Structural and Electronic Rearrangements in Fe2S2, Fe3S4, and Fe4S4 Atomic Clusters under the Attack of NO, CO, and O2

We report results, based on density functional theory-generalized gradient approximation calculations, that shed light on how NO, CO, and O₂ interact with Fe₂S₂, Fe₃S₄, and Fe₄S₄ clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O₂ is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe₄S₄ that splits into two Fe₂S₂ subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe-Fe interaction inducing antiparallel couplings that, in the case of Fe₄S₄, clearly arise from indirect Fe-Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron-sulfur clusters. This increase is noticeably more pronounced for Fe₂S₂ than for the other iron-sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with Fe_mS_m (m = 1-4), the Fe₂S₂CO product results as a prominent one.

Tris(2-mercaptoimidazolyl)hydroborato Cadmium Thiolate Complexes, [TmBut]CdSAr: Thiolate Exchange at Cadmium in a Sulfur-Rich Coordination Environment

A series of cadmium thiolate compounds that feature a sulfur-rich coordination environment, namely [TmBut]CdSAr, have been synthesized by the reactions of [TmBut]CdMe with ArSH (Ar = C6H4-4-F, C6H4-4-But, C6H4-4-OMe, and C6H4-3-OMe). In addition, the pyridine-2-thiolate and pyridine-2-selenolate derivatives, [TmBut]CdSPy and [TmBut]CdSePy have been obtained via the respective reactions of [TmBut]CdMe with pyridine-2-thione and pyridine-2-selone. The molecular structures of [TmBut]CdSAr and [TmBut]CdEPy (E = S or Se) have been determined by X-ray diffraction and demonstrate that, in each case, the [CdS4] motif is distorted tetrahedral and approaches a trigonal monopyramidal geometry in which the thiolate ligand adopts an equatorial position; [TmBut]CdSPy and [TmBut]CdSePy, however, exhibit an additional long-range interaction with the pyridyl nitrogen atoms. The ability of the thiolate ligands to participate in exchange was probed by 1H and 19F nuclear magnetic resonance (NMR) spectroscopic studies of the reactions of [TmBut]CdSC6H4-4-F with ArSH (Ar = C6H4-4-But or C6H4-4-OMe), which demonstrate that (i) exchange is facile and (ii) coordination of thiolate to cadmium is most favored for the p-fluorophenyl derivative. Furthermore, a two-dimensional EXSY experiment involving [TmBut]CdSC6H4-4-F and 4-fluorothiophenol demonstrates that degenerate thiolate ligand exchange is also facile on the NMR time scale.

Advanced paramagnetic resonance spectroscopies of iron-sulfur proteins: Electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM)

The advanced electron paramagnetic resonance (EPR) techniques, electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, provide unique insights into the structure, coordination chemistry, and biochemical mechanism of nature's widely distributed iron-sulfur cluster (FeS) proteins. This review describes the ENDOR and ESEEM techniques and then provides a series of case studies on their application to a wide variety of FeS proteins including ferredoxins, nitrogenase, and radical SAM enzymes. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Bis[1,2-bis(ethoxycarbonyl)ethene-1,2-dithiolato-κ2S,S′]bis(η5-pentamethylcyclopentadienyl)tetra-μ3-sulfido-diiron(IV)diiron(III)(3Fe—Fe)

The title compound, [Fe₄(C₁₀H₁₅)₂(C₈H₁₀O₄S₂)₂S₄], contains a twisted Fe₄S₄ cubane-like core. A twofold rotation axis passes through the Fe₄S₄ core, completing the coordination of the four Fe atoms with two penta­methyl­cyclo­penta­dienyl ligands and two chelating dithiol­ate ligands. There are three short Fe—Fe and three long Fe⋯Fe contacts in the Fe₄S₄ core, suggesting bonding and non-bonding inter­actions, respectively. The Fe—S bonds in the Fe₄S₄ core range from 2.1523 (5) to 2.2667 (6) Å and are somewhat longer than the Fe—S bonds involving the dithiol­ate ligand.

Solvent-Dependent Access to Two Different Ni4II Core Topologies from the First Use of Pyridine-2,6-dimethanol in Nickel(II) Cluster Chemistry

The use of pyridine-2,6-dimethanol, pdmH2, in reactions with nickel(ii) acetate has led to two Ni4 clusters, depending on the solvent. [Ni4(O2CMe)4(pdmH)4]·MeCN (1·MeCN) can be obtained from MeCN and [Ni4(O2CMe)6(pdmH)2(EtOH)2]·1.2EtOH (2·1.2EtOH) from EtOH. Each cluster can be converted into the other in the appropriate solvent. The tetranuclear cluster molecule 1 possesses a distorted cubane {Ni4(μ3-OR)4}4+ core (RO– = pdmH–) with the NiII atoms and the alkoxide-type oxygen atoms from the η3 : η1 : μ3 pdmH– ligands occupying alternate vertices of the cube; four η1 : η1 : μ MeCO2– groups cap four faces of the cube. The four NiII atoms in molecule 2 are located at four vertices of a defective dicubane and are bridged by six oxygen atoms, two μ3 from the η3 : η1 : η1 : μ3 pdmH– ligands and four from four monoatomically bridging MeCO2– groups; peripheral ligation is provided by two η1 : η1 : μ MeCO2– groups and two terminal EtOH ligands. IR data are discussed in terms of the coordination modes of the ligands. Variable-temperature direct-current magnetic susceptibility data of 1 and 2 were modelled with two and three J values respectively, indicating diamagnetic ground states (S = 0). The sign and the magnitude of the J values are discussed in terms of structural features of the complexes.

Reactions of site-differentiated [Fe4S4]2+,1+ clusters with sulfonium cations: reactivity analogues of biotin synthase and other members of the S-adenosylmethionine enzyme family

The first examples of reduced 3:1 site-differentiated Fe(4)S(4) clusters have been synthesized as [Fe(4)S(4)(LS(3))(SR')](3-) (R=Et, Ph) by chemical reduction of previously reported [Fe(4)S(4)(LS(3))(SR')](2-) clusters, and isolated as NBu(4)(+) salts. The reduced clusters were characterized by electrochemistry and EPR, 1H NMR, and Mössbauer spectroscopies. The reaction of oxidized clusters with the sulfonium ions [PhMeSCH(2)R](+) (R=COPh, p-C(6)H(4)CN) in acetonitrile results in electrophilic attack on coordinated thiolate and production of PhSMe and R'SCH(2)R when the reaction occurs at the unique cluster site. The reactions of reduced clusters with these substrates were examined in relation to the reductive cleavage of the cofactor S-adenosylmethionine, the first step in the catalytic cycle of biotin synthase. Product analysis indicated a approximately 4:1 ratio of reductive cleavage to electrophilic attack. The cleavage products are PhSMe, R'SCH(2)R, and RCH(3) for both clusters, and also PhMeS=CHR and RCH(2)CH(2)R from secondary reactions when the sulfonium cation is [PhMeSCH(2)COPh](+) and [PhMeSCH(2)-p-C(6)H(4)CN](+), respectively. Reaction schemes for reductive cleavage based on product distributions are presented. These results parallel those previously reported for homoleptic [Fe(4)S(4)(SR')(4)](2-,3-) clusters and demonstrate that site-differentiated clusters sustain a high percentage of reductive cleavage, a necessary result in the context of biotin synthase activity preceding an investigation of the mode of binding of sulfonium substrates and inhibitors at the unique iron site. [LS(3)=1,3,5-tris[(4,6-dimethyl-3-mercaptophenyl)thio]-2,4,6-tris(p-tolylthio)benzene(3-)].

Environmental dehalogenation: chemistry and mechanism

The halogen cycle is one of the great chemical cycles on earth. Haloorganics are both synthesized and destroyed by the chemistry that controls their flux and form. The synthetic leg of the cycle is both biotic and abiotic in nature. The biotic synthesis results primarily from the biochemical activity of marine algae and kelp, although these are by no means the only sources. The abiotic process is vested in large part in volcanic eruption and emission of gases synthesized as a thermal consequence of venting the earth's core.