Large Reorganization Energies in Electron Transfer: Oxidation of Hydroxylamine by Hexachloroiridate(IV)

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Temperature dependent iodide oxidation by MLCT excited states

Temperature dependent excited state iodide oxidation by two heteroleptic Ru polypyridyl compounds was quantified for the first time.

Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media

In this work we report the kinetics of oxidation of hydroxylamine by a trinuclear Mn(IV) oxidant, [Mn(3)(μ-O)(4)(phen)(4)(H(2)O)(2)](4+) (1, phen = 1,10-phenanthroline), in aqueous solution over a pH range 2.0-4.0. The trinuclear Mn(IV) species (1) deprotonates in aqueous solution at physiological pH: 1 ⇌ 2 + H(+); pK(1) = 4.00 (± 0.15) at 25.0 °C, I = 1.0 (M) NaNO(3). Both 1 and 2 are reactive oxidants reacting with the conjugate acid of hydroxylamine, viz. NH(3)OH(+) where the deprotonated oxidant 2 reacts faster. This finding is in contrast to a common observation and belief that protonated oxidants react quicker than their deprotonated analogues. Mn(IV)(3) to Mn(II) transition in the present reaction proceeds through the intervention of a spectrally detected mixed-valent Mn(III)Mn(IV) dimer that quickly collapses to Mn(II). The rate of the reaction was found to be lowered in D(2)O-enriched media in comparison to that in pure H(2)O media. An initial one electron one proton transfer to Mn(IV)(3) (electroprotic; 1e, 1H(+)) could be mechanistically conceived as the rate step. We also demonstrate by means of high level DFT studies that, among the two sets of Mn(IV) atoms in the trinuclear oxidant, the unique one that is coordinated with two phen ligands and two oxo-bridges is reduced to Mn(III) at the rate step. This is explained based on energetic and spin density calculations. Moreover, this result agrees with the charge distribution on the Mn atoms of the trinuclear complex.

Disproportionation of O-methylhydroxylamine catalyzed by aquapentacyanoferrate(II)

The aquapentacyanoferrate(II) ion, [Fe(II)(CN)(5)H(2)O](3-), catalyzes the disproportionation reaction of O-methylhydroxylamine, NH(2)OCH(3), with stoichiometry 3NH(2)OCH(3) → NH(3) + N(2) + 3CH(3)OH. Kinetic and spectroscopic evidence support an initial N coordination of NH(2)OCH(3) to [Fe(II)(CN)(5)H(2)O](3-) followed by a homolytic scission leading to radicals [Fe(II)(CN)(5)(•)NH(2)](3-) (a precursor of Fe(III) centers and bound NH(3)) and free methoxyl, CH(3)O(•), thus establishing a radical path leading to N-methoxyamino ((•)NHOCH(3)) and 1,2-dimethoxyhydrazine, (NHOCH(3))(2). The latter species is moderately stable and proposed to be the precursor of N(2) and most of the generated CH(3)OH. Intermediate [Fe(III)(CN)(5)L](2-) complexes (L = NH(3), H(2)O) form dinuclear cyano-bridged mixed-valent species, affording a catalytic substitution of the L ligands promoted by [Fe(II)(CN)(5)L](3-). Free or bound NH(2)OCH(3) may act as reductants of [Fe(III)(CN)(5)L](2-), thus regenerating active sites. At increasing concentrations of NH(2)OCH(3) a coordinated diazene species emerges, [Fe(II)(CN)(5)N(2)H(2)](3-), which is consumed by the oxidizing CH(3)O(•), giving N(2) and CH(3)OH. Another side reaction forms [Fe(II)(CN)(5)N(O)CH(3)](3-), an intermediate containing the nitrosomethane ligand, which is further oxidized to the nitroprusside ion, [Fe(II)(CN)(5)NO](2-). The latter is a final oxidation product with a significant conversion of the initial [Fe(II)(CN)(5)H(2)O](3-) complex. The side reaction partially blocks the Fe(II)-aqua active site, though complete inhibition is not achieved because the radical path evolves faster than the formation rates of the Fe(II)-NO(+) bonds.

Thermochemical Properties of HxNO Molecules and Ions from ab Initio Electronic Structure Theory

Coupled-cluster calculations through noniterative triple excitations were used to compute optimized structures, atomization energies at 0 K, and heats of formation at 0 and 298 K for NH2O, HNOH, NH2O-, NH2OH+, NH3OH+, HNO-, and HON. These molecules are important in the gas-phase oxidation of NH3, as well as its solution-phase chemistry. The O-H, N-H, and N-O bond energies of these molecules are given and compared. The N-H and O-H bond energies are quite low, and, for NH2OH, the O-H bond is weaker than the N-H bond (by 7.5 kcal/mol). The energetics for a variety of ionic chemical processes in the gas phase, including the electron affinities of NH2O and HNO, the proton affinities of NH2O and NH2OH, and the acidities of NH2OH and NH2O, are given. The compounds are weak bases and weak acids in the gas phase. Solvation effects were included at the PCM and COSMO levels. The COSMO model gave better values than the PCM model. The relative values for pKa for NH2O and NH2OH are in good agreement with the experimental values, showing both compounds to be very strong bases in aqueous solution with NH2OH being the stronger base by 1.8 pK units at the COSMO level, compared to the experimental pK difference of 1.1+/-0.3 pK units. We predict that NH2OH+ will not be formed in aqueous solution, because it is a very strong acid. Based on the known acidity of NH3OH+, we predict pKa(NH2OH+)=-5.4 at the COSMO level, which is in good agreement with the experimental estimate of pKa(NH2OH+)=-7+/-2.

The Kinetics and Mechanism of the Ferrate(VI) Oxidation of Hydroxylamines

Aqueous solutions of potassium ferrate(VI) cleanly and rapidly oxidize hydroxylamine to nitrous oxide, N-methylhydroxylamine to nitrosomethane, N-phenylhydroxylamine to nitrosobenzene, and O-methylhydroxylamine to methanol and nitrogen. The kinetics show first-order behavior with respect to each reactant and a two term component representing acid dependent and independent pathways. A general mechanism involving intermediate formation coupled with a two-electron oxidation is proposed.

Kinetics and mechanism of hydroxylamine oxidation by [Fe2III(µ-O)(phen)4(H2O)2]4+ in aqueous media

Equilibrium studies show that in aqueous solutions containing excess 1,10-phenanthroline (phen) in the range pH 3-9, the complex ion [Fe2III ( μ-O)(phen)4(H2O)2]4+ (1) undergoes rapid but partial hydrolysis and coexists with [Fe2III ( μ-)(phen)3(H2O)4]4+ (1d),[Fe2 III ( μ-O)(phen)4(H2O)(OH)]3+ (2), and [Fe2III ( μ-O)(phen)4(OH)2]2+ (3). The solution oxidizes hydroxylamine quantitatively to N2O and is itself reduced to [Fe(phen)3]2+. The reactions in the range pH 3-6 are first-order in concentrations of complex and hydroxylamine but exhibits complex [H+] dependence, suggesting kinetic contributions from 1, 1d, and 2 but not from 3. Rapid formation of inner-sphere adducts between NH2OH and different {Fe2O}4+ species followed by rate-determining one-electron transfer to produce NHOH and {Fe2O}3+ is proposed. All subsequent steps are rapid. Ambient light does not affect kinetics and reaction products.Key words: kinetics, equilibrium, oxo bridge, iron (III), hydroxylamine.