Stéphanie Frantz

Multistep redox sequences of azopyridyl (L) bridged reaction centres in stable radical complex ions {(μ-L)[MCl(η5-C5Me5)]2}˙+, M = Rh or Ir: spectroelectrochemistry and high-frequency EPR spectroscopy

The dinuclear complex cations {(μ-L)[MCl(η5-C5Me5)]2}n, M = Rh or Ir and L = abpy (= 2,2′-azobispyridine) or abcp (= 2,2′-azobis(5-chloropyrimidine)), could be isolated as paramagnetic hexafluorophosphates (n = 1+) or, for M = Ir, as diamagnetic bis-hexafluorophosphates (n = 2+). In addition to the reversible one-electron process as indicated by this convertibility there are two successive chloride-releasing reduction steps, separated by unusually large potential differences ΔEEC between 0.75 V (Rh2/abpy) and 1.14 V (Ir2/abcp), leading to the spectroelectrochemically characterised complexes {[(η5-Me5C5)M](μ-L)[MCl(η5-C5Me5)]}+ and (μ-L)[M(η5-C5Me5)]2. This large splitting of ΔEEC establishes the capability of azopyridyl bridges for mediating efficient metal–metal communication beyond mere electron transfer. The neutral complexes (μ-L)[M(η5-C5Me5)]2 are distinguished by a series of intense absorption bands in the near infrared, the lowest absorption energies being displayed by the Ir2/abcp combination. The stable electron reservoir intermediates {(μ-L)[MCl(η5-C5Me5)]2}+ were identified as complexes of L˙− anion radicals via their small g anisotropy as measured through high-frequency (>200 GHz) EPR spectroscopy.

High-frequency EPR study of reduced diruthenium and dirhenium polypyridine complexes based on the 1,2,4,5-tetrazine radical bridge

The radical complexes [(micro-L)[Ru(bpy)(2)](2)]*(3+), [(micro-bmtz)[Ru(cym)Cl](2)]*(+) and [(micro-L)[Re(CO)(3)Cl](2)]*(-), where L are 3,6-disubstituted 1,2,4,5-tetrazines such as 3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine (bmtz) and cym =p-cymene, were studied by X-band EPR in fluid solution and by 285 GHz EPR in glassy frozen solution. A comparison with other transition metal complexes (Cu, Rh, Os, Ir, Pt) involving tetrazine radical ligands reveals that the g anisotropy reflects (i) the pi acceptor effect of the tetrazine substituents, (ii) the competition from ancillary pi acceptor ligands for back donation from the metal, and (iii) the spin-orbit coupling contributions from the transition metal.