The single-crystal electronic and electron spin resonance spectra of copper(II) doped bis(2,2′-bipyridyl)nitritozinc(II) nitrate and bis(2,2′-bipyridyl)nitritocopper(II) tetrafluoroborate: a fluxional CuN2N′2O2 chromophore

Abstract

The single-crystal e.s.r. spectra and polarised electronic spectra of the copper(II) doped [Zn(bipy)₂(ONO)][NO₃] system are shown to be closely comparable to those previously reported for [Cu(bipy)₂(ONO)][NO₃]. The observation of more than four copper hyperfine lines in the single-crystal e.s.r. spectrum measured in the approximate bc plane is consistent with a two-dimensional misalignment of the CuN₂N′₂O₂ chromophore and the temperature variability of the g and A factors are consistent with a two-dimensional fluxional model of the CuN₂N′₂O₂ chromophore with a distorted square-pyramidal 4 + 1 + 1* structure and not with static disorder. This suggests that the structures of [Cu(bipy)₂(ONO)][NO₃] and [Cu(bipy)₂(ONO)][BF₄] are not genuine static stereochemistries of the copper(II) ion, but arise as a consequence of the fluxional model and are best referred to as pseudo cis distorted-octahedral structures. The electronic properties of 0.1–100% copper(II) doped [Zn(bipy)₂(ONO)][NO₃] are shown to be independent of the copper(II) concentration, and this suggests that the structure of the doped CuN₂N′₂O₂ chromophore is independent of the structure of the ZnN₂N′₂O₂ chromophore of the zinc host lattice, but correlates with the known structure of the [Cu(bipy)₂(ONO)][NO₃] complex, an example of the Non-co-operative Jahn–Teller Effect. The electronic reflectance spectrum of [Cu(bipy)₂(ONO)][BF₄] is closely comparable to that of the nitrate, despite having a significantly different, (4 + 1 + 1*) type, fluxional CuN₄O₂ chromophore structure.