Xiuhui Zhang

Possibilities for titanium-titanium multiple bonding in binuclear cyclopentadienyltitanium carbonyls: 16-electron metal configurations and four-electron donor bridging carbonyl groups as alternatives.

The structures for the binuclear Cp(2)Ti(2)(CO)(n) derivatives (Cp = eta(5)-C(5)H(5); n = 8, 7, 6, 5, 4, 3, 2) have been optimized using density functional theory. Furthermore, the thermodynamics of CO dissociation, disproportionation into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1), and dissociation into mononuclear fragments of these Cp(2)Ti(2)(CO)(n) derivatives have been studied. An unbridged Cp(2)Ti(2)(CO)(8) structure with a long approximately 3.9 A Ti-Ti bond is found. As expected from the long Ti-Ti bond, the predicted dissociation energy of this dimer into CpTi(CO)(4) fragments is relatively low at 7 +/- 3 kcal/mol. The lowest energy Cp(2)Ti(2)(CO)(6) structure has two CpTi(CO)(3) units linked by a formal approximately 2.8 A Ti identical withTi triple bond and thus is the next member of the M identical withM triply bonded series Cp(2)V(2)(CO)(5), Cp(2)Cr(2)(CO)(4), Cp(2)Mn(2)(CO)(3), all three of which are stable compounds. The lowest energy structures of Cp(2)Ti(2)(CO)(7), Cp(2)Ti(2)(CO)(5), and Cp(2)Ti(2)(CO)(4) all contain one or two four-electron donor bridging eta(2)-mu-CO groups. However, they are not likely to be stable molecules since their disproportionation energies into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1) are either nearly thermoneutral (n = 5) or exothermic (n = 7 and 4). The lowest energy structure of Cp(2)Ti(2)(CO)(3), in which all three carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a approximately 3.05 A formal Ti-Ti single bond, is a promising synthetic target since it is thermodynamically stable with respect to both CO dissociation and disproportionation into Cp(2)Ti(2)(CO)(4) + Cp(2)Ti(2)(CO)(2). In the lowest energy Cp(2)Ti(2)(CO)(2) structure both carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a formal 2.74 A Ti[triple bond]Ti triple bond. These low energy Cp(2)Ti(2)(CO)(n) (n = 3, 2) structures have only a 16-electron titanium configuration rather than the usually favorable 18-electron configuration for metal carbonyl complexes.

Connexin 43 promotes ossification of the posterior longitudinal ligament through activation of the ERK1/2 and p38 MAPK pathways

Although cervical ossification of the posterior longitudinal ligament (OPLL) is one of the most common spinal diseases, the pathogenic mechanism is still not fully understood. Abnormal mechanical stress distribution is believed to be one of the main causes of OPLL. We have previously found that mechanical stress can up-regulate connexin 43 (Cx43) expression in ligament fibroblasts; this transduces mechanical signals to promote osteoblastic differentiation. In the present study, in order to explore further the intracellular mechanisms of Cx43-induced osteoblast differentiation of ligament fibroblasts, we investigate the potential roles of the osteogenic signaling pathway components ERK1/2, p38 MAPK and JNK in Cx43-mediated mechanical signal transduction. We first confirm higher Cx43 levels in both in vivo ligament tissue from OPLL patients and in vitro cultured OPLL cells. We find that ERK1/2, p38 MAPK and the JNK pathway are all activated both in vivo and in vitro. The activation of these signals was dependent upon Cx43, as its knock-down resulted in diminished mechanical effects and reduced signaling. Moreover, its knock-down almost reversed the osteogenic effect of mechanical stress on ligament fibroblasts and the blocking of the ERK1/2 and p38 MAPK pathways but not the JNK pathway, partly diminished this effect. Therefore, Cx43, which is up-regulated by mechanical stress, seems to function partly via the activation of ERK1/2 and p38 MAPK signals to promote the osteoblastic differentiation of ligament fibroblasts.

Coaxial versus perpendicular structures for a range of binuclear cyclopentadienylpalladium derivatives

Binuclear palladium complexes of planar hydrocarbon ligands are of particular interest since both the coaxial structure (η5-Me5C5)2Pd2(μ-CO)2 and the perpendicular structure (μ-C6H6)2Pd2(Al2Cl7)2 have been synthesized as stable compounds. Here we report theoretical studies on a family of related compounds. For the formal Pd(II) derivatives Cp2Pd2X2 (Cp = η5-C5H5; X = F, Cl, CN) the perpendicular structures with direct Pd–Pd bonds are predicted to lie in energy below the isomeric coaxial structures. These coaxial structures have long Pd⋯Pd distances indicating the absence of palladium–palladium bonds. The lowest energy perpendicular Cp2Pd2X2 structures (X = F, CN) and a higher energy similar Cp2Pd2Cl2 structure contain a substituted η4-C5H5X cyclopentadiene ligand obtained by the addition of X to one of the Cp ligands. For the formal Pd(I) complexes Cp2Pd2L2 (L = CO and CS) the coaxial structures lie in energy below the isomeric perpendicular structures with a particularly large energy separation of ∼19 kcal mol−1 for the thiocarbonyl derivatives. However, for Cp2Pd2(CNCH3)2 the coaxial and perpendicular isomers have essentially the same energies.

Characterizations of novel binuclear alkaline-earth metallocenes: M2(η5-E5)2 (M = Be, Mg and Ca; E = P and As)

A series of novel binuclear alkaline-earth metallocenes M2(η5-E5)2 (M = Be, Mg and Ca; E = P and As) have been studied at the B3LYP/6-311G* and BP86/6-311G* levels of density functional theory. The bonding between the two alkaline earth metal atoms and the bonding between the alkaline earth metal and the five-membered ring are investigated. Natural Bonding Orbital (NBO) analysis shows that the bonding between the metal atom and the five-membered ring is predominantly ionic and each metal atom is in its +1 oxidation state. The lighter alkaline earth metal has stronger bonding in both M2(η 5-P5)2 and M2(η 5-As5)2. A single metal-metal bonding exists between the two metal atoms. For the same alkaline earth metal, the metal–metal bond in M2(η 5-As5)2 is weaker than that in M2(η 5-P5). Nucleus independent chemical shift (NICS) values confirm that the planar exhibits characteristics of aromaticity in these M2(η n-E5)2 species. The NICS values M2(η5-P5)2 are larger than those of the M2(η5-As5)2 analogues. The NICS(0.0) and NICS(1.0), values are in the order of Be2(η 5-E5)2 > Mg2(η 5-E5)2 > Ca2(η 5-E5)2.

The first nonmetal-centered binuclear sandwich-like complexes based on the tetraatomic species E2−4 (E = N, P, As, Sb, Bi) and boron atoms

The first nonmetal-centered binuclear sandwich-like complexes, B2(η4-E4)2 (E = CMe, N, P, As, Sb, Bi; Me = CH3), have been investigated by density functional theory (DFT) to explore whether the nonmetal element can be at the center to form stable sandwich-like complexes. The stable conformer for each species is the D4d staggered one, in which there exists strong interaction between the two boron atoms. Natural bonding orbital (NBO) analysis indicates that the boron–boron bonds are all σ single bonds, which are predicted to be derived mostly from the s and pz orbitals of the boron atoms. The boron–boron bond dissociation energies are higher than that of the Zn–Zn bond in the synthesized complex Cp*ZnZnCp* (Cp* = C5Me5; Me = CH3) suggesting that these nonmetal-centered sandwich-like complexes may be synthesized in future experiments. According to the energy decomposition analysis (EDA), the boron–boron bond is much weaker than the boron–ligand bond and the ability of the E2−4 ligands to stabilize the boron–boron bonds is in the order of N2−4 > P2−4 > As2−4 > Sb2−4 > Bi2−4. Nucleus-independent chemical shift (NICS) values reveal that E2−4 (E = CMe, N, P, As, Sb) rings of B2(η4-E4)2 exhibit characteristics of π aromaticity at 1.0 A above the ring center, whereas Bi2−4 rings of B2(η4-Bi4)2 possess antiaromaticity. In addition, the differences in the bonding nature between the nonmetal-centered and metal-centered binuclear sandwich-like complexes were also investigated. In these nonmetal-centered complexes B2(η4-E4)2, the interactions between the boron atoms and the E2−4 ligands are more than half covalent, while in the metal-centered binuclear sandwich-like complexes, the metal–ligand interactions are mainly ionic.

Quantum chemical and molecular dynamics studies of imidazoline derivatives as corrosion inhibitor and quantitative structure–activity relationship (QSAR) analysis using the support vector machine (SVM) method

The inhibition performance of 10 imidazoline molecules with number of carbon from 15 to 21 of hydrocarbon straight-chain was studied by weight-loss method and theoretical approaches. The main purpose was to build a quantitative structure–activity relationship (QSAR) between the structural properties and the inhibition efficiencies, and then to predict efficiencies of new corrosion inhibitors. The quantum chemical calculation suggested that the active region of imidazoline molecules was located on the imidazoline ring and hydrophilic group, and active sites were concentrated on the nitrogen atoms of the molecules and carbon atoms of hydrophilic group. A model in accordance with the real experimental solution was built in the molecular dynamics, and the equilibrium configuration indicated that the imidazoline molecules were adsorbed on Fe(110) surface in parallel manner. Descriptors for QSAR model building were selected by principal component analysis (PCA) and the model was built by the support vector machine (SVM) approach, which shows good performance since the value of correlation coefficient (R) was 0.99 and the root mean square error (RMSE) was 0.94. Additionally, six new imidazoline molecules were theoretically designed and the inhibition efficiencies of three molecules were predicted to be more than 86% by the established QSAR model.

The lowest triplet electronic states of polyacenes and perfluoropolyacenes

The optimized geometries, total energies, and vibrational frequencies of the polyacenes and perfluoropolyacenes in their closed-shell singlet and lowest triplet states have been studied using the B3LYP method in conjunction with double-ζ plus polarization (DZP) basis sets. The equilibrium structures of all the polyacenes and perfluoropolyacenes are planar, with D2h symmetry, and the singlet and triplet geometries display interesting patterns for each family of molecules. The largest singlet–triplet structural changes appear in the outermost C–C distances. The small perfluoropolyacene systems have ground state singlet states, and the singlet–triplet energy gaps decrease as the number of linearly fused benzene rings increases. With the number of rings greater than seven, the triplet state of the perfluorinated species falls below the closed-shell singlet. For the parent polyacenes, the same shift occurs between n = 8 and n = 9. Correspondingly, the LUMO–HOMO gap for the singlet state decreases with increasing number of linearly fused perfluoro benzene rings. The predicted singlet–triplet separations for the perfluorinated compounds range from 54 kcal mol−1 (perfluoronaphthalene) to −7 kcal mol−1 (n = 10).

Binuclear nickel carbonyls with the small bite chelating diphosphine ligands methylaminobis(difluorophosphine) and methylenebis(dimethylphosphine): formation of NiNi double bonds in preference to ligand cleavage

The structures and thermochemistry of the triads of binuclear nickel carbonyl complexes (bid)Ni2(CO)n (n = 6, 5, 4) and (bid)2Ni(CO)n (n = 4, 3, 2) of the small-bite bidentate chelating diphosphines CH3N(PF2)2 and (Me2P)2CH2 have been investigated using density functional theory. The lowest-energy structures of the carbonyl-richest (bid)Ni2(CO)6 and (bid)2Ni2(CO)4 structures have long Ni⋯Ni distances indicating the lack of direct nickel bonds. Similarly, the lowest energy structures of the intermediate (bid)Ni2(CO)5 and (bid)2Ni2(CO)3 systems have Ni–Ni distances of ∼2.7 A and intact diphosphine ligands. Furthermore, the lowest energy structures of the carbonyl-poorest (bid)Ni2(CO)4 and (bid)2Ni2(CO)2 systems have shorter NiNi distances of ∼2.5 A suggesting formal double bonds and retain the intact diphosphine ligands. This contrasts with the previously studied binuclear iron carbonyls [CH3N(PF2)2]Fe2(CO)6 and [CH3N(PF2)2]2Fe2(CO)4 for which ligand cleavage to separate CH3NPF2 and PF2 units rather than FeFe double bond formation occurs in the lowest energy structures. The experimental [(Me2P)2CH2]2Ni2(CO)4 structure with the boat form of the NiPCPNiPCP eight-membered lies ∼0.5 kcal mol−1 in energy below the higher energy isomer with the chair form of the NiPCPNiPCP ring at the M06-L/TZP level of theory.

Are formal oxidation states above one viable in cyclopentadienylcopper cyanides

AbstractRecent experiments have led to the discovery of the thermally unstable organocopper compounds (η3-C3H5)CuMe2, [(η3-C3H5)CuMe3]–, and CuMe4– in which the copper atom is in the +3 formal oxidation state. In a quest for more stable organocopper compounds with copper in formal oxidation states above one, the binuclear cyclopentadienylcopper cyanides Cp2Cu2(CN)n (Cp = η5-C5H5; n = 1, 2, 3) have been studied using density functional theory (DFT). The lowest energy structures are found to have terminal Cp rings and bridging cyanide ligands up to a maximum of two bridges. Higher-energy Cp2Cu2(CN)n (n = 1, 2, 3) structures are found with bridging Cp rings. The Cp2Cu2(CN)3 derivatives, with the copper atoms in an average +2.5 oxidation state, are clearly thermodynamically disfavored with respect to cyanogen loss. However, Cp2Cu2(CN)2 and Cp2Cu2(CN), with the copper atoms in the average oxidation states +1.5 and +2, respectively, are predicted to have marginal viability. The prospects for the copper(II) derivative Cp2Cu2(CN)2 contrast with that of the “simple” Cu(CN)2, which is shown both experimentally and theoretically to be unstable with respect to cyanogen loss to give CuCN. FigureCp2Cu2(CN),Cp2Cu2(CN)2, Cp2Cu2(CN)3 

The potential role of malonic acid in the atmospheric sulfuric acid - Ammonia clusters formation

Malonic acid (MOA), one of the major dicarboxylic acids (DCAs) in aerosols, has been identified experimentally and computationally to be a strong acid. However, its potential role in the atmospheric clusters formation is still ambiguous. Hence, the participant mechanism of MOA on the formation of atmospheric sulfuric acid (SA)- ammonia (A) clusters was investigated by combining computational methods with atmospheric cluster dynamics code (ACDC). The most stable molecular structures obtained at the M06-2X/6-311++G(3df,3pd) level of theory shows that the added MOA molecule in the SA-A-based clusters presents a promotion on the interactions between SA and A molecules. ACDC simulations indicate directly an obvious enhancement strength RMOA on the clusters formation rates at 218 K and the concentration of MOA ([MOA]) larger than 108 molecules cm-3, up to five orders of magnitude. Meanwhile, enhancement strength of MOA is compared with that of glycolic acid, and as expected, MOA presents a superior enhancement strength. Both RMOA and the compared enhancement strength (rcom) present a positive dependency on [MOA] and a negative dependency on [SA]. With the increase of [A], both RMOA and rcom (except at [SA] = 104 molecules cm-3) first increase, reaching the maximum value and then decrease. Finally, a catalytic participant mechanism of MOA where MOA acts as a mediate bridge for the formation of pure SA-A-based clusters has been identified by tracing the main growth pathways of the system.