[Show abstract][Hide abstract] ABSTRACT: Three biomimetic iron(II) α-hydroxy acid complexes, [(Tp(Ph2))Fe(II)(mandelate)(H2O)] (1), [(Tp(Ph2))Fe(II)(benzilate)] (2), and [(Tp(Ph2))Fe(II)(HMP)] (3), together with two iron(II) α-methoxy acid complexes, [(Tp(Ph2))Fe(II)(MPA)] (4) and [(Tp(Ph2))Fe(II)(MMP)] (5) (where HMP = 2-hydroxy-2-methylpropanoate, MPA = 2-methoxy-2-phenylacetate, and MMP = 2-methoxy-2-methylpropanoate), of a facial tridentate ligand Tp(Ph2) [where Tp(Ph2) = hydrotris(3,5-diphenylpyrazole-1-yl)borate] were isolated and characterized to study the mechanism of dioxygen activation at the iron(II) centers. Single-crystal X-ray structural analyses of 1, 2, and 5 were performed to assess the binding mode of an α-hydroxy/methoxy acid anion to the iron(II) center. While the iron(II) α-methoxy acid complexes are unreactive toward dioxygen, the iron(II) α-hydroxy acid complexes undergo oxidative decarboxylation, implying the importance of the hydroxyl group in the activation of dioxygen. In the reaction with dioxygen, the iron(II) α-hydroxy acid complexes form iron(III) phenolate complexes of a modified ligand (Tp(Ph2)*), where the ortho position of one of the phenyl rings of Tp(Ph2) gets hydroxylated. The iron(II) mandelate complex (1), upon decarboxylation of mandelate, affords a mixture of benzaldehyde (67%), benzoic acid (20%), and benzyl alcohol (10%). On the other hand, complexes 2 and 3 react with dioxygen to form benzophenone and acetone, respectively. The intramolecular ligand hydroxylation gets inhibited in the presence of external intercepting agents. Reactions of 1 and 2 with dioxygen in the presence of an excess amount of alkenes result in the formation of the corresponding cis-diols in good yield. The incorporation of both oxygen atoms of dioxygen into the diol products is confirmed by (18)O-labeling studies. On the basis of reactivity and mechanistic studies, the generation of a nucleophilic iron-oxygen intermediate upon decarboxylation of the coordinated α-hydroxy acids is proposed as the active oxidant. The novel iron-oxygen intermediate oxidizes various substrates like sulfide, fluorene, toluene, ethylbenzene, and benzaldehyde. The oxidant oxidizes benzaldehyde to benzoic acid and also participates in the Cannizzaro reaction.
[Show abstract][Hide abstract] ABSTRACT: An iron(III)-catecholate complex [L(1)Fe(III)(DBC)] (2) and an iron(II)-o-aminophenolate complex [L(1)Fe(II)(HAP)] (3; where L(1) = tris(2-pyridylthio)methanido anion, DBC = dianionic 3,5-di-tert-butylcatecholate, and HAP = monoanionic 4,6-di-tert-butyl-2-aminophenolate) have been synthesised from an iron(II)-acetonitrile complex [L(1)Fe(II)(CH(3)CN)(2)](ClO(4)) (1). Complex 2 reacts with dioxygen to oxidatively cleave the aromatic C-C bond of DBC giving rise to selective extradiol cleavage products. Controlled chemical or electrochemical oxidation of 2, on the other hand, forms an iron(III)-semiquinone radical complex [L(1)Fe(III)(SQ)](PF(6)) (2(ox)-PF(6); SQ = 3,5-di-tert-butylsemiquinonate). The iron(II)-o-aminophenolate complex (3) reacts with dioxygen to afford an iron(III)-o-iminosemiquinonato radical complex [L(1)Fe(III)(ISQ)](ClO(4))(3(ox)-ClO(4); ISQ = 4,6-di-tert-butyl-o-iminobenzosemiquinonato radical) via an iron(III)-o-amidophenolate intermediate species. Structural characterisations of 1, 2, 2(ox) and 3(ox) reveal the presence of a strong iron-carbon bonding interaction in all the complexes. The bond parameters of 2(ox) and 3(ox) clearly establish the radical nature of catecholate- and o-aminophenolate-derived ligand, respectively. The effect of iron-carbon bonding interaction on the dioxygen reactivity of biomimetic iron-catecholate and iron-o-aminophenolate complexes is discussed.
Chemistry - A European Journal 09/2012; 18(37):11778-87. DOI:10.1002/chem.201200886 · 5.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bond cleavage: An iron(II)-α-hydroxy ketone complex of a facial tridentate nitrogen donor ligand undergoes a CC bond cleavage reaction in the presence of dioxygen to form two equivalents of carboxylic acids. This reaction is a functional model of 2,4'-dihydroxyacetophenone dioxygenase.
[Show abstract][Hide abstract] ABSTRACT: Two zinc(II) complexes, [ZnII 2(L1)2] (1) and [ZnII 2(L2)2] (2), and two cobalt(II) complexes,
[CoII 2(L1)2] (3) and [CoII 2(L2)2] (4), (H2L1= iminophenol derived from the condensation of 4-
aminobenzylamine and salicyldehyde, H2L2 = Schiff-base of 4-aminobenzylamine and 3,5-ditert-
butylsalicyldehyde) supported by bis(N,O-bidentate) ligands have been synthesized and
characterized. X-ray single crystal structures of 3 and 4 revealed the dimeric nature of the
complexes with a 1:1 ratio of metal and ligand. The zinc(II) complexes have a very similar
composition as established from different analytical and spectroscopic techniques. The
iminophenols exhibit highly selective fluorescence enhancement with high quantum yield upon
binding with Zn2+ in solution. A fifty-fold enhancement of emission in zinc(II) complexes is
observed with respect to free iminophenols.
[Show abstract][Hide abstract] ABSTRACT: The mononuclear copper(II) complex [Cu(H(2)L(1))(2)(H(2)O)](ClO(4))(2) (1) (where H(2)L(1) = 1,10-phenanthroline-5,6-dioxime) reacts with copper(II) perchlorate in acetonitrile at ambient conditions in the presence of triethylamine to afford a copper(II) complex, [Cu(L(3))(2)(H(2)O)](ClO(4))(2) (2a), of 1,10-phenanthroline furoxan. A similar complex [Cu(L(3))(2)Cl](ClO(4)) (2) is isolated from the reaction of H(2)L(1) with copper(II) chloride, triethylamine, and sodium perchlorate in acetonitrile. The two-electron oxidation of the vic-dioxime to furoxan is confirmed from the X-ray single crystal structure of 2. An intermediate species, showing an absorption band at 608 nm, is observed at -20 °C during the conversion of 1 to 2a. A similar blue intermediate is formed during the reaction of [Cu(HDMG)(2)] (H(2)DMG = dimethylglyoxime) with ceric ammonium nitrate, but H(2)DMG treated with ceric ammonium nitrate does not form any intermediate. This suggests the involvement of a copper(II) complex in the intermediate step. The intermediate species is also observed during the two-electron oxidation of other vic-dioximes. On the basis of the spectroscopic evidence and the nature of the final products, the intermediate is proposed to be a mononuclear copper(II) complex ligated by a vic-dioxime and a dinitrosoalkene. The dinitrosoalkene is generated upon two-electron oxidation of the dioxime. The transient blue color of the dioxime-copper(II)-dinitrosoalkene complex may be attributed to the ligand-to-ligand charge transfer transition. The intermediate species slowly decays to the corresponding two-electron oxidized form of vic-dioxime, i.e. furoxan and [Cu(CH(3)CN)(4)](ClO(4)). The formation of two isomeric furoxans derived from the reaction of an asymmetric vic-dioxime, hexane-2,3-dioxime, and copper(II) perchlorate supports the involvement of a dinitrosoalkene species in the intermediate step. In addition, the oxidation of 2,9-dimethyl-1,10-phenanthroline-5,6-dioxime (H(2)L(2)) to the corresponding furoxan and subsequent formation of a copper(I) complex [Cu(L(4))(2)](ClO(4)) (3) (where L(4) = 2,9-dimethyl-1,10-phenanthroline furoxan) are discussed.
[Show abstract][Hide abstract] ABSTRACT: O 2-dependent transformation: An iron(II)-benzilate complex of a tridentate N3 donor ligand reacts with O 2 to undergo oxidative decarboxylation. Cyclohexene is selectively converted into cis-cyclohexane-1,2- diol in the reaction.
[Show abstract][Hide abstract] ABSTRACT: Two biomimetic iron(II)-catecholate complexes, [(Tp Ph2)Fe II(CatH)] (1) and [(Tp Ph2)Fe II(DBCH)] (2) (where Tp Ph2 = hydrotris(3,5-diphenylpyrazole-1-yl)borate, CatH = monoanionic pyrocatecholate and DBCH = monoanionic 3,5-di-tertbutyl catecholate), have been isolated and characterized to study their reactivity towards dioxygen. The single-crystal X-ray structure of (1) reveals a high-spin iron(II) center ligated by the monoanionic facial N 3 ligand and a monoanionic catecholate, giving rise to a trigonal bipyramidal coordination geometry. Complex (1) represents the first structurally characterized five-coordinate iron(II)-catecholate complex with an asymmetric bidentate binding motif of monoanionic catecholate. While (1) reacts with dioxygen to form the corresponding iron(III)-catecholate, (2) reacts with dioxygen to give 75 % extradiol and 25 % intradiol cleavage products via an iron(III)-catecholate intermediate species. Complex (2) is a potential functional model of extradiol cleaving catechol.
Indian journal of chemistry 04/2011; 50A(3):420-426.
[Show abstract][Hide abstract] ABSTRACT: The synthesis and characterization of an iron-catecholate model complex of a tridentate 2-N-1-carboxylate ligand derived from L-proline are reported. The X-ray crystal structure of the complex [(L)(3)Fe(3)(DBC)(3)] (1) (where L is 1-(2-pyridylmethyl)pyrrolidine-2-carboxylate and DBC is the dianion of 3,5-di-tert-butyl catechol) reveals that the tridentate ligand binds to the iron center in a facial manner and mimics the 2-his-1-carboxylate facial triad motif observed in extradiol-cleaving catechol dioxygenases. The iron(III)-catecholate complex (1) reacts with dioxygen in acetonitrile in ambient conditions to cleave the C-C bond of catecholate. In the reaction, an equal amount of extra- and intradiol cleavage products are formed without any auto-oxidation product. The iron-catecholate complex is a potential functional model of extradiol-cleaving catechol dioxygenases.
[Show abstract][Hide abstract] ABSTRACT: Iron(II)-alpha-hydroxy acid complexes of a tripodal N4 ligand undergo oxidative decarboxylation upon exposure to O(2) and mimic the aliphatic C1-C2 cleavage step catalyzed by CloR.
Chemical Communications 03/2010; 46(11):1830-2. DOI:10.1039/b925389k · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 1,2-Dioximes undergo oxidative transformation mediated by copper(II) ions in acetonitrile to form the corresponding furoxans in high yields. A series of 1,2-dioximes including aliphatic, aromatic, and heterocyclic dioximes were oxidized using these mild conditions.