On the reduced and oxidized forms of the FeMo- cofactor of Azotobacter vinelandii nitrogenase

Reduced and oxidized forms of the FeMo- cofactor of Azotobacter vinelandii nitrogenase are examined theoretically within the intermediate neglect of differential overlap model. The results obtained favor one of the experimentally suggested modes of contraction of the metal system which results in an expansion of the central cavity of the cofactor. The bond index analysis indicates marked changes in the Mo coordination upon electron addition which may contribute to an opening of the Mo atom as a possible binding site at the advanced stages of the reduction process. In this work we also compare the 39- and 41-electron [MoFe₇] core as possible native resting states, both compatible with known spin and M?ssbauer spectroscopies. Received: 19 March 1997 / Accepted: 8 May 1997     

Effect of the potential of an external electron donor on C<Subscript>2</Subscript>H<Subscript>2</Subscript> reduction catalyzed by the nitrogenase active center (FeMoco) isolated from the enzyme

The effect of potential value and chemical properties of an external electron donor on C₂H₂ reduction catalyzed by nitrogenase active center (cluster [(₆-N)Fe₇MoS₉·homocitrate] FeMoco isolated from the enzyme) has been investigated in the presence of proton donors of different acidity. The temperature—reaction rate dependences of these reactions have been studied. It has been shown that the rate-limiting steps of the reactions differ depending on the proton donor used. When thiophenol or water are used as proton donors, and electrochemical step — the electron transfer from cathode to adsorbed catalytic cluster — has been found to be a rate-limiting one. The effective activation energy of ethane formation as a product of four-electron C₂H₂ reduction is found to be 1.5 times lower than that of ethylene, namely, 13 kcal mol^–1. When stronger acid, pentafluorothiophenol, is used as a proton donor, the chemical step of intramolecular rearrangement of the catalyst—substrate complex taking place in solution becomes a rate-limiting one. The effective activation energies of both ethylene and ethane become equal to 32 kcal mol^–1.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1583–1591, August, 2004.     

Catalytic reduction of acetylene and dinitrogen with the participation of the iron-molybdenum cofactor of nitrogenase and synthetic polynuclear molybdenum(iii) complexes

Catalytic reduction of acetylene and dinitrogen was carried out by sodium, zinc, and europium amalgams in the presence of polymolybdenum clusters and the iron-molybdenum cofactor of nitrogenase isolated from the enzyme. The activity of both catalysts toward acetylene changes in the sequence Zn(Hg)<Eu(Hg)<Na(Hg), increasing as the redox potential of the reducing agent is shifted to the negative region. The catalytic reduction of N₂ occurs only by the action of sodium and europium amalgams and only in the presence of synthetic polymolybdenum complexes; in the case of Na(Hg), the main product is hydrazine; in the case of Eu(Hg), it is ammonia. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 890–896, May, 1998.     

A new approach to the synthesis of carbon chains capped by metal clusters

Reactions of Co₃(μ₃-CBr)(μ-dppm)(CO)₇ with {Au[P(tol)₃]}₂{μ-(CC)_n} (n=2–4) have given {Co₃(μ-dppm)(CO)₇}{μ₃:μ₃-C(CC)_nC} [n=2 (1), 3 (2), 4 (3)] containing carbon chains capped by the cobalt clusters. Tetracyanoethene reacts with 2 to give {Co₃(μ-dppm)(CO)₇}₂{μ₃:μ₃-C(CC)₂C[=C(CN)₂]C[=C(CN)₂]C} (4). X-ray structural characterisation of 1, 3 and 4 are reported, that for 3 being the first of a cluster-capped C₁₀ chain.     

Unique peculiarities of the structure of the Fe-Mo cofactor of nitrogenase favorable for multicenter coordination of a nitrogen molecule

The probable coordination mode of a nitrogen molecule on the Fe-Mo cofactor of nitrogenase has been theoretically considered, taking into account both the well-known data on the structure of the Fe-Mo cofactor and the substrate selectivity of nitrogenase and the results of semiempirical calculations of the electronic structures of the cofactor and its complexes with molecular nitrogen. The distances between the Fe atoms in the cofactor are favorable for different multicenter coordination modes of a nitrogen molecule: above the Fe₄ face along its diagonal, through this face, and inside the Fe₆ prism perpendicularly to its axes. It is important that the nitrogen atoms are open for protonation in all coordination modes. The first mode is disadvantageous due to steric hindrances. Of the other variants, the latter is the most favorable both energetically and from the viewpoint of weakening of the N-N bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1928–1933, August, 1996.     

Improving the use of mercury bis(phenyltellurolate) as efficient start for the synthesis of ternary clusters and polymers

Hg(TePh)₂ reacts with AgCl/PMe₂Ph and AgClO₄/PPh₃ to give the ternary clusters [(PhTe)₄Ag₂Hg₂(Cl)₂]_n (1) and [(PhTe)₁₆Ag₄Hg₆Py₄] · H₂O (2). While 1 assembles polymeric, bidimensional layers, 2 attains high symmetric moieties in which the occupational relationship is 40%/60% for the Ag⁺ and Hg²⁺ ions, respectively.     

Electron-transfer chemistry of the iron-molybdenum cofactor of nitrogenase: delocalized and localized reduced states of FeMoco which allow binding of carbon monoxide to iron and molybdenum

The electron-transfer chemistry of the isolated iron-molybdenum cofactor of nitrogenase (FeMoco) has been studied by electrochemical and spectroelectrochemical methods. Two interconverting forms of the cofactor arise from a redox-linked ligand isomerism at the terminal iron atom; this is attributed to rotamerism of an anionic N-methyl formamide ligand bound at this site. FeMoco in its EPR-silent oxidised state is shown to undergo three successive one-electron transfer steps. We argue that the first and second redox processes are associated with electron-transfer delocalised over the iron-sulfur core of the cofactor, whilst the third irreversible process is localised on molybdenum. This is strongly reinforced by spectroelectrochemical studies under (12)CO and (13)CO which reveal two independent carbon monoxide binding sites that are specifically associated with the second (iron core) and third (molybdenum) electron-transfer processes and which give rise to terminal nu((12)CO) bands at 1885 and 1920 cm(-1) respectively. Moreover, in parallel with earlier studies on the enzyme system, it is shown that at low CO concentration, carbon monoxide binds to the cofactor in bridging modes, with nu(CO) bands at 1835 and 1808 cm(-1) that are interconverted by single-electron transfer. Importantly we show that the contentious overall 2e difference in the assignment of the metal oxidation levels in the resting state of the enzyme-bound cofactor, arising from analysis of (57)Fe ENDOR and M?ssbauer data, can be resolved in the light of the electron-transfer chemistry of the isolated cofactor described herein.     

A divergent approach to the precise synthesis of giant organometallic dendrimers using platinum-acetylides as building blocks

Giant platinum-acetylides dendrimers were precisely synthesized by a divergent method; the sixth generation dendrimer, the diameter of which is larger than 10 nm, has 189 Pt atoms per molecule, and its molecular weight is as high as 139750.     

Distinguishing metal halide molecular clusters from perovskite magic sized clusters in the synthesis of metal halide perovskite nanostructures

Background

Research into perovskite nanocrystals (PNCs) has uncovered interesting properties compared to their bulk counterparts, including tunable optical properties due to size-dependent quantum confinement effect (QCE). More recently, smaller PNCs with even stronger QCE have been discovered, such as perovskite magic sized clusters (PMSCs) and ligand passivated PbX₂ metal halide molecular clusters (MHMCs) analogous to perovskites.

Objective

This review aims to present recent data comparing and contrasting the optical and structural properties of PQDs, PMSCs, and MHMCs, where CsPbBr₃ PQDs have first excitonic absorption around 520 nm, the corresponding PMSCS have absorption around 420 nm, and ligand passivated MHMCs absorb around 400 nm.

Results

Compared to normal perovskite quantum dots (PQDs), these clusters exhibit both a much bluer optical absorption and emission and larger surface-to-volume (S/V) ratio. Due to their larger S/V ratio, the clusters tend to have more surface defects that require more effective passivation for stability.

Conclusion

Recent study of novel clusters has led to better understanding of their properties. The sharper optical bands of clusters indicate relatively narrow or single size distribution, which, in conjunction with their blue absorption and emission, makes them potentially attractive for applications in fields such as blue single photon emission.     

MVPACK: A package to calculate energy levels and magnetic properties of high nuclearity mixed valence clusters

We present a FORTRAN code based on a new powerful and efficient computational approach to solve the double exchange problem for high‐nuclearity MV clusters containing arbitrary number of localized spins and itinerant electrons. We also report some examples in order to show the possibilities of the program. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

On the development of an effective model potential to describe water interaction in neutral and ionic clusters

Various effective components of the intermolecular interaction of water containing aggregates are examined and their modeling, in terms of the fundamental physical properties of the involved partners, is discussed. We focus, in particular, on the evolution of these components in going from the simplest neutral rare gas–water aggregates to bulk water and ionic water solutions. The analysis singled out that the model chosen to represent the van der Waals interaction as the composition of the action of three dispersion/induction–attraction centres and found to be appropriate to describe the lighter He–H₂O and Ne–H₂O systems, is not adequate to describe the heavier Ar–H₂O aggregate. It was found, instead, that by increasing the mass of the rare gas, other short range contributions to the interaction come into play. Moreover, it was also found that the water molecule tends to behave as a single centre as the strength of the interaction increases. This led to the development of an effective model potential suitable to describe water clusters in the range going from gaseous to condensed phase. The role of electrostatic contributions is also evaluated. The proposed potential model is tested by comparing molecular beam scattering and neutron diffraction experiments with results of molecular dynamics (MD) calculations.     

An approach to the total synthesis of welwistatin

An approach to the total synthesis of the antimicrotubule agent welwistatin is described. Key transformations include (1) a 7-endo intramolecular conjugate addition reaction of enone 6 to deliver the strained bicyclo[4.3.1]decanone 5; (2) a 6pi electrocyclic ring closure of trienecarbamate 16 followed by oxidation to afford protected aniline 17; and (3) an intramolecular cyclization of acetic acid derivative 18 to give rise to indole 19. [reaction: see text]     

In situ synthesis of trisubstituted methanol ligands and their potential as one-pot generators of cubane-like metal complexes

Two different one pot routes to a variety of metal cubane compounds are reported; one route is based on an in situ benzilic acid type rearrangement and the other involves in situ nucleophilic attack at a ketone. Diketosuccinic acid in basic solution in the presence of certain divalent metal ions undergoes a benzilic acid type rearrangement to generate the carbon oxyanion, C(CO(2) (-))(3)O(-), which serves as a cubane-forming bridging ligand in a series of octanuclear complexes of composition [M(8){C(CO(2))(3)O}(4)](H(2)O)(12) (M=Mg, Mn, Fe, Co, Ni, Zn). At the heart of each of these highly symmetrical aggregates is an M(4)O(4) cubane core, each oxygen component of which is provided by the alkoxo centre of a C(CO(2) (-))(3)O(-) ligand. Reaction of 2,2'-pyridil, (2-C(5)H(4)N)COCO(2-C(5)H(4)N), and calcium nitrate in basic alcoholic solution, which proceeds by a similar benzilic acid type rearrangement, gives the cubane compounds, [Ca(4)L(4)(NO(3))(4)] in which L=(2-C(5)H(4)N)(2)C(COOR)O(-) (R=Me or Et). Nucleophilic attack by bisulfite ion at the carbonyl carbon atom of 2,2'-dipyridyl ketone in the presence of certain divalent metals generates the electrically neutral complexes, [{(C(5)H(4)N)(2)SO(3)C(OH)}(2)M] (M=Mn, Fe, Co, Ni, Zn and Cd). Cubane-like complexes [M(4){(C(5)H(4)N)(2)SO(3)C(O)}(4)] (M=Zn, Mn) can be obtained directly from 2,2'-dipyridyl ketone in one-pot reaction systems (sealed tube, 120 degrees C) if a base as weak as acetate ion is present to deprotonate the OH group of the initial [(C(5)H(4)N)(2)SO(3)C(OH)](-) bisulfite addition compound; the [(C(5)H(4)N)(2)SO(3)C(O)](2-) ligand in this case plays the same cubane-forming role as the ligands C(COO(-))(3)O(-) and (2-C(5)H(4)N)(2)C(COOR)O(-) above. When excess sodium sulfite is used in similar one-pot reaction mixtures, the monoanionic complexes, [M(3)Na{(C(5)H(4)N)(2)SO(3)C(O)}(4)](-) (M=Zn, Mn, Co) with an M(3)NaO(4) cubane core, are formed directly from 2,2'-dipyridyl ketone.     

New convenient synthesis of iridol. An approach to the synthesis of ubiquinones

A strategy for the synthesis of ubiquinones, in which iridol is the key intermediate, has been developed, together with a new convenient synthesis of iridol (2,3-dimethoxy-5-methylphenol) starting from the easily available 4-methylphenol and using mild conditions and friendly and high-yielding reactions.     

An efficient approach for the total synthesis of balticolid

An efficient stereoselective total synthesis of balticolid has been accomplished starting from known aldehyde. The key steps involved in this synthesis are Sharpless asymmetric epoxidation, Wittig olefination, alkylation of 1,3-dithiane and Yamaguchi macrolactonization.     

An iminophosphorane-based approach for the synthesis of spiropyrrolidine-imidazole derivatives

A method based on the reaction of an E-phosphazide, an intermediate in the Staüdinger reaction between triphenylphosphine and an azide, with heterocumulenes allows the one-pot, two-component synthesis of a number of pyrrole-imidazole derivatives. The procedure, which involves sequential treatment of the appropriate α-azido ester with triphenylphosphine and isocyanate leads to the hydantoin product after aqueous work-up. The cyclization conditions can also be adapted for the synthesis of thiohydantoins by using isothiocyanates. These hybrids pyrrole-thiohydantoins undergo a novel oxidative spirocyclization by action of DDQ to give a tricyclic derivative (pyrrole-pyrrolidine-imidazole), which displays an interesting cytotoxic activity.     

An approach to the synthesis of furanoheliangolides through Diels-Alder reactions

The core structure of the natural sesquiterpene lactones furanoheliangolides, an 11-oxabicyclo[6.2.1]undecane system, was synthesized through a pathway involving two Diels-Alder reactions.