David C. Apperley

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.

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.

Molecular Rigidity in Dry and Hydrated Onion Cell Walls

Solid-state nuclear magnetic resonance relaxation experiments can provide information on the rigidity of individual molecules within a complex structure such as a cell wall, and thus show how each polymer can potentially contribute to the rigidity of the whole structure. We measured the proton magnetic relaxation parameters T2 (spin-spin) and T1p (spin-lattice) through the 13C-nuclear magnetic resonance spectra of dry and hydrated cell walls from onion (Allium cepa L.) bulbs. Dry cell walls behaved as rigid solids. The form of their T2 decay curves varied on a continuum between Gaussian, as in crystalline solids, and exponential, as in more mobile materials. The degree of molecular mobility that could be inferred from the T2 and T1p decay patterns was consistent with a crystalline state for cellulose and a glassy state for dry pectins. The theory of composite materials may be applied to explain the rigidity of dry onion cell walls in terms of their components. Hydration made little difference to the rigidity of cellulose and most of the xyloglucan shared this rigidity, but the pectic fraction became much more mobile. Therefore, the cellulose/xyloglucan microfibrils behaved as solid rods, and the most significant physical distinction within the hydrated cell wall was between the microfibrils and the predominantly pectic matrix. A minor xyloglucan fraction was much more mobile than the microfibrils and probably corresponded to cross-links between them. Away from the microfibrils, pectins expanded upon hydration into a nonhomogeneous, but much softer, almost-liquid gel. These data are consistent with a model for the stress-bearing hydrated cell wall in which pectins provide limited stiffness across the thickness of the wall, whereas the cross-linked microfibril network provides much greater rigidity in other directions.

Solid state 29Si NMR studies of apatite-type oxide ion conductors

Apatite-type silicates have been attracting considerable interest as a new class of oxide ion conductor, whose conduction is mediated by interstitial oxide ions. We report here the first 29Si solid state NMR studies of these materials with a systematic investigation of thirteen compositions. Our results indicate a correlation between the silicon environment and the observed conductivity. Specifically, samples which show poor conductivity demonstrate a single NMR resonance, whereas fast ion conducting compositions show more complex NMR spectra. For the oxygen excess samples La9M(SiO4)6O2.5 (M = Ca, Sr, Ba) two peaks are observed at chemical shifts of ≈−77.5 and −80.5 ppm, with the second peak correlated with a silicate group adjacent to an interstitial oxygen site. On Ti doping to give La9M(SiO4)6−x(TiO4)xO2.5 (x = 1,2) the second peak disappears, which is consistent with the “trapping” of interstitial oxygens by Ti and the consequent lowering in oxide ion conductivity. The samples La9.33(SiO4)6O2 and La9.67(SiO4)6O2.5 show a further third weak peak at a chemical shift (≈−85.0 ppm) consistent with the presence of some [Si2O7]6− units in these samples, due to condensation of two [SiO4]4− units. The effect of such condensation of [SiO4]4− units will be the creation of additional interstitial oxide ion defects, i.e. 2 [SiO4]4− → [Si2O7]6− + Oint2−. Overall, the results further highlight the importance of the [SiO4]4− substructure in these materials, and additionally suggest that 29Si NMR could potentially be used to screen apatite silicate materials for oxide ion conductivity

Spectroscopic and Structural Characterization of the CyNHC Adduct of B2pin2 in Solution and in the Solid State

The Lewis base adduct of B(2)pin(2) and the NHC (1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the β-borylation of α,β-unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process.

Synthesis, Structure, and Reactivity of Anionic sp(2) -sp(3) Diboron Compounds: Readily Accessible Boryl Nucleophiles.

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.

Chain conformation in concentrated pectic gels: evidence from 13C NMR

Abstract 13C NMR spectroscopy is a sensitive method of exploring the conformation of polysaccharides in both solid and gel states, because the positions of the peaks for the two carbons on either side of the glycosidic linkage, C-1 and C-4 for pectins, depend on the chain conformation. This approach was used to determine the conformation of calcium pectate at gel concentrations of ca. 0.3 g cm−3, comparable with those found in vivo. The solid acid, methyl-esterified, and sodium forms of pectate, which are known to have a right-handed, threefold helical (31) conformation, were used to define the spectral features of this conformation. These features were also present in both gel and solid calcium pectate, and were more conspicuous in the solid form. When dilute calcium pectate gels are dried they are known from CD and EXAFS evidence to undergo a conformational transition from the “egg-box” twofold helical (21) conformation to the 31 conformation. Calcium guluronate does not undergo this change, nor does calcium pectate in the presence of excess of monovalent cations. Changes in the 13C NMR spectrum marking this transition were observed on drying calcium pectate gels, were prevented by excess of K+, and were not found for guluronate. This enabled the spectral features corresponding to the 21 conformation to be identified. The spectral assignments were in agreement with the stereoelectronic theory of conformation-dependent chemical shifts, which also allowed a third set of peaks to be assigned to conformations dispersed between the 31 and 21 helices. About 70% of the pectate was visible in the gel spectra, the remaining chains being too mobile to be detected. It was concluded that the egg-box form, as dimers or larger aggregates, was the largest single component of the gels studied here, but that substantial quantities of 31 and intermediate helical aggregated forms were also present. Intact, pectin-containing plant cell walls also show spectral features characteristic of these components.

Synthesis and Characterization of a Rhodium(I) σ-Alkane Complex in the Solid State

Solid View of a Sigma Complex For decades, it has been clear from kinetic studies that saturated hydrocarbons can also act as weak ligands, often just prior to bond cleavage reactions. These short-lived intermediates—termed “σ complexes” because the donated electrons reside in single (sigma symmetry) C-H bonds—have been glimpsed spectroscopically but have largely eluded full structural characterization. Pike et al. (p. 1648, published online 23 August) now present the crystallographic characterization of an alkane bound to rhodium, which they captured by direct hydrogenation of a more stable crystalline precursor incorporating an alkene. Hydrogenation of a crystalline precursor enables structural characterization of a commonly evoked reaction intermediate. Transition metal–alkane complexes—termed σ-complexes because the alkane donates electron density to the metal from a σ-symmetry carbon–hydrogen (C–H) orbital—are key intermediates in catalytic C–H activation processes, yet these complexes remain tantalizingly elusive to characterization in the solid state by single-crystal x-ray diffraction techniques. Here, we report an approach to the synthesis and characterization of transition metal–alkane complexes in the solid state by a simple gas-solid reaction to produce an alkane σ-complex directly. This strategy enables the structural determination, by x-ray diffraction, of an alkane (norbornane) σ-bound to a d8–rhodium(I) metal center, in which the chelating alkane ligand is coordinated to the pseudosquare planar metal center through two σ-C–H bonds.

One-pot two-step mechanochemical synthesis: ligand and complex preparation without isolating intermediates

Although the use of ball milling to induce reactions between solids (mechanochemical synthesis) can provide lower-waste routes to chemical products by avoiding solvent during the reaction, there are further potential advantages in using one-pot multistep syntheses to avoid the use of bulk solvents for the purification of intermediates. We report here two-step syntheses involving formation of salen-type ligands from diamines and hydroxyaldehydes followed directly by reactions with metal salts to provide the corresponding metal complexes. Five salen-type ligands 2,2′-[1,2-ethanediylbis[(E)-nitrilomethylidyne]]bis-phenol, ‘salenH2’, 1; 2,2′-[(±)-1,2-cyclohexanediylbis-[(E)-nitrilomethylidyne]]bis-phenol, 2; 2,2′-[1,2-phenylenebis(nitrilomethylidyne)]-bis-phenol, ‘salphenH2’ 3; 2-[[(2-aminophenyl)imino]methyl]-phenol, 4; 2,2′-[(±)-1,2-cyclohexanediylbis[(E)-nitrilomethylidyne]]-bis[4,6-bis(1,1-dimethylethyl)]-phenol, ‘Jacobsen ligand’, 5) were found to form readily in a shaker-type ball mill at 0.5 to 3 g scale from their corresponding diamine and aldehyde precursors. Although in some cases both starting materials were liquids, ball milling was still necessary to drive those reactions to completion because precipitation of the product and or intermediates rapidly gave in thick pastes which could not be stirred conventionally. The only ligand which required the addition of solvent was the Jacobsen ligand 5 which required 1.75 mol equivalents of methanol to go to completion. Ligands 1–5 were thus obtained directly in 30–60 minutes in their hydrated forms, due to the presence of water by-product, as free-flowing yellow powders which could be dried by heating to give analytically pure products. The one-armed salphen ligand 4 could also be obtained selectively by changing the reaction stoichiometry to 1 : 1. SalenH21 was explored for the one-pot two-step synthesis of metal complexes. In particular, after in situ formation of the ligand by ball milling, metal salts (ZnO, Ni(OAc)2·4H2O or Cu(OAc)2·H2O) were added directly to the jar and milling continued for a further 30 minutes. Small amounts of methanol (0.4–1.1 mol equivalents) were needed for these reactions to run to completion. The corresponding metal complexes [M(salen)] (M = Zn, 6; Ni, 7; or Cu, 8) were thus obtained quantitatively after 30 minutes in hydrated form, and could be heated briefly to give analytically pure dehydrated products. The all-at-once ‘tandem’ synthesis of [Zn(salen)] 6 was also explored by milling ZnO, ethylene diamine and salicylaldehyde together in the appropriate mole ratio for 60 minutes. This approach also gave the target complex selectively with no solvent needing to be added. Overall, these syntheses were found to be highly efficient in terms of time and the in avoidance of bulk solvent both during the reaction and for the isolation of intermediates. The work demonstrates the applicability of mechanochemical synthesis to one-pot multi-step strategies.

Thermal conversion of a layered (Mg/Al) double hydroxide to the oxide

A new type of experiment involving temperature-variable in situ(one-dimensional)27Al MAS NMR studies, in which the sample is heated and studied directly in the NMR tube, has been developed. In the particular case chosen for study, the thermal conversion of the layered double hydroxide Mg6Al3.4(OH)18.82(CO3)1.5(NO3)0.364.5H2O to its corresponding oxide showed a diminution of the octahedral site and the formation of the tetrahedral site as the sample was heated. The experiment clearly showed that a tetrahedral site was formed by heating to 100 °C but the existence of a fivecoordinate site at elevated temperatures remains in doubt. Nutation experiments, however, on the layered double hydroxide showed that there are two types of octahedral environments which implies that the structure is more complex than has been considered previously. On slow heating to the oxide, the intensity of the peaks associated with one of the octahedral sites, in which the aluminium is probably associated with the carbonate, is decreased. Rapid heating in an oven at 450 °C produces a different oxide in which there are two octahedral and two tetrahedral sites. The rate of heating is clearly an important factor in the conversion of the layered double hydroxide (LDH) to the corresponding oxide (LDO).