Nanosized {Pd₄(μ₄‑C)}Pd₃₂(CO)₂₈(PMe₃)₁₄ Containing Tetrahedrally Deformed Pd₄ Cage with Encapsulated Carbide Atom: Formal Substitution of Geometrically Analogous Interior Au₄ Entity in Isostructural Au₄Pd₃₂(CO)₂₈(PMe₃)₁₄ by Electronically Equivalent Pd₄(μ₄‑C) and Computational/Catalytic Implications

This first homopalladium carbido cluster, {Pd₄(μ₄-C)}­Pd₃₂(CO)₂₈(PMe₃)₁₄ (<b>1</b>), was isolated (3–7% yields) from an ultimately simplified procedurethe reaction of CHCl₃ under N₂ with either Pd₈(CO)₈­(PMe₃)₇ or Pd₁₀(CO)₁₂­(PMe₃)₆ at room temperature. Charge-coupled device (CCD) X-ray diffraction data at 100 K for <b>1</b>·2.5 C₆H₁₄ (<b>1a</b>) and <b>1</b>·3 CHCl₃ (<b>1b</b>) produced closely related molecular parameters for <b>1</b>. This {Pd₄C}­Pd₃₂ cluster (<b>1</b>) possesses a highly unusual tetracoordinated carbide atom that causes a major distortion of a central regular Pd₄ tetrahedron into a new symmetry type of encapsulated Pd₄ cage of pseudo-<i>D</i>₂ (222) symmetry. Mean Pd–Pd distances for the three pairs of opposite twofold-equivalent Pd–Pd tetrahedral-like edges for <b>1a</b> are 2.71, 2.96, and 3.59 Å; the mean of the four Pd–C distances [range, 1.87(2)–1.94(2) Å] is 1.91 Å. An astonishing molecular feature is that this {Pd₄C}­Pd₃₂ cluster (<b>1</b>) is an isostructural and electronically equivalent analogue of the nanosized Au₄Pd₃₂(CO)₂₈­(PMe₃)₁₄ (<b>2</b>). Cluster <b>2</b>, likewise a pseudo-<i>D</i>₂ molecule, contains a geometrically analogous tetrahedrally deformed interior Au₄ entity encapsulated within an identical Pd₃₂(CO)₂₈(PMe₃)₁₄ shell; mean distances for the three corresponding symmetry-equivalent pairs of slightly smaller opposite tetrahedral-distorted Au–Au edges are 2.64, 2.90, and 3.51 Å. A computational study by both a natural population analysis (NPA) and an atoms-in-molecules (AIM) method performed on model analogues {Pd₄C}­Pd₃₂­(CO)₂₈(PH₃)₁₄ (<b>1-mod</b>) and Au₄Pd₃₂­(CO)₂₈(PH₃)₁₄ (<b>2-mod</b>) suggested that the negatively charged Au₄ entity in <b>2-mod</b> may be described as two weakly interacting electron-pair Au₂ intradimers. In contrast, an NPA of the {Pd₄C} entity in <b>1-mod</b> revealed that two similarly oriented identical Pd₂ intradimers of 2.71 Å are primarily stabilized by Pd–C bonding with a negatively charged carbide atom. The isostructural stabilizations of <b>1</b> and <b>2</b> are then attributed to the similar sizes, shapes, and overall negative charge distributions of the electronically equivalent interior {Pd₄C} and Au₄ entities. This resulting remarkable structural/electronic equivalency between <b>1</b> and <b>2</b> is consistent with the greatly improved performances of commercial palladium catalysts for vinyl acetate synthesis by gold-atom incorporation to suppress carbonization of the Pd atoms, namely, that the extra Au 6s¹ valence electron of each added Au atom provides an effective “negative charge protection” against electron-donating carbon atoms forming Pd carbido species such as {Pd₄C}

Nanosized {Pd₄(μ₄‑C)}Pd₃₂(CO)₂₈(PMe₃)₁₄ Containing Tetrahedrally Deformed Pd₄ Cage with Encapsulated Carbide Atom: Formal Substitution of Geometrically Analogous Interior Au₄ Entity in Isostructural Au₄Pd₃₂(CO)₂₈(PMe₃)₁₄ by Electronically Equivalent Pd₄(μ₄‑C) and Computational/Catalytic Implications

This first homopalladium carbido cluster, {Pd₄(μ₄-C)}­Pd₃₂(CO)₂₈(PMe₃)₁₄ (<b>1</b>), was isolated (3–7% yields) from an ultimately simplified procedurethe reaction of CHCl₃ under N₂ with either Pd₈(CO)₈­(PMe₃)₇ or Pd₁₀(CO)₁₂­(PMe₃)₆ at room temperature. Charge-coupled device (CCD) X-ray diffraction data at 100 K for <b>1</b>·2.5 C₆H₁₄ (<b>1a</b>) and <b>1</b>·3 CHCl₃ (<b>1b</b>) produced closely related molecular parameters for <b>1</b>. This {Pd₄C}­Pd₃₂ cluster (<b>1</b>) possesses a highly unusual tetracoordinated carbide atom that causes a major distortion of a central regular Pd₄ tetrahedron into a new symmetry type of encapsulated Pd₄ cage of pseudo-<i>D</i>₂ (222) symmetry. Mean Pd–Pd distances for the three pairs of opposite twofold-equivalent Pd–Pd tetrahedral-like edges for <b>1a</b> are 2.71, 2.96, and 3.59 Å; the mean of the four Pd–C distances [range, 1.87(2)–1.94(2) Å] is 1.91 Å. An astonishing molecular feature is that this {Pd₄C}­Pd₃₂ cluster (<b>1</b>) is an isostructural and electronically equivalent analogue of the nanosized Au₄Pd₃₂(CO)₂₈­(PMe₃)₁₄ (<b>2</b>). Cluster <b>2</b>, likewise a pseudo-<i>D</i>₂ molecule, contains a geometrically analogous tetrahedrally deformed interior Au₄ entity encapsulated within an identical Pd₃₂(CO)₂₈(PMe₃)₁₄ shell; mean distances for the three corresponding symmetry-equivalent pairs of slightly smaller opposite tetrahedral-distorted Au–Au edges are 2.64, 2.90, and 3.51 Å. A computational study by both a natural population analysis (NPA) and an atoms-in-molecules (AIM) method performed on model analogues {Pd₄C}­Pd₃₂­(CO)₂₈(PH₃)₁₄ (<b>1-mod</b>) and Au₄Pd₃₂­(CO)₂₈(PH₃)₁₄ (<b>2-mod</b>) suggested that the negatively charged Au₄ entity in <b>2-mod</b> may be described as two weakly interacting electron-pair Au₂ intradimers. In contrast, an NPA of the {Pd₄C} entity in <b>1-mod</b> revealed that two similarly oriented identical Pd₂ intradimers of 2.71 Å are primarily stabilized by Pd–C bonding with a negatively charged carbide atom. The isostructural stabilizations of <b>1</b> and <b>2</b> are then attributed to the similar sizes, shapes, and overall negative charge distributions of the electronically equivalent interior {Pd₄C} and Au₄ entities. This resulting remarkable structural/electronic equivalency between <b>1</b> and <b>2</b> is consistent with the greatly improved performances of commercial palladium catalysts for vinyl acetate synthesis by gold-atom incorporation to suppress carbonization of the Pd atoms, namely, that the extra Au 6s¹ valence electron of each added Au atom provides an effective “negative charge protection” against electron-donating carbon atoms forming Pd carbido species such as {Pd₄C}

High-Yield Synthesis of PPh₃-Ligated Decanuclear Tl–Pd Cluster, Pd₉[Tl(acac)](CO)₉(PPh₃)₆: Comparative Analysis of Tl(I)–Pd(0) Bonding Connectivities with Known Tl–Pd Clusters and Resulting Insight Concerning Their Dissimilar Dynamic Solution Behavior

The new Tl­(I)–Pd(0) cluster Pd₉[μ_3/3-Tl­(acac)]­(μ₂-CO)₆(μ₃-CO)₃(PPh₃)₆ (<b>1</b>) was prepared in high yields (over 90%), both by reaction of Pd₁₀(CO)₁₂(PPh₃)₆ (<b>4</b>), PPh₃, and TlPF₆ in THF in the presence of acetylacetone (Hacac) and base (NEt₃) and by direct reaction of Pd₁₀(CO)₁₂(PPh₃)₆ with PPh₃ and Tl­(acac). The composition and molecular structure of <b>1</b> were unambiguously established from 100 K CCD X-ray diffractometry studies of two solvated crystals, <b>1</b>·1.5Hacac·0.5THF (<b>1A</b>) and <b>1</b>·0.3THF (<b>1B</b>), which showed essentially identical geometries for the entire Pd₉Tl­(CO)₉P₆ fragment of pseudo-<i>C</i>_3<i>v</i> symmetry; its composition is in agreement with X-ray Tl/Pd field-emission microanalysis with a scanning electron microscope for crystals of <b>1B</b>. This cluster can be viewed as a markedl<i></i>y deformed Pd₆ octahedron (oct) with the three Pd­(oct) atoms of one of its eight triangular faces connected both by three edge-bridging wingtip (wt) Pd­(μ₂-CO)₂PPh₃ fragments and by a symmetrical capping Tl­(I). Three triply bridging carbonyl ligands asymmetrically cap the lower alternate 3-fold-related triangular faces of the Pd₆ octahedron, and the three other PPh₃ ligands are each coordinated to Pd atoms in the geometrically opposite staggered Pd­(oct)₃ face. The 6s²5d¹⁰ Tl­(I) is also equivalently attached to both chelating O atoms of a bidentate acetylacetonate (acac) monoanion. Although the <i>C</i>₂ axis of the pseudo-<i>C</i>_2<i>v</i> planar Tl­(acac) fragment is approximately parallel to the pseudo-<i>C</i>₃ axis of the TlPd₉ core, the orientation of the Tl­(acac) plane relative to the octahedral-based Pd₉ geometry is considerably different for each of the three independent nondisordered molecules of <b>1</b> in <b>1A</b> and <b>1B</b>; these different planar Tl­(acac) orientations may be mainly attributed to anisotropic crystal-packing effects. Coordination of the Tl­(I) atom to the three Pd­(oct) atoms of the Pd₉ core presumably occurs via its so-called “inert” 6s² electron pair with resulting three short Tl–Pd­(oct) connectivities of mean distance 2.83 Å; these connectivities together with three longer Tl–Pd­(wt) ones of mean distance 3.15 Å give rise to a (crown-like)­Pd₆ sextuple (μ_3/3-Tl) coordination mode. Of particular stereochemical interest is a comparison of solution behavior of <b>1</b> with that for the known structurally related analogue, Pd₉[μ₃-TlCo­(CO)₃L]­(μ₂-CO)₆(μ₃-CO)₃L₆ (<b>2</b>) (with L = PEt₃ instead of PPh₃). In <b>2</b> the Tl­(I) is alternatively attached to a trigonal-bipyramidal Co­(CO)₃L monoanion and primarily coordinated to the three inner Pd­(oct) atoms of a similar PR₃/CO-ligated octahedron; corresponding Tl–Pd­(oct) and Tl–Pd­(wt) mean distances for two independent molecules in <b>2</b> are 2.77 and 3.31 Å, respectively. Variable-temperature ³¹P­{¹H} NMR solution data of <b>1</b> indicate the occurrence of presumed fast wobbling-like motion of the [μ_3/3-Tl­(acac)] entity about the pseudo-<i>C</i>₃ axis of the Pd₉(μ₂-CO)₆(μ₃-CO)₃P₆ fragment <i>without Pd–Tl detachment</i> (i.e., the entire cluster of <b>1</b> remains intact). In direct contrast, corresponding temperature-dependent ³¹P and ¹³C NMR data of <b>2</b> instead are consistent with <i>rapid, reversible dissociation/association of the entire</i> [μ₃-TlCo­(CO)₃L] ligand from the analogous Pd₉(μ₂-CO)₆(μ₃-CO)₃P₆ fragment of <b>2</b>. This highly dissimilar dynamic solution behavior that points to a stronger Tl­(I) attachment to the Pd₉ core in <b>1</b> than that in <b>2</b> may be attributed from the above crystallographic evidence to greater involvement of the outer three wingtip Pd­(wt) atoms in bonding connectivities to the Tl­(I) in <b>1</b> compared to predominant bonding connectivities of only the three inner Pd­(oct) atoms to the Tl­(I) in <b>2</b>. ¹H NMR solution spectra of <b>1</b> also suggest significant covalent character in the bidentate Tl–O­(acac) bonding in <b>1</b> based upon the observation of H­(acac)–Tl coupling; this premise is consistent with its Tl–O distances of 2.35 Å (av) being ca. 0.2 Å shorter than those of 2.52 Å (av) found in crystalline Tl­(acac), which with no observed H–Tl NMR coupling in solution implies ionicity of its bidentate Tl–O bonding. Both <b>1</b> and <b>2</b> conform to an 86 CVE count expected for an octahedral metal polyhedron based upon the Tl­(I) and each wingtip Pd­(μ₂-CO)₂L fragment contributing 2 and 4 CVEs, respectively

Isolation and Structural Characterization of a Mackay 55-Metal-Atom Two-Shell Icosahedron of Pseudo‑<i>I</i>_<i>h</i> Symmetry, Pd₅₅L₁₂(μ₃‑CO)₂₀ (L = PR₃, R = Isopropyl): Comparative Analysis with Interior Two-Shell Icosahedral Geometries in Capped Three-Shell Pd₁₄₅, Pt-Centered Four-Shell Pd–Pt M₁₆₅, and Four-Shell Au₁₃₃ Nanoclusters

We present the first successful isolation and crystallographic characterization of a Mackay 55-metal-atom two-shell icosa­hedron, Pd₅₅L₁₂­(μ₃-CO)₂₀ (L = PPrⁱ₃) (<b>1</b>). Its two-shell icosa­hedron of pseudo-<i>I</i>_<i>h</i> symmetry (without isopropyl substituents) enables a structural/bonding comparison with <i>interior</i> 55-metal-atom two-shell icosa­hedral geometries observed within the multi-shell capped 145-metal-atom three-shell Pd₁₄₅­(CO)₇₂­(PEt₃)₃₀ and 165-metal-atom four-shell Pt-centered (μ₁₂-Pt)­Pd_{164‑<i>x</i>}Pt_<i>x</i>­(CO)₇₂­(PPh₃)₂₀ (<i>x</i> ≈ 7) nanoclusters, and within the recently reported four-shell Au₁₃₃(SC₆H₄-<i>p</i>-Bu^t)₅₂ nanocluster. DFT calculations carried out on a Pd₅₅­(CO)₂₀(PH₃)₁₂ model analogue, with triisopropyl phosphine substituents replaced by H atoms, revealed a positive +0.84 <i>e</i> charge for the entire Pd₅₅ core, with a highly positive second-shell Pd₄₂ surface of +1.93 <i>e</i>