Catalytic addition of amines to carbodiimides by bis(β-diketiminate)lanthanide(II) complexes and mechanistic studies

Reduction reactions of bis(β-diketiminate)lanthanide(III) chlorides formed in situ by reactions of anhydrous LnCl3 with 2 equiv. of sodium salt of the β-diketiminate ligand in THF with a Na/K alloy afforded a series of bis(β-diketiminate)lanthanide(II) complexes LnL2(THF)n (L = L(2,6-Me2) = [N(2,6-Me2C6H3)C(Me)]2CH(-), n = 1, Ln = Eu (1); L = L(2,4,6-Me3) = [N(2,4,6-Me3C6H2)C(Me)]2CH(-), n = 1, Ln = Eu (2); L = L(2,6-iPr2) = [N(2,6-(i)Pr2C6H3)C(Me)]2CH(-), n = 0, Ln = Eu (3), Sm (4); L = L(2,6-ipr2)(Ph) = [(2,6-(i)Pr2C6H3)NC(Me)CHC(Me)N(C6H5)](-), n = 0, Ln = Eu (5), Yb (6); L = L(2-Me) = [N(2-MeC6H4)C(Me)]2CH(-), n = 1, Ln = Yb (7)) in high yields. All the complexes, especially the complexes of Sm(II) (4) and Eu(II) (5), were found to be excellent pre-catalysts for catalytic addition of amines to carbodiimides to multi-substituted guanidines with a wide scope of substrates. The activity depends both on the central metals and the ligands with the active sequence of Yb(II) < Eu(II) and Eu(II) < Sm(II) and L(2,6-Me2) < L(2,4,6-Me3) ∼ L(2,6-iPr2) < L(2,6-ipr2)(Ph) for the ligands. The mechanistic study by the isolation of guanidinate species and their reactivity revealed that Eu(II) monoguanidinate complexes Eu(L(2,6-Me2))[(C6H5N)C(NHCy)(NCy)](DME) (8) and Eu(L(2,6-ipr2)Ph)[(C6H5N)C(NHCy)(NCy)](THF)2 (9) should be the key active intermediates for the systems with Eu(II) complexes and a Yb(III) bis(guanidinate) complex Yb(L(2-Me))[(C6H5N)C(NHCy)(NCy)]2 (11) for the system using a Yb(II) complex.