New nongeminal cyclophosphazenes

Article abstract of DOI:10.1080/10426500490427222

A series of new nongeminally-substituted cyclic phosphazenes with various substituents has been prepared via deprotonation-substitution reactions at the Me groups of both the cis and trans isomers of [(Me)(Ph)PN]₃. Treatment of [(Me)(Ph)PN]₃ with n-BuLi followed by reaction with organic electrophilic reagents affords a variety of cyclic derivatives, [(RCH₂)(Ph)PN]₃, [R = Me, Cl, Br, I, (CH₂) ₂Br, CH₂CH=CH₂, SR, C(=O)OLi, C(=O)OMe, C(=O)OEt]. The structures of theses cis cyclic phosphazenes, which were obtained by x-ray diffraction, illustrate the basket-like shape of the molecules. Heating the cis and trans isomers of the parent [(Me)(Ph)PN]₃ produced mixtures of cyclic trimers and tetramers. The latter were isolated and characterized by x-ray crystallography. Nanoparticles of gold and silver were prepared by reduction of metal salts with a reducing agent in the presence of selected trimers.

Full text of DOI:10.1080/10426500490427222

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Phosphorus, Sulfur, and Silicon and the Related
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NEW NONGEMINAL CYCLOPHOSPHAZENES
Patty Wisian-Neilson a , June-Ho Jung a , Srinagesh K. Potluri a , Clayton E. Mauldin a &
Hongming Zhang a
a Southern Methodist University, Dallas , Texas, USA
Published online: 12 Aug 2010.
To cite this article: Patty Wisian-Neilson , June-Ho Jung , Srinagesh K. Potluri , Clayton E. Mauldin & Hongming Zhang (2004)
NEW NONGEMINAL CYCLOPHOSPHAZENES, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:4-5, 817-820, DOI:
10.1080/10426500490427222
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Phosphorus, Sulfur, and Silicon, 179:817–820, 2004
ꢀC
Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490427222
NEW NONGEMINAL CYCLOPHOSPHAZENES
Patty Wisian-Neilson, June-Ho Jung, Srinagesh K. Potluri,
Clayton E. Mauldin, and Hongming Zhang
Southern Methodist University, Dallas, Texas, USA
(Received August 21, 2003; accepted October 3, 2003)
A series of new nongeminally-substituted cyclic phosphazenes with
various substituents has been prepared via deprotonation-substitution
reactions at the Me groups of both the cis and trans isomers of
[(Me)(Ph)PN]3. Treatment of [(Me)(Ph)PN]3 with n-BuLi followed by
reaction with organic electrophilic reagents affords a variety of
cyclic derivatives, [(RCH2)(Ph)PN]3, [R = Me, Cl, Br, I, (CH2)2Br,
CH2CH CH2, SR, C( O)OLi, C( O)OMe, C( O)OEt]. The structures
of theses cis cyclic phosphazenes, which were obtained by x-ray diffrac-
tion, illustrate the basket-like shape of the molecules. Heating the cis
and trans isomers of the parent [(Me)(Ph)PN]3 produced mixtures of
cyclic trimers and tetramers. The latter were isolated and character-
ized by x-ray crystallography. Nanoparticles of gold and silver were
prepared by reduction of metal salts with a reducing agent in the pres-
ence of selected trimers.
Keywords: Nanoparticles; nongeminal; P C substituents; phosp-
hazene; tetramer; trimer
Although a number of cyclic phosphazenes with alkyl and aryl groups
have been prepared and studied,1 nongeminal substitution of alky
and aryl groups at phosphorus is relatively new. By contrast, the
preparation of phosphazene polymers with nongeminal alkyl and aryl
substituents, e.g., [(Me)(Ph)PN]n, via condensation polymerization of
N-silylphosphoranimines is well-established.2 Cyclic phosphazenes
with nongeminal substituents present interesting stereochemical va-
riety since two isomers are possible for nongeminal trimers, [RRꢁPN]3
and four isomers may exist for the corresponding tetramers. In this
The authors thank the Robert A. Welch Foundation, the Petroleum Research Fund
administered by the American Chemical Society, and the National Science Foundation
for generous financial support.
Address correspondence to Patty Wisian-Neilson, Department of Chemistry, Southern
Methodist University, Dallas, TX 75275. E-mail: pwisian@smu.edu
817

818
P. Wisian-Neilson et al.
article we discuss the preparation, characterization, and selected reac-
tions of cyclic phosphazenes with nongeminally substituted methyl and
phenyl groups.
RESULTS AND DISCUSSION
The diverse reactivity of a class of silicon-nitrogen-phosphorus com-
pounds known as N-silylphosphoranimines, Me3SiN P(X)RꢁRꢁꢁ, has
been exploited to make a number of new compounds, including the
polyphosphazenes with P C bonded alkyl and aryl substituents.2
In the course of these studies, it was found that treatment of
Me3SiN P(X)(Me)(Ph), where X = OPh or OCH2CF3, with simple al-
cohols facilitates cleavage of the silicon-nitrogen bonds as well as the
phosphorus-oxygen bonds. While mixtures of products were obtained
with most alcohols, treatment of Me3SiN P(OPh)(Me)(Ph), 1, with
CF3CH2OH in a 6:1 mole ratio in the absence of solvent gives ca. 90%
yields of a mixture of the cis and trans isomers of [(Me)(Ph)PN]3, 2.3
A large amount of phenol was also formed suggesting that the initial
reaction might have produced the intermediate N H phosphoranimine
which then lost PhOH (Eq. 1). It was necessary to remove the PhOH
by washing with aqueous base before these isomers were separated
by column chromatography. The ratio of trans to cis isomers was ca.
2.8 to 1. Both isomers were fully characterized by NMR spectroscopy,
x-ray crystallography, and elemental analysis. The cis isomer is partic-
ularly interesting since the phenyl groups on the same side of the nearly
planar PN ring resemble a basket.
(1)
Detailed studies of the thermolysis of cyclic phosphazenes fully sub-
stituted with phenyl or methyl groups have shown that while ring
opening allows for the interconversion of trimers and tetramers, no

New Nongeminal Cyclophosphazenes
819
polymer formation can be observed.4 The same type of equilibrium be-
tween trimers and tetramers occurs when either the cis or the trans
isomers of 2 are heated. At 220◦C the trimers predominate with the
trans isomer favored over the cis by at least 3:1. Less than 15% of the
trimers were converted to tetramers at this temperature as determined
by 31P NMR spectroscopy. However, at 250◦C, up to 70% of the trimers
were converted to tetramers after almost two weeks. Thus it was possi-
ble to isolate significant quantities of the tetramers. Indeed, each of the
four possible isomers were separated using a sequence based on solubil-
ity differences, column chromatography, and formation of HCl adducts.
Crystallographic analysis of each isomer shows that these have geome-
tries that may be identified as cone, partial cone, 1,2-alternate, and
1,3-alternate by analogy to calix[4]arene or as cis, α-trans, β-trans, and
γ -trans respectively.5
Like the well-studied polymer, poly(methylphenylphosphazene),
[Me(Ph)PN]n, there are three reactive sites in the simple cis and trans
isomers of the trimer [(Me)(Ph)PN]3, 2. Although the aryl groups are
susceptible to electrophilic aromatic substitution6 and the lone pairs of
electrons on the backbone nitrogen atom coordinate readily to many
Lewis acids,7 the methyl group provides the richest source of new
polymers via deprotonation-substitution reactions.8 Hence, we have fo-
cused on the derivatization of methyl groups1 to form new nongeminal
substituted cyclic phosphazenes.9 Simple deprotonation with n-BuLi
followed by treatment with the simple electrophile MeI was used to
demonstrate the feasibility of the process (Eq. 2). The trans isomer
cleanly goes to the tri-substituted derivative with three equivalents
of the reagents, but the cis isomer yields both di- and tri-substituted
products under similar conditions. This is likely due to the fact that
the deprotonated ion formed from the cis isomer is less soluble. A vari-
ety of other electrophiles have been used since the initial report9 and
afford variations of mono-, di-, and tri-substituted nongeminal cyclics,
[(RCH2)(Ph)PN]3, where R = Cl, Br, I, (CH2)2Br, CH2CH CH2, SR,
C( O)OLi, C( O)OMe, and C( O)OEt.
(2)

820
P. Wisian-Neilson et al.
Several of these new cyclic phosphazenes have been used to stabilize
metal nanoparticles as has been observed for the analogous polymer,
[(Me)(Ph)PN]n.10 Nanoparticles of gold and silver were prepared by re-
duction of metal salts such as NaAuCl4 or AgNO3 with a reducing agent
(e.g., NaBH4) in the presence of the trimer [(LiOOCCH2)(Ph)PN]3 in
water. The metal nanoparticle composites were characterized by UV-
vis spectroscopy and transmission electron microscopy (TEM).
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