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Group 13 and Lanthanide Complexes Supported by Tridentate Tripodal Triamine Ligands:  Structural Diversity and Polymerization Catalysis

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posted on 02.04.2020, 17:03 by Hongping Zhu, Eugene Y.-X. Chen
Several different synthetic approaches to a total of 13 novel B, Al, and Sm complexes derived from the tridentate tripodal triamine ligand [N3]H3 with a neopentane, trisilylmethane, or trisilylsilane backbone and different N-substituents, as well as applications of the selected complexes to polymerization catalysis, are reported. Salt metathesis between HC[SiMe2N(CH2Ph)]3Li3(THF)2 (THF = tetrahydrofuran) and AlCl3 in Et2O/hexanes leads to complete elimination of LiCl and formation of the corresponding tripodal triamido alane HC[SiMe2N(CH2Ph)]3Al·(THF) (1). On the other hand, the reaction of {MeC[CHN(SiMe3)]3Li3}2 and AlCl3 in Et2O/hexanes yields a LiCl-containing compound MeC[CH2N(SiMe3)]3AlCl[Li(Et2O)] (2). Alkane elimination involving [N3]H3 and 1 AlMe3 produces diamido−amino aluminum methyl HC[SiMe2NHAr][SiMe2NAr]2AlMe [Ar = 4-MeC6H4 (3), CH2Ph (4)], while the reaction using ≥2 AlMe3 gives amido−amino aluminum dimethyl [ArHNMe2Si](H)C[SiMe2NAr]2(AlMe2)2 (Ar = 4-MeC6H4, 5) and [(Me3Si)HNCH2](Me)C[CH2N(SiMe3)]2(AlMe2)2 (6). The H2-elimination route involves treatment of [N3]H3 with LiAlH4 and AlH3, affording [{HC[SiMe2N(4-MeC6H4)]3AlH}Li]2 (7) and MeSi[SiMe2N(4-MeC6H4)]3AlH(AlH2) (8), respectively. There is no reaction between [N3]H3 and Al[N(SiMe3)2]3; however, the amine-elimination reaction using Sm[N(SiMe3)2]3 produces tripodal triamido Sm complex {MeSi[SiMe2N(4-MeC6H4)]3Sm}2 (9). Ligand exchange between tripodal borane HC[SiMe2N(4-MeC6H4)]3B and AlR3 (R = Me, H) offers the first-step ligand exchange product HC[SiMe2N(4-MeC6H4)]3BMe(AlMe2) (10) or the second-step ligand exchange product HC[SiMe2N(4-MeC6H4)]3AlH(BH2) (11). Activation of dimethyl metallocenes LZrMe2 by HC[SiMe2N(4-MeC6H4)]3B produces ligand redistribution products LZrMe[N(4-MeC6H4)SiMe2](H)C[SiMe2N(4-MeC6H4)]2BMe [L = Cp2 (12), rac-Et(Ind)2 (13)]. Besides characterizations by NMR and elemental analysis of the above new complexes, six of them (2, 4, 5, 8, 9, and 13) have also been structurally characterized by X-ray single-crystal diffraction studies. “Activated” metallocene complexes 12 and 13 are inactive for ethylene or propylene polymerization. Complex 1 exhibits low activity for ring-opening polymerization (ROP) of propylene oxide, but high activity for ROP of ε-caprolactone (CL). Significantly, tripodal aluminum hydride 8 effects catalytic ROP of CL upon addition of benzyl alcohol as a chain-transfer reagent.