Synthesis and Structural Characterization of (1,4-Dihydropyrid-1-yl)aluminum Complexes

The reaction between LiAlH(4) and pyridine, 4-methylpyridine, or 3,5-dimethylpyridine results in hydride transfer to the pyridine ring to give tetrakis(pyridine)lithium tetrakis(1,4-dihydropyrid-1-yl)aluminate(III), 1, tetrakis(4-methylpyridine)lithium tetrakis(1,4-dihydro-4-methylpyrid-1-yl)aluminate(III), 2, or tetrakis(3,5-dimethylpyridine)lithium tetrakis(3,5-dimethyl-1,4-dihydropyrid-1-yl)aluminate(III), 3, respectively. We claim that 1, instead of lithium tetrakis(1,4-dihydropyrid-1-yl)aluminate(III), is the compound which is known as Lansbury's reagent. Treatment of trimethylamine-alane, AlH(3).NMe(3), with pyridine yields tris(1,4-dihydropyrid-1-yl)(pyridine)aluminum, 4. It could be shown that AlH(3).NMe(3) initially reduces pyridine to 1,2-dihydropyridine, which is subsequently converted into its 1,4-isomer. The X-ray crystal structures of 1-4 were determined. While the differences between Al-N distances within each of the compounds 1-3 are not significant, 4 exhibits two distinctly different types of Al-N bonds, the dative bond between Al and N(pyridine), d(Al-N) = 1.959(2) Å, and the covalent bonds between Al and N(1,4-dihydropyrid-1-yl), d(av)(Al-N) = 1.833 Å.