Synthesis of new fused heterocyclic aromatic hydrocarbons via C–S and C–C bond formation by C–H bond activation in the presence of new Pd(II) Schiff's base complexes

Abstract

The Schiff's base aza-macrocyclic Pd(DPTTP)Cl₂ and Pd(TPTTP)Cl₂ complexes are efficient photocatalysts for the activation of the C–H bond and the formation of both intramolecular C–C bond and C–S bonds to obtain the fused heterocyclic system. Both ligands and Pd(II) complexes are well characterized by experimental and theoretical techniques such as microanalyses, mass, surface area, SEM, TGA, XPS, IR, UV-visible, ¹H-NMR, DFT and TDDFT methods. C–H bond activation was confirmed by sequential reactions and the data were supported by a one-pot synthesis product. Optimization of the intensity of visible light of the photocatalytic reactions. Finally, the sp²-H bond activation via the formation of the C–S and intramolecular C–C bonds in the presence of both the complexes were performed under visible light irradiation to obtain the new fused system, pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine. It is also shown that the performance of the Pd(TPTTP)Cl₂ is better than that of Pd(DPTT)Cl₂ due to its high surface area and low bandgap energy.

---

Introduction

Linear fused heterocycles have a very broad range of applications in the research fields of medicinal and materials chemistry.¹ The synthesis of new compounds and their applications are important in various areas of the chemical sciences.^2–6 C–H bond activation is a key part of organic syntheses.⁷ Most frequently, C–C bond formation and cross coupling reactions^8–16 are carried out in the presence of Pd(0) complexes for the synthesis of new fused heterocyclic systems. However, several coupling reactions are performed in the presence of phosphorous based ligands and PdX₂ (X = OAc or Cl) in which Pd plays a vital role in the construction of the C–C bond between aryl rings. In coupling reactions, organometallic intermediates are developed for the formation of C–C bond; however, Mg, Zn, Sn and phosphorous containing byproducts are obtained.¹⁷ Some of the byproducts are toxic and may contaminate water and soil in the environment. Therefore, it is essential to reduce the usage of Sn and P based compounds.¹⁸ Moreover, it is critical to design new ligands and their Pd-complexes. To avoid possible toxic byproducts, we designed a new macrocyclic system.

Macrocycles are very promising materials for various fields such as medicinal chemistry, metal–organic frameworks, catalysis, photocatalysis, transistors, and sensors.^19–23 Macrocyclic ligands are designed as preorganized systems for several applications as sensors for cations, anions, and neutral species.^24–26 In most of the cases, macrocyclic Schiff base complexes are used as heterogeneous catalysts in numerous organic transformations such as C–H bond activation, C–X (X = C & S) bond formations and multi-component reactions.²⁷ Furthermore, aza-macrocyclic complexes act as a photocatalyst for the splitting of water molecules under visible light irradiation.²⁸ In general, the bandgap energies of the complexes are lower than the respective ligands. The bandgap energy values of complexes fall into the visible region that is less than 3.0 eV. Hence, macrocyclic Schiff base complexes play a significant role in photocatalysis applications.²⁹ However, Pd(II) complexes are used in various types of organic coupling reactions under different thermal conditions.³⁰ Over the period, Pd(II) complexes have been designed with phosphorous-containing ligands.³¹ On the other hand, in the coupling reactions, phosphorous containing ligands were easily converted into oxides as byproducts, which are difficult to remove from the products. Hence, special methods are required to purify the final products. From the literature survey on Pd(II) complexes of various ligands for variety of organic coupling reactions revealed that few gaps are present in the visible-light-driven conditions.

In this study, very simple macrocyclic ligands were used for the formation of Pd(II) complexes and were utilized for C–H bond activation and C–S and C–C bond formation of new polycyclic heterocycles. Synthesis of fused and linear heterocyclic system consisting of pyrido[3,2-f][1,4]thiazepine and naphthyridine moieties have not been conducted. Recently, a naphthyridine containing scaffold has been established as s prominent compound used for a broad range of biological activities.³² Various types of naphthyridine isomers have been developed for different types of applications such as medicinal chemistry, herbicides, and materials chemistry.^33,34

In the database shows that the preparation of benzo[f][1,4]thiazepine and its analogs have been reported;³⁵ however, the synthesis of pyrido[3,2-f][1,4]thiazepine based compounds consisting of N-cyanomethyl-N-methyl amide and carbon disulphide, which are used as reagents, have not been reported. Very few scaffolds such as these are reported in the literature.³⁶ Thiazepine ring containing heterocycles are well-established compounds with fascinating pharmacological activity.³⁷ The best example (Fig. 1), compound 1, is structurally equivalent to furosemide and shows highly efficient diuretic activity that is intensely influenced by the conformational flexibility of the 4-substituents in 3-amino-5-sulfamylbenzoic acid,³⁸ revealing the future medicinal use of compound 1. The conformationally strained compound 2 is a scaffold of thiazepine-analog of the antidepressant nitroxazepine (Sintamil).³⁹ The tetracyclic compound 3 is a non-steroidal antagonist of the progesterone receptor.⁴⁰ Moreover, compound 4 is valuable for use in neurogenic and migraine inflammation. Several synthetic techniques have been reported for the benzothiazepine derivatives for various types of diseases in medicinal chemistry.⁴¹ While, synthetic routes for the synthesis of new pyrido[3,2-f][1,4]thiazepine containing scaffolds are limited in the literature.

Fig. 1 Important frameworks based on benzothiazepine compounds.

There are a few reports on the synthesis of new fused 1,6-naphthyridine derivatives.⁴² We are interested in a study on fused heterocyclic compounds composed of two active pharmaco moieties, such as pyrido[3,2-f][1,4]thiazepine and 1,6-naphthyridine, in a hydride molecular scaffold because such fused molecules are endowed with a variety of biological activities and have a broad range of therapeutic properties. As a result, the present work has no phosphorous atom present in the ligand and very simple reaction conditions for the formation Pd(II) complexes with macrocyclic Schiff base ligands. C–H bond activation is one of the important strategies for the formation of C–C and C–S bonds and were achieved with new Pd(II) based photocatalysts. The new photoreaction conditions are optimized through sequential reactions and one-pot method for establish the reaction pathway and explains the mechanism of the reaction under visible light irradiation.

Materials and experimental methods

The starting materials used were of analytical grade and purchased from Sigma-Aldrich and were used without further purification such as 2-bromonicotinaldehyde,⁴³ 2-aminoethanethioamide, N-ethyl-N-isopropylpropan-2-amine, piperazine, chalcone, cinnamaldehyde, PdCl₂, toluene and other solvents.

Preparation of ligands

Stage-1. According to a reported method,^44,45 aza-Michael products have been prepared by the following procedure.

To a solution of piperazine (0.86 g, 10 mmol) and cinnamaldehyde or 1,3-diphenyl-propenone (1.32/2.08 g, 10 mmol) in 100 mL of dichloromethane, K₂CO₃ (20 mol%) was added. Then, the reaction mixture was continuously stirred at room temperature for 48 h. The reaction was monitored by TLC and the reaction mixture was worked up to obtain the crude product and recrystallized from ethanol to give pure compounds such as 3,3′-(piperazine-1,4-diyl)bis(3-phenylpropanal) (PBPP) and 3,3′-(piperazine-1,4-diyl)bis(1,3-diphenylpropan-1-one) (PDPP).

Stage-2. The compound PBPP or PDPP (10 mmol) and 1,2 diaminobenzene (12 mmol) were dissolved in 30 mL of absolute alcohol and 10 mL of acetic acid in a round bottom flask. The mixture was refluxed for 16 h. The reaction was monitored by TLC. After removal of the solvent from the reaction mixture by a rotary evaporator, the obtained dark yellow residue was purified by recrystallization with chloroform and methanol (9 : 1) to obtain the pure product of 2,15-diphenyl-1,5,12,16-tetraaza-tricyclo[14.2.2.06,11]eicosa-4,6(11),7,9,12-pentaene (DPTTP) or 2,4,13,15-tetraphenyl-1,5,12,16-tetraaza-tricyclo[14.2.2.06,11]eicosa-4,6(11),7,9,12-pentaene (TPTTP) (Scheme 1).

Scheme 1 Preparation of ligands.

Synthesis of complexes

To 10 mL of a warm solution of Pd(CH₃CN)₂Cl₂ (10 mmol), a solution (10 mL) of the respective ligand (10 mmol) (DPTTP/TPTTP) in toluene was added and refluxed. After 30 minutes, the reaction mixture's color changed from brown to orange. Nonetheless, the reaction was continued for four hours at 120 °C and a sample of the reaction was analyzed by LC-MS. After 4 h, the starting materials disappeared in the LC-MS data. The resulting precipitate was collected on a fine frit and washed with cold methanol, acetone and then with dichloromethane. Both the Pd-complexes were dried in vacuo over fused calcium chloride until no change in weight was noticed (Scheme 2).

Scheme 2 Preparation of Pd(II) complexes.

Catalytic properties

Synthesis of pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine and its derivatives was conducted by the following procedure.

The quartz tube charged with 2-bromonicotinaldehyde (1.86 g, 10 mmol) and 2-aminoethanethioamide (5.5 mmol) in 100 mL of water and toluene (9 : 1) was stirred for 15 minutes. Then, 5.16 g of N-ethyl-N-isopropylpropan-2-amine (DIPEA) (40 mmol) and 0.200 mmol of Pd(DPTTP)Cl₂ or Pd(TPTTP)Cl₂ was added to the reaction mixture and exposed to visible light irradiation (tungsten lamp at 500 watts). After 6 hours, TLC (50 : 50 ethyl acetate–hexanes) showed a product spot at R_f 0.45. The reaction mass was cooled to ambient temperature and then diluted with ethyl acetate and extracted. The organic layer was dried over MgSO₄, filtered and the solvent was removed by rotary evaporation. The product was purified by flash chromatography on a silica gel using 40 : 60 ethyl acetate–hexanes as an eluent (Scheme 3).

Scheme 3 Synthesis of pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine.

Computational details

The ground state geometries of ligands (DPTTP & TPTTP) and palladium metal complexes (Pd(DPTTP)Cl₂ and Pd(TPTTP)Cl₂) were fully optimized in the gas phase without any symmetric considerations by a DFT method at the B3LYP level of theory (Becky's three-parameter hybrid function with Lee–Yang–Parr correlation functions).^46,47 The standard basis set of atomic functions 6-31G(d,p) was used for ligands and H, C, N and O-atoms of metal complexes and a LANL2DZ effective core potential basis set was used for palladium.⁴⁸ The calculated vibrational spectrum in all the optimized structures had 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 determined using the TDDFT methodologies for the optimized geometries, and frontier molecular orbitals responsible for the excitations, leading to the absorption maximum, were pictorially visualized from the population analysis. To obtain the HOMO–LUMO gap, LUMO energies were taken from the sum of the transition energy (E_TDDFT) obtained from TDDFT plus the HOMO energy (E_HOMO) obtained from the optimization.⁴⁹ All the calculations were performed using the Gaussian09 software.⁵⁰

Physical measurements

CHN analyses were performed by an Elemental Analyzer Flash EA 1112. The conductance of the metal complexes was measured on a Digisun digital conductivity meter model DI-909. Mass spectra were obtained on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan). Thermograms of all the samples were obtained using a Shimadzu differential thermal analyzer (DTG-60H) with a heating rate of 10 °C min⁻¹. The SEM-EDS images were captured on a HITACHI SU-1500 variable pressure scanning electron microscope (VP-SEM). X-ray photoelectron spectroscopy (XPS) measurements were performed on a KRATOS AXIS165 X-ray photoelectron spectrometer with an excitation energy of 1253.6 eV (Mg Kα) and pass energy of 80 eV. FT-IR spectra were acquired using a Shimadzu spectrometer in the form of KBr pellets. Infrared spectra (KBr disc technique) has recorded with a PerkinElmer BX series; Fourier transforms infrared spectrophotometer. ¹H and ¹³C{1H} NMR spectra were obtained on a Bruker AV400 MHz spectrometer with chemical shifts referenced using the ¹H resonance of residual CHCl₃. Melting points were verified on a Cintex apparatus. Electronic spectra were obtained in chloroform solutions on a JASCO V-650 UV-Vis spectrophotometer. 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. Interfit INT160 EZ LITE Tungsten 2500 watt light was used for photocatalytic studies.

Results and discussion

The elemental analysis and spectral data are presented in Table 1. The mass spectra of DPTTP and TPTTP (Fig. S1 and S2†) confirm the proposed formula by showing a peak at m/z 446 (M + 23)⁺ (65%) corresponding to the molecular ion [C₂₈H₃₀N₄ + 23]⁺. It also shows a peak at m/z 332 corresponding to M − C₇H₇⁺. Similarly, the mass spectrum of the TPTTP displays a peak at m/z 575 corresponding to the macrocyclic moiety [(C₄₀H₃₈N₄) + 1]⁺. These data are in full agreement with a Schiff's base with 1,2-diaminobenzene. The electronic absorption spectrum shown in Fig. S3 and S4† revealed the two maximum absorptions at 348 and 432 nm, which are assigned to the n → π* of DPTTP and TPTTP, respectively. The IR spectra of the DPTTP and TPTTP shown in Fig. S5 and S6† exhibit absorptions at 3145 cm⁻¹, which are assignable to ν(C–H) of a piperazine ring. Moreover, another peak at 1348 cm⁻¹ corresponds to a tertiary amine ν(C–N), and 1673 cm⁻¹ was assigned to ν(C=N) groups.^{51 1}H NMR spectral data of both DPTTP and TPTTP (Fig. S7 and S8†) show the signals of protons corresponding to the proposed structure, and it does not show any signal corresponding to primary amine protons that are present in the reactants. It gives a triplet at 8.937 ppm, which is relevant to azomethine protons⁵² (CH=N, 2H's, J_1,3 = 5.78 Hz). The double doublets merged with the middle signals and appeared as a triplet at 3.534 ppm for tertiary methyl protons (Ar–CH–N–, 2H's, J_1,3 = 8.7 Hz) and another double doublets between 1.989 and 1.857 ppm may be assigned as CH₂ protons attached to the imine group (CH₂–CH=N, 4H's, J_1,2 = 8.7 Hz and J_1,3 = 17.3 Hz). A singlet at 2.676 ppm was assigned to the piperazine –CH₂ protons and also it gives a singlet in the region of 7.861–7.065 ppm for aromatic protons (14H's). Similarly, the ¹H-NMR spectrum of TPTTP (Fig. S6†) recorded in CDCl₃ gave a multiplet at 7.985–7.296 ppm corresponding to aromatic protons (24H's). In TPTTP ligand, two azomethyine are replaced with corresponding two phenyl rings therefore, no azomethine protons but two phenyl rings protons additionally exhibited in ¹H-NMR spectrum of TPTTP as compared with DPTTP.

Table 1 Analytical and physicochemical data of ligands and Pd(II) complexes

Compound	Mol. w (calcd)	m/z	% yield	M.P./D.P.^a (°C)	% C	% H	% N	% Cl	% Pd	Λm (ohm^−1 mol^−1 cm^2)
a M.P. = melting point/decomposition point.
DPTTP	422.4	422.56	52	174	79.54	7.26	13.21	—	—	—
C28H30N4	(422.5)				(79.59)	(7.16)	(13.26)
TPTTP	574.3	574.54	48	206	83.53	6.74	9.72	—	—	—
C40H38N4	(574.7)				(83.59)	(6.66)	(9.75)
[Pd(DPTTP)Cl2]	599.89	600.18	65	250	55.96	5.12	9.41	11.76	17.51	122.4
C28H30Cl2N4Pd					(56.06)	(5.04)	(9.34)	(11.82)	(17.74)
[Pd(TPTTP)Cl2]	752.08	752.12	68	281	63.82	5.16	7.54	9.35	14.08	119.1
C40H38Cl2N4Pd					(63.88)	(5.09)	(7.45)	(9.43)	(14.15)

---

The Pd(II) complexes are air stable and freely soluble in THF, DMF, and DMSO but sparingly soluble in methanol, ethanol, acetone, and benzene. The analytical data of the metal chelates presented in Table 1 indicate that the metal ions are coordinated to one ligand molecule. The large molar conductivities of all the complexes in DMF shows that Pd(DPTTP)Cl₂ and Pd(TPTTP)Cl₂ are ionic in nature.⁵³ The mass spectra of the Pd(II) complexes reveal that base peak is [PdL]²⁺ ion (Fig. S9†). The mass spectra, analytical and thermal data authenticate the formulae of the complexes as [PdL].⁵⁴ Furthermore, all the species containing the metal ion were confirmed by good agreements between the observed and calculated isotopic distributions.⁵⁵

The morphology of the complexes compared with the ligands was entirely different in shape and size. The SEM images of the Pd(II) complexes revealed that the particles were converted into structures such as nanorods and nanosticks compared to the ligands, as shown in Fig. 2. The surface areas of the complexes were far better than the ligands (Table 1).

Fig. 2 SEM images of DPTTP, TPTTP, Pd(DPTTP)Cl₂ and Pd(TPTTP)Cl₂.

The thermograms (Fig. S10 and S11†) of the Pd-complexes first show weight loss in the temperature range of 260–560 °C, related to one ligand molecule. Therefore, single stage decomposition suggested that 1 : 1 complexes were formed.⁵⁶ On further heating, the complexes reveal a mass loss curve beyond 600 °C and continued to a temperature around 800 °C, after which the TG becomes parallel to the temperature axis. The percentage of the residue left was established to be a metal oxide PdO. The obtained residue oxide was further analyzed with XPS and data revealed the formation of PdO,⁵⁷ as shown in Fig. 3. The proper assignment of the majority of the IR bands of the Schiff base macrocycles and its corresponding metal complexes are recorded in the experimental section. The ligand contains two potential donor sites, the azomethine nitrogen and the tertiary nitrogen atoms. An active band observed in the IR spectrum of the free ligand is at 1673 cm⁻¹ attributed to the ν(C=N). This band shifts to 1598–1612 cm⁻¹ in the spectra of the complexes, indicating coordination of the imine nitrogen atoms to the metal.^58–61 Moreover, the nitrogen atoms of piperazine are coordinated to the Pd ion. As a result, methylene bands are shifted towards the lower frequency side in the complexes as compared to the free ligand. A new band appeared in the far-IR spectra of the complexes at 386 and 378 cm⁻¹ and is due to Pd–N coordination, and the absorption bands are assigned to be ν(Pd–N),⁵³ as represented in Fig. S12.†

Fig. 3 XPS image of the PdO residue.

The ¹H-NMR spectra of Pd(DPTTP)Cl₂ and Pd(TPTTP)Cl₂ complexes were obtained in d₆-DMSO. The resonance signals of azomethine⁵³ and the tertiary nitrogen atom attached methylene protons are shifted to the down field side, which indicated that the Pd(II) metal is coordinated with the DPTTP ligand. In the case of the Pd(TPTTP)Cl₂ complex, the spectrum exhibited two significant changes compared with the ligand spectrum wherein the imine and tertiary group nitrogen atoms are coordinated with the Pd(II) ions. Then, the Ar–Hs and tertiary –CH– protons are changed to the deshielding side, as shown in Fig. S13 and S14.† The rest of the proton signals are shifted to the down field side compared to the ligands spectra. Therefore, the complexes are very stable and coordinated with nitrogen atoms. The Pd(II) complexes show three electronic spectral bands (Fig. S15†). These may be assigned to the three spin-allowed transitions,⁶² which are ¹A_1g → ¹A_2g, ¹A_1g → ³T_1g, and ¹A_1g → ¹T_g. Based on the spectral data acquired from the analysis of the Pd(II) complexes, it may be proposed to have a square planar geometry.⁶³ The bandgap energies and surface areas of the complexes are lower than the respective ligands, as shown in Table 2.

Table 2 Surface area and bandgap energies of ligands and Pd(II)-complexes

Compound	Surface area (m^2 g^−1)	Bandgap energy (eV)		Onset value from UV spectra (nm)
		Experimental value	Theoretical value	Experimental value	Theoretical value
DPTTP	8	3.96	3.67	348	337
TPTTP	14	3.48	3.23	432	383
Pd(DPTTP)Cl2	18	3.02	3.25	411	381
Pd(TPTTP)Cl2	26	2.30	2.60	539	476

---

To gain insights into the absorption energies and optical band gap, we carried out TDDFT calculations for these molecules. The calculated absorption energies for the ligands and metal complexes are shown in Tables S1 and S2,† respectively. The calculated absorptions are in good agreement with the experiment (spectra obtained from TDDFT, shown in Fig. S16†). The ligand DPTTP shows an absorption maxima, S₀–S₁ at 338 nm with a low intensity and significant contribution between the HOMO and LUMO. However, further excitations are consisting at lower energy levels. S₀–S₂ is at 319 nm and a significant contribution between the HOMO−2 and LUMO. TPTTP shows absorption maxima at 382 nm with an oscillator strength of 0.019 and a meaningful contribution between the HOMO−1 to LUMO (51%) and HOMO to LUMO (45%). Other excitations, S₀ to S₂, S₃, S₄, and S₅, have mixed contributions. Detailed excitations with corresponding energies are shown in Fig. 4.

Fig. 4 Calculated electronic excitations along with molecular orbitals for (a) DPTTP and (b) TPTTP.

The calculated optical bandgap is consistent with the experiment, and it is estimated using the LUMO values from eqn (1). It is evident from Table 3 that the HOMO and LUMO levels of the metal complexes are stabilized around 6 eV when compared with their corresponding ligands. Similar energy levels are shown graphically in Fig. 5. The bandgap energy of the DPTTP is 3.67 eV whereas its Pd(II) complex bandgap energy is 3.25 eV, the decreased the bandgap energy explained that shifting of absorption band towards longer wavelength region and present of d–d transitions.

Table 3 Calculated energy levels of HOMO, LUMO and HOMO–LUMO gap (HLG, S₀ → S₁) shown in eV

Compounds →	L1	L2	L1-Pd	L2-Pd
HOMO	−5.21	−5.20	−11.89	−11.28
LUMO	−1.54	−1.97	−8.64	−8.68
HLG	3.67	3.23	3.25	2.60

---

Fig. 5 Schematic energy levels of ligands and metal complexes.

This decrease in band gap from the ligand to its metal complex is due to the stabilization HOMO and LUMO levels of complexes.

E_LUMO = E_TDDFT + E_HOMO (1)

Photocatalytic studies

The synthesis of new fused heterocyclic compounds via C–C and C–S bond formation in situ reactions focused on the new catalyst, harsh conditions, time and yields. In addition, catalyst performance and sustainability play an important role in catalytic properties.⁶⁴ Therefore, the synthesis of pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine and its derivatives in the presence of new Pd(II) complexes under visible light irradiation was performed. In this study, the focus was mainly on three aspects such as (i) the intensity of visible light energy, (ii) comparisons of the reaction performance with known catalysts and (iii) reproducibility of the reaction conditions and sustainability of the new Pd-complexes.

The intensity of the visible light also plays a vital role to activate the C–H bond in the presence of a photocatalyst. The variation of the intensity of the visible light is directly proportional to time, whereas there is not much effect on the yield of the final products, as shown in Table 4. Therefore, all the reactions were performed in the presence of 500 watts of light irradiation.

Table 4 Optimization of the intensity of visible light

S. no.	Intensity of the visible light (tungsten lamp)	Time (hours)	% yield
1	300 watts	24	85
2	400 watts	16	88
3	500 watts	6	90

---

In the absence of a new photocatalyst, the product obtained was N-((2-bromopyridin-3-yl)methylene)pyrido[3,2-f][1,4]thiazepin-2-amine, as shown in Scheme 4. However, in the case of the presence of Pd(II), a complex reaction proceeds via Pd(II) to Pd(IV) and C–H bond activation⁶⁵ of thiazepine ring to produce the polycyclic aromatic fused heterocyclic compound, as shown Scheme 5.

Scheme 4 Absence of the Pd(II) complexes.

Scheme 5 Presence of the Pd(II) complexes.

In Scheme 4 synthesis of the pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine follows a unique pathway such as intramolecular C–C bond formation.⁶⁶ However, optimization of the amount of Pd-complexes and various substrates has been studied under different visible light sources. Initially, various amounts of photocatalyst were used to stabilize the condition to produce the same yields and time for the completion of the reaction. Finally, it is optimized with a 0.005 mmol amount of catalyst, which is enough to produce pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine in four hours with an isolated yield of 90% in the presence of a 500 W tungsten lamp. Once the light source and reaction conditions were optimized, the reaction was monitored with UV-visible spectra at fixed time intervals. Every hour, an increase in peak intensities of the spectra was observed, as shown in Fig. 6.

Fig. 6 UV-visible spectra of pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine.

The performance of the photocatalyst was checked with all the substrates and then the results revealed that Pd(TPTTP)Cl₂ was more active than Pd(DPTTP)Cl₂ due to a lower bandgap energy and high surface area (Table 2). All the substrates and products are presented in Table 5. The analytical and spectral data are presented in the ESI.† Therefore, a plausible mechanism is proposed, as given in Fig. 7. Moreover, the establishment of exact mechanism of the formation of final product then the stepwise reactions are performed. Then, initially, the reaction between 2-bromonicotinaldehyde and 2-aminoethanethioamide obtained (E)-2-(((2-bromopyridin-3-yl)methylene)amino)ethanethioamide (Scheme 4, compound 5) and was confirmed from the ¹H-NMR spectral data (Fig. S17†). The second stage reaction in the presence of Pd(II) complexes and DIPEA base under visible light irradiation for 3 hours gave pyrido[3,2-f][1,4]thiazepin-2-amine (Scheme 6, compound 6 and Fig. S18†). Then, the final stage reaction among compound 6 and 2-bromonicotinaldehyde or various aromatic aldehydes under the same conditions of the second stage reaction achieved the final product, as shown in Fig. S19–S35.† Compound 6 is a key material to obtain the various products mentioned in Table 6. Moreover, the direct reaction between 2-bromonicotinaldehyde and 2-amino-ethanethioamide in the presence of Pd(DPTTP)Cl₂/Pd(TPTTP)Cl₂ obtained pyrido[3,2-f][1,4]thiazepin-2-amine, as shown in Scheme 5.

Table 5 Synthesis of new pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine and its derivatives (symmetrical system)

Compound	Catalyst	Time (h)	Yield (%)
	Pd(OAc)2	24	Trace
	Pd(PPh3)4	24	10
	Pd(PPh3)2Cl2	24	5
	Pd(DPTTP)Cl2	6	62
	Pd(TPTTP)Cl2	6	90
	Pd(DPTTP)Cl2	6	54
	Pd(TPTTP)Cl2	6	78
	Pd(DPTTP)Cl2	6	52
	Pd(TPTTP)Cl2	6	69
	Pd(DPTTP)Cl2	6	56
	Pd(TPTTP)Cl2	6	74

---

Fig. 7 Plausible mechanism for the formation of pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine.

Scheme 6 Synthesis of pyrido[3,2-f][1,4]thiazepin-2-amine.

Table 6 Synthesis of new pyrido[3,2-f][1,4]thiazepine based compounds with formyl group (unsymmetrical system)

Aldehyde	Compound	Catalyst	Time (h)	Yield (%)
		Pd(DPTTP)Cl2	6	38
		Pd(TPTTP)Cl2	6	56
		Pd(DPTTP)Cl2	6	53
		Pd(TPTTP)Cl2	6	88
		Pd(DPTTP)Cl2	6	49
		Pd(TPTTP)Cl2	6	84
		Pd(DPTTP)Cl2	6	42
		Pd(TPTTP)Cl2	6	68
		Pd(DPTTP)Cl2	6	20
		Pd(TPTTP)Cl2	6	36

---

Pyrido[3,2-f][1,4]thiazepin-2-amine was reacted with a variety of aldehydes to form fused aromatic heterocyclic compounds based on the naphthyridine moiety, and ¹H-NMR spectra of all the final products are presented in the ESI (Fig. S19†).

In the case of 2-formylpyridine, 3-formylpyridine and 4-formylpyridine reacted with pyrido[3,2-f][1,4]thiazepin-2-amine in the Pd(II) complexes under visible light irradiation to obtain the tetracyclic heterocyclic compound. Whereas the reactivity of the 3-formylpyridine is slow than 2-formylpyridine and 4-formylpyridine to formation of the naphthyridine derivatives. Moreover, intramolecular C–H bond activation between pyridine and thiazepine rings in the presence of the Pd(II) complex and visible light irradiation is presented in Table 6.

When comparing the photocatalytic activity between two photocatalysts, Pd(TPTTP)Cl₂ is better than Pd(DPTTP)Cl₂ due to the large plate type morphology, high surface area, and low bandgap energy. The photocatalytic activity and photo stability of the Pd(II) complexes are checked with ¹H-NMR spectrum (pure and after photocatalysis) and reuse the complexes for the synthesis of pyrido[3′,2′:6,7][1,4]thiazepino[2,3-h][1,6]naphthyridine. After the second cycle of the reaction, all the peaks were slightly shifted and broadened in the ¹H-NMR spectra (Fig. 8).

Fig. 8 ¹H-NMR spectra of Pd(TPTTP)Cl₂ (a) pure (b) after photocatalysis.

Nevertheless, the complexes were used to perform the next cycle and the yields of the compound decreased by 45% due to the decomposition of Pd complexes and the formation of Pd/PdO nanoparticles.⁶⁷ These nanoparticles were confirmed from P-XRD data, as shown in Fig. 9.

Fig. 9 PXRD pattern of the Pd/PdO nanoparticles.

The comparative studies for the solution stability of compound 1a and pentacene were determined from UV-visible spectra. Compound 1a is highly stable in the solution state compared with pentacene, as shown in Fig. 10. After 40 min 90% pentacene was degraded, whereas compound 1a is highly stable with no change in UV-visible spectra (Fig. 9).

Fig. 10 Photostability studies of compound 1a and pentacene at different time intervals under UV-visible irradiation.

The thermal stability of compound 1 was recorded, and the thermogram revealed that decomposition of compound 1 is 5% at 363 °C (Fig. 11).

Fig. 11 Thermogram of compound 1a.

Therefore, the synthesis of both symmetrical and unsymmetrical acene compounds was well characterized with ¹H-NMR spectra and presented in ESI.† Finally, all these compounds have very useful applications as organic field effect transistors⁶⁸ and further studies are under progress.

Conclusions

We developed new Pd(II) complexes for the formation of C–S and C–C bonds under a visible light-driven technique. The bandgap energy of the Pd(TPPTP)Cl₂ exhibited lower energy than Pd(DPTTP)Cl₂ and these values are also estimated by DFT and TDDFT calculations. The bandgap energy of the Pd(TPTTP)Cl₂ exhibited lower energy than Pd(DPTTP)Cl₂ and these values are also estimated by DFT and TDDFT calculations. The experimental values and theoretically calculated values are coincide. The reaction proceeds under base conditions and can be applied to generate a broad range of substrates that can be converted into polycyclic fused linear heterocycles. Mechanistic studies revealed that this reaction was specifically catalyzed by a direct arylation with the formation of the Pd(IV) complex in an oxidative cyclization step. Moreover, the position of the formyl group on the pyridine ring is play vital role in the formation of the thiazepine ring and followed by formation C–H bond activation. Finally, Pd(TPTTP)Cl₂ has a better photocatalytic performance than of Pd(DPTTP)Cl₂ due to higher surface area and lower bandgap energy.

Acknowledgements

VG would like to thank UGC, New Delhi, for the Junior Research Fellowship award. SP & ChP thank SERB – New Delhi, India for financial support under EMR & Young Scientist Start-Up Research Grant, respectively and special thanks to DST-FIST, New Delhi. We thank Prof. Yu-Tai Tao, Institute of Chemistry, Academia Sinica, Taipei, Taiwan (ROC).

Notes and references

1. D. G. Musaev, T. M. Figg and A. L. Kaledin, Chem. Soc. Rev., 2014, 43, 5009.
2. (a) A. R. Dick and M. S. Sanford, Tetrahedron, 2006, 62, 2439; (b) B.-J. Li, S.-D. Yang and Z.-J. Shi, Synlett, 2008, 947; (c) A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507; (d) B. Yao, D. X. Wang, Z. T. Huang and M. X. Wang, Chem. Commun., 2009, 2899; (e) S. Pola, M. Subburu, R. Guja, V. Muga and Y.-T. Tao, RSC Adv., 2015, 5, 58504.
3. A. R. Dick, K. L. Hull and M. S. Sanford, J. Am. Chem. Soc., 2004, 126, 2300.
4. (a) W. C. P. Tsang, N. Zheng and S. L. Buchwald, J. Am. Chem. Soc., 2005, 127, 14560; (b) W. C. P. Tsang, R. H. Munday, G. Brasche, N. Zheng and S. L. Buchwald, J. Org. Chem., 2008, 73, 7603.
5. K. Inamoto, C. Hasegawa, K. Hiroya and T. Doi, Org. Lett., 2008, 10, 5147.
6. X. Wan, Z. Ma, B. Li, K. Zhang, S. Cao, S. Zhang and Z. Shi, J. Am. Chem. Soc., 2006, 128, 7416.
7. (a) Handbook of C–H Transformations, ed. G. Dyker, Wiley-VCH Verlag GmbH & Co. Kga A, Weinheim, Germany, 2005; (b) D. G. Musaev, T. M. Figg and A. L. Kaledin, Chem. Soc. Rev., 2014, 43, 5009; (c) Z. i. Chen, B. Wang, J. Zhang, W. Yu, Z. Liu and Y. Zhang, Org. Chem. Front., 2015, 2, 1107; (d) V. Ritleng, C. Sirlin and M. Pfeffer, Chem. Rev., 2002, 102, 1731; (e) A. E. Shilov and G. B. Shulpin, Chem. Rev., 1997, 97, 2879; (f) B. Li and P. H. Dixneuf, Chem. Soc. Rev., 2013, 42, 5744.
8. (a) B. M. Trost, Science, 1991, 254, 1471; (b) P. A. Wender, Chem. Rev., 1996, 96, 1; (c) J. Magano and J. R. Dunetz, Chem. Rev., 2011, 111, 2177; (d) A. de Meijere and F. Diederich, Metal-Catalyzed Cross-Coupling Reactions, Wiley-VCH, Weinheim, 2004; (e) J. Cornella and I. Larrosa, Synthesis, 2012, 653; (f) N. Rodriguez and L. J. Goossen, Chem. Soc. Rev., 2011, 40, 5030; (g) C. Pan, X. Jia and J. Cheng, Synthesis, 2012, 677; (h) X. Jia, S. Zhang, W. Wang, F. Luo and J. Cheng, Org. Lett., 2009, 11, 3120; (i) O. Baslé, J. Bidange, Q. Shuai and C.-J. Li, Adv. Synth. Catal., 2010, 352, 1145; (j) F. Xiao, Q. Shuai, F. Zhao, O. Basle, G. Deng and C.-J. Li, Org. Lett., 2011, 13, 1614; (k) C.-W. Chan, Z. Zhou, A. S. C. Chan and W.-Y. Yu, Org. Lett., 2010, 12, 3926; (l) Y. Yang, B. Zhou and Y. Li, Adv. Synth. Catal., 2012, 354, 2916; (m) Y. Wu, B. Li, F. Mao, X. Li and F. Y. Kwong, Org. Lett., 2011, 13, 3258; (n) C.-W. Chan, Z. Zhou and W.-Y. Yu, Adv. Synth. Catal., 2011, 353, 2999; (o) C. Li, L. Wang, P. Li and W. Zhou, Chem.–Eur. J., 2011, 17, 10208; (p) B. Zhou, Y. Yang and Y. Li, Chem. Commun., 2012, 48, 5163.
9. (a) K. Tamao, K. Sumitani and M. Kumada, J. Am. Chem. Soc., 1972, 94, 4374; (b) R. F. Heck and J. P. Nolley, J. Org. Chem., 1972, 37, 2320; (c) A. O. King, N. Okukado and E. Negishi, J. Chem. Soc., Chem. Commun., 1977, 19, 683; (d) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457; (e) Y. Hatanaka and T. Hiyama, J. Org. Chem., 1988, 53, 918; (f) K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975, 50, 4467; (g) J. K. Stille, Angew. Chem., Int. Ed. Engl., 1986, 25, 508.
10. R. F. Heck, Acc. Chem. Res., 1979, 12, 146–151; I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009–3066.
11. A. Yamamoto, J. Organomet. Chem., 2002, 653, 5–10.
12. (a) E. Negishi, Angew. Chem., Int. Ed., 2011, 50, 6738; (b) E. Negishi, J. Organomet. Chem., 2002, 653, 34–40.
13. (a) A. Suzuki, J. Organomet. Chem., 2002, 653, 83; (b) A. Suzuki, Angew. Chem., Int. Ed., 2011, 50, 6722.
14. K. Sonogashira, J. Organomet. Chem., 2002, 653, 46–49.
15. (a) B. M. Rosen, K. W. Quasdorf, D. A. Wilson, N. Zhang, N. K. Gary and V. Percec, Chem. Rev., 2011, 111, 1346; (b) I. P. Beletskaya and V. P. Ananikov, Chem. Rev., 2011, 111, 1596.
16. J. Wencel-Delord, T. Droge, F. Liu and F. Glorius, Chem. Soc. Rev., 2011, 40, 4740.
17. J. Yuan, C. Liu and A. Lei, Chem. Commun., 2015, 51, 1394.
18. (a) F.-S. Han, Chem. Soc. Rev., 2013, 42, 5270; (b) F. Hu, Y. Xia, C. Ma, Y. Zhang and J. Wang, Chem. Commun., 2015, 51, 7986; (c) B. Yao, D. Wang, Z. T. Huang and M. X. Wang, Chem. Commun., 2009, 2899; (d) F. Collet, R. H. Dodd and P. Dauban, Chem. Commun., 2009, 5061; (e) T. Xiong, Y. Li, L. Mao, Q. Zhang and Q. Zhang, Chem. Commun., 2012, 48, 2246; (f) J. Park, M. Kim, S. Sharma, E. Park, A. Kim, S. H. Lee, J. H. Kwak, Y. H. Jung and I. S. Kim, Chem. Commun., 2013, 49, 1654.
19. L. F. Lindoy, Chem. Soc. Rev., 1975, 4, 421.
20. S. Kumar, R. R. Jha, S. Yadav and R. Gupta, New J. Chem., 2015, 39, 2042.
21. S. Akine and T. Nabeshima, Dalton Trans., 2009, 10395.
22. (a) S. P. Gavrish, Y. D. Lampeka, H. Pritzkow and P. Lightfoot, Dalton Trans., 2010, 39, 7706; (b) B. Castano, S. Guidone, E. Gallo, F. Ragaini, N. Casati, P. Macchi, M. Sisti and A. Caselli, Dalton Trans., 2013, 42, 2451; (c) T. Ren, Chem. Commun., 2016, 52, 3271.
23. A. Granzhan, N. Kotera and M. P. T. Fichou, Chem. Soc. Rev., 2014, 43, 3630.
24. (a) P. J. Dijkstra, M. Skowronska-Ptasinska, D. N. Reinhoudt, H. J. Den Hertog Jr, J. V. Eerden, S. Harkema and D. D. Zeeuw, J. Org. Chem., 1987, 52(22), 4913; (b) M. Wenzel, J. R. Hiscock and P. A. Gale, Chem. Soc. Rev., 2012, 41, 480.
25. Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39, 3007.
26. S. M. Ng, M. Koneswaran and R. Narayanaswamy, RSC Adv., 2016, 6, 21624.
27. (a) L. F. Lindoy, K. M. Park and S. S. Lee, Chem. Soc. Rev., 2013, 42, 1713; (b) A. Gualandi, L. Cerisoli, H. S. Evans and D. Savoia, J. Org. Chem., 2011, 76, 3399.
28. L. Chen, G. Chen, C. F. Leung, S. M. Yiu, C. C. Ko, E. A. Mallart, M. Robert and T. C. Lau, ACS Catal., 2015, 5, 356.
29. L. Mintrop, J. Windisch, C. Gotzmann, R. Alberto, B. Probst and P. Kurz, J. Phys. Chem. B, 2015, 119, 13698.
30. (a) I. V. Seregin and V. Gevorgyan, Chem. Soc. Rev., 2007, 36, 1173; (b) J. Wencel-Delord, T. Droge, F. Liu and F. Glorius, Chem. Soc. Rev., 2011, 40, 4740; (c) F.-S. Han, Chem. Soc. Rev., 2013, 42, 5270.
31. G. Dyker, Angew. Chem., Int. Ed. Engl., 1992, 31, 1023.
32. V. Massari, D. Daelemans and M. L. Barreca, J. Med. Chem., 2010, 53, 641.
33. (a) V. P. Lirvinov, Adv. Heterocycl. Chem., 2006, 91, 222; (b) K. W. Garey and G. W. Amsden, Pharmacotherapy, 1999, 19, 21.
34. Z. Chen, B. Qu, H. Lan, Q. Gong and J. Kido, Adv. Funct. Mater., 2013, 23, 1323.
35. J. R. Housley, J. E. Jeffery, K. J. Nichol and B. J. Sargent, (Boots Co PLC), PCT Int. Appl. 9411360, 1994Chem. Abstr., 1994, 121, 134167.
36. (a) A. Couture, E. Deniau, P. Grangclaudon and C. Simion, Synthesis, 1996, 986; (b) W. Dölling, M. Biedermann and H. Hartung, Eur. J. Org. Chem., 1998, 1237.
37. (a) J. M. Schaus and F. P. Bymaster, Annu. Rep. Med. Chem., 1998, 33, 1; (b) Y. Liao, B. J. Venhuis, N. Rodenhuis, W. Timmerman and H. Winkström, J. Med. Chem., 1999, 42, 2235.
38. (a) A. Hadou, A. Hamid, H. Mathouet, M.-F. DeÏda and A. DaÏch, Heterocycles, 2008, 76, 1017; (b) K. Nagarajan, J. David, Y. S. Kulkarni, S. B. Hendi, S. J. Shenoy and P. Upadhyaya, Eur. J. Med. Chem., 1986, 21, 21.
39. (a) R. C. Allen, P. A. Reitano and H. Urbach, J. Med. Chem., 1978, 21, 838; (b) P. Catsoulacos, J. Heterocycl. Chem., 1970, 7, 409.
40. (a) T. K. Jørgenson, R. Hohlweg, K. E. Andersen, U. B. Olsen, Z. Polivka and K. Sindelar, U.S. 6187770B1, 2001; (b) J. Rewinkel, M. Bernardus, B. Folmer, B. Johanna, M. L. Ollero and H. Ibrahim, WO 2008135291A1, 2008.
41. (a) P. P. M. A. Dols, B. J. B. Folmer, H. Hamersma, C. W. Kuil, H. Lucas, L. Ollero, J. B. M. Rewinkel and P. H. H. Hermkens, Bioorg. Med. Chem. Lett., 2008, 18, 1461; (b) P. Hermkens, H. Harold, H. Lucas, J. Rewinkel, M. Bernardus, B. Folmer and B. Johanna, WO 03084963A1, 2003.
42. (a) H. Nakamura, J. Kobayashi, Y. Ohizumi and Y. Hirata, Tetrahedron Lett., 1982, 23, 5555; (b) H. Nakamura, J. Kobayashi, Y. Ohizumi and Y. Hirata, J. Chem. Soc., Perkin Trans. 1, 1987, 173; (c) A. Rudi and Y. Kashman, Tetrahedron Lett., 1993, 34, 4683.
43. (a) A. C. Spivey, L. Shukla and J. F. Hayler, Org. Lett., 2007, 9, 891; (b) P. Melnyk, J. Gasche and C. Thal, Synth. Commun., 1993, 23, 2727.
44. V. E. Stewart and C. B. Pollard, J. Am. Chem. Soc., 1936, 58, 1980.
45. K. Hideg and D. Lloyd, J. Chem. Soc. C, 1971, 3441.
46. A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
47. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785.
48. P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270.
49. Z. Fu, W. Shen, R. He, X. Liu, H. Sun, W. Yin and M. Li, Phys. Chem. Chem. Phys., 2015, 17, 2043.
50. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision E.01, Gaussian, Inc., Wallingford CT, 2009.
51. N. E. Borisova, M. D. Reshetova and Y. A. Ustynyuk, Chem. Rev., 2007, 107, 46.
52. J. Fonseca, J. Martinez, L. Cunha-Silva, A. L. Magalhães, M. T. Duarte and C. Freire, Inorg. Chim. Acta, 2010, 363, 4096.
53. I. Masih and N. Fahmi, Spectrochim. Acta, Part A, 2011, 79, 940.
54. J. Bogojeski, J. Volbeda, M. Freytag, M. Tamm and Ž. D. Bugarčić, Dalton Trans., 2015, 44, 17346.
55. H. Sato, K. Morimoto, Y. Mori, Y. Shinagawa, T. Kitazawa and A. Yamagishi, Dalton Trans., 2013, 42, 7579.
56. (a) A. H. Chughtai, N. Ahmad, H. A. Younus, A. Laypkov and F. Verpoort, Chem. Soc. Rev., 2015, 44, 6804; (b) P. Sharma and A. P. Singh, RSC Adv., 2014, 4, 43070; (c) M. Azam, Z. Hussain, I. Warad, S. I. Al-Resayes, Md. S. Khan, M. Shakir, A. T. Kruszynska and R. Kruszynski, Dalton Trans., 2012, 41, 10854.
57. (a) J. Fonseca, J. Tedim, K. Biernacki, A. L. Magalhães, S. J. Gurman, C. Freire and A. R. Hillman, Electrochim. Acta, 2010, 55, 7726; (b) Y. Holade, C. Canaff, S. Poulin, T. W. Napporn, K. Servat and K. B. Kokoh, RSC Adv., 2016, 6, 12627.
58. (a) B. Vats, S. Kannan, M. Sundararajan, M. Kumar and M. G. B. Drew, Dalton Trans., 2015, 44, 11867; (b) A. Bansal, N. Fahmi and R. V. Singh, Main Group Met. Chem., 2003, 26, 335; (c) S. Liu, R. Peloso and P. Braunstein, Dalton Trans., 2010, 39, 2563.
59. J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C.-Y. Su, Chem. Soc. Rev., 2014, 43, 6011.
60. S. Panda, G. R. Krishna, C. M. Reddy and S. S. Zade, Dalton Trans., 2011, 40, 6684.
61. C. Icsel, V. T. Yilmaz, Y. Kaya, H. Samli, W. T. A. Harrison and O. Buyukgungor, Dalton Trans., 2015, 44, 6880.
62. H. B. Gray and C. J. Ballhausen, J. Am. Chem. Soc., 1963, 85, 260.
63. K. Sharma, R. Singh, N. Fahmi and R. V. Singh, Spectrochim. Acta, Part A, 2010, 75, 422.
64. (a) T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2, 527; (b) D. P. Hari, P. Schroll and B. Koenig, J. Am. Chem. Soc., 2012, 134, 2958; (c) D. Ravelli, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2013, 42, 97; (d) J.-T. Yu and C. Pan, Chem. Commun., 2016, 52, 2220.
65. (a) M. Catellani, F. Frignani and A. Rangoni, Angew. Chem., Int. Ed. Engl., 1997, 36, 119; (b) M. Catellani and M. C. Fagnola, Angew. Chem., Int. Ed. Engl., 1995, 33, 2421; (c) M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759; (d) D. Morales–Morales, R. Redón, C. Yung and C. M. Jensen, Chem. Commun., 2000, 1619.
66. A. Modak, J. Mondal, M. Sasidharan and A. Bhaumik, Green Chem., 2011, 13, 1317.
67. (a) V. P. Ananikov, N. V. Orlov, I. P. Beletskaya, V. N. Khrustalev, M. Y. Antipin and T. V. Timofeeva, J. Am. Chem. Soc., 2007, 129, 7252; (b) A. K. Rathi, M. B. Gawande, J. Pechousek, J. Tucek, C. Aparicio, M. Petr, O. Tomanec, R. Krikavova, Z. Travnicek, R. S. Varma and R. Zboril, Green Chem., 2016, 18, 2363.
68. (a) S. Pola, C.-H. Kuo, W.-T. Peng, Md. M. Islam, I. Chao and Y.-T. Tao, Chem. Mater., 2012, 24, 2566; (b) Md. M. Islam, S. Pola and Y.-T. Tao, Chem. Commun., 2011, 47, 6356; (c) Y. Zhen, C. Wang and Z. Wang, Chem. Commun., 2010, 46, 1926; (d) Md. M. Islam, F. Valiyev, H.-F. Lu, M.-Y. Kuo, I. Chao and Y.-T. Tao, Chem. Commun., 2011, 47, 2008; (e) C.-H. Kuo, D.-C. Huang, W.-T. Peng, K. Goto, I. Chao and Y.-T. Tao, J. Mater. Chem. C, 2014, 2, 3928.

---

Footnote

† Electronic supplementary information (ESI) available: Spectral data of all ligands and Pd(DPTTP)Cl₂ and Pd(TPTTP)Cl₂. See DOI: 10.1039/c6ra15609f

---