Group VI Metal Research Articles

ChemInform Abstract: New Organometallic Derivatives of 1,6-Methano(10)annulene.

The reverse addition of 2-lithio-1,6-methanol[10]annulene (2-lithio-1,6-methanocyclodecapentaene) to some group VI metal crabony compounds followed by alkylation with meerwein salts gave the E. O. Fischer transition metal complexes in good to excellent yields. Cyclization of the chromium complex with diphenylacetylene is achieved in moderate yield

Diphosphine Compounds, Part III: UV/Visible Spectroscopy and Novel Routes to Functionalized Diphosphine-M(CO)6 Complexes (M = W, Mo, or Cr)

Reaction of coordinated (diphenylphosphino)methane and ketones or aldehydes have been characterized by 31P{H1}-NMR, 1H{31P}-NMR, and UV/vis spectroscopy in dichloromethane. Group VI metals hexacarbonyl [M(CO)6 where M = Cr, Mo, and W] reacted with (diphenylphosphino)methane, [(Ph2P)2CH2], to give [(OC)4M{(Ph2P)2CH2}] depending upon the reaction conditions. Condensation of [(CO)4M{(Ph2P)2CH2}] with different ketones or aldehydes forms [(CO)4M{(Ph2P)2C = CR1R2}]. Complexes of the types [(OC)4M{(Ph2P)2C = CR1R2}] reacted with hydrazine in a Michael addition to give [(CO)4M{(Ph2P)2CHC(R1R2)NHNH2}](1.3a–e), which condensed with different ketones and aldehydes to give complex of the type [(CO)4M{(Ph2P)2CHC(R1R2)NHN = C(R3)] (1.4a–e). The structures of the complexes are discussed on the basis of elemental analysis (EA), IR,1H-NMR, 31P-NMR spectroscopic data, and FAB mass spectra. The UV/vis spectra show two absorption bands with the low energy band moving to lower energy with increasing substitution on the (diphenylphosphino) methane (dppm) (a bathochromic effect).

Properties of bcc metals by tight-binding total energy simulations

Monovacancies of six bcc d-transition metals V, Cr, Nb, Mo, Ta and W have been studied by tight-binding (TB) simulations using the NRL-TB method. This method satisfactorily reproduces the electronic properties, phonon frequencies, thermal expansion coefficients and atomic mean-squared displacements for pure bulk systems. The TB method shows that for group-V metals the atoms in the first neighbor shell of the vacancy relax inward by about 5% and lose charge from d orbitals, while second neighbor shell atoms relax outward by about 1%. For group-VI metals both first and second neighbor shells relax inward and gain d orbital charge, and for Mo and W the relaxation is in fact stronger for the second than the first neighbor shell. These results are not significantly affected by the use of charge self-consistent terms in the TB model. The structural and charge transfer changes are accompanied by new energy bands at selected k-points for group-VI while they cover the whole Brillouin zone (BZ) for the group-V metals. Comparison to first-principles calculations for one example, Nb, shows good agreement for the presence of new energy bands as well as the charge and structural relaxation of the first and second neighbor shells.

Effect of combining the metals of group VI supported on H-ZSM-5 zeolite as catalysts for non-oxidative conversion of natural gas to petrochemicals

The most prestigious catalyst applied in natural gas (methane) non-oxidative conversion to petrochemicals is 6%Mo/H-ZSM-5. Chromium, molybdenum and tungsten are the group VI metals. Hence, in this work, 6%Mo/H-ZSM-5 was correlated with 3%Cr+3%Mo/H-ZSM-5 and 3%W+3%Mo/H-ZSM-5 as catalysts to examine their promoting or inhibiting effects on the various reactions taking place during methane conversion. The catalytic activities of these catalysts were tested in a continuous flow fixed bed reactor at 700 °C and a GHSV of 1500 ml·g. −1·h −1 Characterization of the catalysts using XRD, TGA and TPD were investigated. XRD and NH 3-TPD showed greater interaction between the W-phase and the Bronsted acid sites in the channels of the zeolite than between Cr-phase and the acid sites in the zeolite.

A new synthetic route to Lindqvist type clusters [(n-Bu4N)x][M′M5O19] [when x = 2, M′ = M = Mo or W; x = 3, M′ = Mo, M = W] from metal carbonyl precursors [(CO)5ML] [M = Mo, W; L= CO, C(OMe)(Me)

Two fully oxidized [(n-Bu4N)2][Mo6O19], 7 and [(n-Bu4N)2][W6O19], 8 and one mixed-metal, mixed-valence one electron reduced [(n-Bu4N)3][MoW5O19], 9 cluster have been synthesized under biphasic reaction conditions from group VI metal carbonyls or Fischer carbene complexes, [(CO)5M=C(OMe)Me] (M = Mo, W) in yields of 73, 77 and 71% respectively. All the clusters have been characterized by IR, 1H and 13C NMR spectra, and crystal structure determinations. Additionally, the composition of 9 has also been supported by mass spectrometry (MALDI-TOF), inductively coupled plasma (ICP) and energy-dispersive X-ray (EDX) analysis. This novel and mild route offers distinct improvements over earlier methods of synthesis for Lindqvist type mixed-valence mixed-metal polyoxometalate cluster.

Group VI metal aminoborylene complex-catalyzed demercuration reactions of bis(alkynyl)mercurials

The first catalytic application of the Group VI metal borylene complexes [(CO)(5)M[double bond]BN(SiMe(3))(2)] involves the demercuration reaction of bis(alkynyl)mercurials, [Hg(C[triple bond]CR)(2)], with formation of a series of buta-1,3-diynes.

Solid solution softening and hardening in the group-V and group-VI bcc transition metals alloys: First principles calculations and atomistic modeling

The interaction of $d$ transition metal additions with dislocations was studied in bcc alloys by using the atomic row model with ab initio parametrization of the interatomic interactions. Opposite trends were obtained for the solute-dislocation interactions in the group-V (Nb, Ta) and group-VI (Mo, W) bcc metals: while additions with extra valence electrons enhance the double-kink nucleation and result in solid solution softening in the group-VI metals, they cannot cause the intrinsic softening in the group-V metals. Oppositely, additions with fewer valence electrons should lead to an intrinsic softening in the group-V metals and a strong hardening in the group-VI metals. The effect of electronic concentration on the solute-dislocation interaction is less pronounced in the group-V alloys, and other mechanisms may be dominant in their softening and hardening behavior.

Electrical properties of BaTi1−x M x O3 (M = Nb, Ta, Mo, W) ceramics

The electrical properties of BaTi1−x M x O3 (M = Nb, Ta, Mo, W) ceramics (partial substitution of Group V (Nb, Ta) or VI (Mo, W) metals on the titanium site) have been studied by impedance spectroscopy. The results indicate that the Group VI metals (Mo and W) are less effective as donor dopants of barium titanate than are Nb and Ta because most of the Mo and W ions reside in the outer layer of the grains, raising its resistance.

(2-(Dimethylammonium)ethyl)cyclopentadienyltricarbonylmetalates: Group VI Metal Zwitterions. Attenuation of the Brønsted Basicity and Nucleophilicity of Formally Anionic Metal Centers

Protonation of (2-(dimethylamino)ethyl)cyclopentadienyl (CpN) group VI metal carbonyl anions with acetic acid proceeds at the amine to provide zwitterionic (2-(dimethylammonium)ethyl)cyclopentadienyltricarbonylmetalates, M(CO)3(η5-CpNH) (M = Cr (1), Mo (2), W (3)). Few zwitterionic organometalates with anionic metal centers and positive charges that cannot be delocalized have been reported to date. The impact of the intramolecular charge separation on the structures and reactivity of zwitterionic organometalates is of current interest. The M(CO)3 units of structurally characterized 1−3 are significantly perturbed in solution and the solid state due to ion-pairing between the pendant ammonium cation and anionic metal center. The concentration and solvent dependence of the 1H NMR ammonium hydrogen chemical shifts of 1−3 and the intramolecular N···M separations of these zwitterions in the solid state are consistent with three-center−four-electron N−H···M hydrogen bonding. One other hydrogen bond with a zerov...

(2-(Dimethylamino)ethyl)cyclopentadienyl Group VI Metal Carbonyl Anions and Divalent Tin(IV) Derivatives

Salts of (2-(dimethylamino)ethyl)cyclopentadienyl (CpN) group VI metal carbonyl anions ([M(CO)3(η5-CpN)]- (M = Cr, Mo, W)), important reagents for studies involving CpN, have been characterized by ...

Electrochemistry of Group VI Metal Carbonyl Compounds Containing 1,1‘-Bis(diphenylphosphino)ferrocene

The oxidative electrochemistry of a variety of homo- and bimetallic group VI metal carbonyl compounds containing 1,1‘-bis(diphenylphosphino)ferrocene (dppf) with the general formulas M(CO)5(dppf) (1a−c), M(CO)4(dppf) (2a−c), and (CO)5M(dppf)M‘(CO)5 (3a−f) (M, M‘ = Cr, Mo, W) was investigated. The effect of the group VI metal and the coordination mode of the dppf ligand on the potential at which the oxidation of the group VI metals and the ferrocene backbone of dppf occurred was examined.

Structural Effects of Group VI Metal Tricarbonyl Binding to Benzenoid Rings: Interruption of Conjugation or Enhanced Aromaticity?

AbstractThe effect of tricarbonyl (group VI metal) complexation on the geometric aromatic character of benzenoid rings is studied as a function of bond‐length alternation (localization) in the parent arene. Good agreement between theory and experiment is established for (η6‐benzene) tricarbonylchromium, ‐molybdenum, and ‐tungsten. It is found that, whereas the electrons of benzene become slightly more localized upon tricarbonyl metal complexation, those of ‘cyclohexatriene' mimics, like in‐starphenylene, become more delocalized. A combination of ab initio quantum‐mechanical and high‐accuracy X‐ray methods leads to a linear structurestructure correlation between the free and metal‐bound arene bond‐alternation geometry. In all cases, the average bond length in the arene increases upon complexation. The computational observation that the average bond length increases more in benzene complexes than in in‐starphenylene implies stronger back bonding in the benzene complexes and coincides with the experimental observation that more‐delocalized arenes form thermodynamically favored complexes. The rotational barriers about the tricarbonylmetal‐to‐arene axis were computed for 1‐Cr, 1‐Mo, and 1‐W as well as for 5‐Cr, 5‐Mo, and 5‐W. Barriers for the former group are characteristically low, almost negligible (0.05 kcal/mol for 1‐Cr; 0.01 kcal/mol for 1‐Mo; 0.27 kcal/mol for 1‐W), whereas for the latter group they are substantial (11.2 kcal/mol for 5‐Cr; 15.2 kcal/mol for 5‐Mo; 13.6 kcal/mol for 5‐W). The higher barriers found in 5‐M compounds are consistent with previous findings.

Preparation and characterization of alumina supported Pd-W and Pd-Mo catalysts photogenerated from group VI metal carbonyls

Tungsten and molybdenum were introduced in γ-Al2O3 and Pd/γ-Al2O3 using metalcarbonyls [W(CO)6 and Mo(CO)6] photoactivation. DRIFTS FT-IR spectra of the surfaces prior to thermal decomposition reveal the presence of unstable metalsubcarbonyl species, in contrast with the ones found previously, when the photoactivation was performed in the presence of triphenylphosphine. The ν(CO) band intensities did not reflect the tungsten and molybdenum uptakes determined by AAS, the loadings being higher in the presence of palladium. After thermal treatment, the catalysts were further characterized by hydrogen chemisorption and FT-IR spectra of adsorbed CO and NO. The results showed that a reaction between the hexacarbonyls and the supported palladium particles takes place leading to bimetallic catalysts, with clear palladium–tungsten and palladium–molybdenum interactions. Pd+Mo(CO)6⇒uvPd+Mo(CO)6−x⇒Pd-Mo(CO)6−x⇒uvPd-Mo. Both group VI metals strongly modify the chemisorption properties of palladium, while molybdenum seems to be present in a reduced state in close association with palladium.

The catalytic synthesis of thiacrowns from thiiranes by Group VI and VII transition metal carbonyl complexes

The synthesis of thiacrowns by the catalytic ring opening cyclooligomerization of thiirane has been investigated by using some Group VI and Group VII metal carbonyl complexes. The Group VI catalyst precursors were transition metal complexes of the form M(NCCH 3)(PR 3) x (CO) 5− x ( x=0, M=Cr, Mo and W; x=1, M=Cr, R=Ph; M=W, R=Ph, or OC 6H 4– p-Me). For Group VII, the manganese carbonyl cations: [Mn(NCCH 3)(L)(CO) 4] +, L=CO, PPh 3, PMe 2Ph, PEt 3, PBu 3 and P(OMe) 3 were studied. The yields of the thiacrowns were significantly improved by the addition of dialkyl acetylenedicarboxylates to the reactions catalyzed by the Group VI complexes, but these had no beneficial effect for thiacrown formation for the manganese catalysts. The principal thiacrown products are 1,4,7,10-tetrathiacyclododecane (12S4), 1,4,7,10,13-pentathiacyclopentadecane (15S5) and 1,4,7,10,13,16-hexathiacyclohexadecane (18S6). Some of the manganese catalysts also yield the cyclic polydisulfides (SCH 2CH 2S) n ( 1– 4) (where n=2–5), but the principal side product with the manganese catalysts is thiirane homopolymer. The phosphine-substituted catalysts give better yields of thiacrowns than the parent carbonyls in all cases.

Hydrodechlorination of CCl4 over group VI transition metal carbides

Hydrodechlorination of carbon tetrachloride (CCl4) was investigated on group VI metal carbides (WC, W2C, Mo2C) prepared by temperature programmed carburization of WO3 and MoO3 in 10–20% CH4/H2 mixtures. Their catalytic performances have been compared with those of 1wt.% Pt/γ-Al2O3 catalyst. Experiments were carried out at atmospheric pressure in a fixed bed flow reactor at 120 and 250°C with a molar ratio H2/CCl4=12. WC shows the best selectivity towards chloroform (CHCl3), whatever the temperature and particularly at 120°C (selectivity above 99%) while W2C and Mo2C produce tetrachloroethylene in substantial amount. Mo2C and W2C deactivate rapidly with time irrespective of the temperature, whereas WC exhibits a good stability at 120°C. Surface characterization indicates that the deactivation on M2C (M=Mo, W) could be correlated to the formation of a chlorocarbonaceous polymer.

Chelated η2-alkene- and η3-allyl-carbene complexes of late transition metals: structure–reactivity relations and preparative use

Abstract Chelated alkene–carbene complexes of Group VI metals typically undergo intramolecular [2+2]-cycloaddition reactions, whereas the iron analogues and related (η3-allyl)–ironcarbene complexes show a more diverse behaviour. They are amenable to reactions at the carbene carbon atom, at the terminus of the π-ligand and even at the tether depending on intrinsic stereoelectronic parameters as well as external factors like solvent polarity and ‘softness’ of attacking nucleophiles. Cascades with extensive reorganization of the organic ligand leading to oligo-enynes, δ-lactones and cyclopentenones may ensue. They are discussed together with some new findings allowing a generalized mechanistic description. The chemical and configurational stabilities of planar-chiral alkene–carbene and allyl–carbene complexes of iron, chromium, manganese and cobalt are compared.

Synthesis, Characterization, and Structure Determination of the Heterobimetallic Polyhydride Complexes (C5Me5)Ru(μ-H)3MH3(C5Me5) (M = Mo, W) Containing Group VI and Group VIII Metals

(C5Me5)Ru(μ-H)3MH3(C5Me5) (4, M = Mo; 5, M = W), the first heterobimetallic polyhydride complexes containing ruthenium and group VI metals, were synthesized, and those structures were determined by X-ray studies. The reactions of 4 and 5 with phosphines, amine, and acetylacetone were examined.

ChemInform Abstract: Sulfur-Alkyne Cyclizations for Formation of Dihydrothiophenes and Annulated Thiophenes.

Cycloisomerization of homopropargylic thiols to dihydrothiophenes is promoted by group VI metal carbonyls. Related thiacyclization transformations under basic and radical conditions are also described, including regioselective formation of benzothiophenes from aryl methyl sulfides and alkynes.

Sulfur-Alkyne Cyclizations for Formation of Dihydrothiophenes and Annulated Thiophenes

Cycloisomerization of homopropargylic thiols to dihydrothiophenes is promoted by group VI metal carbonyls. Related thiacyclization transformations under basic and radical conditions are also described, including regioselective formation of benzothiophenes from aryl methyl sulfides and alkynes.

Syntheses, Molecular Structures, and Bonding of Molybdenum and Tungsten Calix[4]arene Imido Complexes

Syntheses, spectroscopic properties, molecular structures, and bonding of novel calix[4]arene imido compounds are described. Treatment of M(NtBu)2(NHtBu)2Ia (M = Mo), Ib (M = W) or M(NMes)2Cl2(dme) IIa (M = Mo), IIb (M = W) (Mes = 2,4,6-Me3-C6H2) with p-tBu-calix[4]arene LH4 affords calix[4]arene metal complexes LM(NR) 1a, b (M = Mo, W; R = tBu) and 2a, b (M = Mo, W; R = Mes). Analytical and spectroscopic data are consistent with monomeric structures for 1 and 2, retaining a local C4v symmetry for the calix[4]arene metal fragment. These complexes are well-suited to bind small molecules like acetonitrile, tBu-isonitrile, or water within their macrocyclic pockets. The spectroscopic data of some inclusion compounds and the crystal structures of LMo(NtBu)(NCMe) 1a(NCMe), LW(NtBu)(OH2) 1b(OH2), LW(NtBu)(CNtBu) 1b(CNtBu), LMo(NMes)(NCMe) 2a(NCMe), and LW(NMes)(NCMe) 2b(NCMe) are reported. All complexes contain a group VI metal imido [M=NR] moiety mounted on the phenoxide rim of the calix[4]arene ligand as well as an incorporated guest molecule within the cavity. Some insights into the structures of complexes of the type L′W(NR′) (L′ = p-H-calix[4]arene; R′ = H, Me) and into bonding in these compounds are provided by density functional theory, applying the B-P86 density functional and an all SVP basis set within the RI-J-DFT approximation. At least one π bond is of importance for calix[4]arene-metal bonding in these compounds. The metal-imido bond can be described as a triple bond. A geometrically optimized minimum structure of L′W(NMe) 4 shows a calix[4]arene ligand only slightly distorted from a local C4v symmetry and an almost linear tungsten-imido moiety.