Synthesis and characterization of chiral bidentate bis(N-heterocyclic carbene)-carboxylate palladium and nickel complexes

Half-sandwich Ni(II) complexes bearing enantiopure bidentate NHC-carboxylate ligands: efficient catalysts for the hydrosilylative reduction of acetophenones

Chiral nickel complexes containing NHC-carboxylate chelate ligands derived from the (S)-isomeric form of amino acids have been synthesised from the corresponding imidazolium salt and nickelocene. The presence of the carboxylate on the N-side arm of the heterocycle results in the competing formation of mixtures of mono- and bis-NHC complexes (i.e., [Ni(η5-Cp)(κ2-C,O-NHC)] and [Ni(κ2-C,O-NHC)2]), both of which retain the (S)-configuration of the stereogenic center and which can be separated by chromatography. Both the 18e- and 16e- complexes are found to be very stable and cannot be interconverted. The composition of the resulting mixtures depends mainly on the entity of the amino acid residue and, of more practical interest, on the reaction conditions. Thus, microwave heating and MeCN as a solvent favor the formation of the half-sandwich nickel complexes, rather than the bis-NHC compounds. Some of the [Ni(η5-Cp)(κ2-C,O-NHC)] complexes turn out to be among the best nickel catalysts for the hydrosilylative reduction of p-acetophenones described to date, although without chiral induction, in the absence of activating additives and under mild catalytic conditions.

Chirality influence on the cytotoxic properties of anionic chiral bis(N-heterocyclic carbene)silver complexes

Complexes Na₃[Ag(NHC^R)₂], 2a-e and 2b'-c', where NHC^R is a N-heterocyclic carbene of the 2,2'-(1H-2λ³,3λ⁴-imidazole-1,3-diyl)dicarboxylate type, were prepared by treatment of compounds HL^R, 1a-e and 1b'-c' (2-(1-(carboxyalkyl)-1H-imidazol-3-ium-3-yl)carboxylate), with silver oxide in the presence of aqueous sodium hydroxide. They were characterized by analytical, spectroscopic (infrared, IR, ¹H and ¹³C nuclear magnetic resonance, NMR, and circular dichroism) and X-ray methods (2a). In the solid state, the anionic part of complex 2a, [Ag(NHC^H)₂]^3-, shows a linear disposition of C_carbene-Ag-C_carbene atoms and an eclipsed conformation of the two NHC ligands. The proposed bis(NHC) nature of the silver complexes was maintained in solution according to NMR and density functional theory (DFT) calculations. The cytotoxic activity of compounds 2 was evaluated against four cancer cell lines and one non-cancerous cell line and several structure-activity correlations were found for these complexes. For instance, the activity decreased when the bulkiness of the R alkyl group in Na₃[Ag(NHC^R)₂] increased. More interesting is the detected chirality-anticancer relationship, where complexes Na₃[Ag{(S,S)-NHC^R}₂] (R = Me, 2b; ⁱPr, 2c) showed better anticancer activity than those of their enantiomeric derivatives Na₃[Ag{(R,R)-NHC^R}₂] (R = Me, 2b'; ⁱPr, 2c').