Linkage Isomerism in Dinuclear Al and Ga Organometallic Complexes: Structural and Reactivity Consequences

The preparation of dinuclear aluminum and gallium complexes with controlled M center dot center dot center dot M proximity (intermetal separation of 3.28-4.35 angstrom) was achieved through direct chelation of the multidentate bis(diphenyl- (1) and bis(diisopropylphosphinoyl)-1,8-diamino naphthalene (2) ligands. The solid-state structure of the dinuclear complexes obtained with 1 suggests that the geometric constraint imposed by the ligand backbone is responsible for the in situ conversion in solution of the kinetic to the thermodynamic isomers. The complexes derived from 1 represent rare examples of linkage isomer couples stable enough to allow their characterization by X-ray diffraction studies, while for 2 only the thermodynamic isomers were stable enough to be characterized by this technique. Kinetic studies revealed a first-order reaction rate for the isomerization process, and an intramolecular mechanism was rationalized in terms of the effects of temperature, solvent, and structural variation. Preliminary catalytic screening on an isomer couple derived from 1 showed that both the kinetic and thermodynamic species are active for the ROP of epsilon-caprolactone.