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

The hydrolytically stable trivalent, pentadentate amine alcohol ligand s 1,1-bis(2-hydroxyethyl)-4-(2-hydroxy-5-R-benzyl)-1 ,4-diazabutane 2 (H(3)hebab-R; a: R = H, b: R=OMe, c: R=Br, d: R=NO2) react with ammoni um metavanadate to form the oxovanadium(V) complexes [VO(hebab-R)] 4, which have been characterized by H-1, C-13, and V-51 NMR, vibrational (IR, Raman, and resonance Raman) and electronic spectroscopy. Accordin g the high flexibility of the ligand systems the complexes 4 possess t wo 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 gr oup trans to the tertiary amine nitrogen atom, whereas mer-4 is charac terized by a meridional configuration of the phenolate oxygen and the two amine nitrogen donors with the oxo group trans to the secondary am ine nitrogen atom. The two isomers are related by an isomerization equ ilibrium proceeding via a rotation about a pseudo C-3 axis of the octa hedral 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-dioxomolybd enum(VI) complexes of flexible high-denticity ligand systems. The elec tronic influence of the para substituent at the phenolate on the isome rization reaction has been investigated by V-51 NMR spectrnscopy. The thermodynamic parameters determined from the variable-temperature V-51 NMR data show the fac-isomers to be more stable in all cases. Althoug h the smallest Delta H degrees is observed for the methoxy substituted derivative 4b, the X-ray crystal structure analysis shows that even i n this case solely the fac-isomer is present in the solid state. For 4 b the H-1 and C-13 NMR data of both isomers can be observed and assign ed. The data reveal a pronounced dependence of the coordination induce d shift for the relevant resonances on the relative orientation of the chelate rings with respect to the oxo group at the vanadium center, l eading to characteristic resonance patterns. Additional information on the electronic structure of the complexes 4 and the analogous Schiff base derivatives [VO(sabhea-R)] 3 (H(3)sabhea-R = cylidene)-2-(bis(2-h ydroxyethyl)-amino)ethylamine; a: R=H, b: R=OMe, c: R=Br, d: R=NO2) ha s been derived from cyclic voltammetry and density functional calculat ions. The methanolysis reaction of the complexes 4 has been studied by V-51 NMR spectroscopy. A penta-coordinate intermediate has been obser ved, which indicates the semilabile character of the diethanolamine fr agment of the ligand systems 2 in the corresponding oxovanadium(V) com plexes.