Ionic 4,4'-Biphenylene-Bridged Bis-ruthenium Complexes [Ru2(4,4'-{C6H2(CH2NMe2)2-2,6}2)(terpy)2]n+(n=2 and 4) and Their Reversible Redox Interconversion: A Molecular Switch

Abstract

Copper(II)-mediated oxidative coupling of the blue, 18-electron mononuclear complex [RuII{C6H3(CH2NMe2)2-2,6}(terpy)]Cl, 3 (terpy = 2,2';6',2''-terpyridine), affords the unique diamagnetic, binuclear 34-electron complex [(RuIII)2(4,4'-{C6H2(CH2NMe2)2-2,6}2)(terpy)2](CuCl2)4, 5a. This intensely green complex has been the subject of an X-ray crystal structure determination, and the molecular geometry of the tetracation shows the C4-C4'-coupled bridging biphenylene dianion to be planar. Two-electron reduction of this tetracation as its PF6- salt, 5b, with N2H4·H2O affords the corresponding dicationic intensely blue, 36-electron complex [(RuII)2(4,4'-{C6H2(CH2NMe2)2-2,6}2)(terpy)2](PF6)2, 8. An X-ray crystal structure determination of 8 shows a molecular geometry in which the bridging biphenylene dianion is nonplanar, with the metal coordination planes twisted by ca. 36 relative to each other. Cyclic voltammetry data show that the reduction of 5b (4+) to 8 (2+), i.e., [RuIII-RuIII] to [RuII-RuII], is reversible. This reduction proceeds through the mixed valence 35-electron [RuII-RuIII] complex [(RuII)(RuIII)(4,4'-{C6H2(CH2NMe2)2-2,6}2)(terpy)2]3+, 9 (3+), which is intensely purple and which can be independently prepared by mixing acetonitrile solutions of equimolar amounts of 5 (4+) and 8 (2+).