Electronic coupling of two Cp*ClM+/Cp*M reaction centers via p conjugated bridging ligands: Similarities and differences between rhodium and iridium analogues
Dinuclear Cp*ClM(m-L)MClCp*(PF6)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 chem. reversible steps by up to six electrons to Cp*M(m-L)MCp*n-. UV-visible/NIR spectroelectrochem. and EPR of the paramagnetic states were used to identify the various intermediates. The complexes clearly show a reversible, ligand-centered 1-electron redn. (E) preceding the 1st chloride-dissociative metal redn. step (EC). Metal-metal interaction via the bridging p acceptor ligand L causes a splitting of 310-710 mV between the potentials for the two Cl--dissociative steps. The 2nd chloride release occurs in EC+E fashion for L = bpip but in a two-electron process for L = bptz. The MIIMI mixed-valent species Cp*M(m-L)MCp*+ could be identified via long-wavelength bands from intervalence charge transfer (IVCT) transitions. All complexes contg. at least one chloride-free Cp*M group display intense long-wavelength absorption bands. The Ir complexes are distinguished by more neg. potentials of the Cp*Ir-contg. forms, by slower formation of the M2I,II mixed-valent intermediate, by larger g anisotropy of the paramagnetic forms, and by triplet absorption features in the UV-visible electronic spectra. on SciFinder(R)
