Journal of Chemical Education, 67 (1990) 347-349

Preparation, Analysis and Reactivity of Bis[N,N-bis(trimethylsilyl)amino]tin(II).
An Advanced Undergraduate Laboratory Project in Organometallic Synthesis

Charles D. Schaeffer, Jr.1, and Lori K. Myers2

Elizabethtown College, Elizabethtown, PA 17022-2298

and

Suzanne M. Coley2, Julie C. Otter, and Claude H. Yoder

Franklin & Marshall College, Lancaster, PA 17604-3303

Although many halogen derivatives of divalent tin and lead are well-known, examples of stable, monomeric, divalent germanium, tin and lead compounds containing directly-attached carbon or nitrogen are extremely rare. The first stable, monomeric, divalent nitrogen derivatives resulted in an additional surprise: when the ligand is N[Si(CH3)3]2, the resulting compounds are intensely colored, and represent some of the few colored monomeric compounds of the main group elements (highly colored Ge(II), Sn(II) and Pb(II) derivatives also result when the isoelectronic ligand CH[Si(CH3)3]2is used). The red color of the title compound is made even more striking by the fact that the N[Si(CH3)3]2ligand, when incorporated in compounds of many other elements,

does not impart color (3-4). As part of our continuing series on advanced undergraduate laboratory projects (5-6) involving synthesis and multinuclear NMR spectroscopy, we offer a project which explores the preparation and analysis of this unique tin(II) amine. This project is an appropriate vehicle to introduce students to many aspects of the synthesis, analysis, and spectroscopic characterization of air- and moisture-sensitive compounds using the fundamental techniques of modern organometallic chemistry. The presence of six commonly studied NMR nuclei makes this compound particularly amenable to analysis by multinuclear NMR spectroscopy.

Experimental

Synthesis

General. The preparation of the title compound is adapted from published procedures (7-11). All equipment, including vessels used in weighing of reagents, syringes used for transfer of n-butyllithium, magnetic stir bars, and reaction glassware, must be thoroughly dried in a 110 °C oven for at least 8 h prior to use. In addition, all reagents and solvents should be anhydrous. The reaction solvent, tetrahydrofuran (THF), is dried and deoxygenated by distillation under inert atmosphere from calcium hydride or from sodium/benzophenone ketal, and is stored over molecular sieves until use, when it is withdrawn from a closed storage vessel by syringe. If students are unfamiliar with syringe-and-needle techniques, we recommend that they first consult several excellent reviews of these methods (12-16), and that they become proficient at dispensing water or acetone by injection through a rubber septum (preferably containing a pressure-equalizing needle connected to a bubbler and inert gas source) into an Erlenmeyer flask.

Preparation of bis[N,N-bis(trimethylsilyl)amino]tin(II). Fit a three-neck, 250 mL, round-bottom flask with a condenser, pressure-equalizing dropping funnel, magnetic stir bar, and inert gas inlet/exit system. Place a solution of hexamethyldisilazane (7.4 g, 0.046 mol) in 30 mL of anhydrous 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 through a rubber septum placed in the center neck. Caution: n-butyllithium will cause burns if it contacts skin, and it may ignite in the presence of water. This reagent should be 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 ice/water bath. Increase the inert gas flow rate to prevent oil in the bubbler from pulling back into the reaction vessel. After 30 min of magnetic stirring at 0 °C, 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, owing to the evolution of n-butane gas. After the addition is complete, allow the resulting yellow solution to warm to room temperature with constant stirring. Prepare a solution of anhydrous tin(II) chloride (4.7 g, 0.024 mol) in 40 mL of dry THF and transfer it to the dropping funnel. Normally, the tin(II) chloride is not completely dissolved at this point, and care must be taken not to allow the dropping funnel to clog during the addition. Once again, cool the reaction flask to 0 °C, and begin a slow, dropwise addition of tin(II) chloride solution with constant magnetic stirring. The reaction mixture will proceed through a variety of color changes, beginning with dark yellow or red, and ending with maroon. After addition is complete, remove the ice/water bath, and stir the reaction mixture for at least 2.5 h. A fine white precipitate of lithium chloride will now be visible.

Filter the dark reaction mixture into a standard-taper Erlenmeyer flask through a fritted, coarse-porosity, 150-mL Büchner funnel protected with a 1-in. layer of oven-dried Celite®under a nitrogen or argon atmosphere in a glove bag previously fitted with tubing for simultaneous inert gas purging and suction. A water aspirator connection fitted with a stopcock and trap is a convenient, adequate and adjustable source of 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 containing a thin layer of phosphorus pentoxide may be included in the event additional protection is desired). While in the glove bag, transfer the dark liquid to a dry round-bottom flask whose capacity is at least twice the volume of this liquid in order to minimize the consequences of "bumping" during solvent removal. It is desirable to clean the filtration apparatus with several small portions of THF prior to exposing the glassware to air. Otherwise, concentrated nitric acid will be needed to remove the resulting tin(IV) oxidation and hydrolysis products.

Remove solvents from the resulting solution at reduced pressure (no greater than 30 torr), while applying a minimum amount of heat. We have found that a rotary evaporator, such as the Büchi/Brinkmann Rotavapor-R system, and a hot water bath is ideal. Once again, a water aspirator is a satisfactory source of reduced pressure, provided that solvent removal can be accomplished in less than 15-20 min (we have fitted our system with a calcium chloride drying tube, inert gas inlet and mineral oil bubbler, to allow back-filling of the flask with argon after completion of solvent evaporation). Removal of the solvents by atmospheric pressure distillation (even under an inert atmosphere) will result in rapid thermal decomposition of the product into finely-divided, potentially pyrophoric tin metal.

Purify the remaining crude maroon liquid by vacuum distillation on a 12-in. column packed with glass beads or helices. We have found it necessary to 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 heating of the distillation pot. Any pressure between 0.01 and 10 torr is adequate to afford a boiling point sufficiently low to prevent substantial thermal decomposition of the product. Collect at 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, bright orange-red liquid, with b.p. of 109-110 °C/0.75 torr (7), with little or no lower-boiling impurities. A dark brown tar remains in the pot (neverdistill to dryness). This tin(II) amine frequently forms orange-red crystals on cooling, which have a reported melting point of 37-38 °C (8, 10). A yield of 75% is typical.

Physical Properties and Reactivity

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(IV) amines (17), and should be avoided.

Elemental analysis. Routine carbon/hydrogen/nitrogen and total ash analyses performed on this sample will lead to satisfactory results, providing that the air- and moisture-sensitivity of the sample is recognized. Anal. Calcd for C12H36N2Si4Sn:C, 32.80; 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 amine (under argon or nitrogen at one atmosphere, or under vacuum) in preparation for mailing to the analytical firm. Discussion should focus on acceptable error limits for the C/H/N analysis after the results are received. Students may perform their own total ash analysis using aqua regia (three volumes of concentrated hydrochloric acid to one volume of concentrated nitric acid) and platinum crucibles. Have them discuss 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 (7-11, 18). Student discussion should include a review of the techniques available for determining molecular weight, as well as the potential problems of performing a determination on 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 molal freezing point depression constants and convenient melting points (cyclohexane: Kf , 20.0 °C kg/mol, m.p. 6.54 °C, and benzene: Kf , 5.12 °C kg/mol, m.p. 5.53 °C). Students may readily determine a cryoscopic molecular weight for the tin(II) amine, and they should be asked to design, calibrate, and test an easily-constructed apparatus with which they can perform this determination in the absence of moisture and oxygen. A Beckmann differential thermometer is required to measure temperature changes. The solvent should be dried and deoxygenated, the melting points of the solution and the pure solvent should be determined by the student, and the apparatus should be calibrated using a reference material such as benzil (diphenylethanedione; 210.22 g/mol). The molecular weight determined in this manner can be quite accurate, since the freezing point depressions will be between 0.5 and 1.0 °C (e.g., 440 g/mol, cryoscopy in benzene or cyclohexane (10) ).

Spectroscopy and Structure

Infrared. Prominent bands in the infrared, absent in the starting materials, have been attributed to nasymSn(II)-N at 402 cm-1, and nsymSn(II)-N as a shoulder at 378 cm-1 (7, 9-10). The nasym Sn(IV)-N is believed to occur between 880-850 cm-1in simple tin(IV) amines (19), although this assignment has been controversial (19-21) (the Si-N stretch in simple silylamines appears between 950-890 cm-1 (22-23) ). Students should obtain infrared spectra (to 200 cm-1) of the tin(II) amine and starting hexamethyldisilazane as liquid films on CsI plates, assign the bands and confirm these assignments using the literature, discuss the difficulty and hazards in making such assignments, and suggest the preparation of an isotopomer (an isotopic isomer (21) ) of the tin(II) amine whose infrared spectrum, when compared with those two obtained above, might lead to a more definitive assignment of the tin(II)-nitrogen stretching frequency, owing to the mass-induced change in stretching frequency caused by 15N relative to 14N (HINT: starting materials are 15NH3(g) and (CH3)3SiCl(l) ). Excellent discussions of the assignments of Group 14-nitrogen stretching frequencies employing IR band shifts in isotopomers are in the literature (19).

NMR. The proton NMR spectrum may be obtained as a neat liquid, but the single resonance is much sharper if the spectrum is obtained as a 50:50 solution in benzene-d6. Table 1 lists chemical shifts and coupling constants from our work, as well values from the literature (24-26). The tin(II) 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 (25) ). All values (except those involving nitrogen-l5) may be obtained on 50% vol/vol mixtures of the amine in benzene-d6. Classroom discussion should include a treatment of chemical shifts and coupling constants, as well as an explanation of the differences between factors which influence proton chemical shifts and those of the other NMR nuclei in the compound (27).

Crystal and molecular structure. The vapor-phase electron diffraction and solid-state X-ray crystal structures have been determined for this tin(II) amine (28-29).

Extension

Synthesis. Have the students discuss the preparation of [(CH3)3Si]2NSnCl. This compound has been prepared and its reactivity has received preliminary attention (7-9). Is the obvious adjustment in reaction stoichiometry a sufficient modification, or are additional changes desirable? For example, would it be advantageous to add the lithium salt of hexamethyldisilazane to the tin(II) chloride solution (inverse addition), and what basic chemical principles would be employed in performing an inverse addition? Discuss the advantages and disadvantages of using SnBr2 rather than SnCl2 in these preparations. Include thermodynamic considerations such as bond energies, lattice energies, solubilities, etc. Students should also investigate the relative stability of [(Me3Si)2N]2Sn vs. (R2N)2Sn (R is alkyl or aryl), speculate on what features allow Me3Si to impart stability to Sn(II) derivatives, and discuss the conflict between kinetic and thermodynamic stability in general. Have the student design a ligand which lacks nitrogen and is isoelectronic with (Me3Si)2N, plan the synthesis of the appropriate tin(II) compound, and speculate on, and then locate in the literature, the physical and chemical properties of this derivative (30-32).

Spectroscopy. Students should be asked to predict the multiplicities in the fully proton-coupled 13C and 29Si NMR spectra of this compound, as well as in the theoretical isotopomer enriched 100.0 atom-% in 29Si and 15N. Does the presence of only single resonances in the room-temperature 1H, 13C, and 29Si NMR spectra imply that the trimethylsilyl protons are always equivalent? Refer those students who answer this question in the affirmative to the preparation (33) and recent X-ray crystal structure (34) of the simplest tin(II) amine, Sn[N(CH3)2]2. A discussion of the timescales of physical techniques will be instructive (27, 35).

Acknowledgment

We are indebted to the donors of the Petroleum Research Fund administered by the American Chemical Society (C. D. S. and C. H. Y.), and the National Science Foundation (C. H. Y.), for partial support of this research.

Literature Cited

1. Author to whom inquiries should be addressed.

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