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 which are directly involved 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 parasubstituents from one ureayl NH group. The ligands have been used to prepare a series of Mn IIIoxido 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 Hbonds involving the Mn III-oxido unit. The complexes were tested for their reactivity toward 9,10dihydroanthracene (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 suggest 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.

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The interplay between the primary and secondary coordination spheres is crucial to determining the properties of transition metal complexes. To examine these effects, a series of trigonal bipyramidal Co–OH complexes have been prepared with tripodal ligands that control both coordination spheres. The ligands contain combination of either urea or sulfonamide groups that control the primary coordination sphere through anionic donors in the trigonal plane and the secondary coordination sphere through intramolecular hydrogen bonds. Variations in the anion donor strengths were evaluated using electronic absorbance spectroscopy and a qualitative ligand field analysis to find that deprotonated urea donors are stronger field ligands than deprotonated sulfonamides. Structural variations were found in the CoII–O bond lengths that range from 1.953(4) to 2.051(3) Å; this range in bond lengths were attributed to the differences in the intramolecular hydrogen bonds that surround the hydroxido ligand. A similar trend was observed between the hydrogen bonding networks and the vibrations of the O–H bonds. Attempts to isolate the corresponding CoIII–OH complexes were hampered by their instability at room temperature.

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A n EPR approach has been developed and applied t o the determination of the rate constants for the decomposition of some arenediazonium ions b y Fell complexes to give aryl radicals. In continuousf l o w experiments, steady-state concentrations have been measured both for the aryl radical-adducts to the fumarate anion and for ' CH,CO,-(obtained b y reaction of Ar' with the iodoacetate anion): analysis of the dependence of these upon [Fe"] and [ArN,'] leads to values of the rate constants for electron transfer.For the reaction between Fell-EDTA and a series of diazonium ions, for which values of k are in the range 2 x 1 04-4 x 1 O5 dm3 mo1-l s-l, it is shown that there is a good dual-parameter Hammett-Taft correlation (involving 0, and oR+, and with p = 1.74), consistent with the occurrence of an outer-sphere electron-transfer process.