The Catalyzed Substitution of CO by Isonitriles on [MICO)G][M = Cr, Mo, W] A Versatile Undergraduate Inorganic Laboratory Experiment Michel 0. Albers' and Eric Singleten National Chemical Research Laboratory. Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, Republic of South Africa Neil J. Covilie University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2001, Republic of South Africa Transition metal carhonylcornplexes have played asignificant role in the development of coordination chemistry ( I ) . Central to much of this chemistry is the metal carbonyl substitution reaction which is found as an important step in many synthetic and catalytic processes (2). Because o f ~ t h e ready availability of many metal carbonyl complexes, and because a number of fundamental laboratory pricedures are normally involved, metal carhonyl substitution reactions lend themselves as undergraduate laboratorv exneriments (3). Thus, most senior unlergraduate laboratory courses in inoreanic chemistrv include a t least one such experiment " (4),entailing the synthesis, isolation, and characterization of a series of metal carbonyl derivatives. In recent years, catalysts have been discovered which facilitate the replacement of carhonyl ligands by phosphine (PR3) and isonitrile (RNC) ligands in a range of metal carbony1 complexes (5). For instance, cobalt(I1) chloride and palladinmoxide have been found to be excellent catalysts for the rapid and selective replacement of carbonyl ligands on the Group 6 metal hexacarbonyl complexes by isonitriles ( 6 4 , viz. catalyst [M(CO),_,(CNR),] + nCO [MCO),] + nRNC
-
(M = Cr, Mo, W, n = 1-3; R = alkyl, aryl). We have now adapted these particular reactions to a series of undergraduate inorganic laboratory experiments. These experiments are designed to familiarize students with a number of fundamental laboratory skills and techniques which include: I ) I'rrforming a simple preparative inor~anicexperiment using
inert atm,sphrre techniques.
2) Jlonimring a rextion hy either thin-layer chromatography or
by IR spectroscopy.
3) Isolation and purification of inorganic complexes by mystalli4)
zation and column chromatography. Usine..soectmsco~ic and other techninues to estimate ~roduct . purity.
51 i'luwacteriratim of rhr rra~tionproducts hy IR and 'WNMH
aprt tru*c> Mo > W), and like all metal carbonyl compounds, should be considered a s toxic. Precautions should thus he taken, particularly with respect to eliminating spillage. t-Butylisonitrile has a pungent odor, and although many isonitriles are r e ~ o r t e dt o exhibit no a ~ ~ r e c i a btoxicitv le to i t should still be handled kith care. The is&mammals (9), trile is best transferred usine a micro-svrinee . - .(0-1000 uL) directly from a container fitted with a rubber septum; ~ y rinees and other elassware contaminated with isonitrile are hest washed firstwith a 1:10 concentrated hydrochloric acid in methanol solution, followed by washing with water (10). A similar procedure is recommended for cleaning of spillage. Carbon monoxide, which is evolved in the substitution reactions, is an odorless, toxic gas, and care should be exercised to carry out the reactions in an efficient ventilation hood with the apparatus venting into the best ventilated region of the hood. As a generalsafety precaution though, we also recommend that the other manipulations described in these experiments he performed in the hood. General
Chromium. molvbdenum. and tungsten hexacarbonvl, and palladium oxide were purchased from Strem chemicals (7 Mulliken Wav, Dexter Industrial Park, P.O. Box 108, Newburypa,rt. MA 01950). t-Butylisunitrile is a\,ailable commercially Iruin Aldrich Chemical Co. ( P . 0 Box 355, .Milwaukee, WI-53201), as well as from other suppliers, e.g., Fluka AG (CH-9470 Buchs, Switzerland). I t may also be synthesized in 50-60-gquantities by the methodof Gokel et al. (10). T h e catalyst CoC12.2H20 was obtained by heating CoC126Hz0 under vacuum (0.1 mm Hg) a t 50 OC for approximately 5 h (6). The catalyst can be handled readily and stored in air. I t does, however, slowly re-absorb atmospheric moisture over a period of several months, necessitating occasional regeneration. Analar grade toluene was de-oxygenated by distillation under nitrogen or by bubbling a rapid stream of nitrogen through the solvent for several minutes. Silica gel (23-660 fim) wasused for both product purification and column chromatopraphy. Thin-layer chromatography plates were of silica gel and contained a fluorescent indicator (Merck, "Silica Gel 60 Fzsa"). All reactions were routinely carried out under a nitrogen atmosphere in a two-necked, round-bottomed flask (Fig. 1). One neck was fitted with a water-cooled reflux condenser. and the other with a rubber septum or a ~la.isstopper. 'l'he reaction sulurion was hearcd b\, bath ore-set ill 110 "C and maeneticall\. - n uaraffin-oil . stirred. ~
~
Table 1. Reactlon Tlmes and Yields for the Catalyzed Synthesis of the Complexes [M(CO),-JCNBul).] (M = Cr, Mo, W; n = 1-3)
Complex
Cobalt Chloride Palladium Oxide Reaction Times Yieldb Reaction Times Yielda (mi") (%) (min) (%)
30 [Cr(COMCNBu')l 3 [Mo(CO)~(CNB~')] 30 [W(CWCNWI 40 cis[Cr(CO)dCNBu'),1 3 cis[Mo(CO)dCNBu')~] 40 ~isis[W(CO)~(CNBu')zl 180 fao[Cr(CO)3(CNBu')~] ~ ~ ~ [ M O ( C O ) ~ C N B U ' ) ~ ~3 50 fae[~(C0)~(CNBu')~l
75 84 82 80 90
3 3
90 80
3
88 84
3 3 3 10 3 3
85 92 90
93 92 93 90 90 92
AS estimated by IR spectmowy.
looiated yields.
Preparation of [M(C0)6n(CNBU')n] ( M = Cr, Mo, W; n = 1-3 ) Using CoC12-2H20 as Catalyst ( 6 , 8 )
The catalyst CoC122H20 (0.033 g; 0.2 mmol), metal hexacarhonyl (3.0 mmol; M = Cr, 0.660 g; M = Mo, 0.792 g; M = W, 1.056 g) and toluene (10 mL) were added to the reaction flask. The stirred mixture was heated to reflux, and the appropriate amount of t-bntylisonitrile required to achieve mono- (3.6 mol, 403 pL), di- (6.6 mmol, 740 pL), or trisubstitution (9.6 mmol, 1076 pL) was added by micro-syringe to the hot reactants. This gave an immediate blue coloration due to the formation of a cobalt chloride-isonitrile complex (11). (Note: I t is important that this sequence of events be adhered to strictly. In particular the isonitrile must be added to the hot solution of metal hexacarhonyl and catalyst, otherwise a competing reaction, the cobalt chloride-catalyzed polymerization of isonitrile (6, 12), occurs. This can lead to catalvst deactivation and incomplete substitution reactions.) Continued reflux resulted in the solution turning bright areen. Completion of the reaction (Table 1) was indicaied by monitoring by IR spectroscopy or thinlayer chromatography [mobile phase hexane (n = I), hexane-diethyl ether 1:l (n = 2), hexane-diethyl ether 1:2 (n = 311. The mixture was cooled to room temperature, silica gel (10-20 g) added to adsorb the catalyst, and the product was extracted with 5-10,lO-mL portions of toluene. The extracts were filtered, giving colorless or pale yellow solutions of the products. Solvent removal using either a rotary evaporator or steam-bath and water-pump techniques gave the crude products as white or pale yellow solids. A small amount of this material (approximately 0.1 g) was set aside for an investigation of its purity by IR spectroscopy. The remaining material was separated into two approximately equal portions. The first portion was purified by recrystallization (n-hexane for the mono- and disubstituted products and dichloromethane-hexane (1:5) for the trisubstituted products). The second portion was purified by means of a 30 X 2cm silica eel cbromatoeraohv - . -column. For the monosuhstituted prozucts, n-hexane wasused as the mobile phase while dietbylether-hexane mixtures were used for the di- and trisubstituted products (4:l and 1:l hexane-diethylether, resoectivelv). The oroducts were obtained from these procedures in-75-90%-~ieldas white or pale yellow crystalline solids. Preparation of [M(CO)&CNBU' )n] ( M = Cr, Mo, W; n = 1-3 ) Using Palladium Oxide as Catalyst (7.8)
The catalyst PdO (0.020 g; 0.16 mmol), [M(CO)G](2.0 mmol; M = Cr, 0.440g; M = Mo, 0.528 g; M = W, 0.704 g) and toluene (40 mL) were combined in the reaction flask. The mixture was heated to hoilingand the appropriate amount of t-hutylisonitrile required to achieve mono- (2.05 mmol, 230 pL), di- (4.1 mmol, 460 pL), or tri-substitution (6.15 mmol,
Figure 1. Experiments apparatus suitable for catalyzed metal carbonyi substitution reactions.
690 pL) was added by micro-syringe. The reaction mixture was heated under reflux until completion of the reaction (as monitored by IR spectroscopy or thin-layer chromatography) (see Table 1). The mixture was allowed to cool to room temperature and filtered through a fluted filter paper to separate the catalyst. The solvent was removed using either a rotary evaporator or steam-bath and water-pump techniques to give the crude product. Product purification entailed the same procedures detailed above for the cobalt(I1) chloride-catalyzed reaction. Results and Discussion
The reaction of the Group 6 metal hexacarhonyls and metal carhonyl derivatives with isonitriles has been documented extensively (13-16). The direct replacement of carhonyl ligands on [M(CO)G](M = Cr, Mo, W) by isonitriles can generally be achieved only with difficulty, as is evidenced by long reaction times and forcing conditions that, in g e n e r a l , give only p o o r y i e l d s of t h e p r o d u c t s [M(CO)s(CNR)] and [M(C0)4(CNR)2](13, 14). In only one instance has the direct thermal replacement of three carbonyl ligands been reported (17); in this study reaction of tungsten hexacarbonvl with t-butvlisonitrile (as solvent) gave the complex [ W ~ C O ) ~ ( C N B Uin~ )80% ~ ] yield, but after 4 davs reaction time. AS a consequence, indirect methods of suhstitution have eenerallv to be em~lovedfor achieving higher substitution in ihese cokplexes. ~ h & normally e entailthe preparation of synthetic precursors containing labile chelating groups, usually di- and tri-olefin ligands, that are readily displaced by the more nucleophilic isonitrile groups (13-151, e.g.,
-
[(NBD)Mo(CO)J+ 2ButNC [Mo(CO)r(CNBut)n]+ NBD (NBD = bicyclo[2.2.l]bepta-2,5-diene). Although the required isocyanide complexes are obtained in good yields, the prior preparation and purification of the substrate olefin-complexes is a major drawback. However, in contrast to these results, the catalyzed substitution of [M(CO)c,] by isonitriles is a direct, rapid synthetic route which gives the complexes [M(CO)G-"(CNR),] (M = Cr, Mo, W; n = 1-3) in high yield (5-8). Thus, reaction of thutylisonitrile with [M(CO)e] (M = Cr, Mo, W) in ratios varying from approximately 1:l to 3:l in the presence of either CoClr2H20 or PdO as catalysts gives the substitution products [M(CO)G-,(CNBut),] (M = Cr, Mo, W; n = 1-31 in Volume 63
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445
Table 2.
Spectrmoplc and Physical Data for the Complexes [M(CO).-dCNBul)J
(M = Cr, Mo, W: n = 1-3) Melting Pointr (OC)
IRqcm-') Complex
WC)
[Cr(CO),(CNBu')] [Mo(CO)dCNBu')] [W(CO)dCNBu')I ~is[Cr(C0)~(CNBu')r] cis[Mo(C0)~(CNB~')~l c!s[W(CO).(CNBu')rI fae[Cr(CO)3(CNBu')3] fae[Mo(CO)3(CNButh] he1wlCoI~ICNB~'l~l
2148(w) 2150(w) 21551~) 2155(w) 2122(w) 2155iw) 212qw) 2158(w) 2120(w) 2145(w) 2103(m) 2150(w) 2105(m) 21501wl21041ml
WJ)
104-105 115-116 131-132 127-128 129-131 145-147 170-171 179-182 201-202
2058(w) 195519) 2065(w) 1958(s) 2082(w) 1952(9) 2009(m) 1939(m) 1927(s) 1922(m) 2015(m) 1940(m) 1929(s) 192O(m) 2005(m) 1935(m) 1922(a) 1917(m) 1932(s) 1864(s) 1934(s) l860(5) 19281s) 18671sl
R d e d in CDCh relative b TMS.
'Recorded in air. uncarected.
75-93% yield, with reaction times ranging from 3 to 180 minutes. Only the kinetic products, viz. cis-[M(CO)a(CNBut)%]and ~ ~ c - [ M ( C O ) ~ ( C N Bare U ~obtained )~], in these reactions. A notable feature of the catalyzed reactions is that thev occur with distinct. steowise reolacement of carhonvl gro;ps (fi). This is manifhsted in the product purity whichis typicallv >go%, and by rhe ubservation that it is possible to prepnrecis-[M(COJ4(CNHu')?Ifrom [M(('O),,(CNBu'l\,and /nc-IM(COJ3(CNHu')~ from C~S-[M(CO)~(CNBU')Y]. The mechanisms of thecobalt(ll)chlnride- and palladium oxide-catalyzed substitution reactions have not been unequivocall.v~determinedbut possibly involve lahiliration of CO ligands t)y a change in the oxidation state of the metal carbonyl substrate (,;I. The catalyst could thus, via electmn transfer to the metal carhonvl substrate, generate odd-electron metal carbouyl species, the latter showing enhanced ligand lability relative t o their diamagnetic precursors (5). The eeneral exoerimental method for the catalvzed svnthesis 2 the complexes [M(CO)s-,(CNBut),] (M Cr, MO, W. n = 1-3) reauires the addition of metal hexacarbonvl and the catnlysr t o toluene at room temperature. 'She mixture is heated to boilinr fdowed hv theaddition of isonitrile. In the case of cobalt(1i) chloride &catalyst, the addition of isonitrile results in an immediate blue coloration due to the formation of [CoC12(CNBut)J ( 6 , lI ) . Continued heating under reflux gives the required isocyanide complex. Carbon monoxide is evolved in the substitution process. The course of the reaction is best monitored by the changes in the 2300-1800-~m-~region of the IR spectrum. (We have found that a 0.05-cm-pathlength cell fitted with sodium chloride windows is suitable.) The reaction may also he monitored by thin-layer chromatography (see Experimental), and qualitatively by observing the evolution of carbon monoxide using the oil-buhhler in the nitrogen line as an indicator of the progress of the reaction. On completion of the reaction, the catalyst (PdO) may convenientlv be removed bv filtration. In the case of cobalt (11) chloridd as catalyst, adsorption of the catalyst on silica eel followed bv extraction of the reaction ~ r o d u cwith t toluene, gives a separation. Product purity at this stage is typically >go%, with the most likely contaminants being other members of the substitution series. IR spectroscopy is a sensitive measure of product purity and may be used t o establish the identity of these contaminants and also to obtain an estimate of the percentage purity on the basis of relative peak intensities. Thin-layer chromatography and melting-point data (Tahle 2) may be used for hoth a qualitative measure of nroduct ouritv .and also for estahlishine" the likely identity of the contaminants. In the case of reactions emulovine cobalt(I1) chloride as catalvst. another m s i b l e in the prodimpurity Ls a trace of catalyst. ~atal~s&e.&dues uct are indicated bv a ale green coloration in the otherwise white or yellow solibs. column chromatography or recrystallization are techniques suitable for the purification of the
crude reaction products (see Experimental). Both procedures generally work equally well provided that the level of contamination is not excessive, in which case chromatography is the best alternative. Useful alternative methods of purification are detailed by King and Saran (14). The pure compounds [M(CO)G-,(CNBU~),](M = Cr, Mo, W:n = 1-3) are white (n = 1, 2) or pale yellow (n = 3)
.
446
Journal of Chemical Education
10
2100 2000
1900
Wavenumbers (cm-'
1
1
Figure 2. Infrared specna of the complexes [Mo(CO).-ACNBu').] = 1.2 recorded in heme: n = 3 recwded In benzene).
(n = 1-3) (n
crystalline, air-stable solids. They are completely characterized on the basis of characteristic patterns of v(C0) and v(NC) absorption bonds in the 2200-1800-cm-' region of the IR spectrum and also by melting point data, 'H-NMR spectroscopy, and mass spectrometry. 1R spectra of IMINCO)~ ..(CNBu'),l in = 1-31, representative of the series IM(CO)~~.,~CNRU'J,I ( M = Cr, Mo, W; n = 1-31 shown in Firmre 2. Generallv. of .. svectra . -, are --[ M ~ o ) ~ ( c N B Uhave ~ ) ~t ]o t e recorded in a more polar solvent such as benzene or chloroform due to the limited solubility of the materialin hexane and other non-polar solvents. IR data for the complexes [M(CO)fi-,(CNBut),] (M = Cr, Mo, W, n = 1-3) are presented in Table 2. Conner et al. (15) h a v e discussed t h e IR spectra of t h e complexes [M(CO)+,(CNR),] (M = Cr, Mo; n = 1-3) in terms of the group theoretical assignments and also qualitatively in terms of the expected metal-isocyanide ligand bonding interactions. The 'H-NMR spectra of the complexes [M(CO)s-JCNBut)),] (M = Cr, Mo, W; n = 1-3) (Table 2) show only single singlet methyl resonances confirming the expected equivalence of the isocyanide ligands. Electron impact mass spectrometry is generally of limited use for t h e c h a r a c t e r i z a t i o n of t h e complexes [M(CO)6-,(CNBut).] (M = Cr, Mo, W; n = 1-3). Molecular ions are observed in all cases, but disproportionation reactions of the type ~~
and oroduct decom~osition(observed especially for [ M ( c ~ ) ~ ( c N B U ~o&ur ) ~ ] )in the mass spectrometer, cimplicating interpretation of the results (8,15). Posslble Reflnernents
The experiments outlined here emphasize simplicity, safety, and ease of execution. The use of two carhonyl substitution catalysts is described, cobalt(I1) chloride (because of
its ready availability and low cost) and palladium oxide (because of its high catalytic activity and ease of separation from t h r~e a c ~ i o ~ ~ r o d u Theexperimenti ct~). have also bren limited IU the suhstitution of theGroup6 metal hexamrbonyls by isonitriles because of the general availability and low cost of the former, and the stability, crystallinity, and spectroscopic properties of the products. The general method could, however, also be applied to other metal carbonyl complexes using other ligands and also other catalysts (5). Further, the use of aryl isonitriles (e.g., 2,6-dimethylphenylisonitrile) enables the synthesis, using palladium oxide as catalyst, of the entire series [M(CO)s-"(CNR),] (M = Cr, Mo, W; n = 1-6) (7) and an investigation of the chemistry of the homoleptic isocyanide complexes [M(CNR)6] (M = Mo, W). The authors can be contacted for further details of such extensions. Literature CRed (1)AM, E. W.: Stone, F. 0. A. Quart. Reu. 1969, 23, 325: Bzatcrman, P. S. Sfruel. Bonding 1972,10,57; Braterman, P. S. 3frucl.Boding1976.26.1. (2) "Organic Syntheses via Metal Carbonyls"; Wender, I.; Pino, P., Eda; Wiley: New York, 1968:Vd1; 1977;Vol2: Gataa,B.C.;Katzer,J. R.; Sehuit,G. C. A. "Chsmistry of CatalyiicProceasea: McGraw-Hill:New York, 1979. (3) Post, E. W . J. Chrm. Edue. 1980,57,619;Hunt,G. R A. J. Chem.Edue.,1916.53.53. (4) Angcliei, R.J. '"Synthesisand Techniquas in Inorganic Chemi8W:Ssunders:Philadelphia, 1969:pp 117-140. ( 5 ) Albers, M. 0.;Coville, N. J. Coard. Cham. Rau. 1384.53, 227. (6) Albers, M. 0.; Coville. N. J.; Ashworth, T. V.: Singleton, E.; Swanepoel, H. E. J. 0rg.namofol. Chem. 1980.199,55. (7) Couille. N. J.; Alhers. M. 0. lnorg Chim. Acfo 1982.65,L7. (8) Alben, M. 0. PhD Thesis.University of the Witwatersrand,Johannesburg, 1981. (9) Green, J. A,; Hoffman, P. T. In "Isonitrile Chemistry" Ugi. I.. Ed.; Academic: New York,197bpP (10)Gakel, G.W.; Widera, R. P.; Weber, W. P. O w Synth. 1916.55.96. (111 Yamamoto. Y.; Hagihara. N. Bull. Cham. Soc. Jpn. 1969.42.2077. (12) Nolte. R. J. M.: Stephany, R. W.: and Drenth. W . Red Trm. Chim. Pwa-Ea 1973. 92.83;Nolta, R,J. M.;Drenth,W. Rccl. Trou. Chim. Pays-Bas 1973.92.788. (13) Malatesta. L.; Bonati, F:'lsayanide Complor~aafMetals": Wiley: Landon, 1969. (14) King. R. B.;Sarsn. M.S.Inorg.Chem. 1974.13.74. (15) Connor, J. A: Jones. E. M.: McEwen, G. K.:Lloyd. M. K.;MeCleuerty.J. A. J. Chem. Sor.,Dolfon Tmns. 1976,1246. (16)Singleton,E.; Omthuizen,H. E. Ad". O?ganom~tnl.Cham. 1983,22,2W. (17) Dreyer, E.B.:Lam,C.T.;Lippard,S. J.Inarg,Chem. 1979,18,1901
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