Nadia C. Mösch-Zanetti

Molybdenum(VI) dioxo complexes employing Schiff base ligands with an intramolecular donor for highly selective olefin epoxidation.

Reaction of [MoO(2)(η(2)-tBu(2)pz)(2)] with Schiff base ligands HL(X) (X = 1-5) gave molybdenum(VI) dioxo complexes of the type cis-[MoO(2)(L(X))(2)] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R(2) = OMe or NMe(2)) in the side chain in complexes 1-4 is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes 1 and 3. Complexes 1, 3, and 5 exist as a mixture of two isomers in solution, whereas complexes 2 and 4 with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe(2) groups in complexes 3 and 4 lead to lower activity.

Novel Pyridazine Based Scorpionate Ligands in Cobalt and Nickel Boratrane Compounds

Heating of 6-methylpyridazine-3-thione (HPn(Me)) and 6-tert-butylpyridazine-3-thione (HPn(tBu)) with potassium borohydride in diphenylmethane in a 3:1 ratio gave two new scorpionate ligands K[HB(Pn(Me))(3)] and K[HB(Pn(tBu))(3)]. Single crystal X-ray diffraction analysis of the methyl derivative K[HB(Pn(Me))(3)] revealed a dimeric species with one potassium atom coordinated by six sulfur atoms of two scorpionate ligands and a second potassium atom coordinated by three nitrogen atoms of one of the two ligands as well as by three water molecules. The reaction of K[HB(Pn(tBu))(3)] with nickel(II) chloride or cobalt(II) chloride in CH(2)Cl(2) led to the new boratrane compounds [M{B(Pn(tBu))(3)}Cl] (M = Ni 1, Co 3) where a formal reduction of the metal ions to Ni(I) and Co(I), respectively, and activation of the B-H bond occurred. Similar reactivity was observed by employing K[HB(Pn(R))(3)] (R = Me, tBu) and nickel(II) chloride in water. Reaction with cobalt(II) chloride in water also gave boratrane compounds [Co{B(Pn(R))(3)}(Pn(R))] (R = tBu 4, Ph 5), but instead of a chloride a bidentate pyridazinethionate ligand from a defragmentated scorpionate is found in the molecules. The molecular structures of all nickel and cobalt compounds were determined by single crystal X-ray diffraction analyses confirming the formation of boratranes in compounds 1-5. Magnetic measurements confirm the reduced oxidation states and the paramagnetic character of the Ni(I) and Co(I) complexes. Supportive DFT studies were carried out for a better understanding of the electronic nature of the metal-boron bond of the boratrane complexes.

Dioxomolybdenum(VI) Complexes with Pyrazole Based Aryloxide Ligands: Synthesis, Characterization and Application in Epoxidation of Olefins

Synthesis, characterization, and epoxidation chemistry of four new dioxomolybdenum(VI) complexes [MoO(2)(L)(2)] (1-4) with aryloxide-pyrazole ligands L = L1-L4 is described. Catalysts 1-4 are air and moisture stable and easy to synthesize in only three steps in good yields. All four complexes are coordinated by the two bidentate ligands in an asymmetric fashion with one phenoxide and one pyrazole being trans to oxo atoms, respectively. This is in contrast to the structure found for the related aryloxide-oxazoline coordinated Mo(VI) dioxo complex 5. This was confirmed by the determination of the molecular structures of complexes 1-3 by X-ray diffraction analyses. Compounds 1-4 show high catalytic activities in the epoxidation of various olefins. Cyclooctene (S1) is converted to its epoxide with high activity, whereas the epoxidation of styrene (S2) is unselective. Internal olefins (S3 and S4) are also acceptable substrates, as well as the very challenging olefin 1-octene (S5). Catalyst loading can be reduced to 0.02 mol % and the catalyst can be recycled up to ten times without significant loss of activity. Supportive DFT calculations have been carried out in order to obtain deeper insights into the electronic situation around the Mo atom.

Oxo-molybdenum and oxo-tungsten complexes of Schiff bases relevant to molybdoenzymes

A series of octahedral dioxomolybdenum(VI) complexes of the type [MoO(2)L(2)] {L = 4-Ar-pent-2-en-ol; L(i-Pr2Ph) with Ar = 2,6-diisopropylphenyl (1); L(Me2Ph) with Ar = 2,6-dimethylphenyl (2), L(MePh) with Ar = 2-methylphenyl (3) and with Ar = phenyl (4)} and dioxotungsten(VI) compounds [WO(2)L(2)] {L(i-Pr2Ph) (5); L(Me2Ph) (6) and L(MePh) (7)} with Schiff bases have been synthesized as models for oxotransferases. Spectroscopic characterization in solution shows with the sterically encumbered ligands L(i-Pr2)Ph and L(Me2)Ph isomerically pure products whereas the ligand with only one substituent in ortho position at the aromatic ring L(MePh) revealed a dynamic mixture of three isomers as confirmed by variable temperature NMR spectroscopy. Single crystal X-ray diffraction analyses of compounds 1, 2, and 4 and showed them to be in the N,N-trans conformation consistent with the larger steric demand at nitrogen. Oxygen atom transfer (OAT) properties towards trimethylphosphine were investigated leading to the isolation of two mononuclear molybdenum(IV) compounds [MoO(PMe(3))(L(Me2Ph))(2)] (8) and [MoO(PMe(3))(L(MePh))(2)] (9) as confirmed by spectroscopic and crystallographic means. The kinetics of OAT between complex [MoO(2)(L(Me2Ph))(2)] (2) and PMe(3) was investigated by UV/Vis spectroscopy under pseudo-first-order conditions revealing single-step reactions with Eyring values of DeltaH(double dagger) = +60.79 kJ mol(-1) and DeltaS(double dagger) = -112 J mol(-1) K(-1) and a first-order dependence of phosphine consistent with a slow nucleophilic attack of the phosphine showing the octahedral geometries of this system to be unfavorable for OAT. Compound 1 showed no OAT reactivity towards PMe(3) emphasizing the influence of sterical properties. Furthermore, the reactivity of the reduced compounds [MoO(PMe(3))(L(Me2Ph))(2)] (8) and [MoO(PMe(3))(L(MePh))(2)] (9) towards molecular oxygen was investigated leading, in the case of 8, to the substitution of PMe(3) by O(2) under formation of the peroxo compound [MoO(O(2))(L(Me2Ph))(2)] (10). In contrast, the analogous reaction employing 9 led to oxidation forming the dioxo compound [MoO(2)(L(MePh))(2)] (3).

Mechanistic Insight into the Reactivity of Oxotransferases by Novel Asymmetric Dioxomolybdenum(VI) Model Complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-flurobenzyl)-1,4-diazepane, and 1,4-bis(2-hydroxy-4-chlorobenzyl)-1,4-diazepane (H(2)(L1)-H(2)(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO(2)(L)] 1-6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO(2)(L)] 1-4 have been successfully determined by single-crystal X-ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis-β configuration. The Mo-O(oxo) bond lengths differ only by ≈0.01 Å. Complexes 1, 2, 5, and 6 exhibit two successive Mo(VI)/Mo(V) (E(1/2), -1.141 to -1.848 V) and Mo(V)/Mo(IV) (E(1/2), -1.531 to -2.114 V) redox processes. However, only the Mo(VI)/Mo(V) redox couple was observed for 3 and 4, suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1, 2, 5, and 6 elicit efficient catalytic oxygen-atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe(3) at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe(3) at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.

Oxorhenium(V) Complexes with Pyrazole Based Aryloxide Ligands and Application in Olefin Epoxidation

We synthesized and characterized a set of new oxorhenium(V) complexes coordinated by various pyrazole containing phenol (L1-L3) and naphthol ligands (L4-L7). Depending on the starting material, we were able to selectively synthesize monosubstituded or disubstituted complexes of the type [ReOBr(2)L(PPh(3))] (1-7; L = L1-L7) and [ReOClL(2)] (L = L1 8; L2 9; L4 10; L6 11), respectively. All complexes are stable to air and moisture, both in solid state as well as in solution. Furthermore, the cationic oxorhenium(V) complex [ReO(L1)(2)(NCMe)](OTf) (8a) was obtained upon chloride abstraction with silver triflate from 8. All new complexes were able to catalyze the epoxidation of cis-cyclooctene in yields up to 64%. The ease of preparation and their tolerance to air and moisture, as well as the simple ligand modifications, make them an interesting class of novel catalysts. An attempted reduction of perchlorate ClO(4)(-) with complex 8 was unsuccessful. Molecular structures of complexes 1, 4, 6, 7, 8, 8a, 10, and 11 were determined by single crystal X-ray diffraction analyses.

Replacement of an Oxo by an Imido Group in Oxotransferase Model Compounds: Influence on the Oxygen Atom Transfer

Treatment of [MoO(N-t-Bu)Cl(2)(dme)] (dme = dimethoxyethane) with 2 equiv of the potassium salts of Schiff base ligands of the type KArNC(CH(3))CHC(CH(3))O afforded oxo imido molybdenum(VI) compounds [MoO(N-t-Bu)L(2)] {1, with Ar = phenyl (L(Ph)), 2 with Ar = 2-tolyl (L(MePh)), 3 with Ar = 2,6-dimethylphenyl (L(Me2Ph)) and 4 with Ar = 2,6-diisopropylphenyl (L(iPr2Ph))}. We have also prepared related bisimido complexes [Mo(N-t-Bu)(2)L(2) (5 with L = L(Ph), 6 with L = L(MePh), and 7 with L = L(Me2Ph)) by treatment of [Mo(N-t-Bu)(2)Cl(2)(dme)] with 2 equiv of the potassium salt of the respective ligand. 1, 3, 5, and 6 were characterized via single crystal X-ray diffraction. The oxo imido complexes exhibit oxygen atom transfer (OAT) reactivity toward trimethyl phosphine. Kinetic data were obtained for 1 and 3 by UV/vis spectroscopy revealing decreased OAT reactivity in comparison to related dioxo complexes with the same Schiff base ligands and decreased reactivity of 1 versus 3. Cyclic voltammetry was used to probe the electronic situation at the molybdenum center showing reversible reduction waves for 3 and [MoO(2)(L(Me2Ph))(2)] at comparable potentials while 1 exhibits a significant lower potential. Density functional theory (DFT) calculations showed a higher electron density on oxygen in the oxo imido complexes.

Molecular oxygen activation by a molybdenum(IV) monooxo bis(β-ketiminato) complex

Molybdenum(IV) monooxo compound that contains bis(β-ketiminato) ligands activates molecular oxygen forming a molybdenum(VI) monooxo peroxo compound, representing a new entry into molybdenum peroxo derivatives.

The first molybdenum dioxo compounds with η2-pyrazolate ligands: crystal structure and oxo transfer properties

Molybdenum dioxo compounds [MoO2Cl(eta 2-pz)] and [MoO2(eta 2-pz)2] with pz = eta (2)-3,5-di-tert-butylpyrazolate have been synthesized; crystallographic data, catalytic activity, and oxo transfer properties are described.

Dimeric μ-oxo bridged molybdenum(VI) dioxo complexes as catalysts in the epoxidation of internal and terminal alkenes

The preparation of the tridentate phenol based amine ligands HL1–HL4 is achieved via a convenient one-pot synthesis by reductive amination in quantitative yield in an autoclave under 7 bar H2 gas. Reaction of [MoO2(acac)2] and the corresponding ligand HLX (X = 1, 2 and 4) in methanol–H2O results in the formation of orange to red dimeric μ-oxo bridged [{MoO2(LX)}2(μ-O)] (X = 1, 2 and 4) complexes 1–3 in high yield and high purity. Complexes 1–3 are stable towards air and water. Both ligands coordinate via the phenolic O atom, the amine N atom and the third donor atom in the side chain (OMe for 1 and NMe2 for 2 and 3) in a fac mode to the metal center. The molybdenum atoms are linked by a bridging μ-oxo moiety to each other as confirmed by X-ray diffraction analyses of complexes 2 and 3. All complexes have been tested in the epoxidation of several internal and terminal alkenes using TBHP as an oxidant. Depending on the nature of the substrate, the epoxides are obtained in moderate to good yields and high selectivities. In the epoxidation of cyclooctene a TOF = 467 h−1 with complex 1 has been observed, significantly higher compared to other dimeric complexes reported in the literature. In the more challenging epoxidation of styrene, complexes 1 and 2 have proven to be highly selective as only the formation of styrene oxide is observed. The OMe based complex 1 has also proven to be more active than the NMe2 based counterparts 2 and 3. The basic conditions induced by the NMe2 groups in complexes 2 and 3 lower their catalytic activity.