Inorganic Syntheses, Volume 36

• Publisher: Wiley
• Year: 2014
• ISBN:
  9781118744840
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

This volume of Inorganic Syntheses spans the preparations of wide range of important inorganic, organometallic and solid-state compounds. The volume is divided into 6 chapters. The first chapter contains the syntheses of some key early transition metal halide clusters and the very useful mononuclear molybdenum(III) synthon, MoCl 3 (THF) 3. Chapter 2 covers the synthesis of a number of cyclopentadienyl compounds, including a novel route to sodium and potassium cyclopentadienide, MC 5 H 5. Chapter 3 details synthetic procedures for a range of metal-metal bonded compounds, including several with metal-metal multiple bonds. Chapter 4 contains procedures for a range of early and late transition metal compounds, each a useful synthon for further synthetic elaboration. Chapter 5 deals with the synthesis of a number of main group compounds and ligands, while Chapter 6 covers teaching laboratory experiments.

Chapter One
Transition Metal Halide Compounds

1. Octahedral Hexatantalum Halide Clusters

Submitted by Thirumalai Duraisamy,¹ Daniel N. T. Hay,¹ and Louis Messerle¹
Checked by Abdessadek Lachgar²

Octahedral hexatantalum halide clusters usually exist as extended structures of the form Ta₆(μ-X)₁₂X₂ with terminal (outer) and bridging (inner) halogen atoms shared between clusters, or as discrete clusters such as Ta₆(μ-X)₁₂X₂·8H₂O that are better formulated as Ta₆(μ-X)₁₂X₂(OH₂)₄·4H₂O. These clusters consist of six tantalums linked through Ta—Ta bonding to form a Ta₆ octahedron with a halide bridge along each of the 12 octahedral edges and one terminal ligand (halide, water, etc.) located apically on each tantalum.¹ A range of cluster oxidation states have been reported.²

Ta₆Cl₁₄ was first reported in 1907 from the reduction of Ta₂Cl₁₀ (denoted as TaCl₅ hereafter) with sodium amalgam,³ and its structure was determined in 1950.⁴ It is prepared typically by high-temperature, solid-state reduction of TaCl₅ in vacuum-sealed quartz ampules.⁵ Microwave heating has also been employed.⁶ Extraction with large volumes of water gives good yields of the discrete cluster⁷ Ta₆(μ-Cl)₁₂Cl₂(OH₂)₄·4H₂O after aqueous reduction of oxidized cluster contaminants with SnCl₂. The most commonly used approach is that developed by Koknat et al., involving reduction at 700°C of TaCl₅ with a four-fold excess of Ta powder.⁸

Ta₆Br₁₄ was first prepared in 1910 by sodium amalgam reduction of TaBr₅.^3b It has since been prepared by using the reductants aluminum^5a and excess tantalum^8a and can be isolated by aqueous extraction as the discrete cluster Ta₆(μ-Br)₁₂Br₂(OH₂)₄·4H₂O.2^a,b A sample was structurally characterized as [Ta₆(μ-Br)₁₂(OH₂)₆](OH)Br·4H₂O,9 and another structure of the hexaaquo ion [Ta₆(μ-Br)₁₂(OH₂)₆]²⁺ was recently reported.¹⁰

There is considerable interest in the coordination¹¹ and catalytic¹² chemistries of these discrete clusters. Because of its high electron count, the hexaaquo ion [Ta₆(μ-Br)₁₂(OH₂)₆]²⁺ has been used frequently for phase determination^9,13 of isomorphous protein derivatives by SIR, MIR, SIRAS/MIRAS, and SAD/MAD methods in biomacromolecular crystallography. This use is growing as larger biomacromolecular structures and assemblies (e.g., membrane proteins, ribosomes, proteasomes) are studied.

We have found that the main group metal and metalloid reductants mercury, bismuth, and antimony are highly effective¹⁴ in reducing WCl₆ or MoCl₅ at surprisingly lower temperatures than commonly used in the solid-state synthesis of early transition metal cluster halides. Borosilicate ampules can be substituted for the more expensive and less easily sealed quartz ampules at these lower temperatures, and the metals and metalloids are not as impacted by oxide coatings that inhibit solid-state reactions with more active metals. These lower temperatures may allow access to kinetic products, such as trinuclear clusters, instead of thermodynamic products.

We report here an extension of this reduction methodology to the convenient preparation¹⁵ of Ta₆(μ-X)₁₂X₂(OH₂)₄·4H₂O by reduction of TaX₅ with gallium dichloride, Ga⁺GaCl₄⁻ (for X = Cl), or gallium (for X = Br). Gallium dichloride has not been used as a preparative-scale reductant in transition metal chemistry. Gallium is an effective reductant, but because of its tendency to agglomerate and to adhere to glass, reductions employing Ga need to be agitated several times during the course of the reaction in order to optimize yields by homogenization of reactants. We have not yet tested the use of gallium dibromide as a reductant for TaBr₅, but expect that it would eliminate the need to homogenize reactants in gallium-based reductions and might improve the yield. The aquated hexatantalum clusters are liberated from the solid-state products by Soxhlet extraction with water, which greatly simplifies the isolation procedure. We also describe the straightforward preparation of a tetraalkylammon...