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Organometallics 2010, 29, 3442–3449 DOI: 10.1021/om1006215
Assembly of Tunable Supramolecular Organometallic Catalysts with Cyclodextrins Loı¨ c Leclercq† and Andreea R. Schmitzer* Department of Chemistry, Universit e de Montr eal, C.P. 6128 Succursale Centre-ville, Montr eal, Qu ebec, H3C 3J7, Canada. †Present address: UFR of Chemistry, Universit e Lille 1, B^ at. C6, F-59655 Villeneuve d’Ascq Cedex, France. Received June 28, 2010
Methylated-β-CDs were used to activate a catalyst by ligand solution trapping, to form secondsphere coordination by ligand complexation, and to increase the regioselectivity of biphasic and homogeneous hydroformylation reactions. Different methylation degrees of the β-CD allow assembly of hydrophobic or hydrophilic catalysts that can be employed in homogeneous or biphasic hydroformylation reactions.
Introduction Over the last three decades, several supramolecular approaches have been developed in the field of organometallic catalysis, where host-guest interactions have proved to be a powerful tool to elaborate new strategies or to improve the performances of various catalytic entities.1 The design of supramolecular ligands is an elegant strategy to stabilize organometallic complexes under catalytic conditions and represents a potential route to produce a library of ligands.2 A straightforward way to assemble a supramolecular catalytic entity is to generate second-sphere coordination by the use of macrocycles such as cyclodextrins (CDs) (Scheme 1).3 The resulting supramolecular assembly can be used to modify the selectivity of the initial catalyst. However, it is difficult to obtain stable, flexible, and active catalytic systems by these means. The triphenylphosphine trisulfonated sodium salt is known to readily form inclusion complexes with randomly dimethylated-β-CD.4 The application of this complex in hydroformylation reactions was previously reported, but it resulted in a decrease of the regioselectivity of the reaction.4 The formation of the inclusion complex favors the formation of low-coordinated rhodium phosphine species.5 In general it is difficult to avoid the modification of *To whom correspondence should be addressed. E-mail: ar.
[email protected]. (1) Monflier, E.; Hapiot, F.; O’Hare, D. In Comprehensive Organometallic Chemistry III, Vol. 12; Crabtree, R. H.; Mingos, D. M. P., Eds.; Elsevier: Oxford, 2006; pp 781-834. (2) (a) Wilkinson, M. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Org. Biomol. Chem. 2005, 3, 2371–2383. (b) Sandee, A. J.; Reek, J. N. H. Dalton Trans. 2006, 3385–3391. (3) (a) Reek, J. N. H. In Supramolecular Catalysis; Van Leeuwen, P. W. N. M., Ed.; Wiley, VCH: Weinheim, 2008; pp 199-234. (b) Hapiot, F.; Tilloy, S.; Monflier, E. Chem. Rev. 2006, 106, 767–781. (4) (a) Monflier, E.; Fremy, G.; Castanet, Y.; Mortreux, A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2269–2271. ; Angew. Chem. 1995, 107, 2450-2452. (b) Dessoudeix, M.; Urrutigoı¨ ty, M.; Kalck, P. Eur. J. Inorg. Chem. 2001, 7, 1797–1800. (c) Leclercq, L.; Sauthier, M.; Castanet, Y.; Mortreux, A.; Bricout, H.; Monflier, E. Adv. Synth. Catal. 2005, 347, 55– 59. (5) Monflier, E.; Bricout, H.; Hapiot, F.; Tilloy, S.; Aghmiz, A.; Masdeu-Bult o, A. M. Adv. Synth. Catal. 2004, 346, 425–431. pubs.acs.org/Organometallics
Published on Web 07/12/2010
these equilibria simply by assembling host-guest complexes. Therefore, Monflier et al. have reported the self-assembly of a bidentate ligand formed by the complexation of an aminoCD and a phosphane.6 The application of this catalyst was limited to aqueous homogeneous or biphasic catalysis in organic solvents. Considering the lack of flexibility of current supramolecular ligands, we applied our experience with CDs and their use in aqueous and organic solutions7 to design water- or organic-soluble supramolecular catalytic systems. Our approach relies on the self-assembly of dior trimethylated-β-CD (β-DIME or β-TRIME) and di(1adamantyl)benzylphosphine (DABP) (Scheme 2). The choice of β-DIME or β-TRIME was dictated by their hydrophilic and hydrophobic properties, respectively.8 The self-assembly of supramolecular complexes between the DABP metalligand and these CDs modifies the hydrophobic or hydrophilic properties of the resulting supramolecular organometallic catalyst. We present here the characterization and the application of these complexes in homogeneous and biphasic catalytic hydroformylation processes.
Results and Discussion Characterization of DABP/Methylated-β-CD Complexes. i. Inclusion Complexes in Dichloromethane. First, DABP complexation by methylated-β-CDs was investigated by (6) (a) Machut, C.; Patrigeon, J.; Tilloy, S.; Bricout, H.; Hapiot, F.; Monflier, E. Angew. Chem, Int. Ed. 2007, 46, 3040–3042. ; Angew. Chem. 2007, 119, 3100-3102. (b) Legrand, F. X.; Hapiot, F.; Tilloy, S.; Guerriero, A.; Peruzzini, M.; Gonsalvi, L.; Monflier, E. Appl. Catal. A: Gen. 2009, 362, 62–66. (c) Ferreria, M.; Bricout, H.; Sayede, A.; Hapiot, F.; Tilloy, S.; Monflier, E. ChemSusChem 2008, 1, 631–636. (7) (a) Leclercq, L.; Lacour, M.; Sanon, S. H.; Schmitzer, A. R. Chem.;Eur. J. 2009, 15, 6327–6331. (b) Leclercq, L.; Schmitzer, A. R. J. Phys. Org. Chem. 2009, 22, 91–95. (c) Leclercq, L.; Schmitzer, A. R. J. Phys. Chem. B 2008, 112, 11064–11070. (d) Leclercq, L.; Noujeim, N.; Sanon, S. H.; Schmitzer, A. R. J. Phys. Chem. B 2008, 112, 14176–14184. (e) Noujeim, N.; Leclercq, L.; Schmitzer, A. R. J. Org. Chem. 2008, 73, 3784–3790. (8) (a) Szejtli, J. Cyclodextrin Technology; Kluwer: Dordrecht, 1988; p 51. (b) Immel, S.; Lichtenthaler, F. W. St€ arke 1996, 48, 225–232. (c) Leclercq, L.; Bricout, H.; Tilloy, S.; Monflier, E. J. Colloid Interface Sci. 2007, 307, 481–487. r 2010 American Chemical Society
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Scheme 1. Concept of Supramolecular Catalyst Assembly by Ligand (L)/Cyclodextrin (CD) Complexation
Figure 1. H(e) and H(g) of DABP NMR titration profile obtained at 25 °C for addition of (a) β-TRIME and (b) β-DIME in CD2Cl2 solution of DABP (5 mM).
Scheme 2. Methylated Cyclodextrins and Phosphorous Ligand Used in This Work and Their Schematic Representation
NMR spectroscopy in deuterated dichloromethane. 1H NMR titrations of the adamantyl protons showed that 2:1 complexes were formed (Figure 1). It is important to note that no significant shifts were observed for the benzyl protons. The binding constants were estimated at 5900 ( 40 M-2 for the β-DIME and 10 100 ( 110 M-2 for the β-TRIME. These values are weak compared to adamantyl inclusion in aqueous solution.9 To gain more insight into the geometry of the host-guest complexes, we performed a 2D NMR study (Figure 2). ROESY experiments are well suited for this purpose; although the COSY peaks were intense at the concentrations used, it was possible to assign the NOE signals between the DABP and the CDs. The distance must be less than 5 A˚ to observe cross-peaks between protons. It is noteworthy that no cross-peaks were detected between the phosphane aromatic residue and the CDs, confirming the absence of benzyl inclusion in all cases (see Supporting Information (SI), Figure S1). For the adamantyl residues, the principal cross-peaks were observed between the H(e), H(f), and H(g) protons and the C(2) and/or the C(3) methoxy of the CD (and C(2), C(3), and C(6) in the case of β-TRIME). This indicates that the inclusion complexes in dichloromethane are formed by the inclusion of the adamantyl residues of DABP into the CDs hydrophobic cavity with insertion at the wide rim. However, the adamantyl residues are not deeply included into the CD cavity, probably due to the steric hindrance of DABP. To confirm this assumption, we performed 31P NMR experiments, and no significant changes (9) (a) Connors, K. A. Chem. Rev. 1997, 97, 1325–1358. (b) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875–1918.
Figure 2. Partial ROESY NMR at 25 °C of (a) β-DIME and DABP and (b) β-TRIME and DABP in CD2Cl2 (5 mM CD and 10 mM DABP).
were observed in the presence of the CD (see SI, Figure S2). Thus the CD is positioned sufficiently far from the phosphorus atom. In order to assess the energy content for the various inclusion complexes formed in dichloromethane and to support the geometries deduced from the ROESY NMR experiments, semiempirical quantum calculations were undertaken.10 The COSMO (conductor-like screening model) method11 was used to approximate the effect of the dichloromethane solvent (10) Stewart, J. J. P. Stewart Computational Chemistry, Version 7.213W, http://openmopac.net/. €mann, G. J. Chem. Soc., Perkin Trans. 2 1993, (11) Klamt, A.; Sch€ uu 799–805.
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Figure 3. Geometry and enthalpy variation obtained by semiempirical calculations of the inclusion complexes of DABP and β-DIME (top) and β-TRIME (bottom).
Figure 4. 1H NMR spectra in D2O at 25 °C of (a) β-DIME (2 mM) and (b) residual aqueous phase of a DABP (1 mM) and β-DIME (2 mM) in 1:1 CD2Cl2/D2O mixture.
surrounding the ligands and the complexes. Enthalpy changes (ΔH) determine whether the complexation can take place spontaneously.12 The formation of all inclusion complexes was exothermic (Figure 3). It is noteworthy that the predicted geometries for the various complexes are in good agreement with the geometries deduced from the ROESY NMR experiment. ii. Inclusion Complexes in Water. To confirm the formation of the inclusion complex between DABP and β-DIME in aqueous solution, we studied the residual aqueous phase of a DABP (1 mM) and β-DIME (2 mM) mixture in CD2Cl2/ D2O (1:1). In the absence of β-DIME, no NMR signals were obtained in the aqueous phase, confirming that DABP is completely insoluble in D2O. The formation of β-DIME/ DABP inclusion complexes was observed by 1H NMR, where characteristic chemical shifts of the DABP complex were obtained (Figure 4). All chemical shifts of the β-DIME in the residual aqueous phase showed significant differences compared to those of the β-DIME alone. In particular, both internal H(3) and H(5) protons of the β-DIME underwent upfield shifts due to the inclusion of the adamantyl residues in the cavity. These results indicate the formation of inclusion complexes between DABP and β-DIME.13 Since (12) Smith, M. B.; March, J. Mechanisms and Methods of Determining Them. In March’s Advanced Organic Chemistry, 6th ed.; John Wiley & Sons: New York, 2007. (13) Schneider, H. J.; Hacket, F.; R€ udiger, V.; Ikeda, H. Chem. Rev. 1998, 98, 1755–1786.
Figure 5. Partial ROESY NMR at 25 °C of the residual aqueous phase of a DABP (1 mM) and β-DIME (2 mM) in 1:1 CD2Cl2/D2O mixture.
complexation was fast on the NMR time scale, the observed chemical shifts were the average of the chemical shifts for all the complexes present at equilibrium. To gain more insights into the molecular interactions and the geometry of the host-guest complex formed in water, we performed a 2D ROESY NMR study. The ROESY spectrum of the residual aqueous phase in D2O gave intense cross-peaks between the protons of adamantyl groups and the internal H(2)-H(6) protons and the C(2) methoxy protons of the β-DIME (Figure 5). These cross-peaks provide evidence of the formation of an inclusion complex between the β-DIME and DABP in aqueous solution. Moreover, the absence of cross-peaks between the hydrogens of the β-DIME and those of the benzyl residue (not shown) showed that the inclusion of the DABP into the β-DIME occurs only by the adamantyl groups. It is noteworthy that similar conclusions can be made with β-TRIME. To estimate the association constants in aqueous solution, we used the phase-solubility method.14 Phase-solubility analysis involves an examination of the effect of the CD on the water solubility of DABP. Experimentally, the DABP was added to several vials in a way to always be in excess. The presence of solid DABP in these systems is necessary to keep (14) Higuchi, T.; Connors, K. A. Adv. Anal. Chem. Instrum. 1965, 4, 117–212.
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Table 1. Effect of Methylated-β-CDs on DABP Partition Coefficient (Porg/aqu) Estimated from UV Absorbance at 265 nma entry
CD
CD/DABP
Porg/aqub
1 2 3 4
β-DIME β-DIME β-TRIME β-TRIME
1 2 1 2
0.8 0.2 1.5 25.7
a Cary Bio 100 UV-visible spectrophotometer, [DABP] = 1 mM, [CD] = 1 or 2 mM, 25 or 50 °C. b Calculated with eq 5.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ 4K½CDT - 8K½CDT þ 1 ½CD2 G ¼ 8K Figure 6. Phase-solubility diagram of DABP/CD for ()) β-DIME and (() β-TRIME recorded at 265 nm and 25 °C. The black line represents the calculated relative absorbance obtained from eqs 3 and 4 (β-DIME: K = 7300 ( 100 M-1, ε = 21 790 ( 145 L mol-1 cm-1, R2 = 0.9999; β-TRIME: K = 4700 ( 50 M-1, ε = 43 980 ( 820 L mol-1 cm-1, R2 = 0.9998).
the thermodynamic activity at a constant level. To the DABP (G) was added a constant volume of water containing progressively larger concentrations of the CD. The vials were mixed at 25 °C until equilibrium was established (1 week). The solid DABP was then removed, and the aqueous solution was studied by UV-visible spectroscopy. A phasesolubility diagram was constructed by plotting the relative absorbance (obtained by multiplying the absorbance of diluted solutions by the dilution factor) and the total concentration of added CD, [CD]T (Figure 6). Mass spectrometry was used to determine the stoichiometry of the DABP/CD complex. Electrospray ionization (ESI) is used to “fish” loosely bonded supramolecular complexes in solution and to transfer them to a mass spectrometer to investigate their assemblies. The positive ions are formed in solution and then transferred by ESI directly to the gas phase. ESI is characterized by the gentleness by which the gaseous ions are formed, and loosely bonded supramolecules can be observed, such as hydrogen-bonded amino-acid assemblies.15 The ESI mass spectra, in the positive ion mode, of DABP/CD solutions prepared by the phase-solubility technique at 3 mM, give m/z peaks at 3083.5 corresponding to (DABP/2 β-DIME)þ and respectively at 3280.7 corresponding to (DABP/2 β-TRIME)þ. On the basis of this results and the complete water insolubility of DABP in the absence of the CD, the complexation equilibrium with CD is dictated by a 2:1 equilibrium:
2CD þ GuCD2 G
K ¼
½CD2 G ½CD2
ð1Þ
With respect to this model, the mass balance for CD provides eq 2.
½CDT ¼ ½CD þ 2½CD2 G
ð2Þ
The free CD concentration, [CD], can be calculated from eq 1 and substituted in the association constants to obtain eq 3. (15) (a) Koch, K. J.; Gozzo, F. C.; Nanita, S. C.; Takats, Z.; Eberlin, M. N.; Cooks, R. G. Angew. Chem., Int. Ed. 2002, 41, 1721–1724. (b) Takats, Z.; Nanita, S. C.; Cooks, R. G. Angew. Chem., Int. Ed. 2003, 42, 3521–3523. (c) Cooks, R. G.; Zhang, D. X.; Koch, K. J.; Gozzo, F. C.; Eberlin, M. N. Anal. Chem. 2001, 73, 3646–3655.
ð3Þ
[CD2G] can be calculated from eq 3 assuming a value of K. For each [CD2G] value, the molar absorption coefficient, ε, was calculated with eq 4.
A ¼ εl½CD2 G
ð4Þ
where l is the cell length and A is the relative absorbance. This algorithm provides estimation of the association constant, K, as well as minimizes the dispersion of ε. The estimated values were K = 7300 ( 100 M-1, ε = 21 790 ( 145 L mol-1 cm-1 for β-DIME and K = 4700 ( 50 M-1, ε = 43 980 ( 820 L mol-1 cm-1 for β-TRIME. It is noteworthy that an increase of the molar absorption coefficient ε was observed comparing the values obtained for the β-DIME and β-TRIME. Similar changes were observed when guest molecules were dissolved in less polar solvents, suggesting that the guest is incorporated in a more hydrophobic environment.16 Therefore we can conclude that β-TRIME provides a more hydrophobic environment to DABP than β-DIME. The energy content for the inclusion complexes in water was also studied by semiempirical quantum calculations, similar to those undertaken in dichloromethane. The formation of 1:1 and 1:2 inclusion complexes was exothermic, -22 kcal/mol and -28 kcal/mol for the β-DIME and -27 kcal/mol and -32 kcal/mol in the case of the β-TRIME. As in dichloromethane, the 1:2 complexes are more stable and favored in solution. iii. Partition of the Complex between Dichloromethane and Water. Complementary experiments were performed by UV spectroscopy measurements. To a CH2Cl2 solution of DABP ligand was added each of the two methylated-β-CDs (1 or 2 equiv). After water addition and vigorous stirring at 25 °C (24 h), the UV spectra of the dichloromethane and the aqueous phase were recorded. In the absence of CDs, no absorbance was observed in the aqueous phase, suggesting that the DABP was absent in water. The CD/DABP complex partition coefficients (Porg/aqu), between the organic and the aqueous phase, were calculated from eq 5.
Porg=aqu ¼
AbsDABPorg 100 AbsDABPaqu
ð5Þ
As shown in Table 1, the supramolecular complex was prevalent in the aqueous phase for the β-DIME and in the organic phase for the β-TRIME. However a clear influence of the CD/DABP ratio is observed: (i) the 2:1 complex (16) (a) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: Berlin, 1978. (b) Miyake, K; Irie, T; Arima, H.; Hirayama, F.; Uekama, H.; Hirano, M.; Okamoto, Y. Int. J. Pharm. 1999, 179, 237–245.
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Scheme 3. Rhodium Hydroformylation Reaction: (a) Homogeneous; (b) Biphasic Reaction of Hydrophilic Substrate (1); (c) Biphasic Reaction of Hydrophobic Substrate (2)
Figure 7. Tolman angle for optimized structures (PM3/COSMO, MOPAC2009) of (a) TPP (140 A˚), (b) DABP (180 A˚), and (c) β-TRIME/DABP (230 A˚).
Table 2. Homogeneous Hydroformylation of Allyl Alcohol (1) in Water and 1-Octene (2) in Dichloromethanea entry 1 2 3 4 5 6 7 8 9 10
CD
CD/Rh substrate
β-DIME β-DIME β-TRIME β-TRIME
1 2 1 2
β-DIME β-DIME β-TRIME β-TRIME
1 2 1 2
1 1 1 1 1 2 2 2 2 2
solv H2O H2O H2O H2O H2O CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2
convb selectivityc l/bd
