Effect of anomalous aryl strengthening in the series of N-phosphorylureas

Article abstract of DOI:10.1016/j.mencom.2009.09.010

Within the series of model N-diorganophosphoryl-N′-n-octylureas, the ability to extract uranium(VI) from nitric acid solutions is maximal for N-diphenylphosphoryl derivative and sharply decreases when phenyl groups are replaced with n-alkyl or cycloalkyl

Full text of DOI:10.1016/j.mencom.2009.09.010

Mendeleev
Communications
Mendeleev Commun., 2009, 19, 263–265
Effect of anomalous aryl strengthening
in the series of N-phosphorylureas
Alfiya M. Safiulina,a Evgenii I. Goryunov,*b Aleksandr A. Letyushov,a
Irina B. Goryunova,b Sof’ya A. Smirnova,b Allan G. Ginzburg,b
Ivan G. Tananaev,a Edward E. Nifant’evb and Boris F. Myasoedova
a A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
119991 Moscow, Russian Federation. Fax: +7 495 955 4641; e-mail: alfiya_safiulina@mail.ru
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5085; e-mail: phoc@ineos.ac.ru
DOI: 10.1016/j.mencom.2009.09.010
Within the series of model N-diorganophosphoryl-N'-n-octylureas, the ability to extract uranium(VI) from nitric acid solutions is
maximal for N-diphenylphosphoryl derivative and sharply decreases when phenyl groups are replaced with n-alkyl or cycloalkyl
substituents; this indicates the presence of ‘anomalous aryl strengthening’ effect also in the series of N-phosphorylureas, a new
promising class of organophosphorus extractants.
A so-called ‘anomalous aryl strengthening’ (AAS) effect was
revealed in the study of structure dependence of extractive
power for a number of bidentate neutral organophosphorus
compounds (BNOPC).1 This effect consists in the substantial
growth of the extractive power of BNOPC when alkyl substi-
tuents at phosphorus atom are replaced with aryl groups, even
though it is accompanied by a simultaneous decrease in the
basicity of these compounds. This phenomenon was discovered
for a series of P,P,P',P'-tetrasubstituted methylenediphosphine
dioxides2 and later was revealed for carbamoylmethylphos-
phine oxides.3 At present, there is no uniform opinion on the
origin of AAS effect;† however, its significance for purposeful
search for new high-throughput extractants within BNOPC
series can scarcely be exaggerated. Thus, in particular, the most
practically important carbamoylmethylphosphine oxide extractants
were revealed just among compounds containing diphenylphos-
phoryl fragments.5,6
It is necessary to emphasize, however, that the AAS effect
was observed until now only for two structurally related types
of BNOPC whose molecules contain liganding groups (phosphoryl
or phosphoryl and carbonyl) connected by identical methylene
bridge, i.e., the existence of this effect for BNOPC with linkers
of another nature remained unclear.
Moreover, the elucidation of this issue is crucial to solve a
number of important problems to optimize the search for the
most efficient extractants among entirely new types of BNOPC,
in particular, to reveal the nature of factors that determine
unusually high extractive power of N-diphenylphosphoryl-N'-
n-alkyl(C6–C10)ureas Ph2P(O)NHC(O)NHCnH2n + 1 1 (n = 6–10)
toward 4f and 5f elements.7
nitrogen atom, and studied their extractive properties toward
uranium(VI).
This set of model compounds made it possible to compare most
correctly their extractive characteristics with those of N-diphenyl-
phosphoryl-N'-n-octylurea 1a (n = 8), the best extractant among
the compounds of type 1, and to reveal the presence or absence
of AAS effect within the series of N-phosphorylated ureas.
To obtain all these compounds, we used a potentially simplest,
efficient and industrially feasible approach to the synthesis of
N-phosphorylated ureas, which was elaborated previously for
preparing various N-diphenylphosphorylureas,8 including ureas
of type 1.9 This approach consists in the use of one-pot processes
involving as many stages as possible, the key stage is the trans-
formation of phosphoryl chlorides into corresponding phosphoryl
isocyanates in the presence of a weak Lewis acid, anhydrous
magnesium chloride, as an electrophilic catalyst.10
N-Methyl(phenyl)phosphoryl-N'-n-octylurea 2 was obtained
starting from commercially available methyl(phenyl)phosphoryl
chloride with the use of two-stage one-pot process (Scheme 1).
NaOCN, [MgCl2]
PhMeP(O)Cl
[PhMeP(O)NCO]
~20 °C, MeCN
5
n-C8H17NH2
PhMeP(O)NHC(O)NH(n-C8H17
)
~20 °C
2
Scheme 1
The first stage of this process is the reaction of the chloro-
phosphinate with sodium cyanate in the presence of 2.5 mol%
MgCl2 in anhydrous acetonitrile medium. The reaction is
complete within 1 h at ambient temperature, while the yield of
methyl(phenyl)phosphoryl isocyanate 5 is 93% according to
31P{1H} NMR spectra. The resultant isocyanate as acetonitrile
solution was further reacted with n-octylamine to give urea 2 in
88% yield.
Phosphorylureas 3 and 4 were also obtained via one-pot
processes (three-stage in this case) using di-n-hexylphosphinic
(6) and dicyclohexylphosphinic (7) acids, respectively, as starting
compounds (Scheme 2).
For this purpose, we have prepared a number of model
N-diorganophosphoryl-N'-n-octylureas RR'P(O)NHC(O)NH-
(n-C8H17) 2 (R = Me, R' = Ph), 3 (R = R' = n-C6H13) and 4
(R = R' = cyclo-C6H11), whose molecules contain hydrocarbon
residues of different nature bound to the phosphorus atom via
P–C bonds and identical n-octyl substituent at the terminal
†
It is the most widespread opinion that this effect is related to the
amphoteric nature of aryl substituents showing both donor and acceptor
properties depending on certain factors such as reaction center nature,
attacking reagent character, solvent nature, etc.4
These acids were treated with oxalyl chloride in methylene
chloride to form di-n-hexylphosphinic (8) and dicyclohexyl-
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© 2009 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2009, 19, 263–265
80
1a
(COCl)2
NaOCN, [MgCl2]
RR'P(O)OH
6, 7
[RR'P(O)Cl]
8, 9
70
60
50
40
30
20
10
~20 °C, CH2Cl2
~20 °C, MeCN
n-C8H17NH2
[RR'P(O)NCO]
10, 11
RR'P(O)NHC(O)NH(n-C8H17
3, 4
)
~20 °C
3
2
3, 6, 8, 10 R = R' = n-hexyl
4, 7, 9, 11 R = R' = cyclohexyl
4
Scheme 2
0
1
2
3
4
5
6
7
8
CHNO /mol dm–3
3
phosphinic (9) chlorides in 98 and ~100% yields, respectively
(according to 31P{1H} NMR spectra), which were further reacted
(without additional purification) with sodium cyanate to produce
di-n-hexylphosphoryl (10) and dicyclohexylphosphoryl (11)
isocyanates. The reaction of these isocyanates with n-octyl-
amine leads to phosphorylureas 3 and 4 in 83 and 72% yields,
respectively.
Note that the developed one-pot processes do not require
the isolation of intermediate hydrolytically unstable compounds
(phosphoryl chlorides 8, 9 and phosphoryl isocyanates 5, 10
and 11) in an individual state, all process stages proceed at
ambient temperature, and target ureas 2–4 are obtained in
rather high yields and in high purity. The structure of the ureas
was confirmed by elemental analysis data, IR and 1H and
31P{1H} NMR spectra.‡
The study of the ability of N-diorganophosphorylureas 1a
and 2–4 to extract UVI from nitric acid solutions showed that
the replacement of one (urea 2) or two phenyl groups (urea 3)
with normal alkyl fragment results in a substantial (almost three-
fold) decrease in extractive power as compared with reference
N-diphenylphosphorylated urea 1a. For urea 4, where both phenyl
fragments are replaced with their fully hydrogenated analogues,
cyclohexyl substituents, the extractive power decreases by almost
40 times (Figure 1).
Taking into account that ureas 1a and 2–4 have identical
terminal nitrogen-containing fragments and differ only in the
nature of organic substituents connected to the phosphorus
atom via C–P bonds, it is safe to associate the high extractive
power of N-diphenylphosphorylated ureas of type 1 toward
uranium(VI) with the AAS effect.
Figure 1 Extraction of UVI with 0.05 mol dm–3 solutions of N-diorgano-
phosphoryl-N'-n-octylureas 1a, 2–4 in chloroform depending on HNO3
concentration (phase ratio of 1:1).
Thus, on the basis of our experimental data, we can draw
a conclusion that the ‘anomalous aryl strengthening’ effect is
observed not only for methylenediphosphine dioxides and car-
bamoylmethylphosphine oxides but also for N-diorganophos-
phorylureas; that is, this effect has a rather general character in
the series of bidentate neutral organophosphorus compounds.
This work was supported by the Russian Foundation for Basic
Research (project nos. 05-03-08017-ofi_e and 08-03-12153-ofi)
N-Di-n-hexylphosphoryl-N'-n-octylurea 3. Freshly distilled oxalyl
chloride (0.9 g, 7.1 mmol) was added dropwise over 25 min to a mag-
netically stirred solution of 1.090 g (4.7 mmol) of phosphinic acid 6 in
10 ml of anhydrous CH2Cl2 in an argon atmosphere at ambient tempera-
ture, the mixture was stirred for additional 1.5 h at the same temperature,
volatile products were removed in a vacuum from the reaction mixture
containing chlorophosphinate 8 (d 31P-{1H} 73.46 ppm). The residue was
dissolved in 10 ml of anhydrous MeCN, 11 mg (0.115 mmol) of finely
divided anhydrous MgCl2 was added to the solution, the mixture was stirred
until complete dissolution of MgCl2, 0.61 g (9.4 mmol) of NaOCN was
added, and the suspension was stirred for 11.5 h at ambient temperature.
n-Octylamine (607 mg, 4.7 mmol) was added dropwise to the resultant reac-
tion mixture containing phosphoryl isocyanate 10 (d 31P-{1H} 44.10 ppm),
the mixture was stirred for 1 h at ambient temperature, 17 ml of distilled
water was added, and the mixture was stirred for 1 h at ambient tempe-
rature. The precipitate was collected by filtration, washed with distilled
water (4×12 ml), and dried in air to give 1.50 g (83%) of compound 3 as
a white crystalline solid, mp 98–99 °C (from heptane). 31P{1H} NMR,
d: 52.74. 1H NMR, d: 0.86 (t, 9H, Me, 3JHH 6.7 Hz), 1.10–1.69 [m, 28H,
Me(CH2)6 + Me(CH2)4], 1.72–2.13 (m, 4H, CH2P), 3.12 (dt, 2H, CH2NH,
‡
The NMR spectra were recorded on a Bruker AV-400 spectrometer
3JCHH 6.7 Hz, JNHH 6.2 Hz), 6.03 (br. s, 1H, CH2NH), 7.94 [br. s, 1H,
3
operating at 400.13 (1H) and 161.98 MHz (31P) in CDCl3 solutions using
the signals of residual protons of the deuterated solvent as an internal
reference for 1H NMR and 85% H3PO4 as an external reference for
31P NMR. The IR spectra were obtained on a UR-20 spectrometer as
KBr pellets. Di-n-hexylphosphinic acid 6 and dicyclohexylphosphinic
acid 7 were obtained according to literature procedures.11,12
NHP(O)]. IR (KBr, n/cm–1): 3320, 3160, 3100 (NH), 1710, 1700, 1680
(C=O), 1165 (P=O). Found (%): C, 64.76; H, 11.63; N, 7.07; P, 8.01.
Calc. for C21H45N2O2P (%): C, 64.91; H, 11.67; N, 7.21; P, 7.97.
N-Dicyclohexylphosphoryl-N'-n-octylurea 4. Freshly distilled oxalyl
chloride (0.9 g, 7.1 mmol) was added dropwise over 25 min to a mag-
netically stirred solution of 1.090 g (4.7 mmol) of phosphinic acid 7 in
10 ml of anhydrous CH2Cl2 in argon atmosphere at ambient tempera-
ture, the mixture was stirred for additional 1.5 h at the same temperature,
volatile products were removed in a vacuum from the reaction mixture
containing chlorophosphinate 9 (d 31P-{1H} 82.22 ppm). The residue was
dissolved in 10 ml of anhydrous MeCN, 11 mg (0.115 mmol) of finely
divided anhydrous MgCl2 was added to the solution, the mixture was stirred
until complete dissolution of MgCl2, 0.61 g (9.4 mmol) of NaOCN was
added, and the suspension was stirred for 17.5 h at ambient temperature.
n-Octylamine (607 mg, 4.7 mmol) was added dropwise to the resultant reac-
tion mixture containing phosphoryl isocyanate 11 (d 31P-{1H} 49.46 ppm),
and the mixture was stirred for 1 h at ambient temperature. The subsequent
treatment as in the synthesis of compound 3 afforded 1.30 g (72%) of
compound 4 as a white crystalline solid, mp 139–141 °C (from chloro-
N-Methyl(phenyl)phosphoryl-N'-n-octylurea 2. Finely divided anhydrous
MgCl2 (14 mg, 0.147 mmol) was added to a solution of 1.015 g (5.7 mmol)
of chlorophosphinate 4 in 10 ml of anhydrous MeCN, the mixture was
stirred until complete dissolution of MgCl2, 0.74 g (11.4 mmol) of NaOCN
was added, and the suspension was stirred for additional 1 h at ambient
temperature. n-Octylamine (0.738 g, 5.7 mmol) was added dropwise
to the resultant reaction mixture containing phosphoryl isocyanate 5
(d 31P-{1H} 26.65 ppm) and the suspension was stirred for 1 h, MeCN
was removed in a vacuum, 20 ml of distilled water was added to the
residue, and the mixture was stirred for 1 h. The precipitate was collected
by filtration, washed with distilled water (4×20 ml), and dried in air to give
1.55 g (88%) of compound 2 as a white crystalline solid, mp 149–151 °C
1
(from chloroform–hexane). 31P{1H} NMR, d: 33.68. H NMR, d: 0.85
[t, 3H, Me(CH2)7, 3JHH 6.7 Hz], 1.10–1.33 [m, 10H, Me(CH2)5], 1.34–1.50
1
form–hexane). 31P-{1H} NMR, d: 52.25. H NMR, d: 0.86 (t, 3H, Me,
2
(m, 2H, CH2CH2NH), 1.91 (d, 3H, MeP, JHP 14.7 Hz), 3.10 (dt, 2H,
3JHH 6.8 Hz), 1.13–1.53 [m, 22H, Me(CH2)6 + cyclo-C6H11], 1.60–2.03 (m,
12H, cyclo-C6H11), 3.15 (dt, 2H, CH2NH, 3JCHH 6.8 Hz, 3JNHH 6.0 Hz),
6.50 (br. s, 1H, CH2NH), 6.96 [br. s, 1H, NHP(O)]. IR (KBr, n/cm–1):
3320, 3080 (NH), 1700 (C=O), 1210, 1180 (P=O). Found (%): C, 65.57;
H, 10.79; N, 7.41; P, 7.92. Calc. for C21H41N2O2P (%): C, 65.59; H, 10.75;
N, 7.28; P, 8.05.
CH2NH, 3JCHH 6.5 Hz, 3JNHH 6.4 Hz), 6.35 (br. s, 1H, CH2NH), 7.35–7.48
3
(m, 2H, m-Ph), 7.52 (t, 1H, p-Ph, JHH 7.1 Hz), 7.78 (dd, 2H, o-Ph,
3JHH 7.7 Hz, 3JHP 12.7 Hz), 8.44 [d, 1H, NHP(O), 2JHP 7.5 Hz]. IR (KBr,
n/cm–1): 3330, 3150, 3100 (NH), 1700, 1680 (C=O), 1180 (P=O). Found
(%): C, 61.94; H, 8.75; N, 9.08; P, 9.92. Calc. for C16H27N2O2P (%): C,
61.92; H, 8.77; N, 9.02; P, 9.99.
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Mendeleev Commun., 2009, 19, 263–265
6
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and the Russian Academy of Sciences (programme no. 18 of
Basic Research of the Presidium of the RAS ‘Development
of Methods for Preparing Chemical Compounds and Design of
New Materials’, subprogram ‘Development of Methodology
for Organic Synthesis and Design of Compounds with Valuable
Properties of Practical Interest’, project ‘Development of Methods
for Synthesis and Studying the Properties of New Mono- and
Polydentate Organophosphorus Compounds for Radioactive
Waste Partitioning’).
7
8
9
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