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 tert-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.