Introduction - Frontiers in Modern Inorganic Synthetic Chemistry

Introduction - Frontiers in Modern Inorganic Synthetic Chemistry

C H A P T E R 1 Introduction - Frontiers in Modern Inorganic Synthetic Chemistry Ruren Xu Jilin University, China from the abundantly available H2 a...

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C H A P T E R

1 Introduction - Frontiers in Modern Inorganic Synthetic Chemistry Ruren Xu Jilin University, China

from the abundantly available H2 and N2 using osmium as the catalyst. Twenty years later, C. Bosch improved the technique by using inexpensive iron instead of expensive osmium as the catalyst, which laid a solid foundation for the human society to maintain a continued increase in food production to keep up with the human population increase; a major challenge that we have been facing since the past century. Because of their profound contributions to science as well as to the human society, Haber and Bosch received Nobel prizes in chemistry in 1918 and 1931, respectively. Health industry is another area where synthetic chemistry has been playing pivotal roles. Outstanding examples since the mid-twentieth century include the successful syntheses of SAS drug, penicillin, a variety of antibiotics and other medicines, which have substantially improved and continue to improve our overall abilities in treating human diseases and fighting against them. Our ability in producing the three major classes of synthetic materials, namely synthetic fiber, synthetic plastic, and synthetic rubber, has paved the way for many of the recent industrial and agricultural advances. There is no doubt that chemistry, especially synthetic chemistry, has been making considerable contributions to improve the living conditions of the human society. From a scientific perspective, a pool of very large number of new materials created by synthetic chemistry has provided plenty of samples for studying the structureefunction (property) relationships of materials as well as their syntheses, facilitating scientists to study the fundamental chemistry of these materials, which has become a driving force in the recent developments of chemistry and related sciences. For example, the successful preparation of single crystalline silicon and

Synthetic chemistry is at the core of modern chemistry; it provides the most powerful means for chemists to create the material foundation for our envisioned world. Its main objective is to create a large variety of compounds, phases, materials, and ordered chemical systems needed by our rapidly advancing society, going considerably beyond just finding and synthesizing naturally existing compounds. According to recently published studies, over 50 million compounds, naturally existing or not, have been discovered or synthesized, some of which have become indispensable to our daily life. These compounds have provided the basis for many scientific and technological advances in the recent history. In turn, these advances have created rapidly increasing needs for new materials with specific structures and functions, posing challenges to, as well as creating opportunities for synthetic chemists. Specifically, we see increasing needs in this new century, for novel synthesis strategies and techniques, as well as for the related scientific understanding, gearing toward green synthesis, biomimetic synthesis, inorganic synthesis under extreme conditions, and molecular and tectonic engineering of inorganic materials, in efficient, rationally designed and economic manners. We believe that these are among the most essential key elements for the continuing and rapid advancement of science and technology in this new century [1,2]. In the past century, advances in synthetic chemistry have often been the key driving force for the industrial revolutions and birth of new science and technologies; examples of this sort have been numerous [2]. For instance, F. Haber, in the early twentieth century, invented a high-pressure technique to synthesize ammonia, the key ingredient of chemical fertilizers,

Modern Inorganic Synthetic Chemistry, DOI: 10.1016/B978-0-444-53599-3.10001-0

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Copyright Ó 2011 Elsevier B.V. All rights reserved.

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1. INTRODUCTION - FRONTIERS IN MODERN INORGANIC SYNTHETIC CHEMISTRY

numerous semiconductive materials has fueled the emergence of information technology; the production and posttreatment of nuclear fuel of uranium and plutonium, the key to the nuclear technology and safe application, have all been built on chemical technologies with roots in synthetic chemistry. Similar can be said about other high technologies such as laser, nanotech, aviation, and space technology. Without a doubt, the so-called six great technology inventions in the twentieth century would have never materialized without the foundational work by generations of synthetic chemists in the past. The same is true about other technological breakthroughs and growth points in related sciences such as semiconductor, super conduction, cluster, and nanotechnology. Modern inorganic synthetic chemistry, an important branch of synthetic chemistry, has evolved considerably from the traditional synthesis and preparation of inorganic compounds, which now includes the synthesis, assembly, and preparation of supramolecular and highlevel ordered structures in its studies. In recent years, we have been witnessing that an increasingly large number of new inorganic compounds, phases, and complex materials are being synthesized and assembled, having made inorganic synthetic chemistry a key driver for many new scientific and technological developments and advancements. We anticipate that inorganic synthetic chemistry will continue to play equally or more important roles in science as well as in our upcoming life.

1.1. DEVELOPMENT OF NEW SYNTHETIC REACTIONS, SYNTHETIC ROUTES, TECHNOLOGIES AND ASSOCIATED BASIC SCIENTIFIC STUDIES 1.1.1. The Basic Inorganic Compounds This basic class includes covalently bonded molecular compounds, coordination compounds, cluster compounds, metal organic compounds, nonstoichiometric compounds and inorganic polymer, among others.

1.1.2. Inorganics and Materials with Specific Structures Study of inorganic compounds and phases with specific structures is becoming increasingly important as the need for materials with specific properties and functions continues to rise. It is well accepted that the properties and functions of materials are determined by their structures and compositions. More specifically, such properties and functions are often determined by the characteristics of high-level molecular structures

such as those of molecular aggregates, ordered molecular assemblies, and structures in condensed states instead of single molecular structures. Take defects for example, the properties and functions of materials often result from various forms of structural defects in their component compounds or phases in condensed state. A key reason that many complex oxides are being used as popular substrates for functional materials is that they can form many types of structural defects in addition to their many adjustable component elements. Hence, it has become a major topic at the forefront of inorganic chemistry research to study the preparation of solid-state matters with specific structural defects and the associated principles as well as related detection techniques. In addition, the key research topics in today’s inorganic chemistry also include preparation of surfaces and interfaces with specific structures and properties, stacking of layered compounds, preparation of specific polytypes and their intergrowths as well as intercalation structures and low-dimensional structures of inorganic compounds, synthesis and preparation of inorganic compounds with mixed valence complexes and clustered compounds with specific structures, as well as the rapidly emerging and increasingly useful porous compounds with specific channel structures such as microporous crystals, meso- and hierarchical porous materials. Also particularly interesting is the preparation of phases that tend to form distinct structures and are able to form large varieties of distinct structures under extreme synthetic conditions like high or ultrahigh pressures. While a few synthesis examples with the aforementioned characteristics have been reported in the literature, such studies have generally been done in rather ad hoc manners, often accomplished through utilizing the particularity of specific reactions or specific synthesis techniques rather than based on new understanding of a general class of synthesis problems and new synthesis technologies. The latter is clearly more important for the future development of synthetic chemistry.

1.1.3. Inorganics and Materials in Special Aggregate States Another important class of materials are the compounds in special aggregate state, such as in nano state, ultrafine particles, clusters, noncrystalline state, glass state, ceramic, single crystal, and other matters with varying crystalline morphologies such as whisker and fiber. The rapid emergence of nanoscience and technology strongly suggests that different aggregate states of the same matter could exhibit different properties and have different functions. The understanding of this could have substantial implications to the future development of science as well as new functional materials.

BASIC RESEARCH IN SUPPORT OF GREEN SYNTHESIS

1.1.4. Assembly of High-level Ordered Structures There is an emerging class of functional inorganic materials, commonly characterized as being highly ordered supramolecular systems, formed via selfassembly among molecules or molecular aggregates through molecular recognition. The key interaction forces in the formation of such large molecular assemblies are intermolecular non- or weak-bond interactions (van der Waals and hydrogen bond). Examples of such materials include coordination polymers, inorganic polymers, and molecular systems with specific structural features such as nanosystems, capsula, ultrathin membrane (monolayer membrane, multilayer composited membrane), interfaces, two-dimensional layered structures, and three-dimensional biological systems; many of which have been widely used for fabricating high-tech microdevices. Self-assembly is increasingly becoming a key and practical technique in the synthesis and preparation of complex functional systems. It has even been suggested that the introduction of self-assembly-based synthesis techniques could fundamentally advance the chemical production processes that are being widely used in the current industries [2].

1.1.5. Composition, Assembly, and Hybridization of Inorganic Functional Materials The following areas have received considerable attention in recent years: (1) multi-phase composition of materials including enhanced or reinforced fiber- (or whisker-)based materials, the second-phase particle dispersion materials, two- or multi-phase composite materials, inorganic and organic materials, inorganics and metals, and functional gradient materials as well as nanomaterials; and (2) composite material-related hosteguest chemistry, which represents a highly interesting and a very challenging research area. The research focuses include, for example, the assembly of different types of chemical entities in hosts with microporous or mesoporous frameworks such as quantum dot or super lattice-forming semiconductive clusters, nonlinear optical molecules, molecular conductors made of linear conductive polymers and electron transfer chains as well as DeA transfer pairs. All these complex composites could be assembled through synthetic routes consisting of ion exchanges, CVDs, “ships in bottle” and microwave dispersion; (3) nanohybridization of inorganics and organics, which represents a rapidly emerging interdisciplinary field. It studies the formation of new hybrid materials through combining polymerization and solegel processes. These hybrid materials possess those properties which are

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generally absent in pure inorganics or pure organics, and are increasingly being used in fiber optics, wave propagation, and nonlinear materials. It is worth noting that the first survey about this emerging field was published in 1996 by P. Judeinstein [3]. As outlined above, a key task in today’s inorganic synthetic chemistry is to develop novel synthetic reactions, synthetic routes, and associated techniques aiming to create new functional materials with specifically desired multilevel structures in condensed states. As per the past experience, the discovery of a novel and effective synthetic route or technique has typically led to the creation of a large class of new matters and materials. For example, the advent of solegel synthetic route has been a key reason for the development and emergence of nano-states and nanocomposite materials, glass states and glass composites, ceramic and ceramic-based composites, fibers and related composites, inorganic membranes and composite membranes, and hybrid materials. The core chemistry of this synthetic route is hydrolysis and polymerization of starting reactant molecules (or ions) in aqueous solution, i.e., from molecular / polymeric state / sol / gel / crystalline state (or noncrystalline state). This synthetic process could possibly be regulated differently at each individual reaction step so as to create solid-state compounds or materials with different structures or in different aggregate states. While highly promising, we are clearly not there yet due to the complexity as well as our limited understanding of polymerization processes of inorganic molecules in both theoretical and experimental executions. Thus, fundamental studies of these issues represent key areas of focus in today’s inorganic synthetic chemistry. In summary, the near and intermediate-term objectives for today’s inorganic synthetic chemists are to develop novel and more effective synthetic technologies and to carry out related theoretical studies aiming to gain better understanding of the desired new synthesis capabilities which are both economical and environmentfriendly.

1.2. BASIC RESEARCH IN SUPPORT OF GREEN SYNTHESIS The vast majority of known synthetic reactions, especially those used in the preparation of a large variety of rare elements from their ores or raw materials, in the production of fine chemicals as well as in medical and pharmaceutical industries, produce large amounts of by-products, which, along with the used chemicals, solvents, additives, and catalysts, often add major pollutants to our environment and have created considerable environmental issues in the past. Thus, it has

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become absolutely essential to study ways to considerably lower or completely remove environmental pollution produced by the current chemical industry. While this has posed substantial challenges for synthetic chemists, it has also created new opportunities to further develop synthetic chemistry toward new and healthier directions. Green chemistry, clean technologies, and environment-friendly chemical processes have now become a common conviction of many chemists. Ideal synthesis, a concept proposed by Wender [4] in 1996, aims to “make complex molecules from simple starting materials in a manner that is operationally simple, fast, safe, environmentally acceptable and resource efficacious.” This definition has essentially defined the general direction for realizing green syntheses. In 2009, Noyori [5] proposed that we should aim at synthesizing target compounds with a 100% yield and 100% selectivity and avoid the production of waste. This process must be economical, safe, resource-efficient, energy efficient, and environmentally benign. In this regard, the atom economy and the E-factor should be taken into account. The 3Rs (reduction, recycling, and reuse) of resources are particularly important. Such “Green Chemistry” is creative and brings about prosperity. The following research directions have received considerable attention in the recent years, from many synthetic chemists [6]: development and applications of green synthetic reactions with efficient atomic economy, environmental friendliness, and energy efficiency; development and application of environment-friendly source materials, reaction media and solvents, additives and catalysts and highly efficient and selective synthetic reactions as well as associated theoretical studies. These have become the major focuses at the forefront of synthetic chemistry research.

1.3. BASIC RESEARCH ON SYNTHETIC AND PREPARATIVE ROUTES UNDER EXTREME CONDITIONS There have been many cases of successfully synthesizing materials under extreme conditions such as ultrahigh pressure, high temperature, high vacuum, ultralow temperature, strong magnetic and electric fields, laser and plasma which are not possible to be synthesized under normal experimental conditions. A large variety of new compounds, phases and materials as well as new synthetic routes and techniques have been synthesized and developed specifically for chemical syntheses. For example, ultrapure crystals with no dislocation defects can be synthesized in ultrahigh vacuum with zero-gravity. It has even been suggested that the Periodic Table of Elements may need to be significantly modified under ultrahigh pressure since

the width of the forbidden band and the distance between the internal and the external electronic orbits for many matters may be changed under such conditions which can lead to significant differences in the stable valances of an element under the normal versus ultrahigh pressures. It has also been observed that changes in reactivity and reaction rules of reactants under ultrahigh pressure have led to the formation of a variety of new species and more interestingly, of new phases. It is also worth noticing that compounds with specific valence, configuration, and crystal morphology can be formed under hydrothermal conditions with medium temperature and pressure, which helps to overcome the issue caused by the lack of successful synthesis routes in solid-states chemistry for many inorganic functional materials under high temperature. Hence, further studies of the general rules and principles of chemical synthesis under extreme conditions have become one of the major research frontiers in synthetic chemistry.

1.4. BIOMIMETIC SYNTHESIS AND APPLICATIONS OF BIOTECHNOLOGY IN INORGANIC SYNTHESIS Biomimetic synthesis typically refers to syntheses that mimic biological synthesis processed in living organisms. An ultimate goal is to develop synthesis techniques and processes that can lead to the creation of new materials with similar or better/improved properties of naturally existing biological materials or to synthesize new materials with specifically desired properties using naturally existing materials. As a rapidly emerging research field, biomimetic synthesis has attracted great interest of researchers from a number of fields and is being considered a new frontier in synthetic chemistry in the twenty-first century. An interesting observation has been that some of the highly complex synthesis processes using traditional approaches become easy and efficient through biomimetic synthesis. Here we use “biominerals” and “biomineralization” as examples to illustrate some basic ideas of biomimetic synthesis. Various biomineralized materials have been formed as parts of living organisms as a result of genetic mutations and selection by evolution such as bones, teeth, pearls, shells, diatoms, and spider silk. The formation of such special tissues, though by accidents, has given special advantages to the relevant organisms and hence has been kept (selected) during evolution. The inorganic components in these special tissues such as calcium carbonate, calcium phosphates, calcium oxalate, metal sulfates, amorphous silica, iron oxide, and iron sulfide are generally called biominerals.

RATIONAL SYNTHESIS AND MOLECULAR ENGINEERING OF INORGANIC COMPOUNDS WITH SPECIFIC STRUCTURES AND FUNCTIONS

Biomineralization refers to the formation process of biominerals inside living organisms. The process typically involves a sequence of chemical reactions leading to the formation of new tissues mostly made of inorganic phases. The fundamental difference between biomineralization and mineralization in general is that in biomineralization the precipitation of inorganic mineral phases is accomplished through interactions between bio-macromolecules and inorganic ions at the interface between cells and body fluids which are controlled at the molecular level. Because of the unique formation process, biominerals often have special multilevel structures distinct from inorganic structures existing outside living organisms. They tend to have specific characteristics of crystals with highly uniform sizes, clear structure and composition boundaries, highly ordered spatial arrangements, complex morphologies, and well-defined crystal orientations and tend to have clearly defined multilevel structures. In a nutshell, biomineralization is a controlled precipitation and deposition process of biominerals with highly ordered, regular and multilevel structures as the final products. The biomineralization process inside a living organism generally consists of four intertwined and interactive steps: supramolecular preorganization, interfacial molecular recognition, vectorial regulation, and cellular regulation and processing. A key characteristic of biomineralization is the nucleation and growth of inorganic minerals around supramolecular templates in a highly regulated manner. During the biomineralization process, the morphology, size, orientation, and structure of the biominerals are controlled in a sophisticated manner by organic components such as bio-macromolecules involved in the process. Understanding the mechanisms of biomineralization can be useful to guide biomimetic syntheses of new functional materials at multi-scales ranging from the meso- to macro-scale. This is rapidly becoming one of the important research directions in material chemistry as well as in inorganic synthetic chemistry. The highly interesting and unique properties of biomineralized materials, such as (a) lotus leaves and insect wings with self-cleaning properties, (b) cameo shells with specially high strength, toughness, and abrasion resistance, (c) rat’s tooth enamel, (d) spider silk with superb strength and elasticity, and (e) iron oxides located inside fish heads serving as natural compasses, are all results of different structural characteristics of the self-assembled biominerals at multiple scales. Fueled by these observations, a new branch of chemistry, biomimetic material chemistry, is being formed and is rapidly growing with the key aims of elucidating relationships between functions and coordination effects among the multilevel structures of biomineralized materials, to design desired multilevel structures, to apply learned mechanisms of

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biomineralization to the synthesis of inorganic materials, and to synthesize materials with specific multilevel structures and desired properties. In addition, more and more attention is being paid to the development of new techniques that directly mimic biochemical processes in inorganic syntheses, preparation, and assemblies. For example, a number of synthesis methods such as widely used enzymatic catalysis, microorganism-mediated (such as virus and bacteria) synthetic reactions, and template effect used in synthesis and assembly of inorganic functional materials are all inspired by biological synthesis processes. Another example is the emergence of combinatorial synthesis technique, which is regarded as a major breakthrough in the recent history of synthesis techniques. By organizing a large number of polypeptides in an array as catalysts, combinatorial synthesis allows rapid syntheses of astonishingly many new compounds within a short period of time. Such techniques have significantly shortened the screening time, for example, for potentially new drugs and new pesticides. As a result, combinatorial syntheses are being extensively used in the preparation and hydrothermal synthesis of inorganic materials.

1.5. RATIONAL SYNTHESIS AND MOLECULAR ENGINEERING OF INORGANIC COMPOUNDS WITH SPECIFIC STRUCTURES AND FUNCTIONS There have been some cases of new material synthesis through molecular design and engineering in recent years. Traditionally, creation of new compounds with desired properties typically involves syntheses of a large number of compounds and a selection process for the desired compounds from these synthesized compounds. Since 1950s, the number of synthesized compounds has increased from 2 million to more than 50 million, which has formed a large and highly useful compound library. The emerging field of molecular engineering takes a rather different approach to chemical synthesis. The basic idea is that it starts with desired functionalities of a to-be-synthesized material, designs the possible structures of the material based on the specified functionalities, and then creates the material through rational synthesis. The biggest impact of the emergence of molecular engineering on chemistry is that it has greatly broadened our view about the relationships among the functionalities, structures, and synthesis processes, allowing us to better appreciate and understand the relationships between functions and highlevel structures beyond single molecular structures.

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While this field is still in its nascent stage, it is already believed that this is the future direction of synthetic chemistry. Researchers have already started synthesizing new materials based on the general principles of molecular engineering in a number of selected fields. Among these studies, molecular design and rational synthesis of microporous crystal systems represent one of the relatively mature research areas. Microporous crystals generally have specific and regular channel structures [7]. The chemical interactions between the guest molecules and the channels and the framework tend to be considerably stronger than those of other porous materials, and hence the structural features and properties of the channels of such materials, such as the pore size, shape, dimension, orientation, composition of the channel walls, cavities, cages, and structural defects, generally have significantly stronger effects on the diffusion, adsorption, and desorption, the formation of intermediates and the selectivity of molecular reactions inside the channels than those for other porous materials. Thus, microporous crystals represent the most unique system, and could potentially become one of the largest classes of catalysts and adsorptioneseparation materials. Microporous crystals as well as other porous materials such as mesoporous, macroporous, and porous metalorganic framework (MOF) materials are being increasingly used in emerging high technologies, showing great potentials in the development of new materials in the future. As of now, 194 framework types of microporous crystals and much more types of inorganic open-framework materials, have been synthesized in laboratories. Over the years, extensive studies about these structures have been done on the structural features, the framework structures, and their effects on the movement and reactivity of the molecules inside their channels, the rules and regularities of the porecreating reactions, crystallization, and modifications of the channels, windows, and internal surfaces. Therefore, it is reasonable to select the microporous crystals as a case study in molecular engineering. While substantial work has been done in this area, it should be noted that only a small number of true success stories have been reported as of now. The design and rational synthesis of microporous zeolites need to be done based on thorough study of the relationships between the specified functions of the to-be-synthesized material and channel structures. Initial channel models of the desired crystal could be done with the help of computer programs. Subsequently ideal structural models will be selected according to the established relationship between properties and structures of the microporous crystals derived from known structures and functions of such crystals in relevant databases. Finally, a rational synthesis plan

of these ideal structures will be made based on the relationship between the structures and the synthesis conditions. But it is generally not possible to achieve true rational syntheses like those done in organic synthesis via analyzing reaction paths and steps, because their formation mechanism remains elusive and relationship between the synthetic parameters and structural characters remains unclear. Despite the difficulties associated with the rational synthesis, considerable efforts have been made to establish ways toward the rational design and synthesis of target zeolitic materials. Our group has built up a ZEOBANK that includes a database of zeolite synthesis and a database of zeolite structures with the aim to explore a novel way to guide the synthesis of zeolitic materials through data mining. Engineering the synthesis of new matters with desired structures and functions has attracted considerable attention in the areas of chemistry and material science. In Chapter 24 of this book, we will describe our efforts toward the rational design and synthesis of zeolitic inorganic open-framework materials. Currently, our group as well as several other research groups has been actively carrying out studies in the following areas: (1) method development for rational design of structures, (2) development and update of the ZEOBANK synthesis and structure database and synthetic approach guided by data mining for microporous compounds, (3) in-depth study of the formation mechanisms of microporous compounds and the structure-directing effect via experiments and computational simulation, (4) derivation of potential synthesis mechanisms as well as empirical relationship between synthesis conditions and resulting structures derived based on known synthesis data and computer simulation results, which could be used to guide rational structure design and directed synthesis of desired materials, (5) performing combinatorial synthesis for microporous compounds with specific structures and properties, and (6) structural modification, fine-tuning of the chemical properties of channels, windows and internal surfaces, and rational addition of specific active sites such as ions, metal particles, oxides or salts, complex ions, and clusters into specific channels or onto the internal surfaces based on the desired functions and properties of the microporous material. I would like to end this chapter with the words of Ryoji Noyori, the winner of Nobel Prize in 2001, in his feature article “Synthesizing our future” [5] that “Synthesis has a central role in chemistry; chemical synthesis has now reached an extraordinary level of sophistication, but there is vast room for improvement; and chemical synthesis must pursue ‘practical elegance’ that is, it must be logically elegant but must at the same time lead to practical application.”

RATIONAL SYNTHESIS AND MOLECULAR ENGINEERING OF INORGANIC COMPOUNDS WITH SPECIFIC STRUCTURES AND FUNCTIONS

References [1] M.B. Rudy, Chemistry’s golden age, C&EN News (1998). Jan 12. [2] Committee on Challenges for the Chemical Sciences in the 21st Century, Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering, National Academy of Sciences, 2003.

[3] [4] [5] [6] [7]

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P. Judeinstein, C. Sanchez, J. Mater. Chem. 6 (1996) 511e525. P.A. Wender, Chem. Rev. 96 (1996) 1e2. R. Noyori, Nat. Chem. 1 (2009) 5e6. J.H. Clark, Nat. Chem. 1 (2009) 12e13. R.R. Xu, W.Q. Pang, J.H. Yu, Q.S. Huo, J.S. Chen, Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure, John Wiley & Sons, Singapore, 2007.