Slow Hydrogen Atom Self-Exchange between Os(IV) Anilide and Os(III) Aniline Complexes: Relationships with Electron and Proton Transfer Self-Exchange

Hydrogen atom, proton and electron transfer self-exchange and cross-reaction rates have been
determined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to the
Os(IV) anilide TpOs(NHPh)Cl2 (OsIVNHPh) gives the Os(III) aniline complex TpOs(NH2Ph)Cl2 (OsIIINH2Ph)
with a new 66 kcal mol-1 N−H bond. Concerted transfer of H• between OsIVNHPh and OsIIINH2Ph is
remarkably slow in MeCN-d3, with kexH• = (3 ± 2) × 10-3 M-1 s-1 at 298 K. This hydrogen atom transfer
(HAT) reaction could also be termed proton-coupled electron transfer (PCET). Related to this HAT process
are two proton transfer (PT) and two electron transfer (ET) self-exchange reactions, for instance, the ET
reactions OsIVNHPh + OsIIINHPh- and OsIVNH2Ph+ + OsIIINH2Ph. All four of these PT and ET reactions
are much faster (k = 103−105 M-1 s-1) than HAT self-exchange. This is the first system where all five
relevant self-exchange rates related to an HAT or PCET reaction have been measured. The slowness of
concerted transfer of H• between OsIVNHPh and OsIIINH2Ph is suggested to result not from a large intrinsic
barrier but rather from a large work term for formation of the precursor complex to H• transfer and/or from
significantly nonadiabatic reaction dynamics. The energetics for precursor complex formation is related to
the strength of the hydrogen bond between reactants. To probe this effect further, HAT cross-reactions
have been performed with sterically hindered aniline/anilide complexes and nitroxyl radical species.
Positioning steric bulk near the active site retards both H• and H+ transfer. Net H• transfer is catalyzed by
trace acids and bases in both self-exchange and cross reactions, by stepwise mechanisms utilizing the
fast ET and PT reactions.