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, LLnNHAr 1(DME) (L = [Me 3SiNC(Ph)N(CH 2) 3NC(Ph)NSiMe 3], Ar 1 = 2,6- iPr 2C 6H 3, DME = dimethoxyethane, Ln = Y (1), Pr (2), Nd (3), Gd (4), Yb (5)), [Yb(μ 2-NHPh)] 2(μ 2-L) 2 (6) and [LYb] 2(μ 2-NHAr 2) 2 (7) (Ar 2 = (o-OMe)C 6H 4), 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 -NHAr 2 < -NHPh < -NHAr 1 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[NPhCNAr 1](PhCN) (Ln = Y (8), Ln = Yb (9)), LYb[NAr 2CNAr 1](Ar 2CN) (10), and amide complex 5 prepared by protonation of 9 by Ar 1NH 2. 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. © The Royal Society of Chemistry 2012.