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 EPRspectroscopy

Abstract

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 ΔE_EC 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 ΔE_EC 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.