The electronic structure of the H-cluster in the [FeFe]-hydrogenase from Desulfovibrio desulfuricans: a Q-band 57Fe-ENDOR and HYSCORE study

The active site of the (57)Fe-enriched [FeFe]-hydrogenase (i.e., the "H-cluster") from Desulfovibrio desulfuricans has been examined using advanced pulse EPR methods at X- and Q-band frequencies. For both the active oxidized state (H(ox)) and the CO inhibited form (H(ox)-CO) all six (57)Fe hyperfine couplings were detected. The analysis shows that the apparent spin density extends over the whole H-cluster. The investigations revealed different hyperfine couplings of all six (57)Fe nuclei in the H-cluster of the H(ox)-CO state. Four large 57Fe hyperfine couplings in the range 20-40 MHz were found (using pulse ENDOR and TRIPLE methods) and were assigned to the [4Fe-4S](H) (cubane) subcluster. Two weak (57)Fe hyperfine couplings below 5 MHz were identified using Q-band HYSCORE spectroscopy and were assigned to the [2Fe](H) subcluster. For the H(ox) state only two different 57Fe hyperfine couplings in the range 10-13 MHz were detected using pulse ENDOR. An (57)Fe line broadening analysis of the X-band CW EPR spectrum indicated, however, that all six (57)Fe nuclei in the H-cluster are contributing to the hyperfine pattern. It is concluded that in both states the binuclear subcluster [2Fe](H) assumes a [Fe(I)Fe(II)] redox configuration where the paramagnetic Fe(I) atom is attached to the [4Fe-4S](H) subcluster. The (57)Fe hyperfine interactions of the formally diamagnetic [4Fe-4S](H) are due to an exchange interaction between the two subclusters as has been discussed earlier by Popescu and Münck [Popescu, C.V.; Münck, E., J. Am. Chem. Soc. 1999, 121, 7877-7884]. This exchange coupling is strongly enhanced by binding of the extrinsic CO ligand. Binding of the dihydrogen substrate may induce a similar effect, and it is therefore proposed that the observed modulation of the electronic structure by the changing ligand surrounding plays an important role in the catalytic mechanism of [FeFe]-hydrogenase.     

A Ferromagnetically Coupled (S=1) Peroxodicopper(II) Complex

Copper enzymes play important roles in the binding and activation of dioxygen in biological systems. Key copper/dioxygen intermediates have been identified and studied in synthetic analogues of the metalloprotein active sites, including the μ‐η²:η²‐peroxodicopper(II) motif relevant to type III dicopper proteins. Herein, we report the synthesis and characterization of a bioinspired dicopper system that forms a stable μ‐η¹:η¹‐peroxo complex whose Cu‐O‐O‐Cu torsion is constrained to around 90° by ligand design. This results in sizeable ferromagnetic coupling between the copper(II) ions, which is detected by magnetic measurements and HF‐EPR spectroscopy. The new dicopper peroxo system is the first with a triplet ground state, and it represents a snapshot of the initial stages of O₂ binding at type III dicopper sites.     

Ligand‐Centred Reactivity of Bis(picolyl)amine Iridium: Sequential Deprotonation,Oxidation and Oxygenation of a “Non‐Innocent” Ligand

Treatment of [Ir(bpa)(cod)]⁺ complex [ 1 ]⁺ with a strong base (e.g., tBuO^?) led to unexpected double deprotonation to form the anionic [Ir(bpa?2H)(cod)]^? species [ 3 ]^?, via the mono‐deprotonated neutral amido complex [Ir(bpa?H)(cod)] as an isolable intermediate. A certain degree of aromaticity of the obtained metal–chelate ring may explain the favourable double deprotonation. The rhodium analogue [ 4 ]^? was prepared in situ. The new species [M(bpa?2H)(cod)]^? (M=Rh, Ir) are best described as two‐electron reduced analogues of the cationic imine complexes [M^I(cod)(Py‐CH₂‐N?CH‐Py)]⁺. One‐electron oxidation of [ 3 ]^? and [ 4 ]^? produced the ligand radical complexes [ 3 ]^. and [ 4 ]^.. Oxygenation of [ 3 ]^? with O₂ gave the neutral carboxamido complex [Ir(cod)(py‐CH₂‐N‐CO‐py)] via the ligand radical complex [ 3 ]^. as a detectable intermediate.     

O,N‐Chelated Vanadium(IV) Oxo Aminophenolate Complexes: the Effect of Steric Bulk on the Vanadium Coordination Geometry. Can this Influence be detected Spectroscopically?

In order to study the effect of steric bulk on the vanadium coordination geometry in O, N‐chelated vanadium oxo (bis)phenolates, six different ortho‐aminophenolate ligands have been used. The ortho‐aminophenolate system was changed at three different places, i.e. 1) the second ortho position (C⁶) of the arene ring (R), 2) the substituents at the amino nitrogen (R′ and R″), and 3) the benzylic carbon atom (R*). The phenols were used in the preparation of the vanadium oxo (bis)phenolate complexes. In order to study whether it is possible to predict geometrical features of these vanadium complexes, UV/Vis, solution and frozen state EPR and ¹⁴N ESEEM spectroscopic data was measured and compared to the structural features of four structurally characterized vanadium oxo (bis)phenolates. Unfortunately, it turned out that is was not possible to correlate the EPR parameters, the UV/Vis HOMO‐LUMO transitions or ¹⁴N hyperfine couplings to the structural parameters.     

A systematic approach for studying Cu imidazole interactions using multi-frequency ESEEM: Application to Cu(II)-bleomycin and model systems

Many copper containing proteins exhibit well defined ESEEM signals detected at X-band and C-band. In these systems the Cu(II) ion is coordinated to one or several histidine residues. The main sharp features measured in the ESEEM spectra originate from the interaction of the unpaired electron with the remote nitrogen nucleus of the histidine ring. The relative intensities of these features contain information about the orientation of the NQI-tensor in the molecular axis frame as defined by the principal axes of theg-matrix. This information can be related to the orientation of the imidazole ring in the complex. We present a systematic approach to determine the constraints of the Euler angles, α, β, ψ of the NQI-tensor in theg-matrix principal axis system. The first step is to analyze the intensity ratios of the quadrupole peaks and the line shape of the double quantum feature measured on the canonical positions in the EPR spectrum. This will lead to a constraint in the angles (α, β) as well as the effective hyperfine interaction. This information is further refined using spectra on other orientation selective positions. We have applied this method to Cu(II)-Bleomycin and two model compounds: Cu(II)-pypep and Cu(II)-PMA of which we have determined the principal quadrupole values and the orientation of the quadrupole tensor with respect to theg-matrix axis system.     

Radical C−N Borylation of Aromatic Amines Enabled by a Pyrylium Reagent

Herein, we report a radical borylation of aromatic amines through a homolytic C(sp²)−N bond cleavage. This method capitalizes on a simple and mild activation via a pyrylium reagent (^ScPyry-OTf) thus priming the amino group for reactivity. The combination of terpyridine and a diboron reagent triggers a radical reaction which cleaves the C(sp²)−N bond and forges a new C(sp²)−B bond. The unique non-planar structure of the pyridinium intermediate, provides the necessary driving force for the aryl radical formation. The method permits borylation of a wide variety of aromatic amines indistinctively of the electronic environment.     

On <Emphasis Type="Italic">g</Emphasis>-functions for Laguerre function expansions of Hermite type

We examine weighted L ^p boundedness of g-functions based on semigroups related to multi-dimensional Laguerre function expansions of Hermite type. A technique of vector-valued Calderón–Zygmund operators is used.