Synthesis and characterization of Bis(β‐diketiminato) rare‐earth amido complexes, their activity for catalytic addition of amines to carbodiimides

Abstract

Reactions of anhydrous RECl3 (RE = Sm, Nd) with 2 equiv. of NaL2-Me (L2-Me = [N(2-MeC6H4)C(Me)]2CH−) afforded the dimeric chlorides [Sm(L2-Me)2(μ-Cl)]2 (1) and [Nd(L2-Me)2(μ-Cl)]2 (2). Salt metathesis reactions between chloride complexes and NaN(SiMe3)2 in THF yielded a series of bis(β-diketiminato) rare-earth amido complexes RE(L2-Me)2[N(SiMe3)2] (RE = Sm (3), Nd (4), Yb (5), Y (6), Pr (7)). Complexes 1−7 were fully characterized including X-ray single crystal structure analyses. Complexes 3−7 can serve as efficient pre-catalysts for catalytic addition of amines to carbodiimides to afford multi-substituted guanidines. Catalytic activity was influenced by the rare-earth metals with an active sequence of Y (6) ∼ Yb (5) < Pr (7) < Nd (4) < Sm (3). The present catalytic addition reaction using complex 3 showed a wide scope of substrates including primary and secondary aromatic amines and aliphatic cyclic amines. The mechanism investigation by the isolation and characterization of the intermediates of the amido complexes RE(L2-Me)2(NHPh)(THF) (RE = Sm (8), Nd (9), Yb (10)) and guanidinate complexes Sm(L2-Me)2[(C6H5NH)C(NCy)2] (11) and Nd(L2-Me)2[(C6H5N)C(NHCy)(NCy)] (12). Their reactivity revealed that the present catalytic addition proceeded through amine-exchange reaction between the pre-catalyst amido complex and an amine to a novel amido complex, nucleophilic addition of the formed amido species to a carbodiimide to yield a guanidinate species, and protonation reaction of the guanidinate species by an amine to regenerate amido species and release the final product.