Five-membered metallacycloalkynes formed from group 4 metals and [n]cumulene (n = 3,5) ligands

In this account [3]- and [5]cumulene complexes of group 4 metallocenes that form five-membered metallacycles are described. These complexes have a “triple bond” despite their five-membered ring structure, showing that they are regarded as 1-metallacyclopent-3-ynes. The molecular structures show their strained alkyne character. These complexes react with transition metals to form alkyne-coordinated bimetallic complexes. They also receive electrophilic attack by protons and boranes resulting in M–C bond cleavage. When a [3]cumulene couples with an alkyne on the metal, the reaction produces seven-membered metallacycloalkynes that have a strained structure showing an interaction between the “triple bond” and the metal center. Hexapentaenes, [5]cumulenes, form conjugated 1-metallacyclopent-3-ynes. The aryl-substituted [5]cumulene complex was reduced by alkali metal to give dianionic species that reacted with protons to give 1-metallacyclopent-3-ene, a cycloalkene, and with iodomethane to give 1-metallacyclopenta-2,3-diene, a cycloallene. The hexapentaene with tert-butyl groups reacts with zirconocene to form an η2-π-coordinated complex in the presence of trimethylphosphine, although it gave a 1-metallacyclopent-3-yne in the absence of the phosphine. The former was transformed into the latter by addition of a phosphine, and vice versa by removing the phosphine, showing a “haptotropic” shift.