Synthesis and protonolysis of neutral aluminum dihydride compounds stabilized by tridentate-substituted pyrrolyl ligands: Synthesis, structural characterization and ring-opening polymerization of ε-caprolactone

A series of aluminum compounds containing tridentate pyrrolyl ligands were obtained from related aluminum dihydride compounds via protonolysis. Treatment of tetranuclear aluminum compound [C₄H₂N{2,5-(CH ₂NMe₂)₂}Al₂H₅] ₂ (1) with two equivalents of [C₄H₃N{2,5- (CH₂NMe₂)₂}] in methylene chloride at 0 °C led to the formation of [C₄H₂N{2,5-(CH₂NMe ₂)₂}]AlH₂ (2). Similarly, when the deuterated aluminum compound 1D was used, the corresponding aluminum compound [C ₄H₂N{2,5-(CH₂NMe₂) ₂}]AlD₂ (2D) could be isolated. The reaction of 2 with one or two equivalents of phenylethyne, triphenylmethanethiol, 2,6- diisopropylaniline, or triphenylsilanol generated mononuclear aluminum compounds [[C₄H₂N{2,5-(CH₂NMe₂) ₂}]AlRR′ (3, R = -CCPh, R′ = H; 4, R = R′ = -CCPh; 5, R = -SCPh₃, R′ = H; 6, R = R′ = -SCPh₃; 7, R = -NH(2,6-ⁱPr₂Ph), R′ = H; 8, R = R′ = -NH(2,6-ⁱPr₂Ph); 9, R = -OSiPh₃, R′ = H; 10, R = R′ = -OSiPh₃). Related Al-D compounds of 3, 5, 7 and 9 were also synthesized and corresponding IR spectroscopic data well matched in comparison of the stretching frequencies of Al-H and Al-D. The molecular structures of 2D, 4, 5, 5D, 7, and 10 have been determined by X-ray crystallography. Compounds 2, 5, and 7 initiated the ring-opening polymerization of ε-caprolactone and produced high-molecular weight of poly-ε-caprolactone.