Axial Coordination of Carboxylate Activates the Non-heme FeIV?O Unit

Theoretical Identification of the Factors Governing the Reactivity of C-H Bond Activation by Non-Heme Iron(IV)-Oxo Complexes

Selective functionalization of C-H bonds provides a straightforward approach to a large variety of well-defined derivatives. High-valent mononuclear iron(IV)-oxo complexes are proposed to carry out these C-H activation reactions in enzymes or in biomimetic syntheses. In this Minireview, we aim to highlight the features that delineate the distinct reactivity of non-heme oxo-iron(IV) motifs to cleave strong C-H bonds in hydrocarbons, primarily focusing on the hydrogen atom transfer (HAT) process. We describe how the structural and electronic properties of supporting ligands modulate the oxidative property of the iron(IV)-oxo complexes. Furthermore, we highlight the decisive role played by spin-state in these biomimetic reactions. We also discuss how tunneling and external perturbations like electric field influence the transfer of hydrogen atoms. Lastly, we emphasize how computations could work as a practical guide to sketch and develop synthetic models with greater efficacy.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex

The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pK_a for the bound H₂ O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [Fe^IV (O)(TMC)(OH₂ )]²⁺ (1-OH₂ ; TMC=1,4,8,11-tetramethylcyclam) by determining the pK_a of the H₂ O ligand. We establish that 1-OH₂ has a pK_a of 6.9±0.5, a value that falls in between those found for [Fe^III (OH₂ )₆ ]³⁺ and [Fe^II (OH₂ )₆ ]²⁺ . This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1-OH₂ , anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1-OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.

Theoretical spectroscopy of model-nonheme [Fe(IV)OL5]2+ complexes in their lowest triplet and quintet states using multireference ab initio and density functional theory methods

The structure, energies and spectroscopic properties of a simple [FeO(NH(3))(5)](2+) model with ground states (3)A(2g) and (5)A(1g) (in approximate C(4v) symmetry) have been studied in some detail using density functional (DFT) and simplified correlated multireference ab initio methods. The results reveal similarities as well as some pronounced differences in the properties of the molecule in the two alternative spin states.