Nanosized Au2Pd41(CO)27(PEt3)15 containing two geometrically unprecedented 13-coordinated Au-centered (μ13-Au)Pd13 polyhedra connected by triangular face-sharing and three interpenetrating 12-coordinated Pd-centered (μ12-Pd)Au2Pd10 icosahedra: geometrical change in centered polyhedra induced by Au/Pd electronegativity-mismatch

Abstract

The synthesis, isolation, and stereochemical characterization of Au₂Pd₄₁(CO)₂₇(PEt₃)₁₅ (1) are described. This nanosized Au₂Pd₄₁ cluster (maximum metal-core diameter, 1.04 nm) was originally obtained with Au₂Pd₂₁(CO)₂₀(PEt₃)₁₀ as low-yield by-products together with Pd₁₄₅(CO)_x(PEt₃)₃₀ (x ∼ 60) from the reaction of Pd(PEt₃)₂Cl₂ and Au(PPh₃)Cl in DMF with NaOH under CO atmosphere. The subsequent preparation of Au₂Pd₂₁(CO)₂₀(PEt₃)₁₀ in greatly improved yields (preceding article) thereby provided the starting material that led to the isolation of 1 in reasonable yields (54%) from an overnight refluxing of the preformed Au₂Pd₂₁ cluster in THF under N₂. Both the composition (subsequently ascertained from elemental analysis) and molecular geometry of 1 were unequivocally established from a low-temperature CCD X-ray diffraction study, which revealed a cubic unit cell of P2₁3 symmetry with four molecules of 1 and four co-crystallized triphenylphosphine oxide molecules each lying on a crystallographic three-fold axis. The entire Au₂Pd₄₁ core of pseudo-C_3h symmetry may be viewed as a central Au₂Pd₂₉ fragment of pseudo-D_3h symmetry composed of two heretofore geometrically unknown 13-coordinated Au-centered (μ₁₃-Au)Pd₁₃ polyhedra that share a common internal Pd(i)₃ triangular face perpendicular to the C₃ principal axis and of three three-fold-related interpenetrating 12-coordinated Pd-centered (μ₁₂-Pd)Au₂Pd₁₀ icosahedra. A comparative analysis of this central Au₂Pd₂₉ fragment in 1 with an internal Au(i)₂Pd(i)₃ trigonal bipyramid vs. the corresponding central Pd₂₉ fragment in the known homopalladium Pd₃₅(CO)₂₃(PMe₃)₁₅ (2) with an internal Pd(i)₅ trigonal bipyramid resulting from five interpenetrating 12-coordinated Pd-centered [(μ₁₂-Pd)Pd₁₂] icosahedra is particularly illuminating; it provides a striking illustration of the remarkable observed difference between Pd- vs. Au-centered polyhedra which is attributed to a large electronegativity-mismatch in radial bonding interactions that occurs upon replacement of the Pd-centered atom with a highly electronegative Au-centered atom. The entire Au₂Pd₄₁ core-geometry is obtained by additional face-condensations of 12 tetracapping Pd(cap) atoms. This cluster is stabilized by 15 PEt₃ ligands and 27 doubly- and triply-bridging CO ligands. A close geometrical resemblance between the three three-fold-related Au₂Pd₁₄ moities within the Au₂Pd₄₁ core in 1 and the entire Au₂Pd₁₄ core in the known [Au₂Pd₁₄(CO)₉(PMe₃)₁₁]²⁺ dication (3) is observed; resulting stereochemical implications are given.