Stable Ionic Rh(I,II,III) Complexes Ligated by an Imidazolium-Substituted Phosphine with π‑Acceptor Character: Synthesis,Characterization, and Application to HydroformylationHongxing You, Yongyong Wang, Xiaoli Zhao, Shengjie Chen,* and Ye Liu*

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, 3663 North Zhongshan Road,200062 Shanghai, People’s Republic of China

*S Supporting Information

ABSTRACT: The stable ionic Rh(I,II,III) complexes [RhI(acac)-(CO)(L)]PF6 (2), [Rh

II2(OAc)4(L)2]2PF6 (3), and [RhIIICl4(L)2]PF6

(4) were synthesized through the complexation of RhI(acac)(CO)2,RhII2(OAc)4(H2O)2, and RhIIICl3·3H2O with the phosphine-function-alized ionic liquid (FIL) 1 ([L]PF6, L = 1-butyl-2-diphenylphosphino-3-methylimidazolium), respectively. The cation of L in 1 is animidazolium-substituted phosphine with a positive charge vicinal to theP(III) atom, which acts as an electron-deficient donor with π-acceptorcharacter to afford the stable complexes 2−4 due to the presence ofretrodonating π-binding between Rh−P linkage. Due to the weakenedreducing ability of L, the redox reaction between L and RhCl3·3H2Oduring the complexation is avoided, leading to the formation of 4, inwhich the Rh center is in the +3 valence state. Single-crystal X-rayanalyses show that 2−4 are all composed of a Rh-centered cation and a PF6

− counteranion. The cation of 2 possesses structuralsimilarity to RhI(acac)(CO)(PPh3), the cation of 3 with a D4h geometry possesses a structural similarity to RhII2(OAc)4(PPh3)2,and the cation of 4 exhibits an ideal RhIII-centered octahedral geometry, in which the Rh(III) (d6) ion is six-coordinated by fourchlorine atoms in the equatorial plane and two L ligands in the axial positions. TG/DTG analyses indicated that the thermalstabilities of 2−4 in air flow were improved dramatically in comparison to the corresponding analogues RhI(acac)(CO)(PPh3),RhII2(OAc)4(PPh3)2, and Rh

ICl(PPh3)3. 2−4 were found to be good to excellent catalysts for homogeneous hydroformylation of1-octene free of any auxiliary ligand; 3 was the best candidate. The “on water” effect in rate acceleration was evidently observedover 2 and 4 due to their insensitivity to moisture and oxygen.

■ INTRODUCTIONNonvolatile ionic liquids (ILs) have been recognized aspromising alternative solvents, which can dissolve transition-metal complexes (homogeneous catalysts) and then preventtheir leaching and deactivation.1−3 On the other hand, ILs canbe functionalized flexibly by varying cations and/or anions thatconstitute the IL structures with specific functionalities. Thus, itis possible to develop abundant functionalized ILs (FILs) duallypossessing the characters of the incorporated functionalities aswell as those of the ILs.4−9 Phosphine-FILs have long beeninvestigated for the design of ionic organometalates andapplication to homogeneous catalysis, in which the positivecharge in the imidazolium ring was either isolated in a positionremote from the P(III) atom10−13 or placed as a neighbor tothe P(III) atom, such as the amidiniophosphine ligandsreported in Canac and Chauvin’s work.14−19

In contrast to the electron-rich phosphine ligands as strong σdonors,20 electron-deficient ligands remain less explored.Typical examples of electron-deficient ligands are phosphitesand fluoroarylphosphines. The electron-withdrawing effects ofphosphites result from both σ(−I)-induction and π-retrodona-tion into the π* orbitals of the P−O linkages.21 However, P−O

bonds of phosphites are unstable and sensitive to proticcleavage.16 Although fluoroarylphosphines with inert P−Clinkages can be considered as alternatives, their structuraldiversity is limited.22 The recently developed phosphine-FILswith the positive charge vicinal to the P(III) atom have becomeattractive candidates for mimicking the electron-deficientligands, due to their advantages of apparent robustness andthe vicinal electrostatic effect derived from the conjugation of apositive charge with the coordinating P(III) atom.14−19,23 Suchelectron-deficient phosphines are of great concern incoordination chemistry and homogeneous catalysis,24 whileproviding an essential difference in coordinating nature as wellas in catalytic performance.In this paper, through complexation between the phosphine-

FIL of [L]PF6 (1, L = 1-butyl-2-diphenylphosphino-3-methylimidazolium) and the corresponding Rh(I,II,III) pre-cursors, three types of ionic Rh complexes (2−4) weresynthesized for the first time, in which the Rh center was inthe oxidative valence of +1, +2, and +3, respectively (Scheme

Received: March 1, 2013Published: April 30, 2013

Article

pubs.acs.org/Organometallics

© 2013 American Chemical Society 2698 dx.doi.org/10.1021/om400171t | Organometallics 2013, 32, 2698−2704

1). In comparison with the Rh(I,II) analogues, there have beenrelatively few reported examples of Rh(III) complexescontaining tertiary phosphines.25 Detailed crystallographicdata of 2−4 are given. The catalytic performance of 2−4toward hydroformylation of 1-octene was investigated. L as anelectron-deficient ligand with a vicinal electrostatic effectprovides nonclassical coordinating behavior. Consequently,the geometric configuration, stability, and catalytic performanceof the resultant ionic Rh complexes completely vary from thoseof the corresponding PPh3-ligated Rh complexes.

■ RESULTS AND DISCUSSIONSynthesis and Characterization. The complexation of

RhI(acac)(CO)2 with 1 led to the formation of the ioniccomplex 2 in the yield of 67%, following the procedures for thepreparation of Rh(acac)(CO)(PPh3).

26 2 was air and moisturestable both in the solid state and in organic solvents for severalweeks under ambient conditions. Single crystals for X-raydiffraction analysis were obtained through recrystallization inCH2Cl2/acetone/n-hexane. The molecular structure depicted inFigure 1 indicates that 2 is composed of the RhI complex cationand PF6

− anion. The structure of the complex cation is similarto that of Rh(acac)(CO)(PPh3),

26 which exhibits a typicalsquare-planar geometry. The Rh(I) (d8) center, lying at thecenter of inversion, is coordinated by L, CO, and the chelated2,4-pentanedionato ligand. The Rh−P bond distance (2.22 Å)observed in 2 is shorter than those in the PPh3-ligated Rh(I)complexes Rh(acac)(CO)(PPh3)

26 and RhCl(PPh3)327,28 (Rh−

P = 2.23−2.4 Å; Table 1). The P1−Rh1−O1 and C25−Rh1−O2 bond angles are 179°, and the C25−Rh1−P1 and O2−Rh1−O1 bond angles are 90°.The 1H NMR, 13C NMR, and 31P NMR spectra further

support the assigned structure of 2. The 1H NMR spectra of 2show that the resonance for o-H in the phenyl ring shifts tolower field (8.10 ppm) in comparison to that in 1 (7.30 ppm),due to the electron-withdrawing effect from the Rh+ ion. Theresonances for C(4)-H and C(5)-H in the imidazolium ring arenearly the same for 1 and 2 (see Supplementary Table 2 in the

Supporting Information). The 13C NMR spectra of 2 show thatthe resonance for P-C in the imidazolium ring shifts to higherfield (138.2 ppm, d, J = 29 Hz) in comparison to that in 1(142.7 ppm, d, J = 52 Hz), probably due to the π-retrodonatedd electron of the Rh(I) ion into the π* orbital of the P atomand the resultant P-C conjugation effect. The 31P NMR spectraof 2 show two types of resonances at 52.8 (doublet) and−144.0 ppm (quintet), which are attributed to the coordinatedL and PF6

−, espectively. In comparison to the 31P NMR signal

Scheme 1. Syntheses of the Ionic Complexes of 2-4 Ligated by the Phosphine-FIL of 1

Figure 1. Single-crystal structures of 1−4. The PF6− anion and the

solvent molecule are omitted for clarity.

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of Rh(acac)(CO)(PPh3) (48.6 ppm) with structural similar-ity,26 a relatively low-field chemical shift of 2 attributed to L(52.8 ppm) is observed, which further indicates that L is in anelectron-deficient environment.The complexation of RhII2(OAc)4(H2O)2 with 1 led to the

formation of the ionic complex 3. Single crystals for X-rayanalysis were obtained through recrystallization in CH2Cl2/ethanol/diethyl ether. The molecular structure depicted inFigure 1 indicates that, in 3, the RhII complex dimer cation iscounteracted by two PF6

− anions. The structure of the complexcation is very similar to that of Rh2(OAc)4(PPh3)2,

29 in whichthe dirhodium nucleus (RhII, d7) with D4h symmetry iscoordinated by two imidazolium-tailed ligands of L in theaxial position and four acetate groups in the equatorial plane.Each Rh(II) center is lying at the center of a square plane in asix-coordinated octahedral geometry. The Rh1A−Rh1−P1 andO4−Rh1−O1A bond angles are 175 and 89°, respectively. Thebond distances Rh1−P1 (2.44 Å) and Rh1−Rh1A (2.43 Å)observed in 3 are both shorter than those inRh2(OAc)4(PPh3)2

29 (Rh−P, 2.48 Å; Rh−Rh, 2.45 Å; Table1), indicating the enhanced dative interaction and the improvedstability of 3. The P1−Rh1−O1 and C25−Rh1−O2 bondangles are 179°, and the C25−Rh1−P1 and O2−Rh1−O1bond angles are 90°, which are within the standard deviations.Since 3 is in a low-spin state with an indication of the

presence of one unpaired electron in the Rh(II) nucleus (d7), itis endowed with a paramagnetic nature. Consequently, the 1HNMR and 31P NMR signals attributed to L in the cationiccomplex are broadened to flatness. Undoubtedly, if 3 could bemeasured by EPR spectroscopy, the signal for the g factor (ge)for the free electron would be observed. Unfortunately, theinstrument for EPR spectroscopy is not available in ouruniversity, and the measurement cannot be conducted at thepresent stage. However, the 31P NMR spectrum still shows thesignal of the PF6

− counteranion at −144.0 ppm, consistent withits solid-state structure, due to the negligible influence of theRh(II) magnetic moments on the PF6

− counteranion.In comparison with the phosphine-ligated Rh(I,II) analogues,

the Rh(III) complexes containing tertiary phosphines such asRhH2Cl(PPh3)3

30 have been rarely reported. 4, in which the Rhion is in the +3 valence state, was obtained herein as an orange-red solid through simple complexation of RhCl3·3H2O with 1.The molecular structure of 4 (Figure 1) indicates that 4 iscomposed of the Rh complex cation [RhCl4(L)2]

+ and a PF6−

anion. The complex cation exhibits an ideal octahedralgeometry. The RhIII (d6) center, which is six-coordinated byfour chlorine atoms in the equatorial plane and twoimidazolium-tailed L groups in the axial positions, is situatedexactly in the center of an octahedron. The P1−Rh1−P1a andCl−Rh1−Cl1a bond angles are 180°, and the Cl−Rh1−P1 andCl1−Rh1−Cl2 bond angles are 89−91° (Table 1). Theidentical Rh−P distances (2.38 Å) observed in 4 are in theclassical range, in comparison to those in the PPh3-ligatedRh(I,III) complexes of Rh(acac)(CO)(PPh3),

26 RhCl-(PPh3)3,

27 and RhH2Cl(PPh3)330 (Table 1). The cation

[RhCl4(L)2]+ is typically a 18-electron configuration which is

thought to be stable. In addition, the highly symmetricaloctahedron further facilitates the stability of [RhCl4(L)2]

+ in 4.The 1H NMR, 13C NMR, and 31P NMR spectra also support

the assigned structure of 4. The 1H NMR spectra of 4 showthat the resonance for o-H in the phenyl ring (8.70 ppm) shiftsto much lower field in comparison to those in 1 (7.49 ppm).Similarly, due to the significantly intense electron-withdrawingeffect from Rh(III) ion on the P atom, in the 13C NMR spectraof 4 all the resonances for the carbons closely related to the Patom shift to lower field (147.3, s, P-C in imidazolium ring;133.0 and 129.8, s, in NC(H)C(H)N+; 140.2, s, P-C in phenylring), in comparison to those in 1 (142.7, d, J = 52 Hz, P-C inimidazolium ring; 127.8 and 125.3, s, in NC(H)C(H)N+; 132.7,d, J = 20 Hz, P-C in phenyl ring). In comparison to the 31PNMR signals for the other phosphine-ligated Rh complexes asgiven in Table 1, L in 4 shows a high-field chemical shift at 13.6ppm, indicating the relatively electron rich character of the Patom, which can be attributed to the π-retrodonation of a d-electron of the Rh(III) ion and the consolidated electrostaticinteraction Rh3+−Pδ−, as well as the convenient octahedralconfiguration. It has been known that, in the preparation ofRhCl(PPh3)3 through the complexation of RhCl3·3H2O withPPh3, the reduction of Rh(III) to Rh(I) is accomplished byPPh3 as a reducing reagent due to its strongly electron richnature.27,31 In contrast to PPh3, L is an electron-deficient ligandwith weak reductive ability. Consequently, the redox reactionbetween L and RhCl3·3H2O in the course of complexation isavoided, leading to the unchanged valence state for the Rh(III)-centered complex of 4.Table 1 summarizes the comparison data of 2−4 with the

PPh3-ligated Rh complexes in terms of the Rh−P distances andthe 31P NMR signals. In contrast to the corresponding Rhanalogues with the same valence state, the Rh−P distances ofthe L-ligated Rh complexes 2−4 were universally shorter thanthose of the PPh3-ligated complexes (2 vs RhI(acac)(CO)-(PPh3) and RhICl(PPh3)3; 3 vs RhII2(OAc)4(PPh3)2; 4 vsRhIIIH2Cl(PPh3)3), indicating an enhanced interaction for theRh−P linkages due to the π-retrodonation of d-electrons of theRh ion to the cationic phosphorus fragment.In comparison to PPh3 as a typical electron-rich donor, L can

mimic the behavior of the electron-deficient ligand phosphitewith the electron-withdrawing effect, which possesses both theσ-donor and π-acceptor characters as a result of σ-inductionand π-retrodonation.21 Consequently, the Rh−P linkages in 2−4 are all strengthened and the stabilities of 2−4 were improveduniversally. In addition to π-retrodonation, the vicinal electro-static effect derived from the conjugation of the positivelycharged imidazolium ring with the coordinating P(III) atommay be another reason to afford the stable complexes 2−4.14On the other hand, the weak reductive ability of L can avoid thepossible redox reaction between the metal center and L,

Table 1. Comparison of the Rh−P Bond Distances and 31PNMR Signals for the Various Rh Complexes

Rh complex

Rh−P bonddistance (Å)

31P NMR ofphosphine (ppm)

Rh(I) [Rh(acac)(CO)(L)]PF6 (2)

2.2218(8) 52.8

Rh(acac)(CO)(PPh3)

262.2418(9) 48.6

RhCl(PPh3)3 (red)27 2.214(4), 2.322(4)

, 2.334(4)46.8, 29.828

Rh(II) [Rh2(OAc)4(L)2](PF6)2 (3)

2.4368(10), 2.43a none

Rh2(OAc)4(PPh3)229 2.4771(5), 2.45a none

[RhCl4(L)2]PF6 (4) 2.3776(15),2.3776(15)

13.6 (br)

Rh(III) RhH2Cl(PPh3)330 2.302(1), 2.331(1)

, 2.458(1)22.0, 40.1

aRh−Rh bond distance.

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especially when the metal center is in a highly oxidative valencestate such as Rh3+, which facilitates the utilization efficiency ofthe ligand (L) and the coordination diversity.On the other hand, 1 with an electron-deficient nature is

more robust against oxidative degradation due to the intensedelocalization of the lone-pair electrons in the P atom to theadjacent positive imidazolium ring, leading to insensitivity tomoisture and oxygen under ambient conditions. Consequently,the L-ligated complexes 2−4 are also moisture and oxygeninsensitive under ambient conditions and possess improvedthermal decomposition temperatures. The TG/DTG analysesin an air flow (Figure 2) showed that initial thermaldecompositions temperatures of 3 and 4 were up to 180 °C,which were much higher than those of Rh2(OAc)4(PPh3)2 (150°C) and RhCl(PPh3)3 (115 °C); the initial thermaldecomposition temperature of 2 (180 °C) was comparable tothat of Rh(acac)(CO)(PPh3) (185 °C).

Catalytic Performance of 2−4 for Hydroformylationof 1-Octene. Supposedly, the differences in the phosphine-ligated Rh complexes, in terms of the coordinating geometry,the valence state of the Rh center, and the thermal stability,could lead to various catalytic behaviors. In this part, thecatalytic performance of 2−4 toward the hydroformylation of1-octene was investigated in comparison to that of thecorresponding Rh(I,II) analogues. It has been reported that asubstantial rate acceleration could be obtained when theinsoluble reactants were stirred in aqueous suspension, whichwas denoted as a remarkable “on water” phenomenon bySharpless32 and other researchers.33 A similar “on water” effecthas also been observed in Rh-catalyzed hydroformylationswhen water is compatible with the specific phosphine-modifiedRh systems without a negative effect on the catalyst stabilityand selectivity.34,35 The insensitivity to moisture and oxygen of2−4 motivated us to investigate the “on water” effect for

Figure 2. TG/DTG analyses of the phosphine-ligated Rh complexes in an air flow.

Table 2. Hydroformylation of 1-Octene Catalyzed by Different Rh Complexes without the Presence of Any Auxiliary Liganda

sel (%)b

entry cat. additive conversn (%)b ald. (n + iso) iso-octenes L/Bc TONald

1 2 69 83 17 0.7 11402 2 H2O 98 97 3 0.7 19003 3 97 93 7 1.0 18004 3 H2O 96 92 8 1.3 17705 4 45 86 14 2.1 7706 4 H2O 97 82 16 2.2 16107 Rh(acaca)(CO)(PPh3) 99 96 4 0.8 19108 Rh(acaca)(CO)(PPh3) H2O 81 83 17 0.9 13409 Rh2(OAc)4(PPh3)2 98 98 2 1.1 191010 Rh2(OAc)4(PPh3)2 H2O 97 99 1 0.8 193011 RhCl(PPh3)3 87 85 15 0.9 105012 RhCl(PPh3)3 H2O 80 82 18 0.8 1140

aConditions: Rh 0.05 mol % (0.01 mmol), H2O 1 mL, 1-octene 20.0 mmol, CO/H2 (1/1) 4.0 MPa, temperature 120 °C, reaction time 2 h.bDetermined by GC. cL/B, the ratio of linear nonanals to branched nonanals.

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hydroformylation over 2−4, with the additional advantage ofeasy manipulations in open air.Free of any auxiliary phosphine and water, when 2−4 were

applied as the catalysts in parallel under the optimaltemperature (120 °C) in Table 2, the best activity with aTON value of 1800 was observed for 3 but with a poor L/Bratio of 1.0 (entry 3), which was competitive with those forRh(acac)(CO)(PPh3) and Rh2(OAc)4(PPh3)2 (entries 7 and9). Reasonably, the less stable complex 3 exhibited bettercatalytic performance in comparison to 2 and 4 (entry 3 vsentries 1 and 5). The relatively more stable 2 and 4 resulted indifficult dissociation of the ligand to accommodate insertion of1-octene for hydroformylation. Under the same conditions, theaddition of H2O (optimized to be 1 mL) spurred the catalyticactivities of 2 and 4 greatly, resulting in the high TONs of 1900for 2 and 1610 for 4 without an influence on L/B ratio (entry 2vs 1; entriy 6 vs 5). In contrast, when the neutral analoguesRh(acac)(CO)(PPh3) and RhCl(PPh3)3 were used as thecatalysts, the addition of H2O deteriorated the catalytic activityand the selectivity to nonanals obviously (entry 8 vs 7; entry 12vs 11). These results suggested that water as an auxiliaryadditive in a suitable amount indeed plays a significant role inpromoting the catalytic behaviors of 2 and 4 as the defined “on-water” effect, while the catalysts were well compatible withwater. Nevertheless, under the applied conditions, the “on-water” effect was not evidently observed for 3 andRh2(OAc)4(PPh3)2, which were both robust against waterand could exhibit remarkable activities even free of any additive.However, the hydroformylations for 2 and 4 were not efficientwith water as the solvent (>2 mL). Because the Rh catalystsand 1-octene were poorly soluble in water, the surrounding ofthe hydrophobic catalyst and substrate by a large amount ofwater limited the accessibility of the Rh catalyst to 1-octene,resulting in the depressed activation of 1-octene.

■ CONCLUSION

The ionic Rh(I,II,III) complexes 2−4, which were ligated bythe phosphine-FIL 1 as an electron-deficient donor with π-acceptor character, were synthesized for the first time and fullycharacterized. Single-crystal X-ray analyses show that 2 iscomposed of the RhI-centered square-planar cation and onePF6

− anion, whose complex-cation possesses the structuralsimilarity to Rh(acac)(CO)(PPh3), that 3 is composed of adinuclear RhII-centered cation with D4h geometry and two PF6

−

anions, whose complex cation possesses a structural similarityto Rh2(OAc)4(PPh3)2, and that 4 is composed of a highlysymmetrical RhIII-centered octahedral cation and one PF6

−

anion. The TG/DTG analyses indicated that the thermalstabilities of 2−4 in an air flow were improved markedly incomparison to the corresponding analogues Rh(acac)(CO)-(PPh3), Rh2(OAc)4(PPh3)2, and RhCl(PPh3)3. When themoisture- and oxygen-insensitive 2−4 were used as the catalystsfor homogeneous hydroformylation of 1-octene free of anyauxiliary ligand, 3 exhibited the best catalytic performance,which was competitive with those of Rh(acac)(CO)(PPh3) andRh2(OAc)4(PPh3)2. The “on water” effect in rate accelerationwas observed for 2 and 4, obviously due to their compatibilitywith H2O. However, the “on-water” effect was negligible in thecase of 3, which exhibited excellent activity regardless of thepresence or absence of water.

■ EXPERIMENTAL SECTIONReagents and Analysis. All synthesis experiments were carried

out under a nitrogen atmosphere using standard Schlenk techniques.All reagents were purchased from Aladin Reagent Co. in Shanghai,People’s Republic of China, and used as received. The FT-IR spectrawere recorded on a Nicolet NEXUS 670 spectrometer. The 1H and31P NMR spectra were recorded on a Bruker Avance 500spectrometer. The 31P NMR spectra were referenced to 85% H3PO4sealed in a capillary tube as an internal standard. Elemental analyses forCHN were obtained using an Elementar Vario EL III instrument. TG/DTG analysis was performed using a Mettler TGA/SDTA 851e

instrument and STARe thermal analysis data processing system.TG/DTG analysis was run in an air flow with a temperature ramp of10 °C min−1 between 50 and 700 °C. GC analysis was performed on aShimadzu-2014 instrument equipped with a Rtx-Wax capillary column(30 m × 0.25 mm × 0.25 μm).

Synthesis. 1-Butyl-2-diphenylphosphino-3-methylimidazoliumHexafluorophosphate ([L]PF6, 1). 1 (L = 1-butyl-2-diphenylphosphi-no-3-methylimidazolium) was prepared according to the reportedmethods23 with some modification. Under a nitrogen atmosphere, asolution of 1-butyl-3-methylimidazolium hexafluorophosphate (15.0 g,53.0 mmol) in 50 mL of dry CH2Cl2 (refluxed with calcium hydrideand distilled freshly before use) was cooled to −78 °C, and then 27mL of n-BuLi (2.2 M, in hexane, 59.4 mmol) was added dropwise.After the mixture was stirred for 1 h, chlorodiphenylphosphine(PPh2Cl, 11.7 g, 53.0 mmol) was added dropwise. The resultantmixture was stirred overnight while the reaction mixture was warmedto room temperature. After quenching excess n-BuLi with deionizedwater, the obtained oily mixture was stripped of solvent in vacuo. Theresidue was recrystallized from CH2Cl2/EtOH to yield 1 as whitesolids (18.6 g; yield, 75 wt %). 1H NMR (δ, ppm, CDCl3): 7.62 (s,1H, NC(H)C(H)N+), 7.59 (s, 1H, NC(H)C(H)N+), 7.49 (m, 6H,p-,m-H, PPh2), 7.30 (m, 4H, o-H, PPh2), 4.26 (t, 2H, J = 7.5 Hz,CH2CH2CH2CH3), 3.48 (s, 3H, NCH3), 1.56 (m, 2H,CH2CH2CH2CH3), 1.15 (m, 2H, CH2CH2CH2CH3), 0.78 (t, J =7.5 Hz, 3H, CH2CH2CH2CH3).

31P NMR (δ, ppm, DMSO-d6): −28.6(s, PPh2), −143.5 (quint, PF6−). 31C NMR (δ, ppm, CDCl3): 142.7 (d,NCN+, J = 52.0 Hz), 132.7 (d, PC, JCP = 20.0 Hz), 131.0 (s, CHp‑Ar),130.1 (d, J = 7.0 Hz, CHo‑Ar), 127.8 (s, NC(H)C(H)N

+), 127.6 (d, J =7.0, CHm‑Ar), 125.3 (s, NC(H)C(H)N+), 50.5 (s, CH2CH2CH2CH3),37 .8 (s , CH3) , 32 .7 (s , CH2CH2CH2CH3) , 19 .3 (s ,CH2CH2CH2CH3), 13.3 (s, CH2CH2CH2CH3).

(1-Butyl-2-diphenylphosphino-3-methylimidazolium)carbonyl-(2,4-pentanedionato)rhodium Hexafluorophosphate ([Rh(acac)-(CO)(L)]PF6, 2). Under a nitrogen atmosphere, [Rh(acac)(CO)2](commercial, 54 mg, 0.21 mmol) dissolved in dry CH2Cl2 (5 mL) wasadded to the ligand 1 (94 mg, 0.2 mmol) and the mixture thenrefluxed for 4 h. During refluxing, the reaction solution changed fromgreen-yellow to yellow rapidly, accompanied by the gas (CO) release.When the solution was cooled to room temperature, a clear solutionwas obtained upon filtration, to which diethyl ether was added toprecipitate yellow solids. The yellow solids were collected after dryingunder vacuum in the yield of 67% (98 mg). A sample suitable for X-raydiffraction analysis was obtained by recrystallization from CH2Cl2/ethanol/n-hexane. FT-IR (KBr pellet): 3737 (w), 3447 (w), 2960 (s),1620 (w), 1395 (m), 1260 (m), 841 cm−1 (P−F, s). 1H NMR (δ, ppm,CDCl3): 8.10 (m, 4H, o-H, PPh2)), 7.61 (m, 6H, p-,m-H, PPh2), 7.54(s, 1H, NC(H)C(H)N+), 7.49 (s, 1H, NC(H)C(H)N+), 5.53 (m, 1H,OC(CH3)CH(CH3)CO), 3.67 (t, 2H, J = 8.0 Hz,CH2CH2CH2CH3), 3.37 (s, 3H, NCH3), 2.17 (s, 3H, trans to P,OC(CH3)CH(CH3)CO), 1.55 (s, 3H, OC(CH3)CH(CH3)-CO), 1.50 (m, 2H, CH2CH2CH2CH3), 0.95 (m, 2H,CH2CH2CH2CH3), 0.72 (t, 3H, J = 7.3 Hz, CH2CH2CH2CH3).

31PNMR (δ, ppm, CDCl3): 52.8 (d, JP−Rh = 450 Hz, PPh2), −144.0(quint, PF6

−). 31C NMR (δ, ppm, CDCl3): 188.9 (s, COacac), 187.8 (d-d, JCRh = 73.5 Hz, JCP = 25.0 Hz, CO), 186.9 (s, COacac), 138.2 (d,NCN+, JCP = 29.0 Hz), 135.1 (d, PC, JCP = 14.0 Hz), 133.2 (d, JCP =3.0 Hz, CHp‑Ar), 130.0 (d, J = 13.0 Hz, CHo‑Ar), 127.5 (d, J = 8.0,CHm‑Ar), 126.9 (s, NC(H)C(H)N+), 125.5 (s, NC(H)C(H)N+), 50.5(s, CH2CH2CH2CH3), 37.8 (s, CH3), 32.7 (s, CH2CH2CH2CH3), 19.3

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(s, CH2CH2CH2CH3), 13.3 (s, CH2CH2CH2CH3). Anal. Found: C,44.27; H, 4.45; N, 3.95. Calcd: C, 44.72; H, 4.47; N, 4.01.Bis(1-butyl-2-diphenylphosphino-3-methylimidazolium)-

tetraacetatodirhodium(II) hexafluorophosphate ([Rh2(OAc)4(L)2]-(PF6)2, 3). A solution of Rh2(OAc)4(H2O)2 (commercial, 96 mg, 0.2mmol) in 5 mL of ethanol was treated with 1 (234 mg, 0.5 mmol) andthen refluxed for 4 h with vigorous stirring. Upon completion, thereaction solution changed from green to blood-red while red solidsprecipitated at ambient temperature. The red precipitates werecollected after washing with ethanol and drying under vacuum, inthe yield of 80% (220 mg). The sample suitable for X-ray diffractionanalysis was obtained by recrystallization from CH2Cl2/ethanol/diethyl ether. FT-IR (KBr pellet): 3737 (w), 3447 (w), 2960 (s), 1620(w), 1395 (m), 1260 (m), 841 cm−1 (P−F, s). Due to theparamagnetic nature of the Rh(II) center complex cation, the 1HNMR and 31P NMR signals of 3 attributed to ligand L were broadenedto flatness. However the 31P NMR signal attributed to the PF6

−

counteranion was observed clearly at −143.5 ppm (sept, δ, acetone-d6). Anal. Found: C, 46.28; H, 4.52; N, 5.91. Calcd: C, 46.35; H, 4.67;N, 5.41.Bis(1-butyl-2-diphenylphosphino-3-methylimidazolium)-

tetrachloridorhodium(III) Hexafluorophosphate ([RhCl4(L)2]PF6, 4).Under a nitrogen atmosphere, the ligand 1 (710 mg, 1.5 mmol)dissolved in 10 mL of methanol at 45 °C was treated withRhCl3·3H2O (80 mg, 0.3 mmol) and tetrabutylammonium chloride(Bu4NCl, 47 mg, 0.17 mmol). The resultant mixture was refluxed for12 h with vigorous stirring. After the reaction mixture was cooled toroom temperature, orange-red precipitates were collected afterwashing with methanol and ethyl ether and drying under vacuum togive the product 4 (292 mg; yield, 94%). A sample suitable for X-raydiffraction analysis was obtained by recrystallization from acetone/n-hexane. FT-IR (KBr pellet): 3435 (w), 3100 (w), 2961 (m), 2930 (w),2873 (w), 1622 (w), 1570 (w), 1483 (m), 1461 (w), 1435 (s), 1382(w), 1086 (m), 833 cm−1 (P−F, s), 750 (s), 698 (s), 558 (s). 1H NMR(δ, ppm, acetone-d6): 8.70 (m, 4 o-H, PPh2), 8.05 (s, 1H,NC(H)C(H)N+), 7.96 (s, 1H, NC(H)C(H)N+), 7.52 (m, 2 p-H,PPh2), 7.46 (m, 4 m-H, PPh2), 3.98 (m, br, 2H, CH2CH2CH2CH3),3.78 (s, 3H), 1.90 (s, br, CH2CH2CH2CH3), 1.05 (s, br,CH2CH2CH2CH3), 0.71 (t, 3H, J = 7.0 Hz, CH2CH2CH2CH3).

31PNMR (δ, ppm, acetone-d6): 13.7 (d, br, PPh2), −142.6 (quint, PF6

−).31C NMR (δ, ppm, acetone-d6): 147.3 (s, NCN+), 140.2 (br, PC),136.9 (s, CHpara‑Ar), 133.7 (br, CHortho,meta‑Ar), 133.0 (s, NC(H)C(H)N+), 129.8 (s, NC(H)C(H)N+), 57.6 (s, CH2CH2CH2CH3),45.9 (s, CH3), 36.6 (s, CH2CH2CH2CH3), 24.3 (s, CH2CH2CH2CH3),18.1 (s, CH2CH2CH2CH3). Anal. Found: C, 46.28; H, 4.52; N, 5.91.Calcd: C, 46.35; H, 4.67; N, 5.41.X-ray Crystallography. Intensity data were collected at room

temperature (296 K) for 1−4 on a Bruker SMARTAPEX IIdiffractometer using graphite-monochromated Mo Kα radiation (λ =0.71073 Å). Data reduction included absorption corrections by themultiscan method. The structure was solved by direct methods andrefined by full-matrix least squares using SHELXS-97 (Sheldrick,1990), with all non-hydrogen atoms refined anisotropically. Hydrogenatoms were added at their geometrically ideal positions and refinedisotropically. Crystal data and refinement details are given inSupplementary Table 1 (see the Supporting Information).General Procedures for Hydroformylation of 1-Octene

Catalyzed by 2−4. In a typical experiment, the isolated crystallinecatalyst of 2 (or 3 or 4, 0.01 mmol) was mixed with 20 mmol of 1-octene and 1 mL of water (if required) sequentially. The obtainedmixture in a sealed Teflon-lined stainless steel autoclave was purgedwith syngas (CO/H2 1/1, 4.0 MPa) and then stirred vigorously at theappointed temperature for 2 h. Upon completion, the reaction mixturewas extracted with diethyl ether (3 mL × 3). The combined organicphase was analyzed by GC to determine the conversion of 1-octene (n-dodecane as internal standard) and the selectivity to nonanals(normalization method).

■ ASSOCIATED CONTENT*S Supporting InformationTables giving crystal data and selected NMR data and CIF filesgiving crystallographic data for 1−4. This material is availablefree of charge via the Internet at http://pubs.acs.org. The filesCCDC-923401, CCDC-923402, CCDC-923403, and CCDC-919910 also contain supplementary crystallographic data for 1−4. These data can be obtained free of charge from theCambridge Crystallographic Database.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: yliu@chem.ecnu.edu.cn (Y.L.); chenshengjieno1@sina.com (S.C.). Fax: +86 21 62233424. Tel: +86 21 62232078.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the National NaturalScience Foundation of China (Nos. 21273077 and 21076083),973 Program from Ministry of Science and Technology ofChina (2011CB201403), the Ministry of Science andTechnology of China for Croatian-Chinese Scientific andTechnological Cooperation, and the Fundamental ResearchFunds for the Central Universities.

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