Unusually slow photodissociation of CO from (η6-C 6H6)Cr(CO)3 (M= Cr or Mo): A time-resolved Infrared, Matrix Isolation, and DFT investigation

Article abstract of DOI:10.1021/om800925s

The photochemistry of η⁶-C₆H₆)M(CO) ₃ (M = Cr or Mo) is described. Photolysis with λexc. > 300 nm of (η⁶-C₆H₆)Cr(CO)₃ in low-temperature matrixes containing CO produce

Full text of DOI:10.1021/om800925s

Organometallics 2009, 28, 1461–1468
1461
Unusually Slow Photodissociation of CO from (η6-C6H6)Cr(CO)3 (M
) Cr or Mo): A Time-Resolved Infrared, Matrix Isolation, and DFT
Investigation
Mohammed A. H. Alamiry,†,‡ Nicola M. Boyle,‡ Christopher M. Brookes,§
Michael W. George,*,§ Conor Long,*,‡ Peter Portius,§, Mary T. Pryce,‡ Kate L. Ronayne,4
Xue-Zhong Sun,§ Michael Towrie,4 and Khuong Q. Vuong§
Department of Chemistry, UniVersity of Nottingham, Nottingham NG7 2RD, U.K., and School of Chemical
Sciences, Dublin City UniVersity, Dublin 9, Ireland
ReceiVed October 22, 2008
The photochemistry of (η6-C6H6)M(CO)3 (M ) Cr or Mo) is described. Photolysis with λexc. > 300
nm of (η6-C6H6)Cr(CO)3 in low-temperature matrixes containing CO produced the CO-loss product, while
lower energy photolysis (λexc. > 400 nm) produced Cr(CO)6. Pulsed photolysis (λexc. ) 400 nm) of (η6-
C6H6)Cr(CO)3 in n-heptane solution at room temperature produced an excited-state species (1966 and
1888 cm-1) that decays over 150 ps to (η6-C6H6)Cr(CO)2(n-heptane) (70%) and (η6-C6H6)Cr(CO)3 (30%).
Pulsed photolysis (λexc. ) 266 nm) of (η6-C6H6)Cr(CO)3 in n-heptane produced bands assigned to (η6-
C6H6)Cr(CO)2(n-heptane) (1930 and 1870 cm-1) within 1 ps. These bands increase with a rate identical
to the rate of decay of the excited-state species and the rate of recovery of (η6-C6H6)Cr(CO)3. Photolysis
of (η6-C6H6)Mo(CO)3 at 400 nm produced an excited-state species (1996 and 1898 cm-1) and traces of
(η6-C6H6)Mo(CO)2(n-heptane) within 1 ps. For the chromium system CO-loss can occur following
excitation at both 400 and 266 nm via an avoided crossing of a MACT (metal-to-arene charge transfer)
and MCCT/LF (metal-to-carbonyl charge transfer/ligand field) states. This leads to an unusually slow
CO-loss following excitation with 400 nm light. Rapid CO-loss is observed following 266 nm excitation
because of direct population of the MCCT/LF state. The quantum yield for CO-loss in the chromium
system decreases with increasing excitation energy because of the competing population of a high-energy
unreactive MACT state. For the molydenum system CO-loss is a minor process for 400 nm excitation,
and an unreactive MACT state is evident from the TRIR spectra. A higher quantum yield for CO-loss is
observed following 266 nm excitation through both direct population of the MCCT/LF state and production
of a vibrationally excited reactive MACT state. This results in the quantum yield for CO-loss increasing
with increasing excitation energy.
Organometallic complexes have many uses in catalytic and
synthetic systems, and (η6-C6H6)Cr(CO)3 complexes have found
many applications in organic syntheses.1,2 The coordination of
an arene to a Cr(CO)3 fragment can activate the arene toward
nucleophilic attack,1 enhance the kinetic acidity of the ring
hydrogens,3 increase the stability of anionic intermediates,4 and
also impart some regiocontrol in substitution reactions.5 The
ability of the arene ligand to undergo ring-slip or haptotropic
shift processes has also been implicated in some nucleophilic
substitution reactions.6
Although there have been extensive investigations, the precise
mechanism of the arene exchange reaction in (η6-C6H6)Cr(CO)3
complexes is still under debate. Proposals ranging from
self-catalysis,7-9 to CO-loss,10,11 to a stepwise ring displacement
process have been presented.6,12 The latter hypothesis involved
an η6 to η4 haptrotropic shift preceding coordination of the
entering arene ligand. A recent theoretical study that examined
* To whom correspondence should be addressed. E-mail: Mike.George@
nottingham.ac.uk; Conor.Long@dcu.ie.
† Present address: Molecular Photonics Laboratory, School of Natural
Sciences, Bedson Building, University of Newcastle, Newcastle upon Tyne
NE1 7RU, UK.
‡ Dublin City University.
(5) Semmelhack, M. F.; Garcia, J. L.; Cortes, D.; Farina, R.; Hong, R.;
§ University of Nottingham.
Carpenter, B. K. Organometallics 1983, 2, 467–469.
Department of Chemistry, University of Sheffield, Sheffield S3 7HF,
England.
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2707–2710.
4 Central Laser Facility, Photon Science Department, Science &
Technology Facilities Council, Rutherford Appleton Laboratory, Harwell
Science and Innovation Campus, Didcot, Oxfordshire, OX11 0QX, UK.
(1) Djukic, J. P.; Rosemunch, F.; Rose, E.; Simon, F.; Dromzee, Y.
Organometallics 1995, 14, 2027–2038.
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29, 339–346.
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6985.
10.1021/om800925s CCC: $40.75
2009 American Chemical Society
Publication on Web 02/12/2009

1462 Organometallics, Vol. 28, No. 5, 2009
Alamiry et al.
various reduced hapticity species indicated that (η4-C6H6)-
Cr(CO)3 is not at a minimum on either the ground-state singlet
or triplet potential energy hypersurfaces.13 However a number
of other reduced hapticity species were found, including (η2-
C6H6)Cr(CO)3 and (η1-C6H6)Cr(CO)3.
The combination of photochemistry and time-resolved
spectroscopy is a powerful tool for probing ligand substitu-
tions and reaction mechanisms. The photochemistry of (η6-
C6H6)Cr(CO)3 has also been extensively investigated; the
dominant photochemical process is CO-loss.14-20 The high
quantum efficiency (ΦCO ) 0.72) of this process has been
reported to be independent of excitation wavelength at room
temperature,10 although close inspection of the published data
does reveal a small wavelength-dependent effect, which is
particularly relevant to this work (see below). In contrast,
the quantum efficiency of CO-loss in the molybdenum
analogue shows a marked wavelength dependence (ΦCO
increases with increasing excitation energy), and the tungsten
complex appears to be photochemically inert in hydrocarbon
solvents.21 Photoinduced arene exchange was also observed
for (η6-C6H6)Cr(CO)3 under appropriate conditions (see
above).22 A number of intriguing observations were made
for this process. For instance addition of CO to the photolysis
solution suppresses arene exchange. Also the quantum
efficiency of the arene exchange process is strongly solvent
dependent.23 Intermediates in the photoinduced arene ex-
change process remain elusive for (η6-arene)Cr(CO)3 systems
with one exception. Low-energy photolysis of (η6-
C5H5N)Cr(CO)3 produced the haptotropic shift product
(η1-N-C5H5N)Cr(CO)3.24,25 Many questions remain unan-
swered, and the application of fast time-resolved spectro-
scopic techniques offers the opportunity to address some of
these issues.
Figure 1. (a) Ground-state UV/vis spectrum of (η6-C6H6)Cr(CO)3
in cyclohexane at room temperature. (b) Ground-state IR spectrum
(νCO region) of (η6-C6H6)Cr(CO)3 in n-heptane at room temperature.
The published studies of particular relevance to this work
are (i) the now generally accepted mechanism for CO-loss from
homoleptic group 6 carbonyls, which involves vibronic coupling
in the initially produced excited state, leading to an unbound
state with respect to the M-CO interaction and fast (∼100 fs)
expulsion of one CO ligand,26,27 (ii) the photoinduced CO
substitution in M(CO)4(R-diimine) complexes (M ) Cr, Mo,
or W), which involves independent reactions from at least two
different metal to ligand charge transfer (MLCT) excited states,28
and (iii) the recent observation of slow CO-loss from an avoided
crossing of a thermally relaxed 3MLCT/XLCT (metal-to-ligand/
halide-to-ligand charge transfer) excited state following irradia-
tion of [RuCl2(CO)2(iPr-dab)] (iPr-dab ) N,N′-diisopropyl-1,4-
diazabutadiene).29 In this paper we report an investigation into
the photochemistry of (η6-C6H6)M(CO)3 systems (M ) Cr or
Mo) using picosecond time-resolved infrared spectroscopy and
matrix isolation techniques in order to gain a greater insight
into the underlying photophysical processes preceding either
CO-loss or arene exchange. We monitor the decay of an excited-
state precursor to the CO-loss process and show that CO-loss
is slow because of an avoided crossing.
(13) Cohen, R.; Weitz, E.; Martin, J. M. L.; Ratner, M. A. Organome-
tallics 2004, 23, 2315–2325.
(14) Strohmeier, W. Angew. Chem., Int. Ed. 1964, 3, 730.
(15) Black, J. D.; Boylan, M. J.; Braterman, P. S. J. Chem. Soc., Dalton
Trans. 1981, 673, 677.
(16) Wang, W. P.; Parr, R. G.; Murphy, D. R.; Henderson, G. A. Chem.
Phys. Lett. 1976, 43, 409–412.
(17) Wang, W. H.; Jin, P.; Liu, Y. W.; She, Y. B.; Fu, K. J. Chin. Phys.
Lett. 1991, 8, 491–494.
(18) Wang, W. H.; Jin, P.; Liu, Y. W.; She, Y. B.; Fu, K. J. J. Phys.
Chem. 1992, 96, 1278–1283.
(19) Hill, R. H.; Wrighton, M. S. Organometallics 1987, 6, 632–638.
(20) To, T. T.; Heilweil, E. J.; Burkey, T. J. J. Phys. Chem. A 2006,
110, 10669–10673.
(21) Breheny, C. J.; Kelly, J. M.; Long, C.; O’Keefe, S.; Pryce, M. T.;
Russell, G.; Walsh, M. M. Organometallics 1998, 17, 3690–3695.
(22) Gilbert, A.; Kelly, J. M.; Budzwatt, M.; Koerner von Gustorf, E.
Z. Naturforsch. 1976, 31 (b), 1091.
(23) Brennan, P.; George, M. W.; Jina, O. S.; Long, C.; McKenna, J.;
Pryce, M. T.; Sun, X.-Z.; Khuong Q. Vuong, K. Q. Organometallics 2008,
27, 3671–3680.
(24) Breheny, C. J.; Draper, S. M.; Grevels, F. W.; Klotzbucher, W. E.;
Long, C.; Pryce, M. T.; Russell, G. Organometallics 1996, 15, 3679–3687.
(25) Draper, S. M.; Byrne, J. J.; Breheny, C. J.; Long, C.; Low, J. N.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1994, 50, 1669–1671.
(26) Pollak, C.; Rosa, A.; Baerends, E. J. J. Am. Chem. Soc. 1997, 119,
7324–7329.
Results
The UV/vis and IR spectra of (η6-C6H6)Cr(CO)3 are presented
in Figure 1, and Table 1 contains spectroscopic information for
the complexes in this study. The time-resolved and matrix
isolation spectra are presented in the form of difference
absorbance in which the positive (upward) peaks represent the
appearance of new species, while negative (downward bleaches)
peaks represent photoinduced depletion.
Matrix Photochemistry of (η6-C6H6)Cr(CO)3. (η6-C6H6)-
Cr(CO)3 was deposited in a low-temperature (12 K) frozen gas
(27) Rosa, A.; Baerends, E. J.; van Gisbergen, S. J. A.; van Lenthe, E.;
Groeneveld, J. A.; Snijders, J. G. J. Am. Chem. Soc. 1999, 121, 10356–
10365.
(28) Vlcek, A. Coord. Chem. ReV. 2002, 230, 225–242.
(29) Gabrielsson, A.; Towrie, M.; Zalis, S.; Vlcek, A. Inorg. Chem. 2008,
47, 4236–4242.

Photochemistry of (η6-C6H6)M(CO)3
Organometallics, Vol. 28, No. 5, 2009 1463
Table 1. Observed νCO Peak Positions for Complexes in This Study
complex
state
wavenumber (cm-1
)
medium
(η6-C6H6)Cr(CO)3
(η6-C6H6)Mo(CO)3
Cr(CO)6
GSa
1983
1987
1984
1966
1966
1996
1930
1933
1916
1916
n-heptane
n-heptane
CH4 matrix
n-heptane
n-heptane
n-heptane
n-heptane
n-heptane
GSa
GSa
MACT1
MACT1
MACT2
b
b
c
(η6-C6H6)Cr(CO)3
(η6-C6H6)Mo(CO)3
(η6-C6H6)Mo(CO)3
1888
d
1894
1870
1876
(η6-C6H6)Cr(CO)2(n-heptane) GSa
(η6-C6H6)Mo(CO)2(n-heptane) GSa
a GS ) ground state. b MACT1 ) metal-to-ligand charge transfer
type 1. c MACT2 ) metal-to-ligand charge transfer type 2 (see text for
d
details). Band obscured by the MACT2 band of (η6-C6H6)Mo(CO)3.
Scheme 1. Proposed Photochemical Behavior of (η6-C6H6)-
M(CO)3 (M ) Cr or Mo) in n-Heptane Solution at Room
Temperaturea
Figure 2. (a) Changes in IR absorption observed following
irradiation of a 5% CO in CH4 matrix containing (η6-C6H6)Cr(CO)3
at 12 K with λexc. > 400 nm for 330 min showing the possible
formation of a rotamer of the parent (*), Cr(CO)6 (∧), and weak
peaks of the CO-loss species (η6-C6H6)Cr(CO)2 (#). (b) Changes
in IR absorption observed following subsequent photolysis (λexc.
> 300 nm for 125 min) of the same matrix previously irradiated
as in (a) showing the formation of (η6-C6H6)Cr(CO)2 (#) and
Cr(CO)6 (∧). Changes in free concentration by reaction to form
Cr(CO)6 result in the spectral features at approximately 2143 cm-1
.
Picosecond Time-Resolved Infrared (TRIR) Studies. TRIR
studies were undertaken on (η6-C6H6)M(CO)3 (M ) Cr or Mo)
in CO-saturated n-heptane solution at room temperature (298
K).
a
(i) λexc. ) 400 nm, M ) Mo; (ii) λexc. ) 400 nm, M ) Cr; λexc.
)
266 nm, M ) Mo (vibrationally excited); (iii) λexc. ) 266 nm, M ) Cr
or Mo; (iv) λexc. ) 266 nm, M ) Cr (partial).
TRIR Studies on (η6-C6H6)Cr(CO)3 (λexc. ) 400 nm).
Photolysis (λexc. ) 400 nm) of (η6-C6H6)Cr(CO)3 depleted the
νCO peaks of (η6-C6H6)Cr(CO)3 within the instrumental time
resolution used in these experiments (1 ps), Figure 3a. Two new
peaks were formed within 1 ps at 1966 and 1888 cm-1. These
bands are to the low-energy side of the parent bands, and this
might indicate a change to the coordination mode of the benzene
ligand in this species. These two peaks decay over the
subsequent 150 ps with concomitant recovery of (η6-
C6H6)Cr(CO)3 and production of (η6-C6H6)Cr(CO)2(n-heptane)
(1930 and 1870 cm-1), which can be assigned by comparison
to the previously reported nanosecond TRIR experiments.32
Approximately 30% of the parent is re-formed following this
decay based on examination of the low-energy νCO band of (η6-
C6H6)Cr(CO)3. The 30% recovery of the parent bands is
consistent with the published quantum yield for CO-loss from
(η6-arene)Cr(CO)3 of 0.72, which was obtained by averaging
the values for arene ) C6H6 or 1,3,5-(CH3)3C6H3 in isooctane
or benzene solvents.10,22 Thus, it is clear that the initially
produced species, absorbing at 1966 and 1888 cm-1, is an
intermediate in both the recovery of the parent (η6-C6H6)Cr(CO)3
complex and also the formation of (η6-C6H6)Cr(CO)2(n-heptane)
(Figure 3b). A comparison of the IR spectrum of the solution
before and after photolysis revealed no significant changes in
the νCO region.
matrix consisting of a mixture of 5% CO in CH4. The IR
spectrum of the resulting matrix showed νCO stretching peaks
at 1978 and 1906 cm-1, consistent with the parent tricarbonyl
species. The CO was added to the matrix in order to trap any
reduced hapticity intermediates that may be produced during
photolysis. Irradiation of this matrix with λexc. > 400 nm
depleted the peaks of the parent (η6-C6H6)Cr(CO)3 complex and
produced new bands at 1982 and 1911 cm-1 (Figure 2a). These
bands are close to those of the parent complex and are assigned
to Cr(CO)6 and possibly a rotamer of (η6-C6H6)Cr(CO)3.30
Evidence for the formation of Cr(CO)6 comes from an examina-
tion of the relative intensities of the parent peaks (Figure 1b).
The higher energy symmetric mode in (η6-C6H6)Cr(CO)3 is less
intense than the degenerate asymmetric stretch. However the
higher energy peak in Figure 2a (λexc. > 400 nm) is significantly
more intense than the lower energy peak, indicating the
production of Cr(CO)6 (νCO ) 1984 cm-1 Scheme 1). In addition
to the features described above, weak peaks (1922 and 1867
cm-1) characteristic of the CO-loss species (η6-C6H6)Cr(CO)2
were formed.31 Increasing the energy of the irradiation (λexc.
>
300 nm) resulted in a greater depletion of the parent absorptions
for a given irradiation time and an increase in the intensity of
the peaks assigned to the CO-loss species (η6-C6H6)Cr(CO)2
and also Cr(CO)6.
TRIR Studies on (η6-C6H6)Cr(CO)3 (λexc. ) 266 nm).
Photolysis of (η6-C6H6)Cr(CO)3 at higher energy (λexc. ) 266
nm) resulted in depletion of the νCO peaks of the parent complex.
(30) Bitterwolf, T. E.; Bays, J. T.; Scallorn, B.; Weiss, C. A.; George,
M. W.; Virrels, I. G.; Linehan, J. C.; Yonker, C. R. Eur. J. Inorg. Chem.
2001, 261, 9–2624.
(31) Rest, A. J.; Sodeau, J. R.; Taylor, D. J. J. Chem. Soc., Dalton Trans.
1978, 651, 656.
(32) Creaven, B. S.; George, M. W.; Ginzburg, A. G.; Hughes, C.; Kelly,
J. M.; Long, C.; McGrath, I. M.; Pryce, M. T. Organometallics 1993, 12,
3127–3131.

1464 Organometallics, Vol. 28, No. 5, 2009
Alamiry et al.
Figure 5. TRIR difference spectra obtained following photolysis
of (η6-C6H6)Mo(CO)3 in n-heptane (S) at room temperature (λexc.
)
1
400 nm) showing formation of two broad peaks of the MACT
excited state at 1996 and 1894 cm-1, which decay over 150 ps
accompanied by almost complete (approximately 93%) recovery
of the parent depletion (1987 and 1916 cm-1). Additional small
product peaks for (η6-C6H6)Mo(CO)2(n-heptane) at 1933 and 1876
cm-1 formed within the excitation pulse, and these peaks do not
increase over the subsequent 150 ps. The arrows indicate the
temporal behavior of the bands.
within 1 ps following 266 nm excitation. These bands increased
in intensity over the subsequent 150 ps (Figure 4). A particularly
noteworthy observation is that the parent peaks in this experi-
ment recovered by ca. 50%, which is significantly greater than
the recovery observed following excitation at 400 nm and
implies that the quantum yield for the CO-loss process is smaller
at 266 nm than at 400 nm. The observation of a reduction in
quantum yield following higher energy excitation was confirmed
by our steady-state quantum yield determinations (see below).
TRIR Studies of (η6-C6H6)Mo(CO)3 (λexc. ) 400 nm).
Photolysis (λexc. ) 400 nm) of (η6-C6H6)Mo(CO)3 depleted the
parent peaks at 1987 and 1916 cm-1. Two peaks at 1996 and
1894 cm-1 were formed within 1 ps (Figure 5). These peaks
are significantly broader than those of the parent; they narrow
and shift to slightly higher energy as they decay over 150 ps,
indicating the formation of this species in a vibrationally excited
state, which then relaxes. Concomitant with the decay of these
peaks, the parent bleach recovers by approximately 93% and
the residual bleach persists for at least 600 ps. It is important
to note that two weak features are also formed within the laser
pulse at 1933 and 1876 cm-1 corresponding to the CO-loss
species (η6-C6H6)Mo(CO)2(n-heptane). In contrast to the chro-
mium system these peaks do not increase in intensity with time.
The formation of (η6-C6H6)Mo(CO)2(n-heptane) accounts for
the persistent parent bleach and provides an estimate of 0.07
for the CO-loss quantum yield for 400 nm excitation.
Figure 3. (a) TRIR spectra obtained following pulsed photolysis
of (η6-C6H6)Cr(CO)3 in n-heptane (λexc. ) 400 nm) at room
temperature (spectra obtained at 7, 15, 20, 40, 75, and 200 ps)
showing depletion of the two peaks of (η6-C6H6)Cr(CO)3 (1983
and 1916 cm-1) and the immediate formation of two peaks at 1966
and 1888 cm-1. The peaks at 1930 and 1870 cm-1 increase in
intensity over 150 ps; the arrows indicate the temporal behavior of
the bands. (b) Plot of the time dependence of the parent peak at
1916 cm-1 (9), the peak at 1966 cm-1 (b), and the 1930 cm-1
peak (O).
TRIR Studies of (η6-C6H6)Mo(CO)3 (λexc. ) 266 nm).
Irradiation of (η6-C6H6)Mo(CO)3 at 266 nm depleted the parent
peaks and produced two very broad peaks at 1971 and 1899
cm-1 along with weak features of (η6-C6H6)Mo(CO)2(n-heptane)
(1933 and 1876 cm-1) within the instrument response time of
1 ps (Figure 6). The broad peaks at 1971 and 1899 cm-1 decay
over 150 ps with concomitant recovery of the parent bleach
(approximately 60%) and a growth of the bands of (η6-
C6H6)Mo(CO)2(n-heptane). In addition a persistent feature
appears at 1966 cm-1. The ultimate yield of (η6-C6H6)Mo(CO)2(n-
heptane) is greater in these experiments compared to those where
400 nm excitation was used.
Figure 4. TRIR spectra obtained following photolysis of (η6-
C6H6)Cr(CO)3 in CO-saturated n-heptane (λexc. ) 266 nm) (spectra
obtained at 3, 13, 30, 44, and 160 ps after excitation). The arrows
describe the temporal behavior of the various bands. Broad peaks
of the excited state at 1966 and 1888 cm-1 indicate that this species
is vibrationally hot when formed. The peaks narrow as they reduce
in intensity. Peaks at 1930 and 1870 cm-1 show the formation of
(η6-C6H6)Cr(CO)2(n-heptane) within the response time of the
instrument and its subsequent formation over 150 ps.
The bands at 1966 and 1888 cm-1 were also observed in the
TRIR spectrum obtained 1 ps following excitation. These two
features were significantly broader than those produced follow-
ing 400 nm excitation (cf. Figure 3a and Figure 4). Another
important difference between TRIR experiments using different
excitation wavelengths is that the CO-loss species, (η6-
C6H6)Cr(CO)2(n-heptane) (1930 and 1870 cm-1), was formed
Time-Dependent Density Functional Calculations. Density
functional theory (DFT) calculations were undertaken on (η6-
C6H6)Cr(CO)3 to provide information on the electronic and
molecular structures for use in subsequent time-dependent

Photochemistry of (η6-C6H6)M(CO)3
Organometallics, Vol. 28, No. 5, 2009 1465
using two excitation wavelengths of 366 and 265 nm, and
cyclooctene was used as a trapping ligand for the photoproduced
(η6-C6H6)Cr(CO)2 species (see Experimental Section and Sup-
porting Information for more details). A 200 W Hg-Xe arc
lamp was used for these studies, and the required Hg spectral
line was selected using interference filters. These excitation
wavelengths were selected because they are those close to the
excitation wavelengths used in the TRIR experiments with
reasonable intensities. The quantum yields measured showed
wavelength dependence with values of 0.71 for 366 nm and
0.56 for 265 nm excitation, respectively.
Discussion
It is clear from Figure 1 that irradiation of (η6-C6H6)Cr(CO)3
with λexc. ) 400 nm represents photolysis at the low-energy
region of the absorption profile, which will populate its low-
energy excited states. The results of the TDDFT calculations
suggest that the lowest energy transition (377 nm) has a metal
to arene charge transfer character (Table 2), the charge drifting
from the Cr(CO)3 fragment to the benzene ligand (labeled
MACT). However there are other transitions between 344 and
359 nm, and these also have MACT character. To distinguish
between these two sets of MACT transitions, we label the
transition at 377 nm MACT1 and the higher energy transitions
MACT2 in Table 2. Transitions that contain significant ligand
field along with metal to CO charge transfer character are
predicted at approximately 313 nm (labeled MCCT/LF in Table
2). Finally a high-energy MACT transition was calculated at
297 nm, and this is labeled MACT3.
Figure 6. TRIR spectra obtained following photolysis of (η6-
C6H6)Mo(CO)3 at 266 nm. Spectra were recorded at 7,16,19, 30,
44, and 160 ps after the excitation pulse. Arrows indicate the
temporal behavior of the bands. Very broad features of an excited
state are centered at 1971 and 1899 cm-1. Peaks corresponding to
(η6-C6H6)Mo(CO)2(n-heptane) (1933 and 1876 cm-1) form within
the laser pulse and increase in intensity over the subsequent 150
ps. A peak at 1966 cm-1 also forms on this time scale. Examination
of the high-energy band of (η6-C6H6)Mo(CO)3 at 1987 cm-1
indicates a 60% recovery within 600 ps.
Table 2. Vertical Excitation Energy (eV) and Equivalent Photon
Wavelength (nm), Oscillator Strengths, and Prevailing Character for
the Accessible Excited States of (η6-C6H6)Cr(CO)3 as Calculated by
Time-Dependent Density Functional Theory (only transitions with
oscillator strengths >1 × 10-4 are included)
oscillator
energy eV (nm)
strength × 103
prevailing charactera
Under λexc. > 400 nm irradiation of (η6-C6H6)Cr(CO)3, the
low-temperature matrix experiments provide evidence for the
formation of a rotamer and Cr(CO)6 by reaction with CO present
in the matrix. The formation of Cr(CO)6 suggests that CO is
trapping either an arene-loss or a reduced hapticity intermediate.
Decreasing the irradiation wavelength to λexc. > 300 nm
increased the yield of the CO-loss species. This result appears
not to be consistent with the published room-temperature
quantum yield measurements, which indicate that the efficiency
of photochemical loss of CO from (η6-C6H6)Cr(CO)3 is inde-
pendent of excitation wavelength.11 The quantum yields of CO-
loss from (η6-C6H6)Cr(CO)3 were measured for excitation
wavelengths of 366 and 265 nm, and values of 0.71 and 0.56,
respectively, were obtained, which indicate that there is in fact
a significant wavelength dependence for CO-loss. We interpret
that these results mean that there is a difference between the
behavior of (η6-C6H6)Cr(CO)3 in a low-temperature matrix and
its behavior in room-temperature solution.
Time-resolved infrared spectroscopy following irradiation at
400 nm shows that there is a precursor to CO-loss characterized
by two νCO bands at 1966 and 1888 cm-1. This species cannot
be a dicarbonyl since it decays to re-form the parent within 150
ps, and it could not react with CO on this time scale even at
the diffusion-controlled limit. This species is therefore assigned
to be an excited state. The rate of formation of the CO-loss
species, (η6-C6H6)Cr(CO)2 (50-100 ps), is very slow and
indicates that there is a thermal barrier for this process. It should
be noted that the kinetic data for the decay of the excited state
relates to the competing decay routes for the excited state and
not the reaction of (η6-benzene)Cr(CO)2 with the solvent. It is
likely that the reaction of (η6-benzene)Cr(CO)2 with n-heptane
will be very fast (<1 ps).33 To the best of our knowledge, there
is only one other example of such a slow CO-loss from an
b
3.2826 (377.70)
3.2827 (377.69)
3.4569 (358.66)
3.4570 (358.64)
3.5966 (344.73)
3.5967 (344.71)
3.9892 (310.80)
3.9892 (310.79)
4.1654 (297.65)
1.4
1.4
MACT1
MACT1
1.6
1.6
1.3
1.3
1.5
1.5
189.3
MACT2
MACT2
MACT2
MACT2
MCCTc/LFd
MCCT/LF
b
MACT3
a Numeric subscripts are used to distinguish between different
transitions within a given manifold. b Metal to arene charge transfer.
c Metal to carbonyl charge transfer. d Ligand field transitions.
density functional theory calculations (TDDFT). A starting
geometry was obtained from molecular mechanics calculations
on (η6-C6H6)Cr(CO)3. This geometry was then optimized using
tight convergence targets at the B3LYP/LANL2DZ+P model
chemistry. The final structural parameters were consistent with
published values (see Supporting Information). The 20 lowest
excited states were characterized by TDDFT. Table 2 contains
information on the optically accessible excited states along with
their prevailing character defined in terms of the general electron
drift, either toward the metal-arene (MACT) fragment or toward
the metal-CO fragment (MCCT) (see Supporting Information
for more details). It should be noted that all of these states derive
from multiple transitions, and the MCCT transitions also contain
significant ligand field (LF) character. The manifold of MCCT/
LF transitions lies between two MACT manifolds.
Quantum Yield Measurement of Photoinduced CO-Loss
from (η6-C6H6)Cr(CO)3. The quantum yield of the photoinduced
expulsion of CO from (η6-C6H6)Cr(CO)3 was measured in
cyclohexane solution at 298 K. These experiments were
undertaken to explain the apparent difference in the recovery
of (η6-C6H6)Cr(CO)3 following excitation at 400 and 266 nm
in the TRIR experiments. Ferrioxalate actinometry was used to
measure the incident light intensity. Measurements were made
(33) Simon, J. D.; Xie, X. L. J. Phys. Chem. 1987, 91, 5538–5540.

1466 Organometallics, Vol. 28, No. 5, 2009
Alamiry et al.
excited state, which was published during the preparation of
this article.29 Vlcek and co-workers demonstrated that slow CO-
loss in [RuCl2(CO)2(iPr-dab)] arose because of an avoided
crossing of a thermally relaxed 3MLCT/XLCT excited state. Our
results on (η6-C6H6)Cr(CO)3 for 400 nm excitation are inter-
preted in terms of formation of an excited state that either decays
to the parent or via an avoided crossing with a higher energy
excited-state ultimately leads to CO-loss (reaction (i) in Scheme
1). The TDDFT calculations allow tentative assignments of these
lower and higher excited states to MACT1 and MCCT/LF,
respectively. The νCO bands of the excited state are shifted to
lower wavenumbers than the parent bands. A transfer of electron
density from the metal to the arene ligand would be expected
to cause a shift of the νCO bands to higher wavenumbers.
However either a significant change to the coordination mode
of the arene ligand or a reduction of the arene ligand’s
π-acceptor capacity could explain the observed shifts.
There are two important differences in the behavior of (η6-
C6H6)Cr(CO)3 following 266 nm compared to 400 nm excitation.
First the CO-loss species (η6-C6H6)Cr(CO)2 is formed via two
processes: the CO loss species is observed immediately (1 ps)
and then grows as the excited-state bands at 1966 and 1888
cm-1 decay. Second the recovery of the parent is higher and
the quantum yield of CO-loss is lower following 266 nm
compared to 400 nm irradiation. These results support our model
of a manifold of MACT and MCCT/LF excited states governing
the photochemistry of (η6-C6H6)Cr(CO)3 as 266 nm directly
populates the MCCT/LF state (reaction (iii) in Scheme 1),
leading to rapid CO-loss and formation of the MACT1 state.
The MACT1 state then decays via an avoided crossing to the
MCCT/LF state, resulting in two pathways for the formation
of the CO-loss species. The TDDFT calculations also predict a
higher energy MACT3 excited state separate from the low-
energy MACT manifold. The quantum yield results and the
parent re-formation data obtained in the TRIR experiments show
the yield of CO-loss decreases with increasing photolysis energy.
We interpret this to indicate that three excited states are required
to account for the observed photochemical behavior of (η6-
C6H6)Cr(CO)3. The partial population of a high-energy nonre-
active MACT3 excited state (reaction (iv) in Scheme 1) at the
expense of the reactive MCCT/LF state (reaction (iii) in Scheme
1) would reduce the quantum yield of the CO-loss process.
We now consider the TRIR data obtained for (η6-
C6H6)Mo(CO)3 using λexc.) 400 nm. In these experiments bands
at 1996 and 1894 cm-1 are formed within 1 ps and are assigned
to an excited state (reaction (i) in Scheme 1). Additional very
small features of (η6-C6H6)Mo(CO)2(n-heptane) are also present
in the 1 ps TRIR spectrum. The excited state decays over 150
ps, regenerating (η6-C6H6)Mo(CO)3 with no further production
of (η6-C6H6)Mo(CO)2(n-heptane), clearly demonstrating that the
CO-loss product is not formed from the excited state character-
ized by νCO bands at 1996 and 1894 cm-1 and must be produced
from a different excited state.
The measured quantum yield (ΦCO) for loss of CO from (η6-
1,3,5-(CH3)3C6H3)Mo(CO)3 is strongly wavelength dependent
and increases with reducing excitation wavelength (0.58, 0.12,
and 0.05 for λexc.) 266, 313, and 334 nm respectively).21 The
quantum yield measurements were made in cyclohexane solution
rather than n-heptane to allow comparisons with other quantum
yield data obtained in our laboratories. It is unlikely that there
will be any significant difference between quantum yield
measurements in these two solvents. This confirms the presence
of at least two, energetically close, excited states, the higher
energy state being responsible for CO-loss. The TRIR measure-
ments using λexc. ) 266 nm on (η6-C6H6)Mo(CO)3 showed a
recovery of some 75% in the parent bleach over 800 ps. This
would indicate an approximate quantum yield of 0.25 for the
CO-loss process. Similar measurements using λexc. ) 400 nm
yielded a value of 0.07. The behavior of the Mo system contrasts
with that of the Cr, where the quantum yield of the CO-loss
process decreases with reducing excitation wavelength.
The TRIR results obtained for (η6-C6H6)Mo(CO)3 should also
be contrasted with those of the chromium system. Slow CO-
loss is observed following both 266 and 400 nm excitation of
(η6-C6H6)Cr(CO)3, whereas slow CO-loss is only observed
following 266 nm irradiation of the (η6-C6H6)Mo(CO)3. Irradia-
tion of (η6-C6H6)Mo(CO)3 at 400 nm produced excited-state
bands that are different from those obtained for (η6-
C6H6)Cr(CO)3, indicating a different excited-state structure. This
is particularly noticeable for the high-frequency excited-state
band that occurs to lower wavenumber relative to the parent in
(η6-C6H6)Cr(CO)3 but to higher energy for (η6-C6H6)Mo(CO)3.
These different excited states have a profound effect on the
subsequent photoreactivity. The (η6-C6H6)Mo(CO)3 excited state
(MACT2) is nonreactive and relaxes to re-form the parent. In
the 400 nm experiment there is some evidence for a weak band
at 1966 cm-1, and this could be because of the additional
formation of a MACT1 excited state similar to that observed in
the chromium system. The second band of the MACT1 state
would be masked by the broad feature at 1894 cm-1. The signal-
to-noise ratio in these experiments does not permit further kinetic
evaluation of the fate of these bands. These results indicate that
the photochemistry of (η6-C6H6)M(CO)3 is complex and three
different excited states may be involved.
The main difference in the TRIR results at 266 nm for (η6-
C6H6)Mo(CO)3 are the production of two broad features at lower
wavenumber than the parent bands. These bands decay to form
(η6-C6H6)Mo(CO)2, and at later times there is evidence for bands
at 1966 and ca. 1910 cm-1, the latter band being obscured by
the parent. These results are interpreted in terms of changes in
the relative energies of excited states in the MACT1 and MCCT/
LF manifolds and a larger thermal barrier at the avoided crossing
for the molybdenum complex compared to the chromium. More
efficient CO-loss following 266 nm excitation can occur either
by direct population of the MCCT/LF state (reaction (iii) in
Scheme 1) or by a higher proportion of the reactive MACT1
state possessing sufficient thermal energy to surmount the barrier
at the avoided crossing; that is, 266 nm irradiation produces
the MACT1 state in a vibrationally excited state, resulting in
CO-loss (reaction (ii) in Scheme 1).
Concluding Comments
These experiments demonstrate that a variety of competing
photophysical processes must be considered to explain the
photochemistry of these simple “half-sandwich” complexes.
Nonradiative relaxation processes, vibrational cooling, and
avoided crossing to unbound potential energy surfaces are all
involved in explaining the remarkable diversity of the photo-
chemical behavior for these systems. In Figure 7 a schematic
representation of the ground- and excited-state potential energy
surfaces are presented. For the chromium system CO-loss can
occur following excitation at both 400 and 266 nm via an
avoided crossing of MACT and MCCT/LF states. This leads
to an unusually slow CO-loss. Rapid CO-loss is observed
following 266 nm excitation because of direct population of
the MCCT/LF state. The quantum yield for CO-loss in the
chromium system decreases with increasing excitation energy
because of the competing population of a high-energy unreactive

Photochemistry of (η6-C6H6)M(CO)3
Organometallics, Vol. 28, No. 5, 2009 1467
the latter for monochromatic irradiations. Suitable interference filters
were used to select the required Hg spectral line for monochromatic
irradiations.
The initial experiments were performed on the TRIR apparatus
based at the Rutherford Appleton Laboratory (RAL), which has
been described in detail elsewhere.36,37 The later experiments and
the ones reported here were performed using TRIR apparatus
located at Nottingham University, which is based on the PIRATE
apparatus. A commercial Ti:sapphire oscillator (MaiTai)/regenera-
tive amplifier system (Spitfire Pro) (Spectra Physics) is used to
generate 800 nm laser pulses with an energy of 2.3 mJ at a repetition
rate of 1 kHz. This output is divided into two parts with
approximately the same energy pumping either a TP-1 harmonic
generator (TimePlate Tripler, Minioptic Technology, Inc.) to
generate UV pulses (400 or 267 nm) or a TOPAS-C OPA (light
conversion) with a DFG (difference frequency generator) unit to
produce a tunable mid-IR pulse with a spectral bandwidth of 180
cm-1 and the pulse energy of ca. 2 µJ at 2000 cm-1. The IR pulse
passes through a Ge beam splitter, and half of the IR pulse is
reflected onto a single-element MCT detector (Kolmar Technology)
to serve as a reference; the other half serves as the probe beam,
which is focused and overlaps with the pump beam at the sample
position. The UV-vis pump pulse is optically delayed (up to 3 ns)
by a translation stage (LMA Actuator, Aerotech) and focused onto
the sample with a quartz lens. The polarization of the pump pulse
is set at the magic angle (54.7°) relative to the probe pulse to recover
the isotropic absorption spectrum. For a measurement with a longer
time delay, a Q-switched Nd:YVO laser (ACE-25QSPXHP/MOPA,
Advanced Optical Technology, UK) is employed as a pump source,
which is synchronized to the Spitfire Pro amplifier. The delay
between pump and probe pulses can be controlled with a pulse
generator (DG535, Stanford Research System) from 0.5 ns to 100
µs. The broadband transmitted probe pulse is detected with a
HgCdTe array detector (Infrared Associates), which consists of 128
elements (1 mm high, 0.25 mm wide). The array detector is mounted
in the focal plane of a 250 mm IR monochromator (DK240, Spectra
Product) with 150 and 300 L/mm gratings, resulting in spectral
Figure 7. Potential energy cross sections for the ground and
optically accessible excited states of (η6-C6H6)M(CO)3 ((a) M )
Cr; (b) M ) Mo). The vertical solid arrows indicate the excitation
processes. The energy barrier at the avoided crossing of MACT1
and MCCT/LF states results in a thermal barrier (∆E), which is
small for M ) Cr but larger for M ) Mo.
MACT state. For the molydenum system CO-loss is a minor
process for 400 nm excitation, and an unreactive MACT state
is evident from the TRIR spectra. A higher quantum yield for
CO-loss is observed following 266 nm excitation through both
direct population of the MCCT/LF state and production of a
vibrationally excited reactive MACT state. This results in the
quantum yield for CO-loss increasing with increasing excitation
energy. No information on the spin multiplicity of the excited
states was obtained during the course of this work however. It
should be noted that it has been proposed that the photoinduced
CO-loss from Cr(CO)6 occurs via a singlet excited state.34 There
is clearly more to learn regarding the photophysics and
photochemistry of these systems including the multiplicity of
the excited state. The combination of DFT and time-resolved
spectroscopy is likely to be useful in this regard.
resolutions of ca. 4 and ca. 2 cm-1, respectively, at 2000 cm-1
.
The signals from the array detector elements and the single-element
detector were amplified with a 144-channel amplifier and digitized
by a 16-bit analog-to-digital converter (IR-0144, Infrared Systems
Development Corporation). The pump-induced change in the
absorbance (∆A) is determined by chopping the pump pulse at half
the repetition frequency of the laser and calculating the ratio
between the pump-on and pump-off transmittance. Chopping the
excitation light pulse greatly reduces long-term instrumental drift.
The signal from the single-element detector serves as reference to
normalize the shot to shot fluctuation. The pump beam size (∼500
Experimental Section
The matrix isolation apparatus has been described in detail
elsewhere.35 It consists of a HC2 closed helium displex refrigerator
supplied by APD Cryogenics in thermal contact with a CaF2 optical
flat, contained within a stainless steel shroud equipped with CaF2
windows. The temperature of the matrix was maintained by a
heating current controlled by a Lakeshore 330 analog temperature
control unit The optical flat was capable of rotation through 90°,
allowing it to alternatively face the sample vapor/isolating gas
streams and the monitoring spectrophotometers. The shroud vacuum
was achieved by a combination of dual-stage rotary pumps backing
an oil diffusion pump fitted with a liquid N2 coldfinger trap.
Typically a shroud vacuum of 2 × 10-6 Torr was achieved at a
sample-plate temperature of 20 K. Perkin-Elmer Spectrum One
FTIR (typically 16 scans at 4 cm-1 resolution) and Lambda EZ201
UV/visible spectrometers were used to monitor the spectral changes
during the experiments. The isolating gas was admitted to the cold
sample-plate via a needle value system connected to a stainless-
steel gas-mixing line. The isolating gases were obtained from
Cryogenics. Oriel Instruments supplied the light sources, which were
either a 200 W Xe or Xe-Hg lamp, the former for broadband and
(36) Towrie, M.; Parker, A. W.; Shaikh, W.; Matousek, P. Meas. Sci.
Technol. 1998, 9, 816–823.
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Barton, R.; Bailey, P. D.; Subramaniam, N.; Kwok, W. M.; Ma, C. S.;
Phillips, D.; Parker, A. W.; George, M. W. Appl. Spectrosc. 2003, 57, 367–
380.
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M. A.; Cheeseman, J. R.; Montgomery, J. J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C. J.; Ochterski, W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
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1468 Organometallics, Vol. 28, No. 5, 2009
Alamiry et al.
µm diameter) is larger than the probe spot (∼200 µm diameter) to
ensure spatially uniform photoexcitation over the area of the probe
pulse. A Harrick flowing solution cell with 2-mm-thick CaF2
windows (path length 0.5 mm) is mounted on a motorized cell
mount, which moves the cell rapidly in x and y dimensions
throughout the experiment. Consequently each laser pulse il-
luminates a different volume of the sample, reducing overheating
and degradation of the sample solution and cell windows.
compatibility with the published data.45,46 Time-dependent DFT
calculations used the structure obtained from an optimization at
the B3LYP/LANL2DZ model chemistry. The TDDFT calcula-
tions examined only the 20 lowest energy singlet states. The
GaussSum package47 was used to obtain the contribution to
specific Kohn-Sham orbitals made by specific molecule frag-
ments, namely, the three CO ligands, the metal atom, and the
benzene ligand (see Supporting Information for details).
All calculations were carried out using the Gaussian 03
program revision C.0238 running on a dual Xeon processor
workstation (3.6 GHz) under Windows XP. All calculations used
the Becke three-parameter hybrid functionals,39 using the
correlation function of Lee, Yang, and Parr, which includes both
local and nonlocal terms.40 The LANL2DZ and LANL2DZ+P
basis set was used for all calculations, which employs the
Dunning/Huzinaga valence double-ꢀ functions for the first-row
elements,41 and the Los Alamos effective core potentials plus
double-ꢀ functions on elements from Na to Bi.42-44 An
additional polarization function (+P) was added to the chromium
atom for calculations on the reduced hapticity species to ensure
Acknowledgment. We thank the EPSRC for funding. The
authors acknowledge the many helpful discussions with Dr.
D. Brougham (DCU) on theoretical aspects of this work. The
authors thank the EU Framework 6 Programme for support
in accessing the PIRATE system at RAL. Louth County
Council is thanked for support for N.M.B. M.W.G. gratefully
acknowledges receipt of a Royal Society Wolfson Merit
Award.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
OM800925S
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