Synthesis,Reaction Of Mono(Amidinate)Rare Earth Metal Bis(Amide) Complexes And Their Performance In Isoprene Polymerization

A series of neutral rare earth metal bis(amide) complexes supported by anionic amidinate ancillary ligands were synthesized and well-characterized. Reaction of these complexes with organo-borate and aluminium trialkyls afforded cationic mono(amidinate) rare earth metal amide complexes and mono(amidinate) rare earth metal/aluminum alkyl heterometallic complexes, respectively. The neutral rare earth metal bis(amide) complexes could serve as cationic catalyst precursors for the regio-selective polymerization of isoprene.1. Amine elimination of rare earth metal tris(amide) complexes Ln[N(SiHMe2)2]3(THF)n (Ln=Sc, n=1; Ln=Y, Lu, La, n=2) with1equivalent of amidine [PhC(N-2,6-R2C6H3)2]H (R=H, Me,’Pr) in toluene afforded a series of neutral mono(amidinate) rare earth bis(amide) complexes [PhC(N-2,6-Me2C6H3)2]Ln[N(SiHMe2)2]2(THF)n(Ln=Sc (1), n=0:Ln=Y (2), La (3), Lu(4), n=1),[PhC(N-2,6-’Pr2C6H3)2]Ln[N(SiHMe2)2]2(THF)n(Ln=Sc (5), n=0; Ln=Y (6), n=1),[PhC(N-C6H5)2]Sc[N(SiHMe2)2]2(THF)(7). These complexes were characterized by FT-IR spectroscopy and NMR spectroscopy. Single crystal structural determination of2,4,5revealed that the metal centers in2and4are five-coordinated to form a distorted trigonal bipyramidal geometry, while that in5is four-coordinated to adopt a distorted tetrahedral geometry.2. In the presence of one equivalent of [Ph3C][B(C6F5)4] in toluene at room temperature,1,5, and7became active for3,4-selective polymerization of isoprene (3,4-selectivity>70%). The active species in the polymerization are confirmed to be cationic mono(amidinate) rare earth metal amide complexes. The resulting polymers were characterized by GPC and ’H NMR.3. Organo-borate and solvent have significant influence on the transformation of mono(amidinate) rare earth metal bis(amide) complexes to cationic rare earth metal amide complexes. Treatment of1,2,5with1equivalent of [Ph3C][B(C6F5)4] in toluene, a concerted process of Si-H σ-bond activation and silylamide migration took place, affording the cationic rare earth metal amide complexes [{PhC(N-2,6-R2C6H3)2}LnN{SiHMe2}{SiMe2N(SiHMe2)2}(THF)n][B(C6F5)4](R=Me, Ln=Sc (8), n=2; Ln=Y (9), n=4. R=’Pr, Ln=Sc (10), n=2). Treatment of1with1equivalent of [PhNMe2H][B(C6F5)4] in toluene gave the anticipated cationic rare earth metal amide complexes [{PhC(N-2,6-Me2C6H3)2}Sc{N(SiHMe2)2}(THF)3][B(C6F5)4](11). In contrast, treatment of1,2,6with1equivalent of [Ph3C][B(C6F5)4] in THF produced the cationic rare earth metal amide complexes11and [{PhC(N-2,6-R2C6H3)2}Y{N(SiHMe2)2}(THF)3][B(C6F5)4](R=Me (12); R=Pr (13)). Complexes11-13could initiate the ring-opening polymerization of THF, and the catalytic activity increased with the decrease of the ionic radius of central metals. These complexes were characterized by FT-IR spectroscopy and NMR spectroscopy. Single crystal structural determination of10,13revealed that the metal centers in10is five-coordinated to form a distorted trigonal bipyramidal geometry, while that in13is six-coordinated to adopt a distorted octahedral geometry.4. Treatment of1,2,5with excess AlMe3in toluene afforded a series of mono(amidinate) Ln/Al heterometallic methyl complexes [PhC(N-2,6-Me2C6H3)2]Ln[(μ-Me)2AlMe2]2(Ln=Sc (14), Ln=Y (15)),[PhC(N-2,6-’Pr2C6H3)2]Sc[(μ-Me)2AlMe2]2(16) via amide-alkyl exchange. These complexes were characterized by FT-IR spectroscopy and NMR spectroscopy. Single crystal structural determination of15,16revealed that the metal centers in15,16are six-coordinated to form a distorted octahedral geometry.5. Addition of AlMe3into1-7/[Ph3C][B(C6F5)4] binary catalyst systems not only increased the catalytic activity toward isoprene polymerization, but also switched regio-selectivity of the polymerization from3,4-specific to1,4-selective. The active species were postulated to be cationic mono(amidinate) Ln/Al heterobimetallic complexes. The resulting polymers were characterized by GPC and1H NMR.