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

The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X sub(1)-2-X sub(2)C sub(6)H sub(2)-4,5- (NCOCMe sub(2)NCO) sub(2)CR sub(2)}(OH sub(2))] super(-), 1: X sub(1) = X sub(2) = H, R sub(2) = Me sub(2) (a), R sub(2) = (CH sub(2)) sub(2) (b); X sub(1) = X sub(2) = Cl, R sub(2) = F sub(2) (c), etc.), which the proton is known to demetalate at pH < 3, are also subject to catalyzed demetalation by Bronsted acid buffer components at pH 4-9 such as H sub(2)PO sub(4) super(-), HSO sub(3) super(-), and CH sub(3)CO sub(2)H, HO sub(2)CCH sub(2)CO sub(2) super(-). 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 (k sub(obs)) are linear functions of the acid concentrations, and the effective second-order rate constants k sub(1,eff) have a hyperbolic dependence on [H super(+)] (k sub(1,eff) = a sub(1)[H super(+)]/(b sub(1)+[H super(+)]). The rate of demetalation of 1a in H sub(2)PO sub(4) super(-)/HPO sub(4) super(2-) buffer is appreciable, but the k sub(obs) values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by the CR sub(2) 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 catal