Indian Journal of ChemistryVol. 23A, December 1984, pp. 992-994

Synthesis & Reactions of Limonenetricarbonyliron

S S ULLAH*, M E MOLLA & N BEGUMDepartment of Chemistry, Jahangirnagar University, Savar, Dhaka, Bangladesh

Received 23 April 1984, revised and accepted 24 July 1984

The complex. Iimonenetricarbonyliron (I) has been synthesized by the photochemical reaction of dl-limonene withFe{COh· Prolonged irradiation of the reactants affords an isomeric complex (II).The complexes have been characterized bytheir IR, PMR and mass spectra, and elemental analyses. On reaction of complex (I)with Ph3C+BFi, hydride ion abstractionoccurs to give dienyl salt, C,oH15Fe(COhBF iIII) of lirnonenetricarbonyliron, which reacts with NaBH. to give backcomplex(l).

Thermal reaction of d-limonene with Fe(CO)s or of dl-limonene with Fe3(CO)l2 gives c-terpinene-Fett.O),complex indicating isomerization of limonene to IX-

terpinene':". Similar isomerization of 1,5-cyclo-octadiene to 1,3-cyclooctadiene on heating withFe(COh has been reported by Arnet et al.3

Photochemical reaction" of 1,5-cyclooctadiene (1,5-COD) with Fe(COh gives a mixture of 1,5-COD-Fe(COh and 1,3-COD-Fe(COh. It has also beenfounds that 1,5-COD-Fe(COh under both thermaland photochemical conditions isomerizes to 1,3-COD-Fe(COh, We have presently studied the photo-chemical reaction between dl-limonene and Fe(CO)5in order to see if limonene also undergoescomplexation as well as isomerization. In addition,hydride ion abstraction followed by hydride ion attackon the -resulting dienyl salt of one of the limonene-Fe(COh complexes thus obtained has also beeninvestigated.

Materials and MethodsPhotochemical reactions were carried out in sealed

tubes under N2 atmosphere by irradiating with a 125watt medium pressure mercury lamp. Infrared spectrawere recorded on a Beckman 4220 spectrophotometer,PMR spectra on Varian EM-360 (60 MHz) and EM-390 (90 MHz) spectrometers and mass spectra on anMS-12 high resolution spectrometer.

Photochemical reaction of dl-limonene with Fe(CO)5A mixture of ironpentacarbonyl (10 g, 0.05 mol) and

dl-Iimonene (lOg. 0.07 mol) in benzene (20 ml) wasirradiated in a sealed tube for 3 hr and filtered onkieselguhr. Removal of the solvent from the filtrateunder reduced pressure furnished a yellow oil. The oilwas purified by column chromatography over silicaemploying pet. ether (40-60°) as an eluent. Removal ofsolvent from the eluate under reduced pressureafforded the limonenetricarbonyliron[C10HI6Fe(COh. designated as complex-I] as an

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yellow oil; yield 80 '1~ based on Fe(CO )s;IR(cyclohexane): 2060, 2010 and 1985 (vCO) cm-l;PMR(CDCI3): r 4.65 (bs, m-like, IH), 5.36 (bs, 2H) and8.0-8.9 (m, 13H); MS: mjz 276 (M+), 248 (M+ -CO),220(M + - 2CO) and 192(M+ - 3CO). (Found: C, 56.6;H, 5.5. C13HI6Fe03 requires C, 56.5; H, 5.7'1~).

In a separate experiment a mixture of ironpenta-carbonyl (5 g, 0.025 mol) and dl-Iimonene (5 g, 0.036mol) was photochemically reacted in benzene (10 ml) ina sealed tube for 20hr. Usual work-up andchromatographic separation over silica gave a yellowcomplex in 50'1~ yield [based on Fe(COh] whichanalysed for C10HI6Fe(COb. This was found to be anisomer of complex(l) and has been designated ascomplex-Il: IR(thin film): 2040 and 1970 (vCO) cm-l;PMR: (CDCI3): r 2.6-2.8 id, IH), 4.5-4.7 id, IH), 5.3-5.4(s, 3H) and 8.1-8-9 (m, II H); MS: m]z 276 (M "), 248(M+ -CO), 220 (M+ -2CO) and 192 (M+ -3CO)(Found: C, 56.6; H, 5.5. CI3HI6Fe03 requires C, 56.5;H, 5.7 '10)'

Reaction of complex-I with trityttetrajluoroborateTo a solution of complex-I (1 g, 0.003 mol) in

minimum volume of dry dichloromethane was added asolution of triphenylmethyltetrafluoroborate (1.324 g,0.004 mol) in the same solvent and the mixture stirredat room temperature under nitrogen for 3 hr when adark precipitate began to appear. Dry ether was addedto complete precipitation. The product was separatedby decantation of the supernatant liquid and washedwith distilled ether. On drying in vacuo dark violetcoloured dienyl salt, C1oHI5Fe(CObBF 4(I1I) wasobtained in 21.810 yield; IR: (CH2C12): 2110 and 2060(vCO) cm -I (Found: C, 51.2; H, 4.7. CI3HIsFe03BF 4requires C, 51.1; H, 4.9 '1~).

Reaction of the dienyl salt (llI) withsodium borohydride

To an aqueous suspension (25 ml) of the dienyl salt(1.5 g, 0.0041 mol). sodium borohydride (0.0156 g,

ULLAH et al.: REACTIONS OF LlMONENETRICARBONYLIRON

0.0041 mol) in water (5 ml) was added and the mixturestirred for 1 hr under nitrogen atmosphere at O°C. Thereaction mixture was extracted with ice cooled dryether, the ether extract dried (anhyd. MgS04) and thesolvent removed under reduced pressure. The yellowoil obtained was purified by column chromatographyover silica, eluting the column with pet. ether (60-80°).Removal of solvent under reduced pressure gave ayellow liquid, identical with complex(I); yield 40.9 ~~;IR(cyciohexane): 2060, 2010 and 1985 (vCO) cm-I;PMR: (CDCI3): T 4.65 (bs, 1H), 5.36 (bs, 2H), and 8.0-8.9(m, 13H); MS: mlz 276 (M +), 248 (M + - CO), 220 (M +-2CO) and 192 (M+ -3CO) (Found: C, 56.6; H, 5.5.CI3HI6Fe03 requires C, 56.5; H, 5.7~~).

Results and DiscussionThe structure of the limonene-Fetf'O), complex(l),

where limonene is bonded to Fe(COh moiety throughtwo of its non-conjugated double bonds with theretention of the ligand configuration, as has beenobserved in other cases", has been confirmed on thebasis of JR, PMR and mass spectra, and elementalanalyses. The signals at T 4.65 (m, IHA) and 5.36 (m,2H8) have been assigned to olefinic protons attached tocoordinated carbon atoms and the signals at T 8.0-8.9(m, 13H) are attributable to methyl and methyleneprotons bonded to the sp3-carbon atoms. Molecularion peak at mjz 276 and other peaks at m]z 248, 220 and192 indicate the successive loss of three CO groups.

As in the case of 1,5-cyclooctadienetricarbonyliron,limonene-Fetf.O}, complex(l) undergoes isomeriz-ation on irradiation for a longer period to afford thecomplextl l), the structure of which has also beenconfirmed on the basis of JR, PMR and mass spectra,and elemental analyses. The cornplextl l) exhibited inits JR spectrum bands at 2040 and 1970 ern - 1,

characteris~ of complexes having Fe(COh moietyattached to a ligand? Molecular ion peak at mjz 276and other peaks at m]z 248, 220 and 192 indicatesuccessive loss of three CO groups. The low fieldsignals at T 2.6-2.8 and 4.5-4.7 in its PMR spectrumhave been assigned to allylic protons HI and H2,

respectively and those atr 8.1-8.9 to methylene andmethyl protons bonded to sp3-carbon atom. The signalat T 5.3-5.4 has been assigned to methyl protons attachedto an allylic system. PMR spectrum of cornplextl l) issignificantly different from that of complextl), though itis isomeric with complextl). From PMR data it appearsthat the limonene ligand is bonded to Fe(COh throughan alJylic and a a-bond. The position of the methylprotons in the allylic system bonded to the metal seemscompatible with the fact that the chemical shift ofmethyl protons occurs at a relatively low field (T "" 5.4).Further it has been observed previously that thecentral carbon atom of allylic group bonded to an

'lL)7"Ft (CO))

(IV)

'''~''1''(Il¥

Fe(COh moiety is particularly electronegative". Forinstance, the central proton (H 1) of the allylic group inthe novel iron complex (lV)9 occurs at T 5.65. Theisomerisation of complex(l) to complexil l) onirradiating it for longer period may be visualised asshown in Scheme 1.

We have examined the reactivity of the coordinatedligand in complextl) by oxidising it with triphenyl-methyltetrafluoroborate (trityl) followed by nucleo-philic attack (H -) on the resulting dienyl salt. Thedienyl salt(III) exhibits in its IR spectrum peaks at 2110and 2060 em - 1 which are higher than those of thecorresponding neutral complextl) (2060,2010 and 1985cm - 1), as expected from the difference in the charge onthe metal. In the cationic complex there is presumablya reduced back donation of charge to the carbonylligand and hence the carbon oxygen bond order isgreater. Numerous examples are available in theliterature 10 of cationic dienyliron-tricarbonyl com-plexes produced by hydride ion abstraction fromcorresponding neutral complexes by Ph3C+ BFi.Cycloocta-2,4-dienyliron-tricarbonyl tetrafluoro-borate, obtained from the reaction of 1,3-cycloocta-dienetricarbonyliron and Ph3C +BF 4" exhibited JRabsorptions at 2103, 2057 and 2048 em -1 whereas forthe neutral complex the peaks occurred at 2043 and1974 cm - I. On the basis of IR and elemental analysesstructure(III) may be proposed for the dienyl salt[CloHlsFe(COh]+BFi. This structure satisfies theI8-e1ectron rule. The hydride ion abstraction occurs at

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INDIAN J. CHEM., VOL. 23A, DECEMBER 1984

the carbon atom adjacent to a coordinated doublebond. This finds support from literature on similartypes of complexes II.

The reaction of NaBH4 with the dienyl salultl)yields a neutral complex whose IR, PMR and massspectral data, and elemental analyses are akin to thoseof complexrl). This means that H - attack on the dienylsalt(III) gives back the original complex(I). It has beenshown that 1,3-cyclohexadieneirontricarbonyl givescyclohexadienylirontricarbonyl cation on treatmentwith trityltetrafluoroborate '? and Pauson et al. 13 haveshown that cations like cyclohexadienylirontri-carbonyl undergo nucleophilic attack by H -, N - atthe exo position to give back the original neutralcomplex or substituted products.

AcknowledgementThe PMR and mass spectra were obtained from the

Department of Chemistry, Manchester University(UK), and the elemental analyses were procured fromthe Department of Chemistry, Tohoku University,Japan.

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