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

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.