At the Borderline between Metal-Metal Mixed Valency and a Radical Bridge Situation: Four Charge States of a Diruthenium Complex with a Redox-Active Bis(mer-tridentate) Ligand

The complex ions [(LRu)-Ru-3(mu,eta(3):eta(3)-BL)RuL3](n+) (1(n+), L-3 = 4,4',4 ''-tri-tert-butyl-2,6,2',6 ''-terpyridine and H2BL2- = 1,2-bis(salicyloyl)-hydrazide(2-)) were isolated with PF6- or ClO4- counterions (n = 1) and as bis(hexafluorophosphate) (n = 2). Structural, electrochemical, and spectroscopic characterization reveals the monocation as intermediate (K-c = 10(8.2)) in the three-step reversible redox system 1(0/+/2+/3+). The 1(+) ion has the molecule bridged (Ru- - -Ru 4.727 angstrom) ruthenium centers involved in five- and six- membered chelate rings, and it exhibits long-wavelength absorptions at lambda(max) 2240, 1660, and 1530 nm (epsilon(max) = 1000, 3000, and 8000 M-1 cm(-1) respectively), which would be compatible with a (RuRuII)-Ru-III mixed-valent situation or with a coordinated radical ion bridge. In fact, EPR and DFT analysis of 1(+) reveals that the spin is equally distributed over the ligand bridge and over both metals. The oxidized paramagnetic ions 1(2+) and 1(3+) have been studied by H-1 NMR and EPR and by TD-DFT supported UV-vis-NIR and MIR (mid-IR) spectroelectrochemistry. The capacity of various kinds of bis(mer-tridentate) bridging ligands (pi donors or pi acceptors, cyclometalated or noncyclometalated) for mediating metal-metal interactions is discussed.