Synthesis, crystal structure, Hirshfeld surface analysis, DFT calculations and characterization of 1,3-propanediylbis(triphenylphosphonium) monotribromide as brominating agent of double bonds and phenolic rings

Article abstract of DOI:10.1016/j.molstruc.2020.127700

This paper presents synthesis and structural characterization of new members of phosphorus-based organic bromides. 1,3-Propanediylbis(triphenylphosphonium) dibromide I and 1,3-propanediylbis(triphenylphosphonium) monotribromide II, as a new brominating agent for double bonds and phenolic rings, were synthesized. ¹H NMR, ¹³C NMR, ³¹P NMR, FT-IR, single crystal X-ray diffraction crystallography, differential scanning calorimetry, thermogravimetric analysis and differential thermal analysis were used to characterize these salts. Thermal and physicochemical stability, simple working up, non-toxicity in comparison to liquid bromine and high yield are some of the advantages of these salts. These salts have good solubility in organic solvents, such as methanol, ethanol, acetone, dichloromethane and THF. Crystallographic data showed that compound I crystallized in the monoclinic crystal system, in P2₁ space group and compound II crystallized in the monoclinic crystal system, in P2₁/c space group and one of the bromide ions was replaced by tribromide ion in II. The crystal packing structures of title compounds were stabilized by various intermolecular interactions, especially of the type C-H???π contacts. The molecular Hirshfeld surface analysis and 2D fingerprint analysis revealed that the C???H (30.4% for the compound I and 28.3% for compound II) contact, which was related to C-H???π interactions, had the major contribution in the crystal architectures. To get more insight about molecular structures of titled compounds, DFT calculations were performed (energy, structural optimization and natural bond orbital analysis). Bromination of double bonds and phenolic rings was carried out to prove the ability of the tribromide salt to bromine such organic substrates.

Full text of DOI:10.1016/j.molstruc.2020.127700

Journal Pre-proof
Synthesis, crystal structure, Hirshfeld surface analysis, DFT calculations and
characterization of 1,3-propanediylbis(triphenylphosphonium) monotribromide as
brominating agent of double bonds and phenolic rings
Seyed Reza Nokhbeh, Mostafa Gholizadeh, Alireza Salimi, Hazel A. Sparkes
PII:
S0022-2860(20)30023-5
DOI:
https://doi.org/10.1016/j.molstruc.2020.127700
MOLSTR 127700
Reference:
To appear in: Journal of Molecular Structure
Received Date: 29 August 2019
Revised Date: 5 January 2020
Accepted Date: 6 January 2020
Please cite this article as: S.R. Nokhbeh, M. Gholizadeh, A. Salimi, H.A. Sparkes, Synthesis,
crystal structure, Hirshfeld surface analysis, DFT calculations and characterization of 1,3-
propanediylbis(triphenylphosphonium) monotribromide as brominating agent of double
bonds and phenolic rings, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2020.127700.
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Synthesis, crystal structure, Hirshfeld surface analysis, DFT
calculations and characterization of 1,3-
propanediylbis(triphenylphosphonium) monotribromide as
brominating agent of double bonds and phenolic rings
†
†,
*
†
‡
Seyed Reza Nokhbeh , Mostafa Gholizadeh , Alireza Salimi , Hazel A. Sparkes
†Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, I.R., Iran
‡School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
Keywords: phosphonium tribromide, Hirshfeld surface analysis, 2D fingerprint plots, Density Function Theory calculations, X-ray
crystallography, brominating agent,
Supporting Information
Abstract: This paper presents synthesis and structural characterization of new members of phosphorus-based organic
bromides.
1,3-Propanediylbis(triphenylphosphonium)
dibromide
I
and
1,3-propanediylbis(triphenylphosphonium)
1
monotribromide II, as a new brominating agent for double bonds and phenolic rings, were synthesized. H NMR, 13C NMR, 31P
NMR, FT-IR, single crystal X-ray diffraction crystallography, differential scanning calorimetry, thermogravimetric analysis and
differential thermal analysis were used to characterize these salts. Thermal and physicochemical stability, simple working up, non-
toxicity in comparison to liquid bromine and high yield are some of the advantages of these salts. These salts have good solubility
in organic solvents, such as methanol, ethanol, acetone, dichloromethane and THF. Crystallographic data showed that compound I
crystallized in the monoclinic crystal system, in P21 space group and compound II crystallized in the monoclinic crystal system, in
P21/c space group and one of the bromide ions was replaced by tribromide ion in II. The crystal packing structures of title
compounds were stabilized by various intermolecular interactions, especially of the type C-Hꢀꢀꢀπ contacts. The molecular Hirshfeld
surface analysis and 2D fingerprint analysis revealed that the CꢀꢀꢀH (30.4 % for the compound I and 28.3 % for compound II)
contact, which was related to C-Hꢀꢀꢀπ interactions, had the major contribution in the crystal architectures. To get more insight
about molecular structures of titled compounds, DFT calculations were performed (energy, structural optimization and natural
bond orbital analysis). Bromination of double bonds and phenolic rings was carried out to prove the ability of the tribromide salt to
bromine such organic substrates.

Introduction
In recent years heteronium salts, especially ammonium, pyridinium and phosphonium1-3, have played an important role in the
synthesis of organic compounds, chemical industry4, 5 and pharmacy6-9. There are a large number of reports on the synthesis of
various types of quaternary salts which have many interesting properties including ease of synthesis, high thermal stability,
solubility in water and organic solvents, such as methanol, acetone, dichloromethane and chloroform, electrochemical durability,
high viscosity, very low vapor pressure, and long shelf life. Due to their individual properties, these quaternary organic salts are
striking in field of organic chemistry as catalysts10, 11, phase transfer catalysts12, 13, halogenating agents14, oxidative15, 16 and
reductive17 reagents in some organic reactions. Organic phosphonium salts are the structural analogs of organic ammonium salts,
with expectedly similar chemical properties. Nevertheless, these reagents may react under milder conditions than organic
ammonium tribromides. Our use of phosphonium salts rather than ammonium or imidazolium analogs is motivated by the fact that
phosphorus atom has a larger atomic radius, a greater polarizability and a lower binding energy with anions than nitrogen atom.
These attributes make phosphonium salts attractive and interesting alternative halogenation reagents. There are many different
cations and their counterions that can be used as oxidative18, reductive or brominating agents in polar and non-polar solvents19.
Considering the diverse uses of these reagents, it becomes important to incorporate newer tribromides into the existing series.
Bromination of alkenes and aromatic substrates, is usually carried out by bromine solution, but organic ammonium and
phosphonium tribromides are preferable since they do not have the hazards that are associated with liquid bromine and they are
regioselective20 and stereoselective21. Other advantages of organophosphorus tribromides are that they are crystalline, easy to
handle and maintain the desired stoichiometry. These salts are sometimes used as a conveniently weighable, green, non-toxic solid
source of bromine in organic synthesis. Therefore several tribromides have been synthesized. The most frequently used quaternary
phosphonium tribromide reagents as efficient brominating agents are benzyltriphenyl phosphonium tribromide22, 23, tridecyl-
methyl phosphonium tribromide21, ethyltriphenyl phosphonium tribromide24, 1,2-ethanediylbis(triphenylphosphonium)
ditribromide25 and methyltriphenylphosphonim tribromide26.
The Hirshfeld surface analysis is a powerful tool for the consideration of intermolecular interactions27 in the crystal. By means
of Hirshfeld surface analysis all interaction types (hydrogen bonding, close and distant van der Waals contacts, C–H⋯π interactions, π–
π stacking) are readily identifiable and it becomes a straightforward method to classify molecular crystals by the nature of interactions
when examining crystal packing diagrams.
We report herein the synthesis, characterization (1H NMR, 13C NMR, 31P NMR, FT-IR, DSC and TG/DTG/DTA), single crystal
X-Ray analysis and DFT calculations of 1,3-propanediylbis(triphenylphosphonium) dibromide
I
and 1,3-
propanediylbis(triphenylphosphonium) monotribromide II as a green, mild, nontoxic and efficient brominating agent for double
bonds and phenolic rings. By using specialized software (Crystal Explorer), we obtained Hirshfeld surfaces 3D maps and 2D
fingerprint plots to explanation of intermolecular short contacts in the crystals. Furthermore, we report bromination of double
bonds and phenolic rings with excellent yields at room temperature by the use of the title salt II.
Results and discussion
In the first step 1,3-propanediylbis(triphenylphosphonium) dibromide I was synthesized in refluxing DMF. The product was
filtered and washed with DMF. The white powder was recrystallized in hot water (95 % yield). In the second step, the bromide salt
I was treated dropwise with potassium tribromide in aqueous solution at room temperature in a 1:1 ratio. The yellow precipitate of
II was formed, filtered and dried overnight under stream of air (97 % yield) (Scheme 1).
Scheme 1. Synthesis of 1,3-propanediylbis(triphenylphosphonium) bromide I and monotribromide II
Crystal structure description
Based on the results of single crystal X-ray analysis of I, this compound crystallizes in the monoclinic system having chiral P21
space group with Z = 4. The asymmetric unit contains two independent molecules of triphenyl phosphonium cations, four bromide
anions (1:2 ratio of cationic to anionic units) and three water molecules. The positional disorders were observed for one of four
bromide anions Br- (Br3) and oxygen atom of water molecule (O1) with an occupancy ratio of 72.28 and 73.27 for Br3 and O1
atoms, respectively. The ORTEP diagram of compound I is shown in Figure 1 (top). The cations located in general positions and
the phosphorous atoms exhibit a slightly distorted tetrahedral geometry. The bond angles around the P atoms are in the range of
107.0(2)˚ to 113.8(3)˚ for P1, 106.2(2)˚ to 111.4(2)˚for P2, 107.5(2)˚ to 111.6(3)˚ for P3 and 107.2(2)˚ to 113.1(3)˚ for P4. The
geometry of dications (bond lengths and angles) is general and comparable with our previously reported structures 25, 28, 29. The 3D
supramolecular network of this compound is dominated by the variety of C-HꢀꢀꢀBr, O-HꢀꢀꢀBr and C-HꢀꢀꢀO hydrogen bonds (Figure

2), as well as C–Hꢀꢀꢀπ interactions which can play a crucial role in the stabilization of supramolecular assemblies (Figure 3 top).
Hydrogen bond geometry parameters are tabulated in supporting information file, Table S10.
In the crystal packing of I, the bistriphenylphosphonium dication and bromide anion are linked together by several C-HꢀꢀꢀBr
hydrogen bonds with HꢀꢀꢀBr distances ranging from 2.697 to 3.134 Å. The water molecules are involved in the hydrogen bond
interactions with the anion via O-HꢀꢀꢀBr interactions (Figure 2). The cation moieties also interact through C-Hꢀꢀπ interactions with
each other. As expected, the bromide anion in I inserts into the 1D chain of bistriphenylphosphonium dications in which each
cation alternately links anion and solvent molecules together (Figure 3, top). Overall, the interaction results in supramolecular
network of ionic moieties producing the three dimensional pattern.
The monotribromide compound II crystallizes in the monoclinic system with P21/c space group with Z = 4. The asymmetric
unit of this compound includes one triphenyl phosphonium cation, one bromide ion and one tribromide ion. Interestingly, there are
-
two different anions (Br- and Br3 ) in the crystal packing of II (Figure 3 down). The ORTEP diagram of compound II is shown in
Figure 1 (down). Similar to I, the structure of the cationic unit shows the distorted tetrahedral geometry for phosphorous atoms.
The bond angles around the P atoms are in the range of 106.5(3)˚ to 113.8(4)˚ for P1 and 105.9(2)˚ to 112.3(2)˚for P2. Investigation
of crystal packing structure of II revealed that C-HꢀꢀꢀBr hydrogen bonds are dominant interaction in the stabilization of
supramolecular architecture. In this structure, the bistriphenylphosphonium dication and tribromide anion are linked together by
-
several C-HꢀꢀꢀBr hydrogen bonds with HꢀꢀꢀBr distances ranging from 2.830 to 3.171Å for Br3 and 2.636 to 3.109 Å for Br-.
Interestingly, the arrangement of cationic units in the crystal packing of II is similar to I. Tribromide anion in II was replaced with
one of bromide anions and solvent molecules. (Figure 3, down). Surprisingly, bromide anion formed the heptafurcated hydrogen bonds
comprised of one Br- acceptor and seven C-H donor moieties (Figure 4).
Comparison of titled compounds indicated that changing accompanying anionic moiety from bromide to tribromide affects
directional interactions between anionic and cationic units and therefore different supramolecular aggregation can be observed for
these structures. In addition, the presence of solvent molecule in I resulted in the various solvent-based interactions, such as O-
HꢀꢀꢀBr and C-HꢀꢀꢀO hydrogen bonds. One of the main interactions in these compounds are C–Hꢀꢀꢀπ interactions which play a
crucial role in their molecular aggregation in all crystallographic directions.
Figures 1. The ORTEP diagram of compound I (top) and compound II (down) with 50% probability displacement ellipsoids and atom
numbering (hydrogen atoms numberings are omitted for more clarity). There are positional disorders for the bromide anion Br- (Br3) and oxygen
atom of water molecule (O1) in compound I with an occupancy ratio of 72.28 and 73.27, respectively.

Figure 2. Hydrogen bonds in compound I
Figures 3. Presentation of crystal packing structures of compound I (top) and II (down) along a and c crystallography axis, respectively
Figure 4. Presentation of heptafurcated hydrogen bonds comprised of one Br- acceptor and seven C-H donor moieties in compound II
Thermal properties of salts I and II
DSC thermograms have been recorded under nitrogen purging (50 mL/min.). The thermogram of compound
endothermic sharp peak at 349-353 C due to melting point and unsymmetrical low and broad peaks in the range of 360-380 C
I shows an
°
°
assigned to decomposition of I (Figure 5).

Figure 5. DSC thermogram of compound I
An endothermic sharp peak at 163-166 °C due to melting point, a broad exothermic peak at 270-330 °C and a broad endothermic
peak at 330-380 °C assigned to decomposition of anion and cation, respectively are observable in the thermogram of compound II
(Figure 6).
Figure 6. DSC thermogram of compound II
°
In the TG/DTG/DTA thermogram of I, an endothermic peak at 350 C assigned to melting point is observed in the DTA blue
curve with a small decrease in the TG red curve (by about 3-4 %) due to evaporation of water molecules. Immediately after that
peak, a broad endothermic shoulder peak with a sharp and significant decrease in the TG curve has been recorded in the DTA
curve (more than 90 %) assigned to decomposition of I. Ultimately, there is a broad endothermic peak in the range of 350-380 °C
in the DTG brown curve due to decomposition of I (Figure 7).
Figure 7. TG/ DTG/ DTA thermogram of compound I
The thermogram for II shows an endothermic sharp peak at about 165 °C in the DTA blue curve and the smallest decrease in the
°
TG red curve (1 %) for melting point. The broad endothermic peak in the range of 320-370 C in the DTA curve, a strong broad
°
peak in DTG brown curve assigned to decomposition of compound II and about 7.7 % at 800 C in TG red curve for remaining
ash were observed (Figure 8). The DSC and TG/DTG/DTA thermograms show that compound I is more thermally stable than
compound II. However, both salts are very stable on the bench-top in solid form or in solution of organic solvent and in contact
with air without any change in color or performance over 6 months.

Figure 8. TG/ DTG/ DTA thermogram of compound II
Computational studies
The theoretical calculations were performed by using the Gaussian 09 software package, at the DFT-B3LYP level and with 6-
311+G* basis sets for compound I, as well as compound II.
The bond lengths, bond angles and dihedral (torsion) angles were compared with X-ray crystallographic data. The calculation
shows a good correlation between the theoretical and SC-XRD experimental structural parameters of compounds (See supporting
information file).
Natural charge distribution on the molecules of I and II were calculated by NBO method. Calculations showed that P and H
atoms have positive character while C and Br atoms have negative ones (Tables S4 and S5).
The interaction between the frontier molecular orbitals (HOMO from one molecule and LUMO from the other) is an important
part of the total orbital interaction between two molecules. This interaction can only occur if the orbitals have the correct
symmetry. The strength of the interaction is directly related to the HOMO-LUMO energy separation. In general, the smaller the
HOMO-LUMO separation, the stronger the interaction and the more favored the reaction becomes30. The energy level of the
HOMO orbital shows tendency toward giving an electron as a donor in reactions while the energy level of LUMO orbital shows
the ability to taking an electron as acceptor. HOMO-LUMO energies and the energy gap were calculated by B3LYP/ 6-311+G*
presented in figures 9 and 10. Calculations indicate that although replacement of bromide by tribromide does not have any
significant change in LUMO energy level, but it decreases the HOMO energy level (From -4.4585 eV to -4.8447 eV) and
increases the HOMO-LUMO energy gap (From 2.3837 eV to 2.7725 eV) and consequently stabilizes the tribromide salt (For
energy levels and energy gaps for I and II see figures S50 and S51).
Figure 9. HOMO (down) and LUMO (top) diagrams for I

Figure 10. HOMO (down) and LUMO (top) diagrams for II
The Hirshfeld surface analysis
In order to visualizing, quantifying and exploring the various intermolecular interactions of molecule in the crystal lattice of
both compounds, Hirshfeld surfaces (HSs) and their associated 2D fingerprint plots (FPs) were calculated using Crystal Explore
3.1 based on results of SC-X-ray analysis data28, 31
.
The function dnorm is a ratio consisting of the di (distance from the point to the nearest nucleus internal to the any Hirshfeld
surface) and de (distance from the point to the nearest nucleus external to the surface) and the van der Waals (vdW) radii of the
atoms. dnorm is a normalized contact distance, which is enable to identification of the regions of particular importance to
intermolecular interactions in the crystal.
ꢀꢆ ꢇ ꢈꢉꢊꢋ ꢀꢍ ꢇ ꢀꢍꢉꢊꢋ
ꢆ
ꢀꢁꢂꢃꢄ
ꢅ
ꢌ
ꢈꢉꢊꢋ
ꢀꢍꢉꢊꢋ
ꢆ
The value of the dnorm is negative or positive when intermolecular contacts are, respectively, shorter or longer than vdW radii of
the atoms. The dnorm values are mapped onto the Hirshfeld surface using a red–blue–white color scheme: red areas correspond to
closer contacts and negative dnorm value, the blue areas correspond to longer contacts and positive dnorm value and the white regions
are those where the distance of contacts is exactly the vdW separation and with a dnorm value of zero27. Hirshfeld surfaces mapped
with dnorm ranging from -0.1219 A˚ (red) to 2.0166 A˚ (blue) for I (Figure 11a) and ranging from -0.1410 A˚ (red) to 1.1896 A˚
(blue) for II (Figure 11d) and with di ranging from 1.0238 A˚ (red) to 3.1738 A˚ (blue) for I (Figure 11b) and ranging from 1.0116
A˚ (red) to 2.7888 A˚ (blue) for II (Figure 11e) and mapped with de ranging from 1.0256 A˚ (red) to 2.9770 A˚ (blue) for I (Figure
11c ) and ranging from 1.0120 A˚ (red) to 2.7395 A˚ (blue) for II (Figure 11f) are calculated and plotted using Crystal Explorer
software. Hirshfeld surface area of compounds I and II mapped with dnorm shows violent red spots on the surface near the Br ions
which are due to BrinsideꢀꢀꢀHoutside contacts and violent red spots near the some of hydrogen atoms of aromatic rings and aliphatic
chain near the surface in molecule which are due to HinsideꢀꢀꢀBroutside contacts in the crystal structures (Figures 11a and 11d). The
phosphorus atoms show no contacts in the crystal packing, as would be expected given their tetrahedral coordination. It can
therefore be seen that the supramolecular architectures of I and II are mainly controlled by interactions between H and Br atoms.
Figures 11. Hirshfeld surfaces mapped with dnorm for the compound I (a) and II (d), mapped with di for the compound I (b) and II (e) and
mapped with de for the compound I (c) and II (f)

A plot of di versus de is a 2D fingerprint plot which recognizes the existence and amount of different type of intermolecular
interactions. The combination of de and di in the form of a 2D fingerprint plot provides summary of intermolecular contacts in the
crystal. The relative contribution of different interactions to the Hirshfeld surface indicates that in cationic unit of compound I, the
HꢀꢀꢀH (55.9 %), CꢀꢀꢀH (30.4 %) and BrꢀꢀꢀH (11.4 %) contacts account for about 97.7 % of the total Hirshfeld surface area and
BrꢀꢀꢀC, BrꢀꢀꢀBr and CꢀꢀꢀC have very little effect on crystal packing. In cationic unit of compound II, the HꢀꢀꢀH (50.1 %), CꢀꢀꢀH (28.3
%) and BrꢀꢀꢀH (18.2 %) are main contacts about 96.6 % of total surface area and other contacts have no any considerable
contribution in total Hirshfeld surfaces in the cation (Figures 12). By comparison, tribromide anion could affect the relative
contribution of interactions and decreases the HꢀꢀꢀH while increases the BrꢀꢀꢀH hydrogen bonds in II.
Figures 12. 2D fingerprint plots for main close contact contributions (in %) in Hirshfeld surface area for a) HꢀꢀꢀH b) CꢀꢀꢀH/HꢀꢀꢀC and c)
BrꢀꢀꢀH/HꢀꢀꢀBr for compound I cation, d) HꢀꢀꢀH e) CꢀꢀꢀH/HꢀꢀꢀC and f) BrꢀꢀꢀH/HꢀꢀꢀBr for compound II cation
The analysis of the 2D finger print plot of cation I shows that BrꢀꢀꢀH and CꢀꢀꢀH contacts include 41.8 % of total Hirshfeld surface
area as characteristic wings of the plot and most surface area belongs to HꢀꢀꢀH contacts with 55.9 % of total surfaces and also in
cation II the BrꢀꢀꢀH and CꢀꢀꢀH contacts include 46.5 % of total Hirshfeld surface area as characteristic wings of the plot and most
surface area belongs to HꢀꢀꢀH contacts with 50.1 % of total surfaces.
It is clearly observable that in 2D fingerprint plots of both cations, HꢀꢀꢀH interactions are most surface area and maximum
interactions in crystal packing. The πꢀꢀꢀπ contacts are almost zero, and there are no significant BrꢀꢀꢀBr interactions in the crystal
structure of cations I or II. Chart 1 versus chart 2.

Chart 1. Percentage contribution of short contacts in the cation of compound I
Chart 2. Percentage contribution of short contacts in the cation of compound II
The molecule I in the unit cell is connected by weak BrꢀꢀꢀH interactions between nearest neighbors. Nearest intramolecular
interactions are Br2ꢀꢀꢀH21A in 2.602 Å, Br1ꢀꢀꢀH19A in 2.653 Å and Br2ꢀꢀꢀH39 in 2.706 Å. Nearest intramolecular interactions in II are
Br1ꢀꢀꢀH21A in 2.545 Å, Br2ꢀꢀꢀH21B in 2.746 Å and Br3ꢀꢀꢀH19A in 2.778 Å (Figures 13). Nearest intramolecular interactions < 3 Å for I
and II are tabulated in Table 1.
Since, the tribromide anion has three bromine atoms as good acceptors to interact with the other hydrogen bond donors, it can
be predicted that the participation of BrꢀꢀꢀH contacts in the Hirshfeld surface area for II is higher than I.
Table 1. Nearest close contacts in I and II
Compound I
Br2ꢀꢀꢀH21A
Br1ꢀꢀꢀH19A
Br2ꢀꢀꢀH39
d in Å
2.602
2.653
2.706
2.776
2.833
2.873
2.955
Compound II
Br1ꢀꢀꢀH21A
Br2ꢀꢀꢀH21B
Br3ꢀꢀꢀH19A
Br1ꢀꢀꢀH33
Br1ꢀꢀꢀH19B
Br2ꢀꢀꢀH23
Br4ꢀꢀꢀH6
d in Å
2.545
2.746
2.778
2.779
2.841
2.847
2.884
2.977
1
2
3
4
5
6
7
8
Br1ꢀꢀꢀH23
Br2ꢀꢀꢀH19B
Br2ꢀꢀꢀH18
Br2ꢀꢀꢀH33
Br1ꢀꢀꢀH8
Figures 13. a) Nearest close contacts in I and b) nearest close contacts in II
In addition to HS’s and fingerprint plots, the quantitative measurements of HS’s show that the molecular volume and surface
area of II are bigger than I because of tribromide anion and the globularity of both compounds are nearly same whereas the
asphericity of I is bigger than II. Significantly the molecular volume and surface area of cation I are bigger than cation II because
of tribromide anion can collect the cation and cause the globularity of cation II to become bigger than cation I whereas asphericity
of both cations are nearly same (Table 2).
Table 2. Quantitative measures of Hirshfeld surfaces for the compound I and II and their cations
Quantitative measures of
Hirshfeld surfaces
molecular volume
(VH) Å3
surface area
(SH) Å2
globularity
(G)
asphericity
(Ω):
Compound I
845.07
875.96
750.53
626.98
644.40
598.96
0.689
0.687
0.667
0.061
0.045
0.087
Compound II
Compound I cation

Compound II cation
722.80
571.79
0.681
0.088
Bromination of alkenes and phenols
The bromination of double bonds and phenolic rings32 can be carried out by bromine or by organic tribromide salts but the
mechanisms, thermodynamics and kinetics of the two reactions are different 33. Bromination of alkenes (cyclohexene in 1,2-
dichloroethane) has a negative entropy (ꢁS‡ = -40.9 eu), a positive enthalpy (ꢁH‡ = +6.0 kcal/ mol) and a positive activation
energy (Ea = +6.6 kcal/ mol). The proposed mechanism for the bromination of alkenes is presented in Scheme 2.
Scheme 2. Proposed mechanism for bromination of alkenes with tribromide anion
A plausible mechanism for bromination of phenolic compounds in the mixed solvent system (CH2Cl2/MeOH, 1:1) can be
outlined in Scheme 325, 32
.
Scheme 3. Proposed mechanism for bromination of phenolic compounds with tribromide anion
In our work, the bromination of double bonds and phenolic compounds was carried out by using organic tribromide II in
dichloromethane and dichloromethane/methanol solvent system at room temperature. Both electron withdrawing and electron
donating groups were placed on the double bond and phenolic ring to investigate the substituents effect on the rate of reactions. It
was found that substituents significantly influenced the reaction time. In general, bromination of alkenes proceeded in good yields,
but electron withdrawing groups, such as nitro group, increased the reaction time and decreased the reaction yields. Also for
phenolic compounds, electron withdrawing groups, such as nitro and chloro groups increased the reaction time and decreased
reaction yields, and electron donating groups, such as methyl group decreased reaction time and increased reaction yields. (Table
S8 and S9).
Conclusion
In conclusion, the present paper describes the synthesis and characterization of two quaternary phosphonium salts. The crystal
structure, the DFT calculations, analysis of Hirshfeld surfaces and fingerprint plots, as well as spectroscopic properties (1H, 13C,
31P NMR, FT-IR) and thermal behavior of the salts (by DSC-TG/DTA) were studied and reported. Furthermore, the tribromide
title salt was used as mild brominating and oxidizing agent to selective bromination of C-C double bonds and phenolic rings,
instead of very active, non-selective and toxic molecular bromine.
Experimental Section
Triphenylphosphine and potassium bromide were purified by recrystallization to obtain purity of 99.5 % in each case. The
purity of N,N-dimethylformamide (99.7%) and dichloromethane (99.8%) was checked by GC. Water as a solvent was HPLC
grade. The reaction progress was monitored by TLC and GC. NMR spectra were recorded by the Bruker AC spectrometer at 500
or 300 MHz (1H NMR, compound I and II, respectively), 75 or 125 MHz (13C NMR, compound I and II, respectively) and 200
MHz (31P NMR for both compounds) using D2O, DMSO-d6 and CDCl3 as solvents
.

DSC thermograms recorded by METTLER TOLEDO DSC-1 instrument. TG/DTG/DTA thermograms were recorded by Perkin
Elmer Diamond TG/DTA thermal analyzer. Melting and boiling points were determined by METTLER TOLEDO FP900 system,
FP81 cell. Infrared spectra were recorded on a Perkin Elmer Spectrum 65-FT-IR spectrometer as KBr disks (4000- 400 cm-1
region). GLC analysis was performed with Agilent Technology 6890N Gas Chromatograph with a FID detector and cp-sil 5 CB
50 m, 0.32 mm, 1.2 µm capillary column. GC/MS spectra were recorded by Varian 3800 GC and Varian Saturn 2000 Ion trap as a
detector and cp-sil 8 CB low bleed/MS 30 m, 0.25 mm, 0.25 µm capillary column.
Preparation of 1,3-propanediylbis(triphenylphosphonium) dibromide (C39H36P2Br2) I
A mixture of the triphenylphosphine (6.55 g, 25 mmol, excess) and 1,3-dibromopropane (2.01 g, 1.01 mL, 10 mmol, d = 1.989
g/cm3) in DMF (30 mL) was stirred and refluxed for 4 h. After reaction was complete, the reaction mixture was cooled and the
crude white precipitate was filtered and washed with DMF (3 × 10 mL). The white powder was dried under stream of air and
recrystallized in water as cubic shape crystals (6.9 g, 95 % yield) (Figure S47 in supporting information file), m.p. 349-353 °C (by
DSC). Single-crystals of compound I were grown by slow evaporation of an aqueous solution at room temperature. 1H NMR (500
MHz, D2O, δ; ppm):1.90 (brs, 2H, C20H2), 3.55 (brs, 4H, C19,21H2), 7.67, 7.86 (2 × brs, 30H, 6 × C6H5). 13C NMR (75 MHz,
DMSO-d6, δ; ppm,): 16.54(C20), 20.63 (C21, C19), 117.78, 118.93, 130.80, 134.09 and 135.52 (5 × s, C6H5). 31P NMR (200
MHz, CDCl3, δ; ppm): 35.69. FT-IR (KBr, ꢎ̅
/cm-1): 537(s), 685, 723(s, C-Haromatic bend ), 993(m), 1108(s, C-P stretch), 1190(w),
1328(m), 1432(s, Ph-P stretch), 1482(m, C-Haliphatic bend), 2801(w), 2865(m, C-Haliphatic stretch), 3009(m, C-Haromatic stretch).
Elemental analyses (%): calcd.: C, 64.46; H, 4.95; Br, 22.03; found: C, 64.71; H, 4.79; Br, 22.24.
Preparation of 1,3-propanediylbis(triphenylphosphonium) monotribromide (C39H36P2Br4) II
Bromine (1.03 mL, 20 mmol) was added dropwise to a mixing solution of KBr (2.38 g, 20 mmol) in H2O (30 mL). Bromine
layer dissolved after 30 min. The KBr3 solution was added to the aqueous solution of 1,3-propanediylbis(triphenylphosphonium)
dibromide I (7.26 g, 10 mmol in 20 mL of water). The solution was mixed for over 30 min. The yellow precipitate was filtered and
washed with cooled water (3 × 10 mL). The product was air-dried overnight and recrystallized in CH2Cl2 as needle shape crystals
(97 % yields), (Figure S48 in supporting information file), m.p. 163-166 °C (by DSC). Single-crystals of compound II were grown
by slow evaporation of dichloromethane solution at room temperature.1H NMR (300 MHz, DMSO-d6, δ; ppm): 1.80 (brs, 2H,
C20H2), 3.88 (brs, 4H, C19,21H2), 7.82 (brs, 30H, 6 × C6H5). 13C NMR (125 MHz, CDCl3, δ; ppm): 16.38(s, C20), 22.49 (s, C21,
C19), 115.58, 116.31, 129.89, 132.68 and 134.42 (5 × s, C6H5). 31P NMR (200 MHz, CDCl3, δ; ppm): 34.83 (s) for P atom. FT-IR
(KBr, ꢎ̅
/cm-1): 508.2(s), 539.7(s), 686.6(s), 721.8(s, C-Haromatic bend), 994(m), 1111.2(s, C-P stretch), 1190.9(m), 1329.6(m),
1436.3(s, Ph-P stretch), 1484(m, C-Haliphatic bend), 2803.8(m), 2867.8(m, C-Haliphatic stretch), 3013.8(m, C-Haromatic stretch).
Elemental analyses (%): calcd.: C, 52.82; H, 4.06; Br, 36.11; found: C, 52.93; H, 4.21; Br, 35.92. 1H NMR spectra of compounds I
and II showed three distinctive peaks corresponding to carbon atoms of the center CH2, terminal CH2 and aromatic groups in a
ratio 1:2:15 respectively. The exchange of the bromide to the tribromide anion caused very little changes in chemical shifts (0.1 -
0.4 ppm in 1H NMR and 0.8 ppm in 31P NMR) (see SI).
Caution: Bromine is toxic, corrosive, and dangerous for the environment. Keep container tightly closed and in a well-ventilated
place. In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. In case of accident or if you
feel unwell, seek medical advice immediately (show the label where possible). Avoid release to the environment.
General experimental procedure for bromination of alkenes
To
a
stirred mixture of alkene (3 mmol) in dichloromethane (5 mL) was added solution of 1,3-
propanediylbis(triphenylphosphonium) mono tribromide II (3 mmol) in dichloromethane (5 mL) at room temperature. The
reaction progress was monitored by TLC and GC. After disappearance of the yellow-orange color of reagent II, the solvent was
evaporated and diethyl ether was added (3 × 5 mL) for extraction. The mixture was filtered and the solvent was evaporated. The
crude product was purified with column chromatography over silica gel (pore size 60 Å, 200-400 mesh particle size) using a
mixture of n-hexane and ethyl acetate (8:2) as the eluent. The main products, reaction times and isolated yields are tabulated in
Table S8. To confirm identity of the products, they were subjected to GC/MS analysis and their spectra were compared and
validated with the NIST library (see SI). All of the isolated products are known and their physical data have been reported in
literature34.
General experimental procedure for bromination of phenolic rings
To a solution of phenolic compound (3 mmol) in dichloromethane/methanol 50 % (5 mL) was added 1,3-
propanediylbis(triphenylphosphonium) monotribromide II (3 mmol) in dichloromethane (5 mL) at room temperature. The reaction
progress was monitored by TLC and GC. After disappearance of the yellow-orange color of reagent II, the solvent was
evaporated. The extraction was carried out with diethyl ether (3 × 5 mL). The mixture was filtered and the solvent was evaporated.
The crude product was purified with column chromatography over silica gel (pore size 60 Å, 200-400 mesh particle size) using a
mixture of n-hexane and ethyl acetate (8:2) as the eluent. The main products, reaction times and isolated yields are tabulated in
Table S9. To confirm identity of the products, they were subjected to GC/MS analysis and their spectra were compared and
validated with NIST library (see SI). All of the isolated products are known and their physical data have been reported in
literature34.
X-ray crystallography analysis

X-ray diffraction experiments on I and II were carried out at 100(2) K on a Bruker APEX II diffractometer using Mo-Kα
radiation (λ = 0.71073 Å). Data collections were performed using a CCD area detector from a single crystal mounted on a glass
fibre. Intensities were integrated in SAINT and absorption corrections based on equivalent reflections were applied using
SADABS35. Both of the structures, I and II were solved using ShelXT36 and refined by full matrix least squares against F2 in
ShelXL37, 38, using Olex239. All of the non-hydrogen atoms were refined anisotropically, while all of the hydrogen atoms were
located geometrically and refined using a riding model. One of the bromide anions (Br3) and oxygen atom of water molecule (O1)
in compound I showed the positional disorder and were refined with an occupancy ratio of 72.28 and 73.27 for these atoms,
respectively. The structural resolution procedure was performed using WinGX crystallographic software package40. Crystal
structure and refinement data are given in Table 3. CCDC 1947472-1947473 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/retrieving.html (or from the Cambridge
Crystallographic Data Centre 12, Union Road, Cambridge, B2 1EZ; UK, fax +441223336033; or deposit@ccdc.cam.ac.uk).
Table 3. Crystal data and structure refinement for I and II
Identificati
on code
I
II
Empirica
l formula
Formula
weight
Temperat
ure/K
C39H36P2 Br2.H2.66O1.33
750.44
C39H36P2Br4
886.26
100(2)
100(2)
Crystal
system
Space
monoclinic
monoclinic
P21/c
P21
group
a/Å
9.3070(3)
37.3087(11)
10.3701(3)
37.4291(12
b/Å
)
c/Å
α/°
10.2239(4)
90
9.4941(3)
90
β/°
γ/°
105.750(2)
90
105.725(2)
90
Volume/
3547.15(19
3416.8(2)
4
Å3
)
Z
4
ρcalcg/cm
1.459
1.660
3
ꢂ/mm-1
F(000)
Radiatio
n
2.498
1533.0
Mo-Kα (λ =
0.71073)
4.656
1760.0
Mo-Kα (λ
= 0.71073)
2θ range
for data
collection/°
4.08 to
55.836
4.14 to 55.824
-13≤ h ≤13,
-44≤ k ≤49, -
12≤ 1 ≤12
Index
ranges
-11≤ h ≤12, -49≤ k
≤49, -13≤ 1 ≤12
Reflectio
ns collected
Data/rest
raints/para
meters
63046
31917
16316/1/818
8500/0/406
Goodnes
s-of-fit on
F2
1.059
1.021
Final R
indexes [I>
R1 = 0.0348, wR2 =
0.0658
R1 =
0.0346, wR2 =

= 2σ (I)]
Final R
indexes [all
data]
0.0663
R1 =
0.0530, wR2 =
0.0709
R1 = 0.0405, wR2 =
0.0672
Largest
diff.
0.74/-0.46
1.83/-0.78
peak/hole /
e Å-3
ASSOCIATED CONTENT
Supporting Information
Copies of NMR spectra, GC chromatograms, GC/MS spectra, FT-IR spectra, DSC and TG/DTA thermograms, X-Ray
crystallography data, computational studies results
X-ray crystallography data of compounds I and II (CIF)
The supporting Information is available free of charge on the ACS publications website at DOI:
AUTHOR INFORMATION
Corresponding Author
*ꢀ (Mostafa Gholizadeh): m_gholizadeh@um.ac.ir
Acknowledgments
We are grateful for partial support of this work (Grant number 3/45675) by Ferdowsi University of Mashhad Research Council.
The authors would like to specially thank Petrochemical Research and Technology Co to provide DSC Thermograms, GC
Chromatograms and Mass spectra.
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For Table of Contents Only

Highlights
• Synthesis, crystal structure, Hirshfeld surface analysis, DFT
calculations and characterization of 1,3-
propanediylbis(triphenylphosphonium) monotribromide as
brominating agent of double bonds and phenolic rings
†
†,
†
‡
*
•
•
Seyed Reza Nokhbeh , Mostafa Gholizadeh , Alireza Salimi , Hazel A. Sparkes
†Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad,
Mashhad, I.R., Iran
•
‡School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
•
Two phosphonium salts with bromide and tribromide anions were synthesized and
characterized
.
•
•
Structures of these salts were characterized by X-ray crystallography and spectroscopic
methods.
Hirshfeld surfaces and 2D fingerprint plots were discussed.
•
Geometries and energies of ground state for these compounds were optimized by density
functional theory.
•
Brominations of double bonds and phenolic rings were carried out with the tribromide
salt.