Chiral alkaline earth metal complexes with M–Se direct bond (M = Mg, Ca, Sr, Ba): syntheses, structures and ε-caprolactone polymerisation

Abstract

We report here a series of enantiomeric pure alkaline earth metal complexes, each with a metallic direct bond of selenium, with {HN(R-*CHMePh)(P(Se)Ph₂)} (1a) and {HN(S-*CHMePh)(P(Se)Ph₂)} (1b), synthesised using two routes. The first route involves a trans metalation reaction of enantiomeric pure potassium phosphinoselenoic amide [K{N(R-*CHMePh)(Ph₂P(Se))}{THF}_n] (2a) or [K{N(S-*CHMePh)(Ph₂P(Se))}{THF}_n] (2b) prepared from the reaction between either 1a or 1b and [KN(SiMe₃)₂], and the corresponding alkaline earth metal diiodies in THF at room temperature to afford the enantiomeric pure complexes of composition [M{N(R-*CHMePh)P(Se)Ph₂}₂(THF)_n] [M = Mg (3a), n = 1; M = Ca (4a), Sr (5a) and Ba (6a), n = 2] and [M{N(S-*CHMePh)P(Se)Ph₂}₂(THF)_n] [M = Mg (3b), n = 1; M = Ca (4b), Sr (5b) and Ba (6b), n = 2]. The same heavier alkaline earth metal complexes (4a–6a and 4b–6b) can also be obtained through the silylamine elimination method using the corresponding metal bis(trimethylsilyl)amides [M{N(SiMe₃)₂}₂(THF)_n] (M = Ca, Sr, Ba) with phosphinoselenoic amine ligands 1a and 1b in ambient conditions. The solid-state structures of the metal complexes 4a–6a and 4b–6b were established using single-crystal X-ray diffraction analysis. In the solid state, all the metal complexes crystallise in the monoclinic P2₁ space group and each phosphinoselenoic amido ligand is ligated to the metal ion in a bidentate fashion. We also report the syntheses and structures of chiral amidophosphine-borane ligands {HN(R-*CHMePh)(P(BH₃)Ph₂)} (7a) and {HN(S-*CHMePh)(P(BH₃)Ph₂)} (7b) and the corresponding homoleptic barium complexes of composition [Ba{N(R-*CHMePh)P(BH₃)Ph₂}₂(THF)₂] (8a) and [Ba{N(R-*CHMePh)P(BH₃)Ph₂}₂(THF)₂] (8b). The molecular structures of 8a and 8b in the solid state confirm the attachment of chiral amidophosphine-borane ligands to the barium ions. The complexes 5 and 6 were tested as catalysts for the ring-opening polymerisation of ε-caprolactone. High activity in relation to the barium complexes 6a and 6b is observed, with moderate to narrow polydispersity index.