Carmina
Alfonso
a,
Marta
Feliz
b,
Vicent S.
Safont
a and
Rosa
Llusar
*a
aDepartament de Química Física i Analítica, Universitat Jaume I, Av. Sos Baynat s/n, 12071 Castelló, Spain
bInstituto de Tecnología Química, CSIC-UPV, Avda. de los Naranjos, s/n, 46022, Valencia, Spain. E-mail: rosa.llusar@uji.es; Fax: +34 964 728066; Tel: +34 964 728086
First published on 11th April 2016
A diastereoselective synthesis of proline containing aminophosphino cubane-type Mo3S4 clusters, (P)-[Mo3S4Cl3((1S,2R)-PPro)3]Cl ((P)-[Mo-(SN,RC)]Cl) and (P)-[Mo3S4Cl3((1S,2S)-PPro)3]Cl ((P)-[Mo-(SN,SC)]Cl), has been achieved in high yields by reacting the corresponding enantiomerically pure PPro ((R)- and (S)-2-[(diphenylphosphino)methyl]pyrrolidine) ligands with the Mo3S4Cl4(PPh3)3(H2O)2 complex. Circular dichroism, nuclear magnetic resonance and X-ray techniques confirm that the (P)-[Mo-(SN,RC)]Cl and (P)-[Mo-(SN,SC)]Cl cluster cations are diastereoisomers which combine three sources of stereogenicity provided by the cluster framework, one carbon atom of the aminophosphine ligand and the nitrogen stereogenic center. The higher stability of the (P)-[Mo-(SN,SC)]+ cation is due to stabilizing vicinal Cl⋯HN interactions as well as due to the cis-fused conformation of the bicyclic system formed upon coordination of the aminophosphine ligand.
Within the last decade, we have developed a stereoselective synthetic route for the synthesis of complexes of the general formula [Mo3S4Cl3(diphosphino)3]+ by reacting polymeric {Mo3S7Cl4}n phases with enantiopure bis(phosphines), namely ((R,R)-Me-BPE ((+)-1,2-bis[(2R,5R)-2,5-(dimethylphospholan-1-yl)]ethane) or (S,S)-Me-BPE ((−)-1,2-bis[(2S,5S)-2,5-(dimethylphospholan-1-yl)]ethane)).8 The (R,R)-diphosphine invariably leads to (P)-[Mo3S4Cl3((R,R)-Me-BPE)3]+ while (M)-[Mo3S4Cl3((S,S)-Me-BPE)3]+ is isolated as the only product from the reaction with the (S,S)-diphosphine, both compounds being enantiomers of each other. This synthetic procedure was further extended to tungsten and selenium as well as to other optically pure diphosphines.9–11 All these complexes combine the cluster framework chirality with that of the diphosphine ligand. In addition, these trimetallic Mo3S4 compounds can act as metalloligands for a second transition metal to afford catalytically active chiral Mo3S4Cu clusters with moderate enantioselection in cyclopropanation reactions while preserving the absolute configuration of the cluster framework.8
Motivated by the important role of the NH group in catalysis, we decided to explore synthetic routes aimed to obtain optically pure aminophosphine derivatives of the Mo3S4 cluster core.12 For that, we have chosen commercially available chiral aminophosphines derived from (R) and (S)-proline, whose structures are depicted in Fig. 2. The reaction outcome invariably leads to (P)-diastereoisomers unlike the results obtained with diphosphines. To rationalize this unexpected result, theoretical calculations on the relative energies among different stereoisomers have been undertaken.
Fig. 2 Structure of (R)-2-[(diphenylphosphino)methyl]pyrrolidine ((R)-PPro, left) and (S)-2-[(diphenylphosphino)methyl]pyrrolidine ((S)-PPro, right). |
Mo3S4Cl4(PPh3)3(H2O)2 + 3PPro → [Mo3S4Cl3(PPro)3]Cl + 3PPh3 + 2H2O | (1) |
The reaction products with both (R)-PPro, and (S)-PPro show one peak centered at 1330 m/z associated with the [Mo3S4Cl3(PPro)3]+ cation on the basis of the m/z value and its characteristic isotopic pattern. However, there are small differences between the chemical shifts of the unique peak observed in the 31P{1H} NMR spectra of the [Mo-(SN,RC)]+ (32.3 ppm) and [Mo-(SN,SC)]+ (32.1 ppm) complexes and those assigned to three equivalent phosphorus atoms. These differences indicate that these two cluster cations are not enantiomers, a conclusion that was supported by circular dichroism (CD) spectroscopy. Fig. 3 shows the CD spectra of sample solutions of the chlorido salts of the [Mo-(SN,RC)]+ and [Mo-(SN,SC)]+ cations in dichloromethane, in which differences in the intensities and signs of the signals are appreciated. The characteristic bands of the [Mo-(SN,RC)]+ cluster appear at 260 and 301 nm for +51.7 and +14.7 mdeg respectively, while two signals of opposite signs at 254 nm for −46 mdeg and at 313 nm for −3.2 mdeg are found in the CD spectrum of [Mo-(SN,SC)]+ so the two bands must involve proline (R or S) based ligand orbitals.8 Interestingly, the observed band around 255–260 nm shows a red shift with regard to that of the free aminophosphine ligand (220 nm) and opposite signs. The most intense bands registered for both complexes below 250 nm have the same sign and can be tentatively assigned to transitions between orbitals with a marked contribution from the S nitrogen stereogenic center.14,15 The specific rotation is approximately one order of magnitude lower than that of the analogous diphosphino complexes, which show values from −400 to +450 mdeg.8
The absolute configuration of the [Mo-(SN,RC)]+ and [Mo-(SN,SC)]+ cations was determined by X-ray crystallography from their tetrafluoroborate salts. Both structures are solved in the non-centrosymmetric P213 cubic group with Flack parameters close to zero, and the Mo3S4 cluster core located on a C3 axis passing through the capping sulphur, the absolute configuration being P for both [Mo-(SN,RC)]+ and [Mo-(SN,SC)]+ cations. An ORTEP representation of the two cations is depicted in Fig. 4. The stereochemistry of the PPro aminophosphine ligand as R for (P)-[Mo-(SN,RC)]+ and S for (P)-[Mo-(SN,SC)]+ could be established with no ambiguity. X-ray confirms that these (P)-[Mo-(SN,RC)]+ and (P)-[Mo-(SN,SC)]+ cluster cations are diastereoisomers that combine the chirality (R or S) of the PPro aminophosphine ligand with that of the cluster framework. In addition, the pyrrolidine ring can adopt two conformations upon N-coordination that allow the stereogenic nitrogen atom to be R or S (S in our case) and the concomitant introduction of a third source of stereogenicity.
Fig. 4 ORTEP diagrams of (P)-[Mo-(SN,RC)]+ (left) and (P)-[Mo-(SN,SC)]+ (right) with the atom numbering scheme. Hydrogen atoms are omitted for clarity. |
Table 1 lists the most relevant bond distances for (P)-[Mo-(SN,RC)]+ and (P)-[Mo-(SN,SC)]+ together with those of a closely related aminophosphino complex. The interatomic Mo–Mo and Mo–S distances follow the tendencies observed for other Mo3S4 cluster compounds.10,16,17 Each molybdenum atom in (P)-[Mo-(SN,RC)]+ and (P)-[Mo-(SN,SC)]+ is in a pseudooctahedral environment defined by three sulphur atoms, one chlorine atom, and the nitrogen and phosphorus atoms of the aminophosphine ligand, where the phosphorus atom is located trans to the capping sulphur. Such preferential spatial disposition of the aminophosphine ligand when coordinated to cuboidal Mo3S4 cluster units is not unprecedented, and considerably diminishes the number of potential stereoisomers, as also observed in W3S4 clusters.7 Coordination of the chiral PPro ligand to the metal results in the formation of a stereogenic nitrogen center incorporated into a [3.3.0] bicyclic system (see Fig. 4 and 5a). This bicyclic system adopts a cis-fused conformation in the (P)-[Mo-(SN,SC)]+ complex, which is known to be thermodynamically more stable than the trans-fused structure by analogy to bicyclooctanes.18 The calculated dihedral angles (C5–N–C2–C1 and C3–C2–N–Mo, see Table 1) confirm the cis conformation of the bicyclic system with values of 96.860° and 14.402° respectively. In the case of the (P)-[Mo-(SN,RC)]+ complex, both rings lie nearly in the same plane with dihedral angles of 11.269° and 179.416°. As a result, a selective formation of an S configuration at each stereogenic nitrogen center takes place for both (P)-[Mo-(SN,SC)]+ and (P)-[Mo-(SN,RC)]+ diastereoisomers. Note that once the stereochemistry of a nitrogen center is defined, that of the other two nitrogen atoms is equally fixed. The interatomic distances in both clusters are similar to those observed for [Mo3S4Cl3(edpp)3]BPh4 (edpp = (2-aminoethyl)diphenylphosphine), as can be seen in Table 1.6 The small deviations among Mo–(μ-S) bond distances in the three complexes suggest a similar trans influence of the nitrogen vs. the chlorine atoms in these compounds in contrast with the differences found in the corresponding [Mo3S4Cl3(diphosphine)3]+ cluster cations.
Distancea (Å) | (P)-[Mo-(SN,RC)]BF4 | (P)-[Mo-(SN,SC)]BF4 | [Mo3S4Cl3(edpp)3]BPh4b |
---|---|---|---|
a Standard deviations for averaged values are given in brackets. b Data taken from ref. 6. c Distance trans to the Mo–N bond. d Distance trans to the Mo–Cl bond. | |||
Mo–Mo | 2.752(1) | 2.757(1) | 2.7463[7] |
Mo–(μ3-S) | 2.364(3) | 2.353(3) | 2.3611[13] |
Mo–(μ-S)c | 2.277(2) | 2.293(2) | 2.2863[12] |
Mo–(μ-S)d | 2.297(2) | 2.294(2) | 2.2960[12] |
Mo–P | 2.547(2) | 2.550(2) | 2.5414[14] |
Mo–N | 2.300(7) | 2.289(7) | 2.2733[4] |
Mo–Cl | 2.461(2) | 2.484(2) | 2.4625[15] |
Dihedral C5–N–C2–C1 | 11.269° | 96.860° | — |
Dihedral C3–C2–N–Mo | 179.416° | 14.402° | — |
The chlorine and nitrogen atoms in (P)-[Mo-(SN,RC)]+ and (P)-[Mo-(SN,SC)]+ are found on the same side of a trimetallic plane, as emphasized in Fig. 5b, allowing multiple Cl⋯HN interactions. Short Cl⋯HN interactions are observed in (P)-[Mo-(SN,SC)]+ between the chlorine atom coordinated to one metal and the hydrogen atom of the amino group coordinated to the adjacent metal, resulting in a vicinal Cl⋯HN bond length of 2.379 Å (see Fig. 5a) vs. the 2.941 Å found in (P)-[Mo-(SN,RC)]+. Similar values, ranging from 2.436 to 2.995 Å, are reported for the aminophosphine [Mo3S4Cl3(edpp)3]+ cluster analogue.6 These vicinal Cl⋯HN interactions are expected to confer to the (P)-[Mo-(SN,SC)]+ cation an additional stability. On the other hand, the Cl⋯N distances associated with the geminal Cl⋯HN interactions, in which the NH and the Cl belong to the coordination sphere of the same metal centre of the cluster unit, (3.055 Å for (P)-[Mo-(SN,RC)]+, and 3.170 Å for (P)-[Mo-(SN,SC)]+) are slightly longer than the corresponding distance reported for the [Mo3S4Cl3(edpp)3]+ cation (3.021 Å).6 This confirms the similar chelating mode for both edpp and (R)- or (S)-PPro aminophosphines.
To summarize, the reaction between Mo3S4Cl4(PPh)3(H2O)2 and (R)-PPro or (S)-PPro is diastereoselective towards the (P)-Mo3S4 configuration with the preservation of the stereochemistry of the aminophosphine and formation of an S stereogenic nitrogen center. This result is unexpected based on our previous observations on diphosphino Mo3S4 clusters that showed the stereoselective formation of the (P)-M3Q4 (M = Mo, W; Q = S, Se) configuration upon coordination of the enantiomerically pure (R,R)-diphosphine, and the (M)-M3Q4 configuration when the (S,S)-ligand was reacted instead. In both cases, the chirality of the diphosphine was preserved.8–11 In order to explain the preferential diastereoselective formation of [Mo3S4Cl3(PPro)3]+ complexes, DFT calculations were performed to study the differences between relative energies among the four possible isomers.
Distances (Å) | (P)-[Mo-(SN,RC)] + | (P)-[Mo-(SN,SC)] + | (M)-[Mo-(SN,RC)] + | (M)-[Mo-(SN,SC)] + |
---|---|---|---|---|
a Distance trans to the Mo–N bond. b Distance trans to the Mo–Cl bond. c Relative to (P)-[Mo-(SN,SC)]+. | ||||
Mo–Mo | 2.807 | 2.805 | 2.834 | 2.845 |
Mo–(μ3-S) | 2.438 | 2.443 | 2.461 | 2.449 |
Mo–(μ-S)a | 2.380 | 2.387 | 2.376 | 2.376 |
Mo–(μ-S)b | 2.379 | 2.367 | 2.356 | 2.371 |
Mo–P | 2.661 | 2.631 | 2.673 | 2.633 |
Mo–N | 2.289 | 2.291 | 2.297 | 2.362 |
Mo–Cl | 2.550 | 2.570 | 2.600 | 2.544 |
Dihedral C5–N–C2–C1 | 8.770° | 95.723° | 24.866° | 81.800° |
Dihedral C3–C2–N–Mo | 179.573° | 14.052° | 6.480° | 4.885° |
Energies (kcal mol−1)c | 4.8 | 0.0 | 20.2 | 20.0 |
The calculated bond distances listed in Table 2 show a good agreement with the experimental data for the (P)-clusters (see Table 1), whereas the longer Mo–Mo and Mo–(μ3-S) distances for (M)-[Mo-(SN,RC)]+ and (M)-[Mo-(SN,SC)]+ suggest a slight distortion of the Mo3S4 cluster core for these theoretically calculated species. The calculated values for the dihedral angles that describe the conformation of the bicyclic system in the ligand are also in good agreement with the experimental data, with differences ranging less than three degrees. Therefore, both the experimental as well as the theoretical studies establish that the (P)-[Mo-(SN,SC)]+ complex exhibits a cis-fused conformation of the bicyclic system that confers additional stabilization to the system. In addition, the stabilizing effect of the vicinal Cl⋯HN interaction can also be observed in the (P)-[Mo-(SN,SC)]+ calculated structure, that shows an interatomic Cl⋯H distance of only ca. 2.32 Å.
A similar Cl⋯HN interaction can also be noticed in the calculated (M)-[Mo-(SN,RC)]+ species, with a Cl⋯H distance of 2.255 Å, although in this case the interaction takes place between the NH and the Cl belonging to the coordination sphere of the same metal centre of the cluster unit. In order to gain a deeper insight into the nature of such Cl⋯HN interactions, we have performed Natural Bond Order (NBO) and topological Electron Localization Function (ELF) analyses. The bonding character of the shortest Cl⋯HN interactions for (M)-[Mo-(SN,RC)]+ and (P)-[Mo-(SN,SC)]+ has been confirmed by their higher bond orders (0.043 and 0.052, respectively) in comparison with those calculated for geminal and vicinal Cl⋯HN interactions found for the respective (P)-[Mo-(SN,RC)]+ and (M)-[Mo-(SN,SC)]+ isomers (bond orders of ca. 0.003). As can be seen, the Cl–H bond order calculated for the (M)-[Mo-(SN,RC)]+ species is ca. 17% lower than the bond order calculated for (P)-[Mo-(SN,SC)]+, which qualitatively agrees with the calculated stabilities: the (P)-[Mo-(SN,SC)]+ species has a higher bond order and is more stable than the (M)-[Mo-(SN,RC)]+ cation.
On the other hand, the ELF analysis of the Cl⋯HN molecule fragment shows the disynaptic V(N–H) basin accounting for the covalent N–H bond, and two monosynaptic V(Cl) basins, one roughly oriented to the hydrogen atom and the other to the molybdenum atom. There is no disynaptic V(Cl–H) basin, as expected due to the ionic character of this Cl⋯H interaction. The population of the V(Cl) basin pointing to the H atom and therefore involved in the Cl⋯HN interaction is higher for the (P)-[Mo-(SN,SC)]+ structure (3.91e−) than the one found in the (M)-[Mo-(SN,RC)]+ case (3.77e−). This indicates a more electronic contribution of the V(Cl) to the vicinal Cl⋯HN interaction in (P)-[Mo-(SN,SC)]+ as compared with the geminal Cl⋯HN in (M)-[Mo-(SN,RC)]+, which supports from a topological point of view the higher stabilizing effect of such an interaction found for the (P)-[Mo-(SN,SC)]+ structure.
Therefore, in spite of the similar Cl⋯HN distance calculated either for the vicinal interaction in (P)-[Mo-(SN,SC)]+ or for the geminal interaction in (M)-[Mo-(SN,RC)]+, both the NBO as well as the ELF analysis suggest a stronger interaction in the vicinal case, thus contributing to its higher stability.
31P {1H} NMR (121 MHz, CD2Cl2) δ (ppm): 32.3 (s, 3P). ESI-MS (20 V, CH3CN) m/z: 1330 (M+).
31P {1H} NMR (121 MHz, CD2Cl2) δ (ppm): 32.1 (s, 3P). ESI-MS (20 V, CH3CN) m/z: 1330 (M+).
Crystal data for (P)-[Mo-(SN,SC)]BF4·(C7H8): C51H60BCl3F4Mo3N3P3S4·(C7H8), M = 1509.27, cubic, space group P213 (no. 198), a = b = c = 19.5506 (3) Å, α = β = γ = 90.00°, V = 7472.7 (19) Å3, T = 200.00 (14) K, Z = 4, μ = 0.832 mm−1. Reflections (collected/unique): 28631/6384 (Rint = 0.0471). The final refinement converged with R1 = 0.0694 and wR2 = 0.1184 for all reflections, GOF = 1.164, max/min residual electron density 0.80/−0.53 e Å−3. Flack parameter: −0.03 (2). Anisotropic displacement parameters were refined for all non-H atoms.
Footnote |
† Electronic supplementary information (ESI) available: Optimized electronic energies and Cartesian coordinates. CCDC 1455624 ((P)-[Mo3S4Cl3((1S,2R)-PPro)3]BF4) and 1455733 ((P)-[Mo3S4Cl3((1S,2S)-PPro)3]BF4). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00755d |
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