A New Type of Phosphaferrocene–Pyrrole–Phosphaferrocene P-N-P

Note pubs.acs.org/Organometallics

A New Type of Phosphaferrocene−Pyrrole−Phosphaferrocene P-N-P Pincer Ligand Rongqiang Tian, Yongxiang Ng, Rakesh Ganguly, and François Mathey* Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 S Supporting Information *

ABSTRACT: A 2-ethoxycarbonylphosphaferrocene was reduced to the corresponding 2-hydroxymethyl derivative, which was condensed with pyrrole in a 2:1 ratio in the presence of BF3 to give a phosphaferrocene−pyrrole−phosphaferrocene pincer ligand. This tridentate ligand, in turn, reacted with [Rh(acac)(CO)2] to yield a rhodium carbonyl pincer complex. This complex was characterized by X-ray crystal structure analysis and tested in the hydroformylation of internal olefins.

■

INTRODUCTION Pincer ligands are ubiquitous in recent organometallic chemistry whenever good thermal stability or good rigidity of the coordination sphere is required for catalytic applications of transition metal complexes.1 A variety of complexing atoms have been used in such structures but, to the best of our knowledge, only a few examples involve electron-accepting sp2 phosphorus atoms incorporated in either phosphaalkenes, phosphinines, or phosphaferrocenes.2−6 We have recently described a simple approach to couple a pyrrole with a phosphaferrocene that gave us access to a new type of azacymantrene−phosphaferrocene chelates7 and to some phosphaferrocene analogues of calixpyrroles.8 We wish to report here the application of this technique to the preparation of a new type of P-N-P pincer ligand.

mixture readily reacts with [Rh(acac)(CO)2] to give the rhodium carbonyl complex 4 as a rac + meso 1:1 mixture (δ31P 14.7 and 14.6 ppm in CDCl3) (eq 2).

The meso complex was crystallized and characterized by Xray crystal structure analysis (Figure 2). The great flexibility of the pincer ligand prevents any significant distortion of the square-planar geometry of rhodium with a P−Rh−P bite angle of 173°. The pyrrole plane is tilted by 33.3° with respect to the coordination plane of rhodium in order to accommodate the structural requirements of the metal. The P−Rh bonds appear to be shorter at 2.25 Å than the typical phosphinine−Rh bonds in trans-[P2RhCl(CO)] at 2.27 Å.9 The two phenyl substituents efficiently protect the metal, which lies in the center of a cradle. This provides a good thermal stability for the complex. No detectable decomposition of 4 was observed upon heating at 90 °C under 40 bar of H2 + CO. The ν(CO) stretching frequency was observed at 1990.2 for rac 4 and 1988.6 cm−1 for meso 4 in dichloromethane. In terms of acceptor properties, this means that 3 lies between triphenylphosphine and aryl-substituted phosphinines.9 Since one patent describes without details the hydroformylation of olefins with rhodium−phosphaferrocene catalysts,10 we decided to perform some tests on 4 (as the isomeric mixture) with the poorly reactive internal olefins. The hydroformylation experiments were first carried out on the difficult case of (−)-α-pinene. They are reported in Table 1 after optimization of the reaction conditions.

■

RESULTS AND DISCUSSION We reinvestigated the condensation of pyrrole with phosphaferrocenylmethyl alcohol (2) as described in our preceding publication7 using a 2/pyrrole ratio of 2:1 (eq 1).

The pincer ligand 3 was obtained in 53% overall yield from 1 as a rac + meso 1:1 mixture (δ31P −53.7 and −53.8 ppm in CDCl3). We were able to crystallize the meso form and to get its X-ray crystal structure analysis (Figure 1). The rac + meso © 2012 American Chemical Society

Received: January 25, 2012 Published: March 9, 2012 2486

dx.doi.org/10.1021/om300061d | Organometallics 2012, 31, 2486−2488

Organometallics

Note

Figure 1. X-ray crystal structure of the meso pincer 3. One molecule of hexane in the crystal is not shown. Main bond lengths (Å) and angles (deg): C19−P1 1.784(4), P1−C14 1.771(4), C14−C13 1.515(5), C13−C12 1.496(5), C12−N1 1.366(4); C19−P1−C14 89.86(18), C12−N1−C12A 110.3(4).

Figure 2. X-ray crystal structure of the meso complex 4. Disordered hydrocarbon molecules in the crystal are not shown. Main bond lengths (Å) and angles (deg): Rh1−C1 1.848(8), Rh1−N1 2.120(5), Rh1−P1 2.251(2), Rh1−P2 2.248(2), C1−O1 1.137(9); C1−Rh1−N1 177.7(3), C1−Rh1−P2 92.6(2), N1−Rh1−P2 86.19(16), C1−Rh1−P1 94.2(2), N1−Rh1−P1 87.07(16), P1−Rh1−P2 173.17(7).

Table 1. Hydroformylation of α-Pinene:

pinene (mmol) 3.15 a

b

L(3)/Rh (mmol/mmol)

P (bar)

T (°C)

time (h)

total yielda (%)

productb 5:6:7

3

75

120

16

49

21.8:4.3:1

1

Isolated yield. Ratio according to H NMR.

phosphite.11 Several other internal olefins were tested with 3 using the same experimental conditions. In most of the cases, we got the least substituted aldehyde as the major product, and the yield was better than with the phosphinine. For example, 2,3-dimethylbutene gives 3,4-dimethylpentanal in 69% yield vs 29% with the triarylphosphinine.9

Whereas a 2,4,6-triarylphosphinine gives exclusively the internal aldehyde 6,9 our ligand 3 gives mostly the terminal aldehyde 5. This probably reflects the very high steric hindrance around rhodium and the medium donor properties of 3 that favor the migration of the double bond from the endo- to the exo-cyclic position. In terms of yields, 3 is, by far, better than the phosphinine, but still significantly inferior to a highly bulky 2487

dx.doi.org/10.1021/om300061d | Organometallics 2012, 31, 2486−2488

Organometallics

Note

C), 84.23 (s, Cp* C), 86.26 (t, J = 5.2 Hz, C), 89.89 (d, J = 4.2 Hz, C), 105.91 (s, Py CH), 125.56 (s, Ph CH para), 127.73 (s, Ph CH), 129.72 (d, 3JCP = 3.4 Hz, Ph CH), 135.83 (t, JCP = 5.0 Hz C), 137.36, 37.42, 137.50 (t, C). HRMS: m/z calcd for C51H60Fe2NOP2103Rh (M + 2H)+ 979.1904, found 979.1886. IR: ν(CO) = 1990.2 cm−1.

Since many applications of phosphaferrocene complexes in catalysis have been reported,12 it is clear that the easily accessible pincer 3 deserves further investigation.

■

■

EXPERIMENTAL SECTION

All reactions were performed under nitrogen using solvents purified and dried by standard methods. NMR spectra were obtained using JEOL ECA 400, Bruker AV400, or Bruker AV300 spectrometers. MS spectra were obtained in ESI mode on a Thermo Finnigan LCQ DECA XP MAX. X-ray crystallographic analyses were performed on a Bruker X8 APEX diffractometer. Phosphaferrocene 1 was prepared according to the literature.7 Pyrrole was distilled before use. Other reagents were commercially available and used without further purification. Synthesis of Phosphaferrocenes 3a,b. Lithium aluminum hydride (35 mg, 0.89 mmol) was added to a 5 mL THF solution of 1 (200 mg, 0.44 mmol) at −15 °C. The reaction mixture was slowly warmed to room temperature and stirred for 2 h. Excess lithium aluminum hydride was quenched with a small amount of ethyl acetate and two drops of deionized water. The solvents were evaporated, and the resulting precipitate was dissolved in dichloromethane (4 mL). Pyrrole (15 μL, 0.22 mmol) was added to the solution and stirred at room temperature for 5 min. BF3·OEt2 (0.1 mL, 0.77 mmol) was added, and the reaction mixture was stirred for 10 min. Et3N was added to quench the reaction, and the mixture was filtered via a silica gel pad quickly and washed with dichloromethane. Purification was performed via chromatography at −8 °C on silica using 2:1 hexane/ dichloromethane. An orange solid (100 mg, 0.118 mmol) was obtained, and the yield was 53%. One of the isomers suitable for NMR analysis was obtained by recrystallization. A crystal of 3 was grown from a solution of the compound in dichloromethane/ methanol. 31 P NMR (CDCl3): δ −53.8. 1H NMR (CDCl3): δ 1.66 (s, 30H, CH3 Cp*), 1.92 (s, 6H, CH3), 2.13 (s, 6H, CH3), 3.18−3.32 (m, 4H, CH2), 5.67 (d, 2H, Py CH), 7.10−7.14 (m, 2H, Ph), 7.19−7.23 (m, 4H, Ph), 7.37−7.39 (m, 4H, Ph), 7.65 (s, 1H, NH). 13C NMR (CDCl3): δ 10.36 (s, Cp* Me), 11.89 (s, Me), 14.41 (s, Me), 26.89 (d, 2 JCP = 21.7 Hz, CH2), 82.44 (s, Cp* C), 89.53 (d, 2JCP = 3.1 Hz,  CMe), 93.43 (d, 2JCP = 4.1 Hz, CMe), 95.81 (d, 1JCP = 54.5 Hz,  CP), 96.94 (d, 1JCP = 53.8 Hz, CP), 104.84 (s, Py CH), 125.19 (s, Ph CH para), 127.82 (s, Ph CH), 129.49 (d, 3JCP = 9.6 Hz, Ph CH), 130.42 (s, Py C), 140.58 (d, 2JCP = 17.6 Hz, Ph C ipso). HRMS: m/z calcd for C50H60Fe2NP2 (M + H)+ 848.2900, found 848.2892. Synthesis of Rhodium Complexes 4a,b. [Rh(acac)(CO)2] (37 mg) was added to a solution of 3a,b (110 mg) in toluene (5 mL). The reaction mixture was stirred at room temperature for 2 h, and the solvent was removed under vacuum. Purification was performed via chromatography at −8 °C on silica using 15:1 hexane/ethyl acetate. The first two, dark red bands collected separately gave 4a (first fraction) and 4b (second fraction), and the total yield was 67% (86 mg). A crystal of 4a was grown from a solution of the compound in dichloromethane/hexane. Complex 4a. 31P NMR (CDCl3): δ 14.63 (1JP−Rh = 173 Hz). 1H NMR (CDCl3): δ 1.63 (s, 30H, CH3 Cp*), 2.14 (s, 6H, CH3), 2.27 (s, 6H, CH3), 3.37−3.53 (m, 4H, CH2), 5.70 (s, 2H, Py CH), 7.10−7.14 (t, 2H, Ph), 7.18−7.22 (t, 4H, Ph), 7.46−7.48 (d, 4H, Ph). 13C NMR (CDCl3): δ 9.81 (s, Cp* Me), 11.08 (s, Me), 13.78 (s, Me), 28.10, 28.18, 28.27 (t, CH2), 80.82 (d, JCP = 4.0 Hz, C), 83.71 (d, JCP = 4.9 Hz, C), 84.30 (s, Cp* C), 86.26 (s, C), 90.08 (s, C), 106.08 (s, Py CH), 125.52 (s, Ph CH para), 127.69 (s, Ph CH), 129.70 (t, 3JCP = 3.6 Hz, Ph CH), 135.65 (t, JCP = 4.8 Hz C), 137.50 (t, JCP = 6.4 Hz C). HRMS: m/z calcd for C51H60Fe2NOP2103Rh (M + 2H)+ 979.1904, found 979.1886. IR: ν(CO) = 1988.6 cm−1. Complex 4b. 31P NMR (CDCl3): δ 14.72 (1JP−Rh = 173 Hz). 1H NMR (CDCl3): δ 1.70 (s, 30H, CH3 Cp*), 2.12 (s, 6H, CH3), 2.23 (s, 6H, CH3), 3.43−3.47 (t, 4H, CH2), 5.70 (s, 2H, Py CH), 7.10−7.14 (t, 2H, Ph), 7.19−7.23 (t, 4H, Ph), 7.42−7.47 (dd, 4H, Ph). 13C NMR (CDCl3): δ 9.89 (s, Cp* Me), 11.08 (s, Me), 13.74 (s, Me), 28.24, 28.31, 28.40 (t, CH2), 81.07 (t, JCP = 5.2 Hz, C), 83.68 (t, J = 5.0 Hz,

ASSOCIATED CONTENT

* Supporting Information S

Complete hydroformylation data; X-ray crystal structure analysis of compound 4. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank the Ministry of Education in Singapore for financial support of this work: grant MOE2009-T2-2-065. REFERENCES

(1) For recent reviews, see: Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761. Selander, N.; Szabó, K. J. Chem. Rev. 2011, 111, 2048. (2) Doux, M.; Bouet, C.; Mézailles, N.; Ricard, L.; Le Floch, P. Organometallics 2002, 21, 2785. Arliguie, T.; Blug, M.; Le Floch, P.; Mézailles, N.; Thuéry, P.; Ephritikhine, M. Organometallics 2008, 27, 4158 , and references therein. (3) Sava, X.; Mézailles, N.; Maigrot, N.; Nief, F.; Ricard, L.; Mathey, F.; Le Floch, P. Organometallics 1999, 18, 4205. (4) Wilms, H.; Frank, W.; Ganter, C. Chem.Eur. J. 2008, 14, 2719. (5) Jouati, A.; Geoffroy, M.; Terron, G.; Bernardinelli, G. Chem. Commun. 1992, 155. (6) Kawanami, H.; Toyota, K.; Yoshifuji, M. Chem. Lett. 1996, 25, 533. (7) Tian, R.; Escobar, A.; Mathey, F. Organometallics 2011, 30, 1738. (8) Tian, R.; Mathey, F. Organometallics 2011, 30, 3472. (9) Breit, B.; Winde, R.; Mackewitz, T.; Paciello, R.; Harms, K. Chem.Eur. J. 2001, 7, 3106. (10) Ahlers, W.; Mackewitz, T.; M. Roeper, M.; Mathey, F.; Ganter, C.; Breit, B. (BASF) Ger. Offen. DE 19 921730, 2000, Chem. Abstr. 133:349,964b. (11) da Silva, J. G.; Barros, H. J. V.; Balanta, A.; Bolaños, A.; Novoa, M. L.; Reyes, M.; Contreras, R.; Bayón, J. C.; Gusevskaya, E. V.; dos Santos, E. N. Appl. Catal., A: Gen. 2007, 326, 219. (12) Melaimi, M.; Mathey, F.; Le Floch, P. J. Organomet. Chem. 2001, 640, 197. Qiao, S.; Fu, G. C. J. Org. Chem. 1998, 63, 4168. Tanaka, K.; Qiao, S.; Tobisu, M.; Lo, M. M-C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 9870. Shintani, R.; Lo, M. M.-C.; Fu, G. C. Org. Lett. 2000, 2, 3695. Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2001, 20, 3913. Shintani, R.; Fu, G. C. Org. Lett. 2002, 4, 3699. Shintani, R.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 10778. Fu, G. C. Acc. Chem. Res. 2006, 39, 853. Ogasawara, M.; Ito, A.; Yoshida, K.; Hayashi, T. Organometallics 2006, 25, 2715. Carmichael, D.; Goldet, G.; Klankermayer, J.; Ricard, L.; Seeboth, N.; Stankevič, M. Chem.Eur. J. 2007, 13, 5492. Willms, H.; Frank, W.; Ganter, C. Organometallics 2009, 28, 3049.

2488

dx.doi.org/10.1021/om300061d | Organometallics 2012, 31, 2486−2488