Iron(II) Carboxylate Complexes Based on a Tetraimidazole Ligand as Models of the Photosynthetic Non-Heme Ferrous Sites: Synthesis, Crystal Structure, and Mössbauer and Magnetic Studies

The preparations, X-ray structures, and detailed physical characterization are presented for new complexes involving
an iron(II) center, a tetraimidazole ligand (TIM), and different carboxylates. [Fe(TIM)(C6H5CH2CO2)](ClO4) (1) crystallizes
in the Pbca space group with a = 10.8947(13), b = 20.343(2), and c = 22.833(3) Å, Z = 8, and V = 5060.6(11)
Å3. [Fe(TIM)(CH3CO2)](ClO4) (2) crystallizes in the Ia space group with a = 17.117(2), b = 10.3358(12), and c =
25.658(3) Å, β = 90.301(13)°, Z = 8, and V = 4539.5(9) Å3. In both structures, the iron(II) is hexacoordinated
to the four Nimidazole donors of the TIM ligand and the two O donors of a bidentate carboxylate. The flexibility of the
carboxylate bidentate coordination, symmetrical or more or less asymmetrical, associated with the steric demand
of the TIM ligand results in a remarkable versatility of the FeIIN4O2 coordination geometry. The diversity in carboxylate
bidentate coordination modes has allowed us to clearly show the importance of the structural and electronic effects,
through IR and Mössbauer spectroscopy, of this apparently tenuous carboxylate shift. Comparison of the structural
and Mössbauer properties of these complexes with the non-heme ferrous site of photosynthetic systems (i) shows
that the metric parameters of site 2b, including the symmetrically chelated bidentate carboxylate, are closer to
those of the non-heme ferrous site in the bacterial reaction centers of Rhodopseudomonas viridis and R. sphaeroides
and (ii) suggests that the ligand environment of the non-heme ferrous center of PS 2 is close to the axially distorted
octahedral symmetry resulting from an asymmetrical bidentate coordination of the −CO2 motif, as in complex 1.