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Journal of Molecular Catalysis A: Chemical 396 (2015) 68–76

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

jou rna l h om epa ge: www.elsev ier .com/ locate /molcata

onic Rh(I)-complexes containing �-accepting and hemilabile,N-ligands as efficient catalysts for hydroformylation of 1-octene

heng-Jie Chen, Yong-Qi Li, Yong-Yong Wang, Xiao-Li Zhao ∗, Ye Liu ∗

hanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, 3663 North Zhongshan Road, 200062 Shanghai, China

r t i c l e i n f o

rticle history:eceived 28 July 2014eceived in revised form5 September 2014ccepted 17 September 2014vailable online 28 September 2014

a b s t r a c t

The ionic hybrid P,N-ligands of 1 and 2 have been synthesized as one type of unconventional tertiary phos-phines with the advantages of both the �-accepting ligands and the hemilabile ligands. The correspondingionic Rh(I)-complexes of 1A and 2A proved to be more efficient catalysts for hydroformylation of 1-octeneover isomerization than the traditional RhI(acac)(CO)(PPh3), due to the �-backdonation interaction inRh → P linkage and the hemilabile ligation of N,P-coordinating sites to Rh-center. The in situ high-pressureFT-IR analysis indicated that: (1) the formation and lifetime of the active Rh H species (2047 cm−1)

eywords:onic rhodium complexes-Acceptoremilabile ligandsydroformylation

derived from 1A are facilitated by the presence of ligand 1; (2) the formation of the active Rh H speciesis depressed without the aid of the P,N-hemilabile ligation; and (3) the active Rh H species derivedfrom RhI(acac)(CO)(PPh3) is unstable and susceptible to the formation of the inactive carbonyl-bridgedRh-dimer.

© 2014 Elsevier B.V. All rights reserved.

n situ high-pressure FT-IR spectra

. Introduction

Hydroformylation of olefins is one of the industrially mostmportant processes relying on homogeneous catalysis with 100%tom economy [1–4]. Mononuclear rhodium complexes are theost efficient catalysts for this reaction and a great deal of work has

een devoted to the improvement of reaction rates and selectivityy ligand design [2,5–10]. In contrast to the electron-rich phos-hine ligands as the strong �-donors, the �-accepting ligands withlectron-deficient effect remain less explored relatively. The typicalxamples of electron-deficient ligands are phosphonites or phos-hites that possess both the �-donor and �-acceptor characters2,11–13]. The phosphonite- or phosphite-involved Rh-catalystsave been reported to be highly active for hydroformylation oflefins even like internal or branched ones [14–19]. Unfortu-ately, the lability of phosphonites or phosphites toward hydrolysisderived from the susceptible P O bonds) and a tendency tondergo degradation has limited their practical uses [19,20]. Inrder to circumvent the shortcomings of phosphonites and phos-

hites, the ionic phosphines, in which the phosphorus(III) atom

s directly vicinal to the positive charged quaternary ammoniumroups through the stable C P linkages [21–39], can be regarded

∗ Corresponding authors. Tel.: +86 21 62232078; fax: +86 21 62233424.E-mail addresses: xlzhao@chem.ecnu.edu.cn (X.-L. Zhao),

liu@chem.ecnu.edu.cn (Y. Liu).

ttp://dx.doi.org/10.1016/j.molcata.2014.09.021381-1169/© 2014 Elsevier B.V. All rights reserved.

as the potential alternatives for mimicking the �-acceptor char-acter of phosphonites or phosphites, due to the most intensiveelectron-withdrawing effect of the neighbored quaternary ammo-nium groups. Such ionic phosphines, which can be regarded asa type of phosphine-functionalized ionic liquids [36–39], possessthe apparent advantages like the robustness (against hydroly-sis and degradation) and the unique electrostatic effect (due tothe conjugation between a positive charge and the coordinatingP(III) atom). The structural features of some of the ionic phos-phines have been reported specifically in Chauvin and Canac’s work[22,31–35], but the related catalysis was not investigated. In ourprevious work, we have reported some examples of the catalyticreactions over the transition metal complexes that are coordi-nated by the imidazolium-neighbored phosphines with �-acceptorcharacter, because of our interests in the functionalization ofimidazolium-based ionic liquids and their applications to catalysis[36–39]. In comparison to phosphonites/phosphites, phosphaben-zenes, phosphaindolizines, and pyrrolyl-substituted phosphines,the imidazolium-neighbored phosphines are believed to possessthe strongest �-accepting ability [15].

Besides the interests in the �-accepting ligands, the hybridP,N-bidentate ligands containing hard (N-) and soft (P-) donorfunctions with multiple ligations to the Rh-center also represent

a very promising class of ligands for hydroformylation [40–43].Such hybrid ligands possess one weakly coordinating group (hardN-donor) that can reversibly release during the catalytic cycle pro-viding unsaturation at the metal center. Thus, the less strongly

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 68–76 69

OO

Me

Me

RhIOC

NNMe NPPh Ph

NNMe

PF6-

NMe2

TfO-

CO/H2

Cat.: 1A - 4A

OH+

linear (normal) branched (iso)H

O

OO

Me

Me

RhIOC

PPh Ph 2A

1A

NNMe NPPh Ph

NNMe

NMe2

PPh Ph

TfO-

PF6-

1

2

OO

Me

Me

RhIOC

NNMe Bu-nPPh Ph

PF6- 4A

NNMe Bu-nPPh Ph

PF6-

Rh(acac)(CO)2

4

NNMe

TfO-

OO

Me

Me

RhIOC

PPh Ph 3A

NNMe

PPh Ph

TfO-

3 NMe2

NMe2

1A–4

bot[

cwhhptotifc2htbispo41i

Scheme 1. Synthesis of the ionic Rh(I)-complexes of

ound moiety of this type of ligand, called hemilabile, is capablef temporarily holding a coordination site on the metal for pro-ection and then being released for the substrate insertion timely44–51].

With our continuing effort to develop the efficient ionic rhodiumomplexes ligated by the imidazolium-neighbored phosphineshich can be used as a new type of robust �-accepting ligands forydroformylation, as well as the great attention to the hemilabileybrid P,N-ligands with multiple ligations to metal center, in thisaper, the imidazolium-neighbored phosphines of 1–3 along withhe additional amino-substituents (as the N-donors) at one N-atomf imidazolium ring were synthesized. As for 2 and 3, the differen-iation of them is the ortho-position of N,N-dimethylamino groupn phenyl ring for 2 and the para-position of the same amino groupor 3, which allows the possible hemilabile P,N-ligation to the metalenter in 2 but prohibits such ligation in 3. In this way, only 1 and

combine the advantages of both the �-accepting ligands and theemilabile hybrid P,N-ligands, whereas 3 possesses the features ofhe �-accepting ligand and the amino-basicity but without possi-le hemilabile P,N-ligation to the metal center. The corresponding

onic Rh(I)-complexes (1A–3A) ligated by 1–3 respectively wereynthesized for the first time (Scheme 1). For comparison, the sim-le imidazolium-neighbored phosphine of 4 without involvement

f any amino group and the corresponding ionic Rh(I)-complex ofA [38] were also prepared in parallel. The catalytic performance ofA and 2A toward hydroformylation of 1-octene was investigated

n comparison to that of 3A, 4A and RhI(acac)(CO)(PPh3) [52], in

Nonanals

A as the catalysts for hydroformylation of 1-octene.

order to elucidate the effects of the �-accepting nature of the ionicphosphines and the hemilabile P,N-ligations to Rh-center on theinherent catalytic nature of 1A and 2A.

2. Experimental

2.1. Reagents and analysis

The chemical reagents were purchased from Shanghai AladdinChemical Reagent Co. Ltd. and Alfa Aesar China, and used asreceived. The IR spectra and in situ IR spectra were recorded ona Nicolet NEXUS 670 spectrometer. The 1H and 31P NMR spectrawere recorded on a Bruker Avance 500 spectrometer. The 31P NMRspectra were referenced to 85% H3PO4 sealed in a capillary tube asan internal standard. CHN elemental analysis was performed on aVario EL III Element Analyzer. Gas chromatography (GC) was per-formed on a SHIMADZU-2014 equipped with a DM-Wax capillarycolumn (30 m × 0.25 mm × 0.25 �m).

2.2. Synthesis

2.2.1. 1-(2′-Piperid-1′-yl-ethyl)-2-diphenylphosphino-3-methylimidazolium hexafluorophosphate (1)

1 was prepared according to our previously published method[37].

7 r Cata

2m

alNmcawvmtu

i−dwwqora(w

aiwooeyNHN(N

2m

msd7H3(f7

2h

Ta

2mh

i[b

In a typical experiment, the isolated crystalline catalyst

0 S.-J. Chen et al. / Journal of Molecula

.2.2. 1-(2′-(Dimethylamino)phenyl)-2-diphenylphosphino-3-ethylimidazolium trifluoromethanesulfonate (2)

Under N2 atmosphere, to the redistilled DMSO (100 mL) wasdded imidazole (4.1 g, 60 mmol), K2CO3 (16.6 g, 120 mmol),-proline (2.3 g, 20 mmol), CuI (1.9 g, 10 mmol) and 2-bromo-,N-dimethylaniline (12 g, 60 mmol) in sequence. The obtainedixtures were then stirred vigorously at 130 ◦C for 24 h. After

ooled to room temperature, the mixtures were diluted with waternd then extracted with ethyl acetate. The combined organic phaseas washed with brine, dried over Na2SO4, and concentrated in

acuo. The oily residue was purified through silica gel column chro-atography with an eluent of petroleum ether/ethyl acetate (3:1)

o give 1-(2′-(dimethylamino)phenyl)-imidazole as the brown liq-id (6.3 g; yield: 56%).

Under N2 atmosphere, 1-(2′-(dimethylamino)phenyl)-midazole (1.87 g, 10 mmol) in 50 mL dry CH2Cl2 cooled down to78 ◦C was added with n-BuLi (4.6 mL, 2.2 M in hexane, 10 mmol)ropwise. After stirring the mixture for 1 h, PPh2Cl (2.2 g, 10 mmol)as added dropwise. The resultant mixture was stirred overnightith the reaction temperature increasing to ambient. After

uenching excess n-BuLi with deionized water, the resultantily mixture was stripped of the solvent in vacuo. The obtainedesidue was purified by silica gel column chromatography withn eluent of petroleum ether/ethyl acetate (3:1) to yield 1-(2′-dimethylamino)phenyl)-2-diphenylphosphinoimidazole as thehite solid (2.2 g; yield: 60%).

Under N2 atmosphere, to a solution of 1-(2′-(dimethyl-mino)phenyl)-2-diphenylphosphinoimidazole (2.2 g, 6.0 mmol)n 50 mL dry CH2Cl2, after cooled down the solution to −78 ◦C,

as added methyl trifluoromethanesulfonate (1.0 g, 6.6 mmol). Thebtained mixture was stirred vigorously for 1 h. After removalf the solvent and dryness, the residue was recrystallized fromthyl acetate/diethyl ether to yield 2 as the white solid (3.1 g;ield: 96 wt%). 1H NMR ı, ppm, acetone-d6): 8.30 (d, 1H, J = 2.5 Hz,CHCHN+), 8.24 (d, 1H, J = 2.5 Hz, NCHCHN+), 7.68–7.60 (m, 7H,Ar), 7.47–7.45 (m, 3H, HAr), 7.34–7.31 (m, 4H, HAr), 3.61 (s, 3H,+CH3), 2.23 (s, 6H, N(CH3)2). 31P NMR (ı, ppm, acetone-d6): −24.0

s, PPh2). CHN-elemental analysis found for 2 (%): C 56.32, H 4.85, 7.39 (calcd: C 56.07, H 4.71, N 7.85).

.2.3. 1-(4′-(Dimethylamino)phenyl)-2-diphenylphosphino-3-ethylimidazolium trifluoromethanesulfonate (3)

1-(4′-(Dimethylamino)phenyl)-2-diphenylphosphino-3-ethylimidazolium triflate (3) was prepared according to the

ame procedures for the synthesis of 2. 1H NMR (ı, ppm, acetone-6): 8.07 (d, 1H, J = 2 Hz, NCHCHN+), 8.00 (d, 1H, J = 2 Hz, NCHCHN+),.56–7.51 (m, 6H, ortho, para-H-PPh2), 7.48–7.45 (m, 4H, meta--PPh2), 7.21 (d, 2H, J = 9 Hz, 2,6-H-aniline), 6.71 (d, 2H, J = 9 Hz,,5-H-aniline), 3.66 (s, 3H, N+(CH3)), 3.03 (s, 6H, N(CH3)2). 31P NMRı, ppm, acetone-d6): −22.8 (s, PPh2). CHN-elemental analysisound for 3 (%): C 55.81, H 4.25, N 8.18 (calcd: C 56.07, H 4.71, N.85).

.2.4. 1-Butyl-2-diphenylphosphino-3-methylimidazoliumexafluorophosphate (4)

4 was prepared according to previously published method [53].hen the corresponding ionic Rh(I)-complex of 4A was preparedccording to the procedures described in our previous work [38].

.2.5. (1-(2′-Piperid-1′-yl-ethyl)-2-diphenylphosphino-3-ethylimidazolium)carbonyl-(2, 4-pentanedionto) rhodium

exafluorophosphate (1A)

1A was prepared as the yellow solids according to the sim-

lar procedures as described in our previous work (yield: 78%)38]. A sample suitable for X-ray diffraction analysis was obtainedy recrystallization from CH2Cl2/diethyl ether. FT-IR (KBr, pellet,

lysis A: Chemical 396 (2015) 68–76

cm−1): 3435 (m), 2934 (m), 1991 (s), 1568 (s), 1525 (m), 1378(m), 1092 (m), 843 (s). 1H NMR (ı, ppm, CDCl3): 8.14–8.08 (m, 4H,ortho-H-PPh2), 7.63 (m, 7H, HAr), 7.45 (s, 1H, NCHCHN+), 5.54 (s,1H, acac), 3.79 (t, 2H, J = 8 Hz, N+CH2CH2N), 3.32 (s, 3H, N+CH3),2.30 (t, 2H, J = 8 Hz, N+CH2CH2N), 2.17 (s, 7H, acac + Hpiperidine), 1.57(s, 3H, acac), 1.47 (m, 4H, Hpiperidine), 1.38 (br, 2H, Hpiperidine). 31PNMR (ı, ppm, CDCl3): 52.0 (d, J = 448 Hz, PPh2), −144.1 (sept, PF6

−).CHN-elemental analysis found for 1A (%): C 46.04, H 4.85, N 5.26(calcd: C 46.04, H 5.20, N 5.55).

2.2.6. (1-(2′-(Dimethylamino)phenyl)-2-diphenylphosphino-3-methylimidazolium) carbonyl-(2,4-pentanedionto)rhodiumtrifluoromethanesulfonate (2A)

2A was prepared as the yellow solids according to the proce-dures as described in our previous work (yield: 80%) [38]. A samplesuitable for X-ray diffraction analysis was obtained by recrystal-lization from CH2Cl2/diethyl ether. FT-IR (KBr, pellet, cm−1): 3441(m), 2930 (w), 1988 (s), 1570 (m), 1522 (m), 1379 (m), 1270 (s),1155 (m), 1031 (m). 1H NMR (ı, ppm, CDCl3): 8.25 (s, 1H, HAr),7.90–7.85 (m, 2H. HAr), 7.78–7.73 (m, 2H, HAr), 7.47–7.44 (m, 4H,HAr), 7.39–7.29 (m, 3H, HAr), 7.15 (d, 2H, J = 8 Hz, HAr), 7.81 (t, 1H,J = 8 Hz, HAr), 6.60 (d, 1H, J = 8 Hz, HAr), 5.58 (s, 1H, acac), 3.63(s, 3H,N+CH3), 2.46 (s, 6H, N(CH3)2), 2.17 (s, 3H, acac), 1.71 (s, 3H, acac).31P NMR (ı, ppm, CDCl3): 54.5 (d, J = 456 Hz, PPh2). CHN-elementalanalysis found for 2A (%): C 48.70, H 4.28, N 5.29 (calcd: C 48.44, H4.59, N 5.47).

2.2.7. (1-(4′-(Dimethylamino)phenyl)-2-diphenylphosphino-3-methylimidazolium) carbonyl-(2,4-pentanedionto)rhodiumtrifluoromethanesulfonate (3A)

3A was prepared according to our previously published method[38]. The yellow precipitates were collected after washing withdiethyl ether and drying under vacuum, as the product of 3A withthe yield of 80%. The sample suitable for X-ray diffraction anal-ysis was obtained by recrystallization from CH2Cl2/diethyl ether.FT-IR (KBr, pellet, cm−1): 3445 (m), 2920 (w), 1988 (s), 1569 (m),1525 (m), 1377 (m), 121 (s), 1157 (m), 1032 (m). 1H NMR (ı, ppm,CDCl3): 8.03–7.98 (m, 4H, ortho-H-PPh2), 7.89 (s, 1H, NCHCHN+),7.40–7.36(m, 7H, HAr), 6.95 (d, 2H, J = 8 Hz, Haniline), 6.29 (d, 2H,J = 8 Hz, Haniline), 5.59(s, 1H, acac), 3.48 (s, 3H, N+CH3), 2.93 (s,6H, N(CH3)2), 2.21 (s, 3H, acac), 1.61 (s, 3H, acac). 31P NMR (ı,ppm, CDCl3): 55.6 (d, J = 452 Hz, PPh2). CHN-elemental analysisfound for 3A (%): C 48.50, H 4.68, N 5.36 (calcd: C 48.44, H 4.59,N 5.47).

2.3. X-ray crystallography

Intensity data were collected at 296 K (or 173 K) for 1A, 2A,and 3A on a Bruker SMARTAPEX II diffractometer using graphitemonochromated Mo-K� radiation (� = 0.71073 A). Data reductionincluded absorption corrections by the multi-scan method. Thestructures were solved by direct methods and refined by full matrixleast-squares using SHELXS-97 (Sheldrick, 1990), with all non-hydrogen atoms refined anisotropically. Hydrogen atoms wereadded at their geometrically ideal positions and refined isotrop-ically. The crystal data and refinement details are given in S.Table 1.

2.4. General procedures for hydroformylation of 1-octene

(0.01 mmol) was mixed with 1-octene (20 mmol). The obtainedmixture in a sealed Teflon-lined stainless steel autoclave waspurged with syngas (CO/H2 = 1:1, 4.0 MPa) and then stirred vig-orously at the appointed temperature. Upon completion, the

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 68–76 71

nd 3A

awms

2

st1fcrt

3

3

pp[cho3qo1do

Fig. 1. The single crystal structures of 1A, 2A, a

utoclave was cooled down to room temperature and the pressureas carefully released. The solution was analyzed by GC to deter-ine the conversions (n-dodecane as internal standard) and the

electivities (normalization method).

.5. In situ high-pressure FT-IR spectral analysis

All the FT-IR spectra were recorded on a Nicolet NEXUS 670pectrometer. The spectral resolution was about 4 cm−1. The reac-ions of 20 mg of 1A (3A, or RhI(acac)(CO)(PPh3)) with 0.1 mL-octene in 1.5 MPa syngas (CO/H2 = 1:1 volume ratio) were per-ormed by single mixing in the specially designed high-pressure IRell, in which the cylindrical CaF2 was used as the sealing sheets. Theeal time monitoring was performed under the different reactionemperatures.

. Results and discussion

.1. Synthesis and characterization

Highlighted by the advantages of the imidazolium-neighboredhosphines with typical �-acceptor character as observed in ourrevious work [36–39], and the hemilability of hybrid P,N-ligands40–51], we purposely prepared the ionic ligands of 1–3 whichombine the advantages of both the �-accepting ligands and theybrid P,N-ligands. In our preparation protocol, the amino groupf 2-ethylpiperidyl (for 1) or N,N-dimethylbenzenamino (for 2 and) was firstly connected to one N-atom of imidazolium throughuarterization and then PPh2 group is introduced at 2C-position

f imidazolium ring through deprotonation by n-BuLi at −78 ◦C.–3 as the �-accepting phosphines are robust against oxidativeegradation due to the confined lone-pair electrons to the nucleif the P-atom which shares the conjugated positive charge like a

(the hydrogen atoms are omitted for clarity).

phosphenium with the adjacent imidazolium ring [35], leading totheir insensitivity to moisture and oxygen under ambient condi-tions.

Subsequently, the complexation of RhI(acac)(CO)2 with 1–3 ledto the formation of the ionic complexes of 1A–3A respectively withgood to excellent yields (75–80%), For comparison the complex of4A which had been reported in our previous work [38] was pre-pared in parallel. 1A–3A are all air- and moisture-stable both inthe solid state and in the organic solvents for several weeks underambient conditions like 4A. The molecular structures of 1A–3A ana-lyzed by the single crystals for X-ray diffraction were depicted inFig. 1, each of which was composed of the Rh(I)-complex cationwith the structural similarity to RhI(acac)(CO)(PPh3) [52] and thecorresponding counteranion. The Rh(I)-centered vectors are all inthe typical square planar geometry. Each of Rh(I) (d8) center, lyingat the center of inversion, is coordinated by one phosphine, one CO,and one chelated 2,4-pentanedionato ligand. The bond distancesof Rh P (2.22–2.23 A) observed in 1A–4A (Table 1) are shorterthan that of RhI(acac)(CO)(PPh3) (Rh P, 2.24 A), indicating the con-solidated interaction between Rh and P linkages, and then theimproved structural stability of these ionic complexes. Besides the�-backdonation attributed to the �-acceptor character of 1–4, thevicinal electrostatic effect derived from the conjugation of the pos-itive charged imidazolium ring with the coordinating P(III) atommay be another reason to afford the stable complexes of 1A–4A [31].Evidently, the incorporation of the amino group in 1A–3A showsno effect on the structural configuration with similarity to 4A andRhI(acac)(CO)(PPh3).

A quantitative description of �-accepting ability of a ligand

(L) can be made by the determination of the FT-IR spectraldata for the Ni(CO)4−xLx complex of this ligand as reported byTolman [54] and the others [2,15,55]. It was believed that thered-shifted CO-vibrational frequencies were positively correlated

72 S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 68–76

Table 1The selected bond distances, vibrational frequency (v, cm−1) of CO in FT-IR spectra, and 31P NMR signals for the various Rh-complexes.

Rh(I)-complex Bond distance ( ´A) v (CO) (cm−1) 31P NMR (ppm)

Rh P Rh C C O (CO)

1A 2.2264(2) 1.812(6) 1.146(7) 1991 52.02A 2.2326(2) 1.814(8) 1.130(9) 1988 54.5

tovootc1

1oei

31

eRpItR4rniit

T(it

(tmtfI1blwtRWCbwtpl

cannot develop the possible P,N-ligation with the phosphinetogether to the metal center, leading to the similar catalyticperformance of 3A to the amino-free 4A (entries 3 vs. 4). Asfor RhI(acac)(CO)2 without ligation of phosphine, the lowest

3A 2.2180(3) 1.830(2)

4A [38] 2.2218(8) 1.812(3)

RhI(acac)(CO)(PPh3) [52] 2.2418(9) 1.807(2)

o the �-accepting ability of a ligand. Although the Ni-complexesf the ligands 1–4 are unavailable in this paper, their CO-ibrational frequencies may be determined from the IR spectraf the obtained Rh-complexes of 1A–4A in comparison to thatf RhI(acac)(CO)(PPh3). As shown in Table 1, 1A–4A correspondo the relatively higher values for the CO-vibrational frequen-ies (� 1988–1991 cm−1) as compared to RhI(acac)(CO)(PPh3) (�980 cm−1), indicating that 1–4 definitely are good �-acceptors.

In spite of the structural similarity, the 31P NMR signals ofA–4A (52–55 ppm) shift to the lower field in comparison to thatf RhI(acac)(CO)(PPh3) (48.6 ppm), further implying the intensivelectron-withdrawing effect of the neighboured positive-chargedmidazolium on the decrease of the electron density of P-atom.

.2. Catalytic performance of 1A–4A for hydroformylation of-octene

It has been widely known that the electronic and stericffects of the involved phosphines will dramatically influence theh-complex catalyzed hydroformylation of olefins, which is accom-anied by isomerization of terminal olefins to internal ones [56].

n this part, the inherent catalytic nature of 1A–3A were inves-igated respectively in comparison to that of 4A and the neutralhI(acac)(CO)(PPh3) without involvement of any auxiliary ligand.A is ligated by the ionic �-accepting phosphine without incorpo-ation of any amino group, and RhI(acac)(CO)(PPh3) is ligated by theeutral electron-rich PPh3. All these Rh-complexes possess the sim-

lar structural configurations and the same +1 valance state of Rhon, which can rule out the effects of the coordinating geometry andhe valance state of the metal center on the catalytic performance.

It was found in Table 2 that the catalytic activities in term ofON were in the ranking of 1A > 2A > 3A ≈ 4A > RhI(acac)(CO)(PPh3)entries 1–5), indicating that both the ionic nature of 1A–4A and thencorporative P,N-ligation to the Rh-center dramatically influencedhe hydroformylation of 1-octene.

It has been widely accepted that the metal–acyl complexScheme 2, d), which is derived from the dissociation of CO andhe subsequent insertion of it into the metal–alkyl (c) bond, is

ostly irreversibly converted into aldehydes. Therefore, all effectshat support the dissociation of CO from the metal center to theormation of (d) favor hydroformylation over isomerization [56].n comparison to RhI(acac)(CO)(PPh3), the Rh P bonds in ionicA–4A were consolidated due to the �-backdonation interactionetween Rh and P linkages. The intensive Rh P bonds 1A–4A did

ead to the weakened Rh CO bond, which have been demonstratedith the longer Rh C bond distances and the red-shifted CO vibra-

ions (1.812–1.830 A; � 1988–1991 cm−1) in comparison to that forhI(acac)(CO)(PPh3) (1.807 A; � 1980 cm−1) as shown in Table 1.eakening of the Rh CO bond indeed favors the dissociation of

O ligand and facilitate the subsequent CO insertion to the Rh–alkylond to form the corresponding Rh–acyl complexes (Scheme 2, d),

hich corresponds to the rapid formation of aldehydes. Meanwhile,

he Rh-complex intermediates (a, c, and d) can be protected tem-orarily by the chelation of the hemilabile hybrid P,N-bidentate

igand, in which the weakly coordinating N-site can reversibly

1.129(2) 1988 55.61.143(4) 1990 52.81.153(3) 1980 48.6

dissociate during the catalytic cycle providing unsaturation at themetal center for 1-octene coordination timely (Scheme 2, b). Con-sequently, the active Rh H species can be well protected by suchhybrid P,N-bidentate ligands against deactivation [42,57,58], cor-responding to the higher yields of nonanals over 1A and 2A thanthose over 4A and RhI(acac)(CO)(PPh3) only with the monophos-phine involved. On the other hand, as shown in Fig. 1, the distancebetween Rh and the amino-nitrogen atom in the structure of 1Ais 4.4661(43) A which is much shorter than that of 6.2516(94) Ain 2A, and the Rh-acac-six-member-ring is in unusual cis-positionto piperidyl ring in the solid state of 1A whereas the Rh-acac-six-member-ring in 2A is in trans-position to dimethylamino-phenylring. All these structural information demonstrates the weak chem-ical interaction between Rh and amino-nitrogen atom in 1A isindeed stronger than that in 2A, which corresponds to the morefavorable hemilabile P, N-ligation to Rh-center and the higher TONof 1880 over 1A than that over 2A (TON of 1680) (entries 1 vs. 2).

The para-positioned amino group in 3A [Rh. . .N, 7.2204(107) A]

Scheme 2. Mechanism of hydroformylation catalyzed by 1A (or 2A) containinghybrid P,N-ligand.

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 68–76 73

Table 2The homogeneous hydroformylation of 1-octene catalyzed by the different Rh(I)-complexes free of any auxiliary ligand and solvent.a

Entry Rh-complex Conv. (%)b Sel. (%)b L/Bc TONald

Nonanals (L + B) Iso-octenes

1 1A 97 97 3 1.6 18802 2A 91 92 8 0.8 16803 3A 91 84 16 1.0 15204 4A 92 85 15 0.8 15605 RhI(acac)(CO)(PPh3) 86 88 12 1.1 15106 RhI(acac)(CO)2 82 80 20 1.5 1300

/H2 (

cod(

Fc

a Rh concentration 0.05 mol% (Rh-complexes 0.01 mmol), 1-octene 20.0 mmol, COb Determined by GC.c L/B, the ratio of linear nonanals to branched nonanals.

onversion of 1-octene and the selectivity to nonanals werebserved, implying the indispensable role of the phosphine toominate the hydroformylation of 1-octene over isomerizationentry 6).

20 40 6020

30

40

50

60

70

80

90

100

Con

v. o

f 1-o

cten

e (%

)

Time (m

a

20 40 60 0

10

20

30

40

50

60

70

80

90

100

Sel

. to

nona

nals

(%

)

Time (

4A

RhI(acac)(CO)(PPh3)

b

ig. 2. The conversion of 1-octene (a) and the selectivity to nonanals and L/B ratio (b)

onditions: Rh concentration 0.05 mol% (Rh-complex 0.01 mmol), 1-octene 20.0 mmol, CO

1:1) 4.0 MPa, 100 ◦C, time 2 h.

In order to have a better insight into the different catalytic natureof 1A, 4A and RhI(acac)(CO)(PPh3) catalyzed hydroformylation of1-octene, the reaction processes were recorded over time respec-tively (Fig. 2). Although at the initial stage (in 20 min), the selectivity

80 100 120

1A4A

RhI(acac)(CO(PPh3)

in)

80 10 0 12 0

RhI(acac)(C O)(PPh3)

1A

4A

1A

min)

0

2

4

6

L /

B

as a function of the reaction time over 1A, 4A and RhI(acac)(CO)(PPh3) [reaction/H2 (1:1) 4.0 MPa, 100 ◦C].

7 r Cata

o1Rfol

F(d

4 S.-J. Chen et al. / Journal of Molecula

ver RhI(acac)(CO)(PPh3) was 53% along with 51% conversion of-octene which is higher than over 1A and 4A, indicating that

h(acac)(CO)(PPh3) was more active than 1A or 4A for the trans-

ormation of 1-octene to aldehydes. However, the better longevityf 1A or 4A which benefits from the function of hemilabile P,N-igand in stabilizing certain active intermediates and the better

ig. 3. In situ high-pressure FT-IR spectra recorded after mixing the Rh(I)-complexes oCO/H2 = 1). (a) The overlapped and successive IR profiles recorded as the temperature incifferent temperature.

lysis A: Chemical 396 (2015) 68–76

stability of imidazoline-neighbored phosphine than PPh3 resultedin the overall superior catalytic performance of 1A or 4A over

Rh(acac)(CO)(PPh3) (Fig. 2a). The conversions of 1-octene over1A, 4A and RhI(acac)(CO)(PPh3) all increased rapidly during thefirst 40 min and then sluggishly climbed at the prolonged reactiontime. It was found that after 40 min most of 1-octene had been

f 1A, 4A, and RhI(acac)(CO)(PPh3) respectively with 1-octene in 1.5 MPa syngasreasing from 30 ◦C to 110 ◦C; (b) the stacked profiles selected from (a) for clarity at

r Cata

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3i

tawuieRficTc1sipppcpsptbogRIp(2p2wimupsmos2rdcrB2f

mation to the active Rh H species is depressed without the aid

S.-J. Chen et al. / Journal of Molecula

onverted but only with moderate selectivity to nonanals, imply-ng that part of 1-octene was converted to iso-octenes throughapid isomerization in the initial course. Afterwards, the formedso-octenes continued to be hydroformylated to the branchedldehydes steadily with the best efficiency over 1A, accompaniedy the gradually decreased L/B ratio as shown in Fig. 2b. Evi-ently, 1A ligated by the �-accepting hybrid P,N-ligand can provideon-classical coordinating behaviors such as �-backdonation andemilabile-ligation to Rh-center. Hence, the catalytic performancef it for hydroformylation over isomerization is more favorable thanhat of 4A and RhI(acac)(CO)(PPh3).

.3. Detection and characterization of active Rh H species byn situ high-pressure IR spectroscopy

In situ IR spectroscopy has become increasingly important forhe research of hydroformylation of olefins because of the valu-ble information on the active Rh H species in a reaction solution,hich can provide the elucidation of the reaction mechanismnder actual reaction conditions [59–66]. The dramatic promot-

ng effect of hemilabile and �-accepting ligations existing in 1Avoked us to understand the formation and stability of the activeh H species or the other rhodium complex intermediates derived

rom 1A. The hydroformylation of 1-octene catalyzed by 1A wasn situ recorded by a Nicolet NEXUS 670 spectrometer, which wasombined with a specially designed in situ high-pressure IR cell.he mixture compositions including 1A, 1-octene, and syngas wereompletely the same as those for the real reaction in Table 2 (entry), except for the much high concentration of 1A required for IRpectral detection and relatively lower syngas pressure (1.5 MPanstead of 4.0 MPa in the real hydroformylation) due to the limitedressure endurance capability of the designed IR cell. For com-arison, the mixtures containing 4A and RhI(acac)(CO)(PPh3) inlace of 1A were recorded in parallel. The in situ spectra upononsecutively mixing the Rh(I)-complex and 1-octene which wereressured with 1.5 MPa syngas is presented in Fig. 3. As can beeen, as the temperature increases from 30 ◦C to 110 ◦C, the strongeaks at 1988–1979 cm−1, which were the characteristic vibra-ions of CO in fresh 1A, 4A, and RhI(acac)(CO)(PPh3), decayed andlue-shifted due to the derivation of the fresh catalysts to thether CO-ligated non-planar Rh-complex intermediates in the syn-as atmosphere (see the FT-IR spectra of the fresh 1A, 4A, andhI(acac)(CO)(PPh3) recorded in air atmosphere in Supplementary

nformation), accompanied by the immediate appearance of theeaks at 2082 and 2013 cm−1 (�, CO Rh) in each case. As for 1AFig. 3, 1A(b)), the decay of the new formed peaks at 2082 and013 cm−1 subsequently happened at ca. 110 ◦C, which was accom-anied by the presence of three new peaks at 2062, 2047, and023 cm−1. Thereinto, the absorption peak at 2047 cm−1, whichas identified as an active Rh H (rhodium hydride) species accord-

ng to the previously reported work [63–65], grew to reach theaximum intensity after holding 21 min at 110 ◦C, and then grad-

ally decayed to void after 40 min. It has been reported in Moser’sioneering work [66] that the electron-withdrawing effect of theubstituents (R) in the active RhH(CO)3(PR3) complexes could dra-atically influence the Rh H infrared frequencies from the range

f 2018 to 2050 cm−1, and that the strong electron-withdrawingubstituent such as CF3 even led to the Rh H peak appeared at050 cm−1 wave number. Consistent to Moser’s observation, theed-shifted Rh H absorption of 2047 cm−1 was found in our workue to the intensive electron-withdrawing effect of the positive-harged imidazolium in 1A in comparison to some previously

eported Rh H infrared absorptions at 2040–2043 cm−1 [63–65].esides Rh H absorption at 2047 cm−1, the new peaks at 2062 and023 cm−1 were attribute to the CO absorptions existing in theormed active RhH(CO)3(1) species. In contrast, as for 4A (Fig. 3,

lysis A: Chemical 396 (2015) 68–76 75

4A(b)), the formation of the active Rh H species with the character-istic absorption at 2050 cm−1 was quite sluggish. After monitoringthe reaction at 110 ◦C for 33 min, the weak absorption peak at2050 cm−1 began to appear, which implied the difficult activationof 4A to the active Rh H species under the same reaction con-ditions in comparison to that of 1A, which was correlated to theinefficient hydroformylation over 4A. The most intensive electron-withdrawing effect in ligand 4 (without possessing N-donor)reasonably corresponded to the most red-shifted Rh H vibration at2050 cm−1 [66].

As for RhI(acac)(CO)(PPh3) (Fig. 3, RhI(acac)(CO)(PPh3), b), onlythe very faint peak at 2045 cm−1 ascribed to the active Rh H specieswas observed after monitoring the reaction at 110 ◦C for 33 min,indicating the poor stability (life-time) of the formed Rh H speciesdifficult to be accumulated to reach the high concentration for thecommon FT-IR detection. However, in the bridging carbonyl regionbelow 1800 cm−1, a suddenly formed strong peak at 1729 cm−1 wasobserved after monitoring the reaction for 6 min at 110 ◦C, whichwas attributed to the formation of a stable carbonyl-bridged Rh-dimer [67,68], derived from the unstable RhH(CO)3(PPh3) species;whereas in the cases of 1A and 4A, the inactive carbonyl-bridgedRh-dimer with the infrared absorption below 1800 cm−1 was unob-servable completely.

As supported by the information in Fig. 3, the most obvious fea-tures were concluded as follows: (1) the formation and lifetime ofthe active Rh H species (2047 cm−1) derived from 1A are facili-tated by the presence of ligand 1 dually featured with �-acceptingability and P,N-hemilability, which can account for the improvedactivity and selectivity to aldehydes over 1A than the others; (2)the transformation of 4A to the active Rh H species is greatlydepressed due to the presence of ligand 4 only featured with the�-accepting ability but without P,N-hemilability, corresponding tothe worse hydroformylation; (3) the active Rh H species derivedfrom RhI(acac)(CO)(PPh3) is relatively unstable and susceptible tothe formation of the inactive carbonyl-bridged Rh-dimer, corre-sponding to the worst hydroformylation.

4. Conclusions

The ionic hybrid P,N-ligands of 1 and 2 dually featured with �-acceptor and hemilabile characters were synthesized on purposeto construct the corresponding ionic Rh(I)-complexes of 1A and 2A.For comparison, the ionic ligands of 3 and 4 only with �-acceptorcharacter but no possible hemilabile P,N-ligation were prepared inparallel to afford the corresponding Rh(I)-complexes of 3A and 4A.The analyses of the single crystal X-ray diffraction indicate that, incomparison to that of RhI(acac)(CO)(PPh3) with structural similar-ity to 1A–4A, the Rh P bonds in the ionic Rh(I)-complexes of 1A–4Aare more consolidated along with the weakened Rh CO bonds dueto the �-backdonation interaction in Rh → P linkages, which facili-tates the dissociation of CO ligand and the subsequent CO insertionfor the rapid formation of aldehydes. In addition, the hemilabilityof the involved hybrid P,N-ligands in 1A and 2A, which can protectthe active Rh-intermediates temporarily against deactivation dur-ing the catalytic cycle is believed to contribute to the more efficienthydroformylation of 1-octene over isomerization. The in situ high-pressure FT-IR spectral analysis supported the facts including: (1)the formation and lifetime of the active Rh H species (2047 cm−1)derived from 1A are facilitated by the presence of ligand 1 duallyfeatured with �-acceptor ability and hemilability; (2) the transfor-

of hybrid P,N-hemilabile ligation such as in the case of 4A; (3)the active Rh H species derived from RhI(acac)(CO)(PPh3) is rel-atively unstable and susceptible to the formation of the inactivecarbonyl-bridged Rh-dimer.

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cknowledgments

This work was financially supported by the National Natu-al Science Foundation of China (Nos. 21473058, 21273077, and1076083), and 973 Program from Ministry of Science and Tech-ology of China (2011CB201403).

ppendix A. Supplementary data

CCDC-1007335 (1A), CCDC-1007336 (2A), and CCDC-10073363A) contain the supplementary crystallographic data in this paper.hese data can be obtained free of charge from the Cambridge Crys-allographic Data Center via www.ccdc.cam.ac.uk/data request/cif.

Supplementary material related to this article can be found,n the online version, at http://dx.doi.org/10.1016/j.molcata.014.09.021.

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