Evaluation of Cobalt Complexes Bearing Tridentate Pincer Ligands for Catalytic C–H Borylation

Cobalt­(II)
dichloride complexes supported by a variety of neutral,
tridentate pincer ligands have been prepared and, following in situ
activation with NaBEt3H, evaluated for the catalytic borylation
of 2-methylfuran, 2,6-lutidine, and benzene using both HBPin and B2Pin2 (Pin = pinacolate) as boron sources. Preparation
of well-defined organometallic compounds in combination with stoichiometric
experiments with HBPin and B2Pin2 provided insight
into the nature and kinetic stability of the catalytically relevant
species. In cases where sufficiently electron donating pincers are
present, such as with bis­(phosphino)­pyridine chelates, Co­(III) resting
states are preferred and catalytic C–H borylation is efficient.
Introduction of a redox-active subunit into the pincer reduces its
donating ability and, as a consequence, the accessibility of a Co­(III)
resting state. In these cases, unusual mixed-valent μ-hydride
cobalt complexes have been crystallographically and spectroscopically
characterized. These studies have also shed light on the active species
formed during in situ activated cobalt alkene hydroboration catalysis
and provide important design criteria in base metal catalyzed C–B
bond forming reactions.

多种由各类中性三齿钳形配体（tridentate pincer ligands）支撑的二氯化钴(II)配合物（Cobalt(II) dichloride complexes）已被合成。随后，该类配合物经三乙基硼氢化钠（NaBEt3H）原位活化后，以频哪醇硼烷（HBPin）和双(频哪醇合)二硼（B2Pin2，其中Pin为频哪醇基（pinacolate））作为硼源，针对2-甲基呋喃（2-methylfuran）、2,6-二甲基吡啶（2,6-lutidine）与苯（benzene）开展催化硼化反应的性能评价。通过制备结构明确的有机金属化合物（well-defined organometallic compounds），并结合HBPin与B2Pin2的计量反应实验，我们得以深入解析催化相关物种的本质与动力学稳定性。当配体为具备足够给电子能力的钳形配体时，例如双(膦基)吡啶螯合配体（bis(phosphino)pyridine chelates），体系更倾向于形成钴(III)休眠态（Co(III) resting states），此时催化C-H硼化反应的效率较高。若在钳形配体中引入氧化还原活性亚基（redox-active subunit），则会降低其给电子能力，进而削弱钴(III)休眠态的可及性。此类情况下，研究人员通过晶体学与光谱学手段表征了一类罕见的混合价态μ-氢桥钴配合物（mixed-valent μ-hydride cobalt complexes）。本研究还阐明了原位活化钴基烯烃硼氢化催化过程中形成的活性物种，并为贱金属催化的C-B键形成反应（base metal catalyzed C–B bond forming reactions）提供了重要的设计准则。