Bridged cyclophosphazenes resulting from deprotonation reactions of cyclotriphophazenes bearing a P-NH group

Article abstract of DOI:10.1039/c1dt10073d

Cyclotriphosphazene derivatives containing a P-NHR group in the side-chain react in the presence of a strong base to form stable intermolecular bridged products. Reaction of sodium hydride with mono-spiro cyclophosphazene derivatives having a P-NH group,

Full text of DOI:10.1039/c1dt10073d

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PAPER
Bridged cyclophosphazenes resulting from deprotonation reactions of
cyclotriphophazenes bearing a P–NH group†
Serap Bes¸li,*a Simon J. Coles,b David B. Davies,c Adem Kılıc¸a and Robert A. Shawc
Received 14th January 2011, Accepted 16th March 2011
DOI: 10.1039/c1dt10073d
Cyclotriphosphazene derivatives containing a P–NHR group in the side-chain react in the presence of a
strong base to form stable intermolecular bridged products. Reaction of sodium hydride with
mono-spiro cyclophosphazene derivatives having a P–NH group, N3P3Cl4[O(CH2)3NH], (1a) or
N3P3Cl4[CH3N(CH2)3NH], (1b) leads to formation of bis-cyclophosphazenes bridged with an
eight-membered cyclophosphazene ring in an ansa arrangement (2a, 2b) whereas reaction of sodium
hydride with mono-amino cyclophosphazene derivatives [N3P3Cl5(NHR), R = n-hexyl, 3a; i-Pr, 3b; Ph,
3c] give bis-cyclophosphazenes bridged with a four-membered cyclophosphazane ring in a spiro
arrangement (4a–c). In the latter reaction P–O–P bridged compounds (5a–c) were also obtained as a
result of hydrolysis reactions associated with the amount of moisture in the solvent tetrahydrofuran. In
addition, it was found that reaction of a mixture of cyclotriphosphazene with either mono spiro
compound, (1a) or (1b), in the presence of sodium hydride lead to formation of the first examples of
asymmetrically-bridged cyclophosphazenes (6a–b).
In order to investigate the generality of the formation of
Introduction
bridged compounds by deprotonation reactions of cyclotriphosp-
hazene derivatives containing a P–NH group in the side chain,
we used a different mono-spiro cyclophosphazene derivative
containing one NH moiety, N3P3Cl4[NMe(CH2)3NH] (1b) and
two other cyclotriphosphazene compounds containing different
amino groups in the side chain [N3P3Cl5(NHR); R = i-Pr (3b);
Ph, (3c)]. In the reactions using cyclophosphazene derivatives
3b,c the unexpected formation of P–O–P bridged compounds
necessitated re-investigation of the reaction of sodium hydride
with [N3P3Cl5(NHR) R = n-hexyl (3a)] and the effect of trace
amounts of moisture in the solvent on the distribution of products.
Furthermore, it was found that reaction of sodium hydride with
mono-spiro cyclophosphazene derivatives (1a,b) in the presence
of hexachlorocyclotriphosphazene lead to the formation of a new
type of bridged compound, the first examples of asymmetrically-
bridged cyclophosphazenes (6a,b). The deprotonation reactions
carried out in this work are summarised in Scheme 1.
Deprotonation reactions of alkanes, amines etc. are important in
organic and organometallic chemistry.1–8 However, deprotonation
reactions are less common in the chemistry of cyclophosphazenes,
an important class of heterocyclic inorganic ring systems.9–12
It has been demonstrated previously that deprotonation of
secondary amine derivatives of cyclotetraphosphazenes leads
to intramolecular displacement of a chloride and formation
of bicyclic compounds.11,12 On the other hand deprotonation
reactions of cyclotriphosphazenes containing an exo-secondary
amino group in the presence of the strong base NaH lead to
intermolecular displacement of a chloride and formation of stable
bis-cyclophosphazene bridged compounds.13 Thus deprotonation
reaction pathways (inter or intra) depend on the size of the
cyclophosphazene ring.
Bridged cyclophosphazene derivatives showing some interesting
properties such as chirality,14–16 and as polycations,17,18 have
usually been prepared using di- or poly-functional linkers via
P-centers.14–16 There are also polycationic systems consisting of
phosphazene rings linked together via ring N-centres.17,18
Results and discussion
All the reactions outlined in Scheme 1 were carried out under a
standard set of conditions (in a 1 : 1 molar ratio in THF at room
temperature for 2 h under an argon atmosphere). The reaction
mixtures were investigated by TLC and proton-decoupled 31P
NMR spectroscopy and the structures of the isolated products
were determined by elemental analysis, MS, and 1H and 31P NMR
spectroscopy. The 31P NMR chemical shifts and phosphorus–
phosphorus coupling constants were obtained by simulation of
the NMR spectra and are summarized in Table 1, whereas all the
aDepartment of Chemistry, Gebze Institute of Technology, Gebze, Turkey.
E-mail: besli@gyte.edu.tr; Fax: +90 2626053101; Tel: +90 2626053120
bDepartment of Chemistry, University of Southampton, Highfield,
Southampton, SO17 1BJ, UK
cSchool of Biological and Chemical Sciences, Birkbeck College (University
of London), Malet Street, London, WC1E 7HX, UK
† CCDC reference numbers 778738–778742 for the structures of (2b), (4b),
(5b), (5c) and (6a), respectively. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10073d
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Table 1 31P NMR parametersa
31P NMR chemical shift/ppm
PCl2 Pspiro PONH
2J(PP)/Hz
4J(PP)/Hz
Cpd
PClN
1,2
1,3
1,4
2,4
Bridge headb
Pspiro–PCl2
1
2
3
4
23.8
24.3
2ac
2b
4ac
4b
4c
22.0
23.0
27.1
26.1
27.7
23.5
23.3
24.2
24.1
22.1
24.2
22.0
5.3
65.5
49.2
49.6
50.4
54.9
59.7
62.9
57.2
42.2
27.3
27.7
48.5
52.1
56.4
21.5
22.2
21.4
21.3
10.5
1.5
-3.5
-6.9
5a
5b
5c
2.4
0.3
-4.4
59.9
59.8
61.9
6ad
0.7
7.6
15.4
15.5
63.5
46.6
54.5
55.1
2.9
2.7
6bd
20.9
a 202 MHz 31P NMR measurements in CDCl3 solutions at 298 K with chemical shifts given with respect to 85% H3PO4 as an external reference.
b Two-bond coupling constant between the bridge-head P atoms of different cyclophosphazene rings. c Data taken from ref. 13 d The PCl2 groups on the
two cyclophosphazene rings are different.
coupling of ca. 27–28 Hz across the cyclophosphazene ring similar
to compound 2a13 and in the range (23–35 Hz) found for other
P–N–P bridged cyclophosphazenes;19 the 31P NMR spectrum
is consistent with formation of a bis-cyclophosphazene bridged
structure.
Secondly, in order to test whether using cyclotriphosphazenes
having different P–NHR groups in the side chain would
give bis-cyclophosphazenes bridged with a four-membered cy-
clophosphazane ring in a spiro arrangement,13 the deprotona-
tion reactions of cyclotriphosphazenes containing a branched
aminoalky side chain, N3P3Cl5[NHPri] (3b) or aromatic amino
side chain, N3P3Cl5[NHPh] (3c) have been compared with pre-
vious work on the derivative having a linear aminoalkyl side
chain [N3P3Cl5(NHR), R = n-hexyl] (3a).13 The isopropylamine
derivative 3b was treated directly with sodium hydride and both
the TLC and the proton-decoupled 31P NMR spectrum of the
reaction mixture (Fig. 1b) showed, surprisingly, that there are two
products (4b, 5b) in approximately equal amounts, in contrast to
our previous work with the n-hexylamine derivative 3a, where
only one product (4a) was obtained.13 It was concluded from
the analytical data that one of the products was the expected
spiro-bridged compound (4b), which formed via a deprotonation
reaction analogous to compound (4a)13 but the structure of the
other product was unknown. Compound 3c was also reacted with
Scheme 1
sodium hydride under the standard conditions used in this work
and the proton-decoupled 31P NMR spectrum of the reaction
mixture (Fig. 1c) showed that two products (4c, 5c) were formed
with similar NMR characteristics to the reaction with 3b (Fig. 1a),
though in this case the amount of the spiro-bridged compound (4c)
was only about one-third of that of the unknown product (5c).
Therefore we repeated the reaction of the n-hexylamine derivative
3a with sodium hydride under the same reaction conditions,
though on a larger scale than previously,13 and observed two
products (4a, 5a), in which the known spiro-bridged compound
(4a) was predominant compared to the small amount of unknown
product (5a), as determined by 31P NMR of the reaction mixture
(Fig. 1a). It should be noted that in the previous study of the
reaction of 3a in the presence of sodium hydride,13 it was found
by TLC that the spiro-bridged compound 4a moved faster than
other analytical information is provided in the synthesis section
for each new compound.
Characterization of reaction products by 31P NMR
Firstly, in order to test whether using a mono-spiro cyclot-
riphosphazene derivative different from 1a would give a bis-
cyclophosphazene bridged with an eight-membered cyclophos-
phazene ring in an ansa arrangement analogous to 2a,13 the mono
spiro compound, N3P3Cl4[NMe(CH2)3NH] (1b) was reacted with
sodium hydride and only one product was observed by TLC and
in the 31P NMR spectrum of the reaction mixture. The proton-
decoupled 31P NMR spectrum of the isolated product (2b) is
observed as a basic AA¢BB¢XX¢-type spin system with further
coupling on the two multiplets, P(NR)Cl and Pspiro, due to geminal
5308 | Dalton Trans., 2011, 40, 5307–5315
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the starting compound and the same behaviour was observed
for the reaction mixtures using 3b,c as reactants, which gave
an initial indication, supplemented by 31P NMR of the isolated
compounds, that compounds 4b,c were analogous spiro-bridged
derivatives. In addition, in the TLC of the reaction mixtures the
products (5a–c) moved behind the starting compounds (3a–c) and
the isolated compounds had similar 31P NMR spectra indicating
similar structures for these unknown products.
Fig. 2 (a) Proton-decoupled 31P NMR of compound 5b, (b) proton-de-
coupled 31P NMR of compound 6a.
two sets of different A2B spin systems as shown for compound 6a
in Fig. 2b as an example. The 1P multiplet to low frequency (ca.
2
1 ppm) with J(PP) of 63.5 Hz is coupled to the high frequency
(ca. 24 ppm) 2P doublet corresponding to a cyclophosphazene
ring containing the Pspiro group and the other 1P multiplet (ca. 15
2
ppm) is coupled to the 2P doublet (ca. 22 ppm) with a J(PP)
of 54.6 Hz. In addition there is a coupling constant of 21.3 Hz
between the two 1P multiplet signals, which indicates a bridged
structure, and there are what looks like small long range coupling
constants on the signals at ca. 1 ppm (probably for the Pspiro group)
and the PCl2 group at ca. 22 ppm. The MS (which indicates that
compounds 6a, b have 9 Cl atoms) and 31P NMR spectra are
consistent with the probable structures as shown in Scheme 1
but X-ray crystallography was needed to confirm the new type of
bridged structure.
Fig. 1 31P NMR spectra of the reaction mixtures of the reaction with the
sodium hydride of (a) 3a, (b) 3b and (c) 3c.
The proton decoupled 31P NMR spectra of compounds 4b,c
have AA¢X2X2¢ spin systems similar to the spectrum of 4a,13 in
which the chemical shifts of the Pspiro groups in the compounds are
to high frequency (about -7 to 2 ppm) and the chemical shifts of
the PCl2 groups resonate at 26–28 ppm as observed for other PCl2
groups in similar derivatives of cyclophosphazenes.13
25 The spectra also exhibited the additional two-bond coupling
constant resulting from two equivalent coupling paths between
the Pspiro groups of different cyclophosphazene rings indicating
that compounds 4b,c are bis-cyclophosphazenes bridged with a
four-membered cyclophosphazane ring in a spiro arrangement
as observed for 4a.13 The proton decoupled 31P NMR spectra
of compounds 5a–c have complex AA¢X2X2¢-type spin systems
somewhat similar to the spectra of 4a–c and an example is shown
in Fig. 2a for compound 5b. Although the MS indicates that
compounds 5a–c are di-cyclophosphazene derivatives having 8
Cl atoms (from the number of Cl isotope peaks and the relative
intensity of each peak) and the 31P NMR spectra indicate that
they are symmetrical, X-ray crystallography confirmed them to be
P–O–P bridged cyclophosphazenes.
X-ray crystal structures
The molecular structures of compounds 2b, 4b, 5b,c and 6a
are shown in Fig. 3–6, respectively, and the data collection and
refinement parameters are summarised in Table 2. In each struc-
ture the cyclophosphazene ring exhibits no unusual deviations in
bond lengths and angles from either that of the parent N3P3Cl6
structure (CSD code KAGKUY)20 or from that observed in similar
structures in the Cambridge Structural Database.21 The structures
are discussed with respect to related cyclotriphosphazene and
cyclodiphosphazane compounds22–24 and to those for compounds
2a and 4a given in our previous work.13
A third type of deprotonation reaction was investigated.
Sodium hydride was added to a 1 : 1 molar ratio mixture of
hexachlorocyclotriphosphazene with either a mono-spiro (1a–b)
or a mono-substituted (3a–c) cyclotriphosphazene containing a
P–NH group in the side chain. The reactions involving mono-
substituted cyclotriphosphazenes (3a–c) just lead to formation of
compounds 4a–c and 5a–c as observed above in reactions that
did not contain any added cyclophosphazene. However, reactions
involving the mono-spiro cyclotriphosphazenes (1a,b) did not
produce compounds 2a,b but gave new compounds 6a,b, which
MS indicated were di-cyclophosphazenes. In this case the proton-
decoupled 31P NMR spectra of 6a,b exhibited four multiplets with
Fig. 3 The structure of compound 2b has a centre of symmetry and is
meso. Displacement ellipsoids are drawn at the 50% probability level.
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Table 2 Crystal data and refinement parameters for (2b), (4b), (5b), (5c) and (6a)
2b
4b
5b
5c
6a
Empirical formula
Formula weight
Crystal system
Space group
C8H18Cl6N10P6
652.84
C6H14Cl8N8P6
667.67
Monoclinic
C6H16Cl8N8OP6
685.69
Orthorhombic
C12H12Cl8N8OP6
753.72
Monoclinic
C3H6Cl9N7OP6
661.02
Triclinic
Triclinic
¯
¯
P1
P21/c
Pna21
Cc
P1
˚
a (A)
8.5998(2)
8.7747(2)
9.4768(2)
114.0400(10)
95.0370(10)
114.3700(10)
566.06(2)
1
11.6755(17)
8.9298(13)
23.758(3)
90
12.52630(10)
12.4818(2)
16.4619(4)
90
14.6358(2)
16.3497(2)
12.34480(10)
90
8.29070(10)
8.95560(10)
15.00720(10)
95.4020(10)
95.1100(10)
101.3340(10)
1081.050(19)
2
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
95.549(2)
90
2465.4(6)
4
1.799
1.317
1328
0.01 ¥ 0.01 ¥0.01
27.50
16 848
6499
0.0350
R1 = 0.0746
wR2 = 0.1749
1.037/-0.874
90
90
107.4550(10)
90
2817.98(6)
4
1.777
1.167
1496
0.50 ¥ 0.02 ¥0.01
27.50
23 887
6203
0.0516
R1 = 0.0303
wR2 = 0.0663
0.353/-0.401
0.24(5)
3
˚
Volume (A )
2573.83(8)
4
1.770
1.267
1368
0.32 ¥ 0.04 ¥ 0.01
27.50
44 787
5883
Z
Density (calc) (Mg m-3)
Absorption coefficient (mm-1)
F(000)
1.915
1.206
328
0.28 ¥ 0.20 ¥ 0.20
27.48
12 378
2577
2.031
1.622
648
0.10 ¥ 0.10 ¥ 0.10
27.50
23 785
4968
Crystal size (mm)
qmax (◦)
Reflections collected
Independent reflections
R(int)
0.0302
0.0656
0.0211
Final R indices
R1 = 0.0222
wR2 = 0.0571
0.358/-0.439
R1 = 0.0337
wR2 = 0.0565
0.333/-0.336
0.03(5)
R1 = 0.0206
wR2 = 0.0642
0.651/-0.797
F2 > 2sF2
-3
˚
Dr max/min (eA )
Absolute structure parameter
˚
˚
and 2b (2.761 A) and ca. 2.77 A for most other unstrained
cyclophosphazene rings.21 The central eight-membered ring of
compound 2b has bond lengths similar to those for compound
2a,13 in which the average P–N bond length is 1.587 A in the
cyclophosphazene moiety and 1.672 A in the cyclophosphazane
˚
˚
bridge. The sum of the bond angles around the N5 atom of
the cyclophosphazane bridge of 2b is 358.6◦ making it trigonal
planar, similar to that found for 2a (358.6◦).13 Puckering analysis
is a useful measure of ring conformation, because calculation
of the mean plane and puckering parameter provides a unique
quantitative descriptor, the total puckering amplitude (Q), which
may be used to compare different ring conformations.26 Using
puckering analysis it is found that the conformation of the central
Fig. 4 The structure of achiral compound 4b. Displacement ellipsoids
are drawn at the 50% probability level.
˚
eight-membered cyclophosphazane ring of 2b [Q, 0.8915(12) A] is
˚
very similar to that found for 2a [0.9047(13) A].
The structure of compound 2b (Fig. 3) shows a five ring
structure in which the two cyclotriphosphazene rings are linked
via the N-atoms of the two spirocyclic moieties to form an eight-
membered PN ring analogous to the structure of compound 2a.13
The molecule has four stereogenic centers, two pairs of different
centers of chirality [Pspiro (P1 = S, P1¢ = R) and P(NR)Cl (P2 = S
and P2¢ = R)] but has a meso configuration as a result of the centre
of symmetry similar to compound 2a.13 It should be noted that
the inversion structure of 2b is very similar to that of 2a, though
the designation of chirality of the Pspiro group is different because
of the ranking of groups in the CIP configurational system.25
The six-membered cyclophosphazene rings of compound 2b are
slightly puckered with the maximum deviation from the mean
There are two independent molecules that lie about inversion
centres in the unit cell of compound 4b and they are shown in
Fig. 4. Compound 4b exhibits a three ring structure in which
two cyclophosphazene rings are linked in a spiro arrangement by
the four-membered substituted cyclophosphazane ring similar to
that observed for compound 4a.13 The cyclophosphazene rings of
compound 4b are nearly planar, the maximum deviation from the
mean plane for the molecular structure in Fig. 4 is only 0.034 (atom
P1) and for the other molecule in the unit cell is 0.026 (atom N6,
analogous to N1 in Fig. 4). The cyclophosphazane ring compound
4b is also nearly planar having bond angles within the ring that are
close to right angles with 84.3◦ at P and 95.7◦ at N for the molecule
in Fig. 4 (and 84.1◦/95.9◦ for the other molecule in the unit cell),
the nitrogen atom in the ring is almost trigonal planar with the
sum of the bond angles around N4 being 358.6◦ (and 359.2◦) and
˚
plane = 0.085 A (for atom P3) and the planes of the phosphazene
rings are in a perfectly anti-parallel arrangement due to space
group symmetry. The distance between the P-atom of the Pspiro
group of one ring and the P-atom of the P(NR)Cl group of the
˚
the average P–N bond length in the phosphazane ring is 1.664 A
(and 1.660 A) The molecular parameters for the four-membered
cyclophosphazane ring of compound 4b are also similar to those
˚
˚
other ring is 2.934 A, which is similar to that observed for 2a
13
˚
(2.905 A), but longer than the average P ◊ ◊ ◊ P distance between
those atoms in the same phosphazene ring for both 2a (2.772 A)
in the literature 13,23
˚
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previously by different methods.27–30 The cyclophosphazene rings
are essentially planar with the maximum deviations from the
˚
˚
mean plane of 0.049 A (P2) and 0.026 A (N6) for compound
˚
˚
5b and 0.107 A (N1), 0.042 A (N7) for compound 5c. The
two cyclophosphazene rings are bridged via O-atoms from the
P(NHR) groups in which the P–O–P bond angles (138.7 for 5b
˚
and 137.2 for 5c) and average P–O bond lengths (1.602 A for
5b and 1.603 A for 5c) are similar to values for analogous P–
O–P compounds in the literature
˚
29–31
The P(NHR) side groups
are arranged in an ‘anti-parallel’ manner with respect to each
other with very similar conformations found for compounds 5b
and 5c as shown by the superposition of their structures in Fig.
5c. It is likely that the P–O–P bridged compounds 5b and 5c are
hydrolysis products resulting from trace amounts of moisture in
the solvent tetrahydrofuran and this has been investigated further
(next section).
The molecular structure of compound 6a (Fig. 6) demonstrates
that it is a new type of P–N bridged derivative, in which the N-
atom of the spirocyclic moiety is linked to one P-atom of the
cyclophosphazene ring. Whilst all the other bridged compounds
obtained in this work are symmetrical structures, compounds 6a is
unsymmetrical, which accounts for the 31P NMR being observed
as two different A2B spin systems and the feasibility of observing
long range coupling constants (Fig. 2b). In the molecular structure
of compound 6a (Fig. 6) both cyclophosphazene rings are
reasonably planar [the maximum deviations from the mean plane
˚
are 0.136 A (for P2) and 0.169 (for P6)] and the average P ◊ ◊ ◊ P
˚
distances (2.735 A for the mono-substituted cyclophosphazene
˚
ring and 2.746 A for the gem-disubstituted cyclophosphazene
ring) are in the normal range found for mono- and bis-substituted
cyclophosphazene rings.23 The single P–N bridge of compounds
6a is in a similar molecular environment to the two P–N bridged
compounds 2a and 2b and explains the similar Pspiro–N–P(Cl) bond
angles (123.6 for 6a, 122.7 for 2b and 121.5◦ for 2a13) and N–P(Cl)
13
˚
bond lengths (1.6574 for 6a, 1.6530 for 2b and 1.6569 A for 2a ).
Influence of moisture on the formation of reaction products
In the original reaction of compound 3a with sodium hydride to
form the spiro-bridged bis-cyclophosphazene 4a13 freshly dried
tetrahydrofuran was used as solvent, whereas the analogous
reactions with compounds 3b and 3c were done about a week
later with the same solvent. In the reactions using compounds
3b and 3c not only were the expected PNP-bridged derivatives
(4b, 4c) formed but also the POP-bridged compounds (5b, 5c) as
shown by the proton decoupled 31P NMR spectra of the reaction
mixtures given in Fig. 1b and 1c, respectively. When the reaction of
compound 3a with sodium hydride was repeated in the same week-
old THF, a small amount of the POP-bridged compound (5a) was
observed in addition to the PNP-bridged compound 4a as shown
by the 31P NMR spectrum of the reaction mixtures given in Fig.
1a. Hence the formation of the P–O–P bridged compounds (5a–
c) occurred as result of serendipity. For the three reactions done
under the same standard conditions it can be seen (Fig. 1) that the
formation of the POP-bridged compound is the major product
(5c) in the reaction using the aniline derivative 3c, the minor
product (5a) using the n-hexylamine derivative 3a and when the
isopropylamine derivative (3b) is used in the reaction there is about
equal formation of the POP-bridged (5b) and PNP-bridged (4b)
Fig. 5 The structures of (a) compound 5b, (b) compound 5c and (c)
comparison of their conformations normalised to the P–O–P bridge.
Displacement ellipsoids are drawn at the 50% probability level.
Fig. 6 The structure of the asymmetrically-bridged compound 6a.
Displacement ellipsoids are drawn at the 50% probability level.
The molecular structures of 5b and 5c (Fig. 5a and b, respec-
tively) confirmed they are bridged compounds as indicated by
analysis of the 31P NMR spectra, though they are unexpected P–
O–P bridged compounds, of a type which has been synthesised
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compounds. The relative proportions of the compounds observed
in the reaction mixtures by 31P NMR spectroscopy (Fig. 1) were
also mirrored by the amounts of pure compounds isolated.
Although the three mono-amino cyclophosphazene derivatives
(3a–c) will have differences in reactivity with sodium hydride,
which may account for the differences in relative proportions in the
formation of the PNP-bridged derivatives (4a–c), it is necessary
to investigate the origin of the formation of the POP-bridged
compounds (5a–c). As the POP-bridged compounds (5a–c) were
observed in reactions using previously-dried but week-old THF,
it is likely that their formation is due to moisture in the THF
and so it was essential to repeat the reactions in rigorously-dried
solvent. THF was refluxed over sodium–potassium alloy until it
gave a blue color with benzophenone32 and then directly distilled
into the reaction flask under an argon atmosphere. Compound
3b reacted with sodium hydride in this freshly-dried THF and
the proton decoupled 31P NMR spectrum of the reaction mixture
(Fig. 7a) showed that only one product was formed and it is the
PNP-bridged compound (4b). Similarly, when compound 3c was
reacted with sodium hydride in the ultra-dried THF, only the PNP-
bridged compounds (4c) formed, although its yield was again low
as observed in the original reaction in normal THF. The effect of
moisture in the solvent on the relative proportions of the reaction
products was investigated by addition of small aliquots of distilled
water to the THF used to react compound 3b with sodium hydride
under standard conditions for 2 h at room temperature. The proton
decoupled 31P NMR spectrum of the reaction mixture containing
3b : NaH : H2O in a 1 : 1 : 1 molar ratio is shown in Fig. 7c and
that with double the amount of water (1 : 1 : 2 molar ratio) in
Fig. 7d. It can be seen that as the amount of water is increased
there is a concomitant increase in the amount of the POP-bridged
compound (5b) and a decrease in the amount of the PNP-bridged
compound (4b). It is obvious that promotion of the formation
of PNP-bridged compounds by deprotonation reactions requires
rigorously-dried solvents such as THF and that formation of POP-
bridged compounds is assisted by addition of a small amount of
water to the reaction mixture.
Relative reactivities
The reactions with starting compounds containing side-chain
mono-amino groups [PCl(NHR), 3a–c] lead to much greater
proportions of isolated products (4a + 5a, 84%; 4b + 5b, 92%; 4c
+ 5c, 78%) than those using cyclophosphazene derivatives having
a P(X)NHR mono-spiro group (X = O, 1a; X = NMe, 1b), which
only yield deprotonation products (2a, 34%; 2b, 29%). Perhaps this
is not surprising because previous work on amino derivatives of
cyclophosphazenes has shown that the P–Cl bonds in P(NHR)Cl
groups are longer than those in PCl2 groups owing to the electron-
releasing capacity of the –NHR group, which has been related to
the greater reactivity of PNHCl compared to PCl2 groups in the
same molecule,33 and a similar situation is observed for molecular
structures of the 3a–c type having a PCl(NHR) motif.34
The P–N–P bridged compounds are probably formed
by
a proton abstraction–chloride ion elimination reaction
mechanism.11,12,33 In this process the base NaH abstracts a proton
from the P–NH group of the cyclophosphazene derivative, which
is followed by loss of a chloride ion giving a coordinatively-
unsaturated unstable transition state that reacts quickly to form
the product, a P–N–P bridged compound in the present work. A
dissociative route based on a conjugate-base mechanism is shown
in Scheme 2.
Scheme 2 Proton abstraction–chloride ion elimination reaction mecha-
nism for formation of PNP-bridged compounds.
The conditions are the same for all reactions with 3a–c, though
the yields of the spiro-bridged bicyclophosphazene compounds
decrease in the order 4a, 4b and 4c, respectively, which is probably
related to the characteristics of the R group in the P(NHR)Cl
moiety. The precursor 3c has an aryl group, which is likely to
deactivate the amino side chain by mesomeric and, possibly,
steric effects.35,36 However, there is also competition between
formation of PNP- and POP-bridged derivatives for the reactions
of compounds 3a–c and so if formation of the spiro-bridged
compound 4c (19%) becomes difficult due to steric or mesomeric
effects, then greater proportions of the POP-bridged compound
31
Fig. 7
P NMR spectra of the reaction mixtures of the reactions with
the sodium hydride of 3b (a) in ‘ultra dried’ THF, (b) in normal THF, (c) in
aqueous THF (0.1 ml distilled water in 30 ml THF), (d) in more aqueous
THF (0.2 ml distilled water in 30 ml THF).
5312 | Dalton Trans., 2011, 40, 5307–5315
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5c is expected, as found (59%). It is likely that steric effects explain
the difference in yields of the isopropylamine spiro-bridged P–N–
P compound 4b (49%) compared to the analogous n-hexylamine
derivative 4a (68%) and so there is concomitantly a smaller
proportion of the P–O–P bridged compound 5a (16%) compared
to 5b (43%).
The hydrolysis reaction mechanism for the formation of P–O–P
bridged compound 5a–c (Scheme 3) shows that it is necessary to
have a labile P–Cl group as in the reactants 3a–c with exocyclic
P–NHR groups, whereas the P-atoms bonded to the NH groups
in 1a,b do not have a directly-bonded Cl atom.
Methods
Analytical Thin Layer Chromatography (TLC) was performed on
Merck silica gel plates (Merck, Kieselgel 60, 0.25 mm thickness)
with F254 indicator. Column chromatography was performed on sil-
ica gel (Merck, Kieselgel 60, 230–400 mesh; for 3 g crude mixture,
120 g silica gel was used). Elemental analyses were obtained using a
Thermo Finnigan Flash 1112 Instrument. The mass analyzer was a
Bruker Daltonics MicrOTOF mass spectrometer equipped with an
orthogonal electrospray ionization (ESI) source. The instrument
was operated in either the positive or negative ion mode using a
range of m/z 50–3000. 1H and 31P NMR spectra were recorded in
CDCl3 solutions on a Varian INOVA 500 MHz spectrometer using
TMS as an internal reference for 1H and 85% H3PO4 as an external
reference for 31P NMR measurements. The free trial version of the
gNMR 4.1 spectral simulation programme (from Adept Scientific)
was used for calculation of all coupling constants. Differential
Scanning Calorimetry (DSC) of compounds 2b, 4b and 4c was
performed on a Mettler Toledo DSC 822 instrument from 25 ◦C
to 350 ◦C at a heating rate of 10 ◦C min-1.
X-ray crystallography
Intensity data were recorded on a Nonius KappaCCD diffrac-
˚
tometer (120 K, Mo-ka radiation = 0.71069 A) driven by
COLLECT38 and DENZO39 software. Structures were determined
using the direct methods procedure in SHELXS-97 and refined by
full-matrix least squares on F2 using SHELXL-97.40 Full details of
data collection and structure determination are presented in Table
2 and have been deposited with the Cambridge Crystallographic
Data Centre. CCDC reference numbers are CCDC 778738–778742
for the structures of (2b), (4b), (5b), (5c) and (6a), respectively.
Scheme 3 Reaction mechanism for formation of POP-bridged com-
pounds (5a–c).
Further evidence on the relative reactivities of P–Cl groups
is given by comparison of the reactions of either the mono-
spiro cyclotriphosphazenes (1a,b) or the mono-substituted cyclot-
riphosphazenes (3a–c) with NaH in the presence of hexachlorocy-
clotriphosphazetriene; reactions with 3a–c just lead to formation
of the same products 4a–c and 5a–c, whereas the reactions
involving the mono-spiro cyclotriphosphazenes (1a,b) gave the
new asymmetrically-bridged compounds 6a,b. In the reactions of a
mixture of cyclotriphosphazene with either mono-spiro compound
1a or 1b, it is possible that the base NaH abstracts a proton from
the secondary amino group of the cyclophosphazene derivative
and the resulting anion attacks a PCl2 center of hexachlorocy-
clotriphosphazetriene to give the asymmetrically PNP-bridged
cyclophosphazene (Scheme 2). This result is consistent with the
known decrease in relative activity of P–Cl bonds as a result of
geminal substitution of the cyclophosphazene ring in compounds
1a and 1b, because greater electron density at the skeletal atoms
retards the elimination of Cl atoms 22,34f,36,37
Reaction of 1b with NaH to give compound 2b. Compound
1b41 (0.73 g, 2 mmol) was dissolved in 30 mL of dry THF in a
100 mL three-necked round-bottomed flask. The reaction mixture
was cooled in an ice-bath and NaH (60% oil suspension, 0.08 g,
2 mmol) in 10 mL of dry THF was quickly added to a stirred
solution under an argon atmosphere. The reaction was stirred for
a further 2 h at room temperature and followed by TLC on silica gel
plates using THF–hexane (1 : 2). The reaction mixture was filtered;
the solvent removed under reduced pressure and the crude product
was subjected to column chromatography using THF–hexane
(1 : 2) as mobile phase. The product 2b was isolated as a white
solid and crystallized from THF to give white crystals (0.19 g,
29%, mp 308 ◦C, DSC on 1.4 mg). Anal Calc. for: C8H18Cl6N10P6:
C, 14.72; H, 2.78; N, 21.45%; (M, 652.9). Found: C, 14.70; H, 2.77;
N, 21.20%; (M+, 652.9). 1H NMR, CDCl3, 298 K: d 1.98, 2.20 [m,
Experimental section
3
4H, -CH2-]; 2.70 [d, 6H, -CH3, J(PNCH) 13.4 Hz]; 3.12, 3.32,
3.49, 3.83 [m, 8H, N–CH2 and CH3–N–CH2].
Materials
Hexachlorocyclotriphosphazene (Otsuka Chemical Co., Ltd) was
purified by fractional crystallization from hexane. 3-Amino-1-
propanol (Merck), N-methyl-1,3-diaminopropane (Merck), n-
hexylamine (Merck), aniline (Fluka) and isopropylamine (Sigma)
were used as received. THF (Merck) was distilled over Na–K
alloy in an argon atmosphere. Sodium hydride, 60% dispersion
in mineral oil (Merck); prior to use the oil was removed by
washing with dry hexane followed by decantation. CDCl3 for
NMR spectroscopy was also obtained from Merck.
Reaction of 3a with NaH to give compound 4a and 5a. Com-
pound 3a13 (2.06 g, 5 mmol) was dissolved in 50 mL of dry THF
in a 100 mL three-necked round-bottomed flask. The reaction
mixture was cooled in an ice-bath and NaH (60% oil suspension,
0.2 g, 5 mmol) in 10 mL of dry THF was quickly added to a
stirred solution under an argon atmosphere. The reaction was
stirred for a further 2 h at room temperature and followed by
TLC on silica gel plates using dichloromethane–hexane (1 : 2)
giving two new products. The reaction mixture was filtered; the
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solvent removed under reduced pressure and the crude product
was subjected to column chromatography using dichloromethane–
hexane (1 : 2) as mobile phase. The first compound eluted from
Found: C, 19.60; H, 1.35; N, 15.05%, ([M + H]+, 736.7). 1H NMR,
CDCl3, 298 K; d 7.21 [overlap one t and one d 6H, o- and p-CH];
7.35 [t, 4H, m-CH, 3J(HCCH) ca. 7.8 Hz]. The second substance
from the column was the starting compound. The third compound
eluted from the column was the product 5c (1.12 g, 59%, mp
148 ◦C). Anal Calc. for C12H12Cl8N8OP6: C, 19.12; H, 1.60; N,
14.87%, (M, 753.7). Found: C, 19.10; H, 1.60; N, 14.70%, ([M
◦
the column was the product 4a (1.28 g, 68%, mp 143 C). Anal
Calc. for C12H26Cl8N8P6: C, 19.17; H, 3.49; N, 14.90%, (M, 751.9).
Found: C, 19.15; H, 3.45; N, 14.75%, ([M + H]+, 752.9). 1H
3
NMR, CDCl3, 298 K; d 0.81 [t, 6H, -CH3, J(HCCH) 7.0 Hz];
1
1.16–1.28 [m, 12H, -CH2-]; 1.49 [br quintet, 4H, N–CH2–CH2-,
- H]-, 752.7). H NMR, CDCl3, 298 K; d 5.43 [br d, 2H, NH,
3J(HCCH) ca. 7.2 Hz]; 2.99 [tt, 4H, N–CH2-, 3J(PNCH) 17.4 Hz
2J(PNH) ca. 9.3 Hz]; 6.98 [d, 4H, o-CH, 3J(HCCH) 8.1 Hz]; 7.02
[t, 2H, p-CH, 3J(HCCH) ca. 7.4 Hz]; 7.21 [t, 4H, m-CH].
3
and J(HCCH) 6.7 Hz]. The second substance from the column
was the starting compound. The third compound◦eluted from the
column was the product 5a (0.31 g, 16%, mp 65 C). Anal Calc.
for C12H28Cl8N8OP6: C, 18.72; H, 3.67; N, 14.55%, (M, 769.9).
Found: C, 18.70; H, 3.66; N, 14.40%, ([M + H]+, 770.9). 1H NMR,
CDCl3, 298 K; d 0.93 [t, 6H, -CH3, 3J(HCCH) 6.8 Hz], 1.05–1.19
[m, 12H, -CH2-], 1.47 [br quintet, 4H, N–CH2–CH2- 3J(HCCH)
ca. 7.3 Hz]; 2.92 [sextet comprising a doublet of quartets, 4H,
Reaction of 1a and hexachlorocyclotriphosphazene with NaH
to give compound 6a. Compound 1a13 (0.35 g, 1 mmol) and
hexachlorocyclotriphosphazene (0.35 g, 1 mmol) were dissolved
in 20 mL of dry THF in a 100 mL three-necked round-bottomed
flask. The reaction mixture was cooled in an ice-bath and NaH
(60% oil suspension, 0.04 g, 1 mmol) in 50 mL of dry THF was
quickly added to a stirred solution under an argon atmosphere.
The reaction was stirred for a further 2 h at room temperature
and followed by TLC on silica gel plates using dichloromethane–
hexane (5 : 2). The reaction mixture was filtered; the solvent
removed under reduced pressure and the crude product was
subjected to column chromatography using dichloromethane-
hexane (5 : 2) as mobile phase. The product 6a was isolated as
a white solid and crystallized from heptane to give white crystals
(0.46 g, 70%, mp 127 ◦C) Anal Calc. for: C3H6Cl9N7OP6: C, 5.45;
H, 0.91; N, 14.83%; (M, 661.0). Found: C, 5.47; H, 0.92; N, 14.75%;
([M + H]+, 661.7). 1H NMR, CDCl3, 298 K: d 2.05 [m, doublet of
quintets, 2H, -CH2-, 3J(HCCH) 6.3, 5.8 Hz, 4J(POCHH) 1.3 Hz];
3.82 [m, 2H, NCH2]; 4.43 [dt, 2H, OCH2 3J(POCH) 14.8 Hz,
3J(HCCH) 6.3 Hz].
3
3
3
N–CH2- J(HCCH) 7,0 Hz, J(HCNH) 6,8 Hz) and J(PNCH)
12.5 Hz]; 3.18 [br m, 2H, NH].
Reaction of 3b with NaH to give compounds 4b and 5b.
Compound 3b42 (1.85 g, 5 mmol) was dissolved in 50 mL of
dry THF in a 100 mL three-necked round-bottomed flask. The
reaction mixture was cooled in an ice-bath and NaH (60% oil
suspension, 0.2 g, 5 mmol) in 10 mL of dry THF was quickly added
to a stirred solution under an argon atmosphere. The reaction
was stirred for a further 2 h at room temperature and followed
by TLC on silica gel plates using dichloromethane–hexane (2 : 1)
giving two new products. The reaction mixture was filtered; the
solvent removed under reduced pressure and the crude product
was subjected to column chromatography using dichloromethane–
hexane (2 : 1) as mobile phase. The first compound eluted from the
column was the product 4b (0.82 g, 49%, mp 288 ◦C, DSC on 2.0
mg). Anal Calc. for C6H14Cl8N8P6: C, 10.79; H, 2.11; N, 16.78%,
(M, 667.7). Found: C, 10.77; H, 2.10; N, 16.60%, ([M + H]+ 668.8).
1H NMR, CDCl3, 298 K; d 1.31 [d, 12H, -CH3, 3J(HCCH) 6.5 Hz];
3.58 [m, 2H, N–CH-]. The second substance from the column
was the starting compound. The third compound◦eluted from the
column was the product 5b (0.74 g, 43%, mp 77 C). Anal Calc.
for C6H16Cl8N8OP6: C, 10.51; H, 2.35; N, 16.34%, (M, 685.7).
Found: C, 10.50; H, 2.36; N, 16.10%, ([M + H]+, 686.8). 1H NMR,
CDCl3, 298 K; d 1.25 [d, 12H, -CH3 3J(HCCH) 6.4 Hz]; 3.14 [br
triplet, 2H, NH with average coupling ca. 9.8 Hz for 3J(HNCH)
and 2J(PNH)]; 3.52 [m, 2H, N–CH-]. Compound 4b and 5b were
crystallized from a dichloromethane–hexane (1 : 3) solvent system.
Reaction of 1b and hexachlorocyclotriphosphazene with NaH
to give compound 6b. Compound 1b41 (0.36 g, 1 mmol) and
hexachlorocyclotriphosphazene (0.35 g, 1 mmol) were dissolved
in 20 mL of dry THF in a 100 mL three-necked round-bottomed
flask. The reaction mixture was cooled in an ice-bath and NaH
(60% oil suspension, 0.04 g, 1 mmol) in 5 mL of dry THF and was
quickly added to a stirred solution under an argon atmosphere.
The reaction was stirred for a further 2 h at room temperature and
followed by TLC on silica gel plates using THF–hexane (1 : 2).
The reaction mixture was filtered; the solvent removed under
reduced pressure and the crude product was subjected to column
chromatography using THF–hexane (1 : 2) as mobile phase. The
product 6b was isolated as white solid and crystallized from THF
Reaction of 3c with NaH to give compound 4c and 5c. Com-
pound 3c43 (2.02 g, 5 mmol) was dissolved in 50 mL of dry THF
in a 100 mL three-necked round-bottomed flask. The reaction
mixture was cooled in an ice-bath and NaH (60% oil suspension,
0.2 g, 5 mmol) in 10 mL of dry THF was quickly added to a stirred
solution under an argon atmosphere. The reaction was stirred for
a further 2 h at room temperature and followed by TLC on silica
gel plates using dichloromethane–hexane (3 : 2) giving two new
products. The reaction mixture was filtered; the solvent removed
under reduced pressure and the crude product was subjected to
column chromatography using dichloromethane–hexane (2 : 1) as
mobile phase. The first compound eluted from the column was
◦
to give white crystals (0.50 g, 75%, mp 150 C). Anal Calc. for:
C4H9Cl9N8P6: C, 7.13; H, 1.35; N, 16.62%; (M, 674.1). Found: C,
7.15; H, 1.35; N, 16.55%; ([M + H]+, 675.0). 1H NMR, CDCl3, 298
K: d 1.95 [quintet, 2H, -CH2- 3J(HCCH) ca. 5.9 Hz]; 2.62 [d, 3H,
-CH3, 3J(PNCH) 14.7 Hz]; 3.13 [dt, 2H, CH3–N–CH2 3J(PNCH)
15.9 Hz, 3J(HCCH) 6.6 Hz]; 3.79 [m, 2H, NCH2].
Acknowledgements
The authors thank the Scientific and Technical Research Council
of Turkey for financial support (Grant 106M334) and the EPSRC
for funding the National Crystallography Service (Southampton,
UK).
◦
the product 4c (0.35 g, 19%, mp 313 C, DSC on 1.2 mg). Anal
Calc. for C12H10Cl8N8P6: C, 19.59; H, 1.37; N, 15.23%, (M, 735.7).
5314 | Dalton Trans., 2011, 40, 5307–5315
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©