Characterizing the thermal degradation mechanism of two bisphosphoramidates by TGA, DSC, mass spectrometry and first-principle theoretical protocols

Article abstract of DOI:10.1016/j.molstruc.2020.128781

The present investigation describes a combined experimental-theoretical strategy to assess the thermal resistance features of two symmetric bisphosphoramidates, tetraphenyl ethane-1,2-diylbis (phosphoramidate) 1 and tetraphenyl propane-1,3-diylbis (phosphoramidate) 5. Therefore, their structural reluctance to thermal decomposition through differential scan calorimetric (DSC) and thermogravimetric (TGA) experiments was evaluated. Then, their molecular degradation path was followed by analysing recorded data from mass spectrometry measurements performed at different temperature conditions. Their corresponding thermal degradation mechanism was then established by searching plausible transition states interconnecting the intermediaries found in our mass spectrometry records using a quantum theoretical protocol based on Coupled-Cluster calculations. Through this strategy, key intermediaries of the two bisphosphoramidates studied during their molecular degradation mechanism were identified, although compound 5 displayed the highest resistance to heat decomposition.

Full text of DOI:10.1016/j.molstruc.2020.128781

Journal Pre-proof
Characterizing the thermal degradation mechanism of two bisphosphoramidates by
TGA, DSC, mass spectrometry and first-principle theoretical protocols
José Luis Castrejón-Flores, Julieta Reyna-Luna, Yazmin M. Flores-Martinez, María
Isabel García-Ventura, Angel Zamudio-Medina, Marco Franco-Pérez
PII:
S0022-2860(20)31106-6
DOI:
https://doi.org/10.1016/j.molstruc.2020.128781
MOLSTR 128781
Reference:
To appear in: Journal of Molecular Structure
Received Date: 27 March 2020
Revised Date: 15 June 2020
Accepted Date: 26 June 2020
Please cite this article as: José.Luis. Castrejón-Flores, J. Reyna-Luna, Y.M. Flores-Martinez,
Marí.Isabel. García-Ventura, A. Zamudio-Medina, M. Franco-Pérez, Characterizing the thermal
degradation mechanism of two bisphosphoramidates by TGA, DSC, mass spectrometry and first-
principle theoretical protocols, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2020.128781.
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CRediT author statement.
José Luis Castrejón-Flores: Conceptualization, writing-Original Draft preparation.
Julieta Reyna-Luna: Writing-Original Draft preparation, Data Curation, Methodology,
Formal Analysis
Yazmin M. Flores-Martinez: Investigation, Visualization
María Isabel García-Ventura: Conceptualization, Investigation
Angel Zamudio-Medina: Methodology, Supervision, Project Administration, Funding
acquisition, Writing-Reviewing & Editing.
Marco Franco-Pérez: Methodology, Supervision, Project Administration, Funding
acquisition, Writing-Reviewing & Editing.

Characterizing the thermal degradation mechanism of two
bisphosphoramidates by TGA, DSC, mass spectrometry and first-
principle theoretical protocols.
José Luis Castrejón-Flores,a Julieta Reyna-Luna,c Yazmin M. Flores-Martinez,b María
Isabel García-Ventura,a Angel Zamudio-Medinaa* and Marco Franco-Pérezc*
a
Instituto Politécnico Nacional, Unidad Profesional Interdisciplinaria de Biotecnología, Av.
Acueducto s/n Barrio la Laguna Ticomán, CDMX. 07340.
b
Programa Institucional de Biomedicina Molecular. Escuela Nacional de Medicina y Homeopatía.
Instituto Politécnico Nacional, Guillermo Massieu Helguera 239, CDMX, 07320.
c
Departamento de Física y Química Teórica, Facultad de Química, Universidad Nacional
Autónoma de México, Ciudad Universitaria, CP 04510 México, Ciudad de México, México.
*Correspondence: jazamudiom@ipn.mx (A.Z); qimfranco@hotmail.com (M.F.-P)
1

Abstract
The present investigation describes a combined experimental-theoretical strategy to
assess the thermal resistance features of two symmetric bisphosphoramidates, tetraphenyl
ethane-1,2-diylbis (phosphoramidate) and tetraphenyl propane-1,3-diylbis
1
(phosphoramidate) 5. Therefore, their structural reluctance to thermal decomposition
through differential scan calorimetric (DSC) and thermogravimetric (TGA) experiments
was evaluated. Then, their molecular degradation path was followed by analysing recorded
data from mass spectrometry measurements performed at different temperature conditions.
Their corresponding thermal degradation mechanism was then established by searching
plausible transition states interconnecting the intermediaries found in our mass
spectrometry records using a quantum theoretical protocol based on Coupled-Cluster
calculations.
Through this strategy, key intermediaries of the two bisphosphoramidates studied during
their molecular degradation mechanism were identified, although compound 5 displayed
the highest resistance to heat decomposition.
Keywords: Bisphosphoramidate, flame-retardant, thermal decomposition.
2

1. Introduction
Bisphosphoramidates are an organophosphorus family of compounds constituted by
oxygen-phosphorus-nitrogen P(O)-N bonds with broad applications in several fields of
sciences and engineering, including molecular biology and materials design. [1-4] For
instance, phosphoramidates are widely used as building blocks of synthetic DNA
oligonucleotides and some of their derivatives are known inhibitors of urease and
cholinesterase enzymes with a low toxicity profile. [1, 5-12] One important characteristic of
bisphosphoramidates, making them suitable for material design, is their resistant to thermal
decomposition, so they are commonly used as flame-retardant additives to different
materials including epoxy resins, [13-15] polyurethane, [16-20] fabrics [21-23]
polycarbonates [24-25], among others. Importantly, the thermal decomposition does not
involve the formation of toxic adducts, as those observed during the thermal degradation of
traditional halogenated aromatic compounds. [26] The vast majority of organophosphurous
flame-retardants are structurally constituted by Nitrogen-Phosphorus (N-P) or Oxygen-
Phosphorus (O-P) bonds [27-40], and it has been demonstrated that organic compounds
constituted by N-P bonds display a higher resistance to heat decomposition [41] when
compared with (O-P) bonds. [42-51] Clearly, the thermal stability of organophosphurous
flame-retardants is related to the N-P and N-O bond strength, where the former displays a
considerably higher dissociation energy (671 kJ·mol-1) versus (301 kJ·mol-1). [52-55]
Previous investigation using bisphosphoramidates 1 and 2 (Figure 1) shown that they are
more resistant to heat decomposition than the bis(diphenylphosphates) derivatives 3 and 4.
[31] It has been suggested that the phosphoramide N-P bond stability is enhanced by the
degree of substitution of the amine group. [56-59] Thus, theoretically, better flame-
retardants might be found by chemically functionalized the amine moiety.
3

Figure 1. Chemical structure of different bisphosphoramidates flame-retardant candidates.
Despite their well-documented efficacy as flame-retardant additives, the thermal
degradation mechanism of bisphosphoramidates is not clear and is focus of current
investigations. Gaan et. al. suggested that such degradation mechanism proceeds by a Cope
syn rearrangement or a Michael reverse mechanism, implying the heterolytic breakdown of
the N-P and/or the O-P bonds. [18] Moreover, using room temperature mass spectrometry
records, Davidowitz et al. suggested that N-P and O-P bonds in phosphate ester (P(O)OR)
and phosphoramidate (P(O)NR'R') dissociate in a homolytic fashion. [11]
Therefore, in the present investigation, we described the thermal decomposition mechanism
of the two bisphosphoramidates 1 and 5 (Figure 2). Compound 1 was synthesized by
Nguyen in 2008 [44], while compound 5 was recently synthetized by our research group.
[60] These two chemical species own symmetrical molecules which only differs on the size
of the alkyl chain interconnecting the phosphoramidate moieties, where compound 1
displays an alkyl chain constituted by two carbons while compound 5 has an alkyl chain
constituted by three carbons. Unlike other bisphosphoramidates commonly used as flame-
retardant additives, our compounds are not constituted by a central aromatic ring, which
allegedly, enhance the flame- retardant properties of symmetrical phosphoramidates. [31]
By TGA and DSC experiments, we monitored the thermal degradation process of both
compounds and the decomposition intermediaries were identified through mass
spectrometry experiments at different temperature conditions. Moreover, the transition
states interconnecting these intermediaries were established following a highly accurate
quantum theoretical protocol. Thus, the combined experimental-theoretical strategy was
used to propose a plausible thermal degradation mechanism of compounds 1 and 5.
4

Figure 2. Chemical structure of bisphosphoramidates 1 and 5.
2. Methods
2.1 Experimental procedures.
2.1.1 General synthesis and characterization procedures for bisphosphoramidates 1 and 5
All reagents were purchased from Sigma-Aldrich and used as received. The molecular
1
structure of the synthetized compounds was corroborated by H, 13C and 31P Nuclear
Magnetic Resonance (NMR) experiments using a Bruker BioSpin GmbH NMR-
spectrometer operating at 400 MHz for 1H, 100 MHz for 13C and 161.98 MHz for 31P, with
a total 5 mm total body irradiation probe. Deuterated chloroform (CDCl3) was used as
solvent in all these experiments. Chemical shifts (δ) are reported in parts per million.
Multiplicities were recorded as: singlet (s), doublet (d), triplet (t), doublet of doublets (dd),
doublet triplet (dt) and multiplet (m). Coupling constants (J) are given in Hz.
Tetraphenyl ethane-1,2-diylbis(phosphoramidate) (1)
To a solution of ethylenediamine (1mmol) in CH2Cl2, we poured 2 ml of triethylamine and
left stirring for 30 min. Then, diphenyl phosphoryl chloride (2.1 mmol) was added
dropwise. The resulting solution was left in agitation overnight. Once the reaction was
completed, all the volatiles were removed by vacuum. The crude product was purified by
flash column chromatography (Hexane /AcOEt 1:1) obtaining 0.4 g (81%) of 1 as solid
1
white; Rf = 0.43; mp: 134-135 °C. H NMR (400 MHz, CDCl3, 25 °C, TMS, ppm): δ =
3.09 (dd, JHH= 6.4 Hz, JPNCH = 6.8 Hz, 4H, -CH2NH), 4.33 (dt, JPNH = 6 Hz, 2H, NH), 7.10-
5

7.28 (m, 20H). 13C NMR (100 MHz, CDCl3, 25 °C, TMS, ppm): δ = 42.95 (d, JPC = 5.3 Hz,
2C, -CH2NH-), 120.20 (s, 8Cortho), 125.01 (s, 4Cpara), 129.72 (s, 8Cmeta), 150.67 (d, JPC = 6.8
Hz, 4Cipso). 31P NMR (161.98 MHz, CDCl3, TMS, ppm): δ = -0.41 (s). HRMS (DART)
C26H27N2O6P2 [M+H]+ calc. 525.1344, found 525.1361.
Tetraphenyl propane-1,3-diylbis(phosphoramidate) (5)
To a solution of 1,3-diaminopropane (1mmol) in CH2Cl2, we poured triethylamine
(2ml) and left stirring for 30 min. Then diphenyl phosphoryl chloride (2.1 mmol) was
added dropwise. The resulting solution was left in agitation overnight. Once the reaction
was completed, all the volatiles were removed by vacuum. The crude product was purified
by flash column chromatography (Hexane /AcOEt 1:1) obtaining 0.5 g (75%) of 5 as solid
white; Rf = 0.21; mp 85-86 °C. 1H NMR (400 MHz, CDCl3, 25 °C, TMS, ppm): δ = 1.49-
1.57 (m, 2H, - CH2CH2CH2-), 3.03 (dd, JHH= 6 Hz, JPNCH = 6.8 Hz, 4H, -CH2NH), 3.80 (dt,
JPNH = 6.4 Hz, 2H, NH), 7.09-7.31 (m, 20H). 13C NMR (100 MHz, CDCl3, 25 °C, TMS,
ppm): δ = 32.47 (s, 1C, - CH2CH2CH2-), 38.06 (s, 2C, -CH2NH), 120.20 (s, 8Cortho), 124.96
(s, 4Cpara), 129.70 (s, 8Cmeta), 150.79 (d, JPC = 6.7 Hz, 4Cipso). 31P NMR (161.98 MHz,
CDCl3): δ = 0.02 (s). HRMS (DART) C27H29N2O6P2 [M+H]+ calc. 539.1422, found
539.1503
2.1.2 Thermal analysis experiments
TGA and DSC experiments were carried out using a Netzsch STA 449 F3 Jupiter
thermogravimetric analyzer, at a heating rate of 10°C/min, from 323.15 K (50 °C) to
773.15 K (500 °C), under nitrogen atmosphere. Our measurements were performed at room
temperature (records are available in the supporting information file). High-resolution mass
spectra were recorded on a JEOL AccuTOF JMS-T100LC mass spectrometer with TOF
mass analyzer at different temperature conditions, using a muffle brand Felisa model FE-
360.
2.2 Computational details.
Gas phase geometry optimizations and vibrational frequencies calculations were carried out
using the B3LYP [61] density functional approximation conjointly with the 6-311++G(d,p)
basis set. [62] Local minimal and transition states were identified by the computed number
of imaginary frequencies (0 or 1, respectively). Vibrational normal modes corresponding to
6

each transition state were visually inspected, corroborating that the imaginary frequency
corresponded to the expected motion along the reaction coordinate. Intrinsic reaction
coordinate (IRC) calculations were then performed to confirm that the located transitions
states properly connect the intended reactants and products. Thermal corrections needed for
the computations of gas phase Gibbs free energies were obtained at 523.15 K (250 °C).
Cartesian coordinates corresponding to the transition states found in this study are available
in the supporting information file. All our DFT calculations were performed with the
package of programs Gaussian 09. [63]
Single point electronic energy calculations were computed at the sophisticated DLNPO-
CCSD [64] approximation conjointly with the aug-cc-pVDZ basis set, over geometries
previously optimized at the B3LYP/6-311++G(d,p) level. Gas phase Enthalpy and Gibbs
free energy corresponding to a particular chemical species was obtained as the sum of the
electronic energy computed at the DLNPO-CCSD/aug-cc-pVDZ level and the proper
thermal correction computed through the B3LYP/6-311++G(d,p) protocol. Coupled-Cluster
calculations were done using the ORCA computational package. [65]
3. Results and discussion
3.1 Thermogravimetric analysis.
The thermal analysis profiles of the two flame retardant candidates under study are
depicted in Figure 3. Both bisphosphoramidates, 1 and 5, exhibit a solid-liquid phase
transition, which occurs at 407.8 K (134.6 °C) and 356.5 K (83.3 °C), respectively. It can
be observed that compound 5 keeps its structural integrity at higher temperatures than
compound 1. Thermal decomposition of compound 1 begins around 523.15 K (250 °C),
while compound 5 is stable at temperatures close to 573.15 K (300 °C) both in liquid state.
Both bisphosphoramidates, 1 and 5, were completely degraded to a solid carbonized state at
723.15 K (450 °C). Thus, according with the experimental evidence bisphosphoramidate 5
is more resistant to thermal degradation than bisphosphoramidate 1.
7

a)
DSC /(mW/mg)
Exo
120
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
TG /%
Peak at 356.4 °C
100
80
60
40
20
0
Peak at 134.6 °C
Peak at 379.6 °C
20
70
120
170
220
270
320
370
420
470
Temperature /°C
b)
Exo
TG /%
DSC /(mW/mg)
120
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
Peak at 83.3 °C
100
80
60
40
20
0
Peak at 371.3 °C
20
70
120
170
220
270
320
370
420
470
Temperature /°C
Figure 3. TGA-DSC spectra of a) bisphosphoramidate 1 and b) bisphosphoramidate 5.
8

3.2 Detection and characterization of the intermediaries formed during thermal
degradation.
Intermediaries involved in the thermal decomposition process of bisphosphoramidate 1
were identified by measuring the mass spectrum of calcined (solid) samples at two
temperature conditions of 473.15 K (200 °C) and 573.15 K (250 °C) (Molecular peaks and
relative abundances are collected in the supporting information file). A total of four and
five molecular peaks were detected at the different temperature conditions, respectively
(see Table 1). However, three molecular peaks, 292 g·mol-1, 326 g·mol-1 and 524 g·mol-1,
named diphenyl (2-aminoethyl) phosphoramidate, triphenyl phosphate and tetraphenyl
ethane -1,2-diylbis (phosphoramidate), respectively, were detected at both temperatures.
When analyzing the molecular peaks detected during thermal decomposition of compound
5 at 523.15 K (250 °C) and 573.15 K (300 °C), four and three peaks are seen, respectively
(Table 1). Contrarily to compound 1, there are not similar intermediaries found during
degradation between both temperatures tested, although two intermediaries, 326 g·mol-1
and 343 g·mol-1, named triphenyl phosphate and hydroxytriphenoxy-phosphanamine,
respectively, were both detected in compounds 1 and 5 at the maximal temperature tested.
Thus, the present of common intermediaries during thermal degradation suggest a similar
decomposition mechanism for both compounds.
Table 1. Molecular peaks detected by mass spectrometry during thermal degradation of
Compounds Temperature (K)
Mass (g·mol-1)
430
326 343
1
473.15
523.15
523.15
573.15
--
524
292 326
249
524
292
5
94 212 306 538 --
250 -- --
326 343
bisphosphoramidates 1 and 5.
9

3.3 Generalized degradation mechanism proposal for bisphosphoramidates 1 and 5.
The thermal degradation mechanism corresponding to bisphosphoramidates 1 and 5 was
systematically revealed by establishing the lowest energy reaction path that properly
interconnected the intermediaries found in our temperature variable mass spectrometry
measurements. At the first instance, we considered that species observed at the “low”
temperature condition were formed at the earlier stages of the process, while new
intermediaries detected at the “high” temperature value correspond to the later stages of the
decomposition reaction. Thus, at the beginning of our mechanism proposal, we consider all
the plausible molecular interconversion between reactants (compounds 1 or 5) and
corresponding “low temperature” intermediaries. Moreover, we found the transition states
structures that provided the lowest energy barrier for each of the proposed molecular
rearrangement. Subsequent reaction steps were established following a similar strategy, i.e.,
by identifying the molecular interconversion process which exhibited the lowest energy
profile. Our resulting proposal is depicted in Figure 4 while the corresponding reaction
Gibbs free energy profile is shown in (Figure 5). Table 2 collects the computed enthalpy
and Gibbs energy variations between stationary points.
In the first degradation step, reactants 1 and 5 release a phenol molecule providing
structures I1-a and I1-b, respectively. The phenoxy fragment comes from a phosphonate
group while the hydrogen atom from the adjacent amidophosphate nitrogen. This process
proceeds in a concerted fashion, involving a unique transition state atom (molecular graphs
and cartesian coordinates of each of the transition states found in this work are available in
the Supporting Information file). In step two, the remnant phenoxy group belonging to the
P(OPh) moiety of intermediaries I1 migrates towards the phosphonate group, releasing the
OP(OPh)3 species, (compound 9) and cyclizing the alkyl chain (intermediaries I2). We
found that the most favored reaction path for this process is constituted by only one
transition state (TS2-a and TS2-b for n =1 and n=2, respectively). In step three, the
10

oxygen atom of a phenol molecule attacks the electrophilic phosphorus center of
intermediaries I2, forming the I3 species. This process is both exergonic and exothermic in
both cases (n = 1 and n = 2) involving low energy barrier (cf. Table 2 and Figure 5), which
may explain why intermediaries I2 were not detected in our mass spectrometry
measurements. Intermediaries I4 are formed by the addition of an extra phenol molecule to
the phosphorus atom of intermediaries I3, breaking a N-P bond and opening the cyclic
frame. The computed energy barrier for the n=2 case (I3-b
→TS4-b) was higher than for
the n =1 case (I3-a
→
TS4-a), by approximately 146.24 kJ·mol-1, allegedly due to the higher
stability of a six-member cycle than a five-member one. Interestingly, only the I3-b
intermediary was detected in our mass spectrometry records. Then, intermediaries I4 are
decomposed into two entities; an aminoalkene and an amidophosphate moieties
(intermediaries I5). This reaction involves the electrophilic attack of the phosphonate
oxygen to the hydrogen atom attached to the beta carbon (Figure 4), providing a free
electron pair available for the formation of a carbon (α)-carbon (β) double bond, which
promotes the scission of the chemical bond between the amidophosphate nitrogen and the
alpha carbon. The reaction proceeds in a concerted way, exhibiting high-energy barriers in
the two cases (321.50 and 272.96 kJ·mol−1 for the I4a→TS5a and I4b→TS5b processes,
respectively) which indicates that intermediaries I4 are highly stable and they may be
crucial for the flame-retardant properties of our compounds. Intermediaries I4 were
detected in all our mass spectrometry experiments (“low” and “high” temperature
conditions). The formation of intermediary I5 is rather simple; it involves the migration of
a proton from the oxygen to the nitrogen atom, given the higher basicity of the second. As
can be observed in Figure 5, this species is the most thermodynamically favored
intermediary found in this study and it was detected by mass spectrometry at the two
highest temperature conditions here established, i.e., once intermediaries I4 were degraded.
The formation of adduct A is easily explained by the addition of a phenol molecule to the
amidophosphate species I5 forming the hydroxytriphenoxy-phosphanamine derivative. The
reaction is endergonic and endothermic, with an energy barrier of 136.30 kJ·mol−1. From
this last step, flame-retardants 1 and 5 must follow exactly the same degradation process
and no additional conclusions regarding the differences between their flame retardant
performances would be obtained.
11

Figure 4. Thermal degradation mechanism proposal for bisphosphoramidates 1 and 5
12

Comp. 1
Comp. 5
470
370
270
170
70
TS4
TS5
TS2
TS1
TS3
TS6
I1
TS7
I2
R
A
I5
I3
I4
I6
-30
Reaction coordinate
Figure 5. Thermal degradation profile of bisphosphoramidates 1 and 5 computed at the
DLNPO-CCSD/aug-cc-pVDZ level of theory using B3LYP/6-311++G(d,p) thermal
corrections.
13

Table 2. Enthalpy and Gibbs free energy variations between each of the stationary points
constituting the degradation mechanism proposal given in Figure 4 (energy units
are kJ·mol−1), for both compound 1 and compound 5. Computations were done at
the DLNPO-CCSD/aug-cc-pVDZ level of theory using B3LYP/6-311++G(d,p)
thermal corrections
Compound 1
Compound 5
Δ
H
Δ
G
Δ
H
ΔG
Process
TS1
R
→
226.51
225.64
237.11
249.94
R
→
I1
258.65
76.68
146.95
131.26
230.84
60.84
139.83
123.31
I1→
TS2
I1
→
I2
27.70
22.27
-40.34
107.33
-117.35
204.82
-15.02
321.50
-3.48
29.07
-60.45
113.65
-112.65
347.37
-39.59
272.96
I2
I3
→TS3
I2
→
I3
-203.77
110.96
48.11
-202.01
257.20
17.91
→TS4
I3→
I4
I4
I5
→TS5
283.30
157.36
I4→
I5
139.70
126.87
-73.80
39.11
55.17
129.23
-72.96
136.30
152.16
126.87
-73.80
39.11
65.41
129.23
-72.96
136.30
→TS6
I5
→
I6
I6
→TS7
I6
→A
-14.26
75.45
-14.26
75.45
3.4 Discussion of the thermal degradation mechanism proposed in this study.
As expected, the proposed thermal degradation process for the two flame-retardant
candidates under consideration proceeds in a similar fashion, involving similar transition
states and comparable energy barriers. Three important exceptions were detected, i.e., the
energy variations corresponding to the R→TS1, the I3→TS4 and the I4→TS5 processes
(cf. Table 2). Thus, it is reasonable to argue that differences between the flame retardant
properties of compound 1 and compound 5 must be attributed to the energy involved in, at
least, one of these three molecular rearrangements. The energy variation regarding the
R→TS1 process corresponds to the first energy barrier involved in our thermal degradation
mechanism, and represent the reluctance of a reactant R (bisphosphoramidate 1 or
bisphosphoramidate 5) to be thermally degraded. This energy barrier is higher for
14

compound 5 than for bisphosphoramidate 1 by almost 25.10 kJ·mol−1, a consistent result
with our TGA and DSC records, where bisphosphoramidate 1 was degraded at lower
temperature conditions than compound 5. Likewise, our mechanism proposal indicates that
intermediary I3 is crucial for the flame-retardant properties of bisphosphoramidate 5. The
energy required to promote the formation of intermediary I4 from intermediary I3
(I3→TS4 process) is higher for bisphosphoramidate 5 than for bisphosphoramidate 1 by
about 142.26 kJ·mol−1. Particularly, intermediary I4 plays a fundamental role in the flame-
retardant properties of these two bisphosphoramidates, being more stable when n =1 than
when n=2 (by 48.53 kJ·mol−1). After intermediary I4 is thermally degraded, the two flame
retardant candidates exhibit the same degradation pattern.
As a final remark, according with our thermochemistry results, each of the steps
constituting the thermal decomposition mechanism of compounds 1 and 5 proceed in a
concerted fashion, with no formation of free radical species, reinforcing the molecular
mechanism proposed by Gaan and co-oworkers [18].
4. Conclusions
In this work, the properties of two flame retardant symmetric bisphosphoramidates
compound 1 and compound 5, was investigated by a combined experimental-theoretical
strategy to elucidate their thermal degradation mechanism. The DSC experiments revealed
the rate of thermal degradation of each of the bisphosphoramidates under study, observing
that bisphosphoramidate 5 maintains its structural integrity at higher temperatures than
bisphosphoramidate 1. However, both exhibited a similar terminal degradation temperature.
Using variable temperature mass spectrometry measurements, several of the intermediaries
involved in the thermal decomposition process of our compounds, under a "low" and a
"high" temperature condition were described. Thus, by associating the elucidated
intermediaries with the results obtained from our first-principle thermochemical
calculations, a plausible molecular degradation mechanism for each flame-retardant was
proposed involving two key intermediaries I3 and I4.
15

In conclusion, bisphosphoramidate 5 is a more reliable flame retardant than
bisphosphoramidate 1, because it showed greater thermal stability and exhibited a
degradation mechanism involving more reluctant intermediaries to de decomposed under
high temperature conditions.
Acknowledgments.
M. Franco-Pérez thanks LANCAD-UNAM-DGTIC-354- A. Zamudio thanks Secretaría de
Investigación y Posgrado del Instituto Politécnico Nacional (SIP-IPN), for financial support
(Grant 20195476 and 20200192) and for the facilities of the supercomputers HPC-cluster
Xiuhcoatl.
5. References
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Organophosphorus Compounds; Hartley, F.R., Ed.; JohnWiley & Sons, Inc.: New York,
NY, USA, 1990; Volume 1, pp. 9–49.
[2] Stawinski, J.; Kraszewski, A. Acc. Chem. Res. 2002, 35, 952–960.
[3] Marinozzi, M.; Pertusati, F.; Serpi, M. Chem. Rev. 2016, 116, 13991–14055
[4] a) Gao, Y.; Tang, G.; Zhao, Y. Phosphorus Sulfur Silicon Relat. Elem. 2017, 192, 589–
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: