Integrated Paramagnetic Resonance of High-Spin Co(II) in Axial Symmetry: Chemical Separation of Dipolar and Contact Electron−Nuclear Couplings

Abstract

Integrated paramagnetic resonance, utilizing EPR, NMR and ENDOR, of a series of cobalt bis-trispyrazolylborates, Co(Tpx)2, are reported. Systematic substitutions at the ring carbons and on the apical boron provide a unique opportunity to separate through-bond and through-space contributions to the NMR hyperfine shifts for the parent, unsubstituted Tp complex. A simple relationship between the chemical shift difference (δH − δMe) and the contact shift of the proton in that position is developed. This approach allows independent extraction of the isotropic hyperfine coupling, Aiso, for each proton in the molecule. The Co··H contact coupling energies derived from the NMR, together with the known metrics of the compounds, were used to predict the ENDOR couplings at gζ. Proton ENDOR data is presented that shows good agreement with the NMR-derived model. ENDOR signals from all other magnetic nuclei in the complex (14N, coordinating and non-coordinating, 11B and 13C) are also reported.

Anisotropic Fermi Couplings Due to Large Unquenched Orbital Angular Momentum: Q-Band 1H, 14N, and 11B ENDOR of Bis(trispyrazolylborate) Cobalt(II)

“…1), as indicated in earlier single-crystal EPR studies,6 are retained in frozen solution 10. At X-band, ENDOR signals could be obtained at g ⊥ ~ 1, but not near g || ~ 8.5.…”

“…All of the complexes studied here were prepared as described previously 10. Samples for ENDOR were 5 mM in 1:1 d 8 -toluene/CD 2 Cl 2 ; samples for EPR were 20mM.…”

“…The proton couplings are largely determined by Co(II)··H, through-space dipolar coupling. The position and orientation of the protons, nitrogens and borons, relative to the Co(II) ion and its g-tensor are known from our previous studies,10 and thus the signs of individual dipolar couplings are known. In general, for protons which have positive dipolar couplings only the ν + resonance, above the free proton frequency, was observed, while for protons which have negative dipolar couplings, only the ν − resonance, below the free proton frequency, was seen.…”

“…1). 10 In the course of determining hyperfine parameters and relating them to underlying electronic structure, we note and explain unusually anisotropic magnetic effects brought on by the large spin and orbital angular momentum of the ground state. Combination of our preceding X-band results with those of the present study shows that intrinsically isotropic Fermi couplings are made to appear highly anisotropic and proton dipolar couplings become non-traceless with extremely large values in the parallel direction.…”

A hyperactive cobalt-substituted extradiol-cleaving catechol dioxygenase

“…5 K) the spectra exhibited an absorption-like appearance due to rapid-passage effects commonly observed for cobalt complexes at 4 K due to slow relaxation [39, 40]. Co-HPCD exhibits a dominant rhombic signal with g values at 6.7, 3.4 and 2.4 (Figure 7), arising from the ground Kramers doublet of a high-spin d 7 Co(II) center ( S = 3/2) [37, 40, 41]. The low field signal exhibits well-resolved 8-fold hyperfine splitting with A = 80 G, due to the nuclear spin of 59 Co ( I = 7/2).…”