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,
H12. Configuration interaction calculations
were also
carried out for each of the
[Fe2(L)3]5+ complexes to
provide theoretical values of H12 and the
effective donor/acceptor separation distances (rDA). Experimental and
theoretical results for H12 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 rDA values are within 0.01 Å of
the
Fe−Fe distances.