Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries

Low-nuclearity nitrides of the late transition metals are rare and reactive molecular species, with little experimental precedent. The first putative examples of dicobalt bridging nitrides, [(PDI₂)Co₂(μ-N)(PMe₃)₂][OTf]₃ ([CoN]; PDI = pyridyldiimine; n = 2 or 3, representing the length of the aliphatic chain linking PDI imino groups), were reported recently and shown to undergo a range of intramolecular reaction pathways, including N-H bond formation, C-H bond insertion, and P═N bond formation at the bridging nitride. The specific mode of reactivity changed with the phase of the reaction and the size of the macrocycle used to support the transient species. The present contribution offers a computational investigation into both the geometric and electronic structures of these nitrides as well as the factors governing their reaction selectivity. The compounds [CoN] exhibit μ-N-based lowest unoccupied molecular orbitals (LUMOs) that are consistent with subvalent, electrophilic nitrides. The specific orientations of the LUMOs induce ring-size-dependent stereoelectronic effects, thereby causing the product selectivity observed experimentally. Notably, the nitrides also exhibit a degree of nucleophilicity at μ-N by way of a high-energy, μ-N-based lone pair. This ambiphilic character appears to be a direct result of the constrained environment imposed by the folded-ligand geometries of [CoN]. When combined with the experimental findings, these data led to the conclusion that the folded-ligand isomers are the reactive species and that the constrained geometry imposed by the macrocyclic ligand plays an important role in controlling the reaction outcome.