Synthesis, characterization of bridged bis(amidinate) lanthanide amides and their application as catalysts for addition of amines to nitriles for monosubstituted N-arylamidines

Abstract

A series of lanthanide amide complexes supported by bridged bis(amidinate) ligand L, LLnNHAr¹(DME) (L = [Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃], Ar¹ = 2,6-ⁱPr₂C₆H₃, DME = dimethoxyethane, Ln = Y (1), Pr (2), Nd (3), Gd (4), Yb (5)), [Yb(μ₂-NHPh)]₂(μ₂-L)₂ (6) and [LYb]₂(μ₂-NHAr²)₂ (7) (Ar² = (o-OMe)C₆H₄), were synthesized by reaction of LLnCl(THF)₂ with the corresponding lithium amide in good yields and structurally characterized by X-ray crystal structure analyses. All complexes were found to be precatalysts for the catalytic addition of aromatic amines to aromatic nitriles to give monosubstituted N-arylamidines. The catalytic activity was influenced by lanthanide metals and the amido groups with the active sequence of Y (1) < Gd (4) < Nd (3) < Pr (2) ∼ Yb (5) for the lanthanide metals and –NHAr² < –NHPh < –NHAr¹ for the amido groups. The catalytic addition reaction with complex 5 showed a good scope of aromatic amines. Some key reaction intermediates were isolated and structurally characterized, including the amidinate complexes LLn[NPhCNAr¹](PhCN) (Ln = Y (8), Ln = Yb (9)), LYb[NAr²CNAr¹](Ar²CN) (10), and amide complex 5 prepared by protonation of 9 by Ar¹NH₂. Reactivity studies of these complexes suggest that the present catalytic formation of monosubstituted N-arylamidines proceeds through nucleophilic addition of an amido species to a nitrile, followed by amine protonolysis of the resultant amidinate species.