Preparation, Analysis, and Reactivity An Advanced Undergraduate Laboratory Project in Organometallic Synthesis Charles D. Schaeffer,Jr.,' and Lori K. M y e d Elizabethtown College, Elizabethtown, PA 17022
Suzanne M. Coley2, Julie C.
Otter, a n d Claude H. Yoder Franklin & Marshall College, Lancaster. PA 17604
Althoueh manv haloeen derivatives of divalent tin and lead are well kn-, examples of stable, monomeric, divalent eermanium. tin. and lead c o m w u n d s contain in^ directly atiached carbon or nitrogen areextremely rare. The first stable, monomeric, divalent nitrogen derivatives resulted in an additional surprise: when the ligand is N[Si(CH)d&, the resulting compounds are intensely colored a n d represent some o f t h e few colored monomeric compounds of t h e main group elements (highly colored Ge(II), Sn(II), and Pb(I1) derivatives also r e s u l t w h e n the isoelectronic l i e a n d CH[Si(CH,)Ji is used). T h e red color of t h e title comp&nd is made even more atrikine -bv. the fact that the NISi(CH,),I, . ... ligand, when incorporated in compounds of many other elements,
.
."
does n o t impart color (1-2). As p a r t of our continuing series on advanced undergraduate laboratory projects (3-4) involving synthesis a n d multinuclear N M R spectroscopy, we offer a project that explores the preparation and analysis of this unique tin(I1) amine. This project is an appropriate vehicle t o introduce students t o many aspects of t h e synthesis, analysis, a n d spectroscopic characterization of air- a n d moisture-sensitive compounds using the fundamental techniques of modern organometallic chemistry. The presence of six commonly studied N M R nuclei makes this compound particularly amenable t o analysis by multinuclear N M R spectroscopy.
Synthesis General. The preparation of the title campound is adapted from published procedures (5-9). All equipment, including vessels used in weighing of reagents, syringesused for transfer of n-butyllithinm, magnetic stir bars, and reaction glassware, must be thoroughly dried in a 110 O C oven for at least 8 h prior to use. In addition, all reagents and solvents should he anhydrous. The reaction solvent, tetrahydmfuran (THF), is dried and deoxygenated by distillation under inert atmosphere from calcium hydride or from sodiumben~o~henone ketal, and is stored over molecular sieves until use, when it is withdrawn from a closed storage vessel by syringe. If students are unfamiliarwithsvrinee-and-needle technioues..we recommend that thev ,. first eun~ultseveral excellent reviews of these methuds ( 1 0 - l 4 ) , and that they become proficient at dispensing water or arelone by injection through a rubber septum \preferably containing a pressure. equalizing needle connected to a bubbler and inert gas source) into an Erlenmeyer flask.
.
~~
.
' Author to whom inaulrles should be addressed.
~
~
'Present address: L'.K.M..~epiltmontof &emism. Princeton Unlvsrsily, Princeton, NJ 08544: S.M.C.. Depanment of Chemistry, The Pennsylvania State Universlly. Unlversily Park, PA 16802.
.
.
PIe~orotionof. BisN.N-bis(trimethvlsilvl)ominoltin(In. . . Fit a three:neck. 250-mL round-bottom flaskwith condenser...nressure~~-~~~~~ equalizing dropping funnel, magnetic stir har, and inert gas inlet exit Jystrm. Place a solut~onof hexamethyldisilaranp (7.4 g, 0.1)46 moll in 30 mL of anhydrou* THF in the dropping funnel. Carefully transfer 20 mL of a commercial 2.4 M solution of n-butyllithium (0.048 mol) in hexanes to the three-neck flask by injection throuah a rubber septum placed in the center neck. ~autian:n-bntyllith~um will cause burns if i t contacts skin, and it may ignite in the presence of water. This reagent should he used with extreme care (gloves and goggles) in the presence of the instructor, and only after becoming thoroughly familiar with its proper handling and disposal. Begin magnetic stirring and cool the flask and contents to 0 "C using an icelwater bath. Increase the inert gas flow ratetoprevent oil in the bubbler from pulling back into thereaction vessel. After 30 min of magnetic stirring at 0 OC, begin a slow dropwise addition (one drop every second) of the amine solution to the organolithium reagent. The bubbling rate observed in the mineral oil will increase. owine to the evolution of n-butane eas. After the addition is comolete. a h w the resultine~ vellow , . solution to warm ~~.~ to roam temperature with conrtant stirring. Prepare a solution of anhydrous tindl, chloride 14.7 g, 0.021 moll in 4U ml. of dry THF, and transfer it to thedropping funnel. Normally, the tin(I1) chloride is not completely dissolved a t this point, and care must he taken not to allow the dropping funnel to clog during the addition. Once again, cool the reaction flask to0 'C, and begin a slow, dropwise addition of tin(I1) chloride solution with constant magnetic stirring. The reaetion mixture will proceed through a variety of color changes, beginnine with dark vellow or red. and endine with maroon. After additron is complete, remove the ice water hath, and stir the reaction mixture form least 2.5h A finewhiteprecipitate oflithium chloride will now he visible. Filter the dark reaction mixture into a standadtaper Erlenmeyer flask through a fritted, coarse-porosity, 150-mL BOchner funnel protected with a l-in. layer of oven-dried Celite under a nitrogen or dove bae oreviouslv fitted with tubine for arean atmosohere in a " " airnultaneous . . inert ~ eas ~ oureine ., and suction. A water asoirator eonnection ritted with a stopcock end trap is a convenient, adequate, and adjustable source ul vacuum, and the product is unaffected if it is exposed for no longer than 15-20 min to the small amount of water vapor and oxygen introduced by this step (a crystallizing dish containinga thinlayer of phosphorus pentoxide may he included in the event additional protection is desired). While in the glove bag, transfer the darkliauid to a drvround-bottom flask whose ca~acitv is a t least twice the-volume ofthis liquid in order to minim& thk cnnseouences of "bumoine" durine" solvent removal. It is desirable u,clean the filtration apparatus with several small portiuns of THF prior toexposing the glassware toair. Otherwise,rancentrated nuric acid wrll be needed tu remove rhr resulting tin(1V) oxidat~onand hydrolysis products. Remove solvents from the resulting solution at reduced pressure (no greater than 30 tom), while applying aminimum amount of heat. We have found that a rotary evaporator, such as the BiichiIBrinkmann Rotavaoor-R svstem. and a hot water bath is ideal. Once again, a water aspirator i~a satiriactury source of reduced pressure, pnwided that solvent removal can be accomplished in leis than 1% 20 min (we hare fitted our system with a calcium chloridc drymg tube, inert gas inlet, and mineral oil bubhler to allow hack-filling of ~
.
~
~~~~
a
~~
~~~
~~~
~
~~~
~
~~
~
~
~~~~. ~
~~~
~~
~
-. " ~ . ~ ~ ~ . ~~~~~~
~
.~..
.
~~~~~
~
~~~
~
~
.
Volume 67
Number 4
April 1990
347
the flask with argon after completion of solvent evaporation). Rem o -~ d~ of .~ -~. t h e solvents hv atmomherio pressure distillation (even under a n inert atmosphere) will result in rapid thermal decomposition of t h e product into finely divided, potentially pyrophoric tin metal. Purify the remaining crude maraon liquid by vacuum distillation an a 12-in. column packed with glass heads or helices. We have found it necessary &surround such a column with a heated jacket, such as an Ace Mini-Lab Heating and Outer Jacket, catalog number 9329-06 (for a 300-mm inner column length), in order to avoid excessive beating of the distillation pot. Any pressure between 0.01 and 10 tom is adequate to afford a boiling paint sufficiently low to prevent substantial thermal decomposition of the product. Collect a t least three fractions, using care to prevent unnecessary exposure of these fractions to air when changing receiving flasks. The use of a distillation head, such as Ace Mini-Lab Distilling Head, catalog number 9357 or 9358, equipped with a sidearm inlet and stopcocks to facilitate isolation of the receiving flasks from the column, and to permit filling of these receivers with inert gas, is desirable. The distillation, conducted under the conditions described here, will produce a viscous, hright orangered liquid, with bp of 109-110 *C/ 0.75 torr (5), with little or no lower boiling impurities. A dark brown tar remains in the pot (never distill to dryness). This tin(I1) m i n e frequently forms orangered crystals on cooling, which have a reported melting point of 37-38 OC (6,8). A yield of 75% is typical.
NMR Dataa
~~~~
Physical Properties and Reactivify Solubility. The product is highly soluble in most common organic ethers and hydrocarbons. Halogenated hydrocarbon solvents, including chloroform and carbon tetrachloride, are known to react explosively with tin(1V) m i n e s (15) and should he avoided. Elemental Analysis. Routine carbon/hydrogen/nitrogen and total ash analysis performed on this sample will lead to satisfactory results, providing that the air and moisture sensitivity of the sample is recognized. Anal. calcd for C I Z H ~ ~ N ~C,S 32.80; ~ ~ S H, ~ :8.26; N, 6.37; Si, 25.56; Sn, 27.01%. The analytical firm must be informed of the need to prepare the sample in a dry box. Instructors may wish to demonstrate the technique of sealing a sample of the m i n e (under argon or nitrogen a t 1 atm or under vacuum) in preparation for mailing to the analytical firm. Discussion should focus onacceptable error limits for the C/H/N analvsis after the results are received. .--.St.ndentsmsv ne~fotheir~ om-total ash analvsis . ~ . ~ ~ ~usine ~aaua . reeia .. (three volumes of eoneentramd hydrochloric acid to one volume of eoncentrawd nitriearidj and platinum crucibles. Have themdiacuss the feasibility of determining either tin or silicon in the presence of the other. Molecular Weight. The calculated molecular weight is 439.48 g/ mol. The tin(II) amine has been shown to be monomeric in the solution and vapor phases by cryoscopy, vapor phase osmometry, and mass spectrometry (5-9,16). Student discussion should include a review of the techniques available for determining molecular weight, as well as the potentialprahlems of performing a determination of an air-sensitive sample for each method. This compound is an ideal candidate for a cryoscopic molecular weight determination, because of its high solubility in cyclohexane and benzene, two easily dried solvents with relatively high mold freezing point depression constants and convenient melting points (cyclohexane: Kt, 20.0 'C kglmol, mp 6.54 "C, and benzene: Kr, 5.12 OC kglmol, mp 5.53 "C). Students may readily determine a cryoscopie molecular weight for the tin(I1) amine, and they should be askedtodesign, calibrate, and test an easily constructed apparatus with which they can perform this determination in the absence of moisture and oxygen. A Beekmann differential thermometer is required to measure temperature changes. The solvent should he dried and dwxygenated, the melting points of the solution and the pure soluent should be determined by the student, and the apparatus should be calibrated using a reference material such as henzil (diphenylethanedione; 210.22 glmol). The molecular weieht determined in this manner can be auite accurate, since the freeiing point depressions will he between 0.5 and 1.0 OC (e.g., 440 glmol, cryoscopy in henzene or cyclohexane (8). ~~
~~~~~~
~
-
~~
Spectroscopy and Structure Infrored. Prominent hands in the infrared, absent in the s t a r t i i Sn(I1)-N a t 402 em-', and materials, have been attributed to u,, u,, Sn(I1)-N as ashoulder a t 378 cm-' (5,7-8). The v,, Sn(N)-N is believed to occur between 880 and 850 cm-' in simple tin(1V) amines ( I n , although this assignment has been controversial (1719) (the Si-N stretch in simple silylamines appears between 950 and 348
Journal of Chemical Education
Chemlcal Shins
Nwlew
Coupling Constants
'Hb
6 0.23 & 0.01 ppm
'9C
6. 5.55 f 0.05 ppm
"N
6.101 -L. 3 ppmC 6. -239.2 ppm
'rNd
intarnal TMS. ~ositivechemical shin values, including mae fmm the literature, are to high frequency of reference. bRoton NMR spectrum was obtained f a s 10% v0l:vol solution in CaDa containing TMS at 60 MHz on a Varian EMJEOA continuous wave spsoborneter. "N shin is relative to oflernai saturated NH&l in DiO (lit. (241 -251 ppm in hexana. 5 ppm in THF, and -210 i 15 ppm neat. all relative m external saturated aqueous NaN03). QAll dafa (261 are for a 40% solution in CaDa: ISNshin is relative to external 0.1 M CHsN02In CDCb. LLlt. (25) -1.9 ppm in CeH~ICaDs. 'Lit. 12s 18.7 1 HZ. e;it.i;;) 776 ppm in C~HK.~DB. "it. (251.
-242
+
*
890 cm-' (20-21)). Students should obtain infrared spectra (tm 200 cm-') of the tin(I1) m i n e . and, staning hexamethyldisilazane as liquid films on CsI plates, assign the bands and mnfirm these assignments using the literature, discuss the difficulty and hazards in making such assignments, and suggest the preparation of an isotopomer (an isotopic isomer (19)) of the tin(I1) amine whose infrared spectrum, when compared with those two obtained above, might Lead to a more definitive assignment of the tin(I1)-nitrogen stretching frequency, owing to the mass-induced change in stretching frequency caused by I5N relative to 14N (Hint: starting materials are ISNH3(g) and (CH&SiCl(I)). Excellent discussions of the assignments of group 14-nitrogen stretching frequencies employing IR band shifts in isotopomers are in the literature (17). NMR. The proton NMR spectrum may be obtained as a neat liquid, but the single resonance is much sharper if the spectrum is ahtsined as a 50:50 solution in benzene-ds. The table lists chemical shifts and coupling constants from our work, as well as values from the Literature (22-24). The tin(I1) amine can be studied from the standpoint of six NMR-active nuclei (excluding deuterium, tin-115 and tin-117!), and it possesses the most highly deshielded tin-119 NMR chemical shift currently on record (an interesting and logical rationalization has been proposed (23)). All values (except those involving nitrogen-15) may be obtained an 50%vollvol mixtures of the m i n e in benzene-ds. Classroom discussion should include a treatment of chemical shifts and coupling constants, as well as an explanation of the differences between factors that influence proton chemical shifts and those of the other NMR nuclei in the compound (25). Crystal and Molecular Structure. The vapor-phase electron diffraction and solid-state X-ray crystal structures have been determrned for this timll) amine (26-27).
Extension Synthesis. Have the students discuss the preparation of [(CH )Bi]:NSnCl. This compound has been prepared and its reac. tivitv has received weliminsrv attention (5-7). la the ohvious adjustment in reaction stoiehiometry a sufficient modification, or are additional ehanees desirable? For examole. would it he advantavenw to the"lithium of heramethvldisilazane to the tin(I1) " .. add .-. ~ ~ . salt -~~~~ . . chloridesolution (inverseaddifion),and what haair chemicalprinciples would be employed in performing an inverse sdditmn7 Discuss the advantages and disadvantages of using SnBrr rather than SnClz in these preparations. Include thermodynamic considerations such as bond energies, lattice energies, solubilities, etc. Students should also investigate the relative stahility of [(Me3Si)zN]2 Sn VS. (R2N)zSn (R is alkyl or aryl), speculate on what features allow M e 8 to stabilitv to Sn(I1) derivatives. and discuss the conflict ~- imnart between kinetic and thermodynamic stability in general. Have the student design a ligand that larks nirrogen nnd is isoelectronic with ~
~
~
~
.~~~~~~ ~~
~~ ~
~~~~
~~ ~
~
~
~
-
p
(Me3Si)lN,plan the synthesis of the appropriate tin(I1) compound, and speculate on, and then locate in the literature, the physical and chemical properties of this derivative (2830). Spectroscopy. Students should be asked to predict the multiplicities in the fully proton-coupled I3C and 29SiNMR spectra of this compound, as well as in the theoretical isotopomer enriched 100.0 atom %in 29Si and 'SN. Does the presence of only single resonances in the room-temperature 'H, I3C, and 29SiNMR spectra imply that the trimethylsilyl protons are always equivalent? Refer those students who answer this question in the affirmative to the preparation (31) and recent X-ray crystal structure (32) of the simplest tin(I1) m i n e , Sn[N(CH3)2]2.A discussion of the time scales of physical techniques will be instructive (25,331.
Acknowledgment
We are indebted to the donors of the Petroleum Research Fund administered hy the American Chemical Society (CDS and CHI'). and the National Science Foundation (CHY), for partial support of this research. Literature Clted 1. Harri8.D. H.; Lappcrt,M. F. J . Orgonomet. Chem.Libr. 1976.2.13-102. 2. Lappert, M. F.; Power, P. P.: Sangor. A. R.:Sriuaatsua, R. C. Metal and Metalloid Amin*$: Wiby-Halsted: New York, 1930;Chapter 5. 3. Sehseffer,C.D., Jr.; Y0der.C.H. J. Chem Edur. 1985.6%537540. 4. Semples,M.S.;Yoder,C. H.; Schseffer,C.D., Jr. J. Chrm.Edur. 1987.64, 177-178. 5. Scheeffer,C.D., Jr.: Zuckerman, J. J. J. Am. Chem.Soc. 1974.96.7160-7162. 6. Harris, D. H.;Lappert,M.F. J. Chem.Soc., Cham. Commun, 1974,895-396. 7. Lappert, M. F.: Power,P.P. In Orgsnofin Compounds:New Chemialryond Appiicorims, Zuckerman, J. J.. Ed;American Chemical Society: Washington. DC, 1976: Chapter 5 (ACS Adv. Chcm. Ser., No.1571.
8. Gmsne, M. J. s.; Harris, D. H.: Lappert, M. F.;Power, P. P.: Ri"i&re P.:RiviareBaudet,M. J. Cham. Soe..Dollon Trans. 1977,2W&2W9. 9. Veith,M.:Recktenwald, 0 . Top. Curr. Chem. 1982.104,165. 10. Lane,C. F.;Krsmer,G. W. Ald~ichim.Acto 1977.10,11-13. 11. Eisch, J. J. Orgonomdollir Synfheria Nontrowilion-Metal Compounds: Academic: New York, L98L: Vol.2. 12. Gil1,G.B.; Whiting, D. A. Aldrichim. Acfo 1986.19, 3141. 13. Shrivor. D. F.; Drezdon, M. A. The Manipulation ol AirSensilive Compounds, 2nd ed.; Wiley-Interscience: New York. 1986. 14. Wayda. A. L.; Daronsbourg, M. Y., Ed.. Expe"m~nto1 Orgonometallic Chamistry A Prorlieum in Synthesis and Characlarimtion; American Chemical Society Wssh~ ington, DC, 1987:Chspters 1-2 (ACSSymp. Ser.. No. 357). 15. Randal1.E. W.:Yoder,C. H.:Zuckermsn, J. J.Inorg.Nuel. Chem.Lett. 1966,1,105. 16. Corvan, P. J.; Zuekermen. J. J. Inorg. Chim. Acta 1979.34,L255-L253. 17. Randal1.E. W.;Zuckerman,J. J. J.Am. Chom.Sac. 1968,90,31673171. 18. Randall, E. W.; Ellner, J. J.; Zuckerman, J. J. Inorg. Nuel. Chem. Lett. 1966, 1, 10% >,?
19. Rsndsll. E. W.: Ellner,J. J.; Zuekerman, J. J . J. Am. Chem Sor. 1966.38.822. 20. Fessenden, R.;Fewenden, J. S. Chsm.Rau. 1961.61.361-388. 21. Wsnnsgat, U.Ado. Inorg. Chem. Radiochem. 1964.6.225-278. 22. Berlos.K.:Huhler.G.:Nath.H.:Wannineer.P.:Wibere.N.:Wra
