An Investigation of Chlorine Ligands in Transition-Metal Complexes via 35Cl Solid-State NMR and Density Functional Theory Calculations

Can simple 'molecular' corrections outperform projector augmented-wave density functional theory in the prediction of 35 Cl electric field gradient tensor parameters for chlorine-containing crystalline systems?

Many-body expansion (MBE) fragment approaches have been applied to accurately compute nuclear magnetic resonance (NMR) parameters in crystalline systems. Recent examples demonstrate that electric field gradient (EFG) tensor parameters can be accurately calculated for 14 N and 17 O. A key additional development is the simple molecular correction (SMC) approach, which uses two one-body fragment (i.e., isolated molecule) calculations to adjust NMR parameter values established using 'benchmark' projector augmented-wave (PAW) density functional theory (DFT) values. Here, we apply a SMC using the hybrid PBE0 exchange-correlation (XC) functional to see if this can improve the accuracy of calculated 35 Cl EFG tensor parameters. We selected eight organic and two inorganic crystal structures and considered 15 chlorine sites. We find that this SMC improves the accuracy of computed values for both the 35 Cl quadrupolar coupling constant (CQ ) and the asymmetry parameter ( η Q ) by approximately 30% compared with benchmark PAW DFT values. We also assessed a SMC that offers local improvements not only in terms of the quality of the XC functional but simultaneously in the quality of the description of relativistic effects via the inclusion of spin-orbit effects. As the inorganic systems considered contain heavy atoms bonded to the chlorine atoms, we find further improvements in the accuracy of calculated 35 Cl EFG tensor parameters when both a hybrid functional and spin-orbit effects are included in the SMC. On the contrary, for chlorine-containing organics, the inclusion of spin-orbit relativistic effects using a SMC does not improve the accuracy of computed 35 Cl EFG tensor parameters.

Structure and bonding in rhodium coordination compounds: a 103Rh solid-state NMR and relativistic DFT study

This study demonstrates the application of 103Rh solid-state NMR (SSNMR) spectroscopy to inorganic and organometallic coordination compounds, in combination with relativistic density functional theory (DFT) calculations of 103Rh chemical shift tensors and their analysis with natural bond orbital (NBO) and natural localized molecular orbital (NLMO) protocols, to develop correlations between 103Rh chemical shift tensors, molecular structure, and Rh-ligand bonding. 103Rh is one of the least receptive NMR nuclides, and consequently, there are very few reports in the literature. We introduce robust 103Rh SSNMR protocols for stationary samples, which use the broadband adiabatic inversion-cross polarization (BRAIN-CP) pulse sequence and wideband uniform-rate smooth-truncation (WURST) pulses for excitation, refocusing, and polarization transfer, and demonstrate the acquisition of 103Rh SSNMR spectra of unprecedented signal-to-noise and uniformity. The 103Rh chemical shift tensors determined from these spectra are complemented by NBO/NLMO analyses of contributions of individual orbitals to the 103Rh magnetic shielding tensors to understand their relationship to structure and bonding. Finally, we discuss the potential for these experimental and theoretical protocols for investigating a wide range of materials containing the platinum group elements.

Exploring the Potential of Multinuclear Solid-State 1 H, 13 C, and 35 Cl Magnetic Resonance To Characterize Static and Dynamic Disorder in Pharmaceutical Hydrochlorides

Crystallographic disorder, whether static or dynamic, can be detrimental to the physical and chemical stability, ease of crystallization and dissolution rate of an active pharmaceutical ingredient. Disorder can result in a loss of manufacturing control leading to batch-to-batch variability and can lengthen the process of structural characterization. The range of NMR active nuclei makes solid-state NMR a unique technique for gaining nucleus-specific information about crystallographic disorder. Here, we explore the use of high-field ³⁵ Cl solid-state NMR at 23.5 T to characterize both static and dynamic crystallographic disorder: specifically, dynamic disorder occurring in duloxetine hydrochloride (1), static disorder in promethazine hydrochloride (2), and trifluoperazine dihydrochloride (3). In all structures, the presence of crystallographic disorder was confirmed by ¹³ C cross-polarization magic-angle spinning (CPMAS) NMR and supported by GIPAW-DFT calculations, and in the case of 3, ¹ H solid-state NMR provided additional confirmation. Applying ³⁵ Cl solid-state NMR to these compounds, we show that higher magnetic fields are beneficial for resolving the crystallographic disorder in 1 and 3, while broad spectral features were observed in 2 even at higher fields. Combining the data obtained from ¹ H, ¹³ C, and ³⁵ Cl NMR, we show that 3 exhibits a unique case of disorder involving the ⁺ N-H hydrogen positions of the piperazinium ring, driving the chloride anions to occupy three distinct sites.

Equatorial Electronic Structure in the Uranyl Ion: Cs2UO2Cl4 and Cs2UO2Br4

Electric field gradient (EFG) tensors in the equatorial plane of the linear UO₂²⁺ ion have been measured by nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments and computed by relativistic Kohn-Sham methods with and without environment embedding for Cs₂UO₂Cl₄ and Cs₂UO₂Br₄. This approach expands the possibilities for probing the electronic structure in uranyl complexes beyond the strongly covalent U-O bonds. The combined analyses find that one of the two largest principal EFG tensor components at the halogen sites points along the U-X bond (X = Cl, Br), and the second is parallel to the UO₂²⁺ ion; in Cs₂UO₂Cl₄, the components are nearly equal in magnitude, whereas in Cs₂UO₂Br₄, due to short-range bromide-cesium interactions, the equatorial component is dominant for one pair of Br sites and the axial component is larger for the second pair. The directions and relative magnitudes of the field gradient principal axes are found to be sensitive to the σ and π electron donation by the ligands and the model of the environment. Chlorine-35 NQR spectra of ²³⁵U-depleted and ²³⁵U-enriched Cs₂UO₂Cl₄ exhibited no uranium-isotope-dependent shift, but the resonance of the depleted sample displayed a 58% broader line width.

Solid-state 11B NMR studies of coinage metal complexes containing a phosphine substituted diboraanthracene ligand

Transition metal interactions with Lewis acids (M → Z linkages) are fundamentally interesting and practically important. The most common Z-type ligands contain boron, which contains an NMR active ¹¹B nucleus. We measured solid-state ¹¹B{¹H} NMR spectra of copper, silver, and gold complexes containing a phosphine substituted 9,10-diboraanthracene ligand (B₂P₂) that contain planar boron centers and weak M → BR₃ linkages ([(B₂P₂)M][BAr^F₄] (M = Cu (1), Ag (2), Au (3)) characterized by large quadrupolar coupling (C_Q) values (4.4-4.7 MHz) and large span (Ω) values (93-139 ppm). However, the solid-state ¹¹B{¹H} NMR spectrum of K[Au(B₂P₂)]^- (4), which contains tetrahedral borons, is narrow and characterized by small C_Q and Ω values. DFT analysis of 1-4 shows that C_Q and Ω are expected to be large for planar boron environments and small for tetrahedral boron, and that the presence of a M → BR₃ linkage relates to the reduction in C_Q and ¹¹B NMR shielding properties. Thus solid-state ¹¹B NMR spectroscopy contains valuable information about M → BR₃ linkages in complexes containing the B₂P₂ ligand.

Chlorine-35 Solid-State Nuclear Magnetic Resonance Spectroscopy as an Indirect Probe of the Oxidation Number of Tin in Tin Chlorides

Ultrawideline ³⁵Cl solid-state nuclear magnetic resonance (SSNMR) spectra of a series of 12 tin chlorides were recorded. The magnitude of the ³⁵Cl quadrupolar coupling constant (C_Q) was shown to consistently indicate the chemical state (oxidation number) of the bound Sn center. The chemical state of the Sn center was independently verified by tin Mössbauer spectroscopy. C_Q(³⁵Cl) values of >30 MHz correspond to Sn(IV), while C_Q(³⁵Cl) readings of <30 MHz indicate that Sn(II) is present. Tin-119 SSNMR experiments would seem to be the most direct and effective route to interrogating tin in these systems, yet we show that ambiguous results can emerge from this method, which may lead to an incorrect interpretation of the Sn oxidation number. The accumulated ³⁵Cl NMR data are used as a guide to assign the Sn oxidation number in the mixed-valent metal complex Ph₃PPd^ImSnCl₂. The synthesis and crystal structure of the related Ph₃PPt^ImSnCl₂ are reported, and ¹⁹⁵Pt and ³⁵Cl SSNMR experiments were also used to investigate its Pt-Sn bonding. Plane-wave DFT calculations of ³⁵Cl, ¹¹⁹Sn, and ¹⁹⁵Pt NMR parameters are used to model and interpret experimental data, supported by computed ¹¹⁹Sn and ¹⁹⁵Pt chemical shift tensor orientations. Given the ubiquity of directly bound Cl centers in organometallic and inorganic systems, there is tremendous potential for widespread usage of ³⁵Cl SSNMR parameters to provide a reliable indication of the chemical state in metal chlorides.

Grafting of a new bis-silylamido aluminum species on silica: insight from solid-state NMR into interactions with the surface

The new bisamido aluminum species [AlCl{N(SiMe3)2}2(THF)] (1) was prepared and fully characterized by 27Al and 35Cl solid-state NMR, along with X-ray diffraction studies. 1 was grafted on silica partially dehydroxylated at 700 °C, affording silica-supported Al species. The resulting material (2) was characterized by IR, elemental analysis and 1H, 13C and 27Al solid-state MAS NMR. The 1D and 2D 27Al MAS NMR studies showed the occurrence of two types of species, where the Al center adopts a tetracoordinated coordination sphere, with as an additional coordinated Lewis base, either a THF ligand or a silica-surface siloxane moiety. DFT calculations allowed understanding the grafting mechanism and the spectroscopic properties of the material.

Solid-state NMR of quadrupolar nuclei for investigations into supported organometallic catalysts: scope and frontiers

The scope, limitations and outlooks of half-integer quadrupolar nuclei NMR as applied to supported catalysts characterization are discussed.

Elimination of the baseline distortions in WURST-CPMG static experiments

The WURST-CPMG pulse sequence enables: (i) observing very broad spectra due to WURST excitation, and (ii) increasing the S/N ratio due to CPMG acquisition. However, strong baseline distortions may be observed, which make the extraction of the tensor information difficult. We propose a slight modification of the sequence, WURST-CPMGM, in which we skip the first M echoes and we only acquire the following ones. This simple treatment mostly eliminates the strong background signal and the ring down effects, leading to a flat baseline.

Natural abundance 14N and 15N solid-state NMR of pharmaceuticals and their polymorphs

¹⁴N and ¹⁵N solid-state NMR at natural abundance are used in tandem for the investigation of pharmaceuticals and their polymorphs.

35Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients

In this work, we show how to obtain efficient dynamic nuclear polarization (DNP) enhanced ³⁵Cl solid-state NMR (SSNMR) spectra at 9.4 T and demonstrate how they can be used to characterize the molecular-level structure of hydrochloride salts of active pharmaceutical ingredients (APIs) in both bulk and low wt% API dosage forms.

Rapid acquisition of wideline MAS solid-state NMR spectra with fast MAS, proton detection, and dipolar HMQC pulse sequences

Fast MAS and proton detection are applied to rapidly acquire wideline solid-state NMR spectra of spin-1/2 and half-integer quadrupolar nuclei.