Culver Damien B, Venkatesh Amrit, Huynh Winn, Rossini Aaron J, Conley Matthew P
Department of Chemistry, University of California Riverside California 92521 USA
Department of Chemistry, Iowa State University Ames Iowa 50011 USA.
Chem Sci. 2019 Dec 19;11(6):1510-1517. doi: 10.1039/c9sc05904k.
Weakly Coordinating Anions (WCAs) containing electron deficient delocalized anionic fragments that are reasonably inert allow for the isolation of strong electrophiles. Perfluorinated borates, perfluorinated aluminum alkoxides, and halogenated carborane anions are a few families of WCAs that are commonly used in synthesis. Application of similar design strategies to oxide surfaces is challenging. This paper describes the reaction of Al(OR)*PhF (R = C(CF)) with silica partially dehydroxylated at 700 °C (SiO) to form the bridging silanol [triple bond, length as m-dash]Si-OH⋯Al(OR) (). DFT calculations using small clusters to model show that the gas phase acidity (GPA) of the bridging silanol is 43.2 kcal mol lower than the GPA of HSO, but higher than the strongest carborane acids, suggesting that deprotonated would be a WCA. Reactions of with NOct show that forms weaker ion-pairs than classical WCAs, but stronger ion-pairs than carborane or borate anions. Though forms stronger ion-pairs than these state-of-the-art WCAs, reacts with alkylsilanes to form silylium type surface species. To the best of our knowledge, this is the first example of a silylium supported on derivatized silica.
弱配位阴离子（WCAs）含有电子缺乏的离域阴离子片段，这些片段具有相当的惰性，能够实现强亲电试剂的分离。全氟硼酸盐、全氟铝醇盐和卤代碳硼烷阴离子是合成中常用的几类WCAs。将类似的设计策略应用于氧化物表面具有挑战性。本文描述了Al(OR)*PhF（R = C(CF)）与在700°C部分脱羟基的二氧化硅（SiO）反应形成桥连硅醇[三键，长度为中划线]Si-OH⋯Al(OR)（）。使用小簇模型进行的DFT计算表明，桥连硅醇的气相酸度（GPA）比HSO的GPA低43.2 kcal mol，但高于最强的碳硼烷酸，这表明去质子化的将是一种WCA。与NOct的反应表明，形成的离子对比经典的WCAs弱，但比碳硼烷或硼酸盐阴离子强。尽管比这些先进的WCAs形成更强的离子对，但与烷基硅烷反应形成甲硅烷基型表面物种。据我们所知，这是在衍生化二氧化硅上负载甲硅烷基的第一个例子。
