Silver(I) And Gold(I) Mono-, Bis- And Tetra-N-Heterocyclic Carbene Complexes: Synthesis, Characterization, Antibacterial And Photophysical Studies

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Date
2021-04
Authors
Haziz, Umie Fatihah Mohamad
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Universiti Sains Malaysia
Abstract
This work describes the synthesis of symmetrical and non-symmetrical benzimidazolium salts as a precursor for the silver(I)-NHC complexes (where NHC = N-heterocyclic carbene). For the first series, seven n-butyl-n’- alkylbenzimidazolium bromide salts (where alkyl = methyl, ethyl, n-propyl, npentyl, n-hexyl, n-heptyl and n-benzyl), 1-7 were successfully synthesized. The second series, salts 8-10 were synthesized from n-alkylbenzimidazole (where nalkyl = ethyl, n-propyl and n-butyl) with 1,2-dibromoethane, while salts 11-13 were synthesized from n-benzylbenzimidazole with 1,n-dibromoalkane (where alkane = ethane, propane and butane). Meanwhile, through the reaction of 3-(2- bromoethyl)-1-butylbenzimidazole bromide, iii with n-alkylbenzimidazole (where n-alkyl = methyl, ethyl, n-propyl, n-pentyl, n-hexyl, n-heptyl and n-benzyl), seven unprecedented non-symmetry dibenzimidazolium bromide salts, 14-20 were successfully obtained in the third series. The last series, tetradentate benzimidazolium salts 21-28 were successfully synthesized through the reaction of 3-(2-bromoethyl)-1-alkylbenzimidazole bromide salts (where alkyl = ethyl, npropyl, n-butyl and n-benzyl), i-iv with either 1,4-butylbisbenzimidazole or 1,3- bis(methylbenzimidazole)benzene. Salts 1-28 were reacted with silver(I) oxide in appropriate molar ratio via in-situ deprotonation reaction to facilitate the formation of silver(I)-NHC complexes Ag1-Ag28, respectively. Furthermore, Ag1-Ag7 and Ag14-Ag20 were selected as ligand transfer agents, reacted with chloro(dimethylsulfide)gold(I) to yield gold(I)-NHC complexes, Au1-Au7 and Au14-Au20, respectively. The successful complexation was suggested by the disappearance of H2’ peaks in 1H NMR and the presence of Ccarbene-M peaks in the 13C NMR of the complexes. Besides NMR study, the formation of the synthesized compounds were supported by melting points, elemental analysis and IR studies. The single crystal X-ray diffraction analysis has revealed that complexes Ag1, Ag11, Ag20, Au3 and Au18 having the expected chemical formula of [Mn(NHC)2]∙nPF6 (where M = Ag or Au, n = 1 or 2). On the other hand, the silver(I)-NHC complexes with tetrabenzimidazol-2-ylidene ligands, Ag23 and Ag27 formed the dinuclear silver(I)-NHC complexes with formula of [Ag2(μ- NHC)]∙2PF6. Antibacteria activity for all benzimidazolium salts 1-28 and silver(I)- NHC complexes, Ag1-Ag28 were evaluated. All silver(I)-NHC complexes show lower to higher activities against E. coli (ATCC 25922) and S. aureus (ATCC 12600) compared to the standard antibiotic drug, Ampicillin. The photophysical properties of crystallized gold(I)-NHC complexes, Au3 and Au18 were determined using photoluminescence technique in solid state. In the absence of significant aurophilic interaction in Au3, the emission band is attributed to the metal-to-ligand charge transfer (MLCT). Meanwhile, Au18 shows interesting temperaturedependent photoluminescence behaviours resulted from aurophilic interaction in the complex.
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Chemistry
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