In this project funded by the Chemical Synthesis Program of the Chemistry Division, Professor Timothy Hanusa of the Department of Chemistry at Vanderbilt University will explore the relationships between early main-group elements and the lanthanide (rare earth) metals. This study will help establish the extent to which inexpensive and abundant early main group metals such as calcium, strontium, and barium can supplement or substitute for the increasingly expensive rare earth elements, which are of enormous importance to modern society. Multidentate ligands with unconventional bridges such as phosphonium groups will be designed so that the metals can be placed in coordination environments that will compensate for differences in oxidation states. Their structures and reactions with polar molecules can then be studied both experimentally and computationally.
As the relationship between the rare earth metals and comparable main group elements is clarified, this research could lead to new uses of the latter in organic transformations and catalysis. Such reactions are potentially broadly applicable in the chemical and polymer industries, especially where inexpensive and non-toxic reagents are required. Furthermore, the proposed research involves the professional training of students, both in instruction in research skills, and in oral and written communications of their findings. In addition, educational efforts are being directed at broadening the participation of women and members of tradtionally underrepresented groups in chemistry and at reaching out to both middle and high school students.
Finding substitutes for materials that are increasingly expensive or that are produced in an inefficient manner is a continuing challenge for modern chemistry. Metals such as platinum, palladium, and rhodium are widely used in the synthetic organic and pharmaceutical industries, but have prices that rival or exceed that of gold. Similar things could be said for compounds of the lanthanide (so-called "rare earth") elements, which are found in many types of electronic and high-tech equipment, including magnets for disk drives, headphones, and power-generating windmills (neodymium), MRI scanners (lutetium), and batteries for hybrid electric cars (lanthanum). Production of the latter metals is currently almost completely (95%) sourced from a single nation (China), and increasing demand for some of the lanthanides (e.g., the 10-15 kg of lanthanum required for the battery of an electric-hybrid automobile), means that issues of sustainable and environmentally responsible production will be prominent for many years to come. In addition, increasing attention must to be given to the manner in which chemical products are generated; an area of particular interest is solvent use (e.g., 85% of the chemicals used in the pharmaceutical industry are solvents, and even recycling might recover no more than 50% of them). Accordingly, emphases of this research were: (1) to determine new types of chemistry, particularly catalytic in nature, that could be achieved using inexpensive, non-toxic metals such as magnesium and calcium, especially if they could substitute for rare-earth metals; and (2) to examine the usefulness of solvent-free (or reduced solvent) methods in the synthesis of organometallic compounds (i.e., those containing metal-carbon bonds), which are a key component of industrial chemical synthesis. The project was designed to combine results from experiment and theory in reaching its goals. The primary organic units used during the research were the allyl anion (C3H5)– (see Figure 1) and bulky substituted derivatives such as that in Figure 2 (abbreviated A´), which can bind in various ways to metals, with potential changes in reactivity. Although changes in bonding can be documented experimentally, the reasons for such differences must be derived from theory. Computational modeling of these complexes provided critical insight into the energetics of different bonding arrangements. New examples of compounds that can be used in polymerization reactions were found. Allyl complexes of potassium (K), magnesium (Mg), and calcium (Ca) can initiate the catalytic polymerization of butadiene (Figure 3), which is a large component of the rubber used in tire production. Interestingly, compounds containing magnesium alone were not active, but mixed metal compounds (i.e., K/Mg or Ca/Mg) were, which suggests that interaction between the metal centers was important. The unprecedented bonding arrangements were confirmed in crystal structures and modeled in computational studies (e.g., Figure 3). It should be noted that more expensive elements (e.g., neodymium, cobalt) are often used in butadiene polymerization, so that the substitution with the inexpensive K/Mg/Ca metals represents a potentially important advance in this area. Another central outcome was the development of low-cost entry points into mechanochemical synthesis, in which the reagents are ground together, rather than being dissolved in a solvent. This is a rarely used approach in organometallic synthesis, partially from the (unfounded) expectation that solvents are virtually always required to promote reactions. We were able to use an inexpensive approach to demonstrate the principle (i.e., stainless steel ball bearings in a laboratory rotary evaporator), equipment that would be widely available to many researchers (Figure 4). More sophisticated devices could provide faster reaction times, but even the ball bearing/flask approach was sufficient to demonstrate the power of the technique. For example, an aluminum allyl complex (A´3Al) was prepared using this method (Figure 5), something that was not possible when organic solvents were used as the reaction media. Furthermore, the compound proved to be exceptionally reactive, and readily added the bulky ketone benzophenone even at -78 °C, a consequence of the metal center’s having only 3 ligands on it. A rare earth complex with scandium was also prepared using this method, and there are in principle a large number of compounds that could be reexamined to see whether they could be made mechanochemically. This would not only be a "greener" approach to synthesis, but could also provide more reactive compounds than would otherwise be the case. In sum, this research project has identified new classes of catalysts for polymerization that contain inexpensive metals, and explored low cost, reduced solvent techniques for synthesis. Both of these advances hold promise for moving sustainable, environmentally conscious chemistry forward.
