Spin‐Forbidden Reactivity of Transition Metal Oxo Species: Exploring the Potential Energy Surfaces

Abstract

Spin‐forbidden reactions are frequently encountered when transition metal oxo species are involved, particularly in oxygen transfer reactivity. The computational study of such reactions is challenging, because reactants and products are located on different spin potential energy surfaces (PESs). One possible approach to describe these reactions is the so‐called minimum energy crossing point (MECP) between the diabatic reactants and products PESs. Alternatively, inclusion of spin‐orbit coupling (SOC) effects allows to locate a saddle point on a single adiabatic PES (TS SOC). The TS SOC approach is rarely applied because of its high computational cost. Recently evidence for a TS SOC impact on significantly lowering the activation barrier in dioxygen addition to a carbene‐gold(I)‐hydride complex reaction (Chem. Sci. 2016, 7, 7034–7039) or even on predicting a qualitatively different reaction mechanism in mercury methylation by cobalt corrinoid (Angew. Chem. Int. Ed. 2016, 55, 11503–11506) has been put forward. Using MECP and TS SOC approaches a systematic analysis is provided here of three prototypical transition metal oxo spin‐forbidden processes to investigate their implications on reactivity. Cycloaddition of ethylene to chromyl chloride (CrO2Cl2+C2H4), iron oxide cation insertion into the hydrogen molecule (FeO++H2) and H‐abstraction from toluene by a MnV‐oxo‐porphyrin cation (MnOP(H2O)++C6H5CH3) are case studies. For all these processes the MECP and TS SOC results are compared, which show that the spin‐forbidden reactivity of transition metal oxo species can be safely described by a MECP approach, at least for the first‐row transition metals investigated here, where the spin‐orbit coupling is relatively weak. However, for the Mn‐oxo reactivity, the MECP and TS SOC have been found to be crucial for a correct description of the reaction mechanism. In particular, the TS SOC approach allows to straightforwardly explore detailed features of the adiabatic potential energy surface which in principle could affect the overall reaction rate in cases where the involved diabatic PESs are tricky.

The Effect of Hartree-Fock Exchange on Scaling Relations and Reaction Energetics for C–H Activation Catalysts

“…For catalysis with a single metal site, a closely related concern is the extent to which HF exchange is known to strongly influence the ground state spin. Indeed, when multiple spin states are known to be favored by a catalyst [91][92][93][94], this can affect fundamental aspects of the potential energy surface [95], catalyst kinetics [96], and mechanism [97]. Two state reactivity (TSR) [98][99][100]91] is a fundamental model that has been invoked to explain experimental observations of sluggish kinetics in reactions with conserved spin in intermediates.…”

Counter‐Intuitive Gas‐Phase Reactivities of [V2]+ and [V2O]+ towards CO2 Reduction: Insight from Electronic Structure Calculations

“…reactivity scenario. [10] As to the bare [V 2 ] + cluster, due to its multireference character [7,11] we applied the ZORA-NEVPT2(9e,12o)/ZORA-def2-QZVPP Scheme (Table S1 in the Supporting Information). [12] According to these high-level calculations, the ground state of [V 2 ] + corresponds to 4 S g À .…”

“…Therefore, it is quite possible that there is a superposition of the doublet with the quartet state and vice versa, facilitating a multi-state reactivity scenario pending on the efficiency of spin-orbit coupling (see below for a further comment). [10] However, our aim is not to provide a quantitatively correct description of the whole PESs; rather, we aim at offering a qualitative understanding of the amazing reactivity behavior in the [V 2 ] + /[V 2 O] + /CO 2 systems. As the reactivity pattern proved to be quite complex, herein we limit ourselves to the main results.…”

Why Conventional Design Rules for C–H Activation Fail for Open-Shell Transition-Metal Catalysts

“…Single-site transition-metal complex reactivity is dominated by the ground state spin of the catalyst when spin transitions are slow 66,93,[103][104][105] Figure S19).…”

“…58 For example, it is known 49,72 that increasing the ligand field or metal-ligand coordination strength favors formation of a stable but relatively unreactive (i.e., for C-H activation) metal-oxo species, whereas distortion of the coordination environment could improve both reaction steps in high-spin Fe(II) compounds 49 . Nevertheless, since tuning the ligand field 68,92 can alter the ground state spin and the corresponding spin-allowed or forbidden nature of the catalytic reaction cycle [93][94][95][96] , it is essential to understand if these design principles 49 extend to other (e.g., low and intermediate) spin states.…”