On the Mechanisms of Degenerate Ligand Exchange in [M(CH₃)]⁺/CH₄ Couples (M=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as Explored by Mass Spectrometric and Computational Studies: Oxidative Addition/Reductive Elimination versus σ‐Complex‐Assisted Metathesis

Abstract

The degenerate ligand exchange in [M(CH(3))](+)/CH(4) couples occurs in the gas phase at room temperature for M=Ni, Ru, Rh, Pd, and Pt, whereas the complexes containing Fe and Co are unreactive. Details of hydrogen-atom scrambling versus direct ligand switch have been uncovered by labeling experiments with CD(4) and (13)CH(4), respectively. The reactivity scale ranges from unreactive (M=Fe, Co) or inefficient (M=Ni, Pd) to moderately (M=Ru) and rather reactive (M=Rh, Pt). Quite extensive, but not complete, H/D exchange between the hydrogen atoms of the incoming and outgoing methyl groups is observed for M=Pt, whereas for M=Ni and Pd a predominantly direct ligand switch prevails. DFT calculations performed at the B3LYP level of theory account well for the thermal nonreactivity of the Fe and Co couples. For [Ni[CH(3))](+)/CH(4), a sigma-complex-assisted metathesis (sigma-CAM) is operative such that, in a two-state reactivity (TSR) scenario, two spin flips between the (3)A ground and (1)A excited states take place at the entrance and exit channels of the encounter complexes. For M=Ru and Rh, only oxidative addition/reductive elimination (OA/RE) is favored energetically, and the reaction is confined to the electronic ground states (3)A and (2)A. In contrast, for the [Pd(CH(3))](+)/CH(4) system, on the (1)A ground-state potential-energy surface both the OA/RE and sigma-CAM variants are energetically comparable, and the small reaction efficiency for the ligand switch is reflected in transition states located energetically close to the reactants. For the [M(CH(3))](+)/CH(4) complexes of the 5d elements, the sigma-CAM mechanism does not play a role. For M=Pt, the energetically most favored path proceeds in a spin-conserving manner on the (1)A potential-energy surface, which accounts for the extensive single and double hydrogen-atom exchange preceding ligand exchange. Although for M=Os and Ir the [M(CH(3))](+) complexes could not be generated experimentally, computational studies predict that both systems may undergo thermal reaction with CH(4), and an OA/RE mechanism will commence on the respective high-spin ground states; however, the bond-activation and ligand-exchange steps will occur on the excited low-spin surfaces in a TSR scenario.

Experimental and Computational Studies on the Iridium Activation of Aliphatic and Aromatic CH Bonds of Alkyl Aryl Ethers and Related Molecules

“…With respect to the intimate mechanism of the C À H bond-activation step, our calculations prove that the s-CAM mechanism [19] is operative in these iridium complexes. This complements previous proposals [20,21] on the applicability of the OA/RE mechanism in systems based on late 5d elements.…”

“…A hydrogen atom is transferred from a s-bonded CÀH of the anisole ligand to a phenyl group, leading to the formation of s-coordinated benzene. Although the viability of such a mechanism in late 5d systems was doubtful, [20,21] our work proves that it can also be operative in iridium complexes, both for aliphatic and aromatic C À H activations. The third C À H activation (21!B) follows a different pathway.…”

A computational study on the intriguing mechanisms of the gas-phase thermal activation of methane by bare [Ni(H)(OH)]+

“…rotation around the Ni-OH bond, exist which are separated by barriers much below the s-metathesis transition state TS2/3. In this s-complex assisted reaction 18 the emerging H 2 molecule of TS2/3 has a bond length of 1.001 Å . TS2/3 leads directly to the ion/molecule complex 4 [(H 2 )Ni(CH 3 )(OH)] + (3); here, formation of the H 2 leaving group is nearly complete as indicated by the close-toequilibrium bond length of 0.766 Å .…”

The “Missing Link”: The Gas‐Phase Generation of Platinum–Methylidyne Clusters PtnCH+ (n=1, 2) and Their Reactions with Hydrocarbons and Ammonia

“…[11b] In the reaction of the methyl complex PtCH 3 + with CD 4 , degenerate H/D exchange occurs, that means the methane isotopologues CH 4Àn D n (n = 1-3) are lost from the adduct complex, thus indicating activation of a D 3 CÀD bond. [39] In contrast, the dinuclear carbene complex Pt 2 CH 2 + as well as the carbides Pt n C + (n = 1, 2) are completely unreactive toward methane. [11b, 12a] A similar situation holds true for the methylidyne complexes PtCH + and Pt 2 CH + .…”