SPECTROELECTROCHEMISTRY OF AROMATIC LIGANDS AND THEIR DERIVATIVES .1. REDUCTION PRODUCTS OF 4,4'-BIPYRIDINE, 2,2'-BIPYRIDINE, 2,2'-BIPYRIMIDINE, AND SOME QUATERNIZED DERIVATIVES

Cathodic reductions and reduction products of 4,4'-bipyridine (I), N-methyl-4,4'-bipyridinium+ (monoquat; II), N,N'-dimethyl-4,4'-bipyridinium2+ (paraquat; III), 2,2'-bipyridine(IV), 4,4'-dimethyl-2,2'-bipyridine (V), the ethylene-bridged diquaternized derivative 6,7-dihydrodipyrido[1,2-a:2,1-c]pyrazinedium2+ (diquat; VI), unbridged diquaternized N,N'-dimethyl-2,2'-bipyridinium2+ (VII), and 2,2'-bipyrimidine (VIII) have been investigated by cyclic voltammetry and spectroelectrochemistry. As expected, quaternization and the incorporation of added nitrogens in the ring both lead to easier reductions. The radicals formed by one-electron reduction of I-IV show very similar spectra, assignable in terms of a simple Huckel molecular orbital scheme. Thus, N-methylation lowers the energy of the LUMO in I but does not alter its nature. The same scheme, after allowing for loss of symmetry, correlates with established treatments of the reduction products of IV, which we have extended to VI, VIII, and the related reduced species. In all cases studied, the reduction orbital is related to pi-(7) of biphenyl, the transition pi-(6) --> pi-(7) moves, where the rings can rotate, to lower energy on reduction, and bands in the near IR-visible region are observed and assigned to transitions from pi-(7) to higher orbitals.