The Comparative Photochemical Behaviour
of D i b e n z e n e c h r o m i u m a n d B e n z e n e t r i c a r b o n y l
ANDREW
MARION
BUDZWAIT*,
GILBERT**,
JOHN
a n d (the late)
M .
ERNST
Chromium
K E L L Y * * * ,
KOERNER
VON
GUSTORF*
* 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 lightinduced 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 k n o w n to
be photo-chemically labile and to undergo facile
light-induced exchange of b o t h benzene and carbon
m o n o x i d e : this has been demonstrated using r e labelled benzene and carbon m o n o x i d e 1 . On the
other hand, the only published work on the p h o t o chemistry o f dibenzenechromium (DBC) appears to
be its photoreaction with alkyl chlorides t o give
(CeHe^Cr+CL and h y d r o c a r b o n s 2 and a comparison
o f its photochemical reactivity in cyclohexane
solution with that of other metallocenes 3 . Lightinduced decompositions o f (CeH6)2Cr + (DBC + ) in
aqueous solution 4 yields D B C and benzene as well
as Cr 2 + and Cr 3 + .
Our interest in D B C was t o investigate the effect
o f coordination on the photochemistry o f benzene
and t o explore the possibility of exchanging the
coordinated benzene of (CöHe^Cr for other aromatic
ligands.
Requests for reprints should be sent to Dr. A. GILBERT, Department of Chemistry, University of
Reading, Whiteknights Park, Reading, RG6 2 AD,
Berkshire, England.
N o ligand exchange could be detected b y mass
spectroscopy after 2 h irradiation (313 n m , 550 n m
or unfiltered light f r o m high pressure H g arc) o f
D B C (0.6-1.0 x 10" 1 M ) i n C 6 D 6 o r C 6 D 6 / c y c l o h e x a n e
solution. Some CßHe was however detected in the
solvent after irradiation, amounting t o approximately 1 0 % D B C d e c o m p o s i t i o n 5 ; this was a c c o m panied b y the f o r m a t i o n o f some precipitate. N o
d e c o m p o s i t i o n occurred in the dark during 48 hours
at 20 °C.
B T C behaved quite differently however, and under
comparable conditions 7 0 % of the (CeH6)Cr(CO)3
underwent p h o t o i n d u c e d exchange o f the benzene
in C Ö D Ö solution. I n this case decomposition was
v e r y slight. I n a further experiment, t w o samples o f
B T C (0.5 X 10- 1 M) in C 6 D 6 were prepared b y
flushing with argon and with carbon monoxide,
respectively. After irradiation through a p y r e x glass
filter for 1 h, the samples contained 6 . 9 % and 0 . 9 %
(CeD6)Cr(CO)3, respectively: the addition o f carbon
m o n o x i d e had thus considerably suppressed the
benzene exchange process. Further n o Cr(CO)6
could be observed, hence ruling out the conceivable
r e a c t i o n l b of Cr(CO) 3 with C O :
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1092
A. G I L B E R T ET AL. • D I B E N Z E N E - A N D B E N Z E N E T R I C A B B O N Y L - C H R O M I U M
(Qj-Cr(C0)3
JUU.
Cr(C0) 3
+ 3 CO
( 0 )
• Cr(C0) 3
W e have n o t been able t o provide any evidence
for an intermediate in the process of deactivation o f
electronically excited DBC. W e considered that it
•
Cr (CO)g
might be possible to trap an intei mediate such as 3
with powerful ligands (L). Attempts with CO, di-
2
In cyclohexane solution B T C undergoes rapid
light induced decomposition t o release benzene and
t o yield a grey-green solid, which was not further
investigated. Carbon monoxide reduces this decomposition substantially and
1 3 CO
is rapidly incor-
porated into BTC. I n contrast CO does not affect
the slow decomposition o f D B C under the same
conditions.
Light-induced (313 nm) ligand exchange in B T C
has been further investigated in the presence o f
1 3 CO and/or C6Ü6 with argon and cyclohexane as
the alternative atmosphere and medium respectively. The resultant mixtures were analysed b y
mass spectrometry and although some difficulty
was experienced in exact reproducibility f r o m experiment t o experiment, it was evident that
incorporation o f CeD 6 was strongly suppressed b y
CO whereas that of 13 CO was little affected b y
benzene. This suggests that (arene)Cr(CO)2 (1) m a y
also be a precursor in the exchange o f the aromatic
ligand as well as the CO exchange. I n particular the
results wrould argue against the competition o f the
arene and CO for a species such as (tetrahaptobenzene) tricarbonyl chromium for which structure
2 is conceivable, although stepwise deco-ordination
o f benzene has been suggested for thermal SN2
reactions of arene tricarbonylchromium 6
and
m o l y b d e n u m 7 . I n the thermal substitution reactions
it is always the aromatic ligand which is replaced 7 .
The measured quantum efficiency for CO exchange
with 313 nm radiation was in accord with that (0.72)
reported b y W R I G H T O N and H A V E R T Y 8 : the efficiency of arene exchange is approximately one sixth
o f this value.
The described photochemical stability o f dibenzenechromium, wdiich contrasts that o f BTC,
finds its parallel in the theimal reactivity difference
o f these t w o compounds. Thermal exchange o f
benzene in D B C for another aromatic ligand is only
observed in the presence o f catalysts 9 , while that
in B T C is easily brought about b y simply heating
the reactants 1 0 - 1 1 .
5
methylfumarate and dimethylmaleate were unsuccessful. However, an intramolecular Sn2 reaction
involving an attack of the uncoordinated double
b o n d in 4 and a concomitant elimination of L cannot
be ruled out. A very rapid backreaction of 3 t o
DBC could provide another explanation for the
failure o f 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 o f the complex and the
anhydride (MA) reacted in the dark to yield an
orange paramagnetic complex having a U V spectrum indicative o f DBC+ (/.max at 271 nm and 333nm).
Strong Lewis bases are known t o yield violet black
1 : 1 complexes with DBC and these are formulated
as DBC+ salts o f radical anions 1 2 . Although the
composition o f 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
M A to DBC: 2 : 1 complexes of a seemingly similar
t y p e have been reported for nickelocene 1 3 .
The possibility of fast benzene valence b o n d
isomerization as a path of radiationless deactivation
also has t o be considered. For example formation o f
6 m a y result on irradiation o f DBC but this would
be expected t o undergo facile exchange of the
aromatic ligand and this was not observed. Further,
elimination o f 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 t o be the case.
Attempts were made to check for intermediates
f r o m these photolyses b y irradiation of DBC and
1093 A. G I L B E R T ET AL. • DIBENZENE- A N D B E N Z E N E T R I C A B B O N Y L - C H R O M I U M
R
R
o
-
V
R
+ decomposition
products
B T C in a 5 0 : 5 0 methylcyclohexane/isopentane glass
at 77 K . This technique, however, proved to be
inapplicable as both D B C and BTC aggregate on
cooling the solution. Cooling of a 10~3 M solution of
BTC, produced crystals. A t lower concentrations
the absorption band (/max at 316 nm at 20 °C)
shifted t o shorter wavelengths (Amax at 306 n m at
— 1 6 5 °C): the shorter wavelength band (/max at
262 nm) also shifted and was obscured b y intense
tail-end absorption. Similarly with DBC, the absorption spectrum showed marked changes (A max
f r o m 307 n m at 20 ° C t o 302 nm at — 1 6 5 °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 manganese 14 .
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
d o not absorb sufficiently strongly in the visible
region o f the spectrum or that their lifetime is less
than 5 //sec. Alternatively, it is possible that the
excited state of D B C undergoes rapid radiationless
deactivation without any change in its chemical
nature 1 5 .
Transients could, however, be observed on flash
photolysis of a 1 0 - 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 (A max at approx. 500 nm). The concentration and rate of reaction of these species were
essentially unaffected b y 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 C O : the optical density at 497 nm
100 jus after the flash was then reduced to 1 5 % 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 o f the second species is at present
uncertain and more detailed experiments would
have t o be carried out before one could differentiate
between possibilities such as dimers, isomers, or
complexes with trace solvent impurities, b y analogy
with the case of Cr(CO) 6 1 6 .
D B C is isoelectronic with ferrocene and a comparative studj 7 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 k J m o l - 1 17 . Further ferrocene is known to
sensitise m-frans-isomerisation o f stilbene and of
piperylene, and to promote the dimerization of
isoprene 1 8 . W e have found that while D B C does not
sensitize any o f the above reactions, it is capable of
quenching the fiuorenone triplet excited state at a
diffusion controlled rate 1 9 .
Conclusions
In comparison with (CeH6)Cr(CO)3, (CeHö^Cr is
relatively light stable. The major part of the light
energy absorbed b y (CöHß^Cr leads neither to its
decomposition or ligand exchange, nor can it be
transferred t o c o m m o n low energy triplet acceptors.
Internal dissipation of energy, either b y rapid
conversion to the ground state or b y rapid reversible
isomerization, must be an important process. B y
analogy with ferrocene and ruthenocene 17d>e it is
possible that the species merely undergoes symmetrical expansion in passing from the ground
state to the excited state.
I n the case o f (CeH6)Cr(CO)3, the main pathway
t o 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 o f the excited molecule,
t o give Cr(CO)3 and b e n z e n e l b : 1 is also of major
importance for the exchange of CO.
1094
A. G I L B E R T ET AL. • D I B E N Z E N E - A N D B E N Z E N E T R I C A B B O N Y L - C H R O M I U M
We
hope that our observations will
catalyse
further studies o f arene c h r o m i u m complexes t o get
a better understanding o f the intimate mechanisms
of
their
photoreactions.
A
search
for
possible
wavelength effects would be especially desirable.
Experimental
(CeHß^Cr and (C6H 6 )Cr(CO)3 were prepared b y
reported procedures 2 0 - 2 1 . A f t e r repeated sublimation, weak fluorescence of B T C samples was r e m o v e d
and that o f D B C samples was reduced. Spectroscopic
data were as reported in the literature, with the
exception o f the U V spectrum o f (CeHe^Cr. W 7 hen
purified, this species showed as well as the band
with ^max at 307 n m , a previously u r e p o i t e d 2 2
shoulder at 392 ( e ~ 1 0 0 0 l x m o H c m " 1 ) and a
deepened valley at 250 n m ( e ~ 1 0 0 1 X m o l - 1 c m - 1 ) .
All steady state experiments reported here were
carried out in well-dried argon degassed c y c l o hexane or benzene. W e have, however, attempted
to use other solvents as reaction media f o r the
irradiation. (CöHe^Cr is o n l y slightly soluble in
methanol, acetonitrile and a c e t o n e : while admission
of trace amounts o f air t o suspensions o f D B C in
these solvents apparently increased the solubility,
spectral data clearly showed that this was due t o
the formation of DBC+ (Amax at 270 n m and 333 nm).
Irradiations of such solutions p r o d u c e d green-grey
solids and a colourless solution containing at least
8 0 % o f the originally c o m p l e x e d 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 1 3 CO
(isotopic purity 9 0 . 9 % f r o m Merck, Sharp and
D o h m e , Canada) and CeD 6 were performed in
S O L I D E X tubes (fitted with a 3 - w a y stopper) of
15 c m length and 1 c m i. d. at a fixed 6 c m distance
f r o m the lamp. Products were analysed b y mass
spectrometry with an A T L A S C H 5 . F o r experiments with monochromatic light, the b e a m was
1
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The trivial mechanism of traces of air or moisture
promoting oxidation to form the (CeH6)2Cr+ ion,
which is subsequently photolysed yielding benzene
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passed through a B a u s c h - L o m b high intensity
m o n o c h r o m a t o r . Samples were contained in a
conventional 1 c m Suprasil cell, fitted with a Teflon
stopper, adapted so that the sample could be flushed
with argon and sample transfer accomplished under
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temperature.
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the previously described apparatus 2 3 . Samples were
degassed b y the reported procedure 1 6 .
Reaction of dibenzenechromium with maleic anhydride
2 ml of a saturated solution o f D B C in dry and
argon degassed benzene were added to 1 ml o f a
saturated benzene solution of maleic anhydride. A n
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 investigation. A c c o r d i n g t o the elemental analysis, addition
o f 2 moles maleic anhydride to 1 mol D B C and
I m o l H2O had occurred.
(C 6 H 6 ) 2 Cr
Calcd
(C 6 H 6 ) 2 Cr
Calcd
(C 6 H 6 ) 2 Cr
Calcd
(C 6 H 6 ) 2 Cr
Calcd
Found
• C4H2O3 (306.3)
C 62.70 H 4.61 Cr 16.99
• 2 ( C 4 H 2 0 3 ) (404.3)
C 59,41 H 3,99 Cr 12,86
• 2 ( C 4 H 2 0 3 ) + 2 H 2 0 (440.4)
C 54.60 H 4.58 Cr 11.82
• 2 ( C 4 H 2 0 3 ) + 1 H 2 0 (422.4)
C 56.87 H 4.30 Cr 12.31
C 56.16 H 4.35 Cr 11.69
C 56.14 H 4.31
Mol. weight f o u n d [crysc. in (CH 3 ) 2 SO]: 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 t o 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 t o Dr.
F . - W . G R E V E L S for valuable discussion.
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