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Syntheses and Characterization of Ruthenium(II) Tetrakis(Pyridine) Complexes An Advanced Coordination Chemistry Experiment or Mini-Project Benjamin J. Coe Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom;
[email protected] This experiment involves the syntheses and characterization of several coordination complexes of ruthenium, the element found directly beneath iron in the middle of the dblock of the periodic table. Ruthenium has an extremely extensive coordination chemistry that combines relatively facile ligand substitution reactions with good product stability (1). Its complexes show well-understood redox and photochemical properties (2) and are of interest for various practical applications, both current (e.g., in catalysis) and future (e.g., in molecular electronics). This experiment is designed for third-year undergraduate students and provides synthetic experience and qualitative interpretation of the spectroscopic properties of the ruthenium complexes. The intention is to demonstrate how physicochemical techniques commonly applied to purely organic compounds can also give useful information about organotransition metal complexes. Furthermore, the chemistry involved is well suited to provide a simple introduction to research-style work. The most successful learning outcomes are achieved by students working in pairs. Experimental Overview Most of the synthetic chemistry involved in this experiment is summarized in Scheme I. The first and second prepa-
rations involve substitutions of the DMSO (dimethylsulfoxide) ligands in the versatile precursor cis-RuIICl2(DMSO)4 (1), the synthesis of which was originally reported in 1971 (3), and subsequently refined (4). Because it is a well-defined compound and easy to handle, 1 is an attractive alternative to the hygroscopic material RuCl3⭈xH2O (x = 2–3). The Xray crystal structure of 1 has been reported (5). The preparation of trans-Ru IICl 2(py) 4 (2, py = pyridine) has been previously published (4), but trans-RuIICl2(acpy)4 (3, acpy = 4-acetylpyridine) is a new compound. The third reaction generates the complex trans-RuII(NCS)2(py)4 (4), which is derived from substitution of both of the chloride ligands in 2, by analogy with similar reactions (6, 7). The crystal structure of 4 has recently been reported (8), but with only brief synthetic details and no spectroscopic or other data. The preparation of 4 requires some initiative in designing a simple experimental procedure. A previous report has described the preparation directly from RuCl3⭈xH2O of a complex assigned as cis-RuII(NCS)2(py)4 (9), but it is likely that this was in fact the trans isomer 4. Further reactions of 2 with chosen salts can also be carried out, if desired. If only complexes 2– 4 are prepared and characterized, then about 10 h of laboratory time should be sufficient. The extension to include a further speculative synthesis requires an additional 4 h for
cis–Ru(II)Cl2(DMSO)4 1
py, reflux
acpy, toluene, reflux
Cl N N
Ru
O
O Cl
Me N
N
N
N Me
Cl
Ru
N Me
Cl O
2
Me N
3
O
ⴚ
NCS , py/H2O, reflux
NCS N N
N Ru
N
NCS 4
Scheme I. Synthetic reactions carried out in this experiment. Complexes 2–4 are characterized using various spectroscopic techniques.
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In the Laboratory
practical work. Because ruthenium is a precious metal, all of the reactions in this experiment are carried out on a relatively small scale, accentuating the need for careful laboratory manipulation. Complexes 2–4 and any further product(s) are characterized by 1H NMR, infrared, and UV–visible spectroscopies and also either positive ion electrospray or FAB mass spectrometry. The data are then correlated with the molecular structures of the complexes, showing how the different techniques can give distinct and often complementary structural information. Several related synthetic and characterization experiments involving Ru(II) complexes with phosphine ligands have been published (10–13), but this experiment features a unique combination of simple synthetic procedures for complexes of pyridine ligands with multiple spectroscopic and spectrometric techniques and the option of extension to include some experience of research. Experimental Objectives A. Prepare and isolate pure, dry samples of the following complexes: 1. Trans-dichlorotetrakis(pyridine)ruthenium(II), trans-RuIICl2(py)4 (2, py = pyridine). 2. Trans-dichlorotetrakis(4-acetylpyridine)ruthenium(II), trans-RuIICl2(acpy)4 (3, acpy = 4-acetylpyridine). 3. Trans-dithiocyanatotetrakis(pyridine)ruthenium(II), trans-RuII(NCS)2(py)4 (4). B. Obtain and interpret the plexes 2–4.
1H
NMR spectra of com-
C. Obtain and interpret the positive ion electrospray or FAB mass spectra of complexes 2–4. D. Record and interpret the electronic absorption (UV– visible) spectra of complexes 2–4. E. Record and interpret the infrared absorption spectra of complexes 2–4. F. Carry out a further reaction of 2 with a chosen salt and characterize the product.
Experimental Section
Special Reagents and Equipment Required Precursor complex cis-RuIICl2(DMSO)4 (1) Small (ca. 10-mL capacity) sintered glass filter funnels
Because acpy is more expensive and difficult to remove than py (acpy boils at 212 ⬚C, compared with 115 ⬚C for py), the reaction is carried out in toluene as a cosolvent. Using the remainder of the sample of 1 provided, trans-RuIICl2(acpy)4 is prepared and the dry product is stored in a glass sample vial. Although also very efficient, this reaction gives lower yields than that to produce 2; a yield of about 85% is good (lower values indicate incomplete product precipitation).
Preparation of Trans-RuII(NCS)2(py)4; 4 Previous studies have shown that 2 reacts with sodium nitrite, NaNO2, or potassium cyanide, KCN, in aqueous pyridine under reflux to afford the derivatives transRuII(NO2)2(py)4 or trans-RuII(CN)2(py)4, respectively (6, 7). By reference to the most recently published procedure (7) for trans-Ru II (NO 2 ) 2 (py) 4 , a preparation of transRuII(NCS)2(py)4 is designed and carried out using a suitable portion of complex 2 prepared previously. This reaction is considerably less efficient than those to produce 2 or 3; a yield of about 50% is typical. Further Preparative Experiments In similar fashion to the synthesis of 4, a further novel derivative is prepared by reacting 2 with the Na+ or K+ salt of another common anion, for example, azide (N3᎑), carbonate (CO32᎑), cyanate (NCO᎑), or diethyldithiocarbamate (Et2NCS2᎑). 1H
NMR Spectroscopy Although this technique is widely used to characterize purely organic compounds, it can also give useful information about organotransition metal complexes of hydrogencontaining ligands. By using appropriate service facilities, the 1 H NMR spectra of complexes 2–4 and any further product(s) are obtained and the observed signals are assigned and tabulated. Mass Spectrometry Various mass spectrometric techniques have been found to be particularly useful for studying organotransition metal complexes having molecular masses less than about 1000 amu, and larger molecules can also often be meaningfully studied. By using appropriate service facilities (where available), the electrospray or FAB mass spectra of complexes 2– 4 and any further product(s) are obtained and the observed signals are assigned and tabulated.
Preparation of Trans-RuIICl2(py)4 (py = pyridine); 2 All four DMSO ligands in 1 are readily substituted by pyridine on heating in neat pyridine, while the chloride ligands remain coordinated. Using a portion of the sample of 1 provided, trans-RuIICl2(py)4 is prepared. The dry product is stored in a glass sample vial. This reaction is essentially quantitative, and typical yields are in excess of 90%.
Electronic Absorption (UV–Visible) Spectroscopy Organotransition metal complexes exhibit a rich variety of electronic absorption spectroscopic properties, and species in which charge-transfer excitations can occur often display particularly intense visible colors. The electronic absorption spectra of complexes 2–4 and any further product(s) are measured and the wavelengths (nm) and molar extinction coefficients (M᎑1 cm᎑1) of the principal bands are determined.
Preparation of Trans-RuIICl2(acpy)4 (acpy = 4-acetylpyridine); 3 This reaction is analogous to that used to prepare 2, but a substituted pyridine ligand (4-acetylpyridine, acpy) is used.
Infrared Absorption Spectroscopy The quantity of useful information that can be obtained from the infrared absorption spectra of organotransition metal complexes depends on the extent to which groups having
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In the Laboratory Table 1. Spectroscopic and Spectrometric Data for Complexes 2–4 Complex
2
1
H NMR δ/ppm; integral; J/Hz d, 8.63; 8; 5.1
UV–Visible λmax/nm; ε/M᎑1 cm᎑1
Infrared (KBr) νmax/cm᎑1
398; 24,000
ES–MS m/z
1478
487
t, 7.65; 4; 7.6
1445
449
t, 7.13; 8; 7.0
759
408
690 3
d, 8.74; 8; 6.8
484; 24,900
d, 7.58; 8; 6.8
1688
655
1269
490
s, 2.65; 12; --4
449
d, 8.37; 8; 5.1
2090
534
t, 7.71; 4; 7.6
377; 22,600
1480
488
t, 7.21; 8; 7.0
1444 804 762 693
NOTE: NMR multiplicities: s = singlet, d = doublet, t = triplet.
Hazards None of the chemicals or procedures involved in this experiment are of an especially hazardous nature, but all normal precautions for handling potentially harmful chemical reagents should be taken when carrying out the practical work. Pyridine may affect fertility. Diethyl ether has an extremely low flash point (᎑40 ⬚C) and hence should be kept well away from any potential sources of ignition. Azide salts are potentially explosive. All reactions and vacuum filtrations should be carried out in an efficient fume cupboard, and particular care should be taken to not release pyridine into the open atmosphere of the laboratory owing to its highly unpleasant odor. Rotary evaporators that have been used to remove pyridine should be thoroughly cleaned with acetone. Results
Spectroscopic and Spectrometric Data Representative 1H NMR, UV–visible, and infrared spectroscopic and electrospray mass spectrometric (ES–MS) data for complexes 2–4 are given in Table 1. Representative UV– visible spectra are shown in Figure 1. Questions To Stimulate Further Reading and Discussion The following may be assigned as homework and may also be discussed (at least in part) in the class. Note that it will be necessary to refer to literature material, such as that cited herein, to arrive at satisfactory answers to some of these questions.
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1. Discuss the syntheses and molecular structures of cisand trans-RuIICl2(DMSO)4. 2. Explain why 1 reacts with pyridine, et cetera, to give substitution of the DMSO but not the chloride ligands, but 2 may react with suitable Na+ and K+ salts in aqueous pyridine to afford chloride-substituted derivatives. 3. Consider how the spectroscopic and spectrometric data obtained for complexes 2–4 and any further product(s) provide stereochemical information. 4. Why do the complexes 2–4 display such different colors? 5. Discuss the outcomes of any further substitution reaction with 2, if attempted.
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ε / (1000 molⴚ1 L cmⴚ1)
readily assignable vibrational frequencies are present. The infrared spectra of complexes 2–4 and any further product(s) are measured and any readily attributable intense bands are assigned.
30 25
4
3
2
20 15 10 5 0 240
290
340
390
440
490
540
590
640
690
740
λ / nm Figure 1. Representative UV–visible spectra of 2–4 in dichloromethane.
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Supplemental Material
Instructions for the students, notes for the instructor and answers to the questions are available in this issue of JCE Online. Literature Cited 1. Schröder, M.; Stephenson, T. A. In Comprehensive Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press: Oxford, United Kingdom, 1987; Vol. 4, pp 277–518. 2. Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85–277. 3. James, B. R.; Ochiai, E.; Rampel, G. L. Inorg. Nucl. Chem. Lett. 1971, 7, 781–784. 4. Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 204–209.
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5. Alessio, E.; Mestroni, G.; Nardin, G.; Attia, W. M.; Calligaris, M.; Sava, G.; Zorzet, S. Inorg. Chem. 1988, 27, 4099–4106. 6. Bottomley, F.; Mukaida, M. J. Chem. Soc., Dalton Trans. 1982, 1933–1937. 7. Coe, B. J.; Meyer, T. J.; White, P. S. Inorg. Chem. 1995, 34, 593–602. 8. Chen, J.-L.; Han, L.; Chen, Z.-N. Acta Crystallogr., Sect. E 2002, 58, m588–m589. 9. Wajda, S.; Rachlewicz, K. Inorg. Chim. Acta 1978, 31, 35– 40. 10. Cifuentes, M. P.; Roxburgh, F. M.; Humphrey, M. G. J. Chem. Educ. 1999, 76, 401–403. 11. Queiroz, S. L.; de Araujo, M. P.; Batista, A. A.; MacFarlane, K. S.; James, B. R. J. Chem. Educ. 2001, 78, 87–89. 12. McDonagh, A. M.; Deeble, G. J.; Hurst, S.; Cifuentes, M. P.; Humphrey, M. G. J. Chem. Educ. 2001, 78, 232–234. 13. Higgins, S. J. J. Chem. Educ. 2001, 78, 663–664.
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