Article pubs.acs.org/Organometallics
Time-Resolved Infrared Spectroscopy Studies of Olefin Binding in Photogenerated CpRu(CO)X (X = Cl, I) Transients Sohail Muhammad,† Samuel J. Kyran,‡ Rajesh K. Raju,† Edward N. Brothers,† Donald J. Darensbourg,*,‡ and Ashfaq A. Bengali*,† †
Department of Chemistry, Texas A&M University at Qatar, Doha, Qatar Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
‡
ABSTRACT: The mechanism and energetics of ligand (L) substitution (L = THF, cyclopentene, cyclohexene, cyclooctene) from photogenerated CpRu(CO)(X)L (X = Cl, I) complexes were studied using time-resolved infrared spectroscopy. The reactions proceed through a dissociative mechanism, and the Ru−L binding enthalpies were estimated. The trend in the bond enthalpies, Ru− (η2-cyclooctene) ≈ Ru−(η2-cyclopentene) > Ru−(η2-cyclohexene), is correlated with the strain energy of the cycloalkene ring. For all ligands investigated, CpRu(CO)(Cl)−L binding enthalpies were lower than those for the analogous CpMn(CO)2−L and BzCr(CO)2−L complexes. DFT calculations indicate that the lower binding enthalpy for the Ru−L complexes is due to a greater reorganizational energy for the CpRu(CO)Cl fragment as it adopts a configuration suitable for interaction with the ligand.
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the alkene with high selectivity and yield.4 The mechanism of the reaction was not determined, although it was found that a key species was most likely the CpRu(CO)2H complex, presumably formed upon reaction of the parent compound with H2. Since this 18-electron coordinatively saturated hydride complex is unlikely to be the active species, it is reasonable to suggest that the active catalyst is the 16-electron complex CpRu(CO)(H), formed upon thermal dissociation of a CO ligand with subsequent binding of the alkene to the vacant coordination site. The possibility of generating the analogous CpRu(CO)Cl(η2-alkene) complex directly by photolysis of CpRu(CO)2Cl presents an attractive opportunity to investigate its reactivity with a view toward obtaining fundamental information about the Ru−(η2-alkene) bond. Furthermore, since η2-alkene species derived photolytically from the Cr(CO)6,6 CpMn(CO)3,7 and (η6-C6H5R)Cr(CO)38,9 (R = CH3, CF3) complexes have been detected, the results of the present study also provide an opportunity to compare the effect of varying the metal upon the strength and reactivity of the metal−(η2-alkene) interaction. In this work, the mechanism and energetics of the displacement of η2-coordinated cycloalkenes from the Ru center were investigated. The results suggest that the trend in the binding enthalpies correlate well with the strain energies of the cyclic olefins. In addition, the strength of the CpRu(CO)Cl−THF bond is also estimated and compared to that in the CpMn(CO)2THF and BzCr(CO)2THF (Bz = η6-C6H6) complexes. In all cases, the Ru− ligand binding enthalpies are lower than those for the Mn and Cr systems. Detailed theoretical studies were conducted to
INTRODUCTION Transition-metal-containing organometallic compounds have been shown to catalyze a variety of organic transformations.1 Complexes containing the ruthenium metal center are particularly active catalysts in a variety of reactions. For example, many key reactions such as alkene isomerization, hydroformylation, C−H activation of arenes, CO2 fixation, and others have been successfully catalyzed by derivatives of the Ru3 (CO) 12 cluster.2 Other Ru carbonyls such as Ru(CO)3(PR3)2 have also been used as catalysts for the hydrodesulfurization of benzothiophenes.3 The hydroformylation of 1-octene by CpRu(CO)2Cl has also been reported, and despite its high catalytic activity, there are no mechanistic studies that have been undertaken to identify and study the chemistry of the relevant intermediates.4 Photoinduced time-resolved infrared spectroscopy is a powerful tool for identifying and studying the reactivity of intermediates that may be formed during a catalytic cycle.5 A vacant coordination site on a metal complex, necessary for catalytic activity, can be generated by photolytic loss of a ligand. Upon binding of the substrate, the reactivity of the resulting intermediate can be probed with infrared light over a range of time scales. Metal carbonyls tend to be the best candidates for such studies, since photolytic loss of CO often proceeds with high quantum efficiency and the remaining CO’s attached to the metal center are ideal infrared tags for probing the temporal profile of the resulting intermediates. This technique is therefore well suited to study the reactions of Ru carbonyl intermediates, given their importance in promoting several catalytic processes. As mentioned above, the reaction of CpRu(CO)2Cl with 1octene in the presence of H2/CO leads to hydroformylation of © 2012 American Chemical Society
Received: March 10, 2012 Published: May 11, 2012 3972
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CpRu(CO)Cl intermediate.17 Our results are consistent with this observation. As shown in Figure 1, photolysis of a THF
better understand the observed reactivity patterns. Bond energy decomposition analysis (BEDA) suggests that the origin of this weaker binding is due to the large reorganizational energy required to prepare the Ru fragment for binding with either cycloalkenes or THF.
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EXPERIMENTAL AND THEORETICAL METHODS
A Bruker Vertex 80 FTIR equipped with both rapid-scan and step-scan capability was used for the kinetic studies. In addition, a laser flash photolysis apparatus with a CW-Quantum Cascade IR probe laser (Daylight Solutions, 1870−1980 cm−1, 100 mW) was used to study the reactivity of reactions on the microsecond time scale. Solution photolysis was conducted using 355 nm light from a Continuum Surelite I-10 laser. A temperature-controlled 0.75 mm IR cell with CaF2 windows was used for sample photolysis. For the step-scan experiments, the solution was flowed through the IR cell using a syringe pump to ensure photolysis of a fresh solution with every laser shot. A low-temperature IR flow cell (Graesby-Specac) with 0.5 mm CaF2 windows was used for experiments at temperatures 99% purity, while cis-cyclooctene was >95% purity. All cycloalkenes were dried and distilled over CaH2 prior to use. Both THF and heptane were anhydrous grade and >99% purity. All calculations were performed in the development version of the GAUSSIAN suite of programs,12 using the ωB97XD functional13 with the def2TZVPP basis set.14 All geometries were confirmed to be energy minima through vibrational analysis. Bond dissociation enthalpies (BDE’s) were calculated using theoretical energies and vibrational corrections at 298 K for the reaction
[M]−ligand(g) → [M](g) + ligand(g)
Figure 1. Difference IR spectrum obtained upon photolysis of a THF solution of CpRu(CO)2Cl. The positive peak at 1960 cm−1 is due to the formation of the CpRu(CO)Cl(THF) solvate, while the negative absorbances are due to destruction of the parent dicarbonyl. In a cyclohexane solution, the product peak is observed at 1962 cm−1 (Table 1).
solution of CpRu(CO)2Cl results in the destruction of the parent complex, evidenced by the bleaching of the parent bands at 2050 and 1994 cm−1. A single new CO stretching absorbance is observed at 1960 cm−1 (Table 1).18 Since it is wellTable 1. Position of CO Stretching Absorbances for the Relevant Ru Complexes at 293 K νCO (cm−1)
(1)
Distortion/preparative energies were determined via the following series of calculations. First, the gas-phase optimized geometries and energies were determined for each of the species in reaction 1. The [M]−ligand bond was then infinitely stretched, and energy calculations were performed on each species at fixed geometries; this effectively gives gas-phase energies of the unbound ligand and [M] at their bound geometries. The difference in energy between the relaxed and unrelaxed geometries is used here as the distortion/preparative energy. Finally, a brief mention should be made about selection of the spin states for these complexes. For CpRu(CO)Cl, the triplet state was calculated to be significantly higher in energy (>20 kcal/mol), and for CpMn(CO)2, there is experimental evidence that the singlet either is the ground state or is very close in energy to the triplet.15 Similarly, for BzCr(CO)2, previous calculations predict that the energy differences between singlet and triplet spin complexes are not significant.16 Thus, only singlet complexes were considered in the calculations.
a
complex
cyclohexane
CpRu(CO)2Cl CpRu(CO)Cl(THF) CpRu(CO)Cl(η2-cyclooctene) CpRu(CO)Cl(η2-cyclopentene) CpRu(CO)Cl(η2-cyclohexene) CpRu(CO)Cl(η2-pyridine) CpRu(CO)I(η2-cyclooctene) CpRu(CO)I(η2-pyridine)
2050, 1994 1962 1988 1986 1983 1959 1981 1957
calcda 1968 1988 1983 1986
Scaled by a factor of 0.94.
established that coordination of a solvent molecule to a vacant site generated upon photolysis of metal carbonyls occurs within picoseconds of CO loss,19 this species is most likely the monocarbonyl solvate complex CpRu(CO)Cl(THF). As shown in Figure 2, at 297 K in the presence of 2 M THF and 0.2 M cyclooctene, this transient reacts within 500 μs to form a species which grows in at the same rate with a CO stretching absorbance at 1988 cm−1 assigned to the CpRu(CO)Cl(η2-cyclooctene) complex. The high reactivity of the THF solvate complex is in marked contrast to that of species such as CpMn(CO)2(THF)20 and BzCr(CO)2THF,21 which are stable for several seconds under similar conditions. As shown in Figure 3, kobs exhibits a linear dependence on
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RESULTS AND DISCUSSION (a). Ru-THF. Previous low-temperature studies demonstrate that photolysis (300 < λ < 400 nm) of CpRu(CO)2Cl in nitrogen, methane, and CO matrices results in photoejection of a CO ligand rather than Ru−Cl bond cleavage to yield the 16e 3973
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The experimental estimates and the calculated values indicate that the Ru−THF bond is significantly weaker than that in analogous compounds such as CpMn(CO)2−THF (24 kcal/ mol)20 and BzCr(CO)2−THF (21 kcal/mol).21 Since the [M]−THF bond arises primarily due to THF → M σ donation, the strength of this interaction is expected to increase as the formal oxidation state of the metal increases. However, this trend is not observed, since the Ru2+−THF bond is weaker than the Mn+−THF and Cr0−THF interactions. The origin of this difference in binding strength is discussed later in light of DFT calculations. (b). Ru-(η2-cycloalkene). As shown in Figure 4, photolysis of CpRu(CO)2Cl in the presence of cyclooctene and pyridine results in the formation of both the CpRu(CO)Cl(η2cyclooctene) and CpRu(CO)Cl(pyridine) complexes. The η2 species converts completely to the pyridine complex over time. As shown in Figure 5, kobs approaches a limiting value at high [pyridine], suggestive of a dissociative mechanism of cyclooctene displacement (Scheme 1). Application of the steady-state assumption to the CpRu(CO) Cl intermediate results in the kobs dependence shown in eq 2.
Figure 2. Difference IR spectra obtained using step-scan FTIR upon photolysis of a cyclohexane solution of CpRu(CO)2Cl in the presence of 2 M THF and 0.2 M cyclooctene at 297 K. The spectra were obtained at 60 μs intervals. The initially formed CpRu(CO)Cl(THF) species absorbing at 1962 cm−1 converts to the relatively more stable CpRu(CO)Cl(η2-cyclooctene) complex absorbing at 1988 cm−1 within 500 μs.
kobs =
k1k 2[pyridine] k −1[cyclooctene] + k 2[pyridine]
(2)
The limiting value of kobs at high [pyridine] is k1, the step related to the dissociation of the Ru-(η2-cyclooctene) bond. A fit of the rate data to eq 2 yields both k1 and the selectivity ratio k2/k−1. Similar saturation behavior was also observed in the displacement of cyclohexene from the Ru center by pyridine (Figure 6). The displacement of cyclopentene, however, fell in a time regime that could not be easily probed by roomtemperature rapid-scan FTIR, necessitating the use of lower temperatures. Unfortunately, the reduced solubility of CpRu(CO)2Cl at these temperatures resulted in poor IR signal quality such that pyridine concentrations high enough to observe saturation behavior (i.e., k 2 [pyridine] > k−1[cyclopentene]) could not be achieved. However, at 288 K, kobs does exhibit limiting behavior, indicating that, as in the case of the other cycloalkenes, the displacement of cyclopentene from the Ru center by pyridine is also dissociative (Figure 6). At lower temperatures a linear dependence of kobs on [pyridine] was observed and, in this case, the slopes of the kobs vs [pyridine]/[cyclopentene] plots are expected to yield the collection of rate constants k1k2/k−1. The relevant rate constants are shown in Table 2. For the cyclooctene and cyclohexene reactions, the range of the selectivity ratios k2/k−1 is ∼2−8, indicating that the CpRu(CO)Cl fragment reacts more quickly with pyridine than with the cyclooalkenes, although the lack of a significant temperature dependence is consistent with either similar or, more likely, low barriers for the reactions. The displacement of cyclohexene is significantly faster than for the other cycloalkenes such that the temperature had to be lowered to 253 K to yield reaction rates on time scales comparable to the room-temperature reactivity of the cyclopentene and cyclooctene complexes. The reaction rates were fast enough (τ1/2 < 1 ms) at room temperature and higher to allow for the use of the flash photolysis technique mentioned above. As shown in Figure 6, the cyclopentene and cyclohexene ligands are displaced from the Ru center almost 10 and 3000 times faster than cyclooctene, respectively. This difference in
Figure 3. Plot of kobs versus [cyclooctene] for the displacement of THF (2.4 M) from CpRu(CO)Cl(THF) by cyclooctene.
[cyclooctene] at all the temperatures studied. Such dependence does not allow for a determination of the reaction mechanism, as it is consistent with an associative, dissociative, and interchange pathway of ligand substitution.22 The temperature dependence of the second-order rate constants (Table 2) yield activation parameters of ΔH⧧ = 13.7 ± 0.3 kcal/mol and ΔS⧧= +8 ± 1 eu. As discussed below and shown in Table 3, this value of ΔH⧧ is in good agreement with a calculated value of 15.6 kcal/mol for the CpRu(CO)Cl−THF bond dissociation enthalpy. It is therefore reasonable to conclude that the Ru− THF bond is broken in the transition state, an observation that is supportive of either a dissociative or Id mechanism of THF substitution by cyclooctene. The positive value of ΔS⧧ is consistent with this conclusion. 3974
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Table 2. Rate Constants for the Displacement of L from CpRu(CO)(Cl)L Complexes L=
THF
T (K)
10−4k2nd (M−1 s−1)
263 268 273 283 288 293 298 303 308 313 323
2.2 ± 0.2 4.7 ± 0.1
cyclooctene
cyclohexene
k1 (s−1)
0.55 1.2 2.1 3.6 6.8
± ± ± ± ±
k2/k−1
0.02 0.1 0.1 0.2 0.2
7.7 5.8 6.4 6.4 5.4
± ± ± ± ±
10.2 ± 0.4 21.5 ± 0.6
0.7 1.6 0.5 0.7 0.3
10−3k1 (s−1)
cyclopentene k1k2/k−1 (s−1)
k2/k−1
0.44 1.1 2.4 6.7 12.2 3.2 ± 0.1
4.4 ± 0.4
9.9 ± 0.4
2.6 ± 0.3
22.3 ± 2.0
2.2 ± 0.4
± ± ± ± ±
0.02 0.2 0.3 1.1 1.7
Table 3. Activation Parameters and Calculated BDE’s for Several [M]−THF and [M]−(η2-cycloalkene) Complexes complex
ΔH⧧ (kcal/mol)
Cp(CO)(Cl)Ru−THF Bz(CO)2Cr−THFa Cp(CO)2Mn−THFb Cp(CO)(Cl)Ru−(η2-cyclooctene) Cp(CO)2Mn−(η2-cyclooctene)c Bz(CO)2Cr−(η2-cyclooctene) Cp(CO)(I)Ru−(η2-cyclooctene) Cp(CO)(Cl)Ru−(η2-cyclopentene) Cp(CO)(Cl)Ru−(η2-cyclohexene) a
13.7 21.4 24.0 21.0 34.9 24.8 22.8 18.8 17.2
± ± ± ± ± ± ± ± ±
0.3 0.8 3.0 0.6 0.7 0.4 0.8 1.1 1.4
ΔS⧧ (eu)
calcd BDE (kcal/mol)
± ± ± ± ± ± ± ± ±
15.6 19.0 22.7 21.7 29.7, 32.9d 23.9
8 20.3 20.7 14 27.5 18 14 12 16
1 2.8 6.3 2 2.0 3 2 4 5
21.4 17.7
Reference 21. bReference 20. cReference 7. dReference 23.
the binding enthalpies with ΔH1⧧ = 21.0 ± 0.6 kcal/mol, 18.8 ± 1.1 kcal/mol, and 17.2 ± 1.4 kcal/mol for cyclooctene, cyclopentene, and cyclohexene, respectively.24 The large
Figure 4. Difference IR spectra obtained using rapid-scan FTIR upon photolysis of a THF/cyclohexane solution of CpRu(CO)2Cl with 0.7 M cyclooctene and 0.03 M pyridine at 298 K. The spectra were obtained at 2 s intervals and demonstrate the reaction of the initially formed CpRu(CO)Cl(η2-cyclooctene) complex to form the CpRu(CO)Cl(pyridine) product. Figure 5. Plot of kobs vs [pyridine] for the reaction CpRu(CO)Cl(η2cyclooctene) + pyridine → CpRu(CO)Cl(pyridine) + cyclooctene. The cyclooctene concentration was held constant at 2.3 M. The reaction solution also had some THF (0.6 M) added to aid in the solubility of the parent CpRu(CO)2Cl complex. The saturation behavior of kobs is indicative of a dissociative mechanism of alkene displacement by pyridine. The solid lines represent fits to the data according to eq 2.
reactivity is suggestive of weaker binding of cyclohexene to the Ru center in comparison to the other cycloalkenes. Given that cycloalkene substitution is dissociative in nature, steric factors are not likely to be the reason for the observed reactivity trend, since then cyclooctene displacement would be the fastest. Eyring analysis (Figure 7) provides an estimate for 3975
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Scheme 1
Figure 7. Eyring plots for the displacement reaction of η2-coordinated cyclohexene and cyclooctene from the CpRu(CO)Cl fragment by pyridine.
However, as discussed below, DFT calculations point to differences in the promotion energy required for the metal fragments to adopt the necessary geometries to bind to the alkenes as the primary factor in determining the relative bond strengths. The displacement of cyclooctene from the iodo complex CpRu(CO)I(η2-cyclooctene) was also studied. Like the analogous chloro complexes, kobs displays saturation behavior as a function of [pyridine] consistent with a dissociative substitution mechanism. However, as shown in Figure 8, the iodo complex reacts almost 25 times more slowly than the chloro complex. The lower CO stretching frequency of the iodo species (Table 2) is indicative of increased electron density on
Figure 6. Plot of kobs vs [pyridine] for the displacement of η2coordinated cycloalkenes from the CpRu(CO)Cl fragment. The cycloalkene concentrations were held at 2.3, 2.2, and 2.0 M for cyclooctene, cyclopentene, and cyclohexene, respectively. Note that kobs is significantly higher for the substitution of cyclohexene by pyridine, which is primarily due to a weaker interaction with the Ru center relative to that with the other cycloalkenes. The solid lines represent fits to the data according to eq 2.
positive ΔS1⧧ values are consistent with a dissociative substitution mechanism, as are the activation enthalpies, which are in good agreement with calculated Ru−(η2cycloalkene) BDE’s (Table 3). To investigate the effect of the metal center on the reactivity of the metal−(η2-alkene) bond, the substitution of cyclooctene from the BzCr(CO)2 fragment by pyridine was studied. This reaction also proceeds by a dissociative mechanism, since kobs shows saturation as a function of [pyridine]. At 303 K, k1 is almost 55 times larger for the Ru in comparison to that for the Cr complex. From the temperature dependence of k1, the BzCr(CO)2−(η2-cyclooctene) bond strength is estimated at 24.8 ± 0.4 kcal/mol, consistent with a calculated value of 23.9 kcal/mol. As in the case of the THF system, the [M]−(η2cyclooctene) bond strengths are 4−14 kcal/mol lower for [M] = CpRu(CO)Cl than for CpMn(CO)2 (34.9 kcal/mol)7 and BzCr(CO)2 (24.8 kcal/mol). The weaker binding of alkenes to the Ru2+ center may be a consequence of reduced M→alkene π back-bonding in the relatively electron deficient Ru complex.
Figure 8. Plot of kobs vs [pyridine] for the displacement of η2coordinated cyclooctene from the CpRu(CO)I and CpRu(CO)Cl fragments at 293 K. The iodo complex reacts almost 25 times more slowly than the chloro complex at this temperature. 3976
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the Ru center, making it tempting to speculate that the origin of this reactivity difference is a stronger π back-bonding interaction in CpRu(CO)I(η2-cycloalkene). A ΔH1⧧ value of 22.8 ± 0.6 kcal/mol is 1−2 kcal/mol higher for X = I than for X = Cl, suggestive of stronger binding in the former case. The trend in the bond enthalpies Ru−(η2-cyclooctene) ≈ Ru−(η2-cyclopentene) > Ru−(η2-cyclohexene) is correlated with the strain energy of the cycloalkene ring. A back-bonding interaction between the alkene and the metal center reduces the sp2 character of the coordinated carbon atoms, affording some degree of strain relief for the cycloalkene ligand. Of the three cycloalkenes, previous calculations26 show that cyclohexene has the lowest while cyclooctene and cyclopentene have the greatest (and similar) strain energies. Consequently, binding of cyclooctene and cyclopentene to Ru results in more strain relief and therefore a larger bond enthalpy. This trend has also been observed in the binding of cycloalkenes to the CpMn(CO)2 and M(CO)5 fragments.7,27 In the hydroformylation of alkenes by CpRu(CO)2Cl, a Ru− (η2-alkene) complex is a likely intermediate. With bond enthalpies of ∼20 kcal/mol, the Ru−(η2-alkene) bond is neither too strong nor too weak to hinder the subsequent reactivity of this intermediate toward H2 addition and CO insertion necessary to generate the aldehyde. To better understand the nature of alkene binding to the Ru center, detailed DFT calculations were conducted.
Table 4. Selected Calculated Bond Distances (Å) for Some [M]−L Complexesa L=
THF
CC (uncoordinated) CC (coordinated) M−C1 M−C2 M−O CC (coordinated) M−C1 M−C2 M−O CC (coordinated) M−C1 M−C2 M−O
cyclopentene
[M] = CpRu(CO)Cl 1.325 1.383 2.266 2.313 2.218 [M] = CpMn(CO)2
cyclohexene
cyclooctene
1.327 1.382 2.321 2.326
1.328 1.383 2.297 2.328
1.386 2.179 2.199 2.103 [M] = BzCr(CO)2 1.381 2.266 2.314 2.203
a
Here M−C1 and M−C2 refer to the distances between the metal center and the coordinated olefinic carbons.
coordinated olefinic carbons is shorter for the more strongly coordinated Mn and Cr systems. (a). Trends in [M]−L Binding. As mentioned earlier, the interaction of THF and cyclooctene with CpRu(CO)Cl is weaker than that with the CpMn(CO)2 and BzCr(CO)2 fragments. To understand the origin of these binding enthalpy differences, a bond energy decomposition analysis (BEDA) was conducted. Previous studies have suggested that the energetic cost of deforming the geometry of the free ligand to one that is suitable for bonding to the metal center can be important in determining the overall trend in metal−ligand binding energies.6d,27,28 Thus, following the example of Frenking and co-workers,29 the overall [M]−(η2-cycloalkene) and [M]−THF bond energies (De) were separated into two terms, ΔEprep and ΔEint:
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THEORETICAL MODELING To lend support to the experimental findings, detailed DFT modeling of the Ru and related Mn and Cr complexes was performed. As shown in Tables 1 and 3, the calculated CO stretching frequencies and BDE’s are in good agreement with the experimental values, indicating that this level of theory is adequate for modeling the relevant complexes. Calculated structures of the [M]−(η2-cyclooalkene) complexes are shown in Figure 9, and the relevant bond distances are given in Table 4.
−De = ΔEprep + ΔE int
(3)
where ΔEprep = ΔEprep,[M] + ΔEprep,L
(4)
The ΔEprep terms reflects the preparative or promotion enthalpy required to distort the ligand (ΔEprep,L) and metal fragment (ΔEprep,[M]) from their equilibrium geometries to those suitable for bonding. The interaction energy between the prepared fragments is given by ΔEint and, as such, reflects the strength of the metal−ligand interaction once the two reactants have achieved their binding confirmations (Figure 10). As shown in Table 5, the primary reason for the weaker [Ru]−L interaction now becomes apparent and can be attributed to the larger ΔEprep,[M] value required to prepare the CpRu(CO)Cl fragment for binding to THF or cyclooctene. As shown in Figure 11, unlike the 16-electron fragments CpMn(CO)2 and BzCr(CO)2, which have pyramidal geometries similar to those in the final products, CpRu(CO)Cl is planar while the CpRu(CO)Cl−L complexes are pyramidal (Figure 9). Therefore, ΔEprep,Ru is more than 25% of the overall interaction energy (ΔEint) between the promoted metal fragment and the ligand. In contrast, ΔEprep,[M] values for the CpMn(CO)2 and BzCr(CO)2 fragments are negligible, implying that little steric reorganization is required for these
Figure 9. Calculated structures for the [M]−(η2-cycloooctene) complexes.
The CC bond length difference between the complexed and uncomplexed alkenes is almost 0.06 Å, suggestive of a modest Ru→η2-alkene back-bonding interaction. Interestingly, η2 coordination of cyclooctene to the relatively more electron rich Mn and Cr centers also results in a similar elongation of the CC bond. On the basis of these observations, it is reasonable to conclude that the covalent contribution to the metal−(η2-alkene) bond in these complexes is primarily the result of L→M σ donation. While there is no correlation between the calculated BDE and the CC bond length in the metal complexes, the distance between the metal center and the 3977
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(b). Trends in [Ru]−(η2-cycloalkene) Binding. The BEDA results for the Ru−(η2-cycloalkene) bond are shown in Table 6. The interaction enthalpies (ΔEint) for all three Table 6. BEDA Results for the CpRu(CO)Cl−(η2cycloalkene) Complexes
Table 5. BEDA Results for [M]−L Complexes CpRu(CO)Cl
CpMn(CO)2
BzCr(CO)2
ΔEprep,[M] ΔEprep,L ΔEint De
7.1 0.3 0.7 0.4 −25.4 −25.1 17.6 24.4 L = Cyclooctene 11.2 1.5 7.9 10.1 −43.0 −43.7 23.9 32.1
ΔEprep,L (kcal/mol)
ΔEint (kcal/mol)
De (kcal/mol)
cyclopentene cyclohexene cyclooctene
10.4 12.7 11.2
8.1 9.2 7.9
−42.0 −41.6 −43.0
23.5 19.7 23.9
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L = THF ΔEprep,[M] ΔEprep,L ΔEint De
ΔEprep,[Ru] (kcal/mol)
cycloalkenes are similar. The ΔEprep,L term is ∼1 kcal/mol higher for cyclohexene than for cyclopentene and cyclooctene, consistent with the larger strain energy of the latter two ligands, which is relieved as the ligands adopt configurations suitable for interaction with CpRu(CO)Cl. Surprisingly, ΔEprep,[Ru] is largest for binding to cyclohexene, which also contributes to the reduced stability of the Ru−(η2-cyclohexene) interaction relative to that of the C5 and C8 systems. It may be speculated that, while cyclooctene is the larger ligand, it is also more easily deformed, requiring less steric reorganization for the metal fragment, as would also be the case for the smaller cyclopentene ligand. In contrast, cyclohexene, which is more rigid than cyclooctene and larger than cyclopentene, results in relatively more geometric reorganization of the Ru fragment. The lower Ru−(η2-cyclohexene) BDE relative to those for the other cycloalkenes is therefore the result of not only the difference in strain energy but also the larger energetic cost of preparing the CpRu(CO)Cl fragment for bonding.
Figure 10. Energy diagram obtained from a bond energy decomposition analysis (BEDA) for the [M]−(η2-cyclooctene) complex ([M] = CpRu(CO)Cl)). Note the difference in geometry between the ground state [M] and the promoted fragment [M]*, which results in a large ΔEprep,[M] value. The overall bond energy (De) is a function of the total promotion energy required to prepare [M] and L for binding (ΔEprep,[M] + ΔEprep,L) and the interaction energy between the promoted species (ΔEint). All values are in kcal/mol.
[M] =
L
0.7 0.3 −21.8 20.8
CONCLUSIONS Photolysis of CpRu(CO)2Cl in the presence of L = cycloalkenes, THF results in CO loss and the generation of the respective CpRu(CO)Cl(η2-cycloalkene) and CpRu(CO)Cl(THF) complexes. The ligand substitution reactions proceed through a dissociative mechanism, and the activation parameters yield estimates for the Ru−L binding enthalpies. The trend in the bond enthalpies Ru−(η2-cyclooctene) ≈ Ru− (η2-cyclopentene) > Ru−(η2-cyclohexene) is correlated with the strain energy of the cycloalkene ring. The strength of the Ru−L interaction was found to be lower than that in the analogous CpMn(CO)2L and BzCr(CO)2L complexes. Unlike the CpMn(CO)2 and BzCr(CO)2 metal fragments, which have pyramidal geometries, ground-state CpRu(CO)Cl is calculated to have a planar structure. Thus, the primary reason for the weaker binding is due to the greater reorganizational energy required to promote the CpRu(CO)Cl fragment to a pyramidal geometry that is suitable for binding to the ligands.
2.2 9.4 −37.6 26.0
Figure 11. Optimized ground-state geometries of the 16-electron metal fragments. The Ru species adopts a planar configuration, while the Mn and Cr fragments have a pyramidal geometry.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
metal systems to bind to the THF and cyclooctene ligands. It is also interesting to note that ΔEint values are similar for the Ru2+ and Mn+ systems while it is less favorable for the Cr0 complexes. This observation is consistent with the relatively higher electron density in the Cr complexes which would tend to reduce the degree of L→M σ donation, resulting in a weaker net bonding interaction between the prepared fragments.
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ACKNOWLEDGMENTS This publication was made possible by NPRP Grant No. 09157-1-024 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. 3978
dx.doi.org/10.1021/om300197b | Organometallics 2012, 31, 3972−3979
Organometallics
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Article
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