in your element Tracing iodine Pierangelo metrangolo and Giuseppe resnati celebrate the bicentenary of the discovery of iodine — a good time to also bring to its conclusion an international project that aims to define and categorize halogen bonding.

wo hundred years ago, the French and higher animals. Their origin, possible chemist Bernard Courtois accidentally biological activity and significance have been discovered iodine while investigating discussed in more than 80,000 original articles

T Y the corrosion of his copper vessels. Shortly published in the past 60 years, according to after that, Humphrey Davy and Louis-Joseph rAr the US National Library of Medicine. LIB o

Gay-Lussac independently identified it as T The reactivity of iodine and its derivatives a new element, and in 1813 Gay-Lussac o has also impacted synthetic and structural PH named it after the Greek wordιώδης meaning chemistry. In the past 10 years, the number violet. A year later, again in the Gay-Lussac of studies on polyvalent iodine compounds SCIENCE

laboratory, the first non-covalent adduct / has increased dramatically. This surge in S of iodine was prepared on reaction with r interest is mainly due to the combination

ammonia — but it was only in 1863 that it INTE of their very useful oxidizing properties . w

was assigned the structure I2···NH3 (ref. 1). D with a benign environmental character and

A number of adducts featuring similar LES their commercial availability. For example,

attractive interactions with electron- Ar chiral hypervalent iodine compounds have CH

donor species have marked the history of © recently proved effective for enantioselective iodine. For example, the observation of oxidative coupling in asymmetric catalysis7.

I2···aromatic compound complexes made as radiocontrast agents in X-ray-imaging Crystal structures reported in the for a key contribution in the understanding techniques; tincture of iodine, a common Cambridge Structure Database that involve of donor–acceptor adducts2. More recently, disinfectant, is a 2–7% solution of iodine iodo-carbons have also more than tripled the development of dye-sensitized solar cells along with iodide salts — the iodides in the past decade, including widely used – – – technology has relied on the I /I3 couple — form the halogen-bonded ion I3 , which is species such as antimicrobial agents and the the preferred redox mediator since these more soluble than elemental iodine in this antifungal drug haloprogin. cells were first prepared, and still the one that ethanol–water mixture. Despite its 200 years of history, iodine is yields the most stable and efficient devices3. Iodine is essential to living organisms still very much part of recent developments Such non-covalent attractive interactions as a trace element. The human body in diverse areas of chemistry, and there is between the electropositive region of a typically contains 10–20 milligrams of little doubt that it will continue to attract covalently bonded halogen atom (including iodine, more than 90% of which is stored attention in the forthcoming decades. On

I2) and the electronegative region of an in the thyroid gland. It is used for the this anniversary, we wish iodine many more atom or group of atoms (such as ammonia, biosynthesis of the thyroid hormone T3 (a years of success. ❐ aromatics, or I– ions) are now interpreted tri-iodo-thyrosine) and pro-hormone T4 (its References in terms of halogen bonding. Iodine and its tetra-iodo- analogue) — both iodo-organic 1. Guthrie, F. J. Chem. Soc. 16, 239–244 (1863). 6 derivatives are the most prone to forming biomolecules relying on halogen bonding . 2. Benesi, H. A. & Hildebrand, J. H. J. Am. Chem. Soc. such bonds, and it is thus an interesting A very complex enzymatic system protects 71, 2703–2707 (1949). 3. Boschloo, G. & Hagfeldt, A. Acc. Chem. Res. 42, 1819–1826 (2009). twist of fate that an IUPAC project aimed at tissues from an excess of thyroid hormones 4. www.iupac.org/web/ins/2009-032-1-100 delivering the first general definition of the through a series of iodination/de-iodination 5. www.halogenbonding.eu halogen bond4,5 should come to an end in reactions — the O···I halogen bonds 6. Sandler, B. et al. J. Biol. Chem. 279, 55801–55808 (2004). the year of the bicentenary of the discovery occurring makes these reactions extremely 7. Sakakura, A., Ukai, A. & Ishihara, K. Nature 445, 900–903 (2007). of iodine. substrate-selective. For the metabolic The structural diversity of iodine- functions controlled by T3 and T4 to be PieRAngeLO MeTRAngOLO and containing species is particularly carried out, it is particularly important that giusePPe ResnATi are in the Department noteworthy, and those are involved in iodine is ingested — a lack of dietary iodine of Chemistry, Materials, and Chemical a wide variety of fields ranging from can cause mental diseases or goitre. engineering “giulio natta”, and at the Center materials science to biomedicine. Iodo The thyroid hormones are far from of nano science and Technology of the aromatic compounds, for example, serve being the only iodinated biomolecules. italian institute of Technology, both at the At present, approximately 120 have been Politecnico di Milano, Milano, 20133, italy. isolated from living organisms — including e-mail: [email protected]; microorganisms, algae, marine invertebrates [email protected]. Sn Sb Te I Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb 260 nature chemistry | VOL 3 | MARCH 2011 | www.nature.com/naturechemistry

© 2011 Macmillan Publishers Limited. All rights reserved