Nanosized Pd_(37)(co)_(28){p(p-tolyl)_3}_(12) Containing Geometrically Unprecedented Central 23-atom Interpenetratingrntri-icosahedral Palladium Kernel Of Double Icosahedral Units:rnits Postulated Metal-core Evolution And Resultingrnstereochemical Implications

摘要

Pd_(37)(CO)_(28){P(p-Tolyl)_3}_(12) (1) was obtained in ～50% yield by the short-time thermolysis of Pd_(10)(CO)_(12){P(p-Tolyl)_3}_6 in THF solution followed by crystallization via layering with hexane under N_2. The low-temperature (100 K) CCD X-ray diffraction study of 1 revealed an unusual non-spheroidal Pd_(37)-atom polyhedron, which may be readily envisioned to originate via the initial formation of a heretofore non-isolated central Pd_(23) kernel composed of three interpenetrating trigonal-planar double icosahedra (Dl) that are oriented along the three bonding edges of its interior Pd_3 triangle. This central Pd_(23) kernel is augmented by face condensations with two additional phosphorus-free and 12 tri(p-C_6H_4Me)phosphine-ligated Pd atoms, which lower the pseudo-symmetry of the resulting 37-atom metal core from D_(3h) to C_2. The 12 P atoms and 28 bridging CO connectivities preserve the pseudo-C_2 symmetry. The central Pd_(23) kernel in 1 provides the only crystallographic example of the 23-atom member of the double icosahedral family of "twinned" interpenetrating icosahedra (II), which includes the 19-atom two II (1 Dl), the 23-atom three II (3 Dl), the 26-atom four II (6 Dl), and the 29-atom five II (9 Dl). The n-atoms of these Dl models coincide exactly with prominent atom-peak maxima of 19, 23, 26, and 29, respectively, in the mass spectrum of charged argon clusters formed in a low-temperature free-jet expansion. The only previous crystallographically proven 26-and 29-atom Dl members are the central pseudo-T_d tetrahedral Pd_(26) kernel (4 II, 6 Dl) in the PMe_3-ligated Pd_(29)Ni_3(CO)_(22)(PMe_3)_(13) (2) and the central pseudo-D_3h) trigonal-bipyramidal Pd_(29) kernel (5 II, 9 Dl) in the PMe_3-ligated Pd_(35)(CO)_(23)(PMe_3)_(15) (3). Two highly important major stereochemical implications are noted:rn(1) The formation of geometrically identical idealized architectures for these three II palladium kernels with corresponding Dl models constructed for the charged argon clusters provides compelling evidence that the nature of delocalized Pd-Pd bonding in these II (and presumably other nanosized) Pd clusters, in which each zerovalent Pd atom individually has a closed-subshell 4d~(10) ground state, may likewise (as in argon clusters) be viewed primarily in terms of (considerably stronger) attractive dispersion interactions.rn(2) The existence of the 23-atom II Pd_(23) kernel in 1 provides an essential heretofore "missing" geometrical link as an intermediate in the same sequential growth pathway to give the 26- and 29-atom II Pd_n kernels found in 2 and 3, respectively. Accommodation of the 12 bulky P(p-Tolyl)_3 ligands around the entire 37-atom palladium core necessitates an extended metal surface that originates from the pseudo-2D trigonal-planar Pd_(23) kernel found in 1. The much smaller PMe_3 ligands in 2 and 3 would sterically allow further sequential transformations of an initially formed 23-atom II intermediate palladium kernel into the 26-atom spheroidal II palladium kernel in 2 or further into the 29-atom semi-spheroidal II palladium kernel in 3, but with smaller total metal-atom nuclearities of 32 and 35, respectively.