Femtosecond infrared study of the dynamics of solvation and solvent caging

Article abstract of DOI:10.1021/ja003344y

The ultrafast reaction dynamics following 295-nm photodissociation of Re₂(CO)_1O were studied experimentally with 300-fs time resolution in the reactive, strongly coordinating CCl₄ solution and in the inert, weakly coordinating hexane solution. Density-functional theoretical (DFT) and ab initio calculations were used to further characterize the transient intermediates seen in the experiments. It was found that the quantum yield of the Re-Re bond dissociation is governed by geminate recombination on two time scales in CCl₄, ～50 and ～500 ps. The recombination dynamics are discussed in terms of solvent caging in which the geminate Re(CO)₅ pair has a low probability to escape the first solvent shell in the first few picoseconds after femtosecond photolysis. The other photofragmentation channel resulted in the equatorially solvated dirhenium nonacarbonyl eq-Re₂(CO)₉(solvent). Theoretical calculations indicated that a structural reorganization energy cost on the order of 6-7 kcal/mol might be required for the unsolvated nonacarbonyl to coordinate to a solvent molecule. These results suggest that for Re(CO)₅ the solvent can be treated as a viscous continuum, whereas for the Re₂(CO)₉ the solvent is best described in molecular terms.

Full text of DOI:10.1021/ja003344y

4204
J. Am. Chem. Soc. 2001, 123, 4204-4210
Femtosecond Infrared Study of the Dynamics of Solvation and
Solvent Caging
Haw Yang,‡ Preston T. Snee, Kenneth T. Kotz, Christine K. Payne, and Charles B. Harris*
Contribution from the Department of Chemistry, UniVersity of California, Berkeley, California 94720,
and Chemical Sciences DiVision, Ernest Orlando Lawrence Berkeley National Laboratory,
Berkeley, California 94720
ReceiVed September 11, 2000
Abstract: The ultrafast reaction dynamics following 295-nm photodissociation of Re2(CO)10 were studied
experimentally with 300-fs time resolution in the reactive, strongly coordinating CCl4 solution and in the
inert, weakly coordinating hexane solution. Density-functional theoretical (DFT) and ab initio calculations
were used to further characterize the transient intermediates seen in the experiments. It was found that the
quantum yield of the Re-Re bond dissociation is governed by geminate recombination on two time scales in
CCl4, ∼50 and ∼500 ps. The recombination dynamics are discussed in terms of solvent caging in which the
geminate Re(CO)5 pair has a low probability to escape the first solvent shell in the first few picoseconds after
femtosecond photolysis. The other photofragmentation channel resulted in the equatorially solvated dirhenium
nonacarbonyl eq-Re2(CO)9(solvent). Theoretical calculations indicated that a structural reorganization energy
cost on the order of 6-7 kcal/mol might be required for the unsolvated nonacarbonyl to coordinate to a solvent
molecule. These results suggest that for Re(CO)5 the solvent can be treated as a viscous continuum, whereas
for the Re2(CO)9 the solvent is best described in molecular terms.
Introduction
energy excitation whereas pathway (ii) is favored by higher
energy excitation.9-12 Experimentally, the short-lived photo-
fragments MM′(CO)9 and M(CO)5 have been identified and
characterized in the gas phase,12,13 in low-temperature matrix
isolation studies,14-19 in liquid Xe,17 and in room-temperature
solutions.7,20-22 These studies have been instrumental in estab-
lishing our current understanding of the photochemistry of MM′-
(CO)10. However, the influence of the surrounding solvent over
reaction pathways (i), (ii), and (iii) remains to be characterized.
This work examines how the solvent participates in the chemical
processes that follow photofragmentation of Re2(CO)10.
For Re2(CO)10 a UV pulse may cleave the Re-Re bond to
form two formally 17-electron Re(CO)5 radicals or the UV pulse
may remove a CO ligand to form the coordinatively unsaturated
Re2(CO)9. The chemistry that follows the generation of the
reactive species Re(CO)5 or Re2(CO)9 is quite different. The
rhenium pentacarbonyl Re(CO)5 that follows dissociation
The reactivity of multinuclear compounds can generally be
photochemically activated, creating one or multiple coordina-
tively vacant sites at the metal.1 Dinuclear metal complexes such
as MM′(CO)10 (M, M′ ) Mn, Re) can be viewed as prototypical
compounds for organometallics of higher nuclearity in potential
catalytic applications.2-4 The primary photochemical reaction
of MM′(CO)10 in solution has been shown to result in (i)
homolytic fission of the metal-metal σ-bond to form M(CO)5
radicals, (ii) dissociation of a CO ligand to form MM′(CO)9,
and (iii) nondissociative relaxation to the electronic ground state:
2,5-8
(9) Wrighton, M. S.; Ginley, D. S. J. Am. Chem. Soc. 1975, 97, 2065.
(10) Gard, D. R.; Brown, T. L. J. Am. Chem. Soc. 1982, 104, 6340.
(11) Kobayashi, T.; Ohtani, H.; Noda, H.; Teratani, S.; Yamazaki, H.;
Yasufuku, K. Organometallics 1986, 5, 110.
(12) Seder, T. A.; Church, S. P.; Weitz, E. J. Am. Chem. Soc. 1986,
108, 7518.
(13) Freedman, A.; Nersohn, R. J. Am. Chem. Soc. 1978, 100, 4116.
(14) Hepp, A. F.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 5934.
(15) Dunkin, I. R.; Harter, P.; Shields, C. J. J. Am. Chem. Soc. 1984,
106, 7248.
(16) Firth, S.; Hodges, P. M.; Poliakoff, M.; Turner, J. J. Inorg. Chem.
1986, 25, 4608.
(17) Firth, S.; Klotzbuecher, W. E.; Poliakoff, M.; Turner, J. J. Inorg.
Chem. 1987, 26, 3370.
(18) Church, S. P.; Poliakoff, M.; Timney, J. A.; Turner, J. J. J. Mol.
Struct. 1982, 80, 159.
(19) Huber, H.; Kunding, E. P.; Ozin, G. A. J. Am. Chem. Soc. 1974,
96, 5585.
The branching ratios have been found to depend on the
excitation wavelength by which pathway (i) is favored by lower
* To whom correspondence should be addressed. E-mail: harris@
socrates.berkeley.edu.
‡ Present address: Harvard University, Chemical Laboratories, Box 225,
12 Oxford St., Cambridge, MA 02138.
(1) Nakamura, A.; Tsutsui, M. Principals and Applications of Homo-
geneous Catalysis; John Wiley & Sons: New York, 1980.
(2) Meyer, T. J.; Caspar, J. V. Chem. ReV. 1985, 85, 187.
(3) Serpone, N.; Pelizzetti, E. PhotocatalysissFundamentals and Ap-
plications; Wiley: New York, 1989.
(4) Adams, R. D.; Cotton, F. A. Catalysis by Di- and Polynuclear Metal
Cluster Complexes; Wiley VCH: New York, 1998.
(5) Wrighton, M.; Bredesen, D. J. Organomet. Chem. 1973, 50, C35.
(6) Wegman, R. W.; Olsen, R. J.; Gard, D. R.; Faulkner, L. R.; Brown,
T. L. J. Am. Chem. Soc. 1981, 103, 6089.
(7) Church, S. P.; Hermann, H.; Grevels, F.; Schaffner, K. J. Chem. Soc.,
Chem. Commun. 1984, 785.
(8) Firth, S.; Hodges, P. M.; Poliakoff, M.; Turner, J. J.; Therien, M. J.
J. Organomet. Chem. 1987, 331, 347.
(20) Rothberg, L. J.; Cooper, N. J.; Peters, K. S.; Vaida, V. J. Am. Chem.
Soc. 1982, 104, 3536.
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10.1021/ja003344y CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/12/2001

Dynamics of SolVation and SolVent Caging
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4205
the output of a Ti:sapphire oscillator was amplified in a series of dye
amplifiers pumped by a 30-Hz Nd:YAG laser. The UV and IR beams
used to initiate and probe the reaction were generated through nonlinear
crystals. The resulting 295-nm UV photons (with energy of ∼5 µJ/
pulse) were focused into a disk of ∼200 µm diameter at the sample to
initiate chemical reactions. The ∼1-µJ IR pulses having a temporal
full-width-half-maximum of about 70 fs and a spectral bandwidth of
about 200 cm-1 were split into a signal and a reference beam to
minimize the shot-to-shot fluctuation and transient heating of the
detector. These two beams were then focused into an astigmatism-
corrected spectrographic monochromator (SpectraPro-150, Acton Re-
search Corp., 150 gv/mm, 4.0 µm blazed) to form two spectrally
resolved images on a focal-plane-array (FPA) IR detector. The two
frequency-resolved images were digitized by two windows of 12 ×
200 pixels, which allowed simultaneous normalization of a ∼70-cm-1
spectrum. The censoring chip of the detector was an engineering grade,
(256 × 256)-element HgCdTe matrix of dimensions 1.28 × 1.28 cm2
(or 50 × 50 µm2 per pixels). During the course of an experiment, the
sensing chip and its immediate circuits were kept in contact with a
4-L liquid nitrogen dewar to increase its sensitivity in the IR range.
The long-time temperature drift was minimized by normalizing the gain
against the FPA readouts from a small region far away from the
illuminated area. Rejection of bad laser shots further improved the S/N
ratio. With a data acceptance ratio of about 0.5 and a 30-Hz laser
repetition rate, it took about 1 min to acquire signals on the order of
1% absorbance change with a 10:1 S/N ratio after signal averaging for
1000 valid laser shots. The typical spectral and temporal resolution
for this setup were ∼4 cm-1 and ∼300 fs, respectively. The polariza-
tions of the pump and the probe pulse were set at the magic angle
(54.7°) to ensure that all signals were due to population dynamics. All
pathway (i) has been shown to assume a C4V, square-pyramidal
geometry from solid CO matrix studies.18,19 Such a 17-electron
Re(CO)5 radical is capable of abstracting a halogen atom from
halogenated alkanes.23,24 This type of one-electron oxidative
addition to a metal center has been thought to be one of the
crucial steps in many catalytic reactions.25 One of the central
questions regarding the reactivity of 17-electron species is the
possible involvement of a 19-electron precursory complex of
the form (CO)5Re‚‚‚Cl-CR3 during the course of reaction. This
question has been addressed in a recent study and it was found
that for the abstraction of a Cl atom from CHnCl4-n by Re-
(CO)5, the rhenium radical is best viewed as an unsolvated 17-
e- species.26 Due to the weak interaction between a Re(CO)5
radical and the surrounding solvent molecules, a geminate Re-
(CO)5 radical pair may recombine to reform the parent Re2-
(CO)10 molecule on an ultrafast time scale. It will be shown
that for this dissociation pathway, the competition of Cl-
abstraction and geminate recombination is regulated by solvent
caging.
For dissociation pathway (ii), Firth et al. have shown in a
low-temperature study that the primary photoproduct Re2(CO)9
has a coordinatively vacant site in the equatorial position,
denoted eq-Re2(CO)9.17 Irradiation with 546-nm light converts
eq-Re2(CO)9 to the other isomer ax-Re2(CO)9 with an axial
vacant site. One possible reaction following CO photolysis is
ligand substitution at the vacant site of Re2(CO)9. Coville and
co-workers have studied a series of substituted Re2(CO)9L (L
) ligand) and concluded that there is a delicate balance in the
electronic and steric effects in the relative stability of the axially
or equatorially substituted isomers.27-29 In room-temperature
solutions, it is observed that a solvent molecule enters the
coordinatively vacant site to form a solvated complex Re2(CO)9-
(solvent) within a few picoseconds following photodissociation.
However, as will be shown later, such solvation may require
structural reorganization of the Re2(CO)9 metal fragment. If the
complexation energy of a solvent molecule and the metal center
is not sufficient to compensate for the energy cost for structural
reorganization, the unsolvated Re2(CO)9 complex may appear
thermodynamically more stable in weak-coordinating solvents.
For instance, the unsolvated eq-Re2(CO)9 complex has been
observed in the Ar and N2 matrices.17 With the aid of quantum-
chemical calculations, the picture of reorganization/solvation
will be used to explain the experimental results.
kinetic data have been corrected for a positive chirp of ∼1.4 fs/cm-1
,
measured from pump-probe cross correlation with use of a silicon
wafer. A broad, wavelength-independent background signal from CaF2
windows has also been subtracted from the transient spectra and kinetic
traces.
Nanosecond Step-Scan FTIR. These measurements were made by
using a Step-Scan FTIR spectrometer described elsewhere.31 The
instrument was based on a Bruker IFS-88 FTIR with a special scanner
module to allow step scanning. An InSb detector with a 40-ns temporal
full-width-half-maximum (fwhm) measured from the IR scatter of 1064-
nm light from a YAG laser was used. The IR light was focused in the
cavity with two 10-mm focal length BaF2 lenses, which gave beam
sizes smaller than comparable curved mirrors, allowing increased IR
throughput. The sample was photoexcited with 10-ns pulses at 295 nm
from the second harmonic of a dye laser.
Theoretical. To represent the chemical species in a practically
tractable way, the solvent molecule in the complexes Re2(CO)9(solvent)
was represented by CH4 for alkane or by CH3Cl for solvation through
the Cl atom in halogenated solvents. For density-functional theoretical
(DFT) calculations, the commercial JAGUAR package was used.32 The
exchange-correlation functional employed was the Becke’s three-
parameter hybrid functional33 combined with the Lee-Yang-Parr
(LYP) correlation functional,34 commonly denoted as B3LYP.35 This
functional has been shown to give very good results for transition metal
complexes.36,37 Except for molecules with apparent symmetry such as
CH4 or Re2(CO)10, no constraint was imposed during geometry
optimization. The basis set consisted of the 6-31G basis functions for
H, C, O, and Cl atoms,38,39 and the Los Alamos Effective Core Potential
Methods
Samples. Dirhenium decacarbonyl Re2(CO)10 (98%) and carbon
tetrachloride CCl4 (99.9%, d ) 1.589 g/cm3) were purchased from
Aldrich, Inc. Hexane (C6H14, 99.9%, d ) 0.664 g/cm3), trichloromethane
(CHCl3, 99%, d ) 1.484 g/cm3), and dichloromethane (CH2Cl2, 99.9%,
d ) 1.3255 g/cm3) were purchased from Fisher Scientific, Inc. All
chemicals were used without further purification. The sample was
enclosed in an airtight, demountable liquid IR flow cell (Harrick
Scientific Corporation). The concentrations of the Re2(CO)10 solutions
were approximately 7 mM in hexanes and 4 mM in chlorine-substituted
methanes.
(30) Lian, T.; Bromberg, S. E.; Asplund, M. C.; Yang, H.; Harris, C. B.
J. Phys. Chem. 1996, 100, 11994.
Femtosecond Infrared Spectroscopy. Details of the femtosecond
(31) Sun, H.; Frei, H. J. Phys. Chem. B 1997, 101, 205.
(32) Jaguar 3.5; Schroedinger, Inc.: Portland, OR, 1998.
(33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(34) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B41, 785.
(35) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623.
(36) Ricca, A.; Bauschlicher, C. W. Theor. Chim. Acta 1995, 92, 123.
(37) Glukhovtsev, M. N.; Bach, R. D.; Nagel, C. J. J. Phys. Chem. 1997,
101, 316.
(38) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(39) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257.
IR (fs-IR) spectrometer setup have been published elsewhere.30 Briefly,
(23) Stiegman, A. E.; Tyler, D. R. Comments Inorg. Chem. 1986, 5, 215.
(24) Tyler, D. R. Acc. Chem. Res. 1991, 24, 325.
(25) Baird, M. C. Chem. ReV. 1988, 88, 1217.
(26) Yang, H.; Snee, P. T.; Kotz, K. T.; Payne, C. K.; Frei, H.; Harris,
C. B. J. Am. Chem. Soc. 1999, 121, 9227.
(27) Harris, G. W.; Boeyens, J. C. A.; Coville, N. J. Organometallics
1985, 4, 914.
(28) Harris, G. W.; Coville, N. J. Organometallics 1985, 4, 908.
(29) Harris, G. W.; Boeyens, J. C. A.; Coville, N. J. J. Chem. Soc., Dalton
Trans. 1985, 2277.

4206 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Yang et al.
considered to follow the diffusion equation in a continuous
medium,
∂
2
w(r,t;r0,R) ) D∇ w(r,t;r0,R)
(2)
∂t
where w(t,r;r0,R) is the time-dependent survival probability
density of a pair of monomers and D is the diffusion coefficient
of the monomer in the solvent. The model assumes that a pair
of Re(CO)5 radicals rest at r0 apart at time t f 0. The two
radicals can react to form Re2(CO)10 when their separation r is
equal to or smaller than a critical distance R. To account for
the finite recombination rate, an additional boundary condition
at r ) R is imposed,
∂
D w(r,t;r0,R)r)R ) bw(r,t;r0,R)
(3)
∂t
Known as the “radiation boundary condition”, eq 3 states that
the recombination rate is proportional to the concentration at
the contact distance R. The relation of the proportionality b in
eq 3 to the recombination rate constant can be expressed by,
Figure 1. Ultrafast kinetics of Re2(CO)10 following 295-nm UV
photolysis in CCl4. The product kinetics in panel a were recorded at
2045 cm-1. Parent bleach kinetics on panel b were recorded at 2071
cm-1. Solids lines are simulations to a diffusion model for geminate-
pair recombination (see text).
kr ) 4πR2b
(4)
The physical parameters needed for the analysis of the
experimental data are the branching ratio Qi for pathway (i),
the solvent viscosity η, the contact distance R, the initial
separation r0, and the recombination rate constant kr. The contact
distance R is assigned as twice the distance from the center of
the Re-Re bond to the center of the axial Re-C-O line.
Similar treatments for the contact distance have been employed
in the analysis of geminate pair dynamics of disulfides.45,46 By
using the crystallographic data of Re2(CO)10,47 R is estimated
to be 6.3 Å. To verify this estimate, the DFT geometry for the
Re(CO)5 radical and the Lennard-Jones parameters for the C
and O atoms are used to construct a Lennard-Jones Re(CO)5
molecule suitable for classical molecular simulation.48 The
contact distance in this approach is defined as the diameter of
a sphere that encloses the Lennard-Jones Re(CO)5. This
procedure gives a contact distance of 6.2 Å, consistent with
the first estimate.
For the Cl-atom abstraction reaction in CCl4 solution (η20°C
) 0.97 cp), the product kinetics and parent bleach signal are
fitted simultaneously since they both draw from the same Re-
(CO)5 source. Three processes are considered to contribute to
the observed product kinetics: vibrational cooling, nonequilib-
rium barrier crossing to form the product, and equilibrium barrier
crossing. Incorporating these ideas, the normalized product
signal is described as,
(ECP) for Re with the outermost core orbitals included in the valence
description (ECP+5s5p6s5d6p).40 To make certain that a proper energy
extremum had been located each geometry optimization was followed
by a frequency calculation at the same level of theory. The resulting
frequency analyses were used to calculate thermodynamic properties
such as the zero-point energy (ZPE) and enthalpy (H) at 298 K. The
interaction energy between a metal fragment and a model solvent
molecule was also calculated, using the counterpoise technique to
account for basis set superposition error (BSSE).41,42
Results and Discussions
The Reaction Dynamics following Dissociation Pathway
(i): Photolysis of the Re-Re Bond. It has been shown in a
previous Communication that the Re(CO)5 radical is only
weakly solvated in alkane or chlorinated alkane solvents.26 The
interaction energies for Re(CO)5/CH4 and Re(CO)5/CCl4 have
been calculated to be ca. -0.2 kcal/mol and ca. -0.6 kcal/mol,
respectively.43 The weak interaction allows the chemically active
Re center to recombine with another Re(CO)5 radical to form
the parent Re2(CO)10 without much energy cost in displacing
the solvent molecule. In the reactive CCl4 solution, the parent
Re2(CO)10 molecule exhibits two reformation time scales of ∼50
and ∼500 ps. The Re(CO)5 radicals are also capable of
abstracting a Cl atom from CCl4 to form Re(CO)5Cl on a time
scale of ∼1.3 ns. In this section, it will be shown that the
competing geminate-recombination and Cl-atom abstraction are
regulated by solvent caging, and that treating the solvent as a
viscous continuum is sufficient to account for the observed
dynamics.
Iproduct(t) ) Avib exp(-t/τvib) + Aneq(1 - exp(-t/τneq)) +
tdt′k M(t′;r ,R) (5)
∫
a
0
0
The kinetic traces of the Re(CO)5Cl and Re2(CO)10 are
reproduced in Figure 1. The biphasic recovery of the parent
bleach shown in Figure 1b has been explained by a modified
diffusion model in the literature.44 For clarity, the model and
required parameters are briefly restated below. The reaction is
The first term of eq 5 describes the relaxation of vibrationally
excited low-frequency modes (hot bands) that overlap with the
product band; the second term models the nonequilibrium
product formation with a phenomenological rise time τneq, and
the last term relates the time-dependent Re(CO)5 concentration
∞
M(t;r0,R) ) ∫Rdr4πrw2(r,t;r0,R) to the macroscopic, pseudo-
(40) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(41) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553.
(42) Liu, B.; McLean, A. D. J. Chem. Phys. 1973, 59, 4557.
(43) The basis set used in these calculations was augmented by placing
polarization functions on all atoms except Re.
(44) Naqvi, K. R.; Mork, K. J.; Waldenstrfm, S. J. Phys. Chem. 1980,
84, 4, 1315.
(45) Scott, T. W.; Liu, S. N. J. Phys. Chem. 1989, 93, 1393.
(46) Bultmann, T.; Ernsting, N. P. J. Phys. Chem. 1996, 100, 19417.
(47) Churchill, M. R.; Amoh, K. N.; Wasserman, H. J. J. Inorg. Chem.
1981, 20, 1609.
(48) Straub, J. E.; Karplus, M. Chem. Phys. 1991, 158, 221.

Dynamics of SolVation and SolVent Caging
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4207
first-order rate constant ka. To account for the nonequilibrium
barrier crossing in the CCl4 case, the normalized parent signal
can be written as,
Iparent(t) ) -1 + (1 - Aneq) × Qi ×
tdt′4πR2bw(r ) R, t′;r ,R) (6)
∫
0
0
With R fixed ()6.256 Å) and Qi ()0.8) obtained from
experiments, the recombination rate is found to be kr ) (1.0 (
0.2) × 1011 M-1 s-1, an order of magnitude greater than the
literature value measured in CH3CN, (1.0 ( 0.2) × 1010 M-1
s-1 49
. This is not surprising since in the current simplified model,
a single parameter kr is used to encompass all fast dynamics
such as nondissociative energy relaxation and primary geminate-
recombination. The initial separation of the two Re(CO)5
monomers was found to be r0 ) 8.6 ( 0.2 Å, about 2.3 Å away
from their contact distance. The Cl-atom abstraction rate ka )
(7.5 ( 0.4) × 108 s-1 ((7.3 ( 0.4) × 107 M-1 s-1) is in very
good agreement with the literature value, which is calculated
to be (1.8-9.1) × 107 M-1 s-1 from a set of Arrhenius
parameters.50 The time constant for the fast rise of product
kinetics τneq is found to be approximately 2-3 ps, with an
amplitude Aneq ) 0.10 ( 0.01. That is, about 10% of the product
is produced through nonequilibrium barrier crossing. Finally,
the hot band intensity Avib is found to be 0.13 ( 0.01 with a
vibrational-cooling time τvib ) 35 ( 14 ps. The resulting traces
for the product and parent kinetics are plotted on top of the
experimental data displayed in Figure 1.
Within the framework of the diffusion model, the physical
origin of the experimental observation is illustrated in Figure
2. In Figure 2, the abscissa is the separation of the two Re-
(CO)5 monomers, in units of Å. The equilibrium Re-Re
distance Req ∼ 3.0 Å, the contact distance R ∼ 6.3 Å, and the
initial separation r0 ∼ 8.6 Å are also marked. The portion to
the left of R is a schematic potential energy surface plot
depicting dynamics that are not susceptible to the current model.
To the right of R are plots of the spatial distribution of a
monomer around a recombination center at various time delays.
At short time delays such as 1 and 5 ps, the distribution maxima
still linger around the initial separation r0. By comparing the
8.6-Å initial separation with the Lennard-Jones radius of CCl4
of 5.8 Å, one can envision that following photodissociation the
monomers are trapped in the first solvent shell for a few
picoseconds. Furthermore, the initial distribution also broadens
significantly on the 1-5 ps time scale. The broadening results
in a fast buildup of the monomer concentration at R. Recall
that the recombination rate kr is defined to be proportional to
the concentration at the contact distance R (eq 3). Consequently,
the fast broadening of the distribution contributes to the
experimentally observed fast recovery of the parent bleach.
Driven by the accumulating population around R, the distribution
diffuses away from the recombination center until the system
reaches equilibrium. This is shown in Figure 2 for time delays
longer than ca. 20 ps, where the distribution maximum starts
to diffuse away noticeably. This diffusion process is identified
with the longer recovery in the parent bleach data.
Figure 2. Illustration of the diffusion model for geminate recombina-
tion. On the abscissa, Req is the equilibrium Re-Re bond length of
Re2(CO)10, R the contact distance, and r0 the initial separation of the
two Re(CO)5 monomers. The schematic potential curve for the ground-
electronic state and for the dissociative excited state are on the left. In
this simplified model, the rate constant kr encompasses energy relaxation
processes resulting from nondissociative energy relaxation and primary
geminate recombination. Time evolution of the probability distributions
(dashed curves) for the geminate pair are on the right. Inset: An
illustration of solvent caging. The thick circles represent the Re(CO)5
monomers and the thin circles CCl4 molecules. A dashed circle roughly
shows the extent of the first solvent shell.
represent the depletion of parent molecules. In the stronger
coordinating CCl4 solution, the 1945, 1985, 1998, 2005, and
2055 cm-1 bands are assigned to the equatorially solvated
nonacarbonyl species eq-Re2(CO)9(CCl4).26 The CO stretching
peaks of Re(CO)5Cl also appear on the ultrafast spectra as a
result of Cl-atom abstraction. The nonacarbonyl solvate eq-Re2-
(CO)9(CCl4) is found to be stable up to the experimental time
window of 2.5 µs.
In the weaker coordinating hexane solution, the early-time
spectra (for example, the 10-ps panel of Figure 3) are typically
broad and featureless which results from spectral overlap and
excitation of low-frequency bands.51,52 As the photoproducts
dissipate excess vibrational energy at later time delays, the IR
bands narrow in and rise to reveal several identifiable bands.
For the Re2(CO)10 molecules that dissociate though channel (i)
in eq 1, the nascent Re(CO)5 radicals appear as the most intense
broad peak centering on 1992 cm-1 in the 660-ps panel of Figure
3. This assignment is in very good agreement with the previously
reported Re(CO)5 band (1990 cm-1) in cyclohexane solution.16
On the basis of matrix-isolation experiments, the peaks at 1942,
1973, 1984, 1999, and 2051 cm-1 on the same panel are
attributed to the equatorially solvated dirhenium nonacarbonyl
eq-Re2(CO)9(hexane),17 in which a solvent molecule occupies
the equatorially vacant site left behind by the leaving CO ligand.
The kinetic trace of eq-Re2(CO)9(hexane) at 2051 cm-1 exhibits
a single-exponential rise of 69 ( 5 ps and is attributed to
The Reaction Dynamics following Dissociation Pathway
(ii): Photolysis of the Equatorial Re-CO Bond. The infrared
spectra in the CO stretch region are presented in the form of
difference absorbance in which positive bands indicate the
appearance of new species while negative bands (bleaches)
(49) Sarakha, M.; Ferraudi, G. Inorg. Chem. 1996, 35, 313.
(50) Meckstroth, W. K.; Reed, D. T.; Wojcicki, A. Inorg. Chim. Acta
1985, 105, 147.
(51) Dougherty, T. P.; Heilweil, E. J. Chem. Phys. Lett. 1994, 227, 19.
(52) Arrivo, S. M.; Dougherty, T. P.; Grubbs, W. T.; Heilweil, E. J.
Chem. Phys. Lett. 1995, 235, 247.

4208 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Yang et al.
Figure 4. Schematic representations of Re2(CO)10, ax-Re2(CO)9, and
eq-Re2(CO)9. Important geometrical parameters (in Å) and symmetry
groups are also shown in the figure. Experimental values are included
in parentheses.47
Figure 3. Transient difference spectra in the CO stretching region for
Re2(CO)10 in hexane solution at 10, 33, 66, 198, and 660 ps following
295-nm UV photolysis. The last panel is an averaged spectrum for
pump-probe delay greater than 50 ns. The equatorially solvated eq-
Re2(CO)9(hexane) is indicated by asterisks, and the Re(CO)5 radical is
marked by an arrow. IR bands due to the proposed [eq-Re2(CO)9]bare
are marked by open circles. It is very likely, however, that the last
panel is a spectrum of [eq-Re2(CO)9]bare and eq-Re2(CO)9(hexane) in
equilibrium, considering the similar stability of the two species (see
main text). No discernible spectral features were observed in the region
2080-2130 cm-1 (not shown).
vibrational relaxation of the binuclear eq-Re2(CO)9(hexane)
complex. As indicated by the nanosecond spectra in Figure 3,
the eq-Re2(CO)9(hexane) solvate is stable on the ultrafast time
scale up to ∼1 ns. But at longer time delays (>50 ns), the eq-
Re2(CO)9(hexane) thermally transforms to another species that
has discernible CO stretches at 1990, 1999, and 2043 cm-1 as
shown in the last panel of Figure 3. This species is assigned as
unsolvated eq-Re2(CO)9, denoted [eq-Re2(CO)9]bare, based on
theoretical considerations to be discussed below. Although the
current experimental setup does not permit an accurate measure-
ment of the transformation rate from the solvated eq-Re2(CO)9-
(hexane) to the unsolvated [eq-Re2(CO)9]bare, the time constraints
(1 ns < rate < 50 ns) allow for an evaluation of the lower and
upper bounds of the free-energy barrier, which is estimated to
be 5.2 < ∆G‡ < 7.5 kcal/mol.
Figure 5. Schematic representations of ax-Re2(CO)9(CH4), eq-Re2-
(CO)9(CH4), ax-Re2(CO)9(ClCH3), and eq-Re2(CO)9(ClCH3). Important
geometrical parameters (in Å) are also shown in the figure.
Quantum-Chemical Characterizations, Geometry Opti-
mization. A series of DFT calculations were carried out in
efforts to identify and further characterize the transient species
that appear in the time-resolved IR spectra. Shown in Figure 4
are the optimized structures53 of the parent molecule Re2(CO)10
and the unsolvated Re(CO)5 and Re2(CO)9. Compared to X-ray
diffraction data,47 the calculations reproduce reasonably well
the Re-C bond lengths in Re2(CO)10, which are on the average
about 0.02 to 0.03 Å longer than experimental values. When
the dissociation proceeds through pathway (ii) in eq 1, the
resulting coordinatively unsaturated Re2(CO)9 can assume two
different conformations that belong to different symmetry point
groups. The structure of the ax-Re2(CO)9 conformer is located
by removing an axial CO ligand from Re2(CO)10 as the initial
guess for geometry optimization. The other eq-Re2(CO)9
conformer is located by removing an equatorial CO ligand from
Re2(CO)10 as the initial guess. For the addition of alkanes, the
structures of ax-Re2(CO)9(CH4) and eq-Re2(CO)9(CH4) are
displayed in Figure 5. Generally, the methane molecule in both
complexes interacts with one metal center in a side-on fashion,54
and exhibits an extended (∼0.04 Å) C-H bond at the binding
site. Figure 5 shows the configurations of solvated ax-Re2(CO)9-
(ClCH3) and eq-Re2(CO)9(ClCH3). In both complexes, the CH3-
Cl molecule appears to interact with the nonacarbonyl through
the Cl atom. It should be noted that the calculated “bare”
dirhenium fragment of eq-Re2(CO)9 exhibits a marked difference
in geometry compared to those of eq-Re2(CO)9(CH4) or eq-
Re2(CO)9(ClCH3). In particular, without a solvent coordination
in the equatorial position, the most stable structure of Re2(CO)9
appears to assume a local C2V symmetry at the coordinatively
unsaturated Re site as shown in Figure 4. This observation
suggests that the bare eq-Re2(CO)9 fragment must reorganize
(53) Bode, B. M.; Gordon, M. S. J. Phys. Chem. A 1998, 1997, 4646.
(54) Zaric, S.; Hall, M. B. J. Phys. Chem. A 1997, 101, 4646.

Dynamics of SolVation and SolVent Caging
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4209
Table 1. Summary of Relative Stability of the Axial and
Equatorial Conformers at Various Levels of Theorya
B3LYP
B3LYP + ∆ZPE
Re2(CO)9
Re2(CO)9(CH4)
Re2(CO)9(ClCH3)
16.9
10.8
11.1
16.3
10.1
11.0
a In the table, ∆ZPE is the zero-point energy correction. The energies
shown here correspond to (Eax - Eeq) in units of kcal/mol.
before accommodating a solvent molecule. That is, a reorga-
nization energy barrier exists for the solvation process:
Figure 6. An illustration of structural reorganization in equatorial
solvation. The structure above the solid arrow connecting eq-Re2(CO)9-
(hexane) and eq-Re2(CO)9 demonstrates schematically the structural
deformation of the Re2(CO)9 fragment and the associated energy cost.
Note that it does not represent the transition-state structure nor does
the solid line indicate the actual reaction pathway.
eq-Re2(CO)9 + solvent f eq-Re2(CO)9(solvent) (7)
Quantum-Chemical Characterizations, Energetics. One
next turns to the energetics of the species discussed above. The
relative stability of the axial and equatorial form of dirhenium
complexes is summarized in Table 1. The unsolvated eq-Re2-
(CO)9 is energetically 16.3 kcal/mol more stable than the
unsolvated ax-Re2(CO)9, in accordance with the proposition that
the electronic effect is the dominating factor for equatorial
substitution in Re2(CO)9 complexes.27 Such electronic influence
is somewhat lessened after solvation, as indicated by the 10.1
and 11.0 kcal/mol ax to eq stabilization energies for Re2(CO)9-
(CH4) and Re2(CO)9(ClCH3), respectively. The decrease in the
(Eax - Eeq) energy gap upon solvation (-6.2 or -5.3 kcal/
mol) can be understood by two factors: the energy requirement
for the aforementioned structural reorganization and the energy
cost due to steric interaction of the solvent molecule and the
metal fragment. While the steric interaction is difficult to
evaluate with these calculations, the reorganization energy can
be estimated by (Esol - Ebare), where Ebare is the energy of the
optimized, unsolvated dirhenium fragment ax-Re2(CO)9 (or eq-
Re2(CO)9) and Esol is the energy of the dirhenium fragment at
the solvated geometry. Calculations show that there is almost
no energy difference (<0.3 kcal/mol) for the conformational
change in the dirhenium fragment from the unsolvated [ax-Re2-
(CO)9]bare to the axially solvated form [ax-Re2(CO)9]sol in either
ClCH3 or CH4. On the other hand, a minimum energy of 5.5 or
7.1 kcal/mol is required for the unsolvated [eq-Re2(CO)9]bare to
rearrange to the equatorially solvated form [eq-Re2(CO)9]sol in
CH4 or CH3Cl, respectively.55 The results suggest that the energy
requirement for structural reorganization contributes significantly
to the reduction of the (Eax - Eeq) energy gap upon solvation.
To examine whether the solvation energy is sufficient to
compensate for structural reorganization, the binding energies
for ax- and eq-Re2(CO)9...(solvent) are also calculated and
summarized in Table 2. One sees in Table 2 that the binding
energies for ax- and eq-Re2(CO)9...(CH4) are similar in mag-
nitude; however, the energy cost for structural reorganization
of the equatorially solvated species (5.5 kcal/mol) is ap-
proximately equal to the equatorially solvated species’ bind-
ing energy. As a result, the overall binding energy of eq-Re2-
(CO)9...(CH4) is on the order of 1-2 kcal/mol. Table 2 also
shows that the binding energy of eq-Re2(CO)9...(ClCH3) is about
9 kcal/mol greater than that of eq-Re2(CO)9...(CH4), providing
a qualitative rationale for experimental observations that the
equatorial solvate eq-Re2(CO)9(ClCCl3) is stable up to the
experimental window 2.5 µs. The calculations predict their
relative stability of dirhenium nonacarbonyl solvates to be, in
order of increasing stability, ax-Re2(CO)9 , ax-Re2(CO)9(CH4)
< ax-Re2(CO)9(ClCH3) ≈ eq-Re2(CO)9 ≈ eq-Re2(CO)9(CH4)
, eq-Re2(CO)9(ClCH3). The relative stability of the ax-Re2-
(CO)9(CH4) and eq-Re2(CO)9(CH4), the weak binding energy
of eq-Re2(CO)9(CH4), and an estimated minimal barrier of 6-7
kcal/mol for solvation of eq-Re2(CO)9 with CH4 lead one to
propose the unsolvated eq-Re2(CO)9 as the thermodynamically
more stable species in hexane solution.
Proposed Solvation Process. Within the context of the
unsolvated eq-Re2(CO)9 being the most stable species, the
experimental observations are explained in Figure 6. Photodis-
sociation of an equatorial CO ligand creates a coordinatively
vacant site at the metal, which is quickly occupied by a solvent
molecule in the dense liquid environment within a few
picoseconds to form the eq-Re2(CO)9(hexane) solvate. The
equatorial solvate then vibrationally cools on a time scale of
69 ps. The vibrationally cooled equatorial solvate is stable on
the ultrafast time scale (<1 ns), but transforms to the unsolvated
eq-Re2(CO)9 within 50 ns after photolysis. That is, the hexane
solvate may traverse a free-energy barrier in the range of 5.2 <
∆G‡ < 7.5 kcal/mol to become an unsolvated eq-Re2(CO)9. The
theoretically estimated reorganization energy of 6-7 kcal/mol
provides a reasonable rationale for the minimum free-energy
barrier, although the nature of the transition state is not clear at
this point. The experimentally observed greater stability of eq-
Re2(CO)9(CCl4) over eq-Re2(CO)9(hexane) then can be under-
stood by the much greater solvation energy exhibited by eq-
Re2(CO)9(CCl4).
Conclusion
The ultrafast events that follow 295-nm photolysis of Re2-
(CO)10 have been studied in detail in the reactive, stronger-
coordinating CCl4, and in the inert, weaker-coordinating hexane
solutions. The experimental results for the Re2(CO)10/CCl4
system show that 22.5% of the initially excited Re2(CO)10
molecules dissociate to Re(CO)5 radicals that are able to escape
cage recombination and abstract a Cl atom from the CCl4
solvent. About 2.5% of the excited Re2(CO)10 molecules
dissociate to form Re(CO)5 radicals and react further to cross
the energy barrier to abstract a Cl atom on a time scale faster
than vibrational cooling. Such a nonequilibrium barrier crossing
can be understood by considering the excess energy the initiation
UV photon deposits on the Re(CO)5/solvent system. About 20%
of the excited Re2(CO)10 molecules dissociate through a Re-
COeq bond to form the equatorial solvate, eq-Re2(CO)9(CCl4),
which is stable up to the experimental time window of 2.5 µs.
Finally, about 55% of the excited parent molecules reverted to
(55) The barrier to solvation is likely to be equal to or greater than the
difference in energy between the two conformations. The relaxation energies
of the CH4 and CH3Cl complexed fragments do not contribute significantly
to the total relaxation energy (<1 kcal/mol) and are thus not included.

4210 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Yang et al.
Table 2. Summary of the Binding Energiesa
ax-Re2(CO)9(CH4)
eq-Re2(CO)9(CH4)
ax-Re2(CO)9(ClCH3)
eq-Re2(CO)9(ClCH3)
B3LYP
∆E
∆E + ∆ZPE
∆E + ∆ZPE + ∆H
CP
CP + ∆ZPE
CP + ∆ZPE + ∆H
-6.7
-7.7
-7.7
-6.1
-7.0
-7.0
-0.5
-1.0
-2.1
-5.2
-5.7
-6.7
-15.1
-15.8
-15.4
-13.5
-14.1
-13.8
-9.3
-10.0
-10.7
-14.3
-14.9
-15.6
a Note that the structural reorganizational energies for the Re2(CO)9 fragments are not included. See the text. In the table, ∆ZPE is the zero-point
energy correction, ∆H is the correction term for finite temperature (298 K) enthalpy, and CP is the counterpoise procedure for correcting the basis
set superposition error.
their electronic ground state, either via an internal relaxation
or through geminate recombination.
tion-solvation may help to better understand the roles of solvent
in photoactivation of multinuclear organometallic compounds.
In hexane solution, the recombination of Re(CO)5 radicals is
also regulated by solvent caging but without the competing
channel of atom abstraction as in the case of CCl4. Following
dissociation pathway (ii), the equatorial solvate eq-Re2(CO)9-
(hexane) is found to further react to form another species on a
time scale between 1 and 50 ns, or equivalently a free-energy
requirement of 5.2 to 7.5 kcal/mol. With the aid of quantum-
chemical calculations, this thermodynamically more stable
species is attributed to the unsolvated eq-Re2(CO)9. In this
picture, the energy requirement can be understood to stem from
the necessary structural rearrangement for eq-Re2(CO)9 to
accommodate a solvent molecule in the equatorial position.
This work may suggest that for metal radical species the
solvent can be treated as a viscous continuum, whereas for the
dinuclear complexes the solvent is best described in molecular
terms. This conclusion together with the notion of reorganiza-
Acknowledgment. This work was supported by a grant from
the National Science Foundation. The Office of Basic Energy
Science, Chemical Science Division, U.S. Department of Energy
contract No. DE-AC03-76FS00098 is acknowledged for special-
ized equipment used in the course of these experiments. The
authors thank H. Frei for the use of the step-scan FTIR
spectrometer and helpful discussions, and A. Rizvi for his
assistance with experiments.
Supporting Information Available: The Cartesian coor-
dinates for structures optimized at the DFT-B3LYP level of
theory (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA003344Y