In the Laboratory
Chromium(III) Acetate, Chromium(III) Acetate Hydroxide, or µ3-Oxo-esakis-(µ2-acetato-O,O 9)-triaqua-trichromium(III) Acetate? Determining the Structure of a Complex Compound by Analytical and Spectroscopic Methods Liliana Strinna Erre and Giovanni Micera* Department of Chemistry, University of Sassari, Via Vienna 2, 07100 Sassari, Italy Tadeusz Glowiak and Henryk Kozlowski Institute of Chemistry, University of Wroclaw, 14 Joliot Curie St., 50383 Wroclaw, Poland
Interest in undergraduate laboratory work is stimuExperimental Procedure lated by experiments illustrating the practical application of spectroscopy for identification of unknown compounds. To Two reagents were selected with compositions assigned reach a conclusion about the unknown’s structure, the stuas follows: dent accumulates physical and chemical information and • chromium(III) acetate hydroxide, from Aldrich Chemimatches it with spectroscopic data. However, experiments cal, with formula Cr3(CH3CO2)7(OH)2 (1) and a metal that do not demand a deep analysis of spectral results could content of ~24 wt % be not very attractive. If the compound is suitable for the • chromium(III) acetate, from Carlo Erba, with formula detailed application of theory and the available data set is Cr(CH3CO2)3 (2), and a minimum metal content of 24 “unique” for the unknown’s structure, more convincing con± 1 wt % clusions may be drawn and the experiment is more challenging. [Cr3(µ2-CH 3COO)6 (µ3 -O)(H2O)3 ]DHB?2H2 O, the model A case study is an experiment illustrating use of infracompound, where DHB is 2,6-dihydroxybenzoate(1-), can be red spectroscopy and model compounds to reassess the obtained by reaction of chromium(III) acetate (ca. 3 × 10{3 structure of chromium(III) acetate, overthrowing the conmol of chromium) and the acid (3 × 10{3 mol) in 20 × 10{3 ventional structure. This chemical is commercially available dm3 of water at 60 °C with stirring. Crystal-like precipitates (Aldrich and Carlo Erba) with different conventional formuform almost immediately. They are filtered off, washed with lations. Unfortunately, chromium(III) acetate is not cited in water and dried at room temperature. common inorganic chemistry textbooks, which instead exAnalysis. Calc. C19 H19O22Cr3: C, 29.66; H, 4.32. Found C, tensively describe carboxylate complexes of Cr(III) (1, 2). 29.76; H, 4.17%. These usually consist of the trimetallic µ3 -O-Cr3 cage wellknown for the “basic” acetates of trivalent ions including CH3COONa, [Zn(CH3COO)2(H2 O)2] and 2,6-dihydroxyFe(III) and Rh(III). Here three metal ions lie at the corners benzoic acid are commercially available reagents. of a nearly equilateral triangle of side ca. 3.3 Å, and the The results presented here were obtained as follows. center is occupied by the triply bridging oxygen. The six carIR spectra were recorded with an FT Bruker IFS-66 interboxylate groups of the complex unit are equivalent, each of ferometer using KBr disks (4000–600 cm{1 ). Powdered them behaving as an O,O9-bridging ligand connecting a pair samples were spread on NaCl plates (1700–1200 cm{1) or of metal ions. A sixth monodentate donor, for example waas Nujol mulls between polyethylene disks (600–150 cm{1). ter or a nitrogenous base, occupies the octahedral position Elemental analyses (C and H) were obtained with a Perkin– trans to the triply bridging oxygen. The best known example Elmer 240 B elemental analyzer. Thermogravimetric data of such an arrangement is represented by [Cr 3 (µ 2 were obtained with a Perkin–Elmer TGS-2 apparatus in niCH3COO)6(µ3-O)(H2O)3 ]Cl?6H2O (3). Owing to the recurrence of such a structural motif in Cr(III) chemistry, it is rather surprising that monomeric formulations are assigned to Table 1. Elemental Analyses chromium(III) acetate. In this paper we describe Cr C H Cr2O3 a simple, attractive laboratory experiment demFormula (%) (%) (%) (%) onstrating that chromium acetate has a compoCr3(CH3CO2)7(OH)2a 27.87 3.84 37.79 25.86 sition different from the conventional ones and a is endowed with a trinuclear structure. The Cr(CH3CO2)3 31.45 3.96 33.16 22.69 method is based on the joint application of anaCr(CH3CO2)3?H2Oa 29.16 4.49 30.75 21.04 lytical and spectroscopic data and may serve to a [ C r (µ C H C O O ) (µ O ) ( H O ) ] ( C H C O O ) 2 6 . 3 0 4.26 35.66 24.40 illustrate the usefulness of symmetry concepts 3 2 3 6 3 2 3 3 and IR-Raman selection rules in the assignment 1b 25.94 3.72 35.0 23.9 of inorganic structures (see, e.g., ref 4). b 2 26.83 3.66 35.0 23.9 *Corresponding author.
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aCalculated. b Found.
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In the Laboratory Next, the samples may be examined by IR spectroscopy. IR spectra are almost identical. Comparison with the spectra of a series of acetate complexes with divalent or trivalent metal ions shows that the tested samples share a great number of bands (Fig. 1) with [Cr 3 (µ 2 -CH3 COO) 6 (µ3 O)(H2 O)3]DHB?2H2 O, which according to single-crystal Xray diffraction involves an oxo-centered triangle structure with bridging acetato ligands (5). The spectra are distinctive enough of such a structure and fit very well the analysis presented by Johnson et al. for trinuclear “basic” acetates and formates (6). Theory predicts that nonlinear polyatomic molecules have 3N – 6 normal vibrations. The molecular symmetry determines the selection rule (e.g., the IR and/or Raman activity of the vibration modes) (4). By using certain approximations, which appear justified by the consistency of the results, the following procedure has been used for the analysis of a [Cr3 (µ2 -CH3 COO)6 (µ3 -O)L3]+ unit (6):
Transmittance, a.u.
a
b
1800
1400 1000 Wavenumber, cm {1
600
Figure 1. IR spectra of (a) “chromium(III) acetate” and (b) [Cr3(µ2-CH3COO) 6 (µ 3-O)(H2O)3]DHB?2H2O.
trogen or air atmosphere. Diffuse reflectance electronic spectra were taken with a Beckman Acta MIV spectrophotometer using the diffuse-reflectance technique with BaSO4 as reference. Results and Conclusions
1. The central molecular framework [Cr 3( µ2 CH3COO)6(µ3-O)L3]+ may be considered as an assembly of 31 atoms with D3h symmetry, by omitting the hydrogen atoms and assuming monatomic L ligands. Further simplification may be introduced by separating the vibrations into three groups, associated with Cr3O and MO4 units and M– L bonds, respectively. 2. The vibrations pertinent to acetate and L ligands, being relatively independent of the central framework, may be separated and recognized by comparison of free ions or molecules.
A planar D3h M3 O unit will have a total of four fundamental vibrational modes belonging to species A91, A02 , and 2E9 (4). The selection rules are A91 (Raman), A02 (IR) and 2E9 (IR-Raman). This means that three peaks are expected in the IR spectra and three in the Raman spectra. One of the IR bands is found at about 660 cm{1 , which is the most distinctive feature of planar µ3 -OCr3 in all trinuclear “basic” acetates, attributable to νas(OCr3 ) vibration mode (E9). The A02 mode δs (OCr3) is found at 287 cm{1, whereas the E9 mode δ as(OCr3) probably occurs outside the region investigated (below 100 cm{1). The vibrational modes of the CrO4 units formed by acetate coordination can be deduced by assuming three planar D4h MO4 moieties interacting with each other. A single MO4 unit has seven fundamental vibrational modes belonging to species A1g(R), B 1g(R), A2u(IR), B 2g(R), B 2u (in) and 2Eu(IR), for a total of 9 modes. Almost all of them can be seen in the spectra of [Zn(CH3 COO)2 (H2O) 2] (7), where the lowering of symmetry to D2h splits the degenerate modes of the square planar unit ZnO4 formed by the coordinated acetate groups and activates the inactive vibration B2u (corresponding to B1u in D2h symmetry). Pertinent literature values are listed in Table 2.
The experiment is ideal for teaching purposes because assignment of the results requires critical evaluation of analytical data, selection of appropriate spectral techniques, and application of theoretical principles. The usual strategy followed in the structure assignment of inorganic compounds or metal complexes entails determination of analytical data and then acquisition of physical and spectral information by a variety of techniques depending on the nature of the metal ion, the ligand, etc. In this case, preliminary considerations lead to the conclusion that the combination of elemental (C and H) and thermogravimetric (H 2 O and residue) analysis and vibrational (IR and, if available, Raman) spectroscopy may be suitable. The quantitative determination of the metal content (e. g., by atomic absorption spectrometry) could complement elemental analyses. The first point is the examination of the analytical data for the two selected Cr(III) acetate samples (Table 1). It can be observed that elemental analyses of 1 and 2 do not satisfactorily fit the calculated values. In addition, thermogravimetric measurements Table 2. Vibrational Modes of Acetate Complexes (cm{1)a indicate the presence of water molecules not included [Zn(CH3COO)2(H2O)2]b in the conventional formuZnO4 D 4h Eu A 1g B 2g Eu B 2u las of the compounds. A D B + B A B B + B B 1u weight loss of at least 5% 2h 2u 3u g 1g 2u 3u may be detected below 150 278 (IR) 264 (R) 229 (R) 195 (IR) 115 (IR) °C. In both cases the resi[Cr3(µ2-CH3COO)6(µ3-O)(H2O)3]+ due is Cr 2 O 3 , as subCr3O (D 3h ) A 02 E9 E9 stantiated by the compara6 6 0 ( I R ) 2 8 7 ( I R ) < 1 00 tive IR examination of an c authentic sample, the CrO4 A 02 E9 E9 amount being ca. 35 wt %, 439 (IR) 408 (IR) 360 (IR) consistent with a Cr content of ca. 24 wt %. aIR = infrared; R = Raman. bReference 7. c Three CrO units under D 4 3h symmetry.
B 1g
A 2u
Ag
B 1u
83 (R)
?
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In the Laboratory
Transmittance, a.u.
a
500
b
c
400 300 200 Wavenumber, cm{1
100
Figure 2. Vibrational modes generated by interaction of three D4h MO4 units under D3h symmetry.
Figure 3. IR spectra (500–150 cm{1 range) of (a) “chromium(III) acetate”, (b) [Cr 3(µ 2-CH 3COO)6 (µ 3-O)(H 2O) 3 ]DHB?2H2 O, and [Zn(CH3 COO)2(H2O) 2] (c).
The interaction of three MO4 units under D3h symmetry generates a number of vibrational modes as can be seen from the correlation scheme shown in Figure 2. The major absorptions may be easily distinguished at 439 and 408 cm{1 (Fig. 3), which are assigned to the IR active components A02 and E9 of the D4h νd(Eu) mode. A third one, due to E9 (from A1g ), occurs at 360 cm{1 as a shoulder. These values are in quite good agreement with those of the literature. Finally, the IR bands ascribed to vibrational modes of carboxylate groups may be examined. Acetate adopting the µ2-bridging mode has C 2v symmetry. The vibrational modes associated with the carboxyl group may be derived by analogy with the tetraatomic formate ion, for which six vibrations, all active in IR and Raman spectra, are predicted: 3A1, B1 , 2B 2. Four of these are associated with the O-C-O framework: B1 or νas(OCO), A1 or νs(OCO), B2 or π(OCO), and A1 or δ s(OCO). According to the literature (8) they are observed at 1578, 1414, 646, and 615 cm{1, respectively, for sodium acetate. The bands most diagnostic for the coordination mode of carboxylate are the symmetric and asymmetric stretches (ν), and both shift to higher frequency values with respect to the free ion, as a consequence of the bridging mode. In chromium(III) acetate the νas and νs stretches are observed at 1620 and 1450 cm{1, being typical for µ2 -bridging bidentate behavior, in fairly good agreement with literature assignments (6). In principle, band splittings due to the interaction of six equivalent acetate groups would be expected, but these are little pronounced and normally can be observed at low temperature. All these experimental findings, particularly the distinctive IR features of a Cr3O unit, lead to the conclusion that “chromium(III) acetates” are endowed with a trimetallic arrangement where six acetates bridge three metal ions connected by a triply bridging oxygen, so that a total
of four carboxylate oxygens surround each chromium ion. Water molecules act as the terminal ligands completing an almost regular octahedral coordination polyhedron at each metal center, as sketched in Figure 4. The last question concerns the identification of the seventh acetate ion, which is demanded to make the compound neutral and is confirmed by elemental analyses. IR patterns exhibit a broad absorption centered at 1545 cm{1 that is missing in the spectrum of [Cr3(µ2 -CH3COO)6 (µ3O)(H2O)3 ]DHB?2H2 O (Fig. 1).1 The absorption is consistent with the νas stretch of an acetate ion outside the trinuclear complex unit, the corresponding νs mode being a shoulder occurring at 1420 cm{1. Accordingly, this frequency value is close to that of sodium acetate which, in KBr pellets, shows bands at 1578 and 1420 cm{1. On the whole, both the Cr(III) acetate samples examined herein may be satisfactorily described as [Cr3 (µ 2CH3 COO)6 (µ3 -O)(H2O) 3](CH3 COO). Calculated analytical data for such a composition are listed in Table 1. The values fit the experimental analytical data much better than those for either Cr(CH 3 CO 2 ) 3 or Cr3 (CH 3 CO 2 )7 (OH) 2 stoichiometry. At the end of the process further questions may be raised—for example, the effect of isotopic substitution (deuteration) on the vibrational properties of the examined structures or the inadequacy of IR spectroscopy to yield definite proof for coordinated water. Concerning the latter topic, it could be shown that, in addition to the three fundamental modes of the free water molecule (antisymmetric and symmetric OH stretches and HOH bending), rocking, wagging, and M–O stretching modes may be expected for coordinated water (9). Usually the former are not very sensitive to metal coordination, whereas the latter are rather difficult to detect. However, on the basis of model
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In the Laboratory fairly good agreement with literature reports on the same chromophore (10). Note 1. In [Cr3(µ 2-CH3COO)6 (µ3-O)(H 2O)3]DHB?2H 2O the carboxylate groups may be distinguished in IR spectra only by νs stretches (1450 cm {1 for acetate and 1390 cm{1 for DHB). The overlap of the corresponding νas modes yields a broad band centered at 1610 cm{1.
Literature Cited Figure 4. The [Cr 3(µ 2-CH3COO)6(µ3-O)(H2O)3] + trinuclear unit.
comparison and the known preference of Cr(III) for octahedral coordination, there is little doubt that the three water molecules take part in the coordination. Finally, examination of reflectance absorption spectra may complement the above results by showing two dominant d–d bands at 22700 and 17250 cm{1 , 4A2g → 2 T2g and 4A → 2T (F), respectively, consistent with an octahedral 2g 1g field at the metal ion. Further, a set of sharp bands around 14,000 cm{1 is assigned to 4 A2g → 2T1g . This data set too is in
1. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 466. 2. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; p 1199. 3. Chang, S. C.; Jeffrey, G. A. Acta Cryst. 1970, B26, 673. 4. Drago, R. S. Physical Methods in Chemistry; Saunders: Philadelphia, 1977; Chapter 6. 5. Glowiak, T.; Kozlowski, H.; Strinna Erre, L.; Micera, G. Inorg. Chim. Acta 1996, 248, 99. 6. Johnson, M. K.; Powell D. B.; Cannon, R. D. Spectrochim. Acta 1981, 37A, 995. 7. Johnson, M. K.; Powell D. B.; Cannon, R. D. Spectrochim. Acta 1981, 37A, 899. 8. Ito, K.; Bernstein, H. J. Can. J. Chem. 1956, 34, 170. 9. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1977; p 227. 10. Blake, A. B.; Yavari, A.; Hatfield, W. E.; Sethulekshmi, C. N. J. Chem. Soc. Dalton 1985, 2509.
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