The Reduction of a Nitrile (CN) Group by Sodium Borohydride The Preparation of Phosphine-Amine and Phosphine-lmidate Complexes of Tungsten Carbonyl Kristen E. Fausl and Bruce N. Stohoff' Ball State University. Muncie, IN 47306 Experiments for advanced-level undergraduate students that are based on reactions of organometallic compounds are useful for developing laboratory skills and extending the concepts taught in introductory-level chemistry courses. We have recently adapted for laboratory and classroom use a series of reactions based on our observation ( I ) that the complexes shown in Figure 1 result from the reaction in ethanol of W(C0)e with (CsH&P(CHz)zCN and NaBHa.
complexesshom in Figure 1areevident immediatelyupon developing, but they are best visualized by using 1%and/or a UV light.
The experiments described here have been designed especially to reinforce and extend student experiences with recording and interpreting IR and NMR spectra as well as with useful laboratory techniques. I n regard to the latter, thinlayer and flash chromatography, inert atmosphere and vacuum distillation techniques are incorporated and emphasized. As presented, however, a major objective is to provide opportunities for students to assign probable structures for complexes based on spectroscopic data.
Experimental General Information The amine-based product is readily ohtained in excellent yield under nearly all reaction conditions that involve a one-to-one mole ratio of W(C0)6 to (C~HS)~P(CH~)~CN along with an excess of NaBH4, all in commercially available absolute ethanol. In contrast, obtaining the imidate-containing product requires attention to the details described immediately below and in the Syntheses section. It should he noted, however, that we routinely dried and distilled the ethanol for all the reactions described. Both the amine- and imidate-containingproducts, as described in the Syntheses section, can he crystalized from mixtures of dichloromethane and hexane. Typical analytical and 3lP NMR data ohtained from crystalized samples are presented below.2Dichloromethaneis frequently observed to he present in the crystalized samples. For the mixtures of amine- and imidate-containing products, all glassware was dried at 130 'C, or more, for at Least 8 b, assembled while hot and cooled under an atmosohere of drv nitroeen. The aodinrn horohvdride 198% or 99%), was oitained fro& ~ l d r i e Chemb ,~ ical Company and was stored in a desiccator. Abdute ethanol was distilled under nitrogen from magnesium ethoxide (2)and handled w ~ t ah dry syringe. The phosphine rCe,H&PICH,)L!N was prepared as described previously (3). J. T. Baker flash-gradesilica gel and 25- X 75-mm plastic strips coated with silica gel (withfluorescent indicator) have been used for the ehromatomaohic analvses. For the TLC analvses. Rr values of 0.8 + 0.1 anlo.? 0.1 &e obtained for the in;ida& k d m i n e eomnlexna. remeetivelv. with diehloromethane as the eluant. The ~
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Journal of Chemical Education
Syntheses W(CO)I[P~ZP(CHZ)~~H~]. Place 35 mL of absolute ethanol in a 1M)-mL round-bottomed flask fitted with a magnetic stirring bar and a reflux condenser topped with a gas inlet tube. Add 1.00 g of W(CO)e, 0.67 g of PhzP(CH2)zCNand 0.64 g of NaBHd, and then gently reflux the mixture for 12 h while maintaining a nitrogen atmosphere. Cool the reaction mixture to -35 OC, and then pour it into a 250-mL senaratorv funnel. Add 75 mL of 20% aoueous NaCl to the funnel, and eatraft the mixture with 2 X 75 m~'ofdlchloromethane Curnhlne the nonaqueous portions, and dry them over magnesium sulfate. Remove the drying agent by filtration, and evaporate the filtrate to dryness with a rotary evaporato~.~ At the point, the product should be pure by TLC analyses (a single spot with an R, of 4 . 7 ) . but it can be recrystallized as follows. Dissolve 0.5 g of the product in 12 mL of refluxing diehloromethane contained in a 50-mL Erlenmeyer flask. Filter if any cloudiness
' Author to whom cwespondence should be addressed. PhoSphorus-31 shifts (relative to 85% H,W,) in CD3C(0)CD,and typical analytical data tor (A) W(CO)4[Ph,P(CH,),NH,].)~CH,C12 and (B) W(CO)4[Ph2P(CY)2C(OC2H5jNH] are presented below.
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containing product. The yellow spots on strips correspondingto the
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Safety h c a u t l o n s Students must be cautioned to handle all the reagents with w e . In particular, the aqlonitrile and diphenylpbosphinemust he handled in a well-ventilatedfume h o d 3It is also recommended that all column chromatographic separations be carried out in a fume hood since the long-term health risk associated with dichloromethane are in question.
%C 6P
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The laboratory instructor may wlsh to prepare in advance enough (CsH&P(CH2)2CNfw the entire class. Since this phosphine is very stable, it may be stored in a sealed container for maw months with no noticeable decomposition. b t h products slowly decompose if lefl dissolved in chlorinated hydrocarbon solvents.
persists. Add 2 mL of heaane, and boil toevaporatesolvent until the rolutionsrartstocloud. Add a few dropsof dichloromethan~toclear the cloudiness, and then cool the solution to -10 O C for several hours. Collect the crystals on s filter, dry them under vacuum, and analyze as described below. and W(COLIPhnPMixtures of W(CO)rlPha~(CHn)sNHal . ... (cHz)zc(oc&&)N~$. Place ' m ~ a fd< deaxygenated ethanol (distilled from Mg(OEt)% under NI) into a dry, nitrogen-filled flask fitted with a reflux condenser topped with a gas inlet. Add in sequence while maintaining a nitrogen atmosphere, 1.M) g of W(CO)E, 0.67 g of (C&)~P(CHZ)~CN, and 0.67 g of NaBH4. Reflux the mixture gently for 1.k2.5 h. Monitor the reaction via TLC:begn as soon as ihe reagents have dissolved and the solution has timed yellow. Immediately, and every 20 min thereafter, sample the hot reaction mixture directly via a small-diametercapillarytube. Reflux until two yellow spots with Rtvdues of 4 . 7 and 4 . 8 are evident, cool the mixture to -35 'C, pour it into a 250-mL separatoryfunnel, and isolate the crude product as described above for the Ph2P(CH2)3NHz-containingproduct. Pack a 30-mm-diameter chromatography column to a depth of 6 in. with flash-gradesilica gel (4). Stir the crude product with 30 mL of CHzCLz for 15 min. Filter the mixture, and add the filtrate to the chromatomaohvcolumn. Chromatomaoh withamixture of 70%and 30%(by v"lukb) diehloromerhane&d ierane, respertively. Collert 20-mL fractions hpginning as soon as the eluant appears slightly yellow. Use TLC todetermine which fractions to rombine. Immediately remove the solvent from the combined fractions by using a rotary evaporator? Analyze the products as described below. Analyses and Structuial Assignments Infrared and 'H NMR soectral data are used to analvze the produds. Since the data obtained are consistent i i t h several structural possihilities, students are informed that the complexes obtained are monomeric, octahedral, and based on a W(C0)4 unit. The information may be provided directly or through the use of a leading reference (5). Since (CfiH&P(CH2)2CN melts below 50 OC, its IR spectrum is readily obtained as a melt between NaCl plates.-~or the tungsten-containing complexes, we typically record two soectra. The first is obtained in KBr so that the bands originating from the phosphine groups are observable, and the second is obtained in verv dilute CHCL solution so that the intense y(C0) hands remain on scale. The latter may he recorded in a sinele cell from 2200 to 1800 cm-' since the solvent does not absorb appreciably in this region. Clearly, the infrared bands which are key to the interpretation are the two types of y(CN) and y(NH) bands. Although weak, the latter are readily observable as lone as the KBr pellets are carefully Specifically, the appearance bf two bands near 3350 em-' and 3300 cm-' signals the presence of a coordinated-NH2 group (6),whereas a single band near 3360 indicates a bonded imidate group (7). Both the imidate- and amineIcontaining complexes are sufficiently soluble in deuterated acetone to yield solutions that are amenable to NMR analysis with standard, permanent-magnet-based CW instruments. Of the two, the imidate-based complex provides the most straightforward information, and it, therefore, provides a good starting point for these analyses. In ~articular,the -OCHCHn m o w provides the expected triplet and quartet pat
