| Abstrakt: |
Two series of octahedral oxovanadium(IV) compounds, containing charged or neutral axial ligands, with the tetradentate amidate molecules Hcapca and H2capcah of the general formulae trans‐[VIVOX(capca)]0/+(where X=Cl−(1⋅CH2Cl2), SCN−(2), N3−(3), CH3COO−(4), PhCOO−(5), imidazole (6⋅CH3NO2), and η‐nBuNH2(7)) and cis‐[VIVOX(Hcapcah)]0/+(where X=Cl−(8⋅0.5 CH2Cl2), SCN−(9), N3−(10⋅2 CH3OH), and imidazole (11)), were synthesized and characterized by X‐ray crystallography (1⋅CH3OH, 8⋅CHCl3, 9⋅2 CH3CN, 10⋅CH3CN and cis‐[VO(imidazole)(Hcapcah)]+) and continuous‐wave electron paramagnetic resonance (cw EPR) spectroscopy. In addition to the synthesis, crystallographic and EPR studies, the optical, infrared and magnetic properties (room temperature) of these compounds are reported. Ab initio calculations were also carried out on compound 8⋅CHCl3and revealed that this isomer is more stable than the transisomer, in good agreement with the experimental data. The cw EPR studies of compounds 1–5, that is, the VIVO2+species containing monoanionicaxial ligands, revealed a novel phenomenon of the reduction of their Azcomponents by about 10 % relative to the N4reference compounds ([VIVO(imidazole)4]2+and [VIVO(2,2‐bipyridine)2]2+, see reference 46). In marked contrast, such a reduction is not observed in compounds 6⋅CH3NO2–11, which contain neutralaxial ligands. Based on the spin‐Hamiltonian formalism a theoretical explanation is put forward according to which the observed reduction of Azis due to a reduction of the electron–nuclear dipolar coupling (P). The present findings bear strong relevance to cw EPR studies of oxovanadium(IV) in vanadoproteins, VIVO2+‐substituted proteins, and in VIVO2+model compounds, since the hyperfine coupling constant, Az, has been extensively used as a benchmark for identification of equatorial‐donor‐atom sets in oxovanadium(IV) complexes. |
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