Haptotropic Shift of [5]Cumulenes in Zirconocene Complexes and Effects of Steric Factors

Zirconium complexes of some [5]cumulene derivatives were studied for their variable coordination modes and haptotropic shifts. Some [5]cumulene compounds reacted with zirconocene(II) species to afford 1-zirconacyclopent-3-yne complexes that have five-membered cycloalkyne structures. Only a few [5]cumulene compounds afforded η 2 -coordinated complexes in the presence of neutral ligands such as trimethylphosphine and tert -butyl isocyanide. Interconversion between the five-membered structure and the η 2 -complex was observed. Investigation of [5]cumulene derivatives of various cycloalkylidene moieties indicated that the η 2 -complex was preferred when the [5]cumulene has bulkier substituents. A [5]cumulene with 2,2,6,6-tetramethylcyclohexylidene groups much preferred the 1-zirconacyclopent-3-yne structure to η 2 -coordination. In sharp contrast, the η 2 -coordinated complex was favored for a [5]cumulene with 2,2,7,7-tetramethylcycloheptylidene groups in the presence of PMe 3 . Small differences in steric environments caused totally different reactivity in [5]cumulene complexes. DFT calculations on the formation enthalpy were consistent with the experimental results, although that cannot fully rationalize the difference.