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 heterocyclic systems 1,1'-dialkyl-4,4'- bipyrimidinium (1(2+/.+/0)), 4,4'-dimethyl-2,2'-bipyrazinium (2(2+/.+/0)) and 1,1 '-dimethyl-3,3'-bipyridazinium (3(2+/.+/0)) which exhibit simultaneously features of the methylviologen two-step redox system, MV(2+/.+/0) and an alpha-diimine chelate 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+, Cu+, 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, ligand preference and the ability to back-donate electrons into the singly occupied pi* orbital of the cationic radical ligand determine the variable 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-methylpyrazinium dimer could be isolated only in coordinated form, bound to [Ru(bpy)(2)](2+) or Re(CO)(3)Cl, both complexes showing a rather irreversible second reduction due to the non-classical(non- Kekule) electronic structure as predicted from the calculations for 2 degrees and 3 degrees.