Cobalt Cluster Chemistry Mark G. ~ u r n ~ h r and e ~ 'Colleen A. Rowbottom University of New England, Armidale, NSW 2351, Australia
Organometallic chemistry is now covered in most undergraduate chemistry curricular, whether as part of a general inorganic course or as a final-year undergraduate course in its own right. Indeed, organometallic chemistry is being taught as an in-depth course a t sophomore level (I). Fundamental to these courses is a discussion of metal clusters, one of the most rapidly expanding areas of contemporary organometallic chemistry. Recently, several research-level books devoted to clusters have appeared ( 2 4 ) and coverage in inorganic and organometallic texts is also good (5-9). Metal clusters are important because of their relevance to modelling heterogeneous catalysis, their potential as homogeneous catalysts, and their unique position spanning the divide between the molecular and bulk metal domains. Interest has been shown in observing the evolution in a range of properties (structure,,magnetic behavior. ionization notential. etc.) as cluster slze increases. The stability of metal clusters is dependent on metal-meta1 bond streneth. and a meat deal of metal cluster chemistry has been earned outusing the heavier groups 8 and 9 'To whom correspondence should be addressed.
metals ruthenium, osmium, rhodium, and iridium, because of their greater bond enthalpies. However, chemistry using these expensive metals does not lend itself readily to undergraduate experiments, and very few experiments suited to teaching laboratories have been reported. We present below an experiment illustrating some diverse principles of metal cluster chemistry and employing the comparatively cheap metal cobalt. Some iron cluster chemistry experiments have been reported previously in this Journal (10,ll). The tricobalt cluster complexes prepared in this experiment are distinguished by their air stability, both in solution and in the solid state, that makes them ideal synthetic targets for students acquiring confidence and skill in inert atmosphere techniques. This integrated experiment is used in our third-year undergraduate course, and the synthetic aspects can be accommodated comfortably in about 5-6, h of laboratory time. The pronounced color changes accompanying the cluster synthesis and transformation make monitoring the reactions a relatively straightforward procedure. The reactions to be carried out are summarized in Figure 1 and illustrate several aspects of cluster chemistry: cluster formation by condensation of precursors, intermo-
CI
(OCh c
/h
\ ~ c 0 ( c o ) 3
+ MeOH
*
H'
- mins
Co (COh Figure 1. Syntheses of CO~(~~,-CCI)(CO)~ and Co3(p3-CC02Me)(CO)~
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Number 11 November 1994
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lecular CO transfer, ligand transformation on clusters, stabilization of reactive intermediates [examples: C', CCO'] on a cluster core and the fluxionalitv of CO ligands. Experimental Cautions and General Considerations
are toxic and Caution: Metalcarbonyl all reactions should be performed in a well-ventilated hood. All reactions should be carried out in nitrogenpurged flasks with nitrogen-saturated solvents. Dicobalt octacarbonyl could be prepared by students (12)) or obtained from commercial sources and used without further purification. I t may be handled briefly in air but should be stored under a n inert atmomhere in a refrieerator. Commercial aluminum chloride a'lso is used a s Gceived; however. it is hverosco~icand should be handled and w e i ~ h e d in a glove b"$ in h;mid conditions. Caution: C a r b o n tetrachloride is a carcinogen and should be handled in the fumehood. Students should wear safetv gloves while using it. Arotaw evauorator should be the fumehood and used to;emove the carbon tetrachloride from the reaction mixture after completion. Carbon tetrachloride (calcium chloride), dichloromethane (calcium hydride), and methanol (magnesiumiiodine) can be dried and distilled prior to the laboratory session and then distributed to students when needed.
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Preparation of C O ~ ( ~ ~ - C C I ) ( C O ) ~
A two-necked 100-mL round-bottomed flask is equipped with a nitrogen supply (T-piece to a n oil bubbler) and magnetic stirrer. Flush the flask with dry nitrogen, briefly flame it (to remove adsorbed water on the surface of the flask), and add dicobalt octacarbonyl (1.0 g, 2.9 mmol), to the flask. Add 20-mL reaxent made carbon tetrachloride (dried over calcium chloride), and stir under nitrogen a t 50 "C (oil bath, thermometer in solution, under nitrogen). This temperature is maintained (but not exceeded) until evolution of carbon monoxide ceases (about 3 h). During this time. the color of the mixture changes from vellowbrown to red-purple. Cool to room temperature, filter through a small pad of Celite-filter aid, and evaporate the filtrate (rotary evaporator). Dissolve the residue in 25 mL hexane, and wash the solution with equal volumes of 10% hydrochloric acid and water until the aqueous layer is colorless. Dry the hexane layer (sodium sulfate), filter and evaporate to dryness to give purple crystalline Co3(p3CCl)(CO)g, mp 120-122 "C (dec.). A typical yield is 0.60 g (97%).
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Preparation of Co3(p3-CCOzMe)(CO)g
over anhydrous sodium sulfate, filtered and evaporated a t reduced pressure. Check the reaction by analytical TLC on silica-coated aluminum sheets. elutine with 1:l dichloromethanchexant*,and comparing the reactlon mixture to the starting cluster CO~I.~-CCIJ~CO ... If the chloro cluster has been reacted compie'tily, proceed straight to the sublimation. If the chloro cluster is still present, extract the residue with hexane and load the solution onto a n alumina chromatography column cm height), The starting material is eluted with hexane, and the product is eluted with 1:1dichloromethan~hexane.The solvent is removed, and the impure product sublimed (50 "C, 0.4 mm Hg) in vacuo to give pure Co3(p3.CC02Me)(C0)9,mp 90-92 "C (dec.). A typical yield is mg (35%).
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Results Students are asked to report yields and percentage yields. They must be careful with the percentage calculations, given the change in cluster nuclearity in the first step, and intermolecular CO scavenging in the second step (for the latter, assume that one cluster serves as CO source for nine product clusters). Students are introduced to the usefulness of IR spectra in carbonyl complex characterization and the dependence on molecular symmetry of the number and intensity of the IR hands. They are asked to obtain solution IR spectra in the carbonyl region (cyclohexane solvent) and decomposition points to confirm the products'identity Solution IR data are listed in the table. We supply the 'H and 13C NMR spectra to the students that they are asked to interpret. Relevant data also are contained in the table. I n the process, students will observe experimental evidence for CO fluxionality ofligands and be exposed to the low-field resonances for alkylidyne carbons. I t may be useful to contrast the chemical shifts observed in purely organic molecules with those containing metal-bound carbons. Discussion The chemistry of (alky1idyne)tricobalt clusters has been investigated intensively and has been the subject of several review articles (13, 14). The original preparation of C O ~ ( ~ - C H ~ C R ) (from C O )C~O Z ( ~ - R C ~ H ) ( C lacks O ) ~general applicability. The synthesis of C O ~ ( ~ & R ) ( C Opresented )~ here, from C O ~ ( C Oand ) ~ RCX3, can be used for R = H, halogen, alkvl. and awl. and has been used to introduce organic fu&ionalitiat the methylidyne carbon (R = C02R', CRO. CR=CHR'). ~ l t h o u g hsome chemistw of the (alkvlidvne)tricobalt clusters (for example, ~ i ~ a n d i u b s t i t u t i o n jt&al is of metal carbonyl complexes, n e a t e s t interest lies with reactivity a t the apicai meth$idyne carbon. I n the experiment (Fig. 11, the Lewis acid aluminum chloride removes the chloride from the apical carbon (15). The cluster-stabilized carhocation scavenges CO from another cluster to afford a cluster-stabilized acvlium ion. This is a~n- examole - -~ - ~ ~ - -of - inter----~-molecular CO transier and, interestingly, no improvement in yield is achieved by carrying out the reaction under an atmosphere of carbon monoxide. The cluster-stabilized
Atwo-necked 100-mL round-bottomed flask is equipped with a nitrogen supply (as above) and magnetic stirrer. Proceeding a s before. flush the flask with d w nitroeen. briefly flake it, and then add CO&~-CCI)(CO~~ (300kg; 0.630 mmol) and 20 mL dichloromethane (dried over calcium hydride). Add reagent grade aluminum chloride (240 mg, 1.78 mmol) a s rapidly a s possible (it is hyand CO~(IL~-CCOZM~)(CO)~ groscopic). The resulting- mixture should be SPectroscopic Data for CO~(W~-CCI)(CO)~ stirred a t room temperature for about 30 min Complex Solution IR data 1H NMR data f j NMR ~ data (the purple solution will become brown in color). CDC13. .D. D ~ CDC13. ,D, D ~ vfCO1. ci.clohexane, cm-' To the brown mixture is added 3 mL anhydrous methanol (dried over magnesiumiiodine). The C O ~ ( ~ - C C I ) ( C O ) ~2108w, 2061~s. 276.5,~-C; 2045%2029w 198.5,CO yellow-brown solution turns reddish-brown immediately. The solution is stirred 5 min and then Co3(~CC0zMe)(C0)9 2108w, 2063vs, 3.89,OCH3, 255.0, 113-C; 2044%2033w 198.6, CO; poured into 60 mL of ice-cold 5% hydrochloric 179.4, COzCH3; acid. The layers are separated (separatory fun52.8,OCH3 nel) and the red-brown organic layer is dried ~
~~
& ~ - -
~
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Journal of Chemical Education
R acylium ion undergoes nucleophilic attack a t the acyl carbon. In the experiment, reaction with methanol affords the methyl ester-substituted (methylidyne)tricobaIt cluster, hut a range of other nucleophiles (e.g., other alcohols, water, thiols, primary and Ni 0 secondary amines, amides, and metal al- O CP kyls) also will attack the acylium carbon. The students can be referred to the relevant literature for other important coR balt chemistry. Interstitial atoms are of interest, and the addition of three equivalents of [ C O ( C O ) ~to] Cos(p3-CCIKCO)gis (CO) reported to give [C0~(11g-C)(C0)~~1~, with a trigonal prismatic cluster core and a n ~i interstitial six-coordinate carbido ligand CP (16). Mixed-metal clusters can be prepared from C O ~ ( ~ ~ - C R ) (precursors CO)~ Figure 2. Structures of [CO~(!L~-C)(CO)I~]~-, C O ~ N ~ ( W - C R ) ( C O ) and S ( ~CoNiW(p3~~H~) by a metal-exchange reaction using a suit- CR)(CO)s(rl-CsHsk. able metal carbonyl anion (17);examples 8. c r a b h e , R. H. ~ i orgnnomptoiiic i ~ chemistry D,'L~P n - a n ~ i t i ~Mn ~ ~ IwSi l e; y - h of products are C O ~ N ~ ( ) * ~ - C R ) ( C O ) ~and ( ~ - CoNiW(p3C~H~) terseienee: N ~ Wyork, 1988:chapter 13. CR)(C0)5(q-C5H5)2, the latter of which is chiral by virtue of 9. Lukehart, C. M. Pundornentals o/ k n s i l i o n M e l d Organomdallic Chemilry: four differing core constituents (Fig. 2). Bmoks-Cole: Montere% CA, 1985. 10. White.A. J.: Cunningham.A. J.J Cham. Edue 198(1,57,317. Literature Cited 11. Glidewell. C.; Hyde, A. R.; McKechnie, J. S.; Pogor~elec,P. J. J. Cham. Educ. 1985. 1. Miess1er.G. L.; Spesssrd,G. 0.J Chem Educ. 1991,68,16. 62,535. 2. S h r k r , D.F;Kaesz, H. D.:Adams, R. D.,Eds. The Chemistry o/Mptol Cluster Com12. Hagen, A. P.; Miller T. S.; l h e l l , D.L.; Hutchinson. B.; Hance. R. L.; Daniels, L. J. plexes; VCH: New Ymk. 1990. Chem. Edur 1979.36.479. 3. Moskouit-. M.. Ed.Meln1 Clusterr: Wdcy:NewYork. 1986. 13. Seyferth, D A d u Orgonomet. Chem 1976.14,97. 4. Gates, B. C.; Guczi. L.: Knbzinger H., Eds. Metal Clusters in Catalysis: Elseuier: Amsterdam. 1986. 14. Penfold, B. R.: Robinson, B. H.Aer Chem. &a 1973.6.73. 5. Huheey, J. E . I n o r ~ n i ChemiFlry, c 3rd ed.; Harper and Row: New York, 1983:Chap15. Seyforth,D.; Williams, G.H. J. Orgonomot. Chsm. 1972,38,C11: Seyferth, D.; WiIters 13and 14. 6. Shrive? D. F.: Atkins, P. W.; Lanqford, C. H. horgonlc Chrmhfry:OUP: Oxford. 1990:Chapter 16. 7. Miesrler, G. L.:Tam, D. A. Inorganic Chemistry; Rentice-Hall: Englewood Cliffs, NJ. 1991,Chapter 14.
liams,G. H.;Nivert. C. L.Inorg. Chem 1377, 16.758. 16. Alhano. V. G.; Chini, P.; MaNnengo, S.; Sansoni, M.; Strumdo, D. Cham. Commun. 1974,299. 17. Vahrenkamp, H. Comments lnorg. Chrm. 1985,253.
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