Attaining Control by Design over the Hydrolytic Stability of Fe-TAML Oxidation Catalysts
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- 作者：Victor Polshin ; Delia-Laura Popescu ; Andreas Fischer ; Arani Chanda ; David C. Horner ; Evan S. Beach ; Jennifer Henry ; Yong-Li Qian ; Colin P. Horwitz ; Gabor Lente ; Istvan Fabian ; Eckard Mü ; nck ; Emile
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：April 2, 2008
- 年：2008
- 卷：130
- 期：13
- 页码：4497 - 4506
- 全文大小：289K
- 年卷期：v.130,no.13(April 2, 2008)
- ISSN：1520-5126
文摘
The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X1-2-X2C6H2-4,5-( NCOCMe2NCO)2CR2}(OH2)]- , 1: X1 = X2 = H, R2 = Me2 (a), R2 = (CH2)2 (b); X1= X2 = Cl, R2 = F2 (c), etc.), which the proton is known to demetalate at pH < 3, are also subject tocatalyzed demetalation by Br
nsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) arecatalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species ofvariable composition. Pseudo-first-order rate constants for the demetalation (kobs) are linear functions ofthe acid concentrations, and the effective second-order rate constants k1,eff have a hyperbolic dependenceon [H+] (k1,eff = a1[H+]/(b1+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable,but the kobs values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by theCR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 dependinsignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordinationof the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend ontheir structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation,its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to resultfrom the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in anintramolecular manner. The reaction order in picolinic acid equals one for 1a and two for 1b. For 1b, "inactive"pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that thesecond order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid.The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurementsand X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAMLoxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.
nsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) arecatalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species ofvariable composition. Pseudo-first-order rate constants for the demetalation (kobs) are linear functions ofthe acid concentrations, and the effective second-order rate constants k1,eff have a hyperbolic dependenceon [H+] (k1,eff = a1[H+]/(b1+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable,but the kobs values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by theCR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 dependinsignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordinationof the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend ontheir structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation,its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to resultfrom the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in anintramolecular manner. The reaction order in picolinic acid equals one for 1a and two for 1b. For 1b, "inactive"pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that thesecond order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid.The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurementsand X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAMLoxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.