DIPROTONATION AND TAUTOMERISM IN OLEFIN-IRON CARBONYL COMPLEXES

At low temperatures (ca. –78°) solutions of the iron tricarbonyl complexes of butadiene, 1-methylbutadiene, and 1,4-dimethylbutadiene in strong acid display a high-field signal in the 1H nmr indicating the presence of hydrogen on the metal. The metal-bonded proton is in rapid exchange with a selected number of olefin protons. Evidence indicates that a second labile proton must also be associated with the complex and is responsible for the shift of first proton from olefin to metal. Through the metal complexes it is possible specifically to deuterate certain olefins. For the iron tricarbonyl complexes of cyclohexadiene and cycloheptadiene, diprotonation (at the metal and at the olefin) is observed under the same conditions. The metal-hydrogen signal in these complexes is also observed to average with certain olefin protons at slightly higher temperatures, owing to rapid exchange between mono- and diprotonated species. © 1969, American Chemical Society. All rights reserved.