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.