FULL PAPER DOI: 10.1002/ejic.200800063 Mixed-Ligand Oxidovanadium(V) Complexes with N'-Salicylidenehydrazides: Synthesis, Structure, and 51 V Solid-State MAS NMR Investigation Simona Nica, [a] Axel Buchholz, [a] Manfred Rudolph, [a] Annika Schweitzer, [b] Maria Wächtler, [b] Hergen Breitzke, [b] Gerd Buntkowsky, [b] and Winfried Plass* [a] Dedicated to Professor Achim Müller on the occasion of his 70th birthday Keywords: 8-Hydroxyquinoline / Schiff bases / Square-wave voltammetry / Vanadium / 51 V NMR spectroscopy The synthesis and spectroscopic characterization of a series of three oxidovanadium(V) complexes with 8-hydroxyquino- line and Schiff-base ligands derived from salicylaldehyde and ω-hydroxy-functionalized carbohydrazides with dif- ferent chain lengths are reported. The complex with the hy- drazone ligand containing the shortest chain length was crystallographically characterized. This complex crystallizes in the triclinic space group P1 ¯ with two structurally similar but crystallographically independent oxidovanadium(V) complexes. Each vanadium atom is six-coordinate in a dis- torted-octahedral geometry. The two molecules are as- sembled through hydrogen-bonding interactions between the hydroxyl groups of the side-chain substituted Schiff-base ligand and the oxido group of one of the two complexes. Electrochemical measurements performed in acetonitrile solution reveal two reversible one-electron reduction steps. Introduction There is a continuous interest in the coordination chem- istry of vanadium due to its catalytic and medicinal impor- tance. [1] Besides the insulin-like activity of oxidovanadi- um(V) and oxidovanadium(IV) compounds, [2,3] its presence in vanadium-dependent haloperoxidases [4–6] in particular has stimulated the search for structural and functional models. In this context pentavalent-vanadium complexes with the {VO 3+ } [7–12] and {VO 2 + } [13–16] core motifs have re- ceived considerable attention. Moreover, the ability of as- cidians to accumulate vanadium has initiated a rich chemis- try related to its reduced states. [1,17] This particularly in- cludes 8-hydroxyquinoline complexes, which exhibit an interesting electrochemistry. [18–20] The corresponding bis(8- hydroxyquinolinato)vanadium(V) complexes have been ex- [a] Institut für Anorganische und Analytische Chemie, Friedrich- Schiller-Universität Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany Fax: +49-3641-948132 E-mail: Sekr.Plass@uni-jena.de [b] Institut für Physikalische Chemie, Friedrich-Schiller-Uni- versität Jena, Helmholtzweg 4, 07743 Jena, Germany © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2008, 2350–2359 2350 The observed pre-wave feature of the second reduction step indicates the presence of dissociation equilibria related to the 8-hydroxyquinoline coligand. Magic-angle spinning solid- state 51 V NMR spectroscopy allowed to characterize the full series of complexes with alkyl and hydroxy alkyl-substituted hydrazone ligands that were used. The quadrupolar coupling constants are small with a value of about 4 MHz and show little variation within the series. The asymmetry of the chemi- cal shift tensor indicates a rather axial symmetric environ- ment around the vanadium(V) center. The isotropic chemical shifts observed in the solid state occur at about 30 ppm, which is in the same order of magnitude as the solvent in- duced variations, about 10 ppm, found for different solvents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) plored leading to a series of derivatives viewed as inorganic analogs of carboxylic acids. [21] We could recently show that N'-salicylidenehydrazides are versatile ligand systems supporting different types of vanadium(V) complexes, [15] which is a result of this ligand system being prone to variable protonation states at the amide group. [22–24] Moreover, these systems can easily be modified by the introduction of various functional groups attached to the ligand core. [25–27] A particular feature is the formation of intramolecular hydrogen-bonding interactions with the vanadate moiety, in this case a hydroxy function- alized side chain. [28,29] In the present work, we report on a series of mixed-li- gand oxidovanadium(V) complexes containing the ω-hy- droxy functionalized N'-salicylidenehydrazide ligand sys- tem together with the electrochemically interesting 8-hy- droxyquinoline as a coligand. In addition to their electro- chemical properties we also investigated these compounds by 51 V solid-state NMR spectroscopy. The latter has proven to be an excellent probe to gain quantitative site-specific information for amorphous and crystalline solids, in bulk, on surfaces, and at the interfaces. [30–34]