Bis(phosphinoselenoic amides) as versatile chelating ligands for alkaline earth metal (Mg, Ca, Sr and Ba) complexes: syntheses, structure and ε-caprolactone polymerisation

Abstract

We report here a series of heavier alkaline earth metal complexes with N,N′-(ethane-1,2-diyl)bis(P,P-diphenylphosphinoselenoic amide) using two synthetic routes. In the first route, the heavier alkaline earth metal bis(trimethylsilyl)amides [M{N(SiMe₃)₂}₂(THF)_n] (M = Ca, Sr, Ba), when treated with phosphinoselenoic amine [Ph₂P(Se)NHCH₂CH₂NHPPh₂(Se)] (1), afforded the corresponding alkaline earth metal complexes of the composition [(THF)₃M{Ph₂P(Se)NCH₂CH₂NPPh₂(Se)}] [M = Ca (2), Sr (3), Ba (4)]. The metal complexes 2–4 were also obtained from a one-pot reaction, where potassium phosphinoselenoic amide was generated in situ by the reaction of compound 1 and [KN(SiMe₃)₂], followed by the addition of the respective metal diiodides in THF at room temperature. The magnesium complex [(THF)₃Mg{Ph₂P(Se)NCH₂CH₂NPPh₂(Se)}] (5) was also prepared. The solid-state structures of alkaline earth metal complexes 2–5 were established by single crystal X-ray diffraction analysis. In the solid state, all the metal complexes are monomeric but in complexes 2–4, ligand 1 is chelated in a tetra-dentate fashion to each metal ion but in complex 5, ligand 1 behaves as a bidentate ligand. Complexes 2–4 were tested as catalysts for the ring-opening polymerisation of ε-caprolactone and a high level of activity for the barium complex 4 was observed, with narrow polydispersity. We also report the synthesis and structure of the bis(amidophosphino borane) ligand [Ph₂P(BH₃)NHCH₂CH₂NHPPh₂(BH₃)] (6) and the corresponding barium complex [(THF)₂Ba{Ph₂P(BH₃)NCH₂CH₂NPPh₂(BH₃)}]₂ (7).