铬氧化物团簇负离子(CrO_(3))_(1-4)O^(-)活化烷烃的尺寸效应

Size-Dependent Reactivity of Chromium Oxide Cluster Anions (CrO_(3))_(1−4)O^(−) towards Alkanes

本文采用自行研制的耦合船闸式反应装置的飞行时间质谱,研究了铬氧化物团簇负离子(CrO_(3))_(1-4)O^(-)与低碳烷烃(C1~C4)在298 K温度下的反应.结果表明CrO4-和Cr2O7-团簇可以分别抽取乙烷和甲烷中的氢原子,而Cr3O10-和Cr4O13-团簇在检测下限范围内仍对正丁烷呈惰性.理论计算结果表明CrO4-和Cr2O7-团簇具有离域的铬氧自由基(Cr-O^(-·)),与实验中观测到的CrO_(3)_(1,2)O^(-)团簇可以与烷烃发生氢抽取反应相一致.然而,CrO_(3)_(3,4)O^(-)团簇的活性位点转化为相对于O^(-·)自由基活性较低的过氧物种(O_(2)^(2-)).团簇尺寸减小导致CrOy单元具有更负的电荷环境,进而诱使Cr-3d轨道能量升高,可能是引起小尺寸CrO_(3)_(1,2)O^(-)团簇中O_(2)^(2-)位点更倾向于被还原并选择性生成高活性O^(-·)自由基的原因.本工作不仅丰富了人们对与金属键合的O一自由基在温和条件下活化烷烃的化学认识,而且提供了金属氧化物中O_(2)^(2-)物种和O^(-·)自由基之间相互转化的新见解.

The reactivity of chromium oxide cluster anions(CrO_(3))_(1−4)O^(−)towards low carbon(C1−C4)alkanes has been experimentally investigated at 298 K by employing a homemade ship-lock-type reactor coupled with a time-of-flight mass spectrometer.The results demonstrate that while CrO4−and Cr2O7−clusters could abstract a hydrogen atom from C2H6 and CH4,respectively,Cr3O10−and Cr4O13−clusters were found to be inert towards n-C4H10 within the detection limit.Theoretical results reveal that CrO4−and Cr2O7−clusters possess delocalized chromium-bonded oxygen radicals(Cr−O−·),which rationalizes the hydrogen atom abstraction reactions between(CrO3)1,2O−clusters and alkanes.However,the active sites of(CrO_(3))_(3,4)O^(−)clusters evolve to peroxide species(O_(2)^(2−)),which exhibit inferior activity compared to O^(−·)radicals.The increase of Cr-3d orbital energy driven by the more negative charge around CrOy unit formed via downsizing the cluster size has been proposed to account for favorable reduction of O_(2)^(2−)and selective generation of reactive O^(−·)radicals in small-sized(CrO_(3))_(1,2)O^(−)clusters.This study not only enriches the chemistry of metal-bonded O−·radicals toward alkane activation under mild conditions,but also provides a new insight into the conversion between O_(2)^(2−)and O^(−·)radicals over metal oxides.