An insight into the catalytic activity of palladium Schiff-base complexes towards the Heck coupling reaction: routes via encapsulation in zeolite-Y

Zeolite-encapsulated copper(II) complexes with NNO-tridentate Schiff base ligands: catalytic activity for methylene blue (MB) degradation under near neutral conditions

Three novel copper Schiff base complexes, L1Cu(OAc)-L3Cu(OAc), bearing NNO tridentate ligands were synthesized and successfully entrapped in zeolite. All free and encapsulated complexes were fully characterized through experiments combined with theoretical calculations, and were subsequently employed as catalysts to activate H2O2 for degradation of methylene blue (MB). The catalytic activity of free complexes was tunable by substitution effects. The complex L3Cu(OAc) displayed enhanced efficiency by adopting bulky and donor substitutions due to the lower oxidation states. However, the free complexes exhibited modified structural and catalytic properties upon encapsulation into the zeolite. The constraint from the zeolite holes and coordination geometry caused the alteration of electronic structures and subsequently modified the reactivity. This study revealed that upon encapsulation, the larger molecular dimension of L3Cu(OAc) resulted in additional distorted geometry, leading to higher catalytic efficiency for MB degradation with more blue shifts in the UV-Vis spectrum. There was high catalytic activity by LnCu(OAc)-Y compared to that of the free complex, and high recyclability under near neutral conditions. In addition, the catalytic efficiency of L3Cu(OAc)-Y was higher or equivalent compared to other catalysts. This work provides new complexes with NNO tridentate ligands encapsulated inside zeolite and explains the relationship between the modified structure and functionality.

Zeolite encapsulated organometallic complexes as model catalysts

Heterogeneities in the structure of active centers in metal-containing porous materials are unavoidable and complicate the description of chemical events occurring along reaction coordinates at the atomic level. Metal containing zeolites include sites of varied local coordination and secondary confining environments, requiring careful titration protocols to quantify the predominant active sites. Hybrid organometallic-zeolite catalysts are useful well-defined platform materials for spectroscopic, kinetic, and computational studies of heterogeneous catalysis that avoid the complications of conventional metal-containing porous materials. Such materials have been synthesized and studied previously, but catalytic applications were mostly limited to liquid-phase oxidation and electrochemical reactions. The hydrothermal stability, time-on-stream stability, and utility of these materials in gas-phase oxidation reactions are under-studied. The potential applications for single-site heterogeneous catalysts in fundamental research are abundant and motivate future synthetic, spectroscopic, kinetic, and computational studies.

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.

Novel Recyclable Pd/H-MOR Catalyst for Suzuki-Miyaura Coupling and Application in the Synthesis of Crizotinib

In this paper, we report an effective ultrasound method for the synthesis of Pd/H-MOR, which was used as a catalyst in the Suzuki-Miyaura coupling of aryl halides with phenylboronic acid. The structure and morphology of the as-prepared catalysts were fully characterized by X-ray diffraction (XRD), N2 sorption isotherms, scanning electron microscopy (SEM), and an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The advantages of Pd/H-MOR in the coupling reaction are green solvents, high yields, absence of ligands, and recyclability. The catalysts were easily reused at least ten times without significant deterioration in catalytic activity. In addition, this protocol was used in the marketed anti-tumor drug crizotinib synthesis.

Synthesis and structures of three isoxazole-containing Schiff bases

The synthesis and structures of three isoxazole-containing Schiff bases are reported, namely, (E)-2-{[(isoxazol-3-yl)imino]methyl}phenol, C10H8N2O2, (E)-2-{[(5-methylisoxazol-3-yl)imino]methyl}phenol, C11H10N2O2, and (E)-2,4-di-tert-butyl-6-{[(isoxazol-3-yl)imino]methyl}phenol, C18H24N2O2. All three structures contain an intramolecular O-H...N hydrogen bond, alongside weaker intermolecular C-H...N and C-H...O contacts. The C-O(H) and imine C=N bond lengths were consistent with structures existing in the enol rather than the keto form. Despite having dihedral angles <25°, none of the compounds were observed to be strongly thermochromic, unlike their anil counterparts; however, all three compounds showed a visible colour change upon irradiation with UV light.