Anion variation on a cobalt(iii) complex of salen-type ligand tethered by four quaternary ammonium salts for CO2/epoxidecopolymerization

Abstract

Anion exchange of BF₄⁻ occurs by stirring a cobalt(III) complex of salen-type ligand tethered by four quaternary ammonium BF₄⁻ salts over a slurry of NaX in CH₂Cl₂, affording a complex containing four X's per cobalt (X = 2,4,5-trichlorophenolate, 6; X = 4-nitrophenolate, 10; X = 2,4-dichlorophenolate, 12). The ¹H and ¹³C NMR spectra are in agreement with an unusual imine uncoordinated structure. The two salen-phenoxys and the two X's persistently coordinate with cobalt(III) to form a square planar cobaltate complex while the other two X's scramble through coordination and decoordination to the axial sites of the square plane. Another form of the complex (X = 2,4,5-trichlorophenolate, 14; X = 4-nitrophenolate, 15; X = 2,4-dichlorophenolate, 16) is also prepared, in which the scrambling two X's in 6, 10, or 12 are replaced with the corresponding [X⋯H⋯X]⁻ homoconjugate. These complexes, which adopt an unusual imine uncoordinated structure, are excellent catalysts for CO₂/propylene oxide copolymerization (turnover frequency (TOF), 8300–16 000 h⁻¹). In all cases, the complex containing the homoconjugate [X⋯H⋯X]⁻ shows higher activity than the corresponding phenol-free complex. Among the prepared complexes, 4-nitrophenol-4-nitrophenolate homoconjugate complex 15 showed the best performance (TOF, 16 000 h⁻¹; selectivity, 98%; M_n, 273 000), allowing for replacement of the explosive 2,4-dinitrophenolate complex.