Early Transition Metal N,N-Dialkylcarbamato Complexes as Precursors to Metal-Functionalized Silica Surfaces and to Lactides Polymerization Catalysts

Chemical Implantation of Group 4 (Ti, Zr, Hf), Nb and Ta metals on silica has been carried out under mild conditions by the reaction of N,N-dialkylcarbamato complexes M(O2CNR2)n (R = Me, Et, iPr) with the silanol groups of amorphous silica, carbon dioxide and secondary amine being released in the process. Group 4 metals M(O2CNR2)4 complexes have evidenced similar reactivity: on the basis of gasvolumetric, IR and CP-MAS NMR analyses we have suggested the formation of two species, mono- [≡SiO-M(O2CNR2)3] and bis-grafted [(≡SiO)2-M(O2CNR2)2] to the silica matrix. Niobium and tantalum carbamato derivatives react with decomposition of two ligands; formation of bisgrafted [(≡SiO)2-M(O2CNR2)3] species has been proposed. Some reduction is observed with Nb(O2CNR2)5. The treatment of Nb(O2CNR2)5 (R = Me, Et) in refluxing heptane (98 °C) causes the reduction of Nb(V) to Nb(IV), with formation of the Nb(O2CNR2)4 derivative. DFT calculations pointed out that the metal-reduction is promoted by an intramolecular H-O interaction between two monodentate ligands. The R group influences the extent of the process in the order Et > Me. The catalytic activity of the N,N-dialkylcarbamato complexes in the polymerization of lactides has been tested. M(O2CNR2)4 complexes (M = Ti, Zr, Hf) showed satisfactory performances in terms of conversion, molecular weight and polydispersity of the polymer. The complexes polymerize the monomer at 100 °C in toluene, without addition of initiators. Contrastingly, M(O2CNR2)5 (M = Nb, Ta) have resulted completely inactive. The mechanistic aspects of the process have been investigated: an uncommon mechanism has been suggested for the first step of the reaction, on the basis of spectroscopic analyses and DFT calculations. The electrophilic attack of the methyne hydrogen of the lactide seems to be responsible for the catalyst activation, which precedes the propagation of the polymer chain through the classical ROP (Ring Opening Polymerization) of cyclic esters