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The synthesis of a series of sterically modulated palladium(II)–NHC complexes ( 9 – 11 ) of the general formula [Pd(NHC) 2 Cl 2 ] (NHC = 1-benzyl-3(2′-methyl)-propylbenzimidazol-2-ylidine, 1-benzyl-3(2′-methyl)-butylbenzimidazol-2-ylidine and 1-benzyl-3-hexylbenzimidazol-2-ylidine) from their respective silver(I) counterparts ( 6 – 8 ) is presented. Two novel triazine-tethered Zwitterionic (benz)imidazolium salts, 4 and 5 , were prepared and tested as NHC precursors for the preparation of silver(I) complexes. However, all our attempts to prepare silver(I) derivatives from salts 4 and 5 ended with negative results due to the low acidity of the C2 protons. The Zwitterionic derivative 4 was additionally characterized by the single crystal X-ray diffraction technique. The molecular structure of 4 evidenced π–π stacking interactions between the triazine rings of two crystallographically independent units. The palladium complexes 9 – 11 showed good catalytic activities in the epoxidation of 1-octene and styrene under homogeneous conditions with aqueous hydrogen peroxide as an oxidant. The effect of temperature and solvent on the epoxidation of the aforementioned olefins using complexes 9 – 11 was also explored. Density functional theory (DFT) was used to model the structures of the isomers of the palladium complexes. Geometry parameters, electronic energy, molecular orbital energies, band gap, vibrational frequencies and the cis–trans energy barrier were calculated. [ABSTRACT FROM AUTHOR]

Abstract: Two mesityl bridged bis-1H-imidazolium salts having bromide/hexafluorophosphate counter ions have been prepared, compared with similar ortho-xylyl bridged salts and characterized by various spectral and X-ray diffraction techniques. The salt having 2-cyanoethyl substituents undergoes hydrolysis to produce compartmental 1H-imidazolium salts. Further, these salts are involved in the Hofmann-type elimination to produce bis-imidazole and Ag(I)- and Hg(II)-coordination compounds when treated with Ag2O and Hg(OAc)2 in different ratios. Furthermore, a 3-nitrile-substituted imidazolium salt was prepared and treated with the aforementioned metal sources to evaluate the elimination reaction. However, it forms stable N-heterocyclic carbene (NHC) complex without eliminating any of the cyano-substitutions. Both, imidazolium salts and Ag(I)- and Hg(II)-coordination/NHC complexes have been characterized by 1H and, 13C NMR, FTIR spectral, and elemental and single crystal X-ray diffraction analyses. [Copyright &y& Elsevier]

Abstract: A new Hg(II)–N-heterocyclic carbene (NHC) complex (1) of the form [(NHC)2-μ-Hg]·2PF6 was synthesized from the reaction of Hg(II) acetate with a meta-xylyl-linked bis-imidazolium salt (η1-areneC–H model). The complex was characterized using elemental analysis, nuclear magnetic resonance (1H, 13C and HMQC), and single-crystal X-ray diffraction techniques. X-ray structural studies on complex 1 revealed a linear coordination of the Hg ion with both carbene centers of the ligand. In addition, a strong interaction between the Hg ion and a carbon atom of the aryl ring of the ligand was also observed. This interesting interaction is further supported by a downfield chemical shift of the 1H NMR signal for the arene-H that interacts with mercury (δ 0.88 compared to the free ligand). In a related meta-arylene-bis(imidazol-2-yliden-1-ylmethyl) Hg(II) complex (2) with a methyl group between the bridges, synthesized in analogy to complex 1, no such interaction between the Hg ion and the aryl ring was observed. [Copyright &y& Elsevier]

Abstract: A series of Hg(II)- and Ag(I)-NHC complexes of N-alkyl/N-alkyl-aryl substituted CNC pincer-type ligands composed of bis-(benz)imidazolium core were synthesized and characterized using different spectral and analytical techniques. Bis-nitrile functionalized (benz)imidazolium salts (8) undergoes rapid hydrolysis to form 1H-bis-(benz)imidazolium salts, which readily experience the Hofmann-type elimination in presence of basic metal sources. Due to elimination reaction, cationic N-coordinated metal complexes were formed in good yields. Structure of 4a, 4d, 6b·2H2O, 9b and Hg(II)-NHC complex 11a was established by the X-ray diffraction studies. Structural studies reveal the existence of diverse hydrogen bonding patterns that involve the (benz)imidazolium-CH units and the anions. Crystallographic study on 11a reveals linear C–Hg–C arrangement for the complex. [Copyright &y& Elsevier]

Abstract: A series of N-allyl substituted xylyl-linked imidazolium salts (7–10) and their respective Ag(I)-N-heterocyclic carbene (NHC) complexes (11–14) have been synthesized and characterized by a number of spectral and analytical techniques. Molecular structure of complexes 13 and 14 were established by single‐crystal X-ray diffraction method. The in vitro anticancer activity of all imidazolium salts and their Ag(I)-carbene complexes were investigated using human colorectal (HCT 116) cancer cell lines. Imidazolium salts displayed no activity for HCT 116 cell lines, except for 9; yielding IC50 value of 15.9μM. Ag(I)-carbene complexes 12–14 showed exceptionally good activity (0.9–1.3μM) against tested cancer cell lines. Furthermore, complex 11 displayed relatively good anticancer activity with IC50 value of 5.2μM, which is almost equal to standard used. [Copyright &y& Elsevier]

Abstract: Non-symmetrically substituted benzimidazolium salts (1 and 2) having bromide counterion undergo metallation with Ag2O at stiochiometric ratio, giving mononuclear bis-carbene Ag(I) complexes (3 and 4), active as anticancer agents against Human Colorectal (HCT 116) cell lines. Both the complexes provide good activity (IC50 =13.9μM for 3 and 14.6μM for 4) in the anticancer studies and so distinctly better than the ligand precursors (IC50 =119.3μM for 1, and 70.0μM for 2). The molecular structure of Ag complexes 3 and 4 is elucidated by X-ray diffraction studies. The Pd(II)– and Au(I)–NHC complexes (5 and 6) were synthesized from 4 via the technique of transmetallation. However, attempts to produce transmetallated complexes using 3 were unsuccessful. [Copyright &y& Elsevier]