Inorganic Chemistry

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

Inorganic chemists often work with highly air- and water-sensitive compounds. The two most common and practical methods for air-free synthesis utilize either Schlenk lines or gloveboxes. This experiment will demonstrate how to perform simple manipulations on a Schlenk line with a focus on solvent preparation and transfer. Through the synthesis of a reactive Ti(III) metallocene complex, we will demonstrate a new, simple method to degas solvent as well as how to transfer solvent by cannula and by syringe on a Schlenk line. The synthesis of a Ti(III) metallocene compound 3 is shown in Figure 1.1 Compound 3 is highly reactive with O2, (see oxidation of compound 3 to Ti(IV) metallocene 4 shown in Figure 1). Therefore, it is important to run the synthesis under anaerobic conditions. The synthesis of target compound 3 can be monitored visually and progresses through one additional color change before arriving at the desired product, which is blue in color. If during the experiment there is an observed color change from blue to yellow (or green = blue + yellow), this is an indication that O2 entered the flask and that undesired oxidation of compound 3 to the Ti(IV) analog (compound 4) has occurred. Figure 1. Synthesis of Ti(III) metallocene compound 3 and it's reaction with O2…

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

The glovebox provides a straightforward means to handle air- and moisture-sensitive solids and liquids. The glovebox is what it sounds like: a box with gloves attached to one or more sides, which allows the user to perform manipulations within the glovebox under an inert atmosphere.

For manipulations under inert atmospheres, chemists can choose between Schlenk or high-vacuum techniques and a glovebox. Schlenk and particularly high-vacuum techniques offer a higher degree of control of the atmosphere, and are thus suitable for reactions that are greatly air- and moisture-sensitive. The glovebox, however, provides greater access for manipulations in an inert atmosphere. Weighing out reagents, filtering reactions, preparing samples for spectroscopy, and growing crystals are all examples of routine procedures that are more readily performed in a glovebox versus a Schlenk/vacuum manifold. Advancements in glovebox design have increased its performance, such as running reactions at reduced temperatures and spectroscopy within the glovebox. This video will demonstrate how to bring items in and out of the glovebox and how to qualitatively ensure a good working environment. Basic manipulations within a glovebox will be demonstrated through the synthesis of sodium benzophenone.…

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

Sublimation, the direct phase transition of a solid into a gas without first becoming a liquid, takes place at temperatures and pressures lower than that of the compound's triple point (Figure 1).The process of sublimation can be utilized to purify both organic and inorganic solids. During the purification technique, a solid is heated directly into the gas-phase. All non-volatile impurities are left behind while the vaporized compound is then collected (deposition) as a solid on a cold surface. Here, we will use sublimation to purify ferrocene, an inorganic solid with a triple point temperature of 183 °C.1 Figure 1. Generic phase diagram. The colored lines represent the pressure and temperature requirements for phase transitions. Distillation of a solid will occur at pressures and temperatures above the triple point, represented by the green line in the phase diagram. The blue line represents the temperature and pressure conditions where sublimation occurs. …

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

While most organic molecules are diamagnetic, wherein all their electrons are paired up in bonds, many transition metal complexes are paramagnetic, which has ground states with unpaired electrons. Recall Hund's rule, which states that for orbitals of similar energies, electrons will fill the orbitals to maximize the number of unpaired electrons before pairing up. Transition metals have partially populated d-orbitals whose energies are perturbed to varying extents by coordination of ligands to the metal. Thus, the d-orbitals are similar in energy to one another, but are not all degenerate. This allows for complexes to be diamagnetic, with all electrons paired up, or paramagnetic, with unpaired electrons. Knowing the number of unpaired electrons in a metal complex can provide clues into the oxidation-state and geometry of the metal complex, as well as into the ligand field (crystal field) strength of the ligands. These properties greatly impact the spectroscopy and reactivity of transition metal complexes, and so are important to understand. One way to count the number of unpaired electrons is to measure the magnetic susceptibility, χ, of the coordination compound. Magnetic susceptibility is the measure of magnetization of a material (or compound) when placed in an applied magnetic field. …

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

X-ray crystallography is a technique that uses X-rays to study the structure of molecules. X-ray diffraction (XRD) experiments are routinely carried out with either single-crystal or powdered samples.

Single-crystal XRD:

Single-crystal XRD allows for absolute structure determination. With single-crystal XRD data, the exact atomic positions can be observed, and thus bond lengths and angles can be determined. This technique provides the structure within a single crystal, which does not necessarily represent the bulk of the material. Therefore, additional bulk characterization methods must be utilized to prove the identity and purity of a compound. Powder XRD: Unlike single-crystal XRD, powder XRD looks at a large sample of polycrystalline material and therefore is considered a bulk characterization technique. The powder pattern is considered a "fingerprint" for a given material; it provides information about the phase (polymorph) and crystallinity of the material. Typically, powder XRD is used to study minerals, zeolites, metal-organic frameworks (MOFs), and other extended solids. Powder XRD can also be used to establish bulk purity of molecular species. Previously, we have seen how to grow X-ray quality crystals (see video in Essentials of Organic Chemistry series). Here we will lear…

Source: David C. Powers, Tamara M. Powers, Texas A&M

In this video, we will learn the basic principles behind Electron Paramagnetic Resonance (EPR). We will use EPR spectroscopy to study how dibutylhydroxy toluene (BHT) behaves as an antioxidant in the autoxidation of aliphatic aldehydes.

Source: Joshua Wofford, Tamara M. Powers, Department of Chemistry, Texas A&M University 

Mössbauer spectroscopy is a bulk characterization technique that examines the nuclear excitation of an atom by gamma rays in the solid state. The resulting Mössbauer spectrum provides information about the oxidation state, spin state, and electronic environment around the target atom, which, in combination, gives evidence about the electronic structure and ligand arrangement (geometry) of the molecule. In this video, we will learn about the basic principles of Mössbauer spectroscopy and collect a zero field 57Fe Mössbauer spectrum of ferrocene.…

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

One of the goals of chemistry is to use models that account for trends and provide insights into the properties of reactants that contribute to reactivity. Substances have been classified as acids and bases since the time of the ancient Greeks, but the definition of acids and bases has been modified and expanded over the years.1 The ancient Greeks would characterize substances by taste, and defined acids as those that were sour-tasting, such as lemon juice and vinegar. The term "acid" is derived from the Latin term for "sour-tasting." Bases were characterized by their ability to counteract or neutralize acids. The first bases characterized were those of ashes from a fire, which were mixed with fats to make soap. In fact, the term "alkaline" is derived from the Arabic word for "roasting." Indeed, it has been known since ancient times that acids and bases can be combined to give a salt and water. The first widely-used description of an acid is that of the Swedish chemist, Svante Arrhenius, who in 1894 defined acids as substances which dissociate in water to give hydronium ions, and bases as substances which dissociate in water to give hydroxide ions. This definition is thus limited to aqueous acids and necessitates that an acid contribute a …

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

In 1951, Kealy and Pauson reported to Nature the synthesis of a new organometallic compound, ferrocene.¹ In their original report, Pauson suggested a structure for ferrocene in which the iron is singly bonded (sigma bonds) to one carbon atom of each cyclopentadiene ligand (Figure 1, Structure I).1,2,3 This initial report led to wide-spread interest in the structure of ferrocene, and many leading scientists participated in the structure elucidation of this interesting new molecule. Wilkinson and Woodward were quick to suggest an alternative formulization where the iron is "sandwiched" between two cyclopentadiene ligands, with equal binding to all 10 carbon atoms (Figure 1, Structure II).4 Here, we will synthesize ferrocene and decide, based on experimental data (IR and 1H NMR), which of these structures is observed. In addition, we will study the electrochemistry of ferrocene by collecting a cyclic voltammogram. In the course of this experiment, we introduce the 18-electron rule and discuss valence electron counting for transition metal complexes. Figure 1. Two proposed structures of ferrocene.…

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University

Metal carbonyl complexes are used as metal precursors for the synthesis of organometallic complexes as well as catalysts. Infrared (IR) spectroscopy is one of the most utilized and informative characterization methods of CO containing compounds. Group theory, or the use of mathematics to describe the symmetry of a molecule, provides a method to predict the number of IR active C-O vibrational modes within a molecule. Experimentally observing the number of C-O stretches in the IR is a direct method to establish the geometry and structure of the metal carbonyl complex. In this video, we will synthesize the molybdenum carbonyl complex Mo(CO)4[P(OPh)3]2, which can exist in the cis- and trans-forms (Figure 1). We will use group theory and IR spectroscopy to determine which isomer is isolated. Figure 1. The cis- and trans-isomers of Mo(CO)4[P(OPh)3]2.…

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University

This protocol serves as a guide in the synthesis of two metal complexes featuring the ligand 1,1'-bis(diphenylphosphino)ferrocene (dppf): M(dppf)Cl₂, where M = Ni or Pd. While both of these transition metal complexes are 4-coordinate, they exhibit different geometries at the metal center. Using molecular orbital (MO) theory in conjunction with 1H NMR and Evans method, we will determine the geometry of these two compounds.…

Source: Corey Burns, Tamara M. Powers, Department of Chemistry, Texas A&M University

Paddlewheel complexes are a class of compounds comprised of two metal ions (1^st, 2^nd, or 3^rd row transition metals) held in proximity by four bridging ligands (most commonly formamidinates or carboxylates) (Figure 1). Varying the identity of the metal ion and the bridging ligand provides access to large families of paddlewheel complexes. The structure of paddlewheel complexes allows for metal-metal bonding, which plays a vital role in the structure and reactivity of these complexes. Due to the diversity of electronic structures that are available to paddlewheel complexes - and the corresponding differences in M-M bonding displayed by these structures - paddlewheel complexes have found application in diverse areas, such as in homogeneous catalysis and as building blocks for metal-organic frameworks (MOFs). Understanding the electronic structure of the M-M bonds in paddlewheel complexes is critical to understanding their structures and thus to application of these complexes in coordination chemistry and catalysis. Figure 1. General structure of paddlewheel complexes, where M can be a 1st, 2nd, or 3rd row transition metal. When two transition metals are held in close proximity the d-orbitals overlap, which can result in the formation of M-M b…

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University

Today's modern world requires the use of a large amount of energy. While we harness energy from fossil fuels such as coal and oil, these sources are nonrenewable and thus the supply is limited. To maintain our global lifestyle, we must extract energy from renewable sources. The most promising renewable source, in terms of abundance, is the sun, which provides us with more than enough solar energy to fully fuel our planet many times over. So how do we extract energy from the sun? Nature was the first to figure it out: photosynthesis is the process whereby plants convert water and carbon dioxide to carbohydrates and oxygen. This process occurs in the leaves of plants, and relies on the chlorophyll pigments that color the leaves green. It is these colored molecules that absorb the energy from sunlight, and this absorbed energy which drives the chemical reactions. In 1839, Edmond Becquerel, then a 19-year old French physicist experimenting in his father's lab, created the first photovoltaic cell. He illuminated an acidic solution of silver chloride that was connected to platinum electrodes which generated a voltage and current.1 Many discoveries and advances were made in the late 19th and first half the 20th century, and it was only in 1954 that the first practical s…

Source: Deepika Das, Tamara M. Powers, Department of Chemistry, Texas A&M University

Bioinorganic chemistry is the field of study that investigates the role that metals play in biology. Approximately half of all proteins contain metals and it is estimated that up to one third of all proteins rely on metal-containing active sites to function. Proteins that feature metals, called metalloproteins, play a vital role in a variety of cell functions that are necessary for life. Metalloproteins have intrigued and inspired synthetic inorganic chemists for decades, and many research groups have dedicated their programs to modeling the chemistry of metal-containing active sites in proteins through the study of coordination compounds. The transport of O2 is a vital process for living organisms. O2-transport metalloproteins are responsible for binding, transporting, and releasing oxygen, which can then be used for life processes such as respiration. The oxygen-carrying cobalt coordination complex, [N,N'-bis(salicylaldehyde)ethylenediimino]cobalt(II) [Co(salen)]2 has been studied extensively to gain understanding about how metal complexes reversibly bind O2.1 In this experiment, we will synthesize [Co(salen)]2 and study its reversible reaction with O2 in the presence of dimethylsulfoxide (DMSO). First, we …

Source: David C. Powers, Tamara M. Powers, Texas A&M

In this video, we will carry out the photochemically initiated polymerization of styrene to generate polystyrene, which is an important commodity plastic. We will learn the fundamentals of photochemistry and use simple photochemistry to initiate radical polymerization reactions. Specifically, in this module we will examine the photochemistry of benzoyl peroxide and its role as a photo-initiator of styrene polymerization reactions. In the described experiments, we will investigate the role of wavelength, photon absorption, and excited state structure on the efficiency (measured as quantum yield) of photochemical reactions.…