In Situ NMR Observations of the Photolysis of Cymantrene and Methylcymantrene in Supercritical Fluids: A New Technique Using High-Pressure NMR John C. Linehan,* ,† Scott L. Wallen, †,⊥ Clement R. Yonker, † Thomas E. Bitterwolf, ‡ and J. Timothy Bays ‡ Contribution from the Chemical Sciences Department, Pacific Northwest National Laboratory, § Richland, Washington 99352, and the Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343 ReceiVed April 14, 1997 X Abstract: The in situ photolytic exchange of ethylene and hydrogen for carbon monoxide on cymantrene (CpMn- (CO) 3 , Cp ) η 5 -cyclopentadienyl) and methylcymantrene (MeCpMn(CO) 3 , MeCp ) monomethyl-η 5 -cyclopenta- dienyl) dissolved in subcritical and supercritical solvents (CO 2 and ethylene) was investigated by high-resolution, high-pressure 1 H NMR over the temperature range from -40 to 150 °C and a pressure range from 35 to 2600 bar. Photolytic substitution of ethylene for CO proceeded to completion under all conditions investigated, and only one ethylene was observed to substitute for CO on the manganese complexes even in neat ethylene under extreme conditions of pressure and temperature. Only small amounts of dihydrogen were observed to substitute for CO on cymantrene at 35 °C in a binary solvent mixture of CO 2 /H 2 during photolysis. The 1 H chemical shifts of the manganese complexes and their ethylene substituted products were found to be linearly dependent on density with temperature and solvent dependence also observed. The spin-lattice relaxation times (T 1 ) of all the solvent and solute species were observed to be inversely proportional to the density of the solvent over the range of conditions investigated. Temperature and concentration dependent phase behavior and solute saturation were also determined for the methylcymantrene and MeCpMn(CO) 2 (η 2 -C 2 H 4 ) solutes in SCF ethylene. These results represent the first NMR detected in situ photolysis study of organometallics in SCF and demonstrate the utility of this technique. Introduction Interest in supercritical fluids (SCF) has grown enormously in the past decade, which is understandable from both funda- mental and technological points of view since the densities and transport properties of a fluid can be continuously varied between gas-like and liquid-like values. This allows the study of intermolecular interactions over a wide range of molecular distances and the tuning of the solvation properties of the fluid. More recently a number of studies have focused on the use of supercritical fluids as reaction media and reactants for organo- metallic reactions: 1-3 in particular, the utility of SCF in the production of normally unstable or hard to synthesize organo- metallic species, 4 the elucidation of mechanistic pathways in hydroformylation, 5 and as solvents for asymmetric catalysis. 6,7 Several of these studies have been directed toward understanding the fundamental properties and reactions of photochemical substitution of ligands on organometallic complexes in SCF. The majority of the experimental evidence concerning the fate of reactants comes from ex situ analysis of the reaction products 1 or indirect in situ observations of the binding of substituted ligands to the metal center by monitoring the carbon monoxide stretching vibrations with FTIR spectroscopy. 2,4 However, the direct observation of the substituted ligands on the metal center through in situ NMR has not heretofore been reported. We have developed an inexpensive pressure vessel compatible with the in situ spectroscopic observation of species in SCF by NMR, 8,9 ESR, 8 and XAFS 10 that can withstand pressures up to 4000 bar. Commercially available fused silica capillary tubing is well suited for the investigations of fluids under extreme conditions due to the capillary’s high tensile strength caused by the high o.d.-to-i.d aspect ratio. A discussion on the strength of glass as a function of the wall thickness-to-inside diameter ratio has previously been given by Yamada. 11 The high-pressure NMR (HPNMR) capillary cell was recently demonstrated to be an excellent vessel for the NMR detected observations of in situ photolysis reactions due to favorable optical, as well as magnetic, properties. 12 In the present study, we investigate the stability and reactivity of cymantrene and methylcymantrene in SCF under extreme conditions and offer the first demonstra- tion of photolysis in high-pressure SCF with in situ NMR detection of the substitution reactions on organometallic com- plexes by ethylene and hydrogen. We also illustrate the use of † Pacific Northwest National Laboratory. ‡ University of Idaho. § Operated by Battelle Memorial Institute. ⊥ Current address: Department of Chemistry, CB# 3290, University of North Carolina, Chapel Hill, NC 27599. X Abstract published in AdVance ACS Abstracts, August 15, 1997. (1) Jessop, P. G.; Ikariya, T.; Noyori, R. Science 1995, 269, 1065-1069. (2) Poliakoff, M.; Howdle, S. M.; Kazarian, S. G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1275-1295. (3) Jessop, P. G.; Ikariya, T.; Noyori, R. Organometallics 1995, 14, 1510-1513. (4) Banister, J. A.; Lee, P. D.; Poliakoff, M. Organometallics 1995, 14, 3876-3885. (5) Klinger, R. J.; Rathke, J. W. J. Am. Chem. Soc. 1994, 116, 4772. (6) Burk, M. J.; Feng, S.; Gross, M. F.; Tumas, W. J. Am. Chem. Soc. 1995, 117, 8277-8278. (7) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 344-355. (8) Spectrometer Capillary Vessel and Method of Making Same, U.S. Patent No. 5,469,061, 1995. (9) Yonker, C. R.; Wallen, S. L.; Linehan, J. C. J. Supercrit. Fluids 1995, 8, 250-254. (10) Wallen, S. L.; Pfund, D. M.; Fulton, J. L.; Yonker, C. R.; Newville, M.; Ma, Y. ReV. Sci. Instrum. 1996, 67, 2843-2845. (11) Yamada In Glass Cell Method for High-Pressure High Resolution NMR Measurements. Applications to the Studies of Pressure Effects on Molecular, Conformation and Structure; Diehl, P., Fluck, E., Gunther, H., Kosfeld, R., Seelig, J., Springer-Verlag: Berlin, 1991; pp 233-263. (12) Yonker, C. R.; Wallen, S. L. Appl. Spectrosc. 1996, 50, 781-784. 10170 J. Am. Chem. Soc. 1997, 119, 10170-10177 S0002-7863(97)01187-6 CCC: $14.00 © 1997 American Chemical Society