Bridged bis(amidinate) lanthanide aryloxides: syntheses, structures, and catalytic activity for addition of amines to carbodiimides

Abstract

Various lanthanide aryloxide complexes supported by bridged bis(amidinate) ligand L, LLnOAr(DME) (L = Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃, DME = dimethoxyethane, Ln = Y, Ar = 2,6-(Me)₂C₆H₃ (1), 2,6-(ⁱPr)₂C₆H₃ (2), 2,6-(^tBu)₂-4-(Me)C₆H₂ (3); Ar = 2,6-(^tBu)₂-4-(Me)C₆H₂, Ln = Nd (4), Sm (5), Yb (6)) were synthesized, and complexes 1, 2 and 4–6 were characterized by single crystal X-ray diffraction. All the complexes are efficient precatalysts for catalytic addition of amines to carbodiimides. The catalytic activity is influenced by lanthanide metals and the aryloxide groups (Nd (4) ∼ Sm (5) < Y (3) ∼ Yb (6) and -2,6-(Me)₂C₆H₃ < -2,6-(ⁱPr)₂C₆H₃ < -2,6-(^tBu)₂-4-(Me)C₆H₂). The catalytic addition reaction with 3 showed a good scope of substrates. The mechanism investigation revealed the real active intermediate being the monoguanidinate complexes supported by an aryloxide and an amidine-functionalized amidinate group, L′Ln[O2,6-(^tBu)₂-4-(Me)C₆H₂][RNCNHRN(Ar′)] (L′ = Me₃SiNHC(Ph)N(CH₂)₃NC(Ph)NSiMe₃, R = ⁱPr, Ar′ = phenyl, Ln = Yb (8), Y (11); R = Cy, Ar′ = phenyl, Ln = Yb (10), Y (12); R = ⁱPr, Ar′ = 4-ClC₆H₄, Ln = Yb (9)), which were isolated from the reactions of 6 (or 3) with amine and carbodiimide in a molar ratio of 1 : 1 : 1 and structurally characterized. The Ln-active group in the present precatalyst is a Ln–amidinate species, not the Ln–OAr group.