Crystal structure, characterization, Hirshfeld surface analysis and DFT studies of two [propane 3-bromo-1-(triphenyl phosphonium)] cations containing bromide (I) and tribromide (II) anions: The anion (II) as a new brominating agent for unsaturated compounds

Article abstract of DOI:10.1016/j.molstruc.2019.05.127

In this study, propane 3-bromo-1- (triphenyl phosphonium) bromide, I, and propane 3-bromo-1- (triphenyl phosphonium) tribromide, II, (II as a new brominating agent) were synthesized and characterized by ¹H NMR, ¹³C NMR, ³¹P NMR, FT-IR, spectroscopy, Thermogravimetric Analysis, Differential thermal analysis, Differential scanning calorimetry and single crystal X-ray analysis. Density functional theory calculations (energy, structural optimization and frequencies, Natural Bond Orbital, absorption energy and binding energy) were performed by using B3LYP/6-311 G++ (d, p) level of theory. Hirshfeld surface analysis and fingerprint plots were utilized to investigate the role of bromide and tribromide anions on the crystal packing structures of title compounds. The results revealed that the change of accompanying anionic moiety can affect the directional interactions of C-H?Br hydrogen bonds between anionic and cationic units in which the H?Br with a proportion of 53.8% and 40.9% have the major contribution in the stabilization of crystal structures of I and II, respectively. Furthermore, the thermal stability of new brominating agent II with tribromide anion was compared with compound I with bromide anion. Nontoxicity, short reaction time, thermal stability, simple working up and high yield are some of the advantages of these salts.

Full text of DOI:10.1016/j.molstruc.2019.05.127

Accepted Manuscript
Crystal structure, characterization, Hirshfeld surface analysis and DFT studies of
two [propane 3-bromo-1-(triphenyl phosphonium)] cations containing bromide (I)
and tribromide (II) anions: The anion (II) as a new brominating agent for unsaturated
compounds
Seyed Reza Nokhbeh, Mostafa Gholizadeh, Alireza Salimi, Hazel A. Sparkes
PII:
S0022-2860(19)30699-4
DOI:
https://doi.org/10.1016/j.molstruc.2019.05.127
MOLSTR 26634
Reference:
To appear in: Journal of Molecular Structure
Received Date: 11 April 2019
Revised Date: 15 May 2019
Accepted Date: 29 May 2019
Please cite this article as: S.R. Nokhbeh, M. Gholizadeh, A. Salimi, H.A. Sparkes, Crystal structure,
characterization, Hirshfeld surface analysis and DFT studies of two [propane 3-bromo-1-(triphenyl
phosphonium)] cations containing bromide (I) and tribromide (II) anions: The anion (II) as a new
brominating agent for unsaturated compounds, Journal of Molecular Structure (2019), doi: https://
doi.org/10.1016/j.molstruc.2019.05.127.
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Crystal structure, characterization, Hirshfeld surface analysis and DFT studies of two
[Propane 3-bromo-1-(triphenyl phosphonium)] cations containing bromide (I) and
tribromide (II) anions: The anion (II) as a new brominating agent for unsaturated
compounds
Seyed Reza Nokhbeh1, Mostafa Gholizadeh1*, Alireza Salimi1, Hazel A. Sparkes2
1Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad I. R, Iran
2School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
ꢀ Corresponding Author (Mostafa Gholizadeh): m_gholizadeh@um.ac.ir
Abstract
In this study, propane 3-bromo-1- (triphenyl phosphonium) bromide, I, and propane 3-bromo-1- (triphenyl
1
phosphonium) tribromide, II, (II as a new brominating agent) were synthesized and characterized by H-
NMR, 13C-NMR, 31P-NMR, FT-IR, spectroscopy, Thermogravimetric Analysis, Differential thermal
analysis, Differential scanning calorimetry and single crystal X-ray analysis. Density functional theory
calculations (energy, structural optimization and frequencies, Natural Bond Orbital, absorption energy and
binding energy) were performed by using B3LYP/ 6-311 G++ (d, p) level of theory. Hirshfeld surface
analysis and fingerprint plots were utilized to investigate the role of bromide and tribromide anions on the
crystal packing structures of title compounds. The results revealed that the change of accompanying
anionic moiety can affect the directional interactions of C-HꢀꢀꢀBr hydrogen bonds between anionic and
cationic units in which the HꢀꢀꢀBr with a proportion of 53.8% and 40.9% have the major contribution in the
stabilization of crystal structures of I and II, respectively. Furthermore, the thermal stability of new
brominating agent II with tribromide anion was compared with compound I with bromide anion.
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Nontoxicity, short reaction time, thermal stability, simple working up and high yield are some of the
advantages of these salts.
Keywords: phosphonium tribromide, crystal structure, brominating agent, Hirshfeld surface analysis, DFT
calculations
1. Introduction
In recent decades quaternary phosphonium [1], ammonium [2, 3], pyridinium [4] and immidazolium [5]
salts are widely used as phase transfer catalysts [6-9] and reagents in the synthesis of organic compounds
[5]. Phosphonium salts, especially their ionic liquids [5, 10], play an important role in organic synthesis
[11-14], different fields of industries [15-18] and medicine [1, 8, 19]. These salts show a range of
interesting properties including ease of synthesis, very good thermal stability, favourable environmental
aspects, miscibility with water and polar organic solvents, electrochemical stability [20], high viscosity and
low vapor pressure. Because of their distinctive properties, these organic salts are attracting increasing
attention in many fields particularly in organic chemistry. Unlike inorganic reagents that are generally
insoluble in most organic solvents, this type of organic salts are completely soluble in common organic
solvents and therefore can be used successfully as surfactants for organic transformations.
Quaternary ammonium and phosphonium salts (R4N+-X or R4P+-X) are especially valuable because they are
somewhat soluble in both water and nonpolar organic solvents. They are used as phase transfer catalysts to
move ionic nucleophiles and bases in to organic media. A phase transfer catalyst facilitates reaction in
which one of the reactants is insoluble in aqueous solutions and another is insoluble in organic solutions.
The cation of phase transfer salt forms an ion pair with an anion, and the large alkyl or aryl groups in
ammonium or phosphonium ion lend solubility in organic phase. In the organic phase anion is more
reactive than in the aqueous phase because it is stripped of its solvating water molecules. Halogenating,
oxidative or reductive anions can be transferred in to the organic phases by the use of a suitable ammonium
or phosphonium cations as phase transfer catalysts. Indeed cation plays as a carrier to transfer anion to
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organic media. As the length of the alkyl groups increases, the water solubility decreases. Therefor we can
tune the solubility of salts with precisely selection of alkyl or aryl groups on the cation.
With a suitable choice from many different cations and related counterions, phosphonium salts can be
utilized as oxidative, reductive or halogenating agents with good selectivity, mild reaction conditions and
acceptable yields. Although bromination of organic substrates are usually carried out by molecular bromine
but the new generation of phosphonium and ammonium tribromides are synthesized and used for this goal
[2, 3, 10, 14, 21-27] so we prepared and used a new phosphonium tribromide for bromination of double
bonds. Tribromides are more suitable than the liquid bromine because of their crystalline nature, hence it
makes it easier to store, transport, and maintain the desired stoichiometry. Instead of bromination with non-
selective elemental bromine under harsh condition, organic phosphonium tribromides as well as
ammonium and immidazolium tribromides were used to brominate the C-C double bonds and aromatic
rings using mild conditions and in a selective manner [10, 26, 27]. The bromination of double bonds by
liquid bromine or by tribromide salts have a different mechanisms, thermodynamics and kinetics [28].
-
Although, bromination of cyclohexene by Br2 and Br3 in 1, 2-dichloroethane have large negative entropies
(ꢁS‡ = -62.0 eu. and ꢁS‡ = -40.9 eu. respectively), for bromination with Br2 there is a negative activation
-
parameters and enthalpy (Ea = -7.8 kcal/ mol, ꢁH‡ = -8.4 kcal/ mol), while Br3 has positive values (Ea =
+6.6 kcal/ mol, ꢁH‡ = +6.0 kcal/ mol). Thus third -order and second -order rate constants are obtained for
the reactions of free bromine and tribromide (respectively) in different solvents [28]. With respect to the
kinetic and thermodynamic evidence, belluchi proposed two different mechanisms for bromination with
Br2 and by tribromide anion.
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Scheme 1. Two different mechanisms for bromination of double bonds with Br2 and by tribromide anion.
Solid nontoxic phosphonium monocationic and dicationic moieties with tribromide anion including benzyl
triphenyl phosphonium tribromide (BTPPT) [21], ethyl triphenyl phosphonium tribromide (ETPPT) [25],
methyl triphenyl phosphonium tribromide (MTPPT) [23], tridecyl methyl triphenyl phosphonium
tribromide (TMTPPT) [24] and 1,2-ethylene bis (triphenyl phosphonium) ditribromide (EBTPPDT) [27],
have all been used as mild brominating and oxidizing agents for the selective bromination of C-C multiple
bonds and aromatic rings, these reactions are more favourable than using the very active, non–selective and
toxic molecular bromine under harsh conditions. Reusability of tribromide salts is one of the interesting
factors that predominate over molecular bromine as these salts can be used and recycled several times
without any significant decrease in their performances
.
The structural aspects of these salts were investigated by several experimental and theoretical methods
including X-ray crystallography and DFT calculations. Hirshfeld surface analysis was also employed. This
type of calculation is a convenient tool for the investigation of intermolecular interactions. Herein, we
13
31
report the synthesis, characterization (1H-NMR, C-NMR, P-NMR, FT-IR, and TG/DTA/DSC), single
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crystal X-ray, and DFT studies of propane 3-bromo-1-(triphenyl phosphonium) bromide, I, and propane 3-
bromo-1-(triphenyl phosphonium) tribromide, II, using mild, inexpensive and efficient brominating agents.
In addition, Hirshfeld surface and 2D fingerprint plots are presented to highlight short intermolecular
contacts in the crystal structures. In addition, the use of a new salt to brominate double bonds is presented
with excellent yields.
2. Results and discussion
Initially, propane 3-bromo-1-(triphenyl phosphonium) bromide, I, was synthesized as a white powder and
o
recrystallized in hot water (85-88% yield, m.p. 230-232 C) (Fig. 1), and then treated with potassium
tribromide in aqueous solution. Yellow precipitate propane 3-bromo-1-(triphenyl phosphonium)
tribromide, II, was formed, filtered and air dried overnight. DSC thermogram was taken (Fig. 2) (94-96 %
o
yield, m.p. 149-151 C) Scheme 1.
Fig. 1. DSC thermogram of compound I.
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Fig. 2. DSC thermogram of compound II.
Scheme 2. Schematic reaction for preparation of propane 3-bromo-1-(triphenyl phosphonium) bromide, I,
and tribromide, II.
2.1. Crystal structure description
The single crystal X-ray analysis of I revealed that this compound crystallizes in the monoclinic crystal
system in the P21/c space group with Z’=1, Z = 4. The asymmetric unit contains one propane 3-bromo-1-
(triphenyl phosphonium) cation and one bromide anion (1:1 ratio of cationic to anionic units). The ORTEP
view of compound I is shown in Fig. 3a. The phosphorous atom of the cation exhibit a slightly distorted
tetrahedral geometry, with the bond angles around the P atoms in the range from 107.23(7)o to 111.19(7)o.
The geometry of cation (bond lengths and angles) is general and comparable with our previously reported
[27]. The 3D supramolecular network of this compound is dominated by the variety of C-HꢀꢀꢀBr hydrogen
bonds. In addition, C-Hꢀꢀꢀπ interactions, which can play a crucial role in the stabilization of supramolecular
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assemblies, are found to be present (Fig. 3b). The bromide anions each formed eight C-HꢀꢀꢀBr hydrogen
bonds (Fig. 4).
Figs. 3. a) The ORTEP and b) molecular crystal packing diagrams for compound I, with 50% probability
displacement ellipsoids.
Fig. 4. C-HꢀꢀꢀBr contacts in I.
The compound II crystallizes in the monoclinic system with P21/c space group with Z’=1, Z = 4. The
asymmetric unit of this compound contains one propane 3-bromo-1-(triphenyl phosphonium) cation and
one tribromide anion. The ORTEP view of compound II is shown in Fig. 5a. Similar to I, the structure of
the cationic unit has a slightly distorted tetrahedral geometry at the phosphorous atom. The bond angles
around the P atom is in the range of 107.13(15)o to 111.90(15)o. The crystal packing of II shows a
significant number of C-HꢀꢀꢀBr hydrogen bonds which appear the dominant interaction in the stabilization
of supramolecular architecture (Fig. 5b). In addition, there is Brꢀꢀꢀπ interaction between the bromine atom
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of the cationic moiety and phenyl ring of adjacent molecule with 3.456(1) Å Br1 to C13-C18 plane
centroid and C21-Br1ꢀꢀꢀCg (Ph) angle of 22.433(2) ° (Fig. 6).
Figs. 5. a) The ORTEP and b) molecular crystal packing diagrams for compound II, with 50% probability
displacement ellipsoids.
Fig. 6. Brꢀꢀꢀπ interaction between bromine atom of cationic moiety and phenyl ring of the adjacent
molecule in compound II.
Comparing the structures of I and II indicates that the different anionic moiety, bromide or tribromide
affect the directional interactions between anionic and cationic units as would be expected and therefore
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different supramolecular aggregation are observed for these structures. In both structures, however, C-Hꢀꢀꢀπ
interactions which play an important role in their molecular aggregation are present.
2.2. Hirshfeld surface analysis
The Hirshfeld surface of a molecule in a crystal is manufactured by partitioning space in the crystal into
areas where the electron distribution of a sum of spherical atoms for the molecule (the promolecule)
dominates the corresponding sum over the crystal (the procrystal). Following Hirshfeld, a molecular weight
ꢁ ꢃ
function ꢀ ꢂ defined:
ꢄꢅꢆꢇꢈꢇꢉꢊꢋꢌꢉꢊꢁꢂꢃ
ꢄꢅꢆꢇꢋꢆꢍꢎꢏꢐꢉꢁꢂꢃ
ꢁ ꢃ
ꢀ ꢂ =
∑
ꢑꢒꢈꢇꢉꢊꢋꢌꢉꢊ ꢄꢑꢁꢂꢃ
ꢁ ꢃ
ꢀ ꢂ =
∑
ꢑꢒꢋꢆꢍꢎꢏꢐꢉ ꢄꢑꢁꢂꢃ
ρA(r) is a spherically averaged atomic electron density centered on nucleus A, and the promolecule and
procrystal are sums over the atoms belonging to the molecule and to the crystal, respectively.
The Hirshfeld surface is then defined in a crystal as that region around a molecule where w(r) ≥ 0.5. That
is, the region where the promolecule contribution to the procrystal electron density exceeds that from all
other molecules in the crystal. The surface shape explains the interactions between molecules in the crystal
just as atoms in the molecule. Hirshfeld surfaces including almost all of the existing space around
molecules
The function dnorm is relates the distances of any Hirshfeld surface point to the nearest nucleus interior (di)
or exterior (de) to the surface taking into account the van der Waals (vdW) radii of the atoms. dnorm is a
normalized contact distance, which is defined in turns of di, de and the van der Waals radii of the atoms
[32].
ꢓꢕ − ꢂꢖꢗꢘ ꢓꢊ − ꢂꢖꢗꢘ
ꢊ
ꢕ
ꢓꢔꢇꢆꢈ
=
+
ꢂꢖꢗꢘ
ꢂꢖꢗꢘ
ꢊ
ꢕ
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The negative value of dnorm indicates the sum of di and de is shorter than the sum of the relevant van der
Waals radii, which is considered to be a close contact and is visualized as red color on the Hirshfeld
Surfaces. The white color denotes intermolecular distances close to van der Waals contact with dnorm equal
to zero whereas contacts longer than the sum of van der Waals radii with positive dnorm values are colored
blue. The combination of de and di in the form of a 2D fingerprint plot provides summary of intermolecular
contacts in the crystal.
The 2D-fingerprint plots of the Hirshfeld surface provide a visual summary of the frequency of each
combination of de and di all over the surface of a molecule, therefore they not only indicate which
intermolecular interactions are present, but also the relative area of the surface corresponding to each kind
of interaction. In this manner, all interaction types (for example, 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.
These plots are unique for a certain crystal structure and polymorph.
The Hirshfeld surfaces and 2D fingerprint plots, calculated using Crystal Explorer 3.1 from the single
crystal structures, were used for visualizing, exploring and quantifying intermolecular interactions [29-31]
on the surface of molecule in the crystal lattice for both compounds.
Hirshfeld surfaces mapped with di ranging from 1.0095 A˚ (red) to 2.5396 A˚ (blue) for I (Fig. 7 aI) and
ranging from 1.0569 A˚ (red) to 2.6822 A˚ (blue) for II (Fig. 7 aII) and mapped with dnorm ranging from -
0.1259 A˚ (red) to 1.1196 A˚ (blue) for compound I (Fig. 7 bI) and ranging from -0.0863 A˚ (red) to 1.1740
A˚ (blue) for II (Fig. 7 bII) are calculated and plotted using Crystal Explorer software. Hirshfeld surface
area of compounds I and II mapped with dnorm show intense red spots on the surface near the Br ions which
are due to BrinsideꢀꢀꢀHoutside contacts and intense 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 (Figs. 7b).
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Figs. 7. Hirshfeld surfaces mapped with di for compound I (aI) and II (aII) and mapped with dnorm for I (bI)
and II (bII).
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. Plots of di versus de are 2D fingerprint plot
which recognizes the existence of different type of intermolecular interactions calculated for I and II (Figs.
8).
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Figs. 8. 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) BrꢀꢀꢀH/HꢀꢀꢀBr and f)
CꢀꢀꢀH/HꢀꢀꢀC for compound II cation.
The relative contribution of the different interactions to the Hirshfeld surface indicates that in the cationic
unit of compound I, HꢀꢀꢀH(53.8%), BrꢀꢀꢀH(17.6%) and CꢀꢀꢀH(24.8%) contacts account for about 96.2% of
the total Hirshfeld surface area and BrꢀꢀꢀC, BrꢀꢀꢀBr and CꢀꢀꢀC have very little effect on the crystal packing.
While in the cationic unit of II, the main contacts are BrꢀꢀꢀH(29.3%), HꢀꢀꢀH(40.9%) and CꢀꢀꢀH(24.7%)
accounting for about 94.9% of total surface area and other contacts have no major contributions to the total
Hirshfeld surfaces in the cation (Figs. 9).
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Contribution of short contacts for I
Contribution of short contacts for II
cation
cation
Br…H,
17.6,
18%
C…C, 0.4,
0%
Br…C,
3.3, 3%
Br…C,
4.4, 4%
Br…Br,
0.1, 0%
C…C, 0.5,
1%
C…H,
24.8,
25%
Br…H
H…H
C…H
Br…H
H…H
C…H
C…C
Br…C
Br…H,
29.3, 29%
C…H,
24.7, 25%
C…C
Br…C
Br…Br
H…H,
53.8,
54%
H…H,
40.9, 41%
Figs. 9. Percentage contribution of short contacts in compound I and II cations.
The analysis of the 2D fingerprint plot of the cation I shows that BrꢀꢀꢀH and CꢀꢀꢀH contacts are illustrated as
characteristic wings of the plot, although the most surface area relates to HꢀꢀꢀH contacts with 53.8% of total
surfaces. Moreover, in cation II the BrꢀꢀꢀH and CꢀꢀꢀH contacts are illustrated as characteristic wings of the
plot, while HꢀꢀꢀH contacts with 40.9 % of total surfaces.
It is noticeable in 2D fingerprint plots that the most important difference between the intermolecular
interactions in I and II is the percentage of BrꢀꢀꢀH and HꢀꢀꢀH interactions.
The triphenyl phosphonium cations of I and II are connected by BrꢀꢀꢀH interactions between nearest
neighbors in the unit cell. Nearest intermolecular interactions in I are Br2ꢀꢀꢀH19B in 2.734 Å, Br2ꢀꢀꢀH21B in
2.951 Å and Br2ꢀꢀꢀH12 in 3.309 Å (Fig. 10a). Nearest intermolecular interactions in II are Br4ꢀꢀꢀH14 in 2.82
Å, Br3ꢀꢀꢀH6 in 3.052 Å and Br4ꢀꢀꢀH15 in 3.222 Å (Fig. 10b).
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Figs. 10. a) Nearest close contacts in I and b) nearest close contacts in II.
Intra and intermolecular close contacts (up to 3.5 Å) for I and II calculated by Crystal Explorer and
Mercury software packages and are tabulated in Table S4 in supporting information.
Moreover, the quantitative measurements of Hirshfeld surfaces for compounds I and II show that the
molecular volume, surface area and asphericity of II are bigger than I because of bigger tribromide anion
and the globularity of both compounds are nearly same and the comparison of bare cations shows that
although the chemical structures and atomic compositions are similar, the tribromide anion enabled an
increase in the molecular volume and surface area and decrease in the globularity and asphericity (Table 1).
In other words, comparing the tribromide anion to the bromide anion could pull the cation toward itself and
cause an increase in the molecular volume and surface area of cation.
Table 1. Quantitative measures of Hirshfeld surfaces for compound I and II and their cations.
Quantitative measures molecular volume surface area
globularity
(G)
asphericity
):
of Hirshfeld surfaces
Compound I
(VH) Å3
460.64
542.97
421.75
(SH) Å2
398.23
442.51
364.06
(Ω
0.724
0.007
0.016
0.013
Compound II
0.727
Compound I cation
0.747
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Compound II cation
426.94
372.24
0.737
0.005
2.3.Computation studies
The DFT calculations were performed by using the Gaussian software package, at the B3LYP level with 6-
311G++ (d, p) basis sets for both compounds I and II. The bond lengths, bond angles and dihedral angles
were compared with those obtained from X-ray crystallographic data. The calculation results showed a
satisfactory correlation between the theoretical and experimental structural parameters of cations.
Significant differences in the optimized DFT and experimental XRD geometries were observed in the
location of anions in I and II. In compound I, DFT calculation optimized Br- anion at the back of molecule
and far from Br1 (dBr1-Br2 = 9.479 Å) while crystallography data shows that the anion is located at front of
-
molecule near to Br1 (dBr1-Br2 = 5.368 Å). While in compound II, although DFT predicted that Br3 anion is
-
located at front of molecule near to Br1 (dBr1-Br3 = 5.185 Å, Br3 is the atom center of Br3 anion), the
experimental results show that the anion is located at back of molecule and far from Br1 (dBr1-Br3 = 9.366
Å) (Table S6 in supporting information).
Vibrational frequencies of title compounds were calculated by B3LYP/6-311++ (d, p) method. There are
129 vibrational modes for I. The high intensity frequencies of I was calculated at 2979.79 and 3008.95 cm-1
which were assigned to asymmetric stretching of C-H in aromatic rings, Moreover, the vibrational modes
at 1468.47, 1284.67, 741.27 and 536.23 cm-1 were assigned to symmetric scissoring, symmetric wagging,
rotational vibration of C-C bonds and stretching of the P-Carom bond, respectively. For II, 135 vibrational
modes were calculated by B3LYP/6-311++ (d, p) method. High intensity frequencies calculated at 2968.31
cm-1 for the symmetric stretching of C-H in propane chain, 3197.54 cm-1 for symmetric stretching of Carom
H, Moreover, vibrational modes at 1117.47, 737.84 and 197.67 cm-1 were identified as stretching vibration
-
of P-Carom
,
in plane rocking vibration of C-H and symmetric stretching of Br-Br in tribromide anion
respectively
.
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Natural charge distribution on the whole molecules of I and II were calculated by NBO method Calculation
indicated that C and Br atoms have a negative character, while P and H atoms have positive ones (Tables
S1,S2 in supporting information). HOMO and LUMO are most important orbitals in chemical stability and
reactivity of organics. The energy of the HOMO orbitals shows a tendency to donate an electron as a donor
in chemical transformations while the energy of LUMO orbitals represent ability to accept an electron as
acceptor. HOMO-LUMO energies were calculated by B3LYP/ 6-311++g (d) presented in Figs. 11 and 12
(Table S3 in supporting information).
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Fig. 11. HOMO-LUMO diagram and energy levels for I.
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Fig. 12. HOMO-LUMO diagram and energy levels for II.
In addition, binding energies and absorption energies were calculated. The geometry optimizations were
performed by B3LYP method which is known to give reliable data on both cations and anions. The
standard basis set was 6-31/G* which was employed for all of the ions. The energetic results were obtained
by single-point calculations at a higher level of theory, being B3LYP/6-311/G* based on B3LYP/6-31/G*
geometries. Binding energies between cation and anion determined as differences between the sum of the
energies of each unoptimized bare anion and cation separately (after disconnection) and the energy of
optimized ion pair [33, 34]. It is noticeable that anion and cation optimized as an ion pair and that no
optimization is needed after disconnection.
Binding Energy of a system consist of ion pairs
AC
A- + C+
∆
EB = (EAnion-unopt +ECation-unopt) - EIon pair-opt
And absorption energy is defined as differences between the energy of optimized ion pair and sum of
energies of each optimized bare anion and cation before pairing:
Absorption Energy of a system consist of ion pairs
A- + C+
AC
∆
EAbs = EIon pair-opt - (EAnion-opt +ECation-opt
)
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The comparison of the absorption energy and binding energy in compounds I and II shows that Br- anion
-
can affect only 0.66 Kcal/mol on the deformation of cation whiles the Br3 cause deformation of anion
about 2.69 Kcal/mol (Table S5 in supporting information).
2.4. Structural characterization
2.4.1. Thermal properties of title salts I and II
DSC thermograms were collected under a nitrogen atmosphere and, show a sharp endothermic peak
appearing at 230-232 oC due to the melting and a broad peak at 330-380 oC as the decomposition range of I
o
(Fig. 1). In the case of II, there is a sharp endothermic peak at 149-151 C due to the melting and a broad
exothermic peak at 310-370 oC the decomposition range for II (Fig. 2).
o
The TG/DTG /DTA diagram of I shows an endothermic peak at 230 C for melting point in DTA curve
with the smallest decrease in TG curve (about 1.4%) and immediately after that an endothermic shoulder
and an exothermic peak with a significant decrease in TG curve for phase transformation in the molecule.
o
Finally, there exist a broad endothermic peak about 330-370 C in DTA curve and a sharp decrease in TG
o
Curve and very broad peak in DTG curve for the decomposition of I and 2.79% remained ash in 520 C
(Fig. 13).
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Fig. 13. TG/DTG/DTA thermogram of compound I.
o
Also for II, thediagram shows an endothermic sharp peak at about 150 C in DTA curve and a smallest
decrease in TG curve (0.9%) for melting point. The broad endothermic peak in the range of 200-250 oC in
the DTA curve and sharp peaks in DTG curve for removal of bromine molecule from tribromide anion
were determined about 16%. An exothermic peak for phase transformation and immediately increase the
baseline in the range of 275 -350 oC in DTA with a sharp decrease in TG curve was observed. In addition,
o
and broad peak in DTG curve for the decomposition of compound II and about 5 % at 520 C for ash was
observable which are completely matched with DSC thermograms (Fig. 14). Both salts have a good
o
thermal stability; they start to decomposing over 300 C. Compound I is slightly more stable than II in
comparison and they are very stable on the bench-top in form of crystal or solution in the organic solvent
and in contact with air without any change in their color or performance over 3 month.
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Fig. 14. TG/DTG/DTA thermogram of compound II.
3. Experimental section
All reagents including triphenyl phosphine, liquid bromine, 1,3-dibromopropane, potassium bromide and
solvents were purchased from Merck Co. 1,3-dibromopropane was purified by distillation and tested by GC
to give purity more than 99.5 %. Solid reagent and salts were recrystallized to give very good purity.
1
The reaction progresses were monitored by GC. H-NMR, 13C-NMR, 31P-NMR spectra were identified by
the Bruker AC 500 MHz spectrometer with D2O and CDCl3 as solvents. DSC thermograms recorded by
METTLER - TOLEDO DSC-1 Instrument. Infrared spectra were recorded on a Perkin Elmer –Spectrum
65-FT-IR spectrometer as a KBr disk (4000- 400 cm-1 region). GLC analysis was performed with Agilent
Technology 6890N Gas chromatograph with a FID detector and cp-sil 5 CB 30 m, 0.32 mm, 1.2µm
capillary column. GC/MS spectra were taken 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, 0.25µm capillary column.
3.1. Preparation of Propane 3-bromo-1-(triphenyl phosphonium) bromide (C21H21PBr2), I
To a solution of triphenylphosphine (5.24 g, 20 mmol) in toluene (30 ml) in a 50 ml round –bottom flask
equipped with a magnetic stirrer and reflux condenser was added 1,3-dibromopropane (3 ml, 30 mmol, d =
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1.989 g/cm3) drop wise by a 2000 µl syringe. Reaction mixture was refluxed for 4 h. After complete
reaction, the white precipitate was appeared. The reaction mixture was cooled and the product was filtered
and washed with toluene (3 × 10 ml). The white powder was air dried overnight and recrystallized in water
1
as needle shape crystals (5.57 g, 60% yield). H-NMR (500 MHz, D2O): (1C, CH2) δ=3.75ppm (m, 2H),
(2C, CH2) δ=2.15 ppm (m, 4H) (3C, CH2) δ=3.96 ppm (m, 2H), (aromatic rings hydrogen’s) δ=7.64,
7.75(m, 15H). FT-IR: υ = 505,537(s), 685, 723(s), 993(m), 1108(s), 1190, 1328(m), 1432(s), 1482,
1582(m), 2801, 2865(m), 3009(m) cm-1.
3.2. Preparation of Propane 3-bromo-1-(triphenyl phosphonium) tribromide (C21H21PBr4), II
To a solution of KBr (2.38 g, 20 mmol) in H2O (30 ml) in a 50 ml beaker, liquid bromine was added (1.24
ml, 20 mmol) drop wise with continuous magnetic stirring. The bromine layer disappeared after 30 min.
The produced KBr3 solution then added to a solution of propane 3-bromo-1-(triphenyl phosphonium)
bromide I (4.176 g, 9 mmol) in water in 2 min. 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 CHCl3 (10.5 g, 94%), m.p. 150oC (by DSC).
1H-NMR (500 MHz, CDCl3): (1C, CH2) δ=3.58 ppm (m, 2H), (2C, CH2) δ=2.26 (m, 2H), (3C, CH2)
δ=3.62 (m, 2H), (aromatic rings hydrogen’s) δ=7.76, 7.87(m, 15H). 13C-NMR (125 MHz, CDCl3): δ=21.8-
22.2(m, C20), δ=26.1(m, C21), δ=33.1- 33.3(m, C19), δ=76.7- 77.2 triplet for CDCl3 (solvent), δ=117,
130, 133, and 135 (Benzene rings carbons), 31P-NMR (200MHz, CDCl3): one strong peak at 35.1 ppm for
1
P atom. H-NMR spectra of two compounds was taken by 500 MHz spectrometer that shows four
distinctive peaks for carbons 1, 2, 3 and aromatic rings in ratio 2:2:2:15 respectively. Exchange of anion
from bromide to tribromide makes very small shifts in peaks (between 0.2- 0.3 ppm).
3.3. General Experimental procedure for bromination of alkenes
In a typical reaction, the alkene (3mmol) was dissolved in dichloromethane (5mL) and stirred well for 2
min. Propane 3-bromo-1-(triphenyl phosphonium) tribromide II (3mmol) dissolved in dichloromethane (5
22

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mL) and was added to alkene solution drop wise with constant stirring at room temperature. The progress
of the reaction was monitored by TLC and GC. After completion of the reaction and disappearance of the
yellow-orange color of reagent II, the solvent evaporated and diethyl ether was added (3×5 ml). The
mixture was filtered and the solvent evaporated. The crude product thus obtained and then subjected to a
short column of silica gel using a mixture of n-hexane and ethyl acetate (8:2) as the eluent. All of the
isolated products are known and physical data have been reported in the literature. The main products,
reaction times and isolated yields are tabulated in Table 3. To confirm, the products were subjected to DSC
and GC/MS and spectra were compared and validated with the NIST library (see supporting information).
The reusability or recyclability of tribromide salt was tested by regenerating of used brominating salt. The
used salt was extracted by water and evaporated under vacuum. The bromide salt, I, was treated with KBr3
and reused 4 times without significant loss of its performance.
Table 3. Bromination of alkenes by compound II in CH2Cl2 at room temperature
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4. 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 ( = 0.71073 Å). Data collections were performed using a CCD area detector from a single crystal
λ
mounted on a glass fibre. Intensities were integrated in SAINT [35] and absorption corrections based on equivalent
reflections were applied using SADABS [36]. Both of the structures, I and II were solved using ShelXT [37] and
refined by full matrix least squares against F2 in ShelXL [38, 39], using Olex2 [40]. All of the non-hydrogen atoms
were refined anisotropically. While all of the hydrogen atoms were located geometrically and refined using a riding
model. In the case of II the data were found to be twinned and refined against an hklf5 file, with a refined twin scale
fraction of 01247(14). The structural resolution procedure was performed using WinGX crystallographic
24

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software package [41]. Crystal structure and refinement data are given in Table 4. CCDC1907510-1907511
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 4. Crystal data and structure refinement for I and II.
Identification code
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
I
II
C21H21Br2P
464.17
100(2)
monoclinic
P21/c
11.0198(2)
10.0935(2)
17.4446(3)
90
C21H21Br4P
623.99
100(2)
monoclinic
P21/c
9.7192(6)
17.1447(11)
13.2553(9)
90
β/°
γ/°
104.7380(10)
90
1876.50(6)
4
1.643
4.405
92.770(4)
90
2206.2(2)
4
1.879
7.373
Volume/Å3
Z
ρcalcg/cm3
ꢂ/mm-1
F(000)
928.0
0.764 × 0.429 × 0.276
MoKα (λ = 0.71073)
1208.0
Crystal size/mm3
Radiation
0.421 × 0.299 × 0.163
MoKα (λ = 0.71073)
3.886 to 55.898
-12 ≤ h ≤ 12,
-22 ≤ k ≤ 22,
0 ≤ l ≤ 17
5266
Rsigma = 0.0452
5266/6/236
1.025
R1 = 0.0347,
wR2 = 0.0671
R1 = 0.0492,
wR2 = 0.0711
0.69/-0.95
2θ range for data collection/° 3.822 to 55.856
Index ranges
-14 ≤ h ≤ 14,
-13 ≤ k ≤ 7,
-23 ≤ l ≤ 22
16880
Rsigma = 0.0210
4491/0/217
1.023
R1 = 0.0200,
wR2 = 0.0437
R1 = 0.0246,
wR2 = 0.0451
Reflections collected
Rsigma
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3 0.42/-0.53
5. Conclusion
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In this study, the synthesis, characterization and uses of a mild and safe mono cationic phosphonium salt
for bromination of the organic substrate are reported. The bromination agent in title salts also has been used
as mild brominating and oxidizing agent to selective bromination of C-C double bonds in comparison with
very active, non–selective and toxic molecular bromine in very harsh conditions. Reusability or
recoverability of tribromide salts is one of the interesting factors that predominate over molecular bromine
as these salts can be used and recycled several times without any significant decrease in their
performances. Ease of getting bromine in aqueous solutions and release of bromine in organic solvents is
another important aspect of phosphonium based tribromide reagents.
Also in the present paper, the crystal structure, the DFT calculations, analysis of Hirshfeld surfaces and
fingerprint plots, as well as spectroscopic properties and thermal behavior of the title salts are reported.
Acknowledgments
The author is grateful for partial support of this work (grant number 3/45675) by Ferdowsi University of
Mashhad Research Council. The author would like to especially thank Petrochemical Research and
Technology Co to provide DSC Thermograms, GC chromatograms and mass spectra.
We hereby acknowledge that part of this computation was performed on the HPC (High Performance
Computing) center of Ferdowsi University of Mashhad.
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