2,6-bis(pyrazolyl)pyridine functionalised with two nitronyinitroxide and iminonitroxide radicals

During the last two decades, growing interest in the field of material chemistry has been focused on the synthetic design of organic high-spin molecules for use as novel magnetic materials. Conjugated radicals based on the bis(pyrazolyl)-pyridine core are very attractive in principle, since they per se combine optical, chelating and magnetic properties on a single molecular entity. The functionalisation of this core via the bis(4'-formyl) derivatives with Ullman radicals results in novel functional biradicals, which may further be used for supramolecular architectures. In this paper we describe the synthesis, structure and properties of 2,6-bis[4'-(3oxo-1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazol-1'- yl]pyridine (Pz(2)PyNN) and 2,6-bis[4-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazolyl]pyridine (Pz(2)PyIN). The two biradicals can clearly be differentiated by UV/Vis, IR and EPR spectroscopy. The optical absorptions of the blue Pz(2)PyNN appear around lambda(max) = 610 nm (epsilon approximate to 1700 m(-1) cm(-1)) while for Pz(2)PyIN the orange-red colour arises from absorptions at lambda(max) = 468 nm (epsilon approximate to 1400 M-1 cm(-1)). A triplet ground state for the bis (nitronylnitroxide) Pz(2)PyNN with DeltaE(S-T) approximate to 11. 8 +/- 4.8 cm(-1) can be deduced from the EPR studies because of the increase of the double integrated intensity times temperature at very low temperatures, indicating complete population of the triplet state. For the related bis(iminonitroxide) Pz(2)PyIN a much smaller (at least one order of magnitude lower) singlet-triplet energy gap with an upper limit of 0.7 cm(-1) has been derived (0.7 greater than or equal to DeltaE(S-T) greater than or equal to -0.07 cm(-1)). The new multifunctional biradicals represent the first example of high-spin molecules in the bis(pyrazolyl)pyridine family. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).