Nanosized Au4Pd28(CO)22(PMe3)16 Containing 32-Atom Au4Pd28 Core-Geometry with Nearly Regular Encapsulated Au4 Tetrahedron: Proposed Multitwinned-Composite Growth-Pattern Involving Four Interpenetrating Au-Centered Cuboctahedral-Based Three-Layer (Pd3)A(AuPd6)B(Au3)C Polyhedra

Abstract

Our synthetic exploratory efforts to obtain new nanosized Au–Pd carbonyl/phosphine clusters by use of the trimethylphosphine precursor Pd10(CO)12(PMe3)6 with smaller-sized PMe3 ligands (versus precursors with relatively larger PEt3 ligands) have previously produced via reaction with Au(SMe2)Cl an unusual Au4Pd32(CO)28(PMe3)14 (2) containing a pseudo-D2 36-atom Au4Pd32 core-geometry with a highly distorted encapsulated Au4 tetrahedron. Herein we report a striking illustration that analogous precursors under different reaction conditions have given rise to another new type of Au–Pd cluster, Au4Pd28(CO)22(PMe3)16 (1), that not only has a completely dissimilar Au4Pd28 core-geometry with a nearly regular encapsulated Au4 tetrahedron but also a totally different postulated multitwinned-composite growth-pattern. This extraordinary cluster, which was obtained from the reaction of Pd10(CO)12(PMe3)6 with Au(PPh3)Cl or Au(SMe2)Cl (estimated yield, ∼20–40%), has a 32-atom Au4Pd28 framework that roughly conforms to cubic T (23) symmetry that is maintained by inclusion of the 16 PMe3 P atoms but is completely reduced to general C1 (1) symmetry by inclusion of the 22 bridging CO ligands. 1 was isolated under different crystallization conditions to give two solvated crystal forms: namely, 1a as diisopropyl-solvated triclinic ( $$\hbox{P}\bar 1 $$ with Z = 2), and 1b as THF/hexane-solvated monoclinic (P2/c with Z = 4). A comparative analysis of resulting low-temperature CCD X-ray diffractometry determinations revealed an amazing molecular similarity between the actual shapes of the highly deformed Au4Pd28 architectures and ligand connectivities of 1 within the two dissimilar crystal structures. These results clearly indicate that the large observed localized geometrical distortions of 1 are primarily induced by intracluster strain-releasing effects and not by crystal-packing interactions. We propose a multitwinned growth-pattern of its Au4Pd28 core involving the formation of a Au4Pd24 composite-twinned framework formed from four markedly deformed interpenetrating three-layer Au-centered (Pd3)A(Au(n)Pd6)B(Au3)C cuboctahedra (n = 1–4) that are oriented along the four localized threefold axes of the Au4 tetrahedron. The other four outermost (external) Pd atoms that are tetrahedrally disposed about the Au4Pd24 composite presumably provide stabilization by face-condensations (i.e., three tetracapped, one tricapped). This new type of multitwinned bimetallic cluster has direct relevance to both ligated and non-ligated (bare) non-crystalline metal nanoparticles, of which many have been postulated to be multitwinned.