Combined experimental and theoretical investigation into C–H activation of cyclic alkanes by Cp′Rh(CO)2 (Cp′ = η5-C5H5 or η5-C5Me5)

Abstract

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rate of C-H activation of alkanes by Cp'Rh(CO) {Cp' = η(5)-C(5)H(5) or η(5)-C(5)Me(5); 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 C-H bonds with a rate governed by a balance between these activations and hopping along the chain, the rate of C-H 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 C-H bond, the rate is much slower than the corresponding reactions with cyclic alkanes because of steric factors with this bulky alkane.

Probing Organometallic Reactions by Time-Resolved Infrared Spectroscopy in Solution and in the Solid State Using Quantum Cascade Lasers

“…This new band can be readily assigned to the formation of the Rh-cyclohexane σ-complex, Cp*Rh(CO)(C 6 H 12 ), by comparison with previous work. 28,–34 The σ-alkane complex reacts to form the C–H-activated product, Cp*Rh(CO)(C 6 H 11 )H, with a lifetime of 26 (63) ns, which is in good agreement to the previously published literature value of 26 (62) ns. 34 The lifetime for this process was determined by deconvoluting the detector response function from the measured kinetic traces.…”

“…The reactions of the photochemically generated CO-loss product, Cp*Rh(CO) with alkanes have been studied in detail and are summarized in Scheme 1. 28,–34 Upon UV photolysis, loss of CO from the parent occurs within a few picoseconds, and the fragment, Cp*Rh(CO), forms a σ-complex with the alkane solvent. This species is characterized by a v(CO) band to lower energy of the parent vibrations.…”

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

“…Many, if not most of the excellent experimental studies have benefitted greatly from the synergy arising from computational studies . The latter provides insight into the process, that cannot be obtained from experiments.…”

CO Displacement in an Oxidative Addition of Primary Silanes to Rhodium(I)

“…Photolytic dissociation of CO is a prerequisite for reactions of Cp*Rh(CO) 2 or Tp*Rh(CO) 2 (Cp* = C 5 Me 5 ; Tp* = tris(3,5-dimethyl-1-pyrazolyl)borate) with C-H or Si-H bonds. [33][34][35][36][37][38][39][40] For example, Tp*Rh(CO) 2 and Et 3 SiH react under photochemical conditions to give Tp*RhH(SiEt 3 )CO and CO (Figure 1). 41 A sequence of photolytic CO dissociation followed by C-H cleavage is invoked here, as well as in rhodium-or iridium-catalyzed carbonylations of benzene, which occur under continuous irradiation.…”

“…41 A sequence of photolytic CO dissociation followed by C-H cleavage is invoked here, as well as in rhodium-or iridium-catalyzed carbonylations of benzene, which occur under continuous irradiation. 42,43 Detailed time-resolved spectroscopic and computational studies of the mechanism of C-H and Si-H bond activation by CpRh(CO), [44][45][46][47] Cp*Rh(CO), 37,38,48 TpRh(CO), 49 Tp*Rh(CO), 46,[50][51][52][53] and Tp*Rh(CNR), 40 formed via photolytic ligand dissociation, reveal details and lifetimes of the activation process. Interestingly, bidentate and tridentate coordination of pyrazolylborate ligands modulate the electronic structure of rhodium to affect C-H bond oxidative cleavage.…”