Erica Brendler

Inorganic Molten Salts as Solvents for Cellulose

Inorganic molten salts can be used as efficient solvents for cellulose in a wide range of degrees of polymerization. Furthermore, molten salts can be applied as reaction medium for the derivatization of cellulose. For both dissolution and derivatization of cellulose, knowledge of the solution state as well as information about chemical interactions with the solvent system is essential. Using the melts of LiClO4·3H2O, NaSCN/KSCN/LiSCN·2H2O and LiCl/ZnCl2/H2O as cellulose solvents, factors which determine the dissolving ability will be discussed. Besides the specific structure of the molten salt hydrate, the cation and the water content of the melt are the most important factors for the dissolving capability of a molten salt hydrate system. FT-Raman spectroscopy, 7Li and 13C NMR spectroscopy were applied to describe solvent–cellulose interactions and the state of cellulose dissolved in the molten salts. Using Raman and solid state NMR spectroscopy it was proved that cellulose is amorphous in the frozen solvent system. The application of inorganic molten salts as a medium for cellulose functionalization is demonstrated for cellulose carboxymethylation and acetylation.

Structural changes of cellulose dissolved in molten salt hydrates

The behavior of cellulose in molten salt hydrates, e.g., LiClO 4 *3H 2 O, ZnCl 2 *4H 2 O, LiCl*5H 2 O and mixtures of thiocyanates was investigated. Melts, which give rise to a swelling or dissolution of the cellulose were the focus of interest. The melt composition and species, the water amount and the structure of the coordination sphere of the cation of the molten salt hydrate as well as the acidity influence the solubility of the cellulose in molten salt systems. The interactions between cellulose and species of the melts were determined by solution state 13 C NMR measurements at temperatures between 65 °C and 140 °C. Each regenerated cellulose showed other but specific properties which were characterized by wide angle X-ray Scattering (WAXS), solid-state NMR ( 13 C CP/MAS NMR), molecular weight distribution, surface-area determinations and scanning electron microscopy (SEM).

29Si DFT/NMR Observation of Spin–Orbit Effect in Metallasilatrane Sheds Some Light on the Strength of the Metal→Silicon Interaction

Relativistic effects in structural chemistry have long fascinated chemists. Moreover, special relativity can induce spectacularly large effects on NMR parameters. A wellknown example is the normal halogen dependence (NHD) of isotropic shifts, which is a spin–orbit (SO) relativistic effect (a “heavy-atom effect” on a light atom, or HALA). Experimental clues for relativistic effects on chemical shift tensors are not as commonly looked for. Forgeron et al. demonstrated that relativistic effects play an important role in xenon NMR spectroscopy. In this case, the heavy-atom effect is on the heavy-atom NMR itself (HAHA): For XeF2, a 1000 ppm Xe chemical shift tensor component parallel to the molecular axis was found experimentally. In the absence of relativistic effects, this component is zero by symmetry. More recently, Kantola et al. showed that the well-known NHD in methyl halides also has a strong impact on the C shielding anisotropies. In addition to these particular examples, considering strong relativistic effects on NMR properties may in general turn out to be essential in the characterization of various classes of compounds, as shown in the literature, for example, for complexes with Ptand Au-bound O atoms, mercury organic compounds, haloboranes, and various phosphine complexes of main group and transition metals (TM). Furthermore, fundamental insights into unusual bonding situations in novel compounds may be achieved only with the aid of models that take spin–orbit interactions into account. As to the latter, one such novel bonding situation is represented by the formal s-donation from an electron-rich TM (d systems: Pd, Pt ; d system: Au) as a Lewis base towards a Lewis acidic Group 14 element center (Si). Recently, Bourissou and co-workers reported on an aurasilatrane with a rather long Au···Si separation (3.090(2) ) and a somewhat upfield-shifted Si resonance (dSi = 21.4 ppm for the formally pentacoordinate Si atom in an Si(F,C(aryl)3,Au) environment). [9] In contrast (from the Si NMR perspective), two of us presented the paddlewheel-shaped compounds 1 and 2 comprising d-donor sites in closer proximity to the Si atom (Figure 1) and Si resonances in the typical range of hexacoordinate Si atoms, which proved the metal atoms to be efficient lone-pair donors.

Metallasilatranes: Palladium(II) and Platinum(II) as Lone-Pair Donors to Silicon(IV)†

top). In their first so-called metallaboratrane II the ruthenium atom has a lone pair directed towards the Lewis acidic boron atom (according to isolobal considerations). Within the past decade numerous examples of metallaboratranes followed and have been reviewed and commented on recently. Although further buttressing clamps (e.g., 7-azaindole instead of methimazole) are under investigation for metallaboratrane syntheses, these still rely on the oxidative addition of a B H bond. The direct formation of metallametallatranes, that is, cage compounds comprising Au!B, Au!Ga, and Au!Al interactions (III), was reported by Bourissou et al. Braunschweig et al. reported on the compounds IV and V exhibiting Pt!Al and Pt!Be interactions (Scheme 1, bottom) and related gallium-containing systems as examples for the straightforward formation of TM!E interactions (E = main-group element), and Fischer et al. reported compounds containing Rh!Ga bonds. Whereas above examples exhibit transition metal sdonation towards electron-deficient Lewis acidic sites (e.g., Be, B, Al, Ga), similar interactions towards Lewis acidic centers which formally contain an octet shell (e.g., Si with at least four additional substituents) should in principle be feasible. First steps date back to the work of Grobe et al., who already proposed the potential capability of electron-rich Ni, Pd, and Pt complexes to establish donor interactions to transannular located silicon atoms (e.g., VI, Scheme 1). Unfortunately, these initial approaches towards metallasilatranes yielded compounds which exhibit rather long M–Si separations (ca. 3.5–4 ). We now report on the first hypercoordinate silicon complexes which contain electron-rich transition-metal atoms as formal lone-pair s-donors in their ligand spheres. These compounds are accessible in good yield, and strong electronic interactions between the Si atom and the Pd or Pt atoms in their donor sphere are demonstrated by X-ray diffraction analyses and Si NMR spectroscopy. Methimazolylsilanes, bearing the 2-mercapto-1-methylimidazolide ligand as a buttress capable of bridging hard and soft metal sites, were already considered as an entry into metallasilatrane chemistry in our recent studies. Whereas in an initial attempt, Pt underwent oxidative addition into the methimazole C=S bond under formation of a carbene complex, the present report shows that Pd and Pt indeed react with methimazolylsilanes under formation of metallasilatranes. Reaction of SiCl4 with 2-mercapto-1-methylimidazole (methimazole, Hmt) supported by triethylamine provides access to silanes ClSi(mt)3 and Si(mt)4 (Scheme 2, top). The molecular structures of these silanes, as determined by X-ray crystallography, clearly show the tetracoordination of the Si atoms. The S-donor sites remain in distances between 3.30 and 3.35 from the Si atoms. Whereas Si(mt)4 appears stable in aprotic solution (e.g., anhydrous chloroform), silane ClSi(mt)3 undergoes ligand exchange in chloroform solution, thus giving the silanes Cl2Si(mt)2, ClSi(mt)3, and Si(mt)4 (approximate molar ratio 1:5:1 as determined from the Si NMR signals at d = 41.2, 50.5, and 59.1 ppm, respectively) as potential reactants for transition-metal complexes. Although ClSi(mt)3 remains the predominant silane species in chloroform solutions of this compound, [PdCl2(PPh3)2] and cis-[PtCl2(PPh3)2] are susceptible to coordination of Si(mt)4 generated in the exchange equilibrium (Scheme 2, Scheme 1. Selected examples of transition-metal base complexes with electrophilic main-group-element sites.

METHYLCHLOROOLIGOSILANES AS PRODUCTS OF THE BASECATALYSED DISPROPORTIONATION OF VARIOUS METHYLCHLORODISILANES

Abstract The methylchlorodisilanes SiCl 2 MeSiCl 2 Me ( 1 ), SiCl 2 MeSiClMe 2 ( 2 ) and SiClMe 2 SiClMe 2 ( 3 ) disproportionate in the presence of a basic catalyst into methylchloromonosilanes and various methylchlorooligosilanes. Oligosilanes involving up to seven silicon atoms were identified by means of 29 Si-, 13 C- 1 H-NMR and GC-MS measurements. Formation of methylchlorooligosilanes is thoughtto take place via silylene intermediates.

Octahedral HSiCl3 and HSiCl2Me adducts with pyridines.

Stable solid adducts of substituted pyridines (Rpy) with HSiCl(3) and HSiCl(2)Me were prepared in high yields under aprotic and anaerobic conditions at room temperature. The octahedral complexes of HSiCl(3) underwent dismutation reactions in polar solvents. In contrast, the HSiCl(2)Me(Rpy)(2) adducts were not susceptible to dismutation under comparable conditions, but they tended to dissociate more easily because of the reduced Lewis acidity of HSiCl(2)Me relative to HSiCl(3). The bonding between silicon and its surrounding ligands is highly ionic, as can be seen from QTAIM and charge distribution analyses. (29)Si CP/MAS spectra in combination with quantum-chemical calculations show that the lowest shielding is along the Cl-Si-Cl axis. The other two components of the shielding tensor are oriented along the N-Si-N and H-Si-Cl/Me axes. It is known that many reactions of (hydrido)chlorosilanes are catalyzed by pyridine bases. Therefore, the results presented here provide a basis for better control of these reactions, especially chlorine substitution and hydrosilylation.

Atomic Contributions from Spin-Orbit Coupling to 29Si NMR Chemical Shifts in Metallasilatrane Complexes

New members of a novel class of metallasilatrane complexes [X-Si-(μ-mt)(4)-M-Y], with M=Ni, Pd, Pt, X=F, Cl, Y=Cl, Br, I, and mt=2-mercapto-1-methylimidazolide, have been synthesized and characterized structurally by X-ray diffraction and by (29)Si solid-state NMR. Spin-orbit (SO) effects on the (29)Si chemical shifts induced by the metal, by the sulfur atoms in the ligand, and by heavy halide ligands Y=Cl, Br, I were investigated with the help of relativistic density functional calculations. Operators used in the calculations were constructed such that SO coupling can selectively be switched off for certain atoms. The unexpectedly large SO effects on the (29)Si shielding in the Ni complex with X=Y=Cl reported recently originate directly from the Ni atom, not from other moderately heavy atoms in the complex. With respect to Pd, SO effects are amplified for Ni owing to its smaller ligand-field splitting, despite the smaller nuclear charge. In the X=Cl, Y=Cl, Br, I series of complexes the Y ligand strongly modulates the (29)Si shift by amplifying or suppressing the metal SO effects. The pronounced delocalization of the partially covalent M←Y bond plays an important role in modulating the (29)Si shielding. We also demonstrate an influence from the X ligand on the (29)Si SO shielding contributions originating at Y. The NMR spectra for [X-Si-(μ-mt)(4)-M-Y] must be interpreted mainly based on electronic and relativistic effects, rather than structural differences between the complexes. The results highlight the sometimes unintuitive role of SO coupling in NMR spectra of complexes containing heavy atoms.

Stannylene or Metallastanna(IV)ocane: A Matter of Formalism

The s basicity of electron-rich transition metals (TMs) plays a crucial role in Brønsted acid–base reactions of TM complexes, such as [H2Fe(CO)4] and [HCo(CO)4] (strong acids, poor s-basicity of the corresponding conjugate bases) and was shown to increase upon coordination of good donor ligands L, such as phosphines; that is, lowered acidity of [H2Fe(CO)3(PPh3)] or [HCo(CO)3(PPh3)]. [2] Thus, P and/or S donors bearing electron-rich TM centers have been shown to support s donation towards other main-group-element (E) Lewis acidic centers, for example in the so-called metallaboratranes I and II and Be, Al, and Ga compounds of type III (Scheme 1). Very recently, we have described compounds IV–VII comprising {L5TM(d )} moieties that exhibit s donation towards electronically saturated Lewis acidic centers E, that is, Si and Sn. Gabba et al. have reported similar intermetallic interactions in the heterobimetallic complexes VIII–X (Scheme 1), which comprise d TM donor sites with an almost square-planar coordination sphere. Whereas compounds IV–X were obtained by a straightforward route starting from sources that comprise TM and E in the desired oxidation states, herein we present a (formal) redox approach, which involves a reaction sequence starting from a stannylene (SnCl2) and yielding hypercoordinate tin compounds that can be regarded as palladastanna(IV)ocanes. In a convenient one-pot synthesis, [PdCl2(PPh3)2] was treated with the potassium salt of 1-methyl-2-mercaptoimidazole (methimazole, Hmt) and [SnCl2(dioxane)] (Scheme 2) to afford compound 1. Substitution of the tin-bound chlorine atoms with a dianionic tridentate ligand afforded compound 2, which comprises a hexacoordinate tin atom (Scheme 2). Reference compounds 3 and 4 (comprising Sn and Sn, respectively, and the same tridentate ONN ligand as 2) were prepared as references for spectroscopic properties. The molecular structures of 1–4 were confirmed crystallographically (see Figure 1 and the Supporting Information). Scheme 1. Selected examples of TM–base complexes with electrophilic main-group-element sites (“Z-type ligands”). Cy = cyclohexyl.

Octahedral Adducts of Dichlorosilane with Substituted Pyridines: Synthesis, Reactivity and a Comparison of Their Structures and 29Si NMR Chemical Shifts

H(2)SiCl(2) and substituted pyridines (Rpy) form adducts of the type all-trans-SiH(2*)Cl(2)2 Rpy. Pyridines with substituents in the 4- (CH(3), C(2)H(5), H(2)C=CH, (CH(3))(3)C, (CH(3))(2)N) and 3-positions (Br) give the colourless solids 1 a-f. The reaction with pyrazine results in the first 1:2 adduct (2) of H(2)SiCl(2) with an electron-deficient heteroaromatic compound. Treatment of 1 d and 1 e with CHCl(3) yields the ionic complexes [SiH(2)(Rpy)(4)]Cl(2*)6 CHCl(3) (Rpy=4-methylpyridine (3 d) and 4-ethylpyridine (3 e)). All products are investigated by single-crystal X-ray diffraction and (29)Si CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the Si-N distances vary between 193.3 pm for 1 c (4-(dimethylamino)pyridine complex) and 197.3 pm for 2. Interestingly, the pyridine moieties are coplanar and nearly in an eclipsed position with respect to the SiH(2) units, except for the ethyl-substituted derivative 1 e, which shows a more staggered conformation in the solid state. Calculation of the energy profile for the rotation of one pyridine ring indicates two minima that are separated by only 1.2 kJ mol(-1) and a maximum barrier of 12.5 kJ mol(-1). The (29)Si NMR chemical shifts (delta(iso)) range from -145.2 to -152.2 ppm and correlate with the electron density at the Si atoms, in other words with the +I and +M effects of the substituents. Again, compound 1 e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the (29)Si NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent Si-H bonds and very polar Si-Cl bonds, with the highest bond polarisation being observed for the Si-N interaction, which must be considered a donor-acceptor interaction. An analysis of the topological properties of the electron distribution (AIM) suggests a Lewis structure, thereby supporting this bonding situation.

Synthesis of novel celluloses derivatives and investigation of their mitogenic activity in the presence and absence of FGF2

Novel cellulose sulfates (CS) with a controlled degree of sulfation (DS(S)) were synthesized through acetosulfation as well as direct sulfation. CS containing carboxyl (CO) or carboxymethyl (CM) groups were prepared by TEMPO oxidation or by carboxymethylation with chloroacetic acid. The derivatization was characterized by nuclear magnetic resonance and Raman spectroscopy. The derivatives were investigated regarding their cytotoxicity and mitogenic activity by modulation of 3T3 fibroblast proliferation with or without exogenous FGF2. All derivatives were non-toxic for 3T3 cells. CS strongly promoted FGF2-induced proliferation, which was positively related to overall DS(S). In the absence of FGF2, minute quantities of CS with intermediate degrees of sulfation exerted stronger mitogenic effects than heparin. No significant promoting effects of CO and CM on cell proliferation were found, though the structure of CO shows similarities to heparin.