Electronic Structure of Three-Coordinate Fe^II and Co^II β‑Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]⁻, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnac^tBu)ML, where M = Fe^II, Co^II and L = Cl, CH₃. These are denoted FeCl, FeCH₃, CoCl, and CoCH₃ and have been previously reported and structurally characterized. The two Fe^II complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the Co^II complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The Co^II complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two Fe^II complexes are similar with nearly axial zfs and large magnitude zfs given by D = −37 ± 1 cm^–1 for both. The two Co^II complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm^–1 while CoCH₃ has negative D = −49 cm^–1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.