The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs)
([Fe{1-X1-2-X2C6H2-4,5-(NCOCMe2NCO)2CR2}(OH2)]-,
1: X
1 = X
2 = H, R
2 = Me
2 (
a), R
2 = (CH
2)
2 (
b); X
1= X
2 = Cl, R
2 = F
2 (
c), etc.), which the proton is
known to demetalate at pH < 3, are also subject tocatalyzed demetalation by Br
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nsted acid buffer components at pH 4-9 such as H
2PO
4-, HSO
3-, and CH
3CO
2H, HO
2CCH
2CO
2-. 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 H
2PO
4-/HPO
42- buffer is appreciable,but the
kobs values for
1b and
1c are immeasurably low, showing that the rates are strongly affected by theCR
2 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.