Ralf Reinhardt

Electron transfer and chloride ligand dissociation in complexes [(C5Me5)ClM(bpy)]+/[(C5Me5)M(bpy)]n (M=Co, Rh, Ir;n = 2+, +, 0, −): A combined electrochemical and spectroscopic investigation

Abstract In contrast to the rapid and chemically reversible two-electron ECE′ reductive elimination reaction[(C 5 Me 5 )ClM(bpy)] + + 2e − → (C 5 Me 5 )M(bpy)+Cl − ,M=Rh or Ir, the analogous cobalt system exhibits two separate one-electron steps (EC + E′ process) with a persistent, EPR-spectroscopically characterized cobalt(II) intermediate [(C 5 Me 5 )Co(bpy)] + . Within the series of coordinatively unsaturated homologous species (C 5 Me 5 )M(bpy), the cobalt derivative exhibits the smallest and the iridium homologue the largest metal(I)-to-bpy electron transfer in the ground state, as evident from electrochemical potentials and long-wavelength absorption data. A comparison within that homologous series indicates why the rhodium system, with its intermediate position, is most suitable for hydride transfer catalysis.

Electronic coupling of two [Cp*ClM]+/[Cp*M] reaction centers via π conjugated bridging ligands: similarities and differences between rhodium and iridium analogues ☆

Abstract The dinuclear complexes [Cp*ClM( μ -L)MClCp*](PF 6 ) 2 , M=Rh or Ir, L=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(phenyliminoethyl)pyrazine (bpip), are reduced in several chemically reversible steps by up to six electrons to the species [Cp*M( μ -L)MCp*] n − . UV–vis/NIR spectroelectrochemistry and EPR of the paramagnetic states were used to identify the various intermediates. The complexes clearly show a reversible, ligand-centered one-electron reduction (E) preceding the first chloride-dissociative metal reduction step (EC). Metal–metal interaction via the bridging π acceptor ligand L causes a splitting of 310–710 mV between the potentials for the two Cl − -dissociative steps. The second chloride release occurs in EC+E fashion for L=bpip but in a two-electron process for L=bptz. The M II M I mixed-valent species [Cp*M( μ -L)MCp*] + could be identified via long-wavelength bands from intervalence charge transfer (IVCT) transitions. All complexes containing at least one chloride-free Cp*M group display intense long-wavelength absorption bands. The iridium complexes are distinguished by more negative potentials of the [Cp*Ir]-containing forms, by slower formation of the M 2 I,II mixed-valent intermediate, by larger g anisotropy of the paramagnetic forms, and by triplet absorption features in the UV–vis electronic spectra.

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.