Endohedral Plumbaspherenes of the Group 9 Metals: Synthesis, Structure and Properties of the [M@Pb12]3- (M=Co, Rh, Ir) Ions

The icosahedral [M@Pb-12](3-) (M=Co(1), Rh(2), Ir(3)) cluster ions were prepared from K4Pb9 and Co(dppe)Cl-2 (dppe=1,2-bis(diphenylphosphino)ethane)/[Rh(PPh3)(3)Cl]/[Ir(cod)Cl](2) (cod=1,5-cyclooctadiene), respectively, in the presence of 18-crown-6/ 2,2,2-cryptand in ethylenediamine/toluene solvent mixtures. The [K(2,2,2-cryptand)](+) salt of 1 and the [K(18-crown-6)](+) salt of 3 were characterized via X-ray crystallography; the ions 1 and 3 are isostructural and isoelectronic to the [Rh@Pb-12](3-) (2) ion as well as to the group 10 clusters [M '@Pb-12](2-) (M '=Ni, Pd, Pt). The ions are all 26-electron clusters with near perfect icosahedral I-h point symmetry. Clusters 1-3 show record downfield Pb-207 NMR chemical shifts due to sigma-aromaticity of the cluster framework. Calculated and observed Pb-207 NMR chemical shifts and Pb-207-M-x J-couplings (M-x=Co-59, Rh-103, Ir-193) are in excellent agreement and DFT analysis shows that the variations of Pb-207 NMR chemical shifts for the [M@Pb-12](2, 3-) ions (M=Co, Rh, Ir, Ni, Pd, Pt) are mainly governed by the perpendicularly oriented sigma(11) component of the chemical shift anisotropy tensor. The laser desorption ionization time-of-flight (LDI-TOF) mass spectra contain the molecular ions as well as several new gas phase clusters derived from the parents. The DFT-minimized structures of these ions are described.