Ion Complexation Induced High-Spin Associations of Spinlabeled Crown Ethers: A Reevaluation

Between 1978 and 1989, Mukai and coworkers demonstrated that a series of spin-labeled benzo-15-crown-5 derivatives gave triplet ESR spectra in ethanol glasses in the presence of diamagnetic alkali and alkaline earth ions. They proposed that the high-spin coupling resulted from a pair of radicals being brought into close contact by a 1:2 complexation of metal ions with the crown ether moieties. The work of Mukai and his colleagues represents an early example of high-spin coupling induced in radical pairs that assemble around diamagnetic metal ions. Herein, we describe ion binding experiments with simple stable free radicals such as TEMPO and galvinoxyl that lack the crown moiety. The triplet state species that we observe require the possibility of unforeseen mechanisms for previously reported high-spin associations in spin labeled crown ethers.