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Hydrogen-bonding effects on the reactivity of [X-Fe(III)-O-Fe(IV)?O] (X = OH, F) complexes toward C-H bond cleavage.

ABSTRACT: Complexes 1-OH and 1-F are related complexes that share similar [X-Fe(III)-O-Fe(IV)?O](3+) core structures with a total spin S of ½, which arises from antiferromagnetic coupling of an S = 5/2 Fe(III)-X site and an S = 2 Fe(IV)?O site. EXAFS analysis shows that 1-F has a nearly linear Fe(III)-O-Fe(IV) core compared to that of 1-OH, which has an Fe-O-Fe angle of ~130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C-H bond cleavage than 2, a related complex with a [OH-Fe(IV)-O-Fe(IV)?O](4+) core having individual S = 1 Fe(IV) units. Interestingly, 1-F is 10-fold more reactive than 1-OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1-OH and 1-F strongly suggest that the H-bond in 1-OH does not significantly change the electrophilicity of the reactive Fe(IV)?O unit and that the lower reactivity of 1-OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety.

SUBMITTER: Xue G

PROVIDER: S-EPMC3632308 | biostudies-literature | 2013 Apr

REPOSITORIES: biostudies-literature

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Hydrogen-bonding effects on the reactivity of [X-Fe(III)-O-Fe(IV)═O] (X = OH, F) complexes toward C-H bond cleavage.

Xue Genqiang G Geng Caiyun C Ye Shengfa S Fiedler Adam T AT Neese Frank F Que Lawrence L

Inorganic chemistry 20130315 7

Complexes 1-OH and 1-F are related complexes that share similar [X-Fe(III)-O-Fe(IV)═O](3+) core structures with a total spin S of ½, which arises from antiferromagnetic coupling of an S = 5/2 Fe(III)-X site and an S = 2 Fe(IV)═O site. EXAFS analysis shows that 1-F has a nearly linear Fe(III)-O-Fe(IV) core compared to that of 1-OH, which has an Fe-O-Fe angle of ~130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C ...[more]

PMID: 23496330

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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.

Dataset Information

Hydrogen-bonding effects on the reactivity of [X-Fe(III)-O-Fe(IV)?O] (X = OH, F) complexes toward C-H bond cleavage.

Publications

Hydrogen-bonding effects on the reactivity of [X-Fe(III)-O-Fe(IV)═O] (X = OH, F) complexes toward C-H bond cleavage.

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