Bioinspired organometallic chemistry: An oxoiron(IV) unit has been trapped within a macrocyclic tetracarbene ligand, merging a key bioinorganic intermediate with a popular organometallic scaffold (see picture). The stability of the new complex has allowed its characterization by a variety of methods, which show a strong σ‐donating tetracarbene coordination leading to an S=1 ground state and unusual properties of the oxoiron(IV) species.

Unprecedented anion-pi interactions are revealed for the electron-poor triazine rings in [L2(CuCl)3][CuCl4]Cl (L = hexakis(pyridin-2-yl)-[1,3,5]-triazine-2,4,6-triamin), where both the chloride ion and a Cl atom of [CuCl4]2- are located approximately 3.15 A above the ring centroids, in excellent agreement with theoretical predictions for a Cl-...triazine complex. This confirms the importance of attractive anion-pi interactions for the supramolecular assembly of complexes with pi-electron-deficient heteroaromatics.

Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H evolution (second-order rate constant of 2.5 × 10 M s; turnover frequency of 250 s at 10 mM H concentration) from mildly acidic solutions.

Flavodiiron nitric oxide reductases (FNORs) are a subclass of flavodiiron proteins (FDPs) capable of preferential binding and subsequent reduction of NO to NO. FNORs are found in certain pathogenic bacteria, equipping them with resistance to nitrosative stress, generated as a part of the immune defense in humans, and allowing them to proliferate. Here, we report the spectroscopic characterization and detailed reactivity studies of the diiron dinitrosyl model complex [Fe(BPMP)(OPr)(NO)](OTf) for the FNOR active site that is capable of reducing NO to NO [Zheng et al., J. Am. Chem. Soc. 2013, 135, 4902-4905]. Using UV-vis spectroscopy, cyclic voltammetry, and spectro-electrochemistry, we show that one reductive equivalent is in fact sufficient for the quantitative generation of NO, following a semireduced reaction mechanism. This reaction is very efficient and produces NO with a first-order rate constant k > 10 s. Further isotope labeling studies confirm an intramolecular N-N coupling mechanism, consistent with the rapid time scale of the reduction and a very low barrier for N-N bond formation. Accordingly, the reaction proceeds at -80 °C, allowing for the direct observation of the mixed-valent product of the reaction. At higher temperatures, the initial reaction product is unstable and decays, ultimately generating the diferrous complex [Fe(BPMP)(OPr)](OTf) and an unidentified ferric product. These results combined offer deep insight into the mechanism of NO reduction by the relevant model complex [Fe(BPMP)(OPr)(NO)] and provide direct evidence that the semireduced mechanism would constitute a highly efficient pathway to accomplish NO reduction to NO in FNORs and in synthetic catalysts.

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Cyclic alkyl(amino) carbene stabilized two- and three-coordinate Fe(I) complexes, (cAAC)2FeCl (2) and [(cAAC)2Fe][B(C6F5)4] (3), respectively, were prepared and thoroughly studied by a bouquet of analytical techniques as well as theoretical calculations. Magnetic susceptibility and Mössbauer spectroscopy reveal the +1 oxidation state and S = 3/2 spin ground state of iron in both compounds. 2 and 3 show slow magnetic relaxation typical for single molecule magnets under an applied direct current magnetic field. The high-frequency EPR measurements confirm the S = 3/2 ground state with a large, positive zero-field splitting (∼20.4 cm(-1)) and reveal easy plane anisotropy for compound 2. CASSCF/CASPT2/RASSI-SO ab initio calculations using the MOLCAS program package support the experimental results.

The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)-azido complex (1), which has been characterized experimentally (magnetically and by electronic d-d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = -10.7 cm(-1), E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)-azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3(-) ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co-N bond distances) of the axial ligand on the N3(-) site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.

The ability of many copper metalloenzymes to activate O2 and transfer it to organic substrates has motivated extensive attention in the literature. Investigations focusing on synthetic analogues have provided a detailed understanding of the structures of potential intermediates, thereby helping to guide mechanistic studies. We report herein a crystallographically characterized synthetic Cu(II)2(μ-η(1):η(1)-O2) complex exhibiting cis-peroxo bonding geometry, known in iron chemistry but previously unobserved for copper. Detailed investigation by UV-vis, resonance Raman, and infrared spectroscopies provides evidence for a significantly diminished copper-oxygen interaction (ε ≈ 3000 M(-1) cm(-1), ν(Cu-O) = 437 cm(-1), ν(O-O) = 799 cm(-1)) relative to those in known 'coupled' Cu2O2 species, consistent with magnetic measurements which show that the peroxide mediates only weak antiferromagnetic coupling (-2J = 144 cm(-1)). These characteristics are comparable with those of a computationally predicted transition state for O2 binding to type 3 copper centers, providing experimental evidence for the proposed mechanism of O2 activation and supporting the biological relevance of the Cu(II)2(μ-η(1):η(1)-O2) cis-species. The peroxide bonding arrangement also allows binding of sodium cations, observed both in the solid state and in solution. Binding induces changes on an electronic level, as monitored by UV-vis spectroscopy (K(a) = 1700 M(-1)), reminiscent of redox-inactive metal binding by iron-oxygen species. The results presented highlight the analogous chemistry these reactive oxygen species undergo, with respect to both their mechanism of formation, and the molecular interactions in which they participate.