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)Ph 2)} (1a) and {HN(S-∗CHMePh)(P(Se)Ph 2)} (1b), synthesised using two routes. The first route involves a trans metalation reaction of enantiomeric pure potassium phosphinoselenoic amide [K{N(R-∗CHMePh)(Ph 2 P(Se))}{THF} n ] (2a) or [K{N(S-∗CHMePh)(Ph 2 P(Se))}{THF} n ] (2b) prepared from the reaction between either 1a or 1b and [KN(SiMe 3) 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)Ph 2 } 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)Ph 2 } 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(SiMe 3) 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 P2 1 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(BH 3)Ph 2)} (7a) and {HN(S-∗CHMePh)(P(BH 3)Ph 2)} (7b) and the corresponding homoleptic barium complexes of composition [Ba{N(R-∗CHMePh)P(BH 3)Ph 2 } 2 (THF) 2 ] (8a) and [Ba{N(R-∗CHMePh)P(BH 3)Ph 2 } 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.