Iron promoted end-on dinitrogen-bridging in heterobimetallic complexes of uranium and lanthanides

Abstract

End-on binding of dinitrogen to low valent metal centres is common in transition metal chemistry but remains extremely rare in f-elements chemistry. In particular, heterobimetallic end-on N₂ bridged complexes of lanthanides are unprecedented despite their potential relevance in catalytic reduction of dinitrogen. Here we report the synthesis and characterization of a series of N₂ bridged heterobimetallic complexes of U(III), Ln(III) and Ln(II) which were prepared by reacting the Fe dinitrogen complex [Fe(depe)₂(N₂)] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [M^III{N(SiMe₃)₂}₃] (M = U, Ce, Sm, Dy, Tm) and [Ln^II{N(SiMe₃)₂}₂], (Ln = Sm, Yb). Despite the lack of reactivity of the U(III), Ln(III) and Ln(II) amide complexes with dinitrogen, the end-on dinitrogen bridged heterobimetallic complexes [{Fe(depe)₂}(μ-η¹:η¹-N₂)(M{N(SiMe₃)₂}₃)], 1-M (M = U(III), Ce(III), Sm(III), Dy(III) and Tm(III)), [{Fe(depe)₂}(μ-η¹:η¹-N₂)(Ln{N(SiMe₃)₂}₂)], 1*-Ln (Ln = Sm(II), Yb(II)) and [{Fe(depe)₂(μ-η¹:η¹-N₂)}₂{Sm^II{N(SiMe₃)₂}₂}], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N₂ complexes of divalent lanthanides which provide relevant structural models for the species involved in the catalytic reduction of dinitrogen by Fe/Sm(II) systems. Computational studies showed an essentially electrostatic interaction of the end-on bridging N₂ with both Ln(III) and Ln(II) complexes with the degree of N₂ activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(III)–N₂Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N₂ to U(III) and Ln(II) complexes is favoured for the iron-bound N₂ compared to free N₂ due to the higher N₂ polarization.