Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements

Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant d_π orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled d_π orbitals. Both ligands readily form bridges that span M? M bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)₃], [CpW(CO)₂], [Co(CO)₃], and CH are related by the isolobal analogy. Thus the compounds [(RO)₃W ? W(OR)₃], [Cp(CO)₂W?W(CO)₂Cp], hypothetical [(CO)₃Co?Co(CO)₃], and HC?CH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W₃(μ₃-CR)(OR′)₉] versus [Co₃(μ₃-CR)(CO)₉] and [W₄(C)(NMe)(OiPr)₁₂] versus [Fe₄(C)(CO)₁₃]—but differ with respect to M? M bonding. The carbonyl clusters use e_g-type orbitals for M? M bonding whereas the alkoxide clusters employ t_2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or M? M bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M₂(OR)₆] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.     

Transition Metal Polysulfides: Coordination Compounds with Purely Inorganic Chelate Ligands

It has been long known that certain transition metal sulfides dissolve in aqueous ammonium polysulfide. Although it was assumed that they thereby formed metal polysulfides, attention was first paid to such compounds only a few years ago. In recent years a host of new complexes with polysulfido chelate ligands have been isolated and characterized. The complexes are of interest not only regarding their structure and reactivity but also in view of their potential uses; they can be used for the directed preparation of sulfur rings of a given size, and there are indications that they will find applications in catalysis.     

Siloxane Compounds of the Transition Metals

Heterosiloxanes of transition metals contain the characteristic grouping Si? O? X; in the compounds known so far, X may be Au, Zn, Cd, Hg, Ti, Zr, Hf, V, Nb, Ta, Cr, U, Re, Fe, Co, Ni, or Pt. Most of these compounds are obtained from silanols, metal silanolates, acyloxysilanes, disiloxanes, or alkoxysilanes, i.e. from compounds that already contain an Si? O bond. The transition metal may be introduced e.g. as a halide, as an organometailic compound, as an alkoxy compound, or as an oxide. Heterosiloxanes are also formed on reaction of many silicon compounds that do not contain oxygen with transition metal compounds containing oxygen. Some of these compounds, e.g. some titanosiloxanes, are very stable, whereas others decompose explosively. Oligomeric and polymeric heterosiloxanes exist in addition to the monomeric compounds.     

Some Recent Developments in the Chemistry of Transition Metal Oxocations

The range of stability of transition metal oxocations in aqueous solution is discussed. Species with a metal-oxygen bond of order higher than unity are particularly interesting. Bond orders can sometimes be determined from bond lengths and angles, but generally only by infrared spectroscopy. – The frequency range of the metal-oxygen stretching vibration, previously assumed as about 900–1100 cm^?1, must according to these investigations be extended at least to 780 cm^?1 if all multiple bonds, particularly those in trans-dioxo complexes, are to be included. Broad bands down to 650 cm^?1 are ascribed to metal-oxygen bridges. — The existence of cations such as ZrO²⁺ and VO, or of the “well-known” titanyl ion, TiO²⁺, has never been proved. In many cases polynuclear, oxygen-bridged oxometal species must be assumed.     

Trinuclear Clusters of the Early Transition Elements

Trinuclear clusters of the early transition elements, i.e. those elements situated on the left-hand side of the transition series, represent the simplest types of clusters. They characteristically have a pronounced formation tendency and high stability; they are therefore produced under a wide variety of conditions and their triangular M₃ skeleton is conserved in ligand exchange reactions. These clusters play a very important role in the chemistry of the respective elements, and especially those of the 4 d and 5 d series. Biochemical implications are of interest in connection with Mo^IV. This progress report presents a systematic account of the chemistry and the molecular and electronic structure of such compounds. A connection is also established with the crystal field treatment of mononuclear metal complexes.     

Magnetic Anisotropy of Dichlorobis(η5‐cyclopentadienyl) Complexes of Vanadium,Niobium, and Tantalum

Abstract. The magnetic behavior of the mononuclear nd¹ systems MCp₂Cl₂ (M = V⁴⁺[3d¹], Nb⁴⁺[4d¹], Ta⁴⁺[5d¹], space group P2₁/c, pseudosymmetry of the molecules C_2v) deviates from pure single ion spin magnetism on account of ligand field effect (H_lf), spin‐orbit coupling (H_so), and intermolecular spin‐spin exchange interactions (H_ex). For both VCp₂Cl₂ and NbCp₂Cl₂ excellent adaptations to the measured susceptibility data were obtained (2 K ≤ T ≤ 300 K) on the basis of spectroscopic data (lf, so) and cooperative metal–metal interactions (ex) of antiferromagnetic nature [molecular field model (mf)]. For TaCp₂Cl₂ experimental term structure data are not available. Therefore, Jørgensen's spectroscopical series (g‐factor of the central ion) was applied to extrapolate the data set for TaCp₂Cl₂. H_lf, H_so, and H_ex (antiferromagnetic) increase in the order 3d¹ → 4d¹ → 5d¹ leading, with rising atomic number of the metals, to a distinct enhancement of the magnetic anisotropy. At 4 K the μ_eff components μ_eff,y (oriented perpendicular to the cg–M–cg plane; “cg” = center of gravity of the Cp ring), μ_eff,z (oriented along the twofold pseudoaxis), and μ_eff,x are 1.73, 1.69, 1.68 (V), 1.73, 1.62, 1.59 (Nb), and 1.71, 1.59, 1.49 (Ta). While μ_eff,y is independent of T, both μ_eff,z and μ_eff,x decrease with decreasing T.     

Synthesis,Structural Characterization and Preliminary Biological Studies of Several Heterocyclic Transition Metal Carbonyl Complexes

The reaction of molybdenum, tungsten and manganese carbonyls with several thiazole heterocycle ligands yielded a number of coordinated transition metal complexes 1 – 10 . Of these complexes 1 – 6 are new compounds which have not been reported to date. The structures of new compounds were characterized by FT‐IR and ¹H‐NMR spectroscopy as well as single‐crystal X‐ray diffraction analysis. Complexes 1 – 10 are carbon monoxide releasing molecules that show structure‐related anti‐cancer activity. The cytotoxicity of all compounds on Hela cells was evaluated by MTT assay, and the results show that carbon monoxide releasing molecules containing such Schiff base ligands may have biomedical applications for their anti‐tumor effect.     

The Catalytic Activity of Transition Metal Compounds in the Liquid-Phase Oxidation of Hydrocarbons

The way in which dissolved transition metal compounds affect the liquid-phase oxidation of hydrocarbons is demonstrated for compounds of cobalt, copper, and manganese. Kinetic investigations on model systems and comparisons with the non-catalyzed reaction show that the catalysts participate in all the oxidation steps.     

Charting No-Man's Land in d0 Transition Metal Six-Coordination: Structure Predictions for the Complexes [WCl5CH3], [WCl4(CH3)2], and [WCl3(CH3)3]

Unusual structures between octahedral and trigonal-prismatic are found by density functional calculations for the “simple” six-coordinate d⁰ title complexes. While a distorted octahedron is still slightly lower in energy than a prismatic structure for [WCl₅CH₃], intermediate structures start to appear for [WCl₄(CH₃)₂] (see picture). In the case of [WCl₃(CH₃)₃], prismatic arrangements are already more favorable. These and many related compounds should be accessible experimentally.     

Oxidation of Alkenes and Sulfides with Transition Metal Catalysts

Alkenes and sulfides were oxidized with transition-metal catalysts. The oxidant sources include molecular dioxygen, air and iodosylbenzene. The metal ions Mn(III), Fe(III), Co(II) and Ni(II) were used. The Catalysts 1-18 of 1,3-dioxo-, β-ketoimine- or salen-types were prepared and their efficacy was examined. 1,2-Dihydronaphthalene is most efficiently epoxidized with O₂/Me₂CHCHO or PhIO in the presence of Mn(III)-salen catalysts. The Ni(II)-, Co(II)- and Fe(III)-catalysts of either β-ketoimine- or salen-types are useful for epoxidation of styrene, (E)-stilbene and (E)-benzalacetone in the O₂/Me₂HCHO system; these epoxidations are stereospecific without formation of corresponding diastereomeric epoxides. Oxidation of methyl p-tolyl sulfide with O₂/Me₂CHCHO is facilitated by the 1,3-dioxo-catalyst Co(II)-1. Monooxidation is achieved with Me₂CHCHO in equimolar proportions to give the corresponding sulfoxide, whereas overoxidation is realized with excess Me₂CHCHO to give the sulfone. These epoxidation and sulfide oxidations all occur at 25 °C and are complete in less than a day.     

Influence of Ligands on the Activity and Specificity of Soluble Transition Metal Catalysts

The reaction of a covalently σ-bonded ligand (alkyl group, hydrogen) with a molecule (olefin, CO) attached to the transition metal by a coordinate bond is the essence of many catalytic processes. The influence of the other ligands in the complex on this reaction is discussed in the present article. Various ways in which the ligands can act, e.g. by electronic effects via the σ and π electron systems of the complex and by steric effects, are first described separately and then illustrated by examples.     

Tri(mesityloxy)silanethiol – The First Structurally Characterized Organoxysilanethiol (Contributions to the Chemistry of Silicon–Sulfur Compounds. 77 [1])

Tri(mesityloxy)silanethiol (TMST) was isolated as the only product of the reaction between SiS₂ and 2,4,6‐trimethylphenol. TMST crystallizes in the triclinic system. Good quality of the crystal allowed the unrestricted refinement of the mercapto group; the resulting S–H distance is 1.29(4) Å and the Si–S–H bond angle is 95.4(17)°. Molecules of TMST show no hydrogen bonds in the crystal – the FT‐IR spectrum of the solid sample exhibits a very sharp, well‐resolved band of isolated –SH group at 2562 cm^–1.     

Synthesis of Nitrido(phthalocyaninato)metal(V) Complexes RnPcMN (M = Re,Mo, W) and New Compounds with Nitrido‐Bridges between Rhenium and Elements of the Third Main Group

New tetra‐ and octasubstituted nitrido(phthalocyaninato)metal(V) complexes R_nPcMN (M = Re, Mo, W) were synthesized to obtain soluble nitrido‐bridged phthalocyanines. Phthalocyanines with nitrido bridges between rhenium and boron, aluminium, gallium and indium, respectively, were synthesized from nitrido(tetra‐tert.‐butylphthalocyaninato)rhenium(V) complex, tBu₄PcReN and suitable electrophilic reagents like BCl₃, B(C₆F₅)₃, BPh₃, BEt₃, AlCl₃, GaCl₃, GaBr₃, InCl₃, etc. The nitrido‐bridged compounds prepared show different stabilities depending on the substituents at the boron atom. Additionally, the possibility to increase the nucleophilicity of (C₅H₁₁)₈PcWN by reducing this complex with C₈K was studied. The reaction of the reduced complex with electrophiles, e.g. with tBuMeSiCl, Ph₃SiCl and Me₃GeCl indicates the formation of nitrogen‐bridged complexes.     

Mixed Ligands from Group 15/16 Elements: Interface of Solid-State and Molecular Chemistry

A wide range of chemical compounds is spanned by heteroatomic ligands from Group 15/16 elements, which in Nature extend from AsS (in realgar) to [AsS₂⁻]_∞ (in sulfosalt minerals). The stabilization of labile molecules or those that do not exist in the free state by incorporation into transition metal complexes like [Cp^*₂Fe₂(AsSe)₂] ( 1 ) and the oligomerization of molecular units under the influence of metal ions or complexes to form solids or hybrid clusters with inorganic cores and peripheral organometallic ligands are the main subjects of this article. Cp*=C₅Me₅.     

Development of a new electrochemical hydride generator with tungsten wire cathode for the determination of As and Sb by atomic fluorescence spectrometry

A novel electrochemical hydride generator has been developed for the determination of As and Sb. This newly devised hydride generator is constructed from a flowing electrolytic cell, in which the tungsten wire is selected as cathode. Compared with some cathode material usually used in electrochemical hydride generator, the tungsten cathode is of better interference tolerance, corrosion-resistant and longer working time. The characteristics of the cathode material, hydride generating efficiency and interferences of concomitant have been studied in detail. The detection limits (3σ) of As and Sb in sample solution were 0.10 μg L⁻¹ and 0.15 μg L⁻¹, the precisions for 11 replicate measurements of 20 μg L⁻¹ As and Sb were 1.3% and 1.7%. The electrochemical hydride generator coupled with atomic fluorescence spectrometry has been applied to the determination of total As and Sb in tobacco samples.     

Addition of Transition Metals to Improve Physico-Mechanical Properties of Radiation-Vulcanized Natural Rubber Latex Films

The effect of transition metals as a promoter of physico-mechanical properties of radiation-vulcanized natural rubber latex (RVNRL) films was investigated. RVNRL films were prepared by the addition of transition metals (Fe, Mn, etc.) of different concentrations (0–30 ppm) in natural rubber (NR) latex and irradiated with various radiation doses (0–20 kGy). The concentrations of metal ions and radiation doses were optimized and found to be 20 ppm and 12 kGy, respectively. Tensile strength, tear strength, and cross-linking density of the irradiated rubber films were increased with increasing concentration of metal ions as well as radiation doses. The mechanical properties of the films were enhanced by approximately 20% at the optimum conditions. In contrast, elongation at break, permanent set, and swelling ratio of the films were decreased under the same conditions. The comparative effect of metal ions can be explained by Fajan's rules, reported in this article.