EPR and endor studies of cobaloxime(II)

Probing the surrounding of a cobalt(II) porphyrin and its superoxo complex by EPR techniques

“…Also the proton ENDOR spectra for the parallel and perpendicular orientations of CoOEP in THF plus toluene (base-on) were taken. Besides the proton signals of the OEP, larger proton couplings for the THF protons are found which are very similar to those measured earlier in a Co(dmgh) 2 complex [39]. For B I z, the couplings of 2.85, 2.00, and 1.50 MHz and for B II z couplings of 2.5 and 1.4 MHz are assigned to THF.…”

“…In the program, provision was made for noncollinear principal axes of the g tensor and the Co hf tensor [51] and an orientationally dependent linewidth variation as described earlier [52]. ENDOR frequencies are best interpreted by a nucleus-specific spin Hamiltonian containing the hf, the quadrupole, and the nuclear Zeeman interactions [8,9,34,[39][40][41][42][43]. First-order expressions for the ENDOR transition frequencies will approximately hold along g tensor principal directions, if the principal axes of the hf and quadrupole tensors are collinear and, at other orientations, if the hf or Zeeman interaction dominates the nuclear spin Hamiltonian. For a situation where the magnetic field is aligned along one of the principal axes of the g tensor (for example, g^), which also happens to coincide with principal axes of the hf (Az) and quadrupole (Qz) tensors, the ENDOR transition frequencies are given by:…”

“…For B I z, the couplings of 2.85, 2.00, and 1.50 MHz and for B II z couplings of 2.5 and 1.4 MHz are assigned to THF. These data were analyzed by us in an earlier publication on Co(dmgH)2 [39]. A very similar orientation of the THF molecule relative to the square-planar Co(II) complex to that determined earlier for THE ligation to Co(dmgH) z, can be assumed [39] for the case of CoOEP.…”

“…They display very small isotropic and anisotropic couplings, whose magnitudes decrease with increasing distance of the observed proton from the metal, i.e., ortho-(py) > meso(OEP) > ethyl(OEP). Our previous EPR/ENDOR powder study [39] on axially ligated Co(II)dmgH2 complexes has shown that the anisotropy is almost exclusively determined by the dipolar interaction between the bulk spin density at the cobalt atom and the distant proton. Not only could the spatial location of the protons in the axes system of the g tensor be determined but also coupling constant sign ambiguities could be usefully resolved.…”

“…In the Co(II) complexes studied here, the g anisotropy dominates the spectra and one obtains hf tensor components for the various hf-coupled nuclei along the principal axes of the g tensor. From this information one can, in principle, obtain the location of the nuclei around the spin-carrying metal center (distance greater than 0.2 nm) [34,38] by use of the point-dipole approximation as demonstrated earlier for axially ligated Cu(II) and Co(II) complexes [39][40][41][42][43]. More recently cw and pulsed multifrequency EPR and ENDOR studies of Co(II) tetraphenylporphyrin complexes (CoTPP) with imidazol and pyridine as axial ligands as well as their superoxo complexes have been published [8][9][10][11].…”