Dipravath Kumar Seth

Mixed- ligand benzaldehyde thiosemicarbazone complexes of palladium containing N,O-donor ancillary ligands . Syntheses, structures, and catalytic application in C–C and C–N coupling reactions

The reaction of a 1 : 1 mixture of 4-R-benzaldehyde thiosemicarbazone [denoted in general as HL–R; where H stands for the dissociable acidic proton and R (R = OCH3, CH3, H, Cl and NO2) for the substituent] and 1-nitroso-2-naphthol (abbreviated as Hnn), with an equivalent quantity of Na2[PdCl4] in ethanolic medium affords a group of mixed-ligand complexes of the type [Pd(L–R)(nn)]. A similar reaction of a mixture of HL–R and quinolin-8-ol (Hq) with Na2[PdCl4] affords another family of mixed-ligand complexes of the type [Pd(L–R)(q)]. Crystal structures of [Pd(L–Cl)(nn)], [Pd(L–CH3)(q)] and [Pd(L–Cl)(q)] have been determined. In all the complexes the thiosemicarbazones are coordinated to the metal center, via dissociation of the acidic proton, as monoanionic bidentate N,S-donors forming five-membered chelate rings. In the [Pd(L–R)(nn)] complexes, the 1-nitroso-2-naphtholate anion is coordinated as a N,O-donor forming a five-membered chelate ring. Similarly in the [Pd(L–R)(q)] complexes, the quinolin-8-olate anion is bound to the metal center in the N,O-mode forming a five-membered chelate ring. All the [Pd(L–R)(nn)] and [Pd(L–R)(q)] complexes show characteristic 1H NMR signals, and in dichloromethane solution they all display intense absorptions in the visible and ultraviolet regions. Catalytic activities of the [Pd(L–R)(nn)] and [Pd(L–R)(q)] complexes have been examined towards some C–C and C–N coupling reactions, where both were found to show notable catalytic efficiency.

Mononuclear palladium and heterodinuclear palladium–ruthenium complexes of semicarbazone ligands: synthesis, characterization, and application in C–C cross-coupling reactions

Reaction of salicylaldehyde semicarbazone (L1), 2-hydroxyacetophenone semicarbazone (L2), and 2-hydroxynaphthaldehyde semicarbazone (L3) with [Pd(PPh3)2Cl2] in ethanol in the presence of a base (NEt3) affords a family of yellow complexes (1a, 1b and 1c, respectively). In these complexes the semicarbazone ligands are coordinated to palladium in a rather unusual tridentate ONN-mode, and a PPh3 also remains coordinated to the metal center. Crystal structures of the 1b and 1c complexes have been determined, and structure of 1a has been optimized by a DFT method. In these complexes two potential donor sites of the coordinated semicarbazone, viz. the hydrazinic nitrogen and carbonylic oxygen, remain unutilized. Further reaction of these palladium complexes (1a, 1b and 1c) with [Ru(PPh3)2(CO)2Cl2] yields a family of orange complexes (2a, 2b and 2c, respectively). In these heterodinuclear (Pd–Ru) complexes, the hydrazinic nitrogen (via dissociation of the N–H proton) and the carbonylic oxygen from the palladium-containing fragment bind to the ruthenium center by displacing a chloride and a carbonyl. Crystal structures of 2a and 2c have been determined, and the structure of 2b has been optimized by a DFT method. All the complexes show characteristic 1H NMR spectra and, intense absorptions in the visible and ultraviolet region. Cyclic voltammetry on all the complexes shows an irreversible oxidation of the coordinated semicarbazone within 0.86–0.93 V vs. SCE, and an irreversible reduction of the same ligand within −0.96 to −1.14 V vs. SCE. Both the mononuclear (1a, 1b and 1c) and heterodinuclear (2a, 2b and 2c) complexes are found to efficiently catalyze Suzuki, Heck and Sonogashira type C–C coupling reactions utilizing a variety of aryl bromides and aryl chlorides. The Pd–Ru complexes (2a, 2b and 2c) are found to be better catalysts than the Pd complexes (1a, 1b and 1c) for Suzuki and Heck coupling reactions.

Unusual chemical transformations of acetone thiosemicarbazone mediated by ruthenium: C–H bond activation, thiolation, and C–N bond cleavage

Upon reaction with Ru(PPh3)3Cl2 in ethanol in the presence of triethylamine, acetone thiosemicarbazone undergoes several interesting chemical transformations, such as thiolation via methyl C–H bond activation, C–N bond cleavage, and conversion of the CS fragment to CO. Two complexes (1 and 2) were obtained from this reaction, both of which contained a modified thiosemicarbazone coordinated in SNS- or SNO-mode, two triphenylphosphines and a N-bound thiocyanate. The crystal structures of both the complexes have been determined. Theoretical and mass spectral studies have been carried out to probe the transformations. These complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on both the complexes show a reversible oxidation near 0.6 V vs. SCE, followed by an irreversible oxidation near 1.2 V vs. SCE. DFT calculations have been carried out to explain the electronic spectra, as well as the electrochemical observations.