Vanadium complexes with side chain functionalized N-salicylidene hydrazides: Hydrogen-bonding relays as structural directive for supramolecular interactions

The synthesis and spectroscopic characterization of dioxidovanadium(V) complexes with hydrazone Schiff-base ligands derived from salicylaldehyde and ωω-hydroxy functionalized carbonic acid hydrazides with three different chain lengths are reported. This includes three series, the ammonium and potassium salts of the complex anions with the twofold deprotonated ligand systems as well as the corresponding neutral complexes [VO2(HL)]. For four complexes within these series crystal structures are reported, namely the full series of complexes with the ligand system containing the shortest chain length (i.e. ammonium salt, potassium salt, and neutral complex) and the ammonium salt with the ligand based on the longest side chain. All complexes posses a dioxidovanadium group with a five-coordinated vanadium atom in a distorted square pyramidal geometry. Within the series of ammonium salts an extensive hydrogen-bonding network including the side chain hydroxyl end group is observed which is virtually independent of the side chain length. Whereas the structure of the potassium salt is governed by ionic contacts resulting in a bilayered packing arrangement with intercalated potassium ions. In the case of the neutral complex a two-dimensional hydrogen-bonding network is observed. Magic angle spinning solid-state 5151V NMR is used to characterize all three series of complexes. The variation of the chemical shift anisotropy parameters is found to be strongly dependent on differences in the supramolecular structure of the compounds, such as hydrogen bonding or crystal packing, which are caused by variation of the protonation state, counterion, and side chain length.