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 {(mu-L)[MCl(eta(5)-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 DeltaE(EC) between 0.75 V (Rh-2/abpy) and 1.14 V (Ir-2/abcp), leading to the spectroelectrochemically characterised complexes {[(eta(5)-Me5C5)M](mu-L)[MCl(eta(5)-C5Me5)]}(+) and (mu-L)[M(eta(5)-C5Me5)](2). This large splitting of DeltaE(EC) establishes the capability of azopyridyl bridges for mediating efficient metal-metal communication beyond mere electron transfer. The neutral complexes (mu-L)[M(eta(5)-C5Me5)](2) are distinguished by a series of intense absorption bands in the near infrared, the lowest absorption energies being displayed by the Ir-2/abcp combination. The stable electron reservoir intermediates {(mu-L)[MCl(eta(5)-C5Me5)](2)}(+) were identified as complexes of L.- anion radicals via their small g anisotropy as measured through high-frequency (>200 GHz) EPR spectroscopy.