The factors determining the electron transfer-induced
halide labilization in complexes (α-diimine)Re(CO)3(Hal), Hal = Cl and Br, were
systematically studied via EPR and cyclic
voltammetry in the presence of substituting ligands such as
triphenylphosphine, cyanide,
or acetonitrile. The α-diimines employed were the four isomeric
bidiazines (bdz) 3,3‘-bipyridazine, 2,2‘-bipyrazine, and 2,2‘- and 4,4‘-bipyrimidine and the
nonaromatic α-diimines
1,4-di-tert-butyl-1,4-diaza-1,3-butadiene (dab) and
1,3-di-tert-butylsulfurdiimine (sdi). For
comparison, the complexes (L)Re(CO3)Cl, L
= 2,2‘-bipyridine, 1,4,7,10-tetraazaphenanthrene,
and η2-2,2‘,2‘‘-terpyridine, and the new cationic species
[(bdz)Re(CO)3(CH3CN)]+
were also
investigated. In a further experiment, in situ EPR
spectroelectrochemistry was employed
to study the primary paramagnetic intermediates during the reduction of
the prototype
compound, (bpy)Re(CO)3Cl, under a CO2
atmosphere. The susceptibility to substitution was
found to be dependent not on the redox potential but on the π
molecular orbital coefficients
at the metal-coordinating nitrogen centers which are reflected by
14N, 185,187Re, and 31P
EPR
coupling constants. The most labile systems were thus found among
the complexes of the
small dab and sdi ligands, despite their facile reduction. In
contrast, the complexes of these
nonaromatic compounds showed an electrochemically reversible
one-electron oxidation which,
in comparison to the absorption maximum, allowed us to estimate
contributions to the
reorganization energy of the MLCT excited state in two cases. For
the reductive labilization,
it is primarily the small but variable and EPR-detectable
ligand-to-metal electron (spin)
transfer at the metal/ligand interface which determines the extent of
activation in 18 + δ
valence electron intermediates.
