Synthesis and Stereochemical/Electrochemical Analyses of Cuboctahedral-Based Pd23(CO)x(PR3)10 Clusters (x=20 with R3=Bun3, Me2Ph; x=20, 21, 22 with R3=Et3): Geometrically Analogous Pd23(PEt3)10 Fragments with Variable Carbonyl Ligations and Resulting Implications

This research was an outgrowth of previous reactions with [Pd13Ni13(CO)34]4− which produced a tetragonal crystal form of Pd23(CO)20(PEt3)10 (1) that has the same cuboctahedral-based Pd23 framework with an identical number of PEt3 ligands but two fewer CO ligands than the monoclinic crystal form of Pd23(CO)22(PEt3)10 (3) originally reported from reactions with Pd10(CO)12(PEt3)6. A subsequent investigation presented herein to establish whether the carbonyl capacity is influenced by the nature of the phosphine ligands has led to syntheses of Pd23(CO)x(PR3)10 [R3=Et3 (1), Bun3 (4), and Me2Ph (5)] with 20 CO ligands (x=20) from corresponding Pd10(CO)12(PR3)6 precursors either by deligation with Pd(OAc)2, CF3CO2H, Ni(1,5-COD)2, [NMe4]2[Ni6(CO)12], or HCO2H or by spontaneous enlargement; yields varied from 15 to 79%. Although attempts to obtain the original Pd23(CO)22(PEt3)10 (3) were unsuccessful, a highly significant outcome was the isolation (one time) of another monoclinic crystal form possessing the triethylphosphine Pd23(CO)x(PEt3)10 cluster with 21 COs (2). Both the compositions and atomic arrangements for each of five Pd23 clusters [1a (solvated); 1b (unsolvated); 2, 4, and 5] were unambiguously established from low-temperature single-crystal CCD X-ray crystallographic determinations in accordance with their nearly identical IR carbonyl frequencies. Solution 31P{1H} NMR spectra of 1 and 4 at room temperature displayed three distinct signals with expected integral ratios of 2/4/4 that are consistent with the solid-state structures of Pd23(CO)20(PR3)10 [R3=Et3 (1), Bun3 (4)] remaining intact in solution. The metal-core geometries of all of these Pd23(CO)x(PR3)10 clusters, including the thermodynamically stable ones with 20 CO ligands and the kinetic products with additional CO ligands (x=21, 22), are essentially the same. The common Pd23 core may be best described as possessing a centered hexacapped cuboctahedral Pd19 kernel (alternatively denoted as a centered ν2 Pd19 octahedron) with four edge-connected exopolyhedral wingtip Pd(exo) atoms that reduce the pseudo metal-core symmetry from Oh to D2h. The 10 PR3 ligands are linked to the six tetracapped Pd(cap) and four edge-capped wingtip Pd(exo) atoms; the latter four Pd(exo) atoms are each composed of four trigonal-planar Pd(μ2-CO)2(PR3) units. These crystallographic results provide compelling geometrical evidence for a heretofore unknown stereochemical example involving variable carbonyl ligation (x=20, 21, 22) of a close-packed nanosized Pdn(CO)x(PR3)y cluster (in this case with identical PEt3 ligands) without significant changes being induced in either the overall metal-core architecture or steric dispositions of the same number of PR3 ligands. These experimental findings have particular relevance to the long-standing Muetterties cluster/surface science analogy in showing that the different number (as well as different modes) of carbonyl ligations observed in these large metal carbonyl clusters are directly related to pressure-induced dissociative/nondissociative migratory coverages in CO chemisorptions on metal surfaces. The observed expanded capacity of CO coordination on the same Pd23 polyhedron without notable changes in geometry is no doubt a consequence of its virtually nanosized metal-core architecture; distances between outermost centrosymmetrically related pairs of Pd(cap) and Pd(exo) atoms in the Pd23 framework are 0.8 and 0.9 nm, respectively. An electrochemical (CV) study revealed that 1 undergoes one quasi-reversible two-electron reduction to 12− (E1/2=−0.91 V) and two consecutive quasi-reversible one-electron oxidations to 1/1+ at E1/2=0.08 V and 1+/12+ at E1/2=0.32 V (THF; Ag/AgCl as reference electrode). A stereochemical/electronic analysis with the isostructural Au2Pd21(CO)20(PEt3)10 analogue (9) and resulting implications are given.