Gas-Phase Rates of Alkane C-H Oxidative Addition to a Transient CpRh(CO) Complex

Abstract

The gas-phase irradiation of CpRh(CO)(2) (Cp = eta(5)-C(5)H(5)) was examined in order to study the rates of reaction of the 16-electron intermediates presumed to be involved in the C-H oxidative addition of alkanes. "Naked" (unsolvated) CpRh(CO) was detected, and direct measurements of the rates of reaction of this very short-lived complex with alkane C-H bonds were made. Activation of C-H bonds occurs on almost every collision for alkanes of moderate size, and intermediates in which the alkanes are bound to the metal centers, without their C-H bonds being fully broken, are implicated as intermediates in the overall reaction.

Formation and reactivity of organometallic alkane complexes

“…This entropic destabilisation of the organometallic alkane has been suggested to be most pronounced in longer chain linear alkane complexes [50]. A correlation between n-alkane chain length and binding energy to organometallic metal centres has also been previously reported, with the lighter alkanes being found to only bind weakly to metal centres [40,71]. Interestingly, a TRIR spectroscopic study on the reaction of Cr(Á 6 -C 6 H 6 )(CO) 2 (n-alkane) (n-pentane, n-heptane, n-decane) with CO, showed an increased rate of reactivity as the alkane chain length was increased attributed to an increase in the entropy of activation with increasing alkane chain length [50].…”

“…Gas phase room temperature TRIR experiments on the photolysis of Rh(Cp)(CO) 2 in the presence of a range of alkane solvents demonstrated that the initially formed Rh(Cp)(CO) rapidly decayed to form the alkyl hydride products Rh(Cp)(CO)(R)(H), at or close to, the gas-kinetic rate [40]. Interestingly, it was postulated that the C-H activation occurred via an organometallic alkane complex, in which the lighter chain alkanes were suggested to be more weakly bound.…”

First row benchmark tests of the parametrized configuration interaction with parameter X (PCI-X) scheme

“…1 This is one of the smallest systems actually used in experiments to activate C-H bonds of alkanes. 2 Yet, it is at present at the absolute limits of what can be treated with current quantum chemical high accuracy methodology even though only double zeta plus polarization basis ͑DZP͒ sets can be afforded for this system. DZP basis sets in coupled cluster-type calculations lead to bond strengths far away from quantitative accuracy with typical errors of 5 kcal/mol for C-H bonds and of 7-10 kcal/mol for transition metal single bonds.…”

Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

“…The intermediacy of -alkane complexes has been found to play a critical role in these activation reactions (11)(12)(13)(14). Binding of alkanes has been studied experimentally by using time-resolved IR (15)(16)(17) and NMR spectroscopies (18).…”