Terminal Oxo Ligands Research Articles

Active Site Structures of the Escherichia coli N-Hydroxylaminopurine Resistance Molybdoenzyme YcbX.

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data have been used to characterize the coordination environment for the catalytic Mo site of Escherichia coli YcbX in two different oxidation states. In the oxidized state, the Mo(VI) ion is coordinated by two terminal oxo ligands, a thiolate S atom from cysteine, and two S donors from the bidentate pyranopterin ene-1,2-dithiolate (pyranopterin dithiolene). Upon reduction, it is the more basic equatorial oxo ligand that is protonated, with a Mo-Oeq bond distance that is best described as either a short Mo4+-OH2 bond or a long Mo4+-OH bond. Mechanistic implications for substrate reduction are discussed in light of these structural details.

Aminopolyol-Dependent Assembly of Heterometallic Lanthanide-Iron-Oxo Clusters.

Lanthanide-iron clusters usually display interesting structures and outstanding magnetic properties. However, due to the high reactivity (acidity) of the Fe3+-H2O bond and the inability to form a terminal oxo ligand, the preparation of high-nuclearity Ln-Fe clusters is a great challenge. Herein, a series of lanthanide-iron-oxo clusters with the formulas [Y6Fe(HL)10(NO3)2(EG)2(μ3-OH)8(H2O)4]·ClO4·N-H2BDEA·2H2O (Y6Fe, 1, H2L = 3-hydroxypivalic acid, EG = ethylene glycol, N-H2BDEA = 2,2'-(butylimino)diethanol), [Ln8Fe3(H2TEOA)2(HTEOA)2(HL)10(μ3-OH)9(μ2-OH)(μ4-O)2(H2O)4]·(NO3)3·xH2O (Ln = Y, x = 13 for 2, Y8Fe3; Ln = Dy, x = 10 for 3, Dy8Fe3; H3TEOA = triethanolamine), and [Ln12Fe14(HL)16(μ3-OH)20(μ2-OH)12(μ4-O)12(H2O)12]·(NO3)6·xH2O (Ln = Y, x = 40 for 4, Y12Fe14; Ln = Dy, x = 30 for 5, Dy12Fe14) were obtained by adjusting the pH with different aminopolyols as organic alkalis. Structural analysis showed that a cubane-like unit was the main structural unit in compounds 1-5. Compound 1 was formed by two {Y3Fe(μ3-OH)4} units with the common vertices, and compounds 2 and 3 were formed by two {Y3Fe(μ3-OH)3(μ4-O)} units with the common vertices bridging a quadrilateral unit {Ln2Fe2(μ3-OH)3(μ2-OH)}. The basic structural units of cubane-like {Ln2Fe2(μ3-OH)(μ4-O)3}, triangular {LnFe2(μ3-OH)2(μ4-O)}, and neutral iron-hydroxyl {Fe(μ3-OH)(μ2-OH)2} were found in compounds 4 and 5. The universality of building blocks for the assembly has been demonstrated in high-nuclearity lanthanide-iron-oxo clusters. Meanwhile, the structural regulation of the lanthanide-iron-oxo clusters 1-5 was realized by adjusting the pH with different organic alkalis, which provided the reference for the effective synthesis of high-nuclearity lanthanide-iron-oxo clusters. Magnetic studies showed that 3 and 5 displayed a slow magnetic relaxation behavior.

Hervé- and Krebs-Type Magnetic Polyoxometalate Dimers

Lacunary polyoxometalates (POMs) are negatively charged metal–oxo clusters, formally obtained from plenary topologies via fragment removal. Owing to the fragment removal, the lacunary POMs archetypes are rich in nucleophilic terminal oxo ligands, making them suitable for post-synthetic coordination with various heterometals. Trilacunary heteropolytungstates (hetero-POTs) based on bowl-shaped {W9O30} framework incorporating a central lone-pair containing {XO3} hetergroup (X = AsIII, SbIII, and BiIII) function as all-inorganic scaffolds that in the presence of d-block metal cations typically construct sandwich-like dimers of Hervé and/or Krebs. Herein we review the preparative approaches, as well as compositional and magnetic versatility of the constructed Hervé- and Krebs-type dimers and discuss prospective uses as POMtronics.

Kinetics and mechanism of self‐decomposition/reduction of oxo bis (peroxo) pyridine chromium(VI) in nonaqueous solvents

AbstractKinetics of oxidation of phenylmethanol, as a model by the oxo bis (peroxo) pyridine chromium(VI), OPPC, has been studied in different nonaqueous solvents. Decomposition of oxidant in solvents is studied and explained. The probable mechanism of the reaction is expounded and is related to the kinetics data. Furthermore, the data show a hydride abstraction by η2‐peroxo, forming a terminal oxo plus hydroxo ligand. The effects of solvent dielectric constant on the reaction rates are then considered. A formal potential (0.87 V) decomposition of OPPC on the Pt electrode is measured, which is accompanied by a one‐electron transition in determining the reaction. In addition, thermodynamic parameters are calculated and discussed based upon the proposed reaction mechanism.

Raman spectroscopic and theoretical studies on the structure of Ta (V) fluoro and oxofluoro complexes in molten FLiNaK

Combined with Raman spectroscopy and theoretical calculations, the tantalum fluoride and oxyfluoride ions in molten FLiNaK were studied. Firstly, the Raman spectra of K2TaF7 were recorded from 298 to 973 K and no structural evolution was observed. The coexistence of TaF72– and TaF83– anions was confirmed in K2TaF7 and molten FLiNaK-K2TaF7 mixtures. With the addition of Li2O (3 mol%), the terminal oxo ligands TaOF52– and TaOF63– were formed at the expense of TaF72– and TaF83– with the release of free F– ions, respectively. When the Li2O content increased to 13 mol%, two new oxyfluorides TaO2F2– and TaO2F32– were observed that contained two non-bridging oxygens. As the content of Li2O further increased to 40 mol%, insoluble solid tantalum oxide species like β-Ta2O5 was found, which indicates the transformation of intermediate oxyfluorides formed by the reaction of TaF72– and TaF83– fluorides with O2– in molten FLiNaK to oxides.

Catalytic H2O2 Activation by a Diiron Complex for Methanol Oxidation.

In nature, C-H bond oxidation of CH4 involves a peroxo intermediate that decays to the high-valent active species of either a "closed" {FeIV(μ-O)2FeIV} core or an "open" {FeIV(O)(μ-O)FeIV(O)} core. To mimic and to obtain more mechanistic insight in this reaction mode, we have investigated the reactivity of the bioinspired diiron complex [(susan){Fe(OH)(μ-O)Fe(OH)}]2+ [susan = 4,7-dimethyl-1,1,10,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraazadecane], which catalyzes CH3OH oxidation with H2O2 to HCHO and HCO2H. The kinetics is faster in the presence of a proton. 18O-labeling experiments show that the active species, generated by a decay of the initially formed peroxo intermediate [(susan){FeIII(μ-O)(μ-O2)FeIII}]2+, contains one reactive oxygen atom from the μ-oxo and another from the μ-peroxo bridge of its peroxo precursor. Considering an FeIVFeIV active species, a "closed" {FeIV(μ-O)2FeIV} core explains the observed labeling results, while a scrambling of the terminal and bridging oxo ligands is required to account for an "open" {FeIV(O)(μ-O)FeIV(O)} core.

High-Valent Metal-Oxo Species at the Nodes of Metal-Triazolate Frameworks: The Effects of Ligand Exchange and Two-State Reactivity for C-H Bond Activation.

Through quantum-chemical calculations, we investigate a family of metal-organic frameworks (MOFs) containing triazolate linkers, M2 X2 (BBTA) (M=metal, X=bridging anion, H2 BBTA=1H,5H-benzo(1,2-d:4,5-d')bistriazole), for their ability to form terminal metal-oxo sites and subsequently activate the C-H bond of methane. By varying the metal and bridging anion in the framework, we show how to significantly tune the reactivity of this series of MOFs. The electronic structure of the metal-oxo active site is analyzed for each combination of metal and bridging ligand, and we find that spin density localized on the oxo ligand is not an inherent requirement for low C-H activation barriers. For the Mn- and Fe-containing frameworks, a transition from ferromagnetic to antiferromagnetic coupling between the metal binding site and terminal oxo ligand during the C-H activation process can greatly reduce the kinetic barrier, a unique case of two-state reactivity without a change in the net spin multiplicity.

High‐Valent Metal–Oxo Species at the Nodes of Metal–Triazolate Frameworks: The Effects of Ligand Exchange and Two‐State Reactivity for C−H Bond Activation

AbstractThrough quantum‐chemical calculations, we investigate a family of metal–organic frameworks (MOFs) containing triazolate linkers, M2X2(BBTA) (M=metal, X=bridging anion, H2BBTA=1H,5H‐benzo(1,2‐d:4,5‐d′)bistriazole), for their ability to form terminal metal–oxo sites and subsequently activate the C−H bond of methane. By varying the metal and bridging anion in the framework, we show how to significantly tune the reactivity of this series of MOFs. The electronic structure of the metal–oxo active site is analyzed for each combination of metal and bridging ligand, and we find that spin density localized on the oxo ligand is not an inherent requirement for low C−H activation barriers. For the Mn‐ and Fe‐containing frameworks, a transition from ferromagnetic to antiferromagnetic coupling between the metal binding site and terminal oxo ligand during the C−H activation process can greatly reduce the kinetic barrier, a unique case of two‐state reactivity without a change in the net spin multiplicity.

O-O Bond Formation and Liberation of Dioxygen Mediated by N5 -Coordinate Non-Heme Iron(IV) Complexes.

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non‐heme, N5‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O−O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Assessing the Metal–Metal Interactions in a Series of Heterobimetallic Nb/M Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties

A one-pot synthetic procedure for a series of bimetallic Nb/M complexes, Cl-Nb( iPrNPPh2)3M-X (M = Fe (2), Ni (4), Cu (5)), is described. A similar procedure aimed at synthesizing a Nb/Co analogue instead affords iPrN═Nb( iPrNPPh2)2(μ-PPh2)Co-I (3) through cleavage of one phosphinoamide P-N bond under reducing conditions. Complexes 4 and 5 are found to have short Nb-M distances, corresponding to unusual metal-metal bonds between Nb and these first row transition metals. For comparison, a series of heterobimetallic O≡Nb( iPrNPPh2)3M-X complexes (M = Fe (7), Co (8), Ni (9), Cu (10)) was synthesized. In these complexes, the NbV center is engaged in sufficient π-bonding to the terminal oxo ligand to remove the driving force for direct metal-metal interactions. A comparison of the cyclic voltammograms of 2 and 4-10 reveals that the presence of a second metal shifts the redox potentials of both Nb and the late metal center anodically, even when direct metal-metal interactions are not present.

Countercation Effect on CO2 Binding to Oxo Titanate with Bulky Anilide Ligands.

This work focuses on nucleophilic activation of CO2 at the anionic terminal oxo titanium tris(anilide) complexes [(Solv)n M][OTi(N[t Bu]Ar)3 ]m with M=Li, Na, K, Mg, MgMe, AlCl2 , AlI2 ; Ar=3,5-Me2 C6 H3 ; Solv=Et2 O, THF, 12-crown-4, 2,2,2-cryptand; n, m=1-2. The CO2 binding strength to the terminal oxo ligand of [OTi(N[t Bu]Ar)3 ]- ([1]- ) and the stability of the resulting carbonate moiety [O2 COTi(N[t Bu]Ar)3 ]- ([2]- ) are highly dependent on the Lewis acidity of the countercation. We report herein on CO2 binding as a function of countercation and countercation coordination environment, and comment in this respect on the bottom and upper limits of the cation Lewis acidity. Thermodynamic parameters are provided for oxo complexes with lithium as countercation, that is, [(Et2 O)2 Li][1] and [(12-c-4)Li][1].

Elucidation of the Conformation of Polyglycine Organo-Polyoxotungstates: Evidence for Zipper Folding.

The conformation of a family of α1 and α2 polyglycine-containing organo-polyoxometalates was determined through a mixed experimental/molecular dynamics approach. The flexible peptide arm folds around the metal oxide surface in a rigid arrangement in low to average polarity solvents. The topology of the hybrid is the main factor that determines which oxos from the metal-oxide surface accept a H-bond from the closest amino acid. The rest of the peptide follows in a zipper mechanism that establishes a H-bond network that locks the system. The covalent constraint leads to a new type of H-bond where two consecutive amino acids clamp down terminal oxo ligands.

Crystal structure of two polymorphs of molybdenyl salicylidene-2-furfuriliminate (HL1) [MoO2(L1)2

The crystal structures of two polymorphs of molybdenyl salicylidene-2-furfuryliminate [MoO2(L1)2] have been solved by X-ray diffraction. Both complexes crystallize in centrosymmetric and non-centrosymmetric space groups (P21/c and Р21, respectively) of monoclinic system and have similar structures and close geometric parameters. The Мо atoms have a distorted octahedral coordination to two terminal oxo ligands in cis-positions to each other and two pairs of the oxygen atoms (cis- to О(oxo)) and the nitrogen atoms (trans- to О(oxo)) of two bidentate chelate ligands (L1)–.

High-Valent Manganese-Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C-H Bond Cleavage.

The addition of Lewis or Brönsted acids (LA = Zn(OTf)2, B(C6F5)3, HBArF, TFA) to the high-valent manganese-oxo complex MnV(O)(TBP8Cz) results in the stabilization of a valence tautomer MnIV(O-LA)(TBP8Cz•+). The ZnII and B(C6F5)3 complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidation state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn-Nave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn-O bond length is elongated compared to the MnV(O) starting material (Mn-O = 1.55 Å). The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C-H substrates was examined, and it was found that H• abstraction from C-H bonds occurs in a 1:1 stoichiometry, giving a MnIV complex and the dehydrogenated organic product. The rates of C-H cleavage are accelerated for the MnIV(O-LA)(TBP8Cz•+) valence tautomer as compared to the MnV(O) valence tautomer when LA = ZnII, B(C6F5)3, and HBArF, whereas for LA = TFA, the C-H cleavage rate is slightly slower than when compared to MnV(O). A large, nonclassical kinetic isotope effect of kH/kD = 25-27 was observed for LA = B(C6F5)3 and HBArF, indicating that H-atom transfer (HAT) is the rate-limiting step in the C-H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of MnIV(O-LA)(TBP8Cz•+) toward C-H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex MnIV(O-H)(tpfc•+) recently reported (J. Am. Chem. Soc. 2015, 137, 14481-14487).

Unexpected Reaction Promoted by NH+···O=Mo Hydrogen Bonds in Nonpolar Solvents

A new synthetic method using [MoIV(S‐tBu)4], 1,2‐benzenedithiol containing bulky hydrophobic acylamino substituents, 3,6‐[(4‐tBuC6H4)3CCONH]2C6H2‐1,2‐(SH)2 (LH2), a tertiary amine [R3N = Et3N or 2,6‐lutidine (Lu)], and an equimolar amount of water afforded (R3NH)2[MoIVO(L)2] [(R3NH)2[1]]. Each compound formed a strong NH+···O=Mo hydrogen bond between the tertiary ammonium cation and the terminal oxo ligand in toluene, which unexpectedly promoted the one‐electron oxidation of MoIV by lutidinium ion or 0.5 equiv. Me3NO to produce [MoVO(L)2]–. In contrast, in polar solvents, typical oxygen atom transfer (OAT) from Me3NO to [1]2– was observed. A possible mechanism is presented and the role of the hydrogen bond in molybdenum enzymes is discussed.

Oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands: The role of counterions and water of crystallization

Crystal structures of three oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands were solved, namely [(CH3CH2)4N]2[Mo2O5(Hmal)2(H2O)2] (H3mal = malic acid) (1), Na6[Mo2O5(cit)2)]·10.5H2O (H4cit = citric acid) (2) and Na2[WO2(H2cit)2]·10H2O (3). In 1, dianionic malate ligands adopt a unique bidentate coordination mode via alkoxy and α-carboxylate groups in the oxo-bridged dinuclear anionic complex, in which two terminal oxo ligands and a water molecule complete the distorted octahedral geometry around the Mo(VI) centre. In compound 2, a similar oxo-bridged dinuclear core, [Mo2O5]2+, is present. However, the distorted octahedral geometry of each Mo(VI) is completed by oxygen atoms originating from a fully deprotonated citrate ligand, adopting a tridentate coordination mode. The mononuclear complex 3, with two terminal oxo ligands and four oxygen atoms originating from two dianionic, bidentately coordinated citrate ligands positioned in a distorted octahedral geometry around W(VI), shows the presence of unique icosameric water clusters trapped within the crystal lattice.

Metallo-Wittig chemistry of an alkylidene to form a terminal titanium oxo complex.

We report the synthesis and structure of a titanium(iv) benzophenone adduct bearing a terminal oxo ligand, (PNP)Ti = O(OTf)(OCPh2) (PNP- = N[2-PiPr2-4-methylphenyl]2) (2). Complex 2 is readily synthesized in 71% yield from the previously reported titanium alkylidene (PNP)Ti[double bond, length as m-dash]CHtBu(OTf) (1) and two equivalents of benzophenone, by extruding the olefin Ph2C[double bond, length as m-dash]CHtBu. Treatment of benzophenone adduct 2 with a bulky aryloxide salt results in formation of (PNP)Ti[double bond, length as m-dash]O(OAr) (3) (OAr- = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide), concurrent with salt elimination and displacement of the benzophenone. Complexes 2 and 3 were characterized by single-crystal X-ray diffraction and a variety of spectroscopic techniques, including NMR, UV-Vis-NIR spectroscopies, and TD-DFT calculations.

Large Ligand Folding Distortion in an Oxomolybdenum Donor–Acceptor Complex

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO(SPh)2((i)Pr2Dt(0)) (where (i)Pr2Dt(0) = N,N'-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° "envelope"-type fold of the five-membered dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ∼18 000 cm(-1) is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL'CT) transition that also possesses Mo(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo(x(2)-y(2)) orbital character is derived from a strong pseudo Jahn-Teller effect, which drives the large fold-angle distortion to yield a double-well potential in the electronic ground state.

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation.

Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

Synthesis and crystal structure of the two-dimensional coordination compound potassium oxo-bis(hydroxylamido)-pyridine-2, 3-dicarboxylato-vanadate(V)

Abstract: The crystal structure of the title compound, K2[VO(C7H3O4N)(NH2O)2(H2O)]2, is composed of KO9 and VO5N2 polyhedra connected through pyridine-2,3-dicarboxylate (2,3-dipicolinate) bridges. The VO5N2 coordination polyhedron is a pentagonal bipyramid, with hydroxylamido and dipicolinato ligands being bidentate. The axial positions are occupied by the terminal oxo ligand and one oxygen atom from a deprotonated carboxylate group. The oxygen atoms from hydroxylamido and the nitrogen atoms belonging either to the pyridine ring or to the hydroxylamido occupy the equatorial positions and form an approximate pentagonal plane. The irregular KO9 polyhedra are linked, sharing edges and planes to form infinite chains. These chains are linked by the dipicolinate bridges to form layers. The distorted VO5N2 polyhedron is grafted on to the layer by the dipicolinate carboxylate O atom. Adjacent layers are connected through N–H···O hydrogen bonds to form a three-dimensional supramolecular structure.