Abiological Ironminus signSulfur Clusters

Addition of pnictogen atoms to chromium(ii): synthesis, structure and magnetic properties of a chromium(iv) phosphide and a chromium(iii) arsenide

Chromium(ii) chloride reacts with LiCp* (Cp* = C₅Me₅) and LiE (SiMe₃)₂ (E = P or As) to give the chromium(iv) phosphide [(η⁵-Cp*Cr)(μ₃-P)]₄ (1) or the chromium(iii) arsenide [(η⁵-Cp*Cr)₃(μ₃-As)₂] (2), respectively.

Structural Analysis of Cubane-Type Iron Clusters

The generalized cluster type [M₄(μ₃-Q)₄L _n ] ^x contains the cubane-type [M₄Q₄] ^z core unit that can approach, but typically deviates from, perfect T_d symmetry. The geometric properties of this structure have been analyzed with reference to T_d symmetry by a new protocol. Using coordinates of M and Q atoms, expressions have been derived for interatomic separations, bond angles, and volumes of tetrahedral core units (M₄, Q₄) and the total [M₄Q₄] core (as a tetracapped M₄ tetrahedron). Values for structural parameters have been calculated from observed average values for a given cluster type. Comparison of calculated and observed values measures the extent of deviation of a given parameter from that required in an exact tetrahedral structure. The procedure has been applied to the structures of over 130 clusters containing [Fe₄Q₄] (Q = S^2-, Se^2-, Te^2-, [NPR₃]^-, [NR]^2-) units, of which synthetic and biological sulfide-bridged clusters constitute the largest subset. General structural features and trends in structural parameters are identified and summarized. An extensive database of structural properties (distances, angles, volumes) has been compiled in Supporting Information.

Assignment of the photoelectron spectra of FeS3(-) by density functional theory, CASPT2, and RCCSD(T) calculations

The geometric structures of FeS(3) and FeS(3)(-) with spin multiplicities ranging from singlet to octet were optimized at the B3LYP level, allowing two low-lying conformations for these clusters to be identified. The planar D(3h) conformation contains three S(2-) atomic ligands (S(3)Fe(0/-)), whereas the C(2v) structure contains, in addition to an atomic S(2-) ligand, also a S(2)(2-) ligand that is side-on-bound to the iron cation: an η(2)-S(2)FeS conformation. Subsequently, energy differences between the various states of these conformations were estimated by carrying out geometry optimizations at the multireference CASPT2 level. Several competing structures for the ground state of the anionic cluster were recognized at this level. Relative stabilities were also estimated by performing single-point RCSSD(T) calculations on the B3LYP geometries. The ground state of the neutral complex was unambiguously found to be (5)B(2). The ground state of the anion is considerably less certain. The 1(4)B(2), 2(4)B(2), (4)B(1), and (6)A(1) states were all found as low-lying η(2)-S(2)FeS(-) states. Also, (4)B(2) of S(3)Fe(-) has a comparable CASPT2 energy. In contrast, B3LYP and RCCSD(T) mutually agree that the S(3)Fe(-) state is at a much higher energy. Energetically, the bands of the photoelectron spectra of FeS(3)(-) are reproduced at the CASPT2 level as ionizations from either the (4)B(2) or (6)A(1) state of η(2)-S(2)FeS. However, the Franck-Condon factors obtained from a harmonic vibrational analysis at the B3LYP level show that only the (4)B(2)-to-(5)B(2) ionization, which preserves the η(2)-S(2)Fe-S conformation, provides the best vibrational progression match with the X band of the experimental photoelectron spectra.

Stepwise construction of manganese-chromium carbonyl chalcogenide complexes: synthesis, electrochemical properties, and computational studies

When trigonal-bipyramidal clusters, [PPN][E(2)Mn(3)(CO)(9)] (E = S, Se), were treated with Cr(CO)(6) and PPNCl in a molar ratio of 1:1:2 or 1:2:2 in 4 M KOH/MeCN/MeOH solutions, mono-Cr(CO)(5)-incorporated HE(2)Mn(3)-complexes [PPN](2)[HE(2)Mn(3)Cr(CO)(14)] (E = S, [PPN](2)[1a]; Se, [PPN](2)[1b]), respectively, were formed. X-ray crystallographic analysis showed that 1a and 1b were isostructural and each displayed an E(2)Mn(3) square-pyramidal core with one of the two basal E atoms externally coordinated with one Cr(CO)(5) group and one Mn-Mn bond bridged by one hydrogen atom. However, when the TMBA(+) salts for [E(2)Mn(3)(CO)(9)](-) were mixed with Cr(CO)(6) in a molar ratio of 1:1 in 4 M KOH/MeOH solutions and refluxed at 60 °C, mono-Cr(CO)(3)-incorporated E(2)Mn(3)Cr octahedral clusters [TMBA](3)[E(2)Mn(3)Cr(CO)(12)] (E = S, [TMBA](3)[2a]; Se, [TMBA](3)[2b]), respectively, were obtained. Clusters 2a and 2b were isostructural, and each consisted of an octahedral E(2)Mn(3)Cr core, in which each Mn-Mn or Mn-Cr bond of the Mn(3)Cr plane was semibridged by one carbonyl ligand. Clusters 1a and 1b (with [TMBA] salts) underwent metal core closure to form octahedral clusters 2a and 2b upon treatment with KOH/MeOH at 60 °C. In addition, 1a and 1b were found to undergo cluster expansion to form di-Cr(CO)(5)-incorporated HE(2)Mn(3)-clusters [HE(2)Mn(3)Cr(2)(CO)(19)](2-) (E = S, 3a; Se, 3b), respectively, upon the addition of 1 or 2 equiv of Cr(CO)(6) heated in refluxing CH(2)Cl(2). Clusters 3a and 3b were structurally related to clusters 1a and 1b, but with the other bare E atom (E = S, 3a; Se, 3b) further externally coordinated with one Cr(CO)(5) group. The nature, cluster transformation, and electrochemical properties of the mixed manganese-chromium carbonyl sulfides and selenides were systematically discussed in terms of the chalcogen elements, the introduced chromium carbonyl group, and the metal skeleton with the aid of molecular calculations at the BP86 level of the density functional theory.

Cubane-type Co4S4 clusters: synthesis, redox series, and magnetic ground states

The recent demonstration that the carbene cluster [Fe(4)S(4)(Pr(i)(2)NHCMe(2))(4)] (9) is an accurate structural and electronic analogue of the fully reduced cluster of the iron protein of Azotobacter vinelandii nitrogenase, including a common S = 4 ground state, raises the issue of the existence and magnetism of other [M(4)S(4)L(4)](z) clusters, none of which are known with transition metals other than iron. The system CoCl(2)/Pr(i)(3)P/(Me(3)Si)(2)S/THF assembles [Co(4)S(4)(PPr(i)(3))(4)] (3), which is converted to [Co(4)S(4)(Pr(i)(2)NHCMe(2))(4)] (5) upon reaction with carbene. The clusters support the redox series [3](1-/0/1+) and [5](0/1+/2+); monocations (4, 6) have been isolated by chemical oxidation. Redox potentials and substitution reactions indicate that the carbene is the more effective electron donor to tetrahedral Fe(II) and Co(II) sites. Clusters 3-6 have the same overall cubane-type geometry as 9. Neutral clusters 3 and 5 have an S = 3 ground state. As with the S = 4 state of 9 with local spins S(Fe) = 2, the septet spin state can be described in terms of the coupling of three parallel and one antiparallel spins S(Co) = 3/2. The octanuclear clusters [Co(8)S(8)(PPr(i)(3))(6)](0,1+) were isolated as minor byproducts of the formation and chemical oxidation of 3. The clusters exhibit a rhomb-bridged noncubane (RBNC) structure, whereas clusters with the Fe(8)S(8) core possess edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M(8)S(8) core in the form of topological isomers whose stability may depend on valence electron count. A conceptual model for the RBNC <--> EBDC interconversion is presented. (Pr(i)(2)NHCMe(2) = C(11)H(20)N(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).

Precursors to [FeFe]-hydrogenase models: syntheses of Fe2(SR)2(CO)6 from CO-free iron sources

This report describes routes to iron dithiolato carbonyls that do not require preformed iron carbonyls. The reaction of FeCl 2, Zn, and Q 2S 2C n H 2 n (Q (+) = Na (+), Et 3NH (+)) under an atmosphere of CO affords Fe 2(S 2C n H 2 n )(CO) 6 ( n = 2, 3) in yields >70%. The method was employed to prepare Fe 2(S 2C 2H 4)( (13)CO) 6. Treatment of these carbonylated mixtures with tertiary phosphines, instead of Zn, gave the ferrous species Fe 3(S 2C 3H 6) 3(CO) 4(PR 3) 2, for R = Et, Bu, and Ph. Like the related complex Fe 3(SPh) 6(CO) 6, these compounds consist of a linear arrangement of three conjoined face-shared octahedral centers. Omitting the phosphine but with an excess of dithiolate, we obtained the related mixed-valence triiron species [Fe 3(S 2C n H 2 n ) 4(CO) 4] (-). The highly reducing all-ferrous species [Fe 3(S 2C n H 2 n ) 4(CO) 4] (2-) is implicated as an intermediate in this transformation. Reactive forms of iron, prepared by the method of Rieke, also combined with dithiols under a CO atmosphere to give Fe 2(S 2C n H 2 n )(CO) 6 in modest yields under mild conditions. Studies on the order of addition indicate that ferrous thiolates are formed prior to the onset of carbonylation. Crystallographic characterization demonstrated that the complexes Fe 3(S 2C 3H 6) 3(CO) 4(PEt 3) 2 and PBnPh 3[Fe 3(S 2C 3H 6) 4(CO) 4] feature high-spin ferrous and low-spin ferric as the central metal, respectively.

The correlation of redox potential, HOMO energy, and oxidation state in metal sulfide clusters and its application to determine the redox level of the FeMo-co active-site cluster of nitrogenase

This paper describes a procedure that permits the total charge state (i.e., oxidation state) of a complex molecule to be obtained from its redox potential data by comparison with good data (both charge state and redox potential) for reference compounds that are chemically similar. The link between the reference data and the unknown compound is made by the calculated energies of the Fermi level or highest occupied molecular orbital (HOMO). The HOMO energies are calculated by unrestricted density functional methods (DMol) for the reference compounds in their known charge states, and a graphical correlation of HOMO energy and redox potential for oxidation (corresponding to loss of an electron from the HOMO) is constructed. The measured redox potential of the unknown is then applied to the correlation to yield the HOMO energy of the unknown, against which the calculated HOMO energies for various charge states of the unknown are assessed. This method is generally applicable. Using 26 reference data, the method is used here to determine the resting redox state, [NFe6MoS9]0, of the core of the FeMo cofactor (FeMo-co, bound to the MoFe protein) which is the active site of nitrogen-fixing enzymes. The analysis also shows that if the atom at the center of FeMo-co is C rather than N, then FeMo-co must be protonated in its resting state, but if FeMo-co is N-centered, it would not be protonated in the resting state.

Synthesis, Crystal Structure, and Characterization of New Tetranuclear Ag(I) Complexes with Triazole Bridges

The self-assembly of Ag(I) ions with 3,5-dimethyl-4-amino-1,2,4-triazole (L1) and 4-salicylideneamino-1,2,4-triazole (L2) gave two novel complexes, [Ag4(mu2-L1)6][Ag4(mu2-L1)6(CH3CN)2](ClO4)8.2H2O (1) and [Ag4(mu2-L2)6(CH3CN)2](AsF6)4.2H2O (2), both of which contain tetranuclearic clusters constructed via Ag(I) ions and six N1,N2-bridged triazoles with a Ag4N12 core. When 4-(6-amino-2-pyridyl)-1,2,4-triazole (L3) was employed, {[Ag4(mu2-L3)4(mu3-L3)2](CF3SO3)4.H2O}n (3), {[Ag4(mu2-L3)4(mu3-L3)2](ClO4)4}n (4), and {[Ag4(mu2-L3)2(mu3-L3)4](PF6)4.CH3CN.0.75H2O}n (5) were isolated. 3 and 4 are 1D polymers, while 5 is a 2D polymer. 1D and 2D coordination polymers are constructed via the self-assembly of Ag4N12 cores as secondary building units (SBUs). The connection of these SBUs can be represented as a ladderlike structure for 1D polymers and a 4.8(2) net for 2D polymers. Electrospray ionization mass spectrometry measurements and NMR (1H and 13C) studies demonstrate that the tetranuclear SBU retains its integrity and the coordination polymers decompose into the tetranuclear Ag4N12 core in solution. 2 exhibits blue emission in the solid state and green emission in solution at ambient temperature. Strong blue fluorescence for complexes 3-5 in the solid state can be assigned to the intraligand fluorescent emission.

Disulfido metal carbonyl complexes containing manganese

An account of recent studies of the chemistry of new disulfido metal carbonyl complexes containing manganese is presented. The coordination of the disulfido ligands, the nature of reactions at the manganese atom(s) and the nature of insertion reactions at the disulfido ligands are discussed.

Sequential oxidation of the cubane [4Fe--4S] cluster from [4Fe--4S](-) to [4Fe--4S](3+) in Fe(4)S(4)L(n)(-) complexes

Gaseous Fe(4)S(n)(-) (n = 4-6) clusters and synthetic analogue complexes, Fe(4)S(4)L(n)(-) (L = Cl, Br, I; n = 1-4), were produced by laser vaporization of a solid Fe/S target and electrospray from solution samples, respectively, and their electronic structures were probed by photoelectron spectroscopy. Low binding energy features derived from minority-spin Fe 3d electrons were clearly distinguished from S-derived bands. We showed that the electronic structure of the simplest Fe(4)S(4)(-) cubane cluster can be described by the two-layer spin-coupling model previously developed for the [4Fe] cubane analogues. The photoelectron data revealed that each extra S atom in Fe(4)S(5)(-) and Fe(4)S(6)(-) removes two minority-spin Fe 3d electrons from the [4Fe--4S] cubane core and each halogen ligand removes one Fe 3d electron from the cubane core in the Fe(4)S(4)L(n)(-) complexes, clearly revealing a behavior of sequential oxidation of the cubane over five formal oxidation states: [4Fe--4S](-) --> [4Fe--4S](0) --> [4Fe--4S](+) --> [4Fe-4S](2+) --> [4Fe-4S](3+). The current work shows the electron-storage capability of the [4Fe--4S] cubane, contributes to the understanding of its electronic structure, and further demonstrates the robustness of the cubane as a structural unit and electron-transfer center.