Construction of oxygen-bridged multimetallic assembly: dual catalysts for hydroamination reactions

Abstract

Herein, we utilized multiple metal centers of a single catalytic system for two different activation processes in hydroamination catalysis. A trimetallic complex, [Cp*₂(Me)Zr(μ-O)Zr(NMe₂)₂(μ-O)Zr(Me)Cp*₂] (2) bearing three zirconium(IV) ions bridged by two oxygen centers was synthesized by the reaction of Cp*₂(Me)Zr(OH) (Cp* = η⁵-C₅Me₅) with Zr(NMe₂)₄ in a 2 : 1 molar ratio at room temperature. The molecular structure of 2 was determined by a single crystal X-ray diffraction study. Complex 2, in addition to the known heterobimetallic complex [Cp*₂(Me)Zr-(μ-O)-Ti(NMe₂)₃] (3), were tested as catalysts for the intramolecular hydroamination of unactivated secondary aminoalkenes and the intermolecular hydroamination of carbodiimides. The methyl zirconium centers of these catalysts are primarily responsible for intramolecular hydroamination catalysis, while the Zr-amide or Ti-amide centers are responsible for intermolecular hydroamination catalysis. Further study unravelled that catalyst 2 stays active for up to five catalytic cycles, as revealed by in situ recycling methodology. The kinetic study of the catalytic hydroamination reaction with model substrates dicyclohexyl carbodiimide (4) and a primary aromatic amine (5) was investigated using 2 as the catalyst by ¹H NMR spectroscopy.