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 nucleophilecatalyzed 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.

Bimetallic polymerization of lactide with binaphthol-derived bis-heteroscorpionate dizinc and dimagnesium complexes

“…1B). 21 Cationic, bimetallic complexes of these ligands were active and selective catalysts for lactide and lactone polymerization, but showed less activity than a neutral monometallic analogue. We attributed this result to the poor nucleophilicity of cationic complexes in lactide polymerization, 22 and thus sought more electron-rich scaffolds.…”