Carbenes coordination chemistry

The Group VI organometallic chemistry is mainly characterized by the occurrence of N- and C-coordination and carbene complex-formation, as well as by some unique cases of Se- (Te-) coordination, ring opening and deselenation. The Group VII organometallic chemistry is known for the carbene and chelate structures of the derivatized thiazoles. 

Organoiron chemistry offers the C-coordination mode, carbene series, ring opening and detelluration. Organoruthenium and especially organoosmium species are sometimes peculiar along with the classical cases of N-coordination and carbene formation, C,N- and S,N-cases can be traced. 

Reports on the coordination chemistry of A-heterocyclic carbene-containing metal complexes started to appear as long ago as 1968,50,51 while metal-free carbenes have only been isolated very recently.52 In view of the fact that the general chemistry and applications of organic carbenes and related metal complexes in chemical synthesis have been reviewed several times recently,53-57 examples limited only to those carbene complexes with silver(i) have been discussed. Nevertheless, it is worth mentioning that the developments in silver(i) A-heterocyclic carbenes have also been reviewed recently by Lin.5... 

In view of the versatility of A-heterocyclic carbenes as ligands and their structural diversity in silver(i) coordination chemistry, an extension of the work to ligands with two or more carbene moieties was reported. A dinuclear silver(i) complex 52 (Figure 21) with an o-phenylenedimethylene-bridged bis(carbene) ligand has been synthesized in 66% yield from silver(i) oxide and the bis(imidazolium) salt.88 The reaction to synthesize 52 has to be carried out in... 

Abstract A-Heterocyclic carbenes (NHCs) have developed into an important class of ligands in transition metal coordination chemistry. They have been employed successfully as spectator ligands in various catalytically active metal complexes and as organocatalysts. In this chapter we present some important synthetic methods for the preparation of various NHCs and their metal complexes. 

Among the polydentate carbene ligands, particular interest has recently been placed on cyclic polycarbenes. ImidazoUum precursors like 23 [89] or 24 [90, 91], which upon C2 deprotonation would lead to tetradentate or even hexadentate double-pincer NHC ligands, have been prepared. Their interesting coordination chemistry will be discussed in Sect. 4. Finally, Arnold et al. developed and reviewed NHC ligands which are functionalized with additional anionic (alkoxide or amido) donor groups [92]. 

Alternatively, NHC ligands can also be transferred from triethylborane-carbene adducts [103] or complexes of the type [M(NHC)(CO)5] (M = Cr, Mo, W) [104], but these procedures are limited to special cases and are of less importance. The coordination chemistry of silver NHC complexes [105] and the advantages and applications of the Ag20 method [106] have recently been reviewed. 

NHCs have become ligands almost as common as phosphines. This is widely exemplified in a book edited by Nolan.23 We will therefore limit this overview of carbenes as ligands to unusual applications and significant publications regarding their coordination chemistry. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

Similar to the abovementioned silver nhc coordination compounds, carbene chemistry has also been dominant in the field of gold organometallic chemistry. Noteworthy examples include a Au(PPh3)-compound derived from tetraaminoallene, that can be rationalised in terms of a dicarbene with ylide character and which, owing to the electron-rich character of the central carbon atom, offers the potential for dimetallation products.108 Non-activated allenes and alkynes have been found by Lavallo to be readily aminated by cationic carbene gold complexes.109 For this purpose, a 2,6-diisopropylphenyl functionalized cyclic alkylaminocarbene gold(I) complex... 

M. F. Lappert, The Coordination Chemistry of Bivalent Group IV Donors Nucleophilic-Carbene and Dialkylstannylene Complexes, J. Organomet. Chem. 100, 139-159 (1975). 

The novel /V-hcicrocyclic carbene (NHC) l,3-dicyclohexyl-l,3-diazepan-2-ylidene 80 and its 5,6-dioxolane derivative 81 were synthesised and their coordination chemistry with Rh(I), Ir(I), and Pt(0) explored. The coordinated carbene ligands display extremely large NCN bond angles in crystal structures <0704800>. The cyclic urea 82 was synthesised and fluorescent properties studied in a search for new DNA/RNA bioprobes <07JOC102>. 

The description of the steric properties of phosphanes using the Tolman cone angle [113] proved to be an excellent concept capable of explaining many phenomena in the coordination chemistry of phosphanes and their applications, especially in homogenous catalysis. That there is a steric influence connected with NHC was noticed very early, in fact it was thought that the steric hindrance introduced by the N-mesityl substituents was a contributing factor in the isolation of the first stable carbene in 1991 as opposed to dimerisation to the known tetraaminoethylenes [1],... 

Phosphinidenes RP (with a sextet of electrons at the P-atom) are isolobal with carbenes R2C, and their dimers (RP)2, with a P=P double bond, are the counterparts of the carbene dimers, the olefins R2C=CR2. The coordination chemistry of the two phosphorus species is far less developed, but the number of stable complexes is growing rapidly, as summarized in review articles. ... 

A widely used precursor for Rh(I) and Ir(I) in coordination chemistry is the [MC1(C0D)]2 dimer (120). In the presence of either free NHC or in situ generated carbene from the alcohol adduct or the combination imidazolium salt and base, the dimer was cleaved giving rise to [(NHC)MCl(COD)] (121) (Scheme 20). The presence of labile COD allowed for further modification of the metal coordination sphere. For example, under CO atmosphere the COD ligand was rapidly removed, leaving two CO binding the metal center. The... 

---