Evidence for a Precursor Complex in CH Hydrogen Atom Transfer Reactions Mediated by a Manganese(IV) Oxo Complex

Abstract

HAT trick: [MnIV(OH)2(H,MePytacn)]2+ (A) and [MnIV(O)(OH)(H,MePytacn)]+ (B) differ in their reactions with CH bonds: compound A engages in typical single‐step hydrogen atom transfer (HAT) reactions, whereas B first forms a substrate–B encounter complex (C; see scheme). This equilibrium alters the relative CH reactivity from that expected from CH bond dissociation energies.

The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations

“…36 Similar to [Mn(Me 2 EBC)(OH) 2 ] 2+ , two hydroxide ligands of this manganese(IV) complex are in a cis arrangement relative to each other, and the N-methyl groups with the pyridine ring provide steric isolation of the metal center, which is the key feature to prevent the formation of oligomer species. The pK a value of this complex is 7.1, corresponding to release of one proton to form [Mn(Pytacn)(O)(OH)] + , which is similar to that of [Mn(Me 2 EBC)(OH) 2 ] 2+ (6.86).…”

Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid

“…The X-band EPR spectrum of 2 reveals a broad signal at g ∥ = 4.32 and a six-line hyperfine pattern ( A ⊥ = 10 mT) centered at g ⊥ = 1.99, indicating S = 3/2 Mn IV ground state (Figure 2a, inset). [23,28,29] In contrast, 1 shows the intense resonance at g ∥ = 6.48 and weak hyperfine pattern ( A ⊥ = 9 mT) at g ⊥ = 2.02, which means 1 possesses S = 5/2 Mn II center. These contrasting spectral intensities of 1 and 2 are originated from the different axial zero-field splitting term D .…”

“…The Mn-O bond distances in 2 are comparable to those in the previously reported Mn IV -bis(hydroxo) complexes, [Mn IV (Me 2 EBC)(OH) 2 ] 2+ (1.811 Å) and [Mn IV ( H,Me Pytacn)(OH) 2 ] 2+ (1.799 Å). [23,32] …”

Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)‐Oxo Complexes

“…[3] These intermediates show varied reactivity, the basis of which is poorly understood. Some Mn IV -oxo centers react slowly with highly activated C-H bonds, [4] while other species attack the strong C-H bonds of cyclohexane. [5] Large variations in supporting ligands and reaction media make it challenging to rationalize reactivity trends.…”

“…[6b] The smaller KIE of 2-2pyN2Q is within the range observed for Mn IV -oxo species (3.1 – 6.8). [4b, 14] …”