Electrostatic Surface Potentials and Chalcogen-Bonding Motifs of Substituted 2,1,3-Benzoselenadiazoles Probed via 77Se Solid-State NMR Spectroscopy.

Chalcogen bonds (ChB) are moderately strong, directional, and specific non-covalent interactions that have garnered substantial interest over the last decades. However, ChB applications are currently hampered by a lack of methods to characterize and control chalcogen bonds. We report on the influence of various substituents (halogens, cyano, and methyl groups) on the observed self-complementary ChB networks of 2,1,3-benzoselenadiazoles. From molecular electrostatic potential calculations, we show that the electrostatic surface potentials (ESP) of the σ-holes on selenium are largely influenced by the electron-withdrawing character of these substituents. Structural analyses via X-ray diffraction reveal a variety of ChB geometries and binding modes that are rationalized via the computed ESP maps, although the structure of 5,6-dimethyl-2,1,3-benzoselenadiazole also demonstrates the influence of steric interactions. 77Se solid-state magic-angle spinning NMR spectroscopy, in particular the analysis of the selenium chemical shift tensors, is found to be an effective probe able to characterize both structural and electrostatic features of these self-complementary ChB systems. We find a positive correlation between the value of the ESP maxima at the σ-holes and the experimentally measured 77Se isotropic chemical shift, while the skew of the chemical shift tensor is established as a metric which is reflective of the ChB binding motif.

Double-rotation (DOR) NMR spectroscopy: Progress and perspectives.

Double-rotation (DOR) solid-state NMR spectroscopy is a high-resolution technique developed in the late 1980s. Although multiple-quantum magic-angle spinning (MQMAS) became the most widely used high-resolution method for half-integer spin quadrupoles after 1995, development and application of DOR NMR to a variety of chemical and materials science problems has endured. This Trend article recapitulates the development of DOR NMR, discusses various applications, and describes possible future directions. The main technical limitations specific to DOR NMR are simply related to the size of the double rotor system. The relatively large outer rotor (and thus coil) used for most applications over the past 35 years translates into relatively low rotor spinning frequencies, a low filling factor, and weak radiofrequency powers available for excitation and for proton decoupling. Ongoing developments in NMR instrumentation, including ever-shrinking MAS rotors and spherical NMR rotors, could solve many of these problems and may augur a renaissance for DOR NMR.

Solid-state NMR spectra of amino acid enantiomers and their relative intensities.

Under normal experimental conditions in an achiral environment, NMR spectra of enantiomers have chemical shifts and J couplings which are not differentiable. In this work, the reproducibility of spectral intensities for pairs of amino acid enantiomers, as well as factors influencing these intensities, is assessed using 13C and 15N cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy. Prompted by a recent literature debate over a possible influence of the chirality-induced spin selectivity (CISS) effect on spectral intensities obtained in CP/MAS NMR experiments carried out on enantiomers, a number of control experiments were performed with recycle delays of at least 5T1. These included the analysis of proton-decoupled Bloch decay solid-state NMR spectra as well as solution NMR spectra where the cross polarization process is absent. Bloch decay and CP/MAS NMR spectra yield the same relative intensities for pairs of enantiomers while solution NMR spectra provide relative intensities closest to unity. Differences of plus-or-minus a few percent in the D/L spectral intensity ratios observed in all solid-state NMR experiments are due to sample preparation (i.e., grinding, particle size, partial amorphization) and limitations on sample purity. As previously described in the literature, more drastic intensity differences on the order of 50% are easily created by ball milling the samples. Finally, apodization is shown to invert the apparent D/L ratio in low signal-to-noise 15N CP/MAS NMR spectra of aspartic acid enantiomers. In summary, no spectral intensity differences attributable to enantiomerism are identified.

Experimental Evidence for Non-Fermi-Contact J Coupling Across Chalcogen Bonds in Ionic Salt Cocrystal Polymorphs.

A pair of novel polymorphic ionic cocrystals of 3,4-dicyanotelluradiazole and tetraphenylphosphonium bromide are synthesized and are characterized by single-crystal XRD. Strong and directional non-covalent chalcogen bonds (ChB) between Te and Br are analyzed via solid-state NMR to reveal large and anisotropic J(125Te,79/81Br) coupling tensors, providing unequivocal evidence for non-Fermi contact contributions across ChBs. Along with large 79/81Br quadrupolar couplings for the Br- anions, these data provide new tools to characterize chalcogen bonds and to differentiate between ChB polymorphs.

Modulation of Rotational Dynamics in Halogen-Bonded Cocrystalline Solids.

Dynamic processes are responsible for the functionality of a range of materials, biomolecules, and catalysts. We report a detailed systematic study of the modulation of methyl rotational dynamics via the direct and the indirect influence of noncovalent halogen bonds. For this purpose, a novel series of cocrystalline architectures featuring halogen bonds (XB) to tetramethylpyrazine (TMP) is designed and prepared using gas-phase, solution, and solid-state mechanochemical methods. Single-crystal X-ray diffraction reveals the capacity of molecular bromine as well as weak chloro-XB donors to act as robust directional structure-directing elements. Methyl rotational barriers (Ea) measured using variable-temperature deuterium solid-state NMR range from 3.75 ± 0.04 kJ mol-1 in 1,3,5-trichloro-2,4,6-trifluorobenzene·TMP to 7.08 ± 0.15 kJ mol-1 in 1,4-dichlorotetrafluorobenzene·TMP. Ea data for a larger series of TMP cocrystals featuring chloro-, bromo-, and iodo-XB donors are shown to be governed by a combination of steric and electronic factors. The average number of carbon-carbon close contacts to the methyl group is found to be a key steric metric capable of rationalizing the observed trends within each of the Cl, Br, and I series. Differences between each series are accounted for by considering the strength of the σ-hole on the XB donor. One possible route to modulating dynamics is therefore via designer cocrystals of variable stoichiometry, maintaining the core chemical features of interest between a given donor and acceptor while simultaneously modifying the number of carbon close contacts affecting dynamics. These principles may provide design opportunities to modulate more complex geared or cascade dynamics involving larger functional groups.

Rapid Access to Encapsulated Molecular Rotors via Coordination-Driven Macrocycle Formation.

Macrocycle formation that relies upon trans metal coordination of appropriately placed pyridine ligands within an arylene ethynylene construct provides rapid and reliable access to molecular rotators encapsulated within macrocyclic stators. Showing no significant close contacts to the central rotators, X-ray crystallography of AgI -coordinated macrocycles provides plausibility for unobstructed rotation or wobbling of rotators within the central cavity. Solid-state 13 C NMR of PdII -coordinated macrocycles supports the notion of unobstructed movement of simple arenes in the crystal lattice. Solution 1 H NMR studies indicate complete and immediate macrocycle formation upon the introduction of PdII to the pyridyl-based ligand at room temperature. Moreover, the formed macrocycle is stable in solution; a lack of significant changes in the 1 H NMR spectrum upon cooling to -50 °C is consistent with the absence of dynamic behavior. The synthetic route to these macrocycles is expedient and modular, providing access to rather complex constructs in four simple steps involving Sonogashira coupling and deprotection reactions.

To what extent do bond length and angle govern the 13C and 1H NMR response to weak CH⋯O hydrogen bonds? A case study of caffeine and theophylline cocrystals.

Weak hydrogen bonds are important structure-directing elements in supramolecular chemistry and biochemistry. We consider here weak CH⋯O hydrogen bonds in a series of cocrystals of theophylline and caffeine and assess to what extent the CH⋯O distance and angle govern the observed 13C and 1H isotropic chemical shifts. Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations consistently predict a decrease in the 13C and 1H magnetic shielding constants upon hydrogen bond formation on the order of 2-5 ppm (13C) and 1-2 ppm (1H). These trends are reproduced using the machine-learning approach implemented in ShiftML. Experimental 13C and 1H chemical shifts obtained for powdered samples using one-dimensional NMR spectroscopy as well as heteronuclear correlation (HETCOR) spectroscopy correlate well with the GIPAW DFT results. However, the experimental 13C NMR response only correlates moderately well with the hydrogen bond length and angle, while the experimental 1H chemical shifts only show very weak correlations to these local structural elements. DFT computations on isolated imidazole-formaldehyde models show that the 13C and 1H chemical shifts generally decrease with the C⋯O distance but show no clear dependence on the CH⋯O angle. These results demonstrate that the 13C and 1H response to weak CH⋯O hydrogen bonding is influenced significantly by additional weak contacts within cocrystal heterodimeric units.

Field-stepped ultra-wideline NMR at up to 36 T: On the inequivalence between field and frequency stepping.

Field-stepped NMR spectroscopy at up to 36 T using the series-connected hybrid (SCH) magnet at the U.S. National High Magnetic Field Laboratory is demonstrated for acquiring ultra-wideline powder spectra of nuclei with very large quadrupolar interactions. Historically, NMR evolved from the continuous-wave (cw) field-swept method in the early days to the pulsed Fourier-transform method in the modern era. Spectra acquired using field sweeping are generally considered to be equivalent to those acquired using the pulsed method. Here, it is shown that field-stepped wideline spectra of half-integer spin quadrupolar nuclei acquired using WURST/CPMG methods can be significantly different from those acquired with the frequency-stepped method commonly used with superconducting magnets. The inequivalence arises from magnetic field-dependent NMR interactions such as the anisotropic chemical shift and second-order quadrupolar interactions; the latter is often the main interaction leading to ultra-wideline powder patterns of half-integer spin quadrupolar nuclei. This inequivalence needs be taken into account to accurately and correctly determine the quadrupolar coupling and chemical shift parameters. A simulation protocol is developed for spectral fitting to facilitate analysis of field-stepped ultra-wideline NMR spectra acquired using powered magnets. A MATLAB program which implements this protocol is available on request.

Double Chalcogen Bonds: Crystal Engineering Stratagems via Diffraction and Multinuclear Solid-State Magnetic Resonance Spectroscopy.

Group 16 chalcogens potentially provide Lewis-acidic σ-holes, which are able to form attractive supramolecular interactions with electron rich partners through chalcogen bonds. Here, a multifaceted experimental and computational study of a large series of novel chalcogen-bonded cocrystals, prepared using the principles of crystal engineering, is presented. Single-crystal X-ray diffraction studies reveal that dicyanoselenadiazole and dicyanotelluradiazole derivatives work as promising supramolecular synthons with the ability to form double chalcogen bonds with a wide range of electron donors including halides and oxygen- and nitrogen-containing heterocycles. Extensive 77 Se and 125 Te solid-state nuclear magnetic resonance spectroscopic investigations of cocrystals establish correlations between the NMR parameters of selenium and tellurium and the local chalcogen bonding geometry. The relationships between the electronic environment of the chalcogen bond and the 77 Se and 125 Te chemical shift tensors were elucidated through a natural localized molecular orbital density functional theory analysis. This systematic study of chalcogen-bond-based crystal engineering lays the foundations for the preparation of the various multicomponent systems and establishes solid-state NMR protocols to detect these interactions in powdered materials.

Direct investigation of chalcogen bonds by multinuclear solid-state magnetic resonance and vibrational spectroscopy.

We report a multifaceted experimental and computational study of three self-complementary chalcogen-bond donors as well as a series of seven chalcogen bonded cocrystals. Bis(selenocyanatomethyl)benzene derivatives were cocrystallized with various halide salts (Bu4NCl, Bu4NBr, Bu4NI) and nitrogen-containing Lewis bases (4,4'-bipyridine and 1,2-di(4-pyridyl)ethylene). Three new single-crystal X-ray structures are reported. 77Se solid-state nuclear magnetic resonance spectroscopic study of a series of cocrystals establishes correlations between the NMR parameters of selenium and the local ChB geometry. For example, the 77Se isotropic chemical shift generally decreases on cocrystal formation. Diagnostic 13C chemical shifts are also described. In addition, all the chalcogen bonded cocrystals and pure tectons are investigated by Raman and IR spectroscopy techniques. Characteristic red shifts of the NC-Se stretching band upon cocrystal formation on the order of 10 to 20 cm-1 are observed, which provides a distinct signature of the chalcogen bond involving selenocyanates. The 125Te chemical shift tensor and X-ray structure of chalcogen-bonded tellurocyanatomethylbenzene are also reported. Insights into the connection between the electronic structure of the chalcogen bond and the experimentally measured 77Se chemical shift tensors are afforded through a natural localized molecular orbital density functional theory analysis. For the systems studied here, the lack of a very strong a correlation between experimental and DFT-computed 77Se chemical shift tensors leads to the conclusion that many structural features likely influence their ultimate values; however, computations on model systems reveal that the ChB alone produces consistent and predictable effects (e.g., the chalcogen chemical shift decreases as the chalcogen bond is shortened).

121/123Sb Nuclear Quadrupole Resonance Spectroscopy: Characterization of Non-Covalent Pnictogen Bonds and NQR Crystallography.

Pnictogen (or pnicogen) bonding is an attractive interaction between the electrophilic region of group 15 elements (N, P, As, Sb, Bi) and a nucleophile. This interaction for which unique applications in catalysis have recently been uncovered continues to gain popularity. Here, we investigate a series of pnictogen-bonded cocrystals based on SbF3 and SbCl3, prepared via mechanochemical ball milling, with 121/123Sb ( I = 5/2 and 7/2, respectively) nuclear quadrupole resonance (NQR) spectroscopy. Observed NQR frequency shifts upon cocrystallization are on the order of 0.1 to 10 MHz and are clearly diagnostic of the formation of pnictogen bonds to antimony. Further evidence for pnictogen bonding is obtained by complementary 13C cross-polarization magic-angle spinning solid-state NMR experiments. DFT calculations of NMR parameters as well as natural localized molecular orbital analyses support the experimental findings and elucidate the electronic origins of the experimental NQR frequency shifts. This work provides insights into the changes in the antimony quadrupolar coupling constant upon pnictogen bonding: strikingly, the decreases noted here parallel those known for hydrogen bonds, but contrast with the increases reported for halogen bonds. The utility of the observed antimony nuclear quadrupolar coupling constants in constraining structural models of cocrystals for which diffraction-based structures are unavailable, i.e., a rudimentary implementation of NQR crystallography, is established. Overall, this work offers a new approach to understand emerging classes of electrophilic interactions and to contextualize them in the broader landscape of established chemical bonding paradigms.

Single-Crystal NMR Characterization of Halogen Bonds.

Oxygen-17-enriched triphenylphosphine oxide and three of its halogen-bonded cocrystals featuring 1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene as halogen bond donors have been characterized by 31P and 17O single-crystal NMR spectroscopy. Single-crystal NMR allows for the measurement of not only the magnitudes of various NMR interaction tensors, but also their orientations relative to the crystal lattice and therefore relative to the halogen bonds themselves. 31P chemical shift tensors, 17O chemical shift tensors, 17O quadrupolar coupling tensors, and 31P-17O indirect nuclear spin-spin (J) coupling tensors are reported here for P═O···I halogen bonds. The angular deviations in the directions of the pseudo-unique components of the 31P chemical shift tensors, the 17O chemical shift tensors, and the 17O quadrupolar coupling tensors from the direction of the oxygen-iodine halogen bond correlate with the deviations in linearity of the P═O···I halogen bond. There is also a clear decrease in anisotropy and an increase in asymmetry of the J(31P,17O) coupling tensors attributable to the formation of iodine-oxygen halogen bonds. The small but quantifiable changes in the tensors are consistent with the weak nature of these halogen bonds relative to the P═O motif. Overall, this work establishes single-crystal NMR as a novel probe of halogen bonds in solids. Analysis of the results has provided insights into the correlations between the magnitude and orientation of various NMR interaction tensors and the local geometry of the halogen bond. Gauge-including projector-augmented wave computations corroborate the experimental findings.

SCFit: Software for single-crystal NMR analysis. Free vs constrained fitting.

The design and implementation of a software package for the analysis of single-crystal NMR data is presented. The SCFit software can treat spectra arising from various interactions: (i) chemical shift tensor only; (ii) chemical shift tensor and quadrupolar coupling tensor; (iii) dipolar and indirect nuclear spin-spin coupling tensors; (iv) all four interactions. The software is demonstrated on recently reported 17O and 31P single-crystal NMR data for triphenylphosphine oxide and for two of its halogen-bonded cocrystals. The 17O single-crystal NMR data represent a case where all four above-mentioned interactions simultaneously affect the spectra. SCFit can fit the chemical shift and quadrupolar coupling in two ways: (i) through an unconstrained fitting process where all tensor parameters are freely optimized or (ii) through a constrained fitting process where the principal components of the tensors may be fixed to values known previously with high precision via the analysis of powder samples. The second strategy is explored in an effort to reduce the number of unknowns in the fitting process; an improvement in the precision of the resulting tensor orientations is noted in some cases.

Rapid Identification of Halogen Bonds in Co-Crystalline Powders via 127 I Nuclear Quadrupole Resonance Spectroscopy.

127 I nuclear quadrupole resonance (NQR) spectroscopy is established as a rapid and robust method to indicate the formation of iodine-nitrogen halogen bonds in co-crystalline powders. Once the relevant spectral frequency range has been established, diagnostic 127 I NQR spectra can be acquired in seconds. The method is demonstrated for a series of co-crystals of 1,4-diiodobenzene. Changes in the 127 I quadrupolar coupling constant (CQ ) by up to 74.4 MHz correlate with the length of the C-I donor covalent bond and inversely with the I⋅⋅⋅N halogen-bond length. The predictive power of this technique is validated on two previously unknown co-crystalline powders prepared mechanochemically. Single-crystal growth via co-sublimation and structure determination by single-crystal X-ray diffraction cross-validates the findings. Natural localized molecular-orbital analyses provide insight into the origins of the quadrupolar coupling constants.

Comparing the Halogen Bond to the Hydrogen Bond by Solid-State NMR Spectroscopy: Anion Coordinated Dimers from 2- and 3-Iodoethynylpyridine Salts.

Halogen bonding is an increasingly important tool in crystal engineering, and measuring its influence on the local chemical and electronic environment is necessary to fully understand this interaction. Here, we present a systematic crystallographic and solid-state NMR study of self-complementary halogen-bonded frameworks built from the halide salts (HCl, HBr, HI, HI3 ) of 2-iodoethynylpyridine and 3-iodoethynylpyridine. A series of single crystal X-ray structures reveals the formation of discrete charged dimers in the solid state, directed by simultaneous X- ⋅⋅⋅H-N+ hydrogen bonds and C-I⋅⋅⋅X- halogen bonds (X=Cl, Br, I). Each compound was studied using multinuclear solid-state magnetic resonance spectroscopy, observing 1 H to investigate the hydrogen bonds and 13 C, 35 Cl, and 79/81 Br to investigate the halogen bonds. A natural localized molecular orbital analysis was employed to help interpret the experimental results. 1 H SSNMR spectroscopy reveals a decrease in the chemical shift of the proton participating in the hydrogen bond as the halogen increases in size, whereas the 13 C SSNMR reveals an increased 13 C chemical shift of the C-I carbon for C-I⋅⋅⋅X- relative to C-I⋅⋅⋅N halogen bonds. Additionally, 35 Cl and 79/81 Br SSNMR, along with computational results, have allowed us to compare the C-I⋅⋅⋅X- halogen bond involving each halide in terms of NMR observables. Due to the isostructural nature of these compounds, they are ideal cases for experimentally assessing the impact of different halogen bond acceptors on the solid-state NMR response.

Structural Insights from 59 Co Solid-State NMR Experiments on Organocobalt(I) Catalysts.

A series of fumarate-based organocobalt(I) [CoCp(CO)(fumarate)] catalysts is synthesized and characterized by X-ray crystallography, multinuclear (13 C and 59 Co) solid-state NMR spectroscopy, and 59 Co NQR spectroscopy. Given the dearth of 59 Co solid-state NMR studies on CoI compounds, the present work constitutes the first systematic characterization of the 59 Co electric field gradient and chemical shift tensors for a series of cobalt complexes in this oxidation state. Using X-ray crystallography, the molecular geometry about the CoI centre is found to be nearly identical in all compounds studied herein. Owing to the 59 Co nucleus' large chemical shift range, solid-state NMR experiments are found to be able to detect small structural differences between the individual organocobalt(I) compounds. With the aid of density functional theory calculations on these complexes, it is shown that the 59 Co chemical shift anisotropy and the 59 Co quadrupolar coupling constant are both extremely sensitive gauges of the Fu-Co-Cp bond angle, providing a link between these 59 Co NMR observables and the catalysts' structures.

Dynamic Disorder and Electronic Structures of Electron-Precise Dianionic Diboranes: Insights from Solid-State Multinuclear Magnetic Resonance Spectroscopy.

The J(11B,11B) coupling constants of various salts of the electron-precise hexacyanodiborane(6) dianion, [B2(CN)6]2-, were obtained using 11B double-quantum-filtered (DQF) J-resolved solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Our results show that the magnitude of the DQF J splitting is influenced by both the crystallographic symmetry of the system and the presence of dynamics. The splittings are amplified by a factor of 3 as compared to the corresponding theoretical J coupling constants for cases where (1) there is an absence of dynamics but the boron pairs are crystallographically equivalent or (2) the boron pairs are crystallographically inequivalent but are rendered magnetically equivalent on the time scale of the experiment due to dynamic disorder, which was identified by 11B and 13C SSNMR experiments. Consequently, molecular motions need to be taken into consideration when interpreting the results of DQF J-resolved experiments, and conversely, these experiments may be used to identify dynamic disorder. Variable-temperature NMR data support the notion of three different motional processes with correlation times ranging from 102 to 106 s-1 over the temperature range of 248-306 K. When molecular motion and crystallographic symmetry are both accounted for, the J(11B,11B) coupling constants for various [B2(CN)6]2- salts were measured to range from 29.4 to 35.8 Hz, and their electronic origins were determined using natural localized molecular orbital and natural bond orbital analyses. The coupling constants were found to strongly correlate to the hybridization states of the boron orbitals that form the B-B bonds and to the strength of the B-B bonds. This study provides a novel tool to study dynamics in ordered and disordered solids and provides new perspectives on electron-precise dianionic diboranes featuring two-center-two-electron bonds in the context of related compounds featuring multiply and singly bonded boron spin pairs.

Prospects for 207Pb solid-state NMR studies of lead tetrel bonds.

The feasibility and value of 207Pb solid-state NMR experiments on compounds featuring lead tetrel bonds is explored. Although the definition remains to be formalized, lead tetrel bonds may be qualitatively described as existing when there is evidence of a net attractive interaction between an electrophilic region associated with lead in a molecular entity and a nucleophilic region in another, or the same, molecular entity. Unambiguous identification of lead tetrel bonds can be challenging due to the hypervalent tendency of lead. We report here a series of 207Pb solid-state NMR experiments on five metal-organic frameworks featuring lead coordinated to hydrazone-based ligands. Such frameworks may be held together in part by lead tetrel bonds. The acquisition of 207Pb solid-state NMR spectra for such materials is feasible and is readily accomplished using a combination of magic-angle spinning and Carr-Purcell-Meiboom-Gill methods in moderate to low applied magnetic fields. The lead centres are characterized by 207Pb isotropic chemical shifts ranging from -426 to -2591 ppm and chemical shift tensor spans ranging from 910 to 2681 ppm. Careful inspection of the structures of the compounds and the literature 207Pb NMR data may suggest that a tetrel bond to lead results in chemical shift parameters which are intermediate between those which are characteristic of holodirected and hemidirected lead coordination geometries. Challenges associated with DFT computations of the 207Pb NMR parameters are discussed. In summary, the 207Pb data for the compounds studied herein show a marked response to the presence of non-coordinating electron-rich moieties in close contact with the electrophilic surface of formally hemidirectionally coordinated lead compounds.

Structural and Crystallographic Information from 61Ni Solid-State NMR Spectroscopy: Diamagnetic Nickel Compounds.

Despite the significance of nickel compounds, NMR spectroscopy of the active nickel isotope 61Ni remains a largely unexplored field. While nickel(0) compounds have been studied by 61Ni NMR in solution, solid-state experiments have been limited to Knight shift studies of nickel metal and nickel intermetallics. In conjunction with an NMR study of their ligands and 61Ni relativistic computations, the first 61Ni solid-state NMR (SSNMR) spectra of diamagnetic compounds are reported here. Specifically, bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)2], tetrakis(triphenylphosphite)nickel(0) [Ni[P(OPh)3]4], and tetrakis(triphenylphosphine)nickel(0) [Ni(PPh3)4] were studied. 61Ni SSNMR spectra of Ni(cod)2 were used to determine its isotropic chemical shift (δiso = 965 ± 10 ppm), span (Ω = 1700 ± 50 ppm), skew (κ = -0.15 ± 0.05), quadrupolar coupling constant (CQ = 2.0 ± 0.3 MHz), quadrupolar asymmetry parameter (η = 0.5 ± 0.2), and the relative orientation of the chemical shift and electric field gradient tensors. A solution study of Ni(cod)2 in C6D6 yielded a narrow 61Ni signal, and the temperature dependence of δiso(61Ni) was assessed (δiso being 936.5 ppm at 295 K). The solution is proposed as a secondary chemical shift reference for 61Ni NMR in lieu of the extremely toxic Ni(CO)4 primary reference. For Ni[P(OPh)3]4, 61Ni SSNMR was used to infer the presence of two distinct crystallographic sites and establish ranges for δiso in the solid state, as well as an upper bound for CQ (3.5 MHz for both sites). For Ni(PPh3)4, line shape fitting provided a δiso value of 515 ± 10 ppm, Ω of 50 ± 50 ppm, κ of 0.5 ± 0.5, CQ of 0.05 ± 0.01 MHz, and η of 0.0 ± 0.2. The study of Ni(PPh3)4, in particular, demonstrates the utility of 61Ni SSNMR given the lack of a previously reported crystal structure and transient nature of Ni(PPh3)4 in solution.

Multinuclear solid-state magnetic resonance study of oxo-bridged diniobium and quadruply-bonded dimolybdenum carboxylate clusters.

Carboxylate paddlewheels and their oxo-bridged analogues constitute ideal building blocks for the assembly of two- and three-dimensional framework materials. Here, we present a multinuclear (1H, 13C, 93Nb, 95Mo) magnetic resonance study of solid samples of Nb2OCl6(O2Ph)2 (1), Mo2(O2CMe)4 (2), and Mo2(O2CCHF2)4 (3). High-resolution proton and 13C CP/MAS NMR spectra provide valuable information on structure and crystal symmetry and on cocrystallized solvent. 93Nb solid-state NMR spectra of 1 provide quadrupolar coupling constants and chemical shift tensors which are characteristic of the axially asymmetric Nb-O-Nb bridging environment. 95Mo solid-state NMR spectra of 2 and 3 provide quadrupolar coupling constants and chemical shift tensors which are directly characteristic of the molybdenum-molybdenum quadruple bonds in these compounds. The quadruple bonds are characterized by particularly large 95Mo chemical shift tensor spans on the order of 5500ppm. Density functional theoretical computations provide good agreement with the 93Nb and 95Mo experimental data, with some exceptions noted. This work demonstrates possible NMR approaches to characterize more complex framework materials and provides key insight into the Mo-Mo quadruple bond.