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The hydrolytically stable dioxime ligand (2Z-6Z)-piperidine-2,6-dione (H3pidiox) acts as a strong chelator mainly with hard metals in high oxidation states, a pre-requisite for potential applications in metal sequestering processes from aqueous solutions. Reaction of ZrCl4 with H3pidiox in methanol gives the mononuclear compound [ZrIV(η1,η1,η2-H2pidiox-O,N,O′)2(OH2)2]Cl2·H2O·CH3OH (1), while the same reaction mixture in the presence of KOH gave the pentanuclear ZrOC [ZrIV5(μ2-OH)4(OH2)4(μ2–η1,η1,η2-Hpidiox-O,N,O′)4(η1,η1,η1-HpidioxO,N,O′)4]·5KCl·3CH3OH·8H2O (2). Compound 1 is formed at very acidic pH = 0, and the pentanuclear ZrOC 2 at higher pH values (pH = 2). Compounds 1 and 2 were characterized by single crystal X-ray structure analysis, multi-nuclear NMR spectroscopy and ESI-MS spectrometry. The single crystal X-ray structure analysis of 1 revealed a mononuclear zirconium(IV) compound containing an eight-coordinate zirconium atom bound to two singly deprotonated H2pidiox− ligands and two water molecules in a severely distorted bicapped octahedral geometry. The pentanuclear ZrOC 2 constitutes the second example of a Zr5 cluster to be reported and the first one in which the four zirconium atoms are arranged in a tetrahedral arrangement with the fifth occupying the center of the tetrahedron. 1D and 2D NMR spectroscopies of the acidic CD3OD solutions of complex 1 reveal a fast equilibrium between 1 and 2. Addition of KOH into a CH3OH solution of 2 results in the controlled fast transformation of 2 to an asymmetric hexanuclear ZrOC 3 as evidenced by the NMR and real-time ESI-MS solution studies. Further addition of KOH to the solution of 3 leads to the ZrOC 4, and on the basis of NMR and ESI-MS data and in comparison with the known hexanuclear titanium(IV)/H3pidiox cluster, it is concluded that the cluster 4 should have a hexanuclear structure. Electrospray ionization mass spectrometry (ESI-MS) demonstrated not only the structural stability 1 and 2 in solution, but also revealed the reversible pH driven dis-assembly/re-assembly process between the monomeric 1 and the pentanuclear ZrOC 2.

The chiral dicobalt(II) complex [CoII2(μ2-L)2] (1) (H2L = N2,N6-di(quinolin-8-yl)pyridine-2,6-dicarboxamide) and its tert-butyl analogue [CoII2(μ2-LBu)2] (2) were synthesized and structurally characterized. Addition of one equivalent of AgSbF6 to the dichloromethane solution of 1 and 2 resulted in the isolation of the mixed-valent dicobalt(III,II) species [CoIIICoII(μ2-L)2]SbF6 (3) and [CoIIICoII(μ2-LBu)2]SbF6 (4). Homovalent 1 and 2 exhibited catalytic activity towards proton reduction in the presence of acetic acid (AcOH) as the substrate. The complexes are stable in solution while their catalytic turnover frequency is estimated at 10 and 34.6 h−1 molcat−1 for 1 and 2, respectively. Calculations reveal one-electron reduction of 1 is ligand-based, preserving the dicobalt(II) core and activating the ligand toward protonation at the quinoline group. This creates a vacant coordination site that is subsequently protonated to generate the catalytically ubiquitous Co(III) hydride. The dinuclear structure persists throughout where the distal Co(II) ion modulates the reactivity of the adjacent metal site by promoting ligand redox activity through spin state switching.

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (H3dihybo) has been used to synthesize one titanium(IV) and two zirconium(IV) compounds that have been characterized by single-crystal X-ray diffraction and 1H and 13C NMR, solid-state UV–vis, and ESI-MS spectroscopy. The reaction of TiCl4 with H3dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K2[TiIV6(μ3-O)2(μ-O)3(OCH3)4(CH3OH)2(μ-Hdihybo)6]·CH3OH (1), while the reaction of ZrCl4 with H3dihybo and either nBu4NOH or KOH also gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively. Compounds 1–3 have the same structural motif [MIV6(μ3-Ο)2(μ-Ο)3] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked contrast to the octahedral arrangement of the six zirconium atoms in all the Zr6 clusters reported thus far, and a unique Zr6 core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand H3dihybo proved to be quite efficient in substantially reducing the band gaps of TiO2 and ZrO2 to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3, respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3 revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M3 (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M6 (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICSzz scan curves of these systems revealed that the triangular M3 (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICSzz(1) values equal to −30 and −42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICSzz(1) value of the benzene ring of −29.7 ppm calculated at the same level of theory.

Mixed-valent polyoxometalate (POM) clusters are one of the most interesting host species, showing a wide range of structural features and properties. The facile preparation and functionalisation of a mixed-valent polyoxofluorovanadates is reported, where two electrons are trapped to antipodal sites of the clusters. The first members of this family of clusters with the general formula, [VV12VIV2O16(?-O)10(?3-O)10(?3-F)2(L)2]6?, where L: py=pyridine?(1) pyr=pyrazine?(2) im=imidazole?(3), are unique organic-inorganic hybrids with the addition of a N-donor ligand at either end of the polyoxofluorovanadate. The composition and connectivity of 1?3 were characterised by single-crystal X-ray diffraction and electrospray ionisation mass spectrometry. Electron paramagnetic resonance spectroscopy revealed that the two well-separated VIV ions in each cluster are fully uncoupled with J=0, giving a degenerate singlet?triplet ground state. This attenuation of the exchange interaction is probed with density functional theoretical calculations that reveal that the inclusion of the fluoride ion in the cluster produces a bond pathway biased toward destructive interference between competing ferromagnetic and antiferromagnetic interactions. These robust molecular materials are the ideal combination of desirable electronic properties, with an organic handle with which they can be integrated into spintronic circuitry for molecular devices. 24 15 3836 3845

The activation of dioxygen by metal ions is critical in chemical and bio-chemical processes. A scientific challenge is the elucidation of the activation site of dioxygen in some copper metalloproteins, which is either the metal center or the substrate. In an effort to address this challenge, we prepared a series of new copper(ii) complexes (1·2H2O, 2·CH3OH, 3) with bio-inspired amidate ligands and investigated their activity towards dioxygen activation. The secondary amine group ligated to copper(ii) of the complex 1·2H2O in methyl alcohol is oxidized (2e-) by air dioxygen in a stepwise fashion to an imine group, affording complex 2. The copper(ii) complex 2 in methyl alcohol induces the 4e- oxidation by air dioxygen of the imine functionality ligated to copper(ii) to an azinate group, resulting in the isolation of a dinuclear azinate copper(ii) compound (4). Experimental and computational studies, including X-band c. w. EPR, UV-vis and ESI-MS spectroscopy and density functional theory computations, indicate a direct attack of the dioxygen on the -HC[double bond, length as m-dash]N- group ligated to copper(ii), and a possible mechanism of the oxidation of the -HC[double bond, length as m-dash]N- functionality ligated to copper(ii) to an azinate group is provided. This unprecedented activation of dioxygen by a copper substrate paves the way for further exploration of the O2 activation mechanisms in enzymes and the development of effective catalysts in O2-involved green organic synthesis.

Although directly relevant to metal mediated biological nitrification and the coordination chemistry of peroxide, the transition metal complexes of hydroxylamines and their functionalized variants remain mainly unexplored except vanadium(V) and molybdenum(VI). Reaction of the chelating hydroxylamine ligand 3,3′-(hydroxyazanediyl)dipropanamide (Hhydia) with [MII(CH3COO)2]·xH2O (M = CoII, ZnII) in methyl alcohol solution yields the complexes [CoII(η1:η1-CH3COO)(η1-CH3COO)(Hhydia)], (1) and [ZnII(η1-CH3COO)2(Hhydia)], (4), while reaction of Hhydia with trans-[NiIICl2(H2O)4]·2H2O yields [NiII(Hhydia)2]Cl2 (3). The X-ray structure analysis of 1 and 4 revealed that the CoII and ZnII atoms are bonded to a neutral tridentate O,N,O-Hhydia ligand and a chelate and a monodentate acetate groups in a severely distorted octahedral geometry for 1 and two monodentate acetate groups for 4 in a highly distorted trigonal bipyramidal geometry (τ = 0.63). The X-ray structure analysis of 3 revealed that the nickel atom in [NiII(Hhydia)2]2+ is bonded to two neutral tridentate O,N,O-Hhydia ligands. The twist angle, θ, in [NiII(Hhydia)2]2+ is 55.1(2)°, that is, very close to an ideal octahedron. The metal/Hhydia complexes were studied by UV–Vis (cobalt and nickel compounds), NMR (zinc compounds), HR-MS spectroscopy. The 1H and 13C NMR spectra of the methyl alcohol or acetonitrile solutions of ZnII-Hhydia complexes show the existence of both the 1:1 and 1:2 metal:ligand species being in dynamic equilibrium. The exchange processes between the ZnII-Hhydia is through complete dissociation-association of the ligand from the complexes as it is evident from the 2D {1H} EXSY NMR spectroscopy. UV–Vis spectroscopy of the CoII-Hhydia in methyl alcohol also shows the existence of both the 1:1 and 1:2 metal:ligand species in contrast to 1:2 complex [NiII(Hhydia)2]2+ which is the only species found in solution. The NMR and UV–Vis observations are additionally supported by the HR-MS studies.

The development of ligands with high selectivity and affinity for uranium is critical in the extraction of uranium from human body, radioactive waste, and seawater. A scientific challenge is the improvement of the selectivity of chelators for uranium over other heavy metals, including iron and vanadium. Flat ligands with hard donor atoms that satisfy the geometric and electronic requirements of the UVIO22+ exhibit high selectivity for the uranyl moiety. The bis(hydroxylamino)(triazine) ligand, 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H2bihyat), a strong binder for hard metal ions (FeIII, TiIV, VV, and MoVI), reacted with [UVIO2(NO3)2(H2O)2]·4H2O in aqueous solution and resulted in the isolation of the complexes [UVIO2(bihyat)(H2O)], [UVIO2(bihyat)2]2–, and {[UVIO2(bihyat)(μ-OH)]}22–. These three species are in equilibrium in aqueous solution, and their abundance varies with the concentration of H2bihyat and the pH. Reaction of H2bihyat with [UVIO2(NO3)2(H2O)2]·4H2O in CH3CN gave the trinuclear complex [UVI3O6(bihyat)2(μ-bihyat)2]2–, which is the major species in organic solvents. The dynamics between the UVIO22+ and the free ligand H2bihyat in aqueous and dimethyl sulfoxide solutions the metal binding ability of the H2bihyat over pyridine-2,6-dicarboxylic acid (H2dipic) or glutarimidedioxime for UVIO22+, and the selectivity of the H2bihyat to bind UVIO22+ in comparison to VVO43– and FeIII in either UVIO22+/VVO43– or UVIO22+/FeIII solutions were examined by NMR and UV–vis spectroscopies. The results revealed that H2bihyat is a superior ligand for UVIO22+ with high selectivity compared to FeIII and VVO43–, which increases at higher pHs. Thus, this type of ligand might find applications in the extraction of uranium from the sea and its removal from the environment and the human body. 57 13 7631 7643

Although directly relevant to metal mediated biological nitrification and the coordination chemistry of peroxide, the transition metal complexes of hydroxylamines and their functionalized variants remain mainly unexplored except vanadium(V) and molybdenum(VI). Reaction of the chelating hydroxylamine ligand 3,3′-(hydroxyazanediyl)dipropanamide (Hhydia) with [MII(CH3COO)2]·xH2O (M = CoII, ZnII) in methyl alcohol solution yields the complexes [CoII(η1:η1-CH3COO)(η1-CH3COO)(Hhydia)], (1) and [ZnII(η1-CH3COO)2(Hhydia)], (4), while reaction of Hhydia with trans-[NiIICl2(H2O)4]·2H2O yields [NiII(Hhydia)2]Cl2 (3). The X-ray structure analysis of 1 and 4 revealed that the CoII and ZnII atoms are bonded to a neutral tridentate O,N,O-Hhydia ligand and a chelate and a monodentate acetate groups in a severely distorted octahedral geometry for 1 and two monodentate acetate groups for 4 in a highly distorted trigonal bipyramidal geometry (τ = 0.63). The X-ray structure analysis of 3 revealed that the nickel atom in [NiII(Hhydia)2]2+ is bonded to two neutral tridentate O,N,O-Hhydia ligands. The twist angle, θ, in [NiII(Hhydia)2]2+ is 55.1(2)°, that is, very close to an ideal octahedron. The metal/Hhydia complexes were studied by UV–Vis (cobalt and nickel compounds), NMR (zinc compounds), HR-MS spectroscopy. The 1H and 13C NMR spectra of the methyl alcohol or acetonitrile solutions of ZnII-Hhydia complexes show the existence of both the 1:1 and 1:2 metal:ligand species being in dynamic equilibrium. The exchange processes between the ZnII-Hhydia is through complete dissociation-association of the ligand from the complexes as it is evident from the 2D {1H} EXSY NMR spectroscopy. UV–Vis spectroscopy of the CoII-Hhydia in methyl alcohol also shows the existence of both the 1:1 and 1:2 metal:ligand species in contrast to 1:2 complex [NiII(Hhydia)2]2+ which is the only species found in solution. The NMR and UV–Vis observations are additionally supported by the HR-MS studies. 151 417 425