Synthesis Of Rate-Earth Metal Based Frustrated Lewis Pairs And Their Application In Polymerization Of Polar Alkenes

The dissertation presents the synthesis of rare-earth metal based frustrated Lewis pair(FLP)and their application in polymerization of polar alkenes,and rare-earth metal cyclopentadienyl/alkyl complexes catalyzed dehydrogenation of amine-boranes.1.Three cationic rare-earth aryloxide complexes[L1RE(OAr)][B(C6F5)4][RE=Sc,(12);Y(13);Lu(14),Ar=2,6-'Bu2-C6H3]supported by a new type of phorsphine containing ?-diketiminato-ligand were synthesized and characterizd.These complexes exhibited frustrated Lewis pair-like 1,4-addition reactions towards ?,?-unsaturated carbonyl substrates,which demonstrates that rare-earth metal can serve as Lewis-acid component in FLP-type compounds with the cooperation of a main-group(phosphorus)Lewis base.These cationic RE/P intramolecular FLP were found to be effective catalysts for the polymerization of conjugated polar alkene monomers(MMA,MBL and MMBL).For all three monomers,the activity trends were Y?Lu>Sc.Mechanistic studies revealed that the rare-earth metal-catalyzed polymerizations were initiated by new FLP-type 1,4-additions rather than traditional and ubiquitous covalent RE-E(E=H,C,N,etc.)bond insertion or single-electron transfer.2.The polymerization of conjugated polar alkene monomers was realized by combination of facile Lewis acid homoleptic rare-earth-metal tris-aryloxide complexes[RE=Sc(21),Y(22),Sm(23),La(24),Ar=2,6-'Bu2-C6H3;RE=La(25),Ar=2,6-'Pr2-C6H3]and commercially available phosphineas well as N-heterocyclic carbene Lewis bases.This rare-earth Lewis pair polymerization system was also found to be effective on more challenging acrylates and acrylamide monomers,such as tert-butyl methacrylate('BuMA),furfuryl methacrylate(FMA),and N,N-dimethylacrylamide(DMAA).For the methyl methacrylate polymerization,a syndiotactic polymer was produced with an rr value of up to 85%.The catalytic activities were observed to be dependent on the ionic radii of the rare-earth metals(La>Sm>Y>Sc)and the steric and electronic profiles of the Lewis bases.FLP-type addition was confirmed as the initiating step by stoichiometric reactions producing the zwitterionic active species and an end-group analysis3.The chemoselective polymerization of polar divinyl monomers,including allyl methacrylate(AMA),vinyl methacrylate(VMA),and 4-vinylbenzyl methacrylate(VBMA),was realized under mild conditions by using homoleptic rare-earth aryloxide complexes based intramolecular FLP,affording polymers with retention of pendant C=C bonds.In the case of PVBMA,it could be post-functionalized with PhCH2SH.Mechanistic study,including the isolation of the zwitterionic active species and the end-group analysis,revealed that the FLP-type addition was the initiating step in the polymerization.Notably,the 1,4-addition reaction occurred through the conjugated C=C bond,leaving the non-conjugated C=C intact,which is at the origin of the high level of chemoselectivity observed in the polymerization reaction4.Scandocene alkyl complexes,(C5Me5)2ScR[R=CH(SiMe3)2(36);CH2SiMe3(41)],are found to be highly active catalysts for the dehydrocoupling of dimethylamine-borane and diamine-boranes,which is the most active single-site catalyst to our knowledge.A ?-B-agostic scandium amidoborane intermediate(42)is isolated from stoichiometric reaction,and shows an unprecedented reversible ?-H elimination in solution.The resulting elimination product,namely scandocene hydride-aminoborane(43),is indicated by variable-temperature NMR spectra and further confirmed by several trapping experiments.Notably,the complex 42 showed similar catalytic performance compared to its parent complexes 36 and 41,which confirmed the complex 42 as the active species for the dehydrocoupling reaction.