Ring-Expanded N-Heterocyclic Carbene Complexes of Rhodium with

Dec 4, 2012 - This same isomeric mixture was formed as the ultimate product from treating Rh(PPh3)3(CO)H with 6-iPr at room temperature, although path...
1 downloads 9 Views 493KB Size
Download PDF
Article pubs.acs.org/Organometallics

Ring-Expanded N‑Heterocyclic Carbene Complexes of Rhodium with Bifluoride, Fluoride, and Fluoroaryl Ligands Candela Segarra,† Elena Mas-Marzá,† John P. Lowe,† Mary F. Mahon,‡ Rebecca C. Poulten,‡ and Michael K. Whittlesey*,† †

Department of Chemistry, and ‡X-ray Crystallographic Unit, University of Bath, Claverton Down, Bath BA2 7AY, U.K. S Supporting Information *

ABSTRACT: Thermolysis of Rh(PPh3)4H in the presence of the sixmembered N-heterocyclic carbene 1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine (6-iPr) gave the monocarbene complex Rh(6-iPr)(PPh3)2H as a 1:2 mixture of the cis- and trans-phosphine isomers 1a and 1b. This same isomeric mixture was formed as the ultimate product from treating Rh(PPh3)3(CO)H with 6-iPr at room temperature, although pathways involving both CO and PPh3 loss were observed at initial times. Treatment of 1a/1b with Et3N·3HF generated the bifluoride complex cis-Rh(6-iPr)(PPh3)2(FHF) (2a), which upon stirring with anhydrous Me4NF was converted to the rhodium fluoride complex cis-Rh(6-iPr)(PPh3)2F (3a). Thermolysis of 1a/1b with C6F6 resulted in C−F bond activation to afford a mixture of 3a and the pentafluorophenyl complex trans-Rh(6-iPr)(PPh3)2(C6F5) (5b). Complexes 1b, 2a, 3a, and 5b were structurally characterized.

■

■

INTRODUCTION In 2005, we reported that substitution of N-alkyl-substituted five-membered ring N-heterocyclic carbenes (NHCs) into the rhodium phosphine hydride complex Rh(PPh3)4H took place at room temperature to yield a mixture of cis- and trans-isomers of the mono- and bis-NHC complexes Rh(NHC)(PPh3)2H and Rh(NHC)2(PPh3)H (NHC = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (IEt2Me2), 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr2Me2), 1,3-dicyclohexylimidazol-2-ylidene (ICy)).1 The formation of such complex mixtures of products precluded any subsequent investigations of Rh−H bond reactivity. Although relatively few NHC-Rh-H complexes have been described in the intervening time, the examples that have been reported all exhibit an ability to coordinate and/or activate inert bonds in small molecules.2,3 With this in mind, we have revisited the substitution chemistry of Rh(PPh3)4H (as well as the related carbonyl precursor Rh(PPh3)3(CO)H) now toward NHCs with ring sizes >5. These so-called ring-expanded carbenes (RE-NHCs) are known to exhibit much wider N−C−N angles than their five-membered counterparts,4 a feature we hoped might reduce the likelihood of generating geometric isomers and/or monoand bis-substitution products. We now report the case of the six-membered N-isopropyl-substituted ligand 1,3-bis(2-propyl)3,4,5,6-tetrahydropyrimidin-2-ylidine (6-iPr), which reacts with Rh(PPh3)4H to give only the monocarbene species Rh(6-iPr)(PPh3)2H (1), albeit still as a mixture of cis (1a) and trans (1b) isomers. Thus far, we have focused on the reactivity of 1a/1b toward fluorine-based reagents because of (i) the interest in combining hard fluoride ligands with soft metal centers such as Rh5,6 and (ii) the strong precedence for using metal hydride/ dihydride precursors for the synthesis of novel organometallic complexes with fluorinated ligands.7 © 2012 American Chemical Society

RESULTS AND DISCUSSION Substitution Reactions of Rh(PPh 3 ) 4 H and Rh(PPh3)3(CO)H with 6-iPr. Addition of 3 equiv of 6-iPr (1,3bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine) to a toluene solution of Rh(PPh3)4H at room temperature led to the rapid, but incomplete, formation of the mono-NHC complex Rh(6-iPr)(PPh3)2H. Running the reaction at 343 K for 2−3 h led to complete conversion to a 1:2 mixture of the cis- and trans-phosphine isomers 1a and 1b (Scheme 1), which could be isolated as an inseparable mixture typically in ca. 70% yield. Increasing the amounts of ligand used to >3 equiv of 6-iPr had no effect on the isomer ratio. Scheme 1

In the 1H NMR spectrum, there was a hydride resonance with a doublet of doublets of doublets multiplicity at δ −5.66 (2JHPtrans = 106.4 Hz, 2JHPcis = 30.4 Hz, 1JHRh = 25.5 Hz) arising from the minor cis-phosphine isomer 1a, along with a lower frequency doublet of triplets signal at δ −10.35 (2JHP = 26.2 Hz, 1 JHRh = 10.2 Hz) for the trans-phosphine species 1b. Both isomers showed methine protons at very high frequencies (>δ Received: October 19, 2012 Published: December 4, 2012 8584

dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590

Organometallics

Article

160.869(13)° (cf. 160.39(3)° for the IiPr2Me2 complex). This distortion most probably helps to minimize steric interactions between the phosphines and carbene. The Rh−CNHC distance in 1b (2.0928(13) Å) was much longer than in Rh(IiPr2Me2)(PPh3)2H (2.068(2) Å), contrary to what might have been expected on the basis of a more strongly σ-donating sixmembered ring NHC being present. As commented on elsewhere,9 it is likely that enhanced steric clashes between the metal and N-substituents (resulting from the widened N− C−N angle in RE-NHCs) result in the carbene ligand being pushed away from the metal. The carbonyl precursor Rh(PPh3)3(CO)H proved to be more substitutionally labile than Rh(PPh3)4H and reacted immediately with 6-iPr at room temperature via two separate pathways, one involving CO and PPh3 loss, the other involving dissociation of two of the PPh3 ligands.11 Thus, the hydride region of a proton NMR spectrum recorded ca. 15 min after addition of 3 equiv of 6-iPr to Rh(PPh3)3(CO)H in C6D6 showed a 1:2.2:6.4:2.9 mixture of 1a and 1b and two new products, which appeared as a doublet of doublets at δ −2.90 (J = 111.3, 29.5 Hz) and δ −4.60 (J = 27.5, 19.5 Hz). The splitting patterns and magnitudes of the coupling constants led us to assign the new species as the trans-H-Rh-P and trans-HRh-CO isomers, respectively, of the monocarbonyl complex Rh(6-iPr)(PPh3)(CO)H.12 Unexpectedly, after 10 days standing at room temperature, the signals for the two isomers of Rh(6-iPr)(PPh3)(CO)H had vanished, leaving 1a/1b (in a 1:2 ratio) as the only products of the reaction. This transformation was achievable in only 2 h with heating at 343 K. Reaction of 1a/1b with Et3N·3HF. Addition of NEt3·3HF (TREAT-HF) to the mixture of 1a/1b in toluene solution at room temperature led to the clean formation of the rhodium bifluoride complex cis-Rh(6-iPr)(PPh3)2(FHF) (2a), which was isolated as a yellow, air-sensitive solid in moderate yield (65%) upon precipitation from solution with hexane. When THF was used as the reaction solvent rather than toluene, 2a was still formed as the major species, although the reaction was not as clean, with other uncharacterized products apparent by 31P NMR spectroscopy.13 The 31P{1H} NMR spectrum of a crystalline sample of the compound dissolved in C6D6 comprised two highly coupled resonances at δ 60.1 (1JPRh = 219 Hz, 2JPF = 180 Hz, 2JPP = 38 Hz) and δ 37.1 (1JPRh = 124 Hz, 2JPP = 38 Hz, 2JPF = 26 Hz),

6.5). This feature has been noted previously in other RE-NHCcontaining species8,9 and rationalized on the basis of the wide N−C−N angle (e.g., 116.79(12)° in 1b), which places the N-iPr groups proximal to the metal center (see further in discussion of the X-ray data below). The 31P{1H} NMR spectrum displayed two doublets of doublets for 1a, at δ 51.0 (1JPRh = 146 Hz, 2JPP = 24 Hz) and δ 43.1 (1JPRh = 144 Hz, 2JPP = 24 Hz), and a doublet for 1b at δ 43.1 (1JPRh = 180 Hz). The carbenic carbon of only the major isomer 1b could be observed by 13C NMR spectroscopy and appeared at a characteristic high frequency10 (223 ppm) with 1JCRh and 2JCP splittings of 46 and 9 Hz, respectively. Slow diffusion of hexane into a concentrated benzene solution of 1a/1b gave orange crystals of the major isomer 1b suitable for X-ray crystallography. The structure (Figure 1)

Figure 1. Molecular structure of 1b. Ellipsoids are shown at the 30% level. Hydrogen atoms except for Rh-H and those attached to C(5) and C(8) are removed for clarity. Selected bond lengths (Å) and angles (deg): Rh(1)−P(1) 2.2467(3), Rh(1)−P(2) 2.2354(3), Rh(1)−C(1) 2.0928(13), P(1)−Rh(1)−C(1) 100.30(4), P(1)− Rh(1)−P(2) 160.869(13).

exhibited a very similar distorted square-planar geometry to that found in the five-membered NHC analogue transRh(IiPr2Me2)(PPh3)2H.1 Thus, the PPh3 groups were bent in toward the hydride ligand with a P−Rh−P angle of Scheme 2

8585

dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590

Organometallics

Article

147.2°, respectively), this value being at the bottom end of the range reported for other M-FHF species.14 A preliminary assessment of the thermal stability of 2a revealed there was no change in the 31P{1H} NMR spectrum of a sample heated in C6D6 at 343 K for 2 h. Synthesis and Characterization of Rh(6-iPr)(PPh3)2F. Stirring 2a with an excess of anhydrous Me4NF in THF solution resulted in the abstraction of HF and formation of the cis-phosphine isomer of the rhodium fluoride complex Rh(6-iPr)(PPh3)2F (3a) (Scheme 2). The 31P{1H} chemical shift data for this species differed very little from those for 2a, with a change of only 0.6 ppm between the two sets of phosphorus signals. The 19F NMR spectrum was more distinctive, with a new single, sharp, low-frequency Rh-F resonance observed at δ −287.5, with trans and cis JFP couplings of 179 and 28 Hz and a JFRh of 66 Hz. Both the chemical shift and coupling constants are very similar to those reported for Rh(PPh3)3F.15 Bond lengths and angles in the solid-state structure of 3a (Figure 3) showed only very minor changes from those in 2a.

indicating that only the cis-phosphine isomer of the complex was formed (Scheme 2). The bifluoride ligand was characterized by the appearance of a very broad, high-frequency doublet (J = 380 Hz in THF-d8) at 12.1 ppm in the room-temperature 1H NMR spectrum. Upon cooling to 204 K, this resonance resolved into a doublet of doublets centered at δ 12.6, with a large splitting of 370 Hz to the distal fluorine of the bifluoride ligand (i.e., RhFHF) and a smaller 42 Hz splitting resulting from coupling to the proximal fluorine, RhFHF.13b The room temperature 19F NMR spectrum displayed two broadened doublets, one at δ −176 for the distal fluorine, the other at lower frequency (δ −273) for the rhodium-bound proximal fluorine. At 204 K, the higher frequency resonance sharpened to a doublet of doublets with JFH and JFF couplings of ca. 372 and 117 Hz, respectively.14 The lower frequency signal resolved to some extent to become more multiplet-like in appearance, but not enough to allow any of the JFH, JFF, JFRh, and JFP splittings to be measured. The X-ray crystal structure of 2a (Figure 2) confirmed the cis-phosphine geometry observed in solution. The four-

Figure 2. Molecular structure of 2a. Ellipsoids are shown at the 30% level. Hydrogen atoms except for Rh-FHF and those attached to C(5) and C(8) are removed for clarity. Selected bond lengths (Å) and angles (deg): Rh(1)−P(1) 2.3148(6), Rh(1)−P(2) 2.1778(6), Rh(1)−C(1) 2.062(2), Rh(1)−F(1) 2.1217(13), P(1)−Rh(1)−C(1) 163.47(6), P(2)−Rh(1)−C(1) 93.02(4), P(1)−Rh(1)−P(2) 102.46(13), P(1)−Rh(1)−F(1) 82.81(4), P(2)−Rh(1)−F(1) 174.34(4), C(1)−Rh(1)−F(1) 81.53(7).

Figure 3. Molecular structure of 3a. Ellipsoids are shown at the 30% level, and hydrogen atoms except for those attached to C(5) and C(8) are removed for clarity. Selected bond lengths (Å) and angles (deg): Rh(1)−P(1) 2.1850(7), Rh(1)−P(2) 2.3162(7), Rh(1)−C(1) 2.051(3), Rh(1)−F(1) 2.0880(15), P(1)−Rh(1)−C(1) 90.68(7), P(2)−Rh(1)−C(1) 166.43(8), P(1)−Rh(1)−P(2) 101.90(2), P(1)− Rh(1)−F(1) 173.99(5), P(2)−Rh(1)−F(1) 83.27(5), C(1)−Rh(1)− F(1) 83.91(8).

coordinate rhodium center was distorted from a regular square plane with trans-C-Rh-P and trans-P-Rh-F angles of 163.47(6)° and 174.34(4)°, respectively. The metrics of the bifluoride ligand can be compared against those of Rh(PPh3)2(PPh2F)(FHF)15 and Rh(PPh3)(COD)(FHF).14g The Rh−F bond length (2.1217(13) Å) is significantly longer than in either of these two compounds (2.082(1) and 2.083(2) Å, respectively), and the F···F distance is identical (2.325 Å vs 2.329 and 2.331 Å). The Rh−F···F angle (127.44°) is reduced (135.6° and

Of most note was the shortening of the Rh−F distance from 2.1217(13) Å to 2.0880(15) Å as a result of the loss of HF. Upon comparing the structure of 3a with those of 1b and 2a, it is apparent that in all cases the carbene approximates a 2-fold rotation symmetry about the Rh(1)−C(1) axis, although there is a difference of ca. 3° between the N(1)−C(1)−Rh(1) and N(2)−C(1)−Rh(1) angles for the three compounds (1b: 123.16(10)°, 120.05(10)°; 2a: 118.92(16)°, 121.85(16)°; 3a: 8586

dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590

Organometallics

Article

119.98(19)°, 122.78(19)o). As a result, one of the methine hydrogen atoms on either C(5) or C(8) is, on average, 0.11 Å closer to the metal center than the other. The thermal stability of 3a was of interest given that Grushin has reported the degradation of Rh(PPh3)3F within 2−3 h at 353 K to give a 1:1 mixture of the F/PPh3 rearrangement product Rh(PPh3)2(PPh2F)F and the cyclometalated complex Rh(PPh3)2(PPh3)′.15 We found no evidence to suggest the formation of a species analogous to Rh(PPh3)2(PPh2F)F upon heating 3a in C6D6 at 343 K, although two products were generated (Scheme 2). Heating for 3 h led first to the appearance of a compound assigned as the trans-phosphine isomer of Rh(6-iPr)(PPh3)2F (3b). This exhibited a doublet of doublets signal in the 31P{1H} NMR spectrum at δ 30.0 (1JPRh = 175 Hz, 2JPF = 29 Hz) and a somewhat broad, low-frequency doublet of triplets resonance in the corresponding 19F spectrum at δ −326, with the same 19F−31P splitting and a JFRh coupling of 80 Hz, comparable to that in 3a. A 31P{1H} NMR spectrum recorded after 20 h heating still contained mostly 3a, but now also showed in addition to 3b a further product (4) that displayed two doublets of doublets at δ 39.4 (1JPRh = 128 Hz, 2 JPP = 28 Hz) and δ −40.7 (1JPRh = 96 Hz, 2JPP = 28 Hz). There were no new signals in the 19F spectrum. We provisionally assign 4 as a cyclometalated phosphine complex, principally on the basis of the characteristic very low frequency phosphorus resonance.16,17 The formation of 3b and 4 as part of a mixture, along with the incomplete nature of the reaction, has thus far prevented their further characterization. C−F Bond Activation of C6F6 by Rh(6-iPr)(PPh3)2H. The possibility of using C−F bond activation18 as an alternative route to 3 prompted us to study the reactivity of Rh(6-iPr)(PPh 3) 2 H toward hexafluorobenzene. No reaction was observed between 1a/1b and C6F6 (1−5 equiv) in C6D6 over 24 h at room temperature, but upon heating at 363 K for 2 h, C−F activation was observed, although this gave the pentafluorophenyl complex trans-Rh(6-iPr)(PPh3)2(C6F5) (5b) (Scheme 3). Only with prolonged heating was 3a formed, along with the hydrodefluorination products C6F5H and 1,4C6F4H2.

Figure 4. Molecular structure of 5b. Ellipsoids are shown at the 30% level, and hydrogen atoms except for those attached to C(5) and C(8) are removed for clarity. Selected bond lengths (Å) and angles (deg): Rh(1)−P(1) 2.2981(7), Rh(1)−P(2) 2.3062(7), Rh(1)−C(1) 2.078(2), Rh(1)−C(29) 2.098(2), P(1)−Rh(1)−C(1) 92.71(7), P(2)−Rh(1)−C(1) 97.43(7), P(1)−Rh(1)−P(2) 157.31(2), P(1)− Rh(1)−C(29) 88.94(7), C(1)−Rh(1)−C(29) 164.38(9).

Rh(1)−C(29), 164.38(9)°; P(1)−Rh(1)−P(2), 157.31(2)°) relative to 1b, 2a, and 3a, presumably because of the ligand “gymnastics” needed to accommodate the sterically demanding fluoroaryl ring. The level of this distortion can be readily evidenced by the maximum deviation of the contributors to the mean plane containing the four atoms directly coordinated to rhodium in each of the four structures. These values are 0.05, 0.04, 0.02, and 0.38 Å for 1b, 2a, 3a, and 5b. Moreover, in 5b there is a substantial tilt of the carbene relative to the Rh(1)− C(1) axis. The N(1)−C(1)−Rh(1) and N(2)−C(1)−Rh(1) angles at 115.27(17)° and 128.55(18)o, respectively, support this observation, with the net effect being that the methine H(5) comes closer to the metal (H(5)−Rh(1), 2.32 Å) than any of the comparative hydrogen atoms in 1b, 2a, or 3a.

Scheme 3

■

SUMMARY AND CONCLUSIONS The six-membered-ring N-heterocyclic carbene 6-iPr (1,3-bis(2propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine) has been used to prepare cis- and trans-phosphine isomers of Rh(6-iPr)(PPh3)2H from either Rh(PPh3)4H or Rh(PPh3)3(CO)H. Treatment of the hydride complex with Et3N·3HF gave the bifluoride complex Rh(6-iPr)(PPh3)2(FHF), which could be converted to the corresponding fluoride analogue upon reaction with a source of anhydrous fluoride. Attempts to prepare Rh(6-iPr)(PPh3)2F via C−F activation of hexafluorobenzene instead allowed isolation of the pentafluorophenyl complex Rh(6-iPr)(PPh3)2(C6F5). The synthesis and initial reactivity studies of Rh(6-iPr)(PPh3)2F (3a) suggest that incorporation of the RE-NHC brings about noticeable differences to the all-phosphine analogues. Unlike Rh(PPh3)3F, there is no evidence for phosphine loss and Rh(μ-F)2Rh complex formation from 3a

Complex 5b was isolated as an analytically pure yellow solid by precipitation from the mixture with hexane. It exhibited a doublet in the 31P{1H} NMR spectrum, consistent with a transphosphine stereochemistry. The 19F NMR spectrum displayed three multiplets in a 2:2:1 ratio at δ −108, −165, and −167, which were assigned to the ortho-, meta-, and para-F atoms of the C6F5 ring by comparison to the literature.19 The X-ray crystal structure of 5b (Figure 4) confirmed the trans-phosphine arrangement present in solution. The presence of the C6F5 ligand resulted in the geometry at the rhodium center being substantially distorted from square planar (C(1)− 8587

dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590

Organometallics

Article

1H, Rh-FHF). 31P{1H} NMR (C6D6, 298 K): δ 60.1 (ddd, 1JPRh = 219 Hz, 2JPF = 180 Hz, 2JPP = 38 Hz), 37.1 (ddd, 1JPRh = 124 Hz, 2JPP = 38 Hz, 2JPF = 26 Hz). 19F NMR (THF-d8, 298 K): δ −175.9 (br d, J = 377 Hz, 1F, Rh-FHF), −272.6 (br d, J = 188 Hz, 1F, Rh-FHF); 204 K: δ −176.8 (dd, 1JFH = 372 Hz, 2JFF = 117 Hz, 1F, Rh-FHF), −274.9 (br m, 1F, Rh-FHF). Selected 13C{1H} NMR (THF-d8, 298 K): δ 208.1 (ddd, 2JCP(trans) = 110 Hz, 1JCRh= 48 Hz, 2JCP(cis) = 12 Hz, Rh-CNHC). Anal. Calcd for C46H51N2F2P2Rh (834.72): C, 66.19; H, 6.16; N, 3.36. Found: C, 66.04; H, 6.17; N, 3.45. cis-Rh(6-iPr)(PPh3)2F (3a). Anhydrous Me4NF (72 mg, 0.77 mmol) was added in a glovebox to a THF (5 mL) solution of Rh(6-iPr)(PPh3)2(FHF) (66 mg, 0.079 mmol) in an ampule fitted with a J. Young resealable valve. After stirring at room temperature for 2 h, the solution was reduced to dryness, extracted with benzene (8 mL), and filtered by cannula. The filtrate was concentrated to ca. 1 mL, and 3a precipitated as a mustard-colored solid upon addition of hexane upon vigorous stirring. Yield: 20 mg (31%). 1H NMR (500 MHz, C6D6, 298 K): δ 8.30−7.76 (br, 10H, P(C6H5)), 7.65 (sept, 3JHH = 6.5 Hz, 2H, (CH3)2CH), 7.04−6.95 (br, 10H, P(C6H5)), 6.93−6.78 (m, 10H, P(C6H5)), 2.60 (m, 2H, NCH2CH2CH2N), 2.16 (m, 2H, NCH2CH2CH2N), 1.71 (d, 3JHH = 6.5 Hz, 6H, N(CH3)2CH), 1.31− 1.11 (m, 2H, NCH2CH2CH2N), 0.70 (d, 3JHH = 6.5 Hz, 6H, N(CH3)2CH). 31P{1H} NMR: δ 60.7 (ddd, 1JPRh = 202 Hz, 2JPF = 180 Hz, 2JPP = 36 Hz), 37.7 (ddd, 1JPRh = 121 Hz, 2JPP = 36 Hz, 2JPF = 28 Hz). 19F NMR: δ −287.5 (ddd, 2JFP(trans) = 179 Hz, 1JFRh = 66 Hz, 2 JFP(cis) = 28 Hz, 1F, Rh-F). Anal. Calcd for C46H51N2F2P2Rh·0.5C6H6 (853.76): C, 68.93; H, 6.26; N, 3.28. Found: C, 68.93; H, 6.33; N, 3.37. trans-Rh(6-iPr)(PPh3)2(C6F5) (5b). Hexafluorobenzene (65 μL, 0.57 mmol) was added to a solution of 1a/1b (90 mg, 0.11 mmol) in benzene (5 mL) in an ampule fitted with a J. Young resealable valve, and the mixture heated at 363 K for 10 h. After cooling, the solvent was removed, and the residue redissolved in toluene (5 mL) and cannula filtered. The filtrate was concentrated to ca. 2 mL, and hexane added with vigorous stirring to bring about precipitation of 5b as a yellow solid. Yield: 43 mg (40%). 1H NMR (500 MHz, C6D6, 298 K): δ 7.84−7.60 (br, 12H, P(C6H5)), 7.10−6.98 (br, 20H, P(C6H5) + 2 × N(CH3)2CH), 2.54 (m, 4H, NCH 2CH2CH2 ), 1.54 (m, 2H, NCH2CH2CH2), 0.51 (d, 3JHH = 7.0 Hz, 12H, N(CH3)2CH). 31 1 P{ H} NMR: δ 29.8 (d, 1JPRh = 172 Hz). 19F NMR: δ −107.8 (br d, J = 36.5 Hz, 2F, Rh-o-C6F5), 164.8 (dd, 3JFF = 31.2 Hz, 3JFF = 20.4 Hz, 2F, Rh-m-C6F5), −166.7 (t, 3JFF = 20.4 Hz, 1F, Rh-p-C6F5). Selected 13C{1H} NMR: δ 214.6 (m, Rh-CNHC). Anal. Calcd for C52H50N2F5P2Rh (962.76): C, 64.89; H, 5.23; N, 2.91. Found: C, 65.0; H, 5.25; N, 3.03. X-ray Crystallography. Single crystals of compounds for 1b, 3a, and 5b were analyzed at 150 K using a Nonius Kappa CCD diffractometer, while those for 2a (collected at 120 K) were obtained on an Oxford-Diffraction machine. Data were all collected using Mo Kα radiation throughout. Details of the data collections, solutions, and refinements are given in the Supporting Information. The structures were solved using SHELXS-9725 and refined using full-matrix leastsquares in SHELXL-97.25 Refinements were generally straightforward, although notable points were as follows. In 1b, the hydride H(1) was located and refined at 1.6 Å from Rh1. Additionally, carbons C(2)− C(4) were each disordered over two sites in a 75:25 ratio. The 75% occupancy fragment atoms were refined anisotropically. The F-H-F hydrogen (H(1)) in the structure of 2a was readily located and refined freely. There was some evidence from the difference electron density Fourier map that this hydrogen may sometimes move closer to F(2) than F(1). However, disorder of H(1) was not modeled due to the uncertainty associated with determining hydrogen positions when using X-rays. The apical carbene atom C(3) in 3a exhibited disorder over two positions in an 80:20 ratio. Hydrogen atoms attached to C(2) and C(4) are included at full occupancy based on the 80% occupancy component of C(3). The motif was also seen to contain half of a molecule of benzene located proximate to a crystallographic inversion center.

in solution at room temperature nor F/Ph rearrangement of a PPh3 ligand at higher temperature.13,15 We are currently pursuing this further in attempts to make other Rh(RENHC)(PR3)2F analogues, which will allow the properties of the Rh−F bond to be investigated as a function of NHC and phosphine. Similarly, if the oxidative addition of H2 or R3SiH reported for Rh(PMe3)3(C6F5) takes place with Rh(6-iPr)(PPh3)2(C6F5),19b,20 this would open up the possibility of probing the influence of RE-NHCs on the catalytic functionalization of fluorinated aromatic substrates. In light of our recent work on catalytic C−F functionalization with RuNHC complexes,21 this is an avenue we are actively pursuing.

■

EXPERIMENTAL SECTION

All manipulations were carried out using standard Schlenk, highvacuum, and glovebox techniques using dried and degassed solvents, unless otherwise stated. NMR spectra were recorded on Bruker Avance 400 and 500 MHz NMR spectrometers and referenced to residual solvent signals for 1H and 13C spectra for C6D6 (δ 7.15, 128.0) and THF-d8 (δ 3.58, 25.4). Unless otherwise quoted, 1H and 13C resonances for the PPh3 ligands and noncarbenic 13C signals arising from the 6-iPr ligands have been excluded. 31P{1H} and 19F spectra were referenced externally to 85% H3PO4 (85%) and CFCl3, respectively (both δ = 0.0). Elemental analyses were performed by Elemental Microanalysis Ltd., Okehampton, Devon, UK, or the Elemental Analysis Service, London Metropolitan University, London, UK. Rh(PPh3)4H,22 Rh(PPh3)3(CO)H,22 6-iPrH·BF4,23 and anhydrous Me4NF24 were prepared according to the literature. Rh(6-iPr)(PPh3)2H (1a/1b). A Schlenk flask was charged with i 6- PrH·BF4 (280 mg, 1.09 mmol) and KN(SiMe3)2 (217 mg, 1.09 mmol) in toluene (10 mL), and the suspension stirred at room temperature for 30 min. This solution of the free 6-iPr carbene was added to a J. Young ampule containing Rh(PPh3)4H (400 mg, 0.35 mmol) in toluene (20 mL), and the mixture heated at 343 K for 3 h. After cooling to room temperature, the resulting deep orange-red solution was filtered by cannula and concentrated to ca. 3 mL, and hexane was added to afford an orange precipitate. The solid was isolated by cannula filtration and dried under vacuum to give a mixture of isomers 1a and 1b. Yield: 232 mg (84%). 1a: 1H NMR (500 MHz, C6D6, 298 K): δ 6.68 (sept, 3JHH = 6.6 Hz, 2H, (CH3)2CH), 2.62 (m, 2H, NCH2CH2CH2N), 2.40 (m, 2H, NCH2CH2CH2N), 1.54 (m, 1H, CH2CH2CH2), 1.47 (m, 1H, CH2CH2CH2), 1.30 (d, 3JHH = 6.6 Hz, 6H, (CH3)2CH), 0.58 (d, 3JHH = 6.6 Hz, 6H, (CH3)2CH), −5.66 (ddd, 2 JHP(trans) = 106.4 Hz, 2JHP(cis) = 30.4 Hz, 1JHRh = 25.5 Hz, RhH). 31 1 P{ H} NMR: δ 51.0 (dd, 1JPRh = 146 Hz, 2JPP = 24 Hz), 43.1 (dd, 1 JPRh = 144 Hz, 2JPP = 24 Hz). 1b: 1H NMR (500 MHz, C6D6, 298 K): δ 6.52 (sept, 3JHH = 6.7 Hz, 2H, (CH3)2CH), 2.54 (m, 4H, NCH2CH2CH2N), 1.53 (m, 2H, CH2CH2CH2), 0.53 (d, 3JHH = 6.7 Hz, 12H, (CH3)2CH), −10.35 (dt, 2JHP(cis) = 26.2 Hz, 1JHRh = 10.2 Hz, RhH). 31P{1H} NMR: 43.1 (d, 1JPRh = 180 Hz). Selected 13C{1H} NMR: δ 222.7 (dt, 1JCRh = 46 Hz, 2JCP = 9 Hz, Rh-CNHC). Anal. Calcd for C46H51N2P2Rh (796.72): C, 69.34; H, 6.45; N, 3.52. Found: C, 69.12; H, 6.41; N, 3.69. cis-Rh(6-iPr)(PPh3)2(FHF) (2a). Et3N·3HF (20 μL, 0.123 mmol) was added by syringe to a vigorously stirred solution of Rh(6-iPr)(PPh3)2H (100 mg, 0.126 mmol) in toluene (7 mL) in an ampule fitted with a J. Young resealable valve. After stirring at room temperature for 2 h, the solution was reduced to dryness, redissolved in a minimum amount of benzene, and precipitated under vigorous stirring with addition of hexane. Yield: 68 mg (65%). 1H NMR (500 MHz, C6D6, 298 K): δ 13.15 (br d, J = 405 Hz, 1H, Rh-FHF), 8.22− 7.67 (br, 10H, P(C6H5)), 7.52 (sept, 3JHH = 6.7 Hz, 2H, (CH3)2CH), 7.06−6.93 (br, 10H, P(C6H5)), 6.91−6.79 (m, 10H, P(C6H5)), 2.89 (m, 2H, NCH2CH2CH2N), 2.23 (m, 2H, NCH2CH2CH2N), 1.73 (d, 3 JHH = 6.7 Hz, 6H, N(CH3)2CH), 1.25 (m, 2H, NCH2CH2CH2N), 0.59 (d, 3JHH = 6.7 Hz, 6H, N(CH3)2CH). Additional selected 1H NMR (500 MHz, THF-d8): 298 K: δ 12.13 (br d, J = 380 Hz, 1H, RhFHF); 204 K: δ 12.64 (dd, 1JHF(distal) = 370 Hz, 1JHF(proximal) = 42 Hz, 8588

dx.doi.org/10.1021/om300984v | Organometallics 2012, 31, 8584−8590

Organometallics

Article

Am. Chem. Soc. 2001, 123, 10973. (c) Ferrando-Miguel, G.; Gerard, H.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 2002, 41, 6440. (d) Jasim, N. A.; Perutz, R. N.; Archibald, S. J. Dalton Trans. 2003, 2184. (e) Clot, E.; Megret, C.; Kraft, B. M.; Eisenstein, O.; Jones, W. D. J. Am. Chem. Soc. 2004, 126, 5647. (f) Salomon, M. A.; Braun, T.; Krossing, I. Dalton Trans. 2008, 5197. (8) Bazinet, P.; Ong, T. G.; O’Brien, J. S.; Lavoie, N.; Bell, E.; Yap, G. P. A.; Korobkov, I.; Richeson, D. S. Organometallics 2007, 26, 2885. (9) Armstrong, R.; Ecott, C.; Mas-Marzá, E.; Page, M. J.; Mahon, M. F.; Whittlesey, M. K. Organometallics 2010, 29, 991. (10) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385. (11) Note that we found that Rh(PPh3)3(CO)H reacted with fivemembered NHCs via a single pathway involving loss of all three phosphine ligands to give Rh(NHC)2(CO)H complexes. See ref 1. (12) The NMR data for the proposed trans-H-Rh-CO isomer was very similar to that of the N-mesityl analogue Rh(6-Mes)(PPh3)(CO) H, which was formed as the only product in the reaction of Rh(PPh3)3(CO)H with 6-Mes. Spectroscopic, analytical, and structural characterization of Rh(6-Mes)(PPh3)(CO)H are provided as Supporting Information. (13) Braun and co-workers have reported that addition of TREATHF to the all-phosphine complex Rh(PEt3)4H, gave the corresponding rhodium fluoride complex Rh(PEt3)3F rather than the bifluoride product Rh(PEt3)3(FHF). The latter could be prepared along with Rh(PEt3)3Cl upon treatment of Rh(PEt3)3F with