The most reactive third-row transition metal: Guided ion beam and theoretical studies of the activation of methane by Ir +

Abstract

The potential energy surface for activation of methane by the third-row transition metal cation, Ir +, is studied experimentally by examining the kinetic energy dependence of reactions of Ir + with methane, IrCH 2 + with H 2 and D 2, and collision-induced dissociation of IrCH 2 + with Xe using guided ion beam tandem mass spectrometry. A flow tube ion source produces Ir + in its electronic ground state term and primarily in the ground spin–orbit level. We find that dehydrogenation to form IrCH 2 + + H 2 is exothermic, efficient, and the only process observed at low energies for reaction of Ir + with methane, whereas IrH + dominates the product spectrum at higher energies. We also observe the IrH 2 + product, which provides evidence that methane activation proceeds via a dihydride (H) 2IrCH 2 + intermediate. The kinetic energy dependences of the cross sections for several endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D 0(Ir +–2H) > 5.09 ± 0.07, D 0(Ir +–C) = 6.59 ± 0.05, D 0(Ir +–CH) = 6.91 ± 0.23, and D 0(Ir +–CH 3) = 3.25 ± 0.18. D 0(Ir +–CH 2) = 4.92 ± 0.03 eV is determined by measuring the forward and reverse reaction rates for I r + + C H 4 ⇌ IrC H 2 + + H 2 at thermal energy. Ab initio calculations at the B3LYP/HW+/6-311++G(3df,3p) level performed here show reasonable agreement with the experimental bond energies and with the few previous experimental and theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surfaces. We also compare this third-row transition metal system with the first-row and second-row congeners, Co + and Rh +. Differences in reactivity and mechanisms can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.