Multifrequency EPR Study and Density Functional g-Tensor Calculations of Persistent Organorhenium Radical Complexes

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Abstract
The dinuclear radical anion complexes {(μ-L)[Re(CO)3Cl]2}•-, L = 2,2‘-azobispyridine (abpy) and 2,2‘-azobis(5-chloropyrimidine) (abcp), were investigated by EPR at 9.5, 94, 230, and 285 GHz (abpy complex) and at 9.5 and 285 GHz (abcp complex). Whereas the X-band measurements yielded only the isotropic metal hyperfine coupling of the 185,187Re isotopes, the high-frequency EPR experiments in glassy frozen CH2Cl2/toluene solution revealed the g components. Both the a(185,187Re) value and the g anisotropy, g1 − g3, are larger for the abcp complex, which contains the better π-accepting bridging ligand. Confirmation for this comes also from IR and UV/vis spectroscopy of the new {(μ-abcp)[Re(CO)3Cl]2}o/•-/2- redox system. The g values are reproduced reasonably well by density functional calculations which confirm higher metal participation at the singly occupied MO and therefore larger contributions from the metal atoms to the g anisotropy in abcp systems compared to abpy complexes. Additional calculations for a series of systems {(μ-abcp)[M(CO)3X]2}•- (M = Tc or Re and X = Cl, and X = F, Cl, or Br with M = Re) provided further insight into the relationship between spin density distribution and g anisotropy.

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