[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.
[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.
Angewandte Chemie International Edition 05/2012; 51(25):6195-9. · 11.34 Impact Factor
[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: 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-tert-butyl 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 dioxygenases. Oxidative C–C bond cleavage of catechol is catalyzed by nonheme iron enzymes, catechol dioxygenases. These enzymes are found in soil bacteria and play an important role in the biodegradation of aromatic toxic compounds. 1 Catechol dioxygenases are classified into two major categories, extradiol and intradiol cleaving catechol dioxygenases, based on the oxidation state of active site iron and on the position of the C–C bond cleavage. 2,3 In the active site of extradiol dioxygenases, 2,4 iron(II) is coordinated by two histidines and one carboxylate donor, either from asparatate or glutamate residue, showing the '2-his-1-carboxylate facial triad' structural motif. 5,6 The rest of the coordination sites are occupied by water molecules. The extradiol dioxygenases selectively cleave the C2–C3 bond of substrate catechols. The intradiol cleaving dioxygenases use iron(III) in the active site coordinated by two histidines and two tyrosinates and cleave the C1–C2 bond of catechols. 7-9 Extensive studies have been carried out using many iron-catecholate model complexes to understand the mechanism of C–C bond cleavage reaction catalyzed by catechol cleaving dioxygenases. 2,10-12 Intradiol cleavage of catechol has been reported in most of the model systems, but examples of biomimetic complexes that show extradiol cleavage
[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. · 6.38 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.