Direct investigation of chalcogen bonds by multinuclear solid-state magnetic resonance and vibrational spectroscopy

Article abstract of DOI:10.1039/c9cp06267j

We report a multifaceted experimental and computational study of three self-complementary chalcogen-bond donors as well as a series of seven chalcogen bonded cocrystals. Bis(selenocyanatomethyl)benzene derivatives were cocrystallized with various halide s

Full text of DOI:10.1039/c9cp06267j

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H.-P. Loock et al.
Determination of the thermal, oxidative and photochemical
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DOI: 10.1039/C9CP06267J
ARTICLE
Direct Investigation of Chalcogen Bonds by Multinuclear Solid-
State Magnetic Resonance and Vibrational Spectroscopy
Vijith Kumar, Yijue Xu, César Leroy, and David L. Bryce*
Received 00th January 20xx,
Accepted 00th January 20xx
We report a multifaceted experimental and computational study of three self-complementary chalcogen-bond donors as
well as a series of seven chalcogen bonded cocrystals. Bis(selenocyanatomethyl)benzene derivatives were cocrystallized
with various halide salts (Bu4NCl, Bu4NBr, Bu4NI) and nitrogen-containing Lewis bases (4,4’-bipyridine and 1,2-di(4-
pyridyl)ethylene). Three new single-crystal X-ray structures are reported. 77Se solid-state nuclear magnetic resonance
spectroscopic study of a series of cocrystals establishes correlations between the NMR parameters of selenium and the local
ChB geometry. For example, the 77Se isotropic chemical shift generally decreases on cocrystal formation. Diagnostic 13C
chemical shifts are also described. In addition, all the chalcogen bonded cocrystals and pure tectons are investigated by
Raman and IR spectroscopy techniques. Characteristic red shifts of the NC−Se stretching band upon cocrystal formation on
the order of 10 to 20 cm-1 are observed, which provides a distinct signature of the chalcogen bond involving selenocyanates.
The 125Te chemical shift tensor and X-ray structure of chalcogen-bonded tellurocyanatomethylbenzene are also reported.
Insights into the connection between the electronic structure of the chalcogen bond and the experimentally measured 77Se
chemical shift tensors are afforded through a natural localized molecular orbital density functional theory analysis. For the
systems studied here, the lack of a very strong a correlation between experimental and DFT-computed 77Se chemical shift
tensors leads to the conclusion that many structural features likely influence their ultimate values; however, computations
on model systems reveal that the ChB alone produces consistent and predictable effects (e.g., the chalcogen chemical shift
decreases as the chalcogen bond is shortened).
DOI: 10.1039/x0xx00000x
Introduction
Chalcogen bonding (ChB) is a non-covalent interaction occurring Ch···LB axes, has recently been examined in some detail.14,15
between the electrophilic region associated with a group 16 Some open questions include how the NMR response is affected
chalcogen atom (e.g., S, Se, Te) and a nucleophilic region such by chalcogen bonding, how general is this response, whether
as a Lewis base (LB) in the same or another molecular entity.1 such a response can be used in a predictive capacity, and what
Similar to halogen bonding (XB),2 chalcogen bonding is a highly parallels can be drawn with the NMR response to related non-
directional secondary bonding interaction with an R―Ch···LB
angle of approximately 180°.3 Even though the ChB has been
less explored than hydrogen bonds (HB)4 or XB, recent studies
demonstrate the ChB has a tremendous potential, with several
applications in various fields such as supramolecular chemistry,5
crystal engineering,6 anion sensing and transport,7
organocatalysis,8 pharmaceutics,9 and biological processes.10
Despite the promising applications in a myriad of different
fields, the nature of the ChB continues to be examined in the
literature
perspectives.11,12,13 For example, the directional deviations
between the two regions of depleted electron density ( -holes),
from
experimental
and
computational

the maxima of the electrostatic surface potential (ESP), and the
Figure 1. Left: Molecular structures of the benzylic selenocyanates 1, 2, and
benzylic tellurocyanate 3. Top Right: Pair of σ-holes on Se atoms visualized
as the electropositive regions on ESP surfaces for the ChB donor 2 with an
isodensity of 0.02 a.u. (B3LYP/Def2TZVP). Red indicates negative charge
density, and blue is positive charge density. Bottom Right: schematic
representation of the observed chalcogen bond synthon in the series of
cocrystals (Ch= Se, Te; X= N, I‒, Br‒ and Cl‒; R = aromatic systems).
Department of Chemistry and Biomolecular Sciences, University of Ottawa, 10 Marie
Curie Private, Ottawa, Ontario K1N 6N5, Canada. E-mail: dbryce@uottawa.ca; Fax:
+1-613-562-5170; Tel: +1-613-562- 5800 ext. 2018.
† Electronic supplementary information (ESI) available: X-ray crystallographic (CCDC:
1956409-1956411), methods, additional NMR data, additional computational
results. See DOI: 10.1039/x0xx00000x
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covalent interactions including hydrogen bonds and halogen solution by using a combination of X-ray crystallography,
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DOI: 10.1039/C9CP06267J
bonds. Due to their direct relationship with the molecular multinuclear solution magnetic resonance spectroscopy and
orbitals centred on the nucleus of interest, chemical shift tensor quantum chemical calculations.15 Herein, seven chalcogen-
measurements, in concert with computational chemistry, can bonded anionic and neutral cocrystals with benzylic
provide direct insight into the electronic structure of the selenocyanates, and the self-complementary benzylic seleno
chalcogen bond (vide infra).
and tellurocyanates (1,2, and 3) are studied. These simple ChB
donors were chosen to be devoid of functional groups that
could interfere with the occurrence of the chalcogen bonds or
modify their features (Fig.1). As far as the ChB acceptors are
concerned, the halides and neutral pyridine based ditopic
acceptors were chosen to examine the changes in the spectral
parameters with various chalcogen-bond synthons. All
cocrystals are characterized by single crystal and powder X-ray
diffraction (SCXRD and PXRD) and multinuclear solid-state
magnetic resonance spectroscopy. Natural localized molecular
orbital (NLMO) DFT analysis is used to reveal the origins of the
observed changes in the 77Se chemical shift tensors. We also
establish Raman and FT-IR spectroscopic techniques to rapidly
probe for the presence of chalcogen bonds in these powdered
materials. This systematic study combines different solid-state
characterization methods to provide both experimental and
theoretical insights into the correlation between the selenium
chemical shift tensors the ChB electronic structure.
Theoretical investigations such as quantum chemical
calculations based on perturbation theory and density
functional theory (DFT) have been carried out to understand the
nature of ChB interactions in the gas phase.16 UV-vis
absorbance and emission spectroscopy as well as NMR
spectroscopy have been recently utilised to examine the ChB in
solution.17 X-ray diffraction has been widely used to investigate
and quantify the ChB geometry in the solid state. However,
complex chalcogenated systems such as metal organic
frameworks, polymers, peptides, and drug molecules etc., often
do not crystallize easily and may exist as multicrystalline or
amorphous solids; in these cases, the use of complementary
techniques like solid-state nuclear magnetic resonance
(SSNMR) and vibrational spectroscopy are crucial and rapid
tools for structural characterization.
In recent years, SSNMR spectroscopy has been well
acknowledged as a powerful tool to characterize various non-
covalent bonds including σ-hole interactions.18 Our group has
reported extensively on the application of SSNMR and nuclear
quadrupole resonance (NQR) spectroscopies to analyse and
provide new structural insights into σ-hole interactions such as
halogen,19 tetrel20 and pnictogen21 bonds and to contextualize
them in the broader setting of established chemical bonding
Results and Discussion
Structural Studies of the ChB Donors and Cocrystals
Table 1. Summary of the C‒Ch···A chalcogen bonding
geometries observed via SCXRD. (C=carbon, Ch= Se or Te, A=N,
I¯, Br¯, or Cl¯).
paradigms. These studies provide
a distinctive way to
characterize σ-hole interactions, and establish relationships
between local bonding geometry and various NMR and NQR
parameters. Selenium-77 and tellurium-125 possess favourable
NMR properties (spin-½ nuclei) including moderate natural
abundances (7.63% for 77Se and 7.07% for 125Te) and
gyromagnetic ratios (= 5.12 x 107 rad T-1 s-1 for 77Se and
= -8.51 x 107 rad T-1 s-1 for 125Te, respectively).22 Selenium and
tellurium SSNMR provides a particularly sensitive probe of
molecular structure in part due to broad chemical shift ranges
spanning over 3000 and 6000 ppm, respectively.23 While 33S is
present in the most chemically and biologically active
organosulfur compounds,24 its low natural abundance of 0.76%,
rather small gyromagnetic ratio (= 2.05 x 107 rad T-1 s-1), and
moderate quadrupole moment of -69.4 mb conspire to make
33S SSNMR quite challenging.25
a
Compound C‒Ch···A (Å) C‒Ch···A (˚)
Nc
Ref code
FEDHUU
1
3.010(2)
3.015(2)
2.965(2)
3.017(2)
2.997(2)
3.020(2)
2.814(9)
2.830(2)
2.792(4)
3.506(1)
3.249(1)
3.191(2)
2.897(4)
2.865(2)
172.9(6)
174.1(6)
175.9(6)
175.7(6)
174.4(8)
171.8(6)
172.1(3)
177.2(9)
173.5(2)
174.9(9)
174.4(2)
174.3(3)
176.7(1)
167.8(9)
0.86
0.87
0.85
0.87
0.86
0.87
0.77
0.81
0.80
0.88
0.86
0.88
0.83
0.82
2
POXYEH
3
1∙BP
1∙DPE
2∙(Bu4N)I
2∙(Bu4N)Br
2∙(Bu4N)Cl
2∙BP
This work
FEDJAC
This work
GIHCOS
GIHDAF
GIHGIQ
FEDJEG
This work
2∙DPE
a Nc = dCh…A
/rvdW
Recent literature reveals the seleno and tellurocyanate
derivatives are plausible ChB donor motifs, due to the strong
electron-withdrawing power of the cyano group, which makes
the σ-hole potentials at chalcogen atoms remarkably positive.26
Fourmigué and co-workers reported that benzylic
selenocyanates are strong and directional ChB donors for
crystal engineering purposes, where the chain-like motifs can
be tuned into 1D extended structures upon cocrystallization
with ditopic ChB acceptors.27 Recently, we have demonstrated
the strong aptitude of benzylic selenocyanates for the ChB
driven recognition of onium halides in the solid state and in
The ChB donors 1,3-bis(selenocyanatomethyl)benzene (1)
and 1,4-bis(selenocyanatomethyl)benzene (2) are prepared
from their respective benzyl bromides and potassium
selenocyanates in DMF under an argon atmosphere as
described earlier (ESI, paragraph S.2).27 Benzyl tellurocyanate
(3) was prepared by treating benzyl bromide with in situ
generated potassium tellurocyanate in dry DMSO under argon
atmosphere according to the previously reported protocol (ESI,
paragraph S.2).28 Solid-state structural studies reveal the crystal
packing of 1 (FEDHUU) and 2 (POXYEH) are mainly driven by
intramolecular Se⋯N ChB interactions, resulting in extended
2 | J. Name., 2012, 00, 1-3
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Figure 2. Partial ball and stick representations (Mercury 4.10.2) show the intra- and inter-molecular chalcogen bonds with series of monotopic and
ditopic linkers containing halide and nitrogen acceptors. (a) Pure ChB donor ; (b) cocrystal ⸱(Bu4N)Cl; (c) ⸱(Bu4N)I; (d) ⸱(Bu4N)Br; (e) ⸱ DPE; (f)
⸱DPE. Colour codes: grey, carbon; blue, nitrogen; white, hydrogen; yellow, selenium; dark brown, tellurium; green, chloride; brown, bromide; iodide,
3
2
2
2
2
1
magenta (dashed black lines denotes ChB).
chain-like structures (ESI, Fig. S5). The pure ChB donor 3
crystallizes in the monoclinic crystal system with space group
P21/c. As expected, molecules are associated through short and
linear Te···N chalcogen bonds forming chains running along the
crystallographic a axis (C−Te···N distance 2.814 Å, which
corresponds to normalized contact (Nc) values of 0.77, and a C‒
Te···N angle of 172.1°). The normalized contact is defined as the
distance between the interacting atoms divided by the sum of
their van der Waals radii. The MESP maps of compounds 1, 2,
and 3 are presented in the ESI (Fig. 1 and Fig. S18) and visualized
with an isodensity of 0.02 a.u. (electron per bohr3). MESP
surfaces of the Ch atoms clearly show the presence of two
electropositive regions (
along the prolongation of the electron withdrawing nitrile group
is more electropositive than the region ( 2) opposite the Ch-
CH2 bond. (On the 0.02 au surface: 1: 1: 608.59 kJ/mol, 2:
522.47 kJ/mol; 2: 1: 629.86 kJ/mol, 2: 522.47 kJ/mol; 3:1:
705.73 kJ/mol, 2: 628.81 kJ/mol. On the 0.001 au surface: 1:
1: 161.7 kJ/mol, 2: 80.9 kJ/mol; 2: 1: 162.0 kJ/mol, 2: 82.2
kJ/mol; 3:1: 180.1 kJ/mol, 2: 94.5 kJ/mol). These calculated
1 and 2 in dark blue). The region (1)











values are consistent with related literature reports on
selenocyanate derivatives.29 These positive regions interact
with incoming electron-rich areas of the halide ions and/or
neutral pyridine-based ditopic acceptor moieties.
The cocrystals of 1 and 2 with 1,2-di(4-pyridyl)ethylene
(DPE) were obtained by dissolving an equimolar ratio of both
tectons in acetone and slow solvent evaporation at room
temperature. Cocrystal formation is preliminarily confirmed by
PXRD analysis (ESI, Fig. S1-S4) and changes in the melting point
of the complex (64° C for 1·DPE, 110° C for 2·DPE) in comparison
with pure individual tectons. The X-ray structure reveals the 1:1
adduct of new chalcogen bonded systems 1·DPE and 2·DPE
crystallized in a monoclinic system with the C2/c space group
and a triclinic crystal system with the P-1 space group,
Figure 3. Examples of NMR spectra of chalcogen-bonded systems. (A)
Experimental 77Se CP/MAS NMR spectra (blue) obtained at 9.4 T for
pure ChB donor
and
2 and selected chalcogen-bonded cocrystals 2·DPE
·(Bu4N)Br. The corresponding simulated spectra are shown at the
bottom (red). (B) Experimental 125Te CP/MAS NMR spectrum (blue
respectively.
bis(selenocyanatomethyl)benzene and DPE are located on
inversion centers, while in 1·DPE, meta-
In
cocrystal
2·DPE,
para-
2
lines) obtained at 9.4 T and simulated spectrum (red lines) for pure
bis(selenocyanatomethyl)benzene is located on a two-fold axis,
ChB donor 3. Orange asterisks indicate the isotropic chemical shifts.
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and the DPE is positioned on an inversion centre. As detailed in distribution of Se…N distances in these ChB synthon motifs were
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DOI: 10.1039/C9CP06267J
Figure 2 and the ESI, Fig. S6-S7, ChB donors 1-3 are self- found to exhibit maxima in the range 2.50-3.50 Å. The
complementary systems, therefore the ditopic ChB acceptor distribution of Se…A¯ distances falls in the range of 2.65 to 3.87
pyridine moiety competes efficiently with the nitrogen atom of Å (Se··· Cl¯=2.65-3.62 Å, Se··· Br¯=2.78-3.75 Å, Se··· I¯=3.10-3.87
the nitrile group to result in short and directional intermolecular Å) and Te…A¯ distances fall in the range of 2.70 to 4.05 Å
ChBs favouring the formation of chain-like structures. In (Te···Cl¯=2.70-3.3.80 Å, Te··· Br¯=3.08-3.91 Å, Te··· I¯=3.19-4.05
particular, ChB brings the value of Nc down to 0.80 in the DPE Å). Histograms summarizing these data may be found in the ESI.
cocrystals. Various ChB geometries and structural
Multinuclear Solid-State Magnetic Resonance
characteristics are listed in Table 1. The cocrystals of 2 with
Spectroscopic Studies and NLMO Analysis
tetrabutylammonium iodide (GIHCOS), bromide (GIHDAF), and
chloride (GIHGIQ) were prepared based on our previous work,15
and the cocrystals of 1 and 2 with 4, 4-bipyridine (FEDJAC,
FEDJEG) were reproduced according to a literature report.27a
An analysis of relevant chalcogen-bonded structures
available in the Cambridge Structural Database (CSD) was
carried out to provide some context for the structural features
presented in Table 1. In total, 511 crystal structures were found
in the CSD exhibiting Se···N ChB synthon motifs, and among
these 120 entries correspond to selenocyanate motifs.
Similarly, there are few structures reported corresponding to
Se···A¯ chalcogen bonding (A¯= I¯ (Se··· I¯: 44 hits), Br¯(Se··· Br¯:
52 hits), Cl¯( Se··· Cl¯: 55 hits) and F¯( Se···F¯: 2 hits)). The
The new crystal structures described above and the previous
literature demonstrates the occurrence of strong and linear
ChBs with different electron donor systems. We utilized 13C,
77Se, and 125Te magic-angle spinning (MAS) SSNMR experiments
to investigate the electronic environment of chalcogen atoms in
the cocrystals and compare these data with those for pure
benzylic selenocyanates 1 and 2. 77Se and 125Te SSNMR spectra
of the ChB donors and their respective cocrystals are shown in
Figure 3 and in the ESI (paragraph S.6, Fig. S8-S16).
Table 2. Experimental 77Se and 125Te chemical shift tensors for pure ChB donors and cocrystals
compound
Ω (ppm)
iso (ppm)

11 (ppm)
22 (ppm)
33 (ppm)
1
365.3 (0.2) (site 1)
352.3 (0.2) (site 2)
336.4 (0.2) (site 3)
321.9 (0.2) (site 4)
342.5 (0.1)
600 (3)
518 (6)
569 (4)
686 (7)
702 (3)
-0.08 (0.02)
0.59 (0.02)
0.32 (0.02)
-0.20 (0.03)
-0.17 (0.02)
673 (5)
560 (6)
591 (6)
688 (7)
713 (4)
349 (4)
454 (4)
397 (4)
276 (6)
301 (6)
73 (4)
42 (4)
22 (5)
2 (6)
1·BP
11 (6)
2
343.0 (0.1) (site 1)
357.2 (0.1) (site 2)
331.7 (0.2)
617 (6)
629 (4)
713 (3)
688 (3)
598 (1)
615 (2)
613 (3)
606 (5)
-0.13 (0.02)
-0.23 (0.02)
-0.18 (0.02)
-0.11 (0.02)
-0.33 (0.01)
-0.31 (0.01)
-0.25 (0.02)
-0.28 (0.01)
665 (4)
696 (6)
709 (5)
696 (6)
660 (1)
657 (2)
646 (5)
650 (4)
316 (3)
309 (4)
289 (5)
314 (4)
263 (1)
255 (1)
263 (3)
262 (3)
48 (4)
67 (4)
-3 (5)
8 (4)
63 (1)
42 (1)
33 (4)
44 (3)
2·BP
2·DPE
2·(Bu4N)Cl
339.4 (0.1)
328.7 (0.1) (site 1)
318.1 (0.1) (site 2)
314.3 (0.2)
2·(Bu4N)Br
2·(Bu4N)I
3
319.0 (0.1)
679.1 (0.3)
1635 (8)
-0.85 (0.04)
1728 (3)
216 (4)
93 (5)
Spectra of all the compounds were recorded with a minimum of decompose in the NMR rotor, even spinning at a moderately
two different spinning speeds in order to distinguish the low speed of 3 kHz.
isotropic peaks from the spinning sidebands and to increase the
Inspection of the 77Se CS tensor data in Table 2 reveals
precision of the fitted parameters. The isotropic chemical shifts
several interesting points. Firstly, an increase in the value of 11
(

iso), spans (Ω = 11
–

33), skews (
 = 3(22-iso)/) and the
relative to pure 1 or 2 is noted upon cocrystal formation with
nitrogen-based electron donors. Conversely, 11 decreases
upon cocrystal formation with halide ion electron donors. This
highlights the point that the pure ChB donor compounds
feature self-complementary chalcogen bonds and thus their
NMR properties may not necessarily fall at one extreme of the
ranges observed. Secondly, the value of 22 decreases upon
principal components (11
,
22 33) are detailed in Table 2. The
, 
77Se NMR spectrum of pure ChB donor 2 reveals two chemical
shifts at 343.0 and 357.2 ppm, indicating the existence of two
crystallographically distinct selenium sites (Table 2, ESI Fig. S8-
S115). Similarly, the presence of four crystallographically
distinct selenium sites in donor 1 gives rise to four different 77Se
isotropic chemical shifts, ranging from 321.9 to 365.3 ppm
(Table 2). Multiple attempts were carried out to measure 77Se
NMR spectra for cocrystal 1·DPE; unfortunately the material
was pressure and temperature sensitive and starts to
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The Se···Br- distance was varied between 2.70 Å and 3.70 Å in
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0.2 Å increments with the C-Se···Br- angle of fixed at the
DOI: 10.1039/C9CP06267J
experimental value of 174.4°. The Se···N distance was varied
between 2.50 Å and 3.50 Å in 0.2 Å increments with the C-Se···N
angle set to the experimental value of 177.2°. A plot of the
principal components of the 77Se chemical shift tensors as a
function of ChB distance is provided in Figure 4. The isotropic
chemical shifts decrease as the ChB is shortened. There is also
a
second order polynomial decrease in the principal
components as the ChB distance reduces. The change in the
most deshielded tensor component, 11, is more significant than

that for the most shielded tensor component
decrease in span when shortening the ChB.
33, resulting in a
The main NLMO contributions to the computed 77Se
isotropic magnetic shielding changes relating to the ChB
Figure 4. Plot of calculated 77Se isotropic chemical shift (a), span (b) and
skew (c) as a function of ChB distance for the models of ·(Bu4N)Br (blue
circles) and
·BP (orange triangles). (d) A plot of calculated 77Se principal
components of chemical shift tensors: 11 (blue circles), 22 (orange
triangles) and 33 (green diamonds) as a function of ChB distance for the
models of ·(Bu4N)Br.
2
2

2
cocrystal formation. Thirdly, the value of 33 generally
decreases upon cocrystal formation with all electron donors. All
of these changes combine to result generally in a decreased
isotropic 77Se chemical shift upon cocrystal formation. The span
of the CS tensor increases upon cocrystal formation with
nitrogen-based electron donors, but decreases upon cocrystal
formation with halide ion electron donors; this is again
reflective of the fact that the pure ChB donors feature
chalcogen bonds of intermediate strength relative to the two
cocrystal subsets (see Table 1). A comparison between the DFT
calculated 77Se chemical shift values and the experimental
values (ESI; paragraph S.8, Fig. S.19) reveals a poor correlation
likely due to the lack of crystal packing in the cluster-based DFT-
GIAO methods and the high sensitivity of the selenium atom to
the local environment.
Figure 5. Nitrile carbon regions of the 13C CP MAS SSNMR spectra (blue
Analogous to our previous work on halogen bonds,30 an
NLMO analysis of the 77Se isotropic magnetic shielding values
was conducted. The main contributions are: the sum of the
selenium core orbitals (CR), two selenium lone pair orbitals
(LPs), bonding orbitals between selenium and nitrile carbon (BD
Se-CN) and bonding orbitals between selenium and methyl
carbon (BD Se-CH2). The values are provided in the ESI (Table
S.3). While the core orbitals have a consistent positive
lines) obtained at 9.4 T for pure ChB donor
2 and selected chalcogen-
bonded cocrystals ·DPE and ·(Bu4N)I. The corresponding simulated
2
2
spectra (see text for discussion) are shown in red.
distance are similar to what was noted above: the sum of the
selenium core orbitals (CR), two selenium lone pair orbitals
(LPs), bonding orbitals between selenium and nitrile carbon (BD
Se-CN) and bonding orbitals between selenium and methyl
contribution to the isotropic magnetic shielding, contributions carbon (BD Se-CH2). A figure summarizing the main NLMO
from the other orbitals vary with the ChB geometry. The main
NLMO contributions to the isotropic magnetic shielding values
are plotted as a function of ChB geometrical features: ChB bond
distance, angle and Nc values (ESI). While there is a degree of
scatter, the magnitude of the contribution from the bonding
contributions and a plot of the main NLMO contributions to the
magnetic shielding tensors as a function of ChB distance is
provided in the ESI, Fig. S.21. Similar to the experimentally-
based NLMO contributions, the sum of the selenium core
orbitals makes a positive contribution to the isotropic magnetic
shielding values. In both cases, one of the selenium lone pair
orbital between Se and C
≡N tends to decrease as the ChB bond
shortens. Therefore, the Se-C bond can be a sensitive probe for
the occurrence of ChB formation.
orbitals (
observed experimentally in most cases, whereas the sign of the
contributions from other lone pair orbitals ( orbitals) varies. In
 orbitals) makes a negative contribution, which was
The dependencies of the 77Se chemical shift tensors on
Se···Br-/N bond length were also calculated using models built
based on the experimental structures of 2·(Bu4N)Br and 2·BP.

both Br- and BP systems, there are decreasing contributions
from CR and BD Se-CN orbitals as the bond distance becomes
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and asymmetry parameter (휂푄) of 14N. The RDD value,푅퐷퐷
=
View Article Online
훾퐶훾푁휇0
ℏ
DOI: 10.1039/C9CP06267J
−3
〈
〉
, which depends on the inverse cube of the
푟퐶푁
4휋 2휋
motionally averaged internuclear distance, may be calculated
directly from the single-crystal X-ray structure. J coupling and
휂푄 were not found to notably influence the simulated peak
shapes; therefore, these two values were fixed to the DFT
calculated values during the fitting process. The only adjusted
parameters during the fitting process were therefore iso and
CQ. It may be noted that as there are two crystallographically
distinct sites in pure 2, there are two sets of NMR parameters
used to fit the spectrum for this compound (see ESI). The effect
from J coupling between 13C and 77Se is omitted due to the
relatively low natural abundance of the latter; nevertheless this
could contribute to spectral broadening near the baseline. The
results are summarized in the ESI (Table S2). Formation of a
chalcogen bond to 2 causes a decrease in the chemical shift
Figure 6. Experimental Raman spectra of ChB donor 2 (blue, bottom) and
related cocrystals 2·BP (red, middle) and 2·(Bu4N)Br (green, top). A clear
red shift and an intensity increase observed for C–Se along the extension
of the covalently bound C≡N atom, as well red shift and decrease
intensity of C≡N stretching band upon occurrence of the chalcogen bond
highlighted in light blue.
(
(13C) ranging from -1.6 to -2.5 ppm) upon the formation of
the NC–Se A motif.
⋯
shorter. The contributions from Se
 lone pair and BD Se-CH2
orbitals increase as the ChB bond gets shorter.
Raman and Fourier Transform Infrared
Spectroscopy
The experimental 125Te CP/MAS NMR spectrum of pure
donor 3 obtained at 9.4 T is shown in Figure 3. Spectral
simulation provides an isotropic chemical shift of 679.1 ppm
and a span of 1635 ppm (Table 2). These data may be
compared, for example, with those available from Collins et al.
for some salts of the trimethyltelluronium (TMT) and
triphenyltelluronium (TPT) cations.31 Isotropic chemical shifts
ranging from 404 to 472 ppm are reported for various TMT salts
and from 731 to 802 ppm for TPT salts. The 125Te CS tensor spans
are only on the order of 100 ppm for the TMT salts. The data
for 3 are largely reflective of the two-coordinate nature of Te in
this sample, as opposed to the three-coordinate geometries
found in TMT and TPT. Notably, in their work, evidence for
chalcogen bonding of halides to tellurium is also obtained
through J coupling measurements.31 Due to the poor thermal
stability of 3, even after several attempts we were unable to
prepare cocrystals. The pure tellurium donor material also
slowly starts to decompose while spinning the sample in the
NMR rotor. Thus, no further 125Te studies were pursued as part
of this study. However, the CS data reported for 3 provide a
valuable benchmark for the chalcogen-bonded benzylic
tellurocyanate motif.
FTIR and Raman spectroscopies have been applied as simple
and fast screening methods to detect the occurrence of ChB
between the benzylic selenocyanates 1 and 2, and a series of
electron donors. It is well-known that the intermolecular
interaction between an electron-donor species and an
electrophile affects the vibrational motions as assessed by
intensity variations and frequency shifts. Due to the limited
literature32 on the vibrational behavior of chalcogen-bonded
systems, we carried out a detailed vibrational analysis by
density functional theory (DFT) calculations (gas phase) for all
the structures studied herein to accurately assign the molecular
vibration bands and to predict detectable changes upon
formation of the chalcogen bond. Selected experimental and
calculated FTIR and Raman absorption frequencies for all the
cocrystals and the individual ChB donors are reported in Table
3. These data show a good agreement between the calculated
and experimental C–Se absorption frequency, with a variation
of ~20 cm-1. Experimental Raman spectroscopy reveals the C–
Se in plane stretching vibrations of benzylic selenocyanate 2
appear at 510 cm-1 and this is in agreement with the calculated
value, i.e., 510 cm-1 (Table 3, Fig. 6).
13C CP/MAS SSNMR spectroscopy was carried out on pure
ChB donors and cocrystals to characterize the interactions in
terms of their 13C chemical shifts. As shown in Figure 5, the
isotropic 13C resonances of the nitrile carbon exhibit an
asymmetric splitting. Additional 13C CP/MAS SSNMR data were
also acquired at 4.7 T; this field-dependent splitting is due to the
presence of residual dipolar coupling between 13C and 14N
instead of the presence of impurity or different sites as shown
in ESI, (Fig. S.17). Simulations in Figure 5 were adjusted by
including the direct dipolar coupling (RDD) and J coupling
between 13C and 14N, and quadrupolar coupling constant (CQ)
Table 3. Selected experimental and calculated IR and Raman
stretching vibration bands (in cm-1) associated with the N
≡C
and N
≡C–Se bonds in pure ChB donors and related cocrystals.
Experimental
Calculated
Compound
νSe―C≡N (IR)
νSe―C≡N (Ra)
νNC―Se (Ra)
ν NC―Se
1
2147
2139
2147
2139
2138
2149
2138
2150
2137
2137
512
496
510
495
496
513
495
510
501
493
1⸱BP
2
2⸱BP
2⸱BPE
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n.d.a
488b
490
546
474
471
2⸱(Bu4N)Cl
2⸱(Bu4N)Br
2⸱(Bu4N)I
complex systems, in excellent complementarity with X-ray
diffraction methods.
2144
2141
2143
2145
2140
2143
View Article Online
DOI: 10.1039/C9CP06267J
Experimental
a Not detected, presumably due to spectral broadening. b An additional peak is
observed at 504 cm-1
.
Preparation of Cocrystals
ChB donor 1, 2, and acceptor systems such as tetrabutylammonium
halides ((Bu4N)I, (Bu4N)Br, (Bu4N)Cl), 4,4′−bipyridine (BP) and 1,2-
di(4-pyridyl)ethylene (DPE) were dissolved separately in acetone (1
equivalent of onium halides and 0.5 eq of BP and DPE for ChB
donors). These two solutions were mixed and allowed to evaporate
slowly at room temperature. The cocrystals of 1 and 2 with DPE are
obtained by dissolving the two tectons in acetone and diethyl ether
was diffused as a second less efficient solvent.
Interestingly, in benzylic selenocyanate-electron donor
complexes, we find that the occurrence of ChB produces a red
shift and intensity decreases in the C–Se stretching region of
488-510 cm-1 (Table 3). A similar redshift and intensity variation
of the C–X stretching band (X= Br and I), which is the distinct
signature of the halogen bond has been reported by Resnati and
co-workers.33 The observed redshift is likely attributable to the
weakening and lengthening of the C–Se bond upon forming a
ChB with electron donor systems. In addition to the C–Se
stretching, vibrations related to nitrile (C≡N) covalently bound
to selenium in the benzylic selenocyanates could be an indirect
indicator for the occurrence of chalcogen bond.
Powder X-ray Diffraction
Pure ChB donors, ChB acceptors and cocrystals were individually
packed in an aluminium or glass sample holder and data sets were
collected on a Rigaku Ultima IV powder diffractometer at 293 K (±2)
(CuKα1 radiation with a wavelength of λ = 1.54056 Å). The
measurements were carried out in focused beam geometry with a
step-scan technique in 2θ range of 5–50°. Data were acquired by
scintillation counter detector in continuous scanning mode with a
step size of 0.02°. The experimental PXRD patterns of pure ChB
donors, cocrystals and simulated patterns from the single crystal are
provided in ESI† S.4
Conclusions
A series of cocrystals has been prepared based on NC–Se⋯X
chalcogen bonds and characterized with X-ray crystallography,
multinuclear solid-state magnetic resonance, and vibrational
spectroscopies. The σ-hole present on the prolongation of the
NC–Se covalent bond interacts with anionic and neutral Lewis
bases to form isolated or extended neutral 1D networks. From
77Se CP/MAS SSNMR spectroscopy, we observe a general
decrease in the isotropic chemical shift of selenium for the
chalcogen bonded cocrystals in comparison to the pure benzylic
selenocyanates. Changes in the principal components of the
77Se chemical shift tensors were correlated to the local ChB
geometry. Through NLMO calculations it was found that these
changes are mainly associated with the sum of the selenium
core orbitals, with further important contributions from two
selenium lone pair orbitals, bonding orbitals between selenium
and nitrile carbon, and bonding orbitals between selenium and
Single Crystal X-ray Studies
The crystals were mounted on glass fibres with glue prior to data
collection. Crystals were cooled to 200 ± 2 K. The data were collected
on a Bruker AXS diffractometer equipped with MoKα radiation
(wavelength of λ = 0.7103 Å) with an APEX II CCD detector. The raw
data collection and processing were performed with the Bruker APEX
II software package. Structure solution and the refinement details
are provided in ESI† S.5.
methyl carbon.
A lack of strong correlation between
Solid-State NMR Spectroscopy
experimental and computed 77Se chemical shift tensors implies
that many structural features likely influence their ultimate
values; however, computations on model systems reveal that
the ChB alone produces consistent and predictable effects.
13C CP/MAS SSNMR revealed that the NC−Se carbon
chemical shifts decrease upon the formation chalcogen-bonded
cocrystals, compared to the pure ChB donors. We have also
demonstrated that the NC−Se vibration band undergoes a clear
redshift and intensity decrease upon the formation of a
chalcogen bond in the cocrystals. This Raman and IR
SSNMR experiments were conducted using a Bruker Avance III NMR
spectrometer (B0 = 9.4 T, L(77Se) = 76.311 MHz and L(125Te) =
126.240 MHz) equipped with a triple-resonance 4 mm MAS probe.
Samples were ground into fine powder and packed in 4 mm o.d.
zirconium oxide rotors. 77Se SSNMR chemical shifts were referenced
to solid (NH4)2SeO4, (iso = 1040.2 ppm) and 125Te chemical shifts
were referenced to solid telluric acid Te(OH)₆ (iso = 685.5 ppm and
692.2 ppm).31 The MAS frequency was varied from 3 to 12.5 kHz to
obtain spectra with a sufficient number of sidebands for spectral
spectroscopic fingerprint of the selenocyanate-based chalcogen fitting purposes (Fig. S9-S17). Spectra were obtained at 298 K for
bonded cocrystals could prove to be a simple and rapid tool to
detect the occurrence of ChB when alternative methods are not
apposite.
The combination of solid-state NMR and vibrational
spectroscopy techniques employed here, as well the insights
gained from computational chemistry, affords new directions
for the characterization of the chalcogen bonds in more
pure ChB donor 1, 2 and respective cocrystals, in few cases the
temperature was set to 288 K or 278 K due to the low melting point
of the cocrystals and to avoid the possible phase transition (Fig. S9-
S17). A standard 1H→77Se/125Te CP pulse sequence was employed for
all cocrystals. The /2 pulse length was optimized to be 3.2 s for
77Se and 1.60 for 125Te. The CP contact time used was 7 ms for 77Se
and 2 ms for 125Te and the recycle delays varied from 10 s to 4 min.
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The total number of transients varied from 512 to 16000. The 77Se
NMR spectra were simulated using a Herzfeld-Berger analysis34 and
dmfit.35 Residual dipolar coupling between 13C and 14N was analyzed
using the WSolids program.36 The experimental and simulated
spectra for the pure ChB donors and the chalcogen-bonded
cocrystals are provided in the ESI, section S.6.
Acknowledgements
View Article Online
DOI: 10.1039/C9CP06267J
We thank Dr. Jeffrey Ovens, Dr. Glenn Facey, Dr. Peter Pallister
and Dr. Yun Liu for technical support and useful discussions. D.
L. B. thanks the Natural Sciences and Engineering Research
Council of Canada for funding.
Conflicts of interest
References
There are no conflicts to declare.
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