Hydrogen Atom Abstraction from an OsII(NH3)2 Complex Generates an OsIV(NH2)2 Complex: Experimental and Computational Analysis of the N–H Bond Dissociation Free Energies and Reactivity

Double hydrogen atom abstraction from (TMP)­OsII(NH3)2 (TMP = tetramesitylporphyrin) with phenoxyl or nitroxyl radicals leads to (TMP)­OsIV(NH2)2. This unusual bis­(amide) complex is diamagnetic and displays an N–H resonance at 12.0 ppm in its 1H NMR spectrum. 1H–15N correlation experiments identified a 15N NMR spectroscopic resonance signal at −267 ppm. Experimental reactivity studies and density functional theory calculations support relatively weak N–H bonds of 73.3 kcal/mol for (TMP)­OsII(NH3)2 and 74.2 kcal/mol for (TMP)­OsIII(NH3)­(NH2). Cyclic voltammetry experiments provide an estimate of the pKa of [(TMP)­OsIII(NH3)2]+. In the presence of Barton’s base, a current enhancement is observed at the Os­(III/II) couple, consistent with an ECE event. Spectroscopic experiments confirmed (TMP)­OsIV(NH2)2 as the product of bulk electrolysis. Double hydrogen atom abstraction is influenced by π donation from the amides of (TMP)­OsIV(NH2)2 into the d orbitals of the Os center, favoring the formation of (TMP)­OsIV(NH2)2 over N–N coupling. This π donation leads to a Jahn–Teller distortion that splits the energy levels of the dxz and dyz orbitals of Os, results in a low-spin electron configuration, and leads to minimal aminyl character on the N atoms, rendering (TMP)­OsIV(NH2)2 unreactive toward amide–amide coupling.