Quantification of the thermodynamic effects of the low-spin - high-spin interaction in molecular crystals of a mononuclear iron(II) spin crossover complex.

A method is proposed to estimate the energetic and entropic effects of spins of neighbouring molecules on the spin transition of a mononuclear spin crossover (SCO) complex in a molecular crystal. Density functional theory (DFT) methods have been used to model the SCO material [FeII(Lnpdtz)2(NCS)2] (Lnpdtz = 2-naphthyl-5-pyridyl-1,2,4-thiadiazole) exhibiting numerous π-π interactions using a 2D arrangement of 15 molecules. The modelling considers only the effects in the crystallographical ac plane with a particularly pronounced stacking but paves the way for future work with 3D arrangements which are computational much more costly. It involves the optimisation and normal mode calculation of the molecules in a rigid matrix of both low-spin (LS) and high-spin (HS) neighbours. This procedure has been used to calculate the previously defined cooperativity parameter Hcoop (S. Rackwitz, W. Klopper, V. Schünemann and J. A. Wolny, Phys. Chem. Chem. Phys., 2013, 15, 15450). For [FeII(Lnpdtz)2(NCS)] we obtain Hcoop = 11 kJ mol-1, a value which is comparable to those found for 3D polynuclear spin crossover materials. A normal mode analysis of the optimised centrally located molecule indicates that the vibrational entropy of the spin transition is somewhat higher (5 J K-1 mol-1) for the LS to HS transition in the LS matrix than in the HS one. The calculations show that the interactions with the neighbours influence the low-frequency modes with wave numbers <65-70 cm-1. These cause the main difference in the vibrational entropy of the spin transition for the vicinity of high- and low-spin molecules. Furthermore, a deformation of the coordination sphere of the central molecule is observed when the spins of the surrounding centres are switched. This deformation is accompanied by a change in the equatorial Fe-N bond lengths.

Applying Nuclear Forward Scattering as In Situ and Operando Tool for the Characterization of FeN4 Moieties in the Hydrogen Evolution Reaction.

Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and is applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron-nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate-determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal-nitrogen-carbon (MNC) catalysts.

Mechanism and structural dynamics of sulfur transfer during de novo [2Fe-2S] cluster assembly on ISCU2.

Maturation of iron-sulfur proteins in eukaryotes is initiated in mitochondria by the core iron-sulfur cluster assembly (ISC) complex, consisting of the cysteine desulfurase sub-complex NFS1-ISD11-ACP1, the scaffold protein ISCU2, the electron donor ferredoxin FDX2, and frataxin, a protein dysfunctional in Friedreich's ataxia. The core ISC complex synthesizes [2Fe-2S] clusters de novo from Fe and a persulfide (SSH) bound at conserved cluster assembly site residues. Here, we elucidate the poorly understood Fe-dependent mechanism of persulfide transfer from cysteine desulfurase NFS1 to ISCU2. High-resolution cryo-EM structures obtained from anaerobically prepared samples provide snapshots that both visualize different stages of persulfide transfer from Cys381NFS1 to Cys138ISCU2 and clarify the molecular role of frataxin in optimally positioning assembly site residues for fast sulfur transfer. Biochemical analyses assign ISCU2 residues essential for sulfur transfer, and reveal that Cys138ISCU2 rapidly receives the persulfide without a detectable intermediate. Mössbauer spectroscopy assessing the Fe coordination of various sulfur transfer intermediates shows a dynamic equilibrium between pre- and post-sulfur-transfer states shifted by frataxin. Collectively, our study defines crucial mechanistic stages of physiological [2Fe-2S] cluster assembly and clarifies frataxin's molecular role in this fundamental process.

Experimental and theoretical evidence for unprecedented strong interactions of gold atoms with boron on boron/sulfur-doped carbon surfaces.

The 16e square-planar bis-thiolato-Au(iii) complexes [AuIII(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)2][NBu4] (Au-1) and [AuIII(4-methyl-1,2-benzenedithiolato)2][NBu4] (Au-2) have been synthesized and fully characterized. Au-1 and Au-2 were encapsulated in the symmetrical triblock copolymer poloxamer (Pluronic®) P123 containing blocks of poly(ethylene oxide) and poly(propylene oxide), giving micelles AuMs-1 and AuMs-2. High electron flux in scanning transmission electron microscopy (STEM) was used to generate single gold atoms and gold nanocrystals on B/S-doped graphitic surfaces, or S-doped amorphous carbon surfaces from AuMs-1 and AuMs-2, respectively. Electron energy loss spectroscopy (EELS) data suggested strong interactions of gold atoms/nanocrystals with boron in the B/S-doped graphitic matrix. Density-functional theory (DFT) calculations, also supported the experimental findings, pointing towards strong Au-B bonds, depending on the charge on the Au-(B-graphene) fragment and the presence of further defects in the graphene lattice.

Structure-based insights into the mechanism of [4Fe-4S]-dependent sulfur insertase LarE.

Several essential cellular metabolites, such as enzyme cofactors, contain sulfur atoms and their biosynthesis requires specific thiolation enzymes. LarE is an ATP-dependent sulfur insertase, which catalyzes the sequential conversion of the two carboxylate groups of the precursor of the lactate racemase cofactor into thiocarboxylates. Two types of LarE enzymes are known, one that uses a catalytic cysteine as a sacrificial sulfur donor, and the other one that uses a [4Fe-4S] cluster as a cofactor. Only the crystal structure of LarE from Lactobacillus plantarum (LpLarE) from the first class has been solved. We report here the crystal structure of LarE from Methanococcus maripaludis (MmLarE), belonging to the second class, in the cluster-free (apo-) and cluster-bound (holo-) forms. The structure of holo-MmLarE shows that the [4Fe-4S] cluster is chelated by three cysteines only, leaving an open coordination site on one Fe atom. Moreover, the fourth nonprotein-bonded iron atom was able to bind an anionic ligand such as a phosphate group or a chloride ion. Together with the spectroscopic analysis of holo-MmLarE and the previously reported biochemical investigations of holo-LarE from Thermotoga maritima, these crystal structures support the hypothesis of a reaction mechanism, in which the [4Fe-4S] cluster binds a hydrogenosulfide ligand in place of the chloride anion, thus generating a [4Fe-5S] intermediate, and transfers it to the substrate, as in the case of [4Fe-4S]-dependent tRNA thiolation enzymes.

TudS desulfidases recycle 4-thiouridine-5'-monophosphate at a catalytic [4Fe-4S] cluster.

In all domains of life, transfer RNAs (tRNAs) contain post-transcriptionally sulfur-modified nucleosides such as 2- and 4-thiouridine. We have previously reported that a recombinant [4Fe-4S] cluster-containing bacterial desulfidase (TudS) from an uncultured bacterium catalyzes the desulfuration of 2- and 4-thiouracil via a [4Fe-5S] cluster intermediate. However, the in vivo function of TudS enzymes has remained unclear and direct evidence for substrate binding to the [4Fe-4S] cluster during catalysis was lacking. Here, we provide kinetic evidence that 4-thiouridine-5'-monophosphate rather than sulfurated tRNA, thiouracil, thiouridine or 4-thiouridine-5'-triphosphate is the preferred substrate of TudS. The occurrence of sulfur- and substrate-bound catalytic intermediates was uncovered from the observed switch of the S = 3/2 spin state of the catalytic [4Fe-4S] cluster to a S = 1/2 spin state upon substrate addition. We show that a putative gene product from Pseudomonas putida KT2440 acts as a TudS desulfidase in vivo and conclude that TudS-like enzymes are widespread desulfidases involved in recycling and detoxifying tRNA-derived 4-thiouridine monophosphate nucleosides for RNA synthesis.

Heme protonation affects iron-NO binding in the NO transport protein nitrophorin.

The nitrophorins (NPs) comprise an unusual group of heme proteins with stable ferric heme iron nitric oxide (Fe-NO) complexes. They are found in the salivary glands of the blood-sucking kissing bug Rhodnius prolixus, which uses the NPs to transport the highly reactive signaling molecule NO. Nuclear resonance vibrational spectroscopy (NRVS) of both isoform NP2 and a mutant NP2(Leu132Val) show, after addition of NO, a strong structured vibrational band at around 600 cm-1, which is due to modes with significant Fe-NO bending and stretching contribution. Based on a hybrid calculation method, which uses density functional theory and molecular mechanics, it is demonstrated that protonation of the heme carboxyl groups does influence both the vibrational properties of the Fe-NO entity and its electronic ground state. Moreover, heme protonation causes a significant increase of the gap between the highest occupied and lowest unoccupied molecular orbital by almost one order of magnitude leading to a stabilization of the Fe-NO bond.

Structural insight into halide-coordinated [Fe4S4XnY4-n]2- clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy.

Iron sulphur halide clusters [Fe4S4Br4]2- and [Fe4S4X2Y2]2- (X, Y = Cl, Br, I) were obtained in excellent yields (77 to 78%) and purity from [Fe(CO)5], elemental sulphur, I2 and benzyltrimethylammonium (BTMA+) iodide, bromide and chloride. Single crystals of (BTMA)2[Fe4S4Br4] (1), (BTMA)2[Fe4S4Br2Cl2] (2), (BTMA)2[Fe4S4Cl2I2] (3), and (BTMA)2[Fe4S4Br2I2] (4) were isostructural to the previously reported (BTMA)2[Fe4S4I4] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe4S4Cl4]2-, we found the prismane-shaped cluster (BTMA)3[Fe6S6Cl6] (6) (P1̄). 57Fe Mössbauer spectroscopy indicates complete delocalisation with Fe2.5+ oxidation states for all iron atoms. Magnetic measurements showed small χMT values at 298 K ranging from 1.12 to 1.54 cm3 K mol-1, indicating the dominant antiferromagnetic exchange interactions. With decreasing temperature, the χMT values decreased to reach a plateau at around 100 K. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg-Dirac-van Vleck (HDvV) as well as the Heisenberg Double Exchange (HDE) formalism confirmed the delocalisation and antiferromagnetic coupling assumed from Mössbauer spectroscopy.

A subclass of archaeal U8-tRNA sulfurases requires a [4Fe-4S] cluster for catalysis.

Sulfuration of uridine 8, in bacterial and archaeal tRNAs, is catalyzed by enzymes formerly known as ThiI, but renamed here TtuI. Two different classes of TtuI proteins, which possess a PP-loop-containing pyrophosphatase domain that includes a conserved cysteine important for catalysis, have been identified. The first class, as exemplified by the prototypic Escherichia coli enzyme, possesses an additional C-terminal rhodanese domain harboring a second cysteine, which serves to form a catalytic persulfide. Among the second class of TtuI proteins that do not possess the rhodanese domain, some archaeal proteins display a conserved CXXC + C motif. We report here spectroscopic and enzymatic studies showing that TtuI from Methanococcus maripaludis and Pyrococcus furiosus can assemble a [4Fe-4S] cluster that is essential for tRNA sulfuration activity. Moreover, structural modeling studies, together with previously reported mutagenesis experiments of M. maripaludis TtuI, indicate that the [4Fe-4S] cluster is coordinated by the three cysteines of the CXXC + C motif. Altogether, our results raise a novel mechanism for U8-tRNA sulfuration, in which the cluster is proposed to catalyze the transfer of sulfur atoms to the activated tRNA substrate.

Iron Insertion at the Assembly Site of the ISCU Scaffold Protein Is a Conserved Process Initiating Fe-S Cluster Biosynthesis.

Iron-sulfur (Fe-S) clusters are prosthetic groups of proteins biosynthesized on scaffold proteins by highly conserved multi-protein machineries. Biosynthesis of Fe-S clusters into the ISCU scaffold protein is initiated by ferrous iron insertion, followed by sulfur acquisition, via a still elusive mechanism. Notably, whether iron initially binds to the ISCU cysteine-rich assembly site or to a cysteine-less auxiliary site via N/O ligands remains unclear. We show here by SEC, circular dichroism (CD), and Mössbauer spectroscopies that iron binds to the assembly site of the monomeric form of prokaryotic and eukaryotic ISCU proteins via either one or two cysteines, referred to the 1-Cys and 2-Cys forms, respectively. The latter predominated at pH 8.0 and correlated with the Fe-S cluster assembly activity, whereas the former increased at a more acidic pH, together with free iron, suggesting that it constitutes an intermediate of the iron insertion process. Iron not binding to the assembly site was non-specifically bound to the aggregated ISCU, ruling out the existence of a structurally defined auxiliary site in ISCU. Characterization of the 2-Cys form by site-directed mutagenesis, CD, NMR, X-ray absorption, Mössbauer, and electron paramagnetic resonance spectroscopies showed that the iron center is coordinated by four strictly conserved amino acids of the assembly site, Cys35, Asp37, Cys61, and His103, in a tetrahedral geometry. The sulfur receptor Cys104 was at a very close distance and apparently bound to the iron center when His103 was missing, which may enable iron-dependent sulfur acquisition. Altogether, these data provide the structural basis to elucidate the Fe-S cluster assembly process and establish that the initiation of Fe-S cluster biosynthesis by insertion of a ferrous iron in the assembly site of ISCU is a conserved mechanism.

Kinetics and mechanism of sequential ring methyl C-H activation in cyclopentadienyl rhodium(III) complexes.

We have studied activation of the methyl C-H bonds in the cyclopentadienyl ligands of half-sandwich Rh(III) complexes [η5-CpXRh(N,N')Cl]+ by observing the dependence of sequential H/D exchange on variations in CpX = Cp* (complexes 1 and 2), Me4PhCp (CpXPh, 3) or Me4PhPhCp (CpXPhPh, 4), and chelated ligand N,N' (bpy, 1; phen, 2-4). H/D exchange was fastest in d4-MeOD (t1/2 = 10 min, 37 °C, complex 1), no H/D exchange was observed in DMSO/D2O, and d4-MeOD enhanced the rate in CD3CN. The proposed Rh(I)-fulvene intermediate was trapped by [4 + 2] Diels-Alder reactions with conjugated dienes and characterized. The Rh(I) oxidation state was confirmed by X-ray photoelectron spectroscopy (XPS). Influence of solvent on the mechanisms of activation and Diels-Alder adduct formation was modelled using DFT calculations with the CAM-B3LYP functional and CEP-31 g basis set, and influence on the reaction profile of the dimiine ligand and phenyl substituent using the larger qzvp basis set. The Rh(III)-OH intemediate is stabilised by H-bonding with methanol and a Cp* CH3 hydrogen. The Rh(I)(Me4fulvene) species, stabilised by interaction of methanol with a coordinated water, again by two H-bonds H2O-HOMe (1.49 Å) and fulvene CH2 (1.94 Å), arises from synchronous transfer of the methanol OH proton to a Rh(III)-OH ligand and Cp* methyl hydrogen to the methanol oxygen. Additionally, the observed trend in catalytic activity for complexes 1-4 was reproduced by DFT calculations. These complexes form a novel class of catalytic molecular motors with a tunable rate of operation that can be stalled in a given state. They provide a basis for elucidation of the effects of ligand design on the contributions of electronic, rotational and vibrational energies to each step in the reaction pathway at the atomic level, consideration of which will enhance the design principles for the next generation of molecular machines.

A free boratriptycene-type Lewis superacid.

Bicyclic pyrazabole-bridged ferrocenes with BH groups at their bridgehead positions were prepared from [Li(thf)]2[1,1'-fc(BH3)2] and pyrazole or 3,5-dimethylpyrazole in the presence of Me3SiCl (1 or 1Me, respectively; 1,1'-fc = 1,1'-ferrocenylene); Me3SiH and H2 are released as byproducts. Treatment of 1 or 1Me with 1 eq. of the hydride scavenger [Ph3C][B(C6F5)4] afforded the borenium salts [2][B(C6F5)4] (72%) and [2Me][B(C6F5)4] (77%). According to X-ray crystallography, [2Me]+ contains one trigonal-planar borenium cation, the cyclopentadienyl (Cp) rings of the 1,1'-fc fragment remain parallel to each other, but the Cp-B bond vector is bent out of the Cp plane by an unprecedentedly large dip angle α* of 40.6°. The Fe⋯B(sp2) distance is very short (2.365(4) Å) and the 11B NMR signal of the cationic B(sp2) center is remarkably upfield shifted (23.4 ppm), suggesting a direct Fe(3d) → B(2p) donor-acceptor interaction. Although this interpretation is confirmed by quantum-chemical calculations, the coupling between the associated orbitals corresponds to an energy of only 12 kJ mol-1. Accordingly, both the experimental (e.g., Gutmann-Beckett acceptor number AN = 111) and theoretical assessment (e.g., Et3PO and F--ion affinities) of the Lewis acidity proves that [2]+ is among the strongest boron-based Lewis acids available to date.

Density functional theory investigation of Ru(II) and Os(II) asymmetric transfer hydrogenation catalysts.

Transition metal ions have a unique ability to organise and control the steric and electronic effects around a substrate in the active site of a catalyst. We consider half-sandwich Ru(II) (Noyori-type) and Os(II) sulfonyldiamine 16-electron active catalysts [Ru/Os(η6-p-cymene)(TsDPEN-H2)], where TsDPEN is N-tosyl-1,2-diphenylethylenediamine containing S,S or R,R chiral centres, which catalyse the highly efficient asymmetric transfer hydrogenation of aromatic ketones to chiral alcohols using formic acid as a hydride source. We discuss the recognition of the prochiral ketone acetophenone by the catalyst, the protonation of a ligand NH and transfer of hydride from formate to the metal, subsequent transfer of hydride to one enantiotopic face of the ketone, followed by proton transfer from metal-bound NH2, and regeneration of the catalyst. Our DFT calculations illustrate the role of the two chiral carbons on the N,N-chelated sulfonyldiamine ligand, the axial chirality of the π-bonded p-cymene arene, and the chirality of the metal centre. We discuss new features of the mechanism, including how a change in metal chirality of the hydride intermediate dramatically switches p-cymene coordination from η6 to η2. Moreover, the calculations suggest a step-wise mechanism involving substrate docking to the bound amine NH2 followed by hydride transfer prior to protonation of the O-atom of acetophenone and release of the enantio-pure alcohol. This implies that formation and stability of the M-H hydride intermediate is highly dependent on the presence of the protonated amine ligand. The Os(II) catalyst is more stable than the Ru(II) analogue, and these studies illustrate the subtle differences in mechanistic behaviour between these 4d6 and 5d6 second-row and third-row transition metal congeners in group 8 of the periodic table.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes.

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [FeII(Lnpdtz)2(NCX)2] (X = S; Se), with Lnpdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Electron inventory of the iron-sulfur scaffold complex HypCD essential in [NiFe]-hydrogenase cofactor assembly.

The [4Fe-4S] cluster containing scaffold complex HypCD is the central construction site for the assembly of the [Fe](CN)2CO cofactor precursor of [NiFe]-hydrogenase. While the importance of the HypCD complex is well established, not much is known about the mechanism by which the CN- and CO ligands are transferred and attached to the iron ion. We report an efficient expression and purification system producing the HypCD complex from E. coli with complete metal content. This enabled in-depth spectroscopic characterizations. The results obtained by EPR and Mössbauer spectroscopy demonstrate that the [Fe](CN)2CO cofactor and the [4Fe-4S] cluster of the HypCD complex are redox active. The data indicate a potential-dependent interconversion of the [Fe]2+/3+ and [4Fe-4S]2+/+ couple, respectively. Moreover, ATR FTIR spectroscopy reveals potential-dependent disulfide formation, which hints at an electron confurcation step between the metal centers. MicroScale thermophoresis indicates preferable binding between the HypCD complex and its in vivo interaction partner HypE under reducing conditions. Together, these results provide comprehensive evidence for an electron inventory fit to drive multi-electron redox reactions required for the assembly of the CN- and CO ligands on the scaffold complex HypCD.

Chiral Resolution of Spin-Crossover Active Iron(II) [2x2] Grid Complexes.

Chiral magnetic materials are proposed for applications in second-order non-linear optics, magneto-chiral dichroism, among others. Recently, we have reported a set of tetra-nuclear Fe(II) grid complex conformers with general formula C/S-[Fe4 L4 ]8+ (L: 2,6-bis(6-(pyrazol-1-yl)pyridin-2-yl)-1,5-dihydrobenzo[1,2-d : 4,5-d']diimidazole). In the grid complexes, isomerism emerges from tautomerism and conformational isomerism of the ligand L, and the S-type grid complex is chiral, which originates from different non-centrosymmetric spatial organization of the trans type ligand around the Fe(II) center. However, the selective preparation of an enantiomerically pure grid complex in a controlled manner is difficult due to spontaneous self-assembly. To achieve the pre-synthesis programmable resolution of Fe(II) grid complexes, we designed and synthesized two novel intrinsically chiral ligands by appending chiral moieties to the parent ligand. The complexation of these chiral ligands with Fe(II) salt resulted in the formation of enantiomerically pure Fe(II) grid complexes, as unambiguously elucidated by CD and XRD studies. The enantiomeric complexes exhibited similar gradual and half-complete thermal and photo-induced SCO characteristics. The good agreement between the experimentally obtained and calculated CD spectra further supports the enantiomeric purity of the complexes and even the magnetic studies. The chiral resolution of Fe(II)- [2×2] grid complexes reported in this study, for the first time, might enable the fabrication of magneto-chiral molecular devices.