C−H Bond Cleavage with Reductants: Re-Investigating the Reactivity of Monomeric MnIII/IV−Oxo Complexes and the Role of Oxo Ligand Basicity

Abstract

The thermodynamic properties of structurally similar Mn^III and Mn^IV complexes have been reinvestigated to understand their reactivity with substrates having C−H bonds. The complexes have the general formula [MnH₃buea(O)]ⁿ⁻, where [H₃buea]³⁻ is the tripodal ligand, tris[(N′-tert-butylureaylato)-N-ethylene]aminato. These complexes are unique because of the intramolecular hydrogen-bonding (H-bond) network surrounding the Mn−oxo units. The redox potentials for the Mn^III/IV(O) couple was incorrectly assigned in earlier reports: the corrected value is −1.0 V vs Cp₂Fe⁺/Cp₂Fe in DMSO, while the Mn^IV/V(O) process is −0.076 under the same conditions. The oxo ligand in the Mn^III(O) complexes is basic with a pK_a of 28.3; the basicity of the terminal oxo ligand in the Mn^IV(O) complex is estimated to be ∼15. These values were used to re-evalulate the O−H bond dissociation energy (BDE_OH) of the corresponding Mn^II/III−OH complexes: BDE_OH values of 89 and 77 kcal/mol were determined for [Mn^IIIH₃buea(OH)]⁻ and [Mn^IIH₃buea(OH)]²⁻, respectively. Both Mn(O) complexes react with 9,10-dihydroanthracene (DHA) to produce anthracene in nearly quantitative yields. This is surprising based on the low redox potiental of the complexes, suggesting the basicity of the oxo ligand is a major contributor to the observed reactivity. In contrast to the thermodynamic results, a comparative kinetic investigation found that the Mn^III(O) complex reacts nearly 20 times faster than the Mn^IV(O) complex. Activation parameters, determined from an Eyring analysis, found that the entropy of activation is significantly different between the two systems (ΔΔS^‡ = −35 eu, where ΔΔS^‡ = ΔS^‡(Mn^IV(O)) − ΔS^‡(Mn^III(O)). This unusual kinetic behavior can be explained in the context of the basicity of the oxo ligands that leads to different mechanisms: for [Mn^IIIH₃buea(O)]²⁻ a proton transfer-electron transfer mechanism is proposed, whereas for [Mn^IVH₃buea(O)]⁻ a hydrogen-atom transfer pathway is likely.