Preparation and properties of a series of structurally diverse aluminium hydrides supported by β-diketiminate and bis(amide) ligands

Abstract

The synthesis of a diverse series of hydride complexes of aluminium coordinated by N,N′-chelating ligands is reported. Reaction of [{2,6-ⁱPr₂C₆H₃}NC(Me)CHC(Me)N(H)CH₂CH₂NMe₂] with either LiAlH₄ or Me₃N·AlH₃ allows isolation of the corresponding five-coordinate aluminium dihydride [κ³-{(2,6-ⁱPr₂C₆H₃)NC(Me)CHC(Me)NCH₂CH₂NMe₂}AlH₂] (2). The latter complex demonstrates trigonal bipyramidal geometry in the solid-state. Correlation of solid and n-hexane solution infrared spectroscopy data reveals that this coordination is retained in solution. To evaluate the observed coordination geometry, the dissociation of the pendant ligand of 2 was investigated by DFT methods conducted with the M06-2X functional and a hybrid 6,31G+(d,p)/Lanl2DZ basis-set. Reaction of Me₃N·AlH₃ with both N,N′-bis(di-iso-propylphenyl)ethylenediamine and N,N′-bis(mesityl)ethylenediamine gave [{κ²-(ArNCH₂)₂}AlH(NMe₃)] (Ar = Mes, 3a; Ar = 2,6-di-iso-propylphenyl, 3b) in moderate yields. Removal of NMe₃ from 3b by heating under dynamic vacuum allowed the isolation of cis-[AlH{μ-N(Ar)CH₂CH₂N(Ar)}] (Ar = 2,6-di-iso-propylphenyl, cis-4b₂2) as a single diastereomer following crystallization. DFT studies in combination with infrared and NMR spectroscopy and single crystal X-ray diffraction data provide a weight of evidence consistent with the robust dimeric structure of cis-4b₂2 remaining intact in solution. An unusual reaction in which the aluminium dihydride, [κ²-{(2,6-Me₂C₆H₃NHCH₂)₂CH}AlH₂], promotes the P–C bond cleavage of Ph₃PCH₂ is also reported.