Theoretical Study of the Relative Stabilities of the α/β₃-[XW₁₁O₃₉]^<i>m</i>− Lacunary Polyoxometalates (X = P, Si)

A computational study of the relative stability of the monolacunary Keggin polyoxotungstates α and β3-[XW11O39]m− (X = P, m = 7; X = Si, m = 8) was performed. The influence of the nature of different grafted cations and of the central anion XO4n− on the relative stabilities of the lacunary isomers was analyzed. From these results, an interpretation of the structural difference in the metallic frameworks of α-[PW11O39{Ru(DMSO)3(H2O)}]5−, α-[PW11O39{Ru(C6H6)(H2O)}]5−, and β3-[SiW11O39{Ru(DMSO)3(H2O)}]6− is proposed, and conclusions are drawn as to how to favor the formation of β3 derivatives in future syntheses

Experimental and Theoretical Study of the Regiospecific Coordination of Ru^II and Os^II Fragments on the Lacunary Polyoxometalate [α-PW₁₁O₃₉]^7-

New RuII and OsII derivatives of the monovacant [α-PW11O39]7- anion ([PW11O39{M(DMSO)3(H2O)}]5- (M = Ru (1), Os (2)) and [PW11O39{Os(η6-p-cym)(H2O)}]5- (3)) have been synthesized and characterized. The binding mode of the d6-{MIIL3(H2O)}2+ moieties in these compounds is similar to that in the previously described [PW11O39{Ru(η6-p-cym)(H2O)}]5- (4) complex:  bidentate, on two nonequivalent oxygen atoms of the lacuna, leading to a loss of the Cs symmetry of the parent anion, which thus plays the role of a prochiral bidentate ligand. The density functional theory (DFT) (B3PW91) computation of the lowest unoccupied molecular orbitals of the {ML3(H2O)}2+ (M = Os, Ru; L3 = fac-(DMSO)3, η6-C6H6) fragments reveals the similarities between their electrophilic properties. The origin of the regioselectivity of the grafting was investigated through a DFT (B3PW91) analysis of (i) the highest occupied molecular orbital of [α-PW11O39]7- and (ii) the relative energies of the different potential regioisomers obtained by a bidentate grafting of the {ML3(H2O)}2+ moiety onto the lacuna of [α-PW11O39]7-. The role of the water ligand in the stabilization of this peculiar structure was studied

Improvement of the Oxidative Stability of Nanodiamonds by Surface Phosphorylation

Surface phosphorylation of nanodiamond was performed by reaction with phosphoryl chloride in dichloromethane. Depending on the reaction conditions, P contents of up to 1.66 mmol/g were reached. Phosphorylation dramatically enhanced the thermal stability of nanodiamond under oxidizing conditions, shifting the oxidation temperature by up to 190 °C and dividing the oxidation rate by a factor of up to 160. The nature of the grafted phosphate species and their evolution during thermal treatment was followed using solid-state NMR

Surface Functionalization of Detonation Nanodiamonds by Phosphonic Dichloride Derivatives

A new method for the functionalization of detonation nanodiamonds (DNDs) is proposed, on the basis of surface modification with phosphonic dichloride derivatives. DNDs were first modified by phenylphosphonic dichloride, and the grafting modes and hydrolytic stability under neutral conditions were investigated using ¹H, ¹³C, and ³¹P solid state NMR spectroscopy, Fourier transform infrared spectroscopy, as well as elemental analysis. Then, in order to illustrate the possibilities offered by this method, DNDs functionalized by mesityl imidazolium groups were obtained by postmodification of DNDs modified by 12-bromododecylphosphonic dichloride. The oxidative thermal stability of the functionalized DNDs was investigated using thermogravimetric analysis

NMR Investigation of the Role of Osteocalcin and Osteopontin at the Organic–Inorganic Interface in Bone

Mechanical resilience of bone tissue decreases with age. The ability to comprehensively probe and understand bone properties could help alleviate this problem. One important aspect of bone quality that has recently been made evident is the presence of dilatational bands formed by osteocalcin (OC) and osteopontin (OPN), which contribute to fracture toughness. However, experimental evidence of the structural role of these two proteins at the organic–mineral interface in bone is still needed. Solid state nuclear magnetic resonance (SSNMR) is emerging as a useful technique in probing molecular level aspects of bone. Here, we present the first SSNMR study of bone tissue from genetically modified mice lacking OC and/or OPN. Probing the mineral phase, the organic matrix and their interface revealed that, despite the absence of OC and OPN, the organic matrix and mineral were well preserved, and the overall exposure of collagen to hydroxyapatite (HA) nanoparticles was hardly affected. However, the proximity to the HA surface was slightly increased for a number of bone components including less abundant amino acids like lysine, suggesting that this is how the tissue compensates for the lack of OC and OPN. Taken together, the NMR data supports the recently proposed model, in which the contribution of OC–OPN to fracture toughness is related to their presence at the extrafibrillar organic–mineral interfaces, where they reinforce the network of mineralized fibrils and form dilatational bands. In an effort toward further understanding the structural role of individual amino acids of low abundance in bone, we then explored the possibility of specific 13C enrichment of mouse bone, and report the first SSNMR spectra of 97% 13C lysine-enriched tissue. Results show that such isotopic enrichment allows valuable molecular-level structural information to be extracted, and sheds light on post-translational modifications undergone by specific amino acids in vivo

A High-Resolution ⁴³Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

High resolution 43Ca solid-state NMR studies of hydroxyapatite (Ca10(PO4)6(OH)2) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using 1H-43Ca rotational echo double resonance and 1H-43Ca cross polarization magic angle spinning experiments

From <i>Operando</i> Raman Mechanochemistry to “NMR Crystallography”: Understanding the Structures and Interconversion of Zn-Terephthalate Networks Using Selective ¹⁷O‑Labeling

The description of the formation, structure, and reactivity of coordination networks and metal–organic frameworks (MOFs) remains a real challenge in a number of cases. This is notably true for compounds composed of Zn2+ ions and terephthalate ligands (benzene-1,4-dicarboxylate, BDC) because of the difficulties in isolating them as pure phases and/or because of the presence of structural defects. Here, using mechanochemistry in combination with operando Raman spectroscopy, the observation of the formation of various zinc terephthalate compounds was rendered possible, allowing the distinction and isolation of three intermediates during the ball-milling synthesis of Zn3(OH)4(BDC). An “NMR crystallography” approach was then used, combining solid-state NMR (1H, 13C, and 17O) and density functional theory (DFT) calculations to refine the poorly described crystallographic structures of these phases. Particularly noteworthy are the high-resolution 17O NMR analyses, which were made possible in a highly efficient and cost-effective way, thanks to the selective 17O-enrichment of either hydroxyl or terephthalate groups by ball-milling. This allowed the presence of defect sites to be identified for the first time in one of the phases, and the nature of the H-bonding network of the hydroxyls to be established in another. Lastly, the possibility of using deuterated precursors (e.g., D2O and d4-BDC) during ball-milling is also introduced as a means for observing specific transformations during operando Raman spectroscopy studies, which would not have been possible with hydrogenated equivalents. Overall, the synthetic and spectroscopic approaches developed herein are expected to push forward the understanding of the structure and reactivity of other complex coordination networks and MOFs

A Solid-State NMR Study of Lead and Vanadium Substitution into Hydroxyapatite

A systematic study on cationic and anionic substitution in hydroxyapatite structures was carried out, with the aim of understanding the impact of ion exchange on the crystalline structure and properties of these materials. Lead and vanadium were chosen for the exchange, due to their known effects on the redox and catalytic properties of hydroxypatites. Hydroxyapatites with variable Pb and V contents, PbxCa10-x(VO4)y(PO4)6-y(OH)2 (x = 0, 2, 4, 6, 8 and 10 for y = 1; y = 0, 0.5, 1, 2, 3 and 6 for x = 10) were synthesized and characterized by NMR spectroscopy. Solid-state NMR allowed an analysis of the chemical environment of every ion after substitution into the hydroxyapatite network. 43Ca and 207Pb NMR spectra at different lead concentrations provided clear evidence of the preferential substitution of lead into the Ca(II) site, the replacement of the Ca(I) site starting at x = 4 for y = 1. Two NMR distinguishable Pb(I) sites were observed in Pb10(PO4)6(OH)2, which is compatible with the absence of a local mirror plane perpendicular to the c direction. In contrast with 31P NMR, for which only small variations related to the incorporation of Pb are observed, the strong change in the 51V NMR spectrum indicates that lead perturbs the vanadium environment more than the phosphorus one. The existence of a wide variety of environments for OH in substituted apatites is revealed by 1H NMR, and the mobility of the water molecules appears to vary upon introduction of lead into the structure

Influence of Magnesium Substitution on the Basic Properties of Hydroxyapatites

Magnesium-substituted hydroxyapatites (HAP) have been prepared to evaluate the influence of cationic substitution on the surface basic properties of HAP. Despite successful introduction of some of the magnesium cations into the HAP structure (as evidenced by XRD, infrared, and Raman), no influence of low magnesium content in Mgx–HAP samples (x ≤ 1) on the basic conversion of 2-methylbut-3-yn-1-ol was found, which can be explained by the surface content of magnesium being low, as evidenced by CO adsorption. At higher magnesium content, a higher amount of magnesium could be detected on the surface, but this resulted in a structural disorder leading to either nonstoichiometry or eventually the formation of the phase whitlockite. In this case, the associated relative decrease of the amount of basic sites, as well as the possible influence of the enhanced surface concentration of acidic POH groups, are responsible for the lower intrinsic basicity of the related samples. In contrast, preliminary results indicate that an enhancement of the basic reactivity is observed on substituting calcium for strontium

Implementation of High Resolution ⁴³Ca Solid State NMR Spectroscopy: Toward the Elucidation of Calcium Sites in Biological Materials

Calcium is one of the most abundant cations in living organisms. It is found in the mineral phase of bone and in proteins like calmodulin. However, its exact environment beyond the first coordination sphere is often unknown, thus hampering the understanding of many biological processes. Here, calcium benzoate trihydrate (Ca(C6H5COO)2·3H2O) was used as a model for the NMR analysis of calcium sites in biological materials, because of the similarity of its calcium coordination, to water and carboxylate ligands, to that in several calcium-proteins. First, calcium-43 magic angle spinning (MAS) and static NMR spectra of a 43Ca enriched sample were recorded at different magnetic fields, to investigate the electronic environment of calcium. Complex static lineshapes were obtained because of the presence of anisotropic NMR interactions of similar magnitude (chemical shift anisotropy and quadrupolar interaction), and the full interpretation of the spectra required simulations and gauge-including projector augmented wave (GIPAW) DFT calculations. An NMR investigation of the coordination environment of Ca2+ was carried out, using high resolution 13C−43Ca MAS NMR experiments such as TRAPDOR (transfer of population double resonance) and heteronuclear J-spin−echoes. It was shown that despite the weakness of 13C−43Ca interactions, it is possible to discriminate carbon atoms according to their calcium environment. Long-range calcium−carbon correlations were even evidenced by TRAPDOR, reaching distances >5.6 Å. This work demonstrates that by combining solid state NMR experiments, DFT calculations, and simulations, it will be possible to elucidate the electronic and coordination environment of calcium in many important and complex materials