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

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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. Clearly 3a is thermally more stable than 2a. (2a) (10), and a singlet at ␦ 4.92 is due to the Cp ligand of 2a. The Resonance AЈ at 215 K shows a splitting attributed to a JPH coupling, alkane in complexes 2a–2c is known not to undergo oxidative confirmed by a 31P decoupling experiment. The 19F and 1H cleavage to form alkyl hydrides under these conditions (10–12). 1 resonances of 3a were connected to the same species by using a pair In addition to the resonances due to 2a in the H spectrum, of shift correlation experiments, 2D 1H-31P and 19F-31P. Both another peak due to a Cp ligand of a species with a higher 19 Ј 1 ␦ resonances A ( F) and A ( H) were shown to correlate to the same concentration was also observed to grow in at 5.21. Intensities 31P frequency at ␦ 124.2, confirming that they are from the same suggested that this peak was likely from the same species that 19 31 1 19 species. The F- P correlation experiment shows the H couplings produces the F resonance A, i.e., 3a/4a. Further photolysis (1 min) 19 31 in both the F and P dimensions seen earlier and reveals that JPH increased the concentration of all of the newly observed 19F and 1H Ϸ 1 26 Hz [see the supporting information (SI)]. signals. Significantly, there was no evidence for any shielded H Key indicators of the presence of an alkane complex from an resonance (␦ Ͻ 0) expected for 3a/4a. As a result, the temperature 1 NMR perspective are a bound CH2 or CH3 unit in which ␦ His of the NMR probe was raised to 215 K in 10-K steps in the absence 1 1 moderately shielded and, most importantly, a JCH value that is of photolysis. As the temperature was increased, the H NMR slightly reduced from that observed in a free alkane. For example, spectra showed the emergence of a broad resonance AЈ at ␦ Ϫ2.92, the protons of the bound CH2 group in 2a appear at ␦ Ϫ2.3 with which sharpened and increased in intensity at higher temperature. 1 ϭ ␦ 1 Ј JCH 113 Hz (cf. cyclopentane 1.50; JCH 129.4 Hz) (10). In all A was assigned to the bound CH2 protons of cyclopentane in of the alkane complexes observed in NMR studies to date, the Re(Cp)(CO)(PF3)(cyclopentane) (3a), and resonance A is also 19 exchange of the complexed and uncomplexed C-H bond has always assigned to 3a (Fig. 3). The intensity of signal A in the F NMR been fast on the NMR time scale (Fig. 4). This means that the spectra was not temperature-dependent. 1 1 observed ␦ H and JCH values of the protons on the bound carbon The apparent increase in concentration of 3a at higher temper- are an average of the values for complexed and uncomplexed ature is an artifact of the selective excitation scheme used to hydrogens. Density functional theory calculations suggest some suppress the resonances of the free alkane solvent. The selective deshielding of the 1H resonance and an increase in 1J of the excitation can cause broad peaks to be attenuated or even ‘‘lost’’ CH uncomplexed hydrogen(s) in Re(Cp)(CO)2(cyclohexane) (12) compared with the free alkane. Therefore, the true shielding and 1 reduction of JCH for the complexed hydrogen may be just over 1 1 twice the observed changes in the averaged H NMR shift and JCH foraCH2 group compared with the free alkane. The trends

Fig. 2. Possible photoproducts obtained from photolysis of 1 in alkanes. Fig. 4. Exchange of complexed C-H bonds in a bound CH2 unit.

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Fig. 5. Exchange process for alkyl and hydride protons via alkane complexes.

observed in both agostic alkyl and dihydrogen complexes suggest that the more stretched the complexed C-H moiety becomes, the 1 1 smaller JCH will become. Typically the JCH of a complexed C-H bond may be reduced by up to 50% relative to the uncomplexed unit (free, Ϸ125 Hz; complexed, 60–120 Hz) based on data from agostic 1 species [although cases of JCH Ͻ 40 Hz are known (15)]. In an 1 exchange averaged situation, one expects JCH in a bound CH2 of 19 Ϸ Fig. 6. Resolution-enhanced, 470.6-MHz F NMR spectrum after photolysis of an alkane complex to be 90–128 Hz. 1 in n-pentane at 190 K showing resonances from complexes of the formula 3b. In the case where complete oxidative cleavage occurs, the *, secondary photoproduct. complexed C-H bond is broken, and now the comparable coupling 2 is a JCH interaction, which is expected to be small, typically Ͻ10 Hz, 1 and likely of opposite sign to JCH. Examples of complexes be increased. A similar approach may be applied to the use of JPH that undergo a rapid exchange of alkyl and hydride protons, likely as well. The averaged value of JPH in the case of a cis-alkyl hydride via an alkane complex intermediate, are known, e.g., 4a is expected to be much larger (estimate Ϸ33 Hz) than for the ϩ Ͻ [Cp*Os(dmpm)(CH3)H] (21). alkane complex 3a, which is estimated to be 10 Hz but increasing It may not be possible to differentiate complexed alkanes from with C-H bond elongation. The observed value of 26 Hz suggests alkyl hydride species (4) that undergo rapid exchange of hydride and that the products have character that is closer to the alkyl hydride/ alkyl protons based on the ␦ 1Hor␦ 13C values alone, because they highly stretched end of the range of possibilities. may be similar in these two scenarios. However, the averaging of a Based on NMR studies alone, it is not possible to differentiate 1 2 large JCH with a small (negative) JCH in the alkyl hydride case these three possibilities while there are exchange processes present CHEMISTRY would be expected to produce an averaged JCH value of Ϸ55–70 Hz, at the lowest useable temperatures. Other methods must therefore significantly less than that observed for an alkane complex. be applied. In the case of IR spectroscopy, separate rather than To gain insight into the nature of the interaction of cyclopentane averaged bands can often be observed for equilibrating mixtures in 3a, photolysis experiments in partially 13C-labeled cyclopentane allowing for the differentiation of single components that would be 13 (25% cyclopentane- C1, 15% cyclopentane-d10, 60% normal cy- averaged in NMR experiments. This approach is described later and clopentane, equivalent to Ϸ7% 13C total) were carried out. A 13C suggests the presence of an equilibrating mixture of 3a and 4a. edited 1D 1H-{31P} experiment, which selects only the resonances Photolysis in n-pentane. The photolysis of 1 was investigated in a 13 of hydrogens directly attached to a C nucleus with simultaneous mixture of 95% n-pentane and 5% n-pentane-d12 at 190 K, using a 31 P decoupling, was used at 210 K, revealing JCH ϭ 75 Ϯ 2 Hz. A procedure similar to that described in the previous section. After 2D 1H-13C correlated experiment provided ␦ 13C ϭϪ6.80 for the photolysis, three 19F resonances were observed: a major product at 13 1 bound carbon of the cyclopentane of 3a compared with ␦ C ϭ ␦ Ϫ3.36 (dq, JFP ϭ 1,212 Hz, 3 ϫ JFH ϭ 6.33 Hz), and two low 13 1 1 Ϫ31.2 for 2a. ␦ Cof3a is also higher than those reported for intensity signals at ␦ Ϫ3.32 ( JFP ϭ 1,213 Hz) and ␦ Ϫ3.20 ( JFP ϭ Ͻ ␦ trans-Re(Cp)(PMe3)2(R)H (R ϭ n-C6H13, c-C3H5,CH3), which 1,214 Hz) for which JFH could not be measured but is 5Hzfor vary from ␦ Ϫ14 to ␦ Ϫ39 (20). In lieu of more data, values of ␦ 13C Ϫ3.32. These three resonances were attributed to species of the of the carbon in the coordinated CH2 unit (C␣) are not aprioriable formula Re(Cp)(CO)(PF3)(1-pentane) (3b-1), and tentatively to differentiate structures in this case. Re(Cp)(CO)(PF3)(2-pentane) (3b-2) and Re(Cp)(CO)(PF3)(3- 1 The observed value of JCH Ϸ75 Hz for the interacting cyclopentane) (3b-3), respectively (Fig. 6). The nomenclature implies pentane in 3a is significantly less than that reported for the that coordination to the metal center occurs through a C-H group complexed cyclopentane in 2a (112.9 Hz). It is also larger than the of the C1, C2, or C3 of n-pentane. 19 maximum value predicted for an averaged coupling in a cyclopentyl The three JFH splittings in the F resonance of the major product hydride complex 4a (Ϸ70 Hz). This finding suggests at least three in n-pentane (3b-1) indicate that this product is formed by inter- possibilities for the mode of coordination of the alkane in 3a.(i) The actionofaCH3 group with the [Re(Cp)(CO)(PF3)] fragment, and cyclopentane unit is coordinating to the metal center in a ␴-alkane because the three protons of the CH3 group couple equally to the complex fashion with a highly stretched complexed C-H bond, 19F nuclei, they are likely exchanging positions rapidly, too. This significantly greater than the 1.15–1.17 Å calculated for the com- assignment is supported by experiments in 2,2,4,4-pentane-d4 that 19 plexed C-H distance of Re(Cp)(CO)2(cyclohexane) (2c) (12). (ii) produced exactly the same F resonance splittings (3 ϫ JFH), The complexed C-H bond of the coordinated cyclopentane has whereas use of pentane-d12 results in no observable JFH splittings undergone oxidative cleavage to form the cyclopentyl hydride 4a, and a small isotope shift. 19F NMR spectra suggest a rapid exchange 1 but the JCH value for the C␣-H is unusually large. (iii) There is rapid between the three hydrogens of the CH3 group even at 165 K. exchange between the ␴-alkane and its corresponding alkyl hydride Exchange of complexed and uncomplexed hydrogens within the complex, and there is a sizeable amount of each component (3a and bound CH3 group in Re(Cp)(CO)2(1-pentane) (2b-1) is known to 1 4a) in equilibrium. In this latter case, the observed JCH would be the be fast at this temperature (11). H NMR spectra were also weighted average from all of the contributing structures in Fig. 5; collected (at 190 K), and these showed three low-frequency 1H 1 1 i.e., JCH of an alkyl group, JCH of an uncomplexed C-H in a peaks due to the three different isomers of 2b (11). As in the case 1 1 ␴-alkane, JCH of a complexed (and possibly stretched) C-H in a of cyclopentane, no H resonances in the ␦ Ͻ 0 region that could 2 1 19 ␴-alkane, and JCH of a hydride. As an example, if the averaged JCH correspond to the largest signals in the F spectrum were observed. value in 3a were the same as 2a, 113 Hz, and if a best estimate However, when the temperature was increased to 205 K, in the 1 averaged value of JCH for 4a, Ϸ61 Hz, were present, an equilibrium absence of UV light, a broad signal at ␦ Ϫ1.67 appeared due to 3b-1. with a ratio of 3a:4a of 27:73 would be required to produce the Resonances attributed to isomers of 2b rapidly disappeared at this observed value of 75 Hz. If more stretched C-H bonds in the alkane temperature. When the temperature was lowered to 160 K in the complex component are present, the relative amount of 3a would absence of UV light, the broad signal disappeared, whereas the 19F

Ball et al. PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6929 Downloaded by guest on September 27, 2021 Fig. 7. Possible equilibria between four different cyclohexane and cyclohexyl hydride complexes. Keq appears to be close to 1 in complex 3c.

signals did not show any major changes. A decoalescence phenom- signals due to 3b-d12 after the cessation of photolysis was observed, enon is clearly operative in the case of 3b-1 as well. indicating a thermodynamic preference for 3c over 3b-d12. 13 1 The experiment was repeated by using n-pentane- C1 at a higher H NMR monitoring of the photolysis showed the appearance of 1 13 31 temperature of 210 K. Two-dimensional H- C-{ P} correlation two new broad signals at ␦ Ϫ2.73 (JPH Ϸ16 Hz) and ␦ Ϫ3.33 (JPH experiments reveal that this broad signal is correlated with a unresolved) that integrated one proton each with respect to five shielded carbon signal at ␦ Ϫ15.1, and a 13C edited 1H NMR protons of a new peak in the Cp region at ␦ 4.93. The signals at ␦ experiment shows JCH ϭ 89 Ϯ 2 Hz. Again, this is an averaged Ϫ2.73 and ␦ Ϫ3.33 were assigned to the equatorial and axial coupling that in this case involves three hydrogens rather than two protons, respectively, of a bound CH2 unit in the cyclohexane ligand in the case of cyclopentane. If this species were a 1-pentane of 3c. An unambiguous confirmation that these assignments are complex, 3b-1, the exchange of one complexed C-H with two correct has not been obtained. Examination of the broad multiplet 1 uncomplexed C-H bonds of a coordinated CH3 unit would be structures in the H NMR spectrum suggests that the resonance at 3 involved. Alternatively, if this species were an n-pentyl hydride, 4b, ␦ Ϫ3.33 contains two larger axial–axial JHH splittings, consistent 2 the coupling would be the average of one JCH value for the hydride with it being an axial hydrogen and 2D exchange experiments favor 1 19 31 and two alkyl JCH values. The observed value of JCH ϭ 89 Hz is this interpretation. A 2D F- P correlation experiment revealed ␦ 31 much lower than that of 117 Hz observed in 2b-1 and is at the upper P ϭ 118.3 for 3c, and the larger JPH Ϸ16 Hz was retained in this limit of the range expected for an averaged coupling in an alkyl experiment. hydride system, predicted to be Ϸ71–89 Hz. Possible scenarios Repeating the experiment by using uniformly labeled cyclohex- 13 1 based on NMR data for 3b-1 are the following: (i)analkane ane-[ C]6 reveals values of JCH of 93 Ϯ 3 and 107 Ϯ 3 Hz for the complex wherein the complexed C-H bond is stretched to near resonances at ␦ Ϫ2.73 and ␦ Ϫ3.33, respectively. A 1H-13C corre- breaking; (ii) the system being pentyl hydride 4b with relatively high lated experiment indicates that both of these 1H resonances are 13 C-H coupling constants; or (iii) an equilibrium mixture of 4b and from the same bound CH2 moiety that has ␦ C ϭϪ17.6. This can 1 3b where the equilibrium favors the alkyl hydride more than in the be compared with a JCH ϭ 125 Ϯ 0.5 and 96.5 Ϯ 0.5 Hz for the case of cyclopentane. TRIR experiments described below confirm equatorial and axial protons, respectively, observed for the bound that this third scenario is indeed the correct one. CH2 unit of the cyclohexane ligand in Re(Cp)(CO)2(cyclohexane) In the case of 2b, there is a small thermodynamic preference for (2c), which has ␦ 13C ϭϪ22.4 (12). For further comparison, free 1 binding to the CH2 sites over the CH3 sites (11). Studies of species cyclohexane at the same temperature has JCH ϭ 128 Hz (both that induce C-H activation, such as [TpЈRhL] [TpЈϭTris-(3,5- hydrogens) and ␦ 13C ϭ 26.8. In the case of 2c, there was a clear dimethylpyrazolyl)borate; L ϭ CNCH2CMe3] have also shown a preference for the complexation of axial over equatorial bonds, kinetic preference for binding at secondary sites (22). However, it leading to a significantly more shielded 1H NMR shift and lower 1 is well known that there is a preference for C-H oxidative cleavage JCH value for the axial proton that complicates the comparison with to occur at primary carbons (5, 6). In the case of [ReCp(CO)(PF3)] 3c. It is possible that there is a steric factor playing a role here, the interacting with n-pentane, it is possible that the availability of an bulkier PF3 ligand tending to disfavor the coordination of the energetically more accessible primary alkyl hydride 4b leads to a sterically more encumbered axial sites relative to the situation in 2c preference for interaction with the primary carbon, the opposite of with the result that there appears to be almost no thermodynamic that which occurs with 2b. An alternative explanation is that preference for axial or equatorial sites in 3c. Regardless of which are incorporation of the sterically more demanding PF3 ligand leads to the axial and equatorial protons, the average of the two JCH preferred binding of the less hindered primary carbon. couplings observed in 3c (99.5 Hz) is significantly less than the Photolysis in cyclohexane. The preferred conformation of a cyclohex- corresponding average in the alkane complex 2c (110.7 Hz). It is ane ring is a chair in which the axial and equatorial hydrogens within likewise clear that the average JCH coupling of the bound CH2 in 3c aCH2 group are chemically distinct. At temperatures around 190 (99.5 Hz) is significantly larger than for the bound CH2 in cyclo- K, the rate of inversion of the chair forms, the process that can make pentane complex 3a (75 Hz). This finding suggests that in 3c,the axial and equatorial protons equivalent from an NMR perspective, complexed C-H bonds may be more stretched in comparison with is slow. By removing the equivalency of the CH2 protons in the those found in 2c but less stretched than in 3a. Alternatively (or alkane molecule, the two dynamic processes described in the simultaneously), there may be a contribution from alkyl hydride cyclopentane case, rotation of the alkane ligand and swapping of isomers, Re(Cp)(CO)(PF3)(cyclohexyl)H 4c, that are in fast ex- coordination of the metal center between the two CH2 hydrogens, change with alkane complex isomers 3c. The relative amount of will never make these two protons equivalent in the 1H NMR alkyl hydride isomers would be significantly lower for 3c than 3a. spectrum. Therefore, discrete resonances for axial and equatorial This proposed equilibrium between 3c and 4c is described in Fig. 7. hydrogens of a bound cyclohexane moiety are expected. The variety of possible orientations and geometries of the alkane, By using a procedure similar to that used with cyclopentane, a alkyl, or hydride ligands with respect to the PF3 and CO ligands solution of 1 in 50% cyclohexane:50% pentane-d12 was cooled to means that the observed signals may be the average of numerous 190 K. Upon photolysis, four new 19F NMR resonances were subtypes of different isomers. 1 observed to grow in simultaneously. One resonance at ␦ Ϫ3.13 ( JPF The value of JCH for the axial hydrogen (107 Hz) is higher 3 ϭ 1,213 Hz), likely with two unresolved JFH couplings, both Ͻ5 Hz, compared with the equatorial hydrogen (93 Hz) in bound cyclo- was assigned to Re(Cp)(CO)(PF3)(cyclohexane) (3c). The other hexane and to a C-H in bound cyclopentane (75 Hz). This finding resonances were due to the three isomers of suggests that complexed axial C-H bonds retain a mostly, if not Re(Cp)(CO)(PF3)(pentane-d12)(3b-d12) previously assigned in the completely, alkane complex character. By comparison, complexed studies in n-pentane. A noticeable increase in the intensity of the 19F equatorial C-H bonds in cyclohexane and bound C-H in cyclopen- resonance of 3c and corresponding decrease in intensity of the tane are becoming either progressively more stretched or have

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Fig. 9. Kinetic traces of ␯(CO) bands at Ϸ1,883–1,887 and 1,948–1,952 cmϪ1 after photolysis of 1 in pentane at 293 Ϯ 1K(a), cyclopentane at 292 Ϯ 1K(b), and cyclohexane 293 Ϯ 1K(c), all under 2 atm CO.

ratio of bands is clearly different from those obtained in the fitting CHEMISTRY of the Re(Cp)(CO)2(PF3) experiment (Fig. 8). Fig. 8. TRIR spectra showing 2b, 3b, and 4b.(a) FTIR of 1 in n-pentane under CO Multilorentzian fitting of the TRIR spectrum obtained after (2 atm). (b) TRIR difference spectra of the same solution 50 ␮s after photolysis (266 irradiation of 1 reveals the presence of an extra band at 1,955 cmϪ1. nm). (c) Expansion (1,965–1,875 cmϪ1) of TRIR spectrum of 1 in n-pentane. (d) We have recently shown that photolysis of Re(Cp)(CO)3 in scCH4 Expansion of TRIR spectrum of 5 in n-pentane under CO (2 atm) after photolysis. generates Re(Cp)(CO)2(CH4) and Re(Cp)(CO)2(CH3)H that exist in a rapid equilibrium at room temperature in solution (14). We Ϫ1 progressively greater contributions from the corresponding alkyl assign the overlapped band at 1,955 cm to Re(Cp)(CO)(PF3) hydride isomer (i.e., KЉϾKЈ in this case; see Fig. 7). (pentyl)H (4b). The similar lifetimes of 3b and 4b also indicate that both may be present in a rapid equilibrium. The assignments of 3b TRIR Studies. Pentane. The TRIR spectrum obtained after photolysis and 4b have been further supported by density functional theory of 1 in n-pentane under CO (2 atm) is shown in Fig. 8. It is clear calculations in which we have computed the IR frequencies (see that the parent ␯(CO) bands (1,941 and 2,004 cmϪ1) are bleached, the SI). and three transient bands can clearly be observed at Ϸ1,887, 1,923, Cyclopentane/cyclohexane. NMR experiments discussed above have and 1,953 cmϪ1. The transient bands are formed within the time indicated that the nature of the alkane solvent has a significant resolution of the apparatus (20 ns). Previous TRIR studies on the effect on the balance between coordination and activation of the alkane solvent. Experiments in cyclopentane have suggested the photolysis of 1 in scXe (17) have shown both Re(Cp)(CO)2Xe and Ϫ1 presence of a significant proportion of Re(Cp)(CO)(PF3) Re(Cp)(CO)(PF3)Xe are formed. The band at 1,923 cm is (cyclopentyl)H (4a), whereas in cyclohexane only a small fraction, assigned to Re(Cp)(CO)(PF3)(pentane) (3b). The band of 3b is not 0 Ϫ1 if any, of Re(Cp)(CO)2(cyclohexyl)H (4c) was indicated to be stable and decays monoexponentially [kobs ϭ 9.1 (Ϯ 0.1) ϫ 10 s , 292 Ϯ 1 K] in the presence of CO to reform 1. Comparison with the present. ␯(CO) band positions for known Re(Cp)(CO)2(alkane) complexes Ϫ1 allows the transient bands at 1,887 and 1,953 cm to be assigned Table 1. ␯(CO) band positions of complexes recorded to Re(Cp)(CO)2(pentane) (2b). However, the bands at 1,887 and at 292–293 K 1,953 cmϪ1 show different kinetic behavior (Fig. 9a). The low- Ϫ1 Ϫ1 Complex Solvent ␯(CO), cm frequency band at 1,887 cm decays monoexponentially [kobs ϭ Ϯ ϫ 2 Ϫ1 Ϯ Ϫ1 1.01 ( 0.01) 10 s , 292 1 K]. The band at 1,953 cm decays Re(Cp)(CO)2PF3 (1) Cyclopentane 1,939, 2,000 with biexponential kinetics. The faster component is in good Pentane 1,941, 2,004 Ϫ1 1 agreement with the decay of 1,887 cm [kobs ϭ 9.6 (Ϯ 0.2) ϫ 10 Cyclohexane 1,939, 2,000 Ϫ1 s , 291 Ϯ 1 K], and this confirms the band’s assignment to 2b. Re(Cp)(CO)3 (5) Cyclopentane 1,939, 2,030 Further evidence for this assignment is that in the presence of CO, Pentane 1,941, 2,031 Cyclohexane 1,939, 2,030 these bands decay to form Re(Cp)(CO)3 (5). The presence of two Ϫ components in the decay of the 1,953 cm 1 band indicates that a Re(Cp)(CO)2(C5H10)(2a) Cyclopentane 1,882, 1,951 second ␯(CO) band is heavily overlapped with the 2b band at 1,953 Re(Cp)(CO)2(C5H12)(2b) Pentane 1,887, 1,952 cmϪ1, and the slower decay is very similar to the band at 1,923 cmϪ1 Re(Cp)(CO)2(C6H12)(2c) Cyclohexane 1,883, 1,948 0 Ϫ1 Re(Cp)(CO)(PF3)(C5H10)(3a) Cyclopentane 1,922 [kobs ϭ 9.4 (Ϯ 0.9) ϫ 10 s , 291 Ϯ 1 K]. To obtain more information regarding this mystery peak, we monitored the pho- Re(Cp)(CO)(PF3)(C5H12)(3b) Pentane 1,923 Re(Cp)(CO)(PF )(C H )(3c) Cyclohexane 1,918 tolysis of 5 in n-pentane by TRIR to determine precisely the ␯(CO) 3 6 12 Ϫ1 Re(Cp)(CO)(PF3)H(C5H11)(4b) Pentane 1,955 bands of Re(Cp)(CO)2(pentane) (1,887 and 1,952 cm ), and the

Ball et al. PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6931 Downloaded by guest on September 27, 2021 To investigate these results further, we carried out analogous alkyl hydride equilibria in Figs. 4, 5, and 7 are fast on the NMR time experiments to those described above for pentane in cyclopentane scale at 215 K, but decoalescence phenomena are observed at lower and cyclohexane (see the SI). In both experiments, temperatures for 3a/4a and 3b/4b, possibly due to a slowing of Re(Cp)(CO)(PF3)(alkane) and Re(Cp)(CO)2(alkane) (alkane ϭ rotation and/or swapping of complexed and uncomplexed hydro- cyclopentane or cyclohexane) were observed. It was not possible to gens in an alkane ligand. establish whether Re(Cp)(CO)(PF )(alkyl)H was formed from Although CO and PF3 ligands are considered similar in terms of 3 ␲ inspection of the band ratios of Re(Cp)(CO)2(alkane) as described their -acceptor properties, there is a general shift toward products above for Re(Cp)(CO)(PF3)(pentyl)H (4b). Re(Cp)(CO)(PF3) that display a higher degree of oxidative cleavage when [Re(Cp) (alkane) reacts with CO to reform the parent [cyclopentane: kobs ϭ (CO)(PF3)] reacts with alkanes in comparison with [Re(Cp)(CO)2], 0 Ϫ1 which favors alkane complexation without activation. The com- 3.3 (Ϯ 0.1) ϫ 10 s , 292 Ϯ 1 K; cyclohexane: kobs ϭ 4.2 (Ϯ 0.2) ϫ 100 sϪ1, 293 Ϯ 1 K], slower than the corresponding reaction of plexes incorporating a PF3 ligand have greater thermal stability and react more slowly with CO. Re(Cp)(CO)2(alkane) to form Re(Cp)(CO)3. We found no evi- ␯ Ϸ Ϫ1 dence from the kinetics of the (CO) band at 1,950 cm of Materials and Methods Re(Cp)(CO)2(cyclohexane) (2c) for the presence of another species. NMR studies suggest that that the amount of cyclohexyl Re(Cp)(CO)2(PF3)(1) was synthesized by using the literature hydride species 4c present is much less than in the case of pentane procedure (17). or cyclopentane. Any possible concentration of 4c would be small NMR/Photolysis Experiments. NMR samples of 1 (Ϸ1 mg) in a and therefore difficult to detect due to the overlapping of the IR mixture of appropriate alkane and pentane-d (530 ␮l) were spectral features. In the TRIR experiment in cyclopentane, we 12 prepared. These samples were precooled (160–223 K) in the probe examined the decay of the two ␯(CO) bands of Re(Cp) of a Bruker (Rheinstetten, Germany) DPX 300, DMX 500, or (CO)2(cyclopentane) (2a) and both bands decay at a similar rate Ϫ1 ϭ Ϯ ϫ 1 Ϫ1 Ϫ1 ϭ DMX 600 NMR spectrometer. Light from a 100-W Hg arc lamp [1,883 cm : kobs 4.4 ( 0.2) 10 s ; 1,951 cm : kobs 3.8 was delivered to the top of the solution by using a single-core (Ϯ 0.1) ϫ 101 sϪ1, both at 292 Ϯ 1 K]. The slightly increased lifetime Ϫ1 fiber-optic as described in refs. 10 and 11. Progress of reactions was of the band at 1,951 cm may be due to a small concentration of monitored by using 19F and 1H NMR spectroscopy either on ␯ 4a, having (CO) bands overlapped with those of 2a again consis- separate, similar samples or by alternating experiment type. Con- tent with NMR studies. However, this assignment cannot be made ventional 19F spectra were obtained and were 1H-coupled. 1H NMR solely on the TRIR kinetics. spectra were obtained by using an excitation sculpting scheme to suppress the resonances of the free alkane (11). Collected NMR Conclusions spectra and further experimental details are given in the SI. Combined NMR and TRIR data reveal that the species produced

by the interaction of n-pentane with [Re(Cp)(CO)(PF3)] are an IR Experiments. Re(Cp)(CO)3 (Strem Chemicals, Newburyport, equilibrium mixture of Re(Cp)(CO)(PF3)(pentane) (3b) and MA; 99%) was used as received. Cyclopentane (Fluka, Buchs, Re(Cp)(CO)(PF3)(pentyl)H (4b). This contrasts with the Switzerland; HPLC grade), pentane (Lancaster, Ward Hill, MA; [Re(Cp)(CO)2] fragment, which only forms alkane complexes with Ͼ99%), and cyclohexane (Aldrich, St. Louis, MO; HPLC grade, n-pentane and has a slight preference for complexation of second- Ͼ99.9%) were distilled from CaH2 and degassed before use. All ary C-H bonds. The relative concentrations of 3b and 4b may be experiments were carried out by using a CaF2 cell (Harrick, temperature-dependent, with lower temperatures leading to an Ossining, NY; pathlength of 0.5–1.5 mm) under CO (2 atm) at room increase of the relative concentration of alkyl hydride, but accurate temperature. Fresh solution was flowed into the cell after every UV equilibrium constants are difficult to extract. Data for the products laser shot. Details of the diode laser-based TRIR apparatus are from the interaction of cyclopentane with [Re(Cp)(CO)(PF3)] described in ref. 23. Briefly, the IR source is a continuous-wave IR again suggest that an equilibrium between the cyclopentyl hydride diode laser (MDS 1100; Mu¨tek, Herrsching, Germany). In these 4a and the isomeric cyclopentane complex 3a exists. What is clear experiments, the change in IR transmission at one IR frequency was is that there is a trend in both the NMR and TRIR data on going measured by a fast MCT detector, after UV excitation of the sample from n-pentane to cyclopentane to cyclohexane with a progressive by a pulsed Nd:YAG laser (Quanta-Ray GCR-12; Spectra Physics, favoring of the proportion of alkane complex present. Hence, in the Mountain View, CA; 266 nm), which initiates the photochemical case of cyclohexane, even at 180 K there is only a small amount, if reactions. A spectrum was built up on a point-by-point basis in any, of the cyclohexyl hydride form 4c present. The NMR data custom software, by repeating this measurement at different IR suggest that there is little preference for the interaction of [Re(Cp) frequencies. (CO)(PF3)] with either axial or equatorial hydrogens in cyclohexane but that interaction with the equatorial site is more likely to This work was supported by FUJIFILM Imaging Colorants (A.J.C.), the European Union (P.P.), the Engineering and Physical Sciences Research result in the formation of 4c, based on the lower JCH and higher JPH Council (M.W.G.), SASOL (C.M.B.), the Australian Research Council values in the equatorial case. Alternatively, the JCH and JPH values (G.E.B.), the University of Nottingham (A.J.C.), the University of New may be indicative of a more stretched C-H interaction in the case South Wales (G.E.B.), and a J. W. T. Jones Fellowship from the Royal of binding of the equatorial hydrogen. All of the alkane complex- Society of Chemistry (to J.P.R.).

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