Proteins carrying an iron-porphyrin (heme) cofactor are essential for biological O2 management. The nature of Fe-O2 bonding in hemoproteins is debated for decades. We used energy-sampling and rapid-scan X-ray Kβ emission and K-edge absorption spectroscopy as well as quantum chemistry to determine molecular and electronic structures of unligated (deoxy), CO-inhibited (carboxy), and O2-bound (oxy) hemes in myoglobin (MB) and hemoglobin (HB) solutions and in porphyrin compounds at 20–260 K. Similar metrical and spectral features revealed analogous heme sites in MB and HB and the absence of low-spin (LS) to high-spin (HS) conversion. Amplitudes of Kβ main-line emission spectra were directly related to the formal unpaired Fe(d) spin count, indicating HS Fe(II) in deoxy and LS Fe(II) in carboxy. For oxy, two unpaired Fe(d) spins and, thus by definition, an intermediate-spin iron center, were revealed by our static and kinetic X-ray data, as supported by (time-dependent) density functional theory and complete-active-space self-consistent-field calculations. The emerging Fe-O2 bonding situation includes in essence a ferrous iron center, minor superoxide character of the noninnocent ligand, significant double-bond properties of the interaction, and three-center electron delocalization as in ozone. It resolves the apparently contradictory classical models of Pauling, Weiss, and McClure/Goddard into a unifying view of O2 bonding, tuned toward reversible oxygen transport.
In situ spectroelectrochemistry demonstrates stability of electrografted diazonium interfaces on conductive oxides & their suitability as anchoring groups for molecular species.
Surface enhanced vibrational spectroscopy shows the correlation between electron transfer kinetics and protonation degree of Fe Hangman complexes on electrodes.
Two
iron porphyrin complexes with either mesityl (FeTMP) or thiophene
(FeT3ThP) peripheral substituents were attached to basal pyrolytic
graphite and Ag electrodes via different immobilization methods. By
combining cyclic voltammetry and in-operando surface-enhanced Raman
spectroscopy along with MD simulations and DFT calculations, their
respective surface attachment, redox chemistry and activity toward
electrocatalytic oxygen reduction was investigated. For both porphyrin
complexes, it could be shown that catalytic activity is restricted
to the first (few) molecular layer(s), although electrodes covered
with thiophene-substituted complexes showed a better capability to
consume the oxygen at a given overpotential even in thicker films.
The spectroscopic data and simulations suggest that both porphyrin
complexes attach to a Ag electrode surface in a way that maximum planarity
and minimum distance between the catalytic iron site and the electrode
is achieved. However, due to the distinctive design of the FeT3ThP
complex, the thiophene rings are capable of occupying a conformation,
via rotation around the bonding axis to the porphyrin, in which all
four sulfur atoms can coordinate to the Ag surface. This effect creates
a dense and planar surface coverage of the porphyrin on the electrode
facilitating a fast (multi) electron transfer via several covalent
Ag–S bonds. In contrast, bulky mesityl groups as peripheral
substituents, which have been initially introduced to prevent aggregation
and improve catalytic behavior in solution, exert
a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverage and less
stable interactions with the surface are formed. Our results underline
the importance of rationally designed heterogenized molecular catalysts
to achieve optimal turnover, which not only strictly applies
to the here discussed oxygen reduction reaction but eventually holds
also true for other energy conversion reactions such as carbon dioxide
reduction.
Through an optimized synthetic procedure, metalloligand 5, which features a hetero-Pacman scaffold comprising a porphyrin and a terpyridine moiety, has been assembled by a double Suzuki reaction. In a subsequent step, a rutheniumbipyridine fragment was introduced at the terpyridine coordination site of metalloligand 5 to form complex 6, which was fully characterized and its potential application in water oxidation catalysis tested. A number of side-products were iso-[a] Institute Scheme 1. Synthesis of compound 6 (mes = mesityl). Reagents and conditions: a) i. BuLi, B(OMe) 3 ; ii. H + /H 2 O; b) 2, [Pd(dppf)Cl 2 ], Rb 2 CO 3 , thf; c) CHCl 3 /MeOH; d) 4, [Pd(dppf)Cl 2 ], Rb 2 CO 3 , dioxane/water; e) i. [Ru(CH 3 CN) 2 (bpy)Cl 2 ], acetone; ii. NH 4 PF 6 /H 2 O.
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