Trinuclear PdII Complexes with a C3 -Symmetric Triethynylbenzene-Based Tris-NHC Ligand: Catalytic Benefits.

A new C3 -symmetric tris-imidazolium tribromide salt 3, featuring 1,3,5-substituted triethynylbenzene, was used for the preparation of a trinuclear PdII pyridine-enhanced precatalyst preparation stabilization and initiation-type (PEPPSI) complex by triple C2 deprotonation followed by the addition of PdCl2 . Trinuclear PdII complex possessing a combination of NHC and PPh3 ligands has also been synthesized. The corresponding mononuclear palladium(II) complexes have also been synthesized for the comparison purpose. All these complexes have been characterized by using NMR spectroscopy and ESI mass spectrometry. The molecular structure of the trinuclear palladium(II) complex bearing mixed carbene and pyridine donor ligands has been established by using single crystal XRD. All the palladium(II) complexes have been used as pre-catalysts, which gave good to excellent yields in intermolecular α-arylation of 1-methyl-2-oxindole and Sonogashira coupling reaction. Catalytic studies indicate an enhanced activity of the trinuclear PdII complex in comparison to the corresponding mononuclear PdII complex for both catalytic transformations. The better performance of the trinuclear complex has also been further supported by preliminary electrochemical measurements. A negative mercury poison test was observed for both the aforementioned catalyses and therefore, it is likely that these organic transformations proceed homogeneously.

A naphthalene-based heterobimetallic triazolylidene IrIII/PdII complex: regioselective to regiospecific C-H activation, tandem catalysis and a copper-free Sonogashira reaction.

Heterobimetallic complexes featuring mesoionic carbene (MIC) donor ligands are gaining enormous popularity in tandem catalysis owing to the combined action of two different metal centers during catalysis. A rare version of the heterobimetallic PdII/IrIII complex possessing a cyclometalated mesoionic carbene (MIC) ligand is presented along with the analogous homodinuclear PdII complex. A sterically controlled regiospecific cyclometalation towards the formation of a six-membered ring complex over a five-membered ring complex has been performed using a naphthalene-based bis-MIC ligand platform. The interplay between regioselective vs. regiospecific C-H bond activation for the synthesis of cyclometalated IrIII complexes has also been demonstrated using the corresponding naphthyl-derived mono-imidazolylidene ligand. Both homodinuclear PdII and heterobimetallic PdII/IrIII complexes have been characterized using standard spectroscopic techniques including 1H, 13C{1H}, 2D correlation NMR spectroscopy and ESI mass spectrometry. The structure of the cyclometalated heterobimetallic complex has been established by single crystal XRD. The heterobimetallic complex has been employed as a pre-catalyst in the tandem Suzuki-Miyaura/transfer hydrogenation reaction and the homobimetallic PdII complex has been successfully employed as a catalyst in both the Sonogashira coupling and α-arylation of 1-methyl-2-oxindole.

Phenothiazines and phenoxazines: as electron transfer mediators for ferritin iron release.

Intracellular ferritin stores iron as ferrihydrite and releases it for various cellular metabolic activities. The reductive approach, one of the possible mechanisms of iron mobilization from ferritin nanocages, requires electron transfer (ET) from reducing agent(s) to the protein encapsulated iron. In vitro, the rate of ET from the physiological reducing agent, NADH, to mineralized ferritin is very slow resulting in a smaller amount of iron release. Therefore, medically relevant phenothiazine (TH/MB/MG/TDB) and phenoxazine (BCB/CRV/NB) dyes were used as ET mediators to facilitate the electron relay and to evaluate their iron releasing ability from ferritin. These dyes have earlier been exploited as ET mediators during electrocatalysis and in the treatment of methemoglobinemia. With the exception of MG, the midpoint potentials (E1/2) and NADH oxidizing abilities of these dyes dictated by their structure and the reaction conditions along with the dye-ferritin interaction govern the kinetics of reductive iron mobilization. A greater amount of iron release was observed in the case of TH, BCB and CRV. In comparison to neutral pH, acidic pH altered E1/2 and protein conformation leading to enhanced iron mobilization, whereas dissolved O2 and the photosensitizing effect of dyes were found to have a negligible impact. In analogy to in vitro, the acidic environment of the lysosome may bring about similar changes in the reducing agents/dye mediators/ferritin to facilitate the iron release process in vivo. Following Marcus theory, our current observations suggest that the dyes with E1/2 values well separated from those of the reducing agents and ferritin's mineral core can be exploited to facilitate iron release during iron overload conditions.