Competitive NH···Ru/Fe Hydrogen Bonding in Ferrocenyl Ruthenocenyl Tosyl Hydrazone

Abstract

High Resolution Image Download MS PowerPoint Slide A strong nonclassical NH···Fe intramolecular hydrogen bond (IHB) is present in the literature-known diferrocenyl tosyl hydrazone ( 1 ). Here, we confirm by NMR and IR spectroscopy as well as by XRD methods that an analogous NH···Ru IHB is present in the heavier homologue diruthenocenyl tosyl hydrazone ( 2 ). The NH···Ru IHB in 2 is stronger than the NH···Fe IHB in 1 by 6 kJ mol –1, as determined by IR spectroscopy. Further, we probed the E / Z isomer directing abilities of NH···M IHBs in the synthesis of the mixed metallocenyl compound ferrocenyl ruthenocenyl tosyl hydrazone ( 3 ). 3 is obtained as a mixture of the Z and E isomers ( 3a, b ) with NH···Ru and NH···Fe IHBs, respectively. At 111 °C, 3a is preferred with an isomeric ratio of 10:1 (91% 3a ). The formation of the isomeric mixture allows a direct comparison of the competitive NH···Ru/Fe hydrogen bonding. The underlying reason for the 3a: 3b ratio is of thermodynamic nature, rooted in the differences in strengths of the NH···M IHBs of Fe and Ru. Thermal and photochemical approaches and protonation to the corresponding iminium/carbenium ions [H-3a] + / [H-3b] + were employed to overcome the barrier for the E → Z isomerization 3b → 3a, with concomitant switching of the NH···M IHB.