Article pubs.acs.org/Organometallics
Synthesis of Low-Valent Nickel Complexes in Aqueous Media, Mechanistic Insights, and Selected Applications Illán Morales-Becerril, Marcos Flores-Á lamo, Adrián Tlahuext-Aca, Alma Arévalo, and Juventino J. García* Facultad de Química, Universidad Nacional Autónoma de México, Circuito Interior, Ciudad Universitaria, México City, 04510, México S Supporting Information *
ABSTRACT: The synthesis of nickel(0) complexes usually requires the employment of strong reducing agents, including powerful hydride donors such as LiHBEt3, LiAlH4, and DIBAL-H. Herein, we have reduced the Ni(II) complex [(dippe)NiCl2] (1a) at room temperature by using KOH in aqueous media to yield the low-valent complex [Ni(dippe)2] (4a) as the main product, along with the formation of dippeO2. In order to gain some insight into the reaction mechanism, a series of intermediates were isolated and characterized at different reaction stages. As a result, both Ni(II) hydroxobridge (2b) and hydride (3b) complexes were identified as key intermediates. Additionally, the use of water as a hydrogen source in nickel-mediated processes was also investigated and successfully applied to the hydrodefluorination, hydrodesulfurization, and hydrogenation of selected organic substrates.
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catalytic processes.16 For this reason, many hydride systems have emerged as model precursors obtained by a series of pincer-type ligands.17 These complexes have been demonstrated to be suitable for reaching low-valent products. However, their synthesis still requires the same powerful hydride sources as the commonly employed Ni(0) methodologies. Thus, novel synthetic approaches for the synthesis of lowvalent nickel complexes are greatly needed to reach greener procedures that involve the use of water as reaction media and avoid strong hydride donors. Herein, we report the use of KOH in aqueous media as an alternative for the ultimate reduction of the Ni(II) precursor [(dippe)NiCl2] (dippe = 1,2-bis(diisopropylphosphino)ethane). We also provide a few mechanistic insights into this reaction and demonstrate some selected applications in the hydrogenation and activation of C−F and C−S bonds, in nickel-mediated processes using water as the hydrogen source.
INTRODUCTION The use of nickel compounds in a variety of applications has gained increasing attention in the past few years. Particularly low-valent nickel complexes have been found to be useful in catalytic applications, including cross-coupling reactions,1 carboxylations,2 hydroaminations,3 hydrodefluorinations,4 hydrodesulfurizations,5 cycloadditions,6 and C−H bond activations.7 Not only the less forcing reaction conditions but also the capacity of activating inert substrates have positioned nickel complexes as powerful tools in organic synthesis8 compared to the other more expensive 10-group metals. However, its use as a routine option has not been extended due to the lack of mild and environmentally friendly synthetic methods. Low-valent nickel complexes are commonly synthesized under an inert atmosphere by reaction with strong reducing agents, including the powerful hydride donors LiHBEt3,9 LiAlH4,10 and [(iBu)2Al(μ-H)]2.11 These reaction conditions are some of the limitations faced by new green methodologies. Since the discovery of the first Ni(0) complex, [Ni(CO)4], relatively few new synthetic approaches have been developed for obtaining Ni(0) species under mild reaction conditions.12 In several cases, auxiliary agents such as phosphines and alcohols are oxidized, allowing a redox process in the presence of a base and “wet solvents”.13 These reaction conditions have become attractive for a variety of catalytic applications and do not require the use of a starting air-sensitive Ni(0) complex.14 However, to the best of our knowledge, virtually no evidence nor mechanistic insights have been provided to explain the overall reduction. Since theoretical studies have invoked hydride intermediates as Ni(0) precursors,15 nickel−hydride complexes have received increasing attention as key intermediates for a wide series of © XXXX American Chemical Society
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RESULTS AND DISCUSSION Synthesis of [Ni(dippe)2]. Inspired by a seminal report by Pearson et al.,18 we proposed the feasible reduction of the complex [(dippe)NiCl2] (1a) in the presence of a base and aqueous media. Recently, Beletskaya and co-workers demonstrated that Ni(0) species can be generated in situ from Ni(II) precursors along with the phosphine oxidation in the presence of water.13b Therefore, we decided to generate low-valent nickel complexes on the basis of the dippeO2 formation in aqueous media to avoid a large excess of KOH. After a set of different Received: July 25, 2014
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dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
Scheme 1. Formation of [Ni(dippe)2]
Figure 1. 31P{1H} NMR monitoring for the [(dippe)NiCl2]/KOH reaction mixture in D2O.
in D2O are shown in Figure 1. At the beginning of the reaction, a narrow singlet labeled (2a) located at 85.0 ppm was slowly transformed into a complex pattern consisting of four phosphorus signals labeled as (3a) (vide infra); the relative ratios of (2a) vs (3a) determined by 31P{1H} NMR are (3.4:1); (1.5:3.6), and (0.7:6.0) at 1, 3, and 7 days, respectively. At this point, it is important to underline that, in Figure 1, only the D2O soluble compounds are observed; consequently, dippeO2 and (4a) cannot be observed due to their null solubility in water. Instead, it yields a final insoluble brown product clearly observed after 7 days identified as (4a). Therefore, if vacuum was not applied, the reaction time to produce (4a) was substantially increased. The intermediate (2a) was assigned to the complex [(dippe)Ni(μ-OH)]2(OH)2 on the basis of the isolation of a yellow precipitate (2b) produced by ionic exchange process adding stoichiometric amounts of [(nBu4N)(PF6)], as shown in Scheme 2. Compound (2b) readily crystallized by evaporation
reaction conditions, we found that 2 equiv of carefully purified KOH19 is necessary for the complete consumption in 15 min of the otherwise insoluble dichloride complex (1a). An orange homogeneous aqueous solution was obtained at room temperature after stirring for a couple of minutes. After evaporation under vacuum, a reddish-brown oily product was obtained. This residue was readily soluble in organic solvents, but insoluble in water, and showed air-sensitive properties (Scheme 1). After the workup, the nickel(0) complex [Ni(dippe)2] (4a) was isolated and characterized by single-crystal X-ray diffraction analysis. It was then compared with the previous report by Vicic and Jones.20 Also the corresponding byproducts dippeO2, Ni(OH)2, and KCl were identified (see the Supporting Information). In addition, if the reaction was carried out in the absence of KOH, none of the products can be obtained. Instead, the starting dichloride complex (1a) was recovered, which makes evident the use of a base as the key reagent for the nickel reduction. Considering the reaction depicted in Scheme 1, a stoichiometric amount of KOH is required to reduce the Ni(II) complex through the phosphine oxidation. If off-theshelf KOH was used, the yield was dramatically reduced to 40% for the nickel product (4a) after workup, due to the formation of Ni−carbonate complexes.21 Pure KOH allowed complex (4a) to be prepared as the only Ni(0) product. This represents an alternative methodology for the synthesis of low-valent nickel complexes in water. However, at this point, not only the KOH participation but also the mechanistic pathway for the reaction remained unknown. In order to investigate further, we decided to monitor the reaction to identify the involved intermediates by 31P{1H} NMR before the water was removed. A monitoring of the reaction was placed at room temperature in a sealed NMR tube under argon for several days to allow detection of some of its key intermediates. Relevant 31P{1H} NMR spectra of the reaction mixture at different reaction times
Scheme 2. Ionic Exchange Process from (2a) to (2b)
of an acetone solution, and allowed full characterization from a single-crystal XRD analysis. The corresponding ORTEP representation of complex [(dippe)Ni(μ-OH)]2(PF6)2 (2b) is depicted in Figure 2. The resulting Ni(II) complex (2b) displays a square-planar structure in a cis-P2-O2 arrangement with averaged (Ni−O), (Ni−P), and (Ni−Ni) bond distances of 1.893, 2.153, and 2.939 Å, respectively. All of these parameters are similar to other [L2Ni(μ-OH)]2 motifs,22 including the closely related [(dippe)Ni(μ-OH)]2(OTf)2 (2c) that has been synthesized, isolated, and crystallized in the presence of AgOTf from a KOH free methodology.23 B
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
The signals labeled as (3a) in Figure 1 account for the following intermediate being formed in the reaction mixture from (2a). This complex displays four sets of signals in the 31 1 P{ H} NMR spectrum in D2O, located at 90.1, 75.9, 65.4, and 45.9 ppm (see the Experimental Section for δ and couplings in acetone-d6). Note that characteristic high-order splitting patterns have been correlated by selective irradiation experiments (see the Supporting Information). The 1H NMR (H2O; see the Experimental Section) allowed us to observe a key signal located at −11.1 ppm (dt, 2JH‑P = 105 and 60 Hz, for the trans and cis couplings, respectively) that is characteristic of a nickel−hydride complex. Once again, suitable crystals for single-crystal X-ray studies could be obtained only by addition of [(nBu4N)(PF6)] to yield compound [(dippe)(dippeO)Ni(H)](PF6) (3b). The ORTEP representation of complex (3b) is depicted in Figure 3. The Ni−H bond length was calculated to be 1.45 Å and was refined in agreement with previously reported nickel− hydride complexes. The molecular structure of (3b) revealed a Ni(II) in a distorted square-planar geometry that is distorted by the coordinated bulky phosphine mono-oxide. In fact, the Ni(1)−P(2) and Ni(1)−P(3) bonds are slightly longer than the Ni(1)−P(1) probably due to steric effects. As previously described for complex (2b), a solution of compound (3b) was evaporated to dryness under vacuum without formation of (4a). However, the unexchanged aqueous [(dippe)(dippeO)Ni(H)](OH) (3a) was transformed to [Ni(dippe)2] (4a) either readily under vacuum or slowly by just stirring at room temperature. In addition, dippeO2 was also identified as a byproduct after workup by direct comparison with the reported 31P{1H} NMR and also confirmed by its single-crystal XRD analysis (see the Supporting Information). A mechanistic proposal for this redox process is depicted in Scheme 3 considering the intermediates isolated described previously during the monitoring of the reaction and as a result of anion exchange crystallization. The proposal starts with the
Figure 2. ORTEP representation of complex (2b) with ellipsoids at the 50% probability level. All H atoms (except hydroxide H1) and PF6− anions are omitted for clarity. Key bond lengths (Å) and angles (deg): Ni(1)−P(1), 2.1534(10); Ni(1)−O(1), 1.900(3); O(1)−H(1), 0.84(2); P(1)−Ni(1)−P(2), 87.55(4); P(1)−Ni(1)−O(1), 97.08(8); O(1)−Ni(1)−O(1)#1, 78.36(12).
It is noteworthy that, when the aqueous solution of [(dippe)Ni(μ-OH)]2(OH)2 (2a) was evaporated to dryness, [Ni(dippe)2] (4a) was obtained as the major product. However, neither the anion exchanged complex [(dippe)Ni(μ-OH)]2(PF6)2 (2b) nor the synthesized [(dippe)Ni(μOH)]2(OTF)2 (2c) could lead to [Ni(dippe)2] (4a) when vacuum was applied. This provides evidence for the participation of the hydroxide anions in the outer Ni(II) coordination sphere in the phosphine oxidation.
Figure 3. ORTEP drawing for nickel−hydride complex (3b) with ellipsoids at the 40% probability level. All H atoms (except the hydride H1), PF6− anion, and solvent molecules are omitted for clarity. Key bond lengths (Å) and angles (deg): Ni(1)−H(1), 1.45(4); Ni(1)−P(1), 2.1357(16); Ni(1)−P(2), 2.2132(16); Ni(1)−P(3), 2.1896(16); P(4)−O(1), 1.487(5); P(1)−Ni(1)−P(2), 90.63(6); P(2)−Ni(1)−P(3), 111.67(6); P(1)− Ni(1)−P(3), 157.70(7); P(1)−Ni(1)−H(1), 76(3). C
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX
Organometallics
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Scheme 3. Mechanistic Proposal for the Formation of (4a)
anion exchange of (1a) and dimerization to yield (2a). Formation of (2e) probably involves an outer OH− nucleophilic attack to a coordinated phosphine in compound (2a), and then (2e) forms the hydride complex (3a) involving the PO bond formation and the hydroxo bridge dissociation and Ni(OH)2 formation. The evaporation to dryness or longer reaction times might account for H2 elimination, particularly for conversion of complex (3a) to yield (4a). Clearly, when a different anion such as PF6− or OTf− was used, release of H2 by reaction with an external OH− is not possible, nor is the formation of (4a). Applications of the Low-Valent Nickel Complex in the Presence of Water. Considering the formation of the Ni(0) complex (4a) in aqueous media, and also the involvement of some hydride intermediates, we decided to apply this knowledge for the coordination and activation of some selected substrates. It was anticipated that water would act as a hydrogen source based on previous reports from our group.24 Thus, as a proof of concept, hexafluorobutyne (HFB) was the first substrate to be evaluated in the presence of aqueousgenerated [Ni(dippe)2] (4a). The resulting Ni(0) complex [(dippe)Ni(HFB)] (5a) was characterized by multinuclear NMR and easily crystallized from hexane. In the solid state, complex (5a) exhibits a distorted trigonal-planar geometry with a coordinated carbon−carbon multiple bond length of 1.301 Å, as depicted in Figure 4. The use of complex (5a) in the presence of excess water promotes the alkyne reduction that generates the corresponding trans alkene isomer in a selective manner (Table 1). Further, water was demonstrated to be the hydrogen source by the use of D2O; consequently, deuterated products were obtained and characterized by a GC-MS analysis. As in our previous report,24 a time-dependent isomerization process was observed at short reaction times. The kinetic cis product was originally obtained, and it was then slowly transformed into the thermodynamic trans isomer (entries 1−3, Table 1).
Figure 4. ORTEP representation of [(dippe)Ni(HFB)] complex (5a) with ellipsoids at the 60% probability level. Key bond lengths (Å) and angles (deg): Ni(1)−P(1), 2.1536(8); Ni(1)−C(9), 1.869(3); C(9)− C(9′), 1.301(7); P(1)−Ni(1)−P(1′), 91.58(5); P(1)−Ni(1)−C(9), 113.88(11); C(8)−C(9)−C(9′), 139.80(19).
We also found that water concentration plays an important role. The use of stoichiometric amounts of water in THF allowed the hydrodefluorination of the cis-alkene under relatively mild reaction conditions. In fact, the isomerization process was a competing reaction vs the hydrodefluorination reaction; i.e., the more water was used, the more the isomerization products were generated (see Table 1, entries 1 and 4). Also, we speculate that the use of lower amounts of water avoids quenching of intermediates, ultimately leading to HDF. D
dx.doi.org/10.1021/om500767p | Organometallics XXXX, XXX, XXX−XXX
Organometallics
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Table 1. Tandem Hydrogenation/HDF Reaction of [(dippe)Ni(HFB)] with Watera
entry 1 2 3 4 5 a
time 6 2 7 5 3
h days days h days
H2O (eq)
(a)
(b)
(c)
(d)
conv.
300 300 300 5 5
33 97 77 11 8
61 1 13 48 0
6 2 9 41 59
0 0 0 0 33
28 48 97 27 51
Reaction mixtures prepared by using 3.5 mL of THF. All conversions were determined by GC-MS.
Chart 1. HDS Conversions of DBTa
a
Experiments were carried out in 3.5 mL of (1:1) H2O:THF (v:v) mixture. 24 h. Using [(dippe)NiCl2]:KOH:DBT:ADS molar proportions (1:2:1:2), respectively. All conversions were determined by GC-MS.
use of an assistant desulfurating agent (ADS) to thermodynamically enhance the HDS process. Chart 1 depicts the main results for HDS conversions at 70 °C.
Apparently, a sequential hydrodefluorination reaction occurs at longer reaction times using more stoichiometric amounts of water (entry 5, Table 1). This result differs from other methodologies that employ electrophilic main-group Lewis acids, where the partially defluorinated products (−CHF2, −CH2F) were not observed (see the Supporting Information).25 Regarding the regioselectivity for the HDF reaction, we speculate that the second −CF3 group acts as an electronwithdrawing group along with the weakening of C−F bonding in every successive HDF step (C−F bond energies: −CF3 > −CHF2 > −CH2F). On the other hand, another selected substrate was dibenzothiophene (DBT), whose reactivity in the presence of low-valent nickel complexes has been previously reported.20,26 However, herein, we first report the use of water as the hydrogen source in a metal-mediated process. The reaction was carried out under relatively mild conditions using complex (4a), yielding biphenyl as the main hydrodesulfurization (HDS) product and chromatographic conversions close to the 84% at 50 °C. Again, the use of D2O corroborated the incorporation of deuterium to yield dideuterobiphenyl. Efforts to perform this reaction with the use of the air-stable [(dippe)NiCl2] (1a) and KOH as basic media to generate in situ the hydride nickel complex (3a) were made. Unfortunately, low conversions were obtained with this method, even with the
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CONCLUSIONS In summary, the synthesis of the Ni(0) complex [Ni(dippe)2] has been achieved with the use of [(dippe)NiCl2] in aqueous basic media. Key intermediates for the nickel reduction were isolated and characterized by anion exchange, which demonstrated that KOH promotes phosphine oxidation. It was also observed that water can be used as a hydrogen source in metalmediated processes and promotes hydrodesulfurization, hydrodefluorination, and hydrogenation of unreactive substrates such as dibenzothiophene and hexafluorobutyne. HFB was hydrogenated and hydrodefluorinated when stoichiometric amounts of water were employed. The metal-mediated HDS process has been achieved using Ni(0) and a Ni(II) system in aqueous media. Efforts are underway in our group to achieve catalytic deep desulfurization in aqueous media.
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EXPERIMENTAL SECTION
General Considerations. Unless otherwise noted, all operations were carried out in an MBraun glovebox (
