ReviewRecent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type
Introduction
The activation of simple organic molecules by a transition metal constitutes an area of research of great importance. The applications of carbenes as active or auxiliary ligands in organic synthesis and catalysis are mostly focused on monocarbene systems. Most of these Fischer monocarbene complex applications center around the reactivity of the metal–carbon double bond or the carbene-bonded heteroatom X (A in Fig. 1), or on modifications of vinyl- (B) or 1-alkynylcarbene ligands (C) for organic synthesis [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].
In this review the focus is mainly on the synthesis, structures, properties and possible applications of bis- and multicarbene, and multimetal carbene complexes; and those areas of this carbene chemistry that have not been reviewed recently. A comprehensive review by Sierra in 2000 accounts for most of the results before 1999 [11]. The first section will deal especially with dinuclear biscarbene complexes and methods to prepare such compounds, whereas the following section covers the area of Fischer carbene complexes containing metal fragments as carbene substituents. Some attention is given to spectral data in support of the carbene ligand character, while specific structural aspects are emphasized. Theoretical calculations in the past have focused mainly on the donor/acceptor nature of the carbene substituents of simple monocarbene complexes [12], [13], [14], [15], [16], [17] or the steric and electronic effects of the heteroatom on the carbene ligand [18]. Reaction mechanisms of thermal and photochemical transformations have also been calculated by DFT methods [19], [20], [21], [22], [23], but reports on the modeling of multimetal carbene complexes are rare [17].
The enormous interest in nitrogen heterocyclic carbene (NHC) ligands, their complexes and applications resulted in this class of Fischer carbene complexes having branched off from mainstream Fischer carbene chemistry and obtaining an own identity. Discrimination between NHC ligands and aminocarbene complexes referred to in this article will be based on steric properties and the criteria that bisaminocarbene ligands must be acyclic [24]. Many subsequent review articles have appeared in literature on NHC chemistry and will not be included in this article. Suffice to say that the inherent stability of NHC complexes make them ideal candidates for the synthesis of multicarbene metal complexes. Stable singlet carbenes [25] and the capture of free hydroxycarbenes [26] are areas of rapidly developing chemistry, but only metal complexes of these carbenes will be included here. Biscarbene cumulene complexes were not considered as they do not represent Fischer carbene complexes [27]. In addition bridging carbene or carbyne ligands were also not considered.
Section snippets
Multicarbene metal complexes
In this section the authors have divided the more recent examples of Fischer multicarbene complexes into different classes as shown in Fig. 2 for biscarbene complexes. Reference will be made to compounds that initiated the research and will be extended to compounds with more than two carbene ligands.
In complexes consisting of a single metal only, two or more Fischer carbene ligands may be coordinated as independent carbene ligands (I(a)) or form part of a chelate ring (I(b)). In complexes with
Multimetal Fischer carbene complexes
The incorporation of different transition metal moieties in complexes has been widely investigated to study the role of different metal fragments on the reactivity of ligands and the chemistry of the complexes [102], [103], [104], [105]. When applied in the area of Fischer carbene complexes of the type [M(CO)5{C(OR′)R}], the carbene properties have either been modified by introducing metal-containing substituents to further activate the carbene carbon [65], [96], [98] or the carbene ligand is
Some structural aspects
The following descriptions apply to the solid state structures of the complexes and may not apply to the structures in solution.
Concluding remarks
During the last decade few examples of carbene unit assemblies have been reported and their chemistry and properties remain to be investigated [93], [215]. One notable example is the pioneer work by Macomber et al. [216] reporting the first polymers containing multicarbene moieties as a forerunner to the evolution of the multimetal carbene field. The potential for macromolecular multicarbenes is increasing in the areas of synthesis, catalysis and materials. The application of Fischer
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