Organometallics 1995,14, 2395-2399
2395
Matrix Photochemistry of Mn2(CO)lo: Reversible Formation of Mn2(CO)8 from Mn2(CO)&-q1:q2-CO) Frank A. Kvietok and Bruce E. Bursten" Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Received January 18, 1995@ The CO-loss photochemistry of Mnz(C0)lo (1) has been investigated using frozen-matrix photochemical techniques. A solution of 1 (0.6 mM) in neat 3-methylpentane is cooled to 96 K to form a frozen matrix. Initial irradiation of the matrix with broad-band light from a medium-pressure Hg lamp produces the known single-CO-loss product Mn2(CO)&-q1:q2CO) (3). Continued irradiation leads to additional loss of CO and the formation of new IR bands. When thermal back-reactions in the matrix a t 96 K are monitored, two additional products are identified. One is Mn2(C0)8 (81, a double-CO-loss product from 1 that has only terminal CO ligands and may possess a Mn-Mn triple bond. The other product is an isomer of 3,Mnz(C0)g' (91, which likely has a semibridging CO (YCO 1728 cm-l). Under thermal conditions a t 96 K, 8 and 9 reversibly reconvert to 3. The photochemistry of dinuclear organometallic complexes generally exhibits two distinct photochemical channels, namely cleavage into mononuclear radicals and loss of a single ligand while maintaining a dinuclear framework. The dinuclear metal carbonyl complex Mnz(C0)lo (1) is a prototypical member of this class of compounds, and its air stability, high symmetry, and incontrovertible metal-metal bond have contributed to the interest in its photochemistry. Early photochemical studies of 1 elegantly developed an understanding of the various products generated upon irradiation. Initially, much attention was focused on homolysis of the Mn-Mn bond, a process that yields 'Mn(C0)5 radicals (2h2 Later, transient absorption experiments by Vaida et al. provided evidence for the existence of an alternative photochemical product, which was suggested to be a dinuclear CO-loss specie^.^ Kobayashi et al. used solution laser flash photolysis to explore the nature of this dinuclear species and proposed it t o be Mnz(C0)g (3),which reacts rapidly with various substrates to form Mnz(C0)gL specie^.^ Detail concerning the structure of 3 was provided by matrix isolation ~ t u d i e swhich ,~ demonstrated that irradiation of 1 leads to a CO-loss complex containing a semibridging CO, formulated as Mn2(C0)s(p-v1:v2-CO)(3). More recent studies have continued exploring the formation and reactivity of the CO-loss species 3 and related @Abstractpublished in Advance ACS Abstracts, April 15, 1995. (1)(a)Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry, Academic Press: New York, 1979. (b) Meyer, T. J.; Caspar, J. V. Chem. Rev. 1985,85,187-218. (2)(a)Wrighton, M.S.; Ginley, D. S. J . Am. Chem. SOC.1975,97, 2065-2072. (b) Wrighton, M. S.; Abrahamson, H. B. J . Am. Chem. SOC.1977,99, 5510-5512. (c) Hughey, J. L., IV, Anderson, C. P.; Meyer, T. J. J . Orgunomet. Chem. 1977,125,C49-C52. (d) Waltz, W.L.; Hackelberg, 0.;Dorfman, L. M.; Wojcicki, A. J . Am. Chem. SOC. 1978,100,7259-7264. (e) Church, S.P.; Poliakoff, M.; Timney, J. A.; Turner, J. J. J.Am. Chem. SOC.1981,103,7515-7520. (0 Wegman, R. W.; Olsen, R. J.; Gard, D. R.; Faulkner, L. R.; Brown, T. L. J . Am. Chem. SOC.1981,103,6089-6092. (g) Walker, H. W.; Herrick, R.; Olsen, R. J.; Brown, T. L. Inorg. Chem. 1984,23,3748-3752. (3)Rothberg, L. J.; Cooper, N. J.; Peters, K. S.; Vaida, V. J . Am. Chem. SOC.1982,104,3536-3537. (4) Yesaka, H.; Kobayashi, T.; Yasufuku, K.; Nagakura, S. J . Am. Chem. SOC.1983,105,6249-6252. ( 5 )(a) Hepp, A. F.; Wrighton, M. S. J . Am. Chem. SOC.1983,105, 5934-5935. (b) Dunkin, I. R.; Harter, P.; Shields, C. J. J . Am. Chem. SOC.1984,106,7248-7249.
species using fast6 and ultrafast' spectroscopy in solution. Gas-phase experiments have also richly contributed t o our present understanding of possible CO-loss pathways and species.8 Very recently, Brown and co-workers have reported new photochemical studies of 1 in a matrix of 3-methylpentane (3-MP)a t 93 K.9 They provided evidence for the formation of the solvent0 species Mnz(C0)g-S(4; S = 3-MP) upon photodissociation of CO in the matrix. Species 4 decays into the semibridged species 3 by a first-order process with a half-life of ca. 140 s at 93 K. Our current interest in the matrix photochemistry of 1 stems from recent studies in our laboratories on photochemical double CO loss from dinuclear organometallic complexes. For example,we have reported that the well-known complex Cp2Fe2(C0)2(p-C0)2(5; Cp = v5-C5H5)undergoes two stepwise CO losses in a 3-MP matrix a t 98 K.I0 Loss of the first CO generates the triply bridged intermediate CpzFe&-CO)a (6).11 Continued irradiation of 6 leads t o loss of a second CO and the formation of [CpFe(CO)lz (71,an unusual species inasmuch as it contains two terminal CO ligands and apparently an unsupported Fe-Fe triple bond. We have found similar behavior in the matrix photochemistry of a methylene-bridged analogue of 5 , namely [Cp*Fe(CO)]Z(~-CO)@-CH~), although in this case, the doubleCO-loss product contains a methylene bridge and a semibridging CO ligand.12 ~~
(6)Church, S. P.; Hermann, H.; Grevels, F.-W.; Schaffner, K. J . Chem. SOC.,Chem. Commun. 1984,785-786. (7)(a) Zhang, J. Z.; Harris, C. B. J . Chem. Phys. 1991,95,40244032. (b) Waldman, A.; Ruhman, S.; Shaik, S.; Sastry, G. N. Chem. Phys. Lett. 1994,230,110-116. (8)(a) Leopold, D. G.; Vaida, V. J.Am. Chem. SOC.1984,106,37203722. (b) Seder, T. A,; Church, S. P.; Weitz, E. J . Am. Chem.SOC.1986, 108,1084-1086. (c) Seder, T.A.; Church, S. P.; Weitz, E. J . Am. Chem. Soc. 1986,108,7518-7524.(d) Prinslow, D. A,;Vaida, V. J . Am. Chem. SOC.1987,109,5097-5100. (9) (a) Zhang, S.; Zhang, H.-T.; Brown, T. L. Organometallics 1992, 11,3929-3931. (b) Zhang, H.-T.; Brown, T. L. J.Am. Chem. SOC.1993, 115,107-117. (lO)Kvietok, F. A,; Bursten, B. E. J . Am. Chem. SOC.1994,116, 9807-9808. (ll)(a)Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1980,102, 7794-7795. (b) Hooker, R. H.; Mahmoud, K. A,; Rest, A. J . Chem. SOC., Chem. Commun. 1983,1022-1024. (c) Hepp, A. F.; Blaha, J. P.; Lewis, C.; Wrighton, M. S. Orgunometallzcs 1984,3,174-177.
0276-733319512314-2395$09.00/00 1995 American Chemical Society
2396 Organometallics, Vol. 14, No. 5, 1995
Kvietok and Bursten
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Figure 1. Progressive IR spectra during the irradiation of Mnz(C0)lo (1)in 3-methylpentane at 96 K (all abscissas are in cm-l; the ordinates are absorbance): (a) spectrum obtained after 0 min of irradiation; (b) spectrum obtained after 1.5 min of irradiation; (c) spectrum obtained after 3.0 min of irradiation; (d) spectrum obtained after 30 min of irradiation. Note that the scale of absorbance for (a) is different from that for (b), (c), and (d).
Figure 2. Progressive differences of the IR spectra presented in Figure 1 (all abscissas are in cm-l; the ordinates are A(absorbance1): (a) difference between spectra obtained after 1.5 and 0 min of irradiation (Figure l b - Figure la); (b) difference between spectra obtained after 3.0 and 1.5 min of irradiation (Figure IC- Figure lb); (c) difference between spectra obtained after 30 and 3.0 min of irradiation (Figure Id - Figure IC).Note that the scale of A(absorbance1 for (a) is different from that for (b) and
We were curious as to what similarities there might be between 5 and its trichoric13 cousin 1, particularly with respect to multiple photochemical CO loss. Previous studies of 1 have discussed the existence of secondary photolytic processes, although detailed characterization data of the resulting species have been scarce.&ydJ4 We present here new matrix photochemistry that indicates the formation of new species from the secondary photolysis of the CO-loss product 3, including a double-CO-loss product, Mnz(CO)s, that contains only terminal CO ligands.
(C).
Experimental Section Standard Schlenk and inert-atmosphere techniques for handling air- and water-sensitive compounds were employed in these studies. Mnz(CO)lo (Strem Chemicals, Inc.) was purified by sublimation and stored under an Ar atmosphere in a refrigerator prior t o use. 3-Methylpentane(Aldrich)was dried over Na/K alloy and stored under an Ar atmosphere. The matrix studies were performed with a Specac P/N 21500 variable temperature cell equipped with CaFz windows. The (12) Spooner, Y. H.; Mitchell, E. M.; Bursten, B. E. Submitted for
publication. (13)King, R. B. Inorg. Chem. 1966,5,2227-2230. (14)Kobayashi, T.; Ohtani, H.; Noda, H.; Teratani, S.; Yamazaki, H.; Yasufuku, K. Organometallics 1986, 5 , 110-113.
apparatus consists of a sample cell held inside a vacuumjacket by a compartment that also serves as the refrigerant holder. The path length of the cell is 1 mm. Liquid NZ was used as the refrigerant for all experiments. The irradiation source was a 200-W broad-spectrum,medium-pressure Hg-vapor UV/vis lamp (Oriel)filtered with water. All IR spectra were collected on a Nicolet Magna 550 FT-IR spectrophotometer (32 scans, 1.0 cm-l resolution). Difference spectra were generated using a scaling factor of 1;i.e., each difference spectrum represents the exact subtraction of two spectra,with no additional scaling. Slow warming of the matrix was achieved through thermal contact with ambient conditions (roomtemperature) following removal of the refrigerant from the apparatus. The warming and recooling process did not affect the band shape or intensity of stable speciesor generate any spurious signals in a matrixsolvent-only control experiment. Comparative peak heights were measured from unsubtracted spectra at a constant temperature. Results and Discussion When a matrix of 0.6 mM 1 in 3-methylpentane (3MP) at 96 K is irradiated with a n unfiltered mediumpressure Hg lamp, we observe significant changes in the IR spectrum of the matrix. Figure 1 presents the IR spectra prior to irradiation and after 1.5, 3, and 30 min
Matrix Photochemistry of MnZ(C0)lo
Organometallics, Vol. 14, No. 5, 1995 2397
0.35-
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cni’ Figure 3. Difference IR spectrum from the irradiation and thermal back-reaction of 1in 3-methylpentane at 96 K. The spectrum shown is the difference A - B, where A and B are defined as follows: (A) the spectrum obtained after irradiation of the matrix for 30 min followed by 70 min of thermal (dark) reaction at 96 K; (B) the spectrum obtained immediately after irradiation of the matrix for 30 min.
of irradiation. The progressive changes in the species in the matrix are clarified by the difference spectra presented in Figure 2. After 1.5 min of irradiation, the difference IR spectrum reveals the conversion of 1 to 3 and the evolution of CO, consistent with previous findings (Figure 2ah5 Additional illumination (3 min) leads to the continued production of 3 and CO a t the expense of 1,along with the emergence of several new peaks, most notably at 2088,2027,1952,1940, and 1728 cm-l (Figure 2b). Further irradiation for a total time of 30 min leads to enhancement of the new peaks and of the signal for free CO with concurrent consumption of 1 and a decrease in the peaks for 3 (Figure 2c). Because there continues to be a loss of 1, it was not initially clear if the new peaks derive from species produced by the photolysis of 1 or of 3. We see no evidence for the production of radicals 2 in the matrix. The above photochemical processes are thermally reversible: when 70 min is allowed for thermal reactions to occur while the matrix is kept 96 K, 3 is regenerated at the expense of species associated with the new peaks and free CO (Figure 3). These photochemical/thermal conversions are reversible upon further irradiatioddark cycles. In order to effect complete thermal conversion of the new species to 3, it was necessary to warm the matrix t o ca. 120 K in the continued absence of light, presumably t o improve the diffusion of CO through the matrix. To gauge the stoichiometry of the thermal backreaction that forms 3, we have used a comparison of the intensities of the absorptions for free CO at 2132 cm-’ and for 3 at 2055 cm-l, as based on their peak heights. We find that the intensity ratio [free CO consumedl:[3 producedl = 0.024 during the thermal reaction (Figure 3) is nearly the same as the ratio [free CO producedl:[3
producedl = 0.026 during the early (1.5 min) stages of photolysis. The latter ratio provides us with a measure of the intensity ratio in a 1:l mixture of 3 and free CO. This result therefore leads us to conclude that 1 equiv of CO is consumed in regenerating 3 from the new species. These observations provide support for the formation of Mnz(C0)g ( 8 ) as a secondary photolysis product. Greater detail concerning the number of new species and their associated CO stretching frequencies was ascertained by monitoring the IR spectrum after shorter periods of thermal back-reaction. The difference spectrum in Figure 4 was obtained by irradiating the matrix for 40 min and then allowing it to react thermally for 20.5 min. The new IR peaks evident in Figure 2 segregate into two sets under these conditions. The peaks at 2088,2027, and 1728 cm-l increase in intensity at the expense of the peaks at 2048, 1952, and 1940 cm-l and of free CO. Note that the first set of peaks show a decrease in intensity under the longer thermal reaction time used in Figure 3. These observations imply the existence of another new species 9, in addition to 8, among the new species observed in Figure 2. Peaks associated with the formation of 3 are also present in Figure 4, most clearly at 2055 cm-l, and over longer periods of thermal back-reaction the peaks for both 8 and 9 decrease in intensity while those for 3 increase. Because of the potential for overlap with peaks of either 1 or 3, the peaks at 2013,2001, and 1984 cm-l are only tentatively assigned to 9. Similarly, the negative peak at 1977 cm-l is guardedly assigned to 8. A summary of peak assignments is given in Table 1. Another species is present during initial thermal back-reactions when the photolysis is carried out for shorter periods of time, namely the known solvent0
Kvietok and Bursten
2398 Organometallics, Vol. 14, No. 5, 1995 0.050
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cm” Figure 4. Difference IR spectrum from the irradiation and thermal back-reaction of 1 in 3-methylpentane at 96 K. The spectrum shown is the difference A - B, where A and B are defined as follows: (A) the spectrum obtained after irradiation of the matrix for 40 min followed bv 20.5 min of thermal (dark) reaction at 96 K; (B) the spectrum obtained immediately after irradiation of the matrix for i 0 min.
Table 1. IR Stretching Frequencies for Photoproducts of Mnz(CO)lo com p 1ex
vco (cm-lp
Mn2(CO)&-?j+q2-CO) (3)b 2105,2055,2016, 2003,2000, 1986,1963,1759 Mn2(CO)s ( 8 ) 2048,1977,1952,1940 Mnz(C0)g‘ (9) 2088,2027,2013,2001,1984,1728 Values i n italics are only tentatively assigned due to overlapping signals. Reference 5.
complex 4 that was proposed by Brown et al.9 Complex 4 was readily identified by its CO stretching frequencies a t 2029, 1996, 1990, 1972, and 1948 cm-l. Although species 4 rapidly decayed with concomitant growth of 3 during thermal reaction periods, as previously reported, it was found to be a relatively minor component of the sample when longer irradiation times were employed (cf. the relatively small peak at 1990 cm-l in Figure 4 as an indicator of consumed 4). Although a detailed kinetic analysis of the thermal reactions of 8 and 9 with CO t o give 3 was not warranted on the basis of the complexity of the system, the potential for overlapping signals, and the viscous nature of the matrices, a plot of normalized spectral peak heights vs thermal reaction time is informative in several respects (Figure 5). First, the close match of the growth and decay behavior for the signals associated with each species is reassuring corroboration of their assignments. Second, the rapid decay of the peaks related to complex 4 reflects the relatively small influence this species has on the system once secondary photolysis products have been produced. Third, the general appearance of the plot suggests the intermediacy of 9 in the thermal conversion of 8 to 3. The relative amounts of complexes 3,8,and 9 present at the beginning of thermal reaction periods were some-
what variable due to minor differences in concentration and the extent of photolysis. Nevertheless, plots from other runs reveal the same general features shown in Figure 5. On the basis of the above data, we propose that complex 8 combines with CO to form 9,which thermally isomerizes to the known species 3.15 We therefore propose that 9 is an isomer of 3 that we will denote Mnz((2019’ (eq 1).The most intriguing feature in the IR Mn,(CO), 8
+ CO - Mn,(CO),’ 9
-
Mn,(CO),@-&$-CO)
(11
3 spectrum of 9 is the observation of the CO stretch a t 1728 cm-l. The position of this peak, like the peak at 1759 cm-l for complex 3, suggests the presence of a semibridging carbonyl in 9.16 If this conclusion is correct, 9 represents yet another photochemical intermediate that possesses a semibridging carbonyl, as has been seen for 3 and for several dinuclear complexes of Fe.12J7 The findings that species 3 and 9 are both isomers of Mnz(CO)g, that both apparently contain at least one semibridging carbonyl, and that there is a ca. 30 cm-l difference in the stretching frequencies of these species in the semibridging region of the spectrum is very striking. We shall be investigating further the structures of these two isomers. Interestingly, two (15)On the basis of the data in Figure 5,we cannot rule out the possibility that 8 reacts with CO to form 3 directly in addition to forming 9. (16)Crabtree, R. H.; Lavin, M. Inorg. Chem. 1986,25,805-812. (17)Zhang, S.; Brown, T. L. J.Am. Chem. SOC.1993,115, 17791789.
Organometallics, Vol. 14,No. 5, 1995 2399
Matrix Photochemistry of MndC0)lo 1.1
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Figure 5. Temporal behavior of the product IR bands following the irradiation of 1 for 30 min in 3-methylpentane at 96 K. The abscissa shows the time of thermal (dark) reaction after the irradiation. Compound numbers in the legend are referenced in the text. Peak intensities were approximated by IR peak heights, and all spectra were recorded at 96 K. For each frequency, the intensity was renormalized so that its maximum relative intensity was unity. isomers of Rez(C0)g have also been proposed, although both of these contain only terminal carbonyl ligandsals Compound 8 exhibits no peaks below 1940 cm-l, an indication that there are no bridging or semibridging CO ligands in Mnz(C0)a. The apparent absence of bridging carbonyls in the structure of 8 is reminiscent of the double-CO-lossproduct [CpFe(CO)lz(7)detected in our earlier matrix photochemical studies of 5.1° Compound 7 exhibited only two CO stretching frequencies (1958 and 1904 cm-l), which clearly indicated (18)(a) Firth, S.; Hodges, P. M.; Poliakoff, M.; Turner, J. J. Inorg. Chem. 1986, 25, 4608-4610. (b) Firth, S.; Klotzbucher, W. E.; Poliakoff, M.; Turner, J. J. Inorg. Chem. 1987, 26, 3370. (19) Brown has suggested to us that 8 could contain two molecules of coordinated 3-MP, in analogy to 4 (Brown, T. L., personal communication). Additionally, he has suggested that the 1728 cm-l peak could be due to a monosolvento complex, Mnp(CO)&, that contains a semibridging CO, which could be generated by loss of solvent from the disolvento species. We believe that these proposals are unlikely. First, we do not see the formation of 4 in the reaction of 8 with CO, as might be expected upon the addition of CO to a disolvento species. Second, the uptake of CO in the thermal reaction of 8 with CO to form 9 is inconsistent with the formulation of 9 as having only eight CO ligands. Nevertheless, this possibility will be examined by exploring this photochemistry in matrices of differing Lewis basicity.
terminal configurations for the carbonyl ligands and, when coupled with MO calculations, led t o the proposition of an Fe-Fe triple bond. Because [CpFe(CO)lzand Mnz(C0)a are isovalent, it is possible that 8 contains an unsupported Mn-Mn triple bond.lg In support of this notion, gas-phase studies suggest a strengthening of the Mn-Mn bond upon stepwise decarbonylation of l.14 We are currently undertaking additional characterization experiments and ab initio calculations20 to explore further the structure of 8.
Acknowledgment. We gratefully acknowledge the National Science Foundation (Grant CHE-9208703)for support of this research. We are also grateful to Drs. T. L. Brown and M. Poliakoff for helpful reviewer comments. OM9500329 (20) Preliminary results: Barckholtz, T. A,; Lavender, H. B.; Kvietok, F. A.; Bursten, B. E. Abstracts ofPapers, 209th National Meeting of the American Chemical Society, Anaheim, C A American Chemical Society: Washington, DC, 1995; INOR 128.
