Reactions of Dimeric Aluminium Hydride Compounds Containing Bidentate Dianionic Pyrrolyl Ligands and Their Applications in Ring‐Opening Polymerization of ϵ‐Caprolactone

A series of dimeric aluminium compounds containing substituted bidentate dianionic pyrrolyl ligands have been synthesized and their reactivity and application in the ring-opening polymerization of ϵ-caprolactone have been studied. The reactions of [{C4H3N(2-CH2NtBu)}AlH]2 (1) with 2 equiv. of 1-indanone and 9-fluorenone in dichloromethane generated [{C4H3N(2-CH2NtBu)}Al(μ-OC9H9)]2 (2) and [{C4H3N(2-CH2NtBu)}Al{μ-OCH(C12H8)}]2 (3), respectively, by hydroalumination. Similarly, the reactions of 1 with 2 equiv. of 2-cyclohexen-1-one, 1-(2,4,6-trimethylphenyl)-1-ethanone, benzophenone, and 1,1-diphenylacetone in dichloromethane afforded NtBu-bridged dialuminium compounds 4–7, [{C4H3N(2-CH2NtBu)}Al(OR)]2 [4, R = C6H9; 5, R = CH(Me)(C6H2-2,4,6-Me3); 6, R = CHPh2; 7, R = CH(Me)(CHPh2)] by insertion. A similar insertion occurred when 1 was treated with 2 equiv. of 2,4-pentandione and dibenzoylmethane in dichloromethane to yield NtBu-bridged dioxylate aluminium dimeric compounds 8 and 9, respectively. The Al atoms in compounds 2 and 4–7 possess a distorted tetrahedral geometry whereas the Al atoms in 8 and 9 have a square-pyramidal environment. All the compounds have been well characterized by NMR spectroscopy and compounds 2 and 4–9 in the solid state were subjected to X-ray diffraction analysis. A study of the polymerization of ϵ-caprolactone revealed that the activity of the Al complexes is largely reliant on the steric nature of the substituents of their alkoxide groups.