Cobalt Sandwich Compounds John E. Sheats' and Gregory Hlatky2 Rider College. Lawrenceville. NJ 08648
Iron sandwich compounds, especially ferrocene, are well known and thousands of derivatives have been prepared (1). On the other hand, related cobalt complexes have received far less attention. Furthermore, the prnperties of these complexes are often in sharp contrast to their iron-group analogs. In this paper we present a hrief review of the chemistry of the major classes of cohalt sandwich compounds (2). Cobaltocene and Cobalticinium Salts Cobaltocene, (+CSHS)~CO, ( 3 )and the cohalticinium ion, [(I$C:,H~)LC~]+.( 4 ) are the two most important dicyclopentadienyl cohalt compounds. Cohaltocene, a purplish-black solid, is a 19-valence-electron compound, one electron over a closed-shell configuration. It is easily oxidized to the yellow, air-stahle cobalticinium ion, isoelectronic with ferrocene. Cohalticinium salts are among the most stable organometallic species known, melting with decomposition only ahove 250-350" C and resistant to attack by strong acids or hases. Cohaltocene is best prepared from Co(NH:,)&12 and sodium cyclnpentadienide (51. An elegant method of' preparing 1,1'disuhstituted cohaltocenes uses substituted cyclopentadienyl sodium salts (eqn. (1)) (6).
Cobalticinium salts are best prepared from (CsHilTI and CoC12 (7). Methylcyclopentadiene has been used to prepare mixtures of methyl- and 1,l'-dimet~hylcohalticinium salts, which can he converted to carbnxylic acid, ester, amine, amide, and nitro derivatives (8). Unlike ferrocene, which readily undergoes electrophilic suhstitution, neither cohaltocene nor cohalticinium salts undergo this reaction. Cobaltocene is rapidly oxidized by electrophilic reagents, and the net positive charge of the cohalticinium ion repels electrophilic species. Nucleophilic addition is a typical reaction of these compounds. Reduction of cobaltocene with sodium amalgam ( 9 ) or cohalticinium salts with NaBH, (10)gives (C5H5)Co(CiHl;), (I, eqn. (2)). Reaction of cohaltocene with alkyl halides (I 1 ) or cobalticinium salts with organolithium reagents (12) gives exo-(CiHslCo(CiHiR) (11). Action of [(CRH;):IC][BFJ] on I1 ( R = Me or Ph) gives the monosuhstituted cobalticinium tetrafluoroborate (126).
Ill11
By virtue of its ability to act as a une-electron reducing agent, a~baltoceneadds molecules such as dioxygen and tetrafluuroethylene to form bridged complexes (111) (13)
Polymers containing metallocene groups have attracted considerable attention in recent years. Applications of organometallic polymers include use as catalysts, bacteriacides, fungicides, and as conductive or semiconductive materials (14). Derivatives of cohalticinium salts have been used as monomers for preparing organometallic polymers. For example, hrittle, flaky polyesters can he prepared from cuhalticinium-1,l'-dicarhoxylicacid chloride
.. Copolymers with alkyl and aryl metal groups (e.g., R2Sn, ( C S H ; ) ~ Thave ~ ) molecular weights of up to 10'and often exhihit high thermal stability. Potential applications of these polymers are currently under investigation ( 1 5 ) . Cyclopentadienone Cobalt Complexes ( 16) Photochemical decomposition of (CiHs)Co(CO),,(V) with
v VI When unsymmetrical or monosubstituted acetylenes are used, varying amounts of the three possible isomers are formed. The size or the electronic character of the substituents may influence the product distrihution. An X-ray diffraction study of the tetrakidtrifluoromethyl) derivative (VII) shows that the cyclopentadienone ring is distorted, with the carhonyl group 21.3' out of the C,-C4 plane (17). The carhonyl group of the tetramethyl derivative (VIII) is only 9' out of the plane and the C-C distances are essentially equal (18). The qY,rr'structure is consistent with VII, the qhmode with VIII.
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Cyclupentadienone cnbalt complexes can he protonated to form hydroxycohalticinium salts. Due to the strong electron-withdrawing effect of the cohalticinium group, suhstituted hydroxycohalticinium salts are 10"lO'i times more acidic than phenol (19). The complexed cyclopentadienone ring can he demetalated under mild conditions to give the free ring, a potential synthon in natural products chemistry (20).The preparation of suhstituted cyclopentadienone moieties via the cohalt complexes could be a useful application of these cohalt sandwich compounds.
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Volume GO
Number 12 December 1983
1015
Related iminocyclopentadienone complexes (X) and Nalkylaminocohalticinium salts have been prepared via the cohalt metallacycle (1x1(21).
XVla
XVlh
Diamagnetic r-cyclohutadiene cohalt arene compounds (XVIII) can be prepared from XVII and the arene (33).
Cyclobutadlene Cobalt Complexes ( 16) Thermal decomposition of (ChHh)Co(C0)2 with substituted acetylenes yields predominantly the cyclobutadiene complexes (XII) (eqn. (7)) (22). X-ray studies show that the cyclobutadiene ring is essentially flat with equal C-C bond lengths (23). An unsubstituted complex (+-CsHs)Co(@ C4H4) (XIII) can also be prepared photochemically from (C5Hs)Co(CO)2and photo-N-pyrone (24).
These cyclobutadiene complexes are isoelectronic with ferrocene and, unlike the cobalticinium ion, have a similar chemistry. Acylation of XI11 results in substitution on the four-membered ring (2ii). Both rings can he metalated, but the cyclohutadiene ring is preferred (26).The cyclopentadienyl ring in XI1 (R = Ph) undergoes a variety of ring suhstituted reactions and many derivatives have been prepared, including the hydroxy, amino, acetyl, formyl, iodo, and chloromercuri complexes (22). The cyclohutadiene complexes also share with ferrocene the ability to stabilize exocyclic positive charge. The carbucation XIV is sufficiently stable to be isolated as the hexafluorophosphate salt (27).
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Reaction of XVIIIa with n-butyllitbium gives the cyclohexadienyl complex (19). Hydride abstraction with N-hromosuccinimide occurs readily to give the substituted arene derivative (201
XX
XIX
Summary The field of cohalt sandwich compounds is sure to expand in the future. Advances in the preparation of functionalized derivatives should give these cohalt complexes a place as interesting counterparts to the more numerous iron analogs. With organometallic compounds finding increasing applications, the properties of cohalt sandwich compounds may make them useful in catalysis and organic synthesis. Literature Cited 11) The rhernisiry and pmpertbs or fermcene dcrivsfivrs are discused ertenaivdy in Roienlilum. M.. "TheChemirfry of the lnrx~-l;n,~ hlotallecenex."Wiles & Smr. NPWYtrk. 1965. Flv n mimc~>mprehenrlve rou*w up k1 I976 uin,hnlL mdwich cc>rnpmmdiee r. Sheat?. .I. R.J. Ormnr~mrlnl. Chrm I.ihmri,7.401 lIch, Sh. :I27 II9S:ll. \Yilk~nrun.l:..J.Amcv l'hrm. Soc...71,614XIIPi2I. l'~rdes,.I.F.,Chm~ Hr~.95.:10IIIYh21. Hnn. W. P.. Miomher. D.W . and Kaorch. M. I).. J A m r r I'hrm. S n r . In), 1196 I19Xlil.
Arene Complexes The cation [(CfiMefi)2Co]'+,isoelectronic with cobaltocene, is obtained when CoC12 is reacted with hexamethylhenzene and AIC1:j (28). Unlike cohaltocene, [(C6Me6)2CoJ2+cannot be oxidized, but it can he reduced to the 20- and 21-valenceelectron compounds [(CfiMe&Co]+ and (C6Med2Co (29). Magnetic data suggest that these rings do not have @coordination hut may have "slipped" ~ " r e n e rings. The arene complex XV can be prepared from (C5Hn)Co(C0)z and 1,s-cyclohexadiene (30).
Shsab,d.E..and Kirsch. T . S m Inc,ig. U r r - O r # . Chrm.3,59 11973). Sheatr,.l. R.. and Raurch. M. D.. J. Or#. l'hm 36,:iPI'Ill9iOl. Vilhcv. A. H.. and W~ilenrak..l.C., ,:!nth MeelinrAmrr.Chem.Sic.. New York.Sepf. 1960. U8virion , , l ' O r g a n i c ~ h ~ m i r l r yAbrtr. . IIIF. Green. M. I.. H.. Pratt. L..and Wilkinrsn, C , J Clwm Sn~.,:ii6:ll19691. ~ e r h e r i r hI:.. and n m r . t i r . . . ~. l i r ~ a ~ , i m w ml'hm i l6,:lnl 11969). In1 Firrher.Y..O.,sndHerherich.C.E..Clirm Iipr . ! I 4 1517lI9fill;lblEIMurr.N.. J Orilon~imdni. l'hrm..l & C 9 lI!lRII. la1 Hcnhn. H. H.. Praft. I. .. Wattein~n.K. F.md \I'ilkins~in.C..i l'hrm.Silc.. 27:lR ll9fill:lhl Kqima. H..Takahaii.S..anrl Hnyrhirn.K..J.Chrm .S~x..Ch~m. C
