Structure and Conformation of the Medium-Sized Chlorophosphazene

Article pubs.acs.org/IC

Structure and Conformation of the Medium-Sized Chlorophosphazene Rings David J. Bowers, Brian D. Wright, Vincenzo Scionti, Anthony Schultz, Matthew J. Panzner, Eric B. Twum, Lin-Lin Li, Bryan C. Katzenmeyer, Benjamin S. Thome, Peter L. Rinaldi, Chrys Wesdemiotis, Wiley J. Youngs, and Claire A. Tessier* University of Akron, Department of Chemistry, Akron, Ohio 44325-3601, United States S Supporting Information *

ABSTRACT: Medium-sized cyclic oligomeric phosphazenes [PCl2N]m (where m = 5−9) that were prepared from the reaction of PCl5 and NH4Cl in refluxing chlorobenzene have been isolated by a combination of sublimation/extraction and column chromatography from the predominant products [PCl2N]3 and [PCl2N]4. The medium-sized rings [PCl2N]m have been characterized by electrospray ionization−mass spectroscopy (ESI-MS), their 31P chemical shifts have been reassigned, and their T1 relaxation times have been obtained. Crystallographic data has been recollected for [PCl2N]5, and the crystal structures of [PCl2N]6, and [PCl2N]8 are reported. Halogen-bonding interactions were observed in all the crystal structures of cyclic [PCl2N]m (m = 3−5, 6, 8). The crystal structures of [P(OPh)2N]7 and [P(OPh)2N]8, which are derivatives of the respective [PCl2N]m, are also reported. Comparisons of the intermolecular forces and torsion angles of [PCl2N]8 and [P(OPh)2N]8 with those of three other octameric rings are described. The comparisons show that chlorophosphazenes should not be considered prototypical, in terms of solid-state structure, because of the strong influence of halogen bonding.

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INTRODUCTION Polyphosphazenes encompass the largest class of inorganic backbone oligomers and polymers, and applications for these systems continue to accumulate.1,2 By way of substitution reactions, chlorophosphazenes are utilized as precursors to most of the hundreds of other phosphazene compounds. The array of promising applications that exist for phosphazene oligomers and polymers is attributed to the wide range of substituents that can be placed on the P−N backbone.1,2 Therefore, chlorophosphazene chemistry is central to the phosphazene field. Irreproducibility and other difficulties associated with the synthesis and storage of chlorophosphazenes, especially high polymeric [PCl2N]n, have limited commercial use of the entire class of polyphosphazenes. Thus, fundamental research of chlorophosphazenes is essential to overcome these problems. Classically, cyclic chlorophosphazenes of the formula [PCl2N]m (where m = oligomer = 2sigma(I)] R indices (all data)

parameter

8877

18909/0/1009 1.034 R1 = 0.0368, wR2 = 0.0727 R1 = 0.0523, wR2 = 0.0771

8609/2/433

1.018

R1 = 0.0247, wR2 = 0.0516

R1 = 0.0275, wR2 = 0.0526

99.8%

99.8%

full-matrix least-squares on F2

18909 [R(int) = 0.0550]

8609 [R(int) = 0.0262]


−16 ≤ h ≤ 17, −23 ≤ k ≤ 24, −42 ≤ l ≤ 42 99054

−23 ≤ h ≤18, −13 ≤ k ≤11, −29 ≤ l ≤30 13073

0.9815 and 0.9308

3360 0.31 mm × 0.20 mm × 0.08 mm 1.30°−28.31°

2688 0.23 mm × 0.14 mm × 0.11 mm 3.52°−27.30°

0.8058 and 0.6512

0.235 mm−1

1.997 mm−1

semiempirical from equivalents

7624.0(5) Å3 4 1.410 Mg/m3


18.0227(7) Å 31.6490(12) Å

10.5874(4) Å 23.5344(9) Å 112.095(2)° 4292.6(3) Å3 8 2.152 Mg/m3

[PCl2N]8

R1 = 0.0681, wR2 = 0.1489

R1 = 0.0557, wR2 = 0.1385

1.025

2367/0/145


0.8243 and 0.0506


93.8%

2367 [R(int) = 0.0594]

−28 ≤ h ≤ 28, −7 ≤ k ≤ 7, −23 ≤ l ≤ 16 7237

1792 0.41 mm × 0.01 mm × 0.01 mm 3.81°−65.78°

18.267 mm−1

6.1382(5) Å 20.3292(15) Å 111.114(5)° 2896.2(4) Å3 4 2.126 Mg/m3

Cl16N8P8 927.04 100(2) K 1.54178 Å monoclinic C2/c 24.8803(15) Å

Value/Remark C84H70N7O14P7 1618.26 100(2) K 0.71073 Å orthorhombic P2(1)2(1)2(1) 13.3660(5) Å

[P(OPh)2N]7

Cl12N6P6 695.28 100(2) K 0.71073 Å monoclinic Cc 18.5934(7) Å

[PCl2N]6

Table 1. Crystal Data and Structure Refinement for Monoclinic [PCl2N]6, [P(OPh)2N]7, [PCl2N]8, and [P(OPh)2N]8 [P(OPh)2N]8

R1 = 0.1932, wR2 = 0.1120

R1 = 0.0705, wR2 = 0.0891

1.113

9867/0/577


0.9919 and 0.9668


99.9%

9867 [R(int) = 0.1546]

−15 ≤ h ≤ 15, −33 ≤ k ≤ 20, −18 ≤ l ≤ 18 37504

1920 0.15 mm × 0.08 mm × 0.04 mm 1.58°−27.30°

0.233 mm−1

25.761(3) Å 14.643(2) Å 94.047(5)° 4385.5(10) Å3 2 1.401 Mg/m3

C96H80N8O16P8 1849.44 100(2) K 0.71073 Å monoclinic P2(1)/c 11.6548(16) Å

Inorganic Chemistry Article

dx.doi.org/10.1021/ic500272b | Inorg. Chem. 2014, 53, 8874−8886

chlorobenzene (Scheme 1). The following separation methods were chosen such that the temperature did not exceed ∼100 °C, because it has been suggested that ring−ring conversions, such as interconversion of [PCl2N]3 and [PCl2N]6, can occur above 100 °C.5a,29 After [PCl2N]3 and [PCl2N]4 were largely removed by sublimation at 60 °C under vacuum, the residue was extracted with hexane in order to isolate the mixed [PCl2N]m rings from the insoluble [Cl3PN(PCl2N)rPCl3+]X−. At this point, the mixture of [PCl2N]m rings can be stored in dry air for long periods. Whereas sublimation and recrystallization are popular techniques for purification of [PCl2N]3 and [PCl2N]4, they are less useful in separating the higher [PCl2N]m rings from each other. We decided to test reports that state that the separation of the higher [PCl2N]m rings can be done via thin-layer chromatography (TLC) on silica (hydrocarbon elutants),30 despite a report that suggests that surface silanol groups could displace Cl atoms in the rings, leading to covalent binding to the surface of the silica.31 We found that both TLC and column chromatography on silica with pure heptane eluent of the mixture of higher [PCl2N]m rings can be used to isolate [PCl2N]5, [PCl2N]6, [PCl2N]7, [PCl2N]8, and [PCl2N]9 in air. Instead of heptane, hexanes also can be used as an elutant, but the separation is not as good, with no separation of [PCl2N]4 and [PCl2N]5. According to NMR, MS and X-ray crystallographic data, which will be described below, the [PCl2N]m (m = 5−9) rings eluted in order of molecular weight, with the smallest ring coming off first. Several reports indicate that Scheme 1 produces cyclic oligomeric chlorophosphazenes that are larger than dodecamers.32 Further elution of the column with heptane and then toluene gave materials of higher molecular weights but in quantities too small for complete characterization. Some decomposition was evident because a yellow band of material remained on the column. The crude product mixture from Scheme 1 appears to contain acidic impurities, and the polymer [PCl2N]n develops acidic impurities upon storage.33 We have observed significantly greater air sensitivity when the rings [PCl2N]m are impure; this may be due to the presence of the more air-sensitive [Cl3PN(PCl2N)rPCl3+]X− or acidic impurities. Upon longterm storage of the impure higher rings in air, degradation begins to occur, as evidenced by the presence of POCl3, H3PO4, and related impurities (by 31P NMR spectroscopy34). Sublimation or filtration through a pad of silica are effective methods to remove any acidic impurities or [Cl3PN(PCl2N)rPCl3+]X− that may be present. Therefore, the success of the column chromatography may be due, in part, to the destruction of some impurities by the silica. Although the purified medium-sized rings are fairly stable to air (∼1−2 months), we recommend that they be stored under a dry gas to eliminate any degradation. Previous reports of 31P chemical shifts for the medium-sized cyclics [PCl2N]m are unclear and several reports are contradictory,19,35 which includes incorrect assignments by our group based on 31P DOSY NMR spectroscopy.36 Because the chromatography allowed us to isolate each species and confirm its identity with MS and X-ray crystallography (see below), the 31 P NMR chemical shifts now can be assigned unambiguously. Figure 1 shows the 31P NMR spectrum of each isolated cyclic [PCl2N]m in order of elution from the chromatographic column. The 31P chemical shifts were assigned as follows: [PCl2N]5 at −15.1 ppm, [PCl2N]6 at −15.3 ppm, [PCl2N]7 at −17.0 ppm, [PCl2N]8 at −17.7 ppm, and [PCl2N]9 at −17.7 ppm. The 31P chemical shift moves upfield as the ring gets

0.900 and −0.829 e Å−3

[PCl2N]8 Value/Remark −0.01(4)

0.304 and −0.370 e Å−3

0.02(5)

0.373 and −0.325 e Å−3

[P(OPh)2N]7

absolute structure parameter largest diff peak and hole

[PCl2N]6 parameter

Table 1. continued

Article

0.388 and −0.486 e Å−3

[P(OPh)2N]8

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[PCl2N]4 are exaggerated, unless a long relaxation delay was used. To the best of our knowledge, this was not done in earlier studies. Except for an ESI-MS study of [PCl2N]3,40 several decades have passed since the last report of the characterization of chlorophosphazene rings by mass spectrometry.41 For this reason, and to obtain unambiguous NMR assignments, the samples used for NMR characterization were further diluted and characterized by high-resolution electrospray ionization mass spectrometry (ESI-MS). A few ions were observed from the various [PCl2N]m in the positive ESI mode, but these data were less useful than data obtained in the negative mode. As an example, Figure 2 shows the high-resolution ESI spectrum of

Figure 1. 31P NMR spectra of (a) the cyclic pentamer [PCl2N]5, (b) the cyclic hexamer [PCl2N]6, (c) the cyclic heptamer [PCl2N]7, (d) the cyclic octamer [PCl2N]8, and (e) the cyclic nonamer [PCl2N]9, in order of elution from a silica gel column.

larger, but it levels off for the higher rings. This general trend is also observed for the rings [P(OPh)2N]m37 and largely for [PMe2N]m and [PF2N]m.14g,38 The reassigned resonances of [PCl2N]m offer evidence that [PCl2N]8 and [PCl2N]9 behave most similarly to the high-molecular-weight polymer [PCl2N]n in solution, because the 31P chemical shifts for the three substances are virtually identical. When several cyclic chlorophosphazene species are present in a sample, integration of 31P NMR spectra has been used to quantify the relative amount of each ring present.5,35,39 However, strained [PCl2N]3 and [PCl2N]4 would be expected to have longer relaxation times than the unstrained larger rings. A spin−lattice relaxation time (T1) 31P NMR spectroscopy experiment for a sample of purified rings [PCl2N]m (m = 3−9) in CDCl3 was completed and these data are shown in the Supporting Information. The relaxation delay value is the minimum relaxation delay time required to obtain accurate integration values, and it is five times the largest T1 value. [PCl2N]4 has the longest relaxation time at 14.0 s. The T1 value for the remaining rings decreases as the rings increase in size: [PCl2N]3, 13.4 s; [PCl2N]5, 11.2 s; [PCl2N]6, 8.2 s; and [PCl2N]7, 7.8 s. The unresolved signal for [PCl2N]8 and [PCl2N]9 relaxes with the shortest T1 value of 3.2 s. Therefore, for accurate quantification of mixtures of [PCl2N]m (m = 3−9), a relaxation delay time of 70 s (5T1 of [PCl2N]4) is required, which drastically lengthens the time required to obtain a spectrum. Based on several runs of the reaction in Scheme 1, 31 P NMR spectra that are integrated without the proper relaxation delay can result in great overestimation of the yields of the higher [PCl2N]m (m > 5). All integrations reported in this paper were run with a 70-s delay. There are some important ramifications that result from the above NMR studies. The reassignment of 31P NMR chemical shifts of the medium-sized rings indicates that some earlier mechanistic suggestions, with regard to the process in Scheme 1, may be in error. Because of the similarity of the T1 values, reported relative quantities of [PCl2N]3 to [PCl2N]4 obtained from the integrations of 31P NMR spectra are relatively accurate, irrespective of the relaxation delay that was used to obtain the spectrum. However, the relative quantities of the medium-sized rings compared to those of [PCl2N]3 or

Figure 2. Negative-ion mode, ESI mass spectrum of the cyclic heptamer [PCl2N]7. The cyclics [PCl2N]m (m = 5, 6, 8, 9) show similar product ion patterns, and their spectra can be found in the Supporting Information.

the [PCl2N]7 in the negative ion mode. The highest mass signal arises from addition of one electron to neutral [PCl2N]7 to give the radical anion [PCl2N]7−. The next highest mass signal at 18.97 Da lower mass has been assigned as the monooxygenated heptamer [P7Cl13N7O]−, which arises by displacement of one Cl atom of [PCl2N]7− by an O atom, whose source presumably would be water. The third signal at 34.97 Da lower mass from that of [PCl2N]7− is formed by loss of a Cl atom and is assigned to [P7Cl13N7]−. All isolated [PCl2N]m rings (m = 5− 9 ) s h o w e d [P m Cl 2 m N m ] − , [P m Cl 2 m − 1 N m O ] − , an d [PmCl2m−1Nm]− species in the negative mode ESI-MS (see the Supporting Information) as the predominant species. This observation suggests that no ring−ring equilibrium occurs in the gas phase with any of the isolated cyclics, even under highenergy conditions, and suggests that the medium-sized cyclic oligomers have a relatively high level of stability toward ionization. All the signals observed for [PCl2N]m rings (m = 5− 9) in the negative ESI mode showed the expected isotope distributions (error between 3−15 ppm). Single-Crystal X-ray Diffraction Studies of [PCl2N]m or Their Derivatives. Crystallographic data were collected for [PCl2N]3, as well as both forms of [PCl2N]4, [PCl2N]5, [PCl2N]6, [PCl2N]8, [P(OPh)2N]7, and [P(OPh)2N]8. The structures of the first three are known,15−17 but they were redetermined in order to make comparisons of crystal structures obtained under the same conditions and with structures that have better R-values (see the Supporting 8879


Inorganic Chemistry

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isomers of the 12-membered ring of the thionyl phosphazene [(PCl2N)2SOClN]4.44 The thermal ellipsoid plot of the crystal structure of [PCl2N]8 is shown in Figure 4, and selected bond distances and angles are given in Tables 2 and 3. The ring is elongated and is nearly planar. This ring shape is rather different from those observed for the crystal structures of other octameric phosphazene rings,14f,45,46 including that of [P(OPh)2N]8 (see Figure 7 below). Some of the larger P−N distances are statistically different from the smaller ones, but there is no alternating or other pattern to the different lengths, including whether the bond is part of the more linear segment of the ring or at the bent ends, which will be referred to as the kinks. Average bond lengths and angles for the crystallographically characterized [PCl2N]m (m = 3−6, 8) rings are shown in Tables 2 and 3, respectively. The P−N bond distances (Table 2) are toward the longer end of the range of distances characteristic of cyclophosphazenes and polyphosphazenes (1.47−1.62 Å).47 With the exception of [PCl2N]6, the P−N bond lengths become shorter as the cyclics become larger. This correlates with the observation that the larger rings have little or no ring strain and thereby more idealized bonding between P and N may occur. Except for [PCl2N]3, the average N−P−N angle decreases as the ring size increases. No clear trend in the average P−N−P angle and the size of the ring was observed, and the average Cl−P−Cl and N−P−Cl angles vary little with ring size. Interesting trends are observed from the ranges of particular angles for the cyclic oligomers. Consistent with the expectation that the larger rings are more flexible than the smaller homologues, the range of N−P−N angles is ∼0° for [PCl2N]3 and [PCl2N]4 (boat), ∼2° for [PCl2N]4 (chair), ∼4° for [PCl2N]5, ∼6°[PCl2N]6, and ∼6° for [PCl2N]8. However, this trend is not observed for the P−N−P angle. The P−N−P angles are highly variable in [PCl2N]5 and [PCl2N]6 and are in the range of 22° and ∼18°, respectively. In contrast, the range of P−N−P angles is ∼10° for [PCl2N]8 and ∼7° for [PCl2N]4 (chair), and the range for either [PCl2N]3 or [PCl2N]4 (boat) is

Structure and Conformation of the Medium-Sized Chlorophosphazene

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