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-D_3h) and bent (pseudo-C_2v) geometries, respectively; the pseudo-D_3h 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-T_d 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.