Tripalladium-based tetrahedral structures

To prevent a potential anion metathesis between [Pd3]⁺X^- clusters and general salts Y⁺X’ -, a library of [Pd3]⁺X^- was previously synthesized varying their counter-anion (Table 9).

118 A. C. Tsipis, C. A. Tsipis, J. Am. Chem. Soc., 2005, 127, 10623-10638.

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Cluster Z X^- Yield

163 F- F3CSO3-

91%

149 F- SbF6-

90%

164 F- BF4-

85%

165 F- F3CCO2-

86%

166 H- SbF6

-92%

Table 9 : Synthesis tripalladium clusters by changing the silver salt.

We first started by performing reactions in dry THF at room temperature involving 163 and lithium triflate. Several experiments were realized by varying the molar ratio of the lithium salt (Scheme 35).

Scheme 35 : Synthesis of pyramidal [Pd3Li]²⁺.

After one hour of reaction and filtration through a celite pad to remove unsolubilized salts, we examined the products by ¹H NMR analyses (Figure 48)

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Figure 48 : ¹H NMR of reactions with X eq. of lithium triflate.

1H NMR showed that with one equivalent of lithium salt, the spectrum did not changed compared to the naked [Pd₃]⁺ 163 (Figure 48 blue spectrum). When 10 equivalents were added, the chemical shifts of H₂ and H₃ stayed unchanged but we observed a slightly shift of 0.01-0.02 ppm to the left of the spectrum for protons on the thiolate H₇ and H₈ (Figure 48 green spectrum). Surprisingly, with only four equivalents of lithium, we distinguished a double signal around thiolate chemical shifts that implies the presence of two species. One was the naked triangle and the other could have a new molecule, maybe the pyramidal cluster we were looking for. The same observation was made by treating compound 164 with an excess of LiBF4 and a shift of 0.02-0.04 ppm of the thiolate signals was identified while phosphine ones remind unchanged. We then turned to HRMS and X-ray analyses to get more information of this new compound. Unfortunately, HRMS analysis gave only the m/z isotopic pattern of the “naked” triangle 163. We thought that the ionization potential used in electrospray apparatus was strong enough to break the weak interaction between lithium ion and [Pd3]⁺ cluster. Sadly, X-ray analyses were as lackluster as HRMS. After many attempts, crystals revealed to be also the classical naked triangle and ¹H NMR of the crystals confirmed it did not showing anymore shifts presented in figure 48. As HRMS, the potential lithium-containing compound possibly degraded during the crystallization process.

Disappointed by the results with lithium, we kept the faith and we turned ourselves to more Lewis acidic compounds such as silver salts. We realized several reactions which turned out to be more fruitful than the previous ones (Table 10).

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Entry Cluster Silver salt Isolated yield

1 149 AgSbF6 76%

2 164 AgBF4 78%

3 165 AgO₂CCF₃ 72%

Table 10 : Synthesis of pyramidal [Pd3Ag]X2 clusters.

We were glad to observe the typical slight shifts of thiolate protons in ¹H NMR for the three presented attempts (0.01-0.11 ppm). ³¹P NMR presented shifts of 0.35 to 2.55 ppm to the lower field. The presence of a cationic atom perpendicularly to the trimetallic plan should disturb the electron density of the aromatic system. In addition to NMR analyses, HRMS measurements gave us a supplementary clue to confirm the presence of the pyramidal clusters (Figure 49).

Figure 49 : HRMS analysis of 167.

HRMS analysis of 167 showed two isotopic patterns which fit to tripalladium compounds. The one at m/z = 2042.6024 fitted with the calculated mass of 167. The other at m/z = 1698.8070 corresponded to the original triangle 149 that was probably afforded by the decomposition of the pyramid in the ionization source as we supposed for lithium

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117 experiments. Mass analysis for compounds 168 and 169 also matched the calculated mass with m/z = 1892.7208 and 1918.7033 respectively, despite the presence of the naked [Pd₃]⁺ cluster. With great luck, we were able to crystallize compounds 167 and 168 (Figure 50).

Figure 50 : X-ray pictures of pyramidal clusters 167 and 168. Hydrogens and substituent on sulfur and phosphorous atoms were omitted for clarity.

X-ray picture of 167 (Figure 50 left) showed that the silver atom is tetracoordinated. It has a [Pd3]⁺ that acts an ancillary ligand, two molecules of THF (crystallization solvent) and one molecule of water. The two non-coordinating anions lied far from the tetranuclear core.

Ag-Pd distances were between 2.824 and 2.853 Å. A slight distortion that breaks the perfect symmetry of the palladium triangle occurred. Two Pd-Pd distances are 2.899 Å and the third one is 2.917 Å, rendering it isosceles. The case of 168 (Figure 50 right) was a little bit different. The silver atom is also tetracoordinated but there was a participation of the counter-anion BF4

into the coordination. The palladium triangle is closer to the silver atom with Ag-Pd distances between 2.793 and 2.819 Å that formed a slightly distorted pyramid.

Interestingly in both of the cases, the silver atom adopts a tetrahedral conformation. Both complexes can be regarded as typical 18 electrons Ag(I) complexes with four ancillary ligands. Therefore, this observation suggests that the tripalladium complex plays the role of L-type ligand and give two electrons to the silver atom as drawn on figure 51. So far this new type of bonding mode has never been described. Several metallic tetrahedral structures have already been published, mostly using neutral partners. We did not find any analogies of present results in the literature. However, we found a single precedent that we associate to this bonding mode. By adding a cation on a 44 cve triplatinum monocationic triangle, Braunstein et al. concluded that their [Pt3]⁺ cluster is electron-rich enough to bind cationic metallic

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species without further explanation of this phenomenon. ¹¹⁹ Even if their triplatinum cluster was not perfectly symmetrical, they probably missed the metal-aromatic character which could adequately justify the cation-cation bonding.

Figure 51 : Two different representations of complex 167.

Encouraged by these probative results, we managed to test several other Lewis acids, such as FeCl₃, Fe(OTf)₂, Fe(OTf)₃, Sc(OTf)₃, Yb(OTf)₃, Bu₂Sn(OTf)₂, Hg(OTf)_2, Bi(OTf)₃, Dy(OTf)₃. In all cases reactions were tested with 4 and 8 equivalents. Unfortunatly, none of these triflates worked and the starting material was recovered.

We also tried the gold complex AuPPh₃SbF₆ with triangular [Pd₃]⁺ 149. This reaction led to a mixture of complexes in which the triphenylphosphine ligand of the gold complex was exchanged with the tri(4-fluorophenyl)phosphine of the tripalladium cluster. To overcome this problem, we tried complex 166 that possessed triphenylphosphine as ligand. Hopefully, the gold atom was coordinated by the triangle of palladium. No small shifts were detected by ¹H NMR however, but HRMS proved the formation the desired pyramid 170 with m/z = 2230.8426 as expected for a Pd3Au core(Figure 52).

119 C. Archambault, R. Bender, P. Braunstein, Y. Dusausoy, R. Welter, Dalton Trans., 2014, 43, 8609-8619.

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Figure 52 : HRMS of compound 170.

Compared to the Lewis acidic silver(I) and gold(I) salts, the introduction of less Lewis acidic copper was more difficult (Table 11).

Entry Cluster Copper salt Isolated yield

1 165 Cu(MeCN)4PF6 -

2 165 CF3CO2Cu -

3 165 (CF3CO2)2Cu -

4 165 CH₃CO₂Cu -

5 163 (CF₃SO₃Cu)₂•PhMe 75%

Table 11 : Attempts to synthetize pyramidal cluster incorporating copper.

We were a little bit frustrated when we saw that the palladium triangle did not coordinate any classical copper salt. The Lewis acidic character of these salt was not strong enough to be bound by [Pd3]⁺ 165. After some struggles, we found that commercially available (CF3SO3Cu)2•PhMe should have been a better Lewis acid compound. Fortunately, through the use of the reagent, the CF3SO3Cu fragment stacked [Pd3]⁺ cluster 163 affording pyramidal compound 171 in 72% yield.

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Figure 53 : HRMS analysis of compound 171.

As for pyramidal gold-containing cluster 170, no shifts of thiolate signals were observed in ¹H NMR but HRMS analysis provides us the correct mass of the desired tetrahedral compound (Figure 53).

2.4. Conclusions

During this work, we developed and optimized a new methodology in order to synthesize previously reported all-metal -aromatic [Pd3]⁺ clusters. This very convenient synthetic way uses commercially available reagents and affords desired clusters in good to excellent yields in one-pot, without chromatography columns. Moreover, it tolerated several other types of ligands (trialkylphosphine or alkyl disulfide) that the first method did not allow to use. This robust method could be extended to the synthesis of homo- and heteronuclear analogues which revealed to have a metal-aromatic character too. One of the most promising properties of all-metal aromaticity was the theoretical and peculiar property of tripalladium clusters to realize sort of cation -interactions mimicking regular donor ligand formed assembling main group elements. In this case, cation d-orbital-interaction seems a more appropriate definition of this bonding mode. Despite the unavoidable charge repulsion, we were the able to prove this ability by the identification of several metallic tetrahedral complexes [M3M’]²⁺.