Side-on Coordination in Isostructural Nitrous Oxide and Carbon Dioxide Complexes of Nickel.

A nickel complex incorporating an N2 O ligand with a rare η2 -N,N'-coordination mode was isolated and characterized by X-ray crystallography, as well as by IR and solid-state NMR spectroscopy augmented by 15 N-labeling experiments. The isoelectronic nickel CO2 complex reported for comparison features a very similar solid-state structure. Computational studies revealed that η2 -N2 O binds to nickel slightly stronger than η2 -CO2 in this case, and comparably to or slightly stronger than η2 -CO2 to transition metals in general. Comparable transition-state energies for the formation of isomeric η2 -N,N'- and η2 -N,O-complexes, and a negligible activation barrier for the decomposition of the latter likely account for the limited stability of the N2 O complex.

Nickel as a Lewis Base in a T-Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand.

Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis- and trans-coordination modes, was used to synthesize (2)Ni(η2 -cod), 3. In reaction with GeCl2 , it produced (2)NiGeCl2 , 4, featuring a T-shaped Ni0 and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)2 , in a reaction that formally involved Ni-Ge transmetalation, followed by coordination of the extruded GeCl2 moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ-donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)2 Ni(cod) complexes 9 and 10, as well as (NHC)2 GeCl2 derivative 11, incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni-Ge complexes. The isolation of (2)Ni(η2 -Py), 12, provides further evidence for the reluctance of the (2)Ni0 fragment to act as a σ-Lewis acid.

Methylammonium Cation Dynamics in Methylammonium Lead Halide Perovskites: A Solid-State NMR Perspective.

In light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ2, for the C-N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, 2H NMR line shapes indicate that the C-N axis is essentially frozen.

Spin-Spin Coupling between Quadrupolar Nuclei in Solids: (11)B-(75)As Spin Pairs in Lewis Acid-Base Adducts.

Solid-state (11)B NMR measurements of Lewis acid-base adducts of the form R3AsBR'3 (R = Me, Et, Ph; R' = H, Ph, C6F5) were carried out at several magnetic field strengths (e.g., B0 = 21.14, 11.75, and 7.05 T). The (11)B NMR spectra of these adducts exhibit residual dipolar coupling under MAS conditions, allowing for the determination of effective dipolar coupling constants, Reff((75)As,(11)B), as well as the sign of the (75)As nuclear quadrupolar coupling constants. Values of Reff((75)As,(11)B) range from 500 to 700 Hz. Small isotropic J-couplings are resolved in some cases, and the sign of (1)J((75)As,(11)B) is determined. Values of CQ((75)As) measured at B0 = 21.14 T for these triarylborane Lewis acid-base adducts range from -82 ± 2 MHz for Et3AsB(C6F5)3 to -146 ± 1 MHz for Ph3AsBPh3. For Ph3AsBH3, two crystallographically nonequivalent sites are identified with CQ((75)As) values of -153 and -151 ± 1 MHz. For the uncoordinated Lewis base, Ph3As, four (75)As sites with CQ((75)As) values ranging from 193.5 to 194.4 ± 2 MHz are identified. At these applied magnetic field strengths, the (75)As quadrupolar interaction does not satisfy high-field approximation criteria, and thus, an exact treatment was used to describe this interaction in (11)B and (75)As NMR spectral simulations. NMR parameters calculated using the ADF and CASTEP program packages support the experimentally derived parameters in both magnitude and sign. These experiments add to the limited body of literature on solid-state (75)As NMR spectroscopy and serve as examples of spin-spin-coupled quadrupolar spin pairs, which are also rarely treated in the literature.

Solid-State 87Sr NMR Spectroscopy at Natural Abundance and High Magnetic Field Strength.

Twenty-five strontium-containing solids were characterized via (87)Sr NMR spectroscopy at natural abundance and high magnetic field strength (B0 = 21.14 T). Strontium nuclear quadrupole coupling constants in these compounds are sensitive to the strontium site symmetry and range from 0 to 50.5 MHz. An experimental (87)Sr chemical shift scale is proposed, and available data indicate a chemical shift range of approximately 550 ppm, from -200 to +350 ppm relative to Sr(2+)(aq). In general, magnetic shielding increased with strontium coordination number. Experimentally measured chemical shift anisotropy is reported for stationary samples of solid powdered SrCl2·6H2O, SrBr2·6H2O, and SrCO3, with δaniso((87)Sr) values of +28, +26, and -65 ppm, respectively. NMR parameters were calculated using CASTEP, a gauge including projector augmented wave (GIPAW) DFT-based program, which addresses the periodic nature of solids using plane-wave basis sets. Calculated NMR parameters are in good agreement with those measured.

Solid-State (63)Cu, (65)Cu, and (31)P NMR Spectroscopy of Photoluminescent Copper(I) Triazole Phosphine Complexes.

The results of a solid-state (63/65)Cu and (31)P NMR investigation of several copper(I) complexes with functionalized 3-(2'-pyridyl)-1,2,4-triazole and phosphine ligands that have shown potential in the preparation of photoluminescent devices are reported. For each complex studied, distinct NMR parameters, with moderate (63)Cu nuclear quadrupolar coupling constant (CQ) values ranging from -17.2 to -23.7 MHz, are attributed to subtle variations in the distorted four-coordinate environments about the copper nuclei. The spans of the copper chemical shift (CS) tensors, δ11-δ33, for the mono- and bisphosphine complexes are also similar, ranging from 1000 to 1150 ppm, but that for a complex with a strained bidentate phosphine ligand is only 650 ppm. The effects of residual dipolar and indirect spin-spin coupling arising from the (63/65)Cu- (31)P spin pairs, observed in the solid-state (31)P NMR spectra of these complexes, yield information about the orientations of the copper electric field gradient (EFG) tensors relative to the Cu-P bond. Variable-temperature (31)P NMR measurements for [Cu(bptzH)(dppe)]ClO4 (bptzH = 5-tert-butyl-3-(2'-pyridyl)-1,2,4-triazole; dppe = 1,2-bis(diphenylphosphino)ethane), undertaken to investigate the cause of the broad unresolved spectra observed at room temperature, demonstrate that the broadening arises from partial self-decoupling of the (63/65)Cu nuclei, a consequence of rapid quadrupolar relaxation. Ab initio calculations of copper EFG and CS tensors were performed to probe relationships between NMR parameters and molecular structure. The analysis demonstrated that CQ((63/65)Cu) is negative for all complexes studied here and that the largest components of the EFG tensors are generally coincident with δ11.

Experimental characterization of the hydride 1H shielding tensors for HIrX2(PR3)2 and HRhCl2(PR3)2: extremely shielded hydride protons with unusually large magnetic shielding anisotropies.

The hydride proton magnetic shielding tensors for a series of iridium(III) and rhodium(III) complexes are determined. Although it has long been known that hydridic protons for transition-metal hydrides are often extremely shielded, this is the first experimental determination of the shielding tensors for such complexes. Isolating the (1)H NMR signal for a hydride proton requires careful experimental strategies because the spectra are generally dominated by ligand (1)H signals. We show that this can be accomplished for complexes containing as many as 66 ligand protons by substituting the latter with deuterium and by using hyperbolic secant pulses to selectively irradiate the hydride proton signal. We also demonstrate that the quality of the results is improved by performing experiments at the highest practical magnetic field (21.14 T for the work presented here). The hydride protons for iridium hydride complexes HIrX2(PR3)2 (X = Cl, Br, or I; R = isopropyl, cyclohexyl) are highly shielded with isotropic chemical shifts of approximately -50 ppm and are also highly anisotropic, with spans (=δ11 - δ33) ranging from 85.1 to 110.7 ppm. The hydridic protons for related rhodium complexes HRhCl2(PR3)2 also have unusual magnetic shielding properties with chemical shifts and spans of approximately -32 and 85 ppm, respectively. Relativistic density functional theory computations were performed to determine the orientation of the principal components of the hydride proton shielding tensors and to provide insights into the origin of these highly anisotropic shielding tensors. The results of our computations agree well with experiment, and our conclusions concerning the importance of relativistic effects support those recently reported by Kaupp and co-workers.

Feasibility of arsenic and antimony NMR spectroscopy in solids: an investigation of some group 15 compounds.

The feasibility of obtaining (75)As and (1)(21/123)Sb NMR spectra for solids at high and moderate magnetic field strengths is explored. Arsenic-75 nuclear quadrupolar coupling constants and chemical shifts have been measured for arsenobetaine bromide and tetraphenylarsonium bromide. Similarly, (121/123)Sb NMR parameters have been measured for tetraphenylstibonium bromide and potassium hexahydroxoantimonate. The predicted pseudo-tetrahedral symmetry at arsenic and the known trigonal bipyramidal symmetry at antimony in their respective tetraphenyl-bromide "salts" are reflected in the measured (75)As and (121)Sb nuclear quadrupole coupling constants, CQ((75)As)=7.8MHz and CQ((121)Sb)=159MHz, respectively. Results of density functional theory quantum chemistry calculations for isolated molecules using ADF and first-principles calculations using CASTEP, a gauge-including projector augmented wave method to deal with the periodic nature of solids, are compared with experiment. Although the experiments can be time consuming, measurements of (75)As and (121)Sb NMR spectra (at 154 and 215MHz, respectively, i.e., at B0=21.14T) with linewidths in excess of 1MHz are feasible using uniform broadband excitation shaped pulse techniques (e.g., WURST and WURST-QCPMG).

An investigation of 1:1 adducts of gallium trihalides with triarylphosphines by solid-state (69/71)Ga and 31P NMR spectroscopy.

Several 1:1 adducts of gallium trihalides with triarylphosphines, X(3)Ga(PR(3)) (X=Cl, Br, and I; PR(3)=triarylphosphine ligand), were investigated by using solid-state (69/71)Ga and (31)P NMR spectroscopy at different magnetic-field strengths. The (69/71)Ga nuclear quadrupolar coupling parameters, as well as the gallium and phosphorus magnetic shielding tensors, were determined. The magnitude of the (71)Ga quadrupolar coupling constants (C(Q)((71)Ga)) range from approximately 0.9 to 11.0 MHz. The spans of the gallium magnetic shielding tensors for these complexes, δ(11)-δ(33), range from approximately 30 to 380 ppm; those determined for phosphorus range from 10 to 40 ppm. For any given phosphine ligand, the gallium nuclei are most shielded for X=I and least shielded for X=Cl, a trend previously observed for In(III)-phosphine complexes. This experimental trend, attributed to spin-orbit effects of the halogen ligands, is reproduced by DFT calculations. The signs of C(Q)((69/71)Ga) for some of the adducts were determined from the analysis of the (31)P NMR spectra acquired with magic angle spinning (MAS). The (1)J((69/71)Ga,(31)P) and ΔJ((69/71)Ga, (31)P) values, as well as their signs, were also determined; values of (1)J((71)Ga,(31)P) range from approximately 380 to 1590 Hz. Values of (1)J((69/71)Ga,(31)P) and ΔJ((69/71)Ga,(31)P) calculated by using DFT have comparable magnitudes and generally reproduce experimental trends. Both the Fermi-contact and spin-dipolar Fermi-contact mechanisms make important contributions to the (1)J((69/71)Ga,(31)P) tensors. The (31)P NMR spectra of several adducts in solution, obtained as a function of temperature, are contrasted with those obtained in the solid state. Finally, to complement the analysis of NMR spectra for these adducts, single-crystal X-ray diffraction data for Br(3)Ga[P(p-Anis)(3)] and I(3)Ga[P(p-Anis)(3)] were obtained.

Nuclear quadrupole coupling constants for N2O: experiment and theory.

The nuclear quadrupole coupling constants (NQCCs) for the nitrogen and oxygen nuclei in N(2)O have been determined using a variety of computational methods (MP2, QCISD, DFT with B3LYP, PBE0, and B3PW91 functionals, CCSD, CCSD(T), CASSCF, and MRCI) combined with correlation-consistent basis sets. When compared to the available experimental determinations, the results demonstrate that only CCSD(T) and MRCI methods are capable of accurately predicting the NQCCs of the central and terminal nitrogen atoms. The spin-rotation and magnetic shielding tensors have also been determined and compared to experimental measurements where available. (14)N and (17)O NMR relaxation data for N(2)O in the gas phase and a variety of solvents is reported. The increase in the ratio of (14)N spin-lattice relaxation times in solvent for the central and terminal nitrogens supports previous reports of the modification of the electric field gradients at these nuclei in van der Waals complexes. Ab initio computations for the linear FH···N(2)O complex confirm the large change in EFGs imposed by a single perturber.

Nuclear magnetic shielding for hydrogen in selected isolated molecules.

We present the results of gas-phase NMR measurements designed to yield a new experimental value for the absolute (1)H magnetic shielding for an isolated hydrogen molecule and its deuterium isotopomers. The results are based on the original method of direct shielding measurements (Jackowski et al., 2010) and the density dependence of (1)H, (2)H, and (3)He NMR frequencies for molecular hydrogen and atomic helium-3. The absolute isotropic magnetic shielding measured for molecular hydrogen, σ(0)(H(2)), is 26.293(5) ppm at 300 K, within experimental error of previous measurements based on spin-rotation data and quantum chemistry computations, 26.289(2) ppm (Sundholm and Gauss, 1997), and recent ab initio calculations. We also report isotope effects in shielding for H(2), HD, and D(2) molecules that are consistent with theoretical predictions. In addition, gas-phase (1)H chemical shifts extrapolated to zero density have been measured for numerous small molecules. Our results yield precise absolute shielding data that will be useful in establishing benchmark computational chemistry methods for calculating rovibrational averaged magnetic shielding.

A study of silver species on silver-exchanged ETS-10 and mordenite by XRD, SEM and solid-state 109Ag, 29Si and 27AI NMR spectroscopy.

Silver zeolites, especially Ag-ETS-10 and Ag-mordenite, actively bind xenon and iodine, two prime contaminants common to nuclear accidents. The evolution of silver species on silver exchanged ETS-10 (Ag/ETS-10) and mordenite (Ag/Mor) has been investigated by exposing the materials to a series of activation conditions in Ar, air and H2. The samples were characterized by XRD, SEM and solid-state 109Ag, 29Si and 27AI MAS NMR. The silver reduction and structural evolution have been illustrated by those techniques. The effectiveness of one sample of each type of sieve was tested for its ability to trap mercury from a gas stream. However, the results from this study demonstrate that the adsorption characteristics of silver-loaded sieves cannot necessarily be predicted using a full complement of structural characterization techniques, which highlights the importance of understanding the formation and nature of silver species on molecular sieves.

Solid-state NMR and TGA studies of silver reduction in chabazite.

Silver-exchanged molecular sieves have shown great promise in applications ranging from antimicrobial materials to the adsorption of xenon and iodide, two key contaminants emitted from nuclear reactors. In this work, solid-state 27Al and 29Si MAS NMR and TGA were used to study silver reduction in silver-exchanged chabazite under various thermal conditions. The solid-state NMR results for both 27Al and 29Si show that there are no major changes in the chabazite during silver reduction in an argon stream; however a progressive structural change does take place in the hydrogen stream. The structural change likely involves breaking the silicon oxygen bond of the Si-O-AI fragment of chabazite, leading to the formation of extra-framework aluminum oxide. The TGA results at temperatures up to 600 degrees C indicate that silver reduction is less complete in an argon stream than in a hydrogen stream. In this paper we propose that silver reduction occurs via the following reactions: 2(Ag + ZO-)+H2O --> 1/2O2+2Ag0 + 2ZOH and nAg + mAg = Ag(m+n)n+ (in an argon stream); and Ag(+) + ZO(-) + 1/2H2 = Ag0 + ZOH and 2ZOH = ZO(-) + Z(+) + H2O (in a hydrogen stream).

Solid-state 115In and 31P NMR studies of triarylphosphine indium trihalide adducts.

Solid-state (115)In and (31)P NMR spectroscopy, relativistic density functional theory (DFT) calculations, and single-crystal X-ray diffraction were used to investigate a series of triarylphosphine indium(III) trihalide adducts, X(3)In(PR(3)) and X(3)In(PR(3))(2) (X = Cl, Br or I; PR(3) = triarylphosphine ligand). The electric field gradient tensors at indium as well as the indium and phosphorus magnetic shielding tensors and the direct and indirect (115)In-(31)P spin-spin coupling were characterized; for complexes possessing a C(3) symmetry axis, the anisotropy in the indirect spin-spin coupling, DeltaJ((115)In,(31)P), was also determined. The (115)In quadrupolar coupling constants, C(Q)((115)In), range from +/-1.25 +/- 0.10 to -166.0 +/- 2.0 MHz. For any given phosphine ligand, the indium nuclei are most shielded for X = I and least shielded for X = Cl, a trend also observed for other group-13 nuclei in M(III) complexes. This experimental trend, attributed to spin-orbit effects of the halogen ligands, is reproduced by the DFT calculations. The spans of the indium magnetic shielding tensors for these complexes, delta(11)-delta(33), range from 40 +/- 7 to 710 +/- 60 ppm; those determined for phosphorus range from 28 +/- 1.5 to 50 +/- 3 ppm. Values of (1)J((115)In,(31)P) range from 550 +/- 20 to 2500 +/- 20 Hz. For any given halide, the (1)J((115)In,(31)P) values generally increase with increasing basicity of the PR(3) ligand. Calculated values of (1)J((115)In,(31)P) and DeltaJ((115)In,(31)P) duplicate experimental trends and indicate that both the Fermi-contact and spin-dipolar Fermi-contact mechanisms make important contributions to the (1)J((115)In,(31)P) tensors.

Synthesis of gold phosphido complexes derived from bis(secondary) phosphines. structure of tetrameric [Au(MesP(CH(2))(3)PMes)Au](4).

Treatment of 2 equiv of Au(THT)Cl (THT = tetrahydrothiophene) with the bis(secondary) phosphines HP(R) approximately PH(R) (linker approximately = (CH(2))(3), R = Mes = 2,4,6-Me(3)C(6)H(2) (1), R = Is = 2,4,6-(i-Pr)(3)C(6)H(2) (2), R = Ph (4); approximately = (CH(2))(2), R = Is (3); HP(R) approximately PH(R) = 1,1'-(eta(5)-C(5)H(4)PHPh)(2)Fe (5)), gave the dinuclear complexes (AuCl)(2)(mu-HP(R) approximately PH(R)) (6-10). Dehydrohalogenation with aqueous ammonia gave the phosphido complexes [(Au)(2)(mu-P(R) approximately P(R))](n) (11-15). Ferrocenyl- and phenylphosphido derivatives 15 and 14 were insoluble; the latter was characterized by solid-state (31)P NMR spectroscopy. Isitylphosphido complexes 12 and 13 gave rise to broad, ill-defined NMR spectra. However, mesitylphosphido complex 11 was formed as a single product, which was characterized by multinuclear solution NMR spectroscopy, solid-state (31)P NMR spectroscopy, and elemental analyses. Mass spectrometry suggested that this material contained eight gold atoms (n = 4). A structure proposed on the basis of the (1)H NMR spectra, containing a distorted cube of phosphorus atoms, was confirmed by X-ray crystallographic structure determination. NMR spectroscopy, including measurement of the hydrodynamic radius of 11 by (1)H NMR DOSY, suggested that this structure was maintained in solution. Density functional theory (DFT) structural calculations on 11 were also in good agreement with the solid-state structure.

Structural characterization of silver dialkylphosphite salts using solid-state 109Ag and 31P NMR spectroscopy, IR spectroscopy and DFT calculations.

High-resolution solid-state (109)Ag and (31)P NMR spectroscopy was used to investigate a series of silver dialkylphosphite salts, Ag(O)P(OR)(2) (R = CH(3), C(2)H(5), C(4)H(9) and C(8)H(17)), and determine whether they adopt keto, enol or dimer structures in the solid state. The silver chemical shift, CS, tensors and |J((109)Ag, (31)P)| values for these salts were determined using (109)Ag (Xi = 4.652%) NMR spectroscopy. The magnitudes of J((109)Ag, (31)P) range from 1250 +/- 10 to 1318 +/- 10 Hz and are the largest reported so far. These values indicate that phosphorus is directly bonded to silver for all these salts and thus exclude the enol structure. All (31)P NMR spectra exhibit splittings due to indirect spin-spin coupling to (107)Ag (I = 1/2, NA = 51.8%) and (109)Ag (I = 1/2, NA = 48.2%). The (1)J((109)Ag, (31)P) values measured by both (109)Ag and (31)P NMR spectroscopy agree within experimental error. Analysis of (31)P NMR spectra of stationary samples for these salts allowed the determination of the phosphorus CS tensors. The absence of characteristic P=O stretching absorption bands near 1250 cm(-1) in the IR spectra for these salts exclude the simple keto tautomer. Thus, the combination of solid-state NMR and IR results indicate that these silver dialkylphosphite salts probably have a dimer structure. Values of silver and phosphorus CS tensors as well as (1)J((109)Ag, (31)P) values for a dimer model calculated using the density functional theory (DFT) method are in agreement with the experimental observations.

Synthesis and reactivity of phosphine-stabilized phosphoranimine cations, [R3P x PR'2=NSiMe3]+.

A series of phosphine-stabilized phosphoranimine cations [R(3)P x PR'(2)=NSiMe(3)](+), which can be regarded as derivatives of the proposed transient reactive intermediate [PR'(2)=NSiMe(3)](+) in the thermal condensation polymerization of phosphoranimines (R''O)PR'(2)=NSiMe(3) to form poly(alkyl/arylphosphazenes) [PR'(2)=N](n) at 180-200 degrees C, have been prepared. The bromide salts [R(3)P x PR'(2)=NSiMe(3)]Br [R' = Me ([6](+)), OCH(2)CF(3) ([8](+)); R(3)P = Me(3)P (a), Et(3)P (b), (n)Bu(3)P (c), dmpm (d, dmpm = dimethylphosphinomethane), dmpe (e, dmpe = dimethylphosphinoethane)] were prepared from the direct reactions between BrMe(2)P=NSiMe(3) (5) and Br(CF(3)CH(2)O)(2)P=NSiMe(3) (7) and the corresponding tertiary phosphines R(3)P or the diphosphines Me(2)P(CH(2))(n)PMe(2) (n = 1, 2). Cations of the type [6](+) and [8](+), with electron-donating and -withdrawing groups at the phosphoranimine phosphorus center, respectively, undergo facile phosphine ligand substitution with the strong N-donor 4-dimethylaminopyridine (DMAP) to yield the corresponding DMAP-stabilized salts [DMAP x PR(2)=NSiMe(3)]Br [R = Me ([9](+)), OCH(2)CF(3) ([10](+))]. Cations [6](+) with Br(-) anions are particularly labile: for example, [6a]Br slowly releases PMe(3), BrSiMe(3), and forms cyclic phosphazenes such as [Me(2)P=N](4). Anion exchange reactions between the salts [6b]Br or [8c]Br and AgOTf (OTf = CF(3)SO(3)) quantitatively afforded the corresponding and more stable triflate salts [6b]OTf and [8c]OTf. Phosphine ligand abstraction reactions with B(C(6)F(5))(3) were observed for the bromide salts [6b]Br and [8c]Br, which regenerated the phosphoranimines 5 and 7, respectively, and formed the adduct R(3)P x B(C(6)F(5))(3). In contrast, the triflate salts [6b]OTf and [8c]OTf were unreactive under the same conditions. X-ray structural analysis of the P-donor stabilized cations revealed longer P-P and P-N bond lengths and smaller P-N-Si bond angles for cations [6](+) compared to analogs [8](+). These structural differences were rationalized using the negative hyperconjugation bonding model. In addition, the (1)J(PP) coupling constants for the cations [6](+) observed by both solution and solid-state (31)P NMR are remarkably small (13-25 Hz), whereas those for [8](+) are substantially larger and positive (276-324 Hz) and are as expected for P(IV)(+)-P(V) systems. DFT studies suggest that the significant difference in (1)J(PP) couplings observed for [6](+) and [8](+) appears to be related to the electronegativity of the R' substituents at the phosphoranimine phosphorus center rather than the strength of the donor-acceptor P-P bond, which is slightly weaker in [6](+) relative to that in [8](+), as indicated by the X-ray data and reactivity studies.

A 13C and 15N solid-state NMR study of structural disorder and aurophilic bonding in AuI and AuIII cyanide complexes.

Solid-state nuclear magnetic resonance has been used to study several cyanoaurates. Carbon-13 and nitrogen-15 NMR spectra of samples enriched with isotopically labeled 13C,15N cyanide ligands were recorded for stationary samples and samples spinning at the magic angle. Several salts of the dicyanoaurate(I) anion, M[Au(CN)2], where M = n-butylammonium, potassium, and thallium, were studied via solid-state NMR. A gold(III) cyanide, K[Au(CN)4], was also investigated. Carbon-13 and nitrogen-15 chemical shift tensors are reported for each salt, as are the measured 13C,15N direct dipolar coupling constants together with the related derived cyanide bond lengths, r(C,N). The value for r(C,N) in [(n-C4H9)4N][Au(CN)2], 1.17(5) A, was determined to be more realistic than a previously reported X-ray diffraction value of 1.03(4) A. Large 13C NMR line widths from Tl[Au(CN)2], 250-315 Hz, are attributed to coupling with 197Au (I = 3/2) and/or 203/205Tl (I = 1/2), as confirmed by measurements of the transverse relaxation constant, T2. Investigation of the carbon-13 chemical shifts for cyanide ligands bound to gold involved in a variety of metallophilic bonding environments demonstrates that the chemical shift is sensitive to metallophilic bonding. Differences in Au-Tl metallophilic bonding are shown to cause a difference in the isotropic carbon-13 chemical shift of up to 15.7 ppm, while differences in Au-Au aurophilic bonding are found to be responsible for a change of up to 5.9 ppm. The disordered polymeric material gold(I) monocyanide, AuCN, was also investigated using 13C and 15N SSNMR. Two-dimensional 13C,13C double-quantum dipolar-recoupling spectroscopy was used to probe connectivity in this material. The 13C NMR site multiplicity in AuCN is explained on the basis of sensitivity of the carbon-13 chemical shift to aurophilic bonding of the directly bonded gold atom. This assignment allows estimation of the position of the linear [-M-CN-]infinity chain's position with respect to the neighboring polymer chain. For the samples studied, a range of 7 +/- 2% to 25 +/- 5% of the AuCN chains are found to be "slipped" instead of aligned with the neighboring chains at the metal position.

Polymorphism of potassium ferrocyanide trihydrate as studied by solid-state multinuclear NMR spectroscopy and X-ray diffraction.

The polymorphism of bulk powder samples of potassium ferrocyanide trihydrate (K(4)Fe(CN)(6).3H(2)O, KFCT) has been studied using (1)H, (13)C, and (15)N NMR spectroscopy in combination with X-ray diffraction. At room temperature, KFCT typically crystallizes in a monoclinic C2/c form, which converts irreversibly to a monoclinic Cc form upon cooling below -25 degrees C. The structure of both of these forms has been determined using single-crystal X-ray diffraction. A less common metastable tetragonal I4(1)/a form is also known to exist at room-temperature. This tetragonal form also converts to the monoclinic Cc form upon cooling, although this phase transition is irreversible and occurs at -60 degrees C. Initial room-temperature (15)N MAS NMR spectra and powder X-ray diffraction patterns of ground powder samples of KFCT prepared using a variety of crystallization methods suggested that only the C2/c form was obtained from a bulk crystallization. The (13)C MAS NMR spectra consisted of six peaks with equal integrated areas, a result that is inconsistent with the (15)N NMR spectra and known crystal structures. When the samples were not ground, the relative areas of the (13)C NMR peaks were altered, indicating that the bulk samples in fact consisted of the two known forms of KFCT. Using the known temperature dependence of these two polymorphs, the (13)C peaks corresponding to each of the C2/c and I4(1)/a forms were assigned. The (13)C NMR spectra and powder X-ray diffraction results demonstrate that upon grinding, a near 50-50 mixture of the two forms is always produced, rather than a new form entirely. The insensitivity of the (15)N NMR spectra to the polymorphism of KFCT is surprising, and likely arises from a fortuitous overlap of the (15)N NMR peaks of the two forms.

A novel method to control the size of silver nanoparticles formed on chabazite.

High density, uniform, surface-supported nanosilver particles can be generated on mineral chabazite by thermal reduction of exchanged silver cations. These nanoparticles have properties unique from those of bulk silver, including antimicrobial activity, which are a function of particle size. In this study, the activation environment is manipulated in order to alter the average size of the silver particles. Nanoparticles with mean diameters of 4.2, 6.1 and 35.0 nm are formed in air, Ar and H2 at 400 degrees C, respectively. Interaction with the surface of highly polarizing chabazite stabilizes the nanoparticles, and unreduced silver ions on the nanosilver surfaces may play a critical role in determining particle size.