Structural flexibility within a sterically hindered ligand platform:: mononuclear iron(II) carboxylate complexes as subsite models for diiron(II) centers

The synthesis and characterization of a series of mononuclear iron(II) carboxylate complexes are described. By using sterically hindered carboxylate ligands, 2,6-di(p-tolyl)benzoate (ArTolCO2 −) and 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2 −), a series of four-, five-, and six-coordinate iron(II) complexes were synthesized. The compounds are [Fe(O2CArTol)2(1-BnIm)2] (3), [Fe(O2CArTol)2(1-MeBzIm)2] (4), [Fe(O2CAr4-tBuPh)2(2,2′-bipy)2] (5), [Fe(O2CArTol)2(TMEDA)] (6), and [Fe(O2CArTol)2(BPTA)] (7). Structural analyses of 3–7 revealed that the overall stereochemistry of the [Fe(O2CAr′)2Ln] units is dictated by electronic and steric factors of the N-donor ligands (L), as well as by the flexible coordination of the carboxylate ligands. Distinctive Mössbauer parameters obtained for these and related compounds facilitated the spectral assignment of a diiron(II) complex having asymmetric metal sites, [Fe2(μ-O2CArTol)3(O2CArTol)(2,6-lutidine)] (2). Well-defined mononuclear iron carboxylate complexes thus may serve as subsite models for higher nuclearity species in both synthetic and biological systems.