Coordination Chemistry Reviews 257 (2013) 2032–2062 Contents lists available at SciVerse ScienceDirect Coordination Chemistry Reviews journa l homepage: www.elsevier.com/locate/ccr Review Tri-s-triazines (s-heptazines)—From a “mystery molecule” to industrially relevant carbon nitride materials ∗ Anke Schwarzer, Tatyana Saplinova, Edwin Kroke Institut für Anorganische Chemie, TU Bergakademie Freiberg, Leipziger Straße 29, 09596 Freiberg, Germany Contents 1. Introduction and historical overview . 2033 1.1. The heterocyclic systems s-triazine C3N3 and s-heptazine C6N7 – a brief comparison . 2033 1.2. Heptazines – a historical sketch. 2034 2. Melem, and melemium salts . 2036 2.1. Melem . 2036 2.1.1. Formation . 2036 2.1.2. Melem – structure and properties. 2038 2.2. Melemium salts . 2040 3. s-Heptazine-based salts and acids . 2041 3.1. Cyameluric acid and cyamelurates . 2041 3.2. Preparation . 2041 3.2.1. Physical and chemical properties . 2041 3.2.2. Thermal stability and spectroscopic properties . 2042 3.2.3. Applications. 2042 3.3. Melonates and hydromelonic acid . 2042 3.3.1. Introduction . 2042 3.3.2. Preparation . 2043 3.3.3. Physical properties . 2043 3.3.4. Vibrational and NMR spectroscopy . 2043 3.3.5. Crystal structures . 2043 3.3.6. Applications. 2045 4. Other molecular tri-s-triazine derivatives . 2045 4.1. C6N7H3 – the parent s-heptazine. 2045 4.2. Cyameluric halides – tri-chloro-tri-s-triazine . 2045 4.3. The azide C6N7(N3)3 . 2046 4.4. C6N7R3 – alkyl- and aryl s-heptazine derivatives . 2047 4.5. Symmetric and asymmetric cyameluric esters . 2047 4.6. Symmetric and asymmetric cyameluric amides . 2048 4.7. Further molecular derivatives of melem . 2048 4.7.1. Trihydrazino-s-heptazine and melem derived iminophosphoranes . 2048 4.7.2. Imides . 2049 4.8. Comparative reflections on spectroscopic properties and crystal structures of molecular tri-s-triazine derivatives . 2049 4.8.1. Crystal structures . ..