Lithium Complexes Derived of Benzylphosphines: Synthesis, Characterization and Evaluation in the ROP of rac-Lactide and ε-Caprolactone.

A series of lithium complexes ([Ph₂P(-C₆H₄-CH₂Li·TMEDA)] (), [PhP(-C₆H₄-CH₃)(-C₆H₄-CH₂Li·TMEDA)] (), [PhP(-C₆H₄-CH₂Li·TMEDA)₂] () and [P(-C₆H₄-CH₂Li·TMEDA)₃] ()) was prepared from mono-, di- and tri-benzylphosphines and varying amounts of BuLi and was characterized extensively by IR and ¹H, ⁷Li, C and P NMR spectroscopy. The molecular structures of complexes and were determined by single-crystal X-ray diffraction studies. The two complexes have monomeric structures in the solid state comprising seesaw lithium atoms. In each case, the ligand exhibits an asymmetric C-C η²-coordination mode and an intramolecular P-Li bond interaction. Theoretical calculations at Density functional theory (DFT) level M06/6111+G(2d,p) show that indeed a P-Li bond is established which can be explained as the P lone pair () being partially delocalized on an available ² orbital on Li () and additional bonding contribution of the phosphorous atom to Li stems from further delocalization of a σ P-C orbital into the ² orbital on Li. The observed short contact distances between an aromatic carbon and Li in the crystal structures of and are explained as due to the interaction of a σ C-Li orbital into the π* orbital of a C-C aromatic bond. Preliminary tests show compounds , , and are active catalysts in the solvent free ring-opening polymerization (ROP) of -caprolactone (-CL) and -lactide (-LA). High conversions to polycaprolactones were obtained in short periods of time: 1-6 min at 25 °C. Additionally, all four lithium complexes behave as moderately good initiators for the ROP of -LA showing high conversions to polylactides at 140 °C in one hour.