Mahabir P. Singh

Palladium(II)-(E,N,E) pincer ligand (E = S/Se/Te) complex catalyzed Suzuki coupling reactions in water via in situ generated palladium quantum dots

The (E,N,E) pincer ligands (ArECH2CH2)2NH (L1/L2: Ar = Ph, E = S/Se; L3: Ar = CH3O-p-C6H4, E = Te) synthesized by reaction of PhS(-)/PhSe(-)/CH3O-p-C6H4Te(-) with bis(2-chloroethyl)amine react with Na2PdCl4 in aqueous ethanol, resulting in nearly square planar diamagnetic complexes [Pd(L)Cl]Cl (1-3), where L = L1-L3. All the ligands (L1-L3) and their complexes (1-3) have been characterised with (1)H, (13)C{(1)H}, (77)Se{(1)H} and (125)Te{(1)H} NMR spectra and high resolution mass spectrometry. The single crystal structures (determined with X-ray diffraction) of 2 and 3 have been solved (Pd-Se: 2.4104(5)/2.4222(6) Å; Pd-Te: 2.560(2)/2.588(2) Å). The conversions for Suzuki-Miyaura coupling (SMC) of various aryl bromides with phenylboronic and 4-formyl/acetyl phenylboronic acid in water using 2-3 mol% of each of the complexes 1-3 have been found good. Complexes 1 and 2 show better catalytic activity than 3, as higher yields were observed with them in a relatively short time. The coupling reactions appear to be catalyzed with Pd(0) nanoparticles (NPs) generated in situ in the course of reaction. The NPs have been isolated and HRTEM studies on them have revealed their size as ∼1-3 nm. The SEM-EDX indicates their protection with organochalcogen fragments. Addition of TBAB was essential in some cases to get good yield of cross coupled product. The isolated NPs show catalytic activity for SMC independently. The yields of cross coupled product were excellent when NPs were reused. The two phase test suggests a relatively low contribution of homogeneous Pd species in catalysis.

Palladium(II) complexes bearing the 1,2,3-triazole based organosulfur/ selenium ligand: synthesis, structure and applications in Heck and Suzuki–Miyaura coupling as a catalyst via palladium nanoparticles

Air and moisture insensitive palladium complexes, [Pd(L)Cl2] (1/2), in which L = 1-benzyl-4-phenylthiomethyl or 1-benzyl-4-phenylselenomethyl-1H-1,2,3-triazole (L1 or L2) catalyze Heck (HC) and Suzuki–Miyaura coupling (SMC) reactions between a series of aryl bromides including deactivated bromides and n-butyl acrylate and phenylboronic acid, respectively. The optimal catalytic loading was found to be in the order of 0.01 mol%. HRTEM, TGA and EDX data indicated that 3–11 nm nanoparticles (NPs) composed of palladium and sulfur or selenium and protected with L or its fragment, were formed during the catalyzed reaction. The isolated NPs displayed catalytic activity and appeared to have a role in the catalysis. A two-phase test indicated that both homogeneous and heterogeneous catalysis took place. The complexes 1 and 2 were synthesized by the reactions of L1 and L2 respectively with [(MeCN)2PdCl2]. Their single crystal X-ray diffraction indicated that the geometry adopted by ligands around Pd in both complexes is distorted square planar with Pd–S and Pd–Se bond lengths of 2.2727(14) and 2.3693(8) A, respectively. DFT calculation gave bond lengths and angles in keeping with the experimental values. The DFT calculated HOMO–LUMO energy difference is lower for 1 than for 2 in accordance with the observed higher catalytic activity of 1.

Complex of 2-(methylthio)aniline with palladium(II) as an efficient catalyst for Suzuki–Miyaura CC coupling in eco-friendly water

2-(Methylthio)aniline (L1), a bidentate (S,N) ligand synthesized by the reaction of o-aminothiophenol with methyl iodide, on reacting with Na2PdCl4 in acetone and water gives a complex [PdL1Cl2] (1). Single crystal X-ray diffraction studies have revealed that the geometry of palladium in 1 is nearly square-planar and the ligand L1 is bound to the palladium through S and N in a bidentate coordination mode forming a five membered chelate ring. This complex functions as a thermally and air stable catalyst of high efficiency for Suzuki-Miyaura CC coupling reactions in water. It catalyzes CC coupling between various aryl bromides and phenylboronic acid under mild reaction conditions in water. TON value up to 93,000 has been obtained.

Efficient catalytic activation of Suzuki–Miyaura C–C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides

n-Bromodocosane reacts with Na2S, generated in situ by the reduction of elemental sulfur with NaBH4, to give n-didocosyl sulfide (L1), which acts as a protector for palladium nanoparticles (2–7) that are prepared using different palladium precursors in the presence of L1 (Pd : L1 ratio 1 : 2 and 4 : 1). The NPs have been characterized with powder X-ray diffraction, SEM, EDX, UV-vis spectroscopy and HRTEM. The size (nm) ranges for the majority of spherical NPs 2–7 are ∼18–19, 4–5, 5–7, 4–6, 7–9 and 4–6 respectively. The precursor of palladium affects the size, shape and dispersion of the NPs. When [Pd(CH3CN)2Cl2]/Na2PdCl4 was used as a precursor, uniformly dispersed NPs of narrow size range were obtained. L1 and its complex [Pd(L1)2Cl2] (1) have also been synthesized by the reaction of Na2PdCl4 with L1 and characterized with 1H and 13C{1H} NMR spectroscopy. The NPs show good catalytic activity for the Suzuki–Miyaura coupling (SMC) of various aryl chlorides/bromides with phenylboronic acid at low catalyst loading (0.1–0.5 mol% of Pd). The conversion is good for some aryl halides in a short reaction time of the order 1–2 h. Among 2–7, the highest activity is observed for Pd NPs obtained from Na2PdCl4, which is probably due to uniformity in their size and dispersion. The distinct advantage of NPs 2–7 is that they can be separated and reused at least up to five times. The complex 1, equivalent to 0.001 mol% Pd, is efficient for the SMC of some aryl halides, as good conversion into coupled products has been observed. Two phase tests, conducted for 1 and 3, suggest the contribution of both homogeneous and heterogeneous catalytic pathways in overall catalysis.

Palladacycles having normal and spiro chelate rings designed from bi- and tridentate ligands with an indole core: structure, synthesis and applications as catalysts

1-Pyridin-2-ylmethyl-1H-indole-3-carbaldehyde and 1-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)-1H-indole-3-carbaldehyde were synthesized. Their condensation with benzyl amine resulted in indole core containing Schiff bases benzyl-(1-pyridin-2-ylmethyl-1H-indol-3-ylmethylene)amine (L1) and benzyl-[1-(1-benzyl-1H-[1,2,3]triazole-4-ylmethyl)-1H-indol-3-yl methylene]amine (L2) respectively, unknown hitherto. The K2CO3-promoted sulfenylation of the indole formed 3-(pyridin-2-ylsulfanyl)-1H-indole (L3), also unknown so far. The yield of L1–L3 was 72–93%. L1 and L2 on reaction with sodium tetrachloropalladate(II) in the presence of CH3COONa give complexes [Pd(L1/L2-H)Cl] (1/2) in which they bind in a tridentate (N, C−, N′) pincer mode. L3 on reaction with [(MeCN)2PdCl2] results in a dimeric palladacycle [Pd(L3-H)Cl]2 (3) with a spiro ring. The precursor aldehydes, L1–L3 and the Pd(II)-complexes derived from them, were characterized using 1H and 13C{1H} NMR and HR-MS. Complexes 2 and 3, ligands L1 and L3 and the precursor aldehydes of L1 and L2 were authenticated with single crystal X-ray diffraction. The Pd–C bond distances (A) are 1.932(8)/2.115(3) (2/3). The Pd–N bond lengths (A) are: 2.063(7) and 2.028(7) for 2 and 2.053(3) and 2.019(3) for 3. These complexes have been found to be efficient as catalysts for the Suzuki–Miyaura coupling of ArCl (3 as a catalyst) and ArBr and allylation of a variety of aldehydes (1 and 2 as catalysts). The optimum loading of the complexes as catalysts is 0.001–0.01 and 1 mol% respectively for the two reactions, which appear to follow a homogeneous pathway.