Substitution Effects in Highly Syndioselective Styrene Polymerization Catalysts Based on Single-Component Allyl ansa-Lanthanidocenes: An Experimental and Theoretical Study

A series of allyl ansa-lanthanidocenes, [{Me2C­(C5H4)­(Flu)}­Nd­(1,3-C3H3(SiMe3)2)2]K (Flu = 9-fluorenyl; 1-Nd-K­(allyl)) and {R2C­(C5H4)­(R′R′Flu)}­Ln­(1,3-C3H3(SiMe3)2)­(THF)x (R = Me, R′ = 2,7-tBu2, Ln = Y (2-Y), Sc (2-Sc), x = 0; Ln = La (2-La), Pr (2-Pr), Nd (2-Nd), Sm (2-Sm), x = 1; R = Me, R′ = oct = octamethyl­octahydro­dibenzo, Ln = Nd, x = 1 (3-Nd); R = Ph, R′ = H, Ln = Nd, x = 1 (4-Nd); R = Me, R′ = 3,6-tBu2, Ln = Nd, x = 1 (5-Nd)), were prepared in good yields and characterized by NMR spectroscopy (for diamagnetic complexes 2-Sc, 2-Y, and 2-La) and by single-crystal X-ray diffraction (1-Nd-K­(allyl), 2-La, 2-Pr, 2-Nd, 2-Sm, and 4-Nd). Those complexes, especially 1-Nd-K­(allyl), 2-Nd, 4-Nd, 2-La, and 2-Sm, act as single-component catalyst precursors for polymerization of styrene (in bulk or in aliphatic hydrocarbon solutions, (nBu)2Mg as scavenger, Tpolym = 60–140 °C), affording highly syndiotactic polystyrene (sPS) ([r]5 = 63–88%; Tm up to 260 °C). High productivities (up to 4560 kg­(sPS) mol­(Ln)−1 h–1) were achieved at 120–140 °C, at low catalyst loadings ([St]/[Nd] = 20 000–76 000 equiv), with 2-Nd and 2-Pr. On the other hand, precursors having bulky substituents on the fluorenyl moieties in 3,6-positions (3-Nd, 5-Nd) or based on small ionic radius metals (2-Y, 2-Sc) were poorly or not active under standard polymerization conditions. These results have been rationalized by DFT computations, which included the solvent, carried out on the putative 1-Nd, and the isolated 2-Nd and 5-Nd complexes. Three consecutive styrene insertions were studied, and it was revealed that (i) the formation of sPS is thermodynamically controlled by two effectsminimization of repulsions between fluorenyl/styrene phenyl ring and (in the initiation phase) fluorenyl/SiMe3 substituents of the allyl ligandand (ii) the presence of bulky substituents on the fluorenyl moiety does not influence the activation barrier of monomer insertion, but it may destabilize thermodynamically the insertion product.