Old Molecules, New Chemistry

Ever since theorists predicted over a decade ago that a form of solid carbon nitride (C₃N₄) could exceed the hardness of diamond, chemists have pursued the material with zeal. While this "über-diamond" remains elusive, interest has spread to other forms of carbon nitride that have numerous potential uses ranging from semiconductors to fuel cells.

Now, the chemistry of an odd family of nitrogen-rich molecules discovered over 150 years ago is enjoying a renaissance, thanks to a new appreciation for its potential in creating carbon nitride materials.

Known as tri-s-triazines, or heptazines, the molecules share a triangular core group, C₆N₇, of three fused triazine rings. Chemists can vary what's attached to the triangle's corners, from hydroxyl groups to azides. These molecules have rigid structures and are frequently photoluminescent, and most are thermally stable.

Scientists have often turned to the widely industrially used s-triazines [including melamine, C₃N₃(NH₂)₃] as synthetic precursors to carbon nitride materials. Heptazines, however, are a bigger starting fragment--in essence, three triazines--and they may have advantages in the synthesis of extended C₃N₄ solids. There's also some evidence now that the graphitic sheets of C₃N₄ may indeed be composed of heptazine groups--not triazines, as originally thought.

THE FIRST MENTION of heptazines dates back to the 1830s, when Swedish chemist Jöns J. Berzelius discovered a crude polymer after igniting mercuric thiocyanate. The German chemist Justus von Liebig dubbed the substance melon, which had the formula (C₂N₃H)_n. The discoveries of other relatives followed, including melem [C₆N₇(NH₂)₃] and melam (C₆N₁₁H₉).

These chemicals were so inert, so insoluble, that deducing their structures was difficult. In the 1920s, chemists proposed several structures for melon, including a series of tri-s-triazines. Then in 1937, Linus Pauling stepped in with crystallography experiments that showed without a doubt that the core heptazine nucleus had a planar structure consisting of three fused s-triazine rings.

Since then, tri-s-triazines have languished in relative obscurity. Though melem, melon, and other related molecules have numerous industrial uses, such as fireproof materials, chemists hadn't, until recently, paid much attention to their chemistry.

There have been a few exceptions: For example, in the early 1980s, chemistry professor Nelson J. Leonard first synthesized the core, unsubstituted heptazine molecule, C₆N₇H₃. Leonard, now a faculty associate at California Institute of Technology, gave the molecule a thorough characterization using crystallography and spectroscopy, also using theory to study its properties.

Pauling apparently retained his interest in tri-s-triazines. After his death in 1994, he left behind on his blackboard a mysterious structure: a heptazine substituted with two hydroxyls and an azide (C&EN, Aug. 7, 2000, page 62).

In the past few years, a flurry of papers on heptazine research have signaled renewed interest in this area. "It's like an old photograph that someone took out of a drawer and put back on the wall," says Edward G. Gillan, chemistry professor at the University of Iowa, whose lab recently created an azide-substituted heptazine.

Heptazines seem to be an ideal building block for extended carbon nitride network structures, notes Edwin Kroke, chemistry professor at the University of Konstanz, in Germany. Kroke's work includes the first comprehensive study of trichloro-substituted tri-s-triazine. He and his colleagues also have synthesized a number of other heptazine derivatives [New J. Chem., 26, 508 (2002); Coord. Chem. Rev., 248, 493 (2004)].

In pursuit of the C₃N₄ diamond, chemist Tamikuni Komatsu at Asahi Chemical Industry, in Japan, demonstrated that graphitic carbon nitride structures likely take the form of linked heptazines [J. Mater. Chem., 11, 802 (2001)]. Recently, his group used tri-s-triazine derivatives such as the copper salt of 2,5,8-tricarbodiimide-tris-s-triazine to create graphitic carbon nitrides that could be shocked into producing a diamond-like structure [Phys. Chem. Chem. Phys., 6, 878 (2004)].

ASTOUNDINGLY, the structures of the 170-year-old molecules melon and melem were not confirmed until recently. Komatsu's lab fully characterized melon [Macromol. Chem. Phys., 202, 19 (2001)]. And Wolfgang Schnick, chair of the inorganic solid-state chemistry department at the University of Munich, and his colleagues confirmed melem's structure and provided still more evidence that graphitic carbon nitride likely exists in a heptazine form [J. Am. Chem. Soc., 125, 10288 (2003)].

Gillan, who is interested in molecules composed entirely of carbon and nitrogen as precursors to carbon nitride materials, had already published work on C₃N₁₂, triazido-s-triazine, when Pauling's mystery molecule caught his eye. Gillan decided to attach azides to all three corners of the heptazine, a goal accomplished by his graduate student Dale R. Miller [J. Am. Chem. Soc., 126, 5372 (2004)]. Unsurprisingly, triazido-heptazine is an energetic molecule that can detonate from a shock, though it's far more stable than the explosive, single-ringed triazido-s-triazine. The heptazine has promise in a number of applications, including the synthesis of metal coordination frameworks, Gillan says.

Chemistry professors Anmin Tian at Sichuan University and Ning-Bew Wong at the City University of Hong Kong and colleagues undertook an exhaustive theoretical study of 10 different heptazine derivatives, including those substituted with OH, NO₂, and CH₃ [J. Phys. Chem. A, 108, 97 (2004)]. "We have found that some of them might be good candidates for HEDMs, nonlinear optical materials, and molecular device templates," Wong notes.

And what of Pauling's mysterious heptazine with the two hydroxyl groups and the azide? Leonard claims Pauling intended it as a potential spectroscopic label for binding to DNA. As Leonard previously noted (C&EN, Oct. 2, 2000, page 8), "Pauling must have returned to the source of his original structural inspiration for a new application."