Homoleptic lanthanide complexes of chelating bis(phosphanyl)amides: synthesis, structure, and ring-opening polymerization of lactones.

Treatment of the bis(phosphanyl)amide (Ph2P)2NH with KH in boiling THF followed by crystallization from THF/n-pentane leads to [K(thf)n][N(PPh2)2] (n = 1.25, 1.5). Reaction of [K(thf)n][N(PPh2)2] with anhydrous yttrium or lanthanide trichlorides in a 3:1 molar ratio afforded homoleptic bis(phosphanyl)amide complexes [Ln[N(PPh2)2]3] (Ln = Y, Er) as large crystals in good yields. [Ln[N(PPh2)2]3] can also be obtained by reaction of the homoleptic bis(trimethylsilyl)amides of Group 3 metals and lanthanides [Ln[N(SiMe3)2]3] (Ln = Y, La, Nd) with three equivalents of (Ph2P)2NH in boiling toluene. The single-crystal X-ray structures of these complexes always show eta 2 coordination of the ligand. Dynamic behavior of the ligand is observed in solution and is caused by rapid exchange of the two different phosphorus atoms. [Ln[N(PPh2)2]3] was used as catalyst for the polymerization of epsilon-caprolactone. Significant differences in terms of correlation of theoretical and experimental molecular weights as well as polydispersities were observed depending on the nature of Ln. On the basis of the crystal structure of the heteroleptic complex [Lu[N(PPh2)2]3(thf)], we suggest that in the initiation step of epsilon-caprolactone polymerization the lactone adds to the lanthanide atom to form a sevenfold coordination sphere around the central atom.