Platinum(II) Complexes with Thioether-Functionalized Benzimidazolin-2-ylidene Ligands: Synthesis, Structural Characterization, and Application in Hydroelementation Reactions

Simply accessible platinum(II) complexes enabling alkene hydrosilylation at ppm catalyst loadings

An efficient olefin hydrosilylation protocol utilising Pt(II)-thioether-based pre-catalysts is reported. These simple and readily available complexes exhibit excellent catalytic performance and offer significant advantages over existing alternatives, enabling rapid and high conversions at ppm-level catalyst loadings.

A green route to platinum N-heterocyclic carbene complexes: mechanism and expanded scope

A sustainable and facile weak-base synthetic route to platinum N-heterocyclic carbene (NHC) complexes is disclosed. The mechanism of this reaction is also elucidated via experimental and computational investigations. This straightforward protocol is then used for the synthesis of novel Pt(II)-NHC complexes and its utility is further explored to access key Pt(0)-NHC precatalysts.

Benzimidazole-based ionic and non-ionic organoselenium compounds: innovative synthetic strategies, structural characterization and preliminary anti-proliferative activities

Rational design of and efficient synthesis of innovative benzimidazole-based ionic and non-ionic organoselenium compounds is described. The compounds were studied for their anti-proliferative activities against triple-negative breast cancer cells.

Benzimidazole- and Imidazole-Fused Selenazolium and Selenazinium Selenocyanates: Ionic Organoselenium Compounds with Efficient Peroxide Scavenging Activities

Three new classes of ionic organoselenium compounds containing cationic benzimidazolium and imidazolium ring systems with selenocyanates as counterions are described. The cyclization of N,N'-disubstituted benzimidazolium and imidazolium bromides having N-(CH₂)₂-Br and N-(CH₂)₃-Br groups in the presence of potassium selenocyanate (KSeCN) led to formation of the corresponding selenazolium selenocyanates (21a, 21b, 22a, and 22b) and selenazinium selenocyanates (21c, 21d, 22c, and 22d). However, the open-chain selenocyanates with additional selenocyanate counterions (21e, 21f, 22e, and 22f) were formed from the N,N'-disubstituted benzimidazolium and imidazolium bromides having N-(CH₂)₆-Br groups. Mechanistic studies were carried out to understand the feasibility of such cyclization processes in the presence of KSeCN. The compounds were studied further for their potencies to catalytically reduce H₂O₂ in the presence of thiols. Interestingly, the cyclic selenazolium (21a, 21b, 22a, and 22b) and selenazinium compounds (21c, 21d, 22c, and 22d) exhibited significantly higher antioxidant activities than the corresponding acyclic selenocyanates (21f, 22e, and 22f). Selected compounds (22d and 22e) were further evaluated for their potencies in modulating the intracellular level of reactive oxygen species (ROS) in a representative macrophage cell line (RAW 264.7). Owing to the cationic nature of compounds, they may target and scavenge mitochondrial ROS in the cellular medium.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Synthesis, characterization, and the antioxidant activity of the acetylated chitosan derivatives containing sulfonium salts

In this study, a new class of chitosan derivatives possessing sulfonium salts was synthesized, and characterized by FT-IR, ¹H NMR, ¹³C NMR, and elemental analyses. IR spectra, ¹H NMR and ¹³C NMR of the structural units of these polymers validated the designed chitosan derivatives were successfully synthesized. In addition, the antioxidant potential of chitosan and chitosan derivatives was assessed in vitro, screened by 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging, hydroxyl radical scavenging, and superoxide radical scavenging, respectively. Results revealed that designed chitosan derivatives could effectively scavenge DPPH radical, hydroxyl radical, and superoxide radical with inhibition rate of more than 90% at 1.6 mg/mL, higher than chitosan. Moreover, in the cytotoxicity assay, no cytotoxicity was observed for the L929 cells with chitosan and its derivatives at all the testing concentrations. These results indicated that the acetylated chitosan derivatives containing sulfonium salts may be a promising natural antioxidant for the pharmaceutics, food, cosmetics and agriculture management.

Synthesis, characterization, catalytic and biological application of half-sandwich ruthenium complexes bearing hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

A series of cationic Ru(ii)(η⁶-p-cymene) complexes with thioether-functionalised N-heterocyclic carbene ligands have been prepared and fully characterized. Steric and electronic influence of the R thioether substituent on the coordination of the sulfur atom was investigated. The molecular structure of three of them has been determined by means of X-ray diffractrometry and confirmed the bidentate (κ²-C,S) coordination mode of the ligand. Interestingly, only a single diastereomer, as an enantiomeric couple, was observed in the solid state for complexes 1c, 1i and 1j. DFT calculations established a low energy inversion barrier between the two diastereomers through a sulfur pyramidal inversion pathway with R donating group while a dissociative/associative mechanism is more likely with R substituents that contain electron withdrawing group, thus suggesting that the only species observed by the ¹H-NMR correspond to an average resonance position of a fluxional mixtures of isomers. All these complexes were found to catalyse the oxydant-free double dehydrogenation of primary amine into nitrile. Ru complex bearing NHC-functionalised S-tBu group was further investigated in a wide range of amines and was found more selective for alkyl amine substrates than for benzylamine derivatives. Finally, preliminary results of the biological effects on various human cancer cells of four selected Ru complexes are reported.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

A general protocol for the synthesis of Pt-NHC (NHC = N-heterocyclic carbene) hydrosilylation catalysts

A simple synthetic route, making use of inexpensive and environment-friendly solvent/reagents, is described leading to [Pt(NHC)(L)Cl₂] and [Pt(NHC)(dvtms)] (L = DMS, DMSO, Py; dvtms = divynyltetramethylsiloxane) catalysts.

Fused azole-thiazolines via one-pot cyclization of functionalized N-heterocyclic carbene precursors

Various bi/tricyclic azole-thiazolines are obtained via a new one-pot protocol from cheap starting materials.

Donor Strengths Determination of Pnictogen and Chalcogen Ligands by the Huynh Electronic Parameter and Its Correlation to Sigma Hammett Constants

The suitability and accuracy of the Huynh electronic parameter (HEP) was further tested to reveal remote substituent effects in pyridines, which are located five or six bonds away from the reporter probe. These values show an excellent correlation to Hammett σ‐constants of the respective substituents with coefficients of R²=0.9856 (σ_m) and R²=0.9857 (σ_p). Based on this observation, a methodology for the re‐evaluation of certain Hammett constants with larger uncertainties has been proposed and demonstrated. Moreover, the scope of HEP was extended to various neutral pnictogen and chalcogen donors during which “transphobia effects” were revealed for mixed NHC complexes containing phosphites, arsine and stibine for the first time.

Synthesis, characterization, and catalytic application in aldehyde hydrosilylation of half-sandwich nickel complexes bearing (κ1-C)- and hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

Ni complexes bearing thioether-functionalised NHCs; hemilabile ligands for catalytic aldehyde hydrosilylation.

Access to stereodefined (Z)-allylsilanes and (Z)-allylic alcohols via cobalt-catalyzed regioselective hydrosilylation of allenes

Hydrosilylation of allenes is the addition of a hydrogen atom and a silyl group to a carbon–carbon double bond of an allene molecule and represents a straightforward and atom-economical approach to prepare synthetically versatile allylsilanes and vinylsilanes. However, this reaction generally produces six possible isomeric organosilanes, and the biggest challenge in developing this reaction is to control both regioselectivity and stereoselectivity. The majorities of the developed allene hydrosilylation reactions show high selectivity towards the production of vinylsilanes or branched allylsilanes. By employing a cobalt catalyst generated from readily available and bench-stable cobalt precursor and phosphine-based ligands, here we show that this reaction proceeds under mild conditions in a regioselective and stereoselective manner, and affords synthetically challenging, but valuable linear cis-allylsilanes with excellent stereoselectivity (generally cis to trans ratios: >98:2). This cobalt-catalyzed (Z)-selective allene hydrosilylation provides a general approach to access molecules containing stereodefined (Z)-alkene units.

Controlling selectivity in the hydrosilylation of allenes poses serious challenges in terms of product stereochemistry. Here the authors show that the title reaction proceeds with excellent (Z)-selectivity by use of a cobalt catalyst in the presence of phosphine-based ligands.

Cobalt-catalyzed regioselective stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes

The first transition metal-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of (E/Z)-dienes was effectively achieved with excellent E-selectivities using a cobalt catalyst.

We report the first stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes using catalysts generated from bench-stable Co(acac)₂ and phosphine ligands. A wide range of E/Z-dienes underwent this Markovnikov 1,2-hydrosilylation in a stereoconvergent manner, affording (E)-allylsilanes in high isolated yields with high stereoselectivities (E/Z = >99 : 1) and high regioselectivities (b/l up to > 99 : 1). Mechanistic studies revealed that this stereoconvergence stems from a σ–π–σ isomerization of an allylcobalt species generated by the 1,4-hydrometalation of Z-dienes. In addition, a cobalt catalyst that can only catalyze the hydrosilylation of the E-isomer of an (E/Z)-diene was identified, which allows the separation of the (Z)-isomer from an isomeric mixture of (E/Z)-dienes. Furthermore, asymmetric hydrosilylation of (E)-1-aryl-1,3-dienes was studied with Co(acac)₂/(R)-difluorphos and good enantioselectivities (er up to 90 : 10) were obtained.

Recent advances in metal free- and late transition metal-catalysed hydroamination of unactivated alkenes

This Perspective article outlines some of the recent advancements in the development of (chiral) metal-free and late transition metal catalysts for hydroamination of unactivated alkenes.

Hydrosilylation reaction of olefins: recent advances and perspectives

This review focuses on the recent development of efficient, selective, and cheaper hydrosilylation catalyst systems appearing in the last decade.