Pincer silver -carbene

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... 

Fig. 11 A pincer silver(I)-carbene complex designed for the controlled release of Ag ions to fight bund wounds...

Nielsen, D.J., CaveU, K.J., Skelton, B.W. and White, A.H. (2006) Silver(l) and palladium(II) complexes of an ether-functionalized quasi-pincer bis-carbene ligand and its alkyl analogue. Organometallics, 25,4850-6. 

Nielsen et al. have introduced a monoether linked bis-carbene [209] modelled on an amino linked bis-carbene ligand that acts as a C,N,C pincer ligand in a corresponding palladium(II) complex [156]. Synthesis of the ether linked bis-carbene is facile and involves the reaction of the l-co-dichloro-diethylether with 2 equiv. of methylimidazole. Subsequent reaction with silver oxide and carbene transfer to suitable transition metal precursor complexes affords the corresponding complexes (see Figure 3.73). 

Simons et al. [487] used these pincer carbene ligands in the synthesis of silver(I), palladium(II) and rhodium(l) pincer carbene complexes (see Figure 3.167). Similar to the Anker-free pincer carbene ligands, [Rh(cod)Cl]2 proved to be an unsuitable starting material to synthesise a rhodium(l) pincer carbene complex [462]. The ligand acted as bridging ligand to two [Rh(cod)Cl] units instead. 

Silver(I) carbene complexes are characterised by a broad range of different structures [314]. Hence, it is not surprising that Melaiye et al. [488] found an infinite chain structure as the underlying motif in a water soluble silver(l) pincer carbene complex (see Figure 3.168). Water solubility was achieved by the introduction of pendant hydroxyalkyl wingtip groups. Water solubility of the silver(l) pincer carbene is desirable when the compound is used as an antimicrobial agent in medical applications [489]. 

A third structural motif for silver(l) pincer carbene complexes is the dimer observed by Nielsen et al. [490] (see Figure 3.169). This dimeric structure becomes possible because of the flexibility introduced into the pincer structure by the linker unit, which acts as a hinge. It makes it possible for the NHC donor ligands to align themselves parallel to each other and perpendicular to the C-Ag-C vector of the silver coordination sphere. The dimer is considered to possess no argentophilic interactions as the Ag-Ag distance is 31 pm longer than the sum of the van der Waal s radii (340 pm) [20]. 

Figure 3.169 A silver I) pincer carbene complex with dimer structure.

Figure 3.173 The silver(l) complex of a potential C,C,C pincer carbene ligand.

In the same way that the controlled release of silver ions is important in the antibacterial activity of silver nitrate and silver(I) n-pincer-type heterocyclic carbene complexes, the release of carbon monoxide in a controlled... 

Further examples of palladium picolyl- and pyridyl-functionalized carbene complexes, and also a methoxy ether-functionalized complex have been reported.The synthesis of C-N chelating and N-C-N pincer carbene complexes of palladium have been described.Complexes were generally prepared by a silver-transfer route, although several were prepared via the free carbene method. Structural studies were undertaken on a number of the complexes. Air and thermally stable tridentate carbene C-N-C 92, 93 and C-C-C 94 palladium pincer complexes have been prepared.Complex 94 is formed by the reaction of the appropriate aryl-bridged precursor with... 

A general synthesis of pincer N-heterocyclic carbenes with pyridines as the bridging unit is presented below. The combination of 2 with 2-iodoethanol or 3-bromopropanol gave 10a and 10b, respectively (10). The combination of the halide salt of 10a or 10b with an equimolar amount of AgaO gives the silver biscarbene polymers 11a and 11b, respectively. Conq)ound 11a has been crystallographically characterized The hydroxide salts 11a and lib are very soluble and slowly decompose in water. The deconq)osition leads to a slow release of silver atoms. The thermal ellipsoid plot (atoms shown isotropically) of 11a is shown in Figure 4. 

---