Barium complexes with crown-ether-functionalised amidinate and iminoanilide ligands for the hydrophosphination of vinylarenes

Abstract

The detailed multistep syntheses of two nitrogen-based sterically congested iminoanilidine and amidine proligands bearing a tethered 15-member aza-ether-crown macrocycle, namely {I^A^crown}H and {Am^crown}H, are reported. These proligands react with [Ba{N(SiMe₂H)₂}₂·(thf)_n] to generate the heteroleptic barium complexes [{I^A^crown}BaN(SiMe₂H)₂] (5) and [{Am^crown}BaN(SiMe₂H)₂] (6) in high yields. These complexes exhibit high coordination numbers (resp. eight and seven) and are in addition stabilised by mild Ba⋯H–Si interactions. Unusually for oxophilic elements such as barium, the amidinate ligand in 6 is only η¹-coordinated. Complexes 5 and 6 mediate the intermolecular hydrophosphination of styrene with primary (PhPH₂) and secondary (HPPh₂) phosphines. Their catalytic performance compares favourably with those of other barium precatalysts for these reactions. During the course of the hydrophosphination of styrene with HPPh₂ catalysed by 5, the phosphide complex [{I^A^crown}BaPPh₂] (7) could be intercepted and crystallographically characterised.