Crystallographic Study of Manganese(III) Acetylacetonate: An

In the Laboratory

Crystallographic Study of Manganese(III) Acetylacetonate: An Advanced Undergraduate Project with Unexpected Challenges

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Silvano Geremia* and Nicola Demitri Centre of Excellence in Biocrystallography, Department of Chemical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy; *[email protected]

The introduction of undergraduate students to a research experience is widely recognized as an essential component of chemical education. However, to make the chemistry lab experience more like a research experiment and less like a routine task, that is, following a cookbook recipe, is a challenge (1). One approach to this challenge is the integration of welltested protocols for synthesis of simple compounds with the characterization of the synthesized material by measurements not reported in the literature, with the goal to address questions raised by specific topics treated in lecture courses. The participation of the students in the excitement of new discovery, which is the essence of science and research, compensates for the possibility that completely original experimentation often leads to results with nontrivial interpretation. Furthermore, even if the new data do not completely satisfy the initial purpose, the likelihood is that new questions will arise, which will need new investigations, and which will produce new conclusions. In other words, the students will thus become involved in the cycle of scientific discovery. An approach is described that was used successfully in an advanced undergraduate inorganic chemistry laboratory

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course. The crystal structure of one of the most frequently synthesized complexes in inorganic chemistry laboratory courses, manganese(III) acetylacetonate, Mn(acac)3 (2, 3), was determined at room temperature and also at low temperature to investigate the Jahn–Teller effect described in the lecture course. Preparation and spectroscopic and magnetic characterization of metal acetylacetonates, M(acac)3, have been shown to be suitable for student laboratory experiments (2, 3). Within the series Cr(III), Mn(III), Fe(III), and Co(III) acetylacetonates, the d 4 (t 2g 3 e g ) high-spin octahedral Mn(acac)3 is the only complex subjected to a Jahn–Teller distortion as a result of the odd number of eg electrons. Detection of the Jahn–Teller distortion in this complex has been an intriguing story and its description contains several educational aspects. The first X-ray structure reported for Mn(acac)3 showed a regular octahedral complex (R), without the anticipated tetragonal distortion and with short Mn⫺O distances (Figure 1) (4). This surprising result led to considerable speculation concerning the nature of this unusual behavior of Mn(III) (5). The subsequent structure de-

TC

*

axial

*

*

*

equatorial

M

*

*

M

*

*

*

*

*

* regular

tetragonal compression

TE

OD

*

* long

axial

* medium

equatorial

M

*

Figure 1. Different structures possible for Mn(acac)3, where the dashed gray lines indicate bonds of longer distance, solid gray lines indicate bonds of shorter distance, and the solid black lines indicate bonds of medium distance.

*

*

M *

*

short

*

* * tetragonal elongation

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* orthorhombic distortion

Journal of Chemical Education

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Vol. 82 No. 3 March 2005

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In the Laboratory Table 1. Cell Dimensions Found for Various Structures of the Mn(acac)3 Complex Form of Mn(acac)3 α a

β/⬚

a/Å

b/Å

c/Å

Space Group

Ref

15.53

13.339

16.726

090

Pbca

7

β

13.875(15)

07.467(7)

16.203(25)

098.427(20)

P21/c

4

β

14.013(1)

07.600(1)

16.373(1)

099.33(1)

P21/c

7

γ

07.786(1)

27.975(4)

08.020(1)

100.34(1)

P21/n

9

δ

19.799

07.618

23.288

099.134

P21

present work

ε

16.117

07.511

27.831

100.03

P21/n

present work

a It has been proposed that this structure corresponds to a cobalt complex. In fact, the cell parameters are very close to those of Co(acac)3: a = 13.951(9); b = 7.470(5); c = 16.222(11) Å; and β = 98.29(5)⬚ (6).

termination of Co(acac)3 (6) revealed an incredible similarity with the previously determined manganese structure, which lead to the suggestion that the Mn(acac)3 sample had in fact been mislabeled and that its structure was actually that of Co(acac)3 (7). It should be noted that the d6 (t2g6) lowspin octahedral Co(acac)3 is not susceptible to Jahn–Teller distortion. It was supposed that a dark-green crystal of Co(acac)3 was accidentally mounted on the diffractometer instead of a dark-brown crystal of Mn(acac)3 by a technician not familiar with the different colors of these materials (8). Several years later, the Jahn–Teller distortion was established by the structure determination of two different monoclinic crystalline forms of Mn(acac)3 (Table 1) (7, 9). The predominant crystalline form, labeled β, exhibited a moderate tetragonal compression (TC) of about 0.05 Å (7), whereas the other, labeled γ, showed a significant tetragonal elongation (TE) of about 0.2 Å (9) (Figure 1). A third form with an orthorhombic unit cell (Table 1), labeled α (7), was also identified but no structural determination is available. In the article reporting the structure of the γ form of Mn(acac)3 (9), a note with unpublished results indicates the possibility of an orthorhombic distortion (OD) of the coordination polyhedron for the β form (Figure 1). The different distortion detected was attributed to the precision of the new study. To experimentally analyze the Jahn–Teller distortion using precise structural determination, it was decided to incorporate into a program for senior undergraduate students the preparation, crystallization, and the structure determination of Mn(acac)3 (1) both at room and low temperature (data not already reported in literature). Among the techniques used for the chemical characterization of compounds, structure determination by X-ray diffraction is one of the most powerful methods since it permits “visualization” of molecules. The possibility to have a direct image of a molecule synthesized by the student is remarkable and it facilitates the link between the three distinct languages used in chemistry courses: the symbolic (formula), the macroscopic (crystal), and the microscopic (molecule). Chemistry students are expected to be able to translate among these languages, to associate the formula of the molecule with its 3D structure, and to associate the latter with macroscopic properties such as solubility, color, reactivity, and so forth. X-ray crystallography gives the students a unique opportunity to have direct contact with the subject of their studies, the molecule.

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Several successful endeavors to integrate single-crystal Xray diffraction into the undergraduate chemistry curriculum have been recently reported (10–12). The availability of area detectors allows dramatic reduction of the time of X-ray data collection, thereby permitting the determination of crystal structures within the time frame of a student’s laboratory project. Furthermore, it facilitates data collection at low temperature. Laboratory Experience The students involved in this experiment had previously taken undergraduate courses in general chemistry and inorganic chemistry and were familiar with the properties of crystals, their basic crystallography, and the chemical information obtainable from the molecular structure. To supplement this background, the students were given three hours of lectures on the properties of chelating ligands, the stereochemistry of coordination compounds, including the Jahn–Teller theorem, and a description of the synthesis of metal acetylacetonates. The students also receive five hours of lectures on symmetry and space groups in crystals, diffraction geometry, structure factors, the phase problem, Patterson maps and the heavy atom method, and Fourier maps and structure refinement. The first three hours of scheduled lab time were dedicated to literature searching. Three different methods were used: Chemical Abstracts, SciFinder, and Cambridge Crystallography Database. Using the original articles on the structure determination of the compounds under investigation, the goals of the current experience were discussed. On the second day, students synthesized and crystallized the compound. The next week, the students observed the crystals under a microscope and, after selection of suitable single crystals, they mounted them on a glass fiber. Using one of these crystals, two complete X-ray diffraction data sets were collected, at room and low temperature (one during the afternoon and the other overnight). During the first data collection, the student built and studied ball and stick models of both enantiomeric isomers (∆ and Λ) of the molecule under investigation. The day after the two crystal structures were determined and, in order to investigate the reproducibility and reversibility of the observed phase transition, further X-


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461

In the Laboratory

ray diffraction trials with crystals at different temperatures, using both cooling and warming processes, were performed. After completing the experiment, each student wrote a laboratory report in the style of scientific article (title, abstract, background, experimental details, results and discussion, conclusions, and references). These articles were subsequently discussed in the oral examination. Experimental Details

Synthesis Mn(acac)3 was prepared as described in ref 3.

Space group

Crystallization

R

Dark-brown platelet-shaped crystals of Mn(acac)3 were obtained as described in ref 2. A significant improvement in the crystal dimensions was obtained using the following variation to the crystallization protocol: 20 mL of cyclohexane with 0.2 g of the compound were heated using a hot water bath; the boiling mixture was filtered on a vacuum pump; 40 mL of hot petroleum ether were added to the solution; the solution was cooled slowly to room temperature and filtered to eliminate a small quantity of an amorphous precipitate. The liquid was then concentrated to a quarter of the initial volume, halving the volume in two successive steps (heating at 60 ⬚C). The solution was then cooled slowly to room temperature leaving the beaker in the water bath. After three days, dark-brown platelet-shaped crystals, of more than 1 cm in length, were formed (Figure 2A).

Table 2. Cr ystallographic Data for δ-Mn(acac)3 and ε-Mn(acac)3 δ-Mn(acac)3

ε-Mn(acac)3

a/Å

19.799

16.117

b/Å

7.618

7.511

c/Å

23.288

27.831

b/°

99.134

100.03

T/K

290

100

P21

P21/n

Parameter

Z dcalc/(g cm᎑3) No. of reflections (I ≥ 4σI) Number of parameters

8

8

1.349

1.410

0.0770

0.0531

4205

6016

554

410

X-Ray Data Collection The crystals obtained in both protocols were selected and mounted in glass fiber by the students (Figure 2B). Diffraction data at room and low temperature were collected using a Nonius DIP1030 area detector and molybdenum radiation with a graphite monochromator. For data collections at low temperature the crystals were flash frozen to 100 K in a N2 gas stream using an Oxford cryosystem.

Data Processing The determination of unit-cell parameters, integration of reflection intensities, and data scaling were performed using Mosflm and Scala from the CCP4 program suite (13). The diffraction pattern collected at room temperature was initially indexed using a monoclinic unit cell similar to that reported in the literature for the β form (7). However, the diffraction pattern clearly showed the presence of unindexed extra spots. These spots were completely indexed using a different and bigger monoclinic unit cell (Table 2). The systematic absences were in agreement with a P21 or P21兾m space group. This finding revealed the characterization of a new crystalline form of Mn(acac)3. This form was labelled δ. Surprisingly, the diffraction images obtained from frozen crystals of the δ form were indexed using yet another monoclinic unit cell with systematic absences in agreement with the P21兾n space group (Table 2). This fifth form of Mn(acac)3 was labeled ε.

A

B

Solution and Refinement of the Structures

Figure 2. (A) Crystals of Mn(acac)3 obtained by the students in the alternative crystallization protocol. (B) Crystal of Mn(acac)3 mounted in glass fiber by the students.

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Students solved the structures of ε-Mn(acac)3 (two crystallographically independent molecules) by the Patterson and Fourier methods. The Patterson map interpretation gave the starting atomic coordinates of Mn atoms and the remaining atoms were localized in the difference Fourier maps. After several unsuccessful attempts to solve the δ-Mn(acac)3 structure in the centrosymmetric P21兾m space group, the structure was finally solved in the non-centrosymmetric P21 space group using four crystallographically independent molecules. The Patterson map was directly interpreted by the Shelxs pro-


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In the Laboratory Table 3. Mn⫺O distances (Å) in the Mn(acac)3 Cr ystalline Forms Form

Mn⫺O Bond Distance/Å 1

2

3

4

5

6

Ref

βa

1.860(8)b

1.880(8)

1.861(8)

1.893(8)

1.868(8)

1.875(8)

4

β

1.931(10)

1.956(7)

1.991(7)

1.984(8)

2.020(7)

2.003(8)

7

β

1.936(5)

1.950(5)

1.987(5)

1.983(5)

2.018(5)

2.014(5)

9

γ

1.933(3)

1.931(3)

1.934(4)

1.942(3)

2.112(4)

2.109(3)

9

δ

1.937(8)

1.895(8)

2.006(10)

2.004(11)

2.034(9)

2.046(8)

present work

ε

1.926(8)

1.951(8)

2.024(9)

2.023(9)

2.010(11)

2.034(11)

1.964(8)

1.966(10)

1.984(8)

1.976(9)

2.020(11)

2.046(12)

1.959(9)

1.984(10)

2.012(8)

1.989(9)

2.028(11)

1.999(11)

1.930(2)

1.901(2)

1.984(2)

1.976(2)

2.111(2)

2.106(2)

1.914(2)

1.926(2)

1.984(2)

1.981(2)

2.090(2)

2.082(2)

present work

a It has been proposed that this structure corresponds to a cobalt complex. In fact, the metal–oxygen distances are very close to those of Co(acac)3, average value 1.898(8) Å (6). b

The root mean squre error is given in the parentheses.

gram (14). Starting from the atomic coordinates of Mn atoms, the students completed the structure by Fourier methods. In the case of the ε form, the structure was refined by the least-squares method with Shelxl (15), using anisotropic thermal factors for all non-hydrogen atoms,. For the δ-form, anisotropic thermal factors were applied only for metal and oxygen atoms. The contribution of the hydrogen atoms was included at the calculated position and held constant in the last cycles of refinement. Refinement parameters for the structures are given in Table 2.

Results and Discussion

Solid–Solid Phase Transition The δ → ε phase transition was further investigated by collecting single X-ray diffraction images with the crystal at different temperatures. The crystal temperature was varied over the range 270 to 180 K, with steps of 10 K. In the temperature range 270–200 K, the diffraction patterns, collected with a 5⬚ rotation angle of the crystal, were indexed using a δ-form unit cell. It was not possible to index the image collected at 190 K as it clearly showed the presence of spots of both the δ and ε forms. The diffraction image collected at 180 K showed the complete phase transition between the δ and ε forms. In fact, it was successfully indexed using the unit cell of the ε form. This diffraction pattern remained unaltered when the crystal was brought to room temperature. In particular, the diffraction pattern maintained the monoclinic symmetry (in a typical frame containing more then 500 reflections, about a dozen of observed reflections confirmed the symmetry and more than 20 systematic absences agreed with the P21兾n space group) and a similar unit cell (with differences ascribable to thermal expansion). Hazards Acetylacetone is a mild irritant to the skin and mucous membranes. It is also a flammable liquid. Manganese(II) chloride tetrahydrate is harmful if swallowed, inhaled, or absorbed through the skin. Sodium acetate trihydrate is not generally considered dangerous. The normal precautions should be observed. Potassium permanganate is a powerful oxidizing agent www.JCE.DivCHED.org

and should be handled with care. It is harmful if swallowed, inhaled, or absorbed through the skin. It is extremely destructive to the mucous membranes and skin. Cyclohexane is irritating to skin. It may cause lung damage if swallowed and vapors may cause drowsiness and dizziness. It is a highly flammable liquid. Petroleum ether is irritating to skin. It may cause lung damage if swallowed and vapors may cause drowsiness and dizziness; serious damage to health is associated with prolonged exposure through inhalation (such as impaired fertility). Petroleum ether is a highly flammable liquid.

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The students synthesized and crystallized the Mn(acac)3 complex using various protocols. They selected and mounted single crystals suitable for X-ray analysis, acquiring the skill for the recognition of crystal properties necessary for diffraction experiments. The diffraction experiments showed that the students obtained a new monoclinic δ form.1 By cooling to 100 K, crystals of this δ form underwent an irreversible solid–solid phase transition to a new monoclinic form, labeled ε. This new ε form appeared at about 180 K and remained unaltered when brought to room temperature. The students solved the structures using Patterson maps and recognized the synthesized molecule from the electron density maps. The analysis of the coordination distances of the ε form with the frozen atomic thermal motions revealed that both the crystallographically independent molecules exhibit an orthorhombic distortion in the coordination distances (Figure 1), with two trans long (mean value of 2.097(7) Å), two trans medium (mean value of 1.981(2) Å), and two trans short distances (mean value of 1.918(6) Å), as shown in Table 3. Such a distortion is consistent with Jahn–Teller behavior for high spin manganese(III) complexes. It is similar to that reported for [Mn(malonate)3]3− (16) and for one of the two crystallographically independent molecules of Mn(tropolonate)3, the other exhibiting a tetragonal elongation (17). It should be noted that the Jahn–Teller theorem only predicts that for degenerate states a distortion must occur; it does not give any indication of what kind of geometri-


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In the Laboratory

Figure 3. Comparison of the ORTEP drawing (50% probability) of the Mn(acac)3 complex obtained at 293 (left) and at 100 K (right). This figure is shown in color on page 340 of the table of contents.

Figure 4. Crystal packing of Mn(acac)3 structures viewed down the b axis. The trace of the unit cells (β = dotted lines; δ = dashed lines; ε = doubled lines) with different numbers of crystallographically independent molecules (β = 1 molecule with dotted lines; ε = 2 molecules, one with dotted lines and one with doubled lines; δ = 4 molecules, one with dotted lines, one with doubled lines, and two with dashed lines) evidences the strict relation between the three polymorphs. This figure is shown in color on page 340 of the table of contents.

cal distortion will occur or how great it will be. The students recognized the possibility of an orthorhombic distortion in the coordination distances, together with the two canonical distortions, the tetragonal elongation and the tetragonal compression, predicted by Jahn–Teller arguments and reported in textbooks. They also observed that the orthorhombic distortion leads to an asymmetric complex with the three chelating ligands no longer equivalent. The structural determination of the δ form is not as precise as that of the ε form (The standard uncertainty on distances of 0.01 Å being about four times those in the ε form) and does not permit a clear detection of the geometrical nature of the distortion. This is a consequence of the low variable or observable ratio (Table 2) owing to the high number of crystallographically independent molecules in the non-centrosymmetric P21 space group and the low resolution limit (largest angle at which a diffraction spot can be experimentally observed) of the crystal. The difference in the precision between the two structures also became apparent when the 464


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students plotted the ORTEP drawing of the two molecular structures (Figure 3). The differences in the thermal ellipsoids (50% of probability) between the 293 and 100 K structures clearly evidence the improvement in the localization of atoms in the frozen structure with the achievement of more precise bond distances. The analysis of the configuration of the four independent molecules indicated that two molecules have ∆ and the other two Λ configurations. The students observed that the chiral crystal structure is composed of a racemic mixture of the chiral pair ∆, Λ. Another important discovery was the detection of the irreversible solid–solid phase transition of the δ phase to the ε phase. The phase transition implies a strict relation between the crystal structures of the δ and ε forms. The students also observed that the crystal structures of the β and ε phases could be related since the unit cells differ only in a twofold increase of the a axis and an exchange of the a and c axes. Furthermore, if the phases β and δ are related to the ε form (due to


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In the Laboratory

the observed phase transition) then, according to the transitive property, the crystal structures of phases β and δ should also be strictly related.2 The relation between the crystal structures of these three forms became apparent when diagrams of the crystal packing of the three structures were overlapped (Figure 4; a color version of Figure 4 is available in the table of contents on page 340). In Figure 4 the crystal packing of the three phases is represented as viewed down the b axis. The trace of the unit cells: β = dotted lines (red); δ = dashed lines (green); ε = doubled lines (blue)

with different numbers of crystallographically independent molecules: β = 1 molecule with dotted lines (red); ε = 2 molecules, one with dotted lines (red) and one with doubled lines (blue); δ = 4 molecules, one with dotted lines (red), one with doubled lines (blue), and two with dashed lines (green)

demonstrates the strict relationship between the three structures. Only small changes in the orientation of the molecules in the crystal produces these three different crystalline forms.

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Supplemental Material

Instructions for the students are available in this issue of JCE Online. Notes 1. Crystals from both crystallization protocols give the same diffraction pattern that can be indexed with the δ-polymorph unit cell. 2. The a and c axes of the δ form are the two diagonals of the (a, c) face of the β form (Figure 4). Successive analysis showed that upon reducing the data of δ form using a β unit cell, removing half of the observed reflection data, it is possible to obtain a crystal structure close to that reported in the literature for the β form (7). In particular, this structure has anomalous anisotropic thermal factors for the carbon atoms of one acetylacetonate ligand, similar to those reported for the published β structure (7).

Literature Cited

Concluding Remarks The advanced undergraduate project described in this article integrates the well-tested protocol for synthesis of Mn(acac)3 with the low-temperature characterization of the crystal structure of the synthesized complex. The goal to investigate the Jahn–Teller behavior of the high-spin Mn(III) complex, Mn(acac)3, was achieved with the recognition of the unusual orthorhombic distortion of the coordination polyhedron. Furthermore, a unique aspect in these experiments was the identification of new crystalline forms of Mn(acac)3 that actually appeared to the students as a chemical discovery, as indeed it is. The students observed that the answer obtained with new investigation is not always straightforward and that very often new question arise, with new problems. They also realized that science often proceeds by a process of successive approximation. In conclusion, the experiment was an exciting experience for advanced undergraduate students. Not only does it introduce crystallography, one of the most powerful technique in chemistry, but more importantly it demonstrates that new experiments allow new knowledge to be gained with respect to that well established in the literature. Finally, this experiment is a good example to demonstrate to students that wellestablished knowledge in chemistry may also result in unexpected challenges. Acknowledgments We thank Lucio Randaccio for helpful discussions, Neal Hickey for critical reading of this manuscript, and the stu-

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dents (Francesco Amoroso, Giovanni Birarda, Gianluca Bossi, Damiano Carnio, Giorgio De Faveri, Sofia Derossi, Rita De Zorzi, Francesco Ravalico) who performed the experiment.

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1. Ruttledge, T. R. J. Chem. Educ. 1998, 75, 1575. 2. Woolins, J. D. Inorganic Experiments; VCH: Weinheim, Germany, 1994; p 117. 3. Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry—A Comprehensive Laboratory Experience; Wiley: New York, 1991; p 224. 4. Morosin, B.; Brathovde, J. R. Acta Crystallogr. 1964, 17, 705. 5. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 3rd ed.; Interscience: New York, 1972; p 851. 6. Hon, P. K.; Pfluger, C. E. J. Coord. Chem. 1973, 3, 67. 7. Fackler, J. P.; Avdeef, A. Inorg. Chem. 1974, 13, 1864. 8. Fackler, J. P. Inorg. Chem. 2002, 41, 6959. 9. Stults, B. R.; Marianelli, R. S.; Day, V. W. Inorg. Chem. 1979, 18, 1853. 10. Hoggard, P. E. J. Chem. Educ. 2002, 79, 420. 11. Hunter, A. D. J. Chem. Educ. 1998, 75, 1297. 12. Crundwell, G.; Phan, J.; Kantardjieff, K. A. J. Chem. Educ. 1999, 76, 1242. 13. Collaborative Computational Project, Number 4. Acta Crystallogr. D Biol Crystallogr. 1994, 50, 760. 14. Robinson, W. T.; Sheldrick, G. M. Crystallographic Computing 4. Techniques and New Technologies; Isaacs, N. W., Taylor, M. R., Eds.; International Union of Crystallography; Oxford University Press: Cambridge, 1988; p 366. 15. Sheldrick, G. M.; Schneider, T. R. Macromolecular Crystallography, Pt B 1997, 277, 319. 16. Lis, T.; Matuszewski, J.; Jezowska-Trzebiatowska, B. Acta Crystallogr., Sect. B 1977, 33, 1943. 17. Avdeef, A.; Fackler, J. P.; Costamagna, J. H. Inorg. Chem. 1974, 13, 1854.


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Crystallographic Study of Manganese(III) Acetylacetonate: An

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