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

Abstract

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 = L2,6-Me2 = [N(2,6-Me2C6H3)C(Me)]2CH−, n = 1, Ln = Eu (1); L = L2,4,6-Me3 = [N(2,4,6-Me3C6H2)C(Me)]2CH−, n = 1, Ln = Eu (2); L = L2,6-iPr2 = [N(2,6-iPr2C6H3)C(Me)]2CH−, n = 0, Ln = Eu (3), Sm (4); L = L2,6-ipr2Ph = [(2,6-iPr2C6H3)NC(Me)CHC(Me)N(C6H5)]−, n = 0, Ln = Eu (5), Yb (6); L = L2-Me = [N(2-MeC6H4)C(Me)]2CH−, n = 1, Ln = Yb (7)) in high yields. All the complexes, especially the complexes of SmII (4) and EuII (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 YbII < EuII and EuII < SmII and L2,6-Me2 < L2,4,6-Me3 ∼ L2,6-iPr2 < L2,6-ipr2Ph for the ligands. The mechanistic study by the isolation of guanidinate species and their reactivity revealed that EuII monoguanidinate complexes Eu(L2,6-Me2)[(C6H5N)C(NHCy)(NCy)](DME) (8) and Eu(L2,6-ipr2Ph)[(C6H5N)C(NHCy)(NCy)](THF)2 (9) should be the key active intermediates for the systems with EuII complexes and a YbIII bis(guanidinate) complex Yb(L2-Me)[(C6H5N)C(NHCy)(NCy)]2 (11) for the system using a YbII complex.