Pyridyl based mono and di-selenoethers: Synthesis, characterization and DFT study

Article abstract of DOI:10.1016/j.molstruc.2019.127449

In this joint experimental-DFT study, we present a robust synthetic methodology, designed to produce Se-C_n-Se and C–Se–C. Three pyridyl based selenoethers were synthesized, characterized and analyzed by applying Density Functional Theory (DFT) calculations and conducting cyclic voltammetry (CV) experiments. Bis(2-(pyridin-2-yl)ethyl)selane (BPSe), 1,3-bis((2-(pyridin-2-yl)ethyl)selanyl)propane (BP3Se₂) and 1,4-bis((2-(pyridin-2-yl)ethyl)selanyl)butane (BP4Se₂) were synthesized from different selenocyanates and characterized by FT-IR and ¹³C, ¹H, ⁷⁷Se NMR spectroscopies and mass spectrometry. DFT (B3PW91/6-311G(2df,p)) optimized geometries and spectra simulations concur well with the experimental data. Theoretical reactivity behavior was assessed, taking into account HOMO-LUMO diagrams and Molecular Electrostatic Potential maps. Cyclic voltammetry experiments reveal that C–Se bond cleavage occurs in the reduction reaction, whereas, the selenium compounds manifest typical two-electron oxidation behavior. In this work, pyridyl rings were selected to act as electrophile fragments, but from a perspective which considers the possibility of using many other electrophiles to implement this novel procedure.

Full text of DOI:10.1016/j.molstruc.2019.127449

Journal Pre-proof
Pyridyl based mono and di-selenoethers: Synthesis, characterization and DFT study
Luis Felipe Hernández-Ayala, Miguel Reina, Marcos Flores-Alamo, Lena Ruiz-Azuara
PII:
S0022-2860(19)31558-3
DOI:
https://doi.org/10.1016/j.molstruc.2019.127449
MOLSTR 127449
Reference:
To appear in: Journal of Molecular Structure
Received Date: 19 August 2019
Revised Date: 17 November 2019
Accepted Date: 18 November 2019
Please cite this article as: L.F. Hernández-Ayala, M. Reina, M. Flores-Alamo, L. Ruiz-Azuara, Pyridyl
based mono and di-selenoethers: Synthesis, characterization and DFT study, Journal of Molecular
Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2019.127449.
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CRediT author statement
Pyridyl based mono and di-selenoethers: Synthesis, characterization and DFT study
Luis Felipe Hernández-Ayala: Methodology, Validation, Investigation, Writting; Miguel
Reina: Software, Formal analysis, Editing; Marcos Flores-Alamo: Investigation and Lena
Ruiz-Azuara: Conceptualization, Reviewing, Supervision, Project Management

Pyridyl based mono and di-selenoethers: Synthesis, characterization and DFT study
Luis Felipe Hernández-Ayala1, Miguel Reina1, Marcos Flores-Alamo2 and Lena Ruiz-
Azuara*1
1Medicinal Inorganic Chemistry Lab., Facultad de Química Universidad Nacional
Autónoma de México, Av. Universidad 3000, Circuito Exterior s/n, CU, P.O. Box 70-360,
04510 Mexico City, México.
2USAI, Facultad de Química Universidad Nacional Autónoma de México, Av. Universidad
3000, Circuito Exterior s/n, CU, P.O. Box 70-360, 04510 Mexico City, México.
Abstract
In this joint experimental-DFT study, we present a robust synthetic methodology, designed
to produce Se-Cn-Se and C-Se-C. Three pyridyl based selenoethers were synthesized,
characterized and analyzed by applying Density Functional Theory (DFT) calculations and
conducting cyclic voltammetry (CV) experiments. Bis(2-(pyridin-2-yl)ethyl)selane (BPSe),
1,3-bis((2-(pyridin-2-yl)ethyl)selanyl)propane (BP3Se2) and
1,4-bis((2-(pyridin-2-
yl)ethyl)selanyl)butane (BP4Se2) were synthesized from different selenocyanates and
characterized by FT-IR and 13C, 1H, 77Se NMR spectroscopies and mass spectrometry. DFT
(B3PW91/6-311G(2df,p)) optimized geometries and spectra simulations concur well with
the experimental data. Theoretical reactivity behavior was assessed, taking into account
HOMO-LUMO diagrams and Molecular Electrostatic Potential maps. Cyclic voltammetry
experiments reveal that C-Se bond cleavage occurs in the reduction reaction, whereas, the
selenium compounds manifest typical two-electron oxidation behavior. In this work,
pyridyl rings were selected to act as electrophile fragments, but from a perspective which
considers the possibility of using many other electrophiles to implement this novel
procedure.
* Corresponding author: lenar701@gmail.com Phone number (0052-55) 56 22 3529
1

Keywords: Synthesis, organoselenium compounds, DFT calculations, cyclic voltammetry,
spectroscopy.
Introduction
In 1960, the versatile open-chain quadridentate “pdto” chelate was synthesized: 1,8-bis(2-
pyridyl)-3,6-dithiaoctane and its first complexes.1The ligand pdto is a facultative
tetradentated donor compound, capable of adapting to metal geometry, frequently resulting
in square planar, tetrahedral and octahedral geometries.2-13 Additionally, several complexes
with different denticity of pdto have been identified, ranging from uni to tetradentate
coordination modes.3,5,11-13 Pdto related systems have been designated for their potential
applications in several fields, including polymers capable of conducting electricity,13
copper complexes such as superoxide dismutase (SOD) mimetics14 and ruthenium
amebicide agents.15
The unusual properties of selenium compared to oxygen and sulfur atoms, when involved
in chemical bonding and complexes, have triggered numerous investigations in several
branches of chemistry, such as nanomaterials,16 synthesis sciences,17, catalysis,18 and
biochemistry19, among others. Selenium compounds are capable of acting as both
antioxidant20 and pro-oxidant systems21; effects which are strictly dependent on their
concentration and which are referred to as the “selenium paradox”. This characteristic has
been made use of in medicinal chemistry research for the development of several anticancer
based selenium agents22, including different selenocyanates23 and selenoethers.24
Anticancer efficacy, as well as the toxicity of these compounds depends on the selenium
speciation, and it is also well known that the most bioactive systems are those that possess
redox active forms.21 In recent years, the upsurge in chemistry related to selenium
compounds has encouraged the development of computational methods to study structural,
2

spectroscopic, and reactivity characteristics.25-30 For these reasons, the generation of these
molecules, which poses much more of a problem than oxygen or sulfur analogs, to a large
extent transcends the synthetic alternative, because of the usefulness and versatility
afforded by the addition of the selenium atom.
Based on the pdto ligand and the interesting chemical properties of selenium compounds,
we designed and synthesized three bis-selenocyanates (selenocyanates), and three pyridyl
based selenoethers. The aim of this work is to present a systematic way of obtaining Se-
carbon chain-Se class of compounds. In this particular work, pyridyl rings were selected to
act as the electrophile fragment of the molecule, although the methodology presented here
should be robust enough to serve as a starting point for the synthesis of this class of
compounds, using many other electrophiles. Furthermore, all compounds were
experimentally characterized using different techniques: FT-IR and NMR spectroscopies,
elemental analysis and mass spectrometry. The structure of the BPSe hydrochloride salt
was revealed by applying single crystal X-ray diffraction, and cyclic voltammetry (CV)
experiments were carried out to identify redox behavior. Moreover, DFT studies were
performed to simulate and corroborate the experimental spectra and to determine the most
important features of the electronic structure.
Experimental
Materials. Synthesis were carried out under nitrogen atmosphere. Solvents were dried
before use. All chemicals were purchased from Sigma-Aldrich®, and were used without
further purification.
Spectroscopic techniques FT-IR: Selenocyanates and selenoethers FT-IR spectra were
recorded in KBr pellets and KBr disks respectively with a Nicolet Avatar 320 FT-IR
spectrometer. Electron impact mass spectra were carried out with a Jeol SX 102A mass
3

spectrometer. NMR: Experiments were recorded at room temperature using CDCl3 as
solvent on a Varian Unity Nova spectrometer, TMS (1H: 500 MHz, 13C: 125 MHz) and
Me2Se (77Se: 95.35 MHz) were employed as references.
Synthesis of selenocyanates. Selenocyanates (NCSe(CH2)nSeCN, n=2-4) were prepared
31
according to the procedure reported for NCSe(CH2)3SeCN
with some minor
modifications. KSeCN salt (see quantities in Supplementary Data, Table S1) and acetone
(15 mL) were placed in a two-necked flask. The mixture was heated under stirring to 60°C
then the corresponding alkyldibromide was added. These reaction conditions was kept for 4
h. The precipitate was filtered and the solution was concentrated in vacuum, then
hydrolyzed (15 mL H2O) and extracted (3 x 15 mL CH2Cl2). The organic layers were
washed (3 x 15 mL H2O) and dried over MgSO4. The solvent was evaporated in vacuum,
leaving the selenocyanate. Other reaction condition attempts to prepare selenocyanates and
their yields are presented in Table S2.
Preparation of pyridyl based selenoethers. A two-necked flask charged with NaBH4 (Table
S3) and ethanol (15 mL) was placed in an ice bath, then suspension was stirred. The
suitable selenocyanate was dissolved, in the minimum amount of ethanol, and it was added
slowly to the NaBH4 suspension. The solution was kept for 0.5 h at 0 °C and at r. t. for 1 h.
At this point, an amount of conc. HCl was added, a white solid appear. Freshly distilled 2-
vinylpyridine was also added and the new mixture was stirred at r.t. overnight. The
precipitate was filtered, and the solution was concentrated in vacuum, then hydrolyzed (30
mL H2O) and extracted (3 x 20 mL CH2Cl2). The organic layers were washed (3 x 15 mL
H2O) and dried over MgSO4. The solvent was evaporated in vacuum and the desired
product is obtained.
4

EtSeCN: 1,2-diselenocyanatoethane. White solid, 0.55 g, yield: 86%. M. P.: 112 °C.
Elemental anal calc. for C4H4N2Se2, 238.00 g/mol: C: 20.2, H: 1.7, N: 11.8, found: C: 20.3,
H: 1.5, N: 11.7. FT-IR (cm-1): 509 (νs –H2C-Se), 563 (νs NC-Se), 1413 (νs -C-H), N.O. (νas -
C-H), 2148 (νs C≡N). NMR: 1H (ppm) δ: 3.46 (s , 2H, -CH2-, 2JH-Se= 5.1 Hz); 13C (ppm) δ:
29.99 (-CH2-, 1JC-Se= - 116.6 Hz), 105.15 (-C≡N); 77Se (ppm) δ: 262.41. EI-MS, (m/z, %):
238 (10%) [M]+•, 240 (11%) [M+2]+•, 134 (100%) [NCSe(CH2)2]+•.
PrSeCN: 1,3-diselenocyanatopropane. White solid, 0.60 g, yield: 95%. M. P.: 39 °C.
Elemental anal calc. for C5H6N2Se2, 252.03 g/mol: C: 24.5, H: 2.6, N: 10.9, found: C: 24.5,
H: 2.4, N: 10.7. FT-IR, KBr disk (cm-1): 513 (νs –H2C-Se), N. O. (νs NC-Se), 1415, 1438
(νs -C-H), 2850-2950 (νas -C-H), 2148 (νs C≡N). NMR: 1H (ppm) δ: 2.33 (t, 1H, C-CH2-C,
2
13
3JH-Se= 10.5 Hz), 3.16 (t, 2H, Se-CH2-C, JH-Se= 15.9 Hz); C (ppm) δ: 29.01 (C-CH2-C),
1
33.29 (Se-CH2-C, JC-Se= -69.6 Hz), 105.13 (-C≡N); 77Se (ppm) δ: 220.90. EI-MS, (m/z,
%): 252 (27%) [M]+•, 254 (30%) [M+2]+•, 41 (100%) [CH3CN]+.
BuSeCN: 1,4-diselenocyanatobutane. White solid, 0.49 g, yield: 40%. M. P.: 70 °C.
Elemental anal calc. for C6H8N2Se2, 266.06 g/mol: C: 27.1, H: 3.0, N: 10.5, found: C: 27.3,
H: 2.8, N: 10.5. FT-IR, KBr disk (cm-1): 509 (νs –H2C-Se), N. O. (νs NC-Se), 1423, 1456
(νs -C-H), 2860-2940 (νas -C-H), 2152 (νs C≡N). NMR: 1H (ppm) δ: 1.95 (t, 1H, C-CH2-C,
2
3JH-Se= 9.7 Hz), 3.13 (t, 2H, Se-CH2-C, JH-Se= 15.6 Hz); 13C (ppm) δ: 29.50 (C-CH2-C),
1
30.55 (Se-CH2-C, JC-Se= -72.4 Hz), 105.06 (-C≡N); 77Se (ppm) δ: 216.86. EI-MS, (m/z,
%): 266 (4%) [M]+•, 268 (5%) [M+2]+•, 55 (100%) [CH3CH2CN]+.
BPSe: bis(2-(pyridin-2-yl)ethyl)selane. Ambar-red oil, 0.37 g, yield: 65%. Elemental anal
calc. for C14H16N2Se, 291.25 g/mol: C: 57.7, H: 5.5, N: 9.5, found: C: 57.8, H: 5.5, N: 9.6.
FT-IR, KBr disk (cm-1): 563 (νs -C-Se), 582 (νas -C-Se), 1473 (νs -C-H), 2931 (νas -C-H),
3006 (νs =C-H), 1567 (νs C=C), 1589 (νs C=N). NMR: 1H (ppm) δ: 2.90 (t , 2H, H-Cc, 3JH-
5

Se= 12.6 Hz), 3.06 (t , 2H, H-Cd, 2JH-Se= 11.3 Hz), 7.21 (d, 1H, H-Cf), 7.28 (d, 1H, H-Ch),
1
7.68 (t, 1H, H-Cg), 8.49 (d, 1H, H-Ci); 13C (ppm) δ: 22.69 (Cc, JC-Se= -72.6 Hz ), 38.90 (Cd,
2JC-Se= -67.2 Hz), 122.00 (Cf), 123.43 (Ch), 136.91 (Cg), 149.46 (Ci), 160.72 (Ce); 77Se
(ppm) δ: 172.94. EI-MS, (m/z, %): 290 (5%) [M]+•, 292 (10%) [M+2]+•, 106 (100%)
[(C5H4N)(CH2)2]+•.
BP3Se2: 1,3-bis((2-(pyridin-2-yl)ethyl)selanyl)propane. Ambar oil, 0.56 g, Yield: 68%.
Elemental anal calc. for C17H22N2Se2, 412.28 g/mol: C: 49.5, H: 5.4, N: 6.8, found: C: 49.7,
H: 5.6, N: 6.8. FT-IR (cm-1): 565 (νs -C-Se), 1473 (νs -C-H), 2927 (νas -C-H), 3006 (νs =C-
H), 1567 (νs C=C), 1591 (νs C=N). NMR: 1H (ppm) δ: 1.87 (q , 2H, H-Ca), 2.50 (t , 2H, H-
2
Cb, JH-Se= 12.8 Hz), 2.90 (t , 2H, H-Cc, 2JH-Se= 12.5 Hz), 3.07 (t , 2H, H-Cd), 7.20 (d, 1H,
13
H-Cf), 7.28 (d, 1H, H-Ch), 7.69 (t, 1H, H-Cg), 8.48 (d, 1H, H-Ci); C (ppm) δ: 22.63 (Cc,
1JC-Se= -125.3 Hz), 23.50 (Cb, JC-Se= -122.5 Hz), 31.28 (Ca), 39.01 (Cd,), 121.98 (Cf),
1
123.41 (Ch), 136.86 (Cg), 149.48 (Ci), 160.69 (Ce); 77Se (ppm) δ: 160.65. EI-MS, (m/z, %):
412 (4%) [M]+•, 414 (5%) [M++2]+•, 106 (100%) [(C5H4N)(CH2)2]+•.
BP4Se2: 1,4-bis((2-(pyridin-2-yl)ethyl)selanyl)butane. Yellow oil, 0.53 g, yield: 60%.
Elemental anal calc. for C18H24N2Se2, 426.31 g/mol: C: 50.7, H: 5.7, N: 6.6, found: C: 50.8,
H: 5.9, N: 6.7. FT-IR, KBr disk (cm-1): 565 (νs -C-Se), 584 (νas -C-Se), 1473 (νs -C-H),
1
2927 (νas -C-H), 3006 (νs =C-H), 1567 (νs C=C), 1591 (νs C=N). NMR: H (ppm) δ: 1.63
(m , 2H, H-Ca), 2.53 (m , 2H, H-Cb, 2JH-Se= 12.0 Hz), 2.89 (t , 2H, H-Cc, 2JH-Se= 12.7 Hz),
3.07 (t , 2H, H-Cd), 7.21 (d, 1H, H-Cf), 7.29 (d, 1H, H-Ch), 7.69 (t, 1H, H-Cg), 8.49 (d, 1H,
H-Ci); 13C (ppm) δ: 22.52 (Cc, 1JC-Se= -125.6 Hz), 23.08 (Cb, 1JC-Se= -121.2 Hz), 30.51 (Ca),
39.03 (Cd), 121.98 (Cf), 123.43 (Ch), 136.87 (Cg), 149.48 (Ci), 160.76 (Ce); 77Se (ppm) δ:
162.90. EI-MS, (m/z, %): 426 (2%) [M]+•, 228 (2%) [M+2]+•, 106 (100%)
[(C5H4N)(CH2)2]+•.
6

Crystallographic studies. Single crystal 2(C14 H18 N2 Se), 4(Cl), 3(H2O), BPSe mounted on
a glass fiber was studied with Oxford Diffraction Gemini "A" diffractometer with a CCD
area detector (λMoKα = 0.71073 Å, monochromator: graphite) source equipped with a sealed
tube X-ray source at 130 K. Unit cell constants were determined with a set of 15/3 narrow
frame/runs (1° in ω) scans. A data set consisted of 314 frames of intensity data collected,
and a crystal-to-detector distance of 55 mm. The double pass method of scanning was used
to exclude any noise. The collected frames were integrated by using an orientation matrix
determined from the narrow frame scans. CrysAlisPro and CrysAlis RED software
packages32 were used for data collection and data integration. Analysis of the integrated
data did not reveal any decay. Final cell constants were determined by a global refinement
of 5497 reflections (θ < 29.6612°). Collected data were corrected for absorbance by using
analytical numeric absorption correction33 using a multifaceted crystal model based on
expressions upon the Laue symmetry using equivalent reflections. Structure solution and
refinement were carried out with the SHELXS-201434 and SHELXL-201435 WinGX
v2014.136 and Mercury CSD 4.1.0 software37 was used to prepare material for publication.
Full-matrix least-squares refinement was carried out by minimising (Fo2–Fc2)2. All non-
hydrogen atoms were refined anisotropically. For H atom of the amine and water groups
were located in a difference map and refined isotropically with Uiso(H) = 1.2 for H–N and
Uiso(H) = 1.5 for H–O. H atoms attached to C atoms were placed in geometrically
idealized positions and refined as riding on their parent atoms, with C–H = 0.95– 0.99 Å
and with Uiso(H) = 1.2Ueq(C) for aromatic and methylene groups. Details of the X-ray
studies for compound BPSe are summarized in Table S4. Crystallographic data have been
deposited at the Cambridge Crystallographic Data Centre as supplementary material
7

number CCDC 1910652 Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. E-mail:deposit@ccdc.cam.ac.uk.
Cyclic voltammetry. The electrochemical experiments were conducted in a solutions 2 mM
of sample in dry DMSO and with 0.1 M of tetrabutylammonium hexafluorophosphate as
support electrolyte. Three-electrode glass cell, at r.t., under a dry nitrogen was employed.
The working electrode was a 2 mm diameter platinum (BASi). It was carefully polished
and ultrasonically rinsed with acetone each time. The pseudo reference electrode was an Ag
wire. All the potentials were referenced to Fc+/Fc system (ꢀEP= 65 mV, ν= 0.1 V/s). The
counter electrode was a platinum wire. A Potentiostat/Galvanostat PAR 263-A was used to
record the experiments. Interrupted current determination was applied to correct the ohmic
drop between the working and pseudo reference electrode.
Computational Methods. Theoretical calculations were carried out with Gaussian 09
implementation.38 Initial geometries were fully optimized at B3PW91/6-311G(2df,p) level
of theory,39,40 in conjunction with the SMD continuum model using ethanol and DMSO to
mimic polar environments.41 To verify optimized minima, harmonic analyzes were
performed and local minima identified (zero imaginary frequencies). Optimized minima
and IR spectra enable characterization and useful comparisons to the experiments. The
isotropic NMR chemical shifts and anisotropic shielding constants have been calculated for
1H-NMR, 13C-NMR and 77Se-NMR nuclei using GIAO method for the optimized structures
with the same methodology. To gain further insight into the reactivity of these molecules,
Frontier Molecular Orbitals: HOMO and LUMO were obtained. The Molecular
Electrostatic Potential (MEP) of a molecule is also a guide to measure the global reactivity
towards nucleophilic or electrophilic reactants and negatively or positively charged species
and is also useful to study changes in charge distribution. Furthermore, local reactivity dual
8

descriptor, ∆f, a useful tool to reveal reactive sites of a system, is also presented.42
Formally, ∆f, has close relationship to the Fukui function:
∆f (r) = f+(r)- f -(r) = [ρN+1(r)- ρN(r)]-[ρN(r)-ρN-1(r)] = ρN+1(r)-2ρN(r)+ρN-1(r)
It is possible, instead of using electron density, obtain ∆f from spin density, ρs. Since ρN+1
-
ρN and ρN-ρN-1 can be assessed to ρsN+1 and ρs respectively, it appears that ∆f (r) ≈
N-1
ρsN+1(r) - ρsN-1(r). It has been proved that there are no quantitative differences by evaluating
∆f from electron density and spin density.42
Contrasting to Fukui functions, employing ∆f, both electrophilic and nucleophilic attacks
can be seen simultaneously in a qualitative manner.
Additionally, to gain a deeper insight about the C-Se bond strength, bond dissociation
energy (BDE) calculations were computed considering the cleavage of H2C-Se and Se-CN
bonds respectively. BDE comes from the difference between the zero-point energy (EZP) of
the molecule and the sum of EZP of the radical fragments.
Results and Discussion
Synthesis
Although the chemistry of Se-Se compounds has been well described and reported 43-46, few
reports study Se-(CH2)n-Se systems. In this research, we present a method for obtaining this
class of compound. Figure 1 presents the six compounds synthetized in this work and
Figure 2 summarizes the synthetic steps (A, B, C, D and E) for the formation of each
compound. During the first stage (A), selenocyanates were obtained from the KSeCN
reaction and the corresponding alkyl dibromide. BP3Se2 and BP4Se2 were successfully
synthesized from the reaction between 2-vinylpyridine and 1,3-diselenopropane and 1,4-
9

diselenobutane, respectively (C), generated in situ by the reduction of the corresponding
selenocyanates (B). Attempts to prepare 1,2-diselenoethane were unsuccessful and during
stage D, the decomposition of 1,2-diselenoethane is evident. Experimentally, a deep brick
red solid, corresponding to the elemental allotropic selenium Se80 47 was detected and a dark
insoluble precipitate was obtained in a Pb(NO3)2 aqueous trap. It seems that the product of
this reaction can be attributed to PbSe, formed from the reaction between H2Se and Pb salt.
Further reaction of H2Se with 2-vinylpiridine produces BPSe, corresponding to stage E.
This new methodology describes a simple procedure for synthetizing selenoether
derivatives and affords a number of important advantages, compared to other similar
methods, for example it avoids the use of metal catalysts or poisonous H2Se gas as a
primary source of selenium.
Figure 1. Schematic structures of compounds under study.
10

Figure 2. Synthesis selenium compounds.
Characterization
Experimental and Simulated FT-IR and NMR spectroscopy
In the following, we will only describe EtSeCN and BPSe, as the principal results related to
synthetized compounds from their respective families (selenocyanates and selenoethers, see
Table S5) are very similar. Table 1 presents the infrared spectra of EtSeCN and BPSe,
together with their principal bands. We undertook band assignments, as described in
previous reports,48,49 and corroborated these with theoretical DFT calculations. Evidently,
there is great correlation between the shape and principal frequencies of the experimental
and calculated spectra.
Table 2 presents the NMR assignments of EtSeCN and BPSe. In each case, identifiers
(letters case) have been used to mark nuclear positions. In 77Se-NMR, selenoether signals
appear around 160-170 ppm and bis-selenocyantate signals were apparent at a lower field,
between 220-250 ppm, due to the greater electronegativity of the nitrile group. Coupling
1
constants were calculated, showing on one side that the JSe-C are all negatives and in the
range of -60 to -130 Hz; the sign of the constant indicates that the most stable interaction
exists between two antiparallel nucleus spins. Contrastingly, nJSe-H (n=2,3) are all positives
11

and in the range of 5 to 15 Hz, thus the sign of the constant in this context indicates that the
most stable interaction is between parallel nucleus spins. Coupling constants present similar
frequency values to those detected in similar selenium compounds.50 In order to establish
1
the true relationship between experimental and calculated H and 13C chemical shifts,
Figure 3 presents BPSe (EtSeCN contains only one type of hydrogen atom and two
different types of carbon atom), so no further analysis was undertaken. For all other
compounds, results from this study can be found in Tables S6, S7 and Figures S1 and S2 of
Supplementary Data). Notably, the linear correlation factor, R2 is very high, indicating that
this level of theory can reliably reproduce spectroscopic experimental results. Furthermore,
this level of calculation maintains a precision that exceeds others manifesting more
complex methodologies.51
12

Table 1. Experimental ( ̶ ) and calculated ( ̶ ) IR spectra of EtSeCN and BPSe.
Vibration
C-Se (ν
ν EXP (cm-1)
563
ν CALC (cm-1)
568
s
)
C-H (νbn
)
1174
1168
C-H (νsc
C≡N
)
1413
2152
1416
2168
Vibration
ν EXP (cm-1)
1434, 1473
2848, 2927
3006, 3062
1567, 1589
ν CALC (cm-1)
1472, 1512
2968, 3000
3056, 3096
1640
C-H (νas
)
C-H (ν
=C-H
C=C, C=N
s
)
13

Table 2. NMR assignation references for EtSeCN and BPSe.
77Se-NMR
1H-NMR
(ppm); int; mult; Jxy (Hz)]
13C-NMR
[δ (ppm)]
[δ (ppm); 3JSe-H
,
Compounds
[
δ
2JSe-H, 1JSe-C (Hz)]
EtSeCN
c
Cb: 29.99
Cc: 105.13
262.41; 5.1, -
116.6
b
Hb: 3.46; 1H, s
BPSe
Cc: 22.69
Cd: 38.90
Ce: 160.72 172.9; 11.3, 12.6,
Cf: 122.00
Cg: 136.91
Ch: 123.43
Ci: 149.46
Hc: 2.91; 2H; t; 7.6cd
Hd: 3.07; 2H; t; 7.6cd
Hf: 7.21; 1H; d; 7.5fg, 4.8fi
Hg: 7.28; 1H; t; 7.7gfh, 1.8gi
Hh: 7.69; 1H; d; 7.8hg, 0.9hi
c
f
-72.6, -67.2 (2J,
Se-Cd)
e
d
g
h
Hi: 8.49; 1H; d; 4.8fi, 1.8gi, 0.9hi
i
10
8
180
140
100
6
y = 1.0784x - 0.2669
60
4
y = 0.9569x + 6.3086
R² = 0.9974
R² = 0.9995
2
2
20
4
6
8
10
20
70
120
170
δexp (ppm)
δexp (ppm)
Figure 3. Plots of theoretical vs experimental 1H (left) and 13C (right) chemical shifts for BPSe
X-ray structure analysis of [H2(BPSe)]Cl2·3H2O
Crystals of [H2(BPSe)]Cl2 (water molecules are omitted for clarity) were grown in ethanol
at room temperature by slow evaporation and the structure was resolved by single crystal
X-ray diffraction analysis, at T = 130 K. A perspective view of the molecular structure is
14

presented in Figure 4. Selected bond lengths and torsion angles are listed in Table S8. The
asymmetric unit of [H2(BPSe)]Cl2 consists of four independent bis(2-(2-pyridinio)ethyl)
selenium cations, eight chloride anions and six water molecules. The dihedral angle
between the two pyridinium rings is 21.82°, 13.14°, 24.63° and 12.42° for molecules 1-4,
respectively. A deviation from coplanarity is apparent. Values for the C-Se-C angle are
around 96.95° and 100°. Torsion angles and dihedral angles show clear geometrical
differences between the four bis (2-(2-pyridinio)ethyl) selenium cations. Some hydrogen
bond interactions (O—H···O, O—H···Cl and N—H···Cl) are evident in the crystalline
array of [H2(BPSe)]Cl2 (Figure 5). Distances and angles are reported in Table S9.
Hydrogen atoms from the solvation water molecules present two hydrogen bond
interactions: firstly, with a chloride anion (O4W—H4C···Cl4) and then with another water
molecule (O4W—H4D··O3W). Furthermore, the oxygen atom in a water molecule
interacts with hydrogen atoms corresponding to alkyl fragments (O5W-H5C...O4W),
forming the D2 (5) and D2 (4) motif’s; whereas the nitrogen atom from the pyridinio group
2
2
acts as a hydrogen atom donor (N6—H6E···Cl7), establishing the D2 (13) motif. It is
2
possible that these interactions are important for stabilizing the array. All laminar
intermolecular interactions take place along the crystalline b-c plane.
15

Figure 4. ORTEP drawing of the asymmetric unit of compound [H2(BPSe)]Cl2 with the thermal
ellipsoids drawn at the 60% of probability.
Figure 5. Crystal array of compound, [H2(BPSe)]Cl2, view along the b-axis and with perspective to
plane formed by a-c axes emphasizing the C1 (9), R1 (6) motif’s.
1
1
16

Mass Spectrometry
Figures 6 and 7 report the electron impact mass spectrum (EI-MS) of EtSeCN and BPSe. In
all cases, the experimental molecular ion [M]+• correlates well with the calculated pattern
presented in the Figures. The isotopic pattern presents pronounced peaks at values of [M-
2]+•, [M+2]+• and [M+4]+•, principally influenced by the abundance of the selenium
isotopic.48 In the EtSeCN spectrum, a signal at m/z= 238 is observed, corresponding to [M]
+•. Upon fragmentation, additional signals appear at 134 [M-104]+• and 106 [M-132]+• m/z
values. These signals can be attributed to [NCSe-(CH2)2]+▪ and [SeCN]+▪ fragments. This
last fragmentation and signal intensity increment indicate that EtSeCN stabilization leads to
cleavage of the H2C-Se bond. The same behavior is observed for the other selenocyanate
compounds (Figure S3). An interesting feature is observed in the case of PrSeCN and
BuSeCN, as signals in [M-26]+, m/z=55 corresponds to [CH3CH2CN]+ and m/z= 41 to
[CH3CN]+. These results suggest the loss of the CN group and thus, the rupture of the Se-
CN bond in these two molecules.
Figure 6. Electron impact mass spectrum of EtSeCN. Inset: calculated molecular ion [M]+•
17

In Figure 7, the spectrum presents a [M]+• low intensity signal (m/z= 292) due to
[py2(CH2)4Se]+▪. Upon fragmentation, high intensity signals appear at [M-112]+• and [M-
186]+• values. These signals correspond to [py(CH2)2Se]+▪ and [py(CH2)2]+▪, respectively,
and are also present in both BP3Se2 and BP4Se2. Again, the low intensity of [M]+• may
signify that the pyridyl-based selenoether fragments are unstable. Relative ratio
fragmentation signals for these molecules suggest that the stability of a compound
decreases as the aliphatic chain length increases.
Figure 7. Electron impact mass spectra of BPSe. Inset: calculated molecular ion [M]+•
DFT Calculations
Figure 8 illustrates the optimized geometries, Molecular Electrostatic Potential maps
(MEP), dual descriptor, ∆f and HOMO-LUMO diagrams for EtSeCN and BPSe,
respectively. We present the same complete study for each system in Figure S4. In the case
of BPSe, calculated distances and X-ray experimental parameters correlate well. MEP
18

enables us to analyze the regions of the molecules that are positively charged (blue) or
negatively charged (red), and therefore more susceptible to nucleophilic or electrophilic
additions. As anticipated, a charged region of selenocyanates and selenoethers corresponds
to nitrogen and selenium atoms, and therefore these sites are disposed to nucleophilic and
electrophilic attacks, respectively. Selenium charge deficiency (sometimes known as σ
hole) enables the formation of a chalcogen bond and makes possible the recognition of
certain anions52,53. This charge modulation on the selenium atom is a known and useful tool
for molecular recognition 54-56. Among the systems studied, a similar trend is observed; a
neutral and small positive charge is located in the vicinity of selenium atoms, and a
negative charge is situated in both the cyano groups and the nitrogen atom of the pyridyl
rings.
In EtSeCN, HOMO is principally distributed in the selenium atoms and the LUMO is found
throughout the entire molecule. In the case of BPSe, HOMO is located in the chalcogen and
LUMO in the pyridyl rings. For the local reactivity dual descriptor, results are as expected.
Firstly that the nucleophilic attack takes place on Se and N atoms (red isosurfaces), and
secondly that the electrophilic attack tends to occur in the carbon atom (green isosurfaces)
between Se and N atoms for EtSeCN. For BPSe, the same tendency is observed: the
nucleophilic attack is likely to occur on the Se atom, whereas the electrophilic attack occurs
on the alpha carbon atom to Se. These observations for EtSeCN and BPSe are valid for
selenocyanates and selenoethers, respectively.
19

EtSeCN
BPSe
dC-Se=1.97 Å, dSe-CN=1.83 Å
dC-Se=1.96 Å, dSe-C’=1.96 Å
Figure 8. Optimizations, MEP’s, ∆f and FMO’s of EtSeCN (left) and BPSe (right)
20

Redox Behavior
Cyclic voltammetry experiments were performed to reveal the redox behavior of these
compounds. For this section, EtSeCN, PrSeCN, BuSeCN voltammograms are presented, as
some interesting differences appear (Table 3). For the PrSeCN voltammogram, two
cathodic irreversible waves were detected. The first reduction peak corresponds to the
cleavage of the RH2C-SeCN bond (IC) and the second reduction wave can be attributed to
the cleavage of the Se-CN bond (IIC). This behavior is also observed in BuSeCN and other
selenocyanates.57,58 In order to truly comprehend redox reactivity, theoretical bond
dissociation energy (BDE) was computed. The RH2C-SeCN bond dissociation energy is
57.3 kcal mol-1, whereas for the Se-CN cleavage, the BDE is 82.9 kcal mol-1. The BDE
results comply with the experimental potential peaks. The driving force may constitute the
charge-dipole interaction between the centered selenium radical RSe· and the cyanide anion
–CN.58 An unexpected result was discerned, concerning the voltammetry of EtSeCN, as
evidently there is only one reduction process; that attributed to the ruptured H2C-SeCN
bond (IC). Likewise, we observed a reversible signal (II’C/II’A) in -0.11 V vs. Fc+/Fc. DFT
calculations suggest two possible explanations for this phenomenon. Firstly, it may be that
the Se-CN bond dissociation energy value is too large (96.4 kcal mol-1) compared to the
H2C-SeCN bond cleavage (55.6 kcal mol-1), and thus not possible. This significant bond
strength increase (around 14 kcal mol-1) for the Se-CN cleavage may be the reason why
only one experimental reduction wave was observed. Secondly, the manifestation of any
minima on the potential energy surface that would indicate the formation of a stable
▪
NCSe(CH2)2 radical intermediate, is not possible. The evident minima indicates a new
▪
fragmentation of NCSe(CH2)2 radical intermediate, resulting in two products: NCSe▪ and
21

▪
H2C=CH2. The theoretical dissociation reaction path for the NCSe(CH2)2 radical
intermediate is presented in Figure S5.
Table 3. Cyclic voltammograms, peak potentials (EP) and estimated bond dissociation
energies (BDE) to EtSeCN, BuSeCN and PrSeCN
CV to 0.001 M of each compound in DMSO and 0.1 M of TBAPF6 at Pt electrode.
v=0.1V/s
EP,IC
(V vs Fc+/Fc)
-1.32
BDEC-SeCN
(kcal mol-1)
55.6
EP,IIC
(V vs Fc+/Fc)
N. O.
BDESe-CN
(kcal mol-1)
96.4
Compound
EtSeCN
PrSeCN
BuSeCN
-1.41
-1.50
56.0
57.4
-1.77
-1.89
82.0
82.9
Table 4 shows the voltammogram of BP3Se2 (voltammograms of BP4Se2 and BPSe are in
Figure S6). In the case of selenoethers, the three compounds present irreversible reduction
peaks (IC / IIC), related to C-Se bond cleavage and also irreversible oxidation waves with
large current intensity (IIIA/IVA), related to common 2/4e- processes that are typical of
selenium compounds.16,59 For BPSe, a single peak in the voltammogram (IIc) is observed,
due to the symmetric cleavage of the molecule: ethylpyridine and selenoethylpiridine. As
22

BP4Se2 and BP3Se2 molecules present two Se atoms, different cleavages can occur. CI-Se
▪
bond cleavage generates a radical ethylpyridine Py(CH2)2 and a selenium centered radical:
RSe▪ (R=Py(CH2)2Se(CH2)3,4). CII-Se bond cleavage produces a radical selenoethylpyridine
Py(CH2)2Se▪ and a carbon centered radical, R▪. As expected, BDE comply with these
observations, and the most thermodynamic favorable process is the bond cleavage to form
ethylpyridine radical, as demonstrated by the EI-MS experiments. Oxidation processes
present very close peak potential values and similar behavior in the voltammogram, i.e. a
larger current value compared to the reduction processes, suggesting a greater number of
electrons involved (2/4e-).
Table 4. Cyclic voltammogram, peak potential (EP) and estimated bond dissociation energy
(BDE) to BP3Se2
CV to 0.001 M of BP3Se2 in DMSO and 0.1 M of TBAPF6 at Pt electrode. v=0.1V/s
EP,IC
(V vs Fc+/Fc)
N. O.
BDECI-Se
(kcal mol-1)
---
EP,IIC
(V vs Fc+/Fc)
-1.75
BDECII-Se
(kcal mol-1)
45.1
Compound
BPSe
BP3Se2
BP4Se2
-1.66
-1.69
44.2
45.5
-1.78
-1.82
46.2
48.1
23

Conclusions
Here we present a synthetic methodology to generate novel selenoethers, derived from 2-
ethyl-pyridine. Three selenoethers were synthesized from the corresponding
selenocyanates. EI-MS experiments suggest that all the compounds studied form stable
fragments upon the cleavage of the C-Se bond. In particular, the stability of selenoether
compounds decreases, together with an increase in the length of the molecule. DFT
calculations indicate that no significant electronic structural differences are apparent, when
comparing members from the same family (selenoethers and selenocyanates). In
voltammetry experiments, the reduction process involves a C-Se bond cleavage. Calculated
BDE concur well with observed peak potentials: for selenocyanates, results from CV and
BDE, suggest that H2C-Se cleavage occurs more easily than that of Se-CN, and in the case
of selenoethers, the most stable fragment corresponds to the radical generated by the loss of
ethyl pyridine. The versatility of synthetic methodology could be used to build up new
mono and di functionalized selenoethers with interesting properties as ligands, in potential
catalytic systems. Additionally, our research group will pursue its efforts to synthetize the
selenium pdto analogous molecule: the 1,2-diselenoethane.
Apendix A
Supplementary Data: Additional tables and figures of stoichiometry, spectroscopic
characterization, DFT calculations and cyclic voltammetry experiments are included in this
section.
Acknowledgments
24

This work was supported by UNAM-PAPIIT IG200616. UNAM-FQ50009047, L.F.H.A
(grant 307725) and M.R. thanks to CONACyT. The authors thank to UNAM-DGTIC for
the resources and supercomputer services. We also thank to Adrian Espinoza-Guillén for
technical support, Dr. Beatríz Quiroz García and Luis Velasco Ibarra from the Instituto de
Química, UNAM for the NMR and MS-EI determinations, and Caroline Karslake (Masters,
Social Anthropology, Cambridge University, England) for reviewing the grammar and style
of the text in English.
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