Imidazoles transition metal complexes

Certain transition metal complexes can serve as templates for the synthesis of chelating NHC ligands. For example, 1-phenylphosphole complexes of pal-ladium(II) are attacked in a Diels-Alder reaction by 1-vinylimidazole. If 1,2-dichloroethane is used as the solvent the imidazole is alkylated in situ and then subjected to a spontaneous carbometallation reaction [Eq. (37)]. 

Different strategies have been used to attach transition metal complexes to proteins for example, the imidazole moiety of histidine can be coordinated to metal centres such as ruthenium(II), osmium(II) and rhenium(I). Many studies have utilised the imidazole of a histidine residue as a ligand for... 

The vast majority of N-heterocyclic carbenes are based on 5-membered ring systems. It was found that sterically demanding substituents on the NHC are not only beneficial for the stability of the NHC, but also for its catalytic properties. Arguably, the most important and most often employed N-heterocyclic carbenes are imidazol-2-ylidenes IMes and IPr and the imidazolidin-2-ylidenes SIMes and SIPr (Fig. 3). The reactivity of the corresponding transition metal complexes is described in detail in the following sections. 

When the ferrocene unit is brought directly to the imidazole ring, without an intervening spacer, the steric bulk of the ferrocene group comes fully to bear upon the carbene carbon atom of the NHC and thus onto the metal centre of its transition metal complex [147,191,192]. Again, ligands with one [147] and two [191,192] ferrocene units are known and so are achiral [191,192] and planar chiral ones [192] (see Figure 4.56). 

Not only has binding of imidazoles and pyridines to Fe protoporphyrin IX been studied, as discussed in Section 4.1.2, but also photodissociation of axial ligands such as pyridines, imidazoles, or piperidines from six-coordinate, low-spin Fe porphyrins, in which the porphyrin is derived from protoporphyrin IX, or proto- or deuteroporphyrin IX dimethyl ester, has been investigated in nonaqueous solvents using picosecond transient absorption spectroscopy (see Photochemistry of Transition Metal Complexes). It has been shown that photodissociation leads to the formation of five-coordinate complexes, that is, only one ligand appears to be released upon excitation of the six-coordinate complex. ... 

Histidine is the best-known bidentate chelating agent here, but there are others, for example, transition metal complexes with 2-(2 -pyridyl)imidazole may involve a live-membered chelate ring... 

Adhesives which are meant to cure at temperatures of 120 or 171°C require curatives which are latent at room temperature, but react quickly at the cure temperatures. Dicyanodiamide [461-58-5], (TH INI is one such latent curative for epoxy resins. It is insoluble in the epoxy at room temperature but rapidly solubilizes at elevated temperatures. Other latent curatives for 171°C are complexes of imidazoles with transition metals, complexes of Lewis acids (eg, boron trifluoride and amines), and diaminodiphenylsulfone, which is also used as a curing agent in high performance composites. For materials which cure at lower temperatures (120°C), these curing agents can be made more soluble by alkylation of dicyanodiamide. Other materials providing latency at room temperature but rapid cure at 120°C are the blocked isocyanates, such as the reaction products of toluene diisocyanate and amines. At 120°C the blocked isocyanate decomposes to regenerate the isocyanate and liberate an amine which can initiate polymerization of the epoxy resin. Materials such as Monuron can also be used to accelerate the cure of dicyanodiamide so that it takes place at 120°C. 

With respect to the ionic hquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, the 1,3-dialkyhmidazolium cation can act as a hgand precursor for the dissolved transition metal. Its transformation under the reaction conditions into a ligand has been observed in three different ways (i) formation of metal carbene complexes by oxidative addition of the imidazolium cation (ii) formation of metal-carbene complexes by deprotonation followed by coordination of the imidazolylidene on the metal center (iii) dealkylation of the imidazolium cation and formation of a metal imidazole complex. These different ways are displayed in a general form in Scheme 5.3-2. 

Alkynyl(phenyl)iodonium salts have found synthetic application for the preparation of various substituted alkynes by the reaction with appropriate nucleophiles, such as enolate anions [980,981], selenide and telluride anions [982-984], dialkylphosphonate anions [985], benzotriazolate anion [986], imidazolate anion [987], N-functionalized amide anions [988-990] and transition metal complexes [991-993]. Scheme 3.291 shows several representative reactions the preparation of Ai-alkynyl carbamates 733 by alkynylation of carbamates 732 using alkynyliodonium triflates 731 [989], synthesis of ynamides 735 by the alkyny-lation/desilylation of tosylanilides 734 using trimethylsilylethynyl(phenyl)iodonium triflate [990] and the preparation of Ir(III) a-acetylide complex 737 by the alkynylation of Vaska s complex 736 [991]. 

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells (pyridine, phenanthroHne, pyrazole, imidazole, triazole derivatives as Hgands) 12AG(E)8178. 

On the basis of the wide catalytic applications of NHC transition metal complexes [25], Nolan and coworkers have thoroughly studied the catalytic activity in CuAAC reactions of well-defined copper(I) complexes with general formula [CuX(NHC)]. Organic solvents, mixtures of EtOH/water, and pure water have been used as reaction media. In particular, it has been reported that complexes [CuBr(SIMes)] (1 in Fig. 15.1, SIMes=iVAf-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)) and [CuI(IAd)] (2 in Fig. 15.1, IAd=iVAf-adamantyl imidazol-2-ylidene) show a remarkable activity for the synthesis of a... 

It has been demonstrated that iV-(trimethylsilyl)imidazole is able to transform transition metal complexes containing a metal-Cl bond to the corresponding metal-imidazole analogues. This transformation has been reported for transition metal complexes of Ni(II), Pd(II), Pt(n), Ru(n), Ru(m), Te(rv) and Ti(IV).50... 

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). ... 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

B. Non-Metal and Non-Transition Metal Organometallic Complexes of Imidazoles. 120... 

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