Reactivity of Me3M (M=Al, Ga, In) with benzylamine to form the dimers, [Me2MN(H)CH2Ph]2, and the hexamers, [MeAlNCH2Ph]6 and [MeGaNCH2Ph]6: molecular structures of trans-[Me2MN(H)CH2Ph]2 and [MeAlNCH2Ph]6