Ruthenium(III) 2-(aminofluoreneazo)phenolate complexes: Synthesis, characterization, catalytic activity in amidation reaction and Fluorescence quenching studies

Article abstract of DOI:10.1016/j.jorganchem.2020.121408

A series of ruthenium(III)2-(aminofluoreneazo)phenolate complexes with general formula [RuCl(PPh₃)₂(L_1-5)] (1–5) (L = 2-(aminofluoreneazo)phenolate ligands) have been synthesized. The characterization of the synthesized complexes was accomplished by elemental analysis, spectroscopic (FT-IR, UV–Vis, Fluorescence and EPR) and ESI-MStechniques. The catalytic performance of one of the synthesized complexes 3 for the amidation of aldehyde in the presence of NaHCO₃/NH₂OH·HCl has been evaluated. The fluorescence emission of complexes [RuCl(PPh₃)₂(L₂)] (2) and [RuCl(PPh₃)₂(L₃) (3)] are effectively quenched by 1,4-benzoquinone and 1,4-naphthoquinone in acetonitrile medium.

Full text of DOI:10.1016/j.jorganchem.2020.121408

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Ruthenium(III) 2-(aminofluoreneazo)phenolate complexes: Synthesis,
characterization, catalytic activity in amidation reaction and Fluorescence quenching
studies
Muniyappan Thirumal, Anbazhagan Venkattappan, Galmari Venkatachalam
PII:
S0022-328X(20)30310-7
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121408
JOM 121408
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 6 March 2020
Revised Date: 13 May 2020
Accepted Date: 15 June 2020
Please cite this article as: M. Thirumal, A. Venkattappan, G. Venkatachalam, Ruthenium(III) 2-
(aminofluoreneazo)phenolate complexes: Synthesis, characterization, catalytic activity in amidation
reaction and Fluorescence quenching studies, Journal of Organometallic Chemistry (2020), doi: https://
doi.org/10.1016/j.jorganchem.2020.121408.
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Ruthenium(III) 2-(aminofluoreneazo)phenolate complexes:Synthesis,
characterization, catalytic activityin amidation reactionand Fluorescence
quenching studies
a
b
a
Muniyappan Thirumal , Anbazhagan Venkattappan and Galmari Venkatachalam *
a
PG & Research Department of Chemistry, Government Arts College, Dharmapuri-636 705,
Tamil Nadu, India.
b
Department of Chemistry, Vinayaka Mission’s Kirupananda Variyar Arts and Science
College, Vinayaka Missions Research Foundation (Deemed to be University), Salem 636308,
Tamilnadu, India,
Corresponding Author: *E-mail address: gvchem@gmail.com[G. Venkatachalam], Tel.: +91
0
4342 230 008.
Abstract
A series of ruthenium(III)2-(aminofluoreneazo)phenolate complexes with general
formula [RuCl(PPh ) (L )] (1-5) (L=2-(aminofluoreneazo)phenolate ligands) have been
3
2
1-5
synthesized. The characterization of the synthesized complexes was accomplished by
elemental analysis, spectroscopic (FT-IR, UV-Vis, Fluorescenceand EPR) and ESI-
MStechniques. The catalytic performance of one of the synthesized complexes 3 for the
amidation of aldehyde in the presence of NaHCO /NH OH·HCl has been evaluated. The
3
2
fluorescence emission ofcomplexes [RuCl(PPh ) (L )] (2) and [RuCl(PPh ) (L ) (3)] are
3
2
2
3 2
3
effectively quenched by 1,4-benzoquinone and 1,4-naphthoquinone in acetonitrile medium.
Keywords: Ruthenium(III) complex; Azo ligands; Synthesis; Characterization; Aldehyde to
amide; Fluorescence Quenching
1

1
. INTRODUCTION
In last decade’s, organometallic chemistry and homogeneous catalysis in ruthenium
arylazo complexes have been extensively explored in various organic transformations[1].
Cyclometallation/orthometallation has involved one ligating group holding a metal center
close to C–H bond and subsequent closure of the ring via the formation of carbon to metal
bond. There has been facile availability, synthetic flexibility, and beneficial impact on
catalytic reactivity of the metal center have been key parameters that have continued to
stimulate research in transition metal complexes and has disclosed highly diverse fields of
applications[2-5]. Particularly, cyclometalated compounds show important applications, such
as their use in regiospecific organic and organometallic reactions [6], insertion reactions [7],
the synthesis of new metal mesogenic compounds [8] and in biologically active compounds
[
9].
Amides are an important class of chemicals that have been widely used as chemical
intermediates in organic synthesis, raw materials for engineering plastics, detergents, and
lubricants. The conversion of carbonyl compounds, such as aldehydes, ketones, and oximes,
is a good candidate for the synthesis of amides [10–12]. Beckmann rearrangement is
commonly used to transform oximes into the corresponding amides [13].
There are many effective catalytic systems based on ruthenium have been developed
[
14] because of the fascinating reactivities exhibited by the resultant complexes and the
nature of the ligand that dictates the property of those complexes. The rearrangement of
aldoximes into amides has been reported using transition metal catalysts containing nickel,
[
15] copper, [16] ruthenium, [17] palladium, [18] iridium, [19] rhodium [20] and zinc [21].
We describe here the synthesis, characterization and catalytic activity of
cyclometallated ruthenium(III) 2-(aminofluoreneazo)phenolate complexes incorporating
triphenylphosphine analogues. The molecular structure of these complexes was confirmed by
2

elemental analysis along with FT-IR, UV-visible, Fluorescence, EPR and ESI-Mass spectral
studies. Further, the synthesized complexes were effectively catalyzes the conversion of
various aldehydes to primary amides in good yields. Also fluorescence quenching
experiments were carried in acetonitrile solvent out for determining the rate constant.
2
. EXPERIMENTAL METHODS
2
.1 Material and Instrumentation
.
Commercial RuCl 3H O, p-substituted phenols and 2-aminofluorene were purchased
3
2
from Himedia and Sigma-Aldrich. All the reagents used were of analytical reagent grade.
Solvents were purified and dried according to standard procedures [22]. The precursor
complex [RuCl (PPh ) ] was prepared by reported literature method [23].The 2-
3
3 3
(
aminofluoreneazo)phenolate ligands were prepared by coupling diazotization reaction of 2-
aminofluorene with corresponding para-substituted phenols [24].
.2 Physical measurments
Microanalytical (C, H, N) data were obtained with a Vario EL III Elemental analyzer
at SAIF, Cochin. The FT-IR spectra of the compounds were recorded in Agilent Resolution
2
-1
pro system spectrometer over the range 4000 to 400 cm . Electronic spectra of the complexes
were recorded in CHCl solution in a Cary 300 Bio UV-Vis Varian spectrophotometer in the
3
range 800-200 nm.EPR measurements were made with JEOL JES-FA200 X-band EPR
spectrometer for powder samples at room temperature and solution at 77 K was calibrated
with the help of DPPH. Electrospray ionization mass spectra (ESI) have been recorded on
ES-MS Q-TOF mass spectrometer.
3

2
.3 Synthesis of ruthenium(III) 2-(aminofluoreneazo)phenolate complexes (1-5)
The ruthenium precursor [RuCl (PPh ) ] (0.1g, 0.15mmol) and 2-
3
3 3
(
aminofluoreneazo)phenolate ligands (H L -H L ) (0.0311 - 0.0356g, 0.15mmol) were taken
2 1 2 5
in 20 mL of benzene and few drops of Et Nwas added. The reaction mixture was heated
3
under reflux for 5 h. The progress of the reaction mixture was checked by TLC. The solvent
was then evaporated under reduced pressure, which was then subjected to purification by
column chromatography using silica gel (60-120 mesh) using chloroform as eluent. The
ruthenium complexes were obtained as crystalline solid with good yields.
[
RuCl(PPh ) (L )] (1). Yield: 60%; Anal. Calc. for C H ClN OP Ru: C, 70.10; H,
3 2 1 56 44 2 2
–
1
2
.92; N, 4.59. Found: C, 69.90; H, 2.34; N, 4.05%; IR: 1315 (νC–O), 1376 (ν N=N) cm ;
UV–Vis λmax (nm): 600, 465, 325, 270.
RuCl(PPh ) (L )] (2). Yield: 65%; Anal. Calc. for C H ClN O P Ru: C, 68.95; H,
[
3
2
2
56 44
2
2 2
–
1
6
.52; N, 4.15. Found: C, 67.98; H, 6.01; N, 3.96%; IR: 1301 (νC–O), 1398 (νN=N) cm ;
UV–Vis λmax (nm): 610, 480, 345, 245.
RuCl(PPh ) (L )] (3). Yield: 78%; Anal. Calc. for C H ClN OP Ru: C, 70.76; H,
[
3
2
3
59 50
2
2
–
1
4
.99; N, 2.79. Found: C, 69.98; H, 4.03; N, 2.06%; IR: 1297 (νC–O), 1385 (νN=N) cm ;
UV–Vis λmax (nm): 640, 470, 320, 268.
RuCl(PPh ) (L )] (4). Yield: 52%; Anal. Calc. for C H Cl N OP Ru: C, 67.37; H,
[
3
2
4
55 40
2
2
2
–
1
4
.08; N, 2.85. Found: C, 66.80; H, 3.60; N, 2.05%; IR: 1310 (νC–O), 1390 (νN=N) cm ;
UV–Vis λmax (nm): 630, 460, 330, 254.
RuCl(PPh ) (L )] (5). Yield: 58%; Anal. Calc. for C H ClN O P Ru: C, 66.69; H,
[
3
2
5
55 40
3
3 2
–
1
4
.04; N, 4.24. Found: C, 65.90; H, 3.55; N, 3.67%; IR: 1290 (νC–O), 1394 (νN=N) cm ;
UV–Vis λmax (nm): 615, 472, 335, 244.
4

2
.4 General procedure for the conversion of aldehyde to amide
The reaction vessel was equipped with magnetic stirring bar, complex catalyst(1
mol%), aldehyde (1 mmol), NH OH·HCl (1 mmol) and NaHCO (1 mmol) was taken and the
2
3
mixture was placed under an atmosphere of N . Dry and degassed acetonitrile (3 mL) was
2
added and the reaction mixture was refluxed for 5 h. The reaction mixture was then cooled to
room temperature and the solvent was evaporated. The residue was dissolved in CH Cl ,
2
2
filtered and the solvent was removed. The amide was then purified using column
chromatography, which gives the amide in high isolated yield. The products were confirmed
1
by H NMR spectra.
3
. Results and discussion
3
.1 Synthesis and characterization
A series of low spin cyclometalated organoruthenium complexes of the type
[
RuCl(PPh ) (L )]; L = 2-(aminofluoreneazo)phenolate ligand swere synthesized in good
3 2 1-5
yields from the reaction of equi-molar ratio of [RuCl (PPh ) ] with 2-
3
3 3
(
aminofluoreneazo)phenol ligands (H L –H L ) in dry benzene (Scheme 1). It has been
2 1 2 5
observed that azo ligands replace two chlorides, one PPh from the starting precursor and the
3
oxidation state of ruthenium metal atom remains unchanged during the formation of
cyclometalated species. The interesting phenomenon observed is the fact that both phenol and
aromatic C-H of the 2-aminofluourene moiety are coordinated to ruthenium as tridenate
ligand forming two five membered rings after deprotonation. The new complexes are
remarkably stable to air and non-hygroscopic. These ruthenium complexes are highly soluble
in common solvents such as chloroform, dichloromethane, toluene and benzene producing
intense green solutions.
5

R
PPh3
O
R
OH
N
N
N
Benzene/ Et N
N
Ru
Cl
3
[
RuCl (PPh ) ]
3 3 3
Reflux /5h
PPh3
R = CH (H L ); OCH (H L );
3
2
1
3
2 2
(
CH ) C (H L ); Cl (H L );
3 3 2 3 2 4
R = CH (1); OCH (2); (CH ) C (3); Cl (4); NO (5)
3
3
3 3
2
NO (H L )
2
2 5
Scheme 1. Synthesis of ruthenium(III)2-(aminofluoreneazo)phenolate complexes.
The infrared spectra of the ruthenium(III) complexes exhibit many sharp and strong
-1
stretching vibration within range 1600-600 cm . On complexation, ν(N=N) appears at lower
-1
frequency in the range 1398-1352 cm and this red shift supports the coordination of N(azo)
to ruthenium ion [25]. The band corresponding to phenolic ν(C-O) stretching is shifted to
-1
higher frequency in the range 1308-1300 cm in the complexes confirming that the other
coordination site is the phenolic oxygen, this is further supported by the disappearance of a
-1
band due to OH groups of the ligands[26]. In addition, the bands around, 1560 cm is related
to the aromatic C=C vibration of the ligand moiety [27-29].
Electronic spectra of the complexes have been recorded in chloroform solution and
the representative spectrum is shown in Fig. 1(Figs. S1-S5 given in supporting information).
In the visible region, the broad absorption band at about 660 nm, which is assigned to d-d
transitions and the absorption around 400-450 nm is probably due to ligand to metal charge
transfer transitions [30-32].More intense bands observed in the range 250-300 nm are
assigned to π-π* and n-π* ligand transitions. The observed pattern of the electronic spectrum
of the complexes indicated the presence of an octahedral environment around ruthenium (III)
ion similar to that of other ruthenium(III) octahedral complexes [33-35].
6

Fig. 1. Electronic spectrum of complexs [RuCl(PPh ) (L )] (2) &[RuCl(PPh ) (L )] (3).
3
2
2
3 2
3
Fig. 2 Steady State fluorescence spectra of [RuCl(PPh ) (L )] (2) in the presence of various
3
2
2
-4
concentration of Benzoquinone (0, 2, 8, 14, 20, 26 x 10 M) in acetonitrile.
7

Fig. 3 Stern-Volmer plot for the fluorescence quenching of [RuCl(PPh ) (L )] (2) complex by
3
2
2
benzoquinone in acetonitrile
The steady state fluorescence quenching spectra of [RuCl(PPh ) (L )] (2) and
3
2
2
[
RuCl(PPh ) (L )] (3) were carried out by adding different quencher concentrations (2
̶
26 x
3
2
3
-4
1
0 M) of benzoquinone and naphthoquinone to the fixed concentration of the complexes
-5
(
10 M) (Fig 2). The fluorescence intensity of ruthenium{[RuCl(PPh ) (L )] (2) and
3 2 2
[
RuCl(PPh ) (L )] (3)}complexes at 613 nm decreases upon increasing the quencher
3
2
3
concentration and the variation in the fluorescence intensity is related to quinone
concentration by the Stern-Volmer equation [36-39]. A plot of I /I versus [Q] gives straight
0
line. From the slope value, Stern-Volmer constant (K ) can be calculated according to Stern-
SV
Volmer equation
I /I = 1 + K [Q]
0
SV
Where, I and I are fluorescence intensity in the absence and presence of quencher,
0
KSV = Stern-Volmer constant, [Q] = concentration of the quencher. Fig 3 shows the Stern-
Volmer plot for the quenching of [RuCl(PPh ) (L )] (2) by benzoquinone as a quencher.
3
2
2
2
Stern-Volmer constants, Regression coefficient (R ) for [RuCl(PPh ) (L )] (2) and
3
2
2
[
RuCl(PPh ) (L )] (3) complexes are given in the Table 1.
3 2 3
8

2
Table 1: Fluorescence Quenching Stern-Volmer constants (K ), Regression coefficient (R )
SV
for [RuCl(PPh ) (L )] (2) and [RuCl(PPh ) (L )] (3) complexes
3
2
2
3 2
3
S. No.
1
Ru Complex
Benzoquinone
1.02
Naphthoquinone
0.99
[RuCl(PPh ) (L )] (2)
136
97
148
116
3
2
2
2
[RuCl(PPh ) (L )] (3)
0.998
1.01
3
2
3
Upon excitation, the Ru Complexes goes to excited state and an electron transfer from
excited Ru complex to quinones since quinones are electron acceptors. Among quinones,
naphthoquinone show slightly higher stern-volmer constant due the aromatic ring attached to
quinone moiety.
The solid state EPR spectrum the complexes was recorded in X-band frequencies at
room temperature. The EPR spectra are shown in supporting information Figs. S6-S10 and
5
spectral data for all the complexes are given in Table2.The g-value for low spin d
configuration is good probe of molecular structure and bonding since the observed ‘values
are very sensitive to small changes in structure and to metal co-valency. The overall position
of line and nature of the EPR Spectrum of the complexes is a characteristic of low spin
ruthenium (III) octahedral complexes. However, the EPR spectrum of the complex 3recorded
in dichloromethane/toluene solution at 77 K shows rhombic spectrum with three distinct ‘g’
values (g ≠ g ≠ g ); g = 2.20, g = 2.10, g = 1.90. The rhombicity of the spectra reflects the
x
y
z
x
y
z
asymmetry of electronic environments around ruthenium in these complexes [40,41].Hence,
the results from the EPR spectral analysis indicate that these ruthenium(III) 2-
(
aminofluoreneazo)phenolate complexes are significantly distorted from an octahedral
geometry [42,43].
In addition, the calculated molecular ion peak (m/z) value of the complexes 2 and 3
are 974.57 and 1000.65 respectively. The calculated mass spectral peaks of the complexes are
9

shown good agreement with their experimental molecular ion peak and are further support
the assigned structure. The mass spectrum of complex 2 and 3 is shown in Fig. S11 and S12
respectively (see supporting information).
Table 2. EPR data of ruthenium(III) 2‒(aminofluoreneazo)phtholate complexes
Complexes
gx
gy
gz
1
2
3
4
5
2.10
2.12
2.20
2.18
2.22
2.02
2.07
2.10
2.09
2.11
1.91
1.95
1.90
1.97
1.93
_
______________________________________________
2
2
2 1/2
<
g>* = [1/3 gx + 1/3 gy + 1/3 gz
]
To get an insight into the catalytic property of these complexes, optimization of
complex catalysis 1-5 and catalyst/substrate ratio was evaluated for the conversion of
aldehyde to amide. Initially the catalytic reaction was performed using 4-nitrobenzaldehyde
(
1 mmol) as model substrate, NH OH·HCl (1 mmol), 1 mol% of complexes 1‒5 and
2
NaHCO (1 mmol) in acetonitrile which were refluxed for 5 h. It has been observed that the
3
all the complexes to be effective catalysts in acetonitrile solvent and NaHCO gave excellent
3
isolated yields of 75%, 82% , 95%, 64% and 55% respectively and the results were
summarized in Table 3.
1
0

a
Table 3. Catalytic activity of the complexes 1‒5 .
CHO
C om p lex catayls t 1 5-
m ol%
1
NH
2
NO
NHO H·HCl/NaHCO
NO
2
2
3
2
CH CN 5/ h
3
_
_
__________________________________________________
b
Entry
Complex (catalyst)
Yields (%)
__________________________________________________
1
2
3
4
5
[RuCl(PPh ) (L )] 1
75
82
95
64
55
3
2
1
[RuCl(PPh ) (L )] 2
3
2
2
[RuCl(PPh ) (L )] 3
3
2
3
[RuCl(PPh ) (L )] 4
3
2
4
[RuCl(PPh ) (L )] 5
3
2
5
_
__________________________________________________
Conditions: Catalyst (1 mol%) 4-nitrobenzaldehyde (1 mmol), NH OH·HCl
a
2
b
(
1 mmol), NaHCO (1 mmol) and 5 mL CH CN; Isolated yield after column
3
3
chromatography (average of two runs).
Further, we have tested the catalytic reactions using [RuCl(PPh ) (L )] (3) with
3
2
3
different ratio of substrate loading (C/S = 1:1200, 1:900, 1:600, 1:300 and 1:100). The
resultof conversion of 4‒nitrobenzaldehyde to 4‒nitrobenzamide was summarized in Table 3.
It has been observed that the C/S ratio 1:100 is the best compromise between optimal reaction
rates and gave good isolated yield of amide product.
1
1

a
Table 4. Effect of substrate using complex (3)
C om p lex cat ay ls t
m ol%
3
CHO
1
NH
2
NHO H·HCl/NaHCO
NO
NO
2
3
2
2
CH CN 5/ h
3
_
______________________________________________
b
Entry
C/S
Yields (%)
_
_______________________________________________
1
2
3
4
5
1:1200
1:900
1:600
1:300
1:100
55
60
78
89
95
_
______________________________________________
a
Conditions: Catalyst (1 mol%) 4-nitrobenzaldehyde (1‒12 mmol), NH OH·HCl
2
b
(
1 mmol), NaHCO (1 mmol) and 5 mL CH CN; Isolated yield after column
3
3
chromatography (average of two runs).
In order to ensure the new catalyst, a variety of substituted aromatic aldehydes were
converted to primary amides using [RuCl(PPh ) (L )] (3) as an active catalyst. The results
3
2
3
were listed in Table5. Benzaldehyde were smoothly converted into corresponding benzamide
gave an excellent yield of 87% (entry 1). Interestingly the presence of electron donating
group present in the para position of the benzaldehyde, such as CH , OCH substituents
3
3
(entries 2 & 3) on benzaldehyde converted to corresponding amides were obtained a good
yield of 80% and 84%. Similarly, 3, 4 di-substituted methoxy groups (entry 4) on
benzaldehyde converted to benzamide in good isolated yield of 75%. The electron
withdrawing groups like –NO , –Cl and Br substituents (entries 5, 6 and 7) offers excellent
2
yields (95%, 90% and 88%) when compared to substrate containing electron donating groups.
Further, the complex 3 efficiently catalyzes heterocyclic aldehydes (entries 8 & 9) to
1
2

respective amides in good yield. We believe that the catalytic transformation proceeds via the
oxidative addition of the aldoxime N–OH bond to Ru(II), followed by nucleophilic attack on
the coordinated imine, then β-elimination of substrate cyclometallated, and finally reductive
elimination, to give the amide using the catalytic cycle (Scheme 2) previously suggested by
Crabtree et al[44]. The present catalytic system exhibited significantly higher degrees of
conversion when compared to other six coordinate ruthenium complexes [45-48].
1
Cl
PPh
3
O
HO
R
H
N
N
NH
2
R
Ru Cl
N
O
PPh
3
Oxidative Addition
Reductive Elimination
Cl
PPh
3
Cl
PPh
3
O
OH
N
O
Ru
N
H
N
H
2
N
R
O
4
Ru
N
N
H
R
PPh
3
PPh
3
Cl
PPh
3
O
O
H
N
Ru
N
N
R
Beta Elimination
Nucleophilic Attack
H
PPh
3
3
Scheme 2. Proposed mechanism for the Ru catalyzed (3) rearrangement of aldoximes.
1
3

a
Table 5. The conversion of aldehydes to amides using complex (3)
b
Entry
Substrates
Products
Yield(%)
O
CHO
NH2
87
1
2
O
O
CHO
CHO
NH2
80
84
H C
H C
3
3
NH2
NH2
3
4
5
H CO
3
H CO
3
O
CHO
7
5
H CO
H CO
3
3
OCH3
OCH3
O
CHO
NH2
NH2
95
O N
2
O N
2
O
CHO
CHO
CHO
9
0
6
7
Cl
Cl
O
O
NH2
NH2
8
5
Br
Br
N
N
70
8
9
O
O
6
5
NH2
CHO
O
a
Conditions: Catalyst (1 mol%) aldehyde (1 mmol), NH OH·HCl (1 mmol), NaHCO (1
2
3
b
mmol) and 3 mL CH CN. Isolated yield after column chromatography (average of two
3
runs).
1
4

4
. Conclusion
In conclusion, we have synthesized and characterized a new series of
organoruthenium(III) complexes containing 2-(amionofluoreneazo)phenolate ligands of
general formula [RuCl(PPh ) (L )]. The bulky fluorenamine based phenolate azo ligands
3
2
1-5
behaves as tridentate CNO donor ligand forming two five membered chelate rings in these
complexes. Further, we evaluated the catalytic activity of the synthesized ruthenium(III)
complexes and show remarkable results in the conversion of aldehyde to amide in good
yield.
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Research Highlights
Ruthenium(III) 2-(aminofluoreneazo)phenolate complexes: Synthesis,
characterization, catalytic activity in amidation reaction and Fluorescence
quenching studies
•
•
•
New Family of ruthenium(III) 2-(aminofluoreneazo)phenolate complexes containing tris-
bidentate O, N donors of ligands have been synthesized.
The characterization of the synthesized complexes was accomplished by spectroscopic
techniques.
Catalytic activity and Fluorescence quenching of the complexes are evaluated.

Declaration of interests
☐
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
No conflict of interest