Novel Urea Derivatives as Two-Step Redox Systems

Abstract

The structurally novel urea derivatives 4−6 have been synthesized by treating the nucleophilic dipyridoimidazol-2-ylidene 1 with elemental chalcogens Y (Y = S, Se, Te). The oxygen analogue 8 has been obtained indirectly by alkylation of the seleno urea 5, followed by alkaline hydrolysis. All of these urea derivatives share two novel structural features: (i) their nitrogen centers represent the termini of the RED form of a two-step viologen-type redox system, and (ii) they can also be viewed as formally resulting from a cheletropic addition of CY to 2,2′-bipyridine. It has been demonstrated electrochemically that for Y = O, S, Se these ureas can be reversibly oxidized to the corresponding radical cations (SEM forms) at remarkably constant and negative potentials. This also applies to the heaviest congener of the series with Y = Te, except that in this case the oxidation process exhibits only quasi-reversibility. Electrochemical generation of the corresponding OX forms is observed quasi-reversibly at increasingly positive potentials with increasing mass of the chalcogen Y. However, the bis-oxidation of the telluro species 6 cannot be observed. These experimental results are discussed on the basis of semiempirical calculations for the RED, SEM, and OX forms under consideration. Semiempirical calculations also suggest that fragmentation of the novel urea derivatives into CY and 2,2′-bipyridine should be mildly endothermic for Y = O, but increasingly strongly endothermic with increasing mass of Y. This result is qualitatively in line with expectation considering the increasing reluctance of the heavier chalcogens to take part in multiple bonding to carbon. This phenomenon is also reflected in the mass spectrometric fragmentation patterns of the urea derivatives 4−6 and 8.