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| - Slow Hydrogen Atom Self-Exchange between Os(IV) Anilideand Os(III) Aniline Complexes: Relationships with Electronand Proton Transfer Self-Exchange
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| - Hydrogen atom, proton and electron transfer self-exchange and cross-reaction rates have beendetermined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to theOs(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 isremarkably 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 processare two proton transfer (PT) and two electron transfer (ET) self-exchange reactions, for instance, the ETreactions OsIVNHPh + OsIIINHPh- and OsIVNH2Ph+ + OsIIINH2Ph. All four of these PT and ET reactionsare much faster (k = 103−105 M-1 s-1) than HAT self-exchange. This is the first system where all fiverelevant self-exchange rates related to an HAT or PCET reaction have been measured. The slowness ofconcerted transfer of H• between OsIVNHPh and OsIIINH2Ph is suggested to result not from a large intrinsicbarrier but rather from a large work term for formation of the precursor complex to H• transfer and/or fromsignificantly nonadiabatic reaction dynamics. The energetics for precursor complex formation is related tothe strength of the hydrogen bond between reactants. To probe this effect further, HAT cross-reactionshave 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 bytrace acids and bases in both self-exchange and cross reactions, by stepwise mechanisms utilizing thefast ET and PT reactions.
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