Mechanism for C–H bond activation in ethylene in the gas phase vs. in solution – vinylic or agostic? Revisiting the case of protonated Cp*Rh(C2H4)2

Abstract

When Cp*Rh(C(2)H(4))(2)H(+) (2) is exposed to C(2)H(4) in the gas phase, inside the cell of an FT-ICR mass spectrometer, the most notable feature is the lack of any bimolecular reactivity. Collisional activation of 2 leads to ethylene loss and formation of Cp*Rh(C(2)H(4)-mu-H)(+) (3). In contrast to the reactivity of 2 in solution, ethylene dimerisation is negligible in the gas phase. Coordinatively unsaturated 3, rather than 2, is the major species in which reactivity is observed to occur. Compound 3 reacts with ethylene in three parallel processes: (a) Slow addition of ethylene to give 2; (b) rapid, intermolecular hydrogen atom exchange (monitored in separate reactions with free C(2)D(4) to give 3-d(1-5)); (c) ligand substitution of ethylene in 3. DFT calculations reproduce these observations, showing low barriers for hydrogen scrambling, high barrier to ligand loss in 2, and even higher barriers to elimination of either H(2) or ethane. Mechanistic models for the elimination and scrambling processes are discussed.

Mechanistic Molecular Motion of Transition-Metal Mediated Beta-Hydride Transfer: Quasiclassical Trajectories Reveal Dynamically Ballistic, Dynamically Unrelaxed, Two Step, and Concerted Mechanisms

“…Overall, our M06 energies and structures are similar to Hall's and Tilset's previous structures (Figure 1b). 2,5 Relative to 1Rh, our estimate for the G ‡ for TS1Rh is 5.5 kcal/mol and 2Rh is 5.4 kcal/mol. At -80 C these structures have nearly identical Gibbs energies.…”

“…2,3 Experiments and DFT calculations have demonstrated that an agostic C-H-to-metal interaction likely occurs prior to and after -hydrogen transfer. [4][5][6][7][8] Scheme 1. a) Outline of two-step and one-step -hydrogen-to-ethylene transfer reaction mechanisms. b) Qualitative energy landscape for the two-step sequence of -hydrogen elimination with a metal-hydride intermediate followed by migratory insertion.…”

Quasiclassical Direct Dynamics Trajectory Simulations of Organometallic Reactions

“…We chose to focus our direct dynamics study on the β-hydrogen transfer reaction of [Cp*Rh III (Et)­(ethylene)] + (and Ir and Co analogues) because previous static DFT calculations showed that this Rh reaction has a two-step β-hydrogen transfer energy landscape (Figure ). , …”

“…We chose to focus our direct dynamics study on the β-hydrogen transfer reaction of [Cp*Rh III (Et)-(ethylene)] + (and Ir and Co analogues) because previous static DFT calculations showed that this Rh reaction has a two-step βhydrogen transfer energy landscape (Figure 6). 48,49 After confirming the two-step potential energy landscape with the M06 functional, quasiclassical trajectories commencing from the β-hydrogen transfer transition state (TS1Rh; Figure 6) revealed complete dynamical skipping of the Rh−H intermediate. 50 Figure 7a shows 3D depictions of time snapshots of a representative dynamically ballistic Rh trajectory.…”

Dehydrogenation and demethanation of 2-methylpropane and propane in the gas-phase by the 16-electron complex [Ru(bipy)2(CO)]2+* chemically activated by the association of [Ru(bipy)2]2+ and CO

“…[49][50][51][52][53] The Uggerud group investigated the mechanism for C-H bond activation of ethylene by 18e − Rh(III) and 16e − Rh(III) complexes inside the ICR cell in which H 2 elimination was observed in the reaction of ethylene and Cp-Rh(C 2 H 4 -μH) + complexes. 54 DFT calculations showed that vinylic C-H bond activation is an energetically unfavorable process. Rather, it was shown that vinylic C-H activation is possible if a collision between C 2 H 4 and [Cp-Rh-(C 2 H 4 -μH)] + forms an energy rich-adduct product, Cp-Rh(C 2 H 4 -μH)(C 2 H 4 ) + *, which undergoes the vinylic C-H cleavage.…”