Aqua Hydroxo Complexes Research Articles

Octahedral molybdenum iodide clusters with pyrazole or pyrazolate ligands.

Due to the combination of useful physicochemical properties (luminescence, X-ray contrast, etc.), octahedral molybdenum halide cluster complexes [Mo6X8L6]n have been the subject of active investigation during the last decades. The most common methods for synthesizing new compounds with organic ligands involve the use of silver salts of organic acids or the substitution of terminal methylate ligands. However, these methods often necessitate the use of special conditions, such as an inert atmosphere, absolute solvents, and silver salts, which can be costly. In contrast, aqua-hydroxo complexes formed by hydrolysis of many complexes are considered final unreactive products, despite the tendency for them to form. This work proposes a simple and affordable method for the preparation of hexaaqua and hexahydroxo iodide clusters of molybdenum from [Mo6I14]2- in a single step. Furthermore, the possibility of using such compounds as starting complexes for the synthesis of clusters with organic ligands such as pyrazole is discussed. The paper presents synthetic approaches, detailed characterization both in solid and in solution, and a study of the reactivity and luminescence properties of the obtained compounds.

A study of the adsorption of titanium dioxide and zinc oxide nanoparticles on polyethylene microplastics and their desorption in aquatic media

In this study, we investigated the interactions between titanium dioxide (nTiO2) and zinc oxide (nZnO) nanoparticles and polyethylene microplastics (MPs) with respect to their adsorption and subsequent desorption in aquatic media. Adsorption kinetic models revealed rapid adsorption of nZnO compared to nTiO2, while nTiO2 was adsorbed to a greater extent – four times more nTiO2 (67%) was adsorbed on MPs than nZnO (16%). The low adsorption of nZnO can be explained by the partial dissolution of zinc from nZnO in the form of Zn(II) and/or Zn(II) aqua-hydroxo complexes (e.g. [Zn(OH)]+, [Zn(OH)3]−, [Zn(OH)4]2−), which were not adsorbed on MPs. Adsorption isotherm models indicated that the adsorption process is controlled by physisorption for both nTiO2 and nZnO. The desorption of nTiO2 was low (up to 27%) and not pH dependent, and only nanoparticles were desorbed from the MPs surface. On the other hand, the desorption of nZnO was pH dependent; at a slightly acidic pH (pH = 6), 89% of the adsorbed zinc was desorbed from the MPs surface and the majority were in the form of nanoparticles; at a slightly alkaline pH (pH = 8.3), 72% of the zinc was desorbed, but the majority were in the soluble form of Zn(II) and/or Zn(II) aqua-hydroxo complexes. These results demonstrated the complexity and variability of interactions between MPs and metal engineered nanoparticles and contribute to a better understanding of their fate in the aquatic environment.

Optical property trends in a family of {Mo6I8} aquahydroxo complexes.

Luminescence is one of the key properties of octahedral molybdenum cluster complexes and the basis for most areas of their possible practical applications. Nevertheless, the factors affecting the optical properties of the clusters are insufficiently studied and establishing them will allow us to tune both absorption and emission more precisely. In this work, we obtained two new cationic [{Mo6I8}(H2O)4(OH)2](An)2·nH2O (An = NO3-, n = 3; An = OTs-, n = 2, OTs- - p-toluenesulfonate), and two neutral [{Mo6I8}(H2O)2(OH)4]·nH2O (n = 2, 12) aquahydroxo complexes. Due to the similar compositions of the clusters obtained, we determined the influence of crystal packing and ligand environment on the absorption and photo- and radioluminescence properties. Thus, the four-component nature of the cluster emission was established using Gaussian deconvolution of the photoluminescence spectra. It was shown that the influence of both ligand type and crystal density decreases when moving to the red (lower-energy) part of the spectra, with only the first two components located in the blue (higher-energy) part of the spectra being strongly affected. Also, it was found that protonation of two hydroxo ligands leads to a significant decrease in absorption in the visible spectral region.

Synthesis of Al2O3-ZrO2-MxOy (M = Mg, La) complex oxide systems using electrogenerated reagents and their physicochemical properties

A method for the synthesis of metal oxide precursors based on the electrogeneration of reagents in a coaxial electrochemical reactor without a membrane with electrodes substantially different in the surface area was proposed. The electrogeneration is accompanied by the subsequent hydrolysis of the products, the formation of primary particles due to the polycondensation of aquahydroxo complexes, and their further enlargement. In the coaxial electrolyzer-reactor, the flow of gas and electrolyte bubbles moves up and to the sides from the central electrode to form a vortex in the confined space. This makes it possible to distribute the components of the reaction mixture and to suspend more uniformly the formed particles of the solid phase in the liquid bulk due to the appeared turbulence. The introduction of Zr4+, Mg2+, and La3+ ions favors the formation of stable positively charged particles. The average particle size of alumina modified with oxides of other metals (Zr, Mg, and La) obtained using electrogenerated reagents was found to be 20-50 nm. Under thermal treatment of the synthesized systems (1100°C), complex phase transformations lead to the formation of spinel Al2MgO4 and stabilization of α-Al2O3 and tetragonal t-ZrO2 stable at high temperatures up to 1600°C

A solid-phase extraction method using acid-modified Transcarpathian clinoptilolite for preconcentration of trace amounts of lead in water samples

The sorption properties of acid-modified Ukrainian Transcarpathian clinoptilolite towards lead aqueous solutions under dynamic conditions have been investigated. The obtained results show that this nanomaterial most effectively sorbs cationic aqua hydroxo complexes of Pb(II) ions. The optimal conditions for the sorption of Pb(II) on the acid-modified Transcarpathian clinoptilolite are as follows: temperature of H-clinoptilolite thermal pre-treatment—600 °C; sorbent grains diameter—0.20–0.31 mm; pH—8.5; flow rate of Pb(II) solution with the concentration of 0.5 µg mL−1 through the sorbent—5 mL min−1. The maximal sorption capacity value of the acid-modified Transcarpathian clinoptilolite towards Pb(II) is equal to 48.79 mg g−1, that is 4.5 times higher than the respective sorption capacity value of the natural form of this zeolite. The best desorbent of Pb(II) from acid-modified clinoptilolite is 1 M KCl (acidified with hydrochloric acid to pH 4.0). This desorbent removes 97% of sorbed Pb(II). The preconcentration method of Pb(II) nanogram amounts in the solid-phase extraction mode during the atomic absorption analysis of natural waters has been proposed. The linearity of the proposed method was evaluated in the range of 0.08–16 ng mL−1 with the detection limit of 0.025 ng mL−1. Due to acceptable recoveries (≥ 96%) and relative standard deviation (RSD) values (1.6–6.6%) from a spiked tap water.

Sorption of terbium on Transcarpathian clinoptilolite

The sorption properties of Ukrainian Transcarpathian clinoptilolite towards aqueous terbium solutions under dynamic conditions have been investigated. The obtained results show that Transcarpathian clinoptilolite most effectively sorbs cationic aqua hydroxo complexes of Tb(III) ions. In addition, it has been found that clinoptilolite samples thermally treated at 350°C possess the most effective specific surface. Since the content of ion exchange complex of natural clinoptilolite and clinoptilolite thermally treated at 350°C has been established, it was shown that Na+ cations play a dominant role in an ion exchange process during sorption of Tb(III) ions. The investigation has demonstrated that lanthanide sorption on the Transcarpathian clinoptilolite is caused by the processes of ion exchange and adsorption on the zeolite surface. As a result, the dynamic sorption capacity of clinoptilolite under optimum conditions (zeolite calcined at 350°C for 2.5h; 0.20–0.31mm size of sorbent grains; 3mL/min flow rate of the 1.0μg/mL Tb(III) ions solution through sorbent; pH 8.25) was 6.1mg of terbium per 1g of clinoptilolite. Moreover, the conditions of terbium desorption from clinoptilolite have been studied. Solutions of HNO3 (7.0M) and NaCl (1M) acidified by solution of HCl to pH 2.5 were used, providing 95–100% desorption of terbium concentrated on zeolite.

Water Exchange on Manganese(III) Porphyrins. Mechanistic Insights Relevant for Oxygen Evolving Complex and Superoxide Dismutation Catalysis

In this work the rate constants (k(ex)) and the activation parameters (DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger)) for the water exchange process on Mn(III) centers have experimentally been determined using temperature and pressure dependent (17)O NMR techniques. For the investigations the Mn(III) porphyrin complexes [Mn(III)(TPPS)S(2)](n-) and [Mn(III)(TMpyP)S(2)](n+) (S = H(2)O and/or OH(-)) have been selected due to their high solution stability in a wide pH range, enabling the measurements of water exchange in the case of both diaqua and aqua-hydroxo complexes. We have experimentally demonstrated that the water exchange on Mn(III) porphyrins is a fast process (k(ex) approximately = 10(7) s(-1)) of an I(d) to I mechanism, strongly influenced by a Jahn-Teller effect and as such almost independent of a porphyrin charge and a trans ligand. This is also supported by our DFT calculations which show only a slight difference in an average Mn(III)-OH(2) bond found for a positively charged model porphyrin with protonated pyridine groups (2.446 A) and for a simple model without any substituents on the porphyrin ring (2.437 A). The calculated effective charge on the Mn center, which is significantly lower than its formal +3 charge (ca. +1.5 for diaqua; +1.4 for aqua-hydroxo), also contributes to its substitution lability. The herein presented results are discussed in connection to a possible fast exchanging substrate binding site in photosystem II and corresponding inorganic model complexes, as well as in the context of a possible inner-sphere catalytic pathway for superoxide dismutation on Mn centers.

Tetranuclear Lanthanide Aqua Hydroxo Complexes with Macrocyclic Ligand Cucurbit[6]uril

AbstractTetranuclear lanthanide aqua hydroxo complexes with two structural types of core – {Ln4(μ3‐OH)4(μ2‐OH)4}4+ [Ln = Pr (1), Nd (2), Eu (3), Gd (4), Tb (5)] and {Ln4(μ3‐OH)4(μ2‐OH)2}6+ [Ln = Er (6), Yb (7)] – were prepared by heating (130 °C) of aqueous solutions of lanthanide(III) chlorides, cucurbit[6]uril (C36H36N24O12, CB[6]), and 4‐cyanopyridine. The formation of Ln4 complexes results from the combined influence of the tetradentate coordination of the polycation by the portals of the macrocyclic ligand CB[6] and the chelating effect of the carboxylate ligands of isonicotinic acid generated as a result of hydrolysis of 4‐cyanopyridine. X‐ray diffraction analysis of 1–7 revealed the sandwich structure with the tetranuclear lanthanide complex situated between two macrocyclic molecules. The compounds were characterized by elemental analysis, IR, and electrospray ionization (ESI) mass spectrometry. Luminescence spectra of 3 and 5 and magnetic properties of 4 were also studied.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Supramolecular approach to crystallization of polynuclear chromium(III) aqua hydroxo complexes: synthesis and crystal structures of complexes [Cr2(OH)2(H2O)8]4+ and [Cr4(OH)6(H2O)12]6+ with cucurbit[n]uril (n = 7, 8)

Supramolecular compounds of the compositions {[Cr2(OH)2(H2O)8](C42H42N28O14)2}-(NO3)4·18.75H2O (1) and {[Cr4(OH)6(H2O)12](C48H48N32O16)3(NO3)6·55H2O (2) were synthesized from aqueous solutions of chromium(III) nitrate and the macrocyclic cavitand cucurbit[n]uril (C6n H6n N4n O2n , where n = 7 or 8, respectively). According to the X-ray diffraction study, the polynuclear chromium aqua complexes are disposed in cavities formed by the cucurbit[n]uril molecules and are linked to these molecules through hydrogen bonds between the hydroxo and aqua ligands of the polycations and the portal oxygen atoms of the macrocycles. Compound 1 is the first example of supramolecular compounds of cucurbit[7]uril with metal aqua complexes. The isolation of the supramolecular adduct with cucurbit[8]uril 2 in the single-crystalline state allows the determination of the structure of the tetranuclear chromium aqua complex having an adamantane-like structure, [Cr4(μ2-OH)6(H2O)12]6+, which has been previously unknown in the solid state.

A Family of Octahedral Rhenium Cluster Complexes [Re6Q8(H2O)n(OH)6 - n]n - 4 (Q = S, Se; n = 0−6): Structural and pH−Dependent Spectroscopic Studies

The conversions of hexahydroxo rhenium cluster complexes [Re6Q8(OH)6]4- (Q=S, Se) in aqueous solutions in a wide pH range were investigated by chemical methods and spectroscopic measurements. Dependences of the spectroscopic and excited-state properties of the solutions on pH have been studied in detail. It has been found that a pH decrease of aqueous solutions of the potassium salts K4[Re6Q8(OH)6].8H2O (Q=S, Se) results in the formation of aquahydroxo and hexaaqua cluster complexes with the general formula [Re6Q8(H2O)n(OH)6-n]n-4 that could be considered as a result of the protonation of the terminal OH- ligands in the hexahydroxo complexes. The compounds K2[Re6S8(H2O)2(OH)4].2H2O (1), [Re6S8(H2O)4(OH)2].12H2O (2), [Re6S8(H2O)6][Re6S6Br8].10H2O (3), and [Re6Se8(H2O)4(OH)2] (4) have been isolated and characterized by X-ray single-crystal diffraction and elemental analyses and infrared (IR) spectroscopy. In crystal structures of the aquahydroxo complexes, the cluster units are connected to each other by an extensive system of very strong hydrogen bonds between terminal ligands.

The study of induced aquation of Rh(III) sulfate complexes

The processes of aquation of Rh(III) complexes in the presence of BaCl2 and Ba(ClO4)2 were studied by the 103RH and 17O NMR methods. In the first case, the final products were found to be the monomeric aqua chloride complexes, while in the second case, aqua hydroxo complexes were formed. The chloride ions present in the system significantly increase the process rate.

Synthesis and crystal structures of supramolecular compounds of polynuclear aluminum(III) aqua hydroxo complexes with cucurbit[6]uril

Supramolecular compounds of the di-, trideca-, and triacontanuclear aluminum aqua hydroxo complexes, viz., [Al2(OH)2(H2O)8]4+, [Al12(AlO4)(OH)24(H2O)12]7+, and [Al30O8(OH)56(H2O)26]18+, respectively, with the organic macrocyclic cavitand cucurbit[6]uril (C36H36N24O12) were prepared by evaporation of aqueous solutions of aluminum nitrate and cucurbit[6]uril after the addition of pyridine, ammonia, KOH, or NaOH at pH 3.1–3.8. X-ray diffraction study demonstrated that the aqua hydroxo complexes are linked to the macrocycle through hydrogen bonds between the hydroxo and aqua ligands of the polycations and the portal oxygen atoms of cucurbit[6]uril.

Heteronuclear iron(III)–chromium(III) hydroxo complexes and hydroxides, and their catalytic activity towards peroxidative oxidation of alkanes

The equilibrium compositions of the products of hydrolysis of Fe(III) and Cr(III) in their single or binary nitrate solutions, as a function of pH and neutralisation degree have been established and Fe(III)–Cr(III) hydroxides and products of their thermolysis have been obtained and characterized by potentiometric titration, thermal analysis, IR spectroscopy and X-ray powder diffraction, and an Fe(III)–Cr(III) interaction recognised in the binary systems. The obtained aquahydroxo complexes and metal hydroxides have been applied as catalysts for the homogeneous or heterogeneous, respectively, peroxidative oxidation of cyclohexane and cyclopentane to the corresponding ketones and alcohols, at room temperature. The effects of various parameters, such as pH, degree of neutralisation and reagents molar ratios, have been investigated and correlations between the catalytic activity and the form of the hydroxo complex were recognised for the homogeneous systems, whereas for the heterogeneous mixed (heteronuclear) hydroxides a synergic effect has been detected for the oxidation of cyclohexane, the catalytic activity (which can reach 30% yields and 30 turnover numbers (TONs)) being above those of the individual hydroxides.

Physical chemistry of the polynuclear hydrolysis of aluminium and two-charged ions of 3d-metals

The distribution diagrams of Al 3+ and M 2+(M 2+Co 2+, Ni 2+, Cu 2+, Zn 2+) aqua hydroxo complexes both in one-compound and binary systems as a function of pH and the neutralization degree have been obtained experimentally. The effective activation energies have been determined and the kinetic advantage of the heteronuclear complexation compared to the homopolynuclear one has been established. The thermodynamic characteristics have been obtained and their correlation with kinetic parameters and electronic structure of the investigated metal ions has been defined. It has been established that there exists a compensation effect in the processes under study. As the result, the scheme of the Al 3+M 2+ polynuclear hydroxo complexation has been proposed and the features of this process have been revealed.

A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution.

A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(H(2)O)(2)]ClO(4) (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK(a1) = 5.78 +/- 0.04 and pK(a2) = 9.45 +/- 0.06 at 25 degrees C). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 degrees C and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k(f(I)) = 2.6 +/- 0.1 M(-1) s(-1), DeltaH(#)(f(I)) = 62 +/- 3 kJ mol(-1), DeltaS(#)(f(I)) = -30 +/- 10 J K(-1) mol(-1), and DeltaV(#)(f(I)) = -2.5 +/- 0.2 cm(3) mol(-1)) suggest the operation of an I(a) mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 +/- 0.06 and 1172 +/- 22 M(-1) s(-1) at 25 degrees C, respectively. The more labile aqua-hydroxo complex is suggested to follow an I(d) or D substitution mechanism on the basis of the reported kinetic data. At pH > or =4.5, the second substitution step also can be monitored (pH 5.5 and 25 degrees C: k(f(II)) = 21.1 +/- 0.5 M(-1) s(-1), DeltaH(#)(f(II)) = 60 +/- 2 kJ mol(-1), DeltaS(#)(f(II)) = -19 +/- 6 J K(-1) mol(-1), and DeltaV(#)(f(II)) = +8.8 +/- 0.3 cm(3) mol(-1)), for which an I(d) or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.

Charge transfer of iron(III) monomeric and oligomeric aqua hydroxo complexes: semiempirical investigation into photoactivity.

Aqueous hydrolyses of iron(III) solutions were studied using electronic spectroscopy. Complete spectra from 200 to 800 nm were obtained for the four ferric aqua hydroxo complexes: Fe(H(2)O)(6)(3+), Fe(OH)(H(2)O)(5)(2+), Fe(OH)(2)(H(2)O)(4)(+), and the dimer Fe(2)(mu-Omicron Eta)(2)(H(2)O)(8)(4+). Semiempirical Zindo/s calculations were employed to assign which types of electronic transfers are involved so that the photoactivity as regards the photoreduction dissociation Fe(III)(aq) Fe(II)(aq) + OH* can be discussed. Fe(3+) exhibits two LMCT from non-bonding p orbitals (nLp) located at 190 and 240 nm. Fe(OH)(2+) shows two major nLp(OH) --> d transitions at 205 and 295 nm. As regards its geometry, computed investigations using an Fe-OH distance of 2.05 A better fit than using a shorter distance ( approximately 1.8 A); the same conclusion remains constant for all hydroxo complexes. The dihydroxo form's spectrum was confronted to its common cis and trans expectable isomers plus an unusual pentacoordinate one. Even if the trans isomer is supposed to be the lowest Gibbs free energy species in solution, there is some evidence of the presence of the cis form; hence, both species must be close in energy. Other isolated nLp(OH) --> d transfer wavelengths are 235, 245, and 335 nm. As for the dimer, this study provides some clue in favor of the bis(mu-hydroxo)) description. Both water and hydroxo ligands are involved along the electronic transitions toward only d(1) metal-centered orbitals at 220 and 260 nm for H(2)O, 335 and 470 nm for OH(-), and 205 nm for both. Charge transfers for the hydrogen oxide bridge form Fe(2)(mu-Eta(3)Omicron(2))(H(2)O)(8)(5+) were also computed. Finally predictions about the two bis(mu-hydroxo) bridge trimer Fe(3)(OH)(4)(H(2)O)(10)(5+) enable one to foresee a huge and broad charge transfer in the UV region (approximately 240 nm) followed by a multi nLp(OH) --> d(1) transfer extending up to approximately 650 nm.

Linear chain complexes linked by hydrogen bonded O2H3– and O3H5– units: structural and magnetic comparison of trans-dihydroxo-, -diaqua-, and -aquahydroxo-chromium(iii) complexes

The syntheses and characterization of a series of chromium(III) complexes of the general types trans-[(A)4Cr(OH)(H2O)]2+, trans-[(A)4Cr(OH)2]+, and trans-[(A)4Cr(H2O)2]3+ are described. The ligands (A)4 used include the tetradentate ligand 1,4,8,11-tetraazacyclotetradecane (cyclam), its hexamethylated derivative C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (cyca), and the bidentate ligand 2-aminomethylpyridine (pico). The cobalt(III) aquahydroxo complexes with cyclam and cyca are also reported. The trans-diaqua and trans-dihydroxo complexes are monomeric, but the trans-aquahydroxo complexes form linear chains. The complexes trans-[Cr(cyclam)(OH)(H2O)](CF3SO3)2 1 and trans- [Co(cyclam)(OH)(H2O)](CF3SO3)2 3 and their corresponding perchlorates (2 and 4, respectively) have been isolated and the structures of the chromium triflate salt 1 and the cobalt perchlorate salt 4 have been determined. Both of these complexes 1 and 4 form infinite chains in the solid state, adjacent metal centers being bridged by hydrogen bonds between hydroxo and aqua ligands. The Cr⋯Cr separation in 1 is 6.085 Å whereas the Co⋯Co separation in 4 is 6.078 Å. The complex trans-[(pico)2Cr(OH2)2](NO3)3·H2O 8 (where pico is picolylamine, 2-aminomethylpyridine) crystallizes with only one monomeric formula in a unit cell. In contrast to the structures of 1 and 4 (and, presumably, of 2 and 3 also), the two complexes trans-[Cr(cyclam)(OH)2]ClO4·H2O 5 and 8 are monomeric in the solid state. The chromium complex trans-[Cr(cyca)(OH)(H2O)](ClO4)2·H2O 6 crystallizes with two monomeric formula units in a unit cell and the cobalt analogue 7 is isomorphous with the chromium complex. The structures of these complexes are also linear chains, but in this case there is an intervening water molecule between the aqua and hydroxo ligands on adjacent chromium centers. EPR spectroscopy of the chromium complex 1 doped in the diamagnetic cobalt host 3 reveals the presence of weak magnetic interactions.

Kinetics and mechanism of the reaction of sulphur(IV) with N,N?-ethylene-bis(sali-cylidiniminato)manganese-(III) in aqueous solution

The reaction of (diaqua)(N,N′-ethylene-bis(salicylidiniminato)manganese(III) with aqueous sulphite buffer results in the formation of the corresponding mono sulphito complex, [Mn(Salen)(SO3)]− (S-bonded isomer) via three distinct paths: (i) Mn(Salen)(OH2)2+ + HSO3− → (k1); (ii) Mn(Salen)(OH2)2+ + SO32− → (k2); (III) Mn(Salen)(OH2)(OH) + SO32− → (k3) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO32−/HSO3− may be attributed to the strong trans-activating influence of the S-bonded sulphite. The values of the rate constants (10−2ki/dm2 mol−1 s−1 at 25°C, I = 0.3 mol dm−3), ΔHi#/kJ mol−1 and ΔSi#/J K−1 mol−1 respectively are: 2.97 ± 0.27, 42.4 ± 0.2, −55.3 ± 0.6 (i = 1); 11.0 ± 0.8, 33 ± 3, −75 ± 10 (i = 2); 20.6 ± 1.9, 32.4 ± 0.2, −72.9 ± 0.6 (i = 3). The trend in reactivity (k2 > k1), a small labilizing effect of the coordinated hydroxo group (k3/k2 < 2), and substantially low values of ΔS# suggest that the mechanism of aqua ligand substitution of the diaqua, and aqua-hydroxo complexes is most likely associative interchange (Ia). No evidence for the formation of the O-bonded sulphito complex and the ligand isomerization in the sulphito complex, (MnIII-OSO2 → MnIII-SO3), ensures the selectivity of the MnIII centre toward the S-end of the SIV species. The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce MnII, S2O62− and SO42−. The formation of dithionate is a consequence of the fast dimerization of the SO3−. generated in the rate determining step and also SO42− formation is attributed to the fast scavenging of the SO3−. by the MnIII species via a redox path. The internal reduction of the MnIII centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO3− and the other SO32−. The electron transfer is believed to be outersphere type. The substantially negative values of activation entropies (ΔS# = −(1.3 ± 0.2) × 102 and −(1.6 ± 0.2) × 102 J K−1 mol−1 for the paths involving HSO3− and SO32− respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 627–635, 1999

Iron−Manganese Redox Processes and Synergism in the Mechanism for Manganese-Catalyzed Autoxidation of Hydrogen Sulfite

The mechanism for manganese-catalyzed aqueous autoxidation of hydrogen sulfite at pH 2.4 has been revised on the basis of previous comprehensive kinetic studies and thermodynamic data for iron-manganese redox processes and manganese(II) and -(III) protolysis equilibria. The catalytically active manganese species is concluded to be an oxo- (or hydroxo-) bridged mixed-valence complex of composition (OH)Mn(III)OMn(II)(aq) with a formation constant beta' of (3 +/- 1) x 10(4) M(-)(1) from kinetics or ca. 7 x 10(4) M(-)(1) from thermodynamics. It is formed via rapid reaction between Mn(H(2)O)(6)(2+) and hydrolyzed manganese(III) aqua hydroxo complexes, and it initiates the chain reaction via formation of a precursor complex with HSO(3)(-), within which fast bridged electron transfer from S(IV) to Mn(III) takes place, resulting in formation of chain propagating sulfite radicals, SO(3)(*)(-). The very high acidity of Mn(3+)(aq), indicating a strong bond Mn(III)-OH(2) in hydrolyzed manganese(III), makes an attack by HSO(3)(-) on substitution labile Mn(II) in the bridged complex more favorable than one directly on manganese(III). The synergistic effect observed in systems containing iron as well as manganese and the chain initiation by trace concentrations of iron(III) of ca. 5 x 10(-)(8) M can also be rationalized in terms of formation of this bridged mixed-valence dimanganese(II,III) complex. The presence of iron(III) in a Mn(II)/HSO(3)(-) system results in rapid establishment of an iron-manganese redox equilibrium, increasing the concentration of manganese(III) and of the catalytically active bridged complex. The bridged complex oxidizes HSO(3)(-) several orders of magnitude faster than does iron(III) itself. Comparison with some previous studies shows that the different experimental rate laws reported do not necessarily indicate different reaction mechanisms. Instead, they can be rationalized in terms of different rate-determining steps within the same complex chain reaction mechanism, depending on the experimental conditions used.

Metal Ion Promoted Hydrolysis of Polyphosphates

Various CoIII amine aqua hydroxo complexes have been evaluated for their ability to hydrolyse pyrophosphate to phosphate rapidly. Two very effective reagents have emerged, [Co( trpn )(OH)(OH2)]2+ ( trpn = 3,3′,3″-nitrilotris(propan-1-amine)) and [Co( tamen )(OH)-OH2)]2+ ( tamen = 6-(4-amino-2-azabutyl)-6-methyl-1,4-diazacycloheptane), and the latter has been examined closely to assess details of the mechanistic path for hydrolysis. The study shows substitution of the reagent on the substrate is fast and not rate determining. Two moles of the reagent are required to bind to [(en)2Co(P2O7)]- (en = ethane-1,2-diamine) to effect rapid hydrolysis. One supplies the coordinated OH- nucleophile to cleave the phosphorus anhydride, and the other binds as a chelate to the leaving group to assist bond rupture. An unproductive side path, to form a symmetrical dichelate of the form [(en)2Co(P2O7)Co( tamen )]2+, becomes rate limiting. However, if this path can be reduced in significance or eliminated, the study implies that polyphosphates and phosphate esters should hydrolyse on the subsecond time scale.