Hydrogen Atom Abstraction from an Os II (NH 3 ) 2 Complex Generates an Os IV (NH 2 ) 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 1 H NMR spectrum. 1 H-15 N correlation experiments identified a 15 N 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 p K a 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 d xz and d yz 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.