Modeling the Diiron Centers of Non-Heme Iron Enzymes. Preparation of Sterically Hindered Diiron(II) Tetracarboxylate Complexes and Their Reactions with Dioxygen

Abstract

A series of diiron(II) complexes, [Fe 2 (µ-L)(µ-O 2 CR)(O 2 CR)(N) 2 ], where L is a dinucleating bis-(carboxylate) ligand based on m-xylylenediamine bis(Kemp's triacid imide) and N is a pyridine-or imidazolederived ligand, were prepared as models for the carboxylate-bridged non-heme diiron cores of the O 2 -activating enzymes, soluble methane monooxygenase hydroxylase (MMOH), and the R2 subunit of ribonucleotide reductase (RNR-R2). X-ray crystallographic studies revealed differences in the coordination geometry of the bridging monocarboxylate ligand, which shifts from monodentate to syn,syn-bidentate bonding modes. The extent of this carboxylate shift depends on both the steric bulk of the monocarboxylate and the basicity of the ancillary N-donor ligands. Exposure of these diiron(II) complexes to O 2 at -77°C in nonpolar solvents (CH 2 Cl 2 , THF, toluene) yielded deep blue solutions (λ max ≈ 580 nm, ≈ 1200 M -1 cm -1 ), consistent with the generation of diiron(III) peroxo species. This reaction was irreversible, and its stoichiometry was determined by manometry to be 1:1 in diiron(II) complex and O 2 . The diiron(III) peroxo complexes exhibited oxygen isotope-sensitive resonance Raman bands at ∼860 cm -1 , which are assigned to the O-O stretching frequency of a µ-1,2-peroxodiiron(III) core. 57 Fe Mössbauer spectroscopy confirmed the assignment of the diiron(III) oxidation level and indicated that the two iron sites have inequivalent environments (δ 1 ≈ 0.47 mm s -1 , ∆E Q1 ≈ 0.88 mm s -1 ; δ 2 ≈ 0.63 mm s -1 , ∆E Q2 ≈ 1.20 mm s -1 ). Kinetics experiments provided rate constants for the reaction and revealed it to be first order in both diiron(II) complex and O 2 . The factors controlling the rate of formation of the blue species and its stability are discussed.

3,5-Lutidine coordinated zinc(II) aryl carboxylate complexes: Precursors for zinc(II) oxide

“…The shift of the carboxylate coordination mode from monodentate in 4 to syn-syn bidentate bridging in 2 and 3 and to the chelating in 5 implied largely the influence of basicity of the lone pair on the carbonyl oxygen of the carboxylate moiety and sterics. Such 'carboxylate shift' process has important implications for metalloenzymes in understanding their catalytic activities [51][52][53][54]. …”

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

“…The XDK 2− ligand also supports carboxylate-bridged diiron(II) units having the general composition [Fe II 2 (XDK)(μ-RCO 2 )(RCO 2 )( N -donor) 2 ], where R = t -Bu- (pivalate), PhCy- (1-phenylcyclohexylcarboxylate), Ph- (benzoate), i Pr- (isobutyrate), or t BuCH 2 - (1- tert -butylacetate); N -donor = py (pyridine), 3-Fpy (3-fluoropyridine), N -MeIm ( N -methylimidazole), or N - t BuIm ( N - tert -butylimidazole) [54–56]. More sterically hindered XDK 2− variants, containing either propyl (PXDK 2− ) or benzyl (BXDK 2− )in place of methyl substituents on the Kemp’s triacid moiety, could also be employed to assemble similar diiron(II) compounds.…”

“…More sterically hindered XDK 2− variants, containing either propyl (PXDK 2− ) or benzyl (BXDK 2− )in place of methyl substituents on the Kemp’s triacid moiety, could also be employed to assemble similar diiron(II) compounds. The asymmetric bridging mode of the ancillary carboxylate to the diiron(II) core is determined by both steric and electronic factors [56]. X-ray structural studies suggested that greater steric repulsion between XDK 2− and the external carboxylate, and more basic N -donors, favor a syn,syn -bidentate bridging mode of the ancillary carboxylate rather than a syn,anti -monodentate one.…”

The synthesis and structural studies of a new Kemp’s triacid oxaalkyl ester and its complexes with Li+, Na+ and K+ cations

“…Kemp's triacid was used for selective recognition studies of carboxylate anions [6][7][8][9] and its some imide derivatives (m-xylenediamine bis(Kemp's triacid imide) as complexones of divalent metal cations such as Zn 2+ , Co 2+ , Mg 2+ [10][11][12][13][14][15][16][17][18]. KTA complexes with Fe +2 were used as models of non-heme iron enzymes [19] and as a porphyrin-linked dicarboxylate ligand in tri-iron complexes [20]. Condensation of Kemp's triacid with aromatic diamines has given new type of ionophore selective toward Hg +2 cations [21].…”