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New investigations of technical rhodium and iridium catalysts in homogeneous phase employing para-hydrogen induced polarization
Solid State Nuclear Magnetic Resonance 40 (2011) 88–90
Contents lists available at ScienceDirect
Solid State Nuclear Magnetic Resonance journal homepage: www.elsevier.com/locate/ssnmr
New investigations of technical rhodium and iridium catalysts in homogeneous phase employing para-hydrogen induced polarization b ¨ Torsten Gutmann a, Tomasz Ratajczyk b, Sonja Dillenberger b, Yeping Xu b, Anna Grunberg , b c c d Hergen Breitzke , Ute Bommerich , Thomas Trantzschel , Johannes Bernarding , Gerd Buntkowsky b,n a
Laboratoire de Chimie de Coordination (LCC) CNRS, 205 Route de Narbonne, F-31400 Toulouse Cedex 4, France Institute of Physical Chemistry, Technical University Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany c Leibniz Institute for Neurobiology, Brenneckestraße 6, D-39118 Magdeburg, Germany d Institute for Biometry and Medical Computer Science, Otto-von-Guericke University Magdeburg, Leipziger Straße 44, D-39120 Magdeburg, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2011 Received in revised form 30 July 2011 Available online 7 August 2011
It is shown that the para-hydrogen induced polarization (PHIP) phenomenon in homogenous solution containing the substrate styrene is also observable employing simple inorganic systems of the form MCl3 xH2O (M¼ Rh, Ir) as catalyst. Such observation confirms that already very simple metal complexes enable the creation of PHIP signal enhancement in solution. This opens up new pathways to increase the sensitivity of NMR and MRT by PHIP enhancement using cost-effective catalysts and will be essential for further mechanistic studies of simple transition metal systems. & 2011 Elsevier Inc. All rights reserved.
Keywords: Para-hydrogen induced polarization Homogeneous catalysis Nuclear magnetic resonance Sensitivity enhancement
1. Introduction The para-hydrogen induced polarization effect (PHIP) [1] was first described in the late 80s of the last century under the acronym PASADENA [2,3] and further investigated in experiments known as ALTADENA [4]. In these experiments Wilkinson’s catalyst RhCl(PPh3)3 [5] as well as Vaska’s complex Ir(CO)Cl(PPh3)2 [6] were employed as well-known homogenous catalysts for hydrogenation reactions. In the following years a variety of catalysts mainly containing Rh and Ir as well as Ru, Pd, Pt and Co metal centers with different types of ligands were tested in catalytic hydrogenation reactions in homogeneous phase [7–10] for PHIP activity. Recently, it was shown that PHIP is also observable in systems containing heterogenized catalysts such as immobilized catalysts or nano-particles [11–14]. Such systems have the strong advantage of a fast separation of hyperpolarized products and catalysts, which initially have high potential for their applicability in medical or technical applications of PHIP. However, in case of immobilized homogeneous catalysts the ‘‘catalyst leaching’’ in specific solvents have to be taken into account as it was shown in our previous work [15]. In this context the analyses of the PHIP active solutions containing the ‘‘leached species’’ also gave a first surprising hint towards the chemical
n
Corresponding author. E-mail address: [email protected] (G. Buntkowsky).
0926-2040/$ - see front matter & 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.ssnmr.2011.08.002
nature of the active species in solution. The only 31P NMR signal in the spectra of these solutions was a pure singlet (assigned to OPPh3) without any JRh,P. Thus it is partially evident that the ‘‘leached species’’ is not the neat Wilkinson’s catalyst, since the latter would exhibit JRh,P couplings in the range of 146 Hz and 192 Hz, respectively, in solution [16]. Moreover, it follows that the catalytically active species does not consist of phosphine groups, which coordinate rhodium or that their amount is below the detection minimum of the NMR. This leads us to the assumption that the phosphorous ligand could be unnecessary for the generation of the PHIP signal enhancement in these systems and that perhaps a solvated form of the rhodium might be the catalytically active species. From this point the question arises whether simple Rh species formed by dissolution of neat RhCl3 xH2O can act as hydrogenation catalyst and generate PHIP signal enhancement in solution. This would confirm our previous assumption and further opens up new pathways to produce hyperpolarized products employing very costeffective catalysts. Moreover this study could be interesting as initial point for mechanistic studies of the PHIP effect in solution. Further, simple transition metal salts can be of interest in the context of signal amplification by reversible exchange (SABRE) [17]. Only recently, a powerful application of this method for the signal enhancement of proteinogenic amino acids has been demonstrated [18]. Thus, since RhCl3 and especially IrCl3 are water solvable, they may by employed for signal enhancement of different biomolecules in water solution, their natural environment.
T. Gutmann et al. / Solid State Nuclear Magnetic Resonance 40 (2011) 88–90
Up to now, only few examples are known where RhCl3 is employed in hydrogenation reactions, which refers to the fact that this catalyst is not selective and has only low activity for hydrogenation in organic phase because of its low solubility. Therefore its application was performed as solvated ion pair in a water/ organic biphasic system as phase transfer catalyst e.g. for the hydrogenation of carbon–carbon double and triple bonds [19,20]. Recently, it has been shown that RhCl3 supported at MCM-41 material can be employed in the hydroformylation reaction of styrene [21,22]. From these primary insights in a first step PHIP experiments are performed employing the simple transition metal salt RhCl3 xH2O as catalyst. As model reaction the hydrogenation of styrene is chosen in different solvents like methanol-d4 and acetone-d6. To prove that the results for RhCl3 xH2O are also transferable to other transition metal salts, in a second step IrCl3 xH2O as typical example of an iridium metal salt is tested for PHIP experiments. For both catalyst systems the PHIP effect is semi-qualitatively investigated as a function of temperature, solvent and substituent at the styrene. Realizing this, experiments at room-temperature (rt) and at the boiling point (bp) of the appropriate solution are carried out in methanol-d4 and acetone-d6, respectively. As substrates styrene and its para-substituted derivatives 4-methylstyrene and 4-chlorostyrene are chosen. For comparison of the signal enhancement for the different systems and conditions a semiquantitative enhancement factor (EF) is defined. Such EF could be used as a measure for the sensitivity enhancement by PHIP in MRI applications.
Samples employing styrene and styrene derivatives (4-methylstyrene or 4-chlorostyrene) as substrates were prepared employing methanol-d4 or acetone-d6 as solvents. To realize the semi-quantitative comparison of the signal enhancement for these different substrate/catalyst systems an enhancement factor (EF) was defined as ratio between the signal intensity (area) of the –CH2 group of the product ethylbenzene and the signal intensity (area) of the –CH group of the substrate styrene. This simple definition allows us to compare the signal intensity of the hyperpolarized product with that of the thermally polarized substrate (Scheme 1).
2. Results and discussion 2.1. RhCl3 xH2O/styrene For first test experiments RhCl3 xH2O was selected as catalyst employing styrene as substrate for the hydrogenation reaction. As shown in Fig. 1 such system displays efficient PHIP signal enhancement (EF¼2.20 resp. 1.31) when employing specific solvents such as methanol-d4 or acetone-d6. From these primary observations it is evident that also simple rhodium species generated by RhCl3 xH2O are able to produce PHIP signal enhancement. Table 1 Semi-quantitative results of PHIP signal enhancement reached in experiments at different temperatures e.g. room temperature (rt) and boiling temperature of the solvent (bt) employing different substrates, solvents and metal catalysts.
1.1. PHIP experiments Para-hydrogen was enriched employing a laboratory-built apparatus described in a previous paper [23]. The hydrogenation reactions were carried out under ALTADENA [4] conditions. The catalysts RhCl3 xH2O (Strem, 45–1880) and IrCl3 xH2O (Strem, 77–3000) were employed without further purification. Samples were prepared in 5 mm screw-cap NMR tubes (Wilmad). In a typical procedure approximately 7–9 mg of the catalyst were dissolved in 600 ml of the appropriate solvent and 50 ml of the substrate were added forming a homogeneous solution in all cases.
Scheme 1. Hydrogenation reaction of styrene derivatives.
89
RhCl3 xH2O
IrCl3 xH2O
PHIP (rt)
PHIP (bt)
PHIP (rt)
PHIP (bt)
Methanol-d4 Acetone-d6
---
þþ (2.20) þ (1.31)
---
o (0.68) þþ (2.03)
Methanol-d4 Acetone-d6
(0.09) (0.05)
þ (1.83) þ (1.08)
---
(0.11) (0.25)
Methanol-d4 Acetone-d6
(0.17) (0.22)
þþ (4.36) þþ (2.84)
---
o (0.50) þ (1.21)
Notes: very strong (þþ), strong (þ ), weak (o), very weak ( ), (- -) no signals. For comparison the semi-quantitative enhancement factors (EF) are given in brackets for the appropriate catalyst/substrate systems in different solvents.
Fig. 1. (a) 1H NMR spectrum of a solution of styrene/RhCl3 xH2O in methanol-d4 and (b) corresponding 1H PHIP spectrum. (c) 1H NMR spectrum of a solution of styrene/ RhCl3 xH2O in acetone-d6 and (d) corresponding 1H PHIP spectrum. Notes: all hydrogenation reactions were performed at the boiling point of the reaction mixtures.
90
T. Gutmann et al. / Solid State Nuclear Magnetic Resonance 40 (2011) 88–90
Fig. 2. (a) 1H NMR spectrum of a solution of 4-chlor-styrene/RhCl3 xH2O in methanol-d4 and (b) corresponding 1H PHIP spectrum. (c) 1H NMR spectrum of a solution of styrene/IrCl3 xH2O in acetone-d6 and (d) corresponding 1H PHIP spectrum. Notes: all hydrogenation reactions were performed at the boiling point of the reaction mixtures.
These species contain only chloro and/or solvent molecules, respectively, reactant molecules as ligands instead of PPh3 ligands like in Wilkinson’s catalyst. However, one has to take into account that the observed enhancement factor for RhCl3 xH2O for the hydrogenation of styrene is approximately 2 magnitudes lower than that for the neat Wilkinson’s catalyst (EFE77), which was tested under similar conditions. This strong difference seems to be an effect of the much lower catalytic activity of RhCl3 xH2O in organic phase. 2.2. RhCl3 xH2O vs. IrCl3 xH2O/styrene derivatives From this starting point as second example IrCl3 xH2O was tested employing styrene and para-substituted styrene derivatives as substrates for the hydrogenation reaction. This was done to show that even different metal systems work for PHIP and that the PHIP activity depends on the chemical properties of these systems, such as solvent or substituent effects. An overview of the semi-quantitative results of these experiments is given in Table 1. The experiments show that PHIP signal enhancement is observed both in RhCl3 xH2O and in IrCl3 xH2O (see Fig. 2) containing systems. The observed enhancement depends strongly on the temperature. At room temperature (rt) the reaction rate is relatively low. Thus no significant signal intensities are detectable. In all cases the enhancement factors are below 0.5. In contrast, at the boiling temperature for all systems significant signal enhancement was observed which, however, depends on the solvation of the catalyst in the specific solvent and the appropriated substrate. The latter observation is in agreement with Duckett et al. [24] who compared the PHIP signal intensities for the RhH2Cl(PPh3)2(styrene-R) (R¼H, Cl, CH3) complexes. They found that the PHIP intensity decrease in the order CH3 ZHbCl, which seems to be a consequence of the reaction rate of the hydrogenation reaction [25].
3. Conclusion In this publication the PHIP effect employing simple metal catalyst like RhCl3 xH2O and IrCl3 xH2O was investigated. For the model system styrene and some styrene derivatives it was shown by a semi-quantitative comparison that the signal enhancement strongly depends on the chemical and physical conditions like solvent, substituent effect at the styrene and temperature. These studies showed that simple transition metals are valuable systems to produce large PHIP enhancement. This result could be of high interest for the applicability of this technique in technical processes from the economic point of view. Finally it has a high
potential to study new reaction pathways [26] of simple transition metal catalysts.
Acknowledgment This work has been supported by the Deutsche Forschungsgemeinschaft (DFG) under Contract Bu-911-15/1. We thank the NMR laboratory of the organic chemistry department (Dr. R. Meusinger) for the generous allocation of measurement time at their Bruker ARX 300 spectrometer. TG thanks the CNRS-MPI cooperation for financial support for a post-doctoral stay in France. TR thanks the European Union for financial support by Marie-Curie Intra European Fellowship, No PIEF-GA-2009-253065.
References [1] T.C. Eisenschmidt, R.U. Kirss, P.P. Deutsch, S.I. Hommeltoft, R. Eisenberg, J. Bargon, J. Am. Chem. Soc. 109 (1987) 8089. [2] C.R. Bowers, D.P. Weitekamp, J. Am. Chem. Soc. 109 (1987) 5541. [3] C.R. Bowers, D.P. Weitekamp, Phys. Rev. Lett. 57 (1986) 2645. [4] M.G. Pravica, D.P. Weitekamp, Chem. Phys. Lett. 145 (1988) 255. [5] J.A. Osborn, F.H. Jardine, J.F. Young, G. Wilkinso., J. Chem. Soc. a—Inorg. Phys. Theor. (1966) 1711. [6] L. Vaska, J.W. Diluzio, J. Am. Chem. Soc. 83 (1961) 2784. [7] R. Eisenberg, T.C. Eisenschmid, M.S. Chinn, R.U. Kirss, Adv. Chem. Ser. (1992) 47. [8] J. Bargon, in: J.G. de Vries, C.J. Elsevier (Eds.), Handbook of Homogeneous Hydrogenation, vol. 1, Wiley VCH, Weinsheim, 2007, p. 313. [9] S.B. Duckett, C.J. Sleigh, Prog. Nucl. Magn. Reson. Spectrosc. 34 (1999) 71. [10] S.B. Duckett, D. Blazina, Eur. J. Inorg. Chem. (2003) 2901. [11] A.M. Balu, S.B. Duckett, R. Luque, Dalton Trans. (2009) 5074. [12] I.V. Koptyug, V.V. Zhivonitko, K.V. Kovtunov, ChemPhysChem 11 (2010) 5764. [13] I.V. Koptyug, K.V. Kovtunov, S.R. Burt, M.S. Anwar, C. Hilty, S.I. Han, A. Pines, R.Z. Sagdeev, J. Am. Chem. Soc. 129 (2007) 5580. [14] K.V. Kovtunov, I.E. Beck, V.I. Bukhtiyarov, I.V. Koptyug, Angew. Chem. Int. Ed. 47 (2008) 1492. ¨ [15] T. Gutmann, T. Ratajczyk, Y. Xu, H. Breitzke, A. Grunberg, S. Dillenberger, U. Bommerich, T. Trantzschel, J. Bernarding, G. Buntkowsky, Solid State Nucl. Magn. Reson. 38 (2010) 90. [16] P. Meakin, C.A. Tolman, J.P. Jesson, J. Am. Chem. Soc. 94 (1972) 3240. [17] R.W. Adams, J.A. Aguilar, K.D. Atkinson, M.J. Cowley, P.I.P. Elliott, S.B. Duckett, G.G.R. Green, I.G. Khazal, J. Lo´pez-Serrano, D.C. Williamson, Science 323 (2009) 1708. ¨ ¨ ¨ [18] S. Gloggler, R. Muller, J. Colell, M. Emondts, M. Dabrowski, B. Blumich, S. Appelt, Phys. Chem. Chem. Phys. (2011). [19] J. Blum, I. Amer, A. Zoran, Y. Sasson, Tetrahedron Lett. 24 (1983) 4139. [20] J. Blum, I. Amer, K.P.C. Vollhardt, H. Schwarz, G. Hohne, J. Org. Chem. 52 (1987) 2804. [21] B. el Ali, Catal. Commun. 4 (2003) 621. [22] Y. He, G. Chen, S. Kawi, S. Wong, J. Porous Mater. 16 (2009) 721. [23] T. Gutmann, M. Sellin, H. Breitzke, A. Stark, G. Buntkowsky, Phys. Chem. Chem. Phys. 11 (2009) 9170. [24] S.B. Duckett, C.L. Newell, R. Eisenberg, J. Am. Chem. Soc. 116 (1994) 10548. [25] J. Halpern, T. Okamoto, Inorg. Chim. Acta—Artic. Lett. 89 (1984) L53. [26] G. Buntkowsky, J. Bargon, H.-H. Limbach, J. Am. Chem. Soc. 118 (1996) 867.
Contents lists available at ScienceDirect
Solid State Nuclear Magnetic Resonance journal homepage: www.elsevier.com/locate/ssnmr
New investigations of technical rhodium and iridium catalysts in homogeneous phase employing para-hydrogen induced polarization b ¨ Torsten Gutmann a, Tomasz Ratajczyk b, Sonja Dillenberger b, Yeping Xu b, Anna Grunberg , b c c d Hergen Breitzke , Ute Bommerich , Thomas Trantzschel , Johannes Bernarding , Gerd Buntkowsky b,n a
Laboratoire de Chimie de Coordination (LCC) CNRS, 205 Route de Narbonne, F-31400 Toulouse Cedex 4, France Institute of Physical Chemistry, Technical University Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany c Leibniz Institute for Neurobiology, Brenneckestraße 6, D-39118 Magdeburg, Germany d Institute for Biometry and Medical Computer Science, Otto-von-Guericke University Magdeburg, Leipziger Straße 44, D-39120 Magdeburg, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2011 Received in revised form 30 July 2011 Available online 7 August 2011
It is shown that the para-hydrogen induced polarization (PHIP) phenomenon in homogenous solution containing the substrate styrene is also observable employing simple inorganic systems of the form MCl3 xH2O (M¼ Rh, Ir) as catalyst. Such observation confirms that already very simple metal complexes enable the creation of PHIP signal enhancement in solution. This opens up new pathways to increase the sensitivity of NMR and MRT by PHIP enhancement using cost-effective catalysts and will be essential for further mechanistic studies of simple transition metal systems. & 2011 Elsevier Inc. All rights reserved.
Keywords: Para-hydrogen induced polarization Homogeneous catalysis Nuclear magnetic resonance Sensitivity enhancement
1. Introduction The para-hydrogen induced polarization effect (PHIP) [1] was first described in the late 80s of the last century under the acronym PASADENA [2,3] and further investigated in experiments known as ALTADENA [4]. In these experiments Wilkinson’s catalyst RhCl(PPh3)3 [5] as well as Vaska’s complex Ir(CO)Cl(PPh3)2 [6] were employed as well-known homogenous catalysts for hydrogenation reactions. In the following years a variety of catalysts mainly containing Rh and Ir as well as Ru, Pd, Pt and Co metal centers with different types of ligands were tested in catalytic hydrogenation reactions in homogeneous phase [7–10] for PHIP activity. Recently, it was shown that PHIP is also observable in systems containing heterogenized catalysts such as immobilized catalysts or nano-particles [11–14]. Such systems have the strong advantage of a fast separation of hyperpolarized products and catalysts, which initially have high potential for their applicability in medical or technical applications of PHIP. However, in case of immobilized homogeneous catalysts the ‘‘catalyst leaching’’ in specific solvents have to be taken into account as it was shown in our previous work [15]. In this context the analyses of the PHIP active solutions containing the ‘‘leached species’’ also gave a first surprising hint towards the chemical
n
Corresponding author. E-mail address: [email protected] (G. Buntkowsky).
0926-2040/$ - see front matter & 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.ssnmr.2011.08.002
nature of the active species in solution. The only 31P NMR signal in the spectra of these solutions was a pure singlet (assigned to OPPh3) without any JRh,P. Thus it is partially evident that the ‘‘leached species’’ is not the neat Wilkinson’s catalyst, since the latter would exhibit JRh,P couplings in the range of 146 Hz and 192 Hz, respectively, in solution [16]. Moreover, it follows that the catalytically active species does not consist of phosphine groups, which coordinate rhodium or that their amount is below the detection minimum of the NMR. This leads us to the assumption that the phosphorous ligand could be unnecessary for the generation of the PHIP signal enhancement in these systems and that perhaps a solvated form of the rhodium might be the catalytically active species. From this point the question arises whether simple Rh species formed by dissolution of neat RhCl3 xH2O can act as hydrogenation catalyst and generate PHIP signal enhancement in solution. This would confirm our previous assumption and further opens up new pathways to produce hyperpolarized products employing very costeffective catalysts. Moreover this study could be interesting as initial point for mechanistic studies of the PHIP effect in solution. Further, simple transition metal salts can be of interest in the context of signal amplification by reversible exchange (SABRE) [17]. Only recently, a powerful application of this method for the signal enhancement of proteinogenic amino acids has been demonstrated [18]. Thus, since RhCl3 and especially IrCl3 are water solvable, they may by employed for signal enhancement of different biomolecules in water solution, their natural environment.
T. Gutmann et al. / Solid State Nuclear Magnetic Resonance 40 (2011) 88–90
Up to now, only few examples are known where RhCl3 is employed in hydrogenation reactions, which refers to the fact that this catalyst is not selective and has only low activity for hydrogenation in organic phase because of its low solubility. Therefore its application was performed as solvated ion pair in a water/ organic biphasic system as phase transfer catalyst e.g. for the hydrogenation of carbon–carbon double and triple bonds [19,20]. Recently, it has been shown that RhCl3 supported at MCM-41 material can be employed in the hydroformylation reaction of styrene [21,22]. From these primary insights in a first step PHIP experiments are performed employing the simple transition metal salt RhCl3 xH2O as catalyst. As model reaction the hydrogenation of styrene is chosen in different solvents like methanol-d4 and acetone-d6. To prove that the results for RhCl3 xH2O are also transferable to other transition metal salts, in a second step IrCl3 xH2O as typical example of an iridium metal salt is tested for PHIP experiments. For both catalyst systems the PHIP effect is semi-qualitatively investigated as a function of temperature, solvent and substituent at the styrene. Realizing this, experiments at room-temperature (rt) and at the boiling point (bp) of the appropriate solution are carried out in methanol-d4 and acetone-d6, respectively. As substrates styrene and its para-substituted derivatives 4-methylstyrene and 4-chlorostyrene are chosen. For comparison of the signal enhancement for the different systems and conditions a semiquantitative enhancement factor (EF) is defined. Such EF could be used as a measure for the sensitivity enhancement by PHIP in MRI applications.
Samples employing styrene and styrene derivatives (4-methylstyrene or 4-chlorostyrene) as substrates were prepared employing methanol-d4 or acetone-d6 as solvents. To realize the semi-quantitative comparison of the signal enhancement for these different substrate/catalyst systems an enhancement factor (EF) was defined as ratio between the signal intensity (area) of the –CH2 group of the product ethylbenzene and the signal intensity (area) of the –CH group of the substrate styrene. This simple definition allows us to compare the signal intensity of the hyperpolarized product with that of the thermally polarized substrate (Scheme 1).
2. Results and discussion 2.1. RhCl3 xH2O/styrene For first test experiments RhCl3 xH2O was selected as catalyst employing styrene as substrate for the hydrogenation reaction. As shown in Fig. 1 such system displays efficient PHIP signal enhancement (EF¼2.20 resp. 1.31) when employing specific solvents such as methanol-d4 or acetone-d6. From these primary observations it is evident that also simple rhodium species generated by RhCl3 xH2O are able to produce PHIP signal enhancement. Table 1 Semi-quantitative results of PHIP signal enhancement reached in experiments at different temperatures e.g. room temperature (rt) and boiling temperature of the solvent (bt) employing different substrates, solvents and metal catalysts.
1.1. PHIP experiments Para-hydrogen was enriched employing a laboratory-built apparatus described in a previous paper [23]. The hydrogenation reactions were carried out under ALTADENA [4] conditions. The catalysts RhCl3 xH2O (Strem, 45–1880) and IrCl3 xH2O (Strem, 77–3000) were employed without further purification. Samples were prepared in 5 mm screw-cap NMR tubes (Wilmad). In a typical procedure approximately 7–9 mg of the catalyst were dissolved in 600 ml of the appropriate solvent and 50 ml of the substrate were added forming a homogeneous solution in all cases.
Scheme 1. Hydrogenation reaction of styrene derivatives.
89
RhCl3 xH2O
IrCl3 xH2O
PHIP (rt)
PHIP (bt)
PHIP (rt)
PHIP (bt)
Methanol-d4 Acetone-d6
---
þþ (2.20) þ (1.31)
---
o (0.68) þþ (2.03)
Methanol-d4 Acetone-d6
(0.09) (0.05)
þ (1.83) þ (1.08)
---
(0.11) (0.25)
Methanol-d4 Acetone-d6
(0.17) (0.22)
þþ (4.36) þþ (2.84)
---
o (0.50) þ (1.21)
Notes: very strong (þþ), strong (þ ), weak (o), very weak ( ), (- -) no signals. For comparison the semi-quantitative enhancement factors (EF) are given in brackets for the appropriate catalyst/substrate systems in different solvents.
Fig. 1. (a) 1H NMR spectrum of a solution of styrene/RhCl3 xH2O in methanol-d4 and (b) corresponding 1H PHIP spectrum. (c) 1H NMR spectrum of a solution of styrene/ RhCl3 xH2O in acetone-d6 and (d) corresponding 1H PHIP spectrum. Notes: all hydrogenation reactions were performed at the boiling point of the reaction mixtures.
90
T. Gutmann et al. / Solid State Nuclear Magnetic Resonance 40 (2011) 88–90
Fig. 2. (a) 1H NMR spectrum of a solution of 4-chlor-styrene/RhCl3 xH2O in methanol-d4 and (b) corresponding 1H PHIP spectrum. (c) 1H NMR spectrum of a solution of styrene/IrCl3 xH2O in acetone-d6 and (d) corresponding 1H PHIP spectrum. Notes: all hydrogenation reactions were performed at the boiling point of the reaction mixtures.
These species contain only chloro and/or solvent molecules, respectively, reactant molecules as ligands instead of PPh3 ligands like in Wilkinson’s catalyst. However, one has to take into account that the observed enhancement factor for RhCl3 xH2O for the hydrogenation of styrene is approximately 2 magnitudes lower than that for the neat Wilkinson’s catalyst (EFE77), which was tested under similar conditions. This strong difference seems to be an effect of the much lower catalytic activity of RhCl3 xH2O in organic phase. 2.2. RhCl3 xH2O vs. IrCl3 xH2O/styrene derivatives From this starting point as second example IrCl3 xH2O was tested employing styrene and para-substituted styrene derivatives as substrates for the hydrogenation reaction. This was done to show that even different metal systems work for PHIP and that the PHIP activity depends on the chemical properties of these systems, such as solvent or substituent effects. An overview of the semi-quantitative results of these experiments is given in Table 1. The experiments show that PHIP signal enhancement is observed both in RhCl3 xH2O and in IrCl3 xH2O (see Fig. 2) containing systems. The observed enhancement depends strongly on the temperature. At room temperature (rt) the reaction rate is relatively low. Thus no significant signal intensities are detectable. In all cases the enhancement factors are below 0.5. In contrast, at the boiling temperature for all systems significant signal enhancement was observed which, however, depends on the solvation of the catalyst in the specific solvent and the appropriated substrate. The latter observation is in agreement with Duckett et al. [24] who compared the PHIP signal intensities for the RhH2Cl(PPh3)2(styrene-R) (R¼H, Cl, CH3) complexes. They found that the PHIP intensity decrease in the order CH3 ZHbCl, which seems to be a consequence of the reaction rate of the hydrogenation reaction [25].
3. Conclusion In this publication the PHIP effect employing simple metal catalyst like RhCl3 xH2O and IrCl3 xH2O was investigated. For the model system styrene and some styrene derivatives it was shown by a semi-quantitative comparison that the signal enhancement strongly depends on the chemical and physical conditions like solvent, substituent effect at the styrene and temperature. These studies showed that simple transition metals are valuable systems to produce large PHIP enhancement. This result could be of high interest for the applicability of this technique in technical processes from the economic point of view. Finally it has a high
potential to study new reaction pathways [26] of simple transition metal catalysts.
Acknowledgment This work has been supported by the Deutsche Forschungsgemeinschaft (DFG) under Contract Bu-911-15/1. We thank the NMR laboratory of the organic chemistry department (Dr. R. Meusinger) for the generous allocation of measurement time at their Bruker ARX 300 spectrometer. TG thanks the CNRS-MPI cooperation for financial support for a post-doctoral stay in France. TR thanks the European Union for financial support by Marie-Curie Intra European Fellowship, No PIEF-GA-2009-253065.
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