Vanadium(V) Complexes with Flexible and Hydrolytically Stable Amine Alcohol Ligands and their isomerization in solution: Models for the interaction of Vanadium(V) with biogenic ligands

Abstract

AbstractThe hydrolytically stable trivalent, pentadentate amine alcohol ligands 1,1‐bis(2‐hydroxyethyl)‐4‐(2‐hydroxy‐5‐R‐benzyl)‐1,4‐diazabutane 2 (H3hebab–R; a: R = H, b: R = OMe, c: R = Br, d: R = NO2) react with ammonium metavanadate to form the oxovanadium(V) complexes [VO(hebab–R)] 4, which have been characterized by 1H, 13C, and 51V NMR, vibrational (IR, Raman, and resonance Raman) and electronic spectroscopy. According the high flexibility of the ligand systems the complexes 4 possess two different possible structures in solution. These two isomers differ in the ligand arrangement: fac‐4 possesses a facial configuration of the phenolate oxygen and the two amine nitrogen donors with the oxo group trans to the tertiary amine nitrogen atom, whereas mer‐4 is characterized by a meridional configuration of the phenolate oxygen and the two amine nitrogen donors with the oxo group trans to the secondary amine nitrogen atom. The two isomers are related by an isomerization equilibrium proceeding via a rotation about a pseudo C3 axis of the octahedral coordination environment. This isomerization mechanism seems to be of general importance, since it can also account for the solution structures of oxovanadium(V), cis‐dioxovanadium(V) and cis‐dioxomolybdenum(VI) complexes of flexible high‐denticity ligand systems. The electronic influence of the para substituent at the phenolate on the isomerization reaction has been investigated by 51V NMR spectroscopy. The thermodynamic parameters determined from the variable‐temperature 51V NMR data show the fac‐isomers to be more stable in all cases. Although the smallest ΔH° is observed for the methoxy substituted derivative 4b, the X‐ray crystal structure analysis shows that even in this case solely the fac‐isomer is present in the solid state. For 4b the 1H and 13C NMR data of both isomers can be observed and assigned. The data reveal a pronounced dependence of the coordination induced shift for the relevant resonances on the relative orientation of the chelate rings with respect to the oxo group at the vanadium center, leading to characteristic resonance patterns. Additional information on the electronic structure of the complexes 4 and the analogous Schiff base derivatives [VO(sabhea–R)] 3 (H3sabhea–R = N‐(5‐R‐salicylidene)‐2‐(bis(2‐hydroxyethyl)‐amino)ethylamine; a: R = H, b: R = OMe, c: R = Br, d: R = NO2) has been derived from cyclic voltammetry and density functional calculations. The methanolysis reaction of the complexes 4 has been studied by 51V NMR spectroscopy. A penta‐coordinate intermediate has been observed, which indicates the semilabile character of the diethanolamine fragment of the ligand systems 2 in the corresponding oxovanadium(V) complexes.