Enhanced Reactivity of Aluminum Complexes Containing P-Bridged Biphenolate Ligands in Ring-Opening Polymerization Catalysis

Abstract

Aluminum complexes containing [RP(O)(2-O-3,5-tBu2C6H2)2]2− [R = tBu (3a), Ph (3b)] have been synthesized, structurally characterized, and their reactivity studied in comparison with those of their [RP(2-O-3,5-tBu2C6H2)2]2− [R = tBu (2a), Ph (2b)] analogs. Treating AlMe3 with one equiv of H2[3a-b] in THF at 0°C affords quantitatively [3a-b]AlMe, subsequent reactions of which with benzyl alcohol in THF at 25°C generate {[3a-b]Al(μ2-OCH2Ph)}2. The methyl [3a-b]AlMe and the benzyloxide {[3a-b]Al(μ2-OCH2Ph)}2 are all active for catalytic ring-opening polymerization (ROP) of ε-caprolactone and rac-lactide (rac-LA). Controlled experiments reveal that {[3a]Al(μ2-OCH2Ph)}2 is competent in living polymerization. Kinetic studies indicate that [3a]AlMe, in the presence of benzyl alcohol, catalyzes ROP of rac-LA at a rate faster than [3b]AlMe and [2a]AlMe(THF) by a factor of 1.8 and 23.6, respectively, highlighting the profound reactivity enhancement in ROP catalysis by varying the P-substituents of these biphenolate complexes of aluminum.

Catalytic Main‐Group Metal Complexes of Phosphine‐Based Pincer Ligands

“…It should be noted that Liang and coworkers have expanded their work on Al(III) complexes of P‐bridged bisphenolate pincer ligands by introducing the phosphoryl variants of OPO ‐ 1 and OPO ‐ 2 , namely, OOO ‐ 1 and OOO ‐ 2 (Scheme 5), respectively [56] . The latter ligands were used to synthesize Al ‐ 8 and Al ‐ 9 , which are analogs of Al ‐ 4 and Al ‐ 5 wherein the soft P‐donors have been replaced by the hard O‐donors of the R 3 P=O groups, rendering the first coordination sphere phosphine‐free and oxygen‐rich.…”

“…It should be noted that Liang and coworkers have expanded their work on Al(III) complexes of P-bridged bisphenolate pincer ligands by introducing the phosphoryl variants of OPO-1 and OPO-2, namely, OOO-1 and OOO-2 (Scheme 5), respectively. [56] The latter ligands were used to synthesize Al-8 and Al-9, which are analogs of Al-4 and Al-5 wherein the soft P-donors have been replaced by the hard Odonors of the R 3 P=O groups, rendering the first coordination sphere phosphine-free and oxygen-rich. The phosphorylcontaining complexes were also examined as ROP catalysts for ɛ-CL and rac-lactide, in the presence of BnOH, and were found to outperform their phosphine-containing congeners in both reactions.…”

“…Synthesis of OOOÀ Al(III) complexes Al-8 and Al-9, reported by Liang and coworkers in 2018. [56] phosphine-based pincer complexes of Mg(II) and Zn(II), which have been demonstrated as active catalysts for the hydrogenation of unsaturated CÀ C, CÀ N and CÀ O bonds, and were also involved in the electroreduction of CO 2 .…”