Synthesis and stereochemical/electrochemical analyses of cuboctahedral-based Pd-23(CO)(x)(PR3)(10) clusters (x=20 with R-3 = Bu-3(n), Me2Ph; x=20, 21, 22 with R-3 = Et-3): Geometrically analogous Pd-23(PEt3)(10) fragments with variable carbonyl ligat

摘要

This research was,in outgrowth of previous reactions with [Pd13Ni13(CO)(34)](4-) which produced a tetragonal crystal form of Pd-23(CO)(20)(PEt3)(10) (1) that has the same cuboctahedral-based Pd-23 framework with an identical number of PEt3 ligands but two fewer CO ligands than the monoclinic crystal form of Pd-23(CO)(22)(PEt3)(10) (3) originally reported from reactions with Pd-10(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 Pd-23(CO)(x)(PR3)(10) [R-3 = Et-3 (1), Bu-3(n) (4), and Me2Ph (5)] with 20 CO ligands (x=20) from corresponding Pd-10(CO)(12)(PR3)(6) precursors either by deligation with Pd(OAc)(2), CF3CO2H, Ni(1,5-COD)(2), [NMe4](2)[Ni-6(CO)(12)], Or HCO2H or by spontaneous enlargement; yields varied from 15 to 79%. Although attempts to obtain the original Pd-23(CO)(22)(PEt3)(10) (3) were unsuccessful, a highly significant outcome Was the isolation (one time) of another monoclinic crystal form possessing the triethylphosphine Pd-23(CO)(x)(PEt3)(10) cluster with 21 COs (2). Both the compositions and atomic arrangements for each of five Pd-23 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 P-31{H-1) 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 Pd-23(CO)(20)(PR3)(10) [R-3=Et-3 (1), Bu-3(n) (4)] remaining intact in solution. The metal-core geometries of all of these Pd-23(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 Pd-19 kernel (alternatively denoted as a centered v(2) Pd-19 octahedron) with four edge-connected exopolyhedral wingtip Pd(exo) atoms that reduce the pseudo metal-core symmetry from O-h to D-2h. 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(mu(2)-CO)(2)(PR3) units.