Imidazolin-2-ylidene carbene ligand

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

Additional factors which lead to an increased stability of the carbene complexes towards reductive ehmination are the type of NHC ligand and the NA -substitution pattern. The stability of NHC complexes depends strongly on the electronic situation at the carbene center. The oxidative addition of p-tolyl chloride to linear Pd° complexes bearing two unsaturated imidazolin-2-ylidenes (type 5, Fig. 6) or two saturated imidazolidin-2-ylidenes (t3q>e 7, Fig. 6) proceeds readily. The Pd complex with the imidazolin-2-yhdene ligands is stable, while the one with the imida-zolidin-2-ylidene ligands reductively ehminates the C2-arylated imidazolidinium salt [134]. 

Homoleptic hexacoordinate carbenecobalt(m) has been recently obtained. The preparation of the tris-carbene tris-(imidazolin-2-ylidene)borate 82, which is the C-isomer analog of Trofimenko s well-known tris(pyrazolyl)borate, has been reported. Two of these tris-carbene ligands have been coordinated onto the cobalt atom (Figure 29). ... 

The ligand l,3-bis(2,2", 6,6"-tetramethyl-m-terphenyl-5 -yl)imidazole-2-ylidene) ITmt was designed to facihtate the formation of low-coordinate species. It has a much higher overall degree of bulkiness than the well-known l,3-bis(2,4,6-trimethylphenyl)imidazoline-2-ylidene (IMes), but in the vicinity of the carbene center it is less severe because of the absence of substituents in the ortho-position. 

Ackermann, L., Fiirsmer, A., Weskamp, T., Kohl, F.J., Herrmann, W.A., Ruthenium carbene eomplexes with imidazolin-2-ylidene ligands allow the formation of tetrasubstituted cycloalkenes by RCM, Tetrahedron Lett 1999, 40 4787-4790. 

A substantial improvement on the above-described class of Ru-vinyUdene complexes has been achieved by Louie and Grubbs through synthesis of a novel generation of complexes coordinating an A-heterocyclic carbene (NHC) ligand (e.g. imidazolylidene). This new set of Ru complexes, e.g. 15 (IMes = l,3-(2,4,6-trimethylphenyl)imidazoT2-ylidene, R = Cy, R = t-Bu) and 16 (iPrlM = l,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene, R = Cy, R = Ph) has been conveniently produced from the bisphosphane-Ru complex 9 (R = Cy) by reaction with free imidazoline carbene or its precursor salts (Scheme 7). 

While up to 1990 all attempts to isolate a stable N-heterocyclic carbene failed, metal complexes of unsaturated imidazol-2-ylidenes were known as early as 1968. The first complexes of this type were obtained by in situ deprotonation of imidazolium salts using mercury(ll) acetate or dimethylimidazolium hydridopentacarbonylchromate(-II) followed by coordination of the carbene to the metal center (Scheme 1.3). Shortly thereafter, the stabilization of the saturated imidazolin-2-ylidene in a metal complex was described by Lappert who treated electron-rich entetraamines of type 6=6 with coordinatively unsaturated transition metal complexes to obtain complexes with imidazolin-2-ylidene ligands (Scheme 1.3). ... 

Reaction of l,3,4,5-tetramethyl-2-methylene imidazoline with [M(CO)e] (M = Mo or W) gives 54, An X-ray crystal structure of the Mo complex shows the alkene to be end-on coordinated. 2 The analogous imidazol-2-ylidene complexes, 55, are obtained from [M(CO)6] (M = Cr, Mo or W) and the corresponding imidazol-2-ylidene (R = Me, Et, Pr ). Spectroscopy and X-ray crystallography suggest that the carbenes are strong donor ligands with no rc-interaction between the carbene and the metal centre. 3... 

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