Samiran Mahapatra

Reversible cleavage and formation of the dioxygen O-O bond within a dicopper complex

A key step in dioxygen evolution during photosynthesis is the oxidative generation of the O-O bond from water by a manganese cluster consisting of M2(μ-O)2 units (where M is manganese). The reverse reaction, reductive cleavage of the dioxygen O-O bond, is performed at a variety of dicopper and di-iron active sites in enzymes that catalyze important organic oxidations. Both processes can be envisioned to involve the interconversion of dimetal-dioxygen adducts, M2(O2), and isomers having M2(μ-O)2 cores. The viability of this notion has been demonstrated by the identification of an equilibrium between synthetic complexes having [Cu2(μ-η2:η2-O2)]2+ and [Cu2(μ-O)2]2+ cores through kinetic, spectroscopic, and crystallographic studies.

Observation of the longst Fe–N(pyridine) bond in an FeIIN6 chromophore. Crystal structure and 1H nuclear magnetic resonance studies of [FeL22][ClO4]2[L2= 2-(3,5-dimethylpyrazol-1-ylmethyl)-6-(pyrazol-1-ylmethyl)pyridine]

The X-ray structural and 1H NMR spectral properties of a bis(ligand) iron(II) complex of a tridentate pyridylpyrazole ligand L2[L2= 2-(3,5-dimethylpyrazol-1-ylmethyl)-6-(pyrazol-1-ylmethyl)pyridine] have been studied. The complex [FeL22][ClO4]2 crystallizes as yellow prisms in the monoclinic space group P21/n with a= 14.122(3), b= 13.612(3), c= 18.393(4)A, β= 99.11(3)°, Z= 4, R= 0.0518 and R′= 0.0634 for 6464 observed reflections. The Fe atom is bound to six nitrogen atoms from two tridentate (N3) heterocyclic ligands. Considerable distortion is observed with the average Fe-N(pyridine) and Fe-N(pyrazole) distances being 2.274 and 2.173 A respectively. These bond lengths are markedly longer than corresponding lengths for complexes of less sterically demanding ligands. As expected, the ligand molecules are not planar, the pyridyl and pyrazole rings being twisted relative to each other. The 3-methyl substituent induces a steric barrier to co-ordination and also greater inter-ligand repulsion and this is presumably responsible for the accessibility of the quintet state for FeII. The 1H NMR spectral measurements in CD3CN reveal that its solid-state structure is retained in solution.

A new mixed-valence binuclear complex containing the [MnIV(µ-O)2(µ-O2CMe)MnIII]2+ core: synthesis, magnetism, electron paramagnetic resonance and redox properties

The complex [Mn2(µ-O)2(µ-O2CMe)L2][ClO4]2·H2O 1{L = methyl[2-(2-pyridyl)ethyl](2-pyridyl-methyl)amine} has been synthesised and isolated in the solid state. In MeCN solution complex 1 exhibits absorption spectral features characteristic of the [Mn2(µ-O)2(µ-O2CMe)]2+ core. Variable-temperature (19.8–300 K) solid-state magnetic susceptibility data are consistent with a doublet ground state with J=–144 cm–1. The X-band EPR spectrum at 77 K exhibits a sixteen-line pattern centred at g= 2. This spectrum is attributed to the overlap of hyperfine splitting of two chemically distinct manganese nuclei (l= 5/2) with one hyperfine coupling constant being roughly twice the magnitude of the other. Cyclic voltammetry of 1 shows a quasi-reversible one-electron oxidation [E½=+1.0 V vs. saturated calomel electrode (SCE)] to the MnIV2 species as well as an irreversible one-electron reduction (Epc=–0.10 V vs. SCE) to the MnIII2 species. Coulometric or perchloric acid oxidation generates the orange MnIV2 species. The redox stability and the absorption spectral properties of this oxidised species have been investigated.

New triply bridged diiron(III) complexes with [Fe2(µ-O)(µ-X)2]2+ cores [X = MeCO2, PhCO2 or (PhO)2PO2]

A group of three diiron(III) complexes having [Fe2(µ-O)(µ-X)2]2+ cores (X = benzoate, acetate or diphenyl phosphate) has been synthesised with the use of unsymmetrical facially capping tridentate ligands (L1 and L2), where L1 and L2 are [2-(2-pyridyl)ethyl](2-pyridylmethyl)amine and methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine respectively. The lability of the bridging acetate groups in [Fe(µ-O)(µ-MeCO2)2L22]2+ has been demonstrated by exchange with diphenyl phosphate and deuterioacetate as revealed by UV/VIS and 1H NMR studies. These complexes exhibit infrared, electronic and Mossbauer spectral features very similar to those of µ-oxo-diiron(III) proteins as well as of µ-oxo-bis(µ-carboxylato) or µ-oxo-bis(µ-phosphato)diiron(III) complexes. The inequivalence in the chelate rings around each iron(III) might have caused the asymmetry in these compounds which is reflected in their distinctively strong antiferromagnetic coupling (J=–127, –125 and –108 cm–1 for the benzoate-, acetate- and phosphate-bridged complexes respectively).

Synthesis, spectroscopy and electrochemistry of ruthenium(II) complexes of tridentate pyridylpyrazole ligands. Predominance of electronic over steric effects

The preparations and properties of bis complexes of ruthenium(II) containing various combinations of the tridentate ligands 2,6-bis(pyrazol-1-ylmethyl)pyridine (L1) and di- and tetra-methyl substituted derivatives (L2 and L3) are described. The absorption spectral properties of the complexes are thoroughly analysed. Full assignments have been made for the 1H NMR spectra of two representative complexes in CD3CN and the origins of the co-ordination-induced shifts are discussed. Cyclic voltammetric experiments (MeCN solutions) reveal reversible one-electron RuIII–RuII redox couples in the potential range 1.00–1.06 V vs. saturated calomel electrode (SCE). The occurrence of ligand-based irreversible reductions at low potentials (Ep,c values lie in the range: –1.90 to –2.20 V vs. SCE) reveals that these pyrazole-rich ligands are very poor π acceptors. The formal potentials of the RuIII–RuII couples decrease (by ca. 8 mV per methyl group) as the number of methyl groups in the ligands are increased. The decreased potential step size for [RuL(L′)]2+(L,L′= L1–L3) relative to those observed for non-sterically hindered ruthenium(II) complexes reveals that in the present system steric effects due to 3-Me substituent(s) contribute to the observed effect but that electronic factors predominate over steric effects.

Catalytic reduction of nitric oxide to nitrous oxide by alcohols mediated by copper(I) complexes

In a reaction relevant to environmetally important processes carried out by copper-containing enzymes and heterogeneous catalysts, copper(I) complexes of 1,4,7-triisopropyl-1,4,7-triazacyclononane dissolved in alcohols (e.g. benzyl alcohol) efficiently promote the reduction of NO to N2O by the solvent at room temperature, yielding the respective carbonyl compounds (e.g. benzaldehyde).