The crucial importance of agostic interactions in intermediates formed in propylene polymerization using neutral salicyladiminato palladium(II) and nickel complexes as catalysts

The structures of intermediates formed in propylene polymerisation using neutral salicyladiminato palladium(II) and nickel(II) complexes as catalysts have been investigated using density functional theory. Calculations show that all low energy intermediates contain agostic interactions either between the metal and a hydrogen from the added propylene forming four- or five-membered chelate rings, or, when a phenyl ring is present, between the metal and an aromatic C-C bond. The agostic interactions with the metal are concomitant with changes in ligand dimensions and electronic properties. In particular when a metal to hydrogen bond is formed, there is a lengthening of the C-H bond. Significant differences are found for the agostic interactions with palladium and nickel in that for Pd there is a clear preference for specific intermediates but for Ni there are several different structures with similar energies which are likely to lead to a greater variety of products on further polymerisation. (c) 2007 Elsevier B.V. All rights reserved.

Structural variations in Ni(II) complexes of salen type di-Schiff base ligands

Three di-Schiff-base ligands, N,N'-bis(salicylidene)-1,3-propanediamine (H(2)Salpn), N,N'-bis(salicylidene)-1,3-pentanedianiine (H(2)Salpen) and N,N'-bis(salicylidine)-ethylenediamine (H(2)Salen) react with Ni(SCN)(2). 4H(2)O in 2:3 molar ratios to form the complexes; mononuclear [Ni(HSalpn)(NCS)(H2O)]center dot H2O (1a), trinuclear [{Ni(Salpen)}(2)Ni(NCS)(2)] (2b) and trinuclear [{Ni(Salen)}(2)Ni(NCS)(2)] (3) respectively. All the complexes have been characterized by elemental analyses, IR and UV-VIS spectra, and room temperature magnetic susceptibility measurements. The structures of la and 2b have been confirmed by X-ray single crystal analysis. In complex la, the Ni(II) atom is coordinated equatorially by the tetradentate, mononegative Schiff-base, HSalpn. Axial coordination of isothiocyanate group and a water molecule completes its octahedral geometry. The hydrogen atom attached to one of the oxygen atoms of the Schiff base is involved in a very strong hydrogen bond with a neighboring unit to form a centrosymmetric dimer. In 2b, two square planar [Ni(Salpen)] units act as bide mate oxygen donor ligands to a central Ni(II) which is also coordinated by two mutually cis N-bonded thiocyanate ligands to complete its distorted octahedral geometry. Complex 3 possesses a similar structure to that of 2b. A dehydrated form of la and a hydrated form of 2b have been obtained and characterized. The importance of electronic and steric factors in the variation of the structures is discussed. (c) 2007 Elsevier Ltd. All rights reserved.

Experimental and theoretical study of the infrared spectra of BrHI- and BrDI-

Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 mum (590-1666 cm-1) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated. (C) 2004 American Institute of Physics.

Nitrogen donor ligands bearing N-H groups: Effect on catalytic and cytotoxic activity of molybdenum eta(3)-allyldicarbonyl complexes

Reactions of [Mo(eta(3)-C3H5)Br(CO)(2)(NCMe)(2)] with the bidentate nitrogen ligands 2-(2'-pyridyl)imidazole (L1), 2-(2'-pyridyl)benzimidazole (L2), N,N'-bis(2'-pyridinecarboxamido)-1,2-ethane (L3), and 2,2'-bisimidazole (L4) led to the new complexes [Mo(eta(3)-C3H5)Br(CO)(2)(L)] (L = L1, 1; L2, 2; L4, 4) and [{Mo(eta(3)-C3H5) Br(CO)(2)}(2)(mu-L-3)] (3). The reaction of complexes 2 and 3 with Tl[CF3SO3] afforded [Mo(eta(3)-C3H5)(CF3SO3)(CO)(2)(L2)] (2T) and [{Mo(eta(3)-C3H5)(CF3SO3)(CO)(2)}(2)(mu-L-3)] (3T). Complexes 3 and 2T were structurally characterized by single crystal X-ray diffraction, showing the facial allyl/carbonyls arrangement and the formation of the axial isomer. In 2T, two molecules are assembled in a hydrogen bond dimer. The four complexes 1-4 were tested as precursors in the catalytic epoxidation of cyclooctene and styrene, in the presence of t-butylhydroperoxide (TBHP), with moderate conversions and turnover frequencies for complexes 1-3 and very low ones for 4. The increasing number of N-H groups in the complexes seems to be responsible for the loss of catalytic activity, compared with other related systems. The cytotoxic activities of all the complexes were evaluated against HeLa cells. The results showed that compounds 1,2,4, and 2T exhibited significant activity, complexes 2 and 2T being particularly promising. (C) 2008 Elsevier B.V. All rights reserved.

Stochiometrically governed molecular interactions in drug: Poloxamer solid dispersions

This study probes the molecular interactions between model drugs and poloxamers that facilitate dissolution rate improvements using solid dispersions. Ibuprofen and ketoprofen solid dispersions were prepared at different mole ratios using poloxamers 407 and 188. The carbonyl stretching vibration of the ibuprofen dimer shifted to higher wavenumber in the infrared spectra of 2:1 drug:carrier mole ratio solid dispersions, indicating disruption of the ibuprofen dimer concomitant with hydrogen bond formation between the drug and carrier. Solid dispersions with mole ratios >2:1 drug:carrier (up to 29:1) showed both ibuprofen hydrogen-bonded to the poloxamer, and excess drug present as dimers. X-ray diffraction studies confirmed these findings with no evidence of crystalline drug in 2:1 mole ratio systems whereas higher drug loadings retained crystalline ibuprofen. Similar results were found with ketoprofen-poloxamer solid dispersions. Thermal analysis of ibuprofen-poloxamer 407 solid dispersions and their resultant phase diagram suggested solid solutions and a eutectic system were formed, depending on drug loading. Dissolution studies showed fastest release from the solid solutions; dissolution rates from solid solutions were 12-fold greater than the dissolution of ibuprofen powder whereas the eutectic system gave a 6-fold improvement over the powder. When designing solid dispersions to improve the delivery of poorly-water soluble drugs, the nature of drug:carrier interactions, which are governed by the stochiometry of the composition, can affect the dissolution rate improvement.

Construction of supramolecular helices and breaking the helicity by forming supramolecular β-sheet structures using suitable self-assembling pseudopeptide building blocks

Bis-valine derivatives or malonamide (Guha,S.; Drew, M.G.B. Small 2008, 4, 1993-2005) and a bis-valine derivative of 1,1-cyclopropone dicarboxamide were used as building blocks for the construction of supramolecular helical structures. The six-membered intramolecular hydrogen-bonded scaffold is formed, and this acts as a unique supramolecular synthon for the construction of a pseudopeptide-based supramolecular helical structure. However, in absence of this intramolecular hydrogen bond. intermolecular hydrogen bonds are formed among the peptide strands. This leads to a supramolecular beta-sheet structure. Proper selection of the supramolecular synthon (six-membered intramolecular hydrogenbonded scaffold) promotes supramolecular helix formation, and a deviation from this molecular structure dictates the disruption of supramolecular helicity. In this study, six crystal structures have been used to demonstrate that a change in the central angle and/or the central core structure of dicarboxamides can be used to design either a supramolecular helix or a beta-sheet.

Conformational polymorphism of the molecular complex of 3-fluorobenzoic acid with 4-acetylpyridine

Two polymorphs of the molecular complex formed between 3-fluorobenzoic acid with 4-acetylpyridine are described and found to be based upon the same dimeric supramolecular construct. The conformational freedom around the hydrogen bond results in a 180 degrees rotation about this intermolecular link, distinguishing the polymorphs and affecting the packing of the dimeric units. The two polymorphs are fully characterised by single crystal X-ray and neutron diffraction and quantum mechanical calculations. There is evidence of structured crystal growth defects in both polymorphic crystals via observation of diffuse scattering and a disorder model for the average structure of Form I, which can be interpreted as a mixing of the two dimer conformations. The similarity of energy of the distinct dimeric units, supporting their likely co-existence, has been verified by periodic quantum chemical calculations.

Conformational studies of mixed-ligand copper(II) chelates. Crystal and molecular structure of (N,N-dimethyl-N′-ethyl-1,2-diaminoethane-(1-phenyl-1,3,-butanedionato)copper(II) perchlorate

The IR and ligand field spectra and the structure of the mixed-ligand compound [N,N-dimethyl-N′-ethyl-1,2-diaminoethane(1-phenyl-1,3-butanedionato)(perchlorato)copper(II)]), [Cu(dmeen)bzac(OClO3)], are reported. The structure was determined by single crystal X-ray diffraction analysis (triclinic, space group ). The structure is square pyramidal with the apical position occupied by one oxygen of the tetrahedral perchlorato group (distance from copper 2.452(5) Å). The plane of the phenyl ring is tilted forming an angle of 16.72(14)° with the plane of the β-dionato moiety. The nitrogenous base adopts the gauche conformation with torsional angle of 108.72(14)°. The ethyl group is cis oriented relative to the phenyl group, occupying the equatorial position with the vector of the carbon-nitrogen bond forming an angle of 143.9(3)° with the CuNN plane. The interactions of the adjacent axial hydrogen with an oxygen of the perchlorato group result in hydrogen bond formation. The IR spectra reveal that in the solid state the Br− or Cl− displace easily the ClO4− group. The shifts in the ligand field spectra indicate that polar solvents participate in donor-acceptor interactions with the metal centre along an axis perpendicular to the CuN2O2 plane.

A very rare self-assembled ribbon of fused cyclic water pentamers encapsulated in a Cu-II based metal-organic framework

A metal organic framework of Cu-II, tartarate (tar) and 2,2'-bipyridyl (2,2'-bipy)], {[Cu(tar)(2,2'-bipy)]center dot 5H(2)O}(n)} (1) has been synthesized at the mild ambient condition and characterized by single crystal X-ray crystallography. In the compound, the Cu(2,2'-bipy) entities are bridged by tartarate ions which are coordinated to Cu-II by both hydroxyl and monodentate carboxylate oxygen to form a one-dimensional chain. The non-coordinated water molecules form ID water chains by edge-sharing cyclic water pentamers along with dangling water dimers. It shows reversible water expulsion upon heating. The water chains join the ID coordination polymeric chains to a 31) network through hydrogen-bond interactions.

Crystal design approaches for the synthesis of paracetamol co-crystals

Crystal engineering principles were used to design three new co-crystals of paracetamol. A variety of potential cocrystal formers were initially identified from a search of the Cambridge Structural Database for molecules with complementary hydrogen-bond forming functionalities. Subsequent screening by powder X-ray diffraction of the products of the reaction of this library of molecules with paracetamol led to the discovery of new binary crystalline phases of paracetamol with trans-1,4- diaminocyclohexane (1); trans-1,4-di(4-pyridyl)ethylene (2); and 1,2-bis(4-pyridyl)ethane (3). The co-crystals were characterized by IR spectroscopy, differential scanning calorimetry, and 1H NMR spectroscopy. Single crystal X-ray structure analysis reveals that in all three co-crystals the co-crystal formers (CCF) are hydrogen bonded to the paracetamol molecules through O−H···N interactions. In co-crystals (1) and (2) the CCFs are interleaved between the chains of paracetamol molecules, while in co-crystal (3) there is an additional N−H···N hydrogen bond between the two components. A hierarchy of hydrogen bond formation is observed in which the best donor in the system, the phenolic O−H group of paracetamol, is preferentially hydrogen bonded to the best acceptor, the basic nitrogen atom of the co-crystal former. The geometric aspects of the hydrogen bonds in co-crystals 1−3 are discussed in terms of their electrostatic and charge-transfer components.

GRID and docking analyses reveal a molecular basis for flavonoid inhibition of src-family kinase activity

Flavonoids reduce cardiovascular disease risk through anti-inflammatory, anti-coagulant and anti-platelet actions. One key flavonoid inhibitory mechanism is blocking kinase activity that drives these processes. Flavonoids attenuate activities of kinases including phosphoinositide-3-kinase (PI3K), Fyn, Lyn, Src, Syk, PKC, PIM1/2, ERK, JNK, and PKA. X-ray crystallographic analyses of kinase-flavonoid complexes show that flavonoid ring systems and their hydroxyl substitutions are important structural features for their binding to kinases. A clearer understanding of structural interactions of flavonoids with kinases is necessary to allow construction of more potent and selective counterparts. We examined flavonoid (quercetin, apigenin and catechin) interactions with Src-family kinases (Lyn, Fyn and Hck) applying the Sybyl docking algorithm and GRID. A homology model (Lyn) was used in our analyses to demonstrate that high quality predicted kinase structures are suitable for flavonoid computational studies. Our docking results revealed potential hydrogen bond contacts between flavonoid hydroxyls and kinase catalytic site residues. Identification of plausible contacts indicated that quercetin formed the most energetically stable interactions, apigenin lacked hydroxyl groups necessary for important contacts, and the non-planar structure of catechin could not support predicted hydrogen bonding patterns. GRID analysis using a hydroxyl functional group supported docking results. Based on these findings, we predicted that quercetin would inhibit activities of Src-family kinases with greater potency than apigenin and catechin. We validated this prediction using in vitro kinase assays. We conclude that our study can be used as a basis to construct virtual flavonoid interaction libraries to guide drug discovery using these compounds as molecular templates.

The fundamental vibration-rotation band of hydrogen chloride

The fundamental vibration-rotational absorption band of hydrogen chloride near 3 45,t has been remeasured using higher resolving power than previously. The wave-lengths of the absorption lines have been determined more precisely, and the isotopic splitting of lines has been completely resolved. The results have provided new and more satisfactory values for the rotational constants Bi, and the centrifugal stretching constants Di, and their relative values for the two isotopic species agree closely with what is to be expected for the difference in mass. The positions of the lines in the pure rotational absorption spectrum have been calculated from the derived data, and agree closely with those recently observed. The bond lengths re for each isotopic species H35C1 and H37C1 is found to be 1-2744A.

Intensities in infrared bands V: Bond polar properties in dimethylacetylene

The absolute intensities of all except one of the infra-red fundamental vibration bands of dimethyl acetylene have been determined, and the results have been used to compute polar properties of the C—H and C—C bonds. It has been found that if the very probable assumption is made that the acetylenic carbon atoms carry a residual negative charge, the hydrogen atoms in the C—H bonds must carry a residual positive charge. The probable value of the C—H dipole is about 04 Debye, and that of the C—C bond about 1 Debye. Comparisons have been made with the results of similar work with related molecules.

Self-assembled healable materials through a combination of pi-pi stacking and hydrogen bonding

The discovery of polymers with stimuli responsive physical properties is a rapidly expanding area of research. At the forefront of the field are self-healing polymers, which, when fractured can regain the mechanical properties of the material either autonomically, or in response to a stimulus. It has long been known that it is possible to promote healing in conventional thermoplastics by heating the fracture zone above the Tg of the polymer under pressure. This process requires reptation and subsequent re-entanglement of macromolecules across the fracture void, which serves to bridge, and ‘heal’ the crack. The timescale for this mechanism is highly dependent on the molecular weight of the polymer being studied. This process is in contrast to that required to affect healing in supramolecular polymers such as the plasticised, hydrogen bonded elastomer reported by Leibler et al. The disparity in bond energies between the non-covalent and covalent bonds within supramolecular polymers results in fractures propagating through scission of the comparatively weak supramolecular interactions, rather than through breaking the stronger, covalent bonds. Thus, during the healing process the macromolecules surrounding the fracture site only need sufficient energy to re-engage their supramolecular interactions in order to regenerate the strength of the pristine material. Herein we describe the design, synthesis and optimization of a new class of supramolecular polymer blends that harness the reversible nature of pi-pi stacking and hydrogen bonding interactions to produce self-supporting films with facile healable characteristics.

The surface geometry of carbonmonoxide and hydrogen co-adsorbed on Ni 111

A combination of photoelectron spectroscopy, temperature programmed desorption and low energy electron diffraction structure determinations have been applied to study the p(2 x 2) structures of pure hydrogen and co-adsorbed hydrogen and CO on Ni {111}. In agreement with earlier work atomic hydrogen is found to adsorb on fcc and hcp sites in the pure layer with H-Ni bond lengths of 1.74Angstrom. The substrate interlayer distances, d(12) = 2.05Angstrom and d(23) = 2.06Angstrom, are expanded with respect to clean Ni {111} with buckling of 0.04Angstrom in the first layer. In the co-adsorbed phase Co occupies hcp sites and only the hydrogen atoms on fcc sites remain on the surface. d(12) is even further expanded to 2.08Angstrom with buckling in the first and second layer of 0.06 and 0.02Angstrom, respectively. The C-O, C-Ni, and H-Ni bond lengths are within the range of values also found for the pure adsorbates.