Pd8(PDip)6: Cubic, Unsaturated, Zerovalent

Atomically precise nanoclusters hold promise for supramolecular assembly and (opto)electronic‐ as well as magnetic materials. Herein, this work reports that treating palladium(0) precursors with a triphosphirane affords strongly colored Pd8(PDip)6 that is fully characterized by mass spectrometry, heteronuclear and Cross‐Polarization Magic‐Angle Spinning (CP‐MAS) NMR‐, infrared (IR), UV–vis, and X‐ray photoelectron (XP) spectroscopies, single‐crystal X‐Ray diffraction (sc‐XRD), mass spectrometry, and cyclovoltammetry (CV). This coordinatively unsaturated 104‐electron Pd(0) cluster features a cubic Pd8‐core, µ4‐capping phosphinidene ligands, and is air‐stable. Quantum chemical calculations provide insight to the cluster's electronic structure and suggest 5s/4d orbital mixing as well as minor Pd─P covalency. Trapping experiments reveal that cluster growth proceeds via insertion of Pd(0) into the triphosphirane. The unsaturated cluster senses ethylene and binds isocyanides, which triggers the rearrangement to a tetrahedral structure with a reduced frontier orbital energy gap. These experiments demonstrate facile cluster manipulation and highlight non‐destructive cluster rearrangement as is required for supramolecular assembly.


Introduction
Cuboid structures are esthetically pleasing for their high symmetry.Symmetry is also important in self-assembly to atomically precise nanoclusters. [1]Such electronic confinement on the molecular level promises access to new materials for (opto-)electronic devices, catalysts, magnets, and sensors. [2]Hitherto, the field focused on clusters of the p-block [3] and heterometallic derivatives, [4] bio-mimetic manganese cubanes relevant for water-splitting, [5] ironsulfide cubanoids present in cytochromes and nitrogenases, [6] as well as the coinage metals [7] with a specific attention for gold. [8]Gold clusters are commonly synthesized through reduction of Au(I) or Au(III) precursors, and hence feature with rare exceptions [9] an oxidation state >0 and coordinative saturation. [9]Conversely, achieving coordinative unsaturation is desirable, as it provides a handle for controlled post-functionalization, site-differentiation, and the tuning of electronic properties. [10]tomically precise palladium clusters have so far attracted less attention than their gold counterparts.In fact, only a few examples beyond Pd 3 are known, [11] despite their emerging relevance for bond activation and electronic materials. [12]aralleling gold, most pallada-clusters feature oxidation states >0, are obtained in low yields, and are coordinatively saturated.Whereas Pd 8 -wires, [13] nanosheets, [14] and body-centered [Pd 8 B(PPh 2 ) 3 (PPh 3 ) 2 (SXyl) 6 )] [15] as well as [Pd 9 As 6 (PPh 3 ) 8 ] [16] (Figure 1a) are known, and notwithstanding computational prediction by Saillard and Halet, [17] a Pd 8 -cube remains elusive.Notably, compound a was obtained following earlier work on nickel by Dahl and Fenske, namely the isolation of cubic [Ni 8 (PPh) 6 (CO) 8 ] (Figure 1b), [Ni 8 (PPh) 6 (PPh 3 ) 4 ], and [Ni 8 (PiPr) 6 (PMe 3 ) 6 ]. [18] Palladium cluster a as well as the related Ni 8 -compounds are stabilized by strongly coordinating and thus difficult-to-replace carbonyl or phosphine ligands.This is because they were synthesized, analogously to the common procedures for gold, in the presence of trapping ligands.
Herewith, we report the isolation, characterization, and derivatization of a cubic Pd 8 cluster (1).Nanocluster 1 is obtained through the redox-neutral reaction of Pd(0) precursors with a phosphorus(I) compound, namely a triphosphirane, [19] in the absence of trapping ligands.Opposed to previous synthetic strategies, this approach provides high yields and access to coordinative unsaturation.Detailed spectroscopic investigations in combination with quantum-chemical calculations reveal zerovalency for the palladium-atoms and 5s/4d mixing.Post-synthetic derivatization is demonstrated through the reaction with an isocyanide.

Results and Discussion
Heating two equivalents of tris(dibenzylideneacetone)palladium, Pd 2 (dba) 3 , with one equivalent of the triphosphirane (PDip) 3 , [19a] (Dip = 2,6-iPr 2 C 6 H 3 ) in benzene to reflux for 1 day gave a mixture of two phosphorus-containing products in an approximate ratio of 3:1 according to quantitative 31 P-(Figure S6, Supporting Information) and 1 H NMR spectroscopy (Figure S7, Supporting Information) using an internal standard.The 31 P signal of the byproduct at  = −102 ppm splits into a doublet ( 1 J P-H = 231.6Hz) without 1 H-decoupling, indicating a PH moiety that likely derives from intramolecular CH insertion of the in situ formed phosphinidene PDip.The major product 1 ( = +533 ppm) could be isolated analytically pure in 72% yield after workup (Scheme 1).High-resolution mass spectrometry (HRMS; Figure S11, Supporting Information) identified 1 as [Pd 8 (PDip) 6 ].
Single-crystals of 1 suitable for single-crystal X-Ray diffraction (sc-XRD) were obtained from benzene solutions (Figure 2).They confirmed a cubic Pd 8 -cluster, which is capped by six μ 4phosphinidene ligands (Figure 2).Compound 1 crystallized in the trigonal space group R " 3 comprising a threefold axis of symmetry and an inversion center.The S 6 symmetry in the solidstate is due to the orientation of the Dip ligands (Figures S29 and S33, Supporting Information), and the Pd 8 cube features almost equidistant Pd─Pd atoms with 2.6997(6) and 2.6866(6) Å.These values are consistent with body-centered Pd 9 cube a Scheme 1. Synthesis of cluster 1 using low-valent P(I)-and Pd(0) precursors.(2.71 and 2.68 Å), and slightly smaller than found in the facecentered cubic structure of the metal (2.75 Å). [20] Palladium μ 4 -phosphinidenes remained hitherto unknown, yet the Pd─P bonds (2.332(2) to 2.358(2) Å) are elongated compared to the nickel complex b (2.20 and 2.22 Å).
Opposed to coordinatively semi-saturated [Ni 8 (PPh) 6 (PPh 3 ) 4 ], which has been described as "exceedingly air-sensitive," unsaturated 1 is thermally and chemically robust.It features a decomposition point of 291 °C, is air-stable in the solid state, and persists [24] in benzene-solutions on moist air over days.Quantum-chemical calculations at the r 2 SCAN-3c and ZORA-DF/def2-TZVPP levels of theory (DF: PBE, TPSSh, PBE0) were conducted to elucidate the electronic structure of 1.17b] The Electron Localization Function (ELF; Figures S44 and S45, Table S8, Supporting Information) [25] indicates a high degree of electron delocalization within the Pd 8 cube and thus parallels in principle elemental metals (ELF value  = 0.2 for the Pd─Pd bond critical point (BCP)), but the maxima are localized exclusively at the metal sites.This suggests, in combination with the significant Pd─ligand interaction ( = 0.4; Pd─P BCP), that the μ 4 -coordinating phosphinidene ligands are crucial for the cluster's stability.
Also, the shapes of pertinent canonical Kohn-Sham molecular orbitals (MOs, Figure 4; see Figure S38, Supporting Information for the complete MO diagram) such as the HOMO−41 (t 2g ), HOMO−11 (t 2u ) and HOMO−3 (e g ) support efficient electron delocalization.The Pd─P covalency and the 5s character in these orbitals is substantial. [26]This is illustrated by the HOMO−41, where two Pd-atoms feature c(5s) = 0.35, and where the Pd:P contribution amounts to 3:1 (Löwdin's Population Analysis).The latter mixing is in line with the XPS measurements (vide supra), which substantiate a marginally higher physical oxidation state than for Pd-metal.
Cluster 1 also forms when replacing Pd 2 (dba) 3 by an exceedingly reactive palladium(0) precursor, namely an iminofunctionalized cyclic(alkyl)(amino)carbene ( fun CAAC) [27] supported complex ( fun CAAC)Pd(py). [28]This ligand with a hemilabile imino-group supports terminal palladium nitrenes, [29] which are isoelectronic with phosphinidenes.Treating ( fun CAAC)Pd(py) with 0.25, 0.33 or 1 equivalents of (PDip) 3 in perdeutero-benzene led to the instantaneous release of the free fun CAAC and quantitative conversion to various intermediates, which converted at 60 °C to 1 (Figure 5; Figure S14, Supporting Information).The dissociation of the fun CAAC, which is only metastable in free form, [27b] is likely due to steric effects. [30]When running the reaction at room temperature in pyridine, single crystals suitable for SC-XRD analysis formed in situ and identified compound 2 [(PDip) 3 Pd(py) 2 ] (Figure 5, left) as pyridine-trapped intermediate en route to 1.The same is true for single crystals of 3 [(PDip) 5 Pd 2 (py) 2 ] (Figure 5, right), which formed in situ at room temperature in toluene.
We conclude that, instead of phosphinidene transfer, the palladium(0) precursor initially inserts into one P─P bond of (PDip) 3 , which is followed by dissociation of the CAAC to give 2. The subsequent cluster growth to 3 presumably [31] involves the association of two monomers of 2 under formal elimination of PDip.Intrigued by the coordinative unsaturation of 1, we tested its stability in the presence of common reagents in organometallic chemistry.Exposure to dihydrogen in an in situ NMR experiment with heating to 60 °C did not lead to signs of decomposition, which further highlights the robustness of 1.In the presence of ethylene, a weak interaction was evidenced by a shift of the signal in the 31 P NMR spectrum from 533 to 537 ppm (Figure S24, Supporting Information) and slightly shifted cluster signals in the 1 H NMR spectroscopic analysis (Figure S25, Supporting Information).Corroborating the weak interaction, ethylene is removed quantitatively in vacuo, thereby cleanly regenerating 1.
Probing the potential for material chemistry and supramolecular assembly, cluster 1 was treated with ortho-xylyl isocyanide (CNoXyl).Indeed, sequential decoration by either two-, three-, or four (Figures S26 and S27, Supporting Information) isocyanide ligands was achieved.Tetracoordinate [Pd 8 (PDip) 6 (CNoXyl) 4 ] 4 was isolated analytically pure.Vapor diffusion of pentane into a saturated solution in benzene afforded single crystals suitable for sc-XRD experiments, which contained two molecules of co-crystallized benzene and one molecule of pentane.The isocyanide ligands distort the Pd 8 -cube toward a triakistetrahedron by pulling out four palladium atoms, whereupon the other four palladium atoms approach each other.The coordination geometry may then be understood as a pair of two dual Pd 4 tetrahedra, whereof the corners of the larger one (4.20 Å versus 3.46 Å edge length) are coordinated by the isocyanides (Figure 6, bottom).Both the closest Pd─Pd (average: 2.74 Å) and Pd─P (average: 2.36 Å) distances (Figure S32, Supporting Information) increase slightly with respect to nanocluster 1.In line with the elongated bonds, the ELF analysis suggests enhanced electron delocalization within the Pd 8 cube (Figures S46 and S47, Table S8, Supporting Information).The T 2 C─N stretching vibration of the isocyanide ligands appears at 2078 cm −1 in the infrared (IR) spectrum (Figure S22, Supporting Information), which is consistent with literature values for palladium(0), but not with palladium(II) complexes, where more than 2200 cm −1 are expected. [32]s anticipated, 4 absorbs light even stronger than 1, and the maxima in the UV-vis spectrum (Figure S23, Supporting Information) are redshifted to 297 ( = 173 500 m −1 cm −1 ) and 431 nm ( = 88 800 m −1 cm −1 ), with tailing beyond 760 nm.The isocyanide ligands lower the optical HOMO-LUMO energy gap by about 0.33 eV, thus demonstrating that the decoration of the vacant metal-sites allows not only for derivatization, yet also the modification of the cluster's electronic properties.Eventually, we explored the reaction of Pd 2 (dba) 3 with (PDip) 3 in the presence of xylyl isocyanide, but found that 4 was obtained only in traces (Figure S21, Supporting Information).This observation further highlights the synthetic value of coordinatively unsaturated 1.

Conclusion
In summary, treating palladium(0) precursor Pd 2 (dba) 3 with the triphosphirane (PDip) 3 affords the nanocluster [Pd 8 (PDip) 6 ] (1) in 72% yield.The cubic Pd 8 core is thereby stabilized by μ 4capping phosphinidene ligands.Intermediates 2 and 3 en route to 1 were trapped as pyridine adducts and provide insights into the cluster growth mechanism.Nanocluster 1 is coordinatively unsaturated, as its synthesis proceeds in the absence of trapping agents, and features an unusual valence electron count of 104.It is strongly colored, and the broad electronic absorption spectrum parallels the ones of palladium nanoparticles, as it extends to the near infrared region and features a band ≈390 nm.Despite of its zerovalent and low-coordinate nature, yet validating predictions by Saillard and Halet, [17b] nanocluster 1 is thermally stable up to almost 300 °C, and chemically robust in the presence of moisture and dioxygen.Cluster 1 weakly coordinates ethylene and binds isocyanides.Isocyanide binding lowers the HOMO-LUMO energy gap more than 0.3 eV and triggers the structural rearrangement to tetrahedral [Pd 8 (PDip) 6 (CN o Xyl) 4 ] (4), which cannot be obtained directly from the precursor molecules.

Figure 5 .
Figure 5. Reactivity of ( fun CAAC)Pd(py) towards (PDip) 3 and molecular structures of 2 (left) and 3 (right) in the solid state with Dip-groups rendered as wireframes, H-atoms and disordered parts omitted for clarity.See Figures S30 and S31, Supporting Information for further details.

Figure 6 .
Figure 6.Molecular structure of 4 in the solid state (top).Dip-and oXyl substituents are given in wireframes, H-atoms and co-crystallized solvent molecules are omitted for clarity, thermal ellipsoids are displayed at 50% probability, structural parameters are given in Figure S32, Supporting Information.Illustration of the cluster's polyhedron from two different angles (bottom; c.f. Figure S33, Supporting Information).