Ahmed F.
Abdel-Magied
a,
Yusuf
Theibich
a,
Amrendra K.
Singh
a,
Ahibur
Rahaman
a,
Isa
Doverbratt
b,
Arun K.
Raha
a,
Matti
Haukka
c,
Michael G.
Richmond
d and
Ebbe
Nordlander
*a
aChemical Physics, Department of Chemistry, Lund University, Box 124, SE-221 00, Lund, Sweden. E-mail: Ebbe.Nordlander@chemphys.lu.se
bCentre for Analysis and Synthesis, Department of Chemistry, Lund University, Box 124, SE-221 00, Lund, Sweden
cDepartment of Chemistry, University of Jyväskylä, Box 35, FI-400 14, Jyväskylä, Finland
dDepartment of Chemistry, The University of North Texas, Denton, Texas 76203, USA
First published on 27th February 2020
Twenty clusters of the general formula [(μ-H)2Ru3(μ3-S)(CO)7(μ-P–P*)] (P–P* = chiral diphosphine of the ferrocene-based Walphos or Josiphos families) have been synthesised and characterised. The clusters have been tested as catalysts for asymmetric hydrogenation of tiglic acid [trans-2-methyl-2-butenoic acid]. The observed enantioselectivities and conversion rates strongly support catalysis by intact Ru3 clusters. A catalytic mechanism involving an active Ru3 catalyst generated by CO loss from [(μ-H)2Ru3(μ3-S)(CO)7(μ-P–P*)] has been investigated by DFT calculations.
In previous studies, we have investigated asymmetric hydrogenation of α-unsaturated carboxylic acids using catalytic systems based on [(μ-H)4Ru4(CO)12] clusters derivatised with chiral diphosphines. We have shown that such diphosphine ligands may effect strong chiral induction,21,22 and that good conversions and enantioselectivities may be achieved. While catalysis by fragmentation products cannot be excluded, the recovery of unaltered starting clusters after catalytic reactions, recycling of catalysts/catalyst precursors with identical catalytic results, and mercury poisoning tests all implicate the presence of an active cluster catalyst.23
Norton24 has identified a criterion which, if fulfilled, provides incontrovertible evidence for a cluster species acting as an active catalyst, viz. the observation of asymmetric induction in a reaction that is catalysed by a cluster that is chiral by virtue of the cluster framework only (i.e. excluding chiral ligands). Based on this concept, the silylation of acetophenone using chiral tetrahedrane clusters has been established, but cluster racemization was found to occur faster than productive catalysis.24 Here we wish to present the fulfilment of a corollary to Norton's criterion, i.e. proof of diastereomeric control of enantioselectivity when diastereomeric pairs of [(μ-H)2Ru3(μ-S)(CO)7(μ-P–P*)] clusters are used as (asymmetric) hydrogenation catalysts and the chirality of the diphosphine ligand is maintained intact while that of the cluster framework changes. Part of these results have been published in an earlier communication.25
For 3, the 1H NMR in the hydride region shows two signals, a doublet of doublet of doublets at δ −18.13 (JH–P = 12.4, JH–P = 11.2, JH–H = 2.9 Hz) and an (apparent) doublet of triplets at δ −18.40 (JH–P = 9.8, JH–H = 2.9 Hz), while for the corresponding diastereomer 4, the 1H NMR in the hydride region shows two signals at δ −17.82 (ddd, JH–P = 12.2, JH–P = 8.8, JH–H = 3.5 Hz) and δ −18.63 (ddd, JH–P = 7.5, JH–P = 3.6, JH–H = 3.5 Hz).
In the 31P NMR spectrum of 3, the signal for phosphorus P1 (cf.Scheme 1) appears as a triplet, indicating coupling to both hydrides, while the signal for P2 is a doublet. In contrast, the signal for P1 in 4 appears as a doublet while that of P2 is a doublet of doublets. The identities of the two diastereomers were further confirmed by the determination of their crystal structures (Fig. 2 and 3). The analogous diastereomeric pair based on S,S-1b, viz.5 (with diphosphine connectivity corresponding to 3) and 6 (connectivity corresponding to 4) were prepared, and characterized by comparing their spectroscopic data with those of 3 and 4. The 1H NMR spectrum of 5 in the hydride region shows two signals at δ −18.12 (ddd, JH–P = 12.6, JH–P = 11.1, JH–H = 3.0 Hz) and at δ −18.40 (d′t′, JH–P = 9.9, JH–H = 3.0 Hz), while for 6 the corresponding signals are found at δ −17.84 (ddd, JH–P = 12.4, JH–P = 9.1, JH–H = 3.0 Hz) and δ −18.65 (ddd, JH–P = 7.1, JH–P = 3.2, JH–H = 3.0 Hz). Similarly, in the 31P NMR spectrum of 5, the P1 signal (cf.Scheme 1) appears as a doublet of doublets, while the signal for P2 is a triplet, indicating coupling to both hydrides. In contrast, the signal for P1 in the corresponding diastereomer 6 appears as a doublet of doublets while that of P2 is a doublet. Again, the identities of the two diastereomers were further confirmed by the determination of their crystal structures (Fig. 2 and 3).
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Fig. 1 Variable-temperature 1H NMR spectra (hydride region) of [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1g)] 16 recorded over the temperature range 298–213 K (top to bottom). |
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Fig. 2 Molecular structures of four diastereomers (two diastereomeric pairs) representing different combinations of cluster framework and ligand chiralities, using pure enantiomeric forms R,R-1a or S,S-1b ligands; (a) [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1a)] 3, CCDC 993899,† selected bond distances [Å] and angles [°]: Ru1–Ru2 2.8952(9), Ru1–Ru3 2.9022(9), Ru2–Ru3 2.7537(8), Ru1–P1 2.359(2), Ru2–P2 2.3332(19), Ru1–S1 2.368(2), Ru2–S1 2.375(2), Ru3–S1 2.346(2), Ru1–H1 1.77(7), Ru1–H2 1.70(6), Ru2–H2 1.75(6), Ru3–H1 1.77(7); (b) [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1a)] 4, CCDC 993900,† selected bond distances [Å] and angles [°]: Ru1–Ru2 2.915(2), Ru1–Ru3 2.746(2), Ru2–Ru3 2.894(2), Ru1–P2 2.335(5), Ru2–P1 2.349(5), Ru1–S1 2.361(5), Ru2–S1 2.358(4), Ru3–S1 2.351(5). Ru1–H1 1.569, Ru1–H2 1.57, Ru2–H2 1.528, Ru3–H1 1.674 (riding on Ru atoms); (c) [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1b)] 5, CCDC 1043616,† selected bond distances [Å] and angles [°]: Ru1–Ru2 2.9054(2), Ru1–Ru3 2.7518(2), Ru2–Ru3 2.8818(2), Ru1–P1 2.3331(6), Ru2–P2 2.3502(6), Ru1–S1 2.3811(5), Ru2–S1 2.3728(5), Ru3–S1 2.35568(5), Ru1–H1 1.71(4), Ru2–H1 1.74(4), Ru2–H2 1.84(4), Ru3–H2 1.69(4), and (d) [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1b)] 6, CCDC 1043617,† selected bond distances [Å] and angles [°]: Ru1–Ru2 2.9210 (12), Ru1–Ru3 2.893(3), Ru2–Ru3 2.7657(12), Ru1–P2 2.341(2), Ru2–P1 2.361(2), Ru1–S1 2.359(2), Ru2–S1 2.357(2), Ru3–S1 2.355(3), Ru1–H1 1.72(5), Ru1–H2 1.678(17), Ru2–H2 1.57(5), Ru3–H1 1.57(4). Thermal ellipsoids are drawn at the 50% probability level and C–H hydrogen atoms have been omitted for the sake of clarity. |
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Fig. 3 Molecular structure of [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1c)] 8 (CCDC 1043618†) with thermal ellipsoids drawn at the 50% probability level. C–H hydrogen atoms have been omitted for clarity. Selected bond distances [Å] and angles [°]: Ru1–Ru2 2.9276(6), Ru1–Ru3 2.7511(6), Ru2–Ru3 2.9123(6), Ru1–P1 2.3376(13), Ru2–P2 2.3833(13), Ru1–S1 2.3649(14), Ru2–S1 2.3653(13), Ru3–S1 2.3635(13), Ru1–H1 1.90(4), Ru2–H2 1.76(7), Ru2–H1 1.78(4), Ru3–H2 1.94(7). |
The 1H NMR of [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1g)] 16 in the hydride region shows a multiplet signal at δ −18.23, which indicates fluxionality of the hydrides at ambient temperature. Variable-temperature 1H NMR spectra in the hydride region for 16 show that the fluxionality may be frozen out at approximately 253 K to give two signals, an apparent triplet of doublets at δ −18.12 (d′t′ = ddd, JH–P = 10.9, JH–H = 3.0 Hz) and a doublet of ‘triplets’ (ddd) at δ −18.19 (JH–P = 9.4 Hz) (Fig. 1).
As may be expected, the different diastereomers crystallize in non-centrosymmetric space groups (3, 4 and 6: P212121). For complex 5, the polar space group Cc was identified. It was found that this structure contained two different isomers of [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1b)] with opposite chiralities of the diphosphine ligand 1b (cf.Scheme 1), i.e. both S,S-1b and R,R-1b were identified in the structure even though enantiopure S,S-1b had been used in the synthesis of cluster 5. Our explanation for this surprising result is that a small amount of R,R-1b was present in the ligand batch used, and the resultant isomer of 5 co-crystallized with the (majority) product [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-S,S-1b)]. Compound 5 was resynthesized several times and several crystallization attempts were made. A second crystal structure of what appears to be diastereomerically pure [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-S,S-1b)] 5 was obtained (CCDC 1982115). For this structure, the non-centrosymmetric space group P21 was identified, but the quality of the diffraction data was unfortunately so poor that this assignment could not be made with absolute certainty. For this reason, the discussion below relates to the data for [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-S,S-1b)] taken from the crystal structure in the polar space group Cc.
The structures of the two diastereomers based on ligand 1a (3 and 4) are discussed in detail below. The molecules consist of triangular ruthenium frameworks involving Ru–Ru bond lengths that are divided into two distinctive classes, the two hydride-bridged Ru–Ru bonds are “long” [3: Ru(1)–Ru(2) 2.8952(9), Ru(1)–Ru(3) 2.9022(9) Å]; 4: [Ru(1)–Ru(2) 2.915(2), Ru(1)–Ru(3) 2.746(2) Å] and the non-bridged Ru–Ru bond is “short” [3: Ru(2)–Ru(3) 2.7537(8); 4: Ru(2)–Ru(3) 2.894(2) Å]. The triply bridging sulfur atom caps the ruthenium triangle quite symmetrically [Ru–S 2.375(2)–2.368(2) Å for 3, and 2.361(5)–2.358(5) Å for 4]. All the carbonyl ligands are coordinated in a terminal monodentate coordination mode. The diphosphine ligands were found to exclusively coordinate in a bridging fashion, giving rise to nine-membered “dimetallacycles”. As previously observed for related tetraruthenium tetrahydrido clusters,22 the Walphos ligand coordinates in an axial-equatorial mode. In the case of 3, P1 is coordinated in an axial position to Ru1, the metal that is coordinated to both of the hydrides, while P2 is coordinated in an equatorial position to Ru2, which is coordinated by one hydride. For the diastereomer 4, P1 is coordinated in an equatorial position to Ru2, which is coordinated to H1, while P2 is coordinated in an axial position to Ru1, which is coordinated to H1 and H2.
In the molecular structures of the analogous diastereomers based on 1b, 5 and 6, the same difference in the coordination mode of the diphosphine ligand S,S-1b is observed – P1 is coordinated in an axial position to Ru2 which is coordinated to both hydrides, in case of 5, and for 6, P1 is coordinated in an equatorial position to Ru2 which is bridged only with one hydride H1.
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Fig. 4 Molecular structure of [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1h)] 17 (CCDC 1043619†) with thermal ellipsoids drawn at the 50% probability level. C–H hydrogen atoms have been omitted for clarity. Selected bond distances [Å] and angles [°]: Ru1–Ru2 2.9173(8), Ru1–Ru3 2.7359(8), Ru2–Ru3 2.8921(8), Ru1–P1 2.339(19), Ru2–P2 2.3558(19), Ru1–S1 2.359(2), Ru2–S1 2.365(2), Ru3–S1 2.358(2), Ru1–H1 1.7945, Ru2–H2 1.7324, Ru2–H1 1.7347, Ru3–H2 1.7658 (riding on Ru atoms). |
Again, two different distinctive classes of Ru–Ru bond lengths can be observed, the two hydrido-bridged Ru–Ru bonds are “long” [Ru(1)–Ru(2) 2.9276(6), and Ru(2)–Ru(3) 2.9123(6)] and the “shortest” [Ru(1)–Ru(3) 2.7511(6)]. A longer P–P distance is observed for 8 (5.61 Å) with a torsion angle of 87.70, in comparison to 5.48 Å and torsion angle of 83.98 for 4. As expected, the angles in 8 are a bit different than 4, where P1–Ru1–Ru2 = 106.85, P2–Ru2–Ru1 = 118.59 and P2–Ru2–Ru3 = 120.54 in 8, and for 4 it is 103.05, 120.06 and 123.84, respectively.
In case of 17, the shortest Ru–Ru distance is the Ru1–Ru3 edge (2.7359(8) Å), which is not bridged by any hydride, while the two Ru–Ru edges that bridged with two hydrides have slightly symmetric longer distances (2.9173(8) Å for Ru1–Ru2 and 2.8921(8) Å for Ru2–Ru3). The two phosphorus atoms of the coordinated ligand are separated by 5.512 Å with a torsion angle of 87.36(8). The Ru–P bonds are symmetric in lengths (2.3539(19) Å for Ru1–P1 and 2.3558(19) Å for Ru2–P2), and the angles are P1–Ru1–Ru2 = 105.41(5), P2–Ru2–Ru1 = 116.98(5) and P2–Ru2–Ru3 = 121.39(5).
The 1H and 31P NMR data confirmed the difference in the bridging mode of the diphosphine ligand 2a in clusters 18 and 19. For 18, the 1H NMR in the hydride region shows two signals, a doublet of ‘triplets’ at δ −18.29 (dt, JH–P = 36.5, JH–H = 2.5 Hz) and a doublet of doublet of doublets at δ −19.46 (ddd, JH–P = 13.3, JH–P = 7.1, JH–H = 2.5 Hz), while the hydride signals for 19 consist of a doublet of doublet of doublets at δ −17.84 (JH–P = 12.7, JH–P = 9.0, JH–H = 3.6 Hz) and a doublet of doublet of doublets at δ −18.65 (JH–P = 7.5, JH–P = 3.6 Hz). In the 31P NMR spectrum of 18, the P1 signal (cf.Scheme 2) appears as a doublet of doublets, indicating coupling to both hydrides, while the signal for P2 is a doublet. In contrast, the signal for P1 in 19 appears as a doublet while that for P2 is a doublet of doublets. The ES+ mass spectrometric data match with the assigned molecular formula [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-P–P)] (m/z = 1401 [M + H]+) for both 18 and 19. The empirical formulae of clusters 20–22 were confirmed by mass spectrometry, and their structures in solution and the solid state were identified by IR and NMR spectroscopy (cf. Experimental section and ESI†). Due to the high degree of hydride fluxionality exhibited by [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-2d)] 22 at ambient temperature, only one sharp multiplet resonance could be detected for the hydride in the 1H NMR. Variable-temperature (VT) 1H NMR of cluster 22 shows that the fluxionality may be frozen out at 253 K, to reveal the two hydride signals that were expected for the cluster (Fig. 5).
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Fig. 5 Variable-temperature 1H NMR spectra in the hydride region of cluster [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-2d)] 22 recorded over the temperature range 298–233 K (top to bottom). |
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Fig. 6 Molecular structure of [(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-2b)] 20 (CCDC 1043620†) with thermal ellipsoids drawn at the 50% probability level. C–H hydrogen atoms have been omitted for clarity. Selected bond distances [Å] and angles [°]: Ru1–Ru2 2.9272(3), Ru1–Ru3 2.7314(3), Ru2–Ru3 2.8947(3), Ru1–P1 2.3295(8), Ru2–P2 2.3887(7), Ru1–S1 2.3701(8), Ru2–S1 2.3871(8), Ru3–S1 2.3667(7), Ru1–H1 1.82(4), Ru2–H2 1.79(4), Ru2–H1 1.76(4), Ru3–H2 1.86(4). |
The catalysis results for clusters 3–22 are summarized in Table 1. The clusters show moderate catalytic activity in terms of conversion. Furthermore, the enantioselectivities are moderate in comparison to mononuclear catalysts, but relatively high by the standards of cluster-based systems. The enantioselectivites reflect an inherent weakness in the chiral induction effected by clusters – the substrate is likely to bind at a metal site with minimum steric hindrance (vide infra) and the chiral induction effected by bulky chiral ligands is therefore diminished. However, we have observed significantly higher enantioselectivities in other cluster-based systems,21,22,28 and it should be borne in mind that ee's exceeding 40% were unprecedented30,31,34–36 in cluster-based asymmetric hydrogenation before we began our studies. Unfortunately, the separation of the hydrogenated products from the organic phase after using clusters 13 and 16 as catalysts (Table 1, entries 11, 14 and 20, respectively) turned out to be unsuccessful in our hands, so we could not determine the enantiomeric excess for these reactions, albeit with full conversion in case of 16. From Table 1, it is clear that in these catalytic systems the stereochemistry and the bridging mode of the coordinated chiral diphosphine ligands strongly affect the outcome of the hydrogenation reactions, both in terms of conversion and enantioselectivity. In earlier work, we have been able to demonstrate that reversal of enantioselectivity can be effected by reversal of phosphine chirality in cluster-based systems.27 It may be noted that the diastereomeric pairs 3/4, 5/6, 7/8, 9/10, 11/12, and 18/19, within which the chirality of the diphosphine remains the same, but the chirality of the cluster framework changes, all show a reversal in enantioselectivity when the chirality of the cluster framework changes. Furthermore, the pairs 3/5 and 4/6 are diastereomers where the chirality of the cluster framework remains intact while that of the ligand changes (cf.Fig. 2), and for these pairs the enantioselectivity is reversed with reversal in chirality of the ligands. It may thus be concluded that both the cluster framework and the ligand chiralities influence the enantioselectivity for a given cluster. These observations provide compelling evidence for the clusters, or immediate derivatives of the clusters with maintained cluster framework chiralities,37 being the active catalysts.
Entry | Ligand | Catalyst | Conv.a [%] | ee [%] | Config.b |
---|---|---|---|---|---|
a The amount of substrate consumed in the catalytic experiment, assed by 1H NMR spectroscopy, p(H2) = 50 bar, T = 100 °C, solvent = EtOH–toluene 1![]() ![]() |
|||||
1 | 1a | 3 | 49 | 23 | R |
2 | 1a | 4 | 79 | 56 | S |
3 | S,S-1b | 5 | 55 | 24 | S |
4 | S,S-1b | 6 | 64 | 52 | R |
5 | 1c | 7 | 83 | 33 | R |
6 | 1c | 8 | 80 | 21 | S |
7 | 1d | 9 | 94 | 9 | R |
8 | 1d | 10 | 77 | 7 | S |
9 | 1e | 11 | 66 | 17 | S |
10 | 1e | 12 | 7 | 8 | R |
11 | 1f | 13 | 14 | ND | ND |
12 | 1f | 14 | 23 | 48 | S |
13 | 1g | 15 | 47 | 21 | S |
14 | 1g | 16 | 100 | ND | ND |
15 | 1h | 17 | 43 | 13 | R |
16 | 2a | 18 | 88 | 31 | R |
17 | 2a | 19 | 66 | 13 | S |
18 | 2b | 20 | 41 | 17 | S |
19 | 2c | 21 | 68 | 37 | S |
20 | 2d | 22 | 38 | 26 | S |
21 | 1a | 3 + Hg | 51 | 26 | R |
A mechanism involving cluster decomposition during catalysis and regeneration of the intact cluster at the end of the catalytic cycle would be accompanied by formation of both diastereomers,38 even if only one form of the cluster was initially used. However, no trace of such racemization was observed at the end of catalytic cycles employing all clusters. All catalysts were recovered in good yields (≈70%) after a complete catalytic experiment, with one exception: only trace amounts (at best) of catalyst 20 could be recovered. For the other catalysts, the recovery may be considered to be quantitative, when the small amount of catalyst used is taken into account. Furthermore, recyclability was proven for cluster 3 – after completion of the reaction (Table 1, entry 1), the cluster was recovered, purified by TLC and reused under identical experimental conditions. Identical conversion rates and enantiomeric excesses for the two catalytic runs were obtained.
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Scheme 3 Computed catalytic hydrogenation cycle of tiglic acid by cluster 4 to give (S)-2-methylbutyric acid. |
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1a)] 3, yield 32 mg (31%); (Rf = 0.6); IR (ν(CO)/cm−1, cyclohexane) 2066(s), 2046(vs), 2010(vs), 1994(s), 1979(m), 1951(m); 1H NMR (500 MHz, CDCl3) δ −18.13 (ddd, J = 12.4, 11.2, 2.9 Hz, 1H), −18.40 (dt, J = 9.8, 2.9 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 49.75 (d, J = 10.6 Hz), 37.74 (t, J = 9.1 Hz), ES+ MS (m/z): 1464 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1a)] 4, yield 39 mg (38%); (Rf = 0.4); IR (ν(CO)/cm−1, cyclohexane) 2066(s), 2048(vs), 2008(vs), 1995(s), 1982(w), 1951(m); 1H NMR (500 MHz, CDCl3) δ −17.82 (ddd, J = 12.2, 8.8, 3.5 Hz, 1H), −18.63 (ddd, J = 7.5, 3.5 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 48.52 (dd, J = 11.1, 6.6 Hz), 34.41 (d, J = 8.2 Hz). ES+ MS (m/z): 1464 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1b)] 5, yield 29 mg (28%); (Rf = 0.6); IR (ν(CO)/cm−1, cyclohexane) 2066(s), 2048(vs), 2008(vs), 1995(s), 1979(m), 1951(m); 1H NMR (500 MHz, CDCl3) δ −18.12 (ddd, J = 12.6, 11.1, 3.0 Hz, 1H), −18.40 (dt, J = 9.6, 3.1, 3.0 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 48.68 (dd, J = 11.8, 2.0 Hz), 36.60 (t, J = 9.8 Hz). ES+ MS (m/z): 1464 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1b)] 6, yield 40 mg (38%); (Rf = 0.5); IR (ν(CO)/cm−1, cyclohexane) 2066(s), 2046(vs), 2010(vs), 1994(s), 1979(m), 1951(m); 1H NMR (500 MHz, CDCl3) δ −17.84 (ddd, J = 12.4, 9.1, 3.0 Hz, 1H), −18.65 (ddd, J = 7.1, 3.2, 3.0 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 48.59 (dd, J = 11.3, 5.9 Hz), 35.55 (d, J = 8.4 Hz). ES+ MS (m/z): 1464 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1c)] 7, yield 27 mg (32%); (Rf = 0.5); IR (ν(CO)/cm−1, cyclohexane) 2057(s), 2038(vs), 1995(vs), 1984(s), 1965(m), 1932(m); 1H NMR (500 MHz, CDCl3) δ −18.51 (ddd, J = 11.0, 9.7, 3.0 Hz, 1H), −18.64 (dt, J = 11.8, 3.0 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 56.47 (dd, J = 9.6, 5.0 Hz), 37.16 (t, J = 9.7 Hz). ES+ MS (m/z): 1228 [M + Na]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1c)] 8, yield 33 mg (39%); (Rf = 0.4); IR (ν(CO)/cm−1, cyclohexane) 2057(s), 2034(vs), 1998(vs), 1985(s), 1969(m), 1942(m); 1H NMR (500 MHz, CDCl3) δ −18.08 (ddd, J = 10.3, 9.0, 3.0 Hz, 1H), −19.02 (‘m’, ddd, J = not resolved 1H, cf. ESI†); 31P NMR (202 MHz, CDCl3) δ 59.08 (dd, J = 9.6, 6.2 Hz), 35.24 (dd, J = 8.6, 2.7 Hz); ES+ MS (m/z): 1228 [M + Na]+.
[(μ-H)2Ru3(μ3-S)((CO)7(μ-1,2-1d)] 9, yield 26 mg (23%); (Rf = 0.5); IR (ν(CO)/cm−1, cyclohexane) 2063(s), 2044(vs), 2008(vs), 1993(s), 1976(m), 1951(m); 1H NMR (500 MHz, CDCl3) δ −18.09 (dt, J = 9.6, 2.8 Hz, 1H), −18.39 (ddd, J = 12.8, 10.2, 2.9 1H); 31P NMR (202 MHz, CDCl3) δ 48.89 (d, J = 11.8 Hz), 35.65 (t, J = 9.6 Hz). ES+ MS (m/z): 1580 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1d)] 10, yield 37 mg (33%); (Rf = 0.4); IR (ν(CO)/cm−1, cyclohexane) 2064(s), 2047(vs), 2006(vs), 1992(s), 1980(m), 1949(m); 1H NMR (500 MHz, CDCl3) δ −17.82 (ddd, J = 12.2, 8.8, 3.4 Hz, 1H), −18.63 (ddd, J = 7.3, 5.0, 3.4 Hz 1H); 31P NMR (202 MHz, CDCl3) δ 48.49 (dd, J = 11.3, 6.5 Hz), 34.37 (d, J = 8.4 Hz). ES+ MS (m/z): 1603 [M + Na]+.
[(μ-H)2Ru3(μ3-S)((CO)7(μ-1,2-1e)] 11, yield 29 mg (33%); (Rf = 0.6); IR (ν(CO)/cm−1, cyclohexane) 2059(s), 2042(vs), 2000(vs), 1985(m), 1970(w), 1939(w); 1H NMR (500 MHz, CDCl3) δ −17.61 (td, J = 9.8, 3.0 Hz, 1H), −18.17 (‘m’, ddd, J = not resolved 1H, cf. ESI†); 31P NMR (202 MHz, CDCl3) δ 43.88 (dd, J = 9.5, 6.2 Hz), 35.63 (dd, J = 8.9, 2.4 Hz). ES+ MS (m/z): 1248 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1e)] 12, yield 38 mg (43%); (Rf = 0.5); IR (ν(CO)/cm−1, cyclohexane) 2059(s), 2042(vs), 2000(vs), 1985(m), 1970(w), 1939(w); 1H NMR (500 MHz, CDCl3) δ −17.61 (ddd, J = 10.0, 3.0 Hz, 1H), −18.16 (‘m’, ddd, J = not resolved 1H, cf. ESI†); 31P NMR (202 MHz, CDCl3 49.09 (dd, J = 10.2, 1.7 Hz), 36.60 (t, J = 9.5 Hz). ES+ MS (m/z): 1248 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1f)] 13, yield 33 mg (31%); (Rf = 0.6); IR (ν(CO)/cm−1, cyclohexane) 2064(vs), 2047(m), 2038(s), 2003(vs), 1992(s), 1983(m), 1976(w), 1945(m), 1938(w); 1H NMR (500 MHz, CDCl3) δ −18.14 (ddd, J = 13.8, 4.5, 2.5 Hz, 1H), −18.31 (‘m’, ddd, J = not resolved 1H, cf. ESI†); 31P NMR (202 MHz, CDCl3) δ 52.62 (m), 49.49 (dd, J = 11.1, 1.3 Hz). ES+ MS (m/z): 1502 [M + Na]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1f)] 14, yield 45 mg (43%); (Rf = 0.6); IR (ν(CO)/cm−1, cyclohexane) 2064(vs), 2047(s), 2037(m), 2003(vs), 1992(m), 1977(w), 1946(w), 1939(w); 1H NMR (500 MHz, CDCl3) δ −18.17 (ddd, J = 11.9, 7.0, 3.6 Hz, 1H), −18.83 (ddd, J = 4.8, 3.6 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 48.49 (dd, J = 11.3, 6.5 Hz), 34.37 (d, J = 8.4 Hz). ES+ MS (m/z): 1479 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1g)] 15, yield 36 mg (39%); (Rf = 0.6); IR (ν(CO)/cm−1, cyclohexane) 2059(s), 2042(vs), 1999(vs), 1983(s), 1970(m), 1938(m); 1H NMR (500 MHz, CDCl3) δ −17.69 (td, J = 9.7, 3.0 Hz, 1H), −18.25 (dt, J = 6.4, 3.0 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 44.05 (dd, J = 9.5, 6.4 Hz), 36.51 (dd, J = 8.8, 2.6 Hz). ES+ MS (m/z): 1327 [M + Na]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1g)] 16, yield 48 mg (52%); (Rf = 0.5); IR (ν(CO)/cm−1, cyclohexane) 2057(s), 2039(s), 2003(s), 2003(vs), 1984(s), 1967(m), 1940(w); 1H NMR (500 MHz, CDCl3) δ −18.16 (m, 2H); (500 MHz, acetone-d6, 253 K) δ −18.12 (dt, J = 11.0, 3.0 Hz, 1H), −18.19 (‘m’, ddd, J = not resolved 1H, cf. ESI†); 31P NMR (202 MHz, CDCl3) δ 49.32 (dd, J = 7.1, 5.1 Hz), 38.10 (t, J = 9.6 Hz). ES+ MS (m/z): 1305 [M + H]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-1h)] 17, yield 36 mg (42%); (Rf = 0.5); IR (ν(CO)/cm−1, cyclohexane) 2060(s), 2043(vs), 2001(vs), 1986(s), 1972(m), 1941(m); 1H NMR (500 MHz, CDCl3) δ −17.75 (ddd, J = 10.1, 9.1, 3 Hz, 1H), −18.47 (dt, J = 6.1, 3.0 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 44.95 (dd, J = 9.9, 5.7 Hz), 35.39 (dd, J = 9.1, 1.8 Hz). ES+ MS (m/z): 1191 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-2a)] 18, yield 14 mg (14%); (Rf = 0.6): IR (ν(CO)/cm−1, cyclohexane) 2066(vs), 2036(s), 2005(vs), 1996(s), 1987(m), 1951(m); 1H NMR (500 MHz, CDCl3) δ −18.29 (dt, J = 36.5, 2.5 Hz, 1H), −19.46 (ddd, J = 13.3, 7.1, 2.5 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 56.94 (dd, J = 34.7, 5.9 Hz), 25.71 (d, J = 11.8 Hz). ES+ MS (m/z): 1400 [M]+.
[(μ-H)2Ru3(μ3-S)(CO)7(μ-1,2-2a)] 19, yield 25 mg (25%); (Rf = 0.5): IR (ν(CO)/cm−1, cyclohexane) 2064(vs), 2039(ms, 2032(w), 2007(vs), 1993(m), 1980(m), 1967(w), 1951(w); 1H NMR (500 MHz, CDCl3) δ −17.84 (ddd, J = 12.7, 9.0, 3.6 Hz, 1H), −18.65 (ddd, J = 7.5, 3.6 Hz, 1H); 31P NMR (202 MHz, CDCl3) δ 48.59 (dd, J = 10.4, 4.7 Hz), 35.54 (d, J = 8.4 Hz). ES+ MS (m/z): 1400 [M]+.
[(μ-H)2Ru3(μ3-S)((CO)7(μ-1,2-2b)] 20, yield 33 mg (44%); (Rf = 0.6): IR (ν(CO)/cm−1, cyclohexane) 2059(vs), 2031(s), 2016(vs), 1998(vs), 1970(w), 1941(m); 1H NMR (500 MHz, CDCl3) δ −18.31 (dd, J = 32.6, 2.0 Hz, 1H), −19.13 (‘t’, ddd, J = not resolved 1H, cf. ESI†); 31P NMR (202 MHz, CDCl3) δ 57.29 (m), 25.41 (m). ES+ MS (m/z): 1174 [M]+.
[(μ-H)2Ru3(μ3-S)((CO)7(μ-1,2-2c)] 21, yield 44 mg (59%); (Rf = 0.6): IR (ν(CO)/cm−1, cyclohexane) 2057(vs), 2033(s), 1994(vs), 1980(m), 1969(w), 1926(w); 1H NMR (500 MHz, CDCl3) δ −18.77 (br, 1H), −19.09 (m, 1H); 31P NMR (202 MHz, CDCl3) δ 65.05 (m), 35.49 (m). ES+ MS (m/z): 1163 [M + Na]+.
[(μ-H)2Ru3(μ3-S)((CO)7(μ-1,2-2d)] 22, yield, 36 mg (48%); (Rf = 0.6): IR (ν(CO)/cm−1, cyclohexane) 2059(vs), 2038(s), 2001(vs), 1988(m), 1970(w), 1943(w); 1H NMR (500 MHz, CDCl3) δ −18.04 (m, 1H); (500 MHz, acetone-d6, 253 K) δ −17.93 (ddd, J = 11.8, 2.0 Hz, 1H), −18.10 (ddd, J = 23.4, 10.4, 2.0 Hz); 31P NMR (202 MHz, CDCl3) δ 41.66 (m), 20.65 (m). ES+ MS (m/z): 1172 [M]+.
Precautions were taken to avoid the possibility of catalytic activity due to contamination of the reaction vessel or the magnetic stir bars: the reaction vessel and the magnetic stir bars were washed with acetone and rinsed with dichloromethane, followed by a careful visual examination. Whenever the magnetic stir bars appeared to be contaminated, they were discarded.
To separate the carboxylic acid from the cluster, the reaction residue was dissolved in 10 mL of diethyl ether and the carboxylic acid was extracted with aqueous sodium hydroxide solution (1 M, 3 × 10 mL) and washed with diethyl ether (3 × 5 mL), leaving the cluster in the organic solvent. The carboxylate was protonated with sulfuric acid and extracted with diethyl ether (3 × 10 mL), washed with water (2 × 5 mL) and dried over magnesium sulfate. Filtration, followed by evaporation of the ether under vacuum, yielded the carboxylic acid quantitatively. The original ether phase, from which the carboxylic acid was extracted, was concentrated under vacuum to recover the remaining cluster. In certain cases, where ester formation was obtained during the catalytic experiment, the recovered catalyst was dissolved in a minimum quantity of dichloromethane and the products were separated by preparative TLC, eluting with dichloromethane/petroleum ether (1:
2). Usually 60–70% of the cluster was recovered after a catalytic experiment and it was analyzed by IR and NMR spectroscopies. The enantiomeric excess of the product was detected by derivatizing 2-methylbutyric acid with S-methyl mandelate and analyzing the diastereomeric product mixture by NMR, as fully described by Tyrrell et al.46 It was found that flash chromatography of the final products was not necessary.
All reported geometries were fully optimized, and the analytical Hessian was evaluated at each stationary point to determine whether the geometry was an energy minimum (no negative eigenvalues) or a transition structure (one negative eigenvalue). Unscaled vibrational frequencies were used to make zero-point and thermal corrections to the electronic energies. The resulting free energies are reported in kcal mol−1 relative to the specified standard. Standard state corrections were applied to all species to convert concentrations from 1 atm to 1 M according to the treatise of Cramer.51 Internal reaction coordinate (IRC) calculations were performed in order to establish the reactant and product species associated with each transition-state structure. The geometry-optimized structures have been drawn with the JIMP2 molecular visualization and manipulation program.52
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, crystallographic details, FT-IR and NMR spectra. Atomic coordinates of all optimized stationary points and transition states. CIF files for complexes 3 and 4. CCDC 993899, 993900 and 1046316–1046320. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt04799a |
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