Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States.
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
J Am Chem Soc. 2022 Jun 15;144(23):10361-10367. doi: 10.1021/jacs.2c02128. Epub 2022 Jun 3.
The catalytic hydrofunctionalization of alkenes through radical-polar crossover metal hydrogen atom transfer (MHAT) offers a mild pathway for the introduction of functional groups in sterically congested environments. For M = Co, this reaction is often proposed to proceed through secondary alkylcobalt(IV) intermediates, which have not been characterized unambiguously. Here, we characterize a metastable (salen)Co(isopropyl) cation, which is capable of forming C-O bonds with alcohols as proposed in the catalytic reaction. Electron nuclear double resonance (ENDOR) spectroscopy of this formally cobalt(IV) species establishes the presence of the cobalt-carbon bond, and accompanying DFT calculations indicate that the unpaired electron is localized on the cobalt center. Both experimental and computational studies show that the cobalt(IV)-carbon bond is stronger than the analogous bond in its cobalt(III) analogue, which is opposite of the usual oxidation state trend of bond energies. This phenomenon is attributable to an inverted ligand field that gives the bond Co-C character and explains its electrophilic reactivity at the alkyl group. The inverted Co-C bond polarity also stabilizes the formally cobalt(IV) alkyl complex so that it is accessible at unusually low potentials. Even another cobalt(III) complex, [(salen)Co], is capable of oxidizing (salen)Co(Pr) to the formally cobalt(IV) state. These results give insight into the electronic structure, energetics, and reactivity of a key reactive intermediate in oxidative MHAT catalysis.
烯烃的催化氢官能化通过自由基-极性交叉金属氢原子转移 (MHAT) 提供了一种在空间拥挤环境中引入官能团的温和途径。对于 M = Co，该反应通常被认为通过次级烷基钴(IV)中间体进行，但其尚未被明确表征。在这里，我们对一种亚稳的 (salen)Co(异丙基)阳离子进行了表征，该阳离子能够与醇形成 C-O 键，正如催化反应中所提出的那样。对这种形式上的钴(IV)物种的电子核双共振 (ENDOR) 光谱研究确立了钴-碳键的存在，伴随的 DFT 计算表明未配对电子定域在钴中心上。实验和计算研究均表明，钴(IV)-碳键比其钴(III)类似物中的类似键更强，这与键能的通常氧化态趋势相反。这种现象归因于反键配位场赋予了键 Co-C 特性，并解释了其在烷基上的亲电性反应性。反式 Co-C 键极性还稳定了形式上的钴(IV)烷基配合物，使其能够在异常低的电位下获得。甚至另一种钴(III)配合物 [(salen)Co] 也能够将 (salen)Co(Pr) 氧化为形式上的钴(IV)态。这些结果深入了解了氧化 MHAT 催化中关键反应性中间体的电子结构、能量学和反应性。
