Singlet versus Triplet Reactivity in an Mn(V)-Oxo Species: Testing Theoretical Predictions Against Experimental Evidence

Abstract

Discerning the factors that control the reactivity of high-valent metal-oxo species is critical to both an understanding of metalloenzyme reactivity and related transition metal catalysts. Computational studies have suggested that an excited higher spin state in a number of metal-oxo species can provide a lower energy barrier for oxidation reactions, leading to the conclusion that this unobserved higher spin state complex should be considered as the active oxidant. However, testing these computational predictions by experiment is difficult and has rarely been accomplished. Herein, we describe a detailed computational study on the role of spin state in the reactivity of a high-valent manganese(V)-oxo complex with para-Z-substituted thioanisoles and utilize experimental evidence to distinguish between the theoretical results. The calculations show an unusual change in mechanism occurs for the dominant singlet spin state that correlates with the electron-donating property of the para-Z substituent, while this change is not observed on the triplet spin state. Minimum energy crossing point calculations predict small spin-orbit coupling constants making the spin state change from low spin to high spin unlikely. The trends in reactivity for the para-Z-substituted thioanisole derivatives provide an experimental measure for the spin state reactivity in manganese-oxo corrolazine complexes. Hence, the calculations show that the V-shaped Hammett plot is reproduced by the singlet surface but not by the triplet state trend. The substituent effect is explained with valence bond models, which confirm a change from an electrophilic to a nucleophilic mechanism through a change of substituent.

Figure 1

Constrained potential energy scan along the Mn–O bond of [Mn(O)(H₈Cz)(CN)]⁻ calculated by NEVPT2:CAS(8,7) with BS5. Singlet scans are shown in blue solid squares. Triplet scans are shown in red solid squares. Energies are shown relative to the minimum of the singlet complex for clarity.

Figure 2

Potential energy landscape for the sulfoxidation of para-Z-substituted thioanisole (SubZ, Z = N(CH₃)₂, NH₂, OCH₃, CH₃, H, Br, CN, and NO₂) by ^1,3[Mn(O)(H₈Cz)(CN)]⁻. The table gives relative energies (ΔE_SO) for TS_SO as calculated with basis set BS2 and given in kcal mol⁻¹. Optimized geometries of TS_SO give bond lengths in Angstroms and the imaginary frequency of the transition state in cm⁻¹. RC is the reactant complex, TS_SO is the sulfoxidation transition state, P_SO is the sulfoxide product complex, and MECP refers to the minimum energy crossing pointbetween the singlet and the triplet spin state.

Figure 3

MECP-optimized geometries for the singlet–triplet transition for [Mn(O)(H₈Cz)(CN)]⁻ with p-NO₂-thioanisole and p-OCH₃-thioanisole. Bond lengths are given in Angstroms.

Figure 4

Computational Hammett plot for the reaction of singlet and triplet [Mn(O)(H₈Cz)(CN)]⁻ with para-Z-substituted thioanisole derivatives. Data calculated at RIJCOSX-TPSSh-D3/def2-QZVPP/ ZORA//RIJCOSX-B3LYP-D3/SDD/BS2 and includes zero-point, thermal, and solvent corrections. (a) Correlation for singlet spin barriers (TS_SO,Z). (b) Correlation for triplet spin barriers (³TS_SO,Z).

Figure 5

VB curve crossing diagram for nucleophilic and electrophilic sulfoxidation reactions. For explanations see text.

Figure 6

VB predicted values of the barrier heights ΔE_nucl and ΔE_elec from first principles. Values are in kcal mol⁻¹ and plotted against the σ_P Hammett parameter.

Scheme 1

Structure of Complexes and Substrates Investigated, and Experimental Hammett Plot with Data Taken from Ref

Scheme 2

High-Lying Occupied and Virtual Molecular Orbitals of [Mn(O)(H₈Cz)(CN)]⁻ and Occupation Levels in Various Electronic States

Scheme 3

Thermochemical Reaction Scheme Highlighting Ligand Binding versus Oxygen-Atom Transfer