Diiminopyrrolide Copper Complexes: Synthesis, Structures, and rac-Lactide Polymerization Activity

Abstract

and Nbenzyl (L9, R = H) were reacted with Cu(OMe) 2 and pyridylmethanol (R 1 OH) or dimethylaminoethanol (R 2 OH) to yield the corresponding dinuclear complexes L 2 Cu 2 (μ-OR) 2 . Reactions in the absence of chelating R 1 OH or R 2 OH only yielded homoleptic L 2 Cu or L 2 Cu 2 . Complexes with R 2 OH were obtained with all ligands but L4, complexes with R 1 OH for all ligands but L3, L4, and L6. All complexes but those with L7 displayed dinuclear crystal structures with pentacoordinated copper centers and bridging alkoxy groups. The alkoxy group and the pyrrolide ligand were consistently found in equatorial positions. Imine, pyridine, or amine occupied the axial position. L7 coordinated both imino groups to copper, yielding a distorted-octahedral coordination geometry. All complexes containing R 1 OH, with the exception of L7, showed an isotactic bias in the polymerization of rac-lactide, with a maximum P m value of 0.7. Complexes containing R 2 OH provided atactic PLA or, in the case of L3 and L6, heterotactic PLA. Reduced stereocontrol in monoiminopyrrolide copper complexes and lack of stereocontrol with octahedrally coordinated L7 indicate that the catalytic site needs to adapt to chain-end chirality and is participating in enantiomer selection.

Catalytic-Site-Mediated Chain-End Control in the Polymerization of rac-Lactide with Copper Iminopyrrolide Complexes

“… a Conditions: C 6 D 6 , RT, [lactide] = 200 mM, [L 2 Cu 2 (OR) 2 ] = 2 mM. The time of final conversion should not be considered a measure of activity but indicates after what time the reaction was quenched. b M n and M w determined by size exclusion chromatography vs polystyrene standards, with a Mark–Houwink correction factor of 0.58. c M n expected if one alkoxide per catalyst dimer initiates polymerization, calculated from [lactide]/[cat]·conversion· M lactide + M ROH d Number of chains per catalyst dimer, calculated from the ratio of expected and obtained polymer molecular weight. e P m determined from decoupled 1 H NMR by P m = 1 – 2· I 1 /( I 1 + I 2 ), with I 1 = 5.20–5.25 ppm ( rmr , mmr / rmm ), I 2 = 5.13–5.20 ppm ( mmr / rmm , mmm , mrm ). f Two experiments. g Taken from ref . h Taken from ref . …”

“…Previously obtained diimino- and monoiminopyrrolide complexes LCu­(OR) typically form dinuclear copper complexes with a square-pyramidal coordination geometry around copper. The pyrrolide nitrogen and the bridging alkoxides were always found in the equatorial plane, while either the pyridyl or the imino group occupied the axial position. ,, Complexes 2 – 5 follow the same structural pattern (Figure ): Despite τ values up to 0.7, the coordination geometry around copper is best described as square-pyramidal with two short Cu–N distances, two short Cu–O distances, and one elongated Cu–N distance to the ligand in the apical position (Table ). Complexes 2 – 4 , as well as 6 , display the imino group in the apical position.…”

“…The N- dipp complex, 5 , coordinates the pyridyl group in the apical position instead of the imine (Figure , Table ). While it is tempting to ascribe this to the increased steric bulk of the diip substituent, coordination of the pyridyl group in the apical position was also favored upon reducing the steric bulk from N- CH­(Me)­Ph in 6 to N -CH 2 Ph in 7 . , In general, bond distances and angles do not show any clear dependence on the bulk of the N-imino substituent. In concurrence with earlier findings, iminopyrrolide complexes 2 – 5 thus show an invariant structural motif (square-pyramidal coordination with the anionic ligands in equatorial positions) combined with a flexibility of the remaining coordination geometry.…”

“… a Two independent molecules in the asymmetric unit. b Taken from ref . c Taken from ref . d Averaged value of the observed disorder. …”

“…Mid- to late d n -transition metal complexes, on the other hand, have been sparingly studied. Very few studies investigated the performance of Cr, Mn, − or Co systems. ,, Next to iron, ,− copper complexes received the highest attention. − We have reported that copper diiminopyrrolide complex 1 polymerizes rac -lactide with an isoselectivity of P m = 0.7 at room temperature (Scheme ). Isoselective stereocontrol was unprecedented for copper complexes.…”