Carbene-silver complexes

An extension of the research on silver complexes with Lewis base-functionalized mono(A-heterocyclic carbene) ligands has been made toward the better-studied and stronger coordinating phosphine systems. The reaction of a diphenylphosphine-functionalized imidazolium salt with silver oxide in dichloromethane affords a trinuclear silver carbene complex 50, as confirmed by electrospray-ionization mass spectrometry.96,97 Metathesis reaction of 50 in methanol using silver nitrate gives 51 in 33% yield. The crystal structures of 51 were found to be different when different solvents were used during crystallization (Scheme 12).97 One NO3- anion was found to be chelated to... 

Youngs and co-workers extended the chemistry toward synthesizing water-soluble silver carbene complexes 68a-68c (Figure 28) by using hydroxyl-containing pincer-type iV-heterocyclic carbenes as ligands.1 4 It is interesting... 

Iridium 2-pyridinylmethyl imidazolylidene C,N-chelates were obtained by transmetallation of the silver carbene complexes and tested for catalytic activity in the TH of benzophenone and nitroarenes by isopropanol [55]. The neutral monodentate complexes [(L-KC)Ir(COD)Clj [61a,b L = l-methyl-3-(6-mesityl-2-pyridinylmethyl)-2-imidazolylidene, l-mesityl-3-(6-mesityl-2-pyridinylmethyl)-2-... 

Scheme 21 Catalytically active silver carbene complex 17...

However, an encouraging result was obtained very recently for the 1,4-conjugate addition of dialkyl zinc to a variety of Michael acceptors catalyzed by copper. Alexakis, Roland and coworkers have investigated the addition of diethyl zinc to cycloheptenone and observed an enantiomeric excess of 93% (95% yield) in the presence of Cu(OAc)2 and the silver carbene derivative of imidazolium 1 (Scheme 3) [10]. Silver carbene complexes are efficient transfer agents to copper(II) and therefore the potentially harmful use of a base to generate the catalytic species is avoided. 

One of the most generally used methods to prepare a NHC transition metal complex is the reaction of an azolium salt with silver oxide to form the silver carbene complex [24,25,72], It is so general that it has its own name, the silver(I) oxide (Ag O) method... 

Of course, oxide is a base and thus it falls under the heading of reactions with basic transition metal compounds, but the silver carbene complexes are usually only synthesised because the silver atom coordinates only weakly and can easily be replaced by another metal of choice. It is therefore known as a carbene transfer agent. 

Advantage The silver carbene complex can usually be synthesised using undried solvents and with exposure to air. 

Silver carbene complexes are the most commonly used carbene transfer complexes [83], Other carbene transfer agents include lithium adducts [56], potassium complexes [53], molybdenum carbene complexes [83,84] or chromium carbene complexes [85],... 

Of course, the heteroaromatic sidegroup is usually not strong enongh as a donor ligand to replace chloride from a palladinm(II) centre [31,34], Coordination of the sidearm can be achieved by silver halide abstraction nsing a snitable silver salt [31], using a silver carbene complex as the silver source [31,34,35] or reacting the imidazolium salt directly with [Pd(OAc)2] [36] and therefore in the absence of halide (PF - was nsed as the imidazolium counterion). 

Figure 3.54 Amide deprotonation during the synthesis of amino functionalised silver ( ) carbene complexes.

Note The palladium(II) complex in Figure 3.73 shows no Pd-0 coordination despite the existence of a free coordination site owing to chloride abstraction by the silver carbene complex. Coordination of the weak donor acetonitrile is preferred. 

Direct arylation of methyl imidazole with 2,7-dichloronaphthyridine leads to a potentially tetradentate and practically tridentate bis-carbene ligand on a naphthyridine scaffold [363]. Reaction of the bis-imidazolium salt with silver(l) oxide in the usual way yields a linear trinuclear silver carbene complex with this tris-bridging hgand (see Figure 3.115). 

Liao et al. also synthesised an imidazolium salt carrying two carboxylic acid amide functionalities and generated its silver carbene complex. It is distinguished by absolute linearity of the C-Ag-Cl vector and rather short Ag-C and Ag-Cl bond lengths. 

A straightforward preparation for an air-stable silver carbene complex using a standard wet chemistry kit was presented. This experiment removed the tedious requirements of a carbene synthesis and allows for the inclusion of carbenes and carbene transfer agents in the undergraduate laboratory curriculum. 

Substitution of ally lie phosphates by alkenyldiisobutylalanes proceeds via the Sn2 route, CuCl2 in combination with a dinuclear silver-carbene complex (28) are responsible for excellent asymmetric induction. ... 

K.M. Hindi, A.J. Ditto, M.J. Panzner, D.A. Medvetz, D.S. Han, C.E. Hovis, J.K. Hilliard, J.B. Taylor, Y.H. Yun, C.L. Cannon, The antimicrobial efficacy of sustained release silver-carbene complex-loaded 1-tyrosine polyphosphate nanoparticles characterization, in vitro and in vivo studies, Biomaterials 30 (2009) 3771-3779. 

C. Ornelas-Megiatto, P. Shah, P. Wich, J. Cohen, J. Tagaev, J. Smolen, B. Wright, M. Panzner, W. Youngs, J. Frechet, C. Cannon, Aerosolized antimicrobial agents based on degradable dextran nanoparticles loaded with silver carbene complexes. Mol. Pharm. 9 (2012) 3012-3022. 

Diphenylphosphinoethyl-functionalised imidazolium salts and their silver-carbene complexes were used to synthesise a series of di- and trinuclear gold complexes able to exhibit multi-band emission spectra. The PL properties of these species are likely significantly influenced by gold-gold interactions. ... 

N-Heterocyclic carbene complexes can be generated from the stable, free carbenes or by reaction of carbene precursors. Three examples of the s)mthesis of N-heterocyclic carbene complexes from carbene precursors are shown in Equations 13.14-13.16. The reaction of an imidazolium salt with a complex containing a basic ligand forms N-heterocydic carbene complexes (Equation 13.14), as does transfer of the carbene from a silver-carbene complex (Equation 13.15), - or reaction of an imidazolium-2-carboxylate, with accompanying decarboxylation (Equation 13.16). ... 

Silver carbene complexes act as very efficient NHC transfer reagents for the synthesis of different metal-NHC complexes. On the other hand, as far as we know, there is only one example described to date where a silver-NHC complex was used for asymmetric catalysis. In 2006, Ferndndez and coworkers reported the first and only asymmetric catalysis using a chiral NHC-silver catalyst. Enantioselective diboration of styrenes was realized by using silver complex 111 as the catalyst (Scheme 3.70) [99], However, the diol was obtained in low yield and low enantioselectivity (less than 10% ee). 

Another methodology is based on silver transmetallation [35]. In the first step, a silver-carbene complex is formed from the imidazolium salt and silver(I) oxide under reflux in dichloromethane. Subsequently, this complex is treated with a rhodium salt at room temperature to form the rhodium- carbene complex in high yields. In a few cases, also enetetramines have been used as precursors, delivering electron-rich carbenes by cleavage of the central C=C bond [36]. 

Another common method of introducing NHG ligands has been to isolate silver carbene complexes first, and then react them with a platinum salt. Normally, silver oxide is reacted with an imidazolium salt, before the introduction of the platinum source at ambient or low temperature insoluble silver byproducts are conveniently filtered off. A conceptually similar approach is the prior formation of a tungsten or molybdenum pentacarbonyl carbene complex, and reaction of this with a platinum precursor. The room-temperature reaction with, for example, PtCl2(NGPh)2 results in the release of carbon monoxide and the precipitation of an undefined tungsten or molybdenum compound, which may easily be removed by filtration. 

Panzner MJ, Deeraksa A, Smith A, et al. Synthesis and in vitro efficacy studies of silver carbene complexes on biosafety level 3 bacteria. Eur J Inorg Chem. 2009 1739-1745. 

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