Synthesis and protonolysis of neutral aluminum dihydride compounds stabilized by tridentate-substituted pyrrolyl ligands: Synthesis, structural characterization and ring-opening polymerization of ɛ-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-(CH2NMe2)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-(CH2NMe2)2}]AlH2 (2). Similarly, when the deuterated aluminum compound 1D was used, the corresponding aluminum compound [C4H2N{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 ɛ-caprolactone and produced high-molecular weight of poly-ɛ-caprolactone.

Graphical abstractA series of aluminum compounds were obtained by reacting [C4H2N{2,5-(CH2NMe2)2}Al2H5]2 with small organic molecules via deprotonation or hydroalumination and their activities toward the ring-opening polymerization of ɛ-caprolactone are studied.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Aluminum dihydride compound, [C4H2N{2,5-(CH2NMe2)2}]AlH2 (2) can be obtained in high yield from [C4H2N{2,5-(CH2NMe2)2}Al2H5]2. ► Reactions of aluminum dihydride compound with one or two equivalents of H-X-R generated mononuclear aluminum compounds. ► Compounds initiate ring-opening polymerization of ɛ-caprolactone and produced high-molecular weight of poly-ɛ-caprolactone.