P(CN)3 Precursor for Carbon Phosphonitride Extended Solids

Communication pubs.acs.org/cm

P(CN)3 Precursor for Carbon Phosphonitride Extended Solids Brian L. Chaloux,†,∥ Brendan L. Yonke,†,∥ Andrew P. Purdy,‡ James P. Yesinowski,‡ Evan R. Glaser,§ and Albert Epshteyn*,‡ †

NRC Postdoctoral Associate, ‡Chemistry Division, and §Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC 20375, United States S Supporting Information *

W

Scheme 1. Putative C3N3P Synthesis from 2,4-Difluoro-6(bis-trimethylsilyl)phosphino-s-triazine9

e report the thermal self-condensation of neat, white crystalline P(CN)3 powder into a black intractable solid and propose the chemical structure for the product material. This new C3N3P carbon phosphonitride is a P-substituted analogue of C3N4a material predicted to exhibit a number of crystalline phases with varied superlative properties.2,3 For example, cubic-C3N4 is a highly sought-after ultrahard material, which remains elusive.2,4−8 C3N3P is predicted to exhibit a stable graphitic polymorph (g-C3N3P, Figure 1) at ambient

WARNING: P(CN)3 readily hydrolyzes to toxic HCN and may burn violently in air.12 Exclusion of air and moisture, combined with proper ventilation, is required when working with this material. We synthesized P(CN)3 in 92% isolated yield as a white crystalline solid according to a literature procedure. 12 Polymerized samples were produced by heating 0.6 g of P(CN)3 overnight at 220 °C in evacuated, sealed Pyrex tubes (see details in the Supporting Information). The resulting black solid recovered under an Ar atmosphere accounted for 98.5% of the initial mass of P(CN)3, with small amounts of material still coating the inner surfaces of the tube. The nearly quantitative isolated yield is consistent with the observed absence of decomposition to gaseous byproducts. Under these conditions, both transformation of solid P(CN)3 to a black solid without melting and deposition of a uniform, black film onto the tube’s inner walls were observed, suggesting that the self-condensation occurs once P(CN)3 molecules attain sufficient mobility, whether in the gas or condensed phase. When P(CN)3 was heated rapidly to 220 °C in a sealed capillary, melting was clearly observed, followed by blackening and solidification. Defects and edge sites in solid P(CN)3 may allow for sufficient local mobility and diffusion at elevated temperatures to enable condensation of nitriles on short length scales. Although logic would dictate that the cyclization of nitriles when physically constrained is likely to be stepwise, from our observations it is not possible to conclusively discern whether the condensation occurs via a stepwise or concerted chemical mechanism. We can, however, rule out a solid−solid transformation of P(CN)3 to g-C3N3P, due to both the lack of an obvious transition state between crystalline P(CN)3 (I42̅ d symmetry13) to graphitic C3N3P (P6̅m2 symmetry9), as well as the isolation of only an amorphous product. An important fundamental consideration is whether the structure of C3N3P produced by self-condensation of P(CN)3 is

Figure 1. Hypothetical transformation of P(CN)3 to g-C3N3P. Side-on view of the structure of g-C3N3P predicted by DFT, plotted in Mercury using data published by Ding and Feng.9

pressure, which may transform into a low band gap high modulus β- or pseudocubic-phase at pressures of ∼12 GPa, one-fifth of the computed pressure required for the transformation of g-C3N4 to cubic-C3N4.9 This, along with C3N3P possibly having interesting photocatalytic and optoelectronic properties,3 provided our motivation to investigate a new means of producing C3N3P extended solid materials. Purdy and Callahan reported the synthesis of C3N4 via the thermal decomposition of metal thiocyanates.4 Using mass spectrometry, they identified a prevalent C3N4 gas-phase species during sublimation and redeposition of C3N4 solids under vacuum.4 To the best of our knowledge, N(CN)3, which is the likely structure of this C3N4 species, has never been isolated, whereas P(CN)3 is an isolable and stable compound.10−13 Hence, we set out to determine whether P(CN)3 can analogously be used as a precursor to C3N3P. The synthesis of a carbon phosphonitride with a C3N3P composition has previously only been reported once,11 where it was synthesized via thermal elimination of fluorotrimethylsilane (TMS-F) from an s-triazine-based monomer (Scheme 1); however, the reported characterization was limited to X-ray diffraction and Rutherford backscattering of this product.11 Volatiles generated during elimination syntheses of C3N4 and C3N3P complicate reaction conditions; thus, precursors capable of polymerizing to C3N4−xPx without byproducts are of interest. © XXXX American Chemical Society

Received: April 27, 2015 Revised: June 11, 2015

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DOI: 10.1021/acs.chemmater.5b01561 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials analogous to that of g-C3N4 (Figure 1). McMurran et al. proposed a similar structure for their material (Scheme 1), a vacuum-deposited C3N3P thin film that was reported to rapidly and stoichiometrically oxidize to C3N3PO in air.11 In contrast, our bulk C3N3P material exhibited slow mass uptake on exposure to air. Complete oxidation of all PIII to PV-oxide would cause a 14.7% increase in mass, but a gain of only 5.2% was observed. This air-exposed and stable end product is henceforth referred to as C3N3PO1−x to account for partial oxidation or hydrolysis. Although X-ray photoelectron spectroscopy (XPS) indicated complete oxidation of surface P to PV (Supporting Information Figure S3), it is not possible to infer the nature of oxidation from these data. It is likely that the oxidation is incomplete due to a large portion of the P being air-inaccessible. As-synthesized C3N3PO1−x consisted of large, irregular 1− 100 μm grains with homogeneous composition and no evidence of mesoporosity (2−50 nm pore size) based on scanning electron microscopy (SEM) (Figure 2, Supporting

corresponding to 48% densification from P(CN)3 (1.35 g cm−3)13 toward g-C3N3P (calculated as 2.39 g cm−3).9 Despite incomplete densification, Brunauer−Emmett−Teller (BET) N2 gas adsorption measurements showed negligible porosity at pore widths ≥5 nm (Supporting Information Figure S4), although low density can also be due to suboptimal packing of repeat motifs leading to long-range disorder, such as the density difference between vitreous carbon (ρ = 1.5 g cm−3) versus crystalline graphite (ρ = 2.2 g cm−3). XPS is unsuitable in the case of this C3N3P material for distinguishing C and N chemical environments, since organic nitriles (e.g., polyacrylonitrile)14 and azines (e.g., 1,3,5triazine)15 exhibit C 1s and N 1s binding energies identical to within tenths of an electronvolt, although it is useful for the determination of the degree of oxidation of surface P (Supporting Information Figure S3). Heterocyclic azine rings do exhibit characteristic vibrational modes that are observable by IR and Raman spectroscopy (Supporting Information Table S2).16−18 The Raman and attenuated total reflectance (ATR) FT-IR spectra of C3N3PO1−x are shown in Figure 4 with the

Figure 2. SEM of C3N3POx pressed into indium foil at 100× magnification. Inset at 1000× magnification.

Information Figure S5) and energy dispersive spectroscopy (EDS) (Supporting Information Figure S6). It is a robust material that is thermally stable up to 750 °C under both N2 and air. Thermal gravimetric analyses (TGA) of this powder (Figure 3) showed substantial decomposition only at ∼800 °C

Figure 4. ATR FT-IR (top) and Raman (bottom) spectra of C3N3PO1−x with overlaid blue (melamine),16,17 green (melem),18 and red (Ph3PO)1 bars indicating vibrational modes from the corresponding, color-coded species depicted below the spectra (also see Supporting Information Table S2).

vibrational modes of several known molecules overlaid in the respective colors. Melamine ring modes and Ph3PO phosphine modes are overlaid in blue and red, respectively; these constituent modes should be the primary components of the vibrational spectra of g-C3N3P based on its predicted structure. The ring modes of melem, a motif frequently encountered in C3N4, are overlaid in green for comparison. Interpretation of IR and Raman intensities is difficult, and local symmetry has greater influence on higher-order, lower frequency modes than simple stretching and bending modes. The presence of constituent modes, rather than their intensities, is most informative in elucidating structural motifs. In C3N3PO1−x, vibrational modes are broad (fwhm ≈ 200 cm−1), similar to vitreous carbon (fwhm ≈ 100 cm−1).19 A

Figure 3. TGA of C3N3PO1−x under N2 and air heated at 10 °C min−1.

under N2. Under air, 3.3% additional mass was lost at ∼550 °C, possibly corresponding to pyrolysis of unreacted, air-accessible nitriles.12 Both powder X-ray diffraction (PXRD) (Supporting Information Figure S7) and transmission electron microscopy (TEM) (Supporting Information Figure S8) showed C3N3PO1−x to be amorphous. By helium pycnometry, its density was measured to be ρ = 1.85 ± 0.01 g cm−3, B

DOI: 10.1021/acs.chemmater.5b01561 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

is substantially broader than would be expected for even highly disordered triazine networks based on DFT calculations.22 This suggests more than one kind of azine carbon, which is consistent with the two observed 31P NMR peaks. We propose that the lower frequency component of this peak and the unexpected 31P resonance at 230 ppm can be accounted for by formation of “phosphohexazines” (Scheme 2) via cyclization incorporating an entire P(CN)3 unit into a

qualitative comparison of the C3N3PO1−x spectra and the same vibrational modes of melamine (an s-triazine), melem (an sheptazine), and Ph3PO shows that a combination of azine ring modes and phosphine vibrational modes can adequately account for the most prominent features of the vibrational fingerprint (200−1700 cm−1) of C3N3PO1−x. The strong, broad band spanning 2500−3500 cm−1 is most likely associated with adsorbed water. We propose that C3N3PO1−x consists of azines bridging pyramidal phosphines, which (aside from partial oxidation) remain relatively unchanged compared to P(CN)3. Unfortunately, there is a significant overlap in frequencies of triazine and heptazine vibrational modes. It is difficult to envision a mechanism for the reaction of nitriles in P(CN)3 to form P-bridged s-heptazines (C6N7 stoichiometry) that would not involve detectable volatile byproducts. Linear polyazines, however, are a plausible alternative to cyclic s-triazine motifs. If cyclic azines and bridging pyramidal phosphines are the predominant structural moieties in C3N3PO1−x, 31P NMR spectra should show peaks near 0−20 ppm (corresponding to P and PO)20,21 and 13C spectra should exhibit only aromatic resonances in the range of 160−180 ppm.22 Figure 5 shows the solid state 31P NMR

Scheme 2. Proposed Mechanism Accounting for 9bPhospho-s-hexazine Formation in C3N3P

larger ring structure that is analogous to heptazines found in C3N4, but substituting P for the central N. Small molecule phosphohexazines have not previously been reported, making it difficult to verify the presence of this proposed moiety by comparing to literature characterization. The puckering of the ring induced by the pyramidalized central P in the proposed phosphohexazine structure may account for two effects seen in the 31P NMR results: (1) a substantial deshielding as well as (2) a highly anisotropic chemical environment. The breadth of the 13 C NMR signal likewise suggests the presence of three distinct carbons in the aromatic region with broad, overlapping chemical shift distributions, where the lower-frequency component arises from carbon attached to the central P supporting the presence of a phosphohexazine chemical environment.22 The acquisition of 31P and natural abundance 13C NMR spectra was aided by short T1 relaxation times, measured for 31P by a saturation-recovery pulse sequence to be ∼10 s. The electron spin resonance (ESR) spectrum of C3N3PO1−x (Supporting Information Figure S9) exhibits a single strong Lorentzian peak at g = 2.003 with a line width of 17 G and a spin density of ∼1019 cm−3, which explains the short NMR T1 values. Fast MAS suppresses spectral spin-diffusion between nuclei with different isotropic chemical shifts, so the similar 31P relaxation times seen for both species (differing widely in their CSAs) most likely arise from paramagnetic species in C3N3PO1−x affecting both P species in a similar fashion. Lack of hyperfine coupling to 14N or 31P and the short 31P and 13C NMR relaxation times suggest delocalization of these radicals across both organic azines and P, which has been observed in triazine tris(phosphonate)s.24 The origin of these unpaired electrons is unknown but is consistent with the ability of triazines to stabilize radical cations and anions.24−26 In summary, we have demonstrated that P(CN)3 may be employed as a precursor to produce C3N3P extended solid materials. Sealed under vacuum, it self-condenses at modest temperatures with total atom economy into an amorphous solid. In light of presented vibrational and NMR spectroscopic evidence, we submit that this amorphous solid is a 3D network polymer of disordered, P-bridged s-triazines, s-heptazine-like “phosphohexazines”, and unreacted nitriles, as illustrated in Figure 6. This material does not exhibit graphitic, long-range order but does contain the moieties that would be expected for such a material. The trimerization of alkyl and aryl nitriles has

Figure 5. 31P NMR spectrum of C3N3PO1−x under MAS. Isotropic peaks are marked with “↓”, spinning sidebands of the high frequency peak with “*”, and of the low-frequency peak with “×”. See Supporting Information for further experimental details.

spectrum of C3N3PO1−x acquired with rapid magic angle spinning (MAS), with the corresponding 13C MAS NMR spectrum illustrated in Supporting Information (Figure S2). As expected, the 31P NMR spectrum exhibits a strong peak with small chemical shift anisotropy (CSA) at ∼3 ppm (vs H3PO4) with a shoulder at lower frequency. Given the breadth of this peak, this resonance can account for both bridging phosphines and phosphine oxides.23 A second peak with a dramatically larger CSA is seen at ∼230 ppm. Thus, a second P species exists in C3N3PO1−xone which has a highly anisotropic magnetic environment that results in a spinning sideband pattern from the chemical shift anisotropy extending over 400 ppm. The natural-abundance 13C MAS NMR spectrum (Supporting Information Figure S2) shows that on the order of 10% of nitriles remain unreacted (visible as a resolved peak at 115 ppm vs TMS), kinetically trapped within C3N 3PO1−x. This information is unavailable from vibrational spectra but is consistent with the oxidation of unreacted nitriles observable in TGA under air. Aside from the nitrile resonance, only broad, overlapping aromatic CN resonances spanning 100−180 ppm are present in C3N3PO1−x. The 13C chemical shift of melamine (168 ppm)18 is in this region, but the observed peak C

DOI: 10.1021/acs.chemmater.5b01561 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

(8) Daasch, L. W.; Smith, D. C. J. Chem. Phys. 1951, 19, 22. (9) Ding, F.; Feng, Y. P. Comput. Mater. Sci. 2004, 30, 364. (10) Wehrhane, G.; Hübner, H. Annalen der Chemie und Pharmacie 1864, 132, 277. (11) McMurran, J.; Kouvetakis, J.; Nesting, D. C.; Hubbard, J. L. Chem. Mater. 1998, 10, 590. (12) Staats, P. A.; Morgan, H. W.; Cohen, H. M. In Inorganic Synthesis; Rochow, E. G., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1960; Vol. 6, p 84. (13) Emerson, K.; Britton, D. Acta Crystallogr. 1964, 17, 1134. (14) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Wiley: 1992. (15) Bu, Y.; Lin, M. C. J. Phys. Chem. 1994, 98, 7871. (16) Wang, Y.-L.; M. Mebel, A.; Wu, C.-J.; Chen, Y.-T.; Lin, C.-E.; Jiang, J.-C. J. Chem. Soc., Faraday Trans. 1997, 93, 3445. (17) Liu, X. R.; Zinin, P. V.; Ming, L. C.; Acosta, T.; Sharma, S. K.; Misra, A. K.; Hong, S. M. J. Phys.: Conf. Ser. 2010, 215, 012045. (18) Jürgens, B.; Irran, E.; Senker, J.; Kroll, P.; Müller, H.; Schnick, W. J. Am. Chem. Soc. 2003, 125, 10288. (19) Wang, Y.; Alsmeyer, D. C.; McCreery, R. L. Chem. Mater. 1990, 2, 557. (20) Miller, P.; Nieuwenhuyzen, M.; Charmant, J. P. H.; James, S. L. CrystEngComm 2004, 6, 408. (21) Zeldin, M.; Mehta, P.; Vernon, W. D. Inorg. Chem. 1979, 18, 463. (22) Sehnert, J.; Baerwinkel, K.; Senker, J. J. Phys. Chem. B 2007, 111, 10671. (23) Moedritzer, K.; Maier, L.; Groenweghe, L. C. D. J. Chem. Eng. Data 1962, 7, 307. (24) Maxim, C.; Matni, A.; Geoffroy, M.; Andruh, M.; Hearns, N. G. R.; Clérac, R.; Avarvari, N. New J. Chem. 2010, 34, 2319. (25) Selby, T. D.; Stickley, K. R.; Blackstock, S. C. Org. Lett. 2000, 2, 171. (26) O’Connell, A.; Podmore, I. D.; Symons, M. C. R.; Wyatt, J. L.; Neugebauer, F. A. J. Chem. Soc., Perkin Trans. 2 1992, 1403. (27) Bengelsdorf, I. S. J. Am. Chem. Soc. 1958, 80, 1442. (28) Cairns, T. L.; Larchar, A. W.; McKusick, B. C. J. Am. Chem. Soc. 1952, 74, 5633. (29) Katekomol, P.; Roeser, J.; Bojdys, M.; Weber, J.; Thomas, A. Chem. Mater. 2013, 25, 1542. (30) Algara-Siller, G.; Severin, N.; Chong, S. Y.; Björkman, T.; Palgrave, R. G.; Laybourn, A.; Antonietti, M.; Khimyak, Y. Z.; Krasheninnikov, A. V.; Rabe, J. P.; Kaiser, U.; Cooper, A. I.; Thomas, A.; Bojdys, M. J. Angew. Chem., Int. Ed. 2014, 53, 7450. (31) Wilkie, C. A.; Parry, R. W. Inorg. Chem. 1980, 19, 1499. (32) Guo, H.; Yonke, B. L.; Epshteyn, A.; Kim, D. Y.; Smith, J. S.; Strobel, T. A. J. Chem. Phys. 2015, 142, 194503.

Figure 6. Proposed structural composition of C3N3P (prior to air exposure) based on vibrational, NMR, and ESR spectroscopies and comparisons to known materials.

been well-studied, but usually such reactions require high temperatures (≥400 °C) in the absence of Lewis acid catalysis, so the relatively low temperature of 220 °C being sufficient to polymerize P(CN)3 was surprising.27−30 It is possible that, for P(CN)3, cyclization reactions are facilitated by the Lewis acid character of P, due to the electron withdrawing strength of the cyanide groups.31 Recently, our colleagues at the Carnegie Institution of Washington and we reported on the pressureinduced polymerization of P(CN)3; both thermo- and barochemical modes of reaction are important for the complete understanding of the reactivity of the P(CN)3 building block in producing new P-substituted CN materials.32 Further studies on the nature of P(CN)3 reactivity and the properties of the isolated C−N−P materials are ongoing.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Experimental procedures and additional figures and tables. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01561. Corresponding Author

*E-mail: [email protected]. Mailing address: 4555 Overlook Ave. SW, Washington, DC 20375. Author Contributions ∥

B.L.C. and B.L.Y. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Defense Advanced Research Projects Agency (DARPA) under ARO Contract Number W31P4Q-13-I-0005. We thank Dr. Timothy A. Strobel and Dr. Arthur W. Snow for technical discussions. B.L.C. and B.L.Y. are grateful to the National Research Council for administering their postdoctoral research associateships.

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b01561 Chem. Mater. XXXX, XXX, XXX−XXX