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

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.