Dioxomolybdenum

Serrette, A.G. and Swager, TM. (1994) Polar Superstructures Stabilized by Polymeric Oxometal Units Columnar Liquid Crystals Based on Tapered Dioxomolybdenum Complexes. Angewandte Chemie (International Edition in English), 33, 2342-2345. 

Six-coordinated dioxomolybdenum(VI) complexes of general formula [Mo02(L)2], where L is the anion of a heterocyclic thiosemicarbazone have been prepared and spectrally characterized [216]. These complexes were marginally more active than the uncomplexed thiosemicarbazones against... 

Finally, with the aim of discovering novel chiral oxomolybdenum catalysts able to perform enantioselective alkene epoxidations, Kuhn et al. have reported the exploration of the catalytic behaviour of a series of dioxomolybdenum(VI) complexes with chiral cw-8-phenylthiomenthol ligands derived from ( + )-pulegone. Therefore, the epoxidation of c -p-methylstyrene using t-butyl-hydroperoxide as the oxidant and performed in the presence of ( + )-(2i ,5i )-2-[1-methyl-l-(phenylthio)ethyl]-5-methylcyclohexanone oxime as the ligand, did not produce, however, a significant optical induction in these conditions. 

Chiral dioxomolybdenum complexes were synthesized from (25,4R)-4-hydroxyproline and connected to the surface of USY zeolite by covalent bonding (2 in Fig. 7.4). 

In the presence of the dioxomolybdenum complex M0O2 (dipic) (HMPA) (dipic = pyridine-2,6-dicarboxylate, HMPA = hexamethylphosphoramide), 2-methylhex-2-ene and phenylhydroxylamine produce the amine 194 in 52% yield other alkenes react analogously199. 

Scheme 2 Direct grafting of dioxomolybdenum complexes Mo02X2(THF)2 onto MCM-41 and MCM-48...

Another useful strategy for the confinement of metal centres in molecular sieves is the covalent attachment of coordination complexes via a spacer ligand (tethering). The use of surface-fixed N-donor ligands (Lewis base Hg-ands) coordinating dioxomolybdenum M0O2 moieties has been described mainly by Thiel et al. [22,23,28,29] and Gonsalves et al. [15,24,40]. 

Table 3 Allylic alcohol epoxidation with TBHP/dioxomolybdenum Mo02(acac)(L ) catalysts, where the catalysts were homogeneous or zeolite-Y-supported... 

Scheme 12 Dioxomolybdenum complexes 1-3 tested in several imidazolium-based RTILs for olefin epoxidation...

TBHP and the molybdenum catalysts are soluble in imidazohiun-based RTlLs. The system becomes biphasic when the olefinic substrate is added. In all cases, the TOFs of the catalytic reactions are considerably lower with the ionic solvent than when performed without the ionic solvent (data reported in Table 9). This slower catalytic reaction may be due to dilution effects and phase transfer problems, especially with the olefin, which is quite insoluble in the RTIL. The conversion appears to be strongly temperature-dependent, as decreasing the temperature from 55 °C to 35 °C reduces the conversion by ca. 50% (entries 7 and 8, Table 9). With the dioxomolybdenum complexes 1 and 2, the epoxidation reaction proceeds with 100% selectivity (Table 9), whereas some diol is formed with the catalyst 3. 

L = dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and triphenylphos-phine oxide (OPPh3). The former illustrates how simple polyethers may stabilize the hydrated dioxomolybdenum(VI) bromide core. The others are representative examples of addition compounds with ligands that are able to displace water from the coordination sphere of molybdenum but at the same time are not sufficiently basic toward the proton to cause formation of hydrobromides and molybdates. 

Oxo complexes dominate the chemistry of molybdenum in its higher oxidation states,5 80,92 93 and play a major role in the catalytic activity of molybdenum in biological systems (see Section 36.6.7). MoIV complexes have been characterized containing the monooxo (MoO 2+ and the trans-dioxomolybdenum(IV) Mo02 cores, but no compound has been isolated analogous to the ds-dioxomolybdenum(VI) species described in Section 36.5. Crystal structure determinations have been accomplished for a number of MoIV complexes, and some typical examples are listed in Table 3.80... 

While some molybdenum complexes such as Mo03(dien) were found to be inactive,236 the rates of molybdenum-catalyzed epoxidation of alkenes were found to be independent of the structure of the complex used, after an induction period representing the time for exchange of anionic ligands by the alkyl hydroperoxide. cis-Dioxomolybdenum(VI) diolates such as (78) were isolated... 

Metal oxides can be used as metal sources. A classic example of such syntheses is the preparation of dioxomolybdenum(VI) fe-acetylacetonate in the absence of a solvent (3.80) [11] ... 

A smaller turn structure is induced in an N,N -bis(2,3-dihydroxybenzoate)-substituted lysine derivative on coordination to the Mo(VI)C>2 fragment E. In E the conformation of the lysine residue is highly constrained, because of intramolecular hydrogen bonding in combination with coordination of the catechol units to the dioxomolybdenum(VI) moiety [13, 14]. 

Clipping of the linear derivative 4-H4 to fix the WAG-sequence in a bent conformation proceeds smoothly by addition of 02Mo(acac)2 and K2CO3 in methanol. The ds-dioxomolybdenum(VI) complex K2[4MoC>2] is characterized by spectroscopic methods like NMR or ESI-MS. The results obtained indicate that only one of the two possible stereoisomers is formed. This shows that the amino acids of the WAG sequence are able to induce chirality perfectly at the metal complex unit. The good resolution and signal dispersion in the H NMR spectrum indicate the peptide turn adopts a well defined conformation. From the present results, however, it cannot be deduced if this conformation is the biologically active one [34],... 

A large range of different ionic liquids have been screened in the epoxidation of cyclooctene with dioxomolybdenum(VI) complexes and ferf-butyl hydrogenperoxide as oxidant, as shown in Table 5.2.[32] With the diazabutadiene complex, 48, as catalyst, inferior turnover frequencies were observed relative to the reaction in neat substrate or in dichloromethane and the recycling potential of the catalyst turned out to be only limited. Catalyst immobilisation was better with the cationic tris(methylaminomethyl)ethane complex, 49, however at the expense of selectivity. Of the ionic liquids tested, [C4Ciim][Tf2N] gave the best results for both molybdenum complexes. 

Figure 5.1 Dioxomolybdenum(VI) complexes used in epoxidation reactions...

Pietsch, M. A., and Hall, M. B., 1996, Theoretical studies on models for the oxo-transfer reaction of dioxomolybdenum enzymes, Inorg. Chem. 35 127391278. 

Only one dioxomolybdenum(Vl) monocalixarene complex, (6) (R = Me) has been reported. The dioxomolybde-... 

Hydroxamic acids and their amino acid derivatives formed a series of dioxomolybdenum(VI) complexes of the type cis-[Mo02(hdx)2] and [Mo02(hdxamc)2] (hdxH, (10) hdxamcH, (11)), which are attractive as models for siderophores and for the development of metal-chelating drugs. 

The distorted octahedral oxomolybdenum(V) complexes [MoO(S-S-X-S-S)] were prepared by reaction of [M0CI4 (NCMe)2] with the free pentadentate ligand (HS S X S SH, (28)) or its Ni + complex in wet thf, or by oxidation in air of [Mo(CO)2(S-S-X-S S)]. Related dioxomolybdenum(VI) species [Mo02(S-S-X-S-S)J were also reported. 

Many of these complexes also participated in oxygen-atom transfer reactions, giving dioxomolybdenum(VI) species and reduced substrates, and associated kinetic studies have been... 

Cw-dioxomolybdenum(Vl) complexes of sterically bulky (thiolato)pyridine (41) and bi(thiolato)pyridine (42) ligands were designed to prevent comproportionation and maintain mononuclearity during oxygen-atom transfer reactions. The complex with (41) had a distorted octahedral structure and achieved its designed objective, whereas that with (42) did not, having trigonal bipyramidal structure with axial thiolate donors. 

Hydroxumie acids. Oxidation of N-trimethylsilylamtdes (I, prepared by the reaction of a secondary amide with hexamethyldisilazane) with the MoOvMMPT complex in methylene chloride at room temperature for several hours np to several days affords dioxomolybdenum complexes (2) in moderate yields (15-40%). The free hydroxamic acids (3) are Hbcrated by treatment of (2) with ethylenediaminetetraacetic acid (EDTA). 

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