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T h e Comparative Photochemical Behaviour

of Dibenzenechromium and Benzenetricarbonyl C h r o m i u m

A N D R E W G I L B E R T * * , J O H N M . K E L L Y * * * ,

M A R I O N B U D Z W A I T * , and (the late) E R N S T K O E R N E R V O N G U S T O R F *

* Max-Planck-Institut für Kohlenforschung, Abteilung Strahlenchemie, Mülheim a.d. Ruhr, Germany ** Department of Chemistry, University of Reading, Whiteknights Park, Reading, RG6 2AD, Berkshire,

England

*** Chemistry Department, University of Dublin, Trinity College, Dublin 2, Ireland

(Z. Naturforsch. 31b, 1091-1095 [1976]; received April 12, 1976)

Photochemical Ligand Exchange

In comparison with (CeH6)Cr(CO)3, (CeHe^Cr is relatively light stable. The major part of the light energy absorbed by (CßHe^Cr leads neither to its decomposition or ligand exchange, nor can it be transferred to common low energy triplet acceptors. Internal dissipation of energy, either by rapid conversion to the ground state or by rapid reversible isomerization, must be an important process.

In the case of (CeH6)Cr(CO)3, the main pathway to the previously reported light-induced exchange of benzene involves an intermediate which is suggested to be (benzene)-dicarbonylchromium (1) and not a one-step dissociation of the excited molecule, to give Cr(CO)3 and benzenelb: 1 is also of major importance for the exchange of CO.

Benzenetricarbonyl chromium (BTC) is known to be photo-chemically labile and to undergo facile light-induced exchange of both benzene and carbon monox ide : this has been demonstrated using r e -labelled benzene and carbon monoxide 1 . On the other hand, the only published work on the photo-chemistry of dibenzenechromium (DBC) appears to be its photoreaction with alkyl chlorides to give (CeHe^Cr+CL and hydrocarbons2 and a comparison of its photochemical reactivity in cyclohexane solution with that of other metallocenes3 . Light-induced decompositions of (CeH6)2Cr+(DBC+) in aqueous solution4 yields D B C and benzene as well as Cr2+ and Cr3+.

Our interest in DBC was to investigate the effect o f coordination on the photochemistry of benzene and to explore the possibility of exchanging the coordinated benzene of (CöHe^Cr for other aromatic ligands.

Requests for reprints should be sent to Dr. A. GIL-BERT, Department of Chemistry, University of Reading, Whiteknights Park, Reading, RG6 2 AD, Berkshire, England.

N o ligand exchange could be detected by mass spectroscopy after 2 h irradiation (313 nm, 550 nm or unfiltered light f rom high pressure Hg arc) of D B C (0.6-1.0 x 10"1 M) inC 6 D6orC 6 D6/cyc lohexane solution. Some CßHe was however detected in the solvent after irradiation, amounting to approxi-mately 1 0 % D B C decomposit ion 5 ; this was accom-panied by the formation o f some precipitate. No decomposition occurred in the dark during 48 hours at 20 °C.

BTC behaved quite differently however, and under comparable conditions 7 0 % of the (CeH6)Cr(CO)3 underwent photoinduced exchange of the benzene in C Ö D Ö solution. In this case decomposition was very slight. In a further experiment, two samples o f BTC (0.5 X 10-1 M) in C 6 D 6 were prepared by flushing with argon and with carbon monoxide, respectively. After irradiation through a pyrex glass filter for 1 h, the samples contained 6 . 9 % and 0 . 9 % (CeD6)Cr(CO)3, respectively: the addition of carbon monoxide had thus considerably suppressed the benzene exchange process. Further no Cr(CO)6 could be observed, hence ruling out the conceivable react ion l b of Cr(CO)3 with CO:

1092 A. GILBERT ET AL. • DIBENZENE- AND BENZENETRICABBONYL-CHROMIUM

( Q j - C r ( C 0 ) 3 J U U . ( 0 ) • Cr(C0)3

Cr(C0)3 + 3 CO • Cr (CO)g

2

In cyclohexane solution BTC undergoes rapid light induced decomposition to release benzene and to yield a grey-green solid, which was not further investigated. Carbon monoxide reduces this decom-position substantially and 13CO is rapidly incor-porated into BTC. In contrast CO does not affect the slow decomposition of DBC under the same conditions.

Light-induced (313 nm) ligand exchange in BTC has been further investigated in the presence of 13CO and/or C6Ü6 with argon and cyclohexane as the alternative atmosphere and medium respec-tively. The resultant mixtures were analysed by mass spectrometry and although some difficulty was experienced in exact reproducibility from ex-periment to experiment, it was evident that incorporation of CeD6 was strongly suppressed by CO whereas that of 13CO was little affected by benzene. This suggests that (arene)Cr(CO)2 (1) may also be a precursor in the exchange of the aromatic ligand as well as the CO exchange. In particular the results wrould argue against the competition of the arene and CO for a species such as (tetrahapto-benzene) tricarbonyl chromium for which structure 2 is conceivable, although stepwise deco-ordination of benzene has been suggested for thermal SN2 reactions of arene tricarbonylchromium6 and molybdenum7 . In the thermal substitution reactions it is always the aromatic ligand which is replaced7.

The measured quantum efficiency for CO exchange with 313 nm radiation was in accord with that (0.72) reported by W R I G H T O N and H A V E R T Y 8 : the effi-ciency of arene exchange is approximately one sixth of this value.

The described photochemical stability of di-benzenechromium, wdiich contrasts that of BTC, finds its parallel in the theimal reactivity difference of these two compounds. Thermal exchange of benzene in DBC for another aromatic ligand is only observed in the presence of catalysts9, while that in BTC is easily brought about by simply heating the reactants10-11.

W e have not been able to provide any evidence for an intermediate in the process of deactivation of electronically excited DBC. W e considered that it might be possible to trap an intei mediate such as 3 with powerful ligands (L). Attempts with CO, di-

5

methylfumarate and dimethylmaleate were un-successful. However, an intramolecular Sn2 reaction involving an attack of the uncoordinated double bond in 4 and a concomitant elimination of L cannot be ruled out. A very rapid backreaction of 3 to DBC could provide another explanation for the failure of the trapping experiments: the same argument would also stand for an intermediate such as 5.

Use of maleic anhydride (MA) as a trap for 3 was also checked, but solutions of the complex and the anhydride (MA) reacted in the dark to yield an orange paramagnetic complex having a UV spec-trum indicative of DBC+ (/.max at 271 nm and 333nm). Strong Lewis bases are known to yield violet black 1:1 complexes with DBC and these are formulated as DBC+ salts of radical anions12. Although the composition of the present orange complex was relatively variable, such results as given in the experimental section were more consistent with the complex, having a 2 :1 rather than a 1:1 ratio o f MA to DBC: 2 :1 complexes of a seemingly similar type have been reported for nickelocene13.

The possibility of fast benzene valence bond isomerization as a path of radiationless deactivation also has to be considered. For example formation of 6 may result on irradiation of DBC but this would be expected to undergo facile exchange of the aromatic ligand and this was not observed. Further, elimination of the substituted benzvalene should occur on irradiation of di(;p-xylene)chromium, and thereby result in the production of isomeric xylenes: this how-ever was found not to be the case.

Attempts were made to check for intermediates from these photolyses by irradiation of DBC and

1093 A. GILBERT ET AL. • DIBENZENE- AND BENZENETRICABBONYL-CHROMIUM

R

R

o - V R

+ decomposi t ion products

BTC in a 50:50 methylcyclohexane/isopentane glass at 77 K . This technique, however, proved to be inapplicable as both DBC and BTC aggregate on cooling the solution. Cooling of a 10~3 M solution of BTC, produced crystals. At lower concentrations the absorption band (/max at 316 nm at 20 °C) shifted to shorter wavelengths (Amax at 306 nm at — 1 6 5 °C): the shorter wavelength band (/max at 262 nm) also shifted and was obscured by intense tail-end absorption. Similarly with DBC, the ab-sorption spectrum showed marked changes (Amax

from 307 nm at 20 °Cto 302 nm at —165 °C, and the appearance of strong structureless absorption between 200 and 300 nm). B R A T E R M A N has reported aggregation on cooling solutions of the analogous .T-cyclopentadienyl tricarbonyl manganese14.

On flash photolysis of a 5 X 10 - 4 M cyclohexane solution of (CeHe^Cr, no transients could be detected and the solution was recovered essentially unchanged after photolysis. These results suggest either, that if isomers such as 3 or 6 are formed, they do not absorb sufficiently strongly in the visible region of the spectrum or that their lifetime is less than 5 //sec. Alternatively, it is possible that the excited state of DBC undergoes rapid radiationless deactivation without any change in its chemical nature15.

Transients could, however, be observed on flash photolysis of a 10 - 3 M cyclohexane solution of BTC. Immediately after the flash, a weakly absorbing species was recorded which reacted further within the first millisecond to form a second transient species and this absorbed throughout the visible spectrum (Amax at approx. 500 nm). The concentra-tion and rate of reaction of these species were essentially unaffected by the presence of 1.1 X 10_1 M benzene in the solution, but the species are strongly quenched if the solution was saturated under one

atmosphere of CO: the optical density at 497 nm 100 jus after the flash was then reduced to 15% of that in the absence of CO. While these studies are insufficient to allow an unambiguous identification of the transient species observed, it is probable that the first species observed is (benzene)Cr(CO)2, and that in the presence of added CO we are observing the combination of this with CO to reform BTC. The nature of the second species is at present uncertain and more detailed experiments would have to be carried out before one could differentiate between possibilities such as dimers, isomers, or complexes with trace solvent impurities, by analogy with the case of Cr(CO)6 1 6 .

DBC is isoelectronic with ferrocene and a com-parative studj7 of their photochemistry should be of interest. In agreement with the results of B O R R E L L

and H E N D E R S O N 3 we have found that D B C is quite light stable and similar in this respect to ferrocene. Ferrocene is however an efficient quencher of triplet excited ketones and aromatic hydrocarbons, even of those with a triplet energy lower than 160 kJ mol - 1 17. Further ferrocene is known to sensitise m-frans-isomerisation of stilbene and of piperylene, and to promote the dimerization of isoprene18. W e have found that while DBC does not sensitize any of the above reactions, it is capable of quenching the fiuorenone triplet excited state at a diffusion controlled rate19.

Conclusions

In comparison with (CeH6)Cr(CO)3, (CeHö^Cr is relatively light stable. The major part of the light energy absorbed by (CöHß^Cr leads neither to its decomposition or ligand exchange, nor can it be transferred to common low energy triplet acceptors. Internal dissipation of energy, either by rapid conversion to the ground state or by rapid reversible isomerization, must be an important process. By analogy with ferrocene and ruthenocene 17d>e it is possible that the species merely undergoes sym-metrical expansion in passing from the ground state to the excited state.

In the case of (CeH6)Cr(CO)3, the main pathway to the previously reported light-induced exchange of benzene involves an intermediate which is sug-gested to be (benzene )dicarbonylchromium (1), and not a one-step dissociation of the excited molecule, to give Cr(CO)3 and benzene l b : 1 is also of major importance for the exchange of CO.

1094 A. GILBERT ET AL. • DIBENZENE- AND BENZENETRICABBONYL-CHROMIUM

W e hope that our observations will catalyse further studies of arene chromium complexes to get a better understanding of the intimate mechanisms of their photoreactions. A search for possible wavelength effects would be especially desirable.

Experimental (CeHß^Cr and (C6H6)Cr(CO)3 were prepared by

reported procedures2 0-2 1 . After repeated sublima-tion, weak fluorescence of BTC samples was removed and that of DBC samples was reduced. Spectroscopic data were as reported in the literature, with the exception of the U V spectrum of (CeHe^Cr. W7hen purified, this species showed as well as the band with ^max at 307 nm, a previously urepoi ted 2 2

shoulder at 392 ( e ~ 1 0 0 0 l x m o H c m " 1 ) and a deepened valley at 250 nm ( e ~ 1 0 0 1X m o l - 1 cm - 1 ) .

All steady state experiments reported here were carried out in well-dried argon degassed cyclo-hexane or benzene. W e have, however, attempted to use other solvents as reaction media for the irradiation. (CöHe^Cr is only slightly soluble in methanol, acetonitrile and acetone: while admission of trace amounts of air to suspensions of D B C in these solvents apparently increased the solubility, spectral data clearly showed that this was due to the formation of DBC+ (Amax at 270 nm and 333 nm). Irradiations of such solutions produced green-grey solids and a colourless solution containing at least 8 0 % of the originally complexed benzene.

Steady state irradiations were carried out using a water-cooled high pressure mercury lamp (Philips H P K 125 W) . The exchange experiments with 13CO (isotopic purity 90 .9% from Merck, Sharp and Dohme, Canada) and CeD6 were performed in S O L I D E X tubes (fitted with a 3-way stopper) of 15 cm length and 1 cm i. d. at a fixed 6 cm distance from the lamp. Products were analysed b y mass spectrometry with an A T L A S CH5. For experi-ments with monochromatic light, the beam was

1 a W . S T R O H M E I E R a n d D . V O N H O B E , Z . N a t u r -forsch. 18b, 770 [1963]; b Z. Naturforsch. 18b, 981 [1963]; c W. STROHMEIER, Angew. Chem. 75, 453 [1963]; Angew. Chem. Int. Ed. 2, 270 [1963].

2 G . A . R A Z U V A E V a n d G . A . D O M R A C H E V , T e t r a -hedron 19, 341 [1963].

3 P. BORRELL and E. HENDERSON, Inorg. Chim. Acta 12, 215 [1975].

4 O . T R A V E R S O , F . S C A N D O L A , V . B A L Z A N I , a n d S . V A L C H E R , M o l . P h o t o c h e m . 1 , 2 8 9 [ 1 9 6 9 ] .

5 The trivial mechanism of traces of air or moisture promoting oxidation to form the (CeH6)2Cr+ ion, which is subsequently photolysed yielding benzene inter alia4 cannot be totally discounted, since the complete absence of such ubiquitous impurities can never be assured. However in this case, the ion would be present in very low concentration as its solubility in very dry cyclohexane is very low.

6 M. CAIS and R. REJOAN, Inorg. Chim. Acta 4, 509 [1970].

passed through a Bausch-Lomb high intensity monochromator. Samples were contained in a conventional 1 cm Suprasil cell, fitted with a Teflon stopper, adapted so that the sample could be flushed with argon and sample transfer accomplished under argon. All experiments were carried out at room temperature.

Flash photolysis experiments were carried out on the previously described apparatus2 3 . Samples were degassed by the reported procedure16 .

Reaction of dibenzenechromium with maleic anhydride 2 ml of a saturated solution of D B C in dry and

argon degassed benzene were added to 1 ml of a saturated benzene solution of maleic anhydride. An orange solid precipitated, which was washed with dry and argon degassed ether (2 x 5 ml) and dried in vacuo. Thereafter the product was handed in air. Its paramagnetism prohibited a 1 H - N M R investi-gation. According to the elemental analysis, addition of 2 moles maleic anhydride to 1 mol DBC and I mol H2O had occurred.

(C6H6)2Cr • C4H2O3 (306.3) Calcd C 62.70 H 4.61 Cr 16.99

(C6H6)2Cr • 2(C 4H 20 3 ) (404.3) Calcd C 59,41 H 3,99 Cr 12,86

(C6H6)2Cr • 2(C 4H 20 3 ) + 2 H 2 0 (440.4) Calcd C 54.60 H 4.58 Cr 11.82

(C6H6)2Cr • 2(C 4H 20 3 ) + 1 H 2 0 (422.4) Calcd C 56.87 H 4.30 Cr 12.31 Found C 56.16 H 4.35 Cr 11.69

C 56.14 H 4.31 Mol. weight found [crysc. in (CH3)2SO]: 437

435

A. G. and J. M. K . thank The Society for the award of European Fellowships during the period in which this work was carried out. The authors are grateful to Dr. D. H E N N E B E R G , H . D A M E N , and W . S C H M Ö L L E R for the MS analyses and to Dr. F . -W. G R E V E L S for valuable discussion.

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8 M . S . W R I G H T O N a n d J . L . H A V E R T Y , Z . N a t u r -forsch. 30b, 254 [1975].

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1 2 J . W . FITCH and J . J . LAGOWSKI, Inorg. Chem. 4, 864 [1965].

1095 A. GILBERT ET AL. • DIBENZENE- AND BENZENETRICABBONYL-CHROMIUM

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2 3 H . H E R M A N N , G . K O L T Z E N B U R G , a n d D . S C H U L T E -FROHLINDE, Ber. Bunsen. Phys. Chem. 77, 677 [1973].