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

The synthesis of a diverse series of hydride complexes of aluminium coordinated by N,N'-chelating ligands is reported. Reaction of [{2,6-(i)Pr2C6H3}NC(Me)CHC(Me)N(H)CH2CH2NMe2] with either LiAlH4 or Me3N·AlH3 allows isolation of the corresponding five-coordinate aluminium dihydride [κ(3)-{(2,6-(i)Pr2C6H3)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 [{κ(2)-(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-4b2) 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-4b2 remaining intact in solution. An unusual reaction in which the aluminium dihydride, [κ(2)-{(2,6-Me2C6H3NHCH2)2CH}AlH2], promotes the P-C bond cleavage of Ph3PCH2 is also reported.