Indenyl Compounds with Constrained Hapticity: The Effect of Strong Intramolecular Coordination

Abstract

A series of cyclopentadienyl and indenyl molybdenum(II) compounds with intramolecularly coordinated pyridine arms, including scorpionate-like species bearing two irreversibly coordinated arms on the indenyl core, were synthesized and characterized. All presented structural types were confirmed by X-ray diffraction analysis. Owing to the strong nucleophilicity of pyridine, the intramolecular interaction was found to be considerably stronger than that in analogous species bearing tertiary amines in the side chain. Although the starting compounds for the syntheses were isostructural, the reaction outcomes differed considerably. The cyclopentadienyl precursor gave a pentacoordinate η5:κN-compound, whereas the indenyl analogue produced a hexacoordinate species with the unprecedented η3:κN-coordination mode of the indenyl ligand and thus represents an unusual example of the so-called indenyl effect. The unusually high stability of the η3:κN-coordination compounds toward haptotropic rearrangement was clarified by theoretical calculations. As the strong intramolecular interaction prevented rotation of the indenyl moiety, it could not reach the conformation suitable for the η3 to η5 rearrangement. As a result, the low hapticity was effectively locked.