In the Laboratory
W
Studying a Ligand Substitution Reaction with Variable Temperature 1H NMR Spectroscopy An Experiment for Undergraduate Inorganic Chemistry Students Jeffery A. Orvis,* Basant Dimetry, Jeffrey Winge, and T. Corbin Mullis Department of Chemistry, Georgia Southern University, Statesboro, GA 30460-8064; *
[email protected] The study of ligand substitution reactions of transition metal complexes is often a significant component of undergraduate courses in inorganic chemistry (1). Since these reactions are often accompanied by color changes, UV–vis spectroscopy is a classic method for studying them. A common inorganic experiment for a number of years has been the aquation of [Co(NH3)5Cl]2⫹, where the reaction flask is heated to an elevated temperature (60 ⬚C) and then aliquots of the reaction mixture are removed for study by UV–vis spectroscopy (2). The reaction is less than ideal: it is relatively slow and the withdrawing of aliquots is cumbersome, especially for a relatively large class. A number of papers published in this Journal have highlighted the growing role of NMR spectroscopy in the study of chemical reactions, beyond its well-known use in structure analysis. For instance, undergraduate laboratory experiences using NMR spectroscopy to study the kinetics of glycolysis (3), the internal rotation barrier of N,N-dimethylacetamide (4), and electron transfer self-exchange rates in ferrocene derivatives (5) have been recently reported. Starting with the well-known complex [Co(NH3)4CO3]NO3, 1, a convenient synthesis of trans[Co(NH3)4Cl2]⫹, 2, was recently reported (6). This improved synthesis of 2 makes it possible to study its aquation reaction, which is approximately 100 times faster than that of [Co(NH3)5Cl]2⫹ at room temperature (7). The aquation of 2 is slow enough to be studied by NMR spectroscopy, though fast enough to be completed in less than an hour, with over three half-lives completed—an important consideration to obtain reliable kinetic information from this reaction, particularly the reaction order (8). Further, aquation of 2 is accompanied by a trans-reactant to cis-product geometry change, clearly observed in the NMR spectra taken over the course of the reaction. Finally, study of the aquation reaction of 2 at several different temperatures gives very clean results for an activation parameter study. Taken together, the result is an interesting extended project that includes inorganic syntheses and reactivity studies, incorporating several spectroscopic techniques important in modern chemistry.
Product of suitable purity for characterization by UV–vis, IR, and NMR (1H and 13C) spectroscopy can be collected in one laboratory period and used in the subsequent synthesis. Complex 1 is then converted to trans-[Co(NH3)4Cl2]Cl, using the procedure reported by Borer and Erdman (6)
[Co(NH3)4CO3]NO3 + 3 HCl 1
(2)
trans-[Co(NH3)4Cl2]Cl⋅H2O + CO2 + HNO3 2
As this reaction is sensitive, the reported procedure must be followed carefully. A laboratory period is required to produce the product, which is filtered from the reaction mixture as a bright green, crystalline solid. (The purple cis-isomer may be obtained from the filtrate.) When dissolved in water 2 undergoes aquation as shown in eq 3
trans-[Co(NH3)4Cl2]Cl + H2O 2
(3)
cis-[Co(NH3)4(H2O)Cl]Cl2 3
This reaction is studied using 1H NMR spectroscopy by carrying out the reaction in D2O directly in an NMR tube. Good results are obtained with at least twelve spectra, each acquired every two to four minutes, depending on the temperature at which the reaction is run. Working in pairs, up to eighteen students can carry out the reaction kinetics experiment over three lab periods. Each group carries out the reaction at a different temperature and the entire class’s data are combined for the activation parameter determination. Hazards
Experimental Procedure The experiment begins with the synthesis of [Co(NH3)4CO3]NO3, 1, found in many standard references (9) NH3, (NH4)2CO3, H2O2
[Co(H2O)6](NO3)2
[Co(NH3)4CO3]NO3 (1)
Caution must be used when handling the hydrogen peroxide and strong acid solutions. Both can cause severe chemical burns. In addition, steps involving strong ammonia solutions should be done in a fume hood as ammonia fumes are very irritating. Gloves and safety glasses should be worn at all times, and the heating steps should be performed in a fume hood as well.
1
JChemEd.chem.wisc.edu • Vol. 80 No. 7 July 2003 • Journal of Chemical Education
803
In the Laboratory
Experimental Results One interesting aspect of eq 3 is the geometry change from the highly symmetric trans-complex 2 (D4h symmetry) to the much less symmetric cis-complex 3 (Cs symmetry). This change is abundantly clear in the proton spectra acquired. While complex 2 gives one resonance, at 3.9 ppm, complex 3 gives three resonances, in a 2:1:1 ratio, at 4.1, 3.5, and 3.0 ppm, respectively. All resonances are broad singlets. This geometry change provides an opportunity for student discovery. At the beginning of the experiment the students are told that they will be studying the aquation reaction of 2 where one chloride ligand is replaced by a water molecule. When asked to draw the structure of the product, they inevitably draw trans-[Co(NH3)4Cl(H2O)]⫹. With C4v symmetry, this product would show only one ammonia resonance in the NMR spectrum. What the students actually observe are the three resonances of complex 3 growing during the course of their experiment. After some prompting they eventually propose the cis structure of the product. It makes a nice illustration of symmetry and its importance in spectroscopy. A sample set of spectra is shown in Figure 1. Each student or group of students comes away with a set of time versus concentration data from the NMR spectra, which they analyze for zero-, first-, and second-order kinetic behavior. The result is invariably first order, with the observed rate constant easily derived from the slope of the natural logarithm of concentration versus time plot. This reaction is highly temperature sensitive. A temperature change of 1 ⬚C causes a clearly measurable change in the observed rate constant. Results from eight groups of students running the aquation reaction, eq 3, over the temperature range of 19–26 ⬚C are shown in Table 1. Using eq 4, the students generate an Eyring plot to derive the entropy and enthalpy of activation for aquation of 2 (10)
k −∆H ‡ ∆S ‡ + + ln B RT R h
Time
4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8
Chemical Shift (ppm) Figure 1. Stack plot showing the resonance of reactant 2 decreasing over time and the three resonances of product 3 growing over time. These spectra were taken from the reaction run at 24 ⬚C. There is a 140 s gap between each spectrum. The product resonance at 3.5 ppm is obscured by the large reactant resonance at 3.9 ppm.
Table 1. Observed Rate Constants (and Error Ranges) from the Aquation Reaction at Various Temperatures kobs /10᎑4 s᎑1
Temperature /⬚C 19
6.6 (0.3)
20
7.3 (0.2)
21
8.2 (0.2)
22
9.6 (0.4)
23
11.6 (0.4)
(4)
24
13.4 (0.5)
25
15.1 (0.6)
In eq 4, ∆H and ∆S represent the enthalpy and entropy of activation, respectively, kobs is the observed rate constant from the first-order plots, R is the gas constant, and kB and h are Boltzmann’s and Planck’s constants, respectively. An example of an Eyring plot using the data in Table 1 is shown in Figure 2. The value for the enthalpy of activation derived from this plot is 101 ± 3 kJ兾mol; the value for the entropy of activation is 41 ± 9 J兾(mol K). The entropy result, in particular, is the most useful in gaining an insight into the mechanism of the aquation reaction of 2. With a large, positive value, the entropy of activation is consistent with a dissociative reaction mechanism, as expected for a ligand substitution reaction of an octahedral complex (10).
26
17.1 (0.8)
‡
= ‡
Conclusions This experiment has been part of a laboratory project in the synthesis and study of coordination compounds in our junior–senior-level inorganic chemistry course for the past three years. It includes the synthesis and characterization of two Co(III) complexes as well as a mechanistic study of the 804
-12.0
-12.2
-12.4
kobs T
kobs T
-12.6
ln
ln
-12.8
-13.0
-13.2 3.34
3.36
3.38
3.40
T ᎑ 1 / (10᎑ 3 K ᎑ 1) Figure 2. Eyring plot of the data from Table 1, r2 = .994.
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
3.42
In the Laboratory
aquation of one of these complexes, using variable-temperature 1H NMR spectroscopy. This project represents a satisfying mix of classic coordination chemistry syntheses and reactivity studies combined with exposure to several spectroscopic techniques. Utilizing NMR spectroscopy for studying a reaction is not a typical application of this technique in the undergraduate laboratory. The geometry change of the reaction studied here is made quite clear by using NMR spectroscopy, and the students get the surprisingly satisfying experience of inserting a green sample into the instrument at the start of the reaction and withdrawing a purple product from the instrument at the end. The aquation reaction of trans-[Co(NH3)4Cl2]Cl gives clean, reproducible results and is easily followed using 1H NMR spectroscopy. By combining the results of the reaction at several different temperatures, a fine example of determining and using activation parameters, particularly the entropy of activation, in elucidating a reaction mechanism is obtained. By overlapping the aquation study via NMR spectroscopy with other laboratory projects, the instructor can keep students who are not acquiring NMR data busy in the lab. A class of 18 students can complete the synthesis and mechanism study project described here in five, three-hour laboratory periods.
W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 2nd ed.; Prentice Hall: New York, 1999; pp 383–421. 2. Angelici, R. J. Synthesis and Technique in Inorganic Chemistry; Saunders: Philadelphia, 1969; pp 25–30. 3. Mega, T. L.; Carlson, C. B.; Cleary, D. A. J. Chem. Educ. 1997, 74, 1474–1476. 4. Jarek, R. L.; Flesher, R. J.; Shin, S. K. J. Chem. Educ. 1997, 74, 978–982. 5. Jameson, D. L.; Anand, R. J. Chem. Educ. 2000, 77, 88–89. 6. Borer, L. L.; Erdman, H. W. In Inorganic Syntheses; Cowley, A. H., Ed.; Wiley: New York, 1997; Vol. 31, pp 270–271. 7. Linck, R. G. Inorg. Chem. 1969, 8, 1016. 8. Urbansky, E. T. J. Chem. Educ. 2001, 78, 921–923. 9. Angelici, R. J. Synthesis and Technique in Inorganic Chemistry; Saunders: Philadelphia, 1969; pp 15–16. 10. Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; Brooks/Cole: Monterey, CA, 1985; pp 16–17.
JChemEd.chem.wisc.edu • Vol. 80 No. 7 July 2003 • Journal of Chemical Education
805
