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Synthesis

Nitroprusside Color Mystery Resolved

Inorganic Chemistry: 17O NMR helps solve 170-year-old mystery surrounding the structural identities of the Gmelin reaction’s two deeply colored intermediates

by Stu Borman
October 26, 2015 | A version of this story appeared in Volume 93, Issue 42

BIG COLOR
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Credit: Chem. Eur. J.
Structures of red-violet [Fe(CN)5N(O)S]4– and blue [Fe(CN)5N(O)SS]4– Gmelin reaction intermediates.
Ball-and-stick models of two intermediates in the Gmelin reaction along with what their corresponding solutions.
Credit: Chem. Eur. J.
Structures of red-violet [Fe(CN)5N(O)S]4– and blue [Fe(CN)5N(O)SS]4– Gmelin reaction intermediates.

A 170-year-old chemical color mystery has been solved with 17O NMR spectroscopy and other analytical techniques. The Gmelin reaction, first observed in the 1840s by German chemist Leopold Gmelin, is considered one of the most intense color-forming reactions. When nitroprusside ([Fe(CN)5NO]2–) and sulfide combine they produce a brilliant red-violet intermediate that converts rapidly to a deep-blue intermediate. The red-violet intermediate has been proposed to be [Fe(CN)5(HSNO)]3– or [Fe(CN)5(SNO)]4–, but its structure was never confirmed. The identity of the blue intermediate has remained unknown, as has that of its decomposition products. Gang Wu and coworkers at Queen’s University, in Ontario, have now used 17O, 15N, and 13C NMR along with UV-visible and infrared spectroscopy and quantum chemical computations to show that the red-violet intermediate is [Fe(CN)5N(O)S]4– and the blue molecule is [Fe(CN)5N(O)SS)]4– (Chem. Eur. J. 2015, DOI: 10.1002/chem.201503353). Nitroprusside is used to treat hypertension, and the new findings could aid understanding of its mode of action, Wu says.

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