Metallacumulenes

Metallacyclic complexes containing two molecules of diyne per MCp2 group have also been isolated, that with titanium containing 2,4-alkynyl substituents (225) while with zirconium, the unusual seven-membered metallacumulene structure 226 is adopted, which has only one alkynyl substituent. The bi- and tricyclic 2/2 complexes 227 and 228 have so far been obtained only from reactions of TiCp2 with PhC=CC=CPh.30 ... 

Detailed studies of these systems have enabled a rationalization of the observed chemistry to be made, involving as possible intermediates (a) the ful-vene complexes MHCp CsMe4(CH2) or MH2Cp CsMe3(CH2)2, (b) the r -diyne complexes M( 7 -RC2C=CR)Cp 2, and (c) the j" -metallacumulenes, M(/) -RCCCCR)Cp 2. i - ... 

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

Despite this seminal work, it has only been recently that these metallacumulenes have really emerged as useful catalyst precursors or catalyst intermediates in organic synthesis. In particular, significant advances have been made in the field of alkene metathesis and propargylation reactions using mainly ruthenium complexes. A survey of this chemistry is presented in the following section. 

Special issue devoted to the chemistry of vinylidene, allenylidene and metallacumulene complexes (2004) Coord Chem Rev 248 1531-1716... 

Abstract In this chapter, aspects related to the synthesis, structure, and reactivity of metallacumulenes [M]=C(=C) =CR R ( = 1, 3, 5) in which one or both substituents R /R are heteroatomic 7t-donor groups (NR2, OR, SR, SeR) are discussed. The coordination chemistry of related heteroatom-cumulated ligands, i.e., phosphonioacetylides and tricarbon monoxide, is also presented. 

As already commented in the introduction of this article, theoretical calculations have suggested an increased stability of odd-chain metallacumulenes bearing Ti-donor substituents when compared to their all-carbon substituted counterparts [16]. However, despite this theoretical prediction, synthetic efforts directed to the preparation of heteroatom-substituted metallacumulenes longer than allenylidenes have been scarce till now. All the reported examples belong to the family of amino-substituted Group 6 cumulenylidenes. In this section a survey of their chemistry is presented. 

On the basis of the resonance forms depicted in Eig. 11, nucleophilic attacks on complexes 72 are expected to occur at the metal-bound C at Cy or at the terminal Ce atom of the carbon chain where positive charges are located along the different structures. As already discussed in our preceding chapter, this prediction is in complete accord with the extensive theoretical calculations performed in metallacumulenes which, regardless of the substituents of the chain and the metal fragment... 

The number of pentatetraenylidene complexes (x= 5) reported in the literature is even less. All in all, 11 complexes have been described. Complexes with an even larger number of carbon atoms in the chain are unknown. Therefore, cumulenylidene complexes with more than three carbon atoms in the chain are still rather elusive classes of metallacumulenes [1] and their involvement in a catalytic process has not yet been observed. 

Metallacumulenes with alkylidynes, 5, 458-459 in molybdenum carbonyl complexes, 5, 401 Metallacycles with bis-Gp Ti(IV), 4, 563 with nickel... 

By the early 1990s, the investigation into the chemistry of the metallacumulenes with rhodium and iridium as the metal centers also provided the synthetic methodology which allowed filling a gap in the systematics of transition metal... 

H. Fischer, and N. Szesni, 7i-Donor-Substituted Metallacumulenes of Chromium and Tungsten, Coord. Chem. Rev. 248, 1659-1677 (2004). 

The allenylidene [M=C=C=CR R ] unit represents a member of the class of carbon-rich unsaturated hydrocarbyl fragments, and it is capable of binding one to four metal centers. In sharp contrast to the rapidly developing chemistry of mononuclear allenylidene complexes, the scarcity of bi- and polynuclear species of this type has hindered the development of an extensive chemistry. Selected examples from the allenylidene literature are cited in this article owing to the close structural analogy with allenyl derivatives. The reader is referred to a review article by Bruce covering the relevant literature of this metallacumulene up to 1989 (la). 

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