Shape-selective interception by hydrocarbons of the O2-derived oxidant of a biomimetic nonheme iron complex

Abstract

Picky ferryl: The complex [Fe(Tp(Ph(2)))(BF)] (Tp(Ph(2)) = hydrotris(3,5-diphenylpyrazolyl)borate; BF = benzoylformate) reacts with O(2) to generate an oxidant (see picture; O red, pink; Fe yellow; N blue; C gray; H white) that oxidizes added hydrocarbons shape-selectively. Discrimination derives from a cleft formed by two phenyl groups of the Tp(Ph(2)) ligand, favoring oblate spheroidal substrates.

Figure 1

Spectral changes observed in the reactions of 1 (1 mM) with O₂ saturated in benzene at room temperature in a 0.5-cm cell after 1 h. Main plot: dashed line, 1 alone; dash-dotted line, 1 + O₂; solid line, 1 + O₂ + 0.1 M DHA. Inset: solid line, 1 + O₂ + 0.1 M cyclohexene; dashed line, 1 + O₂ + 0.1 M thioanisole.

Figure 2

Substrate concentration dependence on the % trapping of the oxidant formed by the reaction of 1 and O₂ in benzene at 25 °C. Legend for each data cluster from left to right: thioanisole (blue), cyclohexene (red), DHA (yellow), ethylbenzene (indigo).

Figure 3

Space filling model of the shape-selective cleft flanking the putative Fe^IV=O oxidant based on DFT calculations. Red: oxo oxygen; pink: carboxylate oxygen; gold: iron; blue: nitrogen; gray: carbon; white, hydrogen.

Scheme 1

Reactions of 1 with O₂ in benzene at 25 °C

Scheme 2

Hydrocarbon substrates used to intercept the putative Fe^IV=O oxidant. % values indicate the extent to which the green chromophore is decreased in the presence of 0.1 M substrate; values in parentheses are the bond dissociation energies (in kcal/mol) for the weakest C-H bond in each molecule.[10]