A Multifrequency High-Field Electron Paramagnetic Resonance Study of CoIIS4 Coordination

Advanced electron paramagnetic resonance (EPR) methods have been employed in the study of two high-spin cobalt(II) complexes, Co[(SPPh2) 2N]2 (CoPh,PhL2) and CoE(SPPh 2)(SPiPr2)N]2 (Co iPr,PhL2), in which the bidentate disulfidoimidodiphosphinato ligands make up for a pseudotetrahedral sulfur coordination of the transition metal. The CoS4 core in the two complexes has slightly different structure, owing to the different peripheral groups (phenyl or isopropyl) bound to the phosphorus atoms. To determine the zero-field splitting, notoriously difficult for high-spin cobalt(II), the two complexes required different approaches. For CoPh,PhL2, the study of the X-band EPR spectrum of a single crystal as a function of temperature revealed a nearly axial character of the zero-field splitting (ZFS; E/D ≈ -0.05). For CoiPr,PhL2, the combination of the EPR spectra at 9, 95, and 275 GHz revealed a rhombic character of the ZFS (E/D ≈ -0.33). The energy difference between the Kramers doublets in Co Ph,PhL2 and CoiPr,PhL2 amounts to 24 cm-1 and 30 cm-1, respectively. From the X-band EPR spectra of diamagnetically diluted single crystals at fields up to 2.5 T for CoPh,PhL2 and 0.5 T for CoiPr,PhL2, the effective g tensors and cobalt hyperfine tensors have been determined, including the direction of the principal axes in the cobalt sites. The values of the EPR observables are discussed in relation to the structural characteristics of the first (CoS4) and second coordination sphere in the complexes. © 2009 American Chemical Society.