Carbene complexes properties

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

The kinetic and thermodynamic properties of Fischer-type carbene complexes have also been addressed by Bernasconi, who relates the strength of the 7r-donor substituent to the thermodynamic acidity [95-101] and the kinetics and mechanism of hydrolysis and reversible cyclization to differences in the ligand X [96,102]. 

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

A series of dinuclear gold(I)-carbene complexes of imidazolium-linked cydo-phanes and related acyclic bis(imidazolium) salts have been synthesized and their spectroscopic properties were examined by Baker and coworkers [31]. X-ray stmctural analysis of the cation in 1 and 2 (Scheme 5.1) revealed intramolecular Au - Au contacts of 3.5425(6) and 3.0485(3) A respectively. The electronic absorption... 

Zhang, S.-W., Ishii, R. and Takahashi, S. (1997) Syntheses and Mesomorphic Properties of Gold(I)-Carbene Complexes. Organometallics, 16, 20-26. 

Carbene complexes have also been prepared by transmetallation reactions. Lithiated azoles react with gold chloride compounds and after protonation or alkylation the corresponding dihydro-azol-ylidene compounds, e.g., (381) or (382), are obtained.22 9-2264 Silver salts of benz-imidazol have also been used to obtain carbene derivatives.2265 Mononuclear gold(I) carbene complexes also form when trimeric gold(I) imidazolyl reacts with ethyl chlorocarbonate or ethyl idodate.2266,2267 The treatment of gold halide complexes with 2-lithiated pyridine followed by protonation or alkylation also yields carbene complexes such as (383).2268 Some of these carbene complexes show luminescent properties.2269-2271... 

The development of the chemistry of carbene complexes of the Group 8a metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. The pioneering studies of E. O. Fischer and co-workers have led to the characterization of many hundreds of carbene complexes in which the heteroatoms N, O, and S are bonded to the carbene carbon atoms. The first carbene ligands coordinated to Ru, Os, and Ir centers also contained substituents based on these heteroatoms, and in this section the preparation and properties of N-, O-, S-, and Se-substituted carbene complexes of these metals are detailed. 

Although transition metal alkylidene complexes, i.e., carbene complexes containing only hydrogen or carbon-based substituents, were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. Neutral and cationic complexes of these Group 8 metals are known for both metal electron configurations d8 and d6. The synthesis, structural properties, and reactivity of these compounds are discussed in this section. 

The reactions of other reactive zero-valent Group 8 substrates M(CO)2(PPh3)3 (M = Ru, Os), RuH(NO)(PPh3)3, and Ir(NO)(PPh3)3 with RCHN2 (R = H, Me,p-tolyl, C02Et) failed to yield carbene complex products, indicating that very specific properties of the substrate complex are required for successful isolation of alkylidene complexes. 

Synthesis, Properties, and Reactions of Carbene Complexes of Silver 203... 

Similar work was carried out with ligands featuring different flexible or rigid spacers (X = -C(0)0-, -C=C-, -CH2-CH2-) and terminal alkoxy groups where n = 9-12. These compounds and the carbene complexes derived therefrom by addition of alcohols also show mesogenic properties (Scheme 49).211 212... 

The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. O, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as a-donors and 71-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept a-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. 

Because of n-electron donation by the heteroatom, these carbene complexes are generally less electrophilic at C than the corresponding non-heteroatom-substituted complexes (Chapter 3). This effect is even more pronounced in bis-heteroatom-substituted carbenes, which are very weak Tt-acceptors and towards low-valent transition metals show binding properties similar to those of phosphines or pyridine. Alkoxycarbenes, on the other hand, have electronic properties similar to those of carbon monoxide, and stable heteroatom-monosubstituted carbene complexes are also usually formed from metals which form stable carbonyl complexes. 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

The heteroatom-substituted carbene complexes most frequently used in organic synthesis are carbonyl complexes of the type (CO)5M=C(X)R (M Cr, Mo, W X OR, NR2 R H, alkyl, aryl, vinyl, alkynyl, etc.). To some extent such complexes behave as carboxylic esters or amides, the (CO)5M=C group having electronic properties similar to those of a carbonyl group (Figure 2.16). 

Depending on the types of substituents and the precise reaction conditions (l,3-butadien-l-yl)carbene complexes can undergo direct cyclization to yield cyclo-pentadienes [337,350]. As mentioned in Section 2.2.5.1, cyclopentadiene formation occurs particularly easily with aminocarbene complexes [351]. Alternatively, in particular at higher reaction temperatures, CO-insertion can lead to the formation of a vinylketene complex, which, again depending on the electronic properties of the substituents and the reaction conditions, can cyclize to yield cyclobutenones, furans [91,352], cyclopentenones, furanones [91], or phenols (Dotz benzannulation) [207,251,353]. 

Because hydrogen, alkyl, or aryl groups can compensate only to a limited extent the electron deficit of the carbene carbon atom, it is mainly the metal and its ligands which provide stabilization in this type of carbene complex. For this reason the reactivity of these compounds depends mainly on the nature and oxidation state of the metal and on the electronic properties of the remaining ligands. 

Non-heteroatom-substituted carbene complexes of almost all transition metals are known. Depending on the oxidation state of the metal, the overall charge of the complex, and the properties of the additional ligands, the reactivity of alkyl or aryl carbene complexes can vary greatly. Some examples of compounds with strikingly different chemical properties are shown in Figure 3.1. 

Non-heteroatom-substituted carbene complexes cover a broad spectrum of different reactivities, largely dependent on the electronic properties of the metal. In Chapter 1 the division of carbene complexes into Fischer-type and Schrock-type carbenes was discussed. This way of grouping carbene complexes, although difficult to apply... 

Because of the enormous synthetic potential of molybdenum- [22] and ruthenium-based [57,806] single-component catalysts, a closer look at the scope and limitations of the most promising compounds known to date is appropriate. The systematic exploration of the synthetic possibilities offered by these new catalysts has just begun, and many new developments are to be expected in the near future [744,746,747,807]. As quick reference for the organic chemist, the most relevant chemical properties of two types of frequently used catalyst (Figure 3.46) are listed below. These carbene complexes are quite robust and well-suited to the metathesis of elaborate organic intermediates. 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

The availability of stable carbenes featuring a broad range of substituents directly adjacent to the carbene center will allow for great variation of their electronic properties to be achieved and therefore catalytic activities of the resulting carbene complexes should be readily tuned. 

The Au-Au distances are 3.1882(1) A and they are considered to be responsible for the emission band that appears at 460 nm at room temperature. When the anion in the carbene complex is BF4, the structure is similar although the Au-Au distances are substantially longer (3.4615(2) A). The different distance leads to a different emission band that is blue-shifted. This indicates a greater orbital interaction in the former, consistent with its shorter Au-Au distance. Nevertheless, the behavior in solution is similar for both. Thus, at room temperature in solution they lose their emissive properties but they recover them in frozen solutions at 77 K. Interestingly, the emission differs in color, depending on the solvent, ranging from orange (acetone) to blue (pyridine), which would seem to result from the self-association... 

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