Ramasamy Mayilmurugan

Iron(III) complexes of tripodal monophenolate ligands as models for non-heme catechol dioxygenase enzymes: correlation of dioxygenase activity with ligand stereoelectronic properties.

The iron(III) complexes [Fe(L)Cl(2)] 1-6 of the tripodal monophenolate ligands N,N-bis(2-pyridylmethyl)-N-(2-hydroxybenzyl)amine H(L1), [(1-methylimidazol-2-ylmethyl)-(pyrid-2-ylmethyl)aminomethyl]-phenol H(L2), 2,4-dimethyl-6-[(1-methylimidazol-2-ylmethyl)(pyrid-2-ylmethyl)aminomethyl]phenol H(L3), N,N-dimethyl-N-(pyrid-2-ylmethyl)-N-(2-hydroxybenzyl)ethylenediamine H(L4), N,N-dimethyl-N-(1-methylimidazol-2-ylmethyl)-N-(2-hydroxybenzyl)ethylenediamine H(L5), and N,N-dimethyl-N-(1-methylimidazol-2-ylmethyl)-N-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine H(L6) have been isolated and studied as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The complexes have been characterized using elemental analysis, electrospray ionization mass spectrometry, and absorption spectral and electrochemical methods. The single crystal X-ray structures of [Fe(L3)Cl(2)] 3 and [Fe(L6)Cl(2)] 6 have been successfully determined, and the rhombically distorted octahedral coordination geometry around iron(III) in them are constituted by the phenolate oxygen and pyridyl/-NMe(2), and N-methylimidazolyl and tertiary amine nitrogens of the tripodal tetradentate ligands H(L3)/H(L6) and two cis-coordinated chloride ions. The sterically demanding -NMe(2) group as in 6 imposes an Fe-O-C bond angle (139.8 degrees) and Fe-O bond length (1.852 A), which are very close to those (Fe-O-C, 133, 148 degrees; Fe-O(tyrosinate), 1.81, 1.91 A) of 3,4-PCD enzymes. The Fe-O-C bond angle observed for 6 is higher than that for 3 (125.1 degrees), and the Fe-O(phenolate) bond distance in 6 is shorter than that in 3 (1.905 A). In methanol solution all the complexes exhibit two phenolate-to-Fe(III) ligand-to-metal charge transfer (LMCT) bands in the ranges 536-622 and 329-339 nm. Further, when 3,5-di-tert-butylcatechol (H(2)DBC) pretreated with two moles of Et(3)N is added to 1-6, two new intense DBC(2-)-to-iron(III) LMCT bands (466-489, 676-758 nm) are observed, which are similar to those observed for 3,4-PCD enzyme-substrate complex. All the complexes elicit oxidative intradiol cleavage of H(2)DBC in the presence of O(2). Interestingly, among the present complexes, 3 containing coordinated N-methylimidazolyl nitrogen shows the highest rate of intradiol cleavage, which correlates with the highest energy of DBC(2-)-to-iron(III) LMCT band and the most negative DBSQ/DBC(2-) redox potential. Also, the catecholate adducts of complexes 4 and 5, both containing a -NMe(2) donor group, react faster and produce higher amounts of intradiol cleavage products (4: 55.3; 5, 50.6%) than the analogous complexes 1 (43.2%) and 2 (32.7%), both containing a pyridyl nitrogen donor, which is consistent with the more negative DBSQ/DBC(2-) redox potentials for 4 and 5. The increase in rate of catechol dioxygenation with increase in the DBC(2-)-to-iron(III) LMCT band energy and decrease in DBSQ/DBC(2-) redox potential is illustrated by invoking a facile alpha-electron transfer from iron(III) to catecholate-bound molecular oxygen in the substrate activation mechanism proposed for the intradiol-cleaving catechol dioxygenases. Also, when the substituents on the phenolate arm are varied to tune the Lewis acidity of iron(III) center, the reaction rate decreases with decrease in Lewis acidity and, interestingly, extradiol cleavage is also observed when the Lewis acidity is decreased further by incorporating a 3,5-dimethylphenolate arm as in 6.

Mechanistic Insight into the Reactivity of Oxotransferases by Novel Asymmetric Dioxomolybdenum(VI) Model Complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-flurobenzyl)-1,4-diazepane, and 1,4-bis(2-hydroxy-4-chlorobenzyl)-1,4-diazepane (H(2)(L1)-H(2)(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO(2)(L)] 1-6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO(2)(L)] 1-4 have been successfully determined by single-crystal X-ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis-β configuration. The Mo-O(oxo) bond lengths differ only by ≈0.01u2005Å. Complexes 1, 2, 5, and 6 exhibit two successive Mo(VI)/Mo(V) (E(1/2), -1.141 to -1.848u2005V) and Mo(V)/Mo(IV) (E(1/2), -1.531 to -2.114u2005V) redox processes. However, only the Mo(VI)/Mo(V) redox couple was observed for 3 and 4, suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1, 2, 5, and 6 elicit efficient catalytic oxygen-atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe(3) at 65u2009°C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130u2009°C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe(3) at 65u2009°C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.

Novel Iron(III) Complexes of Sterically Hindered 4N Ligands: Regioselectivity in Biomimetic Extradiol Cleavage of Catechols

The iron(III) complexes of the 4N ligands 1,4-bis(2-pyridylmethyl)-1,4-diazepane (L1), 1,4-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L2), and 1,4-bis(2-quinolylmethyl)-1,4-diazepane (L3) have been generated in situ in CH 3CN solution, characterized as [Fe(L1)Cl 2] (+) 1, [Fe(L2)Cl 2] (+) 2, and [Fe(L3)Cl 2] (+) 3 by using ESI-MS, absorption and EPR spectral and electrochemical methods and studied as functional models for the extradiol cleaving catechol dioxygenase enzymes. The tetrachlorocatecholate (TCC (2-)) adducts [Fe(L1)(TCC)](ClO 4) 1a, [Fe(L2)(TCC)](ClO 4) 2a, and [Fe(L3)(TCC)](ClO 4) 3a have been isolated and characterized by elemental analysis, absorption spectral and electrochemical methods. The molecular structure of [Fe(L1)(TCC)](ClO 4) 1a has been successfully determined by single crystal X-ray diffraction. The complex 1a possesses a distorted octahedral coordination geometry around iron(III). The two tertiary amine (Fe-N amine, 2.245, 2.145 A) and two pyridyl nitrogen (Fe-N py, 2.104, 2.249 A) atoms of the tetradentate 4N ligand are coordinated to iron(III) in a cis-beta configuration, and the two catecholate oxygen atoms of TCC (2-) occupy the remaining cis positions. The Fe-O cat bond lengths (1.940, 1.967 A) are slightly asymmetric and differ by 0.027 A only. On adding catecholate anion to all the [Fe(L)Cl 2] (+) complexes the linear tetradentate ligand rearranges itself to provide cis-coordination positions for bidentate coordination of the catechol. Upon adding 3,5-di- tert-butylcatechol (H 2DBC) pretreated with 1 equiv of Et 3N to 1- 3, only one catecholate-to-iron(III) LMCT band (648-800 nm) is observed revealing the formation of [Fe(L)(HDBC)] (2+) involving bidentate coordination of the monoanion HDBC (-). On the other hand, when H 2DBC pretreated with 2 equiv of Et 3N or 1 or 2 equiv of piperidine is added to 1- 3, two intense catecholate-to-iron(III) LMCT bands appear suggesting the formation of [Fe(L)(DBC)] (+) with bidentate coordination of DBC (2-). The appearance of the DBSQ/H 2DBC couple for [Fe(L)Cl 2] (+) at positive potentials (-0.079 to 0.165 V) upon treatment with DBC (2-) reveals that chelated DBC (2-) in the former is stabilized toward oxidation more than the uncoordinated H 2DBC. It is remarkable that the [Fe(L)(HDBC)] (2+) complexes elicit fast regioselective extradiol cleavage (34.6-85.5%) in the presence of O 2 unlike the iron(III) complexes of the analogous linear 4N ligands known so far to yield intradiol cleavage products exclusively. Also, the adduct [Fe(L2)(HDBC)] (2+) shows a higher extradiol to intradiol cleavage product selectivity ( E/ I, 181:1) than the other adducts [Fe(L3)(HDBC)] (2+) ( E/ I, 57:1) and [Fe(L1)(HDBC)] (2+) ( E/ I, 9:1). It is proposed that the coordinated pyridyl nitrogen abstracts the proton from chelated HDBC (-) in the substrate-bound complex and then gets displaced to facilitate O 2 attack on the iron(III) center to yield the extradiol cleavage product. In contrast, when the cleavage reaction is performed in the presence of a stronger base like piperidine or 2 equiv of Et 3N a faster intradiol cleavage is favored over extradiol cleavage suggesting the importance of bidentate coordination of DBC (2-) in facilitating intradiol cleavage.

Faster oxygen atom transfer catalysis with a tungsten dioxo complex than with its molybdenum analog

The synthesis and characterization of a series of molybdenum ([MoO(2)Cl(L(n))]; L(1) (1), L(2) (3)) and tungsten ([WO(2)Cl(L(n))]; L(1) (2), L(2) (4)) dioxo complexes (L(1) = 1-methyl-4-(2-hydroxybenzyl)-1,4-diazepane and L(2) = 1-methyl-4-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane) of tridentate aminomonophenolate ligands HL(1) and HL(2) are reported. The ligands were obtained by reductive amination of 1-methyl-1,4-diazepane with the corresponding aldehyde. Complexes 3 and 4 were obtained by the reaction of [MO(2)Cl(2)(dme)(n)] (M = Mo, n = 0; W, n = 1) with the corresponding ligand in presence of a base, whereas for the preparation of 1 and 2 the ligands were deprotonated by KH prior to the addition to the metal. They were characterized by NMR and IR spectroscopy, by cyclic voltammetry, mass spectrometry, elemental analysis and by single-crystal X-ray diffraction analysis. Solid-state structures of the molybdenum and tungsten cis-dioxo complexes reveal hexa-coordinate metal centers surrounded by two oxo groups, a chloride ligand and by the tridentate monophenolate ligand which coordinates meridionally through its [ONN] donor set. In the series of compounds 1-4, complexes 3 and 4 have been used as catalysts for the oxygen atom transfer reaction between dimethyl sulfoxide (DMSO) and trimethyl phosphine (PMe(3)). Surprisingly, faster oxygen atom transfer (OAT) reactivity has been observed for the tungsten complex [WO(2)Cl(L(2))] (4) in comparison to its molybdenum analog [MoO(2)Cl(L(2))] (3) at room temperature. The kinetic results are discussed and compared in terms of their reactivity.

Fixation and sequestration of carbon dioxide by copper(II) complexes

AbstractThe fixation of carbon dioxide (

Iron(III) complexes of tridentate 3N ligands as functional models for catechol dioxygenases: the role of ligand N-alkyl substitution and solvent on reaction rate and product selectivity

hbox {CO}_{2})

Dioxidomolybdenum(VI) Complexes Containing Ligands with the Bipyrrolidine Backbone as Efficient Catalysts for Olefin Epoxidation

CO2) is an important global challenge. A significant increase of the atmospheric