Spin-spin Interaction Between Phenyl Nitroxides Through the Σ-Π System

The spin states of a series of silicon- and carbon-bridged phenyl nitroxides were examined with respect to the temperature dependent ESR and SQUID measurements. Of those, a linear relationship between the ESR signal intensity and 1/T (T = absolute temperature) and an increase of χ mol T along lowering T were observed for a compound having disilanylene-bridged m- and p-phenyl nitroxide units ( Si2mp ), indicating ferromagnetic spin-spin interaction in this molecule. In contrast to this, no clear intramolecular spin-spin interaction took place in the monosilanylene analogue ( Si1mp ). Mono- and disilanylene-bridged phenylnitroxides with p-, p- or m-, m-substitution ( Si1pp , Si1mm , Si1pp , and Si2mm ) exhibited the singlet ground state. The trisilanylene and siloxanylene bridges did not play an obvious role in the spin interaction, in either a ferro- or antiferromagnetic fashion, regardless of the substitution modes of the phenylenes. MO calculations on the model compounds provided a mechanistic interpretation for the high-spin interaction through the σ-π system.