title: Effect of heme-heme interactions and modulation of metal spins by counter anions in a series of diiron(III)-μ-hydroxo bisporphyrins: unusual stabilization of two different spins in a single molecular framework.
abstract: A new family of five ethene-bridged diiron(III)-μ-hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe2 (bisporphyrin)]OH·X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo-bridged dimer 1 by strong Brønsted acids such as HI, HBF4, HPF6, HSbF6 , and HClO4 produces the μ-hydroxo complexes with I5(-)(2), BF4(-)(3), PF6(-)(4), SbF6(-)(5), and ClO4(-)(6) as counter anions, respectively. The X-ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed-intermediate spins in both the solid state and solution. Moreover, the two different Fe(III) centers of 3-5 are best described as having admixed-high and admixed-intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed-intermediate spins in solution. In contrast, the two Fe(III) centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X-ray structures reveal that the Fe-O bond length increases on going from the μ-oxo to the μ-hydroxo complexes, and the Fe-O(H)-Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6, respectively. Variable-temperature magnetic data have been subjected to a least-squares fitting using the expressions derived from the spin Hamiltonians H=-2JS1·S2 -μ·B+D[S(2)(z) - 1/3S(S + 1)] (for 2, 3, 4, and 5) and H=-2JS1·S2 (for 6). The results show that strong antiferromagnetic coupling between the two Fe(III) centers in 1 is attenuated to nearly zero (-2.4 cm(-1)) in 2, whereas the values are -46, -32.6, -33.5, and -34 cm(-1) for 3, 4, 5, and 6, respectively.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016 (India), Fax: (+91) 512-259-7436.
authors.forename: Sudip Kumar
authors.initials: SK
authors.lastname: Ghosh
authors.forename: Susovan
authors.initials: S
authors.lastname: Bhowmik
authors.forename: Debangsu
authors.initials: D
authors.lastname: Sil
authors.forename: Sankar Prasad
authors.initials: SP
authors.lastname: Rath
mesh_terms: D000838:Anions; D005290:Ferric Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D036103:Nanotechnology
chemical_list: D000838:Anions; D005290:Ferric Compounds; D006418:Heme; D007501:Iron
keywords: anions; bridging ligands; iron; porphyrinoids; spin states