New Synthetic Approach to Molecular Rods Using Cyclophosphazene

Aug 1, 2017 - The molecular structure of compound 3a was unambiguously established by X-ray structure analysis. The appropriate crystallographic data ...
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New Synthetic Approach to Molecular Rods Using Cyclophosphazene-Based Oligospiranes Serap Beşli,*,† Ceylan Mutlu Balcı,† Aylin Uslu,*,† and Christopher W. Allen‡ †

Department of Chemistry, Gebze Technical University, Gebze Kocaeli 41400, Turkey Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125, United States

‡

S Supporting Information *

and the difficulties of functionalizing these types of molecules demonstrate the need for new synthetic routes and materials. Therefore, the search for new types of rod structures and the development of new efficient synthetic methods have been intensively studied research areas in the last decades.2 Cyclic phosphazenes are a remarkable group of inorganic heterocycles that can serve as templates for the construction of new derivatives having properties that lead to a large variety of applications.3−14 New derivatives can be easily obtained by the substitution reactions of the halogens (usually chlorine) attached to phosphorus atoms with various nucleophiles.6,8−11 It is known that the reaction of cyclic phosphazenes with a tetrafunctional alcohol such as pentaerythritol gives spirane-bridged compounds whose structures consist of four six-membered rings, each joined at a tetrahedral phosphorus or carbon atom to the next ring.15 Recently, deprotonation reactions were used to form fused cyclophosphazene rings joined by a phosphazene or phosphazane ring. The new types of molecular structures that were synthesized via these routes were found to be very stable and promising as potential building blocks for larger structures.16 These results prompted us to build up new types of molecular rods by combining the two strategies described above. For this purpose, new types of oligospiranes, with 3a and 3b being the first examples, were achieved, joining the two spirane-bridged cyclophosphazenes already reported. The newly designed molecular rods were constructed by a stepwise synthesis. Initially, the spirane-bridged cyclophosphazene derivative that linked the spiro−spiro structure (1) was obtained from the reaction of pentaerythritol with hexachlorocyclotriphosphazene, N3P3Cl6, according to a literature procedure.15 Compound 1 was allowed to react with n-butylamine in a 1:0.8 molar ratio in tetrahydrofuran (THF) in the presence of triethylamine for 24 h at room temperature, which provided compound 2, as shown in Scheme 1. Compound 2, which contains one monobutylamino side chain, was directly treated with sodium hyride in a 1:1 molar ratio in THF at room temperature for 24 h under an argon atmosphere, and oligospiranes with a four-membered cyclophosphazane ring (3a and 3b) in a spiro arrangement were obtained from the deprotonation reaction by a proton abstraction/chloride ion elimination mechanism (Scheme 1). The similarity of the proton-decoupled 31P NMR spectra of compounds 3a and 3b suggests that they are configurational

ABSTRACT: A synthetic approach was developed to prepare a new type of highly functionalized inorganic− organic heterocyclic molecular rod (3a and 3b) consisting of cyclophosphazene and carbocyclic units. Single-crystal X-ray diffraction analysis of 3a revealed that the molecular length was ca. 2.5 nm. The products of this high-yield process have the potential for derivatization via known phosphazene reactions.

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urrently, one of the most challenging tasks facing synthetic chemists is the design of novel molecules mimicking nature

Scheme 1. Synthesis of Cyclophosphazene-Based Oligospiranes (3a and 3b)

and improving strategies for the construction of these structural motifs in the nanoscale. In this context, molecular rods have been of growing interest because of their advantages as elementary functional building blocks for the construction of supramolecular structures for diverse applications in materials science, nanoelectronics, and biosciences.1 However, some synthetic obstacles © 2017 American Chemical Society

Received: July 10, 2017 Published: August 1, 2017 9413

DOI: 10.1021/acs.inorgchem.7b01715 Inorg. Chem. 2017, 56, 9413−9416

Communication

Inorganic Chemistry

Figure 1. Proton-decoupled 31P NMR spectra of compounds (a) 3a and (b) 3b.

The molecular structure of compound 3a was unambiguously established by X-ray structure analysis. The appropriate crystallographic data (Table S1), selected bond lengths, bond angles, and torsion angles (Table S2), and conformational parameters (Table S3) are summarized in the Supporting Information. The unique compound 3a (Figure 2) consists of four phosphazene rings, two terminal and two internal. The observed structure of 3a confirms that each pentaerythritoxy bridge in the compound is linked spiro−spiro to the two cyclophosphazene units that are joined by a N,N-spiro bridge in the center of the molecule (Figure 2). In compound 3a, the terminal phosphazene rings are approximately planar; the maximum deviation from the mean plane is 0.059(5) Å (for atom N5), while the internal phosphazene rings are in a slightly twisted conformation; the maximum deviation from the mean plane is 0.060(4) Å (for atom N3; Table S3). The new compound 3a having an anti configuration is achiral because of its center of symmetry. The bond lengths and bond angles in these rings are found within the normal range typically found in cyclotriphosphazenes.17 All six-membered spirane rings have a chairlike conformation, and the maximum deviations from the mean plane are 0.251(5) and 0.243 (5) Å (for atoms C3), respectively (Table S3). The molecular structure of 3a, as well as

Figure 2. View of the molecular structure for 3a with an atomnumbering scheme. Displacement ellipsoids were drawn at the 50% probability level, and the hydrogen atoms were omitted for clarity.

isomers that may be described as anti and syn, respectively, depending on the relative disposition of the two groups next to the four-membered cyclophosphazane ring, as shown in Scheme 1. Compounds 3a and 3b were observed as AX2 (two terminal phosphazene rings) and AA′MM′XX′ (two internal phosphazene rings) spin systems because of the different environments for the five phosphorus nuclei within the molecules in the 31 1 P{ H} NMR spectra, as given in Figure 1.

Figure 3. Schematic illustration of the possible molecular structure for longer cyclophosphazene-based oligospiranes. 9414

DOI: 10.1021/acs.inorgchem.7b01715 Inorg. Chem. 2017, 56, 9413−9416

Communication

Inorganic Chemistry

Chem. Soc. 2011, 133, 4896−4905. (d) Müller, P.; Nikolaus, J.; Schiller, S.; Herrmann, A.; Möllnitz, K.; Czapla, S.; Wessig, P. Molecular rods with oligospiroketal backbones as anchors in biological membranes. Angew. Chem., Int. Ed. 2009, 48, 4433−4435. (e) Petrukhina, M. A.; Henck, C.; Li, B.; Block, E.; Jin, J.; Zhang, S.-Z.; Clerac, R. Spirocyclic sulfur and selenium ligands as molecular rigid rods in coordination of transition metal centers. Inorg. Chem. 2005, 44, 77−84. (f) Kang, Y.; Seward, C.; Song, D.; Wang, S. Blue luminescent rigid molecular rods bearing n-7-azaindolyl and 2,2′-dipyridylamino and their Zn(II) and Ag(I) complexes. Inorg. Chem. 2003, 42, 2789−2797. (3) Gleria, M.; De Jaeger, R. Phosphazenes: A Worldwide Insight; Nova Science Pub Inc.: New York, 2004. (4) Adrianov, A. K. Polyphosphazenes for Biomedical Applications; Wiley: New York, 2009. (5) Uslu, A.; Yeşilot, S. Chiral configurations in cyclophosphazene chemistry. Coord. Chem. Rev. 2015, 291, 28−67. (6) Ma, H. X.; Li, J. J.; Qiu, J. J.; Liu, Y.; Liu, C. M. Renewable cardanolbased star-shaped prepolymer containing a phosphazene core as a potential biobased green fire-retardant coating. ACS Sustainable Chem. Eng. 2017, 5, 350−359. (7) Hayes, R. F.; Allen, C. W. The mechanism of a phosphazene− phosphazane rearrangement. Dalton Trans. 2016, 45, 2060−2068. (8) Kumar, D.; Singh, J.; Elias, A. J. Chiral multidentate oxazoline ligands based on cyclophosphazene cores: synthesis, characterization and complexation studies. Dalton Trans. 2014, 43, 13899−13912. (9) Kagit, R.; Yildirim, M.; Ozay, O.; Yesilot, S.; Ozay, H. Phosphazene based multicentered naked-eye fluorescent sensor with high selectivity for Fe3+ Ions. Inorg. Chem. 2014, 53, 2144−2151. (10) Elmas, G.; Okumuş, A.; Kılıç, Z.; Hökelek, T.; Açık, L.; Dal, H.; Ramazanoğlu, N.; Koç, L. Y. Phosphorus−nitrogen compounds. part 24. syntheses, crystal structures, spectroscopic and stereogenic properties, biological activities, and DNA interactions of novel spiro-ansa-spiro- and ansa-spiro-ansa-cyclotetraphosphazenes. Inorg. Chem. 2012, 51, 12841− 12856. (11) Ainscough, E. W.; Brodie, A. M.; Edwards, P. J. B.; Jameson, G. B.; Otter, C. A.; Kirk, S. Zinc, cadmium, and mercury complexes of a pyridyloxy-substituted cyclotriphosphazene: syntheses, structures, and fluxional behavior. Inorg. Chem. 2012, 51, 10884−10892. (12) Tun, Z.-M.; Heston, A. J.; Panzner, M. J.; Scionti, V.; Medvetz, D. A.; Wright, B. D.; Johnson, N. A.; Li, L.; Wesdemiotis, C.; Rinaldi, P. L.; Youngs, W. J.; Tessier, C. A. Group 13 superacid adducts of [PCl2N]3. Inorg. Chem. 2016, 55, 3283−3293. (13) Bowers, D. J.; Wright, B. D.; Scionti, V.; Schultz, A.; Panzner, M. J.; Twum, E. B.; Li, L.-L.; Katzenmeyer, B. C.; Thome, B. S.; Rinaldi, P. L.; Wesdemiotis, C.; Youngs, W. J.; Tessier, C. A. Structure and Conformation of the Medium-Sized Chlorophosphazene Rings. Inorg. Chem. 2014, 53, 8874−8886. (14) Craven, M.; Yahya, R.; Kozhevnikova, E.; Boomishankar, R.; Robertson, C. M.; Steiner, A.; Kozhevnikov, I. Novel polyoxometalate− phosphazene aggregates and their use as catalysts for biphasic oxidations with hydrogen peroxide. Chem. Commun. 2013, 49, 349−351. (15) Coles, S. J.; Davies, D. B.; Eaton, R. J.; Hursthouse, M. B.; Kılıç, A.; Shaw, R. A.; Uslu, A. The structural and stereogenic properties of pentaerythritoxy-bridged cyclotriphosphazene derivatives: spiro−spiro, spiro−ansa and ansa−ansa isomers. Dalton Trans. 2006, 1302−1312. (16) (a) Beşli, S.; Coles, S. J.; Davies, D. B.; Erkovan, A. O.; Hursthouse, M. B.; Kılıç, A. Stable P−N bridged cyclophosphazenes with a spiro or ansa arrangement. Inorg. Chem. 2008, 47, 5042−5044. (b) Beşli, S.; Coles, S. J.; Davies, D. B.; Kılıç, A.; Shaw, R. A. Bridged cyclophosphazenes resulting from deprotonation reactions of cyclotriphophazenes bearing a P−NH group. Dalton Trans. 2011, 40, 5307− 5315. (c) Beşli, S.; Yuksel, F.; Davies, D. B.; Kılıç, A. Conversion of a cyclotriphosphazene to a cyclohexaphosphazene by ring expansion. Inorg. Chem. 2012, 51, 6434−6436. (d) Beşli, S.; Mutlu, C.; Ibişoğlu, H.; Yuksel, F.; Allen, C. W. Synthesis of a new class of fused cyclotetraphosphazene ring systems. Inorg. Chem. 2015, 54, 334−341. (e) Beşli, S.; Mutlu Balcı, C.; Canturk, H.; Dogan, S.; Yuksel, F.; Allen, C. W. Unexpected ring expansion of a four-membered cyclophosphazane. Inorg. Chem. 2016, 55, 7832−7834.

the distance (nearly 2.5 nm) defining the size of the molecule, is shown in Figure S1. It is clearly discernible that 3a adopted a straight geometry. New inorganic−organic heterocyclic compounds in rod form were prepared (3a, 48%; 3b, 34%) by a deprotonation reaction in high yield. They have four and two PCl2 groups in the outer and inner phosphazene rings, respectively, which make them potential precursors for the preparation of macromolecular and supramolecular systems or the longer cyclophosphazene-based oligospiranes, as illustrated in Figure 3. In conclusion, a reliable, simple synthetic route was developed for the preparation of inorganic−organic heterocyclic rodlike compounds. These unique oligomers, which are very stable and soluble in most organic solvents, can be obtained with short steps, high yield, and high purity as opposed to the case for most of the molecular rods in the literature. Further work is underway to obtain the longer cyclophosphazene-based oligospiranes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01715. Syntheses, analyses, mass spectra, X-ray diffraction, and tables (PDF) Accession Codes

CCDC 1536440 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Serap Beşli: 0000-0003-3203-4689 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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DOI: 10.1021/acs.inorgchem.7b01715 Inorg. Chem. 2017, 56, 9413−9416

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DOI: 10.1021/acs.inorgchem.7b01715 Inorg. Chem. 2017, 56, 9413−9416