Oxido Ligand Research Articles

Chemistry of a series of heterobimetallic complexes MnIII2(CaII/SrII)X2 (X = Cl-, Br-).

This manuscript describes the syntheses, structures and magnetism of MnIII-CaII/SrII complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions CaII or SrII were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the CaII/SrII ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes.

Stereospecific formation of vanadium mandelato complexes with [Fe(2,2′-bipyridine)3]2+ as a counter ion

Two vanadium complexes of mandelic acid having [Fe(bpy)3]2+ as counterion, [Fe(bpy)3][V2O4(rac-mand)2]·4.9H2O·0.1CH3CN (1, FeV2L2) and (H3O)[Fe(bpy)3]4[V3O7(S-mand)2]3·28H2O (2, FeV3L2) (bpy = 2,2’-bipyridine, mand2– = mandelato ligand, C8H6O32–) have been synthesized and characterized by single crystal X-ray diffraction and spectral methods. The FeSO4—bpy—KVO3—H2mand—H2O—CH3CN system exhibits a stereospecific behaviour: while from the system including racemic mandelic acid only the complex of the V2L2 type (1) could be obtained in crystalline form, the system with S-mandelic acid afforded the V3L2 (2) complex as the single crystalline product. All vanadium atoms exhibit tetragonal pyramidal coordination geometry with oxygen donor atoms of the oxido ligands and carboxylate anion. The stereospecific behaviour was investigated using the 51 V NMR spectroscopy, which revealed different composition of systems with racemic mandelic acid and S-mandelic acid after some preliminary period (≈ 15 days). The compound 2 is chiral non-racemic compound (space group P21212), the structure of which contains Δ-[Fe(bpy)3]2+ cations and [V3O7(S-mand)2]3– anions.Graphical abstract

Stabilization of 2-Pyridyltellurium(II) Derivatives by Oxidorhenium(V) Complexes

Zwitterionic compounds such as pyridine-containing tellurenyl compounds are interesting building blocks for heterometallic assemblies. They can act as ambiphilic donor/acceptors as is shown by the products of reactions of the zwitterions HpyTeCl2 or HCF3pyTeCl2 with the rhenium(V) complex [ReOCl3(PPh3)2]. The products have a composition of [ReO2Cl(pyTeCl)(PPh3)2] and [ReO2Cl(CF3pyTeCl)(PPh3)2] with central {O=Re=O…Te(Cl)p⏠y}+ units. The Re-O bonds in the products are elongated by approximately 0.1 Å compared with those to the terminal oxido ligands and establish Te…O contacts. Thus, the normally easily assigned concept of oxidation states established at the two metal ions becomes questionable (ReV/TeII vs. ReIII/TeIV). A simple bond length consideration rather leads to a description with the coordination of a mesityltellurenyl(II) chloride unit to an oxido ligand of the Re(V) center, but the oxidation of the tellurium ion and the formation of a tellurinic acid chloride cannot be ruled out completely from an analysis of the solid-state structures. DFT calculations (QTAIM, NBO analysis) give clear support for the formation of a Re(V) dioxide complex donating into an organotellurium(II) chloride and the alternative description can at most be regarded as a less favored resonance structure.

Reactions of a Dioxidomolybdenum(VI) Complex with Thionation Reagents-Formation of Mo(IV) Species with Sulfur Donors.

Molecular molybdenum complexes with sulfur donor ligands are generally studied as soluble model compounds for molybdenum enzymes essential for life. The dioxidomolybdenum(VI) complex with tetradentate aminobisphenolate ligand undergoes a reaction with thionation reagent P2S5 or its organic derivative, Lawesson's reagent, to yield stable Mo(IV) aminobisphenolate complexes, where pristine oxido ligands have been replaced by bidentate sulfur donors tetrasulfide, S42- or (4-methoxyphenyl)phosphonotrithioate residue derived from Lawesson's reagent. This is in contrast to the behaviour of analogous dioxidotungsten(VI) complex, which, under similar conditions, yields W(VI) S2 systems. The overall cis,trans,cis geometry of the parent dioxidomolybdenum(VI) aminobisphenolate is retained, namely, the neutral nitrogen donors are in cis positions, phenolate oxygens are trans to each other and sulfur donors are cis. Although formally Mo(IV), thus d2 system, the studied complexes have diamagnetic singlet electron configurations as a result of the axially compressed octahedral structures.

μ-Oxido-bis-[(5,10,15,20-tetra-phenyl-porphyrinato-κ4 N,N',N'',N''')manganese(III)

In the crystal structure of the title oxido-bridged binuclear complex, [MnIII(TPP)]2O (TPP = tetra-phenyl-porphyrinate, C44H28N4) or [Mn2(C44H28N4)2O], the two penta-coordinate manganese(III) ions are bridged by a single oxido ligand, with an Mn-O distance of 1.7600 (3) Å and an Mn-O-Mn bridging angle of 176.1 (2)°. The bridging O2- ligand is located on a twofold rotation axis, resulting in point group 2 for the entire complex. The MnIII atom is displaced out of the 24-atom mean plane of the porphyrine entity by 0.52 Å. C-H⋯π and π-π inter-actions help to stabilize the mol-ecular packing within the crystal structure.

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer.

The uptake of hydrogen atoms (H-atoms) into reducible metal oxides has implications in catalysis and energy storage. However, outside of computational modeling, it is difficult to obtain insight into the physicochemical factors that govern H-atom uptake at the atomic level. Here, we describe oxygen-atom vacancy formation in a series of hexavanadate assemblies via proton-coupled electron transfer, presenting a novel pathway for the formation of defect sites at the surface of redox-active metal oxides. Kinetic investigations reveal that H-atom transfer to the metal oxide surface occurs through concerted proton–electron transfer, resulting in the formation of a transient VIII–OH2 moiety that, upon displacement of the water ligand with an acetonitrile molecule, forms the oxygen-deficient polyoxovanadate-alkoxide cluster. Oxidation state distribution of the cluster core dictates the affinity of surface oxido ligands for H-atoms, mirroring the behavior of reducible metal oxide nanocrystals. Ultimately, atomistic insights from this work provide new design criteria for predictive proton-coupled electron-transfer reactivity of terminal M=O moieties at the surface of nanoscopic metal oxides.

Bioinspired Di-Fe Complexes: Correlating Structure and Proton Transfer over Four Oxidation States.

Metalloproteins with active sites containing di-Fe cores exhibit diverse chemical reactivity that is linked to the precise transfer of protons and electrons which directly involve the di-Fe units. The redox conversions are commonly corroborated by spectroscopic methods, but the associated structural changes are often difficult to assess, particularly those related to proton movements. This report describes the development of di-Fe complexes in which the movements of protons and electrons are pinpointed during the stepwise oxidation of a di-FeII species to one with an FeIIIFeIV core. Complex formation was promoted using the phosphinic amido tripodal ligand [poat]3- (N,N',N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido)) that provided dynamic coordination spheres that assisted in regulating both electron and proton transfer processes. Oxidation of an [FeII-(μ-OH)-FeIII] complex led to the corresponding di-FeIII species containing a hydroxido bridge that was not stable at room temperature and converted to a species containing an oxido bridging ligand and protonation of one phosphinic amido group to form [Hpoat]2-. Deprotonation led to a new species with an [FeIII-(μ-O)-FeIII] core that could be further oxidized to its FeIIIFeIV analogue. Reactions with phenols suggest homolytic cleavage of the O-H bond to give products that are consistent with the initial formation of a phenoxyl radical─spectroscopic studies indicated that the electron is transferred to the FeIV center, and the proton is initially transferred to the more sterically hindered oxido ligand but then relocates to [poat]3-. These findings offer new mechanistic insights related to the stability of and the reactions performed by di-Fe enzymes.

Synthesis, structure characterization and properties of a new oxidovanadium(IV) coordination polymer incorporating bridging (MoO4)2– and (Mo8O26)4– ligands

Abstract Hydrothermal synthesis, crystal structure and properties of a new heterometallic coordination polymer [(VO(terpy))4(MoO4)2(Mo8O26)]·2H2O (1) (terpy = 2,2′;6′,2″-terpyridine) are reported. Compound 1 contains two crystallographically unique vanadium(IV) atoms, bonded to a terminal oxido ligand and further coordinated to a terpy ligand. The three N atoms of terpy occupy the meridional sites of a distorted {VN3O3} octahedron. A γ-octamolybdate (Mo8O26)4– located on an inversion centre and a tetraoxidomolybdate (MoO4)2– function as bridging ligands. The μ3-bridging tridentate binding of (MoO4)2– leads to the formation of a {V4Mo2O12}4+ cationic unit consisting of an eight-membered heterometallic {Mo2V2O4} ring with protruding oxidovanadium handles. A pair of {V4Mo2O12}4+ units are bridged by the centrosymmetric (Mo8O26)4– ligand, resulting in the formation of an infinite chain of alternating {V4Mo2O12}4+ cations and (Mo8O26)4– anions.

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ3-bridging amino alcohol

Abstract The reaction of Na2MoO4·2H2O with 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH) in water at room temperature results in the formation of the heterometallic coordination polymer [Mo2O6L2(Na2(H2O)4)]·2H2O 1 (L = 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato). The structure of 1 consists of a neutral (Mo2O6) unit located on an inversion center. The Mo atoms exhibit hexa-coordination and are bonded to two terminal and two bridging oxido ligands, an alkoxide oxygen and the amine N atoms of an anionic ligand L– resulting in the formation of an edge-sharing {Mo2O8N2} bioctahedron. The Na+ cations of a centrosymmetric bis(μ2-aqua)-bridged (Na2(H2O)4)2+ unit are penta-coordinated and bonded to two symmetry related L– ligands via the oxygen atoms of their OH groups. The µ3-bridging tetradentate binding mode of L– results in the formation of a two-dimensional heterometallic coordination polymer. The constituents of 1 viz. (Mo2O6), (L)–, (Na2(H2O)4)2+ and lattice water molecules are interlinked with the aid of three varieties of hydrogen bonding interactions. The corresponding tungstate reported recently has been obtained through a similar synthetic protocol and is isostructural.

Tetra-ammonium μ-ethyl-enedi-amine-tetra-acetato-1κ3 O,N,O':2κ3 O'',N',O'''-bis-[trioxidotungstate(VI)] tetra-hydrate.

The title compound, (NH4)4[W2(C10H12N2O8)O6]·4H2O, was obtained from a mixture of tungstic acid, ammonia and ethyl-enedi-amine-tetra-acetic acid (H4edta) in a 2:4:1 ratio. The anion of the complex contains two WO3 units and one bridging edta4- ligand. Each central metal atom is tridentately coordinated by nitro-gen and two carboxyl-ate groups of the edta4- ligand, together with the three oxido ligands, producing a distorted octa-hedral coordination environment around each tungsten atom. The center of the carbon-carbon bond of the ethyl-ene bridge represents a crystallographic inversion center. The crystal structure consists of a three-dimensional supra-molecular framework built up by the dinuclear cations, the ammonium counter-ions and the solvent water mol-ecules via hydrogen bonds of the N-H⋯O and O-H⋯O type.

Bis(2,4-dioxo-pentan-3-ido-κ2 O,O')dioxidomolyb-denum(VI): a redetermination.

The title compound, [Mo(C5H7O2)2O2] or cis-[MoO2(acac)2] (acac is acetyl-acetonate), contains a molybdenum(VI) atom coordinated by two acetyl-acetonate ligands and two doubly bonded oxido ligands in a distorted octa-hedral shape. The mol-ecule is chiral and the asymmetric unit contains two independent mol-ecules (one Δ, one Λ). Extensive C-H⋯O contacts are present throughout the structure. Data were collected at 100 K, providing higher precision of unit-cell parameters and atomic positions than previous determinations [Kamenar et al. (1973 ▸). Cryst. Struct. Commun. 2, 41-44.; Krasochka et al. (1975). Zh. Strukt. Khim. 16, 696-698].

Artificial Metalloproteins with Dinuclear Iron-Hydroxido Centers.

Dinuclear iron centers with a bridging hydroxido or oxido ligand form active sites within a variety of metalloproteins. A key feature of these sites is the ability of the protein to control the structures around the Fe centers, which leads to entatic states that are essential for function. To simulate this controlled environment, artificial proteins have been engineered using biotin-streptavidin (Sav) technology in which Fe complexes from adjacent subunits can assemble to form [FeIII-(μ-OH)-FeIII] cores. The assembly process is promoted by the site-specific localization of the Fe complexes within a subunit through the designed mutation of a tyrosinate side chain to coordinate the Fe centers. An important outcome is that the Sav host can regulate the Fe···Fe separation, which is known to be important for function in natural metalloproteins. Spectroscopic and structural studies from X-ray diffraction methods revealed uncommonly long Fe···Fe separations that change by less than 0.3 Å upon the binding of additional bridging ligands. The structural constraints imposed by the protein host on the di-Fe cores are unique and create examples of active sites having entatic states within engineered artificial metalloproteins.

Influence of temperature on the equilibria of oxidovanadium(IV) complexes in solution.

The equilibria in the solution of three different oxidovanadium(IV) complexes, VO(dhp)2 (dhp = 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonato), VO(ma)2 (ma = maltolato) and VO(pic)2(H2O) (pic = picolinato), were examined in the temperature range of 120-352 K through a combination of instrumental (EPR spectroscopy) and computational techniques (DFT methods). The results revealed that a general equilibrium exists: VOL2 + H2O ⇄ cis-VOL2(H2O) ⇄ trans-VOL2(H2O), where cis and trans refer to the relative position of H2O and the oxido ligand. The equilibrium is more or less shifted to the right depending on the ligand, the temperature, the ionic strength and the coordinating properties of the solvent. With VO(dhp)2, only the square pyramidal species exists at 298 K in aqueous solution, while at 120 K the cis- and trans-VO(dhp)2(H2O) species are also present. The complex of maltol exists almost exclusively in the form cis-VO(ma)2(H2O) in aqueous solution at 298 K, while the trans species can be revealed only at higher temperatures, where the EPR linewidth significantly decreases. The equilibria involving 1-methylimidazole (MeIm), a model for the side chain His coordination, are also influenced by temperature, with its coordination being favored by decreasing the temperature. The implications of these results in the study of the (vanadium complex)-protein systems are discussed and the interaction with myoglobin (Mb) is examined as a representative example.

Reactions of Molybdenum and Tungsten Oxide Tetrafluoride with Sulfur(IV) Lewis Bases: Structure and Bonding in [WOF4]4, MOF4(OSO), and [SF3][M2O2F9] (M = Mo, W).

The structure of [WOF4]4 has been reinvestigated by low-temperature X-ray crystallography and DFT (MN15/def2-SVPD) studies. Whereas the W4F4 ring of the tetramer is planar and disordered in the solid state, the optimized gas-phase geometry prefers a disphenoidally puckered W4F4 ring and demonstrates asymmetric fluorine bridging. Dissolution of MOF4 (M = Mo, W) in SO2 and SF4 results in the formation of MOF4(OSO) and [SF3][M2O2F9], respectively. Both SO2 adducts and [SF3][Mo2O2F9] have been characterized by X-ray crystallography. The crystal structure of [SF3][Mo2O2F9] reveals dimerization of the ion pair that results in a rare heptacoordinate sulfur center. Optimization of the {[SF3][M2O2F9]}2 dimers in the gas phase, however, results in the elongation of one contact such that the sulfur centers are effectively hexacoordinate. Meanwhile, the crystal structure of [SF3][W2O2F9]·HF instead demonstrates hexacoordinate sulfur centers and a highly unusual coordination to [SF3]+ from [W2O2F9]- through an oxido ligand. While [SF3][W2O2F9] does not decompose at ambient temperature, MOF4(OSO) and [SF3][Mo2O2F9] are unstable toward evolution of SO2 or SF4. Computational studies reveal that the monomerization of [WOF4]4 in the gas phase at 25 °C is thermodynamically unfavorable using SO2, but favorable using SF4, consistent with the relative thermal stabilities of WOF4(OSO) and [SF3][W2O2F9].

Crystal structure of an ordered [WOF5]- salt: (1,10-phen-H)[WOF5] (1,10-phen = 1,10-phenanthroline).

Crystals of 1,10-phenanthrolinium penta-fluorido-oxidotungstate(VI), (1,10-phen-H)[WOF5] (1,10-phen = 1,10-phenanthroline, C12H8N2), were obtained upon hydrolysis of WF6(1,10-phen) in CH3CN at 193 K. The (1,10-phen-H)[WOF5] salt contains a rare example of a [WOF5]- anion in which the oxygen and fluorine atoms are ordered. This ordering was verified by bond-valence determinations and structural comparisons with [Xe2F11][WOF5] and Lewis acid-base adducts of WOF4 with main-group donor ligands. The crystal packing is controlled by N-H⋯F hydrogen bonding that is directed exclusively to the axial F atom as a result of its increased basicity caused by the trans influence of the oxido ligand.

Effects of Noncovalent Interactions on High-Spin Fe(IV)-Oxido Complexes.

High-valent nonheme FeIV-oxido species are key intermediates in biological oxidation, and their properties are proposed to be influenced by the unique microenvironments present in protein active sites. Microenvironments are regulated by noncovalent interactions, such as hydrogen bonds (H-bonds) and electrostatic interactions; however, there is little quantitative information about how these interactions affect crucial properties of high valent metal-oxido complexes. To address this knowledge gap, we introduced a series of FeIV-oxido complexes that have the same S = 2 spin ground state as those found in nature and then systematically probed the effects of noncovalent interactions on their electronic, structural, and vibrational properties. The key design feature that provides access to these complexes is the new tripodal ligand [poat]3-, which contains phosphinic amido groups. An important structural aspect of [FeIVpoat(O)]- is the inclusion of an auxiliary site capable of binding a Lewis acid (LAII); we used this unique feature to further modulate the electrostatic environment around the Fe-oxido unit. Experimentally, studies confirmed that H-bonds and LAII s can interact directly with the oxido ligand in FeIV-oxido complexes, which weakens the Fe═O bond and has an impact on the electronic structure. We found that relatively large vibrational changes in the Fe-oxido unit correlate with small structural changes that could be difficult to measure, especially within a protein active site. Our work demonstrates the important role of noncovalent interactions on the properties of metal complexes, and that these interactions need to be considered when developing effective oxidants.

Intercalation of (η5‐Pentamethylcyclopentadienyl)trioxomolybdenum(VI) in a Layered Double Hydroxide

A Zn,Al layered double hydroxide (LDH) with [Cp*MoO3]– (Cp* = η5‐C5Me5) anions in the interlayer has been prepared by a direct coprecipitation method. The intercalated LDH and the mixed metal oxide (MMO) obtained after its calcination were characterized by elemental and thermogravimetric analyses, powder X‐ray diffraction (PXRD), FT‐IR and FT‐Raman spectroscopy, 13C and 27Al MAS NMR, and scanning electron microscopy. The observed basal spacing of 17.1 Å for the LDH Zn,Al‐Cp*MoO3 (corresponding to a gallery height of 12.3 Å) suggests that the guest anions self‐assemble into dimers via offset face‐to‐face Cp*···Cp* interactions, which facilitates hydrogen‐bonding interactions between the oxido ligands of the complexes and the layer hydroxyl groups, while positioning the hydrophobic aromatic rings towards the center of the galleries. Calcination of Zn,Al‐Cp*MoO3 at 550 °C under air gave an MMO comprising well‐dispersed ZnO, α‐ZnMoO4 and ZnAl2O4 (spinel) oxides in a molar of ca. 1.4:1.6:1. The MMO exhibited catalytic activity for the epoxidation of cis‐cyclooctene with tert‐butylhydroperoxide (TBHP) as terminal oxidant, with quantitative yield of the epoxide being achieved within 24 h at 85 °C.

CO2 Activation with Formation of Uranium Carbonate Complexes in a Closed Synthetic Cycle

A closed synthetic cycle for the transformation of carbon dioxide to carbonate with a mid-valent tris(aryloxide)-ligated uranium(IV) terminal oxo complex is presented herein. Starting from the previously reported uranium(III) complex, [UIII(OArAd,Ad,Me)3], the uranium(V) terminal oxo complex, obtained by oxidation with N2O or NO, can be reduced with KC8 to yield CO2 activating anionic uranium(IV) oxo complexes [(Ad,Ad,MeArO)3UIV(O)]−. The cycle proceeds with the formation of [(Ad,Ad,MeArO)2(κ2-Ad,Ad,MeArOCO2)UIV(μ-κ1:κ2-CO3)]− upon reaction of the oxo anion with excess CO2 that adds to the terminal oxido ligand and inserts into the uranium–aryloxide bond. Treating this complex with trimethylsilyl halide eliminates both carbonates and generates, for instance, [K(2.2.2-crypt)][(Ad,Ad,MeArO)3UIV(I)2], which can ultimately be reduced with KC8 to recover the trivalent starting material, thus closing the cycle. In contrast, reaction of the 18-crown-6 stabilized U(IV) oxo analogue with CO2 yields a rare and isol...

Synthesis and Crystal Structure of [Benzyl N'-[[2-(hydroxy-kO)phenyl]ethylene]-N-[[2-(hydroxy-kO)phenyl]methylene]-carbamohydrazonothioato-kN,kN']-ethanol-dioxido-uranium

The uranyl compound UO2(L')EtOH (L' = benzyl N-(2-(oxidanyl)benzylidene)-N'-((2-(1-oxidanyl)phenyl)ethylidene) carbamohydrazonothioate) was synthesized and its molecular and crystal structure was determined by X-ray diffraction. The compound crystallizes in the triclinic sp. gr. P$$\bar {1}$$, Z = 2. The coordination geometry about the uranium atom is distorted pentagonal bipyramidal consisting of a UO2O4N2 polyhedron where the oxido ligands occupy axial positions with a symmetric linear structure (O=U=O angle = 179.11(12)°). The pentagonal plane (N2O3) is formed by the dinegative, tetradentate Schiff base ligand (N2O2) and the oxygen atom of the coordinated alcohol ligand. In the complex, the ligand adopts a propeller-like conformation.

Vanadium(V) complex based supramolecular metallogel: self-assembly and (Metallo)gelation triggered by non-covalent and N+[sbnd]H…O hydrogen bonding interactions

Metallogel (VLG) was obtained by reaction of tridentate dibasic donor’s ligand with vanadium pentoxide in 1:1 M ratio in H2O\\DMF solution. Metallogel formation was studied using IR, UV–Visible and 1H NMR spectroscopies. Single crystal structure suitable for XRD analysis was obtained by repeated crystallization of VLG from DMF/H2O solution after few days. VLG crystallized with monoclinic P21/n space group. The ligand bonded to metal ions in tridentate dibasic fashion, together with two oxido ligand forming a square pyramidal structure. Hence, the metal ions adopted a square pyramidal geometry with slight distortion with geometrical index, τ = 0.13.