Applications of Organometallic Chemistry in the Preparation and Processing of Advanced Materials

The sol-gel route to oxide materials is based on inorganic polymerization reactions. A solution of molecular precursors is converted by a chemical reaction into a sol or a gel which on drying and densification give a solid material. This allows the production of multicomponent materials with high purity and potentially greater chemical homogeneity at lower temperature. The molecular design of alkoxide precursors allows a chemical control over condensation reactions and the production of tailored microstructures. Based on soft chemistry, the sol-gel process is highly amenable to incorporating organic molecules and even biological species into oxide matrices. A form of molecular engineering is springing up. It leads to novel hybrid organic-inorganic materials and opens new opportunities to the development of optical devices and chemical sensors.

The objective of this article is to review recent advances in sol-gel chemistry and to identify areas where further developments are likely to occur.

Ecological as well as economical aspects influence the development of chemical processes. To meet the increasing regulations new heterogeneous catalysts are required, which should improve selectivity and utilize cheaper feedstocks. Zero waste processes are a clear goal of the chemical industries [1]. Most “new” catalysts will be based on gradual improvements of known materials, but there is a demand for new catalysts with properties different from those of the known catalytic materials. Among the “new” catalysts of the past that have successfully replaced traditional catalytic materials, zeolites, bimetallic and bifunctional catalysts might be mentioned.

The sol-gel process represents a chemical synthesis route to fabricate oxidic materials like glasses and ceramics. It is characterized that molecular, oligomeric or colloidal systems can be used as precursors. The synthesis reaction is mainly based on a hydro- lysis and condensation reaction. Through the condensation reaction metal oxygen metal bonds are formed leading to a three-dimensional network. While in silica systems in the majority of the cases under acid catalysis a more or less polymeric type of network is built up, in most other cases like alumina, titania, zirconia the first condensation step leads to colloidal systems which may be amorphous or partially crys- talline like in titania. The gel formation takes place by aggregation of colloidal particles or further growth of polymer networks [1].

We recently discovered methods of dissolving silica in ethylene glycol in the pres- ence of alkali and alkaline base to form anionic pentacoordinated or dianionic hexacoordinated silicon complexes. We report here that the use of strong amine bases can be used in catalytic or stoichiometric quantities to form neutral tetracoordinate siloxanes containing only silicon and ethylene glycol or silicon, triethanolamine and ethy- lene glycol (silatrane glycol). We also find that aluminum hydroxide will dissolve under similar conditions to form alumatranes.

M(CO)₆ (M= Mo, W) and (CH₃)₂M (M = Zn, Cd) have been used as precursors in the synthesis of intrazeolite semiconductor clusters of metal oxides and metal chalcogenides, respectively. These nanoclusters show optical and electronic properties different from the parent bulk semiconductors (quantum size effects). In the synthesis of these novel materials, metal carbonyls and organometallics are sublimed from the vapor phase into supercages of zeolite Y where they are anchored to either framework oxygen or extraframework cation sites. The photooxidation of α-cage encapsulated M(CO)6 provides a mild, clean and quantitative synthetic pathway to molecular dimension monomeric molybdenum(VI) oxide and dimeric tungsten(VI) oxide moieties encapsulated within the void structure of zeolite Y. Thermal vacuum treatment of these materials results in a clean reductive elimination of O2 in two distinct steps around 300 and 400°C for W₂O₆ and one distinct step around 300°C for MoO₃ leaving dimeric W₂O₅ intermediate phase, monomeric WO₂ and MoO₂ final phase products, which can be reversibly oxidized in O₂ at 300°C back to starting metal(VI) oxides. Dimethylmetal precursors are anchored to the BrØnsted acid sites in the α-cages of zeolite Y by releasing methane. Exposure of (CH₃M)₄₈Na₈Y to H₂X (X = S, Se) induces a transformation of the MOCVD type precursors into supralattices ofsemiconductor charge balancing [M₆X4]⁴⁺ nanoclusters housed within the diamond network of supercages in the zeolite Y host.

A large number of ceramics can now be produced via the pyrolysis of polymers [1–3]. These new synthetic routes to ceramics are particularly suitable for the production of fibers [4], coatings [5] or reactive amorphous powders for sintering [6]. Two main routes are used to prepare the preceramic polymers, either the sol-gel process to get oxo-polymers, or organo-metallic chemistry to prepare essentially silicon-based polymers such as polysilanes, polycarbosilanes or polysilazanes. Depending on the pyrolysis atmosphere, oxo-polymers can be transformed into oxides, but also into oxycarbides or oxynitrides, while polysilanes, polycarbosilanes and polysilazanes can be converted into silicon carbide, silicon nitride or silicon carbonitride.

Polymethylsilane (PMS) is an excellent precursor for the preparation of thin films of amorphous SiC on useful substrates, such as silicon wafers, quartz plates and polycrystalline alumina plates. The progress of pyrolysis chemistry can be easily monitored by FTIR on films supported on silicon, down to thicknesses as low as 10- 20 nm. The reactivity of PMS, or of polycarbosilane, with ammonia permits the homogeneous N-doping of these films and they exhibit n-type semiconductivity in the range of 10-30 (Ω.cm)^-1. N-doping can also be achieved by using the PMS prepared by Na coupling of MeSiHCl₂. This polymer, which contains residual Si-Cl bonds, reacts readily with ammonia to produce SiNH₃ ⁺Cl^- groups, which lose NH₄C1 on pyrolysis to produce silazane linkages in the product preceramic. Poly(hydrazinomethylsilane) is produced by reaction of MeSiHCl₂ with N₂H₄, in the presence of an HCl trap. This polymer may be pyrolysed to give a silicon carbonitride product with an electronic conductivity in the same range as glassy carbon (10³ -10⁴ (Ω.cm)^-1. Both FTIR and NMR spectrocopies have been used to determine the structure of the polymer and of the series of intermediate materials that are produced by pyrolysis between 100 and 1500°C.

A method based on the sodium copolycondensation of substituted dichlorosilanes with bis(chlorosilyl)methanes or 1,3-dichlorodisilazanes afforded soluble polysilacarbosilanes (PSCS) or polysilasilazanes (PSSZ) which were easily transformed into polycarbosilanes (PCS) or polycarbosilazanes (PCSZ) respectively. This strategy provides an access to SiC and SiCN fibre precursors with variable Si/C/N ratios. The mechanisms involved in the preparation of the monomers and the copolymers were investigated, as well as the thermal conversion of these polymers into preceramic precursors by using PCSZ and N-rich PSSZ models.

Organopolysilanes and -carbosilanes are already used as starting materials for the commercial production of SiC-based fibers. The process, originally derived from the method described by Yajima involves the melt-, wet- or dry- spinning of organosilicon polymers followed by a curing step and subsequent pyrolysis of the polymer fiber providing the ceramic fiber [1,2]. The intermediate curing step results in an increased cross-linking of the polymer and hence in the formation of an infusible polymer fiber necessary for the shape retaining thermal conversion into the ceramic material.

Two different polycarbosilanes that have essentially a “SiH₂CH₂” composition have been examined as potential precursors to stoichiometric silicon carbide. One of these is a high molecular weight, linear polymer with a [SiH₂CH₂]_n repeat unit (PSE). This polymer is the monosilicon analog of polyethylene and was prepared by ring-opening polymerization of tetrachlorodisilacyclobutane, followed by reduction with LiAlH₄. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 0–25 °C. On pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined in this case by using TGA, mass spec., solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The second polymer is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of Cl₃SiCH₂Cl followed by LiAlH₄ reduction. This polymer thermosets on heating at 200–400 °C (or at 100 °C with a catalyst) and gives near stoichiometric SiC in ca. 80% yield on pyrolysis to 1000 °C. Unlike PSE which is difficult to prepare and only available in small quantities at present, HPCS and its allyl and vinyl derivatives (XHPCS, X= allyl or vinyl) are readily available in high yield from a relatively inexpensive starting material. The application of a 5% AHPCS derivative as a source of SiC matrices for SiC-fiber reinforced composites via a Vacuum Polymer Infiltration and Pyrolysis process is described. The resulting composites exhibit higher flexural strengths than comparably reinforced CVI-SiC matrix composites.

Since the early work of Yajima and Verbeck¹, a great deal of attention has been devoted to the polymer route to silicon based materials. Efforts have been made to prepare new polymeric precursors with various elemental compositions, with rheological properties allowing to fabricate them into useful forms and also to obtain a better understanding of the ceramisation processes^2,3. Whatever is the use of a preceramic polymer: fiber, coating, binder, matrices, it must have at least two important properties which are sometimes difficult to obtain simultaneously. First of all, the polymer must have the appropriate rheology for the fabrication step. Secondly, the elimination of volatile gas must be limited because the yield of residue must be as high as possible and because evolution of gas can damage the ceramic material. Most of the time, this is achieved by adjusting the degree of cross-linking of the preceramic polymer⁴

Polymers containing unsaturated organic units bridged by substituted silicon atoms are of actual interest : The electronic effects of silicon attached to unsaturated organic moieties has incited the chemists to synthesise such polymers in order to explore their physical properties : conductivity and non linear optic properties [1] have been reported. The present paper describes the synthesis (conductivity experiments, and transformation into ceramics) of polymers containing disubstituted silicon atoms alternated with bisacetylenic moieties [2].

The use of molecular design for optimised molecular recognition and self-assembly for improved materials for chemical sensing is discussed. Peripheral substitution of phthalocyanines by alkyl, alkoxy, or crownether groups promotes self-assembly and provides control of local polarisability, optimising charge-transfer interaction energies for rapid response and reversal on exposure to nitrogen dioxide. The use of semiconductivity and surface plasmon resonance to monitor these responses, and their modification by humidity, is described. Initial experiments to develop a new generic class of materials for optical chemical sensing by entrapping molecular recognition agents in porous silicate glasses are described. Crown-ether substituted phthalocyanines and a cyclodextrin with a covalently attached pendant fluorophore have been entrapped in sol-gel glasses. Optical absorption changes in the former on exposure to NO₂ and a decrease in fluorescence of the latter on exposure to toluene have been demonstrated.

A macroscopic “Tinkertoy” [1] construction kit consists of rods and connectors that permit the production of nearly arbitrary constructs in the hands of a child. The current state of our attempts to develop a nanoscopic construction kit on a molecular scale, suitable for the synthesis of designer solids, is reviewed.

Matter of nanoscale dimensions often displays properties unlike those of isolated gas-phase molecules, liquids or solids. Sometimes this is due to their bonding, structure and morphology, while in other cases it is because the small dimensions bring with it new phenomena arising from quantum confinement. Work is in progress in our group to investigate the changing properties of matter at different degrees of aggregation to provide a physical basis for interpreting the unique behavior of such systems. Studying cluster materials comprised of metals and metal compounds is a particularly promising avenue of approach to ascertain when discrete molecular properties begin to coalesce and display collective behavior characteristic of solids.

The fullerene family of highly symmetrical carbon molecules (C₆₀, C₇₀, etc.) offers considerable potential for the construction of new solids with novel properties some of which can be predicted, others of which can only be discovered. Since the revelation that these molecules, which were originally only available as exotic gas phase entities [1], can be made and chemically purified in macroscopic amounts [2,3]; there has been a flurry of interest in developing their chemistry. A number of significant aspects of the chemistry of the fullerenes have emerged and are of particular relevance to the chemistry developed in this chapter [4–6]. The shorter bonds at 6:6 ring junctions have olefinic reactivity. Multiple additions to these sites occur readily and frequently reversibly. Because of the high symmetry of these molecules, the distinction between reactive sites is low. Frequently, when multiple additions occur, it is difficult to control the level (or stoichiometry) of addition and the regiochemistry of the addition. Separation of pure derivatives of the fullerenes frequently requires extensive chromatography. Crystallization of the fullerenes and their simple derivatives produces solids in which the fullerenes exhibit orientational disorder. This greatly complicates studies of fullerenes and their derivatives by single crystal X-ray crystallography, a technique which is usually definitive for the characterization of simple organometallic compounds. The fullerenes readily undergo electron transfer reactions. For example, C₆₀ undergoes six reversible reductions [7]. Certain fullerene salts, for example K₃C₆₀, exhibit superconductivity with remarkably high T_c values [8].

The development of new synthetic routes to both oligomers and polymers constructed from inorganic or organometallic units is of considerable interest as a means of preparing materials with unusual and potentially useful properties. In this paper we report the synthesis of oligomeric and polymeric permethylmetallocenes. We also report on their electrochemical properties and the use of these complexes in the synthesis of magnetic charge transfer complexes.

Materials science in the last few years has become more and more interested at nanoscale phases, where the volume-to-surface area ratio of the bulk material rapidly decreases, conferring to the materials novel properties [1]. From the chemical point of view new synthetic techniques are being developed, aiming at obtaining new materials with varied structural, optical, and transport properties. This new branch of chemistry has also found a new name: nanochemistry. As in normal chemistry the object of interest is the molecule, in nanochemistry it is the nanocluster, intended as a large array of atoms kept together by chemical bonds, which is isolated from other identical or similar objects. Often the nanoclusters are incorporated into a processable matrix, and the combination of the two defines a nanocomposite.

Rigid-rod shaped materials are of interest due to their propensity to form liquid crystalline phases. In addition, if these molecules allow electronic communication along their backbone, they can also exhibit linear optical properties such as luminescence as well as second- and/or third-order nonlinear optical properties. We have employed a palladium and copper catalyzed cross-coupling methodology to prepare a wide variety of well defined and well characterized rigid-rod molecules composed of alternating aryl and alkynyl moieties. Using this chemistry, we were able to alter the electronic properties of the molecules not only by altering the aromatic groups, but also by incorporation of selected π-donor or π-acceptor substituents as end groups. This also allowed the synthesis of both symmetric (D/D and A/A) molecules and unsymmetric (D/A) molecules. The latter are of particular interest in terms of their second-order nonlinear optical properties. In addition, we also prepared related symmetric molecules with ethynyl end groups which were subsequently employed in the synthesis of a series of rigid-rod platinum acetylide polymers. The synthetic strategies employed are discussed, as are preliminary studies of the linear and nonlinear optical properties of some of the chromophores, and the phase behavior of some of the symmetric rods.