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

Abstract

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 pho...