Tris-carbene complexes

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

Monomeric copper(I) tris(carbene) complexes were prepared by reacting with copper(I) salts. The X-ray structures... 

Reaction of Ni(cod)2 with a nitrogen-anchored tripodal NHG ligand TIMEN = tris[2-(3-alkylimidazol-2-ylidene)-ethyl]amine, R = Bu leads to an Ni(0) 16-electron tris-carbene complex NKTIMEN " )" (Equation (11)). 

Using a new tetradendate ligand, Mcyct et al. obtained a tris-carbene complex 57a, which reacted with CO, one-electron oxidizers such as CHgClg and Og to afford complexes 57b-d (Figure 20). ... 

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Electron-rich olefins such as 36 have been used by Lappert in the synthesis of a great number of mono-, bis-, tris-, and tetrakiscarbene complexes from various transition metal species (62). Ru, Os, and Ir carbene complexes have been prepared from reactions with these olefins, e.g.,... 

Monomeric carbene complexes with 1 1 stoichiometry have now been isolated from the reaction of 4 (R = Bu, adamantyl or 2,4,6-trimethylphenyl R = H) with lithium l,2,4-tris(trimethylsilyl)cyclo-pentadienide (72). The crystal structure of one such complex (R = Bu) revealed that there is a single cr-interaction between the lithium and the carbene center (Li-C(carbene) 1.90 A) with the cyclopentadienyl ring coordinated in an if-fashion to the lithium center. A novel hyper-valent antimonide complex has also been reported (73). Thus, the nucleophilic addition of 4 (R = Mes R = Cl) to Sb(CF3)3 resulted in the isolation of the 1 1 complex with a pseudo-trigonal bipyramidal geometry at the antimony center. 

Fig. 9. Structural models of carbon- and nitrogen-anchored tris(carbene) metal complexes.

The first non-heteroatom-substituted carbene complex was prepared by Schrock in 1974 [392] (Figure 3.4). Treatment of tris(neopentyl)tantalum dichloride with neopentyllithium led to the formation of neopentane and (2,2-dimethyl-1-propylidene)tris(neopentyl)tantalum. This carbene complex reacts violently with water or oxygen, but can be sublimed (80 °C) and stored indefinitely at room temperature under argon. 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

Scheme 4 Synthesis and oxygenation of Tris-carbene cobalt complex...

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). 

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). 

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