Compositional and 2H NMR studies of bis(benzene)chromium composites of mesoporous vanadium-niobium mixed oxides

New mesoporous niobium oxides with 5, 10, and 15 mol% vanadium(V) doped into the walls of the structure were synthesized by the ligand-assisted templating method with an octadecylamine template. These materials were characterized by XRD, XPS, EPR, elemental analysis, and nitrogen adsorption before being treated with excess bis(benzene)chromium to give new composites with an organometallic phase in the walls. All materials were also characterized by EPR, XRD, nitrogen adsorption, XPS, SQUID magnetometry, and elemental analysis. The materials with higher percentages of vanadium absorbed more bis(benzene)chromium, because this process depends largely on the electron transfer between the organometallic and the walls of the mesostructure and vanadium(V) is a stronger oxidant than niobium(V). Conductivity studies on these materials revealed that the ratio of Cr(0) to Cr(l) in the pores was more important than the absolute Cr loading level in governing electron transport properties but that increasing the V content led to more insulating behavior regardless of the Cr concentration. Solid-state 2H NMR studies on perdeuteriobenzene analogues of these composites showed the presence of the neutral and cationic Cr species in different ratios depending on the loading. Tumbling of these species was also slow on the NMR time scale, indicating that the charge-carrying Cr species are not rapidly moving through the pore channels of the mesostructure. This suggests that the walls of the structure may play a key role in charge transfer in these composites, contrary to what was previously believed.     

A solid-state 39K and 13C NMR study of polymeric potassium metallocenes

The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) and double frequency sweep (DFS)/QCPMG pulse sequences are applied in order to acquire the first solid-state 39K NMR spectra of organometallic complexes, the polymeric main group metallocenes cyclopentadienyl potassium (CpK) and pentamethylcyclopentadienyl potassium (Cp*K). Piecewise QCPMG NMR techniques are used to acquire a high S/N 39K spectrum of the broad central transition of Cp*K, which is ca. 200 kHz in breadth. Analytical and numerical simulations indicate that there is a significant quadrupolar interaction present at both potassium nuclei (C(Q)(39K) = 2.55(6)/2.67(8) MHz and 4.69(8) MHz for CpK (static/MAS) and Cp*K, respectively). Experimental quadrupolar asymmetry parameters suggest that both structures are bent about the potassium atoms (eta(Q)(39K) = 0.28(3)/0.29(3) for CpK (static/MAS) and eta(Q)(39K) = 0.30(3) for Cp*K). Variable-temperature (VT) 39K NMR experiments on CpK elucidate temperature-dependent changes in quadrupolar parameters which can be rationalized in terms of alterations of bond distances and angles with temperature. 13C CP/MAS NMR experiments are conducted upon both samples to quantify the carbon chemical shielding anisotropy (CSA) at the Cp' ring carbon atoms. Ab initio carbon CSA and 39K electric-field gradient (EFG) and CSA calculations are conducted and discussed for the CpK complex, in order to correlate the experimental NMR parameters with molecular structure in CpK and Cp*K. 39K DFS/QCPMG and 13C CP/MAS experiments prove invaluable for probing molecular structure, temperature-dependent structural changes, and the presence of impurities in these systems.     

A 93Nb Solid‐State NMR and Density Functional Theory Study of Four‐ and Six‐Coordinate Niobate Systems

A variable B₀ field static (broadline) NMR study of a large suite of niobate materials has enabled the elucidation of high‐precision measurement of ⁹³Nb NMR interaction parameters such as the isotropic chemical shift (δ_iso), quadrupole coupling constant and asymmetry parameter (C_Q and η_Q), chemical shift span/anisotropy and skew/asymmetry (Ω/Δδ and κ/η_δ) and Euler angles (α, β, γ) describing the relative orientation of the quadrupolar and chemical shift tensorial frames. These measurements have been augmented with ab initio DFT calculations by using WIEN2k and NMR‐CASTEP codes, which corroborate these reported values. Unlike previous assertions made about the inability to detect CSA (chemical shift anisotropy) contributions from Nb^V in most oxo environments, this study emphasises that a thorough variable B₀ approach coupled with the VOCS (variable offset cumulative spectroscopy) technique for the acquisition of undistorted broad (?1/2?+1/2) central transition resonances facilitates the unambiguous observation of both quadrupolar and CSA contributions within these ⁹³Nb broadline data. These measurements reveal that the ⁹³Nb electric field gradient tensor is a particularly sensitive measure of the immediate and extended environments of the Nb^V positions, with C_Q values in the 0 to >80 MHz range being measured; similarly, the δ_iso (covering an approximately 250 ppm range) and Ω values (covering a 0 to approximately 800 ppm range) characteristic of these niobate systems are also sensitive to structural disposition. However, their systematic rationalisation in terms of the Nb? O bond angles and distances defining the immediate Nb^V oxo environment is complicated by longer‐range influences that usually involve other heavy elements comprising the structure. It has also been established in this study that the best computational method(s) of analysis for the ⁹³Nb NMR interaction parameters generated here are the all‐electron WIEN2k and the gauge included projector augmented wave (GIPAW) NMR‐CASTEP DFT approaches, which account for the short‐ and long‐range symmetries, periodicities and interaction‐potential characteristics for all elements (and particularly the heavy elements) in comparison with Gaussian 03 methods, which focus on terminated portions of the total structure.     

“Crowned” Univalent Indium Complexes as Donors? Experimental and Computational Insights on the Valence Isomers of EE′X4 Species

The use of the univalent indium reagent [In([18]crown-6)][OTf] as a donor is investigated by its reactions with acceptors including InX(3) (X=Cl, Br, I). The donor-acceptor complexes of the form [X([18]crown-6)In-InX(3)] obtained in this manner represent the first new isomeric form of indium(II) halides identified for at least five decades. The formation of such complexes appears to be particularly favorable and they are isolated as products in many reactions involving low-valent indium, a halide source, and [18]crown-6. A convenient solution-phase synthesis of In[ECl(4)] salts is reported. This facile and direct syntheses of In[ECl(4)] (E=Al, Ga, In) salts allows for the in situ preparation and isolation of crown-ether complexes of the form [In([18]crown-6)][ECl(4)], whose existence had been postulated but never confirmed. Solution-phase and solid-state NMR experiments reveal that these compounds can exist as either donor-acceptor complexes or ionic salts, depending on the phase of the system, the nature of the solvent employed, and the identity of the metalate anion involved. Similar investigations into the effect of a smaller crown ether allow for the isolations of salts containing the cation [In([15]crown-5)](+). Computational investigations into the nature of the crowned univalent indium donor fragments, and on the donor-acceptor complexes produced, demonstrate the influence of anionic substituents on the reactivity of lone pair of electrons of the In(I) center. Natural bond orbital (NBO) analysis of donor-acceptor models shows that the composition of the E-E bond MO should provide the ability to predict which models should form stable complexes.     

Octaboraneyl Complexes of Nickel: Monomers for Redox-Active Coordination Polymers

Herein, we establish the preparation, characterization, and reactivity of a new diphosphine ligand, 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P₂B^Cy₄), a scaffold that contains four pendant boranes. An entryway into the coordination chemistry of P₂B^Cy₄ is established by using nickel, providing the octaboraneyl complex [Ni(P₂B^Cy₄)₂]—this species contains a boron-rich secondary coordination sphere that reacts readily with Lewis bases. In the case of 4,4′-bipyridine, an air-sensitive coordination polymer is obtained. Characterization of this material by solid-state NMR and EPR spectroscopy reveals the presence of a charge-transfer polymer, which forms as a function of intramolecular Ni→4,4′-bpy electron transfer (ET), providing an array of oxidized nickel sites and reduced 4,4′-bpy radical anion sites. Notably, the related intermolecular reaction between the model fragments [Ni(dnppe)₂] (dnppe=1,2-bis(di-n-propylphosphino)ethane) and a bis(boraneyl)-protected 4,4′-bpy, provides no ET. Overall, the P₂B^Cy₄ fragment provides a unique opportunity for Lewis base activation, in one case allowing for the facile construction of monomers for incorporation into redox-active macromolecules.     

Multinuclear solid-state NMR spectroscopy of doped lanthanum fluoride nanoparticles

Multinuclear solid-state NMR spectroscopy and powder X-ray diffraction (XRD) experiments are applied to comprehensively characterize a series of pure and lanthanide-doped LaF3 nanoparticles (NPs) that are capped with di-n-octadectyldithiophosphate ligands (Ln3+ = diamagnetic Y3+ and Sc3+ and paramagnetic Yb3+ ions), as well as correlated bulk microcrystalline materials (LaF3, YF3, and ScF3). Solid-state 139La and 19F NMR spectroscopy of bulk LaF3 and the LaF3 NPs reveal that the inorganic core of the NP retains the LaF3 structure at the molecular level; however, inhomogeneous broadening of the NMR powder patterns arises from distributions of 139La and 19F NMR interactions, confirming a gradual change in the La and F site environments from the NP core to the surface. 139La and 19F NMR experiments also indicate that low levels (5 and 10 mol %) of Ln3+ doping do not significantly change the LaF3 structure in the NP core. Similar doping levels of paramagnetic Yb3+ ions severely broaden 19F resonances, but only marginally effect 139La powder patterns, suggesting that the dopant ions are uniformly distributed throughout the NP core and occupy vacant La sites. Measurements of 139La T1 and T2 relaxation constants are seen to vary between the bulk material and NPs and between samples with diamagnetic and paramagnetic dopants. 45Sc NMR experiments confirm that the dopants are integrated into the La sites of the LaF3 core. Solid-state 1H and 31P magic-angle spinning (MAS) NMR spectra aid in probing the nature of the capping ligands and their interactions at the NP surface. 31P cross-polarization (CP)/MAS NMR experiments identify not only the dithiophosphate head groups but also thiophosphate and phosphate species which may form during NP synthesis. Finally, 19F-31P CP/MAS and 1H MAS experiments confirm that ligands are coordinated to the NP surface.