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

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 [MnH(3)buea(O)](n-), where [H(3)buea](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, white 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 similar to 15. These values were used to re-evalulate the O-H bond dissociation energy (BDEOH) of the corresponding Mn-II/III-OH complexes: BDEOH values of 89 and 77 kcal/mol were determined for [Mn(III)H(3)buea(OH)](-) and [Mn(II)H(3)buea(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 tow 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 (Delta Delta S-double dagger = -35 eu, where Delta Delta S-double dagger = Delta S-double dagger(Mn-IV(O)) - Delta S-double dagger(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(III)H(3)buea(O)](2-) a proton transfer-electron transfer mechanism is proposed, whereas for [Mn(IV)H(3)buea(O)](-) a hydrogen-atom transfer pathway is likely.