Selective Formation of Mononuclear Palladium and Acetonitrile-Bridged Dinuclear Palladium Complexes Containing a Chiral Tridentate Ligand

From the chiral Schiff base tridentate ligand L_Ph, unusual acetonitrile-bridged dinuclear palladium complex 1 and organopalladium complex 1' were synthesized selectively by using acetonitrile and ethanol, respectively. The chiral tridentate Schiff base ligand was bound to the palladium metal center with different chelation modes ([ONO] for 1 and [CNO] for 1'). Complex 1 constitutes the first example of dinuclear metal complexes connected only by a bridging acetonitrile, in which an exceptionally short C≡N bond distance [0.945(12) Å] of bridged acetonitrile was observed. To study the influence of a phenyl group attached to an imine, the phenyl-free ligand L_H was prepared and used. In that case, an acetonitrile bridge was not observed. Theoretical calculation studies supporting the formation of 1 and 1' are favored.

Amino acid-derived bisphenolate palladium complexes as C-C coupling catalysts

A series of amine bisphenol (ABP) pro-ligands featuring amino acid ester pendant arms were prepared. Optimisation of the synthetic method allowed the facile incorporation of naturally occurring, chiral amino acids into the ABP scaffold with minimal racemisation. Reaction of the pro-ligands (LH₂) with Pd(OAc)₂, in the presence of amines, led to the formation of complexes with an unprecedented pincer-like O,N,O coordination mode around the Pd^II centre. The complexations in the presence of trialkylamines (NR₃) afforded a mixture of LPdNR₃ and LPdNHR₂ species. The latter was shown to form via an ambient-temperature C-N cleavage involving unstable Pd(OAc)₂(NHR₂)₂ intermediates. Using pyridine as base eliminated this dealkylation and resulted in the exclusive formation of LPd(py) complexes in high yields. In total, seven novel Pd^II ABP complexes were prepared, exhibiting distorted square-planar geometries with the asymmetric ligand moieties orientated towards the metal centre. The air- and moisture-stable LPd(py) complexes were successfully employed as catalysts in two types of C-C coupling reactions. The Suzuki-Miyaura coupling of 4'-bromoacetophenone and phenylboronic acid reached high yields (up to 81%), while a scope of further alkyl bromides was also efficiently converted using low catalyst loadings (1 mol%) and mild temperatures (40 °C). Furthermore, a Pd-pyridine complex achieved high activity in the Mizoroki-Heck coupling of styrene and 4'-bromoacetophenone.

Unprecedented formation of palladium(II)-pyrazole based thiourea from chromone thiosemicarbazone and [PdCl2(PPh3)2]: Interaction with biomolecules and apoptosis through mitochondrial signaling pathway

Two novel pyrazole based thiourea palladium(II) complexes, [PdCl(PPh₃)(C₉H₈NO₂S-pz)] (1) and [PdCl(PPh₃)(C₁₄H₁₀NO₂S-pz)] (2) [pz = pyrazole (C₃H₂N₂)] have been obtained unexpectedly from chromone thiosemicarbazones (L1 and L2) and [PdCl₂(PPh₃)₂]. The compounds have been fully characterized by physicochemical studies. The single crystal X-ray diffraction and spectral studies revealed square planar geometry for the complexes. The conversion of chromone thiosemicarbazone into pyrazole based thiourea might have happened through coordination to palladium(II) ion after enolization, Michael addition and ring opening followed by cyclization. To the best of our knowledge, this is the first report for the conversion of chromone thiosemicarbazone into pyrazole based thiourea moiety. Plausible mechanism was proposed based on the spectroscopic studies. Calf thymus (CT) DNA binding of the compounds was explored using various spectroscopic and molecular docking methods. DNA cleavage studies suggested that complexes 1 and 2 had the capacity to cleave the supercoiled DNA (pUC19) to its naked form. In vitro cytotoxic property of the ligands and complexes has been evaluated against three human cancer cells such as A549, HepG-2 and U937. Complex 2 exhibited potent cytotoxic activity against HepG-2 cells with the IC₅₀ value of 10.4 μM. In addition, mechanistic studies showed that complex 2 induced apoptosis through mitochondrial signaling pathway in HepG-2 cells. Beneficially, complex 2 showed less toxicity against human lung (IMR90) normal cells and hence it emerges as a potential candidate for further studies.

Ferrocenyl palladacycles derived from unsymmetrical pincer-type ligands: evidence of Pd(0) nanoparticle generation during the Suzuki-Miyaura reaction and applications in the direct arylation of thiazoles and isoxazoles

A new family of ferrocenyl-palladacycle complexes Pd(L¹)Cl (Pd1) and Pd(L²)Cl (Pd2) were synthesized and characterized by UV-visible, IR, ESI-MS, and NMR spectral studies. The molecular structures of Pd1 and Pd2 were determined by X-ray crystallographic studies. Palladacycle catalyzed Suzuki-Miyaura cross-coupling reactions were investigated utilizing the derivatives of phenylboronic acids and substituted chlorobenzenes. Mechanistic investigation authenticated the generation of Pd(0) nanoparticles during the catalytic cycle and the nanoparticles were characterized by XPS, SEM and TEM analysis. Direct C-H arylation of thiazole and isoxazole derivatives employing these ferrocenyl-palladacycle complexes was examined. The reaction model for the arylation reaction implicating the in situ generation of Pd(0) nanoparticles was proposed.

Thiazole-based non-symmetric NNC–palladium pincer complexes as catalytic precursors for the Suzuki–Miyaura C–C coupling

New non-symmetric pincer palladacycles, containing [N,N,C] tridentate ligands are synthesized and their catalytic activities in the Suzuki–Miyaura cross coupling reaction are tested using water under aerobic conditions and IR irradiation.

Conversion of Arylboronic Acids to Tetrazoles Catalyzed by ONO Pincer-Type Palladium Complex

A convenient synthesis of a library of tetrazoles through a novel and operationally simple protocol effecting the direct conversion of arylboronic acids catalyzed by a new ONO pincer-type Pd(II) complex under mild reaction conditions using the readily available reagents is reported. The palladium complex was reused up to four cycles in an open-flask condition.

New hybrid materials based on the grafting of Pd(ii)-amino complexes on the graphitic surface of AC: preparation, structures and catalytic properties

Two novel hybrid catalysts have been prepared by anchoring Pd(ii)-amino complexes on the graphitic surface of ACs and their catalytic activities on the 1-octene hydrogenation are presented.

ONO pincer type Pd(ii) complexes: synthesis, crystal structure and catalytic activity towards C-2 arylation of quinoline scaffolds

Four new palladium(ii) complexes featuring ONO pincer type hydrazone ligands were synthesized and characterized by spectroscopic and single-crystal XRD analysis. Utility of these complexes to catalyze C–C bond formation in quinoline scaffolds were assessed.