Modeling the Syn Disposition of Nitrogen Donors in Non-Heme Diiron Enzymes. Synthesis, Characterization, and Hydrogen Peroxide Reactivity of Diiron(III) Complexes with the Syn N-Donor Ligand H[subscript 2]BPG[subscript 2]DEV

In order to model the syn disposition of histidine residues in carboxylate-bridged non-heme diiron enzymes, we prepared a new dinucleating ligand, H[subscript 2]BPG[subscript 2]DEV, that provides this geometric feature. The ligand incorporates biologically relevant carboxylate functionalities, which have not been explored as extensively as nitrogen-only analogues. Three novel oxo-bridged diiron(III) complexes, [Fe[subscript 2](μ-O)(H[subscript 2]O)2(BPG[subscript 2]DEV)](ClO[subscript 4])[subscript 2] (6), [Fe[subscript 2](μ-O)(μ-O[subscript 2]CAr[superscript iPrO])(BPG[subscript 2]DEV)](ClO[subscript 4]) (7), and [Fe[subscript 2](μ-O)(μ-CO[subscript 3])(BPG[subscript 2]DEV)] (8), were prepared. Single-crystal X-ray structural characterization confirms that two pyridyl groups are bound syn with respect to the Fe−Fe vector in these compounds. The carbonato-bridged complex 8 forms quantitatively from 6 in a rapid reaction with gaseous CO[subscript 2] in organic solvents. A common maroon-colored intermediate (λ[subscript max] = 490 nm; ε = 1500 M[superscript −1] cm[superscript −1]) forms in reactions of 6, 7, or 8 with H[subscript 2]O[subscript 2] and NEt[subscript 3] in CH[subscript 3]CN/H[subscript 2]O solutions. Mass spectrometric analyses of this species, formed using [superscript 18]O-labeled H[subscript 2]O[subscript 2], indicate the presence of a peroxide ligand bound to the oxo-bridged diiron(III) center. The Mössbauer spectrum at 90 K of the EPR-silent intermediate exhibits a quadrupole doublet with δ = 0.58 mm/s and ΔE[subscript Q] = 0.58 mm/s. The isomer shift is typical for a peroxodiiron(III) species, but the quadrupole splitting parameter is unusually small compared to those of related complexes. These Mössbauer parameters are comparable to those observed for a peroxo intermediate formed in the reaction of reduced toluene/o-xylene monooxygenase hydroxylase with dioxygen. Resonance Raman studies reveal an unusually low-energy O−O stretching mode in the peroxo intermediate that is consistent with a short diiron distance. Although peroxodiiron(III) intermediates generated from 6, 7, and 8 are poor O-atom-transfer catalysts, they display highly efficient catalase activity, with turnover numbers up to 10 000. In contrast to hydrogen peroxide reactions of diiron(III) complexes that lack a dinucleating ligand, the intermediates generated here could be re-formed in significant quantities after a second addition of H[subscript 2]O[subscript 2], as observed spectroscopically and by mass spectrometry.National Institute of General Medical Sciences (U.S.) (Grant GM032134

Cadmium-Furandicarboxylate Coordination Polymers Prepared with Different Types of Pyridyl Linkers: Synthesis, Divergent Dimensionalities, and Luminescence Study

Five new metal organic frameworks (MOFs) have been synthesized by using cadmium ion and 2,5-furandicarboxylic acid in presence of a variety of bridging amine ligands, [Cd(fdc)(2,2'-bpy)(H2O)](n) (1), {[Cd(fdc)(pyz)(H2O)(2)][Cd(fdc)](H2O)(2)]center dot H2O}(n) (2), {[Cd(fdc)(4,4'-bpy)(H2O)(2)]center dot EtOH}(n) (3), [Cd(fdc)(1,2-bpe)(H2O)](n) (4), and [{Cd-2(fdc)(2)(H2O)(4)}center dot(1,2-bpe)](n) (5), where fdc = 2,5-furandicarboxylic acid, 2,2'-bpy = 2,2'-bipyridyl, pyz = pyrazine, 4,4'-bpy = 4,4'-bipyridyl, 1,2-bpe = 1,2-di(4-pyridyl)ethylene. All the compounds were characterized by single-crystal X-ray analysis and show diversities in their structures. Compound 1 shows linear topology propagating along the crystallographic b-axis. Compound 2 shows supramolecular structure, where two types of ID double chains (ladder type) are present. These chains propagate along the crystallographic a-axis and are tightly held with each other by strong hydrogen bonds. Compound 3 reveals a ID + ID -> 2D polycatenated MOF, where four cadmium centers form a perfect square and these squares are further linked by the carboxylate ligand, forming a ID tube. These tubes are interpenetrated with each other forming a polycatenated 3D MOF. Compound 4 also possesses a polycatenated MOF, but ID sheets are polycatenated with each other forming the 113 + ID -> 3D MOF. Compound 5 is a 2D-based supramolecular 3D MOF, where 1,2-bpe ligands are entrapped within the layer of the 2D by strong hydrogen bonds and pi center dot center dot center dot pi interaction. Luminescence of all the compounds has been investigated