Binucleating Bis(pyrazolyl)alkane Ligands and Their Cationic Dizinc Complexes: Modular, Bimetallic Catalysts for Ring-Opening Polymerization

Abstract

Binucleating ligands enable the rational study and utilization of metal–metal cooperativity in catalysis through discrete bimetallic complexes. However, binucleating ligands rarely show the degree of structural modularity required for detailed optimization or structure–activity analysis, which are fundamental to homogeneous metal catalysis. This report introduces a highly modular family of binucleating proligands (1-R) consisting of two bis(pyrazolyl)alkanes bridged by a phenol. We prepared this series of proligands with variable substituents (R = −H, −Me, −Ph, −iPr) at the pyrazole 3-position in good yields by nucleophile-catalyzed condensation between 2,6-diformyl-4-(tert-butyl)-phenol and the corresponding bis(pyrazolyl)methanones. Cationic dizinc complexes 6-R were then prepared by treating proligands 1-R with diethylzinc and trityl salts. We fully characterized complex 6-H by single-crystal X-ray diffraction, identifying a twisted structure and modeling its conformational dynamics by density functional theory. The zinc atoms in 6-H are distorted tetrahedral, favorable for main group metals. Catalytic comparison identified 6-H as the most active and controlled catalyst for lactide and ε-caprolactone polymerization, resulting in low dispersity, end-group incorporation, and block copolymerization. Complexes 6-R represent the first atom-bridged bis(pyrazolyl)alkane bimetallics and constitute a highly modular platform for the rational investigation of dinuclear main group catalysis.