Multifrequency EPR study and density functional g-tensor calculations of persistent organorhenium radical complexes

The dinuclear radical anion complexes {(mu-L)[Re(CO)(3)Cl](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 Re-185,Re-187 isotopes, the high-frequency EPR experiments in glassy frozen CH2Cl2/toluene solution revealed the g components. Both the a(Re-185,Re-187) value and the g anisotropy, g(1) - g(3), are larger for the abcp complex, which contains the better pi-accepting bridging ligand. Confirmation for this comes also from IR and UV/vis spectroscopy of the new {(mu-abcp)[Re(CO)(3)Cl](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 {(mu-abcp)[M(CO)(3)X](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.