Heteroscorpionate aluminium complexes as chiral building blocks to engineer helical architectures

Abstract

Treatment of heteroscorpionate ligand precursors pbptamH, pbpamH, sbpamH and (S)-mbpamH with 2 equivalents of AlR₃ (R = Et, Me) yielded the corresponding binuclear organoaluminium complexes [Al₂R₄(μ-pbptam)] (R = Me 1, Et 2), [Al₂R₄(μ-pbpam)] (R = Me 3, Et 4), [Al₂R₄(μ-sbpam)] (R = Me 5, Et 6) and [Al₂R₄{μ-(S)-mbpam}] (R = Me 7, Et 8). These complexes have helical chirality due to the demands of the fixed pyrazole rings. The stereoisomerism and the self-assembly processes of these helicates have been studied in some detail in solution by NMR and in the solid state by X-ray diffraction. Mixtures of M- and P-handed enantiomers and mixtures of M- and P-handed diastereoisomers were obtained when achiral (1–4) and chiral (5–8) heteroscorpionate ligands were used as scaffolds, respectively. Re-crystallization from hexane allowed us to obtain M-homochiral architectures in the solid state for the helical complexes [Al₂Et₄(μ-sbpam)] (6) and [Al₂Et₄{μ-(S)-mbpam}] (8). The reaction of heteroscorpionate ligands with 3 equivalents of AlR₃ (R = Me, Et) led to the corresponding trinuclear organoaluminium complexes [Al₃R₇(μ₃-pbptam)] (R = Me 9, Et 10), [Al₃R₇(μ₃-pbpam)] (R = Me 11, Et 12), [Al₃R₇(μ₃-sbpam)] (R = Me 13, Et 14) and [Al₃R₇{μ₃-(S)-mbpam}] (R = Me 15, Et 16). The extra AlR₃ molecule contributes to the formation of a diastereomeric excess of the PS helicate for complexes 15 and 16. X-ray determination of some of the helical complexes allowed us to witness a versatile and efficient self-assembly process of the building blocks (heteroscorpionate aluminium complexes) directed by noncovalent intermolecular CH–π interactions. The structures of these complexes have been determined by spectroscopic methods and the X-ray crystal structures of 2, 6, 8, and 16 have also been established. Concentration-dependent ¹H pulsed field-gradient spin echo (PFGSE) NMR experiments provided evidence for the self-assembly of the single molecular species of complex 2 in solution. The degree of aggregation was calculated for complex 2, with the average number of units constituting the aggregate (N) estimated to be a maximum of 4 molecules in solution before reaching the solid state.