Ring opening polymerisation of lactide with uranium(iv) and cerium(iv) phosphinoaryloxide complexes

The C₃-symmetric uranium(iv) and cerium(iv) complexes Me₃SiOM(OAr^P)₃, M = U (1), Ce (2), OAr^P = OC₆H₂-6-^tBu-4-Me-2-PPh₂, have been prepared and the difference between these 4f and 5f congeners as initiators for the ring opening polymerisation (ROP) of l-lactide is compared. The poorly controlled reactivity of the homoleptic analogue U(OAr^P)₄ (3) demonstrates the importance of the M-OSiMe₃ initiating group. The incorporation of a nickel atom in 1 to form the U-Ni heterobimetallic complex Me₃SiOU(OAr^P)₃Ni (4) may be the first example of the use of the inverse trans influence to switch the reactivity of a complex. This would imply the formation of the U-Ni bond strengthens the U-OSiMe₃ bond to such an extent that the ROP catalysis is switched off. Changing the conditions to immortal polymerisation dramatically increases polymerisation rates, and switches the order, with the Ce complex now faster than the U analogue, suggesting ligand protonolysis to afford a more open coordination sphere. For the ROP of rac-lactide, uranium complex 1 promotes heterotacticity at the highest levels of stereocontrol yet reported for an actinide complex.