Preparation and Reactivity of Higher Metal Cumulenes Longer than Allenylidenes

Preparation and Reactivity of Higher Metal Cumulenes Longer than Allenylidenes 

Cumulenylidene complexes constitute a class of organometallic compounds in which a chain of sp-hybridized carbon atoms is terminated by a sp -hybridized carbon atom (CR2 group) at one end and by a metal-ligand fragment (Lr,M) at the other end. 

The metal-ligand fragment L M, the number of carbon atoms x, and the substituents at the terminal sp -carbon may vary considerably and, correspondingly, the properties and reactivities. The early members of the series of cumulenylidene complexes (x=l, 2, 3 carbene, vinylidene and allenylidene complexes) have established themselves as invaluable building blocks in stoichiometric synthesis and as highly potent catalyst precursors. The higher members might potentially be very useful candidates for application as one-dimensional wires and in opto-electronic devices. 

The number of known, isolated and characterized complexes depends strongly on the length of the chain and drastically decreases with the number of carbon atoms in the chain. A great number of vinylidene complexes of many metals, with different terminal substituents R and various co-ligands have been synthesized and the reactivity has been studied extensively. At present, the solid-state structure of more than 230 vinylidene complexes has been determined by X-ray structure analyses. The number of isolated allenylidene complexes is somewhat smaller. 

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. 

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