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

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 N 2 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 N 2 bridged heterobimetallic complexes of U(iii), Ln(iii) and Ln(ii) which were prepared by reacting the Fe dinitrogen complex [Fe(depe) 2 (N 2 )] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [M III {N(SiMe 3 ) 2 } 3 ] (M = U, Ce, Sm, Dy, Tm) and [Ln II {N(SiMe 3 ) 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-η 1 :η 1 -N 2 )(M{N(SiMe 3 ) 2 } 3 )], 1-M (M = U(iii), Ce(iii), Sm(iii), Dy(iii) and Tm(iii)), [{Fe(depe) 2 }(μ-η 1 :η 1 -N 2 )(Ln{N(SiMe 3 ) 2 } 2 )], 1*-Ln (Ln = Sm(ii), Yb(ii)) and [{Fe(depe) 2 (μ-η 1 :η 1 -N 2 )} 2 {Sm II {N(SiMe 3 ) 2 } 2 }], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N 2 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 N 2 with both Ln(iii) and Ln(ii) complexes with the degree of N 2 activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(iii)-N 2 Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N 2 to U(iii) and Ln(ii) complexes is favoured for the iron-bound N 2 compared to free N 2 due to the higher N 2 polarization.