In the Laboratory
Synthesis of Molybdenum–Molybdenum Quadruple Bonds W A Multistep Advanced Synthesis Laboratory Experiment Laura E. Pence,* Amy M. Weisgerber, and Florence A. Maounis Department of Chemistry, University of Hartford, West Hartford, CT 06117
The concept of quadruple bonds formed between transition metal atoms is often not encountered until students are juniors or seniors, and it is rarely reinforced by laboratory experience, owing to a paucity of easily reproducible experiments at the advanced level. We describe an inorganic experiment that focuses on the formation of the quadruple bond and the substitution chemistry of the assembled dimetal unit for the relatively unfamiliar second-row transition element, molybdenum. The intense and varied colors of the products demonstrate the influence of π-donor, σ-donor, and π-acceptor ligands on the energies of the frontier molecular orbitals; shifts in orbital energies alter the wavelength of the δ– δ * transition in the visible region. The experiment requires students to learn inert atmosphere techniques, but affordable methods are presented as alternatives to glove boxes and vacuum lines. This multistep inorganic experiment is appropriate for an advanced synthetic or integrated laboratory course and is designed to include common aspects of industrial and academic working environments. By carrying out three sequential reactions in which the product of one reaction is the starting material for the next, students are introduced to the complex process of chemical research, which will ease the transition that they make from single-step introductory laboratory experiments to their postgraduate experiences. The molybdenum experiment also may be adapted for a team or “research group” of students who collaborate on the final results. Because up to nine tertiary phosphine complexes may be made in the final step, students can work on parallel syntheses while making different products. Spectroscopic data for the final products may be compiled by the group to give a greater understanding of features that vary or remain constant among closely related compounds. The teamwork skills fostered by collaboration are important to the students’ professional futures. An advanced synthetic experiment should challenge students with new techniques and new responsibilities. The sequence of reactions in this experiment achieves both of these goals. In the first reaction (Scheme I), which is used to prepare the metal–metal bonded compound Mo2(O2CCH3)4, students learn proper procedures for handling the toxic substance Mo(CO)6 as well as how handle compressed gases and how to run a reaction under a positive pressure of nitrogen. CO 2
OC OC
CO
Mo CO
Step 1
CO
H3 H3 C C O O O O Mo Mo O O O O C C H3 H3 Step 2
Cl Mo R3P Cl
PR3
PR3 Cl Mo
Cl Step 3
Cl PR3
Scheme I
404
Cl
Cl
Mo
Mo
Cl Cl
Cl Cl
Cl
4-
HCl gas is required for the preparation of [Mo2Cl8] 4{ in step 2, but rather than purchasing a tank of the corrosive gas, students learn to generate the gas through the dropwise addition of H2SO4 to HCl. This procedure has the merits of not damaging regulators, and it allows the residual acids to be discarded down the drain after neutralization and dilution. The reaction to form the phosphine complex, Mo2Cl4(PR3)4, in step 3 builds on several techniques learned in step 1. If proper handling techniques were acquired in the first reaction, then dispensing liquid phosphines without polluting the laboratory with the pungent stench of the chemicals will be easy. Unlike the first reaction, which may be assembled in air, the third reaction must be assembled under an inert atmosphere and requires deoxygenated solvent to prevent oxidation of the phosphine. Therefore, the students gain experience with glove-bag manipulation and learn the techniques of keeping a system free of air. The final molybdenum–phosphine compound is relatively unreactive in the solid state but decomposes in solution. Therefore samples for UV-vis or NMR analysis must also be prepared by using deoxygenated solvents in a glove bag. We have tried to minimize the cost of implementing this new experiment. Most academic laboratories will have access to N2/Ar gas regulators, bubblers, 1-mL plastic or glass syringes, and most common glassware. The pressure-equalizing dropping funnels for the HCl gas generation apparatus may be obtained from Ace Glass if necessary. If a still is not available for distillation of methanol under an inert gas, then the solvent may be deoxygenated by purging with Ar or N2 for 20 minutes before use. The required chemicals that are not typically found in academic laboratories may be obtained from Aldrich, including the molybdenum hexacarbonyl, Mo(CO)6, used in the first step and the tertiary phosphines, PR3, used in step 3. In our experiments we tested PEt3, PMePh2, and PEtPh2; PMe3, PEt2Ph, PMe2Ph, P(n-Pr)3, P(n-Bu)3, and P(OMe)3 are other phosphines whose molybdenum complexes are known. Students filling out anonymous evaluations at the end of Advanced Synthesis enthusiastically indicated that the experiment should be repeated in subsequent years. An informal survey indicated that the array of colors was one of the most appealing components of the experiment, although the students also displayed an avid interest in the variety of reactions and techniques that they learned. By carrying out three sequential reactions on the molybdenum system, students gained a better appreciation of the reactivity of the compounds in this field of chemistry compared to the experience obtained in a more common single-step experiment. The differing time scales and temperature requirements of the three reactions allowed students to appreciate the difference in the energy W Supplementary materials for this article are available on JCE Online at http://JChemEd.chem.wisc.edu/Journal/issues/1999/ Mar/abs404.html.
*Corresponding author. Email:
[email protected].
Journal of Chemical Education • Vol. 76 No. 3 March 1999 • JChemEd.chem.wisc.edu
In the Laboratory
barrier to metal–metal bond formation compared to ligand substitution. The vivid colors of the three products, bright yellow, purple, and royal blue, were visual indications of the relationships among color, electronic spectrum, orbital energies,
and ligand type. The wide variety of techniques presented in this set of reactions combined with exposure to an unusual set of compounds makes this a valuable experiment that occupies a formerly unfilled niche in our curriculum.
JChemEd.chem.wisc.edu • Vol. 76 No. 3 March 1999 • Journal of Chemical Education
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