Phosphasalen group IV metal complexes: synthesis, characterization and ring opening polymerization of lactide

We report the synthesis of a series of Zr and Ti complexes bearing phosphasalen which differs from salen by the incorporation of two P atoms in the ligand backbone. The reaction of phosphasalen proligands(1a-1c)H₂with Zr(CH₂Ph)₄led to different products depending on the nature of the N,N-linker in the ligand. In the case of ethylene-linked phosphasalen, octahedral Zr complex2aformed as a single stereoisomer intransgeometry. With the phenylene linker, it was shown by dynamic NMR spectroscopy that complex2bexists as a mixture oftransandcis-ß isomers in solution, both enantiomers (? and ?) of thecis-ß isomer being in fast equilibrium with respect to the NMR time-scale. The use of a propylene-linked phosphasalen proligand1cH₂led to a mixture of complexes among which a binuclear Zr complex2cbridged only by one phosphasalen ligand could be isolated and characterized. Addition of 2 equiv. ofⁱPrOH to2aand2bafforded diisoproxy Zr complexes3aand3bas a mixture oftransandcis-ß isomers, the latter undergoing fast ?/? isomerization in solution. Addition of B(C₆F₅)₃to2aand2bgave cationic monobenzyl Zr complexes4aand4bwhich have been further converted into cationic alkoxy Zr complexes5a-band6a-bby alcoholysis withⁱPrOH and (S)-methyl-lactate, respectively. The reaction of the phosphasalen proligands with Ti(NMe₂)₄proceeded diastereoselectively giving rise to Ti complexes7a-cin octahedral geometry withcis-ß wrapping of the ligand. The complexes have been tested for the ROP ofrac-lactide. The neutral phosphasalen Ti and Zr complexes showed only poor activity probably due to the encumbered and electron donating nature of the phosphasalen ligand. In contrast, the cationic Zr alkoxides5a,6aand6bare effective initiators for the controlled and hetero-selective ROP ofrac-lactide.