17 Jul 2010

Preparation and Spectroscopic Characterization of the Classical Coordination Compounds Copper(II) and Silver(II) Picolinate.

Lab Experiment

Submitted by Thomas Smith, Kalamazoo College

This experiment, intended for an upper-level inorganic chemistry course, involves classical transition-metal coordination compounds. The purpose of the exercise is to compare the physical and chemical properties of coordination complexes containing copper(II) and silver(II) ions bound to the anion of pyridine–2–carboxylic acid, also known as picolinic acid, picH. The metallic elements copper and silver are in the same family in the periodic table, but their chemical properties are quite different. Although Cu(II) is the stable oxidation state in aqueous solution, Ag(II) is powerfully oxidizing in water. The conjugate base of picH, pyridine–2–carboxylate or picolinate ion, acts as a ligand toward these metal ions, binding to them in chelating mode through the pyridine ring nitrogen atom and one of the carboxylate oxygen atoms. The compounds are synthesized in water at room temperature. In both cases picolinic acid is deprotonated to give picolinate ion, which then binds to Cu2+ and Ag2+, yielding products formulated as M(pic)2. For silver, Ag+ must be oxidized to Ag2+. Molecular and electronic structural characterization is accomplished through infrared, electronic absorption, and electron spin resonance spectroscopy and density functional calculation. Available crystal and molecular structure information is surveyed using the Cambridge Structure Database. 

Learning Goals: 

• Students will discover that two structurally similar transition metal compounds can be synthesized cleanly from water solutions.

• Students will engage in the molecular and electronic structural characterization of both compounds. They should appreciate the need to employ a variety of physical measurements to develop a comprehensive structural understanding of a molecule.

• Students will develop an appreciation of differences arising from position in the first vs. second vs. third transition series (the gold(II) compound does not exist).

• Students will learn to do an electronic structure calculation on a transition metal compound and to explore a crystallographic database.

Equipment needs: 

• Pyridine-2-carboxylic acid (picolinic acid), copper(II) acetate hydrate, silver(I) nitrate, ammonium peroxodisulfate, sodium carbonate, deionized water

• Beakers, magnetic stirrers, magnetic stirring bars, filtering funnels, filtering, vacuum drying capability, vials

• FTIR spectrometer (mineral-oil/NaCl or KBr disks or KBr and press for pellet making), UV/Visible spectrometer (quartz cell, water), ESR spectrometer (quartz tubes)

• Access to density functional theory computation software (Spartan, for example) and to Cambridge Structure Database

Implementation Notes: 

Students do this experiment in pairs. The synthetic reactions are easily accomplished. The copper reaction gives either the anhydrous material or the monohydrate. The reaction filtrate will yield large crystals, but this requires several weeks at room temperature. The silver compound that does not precipitate from the initial reaction will decompose in solution within a few days at ambient temperature. We store the silver compound in a refrigerator and allow the storage vial to warm to room temperature before removing a sample for physical measurements. The IR measurements are straightforward. We record both solid-state and solution electronic absorption spectra for comparison with the IR spectra (solid state) and to obtain molar absorptivity values. Both compounds give strong ESR spectra for solid-state samples, and a procedure for data analysis is provided. The density functional calculation for the copper compound works well using low-level Spartan software; for silver the number of electrons is too large for successful calculation. Students should understand that generally the simple Spartan DF computation applies to the molecules in the gas phase.

Time Required: 
• One and a half hours for syntheses by a pair of students. • Two-three hours for spectroscopy.
Evaluation Methods: 

Upon completion of the experiment the student pairs submit a brief summary of their results and plans to developing a formal report. A draft report is then written in the style and format of an American Chemical Society journal article; the draft must be thoroughly referenced. The instructor reads the draft and returns it to the students with suggestions for revisions. The final version of the report is evaluated in accord with criteria presented in the policy section of the laboratory manual.


Evaluation Results: 

Given the integrated nature of this exercise, the laboratory reports indicate whether or not the students have mastered the essential ideas of coordination chemistry. The reports reveal skill in laboratory technique through the percent yield and quality of the products and recording infrared and electronic absorption spectra and in interpretation of the spectra. Although reports are often of high quality and reflect considerable insight, some students seem not to grasp the distinction between molecular and electronic structure. A somewhat larger number have difficulty synthesizing the reaction observations and the measurements, computation, and database results into a comprehensive narrative. That requires further discussion with the instructor. Many students need to learn when to reference statements appropriately.

Creative Commons License: 
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