Reduction of Hexaazatrinaphthylenes by Masked Divalent Lanthanide Dinitrogen Reagents

The oxidation state +2 is of interest in rare-earth chemistry since it allows these conventionally redox-inactive metals to be used as reducing agents. However, the divalent oxidation state is difficult to form for most rare-earth elements, and the ensuing compounds are often unstable. Here, we describe an approach to rare-earth reduction chemistry that circumvents the divalent oxidation state by using compounds of trivalent rare earths that store reducing electrons on the dinitrogen ligand [N₂]^2-, akin to "masked" divalent reactivity. Thus, the dinitrogen complexes $\left[{\left({\mathrm{Cp}}_2^{\mathrm{ttt}}M\right)}_2\left(\mu {\mathrm{-1},\mathrm{2-N}}_2\right)\right]$ (1_M, M = Y, Gd, Tb, Dy, Cp^ttt = 1,2,4-C₅^tBu₃H₂) reduce hexaazatrinaphthylene and its hexamethyl derivative to give trimetallic $\left[{\left({\mathrm{Cp}}_2^{\mathrm{ttt}}M\right)}_3\left(R_6\mathrm{HAN}\right)\right]$, where the [R₆HAN]^3- ligands (R = H, 2_M; R = Me, 3_M) form with S = 1/2, and with elimination of N₂. The structures of 2_M and 3_M reveal that the tert-butyl substituents strongly influence the core geometry of these trimetallic complexes. Analysis of the magnetism and electronic structure of 2_Gd and 3_Gd identifies ferromagnetic metal-radical exchange, with coupling constants of J = +2.87 cm^-1 and +3.07 cm^-1, respectively (-2J formalism). The unusual ferromagnetic exchange is a consequence of charge transfer to the gadolinium 5d, 6s, and 6p orbitals from the radical ligands.