ARTICLE pubs.acs.org/Organometallics
Synthesis, Characterization, and Reactivity of Arene-Stabilized Rhodium Complexes Abby R. O’Connor,† Werner Kaminsky,‡ D. Michael Heinekey,‡ and Karen I. Goldberg*,‡ † ‡
Department of Chemistry, The College of New Jersey, 2000 Pennington Road, Ewing, New Jersey 08628-0718 Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States
bS Supporting Information ABSTRACT: The synthesis and characterization of (COD)Rh(I) and (NBD)Rh(I) (COD = cyclooctadiene; NBD = norbornadiene) chloride complexes containing the 2-(dicyclohexylphosphino)biphenyl (PCy2biPh) ligand are reported. Abstraction of the halide with Na(BArF)4 yields cationic Rh(I) complexes [(NBD)Rh(PCy2biPh)][B(ArF)4] (2) and [(COD)Rh(PCy2biPh)][B(ArF)4] (7) (ArF = 3,5-bis(trifluoromethyl)phenyl). In complex 2, the pendent arene of the ligand is coordinated in an η2-fashion to rhodium. Complex 7 exists in two configurations that were characterized by low-temperature NMR spectroscopy. One structure is analogous to 2 with η2-coordination of the arene, and the other exhibits η6-coordination. These structures interconvert on the NMR time scale at room temperature. Addition of H2 to complex 2 yields the Rh(III) dihydride complex [(PCy2biPh)RhH2][B(ArF)4] (5), while the addition of H2 to 7 generates the Rh(I) olefin complex [(COE)Rh(PCy2biPh)][B(ArF)4] (8). In both 5 and 8, the pendent arene of the ligand is bound η6 to Rh. Benzene hydrogenation to cyclohexane using 2 as a catalyst precursor is described. Poisoning experiments indicate that heterogeneous rhodium is likely to be the active catalyst in this arene hydrogenation reaction.
’ INTRODUCTION Transition metal arene complexes have shown utility as catalysts for organic transformations. Most commonly, metals are observed to coordinate all six carbons of an arene ring (η6), but coordination of only four carbons (η4) or two carbons (η2) has also been observed. In some catalytic reactions, the ability of the arene to change hapticity and even dissociate has been implicated as key to the performance of the catalyst. Thus, developing strategies to encourage arene coordination as well as to promote partial or full dissociation are valuable for the design of transition metal catalysts bearing arene ligands. There have been several recent reports of transition metal complexes containing phosphine ligands bearing biaryl rings where coordination of the metal occurs through the phosphine and also through the π-system of the arene. The arene binding mode observed in the complexes depends on the donor ability of the other ligands in the coordination sphere of the metal.1,2 Faller reported η6 -arene coordination of an arene group of a biphenyl phosphine ligand to ruthenium forming a half-sandwich complex.3,4 In contrast, with the same biphenylphosphine ligand, an allyl Pd complex exhibits only η2 -coordination to the arene. 5 These examples suggest that arene coordination can be used to stabilize low-coordinate r 2011 American Chemical Society
phosphine intermediates that are generated within a catalytic cycle.
Such arene coordination is important in Buchwald’s successful application of bulky biaryl dialkyl phosphine ligands in palladium-catalyzed CC cross-coupling and amination reactions.6 While the steric protection provided by the bulky biphenyl group appears to inhibit the oxidation of these biphenyldialkyl phosphines,7 it is also postulated that Pdarene interactions help to stabilize reactive species and increase the electron density at the metal center during catalytic reactions.6,8 Bulky biaryl dialkyl phosphine ligands have also been employed in Rhcatalyzed additions of alkynes to activated aldehydes and ketones,9,10 Rh-catalyzed hydroamination of alkenes,11,12 and Received: September 30, 2010 Published: March 29, 2011 2105
dx.doi.org/10.1021/om1009473 | Organometallics 2011, 30, 2105–2116
Organometallics
ARTICLE
Scheme 1. Synthesis of (NBD)Rh Phosphine Complexes 1 and 2
Ir-catalyzed additions of acyl chlorides to terminal alkynes.13 The electronic and steric parameters for bulky biaryl dialkyl phosphines coordinated to iridium carbonyl fragments have been recently described by Nolan.14 We considered that similar metalarene interactions using these bulky phosphine ligands might stabilize intermediates in hydrogenation reactions. There are many known homogeneous complexes that serve as catalysts for the hydrogenation of alkenes, dienes, and alkynes. In contrast, for the hydrogenation of arenes only a few homogeneous catalysts have been reported. Notably, several heterogeneous catalysts for arene hydrogenation have been reported, and it has been suggested, based on detailed studies of several systems, that most of the reported homogeneous arene hydrogenation catalysts may actually be heterogeneous in nature.1517 The additional stabilization of the arene coordination could contribute to maintaining a homogeneous catalytic species. Reversible coordination of a pendent arene on the phosphine could allow for substrate coordination and also stabilize any coordinatively unsaturated intermediate species that are generated. In this report, we disclose our findings of metalarene interactions between biphenyl phosphine ligands and rhodium. Syntheses are described for several new rhodium complexes containing bulky biaryl phosphine ligands and norbornadiene (NBD) or 1,5-cyclooctadiene (COD) ligands. Coordination of the pendent phenyl ring of the biphenyl group in an η2- or η6fashion to the cationic metal center stabilizes the low-coordination environment at rhodium. The NBD complex exhibits a squareplanar geometry about the metal with the arene coordinated in an η2-fashion. The addition of H2 to this Rh(I) NBD complex affords an 18-electron Rh(III) dihydride complex in which the arene moiety is coordinated in an η6-fashion. In contrast, the (COD)Rh(I) analogue exists as an equilibrium mixture of a square-planar species with an η2-coordination of the arene and a piano stool complex where the arene is bound η6. The reaction of this COD rhodium complex with H2 results in hydrogenation of one CdC bond of the cyclooctadiene to yield cyclooctene (COE). A Rh(I) two-legged piano stool complex is formed, with an η6-arene moiety. The phosphine and COE serve as the two legs. While the demonstrated variable hapticity of the pendent arene of the biaryl group in these phosphine complexes may prove useful in the design of new homogeneous catalysts, our results concerning arene hydrogenation were consistent with the formation of a heterogeneous catalyst.
Figure 1. ORTEP diagram of complex 2. Counteranion and hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.
’ RESULTS AND DISCUSSION Synthesis of (NBD)Rh(PCy2biPh)Cl (1) and [(NBD)Rh(PCy2biPh)][B(ArF)4] (2). Reaction of the [(NBD)RhCl]2 dimer
with 2 equiv of 2-(dicyclohexylphosphino)biphenyl (PCy2biPh) in methylene chloride generated (NBD)Rh(PCy2biPh)Cl, 1 (Scheme 1). The chloride complex 1 was characterized by 1H, 31 P, and 13C NMR spectroscopy and elemental analysis. A doublet at 28.3 ppm (1JRhP = 162 Hz) was observed in the 31P NMR spectrum. Two broad resonances at 4.78 and 3.18 ppm in the 1H NMR spectrum are assigned to the η4-bound norbornadiene. Downfield 1 H NMR signals (8.037.35 ppm) were observed for the free pendent aryl group. All of the aromatic carbon resonances appear from 126 to 147 ppm in the 13C NMR spectrum. These downfield chemical shifts are indicative of an uncoordinated arene group and support the structure of 1 pictured in Scheme 1. Salt metathesis of chloride complex 1 with NaB(ArF)4 (ArF = 3,5-bis(trifluoromethyl)phenyl) yielded cationic [(NBD)Rh(PCy2biPh)][B(ArF)4], 2, quantitatively (Scheme 1). X-ray 2106
dx.doi.org/10.1021/om1009473 |Organometallics 2011, 30, 2105–2116
Organometallics
ARTICLE
Table 1. Selected Interatomic Distances for Complex 2 bond
distance (Å)
bond
Scheme 2. η1-Arene Coordination Proposed for Complex 4
distance (Å)
Rh1C7
2.476(3)
C7C8
1.409(5)
Rh1C8
2.411(3)
C8C9
1.404(5)
Rh1C9
3.077a
C9C10
1.382(6)
Rh1C10
3.644a
C10-C11
1.390(5)
Rh1C11
3.632a
C12C7
1.400(5)
Rh1C12
3.199a
Rh1P
2.2754(9)
Rh1C25
2.111(3)
Rh1C27
2.272(3)
Rh1C26
2.110(3)
Rh1C28
2.271(3)
a
Distances without ESDs were calculated using Mercury visualization software.18
quality crystals of 2 were obtained by vapor diffusion. The structure of 2 (Figure 1) exhibits a square-planar geometry about the rhodium center with η2-coordination of the arene moiety. A list of selected bond lengths is found in Table 1. The RhColefin bond distances are longer for those trans to phosphorus than those trans to the η2-arene (2.11 vs 2.27 Å). The η2-coordination of the arene ring is evident in the comparison of the RhCbiphenyl distances. The RhC7 and RhC8 bond distances are 2.47 and 2.41 Å, respectively. The remainder of the RhCarene distances are in the range 3.203.64 Å. Additionally, the arene CC bond distances that are closer to the metal center (C7C8, C8C9, and C12C7, 1.41, 1.40, and 1.40 Å, respectively) are slightly elongated relative to the arene CC bond distances further away (C9C10, C10C11, and C11 C12, 1.38, 1.38, and 1.39 Å, respectively). The NMR data obtained for complex 2 support a similar structure in solution. Singlet resonances were observed at 4.32 and 3.53 ppm for the olefinic CH moieties of the coordinated NBD. A broad doublet integrating to 2H for the ortho hydrogens attached to C8 and C12 of the pendent ring at 7.03 ppm (3JHH = 10 Hz) is supportive of η2-arene coordination as observed in the solid state. An integration of 2H for this signal implies fast arene rotation at room temperature. This assignment is further supported by the observation of a single upfield resonance at 118.0 ppm in the 13C NMR spectrum for the ortho carbons on the pendent ring, also implying fast exchange of the arene ring on the NMR time scale. Assignment of the individual arene 1H resonances was achieved using 1H1H-COSY experiments, while 1 H13C-HMQC aided in the assignment of some of the 13C resonances. Complex 2 is thermally stable at 100 °C. Heating of 2 in C6D6 or C6H6 at 100 °C in a J. Young-type NMR tube for 4 days resulted in no change in the 1H and 31P NMR spectra. However, heating a solution of 2 in either of these solvents at 130 °C resulted in darkening of the solution and decomposition to unidentified products. The analogous chloride (3) and cationic (4) compounds containing the 2-dicyclohexylphosphino-20 ,60 -diisopropoxy1,10 -biphenyl (RuPhos) ligand were synthesized using a similar procedure. Details of the characterization of 3 and 4 are found in the Experimental Section. Interestingly, the bulky isopropyl groups in the 20 and 60 positions of the pendent ring in complex 4 prevent the η2-interaction with the arene. In the 13C NMR spectrum of 4, only a single resonance for the quaternary carbon of the pendent ring shifts upfield (to 106.2 ppm), while the quaternary carbon bound to the OiPr group does not shift. This suggests the closest contact between the arene ring and Rh is only at the ipso carbon (Scheme 2), which indicates η1-arene coordination.19 A similar arene η1-bonding mode
with the ipso carbon is observed in the (dba)Pd(SPhos) (SPhos = 2-dicyclohexylphosphino-20 ,60 -dimethoxybiphenyl) complex reported by Buchwald.20 DFT calculations by the Buchwald group also show low-energy structures for Pd(SPhos) complexes that possess an η1-interaction with the ipso carbon of the lower pendent ring.8 Synthesis of Rhodium Dihydride Complex (5). Exposure of complex 2 to 90 psi of H2 at room temperature in C6D5Cl or CD2Cl2 affords a new species, 5, after 4 days (Scheme 3). During this time, the initially bright yellow solution becomes colorless. A new upfield resonance at 11.9 ppm integrating for 2H (doublet of doublets, 2JPH = 20 Hz, 1JRhH = 25 Hz) was observed in the 1 H NMR spectrum. The chemical shift of this signal and the couplings to both Rh and P are consistent with the assignment of the product as a Rh(III) dihydride complex. A new doublet at 97.4 ppm (1JRhP = 150 Hz) was observed in the 31P NMR spectrum, shifted downfield relative to the starting material. Three upfield resonances (5.78 ppm, t, 1H; 5.99 ppm, d, 2H; and 6.37 ppm, t, 2H) were also observed in the 1H NMR spectrum. These upfield shifts are characteristic of η6-coordination of an arene moiety.21,22 Additionally, three upfield signals at 97.6, 99.9, and 104.5 ppm were observed for the bound pendent arene carbons in the 13C NMR spectrum, consistent with η6coordination to rhodium. These assignments were further substantiated by 1H1H-COSY and 1H13C-HMQC experiments. Free norbornane was observed in the reaction mixture by both 1 H and 13C NMR spectroscopy. The production of the Rh dihydride complex 5 varied with reaction conditions. When scrupulously dry C6D5Cl or CD2Cl2 solvent was used, a black precipitate and/or a metallic mirror was often observed after 1 day at room temperature. In contrast, the use of solvent that was not dried over CaH2 led to consistently high yields of 5. Reaction of 2 with H2 in the more coordinating solvent acetone-d6 did not yield the dihydride complex 5. Instead, acetone displaced the η2-bound arene moiety and coordinated to the metal center to form a square-planar Rh(I) NBD complex with phosphine and acetone occupying the remaining two sites. This is supported by observation of a resonance at 27.6 ppm in the 31P NMR spectrum and a downfield shift to 8.10 ppm for the ortho hydrogens of the pendent ring in the 1H NMR spectrum. These observations suggest that acetone binds strongly to the metal center, preventing hydrogenation. With the intention of accelerating the formation of 5, a sample of 2 generated in situ from 1 and NaB(ArF)4 was heated to 60 °C in the presence of 90 psi of H2. Upon heating, rapid degradation was observed, with formation of a dark precipitate within 3.5 h. Complete decomposition to unidentified products was observed after 24 h. The addition of 90 psi of H2 to the cationic Rh(RuPhos) complex 4 also generated a dihydride complex analogous to 5. This dihydride product was also characterized by 1H and 31P 2107
dx.doi.org/10.1021/om1009473 |Organometallics 2011, 30, 2105–2116
Organometallics
ARTICLE
Scheme 3. Reaction of Complex 2 with H2 to Form Dihydride Complex 5
Scheme 4. Synthesis of (COD)Rh(I) Complexes 6 and 7
NMR spectroscopy. Full details are provided in the Experimental Section. Synthesis of (COD)Rh(PCy2biPh)Cl (6) and [(COD)Rh(PCy2biPh)][B(ArF)4] (7). The COD analogues of NBD complexes 1 and 2 have also been prepared and characterized. The chloride complex 6 was prepared through reaction of [(COD)RhCl]2 dimer23 with 2 equiv of the bulky PCy2biPh phosphine ligand in CH2Cl2 at room temperature (Scheme 4). The NMR spectroscopic data for 6 are very similar to those found for the NBD analogue 1. A doublet at 27.4 ppm (1JRhP = 138 Hz) was observed in the 31P NMR spectrum. In the 1H NMR spectrum, signals in the range 8.16 to 7.22 ppm were observed for all of the arene hydrogen atoms, suggesting that the pendent arene group is not coordinated to the Rh center. In the 13C NMR spectrum, all of the arene carbon signals appear from 126 to 133 ppm, which is in the typical range of aromatic carbon resonances that are not coordinated to a metal center. Two broad singlets, each integrating for 2H at 5.19 and 3.27 ppm, were observed for the bound COD ligand by 1H NMR spectroscopy. The chloride complex 6 is air and moisture stable. Salt metathesis of chloride complex 6 with NaB(ArF)4 in CH2Cl2 yielded air- and water-stable cationic Rh complex 7 quantitatively (Scheme 4). Complex 7 was characterized by 1H, 31 P, and 13C NMR spectroscopy and elemental analysis. X-ray crystallography showed that the aryl moiety of the biphenyl group is η2-bound in the solid state. Crystallographic details are
Table 2. Selected Interatomic Distances for Complex 7 bond
distance (Å)
bond
distance (Å)
Rh1C7
2.599(2)
C7C8
1.406(4)
Rh1C8
2.528(3)
C8C9
1.405(4)
Rh1C9
3.291a
C9C10
1.372(5)
Rh1C10
3.904a
C10-C11
1.388(5)
Rh1C11 Rh1C12
3.883a 3.292a
C11C12 C12C7
1.379(4) 1.410(4)
Rh1C25
2.135(3)
Rh1C28
2.252(3)
Rh1C32
2.133(2)
Rh1C29
2.269(2)
Rh1P
2.3020(6)
a
RhC distances without ESDs were calculated using Mercury visualization software.18
located in Table 2 and the Supporting Information. An ORTEP diagram for 7 (Figure 2) demonstrates that the structure is similar to that of the NBD complex 2 (Figure 1). The contacts between Rh and C7 and C8 in 7 (2.53 and 2.60 Å, respectively) are significantly shorter than the RhC distances to the other carbons in the arene ring (3.293.90 Å). This difference is indicative of an η2-bound arene similar to that found for 2. The arene ring is closer to the rhodium center in complex 2, as shown by the MCarene distances of 3.203.65 Å. The arene CC bonds that are closer to the metal center are slightly elongated 2108
dx.doi.org/10.1021/om1009473 |Organometallics 2011, 30, 2105–2116
Organometallics (1.41 Å for C7C8, C8C9, and C12C7 compared to 1.37, 1.39, and 1.38 Å for C9C10, C10C11, and C11C12, respectively). The 1H NMR spectrum of a solution of 7 in CD2Cl2 at room temperature is quite different from that observed for the NBD analogue 2. Two broad signals were observed in the 1H NMR spectrum of 7 at room temperature. The resonance at 7.02 ppm is attributed to the pendent arene moiety, and the olefinic hydrogens are observed at 4.09 ppm. No signal was discernible in the 31P NMR spectrum. These data suggest that complex 7 exhibits fluxional behavior on the NMR time scale at room temperature. Low-temperature NMR spectroscopy was used to examine the fluxionality of 7. Sharp 1H and 31P NMR spectra were obtained
Figure 2. ORTEP diagram of (COD)Rh(PCy2biPh)þ. Ellipsoids are drawn at 50% probability. Hydrogen atoms and counteranion are omitted for clarity.
ARTICLE
upon cooling a sample of 7 in CD2Cl2. Two distinct Rh complexes were observed at 70 °C in approximately a 2:1 ratio. There were two doublets in the 31P NMR spectrum at 75.4 and 41.7 ppm exhibiting phosphorusrhodium coupling of 186 and 135 Hz, respectively. From 0 °C through room temperature, the two phosphorus doublets broadened into the baseline. A stacked plot of 1H NMR spectra obtained from 90 to 10 °C is shown in Figure 3. Two sets of olefinic COD signals in approximately a 2:1 ratio were observed at low temperature by 1 H NMR spectroscopy. One set of COD resonances appears at 5.52 and 3.75 ppm, while the other set is at 4.02 and 3.53 ppm. While the signals between 3.5 and 4 ppm are consistent with olefinic CH groups bound to a metal center, the downfield shift of 5.52 ppm suggests an unbound olefinic COD moiety. Additionally, there are two sets of distinct arene resonances in different environments. Three upfield signals at δ 5.89 (t, 3JHH = 5.0 Hz, 1H), 6.78 (d, 3JHH = 5.0 Hz, 2H), and 6.89 (d, 3JHH = 5.0 Hz, 3H) are consistent with the assignment of an η6-bound arene. The remainder of the signals appear downfield as a multiplet at 7.45 ppm under the [B(ArF)4] counteranion resonances. A signal shifted upfield at δ 7.00 (d, 3JHH = 5.0 Hz, 1H) is correlated only to the downfield arene resonances, suggestive of η2-binding to Rh. These spectra are consistent with two rhodium species, one having an η6-bound arene and the other an η2-bound arene, rapidly interconverting on the NMR time scale at room temperature (Scheme 5). Support for these assignments was provided by a 2D 1H13C-HMQC experiment carried out at 70 °C. Three 13C resonances at 98.7, 103.7, and 111.2 ppm, which correspond to a complex that possesses an η6-bound arene, were located by HMQC. Additionally, a signal at 121.7 ppm was observed and this suggests another species that exhibits η6-coordination to rhodium. In 7-η6 the downfield signal at 5.52 in the 1H NMR spectrum suggests that only one of the CdC moieties of the COD is coordinated. By HMQC, the carbon resonance for the unbound CdC of COD was found to be downfield at 130.3 ppm, while the bound olefinic group was
Figure 3. Variable-temperature 1H NMR spectra depicting the fluxionality of η6- and η2-coordination of the arene to rhodium in complex 7 (500 MHz, CD2Cl2). 2109
dx.doi.org/10.1021/om1009473 |Organometallics 2011, 30, 2105–2116
Organometallics
ARTICLE
Scheme 5. η6- and η2-Coodination to Rhodium(I) Observed at Low Temperature
shifted upfield to 71.6 ppm. Complex 7-η6 adopts a two-legged piano-stool geometry, similar to neutral 18-electron CpML2 (M = Ir, Rh, Co) d8 complexes. In contrast, when the arene ring coordinates in an η2-mode, the 16-electron Rh(I) complex adopts a square-planar geometry. This suggests that the more flexible cyclooctadiene ligand facilitates both η6- and η2-coordination modes of the arene moiety at the metal center, in contrast to the more rigid NBD ligand. The downfield-shifted 31P NMR signal at 75.4 ppm with the larger RhP coupling constant of 186 Hz is assigned to the η6-coordinated species, while the doublet at 41.7 ppm (1JRhP = 135 Hz) is assigned to the η2-coordinated species. Reaction of Cationic Rh Complex 7 with H2. It is well documented that cyclooctadiene can be hydrogenated to cyclooctane (COA) and liberated from rhodium bisphosphine cations.24,25 Similarly, it was expected that the addition of H2 to complex 7 would release COA and form a Rh dihydride complex, as had been observed in the reaction of the NBD analogue 2 with H2. In fact, the reaction of 7 with H2 led to complex 8, which has a different structure, as described below. A solution of 7 in CD2Cl2 at room temperature was pressurized with H2 (90 psi), and the reaction was studied by NMR spectroscopy. The 1H NMR spectrum obtained is sharp, in contrast to the 1H NMR spectrum observed for cationic complex 7 in CD2Cl2 at room temperature. The broad signals for the η2bound cyclooctadiene (5.19 and 3.26 ppm) are no longer observed. The 1H NMR spectrum of the product, complex 8, exhibits a new bound olefin resonance at 3.72 ppm integrating for 2H. The three arene resonances for 8 have shifted upfield to 6.03 (t, 3JHH = 5.0 Hz, 1H), 6.91 (d, 3JHH = 5.0 Hz, 2H), and 6.95 (t, 3JHH = 5.0 Hz, 2H) ppm, indicative of η6-arene coordination to rhodium.21 In the 13C NMR spectrum, the observation of three upfield signals (112.4, 103.3, 98.6 ppm) also supports η6coordination to Rh. In the 31P NMR spectrum, a new downfield doublet at 75.2 ppm with a large rhodium coupling constant of 188 Hz was observed. This 31P NMR shift and RhP coupling constant are similar to those observed for the (COD)Rh(I) η6-arene complex 7-η6 described above. Compound 8 can also be prepared independently by the reaction of [(COE)2RhCl]2 with 2 equiv of PCy2biPh and NaB(ArF)4 in CH2Cl2 at room temperature. X-ray quality crystals were obtained by vapor diffusion of pentane into a solution of 8 in CH2Cl2 at 30 °C. The ORTEP diagram for 8 is shown in Figure 4, and selected bond lengths are listed in Table 3. Additional crystallographic parameters can be found in the Supporting Information. The complex adopts a twolegged piano-stool geometry in which the phosphine and the olefin are the legs, similar to complex 7-η6. This coordination environment mirrors the well-known neutral 18e CpML2 complexes. The rhodium to arene carbon bond distances range from 2.19 to 2.38 Å, which is consistent with η6-coordination.21,22 The
Figure 4. ORTEP of [(COE)Rh(PCy2biPh)]þ. Ellipsoids are drawn at 50% probability. Hydrogen atoms, solvent, and counteranion are omitted for clarity.
Table 3. Selected Bond Distances for Complex 8 bond
distance (Å)
bond
distance (Å)
Rh1C7 Rh1C8
2.192(12) 2.300(12)
C7C8 C8C9
1.439(8) 1.381(19)
Rh1C9
2.376(12)
C9C10
1.40(2)
Rh1C10
2.347(13)
C10C11
1.430(19)
Rh1C11
2.354(13)
C11C12
1.364(19)
Rh1C12
2.302(12)
C12C7
1.441(17)
Rh1C25
2.147(12)
Rh1P1
2.252(3)
Rh1C26
2.140(12)
RhColefin bond lengths are 2.15 and 2.14 Å, consistent with an η2 π-type coordination to the COE ring. The CC arene bond distances for C11C12, C8C9, and C9C10 (1.36, 1.38, and 1.40 Å, respectively) are slightly shorter than the C12C7, C10C11, and C7C8 arene bond distances (1.431.44 Å). Werner and co-workers reported an analogous structure, in which a bulky dialkylphosphine bearing a pendent arene (PiPr2(CH2CH2C6H5)) coordinates both through the P and via the η6-arene to a rhodium(I) center.21 Scheme 6 describes the reaction of 7 with dihydrogen. No further reaction of complex 8 with dihydrogen was observed at room temperature. A small amount of hydride 2110
dx.doi.org/10.1021/om1009473 |Organometallics 2011, 30, 2105–2116
Organometallics
ARTICLE
Scheme 6. Synthesis of 8 by Reaction of Complex 7 with H2
Table 4. Selected Bond Distances for Complex 9
Figure 5. ORTEP diagram of complex 9. Ellipsoids are drawn at 50% probability. Hydrogen atoms and two counteranions are omitted for clarity.
complex 5 was observed upon heating complex 8 to 60 °C in the presence of H2 after 1 day. The solution darkened after 2 days at 60 °C under 90 psi of H2, and after 3 days at 60 °C, complete decomposition was observed. Stability of Rhodium(III) Dihydride Complex 5. When solvent was evaporated from samples of 5, a new species was formed, which was also observed during recrystallization of complex 8. X-ray quality crystals of this product were grown from a CH2Cl2 solution of the resulting powder by vapor diffusion with pentane at 30 °C. The new complex is characterized as a RhRh dimer with the formula [Rh(PCy2biPh)]2[B(ArF)4]2 (9) (Figure 5). Selected bond lengths are found in Table 4, and additional crystallographic information can be found in the Supporting Information. Each of the pendent arene rings is spanned by two rhodium centers. Both of the coordinated arene moieties adopt a boat-like conformation. The distance between the two rhodium atoms is 2.61 Å. This structure is a unique example in which rhodium coordinates to two different arene moieties in an η3-fashion. In contrast, Halpern has reported the crystal structure of a dimeric rhodium complex in which each rhodium center is bound to phosphorus and is coordinated η6 to one arene of the biarylphosphine ligand from the adjacent rhodium center.26 Additionally, dimeric
bond
distance (Å)
Rh1C19 Rh1C20
2.091(5) 2.261(5)
Rh1C24
2.298(5)
Rh1C21
2.302(5)
Rh1C22
2.121(5)
Rh1C23
2.290(5)
Rh1Rh
2.6099(8)
Rh1P1
2.3456(14)
rhodium complexes described by Noyori27 and Pringle28 also show a similar η6-arene coordination to rhodium. Benzene Hydrogenation with Norbornadiene Rhodium Complexes. The hydrogenation of C6H6 was studied with NBD precatalyst 2 in CD2Cl2. Heating a 0.056 M solution of C6H6 in CD2Cl2 with 8 mol % 2 and 90 psi of H2 at 60 °C resulted in the hydrogenation of benzene to cyclohexane. After 5 h, the solution had turned to pale yellow and a minor amount of black residue was observed. Cyclohexane and the dihydride complex 3 were observed by 1H NMR spectroscopy. After 1 day, signals for complex 3 were no longer present in the 1H NMR spectrum, and a large amount of black precipitate was observed. After 38 h at 60 °C, cyclohexane was observed by 1H NMR spectroscopy (60% yield, as determined by integration against an internal standard) with a substantial amount of black precipitate. A Hg poisoning test29 was performed to investigate the homogeneity of the catalytic system. Addition of a drop of Hg to a sample of C6H6 in CD2Cl2 with 8 mol % 2 and 90 psi of H2 heated at 60 °C resulted in the production of only a small amount of C6H12 (
