Department of Chemistry , University of Washington , Campus Box 351700 , Seattle , Washington 98195-1700 , United States.
Department of Chemistry , Tufts University , 62 Talbot Avenue , Medford , Massachusetts 02155 , United States.
J Am Chem Soc. 2019 Sep 25;141(38):15046-15057. doi: 10.1021/jacs.9b04729. Epub 2019 Sep 13.
Herein we quantitatively investigate how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of dioxygen binding versus release from structurally analogous Mn-O complexes, as well as the barrier to Mn peroxo O-O bond cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that the steric and electronic properties of Mn-OOR complexes containing N-heterocyclic (N) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen reactivity of a new Mn complex containing a more electron-rich, less sterically demanding N ligand scaffold, and compare it with previously reported Mn complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N ligands. The reaction landscape for the dioxygen chemistry of the more electron-rich complexes is shown to be relatively flat. A total of four intermediates, including a superoxo and peroxo species, are observed with the most electron-rich complex. Two new intermediates are shown to form following the peroxo, which are capable of cleaving strong X-H bonds. In the absence of a sacrificial H atom donor, solvent, or ligand, serves as a source of H atoms. With TEMPOH as sacrificial H atom donor, a deuterium isotope effect is observed (/ = 3.5), implicating a hydrogen atom transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced prior to the formation of an EPR detected MnMn bimetallic species, and 0.5 equiv after its formation.
在此，我们定量研究了金属离子路易斯酸度和空间性质如何影响结构类似的 Mn-O 配合物对氧气的结合与释放的动力学和热力学，以及 Mn 过氧 O-O 键断裂的能垒和 Mn 氧中间体的反应性。以前我们证明了含有 N-杂环（N）配体支架的 Mn-OOR 配合物的空间和电子性质可以对烷基过氧 O-O 键长度和烷基过氧 O-O 键断裂的能垒产生巨大影响。在此，我们研究了一种含有更富电子、空间要求较低的 N 配体支架的新 Mn 配合物的氧气反应性，并将其与以前报道的 Mn 配合物进行了比较。结果表明，含有更富电子金属离子的配合物中氧气的结合是可逆的。O 结合和过氧 O-O 键断裂的动力学障碍与氧化还原电位以及支撑 N 配体的空间性质相关。结果表明，较富电子配合物的氧气化学的反应势垒相对平坦。总共观察到四个中间体，包括超氧和过氧物种，其中最富电子的配合物中观察到。结果表明，在过氧之后形成了两种新的中间体，它们能够断裂强 X-H 键。在没有牺牲 H 原子供体、溶剂或配体的情况下，溶剂充当 H 原子的来源。用 TEMPOH 作为牺牲 H 原子供体时，观察到氘同位素效应（/ = 3.5），暗示了氢原子转移（HAT）机制。用 1,4-环己二烯作为牺牲 H 原子供体时，在形成可检测到 EPR 的 MnMn 双金属物种之前生成 0.5 当量的苯，在其形成之后生成 0.5 当量的苯。
