Photochemistry and Photophysics of Coordination Compounds

78 N. Armaroli et al.
In the CuA mixed-valence binuclear site (Fig. 7), present for example in
the terminal aerobic respiration enzyme (cytochrome c oxidase), a similar
situation occurs [23]. The efficient electron transfer is promoted by the threedimensional
organization and by electronic factors. The presence of two
coordinated anionic cysteine thiolates in a monomeric Cu complex, however,
would severely decrease the rate of the electron transfer process by stabilizing
the oxidized Cu 2+ state and making the reduction potential too negative
(Fig. 7).
Fig. 7 Schematic structure of the CuA mixed valence dinuclear site, present in some
terminal aerobic respiration enzymes
In CuA this is avoided by weakening axial bonding interactions and by
delocalizing the charge over two Cu ions [24].
In conclusion, Nature makes extensive use of the coordination flexibility
of copper complexes and of the related tuning of electronic properties, to
optimize processes of crucial importance in living organisms.
1.4
Cu(I) in Supramolecular Chemistry
In the frame of the spectacular development of synthetic supramolecular
chemistry over the last two decades [25], coordination chemistry has played
a primary role [26] and, in this context, bisphenanthroline Cu(I) complexes
(hereafter indicated as [Cu(NN)2] + ) have been major players [15]. Cu(I)