Dinuclear zinc complexes based on parallel β-diiminato binding sites:: Syntheses, structures, and properties as CO2/epoxide copolymerization catalysts

With the background that beta-diketiminato zinc complexes efficiently catalyze the CO2/epoxide copolymerization via a mechanism involving two catalyst molecules, a ligand system containing two parallel beta-diiminato binding sites linked by a xanthene backbone ([(R)Xanthdim](2-) with residues R = 2,3-dimethylphenyl and 2,4-difluorophenyl at the iminato units, respectively) was investigated with respect to its zinc coordination chemistry. The corresponding diimines [(R)Xanthdim]H-2 were treated with diethylzinc to yield the complexes [(Me2C6H3)Xanthdim](ZnEt)(2), 4, and [(F2C6H3)Xanthdim](ZnEt(thf))(2), 5, respectively. In order to convert these compounds into polymerization catalysts, they were subsequently treated with SO2, which indeed resulted in the corresponding ethylsulfinates. Due to aggregation via intermolecular bridging of ethylsulfinate ligands, the product after the reaction of 5 represents an insoluble coordination polymer. Aggregation does take place also for the primary product obtained from the reaction of 4, as evidenced by the isolation of the tetramer [{[(Me2C6H3)Xanthdim]Zn-2(mu-O2SEt)}mu-O2SEt](4), 6, and the precipitation of an insoluble solid on storing of such mixtures. 4, 5, and 6 display moderate activities as catalysts for the copolymerization of cyclohexene oxide and CO2. A bimodal molecular weight distribution points to two effective mechanisms, one of which probably involves two cooperating Zn centers as anticipated. Possible structural reasons for these catalytic results are discussed.