²⁵Mg Solid-State NMR: A Sensitive Probe of Adsorbing Guest Molecules on a Metal Center in Metal–Organic Framework CPO-27-Mg

Metal–organic frameworks (MOFs) have excellent adsorption capability. To understand their adsorptive properties requires detailed information on the host–guest interaction. The information on MOF desolvation (or activation) is also crucial because the very first step of many applications requires removal of the solvent molecules occluded inside of the pores. Unfortunately, such information is not always available from powder XRD data. Solid-state NMR is an excellent complementary technique to XRD. CPO-27-Mg is a MOF with unusual adsorption ability. The adsorption involves a direct interaction between Mg and guest species. Herein, we present, for the first time, a natural abundance ²⁵Mg solid-state NMR study of CPO-27-Mg at an ultrahigh magnetic field of 21.1 T. The results provide new physical insights into the effects of dehydration/rehydration and adsorption of guest species on the Mg local environment

Chlorine-35 Solid-State NMR Spectroscopy as an Indirect Probe of Germanium Oxidation State and Coordination Environment in Germanium Chlorides

Due to the prevalence of Ge–Cl bonds in germanium chemistry and the inherent challenges of germanium-73 NMR spectroscopy, chlorine-35 NMR spectroscopy was investigated as an indirect method of characterization for these ubiquitous and useful compounds. Chlorine-35 NMR parameters were correlated with structural metrics as well as the oxidation state of germanium

Mapping Out Chemically Similar, Crystallographically Nonequivalent Hydrogen Sites in Metal–Organic Frameworks by ¹H Solid-State NMR Spectroscopy

Metal–organic frameworks (MOFs) are important materials with many actual and potential applications. Crystal structure of many MOFs is determined by single-crystal X-ray diffraction. However, due to the inability of XRD to accurately locate hydrogen atoms, the local structures around framework hydrogen are usually poorly characterized even if the overall framework has been accurately determined. ¹H solid-state NMR (SSNMR) spectroscopy should, in principle, be used as a complementary method to XRD for characterizing hydrogen local environments. However, the spectral resolution of ¹H SSNMR is severely limited by the strong ¹H–¹H homonuclear dipolar coupling. In this work, we demonstrate that high-resolution ¹H MAS spectra of MOF-based material can be obtained by ultrafast sample spinning at high magnetic field in combination with isotopic dilution. In particular, we examined an important MOF, microporous α-Mg₃(HCOO)₆ and α-Mg₃(HCOO)₆ in the presence of several guest species. All six chemically very similar, but crystallographically, nonequivalent H sites of these MOFs were resolved in a chemical shift range as small as 0.8 ppm. Although the assignment of ¹H peaks due to crystallographically nonequivalent hydrogens is difficult due to that they all have almost identical chemical environments, we are able to show that they can be assigned from ¹H–¹H proximity maps obtained from 2D ¹H–¹H double quantum (DQ) experiments in conjunction with theoretical calculations. ¹H MAS spectra of framework hydrogen are very sensitive to the guest molecules present inside the pores and they provide insight into host–guest interaction and dynamics of guest molecule. The ability of achieving very high resolution for ¹H MAS NMR in MOF-based materials and subsequent spectral assignment demonstrated in this work allows one to obtain new structural information complementary to that obtained from single-crystal XRD

Wobbling and Hopping: Studying Dynamics of CO₂ Adsorbed in Metal–Organic Frameworks via ¹⁷O Solid-State NMR

Knowledge of adsorbed gas dynamics within microporous solids is crucial for the design of more efficient gas capture materials. We demonstrate that ¹⁷O solid-state NMR (SSNMR) experiments allow one to obtain accurate information on CO₂ dynamics within metal–organic frameworks (MOFs), using CPO-27-M (M = Mg, Zn) as examples. Variable-temperature (VT) ¹⁷O SSNMR spectra acquired from 150 to 403 K yield key parameters defining the CO₂ motions. VT ¹⁷O SSNMR spectra of CPO-27-Zn indicate relatively weaker metal–oxygen binding and increased CO₂ dynamics. ¹⁷O SSNMR is a sensitive probe of CO₂ dynamics due to the presence of both the quadrupolar and chemical shielding interactions, and holds potential for the investigation of motions within a variety of microporous materials

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

The results of a solid-state ^63/65Cu and ³¹P 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 ⁶³Cu nuclear quadrupolar coupling constant (<i>C</i>_Q) 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 ^63/65Cu– ³¹P spin pairs, observed in the solid-state ³¹P 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 ³¹P NMR measurements for [Cu­(bptzH)­(dppe)]­ClO₄ (bptzH = 5-<i>tert</i>-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 ^63/65Cu 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 <i>C</i>_Q(^63/65Cu) 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 ¹H NMR signal for a hydride proton requires careful experimental strategies because the spectra are generally dominated by ligand ¹H 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

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 ¹H NMR signal for a hydride proton requires careful experimental strategies because the spectra are generally dominated by ligand ¹H 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

Solid State Complex Chemistry: Formation, Structure, and Properties of Homoleptic Tetracyanamidogermanates RbRE[Ge(CN₂)₄] (RE = La, Pr, Nd, Gd)

Tetracyanamidometallates with the general formula RbRE­[T­(CN₂)₄] (RE = La, Pr, Nd, Gd; T = Si, Ge) were prepared by solid state metathesis reactions starting from stoichiometric mixtures of RECl₃, A₂[TF₆], and Li₂(CN₂). Reactions were studied by differential thermal analysis that showed ignition temperatures between 360 and 390 °C for the formation of RbGd­[T­(CN₂)₄] with T = Si and Ge. The powder diffraction patterns of RbRE­[Ge­(CN₂)₄] were indexed isotypically to the already known RbRE­[Si­(CN₂)₄] compound. IR spectra of RbLa­[Ge­(CN₂)₄] were measured and compared with those of RbLa­[Si­(CN₂)₄]. ⁷³Ge, ⁸⁷Rb, and ¹³⁹La solid state NMR measurements and density functional theory calculations were used to verify the novel homoleptic [Ge­(CN₂)₄]^4– ion. Luminescence properties of Eu³⁺, Ce³⁺, and Tb³⁺ doped samples are reported

Cold, Hot, Dry, and Wet: Locations and Dynamics of CO₂ and H₂O Co-Adsorbed in an Ultramicroporous MOF

Climate change from anthropogenic carbon dioxide (CO2) emissions poses a severe threat to society. A variety of mitigation strategies currently include some form of CO2 capture. Metal–organic frameworks (MOFs) have shown great promise for carbon capture and storage, but several issues must be solved before feasible widespread adoption is possible. MOFs often exhibit reduced chemical stabilities and CO2 adsorption capacities in the presence of water, which is ubiquitous in nature and many practical settings. A comprehensive understanding of water influence on CO2 adsorption in MOFs is necessary. We have used multinuclear nuclear magnetic resonance (NMR) experiments at temperatures ranging from 173 to 373 K, along with complementary computational techniques, to investigate the co-adsorption of CO2 and water across various loading levels in the ultra-microporous ZnAtzOx MOF. This approach yields detailed information regarding the number of CO2 and water adsorption sites along with their locations, guest dynamics, and host–guest interactions. Guest adsorption and motional models proposed from NMR data are supported by computational results, including visualizations of adsorption locations and the spatial distribution of guests in different loading scenarios. The wide variety and depth of information presented demonstrates how this experimental methodology can be used to investigate humid carbon capture and storage applications in other MOFs

Mechanochemical Synthesis of Methylammonium Lead Mixed–Halide Perovskites: Unraveling the Solid-Solution Behavior Using Solid-State NMR

Mixed-halide lead perovskite (MHP) materials are rapidly advancing as next-generation high-efficiency perovskite solar cells due to enhanced stability and bandgap tunability. In this work, we demonstrate the ability to readily and stoichiometrically tune the halide composition in methylammonium-based MHPs using a mechanochemical synthesis approach. Using this solvent-free protocol we are able to prepare domain-free MHP solid solutions with randomly distributed halide ions about the Pb center. Up to seven distinct [PbCl_<i>x</i>Br_6–x]^4– environments are identified, based on the ²⁰⁷Pb NMR chemical shifts, which are also sensitive to the changes in the unit cell dimensions resulting from the substitution of Br by Cl, obeying Vegard’s law. We demonstrate a straightforward and rapid synthetic approach to forming highly tunable stoichiometric MHP solid solutions while avoiding the traditional solution synthesis method by redirecting the thermodynamically driven compositions. Moreover, we illustrate the importance of complementary characterization methods, obtaining atomic-scale structural information from multinuclear, multifield, and multidimensional solid-state magnetic resonance spectroscopy, as well as from quantum chemical calculations and long-range structural details using powder X-ray diffraction. The solvent-free mechanochemical synthesis approach is also compared to traditional solvent synthesis, revealing identical solid-solution behavior; however, the mechanochemical approach offers superior control over the stoichiometry of the final mixed-halide composition, which is essential for device engineering