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

Abstract

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.

Characterization and reactivity study of non-heme high-valent iron–hydroxo complexes

“…The experimentally obtained BDFE value of 2 is higher than those of Fe III -OH (70 kcal mol À1 ), 30 Fe III ]O (66 kcal mol À1 ), 60 and Mn III ]O (72 kcal mol À1 ) 61 complexes, but lower than those of Fe IV ]O complexes (87 kcal mol À1 ). 62 de Dinamización "Europa Investigacion" grant (EIN2019-103399), and the Spanish Ministerio de Ciencia, Innovacion y Universidades grant (PID2019-111086RA-I00).…”

“…The experimentally obtained BDFE value of 2 is higher than those of Fe III –OH (70 kcal mol −1 ), 30 Fe III O (66 kcal mol −1 ), 60 and Mn III O (72 kcal mol −1 ) 61 complexes, but lower than those of Fe IV O complexes (87 kcal mol −1 ). 62 …”

From Synthetic to Biological Fe4S4 Complexes: Redox Properties Correlated to Function of Radical S‐Adenosylmethionine Enzymes

“…As demonstrated in Refs. [37][38][39][40][41][42][43][44], H-atom-abstraction (HAA) reactivity and selectivity are strongly influenced by the interplay of redox and acidobasic components of the HAA thermodynamic force, which are usually inter-dependent so that a lower E°is compensated to some extent by a higher pK a and vice versa. Thus, to probe the 5'-dAdo * properties relevant for enzymatic HAA reactivity, we first analyze electrochemical behavior of 5'-dAdo * in aqueous solution and then we compare it with its behavior in models of the [Fe 4…”

Aromatic foldamers as scaffolds for metal second coordination sphere design

“…For example, functional groups covalently appended to the rst sphere ligands may engage in useful second sphere interactions. [5][6][7][8][9][10][11][12][13] In other approaches, a cavity has been built around the metal center to surround some rst sphere ligands 14 using for example calixarenes [15][16][17] or cyclodextrins. 18,19 As an extension, an entire metal complex, that is, including its rst sphere ligands, may be encapsulated within the cavity of a synthetic receptor based, for example, on resorcinarenes 20,21 or self-assembled metallo-organic cages.…”