Synthesis, characterization, and photophysical properties of a series of supramolecular mixed-valence compounds

The synthesis and characterization of 10 cyano-bridged trinuclear mixed-val ence compounds of the form [(NH3)(5)MNC- Fe-II(CN)(4)-CN-M'(NH3)(5)](n+) (M = Ru-III, Os-III, Cr-III, or Pt-IV; n = 2, 3, or 4) is reported. The elect ronic spectra of these supramolecular compounds exhibit a single intervalen t (IT) absorption band for each nondegenerate Fe --> M/M' transition. The r edox potential of the Fe(II) center is shifted more positive with the addit ion of each coordinated metal complex, while the redox potentials of the pe ndant metals vary only slightly from their dinuclear counterparts. As a res ult, the Fe --> M IT bands are blue shifted from those in the corresponding dinuclear mixed-valence compounds. The energies of these IT bands show a l inear correlation with the ground-state thermodynamic driving force, as pre dicted by classical electron transfer theory. Estimates of the degree of el ectronic coupling (H-ab) between the metal centers using a theoretical anal ysis of the IT band shapes indicate that most of these values are similar t o those for the corresponding dinuclear species. Notable exceptions occur f or the Fe --> M IT transitions in Os-Fe-M (M = Cr or Pt). The enhanced elec tronic coupling in these two species can be explained as a result of excite d state mixing between electron transfer and/or ligand-based charge transfe r states and an intensity-borrowing mechanism. Additionally, the possibilit y of electronic coupling between the remote metal centers in the Ru-Fe-Ru s pecies is discussed in order to explain the observation of two closely spac ed redox waves for the degenerate Ru(III) accepters.