New Rare Earth Metal Bis(alkyl)s Bearing an Iminophosphonamido Ligand. Synthesis and Catalysis toward Highly 3,4-Selective Polymerization of Isoprene

Abstract

New rare earth metal bis(alkyl) complexes [(NPN Ph )Ln(CH 2 SiMe 3 ) 2 (THF)] (NPN Ph: N(Ph)PPh 2 ═NC 6 H 2 Me 3 -2,4,6; Ln = Sc ( 3a ), Ln = Y ( 3b ), Ln = Lu ( 3c )) and [(NPN Py )Sc(CH 2 SiMe 3 ) 2 (THF)] (NPN Py = N(Py)PPh 2 ═NC 6 H 2 Me 3 -2,4,6) ( 3d )) have been prepared via protonolysis reaction between rare earth metal tris(alkyl)s and the corresponding iminophosphonamines. Complexes 3a − d are analogous monomers of THF solvate. Each metal ion coordinates to a η 2 -chelated NPN ligand and two cis -located alkyl groups, adopting tetrahedron geometry. The scandium complex [(NPN Ph )Sc(CH 2 SiMe 3 ) 2 (THF)] in combination with [PhNHMe 2 ][B(C 6 F 5 ) 4 ] and Al i Bu 3 forms the first non-Cp-ligated rare earth metal based catalyst system to provide extremely high activity and 3,4-selectivity for the polymerization of isoprene. The resultant high molecular weight polymer ( M n = 99 × 10 4 ) with narrow molecular weight distribution (PDI = 1.55) has a 3,4-regularity up to 94.7% (Tp = –40 °C, toluene, 2 h, 100% yield). The complexes 3b and 3c, with a larger ionic radius, are less regioselective, and 3d, with an electron-donating pyridyl moiety, is inert. The probable initiation species and mechanistic scenario are also presented.