Evolution of physical and photocatalytic properties of new Zn(II) and Ru(II) complexes

Article abstract of DOI:10.1016/j.poly.2019.05.065

Synthesis of Zn(II) and Ru(II) complexes were reported by using N₄-macrocyclic Schiff base ligands under solvothermal conditions. The newly synthesized Zn(II) and Ru(II) complexes have been characterized by various physico-chemical techniques such as elemental analysis, molar conductance, HRMS, TGA, FESEM, UV–Vis, FT-IR, ¹H NMR, and cyclic voltammetry. By using molar conductance studies, the complexes are formulated as [Zn(TPTTP)]Cl₂ and [Ru(TPTTP)Cl₂]. C–H bond activation of an sp³ group of methylstyrenes (converted into cinnamaldehydes) and C–H bond activation of the sp² bond of polycyclic aromatic hydrocarbons through photooxidation was examined in the presence of Zn(II) and Ru(II) complexes. Reusable activity studies and photostability of catalyst are investigated by using UV–Vis spectra. Based on the results, higher catalytic activity of [Ru(TPTTP)Cl₂] complex than [Zn(TPTTP)]Cl₂ complex in both C–H bond activation and photooxidation of aromatic hydrocarbons has been reported.

Full text of DOI:10.1016/j.poly.2019.05.065

Accepted Manuscript
Evolution of Physical and Photocatalytic Properties of New Zn(II) and Ru(II)
complexes
Venkanna Gugulothu, Jakeer Ahemed, Mahesh Subburu, Bhongiri Yadagiri,
Ritu Mittal, Chetti Prabhakar, Someshwar Pola
PII:
S0277-5387(19)30403-6
https://doi.org/10.1016/j.poly.2019.05.065
POLY 13990
DOI:
Reference:
Polyhedron
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Revised Date:
Accepted Date:
9 April 2019
28 May 2019
30 May 2019
Please cite this article as: V. Gugulothu, J. Ahemed, M. Subburu, B. Yadagiri, R. Mittal, C. Prabhakar, S. Pola,
Evolution of Physical and Photocatalytic Properties of New Zn(II) and Ru(II) complexes, Polyhedron (2019), doi:
https://doi.org/10.1016/j.poly.2019.05.065
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Evolution of Physical and Photocatalytic Properties of New Zn(II) and Ru(II) complexes
Venkanna Gugulothu,a Jakeer Ahemed,a Mahesh Subburu,a Bhongiri Yadagiri,a Ritu Mittal,b
Chetti Prabhakarb and Someshwar Polaa*
Abstract
Synthesis of Zn (II) and Ru (II) complexes were reported by using N4-macrocyclic Schiff base
ligands under solvothermal conditions. The newly synthesized Zn (II) and Ru (II) complexes
have been characterized by various physico-chemical techniques such as elemental analysis,
molar conductance, HRMS, TGA, FESEM, UV–vis, FT-IR, 1H-NMR, and cyclic
voltammetry. By using molar conductance studies, the complexes are formulated as
[Zn(TPTTP)]Cl2 and [Ru(TPTTP)Cl2]. C-H bond activation of an sp3 group of methylstyrenes
(converted into cinnamaldehydes) and C-H bond activation of the sp2 bond of polycyclic
aromatic hydrocarbons through photooxidation was examined in the presence of Zn(II) and
Ru(II) complexes. Reusable activity studies and photostability of catalyst are investigated by
using UV-visible spectra. Based on the results, higher catalytic activity of [Ru(TPTTP)Cl2]
complex than [Zn(TPTTP)]Cl2 complex in both C-H bond activation and photooxidation of
aromatic hydrocarbons has been reported.
Keywords: Ru(II) and Zn(II) Complexes, C-H bond activation, Methylstyrenes,
Photooxidation, Polycyclic aromatic hydrocarbons
aDepartment of Chemistry, Osmania University, Hyderabad, India
bDepartment of Chemistry, National Institute of Technology, Kurukshetra, India.
*Corresponding author: E-mail address: somesh.pola@gmail.com Tel: + 919959972288
Introduction
Tetradentate macrocyclic Schiff base ligands and their metal complexes are of very
attention in coordination chemistry due to broad applications in several fields such as material
chemistry [1] and medicinal chemistry [2]. These ligands and metal complexes are examined
due to their essential properties, such as their capability to photooxidation [3], and
applications as C-H bond activation for generation of new organic functional materials [4].
Piperazine itself is achiral, but the usage of the simply existing chiral piperazines as building
blocks mains to chiral macrocycles. Hence, piperazine is an outstanding choice as a
multipurpose building block for the synthesis of achiral or chiral macrocycles [5]. Due to the
overall structural resemblance of piperazine and glucose, the macrocycle consisting of
piperazine as a significant building block would be structurally mimic to the cyclodextrins
1

[6]. There is a need to design an efficient and green catalytic system, which would activate
under milder reaction conditions for many technologically important processes [7]. Jacobsen’s
Mn(Salen) complex has appeared as a significant catalyst for the oxidation of alkenes in the
pharmaceutical industry [8, 9]. Oxidation of natural products such as terpenes to oxygenated
products is of attention; subsequently, these products are converted into natural products [10].
The allylic oxidation of α-pinene forms several products in that one of the products is d-
verbenone which is significant due to its utilization in the synthesis of Taxol [11,12]. The
Cu(II) 2-Quinoxalinol Salen is also reported as a very efficient catalyst for allylic oxidations
[13-16]. Whereas these studies are originated to be appropriate, still several restrictions like
harsh reaction conditions, complex workup and/or purification process, the formation of
destructive wastes, low efficient group easiness, and high costs remain.
As part of our sustained attempts in the synthesis of metal complexes based on novel
N, N and N, O donors as ligands and to investigate their applications in organic synthesis
[17], studies have been led to exploring and know the photophysical, electrochemical and
catalytic performance of macrocyclic ruthenium complexes [18-23]. These specific metal
macrocyclic units can be utilized as preliminary building blocks to build the photo- and
redox-active supramolecular systems. Therefore, complexes of different transition metal ions
binding through macrocyclic ligands exhibited some significant properties such as a) the
photophysical properties with bound metals, and b) the electrochemical communiqué between
metal and ligand center. Lately, the amount of advancement in the chemistry of
tetraazamacrocyclic ligands and their complexes in specific has been noticeable, since these
structural motifs are engaging in various fields such as homogenous or heterogeneous
catalysis [24-29]. In this connection, we report here the synthesis of new Zn(II), and Ru(II)
macrocyclic Schiff’s base complexes and their applications in C–H bond activation through
sp3 and sp2 functional groups.
2. Experimental section and Characterization
Materials and methods
All the starting materials procured from Sigma-Aldrich with the analytical grade. Solvents
were distilled and dried by using standard techniques. [Ru(DMSO)4Cl2], substituted
methylstyrenes, methyl magnesium iodide, and other chemicals were purchased from TCI,
India and all the solvents are analytical grade products from Merck. The purity of the
compounds was monitored by TLC using Merck 60F254 silica gel plates.
2

All the starting materials and solvents were directly used as obtained from the suppliers and
recrystallized/redistilled as necessary. The SEM images captured on HITACHI SU-1500
variable pressure scanning electron microscope (VP-SEM). Thermograms of all samples
were obtained using Shimadzu differential thermal analyzer (DTG-60H) with a heating rate of
10°C min-1 in the range from 50 °C to 1000 °C under Nitrogen purging rate of 20 mL/minute.
Brunauer-Emmett-Teller (BET) surface areas were determined by nitrogen adsorption-
desorption isotherm measurements at 77 K on a Quantachrome autosorb automated gas
sorption system. CHN analyses were supported out utilizing Elemental Analyzer Flash EA
1112.Infrared spectra in KBr discs were recorded with a PerkinElmer BX series, Fourier
Transforms infrared spectrophotometer. 1H and 13C- NMR spectra were recorded on a
Bruker AV500 MHz Spectrometer. The HR FAB and HR documented mass spectra-EI
methods on JMS-700 double focusing mass spectrometer with a high resolution of Mass
Spectrometry Facility at IICT, Hyderabad, India. The conductance of the metal complexes
was measured on Labline digital conductivity meter. Melting points were recorded on a
Digital Melting Point Apparatus – 934 from Electronics India. The phases of the new
materials were established by powder X-ray diffractometer (Miniflex, Rigaku) (Cu K훼,
휆=1.5406 Å angle range of 2θ = 10° to 30°). The electronic spectra were measured in
C2H4Cl2/CH3CN (1:1) solutions on LAMBDA 1050 UV/Vis Spectrophotometer. Cyclic
voltammetry measurements were conducted on a PC-controlled CHI 62C electrochemical
analyzer in a C2H4Cl2 solvent at a scan rate of 100mVs-1 using tetrabutylammonium
perchlorate (0.1 M) as the supporting electrolyte. The glassy carbon, standard calomel
electrode (SCE) and platinum wire were used as working, reference and counter electrodes,
respectively. All the photoreactions were performed by using a multi-tube photo reactor
system with LED visible light, Lelesil Innovative Systems, India. Preparation of M(II)
complexes
3

Scheme-1. Preparation of new Ru(II) and Zn(II) complexes
2.1. Preparation of the Ru(2,4,13,15-Tetraphenyl-1,5,12,16-tetraaza-
tricyclo[14.2.2.06,11]eicosa-4,6(11),7,9,12-pentaene)Cl2
To 20 ml tetrahydrofuran (THF) solution of macrocyclic Schiff base ligand (2.00 mmol)
corresponding Tetrakis(dimethylsulfoxide)dichlororuthenium(II) (2.00 mmol) was added with
constant stirring at room temperature. Then the mixture was heated for five hours at 50oC. On
cooling, the solid product was precipitated out and filtered, washed with cold THF and dried
in a vacuum desiccator over fused CaCl2.
Rf 0.22 (10% MeOH/CHCl3); elemental analysis expt. (calcd): C% 64.28 (64.34), H% 5.11
(5.13) N%
7.46 (7.50)
Cl% 9.46 (9.50), and Ru% 13.34 (13.53); 1H NMR (400MHz,
d6-DMSO ) δ (ppm) = 8.229 (m, 2H), 8.026 (m, 2H), 7.977 - 7.909 (m, 10H), 7.815 – 7.725
(m, 10H), 4.990 (m, 2H), 3.045 (s, 8H), 2.744 (d, 2H), and 2.324 (d, 2H); 13C NMR (100
MHz d6-DMSO) δ (ppm) = 160.88, 143.24, 142.96, 137.50, 134.42, 128.57,126.65, 124.79,
123.01, 122.07, 120.21, 120.12, 112.58, 108.40, 58.40, 34.21 and 20.22 MS (FAB): 747.19
(M+1)+; UV-Vis: 376.92, 402.48, and 418.24 nm.
2.2. Preparation of the Zn(2,4,13,15-Tetraphenyl-1,5,12,16-tetraaza-
tricyclo[14.2.2.06,11]eicosa-4,6(11),7,9,12-pentaene)Cl2
To 20 ml tetrahydrofuran (THF) solution of macrocyclic Schiff base ligand (2.00 mmol)
corresponding ZnCl2 (2.00 mmol) was added with constant stirring at room temperature. Then
the mixture was refluxed for 5 h at 50oC. On cooling, the solid product was precipitated out
and filtered, washed with cold THF and dried in a vacuum desiccator over fused CaCl2.
Rf 0.28 (10% MeOH/CHCl3); elemental analysis expt. (calcd): C% 67.51 (67.57), H% 5.36
(5.39), N% 7.84 (7.88), Cl% 9.92 (9.97) and Zn% 8.99 (9.19); 1H NMR (400MHz, d6-
DMSO ) δ (ppm) = 8.192 (m, 2H), 7.994 (m, 2H), 7.865 - 7.798 (m, 10H), 7.785 – 7.691 (m,
10H), 4.504 (m, 2H), 3.742 (s, 8H), 2.702 (d, 2H), and 2.318 (d, 2H); 13C NMR (100 MHz
4

d6-DMSO) δ (ppm) = 160.79, 143.52, 141.83, 138.83, 135.93, 133.48, 131.86,128.77, 128.55,
126.51, 125.42, 125.12, 122.33, 118.72, 118.55, 111.47 57.41, 34.44 and 20.65 MS (FAB):
712.04 (M+1)+; UV-Vis: 362.13, 416.82, and 482.46 nm.
2.3. Catalytic activity studies
C-H bond activation of methylstyrenes
To a solution of Zn(II)/Ru(II) complex (1.25 μmol), methylstyrenes (0.5 mmol) and TBHP
(1.0 mmol) in 10 mL of acetonitrile was added at room temperature. After the reaction was
stirred under visible light for three hours, (monitored the reaction by LC-MS) solvent was
distilled under reduced pressure. Purification of crude products by column chromatography
with hexane/dichloromethane (9:1, v/v) as eluent to an afforded pure product.
Scheme 2.
General procedure for photooxidation
To a solution of Zn(II)/ Ru(II) complex (1.25 μmol), pyrene (0.5 mmol) and TBHP(1.0 mmol)
in 10 mL of acetonitrile was added. After the reaction was stirred under visible light for 6 h,
(monitored the reaction by LC-MS) solvent was evaporated using rotavapor. Purification of
the crude compounds by column chromatography with dichloromethane/ethyl acetate (9:1,
v/v) as eluent to an afforded pure product.
3. Result and Discussion
3.1. Molar conductivity measurements
The molar conductivities of the Zn(II) and Ru(II) complexes under investigation are measured
for 10−3M solutions in various solvents such as DMF, DMSO, DMA, and NMP as presented
in Table1 and Table S1. For Ru (II) complexes have ΩM values in the range 8.0–12.0 ohm−1
cm2 mol−1 which indicate that the complex is non-electrolytes [30]. However, Zn(II)
5

complexes exhibited higher values in the range 121 – 124.2 ohm-1cm2mol-1 which reveal that
the complex is 1:2 electrolytic in nature [31]. The existence of chloride ions in Zn(II) complex
is sensed by adding silver nitrate solution, immediately white precipitate was formed [32] and
molar conductance was dropped.
Table 1. Analytical and Physicochemical data of Zn(II) and Ru(II) complexes
Calcd.
Mol.w
Measured
Mol. w
m/z
%
aM.P/
Λm ohm-1
mol-1 cm2
(DMSO)
---
Compound
Yield D.P.
%C
%H
%N
%Cl
%M
(oC)
TPTTP
C40H38N4
[Ru(TPTTP)Cl2
C40H38Cl2N4Ru
[Zn(TPTTP)]Cl2
C40H38Cl2N4Zn
574.7
574.54
48
51
56
206
312
284
83.53
(83.59)
64.28
(64.34)
67.51
(67.57)
6.74
9.72
--
--
(6.66) (9.75)
5.11 7.46
(5.13) (7.50)
5.36 7.84
(5.39) (7.88)
746.73
711.05
747.19
712.04
9.46
(9.50)
9.92
13.34
(13.53)
8.99
8.34
118.29
(9.97)
(9.19)
3.2 Thermal Analysis
All the metal complexes were pure and dried under vacuum at ambient conditions
before studying their thermal stability. A weighed quantity of the complexes is heated at
200oC in the hot-air oven for 2 h. No weight loss or the change in the appearance of the
complexes was observed. The thermal data is presented in Table 2, and typical thermograms
are revealed in Figure 1. The single stage of decomposition occurs between 230 and 570 ºC
for both the complexes with a weight loss of 90.89 and 65.93 for [Zn(TPTTP)]Cl2 and
Ru(TPTTP)Cl2 respectively, which matches to the loss of ligand molecule [32].The weight of
the final oxide left in the crucible was signifying the formation of respective metal oxide and
the corresponding data shown in Table-2. The results of thermal analyses confirms the
formulae of the complexes to be [M(L)] [where L = TPTTP and M = Zn(II) andRu(II)] and
conforms with the formula derived from elemental analyses data.
Table –2: Thermal data of M(II) complexes
Pyrolysis
product, %
Found Calcd
Temperature
Complex
Thermal Process
Range, 0C
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2 [ZnO]
[Ru(TPTTP)Cl2] [Ru2O3] 306.8 – 562.5
232.5 – 505.6
90.89
65.93
91.23
66.50
3.3 FESEM analysis
The morphology of the complexes compared with ligand is completely changed in
shape and size [33]. The SEM images of [Zn(TPTTP)]Cl2and Ru(TPTTP)Cl2complexes
exposed that the particles are converted into like nanoplates type particles with variable shape
and nano-octahedra as correlated with the ligand as shown in Figure 2. In this connection
6

surface area of the complexes are far higher than the ligands and values are presented in
Table-3.
Table 3. Surface area and Bandgap energies of ligands and Pd(II)-complexes
Compound
Surfac Bandgap energy (eV)
Onset value from UV
spectra (nm)
e area
(m2/g)
Experimental Theoretical Experimental Theoretical
Value
3.48
Value
4.08
Value
356
Value
382
TPTTP
14
[Ru(TPTTP)Cl2 68
[Zn(TPTTP)]Cl2 32
2.23
2.84
2.39
3.51
556
436
518
353
3.4 Spectral analysis of complexes
The mass spectra of the metal complexes were verified, and the data presented in
Table 1. The mass spectra metal complexes were characterized by a peak corresponding to
Schiff base fragmentation. The mass spectra of the complexes show some important peaks
along with molecular ion peak (base peak) and these peaks matched with ligand mass and
ligand fragments in the spectra (Figure 3). In the mass spectra of the complexes, the peaks at
m/z 712.04, and 747.74 can be represented to the molecular ion peaks ([ML]+1) (L = TPTTP
and M = Zn(II)and Ru(II) complexes, respectively] [34, 35]. The fragmentation pathway of
Zn(TPTTP)Cl2 complex as shown in Figure S1 and S2 and some of the significant peaks are
identified which are matching with the spectral data. The IR spectra of the complexes were
presented in Figure 4. There are some valuable peaks in the spectra of the complexes such as
ν(C=N), ν(M-N) and ν(M-Cl). The ν(C=N) stretching vibration is shown in the free ligand
TPTTP at 1628 cm-1 [36]. This peak was shifted to lower wavenumber region by 25 cm-1 in
both the complexes, signifying that the sharing of azomethine nitrogen in coordination to the
metal ions. Similarly, the tertiary amine band also found to have shifted by 40 cm-1confirming
that the amine nitrogen atoms are involved in coordination [37]. New and most typical peaks
found in the spectra of the metal complexes in the far-IR region and exhibited two important
peaks at 403, and 436cm-1 are assignable to stretching vibrations of the ν (M–N) [38]. A band
showed in the range of 328 cm-1 in the spectra of the Ruthenium complex designates as the
existence of two chloride ions positioned trans to each other around metal center [38]. The IR
spectral data confirm that TPTTP acts as neutral tetradentate ligand coordinating to the metal
ions through azomethine-N, and tertiary amine-N atoms. This spectral data indicates that both
the complexes (Zn(II) and Ru(II)) are mononuclear.1H-NMR spectra of Zn(TPTTP)Cl2 and
Ru(TPTTP)Cl2 complexes recorded in d6-DMSO. The resonance signals of azomethine [35]
7

and tertiary nitrogen atom attached methylene protons are shifted to the downfield side which
indicating that the Zn(II) metal is coordination of TPTTP ligand. In the case of
Ru(TPTTP)Cl2 complex spectrum exhibited two significant changes compared with the ligand
spectrum,i.e., imine and tertiary groups nitrogen atoms are coordination with Ru (II) ions.
Then the Ar-Hs and tertiary –CH- protons are changed to the deshielding side as shown in
Figure S3 and Figure 5. Similarly, all the peaks are shifted in 13C-NMR spectra of both the
complexes compared with ligand as shown in Figure 6 and 7.
The powder XRD outlines of the TPTTP ligand, [Ru(TPTTP)Cl2] and [Zn(TPTTP)]Cl2
complexes showed 2θ peaks are presented in the range of 5 to 30 and high-intensity 2θ peaks
at 16.5o, 10.2o and 9.3o respectively. However, [Ru(TPTTP)Cl2] and [Zn(TPTTP)]Cl2 pattern
were varied when compared with the pure ligand. Therefore, Ru2+and Zn2+ions have been
coordinated with the N-atoms of the ligand. The powder XRD pattern for TPTTP ligand,
[Ru(TPTTP)Cl2] and [Zn(TPTTP)]Cl2 complexes are shown in figure S4. Therefore,
complexes are very stable and coordinated with nitrogen atoms. Finally, based on
spectroscopic data the Zn(II) and Ru(II) are coordinated as follows:
Electrochemical Properties
The redox properties of [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 were examined using cyclic
voltammetry in C2H4Cl2/CH3CN (1:1 ratio,0.1 mol L–1 n-Bu4NPF6). As shown in Figure 8,
one-electron oxidation waves were noticed in the potential region -2.0 to + 2.0 V (vs
Ag/AgCl). Estimation of their redox behaviors to that of TPTTP designates that the two
oxidations around 0.695 V (퐸11/2) and 1.256 V (퐸12/2) are tertiary amine and imine based,
which are derived from the sequential oxidation of neutral TPTTP to the radical cation
(TPTTP+) and then to the dication (TPTTP2+). Upon complexation ([Zn(TPTTP)]Cl2, and
8

[Ru(TPTTP)Cl2] complexes), the first oxidation potential is moved to the slightly more
positive side [39-41], due to the electron-withdrawing inductive effect of the metal centric
nature. However, the second oxidation potential also shifted towards negatively by 0.892 V.
This shift can be explained by the electronic interactions between the TPTTP unit and the
RuCl2 produced by the shorter distance among them [42]. The third redox procedure at about
퐸13/2=-1.452 V can be designated to the Ru(II)-centered one-electron oxidation technique [43].
Electronic spectra
The electronic spectra of the complexes were measured in C2H4Cl2/CH3CN (1:1 ratio).
The electronic spectra of the free ligand exhibited three bands at 321, 337 and 361 nm,
indicates the π–π* and n–π* transitions in the aromatic ring and imine group [44]. The
absorption spectra of complexes [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 in C2H4Cl2/CH3CN (1:1
ratio) at room temperature were recorded. As represented in Figure 9, intense absorption
peaks are involving to the admixture of intraligand (π–π*) transitions of the TPTTP moiety
and the Zn2+ ions and are noticed in the range of 350–450 nm, which are red-shifted and more
intense compared with the free ligand TPTTP. The redshift is sensible since the energy of the
LUMO confined on the TPTTP is lowered upon coordination to Ru(II) [45]. In addition,
complexes [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 also show broad bands at lower energy (350–
600 nm), which could be indicated to the admixture of metal-to-ligand charge-transfer
(MLCT, dπ(Ru)→π*(TPTTP) and ligand-to-ligand charge-transfer (LLCT) transitions
[41,46].
TDDFT Calculations
The ground state geometry optimization of ligand (TPTTP), Zn(TPTTP)]Cl2 and
Ru(TPTTP)Cl2 metal complexes are carried out in gas phase without any symmetric
considerations by using DFT method at B3LYP level. The LANL2DZ effective core potential
basis set was used for Zn and Ru metals [47], and 6-31G (d, p) basis set was used for ligand
and H, C, N, and O-atoms of metal complexes. The calculated vibrational spectrum in all the
optimized geometries has no imaginary frequencies, which indicates that all the optimized
structures are located at the minimum point of the potential energy surface. Electronic
absorption properties were carried out using the TDDFT methodologies at B3LYP functional
for the optimized geometries. All the calculations were performed using the Gaussian09
software [48].
To get an understanding of absorption energies and optical band gap, we have
supported out TDDFT calculations for these molecules. Calculated absorption energies for
9

ligand and metal complexes are shown in Table 4a and are well matched with the experiment.
TPTTP shows maxima absorption at 382 nm with 0.019 oscillator strength, and meaningful
contribution is between HOMO-1 to LUMO (51%) and HOMO to LUMO (45%). Zn (II)
metal complex has an absorption maximum of 422 nm with 0.340 oscillator strength, and the
main transition is between HOMO to LUMO (93%). In the case of Ru (II) complex, the first
excited state is with low oscillator strength (0.001) showing absorption at 792 nm, and the
transition is between HOMO to LUMO (92 %). The second excitation for Ru (II) complex is
at 709 nm with 0.030 oscillator strength, and the major transition is from HOMO-1 to LUMO
(92 %). Calculated absorption energies for ligand and metal complexes are shown in Table 4a
and corresponding molecular orbital pictures are shown in Figure 10.
The calculated HOMO-LUMO gap (HLG) is dependable with the experimental band
gap. It is evident from Table 4b. HOMO and LUMO levels of metal complexes are
stabilized/destabilized when compared with its corresponding ligands, and the energy levels
are shown graphically in Figure 11. The calculated HLG for TPTTP is 4.08 eV, and
Zn(TPTTP)Cl2 is 3.51 eV, it decreased the gap of 0.57 eV. Similarly, in the case of
Ru(TPTTP)Cl2 complex, the HLG is 2.39 eV, the difference in the gap concerning the ligand
is of 1.69 eV. This decline in band gap from ligand to its metal complexes is due to the
stabilization/ destabilization of HOMO and LUMO levels of complexes, and corresponding
values are shown in Table 4b.
Table 4a. Absorption energies ( in nm), Oscillator strength (), Transitions and Weight
in % calculated at TD-B3LYP/6-31G (d, p) level of theory for B3LYP/6-31G (d, p)
optimized geometries (for metals LANL2DZ basis set was used).
Name
TPTTP
λabs
ƒ
Transition
% Ci
51
45
93
3
382 0.019 H-1 -> L
H -> L
422 0.340 H -> L
792 0.001 H-2 -> L
H-1 -> L+1
[Zn(TPTTP)]Cl2
[Ru(TPTTP)Cl2]
2
H -> L
709 0.030 H-1 -> L
92
92
Table 4b: Calculated energy levels of HOMO, LUMO and HOMO-LUMO Gap (HLG)
are shown in eV.
TPTTP [Zn(TPTTP)]Cl2 [Ru(TPTTP)Cl2]
Compounds

HOMO
LUMO
-5.20
-1.12
-11.34
-7.83
-4.29
-1.90
10

HLG
4.08
3.51
2.39
6. C-H bond activation of sp3- C of methylstyrenes (oxidation)
α, β-Unsaturated aldehydes, mostly cinnamaldehydes, play a significant role in various
fields such as food, pharmaceutical, and cosmetic industries [48–53] as well as substantial
precursors for organic synthesis [54]. Hence, many applied approaches, including allylic
alcohols oxidations [55], Peterson olefinations [56], formylation [57], Mannich reactions and
cross-aldol condensations [58] and Saegusa oxidations [59,60] have been established for their
preparation process. Still, these approaches are limited, comparatively harsh reaction setups,
unapproachable reactants, high-cost catalysts, and/or unacceptable selectivity (Chemo- and
regioselectivity). To overwhelmed these disadvantages, a predominantly prominent another
approach is C-H bond activation of sp3 carbon of methylstyrenes under visible light
irradiation technique.
The Zn(II) and Ru(II) complexes of TPTTP were used as catalysts in the oxidation
reactions to get cinnamaldehydes. The oxidation reactions were carried out on a millimole
scale using 0.5 to 1.5 mol % catalyst charging. The oxidation reactions were examined in the
presence and absence of various oxidizing agents such as H2O2/TBHP/m-CPBA with the
catalyst in different solvents to optimize the conditions for better yields. In a typical reaction,
the yield of the product, cinnamaldehyde, increased from 52% to 88% with TBHP as an
oxidant in acetonitrile.
Further, a similar improvement in the yield was observed when TBHP was increased
from 1 equiv. to 2 equiv. In other catalytic protocols reported [61], 10 to 12 equivalents of
TBHP was used to complete the reaction. As anticipated, in the reaction mechanism only 2
equiv. of TBHP is essential for the oxidation of methylstyrenes to cinnamaldehydes.
However, an excess of oxidant is used as some of the oxidants gets cinnamic acid as a
byproduct. Therefore, evident from the investigations using H2O2 which is known to
decompose to a greater extent and required more than ten equivalents of H2O2in the oxidation
reactions.
In the oxidation reactions using m-CPBA and [Zn(TPTTP)]Cl2 complex as a catalyst,
it was observed that very less yield of products was formed and the mixture was too complex.
Therefore, using two equiv. of TBHP and 1.25 mol % of the complex were used to study the
influence of solvent systems on the oxidation of methylstyrene. Reactions progressed very
slowly in CH2Cl2, CHCl3, DMSO, toluene, and DMF than CH3CN as shown in figure 12. In
this case, under inert conditions, only 26% yield was found in 3 h of reaction time as
11

compared to 92% yield in CH3CN solvent at normal atmospheric conditions. Therefore,
which indicates that the contribution of O2 in the oxidation of the various reactants in the
presence of different complexes as catalysts.
C-H bond activation of a diversity of methylstyrenes was studied using 1.25 mol % of
Zn(II) and Ru(II) complexes two equiv. of TBHP in CH3CN under visible light irradiation
(Table 5). All of the reactants were transformed into the corresponding cinnamaldehydes in
excellent yields within an average reaction time in the presence of Ru(TPTTP)Cl2 when
compared with the complexes of [Zn(TPTTP)]Cl2 (Figure 13). The products were isolated and
purified using flash column chromatography using hexane/dichloromethane (3:1) as eluent,
resulting in a quite impressive isolated yield of 92% for liquid products, and the solid
products were purified by recrystallization process.
The C-H activation of methylstyrenes by the Ru(TPTTP)Cl2 and [Zn(TPTTP)]Cl2
complexes were performed as photocatalysts and based on results, Ru(TPTTP)Cl2 exhibited
more activity than [Zn(TPTTP)]Cl2. The optimized reaction conditions were used to carry out
the oxidation reactions of methylstyrene by the Ru(TPTTP)Cl2 and [Zn(TPTTP)]Cl2
complexes as a catalyst, and the obtained data are given in Table 5. The Zn(II) complex
showed low catalytic activity on all the substrates. The catalytic performance of the Ru(II)
complex is similar or even improved than the activity demonstrated by some of the complexes
reported earlier [62]. Ru(II) complex exhibited better catalytic activity than Zn(II) complex
due to high surface area and low bandgap energy.
Scheme 3 Proposed Catalytic Cycle for Oxidation Catalyzed by Ru(II) complex
The proposed mechanism of oxidation of allylic C–H group with TBHP involves binding of
TBHP to the metal ion, homolytic cleavage of O-O bond followed by the formation of Ru(II)-
oxo or Ru(III)-oxo bonds [63]. The plausible mechanism of oxidation reactions catalyzed by
the Ru(II) complex is shown in scheme 3. Similarly, the plausible mechanism of oxidation
reactions catalyzed by the Zn(II) complex through Zn(II)-oxo or Zn(I)-O· radials and shown
scheme S5 and another possible mechanism also possible and mentioned in scheme S6.
12

Table 5. M(II) complexes of TPTTP Catalyzed C-H bond activation of sp3 – C of
methylstyrenes (M = Zn and Ru) under visible light irradiation
S.
No
.
Complex
Reactant
Product
Tim % of
e (h) Yiel
d
1
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
38
92
36
85
29
78
31
76
35
81
37
83
31
76
2
3
4
5
6
7
8
9
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
27
79
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
3
3
24
73
33
74
10 [Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
11 [Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
21
69
13

12 [Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
26
81
13 [Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
3
3
23
78
21
73
14 [Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
15 [Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
3
3
36
85
Sp2 C-H bond activation
Oxidation of polycyclic aromatic hydrocarbons (PAHs) is one of a very significant process for
generating new heterocyclic fused PAHs [64-71]. Oxidation of PAHs with reported protocols
was meager yield, and various byproducts are formed, and the purification process is very
tough for required compounds. For this reason, using new Ru(TPTTP)Cl2 complex acts as a
photocatalyst for photooxidation of PAHs under visible light irradiation technique. Based on
the above allylic oxidation of methylstyrenes, photooxidation of PAHs also examined under
the same conditions for the formation of diones and presented in Table 6.
Table 6. Photooxidation of PAHs in the presence of Ru(TPTTP)Cl2 under visible light
irradiation
S.No. Reactant
Product
Complex
Time % of
(h)
Yield
1
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
74
36
2
3
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
78
32
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
73
27
14

4
5
6
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
85
41
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
80
39
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
78
33
7
8
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
73
28
[Ru(TPTTP)Cl2] 6
[Zn(TPTTP)]Cl2
65
21
Optimization of reaction conditions with pyrene and tetraphene in the presence of various
catalysts commercial and present catalysts such as [Zn(TPTTP)]Cl2, and Ru(TPTTP)Cl2 (5%
mol) under visible light irradiation and shown in figure 14.
Conclusion
In this work, we summarized the synthesis and characterization of new two Schiff
base macrocyclic ligand based complexes with Zn(II) and Ru(II). The newly synthesized
metal complexes are investigated by analytical, thermal, spectroscopic and electrochemical
studies. We studied sp3 and sp2 C-H bond activation via photooxidation conditions using
[Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 complexes for methylstyrenes and polycyclic aromatic
hydrocarbons different substrates and solvent effect. The Ru(II) complex exhibited excellent
catalytic activity in the oxidation of all the substrates to convert the desired oxidized products.
Supporting Information
15

Spectral data of complexes and α, β-unsaturated aldehydes such as 1H-NMR, and Mass
spectra included in are inserted supporting information part (Figure S7 - S47).
ACKNOWLEDGMENT
SP and CP thanks to SERB - New Delhi, India for financial support under UGC-MRP
6417/16 (SERO/UGC) and SB/FT/CS-101/2014 respectively and thanks to DST-FIST, New
Delhi. CP thank CSIR, New Delhi, India for financial support under EMR-II grant (No.
02(0339)/18/EMR-II). Special thankful to Prof. Yu-Tai Tao, Institute of Chemistry, Academia
Sinica, Taipei, Taiwan, for providing FESEM data.
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19

Fig. 1 Thermograms images for a) [Zn(TPTTP)]Cl2 and b) Ru(TPTTP)Cl2 of complexes.
Fig. 2 FESEM images of a) ligand TPTTP; b) [Zn(TPTTP)]Cl2 and c) Ru(TPTTP)Cl2 of
complexes.
Fig. 3 [Zn(TPTTP)]Cl2 of mass spectral fragmentation pathway.
Fig. 4 IR spectral pattern of [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 of complexes.
Fig. 5 1H-NMR spectra of ligand and Ru(TPTTP)Cl2
Fig. 6 13C-NMR Spectra of [Ru(TPTTP)Cl2]
Fig. 7 13C-NMR spectra of ligand and [Zn(TPTTP)]Cl2
Fig. 8 Cyclic voltammograms for a) ligand, b) [Zn(TPTTP)]Cl2 and c) Ru(TPTTP)Cl2 of
complexes.
Fig. 9 Electronic spectra of a) ligand; b) [Zn(TPTTP)]Cl2 and c) Ru(TPTTP)Cl2 of
complexes.
Fig. 10 HOMO-LUMO images of TPTTP, [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2.
Fig. 11 Graphical representation of HOMO-LUMO bandgap energies of TPTTP,
Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2
Fig. 12 Effect of solvent and their rate of conversion under visible light irradiation.
Fig. 13 Effect of complex on rate of conversion each reactant under visible light irradiation.
Fig. 14 Optimization reaction conditions in the presence of various catalytic system under
visible light irradiation.
Fig. 1 Thermograms images for a) [Zn(TPTTP)]Cl2 and b) Ru(TPTTP)Cl2 of complexes.
20

Fig. 2 FESEM images of a) ligand TPTTP; b) [Zn(TPTTP)]Cl2 and c) Ru(TPTTP)Cl2 of
complexes.
Fig. 3 [Zn(TPTTP)]Cl2 of mass spectral fragmentation pathway.
21

Fig. 4 IR spectral pattern of [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 of complexes.
Fig. 5 1H-NMR spectra of ligand and Ru(TPTTP)Cl2
22

Fig. 6 13C-NMR Spectra of [Ru(TPTTP)Cl2]
23

Fig. 7 13C-NMR Spectra of [Zn(TPTTP)]Cl2
Fig. 8 Cyclic voltammograms for a) ligand, b) [Zn(TPTTP)]Cl2 and c) Ru(TPTTP)Cl2 of
complexes.
24

Fig. 9 Electronic spectra of a) ligand; b) [Zn(TPTTP)]Cl2 and c) Ru(TPTTP)Cl2 of
complexes.
HOMO
LUMO
TPTTP
[Zn(TPTTP)Cl2
[Ru(TPTTP)Cl2
Fig. 10 HOMO-LUMO images of TPTTP, [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2.
LUMO
LUMO
-2
HLG 2.39
HOMO
HLG 4.08
-4
-6
HOMO
LUMO
-8
HLG 3.51
-10
-12
HOMO
[Zn(TPTTP)]Cl2
[Ru(TPTTP)]Cl2
TPTTP
Fig. 11 Graphical representation of HOMO-LUMO bandgap energies of TPTTP,
[Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2
25

100
80
60
40
20
0
DCM
Chloroform
Ethylacetate
Toluene
DMF
DMSO
Methanol
Ethanol
acetonitrile
[Zn(TPTTP)]Cl2
Ru(TPTTP)Cl2
Effect of Catalysts
Fig. 12 Effect of solvent and their rate of conversion under visible light irradiation.
100
[Zn(TPTTP)]Cl2
[Ru(TPTTP)Cl2]
80
60
40
20
0
0
-1
1
-1
2
-1
3
-1
4
-1
5
-1
-1
-2
-3
-4
-5
-6
-7
-8
-9
EntryEntryEntryEntryEntryEntryEntryEntryEntry
EntryEntryEntryEntryEntryEntry
Fig. 13 Effect of complex on rate of conversion each reactant under visible light irradiation.
26

100
80
60
40
20
Pyrene
Tetraphene
0
3
3
4
2
)2
y
e])
men
-cy
Cl2[p
h3)
pp
so)
bip
dm
PTTP)Cl2]
(T
RuCl3/NaIO
RuCl2(
RuCl2(
RuCl2(
[Ru
(Ru
Fig. 14 Optimization reaction conditions in the presence of various catalytic system under
visible light irradiation.
27

Synthesis and characterization of new [Zn(TPTTP)]Cl2 and Ru(TPTTP)Cl2 and
their photocatalytic activity studies through C-H bond activation (sp3 and sp2)
under visible light irradiation.
28