Small-Molecule Intramolecular Electron-Transfer Reactions

Small-Molecule Intramolecular Electron-Transfer Reactions 

The molecule [ReI(MQ+)(CO)3(dmb)]2+ has been a model system for studying intramolecular electron transfer over the last two decades. Here, MQ+ is the monodentate ligand Af-methyl-4,4/-bipyridinium, dmb is the bidentate ligand 4,4r-dimethyl-2,2r-bipyridine, and the three CO ligands are facially coordinated. Irradiation of this complex at room temperature in solution with near-UV light leads to a sequence of intramolecular electron-transfer events as shown in Fig. 7. 

In brief, irradiation leads to the sub-picosecond oxidation of the Re(I) center and the production of a mixture of two reduction sites i.e. either the MQ+ or dmb ligand can be reduced. On the time-scale of several picoseconds inter-ligand electron transfer occurs, converting the mixture of excited states to [Ren(MQ )(CO)3(dmb)]2+. 

Structural Models of da/da Inner-Sphere Electron-Transfer Transition States 

It has long been assumed that the rates of inner-sphere electron-transfer reactions for transition-metal complexes should be sensitive to the nature of the donor and acceptor orbital symmetries. Efforts to 

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IV. Small-Molecule Intramolecular Electron-Transfer Reactions... 

A simple approach to understanding the factors which control the "conductivity of proteins towards electron tunneling is to develop "small molecule model systems to mimic intramolecular electron transfer in the protein systems. Appropriate models obviously require that the donor and acceptor be held at fixed distances and orientations which correspond to those in the protein-protein complexes. Models of this type have recently been obtained and investigated [103,104]. In these models the protein matrix is replaced by a simple synthetic spacer which separates two porphyrin molecules. By changing the chemical structure of the spacer, a series of molecules with different reaction distances and geometries has been synthesized. Typical examples of such molecules are presented in Fig. 21. 

Intramolecular electron transfer in cytochrome c has been investigated by attaching photoactive Ru complexes to the protein surface. Ru(bpy)2(C03) (bpy = 2,2 -bipyridine) has been shown to react with surface His residues to yield, after addition of excess imidazole (im), Ru(bpy)2(im)(His) +. The protein-bound Ru complexes are luminescent, but the excited states ( Ru ) are rather short lived (r 100 ns). When direct electron transfer from Ru to the heme cannot compete with excited-state decay, electron-transfer quenchers (e.g., Ru(NH3)6 + ) are added to the solution to intercept a small fraction (1-10%) of the excited molecules, yielding (with oxidative quenchers) Ru ". If, before laser excitation of the Ru site, the heme is reduced, then the Fe to Ru + reaction (ket) can be monitored by transient absorption spectroscopy. The ket values for five different modified cytochromes have been reported (Ru(His-33), 2.6(3) xlO Ru(His-39), 3.2(4) xlO Ru(His-62), 1.0(2) x 10 Ru(His-... 

The END theory is a general approach [38] to approximate the time-dependent Schrodinger equation and is not limited to treatments of molecular reaction dynamics. Other problems that have been studied include intramolecular electron transfer [16], the effect of intense laser light on the vibrational dynamics of small molecules [39], and solitonic charge transport in polyenes [40]. 

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