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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 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.

SUBMITTER: Barman SK

PROVIDER: S-EPMC6776423 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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

Barman Suman K SK Jones Jason R JR Sun Chen C Hill Ethan A EA Ziller Joseph W JW Borovik A S AS

Journal of the American Chemical Society 20190705 28

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 activ ...[more]

PMID: 31274298

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Project description:The thermodynamic properties of structurally similar Mn(III) and Mn(IV) complexes have been reinvestigated to understand their reactivity with substrates having C-H bonds. The complexes have the general formula [MnH(3)buea(O)](n-), where [H(3)buea](3-) is the tripodal ligand, tris[(N'-tert-butylureaylato)-N-ethylene]aminato. These complexes are unique because of the intramolecular hydrogen-bonding (H-bond) network surrounding the Mn-oxo units. The redox potentials for the Mn(III/IV)(O) couple was incorrectly assigned in earlier reports: the corrected value is -1.0 V vs Cp(2)Fe(+)/Cp(2)Fe in DMSO, while the Mn(IV/V)(O) process is -0.076 under the same conditions. The oxo ligand in the Mn(III)(O) complexes is basic with a pK(a) of 28.3; the basicity of the terminal oxo ligand in the Mn(IV)(O) complex is estimated to be approximately 15. These values were used to re-evalulate the O-H bond dissociation energy (BDE(OH)) of the corresponding Mn(II/III)-OH complexes: BDE(OH) values of 89 and 77 kcal/mol were determined for [Mn(III)H(3)buea(OH)](-) and [Mn(II)H(3)buea(OH)](2-), respectively. Both Mn(O) complexes react with 9,10-dihydroanthracene (DHA) to produce anthracene in nearly quantitative yields. This is surprising based on the low redox potiental of the complexes, suggesting the basicity of the oxo ligand is a major contributor to the observed reactivity. In contrast to the thermodynamic results, a comparative kinetic investigation found that the Mn(III)(O) complex reacts nearly 20 times faster than the Mn(IV)(O) complex. Activation parameters, determined from an Eyring analysis, found that the entropy of activation is significantly different between the two systems (DeltaDeltaS(++) = -35 eu, where DeltaDeltaS(++) = DeltaS(++)(Mn(IV)(O)) - DeltaS(++)(Mn(III)(O)). This unusual kinetic behavior can be explained in the context of the basicity of the oxo ligands that leads to different mechanisms: for [Mn(III)H(3)buea(O)](2-) a proton transfer-electron transfer mechanism is proposed, whereas for [Mn(IV)H(3)buea(O)](-) a hydrogen-atom transfer pathway is likely.

Project description:Heme hydroperoxidases catalyze the oxidation of substrates by H2O2. The catalytic cycle involves the formation of a highly oxidizing species known as Compound I, resulting from the two-electron oxidation of the ferric heme in the active site of the resting enzyme. This high-valent intermediate is formed upon facile heterolysis of the O-O bond in the initial FeIII-OOH complex. Heterolysis is assisted by the histidine and arginine residues present in the heme distal cavity. This chemistry has not been successfully modeled in synthetic systems up to now. In this work, we have used a series of iron(iii) porphyrin complexes (FeIIIL2(Br), FeIIIL3(Br) and FeIIIMPh(Br)) with covalently attached pendent basic groups (pyridine and primary amine) mimicking the histidine and arginine residues in the distal-pocket of natural heme enzymes. The presence of pendent basic groups, capable of 2nd sphere hydrogen bonding interactions, leads to almost 1000-fold enhancement in the rate of Compound I formation from peracids relative to analogous complexes without these residues. The short-lived Compound I intermediate formed at cryogenic temperatures could be detected using UV-vis electronic absorption spectroscopy and also trapped to be unequivocally identified by 9 GHz EPR spectroscopy at 4 K. The broad (2000 G) and axial EPR spectrum of an exchange-coupled oxoferryl-porphyrin radical species, [FeIV[double bond, length as m-dash]O Por˙+] with g eff ⊥ = 3.80 and g eff ‖ = 1.99, was observed upon a reaction of the FeIIIL3(Br) porphyrin complex with m-CPBA. The characterization of the reactivity of the FeIII porphyrin complexes with a substrate in the presence of an oxidant like m-CPBA by UV-vis electronic absorption spectroscopy showed that they are capable of oxidizing two equivalents of inorganic and organic substrate(s) like ferrocene, 2,4,6-tritertiary butyl phenol and o-phenylenediamine. These oxidations are catalytic with a turnover number (TON) as high as 350. Density Functional Theory (DFT) calculations show that the mechanism of O-O bond activation by 2nd sphere hydrogen bonding interaction from these pendent basic groups, which are protonated by a peracid, involves polarization of the O-O σ-bond, leading to lowering of the O-O σ*-orbital allowing enhanced back bonding from the iron center. These results demonstrate how inclusion of 2nd sphere hydrogen bonding interaction can play a critical role in O-O bond heterolysis.

Dataset Information

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

Publications

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

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