfield shifted with
4
2
3 2
respect to those of chloro complex [1](ClO ) (Fig. S3(a)
†
). The NH (bqdi) and CH
4
(
methine of tpm) protons of [2](ClO ) disappear in D O due to H/D exchange. The
4
2
2
/
/
aromatic ring protons of bqdi appear separately as an AA BB system. The spinꢀspin
coupling constant values (J) of the aromatic protons of tpm are rather small
3
h,17d
(Experimental), as reported earlier
which distinguish the protons associated with the
1
coordinated tpm and bqdi ligands in the complexes. The H NMR spectra show two
disparate sets of pyrazolꢀ1ꢀyl resonances (Fig. S3(a)†, Experimental) of coordinated tpm
with the peak integrals ratio of 2:1 corresponding to α′, β′, γ′ and α, β, γ protons,
respectively (Scheme 2). The protons associated with the coordinated tpm and bqdi
9

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1
1
ligands in the complexes have been assigned based on the Hꢀ H COSY and 1Dꢀ
homonuclear NOE (Nuclear Overhauser Enhancement) difference experiments of
+
representative 1 in CD CN (Fig. S3bꢀc
†
) as well as with respect to the reported NMR
3
24
spectral features of analogous rutheniumꢀtpm complexes. The cross peaks of the proton
1
1
at δ 6.16 ppm with the protons at δ 8.18 ppm and δ 5.87 ppm in Hꢀ H COSY (Fig.
β
γ
α
S3(b)
†
†
) suggest their correspondences to the H , H and H , respectively (Scheme 2, Fig.
β′
S3(b)
). Also, the protons at δ 6.70 ppm (corresponding to H ) show cross peaks with the
γ′
α′
protons at δ 8.42 ppm and δ 8.44 ppm which correspond to H and H

3
β′ γ′
3
β′ α′ 24
S3(b)
†
γ′
α′
S3(b)
†). Further, NOE experiment was performed to differentiate between H and H as
γ
α
well as H and H protons (Scheme 2, Fig. S3(c)
†
). The enhancement in intensity of the
peaks at δ 8.42 ppm and δ 8.18 ppm along with the disappearance of other peaks (Fig.
δ
S3(c)
†
) on irradiation at δ 9.11 ppm (corresponding to H ) implies their correspondences
γ′
γ
to H and H (Scheme 2), respectively. The assignment of peaks of tpm based on the
3
β′ γ′
3
β′ α′ 24
criterion J(H H ) > J(H H ) has led to reveal the swapping of the chemical shift
γ′
α′
positions of H and H protons in CD CN with respect to those in other solvents (Fig.
3
S3(a)
†
, Experimental). Therefore, NOE experiment was also carried out for
+
representative 1 in (CD ) CO and it was found that on irradiation at δ 9.89 ppm,
3
2
δ
(
corresponding to H ) the intensity of the peaks at δ 8.64 ppm and δ 8.45 ppm are
γ′
γ
enhanced (Fig. S3(d)†) which leads to their correspondences to H and H (Scheme 2),
3
β′ γ′
3
β′ α′ 24
respectively, further validating the aforementioned criterion J(H H ) > J(H H ).
1
0

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α
β
Except H and H protons (Scheme 2), all other protons of tpm are deshielded on
2
4a
coordination to the metal ion. The deshielding may be due to the inductive effect
exerted by the bqdi ligand whose nitrogen donors are trans to the donor atoms of the two
α
magnetically equivalent pyrazolꢀ1ꢀyl groups. The H proton undergoes significant
shielding due to its closeness to the bqdi ring, thus lying in the shielding cone of the bqdi
2
4a
β
ring. Some of the magnetic anisotropy effect has also been transmitted to the H proton
which causes its little upfield shifting as compared to the uncoordinated ligand.
n
Electrochemistry, EPR, UV-vis spectroelectrochemistry, and DFT calculations of 1
(n = 2+, 1+, 0)
−
Complex [1]ClO exhibits reversible 1e oxidation and reduction processes at E° , V
4
298
(
ꢁE , mV): 0.82(80) and ꢀ0.79(80) in CH CN versus SCE/0.1 M Et NClO (Fig. 3).
p 3 4 4
However, the second reduction at E , V = ꢀ1.39 is irreversible in nature at the cyclic
pc
voltammetric time scale.
+
The orbital contributions to the MOs of 1 (Table S2
†
) predict that the HOMO is
composed of 58% Ru with appreciable contribution from Cl (24%) which confirms metal
II
III
III
0
2+
based oxidation, Ru →Ru , leading to the {Ru ꢀbqdi } valence formalism for 1 . This
is further supported by the DFT calculated CꢀNH bond distance of 1.312 Å (Table S5
†
)
0
22,14d
2+
2+
corresponding to a bqdi state
in 1 . The Mulliken spin density distribution in 1
(Ru: 0.767, Cl: 0.173, tpm: 0.047, bqdi: 0.014 (Fig. 4a)) and 65% contribution of Ru in
2
+
III
0
the βꢀLUMO of 1 (Table S3
†) are further in agreement with the {Ru ꢀbqdi }
2
+
configuration. Accordingly, the oxidised species (1 ) exhibits an anisotropic EPR
2
5
spectrum at 110 K in glassy frozen CH CN/0.1 M Bu NPF with g = 2.463, g = 2.248,
3
4
6
1
2
1
1

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2
2
2
3
1/2
2a
g = 1.883, <g> = [(1/3)(g + g + g )] = 2.211, and ꢀg = g ꢀg = 0.580 (Fig. 5a),
3
1
2
1
3
III
suggesting largely metalꢀcentred spin (Ru ).
+
On the other hand, LUMO of 1 is dominated by bqdi (68%) based orbitals (Table S2
†
)
which suggests a bqdi based reduction resulting in the valence configuration of
II
•−
[
Ru (tpm)(bqdi )(Cl)] for 1. Consequently, αꢀSOMO of 1 (Table S4†) is dominated by
bqdi (70%) based orbitals. In accordance, the DFT calculated CꢀNH bond distance of
0
bqdi increases from 1.318 Å (corresponding to bqdi ) to 1.342 Å (corresponding to
•
−
+
bqdi ) (Table S5†) on moving from 1 to 1. The presence of 23% Ru contribution in the
+
II
*
0
14d
LUMO of 1 (Table S2
†
) implies appreciable (dπ)Ru →π (bqdi ) backꢀbonding. The
Mulliken spin density distribution in 1 (bqdi: 0.857, Ru: 0.128, Cl: 0.008, tpm: 0.006
•
−
(
Fig. 4b)) signifies that bqdi is the primary spin bearing centre. This has been further
3
cꢀf
corroborated by the experimentally obtained unresolved free radical EPR signal of the
II
reduced species, 1 at g = 1.988 (Fig. 5b), pointing to a Ru ꢀcoordinated anion radical
2
a
ligand.
+
In the visible region dissolved 1 displays one weak absorption at 760 nm, followed by
a moderately intense band at 514 nm besides other intense higher energy transitions in the
+
UV region (Fig. 6, Table 3). The TDꢀDFT calculations on the optimised structure of 1 in
the S = 0 state also predict one weak transition (at 674 nm), followed by an intense
II
0
0
transition at 480 nm, corresponding to mixed dπ(Ru )→π*(bqdi )/dπ(Cl)→π*(bqdi )
MLCT (metalꢀtoꢀligand charge transfer)/LLCT (ligandꢀtoꢀligand charge transfer) and
II
0
14d
dπ(Ru )→π*(bqdi ) MLCT transitions, respectively (Table 4).
On the other hand,
II
17d
dπ(Ru )→π*(tpm) based MLCT transitions appear only around 300 nm.
1
2

Dalton Transactions
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II
0
+
+
III
0
2+
On oneꢀelectron oxidation of [Ru (tpm)(bqdi )(Cl)] (1 ) to [Ru (tpm)(bqdi )(Cl)]
2
+
(
1 ) the lowest energy band at 760 nm disappears and one weak band appears at 530 nm
followed by two moderately intense nearby bands at 455 nm and 436 nm (Fig. 6, Table
). TDꢀDFT calculations predict several transitions in that region which primarily consist
of tpm(π)→Ru(dπ*) and tpm(π)→bqdi(π*) based LMCT (ligandꢀtoꢀmetal charge
3
3
h
transfer) and LLCT (ligandꢀtoꢀligand charge transfer) transitions (Table 4). TDꢀDFT
calculations also predict weak transitions at 600ꢀ800 nm which have not been observed
experimentally (Fig. 6).
II
•−
The oneꢀelectron reduced [Ru (tpm)(bqdi )(Cl)] (1) exhibits one weak band at 780 nm
followed by one intense band at 637 nm and several moderately intense absorptions in the
higher energy region. The transitions are assigned based on the TDꢀDFT calculations as
MLCT transitions involving filled Ru and singly occupied bqdi•− or empty tpm(π*)
II
3
f
orbitals as well as interꢀligand transitions (Tables 3 and 4, Fig. 6).
II
0
2+
2+
The aqua complex [Ru (tpm)(bqdi )(H O)] (2 ) in water displays one weak band at
2
7
20 nm, followed by an intense band at 497 nm which are assigned to Ru(dπ)→bqdi(π*)
MLCT transitions based on the TDꢀDFT calculations (Table S6). A Ru(dπ)→tpm(π*)
based absorption has been observed at higher energy, at 316 nm (Table S6 and
+
Experimental). On moving from the chloro complex (1 ) to the corresponding aqua
2
+
derivative (2 ) the MLCT bands undergo blue shifts from 760 nm and 514 nm to 720 nm
26
and 497 nm, respectively, due to the stabilisation of dπ orbitals of ruthenium.
Besides the establishment of valence and spin configurations of the complexes, as
discussed in the preceding sections, the study has further been extended to catalytic
II
epoxidation of olefins. Unlike earlier reported isomeric complexes [Ru (trpy)(L)Cl] (trpy
1
3

Page 15 of 58
Dalton Transactions
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/
/
//
21
=
2,2 :6 ,2 ꢀTerpyridine and HL = Quinaldic Acid), the analogous chloro species
+
+
[
Ru(tpm)(bqdi)Cl] (1 ) failed to show any epoxidation activity in presence of oxidants
t
such as H O , BuOOH or PhI(OAc) , possibly due to the lack of necessary lability of the
2
2
2
0
2+
2+
chloro group. Therefore, the corresponding solvato species [Ru(tpm)(bqdi )H O] (2 )
2
has been explored for its feasibility to function as a preꢀcatalyst for the epoxidation
process.
n
Catalytic epoxidation reaction with system 2
II
0
2+
2+
The MOs of the DFT optimised aqua complex [Ru (tpm)(bqdi )(H O)] (2 ) (Fig. S2,
2
Table S7
†) suggest that HOMO and HOMOꢀ1 are primarily composed of metal based
2
+
orbitals, 63% and 70%, respectively. Accordingly, 2 exhibits two successive metal
based oxidation processes at 0.44 V and 0.64 V in buffer solution of pH~7 (Fig. S4†)
IV
which may lead to the eventual formation of a {Ru =O} species as shown in eqns. (1)
2
1q,r
and (2).
II
0
2+
III
0
2+
+
−
[
Ru (tpm)(bqdi )(H O)] → [Ru (tpm)(bqdi )(OH)] + H + e
(1)
(2)
2
III
0
2+
IV
0
2+
+
−
[
Ru (tpm)(bqdi )(OH)] → [Ru (tpm)(bqdi )(O)] + H + e
2
+
The aqua complex (2 ) has thus been tested as a preꢀcatalyst for the epoxidation of a
wide variety of alkene functionalities using PhI(OAc) as the oxidant in CH Cl solution
2
2
2
under aerobic reaction conditions at 298 K. The data for the epoxidation results are
summarised in Table 5. Terminal alkenes were found to be the most effective substrates
towards the said epoxidation process, except for αꢀmethyl styrene where “overꢀoxidised”
acetophenone is found to be the sole product (Table 5).
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4

Dalton Transactions
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2
+
The addition of PhI(OAc) to the CH Cl solution of 2 leads to the spontaneous
2
2
2
change in colour of the solution from reddish orange to yellow due to the in situ
IV
2+
2+
formation of the corresponding rutheniumꢀoxo species [Ru (tpm)(bqdi)(O)] (3 ) as
has also been evidenced from the change in the UVꢀvis spectral features: a shift from
21r,27
5
00 nm to 445 nm (Fig. S5
†
).
The in situ generation of the oxoꢀspecies has further
been established via the following experiments:
(i)
On addition of methyl pꢀtolyl sulfide to the above yellow solution the colour of
the solution reverts back to the initial reddish orange and the MLCT band of the
aqua species at 500 nm reꢀappears (Fig. S5
†) which suggests the oxidation of
IV
2+
sulfide to sulfoxide with the concomitant reduction of [Ru (tpm)(bqdi)(O)]
2
+
II
2+
2+
(
3 ) to [Ru (tpm)(bqdi)(H O)] (2 ). The conversion of pꢀtolyl sulfide to pꢀ
2
tolyl sulfoxide instead of possible pꢀtolyl sulfone has been confirmed by GC
experiments.
2
+
(ii)
The addition of PPh to the CDCl solution of 2 in the presence of
3 3
stoichiometric amounts of PhI(OAc) results in the in situ formation of O=PPh as
2
3,
3
1
confirmed by the P NMR spectrum of the resulting solution which displays a
2
1r
peak at ~30 ppm, typical of O=PPh (Fig. S6†).
3
(iii)
The peak at m/Z (Z = 2) 331.04 (Fig. S1
†) in the mass spectrum, obtained on
addition of PhI(OAc) to the mixture of [2](ClO ) and styrene in CH Cl suggests
2
4 2
IV
2
2
2+
21r
the in situ formation of an adduct between [Ru (tpm)(bqdi)(O)] and styrene.
1
5

Page 17 of 58
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0
The fully oxidised form of the “nonꢀinnocent” quinonoid ligand (bqdi ) in the precursor
2
+
aqua complex (2 ) allows the ruthenium ion to adopt a higher oxidation state via the
formation of an oxoꢀspecies in the presence of a suitable oxidant, PhI(OAc)2 which in
,
turn makes it highly electrophilic to react with the incoming alkene functionalities.
IV
Consequently, DFT calculations on the optimised rutheniumꢀoxo species, [Ru (tpm)
0
2
+
2+
(
bqdi )(O)] (3 ) (Fig. 7) reveal that the triplet (S = 1) state is more stable (~21 kcal
−
1
mol ) than the corresponding singlet (S = 0) ground state.
Metal complex catalysed epoxidation of alkenes is known to proceed via the formation
of an active metalꢀoxo intermediate through the mediation of a suitable oxidant. The
electrophilic metal bound oxo group subsequently interacts with the incoming olefinic
1
9,21
double bond with eventual transfer of the oxo group
to the olefin as shown in Scheme
3
.
O
IV
[
Ru ]
[O]
O
[
O]
H O
2
IV
[
Ru] O
[Ru ] O
II
[
Ru ]
O
[
Ru]
Scheme 3. Proposed reaction pathway.
1
6

Dalton Transactions
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R
R'
R
R'
R
R'
R
R'
R
.
+
.
+
O
O
O
O
O
R'
M
M
M
M
M
(i)
(ii)
(iii)
(iv)
(v)
Scheme 4. Possible transition states/intermediates for the transfer of oxygen from the
metalꢀoxo species to the olefinic double bond.
It has been proposed earlier that the transfer of oxygen atom from the metalꢀoxo complex
to the olefinic double bond can proceed via any of the following five possible different
pathways: (i) concerted transition state, (ii) carbon radical, (iii) carbocation, (iv) πꢀradical
2
1r,28,29
cation, (v) metalaoxetane (Scheme 4).
However, DFT calculations on the optimised adduct (A, Fig. 7) comprising of a
2
+
rutheniumꢀoxo species (3 ) and styrene as the model substrate reveal that the concerted
transition state pathway is preferred in the present case. Upon approach of styrene
towards the rutheniumꢀoxo species, the RuꢀO1 bond length increases to 2.17 Å from 1.78
Å while the C ꢀC bond of styrene lengthens to 1.44 Å from the standard carbonꢀcarbon
α
β
double bond length of ~1.34 Å, with the simultaneous formation of C ꢀO1 and C ꢀO1
α
β
bonds (Fig. 7, Table S8
to the intermediate oxo complex. Thus, the natural charge on Ru decreases to 0.62 from
.89 and that on O1 increases to ꢀ0.56 from ꢀ0.26, implying less electrophilic character of
the RuꢀO bond in A as has also been reflected by the single bond character of the RuꢀO1
bond (Table S8 ). The absence of α and/or β spin on the C ꢀC bond rules out the
†). These in effect alter the electronic property of A with respect
0
†
α
β
1
7

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Dalton Transactions
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possibility of formation of the other alternatives, iiꢀiv, in Scheme 4. The spin density
analysis of the adduct (A) and the intermediate oxoꢀspecies reveals a large delocalisation
of charge around the Ruꢀbqdi plane in A while in the oxoꢀspecies it is rather localised on
the RuꢀO1 bond (Ru: 0.83 and O1: 1.11) (Fig. 8). The complete disappearance of spin on
O1 provides the necessary justification in favor of the existence of transition state “i” in
the present case (Scheme 4). Moreover, energy calculations reconfirm that “i” (Scheme
−1
2+
4
) is situated only ~8 kcal mol higher than the preꢀcatalyst, 2 .
The formation of a concerted transition state “i” (Scheme 4) has also been supported by
1
9
a F NMR spectral study in the case of 4ꢀfluorostyrene as the substrate where the peak of
free 4ꢀfluorostyrene at −114 ppm remains unchanged on addition of PhI(OAc) to the
2
2
+
reaction mixture containing the substrate 4ꢀfluorostyrene and the preꢀcatalyst 2 in
CDCl . The existence of any of the other intermediates, iiꢀiv, in Scheme 4 would have
3
resulted in shifting along with broadening, due to the paramagnetic nature of the alkene,
of the peak during the said catalytic process.
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8

Dalton Transactions
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Conclusion
The experimental and DFT calculations establish that the redox nonꢀinnocent oꢀ
benzoquinonediimine (bqdi) has been stabilised in its fully oxidised quinone state in both
II
0
+
+
the
chloro
complex
([Ru (tpm)(bqdi )(Cl)] ) (1 )
and
in
the
aqua
II
0
2+
2+
(
[Ru (tpm)(bqdi )(H O)] ) derivative (2 ). The chloro complex exhibits metal based
2
II
0
+
+
III
0
2+
2+
oxidation: [Ru (tpm)(bqdi )(Cl)] (1 )→[Ru (tpm)(bqdi )(Cl)] (1 ) and bqdi centred
II
0
+
+
II
•−
reduction ([Ru (tpm)(bqdi )(Cl)] (1 )→[Ru (tpm)(bqdi )(Cl)] (1)). The aqua complex
2
+
2
functions as an efficient preꢀcatalyst for the selective epoxidation of a wide variety of
alkenes in the presence of PhI(OAc) as the sacrificial oxidant in CH Cl where the
2
2
2
IV
0
2
+
oxidation proceeds through an inferred intermediate oxoꢀspecies [Ru (tpm)(bqdi )(O)]
2
+
(
3 ). Terminal alkenes are found to be the most effective substrates, except αꢀmethyl
styrene where “overꢀoxidised” acetophenone is found to be the sole product. The
mechanistic pathway for the epoxidation process has been deciphered via DFT
calculations which suggest the involvement of a concerted transition state.
1
9

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Experimental
Materials
1
7b
The ligand tris(1ꢀpyrazolyl)methane (tpm)
and the starting material
17c
[
Ru(tpm)(Cl) ].1.5H O were prepared as reported previously in the literature. The
3 2
chemicals and solvents were of reagent grade and used as received. Deionised water of
high purity was obtained by passing distilled water through a nanopure MilliꢀQ water
purification system. For solution of pH 7.0 a hydrogen phosphate/dihydrogen phosphate
buffer was employed. For spectroscopic and electrochemical studies, HPLC grade
solvents were employed.
Physical Measurements
Electrical conductivities were checked using a Systronic conductivity bridge 305. Solid
state infrared spectra with samples prepared as KBr pellets were taken on a Nicolet
1
1
1
31
19
spectrophotometer. H NMR, Hꢀ H COSY, 1Dꢀhomonuclear NOE, P NMR and
F
NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer.
Electrochemical measurements were carried out using a PAR model 273A
electrochemistry system. Platinum wire working and auxiliary electrodes and an aqueous
saturated calomel reference electrode (SCE) were used in a threeꢀelectrode configuration.
−
3
The supporting electrolyte was 0.1 mol dm Et NClO and the solute concentration was
4
4
−
3
−3
o
2
∼
10 mol dm . The halfꢀwave potential E
was set equal to 0.5(E + E ), where E
98 pa pc pa
and Epc are anodic and cathodic cyclic voltammetric peak potentials, respectively. A
glassy carbon electrode was used for the electrochemical measurements of [2](ClO ) . A
4
2
+
PerkinꢀElmer Lambda 950 spectrophotometer was used for recording the spectra of 1
2
0

Dalton Transactions
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2
+
−3
and 2 . UVꢀvis spectroelectrochemical studies were performed in CH CN / 0.1 mol dm
3
3
0
Bu NPF at 298 K using an optically transparent thinꢀlayer electrode (OTTLE) cell
4
6
mounted in the sample compartment of a J&M TIDAS spectrophotometer. All the
spectroelectrochemical experiments were performed under dinitrogen atmosphere. The
3
1
EPR measurements were made in a two electrode capillary tube with an Xꢀband (9.5
GHz) Bruker system ESP300 spectrometer. Elemental analyses were done on Perkinꢀ
Elmer 240C elemental analyser. Micromass QꢀToF mass spectrometer was used for
recording electrospray mass spectra. The catalytic reactions were monitored using gas
chromatographic techniques on Shimadzu GCꢀ2014 gas chromatograph with an FID
detector using a capillary column (112ꢀ2562 CYCLODEXB, from J&W Scientific, length
6
0 m, inner diameter 0.25 mm, film 0.25 ꢁm).
Crystallography
Single crystals of [1]ClO .2H O and 2([2](ClO ) ).H O were grown by slow evaporation
4
2
4 2
2
of 1:3 acetonitrileꢀtoluene and 1:3 H Oꢀtoluene solutions, respectively. The structures
2
2
were solved and refined by fullꢀmatrix leastꢀsquares techniques on F using the SHELXꢀ
3
2
9
7 program. The absorption corrections were done by the multiꢀscan technique. All data
were corrected for Lorentz and polarisation effects, and the nonꢀhydrogen atoms were
refined anisotropically. Hydrogen atoms were included in the refinement process as per
the riding model. Hydrogen atoms of the two H O molecules in the unit cell of [1]ClO
2
4
could not be located, however, these have been included in the composition in Table 1.
CCDC numbers for [1]ClO .2H O and 2([2](ClO ) ).H O are 904797 and 904798,
4
2
4 2
2
respectively.
2
1

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Computational details
Full geometry optimisations were carried out at the (R)B3LYP and (U)B3LYP levels
3
3,34
+
2+
2+
2+
for 1 , 2 and 1 , 1, 3 , respectively, using the density functional theory method with
3
5
Gaussian 03 (revision C.02). All elements except ruthenium were assigned the 6ꢀ
3
1G(d) basis set. The LANL2DZ basis set with the effective core potential was employed
3
6,37
for the ruthenium atom.
Vertical electronic excitations based on B3LYP optimised
geometries were computed for the timeꢀdependent density functional theory (TDꢀDFT)
3
8
formalism in acetonitrile using the polarisable continuum model (PCM) of Tomasi and
coꢀworkers; specifically, the conductor like PCM (CPCM) in conjugation with the united
3
9ꢀ41
atom topological model (using UAO radii, implemented in Gaussian 03) was applied.
4
2
GaussSum was used to calculate the fractional contributions of various groups to each
molecular orbital. No symmetry constraints were imposed during structural
optimisations, and the nature of the optimised structures and energy minima were defined
by subsequent frequency calculations. Natural bond orbital (NBO) analyses were
4
3
performed using the NBO 3.1 module of Gaussian 03 on optimised geometry. All the
4
4
calculated structures were visualised with ChemCraft.
General procedure for catalytic epoxidation studies
3
The preꢀcatalyst [2](ClO ) (1.50 mg, 0.0016 mmol) in 5 cm CH Cl was taken in a glass
4
2
2
2
vial and stirred for 15 min at 298 K. The olefin (0.24 mmol) followed by the oxidant
iodobenzene diacetate (0.64 mmol) were added to the above solution under continuous
stirring condition at 298 K. The progress of the reaction was monitored by gas
2
2

Dalton Transactions
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chromatography. The conversions were determined from the gas chromatograms using
1
standard samples and/or H NMR.
Preparation of complexes
Synthesis of [Ru(tpm)(bqdi)(Cl)]ClO ([1]ClO ). Ru(tpm)(Cl) (100 mg, 0.24 mmol)
4
4
3
3
and 1,2ꢀphenylenediamine (38.93 mg, 0.36 mmol) were taken in 25 cm ethanol and NEt
3
3
(
0.1 cm , 0.72 mmol) was added to the mixture. The mixture was heated to reflux with
constant stirring for 6 h which resulted in a pink solution. The removal of solvent under
reduced pressure yielded a dark coloured solid mass. The solid mass was dissolved in
minimum volume of acetonitrile and saturated aqueous NaClO solution was added to it
4
and it was kept in a refrigerator overnight. The residue thus obtained on filtration was
washed with minimum volume of chilled water and then dried in vacuo over P O . The
4
10
crude solid was purified using a silica gel (60ꢀ120 mesh) column. The pink band
corresponding to [1]ClO was eluted by 5:1 dichloromethaneꢀacetonitrile mixture.
4
Evaporation of the solvent mixture under reduced pressure yielded the pure complex
[
1]ClO . Yield: 79 mg (60%). Anal. Calcd for C H N Cl O Ru: C, 34.54; H, 2.90; N,
4 16 16 8 2 4
−
1
2
3
−1
2
0.15. Found: C, 34.62; H, 2.80; N, 19.95. Molar conductivity (Λ (ꢂ cm dm mol ),
M
CH CN): 140. ESIꢀMS(+) (m/Z, CH CN): 457.02 (Calcd 457.02 for
3
3
+
−1
−
1
[
Ru(tpm)(bqdi)(Cl)] ). IR (cm , KBr pellet): 1119, 622 (corresponding to
ν
(ClO )). H
4
δ
3
NMR: (400 MHz): δ (ppm): CDCl : 11.48 (2H, s, NH), 10.02 (1H, s, H ), 8.58 (2H, d, J
3
β′ γ′
γ′
3
β
γ
γ
3
β′ α′
(
H H ) 2.92 Hz, H ), 8.46 (1H, d, J (H H ) 3.56 Hz, H ), 8.45 (2H, d, J (H H ) 2.24
α′
a
b
Hz, H ), 7.34 (2H, dd, J 7.11, 3.11 Hz, H ), 6.99 (2H, dd, J 7.00, 3.12 Hz, H ), 6.66 (2H,
β′
β
3
α
γ
α
pseudo t, H ), 6.13 (1H, pseudo t, H ), 5.72 (1H, d, J (H H ) 2.16 Hz, H ); (CD ) SO:
3
2
2
3

Page 25 of 58
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δ
3
β′ γ′
γ′
3
1
3.64 (2H, s, NH), 9.81 (1H, s, H ), 8.59 (2H, d, J (H H ) 2.82 Hz, H ), 8.45 (2H, d, J
β′ α′
α′
3
β
γ
γ
(
H H ) 2.16 Hz, H ), 8.36 (1H, d, J (H H ) 2.84 Hz, H ), 7.38 (2H, dd, J 6.56, 2.92 Hz,
a
b
β′
β
H ), 6.94 (2H, dd, J 7.04, 3.16 Hz, H ), 6.86 (2H, pseudo t, H ), 6.26 (1H, pseudo t, H ),
3
α
γ
α
δ
5
.96 (1H, d, J (H H ) 2.20 Hz, H ); CD CN: 12.48 (2H, s, NH), 9.11 (1H, s, H ), 8.44
3
3
β′ α′
α′
3
β′ γ′
γ′
3
(
2H, d, J (H H ) 2.24 Hz, H ), 8.42 (2H, d, J (H H ) 2.84 Hz, H ), 8.18 (1H, d, J
β
γ
γ
a
(
H H ) 2.88 Hz, H ), 7.36 (2H, dd, J 6.92, 3.16 Hz, H ), 6.94 (2H, dd, J 6.88, 2.92 Hz,
b
β′
β
3
α
γ
H ), 6.70 (2H, pseudo t, H ), 6.16 (1H, pseudo t, H ), 5.87 (1H, d, J (H H ) 1.92 Hz,
α
δ
3
β′ γ′
γ′
H ); (CD ) CO: 13.26 (2H, s, NH), 9.89 (1H, s, H ), 8.64 (2H, d, J (H H ) 2.84 Hz, H ),
3
2
3
β′ α′
α′
3
β
γ
γ
8
7
.54 (2H, d, J (H H ) 2.20 Hz, H ), 8.45 (1H, d, J (H H ) 2.96 Hz, H ), 7.46 (2H, dd, J
a
b
β′
.22, 2.98 Hz, H ), 6.98 (2H, dd, J 7.00, 3.2 Hz, H ), 6.81 (2H, pseudo t, H ), 6.31 (1H,
β
3
α
γ
α
3
−1
−1
pseudo t, H ), 6.12 (1H, d, J (H H ) 2.20 Hz, H ). λ/nm (
ε / dm mol cm ) in CH CN:
3
7
60 (420), 514 (12200), 332 (3090, sh), 302 (5550, sh), 216 (9680).
Synthesis of [Ru(tpm)(bqdi)(H O)](ClO ) ([2](ClO ) ). [1]ClO (100 mg, 0.18
2
4 2
4 2
4
3
mmol) was taken in 25 cm acetoneꢀwater (1:4) and AgNO (306 mg, 1.8 mmol) was
3
added to it. The mixture was heated to reflux under stirring condition for 4 h. The pink
solution turned to reddish orange with the precipitation of AgCl. The mixture was then
cooled to room temperature and the AgCl precipitate was filtered off through a sintered
3
glass crucible G4. NaClO was added to the concentrated filtrate (5 cm ) and it was then
4
refrigerated overnight. The solid [2](ClO4)2 thus obtained was filtered off and washed
with a few drops of chilled water and dried in vacuo over P O . Yield: 99 mg (87%).
4
10
Anal. Calcd for C H N Cl O Ru: C, 30.10; H, 2.85; N, 17.56. Found: C, 29.95; H, 2.80;
1
6
18
8
2
9
N, 17.32%. Molar conductivity (Λ (ꢂ− cm dm mol ), H O): 225. ESIꢀMS(+) (m/Z,
1
2
3
−1
M
2
2
4

Dalton Transactions
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+
−1
H O): 520.9 (Calcd 520.8 for {[Ru(tpm)(bqdi)]ClO } ). IR (cm , KBr pellet): 3425,
2
4
−
1
1
632 (corresponding to
ν
(OH)); 1092, 625 (corresponding to
ν
(ClO )). H NMR: (400
4
δ
3
β′ γ′
MHz): δ (ppm): (CD ) CO: 13.54 (2H, s, NH), 9.96 (1H, s, H ), 8.78 (2H, d, J (H H )
3
2
γ′
3
β′ α′
α′
3
β
γ
γ
2
7
.76 Hz, H ), 8.69 (2H, d, J (H H ) 1.96 Hz, H ), 8.47 (1H, d, J (H H ) 2.90 Hz, H ),
a
b
.51 (2H, dd, J 7.14, 3.18 Hz, H ), 7.05 (2H, dd, J 7.12, 3.20 Hz, H ), 6.92 (2H, pseudo t,
β′
β
α
3
β′ γ′
γ′
3
H ), 6.30 (2H, m, H , H ); D O: 8.43 (2H, d, J (H H ) 2.80 Hz, H ), 8.39 (2H, d, J
2
β′ α′
α′
3
β
γ
γ
(
H H ) 2.24 Hz, H ), 8.10 (1H, d, J (H H ) 2.96 Hz, H ), 7.38 (2H, dd, J 7.10, 3.10 Hz,
a
b
β′
β
H ), 6.96 (2H, dd, J 7.10, 3.10 Hz, H ), 6.75 (2H, pseudo t, H ), 6.10 (1H, pseudo t, H ),
α
3
−1
−1
5
.79 (1H, br s, H ). λ/nm (
ε
/dm mol cm ) in CH CN: 720 (1210), 497 (19800), 316
3
(5500, sh), 274 (13070, sh), 267 (13800, sh), 255 (14390).
(CAUTION! Perchlorate salts of metal complexes are generally explosive. Care
needs to be taken while handling such complexes).
Acknowledgements
Financial support received from the Department of Science and Technology and Council
of Scientific and Industrial research (Fellowship to HA), New Delhi, India, the DAAD,
FCI and DFG (Germany) are gratefully acknowledged. Xꢀray structural studies were
carried out at the National Single Crystal Xꢀray Diffraction Facility, Indian Institute of
Technology Bombay.
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5

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