Synthesis and Structure of 2,5,8-Triazido-s-Heptazine: An Energetic and Luminescent Precursor to Nitrogen-Rich Carbon Nitrides

Article abstract of DOI:10.1021/ja048939y

Derivatized s-triazine (C₃N₃) precursors have seen significant recent use in the production of carbon nitride materials. Larger polycyclic molecular precursors, such as those containing an s-heptazine core (C₆N₇ or tri-s-triazine), may improve stability and order in carbon nitride products. In this Communication, we describe the synthesis and crystal structure of 2,5,8-triazido-s-heptazine (2). Synthesis of 2 was achieved from melon, an oligomeric s-heptazine synthesized by the pyrolysis of NH₄SCN. Melon was converted to molecular 2,5,8-trichloro-s-heptazine, which was then transformed to the triazide upon reaction with (CH₃)₃SiN₃. The crystal structure of 2 verifies that the s-heptazine is planar and the azides adopt a pinwheel-like C_3h arrangement around the periphery. The s-heptazine core shows π delocalization in the C-N bonds around the periphery (av. 1.33 ), while the internal planar C-N bonds are longer (1.40 ). The heptazine units pack into parallel, but offset, layered sheets in the crystal. The triazide 2 exhibits photoluminescence at 430 nm and rapidly and exothermically decomposes upon heating at 185 °C to produce a tan thermally stable carbon nitride powder with a formula near C₃N₄. Copyright

Full text of DOI:10.1021/ja048939y

Published on Web 04/09/2004
Synthesis and Structure of 2,5,8-Triazido-s-Heptazine: An Energetic and
Luminescent Precursor to Nitrogen-Rich Carbon Nitrides
Dale R. Miller, Dale C. Swenson, and Edward G. Gillan*
Department of Chemistry and the Optical Science and Technology Center, UniVersity of Iowa,
Iowa City, Iowa 52242
Received February 25, 2004; E-mail: edward-gillan@uiowa.edu
Aromatic nitrogen heterocycles have a wide variety of uses in
coordination chemistry and optical science. As one example, the
1,3,5-triazine ring motif (s-triazine, C3N3) has shown extensive
utility in synthetic chemistry,1 coordination chemistry,2 and optical
and magnetic studies.3 Over the past few years, s-triazine has played
a key role in molecular routes to carbon nitride (CNx) materials
investigated by various groups,4 including ours.5 Several of our
triazine ring precursors contrast with those of others in that they
are energetically unstable and rapidly decompose to CNx materials.
In an effort to incorporate larger C-N fragments in a precursor
structure, several groups have begun to examine the larger, related
1,3,4,6,7,9,9b-heptaazaphenalene ring system (tri-s-triazine or s-
heptazine, C6N7, see Scheme 1). The s-heptazine structure was first
postulated as a component of the polymer melon, [-C6N7(NH2)-
NH-]n, by Pauling and Sturdivant over 75 years ago.6a The
posthumous discovery of an azidoheptazine structure, [C6N7-
(OH)2N3], on Pauling’s chalkboard verifies his continued interest
Figure 1. Crystal structure representation of 2. Thermal ellipsoids drawn
in s-heptazines.6b Contemporary synthetic work on s-heptazine-
at 35% probability and bond lengths (Å) are noted.
derived materials includes: (i) the isolation and structure of melem,
[C6N7(NH2)3], which decomposes and graphitizes above 560 °C,7
The synthesis of 2 was accomplished via a melon intermediate
(ii) the pyrolysis of tricarbodiimide-s-heptazine, [C6N7(NCNH)3],
produced by the pyrolysis of NH4SCN.8,12 Melon was converted
at 550 °C to form a C3N3H extended network material,8 (iii) the
to the potassium salt of 2,5,8-trihydroxy-s-heptazine, [C6N7(OK)3],
pyrolysis of a mixture of C6N7(NCNH)3 and C6N7Cl3 (1) at 600
which was then treated with PCl5 to generate 2,5,8-trichloro-s-
°C to form an oligomeric C91N124H14 product,9 and (iv) the crystal
heptazine (1, Scheme 1).10 After purification by Soxhlet extraction
structure and optical properties of C6N7Cl3 (1).10 An extensive
in benzene, 1 was heated at 100 °C in neat trimethylsilyl azide
theoretical study on the structure, stability, and optical properties
under N2 for 12 h, yielding a quantitative conversion to 2,5,8-
of 10 heptazine molecules, including the title compound, was
recently reported.11 These studies show that, like their s-triazine
counterparts, s-heptazine-based precursors are promising, thermally
triazido-s-heptazine (2). Soxhlet extraction in dry acetone was used
to obtain the purified orange-tan product.
An FT-IR comparison of 1 with 2 clearly shows the appearance
robust candidates as precursors to nitrogen-rich, sp2-bonded carbon
of the azide vibrations centered at 2168 cm-1 and several peak shifts
nitride materials.
This Communication describes the synthesis and crystal structure
of 2,5,8-triazido-s-heptazine (2). This polycyclic, completely con-
jugated molecule is comprised of only carbon and nitrogen and is
and intensity changes in the 600-1000 cm-1 region (see Supporting
Information). Mass spectrometry data on 2 show the parent ion
peak at 296 amu and minor peaks consistent with successive N2
loss. The solution 13C NMR spectrum of 2 shows resonances at
energetically unstable due to its high azide content. The molecule
158.7 ppm (Scheme 1, carbon A) and 171.4 ppm (Scheme 1, carbon
2 exhibits visible light photoluminescence and rapidly decomposes
B) that agree very well with those of other s-heptazines.7,10,13
at 185 °C to nitrogen-rich CNx materials.
Scheme 1. Synthesis of 2,5,8-Triazido-s-heptazine (2)a
Single-crystal X-ray diffraction analysis of crystals grown from
dry acetone resulted in the structure of 2 represented in Figure 1.
The molecule is planar with bent azide groups giving rise to C3h
symmetry. The bond distances around the heptazine periphery are
very similar, indicating significant π bond delocalization. The
central nitrogen (N1) lies in the plane of the heptazine ring and
has sp2-like character, so its lone pair has some π overlap with
neighboring carbons. There are angular distortions along the outer
ring, such that the C2-N3-C4 angle is 116.6° while the N3-
C4-N5 angle is 128.8°. These results closely agree with data on
other s-heptazines7,10,13 and with theoretical predictions on 2.11
The azide groups show π bond localization with a short N7-
N8 bond distance (bond order ≈ 2.5) and a longer N6-N7 bond
distance (bond order ≈ 1.5). The C4-N6-N7 angle is 111.9°, and
a Reaction and conditions: (a) (1) 300 °C, air, (2) 400 °C, N2; (b) (1)
2.5 M KOH(aq), reflux, 4 h, (2) PCl5, 130 °C, N2, 10 h; (c) neat (CH3)3SiN3,
100 °C, N2, 12 h.
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C O M M U N I C A T I O N S
behind a tan-brown residue. It also has an impact sensitivity near
6 N‚m, which is similar to that of lead azide, an established primary
explosive.16
In summary, we report the synthesis and crystal structure of 2,5,8-
triazido-s-heptazine (2), a rare example of a fully conjugated,
polycyclic all carbon and nitrogen molecular compound. This planar
heptazine is photoluminescent near 430 nm and shows potential as
an energetic single-source precursor for the rapid synthesis of
nitrogen-rich C3N4 network materials. The large number of nitrogen
lone pairs present in 2 also makes it an attractive candidate as a
component in supramolecular metal coordination frameworks.
Acknowledgment. We thank the Research Corporation (Re-
search Innovation Award) and the University of Iowa for funding
this research.
Figure 2. UV-visible (red curve, left) and photoluminescence (blue curve,
right) spectra for 2.
the N6-N7-N8 angle is slightly bent at 172.6°. The azides lie in
the plane of a flat s-heptazine core. These results agree very well
with structural data from other conjugated nitrogen heterocycles
with azides14 and with theoretical structure calculations on 2.11
The molecules of 2 pack into an AB layer-like structure. The
azides and heptazines are offset from one layer to another leading
to a C3 symmetric channel at the origin running down the c axis
(see Supporting Information). The molecules in adjacent layers are
separated by roughly twice the nitrogen van der Waals distance
(3.08 Å), consistent with near molecule contacts.
The UV-visible absorption spectrum for 2 in ethanol is shown
in Figure 2. The peaks at 275 and 295 nm are likely due to π-π*
and n-π* transitions. Two very weak absorptions are also observed
at 360 and 385 nm and may contribute to the orange-tan appearance
of 2. Theoretical studies predict a HOMO-LUMO gap of 4.14 eV
(300 nm) for 2,11 consistent with the UV-visible data. Under 290
nm excitation, 2 shows a broad photoluminescence peak in ethanol
at 430 nm. This emission is in the range of luminescence observed
for other s-heptazines10,13 and π-conjugated nitrogen-containing
polycyclic systems.15 Note that extended illumination of 2 below
270 nm results in slow degradation, likely via photolytic azide
decomposition.
The thermal stability of 2 was examined by thermogravimetric-
differential thermal analysis (TG-DTA). Under argon flow and a 2
°C/min ramp rate, TG-DTA revealed a rapid weight loss at ∼185
°C accompanied by a sharp exothermic event (see Supporting
Information). The decomposition product was a tan-colored, visibly
porous solid with thermal stability up to ∼500 °C. An isothermal
150 °C TG-DTA experiment shows that 2 is relatively stable for 3
h (<5 wt % loss), then over the next 5 h it steadily loses 23 wt %,
achieving a constant weight equivalent to the loss of 3 N2 per
molecule. The material at this point no longer rapidly decomposes
upon heating at higher temperatures. The large-scale rapid decom-
position of 2 was initiated with a heated filament in a closed
stainless steel reactor under argon as previously described.5c The
product obtained by this method has a bulk elemental formula near
C3N4 and an FT-IR spectrum typical for sp2-bonded CNx materi-
als.4,5 Further characterization of this carbon nitride product is
ongoing.
Molecular azides are often thermodynamically unstable, shock
and impact sensitive, and should be handled with caution. For
example, triazido-s-triazine has explosive properties,16 violently
detonates during vigorous grinding in a polished agate mortar, and
produces pure carbon nanoflakes5a or carbon nanotubes17 when it
explodes. In contrast, the larger polycyclic triazide 2 can be safely
ground in a polished agate mortar, but visibly and moderately
decomposes when ground in an unpolished ceramic mortar, leaving
Supporting Information Available: Experimental procedures for
preparation and characterization of 2. FT-IR spectrum, TG-DTA plot,
crystallographic data, and packing structures for 2. Table comparing
data on 2 and related s-heptazines (PDF and CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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