Aluminum Salen Complexes and Tetrabutylammonium Salts:  A Binary Catalytic System for Production of Polycarbonates from CO₂ and Cyclohexene Oxide

Abstract

A series of complexes of the form (salen)AlZ, where H2salen = N,N'-bis(salicylidene)-1,2-phenylenediimine and various other salen derivatives and Z = Et or Cl, have been synthesized. Several of these complexes have been characterized by X-ray crystallography. An investigation of the utilization of these aluminum derivatives along with both ionic and neutral bases as cocatalysts for the copolymerization of carbon dioxide and cyclohexene oxide has been conducted. By studying the reactivity of these complexes for this process as substituents on the diimine backbone and phenolate rings are altered, we have observed that aluminum prefers electron-withdrawing groups on the salen ligands, thereby producing an electrophilic metal center to be most active toward production of polycarbonates from CO2 and cyclohexene oxide. For example, the complex derived from H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine is essentially inactive when compared to the analogous derivative containing nitro substituents in the 3-positions of the phenolate groups. This is to be contrasted with the catalytic activity observed for the (salen)CrX systems, where electron-donating salen ligands greatly enhanced the reactivity of these complexes for the coupling of CO2 and epoxides. While (salen)AlZ complexes are capable of producing poly(cyclohexene oxide) carbonate with low amounts of polyether linkage along with small quantities of cyclic carbonate byproducts, their reactivities, covering a turnover frequency range of 5.2-35.4 mol of epoxide consumed/(mol of Al x h), are greatly reduced when compared to their (salen)CrX analogues under identical reaction conditions.

A Cryptand-Type Aluminum Tris(salophen) Complex: Synthesis, Characterization, and Cell Imaging Application

“…Given that there is a symmetry plane perpendicular to the C3 axial, the LAl3 molecule belongs to C3h point group. Each salophen moiety shows great distortion, with the angle between two phenyl planes being near 58.57°, larger than that in the crystal of mononuclear LAl reported by Darensbourg et al [35]. The octahedral coordination geometry as well as the distortion of salophen moieties inhibit the aggregation of the molecules.…”

“…Mononuclear salophen ligand and Al salophen complexes were synthesized according to the reported methods [35][36][37][38][39][40]. Salicylaldehyde and o-phenylenediamine were refluxed in ethanol for 16 h to give the orange salophen ligand.…”

Aluminium coordination complexes in copolymerization reactions of carbon dioxide and epoxides

“…43 Several of the compounds were structurally characterized and all contained Al in a square-based pyramidal environment as expected. The most active of the Al-salen complexes reported in this study contained nitro groups in the 5-position and t-butyl groups in the 3-position, Fig.…”

Catalytic Processes for Chemical Conversion of Carbon Dioxide into Cyclic Carbonates and Polycarbonates

“…After the work of Chisholm and Zhou [44], Darensbourg and coworkers [45] investigated a series of (salen)AlX as the catalysts for the copolymerization reaction. Aluminum prefers electron-withdrawing groups on the salen ligands, thereby being active toward production of polycarbonates.…”