The role of the maturase HydG in [FeFe]-hydrogenase active site synthesis and assembly

[FeFe]-hydrogenases catalyze the protons/hydrogen interconversion through a unique di-iron active site consisting of three CO and two CN ligands, and a non-protein SCH₂XCH₂S (X = N or O) dithiolate bridge. Site assembly requires two “Radical-S-adenosylmethionine (SAM or AdoMet)” iron-sulfur enzymes, HydE and HydG, and one GTPase, HydF. The sequence homology between HydG and ThiH, a Radical-SAM enzyme which cleaves tyrosine into p-cresol and dehydroglycine, and the finding of a similar cleavage reaction catalyzed by HydG suggests a mechanism for hydrogenase maturation. Here we propose that HydG is specifically involved in the synthesis of the dithiolate ligand, with two tyrosine-derived dehydroglycines as precursors along with an [FeS] cluster of HydG functioning both as electron shuttle and source of the sulfur atoms.     

A theoretical study of spin states in Ni-S<Subscript>4</Subscript> complexes and models of the [NiFe] hydrogenase active site

We have applied density functional theory, using both pure (BP86) and hybrid (B3LYP and B3LYP*) functionals, to investigate structural parameters and reaction energies for nickel(II)-sulfur coordination compounds, as well as for small cluster models of the Ni-SI and Ni-R redox state of [NiFe] hydrogenases. Results obtained investigating experimentally well-characterized complexes show that BP86 is well suited to describe the structural features of this class of compounds. However, the singlet-triplet energy splitting and even the computed ground state are strongly dependent on the applied functional. Results for the cluster models of [NiFe] hydrogenases lead to the conclusion that in the reduced protein structures characterized by X-ray diffraction a hydride bridges the two metal centres. The energy splitting of the singlet and triplet states in Ni-R and Ni-SI models is calculated to be very small and may be overcome at room temperature to allow a spin crossover. Moreover, the relative stability of the Ni-SI and Ni-R structures adopted in the present investigation is fully compatible with their involvement in the reversible heterolytic cleavage of H(2).     

Synthesis and characterization of hydroxy-functionalized models for the active site in Fe-only-hydrogenases

The reactions of DL-1,4-disulfanylbutane-2,3-diol and 1,3-disulfanylpropan-2-ol with dodecacarbonyltriiron have been investigated. As main products, the iron complexes 1 and 2 were isolated and characterized by spectroscopic methods, as well as single crystal X-ray analysis. Additionally, the unusually large bond angles in the dithiolato bridge was investigated via density-functional theory (DFT) calculations. Moreover, the electrochemical features have been studied by cyclic voltammetry.     

A new cumulene diiron complex related to the active site of Fe-only hydrogenases and its phosphine substituted derivatives: synthesis, electrochemistry and structural characterization

A new cumulene diiron complex related to the Fe-only hydrogenase active site [(μ-SCH₂C(S)CCH₂)Fe₂(CO)₆] (1) was obtained by treatment of (μ-LiS)₂Fe₂(CO)₆ with excess 1,4-dichloro-2-butyne. By controllable CO displacement of 1 with PPh₃ and bis(diphenylphosphino)methane (dppm), mono- and di-substituted complexes, namely [(μ-SCH₂C(S)CCH₂)Fe₂(CO)₅L] (2: L = PPh₃; 3: L = dppm) and [(μ-SCH₂C(S)CCH₂)Fe₂(CO)₄L₂] (4: L = PPh₃; 5: L = dppm) could be prepared in moderate yields. Treatment of 1 with bis(diphenylphosphino)ethane (dppe) afforded a double butterfly complex [(μ-SCH₂C(S)CCH₂)Fe₂(CO)₅]₂(μ-dppe) (7). With dppm in refluxing toluene, a dppm-bridged complex [(μ-SCH₂C(S)CCH₂)Fe₂(CO)₄(μ-dppm)] (6) was obtained. These model complexes were characterized by IR, ¹H, ³¹P NMR spectra and the molecular structures of 1, 2 and 5-7 were determined by single crystal X-ray analyses. The electrochemistry of 1-3 was studied and the electrocatalytic property of 1 was investigated for proton reduction in the presence of HOAc.     

Synthesis, characterization and electrocatalysis of diiron propanediselenolate derivatives as the active site models of [FeFe]-hydrogenases

As an extension of our study on the H-cluster model compounds, a series of diiron propanediselenolate (PDS)-type models have been successfully synthesized. Reaction of diselenol HSe(CH₂)₃SeH with Fe₃(CO)₁₂ in THF (tetrahydrofuran) at reflux gave the parent model compound [μ-Se(CH₂)₃Se-μ]Fe₂(CO)₆ (1) in 48% yield. Further reaction of 1 with PPh₃ or PPh₂H in the presence of Me₃NO in MeCN at room temperature afforded the phosphine-monosubstituted model compounds [μ-Se(CH₂)₃Se-μ]Fe₂(CO)₅(L) (2, L = PPh₃; 3, L = PPh₂H) in 76% and 68% yields, respectively. Similarly, the N-heterocyclic carbene I_Mes-monosubstituted model compound [μ-Se(CH₂)₃Se-μ]Fe₂(CO)₅(I_Mes) (4) could be prepared in 46% yield by reaction of imidazolium salt I_Mes · HCl with n-BuLi followed by treatment of the resulting I_Mes ligand with 1 in THF at room temperature. Compounds 1-4 were fully characterized by elemental analysis and various spectroscopic methods. While the structures of 1-4 were further confirmed by X-ray crystallography, the comparative study of 1 and its analog [μ-S(CH₂)₃S-μ]Fe₂(CO)₆ demonstrates that 1 is a better catalyst for TsOH proton reduction to hydrogen under electrochemical conditions.     

The active site of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans. II. Redox properties, light sensitivity and CO-ligand exchange as observed by infrared spectroscopy

In [FeFe]-hydrogenases, the H cluster (hydrogen-activating cluster) contains a di-iron centre ([2Fe]_H subcluster, a (L)(CO)(CN)Fe(μ-RS₂)(μ-CO)Fe(CysS)(CO)(CN) group) covalently attached to a cubane iron-sulphur cluster ([4Fe-4S]_H subcluster). The Cys-thiol functions as the link between one iron (called Fe1) of the [2Fe]_H subcluster and one iron of the cubane subcluster. The other iron in the [2Fe]_H subcluster is called Fe2. The light sensitivity of the Desulfovibrio desulfuricans enzyme in a variety of states has been studied with infrared (IR) spectroscopy. The aerobic inactive enzyme (H_inact state) and the CO-inhibited active form (H_ox–CO state) were stable in light. Illumination of the H_ox state led to a kind of cannibalization; in some enzyme molecules the H cluster was destroyed and the released CO was captured by the H clusters in other molecules to form the light-stable H_ox–CO state. Illumination of active enzyme under ¹³CO resulted in the complete exchange of the two intrinsic COs bound to Fe2. At cryogenic temperatures, light induced the photodissociation of the extrinsic CO and the bridging CO of the enzyme in the H_ox–CO state. Electrochemical redox titrations showed that the enzyme in the H_inact state converts to the transition state (H_trans) in a reversible one-electron redox step (E _{m, pH 7}=–75 mV). IR spectra demonstrate that the added redox equivalent not only affects the [4Fe-4S]_H subcluster, but also the di-iron centre. Enzyme in the H_trans state reacts with extrinsic CO, which binds to Fe2. The H_trans state converts irreversibly into the H_ox state in a redox-dependent reaction most likely involving two electrons (E _{m, pH 7}=–261 mV). These electrons do not end up on any of the six Fe atoms of the H cluster; the possible destiny of the two redox equivalents is discussed. An additional reversible one-electron redox reaction leads to the H_red state (E _{m, pH 7}=–354 mV), where both Fe atoms of the [2Fe]_H subcluster have the same formal oxidation state. The possible oxidation states of Fe1 and Fe2 in the various enzyme states are discussed. Low redox potentials (below –500 mV) lead to destruction of the [2Fe]_H subcluster.     

The active site of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans. I. Light sensitivity and magnetic hyperfine interactions as observed by electron paramagnetic resonance

The hydrogen-activating cluster (H cluster) in [FeFe]-hydrogenases consists of two moieties. The [2Fe]_H subcluster is a (L)(CO)(CN)Fe(μ-RS₂)(μ-CO)Fe(CysS)(CO)(CN) centre. The Cys-bound Fe is called Fe1, the other iron Fe2. The Cys-thiol forms a bridge to a [4Fe–4S] cluster, the [4Fe–4S]_H subcluster. We report that electron paramagnetic resonance (EPR) spectra of the ⁵⁷Fe-enriched enzyme from Desulfovibrio desulfuricans in the H_ox–CO state are consistent with a magnetic hyperfine interaction of the unpaired spin with all six Fe atoms of the H cluster. In contrast to the inactive aerobic enzyme, the active enzyme is easily destroyed by light. The [2Fe]_H subcluster in some enzyme molecules loses CO by photolysis, whereupon other molecules firmly bind the released CO to form the H_ox–CO state giving rise to the so-called axial 2.06 EPR signal. Though not destroyed by light, the H_ox–CO state is affected by it. As demonstrated in the accompanying paper [49] two of the intrinsic COs, both bound to Fe2, can be exchanged by extrinsic ¹³CO during illumination at 2 °C. We found that only one of the three ¹³COs, the one at the extrinsic position, gives an EPR-detectable isotropic superhyperfine interaction of 0.6 mT. At 30 K both the inhibiting extrinsic CO bound to Fe2 and one more CO can be photolysed. EPR spectra of the photolysed products are consistent with a 3d ⁷ system of Fe with the formal oxidation state +1. The damaged enzyme shows a light-sensitive g=5 signal which is ascribed to an S=3/2 form of the [2Fe]_H subcluster. The light sensitivity of the enzyme explains the occurrence of the g=5 signal and the axial 2.06 signal in published EPR spectra of nearly all preparations studied thus far.     

Desulfovibrio vulgaris Hildenborough HydE and HydG interact with the HydA subunit of the [FeFe] hydrogenase

HydE, HydF, and HydG participate in the synthesis of the complex di-iron center of [FeFe] hydrogenases. The hydE, hydF, hydG, hydA, and hydB genes of Desulfovibrio vulgaris Hildenborough were cloned and His-tag pull-down assays were used to study the potential interaction between HydE, HydF, and HydG with the HydA and HydB protein subunits of the D. vulgaris [FeFe] hydrogenase. Interaction of HydE and HydG with HydA was demonstrated. HydF did not interact with HydA, and none of the accessory proteins appeared to interact with HydB. This suggests that specific protein-protein interactions may be required during [FeFe] cluster synthesis and/or insertion.     

Characterization of the active site of catalytically inactive forms of [NiFe] hydrogenases by density functional theory

The inactive forms, unready (Ni-A, Ni-SU) and ready (Ni-B), of NiFe hydrogenases are modeled by examining the possibility of hydroxo, oxo, hydroperoxo, peroxo, and sulfenate groups in active-site models and comparing predicted IR frequencies and g tensors with those of the enzyme. The best models for Ni-A and Ni-SU have hydroxo (μ-OH) bridges between Fe and Ni and a terminal sulfenate [Ni–S(=O)Cys] group, although a hydroperoxo model for Ni-A is also quite viable, whereas the best model for Ni-B has only a μ-OH bridge. In addition, a mechanism for the activation of unready hydrogenase is proposed on the basis of the relative stabilities of sulfenate models versus peroxide models.     

Hydrogen and oxygen trapping at the H-cluster of [FeFe]-hydrogenase revealed by site-selective spectroscopy and QM/MM calculations

[FeFe]-hydrogenases are superior hydrogen conversion catalysts. They bind a cofactor (H-cluster) comprising a four-iron and a diiron unit with three carbon monoxide (CO) and two cyanide (CN^?) ligands. Hydrogen (H₂) and oxygen (O₂) binding at the H-cluster was studied in the C169A variant of [FeFe]-hydrogenase HYDA1, in comparison to the active oxidized (Hox) and CO-inhibited (Hox-CO) species in wildtype enzyme. ⁵⁷Fe labeling of the diiron site was achieved by in vitro maturation with a synthetic cofactor analogue. Site-selective X-ray absorption, emission, and nuclear inelastic/forward scattering methods and infrared spectroscopy were combined with quantum chemical calculations to determine the molecular and electronic structure and vibrational dynamics of detected cofactor species. Hox reveals an apical vacancy at Fe_d in a [4Fe4S-2Fe]^3 ? complex with the net spin on Fe_d whereas Hox-CO shows an apical CN^? at Fe_d in a [4Fe4S-2Fe(CO)]^3 ? complex with net spin sharing among Fe_p and Fe_d (proximal or distal iron ions in [2Fe]). At ambient O₂ pressure, a novel H-cluster species (Hox-O₂) accumulated in C169A, assigned to a [4Fe4S-2Fe(O₂)]^3 ? complex with an apical superoxide (O₂^?) carrying the net spin bound at Fe_d. H₂ exposure populated the two-electron reduced Hhyd species in C169A, assigned as a [(H)4Fe4S-2Fe(H)]^3 ? complex with the net spin on the reduced cubane, an apical hydride at Fe_d, and a proton at a cysteine ligand. Hox-O₂ and Hhyd are stabilized by impaired O₂^– protonation or proton release after H₂ cleavage due to interruption of the proton path towards and out of the active site.     

Determination of the absolute configurations of chiral organometallic complexes via density functional theory calculations of their vibrational circular dichroism spectra: The chiral chromium tricarbonyl complex of N-pivaloyl-tetrahydroquinoline

The racemate of the chiral tricarbonyl-η⁶-arene-chromium(0) complex, tricarbonyl-η⁶-N-pivaloyl-tetrahydroquinoline-chromium(0), 1, has been synthesized and resolved using chromatography on a (R,R)-Whelk-O1 column. The Absolute Configuration (AC) of 1 has been determined using vibrational circular dichroism (VCD) spectroscopy. The VCD spectrum of 1 has been predicted using the Stephens equation for vibrational rotational strengths, implemented using density functional theory (DFT) in the gaussian program. Using the B3PW91 functional and the 6-311++G (2d,2p) basis set, the predicted VCD spectrum of S-1 is in excellent agreement with the experimental VCD spectrum of (+)-1, leading unambiguously to the AC S-(+). It is concluded that VCD is a useful technique for determining the ACs of chiral organometallic complexes, given the use of optimum functionals and basis sets.     

Photochemical and photophysical properties of ruthenium(II) bis-bipyridine bis-nitrile complexes: Photolability

The electrochemical and photophysical properties of two bis-nitrilo ruthenium(II) complexes formulated as [Ru(bpy)₂(L)₂](PF₆)₂, where bpy is 2,2′-bipyridine and L is AN = CH₃CN and sn = NC-CH₂CH₂-CN, have been investigated. Electrochemical data are typical of Ru-bpy complexes with two reversible reduction peaks located near −1.3 and −1.6 V assigned to each bipyridine ligand and one Ru^II/Ru^III oxidation wave centered at approximately +1.5 V. The sn derivative is both IR and Raman active with its coordinated CN stretch appearing at 2277 cm⁻¹ and 2273 cm⁻¹, respectively. The UV/Vis absorption spectrum of the sn derivative is dominated by an intense (ε_max ∼ 58700 M⁻¹ cm⁻¹) absorption band at 287 nm assigned as a LC (π → π∗) transition. The peak observed at 418 nm (ε ∼ 10 400 M⁻¹ cm⁻¹) is an MLCT band while the one at 244 nm (ε ∼ 23 600 M⁻¹ cm⁻¹) is of LMLCT character. The AN derivative behaves similarly. Both complexes show low-temperature emission at around 537 nm with a lifetime near 10.0 μs. ¹H and ¹³C assignments are consistent with the formulation of the complexes. The complexes undergo photosubstitution of solvent with quantum efficiencies near one. Calculated and experimental results support replacement of the nitrile ligands by solvent. Based on DFT calculations, the electron density of the HOMO lies on the metal center, the bipyridine ligands and the nitrile ligands and electron density of the LUMO resides primarily on the bipyridine ligands. The electronic spectra obtained from TDDFT calculations closely match the experimental ones.     

X-ray absorption spectroscopy of cuprous-thiolate clusters in Saccharomyces cerevisiae metallothionein

Copper (Cu) metallothioneins are cuprous-thiolate proteins that contain multimetallic clusters, and are thought to have dual functions of Cu storage and Cu detoxification. We have used a combination of X-ray absorption spectroscopy (XAS) and density-functional theory (DFT) to investigate the nature of Cu binding to Saccharomyces cerevisiae metallothionein. We found that the XAS of metallothionein prepared, containing a full complement of Cu, was quantitatively consistent with the crystal structure, and that reconstitution of the apo-metallothionein with stoichiometric Cu results in the formation of a tetracopper cluster, indicating cooperative binding of the Cu ions by the metallothionein.     

DFT study of a series of crown-4 ethers and their selectivity trend for alkali metal cations: Li+ and Na+

The molecular and electronic structures for 12- to 16-crown-4 (named 12C4, 13C4, 14C4, 15C4, 16C4, respectively) and 2,3,5,6,8,9-hexahydrobenzo[b][1,4,7,10]tetraoxacyclododecine (B12C4) 3,5,6,7,9,10-hexahydro -2H-benzo[e][1,4,7,10]tetraoxacyclotridecine (B13C4) and their complexes with alkali metal cations Li⁺ and Na⁺ have been explored using the density functional theory (DFT) with B3LYP/ 6–31G* method. The nucleophilicity of crown-4 ethers has been investigated by the Fukui function. Their selectivity trend shows that of all the crown-4 ethers, 14C4 shows the highest cation selectivity for Li⁺ over Na⁺, has been achieved on the basis of thermodynamic analysis. In addition, Li⁺/crown-4 series are more stable than Na⁺/crown-4 series in the gas phase. The calculated results are in good agreement with the experimental observation.     

Synthesis and Fungicidal Activities of New 1,2,4‐Triazolo[1,5‐a]pyrimidines

A series of new acetohydrazone‐containing 1,2,4‐triazolo[1,5‐a]pyrimidine derivatives were designed and synthesized for the purpose of searching for novel agrochemicals with higher fungicidal activity. Their in vitro fungicidal activities against Rhizoctonia solani were evaluated, and the most promising compound, 2‐[(5,7‐dimethyl[1,2,4]triazolo[1,5‐a]pyrimidin‐2‐yl)sulfanyl]‐2′‐[(2‐hydroxyphenyl)methylidene]acetohydrazide ( 2‐17 ), showed a lower EC₅₀ value (5.34 μg ml^?1) than that of commercial carbendazim (EC₅₀=7.62 μg ml^?1). Additionally, compound 2‐17 was also found to display broad‐spectrum fungicidal activities, and its EC₅₀ value (4.56 μg ml^?1) against Botrytis cinereapers was very similar to that of carbendazim. Qualitative structure–activity relationships (QSARs) of the synthesized compounds were also discussed.     

Compounds of the 1,5-di(4-R-phenyl)-3-selenopentanediones-1,5 series interaction with the Basidiomycete Lentinula edodes,lectins: Computations and Experiment

Abstract

The role of spatial and electron structure, hydrophobic properties and concentration of organoselenium compounds on their interaction with fungal metabolites - extracellular lectins of Lentinula edodes, (shiitake mushroom) has been considered. By the hybrid method of density functional theory at the B3LYP/6-31G(d,p) theory level, spatial and electronic structure of the 1,5-diphenyl-3-selenopentanedione-1,5 (preparation DAPS-25), 1,5-di(4-methoxyphenyl)-3-selenopentanedione-1,5 and 1,5-di(4-ethoxyphenyl)-3-seleno- pentanedione-1,5 molecules has been studied. The above molecules have been stated to be substantially similar to each other by their electronic and spatial characteristics. By means of the QSAR properties evaluation by the atomic-additive schemes, it has been shown that the molecules of the preparation DAPS-25, its dimethoxy- and diethoxy-substituted are close to each other by the hydrophilic-lipophilic balance, whereas di-n,-octoxy derivative DAPS-25 is explicitly hydrophobic. The hemagglutinating activity of lectins in the presence of the preparation DAPS-25 and its alkyloxy-substituted increases, therewith the most effective addition is 1,5-di(4-ethoxyphenyl)-3-selenopentanedione-1,5. Apparently, the greater effectiveness of the said substance compared to DAPS ?25 is caused by the formation of hydrogen bonds with a participation of unshared electron pairs of oxygen atoms from the ethoxy groups and mobile hydrogen atoms from the OH groups of glycoconjugates on erythrocytes surface. The positive effect of 1,5-di(4-n-octoxyphenyl)-3-selenopentanedione-1,5 is not so prominent, since the enlarged alkyl chain shields the aromatic fragments of organoselenium molecule participating in the binding with lectin.     

Influence of (hydroxymethyl)pyridine and pyridine-carboxylic acids, in trans-position to the isopropylamine and amine ligands, on the cytotoxicity of platinum complexes

trans-Pt(II) Complexes with aliphatic amines and planar amines such as (hydroxymethyl)pyridines, and pyridine-3- and pyridine-4-carboxylic acids were synthesized and screened for their potential cytotoxic activity in different cancer cell lines used at the NCI for in vitro screens, i.e., MCF7, NCIH460, and SF268. The complexes studied were designed to differ in geometrical parameters such as the position of the phenyl-group substituents and the nature of the substituents themselves for gathering information about the structure-activity relationships in the trans-complexes. The variation of the substituents turns to be crucial for their biological activity, as both pyridine-3- and pyridine-4-carboxylic acids in trans-position to both amine and isopropylamine ligands provided complexes which displayed no specificity toward any type of cell tested, while (hydroxymethyl)pyridine in trans-position to isopropylamine ligands led to complexes that were clearly more effective against the cell lines tested.     

Ab initio prediction of optical rotation: comparison of density functional theory and Hartree-Fock methods for three 2,7,8-trioxabicyclo[3.2.1]octanes

We report ab initio calculations of the frequency-dependent electric dipole-magnetic dipole polarizabilities, beta(nu), at the sodium D line frequency and, thence, of the specific rotations, [alpha](D), of 2,7,8-trioxabicyclo[3.2.1]octane, 1, and its 1-methyl derivative, 2, using the Density Functional Theory (DFT) and Hartree-Fock/Self-Consistent Field (HF/SCF) methodologies. Gauge-invariant (including) atomic orbitals (GIAOs) are used to ensure origin-independent [alpha](D) values. Using large basis sets which include diffuse functions DFT [alpha](D) values are in good agreement with experimental values (175.8 degrees and 139.2 degrees for (1S,5R)-1 and -2, respectively); errors are in the range 25-35 degrees. HF/SCF [alpha](D) values, in contrast, are much less accurate; errors are in the range 75-95 degrees. The use of small basis sets which do not include diffuse functions substantially lowers the accuracy of predicted [alpha](D) values, as does the use of the static limit approximation: beta(nu) approximately beta(o). The use of magnetic-field-independent atomic orbitals, FIAOs, instead of GIAOs, leads to origin-dependent, and therefore nonphysical, [alpha](D) values. We also report DFT calculations of [alpha](D) for the 1-phenyl derivative of 1, 3. DFT calculations find two stable conformations, differing in the orientation of the phenyl group, of very similar energy, and separated by low barriers. Values of [alpha](D) predicted using two different algorithms for averaging over phenyl group orientations are in good agreement with experiment. In principle, the absolute configuration (AC) of a chiral molecule can be assigned by comparison of the optical rotation predicted ab initio to the experimental value. Our results demonstrate the critical importance of the choice of ab initio methodology in obtaining reliable optical rotations and, hence, ACs, and show that, at the present time, DFT constitutes the method of choice.     

Synthesis and theoretical analysis of the structures and bonding in five new neutral square planar bis[2,4-di(aryl)-1,3,5-triazapentadienato]nickel(II) complexes

Solvothermal reactions in methanol of nickel acetate tetrahydrate, Ni(OAc)₂ · 4H₂O, with benzonitrile derivatives NC(C₆H₄)X, where X is one of the electron withdrawing substituents -CN, -NO₂, or -CF₃, located at the m- or p-positions relative to -CN, yield complexes of the general formula Ni{HNC(R)-NC(R)-NH}₂. More specifically, 3-nitrobenzonitrile, 4-nitrobenzonitrile, 1,3-dicyanobenzene, 1,4-dicyanobenzene, and ααα-trifluoro-p-toluonitrile are found to react with Ni(OAc)₂ · 4H₂O to yield Ni{HNC(R)-NC(R)-NH}₂, where R = 3-(NO₂)C₆H₄, 4-(NO₂)C₆H₄, 3-(CN)C₆H₄, 4-(CN)C₆H₄, or 4-(CF₃)C₆H₄, respectively. Analogous reactions of nitriles lacking electron withdrawing groups do not occur under similar conditions. Solid-state structures have been determined for the complexes with p-NO₂, p-CN, and p-CF₃ substituents on the phenyl rings. In addition, we describe density functional theory (DFT) and natural bonding orbital theory (NBO) studies on a simplified analog of these compounds, aimed at understanding their molecular bonding. It is shown that the new compounds for which solid-state structures have been determined are model examples of coordination compounds containing robust ω-bonds.     

Mixed chloride-phosphine complexes of the dirhenium core. Part 11. Reactions of [Re2Cl8] with secondary phosphines, PCy2H and PPh2H

The reaction between the dirhenium(III,III) anion, [Re₂Cl₈]²⁻, and the secondary phosphine, PCy₂H, yields a mixture of products as a result of disproportionation, namely, a dirhenium(II,III) chloride-phosphine complex 1,3,6-Re₂Cl₅(PCy₂H)₃ (1) and a dirhenium(IV) face-sharing bioctahedral compound with bridging phosphido groups, [Buⁿ ₄N][Re₂(μ-PCy₂)₃Cl₆] (2). The diphenylphosphine analogue of 2, [Buⁿ ₄N][Re₂(μ-PPh₂)₃Cl₆] (3) has been similarly prepared from the reaction of [Re₂Cl₈]²⁻ with PPh₂H. An interesting dirhenium(III,III) complex, [Buⁿ ₄N]₂[Re₂(μ-PPh₂)₂(PPh₂H)₂Cl₆] (4) having both neutral terminal phosphines and anionic phosphido bridges, has also been isolated as an intermediate in the latter system. Crystal structures of 1-4 have been determined by X-ray crystallography. The compounds were also characterized by cyclic voltammetry, IR and ³¹P NMR spectroscopy.