Iron-Sulfur-Carbonyl and -Nitrosy1 Complexes A Laboratory Experiment Christopher Glidewell, Andrew R. Hyde, James S. McKechnie, a n d Peter J. Pogorzelec University of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland, U.K. The reaction between Fe3(C0)12and MezSzwas first shown hy King (1) to yield two isomeric forms of Fez(CO)&SMe)z. Both isomers have structures closely related to that of FeACOMI): themore abundant anti isomer (11. . ,R = CH?) ". (2) .. h& twod&ct methyl environments, whereas the s G isomer (111, R = CH3) has two identical methyl groups.
agent, yields of around 90%of the nitrosyl complex are readily obtained; if, however, an NO cylinder is not availahle, the nitrosylation can he effected using sodium nitrite instead, hut here the yields are typically around 30%. The preparations of Fe3(CO)lz and Fez(CO)&3Me)z are based initially upon those described by King (121, hut they have been the subject of extensive modification and simplification for class use, as descrihed below. Experimental
In the series Fe9(CO)c(SR)~ -. -. .- the antilsvn ratio increases with the increase in the steric requirements of the group R so that the ratio is -4 when R = CH3, hut when R = Me& only the anti isomer is detectable (3): on the other hand, when R = H, a third isomer (IV) can he detected in solution (4). Although the pure anti and syn isomers can he isolated, they rapidly re-equilibrate in solution (31, hut a t a rate sufficiently slow on the NMR timescale for the detection of individual 'H and '3C resonances a t room temperature: on the other hand, the carhonyl groups are rapidly fluxional a t room temperature, giving only a single 13C resonance for each isomer ( 5 , 6 ) .The compounds Fez(CO)&3R)z can all he readily converted ( 6 ) to F~Z(NO)&R)Z,the esters of Roussin's (7, 8 ) red anion (FezS~(N0)4)-~. We have developed a comprehensive experiment hased noon this chemistrv that we have used during the nast six $ars in the third (penultimate) year of our undergraduate course (9-11). This exweriment involves the use of inert atmosphere teehiques, and of thin-layer and flexihle-column chromatoerawhv. and the internretation of infrared. 'H and I.'(: I\'MR','~I;~ k a i s spectral d ; t n l'he column chromarugraphv - .is .t~articularlvwniiirant. first Iwrat~se[he use of silira instead of alumina (1; 12) allows the easy separation and isolation of a third product in the Fe3(CO)lz/MezSz reaction, readily identified as the known (13) complex [MeSFez(CO)6]2S, hut not previously detected in this reaction, and secondly because the use of flexible column (e.g., of dialysis tubing) instead of a conventional glass column aeatlv reduces both the time required for the separation and-the volume of solvent consumed. In the experimt!nt, all students prepare FentCO)~yfrom Fe((:OJ, and react this with MeS2, and then determine the antilivn ratio hv ' H NMR. Thereafter there are tw, ot~riuns. ith he; the crude mixture is subjected to chromatography, allowing.. separation and characterization of three Dure com. ponents, followed hy n study of tht: anti = syn eqttilihrium in Fr.rlC'OJdS\leJr; m the rrude mixture is nitrusylated 16) to yield Fez( U O ~ , ~ S h l eIlf ~NO . gas is used as rhe ni~rosylating
" .
a
' To oe performed under a fume hood.
We employ 2.8-cmdiameter plastic d;alysis tuoing: the lower en0
is taped to a 241. long glass tuoe. termhatea oy a geaseless stopcock
the upper end is taped to a wide-bore, short-stem glass funnel for ease of filling the column. These two glass attachments are adequate for supporting the whole column on clamps.
534
Journal of Chemical Education
Preparation of Fe3(C0),~' Set up a three-necked 1-1flaskwith reflux condenser,mechanical stirrer, and nitrogen supply: pass Nz through the flask throughout the preparation. Add to the flask iron pentacarbonyl(21ml) and methanol (85 ml): then add a cold solution of 22.5 g NaOH in 45ml water. Stir the mixture for half an hour. Meanwhile prepare activated manganese(1V)oxide: add ethanol 1150 ml) with caution to a solution of KMnOa (33.5 e ) in water (150 may start spontaneously 5-10 min after the addition of the ethanol.) When the reaction has subsided, stir the mixture thoroughly until no purple rdur rrmaini, and rod tu rouw trmperatwc. After stirring the original rmrtwn noixturr fur 30 mi". add G!5 ml ot'ii~turnredUH.t:I iolutiun. fdlowrd hv the MnO, onste: then stir for 2 h. Add erad"al1v, a solution of F ~ S~. O ; . ~-H ~(202 O ... in 1M H&Oa - . (125 ml!, and r h r n 1511ml id 5(1$ sulfuric iwid Sru i o r halfan h m r and then, if nrceraar), rod t o r . m n tempcmrure. k'lltrr the hlsr k
the experiment Conversion of Fe3(C0),2 to Fe2(CO)6(SMe)2' Dry the Fe3(COj12by washing on a Hirseh funnel with light petroleum (2 X 100 ml) and then desiccating it briefly.Record the yield at this point. Set up a 500-ml three-necked flask with condenser, nitrogen supply, and magnetic stirrer: add FedCO)12 (16.8 g), Me& (30 ml) and benzene (200 ml). Stir the mixture and, using an oil bath, reflux under nitrogen for 5 h. Cool to room temperature, and filter using a sintered glass funnel. N.B.: the residue is pyrophoric and should not he allowed to become drv: do n o t suck air through the residue. Wash the residue with henzen'(4 X 50 ml) and evaporate the combined benzene fraetions to dryness; pump the dry product on the oil pump for 2 h. Meanwhile dispose of the black pyrophoric residue by washing it down the fume-cupboard sink with plenty of water, having previously removed all flammable chemicals from the fume cupboard. All glassware contaminated with Me& should be soaked in sodium hypochlorite solution. Weigh the crude product, run a TLC plate (use light petroleum), and record its NMR soectrum in CDCL solution.
Chromatography Set up a silica chromatography column (2.8 X 140 em)? dissolve the crude mixture in the minimum volume of light petroleum and apply it tothe column, and develop with petroleum. Three hands will be apparent by the time the front of the first hand is halfway down the column. Now allow the solvent to drain from the column, and cut the column with a scalpel to separate the bands. Extract each hand
with petroleum, and remove the solvent on the rotary evaporator at room temperature. Check the purity of each fraction by TLC. Record the vield. meltine- .ooint.. infrared soeetrum. and 'H NMR spectrum ('DU I solur~.m~ 01 each twtion. ~ T ~ p i (yeldsofnnr~al and e).wt'crtCOlr >\lrl. and IM~SF~.?ICO,~I.S are 'LO'r, S r , and 1'0, respertirel?.: m.p.s. ~fi-(i;. 101 10;i. 11% lfi3Y'. respcrtwr1y.l Assign the anti and syn structures to the two major components based upon their 'H NMR spectra: compare the antilsyn ratio deduced from the recovered vields with that from the NMR soeetrum of the crude mixture.
resonances at low temperature. :if How ii the nitruiyl wmplrx F t ~ l ~ O l ~ l S M related c ~ z electronically t b the carlxmvl crmpkx F r ~ l C O ~ r , l S \ l rI)urs ~ ~ ' rirhrr obrv the 18-electronrule? Discussion
T h e reaction sequence carried out in this experiment is: Fe(C0)5
Anti + syn Equilibrium
Take a small (-100 mg) portion of each of the anti and syn isomers and reflux each separately for 15 min in 50 ml of solvent (consult a teacher for the solvent to be used). After cooling the removal of the solvent,examine each sample by TLC and by 'H NMR spectroscopy. Deduce the equilibrium constant K = [anti]/[gvn]. Nitrosylation of Fe2(C0)6(SMe)2 Using NO gas. Set up a 250-ml three-necked flask equipped with reflux condenser, and inlets for both Nz and NO. Dissolve 3.7 g (10 mmol) of Fez(CO)&Me)z in 50 ml CH2CI2,add this solution to the reaction flask and thorouehlv ouree the svstem with Nn. Reflux the mixrure for I h d ~ r i n xnhi& NO',.: t,ut&d thnmgh ;he refluxing solurion. Cool, and iilter the iolutim through a sintered glnrs filnn~l. tN.H..thr rrsidue may be pyrophoril.:dispme ofm prewourly.~Rrduce the volume on the rotary evaporator until crystals just begin to form, then cool in ice and add cold methanol to precipitate the FedN0)dSMe)z. Filter, wash with a little cold methanol and dry in vacuo. Note the yield, and check the purity by TLC. Record the infrared spectrum and the 'H NMR spectrum. Using sodium nitrite. Set up a 2%-ml three-necked flask equipped with NZsupply, reflux condenser, pressure-equalizeddropping funnel, magnetic stirrer, and oil bath. To the flask add 10 mmol Fez(CO)&~R)Sand 50 ml ethanol. Stir under Nz for 5-10 min and then add a solution of 2.8 g NaNOz in 25 ml water and a solution of 4.6 g NaOH in 25 ml water. Place 50 ml of a 1:l mixture of glacial acetic acid and water in the dropping funnel. Reflux the reaction mixture under Nz for 2 h; on adding the nitrite and hydroxide the reaction mixture turns a bright orangelyellow; after 15-20 min heating it should be almost black. After 2 h, allow the mixture to m l , then add the acetic acid solution dropwise with maximum stirring. Vigorous evolution of CO and C02 soon begins; continue adding the acid until this stops. Check the pH of the reaction mixture; if it is still alkaline, continue adding the acid until the mixture is neutral or slightly acidic.
Fez(NO)a(SMe)z:
a cm& v.~rhonylresonnncp in the ' ' C NMR spectrum inf each isomer n t n.cm temperarurz,nnd predat ihr numherofenrhonyl
6('H)/ppm (very solvent dependent, see discussion). v(NO)/cm-': 1760(s),1730(s).
Questions 11 Draw the a d d s t a t e structure of P ~ ~ I C O and J I Zexplain the observatiun 1141rhnr the "C UMH sprrirum shuwsonlyunc reiunanee.
In CDCI,. ani&Fe2(C016(SE1)2. 6(CH3).1.11 and 1 38; dCH,). 2.15 and 2.45: fi(CH,l. 17.5and 17.7: h(CH,l. 19.6and 33.9: 6i~01.209.5. syn-Fe2(C0)e(SEt)2,6(CH3), 1.33: ~ ( c H ~2.45; ) , 6(m3),18.0; 6(CH2), 32.9: d(CO),210.2. Fe2(NO)&Et)2,6FH3), 1.53 and 1.58;6(CH2),3.07 and 3.10; 6(CH3), 19.1; 6(CH2).39.5 and 40.2.
-
Fe3(CO),z
-
Fez(CO)&Me)z
-+
Fez(NO)r(SMe)z (MeSFez(C0)6)zS
All t h e ~ r o d u c t are s to a greater or lesser extent air-sensitive tn iolution, st, that N2 atmosl~hereiare required throughout: in addition the two isomersuf FeJtCOJ,\Shlel:, readily re-form an equilibrium mixrurr in solufion, so that nmaiderable care i i ret~uiredin handling the solutions. B r i d rrflux of either the anti b r t h e syn isomer in hydrocarbon solvents yield t h e equilibrium mixture for which K can readily be determined by 'H NMR; this is slightly solvent dependent and typical values of K are, for benzene, 4.0 f 0.2, and for cyclohexane, 1.6 f 0.2. Because of t h e ease of this interconversion, t h e removal of solvent after chromatography must he performed a t room temperature. Our own students are told the molecular formulae of all the ~ r o d u c t which s are formed subseauent t o F e d 2 0 ) and ~ ~ are iequired t o use t h e spectral data'to deduce-ktru%ral formulae. T h e whole exercise can b e made more challeneine for more able students by withholding molecular formza&d providing instead mass spectral data. Similarly, t h e NMR assignment can be made more difficult by substituting ethyl for methyl throughout3, although in this case we have never obsewed a third compound. T h e minor product isolated in the chromatographic separation, (MeSFe2(CO)&S (V),
has been observed a s a low-yield product in a number of reactions, including those between Fe&0)12 and CHaSCN (13) a n d F e ~ ( c 0 )and ~ cyclooctatetraenyl methyl sulfide, CaH7SCH3 (15). A similar compound, (t-BuSFez(CO),&3 is formed along with many other products when t-BUSH r e a d s over a long period with Fe3(C0)12; less sterically hindered thiols d o not appear to react in this way (16). T h e mechanism of formation of (V) is uncertain (17); neither of Fez(SMe)z(Cole and (V) is converted t o the other on reflux in henzene, nor is (V) formed from the reaction of Me& with F ~ Z S ~ ( CorO Fe&(CO)s, )~ both of which are early products (17, 18) in the reaction of F e ~ ( c 0 with ) ~ ~ organo-sulfur compounds. These tetranuclear complexes are of particular interest in having rigid b u t chiral Fe& frameworks: the overall molecular symmetry is only C2 (13), b u t the framework is rigid in solution u p t o a t least +150°C (19). T h e nitrosyl complex Fez(NO)a(SMe)z, like most of the family of complexes Fe2(N0).,(SR)2 exists (6) a s 1:l mixture of Cz., and C2h isomers (VI and VII, respectively) in solution.
T h e 'H NMR spectra of all of these nitrosyl species are solvent-dependent (6,20), and for R = CHa, the chemical shifts Volume 62
Number 6
June 1985
535
range fidlii 2.20 ppm in toluene-ds to 3.54 ppm in DMF.d7, in CDCln it is 2.83 -+ 0.01 ppm for both isomers. . ~
Literature Cited (1) King.R. B.. J Amer Chem Soe.,84.2460(19621. (2) Dahl, L. L a n d Woi,C. H.,lnorg. Chsm..2,328(19631. 131 Mamsca, L.. Greggio. F.. Sbrignadello. G..and Ror. G.. Inorp. Chim. Acto. 5. 667 11471) ,. ...,.
(41 Seyferth, D., and Hendersnn, R. S.. il Oreanomelollic Chem.. 218. C34 ll9811. (51 Adarnr, R. D..Cotfon. F. A.,Cullen, W. R., Hunter. D. L.,and Michichuk, L.,lnor#. Chem., 14,1395 (19751. (61 Butler, A. R., Clideue1l.C.. Hyde,A. R., McCinnis, J..and Seymour. J.E.. Polyhedmn. 2.1045 119831. (7) Roussin, F.Z.,Ann. Chim. Phy.~.,22.285(18581. I81 For a note on Roursin. see: Butler. A. R.. J. CHeM. Eouc..19,549 (19821. (91 G l i d e w e l l , C . , J . C ~ E~uc.,51.236 ~~. 11980).
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Journal of Chemical Education
(101 Glidewell,C.,andPogonelee,P.J.,J.C~e~.Eou~.,s1,740l1980). I111 Glidewell. C.. MeKechnio, J. S., and Pngonelee, P. J., J. C H M EDUC.,61, 78 ,,ax", \.""-,.
(12) King, R. R., "Organornetallic Syntheses: Transition-Metal Compounds." ~ e a d e m i e Press, h n d o n , 196s. D. 95 and 180. (131 Cdeman, .I. M., Wojcieki. A , Pollick. P. J., and Dahl, L. F., lnorg. Chem.. 6, 1236 (19671. (141 Cotton, F. A., and Hunter, D. L., Inorp. Chim. Act., 11, L9 (1974). 115) Carleton, S. C., Kennedy, F. G., and Knar. S. A. R., J. chem. Soc. ~ o l t o n 2230 . ,>om>
