In many metalloenzymes, sulfur‐containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S‐ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four‐nitrogen bound [Co(cyclam)]2+ (1; cyclam=1,5,8,11‐tetraaza‐cyclotetradecane) and the S‐containing analog [Co(S2N2‐cyclam)]2+ (2; S2N2‐cyclam=1,8‐dithia‐5,11‐diaza‐cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the CoII centers in 1 and 2 point to the involvement of sulfur in the O2 reduction process as the key for the improved catalytic ORR capabilities of 2.

Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Moßbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N 5 O} to {N 6 } and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Moßbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch.

Copper(II) coordination compounds have been investigated for their anticancer properties for decades, however, none have reached advanced human clinical trials. The poor translation of copper(II) complexes from in vitro studies...

In this study, a synthesis route of tri(quinolin-8yl)amine (L), a recent member of the tetradentate tris(2pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ 4 mode, two cisoriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations�bis(triflato), bis(acetonitrile), and mixed coligand species�could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mossbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time-and temperature-resolved UV/vis spectroscopy, Mossbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between highand low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)] 2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment�all of the {N 6 }-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

In vielen Metallenzymen sind Schwefel-enthaltende Liganden an Elektronen-Transfer-Reaktionen beteiligt. In dem hier diskutierten biomimetischen Ansatz wird der Einfluss einer Schwefelkoordination auf eine Kobalt-katalysierte Sauerstoff-Reduktionsreaktion (ORR) demonstriert. Ein Vergleich des ORR-Vermögens eines vierfach Stickstoff-koordinierten [Co(Cyclam] 2 + -Komplexes (1; Cyclam = 1,5,8,11-Tetraaza-cyclotetradecan) und dessen Schwefel-Analogons [Co(S 2 N 2 -Cyclam)] 2 + (2; S 2 N 2 -Cyclam = 1,8-Dithia-5,11-diazacyclotetradecan) zeigt verbesserte katalytische Eigenschaften mit dem in die Ligandensphäre am Kobalt eingeführten Chalkogen. Isolierung und Charakterisierung der Intermediate, die sich im Zuge der Sauerstoffaktivierung an den Kobalt(II)-Zentren von 1 und 2 bilden, identifizieren eine Beteiligung des Schwefels am O 2 -Reduktionsprozess als entscheidenden Faktor für die verbesserten Eigenschaften von 2 bei der katalytischen ORR.

Continued efforts are made on the development of earth-abundant metal catalysts for dehydrogenation/hydrolysis of amine boranes. In this study, complex [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)(μ-CO)Fe(NO)2] (3-K-crown, MePyr = 3-methylpyrazolate) was explored as a pre-catalyst for the dehydrogenation of dimethylamine borane (DMAB). Upon evolution of H2(g) from DMAB triggered by 3-K-crown, parallel conversion of 3-K-crown into [(NO)2Fe(N,N′-MePyrBH2NMe2)]− (5) and an iron-hydride intermediate [(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2]− (A) was evidenced by X-ray diffraction/nuclear magnetic resonance/infrared/nuclear resonance vibrational spectroscopy experiments and supported by density functional theory calculations. Subsequent transformation of A into complex [(NO)2Fe(μ-CO)2Fe(NO)2]− (6) is synchronized with the deactivated generation of H2(g). Through reaction of complex [Na-18-crown-6-ether][(NO)2Fe(η2-BH4)] (4-Na-crown) with CO(g) as an alternative synthetic route, isolated intermediate [Na-18-crown-6-ether][(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2] (A-Na-crown) featuring catalytic reactivity toward dehydrogenation of DMAB supports a substrate-gated transformation of a pre-catalyst [(NO)2Fe(μ-MePyr)(μ-CO)Fe(NO)2]− (3) into the iron-hydride species A as an intermediate during the generation of H2(g).