Carbon Monoxide Activation via O-Bound CO Using Decamethylscandocinium–Hydridoborate Ion Pairs

Ion pairs [Cp*2Sc]+[HB­(p-C6F4R)3]− (R =
F, 1-F; R = H, 1-H) were prepared and shown
to be unreactive toward D2 and α-olefins, leading
to the conclusion that no back-transfer of hydride from boron to scandium
occurs. Nevertheless, reaction with CO is observed to yield two products,
both ion pairs of the [Cp*2Sc]+ cation with
formylborate (2-R) and borataepoxide (3-R) counteranions. DFT calculations show that these products arise
from the carbonyl adduct of the [Cp*2Sc]+ in
which the CO is bonded to scandium through the oxygen atom, not the
carbon atom. The formylborate 2-R is formed in a two-step
process initiated by an abstraction of the hydride by the carbon end
of an O-bound CO, which forms an η2-formyl intermediate
that adds, in a second step, the borane at the carbon. The borataepoxide 3-R is suggested to result from an isomerization of 2-R. This unprecedented reaction represents a new way to activate
CO via a reaction channel emanating from the ephemeral isocarbonyl
isomer of the CO adduct.

离子对[双(五甲基环戊二烯基（Cp*）)钪]+[三(对-R四氟苯基)硼氢根]−（R=F时记为1-F；R=H时记为1-H）已被成功合成，实验证实其与氘气（D₂）、α-烯烃均不发生反应，由此可得出结论：不存在氢负离子从硼到钪的反向转移过程。不过，该离子对与一氧化碳（CO）的反应可生成两种产物，二者均为[双(五甲基环戊二烯基（Cp*）)钪]+阳离子与甲酰基硼酸根（2-R）、硼杂环氧阴离子（3-R）抗衡阴离子形成的离子对。密度泛函理论（DFT）计算表明，上述产物均源自该阳离子的羰基加合物，其中一氧化碳（CO）通过氧原子而非碳原子与钪中心配位。甲酰基硼酸根产物2-R通过两步反应生成：首先，与氧配位的一氧化碳的碳端夺取氢负离子，形成η²-甲酰基中间体；随后，该中间体的碳位点与三(对-R四氟苯基)硼烷发生加成反应。硼杂环氧阴离子产物3-R被认为是由2-R经异构化反应得到。该前所未有的反应为一氧化碳的活化提供了一条新途径，其反应路径源自一氧化碳加合物的瞬态异羰基异构体。