Molybdenum Tricarbonyl Complexes Research Articles

Molybdenum(0)-Tricarbonyl Complex Supported by an Azacalix-pyridine Ligand: Synthesis, Characterization, Surface Deposition and Conversion to a Molybdenum(VI)-Trioxo Complex with O2.

Adsorption of metal-organic complexes on metallic surfaces to produce well-defined single site catalysts is a novel approach combining the advantages of homogeneous and heterogeneous catalysis. To avoid the "surface trans-effect" a dome-shaped molybdenum(0) tricarbonyl complex supported by an tolylazacalix[3](2,6)pyridine ligand is synthesized. This vacuum-evaporable complex both activates CO and reacts with molecular oxygen (O2) to form a Mo(VI) trioxo complex which in turn is capable of catalytically mediating oxygen transfer. The molybdenum tricarbonyl- and trioxo complexes are investigated in the solid state, in homogeneous solution and on noble metal surfaces (Cu, Au) employing a range of spectroscopic and analytical methods.

Front Cover: Molybdenum Tricarbonyl Complexes Supported by Linear PNP Ligands: Influence of P‐ and N‐Substituents on Relative Stability, Stereoisomerism and on the Activation of Small Molecules (Eur. J. Inorg. Chem. 33/2023)

Front Cover: Molybdenum Tricarbonyl Complexes Supported by Linear PNP Ligands: Influence of <i>P</i>‐ and <i>N</i>‐Substituents on Relative Stability, Stereoisomerism and on the Activation of Small Molecules (Eur. J. Inorg. Chem. 33/2023)

Molybdenum Tricarbonyl Complexes Supported by Linear PNP Ligands: Influence of P‐ and N‐Substituents on Relative Stability, Stereoisomerism and on the Activation of Small Molecules

AbstractSeries of linear tridentate PNPhPR‐ligands (R=Me, Et, Pln, Ph, Cyp, iPr, Cy, tBu) and molybdenum tricarbonyl complexes [Mo(CO)3PNPhPR] (R=Ph, Et, Cyp, iPr, Cy,) were synthesized and characterized using NMR‐, IR‐, and Raman spectroscopy as well as X‐ray crystallography. The influence of the different phosphine donor groups of the PNPhPR ligands on the bonding and activation of CO ligands is investigated. Importantly, all complexes are found to adopt a fac geometry, both in solution and in the solid state. This is in contrast to analogous complexes supported by PNHP ligands. DFT calculations reveal that the phenyl ring at the central amine function is the cause of the preferred geometry, hindering isomerization to a mer geometry.

Influence of a Metal Substrate on Small-Molecule Activation Mediated by a Surface-Adsorbed Complex.

Activating small molecules with transition metal complexes adsorbed on metal surfaces is a novel approach combining aspects of homogeneous and heterogeneous catalysis. In order to study the influence of an Au(111) substrate on the activation of the small-molecule ligand carbon monoxide, a molybdenum tricarbonyl complex containing a PN3 P pincer ligand was synthesized and investigated in the bulk, in solution, and adsorbed on an Au(111) surface. By means of a platform approach, a perpendicular orientation of the molybdenum complex was achieved and confirmed by IRRAS and NEXAFS. By using vibrational spectroscopy (IR, Raman, IRRAS) coupled to DFT calculations, the influence of the metal substrate on the activation of the CO ligands bound to the molybdenum complex was determined. The electron-withdrawing behavior of gold causes an overall shift of the CO stretching vibrations to higher frequencies, which is partly compensated by dynamic charge transfer from the substrate to the molybdenum center, which increases its (dynamic) polarizability.

Desulfurization of liquid fuels by extraction and sulfoxidation using H2O2 and [CpMo(CO)3R] as catalysts

Efficient and recyclable liquid–liquid extraction and catalytic oxidative desulfurization (ECODS) systems for the removal of refractory sulfur compounds from liquid fuels are reported that use the cyclopentadienyl molybdenum tricarbonyl complexes [CpMo(CO)3Me] (1), [CpMo(CO)3(CH2-pC6H4-CO2Me] (2) and [CpMo(CO)3CH2COOH] (3) as catalyst precursors. An ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, was used as both extractant and reaction medium, entrapping the active homogeneous MoVI catalysts that are formed in situ under the operating catalytic conditions (aqueous H2O2 as oxidant, 50 °C). The high sulfoxidation activity of the catalyst formed from 1 was largely responsible for enabling >99% desulfurization within 1 h of a model oil containing 1-benzothiophene, dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene (2000 ppm S). The IL/catalyst phase could be repeatedly recycled with no loss of desulfurization efficiency. By sequentially performing extractive desulfurization and ECODS steps, 83–84% sulfur removal was achieved for untreated real diesel and jet fuel samples with initial sulfur contents of ca. 2300 and 1100 ppm, respectively.

Molybdenum(0) tricarbonyl and tetracarbonyl complexes with a cationic pyrazolylpyridine ligand: synthesis, crystal structures and catalytic performance in olefin epoxidation.

The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)3(ptapzpy)Br] (1) and cis-[Mo(CO)4(ptapzpy)]Br (2) is reported, where ptapzpy = 2-(1-propyltrimethylammonium-3-pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal replacement of CO in Mo(CO)6 (for 1) or substitution under milder conditions of piperidine ligands in the precursor cis-[Mo(CO)4(pip)2] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1 and 2 were determined. Thermal treatment of 2 at 125–150 °C leads to mono decarbonylation and formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-butylhydroperoxide (TBHP, 10 equiv.) gives a molybdenum oxide hybrid material formulated as [Mo3O9([ptapzpy]Br)2]·nH2O (3), which was characterised by FT-IR and Raman spectroscopy, thermogravimetric analysis, and 13C{1H} CP MAS NMR spectroscopy. Compounds 1–3 were effective (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with aqueous H2O2 or TBHP (slightly better results were obtained with the former). The characterisation of the Mo-containing solids isolated after the catalytic reaction showed that poorly soluble β-octamolybdate salts, (L)x[Mo8O26], were formed from 1–3 with TBHP and from 1 with H2O2, while soluble oxoperoxo species were formed from 3 with H2O2. These findings helped to explain the different catalytic performances obtained.

Quantum chemical study of the structures and dynamic behavior of tricarbonyl complexes of Group 6 metals (Cr, Mo, W) with polyaromatic hydrocarbons using the density functional theory

The quantum chemical study of the mechanism was performed for tricarbonyl η6-complexes of coronene I-M and kekulene II-M (M = Cr, Mo, W) by the density functional method. The activation barriers of η6,η6-interring haptotropic rearrangements (IHR), being the migration of the metaltricarbonyl group M(CO)3 from one six-membered aromatic ring to another, were determined. The processes of η6,η6-IHR in the metal tricarbonyl complexes with relatively high polycyclic aromatic hydrocarbons (PAH) I and II occur with close energy barriers (ΔG≠ ≈ 20—25 kcal mol–1), which are lower than the barriers (ΔG≠ ~ 30 kcal mol–1) of similar transformations measured or calculated earlier for the chromium tricarbonyl complexes of naphthalene and its derivatives and other PAH. For the molybdenum tricarbonyl complexes the activation barriers of η6,η6-IHR decrease additionally by ~ 5 kcal mol–1 compared to those for the chromium tricarbonyl complexes, whereas for the tungsten tricarbonyl complexes they increase again and become approximately equal to the activation barriers of similar chromium tricarbonyl complexes. All stationary states on the potential energy surface determining the mechanism of η6,η6-IHR are characterized by a decrease in hapticity compared to the initial and final complexes.

Kinetic studies of fluorinated aryl molybdenum(ii) tricarbonyl precursors in epoxidation catalysis

Benzyl substituted molybdenum tricarbonyl complexes displaying CF3 groups are synthesized and applied as catalyst precursors in olefin epoxidation reactions.

ChemInform Abstract: Catalytic Olefin Epoxidation with a Carboxylic Acid‐Functionalized Cyclopentadienyl Molybdenum Tricarbonyl Complex.

AbstractThe catalyst can be recyled, but its activity decreases significantly in consecutive catalytic cycles.

Catalytic olefin epoxidation with a carboxylic acid-functionalized cyclopentadienyl molybdenum tricarbonyl complex

The complex CpMo(CO)3CH2COOH (1) (Cp = η5-C5H5) has been examined as a precatalyst for the epoxidation of cis-cyclooctene and α-pinene using tert-butylhydroperoxide (TBHP) as oxidant. A high turnover frequency of ca. 600 mol molMo−1 h−1 was achieved in the epoxidation of cyclooctene, giving the epoxide as the only reaction product. With α-pinene as substrate, the added-value products α-pinene oxide and campholenic aldehyde were obtained. Two different approaches to facilitate catalyst recovery and reuse were explored: (1) use of an ionic liquid (IL) as solvent, and (2) intercalation of 1 in a Zn,Al layered double hydroxide (LDH) by a direct synthesis (coprecipitation) route. Characterization of the LDH by powder X-ray diffraction, thermogravimetric analysis, FT-IR and 13C CP MAS NMR spectroscopies showed that the CpMo(CO)3CH2COO− anions intercalate in a bilayer arrangement, resulting in an interlayer spacing of 20.7 Å. In the epoxidation of cyclooctene, catalytic activity in the first batch run was very high for the catalyst/IL mixture and moderate for the LDH. Characterization of the LDH after catalysis indicated that nearly complete oxidative decarbonylation of supported complexes had occurred (by reaction with TBHP), resulting in the presence of immobilized oxomolybdenum species. However, catalytic activities for both the recovered LDH and catalyst/IL decreased in consecutive runs, due in part to progressive removal of active species during either the catalytic reaction (for the LDH) or the solvent extraction/work-up (for the catalyst/IL mixture).

Tris(pyrazolyl)methane molybdenum tricarbonyl complexes as catalyst precursors for olefin epoxidation

The molybdenum tricarbonyl complexes [Mo(CO)3(HC(3,5-Me2pz)3)] (1) and [Mo(CO)3(HC(pz)3)] (2) (HC(3,5-Me2pz)3=tris(3,5-dimethyl-1-pyrazolyl)methane, HC(pz)3=tris(1-pyrazolyl)methane) were obtained in good yields by the microwave-assisted reaction of Mo(CO)6 with the respective organic ligand. Complete oxidative decarbonylation of 1 and 2 was achieved by reaction with excess tert-butylhydroperoxide (TBHP) in 1,2-dichloroethane at 55°C. For complex 1, the (μ2-oxo)bis[dioxomolybdenum(VI)] hexamolybdate of composition [{MoO2(HC(3,5-Me2pz)3)}2(μ2-O)][Mo6O19] (3) was obtained in good yield, and its structure was determined by single crystal X-ray diffraction. The compound (4) obtained by oxidative decarbonylation of 2 was not unambiguously identified, but may be chemically analogous to 3. Compounds 1–4 were examined for the first time as homogeneous (pre)catalysts for the epoxidation of olefins with TBHP, using different types of cosolvents at 55°C. During the catalytic reactions 1 and 2 transform in situ into 3 and 4, respectively, and the latter two are fairly stable catalysts. Catalytic tests and characterization studies of the recovered catalysts were carried out in an attempt to understand the kinetic differences observed between the compounds prepared in situ during the catalytic reaction and those prepared prior to the catalytic reaction, from the same precursor complex.

Synthesis and characterization of CpMo(CO)3(CH2–pC6H4–CO2CH3) and its inclusion compounds with methylated cyclodextrins. Applications in olefin epoxidation catalysis

The cyclopentadienyl molybdenum tricarbonyl complex CpMo(CO)3(CH2–pC6H4–CO2CH3) (1) (Cp = η5-C5H5) and its 1:1 inclusion complexes with heptakis(2,6-di-O-methyl)-β-cyclodextrin (1@DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (1@TRIMEB) were prepared and characterized by elemental and thermogravimetric analyses, powder X-ray diffraction, NMR, FT-IR and Raman spectroscopy. All three compounds were investigated as catalyst precursors for olefin epoxidation using cis-cyclooctene (Cy) as a model substrate, a reaction temperature of 55 °C, and different types of oxidants (tert-butylhydroperoxide in decane (TBHPdec) or in water (TBHPaq), and aqueous hydrogen peroxide). With the aim of facilitating recycling of the homogeneous catalytic systems, different solvent mixtures were tested: biphasic liquid–liquid solvent systems [(water, glycerol, ionic liquid)–(n-hexane, α,α,α-trifluorotoluene (TFT), iso-butyl methyl ketone)], and in some cases no cosolvent was added. Excellent epoxide selectivity (100%) was observed for all tested catalytic systems. An example of an interesting catalytic system is that of 1@TRIMEB/TBHPaq/TFT/H2O since the catalyst can be easily separated in the form of an aqueous solution, by decantation, from the organic phase containing the reaction products and recycled.

Molybdenum(II) Diiodo-Tricarbonyl Complexes Containing Nitrogen Donor Ligands as Catalyst Precursors for the Epoxidation of Methyl Oleate

The molybdenum(II) tricarbonyl complexes [Mo(CO)3I2L n ] (n = 1, L = 2,2′-bipyridine, 4,4′-di-tert-butyl-2,2′-bipyridine; n = 2, L = pyridine, 4-tert-butylpyridine) have been examined as catalyst precursors for the epoxidation of the biorenewable olefin methyl oleate with tert-butylhydroperoxide. In situ oxidative decarbonylation of the precursors gives highly active and selective molybdenum(VI) catalysts, which were identified as the one-dimensional molybdenum oxide/bipyridine polymer [MoO3(2,2′-bipyridine)], octanuclear [Mo8O24(4,4′-di-tert-butyl-2,2′-bipyridine)4], and the pyridinium β-octamolybdates (LH)4[Mo8O26] for L = pyridine or 4-tert-butylpyridine.

Epoxidation of DL-limonene using an indenyl molybdenum(II) tricarbonyl complex as catalyst precursor

Abstract The complex IndMo(CO) 3 Me (Ind = η 5 -C 9 H 7 ) is an effective catalyst precursor for the epoxidation of DL-limonene using the following oxidant solutions: (i) commercial tert -butylhydroperoxide in decane (TBHPdec), (ii) commercial aqueous TBHP (TBHPaq) pre-mixed with limonene (TBHP_lim), or (iii) TBHPaq pre-mixed with 1,2-dichloroethane (TBHPdce); simple pre-drying treatments of the reaction solutions were applied prior to feeding the catalyst precursor to the batch reactor. The best results were found for the efficiently pre-dried reaction system (iii), which gave higher 1,2-epoxy-p-meth-8-ene yield at 35 min/55 °C than (i) (82% and 73% yield, respectively). This approach avoids the undesirable partial oxidation of decane, which would imply costly work-up procedures to remove high boiling point impurities from the epoxides. These results together with studies on the reactivity of different olefins indicate fairly high regioselectivity toward the epoxidation of the internal double bonds.

Structural and bonding aspects of molybdenum tricarbonyl complexes of 2,4,6-tritertiarybutyl-1,3,5-triphosphabenzene, P3C3But3 and some λ3,λ3,λ5- and λ3,λ5,λ5-alkylated derivatives

Abstract The molecular structure of the previously reported compound [Mo(CO)3(η6-P3C3But3)] has been determined by a single-crystal X-ray diffraction study. Syntheses and molecular structures are also described for the structurally related compounds [Mo(CO)3(η5-P3C3But3)(Me)(Bun)], [Mo(CO)3(η5-P3C3But3)(H)(Bun)] and [Mo(CO)3(η4-P3C3But3(Me)(Bun)(H)(O)Li(THF)3]. Density functional calculations at the B3LYP/cc-pVDZ(-PP) and BP86/cc-pVDZ(-PP) levels have been carried out on the above complexes and the nature of the bonding between the different rings and molybdenum is discussed. 31P NMR spectroscopic evidence is presented for the existence of the novel complex [Mo(CO)3(η6-P3C3But3)PtCl2(PEt3)] in which the triphosphabenzene ring acts as an overall 8-electron donor to the two metal centres.

Amino acid-functionalized cyclopentadienyl molybdenum tricarbonyl complex and its use in catalytic olefin epoxidation

The complex [(N-benzyloxycarbonylprolyl)cyclopentadienyl]-Mo(CO) 3 Me was prepared by the reaction of the amino acid-functionalized cyclopentadienyl-lithium reagent with Mo(CO) 6 , and subsequent methylation with CH 3 I. The complex was characterized by FTIR, 1 H and 13 C NMR, and elemental analysis. A single-crystal X-ray diffraction study showed that the complex is chiral, crystallizing in the orthorhombic P 2 1 2 1 2 1 space group with the Flack parameter refining to –0.007(16), which unequivocally confirms the presence of an enantiomerically pure compound. The complex was examined as a catalyst precursor in the liquid-phase epoxidation of trans -β-methylstyrene at 280–330 K, using either tert -butylhydroperoxide ( tert -BuOOH), cumylhydroperoxide or urea-hydrogen peroxide adduct as oxidant, and, optionally, a co-solvent. With chloroform and tert -BuOOH, the catalytic activity surpasses that previously reported for chiral complexes of the type CpMo(CO) 3 X, giving the epoxide isomers ((R,R)-(+) and (S,S)-(−)-1-phenylpropylene oxide) in excellent selectivity, albeit with negligible enantiomeric excess. FTIR spectroscopy showed that the oxidative decarbonylation of the tricarbonyl complex with tert -BuOOH is fast under the reaction conditions used, and that the structural integrity of the amino acid-functionalized cyclopentadienyl group is retained during this process. The use of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate as a solvent for the catalyst generates a biphasic liquid–liquid reaction system that enables, at the end of a catalytic cycle, the separation and reutilization of the catalyst.

β-Cyclodextrin and permethylated β-cyclodextrin inclusion compounds of a cyclopentadienyl molybdenum tricarbonyl complex and their use as cyclooctene epoxidation catalyst precursors

The complex CpMo(CO) 3CH 2CONH 2 was immobilized in plain β-cyclodextrin (β-CD) and permethylated β-CD (TRIMEB), and the resultant inclusion compounds were characterized in the solid-state by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), 13C{ 1H} CP/MAS NMR spectroscopy and FTIR spectroscopy. The non-included tricarbonyl complex can be used directly as a precursor to an active (TOF ca. 250 mol mol Mo - 1 h - 1 ) and selective homogeneous catalyst for the epoxidation of cyclooctene. Under the reaction conditions, rapid oxidative decarbonylation of the complex by the oxidant tert-butyl hydroperoxide ( t-BuOOH) takes place to give the active Mo VI catalyst. The cyclodextrin inclusion compounds were also tested as pre-catalysts for homogeneous or heterogeneous cyclooctene epoxidation. With no additional co-solvent, the TRIMEB inclusion compound was soluble under the reaction conditions and gave rise to catalytic activity similar to that observed for the pure non-included complex. The β-CD adduct was insoluble and exhibited comparatively lower activity, but could be recycled without loss of activity. The catalytic behavior of the two inclusion compounds was also tested in the presence of the ethanol, n-hexane and 1,2-dichloroethane solvents, as well as in a biphasic system comprising water and n-hexane.

Chiral Nonracemic α‐Alkylidene and α‐Silylidene Cyclopentenones from Chiral Allenes Using an Intramolecular Allenic Pauson—Khand‐Type Cycloaddition.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stationary phases composed of sol–gel processed organometallic complexes functionalized with polyethylene glycol

A tripodal phosphine molybdenum tricarbonyl complex has been functionalized with polyethylene glycol spacers of average chain lengths ranging from 0 to 90. Hydrolysable end groups were attached to the spacers via a platinum catalysed hydrosilylation, a Williamson ether synthesis Cl(CH2)3Si(OEt)3 and an addition reaction of OCN(CH2)3Si(OEt)3. The resulting complexes were sol–gel processed by two different methods with varying amounts of Si(OMe)4 and Si(OEt)4 as cocondensation agents. This generated organometallic/inorganic hybrid materials with characteristic features depending on the spacer length and the amount of cocondensation agent. A detailed multinuclear NMR spectroscopic investigation established the structure of the metal complexes in the polymer and revealed the cross-linking and degree of condensation of the materials. Furthermore, relaxation studies gave insight into the dynamics of the metal complexes, within the hybrid polymers. Chemistry in the interphase was performed with selected materials. The molybdenum tricarbonyl fragments were removed by a combined photochemical/oxidative method leaving the tripodal phosphine in the polymer. These polymerised phosphines served as starting materials for new rhodium and iridium complexes as reactive centres in stationary phases.

Chiral nonracemic alpha-alkylidene and alpha-silylidene cyclopentenones from chiral allenes using an intramolecular allenic Pauson-Khand-type cycloaddition.

We have successfully effected a transfer of chirality from a chiral nonracemic allene to an alpha-alkylidene and an alpha-silylidene cyclopentenone. The molybdenum-mediated examples possessing a silyl group on the terminus of the allene show good facial selectivities, but isomerization of the (E)-silylidene cyclopentenone to the (Z)-silylidene cyclopentenone occurs upon purification of these products. Alternatively, an alkyl group on the terminus of the allene undergoes cycloaddition with moderate selectivities but gives products that undergo an isomerization of the (Z)-alkylidene cyclopentenone to the (E)-alkylidene cyclopentenone when exposed to acidic conditions. Thus, erosion of the enantiomeric excesses is observed for one of the two products as a result of this isomerization. The allenic Pauson-Khand-type cycloaddition has also been effected by first isolation the (eta(6)-arene)molybdenum tricarbonyl complex, demonstrating for the first time that this is most likely the active complex in the molybdenum-mediated reactions. Attempts to increase the facial selectivity by increasing the size of the arene moiety resulted in a marginal increase in the selectivity at the expense of the yield. Based upon these results, we have concluded that altering the approach for the preparation of nonracemic alpha-alkylidene cyclopentenones is necessary in order to obtain synthetically useful levels of stereocontrol.