Synthesis and Variable-Temperature 'H NMR Conformational Analysis of is($-cyclopentadienyl)titanium Pentasulfide An Experiment for an Integrated, Advanced Laboratory Course Anthony Diaz, Catherine Radzewich, and Mark ~ i c h o l a s ' Western Washington University, Bellingham, WA 98225 I n a n attempt to move beyond the compartmentalized approach to laboratory instruction for chemistry majors, we recently combined our advanced inorganic and physical chemistry laboratory courses into one integrated, twoquarter sequence. Each course is scheduled for 8 h of laboratory per week (two 4-h session) plus I h of labflecture. The dual themes are synthesis and physical measurement, and four or five experiments are completed in each 10week quarter. One of the more common experiments using NMR in t h e traditional physical chemistry laboratory i s the s t u d y of t h e h i n d e r e d r o t a t i o n of N,N-dimethylacetamide (1). This compound is obviously unsuitable for the integrated lab a s defined above because i t is orpanic. However. the studv of a temoerature-deoendent physical process is Indeed germane.' As an alternative to NN-dlmcth\.lacctnmide. our stud e n t s synthesize t h e organomet"a1lic compound CsHdzTiSs. Then variable-temperature 'H NMR spectroscopy in da-toluene solution is used to follow quantitatively the chair-to-chair conformational change of the six-membered TiSs ring. As a n added benefit, the temperature range for this experiment, 293-400 K, is indeed very convenient and does not require liquid nitrogen for sample , cooling. Students calculate first-order rate constants, k's, a t each temperature and the transition-state thermodynamic parameters from the h h e n i u s and Eyring plots.
Synthesis of (q5-Gj~5)2~i.Ss Sulfur (0.321 g, 10.0 mmol) and a magnetic stir bar were introduced into a Schlenk tube, which was then fitted with a rubber septum, evacuated, and filled with argon. The evacuation and filling were repeated three times. With simultaneous stirring, Super Hydride solution (4.0 mL, 4.0 mmol) was injected, causing a n immediate reaction with Hz liberation and formation of the polysulfide anion Ss2-. After the evolution of Hz was complete, the septum was temporarily removed, and (C5Hs)zTiClz (0.500 g, 2.0 mmol) was added under a strong flow of argon. After stirring overnight under argon, the deep-red mixture was transferred to a round-bottom flask, washed with a few mL of THF, and roto-evaporated to dryness. The red solid was then dissolved in 50 mL of CHzClz and filtered through a bed of Celite. The filtrate was roto-evaporated to dryness to yield 0.58 g of dark-red crystalline product (85% yield). The equations for this two-step sequence are 2Li(C2H&BH + 5&S8+ Li& + 2(C2H5)3B+ H2 (C5HS1,TiCl2+ Li2Ss+ (CsHs12TiS5+ 2LiCI Results The Synthesis
Our synthesis of ( q 5 - ~ s ~ s h Tisi ~a smodification of the procedure reported by Shaver and McCall(2, 3). With our vacuum manifold, four Schlenk tube reactions can be run simultaneously, all con\ 1s-s nected t~ one tank of inert gas. This is a one-pot synthesis and does not require sodium-dried and distilled THF. (The reaction can be done with higher yield in the style reported by Shaver and McCall by first preparing a separate soluExperimental tion of (CsHdzTiClz in dry, distilled THF and then cannulac ing this into the LizSs solution.1 Caution: Use a fume hood for all procedures camed out using bis(q5-cydopentadienyl)titaniumpentasulfide after its In our exoerimental orocedure. the THF from the S u ~ e r synthesis to minimize exposure to this malodorous product. Hydr~desolution srrvss as the solvent for the reaction. Its volume is sulficient for efficient reaction to occur. Alrhoueh The synthesis of ( q 5 - ~ s ~ s ) z Twas i ~ 5 carried out in a we have worked on a macroscale level, this could also Ybe Scblenk tube under inert atmosphere, using an argon atscaled down to 100 mg or less of titanocene dichloride. mosphere and a vacuum line with a double-tube manifold The purity of the product can quickly be assessed by 'H vented to the fume hood. The product, once formed, apNMR spectroscopy. Two clean singlets are observed a t 6.39 pears to be relatively air-stable, so subsequent procedures and 6.10 ppm in CDC13 a t 297 K (with protic CHC4 as-rob-evaporation and filtration-are done in air in a n efsigned a s 7.27 ppm); they reflect the magnetic nonequivaficient hood. We purchased from Aldrich lency of the two sets of cyclopentadienyl ring protons. I n addition, the IR and W-vis spectra are in agreement with titanocene dichloride, (CSH5),TiCI, those reported (2,4). 1M Li(C2H5I3BHsolution in THF (Super Hydride) d8-toluene(0.5-mLampoules) The Spectroscopy
CJ
\TiLs\s/
.
.
41 other chemicals were reagent-grade. Solvents were dried over molecular sieves before use. NMR spectra were run on a Bruker AC 300 spectrometer, and temperatures were calibrated against ethylene glycol.
The 'H NMR spectrum of ( q 5 - c s ~ s ) z T iis~temperature s dependent and exhibits the expected line-broadening cbar'Author to whom correspondence should be addressed Volume 72 Number 10 October 1995
937
Thermodynamic Resultsa acteristics associated with a two-site exchange mechanism. The kinetics of the chair-to-chair conformational change can best be followed in Ea AG AH AS ds-toluene (5) due to i t s high boiling point. Spectra are recorded a t 10" intervals from 320- Student datab 62,1 2,2 76.5 f 4.1 59.2 2.2 -58.0 f 6.3 400 K;linewidths and intensities are measured. For tvmical data, the linewidth increases Abel et al. 69.1 + 2.3 76.4 + 4.2 66.2 + 2.3 -33.9 f 6.5 from appro&nately 0.6 Hz at 293 K to 20 112 at the c~alelicencepomt at approximatel? 360 K; ~ : , " ~ ~ . ~ ~ ~ , "I ~a ~~~ O H C , ~ ~ ~ ~ a ~ o e resonances ~ ~ l r p ' , are ~ , oatn 59 and ppm then it narrows a s the tt!mpCrature is increased melny pear of to .me F ~ s e oas m e nternal referenceano assqncd as 2 09 ppm further.
*
g6EK),
Calculations The first-order rate constants can be calculated by one of two methods: the approximate method using linewidths and intensities, or a more precise computer simulation of the total lineshape. Due to its relative simplicity, we have chosen the approximate method, described in detail by Gasparo and Kolodny ( I ) . Three different equations, each approximate in nature, are used to calculate values of k in three different temperature ranges, for slow, intermediate, and fast exchange. Although this method is only approximate, the measurements can be completed in a 4-h lab period, and with care the results are reasonable. Linewidths are calculated by a Lorentzian line-fitting routine, which is part of the Bruker software, and intensities are read from the monitor. As done on the Bruker spectrometer, there is no need to print out and measure spectra by hand. A typical Arrhenius plot is shown in Figure 1.Although the three sets of data points (slow, intermediate, and fast exchange) do not fit seamlessly, a satisfactory least-sauares fit is obtained with a n activation energy, E., in good'agreemmt with that reported by Abel and coworkers (51 and obtained hy totnl lineshape analysis. Adimilarly good correspondence,- save for the ieast accurately cal& lated A S , is found for the transition-state thermodynamic parameters obtained from the Eyring plot of in (kll') vs. LIT, shown in Figure 2. These values are listed for comparison in Table I. The goodness of fit is very much dependent on the shimming a t each temperature and the phasing of each spectrum. I n five separate temperature runs with five separ a t e l y p r e p a r e d samples, t h e following r a n g e of parameters were observed: 56 < E, < 69 kJImol 74 < AG < 79 kJ1mol 52 < A H < 66 kJ1mol -41 < A S < -77 JIK-mol
0 2.0
2.5
3.0 10001T
3.5
Figure 1. Arrhenius plot of In kvs. 1,0001Tfor(CsHs)iTiSs.
..
Standard deviations were in some cases twice a s large a s those reported in the table. Summary The synthesis of ( q 5 - ~ s ~ s h Tisi ~well-suited s for an advanced laboratory course that emphasizes the integration of synthesis and measurement. The synthesis demonstrates inert atmosphere and Schlenk techniques and has the virtue of yielding a n air-stable product. The NMR measurements are done above ambient temperature without need for cooling with liquid nitrogen. The physical process, that is, the chairto-chair conformational change of ( q 5 - ~ s ~ s j z T can i ~ salso . be discussed from an energetics viewpoint, stressing the similarity with cyclohexane, a molecule undoubtedly encountered previously in organic chemistry where conformational analysis is first introduced. At this point molecular mechanics can be introduced by discussing t h e recent study of (115C5H5j2TiS5 by Lawless and Marynick (6)or by allowing students to calculate energies for various conformations (boat, chair, twist-boat, skew-boat) if a suitable computer program is available.
938
Journal of Chemical Education
Figure 2. Eyring plot of In ( W l ) vs. 1,0001Tfor(CsHs)zXSs. Acknowledgment Purchase of the NMR s~ectrometerwas facilitated hv a n NSF-11.1 Grant and funding from the M. .I. Murdock ~ h a r i tahlr l h i t . We are rrratcful to D. Bicklev. Uni\.ers~tvCollege of the caribou,for first informing u s of the existence of the title compound. Literature Cited 1. Gaspara, F P.; Kolodny, N. H. J
Chem Educ 1917,54.258-261.
2. Shaver. A,; McCall, J. M. Orgonometollics 1984.3,1823.1829. 3. Shaver A.: Mecall,J. M.: Marmolejo, G. lnorg. Synth. 1990.27. 59-65. 4. Muller, E. G.; Petersen, J. L.; Dahl, L. F. J Orgonomat C k m . 1976,111,91-112. 5. Abel E.W.; Booth, M.; Omll. K G. J Orgonomet. Chem. 1978.160,75~79. 6. Lawless.M . S.:Marynick.D. S. Inorg Chem. 1891,30,3547-51.
