A Geomimetic Synthesis of Mellite: A Mineral Containing a Benzene Ring

Abstract

Organic minerals that contain carbon, such as carbonates and oxalates, are well known. 1 Calcium carbonate can form on a geological timescale in the carbon cycle 2 and oxalates are present in small quantities in plants. 3 There are however, many other crystalline organic compounds found in the earth that are named after their place of discovery or a minerologist. These include abelsonite (a nickel(II) containing geoporphyrin), 4 carpathite (coronene), 5 fichtelite (a terpene derived hydrocarbon), 6 hoelite (9,10-anthraquinone), 7 kladnoite (phthalimide), 8 kratochvilite (fluorene) 9 and simonellite (a terpene derived hydrocarbon). 10 The structures are shown in the Electronic Supplementary Information (ESI) with pictures of the minerals. These compounds are much less polar than salts, but an additional polar organic mineral is known called mellite or honeystone. [11][12][13][14][15][16] Mellite is presumably formed from mellitic acid and Al 3+ ions. It is unusual in being the only crystalline mineral salt to contain a benzene ring. A search of the Cambridge Crystallographic Data Base revealed many metal-ion complexes of mellitic acid as a hexa-anion, 17-21 but only two that contained aluminium. One paper was the first in which the natural mineral was characterised by a single crystal structure determination, 11 and the other a later paper with a different crystal structure, in which the carboxylic acid groups were not fully deprotonated. 22 A Web of Knowledge search also illustrated the different approach of the two fields of chemistry and geology, since there were only papers on the characterisation and properties of the natural mineral, but not its synthesis. This paper reports our synthesis of mellite and our first foray into the field of organic geochemistry. Mellitic acid 1 can be prepared by the oxidation of graphite, firstly by treatment with HNO 3 , then KMnO 4 , 23 or by anodic oxidation. 24 Linus Pauling makes reference to the graphitemellitic acid transformation. 25 Mellitic acid can also be made by HNO 3 oxidation of hexamethylbenzene 26 and there is an early report of the oxidation of hexamethylbenzene by KMnO 4 taking two months. 27 The oxidation of organics to mellitic acid is considered as a possibility on the Martian subsurface as it occurs at varying depths of the Atacama Desert. 28 Mellitic acid is also a precursor of mellitic anhydride, C 12 O 9 , an interesting oxide of carbon. 29 DiscussionMellitic acid 1 is insoluble in water and so the parent compound can only be crystallised hydrothermally with metal ions. 22 However, it readily dissolves in water when treated with six equivalents of KOH which allows mixing with solutions of other metal ions. Two solutions in water were prepared, one of the hexacarboxylate anion 2, and one of the Al 2 (SO 4 ) 3 . 18H 2 O. These were then mixed (Scheme 1). The product was harvested in good yield and the single-crystal structure (Fig. 1) confirmed it to be identical with that of the natural mineral. 11 It was also identical with a modern data set obtained o...

Mellitate: A multivalent anion with extreme charge density causes rapid aggregation and misfolding of wild type lysozyme at neutral pH

“…Structure of MA 5- ion was obtained from ZINC database (ZINC23329815 record). Structure of MA 6- was obtained from CDS database (mellite crystallographic structure, SOGGEA01 record [ 21 ], CCDC number 1052513). Dockings were performed using Autodock 4.2.6 software [ 22 ].…”

Understanding geology through crystal engineering: coordination complexes, coordination polymers and metal–organic frameworks as minerals

“…There are a vast number of biological sources of formate and acetate species found in such minerals, as the corresponding acids appear in many plant, animal and bacterial metabolic processes (Vranova et al, 2013). On the other hand, the origin of mellitic acid in geological environments is not completely understood, but it was recently proposed that mellitic acid can form by oxidative degradation of graphite in coal (Plater & Harrison, 2015). Indeed, it is noted that the mineral mellite, a hydrated aluminium salt of mellitic acid, is often found in association with lignite coal deposits (Robl & Kuhs, 1991).…”