Anisotropic Fermi Couplings due to Large Unquenched Orbital Angular Momentum: Q-band 1H 14N and 11B ENDOR of bistrispyrazolylborate Co(II)

摘要

We report Q-band ENDOR of ¹H, ¹⁴N, and ¹¹B at the g|| extreme of the EPR spectrum of bistrispyrazolylborate Co(II), Co(Tp)2 and two structural analogs. This trigonally symmetric, high-spin (hs) S = 3/2 Co(II) complex shows large unquenched ground–state orbital angular momentum, which leads to highly anisotropic electronic g-values [g|| = 8.48, g⊥ = 1.02]. The large g-anisotropy is shown to result in large dipolar couplings near g|| and uniquely anisotropic ¹⁴N Fermi couplings, which arise from spin transferred to the nitrogen 2s orbital (2.2 %) via anti-bonding interactions with singly occupied metal dx²−y² and dz² orbitals. Large, well-resolved ¹H and ¹¹B dipolar couplings were also observed. Taken in concert with our previous X-band ENDOR measurements at g⊥ (Myers, et al, Inorg. Chem. 2008, 47, 6701–6710), the present data allow a detailed analysis of the dipolar hyperfine tensors of two of the four symmetry distinct protons in the parent molecule. In the substituted analogs, changes in hyperfine coupling due to altered metal-proton distances give further evidence of an anisotropic Fermi contact interaction. For the pyrazolyl 3H proton, the data indicate a 0.2 MHz anisotropic contact interaction and ~ 4 % transfer of spin away from Co(II). Dipolar coupling also dominates for the axial boron atoms, consistent with their distance from the Co(II) ion, and resolved ¹¹B quadrupolar coupling showed ~ 30 % electronic inequivalence between the B-H and B-C sp³ bonds. This is the first comprehensive ENDOR study of any hs Co(II) species and lays the foundation for future development.