High-nuclearity palladium carbonyl trimethylphosphine clusters containing unprecedented face-condensed icosahedral-based transition-metal core geometries: proposed growth patterns from a centered Pd13 icosahedron†

Abstract

The preparation, isolation, and structural/bonding characterization of four high-nuclearity neutral homopalladium clusters, Pd16(CO)13(PMe3)9 1, Pd35(CO)23(PMe3)15 2, Pd39(CO)23(PMe3)16 3 and Pd59(CO)32(PMe3)21 4, and a minor bimetallic product, Pd29Ni3(CO)22(PMe3)13 5, are given. The four homopalladium clusters were characterized by CCD X-ray crystallographic determinations, elemental analyses, IR, multinuclear NMR, and cyclic voltammetry; because 5 was obtained in very low yields, both its molecular geometry and composition were established from the X-ray crystallographic analysis. These five clusters, obtained from reactions of a ccp Pd–Ni carbonyl cluster precursor and PMe3 (with or without acetic acid), exhibit five different types (four unprecedented) of centered icosahedral-based transition-metal frameworks: (1) the Pd16 core in 1 possesses a centered Pd13 icosahedron. (2) The Pd35 and Pd39 cores in 2 and 3 each have a face-fused centered Pd23 biicosahedron with linear (pseudo-D3h) and bent (pseudo-C2v) geometries, respectively; the pseudo-D3h central Pd29 polyhedron of the Pd35 core in 2 approximately conforms to five interpenetrating centered icosahedra. (3) The crystallographic-D3 (32) Pd59 core in 4 has two centered Pd13 icosahedra that are indirectly connected viatrans double face-sharing with an inner face-fused Pd9 bioctahedron; the entire nanosized face-condensed Pd59 core has 11 interior Pd(i) atoms. (4) The Pd29Ni3 core in 5 contains a pseudo-Td central Pd26 polyhedron comprised of four interpenetrating centered icosahedra. The existence of these highly condensed icosahedral-based metal carbonyl clusters, found only for Pd but not for the other eight Group 8–10 transition metals, may be ascribed to Pd metal having the weakest metal–metal bonding (i.e., smallest cohesive energy). Electronic closed-shell stabilization of each of these clusters is indicated by electron-counting condensation rules giving calculated values in exact agreement with observed electron counts for the metal cores in 1, 2, 4, and 5 (i.e., irregular condensations prevent a reliable electron count in 3). Proposed growth sequences provide logical pathways in the formation of the central palladium fragments in 2, 3, 4 and 5 from the centered Pd13 icosahedral fragment in 1.