Transition Metal Complexes with 1,4,7-Trithiacyclononane A Laboratory Experiment in Coordination Chemistry Gregory J. Grant' and Penny L. Mauldin2 The University of Tennessee at Chattanooga, Chattanooga, TN 37403 William N. Setrer The University of Alabama in Huntsville. Huntsville. AL 35899 Several laboratory experiments dealing with the synthesis of coordination complexes have recently appeared in this Journal (1). We wishto renort a lahoratorv exneriment that deals with'the synthesis or four t r a n s i t i o n m e h complexes containine a crown thioether lieand. For some time macrocyclic complexes of transition metals have generated exceptional interest by their similarities to biological systems and their unusual kinetic and thermodynamic properties (2). Although the coordination chemistry of macrocyclic amines, especially tetradentate m i n e lieands, and crown ethers has been extensively studied. the coordination chemistry of crown thioethers has, until recently, remained relatively unexplored. This situation has dramatically changed in the past few years due t o the intense research activity centered around the tridentate thioether, 1,4,7-trithiacyclononane,9S3. ~~~
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A Several excellent reviews of the coordination chemistry of this important lieand and other thioethers have recentlv appearid (3-6). The 953 ligand has currently been cornplexed t o over 20 transition metal and p-block metal ions, and the list continues to expand a t a rapid pace. The metal ions that currently have been complexed with 9S3 are highlighted in the periodic table shown below in the figure. In 1983Setzer and co-workers reported the first three examples of 9S3 coordination behavior with the svntheses of the his This experiment complexes of Cu(II), ColII), and Ni(I1) utilizes their svnthesis for the nrenaration and characteriza. . tion of four metal crown thioether complexes. Although simole thioethers are usuallv considered to be weak lieands. 9S3 binds metal ions stro&y and exerts a surprishgly high ligand field affect (3,4). The octahedral structure of the his complexes of 9S3 is shown below.
(5.
' Author to whom WrresDondence should be addressed.
Masters of Science ~ d k t i o nstudent, The Univenity of Tennessee at Chattanooga.
The high ligand field effect of 9S3 can be seen in a laboratory experiment. As a lahoratorv teachine exercise, the e x ~ e r i m i n t can be used to reinforce several import& concepts regarding strong and weak field ligands and their relationships to the electronic structures of high spin and low spin complexes. In addition, the characterization of the complexes affords the student the opportunity to apply the analytical techniques of UV-visible and infrared spectroscopy as well as electrochemical and magnetic measu~ements;the study of four transition metal complexes. Complexation studies of 9S3 were initially hampered by the low yield in itssynthesis, but the yield has been dramatically improved through the use of cesium salts (8).In fact, the compound can be purchased commercially, and small amounts may he used that will supply a sufficient quantity of the complexes for the characterizations as descrihed helow. All of the complexes are isolated as the tetrduoroborate salts rather than the more hazardous perchlorate salts, and the descrihed synthesis employs ethanol as a reaction solvent. Spectroscopic, electrochemical, and magnetic susceptibility data for the four complexes are included in Tables 1and 2. Experlrnental All materials were used as received and are commercially availahle. It is not necessary to carry out the ethanol refluxes under a hood. The tetrafluorohorate salts of cohalt(II),copper(II),and nickel(I1)were purchased from Alfa Catalog Chemicals. The tetrafluorohorate salt of iron(I1) was purchased from Strem Chemicals, Ine. The ligand 1,4,7-trithiacyclononane,953, can be purchased from Aldrich Chemical Company. Caution: Diethyl ether and ethanol are flammable. There should he no open flames during the experiment. The 9S3 ligand is a skin irritant, and usen should he wearing protective gloves and gaggles. Measurements UV-visible spectra were obtained with a Varian DMS-200 UVvisible spectrophotometer using water as a solvent. FT-IR spectra were obtained with a Beckmann FT-1100 spectrophotometerusing the KBr disk method. Magnetic moments were obtained using a
Periodic table wim elem&
that have been complexed with 9S3 shaded.
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Number 7 July 1991
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Johnson-Matthey Magnetic Susceptibility Balance and employing standard diamagnetic correction factors (9).Cyclic voltammagrama were recorded using a Princeton Applied Research model 384A Polarographie Analyzer. The supporting electrolyte was 0.1 M (BU)~NPFI; in CH3CN,and sample concentrations were approximately 1 mM. All vojtammograms were recorded at a scan rate of 100 mV/s. The standard three-electrode configuration was as follows: glassy carbon working electrode, Pt wire auxiliary electrode, and AgIAgCl reference electrode. Preparation of Bis( l,4,7-trlthiacyclononene)nlckel(ii) Tetrafluoroborate A solution of 0.127 g (0.400 mmol) of niekel(JI) tetrafluoroborate hexahydrate in 10mL of absolute ethanol was added to a solution of 0.212 g (1.18 mmol) of 9S3 in 15 mL of absolute ethanol. The solution immediatelyturned violet. It was then refluxed gently for 1 b. The dark pink solid obtained from the reflux was filtered and washed with 20 mL absolute ethanol and 15 mL dietbyl ether. The yield was 0.169 g or 71.2%. Preparation of Bis( 1,4,7-trithiacyclononane)cobait(ii) Tetrafluoroborate A solution of 0.102 g (0.300 mmol) of cohdt(I1) tetrafluoroborate hexahydrate in 15mL of absolute ethanol was added to a solution of 0.108 g (0.600 mmol) of 953 in 15 mL of absolute ethanol. The solution turned violet immediately. The solution was then refluxed gently for 1 h, and a violet precipitate formed. The violet solid obtained from the reflua was filtered and washed with 20 mL absolute ethanol and 15 mL diethyl ether. The yield was 0.070 g or 39%. Preparation of Bls( 1,4,7-trithiacyciononane)copper(ll) Tetrafiuoroborate A solution of 0.444 g (0.560 mmol) of eopper(I1) tetrafluoroborate hydrate in 14 mL of absolute ethanol was added to a solution of 0.150 g (0.830 mmol) of 953 in 16 mL of absolute ethanol. A dark brown precipitate immediately formed. The solution was then refluxed gently far 1 h. The brown solid obtained from the reflux was filtered and washed with 20 mLabsolute ethanoland 15mL diethyl ether. The yield was 0.104 g or 83.8%. Preparation of Bis( 1,4,7-trlthiacyclononane)lron(ll) Tetrafluoroborate A solution of 0.127 g (0.390 mmol) of iron(I1) tetrafluoroborate hexahydrate in 10mL of absolute ethanol was added to a solution of 0.200 g (1.10 mmol) of 983 in 15 mL of absolute ethanol. The solution immediately turned violet. It was then refluxed gently for b. The purple solid obtained from the reflux was filtered and washed with 20 mL absolute ethanol and 15 mL diethyl ether. The yield was 0.171 g or 74.3%. Dlscusslon The spectroscopic and magnetic susceptibility data are presented in Table 1. Note that the Co(I1) and the Fe(I1) complexes are present in their low-spin configurations. Such behavior is typical of thioether complexes in general (4). Thioethers tend to form rare examples of octahedral Co(I1) low spin behavior. T h e FelII) complex is also low spin h u t does have some residual paramagnetism as noted previously (10). T h e Cu(I1) and Ni(I1) magnetic moments are typical values for octahedral complexes of those transition metal ions (9). The strong hands present in the FT-IR spectrum form 115&1030 em-' are due to B-F stretching frequencies, which are typical of noncoordinated tetrafluoroborate ions. In the UV-visible spectrum of the Ni(I1) complex the3Az,3Tzstransition is seen a t 784 nm, and the 3Az, 3Tu(F) is seen a t 529 nm. T h e intense hand a t 325 nm is a charge transfer transition that obscures the 3Az, 3Tu(P) transition. In the UV-visible spectrum of the Fe(I1) complexes the 'A1, 'TI, transition is seen at 523 nm, and the 'A,, transition is seen a t 396 nm. The spectra of the Cu(I1) and Co(I1) complexes are more difficult to interpret due to charge transfer transitions and the splitting of d-d transitions due to Jahn-Teller distortions. T h e electrochemical data for the four complexes are summarized in Table 2.
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Journal of Chemical Education
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Concluslons T h e syntheses described herein are straightforward and present n o significant hazards. They allow the student the opportunity to characterize four interesting coordination com~lexesand ~ r o v i d einsieht into the interrelationshio between coordination chemistry theory and experiment. In addition. thev hiehlieht one of the most currentlv researched ligands, 1,4,?-tr~hi&clononane. This experiment can be e x ~ a n d e dto incor~oratethe svntheses of coordination complexes containing the tridentate thioether ligand, 1,4,7-trithiacvclodecane (10S3J. or t h e hexadenrate lipand, 1,4,7,i0,13,16-he~athiac~cl~ctadecane (1856) (3,5,li). A
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(luestlons for Furlher Study T h e following list of questions incorporates literature references to assist with the explanation and the discussion of the student's results. T h e number of the literature reference is indicated. I. Which of the four complexes will exhibit .lahn-Teller disrurrions from octahedral s)mmetry" (7, 10) 2. \$'hat is the point group symmetry of the ligand 1.4,:-rrithiacyclononane, and why is a conformational change unnecessary when metal ions bond to this ligand? How does this compare to macrocvcles such as 14.34 (1.4.8.11-tetrathiacyclotetradecane)? . . . (7,12) 3. What do the positive formal electrode potentials indicate for the Cu(I1) complex? (13) 4. How do the magnetic susceptibility data show that 9.93 is a strong field ligand? Draw electronic configurations d-orbital splitting diagrams for all four complexes showing the number of unpaired eleitrons in each case. (10) 5. Using your UV-visible spectra data, calculate the values for Dq and B in cm-' for the NI(I1) and Fe(I1) comdexes. (10) 6. Identifv which of the observed electronictransitions are d-d rransitionaand which onrsarp charge-transfer transition$. (4,101 7. What color would you anticipate for the his complex of 9S3 with Zrull). IZn(SS:O.]-., and why? (31 ~
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Literature Cited
Table 1. Spectroscopic and Magnetlc Susceptlblllty Data Complex
Table 2.
iR Absorptions. cm-'
UV-VIS
A,,(e).nm
pen
B.M. n
Eleetrochemlcal Data tor Complexes E112, V
Complex
"6.
AgIAgCI
[NI(9S31dBF& +I36 quasi-rev [CO(~S~)~]IBFJ~+0.34 rev -0.51 rev + O) ~m rev [CU(~S~)~](BF~ [Fe(9S3)21(BF& +1.37 rev
Couple N13+lNIZt
co3+lco2+ Co2+1Cot Cu2+1Cu+ Fe3+lFeZ+
2. Mdson, G. A. In Coordinolion Chemistry 01 Maerocyclic Compounds: Melson, G. A.. Ed.: Plenum: New York: 1 9 7 9 ; ~17. 3. Coopor. S. R.: Rewle, S. C. Structure Bonding 1990.12.1, 4. Conper,S. R.Acc. Chem.Res. 1388.21,141. 5. Sehroder. M. Pure Appl. Cham. 1988,60,517. 6. Murray, S. G.: Hart1oy.F. R. Chem.Reu. 1981,81,365. 7. Setzer. W. N.: 0gle.C.A.: Wi1son.G.S.; Glass. R. S.lnorg. Chem. 1383,22,266. 8. Blower, P. J.; Caoper, S. R. Inorg. Cham. 1987.26.2W9.
9. Figgir. B. N.: Lewis, J. In Modern Coardinotion C h ~ m i s t rLewis, ~; J.: Wilkina, R. G.. Edr: Interscience: New York, 1960:p4M). 10. Wieghardt, K.: Kuppers, H.J.; Weiss, J. Inor6 Chem. 1985,24,3061. 11. Setrcr. W. N.: Cacioppo, E. L.; Guo, Q.: Grant, 6. J.; Kim, D. D.: Hubbard, J. L.: VanDerveer.D. G.lnorg. Chem. 1990.29.2672. 12. Glass, R. S.: Wilson, G.S.:Setzer, W. N J . A ~ them. . soe. 1980,102,5068, 13. Docksl. E. R.: Jones. T. E.: Sokol. W. F.:Engerer. R. J.: Rorabaehar. D. B.:Ochrymowycz.L.A. J.Am. Chm.Soc. 1976.98.4322.
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July 1991
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