Metals in Contrived Environments

What defines chemistry over the past century has been our growing capacity to design and construct molecules. The number of new molecules that have been synthesized now number in the millions, and that number continues to grow at an astounding pace, along with continuing growth in synthetic sophistication; we have reached the era of the 'designer' molecule. Many of the new organic molecules prepared can bind to metal ions, or else can be readily converted to other molecules that can do so. This, along with the diversity caused by the capacity of a central metal ion to bind to a mixture of molecules at one time, means that the number of potential metal complexes that are not natural species is essentially infinite. Chemistry has altered irreversibly the composition of the world, if not the universe. Discovering when the first synthetic metal complex was deliberately made and identified is not as easy as one might expect, because so much time has passed since that event. One popular candidate is Prussian blue, a cyanide complex of iron, developed as a commercial artist's colour in the early eighteenth century. A more reliable candidate is what we now know as hexaamminecobalt (III) chloride, discovered serendipitously by Tassaert in 1798, which set under way a quest to interpret its unique properties, such as how separately stable species NH₃ and CoCl₃ could produce another stable species CoCl₃.6NH₃, and to discover similar species. As new compounds evolved, it was at first sufficient to identify them simply through their maker's name. Thus came into being species such as Magnus's green salt (PtCl₂-2NH₃) and Erdmann's salt (Co (NO₂)₃ KNO₂-2NH₃). This first attempt at nomenclature was doomed by profligacy, but as many compounds isolated were coloured another way of identification arose based on colour; thus Tasseart’s original yellow com pound CoCl₃·6NH₃ became luteocobaltic chloride, and the purple analogue CoCl₃·5NH₃ was named purpureocobaltic chloride. This nomenclature also dealt with isomers, with two forms of CoCl₃·4NH₃ identified and recognized– green praseocobaltic chloride and violet violeocobaltic chloride. Suffice to say that this nomenclature soon ran out of steam (or at least colours) also, and modern nomenclature is based on sounder structural bases, demonstrated in Appendix 1.

While some may quail at the outcomes of all this profligate molecule building, what re mainsaconstant are the basic rules of chemistry. A synthetic metal complex obeys the same basic chemical ‘rules’ as a natural one. ‘New’ properties result from the character of new assemblies,not from a shift in the rules. As a classic example of how this works, consider the case of Vitamin B12, distinguished by being one of a limited number of biomolecules centred on cobalt, and one of a rare few natural organometallic (metal–carbon bonded) compounds. This wasdiscovered to exist with good stability in three oxidation states, Co(III), Co(II) and Co(I). Moreover, it was found toinvolve a C Co(III)bond.At the time of these discoveries, examples of low molecular weights ynthetic cobalt(III) complexes also stable in both Co(II) and Co(I) oxidation states were few if any in number, nor had the Co(III)–carbon bond been well defined.Suchobservationslentsomesupporttoaviewthatmetalsinbiologicalentities were ‘special’. Of course, time has removed the discrepancy, with synthetic Co complexes stable in all of the (III), (II) and (I) oxidation states well established, and examples of the Co(III)–carbon bond reported even with very simple ligands in other sites around the metal ion. The ‘special’ nature of metals in biology is essentially a consequence of their usually very large and specifically arranged macromolecular environments. While it is demanding to reproduce such natural environments in detail in the laboratory, it is possible to mimic them at a sufficient level to reproduce aspects of their chemistry. Of course, the synthetic coordination chemist can go well beyond nature, by making use of facilities that don’t exist in Earth’s natural world. This can include even re-making elements that have disappeared from Earth. Technetium is radioactive in all its isotopic forms, and consequently has been entirely transmuted to other elements over time. However, it can be made readily enough in a nuclear reactor, and is now widely available. All of its chemistry, consequently, is synthetic or contrived. The element boron has given rise to a rich chemistry based on boron hydrides, most of which are too reactive to have any geological existence. Some boron hydrides as well as mixed carbon–boron compounds (carboranes) can bind to metal ions. Nitrogen forms a vast array of carbon-based compounds (amines) that are excellent at binding to metal ions; Nature also makes wide use of these for binding metal ions, but the construction of novel amines has reached levels that far exceed the limitations of Nature. After all, most natural chemistry has evolved at roomtemperature andpressurein near-neutral aqueous environments– limitations that do not apply in a chemical laboratory. What the vast array of synthetic molecules for binding metal ions provides is a capacity to control molecular shape and physical properties in metal-containing compounds not envisaged possible a century ago. These have given rise to applications and technologies that seem to be limited only by our imagination.

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المزيد

الآخبار الصحية

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عرض شائع في الفم قد يكون مرتبطا بخطر الإصابة بأمراض خطيرة

نصائح عامة لاستخدام سماعات الرأس بأمان

تأثير اللياقة البدنية على الدماغ

المزيد

الآخبار التكنلوجية

ديدان الأرض تكشف تلوث التربة بالمعادن الثقيلة

خبير أرصاد جوية: ظاهرة النينيو القادمة قد تكون الأقوى خلال 10 سنوات

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علماء الجيولوجيا يفكون لغز الصدع العملاق تحت الماء في المحيط الأطلسي

المزيد

مواضيع ذات صلة

Meeting More Metals - Bridging Ligands

More Teeth Stronger Bite - Polydentates

Greed is Good– Polydentate Ligands The Simple Chelate

Monodentate Ligands– The Simple Type Basic Binders

Ligands with More Bite– Denticity

Different Tribes– Donor Group Variation

Do I Look Big on That?– Chelate Ring Size

Putting the Bite on Metals– Chelation

Making Attachments– Coordination

The Road Ahead

Natural or Made-to-Measure Complexes

Metals in the Natural World