Phosphine-functionalized amidinate ligated rare-earth metal complexes for highly 3,4-selective living polymerization of 1,3-conjugated dienes

Abstract

A series of rare-earth metal bis(alkyl) complexes have been prepared via protonolysis reactions of tris(aminobenzyl) complexes (Ln(CH₂C₆H₄N(Me)₂-o)₃) with phosphine-functionalized amidinated ligands (^DippNCN(CH₂)_nPPh₂, n = 2 (L1-H) and n = 3 (L2-H)). The X-ray diffraction of P2-Sc (^DippNCN(CH₂)₃PPh₂Sc(CH₂C₆H₄N(Me)₂-o)₂) showed the un-coordination of the diphenylphosphine group due to the inherent saturation of the central metal ion. In conjunction with [Ph₃C][B(C₆F₅)₄], all the rare-earth metal complexes showed a high catalytic activity for the polymerization of 1,3-conjugated dienes (isoprene, β-myrcene and β-farnesene), affording highly 3,4-regular polymers (up to 100% 3,4-) with high molecular weight and narrow molecular weight distribution. After the abstraction of the alkyl moiety –CH₂C₆H₄N(Me)₂-o of P1-Sc by [Ph₃C][B(C₆F₅)₄], the species with the coordination of the diphenylphosphine group to the central metal probably formed, as shown in the ³¹P NMR spectra and DFT calculation results, and it might serve as the true active species in the 3,4-selective polymerization of 1,3-conjugated dienes.