OXYGEN TRANSFER REACTIONS: CATALYSIS BY RHENIUM COMPOUNDS

Crystal structures of three zinc(II) halide coordination complexes with quinoline N-oxide

The reaction of one equivalent of zinc(II) halide with two equivalents of quinoline N-oxide (QNO) in methanol yields compounds as ZnX 2(QNO)2, where X = Cl (I), Br (II) and I (III), namely, di-chlorido-bis-(quinoline N-oxide-κO)zinc(II), [ZnCl2(C9H7NO)2], di-bromido-bis-(quinoline N-oxide-κO)zinc(II), [ZnBr2(C9H7NO)2], and di-iodido-bis-(quinoline N-oxide-κO)zinc(II) [ZnI2(C9H7NO)2]. In all three complexes, Zn cations are coordinated by two QNO ligands bound through the oxygen atoms and two halide atoms, with X-Zn-X bond angles ca 20° wider than the O-Zn-O, giving rise to a distorted tetra-hedral geometry. Crystals of (II) and (III) are isostructural and both show pairwise π-stacking of QNO ligands and weak C-H⋯X hydrogen bonds, while (I) packs differently, with a shorter C-H⋯Cl bond and without π-stacking.

Pyridine-4-carboxamidoxime N-oxide

Our work in the area of synthesis of metal-organic frameworks (MOFs) based on organic N-oxides led to the crystallization of pyridine-4-carboxamidoxime N-oxide. Herein we report the first crystal structure of the title compound, C6H7N3O2 [systematic name: (Z)-4-(N'-hy-droxy-carbamimido-yl)pyridine N-oxide]. The hy-droxy-carbamimidoyl group is essentially coplanar with the aromatic ring, r.m.s.d. = 0.112 Å. The compound crystallizes in hydrogen-bonding layers built from the formation of strong O-H⋯O hydrogen bonds between the oxime oxygen atom and the oxygen atom of the N-oxide, and the formation of N-H⋯O hydrogen bonds between one amine nitro-gen atom and the N-oxide oxygen atom. These combined build R 3 4(24) ring motifs in the crystal. The crystal structure has no π-π inter-actions.

Crystal structures of four dimeric manganese(II) bromide coordination complexes with various derivatives of pyridine N-oxide

Four manganese(II) bromide coordination complexes have been prepared with four pyridine N-oxides, viz. pyridine N-oxide (PNO), 2-methyl-pyridine N-oxide (2MePNO), 3-methyl-pyridine N-oxide (3MePNO), and 4-methyl-pyridine N-oxide (4MePNO). The compounds are bis-(μ-pyridine N-oxide)bis-[aqua-dibromido-(pyridine N-oxide)manganese(II)], [Mn2Br4(C5H5NO)4(H2O)2] (I), bis-(μ-2-methyl-pyridine N-oxide)bis-[di-aqua-dibromido-manganese(II)]-2-methyl-pyridine N-oxide (1/2), [Mn2Br4(C6H7NO)2(H2O)4]·2C6H7NO (II), bis-(μ-3-methyl-pyridine N-oxide)bis-[aqua-dibromido-(3-methyl-pyridine N-oxide)manganese(II)], [Mn2Br4(C6H7NO)4(H2O)2] (III), and bis-(μ-4-methyl-pyridine N-oxide)bis-[di-bromido-methanol(4-methyl-pyridine N-oxide)manganese(II)], [Mn2Br4(C6H7NO)4(CH3OH)2] (IV). All the compounds have one unique MnII atom and form a dimeric complex that contains two MnII atoms related by a crystallographic inversion center. Pseudo-octa-hedral six-coordinate manganese(II) centers are found in all four compounds. All four compounds form dimers of Mn atoms bridged by the oxygen atom of the PNO ligand. Compounds I, II and III exhibit a bound water of solvation, whereas compound IV contains a bound methanol mol-ecule of solvation. Compounds I, III and IV exhibit the same arrangement of mol-ecules around each manganese atom, ligated by two bromide ions, oxygen atoms of two PNO ligands and one solvent mol-ecule, whereas in compound II each manganese atom is ligated by two bromide ions, one O atom of a PNO ligand and two water mol-ecules with a second PNO mol-ecule inter-acting with the complex via hydrogen bonding through the bound water mol-ecules. All of the compounds form extended hydrogen-bonding networks, and compounds I, II, and IV exhibit offset π-stacking between PNO ligands of neighboring dimers.

Manganese(II) chloride complexes with pyridine N-oxide (PNO) derivatives and their solid-state structures

Three manganese(II) N-oxide complexes have been synthesized from the reaction of manganese(II) chloride with either pyridine N-oxide (PNO), 2-methyl-pyridine N-oxide (2MePNO) or 3-methyl-pyridine N-oxide (3MePNO). The compounds were synthesized from methano-lic solutions of MnCl2·4H2O and the respective N-oxide, and subsequently characterized structurally by single-crystal X-ray diffraction. The compounds are catena-poly[[aqua-chlorido-manganese(II)]-di-μ-chlorido-[aqua-chlorido-manganese(II)]-bis-(μ-pyridine N-oxide)], [MnCl2(C5H5NO)(H2O)] n or [MnCl2(PNO)(H2O)] n (I), catena-poly[[aqua-chlorido-man-gan-ese(II)]-di-μ-chlorido-[aqua-chlorido-manganese(II)]-bis-(μ-2-methyl-pyridine N-oxide)], [MnCl2(C6H7NO)(H2O)] n or [MnCl2(2MePNO)(H2O)] n (II), and bis-(μ-3-methyl-pyridine N-oxide)bis-[di-aqua-dichlorido-manganese(II)], [Mn2Cl4(C6H7NO)2(H2O)4] or [MnCl2(3MePNO)(H2O)2]2 (III). The MnII atoms are found in pseudo-octa-hedral environments for each of the three complexes. Compound I forms a coordination polymer with alternating pairs of bridging N-oxide and chloride ligands. The coordination environment is defined by two PNO bridging O atoms, two chloride bridging atoms, a terminal chloride, and a terminal water. Compound II also forms a coordination polymer with a similar metal cation; however, the coordination polymer is bridged between MnII atoms by both a single chloride and 2MePNO. The distorted octahedrally coordinated metal cation is defined by two bridging 2MePNO trans to each other, two chlorides, also trans to one another in the equatorial (polymeric) plane, and a terminal chloride and terminal water. Finally, complex III forms a dimer with two bridging 3MePNOs, two terminal chlorides and two terminal waters forming the six-coordinate metal environment. All three compounds exhibit hydrogen bonding between the coordinating water(s) and terminal chlorides.

New avenues for ligand-mediated processes--expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

Redox-active ligands have evolved from being considered spectroscopic curiosities - creating ambiguity about formal oxidation states in metal complexes - to versatile and useful tools to expand on the reactivity of (transition) metals or to even go beyond what is generally perceived possible. This review focusses on metal complexes containing either catechol, o-aminophenol or o-phenylenediamine type ligands. These ligands have opened up a new area of chemistry for metals across the periodic table. The portfolio of ligand-based reactivity invoked by these redox-active entities will be discussed. This ranges from facilitating oxidative additions upon d(0) metals or cross coupling reactions with cobalt(iii) without metal oxidation state changes - by functioning as an electron reservoir - to intramolecular ligand-to-substrate single-electron transfer to create a reactive substrate-centered radical on a Pd(ii) platform. Although the current state-of-art research primarily consists of stoichiometric and exploratory reactions, several notable reports of catalysis facilitated by the redox-activity of the ligand will also be discussed. In conclusion, redox-active ligands containing catechol, o-aminophenol or o-phenylenediamine moieties show great potential to be exploited as reversible electron reservoirs, donating or accepting electrons to activate substrates and metal centers and to enable new reactivity with both early and late transition as well as main group metals.

Kinetics and mechanism of the reaction of Cr(ii) aqua ions with benzoylpyridine N-oxide

Aqueous chromium(II) ions, Cr(aq)(2+), react with benzoylpyridine oxide (BPO) much more rapidly than with other pyridine N-oxides previously explored. The kinetics were studied under pseudo-first order conditions with either reagent in excess. Under both sets of conditions, the major kinetic term exhibits first order dependence on limiting reagent, and second order dependence on excess reagent, i.e.k(Cr) = k2(Cr)[BPO][Cr(aq)(2+)]2 (excess Cr(aq)(2+)), and k(BPO) = k2(BPO)[Cr(aq)(2+)][BPO](2) (excess BPO), where k2(Cr) = (6.90 +/- 0.27) x 10(4) M(-2) s(-1) and k2(BPO) = (3.32 +/- 0.28) x 10(5) M(-2) s(-1) in 0.10 M HClO4. The rate constant k2(Cr) contains terms corresponding to [H+]-independent and [H+]-catalyzed paths. In the proposed mechanism, the initially formed Cr(aq)(BPO)(2+) engages in parallel oxidation of Cr(aq)(2+) and reduction of BPO. The latter reaction provides the basis for a convenient new preparative route for the BPO complex of Cr(III).

Mechanistic Investigation of the Oxygen-Atom-Transfer Reactivity of Dioxo-molybdenum(VI) Complexes

The oxygen-atom-transfer (OAT) reactivity of [LiPrMoO2(OPh)] (1, LiPr=hydrotris(3-isopropylpyrazol-1-yl)borate) with the tertiary phosphines PEt3 and PPh2Me in acetonitrile was investigated. The first step, [LiPrMoO2(OPh)]+PR3-->[LiPrMoO(OPh)(OPR3)], follows a second-order rate law with an associative transition state (PEt3, DeltaH not equal=48.4 (+/-1.9) kJ mol-1, DeltaS not equal=-149.2 (+/-6.4) J mol-1 K-1, DeltaG not equal=92.9 kJ mol-1; PPh2Me, DeltaH not equal=73.4 (+/-3.7) kJ mol-1, DeltaS not equal=-71.9 (+/-2.3) J mol-1 K-1, DeltaG not equal=94.8 kJ mol-1). With PMe3 as a model substrate, the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide-coordinated intermediate were calculated. The latter, 95 kJ mol-1, is in good agreement with the experimental values. An unexpectedly large O-P-C angle calculated for the TS suggests that there is significant O-nucleophilic attack on the P--C sigma* in addition to the expected nucleophilic attack of the P on the Mo==O pi*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [LiPrMoO(OPh)(OPR3)]+MeCN-->[LiPrMoO(OPh)(MeCN)]+OPR3, follows a first-order rate law in MeCN. A dissociative interchange (Id) mechanism, with activation parameters of DeltaH not equal=93.5 (+/-0.9) kJ mol-1, DeltaS not equal=18.2 (+/-3.3) J mol-1 K-1, DeltaG not equal=88.1 kJ mol-1 and DeltaH not equal=97.9 (+/-3.4) kJ mol-1, DeltaS not equal=47.3 (+/-11.8) J mol-1 K-1, DeltaG not equal=83.8 kJ mol-1, for [LiPrMoO(OPh)(OPEt3)] (2 a) and [LiPrMoO(OPh)(OPPh2Me)] (2 b), respectively, is consistent with the experimental data. Although gas-phase calculations indicate that the Mo--OPMe3 bond is stronger than the Mo--NCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [LiPrMoO(OPh)(MeCN)] (3).

Rhenium oxo compounds containing eta2-pyrazolate ligands

Reaction of potassium salts of sterically demanding pyrazolates (pz = bis-3,5-tert-butylpyrazolate, pz= bis-3,5-tert-butyl-4-methylpyrazolate) with Re2O7 affords soluble eta2-pyrazolate complexes of the type [(eta2-pz)ReO3(THF)n](1: pz, n= 1 and 2: pz, n= 0). They were characterized by spectroscopic methods and by X-ray crystallography confirming the eta2-coordinate ligands. Complex 1 employing the ligand with a proton in the 4-position retains one molecule of THF, whereas the additional methyl group in 2 leads to the base-free compound 2. Compound 1 reacts with pyridine and 3,5-dimethylpyridine to form Lewis base adducts of the type [(eta2-pz)ReO3(L)](3: L = py; 4: L = 3,5-Me2py). The pronounced sensitivity towards water of these complexes is demonstrated by the reaction of 1 with one equivalent of water forming the corresponding pyrazolium perrhenate [ReO4][pzH2](5). Its solid state structure shows a hydrogen bonded dimeric assembly. Catalytic activity of 1 is established in oxygen atom transfer-reactions (OAT) from dimethylsulfoxide to triphenylphosphine, and in epoxidations of cyclooctene employing bis(trimethylsilyl) peroxide (BTSP).

Cooperative Bimetallic Reactivity: Hydrogen Activation in Two-Electron Mixed-Valence Compounds

Reversible dihydrogen uptake by a two-electron mixed-valence di-iridium complex is examined with nonlocal density-functional calculations. Optimized metrics compare favorably with crystal structures of isolated species, and the calculated activation enthalpy of acetonitrile exchange is accurate within experimental error. Dihydrogen attacks the Ir(2) core at Ir(II); the Ir(0) center is electronically saturated and of incorrect orbital parity to interact with H(2). Isomeric eta(2)-H(2) complexes have been located, and harmonic frequency calculations confirm these to be potential energy minima. A transition state links one such complex with the final dihydride; calculated atomic charges suggest a heterolytic H(2) bond scission within the di-iridium coordination sphere. This investigation also establishes a ligand-design criterion for attaining cooperative bimetallic reactivity, namely, that the supporting ligand framework has sufficient mechanical flexibility so that the target complex can accommodate the nuclear reorganizations that accompany substrate activation.