Dialane with a Redox-Active Bis-amido Ligand: Unique Reactivity towards Alkynes

The treatment of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) with one equivalent of AlCl3 and three equivalents of sodium in toluene at 110 degrees C produced a stable dialane, (dpp-bian) Al-Al(dpp-bian) (1). The reaction of compound 1 with pyridine gave Lewis-acid-base adduct (dpp-bian)(Py)Al-Al(Py) (dpp-bian) (2). Acetylene and phenylacetylene reacted with compound 1 to give cycloaddition products [dpp-bian((RR2)-R-1)] Al-Al[((RR1)-R-2)dpp-bian] (3:R-1=R-2=CH; 4:R-1=CH, R-2=CPh). These addition reactions occur across Al-N-C moieties and result in the formation of new C-C and C-Al bonds. At elevated temperatures, compound 4 rearranges into complex 5, which consists of a radical-anionic dpp-bian ligand and two bridging alken-1,2-diyl moieties, (dpp-bian) Al(HCCPh)(2)Al(dpp-bian). This transformation is accompanied by cleavage of the dpp-bian-ligand-alkyne C-C bond, as well as of the Al-Al bond. In contrast to its analogous gallium complex, compound 1 is reactive towards internal alkynes. In the reaction of compound 1 with PhC CMe, besides symmetrical addition product [dpp-bian((RR2)-R-1)] Al-Al[((RR1)-R-2)dpp-bian] (R-1=CMe, R-2=CPh; 6), mono-adduct [dpp-bian((RR2)-R-1)] Al-Al(dppbian) (R-1=CMe, R-2=CPh; 7) was also isolated. Complexes 1-7 were characterized by IR, H-1 NMR (1-4), and electronic absorption spectroscopy (3-5); the molecular structures of compounds 1-7 were determined by single-crystal X-ray diffraction.