Group 1 and group 2 metal complexes supported by a bidentate bulky iminopyrrolyl ligand: synthesis, structural diversity, and ε-caprolactone polymerization study

Abstract

We report here a series of alkali and alkaline earth metal complexes, each with a bulky iminopyrrolyl ligand [2-(Ph₃CNCH)C₄H₃NH] (1-H) moiety in their coordination sphere, synthesized using either alkane elimination or silylamine elimination methods or the salt metathesis route. The lithium salt of molecular composition [Li(2-(Ph₃CNCH)C₄H₃N)(THF)₂] (2) was prepared using the alkane elimination method, and the silylamine elimination method was used to synthesize the dimeric sodium and tetra-nuclear potassium salts of composition [(2-(Ph₃CNCH)C₄H₃N)Na(THF)]₂ (3) and [(2-(Ph₃CNCH)C₄H₃N)K(THF)_0.5]₄ (4) respectively. The magnesium complex of composition [(THF)₂Mg(CH₂Ph){2-(Ph₃CNCH)C₄H₃N}] (5) was synthesized through the alkane elimination method, in which [Mg(CH₂Ph)₂(OEt₂)₂] was treated with the bulky iminopyrrole ligand 1-H in 1 : 1 molar ratio, whereas the bis(iminopyrrolyl)magnesium complex [(THF)₂Mg{2-(Ph₃CNCH)C₄H₃N}₂] (6) was isolated using the salt metathesis route. The heavier alkaline earth metal complexes of the general formula {(THF)_nM(2-(Ph₃CNCH)C₄H₃N)₂} [M = Ca (7), Sr (8), and n = 2; M = Ba (9), n = 3] were prepared in pure form using two synthetic methods: in the first method, the bulky iminopyrrole ligand 1-H was directly treated with the alkaline earth metal precursor [M{N(SiMe₃)₂}₂(THF)_n] (where M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent at ambient temperature. The complexes 7–9 were also obtained using the salt metathesis reaction, which involves the treatment of the potassium salt (4) with the corresponding metal diiodides MI₂ (M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent. The molecular structures of all the metal complexes (1-H, 2–9) in the solid state were established through single-crystal X-ray diffraction analysis. The complexes 5–9 were tested as catalysts for the ring-opening polymerization of ε-caprolactone. High activity was observed in the heavier alkaline earth metal complexes 7–9, with a very narrow polydispersity index in comparison to that of magnesium complexes 5 and 6.