6168
J. Phys. Chem. 1987, 91, 6168-6172
Competition between Radiative Pumping and Nonradiative Processes in the Multiphoton Dissociation of Organometaltic Iron Complexes Joseph J. BelBruno,* Peter H. Kobsa, Richard T. Carl, and Russell P. Hughes Department of Chemistry, Dartmouth College, Hanouer, New Hampshire 03755 (Received: February 2, 1987: In Final Form: July 10, 1987)
The multiphoton ionization (MPI) spectra of Fe(CO),, Fe(CO),(v-C&6), Fe(CO)3(v4-C6H8),Fe(CO),(v4-C8H8),two isomers of Fe(C0),(C8F8),and Fe(C0),(v2-C8H8) have been recorded over the laser wavelength region from 445 to 451 nm in order to clarify the relationship between the organic ligand and the final electronic states of the photochemical products. In ail cases, the spectra of the final products were primarily assigned to two-photon resonances originating in the three lowest electronic levels of Fe. The existence of a background current was also established. The availability of C8F8complexes having nearly identical densities of vibrational states, but different bonding characteristics, permitted an examination of the relative importance of the density of states versus bond energetics in controlling the final disposition of the excitation energy in multiple-photon-induced dissociation of organometallic complexes. The distribution of Fe electronic states was shown to be dependent upon the nature of the bonding interaction, in addition to intramolecular energy transfer due to the availability and density of ligand vibrational modes. These results are in agreement with and extend the conclusions of an earlier report describing a similar study of d6 complexes.
Introduction Multiphoton ionization (MPI) and multiphoton dissociation (MPD) studies of organometallic compounds, particularly the metal carbonyls, have been the subject of a series of recent research reports.l-1° The nature of the dissociation mechanism for metal carbonyls has also been examined in solution,” although the order of the absorption process in the latter is probably unity. In these experiments, fragmentation was favored over further up-pumping and parent molecule ionization. The primary product was found to be neutral metal atoms, in a variety of electronic states, from which ionization later occurred. This was in stark contrast to the majority of organic molecules which were initially ionized with subsequent dissociation, often to atomic fragmentsi2 Recent studies have described the relative populations of the metal atoms produced by MPD/MPI as a function of the organic ligand(s) coordinated to the metal atom. The first report6 described the MPD/MPI products Of Cr(CO)6, Cr(C0)3C&6, and cT(c6H6)~.The ratio of the relative excited-state to ground-state Cr populations was discussed in terms of the decreasing bond exiergy across the members of the organometallic series. A more recent study’, reported preliminary data regarding the effect of changing the single ligand in a series of q6 Cr carbonyl complexes. Although no quantitative values were reported, the authors attributed the resultant distribution of neutral chromium atoms to the effects of intramolecular vibrational relaxation (IVR), suggesting that the density of ligand vibrational modes is the controlling factor in the final energy partitioning. Later, a series of iron complexes were examined14 and the highest electronic level (1) Liou, H. T.; Ono, Y.; Engelking, P. C.; Moseley, J. T. J . Phys. Chem. 1986, 90, 2888. (2) Huang, S. K.; Gross, M. L. J . Phys. Chem. 1985, 89, 4422. (3) Gerrity, D. L.; Rothberg, L. J.; Vaida, V. J . Phys. Chem. 1983, 87,
2222. (4) Welch, J. A.: Vaida, V.; Geoffroy, G .L. J . Phys. Chem. 1983, 87. 3635. (5) Leopold, D. G.; Vaida, V. Loser Chem. 1983, 3 , 49. (6) Fisanick, G . J.; Gedanken, A.; Eichelberger, T. S.; Knuebler, N. A,; Robin, M. R. J . Chem. Phys. 1981, 75, 5215. (7) Engelking, P. C. Chem. Phys. Lett. 1980, 74, 207. (8) Gerrity. D. P.; Rothberg, L. J.; Vaida, V. Chem. Phys. Lett. 1980, 74, 1. (9) Duncan, M . A.; Dietz, T. G.; Smalley, R. E. Chem. Phys. 1979, 44, 415. (10) Karny, Z.; Naaman, R.; Zare, R. N . Chem. Phys. Lett. 1978,59, 33. ( 1 1) See, for example: Welch, J. A,: Peters, K. S.; Vaida, V. J . Phys. Chem. 1982, 86, 194. (12) See, for example: Michelsen, H. A,; Guigliano, R. P.; BelBruno, J. J. J . Phys. Chem. 1985, 89, 3034 or Giugliano, R. P.; BelBruno, J. J. J . Photochem. 1987, 37, 263. (13) Hossenlopp, J. M.; Rooney, D.; Samoriski, B.: Bowen, G.: Chaiken, J. Chem. Phys. Lett. 1985, 116, 380.
0022-3654/S7/2091-616S$01.50/0
of Fe populated in the photolysis was found to correlate with the density of states of the ligand, just as with the d6 Cr complexes. Unfortunately, the molecules chosen for the iron study were structurally unrelated, so that it was difficult to more than qualitatively connect the results to a molecular property. Chaiken and co-workers have recently expanded upon their early work involving the chromium complexes and reported quantitative data for the relationship between the density of vibrational states and the final distribution of metal atom quantum levels.15 Intuitively, one might expect that both the total dissociation energy and the availability of “bath” modes for the redistribution of the initial molecular excitation would be significant factors in the dissociatin dynamics, but this hypothesis has not yet been directly tested. One way in which the current work differs from earlier Fe studies is that we have examined a related series of monosubstituted organometallics. A second difference, particularly with respect to the Cr work, is that the initial excitation of the target molecules in the current work was via two-photon, rather than single-photon, excitation. Finally, a novel aspect of the current work is that the complexes under study include three molecules with the same organic ligand, but very different in the nature of the bonding of that ligand to the metal atom. The goal of this study was to, by this judicious choice of subject molecules, evaluate the relative importance of intramolecular energy redistribution and dissociation energy to the branching ratios in MPD/MPI and to test whether or not the previously reported conclusions with respect to d6 complexes were applicable to d8 molecules as well. The iron carbonyl complexes included in this study were Fe(CO),, as a reference system, Fe(Co),(~-C,H,), Fe(C0),(v4C6H8),Fe(C0),(v4-C8H8),two isomers of Fe(CO),C8F8, and Fe(C0),(v2-C8F8). The availability of the C8F8complexes was ideal for this study, since the density of states for these molecules remains constant, if compared at the same excitation energy, while the bond strengths vary. The structures of the complexes containing CsF8 ligands are shown in Figure 1. The density of states for 1-111, and especially for I and 11, is not significantly different. Certainly, the difference is small in the comparison of 1-111 with the remaining complexes. However, the nature of the bonding in these three molecules was substantially different. Complex I was an v4 molecule, as were the other monosubstituted carbonyls. Compound I1 contained an allyl (v3)plus a u ( a ’ ) interaction. Finally, 111 was an v2 molecule. The finer details of the metal(14) Nagana, Y.; Achiba, Y.; Kimura, K. J . Phys. Chem. 1986,90. 1288. ( I 5) Samoriski, B.; Hossenlopp, J. M.; Rooney, D.; Chaiken, J. J . Chem. Phys. 1986,85, 3326. Hossenlopp, J. M.; Samoriski, B.; Rooney, D.; Chaiken, J. J Chem. Phys. 1986, 85. 3331.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Dissociation of Organometallic Iron Complexes F
F
TABLE I: Spectral Intensity Ratios for the eSD,
Seriesa molecule Fe(CO)5 Fe(CO)&H6 Fe(CO)3C6H8 Fe(C0)3C8H8 Fe(CO),C8F8 (1) Fe(CO)3-
F
I
I1
F
F F
C8F8
5D4
5D3
-
6169
aSDJMultiplet
5D*
9
0
100 32.1 (1.9) 15.0 (2.5) 5.87 (2.17) 3.90 (1.61) 100 IO0 100 100
35.7 32.5 31.2 33.0
(2.7) (6.4) (2.5) (0.8)
100 29.0 (3.2)
14.4 (2.6) 14.2 (2.7) 10.1 (1.1) 11.5 (1.2)
4.87 5.79 3.41 5.63
(0.93) (0.50) (0.36) (0.82)
2.66 2.73 2.34 2.87
(1.10) (0.63) (0.32) (0.68)
13.7 (1.7) 7.23 (3.64) 2.12 (1.41)
(I1)
Fe(C0)4C8F8 100 37.0 (4.4) 16.3 (2.3) 6.50 (1.32) 2.23 (0.56) I11
Figure 1. Structural representations of the C8F8containing iron carbonyls.
ligand bonding will be reserved for discussion in the appropriate section.
Experimental Section The output from an excimer-pumped dye laser (Lumonics T861-S/EPD-330) passed through a 15 cm focal length quartz lens into the cell containing the sample vapor. The experimental cell was of the now standard parallel plate design,16having a plate spacing of 2.5 cm with an applied potential of 300 V. Laser pulse energies were of the order of 1 mJ over a pulse duration of approximately 15 ns. For any given sample, the relative intensities of the transitions were independent of laser energy for pulses in the range of -400 MJto 1 mJ, but the data in this work were obtained at the higher energy. Samples were routinely employed at a pressure of 0.02 Torr, with occasional experiments at pressures up to 0.1 Torr. Increasing the sample concentration to this level had no observable effect on the recorded relative peak areas. Spectra were obtained over the single-photon wavelength region from 445 to 451 nm by using a Coumarin 450 laser dye. The current, in wavelength steps of 0.005 nm (laser line width -0.003 nm), was recorded by a Stanford Research Systems boxcar averager and stored in a laboratory microcomputer for later analysis. Peak areas rather than heights were used for comparison of the relative Fe populations. Special care was taken to ensure that all reagents were studied under identical conditions of applied electric field, laser focusing, laser polarization (linear) and total pressure. Fe(CO)5, Fe(CO),(v-C,H,), Fe(C0)3(v4-C6H8), and Fe(C0),(v4-CsHs) were purchased from Strem Chemical and used without any further purification. The Fe(CO),C8FS isomers and Fe(Co),(v2-C8FS) were synthesized for use in these experiments. Details of their preparation may be found e1sewhere.I’ Ultraviolet absorption spectra were recorded on a HewlettPackard 8452B diode array spectrometer at a resolution of 2 nm. Results The gas-phase UV spectra of the target molecules, with the exceptionI8 of Fe(CO)5, have not previously published. We have recorded spectra for many of the complexes used in this study and present a representative sampling of this information in Figure 2. The general spectral features of all of the complexes, except 11, were identical. These spectra consisted of a weak shoulder with a maximum at -40000 cm-’ and a stronger absorption peaking at -47000 cm-’. In Fe(CO)5, the first absorption band has been assigned to a ligand field transition (dxy, d,2+ dZ2) and the higher energy transition to a Fe a* C O absorption.18 Analogous arguments lead to similar identificiation of the tran-
-
-
(16) Johnson, P. M.; Otis, C. E. Annu. Reu. Phys. Chem. 1981, 32, 139. (17) (a) Fe(C0)3(?3-CsF8):Barefoot, A. C.; Corcoran, E. W.; Hughes, R. P.; Lemal, D. M.; Saunders, W. D.; Laird, B. B.; Davis, R. E. J. Am. Chem. SOC.1981, 103, 970 and Hughes, R. P.; Samkoff, D. E.; Davis, R. E.; Laird, B. B. Organometallics 1983, 2, 195. (b) Fe(C0)4(v2-C8F8):ref 17a and Carl, R. T.; Hughes, R. P.; Davis, R. E., to be published. (c) Fe(C0)3($-C8F8): Carl, R. T.; Hughes, R. P.; Davis, R. E., to be published. (18) Goeffrry, G.L.; Wrighton, M. S . Organometallic Photochemistry; Academic: New York, 1979 and references therein.
“he values represent the ratio of the area of the transition relative to that of the 5D4 5D4. The numbers in parentheses are the standard deviations. +-
TABLE 11: Relative Spectral Intensities“for the Higher Electronic
States of Fe m o1ecu 1e Fe(CO15
5F
Fe(C0)3C4H6
Fe(CO)3Cd& Fe(CO),C8Hs Fe(C0)3C8F8
(I)
Fe(CO)&F8 (11) Fe(CO)&F8
1.11 0.25 1.27 1.19 0.27 1.94 1.21
(0.83) (0.14) (0.14) (0.52) (0.08) (0.65) (0.26)
3F 6.58 6.65 3.36 2.71 1.63
(2.03) (1.32) (0.67) (0.65) (0.27)
b 2.75 (0.52)
-
“Specific transitions are discussed in the text. The results are relative to the 5D4 5D4transition, and the standard deviations are given in the parentheses. *Not measurable. sitions shown in Figure 2. The high-energy ligand field transitions are a reflection of the Fe(0) oxidation state and the electron-rich nature of the molecules. The exception to this general trend, in which the spectral features varied in energy according to the substitution of a different ligand, was the Fe(CO),CsF, isomer, 11. This molecule contains an iron atom which is formally Fe(+2) d transition appears at -600 nm, a region typical and the d for this type of absorption. Excitation of the parent molecule for MPD/MPI was always at a one-photon energy of -22 000 cm-’ (450 nm). The complexes have been shown to lack absorption bands at this wavelength, and the initial excitation is inferred to be two-photon. The two-photon energy is noted by an arrow in Figure 2, which indicates that, in all cases, the initial absorption probably resulted in the promotion an electron to a a* orbital of the molecule. Based upon published energy levelsI9for Fe and bond energiesz0 for Fe(CO)5, at least six photons are required to supply sufficient energy to both dissociate Fe from the complex and ionize the atom. Measured power indexes were consistently less than this value, indicating that one or more steps in the process were saturated. The MPI technique did not allow for the extraction of absolute electronic populations due to the unknown two-photon transition moments and ionization probabilities. It was feasible to use the ratio of the areas of two particular bands to construct relative population data. These relative populations could then be compared as a function of organic ligand to discern the dynamical constraints on the photosystem; Le., the ratio of the ratios as a function of the ligand is the significant quantity. Experimental Data. The most intense spectral lines were observed between 447.2 and 448.4 nm, and quantitative efforts were concentrated in this region of the spectrum, although data were obtained over a much broader spectral region. A typical scan of this wavelength region is shown in Figure 3, which presents the MPD/MPI spectrum of Fe(C0)3(q4-C&s) at 0.02 Torr. The e5DJ a5DJtransitions, the fsF4 a5F4transition, and an unidentified transition which originates on the a3F3level have been identified in that figure.z’ The areas under the spectral features +
-
-
( 1 9) Moore, C. E. “Atomic Energy Levels”. Narl. Bur. Sfand. ( U S . )Circ. 1952, No. 467, Vol. 11.
(20) Deeming, A. J. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1982.
6170 The Journal of Physical Chemistry, Vol, 91, No. 24, 1987
\
V W
z
2a 0
v,
m
a
i W
\
a
FdC O)3C4H6
b 0 0 m
0 0 Ln
0
s
0
0
a
X.nm
Figure 2. Gas-phase UV absorption spectra for several of the molecules used in the MPD/MPI experiments. The spectra have been offset for
clarity of presentation, so that the ordinate represents relative optical density for a single curve. No relationship exists among the scales for each absorption curve.
I
m 0
%
m
0
447.69
447.88
N
4 48.07
448.26
1. nm Figure 3. A portion of the MPI spectrum of Fe(C0)3(q4-C6Hs) with the transitions used in this work indicated.
originating from excited Fe atomic levels were strongly dependent on the specific iron molecule subjected to MPD/MPI. In Table I, the values of the integrated areas of the lines belonging to the a5D, multiplet are presented, relative to the e5D4 a'D, e'D, area. The ratios appear to be independent of parent molecule. No correction for the J dependence of the oscillator strengths has been made in Table 1. Indeed, the actual dependence is still a matter of current research. Our concern is with the variation (or lack of such) from molecule to molecule. The values reported in Table I (and Table 11) represent the average of a minimum of three spectral scans, and typically, data from six or more determinations were used to arrive at the ratios reported in the tables. Table I1 contains areas for the transitions originating at the next two electronic states of neutral Fe, relative to the transition discussed above. The data in the table have been divided into four categories according to the nature of the metal-ligand bonding. In contrast to the ground electronic state, the population of the excited levels is molecule-specific. A quick glance at the 3F results
-
+-
(21) The assignment of this transition as originating at the a3F3level was made in ref 14 via photoelectron measurements. No definite assignment of the resonant intermediate state could be made.
BelBruno et al. indicates that a trend is present, but the actual structure-population relationship must still be elucidated. The SFdata are less conclusive. The numbers in parentheses represent standard deviations, but for the SFdata, the actual uncertainty is generally somewhat larger since the MPI signal intensity is approaching the intensity of the background. No trends may be discerned from the SFdata, but in a qualitative sense, the ratios impart information consistent with the conclusions derived below from the triplet-state results. No other Fe electronic states were observed in the experiments. The available energy from an overall six-photon process would not be sufficient to populate any higher states. Certainly, a higher order process is possible, but since the probability of such a transition decreases rapidly with the required number of photons, the likelihood of detecting the small number of Fe atoms resulting from such a photolysis is limited. Indeed, other workers have detected small quantities of iron atoms in levels as high as the twelfth electronically excited stateI4 by studying the source of the background ion signal and the origin of the numerous weak, unassigned lines that appear in the spectrum obtained by multiphoton-induced photoelectron spectroscopy. The apparatus used in the current experiments was not of sufficient sensitivity to record such a small current, and the source of our background was not directly determined. The laser pulse energy used in these experiments (- 1 mJ) was chosen so that a maximum signal intensity could be obtained while avoiding the broadening which accompanies higher energy pulses. At energies just greater than 1 mJ the line widths significantly increased due to the ac Stark effect and lifetime broadening. The power indexes for the iron tricarbonyls were determined to be in the range from 2.1 to 2.3, indicating that several of the steps in the MPD/MPI process were saturated under the energy and focusing conditions of the study. Logically, any one-photon steps as well as the strongly allowed two-photon resonant transition in the neutral iron atom would be expected to be saturated. The controlling factor in the overall process would then be the initial two-photon absorption and MPD of the parent molecule. Under such conditions, the iron MPI peaks would be indicative of the molecule specific nature of the dissociative process.
Discussion The most obvious conclusion from Table I, which the contains the experimental ratios without any statistical factors, is that neither the nature of the organic ligand nor the particular bonding of that ligand to the iron atom is significant with respect to the dissociative process leading to ground-state iron atoms. The distribution of Fe among the five J states of the quintuplet D is almost constant from one iron complex to another. The most reasonable explanation for this behavior is that the absorption process which leads to bare iron atoms prepares a system which is always well above the threshold for production of any of these angular momentum states as final products. This conclusion is consistent with the fact that even the highest energy level of this multiplet, the 'Do, is only 1000 cm-I above19the 5D4ground state. In the case of Fe(CO)S,the absorption of a total of three photons would result in an excess energy of 2.2 eV after dissociation of all the carbonyl groups. This energy is well above the threshold for population of any of the a5D levels. The substituted tricarbonyls would result in excess energies somewhat less than 2.2 eV but still well above the highest ground electronic state multiplet a5D threshold. Assuming, arbitrarily, that all of the eSD transitions are saturated (a reasonable assumption based on the power indexes), then the areas under the spectral lines represent populations of the aSDjstates. A plot of In ( N J / g J )versus E j indicated that, within experimental error, the population was statistical and characteristic of a temperature of -900 K, regardless of the molecular precursor. These results are qualitatively consistent with the recent report involving MPD/MPI of ferrocene.' In that study, most of the iron produced in the dissociation was found to be in the ground state and the statistical distribution of those molecules was representative of a system at 1200 K. The degeneracy factor, gj, in this calculation contains only the
-
-
Dissociation of Organometallic Iron Complexes
+
25 1 degeneracy. This assumes that the line strengths are independent of spin-orbit state, an assumption made necessary by the absence of any theoretical relationship for those quantities. A similar statistical population was observed in a previous study3 involving Cr(C0)6. Significant differences may be observed in the relative populations shown in Table 11. For the moment, the discussion will be restricted to the 3Fstate. Concentrating on the four q4 complexes grouped in the center of the table, one observes that the relative population of Fe(C0),(q4-L) decreased with increasing complexity of L. Structurally, these are equivalent molecules with only a different organic ligand. KimuraI4 used the ratio of ligand vibrational modes to the number of metal-ligand bonds as a measure of the density of states in the complex. Since these are all q4 carbonyls, such a ratio is unnecessary and one may, for the first three, use the number of ligand modes as a crude measure of the density of states. Obviously that value decreases down Table 11. The C8F8 complex may not be treated so simply since it has the same number of vibrational modes as C8H8. However, the energy of C-F vibrational motions is considerably less than that for C-H and the density of states in the C8H8 complex may be roughly assigned a value several times greater than that of C8H8. This is a conservative estimate since the true density of states may be estimated classically as proportional to ( l/hw)s-l,where s is the number of vibrational modes. Therefore, the trend was consistent across the series of four molecules and the initial conclusion is that the relative populations of the accessible electronic levels of Fe are indeed controlled by the availability of “bath” modes for redistribution of the initial excitation energy. This view is reinforced if one examines the total dissociation energies for the molecules in question. The bond dissociation energy is expected to influence the final energy disposal, perhaps a significantly as geometry or the nature of the ligand substituent. The bond enthalpies for Fe(CO),(q4-L) where L = C,H6, C6H8, and C8H8 are 5 . 8 , 5.7, and 5.6 e v , respectively.zz A crude argument based solely on the metal-organic ligand bond energy would predict a small effect, but in the opposite direction of that which was experimentally observed. The Fe(Co),(q4-C8F8) molecule (1) does, however, complicate this interpretation. While the density of states is considerably larger than the other three complexes and the relative population of the 3F excited state is lower, the bond energy is also increased over that for the L = C8H8 molecule. In this case, the estimate is that the fluorocarbon has a dissociation energy of approximately 6.1 eV,z3and the basis for the further decrease in the population of the upper energy level might seem unclear. However, using the results in Table I1 for Fe(CO),, one may draw the conclusion that the bath modes were the major factor, since the total bond enthalpy for the iron pentacarbonyl, 6.1 eV,20 is identical with that of molecule I and MPD/MPI of Fe(CO)5 results in significant population of the higher electronic states. This reasoning is consistent with the 5 F relative population, which appears to be much less for I than for the pentacarbonyl. The argument presented above appears to support the theory that the density of vibrational states is the controlling factor in the disposition of initial excitation energy and that the redistribution of that energy throughout the organic ligand leads to selectivity in the final atomic energy levels. However, the data in Table I1 permit a further test of the importance of bond dissociation energy. Molecules I, 11, and I11 all contain only a C8F8 ligand in a substituted carbonyl. However, the detailed molecular bonding in these complexes is very different. I is an q4 carbonyl that is in the same category as the hydrocarbons discussed above. I1 has the identical molecular formula but may be regarded as (22) Connor, J. A,; Demain, C. P.; Skinner, H. A,; Zafarani-Moattar, M. T. J . Organomet. Chem. 1979,170, 117. An average Fe-CO bond strength of 1.22 eV was employed. (23) This value has a much larger uncertainty since the total bond enthalpy has not been measured and could only be estimated based on the reported value for L = C,H8 and the ‘typical” strengthening of the metal-ligand bond when fluorines are substituted for hydrogens.20 This estimate is most likely a lower limit.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6171 the oxidative product of I; Le., the iron atom may be though of as having the +2, rather than the 0, oxidation state. The two bonds simply denoted in Figure 1 are actually an q3 allyl interaction and a metal-ligand u bond. The total bond dissociation energy for this molecule is very difficult to estimate, but based on ref 20, 22, and 24, the following approximation may be used. The Feligand bond enthalpy for an q3 allyl interaction is not significantly different, on the average, from that of an q4 interaction.z0 This infers that the estimated dissociation energy of I1 is 6.1 eV (the bond energy of I) plus the bond energy of the Fe-C u bond; an estimate of 7.5 eV is conservative. Finally, I11 is an qz olefin complex and the Fe-C bond energy is approximately one-half of that for an q4 molecule.z0 The energies of the M-CO bonds for the pentacarbonyl are acceptable values for substituted carbonyls as so the total bond enthalpy of I11 is estimated to be 5.9 eV. In all three of these molecules the density of vibrational states is approximately the same, and to the first approximation, any differences in product-state distributions should be attributed to the changes in the energy required to completely dissociate the complex. Table I1 indicates that the results bear out this prediction. The populations of both the SFand 3Flevels depend upon the bond energy, increasing with a concomitant decrease in total bond enthalpy. As the extreme example, I1 exhibited no observable population of the high energy 3F state. Earlier reports, from other laboratories, have attributed the final product distributions in the MPD/MPI of organometallic complexes either to the total bond energy6 or to energy redistribution among the ligand vibrational modes.14J5 The current research explored the effect of these two factors through the careful selection of a series of Fe tricarbonyl complexes. Not unreasonably, the result is that both are important and the dominant factor clearly depends upon the specific situation. It is, therefore, not possible to make sweeping generalizations. Several of the earlier papers in this field have contained possible mechanisms for gas-phase MPD of organometallic complexes.15925,z6 In the case of the arene chromium tricarbonyls, ChaikenI5 has suggested two possible mechanisms. One involves competition between two-photon ( 1 + 1 ) dissociation to produce ground-state metal atoms and fragmentation following the initial single-photon excitation with subsequent two-photon dissociation of the coordinately unsaturated complex. The latter process leads to production of excited-state chromium atoms. The authors were not able to differentiate this mechanism from one involving competition between a ( 1 + 2) dissociation of the parent complexes leading to excited-state chromium production and IVR after initial excitation followed by absorption of a second photon with production of ground-state metal atoms. These two mechanisms are suggestive of possible pathways in our own experiments, but our initial excitation involves a coherent two-photon absorption so that significant modifications would be necessary. Grantz5has published the most complete report on the gas-phase decomposition of iron pentacarbonyl. The experiments again involved excitation in a region of the spectrum leading to a ( 1 1 ) type of two-photon process. The results indicate that production of species such as Fe(CO), (via detection of the appropriate mass selected ion) occurred via parent ion fragmentation or iron-molecule reactions. The atomic iron production was postulated to be a result of fragmentation in the neutral ladder. The competitive mechanisms were molecular ionization followed by dissociation and molecular fragmentation with subsequent atomic ionization. This report is of direct relevance to the current work, even though our initial excitation is two-photon. Finally, Engelking26 has recently described the decomposition of ferrocene in terms of a direct excitation to a dissociative potential surface. These experiments were also initiated via two-photon excitation and might be applicable in our work. However, the repulsive states correlating with the
+
(24) Brown, D. L. S . ; Connor, J. A,; Leung, M. L.; Paz-Andrade, M. I.; Skinner, H. A. J . Organomet. Chem. 1976, 110, 79. ( 2 5 ) Whetten, R. L.; Fu, K. J.; Grant, E. R. J . Chem. Phys. 1983, 79, 4x99. . .
(26) Liou, H. T.; Engelking, P. C.; Ono, Y.;Moseley, J. T. J . Phys. Chem. 1986, 90, 2892.
6172 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
-
120
-
100
-
80
-
m
>-'
0
60
a
40
~
F e L + 3CO
20
-
-Fe(CO)2L
+
CO
F i W e 4. Energy level heme for F ~ ( C O ) S ( V - C ~ HThe ~ ) . solid arrows represent the energy of a 450-nm photon. Nonradiative processes are represented by wavy arrows. Two possible pathways are shown. are contained in the text.
various neutral iron electronic levels require a minimum energy of 1.5 eV. The hypothetical potential energy curves used in that report should be equally valid in the present case if the mechanism were applicable. Certainly, the absolute intensity of the Fe spectral features per unit detection time decreases with decreasing excess excitation energy, but in our examples it never approaches zero, even at 0.8 eV of excitation. For this reason, we cannot extend the validity of the direct reaction to the our tricarbonyl systems. It is difficult to firmly establish a mechanism for the dissociative process based on the results we have presented above. The reported trends lend credence to the conclusion that the complexes all decompose according to the same process, even though the total (minimum) excess energy varies from -2.7 eV to as little as -0.8 eV. The strong influence of the ligand density of states might indicate that the organic species remains bonded to the metal for a substantial period of time during the fragmentation process. A possible mechanism, consistent with our data and earlier report^*^*^^ on organometallic MPD, is shown in Figure 4. Although the energetics of the 1,3-butadiene complex are represented, the remaining molecules would have similar energy level diagrams. The initial simultaneous, two-photon excitation populates S2 in the parent molecule. Based upon the work of Whetten et a1.,25the most likely scenario is that the C O ligands are boiled off leaving a Fe-L fragment behind. The absorption of an additional photon by the Fe-L fragment results in formation of iron atoms which then experience MPI. Given similar bond energies for the Fe-L fragment, the transfer of energy to the ligand becomes the determining factor in the availability of energy for production of excited-state Fe. Figure 4 shows formation of Fe-L in the lowest energy state. The actual distribution of energy in the fragment is unknown, but some of the energy distributed into ligand modes
BelBruno et al. would then be unavailable for deposition into atomic iron. The influence of increasing bond dissociation energy in this scheme is obvious. The mechanism is consistent with the absence of any significant laser energy dependence (at modest energies). The Fe ionization is via allowed transitions, and only the initial excitation requires multiple photons. The total intensity of any transition depends on the laser energy, but not the population ratios. Since the areas of the various lines examined in this study vary with precursor and all exhibit power indices of -2, the most likely controlling step in the dissociation (the unsaturated step) is the initial two-photon absorption. The competitive processes for this fragmentation are (1) the absorption of additional photons from Szof the Fe(CO)3L molecule leading to molecular ionization and then dissociation (not shown in Figure 4) and (2) absorption from the S2state of Fe(CO),L followed by rapid dissociation of all of the ligands (shown in Figure 4). The fact that the nonresonant background currents, which are believed to result from molecular ionization,zs in the MPD/MPI spectra are relatively small is indicative of the absence of molecular ionizationzs and the rapid nature of the dissociative process. It is difficult to rule out the (2 + 1) resonant multiphoton absorption with subsequent dissociation of superexcited Fe(CO)&. However, this alternative would also exhibit the Same dependences on laser energy and ligand observed in our experiments, Experiments which identify the transients involved in this Drocess would helo to chose from the two neutral dissociative patiways. The mechakstic discussion has not included the final deposition of the CO or the organic ligands. Clearly, some of the energy may be carried in the diatomic moleculezs or be involved in fragmentation of the organic molecule. With the exception of simple diatomic molecules?5 the fate of the ligands in the MPD/MPI scheme is unknown.
Conclusions The branching ratios of MPD/MPI products of substituted iron tricarbonyls vary with changes in both the nature of the single ligand and the amount of energy required to completely dissociate the molecule. Increasing the density of states in the organic ligand serves to increase the extent of energy redistribution and concomitantly reduce the energy deposited in Fe electronic levels. For complexes with the same organic ligand, but different bonding interactions, the relative populations of the excited states of Fe increase with decreasing total bond dissociation energy. However, even in these circumstances, energy redistribution remains the dominant factor if population comparisons are made with complexes having similar dissociation energies. These results are consistent with an earlier study of a series of arene chromium tricarbonyls. The implication is that the same factors which are relevant to the molecular dynamics of d6 molecules are equally important in d8 systems. While the evidence is certainly not conclusive, the results were consistent with a stepwise dissociation along the neutral dissociative ladder leading to Fe.
Acknowledgment. R.P.H. acknowledges partial support of this work by the Air Force Office of Scientific Research under Contract No. AFOSR-86-0075. Registry No. Fe(C0)5, 13463-40-6; Fe(co),(?~-c,H,),12078-32-9; F ~ ( C O ) ~ ( ? J ~ - C12152-72-6; &+), Fe(CO)3(~4-CsH8), 12093-05-9;Fe(CO),(C,F,), 76624-81-2; Fe(CO),(q2-C8Fs), 76634-42-9.
