Room-Temperature Reactions of the Intermediate(s) Generated by Flash Photolysis of (η 5 -C 5 H 5 )Fe(CO) 2 CH 3 Karen L. McFarlane 1 and Peter C. Ford* Department of Chemistry, University of California, Santa Barbara, California 93106 Received September 12, 1997 The transient formed by the photolysis of the methyl complex CpFe(CO) 2 CH 3 (M, Cp ) (η 5 -C 5 H 5 )) has been studied by FTIR spectroscopy in methylcyclohexane solution at 77 K and by time-resolved infrared detection in ambient-temperature solutions in cyclohexane and tetrahydrofuran. The transient IR spectrum of the intermediate I M displays a single ν co absorption band and can be characterized as a monocarbonyl. In 295 K solution, the reaction of I M with CO occurs with second-order kinetics (rate ) k co [I M ][CO]) with k co values of (6.3 ( 0.2) × 10 8 and (3.4 ( 0.4) × 10 6 M -1 s -1 in cyclohexane and THF, respectively. On the basis of these data, I M is concluded to be the solvento complex CpFe(CO)(Sol)CH 3 . Introduction Near-UV photolysis of CpFe(CO) 2 CH 3 (M, Cp ) (η 5 - C 5 H 5 )) has been argued to effect decarbonylation to give the “unsaturated” intermediate species “CpFe(CO)CH 3 ” (I M ) (eq 1) based on evidence from trapping studies. 2,3 The same intermediate has been proposed in thermal reactions of M with various ligands. 4 I M is also of interest as a model for the reactivity expected for intermediates generated by photolysis of other CpFe- (CO) 2 R compounds in the absence of specific interactions between the coordination site vacated by the photolabi- lized CO and the ligand R- (e.g., for R )-C(O)CH 3 ). 5 However, direct detection of I M in ambient-temperature solutions has not been previously reported. Surpris- ingly, this species has been proven to be somewhat elusive, even in low-temperature experiments in noble gas matrices, 2,7,8 although a monocarbonyl species has been detected in a poly(vinyl chloride) film at 12 K, 9 the difference in behavior being attributed to rapid cage recombination in the more closely confined gas matrix environment. 9 Described here are time-resolved infra- red (TRIR) experiments examining the spectra and reaction dynamics of the intermediate formed by laser flash photolysis of M in two common solvents at ambient temperature. Also reported is the FTIR spectrum of I M recorded after photolysis of M in 77 K methylcyclohex- ane. Experimental Section Reagents. Methylcyclohexane (Aldrich) was treated by standard procedures to remove alkenes and then distilled from sodium metal. Tetrahydrofuran (THF, Aldrich) was distilled from sodium-benzophenone ketyl solution under dinitrogen. Cyclohexane (spectrophotometric grade, B & J Brand, Baxter) was distilled from calcium hydride under dinitrogen. All gases were passed through an Alltech Associates Oxy-trap and a column of 4 Å molecular sieves and Drierite before use. CpFe- (CO)2CH3 was prepared by published methods, 10 and then it was chromatographed on a silica gel column in the inert atmosphere box with pentane. Low-Temperature FTIR Spectra. These were recorded on a Bio-Rad FTS-60 FTIR spectrometer using a R. G. Hansen PFD-FT12.5 fixed temperature pourfill Dewar which was custom fitted with a sample IR cell. The IR cell was first flushed with argon for 10 min and then capped with silicone rubber GC septa. Solutions were prepared by dissolving CpFe- (CO)2CH3 (10 -3 M) in freshly distilled solvent (under Ar), entrained further with argon, and then transferred to the IR cell using a gastight syringe (with an outlet needle in the second port). Initial FTIR spectra were measured at room temperature and again at low temperature after filling the Dewar flask with the cryogen (dry ice/acetone or LN2). The sample was then subjected to 5-10 pulses from a XeCl excimer laser (308 nm), and the spectrum was again measured. TRIR Spectra. The time-resolved infrared instrumenta- tion with modifications in sample handling, has been described. 1b,11 After flash photolysis by XeCl excimer laser (1) Taken in part from the Ph.D. Dissertation of K. L. McFarlane, University of California, Santa Barbara, 1996. (2) Kazlauskas, R. J.; Wrighton, M. S. Organometallics 1982, 1, 602-611. (3) Ryba, D. W. Ph.D. Dissertation, University of California, Santa Barbara, 1991; Chapter 4. (4) Hersh, W. H.; Hunte, F.; Siegel, S. Inorg. Chem. 1993, 32, 2968- 2971. (5) Belt, S. T.; Ryba, D. W.; Ford, P. C. J. Am. Chem. Soc. 1991, 113, 9524-9528. (6) McFarlane, K. L.; Lee, B.; Fu, W.; van Eldik, R.; Ford, P. C. Submitted for publication. (7) Alt, H. G.; Heberhold, M.; Rausch, M. D.; Edwards, B. H. Z. Naturforsch., B: Anorg. Chem., Org. Chem. 1980, 34B, 1070. (8) Fettes, D. J.; Narayanaswamy, R.; Rest, A. J. J. Chem. Soc., Dalton Trans. 1981, 2311-2316. (9) Hooker, R. H.; Rest, A. J.; Whitwell, I. J. Organomet. Chem. 1984, 266, C27-C30. (10) Organometallic Syntheses; Eisch, J. J.; King, R. B., Eds.; Academic Press: London, 1965; Vol. 1, pp 151-152. (11) (a) DiBenedetto, J. A.; Ryba, D. W.; Ford, P. C. Inorg. Chem. 1989, 27, 3503-3507. (b) Ford, P. C.; DiBenedetto, J. A.; Ryba, D. W.; Belt, S. T.; SPIE Proc. 1992, 1636,9-16. (c) Boese, W. T.; Ford, P. C. J. Am. Chem. Soc. 1995, 117, 8381-8391. (1) 1166 Organometallics 1998, 17, 1166-1168 S0276-7333(97)00810-8 CCC: $15.00 © 1998 American Chemical Society Publication on Web 02/26/1998