Iron(III) Ejection from a 'Beheaded' TAML Activator: Catalytically Relevant Mechanistic Insight into the Deceleration of Electrophilic Processes by Electron Donors

摘要

Kinetic studies of the acid-induced ejection of iron(III) show that the more electron-rich tetra-amido-N macrocyclic ligand (TAML) activator [Fe-III{(Me2CNCO-CMe2NCO)(2)CMe2}OH2](-) (4), which does not have a benzene ring in its head component ("beheaded" TAML), is up to 1 x 10(4) times more resistant than much less electron rich [Fe-III{1,2-C6H4(NCOCMe2NCO)(2)CMe2}OH2](-) (1a) to the electrophilic attack. This counterintuitive increased resistance is seen in both the specific acid (k(obs) = k(1)[H+]/(K + [H+])) and phosphate general acid (k(II) = (k(di)K(al) + k(tri)[H+)/(K-a1+[H+])) demetalation pathways. Insight into this reactivity puzzle was obtained from coupling kinetic data with theoretical density functional theory modeling. First, although la and related complexes are six-coordinate in water, 4 has a strong tendency to repel the second aqua ligand favoring [LFe(OH2)](-) and making appropriate the comparison of monoaqua-4 with diaqua-la in the demetalation process. Second, dearomatization exerts a strong effect on the highest occupied molecular orbital (HOMO) energy of five-coordinate monoaqua-4, the presumed target in proton-induced demetalation, stabilizing it by ca. 51 kJ mol(-1) compared with monoaqua-1a. Third, the monoaqua-4 HOMO is localized over the N-p(pi) system of all four N donors in contrast with monoaqua-1a, where N-p(pi) contributions from the head amides only mix with the aromatic ring pi system. Fourth, addition of a second water ligand to monoaqua-1a giving [LFe(OH2)(2)](-) reshapes the monoaqua-1a HOMO by shifting its entire locus from the head to the tail diamido-N section-this HOMO is by 54 kJ mol(-1) less stable than the monoaqua-4 HOMO. These features provide the foundations for mechanistic conclusions concerning demetalation that (i) axial water ligands enable a favored path in the six-coordinate case of 1a, where a proton "slides" toward the Fe-N bond and (ii) early and late transition states are realized for 4 and 1a, respectively, with a larger free energy of activation for the beheaded TAML activator 4.