Forced proximity of nitroxide groups in pincer compounds with a xanthene spacer.

We designed bisnitroxide compounds where the radical sites are located close to each other in a molecule. Two new pincer-type bisnitroxide compounds have been synthesized, involving xanthene-4,5-diyl as a spacer and tert -butyl phenyl nitroxides as arms. From the X-ray crystal structure analysis, the shortest intramolecular interatomic N⋯O and O⋯O distances respectively are 5.074(6) and 5.258(6) Å for the m , m ′-derivative and 3.624(3) and 3.771(3) Å for the p , p ′-derivative. The N⋯O distance in the latter satisfies the empirical criterion for possible dimerization/degradation reaction accompanied by dia-/paramagnetic transition. However, the magnetic study clarified paramagnetic behavior in all the temperature range. According to a singlet-triplet model, antiferromagnetic couplings were characterized with 2 J / k B = −7.71(2) and −8.83(4) K for the m , m ′ and p , p ′-derivatives, respectively. The present result suggests that a more flexible spacer is required for realization of possible dia-/paramagnetic transition. [ABSTRACT FROM AUTHOR]