Attaining Control by Design over the Hydrolytic Stability of Fe-TAML Oxidation Catalysts

Abstract

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 to catalyzed demetalation by Brønsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) are catalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species of variable composition. Pseudo-first-order rate constants for the demetalation (kobs) are linear functions of the acid concentrations, and the effective second-order rate constants k1,eff have a hyperbolic dependence on [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 the CR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 depend insignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordination of the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend on their 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 result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular 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 the second 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 measurements and X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAML oxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.

Chemistry and Applications of Iron–TAMLCatalysts in Green Oxidation Processes Based on Hydrogen Peroxide

“…The rate of acid-induced demetalation depends only slightly on the nature of the head substituents X ( (Figure 3.3a) and the effective secondorder rate constants k 1,eff depend on pH as shown in Figure 3.3b. The inflection point at pH % 6.5 is coincident with the pK a of dihydrogenphosphate, implying that H 2 PO 4 À is reactive [19].…”

“…In water, only bis-ligated species are observed, which are, however, formed from iron-porphyrin dimers [23,24]. The X-ray structure of the monoligated 1-methylimidazole (MeIm) adduct (Ia-MeIm) is shown in Scheme 3.4 [19]. A square pyramidal environment is typical of Fe III -TAMLs in the solid state [8,14,25].…”

“…The imidazole plane is close to parallel to the plane through atoms Fe1, N2 and N4 [dihedral angle 11.2(2) ]. ; the solid line is the calculated curve using best-fit parameters of Equation 3.2; the broken line is a calculated curve with K 2L ¼ 0 validating binding of the second axial ligand [19]. [19].…”

“…The rate of demetalation of 1a in H 2 PO 4 À /HPO 4 2À buffer is appreciable, but the k obs values for 1k and 1m are immeasurably low, showing that the rates are strongly affected by (b) Second-order rate constants k 1,eff for demetalation of 1a as a function of pH at 45 C. The solid line is a theoretical curve calculated using the best-fit values of the parameters of (K ML ÂK LH ) and K a2 [19]. the tail CR 2 fragments.…”

The Impact of Surfactants on FeIII–TAML‐Catalyzed Oxidations by Peroxides: Accelerations, Decelerations, and Loss of Activity

“…The Fe III -TAML activators are predominantly five-coordinate complexes in the solid phase. [23][24][25][26] In water, they turn into six-coordinate species of À1 charge with two axial aqua ligands. One aqua ligand loses a proton to form a species of À2 charge in the range of pH 9.0-10.5 depending on the nature of 1 (Scheme 4 a).…”