STRUCTURAL INFORMATION ON CHROMIUM(V) COMPLEXES OF 1,2-DIOLS IN SOLUTION, AS DETERMINED BY ISOTROPIC AND ANISOTROPIC H-1 ENDOR SPECTROSCOPY

The H-1 ENDOR study of complexes formed by 1,2-diols, namely ethylene glycol, (S)-(+)-1,2-propanediol, and (2R,3R)-(-)-2,3-butanediol, has demonstrated the potential of this technique as a structural probe of Cr(V) complexes in solution. The spectra recorded in liquid CD3OD solution demonstrate the H-1 superhyperfine (shf) interaction, which, except for the case of the glycolate complex, is not resolved in the EPR spectra. The comparative analysis provides unambiguous evidence for the presence of two classes of protons in the chelated ethylene bridge of the complexes. The analysis shows also that the single H-1 ENDOR coupling observed for the bis(ethylene glycolato(2-))oxochromate(V) complex is really an average signal due to a fast axial-equatorial interconversion. Crystallike ENDOR information has been obtained on the anisotropic spectra recorded in frozen solution by using both Cr-52 and Cr-53 isotopes. The complementary use of the isotropic and anisotropic couplings allow discrimination among all the protons of the ligands and calculation of their distances from the metal ion. The proton ENDOR analysis has been supported by a theoretical simulation of the spectra.