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.

Abstract

This first homopalladium carbido cluster, {Pd4(μ4-C)}Pd32(CO)28(PMe3)14 (1), 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 1·2.5 C6H14 (1a) and 1·3 CHCl3 (1b) produced closely related molecular parameters for 1. This {Pd4C}Pd32 cluster (1) 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-D2 (222) symmetry. Mean Pd-Pd distances for the three pairs of opposite twofold-equivalent Pd-Pd tetrahedral-like edges for 1a 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 (1) is an isostructural and electronically equivalent analogue of the nanosized Au4Pd32(CO)28(PMe3)14 (2). Cluster 2, likewise a pseudo-D2 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 (1-mod) and Au4Pd32(CO)28(PH3)14 (2-mod) suggested that the negatively charged Au4 entity in 2-mod may be described as two weakly interacting electron-pair Au2 intradimers. In contrast, an NPA of the {Pd4C} entity in 1-mod 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 1 and 2 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 1 and 2 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 6s(1) 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}.