Donor/acceptor coupling in mixed-valent dinuclear iron polypyridyl complexes: Experimental and theoretical considerations

Coupling between donor and acceptor orbitals for optically-induced intervalence electron transfer processes has been considered for a series of rigid mixed-valent dinuclear tris(2,2'-bipyridine)iron complexes, Each of the four complexes considered contains three saturated bridges which link the two tris(2,2'-bipyridine)iron moieties. The bridging linkages are -CH2CH2- -CH2CH2CH2-, -CH2OCH2-, and -CH2SCH2-. Despite differences in the composition of the bridges X-ray diffraction and/or molecular dynamics calculations show that the metal-metal separation and relative bipyridine orientations among all four complexes are nearly identical. Consequently, the only factor which differs significantly among these complexes and which might affect the donor-acceptor coupling in the mixed-valent forms is their connectivity. These complexes thus provide a unique opportunity to focus on potential superexchange coupling in the absence of ambiguities introduced by other structural and energetic considerations. Theories developed by Mulliken and Hush have been applied to intervalence charge-transfer transitions in order to obtain values of the coupling matrix elements, H-12. Configuration interaction calculations were also carried out for each of the [Fe-2(L)(3)](5+) complexes to provide theoretical values of H-12 and the effective donor/acceptor separation distances (r(DA)) Experimental and theoretical results for H-12 are in excellent agreement and indicate that the bridging moieties are either unimportant in the donor/acceptor coupling or, in one case, actually reduce the coupling compared to the ''bridge-free'' system. The calculated r(DA) values are within 0.01 Angstrom of the Fe-Fe distances.