Indian Journal of ChemistryVol. 27A, December 1988, pp. 1050-1052

Synthesis & Reactions of Isotetralenetricarbonyliron

M N ABSER, M A HASHEM, S E KABIR & S S ULLAH·Department of Chemistry, Jahangimagar University, Savar, Dhaka, Bangladesh

Received 23 November 1987; revised and accepted 14 March 1988

The complex isotetralenetricarbonyliron (I) has been synthesized by the photochemical reaction of isotetralene".;,1, Fe(CO)s. Complex(l) on reaction with Ph)C+BF; affords the trienyl salt [CIOHIIFe(COhIBF. (II) via hydride

,)straction. Complex(II) reacts with NaBH. to give back complex (I). The complexes have been characterizedasis of IR, PMR and mass spectra and elemental analyses.

Photochemical synthesis of metal-olefin complexesusing Fe(CO)5 and cycloolefins precursors hasbeen the subject of considerable research mainlybecause this method operates under considerablymilder conditions than the conventional thermalmethod!">. In such complexes the bonding of anorganic group to a transition metal can be used tomodify the reactivity and regiospecificity" and thisproperty is particularly useful in organic synthesis.The synthesis and hydride abstraction reactions oftrans, trans, trans-l,5,9-cyclododecatrienetricarbo-nyl-iron", trans-l ,4,9-decatrienetricarbonyliron 7

and limonenetricarbonyliron" have been recentlyreported from our laboratory. Iron carbonyl deriv-atives of polycyclic polyenes are, however, rare.Herein we report the photochemical synthesis ofthe title compound (I) and its reaction with tri-phenylmethyltetrafluoroborate.

Materials and MethodsAll operations were carried out in vacuo or un-

der N2 atmosphere. Infrared spectra were re-corded on a PE 983 spectrophotometer, PMRspectra on Bruker WH 360 and Varian XL 200instruments and mass spectra on a high resolutionMS-12 spectrophotometer. Photochemical reac-tions were carried out in sealed glass tubes by ir-radiating with a 125 watt medium pressure mercu-ry lamp. Solvents were distilled under nitrogenand kept over molecular sieves prior to use.

Triphenylmethyltetrafluoroborate was preparedaccording to the published method". Isotetralene(Aldrich) was used as received.

Photochemical reaction of isotetralene with Fe...COkFormation of isotetralenetricarbonyliron.li

A mixture of pentacarbonyliron (0.9 g, 4.5rnmol) and isotetralene (0.6 g, 4.5 mmol) in ben-zene (30 mI) was irradiated in a sealed tube for 10

1050

hr and filtered through kieselguhr. Removal of sol-vent in vacuo afforded a yellow oil, which wassubjected to column chromatography over silicaemploying pet. ether (40-60°) as an eluent. Remo-val of solvent under reduced pressure afforded Ias yellow oil in 73% yield; IR(cyclohexane): vCO2040 and 1956 cm-I; PMR(CDCI3): 6 5.0-6.0 (m,2H) and 0.8-3.6 (m, lOH); MS: mJz 272 (M+), 244(M+ - CO), 216 (M+ - 2CO) and 188 (M+ - 3CO).(Found: C, 57.9; H, 4.8. C13HI203Fe requires C,57.4; H, 4.5%).

A similar reaction using excess pentacarbonylir-on (2.7 g, 13.5 mmol) and isotetralene (0.6 g, 4.5mmol) followed by similar work-up also affordedthe same complex (I); no other product was isolat-ed.

Reaction of complex- I with triphenylmethyltetraj7.u-oroborate: Formation of complex- II

To a dry dichloromethane solution (5 mI) ofcomplex-I (0.5 g, 1.4 mmol), was added a solutionof triphenylmethyItetrafluoroborate (0.60 g, 1.8mrnol) in the same solvent. The mixture wasstirred at room temperature for 1 hr under nitrog-en when a pale yellow precipitate began to appear.Dry ether was added for complete precipitation.The product was separated by decantation of thesupernatant liquid, washed with dry ether and dri-ed under reduced pressure to afford the fairly air-stable pale yellow crystalline trienyl salt,[CIOHllFe(CO)3]BF4(Il) (0.42 g, 63%), which crys-tallized from dichloromethanel ether, m.p. 271-73°(d); IR(nujol): vCO 2120 and 2070 cm " ";PMR(CD3CN): 6 7.87 (dd, IH, J= 6.0, 4.0 Hz),7.45 (dd, IH, J=6.0, 4.0 Hz), 6.90 (d, IH, J=6.0Hz), 5.6-6.0 (m, 2H), 3.95-4.35 (m, 2H) and 2.6-3.4 (m, 4H); MS: m/z 271 (M+), 243 (M+ -CO),215 (M+ - 2CO) and 187 (M+ - 3CO) (Found:

ABSERet al.:SYNTHESIS& REACflONSOFISOTETRALENETRICARBONYLIRON

C,44.0; H, 3.6. C13HllFe03BF4 requires C, 43.6;H,3.1%).

Reaction of complex-Il with NaBH4: Formation ofcomplex-I

To an aqueous suspension (10 mI) of II (0.1 g,0.3 mmol) was added sodium borohydride (0.03 g,0.3 mmol) in water (5 mI). The mixture was stirredfor 1 hr, extracted with ether, dried (MgS04) andthe solvent removed under reduced pressure. Theyellow oil obtained was purified by columnchromatography over silica, eluting with pet. ether(40-60°). Removal of solvent in vacuo afforded ayellow liquid, identical (IR, PMR, mass) with com-plex-I.

Results and DiscussionThe structure of complex-I, in which isotetra-

lene is bonded to Fe(CO)3 moiety through two ofits non-conjugated double bonds, has been con-firmed as follows: The lowfield PMR signals at 05.02-6.0 have been assigned to the free olefinicprotons and these at higher field at 0 0.66-3.4 areattributable to the olefinic protons coordinated toFe(CO h moiety and the methylene protonsbonded to Sp3 carbon atoms. The mass spectrumshows the molecular ion peak at mlz 272 (M ;-and other peaks at mlz 244, 216, 188 and 132 in-dicate successive loss of three CO units and Fe;the presence of Fe(CO)3 moiety in the complex isthus evident",

A common tendency in tricarbonyl( diene ortriene )iron complexes is the formation of cationsby hydride abstraction when treated with triphen-ylmethyltetrafluoroborate" -7,10-13.Accordingly thetrienyl salt (II) obtained by reacting complex-I withPh3C+BFi exhibits in its IR v(CO) bands at 2120and 2070 em - 1 which are of similar pattern butshifted to higher frequencies than those of thecorresponding neutral complex (2048 and 1956em - 1) indicating that I and II have similar struc-tures but with greater positive charge at the metalatom of complex-II. Similar observations were re-ported for a number of dienyl- or trienyl-tricarbo-nyl-iron complexesv'". The structure of salt (II) isfurther supported by PMR and mass spectra andelemental analyses. The low field PMR signals at 06.90 (d, 1H, lAB=6 Hz), 7.45 (dd, 1H, lAB=6 Hz,lBC=4.0 Hz) and 7.87 (dd, 1H, lCD=6 Hz,lBC= 4.0 Hz) have been assigned to allylic protonsHA, HB and He respectively (Scheme 1). The sig-nals at 0 5.6-6.0 im; 2H) 'and 2.6-3.4 (m, 4H)have been assigned to the free olefinic protonsand the methylene protons of the same ring re-spectively and those at 0 3.95-4.35 (m, 2H) to the

0) +FoCCOls CQ

(II I(I(J:: ]+ BF,

L~;co~(IIIScheme I

methylene protons adjacent to the a~lylicsystem. ~hisassignment is in contrast to our preVIOUSobservatlO~saccording to which the central carbon atom of an allylicgroup bondedtoanFe( CO )3moietyisparticularlyelec-tronegative and consequently the proton attachedto it resonate at higher field. In complex-II the HAproton is more shielded than HB and He protonsand hence appears as a doublet at higher field (06.90) showing coupling to HB. A plausible expla-nation for this may be that the stereochemical ar-rangement of HA is such that it is more closer tothe metal than HB and He.

The mass spectrum of II showed the highest, 'to al. at mlz 271 corresponding to the cationic partlClOHllFe{CO)3l+ of the complex-Il. O~er peaksat 243, 215 and 181' indicate successive loss ofthree CO units from [ClOHnFe(CO)3l+. Elementalanalyses are also in agreement with the formulaproposed for complex II, which also satisfies the18-electron rule.

Treatment of the trienyl salt(lI) with NaBH4leads to I. It is suggested that H - attack on IIleads to the reformation of the original neutralcomplex-I. Similar observations have been report-ed in literaturelO,14,15.

AcknowledgementThe PMR and mass spectra were obtained

through the courtesy of the Department of Chem-istry, Tasmania University, Australia, TechnicalUniversity, Berlin, West Germany and UniversityCollege, London.

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