On the non-innocence and reactive: Versus non-reactive nature of α-diketones in a set of diruthenium frameworks

Article abstract of DOI:10.1039/d0dt03400b

α-Diketones are an important class of building blocks employed in many organic synthetic reactions. However, their coordination chemistry has rarely been explored. In light of this, our earlier report on [(acac)2RuII(μ-2,2′-pyridil)RuII(acac)2] (acac = acetylacetonate) showcased the sensitivity of a diketone fragment towards oxidative C-C cleavage. Following the lead, the synthesis of similar but stable diketo fragments containing diruthenium compounds was attempted. Three diruthenium compounds with the bridge 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione (L) were prepared: diastereomeric [(acac)2RuIII(μ-L2-)RuIII(acac)2], 1a(rac)/1b(meso), [(bpy)2RuII(μ-L2-)RuII(bpy)2](ClO4)2, [2](ClO4)2 and [(pap)2RuII(μ-L2-)RuII(pap)2](ClO4)2, [3](ClO4)2 with ancillary ligands of different donating/accepting characteristics. The metal is stabilised in different oxidation states in these complexes: Ru(iii) is preferred in 1a/1b when σ-donating acac is used as the co-ligand whereas electron rich Ru(ii) is preferred in [2](ClO4)2 and [3](ClO4)2 when co-ligands of moderate to strong π-Accepting properties are employed. The oxidative chemistry of these systems is of particular interest with respect to the participation of varying bridging-ligands which contain phenoxide groups. On the other hand, the reduction processes primarily resulting from the metal or the ancillary ligands are noteworthy as the normally reducible 1,2-diketo-group remains unreduced. These results have been rationalised and outlined from thorough experimental and theoretical investigations. The results presented here shed light on the stability of metal coordinated α-diketones as a function of their substituents.

Full text of DOI:10.1039/d0dt03400b

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Page 1 of 32
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On the non-innocence and reactive versus non-reactive natu
D
r
O
e
I: 10.
o
10
f
39/D
α
0DT03400B
-
diketones in a set of diruthenium frameworks
‡
‡
#
#
Farheen Fatima Khan, Saikat Mondal, Shubhadeep Chandra, Nicolas I. Neuman, Biprajit
,
#
*,‡
Sarkar,* and Goutam Kumar Lahiri
‡Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076,
India. Email: lahiri@chem.iitb.ac.in
#Lehrstuhl für Anorganische Koordinaitonschemie, Institut für Anorganische Chemie,
Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany. Email:
biprajit.sarkar@iac.uni-stuttgart.de
†
Electronic supplementary information (ESI) available: X-ray crystallographic files in CIF
1
format for [2](ClO
4
)
2
and [3](ClO
4
)
2
, mass spectra (Fig. S1), H NMR (Fig. S2), DFT
optimised structures (Fig. S3), crystallographic and DFT calculated bond parameters (Tables
S1-S7), MO compositions (Tables S8-S22), energies (Table S23) and TD-DFT (Table S24)
difference of optimised structures (PDF). CCDC- 2026821 ([2](ClO
[3](ClO
crystallographic data in CIF or other electronic format see DOI: xxxxxx.
4
)
2
) and 2026822
(
4
) ), contain the supplementary crystallographic data for this paper. For ESI and
2
1

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Page 2 of 32
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DOI: 10.1039/D0DT03400B
Abstract
α-Diketones are an important class of building blocks employed in many organic synthetic
reactions. However, their coordination chemistry has rarely been explored. In this light, our
II
II
earlier report on [(acac)
2
Ru (μ-2,2′-Pyridil)Ru (acac) ] (acac=acetylacetonate) showcased
2
the sensitivity of the diketone fragment towards oxidative C-C cleavage. Following the lead,
the synthesis of similar but stable diketo fragment containing diruthenium compounds were
attempted. Three diruthenium compounds with the bridge 1,2-bis(2-hydroxyphenyl)ethane-
III
2−
III
1
,2-dione
(L)
were
prepared:
diastereomeric
[(acac)
2
Ru (μ-L )Ru (acac) ],
2
II
2−
II
II
1
a(rac)/1b(meso), [(bpy)
2
Ru (μ-L )Ru (bpy)
2
](ClO
4
)
2
, [2](ClO
4
)
2
and [(bpy)
2
Ru (μ-
2
−
II
L )Ru (pap)
2
](ClO
4
)
2
, [3](ClO
4
)
2
with ancillary ligands of different donating/accepting
characteristics. The metal is stabilised in different oxidation states in theses complexes:
Ru(III) is preferred in 1a/1b when an σ-donating acac is present as the co-ligand whereas
electronically rich Ru(II) is preferred in [2](ClO
4
)
2
and [3](ClO
4
2
) when co-ligands of
moderate to strong π-accepting properties are employed. The oxidative chemistry of these
systems is of particular interest with respect to the varying bridging-ligand participation
which contains phenoxide groups. On the other hand, the reduction processes being primarily
resulting from metal or the ancillary ligands are noteworthy as the normally reducible 1,2-
diketo- group remains unreduced. These results have been rationalised and outlined from
thorough experimental and theoretical investigations. The results presented here shed light on
the stability of metal coordinated α-diketones as a function of their substituents.
2

Page 3 of 32
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Introduction
DOI: 10.1039/D0DT03400B
Quinones are a class of oxidised aromatics that are indispensably pivotal in the aerobic
1
metabolism of cells and are constituents of a variety of enzymes. This class of organic
functionality is frequently researched due to their well-established capability of sequential
one electron transfer events which allow access to singly reduced semiquinone and doubly
2
reduced catecholate forms. Nature often takes advantage of these electron shuttles in the
functioning of metalloenzymes, which usually occurs through inner sphere electron transfer
3
in the catalytic steps of the enzymes. In recent years, our group has extensively explored
quinone type ligands for studying various electron transfer aspects and ligand assisted
4
transformations that encompass biological and fundamental interests. A recent report from
the group in this regard inspected the electronic aspects of ortho-quinone type 2,2'-Pyridil
with the Ru(acac)
2
(acac = acetyacetonate) metal fragment and its consecutive ligand assisted
reactivity leading to dioxygen activation followed by C-C cleavage (Fig. 1).5
N
N
II
II
RR uu (( AA
)
OO
2
2
2
2
II OO
II
N
N
III
III
O
O
(A
2
)Ru
22 (A )Ru
2
−
22
N
N
A= acac
A= acac
−
O
O
O
O
Fig. 1 Previous report from the group depicting C-C cleavage in diruthenium complexes of
2
,2'-Pyridil.5
The observation of such dioxygen driven cleavage activity in 2,2′-Pyridil obviously raises
the question of the generality of such C-C bond cleavage reactions in related diketo-
containing compounds. In this work, the stability/instability of 1,2-bis(2-hydroxyphenyl)
6
ethane-1,2-dione (L) (Scheme 1), a symmetrical bridging ligand comprises of a mixed donor
(
redox active phenoxide, upon deprotonation) – acceptor (keto groups) system, and its
activity/inactivity towards dioxygen is investigated in a set of ruthenium complexes.
The ligand (L) can in principle exhibit the following electronic forms as shown in Scheme
3

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Page 4 of 32
•−
7
View Article Online
DOI: 10.1039/D0DT03400B
1
: a) L form upon reduction and the electron being accommodated at the diketo unit and b)
•
+
L form upon oxidation where the oxidation may be assigned to take place at the phenol.
H
H
H
H
O
O
O
O
O
O
O
O
O
O
O
O
+
e
(a)
H
H
H
−
O
O
O
O
e
(b)
−
+
H
Scheme 1 (a) Reduction and (b) oxidation possibilities for L.
While the deprotonated phenols are established in their commonly observed phenoxide
form, their uncommon oxidised forms phenoxyl radical and phenoxium cation have also been
8
documented. Such processes are of interest since many metalloenzymes like galactose
oxidase contain tyrosinyl radical and such radical systems are known to be stabilised by metal
coordination.9
Herein, we present the syntheses, characterisation, experimental and theoretical evaluation
of the diruthenium complexes 1a/1b, [2](ClO
4
)
2
and [3](ClO
4
2
) (Fig. 2). We also address the
stability/reactivity of 2,2′-Pyridil versus L in the corresponding ruthenium complexes, and
provide a rationale for the experimental observations (Fig. 1).
4 2
(ClO )
4 2
(ClO )
Ph Ph
N
O
N N N N
N
O
O
O
O
N
N
N
Ru
N
Ru
O
Ru
O
O
O
O
O
O
O
O
O
O
Ru
N
Ru
N
N
O
O
N
Ru
N
O O
N N N
Ph Ph
N
N
1
a : rac ∆∆/ΛΛ
( )
4 2
[3](ClO )
∆
Λ
[2](ClO
4 2
)
1
b : meso
(
)
Fig. 2 Representation of complexes.
4

Page 5 of 32
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Results and discussion
DOI: 10.1039/D0DT03400B
Syntheses, characterisation and molecular structure
The dinuclear complexes 1a/1b, [2](ClO and [3](ClO
4
)
2
4
2
) have been prepared following the
protocols developed earlier4b-4d for a variety of diruthenium complexes. The reaction of 1,2-
II
bis(2-hydroxyphenyl)ethane-1,2-dione (L) and the precursor complex Ru (acac)
2
(CH
3
CN)
2
in refluxing toluene in the presence of NEt
3
as a base under dinitogen atmosphere followed
III
by chromatographic separation generated paramagnetic diastereomers [(acac)
2
Ru (μ-
2
−
III
L )Ru (acac)
2
], 1a (rac(ΔΔ/ΛΛ)) and 1b (meso(ΔΛ)) due to the presence of two chiral
II
metal
centres.5
The
, [2](ClO
prepared from the in-situ generated cis-[Ru-(bpy)
corresponding
diamagnetic
complexes
Ru (μ-L )Ru (pap) ](ClO
] and ctc-[Ru(pap)
[(bpy)
2
Ru (μ-
2
−
II
II
2−
II
L )Ru (bpy)
2
](ClO
4
)
2
4
)
2
and [(pap)
2
2
4
)
2
, [3](ClO
4
)
2
were
2
+
2+
2
(EtOH)
2
2
(EtOH) ] ,
2
respectively,R (bpy = 2,2′-bipyridine, pap = 2-phenylazopyridine, ctc = cis-trans-cis
configuration with respect to EtOH, pyridine, and azo nitrogen donors of pap ligands,
1
0
respectively ) and L in presence of NEt
3
base in refluxing toluene followed by
chromatographic purification on a neutral alumina column. As is not uncommon, the
1
1
concurring stabilisation of the high valent Ru(III) state in the complexes 1a/1b and that of
Ru(II) in [2](ClO /[3](ClO due to the effects of the dianionic bridge, the σ-donating
4
)
2
)
4 2
nature of acac ancillary fragment and the π-accepting nature of bpy/pap co-ligands,12
respectively, has been observed and established (vide infra).
The electrically neutral 1a/1b and 1:2 conducting [2](ClO
4
)
2
and [3](ClO
4
2
) were subjected
to multiple characterisation techniques and these yielded satisfactory microanalytical and
mass data (Experimental Section and Fig. S1†).
13
The appearance of the forbidden half-field signals along with anisotropic (rhombic) EPR
(
Fig. 3) at 100 K for 1a/1b affirmed the presence of two metal-centred spins which contribute
to the triplet (RuIII(t2g5)-RuIII(t2g5)) spin state situation (S=1) in these complexes.
5

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Page 6 of 32
g1 = 2.340
g1 = 2.347
View Article Online
DOI: 10.1039/D0DT03400B
1
a
1b
<
g> = 2.06
<
g> = 2.10
∆g= 0.67
g2 = 2.092
∆g= 0.62
g2 = 2.202
g = 4.792
1
/2
g = 4.872
1/2
g3 = 1.675
g3 = 1.715
500
2000
3500
5000
500
2000
3500
5000
H/ G
H/ G
g = 2.500
g = 2.500
1
1
23+
2+
g = 2.012
g = 2.012
<g> = 2.18
<g> = 2.18
∆g = 0.73
∆g = 0.73
2
2
g = 2.215
g = 2.215
3
3
g = 1.765
g = 1.765
2
2
0
0
0
0
0
0
3
3
0
0
0
0
0
0
4
4
0
0
0
0
0
0
3250
3250
3325
3325
3400
3400
3475
3475
H
H
/
/
G
G
H / G
H / G
33+
3+
g = 2.000
g = 2.000
g = 1.996
g = 1.996
3
3
3
3
0
0
0
0
3
3
3
3
6
6
0
0
3
3
4
4
2
2
0
0
3
3
4
4
8
8
0
0
3280 3340 3400 3460
3280 3340 3400 3460
H
H
/
/
G
G
H / G
H / G
3
g
g
= 3.972
= 3.972
1/2
/2
1
g = 1.986
g = 1.986
1000
1500
2000
2500
1
1
5
5
0
0
0
0
3
3
5
5
0
0
0
0
5500
5500
H / G
H / G
Fig. 3 X-band EPR spectra in CH
2
Cl /toluene (1:2) mixture at 5 K.
2
6

Page 7 of 32
Dalton Transactions
The characteristic paramagnetic contact sifted14 1H- NMR resonances corresponding to full
View Article Online
DOI: 10.1039/D0DT03400B
and half molecule over a wide chemical shift range from 20 ppm to -30 ppm approximately
for the diastereomeric pair 1a and 1b, respectively, further establish the presence of unpaired
electronic spins (Fig. S2†). Multiple overlapping proton resonances corresponding to full
molecule and half molecule were observed in the aromatic chemical shift range for the
diamagnetic [2](ClO
4
)
2
and [3](ClO
4
2
) (Fig. S2†), respectively, as could also be corroborated
from their single crystal X-structures (Fig. 4).
Several attempts towards crystallisation of the diastereomeric pair unfortunately only led to
very thin and fragile crystals that did not diffract. However, successful diffraction of single
crystals of [2](ClO
4
)
2
and [3](ClO
4
2
) helped in gaining insights into the structural and
electronic details in these complexes. The structural parameters could largely be
approximated and reproduced by the theoretical calculations (Fig. 4, Fig. S3, Table S1- S7†).
(
a)
(
b)
Fig. 4 Perspective views of the cations of (a) [2](ClO
4
)
2
and (b) [3](ClO
4
2
) . Ellipsoids are
drawn at 50% probability levels. Hydrogen atoms and perchlorates are removed for clarity.
7

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DOI: 10.1039/D0DT03400B
MM
MM
MM
OO
OO
MM
OO OO
MM
OO
OO
OO
OO
OO
OO
OO OO
MM
OO
MM
O
O
OO OO
MM
(a)
(b)
(c)
(d)
Fig. 5 Probable binding modes of L in a dinuclear framework.
Considering the various probabilities around the dinuclear chelation with the tetradentate
ligand L, which here acts as a bis-bidentate chelating ligand, an outlining sketch may be
referenced from Fig. 5. The possibilities a and b are less likely, and since the crystal structure
shows six-membered chelates, these are being ruled out as any major isolable fraction of the
reaction mixture. This may be rationalised due to the strong preference of formation of six-
membered over seven or higher-membered ring chelates due to the stability reasons. The
occurrence of c and d forms may be recognised as two distinct extremes of cis- and trans-
forms which are likely but the adoption of a cis-type configuration would be sterically
demanding owing to the presence of spatially encumbered metal centres with two ancillary
fragments and similarly a strict trans- form would be difficult to attain due to repulsions that
may arise between pendant phenyl rings of the ligand and the neighbouring ancillary groups.
Thus, the molecular structures (Fig. 4) of the complexes [2](ClO
4
)
2
and [3](ClO
4
2
) nearly
display a situation that is rather intermediate of the cis- and trans- forms with the angles
between the two chelate rings being 82.87° and 84.56°, respectively, for the bis-chelated
complexes. The geometry around each metal centre is pseudo octahedral as is commonly
observed due to the differing chelating environment extended by the bridging ligand, the co-
1
5
ligands and their different donor atoms. The metrical parameter involving C=O bond
lengths of 1.274(9) Å and 1.263(8) Å for [2](ClO and [3](ClO , respectively, are
4
)
2
)
4 2
suggestive of the presence of an unreduced coordinated carbonyl function and similar
conclusions for the complex 1 may also be drawn from the computational values (Tables S2,
8

Page 9 of 32
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DOI: 10.1039/D0DT03400B
S4, S6†). The average azo bond distance of pap in [3](ClO
unreduced state.10b
4
) of 1.291(7) Å also reflects its
2
Rationale for the stability of complexes
As can be seen from the above discussion that deprotonated 1,2-bis(2-hydroxyphenyl)ethane-
,2-dione (L) binds to the metal ions and remains stable and intact upon metallation. Initially
the ligand was screened with {Ru(acac) } framework for stability. Upon obtaining stable
complex 1, the ligand was further explored in diruthenium complexes having varying
electronic feature of the ancillary ligands by employing {Ru(bpy) } and {Ru(pap) } metal
fragments and interestingly that also led to the stable complexes [2](ClO and [3](ClO ) ,
1
2
2
2
4
)
2
4
2
respectively. These results are strikingly different from the corresponding 2,2′-Pyridil
containing complexes in which the fragile 1,2-diketo fragment cleaves at the C-C bond upon
5
exposure to air. Intrigued by the sensitivity of diketo fragment in 2,2′-Pyridil which was
proposed to be majorly activated by redox tautomerism, we therefore chose L to affirm the
mechanistic outlines reported earlier - by eliminating the possibility of redox tautomerism
that should ultimately lead to stable complexes with intact diketo moiety.
Gratifyingly, replacement of the pyridine ring in 2,2′-Pyridil with a phenolic group in L
indeed gave us access to air stable complexes 1-[2](ClO
4
)
2
/[3](ClO
4
2
) even when the metal
centre bore different electronic environments around it. These successful results could be
justified by the following two reasons: i) the fact of the elimination of the probability of the
formation of a redox tautomer and ii) increase in energy of the ligand LUMO on moving the
electronegative group away from the pendant ring as well as an overall increase of electron
density in its deprotonated form as can be visualised from a DFT calculated energy
comparison diagram (Fig. 6). The latter reason also strengthens the prior.
Furthermore, attempts towards small molecule activation (O ) by chemical reduction of the
2
bridge (L) in the complexes were also not successful. This could also be comprehended on
9

Dalton Transactions
Page 10 of 32
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the same lines that the energy of the LUMO for the chosen ligand (L) increased to such a
DOI: 10.1039/D0DT03400B
high extent that any reductions on the complex were rather ruthenium, bpy or pap targeted in
the respective complexes 1, [2](ClO and [3](ClO (LUMO compositions in 1: 67%
participation of Ru; [2](ClO : 86% participation of bpy and [3](ClO : 85% participation of
pap) and thereby led to the decomposition of the complexes into unidentifiable products.
4
)
2
)
4 2
4
)
2
)
4 2
O
O
O
O
46135
)
24130
1
−
OH O
O
( c m
N
N
O
OH
O
-16900
E n e r g y
-21080
-
40600
-
42420
=
21340
= 23700
= 22005
∆
E
∆E
∆E
Fig. 6 Comparative energy levels of free ligands. Blue and red lines represent the HOMO and
LUMO levels, respectively. ΔE = HOMO−LUMO gap.
Electrochemistry and electronic structure: an experimental and theoretical case study
The bis-chelated diruthenium complexes 1, [2](ClO
4
)
2
and [3](ClO
4
2
) exhibited a series of
successive electron transfer responses upon being screened with cyclic voltammetry and
differential pulse voltammetry in acetonitrile within the accessible solvent window of ± 2V
(
Fig. 7, Table 1).
As previously mentioned, complex 1 exists as a diastereomeric pair, 1a (rac(ΔΔ/ΛΛ)) and
1b (meso(ΔΛ)), which is not uncommon. However, it has been quite recurrently observed
that such isomerism hardly impacts the electrochemical or electronic properties by much and
1
0

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DOI: 10.1039/D0DT03400B
R2
R2
R1
(
a)
*
O1
O2
R1
(
b)
R3
*
O1
R2
R1
O2
(
c)
O1
O2
R2
R1
(
d)
O1
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
E / V versus SCE
Fig. 7 Cyclic and differential pulse voltammograms for (a) 1a, (b) 1b, (c) [2](ClO
4
)
2
and (d)
[
3](ClO
4
)
2
in CH
3
CN.
Table 1 Electrochemical dataa
E°298[V] (ΔE[mV])b
K
c
c
d
e
f
g
O2
O1
R1
R2
R3
K
c1
K
c2
K
c3
K
c4
1
a
1.22 (60) 1.11(57)
1.22 (60) 1.11(57)
0.81(65) 0.71(66)
−0.49 (80)
−0.49 (80)
−1.12(70)
−0.68 (73)
−0.68 (73)
−
−
0.73×102 1.3×1027 1.6×103
0.73×102 1.3×1027 1.6×103
−
−
1
b
22+
−1.26(67) −1.78(120) 0.49×102 1.0×1031 2.3×102 6.5×108
32+
8.2×1034 1.0×1010
−
1.49(170) −0.27 (109) −0.86 (130)
−
aFrom cyclic voltammetry in CH
CN/0.1 M Et
NClO
/GC at 100 mVs . Potential in V
−1
b
3
4
4
c
versus SCE; peak potential differences ΔE[mV] (in parentheses). Comproportionation
1
7
d
e
f
constant from RTlnK = nF(ΔE). Kc1 between O2 and O1. Kc2 between O1 and R1. Kc3
c
g
between R1 and R2. Kc4 between R2 and R3.
1
6
the pair rather displays a similar behaviour, as can also be observed for 1a and 1b. The
responses marked by * in Fig. 7 (a and b) were observed repeatedly even after many attempts
of purification. Their exact nature or origin cannot be authenticated and may be ascribed to
some inherent electrode deposition for this set of complexes. Changing the scanning
directions also had no impact on the * marked responses and the cyclic voltammograms for
1
1

Dalton Transactions
Page 12 of 32
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1
a and 1b were essentially the same as that of the equilibrium scan. Dicationic complex
DOI: 10.1039/D0DT03400B
[
2](ClO
events shifted to more negative potentials in comparison to that of neutral 1 (For instance, the
first redox events O1/R1 were observed for 1 at 1.11/−0.49 V and for [2](ClO at 0.71/-1.12
V, respectively). On the other hand, more positively shifted potentials were observed for the
dicationic complex [3](ClO containing strongly π-accepting pap co-ligand with respect to
that of bpy in [2](ClO (For instance, the first redox events O1/R1 were observed for
2](ClO at 0.71/-1.12 V and for [3](ClO at 1.49/-0.27 V, respectively). The shifts in the
potentials on moving from complexes 1 to [2](ClO to [3](ClO may be attributed to an
interplay between the differing metal oxidation states and the π-acidity of the changing
ancillary ligands. The second oxidation for the complex [3](ClO could not be perceived
4
)
2
with a moderately π-accepting bpyridine terminal ligand shows multiple redox
)
4 2
)
4 2
)
4 2
[
4
)
2
)
4 2
4
)
2
)
4 2
)
4 2
within the solvent window due to its probable anodic shift as an effect of the strong π-acidic
nature of the pap ligands.
With this data in hand, attempts were made to explore the electronic distribution in the
varying accessible redox states (Scheme 2). In this context, considering together Mulliken
spin density (MSD) and molecular orbital (MO) compositions obtained from optimised
molecular geometries for reversible redox states (Fig. 8, Table 2 and Tables S8-S22†) and
EPR at the paramagnetic intermediate states (Fig. 3) guide in assigning the most suitable
oxidation state scenarios. Due to similar experimental properties of the diastereomic pair 1a
and 1b, the discussion shall be furthered considering the case of 1a, for maintaining clarity.
The EPR data for the complex 1 can be interpreted in terms of a triplet state and together with
III
III
MSD of around 0.801 at each ruthenium centre suggests a Ru /Ru configuration in the
−
1
+
native state (ES=1-ES=0 = 10360 cm , Table S23†). For the oxidised species 1a , no EPR
response could be recorded, which is not too unlikely, due to the more stable but
electronically complicated quartet state for the intermediate.10b DFT calculated spin densities
1
2

Page 13 of 32
Dalton Transactions
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DOI: 10.1039/D0DT03400B
+:
-L )Ru
2Ru µ-L )RuIVA ] 2
III
12
RuIV
2
IVA
2
µ
[
A
2
(
2
]
[A
e
(
2
=
2)
S
(
e
e
-
L )Ru
RuIII
-L )RuIIIA2]
µ
(
1+:
3/2)
RuIII
2
IVA2]
µ
[A
2
(
[A2
e
=
S
(
1
:
RuIII µ-L )Ru
2
III
=
1)
[(A)
2
(
(A)
2
]
S
(
e
e
−
II+0.5
II+0.5
:
RuIII µ-L )RuII
2
2
1
Ru (µ-L )Ru
[(A)
2
(
(A)
2
]
[(A)
2
(A)
2
]
=
1/2)
S
(
e
e
−
2
:
2
2
1
II
II
Ru µ-L )Ru
(A)
2
]
[(A)
2
(
=
1/2)
S
(
A = Ancillary Ligand
+:
23
II+0.5 -L )RuII+0.5
4
2Ru
(µ
A
2
]
[A
=
1)
S
(
e
e
23+
3
+
Ru µ-L )RuIIA2
II
:
II
2
III
3
3
/3
Ru µ-L )Ru A
(
2
]
[A
2
[A
2
(
]
=
1/2)
3
+
3+
S
(
(predominant for 2 )
(predominant for 3 )
e
e
+
+
22
2
2
:
/3
II
II
2
Ru µ-L )Ru A2
]
[A
2
(
=
0)
S
(
e
e
2+
+:
Ru µ-L )Ru A (A )]
A
2
(
II
2
II
/3
[
=
1/2)
S
(
e
e
2
S
/3:
II
2
II
µ-L )Ru A(A )]
=
1)
[(A )ARu (
(
e
e
−
:
2
II
2
II
µ-L )Ru
(A )
2
]
[
(A )ARu (
=
3/2)
S
Scheme 2 Electronic forms. Blue represents active site for the redox processes.
III
IV
point towards a metal based oxidation (Ru /Ru ) with slight participation from the ligand
(
that contains electron rich phenoxyl groups) which is also corroborated by the MO
−
composition of SOMO having significant ligand contribution (Ru/L/acac : 0.23/0.54/0.23),
1
3

Dalton Transactions
Page 14 of 32
III •−
III
View Article Online
DOI: 10.1039/D0DT03400B
suggestive of an alternate resonating electronic form (Ru /L /Ru ). Close by excited state
S=3/2- ES=1/2 = 285 cm−
1
Table S23†) is suggestive of the presence of antiparallel spin
(
E
,
18
alignment (up-up-down) in a three spin state situation. For these complexes, the mixed
valency through the mixed acceptor donor bridge may be an outcome of both the hole- and
16
2+
electron-transfer mechanisms operating simultaneously. The spin situation for 1a can also
IV
IV
III •−
IV
be rationalised as a resonating situation between Ru /Ru ↔Ru /L /Ru (S=2). Due to the
presence of ruthenium centres in relatively high valent oxidation state (III), the reductions are
primarily metal centred and rather delocalised as may be inferred from the Mulliken spin
3
density calculations. The comproportion constant (K
c
= 10 ) however suggests a more
localised situation particularly from electrochemical sense.10b
II
II
The closed-shell (S=0) bipyridine complex [2](ClO
4
) (Ru /Ru ) upon oxidation displays a
2
II
III
metal based rhombic EPR spectrum (corresponding to odd electron containing Ru /Ru
intermediate as the major form) which can be rationalised in terms of the competition
between the increased electron density at the metal and the increased π-acceptor
characteristic of the terminal ligand (with respect to the electron donating acetylacetonate
ancillary ligand). However, Mulliken spin density at the ligand may originate from a minor
alternate ligand-bound radical containing intermediate as depicted in Scheme 2. The second
oxidation leads to a delocalised radical bridged RuII+0.5/L /Ru
•−
II+0.5
mixed valent state.12a On
3
+
the other hand, the electrogenerated species, 3 displays a clean ligand based EPR response
with a g value of 2.000. The EPR spectra, Mulliken spin density and MO compositions
2
+
2+
unequivocally establish the reduction events on both the dicationic complexes (2 and 3 ) to
be co-ligand targeted (bipyridine and phenylazopyridine, respectively) as they possess
suitable π-acceptor orbitals to receive the incoming electron density. It is interesting to note
that the otherwise commonly reducible carbonyl groups in the diketo moiety remained
2+
unaltered upon reduction (vide supra). A second electron reduction of 3 leading to a ligand
1
4

Page 15 of 32
Dalton Transactions
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based EPR with a triplet signal (g1/2 = 3.972) as is expected for the second electron to reduce
DOI: 10.1039/D0DT03400B
another pap unit in 3 (S=1).
Table 2 DFT calculated (UB3LYP/LanL2DZ/6-31G*) Mulliken spin densities
acac−
0.834
0.515
0.200
Complex
Ru1
Ru2
L
bpy
pap
2
+
1
1
a (S=2)
1.227
1.050
1.216
0.967
0.802
0.838
0.472
0.200
-
-
-
-
-
-
a+ (S=3/2)
0
.801
1
1
a (S=1)
a− (S=1/2)
0.354
0.699
0.272
0.505
0.717
0.322
-0.003
-0.030
0.022
0.222
-0.042
-0.178
0.071
0.575
0.424
0.004
0.001
0.021
0.614
-0.002
-0.018
0.075
-
-
24+
23+
(S=1)
-
-
-
-
-
-
-
-
0.024
-0.013
1.027
2.069
2.995
-
-
(S=1/2)
-
2+
(S=1)
2−
(S=3/2)
33+
(S=1/2)
3+
(S=1/2)
(S=1)
(S=1/2)
0
.014
-
2
-0.031
0.049
-
-
0
.222
-0.045
1.081
2.224
-0.042
-0.178
-
3
-
12+
(S=2)
1− (S=1/2)
2 (S=1)
1+ (S=3/2)
1
(S=1)
2+
(S=1/2)
23+
24+ (S=1)
(S=1/2)
2−
(S=3/2)
33+(S=1/2)
3+(S=1/2)
3(S=1)
Fig. 8 Calculated Mulliken spin density plots for the various accessible redox states.
1
5

Dalton Transactions
Page 16 of 32
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DOI: 10.1039/D0DT03400B
UV-vis-NIR spectroelectrochemistry
Ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry (Figs. 9-11 and Figs.
S4a-c (on expanded scale for clearer visible region)) along with theoretical results assists in
the assignment and authentication of oxidation state combinations at the accessible redox
states (Scheme 2). The changing absorption characteristics of the complexes 1, [2](ClO
4
)
2
and [3](ClO were monitored through UV-vis-NIR spectroelectrochemistry using an
4 2
)
optically transparent thin-layer electrochemical (OTTLE) cell on electrochemically reversible
redox states with well defined isosbestic points. Attempts to assign origins of these transitions
have been made through time dependent density functional theory (TD-DFT) calculations
(
Table 3). However, since each absorption band constitutes several close lying transitions, the
discussion is therefore based on the key transitions. Several mixed metal-ligand charge
1
9
transfer transitions exist (Table S24†) primarily due to the inherent covalency factor arising
out of the high spin-orbit coupling constant of ruthenium (Ru(λ) = 1000 cm−1 4a,16).
The spectroscopic features and the following behaviour of the multiple redox states on
electrolysis of the diastereomeric complexes 1a/1b were nearly identical in nature. 1a in its
III
III
native Ru /Ru form exhibited ligand to metal charge transfer (LMCT) transitions in the
high energy region. Mixed MLCT (metal to ligand charge transfer) and LMCT transitions
+
2+
were observed for both the one electron and two electron oxidised forms, 1a and 1a ,
respectively, that may be originating from the resonating electronic configurations. Any
significant increase in intensity or a well defined absorption band in the low energy region,
arising due to mixed valency, could not be spotted, which is not uncommon for complexes
with acetylacetonate peripheries.20 Electronically reduced species 1a displayed many
transitions which enveloped LMCT transitions in the visible region along with a very weak
and broad increase in intensity in the NIR region corresponding to metal to metal charge
transfer (MMCT) transitions as an inter valence charge transfer (IVCT) band implicating the
−
1
6

Page 17 of 32
Dalton Transactions
II
III
View Article Online
DOI: 10.1039/D0DT03400B
existence of a localised Ru /Ru class II mixed valent situation. Further reduction on the
complex led to the extinguishing of the weak and broad NIR band corresponding to the
II
II
2−
formation of an isovalent diruthenim (Ru /Ru ) species, 1a .
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
+
+
1b
1a
1a
1b
ε
ε
4
00
800 1200 1600 2000
400
800 1200 1600 2000
λ / nm
λ / nm
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
+
2+
+
2+
1
a
1a
1b
1b
ε
ε
4
00
800 1200 1600 2000
400
800 1200 1600 2000
λ / nm
λ / nm
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
1a−
0.8
1
a
1b−
1
b
0.6
0.4
0.2
0.0
ε
ε
4
00
800 1200 1600 2000
400
800 1200 1600 2000
λ / nm
λ / nm
0.8
a− 1a2
−
b− 1b2
−
1
1
0.6
0.4
0.2
ε
ε
0.0
4
00
800 1200 1600 2000
400
800 1200 1600 2000
λ / nm
λ / nm
Fig. 9 UV-vis-NIR spectroelectrochemical response in CH
3
CN/0.1 M nBu
4
NPF
for 1n.
6
1
7

Dalton Transactions
Page 18 of 32
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DOI: 10.1039/D0DT03400B
0.6
0.4
0.2
0.0
2+
+
2
2
+
3
+
2
2
2
0.6
0.4
0.2
0.0
ε
ε
4
00
800 1200 1600 2000
4
00
800 1200 1600 2000
λ / nm
λ / nm
0.6
0.4
0.2
0.0
3
+
4+
1
.0
.8
.6
.4
+
2
2
2
2
0
0
0
ε
ε
0.2
0.0
4
00
800 1200 1600 2000
400
2
800 1200 1600 2000
λ / nm
λ / nm
2−
0
0
0
0
0
.8
.6
.4
.2
.0
ε
4
00
800 1200 1600 2000
λ / nm
Fig. 10 UV-vis-NIR spectroelectrochemical response in CH
3
CN/0.1 M nBu
4
NPF
for 2n.
6
1
8

Page 19 of 32
Dalton Transactions
0
0
0
0
.6
.4
.2
.0
View Article Online
DOI: 10.1039/D0DT03400B
2
+
3+
3
3
ε
4
4
4
00
00
00
800 1200 1600 2000
λ / nm
0.6
0.4
0.2
0.0
2
+
+
3
3
ε
800 1200 1600 2000
λ / nm
0.6
0.4
0.2
0.0
+
3
3
ε
800 1200 1600 2000
λ / nm
Fig. 11 UV-vis-NIR spectroelectrochemical response in CH
MLCT transitions targeted towards ligand and moderately π-accepting bipyridine are
observed for the dicationic complex, [2](ClO . For these set of accessible redox states, no
3
CN/0.1 M nBu
4
NPF
6
for 3n.
)
4 2
expected IVCT band in the NIR region was seen which could be interpreted in terms of
weaker electronic coupling between the metals. Although this is less common but a few
2
1
examples similar to this situation are documented in the literature. The successive oxidation
processes display the expected MLCT transitions whereas the reductions are mainly assigned
1
9

Dalton Transactions
Page 20 of 32
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DOI: 10.1039/D0DT03400B
to the L/bpy targeted LMCT transitions.
Unlike [2](ClO , the transitions in [3](ClO
4
)
2
4
) are MLCT transitions but solely to the π*-
2
3
+
orbital of the stronger π-acidic pap co-ligand. The one electron oxidised 3 , with ligand
centred oxidation, revealed a strong low energy band at 1033 nm with underlying MLCT and
intraligand (IL) transitions, largely targeted to the oxidised ligand. Upon successive
reductions in [3](ClO
4
2
) , a weak and broad increment in intensity is observed with first
reduction which develops into a strong and sharp band at 1009 nm. These low energy bands
originate from mostly IL transitions or mixed ligand metal to ligand transitions. The visible
range bands are of mixed MLCT or IL character but are pap targeted only.
Conclusion
Homometallic diruthenium complexes bridged by 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione
(
L) have been probed for redox non-innocence and any reactivity resulting from the diketo
fragment of the ligand. Conventional yet prototypical metal fragments consisting of co-
ligands with differing electronic properties, [Ru(acac) ], [Ru(bpy) ] and [Ru(pap) ] bis-
chelated around the ligand led to the formation of air stable 1a/1b, [2](ClO and [3](ClO
respectively. The following concluding remarks may be summed up from an extensive
experimental and theoretical analysis of the complexes 1a/1b, [2](ClO and [3](ClO
. The chelated ligand containing a phenoxyl moiety actively participates in reversible
2
2
2
4
)
2
4
2
) ,
4
)
2
4
2
) :
1
oxidation processes and thereby leads to resonating redox state formalisms. Thereby, this
redox active ligand can be unambiguously called as a redox non-innocent ligand in the
context of these redox non-innocent compounds. However, the ligand contribution on
oxidation in each complex is not the same. It is rather a function of two factors: (i) the
oxidation state of the metal and (ii) the differently donating/accepting capacities of the co-
ligands. Higher the oxidation state of the metal, more would be the probability of the ligand
to participate in the oxidation process. Whereas, higher the π-accepting capability of the
2
0

Page 21 of 32
Dalton Transactions
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terminal ligand, the more would the electron density of the metal be drifted towards the
DOI: 10.1039/D0DT03400B
ancillary ligand and hence the bridging ligand shall be available for a greater participation in
the oxidation process. This observation of differing ligand contribution upon oxidation in the
complexes 1a/1b, [2](ClO
4
)
2
and [3](ClO
4
2
) allows for the fine tuning of the redox potentials
by changing the terminal ligand handles.
2
. Though the diketo moiety has been previously reported to be fragile with respect to its
5
sensitivity towards air, these set of complexes with a judiciously chosen ligand yield stable
complexes. Thereby, with a slight re-designing or modification of the ligand framework, the
molecular properties may be controlled to such an extent that a fragile and reactive site may
be rendered stable.
These results are of fundamental interest as they extend the scope to investigate similar
ligand frameworks in coordination compounds and check for their varying response towards
stability/reactivity by changing the sterics and electronics in and around the ligand. Since
many of the redox active complexes find place in biomimetic chemistry or even catalysis,
more generalisations in this regard would be an added impetus.
2
1

Dalton Transactions
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Experimental section
DOI: 10.1039/D0DT03400B
Materials
2
2
Cl
,23 and cis−trans−cis-
The precursor complexes cis-Ru(acac)
2
(CH
3
CN)
2
, cis-Ru(bpy)
2
2
2
4
Ru(pap)
Oxalyl chloride and phenol were purchased from Sigma Aldrich. Phenol was added to a
suspension of AlCl in CH Cl followed by the addition of oxalyl chloride to yield 1,2-bis(2-
2
Cl
2
were prepared by following the previously reported literature procedure.
3
2
2
hydroxyphenyl)ethane-1,2-dione(L) following a reported literature procedure.25 Other
chemicals and solvents were of reagent grade and were used as received. For spectroscopic
and electrochemical studies, HPLC grade solvents were used.
Physical measurements
The electrical conductivity of solutions for the corresponding complexes 1, [2](ClO
4
2
) ,
[
3](ClO was checked using an autoranging conductivity meter (Toshcon Industries, India).
4 2
)
Cyclic voltammetric and differential pulse voltammetric measurements of the complexes
were done using a PAR model 273A electrochemistry system. Glassy carbon working,
platinum wire auxiliary electrode and saturated calomel reference electrode (SCE) were used
in a standard three-electrode configuration with tetraethylammonium perchlorate (TEAP) as
−
3
−1
the supporting electrolyte (substrate concentration ≈ 10 M; standard scan rate 100 mV s )
(
Caution! Perchlorate salts are explosive and should be handled with care). All
electrochemical experiments were carried out under dinitrogen atmosphere. The half wave
potential E°298 was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic
cyclic voltammetry peak potentials, respectively. The EPR measurements were made in a two
26
electrode capillary tube with an X-band (9.5 GHz) Bruker system ESP300 spectrometer.
UV-vis-NIR spectroelectrochemical studies were performed in CH
3
CN/0.1 M Bu
4
NClO
4
at
2
98 K using an optically transparent thin-layer electrode (OTTLE) cell27 mounted in the
1
sample compartment of a J&M TIDAS spectrophotometer. H NMR spectra were recorded
2
2

Page 23 of 32
Dalton Transactions
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on a Bruker Avance III 500 MHz spectrometer. The elemental analyses were recorded on a
DOI: 10.1039/D0DT03400B
Perkin-Elmer 240C elemental analyser. Electrospray mass spectral (ESI-MS) measurements
were done on a Bruker’s Maxis Impact (282001.00081) spectrometer.
Preparation of complexes
III
2−
III
Synthesis of [(acac)
2
Ru (µ-L )Ru (acac)
2
], 1a and 1b. The ligand L (31.75 mg, 0.13
II
mmol), the precursor cis-Ru (acac)
2
(CH
3
CN)
2
(100 mg, 0.26 mmol) and the base Et N (33
3
3
mg, 0.32 mmol) were refluxed in 20 cm of toluene under dinitrogen atmosphere for 5 h. The
solvent was evaporated to dryness under reduced pressure and the solid mass was subjected
to purification on a neutral alumina column, which eluted brown coloured products 1a and 1b
with 1:2 and 1:4 pet ether:CH
2
Cl
2
mixtures, respectively.
+
+
1
a: Yield, 50.54 mg, 46%. MS (ESI , CH
3
CN), m/z calcd for {1a} : 840.03; found: 840.13.
1H NMR (500 MHz, CDCl
, 298K): δ(ppm, J(Hz)): 19.05 (d, 1H), 17.55 (s, 1H), 1.85 (t, 2H),
3
1
.62 (m, 1H), 1.18 (d, 2H), 0.80(s, 2H), -3.91 (d, 5H), -10.28 (s, 1H), -17.92 (s, 2H), -26.15
−
1
2
-1
(
s, 1H). Λ
M
(Ω cm M ) in CH
3
CN at 298 K = 3. Anal. calcd for C34
H
36
O
12Ru : C, 48.68;
2
H, 4.33; found: C, 48.75; H, 4.30.
+
+
1
b: Yield, 39.56 mg, 36%. MS (ESI , CH
3
CN), m/z calcd for {1b} : 840.03; found: 840.07.
1H NMR (500 MHz, CDCl
, 298K): δ(ppm, J(Hz)): 18.88 (s, 1H), 1.84 (m, 4H), 1.10 (m,
3
4
H), -0.04 (s, 2H), -4.01 (s, 2H), -8.64(d, 1H), -12.51 (m, 2H), -17.65 (s, 1H), -26.36 (s, 1H).
−
1
2
-1
Λ
M
(Ω cm M ) in CH
3
CN at 298 K = 7. Anal. calcd for C34
H
36
O
12Ru : C, 48.68; H, 4.33;
2
found: C, 48.76; H, 4.30.
II
2−
II
Synthesis of [(bpy)
2
Ru (μ-L )Ru (bpy)
2
](ClO
4
)
2, [2](ClO
4
2
) .The precursor cis-
Ru(bpy)
2
Cl
2
(100 mg, 0.21 mmol) and AgClO
4
(93 mg, 0.45 mmol) were taken in a 30 cm3
of EtOH and heated to reflux for 2 h for dechlorination. The precipitated AgCl was removed
by filtering of the mixture through a sintered Gooch crucible. The filtrate was mixed with a
3
2
0 cm of a hot ethanolic solution of L (24.9 mg, 0.10 mmol) and Et
3
N (31.36 mg, 0.3 mmol)
2
3

Dalton Transactions
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and heated to reflux for 12 h under dinitrogen atmosphere. The solvent was evaporated to
DOI: 10.1039/D0DT03400B
dryness and the product was purified using a neutral alumina column, and the dark pink
complex [2](ClO
4
)
2
was eluted by a CH
2
Cl
2
:CH CN (3:1) solvent mixture.
3
+
CN), m/z calcd for {[2]ClO
}+: 1168.13;
[
2](ClO
4
)
2
: Yield 91.69 mg, 70%. MS (ESI , CH
3
4
1
found 1168.16. H NMR (500 MHz, CD
3
CN, 298K): δ(ppm, J(Hz)): 8.95 (d, 2H), 8.84 (d,
2
3
1
H), 8.65 (d, 2H), 8.64 (d, 2H), 8.57 (d, 2H), 8.43(d, 2H), 8.29 (t, 2H), 8.18 (t, 2H), 7.91 (m,
H), 7.82 (d, 3H), 7.77 (t, 2H), 7.71 (t, 1H), 7.49(t, 2H), 7.26 (m, 3H), 7.16 (t, 2H), 7.12 (d,
−
1
2
-1
H), 6.92 (d, 2H), 6.73 (t, 3H), 6.67 (t,1H), 6.38 (s, 1H). Λ
M
(Ω cm M ) in CH
3
CN at 298
K = 262. Anal. calcd for C54
H
40
O
12
N
8
Ru
2
Cl
2
: C, 51.23; H, 3.18; N, 8.85; found: C, 51.51; H,
3
.30; N, 8.98.
II
2−
II
Synthesis of [(pap)
2
Ru (μ-L )Ru (pap)
2
](ClO
4
)
2, [3](ClO
. The precursor cis-trans-cis-
(100 mg, 0.19 mmol) was dechlorinated with AgClO (83 mg, 0.40 mmol) in 30
4
)
2
. Complex [3](ClO
4
) was
2
synthesised by following the similar protocol for [2](ClO
)
4 2
Ru(pap)
2
Cl
2
4
3
cm of EtOH by reflux for 2 h. The precipitated AgCl was filtered off through a sintered
3
Gooch crucible. The filtrate was added to 20 cm of a hot ethanolic solution of the ligand L
(
34 mg, 0.095 mmol) and base Et
atmosphere for 12 h. Post removal of solvent under vacuum, the reaction mixture was
chromatographed on a neutral alumina column. The pink compound [3](ClO was eluted by
a CH Cl :CH CN (2:1) solvent mixture.
3](ClO
3
N (20 mg, 0.20 mmol) and refluxed under dinitrogen
)
4 2
2
2
3
+
CN), m/z calcd for {[3]ClO
}+: 1274.10;
[
4
)
2
: Yield 85.2 mg (67%). MS (ESI , CH
3
4
1
found 1274.07. H NMR (500 MHz, (CD
3
2
) SO, 298K): δ(ppm, J(Hz)): 9.08 (m, 1H), 9.02
(
(
m, 1H), 8.47 (m, 2H), 8.10 (m, 1H), 7.78 (t, 1H), 7.71(t, 1H), 7.63 (d, 1H), 7.49 (q, 3H), 7.32
−
1
2
-1
m, 6H), 6.89 (d, 2H), 6.77 (d, 1H), 6.69 (m, 1H), 6.59(d, 1H). Λ
M
(Ω cm M ) in CH
3
CN at
2
5
98 K = 230. Anal. calcd for C58
0.95; H, 3.29; N, 12.41.
H
44
O
12
2
N12Ru Cl
2
: C, 50.70; H, 3.23; N, 12.23; found: C,
2
4

Page 25 of 32
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Crystal structure determination
Single crystals were obtained by slow evaporation of 1:1 solution of CH
:1:1 solution of CH OH-CH CN-C for [2](ClO and [3](ClO , respectively. X-ray
diffraction data were collected using a Rigaku Saturn-724+ CCD single crystal diffractometer
using Mo-K radiation. The data collection was evaluated by using the CrystalClear-SM
DOI: 10.1039/D0DT03400B
2
Cl
2
-CH CN and
3
1
3
3
6
H
6
4
)
2
)
4 2
α
Expert software. The data were collected by the standard ω-scan technique. The structure was
solved by direct method using SHELXT-2015a and refined by full matrix least-squares with
2
28
SHELXL-2018/3, refining on F . All data were corrected for Lorentz and polarization
effects and all non-hydrogen atoms were refined anisotropically. The remaining hydrogen
atoms were placed in geometrically constrained positions and refined with isotropic
temperature factors, generally 1.2 Ueq of their parent atoms. Hydrogen atoms were included in
the refinement process as per the riding model. The disordered groups in [2](ClO
3](ClO were modelled using PART command. In addition to the benzene solvate
molecule that can be seen in the data of [2](ClO , there are two other benzene solvent
molecules present. However they are both disordered and removed applying SQUEEZE using
4
)
2
and
[
)
4 2
)
4 2
2
9
PLATON package to achieve better convergence. The Cambridge Crystallographic Data
Centre (www.ccdc.cam.ac.uk/data_request/cif) using the CCDC numbers 2026821 and
2
026822 for [2](ClO
Computational details
Full geometry optimisations were carried out using the density functional theory method at
4
)
2
and [3](ClO
4
)
2
,respectively.
n
n
n
n
the (U)B3LYP level for 1a /1b (n = 2+, 1+, 0, 1−), 2 (n = 4+, 3+, 1+, 0, 1−), 3 (n = 3+, 1+,
n
n
n
n
30
0
) and (R)B3LYP for 1a /1b (n = 2−),2 (n =2+), 3 (n =2+). All elements except ruthenium
were assigned the 6-31G(d) basis set. The LanL2DZ basis set with effective core potential
was employed for the ruthenium atom.31 All calculations were performed with the
3
2
Gaussian09 program package. Vertical electronic excitations based on (U)B3LYP optimised
2
5

Dalton Transactions
Page 26 of 32
View Article Online
geometries were computed using the time-dependent density functional theory (TD-DFT)
DOI: 10.1039/D0DT03400B
3
3
34
formalism in acetonitrile using the conductor-like polarizable continuum model (CPCM).
3
5
Chemissian 1.7 was used to calculate the fractional contributions of various groups to each
molecular orbital. All calculated structures were visualised with ChemCraft.36
Acknowledgements
Financial support received from the Science and Engineering Research Board (SERB,
DST), J. C. Bose Fellowship (to G.K.L., SERB), New Delhi, India, DAAD-Germany and IIT
Bombay (fellowship to F.F.K), is gratefully acknowledged. The help of Prof. R. J. Butcher,
Department of Chemistry, Howard University, USA, in solving the crystal structures of
[
2](ClO
4
)
2
and [3](ClO
4
2
) , is also gratefully acknowledged.
Conflict of interest
The authors declare no competing financial interest.
2
6

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