We have isolated and structurally characterized a terminal iron nitrido complex supported by a bulky tris(carbene)borate ligand. The electronic structure of this complex reveals that the a1 LUMO (formerly Fe(dz2)) is strongly stabilized by reduced antibonding interactions with the carbene sigma-donor ligands and configurational mixing (hybridization) with higher lying Fe 4s and 4p atomic orbitals. This unusual bonding interaction results in a low-lying Fe nitrido acceptor orbital (LUMO) that possesses electrophilic character. Reaction with PPh3 results in nitrogen atom transfer to the phosphine, supporting a reaction mechanism involving nucleophilic attack of the triphenylphosphine HOMO at the electrophilic LUMO of the iron nitrido complex.

Radical ideas: Reaction of the iron(IV) nitrido complex [PhB(MesIm)(3)Fe[triple chemical bond]N] (see picture, Mes=2,4,6-Me(3)C(6)H(2)) with TEMPO-H (1-hydroxy-2,2,6,6-tetramethylpiperidine) results in high yields of ammonia and quantitative formation of [PhB(MesIm)(3)Fe(tempo)]. The mechanism likely involves hydrogen-atom transfer from TEMPO-H to the nitrido complex. Similar reaction with the triphenylmethyl radical yields [PhB(MesIm)(3)Fe[triple chemical bond]N--CPh(3)].

Thermodynamic investigations relevant to hydrogen atom transfer by the high-valent iron imido complex [LMesFe[triple bond]NAd]OTf have been undertaken. The complex is found to be weakly oxidizing by cyclic voltammetry (E1/2 = -0.98 V vs Cp2Fe+/Cp2Fe in MeCN). A combination of experimental and computational studies has been used to determine the acidity of LMesFe-N(H)Ad+ (pKa = 37 in MeCN), allowing the N-H BDFE (88(5) kcal/mol) to be calculated from a thermodynamic cycle. Consistent with this value, [LMesFe[triple bond]NAd]OTf reacts with 9,10-dihydroanthracene (C-H BDE = 78(1) kcal/mol) to form anthracene.