Two geometrical isomers, exo- and endo-[VO(3-EtOsal-meso-stien)] (H2(3- EtOsal-meso-stien)=N,N'-di-3-ethoxysalicylidene-(R,S)(S,R)-1,2-diphenyl-1,2- ethanediamine), were selectively prepared separately. X-Ray structure analyses were carded out for two different crystals of the endo-isomer: endo- [VO(3-EtOsal-meso-stien)] · 3H2O, (A), monoclinic P21/c, a = 10.435(3), b = 9.696(3), c = 30.948(3) Å, β = 94.87(2)°, V = 3120(1) Å3 and Z = 4; endo-[VO(3-EtOsal-mesostien)] · CH3CN, (B), monoclinic P21/c, a = 15.935(3), b = 9.510(2), c = 20.891(2) Å, β = 100.58(1)°, V = 3112(1) Å3 and Z = 4. Under an argon atmosphere, the endo-isomer in both crystals A and B isomerized to the exo-isomer completely upon heating at 210°C for 20 h. The X-ray powder diffraction patterns of the heated crystals A' and B' were approximately identical with that of exo-[VO(3-EtOsal-meso-stien)] · H2O (C). In the latter the V=O bonds are arranged in crystals to form a fairly weak linear chain structure (V=O ··· V=O ···). The drastic rearrangement of the metal complexes in the solid state accompanied by the isomerization reaction indicates that the thermal reaction proceeds in a non- topochemical fashion.
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