J. Phys. Chem. 1985, 89, 5399-5401
5399
Multiphoton Ionization and Fragmentation of Transltion-Metal and Lanthanide ,&Diketonate Complexes J. B. Morris and M. V. Johnston* Department of Chemistry and C.I.R.E.S.(Cooperative Institute for Research in Environmental Sciences), University of Colorado, Boulder, Colorado 80309 (Received: June 26, 1985)
The ionization and fragmentation behavior of transition-metal and lanthanide @-diketonatecomplexes undergoing multiphoton absorption at 266 nm is presented. The dominant mechanism appears to be dissociation of the parent complex followed by ionization of neutral fragments. In most cases, the bare metal ion is the base peak in the mass spectrum. MO’, MF’, and MF2’ clusters (M = metal) may also be efficiently produced. Formation of these clusters can be rationalized on the basis of ligand rearrangement processes observed in electron impact ionization mass spectrometry although the product ions are different.
Introduction The mechanism of multiphoton ionization (MPI) of organometallic compounds in the gas phase has been extensively studied over the past few Metal complexes spanning a wide range of ligands have been investigated including carbonyls,’-4 metallocenes and other r-bonded c~mplexes,~” and a-bonded c o m p l e x e ~ . All ~ of these compounds appear to follow similar photochemical routes when subjected to intense visible or nearultraviolet radiation. Initially, one or more photons are absorbed by the parent complex causing extensive ligand dissociation. Photodissociation usually proceeds to the bare metal atom which is then nonresonantly ionized to give the bare metal ion. Wavelength dependence studies of the metal ion current have confirmed the presence of an intermediate metal atom through atomic r e s o n a n c e ~ . ~In* ~addition , ~ ~ to the bare metal ion, small metal clusters may also be generated., Although the mechanism of cluster ion formation is not as well understood, these observations suggest that MPI of organometallic compounds can provide a synthetic route for molecular as well as atomic species. In this paper, we present a study of the multiphoton dissociation and ionization pathways of transition-metal and lanthanide Pdiketonate complexes. Our motivation for this work is twofold. First, these complexes constitute a new class of ligands which are known to engage in rearrangement processes that leave new species bonded to the central metal atom during electron impact ioni~ a t i o n . If ~ these same rearrangements occur during MPI, new types of inorganic clusters could be produced. Second, @-diketonates find many applications in analytical chemistry including preparative extractions,8 separations by gas chromatography9 and liquid chromatography,I0 and trace metal determinations by gas (1) M. A. Duncan, T. G. Dietz, and R. E. Smalley, Chem. Phys., 44, 415 (1979). (2) (a) D. P. Gerrity, L. J. Rothberg, and V. Vaida, Chem. Phys. Lett., 74, 1 (1980); (b) L. J. Rothberg, D. P. Gerrity, and V. Vaida, J. Chem. Phys., 74, 2218 (1981); (c) J. A. Welch, V. Vaida, and G. L. Geoffroy, J . Phys. Chem.,, 87, 3635 (1983). (3) (a) D. A. Lichtin, R. B. Bernstein, and V. Vaida, J . Am. Chem. Soc., 104, 1830 (1982); (b) D. G. Leopold, and V. Vaida, J. Am. Chem. Soc., 105, 6809 (1983); (c) D. G. Leopold, and V. Vaida, J . Am. Chem. Soc., 106,3720 (1984). (4) P. C. Engelking, Chem. Phys. Lett., 74, 207 (1980). ( 5 ) (a) G. J. Fisanick, A. Gedanken, T. S. Eichelberger IV, N. A. Kuebler, and M. B. Robin, J . Chem. Phys., 75, 5215 (1981); (b) A. Gedanken, M. B. Robin, and N. A. Kuebler, J. Phys. Chem. , 86, 4096 (1982). (6) (a) S. Leutwyler, U. Even, and J. Jortner, Chem. Phys. Lett., 74, 11 (1980); (b) S. Leutwyler, U. Even, and J. Jortner, J . Phys. Chem., 85, 3026 (1981). (7) M. R. Litzow and T. R. Spalding, “Mass Spectrometry of Inorganic and Organometallic Compounds”, Elsevier, Amsterdam, 1973, pp 563-576. (8) K. W. M. Slu, M. E. Bednas, and S. S. Berman, Anal. Chem., 55,473 (1983). (9) R. W. Moshier and R. E. Sievers, “Gas Chromatography of Metal Chelates”, Pergamon Press, Oxford, 1965, p 124-1 30. (10) M. Saito, R. Kuroda, and M. Shibukawa, Anal. Chem., 55, 1025 (1983).
chromatography/mass spectrometry with isotope dilution.* The information gained from this study should aid in the development of new analytical methods based upon MPI of P-diketonate complexes. The ligands studied are all of the form:
where for acetylacetonate (acac), R, = R2 = CH,; trifluoroacetylacetonate (tfa), R1 = CH3, R2 = CF,; hexafluoroacetylacetonate (hfa), R1 = R2 = CF,; 2,2,6,6-tetramethyl-3,5-hepatanedionate (thd), R , = R2 = (CH,),C; 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate (fod), R, = (CH3)$, R2 = CF2CF2CF3. Experimental Section MPI spectra were generated by using the fourth harmonic of a Quanta-Ray DCR-2A Nd:YAG laser at 266 nm. All of the complexes studied exhibit broad charge-transfer transitions throughout this wavelength region ( E > IO3 in methanol) except for divalent manganese acetylacetonate which does not absorb appreciably ( E < 1 in methanol). The laser radiation of nominal pulse length of 5 ns was focused to a ca. 8 mm by 0.25 mm rectangle or to a ca.0.25 mm diameter spot in the ionization region of a time-of-flight mass spectrometer. Mass spectra were obtained with a modified CVC MA-002 time-of-flight mass spectrometer. The ion current vs. time profile from the electron multiplier was sampled by a LeCroy 3500SA signal averager at IO-ns intervals over a m / q range greater than 700 daltons. Mass spectra were averaged for 200 laser pulses and converted from the time domain to the mass domain by the calibration method described below. Relative ion abundances were measured by comparing peak areas. All of the compounds studied can be sublimed at elevated temperatures under vacuum. The thermal stabilities and volatilities of many of these complexes have been investigated by thermogravimetric analysis.” In no case is there evidence for significant thermal decomposition. However, irreversible adsorption and/or minor decomposition of some of the complexes has been observed during gas chromatographic analysis.12 To minimize the possibility of thermal decomposition in our work, chelate samples were vaporized off a glass insertion probe. This procedure avoids prolonged heating or exposure to reactive surfaces. Typically, 10 mg of sample was vaporized from the glass capillary over a period of several minutes. Nonfluorinated chelates (1 1) K. J. Eisentraut and R. E. Sievers, J . Inorg. Nucl. Chem., 29, 1931 (1967). (12) G. Guiochon and C. Pommier, “Gas Chromatography in Inorganics and Organometallics”, Ann Arbor Science Publishers, Ann Arbor, MI, 1973,
Chapter VIII.
0022-3654/85/2089-5399$01 .50/0 0 1985 American Chemical Society
5400 The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
were heated at a rate of ca. 50 'C/min to a final temperature of 175 "C while the more volatile fluorinated chelates were heated at ca. 30 "C/min to a final temperature of 75 'C. To confirm that extensive thermal decomposition was not occurring, several samples were vaporized by a similar procedure into a conventional magnetic mass spectrometer equipped with electron impact ionization. The mass spectra were found to agree well with published spectra.'.l3 A significant disadvantage of the insertion probe technique is the irreproducibility of vaporization kinetics from run to run which causes absolute ion abundances to vary. This variation constrains us to interpret our results on the basis of relative ion abundances only. Relative ion abundances were quite reproducible for spectra taken under similar conditions. Another problem encountered in this work was the poor effective resolution of the mass spectrometer. Although unit mass resolution up to m / q 200 is obtained on our instrument with organic molecules admitted via a molecular leak, unit mass resolution could only be obtained up to ca. m / q 100 with the chelates. Loss of resolution may have originated from the elevated temperatures required for vaporization, large kinetic energy releases accompanying ligand photodissociation, space-charge broadening, or ion-molecule reactions. In an effort to rule out space-charge broadening and ion-molecule reactions, we performed a qualitative pressure dependence study of the MPI mass spectra by varying the time at which mass spectra were recorded during the probe temperature ramp. This process allowed us to span greater than one order of magnitude of the chelate partial pressure. Both instrumental resolution relative ion abundances were found to be independent of chelate partial pressure. The resolution limitation precludes us from making an unambiguous assignment of "high" mass peaks. Below ca. m / q 100, unambiguous assignments can be made by calibrating the flight time ( T ) according to the equation m / q = E(T- b)/kI2
(1)
In this work, the constants k and b were determined from reference compounds having known mass spectra or from the isotopic distributions of bare metal ions observed in the chelate mass spectra. The compounds Cr(tfa),, Cr(hfa),, Fe(tfa),, C ~ ( t f a )Ni(hfa),, ~, La(thd),, Ce(thd)4, and Er(thd), were prepared by standard methods.I4 The compounds Cr(acac),, Mn(acac),, Fe(acac),, Co(acac),, Co(acac),, Ni(acac),, Eu(thd),, and Eu(fod), were purchased from various sources (>97% pure). All were used without further purification. Results and Discussion General Features of MPZ Mass Spectra. All of the P-diketonate complexes studied exhibit extensive fragmentation. In most cases, the bare metal ion is the base peak. Figure 1 shows the 266-nm multiphoton ion mass spectra of Cr(acac),, Cr(tfa)3, and Cr(hfa),, which are typical of the P-diketonates. Only Cr', CrO', and, with the fluorinated ligands, CrF' are produced. "Satellite" ions are due to the natural isotopic distribution of chromium. No other ions are observed between m / q 12 and 700. Ions observed in the mass spectra of other P-diketonate complexes are summarized in Table I. All of the compounds studied were found to have relative ion abundances independent of laser intensity from threshold (ca. 5 MW/cm2) to 200 MW/cm2. These abundances are also given in Table I. Even at the lowest laser intensities, no ions other than those listed in Table I were observed. While relative ion abundancies were found to remain constant, the total ion current increased with increasing laser power for a given beam size. However, no precise laser power dependence could be performed due to the irreproducibility of sample vaporization from run to
Morris and Johnston
9)
m/q
Figure 1. MPI mass spectra of (a) Cr(acac),, (b) Cr(tfa),, and (c) Cr(hfa), taken at a laser intensity of 130 MW/cm2.
TABLE I: Relative Ion Abundances in the MPI Mass Spectra of Several @-DiketonateComplexes compd
%M+
%MO+
Cr(acac), Cr(tfa), Cr(hfa), Mn(acac), Fe( acac), Fe( tfa), Co(acac), Co(acac), Co(tfa), Ni(acac), Ni(hfa), La(thd), Ce(thd), Eu(thd), Eu (fod), Er( thd),
100 100
16
100 a 100 100 IO0
6 2 a
%MF+
%MF2+
71 68 a
a
16
100
100 100
20
100 100
39
27 100
100
65 100
100
25
30
No ions were detected from this compound at any laser intensity up to 200 MW/cm2
run. It is interesting to note than an ion current could not be detected for Mn(acac), even at the highest laser intensities. This behavior is probably due to the low molar absorptivity of this compound in the near-ultraviolet region and suggests that a strong resonance enhancement at the excitation wavelength is required for efficient ionization. Ionization/Dissociation Mechanisms. Insight into the MPI mechanism of these complexes can be gained through metal-ligand binding energies and ionization potentials. The total ligand binding energy can be written as
(1966).
AH0(ML,+M0+3L.) = 3AHfo(Hacac) - 3AHf0(H.) + 3D(Hacac) - AHfo(ML3)+ AH?(M')
(14) R. C. Mehrotra, R. Bohra, and D. P. Gaur, "Metal @-Diketonatesand Allied Derivatives", Academic Press, London, 1978, pp 17-31,
for the example of trivalent metal acac complexes. Given standard
(13) C. G. MacDonald and J. S. Shannon, Aust. J . Chem., 19, 1545
(2)
Transition-Metal and Lanthanide @-DiketonateComplexes
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5401
heats of formation15 for the species in eq 2, the only uncertain term is the dissociation energy of a hydrogen atom (H.) from 2,4-pentanedione (Hacac). If we assume that this energy is roughly 75 f 10 kcal/mol, a value of 8.4 f 1.4 eV is estimated as the total ligand binding energy for F e ( a ~ a c ) ~yielding , an average binding energy of 2.8 eV per ligand. The energy required to remove the first ligand from a trivalent metal complex can be directly obtained from electron impact appearance potential data. In order to use this approach, the metal must form stable di- and trivalent complexes with the same ligand. The ligand dissociation energy is given by DP = A P - I P (3) where D P is the energy required to remove one ligand from a neutral trivalent metal complex, A P is the appearance potential of ML2+from the neutral ML3 complex, and IP is the ionization potential of the corresponding ML2 complex. Using published data for the cobalt and iron complexes,16one calculates first ligand binding energies of 2.2 f 0.2 and 1.3 f 0.2 eV, respectively. It should be noted that these are minimum values since thermal decomposition of the sample during vaporization into the ionization region could give incorrect ML2+ appearance potentials. If the ligand binding energies of Fe(acac), and Co(acac), are representative of other di- and trivalent metal 0-diketonates, the absorption of one photon at 266 nm (4.66 eV) should provide enough energy to dissociate at least one ligand. The exclusive production of M+ and other small ions from these complexes suggests that the dominant mechanism for MPI of transition-metal P-diketonates is dissociation of the complex to the bare metal atom or a small rearrangement cluster followed by ionization. For this process to occur, the ligand dissociation rate upon the absorption of one photon must be greater than the up-pumping rate of a second photon. (Absorption of two photons by the intact complex would exceed the molecular ionization potential for acac and tfa complexes and allow large molecular ions to be produced.) The total energy required to dissociate the complex and ionize the bare metal atom can be calculated for Fe(acac), by summing the total ligand binding energy given above and the ionization potential of atomic iron. This value, 16.3 f 1.4 eV, indicates that at least four photons are required to achieve ionization. In previous work involving other metal complexes, the dissociation followed by ionization mechanism was confirmed by the observation of atomic resonance^.^^^^^^ We could not perform this experiment since we did not have a stable vapor source over time. In contrast to MPI, electron impact causes ionization of the intact complex followed by dissociation to smaller units.' While the bare metal ion frequently appears with electron impact ionization, its abundance is low relative to ions of the form ML,+. Ligand fragments (L+ and smaller organic ions) are also observed in electron impact mass spectra. The absence of organic ions from the MPI mass spectra is surprising since we have found that the "parent" molecules of these ligands (for example, 2,4-pentanedione) ionize efficiently at similar laser powers and wavelength. These observations can be rationalized for the dissociation followed by ionization mechanism if the neutral ligand radicals produced by photodissociation do not undergo efficient MPI. Inorganic Clusters. Table I shows that P-diketonate complexes undergoing MPI can produce significant amounts of M P , MF2+, and MO+. Formation of these ions requires ligand fragmentation in addition to photodissociation. Although the exact mechanism is unclear, analogous processes are found in the fragmentation of chelate ions produced by electron impact i ~ n i z a t i o n . ' ~ , ' ~ (15) (a) J. D. Cox, and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds", Academic Press, London, 1970. (b) R. C. Weast, Ed., 'CRC Handbook of Chemistry and Physics", 58th ed,CRC Press, West Palm Beach, 1977. (16) C. Reichert and J. B. Westmore, Inorg. Chem., 8, 1012 (1969). (17) (a) A. L. Clobes, M. L. Morris, and R. D. Koob, J . Am. Chem. SOC., 91, 3087 (1969); (b) M. L. Morris and R. D. Koob, Inorg. Chem., 20, 2737 (1981).
Migration of fluorine atoms to the central atom in hfa and tfa complexes is manifested in electron impact ionization by the presence of ions such as LMF+ and LMF2+,and by the loss of neutral di- and trivalent metal fluorides." In MPI, MF+ was observed for every complex containing a fluorinated ligand except Co(tfa)2. These ions could originate from neutral mono-, di-, or trifluoride molecules formed during photodissociation. A difluoride ion is observed only for E ~ ( f o d ) ~ . Formation of MO+ requires cleavage of a C-0 bond. In electron impact ionization, C-O bond cleavage is indicated by the presence of ions such as LMO', LMOH', and, in the case of La(acac),, LaO+.I3 These ions are not observed in the electron impact mass spectra of the first row transition series and are most prevalent for complexes containing heavy metals. In MPI, C-0 bond cleavage is observed for C r ( a ~ a c )Cr(tfa)3, ~, Cr(hfa)3, La(thd),, Er(thd),, and Ce(thd),. For the chromium complexes, the ion is unambiguously assigned as CrO'. For the lanthanides, poor instrumental resolution and the resultant uncertainty in m / q scale calibration prohibits us from distinguishing between MO' and MOH'. When fluorinated ligands are present, fluorine migration completes favorably with C - 0 bond cleavage. Figure 1 shows that the ion abundance of CrO+ relative to Cr+ is smaller for the tfa and hfa complexes than for the nonfluorinated acac. The exact precursors to metal oxide and metal fluoride ions are not known. Since the electronic absorption spectra of many vapor-phase diatomic metal oxide and fluoride neutrals are available,'* it should be. possible to search for molecular resonances to confirm the presence of these species. However, these experiments will require a new vapor source of the chelates which is stable over time. Conclusion Transition-metal and lanthanide @-diketonatesundergo extensive dissociation when irradiated at 266 nm. In addition to the bare metal ion, inorganic clusters can be efficiently produced. Formation of these clusters requires similar types of ligand fragmentation that are found in electron impact ionization, but the end products are different. MPI tends to produce small clusters between the central metal atom and electronegative species while electron impact yields large organometallic ions. Although atomic and diatomic molecular resonances could not be investigated, it would appear that the dominant photochemical pathway of these species is photodissociation to the bare metal atom or small metal cluster followed by ionization. We are currently developing a continuous liquid introduction interface to our vacuum chamber which should provide a stable source over time and allow wavelength dependences to be checked. Note Added in Proof. Recently, Jones and c o - w ~ r k e r s 're~ ported the use of transition-metal P-diketonates in photochemical metal vapor deposition. They find that atomic copper is produced with a large amount of translational energy upon the irradiation of Cu(hfa), at 249 nm.lgc Acknowledgment. The authors thank Professor R. E. Sievers and co-workers for providing many of the P-diketonate metal complexes. This work was supported by the National Science Foundation under Grant CHE-8308049. Registry No. Crnacac)3, 21679-31-2; Cr(tfa),, 14592-89-3; Cr(hfa),, 14592-80-4; Mn(acac)2, 14024-58-9; Mn(acac),, 14284-89-0; Fe(acac),, 14024-18-1; Fe(tfa),, 14526-22-8; Co(acac)2, 14024-48-7; Co(acac),, 21679-46-9; Co(tfa),, 16092-38-9; Ni(acac)*, 3264-82-2; Ni(hfa)3, 14949-69-0; La(thd),, 14319-13-2; Ce(thd),, 18960-54-8; Eu(thd),, 15522-71-1; Eu(fod),, 17631-68-4; Er(thd),, 34750-80-6. (18) See, for example, K. P. Huber and G. Herzberg, "Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules", Van Nostrand-Reinhold, New York, 1979. (19) (a) C. R. Jones, F. A. Houle, C. A. Kovac, and T. H. Baum, Appl. Phys. Lett., 46, 97 (1985); (b) F. A. Houle, C. R. Jones, T. Baum, C. Pico, and C. A. Kovac, Appl. Phys. Lett., 46, 204 (1985); (c) E. E. Mariner0 and C. R. Jones, J . Chem. Phys., 82, 1608 (1985); (d) T.H. Baum and C. R. Jones, Appl. Phys. Let?., 47, 538 (1985).
