Bridging Oxo Research Articles

Iridoid Constituents of Viburnum brachybotryum.

Eleven new iridoids, brachybones A-K (1-11), were isolated from the twigs of Viburnum brachybotryum. Their structures including absolute configurations were determined by spectroscopic data analysis and from the electronic circular dichroism (ECD) spectra. All of the compounds 1-11 possess one or two acetoxysenecioate substituents. Furthermore, compounds 5-7 and 11 feature a Cl atom in the molecule, while compounds 9-11 exhibit a cagelike rigid skeleton through an unusual oxo bridge from C-3 to C-8 or C-10. The isolates were evaluated for cytotoxic activity against the HCT-116, A549, and Hela cell lines, and the results showed compounds 10 and 11 to be active against HCT-116 cells.

Effect of 3d/4p Mixing on 1s2p Resonant Inelastic X-ray Scattering: Electronic Structure of Oxo-Bridged Iron Dimers.

1s2p resonant inelastic X-ray scattering (1s2p RIXS) has proven successful in the determination of the differential orbital covalency (DOC, the amount of metal vs ligand character in each d molecular orbital) of highly covalent centrosymmetric iron environments including heme models and enzymes. However, many reactive intermediates have noncentrosymmetric environments, e.g., the presence of strong metal-oxo bonds, which results in the mixing of metal 4p character into the 3d orbitals. This leads to significant intensity enhancement in the metal K-pre-edge and as shown here, the associated 1s2p RIXS features, which impact their insight into electronic structure. Binuclear oxo bridged high spin Fe(III) complexes are used to determine the effects of 4p mixing on 1s2p RIXS spectra. In addition to developing the analysis of 4p mixing on K-edge XAS and 1s2p RIXS data, this study explains the selective nature of the 4p mixing that also enhances the analysis of L-edge XAS intensity in terms of DOC. These 1s2p RIXS biferric model studies enable new structural insight from related data on peroxo bridged biferric enzyme intermediates. The dimeric nature of the oxo bridged Fe(III) complexes further results in ligand-to-ligand interactions between the Fe(III) sites and angle dependent features just above the pre-edge that reflect the superexchange pathway of the oxo bridge. Finally, we present a methodology that enables DOC to be obtained when L-edge XAS is inaccessible and only 1s2p RIXS experiments can be performed as in many metalloenzyme intermediates in solution.

Interfacial coordination chemistry for catalyst preparation

The concept of interfacial coordination chemistry was one of the innovation for which Michel Che became internationally known during his tenure as full professor at the Université Pierre et Marie Curie. Isolated metal ions on oxide surfaces behave like coordination compounds to which the surface contributes not only via neutral oxo bridges but also via hydroxyl groups. The latter being pH sensitive, the surface can be charged either positively or negatively, allowing selective adsorption of transition metal ions of opposite charge. In the early eighties, examples exploiting these properties for the preparation of oxide/metal supported catalysts were still rare. This review presents Michel Che’s early studies, mostly on molybdenum and nickel ions, on which he built, developed and promoted the concept of interfacial coordination chemistry, now widely used in the field of heterogeneous catalysis. This allowed him and his close collaborators to achieve a molecular-scale understanding of several catalyst preparation procedures, such as impregnation, selective adsorption, grafting, deposition–precipitation, and zeolite functionalization. The impact and legacy of the concept of Interfacial Coordination Chemistry on the current design of improved catalytic formulations is also reviewed.

Early-stage formation of (hydr)oxo bridges in transition-metal catalysts for photosynthetic processes

Ab initio simulations have been used to assess reaction pathways for the formation of M–(hydr)oxo–M (M = Co, Mn, Ni) bridges from M(ii) aqueous solutions, as early-stage building blocks of transition-metal catalysts for oxygen evolution.

A Mo(VI) based coordination polymer as an antiproliferative agent against cancer cells.

Metal ions being an important part of biological systems are of great interest in the designing of new drugs. Molybdenum is an essential trace element for humans, animals, and plants and naturally present in many enzymes hence its complexes can be expected to serve as potential candidates for biomedical applications. A novel molybdenum-based coordination polymer, [Mo2(μ2-O)O4(2-pyc)2(H2O)], is synthesized by a hydrothermal route and structurally characterized by using single crystal X-Ray diffraction. The structure consists of molybdenum octahedra connected by a bridging oxo ligand and 2-pyc forming a one-dimensional coordination polymer. This Mo coordination polymer was found to show a considerable inhibitory effect with IC50 values of 22.63 μmol L-1, 28.19 μmol L-1, and 20.97 μmol L-1, against HepG2 (human liver cancer), A549 (human lung cancer), and MCF-7 (human breast cancer) cell lines respectively. This is the first attempt at exploring the molybdenum-based coordination polymer for antitumor applications. The cell cytotoxicity analysis revealed that the anti-tumor potential of the compound is governed by arresting of the A549, HepG2, and MCF-7 cancer cells in the S phase of the cell cycle. UV-Visible absorption spectroscopy further revealed the binding interaction between the Mo coordination polymer and ctDNA and the binding constant was found to be 5.9 × 103 L mol-1, which is in agreement with those of well-known groove binders. This binding interaction in turn induces apoptosis and necrosis pathways leading to the death of the cancer cells.

Engineering the surface of cuprous oxide via surface coordination for efficient catalysis on aerobic oxidation of benzylic alcohols under ambient conditions

The surface organometallic chemistry or surface coordination is considered as an important tool for modifying the surface properties and electronic structure of metal centres to achieve enhancement in catalysis. In this piece of work, we report how cuprous oxide (Cu2O) was turned into an efficient catalyst alongside with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and N-methylimidazole (NMI) for catalytic aerobic oxidation of benzylic alcohols into aldehydes under ambient conditions through surface coordination. The coordination not only enhanced the capability of O2-activation via the ligation of NMI to the metal sites, but also the surface structure was engineered in such a way that a vacancy was created for O2-binding, one of the key steps as revealed by DFT calculations. Both experimental and theoretical results suggested that the coordination to the metal sites at the surface by NMI activated cuprous oxide via breaking up its oxo bridge at the surface to form hydroxyl group to allow O2-binding, and increased also the electron density on the metal site to enhance its capability of O2 activation. A plausible mechanism was proposed by using a variety of spectroscopic and electrochemical techniques as well as DFT calculations.

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.

Roles of D1-Glu189 and D1-Glu329 in O2 Formation by the Water-Splitting Mn4Ca Cluster in Photosystem II.

During the catalytic step that precedes O-O bond formation in Photosystem II, a water molecule deprotonates and moves next to the water-splitting Mn4Ca cluster's O5 oxo bridge. The relocated oxygen, known as O6 or Ox, may serve as a substrate, combining with O5 to form O2 during the final step in the catalytic cycle, or may be positioned to become a substrate during the next catalytic cycle. Recent serial femtosecond X-ray crystallographic studies show that the flexibility of D1-E189 plays a critical role in facilitating the relocation of O6/Ox. In this study, the D1-E189G and D1-E189S mutations were characterized with FTIR difference spectroscopy. The data show that both mutations support Mn4Ca cluster assembly, substantially inhibit advancement beyond the S2 state, and alter the network of H bonds that surrounds the Mn4Ca cluster. Previously, the D1-E189Q, D1-E189K, and D1-E189R mutations were shown to have little impact on the activity, electron transfer rates, or spectral properties of Photosystem II. A rationale for this behavior is presented. The residue D1-E329 interacts with water molecules in the O1 water network that has been suggested recently to supply substrate during the catalytic cycle. Characterization of the D1-E329A mutant with FTIR difference spectroscopy shows that this mutation does not substantially perturb the structure of PSII or the water molecules whose O-H stretching modes change during the catalytic cycle. This result provides additional evidence that the water molecules whose vibrational properties change during the S1 to S2 transition are confined approximately to the region bounded by D1-N87, D1-N298, and D2-K317.

CsVO2 F(IO3 ): An Excellent SHG Material Featuring an Unprecedented 3D [VO2 F(IO3 )]- Anionic Framework.

The first alkali-metal vanadium iodate fluoride, CsVO2 F(IO3 ), with a novel 3D anionic framework, has been rationally designed and hydrothermally synthesized. The 3D [VO2 F(IO3 )]- framework in CsVO2 F(IO3 ) is built from 0D Λ-shaped cis-[VO3 F(IO3 )2 ]4- polyanions via corner-sharing of oxo anions and bridging of the iodate groups. CsVO2 F(IO3 ) displays both a strong second-harmonic generation (SHG) 1.1 times as strong as KTiOPO4 (KTP) under 2.05 μm laser radiation and high laser-induced damage threshold (LIDT) of 107.9 MW cm-2 . This work provides a new route to design SHG crystals with stable 3D anionic structures from low-dimensional structural building units.

CsVO2F(IO3): An Excellent SHG Material Featuring an Unprecedented 3D [VO2F(IO3)]− Anionic Framework

AbstractThe first alkali‐metal vanadium iodate fluoride, CsVO2F(IO3), with a novel 3D anionic framework, has been rationally designed and hydrothermally synthesized. The 3D [VO2F(IO3)]− framework in CsVO2F(IO3) is built from 0D Λ‐shaped cis‐[VO3F(IO3)2]4− polyanions via corner‐sharing of oxo anions and bridging of the iodate groups. CsVO2F(IO3) displays both a strong second‐harmonic generation (SHG) 1.1 times as strong as KTiOPO4 (KTP) under 2.05 μm laser radiation and high laser‐induced damage threshold (LIDT) of 107.9 MW cm−2. This work provides a new route to design SHG crystals with stable 3D anionic structures from low‐dimensional structural building units.

A Chiral Macrocyclic Tetra-N-Heterocyclic Carbene Yields an "All Carbene" Iron Alkylidene Complex.

The first chiral macrocyclic tetra-N-heterocyclic carbene (NHC) ligand has been synthesized. The macrocycle, prepared in high yield and large scale, was ligated onto palladium and iron to give divalent C2 -symmetric square planar complexes. Multinuclear NMR and single crystal X-ray diffraction demonstrated that there are two distinct NHCs on each ligand, due to the bridging chiral cyclohexane. Oxidation of the iron(II) complex with trimethylamine N-oxide yielded a bridging oxo complex. Diazodiphenylmethane reacted with the iron(II) complex at room temperature to give a paramagnetic diazoalkane complex; the same reaction yielded the "all carbene" complex at elevated temperature. Electrochemical measurements support the assignment of the "all carbene" complex being an alkylidene. Notably, the diazoalkane complex can be directly transformed into the alkylidene complex, which had not been previously demonstrated on iron. Finally, a test catalytic reaction with a diazoalkane on the iron(II) complex does not yield the expected cyclopropane, but actually the azine compound.

Phthalocyanine-Cored Fluorophores with Fluorene-Containing Peripheral Two-Photon Antennae as Photosensitizers for Singlet Oxygen Generation.

A series of free base and Zn(II) phthalocyanines featuring fluorenyl antennae linked by methoxy or oxo bridges to the phthalocyanine core (Pc) were synthesized and characterized. Selected linear and nonlinear (two-photon absorption) optical properties of these new compounds were subsequently studied. As previously observed for related porphyrin dendrimers bearing 2-fluorenyl peripheral dendrons, an efficient energy transfer occurs from the peripheral antennae to the central phthalocyanine core following excitation in the fluorenyl-based π–π* absorption band of these chromophores. Once excited, these compounds relax to the ground state, mostly by emitting intense red light or by undergoing intersystem crossing. As a result, the tetrafunctionalized Zn(II) phthalocyanines are fluorescent, but can also efficiently photosensitize molecular oxygen in tetrahydrofurane (THF), forming singlet oxygen with nearly comparable yields to bare Zn(II) phthalocyanine (ZnPc). In comparison with the latter complex, the positive role of the fluorenyl-containing antennae on one- and two-photon brightness (2PA) is presently demonstrated when appended in peripheral (β) position to the phthalocyanine core. Furthermore, when compared to known porphyrin analogues, the interest in replacing the porphyrin by a phthalocyanine as the central core to obtain more fluorescent two-photon oxygen photosensitizers is clearly established. As such, this contribution paves the way for the future development of innovative biphotonic photosensitizers usable in theranostics.

(Hydr)oxo-bridged heme complexes: From structure to reactivity

Iron–porphyrins ([Formula: see text] hemes) are present throughout the biosphere and perform a wide range of functions, particularly those that involve complex multiple-electron redox processes. Some common heme enzymes involved in these processes include cytochrome P450, heme/copper oxidase or heme/non-heme diiron nitric oxide reductase. Consequently, the (hydr)oxo-bridged heme species have been studied for the important roles that they play in many life processes or for their application for catalysis and preparation of new functional materials. This review encompasses important synthetic, structural and reactivity aspects of the (hydr)oxo-bridged heme constructs that govern their function and application. The properties and reactivity of the bridging (hydr)oxo moieties are directly dictated by the coordination environment of the heme core, the nature and ligation of the second metal center attached to the (hydr)oxo group, the presence or absence of a linker, and the degree of flexibility around that linker within the scaffold. Here, we summarize the structural features of all known (hydr)oxo-bridged heme constructs and use those to categorize and thus, provide a more comprehensive picture of structure–function relationships.

Conductive Self-Assembled Monolayers of Paramagnetic {CoII } and { } Coordination Clusters on Gold Surfaces.

Two polynuclear cobalt(II,III) complexes, [Co5(N3)4(N-n-bda)4(bza·SMe)2] (1) and [Co6(N3)4(N-n-bda)2(bza·SMe)5(MeOH)4]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H2bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C2-symmetric {CoII} core of two isosceles Co3 triangles with perpendicularly oriented planes, sharing a central, high-spin CoII ion residing in a distorted tetrahedral coordination environment. This central CoII ion is connected to four outer, octahedrally coordinated low-spin CoIII ions via oxo bridges. Compound 2 comprises a semi-circular {} motif of four non-interacting high-spin CoII and two low-spin CoIII centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {CuIILnIII} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co5}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral CoII centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co10(N3)2(N-n-bda)6(bza·SMe)6] (3), containing 10 virtually non-interacting high-spin CoII centers.

One of the Substrate Waters for O2 Formation in Photosystem II Is Provided by the Water-Splitting Mn4CaO5 Cluster's Ca2+ Ion.

During the catalytic step immediately prior to O-O bond formation in Photosystem II, a water molecule deprotonates and moves next to the water-splitting Mn4CaO5 cluster's O5 oxo bridge. Considerable evidence identifies O5 as one of the two substrate waters that ultimately form O2. The relocated oxygen, known as O6 or Ox, may be the second. It is currently debated whether O6 or Ox originates as the Mn-bound water denoted W2 or as the Ca2+-bound water denoted W3. To distinguish between these two possibilities, we analyzed the D-O-D bending mode of the water molecule that deprotonates/relocates to become O6/Ox. We show that this D-O-D bending mode is not altered by the D1-S169A mutation. Previously, we showed that this D-O-D bending mode is altered substantially when Sr2+ is substituted for Ca2+. Because Sr2+/Ca2+ substitution alters this D-O-D bending mode but the D1-S169A mutation does not, we conclude that the water-derived oxygen that relocates and becomes O6/Ox derives from the Ca2+-bound W3. This conclusion provides an important constraint for proposed mechanisms of O-O bond formation in Photosystem II.

A Gas‐Phase CanMn4−nO4+ Cluster Model for the Oxygen‐Evolving Complex of Photosystem II

AbstractOne of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3O4⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand‐free tetrameric CanMn4−nO4+ (n=0–4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas‐phase reactivity experiments with D2O and H218O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono‐calcium cluster CaMn3O4+ is observed to oxidize water via elimination of hydrogen peroxide. First‐principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn3O4+ as well as the role of calcium.

A Gas-Phase Can Mn4-n O4+ Cluster Model for the Oxygen-Evolving Complex of Photosystem II.

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3 O4 ⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand-free tetrameric Can Mn4-n O4+ (n=0-4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas-phase reactivity experiments with D2 O and H218 O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono-calcium cluster CaMn3 O4+ is observed to oxidize water via elimination of hydrogen peroxide. First-principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn3 O4+ as well as the role of calcium.

Trinuclear and Hexanuclear Lanthanide(III) Complexes of the Chiral 3+3 Macrocycle: X-ray Crystal Structures and Magnetic Properties

A new triphenolic hexaaza chiral macrocyclic amine L forms trinuclear complexes 1-3 with rare earth metal lanthanide(III) ions (Ln = Dy, Eu, and Y) with the general formula [Ln3L(μ3-OH)2(NO3)4(H2O)2]· xCH3OH. The crystal structures of the nitrate derivatives of this type reveal the presence of a {Ln3(μ3-OH)2} core within the macrocycle. For the chloride derivative of dysprosium(III) 4, a duplex of the trinuclear compound is formed to give the hexanuclear [Dy6L2(μ3-OH)3(μ3-O)(μ2-Cl)3Cl4(H2O)2] compound, in which two trinuclear macrocyclic units are linked by bridging chloride anions, supplemented by a hydrogen bond connecting the central oxo and hydroxo bridges as well as by weak interactions at the periphery of the macrocycle. The nuclear magnetic resonance spectra of these complexes reveal a dynamic behavior in solution related to exchange of axial ligands and hindered rotation of phenyl substituents. Magnetic studies of the nitrate (1-3) and chloride (4) dysprosium(III) complexes suggest the presence of weak ferromagnetic interactions between neighboring metal centers. The interaction is strongest for compound 1, and for the related duplex compound 4, it appears to be somewhat weaker. The ac susceptibility measurements for complexes 1 and 4 confirm their field-induced single-molecule magnet behavior with the following characteristics: Ueff = 10.6 cm-1 (15.2 K), τ0 = 2.05 × 10-4 s under 2500 Oe dc fields for 1; Ueff = 7.9 cm-1 (11.4 K), τ0 = 1.68 × 10-4 s under a 3000 Oe dc field for 4.

Octasodium bis-(Nitrilo-tris-Methylenephosphanatooxovanadate(IV))-Dioxo-bis-Oxovanadium(IV) Icosohydrate Na8[(VO)2(μ−O)2{(VO)μ3−N(CH2PO3)3}2]·20H2O

Crystal hydrate of sodium salt of an oxovanadium(IV) complex with nitrilo-tris-methylenephosphonic (NTP) acid Na8[(VO)2(μ−O)2{(VO)μ3−N(CH2PO3)3}2]·20H2O is synthesized, isolated, and studied: space group P1, Z = 2, a = 12.5829(2) A, b = 13.9297(2) A, c = 15.1051(3) A, α = 72.233(2)°, β = 76.907(2)°, γ = 70.388(2)°. The NTP ions are completely deprotonated and hexadentate. In coordinating the VO2+ ion each NTP ion closes three N–V–O–P–C cycles with a common N–V bond. By means of bridging vanadyl ions and oxo ligands the four-membered V–O–V–O cycle and the [–V–O–P–O–]4 macrocycle are closed. Coordination V polyhedra are distorted tetragonal pyramids. With regard to the occurrence of weak V–N bonds the polyhedra of two V atoms can be completed to strongly distorted octahedra.

The role of bridging ligands in dinitrogen reduction and functionalization by uranium multimetallic complexes.

Cooperativity between metal centres is identified as a crucial step in dinitrogen reduction both for the industrial Haber-Bosch process and for the natural fixation of nitrogen by nitrogenase enzymes, but the mechanism of N2 reduction remains poorly understood. This is in large part because multimetallic complexes that reduce and functionalize dinitrogen in the absence of strong alkali reducing agents are crucial to establish a structure-activity relationship, but remain extremely rare. Recently, we reported a multimetallic nitride-bridged diuranium(III) complex capable of reducing and functionalizing dinitrogen. Here we show that an analogous complex assembled with an oxo instead of a nitride linker also effects the four-electron reduction of dinitrogen, but the reactivity of the resulting oxo-(N2) complex differs significantly from that of the nitride-(N2). Computational studies show a different bonding scheme for the dinitrogen where the bridging nitride does participate in the binding and consequent activation of N2, while the oxide does not.