Ionic 4,4 '-biphenylene-bridged bis-ruthenium complexes [Ru-2(4,4 '-(C6H2(CH2NMe2)(2)-2,6}(2))(terpy)(2)](n+) (n = 2 and 4) and their reversible redox interconversion: A molecular switch

Copper(II)-mediated oxidative coupling of the blue, 18-electron mononuclear complex [Ru-II{C6H3(CH2NMe2)(2)-2,6}(terpy)]Cl, 3 (terpy = 2,2';6',2 "-ter pyridine), affords the unique diamagnetic, binuclear 34-electron complex [( Ru-III)(2)(4,4'-{C6H2(CH2NMe2)(2)-2,6}(2))(terpy)(2)](CuCl2)(4), 5a. This i ntensely green complex has been the subject of an X-ray crystal structure d etermination, 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 corre sponding dicationic intensely blue, 36-electron complex [(Ru-II)(2)(4,4'-{C 6H2(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 bipheny lene dianion is nonplanar, with the metal coordination planes twisted by ca . 36 degrees relative to each other. Cyclic voltammetry data show that the reduction of 5b(4+) to 8 (2+), i.e., [Ru-III-Ru-III] to [Ru-II-Ru-II], is r eversible. This reduction proceeds through the mixed valence 35-electron [R u-II-Ru-III] complex [(Ru-II)(Ru-III)(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+) a nd 8 (2+).