Indian Journal of Chemistry Vol. 21A, June 1982, pp. 579·582 Some Reactions of Tris(Triphenylphosphine )-dicarbonyliron( 0) S. VANCHEESAN Chemistry Department, Indian Institute of Technology, Madras 600 036 Received 20 October 1981; revised and accepted 15 February 1982 Tris(triphenylphospbine)-dicarbonyliron(O)(I) undergoes substitution reactions with trimethylphosphite, pyri- dine, dimethyl sulphoxide and methylisocyanide. Substitution takes place via dissociation of I to a 1 coordinativel1 unsaturated 16 electron complex, which is a highly reactive unstable intermediate. Both steric and electronic factors playa prominent role in deciding the feasibility of the reaction. Steric factor is expressed in terms of e, the cone angle of the ligand, and electronic factor in terms of Al mode of CO stretching frequency in Ni(CO)aL, where L is the ligand for which the electronic factor is expressed in terms of "CO. Ligands with cone angle e, greater than that of triphenyl- phosphine e.g. t-butylphosphine, do not react. In the reaction of I with molecular hydrogen and bromine, oxidative addition takes place. Diphenylacetylene forms two isomers, whereas carbon disulphide forms a n-complex on reaction with L MONG the d8 iron-phosphine complexes of (PPh3)2]+BF~ in absolute ethanol was allowed to the type fe(CO)6_" (PPh3) •• (where x = 1 to react overnight with triphenylphosphine in the A 3), the complexes with x = 1 and 2 had been presence of lithium metal. The resulting micro- studied to some extent>", Mono- and bis-phosphine crystalline solid was filtered and freed from phos- complexes can be prepared= by the reaction of phos- phine using hot ethanol and filtered. The solid phine with Fe(CO)5 or Fe3(COh2. Synthesis of complex was recrystallised from 1:1 CH2CI2-n-pen- trisphosphine complex, Fe(COMPPh3)3 (I) involves tane. a somewhat sophisticated procedure since substi- Preparation of bisitriphenylphosphineiitrimethyl- tution of the third carbonyl group by phosphine is phosphite )-dicarbonyliron(O), (l); bis( triphenylphos- relatively difficult. Three methods of preparation phine)(methyl-isocyanide)dicarbonyliron(O), (2); bis- have been reported in the literature+". (triphenylphosphiney-tdtmethyl sulphoxideydicarbonyl- Of the three phosphine complexes, the bisphos- fron(O), (3); bfs(tripheny/phosphille)(pyrfdine)dicarbo- phine complex is quite inert to attack by most of nyliron(O), (4) - To 10 mmol of (I) in 50 ml of THF the nucleophilic reagents; however, Lewis acids like was added 25 mmol of trimethylphosphite, methyl S02 (ref. 8), strong nucleophiles like tetracyano- isocyanide, dimethyl sulphoxide or pyridine in 20 ethylene and oxidising agents like bromine react ml ofTHF. The contents were stirred for 15 min and readily. The trisphosphine complex is more labile refluxed for 30 min. The total volume of THF was to substitution by Lewis bases. This reactivity reduced to one-third of the initial volume by evapo- could be attributed to the fact that the third phos- ration in vacuo and n-pentane was added to preci- phine in the equatorial position is sterically hindered. This makes the phosphine group labile to substi- tution by less bulky ligands. Thus, it was thought TABLE I, - ANALYTICAL DATA OP THE IRON CoMPLEXES worthwhile to study the substitution of equatorial Sl Complax Found (calc.), % N/3 phosphine by different ligands as also the oxidative No. addition of hydrogen and bromine. C H Materials and Methods 1. Fe(CO>s(PPhJ: r(MeO)~Pl G4.8 5.12 (64.14) (5.13) Iron pentacarbonyl obtained from MIs Alfa 2. Fe(CO).(PPhJs(MeNC) 11.2 4.91 2.74· Inorganic (U.S.A.) was filtered and used. AR grade (10.9) (4.81) (2.68) 3. Fe(CO)z(PPhJs (CH3)sSOl 67.25 5.06 4.55t solvents (BDH) were further purified by standard (67.22) (5.042) (4.48) procedures. Methylisocyanide was synthesised by 4. Fe(COMPPh3)z(C.H,N) 72.2 4.91 2.01· the literature methode. Other commercially available (12. IIi) (4.895) (1.99) ligands were used as such. AU the preparations 5. Fe(CO)z(PPhJsBr. 57.31 3.78 were carried out under an atmosphere of nitrogen. (57.28) (3.71) G. (a, b) Fe(CO).(PPh3M (C.HJ:CJ 16.11 4.93 Infrared spectra were run on a Perkin-Elmer 337 (76.66) (4.91) spectrophotometer. AU the peaks were calibrated 1. cis-Fe(HJ(CO)z(PPhJ. 71.5 5.01 using polysterene (1601.8 em-I). Analytical and in- (71.41) (S·<m frared data are presented in Tables 1 and 2 respec- 8. Fe(COMPPhJs(CSJ 65.8 4.2 9.02t tively. (6S.73) (4.21) (9.00) Preparation of tris(triphenylphosphine)dicarbonyl· 'N itrogen, tsulphur. ,iron(O)(/)" - A suspension of [Fe(ArNz)(CO)2- 57? INDIAN J. CHEM .• VOL. 21A. JUNE 1982 P~~3 TABLE iMPoRTANT INFRARED SPECTRAL BANDS OF IRON co 2 - , I COMPLEXES· ,P\Ph3 I CO , \ \ ,~ " \\ SI Complex Band position Assignment ;/ Fe \ ,/Fe.' \ , \ No (cnr ') , . \ ~/ I \ ~p----t---p~ 1. Fe(CO).(pPh3). [(MeO)3P] 1985(s), 1925(s) "CO oc:---+-- ~. 2. Fe(CO)2(PPh3).(MeNC) 1984(s), 1925(s) "CO CO PPh3 2125(vs) "CN (a) (e) 3. Fe(CO).(pPh3).(DMSO) 19&8(s), 1930(s) "CO 1045(m) "SO Besides this, presence of three phosphines in the 4. Fe(CO).(PPh3).(C1H1N) 19&2(s), 1925(s) "CO 5. Fe(CO).(pPh3).Br. 205O(s), 198C(s) "CO equatorial position is sterically not favoured. Hence 6a. Fe(COMPPh3).(Ph.CJ 1980(5), 1927(s) .•CO (b) and (c) are the favoured structures of which (c) 1235(m) vC=ct is more probable due to favourable placing of two b. Fe(CO).(PPh.M'Il2-Ph.CJ 2045(s), 1966(s) vCO phosphines in the trans positions. A mixture of 153C(m) "C=Ct 7. cis-Fe(H.)(CO)2(PPh3). 2043(s), 1985(m) "CO (b) and (c) was invariably obtained irrespective of I965(sh) "Fe-H the method of preparation employed an.d it was not 8. Fe(CO).(PPh3).(CSJ 2012(s), 1952(m) ••COt possible to effect 100% separation of the isomers 1151(m), U20(m) "CSt even after a few successive runs through a column ·Chloroform solutions were used unless otherwiso mentioned. of alumina-. As a result IR spectrum does not tKBr disc. tCS. solvent. exhibit vCO peaks in the same position for the complex (I) prepared and purified by different methods. However, in the compound obtained pitate the complexes. The complexes Were recrys- using the method of Lalor and coworkers? the per- tallised from 1: 1 dichlorom ethane-n-pentane. centage of (c) is far greater than that of (b) and 1 and 2 were reddish-orange. 3 was brownish while there is consistency in IR spectrum. This method 4 was a reddish-brown crystalline compound. was used for preparation of (I) in the present study. Bis (triphenylphosphine) dicarbonyldibromoiron (ll). Fe(COMPPh3)!l undergoes two types of reactions : (5) - To 10 mmol of (I) in 20 ml of CH2CI2 was (i) simple substitution of equatorial phosphine by a added 20 mmol of bromine diluted with 15ml of n- Lewis base L, and (ii) oxidative addition. Substi- hexane and the contents were stirred for 20 min. tution of phosphine takes place via dissociation of The resulting yellow precipitate was filtered and (I) to a coordinatively unsaturated 16 electron recrystallised from 1:1 dichloromethane-n-pentane. complex, Fe(COMPPh3)2, and PPh3• The unsatu- Bis (triphenylphosphine) (dtphenylacetyleney dicar- rated complex readily accepts L, a two-electron bonyliron(O), (6a) ;bis(tripheny/phosphine) (Tj~-diphenyl- donor since five coordination is favoured for Fe(O)lO. acetylene)dicarbonyliron(JI), (6b) - Compound (I) -PPh3 (10 mmol) was dissolved in 25 ml of CH2Cl2 and refluxed for 60 min with 10 mmol of diphenyl- Fe(COMPPh3)3 - Fe(COMPPh3)2L + PPh3 acetylene. This gave complex (6a). When part of +L the solution was irradiated under UV for 6 hr, a The dissociative mechanism was confirmed by carry- mixture of complexes (6a) and (6b) was obtained. ing out the reactions in the presence of excess phos- Compounds (6a) and (6b) were separated by column phine when (I) was obtained unchanged. Only chromatography using alumina and eluted by ben- ligands with cone angle 8 less than that of the zene; 6(a) was yellowish-orange while 6(b) was coordinated triphenylphosphine, i.e. 145°, can subs- _reddish-orange in colour. titute the equatorial phosphine-', Thus, our attempts _ cis-Bisitriphenylphosphine) dihydridodicarbonyliron- to substitute PPh3 by PCY3and P(t-Bu)3 (0 = 172° (II), (7) - Compound (I) (20 mmol) was dissolved and 182" respectively) ended in failure. However, in 100 ml of CH2Cl2 and heated to 35" under a pre- p(OMe)3 (8 = 107") readily substituted phosphine. ssure of 25 atmospheres of hydrogen for 1 hr. The electronic effects of the ligands are also very After cooling to room temperature, two thirds of the important. [The electronic parameter v cm-I, solvent was evaporated and n-pentane was added. measured on the basis of A}("CO) mode in Ni(CO)3L .. Orange crystals of the complex were obtained. (L = phosphine) decreases in the order-! : Bis(triphenylphosphine) (r.-carbondisulphide)dicar- P(OMe)a > PPh3 > PCY3 > p(t-Bu)3 bonyliron(ll), (8) - Compound (I) (10 mmol) was (2079.8) (2068.9) (2056.4) (2056.1) dissolved in 15 ml of CS2 and the solution was Thus, it is clear that in the case of trimethylphos- evaporated to dryness. The resulting dark-brown phite both steric (8) and electronic (v) factors are complex was washed with n-pentane and dried. favourable for the substitution, whereas the factors are not favourable in the cases of cyclohexyl and Results and Discussion r-butylphosphines, Oxidative additions of Hz and Br, take place by There are three possible structures for the title a stepwise mechanism which is typical of 5 coordi- compound (I) of which (a) with carbonyl groups in trans position is ruled out, for such as narrange- ment will exhibit only one ..cO peak in the IR tFor expressing tho electronic parameter, Ni(CO).L is spectrum, whereas (I) actually exhibits two peaks taken as standard.