Tetranuclear Silver Complex Research Articles

Synthesis of silver complexes with chelating bidentate N‐heterocyclic ligands, their application in catalytic A3 coupling, and as antimicrobial agents

We present a synthesis of novel silver complexes stabilized by bidentate ligands based on N‐heterocyclic carbenes (NHC) linked with a bisamide linker. The ligand stabilizes the silver ion in a rare chelating mode. The synthesis of the complex depends on the equimolar ratio of the silver source and the ligand precursor. In case the excess of the silver source is used, the reaction leads to the formation of an unprecedented tetranuclear silver complex, stabilized by two equivalents of the ligand, where each of the silver atoms is coordinated by one NHC and one amide moiety. The silver complexes were applied as a catalyst in a multicomponent A3 coupling and proved to be a very efficient catalyst. The reaction provided desired products in yields up to 96%, and the use of low catalytic loading, as low as 0.1 mol%, was possible without significantly compromising the effectivity of the reaction. Moreover, the complexes showed broad spectra of antimicrobial activity, with minimal inhibitory concentrations in the range of 1 to 31 μg/ml against several Gram‐positive and Gram‐negative bacteria and fungi. Presented complexes represent synthetically challenging molecules, which show great applicability in catalysis and outstanding potential as antimicrobial agents.

Gold(I) and Silver(I) Complexes with Thioether Functionalized Silylamido Ligands

The thioether functionalized aminosilanes Me2Si(NH-C6H4-2-SR)2 (R = Ph, Me) were lithiated with nBuLi and subsequently reacted with AgCl in the presence of PMe3 or with [AuCl(PMe3)]. In the case of Me2Si(NH-C6H4-2-SPh)2 the dinuclear complexes [M2{Me2Si(NC6H4-2-SPh)2}(PMe3)2] (M = Ag; Au) were isolated. The analogous reactions starting from Me2Si(NH-C6H4-SMe)2 afforded the dinuclear gold complex [Au2{Me2Si(NC6H4-2-SMe)2}(PMe3)2] and the tetranuclear silver complex [Ag4{Me2Si(NC6H4-2-SMe)2}2(PMe3)2]. In the dinuclear compounds of the type [M2{Me2Si(NC6H4-2-SR)2}(PMe3)2], each of the silylamide N atoms is connected to a M(PMe3) group to give a nearly linear N–M–P arrangement with Ag–N and Au–N bonds in the range of 212.0(4)–213.3(4) pm and 205.3(3)–208.1(9) pm, respectively. [Ag4{Me2Si(NC6H4-2-SMe)2}2(PMe3)2] consists of a central Si2N4Ag2 ring with linearly coordinated Ag atoms (Ag-N: 223.1(4)–222.1(4) pm) and two peripheral Ag(PMe3) units, which are connected to the amido N atoms in a chelating mode. The relatively short transannular Ag···Ag separation (277.6(1) pm) within the Si2N4Ag2 ring hints for argentophilic interactions. The peripheral Ag atoms are three coordinated with Ag–N distances of 233.9(4)–242.8(4) pm.

Discrete Di‐ and Tetranuclear Silver Complexes Based on ortho‐Imino‐ or ortho‐Amino‐methylpyridyl‐Appended p‐tert‐Butylcalix[4]arene or p‐tert‐Butylthiacalix[4]arene in 1,3‐Alternate Conformation

Four new multivalent ligands 5–8 derived from calix[4]arene and thiacalix[4]arene backbones in 1,3‐alternate conformation bearing ortho‐imino‐ and ortho‐amino‐methylpyridine chelating sites have been designed, prepared and characterized. Owing to the imposed 1,3‐alternate conformation of the backbone, ligands 5–8 offer two coordinating pockets composed of two sets of chelates located above and below the mean plane of the macrocyclic framework. Ligands 6–8, independent of the nature of the anion associated with the Ag+ cation (AgX; X = BF4–, PF6–, SbF6–, AsF6–, NO3–), form dinuclear silver complexes in solution and in the solid state. Ligand 7, in the presence of an excess of Ag+, also forms tetranuclear silver complexes 7‐(AgBF4)4 and 7‐(AgNO3)4 in the presence of two different auxiliary ligands (H2O or NO3–) that have been characterized in the solid state. Due to a rather short Ag–Ag distance of about 2.89 Å, the tetranuclear complex 7‐(AgBF4)4 is emissive in the solid state, at 298 K (λexc = 295 nm, λem = 425 nm), which results from argentophilic interactions between the closely positioned silver cations.

Syntheses and Structural Characterization of Mono-, Di-, and Tetranuclear Silver Carbone Complexes

Mono-, di-, and tetranuclear silver complexes 1–3 were successfully synthesized and isolated using a bis(chalcogenane)carbon(0) (BChC)-bearing iminosulfane ligand. In these complexes, BChCs exhibit κ2C,N and κ3C,C′,N coordination modes. The molecular structure of [Ag4(BChCs)2](TfO)4 (3) reveals the peculiar electronic structure of BChC, which features two electron lone pairs on the central carbon atom. Furthermore, the structure of 3 provides the first experimental proof that carbones can act as multidentate ligands in multication clusters.

Di-, tri-, and tetranuclear silver(I) complexes of phenanthroline-functionalized NHC ligands

A few polynuclear silver(I)–NHC complexes have been synthesized via the reactions of phenanthroline-functionalized imidazolium salts with Ag2O at room temperature. The structures of di-, tri-, and tetranuclear silver complexes were fully characterized. The two N-phenanthrolinylimidazolylidene ligands in [Ag4(L1)4](PF6)4 (1), [Ag2(L2)2](PF6)2 (2), and [Ag2(L3)2](PF6)2 (3) are head-to-head arranged. Complex 1 contains a short ligand-unsupported Ag–Ag bond being 3.152(1) Å representing the first example of strictly linear Ag4 cluster stabilized by NHCs. Trinuclear [Ag3(L4)2](PF6)3 (4) of phenanthroline-bridged biscarbene ligands contains a linearly arranged Ag3 core. [Ag3(L5)2](PF6)3 (5) and [Ag3(L4)2](PF6)3 (7) are also trinulear Ag(I) complexes. Binuclear [Ag2(L6)2](PF6)2 (6) exhibits a twisted macrocyclic structure with the Ag–Ag separation at 3.162(1) Å. In 1–4, the silver ions are held together by two ligands in close proximity with the Ag–Ag distances between 2.810(1) and 2.922(1) Å. The fluorescent properties of these silver complexes have also been briefly studied.

Synthesis and structures of mono- and tetra-nuclear silver complexes of styrene-containing N-heterocyclic carbene

Two tetra-nuclear Ag(I) complexes with styrene-functionalized N-heterocyclic carbene [AgL 2] 2[Ag 2X 4] (L = 1-methyl-3-(4-vinylbenzyl)imidazol-2-ylidene, X = Cl, 2a; X = I, 2b) were prepared by the reactions between the corresponding imidazolium salts with Ag 2O. The reaction mixture was further treated with AgBF 4 to give a mononuclear ion-pair complex [AgL 2][BF 4] ( 3). The molecular structures of these new Ag(I) complexes have been determined by X-ray diffraction analyses. 2a and 2b consist of two [Ag(L) 2] + fragments with the central [Ag 2X 4] 2− anion held together by the close Ag(I)–Ag(I) interactions. Complex 3 is a mononuclear ion-pair complex with a linear bi-coordinate Ag fragment.