New Pd(II) complexes of sulfur ylides; synthesis, X-ray characterization, a theoretical study and catalytic activity toward the Mizoroki–Heck reaction

Abstract

The reaction of sulfur ylides (L) SMe₂C(H)C(O)R (R = 4-nitrophenyl, phenyl, and 3-nitrophenyl) with the dichloro(1,5-cyclooctadiene)palladium(II) complex, [PdCl₂(cod)], in a 2 : 1 ratio gives the new Pd(II) complexes of type cis- and trans-[PdCl₂(SMe₂C(H)C(O)R)₂] (R = phenyl (1), 3-nitrophenyl (2), and 4-nitrophenyl (3)). Characterization of the obtained complexes was performed by elemental analysis, IR, ¹H, ¹³C NMR and mass spectroscopies. Also, the structure of complex trans-[PdCl₂(SMe₂C(H)C(O)C₆H₄-p-NO₂)₂] (3) was characterized by single crystal X-ray analysis. The X-ray crystallography results reveal that the structure of complex 3 contains two Cα-coordinated sulfur ylide ligands in trans geometry. These air/moisture stable complexes were also employed as efficient catalysts for the Mizoroki–Heck cross-coupling reaction of several aryl halides with olefins. The coupled products of these reactions were obtained in good to excellent yields and purity, short reaction times and low catalyst loading. Also a theoretical study on the structure of the complexes 1–3 have been investigated at the BP86/def2-SVP level of theory. The bonding situation between the [PdCl₂] and L₂ fragments [(Ylide)₂] in the [L₂PdCL₂] complexes, were carried out by NBO and energy-decomposition analysis (EDA), as well as their natural orbitals for chemical valence variation (EDA-NOCV). The results confirmed that the contribution of the electrostatic interactions in the Pd–C bond in the complexes is about 50%.

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Introduction

It is a fact that the sulfur ylides R₂S=C(R′)(R′′) are interesting ligands in organometallic chemistry; because these compounds are powerful nucleophilic reagents and provide versatile ligands for metals in their various oxidation states.^1–6 The coordination chemistry of some sulfur ylide complexes have been investigated extensively and α-keto stabilized sulfur ylides, because of their resonating character, show more interesting properties such as high stability and ambidentate character as ligands. Due to the proximity of the keto group and carbanion in these ylides, resonance delocalization of the ylidic electron density was observed, providing additional stabilization and leading to the formation of some useful chelates and complexes.⁷

Coordination modes of these compounds in several metal complexes have been studied and it was found that these ylides can coordinate to metal ions in three modes as depicted in Chart 1: mononuclear, dinuclear and polymeric structure modes. When a metal ion such as Pd(II) interacts only with the Cα atoms of two ylidic groups, these ligands can be placed in the cis or trans positions to form the mononuclear structure.⁸ Dinuclear structure was also observed in case of Hg(II) halides, in which two ylides were coordinate from the Cα atoms to dimeric Hg₂X₄ (X = Cl, Br, I) unit.^9,10 Finally, the polymeric structure was seen when metal ions have bridging ligands. For example, it was reported a polymeric structure of sulfur ylide with AgNO₃ by our group and the X-ray analysis shows a structure with a metallic skeleton based on the C-coordinated sulfur ylide and bridged nitrate group.¹¹

Chart 1 The possible bonding modes of sulfur ylides to metal.

It was reported that some of phosphorus ylide palladium complexes have useful applications in catalytic C–C coupling reactions.^12–15 Although these complexes have some advantages such as high efficiency, low catalyst loading and moisture/air stability, but most of them are expensive and have environmental considerations. In the past years, phosphine free catalytic reactions were developed to eliminate or reduce costs, operational hazards and environmental pollution.^16–19 Among these, there are some catalytic reactions that sulfur containing ligands and their Pd(II) complexes were employed as catalyst in the reaction.^20–22

Heck reaction is one of the most widely adopted methods for the construction of carbon–carbon bonds in modern organic chemistry and it is commonly carried out in the presence of a palladium complex.^23–28 For example, in the reported work by the group of Gruber et al. palladium catalysts containing modified sulfur ligands could activate the coupling reactions with less reactive aryl bromides and aryl chlorides as well as nonactivated alkenes.²⁹ It appears that the release of Pd(0) species in Heck reactions is depended on the ligands around the palladium complex. Thus, organochalcogen ligands such as sulfur ylides, show rivalry with their respective phosphorus ylide analogues for releasing of Pd(0) species in the Heck C–C coupling reaction.

Although it has been published the synthesis of palladium complexes with sulfonium ylides by Bravo et al.,⁸ but only the trans structure of complexes was reported.

In this paper, we report the synthesis and theoretical study of the both cis- and trans-[Pd((Me)₂SCHC(O)R)₂] (R = phenyl, 3-nitrophenyl and 4-nitrophenyl) complexes (1–3). In addition, the X-ray crystal structure of complex 3 and further spectroscopic characterization of complexes was performed. Finally, Mizoroki–Heck reactions of various aryl halides with styrene and ethyl acrylate using these complexes as efficient catalysts were performed, giving good to excellent yields of products under aerobic conditions.

Experimental

Materials and methods

The sulfur ylides [(Me₂SCHC(O)C₆H₅)₂] (Y1), [Me₂SCHC(O)C₆H₄-m-NO₂] (Y2) and [Me₂SCHC(O)C₆H₄-p-NO₂] (Y3) and complex [PdCl₂(cod)] were prepared according to the previously published procedures.^11,30 Dichloromethane was used as reagent grade and dried over CaCl₂. Elemental analysis was performed on a Leco, CHNS-932 apparatus. The ¹H and ¹³C NMR spectra were recorded on Bruker Avance 500 MHz, 400 MHz, 250 MHz and Jeol 90 MHz spectrometers in CDCl₃ or DMSO-d₆ as solvents at 25 °C. IR spectra were recorded on KBr pellets using a Shimadzu 435-U 04 spectrophotometer in the region of 4000–400 cm⁻¹.

Crystallography

A suitable crystal was selected and mounted on a SuperNova, Dual, Cu at zero, Atlas diffractometer. The crystal was kept at 130.00(10) K during data collection. Using Olex2,³¹ the structure was solved with the ShelXT³² structure solution program using direct methods and refined with the ShelXL³³ refinement package using least squares minimisation. All non-hydrogen atoms were refined with anisotropic displacement parameters; H-atoms were constrained to geometrical estimates, with an isotropic displacement parameter of 1.5 times (Me) or 1.2 times (other) the parent carbon atom.

Synthesis of the Pd(II) complexes

General procedure. To a [PdCl₂(cod)] (0.5 mmol) dichloromethane solution (5 ml), a solution of ylide (1 mmol) (5 ml, CH₂Cl₂) was added. The resulting solution was stirred for 2 h at room temperature and then concentrated to a ca. 2 ml under reduced pressure and treated with n-hexane (15 ml) to afford the Pd(II) complex of desired sulfur ylide.

Data for [PdCl₂((Me)₂SC(H)C(O)C₆H₅)₂] (1). Yield: 0.26 g (75%). Anal. calc. for C₂₀H₂₄Cl₂O₂PdS₂ (%): C, 44.66; H, 4.60; S, 11.92. Found: C, 44.95; H, 4.77; S, 12.28. Mp 189–191 °C. Selected IR absorption in KBr (cm⁻¹) ν(CO): 1646. ¹H NMR (250.13 MHz, DMSO-d₆) δ_H (ppm): 2.46–2.74 (m, 12H, SMe₂) (trans isomer), 2.67–3.12 (m, 12H, SMe₂) (cis isomer), 4.93 (s, 1H, SCH, trans major diastereoisomer), 5.15 (s, 1H, SCH, trans minor diastereoisomer), 5.62 (s, 1H, SCH, cis major diastereoisomer), 5.69 (s, 1H, SCH, cis minor diastereoisomer), 7.30–8.30 (m, 20H, Ph). ¹³C NMR (62.90 MHz, DMSO-d₆) δ_C (ppm): 194.48 (s, CO), 128.30–156.40 (m, Ph), 49.94 (s, SCH) (cis isomer), 49.39 (s, SCH) (trans isomer), 27.31–27.79 (d, SMe₂) (cis isomer), 26.51–26.73 (d, SMe₂) (trans isomer).

Data for [PdCl₂((Me)₂SC(H)C(O)C₆H₄-m-NO₂)₂] (2). Yield: 0.27 g (70%). Anal. calc. for C₂₀H₂₂Cl₂N₂O₆PdS₂ (%): C, 38.26; H, 3.53; N, 4.46; S, 10.21. Found: C, 38.52; H, 3.80; N, 4.75; S, 10.55. Mp 195–198 °C. Selected IR absorption in KBr (cm⁻¹) ν(CO): 1630. ¹H NMR (500 MHz, DMSO-d₆) δ_H (ppm): 2.55–2.79 (m, 12H, SMe₂) (trans isomer), 2.69–3.05 (m, 12H, SMe₂) (cis isomer), 5.02 (d, 1H, SCH, trans major diastereoisomer), 5.11 (s, 1H, SCH, trans minor diastereoisomer), 5.66 (s, 1H, SCH, cis major diastereoisomer), 5.73 (s, 1H, SCH, cis minor diastereoisomer), 7.40–8.20 (m, 16H, Ph). ¹³C NMR (125.77 MHz, DMSO-d₆) δ_C (ppm): 192.17–192.21 (d, CO), 119.40–149.20 (m, Ph), 50.08 (s, SCH) (cis isomer), 49.65 (s, SCH) (trans isomer), 28.08–28.39 (d, SMe₂) (cis isomer), 27.10–28.00 (d, SMe₂) (trans isomer).

Data for [PdCl₂((Me)₂SC(H)C(O)C₆H₄-p-NO₂)₂] (3). Yield: 0.32 g (82%). Anal. calc. for C₂₀H₂₂Cl₂N₂O₆PdS₂ (%): C, 38.26; H, 3.46; N, 4.75; S, 10.21. Found: C, 38.50; H, 3.81; N, 4.78; S, 10.35. Mp 199–201 °C. Selected IR absorption in KBr (cm⁻¹) ν(CO): 1637. ¹H NMR (250 MHz, DMSO-d₆) δ_H (ppm): 2.62–2.86 (m, 12H, SMe₂) (trans isomer), 2.78–3.14 (m, 12H, SMe₂) (cis isomer), 5.10 (s, 1H, SCH, trans major diastereoisomer), 5.24 (s, 1H, SCH, trans minor diastereoisomer), 5.71 (d, 2H, SCH, cis two diastereoisomer), 7.90–8.40 (m, 16H, Ph). ¹³C NMR (62.90 MHz, DMSO-d₆) δ_C (ppm): 192.25–193.87 (d, CO), 123.90–150.10 (m, Ph), 55.89 (s, SCH) (cis isomer), 55.33 (s, SCH) (trans isomer), 28.20–29.41 (d, SMe₂) (cis isomer), 27.37–27.61 (d, SMe₂) (trans isomer).

Computational studies

The geometries of complexes 1–3 were fully optimized at BP86 (ref. 34 and 35)/def2-SVP³⁶ level of theory. It has been shown that BP86 is suitable level for calculation of bonding situation between the M→L in such as these complexes.^37–42 The geometry of the metal complex 3, as determined by X-ray crystal structure analysis (Fig. 1), was fully optimized at above mentioned level of theory. The observed geometry of compound 3 was used as a basis for DFT calculations of compound 1 and 2. All calculations were performed using the Gaussian 09 set of programs⁴³ natural bond orbital (NBO) analysis⁴⁴ were also carried out with the internal model Gaussian 09.

Fig. 1 ORTEP view of X-ray crystal structure 3.

Typical procedure for the Mizoroki–Heck reaction

Palladium complex 3 (0.005 mmol), olefin (0.75 mmol), aryl halide (0.5 mmol), Cs₂CO₃ (1.5 mmol) were added to 2 ml of solvent in a small tube and the mixture was heated to reflux temperature for 4 h in the presence of air. The reactions were monitored by thin-layer chromatography (TLC). At the end of the cross-coupling reactions, the mixture was cooled, extracted with n-hexane : EtOAc (80 : 20), filtered and purified by recrystallization from ethanol and water or purified by silica gel column chromatography (n-hexane : EtOAc, 80 : 20).

Results and discussion

Synthesis

According to our previously report, SMe₂ reacts with 1 equiv. of the proper ketone BrCH₂C(O)R (R = phenyl, and 3-nitrophenyl, 4-nitrophenyl) forming the desired sulfonium salts. Further treatment of these salts with NaOH 10% led to elimination of HBr, giving the desired sulfur ylides. Reaction of these ligands with [PdCl₂(cod)] in 2 : 1 ratio yielded the both cis- and trans-[Pd((Me)₂SCHC(O)R)₂] (R = phenyl, 3-nitrophenyl and 4-nitrophenyl) complexes 1–3 in 80–90% yields (Scheme 1).

Scheme 1 Synthesis of Pd(II) complexes 1–3.

Spectroscopy

The structure of complexes 1–3 was characterized successfully by ¹H and ¹³C NMR spectroscopic methods and other conventional techniques as well as IR and elemental analysis. Table S1† shows the brief summary of these collected data sets (see ESI†). trans structure of complex 3 was being unequivocally determined by single crystal X-ray analysis. The CHNS elemental analysis of palladium complexes indicates 1 : 2 stoichiometry between the PdCl₂ and sulfur ylide. Also, the mononuclear structure of these complexes is supported by ESI-mass spectra, with the molecular ion peaks at m/z 537, 628 and 629, respectively, for complexes 1, 2 and 3.

In the far IR spectra of complex 3, two Pd–Cl stretching band was observed in the 327 and 258 cm⁻¹ that is in accord with both cis- and trans-dichlorobis(sulfur ylide) structure in the solid state. IR spectra of complexes 1–3 show a significantly frequency shift of ν(CO) than those of the related sulfur ylides. An increase in ν(CO), indicates the coordination of the ylide was occurred through carbon; whereas decrease of ν(CO) was expected when the ylide coordinates to metal from oxygen atom. The observed absorption bands about 1514–1540 cm⁻¹ in ylides Y1–Y3, were shifted to 1629–1647 cm⁻¹ in complexes 1–3, indicating the coordination of ylide through the Cα atoms. The ν(S⁺–C) band of the ylides is also sensitive to the coordination. It was seen that a lower frequency shift was occurred to about 836–860 cm⁻¹ in comparison to the parent ylides (810–823 cm⁻¹), that is in consistent with some removal of electron density in the S–C bonds. However, infrared spectroscopy could not differentiates between geometric isomers (cis and trans) of each complexes.

The ¹H NMR signal for the SCH group of the complexes 1–3 shows a downfield shift compared to those of the free ylide. The coexistence of the cis and trans isomers is confirmed by the ¹H NMR spectra. Complexes 1–3 in dimethyl sulfoxide displayed two doublets for SMe₂ and two doublet of methine signals. The higher field pair of SMe₂ and methine proton signals may be assigned to the trans isomer. Since methinic carbons are chiral, two (RR/SS or RS/SR) are possible for each cis- and trans-structure of these complexes. In addition, integrated intensity of the SMe₂ signals revealed that the ratio of the cis to trans isomer was about 1 : 2. The fact that PdCl₂(ylide)₂ has a larger portion of the trans isomer would be mainly attributed to a smaller steric repulsion between ylide ligands.

In the ¹³C NMR spectra, a similar downfield shift for characteristic groups SCH and carbonyl was observed. Coordination of ylide through the Cα causes more electron density on this atom, led to the appearance of related ¹³C signal in higher field. In addition, presence of two geometric isomers in these complexes was also confirmed by ¹³C NMR. Observation of four CH₃ signals for the two SMe₂ groups and two methinic carbon signals indicates the presence of such geometrical and optical isomers. However, since carbonyl carbons do not have a hydrogen attached to them, they appear less intense than other carbons in the spectra and the signals were not seen as two separate peaks.

Crystallography

Yellow-orange single crystals of C₂₀H₂₂Cl₂N₂O₆PdS₂ were grown by vapour diffusion of methanol into DMSO. The molecular structure of 3 was shown in Fig. 1. Relevant parameters concerning data collection and refinement were given in Table 1. Selected bond distances and angles for the unit cell of complex 3 are displayed in Table S2.†

Table 1 Crystal data and structure refinement for 3

Empirical formula	C20H22Cl2N2O6PdS2
Formula weight	627.81
Temperature (K)	130.01(10)
Crystal system	Triclinic
Space group	P1̄
a (Å)	7.1934(5)
b (Å)	7.5432(5)
c (Å)	11.4096(8)
α (°)	94.723(6)
β (°)	103.384(6)
γ (°)	91.985(6)
V (Å^3)	599.32(7)
Z	1
Dcalc g cm^−3	1.739
Absorption coefficient (mm^−1)	10.279
F(000)	316.0
Crystal size (mm^3)	0.1272 × 0.0933 × 0.0452
θ range for data collection (°)	CuKα (λ = 1.54184)
Reflections collected	8 to 154.03
Index ranges	−9 ≤ h ≤ 7, −9 ≤ k ≤ 8, −11 ≤ l ≤ 14
Independent reflections	5152
Data/restraints/parameters	2488 [Rint = 0.0419, Rsigma = 0.0562]
Goodness-of-fit on F^2	2488/0/153
Final R indices [I > 2σ(I)]	1.090
Final R indices (all data)	R1 = 0.0475, wR2 = 0.1268
Largest diff. peak and hole (e Å^−3)	R1 = 0.0532, wR2 = 0.1331, 1.96/−1.09

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The Pd atom is in a square planar arrangement, coordinated by two ligands and two chlorides, both ligands and both chlorides are in a trans configuration. The ylide aromatic ring is almost perpendicular to the PdCl₂ C2 plane. The nitro group and the keto group are oriented slightly out of the aromatic ring, the dihedral angles being 13.7(3)° and 7.2(7)° respectively. There are π⋯π interactions between the aromatic rings of two adjacent molecules, the interaction being such that C5 is superimposed over C7 (C5–C7′: 3.550(8) Å, C4–C5–C7′: 86.3(3)°) and C6 is superimposed over the center of the other aromatic ring. The molecules are linked together by weak C–H⋯X interactions into a 3D network.

Theoretical studies

A theoretical study on structures and nature of bonds for complexes 1–3 have been reported at the BP86/def2-SVP level of theory. The X-ray crystal structure confirmed that the complex 3 exhibit the square planar, structure. Thus we can consider two isomers both cis like and trans like for latter complexes. The structures and relative energy for both cis and trans like complexes are presented in Fig. 2. The result in good agreement with experimental data show that, the trans like structure is more stable than cis like structure for all complexes investigated here (Fig. 2). Thus further calculations including structure and nature of bond in the complexes were performed on trans like structure.

Fig. 2 Optimized structures of trans-like (a, c, and e) and cis-like (b, d, and f) of complexes 1–3 as well as the energy of cis-like isomer relative to trans-like isomer at BP86/SVP level of theory.

The optimized structures of both trans like and cis like complexes 1–3 at the BP86/def2-SVP level of theory are shown in Fig. 2, and trends for the variation of the corresponding bond lengths and bond angles extracted from optimizing geometries and are given in Table S3.† The calculated bond lengths and bond angles of complex 3 are in good agreement with the experimental values (Table S3†).

The computed Pd–C and Pd–Cl bond distances are about 0.1 Å longer than the X-ray values. Also the result showed that the R substituent has an insignificant effect on both the Pd–C and Pd–Cl bond distances and C–Pd–C, C–Pd–Cl and Cl–Pd–Cl bond angles in the complexes 1–3 (Table S3†).

For studying the nature of metal–ligand bonds and/or possible interactions in these complexes, the natural bond orbital (NBO) analysis was performed and used to estimate the delocalization of electron density between occupied Lewis-type orbitals and formally unoccupied non-Lewis NBOs (antibonding or Rydberg), which corresponds to stabilizing donor–acceptor interaction.⁴⁵ The values of the partial charge on the Pd, C and Cl atoms involved in interaction between the PdCl₂ and L₂ in of the complexes 1–3 were investigated at BP86/def2-SVP level of theory (Table 2). The results show that the Pd and C atoms of the Pd–C bonds carry positive and negative partial charges and this confirm that there is σ donation between the lone pair of C atoms and empty orbitals of Pd in Pd–C bonds (Table 2).

Table 2 Wiberg bond indices (WBI) of Pd–C and Pd–Cl bonds and natural charges of Pd, C and Cl atoms

Compound	WBIs		NPA
	Pd–C	P–Cl	Pd	C	Cl
1	0.406	0.6453	0.067	−0.67	−0.397
2	0.410	0.6751	0.034	−0.634	−0.390
3	0.402	0.6488	0.067	−0.671	−0.390

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Also the results show that the R substituent has an insignificant effect on the values of WBIs for Pd–C bonds (Table 2). A second-order perturbation theory analysis of the Fock matrix was carried out to evaluate the donor–acceptor interactions on the base of NBO analysis. The values of donor–acceptor interactions in the compounds are given in Table S4.† In these compounds, the important donor–acceptor interactions in the case of Pd–C bonds in complexes 1–3 were carry out from the σ(Pd–C) and σ(Pd–Cl) as donors to σ*(Pd–C), σ*(Pd–Cl), σ*(S–C) and σ*(O–C) as acceptors (Table S4†).

The calculated energies for HOMO and LUMO of 1, 2 and 3 complexes are also given in Table 3. The result showed that the calculated energy gap (ΔE) between the latter orbitals for the complex 3 containing NO₂ as a substituent in para position is smaller than those for other complexes. Also, the latter complex is softer than the other complexes described here. The HOMO and LUMO for all three mononuclear complexes are illustrated in Fig. S1 (see ESI†).

Table 3 Calculated energies for HOMO and LUMO molecular orbitals, hardness and energy gap (ΔE) between the HOMO and LUMO of trans like structures of complexes 1–3

Compound	LUMO (hartree)	HOMO (hartree)	η (eV)	ΔE (eV)
1	−0.0989	−0.1700	0.97	1.94
2	−0.1261	−0.1836	0.78	1.56
3	−0.1364	−0.1886	0.71	1.42

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To better understand the nature of the Pd–C bonding between the L2 fragments [(Ylide)₂] and [PdCl₂] fragment (see Scheme 1) in 1–3 complexes, we carried out quantum chemical calculations in the terms of energy-decomposition analysis at the BP86/TZ2P(ZORA)//BP86/def2-SVP with C₁ symmetry with the program package ADF2009.01. The ΔE_int between two investigated fragments in complexes 1–3 (see Scheme 1) are about 149.4, 145.83 and 139.26 kcal mol⁻¹ respectively (Table 4).

Table 4 EDA analysis (BP86/TZ2P(ZORA)//BP86/def2-SVP) of the [L₂PdCl₂] complex with the C₁ symmetry

	Compound
	1	2	3
ΔEint	−149.38	−145.83	−139.26
ΔEPauli	384.16	378.20	367.60
ΔEelstat	−264.61(49.6%)	−261.42(49.9%)	−257.42(50.8%)
ΔEorb	−246.98(46.3%)	−240.51(45.9%)	−227.31(44.8%)
ΔEdisp	−21.94(4.1%)	−22.09(4.2%)	−22.13(4.3%)

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The results also show that there is a decreasing trend in the interaction energies in the complexes as following 1 > 2 > 3. Thus the interaction energies of complex 1 with H as substituent is greater than complex 2 with NO₂ as substituent in meta position and both of them are greater than complex 3 with NO₂ as substituent in para position. The breakdown of the ΔE_int values into the Pauli repulsion ΔE_Pauli and the three attractive components shows that roughly 50–51% comes from the electrostatic attraction ΔE_elstat while ∼45–46% comes from the orbital term ΔE_orb and the remaining concern to ΔE_dispersion. Thus the values of ΔE_elstat the covalent bonding between the [PdCl₂] fragment and L₂ fragments [(Ylide)₂] in 1–3 complexes becomes visible by the calculated deformation densities Δρ. Fig. 3 show the important deformation densities Δρ and the associated energy values which provide about 94% of the overall orbital interactions for the complex 1. Visual inspection of Fig. 3a indicates the Δρ₁, which show the mainly contribution to ΔE_orb and comes from the Cl₂Pd←L₂ σ donation. Fig. 3c displays the deformation density Δρ₃ which come from charge transfer from L₂ to [PdCl₂] fragment. Also Fig. 3b, d and e display the deformation densities Δρ₂, Δρ₄ and Δρ₅ which comes from the [PdCl₂] fragment to Y₂ ligands σ-back donation.

Fig. 3 Deformation densities Δρ associated with the most important orbital interactions in [L₂PdCl₂]. Note that the colour in this figure, denotes the charge flow, which is from the red to the blue region.

Mizoroki–Heck cross-coupling reactions of aryl halides

The use of Pd(II) complexes 1–3 as a catalyst for Mizoroki–Heck cross-coupling reaction was then examined where both solvent and base effects were carefully examined in the reaction of styrene with bromobenzene under ambient atmosphere. We chose the complex 3 as default catalyst for the optimization of Mizoroki–Heck reaction. Various reaction conditions including base, solvent, temperature and catalyst loading were examined to achieve the best condition (Table S5†). First, reaction of styrene with bromobenzene in DMF at 130 °C for 4 h in the presence of K₂CO₃ (1.5 mmol) and 0.05 mmol of catalyst 3 was chosen as model reaction. The coupled product of this reaction was formed in 83% yield (Table S5,† entry 1). Then, a series of experiments was performed to find optimum conditions (see ESI†).

Using the optimized reaction conditions (Cs₂CO₃ (1.5 mmol), DMF (2 ml), catalyst 3 (0.005 mmol), reflux temperature) we explored the general applicability of Pd–sulfur ylide complex 3 with different olefins and aryl halides containing electron withdrawing to electron donating substituents. It is well known that the catalytic activity was extensively depends on the type of aryl halides substituents. Aryl halides with electron-withdrawing groups on the aryl ring show more reactivity, while electron donating groups make the aryl halides relatively inactive.⁴⁶ As expected, the Mizoroki–Heck cross-coupling reaction of aryl iodides with olefins gave better yields of the desired product than that of aryl bromides and aryl chlorides (Table 5). Both electron-neutral and electron-rich aryl iodides were converted efficiently to the desired coupled products in excellent yields (Table 5, entries 1, 4 and 7). The high reactivity of aromatic iodides is attributed to their small C–I bond dissociation energy and easy oxidative addition to Pd(0) species.⁴⁷ As shown in Table 5, reaction of activated aryl bromides such as 4-bromonitrobenzene, 4-bromobenzaldeyde and 4-bromoacetophenone with styrene and ethyl acrylate afford the corresponding coupled products in high yields (Table 5, entries 9, 10, 11, 14 and 17).

Table 5 Mizoroki–Heck cross-coupling reaction of aryl halides catalyzed by Pd complex^a

Entry	Arylhalide	Olefin	Product	Yield^c (%)	Time (h)
a Reaction conditions for Mizoroki–Heck cross-coupling reaction: arylhalide (0.5 mmol), olefin (0.75 mmol), Cs2CO3 (1.5 mmol), DMF (2 ml), catalyst 3 (0.005 mmol), in the air.b TBAB (1 mmol) was added.c All are isolated yields.
1	Iodobenzene	Styrene	1 g	92	4
2	Bromobenzene	Styrene	1 g	89	5
3	Chlorobenzene	Styrene	1 g	60	6
				86^b
4	4-Iodotoluene	Styrene	2 g	88	5
5	4-Bromotoluene	Styrene	2 g	85	5.5
6	4-Chlorotoluene	Styrene	2 g	58	7
				83^b
7	4-Iodoanisole	Styrene	3 g	89	5
8	4-Bromoanisole	Styrene	3 g	88	5
9	4-Bromonitrobenzene	Styrene	4 g	90	4
10	4-Bromobenzaldehyde	Styrene	5 g	82	4.5
11	4-Bromoacetophenone	Styrene	6 g	86	5
12	1-Bromonaphthalene	Styrene	7 g	81	16
13	Bromobenzene	Ethyl acrylate	8 g	92	3.5
14	4-Bromonitrobenzene	Ethyl acrylate	9 g	93	3
15	4-Bromotoluene	Ethyl acrylate	10 g	90	4
16	1-Bromonaphthalene	Ethyl acrylate	11 g	85	12
17	4-Bromobenzaldehyde	Ethyl acrylate	12 g	88	4

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Whereas, deactivated aryl bromides such as 4-bromoanisole and 4-bromotoluene gave lower yields (Table 5, entries 5 and 8). Also, the electronically neutral bromobenzene produced good amounts of desired coupling products (Table 5, entry 2). Additionally, trying to apply 1-bromonaphthalene as efficient aryl bromide resulted eligible yield of coupling product (Table 5, entries 12 and 16). The usually unreactive aryl chlorides were further explored as the coupling partner with styrene using the complex 3 catalyst system. Although, a number of reported catalysts in literature need to be used in high loadings, and showed little or no activity with aryl chloride substrates,⁴⁸ due to the economical aspect of aryl chlorides and the challenge to activate the C–Cl bond, there was much attention from the academic communities to couple aryl chlorides with olefins.⁴⁹ Both electron rich and electron neutral 4-chlorotoluene and chlorobenzene show the low reactivity in coupling reactions, but we achieved more yield by using of TBAB as additive (Table 5, entries 3 and 6). TBAB is proposed to increase the solubility of the inorganic base Cs₂CO₃ in the organic reaction medium and strengthened the stabilizations of Pd(0) species.⁵⁰

To extend the scope of our work, we next investigated the coupling reaction of various electron rich and deficient aryl halides with activated ethyl acrylate olefin. Palladium complex 3 exhibited a better catalytic activity for the Mizoroki–Heck reaction of activated ethyl acrylate than that of inactive styrene under same reaction conditions. As expected, electron-withdrawing substituent on olefin gives the desired coupling products in more purity and less reaction time (Table 5, entries 13–17).

To evaluate the presence of Pd(0) nanoparticles (NPs) in catalytic system, we performed the mercury drop test,⁵¹ was first introduced by Whitesides. Since mercury leads to amalgamation of the surface of a heterogeneous catalyst, and in contrast, Hg(0) is not expected to have a poisoning effect on homogeneous palladium complexes, the results of mentioned test help us to determine the nature of the active species. Addition of 100 equiv. Hg(0) to the reaction solution at t = 0, was not affected the conversion of the reaction, which suggests that the catalyst is homogeneous in nature.

Also, we analyzed an aliquot of the reaction mixture by SEM analysis. After completion of the reaction, 0.5 ml of the solution was separated and evaporated under reduced pressure. The crude residue was washed with water to remove base and then analyzed by SEM. As expected, the SEM images does not showed presence of any Pd nanoparticles in recovered catalyst (see ESI†).

Conclusions

In summary, we report the synthesis, characterization, theoretical study on structures and nature of bonds and application of cis- and trans-[PdCl₂(SMe₂C(H)C(O)R)₂] (R = phenyl (1), 3-nitrophenyl (2), and 4-nitrophenyl (3)). Complexes toward Mizoroki–Heck reaction. These complexes were characterized fully by spectroscopic methods as well as X-ray crystallography method. Results showed that the cross-coupling reaction of various aryl halides with olefins was performed in good to excellent yields. The ease of preparation of the complexes, low catalyst loading and stability toward air and moisture make these complexes as an ideal catalytic system for Mizoroki–Heck cross-coupling reactions. The results of theoretical study in good agreement with experimental data show that, the trans like structure is more stable than cis like structure for all complexes investigated here. The NBO analysis show that Pd and C atoms of the Pd–C bonds carry positive and negative partial charges and this confirm that there is σ donation between the lone pair of C atoms and empty orbitals of Pd in Pd–C bonds. Also the results showed that the important donor–acceptor interactions in the case of Pd–C bonds in complexes 1–3 were carry out from the σ(Pd–C) and σ(Pd–Cl) as donors to σ*(Pd–C), σ*(Pd–Cl), σ*(S–C) and σ*(O–C) as acceptors. The ETS-NOCV analyses confirm that the mainly contribution to ΔE_orb in Pd–C bonding in [L₂PdCl₂] complexes comes from Cl₂Pd←L₂ σ-donation.

Acknowledgements

Funding of our research from the Bu-Ali Sina University is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Physical measurements and selected ¹³C and ¹H NMR spectra of some compounds. CCDC 1438729 contains the supplementary crystallographic data for the complex 3. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra01390b

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