Multistep redox sequences of azopyridyl (L) bridged reaction centres in stable radical complex ions {((mu)-L)[MCl((eta)^5-C5Me5)]2}+, M = Rh or Ir: spectroelectrochemistry and high-frequency EPR spectroscopy

The dinuclear complex cations {(µ-L)[MCl((eta)^5-C5Me5)]2}n, M = Rh or Ir and L = abpy (= 2,2ʹ-azobispyridine) or abcp (= 2,2ʹ-azobis (5-chloropyrimidine)), could be isolated as paramagnetic hexafluorophosphates (n= 1+) or, for M = Ir, as diamagnetic bishexafluorophosphates (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 (delta)EEC between 0.75 V (Rh2/abpy) and 1.14 V (Ir2/abcp), leading to the spectroelectrochemically characterised complexes {[((eta)^5-Me5C5)M](µ-L)[MCl ((eta)^5-C5Me5)]}+ and (µ-L)[M((eta)^5-C5Me5)]2. This large splitting of (delta)EEC 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 Ir2/abcp combination. The stable electron reservoir intermediates {((mu)-L)[MCl((eta)^5-C5Me5)]2}+ were identified as complexes of L (bullet)- anion radicals via their small g anisotropy as measured through high-frequency (>200 GHz) EPR spectroscopy.