Nanoporosity of an interpenetrated NbO-type molecular framework studied by single crystal X-ray diffraction

The molecular framework [Fe(NCS)(2)(tmbpz)(2)](tmbpz = 3,3',5,5'-tetramethyl-4,4'-bipyrazolyl) consists of a robust doubly interpenetrated NbO-type network that remains stable on the removal of solvent guest molecules.     

Solvation of uranyl-CMPO complexes in dry vs. humid forms of the [BMI][PF6] ionic liquid. A molecular dynamics study

The solvation of the [UO(2)(NO(3))(CMPO)](+) and [UO(2)(NO(3))(2)(CMPO)(2)] complexes (CMPO = octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide) is investigated by molecular dynamics in the "dry" and "humid" forms of a room temperature ionic liquid (IL) based on the 1-butyl-3-methylimidazolium (BMI(+)) cation and the hexafluorophosphate (PF(6)(-)) anion. The simulations reveal the importance of the solvent anions in "dry" conditions and of water molecules in the "humid" solvent. For the [UO(2)(NO(3))(CMPO)](+) complex, the monodentate vs. bidentate coordination modes of CMPO are compared, and the first solvation shell of uranyl is completed by 1-3 PF(6)(-) anions in the dry IL and by 2-3 water molecules in the humid IL, leading to a total coordination number close to 5. The energy analysis shows that interactions with the IL stabilize the [UO(2)(NO(3))(bi)(CMPO)(mono)](+) form (with bidentate nitrate and monodentate CMPO) in the dry IL and the [UO(2)(NO(3))(mono)(CMPO)(mono)](+) form (with monodentate nitrate and CMPO) in the humid IL. The extracted compound characterized by EXAFS is thus proposed to be the [UO(2)(NO(3))(mono)(CMPO)(mono)(H(2)O)(3)](+) species. Furthermore we compare the [UO(2)(NO(3))(2)(CMPO)(2)] complex in its associated and dissociated forms ([UO(2)(NO(3))(mono)(CMPO)(mono)](+) + CMPO + NO(3)(-)) and discuss the results in the context of uranyl extraction by CMPO to ionic liquids.     

Structural and magnetic properties of vanadyl dichloride solvates: from molecular units to extended hydrogen-bonded solids

The preparation, structural characterization and magnetic properties of three solvent adducts of VOCl(2), trans-VOCl(2)(THF)(2)(H(2)O) (1), trans-VOCl(2)(H(2)O)(2).2Et(2)O (2) and cis-VOCl(2)(MeOH)(3) (3) are described. In these solids, hydrogen bonding among the inorganic complexes is the critical determinant of the formation of extended magnetic networks. Compound forms one-dimensional double chains where alternating monomers from the two branches of the chain are hydrogen bonded via the V-Cl ... H-O-V network (with an axial water molecule and equatorial chloride ions). Magnetic studies indicate no interaction among the vanadyl centers. The paramagnetism of 1 is consistent with the extension of the network from the hydrogen donor site of the axial water, which is orthogonal to the d(xy) magnetic orbital. Compound 2 forms one-dimensional chains with water molecules of adjacent monomers held together by hydrogen bonds to ether molecules (V-O-H ... O(ether) ... H -O-V). The chain network radiates only through the equatorial plane of the complex where the water molecules are located. The presence of the intervening solvent molecule between hydrogen bonds of the primary coordination sphere magnetically insulates metal centers and compound is also a simple paramagnet. Removal of the solvent turns on the magnetic interaction and neighboring spin centers couple antiferromagnetically. Compound 3 forms a layered structure via V-Cl ... H-O-V hydrogen bonding, where all the hydrogen donor sites participate in the formation of the network. The vanadyl spin centers, at distances of 5.5 and 6.5 A from each other, couple antiferromagnetically (J/k=-0.7 K). Thus, magnetic coupling among metal centers is achieved when the hydrogen bond network directly radiates from the coordination plane containing the magnetic orbital. These results further support the utility of hydrogen bond as a viable design element in the construction of low dimensional, magnetic solids.     

Long hydrogen-bonded rod of molecular oxide: a hexatantalate tetramer

A tetra-n-butylammonium (TBA) salt of [H(4.5)(Ta(6)O(19))](3.5-) was synthesized by reacting hydrous tantalum oxide with TBAOH. X-ray structural analysis of TBA(3.5)[H(4.5)(Ta(6)O(19))]·2THF·5.5H(2)O (THF = tetrahydrofuran) revealed that this compound consists of a hydrogen-bonded, rod-shaped tetramer of hexatantalate that is almost 30 ? long together with TBA cations and solvent molecules of crystallization [a = 20.6354(5) ?, b = 25.5951(7) ?, c = 37.2058(8) ?, α = 77.092(1)°, β = 86.177(1)°, γ = 88.683(1)°, V = 19110.9(8) ?(3), Z = 8, and space group P ?1]. (1)H NMR spectra showed that this tetrameric structure is maintained in solution.     

Assembly of large simple 1D and rare polycatenated 3D molecular ladders from T-shaped building blocks containing a new, long N,N'-bidentate ligand

Molecular ladders [Co(2)(nbpy4)(3)(NO(3))(4)]*solvents and [Cd(2)(nbpy4)(3)(NO(3))(4)](nbpy4 =N,N'-bis-(4-pyridinylmethylene)-1,5-naphthalenediamine) were synthesized via self-assembly; the former is a large, simple, noninterpenetrated 1D ladder that contains guest solvent molecules between the rungs, while the latter exists as 1D ladders in a rare four-fold interlocked 3D structure.     

Solvent coordination in gas-phase [Mn.(H(2)O)(n)](2+) and [Mn.(ROH)(n)](2+) complexes: theory and experiment

An experimental gas-phase study of the intensities and fragmentation patterns of [Mn.(H(2)O)(n)](2+) and [Mn.(ROH)(n)](2+) complexes shows the combinations [Mn.(H(2)O)(4)](2+) and [Mn.(ROH)(4)](2+) to be stable. Evidence in complexes involving the alcohols methanol, ethanol, 1-propanol, and 2-propanol favors preferential fragmentation to [Mn.(ROH)(4)](2+), whereas the fragmentation data for water is less clear. Supporting density functional calculations show that both [Mn.(H(2)O)(4)](2+) and [Mn.(MeOH)(4)](2+) adopt stable tetrahedral configurations, similar to those proposed for biochemical systems where solvent availability and coordination is restricted. Calculated incremental binding energies show a gradual decline on going from one to six solvent molecules, with a step occurring between four and five molecules. The addition of further solvent molecules to the stable [Mn.(MeOH)(4)](2+) unit shows a preference for [Mn.(MeOH)(4)(MeOH)(1,2)](2+) structures, where the extra molecules occupy hydrogen-bonded sites in the form of a secondary solvation shell. Very similar behavior is seen on the part of water. As part of an analysis of the experimental data, the calculations have explored the influence different spins states of Mn(2+) have on solvent geometry. It is concluded that the experimental observations are best reproduced when the central Mn(2+) ion is in the high-spin (6)S ground state. The results are also considered in terms of the biochemical activity of Mn(2+) where the ion is capable of isomorphous substitution with Zn(2+), which itself exhibits a preference for tetrahedral coordination.     

Rhodium-catalyzed hydroformylation of 1-hexene in an ionic liquid: a molecular dynamics study of the hexene/[BMI][PF6] interface

We report a molecular dynamics study of biphasic systems involved in the rhodium-catalyzed hydroformylation of 1-hexene in the 1-butyl-3-methyl-imidazolium hexafluorophosphate ionic liquid ([BMI][PF(6)] IL). We first describe the neat [BMI][PF(6)] interfaces with hexene (the substrate) and heptanal (the linear reaction product) as organic phases. The former interface is molecularly sharp with BMI+ cations preferentially oriented "perpendicular" (i.e., pointing their butyl chains toward the organic phase), whereas hexene molecules tend to be somewhat parallel to the interface. The interface with heptanal is approximately twice as broad, due to BMI+...O(heptanal) attractions, and the solvent molecules are disordered at the interface. No IL ions solubilize in the organic phase(s) whereas ca. 2-3 hexene or heptanal molecules diffused into the IL phase. The presence of the CO and H2 gases does not modify the nature of the hexene/IL interface, as these gases are mainly solubilized in the organic phase, respectively, as diluted species and in the form of a "gaseous" droplet. In the IL phase, one finds a few CO monomers, whereas the less soluble H2 molecules spend only transient excursions. We next simulate the phase separation of "randomly mixed" IL/hexene liquids with the [RhH(CO)L(3)] precatalyst as a solute, comparing the PPh(3) to the TPPTS(3-) ligands (L). The phases separate much more slowly than in the case of classical liquids, and the neutral complex with PPh(3) ligands solubilizes in the hexene phase, displaying loose dynamical contacts with the IL interface. This contrasts with the -9 charged [RhH(CO)(TPPTS)(3)](9-) complex that sits "immobilized" on the IL side of the interface and is mainly solvated by BMI+ cations. Finally, we characterize the solvation of -6 charged [RhH(CO)(TPPTS)(2)](6-), [RhH(CO)(2)(TPPTS)(2)](6-), and [RhH(CO)(TPPTS)(2)(hexene)](6-) complexes involved as reaction intermediates in the hydroformylation reaction and of the free TPPTS(3-) ligand itself in the bulk IL.     

Anionic gallium-based metal-organic framework and its sorption and ion-exchange properties

A gallium-based metal-organic framework Ga(6)(C(9)H(3)O(6))(8)·(C(2)H(8)N)(6)(C(3)H(7)NO)(3)(H(2)O)(26) [1, Ga(6)(1,3,5-BTC)(8)·6DMA·3DMF·26H(2)O], GaMOF-1; BTC = benzenetricarboxylate/trimesic acid and DMA = dimethylamine], with space group I43d, a = 19.611(1) ?, and V = 7953.4(6) ?(3), was synthesized using solvothermal techniques and characterized by synchrotron-based X-ray microcrystal diffraction. Compound 1 contains isolated gallium tetrahedra connected by the organic linker (BTC) forming a 3,4-connected anionic porous network. Disordered positively charged ions and solvent molecules are present in the pore, compensating for the negative charge of the framework. These positively charged molecules could be exchanged with alkali-metal ions, as is evident by an ICP-MS study. The H(2) storage capacity of the parent framework is moderate with a H(2) storage capacity of ~0.5 wt % at 77 K and 1 atm.     

Observation of entropic effect on conformation changes of complex systems under well-controlled temperature conditions

We report direct observation of an entropic effect in determining the folding of a linear dicarboxylate dianion with a flexible aliphatic chain [(-)O(2)C-(CH(2))(6)-CO(2)(-)] by photoelectron spectroscopy as a function of temperature (18-300 K) and degree of solvation from 1 to 18 water molecules. A folding transition is observed to occur at 16 solvent water molecules at room temperature and at 14 solvent molecules below 120 K due to the entropic effect. The (-)O(2)C-(CH(2))(6)-CO(2)(-)(H(2)O)(14) hydrated cluster exhibits interesting temperature-dependent behaviors, and its ratio of folded over linear conformations can be precisely controlled as a function of temperature, yielding the enthalpy and entropy differences between the two conformations. A folding barrier is observed at very low temperatures, resulting in kinetic trapping of the linear conformation. The current work provides a simple model system to study the dynamics and entropic effect in complex systems and may be important for understanding the hydration and conformation changes of biological molecules.     

Ab initio molecular dynamics study of the hydrolysis reaction of diborane

The hydrolysis reaction of the diborane molecule in aqueous solution has been studied by a series of Car-Parrinello Molecular Dynamics simulations in the Blue Moon Ensemble. The total reaction has been divided into two parts: one dealing with the breaking of B(2)H(6) molecule and the formation of a BH(4)(-) ion, a H(2)BOH molecule and a H(+) ion; the second leads to the formation of two hydrogen molecules and another H(2)BOH molecule, starting from BH(4)(-), two water molecules and a H(+) ion. The total reaction studied in this work has been B(2)H(6) + 2H(2)O --> 2H(2)BOH + 2H(2). We have described both structurally and electronically the reagents and the products through the radial distribution functions and the Wannier Function Center positions calculations, with attention to the solvent effects on the compounds. The free energy barrier value for the first part of the reaction and a detailed mechanisms for both parts have been reported. An interesting behavior of BH(3) and H(2) molecules in solution has been observed. They form a quite stable three center bond between the electron pair of the hydrogen molecule and the empty orbital of the boron atom in BH(3), which has been described from both a structural and electronic point of view.     

On the role of tetramethylammonium cation and effects of solvent dynamics on the stability of the cage-like silicates Si6O156- and Si8O208- in aqueous solution. A molecular dynamics study

We have undertaken explicit solvent molecular dynamics simulations to investigate the preferential stabilization of the silicate octamer Si(8)O(20)(8-) over the hexamer Si(6)O(15)(6-) in relation with the ability of tetramethylammonium (TMA) to form an adsorption layer around these cage-like polyions. We have found that the hexamer cannot support such a layer and as a result is vulnerable to hydrolysis. The dynamics of TMA desorption off the surface of the hexamer is investigated in connection with the solvent dynamics. We have studied the energetics of this preferential stabilization by calculating the relative change in the free energies of formation between the complexes Si(8)O(20)(8-).8TMA and Si(6)O(15)(6-).6TMA and found the former to be more stable by 70 kcal/mol. We also find that the energetics are consistent with experimental data, suggesting that the hexamer is a long-lived metastable species. Furthermore, we have studied the solvent structure and dynamics in the vicinity of both the bare polyions and their complexes with TMA. We have found that, as anticipated, both the octamer and the hexamer participate in hydrogen bonds with the water molecules, regardless of whether a TMA adsorption layer exists or not. In fact, we find that the presence of a TMA adsorption layer has a rather profound effect on the stability of these hydrogen bonds-it increases their lifetime by at least a factor of 2 relative to that of the hydrogen bonds between water and the bare polyions.     

Crystalline molecular alloys

The dicationic tecton 1-2H(+) leads in the presence of anionic M(CN)(6)(3-) complexes to two almost identical crystalline systems A (M = Fe) and B (M = Co) composed of 2-D H-bonded networks and water molecules. The epitaxial growth of B on A used as seed or A on B generates crystalline molecular alloys.     

Syntheses and structural characterizations of 24-membered dimetal (Mn, Ni, Fe) macrocyclic complexes and the C-S bond formation between acetylacetone and a mercapto N-heterocycle

An organic ligand 2,5-di(3-pentanedionylthio)-1,3,4-thiadiazole (H2L) reacts with metal (Mn, Ni, Fe) salts, resulting in 24-membered dimetal macrocyclic complexes [MnL(H2O)(dmso)](2).2dmso, [NiL(H2O)(dmf)](2).2dmf, [MnL(dmf)2]2 and [Fe2L2(solvent)2(SO4)] (solvent=dmso; H2O ; dmf). Di-manganese macrocyclic complexes [MnL(dmf)(dmso)]2 and [MnL(H2O)2](2).6H2O can also be obtained directly by aerobic assembly reaction of MnCl2, dipotassium 1,3,4-thiadiazole-2,5-dithiolate (K2tdadt) and acetylacetone (H2acac) in various solvents, accompanying a C-S bond formation between acetylacetone and the mercapto N-heterocycle. Disulfide has been considered as the intermediate in the assembly reaction. Meanwhile an assembly reaction including MnCl2, 2-mercaptobenzimidazole and H2acac has produced an organic compound 2-(3-pentanedionylthio)benzimidazole with a new C-S bond. These dimetal complexes have similar macrocyclic structures, in which solvent molecules and sulfate coordinate to the octahedral metal in trans-configuration, whereas a pair of water molecules are located in octahedral cis-positions for owing to a small steric effect. A host cavity of sufficiently large size exists in the macrocyclic structure to trap the solvent molecules and the sulfate anion. The IR spectra have been used to assign the solvent molecules trapped and the sulfate anion which is shown as a bridged bidentate ligand. Thermal analyses show the stability of the macrocyclic backbone below 200 degrees C and gradual release processes of the trapped solvent molecules. Decomposition and oxidation of the dimetal macrocycle backbone occur at 300-500 degrees C, resulting in a metal sulfate. Further decomposition led to metal oxide at 500-600 degrees C.     

Solvent effect on the synthesis of clarithromycin: A molecular dynamics study

Clarithromycin (6-O-methylerythromycin A) is a 14-membered macrolide antibiotic which is active in vitro against clinically important gram-positive and gram-negative bacteria. The selectivity of the methylation of the C-6 OH group is studied on erythromycin A derivatives. To understand the effect of the solvent on the methylation process, detailed molecular dynamics (MD) simulations are performed in pure DMSO, pure THF and DMSO:THF (1:1) mixture by using the anions at the C-6, C-11 and C-12 positions of 2',4"-[O-bis(TMS)]erythromycin A 9-[O-(dimethylthexylsilyl)oxime] under the assumption that the anions are stable on the sub-nanosecond time scale. The conformations of the anions are not affected by the presence of the solvent mixture. The radial distribution functions are computed for the distribution of different solvent molecules around the 'O-' of the anions. At distances shorter than 5 A, DMSO molecules are found to cluster around the C-11 anion, whereas the anion at the C-12 position is surrounded by the THF molecules. The anion at the C-6 position is not blocked by the solvent molecules. The results are consistent with the experimental finding that the methylation yield at the latter position is increased in the presence of a DMSO:THF (1:1) solvent mixture. Thus, the effect of the solvent in enhancing the yield during the synthesis is not by changing the conformational properties of the anions, but rather by creating a suitable environment for methylation at the C-6 position.     

Ab initio molecular dynamics simulations of an excited state of X(-)(H(2)O)(3) (X = Cl, I) complex

Upon excitation of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters, the electron transfers from the anionic precursor to the solvent, and then the excess electron is stabilized by polar solvent molecules. This process has been investigated using ab initio molecular dynamics (AIMD) simulations of excited states of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters. The AIMD simulation results of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) are compared, and they are found to be similar. Because the role of the halogen atom in the photoexcitation mechanism is controversial, we also carried out AIMD simulations for the ground-state bare excess electron -- water trimer [e(-)(H(2)O)(3)] at 300 K, the results of which are similar to those for the excited state of X(-)(H(2)O)(3) with zero kinetic energy at the initial excitation. This indicates that the rearrangement of the complex is closely related to that of e(-)(H(2)O)(3), whereas the role of the halide anion is not as important.     

Self-assembly of PcOC8 and its sandwich lanthanide complex Pr(PcOC8)(2) with oligo(phenylene-ethynylene) molecules

Self-assemblies of octakis(octyloxy) phthalocyanine (PcOC8) and its sandwich lanthanide complex, substituted praseodymium bis(phthalocyanine) (Pr(PcOC8)(2)), with oligo(phenylene-ethynylene) (OPE) have been investigated by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface. The assemblies were prepared by dissolving the molecules in phenyloctane solution. It was found that both PcOC8 and Pr(PcOC8)(2) can form 4-fold or 6-fold symmetrical adlayers on HOPG. The intramolecular structure of Pr(PcOC8)(2) molecule was revealed by a high-resolution STM image. When OPE molecules are added into phenyloctane solvent, Pr(PcOC8)(2) molecules prefer to form an ordered adlayer at the top of the OPE adlayer, while PcOC8 molecules adsorb on HOPG surface directly and form separated domains with OPE. These results may be helpful to construct surface assemblies and develop molecular electronic devices in the future.     

Polymer brushes in solvents of variable quality: molecular dynamics simulations using explicit solvent

The structure and thermodynamic properties of a system of end-grafted flexible polymer chains grafted to a flat substrate and exposed to a solvent of variable quality are studied by molecular dynamics methods. The macromolecules are described by a coarse-grained bead-spring model, and the solvent molecules by pointlike particles, assuming Lennard-Jones-type interactions between pairs of monomers (epsilon(pp)), solvent molecules (epsilon(ss)), and solvent monomer (epsilon(ps)), respectively. Varying the grafting density sigma(g) and some of these energy parameters, we obtain density profiles of solvent particles and monomers, study structural properties of the chain (gyration radius components, bond orientational parameters, etc.), and examine also the profile of the lateral pressure P( parallel)(z), keeping in the simulation the normal pressure P( perpendicular) constant. From these data, the reduction of the surface tension between solvent and wall as a function of the grafting density of the brush has been obtained. Further results include the stretching force on the monomer adjacent to the grafting site and its variation with solvent quality and grafting density, and dynamic characteristics such as mobility profiles and chain relaxation times. Possible phase transitions (vertical phase separation of the solvent versus lateral segregation of the polymers into "clusters," etc.) are discussed, and a comparison to previous work using implicit solvent models is made. The variation of the brush height and the interfacial width of the transition zone between the pure solvent and the brush agrees qualitatively very well with corresponding experiments.     

Spectroscopic studies on Solvatochromism of mixed-chelate copper(II) complexes using MLR technique

Mixed-chelate copper(II) complexes with a general formula [Cu(acac)(diamine)]X where acac=acetylacetonate ion, diamine=N,N-dimethyl,N'-benzyl-1,2-diaminoethane and X=BPh(4)(-), PF(6)(-), ClO(4)(-) and BF(4)(-) have been prepared. The complexes were characterized on the basis of elemental analysis, molar conductance, UV-vis and IR spectroscopies. The complexes are solvatochromic and their solvatochromism were investigated by visible spectroscopy. All complexes demonstrated the positive solvatochromism and among the complexes [Cu(acac)(diamine)]BPh(4)·H(2)O showed the highest Δν(max) value. To explore the mechanism of interaction between solvent molecules and the complexes, different solvent parameters such as DN, AN, α and β using multiple linear regression (MLR) method were employed. The statistical results suggested that the DN parameter of the solvent plays a dominate contribution to the shift of the d-d absorption band of the complexes.     

A carboxylate-based dinuclear dysprosium(III) cluster exhibiting slow magnetic relaxation behaviour

Herein, we present a carboxylate-based dinuclear dysprosium compound, namely [Dy(2)L(6)(MeOH)(2)(H(2)O)(2)] (LH = n-butyric acid) from the reaction of Dy(NO(3))(3)·xH(2)O with n-butyric acid and triethylamine in MeOH solvent. The single crystal X-ray diffraction analysis demonstrate that a total of six monocarboxylate ligands formed this dimeric compound by carboxylate bridging along with coordination from solvent molecules (water and methanol). Each Dy(III) ion is coordinated by nine donor atoms forming a mono-capped antiprismatic coordination environment. Alternating current (AC) magnetic measurements show a frequency dependence of the out-of-phase magnetic susceptibilities (χ') indicating a slow relaxation behaviour of the magnetization.     

An efficient synthesis of acetylide/trimetal/acetylide molecular wires

Efficient syntheses are reported for incorporating trimetal units of the type M(3)(dpa)(4)(2+) (M = Cr, Co, Ni, and dpa = 2,2'-dipyridylamide) into polyalkynyl assemblies to give the prototypical bis-phenylacetylide complexes M(3)(dpa)(4)(CCPh)(2). Reactions of M(3)(dpa)(4)Cl(2) with LiCCPh have led only to mixtures of products which cocrystallize forming materials of the composition M(3)(dpa)(4)(CCPh)(x)()Cl(2)(-)(x)(). Here we report that acetonitrile complexes [M(3)(dpa)(4)(NCCH(3))(2)](PF(6))(2) react cleanly with LiCCPh in MeCN to afford the desired target molecules in 40-60% yield and in excellent purity. Isolation of the mixed ligand complex [Co(3)(dpa)(4)(NCCH(3))(CCPh)]PF(6) has been accomplished, which suggests that these reactions are stepwise and that it will be possible to synthesize mixed acetylide complexes (i.e., M(3)(dpa)(4)(CCR)(CCR')) via this method.