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

Abstract

Chalcogenated 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 5chloroisatin appear to be reactive (noticed for the first time) for making >C=N bond, of course one at a time only. The 1 H, 13 C{ 1 H}, 77 Se{ 1 H} and 125 Te{ 1 H} NMR spectroscopy have been used to establish the coexistence 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][ClO 4 ] (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 × 10 4 .

Experimental and theoretical insights into thetransinfluence of organo-sulfur and -selenium ligands in 5,6-membered palladium(ii) cationic pincer complexes based on iminophosphoranes

“…22,23 The group of Singh reported a series of flexible tridentate Schiff bases and amine derivatives including different combinations of group 16 donors (S, Se, and Te) and described their Pd( ii ) unsymmetrical complexes containing 5,6-membered fused rings, which were found to be promising as catalysts for Suzuki–Miyaura coupling. 24,25…”

“…22,23 The group of Singh reported a series of flexible tridentate Schiff bases and amine derivatives including different combinations of group 16 donors (S, Se, and Te) and described their Pd(II) unsymmetrical complexes containing 5, 6-membered fused rings, which were found to be promising as catalysts for Suzuki-Miyaura coupling. 24,25 There have been a number of quantum mechanical studies at the DFT level concerning transition metal pincer ligands, to explore the structure, reactivity and spectroscopic properties of the compounds. Most of these explore the catalytic mechanism of action of symmetrical 5,5-membered cyclometallated systems, 26 fewer focus on unsymmetrical pincer 5,5-membered structures containing ancillary ligands of varying flexibility, and/or alteration of the hard/soft acid base properties of the donor atoms.…”

Electrochemical Synthesis of Pd Nano Particles on Pencil-Graphite and Application for Suzuki Coupling Reactions

“…Various palladium based homogeneous catalysts such as palladium diaminocarbene complexes [6], oxime based palladacycle [7], chalcogenated Schiff base [8], some nitrogen based ligands [9], N-H heterocyclic carbene complex [10] and morpholine-Pd(OAc) 2 [11] were employed in the Suzuki cross coupling reactions. However, these reported catalysts could not be recovered and hence, researchers moved towards heterogeneous supports like alumina based catalyst [12], MCM-41 [13], MIL-101 [14], Al-MOF [15], Montmorillonite clay [16], sepiolite clay [17], SBA-15/ CCMet/Pd(II) [18], Pd acetate in imine functionalized silica gel [19], and nanoporus polymeric ionic liquid loaded with palladium [20].…”