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

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Abstract
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 (12+/˙+/0), 4,4′-dimethyl-2,2′-bipyrazinium (22+/˙+/0) and 1,1′-dimethyl-3,3′-bipyridazinium (32+/˙+/0) which exhibit simultaneously features of the methylviologen two-step redox system, MV2+/˙+/0, and an α-diimine chelate site for metal coordination. While metal-free species 2n+ and 3n+ could not be synthesized as pure compounds, the ‘diazamethylviologen’ system 1n+, a hybrid structure between MVn+ 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 +, Inn+, Tl +, Mo(CO)4, [Re(CO)3(PPh3)]+ 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 π* orbital of the cationic radical ligand determine the variable extent of ion-pair association via the EPR/ENDOR-detectable spin transfer. System 2n+ 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)3Cl, both complexes showing a rather irreversible second reduction due to the non-classical (non-Kekulé) electronic structure as predicted from the calculations for 2° and 3°.
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