Sterically hindered selenoether ligands: palladium(II) complexes as catalytic activators for Suzuki–Miyaura coupling

Abstract

2-Hydroxy/(benzyloxy)-3,5-di-tert-butyl benzaldehyde reacts with PhSeCH₂CH₂NH₂ resulting in a sterically hindered selenoether ligand (Schiff base) [2–HO–3,5–(C(CH₃)₃)₂–C₆H₂–C=N–(CH₂)₂SePh] (L1)/[2–PhCH₂O–3,5–(C(CH₃)₃)₂–C₆H₂–CH₂–NH–(CH₂)₂SePh] (L2). The reactions of L1 and L2 with Na₂PdCl₄ in methanol and acetone–water mixture at room temperature have resulted in complexes [PdCl (L1-H)] (1) and [PdCl₂(L2)] (2)], respectively. Both the complexes and their ligands have been characterized with ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectroscopy. The molecular structures of complexes 1 and 2 have been determined with single crystal X-ray diffraction. The Pd–Se bond lengths in 1 and 2 are 2.370(1) and 2.366(1) Å, respectively. The geometry around palladium in both the complexes is nearly square planar. Complexes 1 and 2 (0.1 mol% Pd) have been found efficient as catalysts for Suzuki–Miyaura C–C coupling reactions in the presence of K₂CO₃ in ethanol. The catalysis in water with complex 1 in the presence of K₂CO₃ was found feasible but with low conversion (up to 40%). The efficiency of 1 in carrying out the coupling is marginally better than that of 2.

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Introduction

The strong electron-donor ability of selenium has led to the synthesis of many transition metal complexes of organoselenium ligands, which have been found promising as catalysts for various organic transformations.¹ This has made organoselenium ligands currently important for designing catalysts for a variety of organic reactions. A wide range of such ligands is known e.g. selenocarbonyl,² pincer type,³ and Schiff bases⁴ promising for catalyst designing. One notable feature of transition metal complexes of organoselenium ligands is that they are much less insensitive to air and moisture in comparison to those which have traditional phosphorous donors. At present palladium complexes of organoselenium ligands in terms of catalytic activity are considered not only rivals of their respective phosphorus and sulphur analogues but have been found in many cases to outperform them.^3a

The steric properties of ligands significantly influence the rate and selectivity of the reactions catalyzed by their transition metal complexes. The steric bulk of a ligand can increase the stability of the catalyst in its coordinatively unsaturated intermediate form or accelerate some steps, such as reductive elimination in cross-coupling reactions which are sterically sensitive.⁵ Thus steric bulk of a ligand may play a role in tailoring efficiency of its complex as a catalyst for Suzuki–Miyaura cross-coupling reactions.⁶ Palladium complexes derived from electron-rich and sterically demanding ligands, of the type monophosphanes,⁷ N-heterocyclic carbenes,⁸ and C₂-symmetric bis-hydrazone,⁹ have been found effective catalyst for Suzuki–Miyaura cross-coupling reactions. However no report on sterically hindered organoselenium ligands is in our knowledge inspite of several reports on activation of Suzuki–Miyaura coupling with palladium complexes and palladacycles¹⁰ of several organochalcogen donors including Se ones,^4a,c–e,11 and chalcogenated carbenes,¹² which have high efficiency and can be modified with ease.

We report herein the synthesis and structural characterization of sterically hindered selenoether ligands, [2–HO–3,5–(C(CH₃)₃)₂–C₆H₂–C=N–(CH₂)₂SePh] (L1) and [2–PhCH₂O–3,5–(C(CH₃)₃)₂–C₆H₂–CH₂–NH–(CH₂)₂SePh] (L2) and their palladium(II) complexes of the type, [PdCl (L1-H)] (1) and [PdCl₂(L2)] (2)], respectively. Two complexes 1 and 2 have been found suitable activators for Suzuki and Miyaura C(sp²)–C(sp²) coupling reactions, which are powerful synthetic tools in organic synthesis,^13,14 along with other transition metal-catalyzed carbon–carbon bond forming reactions such as Heck, Sonogashira, Hiyama, Stille and Kumada coupling reactions. Suzuki and Miyaura coupling has made a significantly higher impact than others in the laboratory and the chemical industries due to three key reasons: (i) the reaction is feasible with a wide range of substrates and many functional groups are tolerated due to mild reaction conditions. This is very helpful in the total synthesis of complex molecules including drugs. (ii) Phenylboronic acid, starting material is readily available, stable and sustainable.¹⁵ (iii) The product biaryl is a very important core component of various biologically and pharmaceutically important compounds (viz. anti-hypertensive, anti-cancer, anti-biotic, anti-inflammatory, and antifungal) and in nonlinear optical materials.¹⁶

Experimental section

Materials and methods

Diphenyl diselenide, 2-hydroxy-3,5-di-tert-butylbenzaldehyde, 2-chloroethyl amine, sodium tetrachloropalladate, phenylboronic acid, potassium carbonate, and aryl bromides were procured from Sigma-Aldrich (USA). Reagents (commercially available from local sources) were used as received without further purification. PhSe(CH₂)₂NH₂ and 2-(benzyloxy)-3,5-di-tert-butylbenzaldehyde were prepared by a following the methods reported in the literature.^17,18 The progress of every coupling reaction was monitored with NMR spectroscopy. The products of Suzuki reactions were authenticated by matching their spectroscopic data with the reported literature values. ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectra were recorded on a Bruker Spectrospin DPX 300 NMR spectrometer at 300.13, 75.47 and 57.24 MHz respectively. The chemical shifts are reported in ppm relative to internal standard (tetramethylsilane in case of ¹H, and ¹³C{¹H} NMR and Me₂Se for ⁷⁷Se{¹H} NMR). Elemental analyses were carried out with a Perkin-Elmer 2400 Series II C, H, N analyzer.

X-ray diffraction data of crystals of 1 were collected on a Bruker AXS SMART-APEX diffractometer with a CCD area detector.¹⁹ Similar data of 2 were collected on an Oxford Xcalibur S diffractometer with Sapphire-3 CCD detector.²⁰ Mo Kα radiations were used in both the cases. Both Crys Alis Pro software suite²¹ and SADABS²² software were used as per requirement. The structures were refined using the SHELX-97 program package and SHELXL97 (within the Win GX program package).^23–26 Non-hydrogen atoms were refined anisotropically. The molecular structures were created with the Diamond program.²⁷ Crystallographic data are given in Table 1.

Table 1 Crystallographic data and structure refinement summary for 1 and 2

	1	2·CH3CN
a R1 = Σ||Fo| − |Fc||/Σ|Fo|.b wR2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2.c S = {Σ[w(Fo^2 − Fc^2)^2]/(n − p)^2]}^1/2.
Formula	C23H30NOPdClSe	C32H41N2OPdCl2Se
Formula weight	557.29	725.93
T/K	298(2)	298(2)
λ/Å	0.71073	0.71073
Crystal system	Orthorhombic	Monoclinic
Space group	Pbca	P21/c
a/Å	10.9228(4)	12.3315(5)
b/Å	10.0631(4)	10.4554(4)
c/Å	43.0466(17)	26.1643(10)
α/deg	90.00	90.00
β/deg	90.00	97.856(4)
γ/deg	90.00	90.00
Vol/Å^3	4731.5 (3)	3341.7 (2)
Z	8	4
Dcalcd/g cm^−3	1.565	1.443
F(000)	2240	1476
θrange/deg	2.91–25.00	3.06–25.00
Reflections measured	4159	5885
Reflections used	3702	5337
Parameters	259	359
μ(Mo Kα) (cm^−1)	2.449	1.830
R1, wR2[I > 2σ(I)]^a	0.0488, 0.0558	0.0357, 0.0407
R1, wR2 (all data)^b	0.1014, 0.1040	0.0818, 0.0840
GooF^c	1.302	1.216

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Synthesis of L1. The selenated amine C₆H₅Se–(CH₂)₂–NH₂ (0.412 g, 2.06 mmol) and 2-hydroxy-3,5-di-tert-butylbenzaldehyde (0.469 g, 2.00 mmol) were reacted in absolute ethanol (20 mL) at room temperature for 12 h. The volatiles from resulting reaction mixture were removed using rotary evaporator which resulted ligand L1 as yellow viscous liquid in 93% yield (0.776 g, 1.86 mmol). ¹H NMR (CDCl₃): δ (ppm) = 1.36 (s, 9H, C(CH₃)₃), 1.51 (s, 9H, C(CH₃)₃), 3.24 (t, 2H, SeCH₂), 3.91 (t, 2H, NCH₂), 7.12–7.58 (7H, Ar–H), 8.35 (S, 1H, CH=N), 13.55 (broads, 1H, OH). ¹³C{¹H} (CDCl₃): δ (ppm) = 28.3 (SeCH₂), 29.5 and 31.5 (C(CH₃)₃), 34.1 and 35.0 (C(CH₃)₃), 59.6 (NCH₂), 117.7, 126.0, 127.1, 129.2, 129.6, 132.9, 136.7, 140.1, 158.0 (Ar–C) 166.9 (N=CH). (⁷⁷Se{¹H} NMR, CDCl₃): δ (ppm) = 281.1.

Synthesis of L2. The selenated amine C₆H₅Se–(CH₂)₂–NH₂ (0.412 g, 2.06 mmol) and 2-(benzyloxy)-3,5-di-tert-butylbenzaldehyde (0.650 g, 2.00 mmol) were reacted in absolute ethanol (30 mL) at room temperature for 12 h. The volatiles were removed using rotary evaporator which resulted in yellow liquid. The yellow liquid formed was reacted with NaBH₄ (0.080 g, 2.11 mmol) in ethanol to obtain ligand L2 as colourless viscous liquid in 91% yield (0.925 g, 1.82 mmol). ¹H NMR (CDCl₃): δ (ppm) = 1.32 (s, 9H, C(CH₃)₃), 1.43 (s, 9H, C(CH₃)₃), 1.74 (broad s, 1H, NH), 2.86 (t, J_HH = 7.05 Hz, 2H, SeCH₂), 2.97 (t, J_HH = 6.45 Hz, 2H, NCH₂CH₂), 3.82 (s, 2H, CH₂N), 5.00 (s, 2H, OCH₂), 7.16–7.49 (broad m, 12H, Ar–H). ¹³C{¹H} (CDCl₃): δ (ppm) = 28.3 (CH₂Se), 31.2 and 31.5 (C(CH₃)₃), 34.4 and 35.3 (C(CH₃)₃), 48.7 (NHCH₂), 48.8 (CH₂NH), 75.3 (OCH₂), 123.2, 125.2, 126.4, 126.6, 127.3, 128.3, 128.8, 129.8, 132.5, 132.8, 138.0, 141.8, 145.8 and 154.1 (Ar–C). (⁷⁷Se{¹H} NMR, CDCl₃): δ (ppm) = 267.8.

Synthesis of complex 1. The Na₂PdCl₄ (0.147 g, 0.5 mmol) was dissolved in 20 mL of methanol. The homogeneous methanolic solution of ligand L1 (0.208 g, 0.5 mmol dissolved in 10 mL of methanol) was added with stirring. The mixture was further stirred for 6 h. The orange red coloured precipitate was obtained which was filtered and dried under vacuo. The single crystals of 1 were grown from CH₃OH–CH₃CN mixture (1 : 1) by slow evaporation of its solution for one week. Yield: 88% (0.245 g, 0.44 mmol). Anal. Calcd for C₂₃H₃₀ClNOPdSe (Mw: 557.29): C, 49.57; H, 5.43; N, 2.51%. Found: C, 49.60; H, 5.24; N, 2.64. ¹H NMR (DMSO-d₆): δ (ppm) = 1.23 (s, 9H, C(CH₃)₃), 1.37 (s, 9H, C(CH₃)₃), 3.10–3.20 (m, 2H, CH₂), 3.72 (m, 1H, CH₂), 4.46–4.50 (m, 1H, CH₂), 7.17–8.18 (broad m, 5H, C₆H₅), 8.09 (s, 1H, N=CH), 7.35 (s, 1H, C₅H₂), 8.19 (s, 1H, C₅H₂). ¹³C{¹H} (CDCl₃): δ (ppm) = 29.9 and 31.7 (C(CH₃)₃), 32.9 (CH₂Se), 34.0 and 35.8 (C(CH₃)₃), 65.7 (N=CH₂), 119.3, 125.6, 128.7, 129.9, 130.4, 130.5, 133.5, 136.1, 138.8, 161.3 (Ar–C), 162.4 (N=CH). (⁷⁷Se{¹H} NMR, CDCl₃): δ (ppm) = 432.1.

Synthesis of 2. The Na₂PdCl₄ (0.147 g, 0.5 mmol) was dissolved in 5.0 mL of water. A solution of ligand L2 (0.254 g, 0.5 mmol) made in 10 mL of acetone was added to homogenized aqueous solution of Na₂PdCl₄ drop wise with vigorous stirring. The mixture was further stirred for 2 h resulting orange-red precipitate. It was poured into cold water (40 mL) and extracted with chloroform (4 × 30 mL). The combined extract was dried over anhydrous sodium sulphate. Its solvent was reduced to 5 mL and complex was precipitated using n-hexane. The precipitate was filtered and dried under vacuo. The single crystals of 2, as 2·CH₃CN were grown from CH₃OH–CH₃CN mixture (1 : 1) by slow evaporation of its solution for one week. Yield: 84% (0.306 g, 0.42 mmol). Anal. Calcd for C₃₀H₃₈Cl₂NOPdSe·CH₃CN (Mw: 725.93): C, 52.94; H, 5.69; N, 3.86%. Found: C, 52.82; H, 5.67; N, 3.78. ¹H NMR (CDCl₃): δ (ppm) = 1.35 (s, 9H, C(CH₃)₃), 1.39 (s, 9H, C(CH₃)₃), 2.63–2.65 (m, 2H, CH₂), 3.55–3.58 (broad m, 2H, CH₂), 4.19–4.27 (m, 1H, CH₂), 4.69–4.73 (m, 1H, CH₂), 4.80–4.90 (m, 2H, OCH₂), 7.20–7.87 (broad m, 12H, Ar–H). ¹³C{¹H} (CDCl₃): δ (ppm) = 29.3 and 29.7 (C(CH₃)₃), 29.6 and 33.4 (C(CH₃)₃), 33.0 (CH₂Se), 33.5 (NHCH₂), 49.0 (NH(CH₂)Ar), 51.0 (OCH₂Ph), 123.4, 124.5, 125.4, 125.6, 125.9, 126.6, 126.8, 128.1, 128.2, 131.2, 135.1, 140.0, 145.9, 152.9 (Ar–C). (⁷⁷Se{¹H} NMR, CDCl₃): δ (ppm) = 489.3.

Procedure for Suzuki reaction of aryl bromides with phenylboronic acid. An oven dried flask was charged with aryl bromide (1.0 mmol), phenylboronic acid (1.2 mmol), K₂CO₃ (2.0 mmol) and ethanol (4.0 mL). The flask was placed on an oil bath at 80 °C under aerobic condition and the reaction mixture stirred until maximum conversion of aryl bromide to coupled product occurred, as revealed with NMR spectroscopy. The mixture was extracted with diethyl ether (100 mL). The extract was washed with water (100 mL) and dried over anhydrous Na₂SO₄.

Results and discussion

Synthesis

The synthetic details of ligands L1 and L2, and their corresponding palladium metal complexes 1 and 2 are summarized in Scheme 1. The L1, L2, 1 and 2·CH₃CN are stable under ambient conditions. The complexes can be stored for six month without noticeable decomposition. The ligands and their complexes 1 and 2 have been characterized by their ¹H and ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectra (see ESI† for the spectra). These data are consistent with the structures depicted for them in Scheme 1.

Scheme 1 Synthesis of ligands L1 and L2 and their Pd(II) complexes.

Crystal structures

Molecular structures of 1 and 2 were determined with single crystal X-ray diffraction. The crystal and structure refinement data are given in Table 1. The Fig. 1 depicts molecular structure of 1 (ellipsoid at 30% probability level; hydrogen atoms have been omitted for clarity). The palladium in 1 is surrounded by O1, N1, Se1 and Cl1 resulting in almost square planner geometry as shown in Fig. 1. The ligand in complex 1, coordinates with Pd in a mono anionic tridentate (Se, N, O⁻) mode forming a six and five membered chelate ring. The six membered ring is formed via O⁻ and N whereas five membered via Se and N. The Pd–N bond length 1.996(4) Å in 1 is comparable to the reported values of 1.985(4) Å for [PdCl{2–O–3–CH(CH₂CH₃)₂–C₆H₃–C=N–(CH₂)₂SeMe}] (I) and 2.003(7) Å for [PdCl{2–O–C₆H₄C(CH₃)=N(CH₂)₂SePh}] (II).^4a,b However, it is slightly shorter than those reported for [PdCl{2–O––C₆H₄C(C₆H₅)=N(CH₂)₂SePh}] (III) 2.010(4) Å and [PdCl{2–O–C₁₀H₆C(CH₃)=N(CH₂)₂SePh}] (IV) 2.010(3) Å.^4c,d The Pd–O distance 2.001(3) Å in 1 is comparable with values reported for (I) 2.017(4) and (III) 1.993(3) Å and longer than those of (II) 1.977(6) and (IV) 1.973(2) Å. Similarly Pd–Se bond distance 2.370(1) Å in 1 is comparable with values for (I) 2.365(1) and (II) 2.367(1) Å and longer than those of (III) 2.358(1) and (IV) 2.360(1) Å. The Pd–Cl distance 2.302(1) Å in 1 is comparable with the value reported for (II) 2.305(2) Å and little shorter than those of (I) 2.323(2), (III) 2.315(2) and (IV) 2.316(1) Å. The molecular structure of 2 is shown in Fig. 2 (Ellipsoid at 30% probability level; hydrogen atoms have been omitted for clarity). The palladium in 2 has nearly square planner geometry constituted by N1, Se1, Cl1 and Cl2. The bond distances: Pd–Se 2.366(1) and Pd–Cl1 2.306(1) Å in 2 are comparable with the values mentioned above, 2.370(1) Å and 2.302(1) Å in case of 1, respectively. However, the Pd–N distance 1.996(4) Å in 1 is shorter than 2.066(3) Å found in case of 2·CH₃CN. Significant non-covalent interactions observed in 1 and 2 are listed in Table 2. In the crystal of complex 1 intermolecular C–H⋯Cl non-covalent interactions exist as shown in Fig. 3. The Cl1 atom acts as a bifurcated hydrogen bond acceptor with H2 (SePh) and H8A (methylene group of adjacent molecule; C8). In crystal of 2·CH₃CN C–H⋯N hydrogen bonding interactions are present in addition to intermolecular C–H⋯Cl non-covalent interactions as shown in Fig. 4. In this crystal, the Cl2 atom acts as a bifurcated hydrogen bond acceptor with H8B (methylene group; C8) and H20 (CH₂Ph) of another adjacent molecule. The Cl1 atom is hydrogen bonded with H31B (CH₃CN) of adjacent molecule. In addition, the N2 atom of CH₃CN acts as hydrogen bond acceptor with H16A (C16 of CH₂Ph group) of adjacent molecule.

Fig. 1 ORTEP representation of 1. Selected bond lengths (Å) and angles (°): Pd1–N1 1.996(4), Pd1–O1 2.001(3), Pd1–Se1 2.370(1), Pd1–Cl1 2.302(1); N1–Pd1–O1 92.5(2), N1–Pd1–Cl1 178.3(1), O1–Pd1–Cl1 88.3(1), N1–Pd1–Se1 89.1(1), O1–Pd1–Se1 177.2(1), Cl1–Pd1–Se1 90.1(1).

Fig. 2 ORTEP representation of 2. Selected bond lengths (Å) and angles (°): Pd1–N1 2.066(3), Pd1–Se1 2.366(1), Pd1–Cl1 2.306(1), Pd1–Cl2 2.331(1); N1–Pd1–Cl1 176.0(1), N1–Pd1–Cl2 88.0(1), Cl1–Pd1–Cl2 95.1(1), N1–Pd1–Se1 88.9(2), Cl1–Pd1–Se1 87.9(4), Cl2–Pd1–Se1 176.8(1).

Table 2 Selected non-covalent interactions of 1 and 2 (inter atomic distances in Å and bond angles in deg)

D–H⋯A	D–H	H⋯A	D⋯A	D–H⋯A
a ½ − x, −1/2 + y, z;b 1 − x, 1 − y, −z;c 1 − x, 1 − y, 1 − z;d x, y, 1 + z;e − 1 + x, y, z;f 1 − x, −y, 2 − z.
1
C8–H8A⋯Cl1^a	0.97	2.68	3.45	140
C2–H2⋯Cl1^b	0.93	2.85	3.69	151
2
C16–H16A⋯N2^c	0.97	2.69	3.55	148
C31–H31B⋯Cl1^d	0.96	2.79	3.71	162
C20–H20⋯Cl2^e	0.93	2.90	3.61	135
C8–H8B⋯Cl2^f	0.97	2.90	3.68	139

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Fig. 3 Packing diagram of 1 illustrating intermolecular C–H⋯Cl hydrogen bonding in the crystal lattice.

Fig. 4 Packing diagram of 2·CH₃CN illustrating intermolecular C–H⋯Cl and C–H⋯N hydrogen bonding in the crystal lattice.

Spectroscopic studies

The ¹H and ¹³C{¹H} NMR spectra of ligand L1 and L2 have been found consistent with their structures depicted in Scheme 1. The signal of OH proton in ¹H NMR spectrum of L1 has been observed at δ 13.55 ppm. The ⁷⁷Se{¹H} NMR spectrum of L1 has a signal at δ 281.1 ppm consistent with the value reported in literature (δ 281.5 ppm for selenoether^4d). In ¹H NMR spectrum of ligand L2, the signal of NH proton appears as a broad singlet at δ 1.74 ppm. The signals for CH₂N and OCH₂ protons in proton NMR spectrum of ligand L2 appear at δ 3.82 and 5.00 ppm, respectively. All expected signals were observed in ¹³C{¹H} NMR spectra of ligands L1 and L2. ⁷⁷Se{¹H} NMR spectrum L2 shows a signal at δ 268.9 ppm. In complex 1, the ligand L1 coordinates to Pd in a mono anionic tridentate (Se, N, O⁻) mode which is corroborated by its single crystal structure. The signal of phenolic proton present at δ = 13.55 ppm in the NMR spectrum of ligand L1 has been found disappeared on complexation. This indicates the deprotonation of phenolic OH on formation of complex 1. In ¹H NMR spectrum of complex 1, each proton of both CH₂ groups becomes diastereotopic and this results in three multiplets at δ, 3.10–3.20, 3.72 and 4.46–4.50 ppm (2H, 1H and 1H, respectively; see Fig. S7 in ESI†), supporting Pd–N and Pd–Se bond formation. The ⁷⁷Se{¹H} NMR spectrum of complex 1 has a signal at δ 432.1 ppm which is highly deshielded (151 ppm) compared to that of free ligand. This may be ascribed due to coordination of selenium to palladium. The large deshielding may be due to formation of five membered chelate ring^4d with Pd. In complex 2 the ligand L2 coordinates to Pd in a neutral bidentate (Se, N) mode as corroborated by its single crystal structure. In ¹H NMR spectrum of complex 2 also, protons of each CH₂ group become diastereotopic and result in five multiplets at δ 2.63–2.65, 3.55–3.58, 4.19–4.27, 4.69–4.73 and 4.80–4.90 ppm (2H, 2H, 1H, 1H, and 2H, respectively see Fig. S10 in ESI†), consistent with Pd–N and Pd–Se bond formation, which makes the NCH₂ and CH₂Se protons rigid. The ⁷⁷Se{¹H} NMR spectrum of complex 2 has a signal at δ 489.3 ppm, highly deshielded (221.5 ppm) compared to that of free ligand, indicating the coordination of selenium with palladium.

Suzuki–Miyaura C–C coupling catalyzed with 1 and 2

Suzuki–Miyaura C–C coupling reactions were carried out in the presence of complexes 1 and 2 as catalyst and results obtained are summarized in Tables 3 and 4. The optimization of reaction conditions with present catalyst 1 was carried out by coupling 4-bromotoulene and phenylboronic acid under aerobic conditions at 80 °C for 5 h using different bases and solvents. The best results were obtained with K₂CO₃ and ethanol (Table 3, entry 6). The conversion was also observed in water (up to 40%) with K₂CO₃ (Table 3, entry 13). The coupling reaction of 4-bromo-nitrobenzene/4-bromobenzo-nitrile/4-bromoacetophenone with phenylboronic acid in the presence of 0.1 mol% of 1 or 2 in 5 h at 80 °C, resulted in corresponding biaryl in 100% yield (Table 4, entry 1–6). The coupling between 4-bromotoluene and phenylboronic acid in the presence of 0.1 mol% of 1 for 5 h at 80 °C, resulted in corresponding biaryl in 88% yield (Table 4, entry 7). However, when catalyst 2 was used for the same coupling under similar reaction conditions yield of corresponding coupling product, biaryl reduced to 82% (Table 4, entry 8). Similar trends were observed in the case of 4-bromobenzaldehyde, 4-bromoanisole and 4-bromobenzoic acid under similar reaction condition which gave 75, 35 and 70% conversion to corresponding biaryl with catalyst 1 and 71, 32 and 61% with catalyst 2. These trends are due to the fact that the catalytic activity is dependent on the nature of the electron withdrawing group on the aryl ring, the reactivity increases in the order of NO₂ > H > OMe. The catalytic efficiency of 1 appears to be slightly higher than that of 2. In comparison to palladium complexes^{4a,c–e,9,11} other than the palladacycles and pincer ligand based ones the performance of 1 and 2 is comparable or better in comparison to those containing N/S donors. Similarly the present complexes are comparable (in some cases favourably) with palladium nanoparticle based catalytic systems.²⁸ Air and moisture insensitivity of 1 and 2 are their additional advantages. However they cannot be recycled for catalytic applications.

Table 3 Optimization of reaction conditions for Suzuki–Miyaura C–C coupling reactions of 4-bromotoluene with phenylboronic acid^a

Entry no	Solvent	Base	Yield^b (%)
a Reaction conditions: 1.0 equiv. of aryl halide, 1.2 equiv. of phenylboronic acid, and 2 equiv. of base and temperature of bath 80 °C. Catalyst 1: 0.1 mol% Pd. Time: 5 h.b NMR(%) yield.
1	1,4-Dioxane (4 mL)	CH3ONa	42
2	1,4-Dioxane (4 mL)	K2CO3	50
3	DMF (4 mL)	CH3ONa	41
4	DMF (4 mL)	K2CO3	47
5	EtOH (4 mL)	CH3ONa	82
6	EtOH (4 mL)	K2CO3	88
7	1,4-Dioxane : water (3 : 1 mL)	CH3ONa	41
8	1,4-Dioxane : water (3 : 1 mL)	K2CO3	42
9	DMF : water (3 : 1 mL)	CH3ONa	33
10	DMF : water (3 : 1 mL)	K2CO3	39
11	EtOH : water (3 : 1 mL)	CH3ONa	52
12	EtOH : water (3 : 1 mL)	K2CO3	68
13	Water (4 mL)	K2CO3	40

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Table 4 Suzuki–Miyaura coupling reaction catalyzed by catalysts 1 and 2^a

Entry no	Aryl halide	Catalyst	Yield^b (%) (TON)	TOF (h^−1)
a Reaction conditions: 1.0 equiv. of aryl halide, 1.2 equiv. of phenylboronic acid, and 2 equiv. of base (K2CO3), solvent: EtOH (4 mL) and temperature of bath 80 °C. Catalyst: 0.1 mol% Pd; time: 5 h.b NMR(%) yield.
1	4-Bromonitrobenzene	1	100(1000)	200
2	4-Bromonitrobenzene	2	100(1000)	200
3	4-Bromobenzonitrile	1	100(1000)	200
4	4-Bromobenzonitrile	2	100(1000)	200
5	4-Bromoacetophenone	1	100(1000)	200
6	4-Bromoacetophenone	2	100(1000)	200
7	4-Bromotoluene	1	88(880)	176
8	4-Bromotoluene	2	82(820)	164
9	4-Bromobenzaldehyde	1	75(750)	150
10	4-Bromobenzaldehyde	2	71(710)	142
11	4-Bromoanisol	1	35(350)	70
12	4-Bromoanisol	2	32(320)	64
13	4-Bromobenzoic acid	1	70(700)	140
14	4-Bromobenzoic acid	2	61(610)	122

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Conclusions

Two sterically hindered selenated Schiff bases and their palladium(II) complexes have been synthesized and structurally characterized with ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectra. In ⁷⁷Se{¹H} NMR spectra deshielding of signal on complexation was up to 221.5 ppm compared to that of corresponding free ligand. The ligand L1 coordinates as tridentate (N, Se, O⁻) mode in complex 1 whereas L2 coordinates as a bidentate (N, Se) ligand in 2 as revealed by single crystal structure. Complexes 1 and 2 are efficient catalysts for Suzuki–Miyaura C–C coupling reactions for aryl bromide with phenylboronic acid in presence of K₂CO₃ in ethanol at 80 °C in 5 h. The reactivity increases on the nature of the substituent on the aryl ring and follows the order NO₂ > H > OMe. The catalytic conversion was also observed up to 40% with K₂CO₃ in water with catalyst 1. The efficiency of 1 in carrying out the coupling is slightly higher than that of 2.

Acknowledgements

The authors acknowledge the Department of Science and Technology, New Delhi (SR/FT/CS-79/2011) (UK) and (SR/WOS-A/CS-57/2012) (VVS) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectra of L1, L2, 1 and 2. CCDC 1009785 and 1009786. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra06313a

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