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

High-yield syntheses of the lanthanide dinitrogen complexes [(Cp₂^tttM)₂(μ-1,2-N₂)] (1, M = Gd, Tb, Dy; Cp^ttt = 1,2,4-C₅Bu₃H₂), in which the [N₂]^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 N₂ is observed in 1, and their Raman spectra show a major absorption consistent with N═N double bonds. Computational analysis of 1 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 [N₂]^2-, a consequence of which is that the dinitrogen ligand occupies a singlet ground state. Magnetic measurements reveal antiferromagnetic exchange in 1 and single-molecule magnet (SMM) behavior in 1. Ab initio calculations show that the magnetic easy axis in the ground doublets of 1 and 1 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 [N₂]^2- ligands in 1 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 with 2,2'-bipyridyl (bipy) to give [Cp₂^tttM(κ²-bipy)] (2, M = Gd, Tb, Dy), in which the lanthanide is ligated by a bipy radical anion, with strong metal-ligand direct exchange coupling.