New polynuclear aluminium oxoalkoxides. Crystal structures of [Al5(μ5-O)(μ-OBui)8(OBui)5] and [Al8(μ4-O)2(μ-OH)2(μ-OBui)10(OBui)8]

Abstract

Treatment of [Al₄(μ₄-O)(OBuⁱ)₁₀(BuⁱOH)] 1 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in boiling toluene produced [Al₅(μ₅-O)(μ-OBuⁱ)₈(OBuⁱ)₅] 2, whereas removal of BuⁱOH from 1 by azeotropic fractional distillation with toluene gave [Al₈(μ₄-O)₂(μ-OH)₂(μ-OBuⁱ)₁₀(OBuⁱ)₈] 3. X-Ray crystallography revealed that in 2 the μ₅-oxo atom is bonded to one apical six-coordinated Al and four basal five-coordinated Al atoms, whilst in 3 the centrosymmetrical molecule comprises two tetranuclear fragments each containing a μ₄-oxo atom bonded to five-coordinated Al atoms and bridged by two μ-hydroxo ligands. Alcohol exchange of 2 with BuⁿOH gave [Al₅(μ₅-O)(μ-OBuⁱ)₈(OBuⁱ)₄(OBuⁿ)] 4 and of 1 with CF₃CH₂OH gave [Al₄(μ₄-O)(OBuⁱ)₆(OCH₂CF₃)₄(CF₃CH₂OH)] 6. Other compounds synthesised were [Al₄(OCH₂CF₃)₁₂] 7, [Al₄(μ₄-O)(OCH₂CF₃)₁₀(CF₃CH₂OH)] 8, [Al{OCH(CF₃)₂}₃(THF)] 9, and [Al{OCH(CF₃)₂}₃(Et₂O)] 10. Using ¹⁷O enriched (10%) water, samples of 1–4, 6 and 8 labelled at the oxo atom were prepared and their ¹⁷O chemical shifts measured in addition to ¹H and ²⁷Al NMR chemical shifts. Possible reaction pathways for the formation of 2 and 3 from 1 are discussed. Structural aspects of aluminium oxoalkoxides are discussed.