Synthesis, structure and oxidation of new ytterbium(II) bis(phenolate) compounds and their catalytic activity towards ε-caprolactone

Two ytterbium(II) bis(phenolate) complexes, [(LYb)-Yb-R] where R = NMe2 1 and OMe 2, have been synthesized and characterized, with 1 being structurally defined to be a dimeric species with an unsymmetrical coordination of the bis(phenolate) ligand which is preserved in solution. Both 1 and 2 have been oxidized by a variety of oxidants (AgX, ROH) to form heteroleptic ytterbium(III) bis(phenolate) complexes: [(L-NMe2)YbPF6] (3), [(L-NMe2)YbOSO2CF3(thf)] (4), [(L-NMe2)YbOBut] (5), [(L-NMe2)YbOPh] (6), [(L-Ome)YbOPh] (7). Compound 4 has been structurally characterized as having a quasi-octahedral environment around ytterbium, with significant inter species hydrogen bonding between CHx and triflate fluorine atoms. Ligand exchange between Yb(N(SiMe3)(2))(3)(thf)(2) and H2LR yielded [(L-NMe2)YbN(SiMe3)(2)] (8) and [(L-OMe)YbN(SiMe3)(2)] (9), while metathesis from YbI2(thf)(2) and K2LOme reproducibly afforded the surprising oxidized product [(L-OMe)(2)YbK(dme)(2)] (10), which was structurally characterized as having a distorted octahedral environment around the ytterbium(III) centre. Compounds 1-9 were used to polymerize epsilon-caprolactone at room temperature in toluene, with only compounds 1, 2, 8 and 9 exhibiting significant catalytic activities. The polycaprolactone formed in these reactions was generally of high molecular weight and polydispersities <1.90 in all but one case.