Azobipyridinium Dications and Radical Monocations as Acceptors

Abstract

Combination of the planar redox-active ions [ML2]2– (L = mnt2– = maleonitrile-1,2- dithiolate; M = Ni (1), Pd (2), Pt (3); dmit2– = 2-thioxo-1,3-dithiol-4,5-dithiolate: M = Ni (4), Pd (5) and trans-4,4′-azobis(1-methyl-pyridinium), (a2+) affords 1:1 ion pairs exhibiting partial and complete electron transfer as evidenced by UV-Vis and EPR spectra. Replacement of planar a2+ by a non-planar dipyridinium ketone b2+ leads to the complexes 1b and 4b. 1a, 2a, and 3a are predominantly composed of dications and dianions while 4a, 5a, and 4b are rare examples of ion pairs consisting of two radical ions. Single crystal X-ray structural analyses was performed on 4a, a(PF6)2, and a(MeSO4)2 while the structure of 1a was resolved by powder X-ray diffractometry. The latter consists of mixed stacks of planar dianions and dications forming a slipped arrangement with the centers of the two ions displaced relative to each other by 250 pm. The short interplanar distances of 340 pm are in agreement with the presence of a weak charge-transfer interaction as indicated by the corresponding absorption band at about 1400 nm. A mean reorganization energy of 0.85 ±0.04 eV is calculated from the Hush equation for complexes 1a, 2a, and 3a. No ion pair charge-transfer band is observable for 4a, 5a, and 4b. Surprisingly, in the solid state structure of 4a the [Ni(dmit)2]− monoanions do not form segregated columns as found in the many ion pairs with redox inert counterions, but prefer a mixed stack arrangement as observed also for 1a.The specific electrical conductivity of pressed powder pellets of complexes exhibiting a charge-transfer band is in the range of 2 × 10−7 to 1 × 10−5 Ω−1cm−1, increasing with increasing driving force of electron transfer in accordance with previous results. Different to that, the conductivity of the other complexes does not depend on driving force and is much higher (2 × 10−4 to 4 × 10−4 Ω−1cm−1).