Nonosized (μ12-Pt)Pd164-xPtx(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, ((mu 12)-Pt)Pd164-xPt(CO)(72)(PPh3)(20) (x approximate to 7), 1, that replaces the geometrically related capped three-shell icosahedral Pd-145 cluster, Pd-145(CO)(chi-)(PEt3)(30) (x approximate to 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 stereochernical 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 Pd-10(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/m3) site symmetry, which is the highest crystallographic subgroup of the noncrystallographic pseudo-icosahedral /(n) (2/m35) symmetry. The "full" four-shell Pd-Pt anatomy of 1 consists of: (a) shell 1 with the centered ((mu 12)-Pt) atom encapsulated by the 12-atom icosahedral PtxPd12-x cage, x = 1.2(3); (b) shell 2 with the 42-atom nu(2) icosahedral Pt-chi Pd42-x cage, x = 3.5(5); (c) shell 3 with the anti-Mackay 60-atom semi-regular rhombicosidodecahedral PtxPd60-x cage, x = 2.2(6); (d) shell 4 with the 50-atom nu(2) pentagonal dodecahedral Pd-50 cage. The total number of crystallographically estimated Pt atoms, 8 3, which was obtained from least-squares (Pt-x/Pd1-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 (PtxPd1-x)-analysis for shells 1-3 originated from the microanalytical results; otherwise, the presumed metal-core composition would have been (mu(12)-Pt)Pd-164. [Alternatively, the (mu(12)-Pt)M-164 core-geometry of 1 may be viewed as a pseudo-/(eta) Pt-centered six-shell successive v, polyhedral system, each with radially equivalent vertex atoms: Pt@M-12(icosahedron)@M30-(icosidodecahedron)@M-12(icosahedron)@M-60(rhombicosidodecahedron)@M-30(icosidodecahedron)@M20- (pentagonal dodecahedron)]. Completely surprising structural dissimilarities between I 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 nu(2) 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 thePd-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.