Synthesis and Reactivity of N-heterocyclic Carbene Stabilized Lanthanide(II) Bis(amido) Complexes

Lanthanide amides are some of the most important reagents and catalysts in lanthanide organometallic chemistry because of their high stability. Nevertheless, the readily available homoleptic Ln[N(SiMe3)(2)](n) (n = 2, 3) complexes lack feasible strategies to modulate their reactivity and selectivity. In this paper, monodentate N-heterocyclic carbenes (NHCs) were applied to synthesize the divalent lanthanide bis(amido) adducts 3,4Sm and 2-4,7,8Eu. sigma-bond metathesis reactions of (IiPr)(2)Yb[N(SiMe3)(2)](2) (2Yb; IiPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with alkyne, aniline, and phenol yielded alkynyl-bridged [(IiPr)Yb[N(SiMe3)(2)]mu-C (math)CtBu)](2) (9), monomeric (IiPr)(2)Yb[N(SiMe3)(2)](NHAr1) (10; Ar-1 = 2,6-iPr(2)C(6)H(3)), and (IiPr)(2)Yb(OAr2)(2) (11; Ar-2 = 2,6-tBu(2)-4-MeC6H2), respectively. All of the NHC adducts of lanthanide(II) complexes were structurally characterized by X-ray diffraction analysis. Moreover, 4Sm has high activity in the ROP of rac-LA without an alcohol initiator. Preliminary mechanism studies reveal that the ROP by 4Sm was initiated through a coordination-insertion mechanism, which involves electrophilic activation of the monomer by the metal center and nucleophilic attack of the amido group on the activated monomer. These studies demonstrate that the diverse NHCs could be potential ligands to not only suppress the ligand redistribution of heteroleptic Ln(II) complexes but also modulate the reactivity of lanthanide amide catalysts.