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

Abstract

Two ytterbium(II) bis(phenolate) complexes, [L^RYb] where R = NMe₂1 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)YbOBu^t] (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 CH_x and triflate fluorine atoms. Ligand exchange between Yb(N(SiMe₃)₂)₃(thf)₂ and H₂L^R yielded [(L^NMe₂)YbN(SiMe₃)₂] (8) and [(L^OMe)YbN(SiMe₃)₂] (9), while metathesis from YbI₂(thf)₂ and K₂L^OMe reproducibly afforded the surprising oxidized product [(L^OMe)₂YbK(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 ε-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.