Carbon–Hydrogen Activation of Cycloalkanes by Cyclopentadienylcarbonylrhodium—A Lifetime Enigma

Abstract

Carbon-hydrogen bond activation reactions of four cycloalkanes (C5H10, C6H12, C7H14, and C8H16) by the Cp'Rh(CO) fragments (Cp' = η(5)-C5H5 (Cp) or η(5)-C5Me5 (Cp*)) were modeled theoretically by combining density functional and coupled cluster theories, and their reaction rates were measured by fast time-resolved infrared spectroscopy. The reaction has two steps, starting with the formation of a σ-complex intermediate, followed by oxidative addition of the C-H bond by the rhodium. A range of σ-complex stabilities among the electronically unique C-H bonds in a cycloalkane were calculated and are related to the individual strengths of the C-H bond's interactions with the Rh fragment and the steric repulsion that is incurred upon forming the specific σ-complex. The unexpectedly large increase in the lifetimes of the σ-complexes from cyclohexane to cycloheptane was predicted to be due to the large range of stabilities of the different σ-complexes found for cycloheptane. The reaction lifetimes were simulated with two mechanisms, with and without migrations among the different σ-complexes, to determine if ring migrations prior to C-H activation were influencing the rate. Both mechanisms predicted similar lifetimes for cyclopentane, cyclohexane, and, to a lesser extent, cycloheptane, suggesting ring migrations do not have a large impact on the rate of C-H activation for these cycloalkanes. For cyclooctane, the inclusion of ring migrations in the reaction mechanism led to a more accurate prediction of the lifetime, indicating that ring migrations did have an effect on the rate of C-H activation for this alkane, and that migration among the σ-complexes is faster than the C-H activation for this larger cycloalkane.

Contrasting electronic requirements for CH binding and CH activation in d6 half‐sandwich complexes of rhenium and tungsten

“…The most favorable process for metal‐walking along the pentane chain amongst all site‐exchange processes studied is, thus, the 1,3‐migration and this result is similar to the theoretical calculations reported for metal migrations in the closely related d 8 –Rh(I) complex [CpRh(CO)(C 4 H 10 )] by George and coworkers . The end‐to‐end migration (1,5‐migration in our model σ‐pentane complexes and 1,4‐migration in George's rhodium–butane σ‐complex) is the least favorable process among all dynamic processes.…”

“…A second aspect that has received significant attention is the intramolecular dynamic process, termed alkane chain walking in many metal‐alkane complexes . The use of 1 H NMR spectroscopy revealed a slow migration of the metal from one CH bond to another in the alkane by Ball and coworkers .…”

“…A second aspect that has received significant attention is the intramolecular dynamic process, termed alkane chain walking in many metal-alkane complexes. [24][25][26][27][28][29][30][31][32][33][34] The use of 1 H NMR spectroscopy revealed a slow migration of the metal from one CAH bond to another in the alkane by Ball and coworkers. [25] They have shown that there is a thermodynamic preference for the coordination of secondary CAH (28) over primary CAH (18) bonds of pentane to the metal center in a complex such as [(g 5 -C 5 H 4 ( i Pr))Re(CO) 2 (C 5 H 12 )] ( i Pr 5 isopropyl) at low temperatures (Scheme 1).…”

A Structurally Characterized Cobalt(I) σ‐Alkane Complex

“…Elegant methods have thus been developed to generate and characterize σ‐alkane complexes in the solution phase (Figure A). Low‐temperature NMR spectroscopy, using in situ photolysis or protonolysis, has revealed σ‐alkane complexes such as 1 and 2 , respectively, while fast time‐resolved spectroscopic studies (TRIR) have probed the formation and onward reactivity of transient σ‐alkane complexes (e.g., 3 ) …”

Calculating singlet excited states: Comparison with fast time-resolved infrared spectroscopy of coumarins

“…Here, we present the predicted infrared spectra of the S 1 excited states of a range of coumarin dyes obtained by fast time-resolved infrared spectroscopy (TRIR) 38,39 and assess the accuracy of computed spectra from TDDFT and eDFT methodologies. These studies demonstrate excellent agreement between experimental and predicted frequencies arising from eDFT calculations at a computational cost sufficiently low to be of potentially wide utility.…”