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 MnIII–oxido complexes, and a Hammett correlation was found between the pKa 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 MnIII–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 pKa 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 pKa(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.