Synthesis and characterisation of alkylaluminium benzimidazolates and their use in the ring-opening polymerisation of ε-caprolactone

The stoichiometric reactions of 2-(benzimidazol-2-yl)-6-methylpyridine (L1) or 8-(benzimidazol-2-yl)quinaldine (L2) with trialkylaluminium reagents R₃Al (R = Me, Et and iBu) afforded the corresponding dialkylaluminium benzimidazolate complexes R₂AlL (L1, R = Me (1), Et (2), iBu (3); L2 R = Me (4), Et (5), iBu (6)). Treatment of L1 with one or two equivalents of Et₂AlCl led to the adducts EtAl(L1)₂·AlEtCl₂ (7) or Et₂AlL1·AlEtCl₂ (8), respectively. Complex 7 was also available via treatment of 8 with one equivalent of L1. Reaction of L1 with two equivalents of AlR₃ (R = Me or Et) afforded R₂AlL1·AlR₃ (R = Me, 9; R = Et, 10), which were also formed when 1 or 2 were reacted with AlR₃. Reaction of L2 with two equivalents of AlR₃ (R = Me or Et) gave the complexes R₂AlL2·AlR₃ (R = Me, 11; R = Et, 12), which were also formed in the stoichiometric reaction of 4 or 5 with AlR₃ (R = Me or Et). Screening of these complexes in the presence of BnOH, for the ring-open polymerisation of e-caprolactone, revealed appreciable activities. Only the aluminium compounds ligated by 2-(benzimidazol-2-yl)-6-methylpyridine maintained high activity in the absence of BnOH. In all cases, polymers with bi- or multi-modal characteristics were produced.