Theoretical studies on the protonation behavior of tropone and its metal complexes

Density functional theory calculations were carried out to investigate structures and stabilities of tropone and troponeiron complexes, (tropone)Fe(CO)₃, (tropone)Fe(CO)₂(PH₃) and (tropone)Fe(PH₃)₃, and their protonated species. The results show that the oxygen-protonated tropone is more stable than the carbon-protonated tropone. On the contrary, in the troponeiron complexes, the carbon protonated species are more stable than the oxygen protonated species. In the neutral and oxygen-protonated complexes, the tropone and oxygen-protonated tropone ligands are η⁴-coordinated. In the carbon-protonated complexes, the carbon-protonated tropone ligand is η⁵-coordinated. The results also show that the metal shift for complexes containing phosphine ligands is more difficult than that for those containing carbonyl ligands. For the neutral methyl-substituted troponeiron complexes, steric effect was found to play a key role in determining the relative stability of the regioisomers. For their protonated species, the electron-donating properties of the methyl substituent(s) were found to be important in determining the relative stability among the different regioiosmers.     

G2 study of triplet [H4, Si,P]+ potential energy surface: Mechanism for reaction of P+ (3P) with silane

A G2 search of the triplet [H₄, Si, P]⁺ potential energy surface (PES) was carried out, along with a study of a number of mechanisms for the reaction of the P⁺ (³P) ion with silane. The most stable isomer, which corresponds to the species resulting from transferring three hydrogen atoms from the silicon to the phosphorus atom, lies 67.3 kcal/mol below the reactants' level. The P⁺(SINGLE BOND)SiH₄ ion-molecule complex also has remarkable stability, 20.4 kcal/mol. Bond properties were calculated and are discussed for all the stable species found on the PES. Various exothermic reaction paths were also fully characterized. The abstractions of a hydrogen molecule and a hydrogen atom, yielding species with P(SINGLE BOND)Si bonds, have comparable kinetic hindrance, although release of molecular hydrogen was found to be more exothermic. Finally, hydrogen and/or charge transfer reactions between P⁺ (³P) and silane are also discussed. © 1997 by John Wiley & Sons, Inc.     

Preparation and olefin-metathesis activity of cyclopentylidene-oxo initiator sites on a molybdenum carbide surface

A low surface area planar sample of bulk β-Mo₂C can be activated for olefin-metathesis reactions through the dissociative chemisorption of cyclopentanone to yield surface alkylidene-oxo species. This methodology allows the formation of a sufficiently high coverage of active sites to permit investigation of heterogeneous metathesis by surface science techniques. Reflectance absorbance infrared spectroscopy measurements were used to isolate η¹ and η²-adsorbed cyclopentanone. The η²-state was found to be the direct precursor for alkylidene-oxo formation. Olefin-metathesis reactions were carried out using propene and cyclopentene. The surface chemistry of cyclopentanone and cyclopentylidene on β-Mo₂C is compared to analogous organometallic systems.     

Bowl-shaped aromatic hydrocarbon precursors of fullerenes as η<Superscript>6</Superscript>-π-ligands in transition metal complexes: Complexes based on sumanene C<Subscript>21</Subscript>H<Subscript>12</Subscript> derivatives

The density functional theory in the PBE approximation was used to explore the geometry and electronic structure of sumanene C₂₁H₁₂ and its five derivatives C₂₁H₁₂R₆ (R = H, F, Cl, Br, and CN). The R groups in C₂₁H₁₂R₆ molecules are attached to carbon atoms in α positions with respect to the central six-membered ring. The possibility of the formation of C₂₁H₁₂R₆η ⁶-π complexes with Cr(C₆H₆), Cr(CO)₃, Mo(C₆H₆), and Mo(CO)₃ was discussed. The relative stability of these complexes was evaluated. The attachment of M(C₆H₆) and M(CO)₃ (M = Cr, Mo) to sumanene C₂₁H₁₂ with the formation of η⁶-π bonds is energetically less favorable than their attachment to sumanene derivatives C₂₁H₁₂R₆. The complexes of sumanene derivatives with Cr(C₆H₆), Cr(CO)₃, Mo(C₆H₆), and Mo(CO)₃ were found to be the most promising objects for synthesis. The C₆₀ and η⁶-(C₆H₆)M-C₆₀ and η⁶-(CO)₃M-C₆₀ (M = Cr, Mo) fullerene complexes were predicted to be much less stable than the η⁶-(CO)₃M-C₆₀R₆ and η⁶-(C₆H₆)M-C₆₀R₆ complexes (M = Cr, Mo; R = H, Hal, CN), where R groups bordered one of the fullerene C₆₀ six-membered rings comprising the atoms to which metal atoms were coordinated.     

D3h‐Symmetric Porphyrin‐Based Rigid Macrocyclic Ligands for Multicofacial Multinuclear Complexes in a One‐Nanometer‐Sized Cavity

The one‐step synthesis of D_3h‐symmetric cyclic porphyrin trimers 1 composed of three 2,2′‐[4,4′‐bis(methoxycarbonyl)]bipyridyl moieties and three porphyrinatozinc moieties was achieved from a nickel‐mediated reductive coupling of meso‐5,15‐bis(6‐chloro‐4‐methoxycarbonylpyrid‐2‐yl)porphyrinatozinc. Although cyclic trimers 1 were obtained as a mixture that included other cyclic and acyclic porphyrin oligomers, an extremely specific separation was observed only for cyclic trimers 1 when using columns of silica gel modified with pyrenylethyl, cyanopropyl, and other groups. Structural analysis of cyclic trimers 1 was carried out by means of NMR spectroscopy and X‐ray crystallography. Treatment of an η³‐allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η³‐allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands.     

Exohedral Complexation of B40, C60 and Arenes with Transition Metals: A Comparative DFT Study

Holes are inevitable in borospherenes. The surface topography of B₄₀ and its π MOs isolobal to benzene allow for better η⁷‐, η⁶‐ and η³‐ exohedral complexation with transition metal fragments than it is possible with C₆₀ and arenes. η⁷‐complexes of B₄₀ is lower in energy than the η⁶‐complexes for metal fragments such as C₅H₅Mn, C₄H₄Fe, and C₃H₃Co that have relatively diffuse frontier orbitals. The fragment C₆H₆Cr prefers η⁶‐coordination. Near‐isodesmic equations based on density functional theory computations of the transition metal complexes of B₄₀, C₆₀ and C₆H₆ support these anticipations. Transition metal complexation increases the stability of B₄₀.     

Kinetic studies on the thermal dissociation of β-cyclodextrin-cinnamyl alcohol inclusion complex

The stability of β-cyclodextrin-cinnamyl alcohol inclusion complex (β-CD·C₉H₁₀·8H₂O) was investigated using TG and DSC. The mass loss took place in three stages: the dehydration occurred between 50–120°C; the dissociation of β-CD·C₉H₁₀O occurred in the range of 210–260°C; and the decomposition of β-CD began at 280°C. The dissociation of β-CD·C₉H₁₀O was studied by means of thermogravimetry, and the results showed: the dissociation of β-CD·C₉H₁₀O was dominated by a two-dimensional diffusion process (D₂). The activation energyE was 161.2 kJ mol^?1, the pre-exponential factorA was 4.5×10¹³ min^?1. Cyclodextrin is able to form inclusion complexes with a great variety of guest molecules, and the interesting of studies focussed on the energy binding cyclodextrin and the guest molecule. In this paper, β-cyclodextrin-cinnamyl alcohol inclusion complex was studied by fluorescence spectrophotometry and infrared absorption spectroscopy, and the results show: the stable energy of inclusion complexes of β-CD with weakly polar guest molecules consists mainly of Van der Waals interaction.     

Molecular orbital studies of hydrogen bonds. IV. Hydrogen bonds in excited states of H2CO with H2O

Ab initio calculations for the interacting system of lower excited states of planar and bent H₂CO with H₂O have been carried out with a minimum basis set, using the recently proposed electron-hole potential method. The blue shifts of the η-π* transition are evaluated as 1100 and 1420 cm^?1 for the singlet and triplet transitions, respectively. In the η-π* states of bent H₂CO, the most stable geometry is one in which an H₂O hydrogen atom is coordinated to the carbon atom.     

DFT study on a fullerene doped with Si and N

DFT calculations are applied for some stable C₆₀, C₅₉Si, and C₅₉N hetero fullerenes. Sn and Ge atoms are doped at the same position of C₆₀. Computations are carried out at the B3LYP/cc pVDZ levels. In this work the effects of the heteroatoms, Si and N, on the structural properties of the fullerene have been studied. The structure, energetic and relative stabilities of the compounds were compared and analyzed with each other. In addition, vibrations spectra of proposed stable neutral species, as well as the infrared intensities are calculated. From the data obtained from calculation, we found that there is strong correlation between the stability of pure C₆₀ fullerene molecule and the numbers of different C-C bonds.     

Binding Matter with Antimatter: The Covalent Positron Bond

We report sufficient theoretical evidence of the energy stability of the e⁺?H₂^2? molecule, formed by two H^? anions and one positron. Analysis of the electronic and positronic densities of the latter compound undoubtedly points out the formation of a positronic covalent bond between the otherwise repelling hydride anions. The lower limit for the bonding energy of the e⁺?H₂^2? molecule is 74 kJ mol^?1 (0.77 eV), accounting for the zero‐point vibrational correction. The formation of a non electronic covalent bond is fundamentally distinct from positron attachment to stable molecules, as the latter process is characterized by a positron affinity, analogous to the electron affinity.     

Stereoanalysis of fullerenes with a chiral molecular framework

Stereoanalysis of three fullerene molecules with a chiral molecular framework C₃₂, C₇₆, and C₇₈ and achiral fullerene C₆₀ molecule was carried out. Comparative quantitative analysis of the degree of chirality showed topology to be the major factor governing the chirality of fullerenes. A procedure for determining the relative contribution of topological chirality to the total chirality of the molecule is proposed. Structural fragments responsible for chirality are found. The title fullerenes are assigned to the corresponding subclasses of homochirality. A classification system of isomeric fullerenes is proposed.     

Molecular orbital calculations on η3- and η5-hexadienyl complexes of the platinum metals; an analysis of their conformational preferences and fluxional characteristics

Extended Hückel molecular orbital calculations are reported for the η³- and η⁵-platinum metal complexes [Pt(η³-C₆H₇)(PH₃)₂]⁺ and [Pt(η⁵-C₆H₇)(PH₃)₂]⁺. The η³-geometry is found to be only 0.56 eV more stable than the η⁵-geometry. This leads to a low energy fluxional process in these molecules. The stereochemistry of this fluxional process is rationalised in terms of the conformational preferences of the [Pt(η⁵-C₆H₇)(PH₃)₂]⁺ ions.     

Coordination of α,β-unsaturated aldehydes on d Ru and Rh complexes: A DFT study

The coordination modes of various α,β-unsaturated aldehydes on d⁶ ML₄ and ML₅ complexes of rhodium and ruthenium have been compared by means of density functional theory (DFT) calculations. The studied aldehydes were acrolein, crotonaldehyde, prenal and cinnamaldehyde and the metallic fragments RuH₂(PH₃)₂, RuH₂(PH₃)₃ and RhH₂Cl(PH₃)₂. On the d⁶ ML₄ Ru fragment, the best coordination geometry is η⁴, where both double bonds are involved. On the d⁶ ML₅ Ru fragment, the η²CC geometry is preferred to the η¹O and to the η²CO ones. Finally on the d⁶ ML₅ Rh fragment, the η²CO mode is not stable and only η¹O and η²CC exist, the former being favored. In conclusion, the η²CO coordination geometry is never favored even if the energy gap between η²CC and η²CO is reduced by the use of bulky ligands (PPh₃) or alkyl substituents on the CC bond. The Ru and Rh complexes behave differently: in the case of Ru, η²CC and η²CO can be in competition and in the case of Rh, the best form is η¹O. This different behavior can explain the results obtained in the hydrogenation reaction of α,β-unsaturated aldehydes.     

THEORETICAL INVESTIGATION OF KETENE BONDING MODES IN BIS(PHOSPHINE) PALLADIUM KETENE COMPLEXES

Abstract

Theoretical studies were carried out on a series of bis(phosphine) palladium ketene complexes (PR₃)₂Pd(CH₂=C=O), and on the related CH₂=C=O and Pd(PR₃)₂ molecular fragments in order to investigate the electronic structure and the bonding of the ketene ligand to the metal fragment in these complexes. An analysis of the frontier MOs has been performed in order to understand the interactions between the ketene and the metal fragments. The calculated results have shown that the η²-(C,C) mode is preferred over the η²-(C,O) mode by 10–15 kcal/mol in bis(phosphine) palladium ketene complexes. The basicity and bulkiness of the phosphine ligands PR₃ have little effect on the bonding mode in (PR₃)₂Pd(CH₂=C=O) complexes. The most stable structure was calculated to be the η²-(C,C) square planar geometry with the CH₂ group of ketene out of the molecular plane. Comparison and discussion between the two bonding modes were also presented in this paper.     

A diketiminate‐bound diiron complex with a bridging carbonate ligand

Reduction of carbon dioxide by a diiron(I) complex gives μ‐carbonato‐κ³O:O′,O′′‐bis{[2,2,6,6‐tetramethyl‐3,5‐bis(2,4,6‐triisopropylphenyl)heptane‐2,5‐diiminate(1−)‐κ²N,N′]iron(II)} toluene disolvate, [Fe₂(C₄₁H₆₅N)₂(CO₃)]·2C₇H₈, a diiron(II) species with a bridging carbonate ligand. The asymmetric unit contains one diiron complex and two cocrystallized toluene solvent molecules that are distributed over three sites, one with atoms in general positions and two in crystallographic sites. Both Fe^II atoms are η²‐coordinated to diketiminate ligands, but η¹‐ and η²‐coordinated to the bridging carbonate ligand. Thus, one Fe^II center is three‐coordinate and the other is four‐coordinate. The bridging carbonate ligand is nearly perpendicular to the iron–diketiminate plane of the four‐coordinate Fe^II center and parallel to the plane of the three‐coordinate Fe^II center.     

Theoretical study of isomerism in the molecules N2O,PNO, P2O,N2S,PNS, and P2S

We have carried out nonempirical calculations of the potential surface for isomeric rearrangements of the molecules N₂O, N₂S, PNO, PNS, P₂O, and P₂S. It was found that for the molecules N₂O and N₂S a linear structure is considerably more favorable than a cyclic one, which lies 60 kcal·mole^–1 higher and has low stability. For P₂O and P₂S the linear and cyclic isomers have similar energies. For PNO and PNS there are two linear isomers and one cyclic isomer. The isomers are separated by appreciable barriers and can exist independently. It is predicted for the ABC molecules with 16 valence electrons that if two or all three of the atoms belong to the third or a later period, then the cyclic isomers should be favored to at least the same extent as the linear isomers.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 794–802, April, 1990.     

Aromaten-Komplexe mit Halbsandwich-Struktur: Die 1:1-Komplexe des Mesitylens mit SbCl3, SbBr3, BiCl3 und BiBr3

Arene Complexes with a Half-Sandwich Structure: The 1:1 Complexes of Mesitylene with SbCl₃, SbBr₃, BiCl₃, and BiBr₃ 1,3,5-Trimethylbenzene forms stable 1:1 complexes with SbCl₃, SbBr₃, BiCl₃, and BiBr₃ of the stoichiometry C₆H₃Me₃·EX₃ ( 1 – 4 ). According to the results of X-ray structure analyses of compounds 2 and 3 , one arene molecule is coordinated to each antimony or bismuth atom characterizing these adducts as half-sandwich species. To a good approximation the mesitylene molecules are centered over the metal atoms, but deviations from strict η⁶-hapticity are larger for antimony than for bismuth. – Despite some obvious analogies in many features of the structures, 2 and 3 are not isostructural. Differences appear with regard to the halogen bridging between the EX₃ moieties giving rise to the formation of two-dimensional networks (EX₃)_n covered above and below by mesitylene molecules. The structural and sequential principles of the layers differ in a characteristic way for 2 and 3 .     

Poly[diaquadi‐μ6‐oxalato‐μ5‐oxalato‐chromium(III)rubidium(I)]: a new supramolecular isomer

The title compound, [CrRb(C₂O₄)₂(H₂O)₂]_n, obtained under hydrothermal conditions and investigated structurally at 100 K, is a three‐dimensional supramolecular isomer of the layered structure compound studied at room temperature. This novel polymer is built up from crosslinked heterobimetallic units. The linkage of alternating edge‐ and vertex‐shared RbO₇(H₂O)₂ and CrO₄(H₂O)₂ polyhedra running along three different directions gives a dense packing. The two independent ligands display two η⁴‐chelation modes and two conventional carboxylate bridges. However, the pentadentate ligand connects the Cr^III and Rb^I ions through one O‐atom bridge, while the hexadentate ligand exhibits an additional η³‐chelation and two O‐atom bridges. Each coordinated water molecule forms an O‐atom bridge between the two metals. Moreover, in the absence of protonated ligands, these water molecules act as donors through their four H atoms in strong‐to‐weak hydrogen bonds. This results in zigzag chains of alternating oxalate and aqua ligands parallel to the twofold screw axis. The six double oxalates known to date containing an alkali and Cr^III all present layered two‐dimensional structures. In the series, this supramolecular isomer is the first three‐dimensional framework.     

Solvent effects on tamoxifen molecule interacting with a single-walled carbon nanotube: a theoretical NMR study

Quantum chemical calculations of the electronic structure of tamoxifen molecule interacting with an open end of a single-walled carbon nanotube (SWCNT) were carried out and the effects of solvents (water, methanol, DMSO, acetone) on the ¹H, ¹³C, ¹⁵N, and ¹⁷O NMR parameters were studied by the GIAO-HF/STO-3G, GIAO-HF/3-21G, and GIAO/B1LYP/3-21G methods using the GAUSSIAN-98 program. The largest σ_iso value was obtained for acetone, whereas the smallest one for water. The opposite trend was obtained for the shielding asymmetry η. According to calculations, atoms at interaction site bear negative charges. The O(43) and N(38) atoms produce negative charge because they have high electron affinities. The dipole moment of tamoxifen molecule in different solvents increases with increasing the dielectric constant of the solvent. The largest dipole moment value was obtained for water by the B1LYP/3-21G method.     

Niederkoordinierte Phosphorverbindungen: LVII. Alkali(diphosphaallyl)ferrate,synthese und Reaktionen

Reaction of the diphosphapropene I with Na₂Fe(CO)₄, or treatment of its lithium salt with Fe(CO)₅, affords 1,3-diphosphaallylferrates(− 1). Their condensation with alkyl halides or chlorophosphanes yields either the η²,η¹- or η¹,η¹-1,3-diphosphapropene-iron complexes. Protolysis with hydrogen chloride in ether leads via the hydrido-iron species to a stable 1,3-diphosphapropene-iron compound.