No abstract

Mononuclear manganese(II) and iron(III) flavonolates were synthesized as synthetic enzyme-substrate complexes, and their oxygenation reactions as biomimetic functional models with relevance to flavonol 2,4-dioxygenases are briefly described.

Stoichiometric quantities of 3,5-di-tert-butylcatechol and aqueous ammonia react in pyridine solution to form 2-amino-4,6-di-tert-butylphenol. Under an atmosphere of dioxygen the aminophenol is oxidized to either the corresponding iminosemiquinone or iminobenzoquinone. In the presence of Cu(II) iminosemiquinone condensation with the aminophenol gives the Cat-N-SQ radical ligand obtained as the Cu(py)2(Cat-N-SQ) complex. Metal and ligand magnetic orbitals are orthogonal and couple ferromagnetically to give a S = 1 molecular spin state at temperatures up to 300 K. In nonpolar solvents the complex undergoes ligand oxidation and disproportionation to give Cu(Cat-N-BQ)2. Crystallographic characterization on crystals obtained as the i-propanol solvate [orthorhombic, C2221, a = 19.548(3) Å, b = 24.536(5) Å, c = 23.655(5) Å, V = 11346(4) Å3, Z = 8, R = 0.068] show that the expected Jahn−Teller distortion appears in the trans Cu−O lengths of the equatorial plane rather than for the axial Cu−N lengths. Reactions carried out with both Cu(I) and Cu metal require metal oxidation to give the Cu(II) products obtained. With metallic Cu this occurs by a reaction with iminoquinone to give bis(iminosemiquinone)copper(II). Further reaction of this product with aminophenol gives Cu(py)2(Cat-N-SQ) by condensation, and, with O2, oxidation gives a coordinated azophenolate ligand in Cu(py)(azophenolate) by a unique N−N bond-forming reaction. Cu(py)(azophenolate) has been characterized crystallographically [monoclinic, P21/n, a = 10.990(2) Å, b = 10.736(3) Å, c = 26.848(4) Å, β = 98.09(1)°, V = 3136(1) Å3, Z = 4, R = 0.048].

Bis(pyridine)(9,10-phenanthrenequinone)(9,10-phenanthrenediolato)copper(II), Cu(py)(2)(PhenCat)(PhenBQ), has been prepared by treating copper metal with 9,10-phenanthrenequinone in pyridine solution. In dilute solution, both Cu(py)(2)(PhenCat)(PhenBQ) and the related complex Cu(tmeda)(PhenCat)(PhenBQ) lose PhenBQ to form Cu(II)L(2)(PhenCat), where L(2)= tmeda, 2 py. EPR spectra recorded at temperatures between 300 and 77 K reveal the presence of species with radical and metal localized spins together at equilibrium. Equilibria between Cu(II)L(2)(PhenCat) and Cu(I)L(2)(PhenSQ) redox isomers are solvent dependent, with a shift to higher temperature for polar solvents. Both complexes are oxygen sensitive, reacting with dioxygen to give complexes of diphenic acid. Structural characterization on products obtained with tmeda show that dioxygen insertion across the C-C bond within the chelate ring leads to dimeric products with adjacent Cu(II) ions bridged by diphenate ligands. The addition of O(2) to Cu(tmeda)(PhenCat) in acetonitrile solution at 0 degrees C appears to form a peroxo complex, tentatively identified as Cu(tmeda)(O(2))(PhenQ) on the basis of iodometric titration, as the precursor to the diphenate complex.

Mesitylcopper reacts with flavonol (flaH) in the presence of 1,3-bis(2-pyridylimino)isoindoline (indH) to yield the diamagnetic complex CuI(fla)(indH), which on reaction with molecular oxygen undergoes oxidative splitting of the C2-C3 bond of the pyranone ring of the flavonolate ligand to give CuI(indH)(O-bs) (O-bs = O-benzoylsalicylate) (orthorhombic, P1, a = 8.048(7) A, b = 8.969(9) A, c = 19.240(2) A, alpha = 85.69 degrees, beta = 80.24(7) degrees, gamma = 77.87(7) degrees, V = 1337(2) A3, Z = 2) and carbon monoxide. The reaction of [CuI(CH3CN)4]ClO4, flaH, and indH with dioxygen at room temperature affords the paramagnetic complex [CuII(fla)(indH)]ClO4 (mu = 2.10 mu B), and after elimination of HClO4, CuII(fla)(ind) (orthorhombic, Pbca, a = 8.888(2) A, b = 19.169(7) A, c = 33.614(10) A, alpha = beta = gamma = 90 degrees, V = 5727(3) A3, Z = 8) with mu = 1.86 mu B is formed. The latter undergoes cleavage of the pyranone ring on oxygenation at 80 degrees C to give CuII(ind)(O-bs) (mu = 1.87 mu B, nu(CO) = 1742 cm-1, and nu(CO2) = 1581, 1387 cm-1) and carbon monoxide. CuII(fla)(ind) and [CuII(fla)(indH)]ClO4 serve as good catalysts for the oxygenation of flavonol to O-benzoylsalicyclic acid.

Mononuclear iron(iii) flavonolate was synthesized as synthetic enzyme-substrate complex, and its direct and carboxylate-enhanced dioxygenation as biomimetic functional models with relevance to flavonol 2,4-dioxygenase are briefly described.

The complex [Fe(indH)(solvent)3](ClO4)2 (1) has been isolated from the reaction of equimolar amounts of 1,3-bis(2′-pyridylimino)isoindoline (indH) and Fe(ClO4)2 in acetonitrile and characterized by X-ray crystallography and several spectroscopic techniques. It is a suitable catalyst for the oxidation of thioanisoles and benzyl alcohols with H2O2 as the oxidant. Hammett correlations and kinetic isotope effect experiments support the involvement of an electrophilic metal-based oxidant. A metastable green species (2) is observed when 1 is reacted with H2O2 at −40 °C, which has been characterized to have a FeIII(μ-O)(μ-O2)FeIII core on the basis of UV-Vis, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopic data.

The preparation and characterisation of two dinuclear iron(III) complexes with tripodal ligands 1,3-bis(2Ј-pyridylimino)isoindoline (indH) are presented. The µ-oxo-bridged di- (2)