Partha Halder

A functional model of extradiol-cleaving catechol dioxygenases: mimicking the 2-his-1-carboxylate facial triad.

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.

Dioxygen Reactivity of Biomimetic Iron–Catecholate and Iron–o-Aminophenolate Complexes of a Tris(2-pyridylthio)methanido Ligand: Aromatic CC Bond Cleavage of Catecholate versus o-Iminobenzosemiquinonate Radical Formation

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.

Molecular and Electronic Structure of a Nonheme Iron(II) Model Complex Containing an Iron−Carbon Bond

The synthesis and molecular structure of a novel iron(II) complex of an acyclic non-NHC bis(2-pyridylthio)carbene ligand containing a S-N-C facial triad are discussed. The iron carbene complex is formed in situ during the reaction of tris(2-pyridylthio)methane with an iron(II) salt. In the six-coordinate iron(II) complex, a strong Fe-C interaction (1.776 A) is observed crystallographically and the complex exhibits a singlet-spin ground state. Density functional theory calculations that reproduce the geometry as well as the spin ground state indicate that the electronic structure of the complex is best described as an iron(II) carbene. The carbene center is stabilized by extensive back-bonding from iron(II) and delocalization into the adjacent thiopyridine units.

Enhanced Reactivity of a Biomimetic Iron(II) α-Keto Acid Complex through Immobilization on Functionalized Gold Nanoparticles†

Immobilization of transition-metal complexes by surface functionalization of gold nanoparticles (AuNPs) has recently attracted the attention for several applications. Thiolprotected AuNPs are stable and soluble in organic solvents. Therefore, immobilization of metal complexes on AuNPs permits reactions in common organic solvents and also induces properties of the metal complex to the NP. AuNPs functionalized by thiol-appended transition metal complexes are expected to find applications as immobilized catalysts to bridge between homogeneous and heterogeneous catalysis. The high surface area of a nanocatalyst increases the contact between the reactant and catalyst dramatically. These catalysts are easy to synthesize through desired surface modification and can be characterized by different analytical and spectroscopic techniques. Moreover, the catalyst can easily be separated from the reaction mixture. Several reports are now available where immobilization of metal catalysts on AuNPs has been shown to increase the catalytic reactivity. Improved catalytic activities of transition-metal catalysts through surface functionalization of AuNPs have inspired us to initiate a project on immobilization of biomimetic iron complexes. In this direction, we have investigated the dioxygen reactivity of nonheme iron complexes on functionalized AuNPs. Dioxygen-activating a-ketoglutarate (a-KG)dependent oxygenases that carry out versatile biological reactions are well-studied in biology and biomimetic chemistry. The iron(II) centers of the enzymes activate dioxygen, upon binding of a bidentate a-KG and the substrate, to form an iron–oxygen intermediate. The heterolytic O O bond cleavage of the intermediate results in formation of an iron(IV)–oxo oxidant to affect substrate oxidations. Iron(IV)–oxo intermediates have been trapped and characterized in studies with several a-ketoglutarate-dependent enzymes. In biomimetic chemistry, a number of iron(II)–aketo acid complexes have been reported as functional models of a-ketoglutarate-dependent oxygenases. The model complexes exhibit versatile reactivity towards dioxygen and, in some cases, iron(IV)–oxo intermediates have been intercepted in the decarboxylation of a-keto acids. Model iron(II)– a-keto acid complexes of N4 donor ligands have been reported to react with O2 for days to undergo stoichiometric decarboxylation of the coordinated a-keto acids. Moreover, they often form m-oxo diiron(III) complex at the end of the decarboxylation reaction, leading to the deactivation of the complex. Such deactivation reaction may be avoided with a properly immobilized complex on the surface of AuNPs and is expected to exhibit improved/catalytic reactivity upon immobilization. As an outcome of our investigation, we report herein an enhanced reactivity of a biomimetic iron(II)–benzoylformate complex of a thiol-appended N4 ligand (TPASH, Scheme 1) towards dioxygen upon immobi-

Rearrangement of the tris(2-pyridylthio)methanido ligand in an iron(II) complex containing an Fe-C bond.

Iron(II) tris(2-pyridylthio)methanido (1) containing an Fe-C bond, obtained from the reaction of tris(2-pyridylthio)methane (HL(1)) and iron(II) triflate, reacts with protic acid to generate iron(II) bis(2-pyridylthio)carbene (1a). The carbene complex is converted to an iron(II) complex (2) of the 1-[bis(2-pyridylthio)methyl]pyridine-2-thione ligand (L(3)) upon treatment with a base. Complex 2 reversibly transforms to 1a in the presence of an acid. During the transformation of 1 to 2, a novel rearrangement of L(1) to L(3) takes place. The iron(II) complexes are reactive toward dioxygen to form the corresponding iron(III) complexes.

Role of α-hydroxycarboxylic acids in the construction of supramolecular assemblies of nickel(II) complexes with nitrogen donor coligands

The topology of the supramolecular self assembly structures of six nickel(II) complexes containing nitrogen donor coligands is described, and the structural features induced by the deprotonated α-hydroxy acids (mandelic and benzilic) used in the complex synthesis are compared. In all the complexes, namely [(6-Me3-TPA)Ni(MA)]B(Ph)4 (1), [(6-Me3-TPA)Ni(R(−)-MA)]B(Ph)4 (2R), [(6-Me3-TPA)Ni(BA)]B(Ph)4 (3), [(AP)2Ni(MA)2] (4), [(AP)2Ni(S(+)-MA)2] (5S) and [(AP)2Ni(BA)2] (6) (where 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine, AP = 2-aminopyridine, MA = racemic mandelate, R(−) and S(+) are enantiomeric mandelates, BA = benzilate anion), the metal ion exhibits a distorted octahedral environment with mandelate or benzilate anions acting as chelating agents through the hydroxyl group and a carboxylate oxygen. The differences in the crystal structures of 2R and 5S (containing enantiomerically pure mandelate anions) are compared with those of 1 and 4, obtained with a racemic mixture of the deprotonated acid. All the nickel(II) complexes form infinite one-dimensional chains built up by hydrogen bonds. The mononuclear building blocks, 1–3, are assembled head-on, with a very similar topology, through H-bond interactions between the hydroxycarboxylate groups of adjacent units, while in neutral complexes 4–6 the cooperative presence of solvent molecules leads to different connections between the complexes. In addition, intramolecular π–π stacking interactions between pyridine rings of the ligand and the phenyl of mandelate or benzilate anions are observed in 1, 3 and 6.