The Nature of Chemical Bonding in Lewis Adducts as Reflected by 27Al NMR Quadrupolar Coupling Constant: Combined Solid-State NMR and Quantum Chemical Approach

Article abstract of DOI:10.1021/acs.inorgchem.8b01009

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 solid-state 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 quantum-chemical 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 (Al^III) and strongly perturbed Al^IV sites.

Full text of DOI:10.1021/acs.inorgchem.8b01009

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
The Nature of Chemical Bonding in Lewis Adducts as Reflected by
27Al 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 solid-
state 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 quantum-
chemical 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.
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.
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 organo-
aluminum 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
Traditionally, structural descriptions of partially and/or fully
crystalline compounds have been conducted using powder X-
ray 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
© XXXX American Chemical Society
A
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Table 1. Selected Crystallographic Information on All the Possible Aluminum Complexes with THF, Mesitylene and Chlorine
Extracted from the CCDC
compound
a
ref code
space group
C2/m
P3
CCDC number
ref
59
AlCl3
AlMes3
FAYKIZ-01
ZOWHEY
YIYCOY
1153248, 275905
12, 60
61
̅
(AlMes2)2OH2·THF
(AlMes2)2Cl2
AlMes2Cl·THF
AlMesCl2·THF
AlMes3·THF
AlCl3·2THF
AlCl3·THF
[AlCl2·4THF]+AlCl4
P1
̅
P21/c
1315535
1304048
62
YIYCIS
P21/a
1304047
62
YANKIH
P1
1299315
63
KAJVUM
ALCTHF-01
TOGSUD
FEYYAJ-01-02
P21/c
1192495
64
Pbcn
1102209, 116803
1273341
65, 66
67
P21/m
−
P21/n, P1121/n, P21/n
1155534, 1155535, 174104
68−70
a
CIF file was downloaded from http://aflowlib.org/CrystalDatabase/AB3_mC16_12_g_ij.html.
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 experi-
ments 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 environ-
ment 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 density-
functional theory (DFT) calculations to enable an identification
of all the aluminum species present in a prepared polycrystal-
line 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
(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.
2. EXPERIMENTAL SECTION
2.1. Sample Preparation. Anhydrous aluminum chloride and 2-
mesitylmagnesium bromide (1 M solution in tetrahydrofuran) were
purchased from Sigma-Aldrich (St. Louis, MO, USA). Toluene (Lach-
Ner; 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 config-
uration. 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
π
Al(NO3)3 (27Al: 0.0 ppm). The CT-selective /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. High-
1
power H decoupling (SPINAL64 for MAS experiments and CW for
static experiments) was used to eliminate heteronuclear dipolar
B
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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
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 DF-
DFT-SAPT procedures in the 2008.1 version of Molpro58 was used.
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
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
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.
dE(AB) = E(AB) − E(A) − E(B)
(1)
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
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 MP2/6-
31G** 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-cc-
pVDZ) basis sets appropriately combined with the cc-pVDZ JK-
fitting52 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 first-
order 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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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.
−
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
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 organo-
aluminum 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
D
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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 DFT-
SAPT (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 counterpoise-
corrected73 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
E
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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
a
total Al WBI
|dE| (kJ/mol)
CQ (MHz)
AlCl3·THF
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
Al−Cl
0.6651
Al−Cl
0.6357
Al−C
Al−Cl
0.6604
Al−Cl
0.6280
Al−Cl
0.5848
Al−C
2.2190
2.0534
1.8678
1.7366
200
158
137
109
6.59
17.7
23.8
28.3
AlMesCl2·THF
AlMes2Cl·THF
AlMes3·THF
0.5106
Al−C
0.4600
Al−C
0.4593
Al−C
0.4264
0.4257
0.4281
a
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.
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.
Here, we have inspected in detail the nature of bonds for
selected aluminum species using quantum chemical calcu-
lations, 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
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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Article
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.
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.
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
27Al 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.
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 frame-
work), 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.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01009.
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
(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)
G
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Inorganic Chemistry
DFT parameters for all possible structural units, (5) the
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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: kobera@imc.cas.cz.
ORCID
Libor Kobera: 0000-0002-8826-948X
Hana Mackova: 0000-0002-8334-4508
Jiri Brus: 0000-0003-2692-612X
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Grant Agency of the Czech
Republic (Grants GA16-13778S and GA16-04109S). Computa-
tional 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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