Reactivity of 1-aza-4-oxo-1,3-butadienes (a-iminoketones) towards triorganoaluminum reagents I. X-ray crystal structures of [(AlMe3)2{s,s-N,O-(MeN=C(Ph)C(Ph)=O}],with a N,O-bridge bonded a-iminoketone, and of Me2Al(t-Bu)N= CHC(Me)2OAlMe3, a carbonyl alkylated product which is O-coordinated to AlMe3