Fariha Saleem

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) 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.

Oxine based unsymmetrical (O−, N, S/Se) pincer ligands and their palladium(II) complexes: synthesis, structural aspects and applications as a catalyst in amine and copper-free Sonogashira coupling

Unsymmetrical pincer ligands having an 8-hydroxyquinoline (oxine) core viz. 2-(phenylthio/selenomethyl) quinolin-8-ol (L1/L2), 2-(N,N-dimethylthiocarbamoyl) quinolin-8-ol (L3) and 2-(pyrrolidin-1-ylthiocarbamoyl) quinolin-8-ol (L4) were synthesized. 2-Methylquinolin-8-ol was converted to 2-bromomethylquninolin-8-ol, which reacted with PhENa (E = S or Se) to give L1 and L2, and the Willgerodt–Kindler reaction on an appropriate aldehyde derivative of quinoline gave L3 and L4. Upon reaction with Na2PdCl4/[Pd(CH3CN)2Cl2], L1–L4 coordinated as a (O−, N, E) donor (E = S/Se) resulting in complexes [Pd(L–H)Cl] (1–4; L = L1–L4). The molecular structures of L1, 1 and 2 were established by single crystal X-ray diffraction. The palladium in 1 and 2 has a nearly square planar geometry. The Pd–S bond distance in 1 is 2.2648(14) A and in 2, the Pd–Se bond distance is 2.3641(7) A. Somewhat rare weak interactions (viz. C–H⋯Pd and Se⋯Cl) were noticed in the crystals of 1 and 2, respectively. Complexes 1 and 2 were found to be efficient in catalyzing Sonogashira coupling under amine and copper free conditions. The catalyst loading of 0.5–1.0 mol% was found to be promising for the conversion of several aryl halides to their coupled products. The yields were lower for ArCl in comparison to ArBr/ArI. The catalytic activity of 1 was marginally lower than that of 2. DFT calculations support the catalytic activity order and bond lengths and angles of 1 and 2.

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.