Gerard R. Dobson
and John R. Paxson North Texos State University Denton, Texas 76203
I
I
The Kinetics and Mechanism of a Metal Carbonyl Substitution Reaction
The areas of organotransition metal chemistry in general ( I ) , and metal carbonyl chemistry in particular (8) have become ones of increasing interest to research workers. Many of the research results are now being incorporated into advanced texts in inorganic chemistry. Unfortunately, relatively few laboratory experiments in this area, particularly experiments suited to a physical chemistry laboratory, are available. An experiment which employs instrumental methods and which is suitable for use either in a physical chcmistry or advanced undergraduate inorganic chemistry laboratory (which has as a prerequisite physical chemistry) is a kinetic investigation of the reaction of (2,5dithiahexane)tetracarbonylchromium(O) or -molybdenum(0) with triethyl phosphite (3) (DTH)M(CO), (yellow)
+ 2 P(OC.H5),
-
IP(OCJ&)d,M(CO), (colorless)
+ DTH
co
HzC,
CO 1,
L,M(CO),
+ DTH
The first mechanism thus yields a rate law first order in both substrate and ligand concentrations, while the rate behavior observed for the second mechanism depends upon the relative magnitudes of ka and k3 [P (OC2H6)3]. Considering only the two limiting cases, for k2 > ka[P(OC2Hs)3], it reduces to
which is kinetically indistinguishable from (4). For any of the three possibilities outlined above, use of large excesses of the phosphite in the kinetic runs, "flooding," will yield pscudo-first-order kinetics. The (DTH)I\l(CO)&substrates are yellow, while the solvent, ligand, and products are colorless. Thus thc reaction can be monitored by obsewation of thc decrease in the intensity of the yellow color as a function of time. Plots of h ( A t AblaRk) versus t should thus be linear. Plots of the rate constants thus obtained, k . (~ = k [P(OC2H&]) versus [P(OC2H&] for scveral kinetic runs under pseudo first-order reaction conditions will also be linear, with slope equal to the second-order ratc constant, k . I t has been found (3) that this reaction proceeds for & = !ICr, largely via rate law (7), while for M = Mo it proceeds largely via ratc law (6). While the two mechanisms are kinetically indistinguishable, differentiation betwecn the two is possible on the basis of the entropies of activation obtained from data at two temperatures. For mechanism (2) the calculated entropy of activation is AS?, which is highly negative (found: - 18.5 0.2 eu) (3) as is expected for a rcaction in which the transition state is much more ordered than is the ground state. For mechanism (3) with k2 >> k3 P(OC2HS)3the calculated entropy of activation is AS1* AS3*-AS2* which is exp~ctedto be somewhat positive (found: +8.1 + 0.3 eu) (3). Thus from thc data obtained the student is able to choose among the three possibilities. Sufficient kinetic data for the purposes described above can be obtained in two 3-hr laboratory periods.
-
(2)
The second involves reversible dissociation of one end of the bidentate ligand followed by nucelophilic attack of the phosphite on the resulting five-coordinate intermediate and other rapid steps. Ha?
and
(1)
There are two simple mechanisms through which the reactions might reasonably be expected to proceed. The first involves nucleophilic attack of the triethyl phosphite, (L), at the octahedron to give a seven-coordinate activated complex or intermediate, which then rapidly yields the observed product. HG
The respective rate laws, the latter being determined through use of a steady-state approximation on the intermediate, (3-A), are
*
+
Volume 49, Number I, January 1972 / 67
mast intense hands in the infrared spectrum in chloroform solution are four carhonyl stretching absorptions ((DTH)Cr(CO),: 2020, 1914, 1898 and 1869 cm-'; (DTH)Mo(CO),: 2030, 1919, 1905 and 1868 cn-') (6).A vibrational analysis of the carbonyl stretching modes of (DTH)MO(CO)~ together with an illustration of its earbonyl stretching spectrum have been given (6). The mass spectra for the two complexes show, among other peaks, those corresponding to the parent ions, (DTH)M(CO).+ and ions derived from successive loss of earhonyls from the parent ion to ultimately give (DTH)Mf. These mass spectra are quite similar to that of (DTH)W(CO),, for which the cracking pattern has been given (7). The nmr spectra. of the compounds in CDCla exhibit singlet resonances a t 2.30 (-CHz) and 2.59 (-CH..) ppm downfield from tetramethylsilane for (DTH)Cr(CO)r and a t 2.35 and 2.63 ppm, respectively, for uncoordinated dithiahexme (neat).
Reaction Conditions
T
Complex
"C
(DTH)Cr(CO),
45 60
(DTH)Mo(C0I4
40 50
[P(OCIHs)sl (mol/llter)
0.3 0.05 0.15 0.3 0.20 0.05 0.15
Approx (sec)
tx/,
5500 5000 1700 850 1600 3000 1000 nnn
Kinetic Runs
Experimental
The substrates, (DTH)Cr(CO). and (DTH)MO(CO)Imay he prepared in advance by the instructor, or they may be prepared and characterized by the students, experiments suitable for a senior level inorganic laboratory. Such an experiment would he similar to the preparation and characterization of (7-mesitylene) Mo(CO)s (4).
Preporotion ond Characterization o f the Substrates Place 4.4 g of hexacarhonyl ehromiam(0)l or 5.3 g of hexacarbonyIm~lybdenum(O)~, together with 5.0 g of DTHa and 50 ml of the appropriate solvent (for Cr(CO)a,xylem; for Mo(CO)., n-heptane) in a 100 ml two-necked flask fitted with an air-cooled reflux condenser, an oil bubbler and a nitrogen inlet.' Caution: 2,5-dithiahexane has an extremely obnoxious odor; the reaction and subsequent work-up must he carried out in an efficient hood. Flush the reaction vessel with a stream of nitrogen for 5 mi". Then heat the reaction solution a t reflux for 4 hr (Cr(COh) . . ~or , 2 hr (Mo(CO).). When the reaction is complete, again sweep nitrogen through the flask as it c o o l to room temperature. Reduce the volume of the reaction solution by two-thirds employing a water aspirator vacuum and then filter the solution under suotian (in s. hood); use a medium porosity sintered glass frit. Recrystallize the precipitate from chloroform excluding as much air as possible: pass a stream of nitrogen aver the hot chloroform solution as the complex is being dissolved, and then rapidly filter the solution by suction under an inverted funnel fastened to the rine stand which hAI< the 1il~eo1.g flnik nr.d thrwtgh ~ h w hn clrrnm of nlrrogrn i i r?piJIy p n c . 4 ,er f i m r r 'l'hert ptrrFr the filter h i k ronluininr ~ ~ ~tubing ~ irom 1 1 1 ~nilrtl1l8c filtrntr with ~ > i t r < . gby m V O I I I I C C tlw gen supply to the side arm and allowing the nitrogen to pass through the flask for a few minutes. Seal the flask with 8. rubber stopper and simultaneously discontinue the nitrogen stream. Remove the nitrogen tube and quickly replace it with the ruhher bulb of a medicine dropper. Place the closed suction flask in a freezer overnight,, and then collect the crystals; wash them with a. small quantity of petroleum ether and dry them under pump vacuum. If the student has prepared the complex he may then characterize i t employing ir, mass and/or proton nmr spectra. The
68
/
Journal o f Chemical Education
Weigh accurately, in a. hood, the amount of triethyl phospbite6 required to give the molarity required (see table) into a 50 ml volumetric flask. Add the solvent, reagent grade 1,2-dichloraethane, to a. total volume of -45 ml. Place the stoppered flask into a constant temperature water bath set a t the required temperature (see table), and allow i t to equilibrate for approximately ten minutes. Using a syringe and needle, carefully add solvent to bring the volume to the inscribed mark. This procedure affords a. ligand solution of the required concentration a t the reaction temperature. Since it is necessary to subtract the value of a ligand-solvent blank from the absorhznce of the reaction solution a t a given time, place the empty cuvet in the spectrophotometer (e.g., a Bausch and Lomb Spectronic40) set a t 8. wavelength of 425 nm for (DTH)Cr(CO), ar 400 nm for (DTH)Mo(CO)., and adjust the instrument to zero and 100% transmittance. Then withdraw a 3 ml sample of the solvent-ligsnd solution from the valumetrio flask, place i t in the empty euvet, and record its absorbance. This value, the absorbance of the blank, must be subtracted from each spectrophotometer reading obtained during the kinetic run. Return the solution to the valumetrio flask. The zero and l0Ogb values for the clean, dry cuvette must be set on the spectrophotometer immediately before each data paint is recorded Weigh the required amount of substrate (for (DTH)Cr(CO)c -2.5 me: . . . ~-30 . mn) -. into a 100 ml volumetric -, for IDTH)MolCO),. . flask. Pour the solvent-ligand solution quickly into this flask and flush the solution and flask quickly with nitrogen using 8. long syringe needle attached to the nitrogen tubing. Quickly close the flask with a ruhher septum, swirl the solution to dissolve the remainder of the substrate, and return the reaction flask to the constant temperature bath. Allow the reaction solution to reequilibrate for about five minutes. After equilibration, withdraw samples from the reaction flask as follows: employing a 5-ml syringe and a syringe needle long enough to reach the bottom of 1
Pressure Chemical Co., 3419 Smallmrsn St., Pittsburgh, Penn.
152Il1
Climax Molybdenum Corp., 1270 Avenue of the Americas, New York, N. Y. 10020. Pfaltz and Bauer, 126-02 Northern Blvd., Flushing, N. Y. 11368. For an illustration of this set-up, see Ref. (4). Aldrich Chemical Co., Inc., 940 West St. Paul Ave., Milwaukee, Wisc. 53233. For hest results, the ligand should be fractionally distilled over Na. before use, and stored under a nitrogen atmosphere.
the reaction flask: purge the syringe with nitrogen. Inject 3 ml of nitrogen into the reaction flask, and remove a 3 ml sample, and record the time at which the sample was withdrawn. Be sure to withdraw a small sample of the nitrogen atmosphere along with the solution to prevent the hot liquid from squirting from the needle as the sample is removed. Record the absorbance of the resction solution. Repeat this procedure for at least ten data points. Dispose of the used reaction solution, and clean and dry the syringe, needle, and cuvet between each determination. At least three points per half life should he obtained. The slower reactions should be monitored over ahont 1.5 half lives; the fester reactions can he monitored to three half lives. Plot the absorbance (from which the velue of the blank has been subtracted) as a function of time on semi-lag paper, and determine the value ~ the plot. From data a t two temperatures7 determine of k , from the rate law and activation parameters for the reaction. From this information, determine the most probable mechanism for the reaction. The instructor might wish to require the student to write and use a least squares computer program to obtain the best fit of the !cobad data, to perform an error analysis an the activation data to ascertain whether or not the values as determined are in good agreement with the published values (8),or to derive various expressions employed in the data treatment, e.g., mte law (5).
Literature Cited (1) Fore. brief introduction, aee. KINO.R. B.. "Tramition Metal Organ.=-
metallie Chemistry." Academic Press. New York, 1969.
(2) For a comprehensive review, see, C h ~ o s n ~ z z oF., , E n c o ~ r .R.. A N D
NATT*,G.. in "Organic Synthesis uio Metal Carbonyla." (I. Wender and P. Pino. Eds.). Intersoienoa Publishers. New York. N. Y... 1968.
.
Vol. 1, PP. 1-272.
(3) F n e m . G. C.. AND DOBBON. G. R.. Inmg. Chem., 7, 5% (1968). 45. 119 (1968). (4) Aaem~cr,R. J., J. CHEM.EDUO., If. C. E.. A N D WILKINBON, G., . I . C h m SOD.. 4454 (5) MANNERSKANTE. (1862). a , Y.. J. CHBM.Eooc., 47, (6) D*nsm=ouna. D. J.. a m D ~ n s ~ s s o n nM. 33 (1970). (7) KIN.. R. B., J. Amel. Cham. SOD., 90. 1429 (1968). (8) Bmsorr, S. W.. The Foundetions of Chemioal Kinetics," MaGrawHill Book Company, Ina., Nsw York, 1960, pp. 86-94.
' '
Obtainable from the Hamilton Co., Box 307, Whittier, Calif. 90608. The student should obtain the one value of k.b.a required at the lower temperature during the first lahoratory period to gain the proficiency at data taking necessary to obtain data for the three required runs at the higher temperature in one laboratory period.
Volume
49, Number 1, Jonuory 1972
/
69
