Rhenium Complexes Containing the Chiral Tridentate Ferrocenyl

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Rhenium Complexes Containing the Chiral Tridentate Ferrocenyl Ligand Pigiphos Esteban Mejía and Antonio Togni* Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Z€urich, CH-8093 Z€urich, Switzerland

bS Supporting Information ABSTRACT: The paramagnetic Re(III) complex [ReCl3(Pigiphos)] (1) ((R)-(S)Pigiphos = bis{(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}cyclohexylphosphine) is accessible from [ReCl3(NCMe)(PPh3)2] and the ligand Pigiphos. 1 reacts with an excess of NaBH4, affording the polyhydride complex [ReH5(Pigiphos)] (2), which was shown to be a classic polyhydride (NMR T1(min) = 158 ms). This complex reacts readily with both Brønsted acids and hydride scavengers to give the corresponding cationic tetrahydride species, which was shown to be active in the catalytic hydrogenation of dimethyl itaconate.

T

he use of Re in homogeneous catalysis, in particular asymmetric catalysis, is still relatively underdeveloped. Recent advances involving Re-based catalysts include C
C, CdC, C
N, C
O, C
S, and C
Si bond formation, functionalization of C
H bonds, oxidation, photoreduction, hydrosilylation, and hydrogenation.1 However, there are only few reports of Recatalyzed hydrogenation processes,2
5 with the recent contributions by Berke and co-workers being arguably the most significant.6
8 Moreover, there appears to be only one reported asymmetric process.9 Metal hydrides are ubiquitous in several catalytic transformations as the catalyst precursors, the active species, or elusive intermediates.10 Among the most stable and hence more studied metal hydride complexes are the Re polyhydrides, especially Re(V) pentahydrides11
13 and Re(VII) heptahydrides.14
16 In spite of their rich chemistry, facile synthesis, and ease of handling, the potential of these kind of derivatives still has not been exploited. Herein we report the synthesis and characterization of the first Re complexes containing the chiral tridentate ligand Pigiphos ((R)-(S)-Pigiphos = bis{(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}cyclohexylphosphine): namely, [ReCl3 (Pigiphos)] (1) and [ReH5(Pigiphos)] (2). Pigiphos, previously reported from our laboratory,17 has been shown to be effective in a number of metal-catalyzed enantioselective transformations such as transfer hydrogenation (Ru),18 acetalization (Rh, Ni),19 hydroamination (Ni),20 hydrophosphination (Ni),21 and Nazarov cyclizations (Ni).22 Furthermore, we show that the pentahydride complex 2 displays activity in the hydrogenation of dimethyl itaconate prior to abstraction of one hydride ligand either by means of a hydride scavenger or by protonation followed by release of a H2 molecule. r 2011 American Chemical Society

’ RESULTS AND DISCUSSION Synthesis and Characterization of [ReCl3(Pigiphos)] (1). The method of choice for the synthesis of this complex is the ligand substitution reaction of the parent complex [Re(NCMe)Cl3(PPh3)2] with Pigiphos, adapting the procedure used for the synthesis of [ReCl3(CH3C(CH2PPh2)3)] (Scheme 1).23 The product was obtained in almost quantitative yield as a dark brown-green microcrystalline solid. Compound 1 is an octahedral Re(III) d4 complex which exhibits a second-order paramagnetism, as is common for [ReX3L3] species.24
26 The large reduction of μeff due to strong spin
orbit coupling26 actually allows the measurement of a Knight-shifted 1H NMR spectrum that is, however, difficult to interpret. This feature, along with its silent 31P NMR spectrum, precludes the effective use of NMR techniques in this case. The complex was then characterized by elemental analysis and by HRMS, and its structure was confirmed by a single-crystal X-ray analysis. An ORTEP view of the complex and selected bond lengths and angles are given in Figure 1. The geometry around the metal center is approximately octahedral with the three chloro ligands in meridional positions. It is noteworthy that, in spite of the bulkiness of the Pigiphos ligand, the distortion from the ideal geometry is rather small in comparison with other mer-(trisphosphine)ReCl3 complexes. In complex 1 the Re atom is displaced only 0.092 Å from the plane defined by the three phosphorus atoms and the angle between the two phosphines in trans positions and the Re atom is fairly Received: July 11, 2011 Published: August 17, 2011 4765

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Scheme 1. Synthesis of [ReCl3(Pigiphos)] (1)a

a

Legend: (i) toluene, reflux, 2 h, 98%.

Figure 1. ORTEP view (50% probability ellipsoids) of [ReCl3(Pigiphos)] (1). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Re
P1 = 2.452(2), Re
P2 = 2.421(2), Re
P3 = 2.444(2), Re
Cl1 = 2.3271(19), Re
Cl2 = 2.4429(19), Re
Cl3 = 2.3684(19); P1
Re
P2 = 87.87(6), P2
Re
P3 = 92.92(6), P1
Re
P3 = 175.57(7), Cl1
Re
Cl2 = 86.36(6), Cl2
Re
Cl3 = 176.22(7), Cl1
Re
Cl3 = 91.78(7), P1
Re
Cl2 = 88.89(6), Cl2
Re
P3 = 90.52(6).

Scheme 2. Synthesis of [ReH5(Pigiphos)] (2)a

a

Legend: (i) ethanol, reflux, 2 h, 99%.

close to 180°: i.e., 175.57(7)°. In mer-(PPh2Me)3ReCl327 and mer-(PPhEt2)3ReCl328 these distances are 0.105 and 0.139 Å while the corresponding angles are 165.8(2) and 167.7(1)°, respectively. A direct comparison of the structural features of 1 with those of other metal complexes bearing the Pigiphos ligand is less straightforward, since this is the first X-ray structure reported for an octahedral complex containing this ligand. Synthesis and Characterization of [ReH5(Pigiphos)] (2). Upon treatment with NaBH4 in refluxing ethanol, the trichloride

Figure 2. Variable-temperature 1H NMR spectra of 2 in toluene-d8 at 500 MHz (hydride region only).

complex 1 is transformed into the title compound 2 in almost quantitative yield (Scheme 2). This procedure is similar to that used to obtain [ReH5(Triphos)].24,29 The product is an orange solid, stable to air and moisture even in solution, though it decomposes slowly in chlorinated solvents to give various still unidentified products. In the FTIR spectrum of 2, apart from the bands corresponding to the ligand vibration modes, ν(Re
H) absorptions are 4766

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Figure 3. Plot of the temperature dependence of T1 for [ReH5(Pigiphos)] (2) recorded at 500 MHz in toluene-d8.

observed at 1931 and 1896 cm
1, similar to those reported for [ReH5(PhP(CH2CH2CH2PCy2)2)].12 The 31P NMR spectrum shows the expected ABX spin system with coupling constants cis JPP = 10.6 and transJPP = 68.3 Hz. This is consistent with the presence of two diastereotopic ferrocenylphosphine moieties. It is noteworthy that the magnitude of the transJPP coupling constant is rather small if compared with those in Ni, Pd, Rh, and Ir complexes with the same ligand in a meridional arrangement, in which it varies from 131 to 411 Hz.17,30 This is a consequence of the small angle subtended by the lateral phosphorus atoms (vide infra). In the 1H NMR spectra at room temperature (in CD2Cl2, C6D6, toluene-d8, or chlorobenzene-d5) the five hydride protons are equivalent and appear at δ
6.05 as a broad featureless singlet, indicating a fast fluxional behavior. Upon heating the toluene-d8 solution to 323 K the fast-movement regime is attained and the hydride signal turns into a broad quartet, suggesting equivalence of the five hydrides and similar coupling constants with the three different phosphorus atoms. Upon cooling, the hydride resonance starts to decoalesce at 273 K, being completely resolved at 213 K into three signals, a quartet at δ
6.05, a triplet at δ
6.2, and a singlet at δ
6.5 with relative intensities of 2:2:1, respectively. Further cooling to 173 K leads to extremely broad signals without further decoalescence. These spectra are shown in Figure 2. With respect to the nature of the Re
H bond, X-ray and neutron diffraction studies on different Re pentahydrides containing chelating12 triphosphine or monodentate31 phosphine ligands have shown that these kind of complexes do not show H
H distances short enough to be accountable by η2-H2 ligands or “nonclassical” hydrides. The same complexes can be also addressed as “classical” in solution on the basis of measurements of spin
lattice relaxation times, T1,32,33 in spite of the ambiguous results given by this method, especially for crowded polyhydrides with bulky ligands34 and for hydrides of metals with high gyromagnetic ratio (such as Re) in which metal hydride dipole
dipole contributions to relaxation are significant.33 Nevertheless, it was necessary to rule out the possible presence or formation of η2-H2 ligands (i.e., as intermediates in the exchange process). To that purpose,

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Figure 4. ORTEP view (50% probability ellipsoids) of [ReH5(Pigiphos)] (2 3 2C6H6). The hydridic protons were not located. The other hydrogen atoms and the solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Re
P1 = 2.355(3), Re
P2 = 2.387(2), Re
P3 = 2.353(3); P1
Re
P2 = 96.20(10), P2
Re
P3 = 97.45(10), P1
Re
P3 = 148.76(9).

Figure 5. Coordination sphere of Re in [ReH5(Pigiphos)] (2). The Re
P core is taken from the actual crystal structure of 2 3 2C6H6 (50% probability ellipsoids), and the hydride ligands (represented by spheres of arbitrary size) are placed assuming an idealized triangulated dodecahedral geometry.

variable-temperature T1 measurements were carried out for complex 2 at 500 MHz, suggesting a “classical” structure. The plot of ln T1 vs 1000/T (K) for the three resonances observed after decoalescence (Figure 3) shows the same behavior for all the hydridic protons with an estimated T1(min) value of 158 ms, which is well in agreement with the reported values for allterminal Re pentahydride complexes.33 X-ray-quality crystals of complex 2, which cocrystallizes with two benzene molecules, were obtained. An ORTEP view of the complex and selected bond lengths and angles are given in Figure 4. Even though the hydrido ligands could not be located in the crystal structure, one can assess the overall coordination geometry around the metal center in the complex by analysis of the geometries of the Re
P bonds.35 The important parameters to take into account are the P
Re
P bond and the distance of the Re atom to the plane defined by the three P atoms (0.498 Å). These sets of parameters are consistent with a triangulated dodecahedral geometry,36 as shown in Figure 5, and are also in accordance with the values obtained for the related compound [ReH5(PhP(CH2CH2CH2PCy2)2)] (95.10, 95.10, 151.80° and 0.486 Å), in which the hydridic protons were actually located.12 The triangulated dodecahedral geometry around the Re atom in 2 may be described by two mutually orthogonal trapezoids 4767

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undertake experiments using either 2 or 3 as catalyst precursors for asymmetric olefin hydrogenation reactions. Using dimethyl itaconate as a model substrate, 100 bar of H2 pressure, and 1 mol % of 2 in CH2Cl2 at 80 °C, no conversion was observed even after a prolonged period of time. However, under the same conditions conversions up to ca. 50% were obtained after activation of 2 with 1 equiv of CPh3[BArF]. A much lower conversion (ca. 20%) was observed when similar hydrogenation experiments were conducted in THF. Thus, one can assume that the coordination ability of the solvent is a drastically limiting factor. These experiments indicate that the cationic 16e complex 3 is catalytically competent, though to a modest extent, as compared to, for example, more typical Rh catalysts. Furthermore, no asymmetric induction took place.

Figure 6. Comparison of 1H NMR spectra at room temperature of 2 upon reaction with different acids (in CD2Cl2 at 250 MHz, hydride region only).

BAAB.37 In complex 2 (see Figure 5), one of these planes contains the central P atom P1 and the hydrogens H1, H2, and H3. The Re atom is also included in this plane. The second plane is defined by the lateral P atoms, P1 and P3, and the hydrides H4 and H5. This geometry implies three different types of hydrido ligands. Two pairs of hydrides occupying the four-neighbor A positions, H1, H2, H3, and H4 (the last two being at the same side of the axially oriented cyclohexyl substituent) and one hydride, H3, occupying the remaining five-neighbor B position. This spatial arrangement accounts for the observed decoalescence pattern in the 1H NMR spectrum when the slow-movement regime is reached upon cooling to 213 K (Figure 2). Reactions of [ReH5(Pigiphos)] (2) with Brønsted Acids and Hydride Scavengers. It has been observed that complexes of the type [ReH5L3] react with Brønsted acids to give [ReH6L3]+ species, partly also described as [Re(H2)H4L3]+ (L = PPh3,38 PMe2Ph,39 PhP(CH2CH2CH2PCy2)232). The stability of these complexes is dependent on the nature of the accompanying phosphine ligands. With monodentate phosphines the cationic complexes react readily with σ-donor ligands, releasing molecular hydrogen, while with the tripodal phosphine PhP(CH2CH2CH2PCy2)2 the resulting η2-H2 complex “exhibits a remarkable lack of reactivity”,38 being practically inert to substitution with common 2e-donor ligands. In contrast, complex 2 reacts in quantitative yield with either HBF4 3 Et2O or H[BArF] 3 2.3Et2O at room temperature in CD2Cl2 to give the corresponding cationic tetrahydride [ReH4(Pigiphos)]+A
(3[BF4] and 3[BArF]). The η2-H2 intermediate [Re(H2)H4(Pigiphos)]+ was never observed, immediately releasing molecular hydrogen at room temperature, as visually indicated by bubbles in the reaction mixture. The reaction of 2 with the hydride ion abstractor CPh3[BArF] gives the cationic tetrahydride 3[BArF], as expected. A comparison of the 1H NMR spectra confirms the formation of 3, which was identified by a clear downfield shift of the hydride resonance from δ
6.67 to
4.72 ppm in all cases, as shown in Figure 6. However, attempts to isolate and fully characterize this highly reactive species failed. Catalytic Hydrogenation of Dimethyl Itaconate. The fact that pentahydride complex 2 can be easily converted into the corresponding “activated species” 3 and that the latter is not prone to strongly bind molecular hydrogen prompted us to

’ CONCLUSIONS In summary, the new chiral enantiopure Re complexes 1 and 2 containing the tripodal phosphine ligand (R)-(S)-Pigiphos have been prepared and fully characterized. Both complexes are remarkably stable to ambient conditions, even in solution. Upon reaction with either Brønsted acids or hydride scavengers [ReH5(Pigiphos)] (2) reacts readily at room temperature to give in all cases the same cationic tetrahydride complex, a 16e species which was shown to be able to catalyze the hydrogenation of dimethyl itaconate under high pressure but without affording any enantiomeric excess. These new complexes are the first of this kind containing a chiral tridentate ligand. Though still nonenantioselective, the potential applications of Re polyhydrides as catalysts in hydrogenation reactions were unveiled. Further efforts are being carried out in our laboratory toward asymmetric Re-catalyzed hydrogenations by exploiting the synthetic modularity of the Pigiphos scaffold. ’ EXPERIMENTAL SECTION [ReCl3(Pigiphos)] (1). The ligand (R)-(S)-Pigiphos (185 mg, 0.204 mmol) and the Re precursor [Re(NCMe)Cl3(PPh3)2]40 (160.3 mg, 0.187 mmol) were dissolved in 30 mL of toluene with stirring. This orange solution was refluxed for 2 h. The resulting dark brown solution was cooled to room temperature and concentrated to a third of its volume under high vacuum, and then 30 mL of n-hexane was added, leading to the precipitation of a brown solid. The precipitate was decanted and washed thoroughly with n-hexane and dried in vacuo to afford the pure product as a green paramagnetic powder. Yield: 220 mg (0.1833 mmol, 98%). FTIR: only bands associated with ligand resonance modes, 3052 (s), 2916 (br), 2848 (s), 1433 (s), 822 (s), 740 (s), 693 (s) cm
1. Anal. Calcd for C54H55P3Cl3Fe2Re: C, 54.00; H, 4.61; P, 7.74. Found: C, 54.09; H, 4.84; P, 7.70. MS (HiResESI): m/z 1165.12 (100, [M
Cl]+); monoisotopic mass calcd 1165.1136, found 1165.1162. [ReH5(Pigiphos)] (2)

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Organometallics A solution of 1 (60 mg, 0.05 mmol) and NaBH4 (111,2 mg, 2,92 mmol) in ethanol was refluxed for 2 h. The resulting orange suspension was allowed to settle, cooled to
20 °C, and filtered through Celite. The resulting orange solid was washed thoroughly with methanol and then taken up in toluene. The toluene solution was concentrated to dryness in vacuo to afford the product as a pale orange powder. Yield: 54.4 mg (0.049 mmol, 99%). 1H NMR (C6D6, 500.23 MHz, 25 °C, TMS; J values in Hz): δ
6.05 (bs, 5H, ReH), 1.12 (m, 1H, HF ax), 1.20
1.30 0 (m, 5H, H1 , HE ax, HD ax), 1.40 (m, 1H, HC ax), 1.50 (dd, 3H, H1, JHH = JHP = 7.5), 1.60
1.75 (m, 2H, HD eq, HB ax), 1.80
1.95 (m, 2H, HE eq, HF eq), HC eq), 2.57 (m, 1H, 0HB eq), 2.75 (m, 1H, HA), 3.01 (m, 1H, 2 2 8 80 , H ), 3.38 (s, 5H, H ), 3.90 (s, 5H, H ), 3.91 (m, 3.15
3.25 (m, 2H, H 0 0 1H, H7 ), 3.98 (dd, 1H, H6, JHH0 = JHH = 2.5), 4.06 (dd, 1H, H6 , JHH = 2.5, JHH = 4.5), 4.18 (bs,0 1H, 0H5 ), 4.23 (bs, 1H, H7), 4.33 (m, 1H,0 H5), 5H, H16, H16 , H11 , 6.90
7.02 (m, 3H, H12 , H11 , H12), 7.03
7.25 (m, 0 150 15 10 140 H , H ), 7.75 (m, 2H, H ), 7.90 (m, 2H, H ), 8.17 (m, 2H, H10 ), 8.38 (m, 2H, H14). 13C{1H} NMR (C6D6, 125.75 MHz, 25 °C,0 TMS; J values in Hz): δ 16.80 (d, 1C, C1, 2JPC = 9.9), 22.66 (d, 1C, C1 , 2JPC = 20 1 2.5), 26.73 (d, 1C, C , JPC = 21.4), 27.47 (s, 1C, CD), 28.50 (d, 1C, CC, 3 JPC = 6.3), 28.83 (d, 1C, CE, 3JPC = 12.6), 30.39 (d, 1C, CF, 2JPC = 1.8), 30.55 (d, 1C, CB, 2JPC = 6.9), 40.45 (dd, 1C, C2, 3JPC = 1.2, 1JPC = 20.7), 42.01 (d, 1C, CA, 1JPC = 17.6), 66.67 (d, 1C, C6, 3JPC = 5.6), 67.01 (dd, 0 3 JPC = 8.2), 1C, C6 , 4JPC = 1.9, 3JPC = 4.4), 68.990 (dd, 1C, C7, 3JPC = 1.2, 8 8 70 3 (s, 5C, C ), 71.80 (dd, 1C, C , J = 3JPC = 70.51 (s, 5C, C ), 70.96 PC 50 40 1 7.8), 72.74 (s, 1C, C ), 74.65 (d, 1C, C , JPC = 50.3), 75.60 (d, 1C, C5, 2 JPC = 1.5), 81.15 (d, 1C, C4, 1JPC = 42.7), 94.88 (dd, 1C, C3, 2JPC = 7.5, 2 30 2 JPC0 = 20.1), 102.23 (dd, 1C, C , JPC0 = 3.9, 2JPC = 18.8), 126.74 (d, 1C, C15 , 3JPC = 9.6), 126.90 (d, 1C, C11 , 3JPC = 10.0), 127.17 (d, 1C, C11, 3 JPC = 9.5), 127.21 (d,0 1C, C15, 3JPC = 8.8),0 127.72 (d, 1C, C12, 4JPC = 1.6), 128.29 (1C, C16 ), 129.03 (d, 1C, C12 , 4JPC = 1.7), 129.08 (d, 1C,0 C16, 4JPC = 1.9), 132.30 (d, 1C,0 C10, 2JPC = 10.0), 133.30 (d, 1C, C14 , 2 JPC = 10.9), 136.27 (d, 1C, C10 , 2JPC = 11.6), 136.96 (d, 1C,0 C14, 2JPC = 90 1 12.3), 140.53 (d, 1C, C , JPC = 51.5), 143.87 (dd, 1C, C13 , 3JPC = 4.6, 1 JPC = 46.5), 146.82 (dd, 1C, C13, 3JPC = 3.4, 1JPC = 44.6), 152.27 (d, 1C, C9, 1JPC = 57.4). 31P{1H} NMR (C6D6, 202.46 MHz, 25 °C, H3PO4 85%; J values in Hz): δ 17.47 (dd, 1P, PPh2, cisJPP = 10.6, transJPP = 68.3), 21.18 (dd, 1P, PPh2, cisJPP = 16.1, transJPP = 68.3), 49.56 (dd, 1P, PPh2, cis JPP = 10.6, cisJPP = 16.1). FTIR: bands associated with ligand resonance modes, 2916 (br), 2847 (s), 1431 (s), 807 (s), 688 (s) cm
1; Re
H, 1931, 1896 cm
1. Anal. Calcd for C54H60P3Fe2Re: C, 58.97; H, 5.50; P, 8.45. Found: C, 59.37; H, 5.57; P, 8.35. MS (HiResESI): m/z 1097.1982 (100, mixture of [M]•+, [M
H]+, [M
2H]+, [M
3H]+, [M
4H]+ and [M
5H]+).

’ ASSOCIATED CONTENT Supporting Information. Text, tables, and figures giving experimental procedures and full analytic and spectroscopic data for complexes 1 and 2 (FT-IR, HiResESI-MS, 1H, 13C, 31P, H
H COSY, C
H HMBC, C
H HMQC, H
H NOESY NMR) and CIF files giving crystallographic data for both 1 and 2 3 2C6H6. This material is available free of charge via the Internet at http:// pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*[email protected].

’ ACKNOWLEDGMENT We thank Nikolas Huwyler and Pascal Engl for some of the experimental work, Katrin Niedermann and Raphael Aardoom for the X-ray structural studies, Dr. Heinz R€uegger and Dr. Aitor

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Moreno for NMR assistance, Dr. Benoît Pugin (SOLVIAS AG, Basel) for hydrogenation experiments, and ETH Z€urich and the Swiss National Science Foundation for financial support.

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