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

Abstract

AbstractHydrogenation 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*FeIV(μ‐SEt)2(μ‐N)FeIVCp*][PF6] (1, Cp* = η5‐C5Me5) and [Cp*CoIII(μ‐SAd)(μ‐N)CoIIICp*] (3, Ad = adamantyl) and linear μ‐nitride [(TPP)FeIV(μ‐N)FeIV(TPP)][PF6] (2, TPP2− = 5,10,15,20‐tetraphenylporphinato), as well as μ‐imide [Cp*CoIII(μ‐SAd)(μ‐NH)CoIIICp*][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.