Synthesis and Stereochemical/Electrochemical Analyses of Cuboctahedral-Based Pd23(CO) x (PR3)10 Clusters (x=20 with R3=Bu n 3, Me2Ph; x=20, 21, 22 with R3=Et3): Geometrically Analogous Pd23(PEt3)10 Fragments with Variable Carbonyl Ligations and Resulting Implications

摘要

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), Bu n 3 (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), Bu n 3 (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 Pd n (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 dissociativeondissociative 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 1 2− (E1/2=−0.91 V) and two consecutive quasi-reversible one-electron oxidations to 1/1 + at E1/2=0.08 V and 1 +/1 2+ 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.