The Nature of Chemical Bonding in Lewis Adducts ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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The Nature of Chemical Bonding in Lewis Adducts as Reflected by 27 Al NMR Quadrupolar Coupling Constant: Combined Solid-State NMR and Quantum Chemical Approach Libor Kobera,*,† Jiri Czernek,† Sabina Abbrent,† Hana Mackova,† Lukas Pavlovec,† Jan Rohlicek,‡ and Jiri Brus† †

Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovskeho nam. 2, 162 06, Prague 6, Czech Republic Department of Structural Analysis, Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Praha 8, 182 21, Czech Republic

‡

S Supporting Information *

ABSTRACT: Lewis acids and Lewis adducts are widely used in the chemical industry because of their high catalytic activity. Their precise geometrical description and understanding of their electronic structure are a crucial step for targeted synthesis and specific use. Herein, we present an experimental/computational strategy based on a solidstate NMR crystallographic approach allowing for detailed structural characterization of a wide range of organoaluminum compounds considerably differing in their chemical constitution. In particular, we focus on the precise measurement and subsequent quantumchemical analysis of many different 27Al NMR resonances in the extremely broad range of quadrupolar coupling constants from 1 to 50 MHz. In this regard, we have optimized an experimental strategy combining a range of static as well as magic angle spinning experiments allowing reliable detection of the entire set of aluminum sites present in trimesitylaluminum (AlMes3) reaction products. In this way, we have spectroscopically resolved six different products in the resulting polycrystalline mixture. All 27Al NMR resonances are precisely recorded and comprehensively analyzed by a quantum-chemical approach. Interestingly, in some cases the recorded 27Al solid-state NMR spectra show unexpected quadrupolar coupling constant values reaching up to ca. 30 MHz, which are attributed to tetra-coordinated aluminum species (Lewis adducts with trigonal pyramidal geometry). The cause of this unusual behavior is explored by analyzing the natural bond orbitals and complexation energies. The linear correlation between the quadrupolar coupling constant value and the nature of bonds in the Lewis adducts is revealed. Moreover, the 27Al NMR data are shown to be sensitive to the geometry of the tetra-coordinated organoaluminum species. Our findings thus provide a viable approach for the direct identification of Lewis acids and Lewis adducts, not only in the investigated multicomponent organoaluminum compounds but also in inorganic zeolites featuring catalytically active trigonal (AlIII) and strongly perturbed AlIV sites.

1. INTRODUCTION Aluminum is one of the most abundant elements in the Earth’s crust usually occurring as oxides or silicates. Aluminum compounds, natural and man-made, have wide technological and industrial importance.1−4 Some of the most important compounds are crystalline aluminosilicates, namely, zeolites, which have an irreplaceable role in catalytic chemistry because of their tunable structures and the targeted preparation of specific catalytic centers.5−8 While these active sites have been described using many spectroscopic methods,9−11 a clear structural description of many important centers is still missing. Lewis acid sites and perturbed aluminum sites represent working examples of these active centers in zeolites. These structurally complicated sites, which are important for many catalytic processes, have been thoroughly described in organoaluminum compounds as units with trigonal planar geometry.12 However, in zeolites, only unconfirmed predictions have been published to date.13 Generally, aluminum Lewis sites have a © XXXX American Chemical Society

great potential use in chemical industry applications due to their high catalytic activity,14−17 while deep understanding of their behavior and structure still remains a challenge. Traditionally, structural descriptions of partially and/or fully crystalline compounds have been conducted using powder Xray diffraction (PXRD) and/or solid-state NMR (ssNMR) spectroscopy. Unfortunately, PXRD analysis can only be used for crystalline systems containing a limited number of structural units. On the other hand, ssNMR spectroscopy allows the structural description of complex systems,18 providing unique information about the local environment of analyzed atoms and can thus be usefully applied for determination of unusual catalytically active aluminum sites.12 The aluminum (27Al) nucleus properties such as high gyromagnetic ratio, 100% isotopic abundance, and noninteger spin number (I = 5/2) have Received: April 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 1. Selected Crystallographic Information on All the Possible Aluminum Complexes with THF, Mesitylene and Chlorine Extracted from the CCDC compound AlCl3a AlMes3 (AlMes2)2OH2·THF (AlMes2)2Cl2 AlMes2Cl·THF AlMesCl2·THF AlMes3·THF AlCl3·2THF AlCl3·THF [AlCl2·4THF]+AlCl4− a

ref code

space group

FAYKIZ-01 ZOWHEY YIYCOY YIYCIS YANKIH KAJVUM ALCTHF-01 TOGSUD FEYYAJ-01-02

C2/m P3̅ P1̅ P21/c P21/a P1 P21/c Pbcn P21/m P21/n, P1121/n, P21/n

CCDC number

ref

1153248, 275905 1315535 1304048 1304047 1299315 1192495 1102209, 116803 1273341 1155534, 1155535, 174104

59 12, 60 61 62 62 63 64 65, 66 67 68−70

CIF file was downloaded from http://aflowlib.org/CrystalDatabase/AB3_mC16_12_g_ij.html.

(NBOs), and complexation energies were analyzed in order to obtain detailed insight into the nature of the aluminum bonds. We believe that the introduced experimental approach combining PXRD and ssNMR spectroscopies with quantum chemical calculations can be extended to the investigation of a wide range of Lewis acids and Lewis adducts in all forms.

made ssNMR investigations both plausible and intriguing. In general, the 27Al NMR isotropic chemical shift depends upon the difference in the Al coordination number with a reduction of approximately 30 ppm observed with coordination number increasing by one.19 Even though 27Al ssNMR spectroscopy is a routine, reliable, and proven technique, known problems of broad, barely detectable 27Al NMR patterns have been described in the literature.20−22 These very broad spectral lines spanning several hundred kHz might be easily overlooked. The recording of these broad NMR spectral lines therefore requires an application of sophisticated approaches. The development of ssNMR techniques such as variable offset cumulative spectra - VOCS,23 (Quadrupolar) Carr−Purcell− Meiboom−Gill - (Q)CPMG,24,25 and wideband uniform rate smooth truncation (WURST)-(Q)CPMG26 enables a relatively easy acquisition of these ultra-wide NMR (UWNMR) signals.27 However, these techniques have a lower spectral resolution. In order to avoid misinterpretation of the resulting spectra, a combination of the UWNMR approach with traditional measurements in this case spin−echo magic angle spinning (MAS)28,29 and multiquantum (MQ)/MAS30 NMR experiments is sometimes needed. Moreover, the combined experimental and computational ssNMR approaches provide additional information about the structural ordering and offer unique and valuable insight into the local electronic environment of the material. In other words, in addition to the structural description, the nature of specific chemical bonds can also be investigated.31−34 In this work, we apply the ssNMR crystallographic approach35 using a series of PXRD, ssNMR (MAS, MQ/ MAS and static WURST-QCPMG) spectra, and densityfunctional theory (DFT) calculations to enable an identification of all the aluminum species present in a prepared polycrystalline system. This complex system was prepared by a reaction of AlCl3 with 2-mesityl magnesium bromide in tetrahydrofuran (see Sample preparation) and contains 3-, 4-, 5-, and 6-fold coordinated aluminum sites. Trimesitylaluminum (AlMes3) was selected as a suitable model for evaluation of the proposed 27Al WURST-QCPMG NMR technique. Therefore, the prepared sample was purposefully not purified, in order to ensure that the sample will contain as many aluminum complexes with tetrahydrofuran (THF), mesitylene (Mes), and chlorine as possible. The aim was to identify all distinct Al sites and the inequivalent structural units present in the system as well as to determine their structural features. It was furthermore shown necessary to combine NMR crystallographic approach with computational chemistry, namely, natural bonding orbitals

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Anhydrous aluminum chloride and 2mesitylmagnesium bromide (1 M solution in tetrahydrofuran) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Toluene (LachNer; Neratovice, Czech Republic) was dried over sodium and purified by distillation. Argon 7.0 (Linde Gas; Prague, Czech Republic) was used without further purification. The AlMes3 is air- and water-sensitive, and thus the Schlenk-line techniques and dry glovebox were employed for the synthesis according to a procedure modified from Schurko et al.12 1 M solution of 2-mesitylmagnesium bromide in tetrahydrofuran (30 mL) was slowly added through a septum to anhydrous aluminum chloride (1.33 g) at −70 °C and vigorously stirred, while the temperature was kept constant to decrease the exothermic dissolution of AlCl3.36 The flask was heated to room temperature, and the reaction proceeded for 16 h. Crystals of magnesium salts were filtered, and tetrahydrofuran was removed by distillation. Solid residuum was pulverized and heated at 140 °C in argon flow for 16 h to remove coordinated tetrahydrofuran. The product was recrystallized once from dried toluene. 2.2. Powder X-ray Diffraction (PXRD). The polycrystalline sample was ground and placed into a 0.5 mm borosilicate glass capillary in a glovebox. Powder diffraction data was measured on the powder diffractometer SmartLab, Rigaku equipped with Cu rotating anode, Johansson monochromator, focusing mirror (CBO-E unit), and D/teX 250 detector in the transmission Debye−Scherrer configuration. Powder pattern was measured from 3° to 100° 2θ and with 0.007° 2θ step size. 2.3. Solid State NMR Spectroscopy (ssNMR). The ssNMR spectra were recorded at 11.7 T using a Bruker AVANCE III HD spectrometer. The 4 mm cross-polarization magic angle spinning (CP/ MAS) probe was used for 27Al experiments at Larmor frequency of ν(27Al) = 130.318 MHz. 27Al solid-echo MAS and 3Q/MAS NMR (with z-filter) experiments were collected at 11 kHz spinning speed. The 27Al chemical shift was calibrated using a 1 M aqueous solution of Al(NO3)3 (27Al: 0.0 ppm). The CT-selective π/2 pulse lengths in the solid echo experiment (90°-nτR-90°-nτR-acq.)29 were 1.75 μs. The 2D 3Q/MAS and WURST-QCPMG experiments were set up using kyanite as a model system. The 27Al WURST-QCPMG NMR experiment was carried out using a 50 μs CT-selective WURST pulse using 1 MHz sweep width, with 64 loops and step 100 kHz. The final 27Al WURST-QCPMG NMR spectrum is the sum of five subspectra. The recycle delay was 2 s for all 27Al ssNMR spectra. Highpower 1H decoupling (SPINAL64 for MAS experiments and CW for static experiments) was used to eliminate heteronuclear dipolar B


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Inorganic Chemistry

exchange correction to it), and the induction, Eind (which collects the second-order induction term, the second-order correction to it, and an estimate of the higher order terms).57 The implementation of the DFDFT-SAPT procedures in the 2008.1 version of Molpro58 was used.

couplings in all measurements. Dried sample was packed into ZrO2 rotors and subsequently stored at room temperature in an inert atmosphere. To compensate for frictional heating of the spinning samples, all NMR experiments were measured under active cooling. The sample temperature was maintained at 298 K, and the temperature calibration was performed on Pb(NO3)2 using a calibration procedure described in the literature.37 All NMR spectra were processed using the Top Spin 3.5 pl2 software package, and spectral lines were simulated using the WSOLID software.38 2.4. Quantum Chemical Calculations. In the part related to the prediction of the NMR parameters, DFT calculations were conducted on individual molecules extracted from each of the chosen crystal structures (Table 1) using ADF software package version 2016.108.39 For these structures, single-point calculations were conducted using the BLYP functional,40 and subsequently the chemical shielding tensors were predicted using the GIAO-BLYP approach.41 DFT calculations were performed using the QZ4P basis set.42 The Euler angles relating the frames of the electric field gradient (EFG) tensor and of the NMR chemical shielding tensor of the relevant 27Al nuclei were extracted from output files using the program INFOR.43 As for the energy parameters, first all the investigated complexes and their components were optimized at the Hartree−Fock (HF) HF/ 6-31G** level of the quantum chemical theory and were verified to be minima of the potential energy surface by computing the harmonic vibrational frequencies and checking they were all real. These geometries were then used to approximate their complete basis set (CBS) total energies using the approach proposed by Truhlar.44 It separately extrapolates the HF portion of the total energy and the MP2 (the second-order Møller−Plesset) correlation energy. The HF energies were obtained employing the standard correlation-consistent polarized-valence double-ζ and triple-ζ (cc-pVDZ and cc-pVTZ) basis sets, while the MP2 correlation energies were estimated using the resolution-of-the-identity (RI) integral approximation45 applied together with the cc-pVDZ and cc-pVTZ auxiliary basis sets46 The resulting RI-MP2/CBS energy was combined with the zero-point vibrational energy (ZPE) value, computed for the corresponding HF/ 6-31G** minimum, to arrive at an estimate of the absolute total energy, E. The complexation energy for a complex AB formed by its components A and B, dE(AB), is then simply the difference

dE(AB) = E(AB) − E(A) − E(B)

3. RESULTS AND DISCUSSION Phase analysis of the obtained PXRD pattern was performed, in which structural data sets extracted from the Cambridge Structure Database (CCDC) for all aluminum complexes as based on the input chemicals - tetrahydrofuran (THF), mesitylene (Mes), and chlorine as well as an AlCl3 phase were considered, see Table 1. From Figure 1 and Figure S1, it

Figure 1. Phase analysis of the measured powder diffraction pattern (red) revealed the presence of the AlMes3·THF phase (blue). The powder diffraction pattern of AlMes3.THF was calculated from the entry KAJVUM of the CCDS database. Intensity is in arbitrary units.

follows that the sample contains only one of the known phases found in the CCDC corresponding to AlMes3·THF (ref code KAJVUM) with another phase(s). Since the indexing of the unknown phase did not lead to a solution, a mixture of several phases must be considered. The number of unidentified reflections in the measured powder diffraction pattern suggests a highly complex system, as supported by 13C MAS NMR spectrum, Figure S2. To elucidate its detailed features, it is necessary to combine traditional and advanced ssNMR techniques with DFT calculations. In the first step, 27Al UW NMR approach was chosen with the resulting spectrum depicted in Figure 2a. This 27Al WURST-QCPMG technique combined with VOCS confirms a multicomponent system, as the powder pattern spans roughly 600 kHz in breadth. This powder pattern is composed of at least three distinct sites: (i) an ultrawide signal spanning 550 kHz; (ii) a broad spectral line which covers ca. 200 kHz; and (iii) the asterisked (narrowest) signal in the 27Al WURST-QCPMG NMR spectrum (Figure 2a) with ca. 50 kHz in width. Due to a somewhat lower spectral resolution of 27Al WURST-QCPMG NMR experiment and its low relative signal intensity, this narrow spectral line could easily be disregarded as impurities. However, when a closer investigation of this “narrow” region in the spectral window from −10 ppm to 150 ppm was carried out using traditional 27Al MAS and 27Al MQ/MAS NMR experiments, several additional structural units present in the material were found. The resulting 27Al spin−echo MAS NMR and 27Al MQ/MAS NMR spectra are depicted in Figure 2, panels b and c, respectively. Thus, the right combination of NMR techniques is crucial for the identification of all species present in the sample. It is worthy to note that by applying the conventional 1D 27Al MAS and 2D 27Al MQ/MAS NMR

(1)

which implicitly includes the deformation energy of A and B. Also using the HF/6-31G** geometries, the natural bond orbitals (NBO) analysis47 was carried out for selected complexes using the MP2/631G** orbitals. The Gaussian 0948 suite of codes was used for the aforementioned HF/6-31G** calculations and for the NBO analysis. The HF and RI-MP2 energies used for extrapolations to the CBS limit were obtained using version 6.5 of Turbomole.49 The coordinates of all the studied structures are provided in the Supporting Information, together with the corresponding RI-MP2/CBS and HF/6-31G** ZPE values. In order to quantify the respective contributions to the interaction energy of the complexes, the symmetry-adapted intermolecular perturbation theory (SAPT)50 was combined with a DFT description of the monomers. The resulting DFT-SAPT treatment was performed in its density-fitting (DF) variant,51 by employing the standard augmented correlation-consistent polarized-valence double-ζ (aug-ccpVDZ) basis sets appropriately combined with the cc-pVDZ JKfitting52 and aug-cc-pVDZ MP2-fitting53 basis sets. The PBE0 exchange54 and PW91 correlation55 DFT functionals were used, and the gradient-regulated asymptotic correction56 was applied to the orbital energies. In particular, for all the monomers, the vertical ionization potential was computed, and the highest occupied molecular orbital (HOMO) energy was added to it to obtain the value of the Molpro parameter “asymp”, which is also included in the Supporting Information (see above). The total interaction energy, ESAPT, is partitioned into the contributions of the following components: the electrostatic, Eelst (which is the sum of the firstorder polarization and first-order exchange terms), the dispersion, Edisp (the sum of the second-order dispersion term and the second-order C


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Inorganic Chemistry

Figure 2. Experimental 27Al ssNMR spectra (black solid line), simulations of the individual aluminum sites (dashed lines) and their sum (red solid line) of the as-synthesized AlMes3 system. (a) Static 27Al WURST-QCPMG NMR experiments with the asterisk denoting the resonance which was dismissed as an impurity by Schurko et al.12 (b) 27Al solid-echo MAS NMR and (c) 27Al 3Q/MAS NMR (see Figure S3 for detail analysis) experiments.

exception of the [AlCl2·4THF]+AlCl4− (FEYYAJ 02) system, where the deviations from the experiments are larger than the typical differences of approximately 5%. This disagreement is most likely caused by a geometrical distortion in this ionic entity with two clearly distinct aluminum sites. Furthermore, based on the literature data,71 where the authors reported that in the presence of ether molecules, the organoaluminum derivatives crystallize as monomeric units, the dimeric organoaluminum species [(AlMes2)2OH2·THF - ZOWHEY and (AlMes2)2Cl2 - YIYCOY] were eliminated from the list of the preselected compounds. Besides the fact that the combined DFT/NMR approach enabled us to reveal and determine six distinct aluminum sites represented in Figure 2 and Table 2, another very interesting result was provided by the static 27Al WURST-QCPMG NMR

experiments the spectroscopic components with CQ > 30 MHz are invisible at moderate magnetic fields (e.g., 11.7 T). However, when the proposed combination of experiments was used an entire set of six distinct structural units was found and described (listed in Table 2). To identify these six distinct aluminum signals, we first used DFT calculations to obtain the 27Al NMR parameters for all preselected structural units listed in Table 1 (DFT calculated NMR parameters of all possible aluminum complexes are given in Table S1 in the Supporting Information). These parameters were further used as the basis for fitting the recorded 27Al ssNMR spectra. Both calculated and experimental NMR parameters of the aluminum signals are listed in Table 2. The DFT computations of the NMR parameters for the 27Al nuclei are in good agreement with the experimental data with the D


Article

AlIV

AlVI

3

6

[AlCl2·4THF]+ AlCl4−

AlIV 4 AlCl3·THF

AlV 5 AlCl3·2THF

Parameter definitions are as follows: δiso = (1/3)(δ11 + δ22 + δ33); Ω = δ11 − δ33; κ = 3(δ22 − δiso)/Ω; CQ = eQV33/h; η = (V11 − V22)/V33; δ11 ≥ δ22 ≥ δ33; |V33| ≥ |V22| ≥ |V11|. Euler angles are defined in ref 72. bδisocal = σisocal(AlCl3) - σisocal − 1.6 (ppm): as explained in Table S1 in the Supporting Information.

528.8

428.0

433.4

476.2

405.9 AlIV 2 AlMes3·THF

experiment. In this spectrum (Figure 2a), two broad resonances were observed and attributed to 3-fold Al in AlMes3 and 4-fold Al in AlMes3·THF with dominant quadrupolar contribution and their CQ reaching 49 and 27 MHz, respectively (see Table 2). The presence of AlMes3·THF complex, a tetra-coordinated Al atom linked to three mesityl groups and one molecule of tetrahydrofuran, was confirmed by PXPD (Figure 1). Our effort now focused on this structural unit and the explanation why this tetra-coordinated aluminum atom (AlIV) exhibits such an unusually large quadrupolar coupling constant observed in the 27Al UWNMR spectrum. Moreover, an attempt was made to describe the role of the respective intermolecular forces in the studied system. First, several variants of the DFTSAPT (symmetry-adapted perturbation theory)50 strategy were applied to the AlCl3 complexes of ethylene and acetylene to establish a reliable computational protocol for the description of the interaction energies of aluminum-containing complexes. An excellent agreement was obtained (Table S2 in the Supporting Information) between the CCSD(T) (coupled cluster calculations with single and double excitations and the perturbative inclusion of triple excitations) benchmark data and the DF-DFT-SAPT procedure outlined in the Experimental Section. On the basis of this, the series of hypothetical complexes of HCN with AlCl3, AlCl2Mes, AlClMes2, and AlMes3 was studied using the same DFT-SAPT strategy. An inspection of the respective contributions revealed an interesting trend (Table S3 in the Supporting Information) of the three energetic contributions (electrostatic, Eelst, dispersion, Edisp, and induction, Eind) becoming of approximately the same magnitude with an increasing number of mesityl groups in the system. This situation holds also for the largest system investigated here (74 atoms), which is the complex of AlMes3 with THF. In this case, the total value of the interaction energy of −112 kJ/mol (which is fairly close to the counterpoisecorrected73 RI-MP2 result of −107 kJ/mol obtained with the aug-cc-pVDZ basis sets) is the sum of the Eelst (+70 kJ/mol), Edisp (−92 kJ/mol), and almost the same value of Eind (−91 kJ/ mol). Consequently, this complex is of the mixed type,74 since neither of the electrostatic, dispersion or induction terms, prevails. These calculations thus illustrate the intricacy of the interactions involving aluminum-containing Lewis acids. Generally, due to their sp3 hybridization, tetra-coordinated aluminum atoms create regular tetrahedra that in the NMR spectra are typically reflected by narrow signals with CQ < 10 MHz19,75 and NMR chemical shifts ranging from 30 to 230 ppm76 (e.g., AlCl3·THF and AlCl4− values in this study). However, in the case of the AlMes3·THF, AlMes2Cl·THF, and AlMesCl2·THF systems listed above, where the aluminum atoms are tetra-coordinated as well, the experimental data and DFT calculations surprisingly indicate CQ values reaching up to 30 MHz (Table S1 in the Supporting Information). This unexpected behavior can be elucidated from a structural viewpoint. The behavior of Lewis acids has been known for many years,77 with one of the most famous examples being F3B···NH3.78 Very briefly, the formation of Lewis acids and Lewis adducts for elements in Group 13 of the periodic table has very particular features. One s orbital and two p orbitals in the valence sphere hybridize to form three sp2 orbitals with one p vacancy, which is then accessible to accepting an electron from a Lewis basean electron-rich molecule containing a lone electron pair (i.e., nitrogen or oxygen). When an electron is accepted into the empty p orbital, a donor−acceptor bond is formed, and the original trigonal planar geometry is changed to

a

102 100(10) 201 200(10) 360 360(20) 354 360(20) 241 240(10) 270 270(20) 0 0(10) 77 80(5) 89 90(10) 90 90(5) 5 5(10) 1 0(10) 184 180(20) 207 205(10) 1 0(15) 178 180(20) 187 190(15) 161 160(15) −0.98 −0.99(0.2) 0.03 0.05(0.1) 0.87 0.87(0.2) 0.92 0.92(0.2) −0.19 −0.19(0.1) −0.91 −0.90(0.2) Al 1 AlMes3

III

Calc. Exp. Calc. Exp. Calc. Exp. Calc. Exp. Calc. Exp. Calc. Exp.

277.9

243.5 240(20) 115.5 120.0(10) 45.2 41.8(5) 88.0 74.2(5) 93.4 80.9(5) −8.4 18.2(5)

50.9 49.2(5) 28.3 27.3(3) 6.14 5.64(0.5) 6.09 6.59(0.5) 1.68 3.09(0.5) 6.28 3.37(1)

0.01 0.01(0.1) 0.13 0.13(0.1) 0.06 0.29(0.1) 0.02 0.58(0.1) 0.60 0.40(0.1) 0.16 0.16(0.1)

121.4 126.5(10) 42.4 46.1(5) 150.5 151.3(10) 72.4 72.9(5) 21.8 23.2(5) 57.7 57.5(5)

α (deg) δiso (ppm)b σiso (ppm) site signal no. compound

Table 2. Calculated and Experimental 27Al NMR Parameters of the Fitted Signalsa

|CQ| (MHz)

η

Ω (ppm)

κ

β (deg)

γ (deg)

Inorganic Chemistry

E


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Inorganic Chemistry

Table 3. Wiberg Bond Index of the Nearest Neighbors Bonded to the Investigated 27Al Sites, the Total Wiberg Bond Index of the Sites and the Complexation Energy, dE (see eq 1), of the Corresponding Complexes AlCl3·THF AlMesCl2·THF AlMes2Cl·THF AlMes3·THF a

Bond WBI Bond WBI Bond WBI Bond WBI

Al−O 0.2049 Al−O 0.1746 Al−O 0.1627 Al−O 0.1519

Al−Cl 0.6658 Al−C 0.5106 Al−C 0.4600 Al−C 0.4257

Al−Cl 0.6651 Al−Cl 0.6357 Al−C 0.4593 Al−C 0.4264

Al−Cl 0.6604 Al−Cl 0.6280 Al−Cl 0.5848 Al−C 0.4281

total Al WBI

|dE| (kJ/mol)

CQ (MHz)a

2.2190

200

6.59

2.0534

158

17.7

1.8678

137

23.8

1.7366

109

28.3

The DFT calculated values of CQ were taken from Table S1 in the Supporting Information.

a trigonal pyramidal or tetrahedral geometry. Simultaneously, an elongation or contraction of the original covalent bonds compensates for the changes in the electron density.79 Moreover, the electrostatic potential at the molecular surface is changed,80 which is also a significant indicator of the formation of the Lewis adducts. Grabowski very exhaustively described an approach regarding how to analyze the nature of substituent bonding to Lewis acid centers using electrostatic (electric field) potential analysis,81 which is helpful for the determination of the strengths of the center interactions with other atoms and/or ions.82 From the literature, it is also clear that the nature of the bonds can be connected to their electric field gradient (EFG).83 Similar results were described by Schurko et al.12 who used 27Al UW NMR spectroscopy for investigation of tri- and penta-coordinated aluminum species and concluded that the EFG tensor orientation and the NMR quadrupolar parameters are dependent upon the distribution of ground-state electron density. These findings suggest that the bond nature of aluminum atoms can be assessed on the basis of their quadrupolar parameters. Here, we have inspected in detail the nature of bonds for selected aluminum species using quantum chemical calculations, namely, the analyses based on electron localization functions (ELF)84,85 and the NBO.86 The selected series of crystal structures (AlCl3·THF, AlMesCl2·THF, AlMes2Cl·THF, and AlMes3·THF) were chosen for a systematic study to determine the dependence of resulting 27Al NMR quadrupolar coupling constants on the bond nature of aluminum Lewis adducts. As a first step, the overall order of the given aluminum bonds was assessed using the Wiberg bond index (WBI) matrix87 based on natural atomic orbitals (NAOs), with the resulting WBI values listed in Table 3. The selected series contain three different bonds (Al−O, Al− Cl, and Al−C), and the NBO analysis clearly confirms that each bond has markedly different strength. While according to the ab initio calculations, a decrease in bond strength follows the trend Al−Cl > Al−C > Al−O, a simple prediction from the electronegativity of the involved atoms suggests the following dependency of bond strength: Al−O > Al−Cl > Al−C. The unexpected lower strength of the Al−O bond obtained from the NBO analysis (Table 3) is also clearly seen in the ELF visualization in Figure 3. In the ELF analysis, the missing contours in the middle of the Al−O bond are in all four molecules instead concentrated around the oxygen atoms indicating a polarized donor−acceptor bond. On the other hand, in the cases of Al−Cl and Al−C bonds the contours are localized in the middle of the bonds, as they are typically covalent. It therefore follows that these aluminum Lewis adducts contain weak Al−O donor−acceptor bonds and strong

Figure 3. Isosurfaces at ELF = 0.5 are plotted with the atomic structures of the investigated organoaluminum compounds. The distribution of ELF shows the distinctly covalent (the contours are localized in the middle of the bonds) and donor−acceptor (the missing contours in the middle of the Al−O bond) nature of bonds for AlCl3·THF, AlMesCl2·THF, AlMes2Cl·THF, AlMes3·THF.

covalent Al−Cl and Al−C bonds. Furthermore, the different nature of bonds can be easily assessed using the ELF analysis. It must be kept in mind that while the ELF analysis is a suitable tool for the visualization of electron localization throughout a molecule for ELF values ranging from 0 to 1, no information is given about the strength of the bonds. However, it is possible to look for that information using correlation between the obtained CQ values and NBO results (Table 3). The CQ values in our case offer an approach how to interconnect the structural motifs with NMR parameters and thus obtain a deeper insight into the type/nature of the individuals bonds. From both the ELF and NBO analyses, the Al−Cl and Al−C bonds were defined as covalent, whereas the Al−−O+ bond as a strongly polarized donor−acceptor bond. Moreover, from Table 3, it is also evident that the magnitude of the Al−−O + bond polarization depends on the other substituents on the aluminum atoms, as reflected from the value of WBI. Additionally, the WBI values and complexation energies (supermolecular (dE) calculations)44 are inversely dependent on the values of the quadrupolar coupling constant (CQ); see Table 3 and Figure 4. The correlations of WBI and/or dE with CQ values show linear dependency in both cases (Figure 4). This dependency clearly indicates that with a weakening of the Al−O bond, the CQ value increases. It follows that the quadrupolar coupling constant reflecting the electron density is a suitable parameter for the study of the bond nature in aluminum Lewis adducts. Moreover, the AlMes3·THF system (KAJVUM) was previously F


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Inorganic Chemistry

Figure 4. Plots showing the correlation of the Wiberg bond index (WBI - dashed line and circles) and the complexation energy of formation of the corresponding complexes (dE - solid line and diamonds) with a quadrupolar coupling constant (CQ) of the Lewis adducts listed in Table 3.

comprehensive information on the electronic structure. The CQ values are shown to be correlated to the energy of the Al−− O+ polarized donor−acceptor bonds using the complexation energy and/or Wiberg bond index. Within a complex compound, the larger CQ values directly correlate with the weakest Al−−O+ bond (see Table 3) and are also directly connected with the geometry of the aluminum Lewis adducts reflecting the structural ordering from trigonal pyramidal to tetrahedral geometry. Along with the quadrupolar coupling constant (CQ) and 27Al chemical shifts, it is then possible to gain experimental insight into the polycrystalline system and define the individual structural features. All the DFT predictions of the assumed structural models correlated very well with experimental NMR parameters from collected NMR spectra. This result suggests that 27Al NMR is a very powerful technique for screening wide ranges of Lewis acids/Lewis adducts in both organic and inorganic materials. To our best knowledge, we have demonstrated for the first time that tetra-coordinated aluminum species can appear in 27 Al NMR spectra as ultrawide signals exceeding 20 MHz due to present weak Al−O donor−acceptor bonds exhibiting the trigonal pyramidal geometry. These results suggest that similar very broad signals can be easily overlooked, and thus the combination of used techniques must be wisely chosen. Additionally, we believe that this approach can be useful especially for the structural analysis of Lewis acids in zeolites, where NMR spectroscopy has an irreplaceable position for structural description of catalytic centers.

described as a system with reversibly leaving/coordinating THF molecule.88 It is well-known that similar bond types between aluminum and oxygen atoms can be found in dehydrated zeolites. As a matter of fact, our laboratory has focused on the investigation of aluminum Lewis acids in zeolites using ssNMR since 2015.89 We have found that not only the interpretation of the recorded spectra is difficult, but also the correct recording of the required spectra can be a challenge. From our previously published paper,89 it is evident that our results were incorrectly recorded (no VOCS and echo based techniques were used) and subsequently slightly misinterpreted. The observed relatively extensive signal broadening was originally attributed to the trigonal Al site in chabazite. However, following our current findings the presence of a fourth much weaker donor−acceptor Al···O bond must be considered. This suggests that an AlIV atom is connected by three covalent bonds to the aluminosilicate framework and by a weak donor−acceptor bond to (i) an oxygen atom from residual water molecules and/ or (ii) an oxygen atom from the aluminosilicate fragment. Nevertheless, this site still has trigonal pyramidal geometry as opposed to the common symmetrical tetrahedron. This trigonal pyramidal geometry was found in this study for the AlMesCl2· THF (YANKIH), AlMes2Cl·THF (YIYCIS), and AlMes3·THF (KAJVUM) systems. Here, the aluminum atoms are sterically hindered (by the mesityl group(s) or aluminosilicate framework), and the Lewis base(s) cannot form the regular tetrahedron geometry. Generally, these aluminum Lewis adducts with weak donor−acceptor bonds have the potential to function as hindered catalytic centers64 activated by withdrawal of an electron-rich molecule.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01009. (1) PXRD pattern with calculated PXRD patterns of selected CCDC entries, (2) experimental 13C MAS NMR spectrum, (3) experimental 27Al 3Q/MAS NMR spectrum with slices and fitting of individual signals, (4)

4. CONCLUSIONS It has been demonstrated that 27Al NMR spectroscopy (combining UW, MAS, and MQ/MAS NMR experiments) is a robust tool for studying a variety of organoaluminum compounds. The values of the quadrupolar coupling constant (CQ) obtained from these methods provide useful and G


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Inorganic Chemistry

(11) Lang, S.; Benz, M.; Obenaus, U.; Himmelmann, R.; Scheibe, M.; Klemm, E.; Weitkamp, J.; Hunger, M. Mechanisms of the AlCl3Modification of Siliceous Microporous and Mesoporous Catalysts Investigated by Multi-Nuclear Solid-State NMR. Top. Catal. 2017, 60, 1537−1553. (12) Tang, J. A.; Masuda, J. D.; Boyle, T. J.; Schurko, R. W. UltraWideline 27Al NMR Investigation of Three- and Five-Coordinate Aluminum Environments. ChemPhysChem 2006, 7, 117−130. (13) Corma, A.; García, H. Lewis Acids as Catalysts in Oxidation Reactions: From Homogeneous to Heterogeneous Systems. Chem. Rev. 2002, 102, 3837−3892. (14) Xu, M.; Lunsford, J. H.; Goodman, D. W.; Bhattacharyya, A. Synthesis of Dimethyl Ether (DME) from Methanol Over Solid-Acid Catalysts. Appl. Catal., A 1997, 149, 289−301. (15) Emeis, C. A. Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347−354. (16) Pagán-Torres, Y. J.; Wang, T.; Gallo, J. M. R.; Shanks, B. H.; Dumesic, J. A. Production of 5-Hydroxymethylfurfural from Glucose Using a Combination of Lewis and Brønsted Acid Catalysts in Water in a Biphasic Reactor with an Alkylphenol Solvent. ACS Catal. 2012, 2, 930−934. (17) Corma, A.; Grande, M. S.; Gonzalez-Alfaro, V.; Orchilles, A. V. Cracking Activity and Hydrothermal Stability of MCM-41 and Its Comparison with Amorphous Silica-Alumina and a USY Zeolite. J. Catal. 1996, 159, 375−382. (18) Bryce, D. L. NMR crystallography: structure and properties of materials from solid-state nuclear magnetic resonance observables. IUCrJ 2017, 4, 350−359. (19) MacKenzie, K. J. D.; Smith, M. E. Multinuclear Solid-State Nuclear Magnetic Resonance of Inorganic Materials; Elsevier: Amsterdam, NL, 2002. (20) van Bokhoven, J. A.; Koningsberger, D. C.; Kunkeler, P.; van Bekkum, H.; Kentgens, A. P. M. Stepwise Dealumination of Zeolite Βeta at Specific T-Sites Observed with 27Al MAS and 27Al MQ MAS NMR. J. Am. Chem. Soc. 2000, 122, 12842−12847. (21) Ernst, H.; Freude, D.; Wolf, I. Multinuclear solid-state NMR studies of Brønsted sites in zeolites. Chem. Phys. Lett. 1993, 212, 588− 596. (22) Hunger, M.; Horvath, T. J. Adsorption of Methanol on Brønsted Acid Sites in Zeolite H-ZSM-5 Investigated by Multinuclear SolidState NMR Spectroscopy. J. Am. Chem. Soc. 1996, 118, 12302−12308. (23) Massiot, D.; Farnan, I.; Gautier, N.; Trumeau, D.; Trokiner, A.; Coutures, J. P. Ga-71 and Ga-69 Nuclear-Magnetic-Resonance Study of Beta-Ga2O3-Resolution of 4-Fold and 6-Fold Coordinated Ga Sites in Static Conditions. Solid State Nucl. Magn. Reson. 1995, 4, 241−248. (24) Meiboom, S.; Gill, D. Modified Spin-Echo Method for Measuring Nuclear Relaxation Times. Rev. Sci. Instrum. 1958, 29, 688−691. (25) Larsen, F. H.; Jakobsen, H. J.; Ellis, P. D.; Nielsen, N. C. Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors. J. Phys. Chem. A 1997, 101, 8597−8606. (26) Dey, K. K.; Prasad, S.; Ash, J. T.; Deschamps, M.; Grandinetti, P. J. Spectral editing in solid-state MAS NMR of quadrupolar nuclei using selective satellite inversion. J. Magn. Reson. 2007, 185, 326−330. (27) Schurko, R. W. Ultra-Wideline Solid-State NMR Spectroscopy. Acc. Chem. Res. 2013, 46, 1985−1995. (28) Hahn, E. L. Spin Echoes. Phys. Rev. 1950, 80, 580−594. (29) Bodart, P. R.; Amoureux, J.-P.; Dumazy, Y.; Lefort, R. Theoretical and Experimental Study of Quadrupolar Echoes for Half Integer Spins in Static Solid-State NMR. Mol. Phys. 2000, 98, 1545− 1551. (30) Frydman, L.; Harwood, J. S. Isotropic Spectra of Half-Integer Quadrupolar Spins from Bidimensional Magic-Angle Spinning NMR. J. Am. Chem. Soc. 1995, 117, 5367−5368. (31) Wiegand, T.; Eckert, H.; Ekkert, O.; Frohlich, R.; Kehr, G.; Erker, G.; Grimme, S. New Insights into Frustrated Lewis Pairs:

DFT parameters for all possible structural units, (5) the comparison of the supermolecular and SAPT interaction energies, (6) the values (in kJ/mol) of the interaction energies, (7) relative contributions to the SAPT-DFT interaction energy of model systems, (8) energetic parameters and final coordinates of all investigated systems (PDF) Accession Codes

All structures used in this paper were downloaded as CIF files from CCDC database, except the AlCl3 structure, which is available at http://aflowlib.org/CrystalDatabase/AB3_mC16_ 12_g_ij.html (see Table 1).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Libor Kobera: 0000-0002-8826-948X Hana Mackova: 0000-0002-8334-4508 Jiri Brus: 0000-0003-2692-612X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant Agency of the Czech Republic (Grants GA16-13778S and GA16-04109S). Computational resources were partially provided under the programme “Projects of Large Infrastructure for Research, Development, and Innovations” LM2010005, and in the Center CERIT Scientific Cloud, part of the Operational Program Research and Development for Innovations, reg. no. CZ. 1.05/3.2.00/ 08.0144.

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