Bioinorganic reaction mechanisms: From high-valent iron to bioorganometallic chemistry

Important aspects of the mechanisms of fundamental bioinorganic reactions have been revealed in recent studies, which have shed light on the structure and properties of high‐valent nonheme Fe centers (left structure) as well as on the biosynthetic formation of the Fe-CN bond in Ni–Fe hydrogenases (right)

Performance of nonrelativistic and quasi-relativistic hybrid DFT for the prediction of electric and magnetic hyperfine parameters in 57Fe Mössbauer spectra

57Fe electric and magnetic hyperfine parameters were calculated for a series of 10 iron model complexes, covering a wide range of oxidation and spin states. Employing the B3LYP hybrid method, results from nonrelativistic density functional theory (DFT) and quasi-relativistic DFT within the zero-order regular approximation (ZORA) were compared. Electron densities at the iron nuclei were calculated and correlated with experimental isomer shifts. It was shown that the fit parameters do not depend on a specific training set of iron complexes and are, therefore, more universal than might be expected. The nonrelativistic and quasi-relativistic electron densities gave fit parameters of similar quality; the ZORA densities are only shifted by a factor of 1.32, upward in the direction of the four-component Dirac−Fock value. From a correlation of calculated electric field gradients and experimental quadrupole splittings, the value of the 57Fe nuclear quadrupole moment was redetermined to a value of 0.16 barn, in good agreement with other studies. The ZORA approach gave no additional improvement of the calculated quadrupole splittings in comparison to the nonrelativistic approach. The comparison of the calculated and measured 57Fe isotropic hyperfine coupling constants (hfcc's) revealed that both the ZORA approach and the inclusion of spin−orbit contributions lead to better agreement between theory and experiment in comparison to the nonrelativistic results. For all iron complexes with small spin−orbit contributions (high-spin ferric and ferryl systems), a distinct underestimation of the isotropic hfcc's was found. Scaling factors of 1.81 (nonrelativistic DFT) and 1.69 (ZORA) are suggested. The calculated 57Fe isotropic hfcc's of the remaining model systems (low-spin ferric and high-spin ferrous systems) contain 10−50% second-order contributions and were found to be in reasonable agreement with the experimental results. This is assumed to be the consequence of error cancellation because g-tensor calculations for these systems are of poor quality with the existing DFT approaches. Excellent agreement between theory and experiment was found for the 57Fe anisotropic hfcc's. Finally, all of the obtained fit parameters were used for an application study of the [Fe(H2O)6]3+ ion. The calculated spectroscopic data are in good agreement with the Mössbauer and electron paramagnetic resonance results discussed in detail in a forthcoming paper

Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion

In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the active catalyst formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion (IC) and vibrational relaxation (VR), also dissipate as much as 20–30% of the absorbed photon energy. Minimization of these energy losses, a holy grail in solar energy conversion and solar fuel production, is a challenging task because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here, we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for IC by means of inter-ligand electron transfer of around 2000 cm–1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligand charge transfer [MLCT(Lm)], and a 2,2′-bipyridine-centered MLCT [MLCT(bpy)], which lies 800–1400 cm–1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA. Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV. Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the corresponding Lm•–-centered and bpy•–-centered reduced photosensitizers, which involve different reducing abilities, that is, −0.79 and −0.93 V versus NHE for Lm•– and bpy•–, respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant for energy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways

Structure and Bonding in Pentacyano(L)ferrate(II) and Pentacyano(L)ruthenate(II) Complexes (L = Pyridine, Pyrazine, and N-Methylpyrazinium): A Density Functional Study

Density Functional Theory (DFT) at the generalized gradient approximation (GGA) level has been applied to the complexes [Fe(CN)5L]n- and [Ru(CN)5L]n- (L = pyridine, pyrazine, N-methylpyrazinium), as well as to [Fe(CN)5]3- and [Ru(CN)5]3-. Full geometry optimizations have been performed in all cases. The geometrical parameters are in good agreement with available information for related systems. The role of the MII-L back-bonding was investigated by means of a L and cyanide Mulliken population analysis. For both Fe(II) and Ru(II) complexes the metal-L dissociation energies follow the ordering pyridine < pyrazine < N-methyl pyrazinium, consistent with the predicted σ-donating and π*-accepting abilities of the L ligands. Also, the computed metal-L bond dissociation energies are systematically smaller in the Ru(II) than in the Fe(II) complexes. This fact suggests that previous interpretations of kinetic data, showing that ruthenium complexes in aqueous solution are more inert than their iron analogues, are not related to a stronger Ru-L bond but are probably due to solvation effects

A new copper(II) di-μ2-carboxylato bridged dinuclear complex: [Cu(oda)phen]2 · 6H2O (oda = oxydiacetate, phen = phenanthroline)

The oxydiacetate-bridged copper(II) complex [Cu(oda)(1,10-phen)] · 3H2O (oda = oxydiacetate dianion, 1,10-phen = 1,10-phenanthroline) has been characterized. The complex is dinuclear and centrosymmetric with each copper atom residing in a CuN2O4 octahedral environment. The Cu(II) ions are bridged by two carboxylate-oxygen atoms in a strictly planar Cu2O2 core with a Cu-Cu distance of 3.417(2) Å. The magnetic susceptibility measurements (2-300 K) show weak ferromagnetic coupling between the copper ions with J = 3.3 cm-1. The results are compared with those of the homologous [Cu(tda)(1,10-phen)]2tda (tda = thiodiacetate dianion). A model proposed for the electronic structures of the complexes based on the density functional theory (DFT) satisfactorily accounts for the magnetic results. © 2007 Elsevier B.V. All rights reserved