Regioselective Protonation of the η4-Anthracene Ligand in [Mn(η4-C14H10)(CO)3]-To Give an η5-Anthracenyl Complex [Mn(η5-C14H11)(CO)3] That Undergoes a Low-Energy 1,4-Hydride Exchange Reaction

Abstract

Protonation of the terminal (η4-anthracene) manganese tricarbonyl anion [Mn(η4-C14H10)(CO)3]- gives the terminal (η5-anthracenyl) manganese tricarbonyl complex [Mn(η5-C14H11)(CO)3]. Single-crystal X-ray diffraction analysis confirms that the proton adds to the hydrocarbon ring, and labeling studies indicate that protonation is regiospecific and occurs at the endo site. Magnetization transfer experiments establish rapid spin population transfer from the exo site to the para ring position. This is proposed to involve an intermediate metal hydride, and dynamic NMR studies have been used to determine that the free energy barrier to this exchange is ca. 14.5 kcal/mol at and below ambient temperatures. This is interpreted in terms of a modification of the mechanism of Lamanna and Brookhart for the isomerization within 6-exo-1H-[Mn(η5-C6HD5)(CO)3].