DFT Calculations (dft + calculation)

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Distribution within Chemistry

Selected Abstracts

The Role of Axial Ligation in Nitrate Reductase: A Model Study by DFT Calculations on the Mechanism of Nitrate Reduction

Kuntal Pal
Abstract The reactivity differences of the model anionic complexes [Mo(mnt)2(X)(PPh3)], [mnt2, = 1,2-dicyanoethylenedithiolate; X = SPh (1a), SEt (1b), Cl (1c), Br (1b)] towards oxygen atom transfer from nitrate, which is a key step performed by nitrate reductase, has been investigated by density functional theory calculations. Unlike complexes 1a and 1b, complexes 1c and 1d do not react with nitrate. Thermodynamically, all these complexes have a similar ability to generate the pentacoordinate active state [Mo(mnt)2(X)], by dissociation of PPh3, although the inaccessibility of the dxy orbital in 1c,d and the instability of the corresponding nitrate-bound enzyme substrate (ES) type complex contributes to their failure to reduce nitrate. The nature of the ES complex for 1a,b is described. The variation in the experimental data due to the change of axial ligation from SPh to SEt on the catalytic pathway has also been addressed. The gas-phase and solvent-corrected potential energy surface for the reaction of 1a,b with nitrate are established with fully optimized minima and transition states.( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

A New Spherical Metallacryptate Compound [Na{Cu6(Thr)8(H2O)2(ClO4)4}]ClO45,H2O: Magnetic Properties and DFT Calculations

Sheng-Chang Xiang
Abstract The hexanuclear copper(II) complex with threoninato acid has been synthesized. Its structure can be described as an octahedron cage with D2h symmetry in which six copper ions are bound by eight threoninato acids with a [3.11223130] coordination mode and one Na+ cation being captured within the center of the cage. In contrast with other hexanuclear copper compounds containing amino acids, the title compound has a prolate Cu6 octahedron with the longest axial distance and a rectangle equatorial plane, as well as special coordinated perchlorate ions. Compared with classic cryptate, hexanuclear copper(II) compounds with amino acids can be regarded as a new topologic type of spherical macrotricyclic metallacryptates [2,2,2,2] whose cages have a high selectivity for sodium ions. The analysis of magnetic susceptibility data shows that the threoninato compound has a ground state with spin S = 3. The computing coupling constant between the equatorial Cu centers and the axial ones is 4.4 cm,1,calculated by using DFT methods for a model compound. This is close to three known experimental values of 1.39, 0.56 or 0.43 cm,1 for complexes with 4-hydroxy- L -prolinato, L -prolinato or L -threoninato acid as ligands, respectively. The dominant ferromagnetic interactions for these complexes can be essentially attributed to the orthogonality between the magnetic orbitals, dxz or dyz orbitals for the equatorial CuII centers and d orbitals for the axial ones. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Structures of the Reactive Intermediates in Organocatalysis with Diarylprolinol Ethers


Abstract Structures of the reactive intermediates (enamines and iminium ions) of organocatalysis with diarylprolinol derivatives have been determined. To this end, diarylprolinol methyl and silyl ethers, 1, and aldehydes, PhCH2CHO, tBuCH2CHO, PhCH=CHCHO, are condensed to the corresponding enamines, A and 3 (Scheme,2), and cinnamoylidene iminium salts, B and 4 (Scheme,3). These are isolated and fully characterized by melting/decomposition points, [,]D, elemental analysis, IR and NMR spectroscopy, and high-resolution mass spectrometry (HR-MS). Salts with BF4, PF6, SbF6, and the weakly coordinating Al[OC(CF3)3]4 anion were prepared. X-Ray crystal structures of an enamine and of six iminium salts have been obtained and are described herein (Figs.,2 and 4,8, and Tables,2 and 7) and in a previous preliminary communication (Helv. Chim. Acta2008, 91, 1999). According to the NMR spectra (in CDCl3, (D6)DMSO, (D6)acetone, or CD3OD; Table,1), the major isomers 4 of the iminium salts have (E)-configuration of the exocyclic NC(1,) bond, but there are up to 11% of the (Z)-isomer present in these solutions (Fig.,1). In all crystal structures, the iminium ions have (E)-configuration, and the conformation around the exocyclic N-CC-O bond is synclinal-exo (cf.C and L), with one of the phenyl groups over the pyrrolidine ring, and the RO group over the , -system. One of the meta -substituents (Me in 4b, CF3 in 4c and 4e) on a 3,5-disubstituted phenyl group is also located in the space above the , -system. DFT Calculations at various levels of theory (Tables,3,6) confirm that the experimentally determined structures (cf. Fig.,10) are by far (up to 8.3,kcal/mol) the most stable ones. Implications of the results with respect to the mechanism of organocatalysis by diarylprolinol derivatives are discussed. [source]

Structural Investigation of a High-Affinity MnII Binding Site in the Hammerhead Ribozyme by EPR Spectroscopy and DFT Calculations.

CHEMBIOCHEM, Issue 10 2003
Effects of Neomycin B on Metal-Ion Binding
Abstract Electron paramagnetic resonance spectroscopy and density functional theory methods were used to study the structure of a single, high-affinity MnIIbinding site in the hammerhead ribozyme. This binding site exhibits a dissociation constant Kdof 4.4 ,M in buffer solutions containing 1,M NaCl, as shown by titrations monitored by continuous wave (cw) EPR. A combination of electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) experiments revealed that the paramagnetic manganese(II) ion in this binding site is coupled to a single nitrogen atom with a quadrupole coupling constant,of 0.7 MHz, an asymmetry parameter,of 0.4, and an isotropic hyperfine coupling constant of Aiso(14N)=2.3 MHz. All three EPR parameters are sensitive to the arrangement of the MnIIligand sphere and can therefore be used to determine the structure of the binding site. A possible location for this binding site may be at the G10.1, A9 site found to be occupied by MnIIin crystals (MacKay et al., Nature 1994, 372, 68 and Scott et al., Science 1996, 274, 2065). To determine whether the structure of the binding site is the same in frozen solution, we performed DFT calculations for the EPR parameters, based on the structure of the MnIIsite in the crystal. Computations with the BHPW91 density function in combination with a 9s7p4d basis set for the manganese(II) center and the Iglo-II basis set for all other atoms yielded values of,(14N)=+0.80 MHz, ,=0.324, and Aiso(14N)=+2.7 MHz, in excellent agreement with the experimentally obtained EPR parameters, which suggests that the binding site found in the crystal and in frozen solution are the same. In addition, we demonstrated by EPR that MnIIis released from this site upon binding of the aminoglycoside antibiotic neomycin B (Kd=1.2 ,M) to the hammerhead ribozyme. Neomycin B has previously been shown to inhibit the catalytic activity of this ribozyme (Uhlenbeck et al., Biochemistry 1995, 34, 11,186). [source]

ChemInform Abstract: (Ph4P)2 [Be3(,-OH)3(H2O)6] Cl5: Crystal Structure and DFT Calculations.

CHEMINFORM, Issue 38 2009
Ralph Puchta
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

ChemInform Abstract: Organocatalytic Asymmetric Hydrophosphination of ,,,-Unsaturated Aldehydes: Development, Mechanism and DFT Calculations.

CHEMINFORM, Issue 52 2008
Ismail Ibrahem
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

Structure of Bromotrinitrosyl Iron, [Fe(NO)3Br], and DFT Calculations of the Structures of [Fe(NO)3X] (X: Cl, Br, I).

CHEMINFORM, Issue 16 2006
Wolfgang Beck
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Electronic Spectra and DFT Calculations of Hexanuclear Chalcocyanide Rhenium Clusters.

CHEMINFORM, Issue 11 2005
S. G. Kozlova
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

DFT Calculations on Group 5 Mixed Metal Tetramers: TaxNbyVz (x + y + z = 4).

CHEMINFORM, Issue 8 2005
Magdalene A. Addicoat
No abstract is available for this article. [source]

DFT Calculations on Adamantane-Like Phosphorus Oxide Sulfides of General Formula P4O10-nSn (n = 1,9).

CHEMINFORM, Issue 41 2004
Nguyen Huu Thong
No abstract is available for this article. [source]

Infrared Spectra and DFT Calculations for the Coinage Metal Hydrides MH, (H2)MH, MH2, M2H, M2H - , and (H2)CuHCu in Solid Argon, Neon, and Hydrogen.

CHEMINFORM, Issue 1 2004
Xuefeng Wang
No abstract is available for this article. [source]

Experimentally Based DFT Calculations on the Hindered Disproportionation of [Al4Cp*4]: Formation of Metalloid Clusters as Intermediates on the Way to Solid Al Prevents the Decomposition of a Textbook Molecule,

Michael Huber Dr.
Clusters are the answer! The formation of metalloid clusters such as [Al50Cp*12] (Cp*=C5Me5) species are responsible for the kinetically hindered decomposition of [Al4Cp*4] to metallic aluminum; that is, metalloid clusters may be considered as nuclei for the formation of metals (see graphic). [source]

Stereoselective Allylation of Ketones: Explanation for the Unusual Inversion of the Induced Stereochemistry in the Auxiliary-Mediated Crotylation and Pentenylation of Butanone by DFT Calculations


Abstract The solvent makes the difference: While attack at the Re face of the intermediate oxocarbenium ion becomes less energy demanding with increasing size of R,, it is the solvent effect that makes this attack the main trajectory, leading to inversed stereochemistry (see picture). Auxiliary-mediated domino crotylations and pentenylations of butanone yield homoallylic ethers with two newly formed stereogenic centers. With our norpseudoephedrine-derived auxiliary, we observed the formation of anti isomers exclusively, and the nature of the major isomer was independent of the substrate double bond geometry. Interestingly, there is a switch in induced selectivity when going from crotylation to pentenylation. Here, we present the computational rationalization for this behavior by identification of the relevant transition states (TSs), the energies of which were determined by using the B3LYP/6-31+G(d) level of theory in combination with the PCM/UAKS method to include the effects exerted by the solvent dichloromethane. To quickly narrow down the number of potentially relevant TSs from the whole set of 288 and 864 TSs for the crotylation and pentenylation, respectively, we employed a screening process based on B3LYP//AM1 energies. The predicted selectivities are in good agreement with experimentally determined ones. Furthermore, the obtained results also facilitate an explanation of the selectivities obtained in hexenylations and heptenylations. Finally, activation energies were determined that account for the significantly longer reaction times than those for the domino allylation with unsubstituted trimethylallylsilane. [source]

Cobalt-Mediated Radical Polymerization of Acrylonitrile: Kinetics Investigations and DFT Calculations

Antoine Debuigne Dr.
Abstract The successful controlled homopolymerization of acrylonitrile (AN) by cobalt-mediated radical polymerization (CMRP) is reported for the first time. As a rule, initiation of the polymerization was carried out starting from a conventional azo-initiator (V-70) in the presence of bis(acetylacetonato)cobalt(II) ([Co(acac)2]) but also by using organocobalt(III) adducts. Molar concentration ratios of the reactants, the temperature, and the solvent were tuned, and the effect of these parameters on the course of the polymerization is discussed in detail. The best level of control was observed when the AN polymerization was initiated by an organocobalt(III) adduct at 0,C in dimethyl sulfoxide. Under these conditions, poly(acrylonitrile) with a predictable molar mass and molar mass distribution as low as 1.1 was prepared. A combination of kinetic data, X-ray analyses, and DFT calculations were used to rationalize the results and to draw conclusions on the key role played by the solvent molecules in the process. These important mechanistic insights also permit an explanation of the unexpected "solvent effect" that allows the preparation of well-defined poly(vinyl acetate)- b -poly(acrylonitrile) by CMRP. [source]

Fe2 and Fe4 Clusters Encapsulated in Vacant Polyoxotungstates: Hydrothermal Synthesis, Magnetic and Electrochemical Properties, and DFT Calculations

Cline Pichon
Abstract While the reaction of [PW11O39]7, with first row transition-metal ions Mn+ under usual bench conditions only leads to monosubstituted {PW11O39M(H2O)} anions, we have shown that the use of this precursor under hydrothermal conditions allows the isolation of a family of novel polynuclear discrete magnetic polyoxometalates (POMs). The hybrid asymmetric [FeII(bpy)3][PW11O39Fe2III(OH)(bpy)2],12,H2O (bpy=bipyridine) complex (1) contains the dinuclear {Fe(,-O(W))(,-OH)Fe} core in which one iron atom is coordinated to a monovacant POM, while the other is coordinated to two bipyridine ligands. Magnetic measurements indicate that the FeIII centers in complex 1 are weakly antiferromagnetically coupled (J=,11.2,cm,1, H=,JS1S2) compared to other {Fe(,-O)(,-OH)Fe} systems. This is due to the long distances between the iron center embedded in the POM and the oxygen atom of the POM bridging the two magnetic centers, but also, as shown by DFT calculations, to the important mixing of bridging oxygen orbitals with orbitals of the POM tungsten atoms. The complexes [Hdmbpy]2[FeII(dmbpy)3]2[(PW11O39)2Fe4IIIO2(dmbpy)4],14,H2O (2) (dmbpy=5,5,-dimethyl-2,2,-bipyridine) and H2[FeII(dmbpy)3]2[(PW11O39)2Fe4IIIO2(dmbpy)4],10,H2O (3) represent the first butterfly-like POM complexes. In these species, a tetranuclear FeIII complex is sandwiched between two lacunary polyoxotungstates that are pentacoordinated to two FeIII cations, the remaining paramagnetic centers each being coordinated to two dmbpy ligands. The best fit of the ,MT=f(T) curve leads to Jwb=,59.6,cm,1 and Jbb=,10.2,cm,1 (H=,Jwb(S1S2+S1S2*+S1*S2+S1*S2*),Jbb(S2S2*)). While the Jbb value is within the range of related exchange parameters previously reported for non-POM butterfly systems, the Jwb constant is significantly lower. As for complex 1, this can be justified considering FewO distances. Finally, in the absence of a coordinating ligand, the dimeric complex [N(CH3)4]10[(PW11O39FeIII)2O],12,H2O (4) has been isolated. In this complex, the two single oxo-bridged FeIII centers are very strongly antiferromagnetically coupled (J=,211.7,cm,1, H=,JS1S2). The electrochemical behavior of compound 1 both in dimethyl sulfoxide (DMSO) and in the solid state is also presented, while the electrochemical properties of complex 2, which is insoluble in common solvents, have been studied in the solid state. [source]

Metal-Induced Tautomerization of p - to o -Quinone Compounds: Experimental Evidence from CuI and ReI Complexes of Azophenine and DFT Studies

Stphanie Frantz
Abstract Azophenine (7,8-diphenyl-2,5-bis(phenylamino)- p -quinonediimine, Lp) reacts with [Cu(PPh3)4](BF4) or [Re(CO)5Cl] to yield the (Ph3P)2Cu+ or [(OC)3ClRe] complex of the tautomeric form 7,8-diphenyl-4,5-bis(phenylamino)- o -quinonediimine, Lo, as evident from structure determinations and from very intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Time-dependent DFT (TD-DFT) calculations on model complexes [(N,N)Re(CO)3Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o -quinonediimine complexes in comparison to compounds with N,N=1,4-dialkyl-1,4-diazabutadiene. The complexes are additionally stabilized through hydrogen bonding between two now ortho -positioned NHPh substituents and one fluoride of the BF4, anion (Cu complex) or the chloride ligand (Re complex). DFT Calculations on the model ligands p -quinonediimine or 2,5-diamino- p -quinonediimine and their ortho -quinonoid forms with and without Li+ or Cu+ are presented to discuss the relevance for metal-dependent quinoproteins. [source]

Diels,Alder Reactions of Symmetrically 1,4-Disubstituted Dienes: Theoretical Study on the Influence of the Configuration of the Double Bonds on the Regio- and Endoselectivity

Samira Baki
Abstract It was experimentally shown in Diels-Alder reactions that symmetrically 1,4-disubstituted dienes exhibit high regio- and endoselectivity induced by the (E,Z) configuration of the double bonds. In order to understand the origin of this selectivity, the transition states associated with the reaction between a series of such dienes (R = OMe, CH3, NH2, F, CN) on substituted ethylene (R, = CO2Me, CN, F, OMe) were determined by DFT calculations. If the regioselectivity confirmed by a single-point MP2 calculation is predicted to be good in most cases, its direction remains difficult to predict; the R, group of the dienophile can orient itself toward either the (Z) or the (E) side of the diene. Neither primary nor secondary frontier orbital interactions appear to be able to rationalize these results. They are more likely explained by a delicate balance between steric and electrostatic interactions. This hypothesis is further supported by the calculated reactivity of the (E,E) and (Z,Z) corresponding dienes. In any case, a standard DFT calculation, confirmed by a single-point MP2 step, is likely to provide a reliable guideline for a synthesis strategy that allows the control of up to four stereogenic centres in a single reaction step.( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

FTIR study of the CO adsorption over Pt/MFI catalysts: Ab initio interpretation

N. S. Nesterenko
Abstract The state and dispersion of Pt supported on MFI catalysts with different contents have been studied using Fourier transform infrared (FTIR) spectroscopy. Experimentally predicted Pt-containing complexes were simulated by density functional theory (DFT) for CO frequency shift estimation and determination of the relative intensities. According to DFT calculation and experimental data, a new interpretation was proposed. A new approach for estimation of the Pt dispersion based on IR spectra of adsorbed CO has been developed. 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]

A new eremophilanolide from Senecio sinuatus Gilib

Eleuterio Burgueo-Tapia
Abstract From the hexane extracts of Senecio sinuatus roots, the new 3,-angeloyloxy-6,-hydroxyeremophil-1(10)-en-8,,12-olide (3), along with the known compounds 3,-angeloyloxy-6,-hydroxyeremophil-1(10)-ene (1), 3,-senecioyloxy-6,-hydroxyeremophil-1(10)-ene (2), and 3,-angeloyloxy-6,,8,-dihydroxyeremophil-1(10)-en-8,,12-olide (4), were isolated. Complete 1H and 13C NMR chemical shift assignments of 1,4 were achieved using one- and two-dimensional NMR techniques, including gHMQC and gHMBC experiments. A Monte Carlo search, followed by B3LYP/6-31G*DFT calculation, provided the theoretical conformations of the eremophilane rings, which were in agreement with results derived from 1H1H NMR coupling constant analysis, and confirmed by NOESY experiments. Copyright 2007 John Wiley & Sons, Ltd. [source]

Tetrahedral Oligothiophenes; Synthesis, X-ray Analysis, and Optoelectronic Properties of Highly Symmetrical, 3D-Branched Oligothiophenes

Kouzou Matsumoto Dr.
Abstract Tetrakis(bithienyl)methane and tetrakis(terthienyl)methane have been synthesized from tetrakis(2-thienyl)methane by use of Suzuki,Miyaura coupling as a key reaction. Their trimethylsilyl (TMS) derivatives are also synthesized. X-ray analysis reveals that each oligothiophene moiety tends to adopt anti -conformations and show relatively small torsion angles between the adjacent thiophene rings. While the longest absorption maxima of these tetrakis(oligothienyl)methanes exhibit only a slight bathochromic shift compared to the corresponding linear oligothiophene derivative, tetrakis(bithienyl)methane and its TMS derivative exhibit an appreciable red-shift in their fluorescence spectra. The intramolecular interaction between thienyl groups of tetrakis(2-thienyl)methane is supported by DFT calculation. [source]

Ruthenium-to-Platinum Interactions in ,6,,1 NCN-Pincer Arene Heterobimetallic Complexes: An Experimental and Theoretical Study

Sylvestre Bonnet
Abstract A series of ,6,,1 -heterobimetallic complexes have been prepared in which a [Ru(,6 -arene)(C5R5)]+ fragment (R = H or Me) and an ,1 -NCN-pincer platinum fragment are combined within the same molecule. In complexes [2]+ and [3]+, the ruthenium and platinum centers are ,6 and ,1 coordinated, respectively, to the same arene ring, whereas in [4A]+ and [5A]+ they are coordinated to two different arene rings that are linked with a covalent bond ([4A]+) or an ethyl bridge ([5A]+). Upon changing the organic manifold between both metal centers, very strong ([2]+) to very weak ([5A]+) ruthenium-to-platinum interactions are obtained. Experimentally, X-ray crystal structures show an increaing steric hindrance when the Ru,Pt distance diminishes, and electrochemical and 195Pt NMR spectroscopic studies show a decreasing electron density on platinum from [5A]+ to [2]+. Theoretical DFT calculations were undertaken, which show an increasing charge on platinum from [5A]+ to [2]+. Our theoretical analysis shows that the particularly strong ruthenium-to-platinum electronic interactions in [2]+ and [3]+ do not come from binding of ruthenium to platinum, but from the pincer Cipso sharing its electron density between both metal centers, which decreases the , donation to platinum, and from increased backdonation of the platinum d electrons into the , system of the arene ring. [source]

Tetranuclear Manganese Complexes with [MnII4] and [MnII2MnIII2] Units: Syntheses, Structures, Magnetic Properties, and DFT Study,

Lucjan B. Jerzykiewicz
Abstract Two tetranuclear manganese compounds, [Mn4(,3,,2 -L)4Br4(LH)4] (1) and [Mn4(,3,,2 -L)2(,,,2 -L)4L2Br2] (2), with cubane and defect dicubane-like cores were synthesized and characterized by single-crystal X-ray diffraction, magnetic measurements, and DFT calculations (LH = 2-methoxyethanol). The magnetic properties of the resulting [Mn4] building blocks are presented and discussed in detail. In particular, in 2 the MnIII,O,MnIII angle 103.12(8) is the largest observed to date for such a system. The conjunction of antiferromagnetic and ferromagnetic interactions within the tetranuclear mixed-valent MnII2MnIII2 complexes results in an unusual ST = 1 ground state. [source]

Mesomerization of S4 -Symmetric Tetrahedral Chelate Complex [In4(L3)4]: First-Time Monitored by Temperature-Dependent 1H NMR Spectroscopy,,

Rolf W. Saalfrank
Abstract VT 1H NMR spectroscopy proved that a non-dissociative and reversible mesomerization process links the tetranuclear indium(III) complexes meso -(,,,,,,,)(P,P,M,M)- 3 and meso -(,,,,,,,)(M,M,P,P)- 3,. During this process four tandem Bailar twists, resulting in the (,)/(,) isomerization at the indium centers, and the (P)/(M) inversion of the four coordinating face-centered, helical ligands (L3)3, are involved. In addition, gas-phase DFT calculations (B3LYP/LANL2DZp) revealed a C1 -symmetric transition state (+21.9 kcal,mol,1) for the mesomerization mechanism which connects 3 and 3,. [source]

Synthesis of 1,3-Diphospha-2,3-dihydro-1H -phenalenes

Arkadii Tarasevych
Abstract The reaction of 1,8-dilithionaphthalene (1) with methylenebis[(dialkylamino)chlorophosphanes] [R2N(Cl)PCH2P(Cl)NR2; R = Me (2a), Et (2b)] leads to the formation of new heterocyclic compounds, 1,3-diphospha-2,3-dihydro-1H -phenalenes 3a,b as a mixture of cis and trans isomers. DFT calculations indicate that the cis isomers are thermodynamically more stable by about 1,3 kcal/mol than the trans isomers. Compounds 3a,b can be converted into dithio the derivatives 5a,b and the borane complexes 6a,b, which were characterized by NMR spectroscopy and investigated by X-ray diffraction analysis. The dialkylamino groups in 3 can be substituted by chlorine to give the chlorophosphane 7. [source]

Rational, Facile Synthesis and Characterization of the Neutral Mixed-Metal Organometallic Oxides Cp*2MoxW6,xO17 (Cp* = C5Me5, x = 0, 2, 4, 6)

Glnur Taban-al
Abstract The reaction of the bis(pentamethylcyclopentadienyl)pentaoxidodimetal complexes Cp*2M2O5 with four equivalents of Na2M,O4 (M, M, = Mo, W) in acidic aqueous medium constitutes a soft and selective entry into neutral Lindqvist-type organometallic mixed-metal oxides Cp*2MoxW6,xO17 [x = 6 (1), 4 (2), 2 (3), 0 (4)]. The identity of the complexes is demonstrated by elemental analyses, thermogravimetric analyses and infrared spectroscopy. Thermal degradation of 1,4 up to above 500 C leads to Mox/6W1,x/6O3. The molecular identity and geometry of compound 2 is further confirmed by a fit of the powder X-ray diffraction pattern with a model obtained from previously reported single-crystal X-ray structures of 1 and 4, with which 2 is isomorphous. DFT calculations on models obtained by replacing Cp* with Cp (I,IV) validate the structural assignments and assist in the assignment of the M,M,,O vibrations. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Molecular Iodine Stabilization in an Extended NI,IN Assembly

Francesco Isaia
Abstract The adduct [bis(quinoxaline)-2,2,,3,3,-disulfideI2], (Q2S2I2), (1) can be easily synthesised from the reaction of Q2S2 and I2 in CH2Cl2 or, in the absence of any solvent, through diffusion of I2 vapours at 60 C. X-ray diffraction analysis shows the presence of an extended NI,IN assembly in which each I2 molecule links a Q2S2 molecule at both ends through a nitrogen atom to form a polymeric species; the d(I,I) and d(N,I) bond lengths confirm a very weak nitrogen,iodine interaction at the base of the NI,IN assembly. DFT calculations provide optimised distances for the NI and I,I bonds and explanation for the zigzag chain formation: the mPW1PW functional and the B3LYP hybrid functional with a variety of basis sets for the I atomic species [CRENBL, LANL2DZ, LANL2DZ(d,p), LANL08(d), SBKJC, SBKJC polarised-LFK and Stuttgart RLC] have been tested. Compound 1 proved stable up to nearly 100 C, and the stability is to be mainly attributed to the lattice energy of its polymeric structure then to donor,acceptor stabilisation. The facile insertion of molecular iodine into the Q2S2 network makes this compound an interesting iodine sponge, suitable for I2 storage; moreover, Q2S2 can easily collect and release I2(g) by a temperature-controlled process (60 and 97 C, respectively). ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Strong Evidence for an Unprecedented Borderline Case of Dissociation and Cycloaddition in Open-Shell 1,3-Dipole Chemistry: Transient Nitrilium Phosphane-Ylide Complex Radical Cations

Holger Helten
Abstract The reaction of 3-ferrocenyl-substituted 2H -azaphosphirene complexes 1a,c in the presence of substoichiometric amounts of ferrocenium hexafluorophosphate yields 3,5-diferrocenyl-substituted 2H -1,4,2-diazaphosphole complexes 3a,c and difluoro(organo)phosphane complexes 4a,c. The reaction of 1a,c and [FcH]PF6 with cyanoferrocene yields 3a,c in a straightforward way. The molecular structures of 3a,c were unambiguously identified by multinuclear NMR spectroscopic experiments, mass spectrometry, and single-crystal X-ray diffraction studies. DFT calculations on model complexes 1d,m and 3d,m reveal a close similarity of Mo and W complexes (vs. Cr) and a strong influence of the ferrocenyl substituent on the geometry, spin, and charge distribution of reactive intermediates and the reaction course. Strong support for the assumption of a dissociation,cycloaddition reaction sequence leading to 3 and thus a surprising "cannibalistic" reaction was obtained.( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Selective B,H versus N,H Bond Activation in Ammonia Borane by [Ir(dppm)2]OTf,

Andrea Rossin
Abstract Ammonia borane (AB, NH3BH3) was activated selectively on the boron hydrogen atoms by [Ir(dppm)2]OTf, as shown by variable-temperature 31P, 11B and 1H NMR studies, to yield cis - and trans -[Ir(H)2(dppm)2]OTf dihydrides. X-ray crystal structures of these complexes were obtained. DFT calculations suggest the key reaction intermediate to be a Lewis base (L) stabilised adduct, cis -[Ir(H)(BH2L)(dppm)2]+( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Sulfur Dioxide and Water: Structures and Energies of the Hydrated Species SO2nH2O, [HSO3],nH2O, [SO3H],nH2O, and H2SO3nH2O (n = 0,8)

Ralf Steudel
Abstract The structures of a large number of hydrates of sulfur dioxide (SO2nH2O), of the sulfonate ion ([HSO3],nH2O), of the tautomeric hydrogensulfite anion ([SO3H],nH2O), and of sulfurous acid (H2SO3nH2O) with up to eight water molecules attached to these species have been optimized at the B3LYP/6-31G(2df,p) level of theory (DFT). The calculated vibrational frequencies allow the definite assignment of certain characteristic modes, and in this way a convincing interpretation of published spectra of aqueous SO2 as well as of SO2 adsorbed on very cold ice crystals has been achieved for the first time. Single-point calculations at the G3X(MP2) level of theory were used to calculate the binding energies of the water molecules in SO2nH2O as well as the relative stabilities of the isomeric anionic species [HSO3],nH2O and [SO3H],nH2O. Generally, the water molecules tend to stick together forming clusters, whereas the particular sulfur-containing molecule remains at the surface of the water cluster, but it is always strongly hydrogen-bonded. Only when there are more than six water molecules are the anions more or less completely surrounded by water molecules. DFT calculations erroneously predict that the gaseous hydrated sulfonate ions are more stable than the isomeric hydrogensulfite ions, even when hydrated with six water molecules. However, if these hydrated species are calculated as being embedded in a polar continuum simulating the aqueous phase, the hydrogensulfite ions are more stable than the sulfonate ions, in agreement with various spectroscopic observations on aqueous sulfite solutions. On the other hand, at the higher G3X(MP2) level, the gaseous hydrated hydrogensulfite anions are more stable than the corresponding sulfonate ions only if the number of water molecules is larger than four, whereas for the weakly hydrated anions the order of relative energies is reversed. The possible implications of these results for the enzymatic oxidation of "sulfite ions" ([HSO3], and [SO3H],) by sulfite oxidase are discussed. The conversion of SO26H2O into its isomer H2SO35H2O is predicted to be exothermic (,H298 = ,56.1 kJ,mol,1) and exergonic (,G298 = ,22.5 kJ,mol,1). ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Synthesis, Protonation and CuII Complexes of Two Novel Isomeric Pentaazacyclophane Ligands: Potentiometric, DFT, Kinetic and AMP Recognition Studies

Andrs G. Algarra
Abstract The synthesis and coordination chemistry of two novel ligands, 2,6,9,12,16-pentaaza[17]metacyclophane (L1) and 2,6,9,12,16-pentaaza[17]paracyclophane (L2), is described. Potentiometric studies indicate that L1 and L2 form a variety of mononuclear complexes the stability constants of which reveal a change in the denticity of the ligand when moving from L1 to L2, a behaviour that can be qualitatively explained by the inability of the paracyclophanes to simultaneously use both benzylic nitrogen atoms for coordination to a single metal centre. In contrast, the formation of dinuclear hydroxylated complexes is more favoured for the paraL2 ligand. DFT calculations have been carried out to compare the geometries and relative energies of isomeric forms of the [CuL]2+ complexes of L1 and L2 in which the cyclophane acts either as tri- or tetradentate. The results indicate that the energy cost associated with a change in the coordination mode of the cyclophane from tri- to tetradentate is moderate for both ligands so that the actual coordination mode can be determined not only by the characteristics of the first coordination sphere but also by the specific interactions with additional nearby water molecules. The kinetics of the acid promoted decomposition of the mono- and dinuclear CuII complexes of both cyclophanes have also been studied. For both ligands, dinuclear complexes convert rapidly to mononuclear species upon addition of excess acid, the release of the first metal ion occurring within the mixing time of the stopped-flow instrument. Decomposition of the mononuclear [CuL2]2+ and [CuHL2]3+ species occurs with the same kinetics, thus showing that protonation of [CuL2]2+ occurs at an uncoordinated amine group. In contrast, the [CuL1]2+ and [CuHL1]3+ species show different decomposition kinetics indicating the existence of significant structural reorganisation upon protonation of the [CuL1]2+ species. The interaction of AMP with the protonated forms of the cyclophanes and the formation of mixed complexes in the systems Cu,L1 -AMP, Cu,L2 -AMP, and Cu,L3 -AMP, where L3 is the related pyridinophane containing the same polyamine chain and 2,6-dimethylpyridine as a spacer, is also reported. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]