A “Beheaded” TAML Activator: A Compromised Catalyst that Emphasizes the Linearity between Catalytic Activity and pK_a

Abstract

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [FeIII{(Me2CNCOCMe2NCO)2CMe2}OH2]− (4) in water in the pH range of 2–13 suggest its pseudo-octahedral geometry with two nonequivalent axial H2O ligands and revealed (i) the anticipated basic drift of the first pKa of water to 11.38 due to four electron-donating methyl groups alongside (ii) its counterintuitive enhanced resistance to acid-induced iron(III) ejection from the macrocycle. The catalytic activity of 4 in the oxidation of Orange II (S) by H2O2 in the pH range of 7–12 is significantly lower than that of previously reported TAML activators, though it follows the common rate law (v/[FeIII] = kIkII[H2O2][S]/(kI[H2O2] + kII[S]) and typical pH profiles for kI and kII. At pH 7 and 25 °C the rate constants kI and kII equal 0.63 ± 0.02 and 1.19 ± 0.03 M−1 s−1, respectively. With these new values for pKa, kI and kII establishing new high and low limits, respectively, the rate constants kI and kII were correlated with pKa values of all TAML activators. The relations log k = log k0 + α × pKa were established with log k0 = 13 ± 2 and 20 ± 4 and α = −1.1 ± 0.2 and −1.8 ± 0.4 for kI and kII, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pKa values.

Predicting Properties of Iron(III) TAML Activators of Peroxides from Their III/IV and IV/V Reduction Potentials or a Lost Battle to Peroxidase

“…The corresponding K a values reflect the Lewisa cidity of the metal center, which determines the oxidative reactivity of TAMLs in aqueous solution.T here are linear dependencies between pK a and log k, in which, as shown in Equations (1) and (2), the k values are rate constants for activation of H 2 O 2 by iron(III) TAMLs (k I )t o form an active TAML or for reactions of the latter with as ubstrate S( k II ). [3] It was therefore tempting to establish ac orrelation of pK a with E8' for the Fe III/IV and Fe IV/V features because the E8' values could be diagnostic for estimating the reactivity of TAMLs in aqueous media [Eqs. (1) and (2)s howg eneral mechanism of catalysis by TAML activators].…”

“…Linear free energy relationship (LFER) methodology [1] is a fundamental tool for analyzing and predicting various properties of iron TAML activators of peroxides (Figure 1). [2] Inter alia, LFERs are particularly useful for establishing relations between the oxidative reactivity of TAMLs (log k II ) and their acid–base properties (p K a ) in the aqueous medium, [3] or between log k II and the operational instability (log k i ) [4] . LFERs have assisted in detailing intimate mechanistic features of C−H bond activation of hydrocarbons by TAML iron(V)oxo species in an organic solvent [5] …”

“…[5] LFERsb ecomep articularlya ttractivea nd useful provided a standard seto fp arameters, whicha re relatedt ot he properties of TAML activators,isavailable.Assuch, forT AMLs,wehavepreviouslyu sedt he free energies of theC ÀHa ctivation( DG ¼ 6 ) [5] andt he pK a values. [3] However, calculations of both DG ¼ 6 and pK a arer ather laborioust asks.T herefore,a lternative,e asier-toaccess parameters were desirablef or analyses andp redictions of reactivity of iron(IV)a nd iron(V)T AMLs pecies involved in TAML-catalyzed or -promotedt ransformations. [2,6,7] Ourc yclic voltammetrys tudies of TAML activators in acetonitrile,t he resultso fw hich arep resentedi nt hisw ork, demonstratedt hat ther eduction potentials of both Fe III/IV andF e IV/V transitionsa re effortlessly measurable andcan be used furtherfor LFER correlations.…”

“…LFERs become particularly attractive and useful provided a standard set of parameters, which are related to the properties of TAML activators, is available. As such, for TAMLs, we have previously used the free energies of the C−H activation (Δ G ≠ ) [5] and the p K a values [3] . However, calculations of both Δ G ≠ and p K a are rather laborious tasks.…”

Bis phenylene flattened 13-membered tetraamide macrocyclic ligand (TAML) for square planar cobalt(III)

“…0.8–1.0 V versus SCE is observed clearly when the TAML of Co III complex contains a benzene unit. No oxidation was reported for 1 , which is an arene-free species, for the “switch” complex 3b , in which benzene ring is replaced by the pyridine fragment, and for the “beheaded” [38, 39] complex 6 (Table 1). Second, the value of E 1/2 for 7b with two benzene rings is much lower than that for all other complexes in Table 1, which have just one ring.…”