Version: March 3, 2010

Synthesis of a Cobalt(III) complex and Variable Temperature Proton NMR Study of its Aquation Kinetics

[This experiment is new for 2010; your comments and suggestions are most welcome.]

Adapted from Orvis, J. A.; Dimetry, B.; Winge, J.; Mullis, T. C. J. Chem. Educ. 2003, 8, 7, 803 Introduction: Complexes of Co(III), including dichlorotetraamminecobalt(III), hold a venerable place in the history of inorganic chemistry. Alfred Werner argued that octahedral arrangement of ligands can explain the existence of the different isomers [Werner 1907]. This is described in his 1913 Nobel Prize lecture. You will synthesize and study the kinetics of a reaction of the cobalt(III) complex, trans-[Co(NH3)4 Cl2 ]Cl: When this bright green complex is dissolved in water, it undergoes an aquation reaction: one chloride ligand is replaced by a water molecule. The product salt is [Co(NH3)4(H2O)Cl]Cl2. The kinetics of this reaction has been studied using a number of techniques (Tsuchida 1936; Pearson 1955, Linck 1969, Borer 1994, Jackson 2006). While often called an acid-catalyzed reaction, Linck observed little dependence of the rate constant with [H+]. You will use proton NMR to study the kinetics of this reaction.

PROCEDURE Synthesis of [Co(NH3)4CO3]NO3 (done by your instructor): Dissolve 20.0 g of ammonium carbonate in 60.0 mL of water, and then add 60.0 mL of concentrated aqueous ammonia. In a second container, dissolve 15.0 g of cobalt (II) nitrate hexahydrate in 30.0 mL of water. While stirring, pour these two solutions together, and note any color changes that may occur. Keep the mixture stirring by using a stir bar. SLOWLY, DROP BY DROP, add 8.0 mL of the 30% hydrogen peroxide solution. If bubbling becomes too vigorous cease additions until the solution settles down. Once the hydrogen peroxide has been added, evaporate the solution to a volume of 90-100 mL. Turn up the heat on the stirring hot plate, but do not let the solution boil. This will take some time, around an hour or so. During the evaporation time, add about 5.0 g of ammonium carbonate to the solution. Add it in small portions, say every ten minutes or so, making the 5.0 g last for the duration of the evaporation step. When the solution volume is down to 90-100 mL, suction-filter the solution and then cool the filtrate on ice. Red/purple crystals of product will form. Collect these crystals by suction filtration, wash them with a few milliliters of ice-cold water and then ethanol, and allow them to dry.

CoNH3

NH3 NH3

NH3

Cl

Cl

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EXPERIMENTAL PROCEDURE: DAY ONE Synthesis of trans-[Co(NH3)4 Cl2 ]Cl: Note: Concentrated acid can cause severe burns. Wear gloves and wash any material spilled on skin immediately. All procedures must be done in the fume hood.

Follow the procedure carefully. Temperature and timing are critical. Prepare a hot oil bath set at 75-80°C. Weigh accurately about 1.0 g of [Co(NH3)4CO3]NO3. Dissolve it in 5.0 mL of water in a 50.0 mL round-bottom flask. Insert a small stir bar and mount a thermometer in the flask. Heat the solution to between 50 and 60°C with stirring. Add 3.3 mL of concentrated HCl very carefully over a period of 15 to 20 seconds. Do not let the solution “boil” over. Heat with vigorous stirring. When the solution reaches about 60°C, start looking for a dark green precipitate of trans-dichlorotetraamminecobalt(III). After you begin to form product, continue stirring for five minutes. Cool the solution to room temperature in an ice bath. Suction filter the crude product with a fritted glass funnel. Remove the stirrer bar. Carefully rinse the green crystals with small portions of ice-cold water. Any purple solid on the filter is probably cis-dichlorotetraamminecobalt(III) chloride. Keep rinsing until all purple solid has been removed, leaving only green crystals. Wash the green crystals with no more than 2.0 mL of ice-cold methanol and air-dry the product. Weigh the product. Place the dried crystals in a stoppered vial. If you would be interested to follow the kinetics by UV-Vis also, see Prof. Chapman for a procedure to replace the following paragraph. UV-Vis spectrum. Weigh 10-15 mg of your solid into a small vial. (Exact concentrations are not important here, so weights and volumes can be approximate.) Since the product reacts immediately when dissolved in water, work in the instrument room. Set up the Evolution 600 spectrophotometer to scan a low resolution spectrum (scan speed 60 nm/min) from 350 to 750 nm. With water in both cuvettes, scan the baseline. Add about 3 mL water to your sample, mix, transfer to a cuvette, and scan. Wait about 20 minutes, and scan again.

Questions (to be answered before the second day of the experiment). Q.1. What was the percent yield of your synthesis? Q.2 You will be using proton NMR to monitor the reaction. The protons you will observe are

on the ammonia ligands. How many proton peaks would you expect to appear in the reactant, trans-dichlorotetraamminecobalt(III).? Explain.

Q.3 In the aquation reaction, a water (actually a D2O) molecule replaces one chloride ligand. Two isomers of the product are possible: the water and remaining chloride may be cis- or trans- to each other. How many peaks would each have in proton NMR? Explain.

EXPERIMENTAL PROCEDURE: DAY TWO Detailed instructions for studying kinetics using 1D Proton NMR spectra on the Bruker Avance 300 are given on a separate sheet. You will do a kinetic run at one temperature assigned by your instructor. You will then pool your data with that of others to explore temperature effects. Since the reaction starts as soon as the sample is dissolved, sample preparation must be done in the NMR room. Bring the following to the NMR room:

Stopwatch, a vial containing about 5 mg of your product, two sealed vials of D2O, two NMR tubes, several Pasteur pipettes, a pipette filter, Kimwipes, and gloves.

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Study the procedure thoroughly and do all preliminary instrument settings before you dissolve your sample. Unnecessary delays in the procedure will compromise the quality of your data. Set up the instrument, including lock and shim, using a sample of D2O. Establish the temperature and compile the kineticzg code with the appropriate time delay. Then mix your sample, put it immediately into the NMR, and start kinetic runs. When all your spectra are collected, process them using the Topspin software. Plot spectra number 1, 6, 11, 16, and 21 on a single page. Which peak(s) show a clear progression in time? Integrate the peaks at approximately 3.2, 3.7, 4.1, and 4.3 ppm. Since the peak at 4.1 ppm overlaps considerably with its neighbors, set the range of integration between the small satellites at about 3.9 and 4.2 ppm. Using manual peak picking, record the heights of the same four peaks. (In some cases, a peak may disappear before the end of the run). Also record the times of the spectra. Remove the sample, turn off the temperature setting, and shut down the instrument.

CALCULATIONS AND QUESTIONS Q.4 What does the NMR spectrum of the aquation product tell you about its structure? Kinetic analysis. Prepare an Excel spreadsheet. Work separately with peak heights and peak integrals. Convert clock times (e.g. 8:43:07) to elapsed times in seconds. Time t = 0 is when the D2O was first added. Prepare plots of the values (heights or integrals) vs. time. Which peak gives a signal (height or area) which is best for further kinetic analysis? Explain. For the remaining kinetic analysis, you may focus on that one peak. Let [R] be concentration of the reactant R. If n is the reaction order, possible rate laws include

–d[R]/dt = k[R]n. The analysis in this experiment is complicated by the fact that the NMR peaks overlap, so the measured signal S (peak height or area) has contributions from both reactant and product. Let S = a [R] + b [P] where a and b are proportionality constants. In an isomerization reaction [R]0 = [R] + [P]. Substituting S = a [R] + b {[R]0 –[R]} = (a–b) [R] + b [R]0. Assuming that the reaction goes to completion, [R] = 0 when t = ∞, so S∞ = b [R]0,. Thus (S – S∞) = (a–b) [R]. Therefore a quantity proportional to [R] used for kinetic analysis is (S – S∞). S∞ may be determined as a fitting parameter. Add a cell to your spreadsheet for S∞, and guess an approximate value based on your data and plot. Then prepare kinetic plots assuming zero, first, and second order kinetics (n= 0, 1, or 2). Focusing on the result that appears closest to linear, adjust S∞ to improve the linearity of fit. This may be done either by trial-and-error or using Excel's solver; be sure to keep a record. Q.5 What is the order of the reaction and what is the rate constant?

Do you need to know values for the parameters a and b above? Explain. Q.6 Which analysis appears to be more reliable, integrals or peak heights? Why?

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Your rate constant k(T) will be combined with others to answer the following. Ask you instructor where to obtain this data. Q.7. The Arrhenius equation is k = A e–Ea/RT. R is the gas constant. What are the Arrhenius

parameters, Ea and A, for this reaction? Q.8 The Eyring equation is ln (k/T) = (–ΔH‡/ RT) + (ΔS‡/ R) + ln(kB/h). kB and h are the

Boltzmann and Planck constants, respectively. What are the Eyring parameters, ΔH‡ and ΔS‡, the enthalpy and entropy of activation respectively, for the reaction?

Q.9 Use you Arrhenius parameters to calculate k at 25°C. Compare with literature values. [Tsuchida, Borer, Pearson, Linck, Jackson].

Additional questions Q.10 What does the entropy of activation suggest about the mechanism of this reaction? Is this

consistent with what is generally observed in substitution reactions of octahedral Co(III) complexes? [Huheey]

Q.11 What is the term symbol for the ground electronic state of the free ion Co3+ ? The ground electronic state of Co(NH)6

3+ and related compounds are singlet. Explain. [Harris] Q.12 Discuss the UV-Vis spectrum of the reactant cation, trans-Co(NH3)4Cl2

+. What electronic transitions are involved? You answer should include a discussion of the orbitals occupied by the d electrons on cobalt and the corresponding states. How would the UV-Vis spectra of the cis- and trans- aquation products differ? [Cotton; Harris]

Q.13 Comment on the stereochemistry of the product, and how it relates to the mechanism of the reaction. [Jackson]

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REFERENCES Orvis, J. A.; Dimetry, B.; Winge, J.; Mullis, T. C. “Studying a Ligand Substitution Reaction with Variable Temperature 1H NMR Spectroscopy: An Experiment for Undergraduate Inorganic Chemistry Students.” J. Chem. Educ. 2003, 80, 803

Werner, Alfred, “Über l.2-Dichloro-tetraammin-kobaltisalse (Ammoniak violeosalze)” Berichte d. D. Chem. Gesellschaff. 1907, 40, 4817-4825.

Werner’s Nobel Lecture (1913) [online]: http://nobelprize.org/nobel_prizes/chemistry/laureates/1913/werner-lecture.pdf

Cotton, F. Albert and Wilkinson, Geoffrey, Advanced Inorganic Chemistry, (Wiley, 5th Ed. 1988; or other editions).

Harris, Daniel C. and Bertolucci, Michael D., Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy (Oxford, 1978).

Huheey, James. E, Inorganic Chemistry: Principles of Structure and Reactivity, (Harper and Row, 3rd Ed, 1983, or other editions).

Riordan, Adam R.; Jansma, Ariane; Fleischman, Sarah; Green, David B.; and Mulford, Douglas R., “Spectrochemical Series of Cobalt(III). An Experiment for High School through College”

The Chemical Educator [online] 2005, 10, 2.

Linck, R. G. “The Rate and Stereochemistry of the Aquation Kinetics of trans-dichlorotetraamminecobalt(III)” “Inorg. Chem. 1969, 8, 1016

Jackson, W. Gregory, “Classic coordination complexes: The kinetics and stereochemistry of acid hydrolysis of cis- and trans-[Co(NH3)4Cl2]+ ”, Polyhedron 2006 25, 1955–1966

Borer, Londa L.; Erdman, Howard W. “An Experiment in trans-cis Isomerization: Synthesis and Kinetics of trans-Dichlorotetraamminecobalt(III) Chloride J. Chem. Educ., 1994, 71 332-

Pearson, R G.; Boston, R.C., and Basolo, F “Mechanism of Substitution Reactions in Complex Ions. V. Effect of Chelation on the Rates of Acid Hydrolysis of Some Cobalt(III) Complex Ions” J. Phys. Chem. 1955.59, 304-307.

Tsuchida, R, “Spectrographic Methods of Studying unstable Compounds. II. The Aquotization of trans-dichloro-tetraammine cobaltic chloride in Aqueous Solutions” Bull. Chem. Soc. Jpn. 1936 11, 721.