Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer

Article abstract of DOI:10.1021/acs.inorgchem.8b01558

In this work, the effect of the electronically different ancillary ligands on the overall properties of the Ru^IIL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic complexes of general formula [RuLQCl]^0/+ was investigated. Four different ancillary ligands (Q) with different electronic effects were used to prepare the heteroleptic compounds from the precursor complex, [RuL(CH₃CN)Cl₂] (1); Q = pcp: 2-(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]⁺ bpy: 2,2′-bipyridyl (moderate π-acceptor), [3]⁺ acac^-: acetylacetonate (strong σ-donor), 4; and DTBCat^2-: 3,5-di-tert-butyl catecholate (strong π-donor), 5. The complexes [2]⁺, [3]⁺, 4, and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic paramagnetic resonance, UV-vis-NIR spectroscopy, and density functional theory. The studies revealed significant effects of the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic effect of the coligand. Accordingly, the complex [2]⁺ (containing a strong π-acceptor coligand, pcp) brings about oxidation efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating coligand) furnished only 15-20% of benzaldehyde under identical reaction conditions. Investigations of the reaction mechanism suggest that an unstable Ru-H species is formed, which is transformed to a Ru-hydrazo intermediate by H-walking as reported by Hall et al. (J. Am. Chem. Soc., 2015, 137, 12330). Aerial O₂ regenerates the catalyst via oxidation of the hydrazo intermediate.

Full text of DOI:10.1021/acs.inorgchem.8b01558

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Effects of Ancillary Ligands on Redox and Chemical Properties of
Ruthenium Coordinated Azoaromatic Pincer
Santi Prasad Rath,† Debabrata Sengupta,† Pradip Ghosh,†,‡ Rameswar Bhattacharjee,#
Mou Chakraborty,† Subhas Samanta,†,§ Ayan Datta,# and Sreebrata Goswami*,†
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
#Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
S
* Supporting Information
ABSTRACT: In this work, the effect of the electronically
different ancillary ligands on the overall properties of the
RuIIL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic
complexes of general formula [RuLQCl]0/+ was investigated.
Four different ancillary ligands (Q) with different electronic
effects were used to prepare the heteroleptic compounds from
the precursor complex, [RuL(CH3CN)Cl2] (1); Q = pcp: 2-
(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]+; bpy:
2,2′-bipyridyl (moderate π-acceptor), [3]+; acac−: acetylacet-
onate (strong σ-donor), 4; and DTBCat2−: 3,5-di-tert-butyl
catecholate (strong π-donor), 5. The complexes [2]+, [3]+, 4,
and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox
partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic
paramagnetic resonance, UV−vis−NIR spectroscopy, and density functional theory. The studies revealed significant effects of
the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these
Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic
effect of the coligand. Accordingly, the complex [2]+ (containing a strong π-acceptor coligand, pcp) brings about oxidation
efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating
coligand) furnished only 15−20% of benzaldehyde under identical reaction conditions. Investigations of the reaction
mechanism suggest that an unstable Ru−H species is formed, which is transformed to a Ru−hydrazo intermediate by H-walking
as reported by Hall et al. (J. Am. Chem. Soc., 2015, 137, 12330). Aerial O2 regenerates the catalyst via oxidation of the hydrazo
intermediate.
duced26,33 a pincer azo-aromatic ligand, 2,6-bis(phenylazo)-
pyridine (L), which in principle can accommodate up to either
INTRODUCTION
■
Over the past two decades, transition metal complexes of the
redox active ligands have drawn significant attention due to their
inherent ability to acquire multiple redox states compared to
four electrons or four electrons plus four protons without
rupture of −N−N− bonds. We showed previously33,34 that
coordination complexes of mere spectator ligands.1−5 As a
Ni(II) and Zn(II) complexes of L are capable of mimicking the
enzymatic pathway for catalytic alcohol oxidation using
primarily ligand-based redox couple (−NN− + 2e− + 2H+
↔ −NH−NH−).
result, this class of compounds is now at the focus of several real-
world applications ranging from development of efficient
chemical catalysts6−9 and materials10−12 for next-generation
digital and optical technologies. The reactivity of metal
complexes bearing redox-active ligands primarily depends on
the nature of metal center,13,14 electronic structure of the ligand
backbone,15−17 and secondary coordination sphere of the
complex.18 Strong metal−ligand π-interactions and redox
activities of the coordinated ligands together modulate the
In this work, we explored the effect of ancillary ligands on the
properties of RuL moiety in a hexacoordinated framework,
[RuLQCl]0/+ (Chart 1). Four ancillary ligands (designated as
Q) of different character (strongly π-accepting 2-(4-
chlorophenylazo)pyridine (pcp), weakly π-accepting 2,2′-
bipyridine (bpy), σ-donating acetylacetonate (acac−), and
strong π-donating 3,5-di-tert-butyl catecholate (DTBCat2−)
ligands) were chosen and allowed to coordinate at the Ru center
in the RuL moiety. We hypothesized that the ancillary ligands, in
́
overall electronic structures vis-a-vis chemical behaviors of such
complexes.
We and others19−32 have long-standing interest in the
coordination chemistry of redox active azo-aromatic ligands.
The neutral azo-aromatics have low lying π* orbital which can
strongly participate in back-bonding as well as can form metal
stabilized azo-anion radical complexes. Recently, we intro-
Received: June 6, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01558
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Article
Chart 1
conjunction with completing the coordination sphere, would
exert tuning effect on their overall physicochemical activities.
Accordingly, the primary aim of this paper is to follow the
ancillary ligand effect on the metal ligand cooperativity between
Ru(II) and the principal ligand L. Besides studying the usual
metal−ligand interplay in determining the electronic structures
of the heteroleptic complexes,35,36 we further scrutinized the
tuning effect on their chemical reactivity.37 We thus set out to
study aerial catalytic oxidation of benzyl alcohol using the above
complexes. An interesting trend was observed: under identical
reaction conditions, the compound with strongest π-acceptor
ligand pcp produced the corresponding aldehydes in an
excellent yield (>85%), whereas the compound with π-donor
ligand, (DTBCat2−), produced the product only in 15% yield.
Mechanistic studies revealed that the coordinating ligand L
directly participates in the hydrogen abstraction process from
alcohol using azo/hydrazo redox couple.
reactions between the complex 1 and the corresponding ligand
(Q) (Chart 1) under refluxing conditions using either
acetonitrile or methanol as solvent. Synthetic procedures
along with their characterization data of the isolated compounds
are collected in the Experimental Section. Micro analytical data,
ESI-MS, and NMR spectral data support their formulations. The
ESI-MS spectra of the complexes [Ru(L)(pcp)Cl]PF6, [2]PF6
and [Ru(L)(bpy)Cl]PF6, [3]PF6 displayed intense molecular
ion peaks at m/z 641 and 580 amu, respectively (Figures S1 and
S2). On the other hand, the complex [Ru(L)(acac)Cl], 4
showed an intense peaks at m/z 488 amu due to (4 − Cl)+
fragment (Figure S3) and complex [Ru(L)(DTBsq•−)Cl], 5
showed two intense peaks at m/z 644 and 424 amu due to (5)+
and (5 − DTBsq•−)+ fragments (Figures S4a and b). All of these
1
isolated complexes except 5 are diamagnetic. The H NMR
resonances of these compounds are overlapping, and so the
unambiguous assignment of individual proton resonance was
not possible. However, proton counts in the spectrum of each of
the compounds corroborated well with the formulation of
RESULTS AND DISCUSSION
■
1
products (Figures S5−S7). Characteristic H NMR signals of
Synthesis and Characterization. The reaction of one
equivalent ligand (L) with RuCl3·xH2O in refluxing ethanol
resulted in a greenish brown compound. The crude mass upon
recrystallization by slow evaporation of its acetonitrile solution
produced a green product of composition Ru(L)(CH3CN)Cl2
(1). Electrospray ionization mass spectrometry (ESI-MS) of the
complex 1 showed an intense peak at m/z 460 amu, which
corroborates with the formulation of (1 − (CH3CN) + H)+. The
complex 1 is diamagnetic at room temperature and showed a
resolved proton NMR spectrum. The aromatic proton
resonances appeared in the region 8.59−7.51 ppm, and a
sharp singlet at 2.70 ppm is noted due to the coordinated
CH3CN moiety. The heteroleptic ruthenium complexes [Ru-
(L)(Q)Cl]0/+ (0 for anionic ligands acac− and DTBsq•−, one
electron oxidized form of DTBcat2−, and + for neutral ligands
pcp and bpy) were prepared as stable compounds from the
acetyl group in the complex 4 appeared at 2.53 and 1.26 ppm as
two separate singlets for the two methyl groups, and one singlet
resonance appeared at 5.08 ppm for the γ-C−H proton; the
13C{1H} NMR spectrum of the compound 4 displayed two
carbonyl carbon resonances at 190.4 and 187.2 ppm. NMR data
of all compounds are included in the Experimental Section, and
their spectra are submitted as Supporting Information (Figures
S5−S7). Solution-state magnetic susceptibility measurement of
the compound 5 by Evans method indicated that it is one
electron paramagnetic38 with μeff (298 K) = 1.68 μB.
Paramagnetic ground state of the complex 5 was further
characterized by its electron paramagnetic resonance (EPR)
spectrum (see below).
Crystal Structure Analysis. All of the molecules ([2]PF6−
5) described herein are crystalline, and their structures were
B
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solved by three-dimensional single crystal X-ray diffraction
analyses. Suitable crystals of these complexes were developed by
slow diffusion of their dichloromethane solution into hexane.
Molecular views along with the atom numbering schemes are
shown in Figure 1. Their ORTEP representations are presented
The ligand L is a strong π-acceptor, and metal to ligand
[Ru(II)dπ − π*(azo)] back bonding in its complexes is a
common phenomenon.24,26 X-ray crystallographic analyses of
the two cationic complexes [2]+ and [3]+ reveal significantly
distorted octahedral geometry about the ruthenium center. The
chelate bite angles are considerably smaller compared to that in
ideal octahedral geometry. The average value of chelate bite
angles of Nazo(L)−Ru−Npy(L) is 76.24° for complex [2]+ and
76.06° for [3]+. In the complexes [2]+ and [3]+, the bite angles
of the ancillary ligands are 75.56° and 77.32°, respectively. In
complex [2]+, the two pyridyl nitrogen atoms of the ligands L
and pcp are disposed trans to each other. In complex [2]+, three
N−N bond lengths are 1.289(11), 1.286(10) (for ligand L), and
1.287(10) Å (for ligand pcp), while the dN−N lengths of complex
[3]+ are 1.299(2) and 1.293(2) Å (Table 2). The two dN−N bond
lengths in the complex 4 are 1.309(3) and 1.300(2) Å. The Ru−
O bond lengths in complex 4 are 2.0428(17) and 2.0821(15) Å,
which are commonly observed39−43 in the other Ru(II)−acac
complexes. The ligand DTBCat coordinates via deprotonation
and can exists in different oxidation states (catecholate,
semiquinone, and quinonoid), C−O bond length of the
coordinated ligand is an indicator44−48 of its oxidation state.
In the complex 5, the C−O bond lengths are 1.293(4) and
1.303(4) Å, which are characteristic48 for the semiquinone
oxidation state of the coordinated ligand. Moreover, the bond
lengths of C18−C19, C19−C20, C20−C21, C21−C22, C22−
C23, and C23−C18 of the phenyl ring of the DTBCat ligand are
1.410(5), 1.365(5), 1.435(5), 1.365(5), 1.439(5), and 1.439(5)
Å respectively. The calculated metric oxidation state (MOS)
value49 is −1.141 (esd 0.0799) indicates the semiquinone state
of the coordinated DTBCat ligand. Two Ru−O bond lengths in
this complex are 2.078(2) Å (trans to Npy of L) and 2.023(2) Å
(trans to Cl atom), respectively. The dN−N of coordinated L are
1.299(4) and 1.300(4) Å, respectively. The dianionic DTBCat2−
ligand was anticipated to have strongest electron donor ability
via σ- and π-donation, but its one electron oxidized semiquinone
Figure 1. Molecular views of the isolated complexes: [2]+, [3]+, 4, and
5. Hydrogen atoms for all the complexes and the counteranion for [2]+
and [3]+ are omitted for clarity.
in Figures S8−S11. All of them adopt distorted octahedral
geometry with the N∧N∧N pincer ligand L coordinating
meridionally in an almost planar configuration.
Table 1. Crystallographic Details of the Complexes [2]PF6, [3]PF6, 4, and 5
compound
CCDC no.
[2]PF6
1846398
[3]PF6
1846399
4
5
1846400
1846401
empirical formula
formula weight
crystal system
space group
C28H21Cl2F6N8PRu
786.47
monoclinic
P21/c
C27H21ClF6N7PRu
725.00
triclinic
C22H20ClN5O2Ru
522.95
monoclinic
P21/c
C31H33ClN5O2Ru
644.14
triclinic
P1
̅
P1
̅
a (Å)
b (Å)
c (Å)
α
8.1668(14)
29.650(5)
12.725(2)
90
95.997(5)
90
8.1792(5)
11.6400(7)
15.7389(10)
87.749(2)
79.667(1)
71.001(2)
2
1393.54(15)
1.728
0.789
13.2910(18)
9.8527(13)
16.737(2)
90
93.249(3)
90
8.7710(11)
9.9940(13)
17.362(2)
83.592(4)
80.279(3)
82.210(4)
2
1480.1(3)
1.445
0.656
β
γ
Z
4
4
V (Å3)
3064.4(9)
1.705
0.810
2188.2(5)
1.587
0.868
D
cal (g/cm3)
μ (mm−1
T (K)
)
293
293
293
293
θ range (deg)
GOF
1.4−27.3
1.04
1.9−32.4
1.18
1.5−27.5
0.80
1.2−27.5
0.84
no. of reflections collected
no. of unique reflections
final R indices ((I > 2σ(I))
R1, wR2
42 137
6945
0.0901
0.2330
22 350
8443
0.0296
0.0816
23 416
4976
0.0251
0.0853
19 992
6512
0.0396
0.1075
C
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Table 2. Selected Bond Distances of the Complexes [2]PF6, [3]PF6, 4, and 5
π-accepting ligands
σ/π-donating ligands
[RuL(acac)Cl]; 4
[RuL(DTBsq•−)Cl]; 5
[RuL(pcp)Cl]PF6; [2]PF6
[RuL(bpy)Cl]PF6; [3]PF6
expt calc
effects on 2,6-bis(phenylazopyridine) ligand (L)
bond parameters
expt
calc
expt
calc
expt
calc
Ru−N1
Ru−N3
Ru−N5
N1−N2
N4−N5
Ru−Cl1
2.044(7)
1.904(6)
2.089(6)
1.289(11)
1.286(10)
2.367(3)
2.140
1.945
2.112
1.283
1.288
2.399
2.0204(15)
1.9075(15)
2.1038(15)
1.299(2)
1.293(2)
2.3653(7)
2.117
1.934
2.120
1.289
1.287
2.399
2.0440(19)
1.8853(18)
2.0852(17)
1.309(3)
1.300(2)
2.3583(7)
2.091
1.898
2.091
1.296
1.296
2.393
2.087(3)
1.894(3)
2.067(3)
1.299(4)
1.300(4)
2.3489(10)
2.097
1.902
2.097
1.296
1.296
2.385
effects on ancillary coligands (Q)
Ru−N6
Ru−N7
2.079(8)
2.068
2.1087(15)
2.0476(16)
2.146
2.103
Ru−N8
N7−N8
Ru−O1
2.015(6)
1.287(10)
2.115
1.282
2.0428(17)
2.0821(15)
1.278(3)
1.267(3)
1.389(3)
1.394(3)
2.081
2.120
1.283
1.272
1.399
1.410
2.023(2)
2.078(2)
1.293(4)
2.123
2.061
1.300
Ru−O2
O1−C18
O2−C20
C18−C19
C19−C20
O2−C23
C18−C23
2.146
2.103
2.117
1.303(4)
1.439(5)
1.304
1.450
state (DTBsq•−) essentially acts as comparatively weak π-
donating ligand.
Being a redox-active ligand, L can adopt multiple oxidation
states upon coordination to the metal center. Such complexes
exhibit ligand-based redox processes which were influenced by
the metal-to-ligand π-back-donations. Presence of the ancillary
́
ligands of different nature influence the back-donation and vis-a-
vis the redox properties of both metal and ligand (see later).
Crystallographic details and selected bond lengths of the
complexes [2]+, [3]+, 4, and 5 are collected in Tables 1 and 2.
Cyclic Voltammetry and EPR Spectroscopy. Cyclic
voltammograms of the isolated complexes [2]+ and [3]+ were
investigated in acetonitrile, whereas those of 4 and 5 were
studied in dichloromethane solvent containing 0.1 M
tetrabutylammonium perchlorate (TBAP) as the supporting
electrolyte. In all cases, platinum was used as the working
electrode and Ag/AgCl as reference electrode. E1/2 of the
ferrocene/ferrocinium couple appeared at 0.4 V under the
experimental conditions. Their cyclic and differential pulse
voltammograms are displayed in Figure 2, and the results are
summarized in Table 3. All of these complexes showed multiple
single electron redox processes in the potential window of 2.5
V.
Coordination of more than one redox active ligand to metal
center is reflected in the cyclic voltammograms of the complexes
[2]+ and [3]+. Each of these complexes exhibits 5 single electron
transfer waves: for [2]+, 5 redox waves appeared at 1.70, 0.08,
−0.42, −0.81, and −1.35 V, while those for complex [3]+
appeared at 1.43, −0.18, −0.70, −1.44, and −1.69 V. In the
case of complex [2]+, the wave that appeared at 0.08 V is
reductive, as evidenced by exhaustive electrolysis at −0.25 V.
Electrochemically generated one electron reduced complex 2
showed a rhombic EPR spectrum (Figure 3(a) and Table 4) at
gav = 1.974 (g1 = 2.003, g2 = 1.974, g3 = 1.945) with notable g
anisotropy,50,51 Δg = 0.058, indicating a ligand-based EPR
spectrum. A similar situation is also observed in the case of
coulometrically generated complex 3 (Figure 3(c) and Table 4) .
Figure 2. Cyclic (black) and differential pulse (red) voltammograms of
the complexes [2]+−5.
In this case, the signal appeared at gav = 1.967 (g1 = 2.015, g2 =
1.957, g3 = 1.929) with Δg = 0.084. Similarities in the EPR
spectrum indicate that the locus of reduction is same in both the
cases. This is further supported by DFT and spectro-electro-
chemical studies (see below). The 2e− reduced complex [2]− is
D
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a
a
Table 3. Cyclic Voltammetry Data
Table 4. EPR Data Table
b
c
b
oxidation E1/2 V
(ΔEp, mV)
complex
2
[2]−
3
[4]−
5
[5]2−
b
complex
reduction E1/2 V (ΔEp, mV)
g1
g2
g3
⟨g⟩
Δg
2.003
1.974
1.945
1.974
0.058
2.009
1.972
1.938
1.973
0.071
2.015
1.957
1.929
1.967
0.085
2.023
1.916
2.002
1.987
1.973
1.987
0.029
[2]PF6 0.08 (70), −0.42(78), −0.81(69),
−1.35 (73)
[3]PF6 −0.18 (86), −0.70 (116), −1.44 (63),
1.70 (126)
c
1.43 (172)
d
e
g
c
1.969
0.107
1.992
−1.69 (143)
f
4
5
−0.49 (130)
1.16 (132)
0.73(67), 1.78 (158)
a
c
−0.13 (64), −0.50 (64), −0.87 (86)
Unless noted otherwise, EPR data were obtained for CH3CN−
toluene glass (containing [Bu4N]ClO4 as supporting electrolyte),
measurements at 77 K. EPR data were obtained for CH2Cl2,
measurements at 77 K (containing [Bu4N]ClO4 as supporting
a
Conditions: cyclic voltammetry was performed in acetonitrile
solution; for complexes [2]PF6 and [3]PF6 and in dichloromethane
solution; for complexes 4 and 5. Supporting electrolyte [Bu4N]ClO4,
working electrode: Platinum, reference electrode: Ag/AgCl, scan rate:
E1/2= (Epa + Epc)/2, where Epa and Epc are anodic and
cathodic peak potentials respectively, ΔEp= (Epa - Epc) (in mV).
b
c
electrolyte). EPR data were obtained for CH2Cl2, measurements at
d
e
77 K. Unless noted otherwise, ⟨g⟩ = [1/3(g12 + g22 + g3 )]1/2. ⟨g⟩ =
2
b
50 mV/s.
f
g
[1/2(g1 + g2 )]1/2. Δg = g1 − g3. No g anisotropy observed.
2
2
c
Quasireversible.
value, Δg = 0.11 (Figure 3(d) and Table 4). Such a high value of
Δg is an indication of high metal-azoaromatic L interactions.
The magnitude of Δg among the series of the complexes [2]+,
[3]+, and 4 are as follows: complex 4 (Δg = 0.11) > complex [3]+
(Δg = 0.084) > complex [2]+(Δg = 0.058). In complex 4,
Ru(dπ)-Lazo(pπ) back-donation is maximum, and the effect is
reflected on the reduction potential of the coordinated ligand L
also EPR active and showed a broad ligand-based spectrum with
gav = 1.973 (g1 = 2.009, g2 = 1.972, g3 = 1.938) (Figure 3(b) and
Table 4). Interestingly, the similar complex [3]− is EPR inactive.
Here, we anticipate that in the complex [2]+, the second
reduction occurs at the ancillary ligand (pcp), resulting in an S =
1 ground state. However, our repeated attempts to identify the
“spin forbidden” signal11,12 (from S = 1 ground state of the
complex [2]−) were unsuccessful. The second reduction in the
complex [3]+, on the other hand, occurs at the same ligand L,
generating a spin coupled system,52−54 [3]−. Our attempts to
study the EPR spectra of further reduced species of the
complexes [2]+ and [3]+ were unsuccessful due to their hyper
reactivity.
red
(E1/2 = 0.49 V) as discussed above. An opposite effect was
observed in the cases of complex [2]+ and [3]+. The ligands pcp
and bpy are both π-acceptors, and so the extent of π-back-
donation to coordinated L in the two complexes are lower. In all
these three complexes, the first reduction occurs at the
coordinated L, which varies systematically as follows: 0.08 V
([2]+) < −0.18 V ([3]+) < −0.49 V (4). While making this
comparison of redox potential, we must note here that the
overall charges of all complexes, discussed herein, are not
identical. The compound 4 is a molecular compound, while the
other two ([2]+ and [3]+) are monocationic. However, they are
all ruthenium(II) complexes, which are isostructural and
The complex 4 exhibits a cathodic response at −0.49 V and an
anodic response at 1.16 V. To ascertain the locus of reduction,
we performed exhaustive electrolysis at −0.6 V of the complex 4.
The reduced complex [4]− exhibits an EPR spectrum at gav
=
1.970 (g1 = 2.023, g2 = 1.916) with significant g anisotropic55,56
Figure 3. EPR spectra of the electrogenerated species (a) 2, (b) [2]−, and (c) 3 at 77 K in acetonitrile-toluene glass; (d) [4]− and (f) [5]2− in
dichloromethane at 77 K; and isolated complex (e) 5 at 77 K in dichloromethane.
E
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isoelectronic compounds, and hence, the comparison appears
reasonable.57 The reduction potential of the complex containing
an ancillary ligand with maximum π-acceptor ability occurs at
lower potential which monotonically shifted cathodic with
increasing donor ability of ancillary ligand.
observed on the L backbone (see Figures 4a,b). Notably,
electrogenerated complex [2]− is also EPR active, which is in
An opposite trend, however, was observed in the oxidation
potential of the above four complexes. In all of these complexes,
oxidation is primarily metal centered (RuII/RuIII), which is
supported by DFT calculations (see Supporting Information,
Theoretical Calculations), and the trend is as anticipated: 1.69 V
([2]+) > 1.42 V ([3]+) > 1.16 V (4). Thus, both L and the
ancillary ligand pcp being strong π-acceptor ligand in the
complex [2]+, the Ru−L delocalization is least; the main ligand L
is most redox noninnocent, and the metal is most electron
deficient among the series of the complexes [2]+, [3]+, and 4.
Electrochemical behavior of the complex 5 is somewhat
different than that of the previously described complexes viz.,
[2]+, [3]+, and 4. It is isolated as a stable free radical complex,
[RuL(DTBsq•−)Cl], which is paramagnetic and displayed a
typical rhombic EPR spectrum (Figure 3(e) and Table 4) for a
Ru(II) coordinated semiquinone radical58 with gav = 1.987 (g1 =
2.002, g2 = 1.987, g3 = 1.973) and with Δg = 0.029. It exhibits
three reductive waves at −0.13, −0.50, and −0.87 V and two
oxidative waves appeared at 0.73 and 1.78 V. Exhaustive
electrolysis at −0.35 V leads to a diamagnetic compound [5]−
due to the reduction of DTBsq•− ligand. This corroborates well
by Density Functional Theory (DFT) calculations (see below).
The two electron reduced complex ([5]2−) is EPR active and
showed a typical azo-anion ligand-based single line EPR
spectrum26 at g = 1.992, indicating the reduction of coordinated
azo-aromatic L (see Figure 3(f) and Table 4). The one electron
oxidized complex [5]+ is EPR inactive, and DFT calculations
suggest that the first oxidation occurs at DTBsq•− ligand.
Density Functional Theory (DFT) Studies. To have an
insight into the electronic description of the isolated complexes,
we performed full geometry optimization using DFT at the
B3LYP level.59−62 The ground state of the complexes [2]+, [3]+,
and 4 are diamagnetic (S = 0), as evidenced by the room
temperature magnetic moment, and thus, optimizations were
performed for the singlet state only. The computed bond
distances were collected in Table 2 and are found to be in good
agreement with those obtained in the X-ray crystallographic
analysis.
DFT calculations for these ruthenium complexes, considering
multiple valence states were performed to discount their
alternative electronic structures. Owing to the presence of two
redox active ligands, the electronic structure landscapes for these
complexes are rather more complex. In the cases of complexes
[2]+ and [3]+, the diamagnetism of the isolated systems is, in
principle, consistent with a range of formulations: (i) [RuII(L)-
(Q)Cl]+ (Q = pcp or bpy), (ii) [RuIII(L•−)(Q)Cl]+, and (iii)
[RuIII(L)(Q•−)Cl]+. DFT analyses indicate that the [RuII(L)-
(Q)Cl]+ electronic description is preferential over its other two
spin-coupled redox congeners. The optimized structures of both
the complexes are also in good agreement with the electronic
configuration. To have an insight into the center of first
reduction, we scrutinized the LUMO of the optimized
structures. In both the complexes, the LUMO was primarily
localized on the ligand L (complex [2]+ (77%) and complex [3]+
(80%)). This further supports the experimentally observed
ligand-based EPR spectra of the coulometrically generated one
electron reduced complexes, 2 and 3. Additionally, these redox
partners were also optimized where major spin population was
Figure 4. Spin density plots of (a) 2, (b) 3, (c) [4]−, and (d) 5.
agreement with the most stable electronic configuration of
[RuII(L•−)(pcp•−)Cl]− (S = 1) state26 (see Supporting
Information, Theoretical Calculations). Thus, the second
electron transfer occurred at coordinated pcp ligand, generating
an S = 1 state which is well-justified26 by the orthogonal
arrangement of two ligands. On the other hand, electro-
generated complex [3]− is EPR inactive. The DFT analysis
indicates a spin pairing situation over the coordinated L. It is
well-documented that the bpy is a poorer12,63 acceptor than pcp,
and thus, the second reduction occurred on coordinated ligand
L•− rather than bpy, resulting in the formation of a spin-coupled
S = 0 state.64 On the other hand, the highest occupied molecular
orbital (HOMO) of the isolated complexes [2]+ and [3]+ are
primarily localized on the metal center. Thus, the oxidative
waves of complexes [2]+ and [3]+ are assigned to Ru(II)/
Ru(III) redox couple. The presence of two strong electron
withdrawing ligands (L and pcp) in [2]+ imparts electron
deficiency on the central ruthenium center. As a result, the
oxidative redox couple in [2]+ appears at a high redox potential,
1.70 V. The corresponding couple for [3]+ appears at a lower
potential of 1.43 V. Complex 4 is diamagnetic and is consistent
with the two electronic structure descriptions: (i) [RuII(L)-
(acac)Cl] and (ii) [RuIII(L•−)(acac)Cl]. DFT calculations
indicate that [RuII(L)(acac)Cl] is energetically favored over
its spin-coupled redox congener. The HOMO of the optimized
structure is located on the metal center while LUMO is
predominated by L contribution. Thus, the oxidation is
associated with the RuII → RuIII, while the reduction occurs at
the azo-aromatic L. This is in line with the EPR spectral feature
of the electrogenerated complex [4]−. Examination of the spin
density plot of the complex [4]− also shows 84% spin is localized
on the L backbone (Figure 4c).
On the contrary to above, complex 5 is an example of stable
uncoupled radical65 with S = 1/2. This feature of the complex is
consistent with the following two formulations: (i) [RuII(L)-
(DTBsq•−)Cl] and (ii) [RuII(L•−)(DTBq)Cl] (DTBq = one
electron oxidation state of DTBsq•−). DFT calculations favor
the former description over the latter, which is also in line56 with
the structural parameters and the characteristic room temper-
ature EPR spectrum of Ru−semiquinone radical systems. The
spin density plot of the complex 5 is also an indicative of
semiquinone radical state and is shown in Figure 4d. To
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Figure 5. UV−vis−NIR spectra of the complexes: in acetonitrile, (a) [2]n (n = +1, 0, −1) and (b) [3]n (n = +1, 0, −1); in dichloromethane, (c) 4 and
(d) [5]n (n = +1, 0, −1, −2).
ascertain the locus of first oxidation/reduction, we examined the
SOMO and LUMO of the complex 5. Both the orbitals are
predominantly localized on the semiquinone ligand, and thus, it
is inferred that the first oxidation and reduction both occur at the
semiquinone ligand. Notably, the single electron reduced state
was also computed, and the [RuII(L)(DTBCat)Cl]− is found to
be energetically favorable over [RuII(L•−)(DTBsq•−)Cl]−
electronic description. The second electron reduction univocally
occurs at the L and is consistent with the observed single line
EPR spectrum. DFT calculations for one electron oxidized
complex [5]+ are consistent with [RuIIL(DTBq)Cl]+ formula-
tion.
reduced complex ([2]−) displays a transition at 565 nm, which is
ascribed as dπ(Ru) → π*(L/pcp) transition.
The complex [3]+, on the other hand, shows an broad
transition at 395 nm. TDDFT analysis indicate that the
absorption is associated with the excitation from orbitals
primarily localized on the phenyl ring of the L to the azo-
backbone. Upon reduction, the complex 3 shows a broad weak
absorption at a NIR region (∼2000 nm), attributed to a L(π) →
L(π*) transition, and MLCT transition appears at 500 nm.
Upon subsequent reduction to [3]−, the L(π) → L(π*) band
diminishes, and the MLCT transition is red-shifted to 530 nm.
The isolated neutral complex 4 displays a strong LLCT
[acac(π) → L (π*)] transition at 430 nm and a relatively broad
MLCT [Ru(dπ) → L (π*)] transition at 628 nm. The 1e−
reduced species [4]− is hyper reactive for recording its
absorption spectrum.
Electronic Spectra. UV−vis−NIR spectra of the com-
pounds [2]+ and [3]+ were recorded in acetonitrile, while those
of compounds 4 and 5 were recorded in dichloromethane
solvent. These data are summarized in Table S1, and their
spectra are depicted in Figure 5. All of the compounds show
multiple absorptions in the range of 200−800 nm. Time-
dependent density functional theory (TDDFT) analyses were
done to assign the major spectral transitions.
Complex 5 contains a semiquinone radical along with a π-
acceptor L. It exhibits a LLCT transition58 (π(DTBsq•−) →
π*(L)) at 635 nm. Upon reduction of the semiquinone ligand,
two broad transitions appeared at 660 and 1100 nm, which are
attributed as MLCT [Ru(dπ) → L(π*)] and DTBCat → L
charge transfer, respectively. Upon two electron reduction, the
electrogenerated complex [5]2− showed two characteristic
transitions at 640 and 1115 nm due to LMCT [π(cat/L) →
Ru(dπ)] and MLCT [Ru(dπ) → L(π*)] transitions,
Beginning with [2]n (n = 1, 0, −1), the isolated complex
([2]+) exhibits an absorption at 560 nm, which is ascribed as a
metal-to-ligand charge transfer (MLCT) transition. Upon
reduction, a broad transition appears at 1860 nm in 2, which
is associated with ligand-to-ligand charge transfer (LLCT)
transition from partially reduced ligand L to pcp. The doubly
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respectively. On the other hand, the one electron oxidized
complex [5]+ exhibits a L(π) → L(π*) transition at 430 nm.
Chemical Catalysis. To understand the chemical tuning of
RuL moiety by different ancillary ligands, we explored a
comparative study on aerobic alcohol oxidation reactions66−68
by using of four heterolaptic complexes viz. [2]PF6, [3]PF6, 4,
and 5 as catalysts. We hypothesized that increasing the π-
acceptor ability of the ancillary ligand would increase the
electrophilic character of RuL moiety and thus promote
enhancement of substrate activation for catalytic alcohol
oxidation reaction.
Table 6. [2]PF6 Catalyzed Aerobic Oxidation of Alcohols into
Ketones/Aldehydes
a
In literature, (re)development of homogeneous catalyst is
commonly made either by changing the metal ion or by
introducing large modification on the primary ligand framework.
However, the effect of using different ancillary ligands in tuning
catalytic activity has been addressed only in recent years.37
Moreover, we wish to note here that precedents for
homogeneous Ru catalyzed aerobic alcohol oxidation reactions
are only few69−77 and mostly substrate specific.
In our study, we chose benzyl alcohol as a model substrate,
and the catalytic reactions using the four aforementioned
catalysts were performed under identical conditions. In a typical
experiment, a mixture of 1 mmol of benzyl alcohol, 2 mol %
(0.02 mmol) of catalyst, 4 mol % (0.04 mmol) of tBuOK, and 10
mL of toluene was heated at 343 K for 6 h under positive
pressure of oxygen. The corresponding benzaldehyde was
purified by preparative thin layer chromatographic technique
using 1:10 dichloromethane and hexane mixture as eluent. The
isolated yields from each of the above reactions are collected in
Table 5.
a
Table 5. Effect of Ancillary Coligand on the Catalytic
Dehydrogenation of Alcohols
Reaction conditions: complex [2]PF6 = 0.02 mmol, substrate = 1
mmol, tBuOK = 0.04 mmol, 10 mL of dry toluene (solvent).
Reactions were studied under positive O2 pressure for 6 h at 343 K.
Detailed reaction procedure is collected in the Experimental Section.
All the yields mentioned herein are isolated yields.
Mechanistic Studies. To have an insight into the
mechanism of the catalytic activity of the complexes, we
performed a series of experiments including radical clock
reaction, isotope labeling experiments. DFT analyses were also
used to discount alternate pathways. Following the literature
reports78−80 on similar catalytic reactions, we propose that the
catalytic cycle begins with the substitution of Ru−Cl by alkoxide
ion (in alkaline condition), to generate a hexacoordinated Ru−
alkoxide complex. The formation of metal alkoxide bond was
monitored81 by IR spectroscopic analysis. An equimolar mixture
Ru catalyst
isolated yield (%)
[2]PF6
[3]PF6
4
5
86
60
20
15
The yields of the above reactions follow the following order:
[2]+ ≫ [3]+ ≫ 4 and 5. Subsequently, the scope of catalytic
activities of the most efficient catalyst in the series ([2]PF6) was
further evaluated by using a variety of primary and secondary
alcohols, including aromatic, polyaromatic, and cyclic aliphatic
alcohols. A series of halogenated benzyl alcohols (Table 6,
entries b−d) afforded the respective aldehydes in high yields
(>70%). This was also found to be equally effective even for
alcohols for strong electron withdrawing CF3 and NO2
substituents (Table 6, entries f and g). All of the above
experiments were performed using an identical protocol as
noted above, and the yields of the products are collected in
Table 6. Optimization table of catalyst loading, nature of bases,
solvent, and temperature are given in Tables S2 and S3. 1H and
13C{1H} NMR spectral data of the isolated oxidized products
(aldehydes/ketones) noted in Table 6 are submitted in the
Supporting Information (Figures S12−S30).
t
of the complex [2]PF6 and BuOK was heated to 343 K in
toluene. The initial brown color of the solution became violet in
1 h. A Ru−O stretching frequency at 755 cm−1 was observed
indicated the formation of the Ru−alkoxide intermediate
(Figures S31 and S32). Its ESI-MS spectrum also signifies the
formulation, an intense molecular ion peak at m/z 679 amu
appeared due to formation of the base adduct of [2]+, i.e.
[RuL(pcp)(tBuO)]+ (Figure S33). In the subsequent step, we
analyzed the most crucial step of migration of β-H of
coordinated alkoxide. We considered two possibilities: (i)
hydrogen atom transfer and (ii) hydride ion transfer. Here,
strategically, we examined oxidation of radical clock substrates82
viz. cyclopropanemethanol and cyclopropyl phenylmethanol.
The reactions yielded cyclopropanecarboxaldehyde and cyclo-
propyl phenylketone, respectively, as exclusive products
(Figures S29 and S34). Absence of any ring opening product
H
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Scheme 1. Possible Two Pathways for the Catalytic Reaction
implies that hydride transfer pathway is operative69 for the above
oxidation reaction. Notably, after the β-H elimination, the C−H
departing from the alkoxide is optimally lined up with the empty
orbital on the ruthenium center and subsequently generated
Ru−H intermediate. This was further supported by the fact that
the radical scavenger like (2,2,6,6-tetramethylpiperidin-1-yl)-
oxyl (TEMPO) did not affect the yield of isolated product.
Next, we attempted to trap the metal hydride intermediate
using NMR spectroscopy which, however, failed possibly due to
its transient existence (see below). To understand the reaction
steps we then plan to follow a stoichiometric reaction between
equimolar amount of catalyst [2]PF6 and benzyl alcohol in an
t
anaerobic condition (in the presence of 2 equiv. BuOK). The
reaction mixture after 3 h was subjected to IR spectral analysis.
Interestingly, the IR spectrum showed two characteristic N−H
vibrations modes at 2924 and 2853 cm−1 (Figure S35). To have
a closer view, we then performed an identical reaction using d12
cyclohexanol as a substrate. Notably, in the later experiment, the
two ND modes were shifted to 2203 and 2105 cm−1 (Figure
S36). From the above isotope labeling experiments, we propose
that the catalytic reaction involves33 the formation of mono
hydrazo intermediate. Here again, we considered the two
plausible acceptors for migratory hydride ion: (i) the β-hydride
transfer to the metal center generating a transient Ru−H
intermediate which follows another migration from Ru−H to an
azo function of coordinated L (pathway I) and (ii) direct H−
migration to the azo function (pathway II), as shown in Scheme
1.
Figure 6. Energy profile diagrams of the two pathways mentioned here
(I and II).
The azo functional group of coordinated L in the catalyst [2]PF6
has low lying π* orbitals with acceptor ability superior to that of
an imine function. Thus, it is most likely that similar consecutive
hydrogen migration occurs in our system. DFT analysis
indicates that in the walking process, hydrogen migration from
Ru−H to adjacent azo-nitrogen occurs over a free energy barrier
of 18.5 kcal/mol via transition state TS2 (Figure S37). Further
electronic analysis also shows that the hydrogen becomes
protonic in TS2 as confirmed by its NBO charge of 0.31. In
either way, the reaction leads to the formation of intermediate V
(from III or II′), where the N−H bond was formed. Computed
Gibbs free energy diagram of the proposed catalytic cycle is
shown in Figure S41. The optimized structures of all
intermediates and transition states involved are shown in
Figures S38−S40.
Finally, catalyst regeneration in the cycle occurs by aerobic
oxidation33 of hydrazo intermediate with the formation of H2O2.
Formation of H2O2 in the catalytic reaction was identified
chemically85−87 (for details, see the Experimental Section and
Figure S41). Considering the above findings, we propose that
the following catalytic cycle (Scheme 2) is operative for alcohol
dehydrogenation reaction.
To discard one of the above two pathways, we considered
DFT calculated energetics for this step. It was observed that the
migration of hydride ion from ruthenium alkoxide intermediate
(I) to ruthenium (II) center necessitates an activation energy of
26.5 kcal/mol and it goes through transition state TS1 (Figure
6). The other possibility of direct H-migration to the azo
function is found to be comparatively more energetic (28.9 kcal/
mol via TS1′). Thus, pathway I is energetically preferred one.
We must admit here that the difference of activation energy
between the two above possibilities is too low (2.4 kcal/mol) to
identify the pathway conclusively.
Similar metal−ligand cooperativity for H− migration was
Effect of Ancillary Ligands on the Reaction Coordi-
nates. In the above catalytic cycle, the abstraction of hydride
from alcohol by the catalyst is thus found to be the rate-
determining step. From our foregoing discussion, it is evident
previously reported83,84 separately by Gru
̈
tzmacher and Hall
group in a Ru−diimine complex. This process was termed as
“hydrogen walking” from Ru−H to a redox acceptor function.
I
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a catalyst bearing a neutral coligand vs anionic coligand is worth
noting, which might be helpful in design and selection of
coordinated ligand for an alcohol oxidation catalyst. To have a
better understanding of the ligand effect on the reaction, we also
compared the energetics of H− migration to ruthenium for
complex [2]PF6 (containing strong π-accepting ligand pcp) with
that of the complex 4 (containing strong σ-donating ligand
acac−). As shown in Figure 7, the energy barrier for the hydride
ion migration to Ru from the attached alkoxide in the complex
[2]PF6 is computed to be 6 kcal/mol lower compared to
complex 4 (TS1 vs TS1acac). Therefore, coligand pcp can
significantly boost the formation of Ru−H intermediate by
stabilizing the associated transition state TS1.
Scheme 2. Proposed Mechanism for the Alcohol Oxidation
Reaction with Catalyst [2]PF6
CONCLUSION
■
Herein, we described an investigation on isostructural
ruthenium complexes where the ancillary ligands do exert
significant electronic effects on the redox noninnocence of RuII
coordinated bis-azoaromatic ligand moiety. The effect of
electron-withdrawing and -donating ability of four different
ancillary ligands systematically tune the overall electronic and
redox properties of the isolated complexes. Our results on
catalytic oxidation of alcohols demonstrate that the properties of
coordinated L, engineered for a chemical reaction, can be
substantially tuned by the incorporation of suitable ancillary
ligand(s). Thus, the ligand pcp with strongest acceptor ability
induced maximum electron deficiency in the complex, [2]PF6 by
effectively engaging itself in the π-system of the principal L.
There was a parallel effect on its redox behavior, accordingly, in
CV, the cathodic reductive and anodic oxidative waves of the
compound appeared at lowest and highest potentials,
respectively. Completely opposite effects were observed in the
case of strongly donating π-donor ligand, [DTBCat]2−, and the
complex 5 displayed most poor catalytic activity in the series.
The results described herein represent a less-explored area in the
participation of redox active ligands in chemical catalysis, which
appears to have a huge scope6,88 in bioinspired catalytic
reactions.
that rate of hydride abstraction from coordinated alkoxide is
directly related to the degree of electrophilicity of the Ru(II)
center in the catalyst. Herein, we considered the Ru−L moiety
which actually participated in the chemical catalysis by
abstraction of hydride ion from the coordinated alkoxide. All
of the isolated complexes are isostructural and most importantly
the central ruthenium ion is in +2 oxidation state, whereas the
azo-aromatic L retains its neutral oxidation state (+0). Although
the complexes [2]+ and [3]+ are cationic, whereas complex 4 and
5 are neutral in nature and might influence the overall
electrophilicity of the ruthenium center, a comparison between
EXPERIMENTAL SECTION
■
Materials. The metal precursor RuCl3 was purchased from Arora
Matthey Limited. All alcohols were purchased from Sigma-Aldrich.
Other chemical and solvents were of reagent grade and used as received.
Figure 7. Gibbs free energy diagram of formation of Ru−H intermediate for the complexes [2]PF6 and 4.
J
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For spectrochemical studies, high-performance liquid chromatography
(HPLC)-grade solvents were used. Tetrabutylammonium perchlorate
was prepared and recrystallized as reported earlier.89 Caution!
Perchlorates have to be handled with care and appropriate safety
precautions.
10.11 (d, J = 5 Hz, 1H), 9.01 (d, J = 8 Hz, 2H), 8.55 (t, J = 8 Hz, 1H),
8.46 (d, J = 8 Hz, 1H), 8.36 (t, J = 8 Hz, 1H), 8.20 (d, J = 8 Hz, 1H), 8.06
(t, J = 7 Hz, 1H), 7.85 (t, J = 7 Hz, 1H), 7.47−7.44 (m, 2H), 7.25−7.20
(m, 8H), 7.11 (t, J = 7 Hz, 1H), 6.70 (d, J = 6 Hz, 1H); 13C{1H}
NMR(125 MHz, CD3CN): 167.6 (2C), 157.2 (1C), 155.0 (1C), 154.2
(1C), 153.5 (1C), 152.9 (1C), 140.6 (1C), 140.2 (1C), 137.3 (2C),
133.8 (2C), 130.3 (4C), 129.3 (1C), 128.4 (1C), 125.2 (2C), 125.0
(2C), 124.1 (4C).
[Ru(L)(acac)Cl]; 4. The precursor complex 1 (100 mg, 0.20 mmol)
and acetylacetone (20 mg, 0.20 mmol) were taken in 20 mL methanol;
2 drops NEt3 were added into it, and the mixture was heated to reflux for
3 h. Then, the mixture was cooled at room temperature, and the
product was purified on a preparative silica gel TLC plate using
dichloromethane as eluent. A green band at the upper part of TLC plate
was collected. The compound, thus obtained, was recrystallized by slow
diffusion of its dichloromethane solution into hexane. Its yield and
characterization data are as follows. Yield: 70 mg (67%). ESI-MS: m/z
488.3649 (4 − Cl)+. IR (KBr, cm−1): 1427 cm−1 (ν(NN)). Anal.
Calcd for C22H20ClN5O2Ru: C, 50.53; H, 3.85; N,13.39. Found: C,
50.82; H, 3.98; N, 13.30.1H NMR(500 MHz, CDCl3): 8.55 (d, J = 8 Hz,
2H), 8.45 (d, J = 8 Hz, 4H), 8.08 (t, J = 8 Hz, 1H), 7.58 (t, J = 7 Hz, 2H),
7.45 (t, J = 8 Hz, 4H), 5.08 (s, 1H), 2.53 (s, 3H), 1.26 (s, 3H) ; 13C{1H}
NMR(125 MHz, CDCl3): 190.4 (1C), 187.2 (1C), 169.3 (2C), 154.0
(1C), 131.8 (2C), 130.8 (2C), 129.3 (4C), 124.8 (4C), 120.2 (2C),
99.8 (1C), 28.8 (1C), 26.0 (1C).
Physical Measurements. A PerkinElmer Lambda 950 spectro-
photometer was used to record UV−vis spectra. Infrared spectra were
obtained using a PerkinElmer 783 spectrophotometer. 1H NMR
spectra were recorded on a Bruker Avance 300, 400, or 500 MHz
spectrometer, and SiMe4 was used as the internal standard. A
PerkinElmer 240C elemental analyzer was used to collect micro-
analytical data (C, H, N). ESI mass spectra were recorded on a
micromass Q-TOF mass spectrometer (serial no. YA 263). All
electrochemical measurements were performed using a PC-controlled
PAR model 273A electrochemistry system. Cyclic voltammetric
experiments were performed under the nitrogen atmosphere using an
Ag/AgCl reference electrode with a Pt disk working electrode and a Pt
wire auxiliary electrode either in dichloromethane or in acetonitrile
solution containing supporting electrolyte, 0.1 M Bu4NClO4. A Pt wire
gauge working electrode was used for exhaustive electrolyses. E1/2 for
the ferrocenium−ferrocene couple under our experimental conditions
was 0.40 V. X-band EPR spectra were recorded with a JEOL JES-FA200
spectrometer.
Synthesis. The ligand 2,6-bis(phenylazo)pyridine was prepared26
according to a procedure reported in the literature.
Preparation of Complexes. [Ru(L)(CH3CN)Cl2]; 1. In a round-
bottom flask equipped with a condenser, a mixture of 165 mg of L (0.57
mmol) and 150 mg RuCl3·xH2O (150 mg,0.57 mmol) in 20 mL of
ethanol was refluxed for 3 h. During this time, the color of the solution
slowly changed from red to green. The resulting green solution was then
evaporated to dryness, and the excess ligand was washed with hexane.
The green product was then extracted with acetonitrile. Yield: 213 mg
(75%). ESI-MS: m/z 459.8470 (1 − (CH3CN) + H)+. IR (KBr, cm−1):
1448 cm−1 (ν(NN)). Anal. Calcd for C19H16Cl2N6Ru: C, 45.61; H,
[Ru(L)(DTBsq•−)Cl]; 5. This was prepared similarly to [Ru(L)(acac)-
Cl] using an appropriate quantity of H2DTBcat in place of Hacac. Its
yield and characterization data are as follows. Yield: 93 mg (72%). ESI-
MS: 643.9383 (5)+, 423.8699 (5 − DTBsq•−)+. IR (KBr, cm−1): 1436
cm−1 (ν(NN)). Anal. Calcd for C31H33ClN5O2Ru: C, 57.80; H, 5.16;
N,10.87. Found: C, 57.45; H, 5.32; N, 11.05. This compound is
paramagnetic in nature, μeff = 1.68 μB (calculated by Evans method), gav
= 1.987.
General Procedure for Catalysis Using Catalyst [2]PF6. The
catalytic reactions were performed following a general procedure. In a
round-bottom flask, a mixture of 1 mmol of substrate in 10 mL of dry
toluene solvent, 0.02 mmol of catalyst (15.7 mg), and 0.04 mmol of
KtBuO (4.5 mg) was heated at 343 K at a positive pressure of oxygen
with continuous stirring for 6 h. The crude product, thus obtained, was
purified on preparative silica gel GF-254 TLC plate using hexane as
eluent.
1
3.22; N, 16.80. Found: C, 45.78; H, 3.34; N, 16.71. H NMR (400
MHz, CDCl3) 8.59 (d, J = 8 Hz, 2H), 8.30 (d, J = 8 Hz, 4H), 8.08 (t, J =
8 Hz, 1H), 7.60 (d, J = 7.5 Hz, 2H,), 7.52 (t, J = 8 Hz, 4H), 2.70 (s, 3H).
13C{1H} NMR (75 MHz, CDCl3): 168.7(2C), 154.8(1C), 133.2(2C),
129.9 (2C), 129.2 (4C), 124.8 (4C), 120.9 (2C), 117.1 (1C), 4.8 (1C).
All the complexes reported below were prepared from the precursor
complex [RuLCl2(CH3CN)], 1.
[Ru(L)(pcp)Cl]PF6; [2](PF6). The precursor complex 1 (100 mg, 0.20
mmol) and pcp, 2-(4-chlorophenylazo)pyridine (50 mg, 0.23 mmol)
were mixed in 20 mL acetonitrile and refluxed for 3 h. The resulting
solution was concentrated to 5 mL followed by saturated aqueous
solution of NH4PF6 was added. The mixture was left in a refrigerator for
1 h; the resulting precipitate was filtered and washed thoroughly with
chilled water to remove excess NH4PF6 and dried in vacuum. The
product was purified on a preparative silica gel TLC plate using
acetonitrile-dichloromethane (1:10) as eluent. A brown band at the
middle part of TLC plate was collected. The compound, thus obtained,
was recrystallized by slow diffusion of its dichloromethane solution into
hexane. Its yield and characterization data are as follows. Yield: 113 mg
(72%). ESI-MS: m/z 640.8040 ([2]PF6 − PF6)+. IR (KBr, cm−1): 1456
cm−1(ν(NN) of L), 1406 cm−1 (ν(NN) of pcp). Anal. Calcd for
C28H21Cl2F6N8PRu: C, 42.76; H, 2.69; N,14.25. Found: C, 42.88; H,
2.64; N, 14.13. 1H NMR(500 MHz, CD3CN): 9.87 (d, J = 5 Hz, 1H),
8.91 (d, J = 8 Hz, 2H), 8.79 (d, J = 8 Hz, 1H), 8.54 (t, J = 8 Hz, 1H), 8.20
(t, J = 8 Hz, 1H), 7.66 (t, J = 8 Hz, 1H), 7.61 (t, J = 7 Hz, 1H), 7.54 (t, J
= 7 Hz, 2H), 7.28(t, J = 8 Hz, 3H), 7.22 (t, J = 7 Hz, 2H), 7.11 (d, J = 8
Hz, 4H), 6.38 (d, J = 7 Hz, 2H); 13C{1H} NMR(125 MHz, CD3CN):
165.0 (2C), 153.2 (1C), 151.7 (1C), 142.1 (1C), 139.1 (2C), 135.1
(2C), 130.8 (2C), 130.7 (4C), 130.3 (1C), 130.0 (2C), 127.8 (2C),
125.5 (1C), 125.1 (1C), 124.4 (4C), 123.9 (1C), 122.2 (1C).
[Ru(L)(bpy)Cl]PF6; [3]PF6. This was prepared similarly to [Ru(L)-
(pcp)Cl]PF6 using appropriate quantity of bpy in place of pcp. Its yield
and characterization data are as follows. Yield: 119 mg (82%). ESI-MS:
m/z 579.8936 ([3]PF6 − PF6)+. IR (KBr, cm−1): 1450 cm−1 (ν(N
N)). Anal. Calcd for C27H21ClF6N7PRu: C, 44.73; H, 2.92; N,13.52.
Isotope Labeling Experiments. In an argon atmosphere Schlenk
tube, 0.1 mmol of the complex [2]PF6 (79 mg) was mixed with the
equimolar quantity of d12-cyclohexanol (22 mg) and 0.2 mmol of
KtBuO (22 mg) in dry and deoxygenated toluene solvent. The solution
was stirred at 343 K for 3 h. The resulting light green colored solution
was then evaporated under vacuum. The IR spectrum of the resultant
compound showed characteristic stretching due to N−D bonds. IR
(KBr disk, cm−1): ν(NN): 1446 cm−1, 1435 cm−1, ν(N−N): 1178
cm−1, ν(N−D): 2203 cm−1, 2105 cm−1.
A similar experiments performed with benzyl alcohol showed
following characteristic bands. IR (KBr disk, cm−1): ν(NN): 1452
cm−1,1420 cm−1, ν(N−N): 1195 cm−1, ν(N−H): 2924 cm−1, 2853
cm−1.
Detection of Hydrogen Peroxide During the Catalytic
Reactions. H2O2, produced in the catalytic reaction, was detected
spectro-photometrically by following the gradual development of the
characteristic band for I3− (λmax (observed) = 360 nm) upon reaction
with I−. In a round-bottom flask containing 1 mmol of benzyl alcohol in
10 mL of dry toluene was mixed with 0.02 mmol of catalyst and 0.04
t
mmol of BuOK, and the mixture was heated at 343 K for 3 h with
continuous stirring. To the reaction mixture an equal volume of water
was added subsequently and extracted with dichloromethane to remove
the leftover reactants and products from the reaction mixture. The
separated aqueous layer was then acidified with H2SO4 to pH 2 to stop
further oxidation. Then, 1 mL of a 10% KI solution and three drops of a
3% of ammonium molybdate solution were added. Hydrogen peroxide
oxidizes I− to I2, which reacts with excess I− to form I3− according to the
following chemical reactions: (i) H2O2 + 2I− + 2H+ → 2H2O + I2, (ii)
1
Found: C, 44.84; H, 2.72; N, 13.33. H NMR (500 MHz, CD3CN):
K
DOI: 10.1021/acs.inorgchem.8b01558
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
I2(aq) + I− → I3 . Increasing acid concentration increases the reaction
AUTHOR INFORMATION
Corresponding Author
*E-mail: icsg@iacs.res.in.
ORCID
Debabrata Sengupta: 0000-0001-7212-3761
Pradip Ghosh: 0000-0002-5364-7604
Ayan Datta: 0000-0001-6723-087X
Sreebrata Goswami: 0000-0002-4380-5656
■
rate. Oxidation reaction is catalyzed by ammonium molybdate solution.
Computational Details. All calculations were performed using
Gaussian 0990 suite of programs. B3LYP/6-31+G(d)91 (C, H, N, O,
Cl), LANL2DZ (Ru)92 level of theory were employed for all structural
optimization and vibrational analysis. The broken-symmetry ap-
proach93−97 was employed to establish the singlet state S = 0 of the
compound(s). The calculations of the ground singlet states were
performed using either spin-restricted or spin-unrestricted approaches
(in G09W, combined with GUESS = MIX). Mulliken spin densities
were used for analysis of the spin populations on ligand and metal
centers.98 Singlet excitation energies based on the solvent-phase
(CH3CN/dichloromethane) optimized geometry of the complexes
were computed using the TDDFT formalism99−101 in acetonitrile/
dichloromethane using the conductor-like polarizable continuum
model (CPCM).102−105 GaussSum106 was used to calculate the
percentage contribution of ligand and metal to the frontier orbital
and the fractional contributions of various molecular orbitals in the
optical spectral transition. Stationary points were characterized by the
absence of any imaginary frequency. All the transition states were
identified by a unique imaginary frequency that connects corresponding
reactant and product. Intrinsic reaction coordinate (IRC) calculations
were carried out for further confirmation of the transition state. For a
better estimation of the electronic energy, single point calculations over
the B3LYP/6-31+G(d)(C, H, N, O, Cl), LANL2DZ (Ru) optimized
geometry were done by using M06107/6-31++G(d,p) (C, H, N, O, Cl),
LANL2TZ(f) (Ru)108 method. Solvent effects were incorporated using
an implicit PCM solvation model (SMD)109 for toluene. All the
reported energies given are in SMD/M06/6-31++G(d,p) (C, H, N, O,
Cl), LANL2TZ(f) (Ru) // B3LYP/6-31+G(d)(C, H, N, O, Cl),
LANL2DZ (Ru) level of theory.
X-ray Crystallography. Crystallographic data for compounds
[2]PF6, [3]PF6, 4, and 5 are collected in Table 1. Suitable crystals for X-
ray diffraction were obtained by slow diffusion of dichloromethane
solution into hexane for rest compounds. All data were collected on a
Bruker SMART APEX-II diffractometer, equipped with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å), and were corrected
for Lorentz polarization effects.
[2]PF6: a total of 42137 reflections were collected, of which 6945
were unique (Rint = 0.194), satisfying the I > 2σ(I) criterion, and were
used in subsequent analysis. [3]PF6: a total of 22 350 reflections were
collected, of which 8443 were unique (Rint = 0.020), satisfying the I >
2σ(I) criterion, and were used in subsequent analysis. 4: a total of
Present Addresses
‡P.G.: Organic Chemistry and Catalysis, Utrecht University,
3584 CG Utrecht, The Netherlands
§S.S.: Department of Chemistry, Indian Institute of Technology,
Jammu 181221, India
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research was supported by the Department of Science
andTechnology (DST), India, SR/S2/JCB-09/2011, EMR/
2014/000520. S.G. sincerely thanks DST-SERB for J. C. Bose
fellowship. S.P.R. and R.B. are thankful to the Council for
Scientific and Industrial Research (CSIR), India for their
fellowship support. M.C. is thankful to DST-INSPIRE for her
fellowship support.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01558.
■
S
Supporting tables and figures for experimental and
computational data (PDF)
Accession Codes
CCDC 1846398−1846401 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
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