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JoVE Science Education

A video library dedicated to teaching the practice and theory of scientific experiments through engaging and easy-to-understand visual demonstrations.

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Synthesis Of A Ti(III) Metallocene Using Schlenk Line Technique

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
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.
<img alt="Figure 1" src="/files/ftp_upload/10428/10428fig1v2.jpg" /> 
Figure 1. Synthesis of Ti(III) metallocene compound 3 and it&#39;s reaction with O2.

Synthesis Of A TiIII Metallocene Using Schlenk Line Technique

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
Inorganic chemists often work with highly air- and water-sensitive ...

Glovebox and Impurity Sensors

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
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.

Glovebox-Operating Procedure and Use of Impurity Sensors

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
The glovebox provides a straightforward means to handle air- and ...

Purification of Ferrocene by Sublimation

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
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&#39;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 &#176;C.1
<img alt="Figure 1" src="/files/ftp_upload/10429/10429fig1v2.jpg" /> 
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.

Sublimation: Concept, Purification of Ferrocene, Applications

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
Sublimation, the direct phase transition of a solid into a gas without ...

The Evans Method

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
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&#39;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, &#967;, of the coordination compound. Magnetic susceptibility is the measure of magnetization of a material (or compound) when placed in an applied magnetic field. Paired electrons are slightly repelled by an applied magnetic field, and this repulsion increases linearly as the strength of the magnetic field increases. On the other hand, unpaired electrons are attracted (to a larger extent) to a magnetic field, and the attraction increases linearly with magnetic field strength. Therefore, any compound with unpaired electrons will be attracted to a magnetic field.1
When we measure the magnetic susceptibility, we obtain information on the number of unpaired electrons from the magnetic moment, &#181;. The magnetic susceptibility is related to the magnetic moment, &#181; by Equation 12:
<img alt="Equation 1" src="/files/ftp_upload/10304/10304eq1.jpg" /> (1)
The constant <img alt="Equation 2" src="/files/ftp_upload/10304/10304eq2.jpg" /> = [(3kB)/N&#946;2)], where &#946;= Bohr magneton of the electron (0.93 x 10-20 erg gauss-1), N = Avogadro&#39;s number, and kB = Boltzmann constant 
XM = molar magnetic susceptibility (cm3/mol) 
T = temperature (K) 
&#181; = magnetic moment, measured in units of Bohr magneton, &#181;B = 9.27 x 10-24 JT-1
The magnetic moment for complexes is given by Equation 21:
<img alt="Equation 3" src="/files/ftp_upload/10304/10304eq3.jpg" /> (2)
g = gyromagnetic ratio = 2.00023 &#181;B 
S = spin quantum number = &#8721;ms = [number of unpaired electrons, n]/2 
L = orbital quantum number = &#8721;ml
This equation has both orbital and spin contributions. For first-row transition metal complexes, the orbital contribution is small and hence can be omitted, so the spin-only magnetic moment is given by Equation 3:
<img alt="Equation 4" src="/files/ftp_upload/10304/10304eq4.jpg" /> (3)
The spin-only magnetic moment can thus directly give the number of unpaired electrons. This approximation can also be made for heavier metals, though orbital contributions may be significant for second and third row transition metals. This contribution may be so significant that it inflates the magnetic moment enough that the compound appears to have more unpaired electrons than it does. Therefore, additional characterization may be required for these complexes.
In this experiment, the solution magnetic moment of tris(acetylacetonato)iron(III) (Fe(acac)3) is determined experimentally using Evans method in chloroform.

The Evans Method: Calculating Unpaired Electrons and Magnetic Susceptibility

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
While most organic molecules are diamagnetic, wherein all their ...

Single Crystal and Powder X-ray Diffraction

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
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 &#34;fingerprint&#34; 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 learn the principles behind XRD. We will then collect both single-crystal and powder data on Mo2(ArNC(H)NAr)4, where Ar = p-MeOC6H5.

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
X-ray crystallography is a technique that uses X-rays to study the ...

Electron Paramagnetic Resonance (EPR) Spectroscopy

Source: David C. Powers, Tamara M. Powers, Texas A&#38;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.

EPR Spectroscopy, Paramagnetic Molecule and Zeeman Effect

Source: David C. Powers, Tamara M. Powers, Texas A&#38;M
In this video, we will learn the basic principles behind Electron Paramagnetic Resonance (EPR). ...

M&#246;ssbauer Spectroscopy

Source: Joshua Wofford,&#160;Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
M&#246;ssbauer spectroscopy is a bulk characterization technique that examines the nuclear excitation of an atom by gamma rays in the solid state. The resulting M&#246;ssbauer 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 M&#246;ssbauer spectroscopy and collect a zero field 57Fe M&#246;ssbauer spectrum of ferrocene.

M&amp;#246;ssbauer Spectroscopy

Source: Joshua Wofford,&#160;Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
M&#246;ssbauer spectroscopy is a bulk ...

Lewis Acid-Base Interaction in Ph3P-BH3

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
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 &#34;acid&#34; is derived from the Latin term for &#34;sour-tasting.&#34; 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 &#34;alkaline&#34; is derived from the Arabic word for &#34;roasting.&#34; 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 proton.2 For example, in water, HCl is an acid, as it dissociates to give the hydronium ion (H3O)+ and the chloride ion. Boron trichloride would not be considered an acid, as in water it hydrolyzes to give B(OH)3 and 3 HCl; the product HCl though is an Arrhenius acid.
In 1923, Johannes Nicolaus Br&#248;nsted and Martin Lowry independently defined acids and bases on their ability to donate and accept hydrogen ions, or protons. Thus came the concept of acid-base conjugate pairs, and the expansion of the definition of acids and bases in solvents other than water. For example, ammonium is an acid, as it can donate a proton and generate ammonia. Ammonia can accept a proton, to give ammonium. Thus, ammonia is the conjugate base of ammonium. This acid-base reaction can occur in water, ammonia, or other solvents.
This video deals with the acid-base definition of the American chemist, Gilbert N. Lewis, who also defined acids and bases in 1923. Indeed, this is the same Lewis from Lewis-dot structures in General Chemistry. His approach focuses not on the ability of acids and bases to donate and accept protons, but rather on their ability to accept and donate electron pairs, respectively. This encompasses the Br&#248;nsted-Lowry definition, as H+ accepts an electron pair from a Br&#248;nsted base during protonation. However, it greatly expands the definition of an acid, now encompassing metal ions and main-group compounds. Here, we compare the 31P NMR of the Lewis acid-base adduct Ph3P-BH3 to free triphenylphosphine.

Lewis Acid-Base Interaction in Ph3P-BH3

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
One of the goals of chemistry is to use models that account for trends ...

Structure Of Ferrocene

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
In 1951, Kealy and Pauson reported to Nature the synthesis of a new organometallic compound, ferrocene.1&#160;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&#160;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 &#34;sandwiched&#34; between two cyclopentadiene ligands, with equal binding to all 10 carbon atoms (Figure 1, Structure II).4&#160;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.
<img alt="Figure 1" src="/files/ftp_upload/10347/10347fig1v2.jpg" /> 
Figure 1. Two proposed structures of ferrocene.

Structure Determination of Ferrocene-an Organometallic Complex

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University&#160;
In 1951, Kealy and Pauson reported to Nature the synthesis of a new ...

Application of Group Theory to IR Spectroscopy

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;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.
<img alt="Figure 1" src="/files/ftp_upload/10442/10442fig1.jpg" /> 
Figure 1. The cis- and trans-isomers of Mo(CO)4[P(OPh)3]2.

Symmetry Elements, Group Theory and IR-Active Vibrations

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
Metal carbonyl complexes are used as metal precursors for the synthesis of ...

Molecular Orbital (MO) Theory

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
This protocol serves as a guide in the synthesis of two metal complexes featuring the ligand 1,1&#39;-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.

Molecular Orbital MO Theory: Predicting Geometry of Transition Metal Complexes

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
This protocol serves as a guide in the synthesis of two metal complexes ...

Quadruply Metal-Metal Bonded Paddlewheels

Source: Corey Burns, Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
Paddlewheel complexes are a class of compounds comprised of two metal ions (1st, 2nd, or 3rd 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.
<img alt="Figure 1" src="/files/ftp_upload/10441/10441fig1v2.jpg" /> 
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 bonds. Overlapping d-orbitals can form three types of bonds - &#963;, &#960;, and &#948; - depending on the symmetry of the orbitals involved. If we assign the molecular z-axis to be coplanar with the M-M bond, a &#963; bond is formed by overlap of the dz2 orbitals and &#960; bonds are formed by overlap of the dxz and dyz orbitals. &#948; bonds are generated by overlap of d-orbitals that have two planar nodes (dxy and dx2&#8211;y2). As a result, all four lobes of the d-orbital overlap and the corresponding &#948; bond has two planar nodes (Figure 2). In theory, with the addition of &#948; bonds, paddlewheel complexes are capable of supporting quintuple bonds, or five bonds between metal atoms.1 In most complexes, the dx2&#8211;y2 forms strong metal-ligand bonds and does not meaningfully contribute to M-M bonding. Thus, quadruple bonds are the maximum bond order in many complexes.
<img alt="Figure 2" src="/files/ftp_upload/10441/10441fig2v2.jpg" /> 
Figure 2. Visual representation of &#963;, &#960;, and &#948; bonding MOs resulting from the linear combination of metal d-orbitals. The dz2 atomic orbitals have the best spatial overlap, followed by the dxz and dyz orbitals. The dxy atomic orbitals have the least amount of spatial overlap.
In this video, we will synthesize the dimolybdenum paddlewheel complex Mo2(ArNC(H)NAr)4, where Ar = p-(MeO)C6H4, which features a quadruple bond. We will characterize the compound by NMR spectroscopy and use X-ray crystallography to study the M-M bond.

Source: Corey Burns, Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
Paddlewheel complexes are a class of compounds comprised of two ...

Dye-sensitized Solar Cells

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
Today&#39;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&#39;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&#160;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 solar cell was built by Bell Laboratories. Starting in the 1950s, solar cells were used to power satellites in space.2
Solar cells are electrical devices that utilize light to create a current. This video demonstrates preparation and testing of one such type of cell, the dye-sensitized solar cell (DSSC). First invented at UC Berkeley by Brian O&#39;Regan and Michael Gr&#228;tzel, Gr&#228;tzel pursued this work at the &#201;cole Polytechnique F&#233;d&#233;rale de Lausanne in Switzerland, culminating in the first highly efficient DSSC in 1991.3&#160;These solar cells, like plants, use a dye to help harness energy from the sun.

Dye-sensitized Solar Cells: Principle, Fabrication and Performance

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
Today&#39;s modern world requires the use of a large amount of energy. While ...

Synthesis of an Oxygen-Carrying Cobalt(II) Complex

Source: Deepika Das, Tamara M. Powers, Department of Chemistry, Texas A&#38;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&#39;-bis(salicylaldehyde)ethylenediimino]cobalt(II) [Co(salen)]2&#160;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 will quantify the amount of O2 consumed upon exposure of [Co(salen)]2 to DMSO. We will then visually observe the release of O2 from the [Co(salen)]2-O2 adduct by exposing the solid to CHCl3.

Synthesis of an Oxygen-Carrying [Cosalen]2 Complex

Source: Deepika Das, Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
Bioinorganic chemistry is the field of study that investigates ...

Photochemical Initiation Of Radical Polymerization Reactions

Source: David C. Powers, Tamara M. Powers, Texas A&#38;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.

Source: David C. Powers, Tamara M. Powers, Texas A&#38;M
In this video, we will carry out the photochemically initiated polymerization of styrene to ...

Science Education - Chemistry

Inorganic Chemistry