Computational Study of Intramolecular Heterocyclic Ring Formation with Cyclic Phosphazenes

International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 Vol. 3 Issue 8, August - 2014 Computational Study of Intramolecular Heterocyclic Ring Formation with Cyclic Phosphazenes Whelton A. Miller III Preston B. Moore Department of Bioengineering Department of Chemistry and Biochemistry School of Engineering and Applied Science University of the Sciences in Philadelphia University of Pennsylvania Philadelphia, PA 19104 USA Philadelphia, PA 19104 USA Abstract—Polyphosphazenes, because of their unique properties, substituted cyclic phosphazenes such as have generated many opportunities to explore a variety of hexachlorophosphazene (NPCl2)3, can yield three types of applications. These applications include areas such as products [13-16]. First, the dinucleophile may replace two biomedical research ( e.g. drug delivery) and material science chlorine atoms at the same phosphorus atom to form geminal (e.g. fire-resistant polymers). Phosphazenes potentially have or spirocyclic structures [17-21]. Second, chlorine units on more variations then benzene analogues because of different substitution patterns. Here we present A computational study adjacent phosphorus atoms may be replaced to produce a of the chemical modifications to a group of cyclic phosphazenes vicinal substituted transannular-bridged cis-structure [20,21]. mainly hexachlorophosphazene ( PNCl2)3. This study focuses on Third, substitution can produce a trans-vicinal bridge (figure the relative energies of reactivity of hexachlorophosphazene to 1) [13,20]. understand their geometry and the complexes they likely form. We compare diols, amino alcohols, and diamines with a carbon linker of 1-7 atoms. These heteroatom chains are attached to a single phosphorus atom or adjoining phosphorus atoms to form ring structures of geminal, vicinal (cis), and vicinal (trans) moieties. We find that the reactivities of “heteroatom caps” are predicted to be O,O (diol) > N,O (amino alcohol) > N,N (diamine). These results can be used to predict energetics and thus the stability of new compounds for biomedical aIJERTndIJERT industrial applications. Keywords—Phosphazenes; Cyclic Phosphazenes; Ring Formation; Quantum Mechanics (QM); Density Functional Theory (DFT); Phosphorus Nuclear Magnetic Resonance (31P NMR); Becke Lee, Yang, and Parr hybride Density Functional Theory (B3LYP) I. INTRODUCTION This Polyphosphazenes have been used in a variety of application, including several areas of biomedical research, such as drug delivery systems [1,2], and in material science, such as fire-resistant polymers [3]. Due to their substituition patterns, phosphazenes potenially have more variations than Fig 1. Substitution patterns for cyclized phosphazenes. X and Y are heteroatoms, i.e., N or O. benzene analogs. Here we present a computational study of the chemical modifications to a group of cyclic phosphazenes In this paper we focus on the relative energies of (figure 1). We report the general conformation of the hexachlorophosphazenes with heteroatom chains, specifically compounds, as well as relative energies, and potential the energy difference between geminal and vicinal structures. applications. By using Quantum Mechanics (QM) to calculate the energy In the past, phosphazenes have been studied for their of formation of the resulting geminal and vicinal substituted properties both as linear polymers [4,5,6], and cyclic structures, we strive to understand the relationship between structures [7,8,9]. Their general electronic structure has also heteroatom chain length, the resulting cyclized compound, been studied [10,11,12], focusing on the interactions between and the likelihood of compound formation. the nitrogen and phosphorus atoms linear or cyclic Phosphorus Nuclear Magnetic Resonance (31P NMR) geometries. Intramolecular reactions of difunctional spectroscopy is widely used to investigate the structure of nucleophiles, amino-alcohols, diols, and diamines with non- IJERTV3IS080927 www.ijert.org 1575 (This work is licensed under a Creative Commons Attribution 4.0 International License.) International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 Vol. 3 Issue 8, August - 2014 phosphorus compounds. We use QM calculations to predict III. RESULTS AND DISCUSSION 31 the P NMR shifts of our structures, and therefore correlate When compared to the cyclic phosphazenes, carbon structural changes to changes in shielding. We analyzed our 31 analogs such as benzene are limited because they permit only results, including the P NMR predictions, to further single substitution at a given position on the ring. Due to the understand the relative energies of dinucleophiles with cyclic substitution pattern of phosphazenes, branching can be phosphazenes. Our analysis allows the prediction of reactivity significantly different than in carbon analogs. One such and the rational design of future reactions and materials based substitution pattern is geminal substitution. The geminal on phosphazene chemistry. “spirocyclic” structures form a bicyclic ring at the II. METHODOLOGY phosphorous. Vicinal structures consist of attachments at two adjacent phosphorous atoms that can be either cis where the We examined three distinct conformations, consisting of attachment is on the same side of the ring, or trans where the geminal (spirocyclic) and vicinal (transannular) bridged attachment is on different sides of the ring. In our compounds (cis or trans) (figure 1). We attached heteroatom investigations we will use a carbon chain capped with chains 1-9 atoms in length, including heteroatom caps, but heteroatoms, where X and Y can be either N or O in any excluding hydrogen (HaX-(CH2)n-YHb), where X and Y can consist of any combination of O or N. Conformational substitution pattern to react with the phosphazene (scheme 1). analysis on the expected products was performed with The result of a secondary intra-atomic reaction made after the Molecular Operating Environment (MOE) [22]. The geometry initial heteroatom bond can give either the geminal or vicinal was optimized using Hartree Fock (HF), and Becke Lee, (cis or trans) substitution pattern. Yang, and Parr hybrid Density Functional Theory (DFT) A. Geminal Cyclic Phosphazene Complexes B3LYP with basis set 6-31g(d,p), and 6-31+g(d,p) [23-25]. Comparative calculations were done under tight optimization Geminal structures (figure 2) can be precursors to larger parameters, with diffusion functions added to the calculation phosphazene macromolecules. These compounds in turn can to account for the possible interaction of the lone pair be incorporated as dendrimers and linear polymers for electrons on N, O, P, and Cl atoms. Calculations were carried numerous uses. Understanding the energetics of these out using Gaussian03 [26]. The lowest energy structures after systems, therefore, is crucial to understanding the structural optimization are reported and used for analysis. We then used properties, preferred products of reactions, and by extension, equation 1 to determine the reaction energy (∆E), allowing for the properties and reactivity of larger phosphazene polymers. discrimination between favorable and unfavorable reactions (scheme 1). 31P NMR calculations were performed with the GIAO [26,27] method from the Gaussian03 package, using DFT with basis set 6-31g(d,p). (A) E(final)=E(PhosphazeneFP) + E(2HCl) (1) (B) E(initial)=E(Phosphazene ring (P3N3Cl6)+Amino Alcohol) + E(HaX-(CH2)n-YHb) IJERTIJERT Fig 2. Geminal Substituted Cyclized Phosphazene. X and Y can equal N or O. (C) ∆E(reaction)=E(final) - E(initial) Fig 3. Plot of energy (∆E) verses chain length for geminal cyclic Scheme 1. Reaction schemes for cyclic phosphazene with Heteroatom chains. phosphazenes. X-axis represents H X-(CH ) -YH , where X and Y can be O X and Y are heteroatom “caps,” i.e., N or O for geminal substitution, vicinal a 2 n b or N. Each product has been calculated using three different levels of theory. (cis) and vicinal (trans) substitution. IJERTV3IS080927 www.ijert.org 1576 (This work is licensed under a Creative Commons Attribution 4.0 International License.) International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 Vol. 3 Issue 8, August - 2014 Figure 3 shows the energy of formation of geminal As mentioned earlier, vicinal structures substituted in a cis substituted phosphazenes. For each product we used three (Figure 4) manner do not have a large additional ring strain different levels of theory to assess basis set dependence, and arising from the carbon chain attempting to traverse cyclic observed that the trend is independent of the theory used. phosphazenes. Without this “bending” at the P-N-P angle, the Specifically, the energy of formation is highly dependent on structures formed relate to chain length as expected. the ability to form cyclic rings and is lowest for the 5-atom (3- Favorable reactions begin at chain lengths of 4 atoms, carbon chain capped with heteroatoms) chain that forms a six- depending on the heteroatom caps, with oxygen being more membered ring, completed by the phosphorus atom. This reactive (Figure 5). Again, we have used three different levels preference parallels the well-known strain of ring formation in of theory to assess the basis set dependence on ∆E, and as carbon analogs such as cyclohexane. The “dip” in energy at before, the trends are independent of the basis set used. Chain the 5-atom chain length is similar to the dip in energy seen in the formation of cyclohexane, when compared to cyclobutane,

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