Oxygen formation through water oxidation catalysis is a key reaction in the context of fuel generation from renewable energies. The number of homogeneous catalysts that catalyze water oxidation at high rate with low overpotential is limited. Ruthenium complexes can be particularly active, especially if they facilitate a dinuclear pathway for oxygen bond formation step. A supramolecular encapsulation strategy is reported that involves preorganization of dilute solutions (10−5
m) of ruthenium complexes to yield high local catalyst concentrations (up to 0.54 m). The preorganization strategy enhances the water oxidation rate by two‐orders of magnitude to 125 s−1, as it facilitates the diffusion‐controlled rate‐limiting dinuclear coupling step. Moreover, it modulates reaction rates, enabling comprehensive elucidation of electrocatalytic reaction mechanisms.
The selective formation of a kinetically stable metallamacrocyclic hexaruthenium complex and its clean conversion into a thermodynamically more stable tetraruthenium isomer as well their molecular structures and redox properties are reported.
The dinuclear complex [(susan){Fe(OH)(μ-O)Fe(OH)}](ClO) (Fe(OH)(ClO); susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane) with two unsupported terminal hydroxido ligands and for comparison the fluorido-substituted complex [(susan){FeF(μ-O)FeF}](ClO) (FeF(ClO)) have been synthesized and characterized in the solid state as well in acetonitrile (CHCN) and water (HO) solutions. The Fe-OH bonds are strongly modulated by intermolecular hydrogen bonds (1.85 and 1.90 Å). UV-vis-near-IR (NIR) and Mössbauer spectroscopies prove that FeF and Fe(OH) retain their structural integrity in a CHCN solution. The OH ligand induces a weaker ligand field than the F ligand because of stronger π donation. This increased electron donation shifts the potential for the irreversible oxidation by 610 mV cathodically from 1.40 V in FeF to 0.79 V versus Fc/Fc in Fe(OH). Protonation/deprotonation studies in CHCN and aqueous solutions of Fe(OH) provide two reversible acid-base equilibria. UV-vis-NIR, Mössbauer, and cryo electrospray ionization mass spectrometry experiments show conservation of the mono(μ-oxo) bridging motif, while the terminal OH ligands are protonated to HO. Titration experiments in aqueous solution at room temperature provide the p K values as p K = 4.9 and p K = 6.8. Kinetic studies by temperature- and pressure-dependent O NMR spectrometry revealed for the first time the water-exchange parameters [ k = (3.9 ± 0.2) × 10 s, Δ H = 39.6 ± 0.2 kJ mol, Δ S = -5.1 ± 1 J mol K, and Δ V = +3.0 ± 0.2 cm mol] and the underlying I mechanism for a {Fe(OH)(μ-O)Fe(OH)} core. The same studies suggest that in solution the monoprotonated {Fe(OH)(μ-O)Fe(OH)} complex has μ-O and μ-OH bridges between the two Fe centers.
We report on five tetranuclear metallamacrocycles with particularly large inner voids of up to 19.4 × 18.9 Å. The macrocyclic complexes were obtained by the self-assembly of spatially extended organic dicarboxylate linkers with two different dinuclear bis(alkenyl) diruthenium precursors. The five complexes include one pair of constitutional isomers, complexes 2-NB and 2-BN, which differ with respect to whether the incorporated triarylamine functionality is part of the "conductive" π-conjugated (2-NB) or the insulating dicarboxylate linkers (2-BN). All macrocyclic complexes were characterized by NMR spectroscopy, UHR ESI mass spectrometry, cyclic and square wave voltammetry, and in two instances by X-ray diffraction studies on single crystals. We also investigated the properties of their various oxidized forms via IR/NIR and UV/vis/NIR spectroelectrochemistry as well as by EPR spectroscopy. DFT studies provide further insight into the structural and electronic properties of these compounds.
Redox-active M6L4 cages display multiple reversible redox-events, enabling switching from overall +12 to −4 charged species with reversible storage of 16 electrons.
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