Hydroxo Bridges Research Articles

Nitrous Oxide Decomposition on the Binuclear [FeII(μ-O)(μ-OH)FeII] Center in Fe-ZSM-5 Zeolite

The reaction mechanism for nitrous oxide (${N}_{2}O$) direct decomposition into molecular nitrogen and oxygen was studied on binuclear iron sites in Fe-ZSM-5 zeolite using the density functional theory (DFT). Starting from the hydroxylated bi-iron complex ${\left}^{+}$, a reductive dehydroxylation pathway was proposed to justify the formation of the active site ${\left }^{+}$. The latter contains two FeII ions linked via oxo and hydroxo bridges, ${{Z}^{-}\left }^{+}$, and for the first time was considered to catalyze the ${N}_{2}O$ ecomposition. The DFT results show the activity of ${\left }^{+}$ complex for the ${N}_{2}O$ decomposition. The first step of the catalytic reaction corresponds to a spontaneous adsorption of ${N}_{2}O$ over FeII sites, followed by the surface atomic oxygen loading and the release of molecular nitrogen. The formation of molecular ${O}_{2}$ occurs through the migration of the atomic oxygen from one iron site to another one followed by the recombination of two oxygen atoms and the desorption of molecular oxygen. The computed reactivity over the binuclear iron core complex ${\left }^{+}$ is consistent with experimental data reported in the literature. Although the dissociation steps of the ${N}_{2}O$ molecules, calculated with respect to adsorbed ${N}_{2}O$ intermediates, are highly energetic, the energy barrier associated with the atomic oxygen migration is the highest one. Up to 700 K, the oxygen migration step has the highest free energy barrier, suggesting that it is the te-limiting step of the overall kinetics. This result explains the absence of ${O}_{2}$ formation in experimental study of ${N}_{2}O$ decomposition at temperatures below 623 K.

Magnetic Interactions in Two Heterobridged Dinuclear Copper(II) Complexes: Orbital Complementarity or Countercomplementarity?

The mechanisms of magnetic exchange interactions in two heterobridged mu-hydroxyl-mu-X dicopper complexes A and B (X = azaindole for A and X = pyrazole for B) are investigated by the calculations based on density functional theory combined with the broken-symmetry approach (DFT-BS). It is found that although the coordination circumstances of the copper centers in the two complexes are very similar, the magnetic magnitudes and signs are diametrically opposed. By the theoretical analyses of magnetic orbital interaction and spin distribution, it is indicated that the difference between the magnetic properties of the two complexes is due to the distinction of orbital interaction of two bridge ligands. Namely, the weak ferromagnetic coupling for complex A arises from the orbital countercomplementarity of the hydroxo and azaindole bridges while the strong antiferromagnetic coupling for complex B arises from the orbital complementarity of the hydroxo and pyrazolato bridges.

A novel hydroxo-bridged cyclic tetranuclear copper(II) complex using the guanazole ligand: Synthesis, crystal structure and thermal analysis

A novel hydroxo-bridged cyclic tetranuclear copper(II) complex with the guanazole ligand (3,5-diamino-1,2,4-triazole = Hdatrz), [Cu 4(datrz) 4(μ 2-OH) 2(NO 3) (H 2O) 6] · (NO 3) · 5(H 2O) ( 1) has been synthesised and characterised by X-ray crystallography. In 1, the four copper(II) centres are linked by hydroxo bridges and the N1, N2-coordination of the deprotonated guanazole ligands. The core has considerably short Cu⋯Cu distances in the range of 3.3550(4)–3.3611(4) Å. Thermal analysis of 1 reveals that the cyclic framework constructed with hydroxo and guanazole ligands is stable up to 285 °C.

Di- and Triheteronuclear Cu−Gd and Cu−Gd−Cu Complexes with Dissymmetric Double Bridge

The study of the Cu-Gd interaction is simplified by the use of heteronuclear Cu-Gd complexes of low nuclearity, such as dinuclear Cu-Gd or trinuclear Cu-Gd-Cu complexes. In the large majority of cases published until now, this interaction presents the advantage of being ferromagnetic. Among the known examples, the complexes with the largest J values have been prepared with use of symmetric Schiff base compartmental ligands, so that the active cores of the molecules involve two phenoxo bridges between the copper and gadolinium ions. Keeping this remark in mind, we decided to synthesize simple complexes in which two different bridges link the copper and gadolinium ions. The structural determinations of a heterodinuclear Cu-Gd and a trinuclear Cu-Gd-Cu complex confirm the asymmetry of the central cores, involving phenoxo and hydroxo bridges. The magnetic studies evidence again the presence of ferromagnetic interactions. These results corroborate preponderance of the planarity of the Cu-O2-Gd core over the dissymmetry of the bridges.

Reporting a New Class of Divanadium(V) Compounds Connected by an Unsupported Hydroxo Bridge

Dinuclear oxovanadium(V) compounds [LV(V)O(mu-OH)OV (V)L](PF6) [H2L = N,N'-tert-ethylene bis(salicylideneimine) (H 2Salen) and its derivatives] ( 1- 3) have been obtained by aerial oxidation of V (IV)OL precursors in THF in the presence of added NH 4PF 6. The oxidized vanadium(V) probably extracts an OH (-) ligand from the residual moisture in the solvent and is retained as an unsupported hydroxo-bridge between the metal centers of these compounds as confirmed by single-crystal X-ray diffraction analyses. The molecules of 1- 3 have centrosymmetric structures with each vanadium center having a distorted octahedral geometry. The bridging OH (-) group is located trans to the terminal VO t bond. The latter exerts strong trans labilizing influence to set the participating vanadium centers apart by about 4.1 A. These separations are by far the largest (e.g., V...V#, 4.131 A in 1) among all binuclear compounds containing an unsupported hydroxo bridge reported to date. The complexes retain their identity also in solution as established by (1)H NMR spectroscopy. Electrochemically, the behaviors of 1-3 are quite interesting as studied by cyclic voltammetry in acetonitrile, each undergoing three (except 3) nearly reversible metal-based reductions, all in the positive potential range (e.g., at E (1/2) = 0.57, 0.39, and 0.04 V versus Ag/AgCl reference for 1) as indicated by steady state voltammetry. The electrode process at 0.39 V appears to involve a single-step two-electron transfer as revealed from the normal and differential pulse voltammetric data and probably includes a combination of V(V)-V(IV) <--> V(III)-V(IV) mixed oxidation states. Compounds 1-3 thus provide a unique example of divanadium compounds in which the metal centers are linked by an unsupported hydroxo-bridge.

Cyclic tetranuclear half-sandwich ruthenium(II) complexes with 4,7-phenanthroline and hydroxo bridges: Crystal structure, solution behaviour and binding to nucleosides

The reaction between [(eta(6)-p-cymene)Ru(H2O)3]X2 and 4,7-phenanthroline (phen) leads to the formation of the rectangular tetranuclear complexes [(eta(6)-p-cymene)4Ru4(mu-4,7-phen-N4,N7)(2)(mu-OH)4]X4 (X=NO3, 1a; SO3CF3, 1b) which have been structurally characterised by X-ray crystallography. 1H NMR spectroscopic studies suggest the presence of a partially dissociated dinuclear species of type [(eta6-p-cymene)2Ru2(mu-4,7-phen-N4,N7)(solv)4]4+ in equilibrium with the tetranuclear cyclic species found in the solid state. The temperature effect for this equilibrium was studied by variable temperature 1H NMR experiments in D2O and MeOD. The results reveal that the proportion of the tetranuclear species increases with the polarity of the solvent which favour stacking interactions between the phenanthroline moieties. In addition, the reactivity of the tetranuclear species towards the nucleosides guanosine (Guo), cytidine (Cyt), 2'-deoxythymidine (Thy) and 2'-deoxyadenosine (dAdo) has been monitored by (1)H NMR as a potential model for the interaction of the 1 species with the probable DNA target. The results reveal that the 1 systems are able to bind the nucleobases endocyclic nitrogen atoms of Guo Cyt, and dAdo.

H4(4,4′-Bipy)3][Sc(OH)(H2O)5]2Cl8: Synthesis, structure, and solid-phase thermal transformation

The reaction of [Sc(OH)(H2O)5]2Cl4 · 2H2O in isopropanol with 4,4′-Bipy in CHCl3 produced a crystalline compound, which was identified as [H4(4,4′-Bipy)3][Sc(OH)(H2O)5]2Cl8 (I) by elemental analysis, IR spectra, and single-crystal X-ray diffraction. In the structure of compound I, the three protonated diimine molecules form a centrosymmetric trimer via N...N hydrogen bonds. The polyhedron around the Sc atom is an octahedron with one split vertex. The excursion of the Sc atom from the plane formed by the oxygen atoms (water molecules) toward the hydroxo bridges is 0.5 A. The thermolysis of compound I generates ScCl3, whereas the final decomposition product of the precursor dimer is ScOCl.

Mild catalytic oxidation of secondary and tertiary amines to nitrones and N-oxides with H2O2 mediated by Pt(ii) catalysts

Bridging hydroxo complexes of Pt(II) can be used as catalysts for the oxidation of tertiary and secondary amines under mild conditions using the environmentally benign hydrogen peroxide as the oxidant. The reaction proceeds with medium to excellent yields particularly in the case of the more electron rich tertiary amines.

Novel gem‐Dithiolato‐Bridged Rhodium Hydroformylation Catalysts: Bridging the Gap in Dinuclear Rhodium Thiolate Chemistry

AbstractThe direct protonation of the bridging hydroxo ligands in [Rh(μ‐OH)(cod)]2 by 1,1‐dimercaptocyclohexane [Chxn(SH)2] yields the gem‐dithiolato‐bridged compound [Rh2(μ‐S2Chxn)(cod)2] (1). The dinuclear framework in 1 is supported by a 1,1‐cyclohexanedithiolato ligand exhibiting a 1:2κ2S,1:2κ2S′ coordination mode. Compound 1 is an active catalyst precursor in the presence of P‐donor ligands for the hydroformylation of oct‐1‐ene under mild conditions of pressure and temperature (100 PSI, 353 K). The best results were obtained with phosphite ligands as modifying ligands. An aldehyde selectivity of 97 %, a regioselectivity towards the linear aldehyde of 81 %, and turnover frequencies of up to 198 h–1 were obtained with the catalytic system 1/P(OMe)3. The dinuclear compound [Rh2(μ‐S2Chxn)(CO)2(PPh3)2] (2) was isolated from the catalytic solutions resulting from the system 1/PPh3 and was characterized by spectroscopic means and by X‐ray diffraction to be the trans isomer. The mixed‐ligand dinuclear complexes 2 and [Rh2(μ‐S2Chxn)(CO)2(PCy3)2] (3) (Cy = cyclohexyl) were independently prepared by reaction of Chxn(SH)2 with the mononuclear complexes [Rh(acac)(CO)(PR3)] in the appropriate molar ratio.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

Acetonitrile Hydration and Ethyl Acetate Hydrolysis by Pyrazolate-Bridged Cobalt(II) Dimers Containing Hydrogen-Bond Donors

The preparation of new CoII-mu-OH-CoII dimers with the binucleating ligands 3,5-bis{bis[(N'-R-ureaylato)-N-ethyl]aminomethyl}-1H-pyrazolate ([H4PRbuam]5-, R=tBu, iPr) is described. The molecular structure of the isopropyl derivative reveals that each CoII center has a trigonal-bipyramidial coordination geometry, with a Co...Co separation of 3.5857(5) A. Structural and spectroscopic studies show that there are four hydrogen-bond (H-bond) donors near the CoII-micro-OH-CoII moiety; however, they are too far away to be form intramolecular H-bonds with the bridging hydroxo ligand. Treating [CoII2H4PRbuam(micro-OH)]2- with acetonitrile led to the formation of bridging acetamidato complexes, [CoII2H4PRbuam(micro-1,3-OC(NH)CH3)]2-; in addition, these CoII-micro-OH-CoII dimers hydrolyze ethyl acetate to form CoII complexes with bridging acetato ligands. The CoII-1,3-micro-X'-CoII complexes (X'=OAc-, [OC(NH)CH3]-) were prepared independently by reacting [CoII2H3PRbuam]2- with acetamide or [CoII2H4PRbuam]- with acetate. X-ray diffraction studies show that the orientation of the acetate ligand within the H-bonding cavity depends on the size of the R substituent appended from the urea groups. The tetradentate ligand 3-{bis[(N'-tert-butylureaylato)-N-ethyl]aminomethyl}-5-tert-butyl-1H-pyrazolato ([H2PtBuuam]3-) was also developed and its CoII-OH complex prepared. In the crystalline state, [CoIIH2PtBuuam(OH)]2- contains two intramolecular H-bonds between the urea groups of [H2PtBuuam]3- and the terminal hydroxo ligand. [nPr4N]2[CoIIH2PtBuuam(OH)] does not hydrate acetonitrile or hydrolyze ethyl acetate. In contrast, K2[CoIIH2PtBuuam(OH)] does react with ethyl acetate to produce KOAc; this enhanced reactivity is attributed to the presence of the K+ ions, which can possibly interact with the CoII-OH unit and ester substrate to assist in hydrolysis. However, K2[CoIIH2PtBuuam(OH)] was still unable to hydrate acetonitrile.

Two Iron-Containing Tungstogermanates: [K(H2O)(β-Fe2GeW10O37(OH))(γ-GeW10O36)]12-and [{β-Fe2GeW10O37(OH)2}2]12-

Interaction of the dilacunary polyanion precursor [gamma-GeW(10)O(36)](8-) with Fe(3+) ions in aqueous buffer medium (pH 4.8) results in the formation of two dimeric tungstogermanates depending on the reactant ratios. When using an Fe3+ to [gamma-GeW(10)O(36)](8-) ratio of 1:1, the asymmetric anion [K(H(2)O)(beta-Fe(2)GeW(10)O(37)(OH))(gamma-GeW(10)O(36))](12-) (1) is formed, whereas [{beta-Fe(2)GeW(10)O(37)(OH)2}2]12- (2) is formed when using a ratio of 2:1. Single-crystal X-ray analyses were carried out on Cs(3)K(9)[K(H(2)O)(beta-Fe(2)GeW(10)O(37)(OH))(gamma-GeW(10)O(36))].19H(2)O (CsK-1), which crystallizes in the triclinic system, space group P1, a = 11.4547(2), b = 19.9181(5), c = 21.0781(6) A, alpha = 66.7977(12), beta = 89.1061(12), gamma = 84.4457(11) degrees, and Z = 2 and on Cs(7)K(4)Na[{beta-Fe(2)GeW(10)O(37)(OH)(2)}(2)].39H(2)O (CsKNa-2), which crystallizes in the monoclinic system, space group C2/m, a = 32.7569(13), b = 12.2631(5), c = 14.2895(5) A, beta = 104.135(2) degrees , and Z = 2. Polyanion 1 consists of (beta-Fe(2)GeW(10)O(37)) and (gamma-GeW(10)O(36)) units linked via two Fe-O-W bridges and a central potassium ion. Two equivalent FeO(6) octahedra complete the belt of the beta-Keggin unit and link to the (gamma-GeW(10)O(36)) fragment. On the other hand, 2 consists of two {beta-Fe(2)GeW(10)O(37)(OH)(2)} units with four bridging hydroxo groups linking the four Fe(3+) ions, forming an eight-membered ring. The magnetic properties of CsK-1 and CsKNa-2 have been studied by magnetic susceptibility and magnetization measurements and fitted according to an isotropic exchange model. Both polyanions 1 and 2 exhibit diamagnetic ground states with a small amount of paramagnetic impurity. Electrochemistry studies on 1 and 2 were carried out in a pH 5 acetate medium. The two polyanions have in common the simultaneous reduction of all of their Fe(3+) centers. This observation suggests the existence of identical or almost-identical influences on these centers and their equivalence, especially in the reduced state. Controlled potential coulometry results indicate the presence of two Fe(3+) centers in 1 and four in 2. The splitting of the tungsten wave of 1 compared to the single tungsten wave of 2 is attributed to a difference in acid-base properties of the polyanions. Voltammetric peak-potential shifts as a function of pH were studied in the case of 2.

Palladium(II) complexes of 1,10-phenanthroline: Synthesis and X-ray crystal structure determination

Two palladium(II) complexes, [Pd(phen)(N CCH 3) 2][O 3SCF 3] 2 ( 1) and [Pd(phen)(μ-OH)] 2[O 3SCF 3] 2 · 2H 2O ( 2) (where phen = 1,10-phenanthroline), have been crystallized following the reaction of Pd(phen)Cl 2 with silver triflate, Ag(O 3SCF 3), in acetonitrile and water, respectively. The structures of both complexes are based on a Pd(phen) 2+ metal core, with two acetonitrile molecules binding in a monodentate fashion in complex 1 and two hydroxo bridges holding together two cores to form a dimer in complex 2. Additionally, both complexes present a hydrogen bonded 3-D network involving the triflate anions in 1, and water and triflate anions in 2. Both complexes have been characterized by infrared and 1H NMR spectroscopy and their crystal structures determined by X-ray crystallography.

Stereoselective Cyclometalation of Planar Pro-chiral (η6-Arene)tricarbonylchromium Complexes withOC-6-[Ru(CO)2Cl2]n

Ortho-metalation of ligand-appended and planar pro-chiral (η6-arene)tricarbonylchromium complexes has been attempted with octahedral [Ru(CO)2Cl2]n. While amino- and oxazolyl-appended substrates displayed no reactivity, 2-[tricarbonyl(η6-phenyl)chromium]pyridine was readily converted into the corresponding cycloruthenated product consisting of a mixture of chloro-bridged heterotetranuclear dimers, which was analyzed by CPMAS 13C NMR and 2D-DOSY 1H NMR techniques in the solid state and in solution, respectively. The main side-products of this cyclometalation reaction were identified as novel dimers containing a chloro and a hydroxo bridge, according to spectroscopic and X-ray diffraction structural determination. The relative configuration of the produced tetranuclear chloro-bridged (Cr,Ru) dimers was assessed by chemical derivatization into (Cr,Ru) monomers, of which the structures were established by X-ray diffraction analysis. Consistent results as to the stereoselectivity of the cycloruthenation reaction were obtained with 2-ferrocenylpyridine. Further reactivity studies of racemic planar-chiral (Cr,Ru) and (Fe,Ru) dimeric ruthenacycles toward anionic bidentate ligands indicated that chelation of the Ru center was rather stereoselective although probably subject to polytopal rearrangements. It was established that the chelation of the ruthenium(II) center by anionic homobidentate ligands was sensitive to steric hindrance, which favors the coordination of the anionic bidentate ligand at the Ru center in an anti fashion with respect to both Cr(CO)3 and FeCp. This property was applied to the synthesis of the first examples of scalemic ruthenacycles possessing ruthenium-centered and planar chiralities. This study is supported by 12 X-ray diffraction structures of relevant new complexes, among which are two unprecedented examples of chloro,hydroxo-bridged dicarbonylruthenium(II) [C,N] chelates.

Characterization of the active site of catalytically inactive forms of [NiFe] hydrogenases by density functional theory

The inactive forms, unready (Ni-A, Ni-SU) and ready (Ni-B), of NiFe hydrogenases are modeled by examining the possibility of hydroxo, oxo, hydroperoxo, peroxo, and sulfenate groups in active-site models and comparing predicted IR frequencies and g tensors with those of the enzyme. The best models for Ni-A and Ni-SU have hydroxo (mu-OH) bridges between Fe and Ni and a terminal sulfenate [Ni-S(=O)Cys] group, although a hydroperoxo model for Ni-A is also quite viable, whereas the best model for Ni-B has only a mu-OH bridge. In addition, a mechanism for the activation of unready hydrogenase is proposed on the basis of the relative stabilities of sulfenate models versus peroxide models.

Sc(III) porphyrins. The molecular structure of two Sc(III) porphyrins and a re-evaluation of the parameters for the molecular mechanics modelling of Sc(III) porphyrins

The crystal structures of two new Sc(III) porphyrins, [Sc(TPP)Cl]·2.5(1-chloronaphthalene), (5,10,15,20-tetraphenylporphyrin)-chloro-scandium(III)·2.5(1-chloronaphthalene) solvate, (Mo Kα, 0.71073 Å, triclinic system P 1 ¯ , a = 9.9530(2) Å, b = 15.4040(3) Å, c = 17.7770(3) Å, α = 86.5190(10)°, β = 89.7680(10)°, γ = 86.9720(10)°, 13101 independent reflections, R 1 = 0.0712) and the dimeric [μ 2-(OH) 2(Sc(TPP)) 2], bis-(μ-hydroxo)-(5,10,15,20-tetraphenylporphyrin) scandium(III) (Mo Kα, 0.71073 Å, monoclinic system C2, a = 24.2555(16) Å, b = 11.1598(7) Å, c = 25.6468(17) Å, β = 91.980(2)°, 13084 independent reflections, R 1 = 0.0485) are reported. In [Sc(TPP)Cl] the metal is five-coordinate and the porphyrin is domed with the metal displaced by 0.63 Å from the mean porphyrin towards the axial Cl − ligand. The average Sc–N bond length is 2.143(3) Å, which is shorter than the average bond length of previously reported structures. Two of the phenyl rings are nearly orthogonal to the porphyrin core and the other two are significantly tilted because of contacts with 1-chloronaphthalene solvent molecules, and the phenyl rings of neighbouring porphyrins. In [μ 2-(OH) 2(Sc(TPP)) 2] both porphyrins are domed, with the metal displaced from the mean porphyrin plane towards the bridging hydroxo ligands. The average Sc–N bond length is 2.197(12) Å, which is in the upper range of Sc–N bond lengths in known Sc(III) porphyrins but not dissimilar to the average Sc–N bond lengths in another other bis-μ 2-hydroxo Sc(III) porphyrin, [μ 2-(OH) 2(Sc(OEP)) 2]. One porphyrin is rotated relative to the upper porphyrin by 25° due to steric contacts between the phenyl substituents. We have used these new structures to re-evaluated our previously reported molecular mechanics force field parameters for modelling Sc(III) porphyrins using the MM2 force field; the training set was augmented from two to seven structures by using all available Sc(III) porphyrin structures and the two new structures. The modelling reproduces the porphyrin core very accurately; bond lengths are reproduced to within 0.01 Å, bond angles to within 0.5° and torsional angles to within 2°. The optimum parameters for modelling the Sc(III)–N bond lengths, determined by finding the minimum difference between the crystallographic and modelling mean bond lengths with the aid of artificial neural network architectures, were found to be 0.90 ± 0.03 mdyn Å −1 for the bond force constant and2.005 ± 0.005 Å for the strain-free bond length. Modelling the seven Sc(III) porphyrins with the new parameters gives an average Sc–N bond length of 2.182 ± 0.018 Å, indistinguishable from the crystallographic mean of 2.181 ± 0.024 Å.

Structural and magnetic studies on mono- and polynuclear chromium ascorbate complexes

Polynuclear chromium ascorbate complexes were isolated and physicochemically analyzed in a comparative manner with their mononuclear analog (1). Characterization by elemental analysis, electronic, vibrational, 13C-n.m.r and mass spectroscopies, and variable temperature magnetic susceptibility studies, allowed structural proposals for the binuclear, [Cr2(μ-OH)2(H2O)(C6H7O6)3(OH)] · 4H2O (2), and trinuclear, [Cr3(μ-O)3(H2O)6(C6H7O6)3] · 4 H2O (3) complexes. The pseudo-octahedral Cr III centers were suggested to be connected through hydroxo bridges in (2) and in (3) by oxo bridges forming a hexocyclic ring.

A new dinuclear heme-copper complex derived from functionalized protoporphyrin IX

A new biomimetic model for the heterodinuclear heme/copper center of respiratory oxidases is described. It is derived from iron(III) protoporphyrin IX by covalent attachment of a Gly-L-His-OMe residue to one propionic acid substituent and an amino-bis(benzimidazole) residue to the other propionic acid substituent of the porphyrin ring, yielding the Fe(III) complex 1, and subsequent addition of a copper(II) or copper(I) ion, according to needs. The fully oxidized Fe(III)/Cu(II) complex, 2, binds azide more strongly than 1, and likely contains azide bound as a bridging ligand between Fe(III) and Cu(II). The two metal centers also cooperate in the reaction with hydrogen peroxide, as the peroxide adducts obtained at low temperature for 1 and 2 display different optical features. Support to this interpretation comes from the investigation of the peroxidase activity of the complexes, where the activation of hydrogen peroxide has been studied through the phenol coupling reaction of p-cresol. Here the presence of Cu(II) improves the catalytic performance of complex 2 with respect to 1 at acidic pH, where the positive charge of the Cu(II) ion is useful to promote O-O bond cleavage of the iron-bound hydroperoxide, but it depresses the activity at basic pH because it can stabilize an intramolecular hydroxo bridge between Fe(III) and Cu(II). The reactivity to dioxygen of the reduced complexes has been studied at low temperature starting from the carbonyl adducts of the Fe(II) complex, 3, and Fe(II)/Cu(I) complex, 4. Also in this case the adducts derived from the Fe(II) and Fe(II)/Cu(I) complexes, that we formulate as Fe(III)-superoxo and Fe(III)/Cu(II)-peroxo exhibit slightly different spectral properties, showing that the copper center participates in a weak interaction with the dioxygen moiety.

A Dimeric Fe(III)-Substituted α-Keggin Tungstogermanate: {[α-GeFe2W10O38(OH)]2}14−

The dimeric Fe(III)-substituted α-Keggin tungstogermanate {[α-GeFe2W10O38(OH)]2}14− (1) was synthesized by reaction of iron(III) with [A-α-GeW9O34]10− in neutral medium and characterized by elemental analysis, IR, UV-Vis, TG-DSC and electrochemistry. An X-ray single-crystal analysis was carried out on K14[α-GeFe2W10O38(OH)]2 ·34H2O, which crystallizes in the monoclinic system, space group P21/c, with a = 20.510(3) A, b = 15.565 (2) A, c = 17.998(2) A, β = 114.672(2)° and Z = 2. The polyanion 1 consists of two [α-GeFe2W10O38(OH)] Keggin moieties linked via two bridging hydroxo groups, leading to a planar Fe4O2(OH)2 cluster. The two half-units are related by an insertion center. The locating of the hydroxo groups is done by the bond valence sum calculations. The TG-DSC results show that the polyanion 1 is stable below 520 °C.

Redox Chemistry of a Dimanganese(II,III) Complex with an Unsymmetric Ligand: Water Binding, Deprotonation and Accumulative Light‐Induced Oxidation

AbstractA dinuclear manganese complex {[Mn2II,IIIL(μ‐OAc)2]ClO4} has been synthesised, where L is the dianion of 2‐{[bis(pyrid‐2‐ylmethyl)amino]methyl}‐6‐{[(3,5‐di‐tert‐butyl‐2‐hydroxybenzyl)(pyrid‐2‐ylmethyl)amino]methyl}‐4‐methylphenol, an unsymmetric binucleating ligand with two coordinating phenol groups. The two manganese ions, with a Mn–Mn distance of 3.498 Å, are bridged by the two bidentate acetate ligands and the 4‐methylphenolate group of the ligand, resulting in a N3O3 and N2O4 donor set of MnII and MnIII, respectively. Electrochemically [Mn2II,IIIL(μ‐OAc)2]+ is reduced to [Mn2II,IIL(μ‐OAc)2] at E1/2(1) = –0.53 V versusFc+/0 and oxidised to [Mn2III,IIIL(μ‐OAc)2]2+ at E1/2(2) = 0.38 V versus Fc+/0. All three redox states have been characterised by EPR, IR and UV/Vis spectroscopy. Subsequent oxidation of [Mn2II,IIIL(μ‐OAc)2]2+ [E1/2(3) = 0.75 V vs. Fc+/0] in dry acetonitrile results in an unstable primary product with a lifetime of about 100 ms. At high scan rates quasireversible voltammetric behaviour is found for all three electrode processes, with particularly slow electron transfer for the II,III/II,II [k°(1) = 0.002 cm s–1] and III,III/II,III [k°(2) = 0.005 cm s–1] couples, which can be rationalised in terms of major distortions of the MnIII centres. In aqueous media the bridging acetates are replaced by water‐derived ligands. Deprotonation of these stabilises higher valence states, and photo‐induced oxidation of the manganese complex results in a Mn2III,IVLcomplex with oxo or hydroxo bridging ligands, which isfurther oxidised to an EPR‐silent product. These results demonstrate that a larger number of metal‐centred oxidations can be compressed in a narrow potential range if build up of charge is avoided by charge‐compensating reactions. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Barium di-μ-hydroxo-bis[(nitrilotriacetato)chromium(III)] hexahydrate

The title compound, Ba[Cr2(C6H6NO6)2(OH)2]·6H2O, is contains two discrete Cr(nta)OH units (nta3− is nitrilo­tri­acetate) in the asymmetric unit, each generating a centrosymmetric dimer with two hydroxo bridges. The ten-coordinate Ba2+ cation provides a link between dimeric units. Each CrIII ion is in a distorted octa­hedral environment and coordinated by two bridging hydroxo O atoms, plus an N atom and three carboxyl­ate O atoms of the nta3− ligand. The important geometrical parameters are Cr—N = 2.0716 (19) and 2.0779 (19) Å, Cr—O = 1.9421 (17)–1.9900 (17) Å, and O—Cr—O = 162.85 (7)–176.90 (7)°.