Communication pubs.acs.org/IC
Unexpected Ring Expansion of a Four-Membered Cyclophosphazane Serap Beşli,*,† Ceylan Mutlu Balcı,† Hakan Cantürk,† Semih Doğan,† Fatma Yuksel,† and Christopher W. Allen‡ †
Department of Chemistry, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125, United States
‡
S Supporting Information *
argon atmosphere for 24 h. The major product in each reaction (compounds 2−4) exhibits the same unique type of fused ring structure (Scheme 1).
ABSTRACT: Nucleophilic substitution reactions of the N(R),N(R)-spiro-bridged octachlorobis(cyclotriphosphazene), N3P3Cl4[N(CH2)5CH3]2N3P3Cl4 (1), with sodium salts of alcohols (1,3-propanediol, 2,2,3,3,4,4-hexafluoro-1,5-pentanediol, and phenol) give ansa products (2−4) via an unexpected rearrangement. These products were characterized by elemental analysis, mass spectrometry, and 1 H and 31P NMR spectroscopy. The molecular structures of compounds 3 and 4 were also established by X-ray crystallography. This new class of phosphazene structures consists of three fused P3N3 rings that arise from expansion of the four-membered phosphazane ring in 1 to a sixmembered N3P3 ring during alcoholysis reactions.
Scheme 1. Synthesis of Compounds 2−4
C
yclic phosphorus−nitrogen compounds have a wide range of derivatives having diverse applications by virtue of their multifunctional nature.1−8 Recently, N(R),N(R)-spiro (R = i-propyl, n-butyl, i-butyl, sec-butyl, n-hexyl, phenyl)-bridged octachlorobiscyclotriphosphazenes, which contain two cyclotriphosphazene rings linked by a four-membered cyclophosphazane ring in a spiro arrangement, were obtained from the deprotonation reaction of aminocyclotriphosphazene derivatives containing an exocyclic P−NHR group.9 Investigations of the reactions of these novel fused ring systems open up opportunities to uncover previously unexplored reactivity pathways and new fused ring architectures in phosphazene chemistry. We have reported the nucleophilic substitution reactions of N(R),N(R)spiro-bridged octachlorobis(cyclotriphosphazene) {N3P3Cl4[N(CH2)5CH3]2N3P3Cl4} (1) with the sodium salt of 1,2-ethanediol in different stoichiometries and obtained only the spiro derivative products monospiro, dispiro (ipsilateral, cis, and trans), and tetraspiro derivatives.10 In this investigation, the nucleophilic substitution reactions of compound 1 with two longer-chain diols (1,3-propanediol and 2,2,3,3,4,4-hexafluoro-1,5-pentanediol), both of which can give spiro and ansa products, is presented. The purpose of this investigation was to see if longerchain diols allow for attack at adjacent phosphorus centers (ansa) rather than, or in addition to, attack at the same phosphorus center (spiro). In order to clarify the role of difunctionality in product selection, compound 1 was reacted with the sodium salt of a monofunctional alcohol, phenol. 1 was allowed to react with the difunctional alcohols 1,3-propanediol and 2,2,3,3,4,4-hexafluoro-1,5-pentanediol and the monofunctional alcohol phenol in a 1:2 molar ratio in tetrahydrofuran, in the presence of sodium hydride under an © XXXX American Chemical Society
The proton-decoupled 31P NMR spectra of compounds 2 and 3 consist of ABX and A′M′X′ spin systems due to the different environments for the six phosphorus nuclei within the molecule. The proton-decoupled 31P NMR spectrum of compound 4 is observed as AX2 and A′M′X types of spin systems due to five different phosphorus nuclei. The proton-decoupled and coupled 31P NMR spectra of compound 2 are given as examples in Figure 1. The molecular structures of compounds 3 and 4 were unambiguously established by X-ray structure analysis. The appropriate crystallographic data (Table S1) and selected bond lengths, bond angles, and conformational parameters (Table S2) are summarized in the Supporting Information. The molecular structure of compounds 3 and 4 (Figures 2) confirmed that they are the unexpected structures, which have been synthesized for the first time in the nucleophilic substitution reactions of compound 1. The main skeleton in both compounds 3 and 4 is made up of three six-membered rings (labeled as A−C in Figure 3) in which two cyclophosphazene rings (A and C) are Received: June 27, 2016
A
DOI: 10.1021/acs.inorgchem.6b01533 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 1. (a) Proton-decoupled 31P NMR spectrum of compound 2. (b) Proton-coupled 31P NMR spectrum of compound 2.
Figure 2. Crystal structures of compounds (a) 3 and (b) 4. Displacement ellipsoids are drawn at the 50% probability level. The hydrogen atoms have been omitted, and only one orientation of the disordered hexylamine chain that is substituted on the N7 atom of compound 3 and only one orientation of the disordered at the C23 and C24 atoms of compound 4 have been presented for clarity.
The unsubstituted cyclotriphosphazene rings (A) in compounds 3 and 4 are approximately planar, and the bond lengths and bond angles in these rings are found within the normal range for many cyclotriphosphazenes.1−10 However, the substituted cyclotriphosphazene rings (C) containing phenoxy or fluorodioxy groups exhibit distortion and compression. The six-membered middle rings (B) also have distortion and compression for compounds 3 and 4, and their maximum deviations from the mean plane are 0.527(3) and 0.518(2) Å for the N6 atom, respectively (Table S2). Whereas the nitrogen atom in the cyclophosphazane ring in 1 is approximately trigonal-planar (the sum of the bond angles around the N4 atom is 346.8°), the N7 and N8 atoms in 3 (the sums of the bond angles around the N7 and N8 atoms are 360.0° and 359.4°, respectively) and 4 (the sums of the bond angles around the N7 and N8 atoms are 359.5° and 359.8°, respectively) are trigonal-planar. B rings have
Figure 3. Conformations of six-membered cyclophosphazene rings. Displacement ellipsoids are drawn at the 50% probability level.
linked in a spiro and an ansa arrangement by the six-membered middle cyclophosphazene ring (B). B
DOI: 10.1021/acs.inorgchem.6b01533 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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two different bond lengths. Two bonds, P4−N6 and P6−N6, which are fused to a cyclophosphazene ring, have normal phosphazene bond lengths in the middle six-membered ring. Four P−N bonds are longer than the other two bonds and are similar to those found in the central phosphazane ring in 1, indicating that these four bonds have single P−N bond character. The spiro bridge in compound 1 is converted to an ansa bridge on the side of the substituted cyclophosphazene ring during alcoholysis reactions. This behavior of 1 in the nucleophilic substitution reactions with long-chain diols and phenol may be explained in terms of the strain and reactivity of the fourmembered phosphazane ring. It is reasonable to presume that alcoholysis reactions proceeded by a common route. The spirobearing phosphorus atom is highly reactive in compound 1 because of strain in the four-membered spiro ring. One of the anionic oxygen ends of the diol attacks the spiro phosphazane site, leading to phosphazane ring opening and the concomitant release of ring strain, and the other end attacks one of the PCl2 groups in a typical phosphazene nucleophilic substitution reaction. The anionic nitrogen center derived from the four-membered spiro moiety is able to attack an adjacent phosphorus atom to form an ansa moiety with significantly reduced ring strain (Scheme 2a).
Communication
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01533. X-ray crystallographic data in CIF format (CIF) Synthesis, analyses, NMR spectra, X-ray diffraction, and tables (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Scientific and Technical Research Council of Turkey for financial support (Grant 113Z304).
Scheme 2. Possible Mechanisms for Formation of (a) Compounds 2 and 3 and (b) 4
REFERENCES
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There is a precedent for a displacement−reattachment process in the well-known phosphazene−phosphazane rearrangement in which the C−O bond in a POR center breaks and the resulting carbocation (because it is positively charged) is reattached at a neighboring nitrogen center.11 A similar mechanism is expected to operate in the formation of compound 4. A phenoxide nucleophile attacks the spiro-phosphorus center and causes the P−N bond of the four-membered ring to break. The anionic nitrogen center thus generated attacks the adjacent phosphorus atom and forms an ansa bridged by the opening of the spiro bridge from the one side. Then, a second phenoxy nucleophile in the medium attacks the bridgehead phosphorus atom, [P(NR)Cl], which is more reactive than the remaining PCl2 groups presumably because of the involvement in the ansa ring resulting in the phenoxy group arrayed in a nongeminal fashion (Scheme 2b). The nucleophilic substitution reactions of compound 1 with mono- and difunctional amines, amino alcohols (unpublished work), and 1,2-ethanediol, which gives only spiro products,10 progress without rearrangement, and normal substituted derivatives form, in contrast to the results reported above. Further work is underway to determine the nucleophilic substitution reaction pathways of compound 1 in order to better understand the factors that control these processes. C
DOI: 10.1021/acs.inorgchem.6b01533 Inorg. Chem. XXXX, XXX, XXX−XXX
