Saucedo-Vázquez Juan Pablo, Kroneck Peter M H, Sosa-Torres Martha Elena
Departamento de Química Inorgánica y Nuclear, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. 04510, México.
Dalton Trans. 2015 Mar 28;44(12):5510-9. doi: 10.1039/c4dt03606a.
A mechanistic study is presented of the oxidative dehydrogenation of the iron(III) complex Fe(III)L(3), 1, (L(3) = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectroscopy and X-ray crystallography as the identical monoimine complex Fe(II)L(4), 2, (L(4) = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanon-1-ene) also formed under an inert nitrogen atmosphere. Molecular oxygen is an active player in the oxidative dehydrogenation of iron(III) complex 1. Reduced oxygen species, e.g., superoxide, (O2˙(-)) and peroxide (O2(2-)), are formed and undergo single electron transfer reactions with ligand-based radical intermediates. The experimental rate law can be described by the third order rate equation, -d[(Fe(III)L(3))(3+)]/dt = kOD[(Fe(III)L(3))(3+)][EtO(-)][O2], with kOD = 3.80 ± 0.09 × 10(7) M(-2) s(-1) (60 °C, μ = 0.01 M). The reduction O2 → O2˙(-) represents the rate determining step, with superoxide becoming further reduced to peroxide as shown by a coupled heme catalase assay. In an independent study, with H2O2, replacing O2 as the oxidant, the experimental rate law depended on [H2O2]: -d[(Fe(III)L(3))(3+)]/dt = kH2O2[(Fe(III)L(3))(3+)][H2O2]), with kH2O2 = 6.25 ± 0.02 × 10(-3) M(-1) s(-1). In contrast to the reaction performed under N2, no kinetic isotope effect (KIE) or general base catalysis was found for the reaction of iron(III) complex 1 with O2. Under N2, two consecutive one-electron oxidation steps of the ligand coupled to proton removal produced the iron(II)-monoimine complex Fe(II)L(4) and the iron(II)-amine complex Fe(II)L(3) in a 1 : 1 ratio (disproportionation), with the amine deprotonation being the rate determining step. Notably, the reaction is almost one order of magnitude faster in the presence of O2, with kEtO(-) = 3.02 ± 0.09 × 10(5) M(-1) s(-1) (O2) compared to kEtO(-) = 4.92 ± 0.01 × 10(4) M(-1) s(-1) (N2), documenting the role of molecular oxygen in the dehydrogenation reaction.
本文介绍了在分子氧存在下，乙醇中铁(III)配合物Fe(III)L(3)，1，（L(3)=1,9-双(2'-吡啶基)-5-[(乙氧基-2''-吡啶基)甲基]-2,5,8-三氮杂壬烷）的氧化脱氢机理研究。通过核磁共振光谱和X射线晶体学确定反应产物为相同的单亚胺配合物Fe(II)L(4)，2，（L(4)=1,9-双(2'-吡啶基)-5-[(乙氧基-2''-吡啶基)甲基]-2,5,8-三氮杂壬-1-烯），该配合物在惰性氮气气氛下也会形成。分子氧在铁(III)配合物1的氧化脱氢反应中起着重要作用。会形成还原态氧物种，如超氧阴离子（O2˙(-)）和过氧化物（O2(2-)），它们与基于配体的自由基中间体发生单电子转移反应。实验速率定律可用三阶速率方程描述：-d[(Fe(III)L(3))(3+)]/dt = kOD[(Fe(III)L(3))(3+)][EtO(-)][O2]，其中kOD = 3.80 ± 0.09 × 10(7) M(-2) s(-1)（60°C，μ = 0.01 M）。O2还原为O2˙(-)是速率决定步骤，如耦合血红素过氧化氢酶测定所示，超氧阴离子会进一步还原为过氧化物。在一项独立研究中，用H2O2替代O2作为氧化剂，实验速率定律取决于[H2O2]：-d[(Fe(III)L(3))(3+)]/dt = kH2O2[(Fe(III)L(3))(3+)][H2O2]，其中kH2O2 = 6.25 ± 0.02 × 10(-3) M(-1) s(-1)。与在N2下进行的反应不同，铁(III)配合物1与O2的反应未发现动力学同位素效应（KIE）或一般碱催化作用。在N2下，配体的两个连续单电子氧化步骤与质子去除相耦合，以1:1的比例生成铁(II)-单亚胺配合物Fe(II)L(4)和铁(II)-胺配合物Fe(II)L(3)（歧化反应），胺去质子化是速率决定步骤。值得注意的是，在O2存在下反应速度快近一个数量级，与kEtO(-)=4.92 ± 0.01 × 10(4) M(-1) s(-1)（N2）相比，kEtO(-)=3.02 ± 0.09 × 10(5) M(-1) s(-1)（O2），这证明了分子氧在脱氢反应中的作用。
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