Nanosized (μ₁₂-Pt)Pd_164-xPt_x(CO)₇₂(PPh₃)₂₀ (x ≈ 7) Containing Pt-Centered Four-Shell 165-Atom Pd−Pt Core with Unprecedented Intershell Bridging Carbonyl Ligands: Comparative Analysis of Icosahedral Shell-Growth Patterns with Geometrically Related Pd₁₄₅(CO)_x(PEt₃)₃₀ (x ≈ 60) Containing Capped Three-Shell Pd₁₄₅ Core

Presented herein are the preparation and crystallographic/microanalytical/magnetic/spectroscopic characterization of the Pt-centered four-shell 165-atom Pd−Pt cluster, (μ₁₂-Pt)Pd_164-xPt_x(CO)₇₂(PPh₃)₂₀ (x ≈ 7), 1, that replaces the geometrically related capped three-shell icosahedral Pd₁₄₅ cluster, Pd₁₄₅(CO)_x(PEt₃)₃₀ (x ≈ 60), 2, as the largest crystallographically determined discrete transition metal cluster with direct metal−metal bonding. A detailed comparison of their shell-growth patterns gives rise to important stereochemical implications concerning completely unexpected structural dissimilarities as well as similarities and provides new insight concerning possible synthetic approaches for generation of multi-shell metal clusters. 1 was reproducibly prepared in small yields (<10%) from the reaction of Pd₁₀(CO)₁₂(PPh₃)₆ with Pt(CO)₂(PPh₃)₂. Its 165-atom metal-core geometry and 20 PPh₃ and 72 CO ligands were established from a low-temperature (100 K) CCD X-ray diffraction study. The well-determined crystal structure is attributed largely to 1 possessing cubic T_h (2/m3̄) site symmetry, which is the highest crystallographic subgroup of the noncrystallographic pseudo-icosahedral I_h (2/m3̄5̄) symmetry. The “full” four-shell Pd−Pt anatomy of 1 consists of: (a) shell 1 with the centered (μ₁₂-Pt) atom encapsulated by the 12-atom icosahedral Pt_xPd_12-x cage, x = 1.2(3); (b) shell 2 with the 42-atom ν₂ icosahedral Pt_xPd_42-x cage, x = 3.5(5); (c) shell 3 with the anti-Mackay 60-atom semi-regular rhombicosidodecahedral Pt_xPd_60-x cage, x = 2.2(6); (d) shell 4 with the 50-atom ν₂ pentagonal dodecahedral Pd₅₀ cage. The total number of crystallographically estimated Pt atoms, 8 ± 3, which was obtained from least-squares (Pt_x/Pd_1-x)-occupancy analysis of the X-ray data that conclusively revealed the central atom to be pure Pt (occupancy factor, x = 1.00(3)), is fortuitously in agreement with that of 7.6(7) found from an X-ray Pt/Pd microanalysis (WDS spectrometer) on three crystals of 1. Our utilization of this site-occupancy (Pt_xPd_1-x)-analysis for shells 1−3 originated from the microanalytical results; otherwise, the presumed metal-core composition would have been (μ₁₂-Pt)Pd₁₆₄. [Alternatively, the (μ₁₂-Pt)M₁₆₄ core-geometry of 1 may be viewed as a pseudo-I_h Pt-centered six-shell successive ν₁ polyhedral system, each with radially equivalent vertex atoms: Pt@M₁₂(icosahedron)@M₃₀(icosidodecahedron)@M₁₂(icosahedron)@M₆₀(rhombicosidodecahedron)@M₃₀(icosidodecahedron)@M₂₀(pentagonal dodecahedron)]. Completely surprising structural dissimilarities between 1 and 2 are: (1) to date 1 is only reproducibly isolated as a heterometallic Pd−Pt cluster with a central Pt instead of Pd atom; (2) the 50 atoms comprising the outer fourth ν₂ pentagonal dodecahedral shell in 1 are less than the 60 atoms of the inner third shell in 1, in contradistinction to shell-by-shell growth processes in all other known shell-based structures; (3) the 10 fewer PR₃ ligands in 1 necessitate larger bulky PPh₃ ligands to protect the Pd−Pt core-geometry; (4) the 72 CO ligands consist of six bridging COs within each of the 12 pentagons in shell 4 that are coordinated to intershell metal atoms. SQUID magnetometry measurements showed a single-crystal sample of 1 to be diamagnetic over the entire temperature range of 10−300 K.