Investigating the reactivity of the (η6-C6H 5R)Cr(CO)2-(η2-C6H5R) [R = H, CH3, CF3] bond: A laser flash photolysis study with infrared detection

Article abstract of DOI:10.1021/om050604h

The (η⁶-C₆H₅R)Cr(CO) ₂-(η²-C₆H₅R) complexes (R = H, CH₃, CF₃) are generated upon photolysis of (η⁶-C₆H₅R)Cr(CO)₃ in the appropriate arene. The energetics and mechanism of the displacement of the η²-coordinated arene from the metal center by piperidine are studied using the technique of laser flash photolysis. The substitution reaction is tentatively assigned as proceeding through an Id mechanism, and the activation enthalpy of 11.5 ± 0.9 kcal/mol provides a lower limit for the strength of the (η⁶-C₆H₆)Cr(CO) ₂-(η²-C₆H₆) bond. The substitution rate decreases as the metal center becomes more electron poor or the arene electron rich, suggesting that L→M σ donation is the primary bonding interaction between the (η⁶-C₆H ₅R)Cr(CO)₂ fragment and the arene ligand. This reactivity is different from that of the Cr - (η²-alkene) bond, where it was found that increasing the electron density on the metal center decreased the rate of substitution of cyclooctene from the (η⁶-C ₆H₅R)Cr(CO)₂-(η²-cyclooctene) complex by pyridine.

Full text of DOI:10.1021/om050604h

Organometallics 2005, 24, 5919-5924
5919
Investigating the Reactivity of the
(η6-C6H5R)Cr(CO)2-(η2-C6H5R) [R ) H, CH3, CF3] Bond: A
Laser Flash Photolysis Study with Infrared Detection
Ashfaq A. Bengali* and Amy R. Grunbeck
Department of Chemistry, Dickinson College, Carlisle, Pennsylvania 17013
Received July 19, 2005
The (η6-C6H5R)Cr(CO)2-(η2-C6H5R) complexes (R ) H, CH3, CF3) are generated upon
photolysis of (η6-C6H5R)Cr(CO)3 in the appropriate arene. The energetics and mechanism of
the displacement of the η2-coordinated arene from the metal center by piperidine are studied
using the technique of laser flash photolysis. The substitution reaction is tentatively assigned
as proceeding through an Id mechanism, and the activation enthalpy of 11.5 ( 0.9 kcal/mol
provides a lower limit for the strength of the (η6-C6H6)Cr(CO)2-(η2-C6H6) bond. The
substitution rate decreases as the metal center becomes more electron poor or the arene
electron rich, suggesting that LfM σ donation is the primary bonding interaction between
the (η6-C6H5R)Cr(CO)2 fragment and the arene ligand. This reactivity is different from that
of the Cr-(η2-alkene) bond, where it was found that increasing the electron density on the
metal center decreased the rate of substitution of cyclooctene from the (η6-C6H5R)Cr(CO)2-
(η2-cyclooctene) complex by pyridine.
Introduction
hand, in the case of electron-deficient metal centers such
as Cr(CO)5, the arene ligand is bound to the metal by a
primarily σ interaction, resulting in a relatively weak
metal-(η2-arene) bond.9-12 Given the interest and rel-
evance of dihapto-coordinated arene complexes in a
number of organic transformations and catalytic pro-
cesses, it is important to investigate the energetic and
mechanistic profile of the arene displacement reaction.
A better understanding of this interaction can be
achieved by studying the effect of varying the electronic
properties of the metal and the arene ring upon the
reactivity of the metal-(η2-arene) bond.
Examples of organometallic complexes with weak
metal-(η2-arene) bonds are relatively few and include
the Cr(CO)5(η2-benzene),9-12 CpMn(CO)2(η2-toluene),13
and Cp*Re(CO)2(η2-benzene) complexes.14,15 Kinetic in-
vestigations yielded a range of 10 to 20 kcal/mol for the
strength of the metal-(η2-arene) bond in these systems.
Although to our knowledge the (η6-C6H6)Cr(CO)2(η2-
arene) complex has not been observed before, it has been
invoked as a possible intermediate in the arene ex-
change reactions of (η6-arene)Cr(CO)3 complexes.16 By
analogy with the weakly bound metal-(η2-arene) com-
plexes mentioned above, which were formed by photo-
lyzing the parent carbonyl in the presence of arene, we
Arene rings η2-coordinated to transition metal centers
are an important class of compounds. Such complexes
have been implicated in a variety of catalytic processes
and are often invoked as intermediates in the activation
of arene C-H bonds, an important reaction in organic
synthesis and industrial processes.1-3 Dihapto coordina-
tion of an arene ring to an electron-rich metal center
has a profound effect on the reactivity of the ring,
“leading to otherwise inaccessible transformations on
the coordinated organic ligand by disrupting the π
system of the bound aromatic molecule”.4
If dihapto coordination is understood as an interaction
between a fixed double bond on the arene ring and the
metal center, it can be described in terms of the
traditional Dewar-Chatt-Duncanson bonding model of
olefins.5,6 Thus, two factors contribute to the strength
of the overall interaction, σ donation from the π cloud
of the aromatic ligand to a metal-centered orbital (LfM
σ donation) and back-donation from the filled metal dπ
to the empty π* orbitals of the organic ligand. If the
metal is a good π base, increased back-bonding results
in a strong metal-(η2-arene) bond and a disruption in
the aromaticity of the arene ligand.4,7,8 On the other
* To whom correspondence should be addressed. E-mail: bengali@
dickinson.edu.
(1) Parshall G. W.; Ittel, S. D. In Homogeneous Catalysis: The
Applications and Chemistry of Catalysis by Soluble Transition Metal
Complexes, 2nd ed.; John Wiley and Sons: New York, 1992; Chapter
7.
(2) Muetterties, E. L.; Bleeke, J. R. Acc. Chem. Res. 1979, 12, 325.
(3) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
(4) Harman, W. D. Chem. Rev. 1997, 97, 1953.
(9) Zhang, S.; Dobson, G. R. Zang, V.; Bajaj, H. C.; van Eldik, R.
Inorg. Chem. 1990, 29, 3477.
(10) Wells, J. R.; House, P. G.; Weitz, E. J. Phys. Chem. 1994, 98,
8343.
(11) Wang, W.; Zheng, Y.; Lin, J.; She, Y.; Fu, K.-J. J. Phys. Chem.
1993, 97, 11921.
(12) Bengali, A. A.; Stumbaugh, T. F. J. Chem. Soc., Dalton Trans.
2003, 354.
(13) Bengali, A. A. Organometallics 2000, 19, 4000.
(14) Bengali, A. A.; Leicht, A. Organometallics 2001, 20, 1345.
(15) Heijden, H.; Orpen, A. G.; Pasman, P. J. Chem. Soc., Chem.
Commun. 1985, 1576.
(16) Traylor, T. G.; Stewart, K. J.; Goldberg, M. J. J. Am. Chem.
Soc. 1984, 106, 4445.
(5) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C79.
(6) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939.
(7) Meiere, S. H. Brooks, B. C.; Gunnoe, T. B.; Sabat, M.; Harman,
W. D. Organometallics 2001, 20, 1038.
(8) Keane, J. M.; Harman, W. D. Organometallics 2005, 24, 1786.
10.1021/om050604h CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/18/2005

5920 Organometallics, Vol. 24, No. 24, 2005
Bengali and Grunbeck
thought it possible to generate the (η6-C6H5R)Cr(CO)2-
(η2-arene) complexes in a similar manner. The proposed
complexes are particularly attractive candidates for
probing the metal-(η2-arene) interaction because a wide
variety of monosubstituted (η6-C6H5R)Cr(CO)3 precursor
complexes can be synthesized, making it possible to
systematically alter the steric and electronic environ-
ment on the metal center.17 In the present study the
technique of laser flash photolysis with infrared detec-
tion is utilized to study the mechanism and energetics
of η2-arene displacement from the (η6-C6H6)Cr(CO)2(η2-
arene) complex (eq 1). To obtain a better understanding
of the metal-(η2-arene) bond in this system, the effect
of changing the electron density on both the metal
center and the arene upon the displacement rate is also
investigated.
Figure 1. Time-resolved IR spectrum obtained 1 µs after
355 nm photolysis of (η6-C6H6)Cr(CO)3 in benzene solvent
at 295 K. The absorption at 1856 cm-1 is assigned to the
CO stretch of the (η6-C6H6)Cr(CO)2(η2-benzene) complex,
while the negative absorption observed at 1896 cm-1 is due
to depletion of the parent tricarbonyl upon photolysis.
Table 1. CO Stretching Frequencies for the
Complexes Investigated in This Study (* ) 4 cm-1
,
all other frequencies are (1 cm-1
complex
νCO (cm-1
)
)
(η6-C6H6)Cr(CO)3
1972, 1896
1966, 1892
1988, 1919
1874, 1815
1880, 1825
1856*
(η6-C6H5CH3)Cr(CO)3
Experimental Section
(η6-C6H5CF3)Cr(CO)3
(η6-C6H6)Cr(CO)2(piperidine)
(η6-C6H6)Cr(CO)2(pyridine)
(η6-C6H6)Cr(CO)2(η2-C6H6)
(η6-C6H6)Cr(CO)2(η2-C6H5CH3)
(η6-C6H6)Cr(CO)2(η2-C6H5CF3)
(η6-C6H5CH3)Cr(CO)2(η2-C6H6)
(η6-C6H5CF3)Cr(CO)2(η2-C6H6)
General Procedures. The experimental apparatus used
in this investigation has been described in detail elsewhere.18
Briefly, a Nd:YAG laser (355 nm, 10 ns pulse width, 1 Hz
repetition rate, 50 mJ/pulse) was used to photolyze a ∼0.6 mM
solution of (η6-arene)Cr(CO)3 in the appropriate arene contain-
ing the required concentration of ligand (either piperidine or
pyridine). The subsequent changes in the absorption of the
transient complexes generated upon photoexcitation were
monitored with IR light from a Pb-salt diode laser (1930-1850
cm-1). The solution was flowed through a temperature-
controlled ((1 °C) IR cell (CaF2 windows, 0.75 mm path length)
to ensure that each laser shot photolyzed a fresh sample. The
signal was averaged over 10 laser shots. All runs were
conducted under pseudo-first-order conditions. The observed
rate constants, kobs, were determined by fitting either the decay
of the reactant (η6-C6H5R)Cr(CO)2(η2-arene) complex or the
growth of the product (η6-C6H5R)Cr(CO)2L complex to a first-
order exponential function. Time-resolved spectra were built
up point by point by recording absorption traces as a function
of the monitoring frequency. The kinetics of the longer time
scale reactions were monitored with either traditonal IR or
UV/vis spectroscopy utilizing a procedure similar to that
described previously.19
see text
1859*
1852*
1877*
∼0.2 ms at 294 K. We assign this species to the (η6-
C6H6)Cr(CO)2(benzene) complex on the basis of the
observation that this species is generated immediately
upon photolysis (within 70 ns) and the fact that previous
studies have shown that UV photoexcitation of transi-
tion metal carbonyls in condensed phase results in the
formation of the solvated complex within picoseconds
of CO loss.20 Specifically, photolysis of (η6-arene)Cr(CO)3
in alkane solvent leads to the immediate generation of
the (η6-arene)Cr(CO)2(alkane) transient.21 Furthermore,
other metal-arene complexes such as Cr(CO)5(benzene),
CpMn(CO)2(toluene), and Cp*Re(CO)2(arene) are also
formed promptly following loss of a CO ligand upon
photolysis of the parent tricarbonyl in the presence of
the appropriate arene. By analogy with similar com-
plexes, benzene is most likely bound to the (η6-C6H6)-
Cr(CO)2 fragment through a fixed double bond on the
arene ring, an η2 interaction. There is precedence for
this type of interaction in other short-lived organome-
tallic complexes,9-15 and several isolable metal-(η2-
arene) complexes have also been synthesized.4,7,8,22 This
structural assignment is also consistent with the ob-
servation that the (η6-C6H6)Cr(CO)2(benzene) complex
is less reactive than the analogous (η6-arene)Cr(CO)2-
(alkane) complex, in which the primary interaction is
between the metal center and the electron density in
All arenes were either anhydrous grade (99%) or dried over
CaH2 prior to distillation. cis-Cyclooctene (95%) was used as
received. The metal complexes, (η6-arene)Cr(CO)3, were either
commercially available ([arene] ) C6H6) or synthesized ac-
cording to literature methods ([arene] ) C6H5CF3, C6H5CH3).17
The photolysis solutions were prepared under an Ar atmo-
sphere.
Results and Discussion
As shown in Figure 1, photolysis of a ∼0.6 mM
solution of (η6-C6H6)Cr(CO)3 in benzene results in the
generation of a species with a CO stretching absorbance
at 1856 cm-1 (Table 1). This transient has a half-life of
(17) Hunter, A. D.; Mozoi, V.; Tsal, S. D. Organometallics 1992, 11,
2251.
(18) Bengali, A. A. J. Organomet. Chem., in press.
(19) Bengali, A. A.; Fehnel, R. Organometallics 2005, 24, 1156.
(20) Simon, J. D.; Xie, X. J. Am. Chem. Soc. 1990, 112, 1130.
(21) 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.

The (η6-C6H5R)Cr(CO)2-(η2-C6H5R) Bond
Organometallics, Vol. 24, No. 24, 2005 5921
Figure 3. Plot of kobs vs [piperidine] at several tempera-
tures for the reaction of (η6-C6H6)Cr(CO)2(η2-benzene) with
piperidine
Figure 2. Decay of the (η6-C6H6)Cr(CO)2(η2-benzene)
complex and concurrent growth of (η6-C6H6)Cr(CO)2-
(piperidine) upon photolysis of a ∼0.6 mM solution of (η6-
C6H6)Cr(CO)3 in benzene in the presence of 0.196 M
piperidine at 318 K. The reactant exhibits residual absor-
bance due to overlap with the product complex. The solid
line represents a first-order exponential fit to the data.
Table 2. Second-Order Rate Constants Obtained
from the Slope of the kobs vs [L] Plots for the
Reaction of (η6-C6H6)Cr(CO)2(η2-benzene) with L )
Piperidine and Pyridine
ksecond × 10-5 (M-1 s-1
piperidine
)
the C-H bond.21,23 The (η6-C6H6)Cr(CO)2(η2-benzene)
complex should have two CO stretching absorbances,
and while only one is observed (at 1856 cm-1), it is
reasonable to conclude that the second band expected
at ∼1900-1910 cm-1 overlaps with the broad parent
T (K)
pyridine
294
303
313
318
6.56 ( 0.24
14.2 ( 0.6
25.2 ( 0.7
31.8 ( 1.2
9.52 ( 0.9
19.7 ( 1.0
31.7 ( 1.7
38.5 ( 0.6
∆Hq ) 11.5 ( 0.9 kcal/mol ∆Hq ) 10.1 ( 1.0 kcal/mol
tricarbonyl absorption at 1896 cm-1 24
.
∆Sq ) +7.6 ( 2.9 eu
∆Sq ) +3.4 ( 3.3 eu
As shown in Figure 2, in the presence of piperidine,
the (η6-C6H6)Cr(CO)2(η2-benzene) transient reacts to
form the previously characterized (η6-C6H6)Cr(CO)2-
(piperidine) complex absorbing at 1873 cm-1.25The
identity of this product was further confirmed by
photolyzing a benzene solution of (η6-C6H6)Cr(CO)3
containing piperidine and acquiring a FTIR spectrum
that showed the presence of CO stretching absorbances
at 1874 and 1815 cm-1. While the higher frequency band
is observed in the flash photolysis experiment, the lower
frequency absorption is beyond the range of our IR diode
laser. The observed rate constant, kobs, obtained by
fitting the decay of the (η6-C6H6)Cr(CO)2(η2-benzene)
complex to a first-order exponential function shows a
linear dependence on the piperidine concentration (Fig-
ure 3). Unfortunately, a linear dependence of kobs on
piperidine concentration does not assist in distinguish-
ing between an associative (A), dissociative (D), or
interchange mechanism (I) of benzene substitution from
the Cr center.26 To further clarify the mechanism, the
displacement reaction was also studied by using pyri-
dine as the incoming ligand, and as shown in Table 2,
the results show only a slight dependence of the rate
constants on the identity of the displacing ligand. Given
the substantial difference in the basicity and hence
nucleophilicity of piperidine and pyridine, the similar
second-order rate constants suggest that the reaction
most likely does not proceed through either an A or Ia
mechanism. We therefore favor a D or an Id mechanism
of benzene displacement from the metal center. It is
difficult to further distinguish between these two mecha-
nistic possibilities. However, since assignment of a D
mechanism is formally contingent upon the identifica-
tion of an intermediate with reduced coordination
number, i.e., the “naked” (η6-C6H6)Cr(CO)2 complex,
which is not observed in the present study, we tenta-
tively suggest that the reaction proceeds through an Id
mechanism. Consistent with this assignment, the dis-
placement of other weakly bound solvent molecules from
(η6-arene)Cr(CO)2(solvent) complexes [solvent ) H2, N2,
Et3Si-H, THF] was also shown to proceed through
either a D or Id pathway.19,27 However, dissociative
substitution of the alkane was ruled out in the case of
the (η6-arene)Cr(CO)2(alkane) complexes.21 The moder-
ately positive activation entropy, ∆Sq ) +7.6 ( 2.9 eu,
obtained from an Eyring analysis (Table 2) of the
reaction with piperidine is also consistent with an Id
mechanistic assignment, which implies that the reaction
proceeds through a single transition state with signifi-
cant Cr-(η2-benzene) bond disruption and the incoming
ligand either loosely bound to the metal center or
present in the first solvation shell. The activation
enthalpy of ∆Hq ) 11.5 ( 0.9 kcal/mol is therefore
(22) See for example: (a) Norris, C. M.; Templeton, J. L. Organo-
metallics 2004, 23, 3101. (b) Tagge, C. D.; Bergman, R. G. J. Am. Chem.
Soc. 1996, 118, 6908. (c) Sweet, J. R.; Graham, W. A. G. J. Am. Chem.
Soc. 1983, 105, 305. (d) Jones, W. D.; Dong. L. J. Am. Chem. Soc. 1989,
111, 8722. (e) Higgitt, C. L.; Klahn, A. H.; Moore, M. H.; Oelckers, B.;
Partridge, M. G.; Perutz, R. N. J. Chem. Soc., Dalton Trans. 1997,
1269. (f) Boese, R.; Stanger, A.; Stellberg, P.; Shazar, A. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1475. (g) Brauer, D. J.; Kru¨ger, C. Inorg. Chem.
1977, 16, 884. (h) Stanger, A.; Boese, R. J. Organomet. Chem. 1992,
430, 235.
(23) Hall, C.; Perutz, R. N. Chem. Rev. 1996, 96, 3125.
(24) The (η6-C6H6)Cr(CO)2(heptane) complex has CO bands at 1927
and 1877 cm-1 (see ref 21). Assuming a similar 50 cm-1 difference in
the symmetric and antisymmetric CO stretching modes of the (η6-
C6H6)Cr(CO)2(η2-benzene) complex, the “missing” peak should be at
∼1906 cm-1
.
(25) Strohmeirer, W.; Hellmann, H. Chem. Ber. 1963, 96, 2859.
(26) Schultz, R. H. Int. J. Chem. Kinet. 2004, 36, 427.
(27) Howdle, S. M.; Healy, M. A.; Poliakoff, M. J. Am. Chem. Soc.
1990, 112, 4804.

5922 Organometallics, Vol. 24, No. 24, 2005
Bengali and Grunbeck
Table 3. Second-Order Rate Constants for the Displacement of the η2-Coordinated Arene from Complexes
1-4 by Piperidine
ksecond × 10-5 (M-1 s-1
)
T (K)
1
2
3
4
294
303
313
318
3.6 ( 0.3
5.4 ( 0.5
12.1 ( 0.6
16.9 ( 0.3
8.4 ( 0.2
5.8 ( 0.1
37.2 ( 2.3
60.2 ( 2.3
115 ( 2.4
18.3 ( 0.6
29.9 ( 0.4
35.0 ( 0.4
10.1 ( 0.6
24.0 ( 2.0
28.7 ( 0.8
∆Hq ) 11.6 ( 1.2 kcal/mol
∆Sq ) +6.4 ( 3.9 eu
∆Hq ) 10.4 ( 1.3 kcal/mol
∆Sq ) +4.3 ( 4.3 eu
∆Hq ) 12.3 ( 1.0 kcal/mol
∆Sq ) +9.8 ( 3.2 eu
∆Hq ) 10.2 ( 0.8 kcal/mol
∆Sq ) +6.4 ( 2.6 eu
expected to provide a lower limit to the strength of the
(η6-arene)Cr(CO)2-(η2-benzene) bond, similar to that of
the CpMn(CO)2-(η2-toluene) interaction, which has
been estimated at ∼14 kcal/mol.13
experimental data. We may therefore conclude that the
aromaticity of the arene ligand is not affected signifi-
cantly upon binding with the metal center.
(η6-C6H5CF3)Cr(CO)2(η2-benzene) vs (η6-C6H5CH3)-
Cr(CO)2(η2-benzene). To investigate the effect of vary-
ing the metal electron density on the reactivity of the
Cr-(η2-benzene) bond, the reaction of (η6-C6H5CF3)Cr-
(CO)2(η2-benzene) (1) and (η6-C6H5CH3)Cr(CO)2(η2-
benzene) (2) with piperidine was studied. These com-
plexes are expected to have considerably different
electron density as indicated by the position of the CO
stretching absorbances for the respective parent tricar-
bonyl complexes (Table 1).17 Importantly, differences in
the reactivity of these two complexes can be readily
attributed to electronic and not steric effects. Through-
out the following discussion we make the reasonable
assumption that all of the displacement reactions
proceed through a similar (Id) mechanism and that
therefore any difference in the reactivity of the metal-
(η2-arene) bond can be related to the stability of this
interaction.
Although 1 and 2 are expected to be the primary
photoproducts upon photolysis of the respective parent
tricarbonyls in benzene solution, previous reports indi-
cate that η6 arene exchange can also occur in the
presence of an arene different from the one bound to
the Cr center.28 Upon prolonged steady-state photolysis
(2 min) of (η6-C6H5CF3)Cr(CO)3 and (η6-C6H5CH3)Cr-
(CO)3 in neat benzene with a 150 W Xe Arc Lamp, the
formation of a small amount of the (η6-C6H6)Cr(CO)3
complex is observed in the FTIR spectrum. However,
because every shot of the laser photolyzes a fresh
sample, and since (η6-C6H6)Cr(CO)3 is not involved in
the reaction of 1 or 2 with piperidine, the presence of a
potentially small amount of (η6-C6H6)Cr(CO)3 is not
expected to be an issue in the flash photolysis experi-
ments.
As shown in Figure 4, the benzene solvent is displaced
by piperidine more rapidly from 2 than from 1. The
second-order rate constants shown in Table 3 suggest
that complex 2 reacts approximately 2-3 times faster
than 1, suggesting that increasing the electron density
on the metal center yields a less stable dihapto interac-
tion. This observation indicates that donation of π
electron density from a localized double bond on the
benzene ligand to the metal (LfM σ bond) is the
primary component of the Cr-(η2-benzene) interaction.
If MfL π back-bonding were the dominant interaction,
it is expected that increasing the electron density on
the metal would result in a stronger and hence less
reactive Cr-(η2-benzene) bond, which is contrary to the
Figure 4. Plot of kobs vs [piperidine] at 318 K for the
displacement of the dihapto-coordinated benzene ligand
from (η6-C6H5R)Cr(CO)2(η2-benzene).
The Cr-(η2-arene) bond in these systems is signifi-
cantly different from that between potent π bases such
as TpRe(CO)(imidazole) and benzene, which involves a
strong π-back-bonding interaction leading to the de-
aromatization of the η2-coordinated arene.7 Further-
more, the importance of π back-bonding in the Re
system was evidenced by the kinetic stabilities of the
Re-(η2-arene) complexes. Thus, unlike the findings of
the present study, the rate of dissociation of the arene
ligand from the Re fragment was found to decrease as
the electron density on the metal increased.29
(η6-C6H6)Cr(CO)2(η2-C6H5CF3) vs (η6-C6H6)Cr-
(CO)2(η2-C6H5CH3). To further investigate the dihapto
interaction, we studied the effect of changing the
electronic characteristics of the η2-coordinated arene
upon the reactivity of the Cr-(η2-arene) bond. Photolysis
of (η6-C6H6)Cr(CO)3 in C6H5CH3 and C6H5CF3 yields the
(η6-C6H6)Cr(CO)2(η2-C6H5CH3) (3) and (η6-C6H6)Cr(CO)2-
(η2-C6H5CF3) (4) complexes, respectively. Since toluene
solvent absorbs IR light strongly in the region where 3
is expected to be observed (≈1856 cm-1), kobs was
determined by monitoring the growth of the product (η6-
C6H6)Cr(CO)2(piperidine) complex. The second-order
rate constants shown in Table 3 indicate that 4 reacts
almost 6 times faster than 3. This finding is consistent
with the earlier conclusion that σ bonding is the primary
interaction between the Cr center and the η2-coordinat-
ed arene. Since C6H5CF3 is a weaker σ donor than C6H5-
CH3, it is expected that the resulting LfM dative bond
will be less stable, and hence more reactive, than that
(28) Gilbert, A.; Kelly, J. M.; Budzwait, M.; Koerner von Gustorf,
E. Z. Naturforsch. 1976, 31B, 1091.
(29) Meiere, S. H.; Brooks, B. C.; Gunnoe, T. B.; Carrig, E. H.; Sabat,
M. S.; Harman, W. D. Organometallics 2001, 20, 3661.

The (η6-C6H5R)Cr(CO)2-(η2-C6H5R) Bond
Organometallics, Vol. 24, No. 24, 2005 5923
Figure 6. Plot of kobs vs [pyridine] at 293 K for the
displacement of cyclooctene from the (η6-C6H5R)Cr(CO)2-
(η2-cyclooctene) complex by pyridine.
Figure 5. FTIR spectra obtained at 150 s intervals
showing the reaction of (η6-C6H5CH3)Cr(CO)2(η2-cyclooctene)
with 0.58 M pyridine at 293 K in cyclooctene solvent. The
peaks marked v are the CO bands of the (η6-C6H5CH3)-
Cr(CO)2(pyridine) complex, while those marked V are as-
sociated with the η2-cyclooctene reactant. The inset shows
the same reaction followed by UV-vis spectroscopy at 60
s intervals with [pyridine] ) 1.1 M.
Scheme 1
between the Cr center and the more electron-rich
toluene ligand.
By contrasting the reactivity of 1 and 2 with that of
3 and 4, it is also evident that changes in the electron
density of the dihapto-coordinated arene effect the Cr-
(η2-arene) bond more than variations in the electronic
characteristics of the metal center. While the differences
in the reactivities are small (less than a factor of 10),
they are nonetheless chemically significant since the
observed trends yield important information about the
metal-(η2-arene) interaction.
(η6-C6H5R)Cr(CO)2(η2-arene) vs (η6-C6H5R)Cr-
(CO)2(η2-alkene). In contrast to the results reported
in the present study, evidence in the literature suggests
that the (η6-C6H5R)Cr(CO)2-(η2-alkene) bond becomes
more reactive as the electron density on the Cr center
is decreased. Thus, reaction of CS2 with (η6-C6H4R2)Cr-
(CO)2-(η2-cyclooctene) to form the (η6-C6H4R2)Cr(CO)2-
(CS) complex proceeds faster when R ) CO2CH3 than
when R ) CH3.30 This difference in reactivity is opposite
that observed in the case of the Cr-(η2-arene) complexes
(1 vs 2) and can be readily attributed to the aromaticity
of the arene ligand, suggesting that a dihapto-coordi-
nated arene binds to the (η6-C6H5R)Cr(CO)2 fragment
in a different manner than does an alkene.
To obtain more quantitative data on the difference
in reactivity between the Cr-(η2-arene) and Cr-(η2-
alkene) bonds, we studied the displacement of the
cyclooctene ligand from (η6-C6H5R)Cr(CO)2(η2-cyclooctene)
(R ) CH3, CF3) by pyridine. As shown in Figure 5,
photolysis of a solution of (η6-C6H5R)Cr(CO)3 in cy-
clooctene in the presence of pyridine results in the
generation of the previously observed (η6-C6H5R)Cr-
(CO)2(η2-cyclooctene) and (η6-C6H5R)Cr(CO)2(pyridine)
complexes.25,31 The cycloocetene complex then converts
to the more stable pyridine complex. The displacement
reaction appears to proceed by a dissociative mechanism
since kobs approaches a limiting value at high pyridine
concentration (Figure 6). Such rate behavior implies the
reversible generation of an intermediate that precedes
the formation of the product complex. A reasonable
mechanism for this reaction is shown in Scheme 1.
Applying the steady-state approximation to the (η6-
C6H5R)Cr(CO)2 fragment yields the following depen-
dence of kobs on [pyridine]:
k1k2[pyridine]
kobs
)
(1)
k-1[cyclooctene] + k2[pyridine]
Since in these reactions cyclooctene is both a solvent
and a reactant, eq 1 must be modified to take into
account the fact that addition of increasing amounts of
pyridine necessarily decreases the concentration of
cyclooctene. The concentration of cyclooctene may be
expressed as:
[cyclooctene] ) [cyclooctene]0 - a[pyridine] (2)
Here [cyclooctene]0 ) 7.67 M and a is the ratio of the
concentrations of pure cyclooctene and pyridine (0.62).
Substitution of eq 2 into 1 yields:
k1k′′[pyridine]
k2
kobs
)
k′′ )
(3)
{
}
k-1
7.67 + (k′′ - a)[pyridine]
(30) Jaouen, G.; Simonneaux, G. Inorg. Synth. 1979, 19, 197.
(31) Butler, I. S.; English, A. M.; Plowman, K. R. Inorg. Synth. 1982,
21, 1.
A fit of the kobs vs [pyridine] data to eq 3 yields k1 values

5924 Organometallics, Vol. 24, No. 24, 2005
Bengali and Grunbeck
of (7.2 ( 0.4) × 10-3 and (12.7 ( 0.2) × 10-3 s-1 at 293
K for the displacement of cyclooctene from (η6-C6H5-
CH3)Cr(CO)2(η2-cyclooctene) and (η6-C6H5CF3)Cr(CO)2-
(η2-cyclooctene), respectively. The ratios k2/k-1 are ∼7
for each complex, suggesting that the (η6-C6H5R)Cr(CO)2
fragment is more selective toward reaction with pyridine
than with cyclooctene but that this relative reactivity
is unaffected by the presence of the different R groups.
Consistent with the earlier literature reports dis-
cussed above, the difference in the k1 values shows that
the reactivity of the Cr-(η2-cyclooctene) bond increases
when the electron density on the metal decreases. This
reactivity is contrary to that observed for the Cr-(η2-
arene) complexes discussed earlier, and it would there-
fore appear that π back-bonding is a relatively more
important factor in the Cr-(η2-cyclooctene) interaction.
It is perhaps not surprising that π back-bonding is
minimized in the Cr-(η2-arene) complexes because the
potential loss of aromaticity would be energetically
costly. Obviously, dihapto coordination of alkenes is not
subject to such considerations and MfL π back-bonding
can lead to a more stable structure.
ment of the η2-coordinated arene ligand by piperidine
is found to proceed through an Id mechanism. The
activation enthalpy yields an estimate of 11.5 ( 0.9 kcal/
mol for the strength of the (η6-C6H6)Cr(CO)2-(η2-C6H6)
bond. The factors influencing the reactivity of the Cr-
(η2-arene) bond were probed by varying the electron
density on both the metal center and the arene. It was
found that when the electron density on the metal
center was increased or that of the arene decreased, the
rate of η2 arene displacement increased, suggesting that
LfM σ donation is the primary bonding interaction in
these complexes. In contrast to the results of the Cr-
(η2-arene) system, it was found that the rate of displace-
ment of η2-bound cyclooctene from the (η6-C6H5R)Cr-
(CO)2(η2-cyclooctene) complex increased as the electron
density on the metal center decreased, suggesting that
π back-bonding is an important factor in the Cr-(η2-
alkene) interaction. It would be of interest to theoreti-
cally model the η2 coordination of arenes and alkenes
to the (η6-C6H5R)Cr(CO)2 fragment to provide more
quantitative insight into the relative importance of σ
and π bonding in these complexes.
Conclusions
Acknowledgment is made to the donors of the
American Chemical Society Petroleum Research Fund
for support of this research.
The (η6-C6H5R)Cr(CO)2(η2-C6H5R) complexes (R ) H,
CH3, CF3) are formed by photolyzing (η6-C6H5R)Cr(CO)3
in the presence of the appropriate arene. The displace-
OM050604H