Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction

Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex [NiII(NNN)(OH)]1- containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction is M-OH + CO2 → M-OCO2H; with variant M, previous rate constants are ≲103 M-1 s-1 in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 105 M-1 s-1 in N,N′-dimethylformamide at 298 K, a value within the range of kcat/KM≈105–108 M-1 s-1 for carbonic anhydrase, the most efficient catalyst of CO2 fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO2, followed by insertion of a CO2 oxygen atom into the Ni-OH bond to generate a four center Ni(η2-OCO2H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Ni(η1-OCO2H) fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [ΔH‡ = 3.2(5) kcal/mol]

Characterization of a High-Spin Non-Heme Fe III

We have generated a high-spin Fe(III)–OOH complex supported by tetramethylcyclam via protonation of its conjugate base and characterized it in detail by various spectroscopic methods. This Fe(III)–OOH species converts quantitatively to an Fe(IV)=O complex via O–O bond cleavage, which represents the first example of such a conversion. This conversion is promoted by two factors: the strong Fe(III)–OOH bond that inhibits Fe–O bond lysis and the addition of protons that facilitate O–O bond cleavage. This example provides a synthetic precedent for how O–O bond cleavage of high-spin iron(III)-peroxo intermediates of nonheme iron enzymes may be promoted

Direct observation of a nonheme iron(IV)–oxo complex that mediates aromatic C–F hydroxylation

The synthesis of a pentadentate ligand with strategically designed fluorinated arene groups in the second coordination sphere of a nonheme iron center is reported. The oxidatively resistant fluorine substituents allow for the trapping and characterization of an FeIV(O) complex at −20 °C. Upon warming of the FeIV(O) complex, an unprecedented arene C–F hydroxylation reaction occurs. Computational studies support the finding that substrate orientation is a critical factor in the observed reactivity. This work not only gives rare direct evidence for the participation of an FeIV(O) species in arene hydroxylation but also provides the first example of a high-valent iron–oxo complex that mediates aromatic C–F hydroxylation