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

The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs)
, 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.