From Stoichiometric to Catalytic Binuclear Elimination in Rh–W

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From Stoichiometric to Catalytic Binuclear Elimination in Rh
W Hydroformylations. Identification of Two New Heterobimetallic Intermediates Chuanzhao Li,* Feng Gao, Shuying Cheng, Martin Tjahjono, Martin van Meurs, Boon Ying Tay, Chacko Jacob, Liangfeng Guo, and Marc Garland* Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Singapore, 627833

bS Supporting Information ABSTRACT: Cyclopentene hydroformylation, both stoichiometric and catalytic, is performed starting with Rh4(CO)12 and CpW(CO)3H as precursors. Synergism is observed. Isotopic labeling experiments confirm 30% of the product formation occurs via a catalytic binuclear elimination reaction. Two new intermediates are spectroscopically identified using band target entropy minimization and DFT, namely, RhW(CO)7Cp and CpW(CO)3H-C5H9CORh(CO)4. The latter intermediate appears to be hydrogen bonded via two simultaneous η5C5H4
H 3 3 3 OdC interactions, one with the CO group on the acyl moiety and the other with a CO on rhodium.

’ INTRODUCTION Synergism in homogeneous catalysis has often been described as the combined application of more than one metal, leading to regio-, chemo-, and stereoselectivities and/or activities that differ significantly from a strictly additive effect.1Although a number of homogeneous catalytic systems display some sort of synergism, the mechanistic origin of this effect is seldom confirmed. Three leading mechanistic candidates are (1) cluster catalysis,2 where each and every intermediate is a mixed-metal species; (2) systems where the second metal assists ligand abstraction to open a coordination site, i.e., the Cativa I/Ru/Ir acetic acid system;3 and (3) catalytic binuclear elimination, where there are two sets of mononuclear complexes MLx and M0 Ly and a set of dinuclear complexes MM0 Lz and where these complexes are involved in one overall mechanism, i.e., catalytic binuclear elimination reactions (CBER).4 The increased use of in situ spectroscopic techniques as well as sophisticated signal processing and chemometric techniques, particularly band target entropy minimization (BTEM),5,6 has allowed a much better understanding of many transition metal homogeneous catalytic systems.7 This approach generally allows the identification of new intermediates, the in situ evaluation of reaction rates, and a better understanding of the relationship between the instantaneous distribution of complexes and the activity of the system. Such in situ studies have been an important contributing factor to a better understanding of synergistic effects, as attested by several studies.4 In the continuing search for CBER,4 we investigate in the present contribution the combined application of Rh4(CO)12 r 2011 American Chemical Society

(1) and CpW(CO)3H (Cp = η5-C5H5; 2) in the hydroformylation of alkenes. In situ FTIR measurements are made, signal processing and chemometric techniques are applied, and spectral results for new intermediates are verified using DFT.

’ RESULTS AND DISCUSSION A stoichiometric hydroformylation8 was first performed by addition of cyclopentene (3) to an n-hexane solution containing 1 and 2 at 298 K. The in situ high-pressure FTIR spectra of the stoichiometric hydroformylation were analyzed using the BTEM algorithm in order to obtain the pure component spectra. Figure 1 shows the pure component spectra of the known solutes present. These spectra are consistent with authentic references as well as spectra obtained from previous in situ high-pressure rhodium-catalyzed hydroformylations.4 In addition, one new and previously unobserved pure component spectrum was reconstructed. This spectrum corresponds to the heterobimetallic complex RhW(CO)7Cp (6). Figure 2 shows the BTEM spectral estimate as well as the DFT-predicted spectrum. The relative intensities as well as band positions of the spectral estimate and the predicted spectrum are quite similar. The observed carbonyl vibrations appear at 1922, 1931, 1994, 2010, 2036, and 2088 cm
1. The geometry for the fully optimized DFT structure is provided in the Supporting Information. Received: April 9, 2011 Published: July 25, 2011 4292

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Scheme 1. Possible Set of Reactions Consistent with the Formation of 4
6 in the Stoichiometric Hydroformylation

Figure 1. BTEM spectral estimates of the known solutes 1
3, RCORh(CO)4 (R = C5H9;4), and cyclopentane carboxaldehyde (5) obtained from analysis of the in situ FTIR data.

Figure 2. BTEM spectral estimate and DFT-predicted spectrum of 6 as well as the optimized geometry of 6.

Figure 3. Relative concentration profiles of the solutes 1
6 during a stoichiometric hydroformylation conducted at 2.0 MPa CO at 298 K. See Supporting Information for details of the experimental design, i.e., changes in CO, 3, etc.

Relative concentration profiles were obtained for the stoichiometric hydroformylation using the above BTEM spectral estimates. Prior to addition of 3, negligible reaction between 1 and 2 was observed. However, upon addition of 3, the formation of 4
6 was observed (Figure 3). Further additions of alkene resulted in even higher yields of 4
6. Throughout the entire

stoichiometric hydroformylation, observable quantities of all species 1
6 were simultaneously observed. After each addition of 3, a new set of concentrations was rapidly achieved. These results suggest that an observable equilibrium was achieved between the various reactants and products. The results also suggest that, in some manner, the hydride 2 is assisting the fragmentation of 1 and its conversion to the acyl 4 as well as its involvement as a hydrogen source for aldehyde formation The hydride-assisted fragmentation of 1 as well as the overall stoichiometric hydroformylation can be explained by a set of metal-mediated steps (Scheme 1). The nonelementary step (a) results in the fragmentation of 1 and formation of HRh(CO)4 (7) as well as 6. Step (b) transforms 7 to 4. Step (c), which is a stoichiometric binuclear elimination,9 cleaves 4 to form aldehyde 5. At ca. 160 min of the stoichiometric hydroformylation, 1.0 MPa hydrogen was added to the system. This resulted in the rapid disappearance of the dinuclear complex 6 and a rapid increase in both 2 and 4. This clearly indicates the rapid activation of molecular hydrogen on the dinuclear complex 6. The concentration of 1 momentarily increased just slightly and then monotonically decreased. Upon addition of hydrogen (Figure 3), the system became catalytic, as clearly indicated by the rapid increase of aldehyde 5. The first turnover of the system occurs at ca. 180 min (20 min after addition of H2). Subsequently, separate catalytic hydroformylations were performed. In this set of experiments similar initial loadings of 1 and different initial loadings of 2 were used. The experiments covered a range of W/Rh from 0 to 2.67. As shown in Figure 4a, increasing concentrations of hydride 2 resulted in increasing rates of aldehyde formation; that is, W/Rh = 2.67 resulted in ca. 50% increase in aldehyde yield at 250 min compared to W/Rh = 0. Moreover, increasing concentrations of 2 resulted in an increased yield of 4 (Figure 4b). This observation further supports the idea that hydride 2 can induce the fragmentation of 1 to acyl 4 in some manner. Figure 4c shows the resulting turnover frequency (TOF) for the systems. Since the TOF is defined in terms of the instantaneous concentration of 4, the increased TOF as a function of increasing 2 suggests that tungsten is having a genuine synergistic effect on the rhodium catalytic cycle. As observed in Figure 4a, 2 alone does not show any catalytic activity. In order to better understand the effect of hydride 2 on the rhodium-catalyzed hydroformylation, and particularly the increased TOF in the bimetallic system, a deutero-formylation was performed. This experiment was conducted with an nhexane solution containing cluster 1 and alkene 3 under 3.5 MPa CO and 1.2 MPa D2 at 298 K. After ca. 330 min, the active system consisted primarily of C5H8DCORh(CO)4 (8) with little observable precursor 1. In other words, at 330 min the induction period was over, the primary intermediate 8 was at a pseudo-steadystate concentration, and 3 was being catalytically transformed to 4293

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Figure 6. (a) New BTEM spectral estimate, (b) composite spectrum created from a 1:1 superposition of the BTEM spectral estimates of 2 and 4, and (c) DFT spectral prediction of the hydrogen-bonded dinuclear complex 10. Arrows indicate split in the major carbonyl vibration of the W moiety.

Figure 4. (a) Concentration of aldehyde 5 (mole fraction), (b) concentration of acyl 4 (mole fraction), and (c) TOF (min
1) versus time for catalytic hydroformylations conducted at 1.7 MPa CO, 1.7 MPa H2, and 298 K.

Figure 5. BTEM spectral estimates of hydride 2: (i) n-hexane solution of 2 alone ( 3 3 3 ) and (ii) during a mixed Rh
W hydroformylation (___). Note the shift and considerable broadening of the major band at 1939 cm
1.

C5H8DCDO (9). RCDO is clearly identified by the CdO vibration at 1721 cm
1 (see Supporting Information for further information). At this point, an n-hexane solution consisting of 2.6  10
4 moles of hydride 2 was injected into the autoclave. Over the next few minutes, an additional 2.1  10
4 moles of RCHO was formed (CdO at 1733 cm
1). Since the H label appears almost exclusively in the formyl group of the aldehyde, the experimental results suggest that a binuclear elimination between 2 and 4 (see Scheme 1c) is responsible for the synergism observed in the present catalytic Rh
W system. Moreover, a comparison of initial rate of RCHO formation indicates that ca. 30% of the aldehyde formation in the mixed-metal system comes from the synergistic pathway (CBER). Additional spectroscopic analysis of the in situ high-pressure FTIR data in this study indicated that the pure component

spectrum of the hydride 2 is not constant. In particular, the major carbonyl band at 1939 cm
1 was shifted to slightly lower wavenumbers in the Rh
W mixed-metal system. In addition, the line-width of the 1939 cm
1 band significantly broadens. Figure 5 shows BTEM spectral estimates of (i) 2 from a solution where no rhodium or hydroformylation product is present and (ii) 2 from a mixed-metal hydroformylation solution. The carbonyl shifts observed in Figure 5 suggest significant interactions between the hydride 2 and another solute. Hydrogen bonding between 2 and the product 5 could be one type of interaction. Indeed, after ca. 30 min of catalytic reaction, the concentration of 5 is much higher than any of the organometallic solutes. Figure 4a,b supports this assertion. Moreover, the concentration of intermediate 4 is more or less constant in the pure rhodium experiment after ca. 120 min, whereas, in the Rh
W experiments, a steady decline in the concentration of 4 is detected after ca. 120 min. If 2 is hydrogen bonding with aldehyde 5, then 2 will probably be less available for the fragmentation of 1 (see Scheme 1). Indeed, the concentration profile of 2 declines as a function of reaction time (see Supporting Information Figure S3). Volume of interaction measurements were made to assess interactions in the three-component system n-hexane/2/5. Statistically significant solute
solute interactions exist between 5
5 and 2
5, but not 2
2. Details can be found in the Supporting Information. BTEM analysis of the catalytic data provided a new and previously unobserved pure component spectrum (Figure 6a). This new spectral estimate shows (i) the major carbonyl vibration of hydride 2, (ii) carbonyl vibrations from acyl 4, and (iii) the acyl moiety at ca. 1698 cm
1. Moreover, the acyl vibration and many of the carbonyl vibrations in Figure 6a have broadened in comparison to the pure component spectra of 2 and 4. Therefore, the BTEM spectral estimate suggests a weak interaction, of some sort, between 2 and 4. Accordingly, the BTEM spectral estimate was compared to a simple 1:1 superposition of BTEM spectral estimates of 2 and 4 and a DFT spectral prediction. The DFT was performed on a hydrogen-bonded heterobimetallic complex with the stoichiometry CpW(CO)3H-C5H9CORh(CO)4 (10), where hydrogen bonding occurs between the Cp ring on tungsten and the acyl carbonyl on rhodium, i.e., η5-C5H4-H 3 3 3 OdC. At this 4294

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Scheme 2. Proposed Catalytic System for Rh
W Hydroformylation Where Both Unicyclic and CBER Catalysis Take Placea

a

The mononuclear species 2 and 4 and the two new hetero-bimetallic complexes 6 and 10 are observable (blue highlight).

point, it should be noted that crystallographic data have been extensively used to support hydrogen-bonding hypotheses on organometallic systems. In many cases, hydrogen bonding of the type C5H4-H 3 3 3 OdC is the predominant scenario in the solid state.10 Finally, the 1:1 superposition and the DFT-predicted spectrum are shown in Figure 6b,c. The 1:1 superposition is very similar to the BTEM spectral estimate. However, the DFTpredicted spectrum shows, and is more consistent with, the clear split apparent for the major carbonyl vibration of the W moiety. In summary, complex spectral changes were observed in the vicinity of 1939 cm
1 in the experimental FTIR spectra obtained during the Rh
W catalytic hydroformylations. These complex changes are due to the simultaneous presence of (i) free 2, (ii) interaction of 2 with the aldehyde 5, and (iii) the formation of the hydrogen-bonded dinuclear complex 10. Hydrogen bonding between the Cp ring on CpMo(CO)3H and 4 has been confirmed in the Rh
Mo hydroformylation system.4f A catalytic reaction scheme consistent with the experimental results is shown in Scheme 2. The main findings were (i) catalytic binuclear elimination occurs between hydride 2 and rhodium intermediate 4 resulting in aldehyde 5 and heterobimetallic intermediate 6, and this mechanism is responsible for ca. 30% of the product formation at these conditions, (ii) molecular hydrogen is readily activated by the new heterobimetallic complex 6, (iii) FTIR and DFT spectroscopic evidence suggests that the weak hydrogen bonded species 10 has two simultaneous interactions of the type η5-C5H4-H 3 3 3 OdC, where OdC represents both an acyl moeity and a carbonyl on rhodium

(see Figure S6), and (iv) independent physicochemical experiments for volumes of interaction confirm that significant solute
solute interactions are present.

’ EXPERIMENTAL SECTION The organometallic reagents used in the present study include 1 (Strem) and 2 (Sigma Aldrich, as well as in-house synthesis). Purity determination and characterization of all reagents were performed by FTIR and NMR (1H, 13C, 183W). All preparations and transfers were carried out using purified gases and glovebox and Schlenk techniques. The in situ spectroscopic studies were performed using a Bruker Vertex70 FT-IR spectrometer and an in-house-constructed 100 mL autoclave. High-pressure injections were made with a Harvard PHD 4400 syringe pump. All FTIR signal processing was performed using in-house algorithms (for BTEM and concentrations). DFT was performed with Gaussian 09. Online volumes of interaction were measured with an Anton Paar DMA 5000. Experimental details including design of experiments, as well as detailed DFT-optimized geometries, are provided in the Supporting Information.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed experimental setup; detailed volumetric measurement results; detailed DFT prediction results. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; li_chuanzhao@ices. a-star.edu.sg.

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