Electron Transfer from Tetrakis(isocyanide)rhodium(I) Monomers, and the Oligomers to Iron(III) and Cobalt(III) Complexes. Enhancement of the Reactivity by the Oligomerization

Electron transfer reactions from tetrakis(isocyanide)rhodium(I) ([RhL4]+ to iron(III) complexes of the [Fe(N–N)3]3+ type (N–N=1,10-phenanthroline, 2,2′-bipyridine) proceed through precursor complexes formed between [RhL4]+ and [Fe(N–N)3]3+ with 1:1, 1:2, and 2:1 stoichiometry. A similar scheme is suggested for the reaction of bis(isocyanide)bis(triphenylphosphine)rhodium(I) with the same oxidants. The intramolecular electron transfer rate constants k1 in the 1:1 precursor complexes have been determined in MeCN at 298 K. In accordance with Marcus theory, the logk1 values are linearly related with the reduction potentials of oxidants with the theoretical slope of 8.5. The [RhL4]+ cations form oligomers in MeCN; [RhL4+]n (n=2,3), and the equilibrium constants for oligomerization have been determined. For a given oxidant [Co(bpy)3]3+ (bpy=2,2′-bipyridine), the logk1 value of [RhL4+]n increases in parallel with the HOMO energy of [RhL4+]n; monomer<dimer<trimer.