Aluminium alkyl and aryloxide complexes of pyrazine and bipyridines: synthesis and structure

Abstract

The reaction of AlMe₃ and [(^tBu)₂Al(μ-OPh)]₂ with pyrazine (pyz), 4,4′-bipyridine (4-4′-bipy), 1,2-bis(4-pyridyl)ethane (bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) yields (Me₃Al)₂(μ-pyz) (1), (Me₃Al)₂(μ-4,4′-bipy) (2), (Me₃Al)₂(μ-bpetha) (3), (Me₃Al)₂(μ-bipethe) (4), Al(^tBu)₂(OPh)(pyz) (5), [(^tBu)₂Al(OPh)]₂(μ-4,4-bipy) (6a), [(^tBu)₂Al(OPh)]₂(μ-bpetha) (7a), [(^tBu)₂Al(OPh)]₂(μ-bipethe) (8a). Compounds 1–4, 6a and 7a have been confirmed by X-ray crystallography. In solution compounds 1–4 undergo a rapid ligand-dissociation equilibrium resulting in a time-average spectrum in the ¹H NMR. In contrast, the solution equilibria for compounds 5–8a are sufficiently slow such that the mono-aluminium compounds may be observed by ¹H NMR spectroscopy: Al(^tBu)₂(OPh)(4,4-bipy) (6b), Al(^tBu)₂(OPh)(bpetha) (7b) and Al(^tBu)₂(OPh)(bpethe) (8b). The inability to isolate [(^tBu)₂Al(OPh)]₂(μ-pyz) and the relative stability of each complex is discussed with respect to the steric interactions across the bridging ligand (L) and the electronic effect on one Lewis acid–base interaction by the second Lewis acid–base interaction on the same ligand.