Targeted Synthesis of End-On Dinitrogen-Bridged Lanthanide Metallocenes and Their Reactivity as Divalent Synthons

[Image: see text] High-yield syntheses of the lanthanide dinitrogen complexes [(Cp(2)(ttt)M)(2)(μ-1,2-N(2))] (1(M), M = Gd, Tb, Dy; Cp(ttt) = 1,2,4-C(5)(t)Bu(3)H(2)), in which the [N(2)](2–) ligands solely adopt the rare end-on or 1,2-bridging mode, are reported. The bulk of the tert-butyl substituents and the smaller radii of gadolinium, terbium, and dysprosium preclude formation of the side-on dinitrogen bonding mode on steric grounds. Elongation of the nitrogen-nitrogen bond relative to N(2) is observed in 1(M), and their Raman spectra show a major absorption consistent with N=N double bonds. Computational analysis of 1(Gd) identifies that the local symmetry of the metallocene units lifts the degeneracy of two 5d(π) orbitals, leading to differing overlap with the π* orbitals of [N(2)](2–), a consequence of which is that the dinitrogen ligand occupies a singlet ground state. Magnetic measurements reveal antiferromagnetic exchange in 1(M) and single-molecule magnet (SMM) behavior in 1(Dy). Ab initio calculations show that the magnetic easy axis in the ground doublets of 1(Tb) and 1(Dy) align with the {M–N=N–M} connectivity, in contrast to the usual scenario in dysprosium metallocene SMMs, where the axis passes through the cyclopentadienyl ligands. The [N(2)](2–) ligands in 1(M) allow these compounds to be regarded as two-electron reducing agents, serving as synthons for divalent gadolinium, terbium, and dysprosium. Proof of principle for this concept is obtained in the reactions of 1(M) with 2,2′-bipyridyl (bipy) to give [Cp(2)(ttt)M(κ(2)-bipy)] (2(M), M = Gd, Tb, Dy), in which the lanthanide is ligated by a bipy radical anion, with strong metal–ligand direct exchange coupling.