Cooperative N2 Reduction across a Multinuclear First-Row Transition Metal Platform

The development of molecular systems capable of dinitrogen (N2) activation and reduction under mild conditions remains a central challenge in inorganic chemistry. Herein, we report the synthesis, structures, and reactivity of a series of multimetallic first-row transition metal complexes supported by sterically encumbered tetraamidodiamine (TADA) ligands. Systematic variation of the silyl substituents enables modulation of ligand sterics while preserving the conformational flexibility of the TADA scaffold, resulting in a diverse set of bimetallic and tetrametallic architectures. Structural studies reveal that increased steric bulk disfavors formation of previously reported syn closed geometries and instead promotes higher nuclearity assemblies, including ate complexes. These complexes were evaluated as precatalysts for N2 reduction under standard silylation conditions (KC8/TMSCl), revealing a dependence of catalytic activity on metal identity, nuclearity, reductant, and ligand sterics. Catalytic activity was found to depend on metal identity in a manner not simply correlated with ligand sterics, with turnover numbers of up to 40. Stoichiometric reduction of a dititanium precursor affords a crystallographically characterized, four-electron reduced product featuring substantial N–N bond elongation and asymmetric Ti environments. Density functional theory calculations support a description of this species as a highly activated N24– unit stabilized by π-backbonding and alkali metal interactions.