Gyandshwar Kumar Rao

Organosulphur and related ligands in Suzuki-Miyaura C-C coupling.

Suzuki-Miyaura C-C cross coupling (SMC), an important synthetic strategy for many organic molecules, has several advantages such as mild reaction conditions, high tolerance toward various functional groups and ease in isolation of the product. Palladium(II) ligated with phosphines (particularly bulky and electron-rich) and N-heterocyclic carbenes (NHCs) has been found to be efficient in the catalysis of SMC. The drawback with many of these catalysts is their air/moisture sensitivity. Since 2000, palladium(II) complexes of organosulphur and related ligands have emerged as viable alternatives to palladium-phosphine/carbene complexes as they have sufficient thermal stability, air and moisture insensitivity. Moreover synthesis of complexes of such ligands is easy. In this perspective Suzuki-Miyaura C-C coupling reactions catalyzed with palladium(II)-complexes of organosulphur ligands have been reviewed. Catalysis of SMC with palladium(II) complexes of organoselenium and tellurium ligands, studied much less in comparison to those of organosulphur ligands, is also included.

Organochalcogen ligands and their palladium(II) complexes: Synthesis to catalytic activity for Heck coupling

Heck cross coupling reactions have been catalyzed with several palladium(II)-complexes of organochalcogen ligands. The complexes applied in molecular and tethered forms appear to be dispensers of catalytically active species, but this aspect has not been thoroughly looked at in all cases. The thermal stability, air and moisture insensitivity and ease of handling and synthesis have made them viable alternatives to phosphine/carbene based Pd-catalysts. This review covers developments from the design of organochalcogen ligands and their palladium(II)-complexes to the applications of these complexes in catalyzing Heck reactions.

Organoselenium ligands in catalysis

Organoselenium ligands are building blocks of several transition metal complexes which catalyze various organic reactions efficiently in solution. In this review the survey of developments pertaining to the designing of such complexes (along with their Se-ligands), and their uses as catalysts for various organic reactions has been presented with critical analysis of present status of the subject. Undoubtedly a new family of catalysts or their precursors has been generated by organoselenium ligands. The catalytic activity and selectivity of their metal complexes can sometimes be fine-tuned efficiently by changing the ligands framework. They show good thermal and air-stability. This, coupled with ease in their handling and synthesis, may make them not only attractive but good alternatives to several popular catalysts.

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.

Tailoring of electro-optical properties of ferroelectric liquid crystals by doping Pd nanoparticles

We demonstrate here the tailoring of electro-optical properties of ferroelectric liquid crystals (FLCs) by doping different concentrations of Selenium Docosane (SD) capped palladium nanoparticles (PdNPs). The operating voltage is lowered by ∼50% in doped FLC as compared to undoped one. The remarkable increment in optical tilt angle of doped FLC is monitored, which is concentration-dependent. The steric interactions among alkyl chains of SD capped PdNPs and FLC molecules could be the probable reason for enhanced optical tilt angle. More importantly, present investigations on doped FLCs are indicative of their indispensible impact on next generation FLCs-based electro-optical devices.

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.

Chalcogenated Schiff bases: Complexation with palladium(II) and Suzuki coupling reactions

AbstractChalcogenated Schiff bases of 5–chloroisatin (L1–L3), 2–(methythio)benzaldehyde (L4), 2–acetylpyridine (L5) and benzaldehyde (L6–L7) have been synthesized. Both the carbonyl groups of 5–chloroisatin appear to be reactive (noticed for the first time) for making >C=N bond, of course one at a time only. The 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR spectroscopy have been used to establish the co-existence of two products, which were found in the ratio 53:47 (E = S), 55:45 (E = Se) and 81:19 (E = Te). The larger amount is of the one in which C=O group away from NH is derivatized. The two products are not separable. Palladium complexes (1–4) of Schiff bases of other three aldehydes were synthesized. The ligands as well as complexes were characterized by multinuclear NMR spectroscopy. The crystal structures of [Pd(L4/L5)Cl][ClO4] (1/2) have been solved. The Pd–Se bond lengths are 2.4172(17) and 2.3675(4) Å, respectively for 1 and 2. The Pd–complexes (3–4) of L6–L7 were explored for Suzuki–Miyaura coupling and found promising as 0.006 mol % of 3 is sufficient to obtain good conversion with TON up to 1.58 × 104. Graphical AbstractChalcogenated Schiff bases of 5–chloroisatin (L1–L3), 2–(methythio)-benzaldehyde (L4), 2–acetylpyridine (L5) and benzaldehyde (L6–L7) and palladium complexes of L4–L7 have been synthesized and characterized by 1H, 13C{1H}, 77Se{1H} and 125Te{1H} NMR. The Pd–complexes of L6–L7 even at 0.006 mol% give good conversions in Suzuki−Miyaura coupling (TON upto 1.58 × 104).