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.

Bifunctional Aluminum Catalysts for the Chemical Fixation of Carbon Dioxide into Cyclic Carbonates

“…The most acute angle of 94.16(10)° for 6 and 95.44(17)° for 8 is observed for N1–Al1–O1 due to the bite of the heteroscorpionate ligand. The Al–C distances (1.870(13)–2.15(1) Å), the Al–N (1.975(3)–1.970(5) Å) and Al–O distances (1.755(2)–1.747(4) Å) are similar to those previously reported. ,− …”

Synthesis of Cyclic Carbonates Catalysed by Aluminium Heteroscorpionate Complexes

“…[27,31] The Al(1)ÀN(1) bond distances of 1.9768(13) f or 12 and1 .956(4) and 1.960(4) f or 13 are similart ot hose in other aluminium pyrazolyl complexes. [27,32] Synthesis of cyclic carbonates:C omplexes 9-14 were initially screened for the conversion of styrene oxide 1a into styrene carbonate 2a at 25 8Ca nd one bar carbon dioxide pressure under solvent free conditions for 24 hu sing 5mol %o fb oth complex 9-14 and TBAB (Scheme 5). Six reactions involving the same epoxide were carriedo ut in parallel in magnetically stirred glass vials within al arge conicalf lask containing cardice pellets with ar ubber stopper pierced by ad eflated balloon.…”

Alternating Copolymerization of Epoxides and Anhydrides Catalyzed by Aluminum Complexes

“…The synthesis of aluminum complexes was performed as previously reported. ,, Solvents were predried over a sodium wire (toluene, n -hexane, THF) and distilled under nitrogen from sodium (toluene, THF) or sodium–potassium alloy ( n -hexane). Deuterated solvents were stored over activated 4 Å molecular sieves and degassed by several freeze thaw cycles.…”