Regulating the Basicity of Metal-Oxido Complexes with a Single Hydrogen Bond and Its Effect on C-H Bond Cleavage

The functionalization of C-H bonds is an essential reaction in biology and chemistry. Metalloenzymes that often exhibit this type of reactivity contain metal-oxido intermediates that are directly involved in the initial cleavage of the C-H bonds. Regulation of the cleavage process is achieved, in part, by hydrogen bonds that are proximal to the metal-oxido units, yet our understanding of their exact role(s) is still emerging. To gain further information into the role of H-bonds on C-H bond activation, a hybrid set of urea-containing tripodal ligands has been developed in which a single H-bond can be adjusted through changes in the properties of one ureayl N-H bond. This modularity is achieved by appending a phenyl ring with different para-substituents from one ureayl NH group. The ligands have been used to prepare a series of Mn^III-oxido complexes, and a Hammett correlation was found between the pK_a values of the complexes and the substituents on the phenyl ring that was explained within the context of changes to the H-bonds involving the Mn^III-oxido unit. The complexes were tested for their reactivity toward 9,10-dihydroanthracene (DHA), and a Hammett correlation was found between the second-order rate constants for the reactions and the pK_a values. Studies to determine activation parameters and the kinetic isotope effects are consistent with a mechanism in which rate-limiting proton transfer is an important contributor. However, additional reactivity studies with xanthene found a significant increase in the rate constant compared to DHA, even though the substrates have the same pK_a(C-H) values. These results do not support a discrete proton-transfer/electron-transfer process, but rather an asynchronous mechanism in which the proton and electron are transferred unequally at the transition state.