Spies Within Metal-Organic Frameworks: Investigating Metal Centers

Article pubs.acs.org/JPCC

Spies Within Metal-Organic Frameworks: Investigating Metal Centers Using Solid-State NMR Peng He,† Bryan E. G. Lucier,† Victor V. Terskikh,‡ Qi Shi,§ Jinxiang Dong,§ Yueying Chu,∥ Anmin Zheng,*,∥ Andre Sutrisno,† and Yining Huang*,† †

Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 § Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P. R. China ∥ State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China ‡

S Supporting Information *

ABSTRACT: Structural characterization of metal−organic frameworks (MOFs) is crucial, since an understanding of the relationship between the macroscopic properties of these industrially relevant materials and their molecular-level structures allows for the development of new applications and improvements in current performance. In many MOFs, the incorporated metal centers dictate the short- and long-range structure and porosity of the material. Here we demonstrate that solid-state NMR (SSNMR) spectroscopy targeting NMRactive metal centers at natural abundance, in concert with ab initio density functional theory (DFT) calculations and X-ray diffraction (XRD), is a powerful tool for elucidating the molecular-level structure of MOFs. 91Zr SSNMR experiments on MIL-140A are paired with DFT calculations and geometry optimizations in order to detect inaccuracies in the reported powder XRD crystal structure. 115In and 139La SSNMR experiments on sets of related MOFs at two different magnetic fields illustrate the sensitivity of the 115In/139La electric field gradient tensors to subtle differences in coordination, bond length distribution, and ligand geometry about the metal center. 47/49Ti SSNMR experiments reflect the presence or absence of guest solvent in MIL-125(Ti), and when combined with DFT calculations, these SSNMR experiments permit the study of local hydroxyl group configurations within the MOF channels. 67Zn SSNMR experiments and DFT calculations are also used to explore the geometry near Zn within a set of four MOFs as well as local disordering caused by distributions of different linkers around the metal. SSNMR spectroscopy of metal centers offers an impressive addition to the arsenal of techniques for MOF characterization and is particularly useful in cases where XRD information may be ambiguous, incomplete, or unavailable.

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INTRODUCTION Metal−organic frameworks (MOFs) are a family of porous solids that emerged approximately two decades ago and have since developed into a flourishing field of research.1 MOFs are inorganic−organic hybrid solids with infinite repeating structures, or frameworks, built from metal cations or clusters connected by organic linkers. Metal-centered secondary building units (SBUs) are commonly used to classify MOF structures according to the geometry and coordination of bound ligands, such as triangular, square-pyramidal, and trigonal-prismatic.2,3 The wide variety of inorganic and organic moieties that may be combined to yield SBUs and ultimately these advanced porous materials allows for customizable structure and tailoring of industrially relevant properties4 such as high thermal stability, permanent porosity, framework flexibility, and exceptionally high surface area. These uniquely tunable traits are important for MOF applications in selective © 2014 American Chemical Society

adsorption, catalysis, hydrogen storage, luminescence, chemical sensors, drug delivery, and magnetism, among others.5 A complete set of structural information facilitates a firm relation of framework to function, helping to link unique MOF properties to their molecular-level origins. In particular, the atomic position, oxidation state, and coordination of the metal centers have an immense influence over the MOF structure and therefore its potential properties. The local environment about the metal cations within MOFs may be influenced through processes such as dehydration,6 desolvation,7 and adsorption of guest molecules,8 although the overall framework structure remains intact. Single-crystal X-ray diffraction (XRD) is a common first choice for MOF characterization, but the Received: June 26, 2014 Revised: August 1, 2014 Published: August 6, 2014 23728

dx.doi.org/10.1021/jp5063868 | J. Phys. Chem. C 2014, 118, 23728−23744

The Journal of Physical Chemistry C

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hardware (ultrahigh magnetic fields) and pulse sequences (WURST-echo and WURST-CPMG), now is the time to explore the feasibility of such studies. Several Zr-based MOFs that exhibit unique properties have been reported.25−27 The structures of several Zr-based MOFs have been solved by single-crystal XRD using traditional or synchrotron sources,26,27 but structures of some other Zr-based MOFs have been refined primarily on the basis of pXRD data.25,27 MIL-140A is a Zr-containing MOF that exhibits high stability and features a significant number of Lewis acid sites.28 The structure of MIL-140A was determined via pXRD methods, rendering the reliability of its fine structure uncertain.28 In such cases, 91Zr SSNMR experiments hold the potential to probe the molecular-level structure and local coordination, providing the final piece to the structural puzzle. Despite the significant challenges involved (vide infra), 91Zr SSNMR spectroscopy has proven to be a sensitive probe of the local environment and structure of zirconium compounds.29−35 However, a thorough 91Zr SSNMR study involving MOFs has not yet been reported. In recent years, indium-containing MOFs have drawn attention because of their potential uses in hydrogen storage, CO2 adsorption, small-molecule separation, and catalysis.36−38 115 In is a quadrupolar nucleus that is often associated with challenging and time-consuming SSNMR experiments that yield broad spectra with low S/N (vide infra). Nevertheless, 115 In SSNMR spectroscopy has been successfully applied to study a variety of systems.39−42 Two In-based MOFs, In(BDC)1.5(bipy) and In(BTC)(H2O)(phen) (BDC = 1,4benzenedicarboxylate; BTC = 1,3,5-benzenetricarboxylate; bipy = 2,2′-bipyridyl; phen = 1,10-phenanthroline), are active and selective acid catalysts in the acetalization of aldehydes. The structures of these MOFs have previously been revealed by single-crystal XRD,37 but a complementary 115In SSNMR study should provide detailed molecular-level information on the local environment around the In centers and may shed light on the catalytic activity in these systems. Compounds incorporating lanthanum have garnered interest because of their intense luminescence and potential applications in luminescent materials and devices, including electroluminescent devices, integrated optics, catalysts, and biological labels. 139La SSNMR experiments in the past were restricted to systems that featured lanthanum in high-symmetry environments (i.e., systems that exhibit smaller magnitudes of the 139La quadrupolar interaction) because of the typically large SOQI and corresponding wide breadth of 139La SSNMR powder patterns. During the last 15 years, significant strides have been made in 139La SSNMR techniques, opening up a diverse array of systems for study.43−48 We are particularly interested in using 139La SSNMR experiments to gain a fuller understanding of La 2 (BDC) 3 (H 2 O) 4 , a luminescent MOF, 4 9 and La2(C4H4O4)3(H2O)2·H2O, a MOF that can catalyze the acetalization of aldehydes and the oxidation of sulfides.50 Titanium compounds have attracted much attention because of their practical applications in diverse fields such as catalysis and electrochemistry. The incorporation of titanium in MOFs should lead to increased catalytic activity due to the porosity of the structure, although reports of Ti-containing MOFs remain sparse.51 Applications of Ti-based MOFs and Ti-functionalized MOFs are diverse and include sensors, 52 epoxidation catalysis,53 and hydrogen storage.54 47/49Ti SSNMR spectroscopy has previously been employed to study the local chemical environment of titanium centers despite the poor 47/49Ti

structures of many MOFs have been refined from their less informative powder XRD (pXRD) patterns because of the difficulties associated with obtaining suitable single crystals. Achieving a highly accurate fit of both the positions and relative intensities of the peaks in a pXRD pattern is often challenging, particularly in cases where large unit cells, trace impurities, low levels of overall crystallinity, and/or a significant distribution of crystallite sizes are involved. Although approximate atomic positions may be established in such a manner, detailed information regarding bond length, bond angle, coordination number, and short-range order about metal centers is often difficult to obtain via pXRD methods, so information from alternate sources is typically required to completely describe the MOF structure.9,10 A wide variety of metal atoms are featured across the diverse field of MOFs, often serving crucial functions related to MOF applications (i.e., catalytic metal centers). Conveniently, many of these metal nuclei are also NMR-active and hold the potential to serve as spies within the framework. There is a wealth of structural information encoded within solid-state NMR (SSNMR) spectra, particularly regarding the local chemical environment and degree of order, rendering the combination of SSNMR and XRD methods a powerful probe of molecular-level structure.11 Several significant challenges are associated with performing SSNMR experiments on metal centers, however. Many of the NMR-active metals incorporated within MOFs are quadrupolar nuclei, which have a nuclear spin greater than 1/2 and are associated with NMR spectra that exhibit low signal-to-noise ratios (S/N). The quadrupolar interaction of the nuclear quadrupole moment with local electric field gradients (EFGs) originating from the surroundings (i.e., other atoms and bonds) typically gives rise to broad SSNMR powder patterns with low S/N. Metal nuclei are dilute in MOFs and typically constitute only a fraction of the total number of atoms within the unit cell; this dilution effect is compounded by the low natural abundances of many NMRactive metal isotopes. Several metal nuclei also possess low magnetogyric ratios (γ), which again reduce the spectral S/N and are associated with undesired phenomena such as acoustic probe ringing that distort or altogether conceal broad NMR signals. Several tools are available to NMR spectroscopists to overcome these obstacles,12,13 such as ultrahigh magnetic fields of 18−23.5 T to enhance S/N and narrow the width of secondorder quadrupolar interaction (SOQI)-dominated powder patterns and pulse sequences based on the wideband uniform-rate smooth-truncation (WURST) and quadrupolar Carr−Purcell−Meiboom−Gill (QCPMG) sequences. 14 WURST pulses afford broadband excitation of powder patterns and may be followed by an echo (WURST-echo) for powder spectra or accompanied by a CPMG echo train (WURSTCPMG) to increase the S/N at the cost of spectral resolution when transverse magnetization relaxation (T2) values are sufficiently long.15−18 Because of the difficulties involved, SSNMR studies of metal centers in MOFs have been largely limited to select nuclei that display favorable NMR properties, including 27Al,19 71Ga,20 and 45Sc.21 Significantly more challenging nuclei, such as 25Mg and 67Zn, have recently been studied with the help of a 21.1 T ultrahigh magnetic field.7,22−24 Many other challenging NMR-active metals, such as 47/49Ti, 91 Zr, 115In, and 139La, appear prominently in several interesting MOF structural motifs and could potentially provide a wealth of structural information. In light of advances in NMR 23729



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nuclear properties (vide infra),55−57 sometimes by using isotopically enriched reagents. To our best of our knowledge, there has not yet been a 47/49Ti SSNMR study of Ti-based MOFs, and 47/49Ti NMR tensor parameters should provide valuable information regarding the local environment of Ti cations within the MOF MIL-125(Ti). Zinc is a prominent metal center in MOFs and is featured in motifs such as IRMOFs,58 ZIFs,59 and others.60,61 Information regarding the local environment around Zn should help clarify the finer structure of such MOFs, particularly where light atoms are involved (i.e., in situations where XRD methods are less informative). As a result of advances in both hardware and pulse sequences in the past two decades, 67Zn SSNMR spectroscopy is now a viable option for characterization of a variety of systems, even at 67Zn natural abundance, including several practical applications in recent years.24 67Zn SSNMR spectroscopy was recently shown to be a useful tool for probing the local environment around Zn in MOFs through the use of a high field of 21.1 T.7 In this study, we have employed 67Zn SSNMR spectroscopy at 21.1 T to study the local environment of several other representative Zn-containing MOFs. Herein we describe detailed multinuclear SSNMR studies of MOFs whose metal centers are zirconium (MIL-140A), indium [In(BDC)1.5(bipy) and In(BTC)(H2O)(phen)], lanthanum [La2(BDC)3(H2O)4 and La2(C4H4O4)3(H2O)2·(H2O)], titanium (MIL-125), and zinc (TIF-1Zn, TIF-5Zn, Zn-zni, and Zndia). SSNMR spectra of natural-abundance metal centers within these MOFs were acquired at a magnetic field of 21.1 T, and additional spectra were collected at fields of 14.1 or 9.4 T where feasible. EFG and chemical shift (CS) tensor parameters extracted from experimental NMR spectra reveal key MOF structural features that may not be evident via complementary methods and demonstrate the feasibility of SSNMR experiments targeting the structure-directing metal centers within MOFs.

pattern was too broad to acquire in a single experiment, so the overall powder pattern was assembled from the coaddition of two or more individual frequency-stepped subspectra in a technique known as variable-offset cumulative spectrum (VOCS) or, more simply, piecewise acquisition.66 Where possible, 1H decoupling was employed in order to increase the observed T2 value and thus boost the observed signal intensity in WURST-echo experiments and the number of CPMG echoes that could be acquired in each scan for WURST-CPMG experiments. Specific information regarding the probes and chemical shift reference used for each nucleus, along with acquisition parameters, are listed in the following sections and in the Supporting Information. 91 Zr SSNMR Spectroscopy. 91Zr NMR spectra of MIL-140A were acquired using a home-built 7 mm HX static (nonspinning) probe at a magnetic field of 21.1 T [ν0(91Zr) = 83.6 MHz] along with the WURST-CPMG pulse sequence.17 Two individual WURST-CPMG experiments were performed on MIL-140A to eliminate selective enhancement based on the sweep direction: one experiment employed a frequency sweep from low to high frequency, and the other used a sweep from high to low frequency. The two individual spectra were then coadded to get the overall undistorted 91Zr WURST-CPMG NMR spectrum. Chemical shifts were referenced to a concentrated dichloromethane (CH2Cl2) solution of Cp2ZrCl2 (δiso = 0.0 ppm)67 using a secondary standard of BaZrO3(s) (δiso = 317.2 ppm).68 Experimental parameters include a WURST pulse length of 50 μs, a spectral width of 1000 kHz, and a pulse delay of 0.5 s. A total of 49 152 scans were collected. All of the experiments employed a proton decoupling field of 30 kHz. 115 In SSNMR Spectroscopy. A home-built 7 mm HX static probe was used to acquire 115In spectra at a field of 21.1 T [ν0(115In) = 197.2 MHz]. 115In NMR chemical shifts were referenced to a solution of 0.1 M In(NO3)3 in 0.5 M HNO3 (δiso = 0 ppm).69 A proton decoupling field of 25 kHz was applied during all of the experiments. The 115In static echo spectrum of In(BDC)1.5(bipy) required 4096 scans utilizing a central transition (CT)-selective 90° pulse length of 1 μs, a spectral width of 200 kHz, and pulse delay of 5 s. For 115In stepwise static echo experiments on In(BTC)(H2O)(phen), a 90° pulse length of 1 μs, a spectral width of 200 kHz, and a pulse delay of 1 s were used to acquire 32 768 scans per subspectrum. The overall spectrum was obtained by coaddition of two individual subspectra separated by a frequency offset of 1 MHz. Stepwise 115In static echo spectra of the two In-based MOFs were also acquired at 9.4 T using a Varian Infinity Plus 400 wide-bore spectrometer [ν0(115In) = 87.5 MHz] equipped with a Varian/Chemagnetics 5 mm HX static probe. A CT-selective 90° pulse length of 0.5 μs, a spectral width of 200 kHz, and a pulse delay is 0.5 s were used. The spectrum of In(BDC)1.5(bipy) was assembled via coaddition of eight subspectra, while the spectrum of In(BTC)(H2O)(phen) was obtained by coaddition of seven pieces. Each subspectrum required 61 440 scans, and the frequency offset between subspectra was 250 kHz. 139 La SSNMR Spectroscopy. A home-built 5 mm HX static probe was used to acquire 139La spectra at a field of 21.1 T [ν0(139La) = 127.1 MHz]. A 1.0 M aqueous LaCl3 solution was used as a reference (δiso = 0 ppm).70 Static spectra were acquired using a standard Hahn-echo pulse sequence along with a 90° CT-selective pulse length of 1 μs, a spectral width of

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EXPERIMENTAL SECTION Synthesis of MOFs. MIL-140A,28 In(BDC)1.5(bipy),37 In(BTC)(H2O)(phen),37 La2(BDC)3(H2O)4,49 La2(C4H4O4)3(H2O)2·(H2O),50 MIL-125(Ti),51 TIF-1Zn,62 TIF-5Zn,63 Zn-zni,64 and Zn-dia65 were prepared according to procedures previously described in the literature. Complete details of MOF syntheses are given in Appendix A in the Supporting Information. Powder X-ray Diffraction Measurements. The identities and purities of the MOFs were confirmed by pXRD characterization (Figure S1 in the Supporting Information). The pXRD patterns of the Zn MOFs (Zn-dia, Zn-zni, TIF-1Zn, and TIF-5Zn) were recorded with a Rigaku MiniFlex II X-ray diffractometer using a wavelength (λ) of 1.5418 Å. Scans were acquired between 2.1° and 40° (2θ) at a rate of 4 deg/min. The pXRD patterns of the other MOFs were acquired with a conventional Rigaku powder X-ray diffractometer using Co Kα radiation (λ = 1.7902 Å). Samples were scanned at 5° ≤ 2θ ≤ 45° at a scan rate of 5 deg/min. The In(BTC)(H2O)(phen) and MIL-125(Ti) pXRD patterns were acquired at 5° ≤ 2θ ≤ 37°. Where necessary, pXRD patterns were converted from Co Kα to Cu Kα radiation wavelengths for comparison with the literature. SSNMR Experiments. A Bruker Avance II NMR spectrometer located at the National Ultrahigh-Field NMR Facility for Solids (Ottawa, ON, Canada) was used to acquire all of the spectra at 21.1 T. In several instances, the powder 23730



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Figure 1. (a−c) Structure of MIL-140A: (a) extended framework; (b) composition of the Zr polyhedra; (c) local Zr environment. Hydrogen has been omitted for clarity. (d−g) Simulated and experimental 91Zr SSNMR spectra of MIL-140A at a field of 21.1 T: (d) experimental spectrum; (e) analytical simulation of the experimental data; (f) 91Zr powder pattern predicted by plane-wave DFT calculations on a fully geometry-optimized structure; (g) simulated 91Zr powder pattern obtained by calculations on the proton-optimized crystal structure.

parameters and conditions for the static echo, static WURSTCPMG, and MAS one-pulse experiments are listed in Table S1 in the Supporting Information. A spikelet spacing of 5 kHz was used for WURST-CPMG spectra. All of the experiments utilized a 1H decoupling field of 60 kHz. Simulation of NMR Spectra. NMR parameters were determined by simulations of the experimental NMR spectra using the WSOLIDS73 and QUEST74 computer programs. The QUADFIT program75 was employed to account for effects of local disorder in simulations of the 47/49Ti and 67Zn spectra of MIL-125(Ti) and TIF-5Zn, respectively. The experimental error for each measured parameter was determined by visual comparison of the experimental and simulated spectra. The parameter of concern was varied bidirectionally from the fitted value, with all other parameters kept constant, until noticeable differences between the experimental and simulated spectra were observed. Theoretical Calculations. Plane-wave density functional theory (DFT) calculations have been shown to be well-suited for the calculation of NMR parameters for a wide variety of nuclei and systems.76 Ab initio calculations based on planewave DFT methods were conducted using the CASTEP software package77,78 within Accelrys Materials Studio. The NMR module79,80 was used to calculate the EFG and nuclear magnetic shielding (MS) tensors. The gauge-including projector-augmented wave (GIPAW) method was utilized to analyze three-dimensional lattices in crystalline materials via pseudopotentials and plane-wave basis sets.77 Unit cell parameters and atomic coordinates were taken from their corresponding crystal structures.28,37,49−51,62−65 The generalized gradient approximation (GGA) along with the Perdew− Burke−Ernzerhof (PBE) functional81 was used. A plane-wave cutoff energy of 550 eV was applied in all of the calculations. SCF convergence criteria and k-point sampling were chosen automatically using the “fine” criteria option set incorporated within CASTEP. All of the geometry optimizations utilized an

500 kHz, and a pulse delay of 1 s. The spectrum of La2(BDC)3(H2O)4 was acquired by collecting 4096 scans, and the spectrum of La2(C4H4O4)3(H2O)2·H2O required 24 576 scans. An 1H decoupling field of 70 kHz was applied in all of the static 139La experiments. Magic-angle spinning (MAS) spectra were acquired at a field of 21.1 T using a Bruker 3.2 mm HX MAS probe. These experiments used a CT-selective 90° pulse length of 2 μs, a spectral width of 500 kHz, and a pulse delay of 1 s to collect 8192 scans while spinning at a frequency of 22 kHz. Stepwise 139La static echo spectra of the two La-based MOFs were also acquired on a Varian Inova 600 NMR spectrometer [ν0(139La) = 84.6 MHz] using a Varian/Chemagnetics 3.2 mm HXY probe. The overall spectrum was constructed via coaddition of two pieces separated by a frequency offset of 67.7 kHz. Each piece required 15 360 scans using a CTselective 90° pulse length of 3.5 μs, a spectral width of 500 kHz, and a pulse delay of 0.5 s. 47/49 Ti SSNMR Spectroscopy. A home-built 7 mm singlechannel static probe was used to acquire 47/49Ti static echo spectra of MIL-125(Ti) at 21.1 T [ν0(49Ti) = 50.75 MHz]. A secondary reference of a Cp 2TiCl 2 solution in CH2 Cl 2 [δiso(49Ti) = −773 ppm relative to neat TiCl4 at 0 ppm]71 was used. The as-made phase required 184 320 scans, while 196 608 scans were collected during experiments on the activated phase. A pulse delay of 0.5 s and a spectral width of 1000 kHz along with a 49Ti CT-selective 90° pulse length of 2 μs were used. No signal was observed using the WURSTCPMG pulse sequence, likely as a result of very short 47/49Ti T2 values. Static spectra were acquired because the peaks were too wide to be properly resolved via MAS methods. 67 Zn SSNMR Spectroscopy. All of the 67Zn spectra were acquired at a field of 21.1 T [ν0(67Zn) = 56.4 MHz] using a home-built 7 mm HX static probe. MAS experiments were performed using a Bruker 7 mm HX MAS probe at a spinning speed of 5 kHz. Chemical shifts were referenced to a 1.0 M solution of Zn(NO3)2(aq) (δiso = 0.0 ppm).72 Specific 23731


The Journal of Physical Chemistry C Table 1. Experimental and Calculated 91Zr, |CQ|/MHz experimental plane-wave DFT (H-optimized) plane-wave DFT (CHO-optimized) plane-wave DFT (fully optimized)

35.0(4) 62.10 43.94 39.80

experimental plane-wave DFTb

79.5(3) 61.14

experimental plane-wave DFTb

145.0(4) 143.96

experimental plane-wave DFTd

18.0(3) 22.83

experimental plane-wave DFTd

28.0(4) 27.17

Article 115

In, and ηQ

0.85(2) 0.63 0.81 0.82

139

La NMR Tensor Parameters δiso/ppm

Ω/ppm

MIL-140A 100(50)

200(100) 328.4 239.5 208.1

a a a

In(BDC)1.5(bipy) −110(25) 130(25) c − In(BTC)(H2O)(phen) 0.18(4) −10(25) 500(25) c 0.74 − La2(BDC)3(H2O)4 0.31(2) 22.5(3) 300(50) c 0.97 − La2(C4H4O4)3(H2O)2·H2O 0.43(3) 125(20) 100(50) c 0.93 − 0.57(4) 0.82

κ

α/deg

β/deg

γ/deg

0(0.35) 0.11 0.10 0.23

35(20) 44.5 33.5 2.8

25(15) 59.2 13.3 19.3

0(20) 55.0 22.5 3.7

1.00(15) −

0(10) −

50(10) −

0(10) −

1.00(15) −

0(10) −

60(10) −

0(10) −

−1.00(30) −

50(10) −

66(5) −

0(10) −

0.20(30) −

30(10) −

45(5) −

30(10) −

a

Theoretical isotropic nuclear magnetic shielding was not converted to a 91Zr chemical shift value because of the lack of an appropriate standard/ referencing method within CASTEP. bGeometry optimization was not performed for these 115In plane-wave DFT calculations. See Table S3 in the Supporting Information for a full tabulation of the 115In calculations. c115In and 139La chemical shielding/shift calculations were not performed because of the dominance of the EFG tensor parameters on the appearance of the spectrum in addition to the very large size of the MOF unit cells and associated high computational costs. dAll of the atomic positions were subjected to geometry optimization prior to 139La plane-wave DFT calculations.

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ultrasoft pseudopotential, while EFG/CS tensor calculations used on-the-fly pseudopotentials. For selected systems involving Ti and Zn, quantum-chemical calculations on isolated clusters or molecular fragments, herein termed “cluster model” calculations, were conducted on the SHARCNET computing network (http://www.sharcnet.ca) using the Gaussian 09 software package.82 Several calculation methods were attempted, including RHF, B3LYP, 83 TPSSTPSS,84 and PBE1PBE.85 The most accurate combinations of methods and basis sets for calculating 47/49Ti and 67Zn NMR parameters are displayed in Table S2 in the Supporting Information. The basis sets used for nonmetal atoms in the MOFs were 6-311+G* for O and N and 6-31G* for C and H. These basis sets were chosen on the basis of previous ab initio studies in the literature that showed good agreement with experimental values in similar systems.86−88 For Ti, the B3LYP method and several different basis sets [3F(4333/43/4), 3F(5333/53/5), 5F(4333/43/4), and 5F(5333/53/5)] were used, since previous literature work on layered Ti compounds showed that this combination yielded results in good agreement with experimental values.56 Calculated EFG tensor components (i.e., V11, V22, and V33) were converted to yield the corresponding quadrupolar coupling constant (CQ) and asymmetry parameter (ηQ) values according to the following definitions: |V11| ≤ |V22| ≤ |V33|, CQ = (eV33Q/h) × (9.7177 × 1021 V m−2), and ηQ = (V11 − V22)/V33, where e is the electric charge, Q is the nuclear quadrupole moment,89 and h is Planck’s constant. The conversion factor of 9.7177 × 1021 V m−2 was used during the calculation of CQ in order to convert from atomic units to hertz. The EFGShield program was used to extract NMR tensor parameters from the results of the Gaussian and CASTEP calculations in most instances.90

RESULTS AND DISCUSSION 91

Zr SSNMR Studies of MIL-140A. 91Zr is considered a challenging nucleus for SSNMR experiments primarily because of its large quadrupole moment of 176 millibarns (mb).89 In addition, the low natural abundance (11.2%) and low γ (−2.49 × 107 rad·T−1·s−1, only 9.3% that of 1H) have limited the potential applications of 91Zr SSNMR spectroscopy, particularly in systems such as MOFs. In MIL-140A, the 91Zr centers are diluted and constitute only ca. 0.67% of the atoms within the complete MOF unit cell. The reported crystal structure of MIL-140A exhibits monoclinic crystal symmetry and resides in the space group C2/c.28 This structure finds Zr in a seven-coordinate local environment, bound to four carboxylate oxygen atoms associated with BDC ligands and three μ3-O2− atoms (Figure 1a). These individual ZrO7 polyhedra are connected to each other and BDC ligands to assemble the MOF (Figure 1b). The backbone of the MOF consists of a chain of ZrO7 polyhedra along the c axis connected via μ3-O2− atoms (Figure 1c). The 91Zr static SSNMR spectrum of MIL-140A at 21.1 T is shown in Figure 1d, and the NMR tensor parameters from the accompanying simulation (Figure 1e) are summarized in Table 1. The most striking detail of the experimental spectrum is its wide breadth (ca. 800 kHz), yielding a CQ value of 35.0(4) MHz, which is near the top of the range established by previous SSNMR experiments on zirconium compounds.29−34 The tight spacing between the two central “horns” in this spectrum indicates that the magnitudes of the individual components of the EFG tensor are markedly dissimilar; this observation is confirmed by the high value of the asymmetry parameter [ηQ = 0.85(2)]. The magnitude of the SOQI far outweighs that of the CS interaction in this system, making quantification of the CS tensor parameters very challenging; nevertheless, it was possible to estimate the values as follows: δiso = 100(50) ppm, Ω = 200(100) ppm, and κ = 0.00(35). In addition, information 23732



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regarding the Euler angles (α, β, and γ) could also be extracted (Table 1). The distinctive 91Zr EFG tensor parameters indicate that Zr4+ cations in this MOF reside in a low-symmetry environment. Zr might be expected to exhibit a relatively low CQ value in this system since it resides at the center of a ZrO7 polyhedron and is surrounded by seven similar oxygen ligands; however, seven-coordinate Zr environments have previously been associated with relatively high 91Zr CQ values.34 In MIL140A, the large bond length distribution (2.018−2.348 Å) and relatively asymmetric distribution of ligands (Figure S2a in the Supporting Information) lower the symmetry of this polyhedron below those of typical high-symmetry shapes such as tetrahedral and octahedral,91 leading to the high CQ and ηQ values observed experimentally. To better understand the relationship between the local structure and the 91Zr EFG tensor parameters, plane-wave DFT calculations were performed on the extended periodic structure of MIL-140A. Prior work has indicated that the results of planewave DFT calculations involving the periodic lattice are in good agreement with experimentally obtained 91Zr NMR tensor parameters.30,76 Since the structure of MIL-140A was originally refined from pXRD data using idealized hydrogen positions,28 geometry optimization of all hydrogen atoms was performed prior to the plane-wave DFT calculations. Surprisingly, the calculated EFG tensor parameters of the H-optimized structure are a poor match to the experimental results (Table 1 and Figure 1g), with CQ overestimated by ca. 90% and ηQ predicted to be 0.63 versus the experimental value of 0.85(2). It is evident that inconsistencies exist between the MIL-140A crystal structure and the 91Zr SSNMR data. Hence, subsequent calculations of the EFG tensor parameters incorporated geometry optimization of all light atoms (C, H, and O) within the unit cell. These calculations produced EFG tensor parameters that approximate the experimental parameters, although a gap still remains between experimental and theoretical CQ values (ca. 25%). Plane-wave DFT calculations incorporating geometry optimization of all atomic positions (i.e., C, H, O, and Zr) were the most successful in reproducing the experimental CQ and ηQ values (Figure 1f), indicating that the optimized structure describes the local Zr environment better than that of the original MIL-140A pXRD structure. The first coordination shell of Zr within MIL-140A, along with corresponding bond lengths and angles, is shown in Figure S2 in the Supporting Information. The bond length distribution within the reported structure (2.018−2.348 Å) is significantly larger than the distribution within our optimized structure (2.038−2.221 Å). In addition, the O−Zr−O bond angles within the first coordination sphere of Zr fall within a broad range in the reported structure (ca. 73−100°) but occupy a much narrower range in our refined structure (six angles are ca. 73−82° and the seventh is 111°). Plane-wave DFT calculations on the reported crystal structure overestimate CQ and underestimate ηQ as a result of exaggerated spherical asymmetry of the ligands about the Zr centers, while calculations on our optimized crystal structure yield good estimates of both EFG tensor parameters. Simulated pXRD patterns based on both the reported and optimized crystal structures of MIL-140A are depicted in Figure S3 in the Supporting Information. Despite the many subtle differences between the two models, including the significant alterations in geometry about the Zr center, these structures cannot be differentiated via pXRD methods because they

present nearly identical diffraction patterns. Unlike pXRD, these subtle changes in local geometry about Zr have a profound effect on the 91Zr SSNMR spectrum, illustrating the sensitivity and utility of SSNMR experiments on MOF metal centers in conjunction with ab initio calculations. 115 In SSNMR Studies of In(BDC)1.5(bipy) and In(BTC)(H2O)(phen). 115In is a spin-9/2 nucleus that has a high natural abundance (ca. 96%) and magnetogyric ratio (5.8972 × 107 rad·T−1·s−1), but it is also associated with a very large quadrupolar moment (770 mb),89 which translates to 115In SSNMR powder patterns that typically exhibit low S/N and range across large frequency breadths. The shape and breadth of 115In SSNMR powder patterns are influenced by both the SOQI and chemical shift anisotropy (CSA), and hence, it is often difficult to isolate individual tensor parameters from a single spectrum. Since the broadening associated with the SOQI scales inversely with the magnetic field and the CSA is proportional to the magnetic field, 115In static NMR spectra were acquired at fields of 21.1 and 9.4 T to improve the reliability of the extracted NMR parameters. In(BDC)1.5(bipy) displays an orthorhombic crystal symmetry, crystallizing in the Pbca space group (Figure 2a).37 The

Figure 2. (a) Extended structure and (b) the local indium coordination environment in In(BDC)1.5(bipy). (c, d) Static 115In SSNMR spectra of In(BDC)1.5(bipy) at (c) 21.1 T and (d) 9.4 T. The blue bottom traces are experimental spectra, while the black top traces are analytical simulations.

eight-coordinate indium center, six oxygen atoms from three bidentate carboxylate groups, and two nitrogen atoms from one bidentate bipy ligand compose the InO6N2 polyhedron (Figure 2b). In contrast, the crystal symmetry of In(BTC)(H2O)(phen) is monoclinic, and its space group is P21/n (Figure 3a).37 Indium resides in a six-coordinate environment within In(BTC)(H2O)(phen), binding to three oxygens belonging to three separate carboxylate ligands, one oxygen from H2O, and two nitrogen atoms from a bidentate bipy ligand (Figure 3b). These InO4N2 polyhedra are interconnected by BTC ligands to weave the extended MOF. If a distant In−O contact of ca. 2.82 Å is included, the indium center within In(BTC)(H2O)(phen) may be considered pseudo-seven-coordinate. The static 115In SSNMR spectra of In(BDC)1.5(bipy) at 21.1 T (Figure 2c) and 9.4 T (Figure 2d) are of significant breadth 23733



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combined with their characteristic low-ηQ appearance, suggest an indium environment of very low local symmetry. The experimental 115In SSNMR spectra of In(BDC)1.5(bipy) yield CQ = 79.5(3) MHz and ηQ = 0.57(4), while the broader spectra of In(BTC)(H2O)(phen) correspond to CQ = 145.0(4) MHz and ηQ = 0.18(4). Despite the similarities in the natures of the coordinated ligands and local geometry, these two indium centers exhibit drastically different EFG tensor parameters, necessitating a more detailed structural analysis of these two MOFs. The CQ values for In(BDC)1.5(bipy) and In(BTC)(H2O)(phen) fall within the range established by previous SSNMR work on indium(III) coordination complexes and adducts.41,42 Since the 115In EFG tensor parameters are exquisitely sensitive to the indium−ligand bond length distribution and geometry, the local structures of these two MOFs may contain clues regarding the origins of their markedly dissimilar EFG tensor parameters. The indium center within In(BDC)1.5(bipy) is coordinated to eight ligand atoms, while that within In(BTC)(H2O)(phen) is pseudo-seven-coordinate if a long In−O contact of ca. 2.82 Å is considered. The local indium−ligand coordination within each MOF is shown in Figure S6 in the Supporting Information. It is clear that In(BDC)1.5(bipy) exhibits a much narrower indium−ligand bond length distribution (2.22−2.55 Å) than In(BTC)(H2O)(phen) (2.12−2.82 Å). Correspondingly, the indium centers within In(BDC)1.5(bipy) correspond to a much lower CQ than those of In(BTC)(H2O)(phen). It appears that the indium CQ value in these MOFs is directly linked to the number of metal−ligand bonds and the range of the metal−ligand bond length distribution, although the ligand geometry must also exert some influence on the EFG tensor. In(BTC)(H2O)(phen) displays an approximate C5 axis of symmetry about In, while no local rotational symmetry is apparent upon inspection of In(BDC)1.5(bipy). Interestingly, the 115In EFG tensor associated with In(BTC)(H2O)(phen) is nearly axially symmetric despite the wide distribution of indium−ligand bond lengths and lower coordination, while that of In(BDC)1.5(bipy) is almost axially asymmetric (Table 1). Within these two indium MOFs, it is clear that the immediate indium−ligand environment has a significant

Figure 3. (a) Extended structure and (b) the local indium coordination environment in In(BTC)(H2O)(phen). (c, d) Static 115 In SSNMR spectra of In(BTC)(H2O)(phen) at (c) 21.1 T and (d) 9.4 T. The blue bottom traces are experimental spectra, while the black top traces are analytical simulations. The experimental peak marked with # is likely due to an impurity in the MOF.

(ca. 375 kHz at 21.1 T and 800 kHz at 9.4 T) but are also welldefined and display sharp features, indicating high purity and crystallinity of the sample. Although the SOQI dominates the spectral appearance, the NMR tensor parameters extracted from simulation of the 115In spectra of In(BDC)1.5(bipy) at both magnetic fields (Table 1) along with the characteristic splitting of the low-frequency or right “horn” of the powder pattern confirm that CSA makes a small but significant contribution to the spectral appearance (Figure S4 in the Supporting Information). In comparison, the static 115In SSNMR spectra of In(BTC)(H2O)(phen) (Figure 3c,d) are much broader (ca. 1 MHz at 21.1 T and 2 MHz at 9.4 T) and,

Figure 4. (a) Extended framework and (b) local La coordination in La2(BDC)3(H2O)4. (c) Static and MAS (νrot = 22 kHz) 139La SSNMR spectra of La2(BDC)3(H2O)4 at a field of 21.1 T and (d) the static spectra at 14.1 T. The bottom blue traces are experimental spectra, while the top black traces are analytical simulations. Asterisks (*) denote spinning sidebands in MAS experiments. 23734



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Figure 5. (a) Extended framework and (b) local coordination of La in La2(C4H4O4)3(H2O)2·H2O. (c, d) Static 139La SSNMR spectra of La2(C4H4O4)3(H2O)2·H2O at fields of (c) 21.1 T and (d) 14.1 T. The bottom blue traces are experimental spectra, while the top black traces are analytical simulations.

site, also of site symmetry C1, which is bound by eight carboxylate oxygens from six BDC ligands and one oxygen from water (Figure 5b). LaO9 clusters are also connected with each other via BDC linkers, illustrating the similarities between these two systems. The MAS spectrum of La2(BDC)3(H2O)4 at 21.1 T is shown in Figure 4c. The MAS powder pattern is indicative of a single site of high symmetry, yielding accurate EFG tensor parameters (CQ and ηQ) as well as δiso. With knowledge of these parameters and data at two different magnetic field strengths, the static spectra from 21.1 T (Figure 4c) and 14.1 T (Figure 4d) may be fit to determine the CSA parameters as well as the Euler angles relating the EFG and CS tensors (Table 1). It is readily apparent from their unique shapes that these static powder patterns are subject to broadening due to both CSA and the SOQI (Figure S7 in the Supporting Information), which are of similar magnitudes. In contrast, the static 139La SSNMR spectra of La2(C4H4O4)3(H2O)2·H2O at both 21.1 T (Figure 5c) and 14.1 T (Figure 5d) bear a close resemblance to typical EFGdominated powder patterns of noninteger quadrupolar nuclei, and the small contributions of CSA to the spectral appearance are limited to a slight splitting of the lower-frequency or right “horn” (Figure S8 in the Supporting Information). In the instance of La2(C4H4O4)3(H2O)2·H2O, only static spectra were acquired because the resonance is too broad to properly resolve with MAS experiments. Both La2(BDC)3(H2O)4 and La2(C4H4O4)3(H2O)2·H2O exhibit CQ values that fall within the range expected from previous work.44,45,47,48,92 As is apparent from their 139La SSNMR spectra, La2(BDC)3(H2O)4 has a lower CQ value [18.0(3) vs 28.0(4) MHz] but a higher Ω value [300(50) vs 100(50) ppm] than La2(C4H4O4)3(H2O)2·H2O, reflecting the different magnitudes of the CS and EFG tensors in these systems. The two MOFs share similar ηQ values that indicate somewhat asymmetric EFG tensors, and these MOFs also exhibit distinct 139La δiso values. As with the In-based MOFs, these La-based MOFs share related ligands and have similar coordination about their La centers, making it particularly

influence on the EFG tensor, through both the number and distribution of bond lengths (CQ) and the geometrical arrangement of ligands in space (ηQ). The relationship between structure and 115In EFG tensor parameters was also probed via DFT calculations (Table 1). Plane-wave DFT calculations were successful in replicating the general disparity between the CQ values for the two systems, with predicted differences of 60−80 MHz. Calculations involving unoptimized atomic positions predict CQ to a high degree of accuracy, particularly for In(BTC)(H2O)(phen). Optimization of the atomic positions results in a negative trend in accuracy: as more atom positions are optimized in these systems, the calculated and experimental CQ values diverge (Table S3 in the Supporting Information). In these instances, it appears that the reported crystal structures of these In(III) compounds accurately describe the local metal environment and that optimization of a larger number of atoms leads to a larger divergence from the experimental results, presumably because these compounds crystallize in an arrangement that simply is not the absolute lowest-energy configuration from a computational perspective. 139 La SSNMR Studies of La 2 (BDC) 3 (H 2 O) 4 and La2(C4H4O4)3(H2O)2·H2O. 139La has a spin of 7/2, a high natural abundance of 99.91%, and a magnetogyric ratio of 3.81 × 107 rad·T−1·s−1 (14.1% of the 1H value). The large nuclear quadrupole moment (200 mb)89 and corresponding sizable powder pattern breadths render 139La a challenging nucleus for NMR studies. The structures of these two MOFs have previously been determined by single-crystal XRD.49,50 The crystal symmetry of La2(BDC)3(H2O)4 is triclinic, and the compound crystallizes in the P1̅ space group (Figure 4a).49 There is one unique eightcoordinate La site within La2(BDC)3(H2O)4, which is bound to six oxygen atoms from six individual carboxylate groups as well as two oxygen atoms from water moieties (Figure 4b) and displays a La site symmetry of C1. In this MOF, LaO8 clusters are connected through BDC linkers. La2(C4H4O4)3(H2O)2· H2O displays monoclinic crystal symmetry and resides in space group C2/c (Figure 5a).50 There is a single nine-coordinate La 23735



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Figure 6. (a) Framework of MIL-125(Ti), with a small portion enhanced for clarity in (b). (c) Environment about individual Ti atoms. (d−f) The three postulated interior structures of MIL-125(Ti), showing the three possible configurations of hydrogen atoms.

value) for 47Ti and −1.5110 × 107 rad·T−1·s−1 (5.64% of the 1H value) for 49Ti. Since the magnetogyric ratios are almost identical, the resonances of the two isotopes often appear adjacent or overlapping in the same NMR spectrum. 49Ti has a larger spin and a smaller quadrupole moment, and hence, it is normally associated with the narrower and more intense signal of the two observed Ti resonances. The crystal symmetry of MIL-125(Ti) is orthorhombic and classified as space group Pbca (Figure 6a).51 Eight TiO6 polyhedra are joined together in this structure to form rings (Figure 6b), which are attached to BDC linkers. Rings on the same crystallographic layer are connected through BDC ligands on the vertexes of the ring, and those of adjacent layers are linked through BDC moieties on the edges. There is a single Ti site with a local site symmetry of C1. Within this MOF, Ti resides in a six-coordinate environment, binding to three oxygen atoms from three carboxylate groups, two μ2-O2− atoms, and one μ2-OH group to form TiO6 polyhedra (Figure 6c). The local environment of Ti in this MOF is uncertain; the structure was reported via pXRD methods that cannot locate hydrogen atoms, leaving the structural assignment of the μ2O2− and μ2-OH ligands ambiguous (vide infra). In addition, dimethylformamide (DMF) solvent molecules remain as postsynthesis guests within the framework and may be removed via the activation process (see the Experimental Section). Both the as-made and activated forms of MIL-125(Ti) were studied via static 47/49Ti SSNMR experiments and yielded fairly broad, complex powder patterns that range across ca. 200 kHz at 21.1 T (Figure 7). These spectra feature a wide 49Ti resonance that exhibits an irregular breadth at the base, which is likely a manifestation of the underlying 47Ti resonance. The line shapes of these MIL-125(Ti) spectra are nearly symmetrical and lack any singularities or sharp edges, appearing quite dissimilar from typical half-integer quadrupole powder patterns and hinting at some local disorder among Ti sites. Appropriately, a distribution of sites was modeled in order to simulate the experimental powder patterns and extract the 47/49 Ti EFG tensor parameters (Table 2). The as-made MIL-125(Ti) powder pattern corresponds to a relatively narrow distribution of Ti sites that exhibit some

important to ascertain the structural origins of their distinct EFG tensor parameters. A careful examination of the La−O bond length distribution reveals a relatively large distribution within the eight-coordinate La2(BDC)3(H2O)4 (average bond length of 2.40 ± 0.12 Å), while the nine-coordinate La2(C4H4O4)3(H2O)2·H2O has a smaller bond length distribution (average bond length of 2.55 ± 0.07 Å) (Figure S9 in the Supporting Information). However, the CQ of La2(C4H4O4)3(H2O)2·H2O is decidedly the larger of the two. In the case of these La-based MOFs, the origin of their disparate CQ values lies within the geometrical arrangement of the La ligands rather than solely on the distribution of La−O bond lengths. Within La2(BDC)3(H2O)4, the ligands organized about La resemble a distorted quadratic antiprism (Figure S10 in the Supporting Information). Since CQ can be viewed as a measure of spherical symmetry, it follows that a metal atom in the center of a high-symmetry shape such as a perfect tetrahedron, octahedron, or quadratic antiprism should have a CQ value of 0.91,93 Since the La center within La2(BDC)3(H2O)4 resides in the center of a distorted (not perfect) quadratic antiprism, its 139La SSNMR spectrum exhibits a low but nonzero CQ value. In this case, the geometric effect of symmetry among the ligands outweighs the effect of a large bond length distribution. Plane-wave DFT geometry optimizations paired with EFG tensor calculations provide accurate estimations of the CQ values for these systems and also correctly predict the large difference between the CQ values of these La-MOFs, likely because of the high accuracy of the single-crystal XRD structures. Plane-wave DFT calculations seem well-suited for accurate predictions of CQ and corresponding powder pattern breadths for La-containing systems in general.45,76 47/49 Ti SSNMR Studies of MIL-125(Ti). Titanium is a challenging nucleus for SSNMR spectroscopy, primarily because there are two NMR-active isotopes that are both quadrupolar. The spins of 47Ti and 49Ti are 5/2 and 7/2, respectively, with natural abundances of only 7.44% and 5.41%. The nuclear quadrupole moment of 47Ti is 302 mb and that of 49 Ti is 247 mb.89 The magnetogyric ratios of the two low-γ isotopes are −1.5105 × 107 rad·T−1·s−1 (5.638% of the 1H 23736



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but the SSNMR results clearly indicate that the local environment of Ti undergoes a noticeable alteration during the activation process. MIL-125(Ti) features large rings or pores within the crystal structure (Figure 6a,b) that contain void space where the polar DMF molecules likely reside, given the potential of strong dipolar interactions with the polar oxo and hydroxyl groups present on the interiors of the rings. The activated MOF corresponds to a lower 49Ti CQ value, indicating that the spherical symmetry around the Ti centers is enhanced by removal of DMF from the framework. It is likely that DMF molecules interact with the interior oxo and hydroxyl groups in the as-made MOF or perhaps make distant contacts to Ti centers, leading to a disruption of the spherical symmetry of the TiO6 polyhedra and consequently to a higher 47/49Ti CQ value. The structure of MIL-125(Ti) was originally determined by pXRD, leaving the assignment of hydrogen atoms unanswered. Of the six oxygens coordinated to Ti, two are μ2-O2− groups and one is a μ2-OH ligand,51 but it is not possible to assign which oxygen atoms belong to oxo and hydroxo moieties. The arrangement of oxygen and hydrogen atoms in the coordination sphere of Ti may influence the 47/49Ti SSNMR spectrum; hence, ab initio calculations were performed in an attempt to see whether the distribution of hydrogen atoms influences the Ti CQ value. Both cluster model and plane-wave DFT calculations were performed on three variants of the reported activated (postcalcination) crystal structure of MIL-125(Ti),51 with each variant representing one of three different possible ways to locate hydrogen atoms (Figure 6d−f). The cluster model calculations incorporated the model depicted in Figure S11 in the Supporting Information, which contains ca. 215 atoms and includes a significant fragment of the MIL-125(Ti) unit cell. In structure A (Figure 6d), four μ2-OH groups reside on the inner side of the ring. In structures B and C (Figure 6e,f), the inner side of the ring is occupied by four μ2-O2− groups. In structure B, the four μ2-OH groups appear on different sides of the ring

Figure 7. Static 47/49Ti SSNMR spectra of (a) as-made and (b) activated samples of MIL-125(Ti) at a field of 21.1 T. The bottom blue traces are experimental spectra, while the top black traces are twonuclei simulations (47Ti, 49Ti).

variance in the 49Ti CQ (16.4 ± 1.2 MHz) and ηQ (0.68 ± 0.15) values. Upon activation, the width of the 49Ti resonance decreases, corresponding to a decrease in CQ to 13.4 ± 1.2 MHz and a drop in ηQ to 0.50 ± 0.15. There is also a slight change in chemical shift, from −590(30) ppm for the as-made sample to −658(30) ppm in the activated form. Despite the differences between the 47/49Ti SSNMR spectra of the as-made and activated forms of MIL-125(Ti), the two forms present quite similar pXRD patterns (Figure S1 in the Supporting Information). XRD proves that the overall framework of MIL-125(Ti) remains intact upon activation,

Table 2. Experimental and Theoretical 47/49Ti SSNMR EFG Tensor Parameters for MIL-125(Ti)a (See Figure 6d−f for Depictions of the Three Possible Structures of MIL-125 Indicated in the “Structure” Column) structureb

modelc

as-made activated Ab

experimental experimental cluster model

Bb

plane-wave DFT cluster model

Cb

plane-wave DFT cluster model

Ti |CQ|/MHz

47

basis set − − 3F(4333/43/4) 3F(5333/53/5) 5F(4333/43/4) 5F(5333/53/5)

20.1 ± 1.5 16.4 ± 1.5 23.97 23.74 25.98 30.18 35.45 19.58 19.82 22.60 26.68 24.05 20.37 20.52 23.37 27.48 24.48

3F(4333/43/4) 3F(5333/53/5) 5F(4333/43/4) 5F(5333/53/5) 3F(4333/43/4) 3F(5333/53/5) 5F(4333/43/4) 5F(5333/53/5)

plane-wave DFT

49

Ti |CQ|/MHz 16.4 ± 1.2 13.4 ± 1.2 19.61 19.42 21.25 24.68 29.01 16.01 24.23 18.48 21.82 19.68 16.66 25.10 19.11 22.48 20.03

ηQ 0.68 ± 0.15 0.50 ± 0.15 0.777 0.657 0.667 0.613 0.595 0.671 0.460 0.433 0.305 0.709 0.662 0.452 0.422 0.288 0.670

Only one CS tensor parameter (δiso) could be extracted: as-made, −590(30) ppm; activated, −658(30) ppm. bSee Figure 6d−f for depictions of the three possible structures of MIL-125, labeled “Structure A”, “Structure B”, and “Structure C”. These are based on an “activated” structure. cAll of the “cluster model” calculations were performed on a fragment of activated MIL-125 using Gaussian 09 (Figure S11 in the Supporting Information). All of the “plane-wave DFT” calculations incorporated the periodic lattice of activated MIL-125 using CASTEP.

a

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Figure 8. (a) Framework of Zn-dia, with an enlarged view shown in (b) and the local coordination environment about the Zn centers pictured in (c). (d−f) Static 67Zn SSNMR spectra of Zn-dia at 21.1 T: (d) spectrum acquired using a WURST-CPMG experiment; (e) spectrum acquired using a standard Hahn-echo experiment; (f) two-site analytical simulation of the experimental spectra.

(150 mb),89 low γ (1.6767 rad·T−1·s−1, 6.3% of the 1H value), and low natural abundance (4.1%). Despite these unfavorable NMR properties, we have applied 67Zn SSNMR spectroscopy to investigate the metal centers within four separate Zn-MOFs. Zn-dia crystallizes in the monoclinic space group P21/c,65 adopting the dia topology (Figure 8a).94 This MOF is assembled from basic units containing six Zn atoms (Figure 8b). There are two crystallographically unique Zn sites within Zn-dia in a 1:1 ratio. Each Zn center is coordinated by four N atoms from four 2-methylimidazolate (MIM) linkers (Figure 8c). The static 67Zn SSNMR spectra of Zn-dia at 21.1 T are shown in Figure 8d,e. This powder pattern is of significant breadth, ranging across ca. 100 kHz. The WURST-CPMG spectrum (Figure 8d) exhibits high S/N but also traces out a powder pattern of relatively low resolution that indicates the presence of multiple Zn sites, a distribution of Zn sites, or a combination of the two within the Zn-dia framework. The static echo spectrum (Figure 8e) provides additional resolution at the cost of S/N, revealing the presence of two unique crystalline Zn sites that are readily simulated (Figure 8f) to yield EFG tensor parameters (Table 3). Each Zn site is associated with distinct CQ and ηQ values, although their chemical shifts are indistinguishable within experimental uncertainty. Plane-wave DFT calculations of 67Zn EFG tensor parameters within the Zn-dia MOF predict both CQ and ηQ values to a high degree of accuracy, particularly for the high-CQ site. A difference of ca. 1.9 MHz between the CQ values of the two Zn sites was calculated, which is close to the experimental difference of 1.3 MHz. These calculations are also able to distinguish the Zn sites on the basis of ηQ values, predicting a difference of 0.14 between the sites compared with the experimental difference of 0.35. Interestingly, the Zn−ligand bond lengths and angles are very similar for the two unique Zn sites, indicating that the differences in their associated EFG parameters arise from subtle differences in long-range structure about each site (i.e., slight differences in ligand orientation or conformation). In this instance, plane-wave DFT calculations provide very accurate 67Zn EFG tensor parameters and are also

alternatively (top, bottom, top, bottom), while in structure C, the four μ2-OH groups are on the same side of the ring (all pointing upward). All of the 47/49Ti EFG tensor parameter calculation results are listed in Table 2. Generally, almost all of the cluster model and plane-wave DFT calculations predict the experimentally observed 47/49Ti CQ and ηQ values reasonably well. Of the cluster model calculations, the 3F(4333/43/4) basis set seems particularly well-suited for this system and gives good predictions of both 47/49Ti CQ and ηQ, consistent with previous reports.56 The plane-wave DFT calculations overestimate 47/49 Ti CQ by a small amount. For each DFT calculation model and basis set combination employed, the predicted 47/49 Ti CQ values of structures B and C are consistently similar, and both are far closer to the experimental values than those of structure A, indicating that the actual arrangement of oxo and hydroxyl groups in MIL-125(Ti) should resemble that within structures B and C. In addition, these calculations also reveal a possible reason for the local disorder observed among Ti sites in the 47/49Ti SSNMR spectra. As we have shown, the arrangement of μ2-OH groups in this structure has a profound effect on the 47/49Ti CQ values. If all of the μ2-OH groups were positioned in an identical configuration within MIL-125(Ti), well-defined 47/49Ti SSNMR powder patterns with clear spectral features would be expected. However, the poorly defined and featureless experimental spectra obtained hint at some measure of disorder among the μ2-OH positions. In view of the multiple ways to distribute hydrogen atoms in this system, structures B and C, and perhaps even A, in addition to the other possible configurations of hydroxyl groups, may coexist in MIL-125(Ti), leading to the observed distribution of 47/49Ti sites and EFG tensor parameters. SSNMR spectroscopy, in combination with ab initio calculations, provides key information regarding the Ti coordination environment and hydrogen positions within the MIL-125(Ti) MOF. 67 Zn SSNMR Studies of Zn-dia, Zn-zni, TIF-1Zn, and TIF-5Zn. The only NMR-active isotope of zinc is the quadrupolar 67Zn, which makes the acquisition of high-quality SSNMR spectra challenging because of its quadrupole moment 23738



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Table 3. Experimental and Theoretical 67Zn EFG Tensor Parametersa compound experimental plane-wave DFT (fully optimized)

ηQ

δiso/ppm

0.21(3) 0.56(4) 0.35

280(10) 268(10) −

9.84 Zn-zni 5.9(4) 4.3(4) 6.06 7.23 6.33 4.71 6.13

0.49

−

0.90(10) 0.35(10) 0.94 0.96 0.77 0.56 0.70

315(10) 305(10) − − − − −

5.04 TIF-1Znb 1 6.8(5) 2 4.2(3) 1 7.2

0.38

−

0.85(15) 0.00(10) 0.94

350(40) 300(40) −61

2 3 4

6.0 14.6 6.6 TIF-5Zna 1 4.0 ± 1.2 1 4.49

0.68 0.66 0

−90 −113 3

0.60 ± 0.10 0.62

303(15) −

1

0.84

−

site 1 2 1 2

experimental plane-wave DFT (H-optimized) plane-wave DFT (optimized CHN) plane-wave DFT (fully optimized)

1 2 1 2 1 2 1

|CQ|/MHz Zn-dia 8.8(3) 10.1(4) 7.90

2

experimental cluster model (fully optimized)b

experimental plane-wave DFT (unoptimized) plane-wave DFT (fully optimized)

4.72

overlapping, corresponding to the two crystallographically inequivalent Zn sites in the framework of Zn-zni. Since for one of the sites the SOQI is far larger in magnitude than the 5 kHz spinning rate and its effects cannot be fully mitigated via MAS, resolution and overlap between isotropic peaks and spinning sidebands are problematic, necessitating static NMR experiments. Static 67Zn SSNMR experiments produce a spectrum resembling that of a superposition of two typical quadrupolar powder patterns and reflect a rather high degree of crystallinity, and the additional fine detail evident in the center as well as on the low-frequency (right) portion of the powder pattern confirms the presence of multiple Zn sites. Two sets of unique 67Zn EFG tensor parameters were readily extracted from the experimental spectra via simulations and are summarized in Table 3, with unique CQ and disparate ηQ values distinguishing the two sites. Ab initio calculations were performed on the Zn-zni system with atomic positions optimized to varying extents (Table 3). As more atomic positons are successively optimized in this system, the calculated EFG tensor parameters increase in accuracy and approach the experimental values. It is clear that an accurate calculation of 67Zn EFG tensor parameters for this MOF is dependent on a complete geometry optimization of all atomic positons within the unit cell, not just the positions of light atoms. Calculations with all of the atomic positions optimized also accurately estimate the significant differences in both the CQ and ηQ values for the two Zn sites. As noted earlier for the case of Zn-dia (vide supra), the two unique Zn sites in Zn-zni have nearly identical Zn−ligand bond lengths and angles but give rise to markedly dissimilar 67Zn EFG tensor parameters. The difference between the EFG tensors of the two Zn centers likely reflects differences in the relative orientations of the four organic linkers bound to each Zn site. TIF-1Zn crystallizes in the space group P42/mnm (Figure S12a in the Supporting Information) with the zea topology,62 incorporating four crystallographically unique Zn sites in a ratio of 4:4:4:1 (Figure S12b) with site symmetries of C1, C1, C1 and C4h, respectively. The acronym TIF within the MOF name stands for tetrahedral imidazolate framework. Each individual Zn atom is coordinated by four N atoms from four different 5,6-dimethylbenzimidazolate (DMBIM) linkers (Figure S12c). In general, four crystallographically identical Zn atoms form a square, while a different set of distinct Zn atoms are situated at the midpoints of the edges. Squares on adjacent layers are woven together via a third set of crystallographically similar Zn atoms, while the last unique Zn site is located at the center of each square. The manner in which these squares are connected leads to tunnels in the TIF-1Zn crystal structure with a diameter of ca. 17 Å. Static 67Zn NMR spectra of TIF-1Zn at 21.1 T are shown in Figure S12d,e. In order to obtain spectra with adequate S/N, the WURST-CPMG pulse sequence was employed, resulting in the spectrum shown in Figure S12d. This spectrum is ca. 50 kHz broad but appears relatively featureless due to the large spacing between individual spikelets and the corresponding low resolution of the powder pattern. The powder pattern obtained via a Hahn-echo experiment (Figure S12e) features higher resolution and somewhat resembles the appearance of a typical half-integer quadrupole powder pattern. Despite the long experimental time (24 h), the S/N of the static echo spectrum is low because of the low weight percent of Zn in this MOF (ca. 18%) and the generally unfavorable NMR properties of the 67 Zn nucleus. The echo spectra indicate that two types of

a

These 67Zn powder patterns are dominated by the second-order quadrupolar interaction. The chemical shielding interaction is comparatively much smaller in magnitude and has a correspondingly small impact on the 67Zn powder pattern. In most cases, the only identifiable CS tensor parameter is the isotropic chemical shift (δiso). Aside from δiso, the only extensive CS tensor parameters obtained were for TIF-5Zn: Ω = 150(30) ppm, κ = 1.0(2), α = 45(20)°, β = 45(20)°, γ = 0(20)°. CS tensor parameters beyond δiso were not considered in these 67Zn NMR ab initio calculations aside from TIF-1Zn. bThe reported crystal structure of TIF-1Zn has organic linkers that exhibit atypical bond lengths and bonding angles. Hence, geometry optimization calculations were performed before the EFG tensor calculations. The only successful method of geometry optimization involved cluster models (using Gaussian 09); plane-wave DFT geometry optimization calculations (using CASTEP) were unable to converge to an optimized geometry. Hence, only cluster model calculations of 67Zn EFG parameters are reported for this MOF.

well-suited for verifying the number of 67Zn SSNMR resonances and their approximate fits. Zn-zni crystallizes in the I41cd space group64 and adopts a zni topology.94 This MOF features square tunnels with a diameter of ca. 6 Å that run through the crystal structure (Figure 9a). Each square within the MOF consists of four Zn atoms at the vertices, which correspond to two crystallographically unique Zn sites that appear alternatingly (Figure 9b). Each Zn center is coordinated by four N atoms from four imidazole linkers (Figure 9c). The static and MAS 67Zn SSNMR spectra of Zn-zni at 21.1 T are shown in Figure 9d,f, respectively, and the corresponding simulations are shown in Figure 9e,g. The MAS spectrum reveals that two individual resonances are present and 23739



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Figure 9. (a) Framework structure of Zn-zni. (b) View of the two crystallographically unique Zn sites. (c) Local coordination environment about each Zn center. (d−g) 67Zn SSNMR spectra of Zn-zni at 21.1 T: (d) static Hahn-echo experiment and (e) its two-site simulation; (f) MAS experiment (νrot = 5 kHz) and (g) its two-site simulation. Asterisks (*) denote spinning sidebands in MAS experiments and simulations.

atoms from two DMBIM ligands and two imidazolate (IM) linkers (Figure S13c). The MAS and static 67Zn SSNMR spectra of TIF-5Zn at a field of 21.1 T are shown in Figure S13d,e,g. As predicted from the crystal structure, only one unique 67Zn NMR resonance initially appears to originate from TIF-5Zn.63 None of the acquired powder patterns resemble those of a typical quadrupolar nucleus; both static powder patterns are rather broad and generally featureless, while the MAS spectrum features a single asymmetric resonance. These peculiar 67Zn SSNMR spectra are likely due to a significant degree of local Zn disorder. Hence, EFG tensor parameters and chemical shifts were extracted via Quadfit simulations75 that incorporate a distribution of Zn sites (Table 3 and Figure S13f,h). The 67Zn EFG tensor parameters extracted from spectra of TIF-5Zn are comparable to those previously reported for other ZIFs.7 The origin of the Zn disorder is likely due to a combination of two factors: (i) unremoved synthesis solvent within the framework (i.e., TGA analysis indicated ca. 1% weight loss upon heating, attributed to guest molecules) and (ii) low crystallinity of the organic linkers, particularly since elemental analyses indicated the composition of our sample to be [Zn(Im)0.73(dmbIm)1.27] versus the expected [Zn(Im)(dmbIm)]. Plane-wave DFT calculations were performed to investigate the origins of disorder within this system. Calculations on the TIF-5Zn crystal structure with no optimization and full optimization of the atomic positions yielded similar results, with CQ and ηQ values falling within or near the experimental distributions for this system. Since the crystal structure is consistent with the SSNMR results, the disorder observed in the 67Zn SSNMR experiments on TIF-5Zn is likely due to a combination of the presence of residual trapped solvent from the synthesis and the distribution of organic linkers around metal center.

powder patterns are present, corresponding to two types of Zn sites with markedly different CQ values (Figure S12f). There are four crystallographically nonequivalent Zn sites within TIF-1Zn (vide supra). In order to help correlate the crystallographic sites to the SSNMR resonances, DFT calculations of 67Zn EFG tensor parameters were performed. The reported crystal structure of TIF-1Zn has organic linkers that exhibit atypical bond lengths and bonding angles, and hence, geometry optimization calculations were performed before the EFG tensor calculations. The only successful method of geometry optimization involved cluster model methods; plane-wave DFT geometry optimization calculations were unable to converge to an optimized geometry. Hence, only cluster model calculations of 67Zn EFG parameters are reported for this MOF. The cluster model calculations of 67Zn EFG tensor parameters were performed on the [Zn(DMBIM)4]2− cluster depicted in Figure S12c, and the results are summarized in Table 3. These calculations agree with the SSNMR observation of two types of powder patterns and the crystallographic observation of four unique Zn sites: three Zn sites exhibit similar EFG tensor parameters, while the fourth site is correlated to a distinctive high CQ value. On the basis of calculations, the intense resonance in the center of the TIF-1Zn 67 Zn powder pattern originates from the three unique sites composing the edges, corners, and linkers of the Zn squares pictured in Figure S12b, while the broad underlying resonance arises from the Zn site at the center of each square. The combination of 67Zn SSNMR experiments and DFT calculations allows the assignment of spectroscopic resonances to crystallographic sites within the TIF-1Zn MOF. TIF-5Zn crystallizes in the space group I41/a and displays a gis topology (Figure S13a in the Supporting Information).63 The extended framework features rectangular SBUs formed by Zn atoms at the vertices (Figure S13b) that create tunnels running through the crystal structure with a diameter of ca. 12 Å. There is only one Zn site, which is coordinated by four N 23740



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CONCLUSIONS In this study, a variety of MOFs featuring different metals have been synthesized and characterized via SSNMR spectroscopy in order to shed new light on the local structure, geometry, and bonding about the metal centers within these intricate materials via correlation of structural features to NMR tensor parameters. In particular, SSNMR experiments involving MOFs can be used to improve the accuracy of a reported structure or provide critical structural information that XRD methods may not. SSNMR experiments also provide valuable information on phase purity and crystallographic features such as the presence of multiple metal sites or disorder within MOFs. Employed in concert with XRD methods and plane-wave DFT calculations, SSNMR experiments involving metal nuclei offer tremendous potential for exploring the molecular-level structure of MOFs. 91 Zr SSNMR experiments, in conjunction with DFT geometry optimizations, reveal that the reported structure of MIL-140A does not adequately describe the local Zr environment; a geometry-optimized ab initio structure is provided that agrees well with the SSNMR results. High-resolution 115In and 139 La SSNMR spectra of In(BDC)1.5(bipy), In(BTC)(H2O)(phen), La2(BDC)3(H2O)4, and La2(C4H4O4)3(H2O)2·H2O acquired at two magnetic fields illustrate the sensitivity of 115 In/139La NMR tensor parameters, particularly the EFG tensor parameters CQ and ηQ, to subtle differences in metal− ligand bond length distributions as well as ligand geometry about the metal center. 47/49Ti SSNMR spectra of as-made and activated samples of MIL-125(Ti) show clear differences that are correlated to the presence of residual weakly coordinated DMF solvent, indicating that the 47/49Ti EFG tensor, and in particular CQ, is a powerful probe of local structure within these systems. The sensitivity of the 47/49Ti EFG tensor to subtle structural changes in this MOF was vividly illustrated via the use of SSNMR experiments and complementary theoretical calculations to determine which of three possible configurations of hydroxyl groups actually exist within the interior channels of MIL-125(Ti). 67Zn SSNMR spectra of Zn-dia, Zn-zni, TIF1Zn, and TIF-5Zn reveal that the 67Zn EFG tensor is sensitive to the number of crystallographically unique Zn sites and local structure about Zn, including the degree of sample crystallinity. SSNMR is a promising tool for exploring the molecular-level structure of MOFs, particularly when used in conjunction with complementary characterization methods. These SSNMR spectra were primarily acquired at a magnetic field of 21.1 T using nonenriched samples and widely available pulse sequences. In light of the increasing access to NMR instruments operating at very high magnetic fields as well as the explosive growth in the field of MOFs, we hope that the structural insights contained within this report will encourage more frequent SSNMR characterization of these unique microporous materials.

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82. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.Z.). *E-mail: [email protected] (Y.H). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Access to the 900 MHz NMR spectrometer and CASTEP software for selected calculations was provided by the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada), a national research facility funded by the Canada Foundation for Innovation, the Ontario Innovation Trust, Recherche Québec, the National Research Council Canada, and Bruker BioSpin and managed by the University of Ottawa (http://nmr900.ca). The Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged for a Major Resources Support Grant. Y.H. thanks NSERC for a Discovery Grant and a Discovery Accelerator Supplements Award. Funding from the Canada Research Chair Program is also gratefully acknowledged. A.Z. and Y.C. are grateful for the financial support from the National Natural Science Foundation of China (21173255). Q.S. and J.D. acknowledge the National Natural Science Funds for Grants 51172153 and 21306126. A.S. acknowledges the Ontario Ministry of Training, Colleges, and Universities for an Ontario Graduate Scholarship. This work was made possible by the facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET) (http://www.sharcnet. ca).

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ASSOCIATED CONTENT

S Supporting Information *

Details of MOF syntheses, pXRD patterns, CSA and EFG tensor contributions to 115In and 139La powder patterns, depictions of metal−ligand bond length distributions, 67Zn SSNMR experimental parameters, structure and 67Zn SSNMR spectra of TIF-1Zn and TIF-5Zn, methods and basis sets involved in cluster model calculations, additional 115In planewave DFT results, and complete citations of refs 7, 28, 49, and 23741



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