Synthesis, characterization and application of nickel(II) complexes modified with N,N′,N″-pincer ligands

A Single Terminal [NiII-OH] Catalyst for Direct Julia-Type Olefination and α-Alkylation Involving Sulfones and Alcohols

A terminal [NiII-OH] complex 1, supported by triflamide-functionalized NHC ligands, showed divergent reactivity for the reaction of sulfone with alcohol, contingent on base concentration, temperature, and time. Julia-type olefination of alcohols with sulfones was achieved using one equiv. of base, whereas lowering base loading to 0.5 equiv. afforded α-alkylated sulfones. Besides excellent substrate scope and selectivity, biologically active stilbene derivatives DMU-212, pinosylvin, resveratrol, and piceatannol were synthesized in high yield under Julia-type olefination conditions. An extensive array of controlled experiments and DFT calculations provide valuable insight on the reaction pathway.

Targeting of G-quadruplex DNA with 99mTc(I)/Re(I) Tricarbonyl Complexes Carrying Pyridostatin Derivatives

The main goal of this work was to elucidate the potential relevance of (radio)metal chelates of 99mTc and Re targeting G-quadruplex structures for the design of new tools for cancer theranostics. 99mTc provides the complexes with the ability to perform single-photon-emission computed tomography imaging studies, while the Re complexes should act as anticancer agents upon interaction with specific G4 DNA or RNA structures present in tumor tissues. Towards this goal, we have developed isostructural 99mTc(I) and Re(I) tricarbonyl complexes anchored by a pyrazolyl-diamine (Pz) chelator carrying a pendant pyridostatin (PDS) fragment as the G4-binding motif. The interaction of the PDF-Pz-Re (8) complex with different G4-forming oligonucleotides was studied by circular dichroism, fluorescence spectroscopy and FRET-melting assays. The results showed that the Re complex retained the ability to bind and stabilize G4-structures from different DNA or RNA sequences, namely those present on the SRC proto-oncogene and telomeric RNA (TERRA sequence). PDF-Pz-Re (8) showed low to moderate cytotoxicity in PC3 and MCF-7 cancer cell lines, as typically observed for G4-binders. Biodistribution studies of the congener PDF-Pz-99mTc (12) in normal mice showed that the complex undergoes a fast blood clearance with a predominant hepatobiliary excretion, pointing also for a high in vitro stability.

Roles of Hydrogen, Halogen Bonding and Aromatic Stacking in a Series of Isophthalamides

The synthesis and spectroscopic characterisation of six bis(5-X-pyridine-2-yl)isophthalamides (X = H, F, Br, Cl, I, NO2) are reported, together with five crystal structure analyses (for X = H, F to I). The isophthalamides span a range of conformations as syn/anti (H-DIP; I-DIP), anti/anti- (F-DIP; Br-DIP) and with both present in ratio 2:1 in Cl-DIP. The essentially isostructural F-DIP and Br-DIP molecules (using strong amide…amide interactions) aggregate into 2D molecular sheets that align with either F/H or Br atoms at the sheet surfaces (interfaces), respectively. Sheets are linked by weak C-H⋯F contacts in F-DIP and by Br⋯Br halogen bonding interactions as a ‘wall of bromines’ at the Br atom rich interfaces in Br-DIP. Cl-DIP is an unusual crystal structure incorporating both syn/anti and anti/anti molecular conformations in the asymmetric unit (Z’ = 3). The I-DIP•½(H2O) hemihydrate structure has a water molecule residing on a twofold axis between two I-DIPs and has hydrogen and N⋯I (Nc = 0.88) halogen bonding. The hydrate is central to an unusual synthon and involved in six hydrogen bonding interactions/contacts. Contact enrichment analysis on the Hirshfeld surface demonstrates that F-DIP, Cl-DIP and Br-DIP have especially over-represented halogen···halogen interactions. With the F-DIP, Cl-DIP and Br-DIP molecules having an elongated skeleton, the formation of layers of halogen atoms in planes perpendicular to the long unit cell axis occurs in the crystal packings. All six DIPs were analysed by ab initio calculations and conformational analysis; comparisons are made between their minimized structures and the five crystal structures. In addition, physicochemical properties are compared and assessed.

Molecular docking-assisted investigation of Pd(II) complexes carrying "SNS" pincer-type pyridine-thioether ligand as potential drug candidates against COVID-19

The coronavirus disease 2019 (COVID-19) pandemic has posed a threat to public health throughout the world since the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) was discovered in late 2019. Since the beginning of the pandemic, scientists have done a tremendous amount of work in this area. However, among these studies, the investigation of the effect of newly synthesized compounds against coronavirus is rather weak. Examining the newly synthesized compounds with a computer-assisted molecular docking study provides quite an advantage in terms of the estimation and analysis of the biochemical activity and binding affinity of existing synthesized compounds against a biological target in a labor, time, and cost-saving way. In this study, the SNS pincer type 2,6-bis[[(4-methylphenyl)thio]methyl]pyridine ligand(L) (1) and its novel Pd(II) complexes ([Pd(κ²-L)(OAc)₂]·3H₂O (2) and [Pd(κ²-L)Cl₂]·3H₂O (3)) were synthesized and characterized by using FT-IR, UV-Vis, NMR, mass and elemental analysis techniques. The synthesized Pd complexes exhibited a square planar structure. The compounds were found to have non-electrolytic behavior. In the meantime, in silico investigations have defined and justified interaction processes between these molecules and Pd(II) at the atomic level. Furthermore, using molecular docking against target proteins of SARS-CoV-2, the efficiency of the SNS pincer type ligand and its Pd (II) complexes produced was studied and discussed for the first time. The experimental data has been supported and illuminated using computational visual methods and molecular docking, and the findings produced indicate compatibility. The binding energy values of the relevant compounds on the four protein model structures of SARS-CoV-2 (Main Protease, Papain-like protease, RdRp without RNA, and RdRp with RNA) are represented. Compound 2 ([Pd(κ²-L)(OAc)₂]·3H₂O) is the structure that exhibits the highest biochemical activity. According to all of the docking studies, Papain-like protease is the SARS-CoV-2 protein with which the three compounds exhibit mutual interaction. The compound 2 structure, in particular, is the most effective in terms of structural and interaction with the targets, as well as binding orientations.

Hydrosilylative reduction of primary amides to primary amines catalyzed by a terminal [Ni-OH] complex

A terminal [Ni-OH] complex 1, supported by triflamide-functionalized NHC ligands, catalyzes the hydrosilylative reduction of a range of primary amides into primary amines in good to excellent yields under base-free conditions with key functional group tolerance. Catalyst 1 is also effective for the reduction of a variety of tertiary and secondary amides. In contrast to literature reports, the reactivity of 1 towards amide reduction follows an inverse trend, i.e., 1° amide > 3° amide > 2° amide. The reaction does not follow a usual dehydration pathway.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

POCN Ni(ii) pincer complexes: synthesis, characterization and evaluation of catalytic hydrosilylation and hydroboration activities

(POCN)Ni(ii) complexes were found to mediate a variety of carbonyl hydroboration reactions, including chemoselective hydroboration of benzaldehyde and hydroborative reduction of amides.

Terminal NiII-OH/-OH2 complexes in trigonal bipyramidal geometries derived from H2O

The preparation and characterization of two NiII complexes are described, a terminal NiII-OH complex with the tripodal ligand tris[(N)-tertbutylureaylato)-N-ethyl)]aminato ([H3buea]3-) and a terminal Ni II-OH2 complex with the tripodal ligand N,N',N″-[2,2',2″-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-). For both complexes, the source of the -OH and -OH2 ligand is water. The salts K2[NiIIH3buea(OH)] and NMe4[NiIIMST(OH2)] were characterized using perpendicular-mode X-band electronic paramagnetic resonance, Fourier transform infrared, UV-visible spectroscopies, and its electrochemical properties were evaluated using cyclic voltammetry. The solid state structures of these complexes determined by X-ray diffraction methods reveal that they adopt a distorted trigonal bipyramidal geometry, an unusual structure for 5-coordinate NiII complexes. Moreover, the NiII-OH and NiII-OH2 units form intramolecular hydrogen bonding networks with the [H3buea]3- and [MST]3- ligands. The oxidation chemistry of these complexes was explored by treating the high-spin NiII compounds with one-electron oxidants. Species were formed with S = 1/2 spin ground states that are consistent with formation of monomeric NiIII species. While the formation of NiIII-OH complexes cannot be ruled out, the lack of observable O-H vibrations from the putative Ni-OH units suggest the possibility that other high valent Ni species are formed.

Synthesis and characterization of metallo-supramolecular polymers from thiophene-based unimers bearing pybox ligands

A series of novel metallo-supramolecular polymers was prepared, based on 2,6-bis(2-oxazolinyl)pyridine chelating groups bridged with thiophene, bithiophene and thienothiophene as a linker, beginning from commercially available (chelidamic) acid.