Non-symmetric pincer ligands: complexes and applications in catalysis

Biological and catalytic potential of sustainable low and high valent metal-Schiff base sulfonate salicylidene pincer complexes

ONO-Pincer Schiff base salicylidene (HSaln ligand) complexes with VO2+, UO2 2+, MoO2 2+ and Mn2+ ions (MSaln complexes = VOSaln, UO2Saln, MoO2Saln and MnSaln, respectively) were synthesized and fully characterized by different physico-chemical tools. The VOSaln complex was further treated with 1,10-phenanthroline which afforded a new VO-complex (VOSaln-Ph). All complexes and their ligands, as eco-friendly reagents, were explored for their biological potential as antibacterial and antifungal agents. Reactivity of MSaln complexes against the tested pathogen strains exhibited a remarkable inhibitory effect compared to the coordinated ligand (HSaln) and applicable standard drugs. Moreover, the MSaln complex-DNA interaction was investigated by ultraviolet-visible spectroscopy, viscosity and gel electrophoresis techniques affording binding strengths in the order: UO2Saln > MnSaln > MoO2Saln > VOSaln-Ph > VOSaln. Additionally, the biological potential of the investigated compounds was further explored by molecular docking to illustrate the nature of the drug-DNA interactions. All MSaln complexes show respectable anti-proliferative potential as anticancer agents against selected human carcinoma cell lines. Aside from the biological activities these complexes (MSaln complexes) were also investigated for catalytic efficiency in the Suzuki-Miyaura cross-coupling system of phenylboronic acid with 2-bromopyridine in water, sustainably. The results indicated that the MnSaln catalyst performed well with high yield. The catalytic potential of MnSaln was compared in water, water-ionic liquid mixtures and ionic liquids.

Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation

Base metal catalysis offers an alternative to reactions, which were once dominated by precious metals in hydrofunctionalization reactions. This review article details the development of some base metals (Fe, Co, and Ni) in the hydroboration and hydrosilylation reactions concomitant with a brief overview of recent advances in the field. Applications of both commercially available metal salts and well-defined metal complexes in catalysis and opportunities to further advance the field is discussed as well.

Cross-Coupling Reaction of Allylic Ethers with Aryl Grignard Reagents Catalyzed by a Nickel Pincer Complex

A cross-coupling reaction of allylic aryl ethers with arylmagnesium reagents was investigated using β-aminoketonato- and β-diketiminato-based pincer-type nickel(II) complexes as catalysts. An β-aminoketonato nickel(II) complex bearing a diphenylphosphino group as a third donor effectively catalyzed the reaction to afford the target cross-coupled products, allylbenzene derivatives, in high yield. The regioselective reaction of a variety of substituted cinnamyl ethers proceeded to give the corresponding linear products. In contrast, α- and γ-alkyl substituted allylic ethers afforded a mixture of the linear and branched products. These results indicated that the coupling reaction proceeded via a π-allyl nickel intermediate.

Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical Pyrrole-Based PNN Pincer and Their Norbornene Polymerization Behaviors versus the Symmetrical NNN and PNP Pincers

Unsymmetrical pincers have been shown to be better than the corresponding symmetrical pincers in several catalysis reactions. A new unsymmetrical PNN propincer, 2-(3,5-dimethylpyrazolylmethyl)-5-(diphenylphosphinomethyl)pyrrole (1), was synthesized from pyrrole through Mannich bases in a good yield. In addition, the new byproduct 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)- N-(hydroxymethyl)pyrrole was also isolated. The reaction of 1 with [PdCl₂(PhCN)₂] and Et₃N in toluene yielded [PdCl{C₄H₂N-2-(CH₂Me₂pz)-5-(CH₂PPh₂)-κ³ P,N,N}] (2). The analogous reaction between 1 and [NiCl₂(DME)] or NiX₂ (X = Br, I) in the presence of NEt₃ in acetonitrile afforded [NiX{C₄H₂N-2-(CH₂Me₂pz)-5-(CH₂PPh₂)-κ³ P,N,N}] (3; X = Cl, Br, I). All complexes were structurally characterized. The norbornene polymerization behaviors of the unsymmetrical pincer complexes 2 and 3 in the presence of MMAO or EtAlCl₂ were compared with those of the symmetrical pincer complexes chloro[2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrolido]palladium(II) (NNN), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II), and chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) (PNP) at different temperatures. The PNN and NNN complexes exhibited far greater activity on the order of 10⁷ g of PNB/mol/h, with quantitative yields in some cases, in comparison to the PNP pincer palladium and nickel complexes. This trend was also supported by the ⁱPr group substituted PNP nickel and palladium pincer complexes. These polymerization behaviors are explained using steric crowding around the metal atom with the support of NMR studies and suggested that the activity increases as the N_pyrazole donor increases. Polymers were characterized by ¹H NMR, IR, TGA, and powder XRD methods.

A non-symmetric sulfur-based O,C,O-chelating pincer ligand leading to chiral germylene and stannylene

New chiral heteroleptic germanium(ii) and tin(ii) metallylenes were obtained using 1-(para-tolylsulfinyl)-3-tosyl-5-tert-butyl-benzene as a non-symmetric O,C,O-chelating pincer ligand.

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.

A dipyrromethane-based diphosphane–germylene as precursor to tetrahedral copper(i) and T-shaped silver(i) and gold(i) PGeP pincer complexes

A dipyrromethane-based PGeP germylene has allowed the synthesis of unusual tetrahedral copper(i) and T-shaped silver(i) and gold(i) germyl complexes.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes

Several pincer ligated nickel mercapto complexes, [2,6-(R₂ PCH₂ )₂ C₆ H₃ ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R₂ PO)₂ C₆ H₃ ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu₂ PO)₂ C₆ H₂ ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH₃ ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH₂ -μ₂ -S(B₂ H₅ ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Treatment of both [CoCl( ^tBuPNP)] and [NiCl( ^tBuPNP)] ( ^tBuPNP = anion of 2,5-bis((di- tert-butylphosphino)methyl)pyrrole) with one equivalent of benzoquinone affords the corresponding chloride complexes containing a dehydrogenated PNP ligand, ^tBudPNP ( ^tBudPNP = anion of 2,5-bis((di- tert-butylphosphino)methylene)-2,5-dihydropyrrole). Dehydrogenation of PNP to dPNP results in minimal change to steric profile of the ligand but has important consequences for the resulting redox potentials of the metal complexes, resulting in the ability to isolate both [CoH( ^tBudPNP)] and [CoEt( ^tBudPNP)], which are more challenging (hydride) or not possible (ethyl) to prepare with the parent PNP ligand. Electrochemical measurements with both the Co and Ni dPNP species demonstrate a substantial shift in redox potentials for both the M(II/III) and M(II/I) couples. In the case of the former, oxidation to trivalent Co was found to be reversible, and subsequent reaction with AgSbF₆ afforded a rare example of a square-planar Co(III) species. Corresponding reduction of [CoCl( ^tBudPNP)] with KC₈ produced the diamagnetic Co(I) species, [Co(N₂)( ^tBudPNP)]. Further reduction of the Co(I) complex was found to generate a pincer-based π-radical anion that demonstrated well-resolved EPR features to the four hydrogen atoms and lone nitrogen atom of the ligand with minor contributions from cobalt and coordinated N₂. Changes in the electronic character of the PNP ligand upon dehydrogenation are proposed to result from loss of aromaticity in the pyrrole ligand, resulting in a more reducing central amido donor. DFT calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.

Acetylacetonato-based pincer-type nickel(ii) complexes: synthesis and catalysis in cross-couplings of aryl chlorides with aryl Grignard reagents

In this work, three different types of acetylacetonato-based pincer-type nickel(ii) complexes (2) were prepared. Complex 2a possessed the tridentate ONN ligand, which was constructed by the condensation reaction of acetylacetone with N,N-diethylethylenediamine. Complex 2b contained the PPh2 donor group in contrast to the NEt2 group in 2a, i.e., an ONP ligand framework. Complex 2c was composed of the NNN ligand, which was prepared by the reaction of 4-((2,4,6-trimethylphenyl)amino)pent-3-en-2-one with N,N-diethylethylenediamine. In addition to X-ray diffraction analysis, these complexes were characterized spectroscopically. Their catalytic activity for a cross-coupling reaction of aryl halides with aryl Grignard reagents was also evaluated. Among these complexes, 2b acted as an effective catalyst for the cross-coupling reaction using aryl chlorides as electrophiles. The electronic properties of these Ni(ii) complexes were investigated by cyclic voltammetry and density functional theory calculations.