Alkali Metal and Alkaline Earth Metal Complexes with the Bis(borane-diphenylphosphanyl)amido Ligand – Synthesis, Structures, and Catalysis for Ring-Opening Polymerization of ϵ-Caprolactone

The syntheses of lithium and alkaline earth metal complexes with the bis(borane-diphenylphosphanyl)amido ligand (1-H) of molecular formulas [{κ2-N(PPh2(BH3))2}Li(THF)2] (2) and [{κ3-N(PPh2(BH3))2}2M(THF)2] [(M = Ca (3), Sr (4), Ba (5)] are reported. The lithium complex 2 was obtained by treatment of bis(borane-diphenylphosphanyl)amine (1-H) with lithium bis(trimethylsilyl)amide in a 1:1 molar ratio via the silylamine elimination method. The corresponding homoleptic alkaline earth metal complexes 3–5 were prepared by two synthetic routes – first, the treatment of metal bis(trimethylsilyl)amide and protio ligand 1-H via the elimination of silylamine, and second, through salt metathesis reaction involving respective metal diiodides and lithium salt 2. The molecular structures of lithium complex 2 and barium complex 5 were established by single-crystal X-ray diffraction analysis. In the solid-state structure of 2, the lithium ion is ligated by amido nitrogen atoms and hydrogen atoms of the BH3 group in κ2-coordination of the ligand 1 resulting in a distorted tetrahedral geometry around the lithium ion. However, in complex 5, κ3-coordination of the ligand 1 was observed, and the barium ion adopted a distorted octahedral arrangement. The metal complex 5 was tested as catalyst for the ring opening polymerization of ϵ-caprolactone. High activity for the barium complex 5 towards ring opening polymerization (ROP) of ϵ-caprolactone with a narrow polydispersity index was observed. Additionally, first-principle calculations to investigate the structure and coordination properties of alkaline earth metal complexes 3–5 as a comparative study between the experimental and theoretical findings were described