Bis(phosphinimino)methanide Borohydride Complexes of the Rare-Earth Elements as Initiators for the Ring-Opening Polymerization of epsilon-Caprolactone: Combined Experimental and Computational Investigations

Rare-earth-metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis-borohydrides [{CH(PPh2NSiMe3)(2)}La(BH4)(2-)(THF)] and [{CH(PPh2NSiMe3)(2)}Ln(BH4)(2)] (Ln=Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH4)(3)-(THF)(3)] and K{CH(PPh2NSiMe3)(2)}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh2NSiMe3)(2)}YCl2](2) and NaBH4. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid-state structures were establised by single-crystal X-ray diffraction. The ring-opening polymerization (ROP) of epsilon-caprolactone initiated by [{CH(PPh2NSiMe3)(2)}La(BH4)(2)(THF)] and [{CH(PPh2NSiMe3)(2)}Ln(BH4)(2)] (Ln=Y, Lu) was studied. At 0 degrees C the molar mass distributions determined were the narrowest values ((M) over bar (w)/(M) over bar (n)=1.06-1.11) ever obtained for the ROP of epsilon-caprolactone initiated by rare-earth-metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B-H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.