Leonardo C.
Moraes
ab,
Rute C.
Figueiredo
ac,
Juan P.
Espinós
d,
Florencia
Vattier
d,
Antonio
Franconetti
e,
Carlos
Jaime
e,
Bertrand
Lacroix
fg,
Javier
Rojo
a,
Patricia
Lara
*ab and
Salvador
Conejero
*ab
aInstituto de Investigaciones Químicas (IIQ), CSIC – Universidad de Sevilla, C/Américo Vespucio 49, 41092, Seville, Spain. E-mail: sconejero@iiq.csic.es; patricia@iiq.csic.es
bCSIC and Universidad de Sevilla, Departamento de Química Inorgánica, Centro de Innovación en Química Avanzada (ORFEO-CINQA), Spain
cUniversidade Federal de Ouro Preto, Departamento de Química, Instituto de Ciências Exatas e Biológicas, Rua Costa Sena, 171, Centro, 35400-000, Ouro Preto, Minas Gerais, Brazil
dNanotechnology on Surfaces Laboratory, Instituto de Ciencia de Materiales de Sevilla (ICMS), CSIC – Universidad de Sevilla, c/Américo Vespucio 49, 41092 Sevilla, Spain
eDepartament de Química, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain
fDepartment of Materials Science and Metallurgical Engineering, and Inorganic Chemistry, University of Cádiz, Spain
gIMEYMAT, Institute of Research on Electron Microscopy and Materials of the University of Cádiz, Spain
First published on 17th March 2020
N-Heterocyclic Thiones (NHT) proved to be efficient ligands for the stabilization of small platinum nanoparticles (1.3–1.7 nm), synthesized by decomposition of [Pt(dba)2], under a H2 atmosphere, in the presence of variable sub-stoichiometric amounts of the NHT. Full characterization by means of TEM, HR-TEM, NMR, ICP, TGA and XPS have been carried out, providing information about the nature of the metal nanoparticles and the interaction of the NHT ligands to the metal surface. Importantly, DFT calculations indicate that some NHT ligands interact with the metal through the CC double bond of the imidazole fragment in addition to the sulfur atom, thus providing additional stabilization to the nanoparticles. According to XPS, TGA and ICP techniques, the surface coverage by the ligand increases by decreasing the size of the substituents on the nitrogen atom. The platinum nanoparticles have been used as catalyst in the hydroboration of alkynes. The most active system is that with a less covered surface area lacking an interaction of the ligand by means of the C
C double bond. This catalyst hydroborates alkynes with excellent selectivities towards the monoborylated anti-Markovnikov product (vinyl-boronate) when one equiv. of borane is used. Very interestingly, aliphatic alkynes undergo a second hydroborylation process leading to the corresponding 1,1- and 1,2-diboroylated species with good selectivities towards the former.
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Fig. 1 Top: Resonant structure of N-heterocyclic thiones (NHTs). Bottom: N-Heterocyclic thiones used in this work. |
In this contribution, we wish to report the synthesis of well-defined, small platinum nanoparticles stabilized by N-heterocyclic thiones, their characterization by different methods (including TEM, HR-TEM, XPS, NMR, ICP and TGA) and their use for the hydroboration of alkynes with hydroboranes. Additionally, DFT calculations have provided some clues about the coordination mode of the NHT to the surface of the NPs. We have found that some of these platinum nanoparticles are very efficient in the hydroboration of alkynes, leading to either mono- or diborylated species.
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Fig. 2 TEM image of platinum nanoparticles PtNP-NHTiPr synthesized from Pt(dba)2 and 0.5 equiv. of NHTiPr. |
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Fig. 3 TEM images and size distribution of platinum NPs: (a) PtNP- NHTC6,(b) PtNP- NHTC14,(c) PtNP-NHTMes and (d) PtNP-NHTC18. |
Finally, the very bulky NHTIPr is not able to stabilize the nanoparticles at any concentration of the ligand. As will be discussed below, it is likely that the bulkiness of this ligand precludes an efficient coordination to the metal surface making this capping ligand inefficient towards stabilization of the metal nanoparticles.
These Pt colloids are rather soluble in organic solvents such as THF and benzene, allowing recording their 1H NMR spectra. As expected from previous results on gold-NHT NPs,13 the very broad signals showed in these spectra are consistent with the presence of platinum nanoparticles stabilized with organic ligands (see ESI, Fig. S2, S5, S8, S11, S14†).19 The broadening of the signals is even more important when the proton atoms are closer to the surface of the metal nanoparticle, to the point that, in some cases, the signals can be lost in the base line. This seem to be the case for most of the colloids, for which no signals due to the back-bone of the imidazolyl fragment or the N–CH2 are clearly discernible in their proton NMR spectra. This is indicating that such fragments are likely in very close proximity to the surface of the metal nanoparticle (see below).
High Resolution Transmission Electron Microscopy (HR-TEM) provides evidence for the crystallinity of the platinum NPs, with interplanar distances characteristic of (111) and (002) planes in the face-centered cubic structure (Fig. 4(a)–(d)). The EDX spectra recorded across various NPs of sample PtNHTC6 using the scanning TEM (STEM) mode shows unequivocally three peaks at 2.1, 9.4 and 11.1 keV characteristic of platinum (besides the corresponding peaks of copper, carbon and Si arising from the grid and the specimen holder) (see Fig. 4(e)). The metal content was analyzed by Inductive Coupled Plasma (ICP) showing and array of values going from 31.65% (NHTC18) to 68.66% (NHTMes) fitting very well with the values obtained for the content of organic material by TGA (see ESI† for details).
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Fig. 4 HR-TEM images of platinum NPs: (a) and (b) PtNP-NHTMes, (c) and (d) PtNP-NHTC6. (e) EDX spectra recorded across various NPs and across the support only (for reference). |
X-ray Photoelectron Spectroscopy (XPS) provided evidence for the coordination of the NHT to the surface of the platinum nanoparticle. In all the cases, the intensities (signal areas) of the signals in the spectra have been normalized by the relative intensity of the N 1s peak. The spectral regions of the Pt 4f, N 1s and S 2p peaks are depicted in Fig. 5.
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Fig. 5 Detailed X-ray photoelectron spectral regions for the main signals of the present elements in the four studied samples: Pt 4f, N 1s and S 2p. |
The N 1s spectra for the four samples consist of a single, very symmetrical and narrow signal (fwhm = 1.7–1.9 eV) which indicates that a single chemical nitrogenated species are present in all the cases. The Pt 4f photoemission signal is a doublet, consisting of two energetic levels, Pt 4f7/2 and Pt 4f5/2, which are separated by 3.34 eV by electron spin–orbit interactions, with an intensity ratio of 0.75. For all the samples, the binding energy of the main peak, Pt 4f7/2, is in the range 71.3–70.9 eV, which are close to the binding energy of bulky metallic platinum (71.2 eV), and far away from the binding energy for Pt(II) or Pt(IV) compounds (in the range 73.6–76.3 eV).20 In this sense, it must be noted that the electronic structure of nanometric particles can be strongly affected by their particle size and by the interaction with the environment. In particular, the Pt 4f7/2 binding energy for nanometric Pt particles increases when the particle size decreases when supported on “inert” substrates (for instance, up to 0.75 eV when supported on graphite).21 Consequently, it can be assumed that platinum is present in metallic state for all the samples, while the small differences in their binding energy (≤0.46 eV) values could be caused by differences in the mean particle sizes or by interaction with the particular organic ligand. In the S 2p spectral region, an asymmetric signal is always found, consisting of a main peak at ∼162.2 ± 0.2 eV with a long tail at the high binding energy side. The intensity, width and shape of this tail slightly differing among the samples (see discussion below).
The relative intensities of the signals provide us with additional pieces of information concerning the surface composition of the samples (Table 1). Importantly, the intensities (N/S atomic ratios) of the global S 2p spectra are quite similar for all samples.
Pt4f | N | S | N/S ratio | Pt/S ratio | Pt/N ratio | |
---|---|---|---|---|---|---|
%At | %At | %At | ||||
PtNP-NHTiPr | 6.55 | 6.41 | 3.43 | 1.87 | 1.91 | 1.02 |
PtNP-NHTMes | 9.39 | 4.73 | 2.55 | 1.85 | 3.68 | 1.99 |
PtNP-NHTC6 | 7.82 | 4.94 | 2.50 | 1.98 | 3.13 | 1.58 |
PtN-PNHTC14 | 3.80 | 3.20 | 1.58 | 2.03 | 2.41 | 1.19 |
A remarkable feature is that the N/S ratios are close to 2, consistent with the composition of the NHT ligands. Additionally, the relative values of the Pt/N ratios range from 1.02 to 1.99. These differences might be related to two factors: the adsorption of the organic molecule and the mean size of the platinum nanoparticles. With respect to the former, higher coverage degree of the surface and higher packing density of the adsorbate molecules, lead to lower Pt/N ratio. Concerning the particle size, for Pt particles with diameters lower than 3 nm, (which is the Pt 4f photoelectrons analysis depth),22 the lower the mean size, the lower the Pt/N ratio (the Pt/N ratio will not be affected for particles diameters larger than 3 nm). Consequently, a lower Pt/N ratio could indicate a higher surface coverage of the Pt particles by the adsorbate or a smaller Pt particle diameter. However, since the mean sizes of the Pt nanoparticles, as determined by TEM, is in a very close range for all the studied samples, between 1.3 and 1.7 nm, it is reasonable to assume that the Pt/N atomic ratio is mainly determined by the adsorption coverage. Therefore, the Pt-NHTiPr are highly covered by the ligand (Pt/N ratio = 1.02) whereas Pt-NHTMes are less covered, in agreement with the higher steric hindrance exerted by the mesityl groups in comparison to the iso-propyl fragments. Nevertheless, the case for platinum nanoparticles PtNP-NHTC6 and PtNP-NHTC14 might be, in principle, counterintuitive. The longer length of the C14 side-chains should give rise to a more sterically hindered environment compared to the C6 alkyl chains. However, the Pt/N ratio observed (Table 1) suggest that the platinum nanoparticles have a higher content of NHTC14 ligands than the NHTC6. It is not entirely understood the reasons underlying to this effect, but one possible explanation could be that the NHTC14 ligands are more packed around the surface as a consequence of lateral packing interactions of the alkyl chains, whereas the alkyl arms in NHTC6 are less ordered, spreading randomly, leading to a less covered surface.23 In fact, according to theoretical calculations the interaction of two C14 chains is favored by ca. 5 kcal mol−1 (ΔEbind = −3.3 vs. −8.1 kcal mol−1 for C6/C6 and C14/C14, respectively), supporting a preference for a more ordered structure.
The S 2p signals have been deconvoluted and fitted with three components (three doublets as shown in Fig. 6; see Experimental section for details). The energy position and the width of the main component is well defined for the four samples; the peak S 2p3/2 is located at 161.8 ± 0.3 eV and its width is relatively low (fwhm = 1.9 ± 0.1 eV). However, the energy positions and widths of the second and third component are poorly defined due to instrumental noise: the peak S 2p3/2 is located at 164.3 ± 0.5 eV and at 166.9 ± 0.5 eV for the second and third component, respectively, while their widths are around 20% wider (fwhm = 2.25 ± 0.15 eV) than that of the main component. Nonetheless, the proportions of the three components vary slightly from one sample to another. The minor component, with a proportion around 10% (with a high binding energy), is very likely related to some that oxidized sulfur species (sulphates, sulphites, …), and it is a regular by-product in the study of adsorbed thiols on metal surfaces. It is worth comparing energy positions of free and coordinated thiones to that observed in the platinum nanoparticles reported herein. The S 2p signal from a free thiourea is located at ∼162.4 ± 0.1 eV, while coordination to metal cations (such as Co(II), Ni(II), Cu(I), Re(IV)) through its sulfur atom leads to small positive S 2p chemical shifts (between +0.3 and +0.6 eV).24 On the contrary, small negative chemical shifts can be expected for coordination of thiourea molecules to metal particles as previously reported on thiourea absorbed on polycrystalline platinum.25 Consequently, the main component in the S 2p spectra observed for PtNPs-NHT is due to chemisorbed molecules to platinum atoms.
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Fig. 6 Experimental and fitted S 2p X-ray photoelectron spectral region for the four studied samples PtNp-BHTC6, PtNP-NHTiPr, PtNP-NHTMes and PtNP-NHTC14. |
Regarding the model, we have considered the stabilization of one Pt nanoparticle (formed by 100 beads) and lineal NHTC14 (50 molecules, 12 beads each one). In addition, Pt nanoparticle has a computed diameter of 1.6 nm in the range of experimental findings. After the simulation time (900 ns), PtNP is surrounded by ca. 20 NHT molecules (Fig. 8a). Radial distribution function (RDF) analysis reveals that both S and N/CC moieties are located on the Pt surface although there is a subtle but clear preference for S beads, and therefore, thione groups. Nevertheless, the calculated distance from mPtNP is around 0.2 nm, in agreement with RS–Pt (and RC–Pt) distance aforementioned in DFT calculations. An analogous simulation has been carried out to study the stabilization of one PtNP by hindered NHTMes (50 molecules, 10 beads each one). Interestingly, this system is stabilized much earlier than NHTC14 along the simulation time (125 vs. 450 ns, respectively). Despite being stabilized faster, the nanoparticle surface is covered by only 10% of NHTMes molecules (Fig. 8b). In this case, the outcomes of the analysis of RD functions provide differences between distances of S and N/C
C beads. In fact, the S beads are on the Pt NP surface while beads for mesityl groups are further away. In addition, the calculated distances of N/C
C beads from mPtNP is even further away (around 1.07 nm; 0.27 nm from the surface) as expected for hindered NHTMes molecules.
Entry | Catalyst | Time (h) | Conversion (%) | Selectivity (2a![]() ![]() ![]() ![]() ![]() ![]() |
---|---|---|---|---|
a Reaction conditions: 0.35 mmol of phenylacetylene (1 M in C6D6), 0.36 mmol of HBpin, 5% PtNP-NHTX, 80 °C. b 20% of styrene is formed. c 5% of styrene is formed. d 15% of styrene is formed. e 14% of styrene is formed. | ||||
1 | PtNP-NHTiPr | 15 | 20 | 50![]() ![]() ![]() ![]() ![]() ![]() |
2 | PtNP-NHTMes | 4 | >99 | 79![]() ![]() ![]() ![]() ![]() ![]() |
3 | PtNP-NHTC6 | 19 | 34 | 72![]() ![]() ![]() ![]() ![]() ![]() |
4 | PtNP-NHTC14 | 20 | 44 | 66![]() ![]() ![]() ![]() ![]() ![]() |
5 | PtNP-NHTC18 | 20 | <5% | –c |
In all cases, both the mono- and di-borylated species 2a–5a have been detected in variable amounts. However, the best system in terms of conversion, selectivity and time appeared to be the colloid PtNP-NHTMes, for which full conversion is achieved in only 4 h with an excellent selectivity towards the monoborylated, anti-Markovnikov trans-isomer 2a (Table 2, entry 2). On the other hand colloids, PtNP-NHTiPr, PtNP-NHTC6 and PtNP-NHTC14 exhibited a rather similar behaviour, but conversions below 45% are observed after 15–20 h (Table 2, entries 1, 3 and 4). The platinum nanoparticles, PtNP-NHTC18 did not show any catalytic activity. A possible explanation for the different performance of the alkyl-substituted NHT colloids compared to the very active PtNP-NHTMes might be related to a different coordination of the NHT to the surface of the platinum nanoparticle. As described above, the N-heterocyclic thiones substituted with alkyl chains seems to bind the surface using both the sulfur atom and the CC double bond, with a flat disposition of the heterocyclic ring towards the metal surface. However, the approach of the C
C double bond of the mesityl derivative NHTMes is hampered by the aromatic rings, leading to coordination of the NHT exclusively through the S atom. This bonding situation might have two possible effects; first, the metal surface is more exposed to the incoming substrates, and second the NHT ligand is less effectively anchored to the surface facilitating its detachment to undergo exchange with the substrates. These effects together with the fact that, according to XPS (see above), the colloid PtNP-NHTMes is less covered by ligands, are likely responsible for the enhanced catalytic activity of PtNP-NHTMes in comparison to the other nanoparticles due to the presence of more active sites.
At this point it's worth mentioning that the morphology and size of the metal nanoparticles are retained after the catalytic process, as shown in the TEM image of Fig. 9 obtained from the content of the crude reaction mixture solutions after hydroboration of phenylacetylene with pinacolborane. This analysis revealed the presence of individual nanoparticles with a similar mean size than the starting material (1.7(0.3) nm; Fig. 9).
Once established the most efficient catalytic system, several reaction conditions were tested. The solvent, temperature and catalyst loadings were modified and the results are summarized in Table 3. The catalyst loading has an influence on the rate of the reaction but only marginally on the selectivity. On the other hand, lower catalyst loadings (1 and 2%, entries 5–8) lead to good selectivities towards the monoborylated product 2a, but slightly longer times are required to achieve full conversion. In this sense an increase of the reaction temperature to 80 °C is clearly beneficial, reducing about 2 h the reaction time (entries 5–7 vs. 6–8), and increasing slightly the monoborylated species 2a to levels even higher than those values found using 5% of catalyst loadings at the same temperature (see Table 2, entry 2). As will be discussed below, the better selectivity observed under lower catalyst loadings is likely related to changes in the active catalyst species during the course of the reaction.
Entry | Catalyst loading (%) | Solv. | T (°C) | Time (h) | Conv. (%) | Selectivity (2a![]() ![]() ![]() ![]() ![]() ![]() |
---|---|---|---|---|---|---|
a In all cases minor amounts of styrene (less than 2%) was observed. | ||||||
1 | 5.0 | C6D6 | 60 | 12 | >99 | 79![]() ![]() ![]() ![]() ![]() ![]() |
2 | 5.0 | C6D6 | 70 | 4 | >99 | 76![]() ![]() ![]() ![]() ![]() ![]() |
3 | 3.0 | C6D6 | 60 | 12 | >99 | 84![]() ![]() ![]() ![]() ![]() ![]() |
4 | 3.0 | C6D6 | 70 | 4 | >99 | 87![]() ![]() ![]() ![]() ![]() ![]() |
5 | 2.0 | C6D6 | 70 | 6 | >99 | 86![]() ![]() ![]() ![]() ![]() ![]() |
6 | 2.0 | C6D6 | 80 | 4 | >99 | 88![]() ![]() ![]() ![]() ![]() ![]() |
7 | 1.0 | C6D6 | 70 | 8 | >99 | 87![]() ![]() ![]() ![]() ![]() ![]() |
8 | 1.0 | C6D6 | 80 | 6.5 | >99 | 88![]() ![]() ![]() ![]() ![]() ![]() |
9 | 1.0 | THF d8 | 80 | 15 | 79 | 89![]() ![]() ![]() ![]() ![]() ![]() |
10 | 1.0 | CD3CN | 80 | 24 | >99 | 83![]() ![]() ![]() ![]() ![]() ![]() |
The effect of the solvent has been also explored with tetrahydrofurane-d8 or acetonitrile-d3 (Table 3, entries 9 and 10).
However, the results were in all cases poorer in terms of activity, selectivity or rate. Therefore, coordinating solvents such as THF or acetonitrile, inhibit to some extent the catalytic process according to their ligand binding properties.
The scope of the reaction has been explored by using, initially, terminal and internal aromatic alkynes bearing electron-donor and withdrawing groups under the optimized reaction conditions (1% cat., 80 °C, benzene-d6). The conversion and selectivity has been determined by means of NMR and/or GC-MS. The monoborylated products (E isomers in the case of terminal alkynes and Z for internal) have been obtained in all cases as the major product in reaction times going from 4 to 60 h (Table 4). The presence of electron–donor or withdrawing groups has an effect on the reaction rate but not significantly on the selectivity (small amounts of diborylated species were detected). In all cases the major product is the one arising from a syn addition of the borane in an anti-Markovnikov manner. The yield of products formed through the anti- or the Markovnikov additions is, typically, below 7%.
a Yields are given for the inseparable mixture of compounds 2–5. |
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The reaction using electron donor groups in the para position of the aromatic ring took place considerably faster than with electron-withdrawing fragments (Table 4, alkynes 1b,cvs.1d,e). Nevertheless, the effect of groups in the ortho position was very diverse and difficult to rationalize. For example, the presence of F and CF3 groups in the ortho position (Table 4, alkynes 1h,i) decreases and increases the rate of the reaction, respectively, in comparison with their para substituted counterparts (alkynes 1d,e). With respect to the selectivity of the process, the ortho substituents slightly favour the amount of the monoborylated products. The borylation process has been extended to both aromatic and aliphatic internal alkynes. Besides obtaining full conversion in all cases, the reactions were considerably faster than those of the terminal alkynes, taking place in as short as 1 h (Table 4, alkynes 1j–l). Although excellent selectivities are obtained for diphenylacetylene and 3-hexyne, the asymmetrical 1-phenyl-1-propyne generates mixtures of isomers 2l and 2l′ in a 1:
3.7 ratio together with a poorer selectivity with respect to the diborylated isomer (Scheme 3).
At this stage, it is important to remark that the amount of the diborylated species formed in the process, generated in principle from the monoborylated derivatives, is not increased when an excess (2 or more equiv.) of pinacolborane is used, even after prolonged periods of heating (see Table S3 in ESI†). Likewise, increasing the catalyst loading or the temperature of the reaction has nearly no effect in the mono-/diborylated ratio.
As for aromatic systems, terminal aliphatic alkynes can be efficiently hydroborated, but as inferred from the data shown in Table 5, the selectivity of the process is slightly poorer, in which the diborylated species are produced in amounts ranging from 4 to 13% of the total amount. In fact, at variance to the results obtained for aromatic alkynes, when the reaction is carried out with an excess (3 equiv.) of pinacolborane, full conversion to the diborylated derivatives is produced, generated as mixtures of the 1,2 and 1,1dihydroboration reaction (Scheme 4 and Table 6). The ratio of these isomers depends strongly on the bulkiness of the alkyl moiety at the 3 position of the alkyne. Thus, the less hindered systems (Table 6, alkynes 1p–r) lead to an approximately 2:
1 ratio in favour to the 1,1 disubstituted alkylborane, whereas the presence of a bulkier group such as cyclopropyl, cyclohexyl or iso-propyl, increase the selectivity up to 100% (alkynes 1m–o). Interestingly, under these reaction conditions (excess of borane) no 1,2-diborylated species is formed for the cyclopropylacetylene 1m, in stark contrast with the reaction of this alkyne with 1.2 equiv. of borane, a reaction that produces small amounts of the 1,2-diborylated derivative 5m with no signs of 6m being detected. We do not have an explanation to this contradictory result, but a possible explanation might be found in a different active surface catalyst, when the reaction is carried out in the presence of an excess of pinacolborane, or in a different active catalyst that is only present at early stages of the reaction. This latter assumption might also explain the formation of small amounts of 1,2-diborylated species in the reaction of aromatic alkynes with pinacolborane that, as stated above, do not increase with either time or in the presence of an excess of the borane.
a Yields are given for the inseparable mixture of compounds 2–5. |
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a Yields are given for the inseparable mixture of compounds 5 and 6. |
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Although the hydroboration of alkynes leading to vinylboronates (monoborylation) has been previously studied using NHC-stabilized Pt nanoparticles,10c this is the first time that diborylation products have been obtained using platinum nanoparticles.
Footnote |
† Electronic supplementary information (ESI) available: Synthesis and characterization of PtNP-NHTs. Catalytic procedures. Spectroscopic data of boron compounds. Computational methods. See DOI: 10.1039/d0nr00251h |
This journal is © The Royal Society of Chemistry 2020 |