A Delicate Balance of Complexation Vs. Activation of Alkanes Interacting with [Re(Cp)(CO)(PF3)] Studied with NMR and Time-Resolv

A delicate balance of complexation vs. activation of SPECIAL FEATURE alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy Graham E. Ball†‡, Christopher M. Brookes§, Alexander J. Cowan§, Tamim A. Darwish†, Michael W. George‡§, Hajime K. Kawanami§, Peter Portius§, and Jonathan P. Rourke¶ †School of Chemistry, University of New South Wales, Sydney 2052, Australia; §School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom; and ¶Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom Edited by John E. Bercaw, California Institute of Technology, Pasadena, CA, and approved March 7, 2007 (received for review November 16, 2006) The organometallic alkane complexes Re(Cp)(CO)(PF3)(alkane) and studies (13), because at Ϸ183 K, the lifetime of these complexes (Ϸ1 Re(Cp)(CO)2(alkane) have been detected after the photolysis of h) is sufficient for NMR experiments. The formation of long lived Re(Cp)(CO)2(PF3) in alkane solvent. NMR and time-resolved IR exper- Re(Cp)(CO)2(alkane) complexes has recently prompted the study iments reveal that the species produced by the interaction of n- of the photolysis of Re(CpЈ)(CO)3 (CpЈϭCp, Cp*) in supercritical pentane with [Re(Cp)(CO)(PF3)] are an equilibrium mixture of methane (scCH4) and liquid ethane. After photolysis in scCH4,a Re(Cp)(CO)(PF3)(pentane) and Re(Cp)(CO)(PF3)(pentyl)H. The interac- rapid equilibrium between Re(CpЈ)(CO)2(CH4) and tion of cyclopentane with [Re(Cp)(CO)(PF3)] most likely results in a Re(CpЈ)(CO)2(CH3)H is observed (14). similar equilibrium between cyclopentyl hydride and cyclopentane There are also many examples of what may be considered model complexes. An increasing proportion of alkane complex is observed complexes for the coordination of alkanes, namely agostic alkyl on going from n-pentane to cyclopentane to cyclohexane, where only complexes (other than ␣-agostic species) (15, 16). In these com- a small amount, if any, of the cyclohexyl hydride form is present. In pounds, the agostic ␩2-C,H interaction is stabilized by the chelate general, when [Re(Cp)(CO)(PF3)] reacts with alkanes, the products effect, because these ligands are tethered to the metal elsewhere. display a higher degree of oxidative cleavage in comparison with Using combined IR, NMR, and theoretical studies, we have CHEMISTRY [Re(Cp)(CO)2], which favors alkane complexation without activation. recently thoroughly characterized an organometallic xenon com- Species with the formula Re(Cp)(CO)(PF )(alkane) have higher thermal i 3 plex Re( PrCp)(CO)(PF3)Xe (17, 18). It was noted that the stability stability and lower reactivity toward CO than the analogous of this complex was slightly higher than that of the closely related Re(Cp)(CO)2(alkane) complexes. i Re( PrCp)(CO)2Xe species, formed in the same reaction mixture. Hence, the question was whether the introduction of a PF ligand ͉ ͉ ͉ 3 alkane complexes CH activation multinuclear NMR photochemistry would stabilize alkane complexation in species of the type Re(Cp)(CO)(PF3)(alkane) compared with Re(Cp)(CO)2(alkane). ntuitively, simple alkane molecules are among the most unlikely Alternatively, would the [Re(Cp)(CO)(PF3)] fragment display re- Iof ligands in coordination chemistry. There are no lone pairs or activity patterns more like those of [Re(Cp*)(CO)(PMe3)] (19) or ␲ electrons available for binding to the metal center. Only strong, [Re(Cp)(PMe3)2] (20) in which the presence of the phosphorus single ␴ bonds are present, and it is through the coordination of C-H donor is known to promote activation of the C-H bond to form alkyl bonds to a metal center that complexation is most likely to occur (1). hydride species? ␴ Complexes containing an intact bond of a covalent species X-H Our studies of the interaction of the [Re(Cp)(CO)(PF3)] frag- coordinated to a metal center are termed ␴-complexes, and there ment with three alkanes cyclopentane, cyclohexane, and pentane has been widespread activity in this field recently (2, 3). described here suggest behavior that is ‘‘in between’’ these two ␴ The prototypical complexes are the dihydrogen (H2) complexes extremes of alkane complex vs. alkyl hydride, and this behavior is first described by Kubas, which contain an H-H unit coordinated to surprisingly dependent on the alkane. a metal (2, 4). In the case of alkanes, the interaction of a C-H bond with a metal is particularly weak, largely because ␴-complexation is Results and Discussion often stabilized by a backbonding component that is highly ineffi- NMR Studies. Photolysis in cyclopentane. A solution of Re(Cp)(CO)2 cient for a C-H bond when compared with a H ligand. An 2 (PF3)(1) in 95% cyclopentane:5% pentane-d12 (added for understanding of this mode of bonding is interesting and important locking purposes) was cooled to 185 K. Before photolysis, there both from a theoretical point of view and to explain the role they was a single peak in the 1H NMR spectrum at ␦ 5.20 due to the play in the reactions of the C-H moiety. Cp protons of 1, and one doublet in the 19F NMR spectrum at The C-H activation reaction is a key step in potential function- 1 ␦ Ϫ2.56 (d, JFP ϭ 1,232 Hz). The sample was irradiated with UV alization of the normally inert alkane molecule, and its integration light for 1 min resulting in the depletion (Ϸ15%) of the 19F into catalytic processes is an important goal (5, 6). The C-H resonance of 1 and the concomitant growth of two new reso- activation of alkanes has been shown to involve alkane complexes as intermediates in a number of elegant IR studies (5–7). After coordination, the C-H bond breaking step is usually fast and Author contributions: G.E.B., M.W.G., and J.P.R. designed research; G.E.B., C.M.B., A.J.C., irreversible, precluding observation of the alkane complex with a T.A.D., M.W.G., H.K.K., P.P., and J.P.R. performed research; and G.E.B., A.J.C., T.A.D., and relatively ‘‘slow’’ technique such as NMR spectroscopy. However, M.W.G. wrote the paper. alkane complexes are not always unstable with respect to C-H The authors declare no conﬂict of interest. activation. Two important examples that are formed photolytically This article is a PNAS Direct Submission. are the long known M(CO)5(alkane) (M ϭ Cr, Mo, W) systems (1, Abbreviation: TRIR, time-resolved IR. 8) and Re(Cp)(CO)2(n-heptane). This latter complex, character- ‡To whom correspondence may be addressed. E-mail: [email protected] or ized by time-resolved IR (TRIR) spectroscopy, had by far the [email protected]. longest lifetime measured (Ϸ25 ms at 298 K) of any alkane complex This article contains supporting information online at www.pnas.org/cgi/content/full/ in solution at the time (9). This crucial result paved the way for 0610212104/DC1. NMR studies of Re(Cp)(CO)2(alkane) (10–12) and further IR © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0610212104 PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6927–6932 Downloaded by guest on September 27, 2021 Fig. 3. Variable-temperature, 300-MHz 1H NMR spectra after photolysis of 1 in 19 Fig. 1. 282.4-MHz F NMR spectra of 1 in cyclopentane at 185 K. (Lower) Before cyclopentane. photolysis. (Upper) After 1 min of photolysis. (Inset) Expansion of the highlighted half of resonance A. with the broadest peaks suffering the most severe attenuation, as ␦ Ϫ 1 ϭ ϫ ϭ was observed at 185 K. Temperature cycling between 215 and 185 nances at 3.28 (dt, JPF 1,211 Hz, 2 JFH 7.9 Hz) labeled K confirms that this is occurring as AЈ disappears again at lower ␦ Ϫ 1 ϭ species A and at 32.4 (d, JPF 1,398 Hz), due to free PF3 temperature. The cause of the broadening of the AЈ resonance is the (Fig. 1). onset of a decoalescence phenomenon that is starting to occur, This observation is consistent with previous experiments (17) leading to increasing linewidths at lower temperatures. For the two where photolysis of 1 led to the loss of either a CO or a PF3 ligand hydrogens in a bound CH2 unit of 3a to be equivalent, it is necessary to form [Re(Cp)(CO)(PF3)] and [Re(Cp)(CO)2]. These fragments for them to average their environments through a combination of can interact with the alkane solvent as outlined in Fig. 2. Resonance both (i) an exchange of complexed and uncomplexed hydrogens as A can be assigned to either a cyclopentane (3a) or a cyclopentyl shown in Fig. 4 and (ii) rotation of the cyclopentane moiety around hydride (4a) complex or to a rapidly equilibrating mixture of both. the metal-ligand bond axis. A slowing of either the rate of exchange The triplet splitting of resonance A, due to JFH, suggests a coupling or rotation could lead to the observed decoalescence. to two effectively equivalent hydrogens. 1 The resonances associated with 2a started to disappear as the H NMR spectra showed a corresponding decrease in the signals temperature increased above 185 K, due to the documented ␦ Ϫ 1 of 1 upon irradiation. A quintet grows in at 2.29 in the H thermal decomposition of 2a (10). At 215 K, signals due to 3a spectrum that is due to the bound CH2 unit of the cyclopentane (including A, AЈ) also decreased with time, due to the thermal ligand in the previously characterized Re(Cp)(CO)2(cyclopentane) decomposition of 3a.

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