Novel lanthanide(ii) complexes supported by carbon-bridged biphenolate ligands: synthesis, structure and catalytic activity

Abstract

[Ln{N(SiMe₃)₂}₂(THF)₂] (Ln = Sm, Yb) reacts with 1 equiv. of carbon-bridged biphenols, 2,2′-methylene-bis(6-tert-butyl-4-methylphenol) (L¹H₂) or 2,2′-ethylidene-bis(4,6-di-tert-butylphenol) (L²H₂), in toluene to give the novel aryloxide lanthanide(II) complexes [{LnL¹(THF)_n}₂] (Ln = Sm, n = 3 (1); Ln = Yb, n = 2 (2)) and [{LnL²(THF)₃}₂] (Ln = Sm (5); Ln = Yb (6)) in quantitative yield, respectively. Addition of 2 equiv. of hexamethylphosphoric triamide (HMPA) to a tetrahydrofuran (THF) solution of 1, 2 and 5 affords the corresponding HMPA-coordinated complexes, [{LnL¹(THF)_m(HMPA)_n}₂(THF)_y] (Ln = Sm, n = 2, m = 0, y = 2 (3); Ln = Yb, m = 1, n = 1, y = 6 (4)) and [{SmL²(HMPA)₂}₂] (7) in excellent yields. The single-crystal structural analyses of 3, 4 and 7 revealed that these aryloxide lanthanide(II) complexes are dimeric with two Ln–O bridges. The coordination geometry of each lanthanide metal can be best described as a distorted trigonal bipyramid. Complexes 1–3, 5 and 7 can catalyze the ring-opening polymerization of ε-caprolactone (ε-CL), and 1–3, along with 5 show moderate activity for the ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC) and the copolymerization of ε-CL and DTC to give random copolymers with high molecular weights and relatively narrow molecular weight distributions.