Further kinetics studies of intermediates formed by flash photolysis of Mo(CO)6

Article abstract of DOI:10.1016/j.ica.2008.01.035

Kinetics studies of the behavior of Mo(CO)₅(CH) in the presence of radical initiators were conducted in cyclohexane (CH) solution by laser flash photolysis with time-resolved infrared detection. Activation parameters were determined for reactio

Full text of DOI:10.1016/j.ica.2008.01.035

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Inorganica Chimica Acta 361 (2008) 3084–3088
www.elsevier.com/locate/ica
Further kinetics studies of intermediates formed
by flash photolysis of Mo(CO)6
Enrique Lozano Diz, Peter C. Ford *
Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara CA 93106-9510, USA
Received 12 December 2007; accepted 21 January 2008
Available online 7 February 2008
Dedicated to Professor Robert J. Angelici.
Abstract
Kinetics studies of the behavior of Mo(CO) (CH) in the presence of radical initiators were conducted in cyclohexane (CH) solution by
5
laser flash photolysis with time-resolved infrared detection. Activation parameters were determined for reactions of Mo(CO)
toluene and with photochemical radical generator dibenzylketone (DBK) in the presence of excess CO.
Ó 2008 Elsevier B.V. All rights reserved.
5
(CH) with
Keywords: Molybdenum; Carbonyl; TRIR; Kinetics; Mechanism
1
. Introduction
chemical behavior of cyclohexane (CH) solutions of
Mo(CO) under a CO atmosphere was recently reexamined
6
Although the chemistry and photochemistry of group
in this laboratory using a high pressure/variable tempera-
ture (HP/VT) flow cell that allows for examining the reac-
tions over much wider ranges of CO pressures (PCO) and
VI metal carbonyls have been extensively studied [1–3],
these and related complexes continue to attract the atten-
tion as catalyst precursors. An example is the carbonyl-
ation of ethylene catalyzed by molybdenum hexacarbonyl
with ethyl iodine [4,5], proposed to proceed by molybde-
num carbonyl radical intermediates. The present study
was undertaken in order to evaluate whether such interme-
diates might be generated by flash photolysis of a mixture
temperatures (T) than previously studied. In this manner,
z
an accurate activation enthalpy ðDH Þ was determined
CO
for the reaction of Mo(CO) (CH) with CO. This proved
5
to be significantly less than the Mo–CH bond enthalpy
determined previously, so an interchange dissociative (Id)
ligand substitution mechanism was proposed [10].
of Mo(CO) and a potential radical precursor in solution.
A question posed when initiating the present study was
whether these pathways would significantly altered by the
simultaneous generation of radicals. For example, dib-
enzylketone (DBK) has been shown to generate radicals
upon photolysis by a step-wise process to give two equiva-
6
As background, previous studies have shown that flash
photolysis of Mo(CO) solutions leads to immediate disso-
6
ciation of one CO (<1 ps) followed by formation of the sol-
vated complex Mo(CO) (sol) as identified by time-resolved
5
Å
Å
infrared (TRIR) and optical (TRO) spectroscopy (Eq. (1))
lents of the benzyl radical (Bz = PhCH ) plus CO with an
2
[
6]. The latter species undergo further reaction with CO
overall quantum yield of ꢀ0.7 [11]. There is significant
and other ligands at rates that are dominated by the
strength of the metal–sol interactions [7–10]. The photo-
overlap of the electronic spectra of DBK and Mo(CO) ,
6
so photolysis of cyclohexane solutions containing both
should lead to simultaneous benzyl radical formation from
the former and CO photodissociation from the latter
*
Corresponding author. Fax: +1 805 893 4120.
E-mail address: ford@chem.ucsb.edu (P.C. Ford).
(rCO ꢀ 0.75) [12,13] to give Mo(CO) (CH). The goal of
5
0
020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.01.035

E.L. Diz, P.C. Ford / Inorganica Chimica Acta 361 (2008) 3084–3088
3085
z
the present study was to evaluate whether such radicals
affected the products or reaction dynamics of the subse-
that DH
was less than the Mo–CH bond energy
CO
ꢁ
1
(45 ± 4 kJ mol ) determined by Burkey et al. [17] using
photoacoustical techniques, an interchange dissociative
quent reactions of the intermediate Mo(CO) (CH).
5
(Id) mechanism was proposed [10].
2
. Experimental
MoðCOÞ ðCHÞ þ COꢀMoðCOÞ þ CH
ð1Þ
5
6
2
.1. Materials
A cyclohexane solution of Mo(CO) and DBK would be
6
characterized by the overlapping absorption spectra of these
two species, so as a result, irradiation at either 308 nm (XeCl
excimer laser) or 355 nm (3rd harmonic of Nd:YAG laser)
results in excitation of both. After flash photolysis, the solu-
Cyclohexane and toluene (Aldrich) were dried and dis-
tilled under N according to published procedures [14].
2
Mo(CO) , and dibenzylketone (DBK) were purchased
6
from commercial sources and dried under vacuum prior
to storing in the glovebox without further purification.
All the solutions were prepared on the glovebox under
tion also include the solvento derivative Mo(CO) (CH) plus
5
Mo(CO) (DBK) and various radical intermediates. For this
5
reason, the flash photolysis of Mo(CO) was first studied in
6
Ar. The Mo(CO) , was weighed and dissolved in cyclohex-
the presence of an aromatic ligand (toluene) for which radi-
cal intermediates were not anticipated.
6
ane (CH), then precise volumes of toluene (TOL) and or
other reactants were measured with micro-syringes, such
that the concentrations were known with accuracy. Solu-
tion concentrations of CO were calculated from published
solubility data and the measured pressures [15]. All gases
used for flash photolysis experiments were passed through
an Oxyclear oxygen scrubber, all the samples were pre-
pared in an inert atmosphere glovebox under argon, and
the CO was 99.999% pure.
3.1. Reactions with toluene
Flash photolysis of the Mo/toluene/CO/cyclohexane
ꢁ
1
system led to a bleach on the signal at 1987 cm owing
to the depletion of Mo(CO) and the appearance of tran-
6
ꢁ1
sient absorbances (TA) at 1964 and 1930 cm correspond-
ing to the ‘‘prompt” formation of Mo(CO) (CH). New
5
ꢁ
1
transient absorbances (TA) at 1954 and 1920 cm
2
.2. Flash photolysis studies
(weaker), not seen for experiments in cyclohexane alone,
were also apparent and these were attributed to formation
Photolysis studies were carried out in a custom-built
of the complex Mo(CO) (Tol) (Tol = toluene) (Eq. (2)) by
5
HP/VT IR flow cell described previously [16]. This cell
allows excellent control of the reactant gas pressures
and the temperature while insuring that the sample will
not be subject to repetitive excitation to generate sec-
ondary photoproducts. Solutions were excited by pulsed
lasers, and the temporal IR absorbances were studied
using the time resolved infrared (TRIR) point-by-point
detection system described before [10,16]. The excita-
tion pump sources were either a Lambda Physik XeCl
excimer laser at (308 nm operating at 2 Hz) or a
Nd:YAG pulse laser using the third harmonic
analogy to similar Mo(CO) L spectra. The TA at
5
ꢁ1
1954 cm reached a maximum in 10–20 ls with a magni-
tude dependant on the toluene concentration [Tol]. In the
absence of CO, no appreciable further changes in the
ꢁ1
1954 cm absorption were observed on the experimental
timescale, which had an upper limit of about 1 s, but under
CO, the signal at decayed in the ms time-scale. The latter
process was assigned to be the reaction of Mo(CO) (Tol)
5
with CO to regenerate Mo(CO) (Eq. (3)).
6
MoðCOÞ ðCHÞ þ Tol ꢁ MoðCOÞ ðTolÞ þ CH
ð2Þ
ð3Þ
5
5
(
355 nm) operating at 2 or 10 Hz. The excitation pulses
MoðCOÞ ðTolÞ þ CO ! MoðCOÞ þ Tol
5
6
had an energy of ꢀ15–20 mJ/pulse with a width of 10–
2
0 ns.
Both the appearance and the slower disappearance of
ꢁ1
the TA at 1954 cm for a solution having a fixed CO pres-
sure of 6.1 atm (0.055 M) proved to be first order giving the
apparent rates constants kobs(2) and kobs(3) that were
dependent on [Tol]. The first step was accelerated by
increasing [Tol] and a plot of kobs(2) values versus [Tol]
3
. Results and discussion
In our recent photochemical study of Mo(CO) in cyclo-
6
hexane [10], the TRIR difference spectrum seen upon flash
photolysis (355 nm) exhibited three features in the mCO
region. These were the prompt transient bleach at
6
ꢁ1 ꢁ1
was linear with a slope ktol = 6.2 (± 0.7) ꢂ 10 M
s
(298 K). Thus the reaction of Mo(CO) (CH) with toluene
5
ꢁ
1
1
987 cm corresponding to depletion of Mo(CO) and
(Eq. (2)) is modestly faster than the analogous reaction
6
the prompt transient absorptions at 1965 (strong) and
with CO (see above). The disappearance of Mo(CO) (Tol)
5
ꢁ
1
1
931 (medium) cm corresponding to formation of the
in the second step was, in contrast, slowed by the higher
toluene concentrations as illustrated by the linear plot of
the reciprocal rate constant 1/kobs(3) versus [Tol] (Fig. 1).
This behavior is consistent with the formation of an inter-
mediate I in the reaction of the toluene complex with CO as
illustrated by Scheme 1.
pentacarbonyl intermediate Mo(CO) (CH). The back reac-
5
tion of Mo(CO) (CH) with CO to regenerate Mo(CO)
5
6
(
Eq. (1)) followed second order kinetics with a rate con-
6
ꢁ1 ꢁ1
stant kCO = 4.6 ꢂ 10 M
s
at 25 °C, and temperature
z
ꢁ1
dependence studies gave DH ¼ 33 ꢃ 3kJmol . Given
CO

3086
E.L. Diz, P.C. Ford / Inorganica Chimica Acta 361 (2008) 3084–3088
à
ꢁ1
ꢁ1
ꢁ1
ꢁ1
-
6
2
60x10
DS (apparent) = ꢁ32 ± 16 J K mol
for kobs(2) and
ꢁ
1
5
4 ± 6 kJ mol and 41 ± 24 J K mol , respectively, for
kobs(3). The DH for Eq. (2) is quite similar to that recorded
for the reaction of Mo(CO) (CH) with CO and sufficiently
less than the Mo(CO) -alkane bond enthalpy [17] of
5 ± 4 kJ mol . This suggests that the mechanism for Eq.
2) is also I . The substantially larger DH for Eq. (3) is likely
due to the more stable molybdenum-to-toluene bond in
Mo(CO) (Tol).
2
2
2
1
1
1
40
20
00
80
60
40
à
5
5
ꢁ
1
4
à
(
d
5
-3
20
30
40
50
60
70
80x10
3.2. Flash photolysis in the presence of dibenzyl ketone
[toluene] M
ꢁ
1
ꢁ1
Fig. 1. Plot of (kobs(2)) versus [Tol] for the slow decay of the 1954 cm
As noted above, the photolysis of DBK generates benzyl
5
band attributed to Mo(CO) (Tol) and generated by 308 nm flash
radicals. The mass spectra of the photoproduct solutions
indicated the formation of diphenylethane consistent with
this process. Furthermore, the FTIR spectrum of these
solutions during the photolysis showed the progressive dis-
appearance of the mCO band at 1718 cm characteristic of
DBK.
photolysis of Mo(CO)
.1 atm CO.
6
(0.72 mM) in 298 K cyclohexane solution under
6
ꢁ
1
k1
Mo(CO) (Tol)
I + Tol
5
k-1
Flash photolysis of a cyclohexane solution of Mo(CO)6
plus DBK under CO with TRIR detection shows prompt
signals corresponding to the formation of the transient
k2
Scheme 1.
I + CO
Mo(CO)6
ꢁ
1
complex Mo(CO) (CH) at 1964 and 1930 cm and the
bleach at 1987 cm
5
ꢁ1
corresponding to depletion of the
Mo(CO) . Subsequently, three new bands appear at 1944
6
Such an intermediate I might be Mo(CO) or, more
ꢁ1
5
(strong) and 1920 (medium) cm that we attribute to the
likely, Mo(CO) (CH). For this scheme, the observed rate
5
formation of Mo(CO) (DBK) by analogy with similar
5
constant would be Eq. (4), and a plot of the reciprocal
ꢁ1
products such as Mo(CO) (Tol), etc. The 1964 cm band
ꢁ1
5
(
kobs(3)) versus [Tol] is predicted to be linear with a
of Mo(CO) (CH) decayed on the ms time scale, at a rate
5
ꢁ
1
slope = k /k k [CO] and an intercept = k . The respec-
ꢁ
1
1
2
1
similar to the appearance of Mo(CO) (DBK) (Fig. 2).
5
tive values of the slope and intercept in Fig. 1 are
ꢁ1
The temporal behavior of the 1944 cm band in the TRIR
ꢁ
3
ꢁ1
ꢁ6
(
2.3 ± 0.1) ꢂ 10 s M
and (93 ± 9) ꢂ 10 s. These
experiment was analogous to that described above for the
toluene complex; that is, it grew in exponentially in the first
few ls, then decayed on the ms time-scale. A key difference
4
ꢁ1
would give k = ꢀ1.1 ꢂ 10 s and a value for the ratio
1
k
/k of 1.33 ± 0.2. This ratio is remarkably close to value
2
ꢁ
1
of 1.35 for the ratio k /kCO, where ktol is the second order
1
tol
was that it appeared faster, and disappeared several orders
rate constant for the formation of Mo(CO) (Tol) (Eq. (2))
5
of magnitude more slowly, than the comparable signals for
the toluene complex under similar conditions. No other
transient IR bands were seen that could be attributed
to prospective intermediates such as the radicals
described above and kCO is the second order rate constant
for the reaction of Mo(CO) (CH) with CO to reform
5
Mo(CO) (Eq. (1)) as independently measured [10]. This
6
agreement strongly suggests that I is Mo(CO) (CH). If this
Å
Å
5
Mo(CO) (Bz) or Mo(CO) (C(O)Bz).
5
5
is the case, it may further be argued that the ratio k /k is
tol
1
Two excitation wavelengths (kirr = 355 nm or 308 nm)
the equilibrium Ktol for constant for the formation of
were utilized for the flash studies, but the kinetics results
were essentially equivalent (within experimental uncertain-
ties), so data from the 355 nm experiments will be reported
here. In addition, varying the CO pressure over the reac-
tion solution from PCO = 0 (argon atmosphere) to
Mo(CO) (Tol) from Mo(CO) (CH) plus toluene in cyclo-
5
5
2
ꢁ1
hexane solution (Eq. (2)), the value being ꢀ6 ꢂ 10 M
.
k
1
k
2
½COꢄ
kobsð3Þ ¼ k
ð4Þ
ꢁ1
½Tolꢄ þ k ½COꢄ
2
6.1 atm had no measurable effect on the rate for appear-
ance of Mo(CO) (DBK) but showed a small, systematic
5
The apparent activation parameters of the reactions indi-
cated in Eqs. (2) and (3) were determined by measuring the
kobs(2) and kobs(3) values at 25, 30, 35, 40 and 45 °C upon
increase in the rate for disappearance of Mo(CO) (CH).
This would be expected given the much slower rate for
5
reaction of the latter with CO. As Table 1 shows, the
à
flash photolysis of a cyclohexane solution of Mo(CO) for
DH s determined for the disappearance of Mo(CO) (CH)
6
5
a specific set of conditions with [toluene] = 18 mM and
PCO = 2.5 atm. Several independent sets of experiments
were carried out under these conditions using different
excitation and monitoring wavelengths, with reasonable
and for the appearance of Mo(CO) (DBK) are
5
1
For example, with [L] = 28 mM and PCO = 6.1 atm, the formation of
5
ꢁ1
6
ꢁ1
Mo(CO)
5
L displayed kobs values of 6.7 ꢂ 10
s
and 6.6 ꢂ 10
s for
à
ꢁ1
agreement for DH (apparent) = 34 ± 4 kJ mol
and
L = Tol and DBK, respectively.

E.L. Diz, P.C. Ford / Inorganica Chimica Acta 361 (2008) 3084–3088
3087
à
DH is about half the enthalpy of Mo–CH bond dissocia-
tion enthalpy, so we again discount a limiting dissociative
pathway as being the dominant mechanism for this substi-
tution reaction.
MoðCOÞ ðCHÞ þ DBKꢀMoðCOÞ ðDBKÞ þ CH
ð5Þ
5
5
In this context we might consider two scenarios. The first
would be that formation of Bzꢅ radicals, which occurs con-
currently with the photodissociation of CO from Mo(CO)
6
owing to the overlapping absorptions at the excitation
-1
0
1
2
µs
3
4
5
2
wavelength, catalyzes the substitution of DBK for CH in
à
the Mo coordination sphere thereby lowering DH . The sec-
ond would be that the mechanism of ligand substitution
with DBK as the entering ligand L is considerably more
associative in character than when L = CO or Tol owing
to its greater nucleophilicity. The higher donor strength of
DBK is evidenced by the lower mCO values seen for IR spec-
trum of Mo(CO) (DBK) relative to those seen for L = Tol
5
or CH [18].
The effect of varying DBK concentration on the kobs(5)
values for the appearance of Mo(CO) (DBK) is summa-
5
rized in Table 2. Notably, increasing [DBK] consistently
leads to increasing kobs(5), although the net effect is not
strictly linear. Nonetheless, at the lower concentrations of
DBK, the apparent second order rate constant derived
0
2
4
µs
8
ꢁ1 ꢁ1
from the ratio kobs(5)/[DBK] is ꢀ3 ꢂ 10 M
s
at
298 K. This would more than a factor of 50 more than that
for L = CO and falls in a reactivity series DKB >
n
PrBr > Tol ꢀ CO consistent with the Lewis base strengths
of such ligands. As a consequence, it would appear that the
reaction of Mo(CO) (CH) with various ligands L is occur-
5
ring by an interchange mechanism with increasing impor-
tance of Mo–L bond formation (associative character)
with the more nucleophilic ligands.
0
.0
0.1
0.2
0.3
0.4
s
With regard to the prospective radical-catalyzed path-
way, the data in Table 2, argue strongly against this possi-
bility, since a larger percentage of the energy from the
initial flash is absorbed by DKB at higher [DBK]. As a con-
sequence, substantial concentrations of Bz radicals are ini-
tially formed (in concentrations comparable to Mo(CO)5-
Fig. 2. TRIR absorbance changes after flash photolysis (kirr = 308 nm) of
a solution of Mo(CO) and dibenzyl ketone in cyclohexane under 6.10 atm
of CO (298 K): (a) at 1964 cm (disappearance of Mo(CO)
time scale; (b) at 1944 cm (formation of Mo(CO)
scale; and (c) at 1944 cm (disappearance of Mo(CO)
timescale.
6
ꢁ1
5
(CH)), 5 ls
(DBK)), 5 ls time
(DBK)), 500 ms
ꢁ
1
Å
5
ꢁ1
5
(CH)) and the generation of such radicals represents an
increasingly larger fraction of the total photoreaction at
these higher [DBK]. Yet the nonlinearity of data in Table
Table 1
2
represents a decreasing net activity of DBK as its concen-
Apparent activation parameters determined for 355 nm flash photolysis of
a cyclohexane solution of Mo(CO)
.1 atm CO
6
(0.0014 M) and DBK (0.014 M) under
tration increases, not the increased activity expected if the
reaction were radical-catalyzed. An even more telling
experiment was one where a 298 K cyclohexane solution
6
à
à
Monitoring frequency
DH
kJ mol
DS
ꢁ1
ꢁ1
ꢁ1
(
)
(J K mol )
of Mo(CO) (0.42 mM) and DBK (18.2 mM) was subjected
6
ꢁ
1
1
1
1
964 cm , disappearance of
18 ± 3
20 ± 3
71 ± 3
ꢁ65 ± 12
to flash photolysis at 308 nm under an argon atmosphere
with the flash intensity varied by a factor of five from
Mo(CO) (CH)
5
ꢁ1
944 cm , appearance of
Mo(CO) (DBK)
ꢁ79 ± 12
5
mJ to 25 mJ. Although this should have raised the
5
ꢁ
1
944 cm , decay of Mo(CO)
5
(DBK)
+37 ± 12
2
The different absorption of the chemicals at 308 and 355 nm shows that
considerably lower than for analogous reactions of
ꢁ3
ꢁ2
our work solution (1.43 ꢂ 10 M in Mo(CO)
6
and 1.4 ꢂ 10 M in DBK)
Mo(CO) (CH) with CO only or with CO and toluene,
5
at 308 nm ꢀ33% of the light is absorbed by the Mo, while at 355 nm ꢀ50%
à
and the DS values are more negative. Furthermore, this
is absorbed.

3088
E.L. Diz, P.C. Ford / Inorganica Chimica Acta 361 (2008) 3084–3088
Table 2
Rate constants kobs(5) measured for the formation of Mo(CO)
from Mo(CO) (CH) in cyclohexane solution (298 K) initiated by flash
photolysis at 355 nm
Acknowledgement
5
(DBK)
5
a
This research was supported by a Grant (DE-FG03-
5ER13317) from Division of Chemical Sciences, Office
8
6
ꢁ1
8
ꢁ1 ꢁ1
[
DBK] (mM) obs(5) (in 10 s
k
)
k
obs(5)/[DBK] (in 10 M s )
of Basic Energy Research, US Department of Energy.
1
1
1
2
2
3
1.4
4.3
7.5
0.7
8.5
8.0
3.0 (± 0.2)
3.6 (± 0.2)
3.9 (± 0.2)
4.8 (± 0.3)
6.6 (± 0.2)
7.7 (± 0.2)
2.7
2.6
2.2
2.3
2.3
2.0
References
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9
4 (1991) 7937.
a
[
Mo(CO)
6
] = 0.76 mM, PCO = 6.1 atm CO. The rate constant kobs(5)
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[3] Y. Shvo, R. Green, J. Organomet. Chem. 675 (2003) 77.
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ꢁ1
was determined for the appearance of the strong mCO band at 1942 cm
assigned to Mo(CO)
5
(DBK).
[
5] J.R. Zoeller, E.M. Blakely, R.M. Moncier, T.J. Dickson, Catal.
Today 36 (1997) 227.
Å
concentrations of Bz fivefold, there was no impact whatso-
ever of flash intensity on the first order rate constant kobs(5)
for the formation of Mo(CO) (DBK) from Mo(CO) (CH).
[6] S. Zhang, V. Zang, H.C. Bajaj, G.R. Dobson, R. van Eldik, J.
Organomet. Chem. 397 (1990) 279.
5
5
[
[
[
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weakly coordinating solvent cyclohexane show that the
first formed intermediate Mo(CO) (CH) reacts with vari-
ous ligands L give the Mo(CO) L complexes via a rate
6
5
[10] C. Kayran, M. Richards, P.C. Ford, Inorg. Chim. Acta 357 (2004)
143.
5
law that is essentially second order. The second order rate
[11] W.K. Robbins, R.H. Eastman, J. Am. Chem. Soc. 92 (1970) 6076.
[12] T. Jiao, G.-L. Leu, G.J. Farrell, T.J. Burkey, J. Am. Chem. Soc. 123
n
constants k follow an order L = DBK > PrBr > toluene
L
(
2001) 4960.
consistent with donor abilities of such ligands. Simulta-
[13] M. Polikoff, Faraday Trans. 2 (73) (1977) 569.
Å
neous generation of benzyl radicals PhCH in significant
2
[14] J.A. Riddick, W.B. Buner, T.K. Sakano, Organic Solvents: Physical
Properties and Methods of Purification, John Wiley & Sons, New
York, 1986.
solution concentrations had little or no influence on this
ligand substitution reaction. Given the well known effects
of radicals on catalyzing other ligand substitution reactions
of metal carbonyls, a likely explanation of the absence of
[15] Carbon Monoxide, IUPAC Solubility Data Series, vol. 43, Pergamon
Press, Oxford, 1990.
[16] S.M. Massick, P.C. Ford, Organometallics 18 (1999) 4362.
similar effect on Mo(CO) (DBK) formation is that the high
5
[17] T. Jiao, G.-L. Leu, G.J. Farrell, T.J. Burkey, J. Am. Chem. Soc. 123
(2001) 4960.
[18] R.J. Angelici, S.M.D. Malone, Inorg. Chem. 6 (1967) 1731.
reactivity of the Mo(CO) (CH) system makes it insensitive
5
Å
to such catalysis by a relatively stable radical such as Bz .