How Do Ring Size and π-Donating Thiolate Ligands Affect Redox-Active, α-Imino-N-heterocycle Ligand Activation?

Enhanced Catalytic Activity via Rapid Two-Electron Transfer in Low-Spin Fe(II) Complex and Spin-State Dependent Proton Reduction Pathways

The growing interest in green hydrogen gas production has brought significant attention to the development of efficient proton reduction catalysts. A comprehensive understanding of proton and electron transfer processes within catalyst complexes is crucial for developing efficient catalysts. While the proton transfer process is influenced by the Brønsted acid used, electron transfer is an intrinsic property determined by the molecular orbitals and spin states of complexes. Complexes that rapidly transfer electrons are associated with high catalytic performance. In this study, we present a first example of low-spin FeII complex that utilizes the π* orbital of ligand for rapid two-electron transfer, resulting in exceptional catalytic performance for hydrogen gas evolution. The consecutive two-electron transfer rate was measured at 33.24 s-1, and in combination with proton transfer, the catalyst achieved an extraordinarily high turnover frequency (TOF) of 224,643 s-1 for hydrogen gas production. Conversely, a high-spin Fe(II) complex produced hydrogen gas at a relatively low TOF of 8848 s-1. These comparative experiments confirmed that the observed high catalytic efficiency is unique to the low-spin FeII complex, attributed to its distinct electron transfer mechanism.

Modulation of the Bonding between Copper and a Redox-Active Ligand by Hydrogen Bonds and Its Effect on Electronic Coupling and Spin States

The exchange coupling of electron spins can strongly influence the properties of chemical species. The regulation of this type of electronic coupling has been explored within complexes that have multiple metal ions but to a lesser extent in complexes that pair a redox-active ligand with a single metal ion. To bridge this gap, we investigated the interplay among the structural and magnetic properties of mononuclear Cu complexes and exchange coupling between a Cu center and a redox-active ligand over three oxidation states. The computational analysis of the structural properties established a relationship between the complexes' magnetic properties and a bonding interaction involving a dx2-y2 orbital of the Cu ion and π orbital of the redox-active ligand that are close in energy. The additional bonding interaction affects the geometry around the Cu center and was found to be influenced by intramolecular H-bonds introduced by the external ligands. The ability to synthetically tune the d-π interactions using H-bonds illustrates a new type of control over the structural and magnetic properties of metal complexes.

Increasing reactivity by incorporating π-acceptor ligands into coordinatively unsaturated thiolate-ligated iron(II) complexes

Reported herein is the structural, spectroscopic, redox, and reactivity properties of a series of iron complexes containing both a π-donating thiolate, and π-accepting N-heterocycles in the coordination sphere, in which we systematically vary the substituents on the N-heterocycle, the size of the N-heterocycle, and the linker between the imine nitrogen and tertiary amine nitrogen. In contrast to our primary amine/thiolate-ligated Fe(II) complex, [Fe^II(S^Me2N₄(tren))]⁺ (1), the Fe(II) complexes reported herein are intensely colored, allowing us to visually monitor reactivity. Ferrous complexes with R = H substituents in the 6-position of the pyridines, [Fe^II(S^Me2N₄(6-H-DPPN)]⁺ (6) and [Fe^II(S^Me2N₄(6-H-DPEN))(MeOH)]⁺ (8-MeOH) are shown to readily bind neutral ligands, and all of the Fe(II) complexes are shown to bind anionic ligands regardless of steric congestion. This reactivity is in contrast to 1 and is attributed to an increased metal ion Lewis acidity assessed via aniodic redox potentials, E_p,a, caused by the π-acid ligands. Thermodynamic parameters (ΔH, ΔS) for neutral ligand binding were obtained from T-dependent equilibrium constants. All but the most sterically congested complex, [Fe^II(S^Me2N₄(6-Me-DPPN)]⁺ (5), react with O₂. In contrast to our Mn(II)-analogues, dioxygen intermediates are not observed. Rates of formation of the final mono oxo-bridged products were assessed via kinetics and shown to be inversely dependent on redox potentials, E_p,a, consistent with a mechanism involving electron transfer.

Structures, Spectroscopic Properties, and Dioxygen Reactivity of 5- and 6-Coordinate Nonheme Iron(II) Complexes: A Combined Enzyme/Model Study of Thiol Dioxygenases

The synthesis of four new FeII(N4S(thiolate)) complexes as models of the thiol dioxygenases are described. They are composed of derivatives of the neutral, tridentate ligand triazacyclononane (R3TACN; R = Me, iPr) and 2-aminobenzenethiolate (abtx; X = H, CF3), a non-native substrate for thiol dioxygenases. The coordination number of these complexes depends on the identity of the TACN derivative, giving 6-coordinate (6-coord) complexes for FeII(Me3TACN)(abtx)(OTf) (1: X = H; 2: X = CF3) and 5-coordinate (5-coord) complexes for [FeII(iPr3TACN)(abtx)](OTf) (3: X = H; 4: X = CF3). Complexes 1-4 were examined by UV-vis, 1H/19F NMR, and Mössbauer spectroscopies, and density functional theory (DFT) calculations were employed to support the data. Mössbauer spectroscopy reveals that the 6-coord 1-2 and 5-coord 3- 4 exhibit distinct spectra, and these data are compared with that for cysteine-bound CDO, helping to clarify the coordination environment of the cys-bound FeII active site. Reaction of 1 or 2 with O2 at -95 °C leads to S-oxygenation of the abt ligand, and in the case of 2, a rare di(sulfinato)-bridged complex, [Fe2III(μ-O)((2-NH2) p-CF3C6H3SO2)2](OTf)2 ( 5), was obtained. Parallel enzymatic studies on the CDO variant C93G were carried out with the abt substrate and show that reaction with O2 leads to disulfide formation, as opposed to S-oxygenation. The combined model and enzyme studies show that the thiol dioxygenases can operate via a 6-coord FeII center, in contrast to the accepted mechanism for nonheme iron dioxygenases, and that proper substrate chelation to Fe appears to be critical for S-oxygenation.