DIQUATERNIZED HETEROCYCLES WITH STRONG ELECTRONIC COUPLING BETWEEN A METAL-CHELATING SITE AND A METHYLVIOLOGEN-TYPE REDOX FUNCTION - EPR ENDOR DETECTED COORDINATION OF METAL-IONS AND COMPLEXES BY RADICAL-CATION INTERMEDIATES/

Calculated (MNDO, AM1) and experimental results from electrochemistry and EPR/ENDOR spectroscopy are reported for the diquaternized heterocy clic systems 1,1'-dialkyl-4,4'- bipyrimidinium (1(2+/.+/0)), 4,4'-dime thyl-2,2'-bipyrazinium (2(2+/.+/0)) and 1,1 '-dimethyl-3,3'-bipyridazi nium (3(2+/.+/0)) which exhibit simultaneously features of the methylv iologen two-step redox system, MV(2+/.+/0) and an alpha-diimine chelat e site for metal coordination. While metal-free species 2(n+) and 3(n) could not be synthesized as pure compounds, the 'diazamethylviologen ' system 1(n+), a hybrid structure between MV(n+) and 2,2'-bipyridine. was studied especially as the radical cation intermediate (n = 1) in the presence of potentially coordinating electrophiles such as Zn2+, C u+, In-n+, Tl+ Mo(CO)(4), [Re(CO)(3)(PPh(3))](+) or [Ru(bpy)(2)](2+). The different character of the metal centres in terms of charge, ligan d preference and the ability to back-donate electrons into the singly occupied pi orbital of the cationic radical ligand determine the vari able extent of ion-pair association via the EPR/ENDOR-detectable spin transfer. System 2(n+) which may be described as a 3,3'-coupled N-meth ylpyrazinium dimer could be isolated only in coordinated form, bound t o [Ru(bpy)(2)](2+) or Re(CO)(3)Cl, both complexes showing a rather irr eversible second reduction due to the non-classical(non- Kekule) elect ronic structure as predicted from the calculations for 2 degrees and 3 degrees.