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 η²-coordinated complexes in the presence of neutral ligands such as trimethylphosphine and <i>tert</i>-butyl isocyanide. Interconversion between the five-membered structure and the η²-complex was observed. Investigation of [5]­cumulene derivatives of various cycloalkylidene moieties indicated that the η²-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 η²-coordination. In sharp contrast, the η²-coordinated complex was favored for a [5]­cumulene with 2,2,7,7-tetramethylcycloheptylidene groups in the presence of PMe₃. 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