Electronic Structure of Three-Coordinate FeIIand CoIIß-Diketiminate Complexes

The ß-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoIIand L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3and have been previously reported and structurally characterized. The two FeIIcomplexes (S= 2) have also been previously characterized by Mo¨ssbauer 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 CoIIcomplexes (with S= 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoIIcomplexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeIIcomplexes are similar with nearly axial zfs and large magnitude zfs given by D= -37 ± 1 cm–1for both. The two CoIIcomplexes likewise exhibit large and nearly axial zfs, but surprisingly, CoClhas positive D= +55 cm–1while CoCH3has 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.