Department of Chemistry, Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States.
J Am Chem Soc. 2011 Aug 31;133(34):13661-73. doi: 10.1021/ja2050474. Epub 2011 Aug 10.
Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO = 2,6-((t)BuC(6)H(3)O)(2)C(6)H(3), THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O(2) activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O(2) activation when dimerized. The product from O(2) activation, [(t)BuOCO]Cr(V)(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr(IV)-O-Cr(IV) dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are g(iso) = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O(2) was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O(2). Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh(3) accelerates O(2) activation by forming the five-coordinate complex trans-[(t)BuOCO]Cr(III)(OPPh(3))(2) (4).
设计了合成和动力学实验，以探究三阴离子钳式铬(III)配合物[(t)BuOCO]Cr(III)(THF)(3)(1)(其中(t)BuOCO=2,6-((t)BuC(6)H(3)O)(2)C(6)H(3),THF=四氢呋喃)激活 O(2)的机理。虽然类似的卟啉和卟啉氧化催化剂在二聚化(形成μ-氧物种)或产物抑制时可能对 O(2)激活失去活性，但当 1 二聚化时，它对 O(2)激活变得更加活跃。O(2)激活的产物[(t)BuOCO]Cr(V)(O)(THF)(2)通过形成μ-O 二聚体{[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O)(3)催化 1 的氧化。复合物 3 与 1 和 2 处于平衡状态，因此不能以纯形式分离。然而，3 和 1 的单晶共沉积，并且使用单晶 X 射线晶体学方法确定了 3 的分子结构。可变(9.5、35 和 240GHz)频率电子顺磁共振波谱支持复合物 3 作为 Cr(IV)-O-Cr(IV)二聚体的分配，根据详细的计算机模拟，具有高(S=2)自旋基态。复合物 3 是第一个明确分配的含有 Cr(IV)二聚体的复合物的例子；其自旋哈密顿参数为 g(iso)=1.976，D=2400G，E=750G。通过紫外-可见分光光度法监测 1 与 O(2)的反应，并确定了试剂的动力学顺序。该反应对复合物 1 和 O(2)的浓度没有表现出一级行为。改变 THF 浓度显示出 THF 的逆序行为。提出了一种自催化机制，其中 3 为关键中间体，用于浓度随时间衰减图的数值模拟，并计算了各个速率常数。模拟与实验观察结果吻合良好。这种加速并不是 2 所独有的；例如，OPPh(3)的存在通过形成五配位复合物反式-[(t)BuOCO]Cr(III)(OPPh(3))(2)(4)加速 O(2)的激活。
