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

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 (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 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°.