Jiao X D, Metelski P D, Espenson J H
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011, USA.
Inorg Chem. 2001 Jun 18;40(13):3228-33. doi: 10.1021/ic0005801.
The oxidation of hydrogen bromide and alkali metal bromide salts to bromine in acetic acid by cobalt(III) acetate has been studied. The oxidation is inhibited by Mn(OAc)(2) and Co(OAc)(2), which lower the bromide concentration through complexation. Stability constants for Co(II)Br(n)() were redetermined in acetic acid containing 0.1% water as a function of temperature. This amount of water lowers the stability constant values as compared to glacial acetic acid. Mn(II)Br(n)() complexes were identified by UV-visible spectroscopy, and the stability constants for Mn(II)Br(n)() were determined by electrochemical methods. The kinetics of HBr oxidation shows that there is a new pathway in the presence of M(II)Br(n)(). Analysis of the concentration dependences shows that CoBr(2) and MnBr(2) are the principal and perhaps sole forms of the divalent metals that react with Co(III) and Mn(III). The interpretation of these data is in terms of this step (M, N = Mn or Co): M(OAc)(3) + N(II)Br(2) + HOAc --> M(OAc)(2) + N(III)Br(2)OAc. The second-order rate constants (L mol(-)(1) s(-)(1)) for different M, N pairs in glacial acetic acid are 4.8 (Co, Co at 40 degrees C), 0.96 (Mn, Co at 20 degrees C), 0.15 (Mn(III).Co(II), Co at 20 degrees C), and 0.07 (Mn, Mn at 20 degrees C). Following that, reductive elimination of the dibromide radical is proposed to occur: N(III)Br(2)OAc + HOAc --> N(OAc)(2) + HBr(2)(*). This finding implicates the dibromide radical as a key intermediate in this chemistry, and indeed in the cobalt-bromide catalyzed autoxidation of methylarenes, for which some form of zerovalent bromine has been identified. The selectivity for CoBr(2) and MnBr(2) is consistent with a pathway that forms this radical rather than bromine atoms which are at a considerably higher Gibbs energy. Mn(OAc)(3) oxidizes PhCH(2)Br, k = 1.3 L mol(-)(1) s(-)(1) at 50.0 degrees C in HOAc.
研究了乙酸钴（III）将氢溴酸和碱金属溴化物盐氧化为溴的反应。该氧化反应受到醋酸锰（II）和醋酸钴（II）的抑制，它们通过络合作用降低了溴化物的浓度。重新测定了在含0.1%水的乙酸中，作为温度函数的Co（II）Brₙ⁻的稳定常数。与冰醋酸相比，这一水量降低了稳定常数的值。通过紫外可见光谱法鉴定了Mn（II）Brₙ⁻络合物，并用电化学方法测定了Mn（II）Brₙ⁻的稳定常数。HBr氧化反应的动力学表明，在M（II）Brₙ⁻存在时存在一条新的反应途径。对浓度依赖性的分析表明，CoBr₂和MnBr₂是与Co（III）和Mn（III）反应的二价金属的主要形式，甚至可能是唯一形式。这些数据的解释基于这一步骤（M、N = Mn或Co）：M（OAc）₃ + N（II）Br₂ + HOAc → M（OAc）₂ + N（III）Br₂OAc。在冰醋酸中，不同M、N对的二级速率常数（L mol⁻¹ s⁻¹）分别为4.8（40℃时Co，Co）、0.96（20℃时Mn，Co）、0.15（20℃时Mn（III）·Co（II），Co）和0.07（20℃时Mn，Mn）。随后，提出发生二溴化物自由基的还原消除反应：N（III）Br₂OAc + HOAc → N（OAc）₂ + HBr₂⁎。这一发现表明二溴化物自由基是该化学反应中的关键中间体，实际上在钴 - 溴化物催化的甲基芳烃自氧化反应中也是如此，在该反应中已鉴定出某种形式的零价溴。对CoBr₂和MnBr₂的选择性与形成该自由基的反应途径一致，而不是与吉布斯自由能高得多的溴原子的反应途径一致。醋酸锰（III）在50.0℃的HOAc中氧化PhCH₂Br，k = 1.3 L mol⁻¹ s⁻¹。
