Monodentate carbene complexes

Chiral monodentate carbene complexes of Rh and Ir of the type [MCl(l,5-COD) (NHC)] (M = Rh, Ir) with the ligands 7-9 (Fig. 2.1) have been stndied as catalysts for the enantioselective hydrogenation of methyl-2-acetamido acrylate. Even though the activities were high, the enantiomeric excesses (ee) were poor [7, 8]. 

Methylation of the dihaptothioacyl complex 22 affords compound 23 containing a bidentate carbene ligand, which on reaction with chloride ion leads to the neutral monodentate carbene complex 24 (50,51). The chelate carbene complex 26 is generated in a novel interligand reaction from the thiocarboxamidothiocarbonyl cation 25. The thiocarbonyl carbon acts as the electrophilic component in this reaction, and 26 is further alkylated to a bidentate dicarbene species (52). 

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-... 

With monodentate carbenes, complexes containing one, two, or three of these ligands in the coordination sphere are known, and quantitative data on the reaction of amines to a coordinated isocyanide ligand have shown the addition to proceed by a stepwise mechanism through an intermediate... 

Hydrazine reacts with [Fe(CNMe)4]2+ to form the chelate (16) in which addition to two adjacent isocyanide ligands has taken place.321 Methylamine reacts322 in a related way to give (17), while reaction with ammonia results in the formation323 of a monodentate carbene complex (18). Methylation of [Fe(CN)2(phen)2] with Me2S04 yields [Fe(CNMe2)(phen)2],26 which reacts with hydrazine to give323 the tris chelate (19) in a similar fashion to the formation of (16). 

In the synthesis of anionic amido-A -heterocyclic carbene complexes of lanthanides (22, Ln = Sm, Y N = (TMS)2N)) from the bridged imidazolium complex 20 via carbene 21, a typical increase in the length of the C-N bond and a decrease in the N-C-N angle were observed going from 20 to 21 <2003AGE5981>. The Sm-Ccarbene distance of 2.588(2) A is the shortest recorded compared to those reported for monodentate Sm-NHC complexes (2.62-2.83 A). 

Tricarbonyl rhenimn (I) bis(carbene) complexes (NHC)2ReBr(CO)3 are easily accessible from [NEtJ [ReBr3(CO)s] precursor and the free carbene source, either a monodentate NHC or a bidentate NHC ligand (75 and 76) (Scheme 14.37)... 

In the rhodium and iridium complexes, the C-coordination, carbene function, and cyclometallated cases prevail. Benzothiazole-2-thione was studied extensively as a ligand and various situations of the exocyclic S-monodentate coordination as well as N,S-combinations in the di-, tri-, and tetranuclear species were discovered. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Simple monodentate NHCs are somewhat susceptible to dissociation when coordinated to early transition metals [6], so in most cases multidentate chelating hgands are employed in which the carbene is tethered to a strongly coordinating anchoring group. This is not universally the case however, and simple monodentate NHC complexes of Zr 1 (Fig. 4.1) have been studied [7]. The complexes were activated with MAO and tested for ethylene polymerisation, leading to moderate activities between 7 and 75 kg mol bar h for linear polyethylene. 

There are a number of reports of NHC complexes of mid-late transition metals being used as catalysts for atom transfer radical polymerisation (ATRP) of acrylates and styrene. Grubbs reported Fe(II) complexes of a simple monodentate carbene. 

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