Electronic Structure Origins of Distinct Hydrogenation Activities Observed for Linear and Bent Bimetallic μ-Nitrides

Hydrogenation of metal nitrides is of particular interest due to the direct relevance to Haber-Bosch ammonia synthesis. Notably, for all bi- and multi-nuclear bridging nitrides reported thus far, only those featuring bent M─N─M cores can react with dihydrogen (H₂) and related H₂-derived species, while the vast majority of linear M─N─M congeners cannot. Herein, we present a detailed electronic-structure study of prototypical bimetallic bent μ-nitrides [Cp*Fe^IV(μ-SEt)₂(μ-N)Fe^IVCp*][PF₆] (1, Cp* = η⁵-C₅Me₅) and [Cp*Co^III(μ-SAd)(μ-N)Co^IIICp*] (3, Ad = adamantyl) and linear μ-nitride [(TPP)Fe^IV(μ-N)Fe^IV(TPP)][PF₆] (2, TPP^2- = 5,10,15,20-tetraphenylporphinato), as well as μ-imide [Cp*Co^III(μ-SAd)(μ-NH)Co^IIICp*][BPh₄] (4), using various spectroscopic techniques, in particular, ¹⁵N solid-state nuclear magnetic resonance, coupled with density functional theory calculations. An in-depth analysis of their distinct ¹⁵N shielding tensors revealed that bent μ-nitrides invariably possess a high-lying proton-accepting molecular orbital (MO) and a low-lying electron-accepting MO. These electronic-structure features are key to the bent μ-nitrides affecting hydrogenolysis via either two-electron oxidation of H₂ or H₂ heterolysis. However, because of symmetry, linear μ-nitrides lack potent proton-accepting MOs, which rationalizes their disparate hydrogenation activities.