Axial Ligand Effects on Electronic Structures and H‑Atom Abstraction by Oxidoiron(IV) Complexes Featuring
Isabelle Becker1, Massimiliano Morganti1, Haowei Chen2
1University of Göttingen, Institute of Inorganic Chemistry, Tammannstraße 4, 37077 Göttingen, Germany.
Abstract:
Oxidoiron-(IV) species are crucial intermediates for the functionalization of C-H bonds by biological oxidases and oxygenases, and understanding the relations between ligation, electronic structure, and reactivity of the FeIVO unit is key to the rational design of bioinspired catalysts. Here, a series of FeIVO complexes based on a macrocyclic tetracarbene ligand and with different anionic trans-axial ligands (Lax = CF3COO-, t BuS-, AdaS-) has been comprehensively characterized, including X-ray crystallography in two cases, as well as zero-field and applied-field 57Fe Mössbauer spectroscopy, magnetic susceptibility measurements, magnetic circular dichroism (MCD), and helium-tagging infrared photodissociation (IRPD). This establishes a detailed picture of the electronic structures of these ferryl complexes, featuring a well-separated S = 1 ground state, and the influence of the trans-ligand. The confinement to triplet-only pathways in C-H bond activations and comparison with the parent FeIVO complex (Lax = MeCN) allow uncovering trends in hydrogen atom abstraction (HAA) ability without perturbations from different extents of two-state reactivity (TSR). Kinetic studies on HAA for C-H substrates with BDEs up to 78 kcal mol-1 show relatively minor effects of the axial ligands, with rates following an electrophilic trend in the order Lax = MeCN > CF3COO- ≈ RS-. This contrasts the antielectrophilic trend found for a related series of tetramethylcyclam (TMC)-based FeIVO complexes for which TSR is dominant. For the present systems, temperature-dependent KIEs, in combination with computations at the DFT and ab initio levels of theory, suggest a semiclassical model with modest tunneling contributions that are most pronounced in the case of a strongly donating trans-thiolate. This study thus provides a combined experimental and computational basis for understanding intrinsic axial ligand effects and the effect of a trans-axial thiolate in S = 1 ferryl intermediates.
More Related Videos
06:53Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks
Published on: June 9, 2023
10:10Application of Elemental Lanthanides in the Selective C-F Activation of Trifluoromethylated Benzofulvenes Providing Access to Various Difluoroalkenes
Published on: July 28, 2018
Related Concept Videos
02:51Metal-Ligand Bonds
In these complexes, transition metals form coordinate covalent bonds, a kind of Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base) to an electron acceptor (Lewis acid). The Lewis acid in...
02:58Crystal Field Theory - Octahedral Complexes
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...
02:42Valence Bond Theory
03:02Colors and Magnetism
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
01:09Complexation Equilibria: Factors Influencing Stability of Complexes
01:19Complexation Equilibria: The Chelate Effect