Nanosized Au4Pd32(CO)28(PMe3)14 Containing a Highly Distorted Encapsulated Au4 Tetrahedron: Proposed Multi-Twinned Growth-Pattern from Two Deformed Au-Centered Double Icosahedral-Based Fragments*

Abstract

As part of an extensive effort to synthesize a variety of nanosized gold–palladium carbonyl phosphine clusters, the neutral Au4Pd32(CO)28(PMe3)14 (1) was isolated and unambiguously characterized by low-temperature CCD X-ray diffraction and IR measurements. This nanosized Au4Pd32 cluster was prepared in low yields (<5%) from the room-temperature reaction of Pd10(CO)12(PMe3)6 (2) with Au(SMe2)Cl in THF/acetone. The heretofore unknown molecular geometry of 1 of pseudo-D2 (222) symmetry (without methyl substituents) may be viewed to arise from a relatively strong (Au–Au)-bonded linkage (2.64 A (av)) of two pentagonal-bipyramidal (μ5-Au)(μ5-Pd)Pd5 polyhedra; this generated 14-atom Au2Pd12 unit may be considered as a markedly deformed part of a 19-atom Au-centered double icosahedron without the inner pentagon (corresponding to five missing inner atoms). In turn, two Au2Pd12 units form a central composite-twinned Au4Pd22 kernel via vertex-fusion of two common Pd atoms along with additional formation of four Pd–Pd bonding, four Au–Pd bonding, and two weaker secondary Au–Au bonding interactions at 2.90 A (av) (versus the other two diagonal Au–Au nonbonding ones at 3.51 A (av)); this resulting Au4Pd22P8 kernel is augmented by the addition of two triangular Pd3P core-fragments and four exopolyhedral PdP groups to give the Au4Pd32P14 framework of 1. This cluster is stabilized by 28 bridging COs, of which 20 are doubly bridging and 8 triply bridging. The largest metal-core diameter of 1 along one pseudo C2 axis is 1.1 nm. This new type of multi-twinned metal cluster has direct relevance to both ligated and non-ligated (naked) non-crystalline metal nanoparticles, many of which possess multiple twinning and/or disorder.