1476
Organometallics 2009, 28, 1476–1479
Rhodium Complexes of Cyclopropenylidene Carbene Ligands: Synthesis, Structure, and Hydroformylation Catalysis Michael Green, Claire L. McMullin, George J. P. Morton, A. Guy Orpen, Duncan F. Wass,* and Richard L. Wingad School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol, BS8 1TS, United Kingdom ReceiVed October 27, 2008
Rhodium(III) cyclopropenylidene complexes of the type [RhCl3(PPh3)2(2,3-di(aryl)cyclopropenylidene)] (Aryl ) C6H5, 4-C6H4F) are synthesized via oxidative addition of 1,1-dichloro-2,3-diarylcyclopropene fragments to rhodium(I) precursors. The molecular structure of these complexes has been determined. Attempted hydroformylation of 1-hexene with these complexes leads to catalysis results which are strongly suggestive of decomposition of the carbene complex. Introduction N-Heterocyclic carbenes have emerged as versatile supporting ligands for homogeneous catalysis,1 their strong σ-donor capabilities resulting in advantages over many other ligand types in a range of reactions. By contrast, the potential of carbocyclic carbene ligands, that is, with no heteroatom, is little explored. Stable transition metal complexes of the carbocyclic carbene 2,3-diphenylcyclopropenylidene have been know for many years;2 such complexes are thermally and chemically robust, this stability attributed to the possibility of a contribution from a resonance form in which a 2π aromatic cationic cyclopropenium moiety is formed (Scheme 1).3 More recently Bertrand and co-workers have isolated stable free 2,3-diaminocyclopropenylidenes.4 Despite these developments, the potential of cyclopropenylidene carbene complexes in catalysis remained unrealized until our recent report of the use of such complexes as highly active and efficient catalysts for C-C and C-N coupling reactions.5 Our results were later confirmed by Herrmann and co-workers.6 Very recently, a rhodium complex of an amine-stabilized cyclopropenylidene has been reported.7 Hydroformylation remains one the most important of catalytic reactions and the preeminence of rhodium complexes supported by basic phosphine donors has naturally led to the investigation of N-heterocyclic carbenes for this application, with promising results.8 We are interested in extending the utility of cyclopropenylidenes as supporting ligands to other Group VIII metals and * To whom correspondence should be addressed. E-mail: duncan.wass@ bristol.ac.uk. (1) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. 1997, 36, 2162. (c) Herrmann, W. A.; Elison, M.; Fischer, J.; Ko¨cher, C.; Artus, G. R. J. Angew. Chem., Int. Ed. 1995, 34, 2371. (d) Bourissou, D.; Guerret, O.; Gabbaı¨, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39. ¨ fele, K. Angew. Chem., Int. Ed. 1968, 7, 950. (b) O ¨ fele, K. J. (2) (a) O Organomet. Chem. 1970, 22, C7. (c) Yoshida, Z. Pure Appl. Chem. 1982, 54, 1059. (d) Konishi, H.; Matsumoto, S.; Kamitori, Y.; Ogoshi, H.; Yoshida, Z. Chem. Lett. 1978, 241. (e) Yoshida, Z.; Kamitori, Y. Chem. Lett. 1978, 1341. (f) Wilson, R. D.; Kamitori, Y.; Ogoshi, H.; Yoshida, Z.; Ibers, J. A. J. Organomet. Chem. 1979, 173, 199. (3) Kawada, Y.; Jones, W. M. J. Organomet. Chem. 1980, 192, 87. (4) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2006, 312, 722. (5) (a) Wass, D. F.; Haddow, M. F.; Hey, T. W.; Orpen, A. G.; Russell, C. A.; Wingad, R. L.; Green, M. Chem. Commun., 2007, 2704. (b) Wass, D. F.; Hey, T. W.; Rodriguez-Castro, J.; Russell, C. A.; Shishkov, I. V.; Wingad, R. L.; Green, M. Organometallics 2007, 26, 4702..
Scheme 1. Cyclopropenylidene Ligands
developing synthetic strategies to access such complexes. We report here that rhodium complexes may be conveniently synthesized Via an oxidative addition process and investigate their activity in hydroformylation of 1-hexene.
Results and Discussion We have found that oxidative addition of 1,1-dichorocyclopropene fragments to soluble, well-defined palladium(0) complexes provides a reliable and versatile route to palladium(II) cyclopropenylidene complexes,5 and reasoned a related route to rhodium(III) complexes could be exploited by oxidative addition to one of the plethora of well-known rhodium(I) starting materials. In this way, 1,1-dichloro-2,3-di(aryl)cyclopropenes (L1-2) react with [RhCl(CO)(PPh3)2] in refluxing toluene, giving the desired rhodium(III) complexes 1-2 in good (70-75%) yield (eq 1). These complexes precipitate from the toluene solution and may be isolated analytically pure without further work up.
Complex 1 was analyzed by NMR spectroscopy, mass spectrometry, CHN elemental analysis and X-ray diffraction study. Distinctive features of the 1H NMR spectrum include a sharp multiplet at 8.22 ppm, indicative of the coordination of this ligand type. A similar downfield multiplet has been observed on coordination of L1 to palladium.5 The 31P{1H} NMR spectrum of 1 shows a sharp doublet at 12.6 ppm (1JPRh ) 89.3 Hz). ESI Nanospray mass spectrometry showed the major peak at 887, corresponding to the loss of chlorine from 1. Complex 2 shows very similar characterizing data (see experimental
10.1021/om801031a CCC: $40.75 2009 American Chemical Society Publication on Web 01/27/2009
Rhodium Complexes of Cyclopropenylidene Carbene Ligands
Figure 1. Thermal ellipsoid plot of 1 in 1.2CHCl3. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity.
section). The 19F{1H} NMR spectrum of 2 shows a single fluorine environment with a characteristic peak at -100.9 ppm, shifted from the starting material L2 at -106.7 ppm. Single crystals of 1 and 2 suitable for X-ray diffraction studies were obtained from chloroform. Crystals of 1.2CHCl3and 2.2CHCl3 crystallized in space group P21/n with one molecule in the asymmetric unit and two cocrystallized chloroform solvent molecules. The molecular structures of 1 and 2 are illustrated in Figures 1 and 2. Selected bond lengths and angles are given in Table 1. Both complexes have a mer-octahedral Rh(III) center coordinated by a cyclopropenylidene ligand (L1 and L2 respectively) which lies trans to a chloride ligand (Cl2) and cis to two trans chloride ligands (Cl1 and Cl3) and two mutually trans triphenylphosphine ligands. The cyclopropenylidene ligands in 1 and 2 exhibit a strong trans influence with the Rh-Cl bond length trans to C1 ca. 0.1 Å longer than cis chlorides. The C3 plane of the cyclopropenylidene ligands lies close to the RhCl3 plane in both cases (interplanar angle 12.0° in 1 and 7.1° in 2). In line with other structural studies for cyclopropenylidene ligands,3,5,7 the similarity of the C3 ring C-C bond lengths suggests the ring is best described as a cyclopropenium fragment, albeit the marginally shorter (ca. 0.02 Å) C2-C3 distances may indicate a small contribution from a cyclopropene resonance form. Hydroformylation of 1-Hexene Using Rhodium Precursors. Complexes 1 and 2 were tested for rhodium catalyzed hydroformylation of 1-hexene. Although rhodium(I) complexes are more often used as precatalysts, the very wide variety of precursors used, the widespread and successful use of in situ catalyst generation and the intermediacy of rhod¨ fele, K.; Herdtweck, E.; HerrmannM, (6) (a) Taubmann, C.; Tosh, E.; O W. A. J. Organomet. Chem. 2008, 693 (13), 2231. (b) Herrmann, W. A.; ¨ fele, K.; Taubmann, C.; Herdtweck, E.; Hoffmann, S. D. J. Organomet. O Chem. 2007, 692, 3854. (7) Kuchenbeiser, G.; Donnadieu, B.; Bertrand, G. J. Organomet. Chem. 2008, 693, 899. (8) For leading reference, see: Praetorius, J. M.; Crudden, C. M. Dalton Trans. 2008, 4079.
Organometallics, Vol. 28, No. 5, 2009 1477
Figure 2. Thermal ellipsoid plot of 2 in 2.2CHCl3. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity. Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1.2CHCl3 and 2.2CHCl3 Rh1-C1 Rh1-Cl1 Rh1-Cl2(trans) Rh1-Cl3 Rh1-P1 Rh1-P2 C1-C2 C1-C3 C2-C3 C1-Rh1-Cl1 C1-Rh1-Cl2 C1-Rh1-Cl3 C3-C1-C2 Cl1-Rh1-C1-C2 Cl3-Rh1-C1-C3
1.2CHCl3
2.2CHCl3
1.959(5) 2.3610(13) 2.4502(13) 2.3250(13) 2.3798(13) 2.3918(13) 1.384(8) 1.381(8) 1.365(8) 89.13(16) 177.18(15) 88.01(17) 59.2(4) -12.7(8) -11.1(7)
1.948(7) 2.3357(19) 2.4528(17) 2.3456(18) 2.3951(19) 2.4003(19) 1.404(9) 1.389(9) 1.356(10) 87.3(2) 177.9(2) 88.5(2) 58.1(5) 8.8(11) 8.9(10)
ium(III) complexes during the catalytic cycle,9 led us to propose that the reducing feedstock gases used would result in a potentially catalytically active species. The conversion of 1-hexene to aldehyde products was evaluated by 1H NMR spectroscopy, with the loss of the olefin peaks at 6.10 and 5.30 ppm, and the appearance of different aldehyde peaks corresponding to each isomer at ∼9.7 ppm. Simple integration of the different environments relative to each other gave a percentage conversion. The standard used was [RhCl(CO)(PPh3)2], a well-known system for successful hydroformylation of such alkenes (run 1). The results of our initial screen are shown in Table 2. No hydroformylation was observed for either 1 or 2 under the experimental conditions (runs 1 and 2). Analysis of the product solutions showed complete retention of the complexes 1 and 2 suggesting a high stability of the rhodium(III)-carbene unit and no formation of the desired rhodium(I) species. This led us to add a reducing agent, namely metallic zinc. Relatively high conversion was seen with both rhodium complexes (runs 4 and 5), with 1 showing conversion to n-, (9) van Leeuwen, P. W. N. M., Claver, C., Eds. Rhodium Catalyzed Hydroformylation; Kluwer Academic Publishers Group: Norwell, MA, 2002.
1478 Organometallics, Vol. 28, No. 5, 2009
Green et al. Table 2. Hydroformylation Resultsa selectivity (%)
a
run
complex
additive
conversion (%)
iso-3-heptanal
iso-2-heptanal
n-heptanal
1 2 3 4 5
[RhCl(CO)(PPh3)2] 1 2 1 2
none none none Zn Zn
100 0 0 91 85
2 1 0
28 24 25
70 75 75
Conditions: 0.25 mol% catalyst; 3 h; 90 °C; 20 bar of CO/H2; [1-hexene] ) 1.75 mol/dm3; toluene diluent. Table 3. Crystallographic Data compound
1.2CHCl3
2.2CHCl3
Color, habit Size/mm Empirical Formula M Crystal system Space group a/Å b/Å c/Å β/° V/A3 Z µ/mm-1 T/K Reflections: total/independent Rint Final R1 Largest peak, hole (eA-3) Fcalc/g cm-3
colorless block 0.22 × 0.11 × 0.08 C53H42Cl9P2Rh 1162.77 monoclinic P21/n 12.897(3) 23.610(6) 17.072(3) 101.853(10) 5087.4(2) 4 0.908 120 59125/11673 0.1256 0.0778 2.33, -1.55 1.52
yellow plate 0.12 × 0.05 × 0.02 C53H40Cl9F2P2Rh 1198.75 monoclinic P21/n 14.195(3) 21.800(5) 16.407(4) 94.971(10) 5058.0(2) 4 0.921 120 48544/11557 0.1175 0.0950 0.76, -0.79 1.57
iso-2 and iso-3 aldehyde products. The effect on hydroformylation of the bound cyclopropenylidene appeared negligible with both complexes 1 and 2 providing very similar conversions. These results are also the same as the standard catalysts within error and, although similar performance to the standard catalyst cannot be fully ruled out, this suggests loss of the cyclopropenylidene fragment leads to the same active catalyst for hydroformylation in all cases. This hypothesis is further corroborated by investigation of the product solutions from runs 4 and 5 by 1H and 31P{1H} NMR spectroscopy; this indicates no coordination of the cyclopropenylidene moiety to the rhodium in each case. A characteristic doublet in the 31P{1H} NMR spectrum at 29.0 ppm indicated the addition of zinc had reduced the Rh(III) complexes to [RhCl(CO)(PPh3)2]. Our working hypothesis is that the affinity of the highly σ-donating cyclopropenylidene for the low oxidation state metal is decreased compared to the rhodium(III) species. Under catalysis conditions, the loss of cyclopropenylidene would make way for the coordination of the excellent π-backbonding carbonyl ligand, leading to a more stable rhodium(I) complex.
Conclusions The synthesis of rhodium(III) cyclopropenylidene complexes of the type [RhCl3(PPh3)2(2,3-di(aryl)cyclopropenylidene)] has been achieved via oxidative addition of 1,1-dichloro-2,3di(aryl)cyclopropene fragments to rhodium(I) precursors. The molecular structure of these complexes reveals the coordinated cyclopropenylidene ligand to have a strong trans influence; the similarity of C-C bonds in the 3-membered rings suggests a significant contribution from a cyclopropenium resonance form. Attempted hydroformylation of 1-hexene with these complexes was unsuccessful and reduction of these species with zinc leads to catalysis results which are strongly suggestive of decomposition of the carbene complex. Further studies to synthesize well-
defined rhodium(I) complexes of these ligands are underway, as is a more wide-ranging investigation into the catalytic properties of cyclopropenylidene complexes. We would like to thank the EPSRC and University of Bristol for funding. We thank the ESPRC National Crystallography Service in Southampton, specifically Richard Stephenson and Louise Male, for collecting the diffraction data for 1 and 2.
Experimental Section General Considerations. All procedures were carried out under an inert (N2) atmosphere using standard Schlenk line techniques or in an inert atmosphere (Ar) glovebox. Chemicals were obtained from the usual suppliers and used without further purification unless otherwise stated. All solvents were purified using an Anhydrous Engineering Grubbs-type solvent system.10 Ligands L1 and L2 were synthesized by a modification of the method of West et al.11 NMR spectra were recorded on JEOL, ECP 300 or Lambda 300 spectrometers. 1H NMR chemical shifts are referenced relative to the residual solvent resonances in the deuterated solvent, 31P{1H} NMR spectra are referenced relative to high frequency of 85% H3PO4 and 19F{1H} NMR spectra are referenced relative to CFCl3. Mass spectra were recorded on an Applied Biosystems QStar XL spectrometer by the School of Chemistry Mass Spectrometry service. Microanalyses were carried out by the Microanalytical Laboratory of the School of Chemistry at the University of Bristol. Catalytic experiments were conducted in a multicell autoclave made from Hastelloy by Baskerville, Manchester, UK. Synthesis of [RhCl3(PPh3)2(cyclo-2,3-C3Ph2)] (1). A solution of 1,1-dichloro-2,3-diphenylcyclopropene (15.0 mg, 0.06 mmol) in toluene (3 mL) was added to a solution of [RhCl(CO)(PPh3)2] (38.7 mg, 0.06 mmol) in toluene (3 mL). The solution was heated at reflux for 2 h during which time a light yellow solid precipitated. After cooling to room temperature the solution was removed by filtration and the yellow solid was washed with cold toluene (2 × 1 mL). The solid was dried in vacuo to give 1 as a yellow, air stable solid (39.8 mg, 0.04 mmol, 72% yield). 1 H NMR (CDCl3): δ 8.22 (m, 4H, carbene ArH), 7.96 (m, 12 H, PPh3), 7.56 (m, 2H, carbene ArH), 7.14 (m, 6H, PPh3), 7.02 (m, 12H, PPh3); 31P{1H} NMR (CDCl3): δ 12.6 (d, 1JRhP ) 89.3 Hz); Nanospray MS: m/z ) 887 [M - Cl]+; Elemental analysis (calcd): C 67.46 (66.29), H 4.29 (4.36). Synthesis of [RhCl3(PPh3)2(cyclo-2,3-C3{4-C6H4F}2)] (2). A solution of 1,1-dichloro-2,3-di(4-fluorophenyl)cyclopropene (15.0 mg, 0.05 mmol) in toluene (3 mL) was added to a solution of [RhCl(CO)(PPh3)2] (34.6 mg, 0.05 mmol) in toluene (3 mL). The solution was heated at reflux for 2 h during which time a deep yellow solid precipitated. After cooling to room temperature the solution was removed by filtration and the yellow solid was washed with cold toluene (2 × 1 mL). The solid was dried in vacuo to give 2 as a yellow, air stable solid (36.0 mg, 0.04 mmol, 75% yield). 1 H NMR (CDCl3): δ 8.27 (m, 4H, carbene ArH), 7.94 (m, 12 H, PPh3), 7.18-7.01 (m, 22H, ArH); 31P{1H} NMR (CDCl3): δ 12.5 (10) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics. 1996, 15, 1518. (11) West, R.; Zecher, D. C.; Tobey, S. W. J. Am. Chem. Soc. 1970, 92, 168.
Rhodium Complexes of Cyclopropenylidene Carbene Ligands (d, 1JRhP ) 88.4 Hz); 19F{1H} NMR (CDCl3): δ -100.9; Nanospray MS: m/z ) 923 [M - Cl]+; Elemental analysis (calcd): C 64.24 (63.84), H 4.36 (3.99). Crystal Structure Determinations. X-ray diffraction experiments on 1 and 2 (both as chloroform solvates) were carried out at 120K on a Bruker-Nonius Kappa CCD diffractometer, using Mo KR radiation (λ ) 0.71073 Å) generated by a Bruker-Nonius FR591 rotating anode. All data collections were performed using a single crystal coated in paraffin oil and mounted on a glass fiber. Intensities were integrated12 from several series of exposures in φ and ω calculated by the COLLECT13 and DENZO12 programs after unit cell determination by the DirAx14 program. Absorption corrections were based on equivalent reflections using SADABS,15 and structures were refined against all Fo2 data with hydrogen atoms riding in calculated positions using SHELXTL.16 Both crystals weakly diffracted at high angle. Crystal 1 contains a small solvent (12) Otwinowski, Z.; Minor, W. HKL package (DENZO, XDisplayF and Scalepack): Methods in Enzymology, Macromolecular Crystallography, Part A; HKL Research Inc.: Charlottesville, VA, 199727307-326. (13) Hooft, R. W. W.; Nonius, B. V., COLLECT data collection software, 1998. (14) Duisenberg, A. J. M. J. Appl. Crystallogr. 1992, 25, 92. (15) Sheldrick, G. M. SADABS V 2007/2; Bruker AXS Inc.: Madison, WI, 2007.
Organometallics, Vol. 28, No. 5, 2009 1479 void for which no sensible model could be derived. The chloroform solvent in 2.2CHCl3 is disordered with the molecules modeled as occupying two sites in both cases. Crystal structure and refinement data are given in Table 3. Catalytic Testing. The rhodium complexes were placed in an autoclave followed by zinc powder, if used. The system was flushed with nitrogen and toluene (5 mL) was added to each vessel. The autoclave was pressurized to 20 bar with a 1:1 mixture of CO/H2 and heated at 90 °C for 1 h in order to preform the catalyst. The autoclave was cooled, vented and refilled with nitrogen. 1-Hexene (1.4 mL) was added to each reaction vessel and the autoclave was heated to 90 °C, pressurized to 20 bar with CO/H2 and sealed. After 3 h the autoclave was cooled and vented to air. The identities of the reaction products were established by 1H NMR spectroscopy and conversion was measured by the integration of product peaks relative to remaining substrate. Supporting Information Available: Crystallographic details. This material is available free of charge via the Internet at http://pubs.acs.org. OM801031A (16) SHELXTL program system, version6.14; Bruker AXS Inc.: Madison, WI, 2000.
