Chapter 6
Primary and Secondary Processes in Organometallic Photochemistry T. R. Fletcher and R. N. Rosenfeld Department of Chemistry, University of California—Davis, Davis, CA 95616
Recent interest in the photochemistry of organometallic compounds is associated in part with the plethora of applications of this technology in chemical synthesis, homogeneous and heterogeneous catalysis and the deposition of metallic films. The field also provides fertile grounds for exploring some fundamental problems in polyatomic photochemistry, e.g. the relationship between electronic structure and reactivity, and the role of radiationless transitions in photochemistry. In fact, it is necessary to address problems of this type in order to develop a rational basis for utilizing the reactions of organometallic species in processes such as those noted. Here, we will discuss recent work from our laboratory on the photactivated dissociation reactions of transition metal carbonyls. The use of time resolved spectroscopic methods in probing photodissociation mechanisms and the structure and reactivity of dissociation products will be described. Several research groups have made significant contributions to the current understanding of metal carbonyl photochemistry and an excellent review has been published. The contributions of Turner, Poliakoff and co-workers have been particularly noteworthy. Their studies of simple metal carbonyls [e.g. Ni(CO) , Fe(CO) , Cr(CO) , etc.] in cryogenic matrices demonstrate that the dominant chemical channel following photoactivation (in condensed phases) is cleavage of a single metal-CO bond, resulting in the formation of CO and a mono-unsaturated metal center. Moreover, they have exploited this observation in obtaining infrared absorption spectra of a variety of unsaturated metal carbonyls. The information obtained from these spectra has dramatically refined our understanding of bonding and electronic structure in the transition metal carbonyls. Studies reported by Grant and co-workers have provided an impetus for much of the current research on gas-phase organometallic photochemistry. They have found that the products of the photolysis of Fe(CO)5 are active catalysts for olefin hydrogenation and geometrical isomerization and that reaction rates for catalyzed processes in the vapor phase can be substantially larger than in condensed phases. Yardley and co-workers have established that highly unsaturated metal centers can be prepared by the photolysis of Fe(CO)5 and Cr(CO)6 in the gas phase. This result is in marked contrast to those obtained in solution phase studies where only mono-unsaturated photoproducts are observed. Moreover, flash photolysis experiments by Breckenridge and co-workers and others indicate that unsaturated transition metal carbonyls can readily associate even 2
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HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY
with relatively "inert" species, e.g. Xe and N2. The findings noted here, as well as others, indicate several reasons why gas phase organometallic photochemical studies provide a useful complement to solution phase work: (i) It is possible to prepare and study mono-, bi- and tri-unsaturated organometallics in the gas phase, while this is generally not possible in solution. (ii) The intrinsic properties (i.e. spectra, kinetics) of unsaturated metal centers can be characterized in the vapor phase. Such properties can be perturbed by facile association with solvent molecules in condensed phases. (iii) The rates of catalyzed reactions in the gas phase can exceed the corresponding solution phase rates. This suggests that, in some cases, it may be advantageous to carry out synthetic procedures in the gas phase. The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved? What is the role of radiationless transitions? Second, what are the primary photoproducts? Are they stable with respect to unimolecular decay? Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent? Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. Consider the photodissociation reaction, (1), where the asterisk denotes rovibrational excitation. We have previously shown how measurements of the ro-
M(CO)
»
6
[M(CO) ]* + CO(v,J)
(1)
5
vibrational energy distribution of the C O product can provide information on the dynamics of fragmentation reactions. If the unsaturated product, M(CO)5, can be spectroscopically observed, then one should be able to obtain both structural information on this species and data on its reaction kinetics. This can all be accomplished, in principle, by infrared absorption spectroscopy, as previously described. * Briefly, M ( C O ) [0.01-0.03 torr] is contained in a one meter pyrex absorption cell where it may be mixed with an added gas, e.g. C O , NH3, etc. at 0-5 torr, and an inert buffer gas (He, Ar at 0-50 torr). The mixture is irradiated with a pulse of ultraviolet (UV) light from an excimer laser (193-351 nm, 1-5 mJ/cm ). The beam of a line tuneable, continuous wave C O laser is directed through the absorption cell, coaxially with respect to the U V laser beam, and then onto an InSb detector. The C O laser oscillates on P +i, (J) transitions, where ν = 0-11 and J = 8
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v
v
1
8-15. Thus, the region 2100-1830 cm" can be covered. This allows us to monitor CO(v,J) by resonance absorption and various M ( C O ) [n = 3-6] as a result of near coincidences between the C O laser lines and the carbonyl stretching vibrations of these species. The temporal response of the detection system is ca. 100 ns and is limited by the risetime of the InSb detector. Detection limits are approximately 10 torr for C O and M ( C O ) . The principal limitation of our instrumentation is associated with the use of a molecular, gas discharge laser as an infrared source. The C O laser is line tuneable; laser lines have widths of ca. l O ^ c m and are spaced 3-4 cm* apart. Thus, spectra can only be recorded point-by-point, with an effective resolution of ca. 4 c m . As a result, band maxima (e.g. in the carbonyl stretching n
-5
n
-1
1
-1
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FLETCHER AND ROSENFELD
Processes in Organometallic Photochemistry
101
1
region) cannot be located to better than 4 c n r and band shapes can be characterized only qualitatively. This is, nevertheless, sufficient to detect the various fragments, M ( C O ) , which can be generated by the photolysis of M(CO)£. To date, we have studied the photochemistry of W ( C O ) at 351 nm and Cr(CO) at 248 and 351 nm. An overview of our findings is presented below. Photochemistry of W ( C O ) at 351 nm. The irradiation of W ( C O ) n
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at 351 nm results in the direct population of the T state. * Dissociation to form W(CO)5 and C O may then occur from the T state or, following intersystem crossing, from the Αχ^ ground state. C O can be observed by time resolved C O laser absorption spectroscopy in vibrational states, ν = 0-2, following the 351 nm photoactivation of W(CO)g (see Figure 1). The vibrational energy distribution of the C O product is shown in Figure 2. Such data can be compared with distributions calculated using models for energy disposal in the fragmentation r e a c t i o n . » In this way, some insight regarding the photophysics of W ( C O ) ^ and its dissociation dynamics can be obtained. l g
3
l g
ι
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13
A simple phase space model can be used to compute the C O product 12
vibrational energy distribution as a function of the available energy, "
14
E
A V
. The
maximum energy which can be partitioned among the products' degrees of freedom is the reaction exoergicity, E
X
= hv-DH°[(CO)5W-CO]. For a 351 nm photolysis,
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E « 35 Kcal/mole. We find that the C O product vibrational distribution calculated using the phase space model with E = 35-40 Kcal/mole is in good agreement with our experimental results (Figure 2). Thus, the measured C O vibrational distribution indicates that vibrational energy disposal to the photolysis products is determined at a point on the potential surface where the full reaction exoergicity is available. This suggests that the 351 nm excitation of W ( C O ) G results in the sequence of events, (2)-(4), where the asterisk denotes vibrational excitation. X
A V
~1
351 ΪΙΪΪΪ
ο
1
WCCO^iX ^)
W(CO) (a T )
)
•
[W(CO) ( X A
(xU^)]*
•
3
W(C0) ( ï T 6
[W(CO)
6
3
•
l g
6
(2)
l g
1
6
)]*
W(CO) ( X ^ j ) +
(3)
1
00(Χ Σ*)
(4)
Photochemistry of C r ( C O ) at 351 nm. The excitation of C r ( C O ) at 6
3
351 nm populates the a T Â!T
l g
l g
6
state either directly, or via intersystem crossing from the
state. The measured C O product vibrational energy distribution is shown in
Figure 3, along with distributions calculated using the phase space model. In this case, the reaction exoergicity is E « 44 Kcal /mole. X
17
However, the calculated C O
product vibrational distribution, when the available energy equals the reaction
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HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY
(0 c
C
Time Figure 1. Time resolved absorption of the C O laser P j Q(10) line following the 351 nm photolysis of W(CO) . [W(CO) ] = 0.025 torr,'[He] = 4.0 torr. 6
6
8.00 9.00 10.00
2
0
V i b r a t i o n a l level, ν Figure 2. Vibrational energy distribution of the C O product formed via the 351 nm photolysis of W(CO)6. Experimental data are indicated a s f l . The lines correspond to results obtained by phase space calculations with an available energy of 40 and 35 Kcal/mole.
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Processes in Organometallic Photochemistry
FLETCHER AND ROSENFELD
103
exoergicity, is significantly hotter than that observed in the laboratory. Agreement between the phase space model and experimental results can be obtained only if the available energy is reduced to ca. 25 Kcal/mole. This indicates that an energy of approximately 19 Kcal/mole is not available to the products' vibrational degrees of freedom. The calculated (X A - a T ) energy splitting in Cr(CO) is 17 Kcal/mole. On the basis of these findings, we can propose that the photodissociation of Cr(CO>6 at 351 nm occurs via (5)-(6). This model is consistent with the results of solution phase quantum 19
l
3
lg
5
351 nm
~i l
CHCO) (X k ) e
l g
lg
Ci(COV.(a T 3
ο
)
•
Ci(CO) ( a T )
(5)
•
Ci
