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

摘要

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 P 2 _(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.