Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rate of CâH activation of alkanes by Cpâ²Rh(CO) Cpâ² = η5-C5H5 or η5-C5Me5; alkane = cyclopentane, cyclohexane and neopentane (Cp only)} in solution at room temperature and allowed the determination of how the change in rate of oxidative cleavage varies between complexes and alkanes. Density functional theory calculations on these complexes, transition states, and intermediates provide insight into the mechanism and barriers observed in the experimental results. Unlike our previous study of the linear alkanes, where activation occurred at the primary CâH bonds with a rate governed by a balance between these activations and hopping along the chain, the rate of CâH activation in cyclic alkanes is controlled mainly by the strength of the alkane binding. Although the reaction of CpRh(CO)(neopentane) to form CpRh(CO)(neopentyl)H clearly occurs at a primary CâH bond, the rate is much slower than the corresponding reactions with cyclic alkanes because of steric factors with this bulky alkane.
快速的时间分辨红外光谱测量使我们能够准确确定Cp'Rh(CO) Cp' = η5-
C5H5或η5-C5Me5;
烷烃 =
环戊烷、
环己烷和
新戊烷(仅限Cp)}在溶液中常温下对
烷烃的C–H活化速率,并探究不同复合物和
烷烃之间氧化裂解速率变化的情况。对这些复合物、过渡态和中间体的密度泛函理论计算提供了对实验结果中观察到的机制和能垒的洞察。与我们之前对线性
烷烃的研究不同,线性
烷烃的活化发生在初级C–H键上,速率受到这些活化与链上跳跃之间平衡的影响,而在环
烷烃中的C–H活化速率主要受
烷烃结合强度的控制。尽管CpRh(CO)(
新戊烷)反应生成CpRh(CO)(新戊基)H明显发生在初级C–H键上,但由于这种体积庞大的
烷烃的立体因素,其反应速率远低于与环
烷烃的相应反应。