Soper Jake D, Mayer James M
Department of Chemistry, Campus Box 351700, University of Washington, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2003 Oct 8;125(40):12217-29. doi: 10.1021/ja036328k.
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)Cl(2) (Os(IV)NHPh) gives the Os(III) aniline complex TpOs(NH(2)Ph)Cl(2) (Os(III)NH(2)Ph) with a new 66 kcal mol(-1) N-H bond. Concerted transfer of H* between Os(IV)NHPh and Os(III)NH(2)Ph is remarkably slow in MeCN-d(3), with k(ex)(H*) = (3 +/- 2) x 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 Os(IV)NHPh + Os(III)NHPh(-) and Os(IV)NH(2)Ph(+) + Os(III)NH(2)Ph. All four of these PT and ET reactions are much faster (k = 10(3)-10(5) 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 Os(IV)NHPh and Os(III)NH(2)Ph 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.
已测定了Os(IV)和Os(III)苯胺及酰苯胺配合物反应中氢原子、质子和电子转移的自交换及交叉反应速率。向Os(IV)酰苯胺TpOs(NHPh)Cl₂(Os(IV)NHPh)中添加一个氢原子，生成具有新的66千卡摩尔⁻¹ N - H键的Os(III)苯胺配合物TpOs(NH₂Ph)Cl₂(Os(III)NH₂Ph)。在MeCN - d₃中，Os(IV)NHPh和Os(III)NH₂Ph之间H⁺的协同转移非常缓慢，在298 K时k(ex)(H⁺) = (3 ± 2)×10⁻³ M⁻¹ s⁻¹。这种氢原子转移(HAT)反应也可称为质子耦合电子转移(PCET)。与该HAT过程相关的是两个质子转移(PT)和两个电子转移(ET)自交换反应，例如ET反应Os(IV)NHPh + Os(III)NHPh⁻和Os(IV)NH₂Ph⁺ + Os(III)NH₂Ph。所有这四个PT和ET反应都比HAT自交换快得多(k = 10³ - 10⁵ M⁻¹ s⁻¹)。这是第一个测量了与HAT或PCET反应相关的所有五个相关自交换速率的体系。Os(IV)NHPh和Os(III)NH₂Ph之间H⁺协同转移的缓慢被认为不是由于大的内在势垒，而是由于形成H⁺转移前体配合物的大的功项和/或显著的非绝热反应动力学。前体配合物形成的能量学与反应物之间氢键的强度有关。为了进一步探究这种效应，已用空间位阻苯胺/酰苯胺配合物和硝酰自由基物种进行了HAT交叉反应。在活性位点附近定位空间位阻会阻碍H⁺和H⁺转移。在自交换和交叉反应中，痕量酸和碱通过利用快速ET和PT反应的分步机制催化净H⁺转移。
