Structural modifications to platinum(II) pincer complexes resulting in changes in their vapochromic and solvatochromic properties.

There is a need to develop rapidly responsive chemical sensors for the detection of low concentrations of volatile organic solvents (VOCs). Platinum pincer complexes have shown promise as sensors because of their colours and vapochromic and solvatochromic properties, that may be related to the non-covalent interactions between the pincer complexes and the guest VOCs. Here we report an investigation into a series of Pt(II) complexes based on the 1,3-di(pyridine)benzene tridentate (N⁁C⁁N) skeleton with the formula [Pt(N⁁C(R)⁁N)(CN)] (R = C(O)Me 2, C(O)OEt 3, C(O)OPh 4) with the fourth coordination site occupied by a cyanide ligand. Solid-state samples of the complexes have been tested with a range of volatiles including methanol, ethanol, acetone, dichloromethane and water, and while 2 displays thermochromism, 3 and 4 display rapidly reversible vapochromism and solvatochromism. These results are correlated with X-ray powder and single crystal X-ray structural data including an assessment of the crystal packing and the void space in the crystalline space. The cyanide ligand and the R substituents are involved in hydrogen bonding that creates the voids within the structures and interact with the solvent molecules that influence the Pt⋯Pt separation in the crystalline state.

Enhancement of Tb(III) -Cu(II) Single-Molecule Magnet Performance through Structural Modification.

We report a series of 3d-4f complexes {Ln2 Cu3 (H3 L)2 Xn } (X=OAc(-) , Ln=Gd, Tb or X=NO3 (-) , Ln=Gd, Tb, Dy, Ho, Er) using the 2,2'-(propane-1,3-diyldiimino)bis[2-(hydroxylmethyl)propane-1,3-diol] (H6 L) pro-ligand. All complexes, except that in which Ln=Gd, show slow magnetic relaxation in zero applied dc field. A remarkable improvement of the energy barrier to reorientation of the magnetisation in the {Tb2 Cu3 (H3 L)2 Xn } complexes is seen by changing the auxiliary ligands (X=OAc(-) for NO3 (-) ). This leads to the largest reported relaxation barrier in zero applied dc field for a Tb/Cu-based single-molecule magnet. Ab initio CASSCF calculations performed on mononuclear Tb(III) models are employed to understand the increase in energy barrier and the calculations suggest that the difference stems from a change in the Tb(III) coordination environment (C4v versus Cs ).

Diffraction evidence for the structure of cellulose microfibrils in bamboo, a model for grass and cereal celluloses.

BACKGROUND: Cellulose from grasses and cereals makes up much of the potential raw material for biofuel production. It is not clear if cellulose microfibrils from grasses and cereals differ in structure from those of other plants. The structures of the highly oriented cellulose microfibrils in the cell walls of the internodes of the bamboo Pseudosasa amabilis are reported. Strong orientation facilitated the use of a range of scattering techniques. RESULTS: Small-angle neutron scattering provided evidence of extensive aggregation by hydrogen bonding through the hydrophilic edges of the sheets of chains. The microfibrils had a mean centre-to-centre distance of 3.0 nm in the dry state, expanding on hydration. The expansion on hydration suggests that this distance between centres was through the hydrophilic faces of adjacent microfibrils. However in the other direction, perpendicular to the sheets of chains, the mean, disorder-corrected Scherrer dimension from wide-angle X-ray scattering was 3.8 nm. It is possible that this dimension is increased by twinning (crystallographic coalescence) of thinner microfibrils over part of their length, through the hydrophobic faces. The wide-angle scattering data also showed that the microfibrils had a relatively large intersheet d-spacing and small monoclinic angle, features normally considered characteristic of primary-wall cellulose. CONCLUSIONS: Bamboo microfibrils have features found in both primary-wall and secondary-wall cellulose, but are crystallographically coalescent to a greater extent than is common in celluloses from other plants. The extensive aggregation and local coalescence of the microfibrils are likely to have parallels in other grass and cereal species and to influence the accessibility of cellulose to degradative enzymes during conversion to liquid biofuels.

Structure of cellulose microfibrils in primary cell walls from collenchyma.

In the primary walls of growing plant cells, the glucose polymer cellulose is assembled into long microfibrils a few nanometers in diameter. The rigidity and orientation of these microfibrils control cell expansion; therefore, cellulose synthesis is a key factor in the growth and morphogenesis of plants. Celery (Apium graveolens) collenchyma is a useful model system for the study of primary wall microfibril structure because its microfibrils are oriented with unusual uniformity, facilitating spectroscopic and diffraction experiments. Using a combination of x-ray and neutron scattering methods with vibrational and nuclear magnetic resonance spectroscopy, we show that celery collenchyma microfibrils were 2.9 to 3.0 nm in mean diameter, with a most probable structure containing 24 chains in cross section, arranged in eight hydrogen-bonded sheets of three chains, with extensive disorder in lateral packing, conformation, and hydrogen bonding. A similar 18-chain structure, and 24-chain structures of different shape, fitted the data less well. Conformational disorder was largely restricted to the surface chains, but disorder in chain packing was not. That is, in position and orientation, the surface chains conformed to the disordered lattice constituting the core of each microfibril. There was evidence that adjacent microfibrils were noncovalently aggregated together over part of their length, suggesting that the need to disrupt these aggregates might be a constraining factor in growth and in the hydrolysis of cellulose for biofuel production.

How cellulose stretches: synergism between covalent and hydrogen bonding.

Cellulose is the most familiar and most abundant strong biopolymer, but the reasons for its outstanding mechanical performance are not well understood. Each glucose unit in a cellulose chain is joined to the next by a covalent C-O-C linkage flanked by two hydrogen bonds. This geometry suggests some form of cooperativity between covalent and hydrogen bonding. Using infrared spectroscopy and X-ray diffraction, we show that mechanical tension straightens out the zigzag conformation of the cellulose chain, with each glucose unit pivoting around a fulcrum at either end. Straightening the chain leads to a small increase in its length and is resisted by one of the flanking hydrogen bonds. This constitutes a simple form of molecular leverage with the covalent structure providing the fulcrum and gives the hydrogen bond an unexpectedly amplified effect on the tensile stiffness of the chain. The principle of molecular leverage can be directly applied to certain other carbohydrate polymers, including the animal polysaccharide chitin. Related but more complex effects are possible in some proteins and nucleic acids. The stiffening of cellulose by this mechanism is, however, in complete contrast to the way in which hydrogen bonding provides toughness combined with extensibility in protein materials like spider silk.

Exchange interactions at the origin of slow relaxation of the magnetization in {TbCu3} and {DyCu3} single-molecule magnets.

New {TbCu3} and {DyCu3} single-molecule magnets (SMMs) containing a low-symmetry Ln(III) center (shape measurements relative to a trigonal dodecahedron and biaugmented trigonal prism are 2.2-2.3) surrounded by three Cu(II) metalloligands are reported. SMM behavior is confirmed by frequency-dependent out-of-phase ac susceptibility signals and single-crystal temperature and sweep rate dependent hysteresis loops. The ferromagnetic exchange interactions between the central Ln(III) ion and the three Cu(II) ions could be accurately measured by inelastic neutron scattering (INS) spectroscopy and modeled effectively. The excitations observed by INS correspond to flipping of Cu(II) spins and appear at energies similar to the thermodynamic barrier for relaxation of the magnetization, ~15-20 K, and are thus at the origin of the SMM behavior. The magnetic quantum number M(tot) of the cluster ground state of {DyCu3} is an integer, whereas it is a half-integer for {TbCu3}, which explains their vastly different quantum tunneling of the magnetization behavior despite similar energy barriers.

Nanostructure of cellulose microfibrils in spruce wood.

The structure of cellulose microfibrils in wood is not known in detail, despite the abundance of cellulose in woody biomass and its importance for biology, energy, and engineering. The structure of the microfibrils of spruce wood cellulose was investigated using a range of spectroscopic methods coupled to small-angle neutron and wide-angle X-ray scattering. The scattering data were consistent with 24-chain microfibrils and favored a "rectangular" model with both hydrophobic and hydrophilic surfaces exposed. Disorder in chain packing and hydrogen bonding was shown to increase outwards from the microfibril center. The extent of disorder blurred the distinction between the I alpha and I beta allomorphs. Chains at the surface were distinct in conformation, with high levels of conformational disorder at C-6, less intramolecular hydrogen bonding and more outward-directed hydrogen bonding. Axial disorder could be explained in terms of twisting of the microfibrils, with implications for their biosynthesis.

Total syntheses of multiple cladiellin natural products by use of a completely general strategy.

The enantioselective total syntheses of 10 cladiellin natural products have been completed, starting from the known allylic alcohol (+)-14, which can be prepared in large quantities. The bridged tricyclic core of the cladiellins has been constructed via three ring-forming reactions: (i) an intramolecular reductive cyclization between an aldehyde and an unsaturated ester, mediated by samarium(II) iodide, to form a tetrahydropyranol; (ii) reaction of a metal carbenoid, generated from a diazo ketone, with an ether to produce an ylide-like intermediate that rearranges to produce E- or Z-oxabicyclo[6.2.1]-5-undecen-9-one; and (iii) a Diels-Alder cycloaddition reaction to construct the third ring found in the core structure of the cladiellins. The key ring-forming reaction, in which a diazo ketone is converted into a bridged bicyclic ether, can be tuned to give either of the isomeric oxabicyclo[6.2.1]-5-undecen-9-ones as the major product by switching from a copper to a rhodium catalyst and selecting the appropriate reaction conditions. The tricyclic products obtained from the three-step sequence involving the Diels-Alder cycloaddition reaction can be employed as advanced intermediates to prepare a wide range of cladiellin natural products.

Utilizing proton transfer to produce molecular salts in bromanilic acid substituted-pyridine molecular complexes - predictable synthons?

Controlled introduction of proton transfer into the design of a series of molecular complexes is described, delivering the systematic production of ionic molecular complexes (molecular salts). The controlled production of molecular salts has relevance as a potential strategy in the design of pharmaceutical materials. In nine molecular complexes consisting of bromanilic acid with the N-heterocyclic compounds 2-, 3- and 4-picoline [bis(2/3/4-methylpyridinium) 2,5-dibromo-3,6-dioxocyclohexa-1,4-diene-1,4-diolate, 2C6H8N(+)·C6Br2O4(2-)], 2,3-, 2,4-, 2,5- and 3,5-lutidine [2,3/2,4/2,5/3,5-dimethylpyridinium 2,5-dibromo-4-hydroxy-3,6-dioxocyclohexa-1,4-dien-1-olate, C7H10N(+)·C6HBr2O4(-)], and 3-bromo-4-methylpyridine [3-bromo-4-methylpyridinium 2,5-dibromo-4-hydroxy-3,6-dioxocyclohexa-1,4-dien-1-olate, C6H7BrN(+)·C6HBr2O4(-)] and 2-bromo-3-methylpyridine [2-bromo-3-methylpyridine-2,5-dibromo-3,6-dihydroxycyclohexa-2,5-diene-1,4-dione (1/1), C6H6BrN·C6H2Br2O4], proton transfer occurs readily between the bromanilic acid molecule and the N heteroatom of the pyridine ring, in all cases producing a charge-assisted bifurcated N-H...O hydrogen bond. This reinforces the value of this motif as a design tool in the crystal engineering of such complexes. The protonation state (and stoichiometry) significantly affect the supramolecular synthons obtained, but 1:2 stoichiometries reliably give rise to PBP synthons and 1:1 stoichiometries to PBBP synthons (where P indicates a methylpyridine co-molecule and B a bromanilic acid molecule). The influence of halogen interactions on the wider crystal packing is also discussed, with C-H...Br and Br...O interactions the most prevalent; only one Br...Br interaction is found.

Temperature dependent solid-state proton migration in dimethylurea-oxalic acid complexes.

The phenomenon of solid-state proton migration within molecular complexes containing short hydrogen bonds is investigated in two dimethylurea-oxalic acid complexes. Extensive characterisation by both X-ray and neutron diffraction shows that proton migration along the hydrogen bond can be induced in these complexes as a function of temperature. This emphasises the subtle features of the hydrogen bond potential well in such short hydrogen bonded complexes, both intrinsically and in the effect of the local crystalline environment. Based on these findings, the synthesis and analysis of a series of solid-state molecular complexes is shown to be a potential route to designing materials with tuneable proton migration effects.

Concise synthesis of the C-1-C-12 fragment of amphidinolides T1-T5.

The C-1-C-12 segment of the amphidinolides T1-T5 has been synthesised in an efficient manner. The key transformations are highly diastereoselective rearrangement of an oxonium ylide, or metal-bound ylide equivalent, produced by intramolecular reaction of a copper carbenoid with an allylic ether, and macrocyclic fragment coupling by one-pot ring-closing metathesis and hydrogenation.

Stereoselective synthesis of polyhydroxylated aminocyclohexanes.

The stereoselective synthesis of a series of di- and tri-hydroxylated aminocyclohexane derivatives has been developed. A one-pot, two step tandem process involving an Overman rearrangement and a ring closing metathesis reaction has been utilised for the asymmetric synthesis of (1S)-1-(2',2',2'-trichloromethylcarbonylamino)cyclohexa-2-ene. Oxidation of this cyclohexene derivative was then studied leading to the preparation of two diol analogues in excellent stereoselectivity. (1S)-1-(2',2',2'-trichloromethylcarbonylamino)cyclohexa-2-ene was then converted to a novel allylic alcohol via a 4,5-dihydro-1,3-oxazole. Functionalisation of this allylic alcohol by Upjohn dihydroxylation conditions or by a directed epoxidation/hydrolysis sequence of reactions allowed the synthesis of two dihydroconduramines in excellent stereoselectivity. The stereochemical assignment of all compounds prepared was confirmed by NOE experiments or X-ray structure determination.

Nanostructural deformation of high-stiffness spruce wood under tension.

Conifer wood is an exceptionally stiff and strong material when its cellulose microfibrils are well aligned. However, it is not well understood how the polymer components cellulose, hemicelluloses and lignin co-operate to resist tensile stress in wood. From X-ray scattering, neutron scattering and spectroscopic data, collected under tension and processed by novel methods, the ordered, disordered and hemicellulose-coated cellulose components comprising each microfibril were shown to stretch together and demonstrated concerted, viscous stress relaxation facilitated by water. Different cellulose microfibrils did not all stretch to the same degree. Attempts were made to distinguish between microfibrils showing large and small elongation but these domains were shown to be similar with respect to orientation, crystalline disorder, hydration and the presence of bound xylan. These observations are consistent with a major stress transfer process between microfibrils being shear at interfaces in direct, hydrogen-bonded contact, as demonstrated by small-angle neutron scattering. If stress were transmitted between microfibrils by bridging hemicelluloses these might have been expected to show divergent stretching and relaxation behaviour, which was not observed. However lignin and hemicellulosic glucomannans may contribute to stress transfer on a larger length scale between microfibril bundles (macrofibrils).

3-Fluoro-benzoic acid-4-acetyl-pyridine (1/1) at 100†K.

In the title compound, C(7)H(5)FO(2)·C(7)H(7)NO, a moderate-strength hydrogen bond is formed between the carboxyl group of one mol-ecule and the pyridine N atom of the other. The benzoic acid mol-ecule is observed to be disordered over two positions with the second orientation only 4% occupied. This disorder is also reflected in the presence of diffuse scattering in the diffraction pattern.

2-Acetyl-pyridinium bromanilate.

In the crystal of the title mol-ecular salt (systematic name: 2-acetyl-pyridinium 2,5-dibromo-4-hydr-oxy-3,6-dioxocyclo-hexa-1,4-dienolate), C(7)H(8)NO(+)·C(6)HBr(2)O(4) (-), centrosymmetric rings consisting of two cations and two anions are formed, with the components linked by alternating O-H⋯O and N-H⋯O hydrogen bonds. Short O⋯Br contacts [3.243 (2) and 3.359 (2) Å] may help to consolidate the packing.

Orientational disorder in 4-chloronitrobenzene.

The crystal structure of 4-chloronitrobenzene, C(6)H(4)ClNO(2), a material that exhibits disorder in the solid state, is re-examined using multiple-temperature single-crystal X-ray diffraction. Our results show a marked improvement on previous crystal structure determinations and our comprehensive multiple temperature measurements help to rationalize the structural anomalies. 4-Chloronitrobenzene exhibits twofold orientational disorder of the NO(2)/Cl substituents, with the molecule lying across an inversion centre. There is also evidence of large thermal motion, which exists at all temperatures and reflects the presence of significant disorder in this material. The nitro group shows possible libration, with one O atom exhibiting larger thermal motion than the other across the whole temperature range. This is explained by a difference in hydrogen-bonding environment.

Selective preparation of elusive and alternative single component polymorphic solid forms through multi-component crystallisation routes.

A transferable, simple, method for producing previously elusive and novel polymorphic forms of important active pharmaceutical ingredients (APIs; paracetamol (acetaminophen), piroxicam and piracetam) is demonstrated. Nitrogen heterocyclic co-molecules are employed to influence the self-assembly crystallisation process in a multi-component environment. Previously unknown solvates have also been synthesised by this method.

Electron density, disorder and polymorphism: high-resolution diffraction studies of the highly polymorphic neuralgic drug carbamazepine.

Analysis of neutron and high-resolution X-ray diffraction data on form (III) of carbamazepine at 100 K using the atoms in molecules (AIM) topological approach afforded excellent agreement between the experimental results and theoretical densities from the optimized gas-phase structure and from multipole modelling of static theoretical structure factors. The charge density analysis provides experimental confirmation of the partially localized π-bonding suggested by the conventional structural formula, but the evidence for any significant C-N π bonding is not strong. Hirshfeld atom refinement (HAR) gives H atom positional and anisotropic displacement parameters that agree very well with the neutron parameters. X-ray and neutron diffraction data on the dihydrate of carbemazepine strongly indicate a disordered orthorhombic crystal structure in the space group Cmca, rather than a monoclinic crystal structure in space group P2(1)/c. This disorder in the dihydrate structure has implications for both experimental and theoretical studies of polymorphism.

Monomeric and dimeric Al(iii) complexes for the production of polylactide.

A series of monometallic and bimetallic Al(iii) complexes with substituted naphthyl based Schiff base ligands have been prepared and characterised. When 1-aminonaphthalene based ligands were reacted with AlMe3 monometallic complexes were isolated, however, with 1,5 and 1,8-diaminonaphthalene based ligands bimetallic complexes were formed. In all cases 4-coordinate tetrahedral Al(iii) centres were observed in the solid state and in solution. There was little difference in rate of polymerisation of rac-lactide between the monometallic and bimetallic complexes based on 1,5-diaminonaphthalene. However, for the 1,8-diaminonaphthalene the complex was an order of magnitude faster than the monometallic and the analogous 1,5-system. Moreover, this complex was active at room temperature, which is rare for aluminium initiators, and PLA with a high degree (Pm = 0.82) of isotacticity was observed.