[FeFe]-hydrogenase from green algae (HydA1) is the most efficient hydrogen (H2) producing enzyme in nature and of prime interest for (bio)technology. Its active site is a unique six-iron center (H-cluster) composed of a cubane cluster, [4Fe4S]H, cysteine-linked to a diiron unit, [2Fe]H, which carries unusual carbon monoxide (CO) and cyanide ligands and a bridging azadithiolate group. We have probed the molecular and electronic configurations of the H-cluster in functional oxidized, reduced, and super-reduced or CO-inhibited HydA1 protein, in particular searching for intermediates with iron-hydride bonds. Site-selective X-ray absorption and emission spectroscopy were used to distinguish between low- and high-spin iron sites in the two subcomplexes of the H-cluster. The experimental methods and spectral simulations were calibrated using synthetic model complexes with ligand variations and bound hydride species. Distinct X-ray spectroscopic signatures of electronic excitation or decay transitions in [4Fe4S]H and [2Fe]H were obtained, which were quantitatively reproduced by density functional theory calculations, thereby leading to specific H-cluster model structures. We show that iron-hydride bonds are absent in the reduced state, whereas only in the super-reduced state, ligand rotation facilitates hydride binding presumably to the Fe-Fe bridging position at [2Fe]H. These results are in agreement with a catalytic cycle involving three main intermediates and at least two protonation and electron transfer steps prior to the H2 formation chemistry in [FeFe]-hydrogenases.

Modification of the Co-oxo cores of cobalt-polyoxometalate water oxidation catalysts is detectable by X-ray absorption spectroscopy (XAS) as demonstrated by comparison of Na10[Co4(H2O)2(PW9O34)2] (1) and Na17[((Co(H2O))Co2PW9O34)2(PW6O26)] (2). XAS reveals the integrity of 1 uncompromised by oxidant-driven water oxidation, which proceeds without formation of catalytic cobalt oxide.

The reaction of rhenocene hydride with bismuth alkoxides leads to complexes containing Re-Bi metal bonds with concomitant formation of the corresponding alcohols. Hence, compounds of the type [Cp 2 Re-BiR 2 ] can be obtained from the monoalkoxide [(MeO)Bi(o-tol) 2 ] n and also from the trialkoxide [Bi{OCH(CF 3 ) 2 } 3 (thf)] 2 , for which in principle also multiple substitution reactions would have been possible, but are prohibited by the electron-withdrawing character of the hexafluoroisopropyl groups. Correspondingly, the trialkoxide [Bi(O t Bu) 3 ] containing electron-rich tert-butyl groups does lead to multiple substitution events: First of all it reacts with two equivalents of [Cp 2 ReH] to give the intermediate [(Cp 2 Re) 2 Bi(O t Bu)], which could not be isolated, since it undergoes an intramolecular alcohol elimination via Cp C-H bond cleavage. This results in the complex [CpRe(μ-η 5 ,η 1 -C 5 H 4 )Bi-ReCp 2 ], featuring a bent Bi-C bond so that one deprotonated Cp ligand bridges a Bi-Re metal bond. All compounds have been fully characterized, and their crystal structures are discussed.

Dedicated to Professor Heinrich Nöth on the occasion of his 85 th birthdayReaction

Nearly phase-pure bismuth ferrite particles were formed by thermolysis of the single-source precursor [Cp(CO)2FeBi(OAc)2] (1) in octadecene at 245 °C, followed by subsequent calcination at 600 °C for 3 h. In contrast, the slightly modified compound [Cp(CO)2FeBi(O2C(t)Bu)2] (2) yielded only mixtures of different bismuth oxide phases, revealing the distinctive influence of molecular design in material synthesis. The chemical composition, morphology, and crystallinity of the resulting materials were investigated by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. In addition, the optical properties were investigated by Fourier transform infrared and UV-vis spectroscopies, showing a strong band gap absorption in the visible range at 590 nm (2.2 eV). The magnetic behavior was probed by vibrating-sample and superconducting quantum interference device magnetometry, as well as (57)Fe Mössbauer spectroscopy.