Synthesis And Characterization Of Bridged Bis(amidinate)s Lanthanide Complexes And Their Catalytic Properties

A series of lanthanide complexes supported by linked bis（amidinate）s （L=[Me₃SiNC（Ph）N（CH₂）₃NC（Ph）NSiMe₃]） were synthesized and fully characterized,including amides, borohydrides, benzyl complexes and anionic heterobimetallic imidocomplexes. Catalytic behaviors of lanthanide amides for addition of aromatic amines toaromatic nitriles to give monosubstituted N-arylamidines were studied, the catalyticactivity was influenced by lanthanide metals and the amido groups, some key reactionintermediates were isolated and structurally characterized. Furthermore, the catalyticactivities of some of these complexes in polymerization of ε-caprolactone and lactidewere tested. The main results obtained are as follows:1. Reactions of LLnCl（THF）₂with （LiNHAr¹, LiNHPh, LiNHAr²; Ar¹=2,6-iPr₂C₆H₃, Ar²=（o-OMe）C6H4） in THF afforded various bridged bis（amidinate）slanthanide amides LLnNHAr¹（DME）（Ln=Y （1）, Pr （2）, Nd （3）, Eu （4）, Gd （5）, Tb （6）,Er （7））,[Yb（μ2-NHPh）]2（μ2-L）2（8） and [LYb]2（μ2-NHAr²）2（9） in high yields. Thesecomplexes were characterized by elemental analysis, IR and NMR spectroscopy in thecase of complex1. The state structures of these complexes were studied by X-raydiffraction. Single crystal structure analysis showed that complexes8-9are bimetalliccomplexes.2. Complexes1-7can serve as efficient catalysts for catalytic addition of aromaticamines to aromatic nitriles selectively to give the monosubstituted amidines in excellentyields with a wide range of aromatic amines. Functional groups such as aromatic C–X（X=F, Cl, CH3, OCH3） bonds can survive under the catalytic reaction conditions. Themechanism studies by the isolation of some key reaction intermediates amidinatecomplexes （LY[NPhCNAr¹]（PhCN）（10）, LYb[NPhCNAr¹]（PhCN）（11）, LYb [NAr2CNAr¹]（Ar2CN）（12）） and their activity for addition of amine to nitrile revealedthat the present catalytic formation of monosubstituted aromatic amidine proceedsthrough nucleophilic addition of an amido species to a nitrile, followed by amineprotonolysis of the resultant amidinate species.3. Reactions of LnCl3（Ln=Nd, Pr and La） with NaBH4afforded ion-pairlanthanide borohydrides [Ln（BH₄）₂（THF）₅]+[Ln（BH₄）₄（THF）₂]ˉ（Ln=La （13）, Pr （14）,Nd （15）） or the neutral Ln（BH₄）₃（THF）₃（Ln=Nd, Pr, La） at different temperature inhigh yields, which provides a straightforward method to prepare ion-pair lightlanthanide borohydrides. The influence of the size of the lanthanide metals on thesynthesis of ion-pair lanthanide borohydrides was observed, the same procedure withthe middle and latter lanthanide metals chlorides as that for the synthesis of13-15didnot afford the ion-pair complexes, but the neutral lanthanide borohydridesLn（BH₄）₃（THF）₃（Ln=Sm （16）, Eu （17）, Yb （18）） were isolated. Complexes13-15showed high activity for the ring-opening polymerizations of ε-caprolactone andL-Lactide. Complex15can also initiate the polymerization of methyl methacrylate（MMA） without cocatalyst. Comparable study on the reactivity between15andNd（BH₄）₃（THF）₃revealed that15showed somewhat higher activity thanNd（BH₄）₃（THF）₃in all the polymerizations studied, but no quite difference in chemicalbehavior between the two complexes.4. Metathesis reaction of LLnCl（THF）₂with NaBH4in a1:1.5molar ratioafforded the monoborohydride complexes LLn（BH4）（DME）[Ln=Y （19）, Nd （20）,Sm（21） and Yb（22）]. Crystal structure analyses revealed1-4are monomers, in whicheach metal is ligated by one L ligand, one η3-BH4group and one DME molecule in atrigonal bipyramid geometry. Complexes19-22were found to be very active single-siteinitiators for the controlled ring opening polymerization of ε-caprolactone and L-lactideas judged by relatively narrow molecular weight distributions （Mw/Mn:1.34-1.50） andexperimental molecular weight Mn（exp） were in good agreement with theoreticmolecular weight Mn（theo）. The highest activity and the best control over the molecularweight for both monomers were found for the system with20. These monoborohydridecomplexes can also initiate the ring opening polymerization of rac-LA to give heterotactically enriched polyLA with Pr （heterotactic enrichment） values in a range of0.69-0.85depending on the lanthanide metals and the most highest heterotacticenrichment （Pr） was found for19（Pr=0.85）. Moreover, complex19initiated thepolymerization of rac-LA in a living fashion.5. Reaction of LYbCl（THF）2with equivalent BnK afforded lanthanidemonobenzyl complex LYbBn（DME）（23）. X-ray crystal structure analysis of23revealed that the complex was monomer with the central metal ligated by one L, oneη2-Bn group and one DME. Treatment of LYCl（THF）2with BnK under the sameconditions as there for the synthese of complex23afforded two complexes. One islanthanide monobenzyl complex LYBn（DME）（24） which structure just like complex23,the other is anionic lanthanide dibenzyl complex[LYBn2]－[Li（DME）₃]⁺（25） with thecentral metal ligated by one L, one η2-Bn group and one η3-Bn. Treatment ofLNdCl（THF）2with BnK at various situation afforded only one product [LNdBn2]－[Li（DME）₃]⁺（26）, cell parameters revealed that complex26has the same single crystalstructure as25. The catalytic activity of complex23in polymerization of L-lactide wastested, preliminary research shows that complex23is very active single-site initiatorsfor the ring opening polymerization of L-lactide.6. Reaction of LYbNHAr¹（DME） with equivalent n-BuLi at-20oC affordedanionic lanthanide heterobimetallic imido complex [LYb（NLiAr¹）NHAr¹]–[Li（DME）3]+（27） in reasonable yield. By the same procedure, the analogues neodymium complex[LNd（NLiAr¹）NHAr¹]–[Li（DME）₃]⁺（28） can also be synthesised. Both complexes werecharacterized by elemental analysis, IR and confirmed by structure analysis.