Compounds which incorporate pseudo-octahedral high-spin cobalt(II) ions often exhibit large magnetic anisotropies and have the potential to function as single molecule magnets and as qubits. This behavior is caused by the spin-orbit coupling of the three lowest orbital states which have a T_2g parentage. Fine tuning these states’ energies is difficult and remains an important challenge in the quest for developing cobalt-based compounds with practical applications. Toward this aim we have synthesized a series of octahedral Co^II(bpte)(NCX)₂ where bpte = S,S′-bis(2- pyridylmethyl)-1,2-thioethane and NCX^- = NCBH₃^-, NCO^-, NCS^-, NCSe^-. The structures of these species have been determined using single-crystal X-ray crystallography. The anisotropies of their ground Kramers doublet were assessed using high-frequency (HF) EPR and X-band EPR spectroscopies. The zero-field splitting of the ground spin quartet of these species was determined using Far-Infrared Magnetospectroscopy (FIRMS), magnetic susceptibility, and magnetization data. The electronic structure of the Co(bpte)(NCX)₂ complexes was investigated using ab initio CASSCF computations which allowed us to rationalize the spectroscopic data. We anticipated that the different energy gaps of the CN-based π_HOMO/π^*_LUMO orbitals of the NCX^- coligands will allow us to control the crystal field splitting of the low-lying orbital states of the Co^II(bpte)(NCX)₂ complexes. Our study revealed that while the selection of the NCX^- coligand affords some control over the magnetic anisotropy of the corresponding cobalt(II) complex, there are additional factors that contribute to the magnetic behavior of these species.

Sebastian Stoian

University of Idaho

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