123Sb Nuclear Quadrupole Resonance Spectroscopy

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry 121/123

Sb Nuclear Quadrupole Resonance Spectroscopy: Characterization of Non-Covalent Pnictogen Bonds and NQR Crystallography Cesar Leroy, Ryan Johannson, and David L. Bryce

J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b11490 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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The Journal of Physical Chemistry

121/123Sb

Nuclear Quadrupole Resonance Spectroscopy: Characterization of Non-Covalent Pnictogen Bonds and NQR Crystallography

César Leroy, Ryan Johannson, and David L. Bryce*

*Author to whom correspondence is to be addressed Department of Chemistry and Biomolecular Sciences & Centre for Catalysis Research and Innovation University of Ottawa 10 Marie Curie Private Ottawa, Ontario K1N 6N5 Canada Tel: +1-613-562-5800 ext.2018; fax: +1-613-562-5170 Email: [email protected]

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Abstract Pnictogen (or pnicogen) bonding is an attractive interaction between the electrophilic region of group 15 elements (N, P, As, Sb, Bi) and a nucleophile. This interaction for which unique applications in catalysis have recently been uncovered continues to gain popularity. Here, we investigate a series of pnictogen-bonded cocrystals based on SbF3 and SbCl3, prepared via mechanochemical ball milling, with

121/123Sb

(I = 5/2 and 7/2, respectively) nuclear quadrupole

resonance (NQR) spectroscopy. Observed NQR frequency shifts upon cocrystallization are on the order of 0.1 to 10 MHz and are clearly diagnostic of the formation of pnictogen bonds to antimony. Further evidence for pnictogen bonding is obtained by complementary

13C

cross-polarization

magic-angle spinning solid-state NMR experiments. DFT calculations of NMR parameters as well as natural localized molecular orbital analyses support the experimental findings and elucidate the electronic origins of the experimental NQR frequency shifts. This work provides insights into the changes in the antimony quadrupolar coupling constant upon pnictogen bonding: strikingly, the decreases noted here parallel those known for hydrogen bonds, but contrast with the increases reported for halogen bonds. The utility of the observed antimony nuclear quadrupolar coupling constants in constraining structural models of cocrystals for which diffraction-based structures are unavailable, i.e., a rudimentary implementation of NQR crystallography, is established. Overall, this work offers a new approach to understand emerging classes of electrophilic interactions and to contextualize them in the broader landscape of established chemical bonding paradigms.


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Introduction In recent years, a consensus has begun to build regarding the nature and nomenclature of a range of relatively understudied non-covalent interactions.1,2,3

Hassel’s Nobel Prize4 on

interactions including those which are now recognized as halogen bonds, along with Alcock’s important work on the identification and classification of so-called secondary bonding interactions,5 have laid the foundations for understanding a range of electrophilic interactions. The similarities and differences of such interactions when compared with the ubiquitous hydrogen bond have been discussed,6 and the halogen bond in particular is now a widely recognized and studied sister interaction.7 Related interactions are named after the electrophilic element.3,8 Along with the halogen bond, pnictogen,9 chalcogen,10 tetrel,11, 12 aerogen,13 and triel14 bonds are among those which are now known to play pivotal roles in catalysis,15 anion recognition,16 drug design,17 and crystal engineering.18 Pnictogen bonding (PnB) in particular has risen in prominence in recent years in part as a result of key studies demonstrating its critical role in the design of new compounds with, in some cases, a strength comparable to hydrogen bonding.19,20,21,22 This non-covalent interaction is between an area of increased electrostatic potential of a pnictogen atom (Pn = N, P, As, Sb, or Bi) in a molecule and an electron-rich moiety such as a Lewis base (Y). This type of interaction may be described using the σ-hole paradigm.23,24 The σ-hole is a region of depleted electron density which typically closely maps to an area of elevated electrostatic potential on the pnictogen.25 To describe these non-covalent interactions, the R-Pn···Y angle, θ, is used along with the normalized contact value, Nc (which corresponds to the ratio of the distance between the bond donor and acceptor (dPnY) to the sum of the van der Waals radii of the atoms or ions involved); θ is typically



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between 155 and 180° and Nc is less than unity if there is a significant interaction. Here, R is one or more substituents covalently bonded to the pnictogen atom. Our group has reported extensively on the application of solid-state multinuclear magnetic resonance (SSNMR) spectroscopy to study and provide new insights into halogen bonds and tetrel bonds.26,27,28,29,30,31 Such studies provide a unique way to characterize non-covalent bonds as well as relationships between spectral parameters and the local bonding geometry. In the case of pnictogen bonds, those involving antimony are the most widely known. Antimony has two NMRactive isotopes, 121Sb and 123Sb, with nuclear spin quantum numbers of 5/2 and 7/2, respectively. Despite possessing some favorable NMR properties (good natural abundances (121Sb: 57.21% and 123Sb: 1)),

42.79%) and moderate gyromagnetic ratios (γ = 6.44×107 rad·s-1·T-1 and 3.49×107 rad·s-1·T-

antimony SSNMR studies are very scarce as a result of the large

121/123Sb

nuclear electric

quadrupole moments (Q = -543 mb and -692 mb for Sb-121 and Sb-123, respectively).32 This means that the central transition (CT; m = 1/2 ↔-1/2) NMR line widths for these isotopes rapidly become intractably broad as a result of the quadrupolar interaction (QI) for all but the most symmetric of chemical environments (e.g., perfect tetrahedral or octahedral environments) even in the highest commercially available magnetic field strengths for NMR spectroscopy. The magnitude of the QI is described by the quadrupolar coupling constant (CQ) and the electric field gradient (EFG) asymmetry parameter (ηQ): CQ = eQV33/h

Equation 1

Q = (V11-V22)/V33

Equation 2


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where V11, V22, and V33 are the principal components of the traceless electric field gradient (EFG) tensor (|V33| ≥ |V22| ≥ |V11|), e is the fundamental charge, h is the Planck constant, and Q is the nuclear quadrupole moment. Only three papers have been published in the last decade on the topic of antimony solidstate NMR spectroscopy.33,34,35,36 In two contributions from Faucher et al., the authors underscore the difficulties inherent in studying these isotopes and consequently, chose to examine antimony centers with relatively symmetric environments, e.g., KSb(OH)6 and SbPh4Br with experimental CQ(121Sb) values measured at about 46.0 and 159.0 MHz, respectively.33,34 The CT line width for SbPh4Br in a magnetic field of 21.1 T is about 5.5 MHz. KSb(OH)6, with a nearly octahedral environment at antimony gives rise to a smaller CQ value and an CT line width of approximately 450 kHz at 21.1 T. Even in this strong magnetic field, where the effects of second-order quadrupolar broadening are reduced, the

121Sb

SSNMR spectrum of SbPh4Br required

approximately 3 days of acquisition time. Prior to this work only a limited set of contributions were focused on antimony NMR, most of them in solution or very symmetric systems.37,38,39,40,41,42 However, antimony nuclei can be probed directly by two other methods in solids: Mössbauer and NQR spectroscopies.43,44,45,46,47,48,49,50,51 Legon has surveyed the experimentally known gas phase complexes featuring pnictogen bonds.2 Herein we report on the characterization of pnictogen bonds between antimony halides (antimony trifluoride (SbF3) and trichloride (SbCl3)) and a set of selected Lewis bases using antimony solid-state NQR spectroscopy. The presence of three regions of elevated electrostatic potential on the surface of the antimony atom (along the extensions of the Sb–F/Sb–Cl bonds, see Figure 1) allows for the formation of cocrystals with nucleophilic entities. Interestingly, the positions of the three σ-holes deviate from the elongation of the X–Sb bonds by about 28° for X = 5 ACS Paragon Plus Environment


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F and 17° for X = Cl. These deviations are important when dealing with the directionality of the non-covalent bonding and thus in crystal engineering. The strong electronegativity of the halogens induces a region of positive electrostatic potential opposite each covalent Sb-X bond. A fourth σhole situated in the middle of the equilateral triangle formed by the halides and in opposition to the lone pair of the antimony has also been investigated.52 Very recently, Yang et al. have shown how to ‘dig’ a deeper σ-hole by oxidizing antimony(III) to the +V state.53

Figure 1. Calculated molecular electrostatic potential maps (MESPs) of (a) SbF3 and (b) SbCl3 on the 0.001 a.u. molecular surface. The most positive values are in blue while the most negative are in red. The black dots demark the most positive regions. The scales used for each molecule have been adjusted in order to facilitate visualization. (c) Deviation (ϕ of the electrostatic potential maxima from the elongation of the X–Sb bonds for X = F (top) and X = Cl (bottom). Calculations are based on structures published by Lipka (SbCl3, 8258-ICSD) and Edwards (SbF3, 16142-ICSD).54,55

Historical work has been reported on the interaction of SbF3 with Lewis bases; however, the resulting compounds have very rarely been examined in the context of non-covalent PnB. A recent review by Scilabra et al. inventories the relevant published structures and discusses them anew in this context.56 In the present contribution, we propose a new and original approach based on antimony NQR experiments and density functional theory (DFT) calculations to study pnictogen bonds. Furthermore, we explore the relationship between the pnictogen bond geometry (i.e., dPnY and θ) and the NQR frequencies and show how this information can be used to constrain


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and propose structural models for cocrystals lacking diffraction-based structures, i.e., NQR crystallography. NMR crystallographic methods now often use NMR data in this context,57 and Bonhomme et al. have previously mentioned NQR data in the framework of NMR crystallography.58

Our work is further complemented by

13C

cross-polarization magic-angle

spinning (CP/MAS) NMR experiments and natural localized molecular orbital analyses to understand the influence of pnictogen bonding on the observed antimony quadrupolar coupling constants. Experimental (i) Synthesis and Sample Preparation Antimony trifluoride (SbF3, 1) and antimony trichloride (SbCl3, 2) were purchased from Alfa Aesar and used as received. Lewis bases (urea, a; phenanthroline, b; nicotinamide, c; trithiane, d; trioxane, e) were purchased from various suppliers and used without further purification. Cocrystals (1.a, 1.b, 1.c, 1.ab, 2.d, 2.e) were prepared by mechanochemistry using a Retsch MM 400 ball mill and 10 mL stainless steel milling jars containing two 5 mm stainless steel grinding balls. Milling was performed at 30 Hz for durations between 30 min and 1 h. Further details on the preparation of each sample are reported in the Supporting Information (SI). Previously reported structures were reproducibly prepared as powders without the use of any liquid, as assessed via powder X-ray diffraction (see SI). All compounds were handled under inert atmosphere to avoid possible decomposition. (ii) NQR Spectroscopy Powdered samples were packed in 5 mm o.d. glass tubes in an argon-filled glovebox. A Bruker Avance III 400 NMR console, without applied external magnetic field, was used to record 7 ACS Paragon Plus Environment


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the 121/123Sb NQR frequencies. A Hahn–echo pulse sequence (/2–––) was used, with typical pulse lengths of 4.6 and 9.2 μs. The recycle delay was set at 0.2 s. The total number of transients acquired varied depending on the sample and the transition under observation (see SI). Data were collected at room temperature using a 5 mm static (solenoid) HX probe. (iii) 13C SSNMR Spectroscopy 1H-13C

CP/MAS SSNMR experiments were performed at 9.4 T on a Bruker Avance III

NMR spectrometer (L(13C) = 100.6 MHz) using a Bruker 4 mm HXY MAS probe. A 4.6 μs proton /2 pulse was used prior to a contact time ranging from 500 to 2000 s depending on the sample.

1H

decoupling was applied during acquisition using the SPINAL64 sequence.59

13C

chemical shifts were referenced to solid glycine (carbonyl signal at 176.4 ppm) as a secondary reference. Further information, such as spinning speeds, recycle delays, and the number of transients, is available in the SI. (iv) Powder X-ray diffraction Powder X-ray diffraction was performed using a Rigaku Ultima IV instrument in BraggBrentano mode with 2θ ranging from 5° to 65° at a rate of 1° per minute (0.02° step size) using CuK radiation. (v) Computational details Density functional theory (DFT) calculations performed on antimony compounds were carried out using the Amsterdam Density Functional (ADF) software package,60,61,62 with the ZORA-QZ4P basis set, a statistically averaged orbital potential (SAOP),63 and relativistic effects (spin-orbit or scalar) using models built from coordinates from X-ray structures obtained from the 8 ACS Paragon Plus Environment

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CCDC without further geometry optimization (except for cocrystal 1.ab, see SI). An assessment of other basis sets and functionals may be found in the SI. Gauge-including projector-augmented wave density functional theory (GIPAW-DFT) calculations on various structures (non-optimized, hydrogens-optimized and all-atoms-optimized structures) were carried out using CASTEP and Materials Studio version 4.3.64,65 Data were parsed from the output files using EFGShield software version 4.3.66 Interconversion between quadrupolar coupling constants and the various observable NQR frequencies was facilitated by QUEST software (version 1.1.7.).67 Models were built using published X-ray structures of SbF3 (16142-ICSD) and SbCl3 (8258-ICSD).54,55 NH3, H2O, and trioxane molecules were added and placed according to particular θ and dPnY values using GaussView 4.1 (see text). When no hydrogens were included in the CCDC structures, they were automatically generated with GaussView with typical distances (N-H: 1.00 Å, O-H: 0.96 Å and C-H: 1.07 Å). For SbF3·xNH3 and SbF3·xH2O, the electron donor (N or O) was chosen to be the closest to the antimony atom (see SI for Cartesian coordinates). No elongation of the Sb–F bonds was included. The PnB distance and the angle were set to physically relevant values of dPnY = 2.58 Å and θ = 165° (inspired from the structure of 1.b) for both NH3 and H2O based models. For the SbCl3·trioxane cocrystal of unknown structure (2.e), coordinates for an isolated molecule of trioxane from its X-ray structure (TROXAN11)68 were used and the molecules were then manually placed according to specific values of θ and dPnY. It is assumed that the three trioxane molecules are in the same plane. See the SI for more information on each model. Calculations of molecular electrostatic potential maps (MESP) were carried out with the Gaussian 09 software at the MP2/def2-TZVP level of theory on the Wooki cluster at the University



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of Ottawa, and visualized using GaussView 4.1 software. The results were traced over electron density surfaces with an isodensity of 0.001 a.u. (electron bohr3). Results and Discussion (i) Description of Structures We have chosen to work with two antimony trihalides, SbF3 and SbCl3, as evidence of PnB to these donors has been recently elucidated.56 The PnB acceptors were selected from the literature (urea, phenanthroline, nicotinamide, and trithiane) or on the basis of similarities with reported structures (trioxane compared to trithiane).69,70,71,72 Compounds studied here are presented in Table 1 along with their respective crystallographic data. Table 1. Crystallographic and structural information on the compounds studied in this work. compound

code

dPnY / Å

Nca

θ/°

Ref

1

SbF3

16142-ICSD

NA

NA

NA

55

1.a

SbF3·urea

UREAAF

2.54 2.61

0.640 0.657

155 154

69

1.b

SbF3·phenanthroline

VAQTUE

2.58 2.77

0.625 0.670

155 137

70

1.c

SbF3·nicotinamide

YUXPUC

2.47

0.598

161

71

1.ab SbF3·phenanthroline·urea

b

2

SbCl3

8258-ICSD

NA

NA

NA

54

2.d

SbCl3·trithiane

TRTHAC

3.26

0.747

171

72

2.e

SbCl3·trioxane

b

Nc corresponds to the normalized contact: Nc = dPnY/(Σ(rvdWPn +rvdWY) with rvdW taken from Alvarez.73

a

b No

reported structures.


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Of the six cocrystals presented here, two are new with unknown crystallographic structures (cocrystals 1.ab and 2.e).

The antimony atom exhibits a distorted octahedral coordination

geometry as a result of three Sb–F covalent bonds (1.90–1.94 Å) and three longer interactions (2.60–2.63 Å) in pure 1. This distortion can arise from the lone pair of the antimony occupying a seventh coordination position.55,74 When forming a complex with an electron donor, a systematic elongation of the Sb–F covalent bond opposite the non-covalent interaction can be noticed. For SbCl3, three covalent Sb–Cl bonds (2.35 to 2.38 Å) can be observed; however, no other particularly strong interactions are noticed in the overall packing.75 The Sb–Cl bonds are lengthened upon cocrystal formation in the reported structures. Below, brief descriptions of the structures of the cocrystals are provided. The success of the mechanochemical syntheses of the compounds (reproduced from literature and new ones) is directly assessed by PXRD experiments (see SI).

Figure 2. Ball and stick representations of known cocrystals along with a schematic representation of the electron donors of (a) SbF3·urea (1.a), (b) SbF3·phenanthroline (1.b), (c) SbF3·nicotinamide (1.c), (d) SbCl3·trithiane (2.d). (Color codes: grey-carbon; blue-nitrogen; red-oxygen; yellow-sulfur; purpleantimony; green-chlorine; yellow/green-fluorine and white-hydrogen). 11 ACS Paragon Plus Environment


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As presented in Table 1, 1,10-phenanthroline forms a bidentate complex in a 1:1 stoichiometry with SbF3 (1.b). The crystallographic structure (Figure 2(b)) reveals that one Sb···N bond follows the typical PnB geometry (with dPnY = 2.58 Å and θ = 155°) while the second one is sterically constrained and thus longer and has a smaller F–Sb···N angle (dPnY = 2.77 Å and θ = 137°). Consequently, the covalent Sb–F bond elongations are different for each bond; 2.04 Å for the one opposite the stronger PnB, and 1.96 Å for the other one with longer Sb···N distance.70 The other nitrogen electron donor considered in this study is nicotinamide; it gives rise to a 1:2 cocrystal, 1.c (see Figure 2(c)). Each SbF3 accommodates two different nicotinamide molecules giving rise to a cocrystal with the following PnB parameters: dPnY = 2.47 Å and θ = 161.21°. Both Sb–F covalent bonds opposite the PnB increase to 1.99 Å while the remaining one is 1.91 Å. Cocrystal 1.c has been studied previously by NQR spectroscopy, but the results have not been discussed in the framework of pnictogen bonding.76 Oxygen-based complexes with SbF3 have been reviewed by Scilabra et al.56 For the present study, we focus on SbF3·urea. The 1:1 cocrystal 1.a is shown in Figure 2(a).69 Two of the three σ-holes participate in pnictogen bonding: dPnY = 2.54 Å and θ = 155° for the shorter one, with an opposing Sb–F bond length of 1.93 Å. whereas the longer PnB causes an Sb–F elongation to 1.96 Å with dPnY = 2.61 Å and θ = 154°. The final known cocrystal studied herein, 2.d, is a 1:1 complex between SbCl3 and trithiane. No complex with SbF3 seems to have been reported yet.77 Three trithiane entities surround the antimony center with equivalent PnB parameters (Sb···S bonds; dPnY = 3.26 Å, Cl–Sb···S angles; θ = 171°) as can be seen in Figure 2(d). While SbCl3 exhibits only very few cocrystals with nitrogen and oxygen-based electron donors, a Cambridge Structural Database (CSD) survey shows that more data have been published with sulfur acting as the Lewis base (see SI). It is remarkable to notice that the angle θ is close to the angle expected with the 12 ACS Paragon Plus Environment

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deviation from linearity for the σ-hole noticed earlier (ϕ in Figure 1); hence, a more linear interaction in the case of antimony trichloride than with SbF3 adducts. The smaller Nc value for SbF3 cocrystals reflects a standard greater proximity between the electron donor and the antimony atom which reflects a more positive electrostatic potential in the case of SbF3.

Figure 3. (a) Top: schematic representation of the PnB donor and acceptor for cocrystal 2.e. The blue areas represent the σ-holes. Bottom: PXRD pattern of cocrystal 2.e (in black); simulated diffractograms of SbCl3 (8258-ICSD) and trioxane (TROXAN11) are presented in green and red, respectively. (b) Top: schematic representation of the PnB donor and acceptor for cocrystal 1.ab. The blue areas represent the σ-holes. Bottom: PXRD pattern of the new cocrystal 1.ab (in black); simulated diffractograms of cocrystal 1.b (VAQTUE) and of urea (UREAXX) are presented in red and green, respectively.

We have successfully obtained two new cocrystals as evidenced via powder X-ray diffraction in Figure 3, namely SbCl3·trioxane and SbF3·phen·urea (cocrystals 2.e and 1.ab). The first is inspired by cocrystal 2.d while the second is based on cocrystal 1.b and urea. Interestingly, 13 ACS Paragon Plus Environment


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Bombieri et al. have previously synthesized the SbF3·phenanthroline·thiourea cocrystal but failed to obtain the equivalent with urea (i.e., SbF3·phenanthroline·urea).46 We have circumvented the issue by using mechanochemical synthesis with the cocrystal 1.b and pure urea. Both unknown structures (1.ab and 2.e) are studied in more detail below (vide infra). (ii) 13C Chemical Shifts are Indirect Probes of Pnictogen Bonding The 13C CP/MAS SSNMR spectra of the pure electron donors and of the pnictogen-bonded cocrystals are presented in Figure 4. Cocrystallization-induced 13C chemical shifts of the donor and the acceptor have been studied previously for halogen-bonded systems. 78,79,80 Presently, as the carbon atoms are removed from the site of interaction (i.e., they are covalently bonded to the electron donor element (N, O, or S)), we do not anticipate a simple correlation between cocrystallization-induced shifts and the local PnB environment. This would be consistent with previous work on halogen-bonded systems, where shifts of several ppm are observed for carbons in the halogen bond acceptor, but there is no simple correlation with structural features.81 Pnictogen-bonding induced 13C chemical shifts are on the order of ppm (Table 2). In the case of cocrystal 1.b the C–N 13C chemical shifts change by 1.4 to 2.1 ppm (Figure 4(b)) while those for cocrystal 1.c change by 2.3 to 3.9 ppm (Figure 4(c)). The shift for the urea carbonyl carbon is smaller at only 0.6 ppm (Figure 4(a)).

The resonances typically broaden upon

cocrystallization (e.g., from 34 to 146 Hz at 9.4 T for cocrystal 2.d, Figure 4(e)). Expectedly, the shifts do not correlate in a simple fashion with pnictogen bond distances and/or angles. DFT calculated carbon chemical shifts can be found in the SI; the magnitudes of the changes in shifts are generally reproduced but there are exceptions and overall the agreement is not exemplary. As a side note, the 1H spin-lattice relaxation time constants, T1, are relatively long for some of the


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compounds studied here. Urea, for example, is known to have a T1 greater than an hour at 295 K which makes it impractical to study.82 The spectrum of pure urea shown here was acquired at 323 K in order to decrease the T1 by a factor of 4 (67.1 min vs. 15.7 min at 295 K and 323 K, respectively). Interestingly, the recycle delay could be shortened from 3600 s for urea to only 60 s for the SbF3.urea cocrystal at 323 K.

Figure 4. 13C CP/MAS NMR spectra of cocrystals (black) and pure starting materials (red and green) at 12.5 kHz, B0 = 9.4 T and room temperature (except where indicated). (a) Cocrystal 1.a at 323 K, (b) cocrystal 1.b, (c) cocrystal 1.c, (d) cocrystal 1.ab with cocrystal 1.b and pure urea (red and green, respectively) at 323 K, (e) cocrystal 2.d and (f) cocrystal 2.e.



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Table 2. 13C chemical shifts of the carbons covalently bonded to the electron donor element Y (O, N, or S). PnB acceptor compound

iso(13C) / ppm

FWHMa / Hz iso(13C) / ppm

FWHMa / Hz

SbF3·urea (1.a)

162.7 ± 0.2

147 ± 3

163.3 ± 0.2

169 ± 4

150.7 ± 0.2

177 ± 4

148.6 ± 0.7

408 ± 9

145.1 ± 0.1

140 ± 5

143.7 ± 0.6

418 ± 20

162.7 ± 0.2 (urea)

147 ± 3

163.8 ± 0.3 (C=O) 245 ± 4

SbF3·phen (1.b)

SbF3·phen·urea (1.ab)

a.

cocrystal

148.6 ± 0.7 (SbF3·phen) 408 ± 9

149.5 ± 0.2 (C-N)

279 ± 6

143.7 ± 0.6 (SbF3·phen) 418 ± 20

142.9 ± 0.2 (C-N)

211 ± 5

SbF3·nicotinamide (1.c)

151.9 ± 0.2 149.1 ± 0.3

177 ± 5 180 ± 3

149.6 ± 0.3 145.2 ± 0.3

203 ± 6 202 ± 6

SbCl3·trithiane (2.e)

37.4 ± 0.1 36.5 ± 0.1

34 ± 2 36 ± 2

38.1 ± 0.2

146 ± 3

SbCl3·trioxane (2.f)

n/ab

n/a

95.6 ± 0.1

101 ± 4

FWHM values correspond to full width at half maximum.

We were unable to obtain a solid-state 13C CP/MAS NMR spectrum for pure trioxane; however, literature data83 give iso(13C) = 93.0 ppm in DMSO-d6, a difference of 2.6 ppm with the shift of solid 2.f.

b.

(iii) 121/123Sb NQR Spectroscopy NQR spectroscopy is performed, contrary to NMR spectroscopy, without any external magnetic field.84,85 Signals arise from the QI itself (the interaction between Q and the electric field gradient at the nucleus), which makes this technique strictly specific to quadrupolar nuclei (I > 1/2).86,87 The nuclear quadrupolar coupling constant can be calculated thanks to the following relations for each antimony isotope88:


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For I = 5/2

𝑒𝑄𝑉33 = 20

For I = 7/2

𝑒𝑄𝑉33 = 28

∑𝜈ij ∑∆𝐸ij ∑𝜈ij ∑∆𝐸ij

Equation 3

Equation 4

Q is the nuclear electric quadrupole moment, V33 is the largest component of the EFG tensor at the nucleus, and e is the fundamental electronic charge. In the event that only one resonance frequency per isotope is detected, we also have, for antimony specifically, the following equation (see SI for a sample calculation): 7/2

7/2

∆𝐸ij

5/2 i'j'

∆𝐸

𝜈ij

= 1.09855

5/2 i'j'

Equation 5

𝜈

where ij corresponds to the NQR frequency of the i–j transition. These transitions are also often labelled with a single subscript as depicted in Figure 5 and as used for the remainder of this manuscript. ΔEij = EiEj (at a specific ηQ) are the corresponding energy differences and are the secular equation roots. Here, i and j are labels for each of the spin states, i.e., ½, 3/2, 5/2, 7/2. More interestingly, numerical solutions (i.e., ΔEij as well as each Ei) have been tabulated for all ηQ values (0 ≤ ηQ ≤ 1).88,89,90,91 Reciprocally, all the NQR frequencies can be obtained from CQ and ηQ. Since antimony possesses two quadrupolar isotopes, a spin-5/2 and a spin-7/2, five different pure NQR frequencies are available (excluding overtones). The reader is referred to the references cited above for further discussion of equations 3, 4, and 5.



Page 18 of 46

Figure 5. Schematic representation of the NQR energy levels for a spin-5/2 nuclide (left) and a spin-7/2 nuclide (right). Overtones are represented in grey.

Shown in Figure 5 are the energy levels for spin-5/2 and spin-7/2 nuclei, which correspond to antimony-121 and -123 respectively (as a side note Q(123Sb)/Q(121Sb) used here is 1.2744 in order to reflect the new quadrupole moments used,32 as opposed to the previous value of 1.2747).51 In practice, thanks to the tables of numerical solutions for the secular equations and the known Q values, it is possible to calculate all five frequencies (and thus obtain CQ and ηQ) with only one of the specific combinations (five possible) of two different frequencies with equations 3, 4, and 5 (see SI for an example). The experimental NQR data recorded at room temperature for the compounds listed in Table 1 are gathered in Table 3 and shown in Figure 6. The probe and spectrometer used in our experiments allowed us to record at least three different frequencies per sample, this enabling the calculation of the other remaining frequencies (see SI).


Page 19 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Pure

121/123Sb

NQR experimental frequencies of SbF3 (black), SbF3·urea (red),

SbF3·phenanthroline (blue), SbF3·nicotinamide (orange) and the new compound SbF3·phenanthroline·urea (green). The bottom spectra correspond to ν1 of 121Sb and the two top ones to ν1 and ν2 of 123Sb.


143.53* 141.20* 133.85* 138.94*

98.4

95.9

87.2

88.2

89.01 ± 0.04

56.5

84.8

105.6

132.6

132.4

143.9

148.2

Calc.

387.52 ± 0.24

326.15 ± 12.52

377.37 ± 4.77

497.11 ± 0.96

508.85 ± 1.52

491.19 ± 7.51

517.43 ± 2.76

525.82 ± 2.01

CQ(121Sb)

493.86

415.64

480.92

633.83

648.47

625.98

659.41

670.10

CQ(123Sb)

Exp

0 ± 0.01

0.02 ± 0.02

0.16 ± 0.02

0.02 ± 0.01

310.68

387.11

486.58

486.23

0.14 ± 0.02 0.04 ± 0.01

527.30

541.38

CQ(121Sb)

0.09 ± 0.01

0.05 ± 0.01

ηQ

* Data calculated from a combination of two found frequencies. See SI for detailed calculations

105.831*

102.78*

70

135.72*

Exp.

ν3

Calc.

ν2

395.93

493.34

620.10

619.65

671.99

689.94

CQ(123Sb)

Calc

0.00

0.09

0.08

0.17

0.03

0.10

ηQ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 123Sb


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Table 3. 121/123Sb NQR frequencies acquired at room temperature, comparison between experimental (Exp.) and calculated (Calc.) (ZORA/QZ4P, relativistic spin orbit, SAOP) values.

20

79.08 ± 0.01 78.27 ± 0.02 75.30 ± 0.03 76.42 ± 0.02 74.66 ± 0.04 58.16 ± 0.01 49.00 ± 0.03 58.11 ± 0.02

1.a 1.b 1.c 1.ab 2 2.d 2.e

Exp.

ν1

1

ν/ MHz

46.6

58.4

73.4

75.0

79.1

82.2

Calc.

116.23*

97.82 ± 0.03

112.64*

149.24*

152.53*

146.61*

154.90*

157.62*

Exp.

ν2

93.2

115.8

145.6

145

158.1

162.6

Calc.

35.27 ± 0.02

29.76 ± 0.03

37.41 ± 0.01

45.33 ± 0.04

46.58 ± 0.02

47.94 ± 0.03

48.44 ± 0.02

48.34 ± 0.01

Exp.

ν1

28.2

36.2

45.4

48.6

48.1

51.0

Calc.

70.55 ± 0.03

59.43 ± 0.04

67.78 ± 0.01

90.46 ± 0.04

92.56 ± 0.03

88.45 ± 0.04

93.79 ± 0.01

95.58 ± 0.01

Exp.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 121Sb

Page 21 of 46 The Journal of Physical Chemistry

21

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Page 22 of 46

Shifts of all NQR frequencies are observed between the pure SbX3 compounds and the pnictogen-bonded cocrystals (see Table 3). In most cases, cocrystals of SbF3 resonate at lower frequencies than does pure SbF3. The shifts are on the order of megahertz. An exception is noted for the lowest NQR frequency of cocrystal 1.a, 1(123Sb), with a slightly higher experimental value (0.2% higher than for 1) as illustrated in Figure 6 (this behavior is reproduced by DFT calculations, vide infra). Typical experimental time per frequency ranged from a few seconds (for pure antimony halides) to about an hour for the compounds with lower antimony content. Generally, the combined frequency shifts translate to a decrease in the value of CQ(121/123Sb) upon cocrystallization, the only exception being for a cocrystal of unknown structure, SbCl3·trioxane (2.e). Interest in 121/123Sb NQR spectroscopy of antimony halides dates back more than 65 years, when Dehmelt and Krüger published a contribution gathering

121/123Sb

and

35/37Cl

NQR

frequencies of SbCl3.92 For antimony trifluoride, a compelling study by Rykovanov et al. focused on the temperature dependence of

121Sb

NQR frequencies.93 They showed that 1(121Sb) varies

from 80.665 MHz at 77 K to 79.170 MHz at 290 K while 2(121Sb) decreases from 160.849 to 158.205 MHz for the same temperature changes and mathematically described the phenomenon with Bayer’s theory.94 This well-known temperature-dependence is taken into consideration when comparing experimental (recorded here at room temperature) to calculated values. A few years later, Zemnukhova and Davidovich published two separate articles on 121/123Sb NQR of antimony trifluoride complexes.76,95 For SbCl3, several studies on the pure compound or on complexes have also been published a number of years ago.96,97,98,99,100 All of these studies show a shifting of the 121/123Sb

NQR frequencies by a few MHz upon cocrystallization; however, these have never been

rationalized in context of pnictogen bonding and no detailed study on the influence of the number 22 ACS Paragon Plus Environment

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of electron donors and their geometries relative to antimony moiety has been conducted to date. Furthermore, in these early studies, diffraction-based structures on which to base correlations of spectral data with structural features were often unavailable. Some frequencies for a few adducts displaying PnB have already been reported. Twenty years ago Zemnukhova and Davidovich reported NQR frequencies for SbF3 and its complexes with glycine as well as with nicotinamide.76 For the four different glycine-based adducts, only two structures were reported, with only one presenting a PnB. They have recorded the five frequencies at 77 K, and important shifts were observed (4.9 MHz for 1(121Sb) and 15.1 MHz for 2(121Sb)). Concerning our experimental data, the cocrystal showing the smallest shift is that featuring an oxygen electron-donor, cocrystal 1.a. In that case, one antimony atom is surrounded by two oxygen atoms (from two different urea molecules). Cocrystal 1.c features the same type of environment around the antimony (i.e., one antimony and two electron donors), but in this case nitrogen atoms are the donors and the frequency shifts are more significant than those of cocrystal 1.a. However, comparing both nitrogen electron-donor cocrystals (1.b and 1.c) in terms of pure NQR resonances is thornier. A better understanding can be obtained by translating these data into CQ and ηQ. Both have a lower CQ than pure SbF3 (525.82 vs. 491.19 and 508.85 MHz for cocrystal 1.b and 1.c, respectively). As mentioned earlier, the phenanthroline cocrystal features a sterically “forced” PnB with an angle θ = 137° that does not allow a good alignment with the σ-hole and thus disturbs the axial symmetry of the EFG at antimony. In contrast, the two molecules of nicotinamide are less constrained and can form two equivalent PnBs with an angle θ = 161°. For cocrystal 1.c, the NQR data presented here are slightly lower than the previously published ones (by 0.55 MHz for 1(121Sb) and 0.34, 1.03 for 1(123Sb) and 2(123Sb) respectively).76 These discrepancies are attributed to temperature effects as the lower the temperature is, the higher the 23 ACS Paragon Plus Environment


Page 24 of 46

NQR frequencies for pure SbF3 are (shifts of 1.50 and 2.64 MHz for 1(121SbF3) and 2(121SbF3) respectively). In our case, the data were acquired at room temperature (296.6 ± 0.5 K). Cocrystal 1.ab is expected to feature both nitrogen and oxygen electron donors interacting with antimony. Compared to cocrystal 1.a we expect only one urea molecule to interact with the antimony atom through a PnB as the two other binding sites are expected to be interacting with the bidentate phenanthroline molecule. For 1.ab, the asymmetry parameter drops from 0.14 (for cocrystal 1.b) to 0.02 due to axial symmetry created by the presence of the urea. The value of CQ(121Sb) for 1.ab is 497.11 MHz, which is lower than that for pure SbF3 and in between the values for 1.a and 1.b (517.30 and 491.19 MHz, respectively).

Interestingly, if we consider

SbF3·phenanthroline as the starting material, the addition of an oxygen electron donor does not decrease the CQ value as seen before but rather counterbalances the effect of the nitrogen Lewis base. Further investigation of such behavior will be discussed in the light of computational studies (vide infra). Concerning cocrystals based on SbCl3, we noted two different cases for CQ: (i) cocrystal 2.d which follows the same trend observed for SbF3 cocrystal with a smaller CQ (by about 14%) compared to compound 2 and (ii) the cocrystal of unknown structure, cocrystal 2.e, that exhibits the only higher CQ value (by 3%). Nevertheless, in both cases, the asymmetry parameter is close to 0 as expected for a nearly axially symmetric environment around the nucleus of interest. Strikingly, CQ(121/123Sb) values in SbF3 and its cocrystals are approximately 1.5 times larger than in the analogous SbCl3 systems; this aspect is investigated via NLMO calculations below.


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Previous Mössbauer spectroscopic data for cocrystal 1.b gave a CQ of about 504 MHz at 4.2 K while 121Sb of SbF3 exhibits a CQ of about 573 MHz at the same temperature.45 For both, the higher values can be attributed to the lower temperature along with the low precision of Mössbauer spectroscopy for CQ values.51 In order to gain a better understanding of why and how the NQR resonances change upon cocrystallization with various electron-donor types, we describe below a computational study of model compounds. (iv) Computational chemistry Overall, a fairly good agreement between the experimental data and DFT calculated quadrupolar coupling constants is observed. The largest discrepancy for CQ is 22 MHz (4.5%) for cocrystal 1.c (Table 3). The root-mean-squared deviation (RMSD) obtained is 13.62 MHz (see Figure 7).

Figure 7. Plot of calculated vs experimental CQ(121Sb) values. Computational data were obtained using ZORA-QZ4P with the SAOP model and relativistic spin-orbit effects. The linear equation of best fit is y = 1.0448x-21.749 with R2 = 0.972 and RMSD = 13.62 MHz.



Page 26 of 46

The experimental data shown in Table 3 clearly suggest a dependence of the NQR frequencies on the nature of the pnictogen bond to antimony. To better understand the relationship between these frequencies and the PnB distance, angle, and identity of the electron donor (e.g., oxygen or nitrogen), we have carried out a series of DFT computations on model systems. Models of SbF3·xNH3 and SbF3·xH2O with x = 1, 2, or 3 were constructed as described in the Experimental section and the 121/123Sb NQR frequencies were calculated. The data are in Table 4. The resulting trends are depicted in Figure 8. Several features of the data are worth discussing. Firstly, it is seen that the 1 and 2 frequencies (for

121Sb)

decrease upon the formation of a pnictogen bond.

Secondly, these frequencies systematically decrease further as the number of pnictogen bonds to antimony increases from 1 to 2 to 3. Finally, it is clear that the impact of a nitrogen-based electron donor (1 decreases by 1.9 to 9.9 MHz) is more pronounced than that of an oxygen-based donor (1 decreases by 0.6 to 2.4 MHz) for a given distance, dPnY, and angle, θ (here dPnY = 2.58 Å and θ = 165°); this behavior is also noted in the experimental data (vide supra). As seen in Table 3, the experimental value of 1(121Sb) for SbF3 decreases by 0.81 MHz upon the introduction of an oxygen-based electron donor (cocrystal 1.a) and by up to 3.78 MHz for a nitrogen-based electron donor (cocrystal 1.b). The experimental values of 2(121Sb) follow the same trend, with reductions of 0.8 to 5.0 MHz and 9.8 to 21.3 MHz for oxygen and nitrogen cocrystals, respectively, relative to pure SbF3. In terms of CQ, a parameter more familiar to NMR spectroscopists, the DFT calculations also mirror the trends seen experimentally and show a decrease in magnitude upon the formation of pnictogen-bonded cocrystals: about 4% upon the addition of 3OH2 and 13% for the addition of 3NH3.


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Figure 8. Representation of ν1(121Sb) (bottom) and ν2(121Sb) (top) NQR frequencies obtained from DFT calculations for SbF3·xNH3 (under the blue bar) and SbF3·xH2O (under the red bar) with x = 1 (red signals), 2 (blue signals) or 3 (green signals). The black signals represent the calculated frequencies for pure SbF3.

Table 4. Calculated 121/123Sb NQR frequencies for SbF3·xH2O and SbF3·xNH3 with x = 1, 2, or 3. SbF3·xH2O ν/ MHz

121Sb

123Sb

SbF3·xNH3 CQ (121Sb)

x

ν1

ν2

ν1

ν2

ν3

0

82.3

162.8

51.0

98.5

148.3

543.74

1

81.7

162.0

50.5

98.0

147.5

2

81.4

161.8

49.9

98.0

3

79.9

157.8

49.7

95.4

121Sb

123Sb

ηQ

CQ (121Sb)

ηQ

ν1

ν2

ν1

ν2

ν3

0.094

82.2

162.7

51.0

98.4

148.2

543.40

0.094

540.82

0.086

80.3

152.9

53.1

91.6

139.7

513.95

0.200

147.3

539.95

0.068

76.5

143.4

51.6

85.7

131.2

483.28

0.229

143.8

527.36

0.102

72.3

141.4

45.8

85.3

129.0

473.29

0.135

Additional DFT calculations on models featuring both nitrogen and oxygen electron donors, i.e., SbF3·NH3·2H2O and SbF3·2NH3·H2O, were carried out to help interpret the experimental results of obtained for cocrystal 1.ab, the detailed structure of which is unknown. Replacing one H2O by one NH3 causes CQ to decline by about 30 MHz and ηQ to increase (from 0.10 to 0.16) compared to SbF3·3H2O. On the other hand, exchanging an ammonia for a water molecule causes ηQ to fall from 0.13 to 0.02 and CQ to drop by about 10 MHz. These observations 27 ACS Paragon Plus Environment


Page 28 of 46

are consistent with the above conclusion that nitrogen donors cause a larger decrease in the value of CQ. For SbF3·phenanthroline·urea (1.ab), the experimental data obtained are CQ(121Sb) = 497.11 MHz and ηQ = 0.02.

On the basis of the bidentate nature of phenanthroline and

monodentate nature of urea, we hypothesize that SbF3·2NH3·H2O, with two nitrogen donors and one oxygen donor, represents a reasonable starting model for 1.ab. The experimental asymmetry parameter for the model is in good agreement with experiment while the CQ value is lower by 33.4 MHz (about 6.7%). This discrepancy led us to further explore the dependence of the computed antimony quadrupolar coupling tensor on the pnictogen bond distance (dPnY) and angle (). Shown in Figure 9 are the DFT-computed dependencies of the antimony NQR frequencies on pnictogen bond length and angle for SbF3·NH3. Each quadrupolar frequency depends more strongly on the distance than on the angle (over the relevant experimental ranges). The frequencies predictably converge to those of pure SbF3 for large enough dPnY values. The value of CQ(121Sb) increases by approximately 40 MHz as the pnictogen bond angle is reduced from 180 to 140 degrees (at a fixed distance of 2.58 Å), and decreases by approximately 75 MHz as the distance is reduced from 3.5 Å to 2.1 Å (at a fixed angle of 165 degrees). Similar trends are observed when using H2O as the electron donor rather than NH3 (see SI).


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Figure 9. Plots of the variation of each of the five calculated 121/123Sb NQR frequencies with dPnY or θ for SbF3·NH3. The curves represent the modification of dPnY when θ is fixed at 165° (black) or 180° (grey) (lower abscissa axis), or the modification of θ for a fixed dPnY of 2.58 Å (green; upper abscissa axis).



Page 30 of 46

Given the number of degrees of freedom for cocrystal 1.ab, rather than searching parameter space for an optimal structure, we have instead constructed an initial model based on the known structure of SbF3·phenanthroline·thiourea. A series of constrained optimizations and crossvalidations against the experimental NQR data led us to the conclusion that the best structure was obtained by fixing the SbF3·phen entity and only optimizing the position of the urea molecule. This model results in computed CQ and ηQ values of 504.61 MHz and 0.07 for dPnY = 2.9 Å and θ = 156° (see SI for the Cartesian coordinates of the optimized structure). These values are in accord with the scaled experimental data of 497.63 MHz and 0.02. The structure of SbCl3·trioxane (2.e) is not known. Using the structure of SbCl3·trithiane (2.d) as a starting point (i.e., θ = 171° and dPnY = 3.26 Å),72 a model for 2.e can be built by first replacing the three trithiane molecules with three trioxane molecules and decreasing the Sb…O distances (from 3.26 Å for 2.d) to reflect those of other known Sb…O pnictogen bonds, i.e., 2.6 to 3.0 Å. We have assumed that the structure remains otherwise similar to the trithiane analogue and modelled the structure as shown in Figure 10.


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Figure 10. SbCl3·trioxane structure based on SbCl3·trithiane. Plots show the variation of calculated CQ(121Sb) as a function of dPnY with fixed θ = 155° and 162° (dark and light blue, respectively) or θ with fixed dPnY = 2.6 and 3.2 Å (dark and light green, respectively). The red line represents the “target” value of 383.13 MHz while the red area reflects the RMSD value, ± 13.62 MHz.

While a complete structural model cannot be obtained based solely on cross-validation against the antimony quadrupolar coupling tensor, a search over the parameter space spanned by experimentally relevant dPnY and  values should provide a set of structures in agreement with the experimental NQR data (CQ(121Sb) = 387.52 MHz and ηQ = 0). Using the calibration of the DFT calculations vs. the experimental data shown in Figure 7, we can anticipate that the correct structural model for 2.e will yield a DFT computed value of CQ(121Sb) = 383.13 ± 13.7 MHz. Interestingly, this is the only compound in the present study where CQ(121Sb) increases experimentally upon the formation of a pnictogen bond to antimony. As shown in Figure 10, agreement with experiment is obtained for a Sb…O distance of approximately 2.7 to 2.8 Å and pnictogen bond angle of approximately 155 to 162 degrees. A more linear pnictogen bond and/or



Page 32 of 46

a shorter distance only serve to decrease the value of CQ, whereas as mentioned for 2.e the experimental value is higher than that for pure SbCl3 and the highest for all compounds studied herein. A survey of relevant structures in the Cambridge Structural Database (see SI) featuring O···SbCl3 pnictogen bonds provides the following distances (Å) and angles (degree): 2.74/168 (COHLAM),101 2.93/161 (TPHALD10) or 2.67/167 & 2.81/171 (DACBZA10).102 These are all consistent with the results of our NQR crystallographic refinement described above and depicted in Figure 10. The best structural model of 2.e, obtained by NQR crystallography and the survey of existing structures, corresponds to dPnY = 2.7 ± 0.1 Å and θ = 158 ± 1° giving rise to CQ(121Sb)= 385.47 ± 7.0 MHz and ηQ= 0.007 ± 0.002. Shown in Table 5 are the results of natural localized molecular orbital (NLMO) calculations of the antimony EFG tensors in SbF3, SbCl3, and two pnictogen-bonded cocrystals. In each case, the lone pair (LP) of the antimony makes the largest magnitude contribution to V33, and thus to the value of CQ(121/123Sb), contributing approximately 70 to 75 % of the total value. The difference in CQ(121Sb) on the order of 150 MHz when comparing SbF3 to SbCl3 is in part due to the effect of the chlorine lone pairs. Upon cocrystallization via PnB, the changes in the quadrupolar coupling constant arise from: (i) a change in the Sb–X bonding contribution by approximately 12% compared to pure antimony halides and (ii) the electron donor LP contribution of 12 to 16 % of opposite sign. This delicate balance between positive and negative changes is consistent with the experimental observation of both increases and decreases in CQ upon the formation of a pnictogen bond (vide supra).


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Table 5. NLMO analysis for antimony EFG tensors for selected compounds (in a.u.).a 1 Sb EFG tensors

2

1.b

V11

V22

V11

V22

V22

V33

V11

V22

Sb–X bonding

0.216

0.381

-0.597

0.523

0.455

-0.979

0.202

0.299

-0.503

0.354

0.353

-0.708

Sb core

0.301

0.528

-0.830

0.276

0.579

-0.857

0.144

0.309

-0.455

0.209

0.210

-0.418

Sb LP

1.459

1.457

-2.915

1.264

1.259

-2.524

1.108

1.104

-2.212

0.873

0.873

-1.745

X LP

-0.276

-0.097

NA

b

0.047

-0.016

NA

b

-0.135

-0.012

0.346

0.002

0.146

-0.136

Y LP

NA

NA

NA

-0.354

-0.094

0.447

NA

NA

NA

-0.191

-0.191

0.383

1.700

2.269

-4.342

1.756

2.183

-3.913

1.319

1.700

-2.824

1.247

1.391

-2.624

1.884 2.265 -4.148 1.610 2.126 SAOP/ZORA QZ4P with scalar relativistic effects b. No X LP contribution above the threshold for printed contributions.

-3.736

1.349

1.617

-2.965

1.176

1.173

-2.349

∑ Analysis Total calculated

V33

V33

V11

2.d V33

a.

When considering the range of element-based electrophilic interactions noted in the introduction (e.g., hydrogen bonds, halogen bonds, pnictogen bonds, etc.), it is important to understand how these interactions compare. How are they similar and how do they differ? The many parallels between halogen bonding and hydrogen bonding have been noted,6 one of the main differences being the stronger directionality of the former. Comparisons involving pnictogen bonds are fewer. The data reported presently allow for a comparison in changes in the electric field gradient at the nucleus of the electrophilic element as a result of secondary bond formation. Deuterium nuclear quadrupolar coupling constants are known to decrease in the solid state and in solution upon the formation of a hydrogen bond; stronger and shorter hydrogen bonds typically have lower CQ(2H) values.103,104,105 Conversely, the quadrupolar coupling constants of covalentlybonded halogens (Cl, Br) increase in solid state upon formation of a halogen bond.106,107 The present work shows that usually, but not always, the antimony quadrupolar coupling constants decrease upon formation of a pnictogen bond (to the tripodal SbF3 and SbCl3 donors). The decreases in CQ(121/123Sb) upon the formation of pnictogen bonds to antimony may be understood 33 ACS Paragon Plus Environment


Page 34 of 46

as resulting from an increase in the symmetry and decrease in the anisotropy of the local environment at the nucleus; i.e., the nominally trigonal pyramidal SbX3 molecules interact with up to three electron donors opposite the covalently-bonded X groups. Conclusions The design and engineering of cocrystals, catalysts, and related materials for particular purposes necessitates an understanding of how to effect specific structural changes. In the case of pnictogen bonds, the ability to alter the geometry and environment about the antimony centre relies on building an understanding of the strength and directionality of interactions with the -hole. There are few experimental methods available to access such information for antimony. In this work, we have demonstrated that antimony NQR spectroscopy is a valuable probe of pnictogen bonds in solids. The 121/123Sb quadrupolar coupling constants generally decrease on the formation of a pnictogen bond, with shorter bonds typically leading to smaller quadrupolar couplings. More linear pnictogen bonds also tend to result in smaller quadrupolar couplings. The quadrupolar coupling constants further decrease systematically as the number of pnictogen bonds to antimony in SbF3 and SbCl3 based systems increases; this effect is more pronounced for nitrogen donors compared to oxygen donors. This work has also demonstrated the viability of mechanochemical approaches to the preparation of air- and water-sensitive pnictogen-bonded cocrystals, including the synthesis of two novel cocrystals.

A rudimentary NQR crystallography approach has been demonstrated to

constrain the local structures of these two new cocrystals. This approach, which is applicable to a wide range of strongly quadrupolar isotopes across the periodic table, offers new opportunities to structurally characterize systems for which diffraction data are unavailable or insufficient.

13C


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chemical shifts have been established as indirect probes of pnictogen bond formation. A natural localized orbital analysis has elucidated the electronic origins of the experimentally observed changes in quadrupolar coupling constant upon the formation of pnictogen bonds. Strikingly, the decreases noted here parallel those known for hydrogen bonds, but contrast with the increases reported for halogen bonds. Overall, this work offers a novel approach to understand emerging classes of electrophilic interactions and to contextualize them in the broader landscape of established chemical bonding paradigms. Acknowledgements We are grateful to the Natural Sciences and Engineering Research Council of Canada for funding. Supporting Information Available Additional details on synthesis and sample preparation; powder X-ray diffractograms; melting points; SSNMR and NQR detailed acquisition parameters; NQR data processing details; further details on DFT calculations. This information is available online at http://dx.doi.org/xxx Conflicts of Interest The authors have no conflicts of interest to declare.



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123Sb Nuclear Quadrupole Resonance Spectroscopy

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