Aluminum Complexes Bearing Bidentate Amido–Phosphine Ligands for Ring-Opening Polymerization of ε-Caprolactone: Steric Effect on Coordination Chemistry and Reactivity

Abstract

The aluminum complexes LAlMe2 (Al1–Al5: LH = 2,6-(R1)2-4-R2-C6H2NHCH2C6H4-2-PPh2; Al1, R1 = H, R2 = H; Al2, R1 = Me, R2 = H; Al3, R1 = i Pr, R2 = H; Al4, R1 = Ph2CH, R2 = i Pr; Al5, R1 = Cl, R2 = H) have been synthesized and characterized by elemental analysis and 1H, 13C, and 31P NMR. NMR analysis in solution reveals an interesting hemilabile coordination of the soft P donor. The molecular structures of Al2–Al4 were defined by X-ray diffraction studies, showing a distorted-tetrahedral geometry around the aluminum center in all structures. Careful comparison of these crystal structures suggested that different substituents on the ligands could lead to unignorable changed coordination environments around the Al center, thus affecting their catalytic properties. In the presence of BnOH, complexes Al1–Al5 efficiently catalyzed the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with high conversions in a controlled manner, and high molecular weights (M n up to 118.6 kg mol–1) of polycaprolactones (PCLs) were readily prepared. Immortal polymerizations by Al4 having bulky Ph2CH groups were also studied with up to 20 equiv of alcohols and 2000 equiv of monomers, without sacrificing polymerization control.

Indium-Catalyzed CO2/Epoxide Copolymerization: Enhancing Reactivity with a Hemilabile Phosphine Donor

“…Analysis of 1 H NMR spectra (CDCl 3 , 25 °C) confirms that the phosphine donors in 1 and 2 are dissociated in solution. The diastereotopic methylene protons, H a and H b , of 1 and 2 appear as overlapping multiplets (Figure -IIb,c) . In contrast, the analogous methylene protons of 3 appear as distinct doublet and triplet peaks with a large Δδ (0.46 ppm) due to restricted rotation around the methylene carbon (Figure -IId).…”

“…The diastereotopic methylene protons, H a and H b , of 1 and 2 appear as overlapping multiplets (Figure 2-IIb,c). 49 In contrast, the analogous methylene protons of 3 appear as distinct doublet and triplet peaks with a large Δδ (0.46 ppm) due to restricted Hemilability of the Phosphine Group in Indium Complex 3. For the pendent phosphine to be truly hemilabile in 3, the phosphine must dissociate upon addition of other coordination ligands or solvent molecules possessing a greater donor ability.…”

“…46−48 Moreover, the 31 P nuclei can serve as probes to study the behavior of complexes in situ by using 31 P{ 1 H} NMR spectroscopy. For example, Li and co-workers used 31 alkyl complexes during the polymerization of ε-caprolactone G. 49 We have been studying 50 neutral and cationic indium complexes as catalysts for the controlled ring-opening polymerization of cyclic esters 51−55 and epoxides 56,57 as well as the copolymerization of lactones and acrylates 58 and coupling of lactones and epoxides to form spiroorthoesters. 59 We showed that introducing hemilabile thiophene, furan, and pyridine donors in series of complexes H can have a profound impact on catalyst stability and reactivity for the polymerization of epichlorohydrin, cyclohexene oxide, and rac-lactide.…”

“…While related ligands containing “soft” P-donors are less common, the M–P interaction may offer the possibility of hemilability which can be useful in stabilizing catalytic transition and resting states. − Moreover, the 31 P nuclei can serve as probes to study the behavior of complexes in situ by using 31 P­{ 1 H} NMR spectroscopy. For example, Li and co-workers used 31 P­{ 1 H} NMR spectroscopy to demonstrate the fluxional coordination of the P-donor in their aluminum alkyl complexes during the polymerization of ε-caprolactone G …”

Robust and efficient hydrogenation of carbonyl compounds catalysed by mixed donor Mn(I) pincer complexes

“…An unusual feature of the CNP ligand, responsible for the formation of complex 6 is the apparent hemilabile nature of phosphine donor arm in MnCNP precatalyst. The phenomenon of ligand hemilability is often employed to rationalize reactivity of organometallic compounds 36 – 41 , especially in the context of hybrid and multidentate ligands 42 – 44 . Invoked mainly for labile donor groups, e.g., oxygen or nitrogen 43 , hemilability is scarce for phosphine donors in general 45 – 47 and for manganese phosphines in particular 48 , 49 .…”